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We report the results of an extensive study of band-to-band optical transitions in Si-Ge ( n:m) superlattices and alloys where n’m. Our samples were grown by molecular-beam epitaxy on ~001! silicon using symmetrically strained layers and characterized by high-resolution x-ray diffraction and transmission electron microscopy. This growth procedure permits the synthesis of continuous Si-Ge superlattices with a thickness of several thousand A. Optical absorption was studied by photocurrent spectroscopy at 300, 77, and 4.2 K. These results were analyzed to determine the dependence of the photocurrent on the photon energy. The energy dependence of absorption was also measured by optical transmission spectroscopy. Analysis of these experiments gives approximate agreement with photoconductivity experiments on the value of the energy gap, but also shows that the energy dependence of the absorption coefficient varies linearly with the photon energy, while photoconductivity experiments show that the photocurrent increases with the fourth power of the energy. The absorption coefficient, and its dependence on the photon energy, are calculated directly from the joint density of states which is extracted from the electronic band structure. Our calculations show that the dependence of optical absorption on photon energy is linear for perfect superlattices: a(\v)5A0(\v2Eg) x , where x51, with the exponent increasing above 1 in the presence of disorder such as from atomic steps, interface roughness, and similar defects. @S0163-1829~98!06715-0#

A novel tandem SiGe-Si device is demonstrated which, based on different photoresponses in two back-to-back diodes, exhibits a voltage tunable spectral responsivity. This feature, in conjunction with a wavelength dependence in the current direction, allows the efficient decryption of a signal when substantial noise is added to distinct carrier frequencies. Time dependent encryption schemes are also possible via a modulated voltage bias. (C) 1997 American Institute of Physics.

We used photocurrent spectroscopy at 300 K, 77 K and 4.2 K to investigate the optical absorption processes of (001) SiGe strained layer superlattices and alloys with an average composition of 50%Si-50%Ge. We observed an evolution toward higher energies of the threshold in the photocurrent spectra as the period of the superlattices decreases, with the spectrum of the shortest period superlattices (2:2) approaching that of the alloy. The energy dependence of absorption in these SiGe heterostructures is quite distinct from that measured in elemental silicon.

We present a voltage-tunable wavelength-selective photodetector (VWP) for the near-infrared (NIR), realized by engineering the optical properties of Si and Ge. The structure exhibits a wide tunability by varying the applied voltage and weighting the photocurrents generated by two back-to-back diodes. We demonstrate its versatility in deconvolving signals of different wavelengths and a preliminary study on the applications of the VWP as a solid-state spectrum analyzer, The spectrum of NIR light impinging the device can be resolved from a set of photocurrent measurements for different bias voltages. (C) 1998 American Vacuum Society.