Plasma etching of surface gratings for laser diodes emitting in the blue-violet spectral region
For the development of GaN-based distributed feedback (DFB) laser diodes in the blue-violet spectral region, specific etching processes are required to realize laterally coupled surface gratings with high reflectivity that generate a narrow bandwidth of the emitting wavelength. Depending on the gained emission wavelength, a periodic structure of alternating materials with refractive index contrast is necessary to realize a grating with high reflectance in laser devices. This can be achieved by etching V-shaped grooves into the AlGaN-cladding layer perpendicular besides the laser ridge as shown in Fig. 1. Inductively coupled plasma (ICP) etching is a proper method for patterning on the sub-micrometer scale and is widely used in semiconductor device fabrication. At FBH, an ICP etch process was developed for the fabrication of laterally coupled 10th order surface grating of GaN-based DFB laser diodes. Lower order (1st, 2nd or 3rd) Bragg gratings would result in extreme short periods, which lead to technical limitations in the process chain.
To manufacture the surface gratings we used i-line stepper lithography with 700 nm thick positive photoresist (PR). The pattern of the photoresist mask was transferred into the AlGaN-based cladding layer by a chlorine-based high density plasma (ICP). A SENTECH SI500 dry etch system equipped with an inductively coupled plasma source was used to etch the V-shaped grooves continuously in one process step. The etching results are shown in Fig. 1(d). Sharp tips of the grooves, smooth side walls and uniform side wall angles are essential to obtain gratings with high reflectance that enable high-efficiency lasing. The upper right hand side schematic of Fig. 1 depicts the mechanism of the formation of the V-groove shape during the ICP etch process. It has been demonstrated that side wall angle q and etch depth can be adjusted by the process pressure of the used BCl3 gas, which controls the etch selectivity of the AlGaN to the photoresist mask and the amount of side wall passivation (redeposition). On the one hand, this offers flexibility for design of the gratings. On the other hand, stable process conditions are very important to maintain reproducible results.
Based on the described key process V-shaped grooves with a side wall angle of 20° were created in the AlGaN surface and DFB laser diodes with an emission wavelength of 404.6 nm and a bandwidth of 0.04 nm fullwidth at half-maximum were demonstrated . According to simulations the obtained etch depth of d = 600 (±10) nm and the side wall angle of θ = 20° allow for a maximum reflectance of around 65% for the etched 10th order grating with a period Λ of 600 nm . The output power is about 46 mW and stable operation under pulsed laser conditions has been achieved.
 J.H. Kang, H. Wenzel, V. Hoffmann, E. Freier, L. Sulmoni, R.-S. Unger, S. Einfeldt, T. Wernicke, M. Kneissl, “DFB Laser Diodes Based on GaN Using 10th Order Laterally Coupled Surface Gratings”, IEEE Photonics Technol. Lett., vol. 30, no. 3, pp. 231-234 (2018).