Hydrothermal growth method for the deposition of ZnO films: Structural, chemical and optical studies

Highlights

• ZnO films were deposited on ZnO seed layer using hydrothermal growth method.

• ZnO seed layers were prepared by RF magnetron sputtering technique.

• The effect of using zinc acetate and chloride on the films properties was studied.

• ZnO films prepared hydrothermally via acetate have better quality.

• PL study demonstrated a potential optical application in optoelectronic field.

Abstract

Hydrothermal growth method was used to grow the zinc oxide (ZnO) thick films on ZnO seed layer previously deposited by RF magnetron sputtering technique. The effect of introducing of zinc acetate and chloride salts on the structure, chemical and physical properties of the obtained films was investigated. Scanning electron microscope (SEM) and atomic force microcopy (AFM) were utilized to both justify the nanostructure growth of the ZnO thick films deposited by hydrothermal growth method, and to identify their morphology. Energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) confirmed the elemental stoichiometry of the prepared ZnO thick films with low contamination. X-ray diffraction (XRD), Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR) characterizations approved the texturation of the ZnO thick films prepared by hydrothermal growth method and verified the nonstructural growth with (002) preferred orientation as well as hexagonal phase. XRD technique indicated that the ZnO films had a good quality (big grain size and preferential orientation) and the Raman and FTIR studies confirmed the XRD results. Moreover, photoluminescence (PL) spectroscopy verified the optoelectronic behavior and demonstrated a potential optical application in this field.

Introduction

Nanostructured zinc oxide (ZnO) materials have attracted noticeable interests due to their remarkable performance in electronics, optics, and photonics [1]. ZnO has interesting physical and chemical properties [2], [3], such as wide-band gap semiconductivity, luminescenсу properties, photoconductivity, antibacterial activity, biocompatibility, biodegradability, and catalytic activity. These properties suggest the use of zinc oxide for many applications in optoelectronic devices, photodetectors, photocatalysts, and as sensors [1], [2]. Semiconductors films especially ZnO and ZnS have been prepared by means of different methods such as thermal evaporation [4], [5], electron gun deposition [6], magnetron sputtering [7], sol-gel technique [8], and hydrothermal method [9], [10]. Among these different methods, the hydrothermal method is attractive technique for the deposition of nanostructures. Indeed, it presents several advantages such as simplicity, low cost, environment friendly conditions, and vast versatility to control the structure and the morphology of the obtained nanostructures by modifying the deposition conditions [3], [11]. The deposition is generally performed in aqueous solution. Therefore, the temperatures are less than the boiling point of water [11]. Vayssieres et al. have used the hydrothermal method, for the first time, for growing of zinc oxide nanostructures on glass and Si substrates [12]. They have applied the aqueous thermal decomposition of zinc nitrate and hexamethylenetetramine (HMT) to obtain ZnO thin films. The hydroxyl anions are released and reacted with the Zn cations to form ZnO [1]. The following chemical equations summarize this process [13]:

$${\left({\mathrm{CH}}_2\right)}_6{\mathrm{N}}_4+6{\mathrm{H}}_2\mathrm{O}\leftrightarrow 6\text{HCHO}+4{\mathrm{NH}}_3$$

$${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{NH}}_4^{+}+{\mathrm{OH}}^{-}$$

$${\mathrm{Zn}}^{2+}+{\mathrm{OH}}^{-}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}^{+}$$

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_2$$

$${\mathrm{Zn}}^{2+}+3{\mathrm{OH}}^{-}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_3^{-}$$

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_4^{2-}$$

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_4^{2-}\to \mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

Several works have reported that the experimental conditions of the growth such as initial solution pH, precursor concentration, and growth temperature have a remarkable effect on the aspect ratio and morphology [2], [14]. Akgun et al. have investigated the use of zinc acetate dihydrate as the source of zinc cations in hydrothermal synthesis of ZnO, as the zinc nitrate hexahydrate is primarily used in many studies, while the zinc chloride is mainly preferred for electrodepositing [9]. The same group has studied the effect of using different zinc salts including acetate, chloride, and nitrate with HMT during hydrothermal growth of ZnO nanowires [15]. The results have revealed that zinc acetate dihydrate salt has allowed the growth of ZnO nanowires with highest aspect ratio. Previously, Govender et al. have found a noticeable morphological effect on the resulting ZnO films by using different counter-ions of zinc in baths containing HMT [16]. While using acetate, formate, and chloride salts mainly have produced rod-like crystallites, whereas nitrate and perchlorate anions mainly have formed wires. Finally, flat hexagonal platelets have been obtained using sulfate anions. However, the optical study comparing the effect of using different zinc salts with HMT during hydrothermal growth of ZnO films is still lacking. In this work, the ZnO thick films were deposited using hydrothermal growth method using HMT and zinc acetate and chloride salts. The effect of introducing zinc acetate and chloride salts on the structure, chemical and physical properties of the obtained films was investigated using different characterization techniques including SEM, EDX, XPS, AFM, XRD, Raman, and FTIR. Finally, the PL spectroscopy was applied to study the optoelectronic behavior of the obtained films.