Characterization of Boron-Incorporated Zinc Oxide Film Chemically Prepared from an Aqueous Solution

Abstract

Zinc oxide (ZnO) film finds use in numerous applications such as transparent electrode in liquid crystal displays, and components in photovoltaic devices and surface acoustic wave devices. ZnO films are prepared by several techniques, such as radio-frequency mag-netron sputtering, chemical vapor deposition, and molecular beam epitaxy. Preparation of oxide film from an aqueous solution presents several advantages over the above-mentioned techniques: (i) the thickness and morphology of film can be controlled by electro-chemical parameters, (ii) relatively uniform film can be obtained on substrates with complex shape, (iii) films can be obtained on sub-strates with a low melting point, below 100C, such as polymer, (iv) the deposition rate is relatively high, (v) the equipment is inexpensive , and (vi) the technique is less hazardous and more environmentally friendly. The direct formation of crystalline ZnO film by elec-trochemical reaction from aqueous solutions has been demonstrated by Izaki et al. 1-4 and Peulon and Lincot. 5,6 Izaki demonstrated for the first time that ZnO film was prepared on a nonconductive substrate by chemical reaction from an aqueous solution containing a zinc nitrate and dimethylamineborane (DMAB) without any postdeposition treatment. 7 Chemical preparation of oxide films has further advantage over electrochemical preparation because they can be prepared on nonconductive substrates such as glasses without an external electric source, which was indispensable for the electrochemical system. The chemical preparation could be exploited very easily in industrial fields, since the deposi-tion system is much simpler than that for electrochemical preparation. According to the electrodeposition mechanism 2 and oxidation reaction of DMAB, 8 the author already proposed tentatively chemical deposition mechanism of ZnO film as described by the following schemes 7 Zn(NO 3) 2 r Zn 2 2NO 3 (CH 3) 2 NHBH 3 H 2 O r BO 2 (CH 3) 2 NH 7H 6e NO 3 H 2 O 2e r NO 2 2 OH Zn 2 2 OH r Zn(OH) 2 Zn(OH) 2 r ZnO H 2 O The reduction reaction of the nitrate ion plays an important role in ZnO formation from the solution and is driven by the oxidation reaction of DMAB contained in the solution. It was found that the chemically prepared ZnO film incorporated small amounts of boron atoms originating from the DMAB. Since the boron atom acts as a donor in semiconducting ZnO film as already reported, 9 it is expected that the amount affects the electrical properties. The aim of this paper is to investigate the effects of incorporated boron on the structural, optical, and electrical characteristics with X-ray diffraction (XRD), observation of surface morphology , measurements of optical transmission spectra, and Hall measurements. Experimental ZnO films were prepared by immersing a substrate in an aqueous solution containing 0.1 mol/L zinc nitrate hydrous and 0.005-0.1 mol/L DMAB kept at 333 K. The solution was prepared with distilled water and the reagent grade chemical. The solution had a nearly constant pH of 6.2. Corning no. 1737 glass was used as the sub-strate. Prior to deposition, the glass substrates (20 20 1.1 mm) were rinsed in acetone and then catalyzed using an industrially employed two-step Pd/Sn activation process composed of a sensitizer and activater (Okuno Chemical Industries, top-sensitizer and top-activater). Boron content in the films was evaluated with an inductively coupled plasma-atomic emission spectrometry (Seiko Instruments, SPS4000). Surface morphology was observed in air using a tapping mode atomic force microscope (AFM, Digital Instruments, Nano-Scope IIIa and Dimension 3000). Film thickness was determined by averaging five measurements obtained from the scanning electron microscopy (SEM) micrographs of film cross sections. XRD measurement was performed using a MacScience MXP18 system with a monochromated Cu K radiation operated at 45 kV and 200 mA. The diffraction angles were referenced to those for high-purity Si powder. 3 Optical transmission and absorption spectra were recorded by scanning wavelength in the range 190-800 nm with reference to air and glass substrate, respectively. The resistivity, carrier concentration, and mobility were evaluated at 296 K using a Hall effects measuring system (Toyo Technica, Resitest 8320) and 0.5 T magnetic field. Results and Discussion Deposition rate.-Figure 1 shows a deposition rate of ZnO films as a function of DMAB concentration. Any film could not be obtained from a simple zinc nitrate aqueous solution in the absence of DMAB. The data represented in Fig. 1 were reproduced even for the solution kept at 333 K for 48 h. Films up to 1 m thick were formed and the thickness was almost proportional to deposition time, regardless of Boron-incorporated ZnO film which had a wurtzite structure and showed optical bandgap energy of 3.3 eV was prepared chemically onto a nonconductive substrate by immersing the substrate in an aqueous solution containing a zinc nitrate and dimethyl-amineborane (DMAB) at 333 K. Effects of the incorporated boron on the structural, optical, and electrical characteristics of ZnO film were investigated using X-ray diffraction, evaluation of surface morphology with an atomic force microscope, measurements of optical transmission spectra, and Hall measurement. Small amounts of boron atoms, which originated from the DMAB, were incorporated into ZnO grain and gave the lattice expansion. A pore-free ZnO film with a smooth surface was obtained from the 0.1 mol/L DMAB solution. The ZnO film showed optical transmission as high as 80% in the visible light region and resistivity of 3.6 10 2 cm with carrier concentration of 1.7 10 16 cm 3 and mobility of 1.0 cm 2 V 1 s 1. It was speculated that the incorporated boron atom acted as a donor in the ZnO film.