Si/Si_1-xGe_x multiquantum wells: A route to infrared detectors

Quantum-well detectors use intersubband transitions for the detection of long-wavelength infrared radiation. Such quantum-well structures provide the electronic and optical properties for a new generation of infrared quantum detectors. It is shown that apart from GaAs/GaAIAs multiquantum-well structures, which were studied extensively recently with respect to their potential application for infrared detection. Si/SiGe structures are suitable for this purpose as well. We have performed a systematic experimental study of intersubband transitions in modulation-doped p-type molecular beam epitaxy grown Si/SiGe quantum wells for well widths between 26 Å and 65 Å and Ge contents in the range from 19% to 50%. Intersubband absorption occurs between 480 cm^-1 and 1830 cm^-1 for such structures. The widths of the absorption lines are only 20 meV as long as the excited states are confined within the SiGe wells. When the excited states lie already in the continuum, a broadband photoresponse can be achieved. For this purpose Si/SiGe structures with sufficiently narrow SiGe wells (about 30 Å) doped to a level of p = 4 × 10¹⁸ cm³ were used. Such structures can be directly grown on Si (001) substrates and are highly compatible with modern Si device fabrication technology which offers a tremendous advantage for the realisation of large scale two-dimensional infrared detector arrays.