Synthesis of Ge1−xSnx alloys by ion implantation and pulsed laser melting: Towards a group IV direct bandgap material

Abstract

The germanium-tin (Ge1−xSnx) material system is expected to be a direct bandgap group IV semiconductor at a Sn content of 6.5−11 at. %. Such Sn concentrations can be realized by non-equilibrium deposition techniques such as molecular beam epitaxy or chemical vapour deposition. In this report, the combination of ion implantation and pulsed laser melting is demonstrated to be an alternative promising method to produce a highly Sn concentrated alloy with a good crystal quality. The structural properties of the alloys such as soluble Sn concentration, strain distribution, and crystal quality have been characterized by Rutherford backscattering spectrometry, Raman spectroscopy, x ray diffraction, and transmission electron microscopy. It is shown that it is possible to produce a high quality alloy with up to 6.2 at. %Sn. 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 Γ valley. Limitations and possible solutions to introducing higher Sn content into Ge that is sufficient for a direct bandgap transition are also discussed.

Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer

“…However, for LN 2 implantation of heavier elements such as Sb or Sn, porosity occurs above about 7 Â 10 15 ion cm À2 and 2:0 Â 10 16 ion cm À2 , respectively, thus limiting obtainable impurity concentrations in Ge. 18,22 In this letter, we illustrate the limit that porosity imposes on the retained implant concentration and demonstrate a simple and yet effective solution to drastically suppress the formation of porosity during Sn implantation at LN 2 temperature. Before implantation, selected Ge substrates were deposited with $40 nm of silicon dioxide (SiO 2 ) by plasma enhanced chemical vapour deposition.…”

“…Recently, there have been several efforts to fabricate the material with a more robust, industrially relevant method using ion implantation in combination with nanosecond pulsed laser melting (PLM). [16][17][18] For example, a good quality Ge-Sn alloy with a Sn concentration greater than 6 at:% has been recently obtained using PLM, 18 demonstrating the potential of this approach. It was shown, however, that achieving Sn concentrations in Ge higher than about 6 at:% through ion implantation is severely hindered by a high sputtering effect in Ge and the onset of ionimplantation induced porosity once the Ge is rendered amorphous (a-Ge).…”

“…It was shown, however, that achieving Sn concentrations in Ge higher than about 6 at:% through ion implantation is severely hindered by a high sputtering effect in Ge and the onset of ionimplantation induced porosity once the Ge is rendered amorphous (a-Ge). 18 It has previously been found that irradiation-induced porosity is favoured in the range of implant temperatures between $ À 80 C and $200 C. [19][20][21] The onset of porosity can be suppressed by undertaking implants outside of this unfavourable temperature window. At elevated temperatures above 200 C, the Ge substrate remains crystalline due to the recombination of mobile vacancies and interstitials.…”

Ion-beam synthesis and thermal stability of highly tin-concentrated germanium – tin alloys

“…Recently, Ge-Sn alloys have also been produced by ion beam synthesis using ion implantation and nanosecond pulsed laser melting (PLM) 6,7 . This fabrication method is potentially advantageous over standard Ge-Sn fabrication via CVD in that it may allow for higher Sn fraction, easier post fabrication pathway for layer lattice relaxation and device implementation.…”

“…% Sn has been achieved using this method, with excellent crystal quality except for some local pore formation that occurs after ion implantation 7 . These defective regions have been shown to be a consequence of ion-beam induced porosity in Ge, which also increases the loss of Sn during implantation 8 .…”