The growth, fabrication, and properties of GaN/AlN/sapphire with periodically poled surface polarity for
second harmonic generation are investigated. The periodic inversion of the surface polarity is achieved by the
growth of a thin AlN buffer layer and subsequent partial removal by using either wet etching with potassium
hydroxide (KOH) or reactive-ion etching (RIE). GaN growth on these substrates by MOCVD leads to Gapolar
GaN on the AlN buffer and N-polar GaN on the bare sapphire. Using atomic force microscopy and
scanning electron microscopy, it is demonstrated that a sufficient combination of H2 and NH3 surface
treatment before the growth of the GaN layers removes surface defects introduced by RIE etching. Thus,
films with comparable quality and properties independent of the etching technique could be grown. However,
in contrast to RIE etching, the interfaces between the Ga-polar and N-polar GaN is rough if KOH etching is
applied. Thus, it is concluded that MOCVD in combination with RIE etched AlN/sapphire substrates can be a
versatile process to fabricate GaN with periodically poled surface polarity as desired for UV light generation
via frequency doubling.
Using high resolution X-ray diffraction and Hall effect measurements, we found that the tensile strain caused by
dislocation inclination in Si doped GaN became immeasurable when carbon co-doping was used to compensate the free
carriers. This result suggested that the tensile strain is related to free carrier concentration instead of Si concentration.
Such an effect could be explained by the Fermi level effect on the dislocation climb at the growth surface mediated by
Ga vacancies, whose concentration is strongly influenced by the Fermi level position. At a high electron carrier
concentration, the formation energy of Ga vacancies is low and Ga vacancy concentration is high. Therefore, the
dislocation climb-rate is enhanced, which results in a higher tensile strain. This phenomenon is similar to the wellknown
Fermi level effect on Ga vacancy governed diffusion in the GaAs system.
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 Al0.81In0.19N/GaN heterojunction structures are compared with conventional Al0.3Ga0.7N/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.
The ferroelectric effect has been demonstrated on an AlGaN/GaN heterostructure field effect transistor using a Pb(Zr0.52Ti0.48)O3 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.
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 Al0.40Ga0.80N and In0.18Al0.82N 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 10-21 cm2. A second trap at 0.35 eV, 10-14 cm2 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 In0.01Ga0.99N
to 3nm In0.01Ga0.99N, the external quantum efficiency (EQE) droop point is increased from 350 Acm-2 to >1000 Acm-2,
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 In0.01Ga0.99N (3nm) to In0.06Ga0.94N (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-2. 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.
Deep levels in thin GaN epilayers grown by metal-organic chemical vapor deposition on different
templates were studied by photocapacitance spectroscopy and deep-level transient spectroscopy
(DLTS) using Schottky barrier diodes. We observed the reduction of electrically and optically active
traps in GaN grown with in situ SiNx nanonetwork and SiO2 striped mask or conventional epitaxial
lateral overgrowth technique (ELO) as compared to a typical control layer on a sapphire substrate.
All samples measured by DLTS in the temperature range from 80 K to 400 K exhibited traps with
activation energies 0.55-0.58 eV and 0.21-0.28 eV. The lowest concentration of both traps was
achieved for the sample with 6 min deposition of SiNx nanonetwork, which was lower than that for
the sample prepared by conventional ELO, and much lower than that in the control. The steady-state
photocapacitance spectra of all samples taken at 80 K over the spectral range 0.75-3.50 eV
demonstrated a similar trend for all the layers. The photocapacitance spectra exhibited defect levels
with optical threshold energies of 1.2-1.3, 1.6, 2.2 and 3.1 eV. The determined concentrations of
traps were compared and the results were consistent with DLTS measurements. The layer with SiNx
nanonetwork has the lowest concentrations of optically active traps with the standard GaN control
layer being the worst in terms of trap concentrations. The consistent trend among the
photocapacitance spectroscopy and DLTS results suggests that SiNx network can effectively reduce
deep levels in GaN, which otherwise can deteriorate both optical and electrical performance of GaN-based
devices.
