We have fabricated and characterized AlInAsSb- and InPAsSb-absorber nBn infrared detectors with 200 K cutoff wavelengths from 2.55 to 3.25 μm. Minority-carrier lifetimes determined by microwave reflectance measurements were 0.2-1.0 μs in doped n-type absorber materials. Devices having 4 μm thick absorbers exhibited sharp cutoff at wavelengths of 2.9 μm or longer and softer cutoff at shorter wavelengths. Top-illuminated devices with n+ InAs window/contact layers had external quantum efficiencies of 40-50% without anti-reflection coating at 50 mV reverse bias and wavelengths slightly shorter than cutoff. Despite the shallow-etch mesa nBn design, perimeter currents contributed significantly to the 200 K dark current. Dark currents for InPAsSb devices were lower than AlInAsSb devices with similar cutoff wavelengths. For unoptimized InPAsSb devices with 2.55 μm cutoff, 200 K areal and perimeter dark current densities at -0.2 V bias in devices of various sizes were approximately 1x10-7 A/cm2 and 1.4x10-8 A/cm, respectively.
Type-II strained-layer superlattices (T2SLs) are receiving increased interest as mid-wave infrared (MWIR) and long-wave infrared detector absorbers due to their potential Auger suppression and ability to be integrated into complex device structures. Although T2SLs show promise for use as infrared detectors, further investigation into the effects of high energy particle radiation is necessary for space-based applications. In this presentation, the effects of both 4.5 MeV and 63 MeV proton radiation on the carrier lifetime of MWIR InAs/InAsSb T2SLs will be shown. The 63 MeV proton radiation study will focus on the carrier lifetime of MWIR InAs/InAsSb T2SL samples of varying donor density. These results reveal a Shockley-Read-Hall (SRH) lifetime associated with a radiation induced defect level, which is not dependent on the donor density of the T2SL. Using 4.5 MeV proton radiation, the dependence of carrier lifetime on relative trap density in MWIR T2SLs samples is studied by varying the particle fluence. A comparison of these two radiation studies shows similar lifetime effects that will be discussed in detail. These results give insight into the viability of Ga-free T2SLs for space applications.
Significantly improved carrier lifetimes in very long-wave infrared (VLWIR) InAs/GaInSb superlattice (SL) absorbers are demonstrated using time-resolved microwave reflectance (TMR) measurements. A nominal 47.0 Å InAs/21.5 ÅGa0.75In0.25Sb SL structure that produces an ∼25 μm response at 10 K has a minority carrier lifetime of 140±20 ns at 18 K, which is an order-of-magnitude improvement compared with previously reported lifetime values for other VLWIR detector absorbers. This improvement is attributed to the strain-engineered ternary SL design, which offers a variety of epitaxial advantages and ultimately leads to the improvements in the minority carrier lifetime by mitigating defect-mediated Shockley–Read–Hall (SRH) recombination centers. By analyzing the temperature dependence of TMR decay data, the recombination mechanisms and trap states that currently limit the performance of this SL absorber were identified. The results show a general decrease in the long-decay lifetime component, which is dominated by SRH recombination at temperatures below ∼30 K and by Auger recombination at temperatures above ∼45 K. Since the strain-balanced ternary SL design offers a reasonably good absorption coefficient and many epitaxial advantages during growth, this VLWIR SL material system should be considered as a competitive candidate for VLWIR photodetector technology.
Significantly improved carrier lifetimes in very long wavelength infrared (VLWIR) InAs/GaInSb superlattice (SL) absorbers are demonstrated by using time-resolved microwave reflectance (TMR) measurements. A nominal 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb SL structure that produces an approximately 25 μm response at 10 K has a minority carrier lifetime of 140 ± 20 ns at 18 K, which is an order-of-magnitude improvement compare to previously reported lifetime values for other VLWIR detector absorbers. This improvement is attributed to the strain-engineered ternary SL design, which offers a variety of epitaxial advantages and ultimately leads to the improvements in the minority carrier lifetime by mitigating defect-mediated Shockley-Read-Hall (SRH) recombination centers. By analyzing the temperature dependence of TMR decay data, the recombination mechanisms and trap states that currently limit the performance of this SL absorber are identified. The results show a general decrease in the long-decay lifetime component, which is dominated by SRH recombination at temperatures below ~30 K, and by Auger recombination at temperatures above ~45 K. This result implies that minimal improvement can be made in the minority carrier lifetime at temperatures greater than 45 K without further suppressing Auger recombination through proper band engineering, which suggests that the improvement to be gained by mitigation of the SRH defects would not be substantial at these temperatures. At temperatures lower than 30 K, some improvement can be attained by mitigated of the SRH recombination centers. Since the strain-balanced ternary SL design offers a reasonably good absorption coefficient and many epitaxial advantages during growth, this VLWIR SL material system should be considered a competitive candidate for VLWIR photodetector technology.
