We present our recent studies on a set of three different type-II InAs/GaSb superlattice interband cascade infrared (IR) photodetectors. Electroluminescence and x-ray diffraction measurements suggest that all the grown structures had comparable material qualities. Two of these detectors were two- and three-stage structures with regular-illumination configurations and the other was a two-stage structure with a reverse-illumination configuration. The 100% cutoff wavelength for these detectors was 6.2 μm at 78 K, extending to 8 μm at 300 K. At T=125 K and higher temperatures, we were able to observe the benefits of the three-stage detector over the two-stage device in terms of lower dark current and higher detectivity. We conjecture that the imperfections from the device growth and fabrication had a substantial effect on the low-temperature device performance and were responsible for the unexpected behavior at these temperatures. We also found that the zero-bias photoresponse increased with temperatures up to 200 K, which was indicative of efficient collection of photogenerated carriers at high temperatures. These detectors were able to operate at temperatures up to 340 K with a cutoff wavelength longer than 8 μm. This demonstrates the advantage of the interband cascade structures to achieve high-temperature operation for long-wave IR photodetectors.
The GaSb-based 6.1 Å lattice constant family of materials and heterostructures provides rich bandgap engineering possibilities and have received considerable attention for their potential and demonstrated performance in infrared (IR) detection and imaging applications. Mid-wave and long-wave IR photodetectors are progressing toward commercial manufacturing applications. To succeed, they must move from research laboratory settings to general semiconductor production, and high-quality GaSb-based epitaxial wafers with diameter larger than the current standard 3-inch are highly desirable. 4-inch GaSb substrates have been in production for a couple of years and are now commercially available. Recently, epi-ready GaSb substrates with diameter in excess of 6-inch were successfully produced. In this work, we report on the MBE (Molecular Beam Epitaxy) growth of generic MWIR bulk nBn photodetectors on 6-inch diameter GaSb substrates. The surface morphology, optical and structural quality of the epiwafers as evaluated by atomic force microscopy (AFM), Nomarski microscopy, low temperature photoluminescence (PL) spectroscopy, and high-resolution x-ray diffraction (XRD) will be discussed. Current density versus voltage (J-V) and photoresponsivity measurements from large-area mesa diode fabricated will also be reported. Material and device properties of these 6-inch epiwafers will be compared to similar structures grown on commercially available 4-inch diameter GaSb substrates.
We present recent studies on a set of three different long wave IR interband cascade infrared photodetectors with Type-II
InAs/GaSb absorbers. Two of these detectors were two- and three-stage structures with regular-illumination
configuration and the other was a two-stage structure with reverse-illumination configuration. The 100% cutoff
wavelength for these detectors was 6.2 μm at 78 K and extended to 8 μm at 300 K. At T=125 K and higher temperatures
we were able to observe the benefits of the three-stage detector over the two-stage device in terms of lower dark current
and higher detectivity. We conjecture that the imperfections from the device growth and fabrication had a substantial
effect on the low-temperature device performance and were responsible for unexpected behavior at these temperatures.
We also found that the zero-bias photo-response increased for temperatures up to 200 K, which was indicative of
efficient collection of photo-generated carriers at relatively high temperatures. Electroluminescence and X-ray
diffraction measurements suggest that all three grown structures had comparable material qualities. However, the twostage
detectors with the reverse-illumination had significantly lower performance than the other two detectors. The
activation energy for the two-stage detectors with the reverse-illumination was 37 meV for T=78-100 K, which was
much lower than the activation energies of the other two detectors (~140 meV). This low activation energy was
attributed to shunt leakage observed in detectors with the reverse-illumination configuration.
