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.
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
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.
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.
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.
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
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 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.
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.