J. H. Lin

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 640 °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 0.32 nm, and then changes to pyramid shapes with {104} and {105} facets at Si capping thicknesses of 0.42 and 0.64 nm, 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 0.18 nm. 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 400 °C, and no significant change in size and shape of islands is observed when Si capping layers are deposited at this temperature.

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.

Formation of planar defects over GeSi islands in Si capping layer grown at low temperature

Coherently strained GeSi/Si(001) islands were overgrown with a Si capping layer of different thicknesses at temperature 300 °C. The structures of the islands and the Si capping layer were investigated by high resolution transmission electron microscopy. The shapes of the embedded islands were well preserved, whereas planar defects were observed exactly over the islands in the capping layers. The strain energy in regions over the islands accumulated with increasing thickness of the Si capping layer, resulting in the formation of the planar defects. By means of a two-step deposition in which 20-nm-thick Si capping layer was first deposited at a low temperature of 300 °C followed by 70-nm-thick Si capping layer deposition at a high temperature of 640 °C, the defect-free Si capping layer with flat surface can be obtained.

Strain analysis of Ge/Si(001) islands after initial Si capping by Raman spectroscopy

The shape of the self-assembled GeSi/Si(001) islands changed from a dome to a pyramid bounded with {103} or {105} facets after initial Si capping at 640 °C. The strains in the islands with initial Si capping are investigated by Raman spectroscopy. Compared with those of the uncapped islands, both peaks of Ge-Ge and Ge-Si vibration modes in the capped islands show blueshifts, corresponding to the Ge content decrease and the compressive strain increase in the capped islands. The total strain energy in an island is found to increase remarkably after Si capping. After simple analysis, it is found that the surface energy change could not overwhelm this large strain energy increase, making the shape transition favorable. It implies that the strain energy in the substrate in association with an island formation as well as evolution should be considered in accounting for the resulting island shape changes after Si capping.

The stability of faceted SiGe quantum dots capped with a thin Si layer

Detailed surface morphologies and facets of partially capped Ge/Si(001) quantum dots were investigated by atomic force microscopy for samples capped with different thicknesses and subsequently annealed in a desiccator for a period of 12 months. The pyramid-shaped quantum dots bounded by {103}, {104} and {105} facets were observed in as-capped samples. After annealing, the {104} and {105} facets remained, while the {103} facets changed their structural profile to {104} facets. Extensive atomic force microscopy and transmission electron microscopy observations confirmed this finding. It is believed that the high surface free energy of the {103} facets and pre-existing Ge atoms in the wetting layer are responsible for this surface evolution.

Increased conductance of individual self-assembled GeSi quantum dots by inter-dot coupling studied by conductive atomic force microscopy

The conductive properties of individual self-assembled GeSi quantum dots (QDs) are investigated by conductive atomic force microscopy on single-layer (SL) and bi-layer (BL) GeSi QDs with different dot densities at room temperature. By comparing their average currents, it is found that the BL and high-density QDs are more conductive than the SL and low-density QDs with similar sizes, respectively, indicating the existence of both vertical and lateral couplings between GeSi QDs at room temperature. On the other hand, the average current of the BL QDs increases much faster with the bias voltage than that of the SL QDs does. Our results suggest that the QDs’ conductive properties can be greatly regulated by the coupling effects and bias voltages, which are valuable for potential applications.