X-ray photoelectron studies of clean and oxidized α-GeTe(111) surfaces

Abstract

Clean and oxidized (104–1015 L of O2) surfaces of α-GeTe have been investigated with x-ray photoelectron spectroscopy by using the synchrotron radiation facility BESSY II as well as an Al Kα source. To understand the first steps of oxidation, complementary quantum chemical calculations were performed. The cleaved surfaces of α-GeTe were found to be rumpled with (111) domains that can be related to the domain (twin) structure of the bulk. Both the Ge 3d and the Te 4d spectra of freshly cleaved surfaces exhibit at least three components, which are explained by a Ge or Te termination of the surface domains with possible contributions of a surface reconstruction. The surface oxidation starts at exposures of 104 L and proceeds via several steps. At low exposures, only changes in the Ge spectra are observed. Consequently, the first step of the reaction is associated with the formation of intermediate peroxidelike structures, wherein both oxygen atoms are bonded to germanium atoms. In the range of exposures between 1010 and 1015 L, a layer of a relatively stable oxidation product with the approximate stoichiometry Ge1+δ+4Te1−δ0O2(1+δ)2− is formed, which shows growth kinetics that obey a time-logarithmic law. At this stage, the peroxidelike structures are still present at the oxide/crystal interface. Once the oxidized layer exceeds a thickness of ≈2.5 nm at ∼1013 L, a transformation of the Te0 state into the Te+4 state is observed at the surface of the oxide layer. The final oxidation product can be described as mGeO2×nTeO2.

Local structure of nitrogen in N-doped amorphous and crystalline GeTe

“…The obtained spectrum is in perfect agreement with previous studies of crystalline GeTe (a-GeTe(111) surfaces, in particular 11 ) and suggests that there are no Ge-N bonds within the crystalline phase, i.e., Ge-N bonds existing in the amorphous phase are broken upon crystallization.…”

“…33 eV corresponds to Ge oxide. 11 This peak increases its intense in the annealed samples and demonstrates that Ge species readily oxidize despite annealing in inert atmosphere. Of higher interest are the features in the 29-32 eV range that correspond to Ge species in GeTe proper.…”

“…Of higher interest are the features in the 29-32 eV range that correspond to Ge species in GeTe proper. 11 The broad spectrum for the amorphous samples is likely to be comprised of two (or more) major contributions. As demonstrated by previous XAFS measurements and simulations, Ge in the amorphous phase may be present in different bonding geometries, namely, bonded to tellurium in Ge(4):Te(2) and/or Ge(3):Te(3) local configurations 12 or forming Ge-Ge bonds.…”

Ab initio study of the high‐temperature phase transition in crystalline GeO2

“…Besides these obvious direct applications, there are also some indirect cases where germanium dioxide is either a beneficial or an unwanted compound. A solid mixture of m GeO 2 × n TeO 2 has been proposed as the final oxidation product of GeTe surfaces, and may well be relevant for other phase‐change data‐storage devices which must be protected from oxidation . Germanium nanowires with an amorphous GeO 2 layer on the outside have been experimentally characterized by Zhang et al, and in fact, oxide species were found to promote growth of silicon and germanium nanostructures .…”

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

“…We did not observe any differences between the two preparation methods also regarding the topographic STM images. Since surface oxidation starts already at ∼10 4 Langmuir of O 2 [53], transfer in UHV or removal of the oxide is mandatory. XPS is also used to cross-check the stoichiometry of the GST-225 samples.…”