Natural charge spatial separation and quantum confinement of ZnO/GaN-core/shell nanowires

Abstract

We performed density-functional calculations to investigate the electronic structure of ZnO/GaN core/shell heterostructured nanowires (NWs) orientating along <0001> direction. The build-in electric filed arising from the charge redistribution at the } 00 1 1 { interfaces and the band offsets were revealed. ZnO-core/GaN-shell NWs rather than GaN-core/ZnO-shell ones were predicted to exhibit natural charge spatial separation behaviors, which are understandable in terms of an effective mass model. The effects of quantum confinement on the band gaps and band offsets were also discussed.Quasi-one-dimensional semiconductor nanostructures are increasingly used in photonic and electronic devices, such as integrated nanodevices, 1 novel field effect transistors (FET) core/shell NWs have demonstrated the accumulation of ZnO-core, Ge-core hole gas and GaN-core electron gas, in agreement with experimental observations. Moreover, the band offset of core-shell NWs can be modulated by the quantum confinement effect and the strain induced by the lattice mismatch between the two components, yielding a board range of band gaps, which facilitates light adsorption of PV devices. The core-shell NWs with type-II band alignment are advantageous in promising solar energy applications such as water splitting, dye-sensitized and even regular solar cells. [13][14][15]

Hybrid density functional study of band alignment in ZnO–GaN and ZnO–(Ga1−xZnx)(N1−xOx)–GaN heterostructures

“…Although, this can be partially overcome by using LDA+U or GGA+U strategy, the calculated band gaps are still much lower than the experimental values. 9,15 Therefore, the ∆E VBO values given by using these functionals remain doubtful.…”

“…7,8 Based on first-principles calculation and an effective mass model, we predicted that ZnO (core)/GaN (shell) h-NWs rather than GaN (core)/ZnO (shell) ones exhibit natural charge spatial separation behaviors in our previous work. 9 Thanks to the unique geometry of core/shell h-NWs which have GaN/ZnO interface extending along the axial direction and carrier separation taking place in axial direction of the h-NWs, photo-generated carriers can reach the interface with high efficiency without substantial bulk recombination, which improves carrier collection and overall efficiency with respect to comparable axially-modulated h-NWs. This provides a novel solar cell structure with high efficiency.…”

“…15 In our previous work, we adopted the Perdew-Wang (PW91) XC functional in the GGA+U strategy and got a ∆E VBO of 1.04 eV for the same interface. 9 Obviously, the ∆E VBO values given by DFT calculations are sensitive to the employed exchange-correlation functionals. However, both LDA and GGA functionals underestimated the band gaps of w-ZnO and w-GaN.…”

Theoretical studies of geometry asymmetry in tellurium nanostructures: intrinsic dipole, charge separation, and semiconductor–metal transition

“…However, charge separation has also been demonstrated in various systems without volume doping. [25][26][27][28][29][30] By the control of morphology, the highest-occupied state and the lowestunoccupied state could be separated along the wire axis in tapered Si NWs. 25 Similar phenomenon was observed in Si NWs under partial strain along the wire axis, 26 as well as Si quantum dots 30 and ZnO NWs 27 with surface reconstructions.…”

Type-II Core/Shell Nanowire Heterostructures and Their Photovoltaic Applications

“…---- There have been numerous attempts and models to predict and calculate the electronic structures and properties of type-II heterostructures [20,23,[38][39][40]. To date, the reported type-II heterostructures are mainly based on II-VI and III-V binary semiconductors, such as ZnO/ZnS [41], ZnO/ZnSe [42], ZnO/ZnTe [23], CdSe/CdTe [43], and GaN/GaP [20], as shown in Table 1.…”