Optimization of Si-C reaction temperature and Ge thickness in C-mediated Ge dot formation

To form Ge dots on a Si substrate, the effect of thermal reaction temperature of sub-monolayer C with Si (100) was investigated and the deposited Ge thickness was optimized. The samples were prepared by solid-source molecular beam epitaxy with an electron-beam gun for C sublimation and a Knudsen cell for Ge evaporation. C of 0.25 ML was deposited on Si (100) at a substrate temperature of 200 °C, followed by a high-temperature treatment at the reaction temperature (T_R) of 650-1000 °C to create Si-C bonds. Ge equivalent to 2 to 5 nm thick was subsequently deposited at 550 °C. Small and dense dots were obtained for T_R = 750 °C but the dot density decreased and the dot diameter varied widely in the case of lower and higher T_R. A dot density of about 2 × 10¹⁰ cm^-2 was achieved for Ge deposition equivalent to 3 to 5 nm thick and a standard deviation of dot diameter was the lowest of 10 nm for 5 nm thick Ge. These results mean that C-mediated Ge dot formation was strongly influenced not only by the c(4 x 4) reconstruction condition through the Si-C reaction but also the relationship between the Ge deposition thickness and the exposed Si (100)-(2 x 1) surface area.