Heteroepitaxy of Ge on (100) Si substrates

The epitaxial growth of GaAs on Si is a subject of growing interest because of the possibility of integrating high-speed and optical devices in Si-based technology. The large lattice mismatch between Si and GaAs makes the growth of high-quality epilayers very difficult. However, Ge and GaAs have similar lattice parameters and thermal expansion coefficients and thus a promising way to improve epitaxial growth of GaAs on Si is to introduce an intermediate Ge buffer layer. This technique will only be effective when the structural defects formed at the Si–Ge interface can be prevented from propagating into GaAs. A series of Ge epilayers have been grown on (100) Si by molecular-beam epitaxy and several methods to reduce dislocation propagation have been examined. These include postgrowth annealing, graded layering, growth interruption, and/or superlattice buffers. The efficiency of these methods was assessed by transmission electron microscopy, Rutherford backscattering/channeling, and double-crystal x-ray diffraction. It was found that the threading dislocation density tends to decrease with increasing thickness. The graded layering technique gave poor results. The introduction of a superlattice buffer reduced the threading dislocation density; however, the best overall quality material was obtained by depositing pure Ge on Si at 600 °C and annealing 30 min at 700 °C.