A. Hoffmann

Gain studies of (Cd, Zn)Se quantum islands in a ZnSe matrix

By inserting stacked sheets of nominally 0.7 monolayer CdSe into a ZnSe matrix we create a region with strong resonant excitonic absorption. This leads to an enhancement of the refractive index on the low-energy side of the absorption peak. Efficient waveguiding can thus be achieved without increasing the average refractive index of the active layer with respect to the cladding. Processed high-resolution transmission electron microscopy images show that the CdSe insertions form Cd-rich two-dimensional (Cd, Zn)Se islands with lateral sizes of about 5 nm. The islands act as quantum dots with a three-dimensional confinement for excitons. Zero-phonon gain is observed in the spectral range of excitonic and biexcitonic waveguiding. At high excitation densities excitonic gain is suppressed due to the population of the quantum dots with biexcitons.

Vertical strain and doping gradients in thick GaN layers

We report on spatially-resolved low-temperature luminescence and Raman experiments on a 220-μm-thick GaN layer grown on sapphire by hydride vapor phase epitaxy. Our measurements reveal that the peak position of the near-band-gap luminescence strongly depends on the distance to the substrate interface. The luminescence shifts continuously to lower energies with decreasing distance but a strong blue shift occurs directly at the interface. We correlate these effects with the inhomogeneous free-carrier distribution and the strain gradient found by our Raman experiments.

Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30u2009meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.

Dynamical study of the yellow luminescence band in GaN

Abstract A comprehensive study of the yellow photoluminescence (YL) in GaN epitaxial films grown by hydrid vapor phase epitaxy and by metal organic vapor phase epitaxy is presented including time-integrated and time-resolved photoluminescence (PL), PL excitation (PLE) and optically detected magnetic resonance (ODMR) experiments. ODMR reveals the participation of shallow and deep double donors based on the analysis of the g -values. This recombination model is supported by time-resolved investigations. PLE spectra show a close connection between the excitation processes of the YL band and of the inner transition of Fe 3+ at 1.293 eV. Two-color stimulation experiments prove energy transfer between YL and the Fe 3+ center by hole transfer, strongly confirming the YL recombination model involving a deep level 1.2 eV above the valence band.

Influence of compensation on the luminescence of nitrogen-doped ZnSe epilayers grown by MOVPE

Abstract The luminescence of ZnSe grown by metalorganic vapour phase epitaxy and doped by a DC nitrogen plasma is investigated. With increasing N 2 flux the donor-acceptor pair (DAP) band continuously develops into a structureless band peaking at 2.62 eV for highest doping levels. This broad band evolves back into a structured DAP band peaking at 2.698 eV with increasing excitation density. At high N concentrations and at large degree of compensation potential fluctuations become important for the spatially indirect DAP recombination. These fluctuations can easily be screened by optically excited carriers making the experimental conditions decisive for luminescence spectra of strongly doped ZnSe: N samples.

Photoluminescence study of the 1.047 eV emission in GaN

We use photoluminescence to study residual transition metal contaminants in GaN layers, which are grown by the sandwich technique either on 6H‐SiC substrate or on sapphire substrate. We observe three no‐phonon lines in the near infrared optical region at 1.3 eV, 1.19 eV, and 1.047 eV caused by 3d transition metals. The appearance of GaN related host modes in the phonon sideband of these emissions proves that the luminescence centers are incorporated in the hexagonal GaN layers. In this paper we especially focus on the luminescence band with the no‐phonon line at 1.047 eV. Temperature dependent photoluminescence measurements reveal an excited state splitting of 8 meV. In photoluminescence excitation spectroscopy we observe a further excited state at 1.6 eV with a fine structure splitting. The appearance of this excited state in the n‐type samples gives evidence that the defect must already exist in its luminescent charge state without illumination. The experimental results on the 1.047 eV emission fit to a...

Hot carrier relaxation in InAs/GaAs quantum dots

Abstract Excited states and energy relaxation processes are studied for self-organized InAs/GaAs quantum dots (QDs). Photoluminescence excitation (PLE) spectra show sample-dependent either multi-LO-phonon resonances or excited state transitions, revealing the dominant carrier relaxation mechanism or the size-dependent excited state splitting of the QDs, respectively. Time-resolved photoluminescence results indicate sample-dependent non-radiative recombination, suggesting a model for the observed PLE behavior, analogous to hot carrier relaxation in higher-dimensional systems. Carrier relaxation in the self-organized InAs/GaAs QDs proceeds by multi-LO-phonon scattering on a 40xa0ps time scale, which is short compared to radiative (>500xa0ps) and non-radiative (>100xa0ps) recombination times, accounting for the absence of a phonon bottleneck effect in PL spectra. However, carrier relaxation might effect the stimulated emission region.

Dynamics of excited states in GaN

Abstract We report a review of the optical properties of GaN epilayers. Photoluminescence, time-resolved and photoluminescence excitation measurements on hexagonal and cubic GaN heterostructures from the band edge region down to the near infrared spectral region provide information about excitonic properties as well as the influence of point and extended defects. Spatially resolved Raman-scattering and photoluminescence experiments allow to analyze the crystal structure, layer orientation and strain contribution to the lattice properties. Luminescence measurements at high excitation densities will be presented giving information about optical gain mechanisms.

Properties of the Biexciton and the Electron-Hole-Plasma in Highly Excited GaN

High-excitation processes like biexciton decay and recombination of an electron-hole-plasma are discussed as efficient mechanisms for lasing in blue laser diodes [1]. Therefore, the investigation of these processes is of fundamental importance to the understanding of the properties of GaN as a basic material for optoelectronical applications. We report on comprehensive photoluminescence and gain measurements of highly excited GaN epilayers grown by metal-organic chemical vapor deposition (MOCVD) over a wide range of excitation densities and temperatures. For low temperatures the decay of biexcitons and the electron-hole-plasma dominate the spontaneous-emission and gain spectra. A spectral analysis of the lineshape of these emissions is performed and the properties of the biexciton and the electron-hole-plasma in GaN will be disscused in comparison to other wide-gap materials. At increased temperatures up to 300 K exciton-exciton-scattering and band-to-band recombination are the most efficient processes in the gain spectra beside the electron-hole-plasma.

Gain Spectroscopy of HVPE-Grown GaN

We report on photoluminescence and optical gain measurements of highly excited GaN crystals grown by hydride vapor physe epitaxy (HVPE). Inelastic scattering processes of excitons dominate the spontaneous emission spectrum under high excitation up to temperatures of 180 K. Towards room temperature phonon-assisted recombination of excitons and free carriers begins to dominate the spectrum. Similar characteristics are observed in temperature-dependent gain measurements.