Due to the unique set of properties possessed by ZnO, thin films of ZnO have received more and more interest in the last 20 years as a potential material for applications such as thin-film transistors, light-emitting diodes and gas sensors. At the same time, the increasingly stringent requirements of the microelectronics industry, among other factors, have led to a dramatic increase in the use of atomic layer deposition (ALD) technique in various thin-film applications. During this time, the research on ALD-grown ZnO thin films has developed from relatively simple deposition studies to the fabrication of increasingly intricate nanostructures and an understanding of the factors affecting the fundamental properties of the films. In this review, we give an overview of the current state of ZnO ALD research including the applications that are being considered for ZnO thin films.

Ab initio calculationsWe investigate crystalline ZnO:hydroquinone (HQ) superlattices grown layer-by-layer by the combined atomic/molecular layer deposition (ALD/MLD) technique; such ALD/MLD layerengineered inorganic-organic thin films form a fundamentally new category of functional coherent hybrid materials that cannot be prepared by any other existing technique. Using quantum chemical methods, we derive atomic-level structural models for the ZnO:HQ superlattices and investigate their structural, spectroscopic, and electronic properties. By comparing the theoretical results with our experimental data we provide a detailed interpretation of experimentally measured infrared spectra, proving the presence of organic interfaces within the crystalline ZnO:HQ superlattices. We moreover show how the band structure of the hybrid material can be tailored by simple and experimentally feasible modifications of the organic constituent. The guidelines for the bandstructure engineering of ZnO:HQ superlattices should be valuable for the systematic enhancement and exploitation of the functional properties of ALD/MLD-grown inorganic-organic superlattices and nanolaminates in general.

Thin ZnO layers doped with aluminum and interspersed with regularly repeating single layers of organic molecules are deposited by a combined atomic layer deposition (ALD) and molecular layer deposition (MLD) method at 220 C using diethyl zinc, trimethyl aluminum, water and hydroquinone (HQ) as precursors. Hydroquinone is found to form distinct layers within the (Zn,Al)O framework such that clear inorganic-organic superlattice structures are realized. Aluminum doping efficiently offsets the decrease in carrier concentration caused by the organic layers, and enables enhanced electrical conductivities for the co-doped (Al plus HQ) thin films. Since superlattice structures in general are known to be highly beneficial in blocking the phonon transport in different materials, our ALD/MLD grown (Zn,Al)O:HQ thin films possess a lot of promise for new types of thermoelectrics.

A significant reduction in thermal conductivity is demonstrated upon incorporation of single layers of hydroquinone within ZnO or (Zn0.98Al0.02)O thin films to form hybrid inorganic–organic superlattice structures.

Nanomaterial interfaces and concomitant thermal resistances are generally considered as atomic-scale planes that scatter the fundamental energy carriers. Given that the nanoscale structural and chemical properties of solid interfaces can strongly influence this thermal boundary conductance, the ballistic and diffusive nature of phonon transport along with the corresponding phonon wavelengths can affect how energy is scattered and transmitted across an interfacial region between two materials. In hybrid composites composed of atomic layer building blocks of inorganic and organic constituents, the varying interaction between the phononic spectrum in the inorganic crystals and vibronic modes in the molecular films can provide a new avenue to manipulate the energy exchange between the fundamental vibrational energy carriers across interfaces. Here, we systematically study the heat transfer mechanisms in hybrid superlattices of atomic-and molecular-layer-grown zinc oxide and hydroquinone with varying thicknesses of the inorganic and organic layers in the superlattices. We demonstrate ballistic energy transfer of phonons in the zinc oxide that is limited by scattering at the zinc oxide/hydroquinone interface for superlattices with a single monolayer of hydroquinone separating the thicker inorganic layers. The concomitant thermal boundary conductance across the zinc oxide interfacial region approaches the maximal thermal boundary conductance of a zinc oxide phonon flux, indicative of the contribution of long wavelength vibrations across the aromatic molecular monolayers in transmitting energy across the interface. This transmission of energy across the molecular interface decreases considerably as the thickness of the organic layers are increased.