Influence From Cross-Sectional Shape On Tunnelling Properties Of Nitride Core-Shell Nanowires

Semiconductor core-shell nanowires(CSNWs)are important candidates for next-generation optoelectronic devices.Quantum tunnelling of carriers through barriers has an initial impact on the nanowires performances.Up to now,the theoretical study on the radial tunnelling in CSNWs has been mostly focused on cylindrical ones,while experimentally prepared nanowires are usually prisms with polygonal cross-sections.Prismatic nanowires have anisotropic tunnelling properties due to their anisotropic radial barrier distributions.The tunnelling characteristics influenced by the cross-sectional shape has theoretical and practical significance for the related applications of nanowire devices.In this thesis,a general theoretical method on the tunnelling characteristics of prismatic nanowires is derived by an effective mass approximattion,and hexagonal nitride CSNWs with double barriers are taken as an example to be discussed in detail.Firstly,the Hamiltonian equation for an electron with given incident energy is solved by combining the transfer matrix and Runge-Kutta methods in column coordinates to obtain the radial wave functions and transmission probability.Then,the radial resonance states of electrons are analyzed according to the peak positions of transmission probabilities.The anisotropy of tunnelling coefficients and their variations with parameters such as the height,thickness and position of the potential barriers are investigated.Further,a tunnelling current model proposed by Ando et al.is applied to obtain the total radial tunnelling current density and its variant relation versus applied bias according to the Fermi distribution and transmission probability of electrons with different incident energy states.Finally,the tunnelling properties in nanowires influenced by the cross-sectional anisotropy are discussed by comparing the tunnelling current-voltage characteristics in several typical prismatic nanowires.The calculation for tunnelling properties in hexagonal nitride CSNWs shows that:1.The radial tunnelling probability of carriers has the same anisotropy as the radial potential barriers in prismatic nanowires.The potential barrier's anisotropy results in the carrier tunnelling in prismatic nanowires occurring in a resonant energy band,which is different from a single energy level in a cylindrical nanowire.2.In a CSNW with equally thick double barriers in the direction perpendicular to the interfaces,as the layer thickness increases with the polar angle,the radial transmission probability decreases due to the thicker barriers,and the resonance levels drop while the number of resonant peaks increases due to the increasing well width.The variation of transmission coefficients in a specific radial direction with the components and thickness of each layer is the same as that in cylindrical CSNWs.3.In a CSNW with unequal thick double barriers,the radial transmission coefficient is smaller than that of a CSNW with equally thick double barriers.The resonance levels drop faster,making it more difficult for electrons to tunnel through the barriers.A comparison of the tunnelling current characteristics among nanowires with triangular,quadrilateral,hexagonal and circular cross-sectional shapes reveals that:1.The radial tunnelling probability has the same symmetry as the cross-section.2.The tunnelling resonance energy band is broadened as the symmetry of cross-sections reduces.3.With the same barrier distribution in the radial direaction vertical to the interface,reducing the cross-section symmetry results in more peaks in the tunnelling I-V curve,with the right-shifting first peak position and a decreasing maximum current density.