Defect Microstructure of Thin Wurtzite GaN Films Grown by MBE
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compared with most other heteroepitaxial semiconductor systems, which probably accounts for the lack of progress in producing long-lived heterostructure devices. Hence, much attention is being given to optimizing growth conditions for higher quality material. In this comparative study, thin films of wurtzite GaN were grown by molecular beam epitaxy (MBE) on substrates of 6H SiC, Si{ 1111 and sapphire. The overall film quality was quantified using double-crystal rocking curves (DCRC) and transmission electron microscopy (TEM) was used to characterize local variations in defect microstructure and to assess the substrate/buffer/GaN interfacial quality. Special attention was given to substrate surface preparation and to the possible benefits of using buffer layers of InN (Si substrate) and AIN (SiC and sapphire substrates) grown at low temperature. EXPERIMENTAL DETAILS The wurtzite GaN films were grown by plasma-enhanced MBE using a Perkin-Elmer 430 system. Details of the system and typical nitride deposition conditions can be found elsewhere[8]. Standard X-ray diffraction double-crystal rocking curves (DCRC) were used to compare the film quality, and transmission electron microscopy was performed in the cross-sectional geometry with a JEM-4000EX high-resolution electron microscope (HREM) operated at 400keV. Prior tc
175 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
deposition, the chemically cleaned, 6H SiC substrates (basal plane) were subjected to hydrogen plasma treatment in situ to ensure oxide removal. All Si {1111 substrates were prepared using standard RCA cleaning, with some later preparations also involving treatment with various buffered oxide etches to produce extended flat terraces, and immersion in HF to produce Hterminated surfaces[9]. As the various InN, AIN or GaN buffer layers were being deposited on the substrate, the temperature was usually ramped upward, typically from 550 to 650 or 750'C. RESULTS Typical low-magnification and high-magnification electron micrographs of wurtzite GaN thin films grown on substrates of 6H SiC (basal plane), Si {1111 and sapphire (c-plane) are shown in figures 1 through 6. For each material, selected area electron diffraction (SAED) patterns confirmed the absence of cubic material and the overall quality of the GaN epitaxial growth. a) 6H SiC (basal plane) The morphology of GaN films grown on 6H SiC was invariably dominated by threading defects that originated at the SiC/GaN or AIN-buffer/GaN interfaces, and extended through to the top surface of the GaN film. Examples of these defects are visible in Fig. 1. These faults, which should be labelled as either double positioning boundaries (DPBs) or stacking mismatch boundaries (SMBs) to reflect their origin, are caused by the coalescense of domains that have nucleated on substrate terraces at the same or different levels. It is significant, as shown by the examples arrowed in Fig. 1, that many defects actually terminate within the GaN epilayer. A marked reduction in defect density with distance fr
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