Ultrahigh photoconductivity of bandgap-graded CdSxSe1-x nanowires probed by terahertz spectroscopy.

Hongwei Liu,Chorng Haur Sow,Jinghua Teng,Xinhai Zhang,Junpeng Lu,Lin Ke,Limin Tong,Zongyin Yang

doi:10.1038/srep27387

Abstract

Superiorly high photoconductivity is desirable in optoelectronic materials and devices for information transmission and processing. Achieving high photoconductivity via bandgap engineering in a bandgap-graded semiconductor nanowire has been proposed as a potential strategy. In this work, we report the ultrahigh photoconductivity of bandgap-graded CdSxSe1−x nanowires and its detailed analysis by means of ultrafast optical-pump terahertz-probe (OPTP) spectroscopy. The recombination rates and carrier mobility are quantitatively obtained via investigation of the transient carrier dynamics in the nanowires. By analysis of the terahertz (THz) spectra, we obtain an insight into the bandgap gradient and band alignment to carrier transport along the nanowires. The demonstration of the ultrahigh photoconductivity makes bandgap-graded CdSxSe1−x nanowires a promising candidate as building blocks for nanoscale electronic and photonic devices.

Highlights

Transient photoconductivity up to 2000 Ω−1cm−1 with the free carrier density up to 3 × 1019 cm−3 is measured
We investigate the ultrafast carrier dynamics to show a clear picture on the bandgap engineering along the composition grading and reveal the effect of the build in “type II” alignment
We have measured the photoconductivity in bandgap-graded CdSxSe1−x nanowires using optical-pump terahertz-probe (OPTP) spectroscopy technique

Summary

Introduction

Transient photoconductivity up to 2000 Ω−1cm−1 with the free carrier density up to 3 × 1019 cm−3 is measured. It should be mentioned that the structural defects in bandgap-graded CdSxSe1−x nanowires can impede free carriers from migrating to surface through scattering or carrier localization. The free carriers start to recombine and the corresponding conductivities were recorded at t = 30, 120, 220, 350 and 440 ps, as presented in Fig. 3b–f, respectively.

Results

Conclusion