Control of Growth Modes by Carbon Mediation in Formation of Ge Quantum Dots on Si(100)

Abstract

Control and mechanism analysis of a Ge quantum dot (QD) formation on a Si(100) substrate by using carbon (C)-mediated c(4 × 4) surface reconstruction (SR) and solid-phase epitaxy (SPE) methods is demonstrated. The Si surface was reconstructed via the formation of C–Si bonds before Ge deposition in the SR method, while the QDs were formed by annealing of an amorphous Ge/C/Si heterostructure in the SPE method. In the SR method, the QDs grew in the Volmer–Wever (VW) mode at C = 0.25 and 0.50 monolayer (ML), and in the Stranski–Krastanov (SK) mode at C = 0.75 ML. The VW-mode QDs were formed owing to the c(4 × 4) surface reconstruction that acted like a virtual partition for Ge nucleation. At C = 0.5 and 0.75 ML, it was confirmed that the C–Ge bonds were formed near the Ge/Si interface because the unreacted excessive C atoms remained at the Ge/Si interface. The formation of C–Ge bonds induced the strain relief of the Ge layer and acted to change the growth mode to the SK mode at C = 0.75 ML. On the other hand, in the SPE method, the QDs grew in the VW mode at C = 0.1 and 0.25 ML due to the Ge aggregation, and in the SK mode at C ≥ 0.4 ML. It was found that a large number of C–Ge bonds owing to the incorporation of C into the Ge layer during the SPE induced the formation of a wetting layer. Therefore, the reduction of the strain energy in the Ge layer occurred at the low C coverage and induced the transition to the SK mode. These results suggest that the growth modes of the QDs via C mediation are controllable in both methods by changing the amount of C used as the mediation layer owing to the change in the C binding states at the Ge/Si interface or in the Ge layer.