Microstructure of InGaN Quantum Wells
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ABSTRACT The microstructure of lnxGalxN quantum wells with intermediate indium concentrations (x = 0.28 and 0.52) has been studied using transmission electron microscopy. High-resolution lattice images and dark-field images taken under high tilt conditions indicate that the quantum wells are inhomogeneous in character. Most of the area of the quantum wells is pseudomorphic with the GaN adjacent layer. However, misfit dislocations are sometimes observed, although with an inhomogeneous distribution. Strained cluster regions are observed in the high-indium concentration quantum wells, with dimensions ranging fi'om 3 to 10 nm in diameter. Evidence is presented suggesting the extent of clustering depends on the exact orientation of the growth surface which is related to the columnar nature of the GaN/sapphire epitaxy. INTRODUCTION The growth of high quality quantum wells requires the production of interfaces that are highly planar and abrupt, with high degrees of chemical homogeneity and crystalline quality. This is very challenging in the case of the nitride semiconductors, since their microstructure contains a high density of dislocations [1]. Furthermore, the growth of InxGal.xN in practice requires different growth conditions than those of the underlying GaN epilayer, such as the use of lower growth temperatures (700-800'C for InGaN vs. 1000-11 000 C for GaN), and much higher V/III ratios in order to achieve significant indium concentration [2]. The incorporation of substantial amounts of indium is inhibited by reported miscibility gaps in the ternary composition diagram [3], high In partial pressure at growth temperature, preferential In etching by H2 and NH 3, and the propensity to form In droplets due to low NH 3 cracking efficiency at low growth temperatures. Recent reports on the emission mechanisms of InxGal-.N SQW suggest that the electroluminescence may be due to recombination of excitons localized at certain potential minima in the quantum wells [4]. One of the possible interpretations of those observations is that InGaN quantum wells are not uniform, and that they may behave more like an array of quantum dots [57]. The microstructure of thin InxGa].xN layers is not easy to probe with standard techniques due to the size of the features and the similarity of the lattices involved. In this paper we report on a study of the microstructure of lnxGal.xN single quantum wells using a combination of transmission electron microscopy (TEM) techniques. EXPERIMENTAL The material under study was grown by metalorganic chemical vapor deposition on (0001) sapphire substrates, using conventional GaN with a 2 ltm-thick Si-doped GaN film grown at 1050'C. Thin layers of InGai.xN were deposited at about 800'C, and capped by GaN layers 30nm thick. The capping layers 453 Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
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Fig. 1. Photoluminescence spectra of In,,Ca 1.)N single quantum wells with x=0.28, and 0.52. were inten
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