While silicon carbide (SiC) is an ideal material for building ultraviolet (UV) photodetectors, the absorbed photons get recombined in the first few nanometers at the surface due to a large absorption coefficient in the 200- to 250-nm wavelength band. This leads to a significantly reduced quantum efficiency near 200 nm. The goal of this work is to design a structure that would increase the responsivity of SiC avalanche photodetectors (APDs) near the 200-nm wavelength.
The key problem is the very small absorption depth of 200-nm photons in SiC. The 200-nm photons typically penetrate only 10-20 nm inside SiC. In this region, there are a large number of surface traps that cause very high recombination. Therefore, the generated electron-hole pairs recombine quickly and cannot navigate to a high field region in the APD. This reduces the responsivity of the photodetector.
Historically, SiC photodetectors have been made with multiple epilayers on a base substrate. These epilayers allow the application of a large bias and creation of a large field for avalanche multiplication of photogenerated carriers. Further, to reduce the surface recombination, passivation of the surface is carried out to reduce the surface traps. While passivating the surface does help somewhat, it does not eliminate the problem of recombination, and has not enabled good quantum efficiencies near 200 nm.
An epilayer stack in 4H-SiC with a shallow surface implantation layer was developed that should allow increase of the responsivity to deep UV. The APDs are then processed using a variety of semiconductor processing techniques including beveling, oxidation, contact etching, and metal deposition. A comprehensive process was designed for building SiC UV photodetectors (PDs) and APDs. In order to achieve this fabrication, a unique process was developed that includes special utilization of a photoresist reflow method prior to beveling. Detailed oxidation methods were developed to passivate the beveled sidewall surface involving two-step, sacrificial, high-temperature oxidation. This is then followed by an etch and a thermal oxidation, an oxide PECVD, and ultimately densification. Silicide-based contacts for low-resistivity ohmic connections to SiC were developed, as well as precise anti-reflection coating by quantified etching.
SiC was identified as the material from which to fabricate these devices because of several unique material characteristics. It is a wide-bandgap semiconductor that absorbs UV, but is transparent to visible light. It has very low intrinsic carrier concentration, and thus the SiC-based APD can have very low dark current and related noise. SiC has the ability to thermally grow an oxide, thus greatly facilitating process development.
The surface doping is gaussian, with its peak right at the surface of the SiC, and extends approximately 200 nm into the device. This n-type doping creates a small surface field that pushes photogenerated electrons away from the surface and into the device. This reduces the time the photogenerated electrons and holes spend in the surface region, and therefore reduces the probability of them recombining through the surface states. This reduction in surface recombination will allow more electrons to reach the high-field avalanche region of the APD, and thereby contribute to the quantum efficiency.
A high-voltage source is required to run the detector. The APD was designed to break down near 200V. Further, a fast oscilloscope or a fast readout circuit is required to measure the number of avalanche events per second, and thereby record the photon count.