Holographic Photoetching of High-Quality Diffraction Gratings in P-GaAs for Distributed Feedback Lasers
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HOLOGRAPHIC PHOTOETCHING OF HIGH-QUALITY DIFFRACTION GRATINGS IN p-GaAs FOR DISTRIBUTED FEEDBACK LASERS RICHARD MATZ Research Laboratories of Siemens AG, Otto-Hahn-Ring 6, D-8000 MUnchen 83, Federal Republic of Germany ABSTRACT Quarter micrometer period diffraction gratings having depth-to-spacing ratios around 0.5 have been photochemically etched in p-GaAs using direct holographic illumination with 257 nm UV light. The results suggest the interaction of the incident light with the etched grating. The etch process is represented with the aid of a potential-pH diagram calculated from literature data. INTRODUCTION Single longitudinal mode lasers are needed for Gbit modulation in optical transmission systems. In spite of tight processing tolerances the distributed feedback (DFB) laser appears to be a promising concept because of its built-in mode selection [IL The central element is a diffraction grating with a period of typically a few tenths of a micrometer. But also passive components (e.g. wavelength demultiplexers) will possibly make use of gratings as planar, wavelength-selective mirrors. Such gratings are usually produced by holographic exposure of a photoresist film which serves as an etch mask. In recent years also several maskless, light-induced etch processes have been studied. Omitting the resist may not only save time and material but also help to improve interfacial quality for subsequent epitaxy or it may even pave the way to device concepts otherwise inaccessible by conventional single-layer resist technology, e.g. structuring of non-planar surfaces. Various diffraction gratings have been produced by this holographic process in n-GaAs [2,3], n-InP and n-GaInAsP [4]. Use has also been made of focussed laser light to fabricate pm-wide via holes and trenches in n-GaAs [5,61. But very few results have been published on processing of undoped or p-type III-Vsemiconductors. Yet several devices would benefit by such a step. For example waveguides are made from undoped material because of free carrier damping and also DFB laser yield may require fabrication of the grating in p-doped material after pn-junction formation. No reports have so far been given of holographic photoetching of diffraction gratings in p-type compound semiconductors. First results are presented here on maskless etching of quarter micrometer gratings in p-GaAs by use of 257 nm UV light. It is pointed out that the process is suitable for very low intensity (100 mW/cm ), large-area (3-4 mm ) direct holographic grating formation, although, as a consequence of unfavourable carrier collection, very low etch rates of 10 nm/min order of magnitude are expected from extrapolating published results obtained with higher intensity [51.
Mat. Res. Soc. Symp. Peroc. Vol- 75- c1987 Mater•ial Research Society
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257nm
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500 Y
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Fig. 1. 1 Laser interferometer for maskless etching of diffraction gratings with period A.
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EXPERIMENT Samples of (001) GaAs with Zn-doping (p=9x1O 1 7 cm-3) were used to investigate the feas
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