D. Rudloff

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.

The origin of optical gain in cubic InGaN grown by molecular beam epitaxy

The optical properties of cubic InGaN samples with varying In content are investigated to provide insight into the processes responsible for optical amplification. The samples were grown by molecular beam epitaxy on GaAs substrates. The structural and optical properties were studied by means of time-resolved and time-integrated photoluminescence spectroscopy and cathodoluminescence microscopy, as well as gain measurements at various temperatures. From these measurements, localized states are proposed to be responsible as recombination mechanism. The cathodoluminescence measurements evidence a direct correlation of the degree of In fluctuation and the efficiency of optical amplification of the samples.

Strain in cracked AlGaN layers

The strain relaxation due to cracks of different depths in AlGaN layers grown on GaN template layers has been investigated using spatially resolved cathodoluminescence spectroscopy, high-resolution x-ray diffraction and two-dimensional finite element simulations. The experimental data consistently show that the relief of tensile stress increases with decreasing crack spacing. The measured strain profiles between the cracks are well described by the theoretical calculations for small crack spacings; whereas, deviations for larger crack spacings have been found. The latter is discussed in terms of inelastic strain relaxation mechanisms, the reliability of the deformation potential for AlGaN employed in this article, and the spatial variations in the properties of the AlGaN, e.g., its composition.

Stress analysis of AlxGa1-xN films with microcracks

Thick AlxGa1−xN epilayer with microcracks grown by metalorganic vapor-phase epitaxy on a GaN buffer above a (0001) sapphire substrate was comprehensively characterized by spatially and spectrally resolved cathodoluminescence (CL) and micro-Raman (μ-Raman) spectroscopy. The variation of the CL line shift and the μ-Raman measurements between the microcracks are consistent with the interpretation that AlGaN is to a large extent stressed like a two dimensional film between the microcracks with nearly full relaxation close to the cracks. A satisfactory theoretical confirmation of this stress distribution was obtained by a three-dimensional finite-element application of the elasticity theory.

Spatially modified layer properties related to the formation of gallium droplets on GaN(0001) surfaces during plasma-assisted molecular-beam epitaxy

The surface morphology and the spatial distribution of defect-related luminescence of GaN(0001) layers grown by plasma-assisted molecular-beam epitaxy under gallium-rich conditions has been investigated. Droplets of liquid gallium form on the surface during growth and lead to distinct spiral hillocks under the droplet. The droplets are surrounded by extended voids which point to an incomplete gallium adlayer on the GaN surface during growth at the droplet boundary. Cathodoluminescence spectra indicate an enhanced intensity in the yellow spectral range for the GaN under the droplets which is attributed to a change in the local density of point defects in the layer.

Comparison of the Mechanism of Optical Amplification in InGaN/GaN Heterostructures Grown by Molecular Beam Epitaxy and MOCVD

We comprehensively studied InGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) and metal-organic vapor deposition epitaxy (MOCVD) using a variety of methods of optical spectroscopy, such as cathodoluminescence microscopy (CL), time-integrated and time-resolved photoluminescence (PL). Micro-photoluminescence and cathodoluminescence results show the variation in emission wavelength at different scales, and this reflects the degree of compositional fluctuations in the samples. We obtain information on the decay times of the main emission lines using time-resolved photoluminescence spectroscopy and models of stretched exponentials, indicating the importance of nanoscale fluctuations for the recombination mechanism. The temperature dependent behavoir of the InGaN emission is explained in terms of a carrier freeze out at local potential fluctuations combined with a thermionic thermalization at elevated temperatures. To correlate the fluctuations in emission wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. From all these results we conclude, that localized carriers at a statistical distribution of potential fluctuations act as recombination centers and that the degree of fluctuations determines the efficiency of optical amplification. The threshold values for lasing and the gain values are compared and discussed with respect to the different growth procedures. From all these findings we draw conclusions concerning the influence of differences in the growth conditions and their impact on the optical properties.

Band Fillling and Energy Relaxation in InGaN/GaN-Multiple Quantum Well Structures

An InGaN/GaN multiple quantum well structure grown by low pressure MOCVD on c-Al2O3 substrate is investigated using spectral-time-resolved cathodoluminescence microscopy (CL). At 6 K the laterally integrated CL spectrum shows a very efficient blue emission from the InGaN multiple quantum well centered at 2.841 eV with an FWHM of 50 meV (σ = 21 meV). Spatially resolved CL wavelength mappings give a standard deviation of σ = 7 meV for the InGaN peak position across an area of 55 × 36 μm2. In time-resolved CL a strong monotonous redshift (ΔE = —60 meV) of the main emission line is observed during 4.5 μs decay following the function Epeak = E0 — 25 meV log (t/t0) which is attributed to a thermalization of carriers within a statistically distribution of localized states. This is supported by a blueshift (ΔE = 75 meV) of the emission line upon increasing cw excitation power by three orders of magnitude (Epeak = E0 + 25 meV log (P/P0).

Luminescence of ZnCdSe/ZnSe ridge quantum wires

Blue light-emitting quantum wire structures fabricated by molecular-beam epitaxial growth on submicrometer prepatterned GaAs substrates were investigated by spatially and time resolved luminescence experiments. The quantum wires are formed due to the different growth rates of ZnCdSe on the (111) and (100) surfaces of the grated substrate. With decreasing wire width, the exciton luminescence splits into two clearly distinguished lines. These lines can be assigned to the emission of the ridge quantum wire and the emission of ZnCdSe quantum wells at the bottom of the grooves. The two-dimensional quantum confinement in the ridge wire is confirmed by a maximum of the decay time at the energy of the ridge luminescence.

Exciton dynamics in ZnCdSe/ZnSe ridge quantum wires

Abstract Blue light-emitting quantum wires are fabricated by molecular beam epitaxy of ZnCdSe/ZnSe on patterned GaAs substrates. In these structures, lateral carrier confinement is obtained by the anisotropic growth of ZnSe. Two luminescence bands are observed, due to emission from the ridge quantum wire and due to luminescence of the QW grown on the bottom of the grooves between the ridges. The exciton decay is studied by time-resolved photoluminescence experiments. At the emission band of the quantum wire, a local lifetime maximum is observed as well as a biexponential exciton decay. Both are attributed to a lifetime elongation caused by the lateral quantum confinement of the excitons within the ridge wires.