Temperature dependent measurements of carrier recombination rates using a time-resolved pump-probe technique are reported for mid-wave infrared InAs/InAsSb type-2 superlattices (T2SLs). By engineering the layer widths and alloy compositions a 16 K band-gap of ~235 ± 10meV was achieved for four doped and five undoped T2SLs. Carrier lifetimes were determined by fitting lifetime models of Shockley-Read-Hall (SRH), radiative, and Auger recombination processes simultaneously to the temperature and excess carrier density dependent data. The contribution of each recombination process at a given temperature is identified and the total lifetime is determined over a range of excess carrier densities. The minority carrier and Auger lifetimes were observed to increase with increasing antimony content and decreasing layer thickness for the undoped T2SLs. It is hypothesized that a reduction in SRH recombination centers or a shift in the SRH defect energy relative to the T2SL band edges is the cause of this increase in the SRH minority carrier lifetime. The lower Auger coefficients are attributed to a reduced number of final Auger states in the SL samples with greater antimony content. An Auger limited minority carrier lifetime is observed for the doped T2SLs, and it is found to be a factor of ten shorter than for undoped T2SLs. The Auger rates for all the InAs/InAsSb T2SLs were significantly larger than those previously reported for InAs/GaSb T2SLs.
The device applications of plasmonic systems such as graphene and two dimensional electron gases (2DEGs) in III-V
heterostructures include terahertz detectors, mixers, oscillators and modulators. These two dimensional (2D) plasmonic
systems are not only well-suited for device integration, but also enable the broad tunability of underdamped plasma
excitations via an applied electric field. We present demonstrations of the coherent coupling of multiple voltage tuned
GaAs/AlGaAs 2D plasmonic resonators under terahertz irradiation. By utilizing a plasmonic homodyne mixing
mechanism to downconvert the near field of plasma waves to a DC signal, we directly detect the spectrum of coupled
plasmonic micro-resonator structures at cryogenic temperatures. The 2DEG in the studied devices can be interpreted as
a plasmonic waveguide where multiple gate terminals control the 2DEG kinetic inductance. When the gate tuning of the
2DEG is spatially periodic, a one-dimensional finite plasmonic crystal forms. This results in a subwavelength structure,
much like a metamaterial element, that nonetheless Bragg scatters plasma waves from a repeated crystal unit cell. A
50% in situ tuning of the plasmonic crystal band edges is observed. By introducing gate-controlled defects or simply
terminating the lattice, localized states arise in the plasmonic crystal. Inherent asymmetries at the finite crystal
boundaries produce an induced transparency-like phenomenon due to the coupling of defect modes and crystal surface
states known as Tamm states. The demonstrated active control of coupled plasmonic resonators opens previously
unexplored avenues for sensitive direct and heterodyne THz detection, planar metamaterials, and slow-light devices.
The two-dimensional plasma resonance excited in the channel of a field effect transistor has recently been utilized as
the frequency-selective absorber in a monolithic far infrared plasmonic cavity detector. In this article we discuss the
relevant parameters pertaining to engineering the plasmonic cavity and an integrated detection element as
constituent elements of a resonant far infrared detector. The spectra of low-order plasmon modes in 18 μm and 34
μm long two-dimensional plasmonic cavities with 4 μm period grating gates have been measured. When the length
of the plasma cavity is significantly larger than the gate length or period, the cavity length rather than grating period
defines the plasmon wavevector. Electronic noise sources are considered; random telegraph noise is suggested as a
dominant noise source when the device is operated as a highly resistive bolometric detector.
Graphene is a promising material for optoelectronics and photonics. Recent experiments demonstrated graphene
photodectectors based on interband transitions working at Mid and Near-IR/Visible regions. Extension of spectral
range to longer wavelengths requires alternative photoresponse mechanisms. One of the mechanisms which has
been proven to be efficient for THz detection in "classical" semiconductor materials is the optically-induced
breakdown of quantum Hall effect. In our work we successfully demonstrated a graphene-based QHE
photodetector. Our result demonstrates the potential of graphene as a material for Far-IR photodetectors. Further
improvement in device design and use of more efficient radiation coupling solutions should enable graphene
photodetectors with broader spectral range, higher sensitivity, and elevated operating temperatures for a variety of
applications.
