【作者】 赵彬；

【导师】 徐丰；

【作者基本信息】 武汉理工大学 ， 物理学， 2019， 硕士

【摘要】 硅作为半导体产业中的重要材料,其常压下的力学性质和电学性质受到了广泛的关注,并得以充分的研究。但在高压下,硅会金属化,并会随压力的变化发生一系列的相变。在对单质的固体物质进行高压相变的研究中,硅是极具代表性的,具备很高的研究价值。到目前为止,科学家们利用X射线衍射技术对硅晶体在高压下的相变及各高压相的晶体结构进行了充分的研究,但对于其弹性力学性质的研究仍然停留在较低压力,因为在更高压力下对样品进行弹性力学性质的研究是极具挑战性的。声速是研究物质弹性力学性质的有力手段。本文利用激光皮秒超声技术结合金刚石压腔（diamond anvil cell,以下简称DAC）静高压技术在室温环境下,对高压加载下硅的纵波声速进行了研究,并对相应的弹性力学性质进行了深入的讨论。在常压下硅为金刚石立方（Si-I）结构,该结构会一直保持稳定到9 GPa。随着压力的进一步升高,硅会接连经历三次相变,并于18 GPa之后稳定在原始六方结构（Si-V）,直至38 GPa。本文对0-9 GPa范围内的Si-I相和18-37 GPa范围内的Si-V相的纵波声速及其相应的弹性力学参数进行了深入地探索与研究。研究发现,初始的样品硅一直保持着其单晶状态直至发生第一次相变,即9 GPa。在这个过程中,我们测量到的声速是声波沿着样品<100>方向上的纵波声速V_L<100>,通过结合该实验结果以及由状态方程得到的体模量B（P）和密度ρ（P）,我们得到了弹性常数C₁₁和C₁₂在不同压力下的值,并将其与我们第一性原理计算结果,以及文献中前人的实验和计算结果进行了比较。之后,伴随着三次相变,样品由最初的单晶状态转变为了各向同性的多晶状态。实验表明,各向同性状态下的Si-V相由18 GPa一直持续到了27 GPa,我们在这个压力范围内测量到的声速即为平均的纵波声速V_L（avg）。以及,结合由状态方程推导出的B（P）和ρ（P）,我们获得了Si-V的剪切模量G（P）。有趣的是,在27 GPa之后,实验测量到的纵波声速明显地偏离了我们计算的平均纵波声速曲线,而朝着理论上的单晶Si-V结构的最快轴c轴方向在发展。该变化趋势表明,多晶状态下的硅随着受到更强的挤压,其晶粒的取向发生了变化,并且是逐渐地转向了c轴。而对于9-18 GPa压力范围内的数据,我们没有作深入地讨论,因为期间发生的多次相变会导致在该压力范围内样品中可能同时存在两种甚至更多种的相结构,并且在无法确定每种结构所占比例的情况下,我们无法对其中某一种相的声速进行定量分析。实验中用来测量样品声速的激光超声技术,是一种先进的全光学泵浦探测技术。相对于其他光学技术,如布里渊散射或者基于同步辐射光源的X射线散射技术,激光超声技术有着其独特的优越性。并且,本文所用到的激光皮秒超声与传统的宽脉冲激光超声技术相比,测量的时间分辨率更高,更适用于小尺寸样品的测量,完全契合金刚石压腔技术对小尺寸样品的要求。该技术起初用于研究薄膜材料的弹性力学性质,随后发展为与高压技术结合,应用于固体物理,地球物理等学科。更多还原

【Abstract】 Silicon has been intensively studied on its mechanical and electronic properties at ambient pressure and it has been widely used in semiconductor field.Upon compression,silicon metalizes and undergoes a series of phase transitions,and thus is considered as a representative elemental solid on pressure-induced phase transitions studies.Up to now,a great deal of studies of silicon about its phase transitions and crystal structures have been performed by X-ray diffraction technique.However,measurement of elastic parameters under high pressures is still a challenge.Only very a few works reported their abilities to study the elastic properties of silicon in low-pressure region.Measurement of sound velocity is an efficient way to probe intrinsic elastic properties of solid matter.In this paper,picosecond laser ultrosonics（PLU）technique combined DAC is used to study the sound velocities as well as elastic properties of silicon under high pressures.Silicon remains stable from 1 atm.to 9 GPa in a diamond cubic（Si-I）structure;for the primitive hexagonal structural silicon（Si-V）,it has been found to be stable and unique phase in the pressure range from 18 GPa to 38 GPa,after intermediate phase transitions from Si-I to Si-V.In this work,sound velocities and elastic properties of silicon have been studied between 0-9 GPa and 18-37 GPa.Firstly,we found that the starting silicon sample remains as single crystal till 9 GPa,and we measured the longitudinal sound velocities along<100>direction.Secondly,with the densities and bulk modulus of silicon under high pressure extracted from the well-known equation of state,the pressure-dependent elastic constants C₁₁ and C₁₂ have been deduced,then compared with our calculations and published experimental and theoretical results.For the Si-V phase,the sample could be regarded to be elastically isotropic due to the appearance of smaller randomly oriented grains after three phase transitions.We found that from 18 GPa to 27 GPa,the measured velocities correspond to the average sound velocity in Si-V.The corresponding shear modulus has thus been obtained.It is worth noting that,above 27 GPa,the measured velocities increase much faster and deviate from the theoretical V_L（avg）,but toward the maximum theoretical velocity in single crystalline Si-V（known to be along the c-axis）,which indicates that the developing texture was characterized by a growing number of grains which c-axis pointed into the propagation direction of the sound pulse,perpendicular to the diamond culet.We did not have a further discussion on data between 9-18 GPa,since two or more phases co-exist during the phase transition processes,under which circumstances we do not know the proportion of each phase,posing difficulties to the quantitative study of a certain phase.The picosecond laser ultrasonics is an advanced all-optical pump-probe technique which demonstrates irreplaceable advantages compared to other optical techniques like Brillouin light scattering and synchrotron–based X-ray diffraction techniques.And the high time resolution of PLU allows measurements on micron-size samples loaded in DAC.The combination of PLU and DAC demonstrate immense potentialities in condensed matter physics and geophysics.更多还原