Elsevier

Vacuum

Volume 84, Issue 7, 4 March 2010, Pages 947-952
Vacuum

Highly transparent conductive and near-infrared reflective ZnO:Al thin films

https://doi.org/10.1016/j.vacuum.2010.01.010Get rights and content

Abstract

Al-doped ZnO (AZO) thin films have been prepared on glass substrates by pulsed laser deposition. The structural, optical, and electrical properties were strongly dependent on the growth temperatures. The lowest resistivity of 4.5 × 10−4 Ωcm was obtained at an optimized temperature of 350 °C. The AZO films deposited at 350 °C also had the high optical transmittance above 87% in the visible range and the low transmittance (<15% at 1500 nm) and high reflectance (∼50% at 2000 nm) in the near-IR region. The good IR-reflective properties of ZnO:Al films show that they are promising for near-IR reflecting mirrors and heat reflectors.

Introduction

Due to the long-time sun exposure in the tropics, there will be 85% solar energy penetrating through the common glass into our rooms. Along with the popularization of the cars, high temperature in the cars due to the sun exposure is attracting more and more attention. To this issue, there are two main problems. One is that the air-condition will waste too much fuel oil. On the other hand, the sun exposure will cause the aging of the materials in the cars. In this regard, a layer of thin film on the glass is necessary in order to reduce the sunlight radiation. The films should be highly transparent in the visible light region and highly reflective in the near-IR region. Tin-doped indium oxide (ITO) and doped zinc oxide (ZnO) thin films can both satisfy the foregoing characteristics. However, ITO has disadvantages including toxicity, high price, and instability in hydrogen plasma [1]. Accordingly, ZnO has attracted much attention to replace ITO because of high transmittance, non-toxicity, and low cost [2]. Undoped ZnO films are highly resistive. In contrast, doped ZnO such as Al-doped ZnO (AZO) can increase the conductivity by several orders of magnitude [3], [4]. AZO thin films can be prepared by many techniques including spray pyrolysis [5], magnetron sputtering [3], sol-gel [6], pulsed laser deposition (PLD) [7], and so on. Among these techniques, PLD seems more advantageous owing to the production of high-quality AZO films at high deposition rates and the similar composition between the films and corresponding targets [7].

To date, the reported reflectance of AZO films on glass was commonly lower than 40% at 2000 nm [8], [9], [10]. To meet the commercial applications, a higher near-infrared reflectance is necessary. In this work, we therefore focus on improving the optical performances in the visible and near-IR regions with high film quality. The influence of growth temperatures on the structural, electrical, and optical properties of AZO thin films has been studied in detail. Highly transparent conductive and near-IR reflective AZO films were deposited by PLD, with a high reflectance of 50% at 2000 nm under optimized growth conditions. To the best of our knowledge, such a high reflectance has not been reported in transparent conducting AZO films before.

Section snippets

Experiments

ZnO:Al thin films were deposited by PLD using a ceramic target with the deposition temperatures ranging from room temperature (RT) to 450 °C. The target was fabricated using high-purity ZnO (99.99%) and Al2O3 (99.99%) powders with an atomic ratio of Zn/Al of 0.97:0.03. The vacuum chamber was evacuated to a base pressure of 6 × 10−4 Pa firstly. Then, the deposition was carried out in high-purity oxygen ambient at a pressure of 0.5 Pa. All the films were deposited on glass substrates. The substrates

XRD analysis

Fig. 1 presents the XRD patterns (θ–2θ) of ZnO:Al films deposited at different substrate temperatures. As shown in this figure, the films exhibit highly preferred orientation with their crystallographic c-axis perpendicular to the substrates except the one deposited at RT. The intensity of (002) peak increases with the growth temperature and reaches a maximum at 350 °C, followed by a reduction with the temperature further increasing.

Fig. 2 shows the 2θ values of (002) diffraction peak of the

Conclusions

In conclusion, highly transparent conductive and near-infrared reflective ZnO:Al films have been deposited by PLD on glass substrates. The influence of growth temperature on the structural, electrical, and optical properties of ZnO:Al films was investigated in detail. All the AZO films were of c-axis preferred orientation, with high transmittance above 87% in the visible region. The lowest resistivity of 4.5 × 10−4 Ωcm was achieved at 350 °C. The highest reflectance at 2000 nm (∼50%) and the lowest

Acknowledgments

This work was financially supported by the National Basic Research Program of China under Grant No. 2006CB604906, the National Natural Science Foundation of China under Grant Nos. 50532060, 60340460439, and 50772099, and the Zhejiang Provincial Natural Science Foundation of China under Grant No. Y40512.

References (19)

  • Y.S. Rim et al.

    Colloid Surf Physicochem Eng Aspect

    (2008)
  • H.T. Cao et al.

    J Mater Sci

    (2004)
  • S.M. Park et al.

    Appl Surf Sci

    (2006)
  • W.W. Wang et al.

    Thin Solid Films

    (2005)
  • T. Tsuji et al.

    Appl Surf Sci

    (2000)
  • T. Schuler et al.

    Thin Solid Films

    (1999)
  • S.H. Bae et al.

    Appl Surf Sci

    (2000)
  • K. Ellmer et al.

    Surf Coat Technol

    (1997)
  • K. Ellmer et al.

    Thin Solid Films

    (1994)
There are more references available in the full text version of this article.

Cited by (106)

  • Fine-grained ZnO ceramic fabricated by high-pressure cold sintering

    2022, Ceramics International
    Citation Excerpt :

    Most of the experimental works have demonstrated a holding time of 1 h or less is sufficient [18]. ZnO ceramics are widely applied in the display screen, solar cells, electronic devices, and other fields [28]. Therefore, fabrication of ZnO ceramics targets with high density, small grain size and high electrical conductivity has attached much attention [29].

View all citing articles on Scopus
View full text