Thermal conductivity of undoped and Sn-doped β-Ga2O3 bulk and single-crystalline thin films have been measured by the 3ω technique. The bulk samples were grown by edge-defined film-field growth (EFG) method, while the thin films were grown on c-plane sapphire by pulsed-laser deposition (PLD). All samples were with (-201) surface orientation. Thermal conductivity of bulk samples was calculated along the in-plane and cross-plane crystallographic directions, yielding a maximum value of ~ 29 W/m-K in the [010] direction at room temperature. A slight thermal conductivity decrease was observed in the Sn-doped bulk samples, which was attributed to enhanced phonon-impurity scattering. The differential 3ω method was used for β-Ga2O3 thin film samples due to the small film thickness. Results show that both undoped and Sndoped films have a much lower thermal conductivity than that of the bulk samples, which is consistent with previous reports in the literature showing a linear relationship between thermal conductivity and film thickness. Similarly to bulk samples, Sn-doped thin films have exhibited a thermal conductivity decrease. However, this decrease was found to be much greater in thin film samples, and increased with Sn doping concentration. A correlation between thermal conductivity and defect/dislocation density was made for the undoped thin films.
Surface structure of the free-standing GaN substrates with polar (000-1), non-polar (1-100), (11-20), and semipolar (20- 21) surface plane were investigated. Clean polar and non-polar GaN surfaces were prepared by annealing under NH3 atmosphere. (1x1) diffraction patterns were observed by low-energy electron diffraction (LEED) for both polar and non-polar GaN surfaces. The polar GaN surface was found well-ordered, while the non-polar GaN surfaces were found less ordered with atomic steps on the surface. Polar angle dependences of the photoelecton diffraction (PED) intensities exited by MgKα radiation from N 1s level were analyzed for all the GaN surfaces, aiming to determine the polarities of the GaN surfaces with polar and semipolar crystal orientations.
The carrier recombination dynamics in bulk m-plane GaN were investigated by excitation and temperature dependent time-resolved photoluminescence (TRPL) spectroscopy. Polarization-resolved measurements of photoluminescence (PRPL) spectra were performed to evaluate the individual contributions of excitons and free carriers to the radiative recombination. The polarization degree and PL lifetime were strongly correlated and dependent on the populations of free carriers and excitons at different excitation density and temperature levels. The free carrier concentration was found to increase due to the dissociation of excitons at high excitation density and temperatures. The excitonic PL life time was found to be ~ 0.7 ns at 10 K at the lowest excitation density used 0.04 μJ/cm2, where no exciton screening was present as confirmed by the 100% polarization degree. The polarization degree obtained at different excitation levels and temperatures by comparing the PL decay times to the excitonic PL lifetime correlated very well with the polarization degree obtained from the excitation dependent PRPL measurements. Finally, it was shown that TRPL and PRPL can be used to separate the excitonic and free carrier contributions to the recombination dynamics in m-plane GaN at any temperature and excitation density.
Optically-injected carrier dynamics were investigated in bulk polar and nonpolar GaN in 1015-to-1020 cm-3 carrier
density range, exploring single- and two-photon photoexcitation conditions. The excitation decay and recombination
rates were monitored by time-resolved photoluminescence and free-carrier absorption techniques, while diffusivity
was investigated by light-diffraction on transient grating technique. Carrier dynamics in c- and m-plane thick
freestanding HVPE GaN revealed nearly linear increase of carrier lifetime with temperature in the 80 - 800 K range
whereas the bipolar carrier diffusivity decreased with temperature. This feature suggests that the measured long
lifetime values of 40-50 ns at RT result from diffusion-governed carrier flow to interface defects at GaN hexagons,
which act as centers of nonradiative recombination. The fast PL transients under carrier injection to submicrometer
thick layer were fitted by using the determined diffusivity and lifetime values and revealed a strong impact of
vertical carrier diffusion, surface recombination, and reabsorption processes. Radiative and nonradiative emission
rates were analyzed by various optical techniques to discriminate contribution of excitons and free carriers at various
temperatures and injected carrier densities.
