Effects of gamma irradiations on reactive pulsed laser deposited vanadium dioxide thin films

Highlights

• Synthesis of VO₂ thin films by Reactive pulsed laser deposition has been achieved.

• Properties VO₂ remain mainly unaffected when subjected to gamma ray doses similar to those encountered during space missions.

• The long range crystal structure of VO₂ remains intact upon irradiation on different doses up to 100 kGy.

• XPS reveals a shift from V⁴⁺ to V⁵⁺ oxidation state upon irradiation, due to the frenkel pair formation on the surface.

• Irradiated films show the characteristic SMT of VO₂, although the electrical and optical properties are slightly affected.

Abstract

Vanadium oxide films are considered suitable coatings for various applications such as thermal protective coating of small spacecrafts because of their thermochromic properties. While in outer space, such coating will be exposed to cosmic radiations which include γ-rays. To study the effect of these γ-rays on the coating properties, we have deposited vanadium dioxide (VO₂) films on silicon substrates and subjected them to extensive γ-irradiations with typical doses encountered in space missions. The prevalent crystallographic phase after irradiation remains the monoclinic VO₂ phase but the films preferential orientation shifts to lower angles due to the presence of disordered regions caused by radiations. Raman spectroscopy measurements also evidences that the VO₂ structure is slightly affected by gamma irradiation. Indeed, increasing the gamma rays dose locally alters the crystalline and electronic structures of the films by modifying the V–V inter-dimer distance, which in turns favours the presence of the VO₂ metallic phase. From the XPS measurements of V2p and O1s core level spectra, an oxidation of vanadium from V⁴⁺ towards V⁵⁺ is revealed. The data also reveal a hydroxylation upon irradiation which is corroborated by the vanishing of a low oxidation state peak near the Fermi energy in the valence band. Our observations suggest that gamma radiations induce the formation of Frenkel pairs. Moreover, THz transmission measurements show that the long range structure of VO₂ remains intact after irradiation whilst the electrical measurements evidence that the coating resistivity decreases with gamma irradiation and that their transition temperature is slightly reduced for high gamma ray doses. Even though gamma rays are only one of the sources of radiations that are encountered in space environment, these results are very promising with regards to the potential of integration of such VO₂ films as a protective coating for spacecrafts.

Introduction

Thermal shielding protective coatings in spacecrafts will be increasingly important with the trend towards the reduction of the spacecraft size. Such low thermal mass spacecrafts, like Cubesats, will thus have to be designed to retain or reject heat more efficiently [1]. The protective coating radiation device (PCR) is a new type of thermal control material for spacecraft that displays high potential to efficiently replace the current space thermal control systems, which rely on heaters with an additional power penalty to maintain spacecraft temperature close to room temperature during cold swings. As its emissivity changes without requiring any electrical instruments or mechanical parts, the PCR allows decreasing the requested power budget [2]. The VO₂-based PCR is one of the most promising parts of this functional thermal control surface and has the advantage of being lighter and free of moving devices. Indeed, VO₂ undergoes a reversible first order semiconductor-to-metal phase transition (SMT) at a critical temperature T_c ≈ 68 °C. Above T_c, VO₂ has a rutile structure with the P4₂/mmm (136) space group, corresponding lattice parameters a_R = b_R = 4.55 Å and c_R = 2.85 Å and all the V–V bonds are equidistant (∼2.87 Å), so the d orbital electrons are shared by all the vanadium ions along the V–V chain. At temperatures below T_c, VO₂ has a monoclinic structure with the P2₁/c space group and lattice parameters a_M = 5.75 Å, b_M = 4.54 Å, c_M = 5.38 Å and β = 122.6° [3]. This structure is characterized by a V–V dimerization and alternatively short (∼2.65 Å) and long (∼3.12 Å) V–V bonds that results in localization of d orbital electrons to individual ions. Both the rutile and monoclinic structures of VO₂ are based on a given oxygen bcc lattice for which the position of the vanadium ions (V⁴⁺) determine the different crystal phase kind. For the rutile structure, the vanadium atoms are located in the center of the oxygen octahedra while in the monoclinic structure, they are slightly shifted from the center. The zigzag-like distortion of the V–V chains leads to two different apical V–O bond distances of 1.77 and 2.01 Å and the metal atom pairing results in two long and two short equatorial V–O distances of 1.86, 1.89, 2.03 and 2.06 Å. The unit cell of the rutile structure contains two formula units while the monoclinic structure contains four formula units due to the Peierls-like pairing of the vanadium atoms. The unit cell is thus doubled as the structure changes from rutile to monoclinic. This transition is accompanied by significant changes in the material electrical resistivity and optical transmission [4], [5], [6], [7], [8], which magnitude depends upon the microstructure and electronic properties of the material [9], [10]. Because of its switching characteristics, VO₂ can thus be used as a thermal shielding coating to limit radiation on sensitive opto-electrical devices [11], [12], [13]. VO₂ thin films could eventually be integrated to PCR devices for the design of ZA Cube 2 [14], a nanosatellite conceived for space weather research missions that is currently in development, demonstrating their potential for space applications.

A large portion of the heat exchange between an object in space and the environment is performed throughout radiation, which is in turn determined by the object’s surface properties. It is therefore crucial to ensure that the object’s surface coating is not modified by the possible irradiations it might encounter in space environment. When solids are bombarded with radiation, electrons may be removed from their orbits and atoms may be knocked out of their sites. Furthermore, impurities may be introduced, either by nuclear transmutation, bombarding ions implanted in the material or by a strong coulomb field that creates charged particles which result in single and multiple point defects. As a result, irradiation of materials can modify their properties, either slightly or drastically [15], [16], [17], [18], [19], [20]. Studies on the irradiation of VO₂ thin films with heavy ions [21], [22], [23], [24], [25] showed that displacement cascades occur, which is the driving mechanism through which disordered regions and defects are inhomogeneously generated in the thin films [26]. These studies also reported that the irradiation-induced defects alter the SMT of VO₂ by lowering both the phase transition temperature and the semiconductor phase resistivity. While studies on the displacement of atoms by gamma rays irradiation remains scarce, previous reports have shown that measurable defects can be produced [27]. At high energy (±1.5 MeV), Compton scattering and pair production are likely to be the interaction processes driving the atomic displacement. In this paper, we thus investigated the effects of the interaction of γ-rays on the displacement of atoms, the electronic structure, the crystal microstructure, the lattice vibrations and phonon modes as well as on the optical transmission and electrical properties of vanadium dioxide thin films irradiated with doses typical to those encountered in space missions.