Type-II superlattice materials for mid-infrared detection

Type-II superlattices composed of alternating thin layers of InAs and GaSb, have been shown to be a highly flexible infrared materials system in which the energy band gap can be adjusted anywhere between 360 meV and 40 meV. These superlattices (SLs) are the III-V equivalent to the well established HgxCd1-xTe alloys used for infrared detection in the short, mid and long wavelength bands of the infrared spectrum. There are many possible designs for these superlattices that will produce the same narrow band gap by adjusting individual layer thicknesses and interface composition. Systematic growth and characterization studies were performed to determine optimum superlattice designs suitable for infrared detection in the 3 to 5 mu m wavelength band. For these studies the individual layer thicknesses were less than 35 angstrom. The effects of adding different thickness InSb-like interfaces were also studied. Through precision molecular beam epitaxy, design changes as small as 3 angstrom to the SL layers could be studied. Significant changes were observed in the infrared photoresponse spectra of the various SL samples. The infrared properties of the various designs of these type-II superlattices were modeled using an 8-band Envelope Function Approximation. The infrared photoresponse spectra, combined with quantum mechanical modeling of predicted absorption spectra, were a key factor in the design optimization of the InAs/GaSb superlattices with band gaps in the range of 200 to 360 meV.