Microstructures of GaN films deposited on (001) and (111) Si substrates using electron cyclotron resonance assisted-mole
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T. Lei and T. D. Moustakas Molecular Beam Epitaxy Laboratory, Department of Electrical Engineering, Boston University, Boston, Massachusetts 02215 (Received 17 September 1993; accepted 26 April 1994)
The microstructures of GaN films, grown on (001) and (111) Si substrates by a two-step method using Electron Cyclotron Resonance assisted-Molecular Beam Epitaxy (ECR-MBE), were studied by electron microscopy techniques. Films grown on (001) Si had a predominantly zinc-blende structure. The GaN buffer layer, grown in the first deposition step, accommodated the 17% lattice mismatch between the film and substrate by a combination of misoriented domains and misfit dislocations. Beyond the buffer layer, the film consisted of highly oriented domains separated by inversion domain boundaries, with a substantial decrease in the defect density away from the interface. The majority of defects in the film were stacking faults, microtwins, and localized regions having the wurtzite structure. The structure of the GaN films grown on (111) Si was found to be primarily wurtzite, with a substantial fraction of twinned zinc-blende phase. Occasional wurtzite grains, misoriented by a 30° twist along the [0001] axis, were also observed. A substantial diffusion of Si was seen in films grown on both substrates.
I. INTRODUCTION GaN films are currently being investigated in a number of laboratories because of their potential for applications in optoelectronic devices as well as high temperature and high frequency electronics.1 These applications require the successful growth of highquality crystals of GaN which can be intentionally doped n- and p-type. Due to the lack of a native substrate, GaN has been grown heteroepitaxially on a variety of substrates by a number of vapor-phase methods.2"4 Heteroepitaxy has been used successfully for the growth of semiconducting films for optoelectronic applications. A common example is GaAs on Si, in which the potential problems of misfit dislocations due to a 4% difference in the lattice parameters of the two cubic unit cells, as well as presence of anti-phase disorder due to the growth of a polar film on a nonpolar substrate, have been successfully addressed.5 The growth of GaN on Si, however, presents additional challenges. Unlike GaAs, which exists only in the cubic zinc-blende structure, GaN exists in two allotropic structures, wurtzite (a) and zinc blende (/3). The majority of work on the growth of GaN has been reported on sapphire substrates, where GaN is grown in its thermodynamically stable wurtzite phase. However, 2370
J. Mater. Res., Vol. 9, No. 9, Sep 1994
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there is a motivation to grow the zinc-blende structure, since cubic structures of the III-V compounds have been more readily doped both n- and /?-type.6 Heteroepitaxial stabilization of /?-GaN on substrates with cubic symmetry, i.e., £ - S i C , 7 (001) MgO, 8 and (001) GaAs,9 have been previously reported. This present study was undertaken to explore ways to heteroepitaxially grow Ga
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