Characterization Of The N-ZnO/p-Si Photodiode Fabricated Using Pulsed Laser Deposition

Photoelectric materials have attracted considerable research attention during recent years because of their wide application in the market. The semiconductive ZnO, which has a wide band gap of 3.37eV, has been recognized as a promising photoelectric material. ZnO/p-Si heterojuncion can show intense photovoltaic effect. In addition, ZnO has many other advantages such as simple preparation, stability and low cost, so, it has huge application in optical and electrical fields. In this thesis, undoped ZnO films on p-Si substrate are deposited to get n-ZnO/p-Si photodiodes and the characteristics of the devices are studied.The ZnO films are prepared by pulsed laser deposition (PLD). X-ray diffractometer (XRD), scanning electron microscopy (SEM) and atom force microscopy (AFM) were used to study the surface morphology of the ZnO films and the structure of the heterojunctions. The photoelectric properties of the devices were characterized by the current-voltage (I-V) measurement.It is concluded that the quality of the diode junction is as important as that of the n-ZnO film deposited on the p-Si for a photodiode. Films grown and post-annealing at higher temperature have better quality; however, the diodes' photovoltaic effects are not ideal because of the thermal diffusion at the interface. The quality of the photodiodes with homobuffer layer is higher than those without buffer layer. Oxygen pressure seems to reduce defects such as oxygen vacancies, but excess oxygen may change the energy band structure of the ZnO films and degrade the quality of the heterojunction.When illuminated under the light with different wavelength, low energy photons in the visible range can transmit through the ZnO films due to the wide band gap of ZnO, and mainly be collected at the depletion region of p-Si. Photo-current is produced due to the light-induced electron-hole generation and will saturate at moderate bias. On the other hand, ultraviolet photo will be absorbed in the depleted n-ZnO layer, and the photo-current increases with the reverse bias. Based on the ultraviolet response of ZnO and the visible-infrared response of the Si, UV-enhanced photodetector can be obtained.