硫化鎘表面重構現象之理論研究及稀磁性半導體高壓下的物性研究

Abstract

本論文可分為兩個部份,第一部份為 CdS 表面重構的理論模擬工作,第二 部份為 ZnSe 為基礎的稀磁性半導體的高壓工作第一部份我們使用的方法 是 first-principles 分子動力 (MD) 方法,利用區域密度近似和非局限 虛位能,來研究陰離子 (S) 終結的 CdS(100) 的重整表面,從我們選取的 幾種重整表面的模型中,經由計算發現,最穩定的構造是包含有二個 dimer 和二個位移 dimer 的 (2x4) 的表面重整構造第二部份利用 micro-Raman 技術,研究 ZnSe 系列的高壓相變的物理行為,從我們的研究中發現有隨著 外加壓力增加,硒化鋅的 TO 拉曼散射譜在 4.7 GPa 第一次分裂成兩支, 在9.1 GPa 處高頻分支,第二次分裂為兩分支(共三支),氯化鈉相的金屬相 變點存在於 14.4 GPa,共有四支拉曼模出現, LO, TO, TO 分裂(I), TO 分裂(II),前三支表現出藍位移,後一支為紅位移,硒化錳鋅與硒化鐵鋅在 高壓下的行為和硒化鋅類似,另外,利用 EDXD 方法,知道稀磁性半導體與 非稀磁性半導體,在高壓下,相轉變壓力有下降的現象,而體積變化佔較重 要的指導因素 This thesis consistes of two parts, one is concerning about theoretical study the surface reconstruction of CdS(100), another one is using Raman and EDXD experimentical methods to study the phase transition pressure of ZnSe base semiconductor which containing dilute magnetic semiconductor (DMS) under high- pressure. For the first part, the surface structure of the strain stablized zinc-blende monolayer anion-terminated CdS(100) surface is studied by the local-orbital density-functional molecular-dynamics method. By analogy with the GaAs(100) surface, four structural models are considered and the structure with a (2x4) unit cell is found, in which there are a two-dimer unit and a two-shifted-dimer unit is the most favorable. These results can be understood in terms of physical properties associated with ionicity.For the second part, Zn1-xFexSe, x=0, 0.035 and 0.16, were studied by Raman scattering spectroscopy up to 35.0 GPa. It was found that the semiconductor-metal phase transition pressures were 14.4, 12.0, and 10.9 GPa, respectively. A visible anomaly of the TO Raman mode splitting vs pressure was observed before the semiconduvtor-metal phase transition at 4.7, and 9.1 GPa for ZnSe and at 4.5, and 7.2 GPa for Zn0.965Fe0.035Se, respectively. While Zn0.84Fe0.16Se shows mode splitting at 4.7 GPa only. For these three samples, one of the TO splitting modes exhibits phonon softening (red shift), while the other manifested frequency increasing (blue shift) with pressure. It was found that the pressure for the splitting of the TO mode, which exhibited blue shifting, decreases as the impurity concentration increases. In addition, the three unidentified TO Raman modes were still observable even above the metallization pressure. For x=0.035 and 0.16, a new Raman mode, which was identified as Fe LO local mode, was observed between the pure ZnSe LO and TO modes. Fe LO local mode exhibites blue shift behavior before metallization and disappeares as the pressure is higher beyond the metallization pressure. From the calculated Gruneisen parameter, it implies that Zn0.84Fe0.16Se has higher ionicity. The reason for the observation of Raman peaks at pressure above the metallizayion pressure may be due to the existence of TO modes in the thin surface of the high pressure metallic phase. For Zn0.76Mn0.24Se crystal, three Raman modes: one TO mode at 197.2 cm-1, one mode at 249.4 cm-1, and a Mn local mode located at 222.5 cm-1 are found at ambient pressure. The Mn local mode is splitted into two modes at 4.7 GPa while visible anomaly splttings of TO mode occur at 6.0 and 8.9 GPa. The semiconductor-metal phase transition of Zn0.76 Mn0.24Se crystal is observed at 9.6 GPa which is lower than that of ZnSe crystal. The reduction of the phase transition pressure is ascribed to the increasing of the volume factor of the impurity atom.The energy-dispersive x-ray-diffraction(EDXD) was employed to study the pressure induced phase transitions of Zn0.9Cd0.1Se, Zn0.84Fe0.16Se, and Zn0.76Mn0.24Se crystals up to 23.3, 21.0 and 24.3 GPa, respectively.Our results show that B3 to B1 structures for these crystals occurred at 10.3, 11.4, and 9.6 GPa, respectively. Comparing to the phase transition pressure(14.4 GPa) of ZnSe, a reduction of about 3 ~ 5 GPa exhibits in these ternary compounds of ZnSe. The exhibitation of the reduction of the phase transition pressure in the ternary compound suggests that the change of the volume at coexist phases (B3 to B1) might be the main reason of this reduction.