The effects of sub-bandgap transitions and the defect density of states on the photocurrent response of a single ZnO-coated silica nanospring

Abstract

The electrical and optoelectronic properties of nanometer-sized ZnO structures are highly influenced by its native point defects. Understanding and controlling these defects are essential for the development of high-performance ZnO-based devices. Here, an electrical device consisting of a polycrystalline ZnO-coated silica nanospring was fabricated and used to characterize the electrical and photoconductive properties of the ZnO layer using near-UV (405 nm) and sub-bandgap (532 and 633 nm) excitation sources. We observe a photocurrent response with all three wavelengths and notably with 532 nm green illumination, which is the energy associated with deep oxygen vacancies. The polycrystalline ZnO-coated silica nanospring exhibits a high responsivity of 1740 A W−1 with the 405 nm excitation source. Physical models are presented to describe the photocurrent rise and decay behavior of each excitation source where we suggest that the rise and decay characteristics are highly dependent on the energy of the excitation source and the trapping of electrons and holes in intermediate defect levels in the bandgap. The energy levels of the trap depths were determined from the photoconductive decay data and are matched to the reported energy levels of singly and doubly ionized oxygen vacancies. A phenomenological model to describe the dependence of the saturation photocurrent on excitation intensity is presented in order to understand the characteristics of the observed breaks in the slopes of the saturation photocurrent versus excitation intensity profile.

Ultrashort and metastable doping of the ZnO surface by photoexcited defects

“…The transient surface doping leads to downward band bending, as opposed to the usually assumed upward surface band bending in photoexcited ZnO. 15,16,22,23 To the extent that we can judge with PE spectroscopy, we conclude that the conceptual analogy between shallow donors and optically excited DX holds true, ranging from quasi-equilibrium metallization and critical behaviour on ultrafast timescales to metastable n-type doping at the ZnO surface. Beyond these fundamental research aspects, our results are also relevant to ZnO-based devices.…”

“…an electron depletion layer with low conductivity. 15,16,22,23 In this picture, upon photoexcitation of deep defects or across the band gap, electrons are driven into the subsurface, while holes accumulate at the surface. This may contribute in two ways to PPC.…”

“…Firstly, photoexcited electrons in the subsurface may lead to conductivity for rather long times, as the recombination is slowed by the energy barrier resulting from the band bending. 16,22,23 Secondly, the electron–hole separation counteracts the equilibrium band bending (a surface photovoltage effect), eventually leading to flat bands, thereby increasing the surface conductivity with respect to the equilibrium. 23 The question is, though, whether upward band bending is at all required (and an adequate assumption) to explain the PPC effect.…”

“…16,22,23 Secondly, the electron–hole separation counteracts the equilibrium band bending (a surface photovoltage effect), eventually leading to flat bands, thereby increasing the surface conductivity with respect to the equilibrium. 23 The question is, though, whether upward band bending is at all required (and an adequate assumption) to explain the PPC effect. DX themselves are already predicted to have long lifetimes.…”

Photothermal Structural Dynamics of Au Nanofurnace for In Situ Enhancement in Desorption and Ionization

“…Because the electrons were swept across the depletion region, most V O + states converted to V O ++ centers by trapping holes at the VB, and radiative recombination occurred between this center and the electrons at the CB, emitting a photon energy of 2.19 eV. [75] In addition, the V O + defect easily trapped a photoexcited electron to form the neutral V O…”