MOVPE homoepitaxy of high-quality GaN: Crystal growth and devices

Epitaxial growth on GaN bulk single crystal substrates sets new standards in GaN material quality. The outstanding properties provide insights into fundamental material parameters (e.g. lattice constants, exciton binding energies, etc.) with a precision not obtainable from heteroepitaxial growth on sapphire or SiC. With metalorganic vapor phase epitaxy (MOVPE) we realized unstrained GaN layers with dislocation densities about six orders of magnitude lower than in heteroepitaxy. By the use of dry etching techniques for surface preparation, an important improvement of crystal quality is achieved. Those layers reveal an exceptional optical quality as determined by a reduction of the low-temperature photoluminescence (PL) linewidth from 5 meV to 0.1 meV and a reduced X-ray diffraction (XRD) rocking curve width from 400 to 20 arcsec. As a consequence of the narrow PL linewidths, new features as, e. g. a fivefold fine structure of the donor-bound exciton line at 3.471 eV was detected. Additionally, all three free excitons as well as their excited states are visible in PL at 2 K.

Dry etching techniques for surface preparation allow morphologies of the layers suitable for device applications. We report on InGaN/GaN multi-quantum-well (MQW)_ structures as well as GaN pn- and InGaN/GaN double heterostructure light emitting diodes (LEDs) on GaN bulk single crystal substrates. Those LEDs are twice as bright as their counterparts grown on sapphire. In addition they reveal an improved high power characteristics, which is attributed to an enhanced crystal quality and an increased p-doping.     

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Interplay of rubber network theory with the damping behavior of crosslinked polyurethanes

Network formation (branching) theory was used to examine the relationships between network structure and concomitant sound and vibration damping. For a series of model polyether-based polyurethane networks with varying stoichiometry and composition, the glass transition temperature T_g, was found to increase with increasing concentration of elastically active network chains, EANCs, as well as the ratio of branch OH group concentration to the total OH group concentration ρ. The values of (tan δ)_max, the peak height of tan δ at T_g, linearly decrease with increasing concentration of EANCs, regardless of the ρ values. However, the loss area (LA), equal to the integral of the linear loss modulus-temperature curve, is independent of the concentration of EANCs and/or ρ. Utilizing group contribution analysis techniques, the value of the main chain -O- group contribution, LA_-O-, is 19.1 GPa·K/g, a rather large value. This finding gives insight into why polyether urethanes are preferred for many damping applications.