Objective As the most common structural unit in electronic devices, the interface contact state of metal-semiconductor heterostructures plays an important role in the performance of electronic devices. Some traditional nanojoining technologies, such as ultrasonic nanojoining, joule heating, and ion and electron beam irradiation, have been used to improve the interface contact state of metal-semiconductor heterostructures. However, these methods easily cause additional damage to nanomaterials and require a complex processing environment, which poses a great challenge to the control of energy inputs. Femtosecond lasers with short pulse time and high peak intensity, as a method of nanojoining, are suitable to process nanomaterials with high melting points and damage thresholds. In recent years, most research studies have focused on the fabrication of metal-semiconductor heterostructure devices using femtosecond laser technology, but the material systems need to be broadened. In this study, the dry transfer method is used to fabricate Au electrode-CuO/ZnO nanowires (NWs) heterostructures and a semiconductor inverter based on p-type CuO NWs and n-type ZnO NWs. The nanojoining of the heterostructures based on the localized energy input of femtosecond lasers caused by the surface plasmon effect is successfully realized. The laser-treated semiconductor inverter had obtained stable voltage regulation capability based on the p-type and n-type field-effect transistor characteristics of CuO and ZnO NWs. This study provides a new idea for the top-down assembly of micro/nanoelectronic devices. We hope that our design provides a new idea for the top-down assembly of micro/nanoelectronic devices.
Methods CuO nanowires (NWs) were synthesized by thermal oxidation at 400 ℃ for 4 h; however, ZnO NWs and lithography electrodes were purchased directly from XFNANO Materials. Au electrode-CuO/ZnO NWs heterostructures and a semiconductor inverter based on p-type CuO NWs and n-type ZnO NWs were fabricated using a continuous dry transfer method. The nanojoining of the heterostructures was achieved using femtosecond laser irradiation (50 fs pulse duration, 800 nm wavelength, and 1 kHz frequency). The surface morphology and crystalline structure of nanowires were analyzed using scanning electron microscopy (SEM, Zeiss Supra 55) and X-ray diffraction (XRD, Bruker D8). The electrical characterization of the heterostructures and the semiconductor inverter in a three-terminal configuration were examined by a probe station (Keithley 2636B). In addition, the commercial finite element analysis (COMSOL Multiphysics 5.4) was used to simulate the electric ?eld distribution of the heterostructures under linearly polarized Gauss light to further elucidate the interfacial modification mechanism of Au electrode-CuO/ZnO under femtosecond lasers.
Results and Discussions The stable welded joints of the Au electrode-CuO/ZnO NWs heterostructures are obtained by femtosecond laser irradiation with a power intensity of 12.9 mJ/cm 2 and 20.2 mJ/cm 2, respectively. The nanowires form a degree of wetting angle on the surface of the Au electrode (the red dotted line area), and the surface organic matter is effectively removed, greatly improving the interface contact state (Fig. 4). The simulation results show that the enhancement region with a high electric field intensity mainly occurred in the interface between the Au electrode and nanowires on both sides, while the suspended nanowires in the middle have lower electric field intensity (Fig. 5). The strong surface plasmon effect due to femtosecond laser irradiation confines the energy to the interface of the heterostructures, promoting the formation of low damage welded joints. Compared with the electrical measurement results of pristine and laser-treated heterostructures, the voltage dependence of the p-type and n-type NW field-effect transistors (FETs) irradiated by femtosecond lasers is significantly improved. When the gate voltage (VG) reaches +20 V and -20 V, there is an obvious trend of current suppression for CuO and ZnO NWs, respectively (Fig. 6). In addition, the semiconductor inverter is fabricated based on the p-type and n-type FET characteristics of the CuO and ZnO NWs (Fig. 7). The laser-treated inverter had obtained stable voltage regulation capability, with a full voltage output swing of ~79.5% (Fig. 8).
Conclusions In this study, the Au electrode-CuO/ZnO NWs heterostructures are successfully fabricated through the dry transfer method. The strong plasmon interaction of metal-semiconductor heterostructural interface induced by femtosecond laser irradiation is demonstrated using COMSOL multi-physical field simulation to elucidate nanojoining mechanism. The laser energy is effectively limited in the interface area between the Au electrode and nanowires, achieving a low damage nanojoining of the heterogeneous materials. The device units of the two kinds of nanowire heterostructures have obvious electrical characteristics of p-type and n-type FETs with a great back gate control performance. The laser-treated semiconductor inverter had obtained stable voltage regulation capability based on p-type CuO NWs and n-type ZnO NWs, with a full voltage output swing of ~79.5%. The fabrication of Au electrode-CuO/ZnO nanowires heterostructures through dry transfer and femtosecond laser irradiation extends the material systems of metal-semiconductor heterostructures. In the end, we hope that the design of the inverter provides a new idea for the top-down assembly of micro/nanoelectronic devices.
