Horizontal Distributed Bragg Reflector Mirrors for Waveguide Lasers

S. W. Pang and S. Thomas III

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

Distributed bragg reflector mirrors have been etched in InP and GaAs. The mirrors were patterned using electron beam lithography and etched with an electron cyclotron resonance source. The Ni lines deposited on InP were 122 nm wide and on GaAs were 59 nm wide for ¦Ë/4 DBR mirrors. This should allow high reflectivity mirrors to be fabricated for lasers emitting at 1.55 and 0.85 µm. This fabrication technique has the advantage that no regrowth step is required and the length of the passive mirror region is confined to a few micrometers. The structure requires etching vertical profiles with smooth surface morphology and high selectivity to the masking material. The effects of rf power, Cl2 percentage in Ar, champer pressure, and stage temperature were studied. Increasing the rf power from 100 to 250 W caused the InP selectivity to Ni to decrease from 147 to 55. A similar effect was observed for etching GaAs. For both InP and GaAs, a vertical profile can be obtained by reducing the Cl2 percentage in Ar and by using low chamber pressure of 1 mTorr. (Figure 1) However, increasing the Cl2 percentage improves selectivity to the Ni mask. With 20% Cl2 in Ar, vertical profiles and high selectivities are obtained for both InP and GaAs. Using high temperature increases the volatility of InClx etch products and improves the selectivity of InP to Ni. For InGaAsP/InP waveguide lasers, 122 nm wide mirrors were etched in InP to a depth of 2.0 µm (Figure 2) for maximum reflectivity at 1.55 µm. For AlGaAs/GaAs waveguide lasers, 59 nm wide mirrors were etched in GaAs to a depth of 1.5 µm. (Figure 3) These etch depths are sufficient to clear the top cladding layer and the active region of most devices and should provide high reflectivity. Etching of n¦Ë/4 mirrors, where ¦Ë is the emission wavelength, was studied and the dependence of gap spacing on etch depth was measured. The variations in etch depth for different gaps were minimized by reducing the pressure.

Figure 1. Effects of linewidth and pressure on the etch rate of InP for various gap widths. Etching at lower pressure reduces the aspect ratio dependence effect.

Figure 2. DBR mirrors etched in InP with ¦Ë/4 period. The mirrors were 122 nm wide and 2 µm deep. The etch conditions were 250 W microwave power, 200 W rf power, Cl2/Ar at 4/16 sccm, 1.0 mTorr pressure, 8 cm source distance, and stage temperature of 350¡ãC.

Figure 3. Three period distributed Bragg reflector mirror structure etched in GaAs. The mirrors were 59 nm wide and 1.5 µm deep. The etch conditions were 50 W microwave power, 100 W rf power, Cl2/Ar gas flow at 4/16 sccm, 1 mTorr pressure, and 8 cm source distance.

References

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Last Updated: November 19, 2007

E-Mail: pang@eecs.umich.edu 

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