Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Department of Engineering Physics

First Advisor

Robert L. Hengehold, PhD


Optical emission and absorption properties of Si1-x Gex/Si superlattices grown on (100), (110), and (111) Si substrates were investigated to determine the optimal growth conditions for these structures to be used as infrared detectors. Fully-strained Si1-x Gex/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°C for all three silicon orientations. Room temperature absorption measurements were conducted on (100) and (110) Si1-x Gex/Si superlattices using Fourier transform spectroscopy while varying incident electric field polarization. Strong intersubband absorption was observed at 7.8 µm from a sample composed of 15 quantum wells of 40 Å Si0.8Ge0.2 separated by 300 Å of Si grown on (100) Si by MBE at 550°C. Valence band wells were doped 5 x 1019 cm-3 with boron. This transition. identified as HH1↔HH2, exhibited strong polarization dependence according to (100) Si selection rules. No subband transitions were observed on similar (110) Si1-x Gex/Si superlattices ranging in boron dopant concentration from 1-8 x 1019 cm-3. Selection rules for (110) Si indicate an HH1↔LH2 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°C. A clear correlation between PL emission and photo-absorption was unattainable since the growth conditions necessary to ameliorate intersubband absorption quenches interband emission.

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