Semiconductor Far-Infrared/THz Upconversion Imaging Devices

In the visible and near-infrared (NIR) range (up to 1.1μm), the silicon charge-coupled devices (Si CCDs) are excellent and mature imaging sensors. Ultraviolet and even beyond can be covered by coating a CCD with appropriate luminescent materials. For the detection imaging of mid-infrared (MIR) light, the arrays based on InSb, InGaAs, HgCdTe or GaAs/AlGaAs quantum well photodetectors are always employed. In the far-infrared (FIR)/THz region, the scanning imaging and real-time imaging mehods are mainly used, but they are costly and only applied in special fields, such as military, astronomy, and so on. Up to now, few widely used and efficient imaging methods for FIR/THz radiation have been available. The heat detectors can be used to detect the FIR light, such as bolometer and pyroelectric, but they have long response time and high prices. Some sensitive semiconductor FIR/THz photodetectors realized these years, like GaAs FIR homojunction photodetector (HIWIP) and GaN/AlGaN heterojunction MIR/FIR dual-band photodetector (HEIWIP), provide a new way for implementing the low-cost and efficient FIR/THz imaging. Based on the concept of photon frequency upconversion, integrating a semiconductor FIR/THz photodetector with a short wavelength light emitting diode (LED), the emission wavelength of which falls in the efficient imaging range of Si CCD, we can carry out the semiconductor FIR/THz upconversion imaging. The FIR/THz imaging mechanism can be described as: under a certain bias, due to the reduction of the photodetector resistance upon FIR/THz radiation, one can expect the increase of the potential drop across the LED, the upconverted image of the FIR/THz object is realized by the short wavelength emission from LED directly imaged by the Si CCD.Integrating a GaAs HIWIP with a GaAs/AlGaAs NIR LED, we can have the GaAs-based semiconductor FIR/THz upconversion imaging device (HIWIP-LED). GaAs HIWIP is an important segment of the HIWIP-LED, the main structure of which is consisted of the multi-period GaAs emitter/intrinsic layers. Through the modulation of doping concentration in the emitter layers and the bias, the cutoff wavelength of GaAs HIWIP can be easily changed. At the same time, we have analyzed the free carrier absorption, hot carrier transportation, and collection process at the interfaces in the HIWIP, as well as the impact ionization process, and the spectral response under different doping concentrations and biases of the multi-period GaAs HIWIP.Upconversion quantum efficiency and image quality are two important performance parameters of the semiconductor FIR/THz upconversion imaging device. Start from the continuity equation of the HIWIP, in combination with the spectral decomposition approach, we can get the spatial distribution of the photocurrent reaching LED. Considering the photon recycling effect in the LED, and analyzing the carrier diffusion equation in the active layer, the output NIR spatial distribution can also be obtained. Based on the theoretical analysis, we have investigated the dependences of modulation transfer function and upconversion quantum efficiency on the device parameters of the HIWIP and LED, and optimized these parameters. The studies show that under the optimized upconverter structure, the FIR/THz upconversion imaging can be realized successfully, and good image quality can be expected.However, the upconversion efficiency of the HIWIP-LED is low, mainly due to the weak FIR absorption in the HIWIP and low photon extraction efficiency of LED. In addition, the photon recycling effect in the LED is harmful to the image quality. In order to improve the performance of GaAs HIWIP-LED, we employ the resonant cavity enhanced structure. Firstly, we use a GaAs HIWIP with a bottom mirror (HIWIP-BM), instead of the normal GaAs HIWIP. Under the resonant condition, the quantum efficiency of the GaAs HIWIP-BM is obviously higher than that of GaAs HIWIP, due to the enhanced light absorption with a bottom mirror. Through the analysis for the modulation transfer function of the FIR/THz imaging system, it is found that the bottom mirror would not influence the image quality, while improving the upconversion quantum efficiency. Secondly, we add a resonant cavity constructed by the top and bottom distributed Bragg reflectors (DBRs) to the GaAs/AlGaAs LED, the dependences of NIR photon extraction efficiency on the DBR parameters have been studied. The calculations show that the NIR photon will have an extraction efficiency of over 25%, when the distance between the active layer and the bottom DBR with 1 pair is 9 times of that between the active layer and the top DBR with 6 pairs. Due to the existence of resonant cavities, the upconversion efficiency of GaAs HIWIP-BM-RCE-LED increases to 5~6 times of that of HIWIP-LED. At the same time, the photon recycling effect has been reduced in the DBR cavity, so that the FIR/THz image quality has an obvious improvement. In addition, the NIR photons from LED have been blocked effectively to propagate over the HIWIP, thus the corsstalk in the FIR/THz upconversion imaging process is negligible.The recently realized single-period GaN/AlGaN HEIWIP MIR/FIR dual-band photodetector provides a new way to implement FIR/THz upconversion imaging. We have simulated the MIR and FIR spectral response of the single-period HEIWIP, through the transfer matrix method and effective mass theory, respectively, the theoretical results agree well with the experimental data. Based on the simulation, we have designed the multi-period GaN/AlGaN HEIWIP, which has better response performance in both MIR and FIR ranges. Integrating the optimized multi-period HEIWIP with a violet GaN/AlGaN LED, we can carry out the MIR/FIR dual-band upconversion imaging. Replace the normal LED with a DBR resonant cavity enhanced GaN/AlGaN LED, good image quality can be expected. At the same time, due to the high responsivity of multi-period GaN/AlGaN HEIWIP, the upconversion quantum efficiency of HEIWIP-LED is high.The proposed semiconductor FIR/THz upconversion imaging mesa can be fabricated by the conventional mesa preparation method. Using the natural carrier density distribution in the upconverter mesa, in combination with Si CCD, the FIR/THz upconversion imaging device can be directly made. Moreover, the upconversion image is obtained through the efficient and mature Si CCD, and no hybrid readout circuit is required. In the GaAs HIWIP-LED, the imaging wavelength of the FIR/THz radiation is easily modulated, due to the tunable cutoff wavelength of GaAs HIWIP. Therefore, the advantages of the semiconductor FIR/THz imaging devices are low-cost, simple, and wavelength-tunable. Due to the wide applications of FIR/THz detection imaging in various areas, such as space astronomy,infrared physics,spectroscopy,new materials,and so on, the semiconductor FIR/THz upconversion imaging devices have potential application values. This work is sponsored by the Natural Science Foundation of China under contract No. 60576067, Shanghai Rising-Star Program under contract No. 05QMH1411, as well as the National Minister of Education Program for ChangJiang Scholars and Innovative Research Team in University of IRT 0524.