An aggregate of Eu-doped GaN columnar crystals was synthesized at 700 C and a N 2 pressure of 5 MPa at the surface of a Na-Ga melt with the addition of Eu metal. The color of the crystals was black at the melt-phase side, and changed to brown to colorless and transparent toward the gas-phase side. Energy-dispersive X-ray analysis revealed that the Eu content was 0.15 at. % at the black part and decreased gradually below <0:1 at. % at the colorless transparent part. The colorless transparent part glowed red under ultraviolet radiation. A strong emission peak associated with the intra-4 f transition of Eu 3þ from 5 D 0 to 7 F 2 was observed at 621 nm in the photoluminescence spectrum and at 624 nm in the cathodoluminescence spectrum.
InP is now becoming an indispensable material not only for longwave length opto-electronic devices but also for the future high frequency devices. Promoted by this requirement, InP bulk crystal technology has been rapidly developed. In the present paper, the development is reviewed in the field of purification, reduction of dislocation densities, semi-insulating InP and polishing. The possibility of undoped semi-insulating InP, a highlight topic of the recent developments, is also discussed on the basis of the recent experimental results.
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