In AlGaN/GaN heterostructure field effect transistors (HFETs), two-dimensional-electron-gas (2DEG), induced by
strong piezoelectric and spontaneous polarization field, has high sheet density, and can be tuned up to 5 ×1013 cm-2 with pure AlN barrier.[Appl. Phys. Lett. 90, 182112 (2007)].For Al compositions larger than 40%, due to the large lattice mismatch between GaN and AlGaN, strain-related issues significantly reduce the mobility for these high sheet carrier densities. Recently, using nearly lattice-matched AlInN/GaN to improve the performance of HFETs has been studied
theoretically and experimentally. A high sheet density (2.42 ×1013 cm2) with >1000 cm2/Vs mobility has been reported by inserting an AlN spacer layer between the AlGaN barrier and GaN channel. However, low-temperature mobilities for AlInN/GaN HFETs are much lower than those for AlGaN/GaN HFETs. In this paper, we study the Al1-xInxN/AlN/GaN (x=0.20 - 0.12) (HFETs) grown by metalorganic chemical vapor deposition. Reduction of In composition from 20% to
12% increased the room temperature equivalent two-dimensional-electron-gas (2DEG) density from 0.90×1013 cm-2 to 1.64 ×1013 cm-2 with corresponding electron mobilities of 1600 cm2/Vs and 1410 cm2/Vs. Furthermore, at 10 K, the mobility reached 17,600 cm2/Vs with a sheet density 9.6 ×1012 cm-2 for the nearly lattice-matched Al0.82In0.18N
/AlN/GaN heterostructure. The HFETs having 1 μm gate length exhibited a maximum transconductance of ~ 250 mS/mm with good pinch-off characteristics.
In this paper, we investigated Schottky and metal-semiconductor-metal (MSM) photodetector structures fabricated on
GaN templates with in situ SiNx nanonetwork, which were shown to reduce the dislocation densities significantly in the
overgrown material. The GaN layers were grown by metalorganic chemical vapor deposition. The peak responsivity of
the Schottky photodetectors on templates with SiNx nanonetwork was measured to be 0.16 A/W, significantly larger
than that for the control samples (0.09 A/W). The MSM photodetectors on templates with SiNx nanonetwork also
showed significantly enhanced photoresponsivity (100 A/W) when compared to the control sample without any SiNx
(30 A/W) and photoconductive gain. The improvement in the photoresponsivity in both Schottky and MSM
photodetector structures with the use of SiNx nanonetwork is due to the reduction of dislocation densities.
Charge trapping resulting in localized band bending on MBE-grown GaN films was investigated using a new
combination of conducting atomic force microscopy (CAFM) and scanning Kelvin probe microscopy (SKPM). CAFM
was first used to locally inject charge at the surface oxide/semiconductor interface, and then SKPM was performed to
monitor the evolution of the resulting surface potential. In a dark environment, the additionally charged interface states
due to CAFM charge injection resulted in an induced additional band bending that persisted for hours. The induced band
bending is nominal (<0.5 eV) for CAFM voltages less than 8 V, and reaches a saturation value of ~3 eV for voltages
greater than 10 V. The saturation band bending corresponds to a total density of charged interface states (2×1012 cm-2)
that is double the value observed for the intrinsic surface. Induced band bending could still be observed up to 4 h after
charge injection, indicating that charge trapping is relatively stable in a dark environment. However, charged interface
states could be rapidly neutralized by illumination with UV light. A phenomenological model based on a tunneling
mechanism was used to successfully describe the CAFM charge injection, where electrons travel from the tip through an
oxide barrier and become trapped at oxide/GaN interface states. Saturation occurs due to the existence of a finite density
of chargeable states at the interface. After charge injection, the decrease in induced band bending with time was found to
be consistent with a thermionic model of charge transfer from the interface to the bulk.
Strong coupling between the exciton and cavity modes were demonstrated in a bulk ZnO-based hybrid microcavity The
hybrid microcavity consisted of a λ-thick bulk ZnO cavity layer sandwiched between a 29 pair Al0.5Ga0.5N/GaN bottom
distributed Bragg reflector (DBR) and an 8 pair SiO2/Si3N4 top DBR grown by molecular beam epitaxy, metalorganic
chemical vapor deposition, and ultra high vacuum plasma-enhanced chemical vapor deposition, respectively. All layer
interfaces were sharp and optical reflectivity measurements were performed to characterize the DBRs. The anti-crossing
behavior in the polarton dispersion, which indicates the system is in the strong coupling regime was observed in angle-resolved
photoluminescence measurements at room temperature, and a vacuum Rabi splitting of ~50 meV in the ZnO-based
hybrid microcavity was obtained.
