High-power and long-lifetime InAs/GaAs quantum-dot laser at 1080 nm

High power and long lifetime have been demonstrated for a semiconductor quantum-dot (QD) laser with five-stacked InAs/GaAs QDs separated by an InGaAs strain-reducing layer (SRL) and a GaAs spacer layer as an active medium. The QD lasers exhibit a peak power of 3.6 W at 1080 nm, a quantum slope efficiency of 84.6%, and an output-power degradation rate of 5.6%/1000 h with continuous-wave constant-current operation at room temperature. A comparative reliability investigation indicates that the lifetime of the InAs/GaAs QD laser with the InGaAs SRL is much longer than that of a QD laser without the InGaAs SRL. This improved lifetime of the QD laser could be explained by the reduction of strain in and around InAs QDs induced by the InGaAs SRL. (C) 2001 American Institute of Physics.

Experimental investigation of photoluminescence dynamics of cavity polaritons under nonresonant excitation

Low-temperature time-resolved photoluminescence (PL) experiments have been performed on a semiconductor planar microcavity, which contains two sets of three In0.13Ga0.87As/GaAs quantum wells embedded in a 3 lambda /2 GaAs cavity. The spontaneous emission dynamics of both lower- and upper-branch polaritons is investigated as a function of exciton-cavity detuning under nonresonant optical excitation. It is found that the PL decay times of both branches are independent of cavity detuning while the PL rising kinetics of the lower- and upper-branch polaritons exhibits a significant difference. The rise time of the upper polarition branch shows a strong dependence on cavity detuning, while the rise time of the lower polarition branch is less sensitive to cavity detuning. Our results can be well understood in the framework of the theoretical prediction of Tassone et al.

Analysis of semiconductor microlasers with an equilateral triangle resonator by rate equations

Semiconductor microlasers with an equilateral triangle resonator (ETR) are analyzed by rate equations with the mode lifetimes calculated by the finite-difference time-domain technique and the Pade approximation. A gain spectrum based on the relation of the gain spectrum and the spontaneous emission spectrum is proposed for considering the mode selection in a wide wavelength span. For an ETR microlaser with the side length of about 5 mum, we find that single fundamental mode operation at about 1.55 mum can be obtained as the side length increases from 4.75 to 5.05 mum. The corresponding wavelength tuning range is 93 nm, and the threshold current is about 0.1 to 0.4 mA.

Photoluminescence properties of a GaN0.015As0.985/GaAs single quantum well under short pulse excitation

Under short pulse laser excitation, we have observed an extra high-energy photoluminescence (PL) emission from GaNAs/GaAs single quantum wells (QWs). It dominates the PL spectra under high excitation and/or at high temperature. By measuring the PL dependence on both temperature and excitation power and by analyzing the time-resolved PL results, we have attributed the PL peak to the recombination of delocalized excitons in QWs. Furthermore, a competition process between localized and delocalized excitons is observed in the temperature-dependent PL spectra under the short pulse excitation. This competition is believed to be responsible for the temperature-induced S-shaped PL shift often observed in the disordered alloy semiconductor system under continuous-wave excitation. (C) 2001 American Institute of Physics.

Quantum dots in glass spherical microcavity

Glass spherical microcavities containing CdSexS1-x semiconductor quantum dots (QDs) are fabricated. The coupling between the optical emission of embedded CdSexS1-x QDs and spherical cavity modes is realized. When the luminescence of QDs is excited by a laser beam, the strong whispering gallery mode resonance with high Q factors is achieved in the photoluminescence spectra. (C) 2001 American Institute of Physics.

Optical spectra of CdSe nanocrystals under hydrostatic pressure

Optical spectra of CdSe nanocrystals are measured at room temperature under pressure ranging from 0 to 5.2 GPa. The exciton energies shift linearly with pressure below 5.2 GPa. The pressure coefficient is 27 meV GPa(-1) for small CdSe nanocrystals with the radius of 2.4 nm. With the approximation of a rigid-atomic pseudopotential, the pressure coefficients of the energy band are calculated. By using the hole effective-mass Hamiltonian for the semiconductors with wurtzite structure under various pressures, we study the exciton states and optical spectra for CdSe nanocrystals under hydrostatic pressure in detail. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbit coupling on the hole states are investigated. The Coulomb interaction of the exciton states is also taken into account. It is found that the theoretical results are in good agreement with the experimental values.

Electronic states of InAs/GaAs quantum ring

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings. (C) 2001 American Institute of Physics.

