ABSTRACT
Title of Thesis: RADIATION EFFECTS IN DIAMOND
SUBSTRATES AND TRANSISTORS
Aayush Thapa
Master of Science, 2019
Thesis directed by: Professor Aris Christou
Department of Materials Science & Engineering
Diamond and diamond devices are potentially radiation damage resistant due
to diamond?s wide bandgap and high displacement energy per atom. Conduct-
ing channels in diamond?necessary for the realization of field effect transistors
(FETs)?are based on hydrogen-terminated surfaces or buried implanted acceptors
(delta doping). The present thesis investigates the susceptibility of these chan-
nels to either ionizing or non-ionizing radiation. Gamma radiation tolerances of
H-terminated based FETs at low (?100 krads) and high doses up to 26 Mrads, and
proton radiation tolerance of H-terminated and delta doped substrates at 152 keV
and fluences of 1.02 ?1012 cm?2 are studied. For gamma radiation, we report a
decrease in drain current and threshold voltage for low dose but increase in both
at high dose. And for proton radiation, we report a change in activation energy for
conductivity, an increase in resistivity, and decrease in both mobility and carrier
concentration.
RADIATION EFFECTS IN DIAMOND
SUBSTRATES AND TRANSISTORS
by
Aayush Thapa
Thesis submitted to the Faculty of the Graduate School of the
University of Maryland, College Park in partial fulfillment
of the requirements for the degree of
Master of Science
2019
Advisory Committee:
Professor Aris Christou, Chair/Advisor
Professor Isabel K. Lloyd
Professor F. Patrick McCluskey
?c Copyright by
Aayush Thapa
2019

Table of Contents
Table of Contents ii
List of Tables iv
List of Figures v
1 Introduction and Statement of Thesis Objectives 1
1.1 Properties of Diamond . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Diamond as an ultra-wide bandgap semiconductor . . . . . . . . . . . 3
1.3 Doped Diamond Substrates . . . . . . . . . . . . . . . . . . . . . . . 7
1.4 Radiation Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2 Materials and Methods 11
2.1 Diamond Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1 H-terminated Diamond . . . . . . . . . . . . . . . . . . . . . . 12
2.1.2 Delta-doped Diamond . . . . . . . . . . . . . . . . . . . . . . 13
2.2 H-terminated Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Temperature Dependent Hall Effect Measurement . . . . . . . . . . . 17
2.3.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.2 Van der Pauw Technique . . . . . . . . . . . . . . . . . . . . . 17
2.3.2.1 Resistivity Determination . . . . . . . . . . . . . . . 18
2.3.2.2 Hall Cofficient Determination . . . . . . . . . . . . . 19
2.3.2.3 Hall mobility and carrier concentration . . . . . . . . 20
2.3.3 Activation Energy Determination . . . . . . . . . . . . . . . . 20
2.3.4 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.4.1 Contact Metal Deposition . . . . . . . . . . . . . . . 22
2.3.4.2 Sample Mounting . . . . . . . . . . . . . . . . . . . . 23
2.4 Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5 Device Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3 Results and Discussion 30
3.1 H-terminated FET Radiation Effects . . . . . . . . . . . . . . . . . . 30
3.2 Pre-irradiation Hall Measurement . . . . . . . . . . . . . . . . . . . . 31
3.2.1 H-terminated diamond . . . . . . . . . . . . . . . . . . . . . . 32
3.2.2 Delta doped diamond . . . . . . . . . . . . . . . . . . . . . . . 39
3.3 Post-irradiation Hall Measurement . . . . . . . . . . . . . . . . . . . 46
ii
3.3.1 H-terminated diamond . . . . . . . . . . . . . . . . . . . . . . 47
3.3.2 Delta doped diamond . . . . . . . . . . . . . . . . . . . . . . . 51
3.4 Device Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4 Conclusions and Future Directions 57
4.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Bibliography 62
iii
List of Tables
1.1 Important Properties of Diamond . . . . . . . . . . . . . . . . . . . . 1
1.2 Johnson figure of merit for semiconductor materials . . . . . . . . . . 5
1.3 Electrical properties of wide- and ultra-wide bandgap semiconductors
[8] [13] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Parameters used in quadratic constant mobility model for MOSFET
IV characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
iv
List of Figures
1.1 Unit cell for diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Baliga figure of merit contours for various semicondcutors . . . . . . . 4
2.1 AFM image of a polished diamond surface . . . . . . . . . . . . . . . 12
2.2 FTIR Spectrum of H-terminated Diamond . . . . . . . . . . . . . . . 13
2.3 I-V characteristics for H-terminated verus O-terminated diamond . . 14
2.4 AFM image of a polished delta-doped diamond surface . . . . . . . . 15
2.5 SIMS profile for double delta-doped diamond sample . . . . . . . . . 15
2.6 Schematic of a MOSFET based on H-terminated diamond . . . . . . 16
2.7 Conductivity curve for a doped semiconductor . . . . . . . . . . . . . 21
2.8 Diamond sample with Van Der Pauw configuration . . . . . . . . . . 23
2.9 Diamond sample prepared for Hall effect measurement . . . . . . . . 24
2.10 Cyclotron chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.11 Diamond substrates being irradiated with protons in the cyclotron . . 28
3.1 Device characteristics at low-dose (?100 kRad) gamma irradiation . . 31
3.2 Drain current versus drain voltage at high-dose (?13 MRad) gamma
irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 I-V curves at 298 K for H-terminated diamond . . . . . . . . . . . . . 33
3.4 I-V curves at 250 K for H-terminated diamond . . . . . . . . . . . . . 34
3.5 Conductivity versus reciprocal temperature for H-terminated diamond 35
3.6 Mobility versus temperature for H-terminated diamond . . . . . . . . 37
3.7 Mobility versus temperature for H-terminated diamond on log-log scale 37
3.8 Sheet carrier density versus reciprocal temperature for H-terminated
diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.9 Pre-anneal I-V curves at 288 K for delta-doped diamond . . . . . . . 40
3.10 Post-anneal I-V curves for delta-doped diamond at 288 K . . . . . . . 41
3.11 Post-anneal I-V curves for delta-doped diamond at 200 K . . . . . . . 42
3.12 Conductivity versus reciprocal temperature for delta-doped diamond . 43
3.13 Conductivity versus reciprocal temperature for double delta-doped
diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.14 Mobility versus temperature for double delta-doped diamond on log-
regular scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
v
3.15 Bulk carrier density versus reciprocal temperature for H-terminated
diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.16 I-V curves at 288 K for H-terminated diamond after proton irradiation 48
3.17 I-V curves at 288 K for H-terminated diamond after proton irradiation
for nanoAmpere input current regime . . . . . . . . . . . . . . . . . . 49
3.18 Resistivity versus temperature for H-terminated diamond . . . . . . . 50
3.19 Conductivity versus reciprocal temperature for H-terminated dia-
mond after proton irradiation . . . . . . . . . . . . . . . . . . . . . . 51
3.20 I-V curves at 288 K for delta-doped diamond after proton irradiation 52
3.21 Resistivity versus temperature for H-terminated diamond . . . . . . . 53
3.22 Conductivity versus reciprocal temperature for delta-doped diamond
after proton irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.23 I-V characteristics for H-terminated diamond pre- and post-irradiation
with protons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.24 I-V characteristics for delta-doped diamond pre- and post-irradiation
with protons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
vi
Chapter 1: Introduction and Statement of Thesis Objectives
1.1 Properties of Diamond
Diamond is one of the most versatile materials with extreme physical proper-
ties. Diamond is made entirely of carbon atoms, and its unit cell comprises two face
centered cubic (FCC) sub-cells displaced 1/4 the body diagonal, as shown in Fig
1.1 [3]. Each carbon atom forms covalent bonds with four adjacent carbon atoms,
leaving behind no free electrons. This rigid structure is what gives diamond its me-
chanical robustness. Table 1.1 lists some of the important properties of diamond [4].
Table 1.1: Important Properties of Diamond
Property Value Units
Hardness 1.0? 104 kg/mm2
Strength, tensile > 1.2 GPa
Strength, compressive > 110 GPa
Young?s modulus 1.22 GPa
Thermal expansion coefficient 1.1? 10?6 K?1
Thermal conductivity 20.0 W/cm?K
Thermal shock parameter 3.0? 108 W/m
Optical index of refraction (at 591 nm) 2.41 Dimensionless
Optical transmissivity (from nm to far IR) 225 Dimensionless
Loss tangent at 40 Hz 6.0? 10?4 Dimensionless
Bandgap 5.47 eV
Diamond?s mechanical robustness and optical properties are well known. Since
1
Figure 1.1: Unit cell for diamond [From [3]]
diamond is the hardest material, it has been used for a variety of mechanical ap-
plications. Furthermore, because of diamond?s ultra-wide bandgap of 5.47 eV, it
can transmit light across the ultraviolet to microwave range. Diamond?s high hard-
ness, high transmissivity, high thermal conductivity, and low coefficient of thermal
expansion (see table 1.1) make it ideal for use for transmission windows and domes
that can withstand severe environmental conditions [5] [6]. Even though diamond
has been widely used for its mechanical, and, to some extent, optical properties, its
excellent electrical properties have not found wide commercial application. This is
mainly due to non-uniformity in naturally occuring and synthetic diamond, which
has impeded the research into using diamond as an electronic-grade semiconductor
material.
