Metal-Organic Chemical Vapor Deposition of Zn-In-Sn-O and Ga-In-Sn-O Transparent Conducting Oxide Thin Films
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transparency versus those produced in equilibrium bulk materials. For TCO film preparation, diverse techniques have been utilized including sputtering [6,7,11,12], pulsed laser deposition [6,7], spray pyrolysis [13,14], and metal-organic chemical vapor deposition (MOCVD) [15-17]. Among these, MOCVD processes offer many attractions for device fabrication, such as in-situ growth under a variety of atmospheres, amenability to larger area coverage with high throughput, conformal coverage, easy control of growth chemistry, and the possibility of creating metastable phases. In this report, we demonstrate effective MOCVD process for Sn-doped Zn-In-O and GaIn-O films, with the goals of defining appropriate growth conditions, the role of dopant and reductive annealing, and the relationship of film processing to microstructure and electrical/optical properties. EXPERIMENTAL The horizontal, low-pressure MOCVD reactor design described previously [ 18] was used
in these studies. The volatile metal-organic precursors In(dpm) 3 , Ga(dpm) 3 , and Zn(dpm) 2 (dpm = dipivaloylmethanate) were prepared from high-purity metal nitrates and were vacuumsublimed twice. Sn(acac) 2 was prepared from SnCl 2 and vacuum distilled. Details of precursor conditions for film growth are shown in Table 1. Films were grown at 400-500 'C on clean quartz and glass slide substrates using oxygen (100-200 sccm) as the oxidant. Some films were then reductively annealed at 2 Torr, 400-425 'C in a 200 sccm flowing gas mixture of H2 (4%) and N2. Characterization of the as-grown and annealed films was carried out using the microstructural and charge transport analysis instrumentation described previously [ 19]. Film composition was analyzed by SEM/EDX and XPS, and was calibrated using inductively coupled plasma (ICP) spectrometry. Optical transparency measurements were carried out with a Cary lE (uv-vis) spectrometer. Film thicknesses were measured with a Tencor P- 10 profilometer after etching a step in the film using concentrated HCl as the etchant. TABLE 1. Process condition for precursors used in film deposition Precursor In(dpm)3 Ga(dpm) 3 Zn(dpm) 2 Temperature 'C 98-104 86-90 78-84 Flow gas (Ar), sccm 50-80 5-20 10-60 w
Sn(acac) 2 RT 5-20
RESULTS AND DISCUSSION In-Situ Growth of Zn-In-O and Zn-In-Sn-O Films ZnIn 2Ox+3 films with compositions in the range x = 0-8 were grown in-situ (without post annealing) on quartz and glass at growth rate -20 kmin using a cold-wall system. Consistent with our earlier results [16,17] on Zn.In 2Ox. 3 films deposited in hot-wall systems, films exhibit the maximum 25 'C conductivity, 1030 S/cm (n-type, N = 4.5 x 1020 cm3 , g = 14.3 cm 2/Ves) at a Zn/In ratio of 0.33. The as-grown films are polycrystalline with x-ray reflection features assignable to cubic In 20 3 at low Zn levels and to ZnO at low In levels. Films with a Sn/(Zn+In) ratio in the range of 0-2.2 were also grown on quartz and glass at substrate temperature of 450 - 500 'C. X-ray diffraction reveals poorly crystalline film microstructures, with broad reflection
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