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.
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
5.8×10<sup>9</sup> cmHz<sup>1/2</sup>/W at 300 K, comparable to the commercial Hg<sub>1-x</sub>Cd<sub>x</sub>Te 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 1x10<sup>9</sup> cmHz<sup>1/2</sup>/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.
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 effect of defects on the dark current characteristics of MWIR, III-V nBn detectors has been studied. Two different types of defects are compared, those produced by lattice mismatch and by proton irradiation. It is shown that the introduction of defects always elevates dark currents; however the effect on dark current is different for nBn detectors and conventional photodiodes. The dark currents of nBn detectors are found to be more tolerant of defects compared to pn-junction based devices. Defects more weakly increase dark currents, and cooling reduces the defect-produced dark currents more rapidly in nBn detectors than in conventional photodiodes.
Quantum-engineered multiple stage photovoltaic (PV) devices are explored based on InAs/GaSb/AlSb interband
cascade (IC) structures. These ICPV devices employ multiple discrete absorbers that are connected in series by widebandgap
unipolar barriers using type-II heterostructure interfaces for facilitating carrier transport between cascade stages
similar to IC lasers. The discrete architecture is beneficial for improving the collection efficiency and for spectral
splitting by utilizing absorbers with different bandgaps. As such, the photo-voltages from each individual cascade stage
in an ICPV device add together, creating a high overall open-circuit voltage, similar to conventional multi-junction
tandem solar cells. Furthermore, photo-generated carriers can be collected with nearly 100% efficiency in each stage.
This is because the carriers travel over only a single cascade stage, designed to be shorter than a typical diffusion length.
The approach is of significant importance for operation at high temperatures where the diffusion length is reduced.
Here, we will present our recent progress in the study of ICPV devices, which includes the demonstration of ICPV
devices at room temperature and above with narrow bandgaps (e.g. 0.23 eV) and high open-circuit voltages.
We show simulation results of the integration of a nanoantenna in close proximity to the active material of a photodetector. The nanoantenna allows a much thinner active layer to be used for the same amount of incident light absorption. This is accomplished through the nanoantenna coupling incoming radiation to surface plasmon modes bound to the metal surface. These modes are tightly bound and only require a thin layer of active material to allow complete absorption. Moreover, the nanoantenna impedance matches the incoming radiation to the surface waves without the need for an antireflection coating. While the nanoantenna concept may be applied to any active photodetector material, we chose to integrate the nanoantenna with an InAsSb photodiode. The addition of the nanoantenna to the photodiode requires changes to the geometry of the stack beyond the simple addition of the nanoantenna and thinning the active layer. We will show simulations of the electric fields in the nanoantenna and the active region and optimized designs to maximize absorption in the active layer as opposed to absorption in the metal of the nanoantenna. We will review the fabrication processes.
We have fabricated low-dark-current InGaAs photodetectors utilizing an epitaxial structure incorporating an InAlGaAs passivation layer and a simple mesa isolation process, and requiring no implant or diffusion steps. At 295 K, areal and perimeter dark current contributions are 15 nA/cm<sup>2</sup> and 9 pA/cm, respectively, in devices with large aspect ratios biased at -0.1 V. High responsivity was achieved even at zero bias in these devices. Devices were modeled using a commercial drift-diffusion simulator. Good fits to reverse dark current-voltage measurements were obtained using a model that included both bulk and interfacial generation mechanisms. Assuming similar electron and hole Shockley-Read-Hall lifetimes, dark current under small reverse bias are consistent with generation at the interface between the absorber and underlying layers. With increasing negative bias a large increase in dark current is associated with depletion near the InAlGaAs/absorber interface, while small increases in current at large reverse bias suggest long Shockley-Read-Hall lifetimes in the absorber. Forward biasing of these devices results in efficient injection of minority carrier holes into the absorber region, mimicking photogeneration and providing a method to predict the performance of illuminated detector arrays.
We demonstrate the effects of integrating a nanoantenna to a midwave infrared (MWIR) focal plane array (FPA). We
model an antenna-coupled photodetector with a nanoantenna fabricated in close proximity to the active material of a
photodetector. This proximity allows us to take advantage of the concentrated plasmonic fields of the nanoantenna. The
role of the nanoantenna is to convert free-space plane waves into surface plasmons bound to a patterned metal surface.
