Anne M. Itsuno

Mid-wave infrared HgCdTe nBn photodetector

A unipolar, barrier-integrated HgCdTe nBn photodetector with all n-type doping and a type-I band lineup is experimentally demonstrated. Planar mid-wave infrared (MWIR) nBn devices exhibit current-voltage (I-V) characteristics that are consistent with band inversion in reverse bias, indicating a barrier-influenced behavior. Dark current saturation is observed beyond a reverse bias of approximately −0.8 V. Bias-dependent photoresponse is observed in the mid-wave infrared with a cut-off wavelength around 5.7 μm. Numerical modeling based on experimental results predicts an internal peak quantum efficiency of approximately 66%.

Predicted Performance Improvement of Auger-Suppressed HgCdTe Photodiodes and

Infrared detectors require cryogenic operation to suppress dark current, which is typically limited by Auger processes in narrow-band-gap semiconductor materials. Device structures designed to reduce carrier density under nonequilibrium reverse-bias operation provide a means to suppress Auger generation and to reduce dark current and subsequent cryogenic cooling requirements. This study closely examines mercury cadmium telluride (HgCdTe) p+/ν/n+ device structures exhibiting Auger suppression, comparing the simulated device behavior and performance metrics to those obtained for conventional HgCdTe p+/ν detector structures. Calculated detectivity values of high-operating-temperature and double-layer planar heterojunction devices demonstrate consistently higher background limited performance (BLIP) temperatures over a range of cutoff wavelengths. BLIP temperature improvements of ΔTBLIP ~ 48 K and 43 K were extracted from simulations for midwavelength infrared and long wavelength infrared devices, respectively. These studies predict that Auger-suppressed detectors provide a significant advantage over conventional detectors with an increased operating temperature of approximately 40 K for equivalent performance for devices with cutoff wavelength in the range of 5-12 μm .

p\hbox{-}n

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 applications.

Heterojunction Detectors

A long-wavelength IR broadband reflector is demonstrated using a high-index-contrast subwavelength grating based on a Si/SiO2 system. The field response has been computationally and experimentally verified to exhibit broadband reflectance in the spectral range of 13-16 μm with Δλ/λ=18.5% for reflectance >70%. The gratings exhibit incident field polarization dependence with an average extinction ratio of 1:1.6.

Theoretical and experimental investigation of MWIR HgCdTe nBn detectors

A nearly universal goal for infrared photon detection systems is to increase their operating temperature without sacrificing performance. For high quality HgCdTe photovoltaic infrared detectors at elevated temperatures, the lowdoped absorber layer becomes intrinsic, carrier concentrations are high and Auger processes typically dominate the dark current. Device designs have been proposed to suppress Auger processes in the absorber by placing it between exclusion and extraction junctions under reverse bias. In this work, we analyze the non-equilibrium operation of very long wavelength infrared (VLWIR) HgCdTe devices and identify the performance improvements (operation temperature, responsivity, detectivity) expected when Auger suppression occurs. We identify critical structure design requirements that must be satisfied for optimal performance characteristics from VLWIR non-equilibrium devices and compare these devices with current state of the art double layer planar heterostructure (DLPH) devices.

Broadband long-wavelength infrared Si/SiO2 subwavelength grating reflector

High sensitivity HgCdTe infrared detector arrays operating at 77 K can be tailored for response across the infrared spectrum (1 to 14 μm and beyond), and are commonly utilized for high performance infrared imaging applications. However, the cooling system required to achieve the desired sensitivity makes them costly, heavy and limits their applicability. Reducing cooling requirements and eventually operating at temperatures that could be reached with thermoelectric coolers can lead to lighter and more compact systems. However, at these elevated temperatures, the absorber layer becomes intrinsic, carrier concentrations are high and Auger processes typically dominate the dark current and noise characteristics. Auger processes can be suppressed by placing the absorber layer between an exclusion junction and an extraction junction at reverse bias. This reduces the minority carrier concentration in the absorber by several orders of magnitude below thermal equilibrium. The majority carrier concentration also drops significantly below thermal equilibrium to maintain charge neutrality, eventually reaching the extrinsic doping level. This device architecture produces a lower dark current density and lower noise at non-cryogenic temperatures than standard p-n junction photodiodes. Due to the precise control of the layers thicknesses and compositions that could be achieved with molecular beam epitaxy (MBE), this technique is the method of choice for implementing these novel non-equilibrium devices. In this work, we analyze Auger suppression in HgCdTe alloy-based device structures and determine the operation temperature improvements expected when Auger suppression occurs. We identified critical material (absorber dopant concentration and minority carrier lifetime) requirements that must be satisfied for optimal performance characteristics. Experimental dark current-voltage characteristics between 120 and 300 K are fitted using numerical simulations. Based on this, the negative differential resistance (NDR) observed in experimental data is attributed to the full suppression of Auger-1 processes and the partial suppression of Auger-7 processes. We will also present an analysis and comparison of our theoretical and experimental device results in structures where Auger suppression was realized.

VLWIR high operating temperature non-equilibrium photovoltaic HgCdTe devices

HgCdTe-based infrared (IR) detectors remain the front-runner for high performance IR focal plane array (FPA) applications due to their favorable material and optical properties. While state-of-the-art HgCdTe p-n junction technology such as the double layer planar heterostructure (DLPH) devices can achieve near theoretical performance in the mid-wave and long-wave infrared (MWIR, LWIR) spectral ranges, the cryogenic cooling requirements to suppress dark current are still much greater than desired. HgCdTe material growth by molecular beam epitaxy (MBE) provides the accurate control over alloy composition and doping required to achieve future detector architectures that may serve to reduce dark current for enhanced operation. However, controllable in situ p-type doping of HgCdTe by MBE is still problematic. As a potential solution to address these issues, we propose a unipolar, type-I barrier-integrated HgCdTe nBn IR detector based on similar principles to the type-II nBn structure used in III-V materials [1] with the intent that it may serve as a basis for advanced HgCdTe-based architectures for reduced cooling requirements.

Non-cryogenic operation of HgCdTe infrared detectors

The performance of leading HgCdTe p-n junction infrared (IR) device technology is limited by thermal generationrecombination (G-R) mechanisms and material processing challenges associated with achieving low, controllable in-situ p-type doping using molecular beam epitaxy (MBE) growth techniques. These aspects are addressed in the proposed hybrid HgCdTe NBνN structure which relies on band gap engineered layers to suppress Shockley-Read-Hall (SRH) and Auger G-R processes contributing to performance degradation. The unipolar NBνN architecture provides the desired advantages of a simplified fabrication process, eliminating p-type doping requirements. Physics-based numerical device simulations incorporating established HgCdTe material parameters and G-R mechanisms are used to study the performance characteristics of a long wavelength infrared (LWIR) NBνN device with a 12 μm cut-off wavelength. The calculated results are compared to those values obtained for an LWIR HgCdTe nBn device. Theoretical dark current density (Jdark) values of the NBνN device are lower by an order of magnitude or more for temperatures between 50 K and 245 K. Calculated detectivity (D*) values of 2.367 x 1014 - 2.273 x 1011 cm Hz0.5/W for temperatures ranging from 50 K and 95 K, respectively, are observed in the NBνN structure.