Abstract
Using reflection high-energy electron diffraction (RHEED), the growth onset of molecular beam epitaxy (MBE) deposited germanium telluride (GeTe) film on Si(111)-(√3 × √3)R30°-Sb surfaces is investigated, and a larger than expected in-plane lattice spacing is observed during the deposition of the first two molecular layers. High-resolution transmission electron microscopy (HRTEM) confirms that the growth proceeds via closed layers, and that those are stable after growth. The comparison of the experimental Raman spectra with theoretical calculated ones allows assessing the shift of the phonon modes for a quasi-free-standing ultra-thin GeTe layer with larger in-plane lattice spacing. The manifestation of the latter phenomenon is ascribed to the influence of the interface and the confinement of GeTe within the limited volume of material available at growth onset, either preventing the occurrence of Peierls dimerization or their ordered arrangement to occur normally.
Highlights
Since the late 60’s, germanium telluride (GeTe) has attracted a lot of attention both from a fundamental and technological perspective; it has been kept under constant scrutiny as a thermoelectric[1,2], ferroelectric[3,4,5], and phase-change material[6,7,8]
Note that the α-GeTe unit cell is described with cubic notation, despite the trigonal distortion of
Pattern. (c) Integrated specular beam intensity oscillations close to growth onset demonstrating the formation of closed layers. (d) {211} lattice planes spacing calculated from reflection high-energy electron diffraction (RHEED) streak spacing showing a larger inplane spacing with respect to α-GeTe during the first 200 seconds of growth
Summary
The preparation of the Si(111)(√3 × √3) R30-Sb surface is described in the cited publication. The growth itself is performed at a substrate temperature of 260 °C, using Ge and Te dual-filament effusion cells with base and tip temperature of Tbase(Ge) = 1120 °C and Ttip(Ge) = 1140 °C for the Ge cell, Tbase(Te) = 335 °C and Ttip(Te) = 470 °C for the Te cell. The cell fluxes are beforehand calibrated by performing X-ray reflectivity measurements on amorphous Ge and Te films grown at room temperature (RT). Cross-sectional TEM specimen were prepared along Si< 110 > or < 211 > crystallographic directions by mechanical polishing, dimple grinding and low-voltage Ar+ ion milling for final thinning using a Gatan PIPS II. Calibration of images is typically performed on the basis of the Si(111) interplanar distance (0.3135 nm).
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