Charge Accumulation Dynamics in Mid-Gap States of Nanocrystals with a Photoresponse Modulation

Colloidal nanocrystals with a narrow band gap offer a low-cost infrared photodetection platform but often suffer from surface defects due to their incomplete surface passivation. Such defects can generate mid-gap trap states and reduce the carrier transport. Thus, revealing the trap-mediated transport could be important to attaining a fast photodetection with the suppression of carrier quenching. Here, charge accumulation in mercury telluride (HgTe) nanocrystals is studied to elucidate the effect of trapped carriers on the photodetection speed. We show that the photoresponse time in a nanocrystal film depends on the applied alternating voltage frequency due to the trapping timescale. Under a continuous applied voltage, the collection of photoexcited carriers is delayed by state filling, while high-power excitation highly occupies the trap states and improves the response time. The trapping process can be overcome by a high-frequency voltage modulation due to the insufficient transport time for trapped photocarriers, leading to a fast response lifetime of 10 mu s. Capacitance measurements using electrochemical impedance spectroscopy reveal that the charge accumulation in the mid-gap states determines the frequency-dependent carrier transport. We further show that the energy level of the mid-gap states can be estimated using ultraviolet photoemission spectroscopy and Mott-Schottky analysis.