High performance infrared sensors are vulnerable to slight changes in defect densities and locations. For example in a space application where such sensors are exposed to proton irradiation capable of generating point defects the sensors are known to suffer performance degradation. The degradation can generally be observed in terms of dark current density and responsivity degradations. Here we report results of MWIR HgCdTe/CdZnTe single element diodes dark current densities before and after exposure to 63MeV protons at room temperature to a total ionizing dose of 100 kRad(Si). We find the irradiated diodes as a group show some signs of proton-induced damage in dark current.
Exposure to proton radiation degrades the performance of wavelength infrared (MWIR) and long wavelength infrared (LWIR) HgCdTe photodetectors to varying degrees depending on the dose and energy of the incident particles. We report an experimental characterization of test devices of multiple sizes and configurations designed to investigate the effect proton radiation has on detector performance. Photodetector devices, from test element devices to fully functional focal plane arrays, are processed into MWIR and LWIR HgCdTe material grown by molecular beam epitaxy (MBE), in both single and two-color architectures, on CdZnTe and CdTe-buffered Si substrates. The devices receive doses of 30 krad(Si) and 100 krad(Si) from an incident beam of 63 MeV protons. The lower dose induces negligible degradation. At the higher dose, MWIR detectors begin to show reduced activation energy for higher temperatures, while LWIR detectors are more strongly affected with the activation energy being halved following proton irradiation.
We report the development of high performance low cost SWIR infrared detectors from MBEgrown HgCdTe on 3-inch CdTe-buffered silicon substrates. The experimental findings demonstrate that despite the large lattice mismatch between HgCdTe and Si substrate, the materials and detector performances are sufficiently better than those reported for III-V mixed crystals. High minority carrier lifetime of the order 3 μs at room temperature was measured on the as grown material. Photodetectors fabricated from this material produced low dark current densities on the order of 10<sup>-6</sup> A/cm<sup>2</sup> and 10<sup>-3</sup> A/cm<sup>2</sup> at 200K and 300K. Quantum efficiency exceeding 70% at 2.0 μm, without antireflective coating, was measured on single element detectors. Further, 320 X 256, 30 μm pitch FPA’s have been fabricated with this HgCdTe on Si material and dark current operability of ~ 99.5% (mean dark current of 30 pA/Pixel) at 200K has been demonstrated.
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.
Spatial noise and the loss of photogenerated current due material non-uniformities limit the performance of long
wavelength infrared (LWIR) HgCdTe detector arrays. Reducing the electrical activity of defects is equivalent to
lowering their density, thereby allowing detection and discrimination over longer ranges. Infrared focal plane arrays
(IRFPAs) in other spectral bands will also benefit from detectivity and uniformity improvements. Larger signal-to-noise
ratios permit either improved accuracy of detection/discrimination when an IRFPA is employed under current operating
conditions, or provide similar performance with the IRFPA operating under less stringent conditions such as higher
system temperature, increased system jitter or damaged read out integrated circuit (ROIC) wells. The bulk passivation of
semiconductors with hydrogen continues to be investigated for its potential to become a tool for the fabrication of high
performance devices. Inductively coupled plasmas have been shown to improve the quality and uniformity of
semiconductor materials and devices. The retention of the benefits following various aging conditions is discussed here.
Inductively coupled plasma (ICP) chemistry based on a mixture of CH<sub>4</sub>, Ar, and H<sub>2</sub> was investigated for the purpose of delineating HgCdTe mesa structures and vias typically used in the fabrication of second and third generation infrared
photo detector arrays. We report on ICP etching uniformity results and correlate them with plasma controlling
parameters (gas flow rates, total chamber pressure, ICP power and RF power). The etching rate and surface morphology
of In-doped MWIR and LWIR HgCdTe showed distinct dependences on the plasma chemistry, total pressure and RF
power. Contact stylus profilometry and cross-section scanning electron microscopy (SEM) were used to characterize the
anisotropy of the etched profiles obtained after various processes and a standard deviation of 0.06 &mgr;m was obtained for
etch depth on 128 x 128 format array vias. The surface morphology and the uniformity of the etched surfaces were
studied by plan view SEM. Atomic force microscopy was used to make precise assessments of surface roughness.