@misc{DasHenkelTallaridaetal., author = {Das, Chittaranjan and Henkel, Karsten and Tallarida, Massimo and Schmeißer, Dieter and Gargouri, Hassan and K{\"a}rkk{\"a}nen, Irina and Schneidewind, Jessica and Gruska, Bernd and Arens, Michael}, title = {Thermal and plasma enhanced atomic layer deposition of TiO2: Comparison of spectroscopic and electric properties}, series = {Journal of Vacuum Science and Technology A}, volume = {2015}, journal = {Journal of Vacuum Science and Technology A}, number = {33}, issn = {1520-8559}, doi = {10.1116/1.4903938}, pages = {01A144-1 -- 01A144-8}, abstract = {Titanium oxide (TiO2) deposited by atomic layer deposition (ALD) is used as a protective layer in photocatalytic water splitting system as well as a dielectric in resistive memory switching. The way ALD is performed (thermally or plasma-assisted) may change the growth rate as well as the electronic properties of the deposited films. In the present work, the authors verify the influence of the ALD mode on functional parameters, by comparing the growth rate and electronic properties of TiO2 films deposited by thermal (T-) and plasma-enhanced (PE-) ALD. The authors complete the study with the electrical characterization of selected samples by means of capacitance-voltage and current-voltage measurements. In all samples, the authors found a significant presence of Ti3+ states, with the lowest content in the PE-ALD grown TiO2 films. The observation of Ti3+ states was accompanied by the presence of in-gap states above the valence band maximum. For films thinner than 10 nm, the authors found also a strong leakage current. Also in this case, the PE-ALD films showed the weakest leakage currents, showing a correlation between the presence of Ti3+ states and leakage current density.}, language = {en} } @misc{DasKotHenkeletal., author = {Das, Chittaranjan and Kot, Małgorzata and Henkel, Karsten and Schmeißer, Dieter}, title = {Engineering of Sub-Nanometer SiOₓ Thickness in Si Photocathodes for Optimized Open Circuit Potential}, series = {ChemSusChem}, volume = {9}, journal = {ChemSusChem}, number = {17}, issn = {1864-5631}, doi = {10.1002/cssc.201600777}, pages = {2332 -- 2336}, abstract = {Silicon is one of the most promising materials to be used for tandem-cell water-splitting devices. However, the electrochemical instability of bare Si makes it difficult to be used for stable devices. Besides that, the photovoltage loss in Si, caused by several factors (e.g., metal oxide protection layer and/or SiO₂/Si or catalyst/Si interface), limits its use in these devices. In this work, we present that an optimized open circuit potential (OCP) of Si can be obtained by controlling the SiOₓ thickness in sub-nanometer range. It can be done by means of a simple and cost-effective way using the combination of a wet chemical etching and the low temperature atomic layer deposition (ALD) of TiO₂. We have found that a certain thickness of the native SiOₓ is necessary to prevent further oxidation of the Si photocathode during the ALD growth of TiO₂. Moreover, covering the Si photocathode with an ALD TiO₂ layer enhances its stability.}, language = {en} } @inproceedings{RouissiHenkelTallaridaetal., author = {Rouissi, Zied and Henkel, Karsten and Tallarida, Massimo and Schmeißer, Dieter}, title = {ALD growth of Al2O3 on Stepped Surface of HOPG and Ag-HOPG}, series = {Verhandlungen der Deutschen Physikalischen Gesellschaft}, booktitle = {Verhandlungen der Deutschen Physikalischen Gesellschaft}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, pages = {S. 419}, abstract = {Atomic Layer Deposition (ALD) is an excellent deposition technique to grow thin film with high homogeneity coverage on ideal surfaces. Here we are interested in the growth properties on non-ideal (stepped, inert) surfaces. Using STM we studied the growth of Al2O3 by ALD on stepped surface of HOPG and silver covered HOPG (Ag-HOPG). The HOPG sample was cleaved to and then cleaned by N2 in vacuum. We obtain regular steps with terrace widths of 50nm - 500nm. Ag was deposited by thermal deposition on the HOPG. The Al2O3 growth at RT was studies after the first, third, and fifth cycle. Silver evaporation leads to step decoration with island of 1nm-7nm. On the terraces we find the formation of 2D domain of Ag chains. The features caused by the individual ALD cycles of Al2O3 depend on the terrace widths of the HOPG substrate. For small terraces (<100nm) we obtain a growth of 2D domains ( 20nm-25nm) on the edge steps and in the middle of the terrace. For large terraces (>100nm) we find agglomeration of the precursors on individual nucleation sites. Here 3D islands are formed with a height of up to 5nm in the fifth ALD cycle.