Contrasting transparent conductive properties of ZnO films on amorphous and crystalline substrates in view of thickness dependence

Abstract

The authors conducted comparative studies on ZnO films deposited on various substrates to elucidate how the different nucleation and crystallization processes affect their transparent conductive properties. The resistivity versus thickness curves of Ga-doped ZnO films deposited on a-SiNx:H films and glass substrates coincided within the experimental error. This result means that as long as the amorphous substrate is neither reactive with the deposited film nor providing crystalline seeds, resistivity is determined only by self-crystallization. In contrast, the resistivity of undoped ZnO films on sapphire c-planes was about half that on glass substrate even when the films were deposited at room temperature, indicating that the crystal template of sapphire stimulates local crystallization of ZnO films, though they are not epitaxial. With regard to the dependence on deposition temperature, a sudden drop in carrier concentration of undoped ZnO films was commonly observed between 200 and 300 °C for both glass and sapphire substrates, as a result of eliminating crystal disorder that facilitates holding donors. A significant difference was manifested between 300 and 500 °C; ZnO films on glass were nearly insulating, whereas those on sapphire were conductive, reflecting higher mobility and more reduced state. On sapphire substrates, the resistivity and sheet resistance versus thickness curves exhibited a monotonic decrease below 200 °C, whereas a plateau region appeared between 20 and 100 nm at 300 and 400 °C. This corresponds to the existence of electrically dead or inactive region near the interface probably because of depletion of carriers in the lattice-matched epitaxial layer. ZnO films became well conductive only when they were sufficiently thick.

Atomic insights in crystallization of liquid Cu on single crystal Ta and amorphous Ta

“…It means that amorphous Ta is not benefit for the nucleation of Cu embryos. The low nucleation potency of some other amorphous materials was also reported in previous studies [2,16]. To further verify our speculation, more targeted simulations were carried out, in which the Ta atoms were fixed to inhibit the recrystallization of the amorphous Ta.…”

“…Heteronucleation is a ubiquitous phenomenon, which has attracted extensive interest because of its importance in fundamental research and materials engineering [1][2][3][4][5][6][7]. In this complex process, the nucleation mechanism may vary dramatically upon change of conditions, such as the nucleant, impurity, undercooling and so on [2,7]. In practical perspectives, the understanding of heteronucleation mechanism in different conditions would inspire the control of materials properties towards diverse applications [8].…”

“…Although some researches paid attention to the heteronucleation on amorphous substrates [13][14][15], few work focused on the differences of heteronucleation on crystalline and amorphous materials, especially at an atomic scale. Akazawa [2] studied the deposition of ZnO on glass and sapphire c-planes, the results suggested that the crystallinity of ZnO near the glass/ZnO interface was lower than that near the sapphire/ZnO interface. Using molecular dynamics simulation, An et al [16] found that spraying high temperature liquid Cu nanodroplets on amorphous substrate (Cu 46 Zr 54 ) obtained amorphous Cu, while spraying the same nanodroplets on crystalline substrate (Cu) obtained crystal Cu.…”

Solution processed ZnO homogeneous quasisuperlattice materials

“…Furthermore, the absence of peak shifts indicates negligible growth-induced strain in or between high ordered layers in the QSL structure. 38 Electron Fig. 3(a), we measured the thickness of both dense and less dense portions of each periodic bilayer in the QSL by TEM of FIB-sectional electron transparent lamella, as a function of the total number of iterative depositions.…”

“…35 Other interstitial and vacancy defects are also formed in ZnO and understanding the nature of these during crystallization of thin films at lower temperatures remains a challenge since stoichiometry and composition determine conductivity and free carrier mobility. 38 Heterostructures involving quasisuperlattice (QSL) of various oxides have demonstrated the formation of structures with spatially varying conduction band characteristics 39 and that carrier mobility can be enhanced at such interfaces, 40 removing dependence on limited Hall mobility in the either of the bulk materials. At oxide interfaces with the dielectric, or between homo-or heterophase interface in quasi or ordered superlattices, trapped charges in an oxide layer or interface from electric field-induced removal of majority carriers that leave a net positive charge density on the surface or interface.…”