Publications

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High temperature operation (250-340 K) of short-wavelength interband cascade infrared photodetectors (ICIPs) with InAs/GaSb/Al0.2In0.8Sb/GaSb superlattice absorbers has been demonstrated with a 50% cutoff wavelength of 2.9 μm at 300 K. Two ICIP structures, one with two and the other with three stages, were designed and grown to explore this multiple-stage architecture. At λ = 2.1 μm, the two- and three-stage ICIPs had Johnson-noise-limited detectivities of 5.1 × 109 and 5.8 × 109cm Hz1/2/W, respectively, at 300 K. The better device performance of the three-stage ICIP over the two-stage ICIP confirmed the advantage of more stages for this cascade architecture. An Arrhenius activation energy of 450 meV is extracted for the bulk resistance-area product, which indicates the dominance of the diffusion current at these high temperatures.

We investigate high-temperature and high-frequency operation of interband cascade infrared photodetectors (ICIPs)-two critical properties. Short-wavelength ICIPs with a cutoff wavelength of 2.9 μm had Johnson-noise limited detectivity of 5.8×109 cmHz1/2/W at 300 K, comparable to the commercial Hg1-xCdxTe photodetectors of similar wavelengths. A simple but effective method to estimate the minority carrier diffusion length in short-wavelength ICIPs is introduced. Using this approach, the diffusion length was estimated to be significantly shorter than 1 μm at high temperatures, indicating the importance of a multiple-stage photodetector (e.g., ICIPs) at high temperatures. Recent investigations on the high-frequency operation of mid-wavelength ICIPs (λc=4.3 μm) are discussed. These photodetectors had 3-dB bandwidths up to 1.3 GHz with detectivities exceeding 1x109 cmHz1/2/W at room temperature. These results validate the ability of ICIPs to achieve high bandwidths with large sensitivity and demonstrate the great potential for applications such as: heterodyne detection, and free-space optical communication.

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 20 K-80 K. 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.

Carrier lifetime and dark current measurements are reported for a mid-wavelength infrared InAs0.91Sb0.09 alloy nBn photodetector. Minority carrier lifetimes are measured using a non-contact time-resolved microwave technique on unprocessed portions of the nBn wafer and the Auger recombination Bloch function parameter is determined to be |F1F2|=0.292. The measured lifetimes are also used to calculate the expected diffusion dark current of the nBn devices and are compared with the experimental dark current measured in processed photodetector pixels from the same wafer. Excellent agreement is found between the two, highlighting the important relationship between lifetimes and diffusion currents in nBn photodetectors.

Epsilon-near-zero (ENZ) modes arising from condensed-matter excitations such as phonons and plasmons are a new path for tailoring light-matter interactions at the nanoscale. Complex spectral shaping can be achieved by creating such modes in nanoscale semiconductor layers and controlling their interaction with multiple, distinct, dipole resonant systems. Examples of this behavior are presented at midinfrared frequencies for ENZ modes that are strongly coupled to metamaterial resonators and simultaneously strongly coupled to semiconductor phonons or quantum-well intersubband transitions (ISTs), resulting in double- and triple-polariton branches in transmission spectra. For the double-polariton branch case, we find that the best strategy to maximize the Rabi splitting is to use a combination of a doped layer supporting an ENZ feature and a layer supporting ISTs, with overlapping ENZ and IST frequencies. This design flexibility renders this platform attractive for low-voltage tunable filters, light-emitting diodes, and efficient nonlinear composite materials.

Superconductivity in topological materials has attracted a great deal of interest in both electron physics and material sciences since the theoretical predictions that Majorana fermions can be realized in topological superconductors. Topological superconductivity could be realized in a type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a conventional superconductor. Here, we report observations of the proximity effect induced giant supercurrent states in an InAs/GaSb bilayer system that is sandwiched between two superconducting tantalum electrodes to form a superconductor-InAs/GaSb-superconductor junction. Electron transport results show that the supercurrent states can be preserved in a surprisingly large temperature-magnetic field (T - H) parameter space. In addition, the evolution of differential resistance in T and H reveals an interesting superconducting gap structure.

We present that temperature-dependent measurements of carrier recombination rates using a time-resolved optical pump-probe technique are reported for mid-wave infrared InAs/InAs_1-xSb_x type-2 superlattices (T2SLs). By engineering the layer widths and alloy compositions, a 16 K band-gap of ~235 ± 10 meV was achieved for five unintentionally and four intentionally doped T2SLs. Carrier lifetimes were determined by fitting lifetime models based on Shockley-Read-Hall (SRH), radiative, and Auger recombination processes to the temperature and excess carrier density dependent data. The minority carrier (MC), radiative, and Auger lifetimes were observed to generally increase with increasing antimony content and decreasing layer thickness for the unintentionally doped T2SLs. The MC lifetime is limited by SRH processes at temperatures below 200 K in the unintentionally doped T2SLs. The extracted SRH defect energy levels were found to be near mid-bandgap. Additionally, it is observed that the MC lifetime is limited by Auger recombination in the intentionally doped T2SLs with doping levels greater than n ~10¹⁶ cm^-3.

