Carrier Transport in Auger-Suppressed Infrared Detector Materials

Abstract

State of the art infrared detectors can operate at high efficiency and low noise throughout the infrared wavelength range. However, MWIR and LWIR detectors are still limited by very low operating temperatures in order to achieve low noise, and SWIR detectors are either prohibitively expensive or limited to small wavelength ranges.

Mercury cadmium telluride (HgCdTe, or MCT) with low n-type indium doping concentration offers a means for high performance infrared detection in the mid-wave and long-wavelength range. Characterizing carrier transport in materials with ultra low doping (ND = 1014 cm−3 and lower), as well as multi-layer material structures designed for infrared detector devices, is particularly challenging using traditional Van der Pauw Hall methods. Hall effect measurements with swept magnetic field were used in conjunction with a multi-carrier fitting procedure and Fourier-domain mobility spectrum analysis (FMSA) to analyze multi-layered MCT samples. Using low temperature measurements (77 K), we were able to identify multiple carrier species, including an epitaxial layer (x = 0.2195) with n-type carrier concentration of n = 1 × 1014 cm−3 and electron mobility of μ = 280,000 cm2/Vs. The extracted electron mobility matches or exceeds prior empirical models for MCT, illustrating the outstanding material quality achievable using current epitaxial growth methods, and motivating further study to revisit previously published material parameters for MCT carrier transport. The high material quality is further demonstrated via observation of the quantum Hall effect at low temperature (5 K and below).

For short-wave absorption, type II superlattices (both lattice matched and strained) based on In0.53Ga0.47As/GaAs0.51Sb0.49 grown on InP substrates were simulated and investigated for short wavelength infrared detection. Eight band k·p simulations were utilized to extract information on the electronic band structure, which were in turn used to calculate the optical absorption spectrum of the superlattice. The effective band gap is calculated, and cutoff wavelengths greater than 2 μm were observed. Quantum efficiency was calculated for a standard InGaAs/T2SL/InGaAs p-i-n device structure, where quantum efficiency exceeding 50% at 2 μm may be achieved. Dark current was calculated considering Auger, radiative, and Shockley-Read-Hall generation-recombination, where Shockley-Read-Hall recombination-generation was found to be the limiting mechanism for a trap density greater than 5×1014 cm−3, and radiatively limited performance is predicted for a lower trap density. The estimated dark current density is expected to be comparable to existing HgCdTe technology, while outperforming extended-range InGaAs by more than an order of magnitude.

The work outlined in this thesis provided some of the first evidence that low doping levels and high mobilities in HgCdTe devices could be measured electrically in a multi-layer structure, which helped to pave the way for Auger-suppressed detectors. The high mobility and early evidence of quantum Hall effects (at temperatures as high as 4K) indicate HgCdTe is a great candidate for future QHE experiments. Also, the work on both lattice matched and strained superlattices provide a roadmap and methodology for future SWIR superlattice detector design.