Growth and Doping of AlGaN Alloys by ECR-assisted MBE
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 10
Growth and Doping of AlGaN Alloys by ECR-assisted MBE D. Korakakis, H.M. Ng, M. Misra, W. Grieshaber, T.D. Moustakas Department of Electrical and Computing Engineering and Center for Photonics Research This article was received on and accepted on September 16, 1996.
Abstract We report the growth of Alx Ga1-xN alloys on (0001) sapphire by the method of Electron Cyclotron Resonance-assisted Molecular Beam Epitaxy (ECR-MBE). The films were doped n-type with silicon at carrier concentration levels from 1016 to 1019 cm-3. SEM studies reveal smooth surface morphology consistent with the observed 3x4 surface reconstruction in the RHEED pattern. Independent determination of the Al-concentration and the lattice constant of the alloys shows that Vegard’s rule is obeyed in the pseudo-binary GaN-AlN system. The bandgap of the alloys, determined by transmission and photoluminescence measurements, was found to depend linearly on Al-concentration.
1. Introduction The family of the III-V nitrides are gradually finding applications in optical devices (LEDs, lasers and detectors) as well as electronic devices [1]. However, the majority of the scientific effort was focused in the growth and characterization of GaN films. Very little effort has been dedicated in the study of the ternary and quarternary alloys. For example, there are conflicting results regarding the bowing parameter in the AlGaN alloys [2] [3] [4] and whether Vegard’s rule is applicable in this system [3]. Also there are only limited intentional doping studies [5]. In this paper, we report the growth and Si-doping of Alx Ga1-xN alloys with x 0.35 by ECR-MBE.
2. Experimental Methods The AlGaN films were deposited in a Varian Gen-II MBE unit, equipped with an ASTEX compact ECR plasma source to supply activated nitrogen gas. This deposition method has been described in detail previously [6] [7] [8] [9]. The films were grown on (0001) sapphire substrates, which after degreasing and acid etching, were thermally outgassed and nitridated by exposing to an ECR-nitrogen plasma. This process was found to convert the surface of the substrate to atomically smooth AlN [6] [7] [8] [9]. The films were grown in two temperature steps [6] [7] [8] [9]. A low temperature AlGaN buffer (approximately 300Å thick) was grown at 550-650C and the rest of the film (approximately 1µm thick) was grown at 700-800C at a growth rate typically 0.2µm/hr. Doping was accomplished by subliming silicon at temperatures between 1200-1400°C. The structure of the films was determined by in-situ 10kV Reflection High Energy Electron Diffraction (RHEED) and X-ray diffraction (XRD), employing Co-Kα radiation, monochromated with a single Ge crystal. The surface morphology and the composition of the films were determined with a JEOL JSM-6100 scanning electron microscope (SEM) equipped with a Kevex detector with a Be-window for energy dispersive x-ray spectroscopy (EDS). EDS measurements at more than 6kV overestimate the concentration of
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