Absorption spectra of nanocrystalline silicon embedded in SiO2 matrix

Nanocrystalline silicon (nc-Si) embedded SiO2 matrix has been formed by annealing the SiOx films fabricated by plasma-enhanced chemical vapor deposition (PECVD) technique. Absorption coefficient and photoluminescence of the films have been measured at room temperature. The experimental results show that there is an "aUrbach-like" b exponential absorption in the spectral range of 2.0-3.0 eV. The relationship of (alpha hv)(1/2) proportional to(hv - E-g) demonstrates that the luminescent nc-Si have an indirect band structure. The existence of Stokes shift between photoluminescence and absorption edge indicates that radiative combination can take place not only between electron states and hole states but also between shallow trap states of electrons and holes. (C) 2000 Elsevier Science B.V. All rights reserved.

Infrared absorption efficiency in AlAs/AlxGa1-xAs type-II multiple-quantum-well structure grown on (211) GaAs substrate

We have made a normal incidence high infrared absorption efficiency AlAs/Al0.55Ga0.45As multiple-quantum-well structure grown on (211) GaAs substrates by molecular beam epitaxy (MBE). A strong infrared absorption signal at 11.6 mu m due to the transition of the ground state to the first excited state, and a small signal at 6.8 mu m due to the transition from the ground state to continuum. were observed. A 45 degrees tilted incidence measurement was also performed on the same sample for the comparison with a normal incidence measurement. Both measurements provide important information about the quantum well absorption efficiency. Efficiencies which evaluate the absorption of electric components perpendicular and parallel to the well plane are eta(perpendicular to) = 25% and eta(parallel to) = 88%, respectively. The total efficiency is then deduced to be eta = 91%. It is apparent that the efficiency eta(parallel to) dominates the total quantum efficiency eta Because an electron in the (211) AlAs well has a small effective mass (m(zx)* or m(zy)*), the normal incidence absorption coefficient is expected to be higher:than that grown on (511) and (311) substrates. Thus, in the present study, we use the (211) substrate to fabricate QWIP. The experimental results indicate the potential of these novel structures for use as normal incidence infrared photodetectors.

Steady and transient optical properties of cubic InGaN epilayers

Photoluminescence (PL) and time-resolved PI, were employed to study the steady and transient optical properties of cubic InxGa1-xN epilayers grown by MBE. The results suggest that the PL transitions in InGaN epilayers are mainly from localized exciton states. The localization energies are estimated to be 60 meV, independent of In composition. The PL decay is characterized by a hi-exponential dependence. The fast process (50 ps at 12K) is related to the fast relaxation of excitons, while a slower contribution (200-270 ps at 12K) is attributed to the decay process of localized excitons.

Electronic characteristics of InAs/GaAs self-assembled quantum dots by deep level transient spectroscopy

A deep level transient spectroscopy technique has been used to determine the emission activation energies and capture barriers for electrons and holes in InAs self-assembled quantum dots embedded in GaAs. The ground electron and hole energies relative to their respective energy band edges of GaAs are 0.13 and 0.09 eV. Measurements show that the capture cross section of quantum dots is thermally activated. The capture barrier of quantum dots for electrons and holes are 0.30 and 0.26 eV, respectively. The results fit well with the results of photoluminescence spectroscopy measurements. (C) 2000 Elsevier Science B.V. All rights reserved.

Investigation of absorption of nanocrystalline silicon

Nanocrystalline silicon embedded SiO2 matrix is formed by annealing the SiO2 films fabricated by plasma enhanced chemical vapor deposition technique. In conjunction with the micro-Ramam spectra, the absorption spectra of the films have been investigated. The blue-shift of absorption edge with decreasing size of silicon crystallites is due to quantum confinement effect. It is found that nanocrystalline silicon is of an indirect band structure, and that the absorption presents an exponential dependance absorption coefficient on photon energy ii! the range of 2.0-3.0 eV, and a sub-band appears in the the range of 1.0-1.5 eV. We believe that the exponential absorption is due to the indirect band-to-band transition of electrons in silicon nanocrystallites, while the Sub-band absorption is ascribed to transitions between the amorphous silicon states existing in the films.

Structural and infrared absorption properties of self-organized InGaAs GaAs quantum dots multilayers

Self-organized InGaAs/GaAs quantum dots (QDs) stacked multilayers have been prepared by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy shows that the InGaAs QDs are nearly perfectly vertically aligned in the growth direction [100]. The filtering effect on the QDs distribution is found to be the dominant mechanism leading to vertical alignment and a highly uniform size distribution. Moreover, we observe a distinct infrared absorption from the sample in the range of 8.6-10.7 mu m. This indicates the potential of QDs multilayer structure for use as infrared photodetector.

