Boron diffusion in Ge+ premorphized and BF2+ implanted Si(001)

The annealing behavior of Si implanted with Ge and then BF2 has been characterized by double crystal X-ray diffraction (DCXRD) and secondary ion mass spectroscopy (SIMS). The results show that annealing at 600 degrees C for 60 minutes can only remove a little damage induced by implantation and nearly no redistribution of Ge and B atoms has occurred during the annealing. The initial crystallinity of Si is fully recovered after annealing at 950 degrees C for 60 minutes and accompanied by Ge diffusion. Very shallow boron junction depth has been formed. When annealing temperature rises to 1050 degrees C, B diffusion enhances, which leads to a deep diffusion and good distribution of B atoms into the Si substrate. The X-ray diffraction (004) rocking curves from the samples annealed at 1050 degrees C for 60 minutes display two SiGe peaks, which may be related to the B concentration profiles.

Pressure behaviour of photoluminescence from InAs submonolayer in GaAs matrix

We have investigated the dependence on hydrostatic pressure of the photoluminescence of an InAs submonolayer embedded in a GaAs matrix at 15 K and for pressure up to 8 GPa. Strong InAs-related emissions are observed in all three samples at ambient pressure. The temperature dependence of the emission intensity for these Peaks can be well characterized by the thermal activation of excitons from the InAs layer to the GaAs matrix. With increasing pressure, the InAs-related peaks shift to. higher energies. The pressure coefficients of these peaks are very close to that of the free exciton in bulk GaAs. Some weak peaks observed at pressures above 4.2 GPa are attributed to indirect transitions involving X states in the InAs layer. These results are similar to the pressure behaviour observed in the InAs/GaAs monolayer structures. A group of new lines has been observed in the spectra when pressure is increased beyond 2.5 GPa, which is attributed to the N isoelectronic traps in the GaAs matrix.

Structure and photoluminescence of InGaAs self-assembled quantum dots grown on InP(001)

Molecular beam epitaxy has been used for growing InGaAs self-assembled quantum dots (QDs) in InAlAs on an InP(001) substrate. Nominal deposition of 9.6 monolayers of In0.9Ga0.1As results in QDs of similar to 6.5 nm high with an areal density of 3.3 X 10(11) cm(-2). Conspicuous bimodal size distribution is identified, and is responsible for the observed QDs photoluminescence (PL) emission with two peaks at 0.627 and 0.657 eV. Good agreement is achieved between the observed PL peak energies and calculated results. (C) 1999 American Institute of Physics. [S00218979(99)00101-2].

Photovoltage and photoreflectance spectroscopy of InAs/GaAs self-organized quantum dots

We present a detailed study of the interband excitonic transitions of InAs/GaAs self-organized quantum dots (QDs) based on photovoltage (PV) photoreflectance (PR) and photoluminescence (PL) spectroscopy. At room temperature, the interband absorption transitions of QDs have been observed by using PV spectrum, which clearly exhibits four well-resolved excitonic absorption peaks. The absorption line shape is Gaussian-like. Furthermore, the corresponding excitonic transitions are also observed in PR experiment at 77 K. The first derivative of a Gaussian profile can fit the experimental data well. (C) 1998 American Institute of Physics. [S0003-6951(98)00743-8]

Structure, fluorescence and stability of CdS nanoparticles prepared in air

CdS nanoparticies were prepared in air and their stability by air annealing was studied. A small change in crystal structure and particle size was observed by air annealing, but a rapid reduction in fluorescence was found. Through investigation, it is revealed that it is the surface change or reconstruction rather than the variation of the size or structure that decreases the fluorescence. The emission of the particles consists with two peaks which are dependent on the excitation energy. The two peaks are considered to be arisen from "two" different sizes of nanoparticles and may be explained in terms of selectively excited photoluminescence. Finally we discuss why the discrete state of nanoparticles are able to be resolved in the photoluminescence excitation spectrum, but could not be differentiated in the absorption spectrum.

Hole trap levels in Mg-doped GaN grown by metalorganic vapor phase epitaxy

Hole trap levels in a Mg-doped GaN grown by metalorganic vapor phase epitaxy (MOVPE) are studied with deep level transient spectroscopy (DLTS). The Mg concentration of the sample was 4.8 x 10(19) cm(-3), but the hole concentration was as low as 1.3 x 10(17) cm(-3) at room temperature. The DLTS spectrum has a dominant peak D-1 with activation energy of 0.41+/-0.05 eV, accompanied by two additional peaks with activation energies of 0.49+/-0.09 eV (D-2) and 0.59+/-0.05 eV (D-3). It was found that the dominant peak D-1 consists of five peaks, each of which has different activation energy and capture cross section. A relevant model for these levels is presented in relation to the Mg-N-H complexes. (C) 1998 American Institute of Physics. [S0003-6951(98)04340-X].

Theory of terahertz-photocurrent resonances in miniband superlattices

The currents of de and ac components and their phase-angle cosines for a superlattice under a direct bias and alternating field are calculated with the balance equations. It is found that the de currents as functions of the direct field show resonance peaks at the fields corresponding to the Bloch frequency equal to n omega. With increasing alternating field intensity the resonance peaks of higher harmonic increase, and simultaneously the first peak caused by the de field decreases. The results are in good agreement with the experimental results, indicating that this resonance can be understood in terms of electron acceleration within the miniband, i.e., it is a bulk superlattice effect, rather than caused by the electric-field localization mechanism (Wannier Stark ladder). The phase-angle cosine for the first harmonic cos phi(1) becomes negative when the Bloch frequency increases to be larger than the frequency of the ac field omega, and it also shows resonance peaks at the resonance frequencies n omega. The negative cos phi(1) may cause the energy transferred to the alternating field, i.e., oscillation of the system.