The GaSb-based family of materials and heterostructures provides rich bandgap engineering possibilities for a variety of infrared (IR) applications. Mid-wave and long-wave IR photodetectors are progressing toward commercial manufacturing applications, but to succeed they must move from research laboratory settings to general semiconductor production and they require larger diameter substrates than the current standard 2-inch and 3-inch GaSb. Substrate vendors are beginning production of 4-inch GaSb, but another alternative is growth on 6-inch GaAs substrates with appropriate metamorphic buffer layers. We have grown generic MWIR nBn photodetectors on large diameter, 6-inch GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures, including bulk GaSb nucleation, AlAsSb superlattices, and graded GaAsSb and InAlSb ternary alloys, were employed to bridge the 7.8% mismatch gap from the GaAs substrates to the GaSb-based epilayers at 6.1 Å lattice-constant and beyond. Reaching ~6.2 Å extends the nBn cutoff wavelength from 4.2 to <5 µm, thus broadening the application space. The metamorphic nBn epiwafers demonstrated unique surface morphologies and crystal properties, as revealed by AFM, high-resolution XRD, and cross-section TEM. GaSb nucleation resulted in island-like surface morphology while graded ternary buffers resulted in cross-hatched surface morphology, with low root-mean-square roughness values of ~10 Å obtained. XRD determined dislocation densities as low as 2 × 107 cm-2. Device mesas were fabricated and dark currents of 1 × 10-6 A/cm2 at 150K were measured. This work demonstrates a promising path to satisfy the increasing demand for even larger area focal plane array detectors in a commercial production environment.
Temperature-dependent minority carrier lifetimes of InAs/InAs1-xSbx type-II superlattices are presented. The longest lifetime at 11 K is 504 ± 40 ns and at 77 K is 412 ± 25 ns. Samples with long periods and small wave function overlaps have both non-radiative and radiative recombination mechanisms apparent, with comparable contributions from both near 77 K, and radiative recombination dominating at low temperatures. Samples with short periods and large wave function overlaps have radiative recombination dominating from 10 K until ~200 K. The improved lifetimes observed will enable long minority carrier lifetime superlattices to be designed for high quantum efficiency, low dark current infrared detectors.
Mega-pixel FPAs in both MWIR and LWIR spectral bands based on Sb strained layer superlattices and nBn epitaxial
structures grown on GaSb substrates have recently demonstrated impressive performances at high operating
temperatures. An essential component of SLS epitaxial growth initiation is the starting wafer flatness, smoothness and
haze. Large diameter GaSb wafers must be manufactured meeting these stringent demands and current state-of-the-art
GaSb substrate manufacturing is focused on 100mm wafer diameters. Using a newly developed polishing process,
100mm GaSb substrate manufacturing has resulted in consistent starting wafer peak-to-valley flatness well below 5μm
and surface roughness below Rms of 0.2nm. Final substrate and epitaxial wafer Surfscan mapping (<1000/cm2 surface
defects) and surface roughness (Rms~0.2nm) are presented and compared with measurements of the starting substrates.
This paper evaluates the manufacturing and epitaxial growth on 100mm GaSb substrates that have been processed to
achieve an MBE grown InAsSb-based nBn MWIR photodetector structure.
InAs/InAs1-xSbx strain-balanced superlattices (SLs) on GaSb are a viable alternative to the well-studied InAs/Ga1-xInxSb
SLs for mid- and long-wavelength infrared (MWIR and LWIR) laser and photodetector applications, but the InAs/InAs1-xSbx SLs are not as thoroughly investigated. Therefore, the valence band offset between InAs and InAs/InAs1-xSbx, a critical
parameter necessary to predict the SL bandgap, must be further examined to produce InAs/InAs1-xSbx SLs for devices
operational at MWIR and LWIR wavelengths. The effective bandgap energies of InAs/InAs1-xSbx SLs with x = 0.28 -
0.40 are designed using a three-band envelope function approximation model. Multiple 0.5 μm-thick SL samples are
grown by molecular beam epitaxy on GaSb substrates. Structural characterization using x-ray diffraction and atomic
force microscopy reveals excellent crystalline properties with SL zero-order peak full-width-half-maximums between 30
and 40 arcsec and 20 x 20 μm2 area root-mean-square roughnesses of 1.6 - 2.7 A. Photoluminescence (PL) spectra of
these samples cover 5 to 8 μm, and the band offset between InAs and InAs/InAs1-xSbx is obtained by fitting the PL peaks to
the calculated values. The bowing in the valence band is found to depend on the initial InAs/InSb valence band offset
and changes linearly with x as CEv_bowing = 1.58x - 0.62 eV when an InAs/InAs1-xSbx bandgap bowing parameter of 0.67 eV is
assumed. A fractional valence band offset, Qv = ΔEv/ΔEg, of 1.75 ± 0.03 is determined and is practically constant in the
composition range studied.
