SiC avalanche photodiodes (APDs) have the merits of visible light blindness, low weight, and high gain, exhibiting obvious advantages in weak ultraviolet detection. SiC APDs are mostly designed as pn junction structures, which can be divided into pin and nip APDs, according to the distribution of the epitaxial layers. When the SiC APD is conducting weak light detection, the device works under the avalanche state; the SiC pin and nip APDs exhibit completely opposite carrier drift directions. Holes in the pin APD drift toward the p-type layer, that is, the device surface, whereas the electrons drift toward the n-type layer, that is, the substrate; the nip APD exhibits the opposite. Cha et al. estimated the absorption coefficient of SiC at room temperature, and the results showed that SiC has a higher absorption coefficient for high-energy photons. Therefore, for the SiC pin and nip APDs, owing to the difference of the carrier drift direction, the types of carriers that cause collision ionization may be different for different wavelength detections . Considering that the collision ionization coefficient of holes in SiC is larger than that of electrons, different types of collision ionization carriers will certainly affect the detection performance of APDs. In this work, SiC nip APDs are designed and fabricated with a higher single photon detection efficiency for short wavelength ultraviolet light, benefitting from the hole dominated collision ionization process. This work is conducive for the in-depth understanding of the working mechanism of SiC APD and provides theoretical guidance for its optimization in the future.
The SiC nip APD is fabricated on an n-type 4H-SiC substrate (Fig. 1). The epitaxial structure is a 10 μm thick p-type contact layer (acceptor doping concentration NA=3×1018 cm-3), a 0.05 μm thick transition layer, a 0.7 μm thick n-type avalanche multiplication layer (donor doping concentration ND=1×1015 cm-3), a 0.2 μm thick n-type transition layer (ND=1×1018 cm-3), and a 0.15 μm thick n-type contact layer (ND=1×1019 cm-3) from bottom to top. The first step of APD fabrication is mesa etching. To suppress the peak electric field at the edge of mesa, the device is designed to be a beveled mesa structure (beveled angle of ～5°), and the etching depth is 0.5 μm. The epitaxial wafer is then passivated by the thermal oxidation layer and the plasma-enhanced chemical vapor deposition SiO2 layer. Finally, both the front and back contact metals (Ni/Ti/Al/Au) are deposited by electron-beam evaporation and annealed by rapid thermal annealing at 850 ℃ for 3 min in N2 ambience.
To study the avalanche mechanism of SiC nip APD, the gain curves of SiC nip APD are plotted (Fig. 3). When the wavelength of the incident ultraviolet light increases from 220 nm to 320 nm, the breakdown voltage of the nip APD increases by 1.3 V. When the over bias is 3 V, the dark count rate is 1.7 Hz/μm2, and the single photon detection efficiency of the SiC nip APD for 240 nm and 280 nm incident UV light is 11.4% and 6.5%, respectively (Fig. 5). In the same working conditions, the single photon detection efficiency of the SiC nip APD for short wavelength ultraviolet light is evidently higher. The quantum efficiency of the SiC nip APD under 240 nm and 280 nm incident UV light is 15% and 45%, respectively (Fig. 6). Assuming that the SiC nip APD operates at 3 V overbias, the estimated photon avalanche probability is 84% and 16% at 240 nm and 280 nm illumination, respectively. It can be observed that the high single photon detection efficiency of the SiC nip APD for high-energy ultraviolet light is mainly related to the large photon avalanche probability. When the 240 nm ultraviolet light is incident on the SiC nip APD surface, photogenerated carriers are generated on the device surface and enter the depletion region through diffusion. Under the avalanche state, when the hole enters the depletion region, it drifts towards the substrate under the strong electric field, and its acceleration distance and collision ionization process span the entire depletion region. When the electrons drift towards the device surface after entering the depletion region, the acceleration distance and collision ionization process can be ignored. Thus, at 240 nm illumination, the avalanche process of the SiC nip APD is dominated by holes. With the increase of the ultraviolet light wavelength, the contribution of the electrons gradually increases. The collision ionization coefficient of holes in SiC is larger than that of electrons; therefore, the hole-dominated collision ionization process exhibits a higher avalanche gain and higher single photon detection efficiency.
In this work, SiC nip APDs are designed, and their avalanche characteristics are studied in detail. Under high energy ultraviolet light illumination, the avalanche multiplication process of the SiC nip APD is dominated by holes. With the increase of the incident light wavelength, the contribution of the electrons gradually increases. As the collision ionization coefficient of holes in SiC is larger than that of electrons, the SiC nip APD exhibits a higher gain and higher single photon detection efficiency for high-energy ultraviolet light illumination. The essential characteristics of the SiC nip APD ensure that the device is more suitable for short wavelength ultraviolet detection.
