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Alumina Thin Film Growth: Experiments and Modeling
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . Linköping University, The Institute of Technology.
2007 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis deals with experimental and theoretical studies related to the growth of crystalline alumina thin films. Alumina, Al2O3, is a polymorphic material utilized in a variety of applications, e.g., in the form of thin films. Many of the possibilities of alumina, and the problems associated with thin film synthesis of the material, are due to the existence of a range of different crystalline phases. Controlling the formation of the desired phase and the transformations between the polymorphs is often difficult.

In the experimental part of this work, it was shown that the thermodynamically stable alpha phase, which normally is synthesized at substrate temperatures of around 1000 °C, can be grown using reactive sputtering at a substrate temperature of 500 °C by controlling the nucleation surface. This was done by predepositing a Cr2O3 nucleation layer. Moreover, it was found that an additional requirement for the formation of the α phase is that the depositions are carried out at low enough total pressure and high enough oxygen partial pressure. Based on these observations, it was concluded that energetic bombardment, plausibly originating from energetic oxygen, is necessary for the formation of α alumina (in addition to the effect of the chromia nucleation layer). Further, the effects of impurities, especially residual water, on the growth of crystalline films were investigated by varying the partial pressure of water in the ultra high vacuum (UHV) chamber. Films deposited onto chromia nucleation layers exhibited a columnar structure and consisted of crystalline α-alumina if deposited under UHV conditions. However, as water to a partial pressure of 1x10-5 Torr was introduced, the columnar growth was interrupted. Instead, a microstructure consisting of small, equiaxed grains was formed, and the gamma-alumina content was found to increase with increasing film thickness. When gamma-alumina was formed under UHV conditions, no effects of residual water on the phase formation was observed. Moreover, the H content was found to be low (< 1 at. %) in all films. Consequently, this shows that effects of residual gases during sputter deposition of oxides can be considerable, also in cases where the impurity incorporation in the films is found to be low.

In the modeling part of the thesis, density functional theory based computational studies of adsorption of Al, O, AlO, and O2 on different alpha-alumina (0001) surfaces have been performed. The results give possible reasons for the difficulties in growing the α phase at low temperatures through the identification of several metastable adsorption sites, and also provide insights related to the effects of hydrogen on alumina growth.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2007. , 41 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1292
Keyword [en]
Alumina, Reactive sputtering, Phase formation, Residual water, Surface adsorption, Density functional theory
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:liu:diva-8461ISBN: 978-91-85715-98-5 (print)OAI: oai:DiVA.org:liu-8461DiVA: diva2:23234
Presentation
2007-02-15, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
Report code: LiU-TEK-LIC-2007:1.Available from: 2007-03-02 Created: 2007-03-02 Last updated: 2013-10-30
List of papers
1. Phase control of Al2O3 thin films grown at low temperatures
Open this publication in new window or tab >>Phase control of Al2O3 thin films grown at low temperatures
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2006 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 513, no 1-2, 57-59 p.Article in journal (Refereed) Published
Abstract [en]

Low-temperature growth (500 °C) of α-Al2O3 thin films by reactive magnetron sputtering was achieved for the first time. The films were grown onto Cr2O3 nucleation layers and the effects of the total and O2 partial pressures were investigated. At 0.33 Pa total pressure and ≥ 16 mPa O2 partial pressure α-Al2O3 films formed, while at lower O2 pressure or higher total pressure (0.67 Pa), only γ phase was detected in the films (which were all stoichiometric). Based on these results we suggest that α phase formation was promoted by a high energetic bombardment of the growth surface. This implies that the phase content of Al2O3 films can be controlled by controlling the energy of the depositing species. The effect of residual H2O (10− 4 Pa) on the films was also studied, showing no change in phase content and no incorporated H (< 0.1%). Overall, these results are of fundamental importance in the further development of low-temperature Al2O3 growth processes.

