Carbon tetrabromide-based in situ etching of GaAs improved by TMAl
Fig. 1: SEM image with secondary electrons (a) and panchromatic CL image (b) of same area of a GaAs surface etched only with CBr4 at 575°C recorded at 80 K
Fig. 2: Experimental values for GaAs etch rate with addition of a) TMGa and b) TMAl at 675°C (empty) and 575°C (filled)
Buried structures produced in multiple growth steps can enhance the functionality of laser diodes. Passive sections can be used, for example, for phase modulation, beam shaping, and reduction of leakage currents. Using conventional ex-situ patterning for AlGaAs results in an oxide layer that disturbs or even inhibits subsequent regrowth. A combination of pattern generation and regrowth by metal organic vapor phase epitaxy (MOVPE) in the same reactor could avoid this oxide formation. Carbon tetrabromide (CBr4) is widely used in MOVPE, also at FBH, to incorporate C as a p-dopant in GaAs and AlGaAs. Addition of CBr4 during growth is also known to reduce the growth rate due to Br-related etching reactions. As such etching reactions could be useful for pattern transfer inside an MOVPE reactor they were studied at FBH.
Etching of about 150 nm of GaAs with CBr4 leads to the formation of etch pits on the surface. Cathodoluminescence (CL) images show dark spots in the center of such pits, indicating that they are seeded by dislocations present in the substrate or in the etched epitaxial layer structure (Fig. 1).
Adding trimethylgallium (TMGa) during the etching of GaAs with CBr4 reduces the etch rate due to competition with growth (Fig. 2a). Both processes, growth and etching, are apparently independent. The development of etch pits is unaffected by the presence of TMGa.
However, adding trimethylaluminum (TMAl) during etching has a much more complex effect: at high temperature the etch rate is strongly reduced, probably due to a "masking" by AlAs forming on the surface, while at low temperature the etch rate is enhanced (Fig. 2b). TMAl not only affects the etching reaction but also prevents the formation of unwanted etch pits; only very shallow hillocks mark the position of the dislocations. These results suggest that TMAl-assisted CBr4 etching could be a useful tool for MOVPE in situ pattern transfer without contact to oxygen.
Publication
P. Della Casa, A. Maaßdorf, U. Zeimer, M. Weyers, "CBr4-based in-situ etching of GaAs, assisted with TMAl and TMGa", J. Crystal Growth 434, pp. 116-122 (2016).