Recent advances in diamond synthesis by chemical vapor deposition (CVD)
and high-pressure, high-temperature (HPHT) processes have led to massive reduc-
2
tion in nonuniformity in synthetic diamond films [7] [4]. This has been key in pushing
the efforts for diamond as an electronic material.
1.2 Diamond as an ultra-wide bandgap semiconductor
Ultra-wide bandgap (UWBG) materials are materials whose bandgaps exceed
those of galium nitride which has a bandgap of 3.4 electronVolts (eV). Some of
the UWBG materials are beta phase of galium oxide (Ga2O3), aluminum nitride
(AlN), aluminum galium nitride (AlGaN), cubic BN and diamond. These mate-
rials offer exciting and challenging area of research in semiconductor electronics,
especially in the field of high power and RF electronics such as for high bandwidth
communications, radar and intelligence applications. This is because many impor-
tant parameters for device performance have highly non-linear dependence with the
materials? bandgap [8]. Thus, UWBG materials have the potential to make possi-
ble devices with higher levels of performance than devices based on silicon, galium
arsenide, silicon carbide or galium nitride prevelant today.
Fig 1.2 helps illustrate the non-linear scaling of device parameters such as
breakdown voltage (VBR) and specific on-resistance (RON?SP ) with bandgap. In the
figure, contours of Baliga figure of merit defined as V 2BR/RON?SP for low-frequency
unipolar vertical power switches is shown in log-log scale. High breakdown voltage
and low specific on resistance are desirable properties. So, the higher the BFOM,
the better the device properties i.e. high breakdown voltage and low specific on
resistance. VBR and RON?SP depend on parameters that depend on critical electric
3
Figure 1.2: Baliga figure of merit contours for various semicondcutors for low-
frequency unipolar vertical power switches [From [8]]
field (EC) at which avalanche breakdown occurs [8] [9]. When expressed in terms
of the critical electric field, the BFOM is directly proportional to E3C . The critical
electric field scales by about square of the bandgap. Hence, the BFOM scales as
the sixth power, which explains why UWBG materials are on the lower right hand
corner of fig 1.2 even on a log-log scale. It should be noted that even among the
UWBG materials, diamond has one of the best BFOM.
For high-frequency, high-power applications, the Johnson figure of merit (JFOM),
given by equation 1.1, is a better index for assessing the suitability of materials [8].
vsatEC
JFOM = (1.1)
2?
vsat is saturated carrier velocity, andEC is the critical electric field at which avalanche
breakdown occurs. vsat, EC , and JFOM of some semiconductors are given in table
1.2 [8]. UWBG materials have higher JFOM than Si and GaN, making them suit-
able for high-frequency, high-power applications. Again, diamond has one of the
4
best JFOM even among UWBG materials.
Table 1.2: Johnson figure of merit for semiconductor materials
Material vsat EC at ND = JFOM JFOM nor-
(107 cm s?1) 1016cm?3 (1012V s?1) malized to Si
(MV cm?1) =1
Si 1.0 0.3 0.48 1
GaN 1.4 4.9 11.0 23
AlN 1.3 15.4 31.9 67
? ?Ga2O3 1.1 10.3 18.0 38
c-BN unknown 17.3 - -
Diamond 2.3 (e-) 13.0 47.6 (e?) 100
1.4 (h+) 29.0 (h+) 61
Another important property for substrates utilized to make high-frequency,
high-power devices is their ability to remove heat. Diamond has the highest known
thermal conductivity of any material, which is what further separates it from other
UWBGs with similarly high JFOM as seen in table 1.2. High thermal conduc-
tivity is of particular importance because in many power electronics and optoelec-
tronics applications, device operation is limited by its capability to remove heat.
Devices based on wide bandgap such as GaN have already encountered problems
with excessive heating at high power levels, so much so that diamond layers have
successfully been integrated for heatsinking [10]. Likewise, when compared to con-
trol AlGaN/GaN HEMT, nanocrystalline diamond-capped AlGaN/GaN HEMTs
reduced device temperature by about 20% from 0.5 to 9 W/mm dc power device
operation [11]. Diamond as heatsink layer will probably also be needed for AlGaN,
AlN, and Ga2O3 based devices [8] [11] [12]. This necessitates research in diamond
5
as a semiconducting material and diamond-based devices.
Furthermore, other electrical properties of diamond such as demonstrated n-
type and p-type dopability, electron and hole mobility, and critical electric field are
comparable or better than other UWBG materials. Critical electrical properties of
common WBG and UWBG semiconductor materials are listed in table 1.3.
Table 1.3: Electrical properties of wide- and ultra-wide bandgap semiconductors [8]
[13]
WBG UWBG
Material GaN 4H-SiC AlGaN/ ? ? Diamond
AlN Ga2O3
Bandgap (eV) 3.4 3.3 Up to 4.9 5.5
6.0
Thermal Conductivity 253 370 253-319 11-27 2290-
(W m?1K?1) 3450
State-of-the-art substrate ? 104 ? 102 ? 104 ? 104 ? 105
quality (dislocations per
cm2)
State-of-the-art substrate 8 (on 8 2 4 1
diameter (inches) Si)
Demonstrated p-type dopa- Good Good Poor No Good
bility
Demonstrated n-type dopa- Good Good Moderate Good Moderate
bility
Electron mobility (cm2/V s) 2000 900 300 4500
Hole mobility (cm2/V s) 200 120 3800
Breakdown field (MV/cm) 3.3 2 8 10
6
1.3 Doped Diamond Substrates
Field effect transistors (FETs) based on diamond rely on conductive channel
formed by unconventional means. This is because substitutional doping in diamond
results in deep donor and acceptor states. The deep acceptor states (0.37 eV for
boron) and deep donor states (0.7 eV for nitrogen, and 1.3 eV for phosphorous)
cause low activation at room temperature [16]. There is heavy interest in two current
solutions: surface terminating with hydrogen, and incorporating a thin heavily boron
doped layer buried few tens of nanometers from the surface. Both of these methods
result in the formation of a 2D conduction channel.
Hydrogen termination of diamond leads to a negative electron affinity (NEA)
of its surface. This NEA surface loses electrons to adsorbed gaseous species such
as atmospheric moisture, NO2, or high electron affinity oxides [14]. These phys-
ically adsorbed species act as surface transfer dopants, generating a conductive
two-dimensional hole gas (2DHG) near the surface [15].
Likewise during epitaxial growth, extremely high concentration of boron i.e.
greater than 1020 cm?3 can be incorporated, which creates a confined hole channel
layer. This thin layer of about 1-3 nm buried a few tens of nanometers from the top
surface is what is termed the delta layer. Based on 2D densities of holes, fraction of
holes located outside of heavily doped region, in the lightly doped and high mobility
region was calculated and where the 2DHG channel is predicted to form.
FETs have been fabricated successfully based on p-channel formation in dia-
mond in the ways mentioned above. To date, the highest reported maximum drain
7
current density for diamond devices is -1.35 A/mm for H-terminated substrates [17].
Likewise, Kawarada et al. have demonstrated stable operation of diamond devices
based on H-terminated substrates at temperatures up to 400?C in vacuum [18].
1.4 Radiation Effects
Diamond is predicted to be radiation hard i.e. resistant to radiation damage.
This is because of its ultra-wide bandgap and high binding energy of its covalent
carbon to carbon bonds. A measure for accessing potential radiation hardness of
materials is the threshold displacement energy, which is the minimum kinetic en-
ergy required by an atom in a solid to be parmanently displaced from its lattice site.
Diamond has a displacement energy of 43 eV, whereas other semiconductors such
as Si has 13-20 eV, 4H-SiC has 21.8 eV, and GaAs, 10 eV [20]. Thus, in addition
to mechanical robustness, and promising electrical properties, diamond potentially
also has high radiation tolerance. So, diamond-based electctronics could be used in
extremely harsh environments subject to high temperature, pressure and/or radia-
tion. These electronics could be subjected to protons, alpha particles and electrons
in the low earth orbit satellites, and to neutrons or gamma rays in nuclear reac-
tors and military systems [22]. While many radiation damage studies have been
conducted with regards to diamond as detectors, not many have been done inves-
tigating radiation effects in diamond transistors [19]. Radiation damage studies in
diamond from device point of view is especially important because diamond devices
are based on unconventional p-channel formed by surface hydrogen termination or
8
by heavily boron doped delta layer, which may be succeptible to radiation damage.
