Structural, optical and electrical properties of single-phase wurtzite ZnMgAlO thin films deposited by ultrasonic spray pyrolysis

Abstract

Incorporation of various elements in ZnO to modulate its optical and electrical properties is crucial for applications of the resulting alloys in optoelectronics. In particular, the co-incorporation of magnesium Mg and aluminum Al in ZnO to form the quaternary Zn_1−x−yMg_xAl_yO makes it possible to modulate the bandgap by Mg incorporation, and to control the electrical conductivity by Al incorporation. In this article, are presented the results of Zn_1−x−yMg_xAl_yO deposition by ultrasonic spray pyrolysis, using nontoxic and easily available precursors, and the study of the thin films structural, optical, and electrical properties. The deposition parameters are optimized in order to obtain wurtzite single-phase thin films with bandgap increasing with composition and transparency, higher than 90%, in a wide wavelength range. The electrical properties variation exhibits two competing regimes due to the co-incorporation of Al and Mg. On the one hand, the Al incorporation tends to increase the free carrier concentration, and, on the other hand, the Mg incorporation induces a bandgap increase and the inhibition of shallow donors by a compensation mechanism decreasing the carrier concentration. The high transparency of the wurtzite single-phase films along with the modulation of the bandgap and electrical properties can be interesting for optoelectronic applications, in particular in all-oxide solar cells and detectors.

Introduction

To modulate the optical and electrical properties of ZnO, mainly for use as a transparent conducting oxide (TCO) or active thin film in optoelectronic devices, various elements were incorporated into it to develop a vast family of alloys [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. In particular, the incorporation of aluminum and magnesium to obtain the quaternary ZnMgAlO has potential applications in solar cells as a window or buffer layer but also in display devices or as an active layer in some optoelectronic devices. For instance, Thapa et al. [8] developed by DC sputtering a luminescent device based on Zn_0.83Mg_0.17O after a post-annealing at a high temperature of 900 °C while Zhang et al. [6] used pulsed laser deposition to deposit ZnAlO and optimized its electrical properties after annealing in H₂/N₂ for use as TCO in, e.g., solar cells. Indeed, incorporating Al in ZnO, where Al³⁺ ions replace Zn²⁺ ions, can induce an increase in the free carrier concentration, while incorporating Mg, with the substitution of Zn²⁺ by Mg²⁺, will increase the bandgap without generating additional free carriers. It is therefore potentially possible to jointly modulate the electrical and optical properties of ZnO with the co-incorporation of Mg and Al. In a recent published work, Tsai et al. [11] prepared the quaternary ZnMgAlO thin films by radio frequency (RF) magnetron sputtering and presented a comprehensive study of their structural, electrical, and optical properties for use in ultraviolet optoelectronic applications. Yang et al. in Ref. [12] used pulsed laser deposition to deposit ZnMgAlO and demonstrated the potential of this material in particular for use as TCO in various optoelectronic devices. Another deposition approach, based on a sol-gel technique, was used by Cakiroglu et al. [13] to develop ZnMgAlO nanoparticles with 2% of aluminum and with magnesium composition varying from 5% to 20% and studied their morphology and structural properties. Recently, Karzazi et al. [14] used standard spray pyrolysis to fabricate ZnMgAlO with a fixed aluminum composition of 2% and a variable magnesium composition from 0% to 6%. They used anhydrous zinc acetate, hexahydrated aluminum chloride and magnesium sulfate as precursors. We propose here the development of ZnMgAlO by using ultrasonic spray pyrolysis (USP) deposition technique with aluminum compositions of 0%, 1% and 2% and magnesium composition from 0% to 7%, with zinc acetate dihydrate, aluminum acetylacetonate and magnesium acetate tetrahydrate as precursors. The main advantages of using USP combined to the use of nontoxic and easily available precursors are the flexibility to deposit thin films with tuneable parameters, its cost effectiveness and the possibility of easily transferring the developed processes to industry. A comprehensive study of the structural, optical and electrical properties of the USP deposited ZnMgAlO thin films is presented. The deposition conditions are detailed in section 2.1 alongside with the various characterization techniques in section 2.2. In section 3 are discussed the effect of aluminum and magnesium compositions on the structural, optical and electrical properties of the deposited thin films.