Dennis T. Norton

Cascaded Superlattice InAs/GaSb Light-Emitting Diodes for Operation in the Long-Wave Infrared

Superlattice InAs/GaSb light-emitting diodes with peak emission wavelength of 8.6 μm and output power approaching 190 μW at 77 K from a 120 × 120 μm2 mesa are demonstrated. Output power in excess of 600 μ.W was demonstrated from a 520 × 520 μm mesa at 1 A drive current and 50% duty cycle. Devices were grown by molecular beam epitaxy on lightly n-doped GaSb substrates and employed a 16-stage cascaded active region configuration to improve current efficiency and increase optical output. Emitting regions were coupled by semi-metallic tunnel junctions consisting of a p-GaSb layer and a thickness-graded InAs/GaSb superlattice stack.

Comparison of tunnel junctions for cascaded InAs/GaSb superlattice light emitting diodes

Tunnel junctions in cascaded structures must provide adequate barriers to prevent carriers from leaking from one emission region to the next without first recombining radiatively, while at the same time remain low in tunneling resistance for current recycling. In this study, a variety of tunnel junction designs are compared in otherwise identical four stage InAs/GaSb superlattice light emitting diodes, which past studies have found hole confinement to be problematic. Here we used GaSb on the p-side of the junction, while varying materials on the n-side. The authors find Al0.20In0.80As0.73Sb0.27 tunnel junctions function best due to the low set of the conduction band; Ga0.75In0.25As0.23Sb0.77 also works well, though is more resistive due to a reduced set of the conduction band; and GaSb, while giving good hole confinement, results in a very resistive junction. Graded superlattice junctions can also work well, though they show sensitivity to doping levels, and present some challenges in growing strain-free.

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Single element 33×33 μm² InAs/GaSb superlattice light-emitting diodes (SLEDs) operating at 77 K with peak emission at approximately 4.6 μm are demonstrated. A peak radiance of 2.2 W/cm²/sr was measured corresponding to an apparent temperature greater than 1350 K within the 3-5 μm band. A 48 μm pitch, 512 × 512 individually addressable LED array was fabricated from a nominally identical SLED wafer, hybridized with a read-in integrated circuit, and tested. The array exhibited a pixel yield greater than 95%.

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Over the last decade, InAs/GaSb superlattice structures have become an increasingly important technology for infrared applications. 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. An 8 × 8 matrix of 48-μm pitch pixels was fabricated using standard photolithography and wet-etch techniques. At 77 K, the emitted wavelengths are in the 3.2-4.2- and 4.2-5.2-μm range, with peak wavelengths at 3.81 and 4.72 μm. In quasi-continuous operation, radiances in excess of 2 W/cm2 · sr from the longer wave and 5 W/cm2 · sr from the shorter wave emitters are achieved.

512 Individually Addressable MWIR LED Arrays Based on Type-II InAs/GaSb Superlattices

InAs/GaSb type-II superlattice light-emitting diodes were fabricated to form a device that provides emission over the entire 3–5 μm mid-infrared transmission window. Variable bandgap emission regions were coupled together using tunnel junctions to emit at peak wavelengths of 3.3 μm, 3.5 μm, 3.7 μm, 3.9 μm, 4.1 μm, 4.4 μm, 4.7 μm, and 5.0 μm. Cascading the structure recycles the electrons in each emission region to emit several wavelengths simultaneously. At high current densities, the light-emitting diode spectra broadened into a continuous, broadband spectrum that covered the entire mid-infrared band. When cooled to 77 K, radiances of over 1 W/cm2 sr were achieved, demonstrating apparent temperatures above 1000 K over the 3–5 μm band. InAs/GaSb type-II superlattices are capable of emitting from 3 μm to 30 μm, and the device design can be expanded to include longer emission wavelengths.

Dual-Color InAs/GaSb Cascaded Superlattice Light-Emitting Diodes

Here, the authors report on the occurrence, cause, and elimination of pyramidal defects in layers of GaSb grown by molecular beam epitaxy on GaSb substrates. These defects are typically 3–8 nm high, 1–3 μm in diameter, and shaped like pyramids. Their occurrence in the growth of GaSb buffer layers can propagate into subsequent layers such as GaSb, GaInAsSb, and GaSb/InAs superlattices. Defects are nucleated during the early stages of growth after the thermal desorption of native oxide from the GaSb substrate. These defects grow into pyramids due to a repulsive Ehrlich–Schwoebel potential on atomic step edges leading to an upward adatom current. The defects reduce in density with growth of GaSb. The insertion of a thin AlAsSb layer into the early stages of the GaSb buffer increases the rate of elimination of the defects, resulting in a smooth surface within 500 nm. The acceleration of defect reduction is due to the temporary interruption of step-flow growth induced by the AlAsSb layer. This leads to a reduc...

Broadband mid-infrared superlattice light-emitting diodes

GaSb-based, 6.1 A lattice-constant, infrared photodetector materials were grown on large diameter, 6-in. GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures, including bulk GaSb nucleation, AlAsSb superlattices, and graded GaAsSb ternary alloys, were investigated to bridge the 7.8% mismatch gap between the GaAs substrates and the GaSb-based epitaxial layers. Unique surface morphologies and crystal structure properties, as revealed by atomic force microscopy and cross-section transmission electron microscopy, pointed to different relaxation mechanisms for different buffer architectures. GaSb nucleation results in a more island-like surface morphology with a mix of 90° misfit and 60°-type threading dislocations, while the graded ternary buffer results in a cross-hatch surface morphology with effective filtering of the threading dislocations. Low root-mean-square roughness values of 5–20 A were obtained for this type of metamorphic epilayer growth. A generic InAsSb/AlAsSb nBn...

Causes and elimination of pyramidal defects in GaSb-based epitaxial layers

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.

MBE growth of GaSb-based photodetectors on 6-inch diameter GaAs substrates via select buffers

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.

MBE growth of Sb-based nBn photodetectors on large diameter GaAs substrates

Next-generation Infrared Focal Plane Arrays (IRFPAs) are demonstrating ever increasing frame rates, dynamic range, and format size, while moving to smaller pitch arrays.1 These improvements in IRFPA performance and array format have challenged the IRFPA test community to accurately and reliably test them in a Hardware-In-the-Loop environment utilizing Infrared Scene Projector (IRSP) systems. The rapidly-evolving IR seeker and sensor technology has, in some cases, surpassed the capabilities of existing IRSP technology. To meet the demands of future IRFPA testing, Santa Barbara Infrared Inc. is developing an Infrared Light Emitting Diode IRSP system. Design goals of the system include a peak radiance >2.0W/cm2/sr within the 3.0-5.0μm waveband, maximum frame rates >240Hz, and >4million pixels within a form factor supported by pixel pitches ≤32μm. This paper provides an overview of our current phase of development, system design considerations, and future development work.