Timothy Ashley

High-performance long-wavelength HgCdTe infrared detectors grownon silicon substrates

Long-wavelength HgCdTe heterostructures on silicon (100) substrates have been grown using metal-organic vapor phase epitaxy. Test diodes have been fabricated from this material using mesa technology and flip-chip bonding. We have demonstrated excellent resistance-area product characteristics for diodes with a 10.2μm cutoff wavelength. R0A values approaching 103Ωcm2 at 80K have been measured and the resistance-area product maintained above 102Ωcm2 at 1V reverse bias. Variable temperature R0A values correspond to expected generation-recombination loss mechanisms between 60 and 120K. Current-voltage characteristics of two diodes at opposite sides of an array indicate that a very uniform imaging long-wavelength infrared array could be fabricated from this material.

Photomultiplication with low excess noise factor in MWIR to optical fiber compatible wavelengths in cooled HgCdTe mesa diodes

Infrared avalanche diodes are key components in diverse applications such as eye-safe burst illumination imaging systems and quantum cryptography systems operating at telecommunications fiber wavelengths. HgCdTe is a mature infrared detector material tunable over all infrared wavelengths longer than ~850nm. HgCdTe has fundamental properties conducive to producing excellent detectors with low noise gain. The huge asymmetry between the conduction and valence bands in HgCdTe is a necessary starting point for producing impact ionization with low excess noise factor. Other factors in the band structure are also favorable. The low bandgap necessitates at least multi-stage thermoelectric cooling. Mesa diode structures with electron initiated multiplication have been designed for gains of up to around 100 at temperatures at or above 80K. Backside illuminated, flip-chip, test diode arrays have been fabricated by MOVPE using a process identical to that required for producing large imaging arrays. Test diode results have been obtained with the following parameters characterized, dark current vs. voltage and temperature, gain vs. voltage, and spectral response as a function of wavelength and bias. The effect of changing active region cadmium composition and active region doping is presented along with an assessment of some of the trade-offs between dark leakage current, gain, operating voltage and temperature of operation.

Lateral light emitting n-i-p diodes in InSb∕AlxIn1−xSb quantum wells

Lateral light emitting diodes have been fabricated in InSb∕AlxIn1−xSb quantum wells using a simple bevel etching technique. The peak in emission was found to be in the range of 4–5μm, confirming that the emission was from the quantum well.

InSb∕AlxIn1−xSb quantum-well light-emitting diodes with high internal quantum efficiencies

The properties of InSb∕AlxIn1−xSb quantum-well light-emitting diodes have been investigated as a function of temperature from 300to15K. Over the whole range, the peak emission occurred at significantly higher energies than the band gap of InSb but below the band gap of the AlxIn1−xSb barriers, confirming that emission is from the quantum wells. Maximum internal quantum efficiencies of 65% and 85% were measured at 15K for diodes containing 40 and 20nm quantum wells, respectively.

Integrated infrared detectors and readout circuits

The standard process for manufacturing mercury cadmium telluride (MCT) infrared focal plane arrays (FPAs) involves hybridising detectors onto a readout integrated circuit (ROIC). Wafer scale processing is used to fabricate both the detector arrays and the ROICs. The detectors are usually made by growing epitaxial MCT on to a suitable substrate, which is then diced and hybridised on to the ROIC. It is this hybridisation process that prevents true wafer scale production; if the MCT could be grown directly onto the ROIC, then wafer scale production of infrared FPAs could be achieved. In order to achieve this, a ROIC compatible with the growth process needs to be designed and fabricated and the growth and processing procedures modified to ensure survival of the ROIC. Medium waveband IR detector test structures have been fabricated with resistance area product of around 3x104 Ω cm2 at 77K. This is background limited in f/2 and demonstrates that wafer scale production is achievable.

Mid-infrared AlxIn1-xSb components for gas sensing

The performance of A1xIn1-xSb LEDs have been investigated for a number of aluminum concentrations between 0% and 8.8%. The devices were designed for use in CO2, CO, CH4, NO and NO2gas sensors since an increasing aluminium concentration produces shorter wavelength LEDs. The sensitivity of the NO gas sensing system was measured and a detection limit of 400ppm was found.

Magnetic Field Effects in InSb/AlxIn1−xSb Quantum-Well Light-Emitting Diodes

The spectral properties of InSb/AlxIn1−xSb quantum-well light-emitting diodes have been investigated at T = 15K in a magnetic field of approximately 250mT, applied by mounting the diodes inside a toroidal permanent magnet. In all cases, the low energy side of the spectra is shifted towards higher energy with the field applied.

Electroluminescence From InSb-Based Mid-Infrared Quantum Well Lasers

AlxGayIn1−x−ySb heterostructures suitable for laser applications have been grown on GaAs substrates. The structures have been analysed using X-ray diffraction and photoluminescence. Laser diodes have been fabricated by wet etching 20μm wide bars and cleaving into either 1mm or 2mm lengths. The electroluminescence of these bars has been investigated over a range of temperatures.