Abstract
We report a rapid, self-catalyzed, solid precursor-based thermal plasma chemical vapor deposition process for depositing a conformal, nonporous, and optically transparent nanocrystalline ZnO thin film at 130 Torr (0.17 atm). Pure solid zinc is inductively heated and melted, followed by ionization by thermal induction argon/oxygen plasma to produce conformal, nonporous nanocrystalline ZnO films at a growth rate of up to 50 nm/min on amorphous and crystalline substrates including Si (100), fused quartz, glass, muscovite, c- and a-plane sapphire (Al2O3), gold, titanium, and polyimide. X-ray diffraction indicates the grains of as-deposited ZnO to be highly textured, with the fastest growth occurring along the c-axis. The individual grains are observed to be faceted by (103) planes which are the slowest growth planes. ZnO nanocrystalline films of nominal thicknesses of 200 nm are deposited at substrate temperatures of 330°C and 160°C on metal/ceramic substrates and polymer substrates, respectively. In addition, 20-nm- and 200-nm-thick films are also deposited on quartz substrates for optical characterization. At optical spectra above 375 nm, the measured optical transmittance of a 200-nm-thick ZnO film is greater than 80%, while that of a 20-nm-thick film is close to 100%. For a 200-nm-thick ZnO film with an average grain size of 100 nm, a four-point probe measurement shows electrical conductivity of up to 910 S/m. Annealing of 200-nm-thick ZnO films in 300 sccm pure argon at temperatures ranging from 750°C to 950°C (at homologous temperatures between 0.46 and 0.54) alters the textures and morphologies of the thin film. Based on scanning electron microscope images, higher annealing temperatures appear to restructure the ZnO nanocrystalline films to form nanorods of ZnO due to a combination of grain boundary diffusion and bulk diffusion.PACS: films and coatings, 81.15.-z; nanocrystalline materials, 81.07.Bc; II-VI semiconductors, 81.05.Dz.
Highlights
Zinc oxide [zinc oxide (ZnO)] is a direct, wide bandgap (Eg = 3.37 eV at room temperature) semiconductor which has a high exciton binding energy (60 meV) [1,2,3,4,5]
Properties of as-deposited ZnO film At a constant argon-to-oxygen (99.67% to 0.33%) ratio and a constant total flow rate (301 sccm), the morphological and dimensional properties of the ZnO nanocrystalline thin films are found to be dependent on factors including source temperature profile, deposition duration above the melting point of zinc (420°C), substrate type and temperature, and thermal annealing temperatures
scanning electron microscopy (SEM) images of ZnO deposited from a nickel heating chamber subjected to saw-toothed temperature profile up to 570° C show a strong positive linear relationship between the heating durations above 420°C and the film thicknesses, as shown in Figures 3 and 4
Summary
A direct, rapid, close-to-ambient pressure vapor phase synthesis method using inexpensive precursors is highly desirable from a synthesis and process development standpoint To address such challenges, this paper reports a rapid, direct, self-catalyzed thermal plasma chemical CVD process for depositing a conformal, nonporous nanocrystalline ZnO thin film on various crystalline and amorphous substrates using solid zinc as the precursor material at 130 Torr. For a high rate of deposition, the RF power and plasma intensity - which is proportional to the rate of change of the temperature of the nickel heating chamber - must be relatively high, yet the nickel heating chamber temperature is to be maintained well below the boiling point of Zn, so that Zn droplets do not form and deposit on the substrate as metallic zinc. Thermal annealing of samples is performed in a tube furnace (MTI GSL-1100X, MTI Corporation, Richmond, CA, USA) at 300 sccm of argon flow at temperatures ranging from 750°C to 950°C for 1 h
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