Pulse repetition rate dependent structural, surface morphological and optoelectronic properties of Ga-doped ZnO thin films grown by pulsed laser deposition

Highlights

• Highly transparent conductive Ga-doped ZnO films were prepared by PLD technique.

• Effect of pulse repetition rate on film properties was studied for the first time.

• All the films are polycrystalline with a hexagonal wurtzite crystal structure.

• Pulse repetition rate greatly affects the optoelectronic properties of the films.

• The GZO film prepared at 5 Hz exhibits optimum figure of merit, i.e. 0.0761/Ω.

Abstract

Transparent conductive Ga-doped ZnO (GZO) thin films were grown on glass substrates by pulsed laser deposition using 2 at% Ga₂O₃ doped ZnO target. A Substrate temperature of 400 °C and oxygen pressure of 7 × 10⁻³ Torr was kept constant during the deposition. The effects of laser pulse repetition rate from 3 to 15 Hz on crystallographic, surface morphological, optical and electrical properties of the GZO films were investigated. All the samples show the hexagonal wurtzite phase with a dominant c-axis orientation and the optimal crystallization of the GZO film occurs at 10 Hz; which is slightly better that of the film deposited at 5 Hz. AFM and FE-SEM analysis revealed that the film growth is profoundly influenced by the pulse repetition rate and the film thickness increases with an increase in the pulse repetition rate due to enhancement in deposition rate and the island density. Hall-effect measurements confirmed that all samples are heavily doped degenerate semiconductors with n-type electrical conductivity. The electrical transport properties of the film are found to be largely dependent on the crystallinity; while their optical transmittance is majorly dominated by the film thickness and roughness. The GZO film deposited at 5 Hz shows the highest figure of merit (76.11 × 10⁻³/Ω) as a result of its optimum sheet resistance (3.63 Ω/□) and optical transmittance (∼88%).

Introduction

Over the years, transparent conductive oxide (TCO) thin films have been extensively applied for optoelectronic devices consisting of solar cells, flat-panel displays, and light emitting diodes, since these films simultaneously ensure high transmittance in the visible region of the electromagnetic spectrum and low resistivity [1], [2], [3], [4], [5]. Particularly, indium tin oxide (ITO) i.e. Sn-doped In₂O₃ films are the leading TCO materials [1]. However, indium in this material has some serious drawbacks such as toxicity and high cost as a result of its limited resources, so it is required to search for its substitute materials. Since last decade or so, the ZnO doped with some other group-III elements such as Al and Ga has been grown as promising alternative material to ITO considering their beneficial features such as low cost, non-toxicity, high transparency, and high chemical stability [6]. Amongst these elements, Al has high reactivity and thus the oxidation of Al would lead to the deterioration of Al-doped ZnO film performance. On the other hand, Ga is more resistant to oxidation [7] and the covalent bond length between Ga–O (1.92 Å) and ZnO (1.97 Å) is relatively well-matching than that of Al–O (2.7 Å) [8]. Therefore, even higher Ga doping concentrations would produce less strain and only a small local deformation of the ZnO lattice. Obviously, many researchers are continuously engaged in investigations on Ga-doped ZnO (GZO) thin films for optoelectronic applications.

To date, a variety of physical and chemical thin film deposition techniques have been demonstrated by researchers to grow transparent conducting GZO thin films, though the pulsed laser deposition (PLD) technique has always proven its uniqueness [9], [10], [11], [12], [13], [14] through its main advantageous features such as low growth temperature, stoichiometric material transfer, excellent adhesion, possibility to coat multi-component materials, high deposition rate and high material yield efficiency [15]. In the PLD method, the preparation parameters such as substrate temperature and oxygen pressure have been reported to strongly influence the Ga-doped ZnO film properties [16], [17]. Additionally, the laser related parameters such as laser fluence, wavelength, pulse duration and repetition rate also have seen to influence the nucleation process and growth kinetics of the film, and hence their properties [18], [19], [20]. Although there were several earlier investigations on the PLD growth of Ga-doped ZnO thin films, most researchers have grown the films with laser pulses of fixed repetition rate [16], [17], [21], [22], [23]. However, there is hardly any exclusive report on the growth of Ga-doped ZnO films by PLD with different laser pulse repetition rates to investigate its influence on the optoelectronic properties of films, to the best of our knowledge. Such report may offer some physical insights for the investigators which may be useful for their studies on the optoelectronic devices.

Therefore, in the present work, transparent and conducting Ga-doped ZnO thin films were synthesized on glass substrates using the PLD technique at different laser pulse repetition rate with an aim of optimizing this key process parameter to tailor the desired optoelectronic properties. All the films are grown at 400 °C substrate temperature using the laser having different laser pulse repetition rates ranging from 3 to 15 Hz of a 248 nm KrF excimer laser, keeping all other experimental parameters unchanged. Dependence of crystallographic, surface morphological, electrical and optical properties of GZO thin films on laser pulse repetition rate were systematically investigated by using X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, Hall Effect measurement and ultraviolet–visible spectroscopy techniques.