Bandgap Engineering of Ge Nanocrystals by Size Controlled Deposition and Doping

Abstract

Germanium nanocrystals (nc) offer promising properties for different applications in electronics and photonics, like non-volatile memories [1] or photodiodes for the visible or infrared wavelength region [2, 3]. For those applications, a spatially and size controlled synthesis is necessary to enable for example electrical isolation between the nanocrystals or the proper adjustment of the optical bandgap. Recent work has shown that the material systems Ge/TaZrOx, Ge/SiO2 and Ge/TaZrOx/SiO2 fulfil these requirements. To form size controlled nc, a Ge rich oxide layer like SiO2 or TaZrOx and pure oxide layer are deposited alternatingly via co-sputtering. Subsequent rapid thermal annealing forms the Ge nc by phase separation and crystallization. During the annealing process the pure TaZrOx interlayers act as a diffusion barrier for Ge, so these nc are confined to the thickness of the deposited mixed layer. The addition of a pure SiO2 interlayer leads to enlarged nc size by growth of the nc into the SiO2 layer. The use of an additional SiO2 layer has the further advantage of improved lateral nc separation in comparison to structures without the SiO2 interlayers. This leads to an improved electrical isolation of each nc. Still the bandgap of these nc can be adjusted easily by their size due to the quantum confinement effect. Another advantage of Ge is the full compatibility to Si based processes. However, both Si and Ge has an indirect band gap, so the efficiency of e.g. light emitting diodes is low. The fabrication of Sn containing Ge can lead to a direct bandgap [4]. In our work we show the formation of Sn doped Ge nc in TaZrOx and SiO2, to create nc of controlled size. Our focus is the crystallization process of the nc, to avoid formation of pure Sn crystals, but still achieve a complete separation of the Ge-Sn alloy from the oxides. Samples with different Sn concentrations are annealed in a range of 500 °C to 800 °C and checked by Raman scattering and TEM. Furthermore, the influence of the used oxide on the nc crystallization temperature is investigated. [1] Tiwari, S. Appl. Phys. Lett. 1996, 69, 1232 [2] Haas, S. J. Appl. Phys. 2013, 113, 44303 [3] Lehninger et al. Phys . Stat. Sol. (a), 2018, 155, 1701028 [4] Slav, A. et al. ACS Appl.NanoMater. 2019, 2, 3626−3635