Phase Separation in InGaN/GaN Multiple Quantum Wells
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M.D. MCCLUSKEY *, L.T. ROMANO *, B.S. KRUSOR *, D.P. BOUR *, C. CHUA *, N.M. JOHNSON *, KIN MAN YU ** *Xerox Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto, CA 94304, mccluske@ parc.xerox.com "**LawrenceBerkeley National Laboratory, MS 2-200, 1 Cyclotron Rd., Berkeley, CA 94720 ABSTRACT Evidence is presented for phase separation in In0.27Ga 0.73N/GaN multiple quantum wells (MQW's). After annealing for 4 min at a temperature of 1100 °C, the absorption threshold at 2.95 eV is replaced by a broad peak at 2.65 eV. This peak is attributed to the formation of Inrich InGaN phases in the active region. X-ray diffraction measurements show a shift in the diffraction peaks toward GaN, consistent with the formation of an In-poor phase. INTRODUCTION The development of blue light-emitting diodes (LED's) [1] and laser diodes (LD's) [2] has focused a great deal of research activity on GaN-based III-V nitrides. The band gaps of InGa1 ._N alloys cover a wide spectral range, from red (InN) to UV (GaN), making this alloy system ideal for numerous optoelectronic applications [3]. The active region in GaN-based LED's and LD's consists of InGa1 _,N/InyGajyN multiple quantum wells (MQW's). In this paper, we report evidence of phase separation in annealed InGaN/GaN quantum wells. The formation of In-rich InGaN precipitates yields a low energy peak in the optical absorption spectrum of the MQW's. Evidence of phase separation was reported previously in polycrystalline InGaN films that were annealed at temperatures below 700'C [4,5]. Thick (0.3 gm) InGaN layers grown by molecular beam epitaxy (MBE) contain regions of pure InN for atomic In concentrations greater than x = 0.3 (Ref. 6). These experimental results are in agreement with theoretical calculations which predict that InN and GaN are not miscible for typical growth temperatures of around 800'C [7]. In GaN/InGaN/GaN double heterostructures, however, atomic In concentrations up to x = 0.8 can be incorporated without phase separation [6]. EXPERIMENT Experimental details In this study, phase separation was investigated in InGaN/GaN MQW structures. The first structure consists of a 0.2 gim GaN:Mg layer, a 10 period superlattice of 20 A In 0 .27 Gao.73N well / 40 A GaN barrier, and a 4 jim GaN:Si layer on a sapphire substrate. The thickness of the well plus barrier was determined by the spacing between satellite peaks in the x-ray diffraction (XRD) spectrum. The barrier-to-well thickness ratio of 2:1 was measured with transmission electron microscopy (TEM). The In concentration in the InGaN quantum wells was determined by Rutherford backscattering spectrometry (RBS), by assuming the absence of In within the 985
Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
GaN barriers. Samples underwent a rapid thermal anneal in a Heatpulse thermal processor. Prior to annealing, a SiN cap was deposited over the GaN to prevent decomposition. RBS was performed with a 1.95 MeV He' beam generated by a 2.5 MeV Van de Graff accelerator. The backscattered He ions were collect
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