The polarization fields in the c-axis-oriented hexagonal GaN system cause spatial separation of electrons and holes in
quantum wells, reducing the quantum efficiency, and resulting in a red shift of the emission as well as a blue shift with
increasing injected carrier density. In this paper, we report on the growth and optical characterization of InGaN/GaN
multiple quantum wells (MQWs) on nonpolar
(112¯0) a- and polar (0001) c-planes, as well as two semipolar planes,
(112¯2) and (11¯01) of GaN. There are two kinds of a-plane used in this study. One of the (112¯0) a-planes was
obtained on
(11¯00) m-plane sapphire substrates during the epitaxial lateral overgrowth (ELO) of
(112¯2) oriented
semipolar GaN films, while the other one was planar a-plane GaN which was grown on
(11¯01) r-plane sapphire
substrates. The semipolar
(112¯2) and (11¯01) planes were obtained as sidewall facets during the ELO of c-plane GaN
with the mask stripes aligned along the GaN m-axis and a-axis, respectively. InGaN/GaN multiple quantum wells
(MQWs) with a nominal well thickness of 4 nm and a barrier thickness of 8 nm were grown on these five GaN
samples by metalorganic chemical vapor deposition. Excitation power dependent photoluminescence (PL)
measurements were carried out on these quantum well structures to study the effect of polarization-induced electric
field on the band-edge emission. The quantum-well emission energy from the two a-plane MQWs showed zero shift,
compared to a 74 meV blue shift for the c-plane MQWs when the excitation power was increased from 1.3 mW to
37.0 mW. The semipolar
(112¯2) showed a blue shift of 35 meV with increased excitation power, suggesting reduced
polarization compared to that of c-plane. No quantum-well emission could be observed for the MQWs on
(11¯01)
semipolar planes. The shift in the quantum-well emission energy was attributed to the change of the screening effect of
photon-generated carriers in the quantum wells at different excitation powers.
For exact notation please see manuscript
The surface morphologies of unintentionally doped epitaxial laterally overgrown c-plane and a-plane
GaN samples subjected to photoelectrochemical (PEC) etching in aqueous KOH is reported. By
maintaining the etch in the carrier-limited regime, elucidation of the optically and electrically active
defects can be achieved. Results correlating surface morphologies after PEC etching with TEM results
verify the reduction of threading dislocations in the overgrown "wing" regions, as compared with the
"windows" (seed regions) for both a- and c-plane GaN ELO samples. Also, within and near the
window regions of the a-GaN ELO samples, PEC etching reveals a significant amount of basal
stacking faults that propagate to the surface. This work represents a systematic evaluation of the effects
of PEC etching on polar and nonpolar surfaces of the GaN layers grown by the conventional ex situ
ELO method. The surface morphology and the whisker densities after PEC etching of c-plane GaN
samples grown using SiNx nanonetwork mask layers, a method referred to as in situ nano-ELO, also
indicates significant improvement of the material quality. The identification of variations in surface
morphology at different times during PEC etching of GaN may have utility in that the assessment of
the material quality can be made and assorted nanopatterns of GaN surfaces can be intentionally
achieved in a controllable, large-scale, and inexpensive manner.
Reduction of deep centers in GaN layers grown employing nano-ELO SiNx porous nanonetworks has been studied by
deep-level transient spectroscopy (DLTS). The obtained concentrations of deep traps in layers with SiNx nanonetworks
were compared with an otherwise identical reference sample and with another sample grown by employing conventional
ELO technique. Two traps, labeled A (0.54-0.58 eV) and B (0.20-0.23 eV), were delineated in all layers with trap A
being dominant in the temperature range 80-400 K. The concentration of trap A in SiNx layers was found to be lower by
2-4 times compare to the reference sample. The minimum concentration 7.5x1014 cm-3 was obtained in the layer grown
on SiO2 stripe pattern which is ~6 times lower compare to the reference sample. We have found the logarithmic capture
mechanism up to ~20 ms for deep center A. Considering that the lateral growth mainly reduces the edge dislocations in
our films it is tempting to suggest that structural defects that may have a direct and or indirect role in the creation of the
dominant trap which we believe are located close to each other along the edge threading dislocation lines. In addition, a
small blue shift, compare to a strain free layers, of the neutral-donor-bound-exciton line (D0XA) observed in the photoluminescence spectra of the samples grown with lateral overgrowth is indicative of partial strain relief.
Growth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase
epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent
on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low
NH3/TMAl ratio resulted in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The
polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4,
namely the etching rate on N-polarity is substantial larger, a method reported earlier. We suggest that the
Al adatoms form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N
leading to a metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface,
are absent on N-polarity GaN grown on off-axis C-SiC owing to high density of terraces on the substrate
surface. The properties of GaN layers grown on C-SiC have been studied by X-ray diffraction and are
reported in this paper.