Intraband absorption in the 8-12 mu m band from Si-doped vertically aligned InGaAs/GaAs quantum-dot superlattice

Normal-incident infrared absorption in the 8-12-mu m-atmospheric spectral window in the InGaAs/GaAs quantum-dot superlattice is observed. Using cross-sectional transmission electron microscopy, we find that the InGaAs quantum dots are perfectly vertically aligned in the growth direction (100). Under the normal incident radiation, a distinct absorption peaked at 9.9 mu m is observed. This work indicates the potential of this quantum-dot superlattice structure for use as normal-incident infrared imaging focal arrays application without fabricating grating structures. (C) 1998 American Institute of Physics. [S0003-6951(98)01151-6].

Normally incident infrared absorption in vertically aligned InGaAs/GaAs quantum dot superlattice

30-period InGaAs/GaAs quantum dot superlattice was fabricated by MBE. Using cross sectional transmission electron microscopy, the InGaAs quantum dots were found to be perfectly vertically aligned in the growth direction (100). Under normally incident radiation, a distinct absorption in the 8.5 similar to 10.4 mu m range peaked at 9.9 mu m was observed. The normally incident infrared absorption in vertically aligned quantum dot superlattice in the 8 similar to 12 mu m range was realized for the first time. This result indicates the potential application of the quantum dot superlattice structure without grating as normally incident infrared detector focal plane arrays.

Linewidth of the infrared absorption spectra due to bound-to-continuum transition in GaAs/AlxGa1-xAs multiple quantum well structures

We have observed an extremely narrow absorption spectrum due to bound-to-continuum transition in GaAs/AlxGa1-xAs multiple quantum wells (MQWs). Its linewidth is only about one tenth of the values reported previously. Our calculation indicates that the broadening of the excited state in the continuum has little contribution to the absorption linewidth. We have grown a sample whose MQW region contains two kinds of wells with a minor thickness inhomogeneity. Its resultant absorption linewidth is six times as large as that of homogeneous well sample, which is in good agreement with our theoretical analysis. Thus we can suggest that the wider absorption spectra reported by many authors may be due to the well width inhomogeneity. (C) 1998 American Institute of Physics. [S0003-6951(98)03430-5]

Conduction-band offset in a pseudomorphic GaAs/In0.2Ga0.8As quantum well determined by capacitance-voltage profiling and deep-level transient spectroscopy techniques

The conduction-band offset Delta E-C has been determined for a molecular beam epitaxy grown GaAs/In0.2Ga0.8As single quantum-well structure, by measuring the capacitance-voltage (C - V) profiling, taking into account a correction for the interface charge density, and the capacitance transient resulting from thermal emission of carriers from the quantum well, respectively. We found that Delta E-C = 0.227 eV, corresponding to about 89% Delta E-g, from the C - V profiling; and Delta E-C = 0.229eV, corresponding to about 89.9% Delta E-g, from the deep-level transient spectroscopy (DLTS) technique. The results suggest that the conduction-band discontinuity Delta E-C obtained from the C-V profiling is in good agreement with that obtained from the DLTS technique. (C) 1998 American Institute of Physics.

Study of microstructure of high stability hydrogenated amorphous silicon films by Raman scattering and infrared absorption spectroscopy

The microstructure, hydrogen bonding configurations and hydrogen content of high quality and stable hydrogenated amorphous silicon (a-Si:H) films prepared by a simple ''uninterrupted growth/annealing" plasma enhanced chemical vapor deposition technique have been investigated by Raman scattering and infrared absorption spectroscopy. The high stability a-Si:H films contain small amounts of a microcrystalline phase and not less hydrogen (10-16 at. %), particularly, the clustered phase hydrogen, Besides, the hydrogen distribution is very inhomogeneous. Some of these results are substantially distinct from those of conventional device-quality n-Si:H film or stable cr-Si:H films prepared by the other techniques examined to date. The stability of n-Si:H films appears to have no direct correlation with the hydrogen content or the clustered phase hydrogen concentration. The ideal n-Si:H network with high stability and low defect density is perhaps not homogeneous. (C) 1998 American Institute of Physics.

