Christoph H. Grein

The design, implementation, and performance of the Atro-H SXS calorimeter array and anti-coincidence detector

The calorimeter array of the JAXA Astro-H (renamed Hitomi) Soft X-ray Spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV, enabling essential plasma diagnostics. The SXS has a square array of 36 microcalorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices have demonstrated a resolution of better than 4.5 eV at 6 keV when operated at a heat-sink temperature of 50 mK. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, resistance function, absorber details, and means of attaching the absorber to the thermistorbearing element. We will also present the thermal characterization of the whole array, including thermal conductance and crosstalk measurements and the results of pulsing the frame temperature via alpha particles, heat pulses, and the environmental background. A silicon ionization detector is located behind the calorimeter array and serves to reject events due to cosmic rays. We will briefly describe this anti-coincidence detector and its performance.

Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrates

The efficiencies of monolithic single-crystal II–VI and III–V, two-junction and three-junction solar cells are calculated. The structures consist of II–VI or III–V homojunctions grown on an active-junction substrate (silicon for II–VI and germanium for III–V) and of inverted three-junction II–VI or III–V structures, with the band gaps chosen to maximize the efficiencies. Our calculations for the II–VI cells give theoretical efficiencies up to 44% under 1 sun and 50% under 500 suns, ∼3% absolute higher than for the III–V cells. Maximum obtainable laboratory and production-line efficiencies for multijunction II–VI cells are predicted. Preliminary laboratory results for II–VI two-junction cells are also presented.

Auger recombination in long-wave infrared InAs/InAsSb type-II superlattices

The Auger lifetime is a critical intrinsic parameter for infrared photodetectors as it determines the longest potential minority carrier lifetime and consequently the fundamental limitations to their performance. Here, Auger recombination is characterized in a long-wave infrared InAs/InAsSb type-II superlattice. Auger coefficients as small as 7.1×10−26 cm6/s are experimentally measured using carrier lifetime data at temperatures in the range of 20u2009K–80u2009K. The data are compared to Auger-1 coefficients predicted using a 14-band K·p electronic structure model and to coefficients calculated for HgCdTe of the same bandgap. The experimental superlattice Auger coefficients are found to be an order-of-magnitude smaller than HgCdTe.

MWIR InAsSb barrier detector data and analysis

Mid-wavelength infrared (MWIR) InAsSb alloy barrier detectors grown on GaAs substrates were characterized as a function of temperature to evaluate their performance. Detector arrays were fabricated in a 1024 × 1024 format on an 18 μm pitch. A fanout was utilized to directly acquire data from a set of selected detectors without an intervening read out integrating circuit (ROIC). The detectors have a cutoff wavelength equal to ~ 4.9 μm at 150 K. The peak internal quantum efficiency (QE) required a reverse bias voltage of 1 V. The detectors were diffusion-limited at the bias required to attain peak QE. Multiple 18 μm × 18 μm detectors were tied together in parallel by connecting the indium bump of each detector to a single large metal pad on the fanout. The dark current density at -1 V bias for a set of 64 × 64 and 6 × 6 array of detectors, each of which were tied together in parallel was ~ 10-3 A/cm2 at 200 K and 5 × 10-6 A/cm2 at 150 K. The 4096 (64 × 64) and 36 (6 × 6) detectors, both have similar Jdark vs Vd characteristics, demonstrating high operability and uniformity of the detectors in the array. The external QE measured using a narrow band filter centered at ~ 4 μm had values in the 65 – 70 % range. Since the detectors were illuminated through a GaAs substrate which has a reflectance of 29%, the internal QE is greater than 90 %. A 1024 × 1024 ROIC on an 18 μm pitch was also designed and fabricated to interface with the barrier detectors. QE at 150 K for a 1024 × 1024 detector array hybridized to a ROIC matched the QE measured on detectors that were measured directly through a fanout chip. Median D* at 150 K under a flux of 1.07 × 1015 ph/(cm2/s was 1.0 x 1011 cm Hz1/2 /W.

MWIR imaging with low cost colloidal quantum dot films

Suspensions of HgTe colloidal quantum dots (CQD) are readily synthesized with infrared energy gaps between 3 and 12 microns. Infrared photodetection using dried films of these CQDs has been demonstrated up to a cutoff wavelength of 12 microns. The synthesis of CQDs and the fabrication of detector devices employ bench-top chemistry techniques, leading to the potential for the easy manufacture of infrared photon detecting imagers at low cost. Recent electrical and optical measurements of these CQD films are discussed. Recent successful prototypes of complete focal plane arrays from CQD films and commercially-available ROICs are also described.