AlGaN/AlGaN distributed Bragg reflectors (DBRs) designed for the ultraviolet spectral region have been attained. The
crack-free structures were grown on c-plane sapphire by plasma assisted molecular beam epitaxy (MBE). To minimize
the built-in strain in DBRs, a thin buffer layer was used directly on c-plane sapphire. A peak reflectivity of 95% at 381
nm with a 21 nm stop band width was obtained at room temperature (RT) using a 32.5 pairs Al0.7Ga0.3N/Al0.15Ga0.85N
DBR. With a driving force for DBRs and emitting regions in wide band gap semiconductor microcavities, such as those
based on GaN and ZnO, is the quest for cavity polariton which is the coupled mode between the exciton and photon
modes. Moreover, the exploitation of cavity polaritons could be expected in the course of the development of extremely
low-threshold optoelectronics devices.
Preliminary results on nanoheteroepitaxy of GaN on silicon face (Si-face) and carbon face (C-face) nano-columnar
SiC (CSC) by metalorganic chemical vapor deposition (MOCVD) are reported. The CSC
substrates are fabricated from standard SiC wafers by photo-enhanced electrochemical etching, with typical
diameter of pores around 20nm. Noticeable reduction of threading dislocations (TDs) in GaN is realized on
the CSC substrates. On the C-face CSC, GaN nuclei have an inverted pyramidal shape which contains high
density of stacking faults (SFs). These SFs block possible extension of TDs into upper portion of the layer.
On the Si-face CSC, TDs are annihilated by forming nanoscale TD half-loops over the surface pores. These
nanoscale TD loops confine the defective layer in GaN to within ~50 nm thickness from the GaN/CSC
interface. High density (~5x108 cm-2) of remnant TDs still presents in GaN grown on CSC, chiefly because
the surface damages on CSC were not properly removed before growth.
GaN epitaxial layers grown on SiC and sapphire suffer from high density of line and point defects. To address this
problem, new growth methods using in situ or ex situ nano-network masks as dislocation filters have been introduced
recently. In this work, we report on metalorganic chemical vapor deposition (MOCVD) of GaN layers on 2-inch
sapphire substrates using in situ SiNx nano-networks intended for defect reduction. SiNx interlayers with different deposition times were employed after ~2 &mgr;m GaN grown on sapphire, which was followed by ~3.5 &mgr;m GaN
overgrowth. With increasing SiNx coverage, full width at hall maximum (FWHM) values of (0002) and (101-2) X-Ray
diffraction (XRD) peaks monotonously decrease from 252 arc sec to 217 arc sec and from 405 ar csec to 211 arc sec,
respectively for a 5.5 &mgr;m thick film. Similarly, transmission electron microscopy (TEM) revealed that screw and edge
type dislocation densities as low as 4.4x107 cm-2 and 1.7x107 cm-2 were achieved. The use of SiNx nanonetwork also increases the radiative recombination lifetimes measured by time-resolved photoluminescence to 2.5 ns from less than
0.5 ns in control GaN. We have also fabricated Ni/Au Schottky diodes on the overgrown GaN layers and the diode
performance was found to depend critically on SiNx coverage, consistent with TEM, XRD and TRPL results. A 1.13eV
barrier height was achieved when SiNx layer was used compared to 0.78 eV without any SiNx nanonetwork.
Furthermore, the breakdown voltage was improved from 76 V to 250 V with SiNx nanonetwork.
Although standard GaN device structures used for FETs, light emitters, and detectors have been investigated reasonably
extensively, the device structures relying on the particulars of current transport over barriers in this material system have
not received as much attention, to a large extend due to the insufficient quality of the layers. Unless special measures are
taken, the defects present in the barrier material induce current conduction paths that preclude any possibility of
observing the fundamental current conduction mechanisms. To overcome this impediment, high quality GaN layers,
followed by the vertical single barrier heterostructures, have been grown on sapphire substrates using epitaxial lateral
epitaxy in a metal organic chemical vapor deposition system with the aid of an in-situ deposited SiNx nanonet. Structural
and optical properties of the films indicate their superior nature. With these templates in hand, n-GaN/i-AlxGa1-xN/n-
GaN structures with varying barrier width and height have been prepared and tested for their IV characteristics. The
rectification observed is consistent with the barrier design. Because the band bending is affected by polarization charge,
which is dependent on pressure, current vs. voltage measurements under pressure have also been recorded. In this
presentation, the details of the measurements and analyses, as well as the pertinent aspects of growth related issues will
be discussed.
