Investigation of the elemental distribution in electronic materials for mobile devices

• Untersuchung der Elementverteilung in elektronischen Materialien für Mobilgeräte

Yatim, Alexandra Katharina; Wuttig, Matthias (Thesis advisor); Mayer, Joachim (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

The ongoing "Digital Revolution" places ever increasing demands on mobile devices. It does not only account for a constant generation of new products, but also requires a continuous optimisation of the individual components of such devices. As a result, the technology sector of smartphones, tablets, wearables etc. is rapidly growing. Inthe current work, specific materials from the classes of transparent conductive oxides (TCOs) and phase-change materials (PCMs) were investigated with respect to their elemental distribution. These material classes are promising to meet the challenges of display and data storage applications, respectively. (i) The material most commonly used in display applications, namely In2O3:Sn, called tin-doped indium oxide or indium tin oxide (ITO), contains indium, which is scarce and thus expensive. Hence, intensive research is focused on finding an alternative material. One of the most promising candidates for substituting ITO is ZnO:Al. This material combines natural abundance, low cost, nontoxicity, and thermal stability. Although its resistivity in the as-deposited state cannot keep up with that of ITO, it can be decreased by post-deposition heat treatment. Prior to this work, it had been found that the same annealing parameters might lead to an enhancement of the electricalproperties if a hydrogenated amorphous silicon (a-Si:H) capping layer is used during heat treatment, while upon annealing without any capping layer the charge carrier mobility as well as the charge carrier density might be decreased. In the frame of the current work, the degradation of the electronic properties upon specific heat treatment without any capping layer could be correlated with aluminium segregation at the grain boundaries by employing atom probe tomography (APT). Further investigation of additional samples confirmed the correlation of segregation of the dopant at the grain boundaries with deterioration of the electronic properties. In addition to ZnO:Al, the non-classical TCO material TiO2:Nb was investigated by means of APT. The fact that for this materialthe evaporation field of the dopant is higher than that of the matrix metal makes APT investigations difficult. However, the yield could be improved by selecting the lowest possible base temperature.(ii) The increasing amounts of data stored on mobile devices require a technology that offers high storage densities. PCMs feature excellent scaling properties and fast switching between the logical states, which arerealised by the amorphous and the crystalline phase. If their endurance can be sufficiently increased, they might replace the Flash memory and maybe even the dynamic random-access memory (DRAM), which would turn them into a "universal memory". While PCMs have already been successfully used in optical data storage devices for more than 20 years, in case of electronic data storage applications, atomic migration of the elements towardsdifferent electrodes had been observed for Ge2Sb2Te5. Furthermore, crystallisation of the PCM GeTe in a furnace had been found to lead to germanium precipitation in case of annealing temperatures of T =250 °C and above. The question arises whether precipitation can be suppressed upon fast switching. No APT measurements on PCMs had been reported in literature so far. For this reason, it was one of the aims of the current work to establish APT on GeTe, which is a parent compound of prominent ternary PCMs. The knowledge gained from these experiments can be used in future investigations of phase-change memory cells. In this context,first the evaporation field of GeTe was determined to be roughly F_GeTe ~ 16 V/nm. Secondly, germanium precipitates were not only found in a sample annealed at T = 290 °C, but also in a sample annealed at T = 220 °C. Since earlier results had been gained employing x-ray diffraction (XRD), which cannot detect amorphous phases,this new result can be explained by the precipitation of amorphous germanium at a temperature of T = 220 °C or below and the crystallisation of these precipitates at T = 250 °C.In summary, in the present work the importance of the atomic arrangement of the different elements on the physical properties of various electronic materials could be highlighted and concretised with regard to their application in mobile devices. For this purpose, atom probe measurements on novel materials were established. The obtained results might lay the foundation for a trendsetting and targeted development of new technologies.

Institutions

• Chair of Experimental Physics I A and I. Institute of Physics [131110]
• Department of Physics [130000]