T. Ashley

High-speed, low-power InSb transistors

High-speed, low-power consumption field-effect transistors fabricated from InSb/In/sub 1-x/Al/sub x/Sb are demonstrated. A 0.7 /spl mu/m gate-length enhancement-mode device shows an f/sub T/, of 74 GHz, and an f/sub max/ of 89 GHz, at a drain voltage below 0.5 V. This is the fastest reported transistor for its gate length, as far as is known.

Optical concentrators for light emitting diodes

The incorporation of non-imaging optical concentrations in uncooled mid-IR LEDs is described. Novel micromachining methods are used to produce optical concentrators in the growth substrate of epitaxial InSb/InAlSb heterostructures. Resultant large area LED arrays, displaying both positive and negative luminescence, are shown to have optical gains of 3.5 over conventional mesas made form the same material. The LED technology shown also relies on the micromachined substrate being transparent to 3-5 micrometers radiation and to act as a low resistance common contact. The use of degenerate doping in InSb is described, resulting in a shift in the room-temperature transmission to the 3-5micrometers atmospheric window and providing high electrical conductivities.

Epitaxial structures for reduced cooling of high-performance infrared detectors

IR detectors are normally cooled to 80K or below to obtain the highest, background limited performance. We present results for indium antimonide/indium aluminium antimonide and mercury cadmium telluride detectors grown by epitaxial processes in order to facilitate high performance with reduced cooling requirements. The epitaxial growth enables structures to be grown which offer precise control of carrier generation and current leakage mechanisms so that the maximum temperature can be achieve din a photodiode operated in a conventional manner, near zero bias. These types of structure offer even greater operating temperature when reverse biased to suppress non-radiative generation mechanisms. The epitaxial growth also has advantages for conventional, 80K operation, which are described.

Direct determination of Shockley-Read-Hall trap density in InSb/InAlSb detectors

Accurate determination of trap density in the active region of mid-infrared narrow-bandgap detectors is crucial in the development towards background-limited performance at higher operating temperatures. We have used both optical and electrical measurements to determine the trap density in InSb/InAlSb nonequilibrium detector structures. Both of these techniques result in very good agreement with trap densities of 5×1014xa0cm-3.

InSb focal plane array (FPAs) grown by molecular beam epitaxy (MBE)

Staring InSb FPAs grown by MBE have been demonstrated. Low growth temperatures have been employed to provide p+-n- n+ photodiodes with a dark, 80 K ROA equals 9 X 105(Omega) cm2. A degenerately doped substrate has been used to provide transparency in the 3.5 micrometer - 5.5 micrometer spectral region. Free carrier absorption necessitates some thinning of the substrate and an anti- reflection coated external quantum efficiency of 62% has been achieved with a final thickness of approximately equals 40 micrometer. 320 X 256 FPAs operating at 90 K and looking at a 295 K scene in f/2 have a noise equivalent temperature (NE(Delta) T) at half well of 10.4 mK. FPA operability exceeds 99.7%.

Optical modelling of cone concentrators for positive and negative IR emitters

InSb optical cone concentrators are modelled to assess their optical performance in positive and negative emission modes. The output distribution from the active layer is shown to be isotropic for thin active regions, tending towards Lambertian distribution as the layer thickness increases. Refraction from the emitting surface is shown to make the distribution more Lambertian. Optical efficiencies of straight-sided and Winston cone concentrators are modelled using a ray-tracing program, and those of straight-sided cones also determined using an analytical approximation, which allows use of the distribution derived earlier. The effect on the active layer thickness on the positive and negative emission is also determined. Normal light emitting diode structures are found to be poor positive emitters and to draw large currents when used as negative emitters. A Winston cone of area gain equal to n 2 is found to be the best option for devices required to work in both positive and negative modes. Intermediate structures are also considered. The optimum active layer thickness is derived for three emitter configurations.

4- to 10-μm positive and negative luminescent diodes

We describe uncooled mid-IR light emitting and negative luminescent diodes made form indium antimonide based III-V compounds, and long wavelength devices made from mercury cadmium telluride. The application of these devices to gas sensing, improved thermal imagers and imager testing is discussed.

Micromachined optical concentrators for IR LEDs

One of the most important factors limiting the optical efficiency of LEDs is total internal reflection of generated light, where photons incident to the surface at angles greater than the critical angle are reflected back into the semiconductor and absorbed. Most semiconductors have a large refractive index and hence a small critical angle. Narrow gap semiconductors, such as InSb, have particularly large refractive indexes and corresponding smaller critical angles. Additionally, strong absorption of light in the 3-5(mu) m range means that epoxy immersion lenses, which are used for GaAs Ir LEDs, cannot be used in InSb based IR LEDs. We have therefore used a novel micromachining technique to fabricate optical concentrators inInSb and HgCdTe layers. Inductively coupled plasma (ICP) etching is used to alternatively eatch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with parabolic profiles. Continuing optimization of the process to reach the theoretical limits of optical gain is described together with some of the main issues associated with the fabrication process.

Micromachined optical concentrators for IR negative-luminescent devices

Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as thermal radiation shields in IR cameras, and as IR sources in gas sensing systems. For many of these applications a large area (>1cm2) device is required, together with as large as possible apparent temperature range. However, under reverse bias significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. These may lead to current heating of the device, which in turn reduces the apparent temperature range. We have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. To fabricate the concentrators parabolic resist masks are first produced, which are approximately 10 μm high and approximately 53 μm wide, by resist reflow at 120 degrees C. Inductively coupled plasma (ICP) etching is then used to alternately etch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Currently, the concentrators are typically 30 μm high, with a top diameter of approximately 15 μm. Continuing optimization of the process to reach the theoretical limits of optical gain is described.