The only prominent study on radiation effects on diamond devices was done
by Verona et al. who conducted neutron irradiation experiments on H-terminated
diamond substrates and devices [19]. Diamond was found to have stable device
performance after irradiation with 14.8-MeV neutron fluence up to 1014 cm?2. The
effects of other types of radiation on H-terminated or delta-doped substrates and
devices are unknown. This thesis addresses part of this issue by investigating the
effects of both ionizing radiation in the form of protons, as well as that of non-
ioninizing radiation in the form of gamma rays on diamond substrates and devices.
These two different types of radiation was selected because the interaction of semi-
conductors with a neutral radiation like gamma rays is fundamentally different than
that with a charged particle like proton [22]. The energy loss mechanisms for protons
are Rutherford scattering, knock-on atoms, and energy loss to ionization, whereas
that for gamma rays are the photoelectric effect, Compton scattering, and pair
production [23].
Rutherford scattering causes lattice damage by displacing atoms from its lat-
tice site. This occurs when an atom in a solid scatters a charged particle like protons
and the reaction on it is strong enough to displace it from its lattice site. Further-
more, if the displaced atom has sufficient energy, it may displace additional atoms,
called knock-on atoms, creating more defects. This effect is prevelant for heavy
particles such as protons. Also, charged particles passing through a material will
ionize it if the velocity of the charged particles is greater than that of the electrons
in the material. Likewise, gamma radiation may cause ionization effects as well as
9
displacement of atoms. Compton scattering is the dominant lattice damage mecha-
nism for gamma rays below 15 MeV. It occurs when a gamma ray photon interacts
with a valence band electron and transfers portion of its energy to the electron,
causing the gamma photon to be scattered and the electron to be ejected from the
atom. This recoil electron is what damages the lattice. Detailed information on the
interactions and effects of different types of radiation on different WGB and UWBG
semiconductor materials and devices can be found in [19] - [25].
10
Chapter 2: Materials and Methods
2.1 Diamond Substrates
New Diamond Technology supplied undoped (100) type IIa single crystal high
pressure high temperature (HPHT) substrates to the University of Maryland. This
type of substrates was selected because of their very low dislocation density, as
dislocations could act as carrier scattering centers. These substrates ranged between
3 mm ? 3 mm to 4.5 mm ? 4.5 mm in length and width, and 0.5 mm in thickness.
These substrates were polished to less than 0.3 nm surface roughness to reduce the
possibility of scattering further. To remove the polishing damage, these substrates
were then etched using a low power plasma to maintain low surface roughness.
Both H-terminated and delta-doped diamond substrates were used in this the-
sis. Hydrogen-plasma treatment was performed to achieve hydrogen termination
on these substrates, whereas delta-doped samples were obtained by incorporating a
heavily doped boron layer during epitaxial growth using Chemical Vapor Deposition
(CVD).
11
Figure 2.1: AFM image of a polished diamond surface with less than 0.3 nm RMS
roughness
2.1.1 H-terminated Diamond
For H-terminated diamond samples, RMS roughness value of less than 3 A?
was determined using atomic force microscopy (AFM), wihch verified the effects of
polishing and etching. An AFM micrograph of a sample shown in Fig. 2.1 whose
RMS surface roughness was found to be less than 0.3 nm.
The substrate surface was exposed to a hydrogen plasma at temperatures
above 700?C to achieve hydrogen termination. To confirm whether hydrogen termi-
nation was achieved, fourier transform infrared spectroscopy (FTIR) was used. The
FTIR spectrum shown in Fig. 2.2 has peaks between 2800 and 3000 cm?1, which
correspond to C-H bonds [26].
12
Figure 2.2: FTIR Spectrum of H-terminated Diamond Sample showing peaks be-
tween 2800 and 3000 cm?1 corresponding to C-H bonds
Likewise, the effect of H-termination i.e. the formation of conductive layer
was observed by performing a simple current-voltage measurement between two gold
contact pads on the hydrogenated sample. Fig.2.3 shows that H-terminated surface
had linear current-voltage response, whereas when the H-termination was replaced
with oxygen, there was barely any current response to applied voltage compared to
that of the H-terminated diamond.
2.1.2 Delta-doped Diamond
As opposed to H-terminated diamond, delta doped diamonds had epitaxial
layer grown on the HPHT substrate with 1-3 nm of heavily doped-boron delta layer
incorporated in the epi-layer. Fig 2.4 shows AFM image of delta-doped diamond
13
Figure 2.3: Current-Voltage (I-V) characteristics for H-terminated versus O-
terminated diamond substrate [From [2]]
whose surface roughness is less than 0.3 nm.
Two different types of delta-doped diamonds were used: single delta-doped
and double delta-doped. Single delta-doped diamond had only one 2-nm delta layer
buried 12 nm from the surface, whereas double delta-doped diamond had two heavily
B-doped delta layers. The first delta layer was at 26 nm from the top surface, and
the second delta layer was right at the surface. The SIMS profile in fig 2.5 shows
the two layers for the double delta doped sample.
2.2 H-terminated Devices
Surface transfer dopants such as H2O, NO2 and NO are unstable, and thus,
problematic in fabricating relaiable devices. High electron affinity dielectrics are
better suited as surface transfer dopants for H-terminated devices. In this work,
14
Figure 2.4: AFM image (1?m ? 1?m) of a polished delta-doped diamond surface
with less than 0.3 nm RMS roughness
Figure 2.5: SIMS profile for double delta-doped diamond sample
15
Figure 2.6: A schematic of H-terminated diamond based MOSFET device with
Al2O3 as surface transfer dopant which leads to the 2DHG channel [From [2]]
aluminum oxide, Al2O3, was used as gate oxide.
A schematic of MOSFETs fabricated using HPHT hydrogen terminated sub-
strates is shown in Fig. 2.6. A blanket Au layer of 100-150 nm was evaporated
on the surface of the hydrogen terminated substrate. This ensured protection of
the surface hydrogen termination responsible for the conducting channel under each
device. Device mesas were created by patterning photoresist on top of the blanket
Au layer and using etchback process with KI/I2 wet etchant. To isolate the devices,
O2 plasma etch was employed which replaced C-H bonds with C-O bonds between
devices, effectively eliminating 2DHG channel underneath. Again, photoresist was
patterened and KI/I2 wet etchback was performed to form discrete Ohmic contacts
from the Au mesas. 25 nm of Al2O3 was deposited by atomic layer deposition (ALD)
at 175?C. Then, using e-beam and liftoff processes, 100 nm of Al gate was deposited.
16
2.3 Temperature Dependent Hall Effect Measurement
2.3.1 Background
The Hall effect is a technique used to distinguish carrier type of a semiconduc-
tor, and to measure majority carrier mobility and majority carrier concentration. It
is based on the Lorrentz force given by equation 2.1 exerted on moving charges.
F = qv ?B (2.1)
When current is passed in one direction along the surface of the semiconductor and
the magnetic field is perpendicular to the surface of the semiconductor, the force
experienced by moving charges will force the charges to accumulate in perpendicular
direction to the direction of motion of the original current. This buildup induces
an electric field in the opposite direction, balancing the magnetic field force. This
induced electric field is called the Hall field, which is responsible for producing the
Hall voltage ? a voltage difference across the semiconductor surface, perpendicular
to the direction of injected current. The polarity of Hall voltage is evidence of the
carrier type. Furthermore, the Hall voltage along with the applied current, magnetic
field, and sample dimensions help determine the majority carrier concentration and
the majority carrier mobility.
2.3.2 Van der Pauw Technique
The Hall effect can be measured using different configurations. This method-
ology presented here is based on the work of Van der Pauw with guidelines from
17
ASTM standard F76-08 [28].
Van der Pauw technique helps determine an arbitrarily shaped sample?s resis-
tivity and, in presence of a magnetic field, the hall coefficient. For this, four point
ohmic contacts must be made at the surface of a homogeneous sample, as close to
the edges (corners, for square samples) as possible. Together, the resistivity and the
hall cofficient are used to obtain hall mobility.
2.3.2.1 Resistivity Determination
Resistivity is determined by measuring voltage response across adjacent con-
tacts by passing current through the other pair of contacts. Measurement accuracy
is improved by repeating the process to get voltage response from each adjacent pair
of hall pads which amounts to 4 total measurements. Furthermore, the polarity of
the current source and voltage for each of these 4 measurements is switched to give
4 additional data points. Thus obtained data points are used in equations 2.2 and
2.3 to calculate resistivity values, the average of which is taken as the resistivity of
the sample.
? fAt
?A = (V21,34 ? V12,34 + V32,41 ? V23,41) (2.2)
4ln(2) I
? fBt
?B = (V43,12 ? V34,12 + V14,23 ? V41,23) (2.3)
4ln(2) I
fA and fB are geometrical factors based on sample symmetry. In case of perfect
symmetry, fA = fB = 1. The geometrical factors fA and fB are determined by
corresponding resistance ratios given by equations 2.4 and 2.5 respectively.
