Two of the key challenges in the realisation of focal plane arrays based on type-II InAs/GaSb superlattices (T2SL) are the difficulty in achieving a good sidewall profile and the increased dominance of surface leakage current as the device dimensions shrink. We report the electrical and morphological results of test pixels for mid-wave infrared T2SL photodiodes etched using a Cl 2 /Ar based inductively coupled plasma reactive ion etching (ICP-RIE) process and passivated using SU-8 epoxy photoresist. The etch rate and sidewall surface morphology of GaSb, InAs, and InAs/GaSb T2SL materials are compared after dry etching under the same conditions, leading to the determination of an optimal etch rate. The effect of surface treatment using selected wet chemical etchants before passivation on the surface leakage current is presented. Limitations of the dry etching recipe and further improvement of the sidewall verticality and smoothness are also discussed. Good sidewall profiles and bulk-limited dark currents are demonstrated for T2SL photodiodes etched to depths between 1.5 and 3.5 μm with a pitch size down to 12 μm.
Next generation infrared photodetector technology will require focal plane array (FPA) systems that have multi-spectral imaging capabilities. One proposed approach to realizing these multicolor devices is to use plasmonic resonators. However, device development and characterization are commonly addressed with large front side illuminated single pixel detectors on a supporting epitaxial substrate. The focal plane arrays on the other hand are backside illuminated. Moreover, in a front side illuminated device, there is significant substrate scattering of the incident light. Here, we propose a method for the accurate measurement of device performance by using a hybridized chip design (hybrid chip) that is similar to the fabrication of an FPA system, with the substrate completely removed through a combination of mechanical polishing and subsequent wet etching techniques. The hybrid chip was also designed to precisely characterize the effects of varying mesa size by incorporating square mesa structures that range from 25 to 200 μm in width. This approach offers an advantage over conventional device characterization because it incorporates mesas that are on the same scale as those normally used in FPA systems, which should therefore provide a fast transition of new photodetector technology into camera based systems. The photodetector technology chosen for this work is a multi-stack quantum dots-in-a-well (DWELL) structure designed to absorb electromagnetic radiation in the mid-infrared spectral range.
We report on the testing of a set of InAs/GaSb multicolor strained-layer superlattice photodetectors and Dotin-Well detectors grown with InAs dots in InGaAs/GaAs wells fabricated by the Center for High Technology Materials at the University of New Mexico . These devices are 2-color devices sensitive to near-IR and mid-IR wavelengths. The wavelength sensitivities of these devices are a function of the applied forward and reverse bias. We present measurements of the dark current and relative spectral response of these photodetectors measured at both cryogenic and room temperatures.
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