InAs/GaSb superlattice light-emitting diodes are a promising technology for progressing the state-of-the art infrared scene projectors. By targeting a specific band of interest, they are able to achieve apparent temperatures greater than that of conventional resistor arrays and settling times on the order of nanoseconds. We report the fabrication of a dual-color infrared InAs/GaSb superlattice light-emitting diode array for operation in the mid-wave infrared. By stacking two superlattice structures back-to-back with a conductive layer separating them, independently operable, dual-color, cascaded InAs/GaSb superlattice light-emitting diodes were grown via molecular beam epitaxy on (100) GaSb substrates. At 77K, the emitted wavelengths are in the 3.2-4.2μm and 4.2-5.2μm range, with peak wavelengths at 3.81μm and 4.72μm. Using photolithography and wet etching, a 512×512 array of 48μm-pitch pixels were fabricated and hybridized to a silicon read-in integrated circuit. Test arrays with an 8×8 matrix of pixels demonstrated greater than 2 W/cm2˙sr for the 4.7μm emitter and greater than 5W/cm2˙sr for the 3.8μm emitter; the lower radiance in the long-wave emitter is due to a small active region volume left after fabrication. These respectively correspond to apparent temperatures greater than 1400K and 2000K in the 3-5μm band including fill factor.
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.
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.
Ga(In)Sb/InAs-based strained-layer superlattices (SLS) have received considerable attention recently for their potential
in infrared (IR) applications. These heterostructures create a type-II band alignment such that the conduction band of
InAs layer is lower than the valence band of Ga(In)Sb layer. By varying the thickness and composition of the constituent
materials, the bandgap of these SLS structures can be tailored to cover a wide range of the mid-wave and long-wave
infrared (MWIR and LWIR) absorption bands. Suppression of Auger recombination and reduction of tunneling current
can also be realized through careful design of the Type-II band structure.
The growth of high-quality Ga(In)Sb/InAs-based SLS epiwafers is challenging due to the complexity of growing a large
number of alternating thin layers with mixed group V elements. In this paper, the development of a manufacturable
growth process by molecular beam epitaxy (MBE) using a multi-wafer production reactor will be discussed. Various
techniques were used to analyze the quality of the epitaxial material. Structural properties were evaluated by
high-resolution x-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM). Optical
properties were assessed by low-temperature photoluminescence measurements (PL). Surface morphology and
roughness data as measured by Nomarski optical microscope and atomic force microscope (AFM) will be presented.
Device characteristics such as dynamic impedance, responsivity, quantum efficiency, and J-V characteristics of
photodiodes fabricated using our SLS epiwafers will be discussed.
We designed and fabricated 64x64 supper lattice light emitting diode (SLED) array with
peak emission wavelength of 3.8 micron. The light emission is observed from the bottom side of
the device through the substrate. The CMOS driver circuit is fabricated in the 130 nm IBM 8HP
SiGe process. The unit cells were designed to source up to 100mA to the LED. These unit cells
can be individually addressable, and have analog drive and memory that can operate at a 1 kHz
array refresh rate. We use supper lattice epitaxial active region LED structures grown on n-type
GaSb substrates. After initial mesa etching and contact metal deposition, the LED array is flip
chip mounted on the LCC package. The light emission is observed from the LED array by InSb
focal plane MWIR camera and the apparent black body temperature is measured.
