Selective epitaxial growth of strained SiGe/Si for optoelectronic devices
Introduction
Selective epitaxial growth of Si by chemical vapour deposition (CVD) on patterned substrates is known to produce well-defined planes since the early 1960 [1]. Facet formation is a common phenomenon in crystal growth because the growth rate is different for different crystallographic orientations [2]Ishitani reported that during the selective epitaxial growth (SEG) facets are formed at an early stage of the growth process being generated from the bottom edge of the side wall and developing in proportion to layer thickness [3]Selective epitaxy has been demonstrated as a method for fabrication of highly regular arrays of dots which exhibit excellent uniformity [4]and good luminescence down to 50 nm lateral size [5].
The relaxation in small holes is retarded or even suppressed for sizes ≤10 μm 6, 7. This phenomenon can be used to grow in small oxide holes strained layers much thicker than on large areas, at least for strain ≤1%.
In this study the facet formation and the reduction of the misfit dislocation density in small areas will be discussed.
Section snippets
Experimental details
The epitaxial growth was carried out at 0.12 Torr in a cold wall, load-locked, high-vacuum LPCVD system using SiCl2H2 and He-diluted GeH4 as source gases and H2 as carrier gas [4]. Before epitaxy the wafers were cleaned by the standard RCA cleaning, while the protective oxide was removed ex-situ in a 2% HF-ethanol solution with subsequent rinse in water. For EL measurements a Fourier transform spectrometer equipped with a cooled Ge detector was used. Atomic force microscopy was performed with a
Facets
For facet investigations multilayer structures with ten periods of Si/SiGe were grown at 700 or 800°C. Each period consisted of a Si layer ∼80 nm thick and a Si1−xGex layer ∼4 nm thick. The SiGe layers were grown as markers so as see in cross section TEM micrographs, the evolution of facets during growth. The substrates were thermally oxidized (001) Si lightly n- or p-type doped and patterned by optical lithography with arrays of rectangular holes along 〈100〉 or 〈110〉 directions [5].
Fig. 1
Reduction of misfit dislocation density in dots and wires
Electroluminescent diodes with strained SiGe layers in the active region [10]show a strong drop of the interband emission at higher temperatures. To increase the electroluminescence intensity the band offset and/or layer thickness have to be increased. However, the increase of the thickness of SiGe is limited by plastic relaxation, while the increase of the Ge content above a certain strain (∼1%) leads to island formation. Concerning the increase of the thickness by maintaining the SiGe layer
Conclusions
In summary, we have studied the facet formation of dots and wires grown by selective epitaxy. Mainly {hhl} facets are observed, however under certain conditions also facets with h≠k≠l ({0 1 12}) form. The formation of the {111} and {119} planes seems to be kinetically controlled, while the {113} facets seem to be the equilibrium configuration for the 〈110〉 zone. The electroluminescence of strained SiGe in finite pads was studied as being dependent on the thickness of the SiGe layer. It was
Acknowledgements
We are thankful to K. Schmidt for the RBS characterization, to R. Apetz for preparing some of the EL diodes and for helpful discussions and to C. Dieker for the TEM analysis.
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