Intrasubband and intersubband transitions in lightly and heavily doped GaAs/AlGa1-xAs multiple quantum wells

We use a polarizer to investigate quantum-well infrared absorption, and report experimental results as follows. The intrasubband transition was observed in GaAs/AlxGa1-xAs multiple quantum wells (MQWs) when the incident infrared radiation (IR) is polarized parallel to the MQW plane. According to the selection rule, an intrasubband transition is forbidden. Up to now, most studies have only observed the intersubband transition between two states with opposite parity. However, our experiment shows not only the intersubband transitions, but also the intrasubband transitions. In our study, we also found that for light doping in the well (4x10(18) cm(-3)), the intrasubband transition occurs only in the lowest subband, while for the heavy doping (8x10(18) cm(-3)), such a transition occurs not only in the lowest subband, but also in the first excited one, because of the electron subband filling. Further experimental results show a linear dependence of the intrasubband transition frequency on the root of the well doping density. These data are in good agreement with our numerical results. Thus we strongly suggest that such a transition can be attributed to plasma oscillation. Conversely, when the incident IR is polarized perpendicular to the MQW plane, intersubband-transition-induced signals appear, while the intrasubband-transition-induced spectra disappear for both light and heavy well dopings. A depolarization blueshift was also taken into account to evaluate the intersubband transition spectra at different well dopings. Furthermore, we performed a deep-level transient spectroscopy (DLTS) measurement to determine the subband energies at different well dopings. A good agreement between DLTS, infrared absorption, and numerical calculation was obtained. In our experiment, two important phenomena are noteworthy: (1) The polarized absorbance is one order of magnitude higher than the unpolarized spectra. This puzzling result is well explained in detail. (2) When the IR, polarized perpendicular to the well plane, normally irradiates the 45 degrees-beveled edge of the samples, we only observed intersubband transition spectra. However, the intrasubband transition signals caused by the in-plane electric-field component are significantly absent. The reason is that such in-plane electric-field components can cancel each other out everywhere during the light propagating in the samples. The spectral widths of bound-to-bound and bound-to-continuum transitions were also discussed, and quantitatively compared to the relaxation time tau, which is deduced from the electron mobility. The relaxation times deduced from spectral widths of bound-to-bound and bound-to-continuum transitions are also discussed, and quantitatively compared to the relaxation time deduced from electron mobility. [S0163-1829(98)01912-2].

Preparation of I-2 clusters and their absorption spectra

Samples have been prepared at different temperatures by loading It molecules into the cages of zeolite 5A, and the measurements of the absorption spectra have been carried out for the prepared samples. It is shown that 12 molecular clusters are formed in the cages of zeolite 5A, and it is also found that molecular clusters which are bonded with intermolecular forces have an important feature, namely, the intermolecular distance in molecular clusters can be changed on different preparing conditions and the blue shift of absorption edges can not be as the criterion of forming molecular clusters.

High energy ion implantation enhanced intermixed quantum well structures and their absorption characteristics in passive optical waveguides

We have shown that high energy ion implantation enhanced intermixing (HE-IIEI) technology for quantum well (QW) structures is a powerful technique which can be used to blue shift the band gap energy of a QW structure and therefore decrease its band gap absorption. Room temperature (RT) photoluminescence (PL) and guided-wave transmission measurements have been employed to investigate the amount of blue shift of the band gap energy of an intermixed QW structure and the reduction of band gap absorption, Record large blue shifts in PL peaks of 132 nm for a 4-QW InGaAs/InGaAsP/InP structure have been demonstrated in the intermixed regions of the QW wafers, on whose non-intermixed regions, a shift as small as 5 nm is observed. This feature makes this technology very attractive for selective intermixing in selected areas of an MQW structure. The dramatical reduction in band gap absorption for the InP based MQW structure has been investigated experimentally. It is found that the intensity attenuation for the blue shifted structure is decreased by 242.8 dB/cm for the TE mode and 119 dB/cm for the TM mode with respect to the control samples. Electro-absorption characteristics have also been clearly observed in the intermixed structure. Current-Voltage characteristics were employed to investigate the degradation of the p-n junction in the intermixed region. We have achieved a successful fabrication and operation of Y-junction optical switches (JOS) based on MQW semiconductor optical amplifiers using HE-IIEI technology to fabricate the low loss passive waveguide. (C) 1997 Published by Elsevier Science B.V.

Observation of photogalvanic current for interband absorption in InN films at room temperature

The linear and circular photogalvanic effects have been observed in undoped InN films for the interband transition by irradiation of 1060 nm laser at room temperature. The spin polarized photocurrent depends on the degree of polarization, and changes its sip when the radiation helicity changes from left-handed to right-handed. This result indicates the sizeable spin-orbit interaction in the InN epitaxial layer and provides an effective method to generate spin polarized photocurrent and to detect spin-splitting effect in semiconductors with promising applications on spintronics.