AlGaN/GaN devices are typically grown on foreign substrates such as SiC and sapphire due to lack of commercial
bulk GaN. Metalorganic chemical vapor deposition (MOCVD) is a widely used method for growth of GaN templates
for these structures even for other growth methods. Because the growth temperature during molecular beam epitaxy
(MBE) is low, dislocation motion is hindered leading to a high dislocation density, particularly pure edge type, when
grown directly on foreign substrates. On the other hand, low background doping, sharp interface and well-controlled
growth rate allow MBE to grow high performance modulation doped field effect transistor (MODFET) structures on
MOCVD GaN templates. However, the regrowth interface in this case has been reported to act as a parallel channel
unless Zn-doped GaN templates were used. [J. Appl. Phys.v92p338] In this paper we report on the control of the
regrowth interface of GaN/AlGaN MODFETs by rf-assisted MBE on GaN templates prepared by MOCVD. We have
found that the defective parallel channel at the regrowth interface could be effectively eliminated by a proper growth
procedure and pre-cleaning using KOH combined with high temperature (800oC) thermal annealing in vacuum.
Reflection high energy electron diffraction (RHEED) was used to monitor the interface quality to a first order during
the initial growth stages. Electrical and structural properties at the regrowth interface were analyzed by capacitance-voltage
(CV) measurements and transmission electron microscopy (TEM). Al0.3Ga0.7N/GaN MODFET structures
grown under the optimized conditions exhibited a maximum transconductance of 230 mA/mm for a 1&mgr;m gate length.
Epitaxial growth of ZrO2 has been achieved on MOCVD-grown GaN(0001) templates by
oxides molecular beam epitaxy using reactive H2O2 for oxygen and organometallic
source for Zr. Utilizing a low temperature buffer layer followed by high temperature
insitu annealing and high-temperature growth, monoclinic (100)-oriented ZrO2 thin films
were obtained. The full width at half maximum of ZrO2 (100) rocking curve was 0.4 arc
degree for 30-nm-thick films and the rms roughness for a 5&mgr;m by 5 &mgr;m AFM scan was 4 Å. The employment of epitaxial ZrO2 layer in the AlGaN/GaN heterojunction field effect
trasnsistor as a gate dielectric has resulted in the increase of the saturation-current density
and pinch-off voltage as well as in near symmetrical gate-drain I-V behavior.
We study AlxGa1-xN/AlN/GaN heterostructures with a two-dimensional-electron-gas (2DEG) grown on different GaN
templates using low-temperature magneto-transport measurements. Heterostructures with different Al compositions are
grown by metal-organic vapor phase epitaxy (MOVPE) on three different templates; conventional undoped GaN (u-
GaN), epitaxial lateral overgrown GaN (ELO-GaN), and in situ ELO-GaN using a SixNy nanomask layer (SiN-GaN).
Field-dependent magneto-resistance and Hall measurements indicated that in addition to 2DEG, the overgrown
heterostructures had a parallel conducting layer. The contact resistance for the parallel channels was large so that it
introduced errors in the quantitative mobility spectrum analysis (QMSA) of the data. Notwithstanding complexities
introduced by parallel conducting channels in mobility analysis in SiN-GaN and ELO-GaN samples, we were able to
observe Shubnikov-de Haas (SdH) oscillations in all samples, which confirmed the existence of 2DEGs. To characterize
the parallel channel, we repeated the transport measurements after the removal of the 2DEG by etching the
heterostructure. The 2DEG carrier density values were extracted from the SdH data, whereas the zero-field 2DEG
conductivity was determined by subtracting the parallel channel conductivity from the total conductivity. The resulting
2DEG mobility was significantly higher (about a factor of 2) in the ELO-GaN and SiN-GaN samples as compared to the
standard control sample. The mobility enhancement is attributed to the threading dislocation reduction by both ELO
techniques.
