Materials Science Reports最新文献

The growth quality of YBaCuO thin films deposited by sputtering on different substrates (Al₂O₃, MgO, SrTiO₃, Zr(Y)O₂) has been studied by X-ray diffraction and channeling experiments as a function of the deposition temperature. Besides the substrate orientation, the substrate temperature is the parameter determining whether films grow in c, a, (110) or mixed directions. Epitaxial growth correlates with high critical current values in the films of up to 5.5 × 10⁶ A/cm² at 77 K. Ultrathin films with thicknesses down to 2 nm were grown revealing three-dimensional superconducting behaviour. Films on (100) SrTiO₃ of 9 nm thickness and below are partially strained indicating commensurate growth. From the analysis of the surface disorder 0.5 displaced Ba atom per Ba₂Y row was obtained indicating that the disordered layer thickness is about 0.3 nm. Tunnel junctions fabricated on these films reveal gap-like structures near ±16 mV and ± 30 mV.

The technique of ion implantation is being investigated as a general method for altering the near-surface properties of insulating materials. The primary motivation behind these investigations is to develop ion implantation as a practical means of controlling and improving the near-surface mechanical, optical, or electronic properties of insulators. Changes in these properties depend on the microstructures and compositions developed in the material during the ion implantation process and subsequent thermal treatments. In many cases, structures and compositions can be produced by implantation and thermal annealing that cannot be achieved by conventional techniques. In this work, the response of a wide range of crystalline oxides to ion implantation and subsequent thermal processing will be reviewed. The materials treated here include Al₂O₃, LiNbO₃, CaTiO₃, SrTiO₃, ZnO, and MgO, as well as the non-oxide materials Si₃N₄ and SiC. The response of these insulators to ion implantation varies widely and depends on the specific material, the implantation species and dose, and the implantation temperature. Ion implantation produces displacement and other damage in the near-surface region, and in many cases, the surfaces of originally crystalline insulators are turned amorphous. Thermal annealing can often be used to restore crystallinity to the damaged near-surface region, and additionally, metastable solid solutions can be produced. For a number of oxide materials, the annealing behavior has been studied in detail using both Rutherford backscattering-ion channeling techniques and transmission electron microscopy. These studies show that, in some materials, the annealing behavior is quite simple and takes place by solid-phase epitaxial crystallization where the amorphous-to-crystalline transformation occurs at an interface that moves toward the free surface during the annealing process. In such materials, the regrowth kinetics have been measured, and the associated activation energies for crystallization have been determined. The formation of metastable solid solutions during crystallization of the amorphous phase will also be discussed.

In this review we have examined the crystallization behavior of a-Si over the temperaturerange from 500 °C to ∼ 1380°C. We have shown that SPE is a thermally activated process which is characterized by a single activation energy (2.7 eV) over the temperature range from ∼ 470 °C to ∼ 1350 °C. The activation energies for intrinsic SPE in ion-implanted and e-beam deposited layers on Si(100) substrates were found to be the same, implying that the interfacial bond breaking and rearrangement processes responsible for SPE in those layers is the same in spite of possible differences in microstructure. We showed that solid phase, transformations can occur in a-Si at temperatures far in excess of the a-Si melting point, T_al, and that the competition between solid phase crystallization and melting is a function of heating conditions (thermal rise-time, heating duration) and properties of the sample (amorphous film thickness). We evaluated kinetics of random nucleation and growth over a temperature range from 650 to 1380°C and showed that the random crystallization process is a well-behaved function of temperature over that temperature range with an activation energy of 4 eV. We found that random nucleation and growth becomes the dominant solid phase crystallization process at high temperatures ( > 1330 °C) in agreement with predictions based on differences between the activation energies for random crystallization and SPE.

The effects of doping and non-doping impurities on the kinetics of SPE and randomcrystallization were investigated as a function of temperature and impurity concentration. We showed that a variety of phenomena, including precipitation and impurity segregation can alter the intrinsic crystallization kinetics, and we identified time-temperature-concentration windows in which specific processes dominate. We concentrated on the investigation of crystallization behavior in layers containing impurities which illustrate the wide range of temperature- and concentration-dependent phenomena that can occur during heating of an amorphous thin film. Simple rate-enhancement and -retardation processes produced by doping (B and P) and non-doping impurities (F) were contrasted with the complex rate changes that can occur when processes such as impurity clustering and phase separation compete either individually or collectively with SPE in layers containing As, In and Au. We showed that random crystallization rates can be enhanced enormously by the presence of certain impurities and that impurity-enhanced nucleation can give rise to strong competition between random crystallization and SPE at temperatures much lower than observed in intrinsic layers. In the case of certain impurities (e.g. fluorine) the effects due to enhanced random crystallization in the high-temperature regime can be predicted from nucleation rate measurements performed at low temperatures. In other cases (e.g. arsenic), a different enhancement mecha