RBS-Channelling analysis into the effect of thermal annealing on GeSn strained layers

(1) Department of Physics, University of Cape Town, Rondebosch 7700, South Africa  and  iThemba LABS, National Research Foundation, P.O. Box 722, Somerset West 7129, South Africa
(2) iThemba LABS, National Research Foundation, P.O. Box 722, Somerset West 7129, South Africa
(3) Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
(4) IMEC, Kapeldreef, B-3001 Leuven, Belgium

Alloying Ge with Sn improves carrier mobility in the material and can also transform the material from an indirect bandgap into a direct bandgap semiconductor.  Unfortunately α-Sn is only stable at temperatures < 13℃ and this, coupled with the large mismatch between Sn and Ge, results in an upper limit of Sn in Ge of around 1%.  It is possible to produce alloys with the higher Sn concentrations but these alloys are metastable and can relax on thermal annealing.  Rutherford Backscattering Spectrometry and channelling have been employed to investigate the effect of thermal annealing on epitaxial Ge Sn (6% Sn) strained layers grown on Ge-buffered Si(100) wafers, with channelling along the [110] axis being used to investigate the strain residing in the layers upon thermal annealing.  Annealing at temperatures below 400℃ for 20 minutes had no noticeable effect on the strain in the epitaxial layers.  Once the temperature was raised above 400℃ however, relaxation of the layer sets in and the GeSn layer has essentially completely relaxed following a 20 minute anneal at 650℃.  The results are in good agreement with similar investigations conducted using X-ray diffraction.  The advantage of the RBS/Channelling approach however, is its ability to provide compositional information as a function of depth.  One is therefore able to monitor the effect of the thermal anneal on the Ge and Sn distribution throughout the layer, and also to extract information about their lattice location.  The results obtained show that that when the initial relaxation sets in both the Ge and the Sn are still situated in substitutional sites, and it is only around 600℃ after substantial relaxation has taken place that the Sn finally breaks free from the lattice sites and diffuses to the surface of the sample.  RBS-Channelling thus provides insight into the manner in which the relaxation process takes place and with this better understanding it might be possible to design more stable strained layers.