Detection of indium segregation effects in InGaAs/GaAs quantum wells using reflectance-difference spectrometry

The influence of the Indium segregation on the interface asymmetry in InGaAs/GaAs quantum wells have been studied by reflectance-difference spectroscopy (RDS). It is found that the anisotropy of the 2H1E (2HH --> 1E) transition is very sensitive to the degree of the interface asymmetry. Calculations taking into account indium segregation yield good agreement with the observed anisotropy structures. It demonstrates that the anisotropy intensity ratio of the 1L1E (1LH --> 1E) and 2H1E transitions measured by RDS can be used to characterize the interface asymmetry. (C) 2002 Elsevier Science B.V. All rights reserved.

Optical transitions and type-II band lineup of MBE-grown GaNAs/GaAs single-quantum-well structures

We have investigated transitions above and below band edge of GaNAs/GaAs and InGaNAs/GaAs single quantum wells (QWs) by photoluminescence (PL) as well as by absorption spectra via photovoltaic effects. The interband PL peak is observed to be dominant under high excitation intensity and at low temperature. The broad luminescence band below band edge due to the nitrogen-related potential fluctuations can be effectively suppressed by increasing indium incorporation into InGaNAs. In contrast to InGaNAs/GaAs QWs, the measured interband transition energy of GaNAs/GaAs QWs can be well fitted to the theoretical calculations if a type-II band lineup is assumed. (C) 2001 Elsevier Science B.V. All rights reserved.

Modification of emission wavelength of self-assembled In(Ga)As/GaAs quantum dots covered by InxGa1-xAs(0 <= x <= 0.3) layer

Red shifts of emission wavelength of self-organized In(Cla)As/GaAs quantum dots (QDs) covered by 3 nm thick InxGa1-xAs layer with three different In mole fractions (x = 0.1, 0.2 and 0.3, respectively) have been observed. Transmission electron microscopy images demonstrate that the stress along growth direction in the InAs dots was reduced due to introducing the InxGa1-xAs (x = 0.1, 0.2 and 0.3) covering layer instead of GaAs layer. Atomic force microscopy pictures show a smoother surface of InAs islands covered by an In0.2Ga0.8As layer. It is explained by the calculations that the redshifts of the photoluminescence (PL) spectra from the QDs covered by the InxGa1-xAs (x greater than or equal to 0.1) layers were mainly due to the reducing of the strain other than the InAs/GaAs intermixing in the InAs QDs. The temperature dependent PL spectra further confirm that the InGaAs covering layer can effectively suppress the temperature sensitivity of PL emissions. 1.3 mum emission wavelength with a very narrow linewidth of 19.2 mcV at room temperature has been obtained successfully from In,In0.5Ga0.5As/GaAs self-assembled QDs covered by a 3-nm In0.2Ga0.2As strain reducing layer. (C) 2001 Elsevier Science B.V. All rights reserved.

High power 980 nm Al-free strained quantum-well lasers for erbium-doped fiber amplifiers

In this paper, we reported on the fabrication of 980 nm InGaAs/InGaAsP strained quantum-well (QW) lasers with broad waveguide. The laser structure was grown by low-pressure metalorganic chemical vapor deposition on a n(+)- GaAs substrate. For 3 mu m stripe ridge waveguide lasers, the threshold current is 30 mA and the maximum output power and the output power operating in fundamental mode are 350 mW and 200 mW, respectively. The output power from the single mode fiber is up to 100 mW, the coupling efficiency is 50%. We also fabricated 100 mu m broad stripe coated lasers with cavity length of 800 mu m, a threshold current density of 170 A/cm(2), a high slope efficiency of 1.03 W/A and a far-field pattern of 40 x 6 degrees are obtained. The maximum output power of 3.5 W is also obtained for 100 mu m wide coated lasers. (C) 2000 Elsevier Science B.V. All rights reserved.

AlGaInP visible quantum well lasers for information technology

650 nm-range AlGaInP multi-quantum well (MQW) laser diodes grown by low pressure metal organic chemical vapor deposition (LP-MOCVD) have been studied and the results are presented in this paper. Threshold current density of broad area contact laser diodes can be as low as 350 A/cm(2). Laser diodes with buried-ridge strip waveguide structures were made, threshold currents and differential efficiencies are (22-40) mA and (0.2-0.7) mW/mA, respectively. Typical output power for the laser diodes is 5 mW, maximum output power of 15 mW has been obtained. Their operation temperature can be up to 90 degrees C under power of 5 mW. After operating under 90 degrees C and 5 mW for 72 hrs, the average increments for the threshold currents of the lasers at 25 degrees C and the operation currents at 5 mW (at 25 degrees C) are (2-3) mA and (3-5) mA, respectively. Reliability tests showed that no obvious degradation was observed after 1400 hours of CW operation under 50 degrees C and 2.5 mW.

