Quantum dot infrared photodetectors (QDIPs) have attracted significant attention due to selective photoresponse, high photoconductive gain, and numerous possibilities for nanoscale engineering of photoelectron processes, which control the detector characteristics. Our approach to improving QDIP performance is based on optimization of three dimensional nanoscale potential profile created by charged quantum dots (QDs). Nanoscale profile around QDs allows us to control photoelectron capture processes, which determines the photoelectron lifetime, detector operating speed, responsivity, the spectral density of noise, noise bandwidth, and the detector dynamic range. The nanoscale potential profile is determined by doping of QDs and inter-dot space. In this work, we study various ways of selective doping and its effects on characteristics of photodetectors. We investigate and compare intra-dot doping, inter-dot doping, and complex bipolar doping. To investigate effects of selective doping, we fabricated AlGaAs/InAs QD structures with n-doping of QD layers, structures with n-doping of barriers, and structures with p-doping of QD layers and n-doping of interdot space. We measured dark current, spectral photoresponse, voltage dependence of responsivity, and noise characteristic. The photoresponse is improved due to photon-electron coupling, which increases with QD filling by electrons. However, the noise current also increases due to increase in QD filling. Therefore, possibilities for improvement of QDIP structures with unipolar doping are very limited. Our results show that spectral photoresponse, responsivity, and detector sensitivity are substantially improved due to bipolar doping, which provides decoupled control of electron filling of QDs and the potential barriers around QDs.
We designed, fabricated, and characterized multi-color IR photodetectors with asymmetrical doping of GaAs/AlGaAs double quantum wells (DQW). We measured and analyzed spectral and noise characteristics to evaluate feasibility of these photodetectors for remote temperature sensing at liquid nitrogen temperatures. The bias voltage controls the charge distribution between the two wells in a DQW unit and provides effective tuning of IR induced electron transitions. We have found that the responsivity of our devices is symmetrical and weakly dependent on the bias voltage because the doping asymmetry compensates the effect of dopant migration in the growth direction. At the same time, the asymmetrical doping strongly enhances the selectivity and tunability of spectral characteristics by bias voltage. Multicolor detection of our QWIP is realized by varying the bias voltage. Maximum detection wavelength moves from 7.5 μm to 11.1 μm by switching applied bias from -5 V to 4 V. Modeling shows significant dependence of the photocurrent ratio on the object temperature regardless of its emissivity and geometrical factors. We also experimentally investigated the feasibility of our devices for remote temperature sensing by measuring the photocurrent as a response to blackbody radiation with the temperature from 300°C to 1000°C in the range of bias voltages from -5 V to 5 V. The agreement between modelling and experimental results demonstrates that our QWIP based on asymmetrically doped GaAs/AlGaAs DQW nanomaterial is capable of remote temperature sensing. By optimizing the physical design and varying the doping level of quantum wells, we can generalize this approach to higher temperature measurements. In addition, continuous variation of bias voltage provides fast collection of large amounts of photocurrent data at various biases and improves the accuracy of remote temperature measurements via appropriate algorithm of signal processing.