Ultraviolet (UV) region is very important as many biological agents (such as anthrax) are luminescent in UV. Scattering of short-wavelengths in atmosphere enables non-line-of-sight secure communications in rugged terrains whereas strong reflection/absorption of UV at ionosphere promises secure space-to-space communications. Where photomultiplier tubes are found to be bulky and fragile, and Si(C)-based photodiodes require external filter elements, our world’s highest performing UV GaN avalanche photodiodes (APDs) (gains of >51000, and external inherit quantum efficiency of 57%) can be employed. As we increased the performance of GaN APDs, newer applications emerge such as UV single photon detection. Via Geiger-mode operation, we have realized (world’s first) UV single photon detectors (SPDs) with single photon detection efficiencies as high as 20%, that could enable quantum computing and data encryption in near future. Compared to photomultiplier tubes or superconducting single photon detectors, the use of Geiger-mode APDs presents key advantages such as lower operation voltages, much reduced sizes, and no need for cooling to very low temperatures, which may enable the fabrication of more compact, lower power, and all-solid-state APD/CMOS integrated arrays.

The current ultraviolet detection technologies are compared in Figure 1. Compared to photocathodes and filtered Silicon detectors, AlGaN APDs achieve superior detection efficiency with a long lifetime. Lower ownership cost, robustness and no toxicity of the detectors make them both affordable and environmentally benign detection solution.

Figure 1: Comparison of Ultraviolet Detection Technologies

An avalanche photodiode (APD) is a semiconductor detector (a diode) composed of p-type layer (for p-contact), intrinsic (i-) layer (for photomultiplication), and n-type layer (for n-contact). The high electric field confined in the i-layer accelerates the photo-generated carriers. Once the photogenerated carriers are accelerated (energetic) enough and hit the lattice, they generate more carriers (electron and hole). The newly generated carriers follow the same pattern leading to an avalanche process. Thus, from one carrier, multiple carriers can be generated, which means "gain" in the photodiode.

Material quality is vital for APD performance. Any crystal defect in the material will lead to scattering of the carrier preventing its energy built-up for multiplication process. Thus, the avalanche gain is strictly dependent on material quality. Besides, defects are mainly non-radiative recombination centers and the significant leakage current source. This leakage current will ultimately limit the quality and limit of detection.

The doping quality and (doping and dopant) profile are other important qualities. The better the electric field is confined, the higher the field under the same voltage bias. Lower bias voltage means lower energy consumption and less leakage current. Thus, the lower bias operation of the APDs are dependent upon the doping quality (especially p-type doping for nitride material system).

World's first avalanche p-i-n photodiodes were fabricated on AlN templates for back illumination. Structures with different intrinsic layer thicknesses were tested. A critical electric field of 2.73  MV/cm was estimated from the variation of the breakdown voltage with thickness. From the device response under back and front illumination and the consequent selective injection of holes and electrons in the junction, ionization coefficients were obtained for GaN. The hole ionization coefficient was found to be higher than the electron ionization coefficient as predicted by theory. Excess multiplication noise factors were also calculated for back and front illumination, and indicated a higher noise contribution for electron injection.

Delta (−) doping is studied in order to achieve high quality p-type GaN. Atomic force microscopy, x-ray diffraction, photoluminescence, and Hall measurements are performed on the samples to optimize the -doping characteristics. The effect of annealing on the electrical, optical, and structural quality is also investigated for different -doping parameters. Optimized pulsing conditions result in layers with hole concentrations near 10¹⁸  cm⁻³ and superior crystal quality compared to conventional p-GaN. This material improvement is achieved thanks to the reduction in the Mg activation energy and self-compensation effects in -doped p-GaN.

Figure 2: (a) (Top bird's eye view) Scanning electron microscope micrograph of an APD, (b,c) Sketch of a Separate Absorption and Multiplication (SAM) GaN APD, and high inherit quantum efficiency p-i-n APDs.

High quality -doped p-GaN is used as a means of improving the performance of back-illuminated GaN avalanche photodiodes (APDs). Devices with -doped p-GaN show consistently lower leakage current and lower breakdown voltage than those with bulk p-GaN. APDs with -doped p-GaN also achieve a maximum multiplication gain of 5.1×10⁴, more than 50 times higher than that obtained in devices with bulk p-GaN. The better device performance of APDs with -doped p-GaN is attributed to the higher structural quality of the p-GaN layer achieved via -doping.

Back-illuminated avalanche photodiodes (APDs) composed of heterojunctions of either p-GaN/i-GaN/n-AlGaN or p-GaN/i-GaN/n-GaN/n-AlGaN were fabricated on AlN templates. At low voltage, an external quantum efficiency of 57% at 352 nm with a bandpass response was achieved by using AlGaN in the n-layer. Dependency of gain and leakage current on mesa area for these heterojunction APDs were studied. Back-illumination via different wavelength sources was used to demonstrate the advantages of hole-initiated multiplication in GaN APDs.