We have measured high resolution thermal conductivity (kappa) and Raman spe
ctra {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 (H
VPE). The former was determined with a scanning thermal microscope while th
e latter was obtained using a micro-Raman system, both having a spatial res
olution of approximate to 2-3 mu m. For all three sets of samples the therm
al conductivity decreased linearly with log n, about a factor of two decrea
se in kappa for every decade increase in n. Also, we found a correlation be
tween film thickness and improved thermal conductivity. Furthermore, kappa
approximate to 1.95 W/cm K for one of the most lightly doped samples (appro
ximate to 6.9x10(16) cm(-3)), higher than previously reported kappa approxi
mate 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, 124
0 (1999)], kappa=1.55 W/cm K on LEO samples using a third-harmonic techniqu
e [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 co
nstant 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 mea
sures n in both the epilayer and GaN/sapphire interfacial region [D. C. Loo
k 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 epila
yer. (C) 2000 American Institute of Physics. [S0021-8979(00)01719-9].