The electron transport properties of AlGaN/GaN heterojunction field effect transistors
(HFETs) were studied by two-dimensional drift-diffusion (DD) modeling method. The model
performs self-consistent numerical computation on the Poisson equation, carrier statistics and
current and continuity equations. The spontaneous and piezoelectircal polarization charges
uniquely for GaN material was taken into account, which influences significantly the electron
distribution and current density. Sliced 1D Schrödinger equation along c direction was solved to
obtain electron distribution. The electrical field at the 2DEG channel was found to have a peak
position locates at the gate edge towards the drain side, reaching 106 V/cm at high bias condition.
The surface potential boundary conditions also were found to have profound influence to the
GaN based micro emitter optoelectronic device array has been proved to be the core
component for wide variety of applications such as microdisplay, biosensor, projection etc.
Etching is one of the key steps to form the GaN micro emitter array device, including inductively
coupled plasma (ICP) dry etch and alkaline solution wet etch. This paper reports the recent
progress made by Ostendo Technologies Inc in fabricating the ultra-high density, large aspect-ratio
etching formed monolithic GaN micro emitter optoelectronic device array. The unit density
reaches 1M per cm<sup>2</sup>, with good uniformity across the whole wafer. Perpendicular etching sidewall
was achieved, with smooth surface roughness which is significance feature used for laser diodes
The ferroelectric effect has been demonstrated on an AlGaN/GaN heterostructure field effect transistor using a Pb(Zr<sub>0.52</sub>Ti<sub>0.48</sub>)O<sub>3</sub> layer deposited by radio-frequency magnetron sputtering. The device with a metal-ferroelectric-metal-semiconductor (MFMS) structure was fabricated with a Schottky contact placed between ferroelectric PZT and AlGaN/GaN 2-dimensional electron gas (2DEG) channel. The Schottky contact performs as a bottom electrode of the ferroelectric PZT and also as a barrier layer to prevent interaction at the interface between PZT and GaN. X-ray diffraction revealed the formation of (111)-oriented perovskite phase PZT on gate patterned AlGaN/GaN heterostructures. Transfer characteristics of the double-gate ferroelectric field effect transistor was determined by measuring its source-drain current as the gate bias applied on the top electrode was swept from -15 V to 15 V and then back to -15 V with a voltage step of 0.1 V. Ferroelectric behavior was observed in the plot of source-drain current versus gate voltage that showed a large counterclockwise hysteresis with a 50 % current modulation at zero gate bias.
GaN-based heterojunction field effect transistors (HFETs) are strong contenders to replace vacuum tubebased
devices in the high power, and high frequency arena. However, the piezoelectric stain, exploited to
generate high density two-dimensional electron gases (2DEGs) in AlGaN/GaN devices is not necessarily
desirable nor might it bode well in terms of device reliability. By using lattice-matched InAlN as the
barrier, even higher densities of 2DEG and now respectable DC and RF performance can be achieved while
at the same time avoiding the strain and subsequent reliability issues in the devices. However, little work
has been done in identification of trapping mechanisms in the InAlN-based devices. The trapping is at the
heart of the reduced RF performance of all the GaN-based devices, limiting the maximum attainable RF
power. In this work, transient current spectroscopy, which observes the dynamics of the drain current
during gate lag measurements, is utilized to ascertain information about the trapping levels, cross sections,
and spatial locations of traps in the InAlN-based devices. Preliminary measurements indicate that one of
the traps identified in this work (at 0.12eV) is similar to one measured by deep level transient spectroscopy
(DLTS) in similar structures. Investigations of this type are imperative for the further development and
implementation of this highly promising material system.
Near lattice matched Al<sub>0.81</sub>In<sub>0.19</sub>N/GaN heterojunction structures are compared with conventional Al<sub>0.3</sub>Ga<sub>0.7</sub>N/GaN heterojunctions in terms of the sheet density and mobility and their dependence on barrier and spacer layer parameters. With the insertion of an AlN spacer, the mobility of both structures is improved dramatically. Self-consistent solution of Poisson-Schrödinger equations was developed in order to determine the band structure and carrier distribution in these GaN based heterostructures in an effort to gain insight into the experimental observations. Surface donor states were included to account for the origin of electrons in 2DEG, which is treated as charge neutralization conditions in the simulation. Also the change in the piezoelectric polarization due to the electromechanical coupling effect, and shift of band gap caused by uniaxial strain were both included in the calculations. The calculated sheet density is close to the measured values, especially for the AlGaN samples investigated, but a notable difference was noted in the AlInN cases. The discrepancy is confirmed to be caused by the existence of a Ga-rich layer on the top of AlN spacer during the growth interruption, which can split the 2DEG into two channels with different mobilities and lower the overall sheet density. When the modifications made necessary by this GaN layer are taken into account in our model for the AlInN barrier case, the calculations match with the experimental data. When the spacer thickness increase from 0.3 to 3 nm, the total sheet density was found to slightly increase experimentally, which agreed with the theoretical prediction.
