Investigation On Zno Nanomaterials Based Field Effect Transistor And Photodetector

In the past decades, ZnO, has attracted much attention due to its superior optoelectronic properties (band gap: 3.37 eV, exciton binding energy: 60 meV). The nano-effects greatly enhance the optoelectronic properties of ZnO nanomaterials, for example, the large surface to volume ratio benefiting from the nano-stucture,greatly enhanced the surface adsorption of gas molecules, which makes them have promising potential applications in the field of light-emitting diodes, ultraviolet (UV) detectors,solar cells and photoelectrochemical water splitting. Among them, a field effect transistor (FET), one of the most fundamental and important building blocks of nanoelectronic devices, has been fabricated, using ZnO nano-structure as the active channel, and extensively investigated. However, due to the complex composition of intrinsic defects, the presence of surface states, and their important role played in the performance of ZnO FET, it is necessary to reveal the physical mechanism and find some way to improve the performance of ZnO FET. In addition, in recent years, with the rise of two-dimensional nanomaterials, many ultra-thin nanomaterials have been fabricated, including ultra-thin ZnO nanobelt and nanofilm. But the optical and electrical properties of these ultra-thin ZnO nanomaterials have been remained unknow.To address the above issues, we have fabricated ZnO nanowires and ultra-thin ZnO nanobelts through chemical vapor deposition method (CVD). And then,based on these ZnO nanomaterials, we have fabaticated FET and UV detector devices. On the basis of deeply studying the optoelectronic of these devices, proposing some effect ways to improve the performance of ZnO FET; including electron mobility,subthreshold swing, on/off ratio, achieving the device type and threshold voltage control of ZnO FET, and optimizing the performance of ZnO UV detector device.Some innovative results have been achieved and listed as follows:(1) Exploring the influence of pre-annealing the dielectric layer in a reduced atmosphere on the performances of single nanowire ZnO FET. The experimental results show that the performances of the single ZnO nanowire FET have been greatly improved after the dielectric layer annealed in a reduce atmosphere, the electron mobility increased from 2.0 cm~2/V·s to 37.9 cm~2/V·s, on/off ratio rised from 10.6 to 2.6×104, and the subthreshold swing droped form 74 V/decade to 1 V/decade. The pre-annealing treatment significantly reduces the number of non-bridging oxygen atoms at the surface of the dielectric layer, which blocks the interaction between ZnO nanowires and dielectric layer,and finally enhances the electrical characteristics of the ZnO nanowire FETs.(2) Adjusting the threshold voltage controlling the device mode of the ZnO FET.Systematic experimental study has found that the carrier concentration of ZnO nanowire was decreased and the corresponding threshold voltage has shifted from negative to positive, which indicating the conversion of device mode from depletion type to enhancement type, when the ZnO nanowire was annealed in an oxygen ambient. In contrast, the carrier concentration of ZnO nanowire was increased and the threshold voltage was shifted form positive to negative when the nanowire was annealed in a reduced ambient. The variation of carrier concentration is mainly related to the defect changing when the ZnO annealed under different conditions. In addition,when the ZnO nanowire was decorated with Au nanoparticles, the threshold voltage of the corresponding FET will also move forward. This phenomenon can be explained by the charge transfer effect between ZnO and the Au nanoparticles once they are contacted to each other.(3) Fabricated and study the electrical properties of the ultra-thin ZnO nanobelt FETs. With respect to the ZnO nanowire FETs, the electrical properties of the FETs based on ultra-thin ZnO nanobelt have greatly improved, the mobility of some devices have reached 200 cm~2/V·s. The enhancement of the FET performance can be explained by the the special ultra-thin structure of the ZnO nanobelts and the passivation effect after the ZnO nanobelts covering on the surface of the dielectric layer.(4) Adjustment of the threshold voltage and the controlling of device operation type of the ultra-thin ZnO nanobelt FETs. ZnO nanobelt FETs using Ti/Au as the electrodes usually exhibit a negative threshold voltage,indicating n-channel depletion mode behavior,whereas ZnO FETs with MoOx/Au electrodes instead of Ti/Au show a positive shift of threshold voltage, exhibiting an n-channel type enhancement mode.In contrast, the decoration on the surface of ZnO channel by MoO, significantly increases the electron carrier concentration, and then negatively shifts the threshold voltage. We propose that MoO, thin film may play a passivation effect role,passivating the large amount of adsorbed species on as-grown ZnO nanobelts,releasing the electron, and finally achieving the control of threshold voltage.(5) Fabricated and study the UV detector devices base on ZnO nanowire, and utilizing the Schottky barrier to improve the detector properties of these UV detector devices. Experiments show that the single ZnO nanowire have a good response to the 350 nm UV light. The response and recover speed can be improved by introding Schottky barrier at the electrodes of the UV detector based on the single ZnO nanowire. In addition, we have fabricated scattered ZnO nanowires structure photodetector, utilizing the potential barrier at the interface of two individual ZnOnanowire to achieve a better UV detect property. Finally, we get a UV detector with a switch onLofl ratio of 103, response and recover time of 12.1 s and 2.3 s,respectively.In conclusion, we have fabricated FETs and UV detectors based on ZnO nanowires and nanobelts. Systematically study the influence of dielectric layer annealing process,ZnO atmosphere annealing process,surface modification process on the performances of FET devices. And also raised some effect way to improve the performance of ZnO UV detectors. These study may facilitate the practical applications of ZnO in semiconductor integrated circuits, atmosphere detection and UV-detection.