Optical and photoluminescent properties of sol-gel Al-doped ZnO thin films

doi:10.1016/j.matlet.2006.06.065

Materials Letters

Volume 61, Issues 4–5, February 2007, Pages 1118-1121

https://doi.org/10.1016/j.matlet.2006.06.065 Get rights and content

Abstract

Al-doped zinc oxide (AZO) thin films have been prepared via a sol-gel process. Optical and photoluminescent properties of the AZO films have been investigated. The UV absorption edge was blue shifted with increasing Al doping concentration. Efficient green–yellow emission was obtained after annealing at 850 °C. For the 850 °C-annealed samples, the green peak was red shifted from 518 to 565 nm as the Al doping concentration increased from 0 to 2.0 at.%. In addition, violet emission in the range of 400–450 nm was observed in the 850 °C-annealed AZO films. The possible origins responsible for these emission bands have been discussed.

Introduction

Zinc oxide has recently gained much interest because of its potential use in many applications, ranging from antireflection coatings, transparent electrodes in solar cells [1], thin film gas sensors [2], varistors [3], spintronic devices [4], photodetectors [5], surface acoustic wave devices [6], and light emitting diodes [7], to nanolasers [8], attributed to its wide and direct band gap, excellent chemical and thermal stability, and specific electrical and optoelectronic property of being a II–VI semiconductor with a large exciton binding energy. ZnO normally forms in the hexagonal structure with a = 3.25 Å and c = 5.12 Å; each Zn atom is tetrahedrally coordinated to four O atoms, where the Zn d-electrons hybridize with the O p-electrons; layers occupied by zinc atoms alternate with layers occupied by oxygen atoms [9].

It is well known that chemical doping greatly influences the electronic and optical properties of ZnO. Control of defects is, therefore, vital in achieving viable applications of ZnO. Doped ZnO thin films are of technological importance because of their great potential for various applications such as transparent conducting electrodes (doping with III A elements) [1], [10], insulating or dielectric layers (doping with Li) [11], and spintronic devices (doping with Mn) [4]. Among them, being noted for the high conductivity and good optical transmittance, Al-doped ZnO (AZO) films have drawn considerable attention for transparent conducting electrodes [10]. Undoped ZnO usually contains various intrinsic defects such as Zn vacancies, interstitial Zn, O vacancies, interstitial O [12], and antisite O (O_Zn) [13]. These intrinsic defects form either acceptor level or donor level in the band gap that would greatly affect the luminescent properties of ZnO [13]. By introducing extrinsic dopant Al, the defect environment is changed whether the Al atom substitutes the zinc atom or it occupies the interstitial site. Therefore, it would be worthy to investigate this doping effect on the luminescent properties of AZO.

A sol-gel technique is adopted in this work as it provides excellent compositional control, homogeneity on the molecular level, and lower crystallization temperature. Recently, Xu et al. [14] and Kuo et al. [15] investigated the electrical and optical properties of c-axis-oriented sol-gel AZO films. In the present study, we explored the influence of aluminum doping and annealing on the optical and photoluminescent properties of sol-gel ZnO films.

Section snippets

Experimental

The starting material used in the present study was zinc acetate 2-hydrate (99.5% purity, Zn(CH₃COO)₂·2H₂O). Absolute ethanol was used as a solvent, and diethanolamine (HN(CH₂CH₂OH)₂, DEA) as a sol stabilizer. Aluminum as a dopant was added in the form of Al(NO₃)₃·9H₂O (extra purity) with three different Al/Zn ratios, i.e., 0, 0.5, and 2.0 at.%. A 0.5 M ethanolic solution of zinc acetate was prepared by dissolving 0.05 mol of zinc acetate 2-hydrate in 0.1 L of absolute ethanol in a 250 mL

Results and discussion

Fig. 1 shows the XRD patterns of AZO thin films annealed at 400 °C in air for 1 h. All samples are polycrystalline and exhibit the hexagonal structure (JCPDS 36-1451) with all peaks in the recorded range identified. No other crystalline phase was found within the phase detectability of XRD. As seen in Fig. 1, the intensity of the diffraction peaks is decreased, and the FWHM of the peaks is increased with increasing doping concentration, indicating that the film crystallinity is deteriorated.

Conclusions

We have prepared transparent AZO thin films by the sol-gel process. The film crystallinity was deteriorated with increasing Al doping concentration. With increasing Al doping concentration, a broadening in the band gap of the AZO films was observed due to the Moss–Burstein shift. A sharp UV peak ascribed to the free exciton emission and defect-related visible luminescence were observed in the AZO films. At a given Al doping concentration, the UV-to-visible emission ratio increased with

Acknowledgement

This work was supported by the 2005 Research Fund of the University of Ulsan.

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Optical and photoluminescent properties of sol-gel Al-doped ZnO thin films

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgement

Thin Solid Films

Sens. Actuators, B, Chem.

J. Eur. Ceram. Soc.

J. Cryst. Growth

Mater. Sci. Semicond. Process.

Prog. Mater. Sci.

J. Cryst. Growth

Thin Solid Films

J. Phys. Chem., B.

Adv. Mater.

Science

J. Vac. Sci. Technol., A.