Growth Mechanisms And Structural Properties Of Semiconductor Nano Materials

Semiconductor nano materials, whose dimensions are confined within tens of nanometers, have attracted tremendous interests due to their superior electronic and optical properties compared with traditional bulk materials. Supported by National High Technology Research and Development Program of China (Grant No2009AA03Z405) and National Natural Science Foundation of China (Grant No.60971068,60908028), the research works presented in this doctoral thesis focus on the growth mechanisms and structural properties of the semiconductor nano materials, espacially of nanowire and quantum dots. The main contents and innovative ideas are listed below:1) In the framework of continuum elasticity, the edge dislocation is simulated by inserting an intial strain, which corresponds to compressing an extra half plane into one original lattice, in the finite element model. When a model contains both nano heterostructure and a misfit dislocation, the residual strain can be obtained. In this thesis, we build an energy balance approach based on the residual strain energy to predict the critical size for plastic relaxation. The critical thickness for wurtzite InGaN/GaN heterostructure is calculated. The results are in agreement with the experimental data. We also investigated the critical thickness and critical radius for the axial heterostructure in nanowires. The critical thickness enhancement derives from the lateral surface relaxation. Moreover, the critical size for InSb/GaSb quantum dots is presented.2) The finite element model of quantum dot with both threading dislocation and interfacial misfit dislocation is built. The dislocation filtering can be achieved by strained quantum dot when the energy of the case that threading dislocation goes through the quantum dot without inclination is large than the energy of the case that it is belt to interfacial dislocation. The energy used in the balance approach includes residual strain energy, dislocation core energy and a barrier for dislocation to incline. We analyze the critical condition for nitride quantum dot to filter threading dislocation. The cylindrical and hexagonal truncated pyramidal shapes are considered.3) In the framework of thermodynamics, the equilibrium composition profile in the alloy semiconductor quantum dot is studied by using finite element method and method of moving asymptotes. We find that the larger (smaller) component tends to occupy the site in tension (compression), to reduce the strain energy. Gibbs free energy contains not only strain energy but also entropic contributions. When the temperature is relative high, the value of entropy is dominant, which prefers the material to be uniform. The entropy confines the alloy segregation. The results shows that the main driving force of the nonuniform composition profile is the strain minization. We systematically investigated the pyramid, dome, barn shaped GeSi/Si quantum dots and pyramid, dome shaped InGaAs/GaAs quantum dots. All the geometry is built based on the experimential observations. Moreover, the edge dislocation induced composition profile is obtaioned.4) In the framework of kinitics, the transient growth model of multi-component alloy nanowire is built. The set of time-dependent differential diffusion equations with "moving boundaries" are numerically solved by the finite element method. an Arbitrary Lagrangian-Eulerian (ALE) method is employed to move the nanowire-catalyst interface. Two scenarios, namely the presence or absence of adsorption on the sidewall of nanowires, are discussed separately. Our simulations reveal that the component with larger (smaller) diffusivity will segregate near the bottom (top) of the nanowire.5) The single band effective-mass Hamiltonians for electron and heavy hole are adopted to investigate the electronic structure of wurtzite hexagonal truncated pyramid GaN/AIN quantum dot. We employ the density matrix approach and perturbation expansion method to calculate the optical absorption coefficients. The misfit induced strain, piezoelectric effect and spontaneous polarization are considered. The band edge deforms and electron’s wavefunction is confined up to the top of the GaN QD, while the hole wavefunction is pushed towards to the wetting layer. Moreover, the adjacent threading dislocation reduces the transition energy, and this change relates to the position of dislocation.6) The significant effect of the nonuniform composition in dome and barn shaped GeSi QD on electronic structure is analyzed in depth. Due to the composition variation, the total band edge of heavy hole is dominated by the band offset and spin-orbit coupling rather than the strain effect. The numerical results reveal that the wave function of heavy hole trends to be localized in the Ge-rich region at the top of the large QD. Moreover, the size effect gradually compensates the composition effect as the size of QD decreases.