Photoluminescence investigation of GaInP/GaAs multiple quantum wells grown on (001) and (311) B GaAs surfaces by gas source molecular beam epitaxy

Photoluminescence (PL) investigation was carried out on GaInP/GaAs multiple quantum wells structures grown on (001) and (311) B surfaces of GaAs by gas source molecular beam epitaxy. Superlattice structures of GaAs/GaInP grown on (001) GaAs substrate were also studied in comparison. Deep-level luminescence was seen to dominate the PL spectra from the quantum wells and superlattice structures that were grown on (001) GaAs substrate. In contrast, superior optical properties were exhibited in the same structures grown on (311) B GaAs surfaces. The results suggested that GaAs/GaInP quantum well structures on (311) B oriented substrates could efficiently suppress the deep-level emissions, result in narrower PL peaks indicating smooth interfaces. (C) 1998 American Institute of Physics.

Quantum-confined Stark effects of exciton states in V-shaped GaAs/AlxGa1-xAs quantum wires

Quantum-confined Stark effects are investigated theoretically in GaAs/AlxGa1-xAs quantum wires formed in V-grooved structures. The electronic structures of the V-shaped quantum wires are calculated within the effective mass envelope function theory in the presence of electric field. The binding energies of excitons are also studied by two-dimensional Fourier transformation and variational method. The blue Stark shifts are found when the electric field is applied in the growth direction. A possible mechanism in which the blueshifts of photoluminescence peaks are attributed to two factors, one factor comes from the asymmetric structure of quantum wire along the electric field and another factor arises from the electric-field-induced change of the Coulomb interaction. The numerical results are compared with the recent experiment measurement.

Electronic and vibrational Raman scattering in resonance with yellow luminescence transitions in GaN on sapphire substrate

We analyze low-temperature Raman and photoluminescence spectra of MBE-grown GaN layers on sapphire. Strong and sharp Raman peaks are observed in the low frequency region. These peaks, which are enhanced by excitation in resonance with yellow luminescence transitions, are attributed to electronic transitions related to shallow donor levels in hexagonal GaN. It is proposed that a low frequency Raman peak at 11.7 meV is caused by a pseudo-local vibration mode related to defects involved in yellow luminescence transitions. The dependence of the photoluminescence spectra on temperature gives additional information about the residual impurities in these GaN layers.

Raman scattering from In1-x-yGaxAlyAs quaternary alloys

Raman scattering studies were reported of In1-x-yGaxAlyAs/InP lattice matched quaternary alloys. The quaternary alloys a.ere grown on (100) oriented InP substrates by MBE method. The composition and intensity dependence of optical phonon mode frequencies show that the quaternary alloys exhibit three-mode behavior, i.e. InAs-like, GaAs-like and AlAs-like modes. Polarization analysis of the Raman spectra shows that the LO phonon modes are Raman active in the depolarized configuration and Raman inactive in the polarized configuration. TO phonon modes were also observed due to disorder effects, resulting in the asymmetrical shapes of the Raman peaks of the optical phonons.

Photoluminescence from surface-oxidized Ge nanoclusters

Photoluminescence from gas-evaporated Ge nanoclusters consisting of a crystalline core encased in an oxide shell are presented. An as-grown sample shows room temperature luminescence with separate peaks around 357 and 580 nm. Prolonged air exposure of the clusters reduces the Ge core dimensions, and the emission initially at 580 nm shifts to 420 nm; however, the violet luminescence at 357 nm displays no difference. These results indicate that there are two mechanisms involved with light emission from Ge nanoclusters, visible light emission associated with the quantum confinement effect, and violet light emission correlated to luminescent centers. (C) 1998 Elsevier Science B.V.

Observation of the third subband population in modulation-doped InGaAs/InAlAs heterostructure

The population of the third (n = 3) two-dimensional electron subband of InGaAs/InAlAs modulation-doped structures has been observed by means of Fourier transform photoluminescence (PL). Three well resolved PL peaks centred at 0.737, 0.908, and 0.980eV are observed, which are attributed to the transitions from the lowest three electron subbands to the n = 1 heavy-hole subband. The subband separations clearly exhibiting the features of the stepped quantum well with triangle and square potentials are consistent with numerical calculation. Thanks to the presence of Fermi cutoff, the population ratio of these three subbands can be estimated. Temperature- and excitation-dependent luminescences are also analyzed.

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.

The third subband population in modulation-doped InGaAs/InAlAs heterostructures

We have observed the population of the third (n=3) two-dimensional electron subband of InGaAs/ InAlAs modulation-doped structures with very dense sheet carrier density by means of Fourier transform photoluminescence (PL). Three well-resolved PL peaks centered at 0.737, 0.908, and 0.980 eV are observed, which are attributed to the recombination transitions from the lowest three electron subbands to the n=1 heavy-hole subband. The subband separations clearly exhibit the features of the stepped quantum well with triangle and square potential, consistent with numerical calculation. Thanks to the presence of the Fermi cutoff, the population ratio of these three subbands can be estimated. Temperature and excitation intensity dependence of the quantum well luminescence intensity is also analyzed. (C) 1997 American Institute of Physics.