Microscopic theory of laser gain in semiconductor quantum wells

Received: 31 October 1997/Accepted: 31 October 1997     

Improvement of photoluminescence and electroluminescence characteristics of MBE-grown In0.53Ga0.47As/In0.53(Ga0.6Al0.4)0.47As quantum well laser structure with InGaAlAs digital alloys by thermal annealing

We investigated the effect of rapid thermal annealing (RTA) on the photoluminescence (PL) and electroluminescence of the In_0.53Ga_0.47As/In_0.53(Ga_0.6Al_0.4)_0.47As multiple quantum well (MQW) laser structure with InGaAlAs barrier layers provided by the digital-alloy technique. The SiO₂- (Si₃N₄-) capped samples followed by the RTA exhibited a significant improvement of PL intensity without any appreciable shifts in PL peak energy for settings of up to 750 °C (800 °C) for 45 s. This improvement is attributed to the annealing of nonradiative defects in InAlAs layers of digital-alloy InGaAlAs and partially those near the heterointerfaces of the digital-alloy layers. The InGaAs/InGaAlAs MQW laser diodes fabricated on the samples annealed at 850 °C show a hugely improved lasing performance. Received: 2 September 2002 / Accepted: 3 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +82-62/970-2204, E-mail: ytlee@kjist.ac.kr     

Optical anisotropy in nonspherical quantum dots

Received: 10 April 1997/Accepted: 12 June 1997     

External radiative quantum efficiency of 96% from a GaAs / GaInP heterostructure

 GaAs/GaInP double heterostructures are index matched with ZnSe hemispheres to increase the coupling of photoluminescence out of the device. We measure external quantum efficiencies as large as 96% at room temperature using a bolometric calibration technique. When the carriers are optically injected near the bandgap energy, the luminescence is blueshifted by up to 1.4 kT. In this case, external efficiencies exceeding 97.5% would yield optical refrigeration in the solid state. Received: 22 July 1996/Accepted: 23 September 1996     

Optical gain and threshold properties of strained InGaAlAs/AlGaAs quantum wells for 850-nm vertical-cavity surface-emitting lasers

The valence subband structures, optical gain spectra, transparency carrier densities, and transparency radiative current densities of different compressively strained InGaAlAs quantum wells with Al_0.3Ga_0.7As barriers are systematically investigated using a 6 × 6 k · p Hamiltonian including the heavy hole, light hole, and spin-orbit splitting bands. The results of numerical calculations show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAlAs quantum wells can be enhanced by introducing more compressive strain in quantum wells. However, further improvement of the optical properties of InGaAlAs quantum wells becomes minimal when the compressive strain is higher than approximately 1.5%. The simulation results suggest that the compressively strained InGaAlAs quantum wells are of advantages for application in high-speed 850-nm vertical-cavity surface-emitting lasers.     

High quality GaAs quantum wires grown by flow rate modulation epitaxy

Accepted: 14 October 1997     

Second-harmonic spectroscopy of electronic structure of Si/SiO2 multiple quantum wells

Size effects in the resonant nonlinear optical response of amorphous Si/SiO₂ multiple quantum wells (MQW) are studied by second-harmonic generation (SHG) spectroscopy in a spectral interval of second-harmonic photon energies from 2.5 to 3.4 eV. The sensitivity of SHG spectroscopy to thickness-dependent electronic structure (sub-band energy position and density of states line shape) of MQW is demonstrated. A monotonic red shift of central energies of SHG resonances by 120 m eV upon increase of the well thickness from 2.5 to 10 ? is observed. This is interpreted as a size dependence of the position of singularities in the combined density of states for a 2D gas of electrons moving in an effective potential well. It is shown that, for agreement with experiment, the simplest (rectangular) shape of the well should be modified in order to take into account the lattice-potential distortion at the interfaces. Received: 16 October 2001 / Revised version: 16 April 2002 / Published online: 6 June 2002     

Hydrostatic pressure effects on impurity states in InAs/GaAs quantum dot

Within the framework of effective-mass approximation, the hydrostatic pressure effects on the donor binding energy of a hydrogenic impurity in InAs/GaAs self-assembled quantum dot(QD) are investigated by means of a variational method. Numerical results show that the donor binding energy increases when the hydrostatic pressure increases for any impurity position and QD size. Moreover, the hydrostatic pressure has a remarkable influence on the donor binding energy for small QD. Realistic cases, including the impurity in the QD and the surrounding barrier, are considered.     

