Quasi-one Dimensional ZnSe Nano-materials With Controllable Doping And Its Applications As Nano-optoelectronic Devices

The outstanding electrical and optical properties of quasi-one dimensionalnano-semiconductor materials have been widely interesting. ZnSe is an importantII-VI semiconductor in the wide band-gap semiconductors. The first blue-greenlaser has been fabricated based on ZnSe material in the1991, which is5yearsearlier than GaN laser. ZnSe is a direct-gap semiconductor with a wide directband-gap of².7eV at room temperature, corresponding to the excitationwavelength of about460nm. ZnSe nanostructures have potential inhigh-performance light emitting diodes （LED）, photovoltaic, and the photodetector.Therefore, quasi one-dimensional nanomaterials and nano-optoelectronic deviceshave an important significance. However, the resistance rate of intrinsic ZnSenanomaterials is virtually insulating and difficult to apply. The synthesis of quasione-dimensional ZnSe nano materials with controlled doping and nanoscale devicesis in great need.This thesis systematically studied the synthesis of n-type ZnSe nanoribbonsand p-ZnSe nanowires by introducing Cl and N element via a thermal evaporationmethod. The morphologies, phase and composition of the as-synthesized ZnSe werecharacterized. In order to further study the electrical and optical properties of ZnSenanostructures, n-type field effect transistors（nMOSFET）, p-type field effecttransistors（PMOSFET）, pn junction, the schottky diodes and junction field effecttransistors（JFET） based on ZnSe nanostructures were fabricated. At the same time,the influence of doping level and device progress on the electrical and optical ofZnSe nanostructures was studied as well. The detailed achievement made in thisthesis is as follows:1. Cl doped ZnSe nanoribbons（NRs） were synthesized via a thermal evaporationmethod by using ZnCl₂powder and ZnSe powder as the dopant and the sourcematerial, respectively. The as-synthesized ZnSe:Cl NRs were characterized byscanning electron microscopy （SEM）, high-resolution transmission electronmicroscopy （HRTEM）, X-ray diffraction spectroscopy （XRD）. It is seen that n-typeZnSe:Cl NRs have single-crystal wurtzite hexagonal structure with the growthdirection of [120] and a uniform morphology with width in the range of30-100μm, length up to seveal tens of micrometers, even more hundreds of micrometers.MOSFETs based on n-ZnSe:Cl NRs were fabricated and electrical andoptoelectronic properties of NRs were characterized. Remarkably, the mobility andcarrier concentration of ZnSe:Cl NRs can be adjusted from0.4cm²V^-1s^-1to17.3cm²V^-1s^-1and1.8×10¹⁶to6.9×10¹⁸cm^-3, respectively. The wavelengthdependent I-V curves obtained of ZnSe:Cl NR exposed by using the monochromaticlight as irradiation source. It is seen that the NR’s conductance strongly depends onthe light wavelength, which increases with decreasing wavelength and reaches themaximum value at approximately460nm. The maximum Responsivity （R） andGain （G） are estimated to be7.9×10⁵AW^-1and2.1×10⁶. The spectral response ofthe ZnSe NR after a rapid thermal anneal process in air indicates that the UVresponse of the ZnSe:Cl NRs were enhanced, due to the formation of a thin ZnOoxide layer on the surface of the NRs. The time response spectra of the ZnSe NRswith various levels of doping were analyzed, it is clear that the photocurrent of theundoped ZnSe NR is much lower than that of the doped NRs. What is more, thelonger response time was observed in doped ZnSe NR, which is due to the carriertraps induced by the Cl doping. Besides, ZnSe nano-MOSFETs fabricated onflexible substrates were discussed, the results show an improved electrical andoptical properties.2. P-type ZnSe nanowires（NRs） were also synthesized via thermal evaporationmethod using NH3as dopant. The as-synthesized ZnSe:N nanowires werecharacterized. It is noting that ZnSe NRs have znic blend structure and a uniformmorphology with a diameter in the range of100nm to500nm and the length up totens of micrometers. Bottom gate-field-effect transistors were fabricated based onp-type ZnSe NRs, the electrical properties of ZnSe:N NRs were characterized andthe result shows an obvious p-channel gate effect. The p-type ZnSe NR,p-ZnSe/n-Si heterojunction was constructed, the obvious characteristics of dioderectifier were observed and the on/off current ratio of106can be obtained, this highon/off ratio for the heterojunction provides a potential application in nano-devices.P-ZnSe:N/n-Si heterojunction PV under the light illumination is characterised, thecalculated open circuit photovoltage （VOC） and the short circuit current （ISC） are0.45V and2.6nA, respectively. Furthermore, the corresponding power conversionefficiency （η） of1.8%, which is expected to be applied in solar cell. Theperformance of the junction field effect transistor （JFET） based on theheterojunction such as low turn-on voltage, the low subthreshold swing, highswitching and high transconductance has been effectively improved compared with bottom gate effect transisitor. In addition, the schottky diode based on Al electrodeand p-type ZnSe NRs were fabricated. Significantly, an obvious hysteresisphenomenon due to charge storage was observed. The long work time can hold aslong as104s, which suggests its potential applications in nano-memorizer...