The study and development of superlattice infrared photodetectors for high temperature and low bias operation

• Main Authors: Jen-Hsiang Lu, 盧仁祥
• Other Authors: 管傑雄
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/24461175712717811831

博士 === 國立臺灣大學 === 電子工程學研究所 === 95 === The large scale focal plane array (FPA) imaging systems based on quantum well infrared photodetectors (QWIPs) have shown the potential use for military, medical and civil applications. However, the drawback of QWIPs under high temperature operation is the high dark current. In this dissertation, the aim is to design the superlattice infrared photodetectors (SLIPs) which are suitable for high temperature (80 K) and low bias operation with the enhanced photocurrent and the lower dark current. The current magnitude can be lowed if we operate the device under low bias range. Due to the low operational bias of SL, we choose superlattice (SL) as the active region of photodetectors. At first, we study the temperature dependence of photoresponse of a single-barrier SLIP whose structure is a 15-period SL with a single barrier to know how to optimize its structure parameters. We conclude that four factors will affect the temperature dependence including doping density in SL, externally applied bias, the single barrier’s thickness and energy height. The doping density and temperature will change electron distribution in the first miniband of SL, and thereby cause the variations of photoresponse. Besides, the scattering during electron transport through the single barrier is expected to increase with the applied bias. The single barrier’s thickness and height influence the ballistic transport behavior and the tunneling mechanism, respectively. We also establish a simple model based on these factors to simulate the temperature dependence of photoresponse. The fitting results show the agreement with the experimental results. By understanding these factors’ effect, we design an infrared photodetector using the structure of a 15-period SL integrated with a 50-period multiple quantum wells (MQWs) to demonstrate its photoresponse under high temperature and low bias operation. The MQWs are utilized to reduce the noise current power and to add the response range. We find that the photocurrent of this device is not reduced by the additional MQWs but the dark current is. Hence, due to the low noise gain and low dark current, the maximum detectivity (D*) can occur at low negative bias. In addition, the photovoltaic response even appears at 80 K. In comparison with the previous sample, a 15-period SL with a single barrier, this device demonstrates not only the higher photocurrent and lower dark current, but also the better temperature dependence of photoresponse because the whole spectrum increases with temperature. By using the MQWs as a noise filter, this device is more suitable for operation under low bias and high temperature condition. Because of the lower electron mobility in the miniband, the photocurrent of SL is relatively lower than that of MQWs. Therefore, we design a double-barrier SLIP whose structure is a SL sandwiched between two different thickness barriers to enhance its photocurrent. The thin barrier adjacent to the collector contact is for electrons to traverse ballistically and to reduce the scattering loss of photocurrent, while the thick barrier is to block electrons moving backward and thereby to increase the photocurrent. However, we find that the current-voltage curves of this device are quite asymmetric. We attribute it to the carrier depletion in the SL and the resulting large built-in electric field to worsen its performance. In order to solve this problem, we fabricate the metallic contact on the SL instead of the emitter contact layer to supply electrons immediately. For this new processed device, we observe that its photoresponse is increased and the dark current is lowered. From our results, we find a backward thick barrier is helpful for photoresponse enhancement in SLIPs, especially under low bias operation. In our future work, we will integrate the above structures such as the thick barrier and MQWs into one device to improve its performance. For the n-type quantum well light coupling problem, we adopt the periodic metal grating layer on the top of the photodetector to absorb the normal incident light. Based on these concepts, we hope that we can fabricate a photodetector array with good performance which can be operated at 80 K and low bias condition.