Avalanche photodiodes based on wide-band-gap semiconductors are of special interest when there is a need for reliable ultraviolet (UV) detection with single photon counting capabilities. Materials such as SiC, (Al)GaN, or diamond present optoelectrical properties with intrinsic advantages for visible-blind UV detection, potentially outperforming other narrow-band-gap counterparts that require extensive filtering. In particular, the tunable response of AlGaN detectors allows us to accomplish solar- to visible-blind performances with the same material system without the need of filters, as well as to match specific bands of biological interest within the 200–360  nm range.

There is a need for single photon detectors for a variety of scientific, military, and civilian applications including free-space optical communications, quantum computing, environment monitoring, astrophysics, or biological agent detection.

Compared to photomultiplier tubes or superconducting single photon detectors, the use of Geiger-mode avalanche photodiodes (APDs) presents some 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.

Geiger-mode operation under gated quenching has been previously demonstrated in front-illuminated GaN APDs with a single photon detection efficiency (SPDE) of 13% at a dark count rate of 400  kHz in devices with an area of 1075  µm.However, two of the major problems with GaN APDs are the rapid increase of the dark current with area and the consequent limitation of the maximum achievable gain, which have prevented the operation of larger area devices in Geiger mode. In contrast, SiC devices have shown a low dark count rate of 28  kHz for 7854  µm² devices but have done this with a lower SPDE of only 3.6%

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).

Geiger-mode operation requires a hard but reproducible breakdown. Thus, sharp interfaces are required with a good device design to ensure fast operability.

The device structure consists of a GaN p-i-n homojunction grown on top of a transparent AlN template. The AlN is grown on a double-side polished sapphire substrate with a low-temperature AlN buffer to allow back illumination. The active region consists of p-type GaN:Mg, unintentionally doped GaN, and n-type GaN:Si layers with thicknesses of 285, 200, and 200  nm, respectively. Capacitance-voltage measurements yielded a donor concentration of ~2×10¹⁸  cm⁻³ in the n-type material and a residual concentration of ~2.5×10¹⁶  cm⁻³ in the intrinsic layer. For the p-type GaN layer, hole concentrations of (1–3)×10¹⁸  cm⁻³ were determined by Hall-effect measurements of test samples.

Figure 1: Single Photon Detector Designs: (a) SAM APD design, (b) P-I-N APD design and SEM view of fabricated device. Inset shows the Geiger-mode operation circuitry

World's first back-illuminated GaN APDs operating in Geiger mode have been presented. These devices showed a flat Geiger-mode response for photon energies above the band gap and a high visible-light rejection ratio. Single photon counting was demonstrated in devices ranging from 225 all the way up to 14063  µm². In the smallest device, SPDE of 20% and the dark count rate <10  kHz were obtained.