From Event: SPIE Optical Engineering + Applications, 2018
Higher operating temperatures can be provided by a system based on HgCdTe infrared detectors utilizing optimized material parameters (i.e. doping and defects) and non-equilibrium device architectures that suppress detector diffusion current. A PIN (P+/Intrinsic/N+) hetero-junction photodiode can eliminate Auger generated diffusion current, resulting in a diode dominated by Shockley-Read-Hall (SRH) depletion current. The limiting dark current in the PIN diode will be determined by the SRH lifetime of the HgCdTe material. We are optimizing processes for material growth and post-annealing, to improve in SRH lifetimes and hence depletion dark current and performance at higher operating temperatures. We report on the growth of high-quality n-type long wavelength infrared (LWIR) HgCdTe (cutoff wavelength 10 μm at 77 K) layers grown on CdTe/Si and CdZnTe substrates by molecular beam epitaxy (MBE). A low indium concentration in the absorber layer of ~1x10-14 is confirmed by secondary ion mass spectrometry (SIMS). In order to reduce potential SRH centers and enable low doping levels, we are applying gettering processes to reduce impurity levels below what is achievable by best practices in MBE growth. Concentration profiles of impurities such as Na and K are seen to getter to top surface and interface with the substrate, and are seen to be dramatically reduced in the absorber layer as annealing temperature is increased. We are also studying the effect of anneals on the sharpness of heterojunctions created by MBE growth. The potential benefits of heterojunction devices in suppressing dark current, particularly structures with lower thickness, can only be realized if interdiffusion can be controlled. We have successfully fit SIMS Cd profiles to a model that incorporates the temperature and composition dependence of the interdiffusion coefficient. These results, along with dependence of As diffusion coefficients vs. temperature and Hg pressure lead us to propose and test alternative annealing profiles that better preserve the heterojunctions while maintaining an appropriate amount of As diffusion for junction formation.
Silviu Velicu, "Material and process development for high operating temperature LWIR PIN HgCdTe focal plane arrays (Conference Presentation)," Proc. SPIE 10766, Infrared Sensors, Devices, and Applications VIII, 107660G (Presented at SPIE Optical Engineering + Applications: August 23, 2018; Published: 18 September 2018); https://doi.org/10.1117/12.2323254.5836576101001.
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