The Large Area Picosecond Photodetector Collaboration is developing large-area fast photodetectors with time resolution ~10 ps and space resolution ~1 mm based on atomic layer deposition-coated glass Micro-Channel Plates (MCPs). We have assembled a facility at Argonne National Laboratory for characterizing the performance of a wide variety of microchannel plate configurations and anode structures in configurations approaching complete detector systems. The facility consists of a pulsed Ti:Sapphire laser with a pulse duration ≈100 fs, an optical system allowing the laser to be scanned in two dimensions, and a computer-controlled data-acquisition system capable of reading out 60 channels of anode signals with a sampling rate of over 10 GS/s. The laser can scan on the surface of a sealed large-area photodetector, or can be introduced into a large vacuum chamber for tests on bare 8 in.-square MCP plates or into a smaller chamber for tests on 33-mm circular substrates. We present the experimental setup, detector calibration, data acquisition, analysis tools, and typical results demonstrating the performance of the test facility.
"Measurement of InAsSb bandgap energy and InAs/InAsSb band edge positions using spectroscopic ellipsometry and photoluminescence spectroscopy" (2015). The structural and optical properties of lattice-matched InAs 0.911 Sb 0.089 bulk layers and strainbalanced InAs/InAs 1Àx Sb x (x $ 0.1-0.4) superlattices grown on (100)-oriented GaSb substrates by molecular beam epitaxy are examined using X-ray diffraction, spectroscopic ellipsometry, and temperature dependent photoluminescence spectroscopy. The photoluminescence and ellipsometry measurements determine the ground state bandgap energy and the X-ray diffraction measurements determine the layer thickness and mole fraction of the structures studied. Detailed modeling of the X-ray diffraction data is employed to quantify unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattices. A Kronig-Penney model of the superlattice miniband structure is used to analyze the valence band offset between InAs and InAsSb, and hence the InAsSb band edge positions at each mole fraction. The resulting composition dependence of the bandgap energy and band edge positions of InAsSb are described using the bandgap bowing model; the respective low and room temperature bowing parameters for bulk InAsSb are 938 and 750 meV for the bandgap, 558 and 383 meV for the conduction band, and À380 and À367 meV for the valence band. V C 2015 AIP Publishing LLC.[http://dx
The molecular beam epitaxy growth and optical properties of the III-V semiconductor alloy InAsSbBi are investigated over a range of growth temperatures and V/III flux ratios. Bulk and quantum well structures grown on the (100) on-axis and offcut GaSb substrates are examined. Bismuth readily incorporates at growth temperatures around 300 °C but results in materials with limited optical quality. Conversely, higher growth temperatures around 400 °C yield improved optical performance but with limited Bi incorporation. Photoluminescence spectroscopy is used to examine the optical properties and bandgap energies of InAsSbBi layers grown at temperatures from 400 to 430 °C using 0.91 and 0.94 As/In flux ratios, 0.10 and 0.12 Sb/In flux ratios, and 0.05 and 0.10 Bi/In flux ratios. Emission is observed from low to room temperature with peaks ranging from 3.7 to 4.6 μm. The relationships between Bi incorporation, surface morphology, growth temperature, and group-V flux are examined. Large concentrations of Bi-rich surface features are observed on samples where the incident Bi flux neither fully incorporates nor desorbs but instead accumulates on the surface and coalesces into droplets.
