J. Holst

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±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.

Time-resolved microphotoluminescence of epitaxial laterally overgrown GaN

Epitaxial laterally overgrown GaN (ELOG) structures are microscopically characterized using spatially resolved microphotoluminescence (micro-PL) and time-dependent spectroscopy. To understand the influence of the different lateral growth mechanisms on the peak position and the temporal behavior of the transition lines, we correlated the different micro-PL emission spectra with results of spatially resolved time-dependent spectroscopy experiments.

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.

Optical-Gain Measurements on GaN and Al x Ga 1-x N Heterostructures

Optical gain processes in thin GaN and AlGaN are compared by means of gain spectroscopy using the stripe length method and high-excitation photoluminescence, both performed at various densities and temperatures. We find that inelastic excitonic scattering processes and biexciton decay are important at low temperatures and low excitation densities Both materials are similar in that increasing the excitation density results in gain spectra dominated by the electron-hole plasma and phonon-assisted band-to-band recombination. These also prevail at high temperatures.

Correlation between Structural Properties and Optical Amplification in InGaN/GaN Heterostructures Grown by Molecular Beam Epitaxy

We comprehensively studied InGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) using a variety of methods of optical spectroscopy, such as cathodoluminescence microscopy (CL), time-integrated and time-resolved photoluminescence. To correlate the fluctuations in emission wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. The lateral homogeneity can be drastically improved using a template of GaN grown on the sapphire substrate by metal-organic vapor phase epitaxy (MOVPE). Gain values up to 62 cm were found in samples with low indium fluctuations, which is comparable to values for high-quality InGaN/GaN heterostructures grown by MOVPE.