Luminescence of MBE Si m Ge n Strained Monolayer Superlattices
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LUMINESCENCE OF MBE SimGe,. STRAINED MONOLAYER SUPERLATTICES M. A. KALLEL, V. ARBET-ENGELS, R. P. G. KARUNASIRI AND K. L. WANG Device Research Laboratory, 7619 Boelter Hall, Electrical Engineering Department, University of California, Los Angeles, CA 90024 ABSTRACT SimGe, strained monolayer superlattices (SMS) have been fabricated by molecular beam epitaxy (MBE) and characterized using photoluminescence (PL). Symmetrically strained structures with different periodicities have been grown on top of a Siji.Ge, alloy buffer layer. Luminescence features below (above) the Si (Ge) energy bandgap have been observed and attributed to either dislocations in the buffer layer or to energy band transitions in the SMS. INTRODUCTION Advances in SiGe-MBE have allowed the growth of high quality SimGe, SMS's. Due to the 4.2% lattice mismatch between Si and Ge, layers of either material can be pseudomorphically grown only up to a critical thickness [1]. This imposes serious limitations in the design and growth of SMS's. The lattice mismatch, on the other hand, causes pseudomorphically grown layers to be strained, which can be positively exploited for engineering the optical properties of SMS's. Because of the artificially introduced periodicity, zone folding in momentum space has been predicted to occur. The concept of achieving a direct bandgap material from an indirect bandgap material by virtue of the superlattice periodicity was first recognized by Gnutzmann and Clausecker [2]. The observation of new optical transitions in a Si 4 Ge 4 SMS by means of electroreflectance spectroscopy [3] triggered a lot of interest and intensified the efforts towards understanding the nature of the bandgap of SMS's. The electronic properties of Si..Gen SMS's have been examined theoretically [4, 5, 6] and were found to be promising candidates for direct bandgap materials if the strain is made to be accommodated by the Si layers, by using a Ge substrate for example. No specific structure has however been proposed as a direct bandgap material. Ciraci and Batra [7], on the other hand, found SiGe,, structures to be indirect, even if the strain is shifted to the Si layers. In fact, all of the structures that have so far been discussed in the literature [8, 9, 10, 11] have been indirect. Lately, PL has become a very important tool in studying the optical properties of Si,,Gen SMS's. After Eberl and coworkers [12] observed luminescence in Si 3Geg superlattices with different thicknesses, a lot of effort has been directed towards obtaining luminescence from SimGe, SMS's and understanding the dependence of the PL signal on the periodicity and the strain conditions of the superlattice [13, 14]. The reported luminescence has been attributed to the artificial periodicity of the superlattice and studied based upon strain conditions and the type of band alignment, but the nature of the transitions has not been identified. Okumura and coworkers obtained a sharp PL peak Mat. Res. Soc. Symp. Proc. Vol. 198. @1990Materials Research Society
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from a Sil 2 Ge 4 SM
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