TEOS layers for low temperature processing of group IV optoelectronic devices

Abstract

The thermal budget is highly critical in processing the emerging group IV Silicon-Germanium-Tin (SiGeSn) optoelectronic devices. These emerging semiconductors exhibit a fundamental direct bandgap covering the mid-infrared range at Sn contents above 10 at. %, which is an order of magnitude higher than the equilibrium solubility. Consequently, the device processing steps must be carried out at temperatures low enough to prevent the degradation of these metastable layers. However, conventional optoelectronic device fabrication methods often require the deposition of dielectric layers at temperatures reaching 400 °C. Although this temperature can be sustained in processing a variety of devices, yet it is sufficiently high to damage GeSn and SiGeSn device structures. With this perspective, the authors investigated the morphological and optical properties of tetraethylorthosilicate (TEOS) layers as an alternative material to conventional dielectric layers. Spin-on-glass deposition on an Si wafer with baking temperatures in the 100–150 °C range leads to high homogeneity and low surface roughness of the TEOS layer. The authors show that the TEOS optical transmission is higher than 90% from visible to mid-infrared wavelengths (0.38–8 μm), combined with the analysis of the real and complex part of the refractive index. Furthermore, the TEOS deposition on GeSn and SiGeSn samples does not affect the material crystallinity or induces clustering of Sn atoms. Therefore, the low deposition temperature and high transparency make TEOS an ideal material for the integration of metastable GeSn and SiGeSn semiconductors in the fabrication of mid-infrared photonic devices.