Self-assembled quantum dots (QDs), one kind of the nano-structures, have attracted intense interests and been widely studied for promising applications in low-dimensional optical and electronic devices. Full understandings of the composition profile of self-assembled QDs and of one single QD are of fundamental importance. Several techniques such as TEM and XRD etc. had been applied to analysis the QDs, but giving area-averaged information containing an ensemble of dots.
In this study we use high-resolution field emission Auger electron microscopy (AES) and Secondary electron microscopy (SEM) to investigate the surface topography and distribution of the lateral composition of Ge QDs grown by molecular beam epitaxy on Si(001) substrate. We have investigated two types of samples grown at 550 degree centigrade (sample A) and 640 degree centigrade (sample B), respectively. Our results demonstrated that the dot lateral composition is neither pure Ge nor Si nor homogeneous GexSi1-x, but the mixture of Si and Ge. The composition distribution is asymmetric in one dome-shaped dot, showed lower Ge concentration in the periphery part and higher Ge concentration in the center part. Domes grown at 640 degree centigrade show different composition distribution features even in same shaped domes, which are supported by the selective etching experiments and conductive atomic force microscopy (C-AFM) results in our pre-work.
This noticeable difference can be attributed to the different degrees of Si alloying into the Ge QDs at different growth temperatures. Sample A was deposited at 550 degree centigrade followed by an immediate cooling down to room temperature after Ge layer deposition, thus only a little amount of Si is alloyed into the Ge dots. On the contrary, sample B was deposited at 640 degree centigrade and held at this temperature for 5 min before cooling down, thus a large amount of silicon was alloyed into the quantum dot.
Mapping the composition distribution of Ge and Si on one single dot by high resolution AES in nano-scale is demonstrated for the first time.
|