Place, publisher, year, edition, pages
Elsevier, 2006
Keyword
Aluminum oxide, Chromium oxide, Sputtering, Ion bombardment, X-ray diffraction
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14318 (URN)10.1016/j.tsf.2006.01.016 (DOI)
Note
Original publication: Andersson, J.M., Wallin, E., Helmersson, U., Kreissig, U. and Münger, E.P., Phase control of Al2O3 thin films grown at low temperatures, 2006, Thin Solid Films, (513), 1-2, 57-59. http://dx.doi.org/10.1016/j.tsf.2006.01.016. Copyright: Elsevier B.V., http://www.elsevier.com/ Available from: 2007-03-02 Created: 2007-03-02 Last updated: 2017-12-13Bibliographically approved
2. Ab initio studies of Al, O, and O2 adsorption on α-Al2O3 (0001) surfaces
Open this publication in new window or tab >>Ab initio studies of Al, O, and O2 adsorption on α-Al2O3 (0001) surfaces
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2006 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 12, 125409-1-125409-9 p.Article in journal (Refereed) Published
Abstract [en]

The interactions of Al, O, and O2 with different α- Al2O3 (0001) surfaces have been studied using ab initio density functional theory methods. All three surface terminations obtainable by cleaving the bulk structure [single Al-layer (AlO), double Al-layer (AlAl), and O terminations] have been considered, as well as a completely hydrogenated O-terminated surface. Adsorbed Al shows strong ioniclike interaction with the AlO - and O-terminated surfaces, and several metastable adsorption sites are identified on the O-terminated surface. On the completely hydrogenated surface, however, Al adsorption in the bulk position is found to be unstable or very weak for the studied configurations of surface H atoms. Atomic O is found to interact strongly with the AlAl -terminated surface, where also O2 dissociative adsorption without any appreciable barrier is observed. In contrast, O adsorption on the AlO -terminated surface is metastable relative to molecular O2. On the O-terminated surface, we find the creation of O surface vacancies to be plausible, especially upon exposure to atomic O at elevated temperatures. The results are mainly discussed in the context of alumina thin film growth and provide insight into phenomena related to, e.g., preferred adsorption sites and effects of hydrogen on the growth.

Place, publisher, year, edition, pages
College Park, MD, United States: American Physical Society, 2006
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-10427 (URN)10.1103/PhysRevB.74.125409 (DOI)000240872500080 ()
Note

Original publication: E. Wallin, J.M. Andersson, E.P. Münger, V. Chirita & U. Helmersson, Ab initio studies of Al, O, and O2 adsorption on α- Al2 O3 (0001) surfaces, 2006, Physical Review B, (74), 125409. http://dx.doi.org/10.1103/PhysRevB.74.125409. Copyright: The America Physical Society, http://prb.aps.org/

Available from: 2007-12-12 Created: 2007-12-12 Last updated: 2017-12-14Bibliographically approved
3. Influence of residual water on magnetron sputter deposited crystalline Al2O3 thin films
Open this publication in new window or tab >>Influence of residual water on magnetron sputter deposited crystalline Al2O3 thin films
2008 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 516, no 12, 3877-3883 p.Article in journal (Refereed) Published
Abstract [en]

The effects of residual water on the phase formation, composition, and microstructure evolution of magnetron sputter deposited crystalline alumina thin films have been investigated. To mimic different vacuum conditions, depositions have been carried out with varying partial pressures of H2O. Films have been grown both with and without chromia nucleation layers. It is shown that films deposited onto chromia nucleation layers at relatively low temperatures (500 °C) consists of crystalline alpha-alumina if deposited at a low enough total pressure under ultra high vacuum (UHV) conditions. However, as water was introduced a gradual increase of the gamma phase content in the film with increasing film thickness was observed. At the same time, the microstructure changed drastically from a dense columnar structure to a structure with small, equiaxed grains. Based on mass spectrometry measurements and previous ab initio calculations, we suggest that either bombardment of energetic negative (or later neutralized) species being accelerated over the target sheath voltage, adsorbed hydrogen on growth surfaces, or a combination of these effects, is responsible for the change in structure. For films containing the metastable gamma phase under UHV conditions, no influence of residual water on the phase content was observed. The amounts of hydrogen incorporated into the films, as determined by elastic recoil detection analysis, were shown to be low. Overall, the results demonstrate that residual water present during film growth drastically affects film properties, also in cases where the hydrogen incorporation is found to be low.

Place, publisher, year, edition, pages
ScienceDirect, 2008
Keyword
Aluminum oxide, Phase formation, Sputtering, Water
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-11476 (URN)10.1016/j.tsf.2007.07.135 (DOI)
Note
Original publication: E. Wallin, J.M. Andersson, M. Lattemann, and U. Helmersson, Influence of residual water on magnetron sputter deposited crystalline Al2O3 thin films, 2008, Thin Solid Films, (516), 12, 3877-3883. http://dx.doi.org/10.1016/j.tsf.2007.07.135. Copyright: Elsevier B.V., http://www.elsevier.com/Available from: 2008-04-03 Created: 2008-04-03 Last updated: 2017-12-13Bibliographically approved

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