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ABSTRACT Photoluminescence spectra of hexagonal GaN were measured in the temperature range T= 2 1200 K. We identify the Free Exciton (FX) as the dominant recombination process in our high quality samples for temperatures above 200 K. From the line shape fit of the FX we determine the excitonic band gap shift with temperature. An analysis according to the empirical Varshni equation gives Eg (T)-Eg(O K) = (-x T 2)/(T + P3),with x = (7.3 ± 0.3)"10-4 eV/K and P = (594 + 54) K. We have detected significant differences in the band gap energy at low and higher temperatures for GaN layers grown on different substrate materials. Heating GaN above 1200 K leads to irreversible changes in the near band gap photoluminescence spectra.

INTRODUCTION The thermodynamically stable form of GaN is the wurtzite (a) modification. The direct fundamental gap Eg makes this material suitable for optoelectronic devices in the near-UV. The temperature dependence of Eg is not well known below 600 K and is unknown above this temperature. In this study we present data for the temperature dependence of Eg up to 1200 K and demonstrate a dependence of Eg on various sample parameters (substrate, growth properties and

thermal history). EXPERIMENTAL High quality hexagonal GaN layers (a-GaN) grown on sapphire (A12 0 3) or SiC substrates by molecular beam epitaxy (MBE) were used in this study. Table I lists the parameters known from these samples. Layers were grown on A120 3 substrates at the Walter-Schottky- Institute at the TU Munich and kindly supplied by M. Stutzmann, while the layers grown on SiC were prepared at the ETL in Tsukuba and generously donated by H. Okumura. sample

substrate

layer thickness [n] (cm"3) (pm)

GaN/A120 3

a-A12 0 3

3

GaN/SiC

3C-SiC

1.5

1.1017

PH

growth

2

(cm /Vs)

method

280

MBE/8000 C

-

MBE

Table 1: GaN-layers investigated in this study 719

Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society

The photoluminescence (PL) of the samples was excited by the 325 nm line of a He-Cd Laser with output powers in the range of 3 - 8 mW. For the temperatures from 1.7 to 300 K, measurements were performed with the samples mounted in a temperature controlled He-cryostat (Oxford). For the PL measurements above room temperature, the samples were mounted in a specially designed, Ar filled quartz ampoule, which was introduced in an oven. The PL was spectrally dispersed at low temperatures with a high resolution double monochromator. A monochromator with 0.25 cm focal length and high throughput was used at higher sample temperatures. The signals were detected and amplified by a photomultiplier tube with GaAs cathode and photon counting technique. PHOTOLUMINESCENCE SPECTRA GaN/A120 3 The photoluminescence spectra of the GaN layers on A120 3 exhibit in the low temperature regime (T=1.7 K - 10 K) a strong bound exciton recombination (12-line) (Fig. 1). For T>20 K a high energy shoulder develops, which can be identified with the Free Exciton (FX) emission.[1,2] The emission above 200 K is dominated by the FX-e