Hyperspectral infrared imagers are of great interest in applications requiring remote identification of complex chemical agents. The combination of mercury cadmium telluride detectors and Fabry–Perot filters (FPFs) is highly desirable for hyperspectral detection over a broad wavelength range. The geometries of distributed Bragg reflector (DBR)-based tunable FPFs are modeled to achieve a desired spectral resolution and wavelength range. Additionally, acceptable fabrication tolerances are determined by modeling the spectral performance of the FPFs as a function of DBR surface roughness and membrane curvature. These fabrication nonidealities are then mitigated by developing an optimized DBR process flow yielding high-performance FPF cavities suitable for integration with hyperspectral imagers.
The development of a broadband IR focal plane array poses several challenges in the area of detector design, material, device physics, fabrication process, hybridization, integration and testing. The purpose of our research is to address these challenges and demonstrate a high-performance IR system that incorporates a HgCdTe-based detector array with high uniformity and operability. Our detector architecture, grown using molecular beam epitaxy (MBE), is vertically integrated, leading to a stacked detector structure with the capability to simultaneously detect in two spectral bands. MBE is the method of choice for multiplelayer HgCdTe growth because it produces material of excellent quality and allows composition and doping control at the atomic level. Such quality and control is necessary for the fabrication of multicolor detectors since they require advanced bandgap engineering techniques. The proposed technology, based on the bandgap-tunable HgCdTe alloy, has the potential to extend the broadband detector operation towards room temperature. We present here our modeling, MBE growth and device characterization results, demonstrating Auger suppression in the LWIR band and diffusion limited behavior in the MWIR band.
The combination of HgCdTe detectors and Fabry-Pérot filters (FPFs) is highly desirable for hyperspectral
detection in the infrared band over a broad wavelength range. The results of comprehensive modeling of distributed-
Bragg-reflector-based tunable FPFs that can be used with HgCdTe array detectors for hyperspectral imaging modules are
presented, focusing on the impact of FPF non-idealities on optical performance. The effects of surface and interface
roughness on the spectral resolution and transmissivity of the cavity was explored to determine if certain thin film
deposition techniques are suitable to economically fabricate FPFs. The impact of varying field-of-view (FOV) and
incident angles are also discussed. Finally, the impact of FPF bowing on spectral resolution is discussed.
We present in this study a theoretical and experimental investigation of the MWIR HgCdTe nBn device concept.
Theoretical work has demonstrated that the HgCdTe nBn device is potentially capable of achieving performance
equivalent to the ideal double layer planar heterostructure (DLPH) detector. Comparable responsivity, low current
denisty Jdark, and high detectivity *D values rival those of the DLPH device without requiring p-type doping. The
theoretical results suggests that the HgCdTe nBn structure may be a promising solution for achieving a simplified MWIR
device structure and addressing problems associated with reducing thermal generation in conventional p-on-n structures
and processing technology limitations such as achieving low, controllable in-situ p-type doping with MBE growth
techniques. Furthermore, the physical mechanisms for selective carrier conduction in the nBn structure may provide a
basis to incorporate into future device structures to suppress intrinsic Auger carrier generation. Likewise, the
experimental demonstration of the MWIR HgCdTe nBn devices introduces a promising potential alternative to
conventional high performance p-n junction HgCdTe photodiodes. The experiments described in this study illustrate the
successful implementation of a HgCdTe barrier-integrated structure. The measured current-voltage characteristics of
planar-mesa and mesa HgCdTe nBn devices exhibit barrier-influenced behavior and follow temperature-dependent
trends as predicted by numerical simulations. Optical measurements of the planar-mesa MWIR HgCdTe nBn device
indicate a bias-dependent spectral response. Further changes to MWIR HgCdTe nBn layer structure has shown an over
105 A/cm2 reduction in Jdark as well as a shift to a lower turn-on operation bias. This experimental investigation highlights
the potential for pursuing similar and related unipolar, type-I barrier devices for high performance infrared detector
The Mid-wave infrared (MWIR) spectrum has applications to many fields, from night vision to chemical and biological
sensors. Existing broadband detector technology based on HgCdTe allows for high sensitivity and wide range, but lacks
the spectral decomposition necessary for many applications. Combining this detector technology with a tunable optical
filter has been sought after, but few commercial realizations have been developed. MEMS-based optical filters have
been identified as promising for their small size, light-weight, scalability and robustness of operation. In particular,
Fabry-Perot interferometers with dielectric Bragg stacks used as reflective surfaces have been investigated. The
integration of a detector and a filter in a device that would be compact, light-weight, inexpensive to produce and scaled
for the entire range of applications could provide spectrally resolved detection in the MWIR for multiple instruments.
We present a fabrication method for the optical components of such a filter. The emphasis was placed on wafer-scale
fabrication with IC-compatible methods. Single, double and triple Bragg stacks composed of germanium and silicon
oxide quarter-wavelength layers were designed for MWIR devices centered around 4 microns and have been fabricated
on Silicon-On-Insulator (SOI) wafers, with and without anti-reflective half-wavelength silicon nitride layers. Optical
testing in the MWIR and comparison of these measurements to theory and simulations are presented. The effect of film
stress induced by deposition of these dielectric layers on the mechanical performance of the device is investigated. An
optimal SOI substrate for the mechanical performance is determined. The fabrication flow for the optical MEMS
component is also determined. Part of this work investigates device geometry and fabrication methods for scalable
integration with HgCdTe detector and IC circuitry.