Amplification and wave mixing of induced and spontaneous emission in semiconductor laser amplifiers

We theoretically describe and experimentally investigate the spatio-spectral wave mixing of induced and spontaneous emission in large-area InGaAs-semiconductor laser amplifiers. The dynamic light-matter-coupling is described by a spatially resolved theory based on Maxwell–Bloch–Langevin equations, taking into account many-body-carrier interactions, energy transfer between the carrier and phonon systems and, in particular, the spatio-temporal interplay of stimulated and amplified spontaneous emission and the noise caused by spontaneous emission. Our numerical model reveals the fundamental physical processes which are responsible for the spectral power distribution of the amplified laser light and predicts the emission properties of high-power semiconductor laser amplifiers, such as emission spectra and input power–output power characteristics. Received:30October2002/Revisedversion:21November2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-711/6862-349, E-mail: Edeltraud.Gehrig@dlr.de RID="**" ID="**"Also at: Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101, Tampere, Finland RID="***" ID="***"Present address: Heidelberger Druckmaschinen AG, Speyerer Strasse 6, 69115 Heidelberg, Germany RID="****" ID="****"Present address: Lightbit Corporation, 411 Clyde Avenue, Mountain View, CA 94043, USA     

Optical properties of GaN/AlN multiple quantum wells

Optical properties of GaN/AlN multiple quantum wells (MQW) have been investigated by Raman scattering, photoluminescence and photoluminescence excitation measurements. A careful examination of the Raman spectrum reveals the fact that the constituent layers of GaN/AlN MQWs are well strained. The experimental results of emission and absorption in MQWs were compared with the calculated solutions of the finite quantum well and the bound states involved in the optical transitions were identified. It is found that the interband transitions up to n=3 bound state can be observed in the strained GaN/AlN MQWs sample. The temperature dependence of the heavy-hole transitions shows an interesting phenomenon, in which the peak energy first increases with increasing temperature and then decreases with the temperature rapidly. The observation can be explained in a consistent way by the strain effects of lattice mismatch due to the interplay between the thermal expansion of GaN and AlN layers. Our results indicate that pseudomorphic GaN/AlN MQWs with good quality can be readily grown, and their applications in optoelectronics can be expected in the near future.     

Mixing kinetics and write-once optical recording characteristics of Sb/Se bilayer films

Bilayer Sb/Se films are irradiated with 12 ns pulses from an ArF laser (extended areas) and from a focused Ar⁺ laser (micron-sized areas). Real-time reflectivity measurements are used to determine if the process occurs within the solid or liquid phase and the transformation time, in addition to measure the optical contrast and the medium sensitivity. Transmission electron microscopy is used to analyze the structure of the transformed areas and the medium resolution. The results show that mixing is initiated by preferential melting at the grain boundaries and an amorphous phase is produced upon irradiation at high energy densities. Finally, the characteristics of the mixing process in Sb/Se films as a write-once optical recording mechanism are discussed in terms of the sensitivity and resolution of the recorded spots and the time required for recording.     

Diode laser soldering using a lead-free filler material for electronic packaging structures

As of today, several lead-free soldering pastes have been qualified for currently used soldering process. Regarding the new potential of laser-assisted soldering processes, the behaviour of the SnAgCu soldering paste requires, however, new investigations. In the first part of this study, the specific temperature profile of a laser soldering process is investigated using a high power diode laser (HPDL). These experimental results are compared to a thermal simulation developed for this specific application. The second part of this work deals with the diffusion of the tin-based filler material through the nickel barrier using the information extracted from the temperature simulations.     

Optical diffuse reflectance spectra of GaSb nanocrystals embedded in SiO2 matrix by radio-frequency magnetron co-sputtering

Nanocrystalline GaSb embedded in SiO₂ films was grown by radio-frequency magnetron co-sputtering. X-ray diffraction pattern and transmission electron microscopy (TEM) confirm the existence of GaSb nanocrystals in the SiO₂ matrix. The average size of GaSb nanoparticles is in the range of 3 to 11 nm. Diffuse reflectance spectra were used to characterize the small change of the band gap of the semiconductor. The diffuse reflectance spectra shows that the absorption peaks have a large blueshift of about 4.0 eV of the absorption relative to that of bulk GaSb. It has been explained by quantum confinement effects. Room temperature optical transmission spectra show that the absorption edge exhibits a very large blueshift of about 2.1 eV with respect to that of bulk GaSb in agreement with quantum confinement. Received: 28 July 1999 / Accepted: 27 October 1999 / Published online: 1 March 2000     

