Interface Effects on the Persistent Photoconductivity in Thin GaN and AlGaN Films
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M. Kelly **, 0. Ambacher **, and M. Stutzmann ** * University of Oldenburg, FB8/EHF, D-26129 Oldenburg •* Technical University of Munich, Walter Schottky Institute, D-85748 Garching
Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G5.5 (1999) Abstract Thin films of GaN and its alloy A1GaN are investigated with respect to their properties of the persistent photoconductivity (PPC). In this work, we show that the film-substrate interface plays an important role for the metastable electrical effect. Strongly absorbed bandgap light causes an increase of photoconductivity which is about one order of magnitude higher when the sample is illuminated from the substrate side near the interface than from the growth side. To access the interface properties at the substrate, we use temperature-dependent Hall effect measurements. The smallest PPC effect was observed for the GaN film with the best interface properties grown on SiC.
I. Introduction GaN and its alloys with In and Al have become the most favorite candidates for blue light emitting diodes, laser diodes, and high-temperature, high-power electronic devices [1,2]. It has been shown very early that intrinsic defects play an important role in the electrical behavior of these materials [3,4]. Extensive studies on defect-related behavior like, e.g., the "yellow band luminescence" or
the consequences of additional defect-rich layers for Hall measurements [5,6] have been performed. Also the persistent photoconductivity (PPC) observed by several groups [7,8], is
discussed as a result of native defects in GaN. However, since the PPC seems to be present independent of the doping, alloying and growth method, other sources for the long-time constants connected with the metastability have to be considered. In this work, we investigate the influence of the interfacial and defect-rich layer on the PPC and Hall effect measurements. II. Experiment The samples were grown by metal-organic chemical vapor deposition (MOCVD) as well as by molecular beam epitaxy (MBE). As substrate material, both sapphire and SIC were used. For some
samples, a 50nm thick GaN low-temperature buffer layer was also grown. Ohmic contacts were prepared in a coplanar van der Pauw geometry by electron beam evaporation of 20nm Ti and 100nm Al and subsequent annealing in N2 atmosphere at 900'C for around 60s. For the lightdependent PPC measurements, an Oxford cryostat CF 1204 was used in combination with a Xenon
lamp and a Bentham M300 monochromator. In this setup, it is possible to mount the sample for front- and backside illumination. Temperature-dependent Hall effect measurements were carried out in a Janis ST300 cryostat and Bruker B-E 10 magnet system. In Table I, the samples under investigation are summarized.
G 5.5 Mat. Res. Soc. Proc. Vol. 537 © 1999 Materials Research Society
Table 1:Samples used in this work for PPC and Hall effect measurements * the grower did not provide any details about the buffer Sample
Alloy
SM3
GaN
SM5
A10 15 Ga 85 N
S13 SMI S16
GaN GaN GaN
Growth
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