From transparent to black amorphous zinc oxide thin films through oxygen deficiency control

Abstract

Despite the fact that zinc oxide is a well-known transparent oxide, several recent studies on “black” ZnO have renewed its potential for photocatalytic applications. We report on the control of oxygen deficiency in ZnO thin films grown at 300 °C on c-cut sapphire single-crystal substrates by pulsed electron beam deposition (PED) through a slight variation of argon pressure in PED. At a pressure of 2 × 10−2 mbar transparent, stoichiometric (ZnO) and crystalline films are obtained, while at 9 × 10−3 mbar black, oxygen-deficient (ZnO0.85) and amorphous films result. Stoichiometry, structural, and optoelectronic properties of transparent and black ZnO thin films were comparatively analyzed as a function of oxygen deficiency. Black ZnO thin films exhibit enhanced absorption in the visible and near-infrared due to oxygen deficiency, thus extending the range of applications of zinc oxide thin films from transparent electronics to solar absorbers and photocatalysis.

Zn Redistribution and Volatility in ZnZrOx Catalysts for CO2 Hydrogenation

“…The color can be attributed to ZnO nanostructures deposited from the gas phase, since XPS analysis (see Figure S4) did not reveal any carbon contamination above what was expected from the pure ZrO 2 surface and both Zn 2p and Zn Auger spectra did not show any sign of metallic Zn. We propose that strong absorption in the visible range can be explained by the formation of in-gap states in partially reduced ZnO, which was previously reported in the literature. , It is interesting to note the sharply contrasting bright white color of ZnO deposits downstream from the reactor (see Figure S1), where the temperature of the substrate ( i.e. , quartz tube walls) was much lower.…”

“…We propose that strong absorption in the visible range can be explained by the formation of in-gap states in partially reduced ZnO, which was previously reported in the literature. 35,36 It is interesting to note the sharply contrasting bright white color of ZnO deposits downstream from the reactor (see Figure S1), where the temperature of the substrate (i.e., quartz tube walls) was much lower. Moreover, no signs of metallic Zn(0) species were detected (by XPS) in either high-or low-temperature zones in the oxygen-free H 2 /He flow.…”

Transparent Structures for ZnO Thin Film Paper Transistors Fabricated by Pulsed Electron Beam Deposition

“…The PED method is an ablation deposition method, similar to pulsed laser deposition (PLD), using a pulsed electron beam instead of the nanosecond ultraviolet laser beam. The focused electron beam is produced in a channel-spark discharge attached to the deposition chamber, as described in detail previously [20]. The electron beam in PED is pulsed (about 100-110 ns at full-width at half-maximum), very intense (approximately hundreds of amperes), and has a polyenergetic energy distribution of electrons, which makes it possible to ablate, i.e., to melt, and vaporize and generate an ablation plasma from a solid target, as is done in the pulsed laser deposition process [20].…”

“…The typical fluence in our experiments varies up to 2.5 J/cm 2 , and the working gas is oxygen in the pressure range of 1-2 × 10 −2 mbar. PED is a versatile and cost-effective thin film deposition method that enables the adjustment of deposition parameters to precisely control the composition, morphology, thickness, and physical properties of various thin films [5,9,20,29,30].…”

“…The fact that ZnO thin films are readily polycrystalline with a hexagonal würtzite structure close to room temperature on various substrates gives them the disadvantage of reducing the mobility of the charge carriers (electrons) due to dislocations or impurities at the grain boundaries, hence influencing their resulting performances in transistors. Some attempts have been made to obtain amorphous zinc oxide thin films and, thus, to eliminate the grain boundaries by pulsed laser deposition and sputtering at cryogenic temperatures [15][16][17] by using chemical methods [18,19] and, recently, by enhancing the oxygen deficiency in PED [20]. However, these attempts are neither expensive in their applications (cryogenic temperatures) nor does the large oxygen deficiency benefit the fabrication of the transistor's components, which require almost ideal ZnO stoichiometry.…”