Growth of Urtra-thin Films and Their Nonlinear Optical Response in Layered Semiconductors

用热壁法在CdI_2单晶的折痕衬底和CaF_2单晶的（1ll）折痕表面制备了极薄的BiI_3微晶。在CdI_2基体中生长出片状、单层厚度的微晶。在这些晶体中，我们利用透射电子显微镜和原子力显微镜技术研究了它们的结构特征与光学性质和激子在高密度下的行为。在CdI_2折痕衬底上生长的BiI_3厚度为300 A的样品中，观察到与本体晶体相似的窄线宽激子吸收带。用热壁法对蒸发速率很小的BiI_3样品在CaF_2单晶（111）折纹表面上观察到新的吸收带和2.02eV的共振发光带。对于CdI_2微晶带的共振激发，发光带出现Stokes位移。建立了激子质心运动受限的量子盘模型，并建立了量子盘中激子-声子相互作用的理论。研究了薄层微晶中激子的非线性光学响应。从发光光谱的激发能依赖性分析出发，讨论了微晶中双激子态的稳定性。

We'v prepared very thin micro-crystals of BiI_3 on the creaved substrate of CdI_2 single crystals and on the (1ll) creaved surface of CaF_2 single crystals by using a Hotwall method. The microcrystallites having a disk-like shape with mono-layer thickness were grown in CdI_2 matrices. In these crystals we have studied structural characteristics by using a transmission electron microscope and AFM technique in connection with optical properties and behavior of the excitons at high density.The exciton absorption band with narrow line width similar in bulk crystals was observed in the samples with the thickness of 300 A of BiI_3 grown on the substrate of creaved CdI_2 crystals. New absorption band appeared and resonant luminescence band at 2.02eV was observed in the case of the sample with very small evaporation rate of BiI_3 on the (111) creaved surface of CaF_2 single crystals by using a Hotwall method.For the resonant excitation at the microcrystallite bands in CdI_2 matrices, luminescence bands appear with Stokes shifts. A quantum disk model in which the center of mass motion of the exciton is confined and a theory of the size-dependent exciton-acoustic phonon interaction in the quantum disks have been developed. Nonlinear optical rsponse of the excitons in such microcrystallites with thin layer thickness have been investigated. From the analysis of the excitation energy dependence of the luminescence spectra, stability of the biexciton states in microcrystaliites was discussed.