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Under pulsed operation, time dependent spectral and electro-optical measurements on GaN-based laser diodes show a considerable red shift in the emission wavelength and a decreasing voltage drop across the device. These changes appear on a rather short time scale in the microsecond range. During a 3.7 microsecond long pulse, a temperature increase of approximately 50 K is obtained using different experimental methods. This value agrees well with numerical simulations based on the thermal properties of the material.

In this work, we investigate the extraction efficiency for UV emitting rectangular 300 Â 300 mm 2 gallium nitride (GaN) based light emitting diodes (LEDs) by simulation with a raytracer tool. It is shown that the extraction efficiency depends strongly on slight variations of the absorption in the GaN layers. Furthermore, the influence of the substrate shape is studied. For standard rectangular sapphire substrate based LEDs the calculated extraction efficiency is 12.4%, whereas for silicon carbide substrate based devices the higher refractive index causes a lower efficiency (4.5%). Using a shaped SiC substrate the extraction efficiency can be improved to 17.2% and 19.9% for a sapphire substrate. The influence of geometric design parameters like sidewall angle are analyzed as well.Introduction High brightness light emitting diodes (LEDs) have proven their suitability for a variety of applications such as large area outdoor displays or the generation of white light. GaN based LEDs offer a high efficiency and cover nearly the full visible wavelength range. However, internal losses and total internal reflections (TIR) limit the device performance. OSRAM-OS invented the ATON technology and improved the external quantum efficiency of GaN LEDs grown on SiC by a modified substrate geometry [1, 2]. The high refractive index of silicon carbide (n % 2:77 at 410 nm [4]) causes a small extraction cone of 42 for an unencapsulated device, whereas for sapphire substrates due to n ¼ 1:7 the cone is 72 . In the visible range both materials are transparent, but the still present low absorption reduces the extraction efficiency. The intensity decay for rays which are within the extraction cone is small for a low absorption coefficient. On the other hand, for TIR rays which propagate long distances within the device, the intensity is strongly reduced. In the ideal case, where no surface roughness and bulk scattering is present, the intensity is decreased to zero. In reality these rays contribute to the extraction efficiency due to scattering. The smaller escape cone and the higher absorption coefficient yield a lower extraction efficiency for SiC based devices compared to sapphire based LEDs. To overcome the problem of extraction, several LED designs have been proposed [5][6][7] for other material systems than IIInitrides. However, the implementation requires material specific processing steps. In this work, a structured substrate geometry is considered which can improve the efficiency in both, more rays in the extraction cone and a shorter average propagation distance.

Abstract-Different experimental and simulation techniques aiming at a better understanding of lateral mode absorption in light-emitting diodes (LEDs) are presented in this paper. A measurement of transmitted power versus propagation distance allows us to derive the absorption losses of LED layer structures at their emission wavelength. Two models for the observed intensity distribution are presented: one is based on scattering, whereas the other relies on selective absorption. Both models were applied to InGaN-on-sapphire-based LED structures. Material absorption losses of 7 cm 1 for the scattering model and 4 cm 1 for the absorbing-layer model were obtained. Furthermore, these values are independent of the emission wavelength of the layer structure in the 403-433-nm range. The losses are most likely caused by a thin highly absorbing layer at the interface to the substrate. In a second step, interference of the modal field profile with the absorbing layer can be used to determine its thickness ( = 75 nm) and its absorption coefficient ( 3900 cm 1 ). This method has also been tested and applied on AlGaInP-based layer structures emitting at 650 nm. In this case, the intensity decay of = 30 cm 1 includes a contribution from the absorbing substrate.

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