Research And Fabrication Of UV SAM 4H-SiC APDs With Low Breakdown Voltage And Its P-type Ohmic Contacts

Low-level and single photon ultraviolet (UV) signal detections are mainly used in laser-induced fluorescence biological-agent detection, non-line-of-sight covert communica- tions, non-destructive material analysis, high energy physics, optical time domain reflectometer, ultra-high sensitivity for air contamination detection, and so on. 4H-SiC APD is the only promising UV detector that satisfies low-level and single photon detection requirements, e.g. high quantum efficiency, low dark current, low excess noise, high speed and visible-blind operation.By far, 4H-SiC APDs based on PN or PIN structure have been reported, which can not effectively resolve the trade-off in the maximum achievable photoresponse, the faster response time and the operating voltage. The separated absorption and multiplication (SAM) 4H-SiC APDs have been reported, which showed a relative high breakdown voltage. In addition, on p-type 4H-SiC, metals usually form a Schottky contact with a large barrier height. Therefore, it's difficult to attain 4H-SiC p-type Ohmic contacts with low contact resistivity. Up to now, the relevant reports on 4H-SiC APDs have not been found in China.The purposes of this work are to fabricate a high-performance 4H-SiC APD and to achieve a good 4H-SiC p-type Ohmic contact. The achieved results are as follows:Firstly, according to the structure properties of APD and the 4H-SiC material properties, a SAM 4H-SiC APD with high spectral responsivity and low breakdown voltage was designed. The influences of various thicknesses of p+ epilayer and the depletion region on spectral response and time response have been theoretically analysed. Considering the factors of spectral response, time response, and breakdown voltage, the optimal parameters of SAM 4H-SiC APDs were determined.Secondly, the SAM 4H-SiC UV APDs with low breakdown voltage were fabricated and characterized successfully. The low breakdown voltage (Vb) of -55 V and the reach through voltage of -27.5 V were obtained from the reverse I-V characteristic. The dark current kept at 10 pA level from 0 V to the reach through voltage. At -50 V (90% Vb), the dark current was 60 nA and a high gain about 1.8×10⁴ was achieved. The typical spectral responsivitives and quantum efficiencies of APDs were measured at different reverse bias voltages ranging from 0 V to -35 V. At 0 V, the peak responsivity was calculated to be about 0.070 A/W at 270 nm, corresponding to a peak external quantum efficiency of 32.6%. To the best of our knowledge, it was the highest quantum efficiency achieved at low reverse bias at present. The UV-to-visible rejection ratio around three orders of magnitude was also extracted from the spectral response. Furthermore, at 0 V, the maximum spectral detectivity about 6.0×10¹³ cm Hz^1/2W^-1 and the corresponding noise equivalent power 3.75×10^-16 W were obtained. With the increasing reverse bias, the peaks of response wavelength shifted from 270 nm to 280 nm and the peak responsivity increased to 0.077 A/W, corresponding to the maximum external quantum efficiency of 35%. The results of spectral response also showed the spectral responsivities were enhanced within the whole UV range, especially in the range of 220 nm to 260 nm which might be due to the decrease of photogenerated carrier recombination in the p+ layer. In conclusion, the fabricated 4H-SiC APDs with low breakdown voltage have excellent performance for UV signal detection.Thirdly, in order to improve the performance of 4H-SiC p-type Ohmic contacts, four kinds of 4H-SiC p-type Ohmic contacts used Al-based multiple metals, including Ti(1200 (?))/Al(600 (?))/Au(850 (?)), Al(1200 (?))/Ti (600 (?))/Au(850 (?)),Ti(600 (?))/Al(1200 (?))/Au(850 (?)) and Al(600 (?))/Ni(600 (?)) /Al(400 (?) )/Au(850 (?)), were fabricated and characterized. The lowest specific contact resistance of 3.6×10-5 (?)cm2 was achieved by the linear transmission line method (LTLM) after Ti(600 (?))/Al(1200 (?))/Au(850 (?)) annealed at 930℃for 12 minutes, and the specific contact resistance of 4H-SiC/Ti(1200 (?))/Al(600 (?))/Au(850 (?)) Ohmic contact was about 4.2×10^-4 (?)cm². Furthermore, in order to reveal the mechanism of Ohmic behavior, the scanning electron microscope (SEM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and Panalytical X'pert PRO x-ray diffraction (XRD) were used to analyse Ohmic contact morphology, chemical composition and the phase formation of 4H-SiC/Ti(1200 (?))/Al(600 (?))/Au(850 (?)) sample before and after annealing. The results showed that the formation of TiC,TiAl₃ and Au(35 at%)+Ti(42 at%) occurred by annealing at 930℃improved the Ohmic behavior of the contacts.