Geoff R. Nash

Mid-infrared AlxIn1−xSb light-emitting diodes

The properties of AlxIn1−xSb light-emitting diodes (LEDs) have been investigated as a function of aluminum concentrations between 0% and 8.8%. By varying the aluminum concentration it is possible to tailor the peak emission wavelength to match the characteristic absorption of CO2, CO, CH4, NO, and NO2, making these diodes suitable for use in infrared gas sensing applications. The total emitted power and internal quantum efficiency were found to have maxima of 27mW∕cm2 and 4.2%, respectively, at a composition of 2.5%, where the peak emission was found to be 5.3μm, making LEDs of this composition particularly suited to the detection of NO.

Recombination processes in midinfrared AlxIn1−xSb light-emitting diodes

Emission characteristics, spectral properties, and quantum efficiencies of AlxIn1−xSb light-emitting diodes, with aluminum compositions between 0% and 8.75%, have been investigated as a function of temperature from 25 to 300 K, and a function of current from 1 to 100 mA. As both current and temperature are varied a change in the dominant recombination mechanism is observed as indicated by changes in the measured emission. An analysis of the light-current characteristics shows that Auger processes become important in all devices at temperatures above 100 K, implying an activation energy of approximately 7–13 meV depending on the aluminum composition.

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.

GaInSb/AlInSb multi-quantum-wells for mid-infrared lasers

Photoluminescence (PL) from GaInSb/AlInSb type I multi-quantum-wells, grown on GaAs, has been investigated as a function of strain in the quantum wells. Luminescence, between 3 and 4u2002μm, was observed for all samples, with good agreement between the measured and calculated peak emission energies. Analysis of the temperature dependence of the luminescence suggests that population of excited quantum well hole subbands occurs at high temperature, leading to a reduction in the PL signal. Room temperature luminescence was obtained from a sample with ∼0.8% strain in the quantum wells. Preliminary results from laser diodes fabricated from companion wafers indicate lasing up to 220 K.

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.

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.

Perspectives on Dynamic Infrared Scene Projection using Positive and Negative Luminescence

A dynamic infrared scene projector based on IR luminescent devices has many potential advantages compared with existing systems based on micro-resistor arrays. These include very fast response times, as individual devices can be driven at frequencies greater than 1 MHz, and no need for cryogenic cooling. Additionally, luminescent sources can not only appear hot to an IR observer when in forward bias, but also appear cold in reverse bias (commonly referred to as negative luminescence), so that a large apparent temperature range around ambient can be simulated. For a scene projector a large array of photodiodes is required, where each photodiode can be biased individually. As a precursor to the manufacture of a scene projector, we have already fabricated large area MW devices, consisting of arrays of photodiodes, suitable for use as calibration sources in IR cameras. To reduce the currents needed to achieve maximum dynamic temperature range, we have used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb devices. We present here recent results from a large area (~0.86cm2) medium wavelength (MW) device, consisting of an array of photodiodes each with an integrated optical concentrator. The reverse saturation current of the device was measured to be ~2.3A/cm2, which is significantly smaller than the value of ~9A/cm2 reported previously for similar devices without optical concentrators. The device also displays a large apparent temperature range in line with device modelling. Finally, we will discuss the perspectives on using similar devices for dynamic infrared scene projection.

Measurement of mid-infrared alinsb light-emitting diodes with surface patterning

3D FDTD modelling is employed to design a surface pattern for mid-IR LEDs. Measured enhancement factors over an un-patterned device of 8% and 14% are found at 300 K and 25 K respectively.

3D modelling of enhanced surface emission using surface roughening

3D FDTD is used to study the effect of surface roughening on the emission of a point source embedded in GaAs with a mirror behind the dipole. Enhancement factors of 10 : 1 are observed.

Slot-grating flat lens for telecom wavelengths

We present a stand-alone beam-focusing flat lens for use in the telecommunications wavelength range. Light incident on the back surface of the lens propagates through a subwavelength aperture and is heavily diffracted on exit and partially couples into a surface plasmon polariton and a surface wave propagating along the surface of the lens. Interference between the diffracted wave and re-emission from a grating patterned on the surface produces a highly collimated beam. We show for the first time a geometry at which a lens of this type can be used at telecommunication wavelengths (λ=1.55 μm) and identify the light coupling and re-emission mechanisms involved. Measured beam profile results at varying incident wavelengths show excellent agreement with Lumerical FDTD simulation results.