Interaction of light with the ZnO surface: Photon induced oxygen “breathing,” oxygen vacancies, persistent photoconductivity, and persistent photovoltage

Abstract

ZnO surfaces adsorb oxygen in the dark and emit CO 2 when exposed to white light, reminiscent of the lungs of living creatures. We find that this exchange of oxygen with the ambient affects the integrity of the ZnO surface. Thus, it forms a basis for several interesting surface phenomena in ZnO, such as photoconductivity, photovoltage, and gas sensing, and has a role in ZnO electrical conduction. Using x-ray photoelectron spectroscopy on ZnO nanowires, we observed a decomposition of ZnO under white light and formation of oxygen-depleted surface, which explains photoconductivity by the electron donation of oxygen vacancies. Our findings suggest that the observed decomposition of the ZnO lattice may only take place due to photon-induced reduction of ZnO by carbon containing molecules (or carbo-photonic reduction), possibly from the ambient gas, accounting in a consistent way for both the reduced demands on the energy required for decomposition and for the observed emission of lattice oxygen in the form of CO 2. The formation of oxygen-vacancy rich surface is suggested to induce surface delta doping, causing accumulation of electrons at the surface, which accounts for both the increase in conductivity and the flattening of the energy bands. Using surface photovoltage spectroscopy in ultra high vacuum, we monitored changes in the deep level spectrum. We observe a wide optical transition from a deep acceptor to the conduction band, which energy position coincides with the position of the so called "green luminescence" in ZnO. This green transition disappears with the formation of surface oxygen vacancies. Since the oxygen vacancies are donors, while the green transition involves surface acceptors, the results suggest that the initial emission of oxygen originates at the defect sites of the latter, thereby eliminating each other. This suggests that the green transition originates at surface Zn vacancy acceptors. Removing an oxygen atom from a Zn vacancy completes the vacancy to become a full ZnO molecule vacancy, which does not produce deep levels. Our results explain why ZnO finds use as an electrical detector for oxygen and for carbon containing gas molecules. They may also shed new light on photocatalytic uses of ZnO. It is suggested that similar surface phenomena may affect other semiconducting oxides. V

Metallic core conduction in unintentionally doped ZnO nanowire

“…Numerous papers reported the interaction mechanisms of ZnO wire surfaces with oxygen and hydrogen species. [1][2][3][4][5][6] When these species are adsorbed to the wire surface, charge transfer from the ZnO NW to the adsorbed species induces band bending that can appear as either an electron depletion layer or an electron accumulation layer. UV illumination or electron beam bombardment could break the bonds between the ZnO NW and the species 7,8) and could even create persistent conduction in ZnO wires.…”

“…UV illumination or electron beam bombardment could break the bonds between the ZnO NW and the species 7,8) and could even create persistent conduction in ZnO wires. 5,[9][10][11] Surface-related phenomena and persistent photoconductivity may produce an additional conduction path (electron accumulation) or a reduced conduction section (electron depletion), making reliable measurement of the transport properties a challenge. 6,12) Knowledge of the electrical/ doping properties of unintentionally doped ZnO NWs is a crucial issue affecting the next step toward optoelectronic applications: the control of p-type doping.…”

Luminescence dynamics of hybrid ZnO nanowire/CdSe quantum dot structures

“…1 take place on a time scale of a few tens of nanoseconds, the PC dynamics need a time-scale of minutes. As shown in previous studies [12,13], desorption and readsorption of oxygen molecules on the oxide as well as the QD surfaces play an important role for the electron transfer processes between both material systems. Faster dynamics were measured in an oxygen rich environment.…”

Влияние Термических И Плазменных Обработок На Фотолюминесценцию Пленок Оксида Цинка

“…Поведение проводимости демонстри-рует хорошо известный эффект замороженной фото-проводимости, наблюдаемый в ZnO при УФ-освеще-нии [17][18][19], природа которого до конца не выяснена.…”