【作者】 王晖；

【导师】 蒋建中； 吴希俊；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2010， 博士

【摘要】 本文主要利用高压原位同步辐射X射线衍射技术和金刚石对顶砧高压装置,对纳米尺寸Ge-Ⅰ（立方金刚石结构）、q-GeO₂（α-石英结构）、和β-Ga₂O₃的高压行为进行了研究,并对比了相应大块材料的高压数据,以期了解晶粒尺寸对相变压力、体弹性模量的影响。本文还总结了ⅣA族单质半导体、ⅢA-ⅤA族和ⅡB-ⅥA族二元半导体材料的高压数据,探究相变压力P_tr和能隙之间的关系。首先利用能量色散X射线衍射技术对晶粒尺寸为100 nm、49 nm和13 nm的Ge进行了高压实验,最高压力为35 GPa。发现在高压下会发生Ge-Ⅰ→Ge-Ⅱ的相变,相变压力分别11.5 GPa、12.4 GPa、16.4 GPa。根据压力-体积数据计算出Ge-Ⅰ相在零压下的体弹性模量分别为88GPa、92 GPa、112 GPa。可以看出,随着晶粒尺寸的减小,其体弹性模量逐渐升高,相转变压力亦随之提高。本文建立了一个描述晶粒尺寸与体弹性模量关系的模型,并利用Jamieson公式计算了相变压力随晶粒尺寸的变化,尽管在数值上存在一定的误差,但理论计算结果成功预示了随着晶粒尺寸的减小,体弹性模量和相变压力增加的趋势,与实验结果相一致。其次又利用能量色散X射线衍射技术研究了晶粒尺寸为40 nm和260 nm q-GeO₂的高压相变,最高压力分别为51.5 GPa和40.6 GPa。在加压过程中发生了两种相变:首先由α-石英结构的GeO₂转变为非晶GeO₂,40 nm q-GeO₂相变的起止压力分别为10.8 GPa和14.9 GPa,而260 nm q-GeO₂的则为9.5 GPa和12.4 GPa;然后会发生非晶相到单斜相的转变,40 nm和260 nm的试样检测到单斜相的压力分别为26.9GPa和28.4 GPa。这种单斜相和非晶相的混合物可以保持到实验的最高压力,并能够保留到卸压以后。这个结果表明非晶相与单斜相的自由能比较接近,因此起始材料中的缺陷对q-GeO₂在高压下发生何种相变有很大的影响。而后又利用角度色散X射线衍射技术对晶粒尺寸为14 nm的β-Ga₂O₃进行了高压研究,最高压力为64.9 GPa。实验发现β-到α-Ga₂O₃相变的起始压力在13.6-16.4 GPa之间,相变进行的很缓慢,为不可逆相变,最高压力下的相仍为α-Ga₂O₃,并可保留到卸压后。根据第一性原理的计算结果以及相变压力和能隙之间的关系,我们判定纳米β-Ga₂O₃的相变压力要高于大块材料。压力提高的主要原因是由于纳米β-Ga₂O₃和α-Ga₂O₃表面能的差异,数值大约为4.6-4.7 J/m²。根据体积压缩数据计算了β-Ga₂O₃和α-Ga₂O₃的体弹性模量,当B₀′=4时β-Ga₂O₃和α-Ga₂O₃的体弹性模量分别为228 GPa和333 GPa,都要高于相应的大块材料的数值。最后我们总结了ⅣA族单质半导体、ⅢA-ⅤA族和ⅡB-ⅥA族二元半导体材料的高压数据。对于纤锌矿结构的二元半导体,在高压下将从直接半导体转变为NaCl结构的间接半导体。而闪锌矿结构的半导体（不包括ZnS和ZnSe）在高压下会转变为六配位或者八配位的金属相。对于可转变为金属相的闪锌矿结构半导体,它们基本满足P_tr⊿V=0.14+0.12E_g;而在高压下无法转变为金属相的二元半导体则符合P_tr⊿V＜0.14+0.12E_g。更多还原

