Phase Separation and Atomic Ordering in AlGaInN Alloys
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ow with Polaroid Corporation, Laser Diode Manufacturing and Development, Norwood, with Nottingham
2 Now
University, Nottingham, UK.
MA.
193 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
Evidence has recently been presented that these phenomena have a significant effect on the optical and electronic properties of the alloys [1]. Similarly, the solid solutions of M-V nitrides were considered until recently to be homogeneous random alloys. However, there is ample recent evidence of phase separation and atomic long range ordering in InGaN and AIGaN alloys [7-10]. The study of such phenomena in these alloys is important since these materials are currently employed in various optical and electronic devices. In this paper, we present our recent results in phase separation and atomic long range ordering in InGaN and AlGaN alloys. EXPERIMENTAL METHODS Both the InGaN and AlGaN alloys were grown by plasma-assisted molecular beam epitaxy (MBE). The deposition system consists of a Varian GEN II MBE unit equipped with an ASTeX compact electron cyclotron resonance (ECR) plasma source for activating the nitrogen. The group HI elements and the dopants (Si,Mg) are evaporated from regular effusion cells. The design of the ECR source, for producing sufficient amount of activated nitrogen for the growth of high quality GaN films at a growth rate of 200-300 nm/h, was reported previously [11]. The films were grown on (0001) sapphire substrates, which were first subjected to plasma nitridation (conversion of the surface of A120 3 to AiN) and then coated with 30nm low temperature GaN buffer [12]. The InGaN films were grown at 650 to 700'C at growth rates between 10-30 A/min, which is of the same order as those of InGaN films grown by the metal-organic chemical vapor deposition (MOCVD) method [13]. The AlGaN films were grown at 750'C at growth rates of 25-35 A/min. During the growth of these films, a number of deposition parameters such as the ratio of the I/V fluxes and the degree of n-type doping, were varied. The structure of the films was determined by x-ray diffraction (XRD) using Cu Ka1 and Co radiation in a four circle diffractometer with a Ge (111) single crystal monochromator that forbids x-ray reflection of the X/2 harmonic. The composition of the films was determined from the XRD peaks assuming that Vegard's law is applicable in these ternary alloy systems. The InGaN films were also characterized by optical absorption and photoluminescence studies as reported earlier [7-91. EXPERIMENTAL RESULTS AND DISCUSSION A. Phase separation in InGaN Alloys Phase separation in InGaN alloys was studied in thick (300400 nm) InGaN films or thinner (
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