Tight control of blood glucose levels has been shown to dramatically reduce the long-term complications of diabetes. Current invasive technology for monitoring glucose levels is effective but underutilized by people with diabetes because of the pain of repeated finger-sticks, the inconvenience of handling samples of blood, and the cost of reagent strips. A continuous glucose sensor coupled with an insulin delivery system could provide closed-loop glucose control without the need for discrete sampling or user intervention. We describe an optical glucose microsensor based on absorption spectroscopy in interstitial fluid that can potentially be implanted to provide continuous glucose readings. Light from a GaInAsSb LED in the 2.2-2.4 μm wavelength range is passed through a sample of interstitial fluid and a linear variable filter before being detected by an uncooled, 32-element GaInAsSb detector array. Spectral resolution is provided by the linear variable filter, which has a 10 nm band pass and a center wavelength that varies from 2.18-2.38 μm (4600-4200 cm-1) over the length of the detector array. The sensor assembly is a monolithic design requiring no coupling optics. In the present system, the LED running with 100 mA of drive current delivers 20 nW of power to each of the detector pixels, which have a noise-equivalent-power of 3 pW/Hz1/2. This is sufficient to provide a signal-to-noise ratio of 4500 Hz1/2 under detector-noise limited conditions. This signal-to-noise ratio corresponds to a spectral noise level less than 10 μAU for a five minute integration, which should be sufficient for sub-millimolar glucose detection.
In this study, we examine processes limiting the performance of 4 micron superlattice pin photodiodes for different temperature and mesa size regimes. We show that the performance of large mesa photodiodes at low temperature is most severely limited by a trap-assisted tunneling leakage current (x300), while small mesa sizes are additionally limited by perimeter leakage (x20). At room temperature, large mesa photodiodes are limited by the diffusion current, and small
mesa photodiodes are further limited by the perimeter leakage (x100). To reduce or eliminate the impact of perimeter leakage, we have tried passivating the mesa sidewalls with SiN, an approach that was only minimally successful. We have also laid the groundwork for another approach to elimination of perimeter leakage currents, namely, elimination of the sidewalls altogether through planar processing techniques. Planar processing schemes require the deposition of a
thick, wide bandgap semiconductor or "window layer" on top of the homojunction. We compare the performance of two otherwise identical InAs/GaSb superlattice homojunction detectors, except one with a GaSb window layer, and one without. We show that inclusion of the thick GaSb window layer does not degrade detector performance.
The performances of a pin versus a pn structure from GaInAsSb materials operating at room temperature are compared both from a theoretical point of view and experimentally. Theoretically, it is found in materials limited by generation-recombination currents, pn junctions have a higher D* than pin junctions. The thinner depletion region of pn junctions results in a lower responsivity but a higher dynamic resistance, giving an overall higher D* compared to a pin structure. A series of five p+pn+ Ga0.80In0.20As0.18Sb0.82 detector structures latticed matched to GaSb substrates and with 2.37 μm cut off wavelength were grown by molecular beam epitaxy and processed into variable size mesa photodiodes. Only the doping of the absorbing (p) region was varied from sample to sample, starting with nominally undoped (~1x1016 cm-3 pbackground doping due to native defects) and increasing the doping until a p+n+ structure was attained. Room temperature dynamic resistance-area product R0A was measured for each sample. A simple method is presented and used to disentangle perimeter from areal leakage currents. All five samples had comparable R0A's. Maximum measured R0A was 30 Ω-cm2 in the largest mesas. Extracted R0A's in the zero perimeter/area limit were about ~50 Ω-cm2 (20-100 Ω-cm2) for all samples. Within uncertainty, no clear trend was seen. Tentative explanations are proposed.
A focal plane array detector sensitive from 2.0-2.5 μm and consisting of 32, 1.0 mm x 50 μm pixels, all functional, is demonstrated. Mean room-temperature R0A is found to be 1.0 Ωcm2, limited by sidewall leakage. The focal plane array is fabricated from an MBE-grown homojunction p-i-n GaInAsSb grown on an n-type GaSb substrate. Back-illumination geometry is compared to front-illumination geometry and is found to be favorable, particularly the improved responsivity (1.3 A/W at 2.35 μm corresponding to 68% quantum efficiency) due to reflection of light off the metal contact. Further, back-illumination is the most convenient geometry for mounting the array onto a compact blood glucose sensor because both contacts can be mounted on one side, while detector illumination occurs on the other.
