T2SL (Type II Super Lattice) or sometimes also called SLS (Strained Layer Super-lattice) is a material / technology that can be used to make high quality cooled infrared photon detectors with a cut-off wavelength ranging from 2 µm to 30 µm. This covers the SWIR, MWIR, LWIR and VLWIR wavelength bands, loosely defined as 2-3 µm, 3-5 µm, 8 -12 µm and > 12 µm, respectively.
A superlattice is a system made of a repeating sequence of thin layers of different materials. If the layer thicknesses are small enough in a quantum mechanical sense, minibands are formed in the material. The result is an artificial material with properties that can be engineered; in the detector case the bandgap energy corresponding to the desired cut-off wavelength. When two semiconductors are brought in contact, there are several ways the valance and conduction bands can align. If both the valence and the conduction band edge of the second material are above the band edges of the first material, it is called a broken type II band alignment. The III/V compound materials InAs and GaSb form such a band alignment (see figure).
Band alignment of InAs GaSb and the forming of minibands.
As can be seen in the next figure, InAs has a lattice mismatch of less than 1 % on GaSb. Starting with GaSb substrates alternating layers of InAs and GaSb with atomic layer precision can be deposited using MBE (Molecular Beam Epitaxy). By interface engineering (create an interface layer of InSb) or using more complicated superlattices like GaxIn1-xSb/InAs thick strain compensated structures with high crystal quality can be grown. If doping in the form of trace amounts of Be, Te or Si is incorporated, photovoltaic p-i-n structures that can be used to detect IR radiation of the desired wavelength are formed.
Lattice constants and band gap energy of several III/V materials
The strain of the individual layers is a key feature of T2SL as it suppresses Auger generation of carriers, one of the main dark current components in other detector types, e.g. MCT (Mercury Cadmium Telluride) detectors.
Surface currents have historically been the limiting factor, but recently this weakness has been overcome by dielectric passivation. Current T2SL devices are therefore mostly limited by Shockley-Read-Hall generation via mid-gap defects in the depletion area. With clever band engineering even this component of the dark current can be minimized and optimal diffusion limited operation is within reach. The use of band engineering will also create opportunities for affordable multi colour / multi band detectors.
MWIR T2SL FPA’s have been reported with NETD < 20 mK and operating temperature up to 120 K, i.e achieving the status of being a “HOT” (High Operating Temperature) detector technology. In the LWIR range 1k×1k focal plane arrays have been demonstrated with NETD < 24 mK at 80 K operating temperature. Not only good numbers but also very importantly good imagery has been demonstrated.
Image taken at 110 K by a 320 x 256 MWIR T2SL detector (IRnova)
When looking specifically at the MWIR window, T2SL is expected to have decisive advantages over competing technologies like MCT or InSb; T2SL is much more uniform than the notoriously difficult material system MCT, where the ratio of mercury to cadmium determines the cut-off wavelength. In superlattices the cut-off wavelength is determined by the thickness of the constituent layers. In modern MBE equipment, the layer thickness variation is very well controlled and can be less than 1 % over a whole 3″ wafer. Theoretically, T2SL detectors are expected to demonstrate lower dark current than corresponding MCT detectors. Although getting near, this has not been experimentally shown yet. Compared to InSb, which needs to be cooled to 80 K, T2SL can operate at a higher operating temperature of 120 K, leading to crucial system advantages of size weight and power (SWaP).