| Summary: | 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.
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