Jm Hayes

Raman scattering studies on single-crystalline bulk AlN under high pressures

We report on the Raman analysis of wurtzite single-crystalline bulk AlN under hydrostatic pressures up to 10 GPa. The pressure dependence of the AlN phonon frequencies was investigated. Mode Gruneisen parameters of 1.39, 1.57, 1.71, 0.93, and 1.26 were determined for the A1 (TO), E1 (TO), E2 (high), A1 (LO), and the quasi-longitudinal optical phonons, respectively. Recent theoretical calculations underestimate the pressure-induced frequency shift of the AlN phonons by about 20%–30%. Mode Gruneisen parameters of AlN were compared to those of GaN.

Phonon lifetimes in bulk AlN and their temperature dependence

We report on the Raman analysis of the phonon lifetimes of the A1(LO) (longitudinal optical) and E2(high) phonons in bulk AlN crystals and their temperature dependence from 10 to 1275 K. Our experimental results show that amongst the various possible decay channels, the A1(LO) phonons decay primarily into two phonons of equal energy (Klemens model), most likely longitudinal-acoustic phonons, whereas the E2(high) phonon decays asymmetrically into a high-energy and a low-energy phonon. Possible decay channels of the E2(high) phonon have been shown to include combinations of E2(low) and acoustic phonons. Phonon lifetimes of the A1(LO) phonon and the E2(high) phonon of 0.75 and 2.9 ps, respectively, were measured at 10 K.

Temperature Dependence of the Phonons of Bulk AlN

Micro-Raman scattering measurements were performed on bulk wurtzite AlN crystals over a temperature range from 10 K to 1275 K. The temperature dependence of the frequency of the AlN phonons follows an empirical relationship previously introduced for diamond. A temperature-induced frequency shift of the E2 phonon of -(2.22±0.02)×10-2 cm-1/K was determined for high temperatures, which is similar to values reported for bulk GaN. The results provide the basis for the non-invasive monitoring of the temperature of (Al,Ga)N by Raman scattering.

Angular dispersion of polar phonons in a hexagonal GaN-AlN superlattice

Abstract Raman spectra of a wurtzite GaN (6.3 nm)–AlN (5.1 nm) superlattice have been recorded under visible excitation. When the orientation of the phonon wavevector is varied, the angular dispersion of polar phonons from the superlattice is clearly evidenced. These experimental data are found to be in good agreement with the results of a previous calculation based on a dielectric continuum model, taking into account the strain of the two types of layers, which predicts dispersive interface and quasi-confined modes.

High-temperature processing of GaN: The influence of the annealing ambient on strain in GaN

Using micro-Raman scattering we have investigated the influence of the annealing ambient on the high-temperature processing of GaN. Compressive strain is found in GaN layers after high-temperature processing in oxygen-containing atmospheres. This strain is significantly enhanced by the addition of water vapor to the annealing ambient, suggesting the enhanced inclusion of oxygen into GaN. Characteristic photoluminescence lines appear at 3.355 and 3.406 eV after annealing in oxygen in the presence of water vapor. No strain is introduced by high-temperature processing in nitrogen ambient, even at temperatures close to the thermal decomposition temperature and in the presence of water vapor.

New technique for sublimation growth of AlN single crystals

Single crystalline platelets of aluminum nitride (AlN) were successfully grown by a new technique. It consists of (1) depositing an AlN buffer layer on a SiC substrate by metal organic chemical vapor deposition (MOCVD) below 1100°C, (2) forming an (AlN) x (SiC) 1−x alloy film on the AlN film by condensing vapors sublimated at a temperature of 1800°C from a source mixture of AlN-SiC powders, followed by (3) condensing vapors sublimated from a pure AlN source (at 1800°C). The necessity of the first two steps for the successful AlN sublimation growth on SiC substrate was illustrated by the initial nucleation studies of alloys on SiC substrates with and without MOCVD AlN buffer layers: an AlN MOCVD buffer layer leads to continuous, single grain growth mode; The (AlN) x (SiC) 1−x alloy film reduces the crack density because its thermal expansion coefficient is intermediate between SiC and AlN. X-ray diffraction (XRD) and Raman spectroscopy studies indicated the high quality of the AlN single crystal.

Raman scattering studies on single-crystalline bulk AlN: temperature and pressure dependence of the AlN phonon modes

Abstract We report on the Raman analysis of single-crystalline bulk AlN. AlN phonon modes were investigated as a function of temperature and hydrostatic pressure. Phonon decay channels were studied via the AlN Raman linewidth. Mode Gruneisen parameters describing the low-pressure behavior of the AlN phonon modes were determined and used to estimate hydrostatic stress in amber discolored AlN substrates.

Photoluminescence spectroscopy on annealed molecular beam epitaxy grown GaN

Photoluminescence (PL) spectroscopy has been used to investigate the effect that annealing temperature and ambient has on annealed molecular beam epitaxy grown GaN. Significant differences induced by the different annealing conditions occur in the PL spectra in the 3.424 eV region as well as the deep level band (2.0–3.0 eV). Power resolved measurements indicate that the 3.424 eV emission is a donor–acceptor pair transition. In the deep level region peaks are observed in all spectra at 2.3 and 2.6 eV. This suggests that the 2.3 and 2.6 eV peaks are related and a model is proposed to explain this luminescence.

Thermal management and device failure assessment of high-power AlGaN/GaN HFETs

Self-heating effects limit the performance of high-power AlGaN/GaN HFETs. Knowledge of the temperature in the active area of AlGaN/GaN HFETs is essential for optimizing device design, performance and reliability, however, direct measurement of this temperature is not readily achieved by IR techniques. Improved temperature information can be obtained by micro-Raman spectroscopy allowing temperature measurements with 1 /spl mu/m spatial resolution, important for local device geometries in the micron/sub-micron dimension range. This novel approach allows fast temperature measurements with minimal influence on device performance. We illustrate the use of micro-Raman spectroscopy for thermal management and device failure assessment by studying effects of device design and substrate on temperature in active high-power AlGaN/GaN HFETs. Temperature evolution up to device failure was investigated.

Determination of the Mode Grüneisen Parameter of AlN using different Fits on Experimental High Pressure Data

To investigate the pressure dependence of the AlN phonon frequencies Raman spectra of single-crystalline bulk AlN under hydrostatic pressure up to 10 GPa were recorded. The Raman peak positions of the A 1 (TO), E 1 (TO), E 2 (high), A 1 (LO) and quasi-longitudinal optical (QLO) phonons were plotted as a function of pressure. The experimental data was fitted using the traditional parabolic fit (M. Kuball et al . (2001) Appl. Phys. Lett., 78, 724 [1]) and fits to physical models, density, volume, etc. The mode Grüneisen parameters of the different phonons were determined for each fit and significant differences are found between the various fits. Results are compared with recent theoretical calculations (J.-M. Wagner et al . (2000) Phys. Rev. B, 62, 4526 [2]).