J. Cui

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 680 °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.

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.

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.

Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown by molecular beam epitaxy

The size uniformity of self-assembled SiGe quantum rings, which are formed by capping SiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growth temperature and the areal density of SiGe quantum dots. Higher growth temperature benefits the size uniformity of quantum dots, but results in low Ge concentration as well as asymmetric Ge distribution in the dots, which induces the subsequently formed quantum rings to be asymmetric in shape or even broken somewhere in the ridge of rings. Low growth temperature degrades the size uniformity of quantum dots, and thus that of quantum rings. A high areal density results in the expansion and coalescence of neighboring quantum dots to form a chain, rather than quantum rings. Uniform quantum rings with a size dispersion of 4.6% and an areal density of 7.8×10(8) cm(-2) are obtained at the optimized growth temperature of 640°C.

Ordered GeSi nanorings grown on patterned Si (001) substrates

AbstractAn easy approach to fabricate ordered pattern using nanosphere lithography and reactive iron etching technology was demonstrated. Long-range ordered GeSi nanorings with 430 nm period were grown on patterned Si (001) substrates by molecular beam epitaxy. The size and shape of rings were closely associated with the size of capped GeSi quantum dots and the Si capping processes. Statistical analysis on the lateral size distribution shows that the high growth temperature and the long-term annealing can improve the uniformity of nanorings. PACS code1·PACS code2·more Mathematics Subject Classification (2000) MSC code1·MSC code2·more

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.

Formation of Nanopits in Si Capping Layers on SiGe Quantum Dots

In-situ annealing at a high temperature of 640°C was performed for a low temperature grown Si capping layer, which was grown at 300°C on SiGe self-assembled quantum dots with a thickness of 50 nm. Square nanopits, with a depth of about 8 nm and boundaries along 〈110〉, are formed in the Si capping layer after annealing. Cross-sectional transmission electron microscopy observation shows that each nanopit is located right over one dot with one to one correspondence. The detailed migration of Si atoms for the nanopit formation is revealed by in-situ annealing at a low temperature of 540°C. The final well-defined profiles of the nanopits indicate that both strain energy and surface energy play roles during the nanopit formation, and the nanopits are stable at 640°C. A subsequent growth of Ge on the nanopit-patterned surface results in the formation of SiGe quantum dot molecules around the nanopits.

Growth and memory effect of Er-stabilized β-MnO2 films grown on Si substrates

A memory effect is reported for Er-stabilized beta-MnO2 films made of highly orientation-aligned textured nanocrystals. The films are composed of nanocrystals with a size of about 20 nm. The crystalline direction along the growth direction is almost along beta-MnO2 , but the one in the plane is disordered. Er doping can effectively enhance the thermal stability of beta-MnO2 up to 850 degrees C, which is essential for its future application in industry. A memory effect has been observed for both as-grown and annealed samples. The mechanism of the memory effect was found by analysis to be charge trapping by carrier injection, from either the bottom or the top electrode. For the annealed sample, a low leakage current was achieved, which is about 5 orders of magnitude smaller than that of the as-grown sample. The results show that beta-MnO2 is a promising candidate material for nonvolatile memory applications.

Hall resistivity of Fe doped Si film at low temperatures

Nonmonotonically magnetic-field-dependent and sensitively temperature-dependent Hall resistivity of Fe doped Si film has been systematically studied at low temperatures. Two-band of holes conduction mechanism is proposed to be responsible for the observed extraordinary Hall resistivity, as well as magnetoresistance characteristics. Holes in the valence band are generated by thermal activation of electrons from the valence band to shallow acceptor levels with an activation energy of 41.2 meV while holes in acceptor impurity band transport by hopping processes with an activation energy of 2.5 meV. This work shows that even very complicated behavior of Hall resistivity may be understood under a two-band conduction mechanism.