Growth and characterization of ZnO and MgxZn1 − xO thin films by aerosol assisted chemical vapor deposition technique
Highlights
►Design of aerosol assisted chemical vapor deposition (AACVD) system for oxide thin films. ►MgxZn1 − xO thin films have been deposited on glass substrate using AACVD system. ►MgxZn1 − xO thin films were polycrystalline and single phase up to x = 0.2. ►The films are optically transparent with low absorbance. ►Variation of lattice parameters of MgxZn1 − xO films follow Vegard's law up to x = 0.2.
Introduction
The growth and characterization of ZnO and ZnO based alloys e.g. MxZn1 − xO (M = Mg, Cd and Mn etc.) have become an active research field due to the wide ranging applications of the above materials in the fabrication of light emitting diodes, UV-photo detectors, UV-blue semiconductor lasers, flat panel displays, solar cells, gas sensors, photo-voltaic cells and transparent electrodes [1], [2], [3]. Since the oscillator strength of excitons is typically much larger than that of direct electron–hole transitions in direct band gap semiconductors, the large exciton binding energy of ZnO (60 meV) compared to GaN (25 meV) makes it a promising material for optical devices utilizing excitonic effects. Optically pumped lasing has been reported in ZnO platelets, thin films, clusters consisting of ZnO nano-crystals and ZnO nano wires [4], [5], [6]. Recently, several near ultraviolet diode sources have been reported that utilize the GaN material system [7]. As an alternative to the GaN material system, ZnO alloys are of great interest. Alloying ZnO films with MgO or CdO permits the band gap to be tailored between 2.8 and 4 eV, which facilitates band gap engineering [8]. This also suggests the possibility of hybrid optoelectronic devices comprising lattice-matched MgZnO/AlGaN heterojunctions. Although the crystal structure of ZnO is hexagonal, whereas that of MgO is cubic, the near equality of ionic radii of Mg++(1.36 Å) and Zn++ (1.25 Å) allows some replacement in either structure [9]. According to the phase diagram, MgO allows a maximum of 56 wt.% ZnO at 1600 °C and maintains its NaCl structure with the lattice constant remaining close to that of pure MgO (4.208 Å) [10].
In the case of bulk ZnO, the bulk solid solubility of Mg is limited to only maximum 2%, and the unit cell retains its hexagonal structure [11]. However, the solubility of Mg in ZnO thin films has been reported to have different values, e.g. Ryoken et. al. [12] have reported the stable wurtzite formation up to x = 0.18 by pulsed laser deposition (PLD) technique. Kang et al. [13] have reported the solubility limit to be up to x = 0.4 by magnetron sputtering technique. Ohtomo et al. [14] have reported this limit to be up to x = 0.36 by PLD technique. Besides the above reports, MgxZn1 − xO films have also been deposited by metalorganic chemical vapor deposition [15], molecular beam epitaxy [16], sputtering [13], [17], spray pyrolysis [18], etc. In the present work, we have employed the aerosol assisted chemical vapor deposition (AACVD) technique for the growth of MgxZn1 − xO films on soda lime glass substrates. AACVD is an attractive technique due to its low cost, simplicity, minimal waste production and the possibility of injecting liquid precursor or solid precursor dissolved in suitable solvents in the CVD reactor for growth of thin films. This technique becomes specifically beneficial for liquid/solid precursors with very low vapor pressure.
There are a few reports on the use of AACVD technique for the deposition of ZnO, ZrC, F:SnO2, α-Fe2O3 thin film electrodes, yttria stabilized zirconia [19], [20], [21], [22], [23] etc. In this technique, a nozzle of extremely fine dimensions is used to inject the solution of the precursor directly onto the heated substrate. Using AACVD technique, we have been able to incorporate up to 20 wt.% Mg in ZnO while maintaining the wurtzite hexagonal structure. The crystalline structure and the surface morphology of ZnO films were investigated using X-ray diffraction (XRD) and atomic force microscopy (AFM) respectively. The optical properties of the films were studied via optical spectrophotometery. In this paper, we report the deposition of single phase wurtzite MxZn1 − xO thin films up to x = 0.2.
Section snippets
Experimental technique
In the present work, we have used an indigenously developed aerosol assisted chemical vapor deposition (AACVD) system for the growth of thin films of ZnO and MgxZn1 − xO. The schematic of the AACVD system is shown in Fig. 1. The system consists of a horizontal SS-304 cylindrical reactor, where the growth is carried out at atmospheric pressure. The precursor was dissolved in a suitable solvent and used as the source of the depositing material. This liquid precursor was injected into the reactor
Optical characterization
The transmittance spectra of the deposited thin films of ZnO and MgxZn1 − xO are shown in Fig. 2. Transmittance of the substrate is also shown in the same figure. The presence of interference fringes and a sharp absorption edge can be seen in all the films. Absorption edges have been highlighted in the inset shown in Fig. 2. Good optical qualities of MgxZn1 − xO films are grown irrespective its Mg content. As expected, the sharp band edge absorption feature shifts to lower wavelengths (blue shift)
Conclusions
We have deposited good optical quality poly-crystalline MgxZn1 − xO thin films using an indigenously developed AACVD system. The transmission data clearly shows a shift in the band edge to lower wavelengths with increasing Mg-content in the films. This information has been used to determine the Mg-content in the films and the Mg incorporation efficiency for an AACVD system has been determined. XRD data shows that in the range of Mg compositions (0 ≤ x ≤ 0.2) studied in this work, there is no phase
Acknowledgment
The authors acknowledge Dr. Meena Tiwari of SGSITS, Indore for the UV–VIS measurement. Authors VK, TG and RK thank Dr. S.M. Oak of RRCAT, Indore for encouragement and support during the course of the work.
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