Single Sige Quantum Dots And Quantum Ring The Electrical Properties Of Its Components Distribution

In this thesis,the electrical properties of individual self-assembled GeSi quantum dots(QDs) and quantum rings(QRs) grown by molecular beam epitaxy were investigated by scanning probe microscopy.The mechanism of the conductance distribution was discussed together with the composition distributions of individual QDs and QRs.The main results are as follows:Firstly,the conductance distribution of individual GeSi quantum dots(QDs) and quantum rings(QRs) during the transformation from QDs to QRs by depositing Si capping layer on QDs are studied by CAFM.Without Si capping,the QDs are dome-shaped and the current is higher at the ring region between the center and boundary than that at the center.With 0.32 nm Si capping,the QDs change to pyramid shape,which have higher current at the center and the arris than that at the facets, forming a bright cross-shape distribution.Further increasing the capping layer to 2 nm, QRs are formed,which have higher current at the rim than that at the central hole.By comparing with the composition distributions obtained by AFM combined with selective chemical etching,it is found that for the dome-shaped and pyramid-shaped QDs,the conductance distribution can be attributed to the composition distribution and also the geometric factor.But for QRs,the conductance distribution can not be attributed to the composition distribution,and the possible explanation may be relied on the ground state electron density distribution and/or the strain distribution.Secondly,the current images of GeSi QRs with different Ge contents were measured by CAFM.It was found that the conductance distributions of QRs with different compositions are all in ring-shape,and show no dependence on composition. The theoretical calculations showed that the ground state electrons in the QRs' adiabatic potential determined by the topography confinement were mainly distributed in the ring part,which will result in a higher conductance distribution at the ring parts.SCM results show that the carrier concentration is higher at the ring part than the ring center,according with the above concept.Also the EFM results show that it's easier to induce charges by an external electric field in the ring part of QRs,suggesting the higher electric field distribution at the ring part,which can further enhance the conductance there.Thirdly,the current distribution of the GeSi QRs was measured by CAFM at different normal forces between the tip and samples.Both the current images and I-V curves show that the conductance of both the QRs(ring region) and the wetting layer increases with the normal force.However when the normal force exceeds a threshold value,the conductance of the QRs decreased with normal force,while the conductance of the wetting layer still keeps on increasing to exceed the QRs.The possible explanation is as follows:the QRs were wear and tear seriously or deformation under the high normal force scanning in ambient,resulting in the change of the electronic states of the QRs.However,the conductance of the dome-shaped QDs sample increases with increasing normal force only.That may be because of the QDs height is much larger than the QRs,and little changes happened for the electronic states in the QDs since that the deformation under normal force or wear is negligible.Finally,some further research methods were suggested for the studies of QDs' properties.Both positioning technologies and cross-section polishing technologies like simplified procedures of TEM sample preparation were talked about.