J. Y. Wang

Wetting and crystallization at grain boundaries: Origin of aluminum-induced crystallization of amorphous silicon

It has been shown experimentally that the grain boundaries in aluminium in contact with amorphous silicon are the necessary agents for initiation of the crystallization of silicon upon annealing temperatures as low as 438K. Thermodynamic analysis has shown (i) that Si can “wet” the Al grain boundaries due to the favorable Si∕Al interface energy as compared to the Al grain-boundary energy and (ii) that Si at the Al grain boundaries can maintain its amorphous state up to a thickness of about 1.0nm. Beyond that thickness crystalline Si develops at the Al grain boundaries.

Mechanism of aluminum-induced layer exchange upon low-temperature annealing of amorphous Si/polycrystalline Al bilayers

The aluminum-induced layer exchange (ALILE) process occurring upon annealing amorphous Si/polycrystalline Al bilayers (a-Si/c-Al) has been observed at a temperature as low as 165u2009°C. The diffusion length of Si along Al grain boundaries is proposed as a tool for determining the annealing conditions, i.e., temperature and time, for the occurrence of the c-Al→c-Si layer exchange. Analysis of the local and global energy changes upon layer exchange reveals that a tiny driving force controls the kinetics of layer exchange and leads to a general interpretation of the mechanism of the ALILE process.

A new method for the determination of the diffusion-induced concentration profile and the interdiffusion coefficient for thin film systems by Auger electron spectroscopical sputter depth profiling

A new Auger electron spectroscopical sputter depth profiling method was developed to determine the interdiffusion coefficient for the initial stage of diffusion annealing of thin films. The method is based on (i) adoption of an interdiffusion model appropriate for the specimen investigated and (ii) convolution of an accordingly calculated diffusion-induced concentration profile with the smearing effects due to atomic mixing, surface/interface roughness, escape depth of the Auger electrons, and preferential sputtering. The diffusion-induced concentration profile and the interdiffusion coefficient are determined by fitting in an iterative least-squares procedure of the calculated Auger electron spectroscopical depth profile to the measured one. The method was applied to bilayered and multilayered structures, exhibiting dominant grain-boundary diffusion and dominant volume diffusion, respectively. A very small extent of interdiffusion, characterized by diffusion distances as small as 1 nm, could be quantified.

The initial stage of the reaction between amorphous silicon and crystalline aluminum

The initial stage of crystallization of amorphous silicon in Al∕Si and Si∕Al bilayers was investigated by x-ray diffraction analysis and Auger electron spectroscopy. The bilayers initially consist of amorphous silicon and crystalline aluminum, produced by sputter deposition. The microstructural and compositional changes occurring in the Al∕Si and Si∕Al bilayers were investigated extensively at 165u2009°C as a function of the time from half an hour to 30 days. Upon annealing, mass transport across the original bilayer interface occurred and amorphous silicon could crystallize into aggregates of nanocrystals with {111} planes oriented preferentially parallel to the surface. The kinetics of the process depends on the sublayer sequence in the bilayers. Residual stress, lattice microstrain, and crystallite size of both the Al phase and the crystallized Si phase were measured quantitatively. These data allowed the assessment of the Gibbs energy changes occurring upon annealing. It was shown that grain boundaries in...

Metal-catalyzed growth of semiconductor nanostructures without solubility and diffusivity constraints

and vapor–solid–solid (VSS) [ 11–14 ] mechanisms, requiring substantial solubility and diffusivity of the semiconductor in the catalyst and thus high growth temperatures. The present study reveals, using in situ heating electron microscopy, that metal-catalyzed growth of semiconductor nanostructures can be realized without these constraints, thereby enabling strikingly low growth temperatures. The growth mechanism has been unraveled at the atomic scale for the Al-catalyzed growth of Si nanostructures at 150 ° C. Growth starts with wetting of high-angle grain boundaries (GBs) in the Al catalyst, by Si in its amorphous form, and continues by nucleation and growth of nanostructured crystalline Si (c-Si) in the template as precisely defi ned by the Al grain boundary network. The disclosed mechanism breaks solubility and diffusivity limits that have hitherto dictated selection of metal catalysts and growth temperatures and thereby opens new perspectives for direct fabrication of nanostructure devices on heat-sensitive substrates. The fundamental understanding of the metal-catalyzed growth mechanisms has been advanced by recent developments of in situ heating electron microscopy techniques. [ 12–15 ] Metalcatalyzed VLS growth of semiconductor nanowires proceeds by the precipitation of semiconductor material out of metal-semiconductor eutectic melt droplets, which are supersaturated by ceaseless exposure to gas-phase semiconductor reactants above the eutectic temperature. [ 8–10 ] More recent works have demonstrated that semiconductor nanowires can also be grown via a VSS growth mechanism below the eutectic temperature, at which the