Structural Study of GaN(As,P) Layers Grown on (0001) GaN by Gas Source Molecular Beam Epitaxy
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Dept. of Materials Science and Engineering, Kwangju Institute of Science and Technology (KJIST), Kwangju 500-712, Korea, c)Electronic mail: [email protected] b) Dept. of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407 Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.11 (1999)
ABSTRACT Transmission electron microscope (TEM) and transmission electron diffraction (TED) examination has been performed to investigate microstructural properties of gas source molecular beam epitaxial GaN(As,P) layers grown on (0001) GaN/sapphire at temperatures in the range 500 – 760 °C. As for the GaNAs, we report the observation of ordering with a space group P3m1 in the layer grown at 730 °C. The layers grown at temperatures below 600 °C are polycrystalline, whist the 730°C GaNAs layer has epitaxial relation to the GaN substrate. It is also shown that the GaNAs layers experience a structural change from a zinc-blende phase to a wurtzite phase, as the growth temperature increases. As for the GaNP, it is shown that the layers grown at temperatures ≤ 600 °C experience phase separation resulting in a mixture of GaN-rich and GaP-rich GaNP with zinc-blende structure. However, the layers grown at temperatures ≥ 730 °C are found to be binary zinc-blende GaN(P) single crystalline materials. The layers grown at temperatures ≥ 730 °C consist of two types of micro-domains, i.e., GaN(P)I and GaN(P)II; the former having twin relation to the latter. I. INTRODUCTION Gallium nitride-based materials are of technological importance because of their applications in short wavelength optical devices, e.g. blue-green light emitting diodes (LEDs) and violet laser diodes (LDs). Due to a large bowing parameter, addition of As or P to gallium nitride could lead to the practical engineering of specific semiconductors having a wide range of wavelengths from ultra violet to larger than 2 µm. Calculations based upon bulk thermodynamics indicated that a large miscibility gap exists for a GaN-GaP system where phase separation may occur by spinodal decomposition during layer growth.[1] The presence of such a miscibility gap would be a major obstacle for the successful growth of ternary GaNP alloy layers. Iwata et al.[2], investigating gas source molecular beam epitaxial (GSMBE) growth of GaN1-xPx (x ≤ 0.015), showed that phase separation occurred for a high PH3 flow rate condition. Bi and Tu investigated GaNxP1-x layers grown on GaP substrates at temperatures in the range 500 − 610 °C by GSMBE using a N radical beam source.[3] They showed that GaNxP1-x with a maximum N concentration of 16% was obtained, but no phase separation was observed. In this article, we describe structural results obtained from transmission electron microscope (TEM) and transmission electron diffraction (TED) studies of GaN(As,P) layers grown on (0001) GaN at temperatures ranging from 500 to 760 °C.
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