Fundamental and functional properties of ultra-thin oxide films grown by atomic layer deposition (ALD) studied in-situ by means of surface sensitive techniques
Final Report Abstract
In this project a systematic in-situ study of fundamental and functional properties of the atomic layer deposition (ALD) growth using surface sensitive methods was performed. The effect of the surface morphology on the growth mechanism was investigated. The occurrence and strength of surface diffusion was characterized by means of in-situ cycle-bycycle scanning tunneling microscopy for ALD films on substrates with regularly stepped surfaces. The distribution of ALD nucleating sites as a function of substrate temperature and average terrace width was determined. Particularly it was found, that the growth of HfO2 on a Si(111)-H terminated surface follows two regimes. Initially (regime I, first and second ALD cycle) the growth is governed by random deposition followed by Mullins diffusion. This process is independent on the ALD substrate temperature. Afterwards (regime II, the layer is closed after the second cycle), the nucleation is not occurring anymore at room temperature (RT) leading to high surface diffusion. In contrast, at 280°C the growth in this regime follows random deposition dynamics with chemisorptions which are not occurring at RT. Synchrotron radiation photoemission and absorption spectroscopy was conducted on several oxide systems (HfO2, Al2O3, TiO2, TiOxNy) prepared by ALD. In particular the electronic structure of the films was resolved by resonant photoelectron spectroscopy. Besides the derivation of the partial density of states and the electronic band schemes of these materials, intrinsic defects states including charge transfer, excitonic and polaronic states were identified by their individual spectroscopic signature. All band schemes of these materials are very similar. The appearance of the intrinsic defect states within the gap of all investigated materials indicates a common mechanism for their formation. Seemingly this is not related with the size of the band gap as these are quite different in these systems. It demonstrates that all the oxide films always have intrinsic charges. The tailoring of ALD film functionalities upon the choice of the appropriate supporting material and the applied ALD process and its parameters were investigated. Here in particular the role of the substrate for the strength of appearance of the different defect states was shown for Al2O3 ALD layers on Ru, Si and pervoskite substrates. For the three systems quite different behaviors were observed. On Ru, the intensity of the excitonic state is the lowest, and the charge transfer states are dominating the electronic structure within the band gap, whereas on the perovskite the excitonic states are dominating and on Si excitonic and charge transfer states are showing similar intensities in the data. On TiO2 and TiOxNy ALD films it was intensively demonstrated how in-gap states attributed to excitonic or polaronic defect states are influencing the electrical conductivity of the films and how these properties can be modified by the choice of the ALD procedure. Finally ALD layers were used as protection layers against photo-corrosion or photo-oxidation in Si photocathodes for water splitting devices as well as humidity barriers in perovksite solar cells. For both systems performance improvements could be realized by the use of the ALD layers. The Si photocathode was covered by a thin TiO2 ALD layer where the aforementioned tuning of the conductivity via the in-gap states enables the high conductivity for the transport of the generated charge carriers when the TiO2 layer protects the electrode against the direct contact to the electrolyte. Furthermore, the energetic position of the charge transfer band of TiO2 coincides with the energetic positions of the band edges of Si photoelectrodes. As a result an enhanced long term stability and optimized onset potential was observed. For perovskite solar cells it was demonstrated that the performance and stability can be improved/recovered by covering the perovskite films with a thin Al2O3 ALD layer where the excitonic defects may play a key role.
Publications
- Quantum size effects in TiO 2 thin films grown by atomic layer deposition. Beilstein J. Nanotechnol. 5 (2014) 77
M. Tallarida, C. Das, D. Schmeißer
(See online at https://doi.org/10.3762/bjnano.5.7) - Si microstructures laminated with a nanolayer of TiO 2 as long-term stable and effective photocathodes in PEC devices. Nanoscale 7 (2015) 7726
C. Das, M. Tallarida, D. Schmeißer
(See online at https://doi.org/10.1039/c5nr00764j) - Electronic properties of atomic layer deposited films, anatase and rutile TiO2 studied by resonant photoemission spectroscopy. J. Phys D: Appl. Phys. 49 (2016) 275304
C. Das, M. Richter, M. Tallarida, D. Schmeißer
(See online at https://doi.org/10.1088/0022-3727/49/27/275304) - Room temperature Atomic Layer Deposition of Al2O3: impact on efficiency, stability and surface properties in perovskite solar cells. ChemSusChem 9 (2016) 3401
M. Kot, C. Das, Z. Wang, K. Henkel, Z. Rouissi, K. Wojciechowski, H. J. Snaith, D. Schmeisser
(See online at https://doi.org/10.1002/cssc.201601186) - Spectroscopic investigation of the electronic structure of thin atomic layer deposition HfO2 films. J. Vac. Sci. Technol. A 34 (2016) 01A117
S. Alberton Corrêa, S. Brizzi, D. Schmeisser
(See online at https://doi.org/10.1116/1.4935338) - Localized defect states and charge trapping in atomic layer deposited-Al2O3 films. J. Vac. Sci. Technol. A 35 (2017) 01B125
K. Henkel, M. Kot, D. Schmeißer
(See online at https://doi.org/10.1116/1.4971991)
