Molecular Beam Epitaxy

Molecular beam epitaxy (MBE) (Fig. 9.9) is a widely used method to prepare high-quality and homogeneous epitaxial crystal layers. It is highly reproducible in terms of both film thickness and composition control. Although various methods to prepare V02 thin films have been studied, a process to prepare V02 thin films with a perfect V-0 stoichiometry and near single-crystal structure is still very attractive. Studies on the preparation of V02 thin films using MBE and their MIT performance have been conducted only in recent years.

There are mainly two methods for V02 thin film preparation by MBE. The first one, developed by Sambi et al. [85, 86], is called the "periodic annealing” method. Here, before deposition, a Ti02(110) substrate is cleaned with cycles of Ar-ion sputtering, followed by annealing at 573 К in 02 (1 x 10-6 mbar), to produce sufficient bulk oxygen vacancies to avoid the charging problem. Then, 0.2-

0.5 monolayers (MLs), corresponding to 5.2 x 1014 vanadium atoms/cm2, of amorphous V metal are deposited in vacuum (5 x 10-11 mbar) at room temperature by electron beam evaporation. A postannealing process (at 423 K) in oxygen atmosphere (7.5 x 10_7-1.5 x 10-6 mbar) is followed to obtain an epitaxial V02 layer. The sample is then cooled to room temperature, and the next 0.2-0.5 ML of amorphous V metal is deposited. This cycle is repeated until a 3-5 ML thick epitaxial V02 layer is obtained [87]. The films prepared by Sambi [87] showed ordered V02 phase grown epitaxially on Ti02(110) with a rutile-type structure, but no MIT was observed in these films.

Tashman et al. [87] followed Sambi’s work and deposited V02 on TiO2(001). The resulting films did not show MIT either. They developed the following process and alterations:

  • • They used distilled ozone instead of 02.
  • • They increased the temperature from room temperature to 395 К and from 423 К to 473 К for deposition and annealing, respectively.
  • • They added a rapid postannealing process at 673 К (3K s_1, 1 x 10-6 mbar of distilled ozone).

With these alterations, obvious MIT was observed in films even as thin as 2.3 nm, with a resistance change of 1.4 orders of magnitude (log[Aft/ft]). Tashman's work also showed that the transition width decreases monotonically with film thickness, while the hysteresis increases monotonically with film thickness [87].

Paik et al. [88] adopted this process for the study V02 thin films, but with an even higher deposition and annealing temperature (523 K) and a postannealing process up to 623 K. This annealing step would enhance the coalescence of (OOl)-oriented V02 islands and improve the abruptness of the MIT [88]. They also developed a process for Ti02 substrate pretreatment [88].

The other method is RF-plasma-assisted oxide MBE, reported by Fan et al. [42]. In this method, a standard RF-plasma source was used to provide reactive oxygen radicals. Pure metallic vanadium powder was used as the target for e-beam evaporation. An А120з crystal slice or Ti02 could be used as the substrates, which were degassed and annealed at 823 К in vacuum (4 x 10-9 mbar). The chamber pressure was maintained at 1.3-4 x 10-5 mbar during the film preparation. The evaporation rate of vanadium was controlled at 0.1 A s-1. An optimized flow rate of 1.8 seem for pure V02 film depositions was used as a lower oxygen gas flow would cause oxygen deficiency and could result in a V203 film being formed instead.

Extremely thin V02 films could be prepared by MBE, the thinnest one with MIT reported being 1.5 nm, by Paik [88]. It seems that the thinner films have lower MIT temperatures [87-89]. Fan et al. studied this phenomenon using interfacial strain dynamics and theoretical calculations and claimed that the electronic orbital occupancy is strongly affected by the interfacial strain, which also changes the electron-electron correlation and controls the phase-transition temperature [89, 90]. Up to now, the lowest MIT temperature of pure V02 thin films reported by MBE was 280.5 K, in which a 3.3 nm thick film has a resistance change of 2.3 orders of magnitude. The thinnest film (1.6 nm) did not show the lowest MIT temperature, probably due to diffusion of the Ti substrate into the thin film layers [88]. However, compared with the epitaxial V02 films prepared by PLD on the similar TiO2(001) substrate with a similar thickness (from 3-15 nm) [32], these thin epitaxial V02 films show lower MIT temperatures and smaller resistance changes during MIT. This may indicate higher stresses in these epitaxial V02 thin films. More work could still be done in the MIT investigation of MBE epitaxial V02.

Besides the advantages in preparing extremely thin films, MBE also shows advantages in the preparation of large-sized thin films, accurate stoichiometiy, and especially low-temperature deposition (350’C [87]). Low-temperature deposition for V02 means a reduced lattice mismatch due to the thermal expansion and diffusion of substrate elements and higher crystal quality, which can be used in basic theory research [89, 90] and in the development of precision components [91, 92]. It could also enable the application of VO2 in electronic devices, such as in uncooled infrared focal plane arrays. However, MBE requires a crystallized substrate with very little lattice mismatch with the deposition materials and expensive equipment, which limit its wide usage.

 
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