Recent developments in HgCdTe photovoltaic devices grown on alternative substrates using heteroepitaxy

One of the principal difficulties in liquid phase epitaxial (LPE) HgCdTe MCT photodiode production is the availability of high quality substrates for epitaxial growth. Rockwell has developed an alternative substrate material which overcomes the disadvantages of bulk CdTe without sacrificing the epitaxial quality of HgCdTe grown on it. This substrate consists of a single crystal sapphire wafer (typically 2" in diameter) prepared appropriately and covered with a thin (10 - 20 µm) LPE grown layer of cadmium telluride. The substrate has been given the acronym PACE for Producible Alternative to CdTe for Epitaxy. Devices made in HgCdTe LPE wafers on PACE have shown excellent electrical and optical properties and superior uniformity when compared to devices fabricated in HgCdTe grown on CdTe substrates. Typical values for midwave infrared (MWIR, 3- 5 µm wavelength) diodes are > 10 Ω-cm²at 195K (cutoff wavelength, µm), > Ω-cm²at 120K ( µm) and > 10⁶Ω-cm²at 77K (λc= 4.6 µm). Typical quantum efficiencies are 60 - 80% without AR coating. Analysis of the detectivity of a 1024 element MWIR hybrid focal plane array shows that the number of defective elements, even under low-to-moderate photon backgrounds (high 10¹²photons cm²s^-1), is less than 5%. This new technology appears very promising for lowering cost and increasing yield of infrared photodetector arrays. It may also lead to new device applications in areas traditionally reserved for more conventional materials such as GaAs and Si.