Growth of Core-Shell Silicon Quantum Dots in Borophosphosilicate Glass Matrix – Raman and Transmission Electron Microscopic Studies

Annealing silicon (Si)-rich borophosphosilicate glass (BPSG) at a high temperature results in the growth of core-shell Si quantum dots (QDs) composed of a boron (B) and phosphorus (P) codoped crystalline Si core and an amorphous shell made from B, Si and P (B and P codoped Si QDs) in a BPSG matrix. The amorphous B_xSi_yP_z shell is responsible for many superior properties of codoped Si QDs such as hydrophilicity, high resistance to hydrofluoric acid (HF) etching, stable luminescence in different environment, robustness of the luminescence for chemical treatments, etc. In this work, we study the growth process of the amorphous shell by Raman spectroscopy and transmission electron microscopy. We show that amorphous Si particles are first grown in a BPSG matrix within 30 s of annealing of Si-rich BPSG. After 50 s annealing, a crystalline Si core appears within an amorphous Si particle. The formation of a crystalline Si core is accompanied by the formation of an amorphous B_xSi_yP_z shell. From the annealing time dependence of the volumes of the core and the shell, we show that supersaturated B and P are expelled to the surface of a crystalline Si core during the growth, which increases B and P concentration in an amorphous B_xSi_yP_z shell.