Dayan Ma

Multi-band silicon quantum dots embedded in an amorphous matrix of silicon carbide.

Silicon quantum dots embedded in an amorphous matrix of silicon carbide were realized by a magnetron co-sputtering process and post-annealing. X-ray photoelectron spectroscopy, glancing x-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy were used to characterize the chemical composition and the microstructural properties. The results show that the sizes and size distribution of silicon quantum dots can be tuned by changing the annealing atmosphere and the atom ratio of silicon and carbon in the matrix. A physicochemical mechanism is proposed to demonstrate this formation process. Photoluminescence measurements indicate a multi-band configuration due to the quantum confinement effect of silicon quantum dots with different sizes. The PL spectra are further widened as a result of the existence of amorphous silicon quantum dots. This multi-band configuration would be extremely advantageous in improving the photoelectric conversion efficiency of photovoltaic solar cells.

Stress-induced annihilation of Stone–Wales defects in graphene nanoribbons

Stress arising from structural or thermal misfit impacts the reliability of graphene-related devices. The deformation behaviour of graphene nanoribbons (GNRs) with Stone?Wales defects under stress studied by molecular dynamics shows that nearly all the SW defects annihilate via inverse rotation of C?C bonds. The fracture stress of defective GNRs is comparable to that of perfect ones and similar to mechanical annealing observed from bulk metals. It is a competition between bond rotation and fracture and depends on the strain rate and temperature. At a lower strain rate, such as 10?5?ps?1, the rotation velocity of C?C bonds of 4.2???ps?1 is three orders of magnitude larger than the velocity of the collective movement of atoms (1.2???10?3???ps?1). There is enough time for the C?C bond rotation to respond to the external load, but it becomes more difficult at higher strain rates. This stress-induced SW defect annihilation can be enhanced at higher temperatures because of enhanced exchange of atomic momentum and energy. The results reveal the dominant influence of SW defects on the mechanical properties of two-dimensional materials.

Diffusion-controlled formation mechanism of dual-phase structure during Al induced crystallization of SiGe

Aluminum induced crystallization of amorphous SiGe at low temperature is studied and a dual-phase stacked structure with different compositions emerges when the annealing temperature is higher than a critical value. This behavior is very sensitive to the oxidization state of the interlayer. A model based on energetics is proposed to elucidate this temperature dependent behavior. Thermodynamically, it can be ascribed to the competition between grain-boundary-mediated and interface-mediated crystallization and kinetically, it stems from the different diffusion rates of Si and Ge. The results are useful to the design and fabrication of high-efficiency solar cells.

Synthesis and stress relaxation of ZnO/Al-doped ZnO core–shell nanowires

Doping nanostructures is an effective method to tune their electrical and photoelectric properties. Taking ZnO nanowires (NWs) as a model system, we demonstrate that atomic layer deposition (ALD) can be adopted for the realization of a doping process by the homo-epitaxial growth of a doped shell on the NW core. The Al-doped ZnO NWs have a layered superlattice structure with dopants mainly occupying the interstitial positions. After annealing, Al(3+) ions diffuse into the ZnO matrix and occupy substitutional locations, which is desirable for dopant activation. The stress accumulated during epitaxial growth is relaxed by the nucleation of dislocations, dislocation dipoles and anti-phase boundaries. We note that the proposed method can be easily adopted for doping different types of nanostructures, and fabricating superlattices and multiple quantum wells on NWs in a controllable way.

Preferred armchair edges of epitaxial graphene on 6H-SiC(0001) by thermal decomposition

Scanning tunneling microscopy is used to study the edge orientation of graphene fabricated by thermal decomposition of 6H-SiC. The exploration on the atomically resolved structures and the patterns in reciprocal space demonstrates that the armchair direction is always parallel to the basic vector of 6 × 6 reconstruction as well as the close-packed direction of 6H-SiC substrate. This can be used as the criterion to characterize the edge direction of graphene. With this method, it is found that armchair edges are preferred in both monolayer and bilayer regions. This special edge certainly will affect the electronic states and consequently the properties.