These plasmonic fields are concentrated in a small volume near the metal surface. Field concentration allows for a
thinner layer of absorbing material to be used in the photodetector design and promises improvements in cutoff
wavelength and dark current (higher operating temperature). While the nanoantenna concept may be applied to any
active photodetector material, we chose to integrate the nanoantenna with an InAsSb photodiode. The geometry of the
nanoantenna-coupled detector is optimized to give maximal carrier generation in the active region of the photodiode, and
fabrication processes must be altered to accommodate the nanoantenna structure. The intensity profiles and the carrier
generation rates in the photodetector active layers are determined by finite element method simulations, and iteration
between optical nanoantenna simulation and detector modeling is used to optimize the device structure.
Interband cascade (IC) infrared (IR) photodetectors (ICIPs) are a new type of infrared detectors based on
quantum-engineered InAs/GaSb/AlSb heterostructures. They combine the features of conventional interband
photodiodes with the discrete nature of quantum-well IR photodetectors (QWIPs). The operation of ICIPs takes
advantage of fast intersubband relaxation and interband tunneling for carrier transport, and relatively slow interband
transitions (long lifetime) for photon generation. As such, ICIPs can be optimized for specific application
requirements, such as higher temperature operation or lower noise. By adopting a finite type-II InAs/GaSb
superlattice (SL) as the absorber, we have demonstrated mid-IR ICIPs with low noise and photovoltaic operation. In
this paper, we report some of our recent efforts in the development of mid-IR ICIPs for high temperature operations.
The ICIP devices with a cut-off wavelength of 3.8 μm exhibit a resistance-area product of 2.65×10<sup>6</sup> and 6.36×10<sup>3</sup>Ωcm<sup>2</sup> at 80 and 160 K, respectively.
Interband cascade (IC) lasers take advantage of the broken band-gap alignment in type-II quantum wells to
reuse injected electrons in cascade stages for photon generation with high-quantum efficiency, while retaining
interband transitions for photon emission without involving fast phonon scattering. Over the past several years,
significant progress has been made in developing efficient IC lasers, particularly in the 3-4 μm region where
continuous wave (cw) operation was achieved at above room temperature with low power consumption. In this
paper, we report our recent efforts in the development of IC lasers at longer wavelengths (4.3-7.5 μm) based on InAs
substrates and plasmon-waveguide structures. Cw operation of plasmon-waveguide IC lasers has been achieved at
temperatures up to 184 K near 6 μm. Also, improved thermal dissipation has been demonstrated with the use of the
plasmon waveguide structure.
Interband cascade (IC) infrared (IR) photodetectors (ICIPs) are a new type of detector that combines features of
conventional interband photodiodes with the discrete nature of quantum-well IR photodetectors (QWIPs) and IC
lasers. The operation of ICIPs takes advantage of fast intersubband relaxation and interband tunneling for carrier
transport, and relatively slow interband transitions (long lifetime) for photon generation. As such, ICIPs can be
tailored to optimize device performance for specific application requirements. We report our initial efforts in the
development of ICIPs. We have observed the photocurrent from an InAs-based IC laser with a cutoff wavelength
near 8 μm at 80 K, and significant photocurrent from GaSb-based ICIPs with cutoff wavelengths near 5 μm at 80 K
and 7 μm at above room temperature.
We have fabricated mid-wave infrared photodetectors containing InAsSb absorber regions and AlAsSb barriers in
n-barrier-n (nBn) and n-barrier-p (nBp) configurations, and characterized them by current-voltage, photocurrent, and
capacitance-voltage measurements in the 100-200 K temperature range. Efficient collection of photocurrent in the nBn
structure requires application of a small reverse bias resulting in a minimum dark current, while the nBp devices have
high responsivity at zero bias. When biasing both types of devices for equal dark currents, the nBn structure exhibits a
differential resistance significantly higher than the nBp, although the nBp device may be biased for arbitrarily low dark
current at the expense of much lower dynamic resistance. Capacitance-voltage measurements allow determination of the
electron concentration in the unintentionally-doped absorber material, and demonstrate the existence of an electron
accumulation layer at the absorber/barrier interface in the nBn device. Numerical simulations of idealized nBn devices
demonstrate that photocurrent collection is possible under conditions of minimal absorber region depletion, thereby
strongly suppressing depletion region Shockley-Read-Hall generation.