We use epsilon-near-zero modes in semiconductor nanolayers to design a system whose spectral properties are controlled by their interaction with multi-dipole resonances. This design flexibility renders our platform attractive for efficient nonlinear composite materials.

We have fabricated a superconductor (Ta)-InAs/GaSb bilayer-superconductor (Ta) junction device that has a long mean free path and can preserve the wavelike properties of particles (electrons and holes) inside the junction. Differential conductance measurements were carried out at low temperatures in this device, and McMillan-Rowell like oscillations (MROs) were observed. Surprisingly, a much larger Fermi velocity, compared to that from Shubnikov-de Haas oscillations, was obtained from the frequency of MROs. Possible mechanisms are discussed for this discrepancy.

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We use cross-sectional scanning tunneling microscopy (STM) to reconstruct the monolayer-by-monolayer composition profile across a representative subset of MBE-grown InAs/InAsSb superlattice layers and find that antimony segregation frustrates the intended compositional discontinuities across both antimonide-on-arsenide and arsenide-on-antimonide heterojunctions. Graded, rather than abrupt, interfaces are formed in either case. We likewise find that the incorporated antimony per superlattice period varies measurably from beginning to end of the multilayer stack. Although the intended antimony discontinuities predict significant discrepancies with respect to the experimentally observed high-resolution x-ray diffraction spectrum, dynamical simulations based on the STM-derived profiles provide an excellent quantitative match to all important aspects of the x-ray data.

Coherent superposition of light from subwavelength sources is an attractive prospect for the manipulation of the direction, shape and polarization of optical beams. This phenomenon constitutes the basis of phased arrays, commonly used at microwave and radio frequencies. Here we propose a new concept for phased-array sources at infrared frequencies based on metamaterial nanocavities coupled to a highly nonlinear semiconductor heterostructure. Optical pumping of the nanocavity induces a localized, phase-locked, nonlinear resonant polarization that acts as a source feed for a higher-order resonance of the nanocavity. Varying the nanocavity design enables the production of beams with arbitrary shape and polarization. As an example, we demonstrate two second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (∼5μm): a beam splitter and a polarizing beam splitter. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.

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We use epsilon-near-zero modes in semiconductor nanolayers to design a system whose spectral properties are controlled by their interaction with multi-dipole resonances. This design flexibility renders our platform attractive for efficient nonlinear composite materials. © OSA 2015.

Time-resolved measurements of carrier recombination are reported for a midwave infrared InAs/InAs0.66Sb0.34 type-II superlattice (T2SL) as a function of pump intensity and sample temperature. By including the T2SL doping level in the analysis, the Shockley-Read-Hall (SRH), radiative, and Auger recombination components of the carrier lifetime are uniquely distinguished at each temperature. SRH is the limiting recombination mechanism for excess carrier densities less than the doping level (the low-injection regime) and temperatures less than 175 K. A SRH defect energy of 95 meV, either below the T2SL conduction-band edge or above the T2SL valence-band edge, is identified. Auger recombination limits the carrier lifetimes for excess carrier densities greater than the doping level (the high-injection regime) for all temperatures tested. Additionally, at temperatures greater than 225 K, Auger recombination also limits the low-injection carrier lifetime due to the onset of the intrinsic temperature range and large intrinsic carrier densities. Radiative recombination is found to not have a significant contribution to the total lifetime for all temperatures and injection regimes, with the data implying a photon recycling factor of 15. Using the measured lifetime data, diffusion currents are calculated and compared to calculated Hg1-xCdxTe dark current, indicating that the T2SL can have a lower dark current with mitigation of the SRH defect states. These results illustrate the potential for InAs/InAs1-xSbx T2SLs as absorbers in infrared photodetectors.

Metallic nanocavities with deep subwavelength mode volumes can lead to dramatic changes in the behavior of emitters placed in their vicinity. This collocation and interaction often leads to strong coupling. Here, we present for the first time experimental evidence that the Rabi splitting is directly proportional to the electrostatic capacitance associated with the metallic nanocavity. The system analyzed consists of different metamaterial geometries with the same resonance wavelength coupled to intersubband transitions in quantum wells.

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