InAs nanowires directly integrated on Si platform show great promise in fabricating next generation mid-infrared optoelectronic devices. In this study we demonstrated the growth of catalyst-free, selective-area InAs nanowire arrays on electron beam patterned Si<sub>3</sub>N<sub>4</sub>/Si(111) by molecular beam epitaxy. Growth parameters were studied, and nanowire growth kinetics dependence on patterned mask opening diameter and interwire distance was investigated. Under certain growth conditions, nanowire diameter was found to be relatively independent of nanohole diameter and pitch. We also realized the growth of randomly-nucleated, self-assembled nanowires on Si(111) and investigated the temperature, flux influence on nanowire morphology.
Cascaded InAs/GaSb superlattice light emitting diodes are being developed for broadband, high radiance light sources for spectroscopy, and advanced technologies using large format arrays. Cascading is shown to allow broadening of the SLED spectral output, tuning of the electrical characteristics, and boosting of the maximum output power and efficiency. Wallplug efficiencies are found to be low, but internal quantum efficiency very high. It is anticipated that external quantum efficiency can be significantly increased by using strategies to help extract the light from these high index materials.
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/cm<sup>2</sup>˙sr for the 4.7μm emitter and greater than 5W/cm<sup>2</sup>˙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.
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.
The exciton binding energy in GaAs-based quantum-well (QW) structures is in the range of ~10 meV, which falls in the
THz regime. We have conducted a time-resolved study to observe the resonant interactions of strong narrowband THz
pulses with coherent excitons in QWs, where the THz radiation is tuned near the 1s-2p intraexciton transition and the
THz pulse duration (~3 ps) is comparable with the exciton dephasing time. The system of interest contains ten highquality
12-nm-wide GaAs QWs separated by 16-nm-wide Al 0.3Ga 0.7As barriers. The strong and narrowband THz pulses
were generated by two linearly-chirped and orthogonally-polarized optical pulses via type-II difference-frequency
generation in a 1-mm ZnTe crystal. The peak amplitude of the THz fields reached ~10 kV/cm. The strong THz fields
coupled the 1s and 2p exciton states, producing nonstationary dressed states. An ultrafast optical probe was employed to
observe the time-evolution of the dressed states of the 1s exciton level. The experimental observations show clear signs
of strong coupling between THz light and excitons and subsequent ultrafast dynamics of excitonic quantum coherence.
As a consequence, we demonstrate frequency conversion between optical and THz pulses induced by nonlinear
interactions of the THz pulses with excitons in semiconductor QWs.
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<sup>-1</sup>) 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/Hz<sup>1/2</sup>. This is sufficient to provide a signal-to-noise ratio of 4500 Hz<sup>1/2</sup> 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.
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+ Ga<sub>0.80</sub>In<sub>0.20</sub>As<sub>0.18</sub>Sb<sub>0.82</sub> 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 (~1x10<sup>16</sup> cm<sup>-3</sup> 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-cm<sup>2</sup> in the largest mesas. Extracted R0A's in the zero perimeter/area limit were about ~50 Ω-cm<sup>2</sup> (20-100 Ω-cm<sup>2</sup>) for all samples. Within uncertainty, no clear trend was seen. Tentative explanations are proposed.
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.
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 R<sub>0</sub>A is found to be 1.0 Ωcm<sup>2</sup>, limited by sidewall leakage. The focal plane array is fabricated from an MBE-grown homojunction <i>p-i-n </i>GaInAsSb grown on an <i>n</i>-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.
Erbium was introduce into GaAs/AlGaAs quantum well structures in the process of growth by MBE in an attempt to enhance semiconductor-Er transfer by means of a resonance between quantum well and Er ion transitions. Instead the quantum well was washed out by efficient interdiffusion of Ga and Al and diffusion of Er. We have demonstrated also that erbium interacts with aluminum in arsenides; this interaction leads to the formation of Er-containing Al- enriched clusters.