【Abstract】 We have studied the high-pressure behavior of nanocrystalline Ge-Ⅰ（cubic diamond structure）,q-GeO₂（α-quartz structure）,andβ-Ga₂O₃ using in situ high-pressure synchrotron x-ray diffraction and diamond anvil cell techniques.The effects of grain size on transition pressure and bulk modulus have been discussed by analyzing the data of nanocrystals and comparing with that of bulk counterpart.And the high-pressure data ofⅣA group elements,ⅢA-VA andⅡB-ⅥA binary compounds were summarried in order to explore the relationship between transition pressure P_tr and energy gap E_g.First,energy-dispersive synchrotron x-ray diffraction measurements of nanocrystalline Ge with grain sizes of 100 nm,49 nm,and 13 nm were carried out up to 35 GPa.Transition pressures from Ge-Ⅰto Ge-Ⅱwere 11.5 GPa,12.4 GPa,and 16.4 GPa,and the bulk modulus were 88 GPa,92 GPa,and 112 GPa for Ge-Ⅰwith grain size of 100 nm,49 nm,and 13 nm, respectively.It is confirmed that the values of bulk modulus and transition pressures would increase with decreasing particle size.A model was established to calculate the bulk modulus with consideration of grain size, and Jamieson equation was used to calculate the transition pressures of samples with different sizes.Theoretical results corresponded well with the tendency of experimental data.Second,high-pressure behaviors of nanocrystalline q-GeO₂ with average crystallite sizes of 40 nm and 260 nm have been studied by in situ high-pressure synchrotron radiation x-ray diffraction measurements up to about 51.5 GPa at ambient temperature.Two phase transformations,from q-GeO₂ to amorphous GeO₂ and from amorphous GeO₂ to monoclinic GeO₂ phase,were detected.The ranges of transition pressure from q-GeO₂ to amorphous GeO₂ were found to be approximately 10.8-14.9 GPa for the 40 nm q-GeO₂ sample,and 9.5-12.4 GPa for the 260 nm q-GeO₂ sample. The mixture of amorphous and monoclinic GeO₂ phases remained up to 51.5 GPa during compression and even after pressure release.This result strongly suggested that the difference of free energy between the amorphous phase and the monoclinic phase might be small.Consequently, defects in the starting material,which could alter the free energies of amorphous phase and the monoclinic phase,may play a key role for the phase transformation of q-GeO₂.Third,free-standing nanocrystallineβ-Ga₂O₃ particles with an average grain size of 14 nm prepared by chemical method was investigated by angle-dispersive synchrotron x-ray diffraction in diamond anvil cell up to 64.9 GPa at ambient temperature.It was found thatβ- toα-Ga₂O₃ transition began at about 13.6-16.4 GPa,and extended up to 39.2 GPa.At the highest investigated pressure,onlyα-Ga₂O₃ was present,which remained after pressure release.Based on the first principle calculation and the relationship between the transition pressure and energy gap,we concluded that the transition pressure of nanocrystallineβ-Ga₂O₃ is higher than that of bulk counterpart.The enhancement of the transition pressure was mainly due to the difference of surface energy betweenβ-Ga₂O₃ andα-Ga₂O₃ phases,which was determined to be about 4.6-4.7 J/m².A Birch-Murnaghan fit to the P-V data yielded a zero-pressure bulk modulus at fixed Bo’= 4:B₀ = 228 GPa and B₀ = 333 GPa forβ-Ga₂0₃ andα-Ga₂O₃ phases,respectively.It could be concluded that the phase transition pressure and bulk modulus of nanocrystallineβ-Ga₂O₃ were higher than that of bulk counterpart.Finally,the high-pressure data ofⅣA group elements,ⅢA-ⅤA andⅡB-ⅥA binary compounds were summarried.It was found that the binary compounds with wurtzite structure will transform from direct to indirect band-gap semiconductor under high pressure.But almost all semiconductors with zinc-blende structure could transform to six-fold or eight-fold coordinated metallic phase,except ZnS and ZnSe.As to ZB-structured semiconductors with metallic high-pressure phase,a linear relationship was obtained:P_tr△V=0.14+0.12E_g.And for semiconductors that could not transform into metallic phase during compression,the appropriate relationship tended to be P_tr△V＜0.14+0.12E_g.更多还原