Structure and growth characterization of biaxially textured PCM thin films and heterostructures

Pohlmann, Marc; Wuttig, Matthias (Thesis advisor); Mayer, Joachim (Thesis advisor)

Aachen (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Chalcogenide-based phase-change materials have been experiencing a trend of increasing importance as data storage media within the past decades. Starting from optical, sequential data storage media, the step towards parallel, electrical storage has been achieved recently with the Intel Obtane memory. While these industrial applications are further improved, the fundamental research on PCMs is gaining more and more attention. In order to understand the strong property contrast between amorphous andcrystalline phases of PCMs these compounds have been investigated intensively. Someof those investigations led to the observation that PCMs might exhibit odd effects upon strong confinement. Some works were already able to show effects like spontaneous crystallization of thin films. However, a lot of questions remain and demand the capabilities to fabricate highly textured thin film structures in order to be answered. Hence, in the frame of this work the structure of different PCMs grown via molecular beam epitaxy (MBE) was investigated. With the goal of depositing thin films and hetero structures, the growth of SnTe, GeTe and Sb2Te3 on different Si surfaces was compared. The growth of binary compounds on 7x7-reconstructed as well as Sb-passivated Si (1 1 1)substrates was compared. These two surfaces offer different achievable thin film properties by changing the growth mode and biaxial texture of the films. Thereby, the growth of binary compounds was investigated in order to employ them as seed layers for the heterostructures. Investigations of Sb2Te3 are presented to characterize the growth on the different surfaces and enable a good comparison of this work with other related publications. The seeding process is observed via scanning electron microscopy in order to understand how the texture formation differs on the surfaces. The incommensurate growth of SnTe (0 0 1) on the trigonal Si substrate surface is investigated. In direct comparison to Sb2Te3, the seeding on different substrates is investigated via scanning electron microscopy. The resulting films are characterized via X-ray diffraction(XRD) studies. Finally, a model is proposed which can explain how the epitaxial relation between the incommensurate symmetries of substrate and film can still resultin a superior biaxial texture. The growth of GeTe on Sb-passivated substrates is reported with a strong degree of twin domain suppression. The suppression is sufficient to observe the misalignment of distortion domains via reciprocal space maps without overlapping reflections from otherrotational domains. Also a growth series is presented, in which GeTe thin films aredeposited on 7x7-reconstructed Si. The GeTe layers are grown with a thickness of 4 to15 nm. XRD studies and Raman measurements are employed to characterize the thickness dependent structural change of GeTe and show how this change does not relate to strain, but to a confinement induced change of the chemical bonding. Then, the in-planetexture of this material is also compared to the model that was introduced for SnTe, inorder to confirm its applicability to other systems. A growth study of Ge-Sb-Te compounds is shown, in which the continuous transition from a cubic to a trigonal structure is shown. Also, on the example of this compound, abrief report of the accuracy of stoichiometry determination via energy dispersive X-raydetection is given. After characterizing the growth of binary structures and proving that they can be deposited highly textured, these are employed as seeds for different superlattices. In this part of the work, the analysis of XRD measurements of heterostructures is described.The results are compared with transmission electron microscopy. The strain developmentwithin these superlattices is described by fitting reflection high-energy electrondiffraction measurements with a spring model. The resulting strain model is then implemented for XRD simulations. Thereby, the in-situ and ex-situ measurements arecompared and the XRD simulation is improved.This procedure is performed to describe the structure of different SnTe-Sb2Te3-superlattices.The growth of different thickness ratios of the two layers is compared in regard to the achievable out-of-plane texture. These results are then compared with the fundamentally different system of the SnTe-GeTe-superlattices. Due to the structural difference that no Van-der-Waals-like layers are present in either SnTe or GeTe, as opposed toSb2Te3, these heterostructures show very different growth conditions. Via XRD and RHEED, these structures are compared with the Sb2Te3-containing superlattices. It is shown that the absence of Sb2Te3 renders intermixing much easier, thereby lowering thehighest possible deposition temperature that is still sufficient to avoid intermixing. However,the successful deposition of superlattices was achieved. Due to the different texture formation of GeTe and SnTe on Si (1 1 1) surfaces, it was possible to also switch the texture of the superlattice by switching only the seed layer. Also, the spring model that was applied for Sb2Te3-containing superlattices fails for these structures. The reasons for this failure are discussed and a roughness-based calculation of the in-plane lattice constant is presented which is much more capable of describing the measured trends.