The Growth Mechanism And Nucleation Theory Of Semiconductor Nanomaterials

The conventional film technologies have run into interfacial lattice mismatch issues that often result in highly defective optical materials. The unique one-dimensionality inherent to nanowires has already solved some of the long-standing technical problems that have plagued the thin film community. In this regard, nanowires growth provides a natural mechanism for relaxing the lattice strain at the interface and enables dislocation-free semiconductor growth on lattice mismatched substrates. The unwanted Au contamination has led to many attempts to develop catalyst-free growth procedures by a lot of research groups in the world. Understanding their growth mechanism as well as nucleation theory is crucial for the fabrication of high-quality materials.The research in this dissertation is supported by grants from The National Basic Research Program of China (No:2010CB314900), which professor Xiaomin Ren is responsible for as a chief scientist, Key Program Project of the National Natural Science Foundation of China(No:90201035), The111Project(No:B07005), The National High Technology Research and Development Program of China(No:2007AA03Z418)。The Russian Federal Agency for Science and Innovation(No:02.740.11.0383), The Russian Foundation for Basic Research(No:11-02-00727-a), The Scientific Programs of Russian Academy of Sciences(No:10-02-93107-a), which professor Dubrovskii is responsible for them.In this dissertation, a great deal of research work can be described as follow:theoretical and experimental studies on the growth of inclined nanowires by Molecular Beam Epitaxy, study of elastic energy relaxation and plastic deformation in nanostructures on lattice mismatched substrates, stress-driven nucleation theory of three-dimensional nanostructures, the nucleation theory of quantum dots on the vicinal substrates. The main achievements are listed as follows.1. Theoretical model for the growth of inclined semiconductor nanowires via Molecular Beam Epitaxy deposition is developed. General expression for the nanowires growth as function of its radius and the growth condition is obtained and analyzed. Growth experiments are carried out on the GaAs(211)A and GaAs(111)B substrates. It is found out that20°inclined nanowires are two times longer in average, which is explained by a larger impingment rate on their sidewalls. Supersaturations and diffusion lengths of surface and sidewall adatoms are also estimated by fitting the theoretical model to the experimental data. The obtained results show the importance of sidewall adatoms in the MBE growth of nanowires.2. An analytical expression of elastic strain energy relaxation in nanostructures of different isotropic geometries grown on lattice mismatched substrates is obtained. The results are derived from large aspect ratio condition. The differences of the elastic strain energy relaxation in cylindrical nanostrucuture on the rigid and elastic substrate are compared. A dislocation model is considered to analyze the competition between the elastic and dislocation energies. We calculate out the critical diameter and critical thickness below which the plastic deformation is energetically suppressed. Our data indicate that the elastic energy is highly depended on the island shape, with the relaxation becoming faster as the contact angle increases. The analytical model is compared with experimental and numerical results.3. The model of stress-driven nucleation of three-dimensional nanostructures in lattice mismatched systems is proposed. The nucleation barrier and the energetically preferred aspect ratio are obtained. The comparison between the two-dimensional and three-dimensional nucleation barrier is carried in order to get the three-dimensional nucleation conditions. The fitting curve is plotted according to the experimental results, and the curve shows the tendency for an increase in the preferred aspect ratio with regard to different material systems. The nucleation theory expects the wurzite crystal structure of nanocrystals under the stress-driven nucleation process, which is consistent with the case of GaAs nanoneedles on sapphire. Overall, the stress-driven nucleation of islands may offer a new growth mechanism for the fabrication of catalyst-free high quality nanomaterials.4. A model of the nanoislands nucleation in Stranski-Krastanov growth mechanism on lattice-mismatched vicinal substrate is proposed. The nucleation barrier and energetically favorable aspect ratio are obtained on different material systems. The favorable aspect ratio as a function of miscut angle is also demonstrated. The theoretical analysis shows the minimum nucleation barrier of island is on the decrease with increment of substrate misorientation, which means the nucleation of nanoislands on vicinal substrate is more favorable than that on singular substrate. This result is consistent with experimental observation. It is suggested that the vicinal substrate can be explored further as an effective way to direct nanoislands nucleation and self-assembly. With regard to miscut, it offers an additional degree of control over the nucleation process of nanoislands.5. A nucleation model of Ge quantum dots on Si vicinal substrate in Stranski-Krastanov growth mechanism is proposed. The variation of shape factor and elastic energy densities of quantum dots with miscut angles is fitted. The influence of miscut angles, the thickness of wetting layers and the densities of quantum dots on the nucleation process is analyzed. The conclusion shows the tendency for a decrease in the nucleation barrier and critical volume of quantum dots with miscut angle and the thickness of wetting layers. The variation of densities of quantum dots is trivial to the nucleatin barrier.