Formation of Resonant Bonding during Growth of Ultrathin GeTe Films

Turning chalcogenide compounds, such as germanium telluride (GeTe), into thermoelectric materials that convert waste heat into electricity requires careful balancing of conflicting material properties. Epitaxial growth of GeTe has revealed some interesting challenges, especially if it comes to producing ultra-thin films. We report that ultrathin films of GeTe have thickness-dependent structures that are tunable using different surface passivation techniques. Often in fact the deposition of a thin crystalline layer, for growth parameters that should yield a crystalline material, is not successful and an amorphous layer is obtained instead. An entire GeTe amorphous film crystallizes once a critical thickness of 4 GeTe bilayers is reached.

Figure: (Top) Integrated reflection high-energy electron diffraction (RHEED) intensity during growth around the GeTe(111)-(1×1) reconstruction streaks, showing that crystallization occurs after formation of 4 GeTe bilayers. (Bottom) Raman spectra acquired on GeTe samples of 2, 4, 6 and 8 bilayers nominal thickness. RHEED images acquired at the end of each growth are also shown. Significant differences in intensity and frequency of the Raman modes are observed for the two thinner samples, which are amorphous, as compared with the two thicker crystalline samples.

Here we report and explain such a scenario. Using Raman spectroscopy we show a remarkable change of vibrational modes above the critical thickness that is attributed to a change in the nature of the bonds: While ordinary covalent bonding is found in ultrathin films, resonant bonding can prevail only once a critical thickness is reached. Resonant bonding in chalcogenides is a unique bonding mechanism, which differs significantly from metallic or ordinary covalent bonding. The atoms in crystalline GeTe have 6 nearest neighbors, but only 3 p-electrons per atom to form saturated bonds, sometimes called 2 center – 2 electron bonds. Therefore, there are too many nearest neighbors for the electrons to form ordinary covalent bonds to each neighbor. Instead, the system employs 3 center – 2 electron bonds, also denoted as resonant bonds. In this situation, neighboring atoms are held together just by a single electron, not an electron pair. Unlike in metals, however, these electrons are still rather localized between two atoms, leading to a non-vanishing band gap. GeTe ultra-thin films try to develop resonant bonds in the direction of the film normal. Yet, electron delocalization in z-direction is impossible, if the substrate provides an electronic barrier for the corresponding p-electrons of GeTe. This scenario is further supported by density functional theory calculations showing that ultrathin films do not utilize resonant bonding in contrast to the bulk phase.  

1 Author R. N. Wang , W. Zhang , J. Momand , I. Ronneberger , J. E. Boschker , R. Mazzarello , B. J. Kooi , H. Riechert , M. Wuttig , R. Calarco

Formation of Resonant Bonding during Growth of Ultrathin GeTe Films

Source NPG Asia Mater. , 9 , e369 ( 2017 )
DOI : 10.1038/am.2017.95 | 2777 Cite : Bibtex RIS
R. N. Wang, W. Zhang, J. Momand, I. Ronneberger, J. E. Boschker, R. Mazzarello, B. J. Kooi, H. Riechert, M. Wuttig, and R. Calarco