| Summary: | 碩士 === 國立中興大學 === 材料工程學研究所 === 92 === III-nitride compound semiconductors are recognized to be the key materials for optoelectronic devices, such as short-wavelength light-emitting diodes (LEDs) and laser diodes. In the conventional epitaxial process for GaN LEDs, threading dislocations are the most common defects, which make the carrier releasing energy without luminescence as a result of nonradiative recombination effect. The reduction of threading dislocations is very important if one wants to fabricate high-efficiency GaN LEDs. Recently, the patterned sapphire substrate was confirmed to be an efficient method to reduce the threading dislocations and to enhance the scattering the emission light from the active layer. Thus the external quantum efficiency of the GaN LED was improved.
In this thesis, the pattern etching process for the sapphire wafer by an inductively-coupled-plasma (ICP) etcher was described. The etch characteristics were investigated by varying the etching parameters, such as gas combination of Cl2/BCl3/Ar, chamber pressure, ICP power, and DC-bias voltage. The increase of BCl3 content (up to 66.7%) in the Cl2/BCl3 gas mixture increases the etch rate and improves the etch selectivity over Ni. In our experiments, the etch rate of sapphire can increase from 35 to 280 nm/min with a surface roughness from 0.2 to 1.9 nm, and the etch profile from 60 to 79°. The etch selectivity over Ni can reach above 6 which is higher than the other mask materials (SiO2 or Cu). However, the SiO2 mask can offer the lower taper angle from 24 to 35° after ICP etching due to the faster etch rate of the SiO2 side wall. Thus one can utilize a suitable mask material to achieve the etching rate, taper angle, and roughness which are desirable for patterned sapphire substrates.
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