Final PhD Seminar

Synthesis of Germanium – Tin Alloys by Ion Implantation and Pulsed Laser Melting: Towards a Group IV Direct Band Gap Semiconductor

Mr Tuan Tran

The germanium-tin (Ge1-xSnx) material system is expected to be a direct bandgap group IV semiconductor at a Sn content of   Such Sn concentrations can be realized by non-equilibrium deposition techniques such as molecular beam epitaxy or chemical vapour deposition. In this PhD work, the combination of ion implantation and pulsed laser melting (PLM) is demonstrated to be an alternative promising method to produce a highly Sn concentrated alloy with good crystal quality. Initially, it is shown that it is possible to produce a high quality alloy with up to . The structural properties of the alloys such as soluble Sn concentration, strain distribution and crystal quality have been characterized by Rutherford backscattering spectrometry (RBS), Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The optical properties and electronic band structure have been studied by spectroscopic ellipsometry. The introduction of substitutional Sn into Ge is shown to either induce a splitting between light and heavy hole subbands or lower the conduction band at the  valley.

However, at implant doses higher than  ion-beam induced porosity in Ge starts to occur, which drastically reduces the attainment of the implanted Sn concentration and hinders a good crystallisation of the material. It will be shown that a nanometer thick SiO2 layer deposited on the Ge substrates prior to the implantation can largely eliminate the formation of porosity. This capping SiO2 layer helps to increase the attained Sn concentration up to  after implantation as well as significantly improves the crystal quality of the Ge-Sn layer after PLM. With the use of the capping layer, a good quality Ge-Sn layer with  has been achieved using Sn implants at an energy of . A thermal stability study showed that the material is metastable up to .

Finally, a possible pathway to the synthesis of strain-relaxed material is studied by implanting Sn at a higher implant energy of . XRD/reciprocal space mapping showed that the material is largely relaxed, which is beneficial for the direct band gap transition and solves the trade-off issue between the higher Sn concentration and the compressive strain. RBS indicates a sub-surface band of disorder which suggests a possible mechanism for the strain relaxation. A preliminary photoluminescence study is able to detect photon emission at a wavelength of  from this material. However, TEM micrographs show the formation of non-equilibrium defects and/or a cellular breakdown phenomenon beginning at a depth corresponding to high Sn concentration. These issues will be discussed along with some possible solutions for them.    

Date & time

Fri 10 Feb 2017, 3.30–4.30pm



RSPE Seminar Room - 414


Members of RSPE welcome