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The optical properties of n-type GaN are investigated for Si doping concentrations ranging from 5ϫ10 16 to 7ϫ10 18 cm Ϫ3. The photoluminescence linewidth of the near-band gap optical transition increases from 47 to 78 meV as the doping concentration is increased. The broadening is modeled in terms of potential fluctuations caused by the random distribution of donor impurities. Good agreement is found between experimental and theoretical results. The intensity of the near-band-gap transition increases monotonically as the doping concentration is increased indicating that nonradiative transitions dominate at a low doping density. The comparison of absorption, luminescence, reflectance, and photoreflectance measurements reveals the absence of a Stokes shift at room temperature demonstrating the intrinsic nature of the near-band edge transition.

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This paper examines the current status of technology and discusses technical options for developing DC transmission grids. The fast advances in VSC HVDC, the recent offshore VSC projects, the experience with multiterminal HVDC and recent development of fast DC circuit breakers bring large meshed DC grids closer to reality. The most important and most difficult remaining technical challenge is the system level protection of DC grids. The article further discusses some of the ongoing research directions like the use of travelling wave detection for fast protection or deployment of DC/DC converters for isolation of DC faults. One of the main work packages in EU funded Twenties project studies the major prerequisites for operation of DC grids. This project has delivered some major studies of DC grids and two hardware demonstration systems are under development: a mock-up DC grid at University of Lille and fast DC Circuit Breaker at ALSTOM.

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The competition between band gap and the 2.2 eV ͑yellow͒ luminescence of epitaxial GaN is studied for excitation densities ranging from 5ϫ10 Ϫ6 to 50 W/cm 2. The ratio of the peak intensities of the band gap-to-yellow luminescence changes from 4:1 to 3000:1 as the excitation density is increased by 7 orders of magnitude. At room temperature, the band gap luminescence linewidth is 2.3kT, close to the theoretical minimum of 1.8kT. A model is developed describing the intensity of the two radiative transitions as a function of the excitation density. This model is based on bimolecular rate equations and takes into account shallow impurities, deep levels, and continuum states. The theoretically predicted dependences of the two different luminescence channels follow power laws with exponents of 1 2 , 1 and 3 2. Thus the intensity of the yellow luminescence does not saturate at high excitation densities. These dependences are in excellent agreement with experimental results. The relevance of the results for optoelectronic GaN devices is discussed. It is shown that the peak intensity of the yellow luminescence line is negligibly small at typical injection currents of light-emitting diodes and lasers.

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Luminescence spectra of GaN epitaxial layers grown on sapphire display a strong intensity modulation of the below-band gap transitions and on the low-energy side of the near-band gap transition. The intensity modulation is attributed to a microcavity formed by the semiconductor-air and semiconductor-substrate interface. The microcavity effect is enhanced by using metallic reflectors which increase the cavity finesse. It is shown that microcavity effects can be used to determine the refractive index of the microcavity active material. Using this method, the GaN refractive index is determined and expressed analytically by a Sellmeir fit. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00421-X͔ Microcavity effects in semiconductor optoelectronic devices have attracted much attention due to the potential of high-efficiency light-emitting diodes ͑LED͒, and low threshold lasers. 1 The enhancement of the spontaneous emission by microcavity effects has been demonstrated for resonantcavity LEDs in organic 2 as well as semiconducting 3 material systems. High-finesse GaN microcavities with distributed Bragg reflectors were recently realized by Redwing et al. 4 In the present study, the microcavity effects occurring in GaN epitaxial layers are analyzed and used for refractive index determination. Due to the refractive index step at the substrate-epilayer interface, the cavity effects are observed in GaN layers with a sufficiently small surface roughness. 5 By using metallic silver reflectors instead of the weakly reflecting semiconductor-air interface, the microcavity effects can be strongly enhanced. It is shown that the near-band gap transition of GaN is modulated on the low-energy shoulder only. In contrast, the entire band of below-band gap transitions are modulated. A new method is developed to determine the refractive index of the optically active material of microcavity structures. The usefulness of this method is demonstrated for GaN and the refractive index of GaN is expressed in analytic form by the Sellmeir equation.The GaN epitaxial layers were grown on ͑0001͒ oriented sapphire in an Emcore metal-organic vapor phase epitaxy ͑MOVPE͒ system. An initial 200-Å-thick GaN buffer layer was grown at 500°C after nitridation of the substrate. A homogenous 3-m-thick Si-doped GaN epitaxial layer (n ϭ2ϫ10 18 cm Ϫ3 ) was grown at 1050°C. After growth, the substrate was polished to allow for transmittance measurements. These measurements were performed using a broadband xenon light source. A polished sapphire substrate was used for reference measurements. The photoluminescence measurements were performed at room temperature with excitation by the 325 nm line of a HeCd laser. The very high luminescence intensity of the samples demonstrates the excellent quality and high radiative efficiency of the GaN epitaxial films. An excitation power density of 10 W/cm 2 on the sample surface was used. The luminescence was dispersed in a 0.75 nm monochromator and detected by a GaAs photo-multiplier connected to phase-sensitive amplifi...