V21,34 ? V12,34
QA = (2.4)
V32,41 ? V23,41
18
V43,12 ? V34,12
QB = (2.5)
V14,23 ? V41,23
Equation 2.6 is then used to find the appropriate geometrical factors, fA and fB,
which are used in equations 2.2 and 2.3 respectively to get resistivity values. If Q
obtained from equation 2.4 or 2.5 is less than 1, then its reciprocal must be used to
find f.
{ ( )}
Q? 1 f 1 0.693
= arccosh exp (2.6)
Q+ 1 0.693 2 f
Finally, the average resistivity, ?avg, is calculated as follows:
?A + ?B
?avg = (2.7)
2
2.3.2.2 Hall Cofficient Determination
To determine Hall Coefficient, magnetic field is applied such that the magnetic
flux is perpendicular to the sample?s surface. Current must be passed diagonally
through the sample, say leads 1 and 3, and voltage difference measured at the
pads on the other diagonal, in this case, leads 2 and 4. Additional measurements
under reverse current is also obtained. Furthermore, the polarity of magnetic field
is switched, and voltage measurements for both forward current and reverse current
are taken. These voltage measurements in V along with magnetic flux in Gauss,
current in A, and thickness of the sample in cm are used in equations 2.8 and 2.9
to get two hall coefficients in cm3C?1 for the two diagonals.
t
RHC = 2.50? 107 [V31,42(+B) ? V13,42(+B) + V13,42(?B) ? V31,42(?B)] (2.8)
BI
? 7 tRHD = 2.50 10 [V42,13(+B) ? V24,13(+B) + V24,13(?B) ? V42,13(?B)] (2.9)
BI
19
The average Hall-coefficient is then calculated as follows:
RHC +RHD
RHavg = (2.10)
2
2.3.2.3 Hall mobility and carrier concentration
Resistivity and hall coefficient values thus evaluated are used to determine hall
mobility in cmV ?1s?1 using the following equation:
|RHavg|
?H = (2.11)
?avg
The drift mobility, ?, in a single carrier sample is related to the average Hall mobility
by a proportionality constant:
?Havg = r? (2.12)
Equation 2.13 shows relation between the Hall coefficient and carrier concentration
for extrinsic semiconductors, given that the measurement temperature is below the
intrinsic region with conduction dominated by a single carrier type.
r
RHavg = (2.13)
nq
where r is proportionility constant on the order of unity, q is elementary charge, and
n is carrier concentration.
2.3.3 Activation Energy Determination
Temperature dependence of conductivity of a material, which is the reciprocal
of resistivity, can be used to extract its activation energy. Fig. 2.7 shows conductiv-
ity curve for most semiconductors [29]. At lower temperatures, conduction through
20
Figure 2.7: Expected behavior of natural log of conductivity versus reciprocal tem-
perature for a doped semiconductor (Taken from [29])
extrinsic carriers dominates, as there is not enough energy for activation of intrinsic
carriers. At high enough temperatures when activation energy becomes equal to or
greater than the intrinsic bandgap, conduction through intrinsic carriers dominates.
Conductivity of a semiconductor is given by:
? = (n?e + p?h)e (2.14)
The electron and hole carrier concentrations are functions of density of states in the
conduction and valence bands for intrinsic conduction, and that of density of states
in donor or acceptor states for extrinsic conduction. Carrier concentrations which
are equal for intrinsic semiconductor are given by equation 2.15 in which ?C? is a
constant.
3 Eg
n = p = CT ?2 e 2kT (2.15)
From equations 2.14 and 2.15, the following relationship is obtained for conductivity.
3
Since ?T? in exponential term overshadows T 2 , this term can be ignored. Taking
natural log of both sides yields activation energy in the form of Eg from the slope
21
of ln? versus 1/T relationship, as described by 2.18 which is seen in Fig 2.7.
[ ]
3 Eg
ln? = ln C(?e + ?h)eT
?
2 e 2kT( ) (2.16)
Eg
ln? = (ln [C(?e + ?h)e] + ln e
?
2kT (2.17)
? )Eg 1
ln? = + ln [C(?e + ?h)e] (2.18)
2k T
2.3.4 Sample Preparation
The HMS-5000 System at the University of Maryland?s Nanocenter was used
to measure the temperature dependent Hall parameters. This system has four gold
probes which measure current-voltage characteristics. Van Der Pauw configuration
was patterned on the samples, and the samples were mounted on HMS-5000 System
for Hall measurements. The details of this process are given below.
2.3.4.1 Contact Metal Deposition
First, the samples were cleaned by sonicating them in acetone for 30 minutes
and blow-drying with nitrogen gun. To deposit contact metal only on the corners,
silicon piece was used as a mask. The silicon mask was attached to the sample
using double sided kapton tape. This Si piece covered the entire surface of the
sample, exposing only the corners. Contact metal was then evaporated using the
Angstrom e-beam evaporator at the Maryland Nanocenter. 25 nm layer of chromium
was first diposited as adhesion layer, followed by 250 nm layer of gold without the
interruption of vacuum, which was around 4 ? 10?6 torr. Fig. 2.8 shows a sample
with the contacts thus deposited.
22
Figure 2.8: Cr/Au contacts on the corner of the diamond sample for Hall Effect
Measurement using the Van Der Pauw Technique
2.3.4.2 Sample Mounting
All of the diamond samples were less than 4.5 mm ? 4.5 mm in size. This
posed a problem, as the HMS-5000 System used for Hall measurements could not
accomodate substrates smaller than 5 mm ? 5 mm. HMS-5000 System?s probes
could not be moved to be placed on the corners of the samples where contact pads
were deposited. Thus, a work-around was established in order to be able to use the
HMS-5000 System. Diamond sample was placed on top of silicon dioxide (SiO2),
which is a great electrical insulator. Bondwires were drawn from the contact pads
on diamond corners to contact pads on silicon dioxide substrate. This effectively
extended the contact pads of diamond samples so that the probes of the HMS-5000
System could reach the small diamond samples and measure their properties. The
sample set up along with the four probes of the HMS System is shown in Fig. 2.9.
The SiO2 substrates were obtained by thermally growing 1 micron layer of
SiO2 on a Siicon wafer. Then, 40 nm layer of chromium was evaporated on the
23
Figure 2.9: Diamond sample on top of the non-conductive Silicon dioxide for Hall
measurements using HMS-5000 System. Wirebonds are drawn from the contact
pads on diamond to contact pads on SiO2 on which probes of HMS-5000 System
are dropped
entire wafer, followed by 400 nm of gold in the Angstrom e-beam evaporator under
high vacuum (i.e. < 5 ? 10?6 torr) without interruption. The 3-inch wafer was
cleaved into several smaller pieces which fit the sample size specification of the
HMS-5000 system. These smaller pieces were prepared such that each had four
distinct regions with metal on top. To do this, photoresist was deposited on the
corners to shield those areas. Metal was etched from the remaining bulk of the area
by a 2-step wet etching process. First, gold was stripped off using KI/I2 solution
(Au Etchant TFA, Transene Co.). Once gold had come off, the pieces were immersed
in ceric ammonium nitrate and nitric acid (Chromium Etchant 1020, Transene Co.)
for Cr etching until the SiO2 surface was exposed. Finally, the sample was cleaned
in acetone and isopropyl alcohol (IPA) to remove the resist from the corners.
LOR7B resist was used to attach a diamond sample each to the bare SiO2
24
part of each of the mount pieces by hardbaking it at 180?C for 45 minutes. Then, a
semi-automatic wire bonder (West Bond 454647E) was used to connect bondpads on
diamond and SiO2 through bondwires. The SiO2 substrate with the diamond thus
mounted was placed in the stage of the wirebonder, which was heated at 100?C. The
stage was heated because bondwires stick to bond pads relatively easily when the
bond pads are at high temperature. LOR7B was selected as an adhesive precisely
for this, as it provided sturdy adhesion even at such temperatures whereas other
available photoresists such as MicroChem?s 1813 and 1805 would start to liquefy.
This is because the LOR?s softening temperature was higher than the stage?s tem-
perature, whereas that of 1813 and 1805 resists were lower. Double sided Kapton
tape would have been easier to work with, however, its adhesion was not sturdy
enough, and the sample would move. This meant that the bondwires would not
stick on the diamond?s contact pads. Once wirebonding was complete, the samples
were ready for Hall measurements.
The probes of the HMS-5000 system were dropped on the metal regions of
SiO2 to probe the diamond sample. Since diamond has lower bandgap than SiO2,
the easiest conduction path is through the metal wire and across the diamond. To
ensure that SiO2 substrates were not affecting the results, HMS-5000 System was
run for SiO2 substrates without diamond on top. The software gave no voltage
readings for the entire input current range, and would read ?Contact fail.? This is
because resistivity of SiO2 is higher than the upper limit for resistivity of HMS-5000
System. Thus, when diamond was mounted on SiO2 substrate, the voltages were
now being measured across the contact pads in the diamond because diamond?s
25
resistivity (much lower than that of SiO2) was within the limits of this instrument.
