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1 Introduction The performance of commercial LEDs has improved tremendously over the past few years. Today commercial LEDs cover the entire spectral range from UV to IR. The brightness of InGaN-LEDs has been increased by more than an order of magnitude over the last 10 years. Internal Quantum Efficiencies (IQE) of 75% with corresponding wall plug efficiencies (WPE) above 50% have been demonstrated for blue LEDs [1]. The OSRAM Opto Semiconductors ThinGaN-technology has pushed the Light Extraction Efficiency (LEE) of LED chips beyond 80%. Thin GaN technology also provides scalability of LED chips: LED brightness and efficiencies can be scaled to larger chip areas without losses. However, the InGaN Internal Quantum Efficiency is neither independent of the energy gap (or emission wavelength) nor of the current density: while IQE of more than 75% can be achieved for blue InGaN LEDs (440 nm, 50 A/cm 2 ) the IQE drops to less than 40% for green InGaN LEDs (540 nm, 50 A/cm²). At high current densities this efficiency loss even worsens especially for long wavelengths ("droop"). Pushing InGaN-LEDs towards red emission, the IQE drops dramatically below 10%. For wavelength above 580 nm the InGaAlP material system provides very efficient yellow, amber and red LEDs. In the wavelength range between 500 nm and 580 nm, InGaAlP LEDs are not efficient anymore due to weak carrier confinement.

A review is given on the results of magnetic resonance studies of transition metal impurities in SiC polytypes. The data are presented for the elements titanium (Ti), vanadium (V), chromium (Cr), molybdenum (Mo), manganese (Mn), scandium (Sc) and copper (Cu). Most of these transition metals were found to occur in multiple charge states, underlining their role as deep level defects in SiC. A compilation of relevant ESR parameters for transition metal defects in various SiC polytypes is presented in the Appendices.

A characteristic infrared luminescence spectrum, dominated by a zero-phonon line at 1.30 eV, has been observed on AlN polycrystalline material. It is assigned to the spin-forbidden internal 3d–3d transition 4T1(G)→6A1(S) of Fe3+Al(3d5). By photoluminescence excitation spectroscopy the (-/0) acceptor level of iron in AlN could be located at EV+3.0 eV. The corresponding value for iron in GaN is EV+2.5 eV. From these values, the valence-band offset in AlN/GaN heterojunctions is predicted as ΔEV=0.5 eV, the conduction-band offset as ΔEC=2.3 eV.