H. Schweizer

Optical transitions and carrier relaxation in self assembled InAs/GaAs quantum dots

We present experimental results concerning optical transitions and carrier dynamics (capture and relaxation) in self assembled InAs/GaAs quantum dot structures grown by metalorganic vapor phase epitaxy.Photoluminescence(PL) measurements at high excitation level reveal optical transitions above the ground state emission. These transitions are found to originate from occupied hole states by solving the quantum doteigenvalue problem. Time‐resolved studies after non‐resonant pulse excitation exhibit a relaxation ladder of the excited carriers from the GaAs barrier down to the ground state of the quantum dots. From both the continuous‐wave measurements and the PL‐decay curves we conclude that the carrier relaxation at non‐resonant excitation is mediated by Coulomb interaction (Auger effect). PL‐decay curves after resonant pulse excitation reveal a longer rise time compared to non‐resonant excitation which is a clear indication of a relaxation bottleneck inside the quantum dots. We interpret the rise time (≊ 400 ps) in this case to originate from relaxation via scattering by acoustic phonons. The PL‐decay time of the ground state emission ≊700 ps is interpreted as the excitonic lifetime of the quantum dot.

Deep‐level defects and n‐type‐carrier concentration in nitrogen implanted GaN

We analyzed the intrinsic defects and the n‐type‐carrier concentration generated by nitrogen ion implantation in n‐type GaN by deep‐level‐transient spectroscopy and by capacitance–voltage measurements, respectively. The samples were grown on sapphire by metalorganic vapor‐phase epitaxy. Nitrogen implantation with different ion doses and postimplantation rapid‐thermal annealing (RTA) were investigated. We observed a growing n‐type‐carrier concentration and increasing defect concentration with increasing nitrogen ion implantation doses. After RTA the concentration of free carriers and deep levels as found in the as‐grown state are restored. We also address contrarily seeming results from measurements of sheet resistance after N implantation published recently.

Long-range ordered lines of self-assembled Ge islands on a flat Si (001) surface

Self-assembled growth in combination with prepatterning yields ordered lines of Ge islands on a planar Si (001) surface. The self-assembled Ge nanostructures are grown on top of a 15-period Si/SiGe superlattice, which is deposited on a prepatterned Si substrate. The pattern consists of 10 nm deep trenches with a period of 250 nm. The superlattice translates the surface modulation of the substrate into a strain-field modulation at the growth front of the superlattice. This strain field modulation provides the template for the ordered nucleation of self-assembled Ge islands. Our method gives rise to the long-range ordering of perfectly passivated nanostructures and can in principle be applied to any other strained material system.

Optical resonances of bowtie slot antennas and their geometry and material dependence

In order to provide a guide for the design and optimization of bowtie slot antennas in the visible and near infrared spectral regime, their optical properties have been investigated with emphasis on geometry and materials. Although primarily theoretical, experimental investigations for reduced thickness cases are also included. As characterized by their field patterns, two types of resonances are discussed: plasmonic and Fabry-Pérot-like resonances. These resonance types show a linear dependence on aperture perimeter and film thickness, respectively, while showing a complementary behavior with near independence of the other respective parameter. Metal properties, as in the Drude model, are also considered. Various metals with respectively different skin depths are studied, showing a nearly linear dependence of the resonance wavelength on skin depth.

Excitonic transitions and exciton damping processes in InGaAs/InP

Detailed absorption measurements and the analysis of the absorption spectra of In1−xGaxAs lattice matched to InP are reported. The lattice matching parameter Δa/a covered a range from +4×10−3 to −1×10−3. From the absorption data of material with small matching parameter we obtain the value of the interband matrix element ( P2=20.7 eV), the excitonic Rydberg (Ex =2.5 meV), and damping constant (Γ0=5.1 meV) in the temperature range from 1.5 to 340 K. From the temperature dependent band‐gap shrinkage and exciton damping constant Γ, information on the carrier‐phonon interaction is obtained. The effect of the biaxial stress in the epitaxial layers caused by the mismatch with the substrate is demonstrated by absorption spectra which directly reveal the valence band splitting due to stress. Absorption measurements on samples with and without substrate indicate that the strained expitaxial layers do not relax completely if the substrate is etched away. The remaining strain field is probably caused by misfit dislo...

Auger carrier capture kinetics in self-assembled quantum dot structures

We establish rate equations to describe Auger carrier capture kinetics in quantum dot structures, calculate Auger capture coefficients for self-assembled quantum dots, and analyze Auger capture kinetics using these equations. We show that Auger capture times can be of the order of 1–100 ps depending on barrier carrier and dot densities. Auger capture rates depend strongly on dot diameters and are greatest at dot diameters of about 10–20 nm.

Resonance hybridization in double split-ring resonator metamaterials

We introduce a plasmon hybridization picture to understand the optical properties of double split-ring resonator metamaterials. The analysis is based on the calculated reflectance spectra from a finite-integration time-domain algorithm. Field distributions of the double split-ring resonators at the resonant frequencies confirm the results from the plasmon hybridization analysis. We demonstrate that the plasmon hybridization is a simple and powerful tool for understanding and designing metamaterials in the near infrared and visible regime.

Systematics of laser operation in GaAs/AlGaAs multiquantum well heterostructures

Important laser parameters of GaAs/AlGaAs MQWHs were measured by means of optical gain spectroscopy. Unsaturated optical net gain spectra are carefully analyzed using a model for band-to-band transitions including momentum conservation and an energy-and density-dependent lifetime broadening. The calculated dependence of the peak gain on the carrier density agrees well with experimental data. The description of energetic positions of the peak gain has to include a bandgap shrinkage with carrier density, present at laser threshold. Temperature-dependent measurements of the onset of optical net gain reveal a pure thermodynamic behavior of the absolute threshold values as a function of L z and of the characteristic temperature T 0 .

Self-assembled InAs/GaAs quantum dots under resonant excitation

The energy structure and the carrier relaxation in self-assembled InAs/GaAs quantum dots (SADs) is investigated by photoluminescence excitation spectroscopy (PLE) and photoluminescence (PL) at resonant excitation (below the GaAs and the wetting layer bandgap). In PLE measurements we find a clear resonance from the first excited hole state as well as resonances from a relaxation via different phonons. From a comparison of the PL-rise times in time resolved spectroscopy, we conclude on a fast electron relaxation (⩽50 ps) and a slow hole relaxation with a time constant of about 400 ps. Different relaxation paths are observed in the InAs/GaAs quantum dot system and allow us to identify the hole relaxation in the SADs as multiphonon assisted tunneling. The PL-decay time in the SADs after resonant excitation (about 600 ps) is attributed to the lifetime of the quantum dot exciton. In agreement with theoretical predictions, we find a constant lifetime of about 600 ps for temperatures below 50 K and a linear incre...

Influence of domain size on optical properties of ordered GaInP2

Using dark‐field transmission electron microscopy images of ordered GaInP samples, we show how the ordering domain size depends on the growth temperature. Samples with different average domain sizes are compared with regard to their photoluminescence (PL) and excitation spectra. We find a close correlation between the size of the ordered domains and the relative intensity of the PL peak from band–band recombination compared with the rapidly shifting, below‐band‐gap luminescence emission.