A team of scientists from Wrocław University of Science and Technology in cooperation with researchers from the URANOS beamline conducted a series of experiments on layered semiconductors GeS, SnS, and SnSe using the angle-resolved photoemission spectroscopy (ARPES). The investigation revealed characteristic features of the electronic band structure, potentially responsible for the enhanced thermoelectric properties.

Compounds of the groups IV and VI of the periodic table (MX, where M = Ge, Sn, and X = S, Se) belong to the family of van der Waals crystals – materials with a unique layered structure analogous to graphene. They attract increasing attention as suitable for applications in two-dimensional optoelectronics and may also exhibit excellent thermoelectric properties, as it has been shown for SnSe (Zhao et al., 2014, Nature 508, 373–377). The thermoelectric effect allows direct conversion of heat into electricity without side products, such as carbon dioxide emission. Unfortunately, the efficiency of the process is relatively low, and the state-of-the-art commercially used materials are complex lead compounds. SnSe and its close relatives make a promising alternative with a simple, non-toxic composition. The thermoelectric properties of SnSe result from several factors, one of which is a characteristic dispersion of the valence band, predicted theoretically for all MXs, but not confirmed experimentally to date.
In the experimental session, a series of ARPES measurements was performed for GeS, SnS, and SnSe crystals, supported by ab-initio calculations of the electronic band structure. The obtained results were analyzed considering the influence of the valence band dispersion on the thermoelectric performance and other properties of the materials. As shown in Fig. 1, taking GeS as an example, images of the valence band in a broad energy range were acquired, confirming its multivalley character. What is more, thanks to the high resolution of the measurement technique, a characteristic band shape near the X ̅ point of the Brillouin zone was revealed, composed of two closely laying hole pockets. The photoemission map of this region, along with calculated band dispersion, is presented in Fig. 2 for SnSe. The presence of multiple band extrema in the proximity of the Fermi level ensures a high Seebeck coefficient, simultaneously preserving good electrical conductivity. The combination of the two parameters leads to high thermoelectric efficiency.
The obtained results also enabled the exploration of material properties important for various applications and heterostructures engineering, such as hole effective mass, directly correlated with the charge carrier mobility, and the Fermi level position relative to valence and conduction bands. The determined values indicate the intrinsic p-type conductivity of all three semiconductors.

Figure 1. The results of ARPES measurements for GeS crystal. Constant energy maps acquired for the energy corresponding to the valence band top E = E_VBM (a) and E = E_VBM – 3 eV (b), and maps of the valence band dispersion along k_x (c) and k_y (d) directions of the wavevector.

Figure 2. SnSe valence band dispersion map near point of the Brillouin zone, revealing characteristic double-valley band shape.

Authors: Agata K. Tołłoczko, Szymon J. Zelewski

Link to the publiction: A. K. Tołłoczko, S. J. Zelewski, J. Ziembicki, N. Olszowska, M. Rosmus, T. Woźniak, S. Tongay, R. Kudrawiec, Photoemission Study of the Thermoelectric Group IV-VI van der Waals Crystals (GeS, SnS, and SnSe), Advanced Optical Materials, 2302049(2023) doi: 10.1002/adom.202302049