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White light-emitting diodes (wLEDs) are the new environmental friendly sources for general lighting purposes. For applications requiring a high-brightness, current wLEDs present overheating problems, which drastically decrease their emission efficiency, color quality and lifetime. This work gives an overview of the recent investigations on single-crystal phosphors (SCPs), which are proposed as novel alternative to conventional ceramic powder phosphors (CPPs). This totally new approach takes advantage of the superior properties of single-crystals in comparison with ceramic materials. SCPs exhibit an outstanding conversion efficiency and thermal stability up to 300°C. Furthermore, compared with encapsulated CPPs, SCPs possess a superior thermal conductivity, so that generated heat can be released efficiently. The conjunction of all these characteristics results in a low temperature rise of SCPs even under high blue irradiances, where conventional CPPs are overheated or even burned. Therefore, SCPs represent the ideal, long-demanded all-inorganic phosphors for high-brightness white light sources, especially those involving the use of high-density laser-diode beams.

β-Ga2O3 is the most transparent conductive oxide, well known since several decades for its large bandgap of 4.8 eV. Its potential as semiconductor material, however, is just emerging in recent years. Present work shows the development of βGa2O3 for semiconductor applications and its current state-of-the-art. The discussion is focused on three different aspects: (1) Advantageous growth from melt of large-size β-Ga2O3single-crystals. High-crystalline quality and carrier control make possible the production of conductive and semi-insulating wafers. (2) β-Ga2O3as substrate for homoepitaxy as well as for heteroepitaxial deposition of GaN-based devices. High-brightness blue-LEDs with vertical current injection are demonstrated. (3) Potential of β-Ga2O3for high-power devices with higher breakdown voltage than GaN and SiC counterparts. The first Schottky barrier diode is shown, as well as first transistors (MESFET and MOSFET) are indicated. Single-crystal phosphors are introduced as novel alternative to currently used powder phosphors. In connection with high-brightness white light-sources, based on LEDs or LDs plus phosphor converters, single-crystal phosphors possess advantageous features. These avoid the use of resins and exhibit a very high internal quantum efficiency, which remains stable with the temperature increase.

Large size single crystals of beta-Ga 2 O 3 with 1 inch in diameter have been grown by the floating zone technique. The stable growth conditions have been determined by the examination of the crystal structure. Wafers have been cut and fine polished in the (100), (010) and (001) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of beta-Ga 2 O 3 as a substrate for optoelectic devices operating in the visible/near UV and with vertical current flow. Epitaxial growth of nitride compounds by the metalorganic vapor phase epitaxy (MOVPE) technique is demonstrated on beta-Ga 2 O 3 single crystal substrates. High-quality (0001) GaN epi-layers with a narrow bandedge luminescence are obtained using a low temperature conductive buffer layer. InGaN multi-quantum well (MQW) structures were also successfully grown. The first blue light-emitting diode (LED) on beta-Ga 2 O 3 with vertical current injection is demonstrated.

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