Abstract: Strain relaxation in the GaN/AlN/6H-SiC epitaxial system grown by vicinal surface
epitaxy (VSE) is investigated and compared with that in on-axis epitaxy. High resolution x-ray
diffraction (HRXRD) measurements show that GaN films grown by VSE have improved crystalline
quality. High resolution transmission electron microscope (HRTEM) studies reveal that there are
two types of misfit dislocations (MDs) at AlN/6H-SiC interfaces: 60˚ complete dislocations along
<1120 > directions with Burgers vector 1/3<1120 > and 60˚ Shockley partials along <10 10 >
directions with Burgers vector 1/3<10 10 >. The latter are usually geometrical partial misfit
dislocations (GPMDs) that are dominant in VSE to accommodate the lattice mismatch and stacking
sequence mismatch simultaneously. In VSE, it is the high-density GPMDs formed at the vicinal
surface steps that facilitate rapid strain relaxation at the initial stage of deposition and hence lead to
superior crystalline quality of the subsequently grown GaN films.
Abstract: The phonon anisotropy property of the GaN wurtzite crystal is studied using angular
dependent Raman spectroscopy both theoretically and experimentally. The polarized Raman
scattering spectra were recorded from cross-sections of c-axis oriented GaN films as a function of
the angle between the incident laser polarization direction and the film normal direction in three
different configurations. The Raman intensity of A1(TO) showed a sinusoidal dependence on the
rotating angle, as also did the E1(TO) mode, while the E2 mode has a quite different behavior. The
theoretical fit takes into account the susceptibility contribution and the phase differential of
different vibrating elements.
Abstract: Using a combination of synchrotron white beam x-ray topography (SWBXT) and high
resolution x-ray diffraction (HRXRD), the structural quality of AlN crystals grown by various
sublimation-based techniques have been non-destructively analyzed. Spontaneously nucleated AlN
crystals are characterized by very low defect densities but their size is small. Self-seeding results in
nucleation of multiple grains of different orientations, a few of which are of good quality while most
are highly strained. Using readily available commercial 4H and 6H-SiC substrates, several growth
runs have been carried out using different growth conditions to obtain thick AlN layers, either
attached to the seed or free-standing. While attached layers are typically cracked and highly strained,
crack-free free-standing layers can be obtained by delamination or SiC decomposition. X-ray
characterization reveals these crystals have good purity but moderately high defect densities.
Abstract: Both n-type and p-type GaN MOS capacitors with plasma-enhanced CVD-SiO2 as the
gate oxide were characterized using both capacitance and conductance techniques. From a n type
MOS capacitor, an interface state density of 3.8×1010/cm2-eV was estimated at 0.19eV near the
conduction band and decreases deeper into the bandgap while from a p type MOS capacitor, an
interface state density of 1.4×1011/cm2-eV 0.61eV above the valence band was estimated and
decreases deeper into the bandgap. Unlike the symmetric interface state density distribution in Si,
an asymmetric interface state density distribution with lower density near the conduction band and
higher density near the valence band has been determined.
Abstract: It is generally accepted that the Schottky barrier height (SBH) is affected by the initial
band bending at the bare nGaN surface as well as by an additional contribution following metal
deposition. In this work the effect of processing used for device fabrication on the surface band
bending of bare c-plane nGaN was studied by surface potential electric force microscopy (SP-EFM).
An increase of the initial upward band bending from 1.0 ± 0.1eV for the as-grown GaN to 1.9 ±
0.1eV after RTA treatment in N2 ambient was observed. No significant dependence of band bending
on N2 or Ar as ambient gas during the RTA treatment was observed. The increase of the initial
upward band bending was also confirmed by photoluminescence (PL) measurements. We suggest
that the RTA treatment causes a high density of surface states, possibly as a result of high
temperature reaction of ambient gas and remnant contamination.
Abstract: Carrier transport properties of AlGaN/GaN heterostructures have been analyzed with the
quantitative mobility spectrum analysis (QMSA) technique. The nominally-undoped
Al0.15Ga0.85N/GaN sample was grown by metal-organic vapor phase epitaxy. Variable-magneticfield
Hall measurements were carried out in the temperature range of 4-160 K and magnetic field
range of 0-6.6 T. QMSA was applied to the experimental variable-field data to extract the
concentrations and mobilities associated with the high-mobility 2DEG and the relatively lowmobility
bulk electrons for the temperature range investigated. For temperatures below 100 K the
calculated mobility and carrier density values were close to the experimental results. No bulk
conduction was observed in this temperature range. At 160 K, QMSA results show that parallel
conduction in 3 mm thick GaN layer started to affect the average electron mobility.
Abstract: The investigated AlGaN epitaxial layers were grown by hydride vapor phase epitaxy
(HVPE) on a commercial P+ SiC substrate or on an N+ SiC Lely substrate with a p+ SiC layer
previously grown by sublimation epitaxy. To investigate the electrical characteristics of the n-p
heterojunction, mesa structures of 100, 200 and 1500 microns in diameter were fabricated by
reactive ion etching. Investigation of electrical characteristics shows good quality of grown n-
AlGaN/p-SiC heterojunctions. This shows applicability of this technological combination for
producing n-AlGaN/p-SiC bipolar or FET transistors.
Abstract: GaN PiN diodes with a 4 μm Si-doped n--GaN drift layer (n~7×1016 cm-3) were grown on
free-standing GaN using metalorganic chemical vapor deposition. Atomic force microscopy showed
smooth surfaces with a step structure indicating good 2D growth. The dislocation density and
impurity incorporation in the drift layer were remarkably reduced compared to a similar diode
structure grown on sapphire. The full width at half maximum of the (0002) rocking curve was 79
arcsec, much smaller than 230 arcsec for the heteroepitaxial structure. The diodes on GaN
demonstrated rectification up to –265 V, corresponding to a critical electric field ~2.7×106 V/cm.
The maximum value of the figure of merit is ~2.4 MW cm-2, which represents a 2.2× improvement
over the diodes on sapphire.
Abstract: Characterization of n+-GaN/p−-SiC and n+-GaN/p+-SiC heterojunctions as well as
fabrication of GaN/SiC heterojunction bipolar transistors (HBTs) using these heterojunctions is
presented. The electroluminescence spectrum from n+-GaN/p+-SiC heterojunction diodes under
forward bias clearly indicates electron injection from n+-GaN into p+-SiC. HBTs consisting of
n+-GaN emitter /p+-SiC base/n−-SiC collector/n+-SiC substrate have been fabricated. Although clear
common-base properties were obtained, the current gain was very low (10-4). SiC homojunction
bipolar junction transistors (BJT) using the same base-collector junction exhibited a current gain
value of 0.5, suggesting the low current gain of GaN/SiC HBTs originates from low emitter
Abstract: We report on simple techniques for extracting the electrical properties of 1-dimensional
semiconductor nanowires using standard ultraviolet (UV) photo-lithography instead of e-beam
lithography (EBL), which is a commonly used technique for the fabrication of nanoscale electrical
devices. For electrical transport measurement the gallium nitride nanowires (GaN NWs) were
prepared by a horizontal hot-wall chemical vapor deposition (CVD) with metallic Ga and NH3 gas for
Ga and N sources, and GaN nanowire field effect transistor (FET) structures on a 8×8 mm2 silicon
wafer were fabricated by ordinary 2-mask photo-lithography processes. The estimated carrier
mobility from the gate-modulation characteristics is on the order of 60 ∼ 70 cm2/V⋅s. We found that
our approach is a powerful and simple technique to extract the electrical properties of semiconductor
nanowires. The material characteristics of GaN nanowires are also discussed.