Dual-carrier multiplication high-gain MWIR strain layer superlattice impact ionization engineered avalanche photodiodes

A novel heterostructured dual carrier multiplication extremely high gain MWIR InAs/InGaSb Type II strained layer superlattice (T2SLS) impact ionization engineered (I2E) APD was designed and simulated. Spatially separated T2SLS electron and hole multiplication regions are designed using 14 band k.p bandstructure modeling. In the novel dual carrier device, the I2E T2SLS electron and hole multiplication regions are placed right next to each other. This allows for a carrier feedback between the electron and hole multiplication regions. This feedback between the electron and hole multiplication regions allows for extremely high gain values for the overall device. While the individual gain of the electron and hole multiplication regions can be kept extremely low, the overall gain can be >103. This can be achieved at a reverse bias of 3.5V. The effective k is designed to be approximately .07. Such low bias operation of the MWIR APD allows for active operation and passive mode operation on the same pixel using standard ROIC and this opens up possibility of large format dual mode imaging arrays.

Molecular dynamics growth modeling of InAs1−xSbx-based type-II superlattice

Abstract. Type-II strained-layer superlattices (T2SL) based on InAs1−xSbx are a promising photovoltaic detector material technology for thermal imaging; however, Shockley–Read–Hall recombination and generation rates are still too high for thermal imagers based on InAs1−xSbx T2SL to reach their ideal performance. Molecular dynamics simulations using the Stillinger–Weber (SW) empirical potentials are a useful tool to study the growth of tetrahedral coordinated crystals and the nonequilibrium formation of defects within them, including the long-range effects of strain. SW potentials for the possible atomic interactions among {Ga, In, As, Sb} were developed by fitting to ab initio calculations of elastically distorted zinc blende and diamond unit cells. The SW potentials were tested against experimental observations of molecular beam epitaxial (MBE) growth and then used to simulate the MBE growth of InAs/InAs0.5Sb0.5 T2SL on GaSb substrates over a range of processes parameters. The simulations showed and helped to explain Sb cross-incorporation into the InAs T2SL layers, Sb segregation within the InAsSb layers, and identified medium-range defect clusters involving interstitials and their induction of interstitial-vacancy pairs. Defect formation was also found to be affected by growth temperature and flux stoichiometry.

Molecular Beam Epitaxy of HgCdTe Materials and Detectors

Hg1xa0−xa0xCdxTe remains the dominant material employed in high-performance infrared photon detectors. This chapter summarizes the current status of single-crystal Hg1xa0−xa0xCdxTe growth by molecular beam epitaxy, including choices of substrates, in situ characterization for control and monitoring of the growth, initial nucleation and subsequent growth, and doping. The resulting structural, optical, and electrical material properties are discussed. We also provide an overview of the device physics of Hg1xa0−xa0xCdxTe-based infrared detectors, including photoconductors, homo- and hetero-junction photodiodes, and specialty devices such as avalanche photodiodes and high-speed detectors. The potential of HgCdTe-based superlattices as an infrared-absorbing material is explored from the perspective of their possible properties in comparison with Hg1xa0−xa0xCdxTe alloys.

Colloidal quantum dots for low-cost MWIR imaging

Monodisperse suspensions of HgTe colloidal quantum dots (CQD) are readily synthesized with infrared energy gaps between 3 and 12 microns. Infrared photodetection using dried films of these CQDs has been demonstrated up to a wavelength of 12 microns, and HgTe CQD single-elemnet devices with 3.6 micron cutoff have bee nreported nad show ogod absorption <(10^4 cm^-1), response time and detectivity (2*10^10 Jones) at at emperature of 175 K; with the potential fo uncooled imaging. The synthesis of CQDs and fabrication of detector devices employ bench-top chemistry techniques, leading to the potential for rapid, wafer-scale manufacture of MWIR imaging devices with low production costs and overhead. The photoconductive, photovoltaic and optical properties of HgTe CQD films will be discussed relative to infrared imaging, along with recent achievements in integrating CQD films with readout integrated circuits to produce CQD-based MWIR focal plane arrays.

Ideal performance of and defect-assisted carrier recombination in MWIR and LWIR InAs/InAsSb superlattice detectors

Detector-relevant material properties are calculated for mid-wavelength infrared and long-wavelength infrared InAs/InAsSb type-II superlattices (T2SLs). The electronic structure, transport, optical and carrier recombination properties are calculated for a series of T2SLs with varying Sb content in the InAsSb layer, and strain balanced for growth on GaSb substrates. The electronic-structure calculations rely on a well-tested envelope-function formalism based on fourteen bulk bands that has been extensively tested for InAs/GaInSb superlattice detectors. Targeted cutoff wavelengths are 5.2 microns and 10 microns. As the Sb composition and the strain in the InAsSb layer is varied the conduction and valence band edges also shift, and the resulting effect of these shifts on the Shockley-Read-Hall recombination rates from defect states in the gap is presented. Anisotropy in the carrier masses can also reduce detector performance; we find that hole mass anisotropy can be moderate for high-performance InAs/InAsSb superlattices.