42 research outputs found
Modeling of Radiation-Induced Defect Recovery in 3C-SiC Under High Field Bias Conditions
In this work, the implications of high field bias conditions in
radiation-induced defect recovery in 3C-SiC crystals is studied. It is well
known that transient heating effects (or thermal spikes) occur when energetic
swift heavy ions (SHIs) deposit energy to the surrounding medium via
ionization. Here, we explore the dynamics of this transient event under high
background electric fields in 3C-SiC, which is what occurs when an ion strike
coincides with field-sensitive volumes. In this study, we use the Ensemble
Monte Carlo method to quantify how the energy deposition of the ionized regions
change in response to high background electric fields. Subsequently, we study
the relationship between the radiation-induced thermal spike and defect
recovery using molecular dynamics simulations. We find that field strengths
below the critical breakdown of wide bandgap devices are sufficient to
exacerbate the localized heating, which subsequently enhances the defect
recovery effect. This work is beneficial for 3C-SiC electronics and materials
used in high radiation environments.Comment: 6 figures, 20 page
High-temperature Ultraviolet Photodetectors: A Review
Wide bandgap semiconductors have become the most attractive materials in
optoelectronics in the last decade. Their wide bandgap and intrinsic properties
have advanced the development of reliable photodetectors to selectively detect
short wavelengths (i.e., ultraviolet, UV) in high temperature regions (up to
300{\deg}C). The main driver for the development of high-temperature UV
detection instrumentation is in-situ monitoring of hostile environments and
processes found within industrial, automotive, aerospace, and energy production
systems that emit UV signatures. In this review, a summary of the optical
performance (in terms of photocurrent-to-dark current ratio, responsivity,
quantum efficiency, and response time) and uncooled, high-temperature
characterization of III-nitride, SiC, and other wide bandgap semiconductor UV
photodetectors is presented
Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering
Over the last decade, progress in wide bandgap, III-V materials systems based
on gallium nitride (GaN) has been a major driver in the realization of high
power and high frequency electronic devices. Since the highly conductive,
two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is based on
built-in polarization fields (not doping) and is confined to very small
thicknesses, its charge carriers exhibit much higher mobilities in comparison
to their doped counterparts. In this study, we show that this heterostructured
material also offers the unique ability to manipulate electrical transport
separately from thermal transport through the examination of fully-suspended
AlGaN/GaN diaphragms of varied GaN buffer layer thicknesses. Notably, we show
that ~ nm thin GaN layers can considerably impede heat flow without
electrical transport degradation, and that a significant improvement (~x) in
the thermoelectric figure of merit () over externally doped GaN is
observed in 2DEG based heterostructures. We also observe state-of-the art
thermoelectric power factors ( ) at room
temperature) in the 2DEG of this material system. This remarkable tuning
behavior and thermoelectric enhancement, elucidated here for the first time in
a polarization-based heterostructure, is achieved since the electrons are at
the heterostructured interface, while the phonons are within the material
system. These results highlight the potential for using the 2DEG in III-V
materials for on-chip thermal sensing and energy harvesting.Comment: Accepted for publication in Advanced Functional Materials, in press
(2018
High responsivity, low dark current ultraviolet photodetector based on AlGaN/GaN interdigitated transducer
An ultraviolet (UV) photodetector employing the two-dimensional electron gas
(2DEG) formed at the AlGaN/GaN interface as an interdigitated transducer (IDT)
is characterized under optical stimulus. The 2DEG-IDT photodetector exhibits a
record high normalized photocurrent-to-dark current ratio (NPDR,
). In addition, we observe a high responsivity ( A/W)
and ultraviolet-visible rejection-ratio (), among the highest reported
values for any GaN photodetector architecture. We propose a gain mechanism to
explain the high responsivity of this device architecture, which corresponds to
an internal gain of . We argue that the valence band offset in the
AlGaN/GaN heterostructure is essential in achieving this high responsivity,
allowing for large gains without necessitating the presence of trap states, in
contrast to common metal-semiconductor-metal (MSM) photodetector architectures.
