Xinju Yang

Self-assembled SiGe quantum rings grown on Si(001) by molecular beam epitaxy

SiGe quantum rings (QRs) were grown by partially capping on Ge quantum dots (QDs) on Si(001). Atomic force microscopy images show the shape transformation from QDs to QRs. Initial capping, with a Si layer thickness less than 2 nm, will result in the decrease of height of QDs and increase of base diameter of QDs. Capped with a Si layer, QDs will change into QRs. The mechanism of transformation from QDs to QRs is discussed. The strain will redistribute after capping, thus the strain energy relief, together with high Ge surface diffusion and Ge surface segregation at a relative high temperature of 680u200a°C, play the dominant role.

Conductive atomic force microscopy studies on the transformation of GeSi quantum dots to quantum rings

Conductive atomic force microscopy has been employed to study the topography and conductance distribution of individual GeSi quantum dots (QDs) and quantum rings (QRs) during the transformation from QDs to QRs by depositing an Si capping layer on QDs. The current distribution changes significantly with the topographic transformation during the Si capping process. Without the capping layer, the QDs are dome-shaped and the conductance is higher at the ring region between the center and boundary than that at the center. After capping with 0.32 nm Si, the shape of the QDs changes to pyramidal and the current is higher at both the center and the arris. When the Si capping layer increases to 2 nm, QRs are formed and the current of individual QRs is higher at the rim than that at the central hole. By comparing the composition distributions obtained by scanning Auger microscopy and atomic force microscopy combined with selective chemical etching, the origin of the current distribution change is discussed.

Layer-dependent nanoscale electrical properties of graphene studied by conductive scanning probe microscopy

The nanoscale electrical properties of single-layer graphene (SLG), bilayer graphene (BLG) and multilayer graphene (MLG) are studied by scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM). The quantum capacitance of graphene deduced from SCM results is found to increase with the layer number (n) at the sample bias of 0 V but decreases with n at -3 V. Furthermore, the quantum capacitance increases very rapidly with the gate voltage for SLG, but this increase is much slowed down when n becomes greater. On the other hand, the magnitude of the EFM phase shift with respect to the SiO2 substrate increases with n at the sample bias of +2 V but decreases with n at -2 V. The difference in both quantum capacitance and EFM phase shift is significant between SLG and BLG but becomes much weaker between MLGs with a different n. The layer-dependent quantum capacitance behaviors of graphene could be attributed to their layer-dependent electronic structure as well as the layer-varied dependence on gate voltage, while the layer-dependent EFM phase shift is caused by not only the layer-dependent surface potential but also the layer-dependent capacitance derivation.

Effects of a native oxide layer on the conductive atomic force microscopy measurements of self-assembled Ge quantum dots

The electrical properties of self-assembled Ge quantum dots were investigated by using conductive atomic force microscopy (CAFM). It was found that the conductive properties of quantum dots were strongly affected by the native oxide layer formed on the quantum dots. With the existing oxide layer, the current–voltage curves of the quantum dots obviously depended on the applied normal forces, and all the curves could be well fitted by the Fowler–Nordheim tunnelling model by changing the oxide layer thicknesses. After the oxide layer was removed by HF etching, the current–voltage characteristics could be well explained by the Schottky emission model. But when the etched sample was exposed to air for more than 150 min, its current–voltage behaviour returned back to that before the etching. Despite the significant effects of the oxide layer on the current–voltage characteristics, the current distributions of individual quantum dots remained almost the same for different thicknesses of the oxide layer or different normal forces applied. Therefore, the effects of the oxide layer on the current distribution can be ignored, though the absolute current values were strongly affected. Our results thus provide important evidence for the reliability of CAFM measurements in ambient conditions.

Shape change of SiGe islands with initial Si capping

The morphologies of self-assembled Ge/Si(001) islands with initial Si capping at a temperature of 640u2009°C are investigated by atomic force microscopy. Before Si capping, the islands show a metastable dome shape with very good size uniformity. This dome shape changes to a pyramid shape with {103} facets at a Si capping thickness of 0.32u2004nm, and then changes to pyramid shapes with {104} and {105} facets at Si capping thicknesses of 0.42 and 0.64u2004nm, respectively. Noteworthy is that islands with one side retained their dome shape while the other three sides that changed to {103} facets are observed at a Si capping thickness of 0.18u2004nm. These observations indicate that island shape change with Si capping is a kinetic rather than thermodynamic process. The atomic processes associated with this island shape change are kinetically limited at a low temperature of 400u2009°C, and no significant change in size and shape of islands is observed when Si capping layers are deposited at this temperature.

