Electron-beam patterning of surface gratings for wavelength-stabilization of semiconductor lasers
Fig. 1: SEM picture of surface gratings patterned by electron-beam lithography
Fig. 2: Measured reflectivities of 3. and 8. order DBR gratings
By implementing gratings into semiconductor laser resonators, the spectral emission width of these lasers can be significantly reduced – from typically a few nanometers down to the femtometer scale. This feature makes the efficient semiconductor light sources also attractive for narrow-band applications such as spectroscopy.
An established method for grating integration is to introduce periodical grooves with defined depth, shape, and period into the laser surface. So far, these gratings were defined by using i-line stepper lithography [1]. Due to the limited resolution of i-line lithography, grating periods could only be defined down to 720 nm. Reasonable reflectivities of high order gratings were obtained by etching V-shaped grooves, which are tapered towards the active region.
However, when using electron-beam lithography (VISTEC SB251) structures with significantly smaller dimensions can be processed. An increased resolution allows us now to produce surface gratings with ≥ 240 nm period. For that purpose an etching technology is developed allowing to transfer resist gratings up to 1.5 µm deep into the semiconductor, see Fig. 1. The resulting low order gratings show increased reflectivities compared to higher order gratings including enhanced wall-plug efficiencies of DBR lasers. Fig. 2 depicts a comparison of determined reflectivities of fabricated gratings of 3rd and 8th orders. In addition, it has been demonstrated that the electron-beam process is more flexible and offers an improved reproducibility.
Meanwhile, the electron-beam patterning of surface gratings is already part of the DBR laser fabrication process at the FBH.
Publication
[1] J. Fricke, W. John, A. Klehr, P. Ressel, L. Weixelbaum, H. Wenzel and G. Erbert "Properties and fabrication of high-order Bragg gratings for wavelength stabilization of diode lasers" Semicond. Sci. Technol., vol. 27, no. 055009 (2012).
FBH research: 11.10.2013