Phononic crystals (PnCs) are acoustic devices composed of a periodic arrangement of scattering centers embedded in a homogeneous background matrix with a lattice spacing on the order of the acoustic wavelength. When properly designed, a superposition of Bragg and Mie resonant scattering in the crystal results in the opening of a frequency gap over which there can be no propagation of elastic waves in the crystal, regardless of direction. In a fashion reminiscent of photonic lattices, PnC patterning results in a controllable redistribution of the phononic density of states. This property makes PnCs a particularly attractive platform for manipulating phonon propagation. In this communication, we discuss the profound physical implications this has on the creation of novel thermal phenomena, including the alteration of the heat capacity and thermal conductivity of materials, resulting in high-ZT materials and highly-efficient thermoelectric cooling and energy harvesting.
We demonstrate the steering of coherent mid-infrared radiation through plasmonic structures consisting of a
single sub-wavelength slit flanked by a periodic array of grooves, fabricated on GaAs substrates. We demonstrate
control of steering angle by tuning the incident radiation, and study beam quality for the transmitted light. In
addition, we demonstrate that small shifts in the refractive index of the GaAs substrate can actively control the
steering angle of the transmitted light, opening a path towards the development of no-moving-parts plasmonic
beam steering devices.
We demonstrate mid-infrared electroluminescence from intersublevel transitions in self-assembled InAs quantum dots
coupled to surface plasmon modes on metal hole arrays. Subwavelength metal hole arrays with different periodicity are
patterned into the top contact of the broadband (9-15 μm) quantum dot material and the measured electroluminescence
is compared to devices without a metal hole array. The resulting normally directed emission is narrowed and a splitting
in the spectral structure is observed. By applying a coupled quantum electrodynamic model and using reasonable values
for quantum dot distributions and plasmon linewidths we are able to reproduce the experimentally measured spectral
characteristics of device emission when using strong coupling parameters.
We demonstrate room temperature electroluminescence from intersublevel transitions in self-assembled InAs quantum
dots in GaAs/AlGaAs heterostructures. The quantum dot devices are grown on GaAs substrates in a Varian Gen II
molecular beam epitaxy system. The device structure is designed specifically to inject carriers into excited conduction
band states in the dots and force an optical transition between the excited and ground states of the dots. A downstream
filter is designed to selectively extract carriers from the dot ground states. Electroluminescence measurements were
made by Fourier Transform Infrared Spectroscopy in amplitude modulation step scan mode. Current-Voltage
measurements of the devices are also reported. In addition, both single period and multi-period devices are grown,
fabricated, characterized, and compared to each other. Finally, we discuss the use of plasmonic output couplers for these
devices, and discuss the unique emission observed when the quantum dot layer sits in the near field of the plasmonic top
contacts.
Straightforward extension of canonical microwave metamaterial structures to optical and IR frequency dimensions is
complicated by both the size scale of the resulting structures, requiring cutting edge lithography to achieve the requisite
line-widths, as well as limitations on assembly/construction into final geometry. We present a scalable fabrication
approach capable of generating metamaterial structures such as split ring resonators and split wire pairs on a micron/sub-micron
size scale on concave surfaces with a radius of curvature ~ SRR diameter. This talk outlines the fabrication
method and modeling/theory based interpretation of the implications of curved metamaterial resonators.
Terahertz detection using excitations of plasmon modes offers a high-speed, high resolution, and frequency-selective
alternative to existing technology. Plasmons in high mobility quantum well two-dimensional electron gas (2DEG)
systems can couple to radiation when either the channel carrier density, or the incident radiation, is spatially modulated
with appropriate periodicity. Grating-gated terahertz detectors having a voltage tunable frequency response have been
developed based on this principle. A continuous wave THz photomixer was used to characterize the resonant absorption
in such devices. At the fundamental 2DEG plasmon frequency, defined by the grating and the quantum well carrier
density, a 20% change in transmission was observed. As the resonance is tuned from the 'natural' plasmon frequency
through application of a gate bias, it shifts as expected, but the transmission change drops to only a few percent.
We demonstrate active control of propagating surface waves on a mid-infrared extraordinary optical transmission
grating. The surface waves are excited at the interface between a GaAs substrate and a periodically patterned metal film
using a dual wavelength quantum cascade laser. The spectral properties of the laser and the transmission grating are
characterized by Fourier Transform Infrared spectroscopy. In addition, the far-field emission from excited surface
waves at the metal/GaAs interface is studied using a novel spatial and spectral imaging technique. By actively
controlling the optical properties of the grating, we demonstrate the ability to control the coupling of incident coherent
radiation to surface waves on the grating. With increased tunability of the grating, directional control of excited surface
waves should be achievable. These results suggest that the development of actively tunable plasmonic structures could
result in plasmonic routers and switches for interconnect or sensing applications.
