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.
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 on the emission properties of GaN/AlN superlattices (SLs) grown by metalorganic chemical vapor deposition on a thick GaN layer. Nominally undoped and Si-doped SL structures with the well/barrier thickness ratio 3:1 and different SL periods are investigated. It is found that in these SLs without capping layer the energy position, intensity and linewidth of the emission are determined by the interplay of the built-in polarization field, the depletion field arising from the pinning of the Fermi level at the surface, and the screening of the electric field in the quantum well due to the both the polarization-induced two-dimension electron gas (2DEG) and the photo-generated carriers. A non-uniform equilibrium electron distribution and an electron accumulation at the bottom AlN/GaN interface are evidenced by the observed recombination of the 2DEG with the photo-excited holes occurring below the GaN bandgap.
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.
We report on low temperature photoluminescence (PL) in InxGa1-xN multiple qunatum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the QW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample 5 QWs of width 3 nm and with 6 nm highly Si doped In0.01Ga0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for n-doped Al0.07Ga0.93N/GaN structures, with near surface MQWs.
Optical parametric oscillator based on LiB3O5 (LBO) crystal and synchronously pumped at 532 nm by second harmonic generation of Nd:YAG mode locked laser, is described. In the xy plane, we use type I nonlinear interaction. Using the Sellmeier equations for the principal refractive indices of LBO, the phase-matching is calculated and the tuning curves within all transmission range are obtained. Temperature-tuning within 120-180 degrees C for the crystal is applied, and a precise thermostabilization of the heating is also realized.
Nonlinear behavior of optical properties of PnSnTe epitaxial structures excited by CO2 laser pulses is investigated. Fast optical switching and bistability are observed. Numerical modeling of the experiment with an account for the dynamics of the noneuqilibrium carriers and the sample temperature is performed. Carrier-induced nonlinear refractive cross-section of the studied compound is determined.
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