Research On Controlled Doping In ZnS Nanoribbons, Its Semiconductor Properties And Nano-Devices Applications

Zinc sulfide (ZnS) is an important group Ⅱ-Ⅵ direct band gap semiconductor material with a wide bandgap of3.7eV. It has shown great applications in diverse fields such as flat panel displays, light-emitting diodes (LEDs), injection lasers and ultraviolet (UV) detectors. Recently,1-dimension (1D) ZnS nanostructures have attracted attention due to their potential application in a new generation of nano-electronics and nano-optoelectronics, as mentioned above. However, some inevitable obstacles remain for the practical application of ZnS nanodevices. First, intrinsic or unintentionally doped ZnS nanostructures are highly insulating due to the high crystal quality with little donor/acceptor detects and are thus not suitable for device applications. Second, ohmic contacts in ZnS nanostuctures, particularly in p-type ZnS nanostructures, are much more difficult due to the surface Fermi level pinning induced by surface states. Third, how to construct desired structure of the devices based on ZnS nanostructures and future enhance the performance of the whole devices is predominant in devices applications. In order to overcome the obstacle of the ZnS nanostuctures applications, in this thesis, n-and p-type controlled doping in ZnS nanoribbons (NRs) and optimization of the electronic and optoelectronic devices based on them are systematically studied. The main conclusions are summarized as follows:(1) The controlled n-type ZnS:Cl NRs/ZnS:Ga NRs and p-type ZnS:Ag NRs/ZnS:Cu NRs were successfully synthesized via a chemical vapor deposition (CVD) method for the first time. Morphologies and structure of the as-synthesized NRs characterization demonstrated that all the NRs are single-crystal wurtzite structure. Moreover, in order to evaluate the electrical transport properties of the doped NRs, back-gate field effect transistors (FETs) based on individual doped NRs were constructed. The conductivity of the Cl-doped ZnS NRs was significantly enhanced with8orders of the magnitude as compared with the undoped ones and could be tuned from～10-3to5.0Scm-1, and the highest carrier concentration as high as3.8×1017cm-3was achieved. The field-effect electron mobility μn was deduced to be130.9cm2V-2s-1, which is much than that of previous Ⅱ-Ⅵ nanostructures. The p-type ZnS:Ag NRs and ZnS:Cu NRs with high hole concentration of4×1017cm-3and1.4×1018cm-3were synthesized, respectively. Moreover, the meta-semiconductor field effect transistors (MESFETs) and flexible MESFETs based on individual ZnS:Ag NR were fabricated by photolithography and subsequently a lift-off process, respectively. The subthreshold swing and transconductance of the MESFETs can be deduced to be20mV/dec and100μS, respectively. The value of the hole mobility (μn) as high as1300cm2V-1s-1. The flexible MESFETs exhibit stable electrical characteristics under different tension. The electrical transport properties of the doped NRs based on FETs clearly proves the successful synthesis of the n-and p-type doped ZnS NRs, and the conductivity and carrier mobility of the doped ZnS NRs were significantly enhanced. The series of efforts mentioned above were dedicated to explore some high-performance of nano-devices.(2) A new method for excellent ohmic contacts to the doped ZnS NRs was first reported by saturating the surface states and then formatting a highly conductive interfacial layer with the same type to NRs. Excellent ohmic contact to n-type ZnS NRs was successfully achieved by using ITO electrode instead of conventional metallic electrodes, which were attributed to saturate the surface states and greatly promote electron tunneling transport from the n-ZnS NR to ITO electrode by thermionic or thermionic-field emission. Additionally, a new metallization scheme for achieving excellent ohmic contact to p-type ZnS NRs at room temperature was firstly demonstrated. Bilayer electrodes consisting of Cu (4nm)/Au (50nm) provided a specific contact resistivity as low as5.6x10-7Ωcm2with p-ZnS NRs. Interface analysis via depth profiling X-ray photoemission spectroscopy, Auger electron spectroscopy, and high-resolution transmission electron microscopy revealed the formation of a highly conductive Cu2S interfacial layer, which could serve as the buffer layer and thus promote the hole transport from NR to electrode.(3) High gain ultraviolet photoconductors based on n-type ZnS NRs were successfully constructed and used to study the low-intensity UV response properties by enhancing the mobilities of the NRs. Nano-photodetectors (PDs) constructed from the controllable ZnS:Cl NRs exhibited excellent device performance with extremely high sensitivity to the UV light while are nearly blind to the visible light. The photoconductive gain (G) of the detectors are remarkably increase with the doping level increasing, and reach to107, which highly improved the devices performance of the UV photodetectors based on ZnS nanostructures. The photoconducting detectors (PCDs) based on individual ZnS:Ga NR was constructed to study the low-intensity ultraviolet (UV) response properties. It was seen that the device exhibits excellent photoconductive properties upon bias voltage as low as-0.01V in terms of high sensitivity to UV light with intensity as1μWcm-2(corresponding to incident power of1×10-14W on NR), gain as high as-2.4×106and relatively fast response time as-0.3s. The high gain and fast response time are mainly attributed to the excellent ohmic contact obtained by using ITO electrode and high carrier mobility of the NRs.(4) An approach of optimizing the device structure was provided to enhance the response speed and sensitivity of the photodetectors. P-n photodiodes has been constructed from the p-ZnS NR-n-Si heterojunction with a response speed as high as-48μs (rise time). Furthermore, the device also exhibits stable optoelectrical properties with high sensitivity to UV-visible-NIR light and an enhancement of responsivities of1.1×103AW-1for254nm under a reverse bias of0.5V. In addition, a room-temperature UV light detector based on ITO/p-ZnS NR schottky barrier diodes (SBDs) were fabricated by photolithography and subsequently lift-off process. The device shows ultra-high detectivity and photoconductive gain of9.8×1020cmHz1/2W-1and7×105, respectively, and is capable of detecting an extremely weak UV light with incident power of6×10-17W (-85photons/s on the NR) at bias of0.01V. It is found that the presence of the trapping states at interface of the ITO/p-ZnS NR Schottky junction play an important role. The electric field distribution of the depletion junction of the device was obtained by device simulation method, which is capable to explain the model of the weak UV detection. That is the trapped photogenerated electrons on interface states significantly enhanced the charge density and generate a high potential.(5) In order to simplify the process of the nano-semiconductor-based memory and enhance the devices performance, the two structures of nonvolatile memories with two-terminal structures were successfully constructed. The nonvolatile memory based on Al/p-ZnS NR SBD exhibits a stable reproducible hysteresis and excellent memory characteristics with a programming speed of100ns, high current on/off ratio of108, long retention time of6×104s and good endurance>12months. The control experiments suggest that the attractive hysteresis can be attributed to the trapping of the carriers in the interfacial layer between Al electrode and ZnS NR which is AlOx measured by HRTEM cross-section forming in the process of the Al electrode deposition. The electrical switching behavior can be attributed to the charge trapping in interface layer. And the trapped and detrapped times of the interface states were estimated with the measurement of conductance-frequency, which is capable to explain the observed operation speed of100ns. Moreover, a new structure nonvolatile memory, with large conductance switching(on/off ratio>106), has been constructed from the p-ZnS NR/n-Si heterojunction. These devices exhibit a stable and reproducible hysteresis and excellent memory characteristics with long retention time of1×105s and good endurance>6months at room temperature. The electrical switching behavior could be attributed to the charge trapping and detrapping in interface states at the junction.