| Summary: | 博士 === 國立臺灣大學 === 物理學系研究所 === 86 === We present our studies on the optical and electrical properties of some low
dimensional III-V and IV semiconductors . These consist of the following
parts:
1.The effective mass of the quaternary alloys in III-V semiconductor.
a. Effect of disorder-induced band mixing on the conduction-band effect
ive mass of InAlGaAs alloys lattice matched to InP.
It is found that the calculation of the effective mass for InAlG
aAs alloys obtained from the five-band theory is smaller than the mea
sured value from the optically detected cyclotron resonance reported re
cently. We point out that the effect of disorder-induced conduction vale
nce band mixing must be considered. This disorder effect which creates p
otential fluctuations is to reduce the matrix element p2 for the conduc
tion valence band coupling in the theoretical expression. The strength of
the potential fluctuations can be described in terms of the electronegativ
ity difference related to chemical disorder. The inclusion of the disorder
effect in the (In0.53Al0.47As)x(In0.53Ga0.47As)1-x quaternary system giv
es a very good fit to the measured data.
b. Compositional dependence of the conduction-band effec
tive mass of InGaAsP lattice matched to InP.
The conduction-band-edge effective masses in InG
aAsP alloys have been determined for several different compositions cover
ing the complete range of alloys grown lattice-matched on InP. The effect
ive mass is obtained from far-infrared optically detected cyclotron resonan
ce (ODCR). In contrast to previous experiments, the ODCR technique provide
s a direct method to determine the band-edge effective mass in undop
ed thin films. Thus, a correction due to nonparabolicity effects is n
ot required. It is found that the experimental values are larger than the e
ffective masses predicted from the five-band calculation. We show that thi
s discrepancy can be satisfactorily removed by the introduction
of the effect of disorder-induce potential fluctuations which causes the wa
ve function mixing between conduction and valence bands. It is found
that the strength of the potential fluctuations can be well descr
ibed in terms of the Phillips''''s electronegativity difference related to
chemical disorder. In addition, the dependence of the band-gap energy
on alloy composition is determined by photoluminescence measurements, an
d it also shows a nonlinear relationship.
c. Enhancement of conduction-band
effective mass in III-V semiconductor alloys induced by chemical di
sorder.
The conduction-band effectiv
e masses in InxGa1-xAs with a complete range of composition, and InAlGaAs
and InGaAsP alloys covering the complete range of lattice matched to
InP have been determined by far-infrared optically detected cyclotr
on resonance and magnetophotoconductivity measurements. It is found
that the measured effective masses as a function of alloy composition are
heavier than the values predicted from the five-band theory. We s
how that this discrepancy can be resolved by including the effect o
f disorder-induced potential fluctuations that causes the wave funct
ion mixing between conduction and valence bands. We find that the str
ength of the potential fluctuations can be well described in terms
of the Phillips electronegativity difference related to chemical disord
er.
2. Photoreflectan
ce study of barrier-width dependence of above-barrier states in GaAs-
AlxGa1-xAs multiple quantum wells.
The optical
transitions of the quasibound states at the above barrier regions in G
aAs/Ga0.77Al0.23As multiple quantum wells have been observed at room tem
perature by photoreflectance measurement. It is found that the barrier-wi
dth dependence of the above-barrier transition energies can be described
quite well by the modified Messiah''''s calculation. However, the simple c
alculation using the constructive interference condition can only explain
the transitions at lower energies, it fails with increasing transi
tion energy.
3. En
hancement of Subnikov-de Haas oscillations by microwave radiation. W
e report for the first time that the Subnikov-de Haas (SdH) pattern ca
n be greatly enhanced by recording the changes in the quantum o
scillations of magnetoresistance due to microwave radiation. In order
to understand the origin of the enhancement, the dependence of the
enhanced SdH signal on temperature, microwave frequency and power has
been studied. It is concluded that the enhancement can be attributed to
the effects of free carrier absorption and the suppression of the
nonoscillatory magnetoresistance. The technique shown here can be used
to detect the SdH oscillations at relatively high temperature and in
samples with moderate mobility without perturbing the carrier
concentration.
4. Observation of persistent photoluminescence in porous silicon:
An evidence of surface emission.
We report the observation of persistent photoluminescence (PPL)
in oxidized porous silicon. The PPL decay can be well described by
a stretched-exponential function, and its decay rate is not sensi
tive to the change of temperature. We point out that the PPL behavior
can be interpreted in terms of the picture that the emission arises
from the excited surface complexes, which are produced by capture of
photocarriers tunneling from the nearest shallow trap in the nanocrystall
ine silicon. To explore the microscopic origin of the surface compounds
, we performed infrared absorption, and found that the PPL intensity cor
relates well with Si-OH vibration mode. Further evidence is provided
by the recent theoretical calculation showing that the Si-OH complex
can emit the photon energy in the range observed here. We thus provide a c
oncrete evidence to support the fact that the PL signal of porous silicon
does contain surface emission.
5. Photoconductivity in self-organized InAs quantum
dots.
Photoconductivity in self-organined InAs/GaAs q
uantum dots is reported. It is found that the ratio of the photoconducti
vity signal caused by transition in InAs dots to that of GaAs matrix
increases with increasing temperature. We point out that the photocondu
ction of InAs dots can be attributed to the thermal activation of phot
ocarriers into GaAs matrix, where the conduction takes place. We have a
lso observed the persistent photoconductivity in the InAs quantum dot.
It is confirmed that the PPC effect is induced to the band bending at
the interface between InAs quantum dot and GaAs matrix due to carri
er transfer can prohibit the return of the electrons back to InAs do
t.
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