Optical Emission and Absorption Properties of Silicon-Germanium SuperlatticeStructures Grown on Non-Conventional Silicon Substrate Orientation

摘要

Optical emission and absorption properties of Si (1-x) Ge(x)/Si superlatticesgrown on (l00), (110), and(111) Si substrates were investigated to determine the optimal growth conditions for these structures to be used as infrared detectors. Fully-strained Si (1-x) Ge(x)/Si superlattices were grown by molecular beam epitaxy (MBE) and examined using low-temperature photoluminescence (PL) to identify no-phonon and phonon-replica interband transitions across the alloy bandgap. Phonon-resolved emission was most intense for undoped quantum wells grown at 710 deg C for all three silicon orientations. Room temperature absorption measurements were conducted on (100) and (110) Si (1-x) Ge(x)/Si superlattices using Fourier transform spectroscopy while varying incident electric field polarization. Strong intersubband absorption was observed at 7.8 micrometer from a sample composed of 15 quantum wells of 40 A Si(0.8)Ge(0.2) separated by 300 A of Si grown on (100) Si by MBE at 550 deg C. Valence band wells were doped 5 x 10 (exp 19)/ cucm with boron. This transition. identified as HH1(Lefe and right arrow)HH2, exhibited strong polarization dependence according to (100) Si selection rules. No subband transitions were observed on similar (110) Si (1-x) Ge(x)/Si superlattices ranging in boron dopant concentration from 1-8 x lO (exp 19)/ cucm. Selection rules for (110) Si indicate an HHI-LH-2 transition is allowed within the transmission bandpass of the experimental apparatus at both parallel and normal incident electric field polarizations; however, this peak was most likely masked by free-carrier absorption which dominated the spectrum. Intersubband absorption transitions were observed only for doped superlattices grown at 550 deg C.