Advances in InAs/GaSb type-II superlattice light-emitting diode arrays

Russell J. Ricker

doi:10.17077/etd.005241

InAs/GaSb superlattice light-emitting diode (SLEDs) arrays have been developed for mid-infrared scene projection, and as high brightness mid-infrared sources. A high-resolution array of 1024x1024 pixels, a dual-color array of 512x512 pixels, and broadband single-element devices have been fabricated. The high-resolution arrays consist of cascaded 16-stage emitters with a 24μm pitch, which has been hybridized to a silicon RIIC. Periphery evaluation chips fabricated in tandem with the array on the same wafer provide a projected peak pixel radiance of 3.91W/cm2/sr peaking at 4.46μm, with an apparent temperature of 1800K in the 3-5μm wave band. Room temperature tests have been performed, indicating a yield exceeding 99%. Tests at 77K are to be performed next. A dual-color array of 512x512 pixels has been hybridized and packaged, and is being prepared for testing. The pixels were formed by growing two SLEDs in opposite orientation to each other, making the two colors independently operable; the top with 16 cascaded stages, and the bottom with 12 cascaded stages. Prototype devices had spectral peaks at 3.81μm and 4.72μm, with good separation on either side of the CO2 absorption band at 4.2μm. Peak pixel radiances of 2.6W/cm2/sr and 0.64W/cm2/sr were achieved for the short- and long-wavelength colors, which correspond to peak apparent temperatures of 1900K in the 3.2-4.2μm band and 1380K in the 4.2-5.2μm bands, respectively. Periphery evaluation chips fabricated concurrently with the array from the same wafer achieved spectral peaks at 4.13μm and 5.0μm, providing projected peak pixel radiances in the array of 1.38W/cm2/sr and 0.3W/cm2/sr, respectively. In the 3.7-4.5μm and 4.4-5.6μm wave bands, the projected peak apparent temperatures are 1810K and 1030K. Broadband devices were formed by cascading eight unique active regions, each designed to emit a distinct color, in a single SLED device. At low injection, the spectral peaks were easily discernible at 3.36µm, 3.61µm, 3.86µm, 4.18µm, 4.45µm, 4.81µm, and 5.33µm; as current density in the devices increased, the spectrum smoothed into a single continuous distribution with a peak located at 3.7μm, spanning the spectrum from 3μm to 6μm. Peak radiances of 1.04W/cm2/sr were achieved in 100μmx100μm SLEDs, with an apparent temperature in the 3-5μm band of 1100K. Five unique electrical contacts were investigated: 200Å Ti/300Å Pt/1000Å Au, 87Å Pd/560Å Ge/233Å Au/476Å Pt/1056Å Au, 100Å Ti/100Å Au/100Å Pt/1000Å Au, 100Å Ti/1000Å Au, and 500Å Au. Contacts containing gold without a diffusion barrier (Pt, Pd) between the gold and n-GaSb performed the best on 4-stage SLEDs, with Ti/Au performing the best, improving wallplug efficiency by a factor of three and external quantum efficiency by fifty percent over traditional Ti/Pt/Au contacts. No noticeable boosts to efficiency were observed by Ti/Au contacts on 16-stage SLEDs over traditional Ti/Pt/Au contacts. Two methods of surface roughening were investigated to improved extraction efficiency. The first method utilized a random pattern transferred via photolithography and Cl2 RIE-ICP etch to a SLEDs device thinned to 20μm. The second method was a maskless method randomized in a Cl2 RIE etch on a SLEDs device thinned to 20μm. The photolithographic method achieved a 33% increase in extraction efficiency, and the maskless method achieved a 100% increase in extraction efficiency over an unthinned, unpatterned SLEDs device. Some specular reflection still occurred, visible in optical modes in the spectral data, suggesting further optimization could be achieved.

Advances in InAs/GaSb type-II superlattice light-emitting diode arrays

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