Study On Phonon And Exciton Properties Of Low-dimensional Ⅲ-Ⅴ Semiconductors And Two-dimensional WSe₂ Materials

In recent decades,low-dimensional materials such as thin films,superlattices,and two-dimensional atomic structures have gradually evolved from traditional bulk materials,because they exhibit many excellent mechanical,optical,and electrical properties that are different from those of bulk materials and have been great concern.For GaAsBi thin film materials,as the Bi composition increases,the material band gap will be greatly reduced.In the III-V semiconductor superlattice,the band structure can be adjusted by thickness.In theory,the range of the detection wavelength covering 2-30μm can be realized.In the new transition metal dichalcogenide material,the band gap of the material can be changed from visible to near-infrared.Extensive research indicates that these materials have potential applications in infrared light emitters,photodetectors and so on.This paper focuses on the exciton and phonon properties of GaAsBi,III-V superlattice and WSe₂,and Several aspects have made some progress:1.We used Riber Molecular Beam Epitaxy to grow different compositions of undoped GaAs_1-xBi_x on GaAs（100）substrates and with epilayer thicknesses 450 nm（x=0.4%,0.8%,and 2.6%,3.7%）.We have measured the room-temperature micro-Raman spectra of four nominally undoped GaAs_1-xBi_x with a small amount of Bi and have observed the LOPC mode as well as the new structure at 286.5 cm^-1 in all samples.Polarization-dependent and excitation energy-dependent Raman spectroscopy confirms the existence of the LOPC mode.With the increase of Bi component,the LOPC mode first appears in the vicinity of the longitudinal optical（LO）phonon frequency,and then shifts towards near the transverse optical（TO）phonon frequency.This variation tendency of the LOPC frequency with Bi concentration in GaAsBi is corresponding to the change trend of LOPC frequency with hole density in p-type semiconductors.The new vibrational modes(²86.5 cm^-1)between the TO and the LO phonons was verified by use of low temperature polarized Raman measurement and the corresponding scattering intensities are found to be linearly proportional to the composition of Bi in GaAsBi,one may consider a Bi-relate vibration mode.Considering the influence of photogenerated carriers,the hole concentrations determined by using the LOPC/LO Raman intensities ratio increase from⁶.5×10¹⁶ to².8×10¹⁷ cm^-33 with Bi content and the measured results are consistent with our Hall test values,explaining the possible source of the large difference in hole carrier concentration obtained by the two methods reported in the literature.Our results show that Raman spectroscopy is a very effective means of determining the concentration of GaAsBi carriers.2.We use angular resolved polarization Raman spectroscopy to study the Raman Brillouin scattering behavior of folded acoustic phonons in III-V semiconductor superlattices.The abnormal in-plane anisotropic behavior related to the period of superlattices are observed and is noticeable in short-period superlattice.We believe that the traditional theoretical model does not take into account the existence of the breakdown of the spatial symmetry due to interfacial stress during the growth of superlattice.Based on the traditional isotropic photoelastic model,the anisotropy of the in-plane stress is qualitatively calculated as a perturbation.We successfully explained that the observed long acoustic phonon Raman vibration is in-plane anisotropic along the vertical growth plane.Our research results not only provide an effective complement to the traditional superlattice low-wavenumber Raman vibration theory,but also provide possibilities for future superlattice-based THz polarization detection and emission applications.3.We present a temperature-dependent and time-resolved PL study for few-layer WSe₂.The indirect band gap blue shift and the increase of luminescence intensity with increasing temperature in multilayer WSe₂ were observed.In monolayer and bilayer WSe₂,the intensities of direct and indirect exciton PL exhibit normal reduction with the increase in temperature.However,abnormal enhancement of light emission from direct and indirect excitons in few-layer WSe₂（≥3 L WSe₂）is observed at high temperatures that range from 300 K to 400 K.Combined with the first-principles calculation,based on the influence of temperature changes on lattice constants and photoacoustic interactions,we theoretically analyzed the effect of temperature on the energy band structure,and explained the blue-shift characteristics of the band gap at low temperatures.We also verified the crossover of and indirect transitions that occurred at elevated temperature based on the temperature-dependent shift of indirect exciton PL peak.A model that involves the intervalley transfer of thermally activated carriers from the point to the K point and the crossover of and indirect transitions is proposed to comprise the high-temperature luminescence enhancement of direct and indirect transitions in few-layer WSe₂.Our experimental results not only show that the temperature-dependent fluorescence intensity is greatly dependent on the electronic structure and number of layers of the material,but also provide a means to effectively improve the luminescence enhancement of two-dimensional materials.In addition,a strong electric field is generated on the double layer WSe₂ by polarizing the P（VDF-TrFE）ferroelectric material with an applied bias.The experimental results show that the different polarization directions of ferroelectric materials lead to different excitonic fluorescence intensity.As the absolute value of negative bias increases,the fluorescence intensity gradually increases.However,as the forward bias increases,The fluorescence intensity gradually decreases.