Influence of the grain boundary barrier height on the electrical properties of Gallium doped ZnO thin films

Abstract

The pulsed laser deposition (PLD) technique is used to deposit Gallium doped zinc oxide (GZO) thin films on glass substrates at 250 with different Gallium (Ga) doping concentration of 0, 1.0, 3.0 and 5.0%. The influence of Ga doping concentration on structure, chemical atomic compositions, electrical and optical properties was investigated by XRD, XPS, Hall measurement and UV spectrophotometer, respectively. The relationship between electrical properties and Ga doping concentration was clarified by analyzing the chemical element compositions and the chemical states on the GZO films. It is found that the carrier concentrations and oxygen vacancies in the GZO films increase with increasing Ga doping concentration. The lowest resistivity (3.63 × 10⁻⁴ Ω cm) and barrier height of grain boundaries (14 mV) were obtained with 3% Ga doping. In particular, we suppose the band gap of 5% Ga doping sample larger than that of 3% Ga doping sample is due to the quantum size effect from the amorphous structure rather than Moss–Burstein shift.

Introduction

Transparent conducting oxides (TCO) with optical transmittance exceeding 80% in the visible region (550 nm) and resistivity less than 10⁻⁴ Ω cm have been widely used in a variety of applications. Most of the previous research on TCOs has been focused on indium-doped tin oxide (ITO) [1], [2]. ITO thin films are often applied on opto-electrical devices due to their excellent conductivity and transparency. However, the ITO films are expensive and not suitable for plasma deposition processes. Thus, the development of alternative TCO materials is necessary to resolve this serious problem. Zinc oxide (ZnO) films have been investigated in recent years as TCOs because of their good electrical and optical properties in combination with their large band gap of 3.3 eV [3], abundance in nature, and lack of toxicity [4]. The electrical behavior of ZnO thin films could be significantly improved by replacing Zn atoms with higher valence elements, such as In, Al and Ga [5], [6], [7], [8], [9], [10]. From many reports which have reported about the doping effect of impurities on ZnO, they were widely demonstrated that Al and Ga were the best dopants for transparent conductive ZnO films. The Gallium doped zinc oxide (GZO) films have more advantages compared with alumina doped zinc oxide (AZO) films. The radius of Ga³⁺ (0.062 nm) is closer to that of Zn²⁺ (0.074 nm) than that of Al³⁺ (0.053 nm) [11], [12], [13]. Gallium is a good dopant because it is less reactive with oxygen [14] and has less moisture resistance [10] than other dopants. For those reason, many researchers gradually focus on the study of GZO thin films.

Various techniques have been used to deposit ZnO-based films on different substrates, including metal-organic chemical vapor deposition (MOCVD) [15], molecular beam epitaxy (MBE) [16], pulsed laser deposition (PLD) [17], and spray pyrolysis deposition (SPD) [18]. Compared to other deposition methods, PLD is characterized by several advantages, such as low deposited temperature, good adhesion on substrates, and the easy deposition of alloys and compounds of materials with different vapor pressures [17], [19]. GZO thin films which were deposited by PLD have shown better resistivity near ∼10⁻⁴ Ω cm [20], [21], [22]. In this paper, high quality GZO transparent conductive films were prepared by 355 nm PLD with various Ga doping concentrations. The dependence of the structural, electrical and optical properties of the GZO films on Ga doping concentration was investigated. The X-ray photoelectron spectroscopy (XPS) of oxygen vacancies have been used to probe the carrier concentration. The barrier height of grain boundaries at GZO thin films, with different percentages of Ga doping concentration, could be calculated from the temperature dependence on electrical mobility.