Influence of delta-doping on the performance of Ge/Si quantum-dot mid-infrared photodetectors

Abstract

The effect of delta-doping on the performance of ten-period Ge/Si quantum-dot (QD) mid-infrared photodetectors (λmax≃3.4 μm) was investigated. Ge QDs fabricated by molecular-beam epitaxy at 500 °C are overgrown with Si at 600 °C. Each Si barrier contains a boron delta-doping layer located near the QD plane to provide holes to the dots. Within the sample set, we examined devices with different positions of the δ-doping layer with respect to the QD plane, different distances between the δ-doping layer and the QD plane, and different doping densities. All detectors show pronounced photovoltaic behavior implying the presence of an internal inversion asymmetry. We observed a reversal of the voltage dependence of responsivity with respect to zero bias when the δ-doping plane is carried from the bottom to the top of the dot layer. This result indicates that the main reason for the asymmetric photoresponse is the existence of a built-in electric field due to the placing dopants in the barriers. Devices with a lower doping density (pB=4×1011 cm−2) or with a shorter distance between the doping layer and QDs (d = 2 nm) are found to operate better in a photoconductive mode with the highest peak detectivity of about 6×1010 cm Hz1/2/W at T = 90 K and 0.2 V bias. The best performance is achieved for the device with pB=12×1011 cm−2 and d = 5 nm in a photovoltaic regime. At a sample temperature of 90 K and no applied bias, a responsivity of 0.83 mA/W and detectivity of 8×1010 cm Hz1/2/W at λ=3.4 μm were measured under normal incidence infrared radiation.