Meltback etching and regrowth characteristics for the fabrication of self-collimating LED

Abstract

A research about the hemispherical lens-attached LED is done numerically and experimentally. Numerical calculation is based upon the ray-tracing method. The effect of the geometrical structure is studied on the field pattern. The beam could be collimated narrowly at optimal condition. The lens was fabricated using the meltback etching and regrowth method. The mechanism of meltback etching was researched for both planar and selective meltback etching. In case of planar meltback etching, the effect of convection was dominant and the etching depth showed t1/2 - 0.7 relation for the etching time. But in the selective etching, the effect of diffusion was dominant although the relation of etching volume and etching time showed similar tendency. The meltback etching shape was different for different etching condition. When etched with Ga melt only, the shape showed (111) planes. The isotropic etching shape could be obtained by reducing the mask opening width, by reducing the degree of undersaturation and by increasing the amount of added Al to the melt. Regrowth showed Ga capturing effect when the degree of supersaturated As concentration was large. The regrowth mechanism showed crystallographic planes when the As was furnished sufficiently. But when grown under small As concentration, the regrown shape showed elliptic. By reducing the As concentration could be obtained by using the homogeneous nucleation effect. But the grown layer using this method had large grading of Al composition. To reduce the As concentration effectively and reproducibly, dummy-substrate was used and the melt height was reduced. LED layer was grown over the lens. It was also grown under the condition of small As concentration. The amount of As concentration was controled by controlling the cooling rate. Although the fabricated LED showed large leakage current, due to the curved surface of the lens, the efficiency was as large as 0.5\%. Due to the Ga carry over effect, the spectrum of the LED w...