Abstract
The optical and electrical properties of transparent conducting oxide (TCO) thin films are strongly linked with the structural and chemical properties such as elemental depth profile. In R&D environments, the development of non-destructive characterization techniques to probe the composition over the depth of deposited films is thus necessary. The combination of Grazing-Incidence X-ray Fluorescence (GIXRF) and X-ray reflectometry (XRR) is emerging as a fab-compatible solution for the measurement of thickness, density and elemental profile in complex stacks. Based on the same formalism, both techniques can be implemented on the same experimental set-up and the analysis can be combined in a single software in order to refine the sample model. While XRR is sensitive to the electronic density profile, GIXRF is sensitive to the atomic density (i. e. the elemental depth profile). The combination of both techniques allows to get simultaneous information about structural properties (thickness and roughness) as well as the chemical properties. In this study, we performed a XRR-GIXRF combined analysis on indium-free TCO thin films (Ga doped ZnO compound) in order to correlate the optical properties of the films with the elemental distribution of Ga dopant over the thickness. The variation of optical properties due to annealing process were probed by spectroscopic ellipsometry measurements. We studied the evolution of atomic profiles before and after annealing process. We show that the blue shift of the band gap in the optical absorption edge is linked to a homogenization of the atomic profiles of Ga and Zn over the layer after the annealing. This work demonstrates that the combination of the techniques gives insight into the material composition and makes the XRR-GIXRF combined analysis a promising technique for elemental depth profiling.
Highlights
One of the challenges in the optoelectronic devices is to find an acceptable compromise between the need to increase the light emitting/absorbing area and the need for low series resistance of the metal contact grid [1]
In this work we demonstrate how the Grazing-Incidence X-ray Fluorescence (GIXRF)/Xray reflectometry (XRR) combined strategies apply for non-destructive characterization of the depth-dependent properties in gallium-doped zinc oxide thin layered films, and how the optical properties of this advanced indium-free transparent conducting oxide (TCO) relate to the XRR/GIXRF-deduced gallium profile in the film
We found that the annealing process affects the optical properties by a homogenization of the mass density and the elemental profile over the thickness
Summary
One of the challenges in the optoelectronic devices is to find an acceptable compromise between the need to increase the light emitting/absorbing area and the need for low series resistance of the metal contact grid [1]. The use of transparent conducting oxides (TCOs) allows to achieve this compromise. Due to the expected scarcity of indium, efforts are underway to develop indiumfree TCOs for the above-mentioned devices. ZnO heavily doped with Ga (GZO) is becoming a very attractive candidate for future generation TCOs [2]. ZnO belongs to groups II–VI semiconductors, featuring a wide band gap (3.3 eV) at room temperature [3]. The effects of both sources are weakened at room temperature [9]. The optical and electrical properties of ZnO can only be enhanced through doping [10]
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