Low-Temperature Spatial Atomic Layer Deposition of Metal Oxides and Their Integration

Abstract

Metal oxide thin films are central to several emerging technologies, including sustainable flexible packaging, perovskite solar cells (PSCs), and next-generation optoelectronic devices that combine two-dimensional (2D) semiconductors with three-dimensional (3D) substrates. The translation of these technologies from laboratory demonstrations to industrial manufacturing requires deposition methods that simultaneously deliver high film quality, atomic-scale thickness control, compatibility with temperature-sensitive substrates, and the throughput needed for continuous large-area production. Atmospheric-pressure spatial atomic layer deposition (AP-SALD) can meet these combined requirements. By physically separating the precursor zones in space, and by operating without vacuum infrastructure, AP-SALD preserves the conformality and pinhole-free coverage of conventional ALD while achieving deposition rates up to two orders of magnitude higher and full compatibility with roll-to-roll processing. However, the low-temperature operation of AP-SALD (50-200 °C) that makes it attractive for thermally sensitive substrates also introduces challenges, including incomplete precursor decomposition, residual carbon and hydroxyl impurities, restricted grain growth, and defect-rich microstructures whose consequences propagate directly into device performance. This thesis establishes the process-property-performance relationships that link AP-SALD growth conditions to the structural, optical, electrical, and mechanical characteristics of the resulting metal oxide films, and to the performance of the devices into which they are integrated. Four interconnected studies are presented. First, scalable Al2O3-ZnO nanolaminate (NL) barrier coatings are demonstrated on compostable polylactic acid (PLA) and recyclable polyethylene terephthalate (PET) packaging films using a sheet-to-sheet AP-SALD system operating at 50 °C. An 8-stack NL of approximately 96 nm total thickness achieves a water vapor transmission rate (WVTR) below 0.5 g·m-2·24hr-1 on PLA and below the 0.005 g·m-2·24hr-1 measurement limit on PET, while retaining a WVTR below 2 g·m-2·24hr-1 when laminated between PET films and subjected to the harshest industry-standard Gelbo flex-durability tests. The continuity of individual nanolaminate layers is identified as essential for full barrier performance, defining an optimal design window for the coating, and the less-than-0.4 wt% additional material contributed by the nanolaminate is shown to be compatible with composting and recycling pathways. Second, a systematic study of how the AP-SALD deposition temperature (50-200 °C) controls the structural, compositional, optical, and electronic properties of SnO2 and ZnO thin films is reported. ZnO is found to crystallize in the wurtzite phase across the entire temperature range, with the grain size increasing from 30 nm at 50 °C to 150 nm at 200 °C, while SnO2 remains amorphous at low temperatures and develops nanocrystalline rutile character at 200 °C. The optical bandgap narrows with increasing deposition temperature (3.40 to 3.29 eV for ZnO; 4.01 to 3.79 eV for SnO2), the bulk carbon and hydroxyl content decrease, and the resistivity decreases by approximately nine orders of magnitude, reaching conductivities of 181 S/cm for SnO2 and 5.41 S/cm for ZnO at 200 °C with closely aligned conduction-band positions suitable for electron-selective contacts. These results provide a quantitative framework for tuning the properties of AP-SALD metal oxides for specific optoelectronic applications. Third, this fundamental understanding is translated into the practical optimization of AP-SALD SnO2 as the electron transport layer (ETL) in perovskite solar cells. By systematically identifying the impact of post-deposition annealing and substrate roughness on perovskite crystallization, a champion power conversion efficiency of 20.2 % is achieved in a simplified n-i-p architecture using AP-SALD SnO2 as the sole ETL, matching the performance of spin-coated SnO2 nanoparticle reference devices. To the best of the author’s knowledge, this represents the first high-efficiency n-i-p PSC employing AP-SALD SnO2 as the sole ETL without additional buffer or surface-modification layers, and advances AP-SALD toward industrial-scale, solvent-free, and cost-effective ETL deposition for perovskite photovoltaics. Fourth, the thesis investigates how the surface morphology, defect density, and hydroxylation of AP-SALD-grown ZnO govern exciton dynamics in mixed-dimensional WS2/ZnO heterostructures. By comparing AP-SALD polycrystalline ZnO with single-crystalline m-plane ZnO references using multi-modal spectroscopy, microscopy, and density functional theory, the rough, defect-rich AP-SALD surface is shown to create a gapped, discontinuous interface that produces approximately threefold slower exciton dissociation, spatially inhomogeneous charge transfer, enhanced electron-phonon coupling, and a unique photoinduced absorption feature at 2.03 eV that is proposed as a spectroscopic diagnostic for AP-SALD ZnO interface quality. Specific surface properties (hydroxylation, defect density, and roughness) are identified as design levers for tailoring exciton dynamics in mixed-dimensional optoelectronic devices. Together, these studies establish AP-SALD as a versatile and scalable low-temperature platform whose process parameters can be tailored to the very different requirements of sustainable packaging, photovoltaics, and 2D-material-based optoelectronics. The fundamental insights and practical demonstrations reported in this thesis lay a foundation for the advancement of low-temperature AP-SALD of metal oxides as a cornerstone technology for the scalable fabrication of functional thin films and their integration into devices that address pressing societal and technological challenges.