Bulk GaN substrates promise to bring the full potential of nitride-based devices to bear since they offer a
low thermal and lattice mismatched alternative to foreign substrates for epitaxial growth. However, due to
the high cost and low availability of bulk GaN substrates, effects such as surface misorientation (offcut),
surface polishing, and preparation of such substrates on subsequent epitaxy are still not well understood. As
such, AlGaN/GaN heterostructures with nominal Al compositions of 25% were grown by MOCVD on
semi-insulating bulk GaN substrates with offcuts ranging from 0.05 to 1.95° in the m-direction (10 10) to
attempt to determine the optimal offcut for bulk GaN substrates for AlGaN-based HFET devices. X-ray
diffraction (XRD) studies indicate that the Al composition does not vary with offcut, however reciprocal
space mapping shows evidence of strain relaxation of the AlGaN in samples grown on substrates with
offcut >1.1°. Additionally, we observed a minimum in sheet resistance of the 2DEGs for substrates with
offcuts near 0.5°, arising from higher mobilities in these samples. Evidence of an optimal substrate
misorientation is important for AlGaN-based devices grown on bulk GaN substrates.
Photoluminescence (PL) properties are reported for a set of m-plane GaN films with Mg doping varied from mid 1018cm-3 to well above 1019 cm-3. The samples were grown with MOCVD at reduced pressure on low defect density m-plane
bulk GaN templates. The sharp line near bandgap bound exciton (BE) spectra observed below 50 K, as well as the
broader donor-acceptor pair (DAP) PL bands at 2.9 eV to 3.3 eV give evidence of several Mg related acceptors, similar
to the case of c-plane GaN. The dependence of the low temperature BE spectra on excitation intensity as well as the
transient decay behavior demonstrate acoustic phonon assisted transfer between the acceptor BE states. The lower energy
donor-acceptor pair spectra suggest the presence of deep acceptors, in addition to the two main shallower ones at about
0.23 eV. Similar spectra from Mg-doped GaN nanowires (NWs) grown by MOCVD are also demonstrated and briefly
discussed.
We report the internal quantum efficiency (IQE) and external quantum efficiency (EQE) of m-plane InGaN-based
LEDs and c-plane LEDs emitting at 400 nm. According to excitation density dependent photoluminescence (PL)
measurements, the IQE values for the m-plane LEDs are approximately 70% higher than that for c-plane reference
LEDs with the same structure (multiple-quantum well active region with 3nm In0.01Ga0.99N barriers) and similar
emission wavelength (400nm) albeit on sapphire substrate. Electroluminescence (EL) measurements reveal that the
external quantum efficiency (EQE) from both LEDs shows negligible droop under electrical injection due to the
employment of thin In0.01Ga0.99N barriers, i.e. coupled quantum wells, in the active regions. However, with
increasing current, the EL intensity of the m-plane LEDs increases more rapidly than that of the c-plane LEDs, with
the EL intensity reaching its peak value at ~240 Acm-2 for the m-plane LEDs and ~420 Acm-2 for the c-plane LEDs.
Consistent with the IQE values obtained from the PL measurements, the m-plane LEDs exhibit ~ 60% higher EQE
values than the c-plane counterparts. It is also worth to mention that increasing the emission wavelength (from 400
nm to 435 nm) for the m-plane LEDs does not affect the efficiency retention at high injection levels.