2.4 Radiation
Radiation hardness of diamond is an appealing prospect for diamond-based
sensors and devices. This works explores diamond?s radiation hardness against
gamma rays in H-terminated devices for change in output characteristics. This
work also delves into the effects of proton irradiation on transport characteristics
through Hall effect measurements.
Firstly, radiation stability of H-terminated diamond field effect transistors were
explored. These FETs were subjected to gamma irradiation at the University of
Maryland Radiation Facilities (UMRF). High capacity dry cell, panoramic gamma
irradiator was used with 60Co as source material for gamma irradiation. 60Co source
decays to release 1.17 and 1.33 MeV gamma rays. The devices were subjected to
irradiations from a total dose of 1 krad(Si) up to 26 Mrad(Si), depending on the
devices available and schedule of radiation facility.
Additionally, radiation hardness of the bare H-terminated and delta-doped
substrates was investigated. For this, UMRF?s cyclotron was used to irradiate the
sample substrates with protons. Fig 2.10 shows the cyclotron chamber. The tube-
like structure on the bottom right extending from outside the chamber to near the
center is the linear positioner for sample stage. The spot size of the proton beam
was just enough to accomodate the entire surface area of the 3.5 mm ? 3.5 mm
sample on the stage. Thus, the samples had to be carefully placed, and parameters
26
optimized in order to get uniform coverage of the entire surface with protons. The
cyclotron frequency was set to 7.15 MHz, and magnetic field was 0.47 T. The sample
was placed 12 cm from the center of the cyclotron chamber. From these parameters
and the kinetic energy relation, energy of the protons was calculated to be 152 keV.
Beam current, spot size of the beam, and irradiation run time were used to determine
the fluence of the protons. The beam current determined using Ohm?s law was 73.6
nA for a voltage drop of 73.6 mV with a resistance of 1 M?. To help estimate the
spot size of the beam, phosphor background was used in the sample holder. This is
because phosphor glows brightly whereever the beam strikes it, making it easy to
determine the spot size. Fig. 2.11 shows the sample holder with an H-terminated
sample and a delta-doped sample being irradiated. The spot size was estimated to
be 269 cm2. For 73.6 nA beam current, 269 cm2 spot size and considering 10% duty
cycle for a run time of 100 minutes, the fluence was calculated to be 1.03 ? 1012
protons/cm2.
2.5 Device Simulation
Modeling of device I-V characteristics was done in order to understand the
effect of radiation on device performance. The quadratic model for constant mo-
bility assumption was used to model DC characteristics of diamond-based FETs.
Mobility values for this model was obtained from Hall measurements for pre and
post-proton irradiation. Modeling is done in Matlab based on equation 2.20 for
27
Figure 2.10: Cyclotron chamber
Figure 2.11: Diamond substrates being irradiated with protons in the cyclotron
28
p-channel depletion mode.
?pWG
[ ? = ] (2.19)diLG
V 2
ID = ? (Vg ? Vth)V ? DSDS (2.20)
2
Where  is dielectric constant of gate insulator, ?p is mobility, WG and WL are gate
width and gate length respectively, di is dielectric thickness, ID is drain current
output, Vg is gate voltage, Vth is threshold voltage, and VDS is drain-source voltage.
29
Chapter 3: Results and Discussion
3.1 H-terminated FET Radiation Effects
H-terminated FETs were subjected to gamma radiation at low dose from 1 to
100 kRad(Si) as well as high dose up to 26.3 MRad(Si). Fig 3.1 shows device charac-
teristics at 0 kRad i.e. pre-irradiation, 1 kRad, 2 kRad, 3 kRad, and 100 kRad dose.
At an initial dose of 1 kRad, drain current, threshold voltage, and transconductance
decreased, while no significant change was observed for on-resistance. These char-
acteristics were stable after irradiation up to a dose of 100 kRad. Campbell and
Mainwood reported 0.03 and 0.09 vacancies/gamma per cm in diamond by gamma
irradiation of 1 and 2 MeV respectively [23]. Thus, the slight decrease in output
characteristics may be because of the introduction of defects acting as scattering
centers.
High dose irradiation led to the opposite effect for drain current output, as
shown in 3.2. At 13.6 Mrad dose, drain current increased significantly. Upon further
irradiation at a total dose of 26.3 MRad, drain current remained the same. High
dose irradiation seems to have resulted in an annealing effect. The increased drain
current response after irradiation could be due to the formation of defects at oxide
interface. These defects form trap states, resulting in enhanced hopping conductivity
30
(a) (b)
Figure 3.1: Device characteristics at low-dose (?100 kRad) gamma irradiation [1]
which led to the increase in current output. Also, threshold voltage to turn off
devices increased significantly from 5 V pre-irradiation to about 9 V post-irradiation.
However, gate leakage current showed no significant increase post-irradiation.
3.2 Pre-irradiation Hall Measurement
Hall measurements were taken for H-terminated and delta doped samples be-
fore irradiating these samples. The upper limit of the temperature range for these
measurements was 350 K because that is the upper limit for HMS-5000 system, and
the lower limit varied based on the sample, as it depended on sample?s resistivity
and HMS system?s limitation.
31
(a) (b)
(c)
Figure 3.2: Drain current versus drain voltage for (a) pre-irradiation, (b) post-13.6
Mrad, and (c) post-26.3 MRad of gamma irradiation [1]
3.2.1 H-terminated diamond
HMS-5000 System was used to determine whether or not the contacts exhibited
ohmic behavior, in other words, linear current-voltage (I-V) relationship. Current
was passed through adjacent contact pads, and voltage difference measured.
H-terminated diamond was annealed after metal deposition at 550?C for 1
minute. Rapid thermal annealing (RTA) was performed using RTA-610 device in
32
Figure 3.3: I-V curves at 298 K substrate temperature for H-terminated diamond
annealed at 550?C for 1 minute in RTA
N2 environment. Fig 3.3 shows I-V behavior at substrate temperature of 298 K
for four adjacent contact pads ab, bc, cd, and da. The linear relationship proves
ohmic behavior, however, the difference in voltage response for the four pairs of
contact pads indicates non-uniformity. This could be because of non-uniform surface
termination with hydrogen.
I-V curves were also obtained at substrate temperature of 250 K as shown
in Fig 3.4. The voltage response of each contact pair increases by an order of
magnitude compared to I-V curves at room temperature. Furthremore, two contact
pad pairs plateau at current higher in magnitude than about 0.6 ?A and 0.7 ?A,
deviating from linear ohmic behavior. Thus, when performing Hall measurements
at temperatures lower than about 288 K, 0.1 ?A current was used to be in the linear
I-V region, whereas for temperatures higher than that, 1 ?A was used for better
accuracy.
33
Figure 3.4: I-V curves at 250 K substrate temperature for H-terminated diamond
annealed at 550?C for 1 minute in RTA
Current-voltage measurements, as outlined in section 2.3.2, were performed
for each temperature step in the range from 250 K to 350 K. For almost all of the
data points, Hall coefficients calculated were positive as expected for semiconductor
with p-type carriers. The occasional negative hall coefficients must be due to non-
uniformity in ohmic contact pads and random fluctuations.
Fig 3.5 shows natural logirathm of conductivity as a function of reciprocal
temperature for two sets of data points taken for temperature range of 250 K to 350
K at an increment of 1 K. HMS-5000 System datapoints are based on conductivity
output from the HMS-5000 System software, and the Calculated (ASTM) datapoints
are based on data analysis according to ASTM standards outlined in [28]. HMS-5000
System software calculated higher resistivity (thus, lower conductivity) than that
calculated using ASTM standards for Van der Pauw configuration. This difference is
about an average of 45% for all data-points. Refering back to equations 2.2 and 2.3,
34
Figure 3.5: Natural logarithm of conductivity versus reciprocal temperature for
H-terminated diamond for data output by HMS-5000 System software and data
analyzed using ASTM standards [28], as well as the trendlines for both data sets
with information about its slope
this difference in the two datasets for resistivity arises from the geometrical factors
fA and fB determined for the two datasets. The method of determination of fA
and fB by the HMS-5000 System software is unknown, whereas that for Calculated
(ASTM) dataset was by using ASTM Standards by satisfying equation 2.6. Thus,
there is more confidence in the accuracy of the dataset obtained by following the
ASTM Standards. The slopes of -4406.3 K and -4273 K for Calculated and HMS-
5000 datasets give activation energy (Eg) of 0.76 eV and 0.74 eV respectively. Since,
conduction is through p-type carriers, it must be through movement of holes as this
sample has no ionic charge carriers. Furthermore, the activation energy of 0.76 eV
indicates that there is shallow acceptor level sitting below the Fermi energy level
and 0.76 eV above the valance band.
