A Theoretical Approach to Investigate Low-Temperature Nanoscale Oxidation of Metals under UV Radiation

Abstract

The oxidation of metals at low temperatures is important not only from a fundamental point of view but also from technological relevance. Recent research has demonstrated that the nanoscale oxidation of metals such as Zr can be significantly altered during photon radiation. While there have been some experimental results to this effect, there are still no quantitative models to explain the enhancement in oxidation and the resulting self-limiting oxide thickness in the presence of radiation. In this paper, we report a detailed theoretical and numerical study of UV light-enhanced low-temperature metal oxidation. This model takes into consideration oxygen adsorption and desorption at the oxide/gas interface, ionic currents within the growing oxides enhanced by the UV-induced high-field migration, as well as electronic tunnel current in the metal-oxide-oxygen systems. Compared to the low tunnel electronic current in natural oxidation ͑without UV light͒, the tunnel electronic current due to excitation in the UV-lightenhanced oxidation process is dramatically larger than the thermionic electron current, leading to an increased oxide thickness. In addition, the model is utilized to calculate the self-limiting oxide thickness as a function of temperature with and without UV radiation, including the effect of oxygen partial pressure. Our numerical calculations indicate trends consistent with reported experimental results.Low-temperature metal oxidation is an important area of research not only from a fundamental point of view to understand mechanisms of metal oxidation but also due to the technological importance of nanoscale oxides. Examples include the use of nanometer thick metal oxides such as MgO and Al 2 O 3 as tunnel barriers in spin tunnel junction devices, 1,2 HfO 2 and ZrO 2 as alternate gate dielectrics in advanced transistor devices, 3-5 and Ta 2 O 5 in corrosion-resistant coatings. 6,7 In such cases, it is extremely important to understand the rate of oxidation and how it can be controlled precisely to enable stoichiometric oxide formation. The thickness of natural oxides at room temperature is typically of the order of 1-3 nm. 8,9 It has recently been demonstrated that oxidation of metals under UV light can dramatically enhance the rate of oxidation and also lead to high-quality dielectrics that can be used in nanoelectronic devices. 10-12 While there is limited experimental data on the initial oxidation rates of different metals under UV radiation, 13,14 there are presently no theoretical models to explain this process quantitatively. Progress in modeling low-temperature oxidation of metals is less pronounced compared to the understanding of intermediate-and high-temperature oxidation regimes. A brief description of some of the original oxidation models discussing lowtemperature oxide growth are described below.1. Wagner theory. 15 The model presented a quantitative description of the oxidation mechanism by linear equations for charged defects through ionic solids. The conclusion is L ϰ ͱ t, where L is the ...

Effect of photon irradiation on structure of yttria-doped zirconia thin films grown on semiconductor substrates

“…Moreover, recent theoretical calculations have demonstrated that the field introduced by UV irradiation enhances the ionic diffusion in the oxide layer. 2,3 These results indicate that the UV irradiation may also be used to assist oxygen incorporation even at room temperature.…”

Compositional tuning of ultrathin surface oxides on metal and alloy substrates using photons: Dynamic simulations and experiments

“…14 Additionally, UV-light-induced high-field migration also enhances the ionic currents within the growing oxide film which comprises the electric field effect. 15 These photon effects result in significantly increased oxidation kinetics which can lead to atomic scale structural and morphological differences when compared to native-oxide films. Figure 10 shows the simulated oxide growth snapshots on Ni in the case of natural and photon-assisted oxidation.…”

“…5 As mentioned earlier, the interaction of UV photons with molecular oxygen leads to the creation of activated atomic oxygen 14 and enhancement of the ionic currents within the growing oxide film by light-induced highfield migration. 15 Therefore, we study molecular and atomic oxidation of 5% Ni-Al substrates in the presence and absence of an external electric field. Oxidation kinetics enhancement due to externally applied electric field in case of metals such as Al is observed at electrostatic potentials approaching the Mott potential and higher.…”

“…14 Additionally, the ionic currents within the growing oxide film are also enhanced by light-induced highfield migration. 15 These photon-induced effects allow us to tune the composition of oxides grown on alloy substrates at room temperature and at the atomic level. While lowtemperature oxidation of pure metal surfaces such as Al͑100͒ have been studied, the oxidation process in metallic compounds such as Ni-Al binary alloys is less clear, even though the latter is of great practical importance.…”

Direct measurement of oxygen incorporation into thin film oxides at room temperature upon ultraviolet photon irradiation

“…7 An important advantage of our approach is that it can be performed in a self-limiting manner by suitably controlling the temperature and oxygen partial pressure down to a monolayer. 14 We chose YDZ thin films as a model system for this study owing to its importance as a model oxygen-ion conductor, extrinsic nonstoichiometry, and band gap close to the photon energy used in this study. Ge was chosen as a model substrate because YDZ/Ge interface is abrupt and stable even after UV exposure.…”