Compound semiconductors based on GaN have multiple functional applications. Useful compositions include GaN, and
ternary and quaternary compositions of (AlGaIn)N. Defects arising from lattice mismatch, point defects, or impurities
may act as electrical trapping centers and degrade device efficiency. Current-voltage, capacitance-voltage, thermal
admittance spectroscopy (TAS), and deep level transient spectroscopy (DLTS) measurements are applied to characterize
the defects in Al<sub>0.40</sub>Ga<sub>0.80</sub>N and In<sub>0.18</sub>Al<sub>0.82</sub>N in this report. Broad peaks with a shoulder at high temperature dominate the
DLTS spectra in each of the materials. An acceptor trap associated with a dislocation appears at 340 K in AlGaN. The
defect has an energy of 0.2 eV and capture cross section of 10<sup>-21</sup> cm<sup>2</sup>. A second trap at 0.35 eV, 10<sup>-14</sup> cm<sup>2</sup> appears in the
TAS measurements in addition to the trap at 0.2 eV. Defects in InAlN are dominated by a peak near 150 K. Two traps
appear in the TAS measurements. Both traps in the InAlN are acceptors, based on a lack of field dependent emission
rates using double pulse DLTS (DDLTS). The two energy levels in InAlN appear to be coupled, with only one state
occupied at a time.
We have investigated the efficiency droop in InGaN based multiple-quantum light-emitting diodes (MQWs-LEDs) and
double hetero-structure light-emitting diodes (DH-LEDs) by changing the barrier (both thickness and barrier height)
within quantum wells. Our results show that for MQW-LEDs, with the decrease of barrier width from 12nm In<sub>0.01</sub>Ga<sub>0.99</sub>N
to 3nm In<sub>0.01</sub>Ga<sub>0.99</sub>N, the external quantum efficiency (EQE) droop point is increased from 350 Acm<sup>-2</sup> to >1000 Acm<sup>-2</sup>,
and the slope of EQE drop is also greatly reduced. When the barrier height of the MQW-LEDs is decreased, i.e. barriers
changed from In<sub>0.01</sub>Ga<sub>0.99</sub>N (3nm) to In<sub>0.06</sub>Ga<sub>0.94</sub>N (3nm), the EL intensity is reduced to half. In the case of DH-LEDs,
6nm DH-LED shows the highest EL intensity and no EQE droop up to 1000 Acm<sup>-2</sup>. When the active region of the DHLED
is increased from 6nm to 12nm, the electroluminescence (EL) intensity is reduced to 70% of that of the 6nm DHLED,
and the EQE shows negligible droop compared to the 6nm DH-LED due to both enhanced hole injection and
reduced electron overflow. These results suggest that heavy effective mass of holes and low hole injection efficiency
(due to relatively lower p-doping) leading to severe electron leakage are responsible for the efficiency droop.
Vertical double heterostructures based on GaN were prepared and investigated for their current voltage characteristics
and compared to theory. In our quest to observe negative differential resistance (NDR) phenomenon based on quantum
mechanical tunneling to, we fabricated resonant tunneling diode (RTD) like structures grown on low defect density and
high quality templates prepared by metal organic chemical vapor deposition using in situ SiN nanonetwork induced
epitaxial lateral overgrowth. The measured threading dislocation density of the template was in the range of 10<sup>7</sup> cm<sup>-2</sup>.
Inductively coupled plasma reactive ion etching (ICP-RIE) where in enhanced chemical etching mode was used for
reducing the detrimental surface defects on the mesa walls which otherwise contribute to current. Double barrier
structures with varying barrier and quantum well thicknesses as well as doping profiles were tested for their I-V
characteristics. The rectifying phenomenon occurred as a result of depletion region in GaN above the top Al(Ga)N layer
and asymmetric barrier shape of GaN RTD-like structure due to polarization. With the aid of calculated band structure
and resultant doping profile optimization, we now observe what appears to be resonant increase in current, the source of
which is not yet clear, at quantum states of the well.
