Multilayered ZnO-based thin films to control heat and electrical transport properties

Thumbnail Image
Journal Title
Journal ISSN
Volume Title
School of Chemical Technology | Doctoral thesis (article-based) | Defence date: 2021-03-26
Degree programme
80 + app. 42
Aalto University publication series DOCTORAL DISSERTATIONS, 26/2021
Interfaces between materials can have properties that differ greatly from the bulk state. In classical materials only a tiny fraction of atoms are at the interface while the vast majority is in the bulk of the material. The capability to engineer materials with an artificially high amount of interfaces opens up a pathway to amplify the interface effects and tailor the material properties by controlling the amount of interfaces. This approach to engineer materials step by step or layer by layer also allows for a controlled combination of very different materials into a hybrid material that would not form naturally and which can show fundamentally different and new properties. In this thesis atomic layer deposition (ALD), molecular layer deposition (MLD) and pulsed laser deposition (PLD) are utilized to engineer ZnO-based thin films with high interface densities. The films are analysed with x-ray reflectivity (XRR), x-ray diffraction (XRD) and transmission electron microscopy (TEM) in regards to their internal structure. Time domain thermoreflectance (TDTR) is utilized to measure the thermal conductivity, the electrical properties are measured with a hall measurement setup. The latter is the focus in layered thin films of polycrystalline ZnO and amorphous InGaZnO4 in which a considerable increase in the charge carrier concentration following the interface density could be demonstrated. The interfaces between a ZnO matrix, ZnO-benzene and AlOx layers are studied in detail in a hybrid ZnO/ZnO-benzene/AlOx system in which this work demonstrates, that these layers in ZnO can be as thin as a single atom/molecule, yet still form distinctive layers. However, these very thin layers of ZnO-benzene and AlOx are found to have little impact on the crystal growth of ZnO, but can act as effective barriers for ZnO crystal growth when 10 or more consecutive ALD/MLD cycles are utilized for each AlOx/benzene layer respectively. Finally the thermal conductivity in ZnO/benzene thin films is characterised, the database for the thermal conductivity in that system is significantly extended and thermal conductivities for irregularly layered structures are reported for the first time in ZnO/ZnO-benzene hybrid thin films. Analysis with multivariate data analysis of the database confirms that the interface density has the most pronounced effect on the thermal conductivity.  
Defense is held on 26.3.2021 12:00 – 15:00
Supervising professor
Karppinen, Maarit, Prof., Aalto University, Department of Chemistry and Materials Science, Finland
Interfaces, hybrid materials, thin film, ALD, MLD, PLD
Other note
  • [Publication 1]: F. Krahl, Yanling Ge and Maarit Karppinen. Characterization of Multilayer ZnO/AlOx/benzene Thin-Film Structures Grown through ALD/MLD. Semiconductor Science and Technology, 36, 025012, 2020.
    Full text in Acris/Aaltodoc:
    DOI: 10.1088/1361-6641/abcee2 View at publisher
  • DOI: 10.1002/aelm.202000404 View at publisher
  • [Publication 3]: Fabian Krahl, Ashutosh Giri, John A. Tomko, Tommi Tynell, Patrick E. Hopkins and Maarit Karppinen. Thermal Conductivity Reduction at Inorganic–Organic Interfaces: From Regular Superlattices to Irregular Gradient Layer Sequences. Advanced Materials Interfaces, 5, 11, 1701692, 2018.
    Full text in Acris/Aaltodoc:
    DOI: 10.1002/admi.201701692 View at publisher
  • [Publication 4]: Fabian Krahl, Ashutosh Giri, Md Shafkat Bin Hoque, Linda Sederholm, Patrick E. Hopkins and Maarit Karppinen. Experimental Control and Statistical Analysis of Thermal Conductivity in ZnO-Benzene Multilayer Thin Films. Journal of Physical Chemistry C, 124, 45, 24731–24739,2020.
    DOI: 10.1021/acs.jpcc.0c06461 View at publisher