Effect of Thermal Annealing on the Optical Properties of Strained Si20Ge20 Superlattices Grown on Si(1 0 0) Substrates

Abstract

Rapid developments and improvements in growth technologies have made possible the fabrication of several kinds of arti®cial quantum structures with interfacial abruptness on the scale of a few lattice constants [1, 2]. Among these quantum systems, strained SimGen superlattices hold considerable interest for both scienti®c and application reasons [3±12]. The SimGen superlattices have emerged as excellent candidates for arti®cially created quasidirect gap materials resulting from the zone-folding effect [11]. Because Si and Ge have an inherent lattice mismatch (Aa=a ˆ 4:2% at 25 8C), however, the Ge can only be commensurately grown on a Si substrate until a thickness of only 9 AE is reached [8]. Although the effects on strain of phonons in SimGen, as well as the dispersion relations for phonons in strained Si and Ge, have been reported [13, 14], the effect of thermal annealing on the optical phonons has not yet to the best of our knowledge been investigated. Because thermal treatments are necessary for the fabrication of semiconductor devices, understanding the effect of annealing processes on the behavior of optical properties is very important for achieving high-quality devices [15]. Furthermore, annealing processes at high temperatures create the interdiffusion of Ge and Si atoms, and the interdiffusion behavior affects the strain of the SimGen superlattices, which is very important for optoelectronic devices. This letter reports the results of double-crystal Xray diffraction (DCXD), transmission electron microscopy (TEM), secondary-ion mass spectroscopy (SIMS) and Raman scattering measurements on SimGen strained superlattices grown on Si substrates by molecular beam epitaxy (MBE). These tests were performed to investigate their structural properties and to investigate the effect of thermal treatment on the behavior of the optical properties. The samples in this study were SimGen superlattices grown by MBE at a substrate temperature of 380 8C. A Si0:5Ge0:5 buffer layer was grown on a Si substrate prior to the growth of the SimGen superlattice to symmetrize the stress. The growth rates of the Si and Ge layers were 2:75 3 10 cmy2 sy1 and 2:20 3 10 cmy2 sy1, respectively. The thickness of the SimGen superlattice was 0.3 im, and its period was 55. The TEM observations were performed in a Jeol 200 CX transmission electron microscope operating at 400 kV. The samples for the TEM measurements were prepared by cutting and polishing with diamond paper to a thickness of approximately 30 im and then argon-ion milling at liquid-nitrogen temperature to electron transparency. Raman scattering measurements were made in backscattering geometry with the 4877 AE Ar‡ line at 200 mW. The SimGen superlattice as-grown by MBE had mirror-like surfaces without any indication of pinholes, which was con®rmed by Normarski optical microscopy and scanning electron microscopy measurements. The results of the DCXD measurements for the SimGen superlattice showed that the SimGen peaks in addition to the Si peak were observed. The values of m and n, determined based on the superlattice formation by opening and closing the respective source shutters, were 20 and 20, respectively, and the determined m and n values were con®rmed by the high-resolution TEM measurements. A bright-®eld TEM image showed the Si20Ge20 superlattice layer, top layer, the Si0:5Ge0:5 buffer layer and the Si substrate bottom layer. Many dislocations and defects existed in the Si0:5Ge0:5 buffer layer due to the lattice mismatch between the Si0:5Ge0:5 buffer layer and the Si substrate. Moreover, dislocations were only occasionally observed along the Si0:5Ge0:5=Si interface [16]. The stability condition of the SimGen superlattice is aSidSi‡ aGedGe ˆ 0, where aSi and aGe are the lateral deformations of the Si and Ge, respectively, and dSi and dGe are the thicknesses of the Si and Ge, respectively. To obtain the symmetry of deformation for the SimGen superlattice, a Si0:5Ge0:5 buffer layer with a 2000 AE thickness was used. When the buffer layer was relaxed perfectly, the deformation values of the Ge and Si in the SimGen superlattice were chosen as y2.00% and ‡2.09%, respectively [9]. In this case, the deformations of the Si and Ge layers compensated each other. A dislocation between the superlattice layer and the buffer layer did not occur as long as the critical thickness of each layer was not exceeded. The cross-sectional high-resolution TEM images of the SimGen superlattices showed that the Si and Ge layers in the superlattice layer were