Optical Properties of Ordered Ge-Si Atomic-Layer Superlattices
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OPTICAL PROPERTIES OF ORDERED Ge-Si ATOMIC-LAYER SUPERLATTICES T. J. *
M. BONAR* AND C. BEAN*, J. BEVK*, J. P. PEARSALL*, J. P. MANNAERTS* AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, 07974
NJ
ABSTRACT
A systematic study of the band-to-band optical transitions in commensurate strainedlayer superlattices of Ge and Si grown on (001) Si substrates show the presence of new electronic energy bands. Our measurements suggest that superlattice structure on the same- scale as the unit cell results in a band structure significantly different from that of a random alloy. L INTRODUCTION Strained-layer epitaxy is the intentional growth of lattice-mismatched heterostructures in which the internal strain from lattice-mismatch is not relaxed through dislocation formation. The technique of strained layer epitaxy is a central feature of the growth of Ge-Si alloys 12 on Si or Ge substrates. ' High-quality. defect free growth is obtained if the thickness of the 34 strained hetero-epitaxial layers is kept below a critical value. ' For the present case of Ge grown directly on Si, the critical thickness is reached when 6 atomic monolayers of Ge are grown on (001) Si. For superlattices of such fine dimensions, on the order of the unit cell, which is 4 monolayers thick, three important perturbations can affect the energy band structure: (1) latticemismatch strain, (2) quantum-size effects and (3) the creation of a new unit cell by the superperiodicity imposed by the ordered strained layer epitaxy. In earlier work, we have studied5 the effects of quantum confinement and strain on the energy levels of Ge-Si heterostructures. Our work showed that these perturbations can shift some levels by as much as 0.5 eV. As significant as these changes are, we also showed that the new spectrum of energy levels can be derived accurately from bulk unstrained levels using well-established calculations for the effects of strain and quantum confinement. In this work, we enter a different world. Our experimental results show that the principal effect of atomic-layer ordered superlattices is the creation of new band-to-band transitions whose characteristic energies are not the result of a straight forward evolution of the levels found in alloys. II. SAMPLE PREPARATION All of the samples studied in this report were grown by molecular beam epitaxy on (001) Si substrates. Because of critical thickness considerations, the maximum quantum-well thickness was fixed at 50A. Five sample structures were grown for these experiments: (a) a single quantum well of alternating monolayers of Ge and Si, (b) a single quantum-well of alternating two monolayers of Ge and Si, (c) a single quantum-well of alternating four monolayers of Ge and Si, (d) a single quantum-well of alternating six monolayers of Ge and Si and (e) a multiple quantum-well consisting of twenty repetitions of the 4x4 quantum well spaced apart by 200A of Si. of study and The successful growth of these samples is one result of several years0worth 7 refinement of strained-layer growth techniques a
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