Remote plasma deposition of metal oxides : routes for controlling the film growth

The second part of this thesis work was dedicated to extending the applicability of the ETP-MOCVD technique to obtain dense Al2O3 films at relatively low substrate temperatures (<400 ºC) compared to other CVD processes. While, initially, the ETP-deposited Al2O3 film properties were found to be rather poor, i.e., low refractive index (30 at%), a key parameter to obtain film densification was identified. Through the addition of ion bombardment to the ETP-MOCVD process by means of an external rf bias applied to the substrate, films with high refractive index (1.6 at 633 nm) and low hydrogen content (=4%) can be obtained at temperatures even below 150 ºC. These films are potentially suitable as water permeation barrier layers on polymers, as preliminaryMetal oxides are a class of materials which plays a major role in many present applications, ranging from optical coatings to microelectronics, photovoltaics and gas/moisture diffusion barrier technology. Thin metal oxide films can be obtained using different deposition techniques, such as physical vapor deposition (i.e., sputtering) and chemical vapor deposition. In the present project, an expanding thermal plasma metal organic chemical vapor deposition (ETP-MOCVD) technique was used for the deposition of zinc oxide (ZnO) and aluminum oxide (Al2O3) thin films. ZnO polycrystalline films have been intensively studied in the recent years as transparent conductive oxides for applications such as, among others, channel/gate layers in thin film transistors or front electrodes in solar cells, as well as applications which require both n- and p-type films, i.e., light emitting diodes. Al2O3 amorphous dielectric films, on the other hand, have shown great potential in high-k applications and, more recently, in gas/moisture diffusion barrier applications. Understanding the thin film growth and controlling it in terms of structure, morphology and opto-electrical properties is a necessary step in order to extend the application range of the deposited layers. In this work the evolution of the Al-doped ZnO (AZO) film properties during growth was investigated by an extensive set of ex situ and in situ techniques. In particular, the dependence of the intrinsic properties (crystallinity, stoichiometry, doping level, etc.) and extrinsic properties (grain size, morphology) on the film thickness was studied and correlated with the electrical characteristics of the deposited layers. As a result, it was shown that the working pressure plays an important role in controlling the development of the electrical and morphological film properties during growth. At 1.5 mbar ("high pressure") the AZO films are characterized by a low nucleation density, a large sheet resistance gradient with film thickness and high root-mean-square values, i.e., >4% of the film thickness. By decreasing the pressure from 1.5 mbar to 0.38 mbar ("low pressure"), the initial layer becomes denser, the sheet resistance gradient is significantly reduced and the films become smoother, i.e., 30 at%), a key parameter to obtain film densification was identified. Through the addition of ion bombardment to the ETP-MOCVD process by means of an external rf bias applied to the substrate, films with high refractive index (1.6 at 633 nm) and low hydrogen content (=4%) can be obtained at temperatures even below 150 ºC. These films are potentially suitable as water permeation barrier layers on polymers, as preliminary investigations have already indicated.