Similarly, fig 3.6 shows temperature dependence of mobility for H-terminated
diamond. At 300 K, mobility of 128.3 cm2V ?1s?1 was obtained from calculations
35
based on ASTM standards [28], whereas that of 64.5 cm2V ?1s?1 was obtained from
calculations based on HMS-5000 System software. This is close to the mobility value
of 68 cm2V ?1s?1 reported by Verona et. al. [14], whereas the ASTM-based value is
about twice as much. For both the datasets mobility increases with temperature,
which is a strong indication that ionized impurity scattering is the dominant scat-
tering mechanism in this temperature range of 250 K to 350 K. Fig 3.7 shows log
of mobility versus log of temperature data-points for Calculated (ASTM) data set.
Two trendlines are drawn for the dataset. The one in blue has high R2 value of 0.95,
and shows that ??T 0.86. The trendline in red has much lower R2 value at 0.42, but
goes through more data points. Thus, if the scatter in the data set, especially at
lower temperature region, is to be reduced, the red trendline might be more fitting
3
for the dataset. The red trendline gives ? ? T 2 . Either way, mobility proportional
to positive power (ideally, 3/2) for temperature range of 250 K to 350 K indicates
that for this temperature range, the most dominant scattering mechanism is ionized
impurity scattering [30] [31].
Carrier concentration of 2.97 ?1011 cm?2 was obtained at 300 K assuming
that the hall proportionality constant (r) is 1. Verona et. al. reported carrier
concentration of 9.50 ?1012 cm?2 which is higher than 1 order of magnitude [14].
One possible explanation for this difference is that less carriers are activated for H-
terminated substrate here since the sheet resistance in this work 32.4 k?/sq which
is about 3 times greater than that reported by Verona et. al. (10.5 k?/sq). The
higher resistance is due to lower surface coverage of hydrogen in the diamond sample
in this work compared to the sample used by Verona et. al. Garrido et al. observed
36
Figure 3.6: Mobility versus temperature for H-terminated diamond for data output
by HMS-5000 System software and data analyzed using ASTM standards [28]
Figure 3.7: Mobility versus temperature for H-terminated diamond for data output
using ASTM standards [28] and trendlines to determine ? and T relationship
37
decrease in carrier concentration of H-terminated diamond after annealing steps at
high temperatures ? 500 K (226?C) [27]. Carrier concentration was reduced from
9 ?1012 cm?2 to 9 ?1011 cm?2 after two annealing steps. Hence, annealing step to
obtain linear I-V behavior could have led to the desorption of some surface hydrogen,
reducing the total carrier concentration.
Sheet carrier concentration for H-terminated diamond increases with increas-
ing temperature in 250 K to 350 K range, as shown in Fig 3.8 . This is not char-
acteristic of 2DHG, which is what is expected to form near H-terminated surface
because of NEA and surface transfer dopants. Sheet carrier concentration was found
to be temperature independent for H-terminated diamond substrates by Hayashi et
al. [32]. The temperature independent behavior is expected for carrier transport
in 2D inversion layers [27]. Garrido et al. reported this temperature indepen-
dence as well for H-terminated diamond substrates; however, upon annealing the
H-terminated substrates at 500 K (226?C), they observed a decrease in carrier con-
centration with decreasing temperature. Fig 3.8 shows about 2 orders of magnitude
change in carrier concentration over 250-350 K temperature range. In contrast,
Garrido et al. reported less than one order of magnitude change over 100-400 K
range. The stronger temperature dependence in this work could be because of the
higher annealing temperature (550 ?C) than that by Garrido et al. Furthermore,
the deviation could also be because, in addition to 2DHG conduction channel, H-
terminated diamond also has bulk conduction channel, and may have conduction
channel facilitated by defects such as unintentional impurity atoms, dislocations,
etc. Variable magnetic field and temperature Hall measurements can be carried out
38
Figure 3.8: Sheet carrier density versus reciprocal temperature for H-terminated
diamond for data output using ASTM standards [28]
to determine effects of different conduction mechanisms [33]. Furthermore, mobility
is expected to decrease with increasing carrier concentration because of increase in
ionized impurity scattering. However, the amount of scattering is also affected by
the interaction time between carriers and impurities [31]. The interaction time in-
creases with temperature, so the amount of scattering decreases, leading to increase
in mobility with increasing temperature, which was observed in this case.
3.2.2 Delta doped diamond
Sample S20 which has the heavily boron-doped delta layer buried about 12
nm from the surface was prepared for Hall measurements as outlined in section
2.3.4. First, current-voltage measurements were performed to determine whether
or not linear behavior was obtained. The I-V profile was not linear near room
39
Figure 3.9: Pre-anneal I-V curves for delta-doped diamond at 288 K substrate
temperature
temperature as shown in fig 3.9. Hall measurements was not performed since reliable
Hall measurement requires formation of good ohmic contacts.
The sample was de-mounted and prepared again for Hall measurements, only
this time an annealing step was added after metal deposition to help form Ohmic
contacts. RTA-600 was used to anneal the sample at 500?C for 5 minutes in N2
environment. IV profile of the annleaned sample was measured again at 288 K.
The annealing step led to almost perfectly linear I-V behavior, as shown in fig 3.10.
There was variation in voltage response of the different pairs of contacts. This may
be the result of non-uniformity in the substrate because of non-uniform delta-layer
doping, or unintentional impurities. I-V profile was also measured after annealing
at lower substrate temperatures. Fig 3.11 shows linear I-V behavior at substrate
temperature of 200 K. At 200 K, the voltage response is about an order of magnitude
higher than that at 288 K for the same applied current. This means that resistivity
40
Figure 3.10: I-V curves at 288 K substrate temperature for delta-doped diamond
annleaned at 500?C for 5 minutes
of the substrate must have increased at lower temperature.
Hall measurements were performed for each temperature step in the range from
210 K to 350 K. Resistivity calculations, as outlined in section 2.3.2, revealed that
there were large random fluctuations in measured voltage data in the temperature
range of 264 K to 292 K. Thus, data points for 264 K to 292 K temperature range
were ignored.
Resistivity of 8.4 ?104 ?? cm was obtained at 300 K. Fig 3.12 shows temper-
ature dependence of conductivity in the form of log of conductivity versus reciprocal
temperature. Using the value for slope in equation 2.18, the activation energy was
calculated to be 94.8 meV. This is much lower than the acceptor level of 0.37 eV
for boron. However, for high impurity concentration levels, Inushima et al. report
conduction through hopping mechanism using excited level of boron [34]. The com-
bination of the excited level of boron and optical phonons forms an impurity band
41
Figure 3.11: I-V curves at 200 K substrate temperature for delta-doped diamond
annleaned at 500?C for 5 minutes
through which carriers can move [35]. This manifests as a trap level between 50
and 70 meV above the valance band at the interface between heavily doped and
low doped regions [34]. Activation energy of 94.8 meV obtained here is only slightly
above this.
Calculation of hall coefficients showed large differences between two diagonal
hall coefficients for the same temperature. For the temperature range tested, the
average hall coefficient randomly fluctuated between positive and negative value.
Thus, the results show mixed carrier conduction for the bulk, instead of just con-
duction through holes due to the delta layer. This also meant that the carrier
concentration and mobility data had a lot of variability, so much so that extraction
of any trend was not possible. Even with large variability, most of the data points
for mobility fell below 15 cm2V ?1s?1. Likewise, sheet carrier density values were
between 1010 cm?2 and 1013 cm?2 for both positive and negative carrier types, with
42
Figure 3.12: Natural logarithm of conductivity versus reciprocal temperature for
delta-doped diamond for data Hall data analyzed using ASTM standards [28] along
with a linear trendline
a weak negative slope for sheet carrier density versus reciprocal temperature.
Double delta-doped sample was also probed using the Van der Pauw con-
figuration. I-V measurements were conducted to make sure that there was linear
behavior. Once linear I-V behavior was ensured, Hall measurements were performed
for temperature range of 125 K to 350 K at input current of 0.5 mA. Measurement
at temperature as low as 125 K was possible in the HMS-5000 System for double
delta doped diamond because of its much lower resistivity than that of H-terminated
or single delta-doped diamond. This was expected because of high doping density
of 4 ? 1021 cm?2 right at the surface. Fig 3.13 shows log of conductivity versus
reciprocal temperature. A linear trendline was drawn for part of the data set at
higher temperature end, as it gives a linear behavior. The slope thus extracted
yielded an activation energy of 53 meV. This is slightly lower than the activation
energy of 94 meV for single delta-doped diamond, but falls within the trap level of
43
Figure 3.13: Natural logarithm of conductivity versus reciprocal temperature for
double delta-doped diamond, along with a linear trendline for the highligted data-
points
50 and 70 meV observed by Inushima et al due to hopping conductivity [34]. Hence,
conduction in double delta-doped diamond is due to hopping mechanism.
