J. C. Fan

TEMPERATURE DEPENDENCE OF PHOTOLUMINESCENCE SPECTRA IN INAS/GAAS QUANTUM DOT SUPERLATTICES WITH LARGE THICKNESSES

In this report, we investigate the thermal relaxation of the photoluminescence (PL) in InAs/GaAs quantum dot superlattices with large thicknesses that have two to more than three times the critical thickness for spontaneous island formation. It is found that the linewidth first decreases and then increases with increasing temperature. In addition to thermionic emission, we suggest that carrier repopulation among quantum dots plays an important role in the PL quenching. The temperature dependence of PL peak energy following a Varshni relation was attributed to the dilation of lattice and electron-lattice interaction. The emission intensity quenches rapidly when the temperature rises to around 60 K, indicating the existence of defect-related centers in the vicinity of InAs/GaAs interfaces. In addition, we performed the measurement of the activation energy of PL quenching at different emission energy. We found that the loss mechanism of PL quenching based on the activation of electron-hole pairs from quantum...

PERSISTENT PHOTOCONDUCTIVITY IN SIGE/SI QUANTUM WELLS

Persistent photoconductivity (PPC) has been observed in boron-doped Si1−xGex/Si quantum wells. The decay kinetics of the PPC effect can be well described by a stretched-exponential function, Ippc(t)=Ippc(0)exp[−(t/τ)β](0<β<1), which is usually observed in many disorder materials. Through the studies of the PPC effect under various conditions, such as different temperature, different photon energy of photoexcitation, and different Ge content, we identify that the alloy potential fluctuations induced by compositional disorder are the origin of the PPC effect in Si1−xGex/Si quantum wells.

Effects of alloy potential fluctuations in InGaN epitaxial films

Results of photoluminescence and photoconductivity measurements in epitaxial films are presented. The photoluminescence peak energy and intensity show several anomalous behaviours. The peak energy changes with temperature exhibiting an inverted S-shape dependence, where it decreases, then increases with increasing temperature in the range 40-100 K and finally decreases with increasing temperature. The intensity shows a temperature dependence similar to that of amorphous semiconductors and disordered superlattices. A blue shift of the photoluminescence energy with increasing excitation intensity is observed. A large Stokes shift between the photoluminescence peak position and the band edge transition energy is found; it decreases with decreasing indium content. A persistent photoconductivity effect has been detected up to room temperature with a stretched-exponential function for its decay rate. All these observations can be explained in a consistent way by alloy potential fluctuations, and these clearly indicate the existence of compositional fluctuations. These two related effects thus appear to constitute the mechanism for the widely observed localized excitons in InGaN-based devices.

Above-barrier states in GaAs–AlGaAs superlattices studied by photoconductivity and photoreflectance

We report that the quasibound states at the above-barrier region in AlGaAs–GaAs superlattices can be clearly observed at room temperature by photoconductivity as well as photoreflectance measurements. We provide concrete evidence to confirm that free-carrier confinement at barrier layer does exist. It is also found that the barrier-width dependence of the above-barrier transition energies can be described quite well by the modified Messiah’s calculation. However, the simple calculation using the constructive interference condition can only explain the transitions at lower energies, and fails with increasing transition energy.

Observation of persistent photoluminescence in porous silicon: Evidence of surface emission

We report on 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 sensitive 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 is produced by capture of photocarriers tunneling from the nearest shallow trap in the nanocrystalline silicon. To explore the microscopic origin of the surface compounds, we performed infrared absorption, and found that the PPL intensity correlates 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 concrete evidence to support the fact that the PL signal of porous silicon does contain surface emission.

Effect of disorder‐induced band mixing on the conduction‐band effective mass of InAlGaAs alloys lattice matched to InP

It is found that the calculation of the effective mass for InAlGaAs alloys obtained from the five‐band k⋅p theory is smaller than the measured value from the optically detected cyclotron resonance reported recently. We point out that the effect of disorder‐induced conduction valence band mixing must be considered. This disorder effect which creates potential fluctuations is to reduce the matrix element P2 for the conduction valence band coupling in the k⋅p theoretical expression. The strength of the potential fluctuations can be described in terms of the electronegativity difference related to chemical disorder. The inclusion of the disorder effect in the (In0.53Al0.47As)x(In0.53Ga0.47As)1−x quaternary system gives a very good fit to the measured data.

High crystalline quality ZnSe films grown by pulsed laser deposition

We have grown epitaxial ZnSe films on (001)GaAs substrates at 300 °C by pulsed laser deposition (PLD). Before the growth, thin buffer layers of GaAs are also grown by PLD at 300 °C. While the pattern of reflection high energy electron diffraction (RHEED) of the buffer layers is spotty, the pattern of the ZnSe films subsequently grown is streaky, and shows distinct Kikuchi lines and bands. The x‐ray rocking curve width of the films is as narrow as 150 arcsec. Photoluminescence (PL) at 10 K of the films shows free and bound excitons, donor‐acceptor pairs (DAP), and is free of any deep level emissions, indicating good crystalline quality of the films. Scanning electron microscopy (SEM) shows that the particulate number density of the films is only about 1 particulate per 400 μm2.

Dependence of electron effective mass on alloy composition of InAlGaAs lattice matched to InP studied by optically detected cyclotron resonance

The electron effective mass of InAlGaAs lattice matched to InP has been determined as a function of Al content. The electron effective mass is obtained from far‐infrared optically detected cyclotron resonance (ODCR). In ODCR, the carriers are provided by optical pumping, and hence no doping is necessary. Unlike previous reports, we are able to detect the cyclotron resonance signal of a thin intrinsic epilayer at low temperature. Thus corrections of nonparabolicity are not required. In addition, from photoluminescence measurement, we determine the band‐gap energy. Both the effective mass and band‐gap energy show a nonlinear variation with Al composition.

Persistent photoconductivity in InGaP/GaAs heterostructures

Photoconductivity spectra have been investigated in InGaP/GaAs heterostructures, and persistent photoconductivity (PPC) has been observed. Through a detailed study of the dependence on excitation wavelengths and the decay kinetics, we identify that the separation of electrons and holes due to the macroscopic potential barrier between the heterointerface and the substrate is the origin of the PPC effect. In addition, we found that the PPC effect can be reduced and the photosensitivity can be enhanced after hydrogenation. We suggest that this behavior can be interpreted by the fact that the incorporation of atomic hydrogen produces donor neutralization and passivates the activity of defective bonds, thus reducing the potential barrier for the recombination of electrons and holes and enhances the photoresponse.

Assessment and validation of CLIGEN-simulated rainfall data for Northern Taiwan

By using CLIGEN and long-term meteorological data, consecutive daily rainfall data can be simulated. At present, the applicability of CLIGEN to regions other than the United States remains to be tested. In this study, CLIGEN was first applied in a subtropical monsoon climate region. Rainfall data from 1961 to 1990 at three weather stations in northern Taiwan were collected, and rainfall parameters and patterns were validated. The results showed no significant difference in the mean and distribution patterns between observed and simulated values for rainfall parameters including precipitation for each month, wet days for each month, precipitation on wet days for each month and standard deviation of daily precipitation for each month. Regarding rainfall pattern, after re-calibration of storm pattern coefficients, the accuracy of the simulated storm duration was greatly enhanced. However, the simulated daily maximum 5- and 30-min peak intensities were overestimated, and there is room for improvement.