Nanoscale electrical property studies of individual GeSi quantum rings by conductive scanning probe microscopy

The nanoscale electrical properties of individual self-assembled GeSi quantum rings (QRs) were studied by scanning probe microscopy-based techniques. The surface potential distributions of individual GeSi QRs are obtained by scanning Kelvin microscopy (SKM). Ring-shaped work function distributions are observed, presenting that the QRs rim has a larger work function than the QRs central hole. By combining the SKM results with those obtained by conductive atomic force microscopy and scanning capacitance microscopy, the correlations between the surface potential, conductance, and carrier density distributions are revealed, and a possible interpretation for the QRs conductance distributions is suggested.

Bias-dependent conductive characteristics of individual GeSi quantum dots studied by conductive atomic force microscopy

The bias-dependent electrical characteristics of individual self-assembled GeSi quantum dots (QDs) are investigated by conductive atomic force microscopy. The results reveal that the conductive characteristics of QDs are strongly influenced by the applied bias. At low (-0.5 to - 2.0 V) and high (-2.5 to - 4.0 V) biases, the current distributions of individual GeSi QDs exhibit ring-like and disc-like characteristics respectively. The current of the QDs central part increases more quickly than that of the other parts as the bias magnitude increases. Histograms of the magnitude of the current on a number of QDs exhibit the same single-peak feature at low biases, and double- or three-peak features at high biases, where additional peaks appear at large-current locations. On the other hand, histograms of the magnitude of the current on the wetting layers exhibit the same single-peak feature for all biases. This indicates the conductive mechanism is significantly different for QDs and wetting layers. While the small-current peak of QDs can be attributed to the Fowler-Nordheim tunneling model at low biases and the Schottky emission model at high biases respectively, the large-current peak(s) may be attributed to the discrete energy levels of QDs. The results suggest the conductive mechanisms of GeSi QDs can be regulated by the applied bias.

Thermally oxidized formation of new Ge dots over as-grown Ge dots in the Si capping layer

A Si-capped Ge quantum dot sample was self-assembly grown via Stranski-Krastanov mode in a molecular beam epitaxy system with the Si capping layer deposited at 300u2009°C. After annealing the sample in an oxygen atmosphere at 1000u2009°C, a structure, namely two layers of quantum dots, was formed with the newly formed Ge-rich quantum dots embedded in the oxidized matrix with the position accurately located upon the as-grown quantum dots. It has been found that the formation of such nanostructures strongly depends upon the growth temperature and oxygen atmosphere. A growth mechanism was proposed to explain the formation of the nanostructure based on the Ge diffusion from the as-grown quantum dots, Ge segregation from the growing oxide, and subsequent migration/agglomeration.

Fabrication of Straight Silicon Nanowires and Their Conductive Properties

Straight Si nanowires (Si NWs) with tens to hundreds of micrometers in length and 40–200xa0nm in diameter are achieved by annealing a Si substrate coated with metallic Fe. The influences of annealing gas and temperature on the formation of Si NWs are investigated. It is found that the annealing gas has significant impacts on the microstructure of the NWs. By increasing the hydrogen ratio in the forming gas, straight and crystal Si NWs with thin oxide shells are obtained. Both the conductive properties along and perpendicular to the NW are investigated by conductive atomic force microscopy (CAFM) on single NWs. The conductance perpendicular to the NW is too poor to be detected, while a weak conductance can be measured along the NW. The results indicate that the measured resistance mainly comes from the contact(s), and the Si NWs exhibit typical semiconductive conductance themselves, which should have potential applications in nanoelectronics.

Electrical properties of individual self-assembled GeSi quantum rings

The nanoscale electrical properties of self-assembled GeSi quantum rings (QRs) were investigated by conductive scanning probe microscopy at room temperature. The current distribution of individual GeSi QRs measured by conductive atomic force microscopy (CAFM) shows a low conductivity at the central hole as compared to the rim; however, the QRs’ composition distribution obtained by selective chemical etching combined with AFM observation reveals that within the QRs’ central holes, the Ge content is high, which should lead to a high conductivity instead of a low one as observed. Together with the results obtained by scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM), it is supposed that the GeSi QRs’ electrical properties are mainly determined by the ring-shaped topography, rather than by the complete oxidation of the QRs’ central hole or their composition distributions.