Antimony-based photodetector materials have attracted considerable interest for their potential and demonstrated
performance in infrared detection and imaging applications. Mid-wavelength infrared detector has been demonstrated
using bulk InAsSb/AlAsSb-based nBn structures. Heterostructures based on InAs/Ga(In)Sb strained layer superlattices
create a type-II band alignment that can be tailored to cover a wide range of the mid- and long-wavelength infrared
absorption bands by varying the thickness and composition of the constituent materials. Through careful design, these
Sb-based detectors can realize desirable features such as higher operating temperature, better uniformity, suppression of
Auger recombination, reduction of tunneling currents, and higher quantum efficiency. The manufacturing challenge of
these structures is the reproducible growth of high-quality Sb-based epiwafers due to their complex designs including
large numbers of alternating thin layers and mixed group-V elements. In this paper, we discuss the manufacturability of
such epiwafers on 3" and 4" diameter GaSb substrates by molecular beam epitaxy using multi-wafer production tools.
Various techniques were used to characterize the material properties of these wafers, including high-resolution x-ray
diffraction, low-temperature photoluminescence, Nomarski optical microscopy, and atomic force microscopy.
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 2.65×106 and 6.36×103Ωcm2 at 80 and 160 K, respectively.
The air quality of any manned spacecraft needs to be continuously monitored in order to safeguard the health of the
crew. Air quality monitoring grows in importance as mission duration increases. Due to the small size, low power draw,
and performance reliability, semiconductor laser-based instruments are viable candidates for this purpose. Achieving a
minimum instrument size requires lasers with emission wavelength coinciding with the absorption of the fundamental
absorption lines of the target gases, which are mostly in the 3.0-5.0 μm wavelength range. In this paper we report on our
progress developing high wall plug efficiency type-I quantum-well GaSb-based diode lasers operating at room
temperatures in the spectral region near 3.0-3.5 μm and quantum cascade (QC) lasers in the 4.0-5.0 μm range. These
lasers will enable the development of miniature, low-power laser spectrometers for environmental monitoring of the
spacecraft.
Laser-based metrology has been identified as an enabling technology in the deployment of large, spaceborne
observatories, where nanometer-level knowledge of fiducial displacement drives overall system performance. In
particular, Nd:YAG NPRO (non-planar ring oscillator) based lasers have received considerable attention in this
application because of their inherent high coherence at wavelengths of interest (1064 and 1319nm). However, the use of
NPRO based lasers in decade long space missions is limited by typical 800nm-band pump laser diode wearout and
random failure rates. Therefore, reliably achieving multi-hundred milliwatt NPRO power over prolonged mission
lifetimes requires innovative pump architectures. In this paper we present a pump architecture capable of supporting
continuous NPRO operation over 5.5yrs at 300mW with reliability exceeding 99.7%. The proposed architecture relies
on a low-loss, high port count, all-fiber optical coupler to combine the outputs of multiple single-mode pump laser
diodes. This coupler is capable of meeting the exacting environmental requirements placed by a space mission, such as
SIM Lite.
