GaN/AlGaN-based UV photodetectors with performances exceeding the PMTS
The recent developments in high Al-content AlxGa1−xN material growth technology made it possible to fabricate high performance solar-blind photodetectors operating in the ultraviolet (UV) spectral region with improved receiver sensitivity, low noise, low dark current density, and high speed. AlGaN-based Schottky, p-i-n, and metal-semiconductor-metal photodetectors (MSM) with very high performances have already been demonstrated. The UVfiltering nature of the atmospheric ozone molecules blocks the solar radiation to reach the earth’s surface for wavelengths shorter than 280 nm. In this case, UV photodetectors with cutoff wavelengths around 280 nm, which are also called solarblind detectors, can detect very weak UV signals under intense background radiation. These devices have important applications including missile plume detection, chemical/biological agent sensing, flame alarms, covert space-tospace and submarine communications, ozone-layer monitoring, and gas detection. Due to their high responsivity (600 A/W), high speed, high cathode gain (on the order of a million), and low dark current properties, photomultiplier tubes (PMTs) are frequently used in such applications. However, PMTs are very expensive and bulky. Besides, they require a cooling system, and they have high operation voltages in excess of 1000 V. To achieve solar-blind detection, PMTs should also be integrated with complex and expensive filters. In order to avoid these disadvantages, high performance solid-state UV photodetectors with high internal gain are needed. Wide band-gap semiconductor photodetectors, such as AlxGa1−xN with x=0.4, are ideal candidates for this purpose. These devices are intrinsically solar blind, in which no additional filters are needed, they have low noise, and fast response times. The lack of high internal gain has been the major limitation for the usage of AlGaN photodetectors for applications that require high sensitivity detectors. There have been several theoretical research works that examined the avalanche effect in GaN and AlGaN-based structures. However, reproducible high gain in AlGaN-based APDs is still a major limitation. We have designed, fabricated, GaN/AlGaN based photodetectors, and according to characterization measurements, the Schottky, p-i-n, and avalanche detectors have high performance in terms of quantum efficiency, dark current, detectivity, high speed response, and high reproducible avalanche gain.