Elsevier

Thin Solid Films

Volume 617, Part B, 30 October 2016, Pages 120-125
Thin Solid Films

Effect of doping concentration and temperature on the morphology, crystallinity and electrical conductivity of Al:ZnO nanostructured films grown from aqueous solution

https://doi.org/10.1016/j.tsf.2016.07.004Get rights and content

Highlights

  • Al:ZnO nanostructures were grown on glass by hydrothermal method (70–95 °C).

  • Al-doping up to 4 wt.% changes the ZnO nanowires to nanoblades and nanosheets.

  • Crystallite size decreases from 23.8 to 21.1 nm for Al-doping between 0 and 4 wt.%.

  • No crystalline structure was observed for 6 wt.% Al:ZnO nanostructures.

  • 2 wt.% Al:ZnO film grown at 95 °C shown the best conductivity of 140.9 Ω 1 cm 1.

Abstract

Al-doped ZnO nanostructures were grown on glass substrate by hydrothermal method, in the temperature range of 70–95 °C, from aqueous solutions of zinc nitrate he333xahydrate and aluminum chloride hexahydrate. Based on scanning electron microscopy, X-ray diffraction, micro-Raman spectroscopy and electrical measurement data, the effect of 2–6 wt.% Al doping on the morphology, microstructure and electrical properties was investigated. The morphology and crystalline structure are strongly dependent on the Al-doping and growth temperature. When Al doping increases from zero to 2 and 4 wt.%, the morphology changes from nanowires with higher optical phonon confinement, to combined morphologies including nanowires, nanoblades and transparent nanosheets. Lowering the temperature of the growth solution and/or increasing the concentration of aluminum dopant above 4 wt.%, 1D (nanowires) and 2D (nanoblades and nanosheets) nanostructures collapse into an amorphous structure with totally changed morphology. Micro-Raman spectra and X-ray diffraction patterns confirmed the wurtzite structure with preferential c-axis growth direction of samples containing up to 4 wt.% aluminum. The average crystallite size decreases continuously from 23.8 to 21.1 nm with increasing the Al concentration from 0 to 4 wt.%. The electrical conductivity of the obtained nanostructures significantly depends on the aluminum doping concentration. 2 wt.% Al-doped ZnO nanostructured layer grown at 95 °C showed the best electrical conductivity of 140.9 Ω 1 cm 1. The ambivalent role of aluminum, donor or acceptor, depending on doping concentration was highlighted.

Introduction

ZnO is a II–VI compound semiconductor with a wide and direct band gap of 3.37 eV and high exciton binding energy of 60 meV. ZnO is an excellent candidate for various electronic and optoelectronic devices due to its remarkable properties: excellent chemical and thermal stability, high-transparency in the visible region, near-UV emission, biocompatibility, and wide electrical conductivity range [1], [2]. Due to quantum confinement, high surface-to-volume ratio, higher optical gain, faster response and specific crystalline orientation, ZnO nanowires are very promising materials to be used in sensors, cantilevers as well as in optoelectronic devices such as light-emitting diodes, excitonic solar cells etc. [1], [2], [3], [4].

The properties and functionality of ZnO nanostructures can be improved and diversified by doping various chemical elements such as Ga, In, Sn, P or Al into ZnO structures [5], [6], [7]. Among them, Al-doped ZnO nanowires, that are capable of reaching tunable band gap and the highest conductivity without deterioration in crystallinity, are considered as an alternative to the indium tin oxide (ITO) thin films as the most currently accepted transparent conductive oxide (TCO) material. Specially, Al:ZnO thin films with high c-axis orientated crystalline structure along (002) plane have potential applications in broadband UV photodetectors with high tunable wavelength resolution. The controlul of point defects is essential in achieving the desired applications of ZnO nanostructures.

Various intrinsic defects such as zinc vacancies, interstitial zinc, oxygen vacancies or interstitial oxygen are present inside the undoped ZnO structure. These intrinsic defects form either acceptor level or donor level in the band gap that would greatly affect the luminescent properties of ZnO. By introducing extrinsic Al dopant, the defect environment is changed whether the Al atom substitutes the zinc atom or it occupies the interstitial site [8], [9], [10], [11], [12].

In the present paper, we report the hydrothermal synthesis of undoped and Al-doped ZnO nanostructures grown on glass substrate from zinc nitrate hexahydrate in the presence of hexamethylenetetramine. The effects of Al doping and the temperature of growth solution on the morphology, crystallinity and electrical properties of the obtained ZnO were investigated in details

Section snippets

Film preparation

The undoped and Al-doped ZnO nanostructured films have been hydrothermally grown on glass substrate, coated with a sol-gel ZnO thin film, using an equimolar (0.025 M) aqueous precursor solution of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (C6H12N4, HMTA). The growth process was carried out at 70, 80 and 95 °C in a sealed Teflon-lined autoclave. The growth time was the same for all samples, namely 3 h. As doping source, aluminum chloride hexahydrate (AlCl3·6H2O) was added

Morphology and microstructure

Fig. 1 presents the SEM images of undoped and Al-doped ZnO nanostructures grown 3 h at 80 and 95 °C. The morphologies of nanostructured films are dependent on the Al-doping and growth temperature. It was observed that different Al doping concentrations give different morphologies. Undoped ZnO film grown at 95 °C consists of only nanowires arrays with the average diameter in 20–30 nm range (Fig. 1(e)). For the undoped ZnO sample grown at 80 °C, the presence of individual nanoblades can be observed,

Conclusions

The effect of aluminum doping and temperature of the growth solution on the morphology, microstructure and electrical properties of undoped and Al-doped ZnO nanostructures has been investigated. By controlled Al-doping, the morphology of the nanostructured films has changed and different kinds of nanostructures were obtained. The undoped ZnO nanostructures consist of nanowires when grown at 90 °C and of nanowires and individual nanoblades when grown at 80 °C. For the samples doped with 2 and 4 

Aknowledgements

This work was supported by PNII-PT-PCCA-2013-4-2104-D1-NANOZON (Contract nr. 27/2014). The authors thank to Dr. M. Danila from IMT- Bucharest for XRD analysis.

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