Theoretical Study of the Surface Reconstruction of CdS(100) and Investigation of Physical Properties of Dilute Magnetic Semiconductor under High-pressure

• Main Authors: Lin, Chih-Ming, 林志明
• Other Authors: Tzong-Jer Yang
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/52028680950729408727

博士 === 國立交通大學 === 電子物理學系 === 85 === 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.