At temperature points below 252 K, however, there were fluctuations in hall
coefficients similar to that for single delta-doped diamond. The hall coefficients fluc-
tuated between positive and negative values below 252 K, but were always positive
for temperatures above 252 K. Thus, above 252 K, conduction occurs predominantly
due to p-type carriers, whereas below this temperature, conduction occurs due to
mixed carriers. At 300 K, resistivity of 10.1 ? ? cm, mobility of 11.4 cm2V ?1s?1,
and sheet carrier density of 5.4 ? 1016 cm?2 was obtained. Fig 3.14 shows mobil-
ity versus temperature relation. The highlighted linear part of the data set was
used to calculate the following dependence of mobility on temperature: ? ? T 12.
This dependency is much greater than the usual T 3/2 relationship for ionized impu-
44
Figure 3.14: Mobility versus temperature for double delta-doped diamond and trend-
line for the highlighted linear part of the data set to determine ? and T relationship
rity scattering. The mobility values fluctuate below 252 K as expected because of
fluctuating hall coefficients at those temperature points.
Fig 3.15 shows bulk carrier concentration versus reciprocal temperature for
temperatures above 252 K. Again, this range was selected due to large spread in data
below 252 K. Carrier concentration decreases with increasing temperature for 252
K to 350 K. The highest bulk carrier concentration in this range is 5.1? 1017 cm?3,
thus, sheet carrier concentration must be below this number. However, boron con-
centration is in excess of 1 ? 1021 cm?2 for both the surface and sub-surface delta
layers. The significantly lower value for carrier concentration could be because of
partial ionization of boron impurities. Also, number of ionized impurities should
increase with increasing temperature as there is more thermal energy, which is op-
posite to what is observed in this case. This means that there must be increasing
45
Figure 3.15: Bulk carrier density versus reciprocal temperature for double delta-
doped diamond
compensation of p-type carriers in this temperature range.
3.3 Post-irradiation Hall Measurement
Measurement of I-V characteristics after proton irradiation showed degrada-
tion of Ohmic behavior for both H-terminated and delta doped diamonds. Because
of this, the processed data showed more scatter in the range than that for pre-
radiation case. Hall measurements were taken in the range of 288 K to 350 K. Hall
measurements at lower temperatures were increasingly difficult because of the in-
crease in resistivity with decreasing temperature. This is because it was harder to
measure the higher voltage differences because of HMS-5000 System limitation.
46
3.3.1 H-terminated diamond
Following irradiation with protons, H-terminated diamond showed degrada-
tion of ohmic contacts as seen from the deviation from linear I-V behavior in fig
3.16(a). This caused there to be greater error in the calculated data for resistivity
and hall coefficient, resulting in greater scatter in temperature dependent resistivity,
mobility and carrier concentration datasets. Thus, to get more relaible temperature
dependence datasets, the contacts were annealed at 550?C for 1 minute. Post-anneal
measurements, shown in fig 3.16(b), showed improvement in linear behavior. But,
at input current in microAmpere regime, the I-V behavior was still poor. I-V mea-
surements taken at input current in nanoAmpere regime, however, show far better
linear relationship, as evidenced by fig 3.17. At this input current domain, annealing
at 550?C for a minute leads to dramatic improvement in linear I-V behavior. Also
seen is that the voltage response for the same input current decreased by an order
of magnitude as a result of annealing.
Hall measurements were performed for both post-irradiation as-is and post-
irradiation annealed cases at input current of 1 nA. Quality of ohmic contact had
a direct effect on resistivity calculations. Fig 3.18 shows comparision of tempera-
ture dependent resistivity values between pre-radiation, post-radiation, and post-
radiation annealed cases. The dramatic increase in post-radiation resistivity values
result from error in resistivity calculation due to non-linear I-V behavior. In con-
trast, the post-irradiation annealed resistivity values are only slightly greater than
pre-irradiation values. Nevertheless, annealing might have led to some degree of
47
(a)
(b)
Figure 3.16: I-V curves at 288 K substrate temperature for H-terminated diamond
after proton irradiation (a) as-is (b) after annealing at 550?C for 1 minute
healing of the displacement damage sustained by the substrate. The increase in
resistivity is due to defects formed in the substrates due to displacement damage
caused by proton irradiation.
Fig 3.19 shows natural log of conductivity versus reciprocal temperature graphs
for post-irradiation as-is and post-irradiation annealed cases. Activation energy (Eg)
of 0.639 eV was obtained from the slope of the dataset for post-irradiation as-is
case, a decrease from 0.76 eV for pre-irradiation case. Activation energy (Eg) for
48
(a)
(b)
Figure 3.17: I-V curves at 288 K substrate temperature for H-terminated diamond
after proton irradiation for nanoAmpere input current regime (a) as-is (b) after
annealing at 550?C for 1 minute
post-irradiation annealed case was even lower at 0.407 eV. Degradation of linear
I-V behavior may have contributed to the deviation from activation energy of pre-
irradiation case. However, this deviation seems more strongly correlated to radiation
damage. This is because when ohmic behavior was improved by post-irradiation
annealing process, the difference in activation energy between pre-irradiation and
post-irradiation annealed cases became even more pronounced than that between
pre-irradiation and post-irradiation as-is case. Thus, proton irradiation introduced
49
Figure 3.18: Resistivity versus temperature for H-terminated diamond before and
after proton irradiation
trap states, which facilitated the electrical conduction.
Diagonal hall coefficients for each temperature point calculated using equations
2.8 and 2.9 had a lot of variation. This is the effect of sample inhomogeniety caused
by proton irradiaiton. Furthermore, for some temperature points, the two diagonal
hall coefficients had opposite signs. This means that conduction in H-terminated
diamond post-irradiation is due to mixed carriers, rather than just a single carrier
type. Moreover, this large spread in hall coefficient data set resulted in large spread
in mobility and carrier concentration data sets. Thus, no trend for mobility and
carrier concentration with respect to temperature could be identified. Even with this
spread, thirty-nine out of sixty-three total mobility data points in the temperature
range of 288 K to 350 K for post-irradiation annealed case had values less than
15 cm2V ?1s?1. Likewise, sheet carrier concentration data set for post-irradiation
50
Figure 3.19: Natural log of conductivity versus reciprocal temperature for H-
terminated diamond after proton irradiation as-is and after annealing at 550?C for
1 minute
as-is case was in 107 to 109 cm?2 range, with most data points in 108 cm?2 regime.
And for post-irradiation annealed case, sheet concentration data set was in 109 to
1011 cm?2 range.
3.3.2 Delta doped diamond
After proton-irradiation, delta-doped diamond showed degradation in ohmic
contacts similar to that for H-terminated diamond. Fig 3.20(a) shows this deviation
from linear behavior. Also, for the same input current, voltage difference increased
by an order of magnitude than for the pre-irradiation case. However, after annealing
the contacts at 600?C for 1 minute, the I-V behavior was linear again, as seen in fig
3.20(b).
Hall measurements were taken for post-irradiation as-is case at input current
51
(a)
(b)
Figure 3.20: I-V curves at 288 K substrate temperature for delta-doped diamond
after proton irradiation (a) as-is (b) after annealing at 600?C for 1 minute
of 1 nA to minimize error. For post-irradiation annealed case, measurements were
taken at 10 nA input current as was done for pre-irradiation case. Post-irradiation
as-is case showed an increase in resistivity values by an order of magnitude. This
effect subsided once the contacts were annealed at 600?C for 1 minute. The re-
sistivity values for post-irradiation annealed case matched very closely to that of
pre-irradiation case, especially at higher temperatures. Fig 3.21 shows the differ-
ence in resistivity values for all three cases. This behavior for resistivity was also
52
Figure 3.21: Resistivity versus temperature for delta-doped diamond before and
after proton irradiation
exhibited by H-terminated diamond. The increase in resistivity is due to defects
formed by displacement damage due to proton irradiation. Nevertheless, the degra-
dation of ohmic contacts also affected the calculations. When ohmic contacts were
annealed to obtain linear I-V behavior, resistivity values were only slightly higher
than for pre-irradiation case. However, annealing might also have led to some degree
of damage healing and structural recovery, yielding lower resistivity.
Fig 3.22 shows natural log of conductivity versus reciprocal temperature graphs
for post-irradiation as-is and post-irradiation annealed cases. For both the data sets,
trendlines are drawn for the linear parts to obtain information about activation
energy. The data points making up the linear region are highlighted for both data
sets. Pre-irradiation case, which has more spread in the data set, gives activation
energy of 0.366 eV. Post-irradiation case with far less spread due to linearity in
53
Figure 3.22: Natural log of conductivity versus reciprocal temperature for delta-
doped diamond after proton irradiation as-is and after annealing at 600?C for 1
minute
I-V measurements gives activation energy of 0.356 eV, very close to that for post-
irradiation as-is case. The activation energy values are close to boron acceptor level
of 0.37 eV. The post-irradiation activation energies are markedly higher than 0.0948
eV for the pre-irradiation case. This means that conduction through the valence
band is dominant after irradiation. This could be due to disruption of the shallow
level of 94.8 meV.