High brightness InGaN light emitting diodes (LEDs) require high quantum efficiency and its retention at high injection
levels. The efficiency drop at a high injection levels in InGaN light emitting diodes (LEDs) has been attributed, e.g. to
polarization field on polar c-plane InGaN and the heavy effective hole mass which impedes high hole densities and
transport in the active quantum wells. In this study, we carried out a comparative investigation of the internal quantum
efficiency (IQE) of InGaN active region in LED structures using resonant optical excitation for layers with polar (0001)
orientation on c-plane sapphire, and nonpolar (1-100) m-plane orientation, the latter on specially patterned Si and on m-plane bulk GaN. Analysis of the resonant photoluminescence (PL) intensity as a function of the excitation power
indicate that at comparable generated carrier concentrations the IQE of the m-plane InGaN on Si is approximately a
factor of 2 higher than that of the highly optimized c-plane layer. At the highest laser excitation level employed
(corresponding carrier concentration n ~ 1.2 x 1018 cm-3), the m-plane LED structure on Si has an IQE value of
approximately 65%. We believe that the m-plane would remain inherently advantageous, particularly at high electrical
injection levels, even with respect to highly optimized c-plane varieties. The observations could be attributed to the lack
of polarization induced field and the predicted increased optical matrix elements.
The internal quantum efficiency (IQE) and relative external quantum efficiency (EQE) in InGaN LEDs emitting at 400
nm with and without electron blocking layers (EBLs) on c-plane GaN and m-plane GaN were investigated in order to
shed some light on any effect of polarization charge induced field on efficiency killer carrier spillover. Without an EBL
the EQE values suffered considerably (by 80 %) for both orientations, which is clearly attributable to carrier spillover.
Substantial carrier spillover in both polarities, therefore, suggests that the polarization charge is not the major factor in
efficiency degradation observed, particularly at high injection levels. Furthermore, the m-plane variety with EBL did not
show any discernable efficiency degradation up to a maximum current density of 2250 Acm-2 employed while that on cplane
showed a reduction by ~ 40 %. In addition, IQE of m-plane LED structure determined from excitation power
dependent photoluminescence was ~80 % compared to 50 % in c-plane LEDs under resonant and moderate excitation
condition. This too is indicative of the superiority of m-plane LED structures, most probably due to relatively larger
optical matrix elements for m-plane orientation.
Defect formation in wurtzite a-plane gallium nitride grown on r-plane sapphire has been studied using transmission
electron microscopy. The observed defect pattern grown along the [11-20] direction shows significant differences
compared to films grown along the [0001] direction. Predominant line defects identified in the a-plane GaN are Frank-Shockley partial dislocations bounding basal plane stacking faults and originating at the film/substrate interface. In order
to understand the impact of the anisotropic elastic properties of the wurtzite structure on the dislocation formation and
the stress around the dislocations anisotropic plane strain elasticity theory was applied and compared with results
obtained from isotropic theory calculations. Furthermore, dislocation properties were calculated for AlN and InN. It was
found that the line energy found for InN amounts only to about one third of the values obtained for GaN and AlN.
We have investigated bulk GaN material grown by HVPE either in the conventional polar [0001] direction and
subsequently sliced with nonpolar surfaces or grown in the nonpolar [11-20] direction. Spatially resolved techniques
such as cathodoluminescence imaging and transmission electron microscopy, as well as profile measuring techniques
such as positron annihilation spectroscopy and secondary ion mass spectroscopy were employed to directly visualize the
extended structural defects, and point defect (impurity and vacancy) distributions along the growth axes. A comparative
analysis of the results shows a distinctive difference in the distribution of all kind of defects along the growth axes. A
significant decrease in the defect density in material grown along the polar direction, in contrast to the constant behavior
of the high defect density in material grown along the nonpolar direction points out the low-defect superior quality of the
former material and indicates the preferable way of producing high-quality GaN substrates with nonpolar surfaces.
We have studied the emission distributions in nonpolar α-plane GaN thick films grown by HVPE using different nucleation schemes. The emission spectra show in addition to the near band edge emission band, also defect related bands due to different structural defects being enhanced/reduced to different extent in samples grown on different templates. Spatially resolved cathodoluminescence imaging reveals the in-plane distributions of the respective emission bands, which allows us to correlate the emissions with particular stacking fault structural defects independently revealed by plan-view transmission electron microscopy. In addition, emission distributions were visualized in vicinity of largescale defects like surface triangle pits, depressions and cracks attributed to prevailing defect formation and/or impurity incorporation.
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