| dc.description.abstract | InGaAs based Single-photon Avalanche Diode (SPAD) instead of Si SPAD is studied for the detection of light with wavelength longer than 1μm. It has great potential in the applications of medical electronics, fiber-optic communication, self-driving control systems and etc. The application of Light Detection and Ranging (Lidar) receives especial attention. In such application, SPAD can acts as a ToF sensor to accurately calculate the safe range of the car from the car in proximity, pedestrians and obstacles. InGaAs/InP based SPAD had been studied for a long time and is currently still active. Along with the multiplication layer using InP, the multiplication using InAlAs recently receives much interest due to its higher avalanche triggering probability, so a higher photon detection efficiency (PDE) is anticipated. This work focuses on the study of InGaAs/InAlAs based SPAD which is composed with absorption, grading, charge and multiplication layer. We use Silvaco technology computer-aided design (TCAD) to simulate the electric field strength and distribution for the determination of breakdown voltage (49 V) as well as punch-through voltage (20 V). In a typical SPAD design, the multiplication layer should be thick enough for reducing the tunneling current and hence suppressing the dark count rate (DCR). However, as the thickness of multiplication layer increases, the afterpulsing effect and the timing jitter will also get worse. Therefore, a unique design in the multiplication layer is proposed to compromise DCR, afterpulsing and jitter. The multiplication layer is separated into two layers with different electric field strength by an additional charge layer, which makes the effective avalanche region narrower, and hence improving the afterpulsing effect and the timing jitter.
SPAD devices are processed into mesa-type to define the active detection region. The sidewall of mesa is first treated by the sulphidation and protected by the passivation for reducing the leakage current. The anode and cathode are plated with p/n metal respectively and then wired bond to a PCB for subsequent measurement, including current-voltage characteristics, capacitance-voltage characteristics, dark count rate analysis, afterpulsing probability, photon detection efficiency, and jitter.
For the sidewall passivation, we chose BCB and PI which can effectively reduce the leakage current according to the literature. The breakdown voltage and punch-through voltage of our SPAD device are 42.7 V and 25 V at room temperature respectively. The temperature coefficient is 49 mV / K. The subsequent measurements are carried out for the SPAD device passivated by BCB since it has better performance in the leakage current and dark count rate. Under the operation condition of gating pulse width of 20 ns, the temperature of 187.5 K and the excess bias of 3 %, the PDE reaches 39 % and the dark count rate is 19 %. Since the dark count rate is really high at such long pulse width for the reason of serious afterpulsing effect, we further reduce the gating pulse width to 5 ns. The afterpulsing is thus greatly reduced to below 1 % while operating at 500 kHz either for room temperature or for 187.5 K. We obtain the best DCR of 0.5 % per gate, the PDE of 26 % and timing jitter is 72 ps at 187.5 K for the excess bias of 7 %. It is worthy to emphasize that our SPAD device can perform well at room temperature, with the DCR of 13 % and PDE of 18 %, under the excess bias of 7 %.
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