High-field cyclotron resonance and electron-phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B > similar to 23 T), the three samples behave very differently. For the most lightly doped sample (3 x 10(11) cm(-2)) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6 x 10(11) cm(-2)) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2 x 10(12) cm(-2)) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR. (C) 1998 Elsevier Science B.V. All rights reserved.

High Characteristic Temperature InGaAsP/InP Tunnel Injection Multiple-Quantum-Well Lasers

We fabricate 1.5 mu m InGaAsP/InP tunnel injection multiple-quantum-well (TI-MQW) Fabry-Perot (F-P) ridge lasers. The laser heterostructures, including an inner cladding layer and an InP tunnel barrier layer, are grown by metal-organic chemical-vapor deposition (MOCVD). Characteristic temperature.. 0 of 160K at 20 degrees C is obtained for 500-mu m-long lasers. T-0 is measured as high as 88K in the temperature range of 15-75 degrees C. Cavity length dependence of T-0 is investigated.

Structure and properties of InAs/AlAs quantum dots for broadband emission

The InAs quantum dots (QDs) on an AlAs layer are grown on GaAs substrates by molecular beam epitaxy technique. The properties of materials and optics of such QD structures have been investigated by cross sectional transmission electron microscopy and photoluminescence (PL) techniques. It is discovered that the inhomogeneous strain filed mainly exists below InAs QDs layers in the case of no wetting layer. The full width at half maximums (FWHMs) and intensities of PL emission peaks of InAs QDs are found to be closely related to the thickness of the thin AlAs layers. The InAs QDs on an eight monolayer AlAs layer, with wide FWHMs and large integral intensity of PL emission peaks, are favorable for producing broadband QD superluminescent diodes, external-cavity QD laser with large tuning range.

Ab initio calculations of differential cross sections for single charge transfer in He-3(2+)+He-4 collisions

The single charge transfer process in He-3(2+)+He-4 collisions is investigated using the quantum-mechanical molecular-orbital close-coupling method, in which the adiabatic potentials and radial couplings are calculated by using the ab initio multireference single- and double-excitation configuration interaction methods. The differential cross sections for the single charge transfer are presented at the laboratorial energies E = 6 keV and 10 keV for the projectile He-3(2+). Comparison with the existing data shows that the present results are better in agreement with the experimental measurements than other calculations in the dominant small angle scattering, which is attributed to the accurate calculations of the adiabatic potentials and the radial couplings.

Effect of phase relaxation on quantum superpositions in complex collisions

We study the effect of phase relaxation on coherent superpositions of rotating clockwise and anticlockwise wave packets in the regime of strongly overlapping resonances of the intermediate complex. Such highly excited deformed complexes may be created in binary collisions of heavy ions, molecules, and atomic clusters. It is shown that phase relaxation leads to a reduction of the interference fringes, thus mimicking the effect of decoherence. This reduction is crucial for the determination of the phase-relaxation width from the data on the excitation function oscillations in heavy-ion collisions and bimolecular chemical reactions. The difference between the effects of phase relaxation and decoherence is discussed.

Rotamers of m-aminophenol cation studied by mass analyzed threshold ionization spectroscopy and theoretical calculations

The vibrationally resolved spectra of selected rotamers of m-aminophenol have been recorded by mass analyzed threshold ionization spectroscopy in connection with two-color resonant two-photon excitation scheme. The adiabatic ionization energies of the cis and trans rotamers are 61460 +/- 5 and 61734 +/- 5 cm(-1), respectively. The frequencies of modes 1 (breathing) and 18a (in-plane CH bending) are measured to be 744 and 1097 cm(-1) for the cis, and 736 and 1104 cm(-1) for the trans rotamer, respectively. This indicates that different orientation of the OH with respect to the NH2 substituent only slightly influences these two modes. (C) 2004 Elsevier B.V. All rights reserved.

The geometry of the NO2- anion: ab initio calculations and Franck-Condon analysis

Geometry optimization and harmonic vibrational frequency calculations have been performed on the (X) over bar (2)A(1) state of NO2 and (X) over bar (1)A(1) state of NO2-. Franck-Condon analyses and spectral simulations were carried out on the NO2((X) over bar (2)A(1))-NO2-((X) over bar (1)A(1)) photo detachment process. In addition, the equilibrium geometry parameters, r(NO)= 1.248 +/- 0.005 Angstrom and angle(ONO) 116.8 +/- 0.5degrees, of the (X) over bar (1)A(1) state of NO2-, are derived by employing an iterative Franck-Condon analysis procedure in the spectral simulation. Our conclusions regarding the anion geometry suggest a reinterpretation of the results of Woo et al. (C) 2004 Published by Elsevier B.V.