Surface characterization of ZnO:Al transparent thin films
Abstract
We present the effect of hydrogen on surface characterization by Rf magnetron sputtered ZnO:Al thin films. ZnO:Al films are self textured and surface roughness of ZnO:Al films varies with H2 dilution ratio and ultimately affects on diffused transmittance varies from 41-27% with the variation of hydrogen (5-40%) with improved resistivity of 3.9×10-4 Ωcm. The light scattering effect of hydrogen textured thin films was evaluated by the measurement of diffused transmittance. Experimental result shows that introduction of hydrogen effect significantly improved the light trapping in terms of diffuse transmittance without affecting electrical and other optical properties of ZnO:Al thin film.
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Introduction
Transparent conductive oxides (TCOs) have been extensively studied because they are essential elements in thin film optoelectronic devices applications such as thin film solar cells, flat-panel displays, and light emitting diodes [13].Transparent conducting oxide (TCO) thin film is a special type of materials with wide band gap oxide (>3eV), high optical transmittance (≥ 85%) over wide range of solar spectrum, and low sheet resistance. Main important TCOs are Fluorine doped tin oxide (FTO), Indium tin oxide (ITO), Aluminum doped zinc oxide (ZnO:Al), Antimony doped tin oxide (ATO), Galium doped Zinc Oxide (ZnO:Ga), Galium doped Indium Zinc Oxide (IZO:Ga), and Indium doped Cadmium Oxide thin films. Other Ternary Compounds based TCO Materials are Zn2SnO4, MgIn2O4, CdSb2O6:Y, ZnSnO3, GaInO, Zn2In2O5 and In4Sn3O12 etc. Among all TCO thin films Fluorine doped tin oxide (FTO), and Indium tin oxide (ITO) are commercially available for different device applications and the others are under lab scale development stage. TCO thin films have wide applications such as in micro-electronic devices, displays, thin film transistor, light emitting diodes (LEDs), solar cells and other photonic devices [4-9]. Though indium tin oxide (ITO) film is extensively applied to photovoltaic devices and flat panel display because of its good electrical and optical properties, it has some problems such as high cost, low stability to H2 plasma and toxicity. Aluminum-doped zinc oxide thin films are make a compete as transparent conductive oxide (TCO) films prepared materials due to the advantages of ZnO:Al films are cheap and abundant elements [10]. In addition, AZO thin films have an excellent chemical stability and specific electronic/optical properties of a wide band gap (Eg=3.4 to 3.9) semiconductor. Therefore, AZO thin films are usually used as transparent conducting electrodes in solar cells. Several studies using different deposition methods have been reported, such as sol-gel processes [11], pulsed laser deposition [12], sputtering [13] and molecular beam epitaxy [14]. RF (Radio-Frequency) Sputtering method is an effective technique due to its ability to produce reasonable quality thin films at a high deposition rate [15]. Based on the progress in the previous works, it is important to better understand the influences of rf magnetron sputtering at different Ar flows and sputtering power to obtain the optimum conductivity and transmittance.
In this study ZnO: Al thin films were prepared by using the rf magnetron sputtering at different hydrogen dilution with sputtering power 150W to examine the optical and electronic properties. Transmittance of all films is about 80 - 90 % in the visible range. The lowest resistivity of 3.9×10-4 Ω-cm (18.5Ω/sq) and highest transmittance of 90 % was obtained at sputtering power of 150 W and Ar flow of 40 sccm. The observed property of the ZnO:Al thin films is suitable for transparent conductive electrode applications.
Conclusion
The electrical, optical and surface morphological properties of ZnO thin films etched with hydrogen were investigated in this paper. Experimental results show that hydrogen gas incorporation with Ar plays an important role in modifying the morphology, optical and electrical properties of ZnO films. The surface roughness of the textured ZnO thin film is increasing with decreasing of the hydrogen dilution concentrating resulting in light scattering, which turn in favors light is trapping to enhance the light absorption. The diffused transmittance increases from 27% to 41% with improved low resistivity from 7.6×10-4 to 3.9×10-4 of ZnO: Al with decreasing of CH value 40%-5%.In this study it was demonstrate that the introduced of more scattering diffused light improved the light absorption and resistivity of ZnO:Al film by proper textured surface through hydrogen gas.