Tight control of blood glucose levels has been shown to dramatically reduce the long-term complications of diabetes. Current invasive technology for monitoring glucose levels is effective but underutilized by people with diabetes because of the pain of repeated finger-sticks and the cost of reagent strips. Optical sensing of glucose could potentially allow more frequent monitoring and tighter glucose control for people with diabetes. The key to a successful optical non-invasive measurement of glucose is the collection of an optical spectrum with a very high signal-to-noise-ratio in a spectral region with significant glucose absorption. Unfortunately, the optical throughput of skin is very small due to absorption and scattering. To overcome these difficulties, we have developed a high-brightness tunable laser system for measurements in the 2.0-2.5 μm wavelength range. The system is based on a 2.3 micron wavelength, strained quantum-well laser diode incorporating GaInAsSb wells and AlGaAsSb barrier and cladding layers. Wavelength control is provided by coupling the laser diode to an external cavity that includes an acousto-optic tunable filter. Tuning ranges of greater than 110 nm have been obtained. Because the tunable filter has no moving parts, scans can be completed very quickly, typically in less than 10 ms. We describe the performance of the laser system and its potential for use in a non-invasive glucose sensor.
We have performed in vivo measurements of near-infrared rat skin absorption in the 4000-5000 cm-1 spectral
range (2.0-2.5 μm wavelength) during a glucose clamp experiment. The goal of this work is to identify the presence
of glucose-specific spectral information in order to evaluate the requirements for a noninvasive transcutaneous
glucose instrument. Skin spectra are collected using an FTIR spectrometer coupled with a fiber-optic interface.
In the experiment, an animal is allowed to stabilize at a euglycemic level for three hours while blood glucose
values are monitored using samples taken from an arterial catheter. The blood glucose level is then increased
above 30 mM by venous infusion of glucose and held for two hours, after which it is allowed to return to normal.
Spectra are recorded continuously during the procedure and are analyzed to identify changes due to the glucose
variations. Because the change in absorbance due to an increase in glucose concentration is small compared
to changes due to other variations (e.g., the thickness of the skin sample), a simple subtraction of absorbance
spectra from the hyperglycemic and euglycemic phases is not instructive. Instead, a set of principal components
is determined from the euglycemic period where the glucose concentration is constant. We then examine the
change in absorbance during the hyperglycemic period that is orthogonal to these principal components. We find
that there are significant similarities between these orthogonal variations and the net analyte signal of glucose,
which suggests that glucose spectral information is present.
We describe on-line optical measurements of urea concentration during the regular hemodialysis treatment of several patients. The spectral measurements were performed in the effluent dialysate stream after the dialysis membrane using an FTIR spectrometer equipped with a flow-through cell. Spectra were recorded across the 5000-4000 cm-1 (2.0-2.5 micrometers at 1-minute intervals. Optically determined concentrations matched concentrations obtained from standard chemical assays with a root-mean-square error of 0.29 mM for urea (0.8 mg/dl urea nitrogen), 0.03 mM for creatinine, 0.11 mM for lactate, and 0.22 mM for glucose. The observed concentration ranges were 0-11 mM for urea, 0-0.35 mM for creatinine, 0-0.75 mM for lactate, and 9-12.5 mM for glucose.
The development of mid-infrared interband diode lasers has been hindered by factors such as Auger recombination and intervalence band absorption, which become increasingly important at longer wavelengths. A number of structures have been proposed in which the effects of these processes are reduced. The maximum gain per unit volumetric current density can be used as a figure of merit for comparing different active region materials. Using this figure of merit, we compare a series of structures with band gaps near 0.3 eV (i.e., wavelengths near 4 microns). The figure of merit is obtained from gain spectra calculated using superlattice K(DOT)p theory and a combination of calculated and measured recombination rates. We show that devices based on active regions incorporating type-I InAsSb/AlInAsSb or InAsSb/InAsP quantum wells should have room temperature threshold currents 7 - 13 times smaller than those of devices based on bulk InAs. However, devices using type-II superlattice active regions should have room temperature threshold currents that are a factor of 3 - 4 times smaller than those of the type-I quantum wells. The figure of merit can also be used to determine the optimal thickness of the active region as a function of waveguide loss and optical mode width.