}, language = {en} } @misc{JanowitzMahmoodinezhadNaumannetal., author = {Janowitz, Christoph and Mahmoodinezhad, Ali and Naumann, Franziska and Plate, Paul and Henkel, Karsten and Flege, Jan Ingo}, title = {XPS and UPS investigation of an ALD prepared Al2O3/ZnO heterojunction}, series = {Verhandlungen der DPG - SurfaceScience21}, volume = {2021}, journal = {Verhandlungen der DPG - SurfaceScience21}, publisher = {Deutsche Physikalische Gesellschaft e.V.}, address = {Bad Honnef}, abstract = {The band alignment of two large band gap oxides was studied by a combination of XPS and UPS using consecutive sputter steps to unravel the electronic structure and elemental composition of each layer and the interface region. An Al2O3/ZnO heterointerface (10 nm Al2O3 on 59 nm ZnO) was grown on top of a Si single crystal substrate by consecutive thermal and plasma-assisted atomic layer deposition (ALD) respectively. The valence band maximum of Al2O3 was found to be 1.1 eV below that of ZnO, the conduction band minimum 2.3 eV above, resulting in a type-I staggered heterojunction. A reduction of ZnO to elemental Zn in the interface region was detected by the apparent shoulder of the Zn 2p and 2s core levels and by the Zn LMM Auger. This suggests an ALD interface formation mechanism different from previous models identified for other heterointerfaces.}, language = {en} } @misc{KostoTschammerMoralesetal., author = {Kosto, Yuliia and Tschammer, Rudi and Morales, Carlos and Henkel, Karsten and Flege, Jan Ingo and Ratzke, Markus and Fischer, Inga Anita and Costina, Ioan and Alvarado Chavarin, Carlos and Wenger, Christian}, title = {Rational design and development of room temperature hydrogen sensors compatible with CMOS technology: a necessary step for the coming renewable hydrogen economy}, series = {Proceedings of iCampus Conference Cottbus 2024}, journal = {Proceedings of iCampus Conference Cottbus 2024}, publisher = {AMA Service GmbH}, address = {Wunstorf}, isbn = {978-3-910600-00-3}, doi = {10.5162/iCCC2024/P21}, pages = {182 -- 185}, abstract = {The transition towards a new, renewable energy system based on green energy vectors, such as hydrogen, requires not only direct energy conversion and storage systems, but also the development of auxiliary components, such as highly sensitive hydrogen gas sensors integrated into mass devices that operate at ambient conditions. Despite the recent advances in nanostructured metal oxide thin films in terms of simple fabrication processes and compatibility with integrated circuits, high sensitivity, and short response/recovery times usually require the use of expensive noble metals or elevated tem-peratures (>250 ºC), which results in high power consumption and poor long-term stability. This article presents the first steps of the work on developing a novel resistive hydrogen gas sensor based on ultrathin cerium oxide films, compatible with complementary metal oxide semiconductor technology and capable of operating at room temperature. Here, we show a multidisciplinary bottom-up approach combining different work areas for the sensor development, such as sensor architecture, sensing mechanism and deposition strategy of the active layer, electrical contact design depending on the desired electrical output, and fast testing under controlled environments.}, language = {en} } @misc{MoralesKostoTschammeretal., author = {Morales, Carlos and Kosto, Yuliia and Tschammer, Rudi and Henkel, Karsten and Flege, Jan Ingo}, title = {Reduction by H2 exposure at room temperature of ceria ultrathin films grown by atomic layer deposition}, series = {Verhandlungen der DPG}, journal = {Verhandlungen der DPG}, publisher = {Deutsche Physikalische Gesellschaft}, address = {Bad Honnef}, issn = {0420-0195}, abstract = {Atomic layer deposition (ALD) exhibits a high potential for integration as a scalable process in microelectronics, allowing well-controlled layer-by-layer deposition and conformal growth on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric films, potentially resulting in modified materials properties that, in the case of ultra-thin deposits, can also be affected by film/substrate interaction. Interestingly, initial in situ X-ray photoemission spectroscopy (XPS) measurements of ceria ALD-deposits on Al2O3/Si, sapphire, and SiO2 substrates confirm a Ce3+/Ce4+ mixture dependent on the substrate interaction, deposit thickness, and morphology. Using near-ambient pressure XPS, we have significantly reduced ultrathin (< 10 nm) ceria films grown by ALD by exposing them to different O2/H2 partial pressures at moderate temperatures (< 525K). Notably, the total amount of reduction to Ce3+ is found to depend on the deposit thickness and initial ceria/substrate interaction. Furthermore, the intrinsic defects related to the ALD method seem to play a critical role in the reversible reduction at room temperature.}, language = {en} }