| dc.description.abstract | The InGaAs/InP single photon avalanche detectors (SPADs) detecting light wavelength of near infrared are suitable for the fiber-based communication. However, the III-V material system has experienced serious issues of defects due to the difficulty of epitaxy, so III-V based SPADs could bear poor dark count characteristics and significant afterpulsing. In this thesis, we present a study of the dark count rate for InGaAs/InP SPADs at different temperature ranges. Besides the thermal generation, SRH carriers, tunneling carriers and afterpulsing, we additionally address the charge persistence effect by the temperature-dependent dark count probability. This effect is further discussed and evidenced by the experiment of light injection.
In this study, for investigating dark count characteristics, the SPAD is operated using the gated mode. The temperature-dependent experiments are implemented by cooling the device down to 77K using liquid nitrogen. The dominant mechanisms at different temperature regions can be categorized as follows. First is the high temperature region, between 225K and 300K, in this region thermal generation and SRH carriers are the dominant sources of dark counts, which can be evidenced by extracting the value of activation energy. Second is the low temperature region, between 77K and 125K, in such region serious afterpulsing effect is observed if there is no sufficient hold-off time for releasing the trapped carriers. The final is the intermediate temperature region between high and low temperature region. There are extremely low dark counts for this region. It is known that the main source of dark counts originates from the tunneling current in such region. In the temperature-dependent dark count rate measurements, an abnormal peak occurs in the intermediate temperature region. In order to clarify the cause of abnormal peak, we further measure the temperature-dependent dark count rate under different dc bias and gate pulse height. It is observed that the curve of dark count rate shifts to the lower temperature with increasing dc bias. We attribute this observation to the charge persistence effect, which can be further evidenced by the following measurements under light injection.
To evidence the charge persistence effect observed under dark conditions, we illuminate the SPAD prior to the gate pulse. With varying the delay time between light pulse and gate pulse, the charge persistence effect can be investigated. The count rate is found to exponentially decay with the delay time. By fitting the curve with exponential decay formula, we can extract two time constants which are involved by two mechanisms. The literatures have addressed that a faster time constant is caused by the twilight effect and the slower time constant is due to the charge persistence effect. The count rate as a function of delay time shows that there is slight charge persistence effect at 300K, where the persistent charges are mainly contributed by thermal generation and SRH carriers besides the photo-generated carriers. At 200K, the dark carriers are effectively suppressed, therefore no obvious charge persistence is observed. At 150K, at which an abnormal peak occurs, serious charge persistence effect is observed. Comparing the results under dark and light conditions, it is found that at the temperature where higher dark count occurs the charge persistence effect is also distinct, which evidences our argument. | en_US |