Atomic layer deposition of Transparent Conductive Films
Begeleiding: Jakob Kuhs
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Transparent Conductive Films (TCFs) play an important role in optoelectronic devices such as thin film solar cells and light emitting diodes (LEDs). Most of these films are n-type wide band gap metal oxides like Al-doped ZnO and Sn-doped indium oxide (ITO). At present, there are only a few p-type TCFs with performance comparable to n-type materials. The development of p-type sulfides could lead to a new range of design concepts for optoelectronic devices and transparent electronics. Recent results suggest that ZnS may be a promising material for inorganic p-type TCFs.
For many optoelectronic applications highly uniform and conformal thin films have to be deposited onto three-dimensional structures. To date, atomic layer deposition (ALD) is the most promising technique for the conformal deposition of nano coatings. ALD is a self-limited deposition method that is characterized by alternating exposure of the growing film to chemical precursors, resulting in the sequential deposition of (sub)mono layers over the exposed sample surface. The self-limiting nature of the vapor-solid reactions ensures pinhole free coatings with a precise thickness controlled at the atomic scale and a superb conformality onto large scale substrates with complex topologies.
The aim of this master thesis is to develop an ALD process for p-type ZnS thin films. As p-type dopants, Cu and N, respectively from copper organic precursors and Ammonia/Ammonia plasma, will be investigated. One of the challenges will be to control the amount of incorporated dopants either by steric hindrance of the used precursor or by occupying free surface sites with alkyl alcohols. In-situ ellipsometry will be used to monitor the growth of the films during ALD. To enable electrical characterization of the deposited material, different metals will be screened in order to determine which one forms the best electrical contact to ZnS.
Atomic Layer deposition will be done in-house on a home-made research type ALD reactor. The physical properties of the deposited films will be characterized by x-ray photoelectron spectroscopy (XPS), x-ray reflectivity, fluorescence and diffraction methods. The electrical and optical properties of p-type ZnS will be investigated by sheet resistance measurements and spectroscopic ellipsometry, respectively.