Parameter Control in AACVD Growth of a VО2 Thin Film

Commonly used precursors for AACVD are VO(acac)₂ [29], V(acac)3 [30], and other kinds of organometallic precursors. Piccirillo et al. investigated the parameters that influence V0₂ deposition for two kinds of precursors, VO(acac)₂ and V(acac)3 [30]. The factors they researched are the type of solvent and the temperature and flow rate of the carrier gas. The only combination that was able to produce a V0₂ thin film was a VO(acac)₂/ethanol combination at a carrier gas flow rate of 1.5 L/min in the temperature range of

Figure 8.8 Phase diagram for the deposition of V0₂/carbon hybrid structures. Adapted from Ref. [28].

500°C-600^CC. V₂0₃ was produced for VO(acac)₂/methanol/water and V(acac)3/methanol/water combinations at the fixed flow rate of 1.5 L/min at 500°C-600°C. The V(acac)₃/ethanol combination deposited a mixing phase of V0₂ and other vanadium oxides when the carrier gas flowrate was set at 1.5 L/min. Lastly, when the carrier gas flow rate increased from 1.5 L/min to 3 L/min, vanadium oxides with a higher valence number were produced.

It is worth mentioning that Naik et al. produced V0₂ though AACVD without employing commonly used oxygen sources such as 0₂ and water [11]. In this case, VO(acac)₂ was dissolved in ethanol. At the temperature of 450°C-600°C, V0₂ was formed with an island growth morphology.

Parameter Control in Hybrid AA/APCVD Growth of a VО2 Thin Film

Hybrid AA/APCVD combines the advantages of both AACVD and APCVD. This system can deposit a high-quality, uniform film. At

Figure 8.9 Schematic diagram of the hybrid AP/AACVD system.

the same time, it can add nanoparticles into the film by using a nanoparticle slurry aerosol. The hybrid CVD method is suitable for producing nanocomposites. A schematic illustration of the hybrid CVD system is shown in Fig. 8.9.

Warwick’s group produced a V0₂ thin film, a V0₂-Ti0₂ nanocomposite, and a V0₂-Ce0₂ nanocomposite by hybrid CVD and compared their performance [31]. The V0₂ film was synthesized by the V0(acac)₂-0₂ APCVD system. At the same time, Ti0₂ or Ce0₂ nanoparticles were transported into the chamber by the AACVD system in the form of an aerosol. All three materials showed lowered r_c compared with that of bulk V0₂, and the reflectance showed a significant change of 30% between 25 C and 80 C. Also, the films had photocatalytic properties similar to those of a titanium dioxide thin film. The group concluded that although the visible transmittance of the nanocomposite still needed to be improved, hybrid CVD was a promising way to produce thermochromic nanocomposites [1].

Parameter Control in ALD Growth of a VО2 Thin Film

Tetrakis(ethylmethylamido)vanadium (TEMAV) [V(NEtMe)₄] is the vanadium-containing precursor commonly used in ALD [32-36].

Other precursors used in ALD are УО(ОС₃Н₇)з [37], and V0(acac)2 [38]. The common oxygen sources used in ALD are water [37], ozone [36, 39], and oxygen plasma [40]. Compared with ozone, the system with water gives a higher deposition rate with a TEMAV precursor [32]. The enhancement of the growth rate is because the hydroxyl group dissociates from water [41]. The hydroxyl group serves as an intermediate for the metal compound to anchor on the substrate surface.

As discussed previously, the key to the ALD process is the selfterminating surface half-reaction. Therefore, the growth conditions, such as the growth temperature, the injection time, and the purging time, have to be carefully planned. To form a saturated monolayer, the binding energy between substrate and precursor should be larger than the binding energy between the monolayer and precursor particles above [42]. An adequate purging time is needed for the excess precursor to re-evaporate and leave the reaction chamber because the injected precursor has to be in excess to ensure a saturated layer is formed. The case reported by Rampelberg et al. [36] demonstrated a typical ALD process for V0₂(M) film deposition (Fig. 8.10]. This group utilized TEMAV and 0₃ as precursors. The pulsing scheme was [5-10-10-5]: TEMAV was injected into the chamber for 5 s, followed by 10 s of argon purging. Then 0₃ was introduced into the chamber for 10 s Finally, another 5 s of argon purging was executed. The growth temperature was set at 150°C. During the TEMAV injection step, an excess amount of TEMAV was introduced into the system. The group has proven that the substrate would reach the TEMAV saturated state in 2 s. Therefore, excess TEMAV would accumulate on the monolayer and re-evaporate during the first purging step. After the first purging step, an excess amount of 0₃ was introduced into the system and reacted with TEMAV. After the reaction, the unreacted 0₃ was carried out from the chamber. An amorphous film was produced and then was annealed at 450°C in a He atmosphere for 30 min. The annealed sample showed a V0₂(M] phase. The x_c for the sample was 67°C, and the hysteresis loop width was 12°C.

The optimum conditions for different CVD methods to synthesis V0₂ are summarized in Table 8.1. It should be pointed out that

Figure 8.10 Schematic diagram to show the ALD process reported by Rampelberg et al. [36]. Adapted from Ref. [43].

an organovanadium precursor such as V0(acac)₂ and VO(OC₃H₇)₃ has the largest process window for APCVD, MOCVD, PECVD, and AACVD systems. Thus, organometallic precursors are fit for systems with rough condition controlling. The VC1₄-H₂0 system is the most suitable system for the deposition of a high-purity V0₂ film. However, due to its toxicity, reactivity, and narrow growth window, the VC1₄-H₂0 APCVD system must be able to control the growth parameters, such as H₂0 concentration, temperature, and total flow rate, precisely. Although currently it is only used for ALD, TEMAV is the precursor with the lowest reaction temperature. The precursor has a high potential to be used in other CVD techniques, such as MOCVD and AACVD. V0C1₃ has been the least used precursor in recent years and is not recommended because of the difficulty to produce a V0₂ pure phase and the highest reaction temperature requirement among the precursors.

Table 8.1 Parameters for various CVD techniques to deposit pure-phase V0₂(M); pure-phase V0₂(B); and a V0₂(M), V0₂(B), and V0₂-V0_v mixing phase

^a"T" stands for growth temperature. ^b"P” stands for chamber pressure.

^cDue to the lackofphase information in someofthe references, we cannot differentiate the optimum conditions for V0₂(M) and V0₂(B) in theVCl₄-H₂0 system.

^dThe pulse plan describes the purging step in ALD. The four values indicate the time for metal precursor injection, the first purging, the oxygen/ozone injection, and the second purging.