The ultraviolet photodetection properties of gallium nitride/ silicon nanoporous pillar array

Abstract

The accurate detection of ultraviolet light (UV) has been playing important roles in a variety of fields such as industrial and agricultural production, environmental monitoring and protection, defense and aerospace industries. Semiconductor materials have been widely used in fabricating various photodectors. As a representative of the first generation semiconductor materials, silicon has been applied to fabricate UV photodetector for a long time. For example, the first solid UV detectors were based on UV-enhanced silicon photodiode. Nevertheless, silicon-based photodiodes usually exhibit responses to visible light, which will cause serious disturbances for the detection of UV. Furthermore, crystal silicon is an indirect bandgap semiconductor, which decides that its conversion efficiency and therefore responsivity during the optoelectronic process would be low. All these shortcomings make the effort on developing novel UV photodetectors being both important and necessary. In the past years, great progresses have been achieved in preparing high-quality compound semiconductors with direct and wide bandgaps, and this provides a good opportunity for developing high-performance UV photodectors. As a III-V compound semiconductor with a direct and wide bandgap (~3.4 eV), gallium nitride (GaN) possesses the properties of high electronic mobility and heat conductivity, good chemical and thermal stability, and excellent anti-irradiation capability, and therefore is thought to be one of the most suitable materials for preparing UV detectors. Currently, high-quality GaN thin films are usually prepared by heteroepitaxy technology utilizing sapphire wafers as substrates. However, sapphire is an insulator, which can be used only as a mechanical support instead of a functional substrate and might bring additional technical difficulty in device integration. Such a problem might be solved by using silicon substrate, provided that the large lattice and thermal mismatch between silicon and GaN can be conquered, or it will cause serious problems relating to the device performance and operation stability. A proved method is through introducing nanostructured silicon substrates to construct GaN/Si heterojunction, by which the mismatches could be greatly diminished through a three-dimensional stress release mode at the GaN and silicon interface. Silicon nanoporous pillar array (Si-NPA) is a silicon hierarchical structure characterized by a regular array of well separated, micron-sized and highly nanoporous silicon pillars, and it has been proved an ideal template for preparing silicon-based heterstructures with high-quality interfaces. Here we will report that utilizing Si-NPA as substrates, GaN/Si-NPA nanoheterojunctions were prepared by growing GaN nanocrystallites using a chemical vapor deposition method. Through changing the deposition time, the surface morphology, chemical constitution, and electrical and photoluminescent properties of GaN/Si-NPA can be tuned effectively, and this provided a solid foundation for optimizing the device performance of the subsequently developed UV photodetectors. Through preparing ITO top transparent electrodes and silver back electrodes on these nanoheterojunctions, prototype UV photodectors with a device structure of ITO/GaN/Si-NPA/ sc -Si/Ag were prepared and the photodetection properties were studied. It was shown that high-quality UV detection can be realized without applying any bias, with the response and recovery time being ~0.12 s and ~0.24 s, respectively. For the monochromatic UV light with a wavelength of 340 nm, a responsivity of ~0.15 mA/W was achieved. These results might have provided a novel route for developing Si-based GaN UV photodetectors.