Hall coefficients for both post-irradiation as-is and post-irradiation annealed
cases showed similar characteristics. There was high fluctuation in the hall coeffi-
cients to the extent that sometimes the diagonal coefficients for the same temper-
ature point had opposite signs. This means that there was mixed type of carriers.
Also, as a result of the spread in hall coeffcients, calculated mobility and sheet con-
centration values had a large spread. This was especially true for post-irradiation
54
as-is case. For post-irradiation annealed case, while the highest mobility value was
608.5 cm2V ?1s?1, thirty-eight out of sixty total data points in the tested temper-
ature range of 291 K to 350 K yielded mobility values less than 10 cm2V ?1s?1.
Thus, activation energy for conduction increased, and carrier concentration and hall
mobility decreased after proton irradiation.
3.4 Device Simulation
Device simulation was performed to understand the post-irradiation degrada-
tion of output current from device point of view. Using the quadratic model and
constant mobility assumption, drain current as a function of source voltage was
modeled. The gate voltage used was from 5 V to -5 V for threshold voltage assumed
to be 5.1 V. Mobility values obtained from Hall measurements (listed in table 3.1)
was used in the model. For H-terminated diamond, Al2O3 is used as gate oxide,
whereas for delta-doped diamond, the intrinsic cap layer serves this purpose. Fig
3.23 and 3.24 show pre- and post- irradiation drain current output versus source
voltage for FETs based on H-terminated and delta doped diamond respectively.
Maximum drain current (ID) values are also listed in table 3.1 for each case for
gate voltage of -5 V. For -5 V gate bias, the maximum drain current decreased by
95% for H-terminated diamond post irradiation. And, for delta-doped diamond, the
maximum drain current decreased by 37%.
55
Table 3.1: Parameters used in quadratic constant mobility model for MOSFET IV
characteristics
H-terminated Delta-doped
Dielectric material Al2O3 Intrinsic diamond
Dielectric constant 9 5.7
Dielectric layer depth (nm) 25 12
Mobility at 300 K 128.3 6.49 2.92 1.84
(cm2V ?1s?1)
Max. drain current at Vg = 695 35.15 20.87 13.15
-5V (mA/mm)
(a) (b)
Figure 3.23: I-V characteristics for H-terminated diamond (a) pre- and (b) post-
irradiation with protons
(a) (b)
Figure 3.24: I-V characteristics for delta-doped diamond (a) pre- and (b) post-
irradiation with protons
56
Chapter 4: Conclusions and Future Directions
4.1 Conclusions
Due to high thermal conductivity, mechanical robustness and ultra-wide bandgap,
diamond shows great promise as material for high frequency, high-power device ap-
plications. Also, due to high binding energy of carbon atoms in diamond, it is
predicted to be radiation hard. Radiation hardness would allow diamond based
sensors and devices to be used in harsh environments such as outer-space, nuclear
reactors, and the Large Hadron Collider, etc. This work explored radiation hardness
of diamond from device point of view.
As such, the diamonds tested had conducting channel activated much below
the ultra wide bandgap energy of diamond. This was possible in two ways: (1)
terminating HPHT diamond surface with hydrogen, and (2) incorporating heavily
boron doped delta layer tens of nanometers underneath the top surface.
FETs were fabricated on H-terminated substrates. Blanket Au layer was de-
posited, then patterened into mesas, and subjected to O2 plasma to remove H-
termination, isolating the devices. More patterning was done to form Ohmic con-
tacts. Finally, gate dielectric, Al2O3, was deposited by ALD, followed by e-beam
and liftoff processes to deposit Al gate on top. The FETs were subjected to gamma
57
irradiation from a total dose of 1 kRad to 26.3 MRad. At low dose up to 100
kRad, the FETs showed slight decrease in drain current output, threshold voltage,
and maximum transconductance. This could be due to the formation of defects
which act as scattering centers. However, Campbell and Mainwood?s simulation
shows that very few i.e. ? 0.09 vacancies/gamma per cm are formed in diamond
by gamma irradiation of 1 and 2 MeV [23]. Thus, further research is necessary to
determine the cause. In contrast, after high dose ? 13.6 MRad gamma irradiation,
the drain current output increased sigificantly. Gamma irradiation can form trap
states by introducing defects, which could result in enhanced hoping conductivity
that is responsible for higher output drain current. However, threshold voltage also
increased significantly, while gate leakage current remained unchanged.
Radiation hardness of diamond was also evaluated for proton irradiation. A
cyclotron was used to bombard the samples with 152 keV protons at fluence of
1.03 ? 1012 cm?2. This time, bare substrates of H-terminated and delta-doped
diamond were used. Hall measurements using the Van der Pauw configuration
were performed to evaluate changes in transport properties. Diamond substrates
were too small, however, for the available Hall measurement system, HMS-5000, to
probe them directly. An alternative approach was designed to get around this size
limitation. Insulating pieces of SiO2 substrate were patterned with Cr/Au contact
pads in the corners. The small diamond substrates also with Cr/Au contact pads
in the corners were glued to these bigger SiO2 substrate using LOR7B resist. This
was followed by wire-bonding to join Cr/Au pads on the diamond substrates with
Cr/Au pads on the SiO2 substrates. Probes of HMS-5000 System could then be
58
dropped on the Cr/Au pads on the SiO2 substrate to perform Hall measurements
on the diamond glued to it.
Appropriate input current was selected for each substrate before and after
proton irradiation to ensure measurements in linear I-V region. At 300 K, H-
terminated diamond showed resistivity of 8.21 ?103 ??cm, carrier concentration of
2.97 ?1011 cm?2, and hall mobility of 128.3 cm2V ?1s?1. Conduction by p-type car-
riers and activation energy of 0.76 eV was obtained. Also, mobility increased with
3
temperature closely matching the relationship with temperature of ? ? T 2 , which
is widely accepted to be due to ionized impurity scattering. After proton irradia-
tion, Hall measurements showed significant differences. At 300 K, sheet resistivity
increased to 2.69 ?105 ?? cm, carrier concentration decreased to 1.8 ?1011 cm?2,
and mobility also decreased to 6.49 cm2V ?1s?1. Hall coefficient showed fluctuation
between positive and negative sign, indicating mixed carrier conduction. Activation
energy decreased to 0.407 eV, indicating formation of a new trap level due to proton
irradiation.
Likewise, for delta-doped diamond, resistivity of 8.37 ?104 ? ? cm, carrier
concentration of 1.27 ?1012 cm?2, and mobility of 2.92 cm2V ?1s?1 was obtained at
300 K. Hall coefficients again showed mixed carrier conduction. Activation energy of
94.7 meV was obtained, which is close to the expected activation energy for hopping
conduction reported by Inushima et al [34]. After proton irradiation, at 300 K,
resistivity increased to 1.33 ?105 ?? cm, sheet carrier concentration was the same
at 1.27 ?1012 cm?2, and mobility decreased to 1.84 cm2V ?1s?1. Hall coefficients
indicated mixed carrier conduction. Activation energy increased significantly to
59
0.356 eV, which is close to the boron acceptor level, indicating disruption of the
shallow trap state.
Simulation of drain current versus source voltage showed degradation of max-
imum drain current output by 95% for H-terminated diamond post proton irradia-
tion. Similarly, for delta-doped diamond, maximum drain current output decreased
by 37% after proton-irradiation. Thus, both H-terminated and delta-doped diamond
substrates show degradation due to proton-irradiation.
4.2 Future Directions
Based on the results of this work, several topics can be investigated to con-
tinue the advancement of knowledge in diamond based devices. First and foremost,
better controls must be implemented for more reliable results through improvement
in ohmic contacts, monitoring surface hydrogen coverage, and fabricating smaller
structures, thus allowing less variability, for Hall measurements. A more robust
and reliable contact scheme must be developed to obtain good ohmic behavior and
avoid delamination of contacts. This can be done through alloyed contact process-
ing for different metal combinations. To explore different contact schemes as well
as measure contact resistance, and sheet resistivity, circular transfer length method
(CTLM) pattern has already been prepared to be patterned using electron-beam
direct write procedure. If successful, this would significantly improve the reliability
of measurements of FETs as well as that of Hall measurements. Likewise, because
hydrogen termination is unstable at high temperatures, surface coverage of diamond
60
with hydrogen should be mapped for the entire surface using Micro-Raman spec-
troscopy. This would also be useful to monitor homogeniety of H-terminated sam-
ples, which is crucial for accuracy of Hall coefficient calculation. To ensure less vari-
ability, Hall measurements can be performed using Hall bar configuration fabricated
at a scale of about 100 microns. Also, variable magnetic field Hall measurement can
be used to determine contributions from bulk and 2D conduction separately. Fur-
thermore, higher dose gamma irradiation and additional proton fluences should be
used in addition to different defect characterization techniques such as deep level
transient spectroscopy (DLTS), capacitance-voltage-temperature (C-V-T), photo-
capacitance, and deep level optical spectroscopy (DLOS), etc. to understand the
fundamental effects of radiation on p-channel diamond substrates and devices.
61
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