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

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.