The formation mechanism of planar defects in compound semiconductors grown epitaxially on {100} silicon substrates
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I. INTRODUCTION For the fabrication of sophisticated electronic devices it is desirable to integrate single crystalline compound semiconductors onto silicon chips. For this reason, considerable efforts are made to grow perfect single crystalline films of these compound semiconductors epitaxially on {100} silicon substrates. The most common techniques are chemical vapor deposition (CVD), metal-organic CVD (MOCVD), and molecular beam epitaxy (MBE). However, the quality of the films is not sufficiently good as yet because they contain high densities of various crystal defects. It is believed that these defects degrade the performance of electronic devices by trapping minority charge carriers. Several of the compound semiconductors that are grown epitaxially on silicon have the zincblende structure. The frequently observed defects in (MO)CVD and MBE films of these materials are line defects and two-dimensional defects. Common line defects are misfit dislocations and threading dislocations. Misfit dislocations accommodate the lattice mismatch 8 = (af — as)/as between the epitaxial film and the substrate with lattice parameters af and as, respectively.1 Threading dislocations penetrate the epitaxial film from the film/substrate interface to the surface. The possible two-dimensional defects are stacking faults, twin boundaries, random grain boundaries, and inversion domain boundaries. The formation of random grain boundaries (i.e., polycrystalline material) is avoided by employing a two-step technique in (MO)CVD or MBE (e.g., see Ref. 2). The first step is basically the nucleation stage at a rather low substrate temperature and the formation of a continuous thin heteroepitaxial film. The second step is homoepitaxial growth on this initial thin film at a relatively high temperature.3 Inversion domain boundaries are possible in the zincblende structure because there is no inversion operation among the symmetry elements of this structure. These 834
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J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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two-dimensional defects separate domains in which the atomic species on the two fee sublattices of the zincblende structure are interchanged. Apparently, inversion domain boundaries are correlated with demi-steps on the surface of the Si substrate.4 It has been shown by scanning tunneling microscopy that the reconstructed surface of a vicinal {100} Si substrate, i.e., a substrate with a surface tilted some degrees away from the exact {100} orientation, is practically free of demi-steps.5 This explains the empirical result that inversion domain boundaries are effectively suppressed by using vicinal {100} Si substrates.6 The persistent two-dimensional defects, however, are stacking faults and twin boundaries. They are commonly attributed to stresses caused by the mismatch 8 of the lattice parameters, or by the mismatch of thermal expansion, between film and substrate. It has been proposed that these mismatch stresses nucleate dislocation half loops at the film surface once the film
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