AlGaN provides filter-free detection

AlGaN has some unique material properties that are ideal for high-performance ultraviolet (UV) detectors. They include a high optical absorption coefficient and a direct bandgap that can be tuned from 200 to 365 nm. This allows the bandgap to be tailored to produce "solar blind" detectors that operate at wavelengths below 280 nm. These devices are "blind" to solar radiation - at such short wavelengths the Sun s emission is strongly absorbed by atmospheric ozone, and so it is relatively straightforward to detect the emission from artificial sources such as UV lamps in daylight.

A wider bandgap gives these solar-blind detectors a much lower thermal noise than those built using narrower bandgap material such as silicon. Additionally, the filters required by the narrower bandgap alternatives to reject solar wavelengths cause drawbacks in terms of extra cost, size and weight. AlGaN is also radiation hard, able to withstand high temperatures, and can be grown n-type or p-type by MOCVD or MBE onto various substrates, including those that are UV transparent. In addition, the material s quality is constantly improving through the enormous investment in AlGaN-based visible and UV emitters and RF electronic components.

These AlGaN detectors could be used for a wide range of tasks, including atmospheric ozone-level monitoring, UV astronomy, flame detection, combustion monitoring, medical imaging, missile warning, and biological agent detection and identification.

Prior to our efforts, there were only two reports of GaN-based hybrid focal-plane arrays (FPAs) - a 256 × 256 FPA fabricated by the NASA Goddard Space Flight Center, which employs GaN photoconductors, and a 32 × 32 FPA featuring a back-illuminated PIN photodiode built by Honeywell, North Carolina State University, and the US Army Night Vision and Electronic Sensors Directorate. In both cases a GaN absorber layer gave the FPA a cutoff wavelength of 365 nm, meaning that these detectors were visible blind but not solar blind.

At BAE Systems, Lexington, MA, we have developed the first solar-blind 256 × 256 AlGaN back-illuminated hybrid UV FPAs under a program sponsored by the Defense Advanced Research Projects Agency (DARPA). Our detector arrays, which are built using epiwafers from Cree, Emcore and the University of Texas at Austin (UT), have 30 μm × 30 μm unit cells, a photosensitivity in the 260-280 nm waveband, and a quantum efficiency of more than 50% at 270 nm. They are formed by attaching a back-illuminated AlGaN detector array to a matching silicon CMOS readout integrated circuit (ROIC) chip with indium-bump interconnects (see figure 1).

Grown on UV-transparent sapphire substrates, the detectors are based on the back-illuminated PIN heterojunction architecture reported by Honeywell. However, we have replaced the GaN absorber layer in the Honeywell design with a wider-gap Al0.47Ga0.53N layer that has the cut-off wavelength of 280 nm required for solar-blind detection (see figure 2), and the composition of the n-doped window layer has been switched from Al0.28Ga0.72N to Al0.6Ga0.4N to give a cut-on wavelength of 260 nm.

The increased aluminum content required to advance from visible-blind to solar-blind detectors has demanded significant AlGaN materials development. MOCVD growth conditions had to be established and optimized for growing crack-free layers of high-quality AlGaN on sapphire, including the development of an innovative buffer. In addition, an approach for producing the silicon-doped Al0.6Ga0.4N window layers with sufficiently high conductivity had to be developed. Both improvements had to be delivered while producing material capable of extremely low leakage currents and high quantum efficiencies when used in back-illuminated PIN photodiodes.

The AlGaN epiwafers were grown on 2 inch c-plane double-side-polished sapphire substrates by MOCVD and featured the basic back-illuminated PIN structure (see figure 2). The 256 × 256 arrays were fabricated using a four-mask process, with mesa etching carried out by inductively coupled plasma chlorine-based dry etching at Boston University Photonics Center.

Material from all three suppliers produced photodiodes with extremely low leakage currents and correspondingly high resistances at zero bias voltage (see figure 3). High resistance is important because the thermal noise is inversely proportional to the square root of the zero-bias resistance. The leakage current at near-zero bias voltage is too small for us to measure in our laboratory, and a reverse bias of several volts is required to raise this current above the probe station s noise floor of 1 × 10-13 A. Under forward bias, the current increases exponentially over three-to-four orders of magnitude and is limited by series resistance at higher voltages.

Our photodiodes have high quantum efficiencies. Figure 4 shows a typical response for a UT photodiode with a 48% peak quantum efficiency at zero bias. None of the devices featured any antireflection coating on the back surface of the sapphire substrate, and without the reflection loss at the air-sapphire interface the UT diode s quantum efficiency would have been 52.3%.

Quantum efficiency increased at a small reverse-bias voltage, such as 5 V. Analytical modeling and numerical simulations suggest that this increase is caused by incomplete depletion of the unintentionally doped absorber layer at zero bias. This allows rapid recombination of photocarriers in the undepleted part of this layer, which is located next to the higher bandgap window layer. A reverse-bias voltage pushes the edge of this depletion region towards the window layer and increases the number of photocarriers collected.

The best 256 × 256 AlGaN photodiode arrays were attached to low-noise 256 × 256 ROIC chips at BAE Systems that were specifically designed to match the ultra-high-resistance low-noise AlGaN photodiodes. The UV response, dynamic resistance and noise in selected FPAs was then measured for each of the 65,536 pixels (see "The best 256 à— 256"). There was a high degree of uniformity of the pixel s performance within the arrays (see figure 5), and their ability to generate UV images of objects is shown in figure 6.

These data show that high-sensitivity solar-blind hybrid FPAs with high uniformity can be realized with AlGaN PIN photodiode arrays grown by MOCVD on sapphire. Material improvements to AlGaN layers incorporating a high aluminum fraction, which includes strain management through innovative buffer layers, have driven record quantum efficiencies at cut-off wavelengths as short as 280 nm. These advances are a solid basis to explore more ambitious AlGaN detectors and arrays, such as devices with avalanche gain with the potential for single-photon counting.

This work was funded by the DARPA Solar Blind Detector Program, through an Office of Naval Research contract. The work at Cree was funded by the Air Force Research Laboratory. We thank our collaborators Milan Pophristic, Shiping Guo, Boris Peres and Ian Ferguson at Emcore; Rajwinder Singh and Charles Eddy at Boston University; Uttiya Chowdhury, Michael Wong and Russell Dupuis at UT; and Ting Li and Steven DenBaars at Cree.

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