Analysis and numerical simulation of eigenmode characteristics for semiconductor lasers with an equilateral triangle micro-resonator

The eigenmodes confined in the equilateral triangle resonator (ETR) are analyzed by deriving the eigenvalues and the mode field distributions and by the finite difference time domain (FDTD) technique. The analytical results show that the one-period-length for the mode light rays inside the ETR is the perimeter of the ETR, and the number of transverse modes is limited by the condition of total internal reflection. In addition, the sum of the longitudinal mode index and the transverse mode index should be an even number, which limits the number of confined modes again. Based on the FDTD technique and the Pade approximation, we calculate the mode resonant frequencies and the quality factors from the local maximum and the width of the spectral distribution of the intensity The numerical results of mode frequencies agree very well with the analytical results, and the quality factor of the fundamental mode is usually higher than that of the higher order transverse modes. The results show that the ETR is suitable to realize single-made operation as semiconductor microcavity lasers.

Novel coupled multi-active region high power semiconductor lasers cascaded via tunnel junction

A novel semiconductor laser structure is put forward to resolve the major difficulties of high power laser diodes. In this structure, several active regions are cascaded by tunnel junctions to form a large optical cavity and to achieve super high efficiency. This structure can solve the problems of catastrophic optical damage of facet, thermal damage and poor light beam quality effectively. Low-pressure metalorganic chemical vapor deposition method is adopted to grow the novel semiconductor laser structures, which are composed of Si:GaAs/C:GaAs tunnel junctions, GaAs/InGaAs strain quantum well active regions. External differential quantum efficiency as high as 2.2 and light power output of 2.5 W per facet (under 2A drive current) are achieved from an uncoated novel laser device with three active regions.

New progress of studies on semiconductor materials

This paper is a review of research and development on semiconductor materials, which covers main scientific activities in this field. The present status acid future prospects of studies on semiconductor materials, such as silicon crystals, GaAs related III-V compound semiconductor materials and GaAs, InP and silicon based quantum well and superlattice materials, quantum wires and quantum dots materials, microcavity and photonic crystals, materials for quantum computation and wide band gap materials, are briefly discussed.

Fabrication and characterization of metal-semiconductor-metal (MSM) ultraviolet photodetectors on undoped GaN/sapphire grown by MBE

Interdigital metal-semiconductor-metal (MSM) ultraviolet photoconductive detectors have been fabricated on undoped GaN films grown by molecular beam epitaxy (MBE), Response dependence on wavelength, applied current, excitation powers and chopper frequency has been extensively investigated. It is shown that the photodetector's spectral response remained nearly constant for wavelengths above the band gap and dropped sharply by almost three orders of magnitude for wavelengths longer than the band gap. It increases linearly with the applied constant current, but very nonlinearly with illuminating power. The photodetectors showed high photoconductor gains resulting from trapping of minority carriers (holes) at acceptor impurities or defects. The results demonstrated the high quality of the GaN crystal used to fabricate these devices. (C) 2000 Elsevier Science B.V. All rights reserved.

Investigation on anisotropy of vertical-cavity surface-emitting lasers

We have studied the spontaneous emission of polarized excitons in the GaInP/AlGaInP vertical-cavity surface-emitting lasers from 50 K to room temperature. It is observed that the spontaneous emission peak enters and leaves the resonant regime. At the resonant regime, the emission intensities of the perpendicularly and horizontally polarized excitons are enhanced and their proportions are different from that in nonresonant regime. These experimental results are explained by the dressed exciton theory of the semiconductor microcavity device. Based on this theory, the intensity enhancement and the polarization dependence are understood as cooperative emission and the microcavity anisotropy. (C) 2000 American Institute of Physics. [S0021-8979(00)05315-9].

Correlated structural and optical investigation of terbium-doped zinc oxide nanocrystals

Tb3+-doped zinc oxide nanocrystals with a hexagonal wurzite structure were successfully prepared by reaction between Zn-O-Tb precursors and LiOH in ethanol. Good incorporation of Tb3+ in ZnO nanocrystals is proved by XRD, FTIR, PL and PLE measurements. The presence of acetate complexes to zinc atoms on particle surfaces is disclosed by FTIR results. Emission from both Tb3+ ions and surface states in ZnO matrix, as well as their correlation were observed. The luminescence mechanism is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

Modeling and optimization of semiconductor manufacturing process with neural networks

A neural network-based process model is proposed to optimize the semiconductor manufacturing process. Being different from some works in several research groups which developed neural network-based models to predict process quality with a set of process variables of only single manufacturing step, we applied this model to wafer fabrication parameters control and wafer lot yield optimization. The original data are collected from a wafer fabrication line, including technological parameters and wafer test results. The wafer lot yield is taken as the optimization target. Learning from historical technological records and wafer test results, the model can predict the wafer yield. To eliminate the "bad" or noisy samples from the sample set, an experimental method was used to determine the number of hidden units so that both good learning ability and prediction capability can be obtained.