Studies On The Bandgap Shift Of Low-dimensional Semiconductor Nanostructures And Related Modulation Of Dielectric Properties Based On Atomic-bond-Relaxation Method

With the development of micro/nano electronics technology,the semiconductor integrated circuits are increasingly integrated,the function of a single chip is getting more powerful,and the package size becomes more and more smaller,which make the microelectronics industry put forward higher requirements on chip performance,reliability,and the reduction of the cost of the chip.When the dimensions of electronic devices of large scale integrated circuits are smaller than 0.1 micron,the function and performance of these devices strongly depend on the reconstruction of the internal electric field.A lot of physical properties are related to the dielectric performance of the dielectric layer material in the device,e.g.,current-voltage characteristics,exciton binding energy,carrier energy,and optical properties,etc.Moreover,the ultrahigh density of components makes the spacing between the wires significantly narrower,which not only increases the resistance of the circuit and the power consumption,but also results in the signal delay and wire cross talk due to distributed capacitance.Thus,some further studies on the electronic and dielectric properties of materials at the nanoscale will be of great importance to the development of micro/nano electronics.In general,the bandgap of semiconductor nanomaterials broadens and the dielectric constant suppresses as the size reduces.Also,some theoretical works had been reported to clarify the novel phenomena,including quantum confined effect and surface effect.However,for the cases of some systems with special morphological features or geometric parameters,including nanocones,nanoporous structures,and core-shell nanowires,the underlying mechanism at atomic level on the band shift and dielectric modulation remains unclear.Compared to the conventional nanostructures such as nanocrystals and nanowires,these kinds of nanostructures have some unique physical properties due to unique geometric features.Therefore,it is an urgent need to establish a theoretical model that can overcome the shortcomings of the current theoretical method and analyze these novel physical phenomena in essence and predict the changing trend.Based on the atomic-bond-relaxation correlation mechanism,the effects of size,shape and geometry parameters of low-dimensional semiconductor nanostructures on electronic structure and related dielectric properties have been investigated from the perspective of atomistic origin.A general model for surface and interface of nanoscale systems including the taper nanowires with positive curvature and the nanoporous Si with negative curvature has been established.Furthermore,the related mechanism has been addressed in detail.Our results not only enhance the scientific understanding of the effects from size,shape and geometry parameters on nanostructures,but also provide a series of dielectric modulation method,suggesting the potential applications in developing new micro/nano devices.The main work and results are listed as follows,?).We investigate the effect of surface atomic relaxation on the band structure of semiconductor nanomaterials.It is found that the ratio of surface-to-volume increases as the size decreases,and the coordination defects of the surface atoms becomes more and more serious,which results in surface atomic bond relaxation.Moreover,a self-equilibrium state with the lowest energy of the nanosystem will be achieved.The physical properties will be different from the corresponding bulk counterparts.In addition,the relationship between the energy bandgap of the semiconductor nanostructures and the dielectric properties has been established in terms of Kramers-Kronig equation.Furthermore,an effective method for the dielectric modulation of semiconductor nanostructures has been proposed.?).We address the physical mechanism of band offset and dielectric suppression of tapered and circular ZnO nanowires.Theoretical model analysis indicates that the self-equilibrium strain of both tapered nanowires and circular nanowires will alter the band structure and further affect the dielectric properties.Importantly,our model shows that the electronic and dielectric properties of tapered nanowires have some particular properties compared to that of the circular nanowires.In the case of tapered nanowires,the size effect gives the major contribution to the dielectric properties,whereas for large scale surface gradient effect dominates as the size closes to the Bohr radius.Our results are validated by comparing them with the available evidence,suggesting that the developed method is helpful for shape design on tunable electronic properties of nanostructures.?).We propose an analytical model to illustrate the band shift and dielectric modulation of nanoporous silicon from the perspective of atomistic origin.It was found that both surface effect caused by surface atomic coordination imperfection and elastic interaction among nanopores,affect the related properties.Our results demonstrate that the dielectric properties depend not only on porosity,but also on the geometric parameters such as pore size and spacing.Also,our predictions are consistent with the available evidence.Therefore,the proposed method can not only shed light on the effects from self-equilibrium strain and elastic interaction among nanopores,but also provide an effective way of shape design on tunable electronic properties of silicon-based porous nanostructures and nanodevices.