We report a comparison of various gate dielectrics including SiO<sub>2</sub>, Si<sub>3</sub>N<sub>4</sub>, ZrO<sub>2</sub> and Pb(Zr, Ti)O<sub>3</sub>
(PZT) on AlGaN/GaN heterojunction field-effect transistors, deposited by PECVD, MBE, and
sputtering respectively. In terms of I-V characteristics, maximum drain-source current could be
enhanced under positive gate voltage and the reverse leakage current level decreases by orders of
magnitude. In terms of DC measurements, very thin SiO<sub>2</sub> layers can improve performance, which
may be due to the passivation effect to remove surface states. No significant difference exists
between control and the Si<sub>3</sub>N<sub>4 </sub>and ZrO<sub>2 </sub>samples. Slightly reduction in transconduction is observed
on the sample with PZT probably because of the much thicker layer was utilized. The thickness of
insulator layers examined from C-V measurements reveals a better crystal quality can be obtained
by PECVD deposition. While the RF S-parameters measurements shows the PZT gate dielectric
brings the highest cut-off frequency or the lowest gate capacitance confirmed also by C-V data,
which makes it a better candidate for microwave applications.
AlGaN/6H-SiC heterojunction bipolar transistors (HBTs) were fabricated, and the device performance as well as the
electrical properties of the n-AlGaN/p-SiC heterojunction were studied by temperature dependent current-voltage
characterization. Current gain β=I<sub>C</sub>/I<sub>B</sub> calculated from <i>I-V</i> characteristics varied from sample to sample in the range of
75-100. A barrier height of 1.1 eV is derived from the Arrhenius plot and its origin is discussed.
The conduction band offset of n-ZnO/n-6H-SiC heterostructures prepared by rf-sputtered ZnO on commercial n-type
6H-SiC substrates has been measured. Temperature dependent current-voltage characteristics, photocapacitance, and
deep level transient spectroscopy measurements led to conduction band offsets of 1.25 eV, 1.1 eV, and 1.22 eV,
Electrical properties of n-ZnO/n-GaN isotype heterostructures prepared by rf-sputtering of ZnO films on GaN layers
which in turn grown by metal-organic vapour phase epitaxy are discussed. Current-voltage <i>(I-V)</i> characteristics of the n-
ZnO/n-GaN diodes exhibited highly rectifying characteristics with forward and reverse currents being ~1.43x10<sup>-2</sup> A/cm<sup>2</sup>
and ~2.4x10<sup>-4</sup> A/cm<sup>2</sup>, respectively, at ±5 V. From the Arrhenius plot built representing the temperature dependent
current-voltage characteristics (<i>I-V-T</i>) an activation energy 0.125 eV was derived for the reverse bias leakage current
path, and 0.62 eV for the band offset from forward bias measurements. From electron-beam induced current
measurements and depending on excitation conditions the minority carrier diffusion length in ZnO was estimated in the
range 0.125-0.175 &mgr;m. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073
Surface properties of GaN subjected to reactive ion etching and the impact on device performance have been investigated by surface potential, optical and electrical measurements. Different etching conditions were studied and essentially high power levels and low chamber pressures resulted in higher etch rates accompanying with the
roughening of the surface morphology. Surface potential for the as-grown <i>c</i>-plane GaN was found to be in the range of 0.5~0.7 V using Scanning Kevin Probe Microscopy. However, after reactive ion etching at a power level of 300 W, it decreased to 0.1~0.2 V. A nearly linear reduction was observed on <i>c</i>-plane GaN with increasing power. The nonpolar <i>a</i>-plane GaN samples also showed large surface band bending before and after etching. Additionally, the intensity of the near band-edge photoluminescence decreased and the free carrier density increased after etching. These results suggest that the changes in the surface potential may originate from the formation of possible nitrogen vacancies and other surface oriented defects and adsorbates. To recover the etched surface, N<sub>2</sub> plasma, rapid thermal annealing, and etching in wet KOH were performed. For each of these methods, the surface potential was found to increase by 0.1~0.3 V, also the reverse leakage current in Schottky diodes fabricated on treated samples was reduced considerably compared with as-etched samples, which implies a partial-to-complete recovery from the plasma-induced damage.
Ferroelectric field effect transistors (FFETs) with hysteretic I-V characteristics were attained with 25 nm thick Pb(Zr<sub>0.52</sub>Ti<sub>0.48</sub>)O<sub>3</sub> (PZT)/Si<sub>3</sub>N<sub>4</sub> gated AlGaN/GaN heterostructure. The PZT films used in the gate of the device was deposited by magnetron rf-sputtering at the substrate temperature of 700 <sup>o</sup>C. Increasing the PZT deposition temperature from that in previous device structures from 600 <sup>o</sup>C to 700 <sup>o</sup>C we obtained much improved device performance in terms of the IV characteristics inclusive of hysteretic behavior. The pinch-off voltage was about 7 V in FFET device compared to 6 V in a the control (conventional) AlGaN/GaN device. Counterclockwise hysteresis appeared in the transfer characteristic curve of a FFET with a maximal drain current shift of about 10 mA at the gate-to-source voltage of -6 V.