High spatial resolution thermal conductivity and Raman spectroscopy investigation of hydride vapor phase epitaxy grown n-GaN/sapphire (0001):: Doping dependence

We have measured high resolution thermal conductivity (kappa) and Raman spectra {E-2 mode [high frequency], A(1) mode [longitudinal optical (LO)], and high frequency LO-plasmon coupled mode [LPP+]} at 300 K of three series of n-GaN/sapphire (0001) samples fabricated by hydride vapor phase epitaxy (HVPE). The former was determined with a scanning thermal microscope while the latter was obtained using a micro-Raman system, both having a spatial resolution of approximate to 2-3 mu m. For all three sets of samples the thermal conductivity decreased linearly with log n, about a factor of two decrease in kappa for every decade increase in n. Also, we found a correlation between film thickness and improved thermal conductivity. Furthermore, kappa approximate to 1.95 W/cm K for one of the most lightly doped samples (approximate to 6.9x10(16) cm(-3)), higher than previously reported kappa approximate to 1.7-1.8 W/cm K on lateral epitaxial overgrown (LEO) material with n approximate to(1-2)x10(17) cm(-3) [V. M. Asnin , Appl. Phys. Lett. 75, 1240 (1999)], kappa=1.55 W/cm K on LEO samples using a third-harmonic technique [C. Y. Luo , Appl. Phys. Lett. 75, 4151 (1999)], and kappa approximate to 1.3 W/cm K on a HVPE sample [E. K. Sichel and J. I. Pankove, J. Phys. Chem. Solids 38, 330 (1977)]. The carrier concentration dependence of kappa is similar to that of other semiconductors in a comparable temperature range. On a log-log scale the linewidth of the observed E-2 Raman mode remained constant up to n approximate to 1x10(18) cm(-3) and then increased linearly. The carrier concentration obtained from the LPP+ mode is less than the Hall effect determination. This is probably due to the fact that the latter measures n in both the epilayer and GaN/sapphire interfacial region [D. C. Look and R. J. Molnar, Appl. Phys. Lett. 70, 3377 (1997); W. Gotz , Appl. Phys. Lett. 72, 1214 (1998)] while the Raman signal is primarily from the epilayer. (C) 2000 American Institute of Physics. [S0021-8979(00)01719-9].