Photoluminescence Properties of ZnO and ZnCdO Nanowires

Abstract

Zinc oxide (ZnO) is a wurtzite-type II-VI compound semiconductor with a wide, direct band-gap (3.37 eV) and has an exciton binding energy of 60 meV, larger than thermal energy at room temperature (RT) of kBT ~ 25 meV. By alloying with CdO, which has a cubic NaCltype structure and a narrower direct band gap of 2.3 eV, the band gap of ZnO can be redshifted to a blue or even a green spectral range. Therefore, ZnO [1] and ZnCdO [2] nanowires have attracted significant attention as promising “building blocks” for nanometerscale optoelectronic devices operating in the green to UV spectral region. We report on the photoluminescence (PL) studies of ZnO and Zn1-xCdxO (x~0.02-0.05) nanowires grown at different temperatures (435, 535, 570°C) by non-catalyst solid-vapor technique on SiO2/Si substrates. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show that the structure of ZnO and Zn1-xCdxO nanowires is of wurtzite-type with good crystal perfection. Surface morphology of samples was determined by scanning electron microscopy (SEM). The PL spectra of as-grown nanowires, nanowires extracted from the substrate and deposited onto Si wafer, and nanowires dispersed in ethanol solution by sonication were collected using 325 nm (He-Cd) or 351 nm (Ar + ) laser lines. The measurements were performed at temperatures from 10 K to 300 K. The temperature dependence of photoluminescence emitted by ZnO and Zn1-xCdxO nanowires was measured and analyzed. The low-temperature PL spectra for ZnO (Zn1-xCdxO) as-grown nanowires are dominated by the emission peak at 3.359 eV (3.361 eV) originating from the radiative recombination of donor-bound excitons (DX). At temperatures 80-100 K, a small peak at 3.369 eV (3.337 eV) assigned to the free-exciton (FX) recombination starts to appear. The FX peak redshifts with increasing temperature, its intensity increases and finally FX line dominates the spectrum at RT. Additionally at low temperature we observed the peak at 3.311 eV (3.328 eV) which most probably is due to a two-electron satellite (TES). In spite of the narrowing of band gap in Zn1-xCdxO, a blueshift (of about 2 meV) of the near-band edge (NBE) emission was observed in the PL spectra. It might be caused by a spinodal decomposition. In order to study PL from a small number of individual nanowires we either dispersed them in ethanol solution by sonication or extracted them from the substrate and deposited onto Si wafer. We present the dependence of PL spectra of Zn1-xCdxO nanowires in ethanol solution on excitation power. Also we measured polarization properties of emitted light. The comparison at RT of μ-PL measurements performed on several (~10) Zn1-xCdxO nanowires resting on Si wafer with the macro-PL of as-grown nanowires shows also a blueshift of the NBE emission. This effect is probably a consequence of selective transfer of nanowires, having smaller diameter, onto the silicon substrate. The research was partially supported by the Ministry of Science and Higher Education (Poland) through grant N515 015 32/0997 and by the Network New materials and sensors for optoelectronics, information technology, energetic applications and medicine.