Our proposed gain mechanism is consistent with measurements of the scaling of
gain with device channel width and incident power. In addition to high
performance, this photodetector architecture has a simple two-step fabrication
flow that is monolithically compatible with AlGaN/GaN high electron mobility
transistor (HEMT) processing. This unique combination of low dark current, high
responsivity and compatibility with HEMT processing is attractive for a variety
of UV sensing applications
Molybdenum Trioxide Gates for Suppression of Leakage Current in InAlN/GaN HEMTs at 300{\deg}C
Because high electron mobility transistors (HEMTs) often exhibit significant
gate leakage during high-temperature operation, the choice of Schottky metal is
critical. Increased gate leakage and reduced ON/OFF ratio are unsuitable for
the design of high-temperature electronics and integrated circuits. This paper
presents high-temperature characteristics of depletion-mode molybdenum trioxide
(MoO)-gated InAlN/GaN-on-silicon HEMTs in air. After a room temperature
oxidation of the Mo for 10 weeks, the leakage of the HEMT is reduced over 60
times compared to the as-deposited Mo. The use of MoO as the Schottky
gate material enables low gate leakage, resulting in a high ON/OFF current
ratio of 1.2 x 10 at 25{\deg}C and 1.2 x 10 at 300{\deg}C in air.
At 400{\deg}C, gate control of the InAlN/GaN two-dimensional electron gas
(2DEG) channel is lost and unrecoverable. Here, this permanent device failure
is attributed to volatilization of the MoO gate due to the presence of
water vapor in air. Passivation of the device with SiN enables operation up to
500{\deg}C, but also increases the leakage current. The suppression of gate
leakage via Mo oxidation and resulting high ON/OFF ratio paves the way for
viable high-temperature GaN-based electronics that can function beyond the
thermal limit of silicon once proper passivation is achieved
Effect of Proton Irradiation Temperature on Zinc Oxide Metal-Semiconductor-Metal Ultraviolet Photodetectors
The electrical and structural characteristics of 50 nm zinc oxide (ZnO)
metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors subjected to
proton irradiation at different temperatures are reported and compared. We
irradiated the devices with 200 keV protons to a fluence of 1016 cm-2.
Examination of the X-ray diffraction (XRD) rocking curves indicates a strongly
preferred (100) orientation for the grains of the as-deposited film, with
decreases in crystal quality for all irradiated samples. In addition, peak
shifts in XRD and Raman spectra of the control sample relative to well-known
theoretical positions are indicative of tensile strain in the as-deposited ZnO
films. We observed shifts of these peaks towards theoretical unstrained
positions in the irradiated films relative to the as-deposited film indicate
partial relaxation of this strain. Raman spectra also indicate increases of
oxygen vacancies (V_O ) and zinc interstitials (Zn_i ) relative to the control
sample. Additionally, photocurrent versus time measurements showed up to 2x
increases in time constants for samples irradiated at lower temperatures months
after irradiation, indicating that the defects introduced by suppression of
thermally-activated dynamic annealing process has a long-term deleterious
effect on device performance.Comment: 5 Pages, 4 figure
Micro-Tesla Offset in Thermally Stable AlGaN/GaN 2DEG Hall-effect Plates using Current Spinning
This letter describes the characterization of a low-offset Hall-effect plate
using the AlGaN/GaN two-dimensional electron gas(2DEG). Four-phase current
spinning was used to reduce sensor offset voltage to values in the range of 20
nV, which corresponds to a low residual offset of 2.6 micro-Tesla when supplied
with low voltages (0.04 to 0.5V). These offsets are 50x smaller than the values
previously reported for GaN Hall-effect plates, and it is on par with
state-of-the-art silicon Hall-effect plates. In addition, the offset does not
exceed 10 micro-Tesla even at higher supply voltage of 2.34V. The sensor also
shows stable current-scaled sensitivity over a wide temperature range of -100C
to 200C, with temperature drift of -125 ppm/C. This value is 3x better than
state-of-the-art Silicon Hall-effect plates. Additionally, the sensor's voltage
sensitivity (57 mV/V/T) is also similar. Because of their low offset values,
AlGaN/GaN Hall-effect plates are viable candidates for low-field and high
temperature magnetic sensing in monolithic GaN systems used in extreme
temperature environments such as power inverter, down-well, combustion, and
space applications
Analysis of the Mobility-Limiting Mechanisms of the Two-Dimensional Hole Gas on Hydrogen-Terminated Diamond
Here we present an analysis of the mobility-limiting mechanisms of a
two-dimensional hole gas on hydrogen-terminated diamond surfaces. The
scattering rates of surface impurities, surface roughness, non-polar optical
phonons, and acoustic phonons are included. Using a Schrodinger/Poisson solver,
the heavy hole, light hole, and split-off bands are treated separately. To
compare the calculations with experimental data, Hall-effect structures were
fabricated and measured at temperatures ranging from 25 to 700 K, with hole
sheet densities ranging from 2 to 6 and typical
mobilities measured from 60 to 100 cm/(Vs) at room temperature.