High-quality, single-layered epitaxial graphene fabricated on 6H-SiC (0001) by flash annealing in Pb atmosphere and mechanism

High-quality epitaxial graphene is produced on silicon carbide by flash annealing of 6H-SiC in a lead (Pb) atmosphere at ∼1400 °C for 30 s. Nearly three top bilayers of SiC are decomposed due to fast heating and cooling, and sublimation of Si atoms from SiC is retarded by the Pb atmosphere. The synergetic effects promote the growth of continuous single-layered graphene sheets on the SiC terraces, and a model is established to elucidate the effects and growth mechanism.

Impact of ultrathin Al2O3 interlayer on thermal stability and leakage current properties of TiO2/Al2O3 stacking dielectrics

Ultrathin TiO2/Al2O3 stacking structures were fabricated using an atomic layer deposition technique. The effect of the ultrathin Al2O3 interlayer on interfacial thermal stability and leakage current properties were studied. After thermal annealing of the TiO2/Al2O3/TiO2/Al2O3/Si structure at 700 °C for 60 s, the Al2O3 double layers remained amorphous, although the layers of TiO2 were crystallized. The amorphous Al2O3 divided the grain boundaries which would otherwise serve as diffusion paths for atoms and as leakage current channels from the TiO2 layers. As a result, atomic diffusion and surface roughness were suppressed, and the leakage current value was reduced by about a 1.5 order of magnitude compared with TiO2/Al2O3/Si. The improved interfacial stability as well as the reduced leakage current density indicates the present stacking structure has potential application in future high-performance microelectronics.

Evidence of atomically resolved 6×6 buffer layer with long-range order and short-range disorder during formation of graphene on 6H-SiC by thermal decomposition

Scanning tunneling microscopy (STM) is performed to study the formation mechanism of graphene on 6H-SiC by thermal decomposition in situ and the evolution of an atomically resolved 6×6 structure in the buffer layer is revealed. The long-range order of the 6×6 structure is maintained during growth, but the short-range arrangement changes with temperature. Based on STM images acquired at different voltages, a structure consisting of triangular silicon clusters with the 6×6 structure and filled by amorphous carbon atoms is proposed. The 6×6 silicon clusters serve as the template and amorphous carbon atoms provide the carbon source for graphene growth.

Selective growth of Pb islands on graphene/SiC buffer layers

Graphene is fabricated by thermal decomposition of silicon carbide (SiC) and Pb islands are deposited by Pb flux in molecular beam epitaxy chamber. It is found that graphene domains and SiC buffer layer coexist. Selective growth of Pb islands on SiC buffer layer rather than on graphene domains is observed. It can be ascribed to the higher adsorption energy of Pb atoms on the 63 reconstruction of SiC. However, once Pb islands nucleate on graphene domains, they will grow very large owing to the lower diffusion barrier of Pb atoms on graphene. The results are consistent with first-principle calculations. Since Pb atoms on graphene are nearly free-standing, Pb islands grow in even-number mode.

Thermodynamics and kinetic behaviors of thickness-dependent crystallization in high-k thin films deposited by atomic layer deposition

Atomic layer deposition is adopted to prepare HfO2 and Al2O3 high-k thin films. The HfO2 thin films are amorphous at the initial growth stage, but become crystallized when the film thickness (h) exceeds a critical value ( hcritical*). This phase transition from amorphous to crystalline is enhanced at higher temperatures and is discussed, taking into account the effect of kinetic energy. At lower temperatures, the amorphous state can be maintained even when h>hcritical* owing to the small number of activated atoms. However, the number of activated atoms increases with the temperature, allowing crystallization to occur even in films with smaller thickness. The Al2O3 thin films, on the other hand, maintain their amorphous state independent of the film thickness and temperature owing to the limited number of activated atoms. A thermodynamic model is proposed to describe the thickness-dependent phase transition.