Research Update: Inhomogeneous aluminium dopant distribution in magnetron sputtered ZnO:Al thin films and its influence on their electrical properties

Abstract

The spatial distribution of Al in magnetron sputtered ZnO:Al films has been investigated in depth. Two different kinds of inhomogeneities were observed: an enrichment in the bulk of the film and an enrichment at the interface to the substrate. This has been correlated to the electrical properties of the films: the former inhomogeneities can lead to trap states at the grain boundaries limiting the free carrier mobility. The latter can promote the formation of secondary phases, which leads to an electrical inactivation of the dopant. Furthermore, this effect can contribute to the thickness dependence of the electrical properties of ZnO:Al films.

Using Atom-Probe Tomography to UnderstandZnO∶Al/SiO2/SiSchottky Diodes

“…AZO=SiO 2 =Si interface for all samples, as also observed by Bikowski et al [37]. This suggests that at the interface the AZO is always highly doped, even for films that are semiconducting in the bulk.…”

“…This makes it unlikely that the AZO=SiO 2 =Si Schottky barrier heights here are fixed by the work function of metallic Al, as in the so-called effective work-function model for Schottky barriers on III-V semiconductors [8]. The presence of insulating Al 2 O 3 or ZnAl 2 O 4 as secondary phases at the SiO 2 =AZO interface has been confirmed by a combination of transmission electron microscopy and transmission electron microscopy [37]. It is hypothesized that the low vapor pressures of Al, Al 2 O 3 , and ZnAl 2 O 4 relative to Zn may be responsible for preferential deposition of Al-rich oxides during the early stages of film growth.…”

“…It is hypothesized that the low vapor pressures of Al, Al 2 O 3 , and ZnAl 2 O 4 relative to Zn may be responsible for preferential deposition of Al-rich oxides during the early stages of film growth. This is a separate mechanism proposed to explain excess Al at the interface, in addition to the thermodynamic driving forces described by theory [37][38][39]. The presence of Alrich oxides at the interface could affect the measured J 0 , Φ n , and Φ p .…”

Possibilities of Increasing the Usability of Sputtered AZO Films as a Transparent Electrode

“…The symbols denote the specific values of R s taken from Li et al (circles), Bikowski et al (stars), Lee et al (triangles up), Novák et al (triangles right), Rahmane et al (triangles down), Novák et al (triangles left), Seo et al (squares), Hao et al (diamonds), Cornelius et al (pentagons) and Kluth et al (hexagons). Suitable sheet resistance for specific application are marked with grey areas.…”

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

“…Al enrichment has been observed by several authors when the substrate temperature was increased above an optimal value [52,37,53]. Even though the substrate temperature was kept constant in this study, the actual temperature of the growing film is given by the substrate temperature plus a temperature shift proportional to the potential energy flux of the particles reaching the substrate.…”