Elsevier

Surface Science

Volume 644, February 2016, Pages 13-17
Surface Science

Texture of Ge on SrTiO3 (001) substrates: Evidence for in-plane axiotaxy

https://doi.org/10.1016/j.susc.2015.08.035Get rights and content

Highlights

  • The texture of the Ge thin-film obtained by MBE growth on SrTiO3(001) substrate is investigated using XRD.

  • All the Ge crystallites have one of the Ge-[110] direction aligned to one of the SrTiO3-[100] directions.

  • This is the first evidence of the axiotaxy occurrence for a semiconductor on an oxide substrate.

Abstract

We report the first experimental evidence of the formation of an axiotaxial texture in a semiconductor/oxide structure, namely Ge deposited by molecular beam epitaxy on a SrTiO3 substrate. The texture of the deposit is carefully analyzed based on X-ray pole figure measurements. We show in particular that the axiotaxial texture is not random, but that the deposit presents a limited number of well defined crystal orientations along axiotaxy locus. We also evidence the presence of twinned zones in the Ge crystal, and we discuss their experimental signature regarding that of the axiotaxy texture. In the end, we show that interface dangling bonds are the main parameter driving Ge crystal orientation, and we compare their influence to that of epitaxial misfit.

Graphical abstract

X-ray diffraction texture pattern of a Ge deposit exhibiting axiotaxial relationship with its SrTiO3 substrate. The diffraction peak pattern corresponds exactly to the axiotaxy locus, showing alignment of the Ge < 1 1 0 > direction parallel to STO < 1 0 0 > direction.

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Introduction

In a classical vision, epitaxy is restricted to homogeneous materials (low mismatch, high adhesion, strong chemical compatibility between substrate and deposit) [1]. In such cases, the local order of the substrate is spontaneously inherited by the deposit and the misfit stress can be relaxed through different mechanisms (surface roughening or morphological transition, dislocations [2]). In a wider perspective, the problem can be extended across the deposit/substrate heterogeneity scale, towards more heterogeneous systems. In the fully disordered case, local order of the substrate is inherited only locally by the deposit resulting in the formation of a polycrystal. A variety of intermediate situations can lead to a variety of situations from monocrystalline to polycrystalline. Several intermediate textures (preferred crytallographic orientation) have been reported in the literature, among them one can cite fiber texture, competition between several preferred orientations and axiotaxy, first evidenced for the NiSi/Si(0 0 1) system [3].

Competition between several orientations it typical for semiconductor (InP, GaAs, InAs, Ge)/SrTiO3(0 0 1) epitaxial systems [4], [5], [6], [7], [8] and has been theoretically described based on ab-initio calculations [9], [10], [11], [12]. It is interesting to notice that in the above mentioned examples, all reported epitaxial relationships involve an alignment of the [1 1 0] in-plane direction of the semiconductor lattice to the [1 0 0] direction of the SrTiO3 lattice.

The formation of axiotaxial texture (equivalent to a fiber texture with an off-normal fiber axis) has attracted specific attention since its first evidence in the NiSi/Si(0 0 1) system [3]. Axiotaxy has up to now been reported for silicide/silicon systems grown by solid state diffusion of the metal in the silicon substrate: NiSi on Si(0 0 1), Si(1 1 1) and Si(0 1 1) [13], CoSi2 on Si(0 0 1), Si(1 1 1) and Si(0 1 1) [15], [16], CrSi2 on Si(0 0 1) [17] and FeSi2 on Si(0 0 1) [14]. Similar behavior has been observed for Ni on Ge surface [18] or NiSiGe on SiGe surface [19], as well as for MAX phase (Ti–Al–C) on Al2 O3(0 0 0 1), Al2 O3(1 0 1 0) and Al2 O3(1 1 0 2) substrates [20] and recently for MnP nanoclusters grown on GaP [21]. In all these cases, axiotaxial texture originates from solid state diffusion induced by annealing at high temperature. Interestingly, the concept of one-dimensional interface underlying axiotaxial texture can be linked to the observation of several competing epitaxial relationships sharing a common alignment along a single in-plane direction reported for the semiconductors/SrTiO3(0 0 1) systems.

In this paper we focus on the determination of the texture of Ge grown on SrTiO3(0 0 1) substrates by molecular beam epitaxy. Indeed, there is a strong interest in combining functional oxides with semiconductors, since material diversification and integration of heterogeneous materials on the same wafer are expected to provide the opportunity for integrating various functionalities on the same chip, which is a key issue for further progress of device performances in the microelectronic field. We provide here evidence for an axiotaxial texture of the Ge grains and give clarifying elements on the origin.

Section snippets

Sample fabrication and morphology

The sample considered in the following was fabricated by molecular beam epitaxy (MBE). A (001)-oriented SrTiO3 (STO) substrate was submitted to a buffered oxide etch chemical treatment to obtain a purely TiO2-terminated surface [22]. The sample was then introduced in the MBE chamber and annealed at 700 °C during 30 min. under an oxygen partial pressure of 5 × 10 6 Torr in order to remove carbon contaminations and to obtain an atomically flat step and terraces morphology. The substrate temperature

Epitaxial relationship for the Ge/STO interface

We have shown that all reflections along the axiotaxy locus correspond to Ge crystallites oriented so that the in-plane [1 1 0] direction in the Ge crystal is parallel to in-plane STO [1 0 0] direction. In this section, we will discuss the epitaxial relationships between the different crystallites, depending on their crystal orientation, and the STO substrate. As all Ge crystallites are in axiotaxial relationship with the substrate, they all present the same misfit with respect to the substrate

Conclusions

Our results provide the first evidence of axiotaxy, i.e. intermediate order dominated by the one-dimensional match, in a semiconductor/oxide epitaxial structure. As can be easily seen from geometric considerations, the one-dimensional order is dictated by the match at the Ge/STO interface, of one 〈1 1 0〉 axis of the Ge crystal always aligned to one STO〈1 0 0〉 interface-plane axis of the STO which is a validation of our previous work based on a 1D Frenkel-Kontorova model [23]. Based on a careful

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  • Cited by (2)

    • Axiotaxy in oxide heterostructures: Preferential orientation of BaCeO<inf>3</inf> nanoparticles embedded in superconducting YBa<inf>2</inf>Cu<inf>3</inf>O<inf>7 −δ</inf> thin films

      2017, Thin Solid Films
      Citation Excerpt :

      This texture was also observed in MnP nanoclusters present in GaP thin films [15]. More recently, semiconductors on top of an oxide matrix [16] were demonstrated to have an axiotaxy grain texture. Nevertheless, no evidence of axiotaxy for oxide nanocomposite has been reported.

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