Optical time-domain reflectometry (OTDR) is an effective technique for locating faults in fiber communication links.
The fact that most OTDR measurements are performed manually is a significant drawback, because it makes them too
costly for use in many short-distance networks and too slow for use in military avionic platforms. Here we describe and
demonstrate an automated, low-cost, real-time approach to fault monitoring that can be achieved by integrating OTDR
functionality directly into VCSEL-based transceivers. This built-in test capability is straightforward to implement and
relevant to both multimode and single mode networks.
In-situ OTDR uses the transmitter VCSEL already present in data transceivers. Fault monitoring is performed by
emitting a brief optical pulse into the fiber and then turning the VCSEL off. If a fault exists, a portion of the optical
pulse returns to the transceiver after a time equal to the round-trip delay through the fiber. In multimode OTDR, the
signal is detected by an integrated photodetector, while in single mode OTDR the VCSEL itself can be used as a
detector. Modified driver electronics perform the measurement and analysis.
We demonstrate that VCSEL-based OTDR has sufficient sensitivity to determine the location of most faults commonly
seen in short-haul networks (i.e., the Fresnel reflections from improperly terminated fibers and scattering from
raggedly-broken fibers). Results are described for single mode and multimode experiments, at both 850 nm and 1.3 μm.
We discuss the resolution and sensitivity that have been achieved, as well as expected limitations for this novel
approach to network monitoring.
Improvements in the performance of InGaAsN quantum well VCSELs operating near 1300 nm are reported. The effects of alloy composition on the photoluminescence intensity, linewidth, and anneal-induced wavelength blueshift of molecular beam epitaxial InGaAsN quantum wells are detailed. VCSELs employing a conventional p-n diode structure are demonstrated and compared to devices using two n-type DBR mirrors and an internal tunnel diode. Room-temperature differential efficiencies as high as 0.24 W/A, output powers of 2.1 mW, and a maximum CW operating temperature as high as 105 degree(s)C have all been demonstrated in these devices.
We review our progress in the development of an optical interconnect technology consisting of optical and optoelectronic switches that integrate vertical-cavity surface-emitting lasers (VCSELs) with other photonic and electronic components, including heterojunction phototransistors (HPTs) and heterojunction bipolar transistors (HBTs). We describe a reconfigurable multi-access optical network architecture that allows many high speed electronic processors to simultaneously communicate with each other and with other shared resources, and for its implementation, an integrated optoelectronic switching technology that combines the functions of an optical transceiver and a spatial routing switch. The network provides parallel and dynamically reconfigurable optical interconnections between nodes, as well as optoelectronic interfaces to each processor. By converting data between the electrical and optical formats, these multi-functional switches can receive or transmit optical data, or to bypass and re-route it to another node. Optical switching has been demonstrated experimentally at a data rate of 200 Mb/s, and electrical-to-optical data conversion has been achieved at a data rate of > 500 Mb/s.
Picosecond pulses are used to measure the hot phonon generation rate ((partial)N<SUB>q</SUB>/(partial)t) of Raman active GaAs LO phonons in several GaAs/Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As superlattices (SL's) and quantum wells (QW's). Drastic increase in (partial)N<SUB>q</SUB>/(partial)t is observed as the barrier width (L<SUB>b</SUB>) decreased below a critical value for SL's with x >= 0.4. This is interpreted as due to a phonon transition from confinement to propagation. In contrast, for x <EQ 0.2 we do not observe this LO phonon transition. In this case the LO phonons only show a bulklike character regardless of the L<SUB>b</SUB>'s considered. We have also observed the existence of a critical x equals x<SUB>0</SUB> below which the LO phonons are no longer confined. Estimate of the LO phonon penetration depth into the barriers are also obtained for the different x values.