Improvement of electrical and optical properties of molybdenum oxide thin films by ultralow pressure sputtering method

Abstract

In this work, we investigated the structural, electrical and optical properties of molybdenum oxide thin films deposited by the reactive dc magnetron sputtering method. The molybdenum oxide films were prepared at sputtering pressures ranging from 6.7 × 10−1 to 6.7 × 10−2 Pa. In order to promote their electrical conductivity, all the deposited MoOx films were annealed in Ar ambient at 450 °C for 8 h. The resistivity of the MoOx films varied from 10−4 to 10−2 Ω cm depending on the O2 content in the sputtering ambient. The lowering of the resistivity of the MoO2 films was mainly attributed to the formation of a monoclinic MoO2 polycrystalline phase. As the sputtering pressure decreased, the content of monoclinic polycrystalline MoO2 phase increased, resulting in low resistivity films. The formation of the dominant MoO2 phase at lower sputtering pressures was attributed to the stress induced crystallization. The post-deposition annealed (PDA) MoOx film, deposited at an ultralow sputtering pressure (6.7 × 10−2 Pa) and O2 content of 40%, had an atomic ratio of O to Mo ≈ 2.85 and was highly transparent and conductive: the transmittance in the visible wavelength range of 400–500 nm was about 73% and the resistivity was 1.05 × 10−3 Ω cm. This result is superior to those of MoOx films epitaxially grown by the pulse laser deposition method.

Tuning the optoelectronic properties of amorphous MoOx films by reactive sputtering

“…Since the argon pressure was kept constant at 2.44 Â 10 À3 mbar during all depositions, it is likely that the oxygen causes target poisoning and therefore reduces the deposition rate. 7 In this regime though, MoO x molecules are sputtered off instead of metallic Mo. The filmgrowth rate increases directly with the applied power in the DC sputtering process, which is in agreement with observations reported in the literature.…”

“…In their work, polycrystalline films were observed independently of the oxygen partial pressure, and in addition, an increase in surface roughness with the oxygen partial pressure was also reported. In the case of as-deposited films formed at room temperature, Oh et al 7 also found amorphous films at similar sputtering pressures as presented here, and they, in addition, reported on poly-crystalline films formed at ultra low sputtering pressures. In this work, non-crystalline films are observed independently of the oxygen partial pressures in the range of 1.00 Â 10 À3 to 3.00 Â 10 À3 mbar.…”

“…1,4 Meanwhile, other applications such as gas and chemical sensors were also reported using MoO x films. 5 An extensive list of deposition techniques including thermal evaporation, 6 DCsputtering, 7 RF-sputtering, 8 pulsed laser deposition, 9 atomic layer deposition, 10 electron beam evaporation, 11 and solution processing 12 have been reported in the last years.…”

“…In the case of MoO x , it has been demonstrated that the film conductivity, the optical transmittance, as well as the microstructure and surface roughness of the films can be tuned by controlling the plasma density, the oxygen partial pressure, and the deposition pressure during the sputtering process. In these studies, the deposition temperature also plays a fundamental role in the growth dynamics, demonstrated by the use of either in situ annealing [14][15][16] or post-annealing under inert atmospheres 7 for the investigated films. Two main principal oxide phases are found to be MoO 2 and MoO 3 ; 7 however, non-stoichiometric and more complex structures are also predicted by the Mo-O phase diagram.…”

Crystalline Molybdenum Oxide Layers as Efficient and Stable Hole Contacts in Organic Photovoltaic Devices

“…Among these metal oxides, MoO 3 shows one of the largest work functions, ,, combined with relatively low deposition temperatures, which makes it an attractive material for device integration. In addition, as in the case for other transition metal oxides, the electrical, optical, and physical properties of MoO x can be tuned over wide ranges via defect engineering, making MoO x particularly interesting for various optoelectronic device applications.…”