We have fabricated and characterized plasmonic terahertz detectors that integrate a voltage controlled planar barrier with
a grating gated GaAs/AlGaAs high electron mobility transistor. These detectors exhibit a narrowband, tunable
plasmonic response. Substantially increased responsivity is achieved by introducing an independently biased, narrow
gate that produces a lateral potential barrier adjacent to the drain when biased to pinch-off. DC electrical characterization
in conjunction with bias-dependent terahertz responsivity and time constant measurements indicate that a hot electron
bolometric effect is the dominant response mechanism over a broad range of experimental conditions. The temperature
dependence of the bolometric response is consistent with the energy relaxation time and absorption coefficient of a
2DEG. Rectification resulting from non-linear current-voltage characteristics also appears to contribute to the response.
Additionally, we have begun investigating the operation of this device with the full grating gate biased to pinch-off to
produce many detection elements in series.
We demonstrate doping-tunable mid-infrared extraordinary transmission through periodic sub-wavelength openings in
thin metal films. This effect, known as extraordinary optical transmission, is thought to result from the excitation of
Surface Plasmon Polaritons at the metal/dielectric interface. The metal aperture arrays studied were fabricated upon
GaAs substrates. Because the dielectric constant of a semiconductor changes with carrier concentration, identical
metallic grating show different spectral characteristics as a function of GaAs epilayer doping. Thus, the resonant
transmission peaks of our grating structures can be shifted by varying the doping of the n-GaAs epilayer upon which
they are fabricated. We demonstrate peak shifts of 37 cm-1, or approximately 0.33 μm, as we move from undoped GaAs
layer to highly doped n-GaAs layers. Additionally, we study the effect of doping layer thickness on the resonant
transmission peak position, which allows for an estimate of the surface plasmon strength as a function of distance from
the metal/dielectric interface. Furthermore, we present calculated results for our samples and compare them to our
experimental results and propose an explanation for the slight discrepancy between theoretical and experimental results.
The devices presented could eventually lead to voltage-tunable mid-IR mirrors, filters, or modulators.
Split grating-gate field effect transistors (FETs) detectors made from high mobility quantum well two-dimensional
electron gas material have been shown to exhibit greatly improved tunable resonant photoresponse compared to single
grating-gate detectors due to the formation of a 'diode-like' element by the split-gate structure. These detectors are
relatively large for FETs (1mm x 1mm area or larger) to match typical focused THz beam spot sizes. In the case where
the focused THz spot size is smaller than the detector area, we have found evidence, through positional scanning of the
detector element, that only a small portion of the detector is active. To further investigate this situation, detectors with
the same channel width (1mm), but various channel lengths, were fabricated and tested. The results indicate that indeed,
only a small portion of the split grating gated FET is active. This finding opens up the possibility for further
enhancement of detector sensitivity by increasing the active area.
Electroluminescence from self-assembled InAs quantum dots in cascade-like unipolar heterostructures is demonstrated.
Initial results show weak luminescence signals in the mid-infrared from such structures, though more recent designs
exhibit significantly stronger luminescence with improved designs of the active region of these devices. Further studies
of mid-infrared emitting quantum dot structures have shown anisotropically polarized emission at multiple wavelengths.
A qualitative explanation of such luminescence is developed and used to understand the growth morphology of buried
quantum dots grown on AlAs layers. Finally, a novel design for future mid-infrared quantum dot emitters, intended to
increase excited state scattering times and, at the same time, more efficiently extract carriers from the lowest states of
our quantum dots, is presented.
A split-grating-gate detector design has been implemented in an effort to combine the tunability of the basic grating-gate detector with the high responsivity observed in these detectors when approaching the pinchoff regime. The redesign of the gates by itself offers several orders of magnitude improvement in resonant responsivity. Further improvements are gained by placing the detector element on a thermally isolating membrane in order to increase the effects of lattice heating on the device response.
Grating gated field effect transistors (FETs) are potentially important as electronically tunable terahertz detectors with spectral bandwidths of the order of 50 GHz. Their utility depends on being able to 1) use the intrinsic high speed in a heterodyne mixer or 2) sacrifice speed for sufficient sensitivity to be an effective incoherent detector. In its present form the grating gated FET will support IF frequencies up to ~10 GHz, an acceptable bandwidth for most heterodyne applications. By separating the resonant plasmon absorption from the responsivity mechanism, it appears that a tuned, narrow terahertz spectral band bolometer can be fabricated with NEP ~ 10-11 watts/√Hz and response times of the order of 30 msecs, useful in a passive multispectral terahertz imaging system.
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