Remote Plasma Deposition of Metal Oxides: Routes for Controlling the Film Growth

Ioana Volintiru
Abstract & Cover

Metal 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 (Al203) 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 it and ptype films, i.e., light emitting diodes. Al203 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 AI-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., <1% of the film thickness. The sheet resistance gradient and the surface roughness development correlate with the grain size evolution, indicating the transition from pyramid-like at high pressure to pillar-like growth mode at low pressure. The in situ use of the spectroscopic ellipsometry (SE) technique, a novelty in the ZnO field, allowed the real time identification of the growth mode by monitoring the thickness development in the initial growth stage and the roughness evolution during film growth. A slower growth rate was observed for the pyramid-like films during the initial growth phase compared to the bulk, while the pillar-like films exhibited a linear increase in thickness at all stages of growth. A saturation behavior in the roughness evolution for films thicker than 150-200 nm was observed for the pyramid-like structure, while for pillar-like films the roughness scales linearly with the film thickness. The relation between these differences and the two growth modes was validated by comparison with ex situ measurements, such as time-of-flight secondary ion mass spectrometry TOF-SIMS (initial growth) and atomic force microscopy AFM (roughness). Moreover, the SE measurements proved to be useful in determining the in grain electronic properties of the AZO films, which is essential to define the role of grain boundaries in limiting the electron transport in ZnO films. The results obtained demonstrate excellent in grain mobility values, i.e., above 100 cm2/Vs (pyramid-like growth) and 50 cm2/Vs (pillar-like growth), independent of the film thickness. These values are much higher than the ones provided by the ex situ measurements (Hall), which indicates that the limiting factor for the electron transport in these films is the scattering at grain boundaries. Controlling the film growth mode is very important from an application point of view. The low resistivity and high roughness of the pyramid-like films make them suitable as front electrodes in a-Si:H and in pc-Si solar cells applications. However, the sheet resistance gradient with thickness and the low nucleation density makes them unsuitable vvlieti the AZO films are deposited on the solar cell as a substrate or in applications where thinner and smoother layers are required, such as thin film transistors. In these cases the pillar-like films or a combination of the two modes might be more appropriate. Moreover, the film growth studies, both ex Sal and in situ, presented in this thesis, indicate that a valid route for further improving the conductivity of the AZO films is to increase the grain size at the initial stage of film growth by, for example, increasing the substrate temperature or using a ZnO buffer layer as substrate. Another challenge in the ZnO field is to obtain ptype conductivity, which, in combination with the more easily obtainable n-type ZnO, would allow the fabrication of ZnO homojunctions. In this work initial studies on the plasma chemistry and its influence on the doping efficiency were performed in the case of nitrogen-doped ZnO. Because the expanding thermal plasma has a high dissociation degree for N2, allowing a large flux of N radicals and/or N-containing species towards the substrate, it could be an excellent source for p-type N-doping of ZnO films. Nitrogen incorporation in the ZnO films was successfully obtained using this technique. The nitrogen was found to incorporate preferentially as -CEN (nitrile bond), which is electrically inactive. As a consequence, no p-type conductivity was generated in the N-doped ZnO films. The detection of CN presence in the film using infrared spectroscopy is a novelty in the field and it is found to be corroborated by the formation of HCN in the plasma, suggesting an inherent limitation in any deposition process which combines the use of a metalorganic precursor with a highly reactive nitrogen environment. Using the knowledge acquired in this project, a valid route to overcome this limitation can be proposed, i.e., to combine the advantages of both ETP and sputtering techniques, by using a metal or metal oxide target, sputtered in an expanding thermal Ar/N2 plasma environment. The second part of this thesis work was dedicated to extending the applicability of the ETP-MOCVD technique to obtain dense Al203 films at relatively low substrate temperatures (< 400 0C) compared to other CVD processes. While, initially, the ETP-deposited Al203 film properties were found to be rather poor, i.e., low refractive index (<1.5 at 633 nm) and high hydrogen content (>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 (540/0) can be obtained at temperatures even below 150 ciC These films are potentially suitable as water permeation barrier layers on polymers, as preliminary investigations have already indicated.

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Eindhoven University of Technology
(Eindhoven, Netherlands)
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