High-performance germanium quantum dot photodetectors: Response to continuous wave and pulsed excitation

Efficiency and response speed are key figures of merit for high-performance photodetectors, with high efficiency often obtained at the expense of slow photoresponse. Here, we report on germanium quantum dot photodetectors (Ge QD PDs) with a 25-nm-thick active layer that possesses both high internal quantum efficiency (IQE) and fast photoresponse, yet is still based on simple design and fabrication. We characterize these devices with continuous wave (CW) and pulsed excitation at room temperature as a function of incident power and applied bias. Under the reverse bias of –4 V, the IQE approaches ∼2000% over a broad spectral range (λ = 500–800 nm). The transient photoresponse speed to a 4.5 ns laser pulse at λ = 640 nm is under 20 ns. Furthermore, we observe an interesting phenomenon: by superimposing a weak CW HeNe laser beam ( λ = 632.8 nm) on the laser pulse, we obtain an optically tunable photoresponse while retaining fast speed. This study elucidates the role of photocarrier generation, trapping, and hopping in the percolative Ge QD oxide matrix and helps explain the observed high gain and fast response speed. The demonstrated IQE and nanosecond response time render our devices suitable for low-light detection and imaging.