SITP has achieved positive progress in the research of photoconductive properties of semiconductor nanowires
In recent years, semiconductor nanowires have attracted comprehensive attentions in various fields, including the energy, biology, micro-electronics and micro-mechanics. Especially, the nanowire based photoelectric devices, such as the photodetecotrs and the solar cells have showed certain superiority. In principle, the performance of these devices is dominated by three factors: the efficiency of light absorption, the mobility, and the lifetime of carriers. By taking the advantage of the light funneling effect, nanowire devices with a physical filling factor of only a few percent can effectively absorb the incident light, even achieving higher efficiency than the thin film counterparts. The electron and hole mobility of nanowires is approaching the bulk value. While these findings prove the unique superiority of nanowires in photoelectric conversion, the carrier lifetime is likely to hamper the further improvement of its performance. Meanwhile, the intentional doping in nanowires with n- or p-type impurities is a key process to realize electric and photoelectric functionalities. However, the consequence of doping on nanowires’ electrical properties could be different from the bulk case due to their high surface-to-volume ratio. Therefore to clarify the micro-mechanism of these factors will be conducive to the practical application of nanowires.
In the guidance of the cooperated supervisors, the doctoral candidates of National Laboratory for Infrared Physics, Hui Xia et al., have embedded GaAs nanowires with polymer and achieved the photoconductive characterizations of individual epitaxial nanowires based on the conductive atomic force microscopy (C-AFM). Compared to the single nanowire devices, the injury to nanowire induced by the photolithography and ion beam etching are avoided in this experimental setup, thus it is beneficial to detect the original properties of nanowires. Different from the conventional bulk material, the novel linear photocurrent response induced by n-type doping is observed in GaAs nanowires. According to the numerical simulation which takes into account the actual structure of nanowires, researchers reproduce the current-voltage curves of nanowires with different doping conditions, and extract the minority carrier (hole) lifetime of individual nanowire. Further analysis reveals that the electron released from the n-type impurity is likely to fill the high density of surface states, and finally results in the drastic recombination of photo-excited holes. This mechanism clarifies the origin of the much lower carrier lifetime of nanowires compared to the bulk material, and is conducive to the control of the surface properties of nanowire and the optimization and application of doped nanowires.
This work is funded by the National Basic Research Program of China, the National Science Foundation of China, and the International Cooperation Partnership Program of the Chinese Academy of Sciences. It is published in ACS Nano (volume 6, pp 6005-6013), whose latest impact factor is 10.774.