Nanocrystals of GeSn alloys in oxide matrix for optoelectronic applications

Obtaining direct bandgap in semiconductor alloys of the group IV elements is highly desired for photonic applications. GeSn alloys show a transition from Ge indirect to direct bandgap GeSn for compositions higher than 8% Sn... [ view full abstract ]

Obtaining direct bandgap in semiconductor alloys of the group IV elements is highly desired for photonic applications. GeSn alloys show a transition from Ge indirect to direct bandgap GeSn for compositions higher than 8% Sn [D. Stange et al. ACS Photon. 3, 1279 (2016)]. Additionally, alloying Ge with Sn features the possibility of tuning the bandgap of Ge to lower energies [P. Moontragoon, et al. Semicond. Sci. Technol. 22, 742 (2007)]. Challenges in obtaining direct bandgap GeSn is related to the low miscibility of Ge and Sn (less than 1%), as well as to the compressive strain in GeSn layers epitaxially grown on substrates as Si and Ge. However, metastable direct bandgap GeSn layers with up to 14%-16% Sn have been obtained and even the lasing effect was demonstrated [S. Wirths et al. Nat. Photonics 9, 88−92 (2015)].

We have investigated the properties of GeSn nanocrystals embedded in SiO₂ matrix. (Ge_1-xSn_x)_1-y(SiO₂)_y alloy layers were obtained by magnetron sputtering co-deposition on c-Si and fused quartz substrates. The Sn composition was varied between 9% and 16%, while the SiO₂ composition lies within a range of 0-17%. The as-deposited amorphous layers were subject to rapid thermal annealing resulting in segregation of GeSn and formation of nanocrystals at temperatures in the range of 350 ^oC-450 ^oC depending on the film composition. In Fig.1, X-Ray diffraction curves for two annealing temperatures at 400 ^oC and 450 ^oC (x=9%; y=9%) are shown. The formation of GeSn crystals with the Ge network structure but higher lattice constant of 5.58 A is evidenced. At 450 ^oC, a segregation of metallic beta Sn is observed. The size of the nanocrystals of 3-7 nm was evaluated from HRTEM imaging and XRD measurements. Optical transmittance and reflectance in VIS-IR range have been measured to study the optical absorption and optical bandgap for different compositions and annealing temperatures. In general, two bandgap values E_g1 and E_g2 are found (Fig.2). The obtained data dependence demonstrates the tunability of the optical bandgap of GeSn nanocrystals in oxide matrix as function of Sn composition, offering great potential for optoelectronic applications in MIR range.