Band-gap and strain engineering in GeSn alloys using post-growth pulsed laser melting

Abstract

The pseudomorphic growth of Ge1-xSnxon Ge causes in-plane compressive strain, which degrades the superior properties of the Ge1-xSnx alloys. Therefore, efficient strain engineering is required. In this article, we present strain and band-gap engineering in Ge1-xSnxalloys grown on Ge a virtual substrate using post-growth nanosecond pulsed laser melting (PLM). Micro-Raman and X-ray diffraction show that the initial in-plane compressive strain is removed. Moreover, for PLM energy densities higher than 0.5 J cm-2, the Ge0.89Sn0.11 layer becomes tensile strained. Simultaneously, as revealed by Rutherford Backscattering spectrometry, cross-sectional transmission electron microscopy investigations and X-ray diffraction the crystalline quality and Sn-distribution in PLM-treated Ge0.89Sn0.11 layers are only slightly affected. Additionally, the change of the band structure after PLM is confirmed by low-temperature photoreflectance measurements. The presented results prove that post-growth ns-range PLM is an effective way for band-gap and strain engineering in highly-mismatched alloys.

Evolution of point defects in pulsed-laser-melted Ge1-x Sn x probed by positron annihilation lifetime spectroscopy

“…Transmission electron microscopy (TEM) results show the formation of Sn-rich Ge 1-x Sn x filaments. The strain transition from compressive to tensile is confirmed by micro-Raman (µ-Raman) spectroscopy and discussed in detail in [7]. The presented post-growth PLM may be applied for the fabrication of strain-relaxed or even tensile-strained virtual substrates for the growth of strain-free Ge 1-x Sn x layers.…”

“…A 290 nm-thick Ge 0.89 Sn 0.11 layer was epitaxially grown by MBE on a (100)-oriented 4 inch p-type Si substrate with a sheet resistance of Rs = 10-20 Ω cm. The entire layer stack is shown in figure 1(a), and a detailed description of the process flow can be found elsewhere [7]. Before the growth of the Ge 0.89 Sn 0.11 layer, we grew a 50 nm-thick Si virtual substrate followed by a 100 nm-thick Ge virtual substrate and a 300 nm-thick Ge-buffer layer and performed an in-situ post-growth thermal treatment at 750 • C for 5 min.…”

“…Ge 1-x Sn x alloys are among the most promising materials to replace Si in future optoelectronics. These alloys enable an effective band gap engineering due to (i) alloying the indirect semiconductor Ge with the negative direct-band-gap material Sn [1,2]; (ii) strong n-type doping [3][4][5] or (iii) strain engineering [6,7]. The conversion from an indirect-to a direct-band-gap material was experimentally shown on partly strain-strain relaxed Ge 0.9 Sn 0.1 by Ghetmiri et al [8].…”

“…Stain relaxation can be achieved in multiple ways: (i) using buffer layers with a lattice parameter in the range of the targeted Ge 1-x Sn x alloy; (ii) the formation of misfit or threading dislocations due to the growth of layers thicker than the critical thickness for pseudomorphic growth [19,20]; (iii) alloy engineering by adding smaller elements into the Ge 1-x Sn x alloy system, e.g. Si [21], P [6], or B [11]; (iv) using post-growth thermal treatment [7]; (v) strain release by the fabrication of nano-structures [1,22,23]. In recent years, first success could be achieved by improving the growth process and using methods to gain partly strain-relaxed Ge 1-x Sn x alloys.…”

“…Recently, first results based on post-growth pulsed laser melting (PLM) on Ge 1-x Sn x alloys with a promising in-plane strain relaxation effect were published [7,24]. The implementation of strain-relaxed PLM Ge 1-x Sn x as a virtual substrate could be a helpful approach for further improvements.…”