Research On Optical Properties Of Two-dimensional Semiconductor GeAs

Two-dimensional(2D)materials with anisotropic crystal structures,as compared to the corresponding bulky systems,have richer and more interesting optical,electrical and thermal properties,and have been widely investigated in the field effect transistors,superconductors,light emitting devices,solar cells,photodetection,and so on.As a typical 2D van der Waals(vd W)layered structure material,the IV–V compound GeAs narrow-gap semiconductor has obvious and intriguing anisotropies that are benefit for applications in micro/nano photonic and electronic devices.In this paper,the optical properties of 2D GeAs are systematically studied by means of the confocal micro-Raman microscopy and Fourier transform infrared(FTIR)spectroscopy.The main results obtained are as follows:1)The vibrational modes,in-plane anisotropy,and temperature-related physical properties of GeAs are investigated using Raman spectroscopy with variable conditions(temperature,polarization,geometric configuration,etc.).At room temperature,12main Raman modes are observed,including 4 B_g modes and 8 A_g modes.The armchair and zigzag orientations in the GeAs crystal are determined by combining with the microstructural characterization and polarized angle-resolved Raman spectroscopy.It is found that with the continuous change of the polarization angle,the Raman intensity and Raman shift of some vibrational modes show different symmetric evolution laws.Among them,the periodic variation of the intensity can be described by the Raman tensor,while the periodic variation of the shift can be explained from the phonon dispersion relation.Simultaneously,in the temperature range from 83 K to 823 K,the temperature dependence of the Raman modes can give a thermal-related constant,based on which some key thermal parameters can be directly derived including the thermal expansion coefficient,the Green-Eisen constant and the quasiparticle lifetime as well as others.These values from experiments are well consistent with the calculation predictions,suggesting the new strategy for understanding the structure-dependent optical anisotropy and thermal property in the 2D vd W GeAs.2)The optical bandgap,valley polarization,optical rotation effect,and the influence of thickness on the optical properties in GeAs are studied by the FTIR-based transmission/absorption spectroscopy.At room temperature under the near-infrared light source and vacuum conditions,the bandgap of bulk GeAs is experimentally measured,pointing to～0.89 e V.The polarization angle-resolved transmission spectroscopy further reveals that GeAs is a kind of spontaneous valley polarization semiconductor.The absorption of different valleys for linearly-polarized light in different directions is obviously different,and the corresponding energies of the two valleys are～0.75 e V and～0.93 e V.Combining with the polarization angle-resolved Raman spectroscopy and TEM images,we find that the smaller bandgap in GeAs corresponds to the armchair direction,and the larger one relates to the zigzag direction.In addition,the transmission spectra under different configurations demonstrate that GeAs has strong optical activity,and its optical axis directions are armchair and zigzag directions.More importantly,GeAs with different thicknesses prepared by the mechanical exfoliation show that the thickness is inversely proportional to the optical bandgap corresponding to the two valleys.The larger the thickness,the smaller the bandgap;however,the energy difference between the two valleys increases and tends to be saturated,indicating that the bulk GeAs has strong spontaneous valley polarization characteristics at room temperature,and simultaneously the thinned GeAs may be a paravalley state.Finally,the thickness-dependent optical activity is explored by transmission spectra at different configurations on GeAs with different thicknesses,of which the results present that the thicker the sample,the more obvious the optical rotation effect.