Existing data from literature was also used, which spans sheet densities above
1. Our analysis indicates that for low sheet
densities, surface impurity scattering by charged acceptors and surface
roughness are not sufficient to account for the low mobility. Moreover, the
experimental data suggests that long-range potential fluctuations exist at the
diamond surface, and are particularly enhanced at lower sheet densities. Thus,
we propose a second type of surface impurity scattering which is caused by
disorder related to the C-H dipoles.Comment: 11 pages, 7 figure
Effect of Geometry on Sensitivity and Offset of AlGaN/GaN and InAlN/GaN Hall-effect Sensors
The current- and voltage-scaled sensitivities and signal-to-noise ratios
(SNR) (with respect to thermal noise) of various octagonal AlGaN/GaN and
InAlN/GaN Hall-effect sensors were examined in this work. The effect of metal
contact lengths on sensitivity and sensor offset was evaluated. Calculations
that take into account the shape of the device show that devices with
point-like contacts have the highest current-scaled sensitivity (68.9 V/A/T),
while devices with contacts of equal length to their non-contact sides have the
highest voltage-scaled sensitivity (86.9 mV/V/T). The sensitivities of the two
other devices follow the predicted trends closely. All the devices have offsets
less than 20 T at low supply current operation (< 300 A) and most
remain below 35 T at higher supply current (up to 1.2 mA). The consistent
low offsets across the devices imply that the choice of Hall-effect sensor
geometry should mainly depend on whether the device is current-biased or
voltage-biased and the frequency at which it will operate. This work
demonstrates that GaN Hall-effect sensor performance can be improved by
adjusting the geometry of the Hall-effect plate specific to its function (e.g.,
power electronics, navigation, automotive applications)
Gallium Nitride Photodetector Measurements of UV Emission from a Gaseous CH4/O2 Hybrid Rocket Igniter Plume
Owing to its wide (3.4 eV) and direct-tunable band gap, gallium nitride (GaN)
is an excellent material platform for UV photodetectors. GaN is also stable in
radiation-rich and high-temperature environments, which makes photodetectors
fabricated using this material useful for in-situ flame detection and
combustion monitoring. In this paper, we use a GaN photodetector to measure
ultraviolet (UV) emissions from a hybrid rocket motor igniter plume. The
normalized photocurrent-to-dark current ratio (NPDR) is a performance metric
which simultaneously captures the two desired characteristics of high
responsivity and low dark current. The NPDR of our device is record-high with a
value of 6 x 10 W and the UV-to-visible rejection ratio is 4 x
10. The photodetector shows operation at high temperatures (up to
250{\deg}C), with the NPDR still remaining above 10 W and the peak
wavelength shifting from 362 nm to 375 nm. The photodetector was placed at
three radial distances (3", 5.5", and 7") from the base of the igniter plume
and the oxidizer-to-fuel ratio (O2/CH4) was varied. The data demonstrates a
clear trend of increasing current (and thus intensity of plume emission) with
increasing fuel concentration and decreasing separation between the
photodetector and the plume. By treating the plume as a black body, and
calculating a radiative configuration factor corresponding to the geometry of
the plume and the detector, we calculated average plume temperatures at each of
the three oxidizer-to-fuel ratios. The estimated plume temperatures were
between 850 and 950 K for all three combustion conditions. The temperature is
roughly invariant for a fixed fuel concentration for the three tested
distances. These data demonstrate the functionality of GaN as a material
platform for use in harsh environment flame monitoring.Comment: Accepted to IEEE Aerospace Conference 201