Due to their unique physical properties GaN-based heterostructures show a great promise for spintronics applications. This stimulates the search for GaN-based ferromagnetic semiconductors which can be used for injection of spin polarized carriers in device structures. In this study, magnetic properties of GaN layers implanted with Gd+ ions to various doses were investigated. Magnetization curves of samples with Gd content nGd = 2x1017 and 2x1018 cm-3 show clear hysteresis, while the samples with nGd = 2x1016 and 2x1019 cm-3 exhibit no ferromagnetism. Most likely, the lowest Gd concentration produced magnetization below the detection limit, whereas the absence of ferromagnetism in the sample with the highest Gd content may be resulted from heavy implantation-induced damage. Curie temperatures for samples with Gd contents of 2x1017 and 2x1018 cm-3 were estimated to be larger than 300 K. Saturation magnetizations of 1550 μB and 1350 μB per Gd-atom were found at 5 K and 300 K, respectively, for the sample with nGd=2x1018 cm-3.
We have used the techniques of atomic force microscopy (AFM) and conductive AFM (C-AFM) to investigate the morphology and localized current conduction of GaN films grown by molecular beam epitaxy (MBE) on metal organic chemical vapor deposition (MOCVD) templates. The most common type of surface morphology consists of undulating spiral "hillocks" terminated by small pits. A low density of holes are interspersed between these hillocks with typical diameters of ~150 nm and densities on the order of 108 cm-2. For C-AFM measurements, a Pt-coated AFM tip was brought into contact with the GaN surface to form a microscopic Schottky contact. In reverse bias, C-AFM shows localized current leakage at the centers of approximately 10% of spiral hillocks, which are presumably associated with screw dislocations. Shifts in forward-bias turn-on voltages and changes in the conduction mechanism are observed in these defect regions. Local I-V curves indicate a Frenkel-Poole mechanism for forward conduction on defect regions.
We report on the structural, electrical, and optical characterization of GaN epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on SiNx and TiNx porous templates in order to reduce the density of extended defects. Observations by transmission electron microscopy (TEM) indicate an order of magnitude reduction in the dislocation density in GaN layers grown on TiNx and SiNx networks (down to ~108 cm-2) compared with the control GaN layers. Both SiNx and TiNx porous network structures are found to be effective in blocking the threading dislocation from penetrating into the upper layer. Supporting these findings are the results from X-Ray diffraction and low temperature photoluminescence (PL) measurements. The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably for the layers grown with the use of SiNx and TiNx layers, which generally suggests the reduction of edge and mixed threading dislocations. In general, further improvement is observed with the addition of a second SiNx layer. The room temperature decay times obtained from biexponential fits to time-resolved photoluminescence (TRPL) data are increased with the inclusion of SiNx and TiNx layers. TRPL results suggest that primarily point-defect and impurity-related nonradiative centers are responsible for reducing the lifetime. The carrier lifetime of 1.86 ns measured for a TiNx network sample is slightly longer than that for a 200 μm-thick high quality freestanding GaN. Results on samples grown by a new technique called crack-assisted lateral overgrowth, which combines in situ deposition of SiNx mask and conventional lateral overgrowth, are also reported.
Ferroelectric field effect transistors (FFETs) with hysteretic I-V characteristics were attained with 25 nm thick Pb(Zr0.52Ti0.48)O3 (PZT)/Si3N4 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 oC. Increasing the PZT deposition temperature from that in previous device structures from 600 oC to 700 oC 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.
Surface profiles of deep levels in GaN sample grown by metal-organic chemical vapor deposition and by hydride vapor phase epitaxy are measured by differential deep level transient spectroscopy (DDLTS). The concentration of acceptor defects at the surface are expected to be lower than the bulk defect concentration because of the shift in Fermi level at the surface, based on theoretical estimates of defect formation energies and the band bending at the surface from spontaneous polarization. Similarly, donor defects are expected to increase in concentration as the surface is approached. The measured concentration profiles of various traps are found to span the range of behavior, from constant, to increasing or decreasing at the interface. Deep level profiling is therefore seen as an important tool to assist in determining defect composition. Although the behavior is as expected, the change in concentration from bulk to surface, is larger than measured values for the defects with the lowest formation energies, based on a conservative estimate of band bending. The difference may reflect a band bending that is different at the growth temperature than predicted, or a consequence of non-equilibrium growth conditions. As growth proceeds, the defects incorporated at the surface are in a non-equilibrium concentration when covered by subsequent layers, unless there is a mechanism whereby equilibrium defects can be formed, e.g. VGa by forming interstitial Ga, or there is enough energy for defect diffusion to take place. Peaks in the defect profile were measured, as would be expected for a donor defect formed at the surface, but with a non-equilibrium concentration in the bulk, driving diffusion toward the surface.
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