Abstract
The ability to grow inorganic thin films with highly controllable structural and optical properties at low substrate temperature enables the manufacturing of functional devices on thermo-sensitive substrates without the need of material postprocessing. In this study, the authors report on the growth of zinc oxide films by direct plasma-enhanced atomic layer deposition at near room temperature. Diethyl zinc and oxygen plasma were used as the precursor and coreactant, respectively. The process was optimized with respect to the precursor and coreactant dosing as well as to the purging times, which ultimately resulted in saturated atomic layer deposition growth. The so-obtained films exhibit a polycrystalline pattern with a (100) texture and low amount of incorporated carbon. Furthermore, the possibility to tune crystallite size, refractive index, and bandgap of the films by adapting the plasma radio-frequency power is demonstrated.
Highlights
In order to ensure self-limited atomic layer deposition (ALD) growth and to avoid a chemical vapor deposition (CVD)-like growth mode, each of the four steps in an ALD cycle has to be saturated; that is, the precursor and plasma exposure time should be sufficient to react with all available surface sites, and the purging steps need to effectively remove unreacted precursor molecules, byproducts, and oxidizing species, in order to avoid reactions in the gas phase
We demonstrate the deposition of polycrystalline Zinc oxide (ZnO) with a (100) texture by plasma-enhanced atomic layer deposition (PE-ALD) at substrate temperatures close to room temperature using DEZ and O2-plasma as the precursor and coreactant, respectively
A few examples are present in literature of studies concerning the combination DEZ, PE-ALD and room temperature deposition, which to the authors’ opinion is the most convenient combination in terms of monomer availability (DEZ as opposed to dimethyl zinc (DMZ) is largely available), and processing parameters
Summary
Zinc oxide (ZnO) is a wide direct band gap semiconductor (Eg $ 3.3 eV) with attractive optoelectronic and piezoelectric properties. These properties are essential for various applications such as solar cells, transparent conductive oxide layers, piezoelectric nanogenerators, and gas sensors. The ability to tune optical and structural properties of the zinc oxide films is crucial for adapting the material characteristics to meet device requirements (i.e., enhanced device efficiency or sensitivity). the application of ZnO on thermo-sensitive substrates such as polymers and biomaterials requires deposition processes operating at low temperatures which should be well below the glass transition temperature of polymers and temperatures that would lead to, e.g., degradation of the biomolecules. Thermal ALD of ZnO at 23 C has been reported using DEZ and water as the metal precursor and coreactant, respectively, resulting in polycrystalline films with a (002) preferential orientation and a GPC of around 0.85 A /cycle.. To the authors’ knowledge, no previous results have been reported for depositions near room temperature by applying DEZ and O2-plasma This low substrate temperature makes it possible to process thermo-sensitive substrates and prevents decomposition effects of the precursor so that controlled ALD-growth can be achieved. In this contribution, the tuning of ZnO properties has been investigated. In the literature, tuning of structural, optical, and electrical properties has been achieved by modifying process parameters such as substrate temperature, O2-plasma dose, or introducing additional process steps. In this study, we investigated the influence of RF-power on the structural and optical properties of the ZnO films and demonstrate a simple approach to tune the crystallite size, refractive index, extinction coefficient, and bandgap for PE-ALD at room temperature
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