Growth and characterization of CdTe/Si heterostructures — effect of substrate orientation

Transmission electron microscopy and small-probe microanalysis have been used to compare the microstructure and compositional profiles of CdTe/Si heterostructures grown by molecular beam epitaxy on (001), (211) and (111) silicon substrates. Overall, our results have demonstrated that the final CdTe growth orientation is determined by careful preparation of the Si substrate surface, the nature of the interfacial layer, and the initial phase nucleation. Initial studies confirmed that growth on (001) was problematical, not only because of the large lattice mismatch between materials (approximately 19%), but also because the double-domain reconstruction of the Si substrate surface degraded epilayer quality. Growth of high quality, domain-free CdTe(111)B was achieved by offcutting the substrate with respect to the [110] surface direction, with an additional rotation about [110]. Alternatively, with intermediary buffer layers of Ge(001), perfect a/2〈110〉 Lomer edge dislocations accommodated the misfit at the CdTe/Ge interface, and the (001) orientation of the Si substrate was retained during CdTe growth. For (211)-oriented substrates a very thin (approximately 2 nm) buffer layer of ZnTe prior to CdTe deposition was sufficient to maintain the substrate orientation, although Zn diffusion was often observed during subsequent annealing. The growth of Cd1−xZnxTe(211)B (with x∼2–4%) with intermediary CdTe buffer layers then provided substrates which were suitably lattice-matched for growth of HgCdTe. Finally, large-area, domain-free CdTe(111)B was achieved using As-passivated Si(111) substrates and thin (approximately 50 nm) ZnTe buffer layers.