Characterization of, and novel architectures for, strained-layer superlattice infrared photodetectors
The current generation of infrared photodetectors, used in imaging and sensing applications, are predominantly made of an expensive II-VI material, HgCdTe (mercury cadmium telluride, MCT). A viable alternative class of materials, the III-V type-II superlattices (T2SLs), have been actively studied due to several potential and predicted advantages over MCT. T2SLs offer an easier path to the design of arbitrarily small effective bandgaps by controlling the layer thickness, more uniform material growth across the wafer, significantly greater flexibility in detector architecture, reduced Auger recombination due to the light-hole and heavy-hole splitting, and theoretically higher operating temperatures at longer wavelengths. However, despite the proposed superior performance offered by T2SL-based detectors, this improved performance has not yet been realized, a fact largely believed to be a result of defects and growth imperfections in the T2SL materials. To improve the material quality, it is important to characterize and understand the carrier dynamics of T2SLs. The primary thrust of this thesis is the description and development of experimental techniques for characterizing T2SL detectors. The first part of this dissertation focuses on establishing a new approach to electron beam induced current (EBIC) measurements, used to study the diffusion characteristics of InGaAs/InAsSb superlattices. By measuring the current generated by the electron beam of a scanning electron microscope (SEM), EBIC allows us to extract the minority carrier diffusion length (L) and the surface recombination velocity to diffusivity ratio (S/D) of a material. When combined with information on minority carrier lifetime (τ), for instance from time-resolved photoluminescence measurements, the minority carrier mobility (μ) of the material can be extracted. By performing TRPL and EBIC, InGaAs/InAsSb photodetectors with varying InGaAs Ga-fraction have been studied to show that the minority carrier mobility increases as the gallium composition increases. In the second part of this dissertation, we investigate InGaAs/InAsSb detectors with varying unit cell thicknesses in order to characterize the minority carrier diffusion length as a function of layer thickness. Carrier transport studies (including EBIC) of infrared (IR) detector materials typically focus on the vertical transport of minority carriers, as the photoexcited charge in these devices is almost always collected across a junction in the vertical (growth) direction. However, with the growing technological importance of IR focal plane arrays (FPAs), understanding the lateral diffusion of photo-excited carriers becomes increasingly important, particularly in quantum-engineered IR absorber materials such as T2SLs, whose layered design would be expected to give anisotropic charge transport properties. In the third portion of the dissertation we develop and discuss a technique for extracting the anisotropic carrier diffusion in T2SL materials. In order to extract both vertical and lateral minority carrier diffusion lengths of InGaAs/InAsSb superlattices, a two-dimensional electron beam induced current (2D-EBIC) technique is developed. The results from our 2D-EBIC studies show the lateral hole mobility to be between 3 to 5 times greater than the vertical hole mobility in 6 ML In [subscript 0.88] Ga [subscript 0.12] As / 6 ML InAs [subscript 0.65] Sb [subscript 0.35] superlattice material. The final part of this dissertation proposes an architecture for enhanced absorption in ultra-thin strained-layer superlattice detectors utilizing a hybrid optical cavity design. The proposed detector architecture utilizes a highly doped semiconductor ground plane beneath the ultra-subwavelength thickness long-wavelength infrared absorber material, upon which we pattern metallic antenna structures. Using realistic material parameters, the detector absorption achieves near 50 % in absorber layers with thicknesses of approximately λo/50. The detector absorption is investigated as a function of wavelength and incidence angle, as well as detector geometry. The proposed device architecture offers the potential for high efficiency detectors with minimal growth costs and relaxed design parameters.