HgCdTe on sapphire — A new approach to infrared detector arrays

Some of the limitations imposed by bulk CdTe substrates on epitaxial HgCdTe, such as wafer size, fragility, and uniformity, have led to the development of an alternate substrate to CdTe for epitaxial HgCdTe growths. Described here are the synthesis and some of the properties of an alternate hybrid CdTe/sapphire substrate, and the material and device properties of liquid phase epitaxial (LPE) grown HgCdTe on CdTe/sapphire substrates. Devices made in LPE grown HgCdTe layers on CdTe/sapphire have shown excellent electrical and optical properties and superior uniformity in diode-to-diode D* in midwave infrared (MWIR) focal planes at low temperature when compared to devices fabricated in HgCdTe grown on CdTe substrates. Diodes have typical resistance area product values of ⩾ 10 Ω cm2 at 195 K (cutoff wavelength λc = 4.2 μm), ⩾ 3 x 104 Ω cm2 at 120 K (λc = 4.45 μm) and ⩾ 1 x 106 Ω cm2 at 77 K (λc = 4.6 μm). Typical quantum efficiencies are 60–80% without anti-reflection 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 1012 photons cm-2 s-1), is less than 5%.

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