Interband cascade lasers are efficient and compact semiconductor mid-infrared (3-6 μm) light sources with low-power
consumptions. We report our recent progress in the development of interband cascade lasers with separate confinement
layers. Broad-area (0.1mmx1mm) lasers have been operated in cw mode at temperatures up to 213K near 3.36 μm. For
narrow ridge-waveguide (0.01mmx1.5mm) lasers, cw operation has been achieved at temperatures up to 266K near 3.43
μm, 260K near 3.7 μm, and 238K near 4.04 μm. The results on both broad-area and narrow-ridge IC lasers are
discussed in comparison with previous regular IC lasers without separate confinement layers.
We propose and have demonstrated a prototype high-reliability pump module for pumping a Non-Planar Ring Oscillator
(NPRO) laser suitable for space missions. The pump module consists of multiple fiber-coupled single-mode laser diodes
and a fiber array micro-lens array based fiber combiner. The reported Single-Mode laser diode combiner laser pump
module (LPM) provides a higher normalized brightness at the combined beam than multimode laser diode based LPMs.
A higher brightness from the pump source is essential for efficient NPRO laser pumping and leads to higher reliability
because higher efficiency requires a lower operating power for the laser diodes, which in turn increases the reliability
and lifetime of the laser diodes. Single-mode laser diodes with Fiber Bragg Grating (FBG) stabilized wavelength permit
the pump module to be operated without a thermal electric cooler (TEC) and this further improves the overall reliability
of the pump module. The single-mode laser diode LPM is scalable in terms of the number of pump diodes and is capable
of combining hundreds of fiber-coupled laser diodes. In the proof-of-concept demonstration, an e-beam written
diffractive micro lens array, a custom fiber array, commercial 808nm single mode laser diodes, and a custom NPRO
laser head are used. The reliability of the proposed LPM is discussed.
We demonstrate an electrically-tuned nematic liquid crystal (LC) infiltrated photonic crystal (PC) laser. The PC laser is encased between two transparent indium tin oxide (ITO) glass plates which serve as the modulating electrodes and also define the LC cell. Applying a voltage across the cell realigns the LC, modifies the laser cavity's optical path length, and blue-shifts the lasing wavelength. The measured tuning threshold voltage agrees well with the experimentally determined LC threshold voltage which confirms the tuning is due to the LC realignment at the onset of the LC's Freedericksz transition. Furthermore, the electrically-tuned PC laser also demonstrates the successful integration of nonlinear optical materials, electronics, and fluidics with PCs and suggests further integration with other materials will lead to photonic devices with increased functionality and utility.
We have investigated the miniaturization of photonic devices for ultimate photon localization, and have demonstrated two-dimensional photonic crystal nanolasers with two important quantum nanostructures-quantum wells (QWs) and quantum dots (QDs). Photonic crystal cavities with QW active material, are simple, but powerful nanolasers to produce intense laser output for signal processing. On the other hand, when located in a high-quality factor (Q) nanocavity, because QD(s) strongly couple with the intense optical field, QD photonic crystal cavities are expected to be good experimental setups to study cavity quantum electrodynamics, in addition to high speed and compact laser sources. Our photonic crystal nanolasers have shown as small thresholds as 0.12mW and 0.22mW for QD-photonic crystal lasers and QW-photonic crystal lasers, respectively, by proper cavity designs and nanofabrication. For QD-photonic crystal lasers, whispering gallery modes in square lattice were used together with coupled cavity designs and, for QW-photonic crystal lasers, quadrapole modes in triangular lattice with fractional edge dislocations were used to produce high-Q modes with small mode volume.
We have fabricated planar photonic crystal nanocavity lasers, based on our new high-quality factor design that incorporates fractional edge dislocations in triangular lattice photonic crystal cavities. Lasers with InGaAsP quantum well active material emitting at 1550nm were optically pumped with 10ns pulses, and lase at room temperature at threshold pumping powers below 220 microWatt. We have attributed this to the small mode volume and the high Q factors inherent to our device design. We have performed detailed numerical analysis of our structures, and have found an excellent agreement between theoretical predictions and experimental results. The optical field of the lasing mode in our nano-laser is localized in the air-hole region and therefore the laser can be used to investigate interaction between light and matter introduced in the cavity and nanospectroscopy.
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