The optical properties of bulk InAs0.936Bi0.064 grown by molecular beam epitaxy on a (100)-oriented GaSb substrate are measured using spectroscopic ellipsometry. The index of refraction and absorption coefficient are measured over photon energies ranging from 44 meV to 4.4 eV and are used to identify the room temperature bandgap energy of bulk InAs0.936Bi0.064 as 60.6 meV. The bandgap of InAsBi is expressed as a function of Bi mole fraction using the band anticrossing model and a characteristic coupling strength of 1.529 eV between the Bi impurity state and the InAs valence band. These results are programmed into a software tool that calculates the miniband structure of semiconductor superlattices and identifies optimal designs in terms of maximizing the electron-hole wavefunction overlap as a function of transition energy. These functionalities are demonstrated by mapping the design spaces of lattice-matched GaSb/InAs0.911Sb0.089 and GaSb/InAs0.932Bi0.068 and strain-balanced InAs/InAsSb, InAs/GaInSb, and InAs/InAsBi superlattices on GaSb. The absorption properties of each of these material systems are directly compared by relating the wavefunction overlap square to the absorption coefficient of each optimized design. Optimal design criteria are provided for key detector wavelengths for each superlattice system. The optimal design mid-wave infrared InAs/InAsSb superlattice is grown using molecular beam epitaxy, and its optical properties are evaluated using spectroscopic ellipsometry and photoluminescence spectroscopy.
Articles you may be interested inAlloying bismuth with InAs provides a ternary material system near the 6.1 Å lattice constant, which covers the technologically important mid-and long-wavelength infrared region. One challenge for this material system is that it is not straightforward to incorporate bismuth into the bulk InAs lattice, since bismuth has a tendency to surface-segregate and form droplets during growth. In this work, the conditions for InAsBi growth using molecular beam epitaxy are explored. A growth window is identified (temperatures կ 270 C, V/III flux ratios 0.98 խ As/In խ 1.02, and Bi/In ffi 0.065) for droplet-free, high-quality crystalline material, where InAsBi layers with compositions of up to 5.8% bismuth (nearly lattice-matched to GaSb) are attained. The structural quality of InAsBi bulk and quantum well samples is evaluated using x-ray diffraction and transmission electron microscopy. The optical quality is assessed using photoluminescence, which is observed from quantum well structures up to room temperature and from thick, low Bi-content bulk layers at low temperatures. Bismuth is also used as a surfactant during the growth of InAs/InAsSb superlattices at 430 C where it is observed that a small bismuth flux changes the surface reconstruction of InAs from (2Â1) to (1Â3), reduces the sticking coefficient of antimony, results in a slight increase in photoluminescence intensity, does not significantly incorporate, and does not alter the surface morphology.
a b s t r a c tThe microstructure of InAs 1 À x Bi x (x $ 4.5% and $ 5.8%) films and Bi-mediated InAs/InAs 1 À x Sb x type-II superlattices grown by molecular beam epitaxy on GaSb (0 0 1) substrates has been investigated by electron microscopy techniques. Lateral compositional modulation exists in both smooth and hazy regions of all InAsBi films observed but no atomic ordering is apparent using current imaging projections. Surface droplets present in hazy regions assume a zincblende crystalline structure that is usually tilted relative to the underlying dilute bismide film. Study of Bi-mediated InAs/InAs 0.81 Sb 0.19 type-II superlattices indicates that the InAs-on-InAsSb interface still appears broadened relative to the InAsSb-on-InAs interface.
Anisotropic carrier transport properties of unintentionally doped InAs/InAs0.65Sb0.35 type-II strain-balanced superlattice material are evaluated using temperature- and field-dependent magnetotransport measurements performed in the vertical direction on a substrate-removed metal-semiconductor-metal device structure. To best isolate the measured transport to the superlattice, device fabrication entails flip-chip bonding and backside device processing to remove the substrate material and deposit contact metal directly to the bottom of an etched mesa. High-resolution mobility spectrum analysis is used to calculate the conductance contribution and corrected mixed vertical-lateral mobility of the two carrier species present. Combining the latter with lateral mobility results from in-plane magnetotransport measurements on identical superlattice material allows for the calculation of the true vertical majority electron and minority hole mobilities; amplitudes of 4.7 ×103 cm2/V s and 1.60 cm2/V s are determined at 77 K, respectively. The temperature-dependent results show that vertical hole mobility rapidly decreases with decreasing temperature due to trap-induced localization and then hopping transport, whereas vertical electron mobility appears phonon scattering-limited at high temperature, giving way to interface roughness scattering at low temperatures, analogous to the lateral electron mobility but with a lower overall magnitude.
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