Synthesis, doping and device applications of II-VI semiconductor nanowires

Abstract

ZnTe is an important group II-VI direct band gap semiconductor material with attractive electronic and optoelectronic properties. Although ZnTe nanowires are very promising building blocks for nanoscaled optoelectronic devices, several issues have to be addressed to enable their practical applications. First, how to synthesize ZnTe nanowires with controlled morphologies at low cost and large scale is an important challenge to be overcome. Second, controllable and uniform doping in ZnTe nanowires is a critical prerequisite for semiconducting applications. Last but not least, how to improve the ZnTe nanowire-based nanodevice performance is predominant in device applications. In the present study, coaxial nanocables with single crystalline ZnTe nanowire cores and amorphous SiOx shells were synthesized via a simple one-step CVD approach. Transport characteristics of field-effect transistors (FETs) fabricated from indvidual ZnTe nanowire cores show typical p-type semiconducting behavior. It was also revealed that the SiOx shell on the nanocables can act as an effective insulating layer. Improved electrical properties of p-type ZnTe nanowire FET using HfO2 as the dielectric layer were also demonstrated. Moreover, doping of ZnTe nanowires via two approaches, ie., i) ex-situ Ag doping, ii) in-situ P doping, were successfully demonstrated. For ex-situ Ag doped ZnTe nanowires, XPS confirmed the existence of Ag in the doped ZnTe nanowires. The Ag-doped ZnTe nanowires exhibited p-type conductivity about three orders of magnitude higher than that of the undoped ZnTe nanowires. Enhanced p-type conduction in Ag-doped ZnTe nanowires was attributed to the ion exchange process on the surface of ZnTe nanowires and diffusion of Ag into ZnTe nanowires. On the other hand, controlled in-situ P-doping in ZnTe nanowires has been achieved and shown to enhanced p-type conduction with a resistivity of 2.5 × 103 Ω·cm, a carrier concentration of 5.46 × 1017 cm-3, and a mobility of 4.47 × 10-3 cm-2 V-1 S-1. The enhanced conductivity can be ascribed to the doped P atoms substituting the Te atoms. The Te P will generate holes at the top of the valence band and thus enhance the p-type conductivity. Lastly, ZnTe nanowire photodetectors with good photoconductive sensitivity in the visible range has been demonstrated. The nanowire photodetector showed the highest photoconductive gain of 15396 among all reported visible-light nanostructured photodetectors, including CdS, CdSe, ZnSe, etc. The ZnTe nanowire photodetector also exhibited a fast response speed of 1.3 s. Finally, the mechanisms of photoconduction of the photodetectors were discussed.