Scanning Tunneling Microscopy Studies of InGaN Growth by Molecular Beam Epitaxy
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G9.5 (1999) ABSTRACT InGaN alloys with indium compositions ranging from 0-40% have been grown by molecular beam epitaxy. The dependence of the indium incorporation on growth temperature and group III/group V ratio has been studied. Scanning tunneling microscopy images, interpreted using first-principles theoretical computations, show that there is strong indium surface segregation on InGaN. Based on this surface segregation, a qualitative model is proposed to explain the observed indium incorporation dependence on the growth parameters. INTRODUCTION Ternary In.Ga _xN alloys are widely used in the development of GaN-based optoelectronic devices.' To achieve high quality layers with desired optical properties, it is important to understand and control the indium incorporation efficiency and uniformity. In conventional III-V semiconductors, surface segregation of metal species (Ga, In, or Al) is known to be an important process. 2 In this paper, we present results for the indium incorporation in InGaN during growth by molecular beam epitaxy (MBE), with the characterization performed by x-ray diffraction (XRD), Auger spectroscopy, and scanning tunneling microscopy (STM). The dependence of the indium incorporation on growth parameters is studied. STM images reveal strong surface segregation of the indium, from which a model is proposed to explain the incorporation kinetics. EXPERIMENT The studies described here were performed in a combined MBE/surface analysis system. The growth chamber contains gallium and indium effusion cells, an RF-plasma nitrogen source, and a reflection high energy electron diffraction (RHEED) system. Growth is performed on solvent-cleaned sapphire (0001) substrates, heated first to 700'C for 15 min, then exposed at 1050'C to the nitrogen plasma with a power level of 550 W and pressure of 7.0 x 105 Torr, for 30 min. First a GaN buffer layer of about 20 nm thickness is grown at 550'C using a nitrogen pressure of 1.5 x 10- Torr. The growth temperature is then increased to 720'C for the remainder of the GaN growth. The total thickness of GaN was in the range 200-400 nm. Following the GaN growth, the substrate temperature is lowered to 620-670'C for the InGaN deposition. Typical growth rates for the GaN and InGaN are 200 nm/h. Gallium and indium flux rates were calibrated with an in situ crystal thickness monitor. The substrate temperature was measured by a optical pyrometer with emissivity set to be 0.7; care was taken to ensure a clean viewport between the pyrometer and the sample. From our previous work,3 it is known that GaN films prepared in the above manner are nitrogen polar.
G 9.5 Mat. Res. Soc. Symp. Proc. Vol. 537 © 1999 Materials Research Society
RESULTS AND DISCUSSION Indium Incorporation Dependence on Growth Parameters Several series of samples have been grown with different growth parameters. After the growth, XRD were performed to measure the indium incorporation. For the analysis of the XRD data, we ass
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