The authors experimentally find that the thermal conductivity of gallium nitride depends critically on dislocation density using the 3-omega technique. For GaN with dislocation densities lower than ${10}^6{\mathrm{cm}}^{-2}$ , the thermal conductivity is independent with dislocation density. The thermal conductivity decreases with a logarithmic dependence for material with dislocation densities in the range of ${10}^7–{10}^{10}{\mathrm{cm}}^{-2}$ . These results are in agreement with theoretical predictions. This study indicates that the hydride vapor phase epitaxy method offers an attractive route for the formation of semi-insulating gallium nitride with optimal thermal conductivity values around $230\mathrm{W}∕\mathrm{m}\mathrm{K}$ and very low dislocation density near $5\times {10}^4{\mathrm{cm}}^{-2}$ .

Conventional and high-resolution electron microscopy have been applied for studying lattice defects in nonpolar $a$-plane GaN grown on a $4H$-SiC substrate with an AlN buffer layer. Samples in plan-view and cross-sectional configurations have been investigated. Basal and prismatic stacking faults together with Frank and Shockley partial dislocations were found to be the main defects in the GaN layers. High-resolution electron microscopy in combination with image simulation supported Drum’s model for the prismatic stacking faults. The density of basal stacking faults was measured to be $\sim 1.6\times {10}^6{\mathrm{cm}}^{-1}$. The densities of partial dislocations terminating $I_1$ and $I_2$ types of intrinsic basal stacking faults were $\sim 4.0\times {10}^{10}{\mathrm{cm}}^{-2}$ and $\sim 0.4\times {10}^{10}{\mathrm{cm}}^{-2}$, respectively. The energy of the $I_2$ stacking fault in GaN was estimated to be $(40\pm 4)\mathrm{erg}∕{\mathrm{cm}}^2$ based on the separation of Shockley partial dislocations.

Thermal conductivity in non-metallic crystalline materials results from cumulative contributions of phonons that have a broad range of mean free paths. Here we use high frequency surface temperature modulation that generates non-diffusive phonon transport to probe the phonon mean free path spectra of GaAs, GaN, AlN and 4H-SiC at temperatures near 80 K, 150 K, 300 K and 400 K. We find that phonons with MFPs greater than 230 ± 120 nm, 1000 ± 200 nm, 2500 ± 800 nm and 4200 ± 850 nm contribute 50% of the bulk thermal conductivity of GaAs, GaN, AlN and 4H-SiC near room temperature. By non-dimensionalizing the data based on Umklapp scattering rates of phonons, we identified a universal phonon mean free path spectrum in small unit cell crystalline semiconductors at high temperature.

Hexagonal AlN is a non-ferroelectric material and does not have any phase transition up to its melting point (>2000°C), which indicates the potential use of AlN for high-temperature sensing. In this work, the elastic, dielectric, and piezoelectric constants of AlN single crystals were investigated at elevated temperatures up to 1000°C by the resonance method. We used resonators of five different modes to obtain a complete set of material constants of AlN single crystals. The electrical resistivity of AlN at elevated temperature (1000°C) was found to be greater than 5 × 10 ¹⁰ Ω · cm. The resonance cm. frequency of the resonators, which was mainly determined by the elastic compliances, decreased linearly with increasing temperature, and was characterized by a relatively low temperature coefficient of frequency, in the range of -20 to -36 ppm/°C. For all the investigated resonator modes, the elastic constants and the electromechanical coupling factors exhibited excellent temperature stability, with small variations over the full temperature range, <;11.2% and <;17%, respectively. Of particular significance is that due to the pyroelectricity of AlN, both the dielectric and the piezoelectric constants had high thermal resistivity even at extreme high temperature (1000°C). Therefore, high electrical resistivity, temperature independence of electromechanical properties, as well as high thermal resistivity of the elastic, dielectric, and piezoelectric properties, suggest that AlN single crystals are a promising candidate for high-temperature piezoelectric sensing applications.

We report the performance of GaN $p\text{-}i\text{-}n$ ultraviolet avalanche photodiodes grown on bulk GaN substrates by metal-organic chemical vapor deposition. The low dislocation density in the devices enables low reverse-bias dark currents prior to avalanche breakdown for $\sim 30\mu \mathrm{m}$ diameter mesa photodetectors. The photoresponse is relatively independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of $\sim 2.8\mathrm{MV}∕\mathrm{cm}$ . The magnitude of the reverse-bias breakdown voltage shows a positive temperature coefficient of $\sim 0.05\mathrm{V}∕\mathrm{K}$ , confirming that the avalanche breakdown mechanism dominates. With ultraviolet illumination at $\lambda \sim 360\mathrm{nm}$ , devices with mesa diameters of $\sim 50\mu \mathrm{m}$ achieve stable maximum optical gains greater than 1000. To the best of our knowledge, this is the highest optical gain achieved for GaN-based avalanche photodiodes and the largest area III-N avalance photodetectors yet reported.

The temperature-dependent electrical characteristics of Schottky rectifiers fabricated with a $\mathrm{Si}{\mathrm{O}}_2$ field plate on a freestanding $n^{-}$ gallium nitride (GaN) substrate were reported in the temperature range of $298–473\mathrm{K}$ . The Schottky barrier heights evaluated from forward current-voltage measurement revealed an increase of Schottky barrier height and series resistance but a decrease of ideality factor $\left(n\right)$ with increasing temperature. However, the Schottky barrier heights evaluated from capacitance-voltage measurement remained almost the same throughout the temperature range measured. The Richardson constant extrapolated from $\ln (J_0∕T^2)$ vs $1∕T$ plot was found to be $0.029\mathrm{A}{\mathrm{cm}}^{-2}{\mathrm{K}}^{-2}$ . A modified Richardson plot with $\ln (J_0∕T^2)$ vs $1∕\mathit{nT}$ showed better linearity, and the corresponding effective Richardson constant was $35\mathrm{A}{\mathrm{cm}}^{-2}{\mathrm{K}}^{-2}$ . The device showed a high reverse breakdown voltage of $560\mathrm{V}$ at room temperature. The negative temperature coefficients were found for reverse breakdown voltage, which is indicative of a defect-assisted breakdown.