The correlation between the radial distribution of high-energetic ions and the structural as well as electrical properties of magnetron sputtered ZnO:Al films

Abstract

The origin of the pronounced radial distributions of structural and electrical properties of magnetron sputtered ZnO:Al films has been investigated. The film properties were correlated with the radially resolved ion-distribution functions. While the positive ions exhibit low energies and a radial distribution with a maximum intensity opposite the center of the target, the negative ions can have energies up to several hundred eV, depending on the target potential, with a radial distribution with two maxima opposite the erosion tracks. The most prominent positive ion is that of the working gas (Ar+), while the highest flux of the negative ions is measured for negative oxygen O−. The radial distribution of the flux of the high-energetic negative ions can clearly be related to the radial variations of the structural (c-axis lattice parameter, crystallite size) and electronic (resistivity) properties for sputtering from the planar target, which points to the decisive role of the high-energetic negative oxygen ions for the film quality. The relation between the negative ion bombardment and the structural as well as electronic properties can be explained by a qualitative model recently developed by us. The same target has also been investigated in the eroded state. In this case, the limited acceptance angle of the mass spectrometer leads to a misinterpretation of the radial distribution of the flux of the high-energetic negative ions. This effect can be explained by a simulation, based on the assumption that the high-energetic negative ions are mainly accelerated in the cathode (target) sheath perpendicular to the uneven substrate surface.

On performance limitations and property correlations of Al-doped ZnO deposited by radio-frequency sputtering

“…In the following, we refer to it as "energy flux" for simplicity, and will assume our measured lattice shift to be roughly proportional to it. This proportionality can be justified experimentally for sputter-deposited AZO based on the results of a recent study, where the measured energy-weighted O − ion flux density and the measured lattice shift were found to be correlated for both DC-and RF sputtering [9].…”

“…According to quantitative models verified against a number of sputterdeposited materials [31], the in-plane compressive stress σ scales with the product of the flux density of the bombarding particles Γ times the square root of their kinetic energy E, that is, σ ∝ Γ √ E. This relation must be rewritten for RF-sputtered AZO, because the O − flux is not monoenergetic, but it has an energy distribution function centered on the energy corresponding to the target self-bias voltage (about 140 V in our study) and is rather uniformly distributed from 0 eV up to approximately twice the value of the self-bias [9,27]. Also, both Γ and E may depend on deposition pressure p and radial position x.…”

“…Resistivity in the 10 −4 Ω cm range can be obtained by sputtering while retaining more than 80% average transmission in the visible region of light [6]. However, an issue related to sputter deposition of AZO is that the resistivity of the deposited films exhibits spatial inhomogeneity [7,8,9,10,11,12,13]. Mainly two explanations exist for this phenomenon: 1) bombardment of the film by inhomogeneously distributed energetic particles during deposition [7]; 2) inhomogeneity in the amount and activity of oxygen reaching the substrate, which results in non-optimal oxygen stoichiometry in certain regions of the film [8].…”

“…Mainly two explanations exist for this phenomenon: 1) bombardment of the film by inhomogeneously distributed energetic particles during deposition [7]; 2) inhomogeneity in the amount and activity of oxygen reaching the substrate, which results in non-optimal oxygen stoichiometry in certain regions of the film [8]. According to hypothesis (1), O − and O − 2 ions (the former being more abundant) [9,14,15] are formed at the target and accelerated through the cathode sheath up to an energy corresponding to the target DC bias voltage. Upon leaving the cathode sheath, such a collimated beam of energetic ions travels mostly perpendicular to the target surface with a small collision cross section with the working gas [16].…”

Morphological and electrical evolution of ZnO: Al thin filmsdeposited by RF magnetron sputtering onto glass substrates

“…ZnO:Al and ZnO:Ga thin films have been produced by a wide variety of techniques, including chemical and physical processes, such as pulsed laser deposition 6 , chemical vapor deposition 7 , spray pyrolysis 8 , sol-gel technique 9 , atomic layer deposition 10 and magnetron sputtering 11 onto a variety of substrates. Among these methods, magnetron sputtering is especially interesting, because it can not only be conducted at low temperatures, but can also produce high-quality crystalline intrinsic and doped ZnO thin films 12 .…”