Investigation of External Charges Effects on Piezoelectric ZnO Nanogenerator

Abstract

Piezopotential generation in semiconductive ZnO nanowire (NW), oriented along the c-axis [0001], is significantly affected by free charge carriers within the ZnO NW. In this paper, the effect of free carriers' distribution in semiconductive ZnO nanowire is investigated, using a Finite Element Method (FEM). The mentioned effect leads to modification of the conduction band variation, carrier concentration profiles, and eventually, the magnitude and distribution of the piezoelectric potential. The impact of free charge carriers shows that the negative potential distributed at the tip of ZnO NW is decreased from V -270 mv for the donor concentration ND  1  10 15 C/m 3 to the V -25 mV, in presence of the donor concentration of ND  1  10 18 C/m 3. With selecting the appropriate electrical boundary conditions and applying the surface charges density at the top of the nanowire, the potential reduction is compensated. The electrostatic effect leads to a significant enhancement of the piezoelectric potential. The results are well shown the interplay of volume and surface charges and their influence on performance of nanogenerator, and so are crucial for designing of nanogenerators with high piezoelectric potential and good efficiency.

Dimensional Roadmap for Maximizing the Piezoelectrical Response of ZnO Nanowire-Based Transducers: Impact of Growth Method

“…These studies reveal that thinner and longer NWs would increase the generated piezoelectric potential when sollicited under compressive forces, but they do not consider their semiconducting properties, which represent a strong limitation. The influence of doping level and free carrier screening on the piezoelectric response of semiconducting (mostly ZnO) NWs has been studied theoretically by means of numerical simulations [ 55 , 56 , 57 ]. The presence of free charge carriers in the core of NWs basically screens the piezoelectric potential induced by the strain generated under mechanical solicitations.…”

“…In Ref. [ 57 ], the authors added an external surface charge density at the top of the NW, extending the depletion region from the top and thus increasing the piezoelectric response of the NW but keeping it independent of length for practical length values. Experimental reports have confirmed the role of doping level on the VING device performances [ 58 ].…”

“…In this case, the application of a pressure to the VING generates a polarization field that depletes only the top of the NWs, while the core of the NW remains neutral, with free carriers screening the polarization field. This is known as the free surface Fermi level assumption, which is the usual assumption in most numerical simulations [ 56 , 57 ]. However, in the presence of a large density of surface traps along the NW (m-plane) sidewalls, which is expected in ZnO NWs [ 63 ], thermodynamic equilibrium results in surface band bending, with a balance between surface charges and depletion charges ( Figure S1 in the Supplementary Materials ).…”

Novel Interfacial Bulk Heterojunction Technique for Enhanced Response in ZnO Nanogenerator

“…The simulation was performed for the multiple CuO nanoparticles around zinc oxide microparticles using the following equations where D , E , ρ, and V are the electrical displacement field, electric field, total charge density, and potential difference. Previous studies in the ZnO-based piezo simulation were taken for the reference voltage. − Figure a shows the variation of electric potential over the distance in the medium. The medium of simulation was chosen as air.…”

Fundamental Definitions for Axially-Strained Piezo-Semiconductive Nanostructures

“…The Dirichlet condition (V = 0) is applied at the center of the nanowire. In order to consider the effect of charge carriers, the equations for semiconductors have been added to the simulations and, similar to [ 22 , 23 , 34 ], for simplicity, the effects of external charges [ 37 , 38 , 39 ] have not been considered. The nanowire is assumed to be uniformly n-type doped, with different doping conditions (intrinsic, 10 16 cm −3 , 10 17 cm −3 ).…”