Self-organized formation of shell-like InAs/GaAs quantum dot ensembles

Formation of a multimodal quantum dot (QD) ensemble by strained layer epitaxy of InAs on GaAs near the critical value for the onset of the 2D-3D transition is studied. Reflection anisotropy spectroscopy is employed to confirm that a smooth surface is maintained during strained layer growth prior to QD formation. Instantaneous capping after deposition leads to InAs quantum wells with some thickness flucuations. Multimodal QD InAs ensembles form after an at least short growth interruption prior to cap layer deposition. The QDs consist of pure InAs with heights varying in steps of complete InAs monolayers. Related exciton energies indicate a simultaneous increase of both height and lateral extension, i.e. a shell-like increase of sizes. The formation of the multimodal QD ensemble is described by a kinetic approach. A growth scenario is presented where QDs having initially shorter base length stop vertical growth at a smaller height, accounting for the experimentally observed shell-like sub-ensemble structure.     

Intrinsic birefringence in (11N)-oriented quantum wells of cubic crystals

The effect of natural birefringence in (11N) oriented Quantum Wells (QWs) due to the in-plane optical anisotropy is investigated. The birefringence is more pronounced, the stronger the anisotropy of hole effective masses and larger the splitting of the heavy- and light-hole sub-bands. The magnitude of the effect is maximum in (110)-oriented QWs. An order-of-magnitude estimation of the effect in QWs of GaAs and of ZnSe is given.     

Auger electron spectroscopy of the kinetics of evaporation of palladium beaded films from sapphire substrate

) surface of sapphire were investigated by Auger electron spectroscopy in the temperature range of 1053–1083 K. It was found that the decrease of the effective thickness during heat treatments under 10^-8 mbar was caused by evaporation from the surface of the substrate. Using the model developed by Kaganovskii and Beke we have established that the process is controlled by surface diffusion. It was shown that the evaporation rate of palladium adatoms from an alumina surface is given by I_o=1.45×10³⁶exp{-(393±46 kJ/mol)/RT}ms, and, on the basis of an estimation of the surface diffusion length, the effective surface diffusion coefficient D_s was evaluated: Received: 30 March 1998/Accepted: 28 May 1998     

Photoluminescence of Er3+ ions in amorphous silicon layers under intensive laser excitation

Transients of the photoluminescence (1.54 μm) of Er³⁺ ions embedded in an amorphous silicon matrix excited with intensive laser pulses are simulated using a phenomenological model which takes into account both the defect-related excitation mechanism and stimulated optical transitions in the ions. The simulated transients are compared with the experimental ones observed in Er-doped amorphous silicon layers under pulsed laser excitation. The modeling and the experimental results demonstrate a possibility to realize a regime of superradiance in the system of Er³⁺ ions pumped via an electronic excitation of the amorphous matrix. Received: 7 August 2001 / Revised version: 1 November 2001 / Published online: 17 January 2002     

Determination of parameters of Cu surface mass transport on sapphire from morphological changes of beaded films caused by evaporation

s , the surface diffusion coefficient, D_s ^′, and the surface reaction rate coefficient, β_s ^′, of Cu on alumina are determined in the temperature range 1048–1198 K. Measuring simultaneously the time dependence of the effective thickness, H_eff(t), the lateral shift of the boundary, y(t) of beaded films (BF) and using vapour pressure data we concluded that the process is controlled by surface reaction at the perimeters of beads. Supposing Arrhenius-type temperature dependence for D_s ^′, β_s ^′ and λ_s the activation energies and preexponential factors have been calculated. Received: 2 October 1996/Accepted: 27 November 1996     

Observations of band structure and reduced group velocity in epitaxial GaN–sapphire 2D photonic crystals

We report experimental results for the band structure of 2-dimensional triangular photonic crystals of air holes in an epitaxial group III–nitride waveguide film. Surface coupling techniques enable the observation of sharp resonance dips in the transmission spectra due to a resonance phenomenon between the incident light and Bloch modes of the photonic crystal. The position of the dips has been measured as a function of angle of incidence and the photonic band structure has been successfully constructed by the measurement. Corresponding Bloch-mode group velocities have also been obtained. Received: 28 May 2001 / Published online: 23 October 2001