Composition Dependence of the Band Gaps of Semiconducting GeS_xSe_1–x van der Waals Alloys

Abstract

Alloying of two-dimensional (2D)/layered chalcogenide semiconductors by forming ternaries with properties that span the range between the binary constituents allows tuning of the electronic and optical properties and achieving the full potential of these materials. While the focus so far has been on transition-metal dichalcogenides, alloying in layered group IV chalcogenidespromising for optoelectronics, photovoltaics, ferroelectrics, etc.remains less understood. Here, we investigate alloying in the GeSe–GeS system and its effect on the fundamental band gap. We synthesize single-crystalline layered GeS x Se1–x alloy micro- and nanowires whose compositions are tunable over the entire range of S content, x, via the GeS and GeSe precursor temperatures. Cathodoluminescence in scanning transmission electron microscopy is used to investigate the composition dependence of the band gaps of GeS x Se1–x alloy micro- and nanowires. The band gaps of bulk-like microwires increase systematically with the sulfur content of the alloys, thereby covering the entire range between GeSe (1.27 eV) and GeS (1.6 eV). The composition dependence of the fundamental band gap is close to linear with a bowing coefficient b = 0.173 eV. Density functional theory calculations support the isomorphous behavior of GeSe–GeS solid solutions and demonstrate that the band gaps are indirect and have similar small bowing as determined experimentally. Finally, we establish pronounced size effects in GeS x Se1–x alloy nanowires that provide access to higher-energy optoelectronic transitions than can be realized in bulk alloys of the same composition. Our results support applications of germanium monochalcogenide alloys in areas such as optoelectronics and photovoltaics.

Photonics in Multimaterial Lateral Heterostructures Combining Group IV Chalcogenide van der Waals Semiconductors

“…The most intense peaks in both bands (I) and (II) appear at ≈1.4 and 1.44 eV, respectively. Valence EELS measurements in the GeS 0.7 Se 0.3 alloy (Figure S7, Supporting Information) confirm an absorption onset (i.e., bandgap, E g ) of ≈1.42 eV, in close agreement with measurements for GeS‐GeSe alloys [ 33 ] that give E g ≈ 1.45 eV for GeS 0.7 Se 0.3 . Thus we assign the most intense peaks in the CL spectra in both GeS 0.7 Se 0.3 bands to emission due to recombination across the alloy bandgap.…”

“…This shift of the Raman lines is consistent with the formation of a GeS 1−x Se x alloy during the growth from mixed GeS and GeSe vapors. [33] Furthermore, we investigated the structure, morphology, and composition of the multimaterial heterostructures by STEM-EDS (Figure 2 and Figures S4 and S5, Supporting Information), (S)TEM imaging, and electron diffraction (Figure 3). These investigations were performed on cross-sectional samples that reveal not only the structure and composition of the heterostructures but also the interfacial connectivity between the different materials.…”

“…This shift of the Raman lines is consistent with the formation of a GeS 1− x Se x alloy during the growth from mixed GeS and GeSe vapors. [ 33 ]…”