Chemical Vapor Deposition of IrTe 2 Thin Films

: Two-dimensional (2D) IrTe 2 has a profound charge ordering and superconducting state, which is related to its thickness and doping. Here, we report the chemical vapor deposition (CVD) of IrTe 2 ﬁlms using di ﬀ erent Ir precursors on di ﬀ erent substrates. The Ir(acac) 3 precursor and hexagonal boron nitride (h-BN) substrate is found to yield a higher quality of polycrystalline IrTe 2 ﬁlms. Temperature-dependent Raman spectroscopic characterization has shown the q 1 / 8 phase to HT phase at ~250 K in the as-grown IrTe 2 ﬁlms on h-BN. Electrical measurement has shown the HT phase to q 1 / 5 phase at around 220 K.


Introduction
IrTe 2 has attracted much interest for its intriguing properties such as structure phase transitions, charge density wave (CDW) ordering and superconductivity arising from the strong spin-orbit coupling (SOC) [1][2][3][4][5][6]. With these unique properties, IrTe 2 can be used in memory, oscillator, superconductor devices [7][8][9], etc. IrTe 2 has a layered hexagonal structure and each layer is a sandwich-like structure with three layers of Te-Ir-Te atoms. At room temperature, IrTe 2 has a trigonal phase (high temperature phase, HT), in which each iridium atom is coordinated with six tellurium atoms, forming edge-shared IrTe 6 octahedrons [10]. As the temperature lowers to about 280 K, the crystal structure changes from the trigonal phase to the monoclinic phase. In the monoclinic phase, charge ordering is formed due to the charge transfer from Ir 5d to Te 5p, forming Ir 4+ -Ir 4+ dimers. This dimerization suppresses the structure of the IrTe 6 octahedron. This monoclinic phase is also called the q 1/5 phase, in which Ir 3+ gives an electron to Te 2 3-, forming the periodic structure of the 33344 (3 refers to Ir 3+ , 4 refers to Ir 4+ ) arrangement of Ir atoms (Figure 1b) with the modulation vector → q = (1/5, 0, − 1/5) ( Figure 1e). As temperature further lowers down to 180 K, the periodic structure of 34433344 forms, i.e., the q 1/8 phase with the modulation vector → q = (1/8, 0, − 1/8) appears (Figure 1c,f) [11]. In the warming process, IrTe 2 changes from the q 1/8 phase to the HT phase directly without an intermediate q 1/5 phase, which is due to the extra pinning energy that existed in the q 1/8 phase. Phase transition is accompanied by a change of resistance and magnetic susceptibility, which is of great research interest [12].
Additionally, IrTe 2 has a metastable superconducting state. In IrTe 2 , the striped charge order competes with the superconducting state [13]. The phase transition from the HT phase to the striped charge-ordered phase of IrTe 2 would be suppressed as the sample thickness decreases, which is favorable for the appearance of the hexagonal superconductive phase [14]. Besides, Pt or Pd doping can also suppress the charge order phase, making the superconductive phase appear [15]. IrTe 2 is Growth of IrTe2 single crystals is reported using the chemical vapor transport (CVT) [10] and self-flux technique [13,16,18]. Mechanical exfoliation is further used to obtain nanometer thick IrTe2 [14]. Mechanical exfoliation is of low yield and poor reproducibility. CVD can grow large-area 2D materials with high uniformity. Graphene and transition metal chalcogenides (TMDs) have been grown by CVD [19][20][21][22][23][24], showing excellent physical properties similar to those obtained from mechanical exfoliation.
Here in this work, we have successfully synthesized the IrTe2 thin films using the CVD method. Different Ir precursors (including Ir(acac)3, IrCl3 and Ir) and different substrates (including h-BN, SiO2/Si, mica and sapphire) have been investigated. At optimized conditions, polycrystalline IrTe2 films with thickness of ~20 nm and domain size of ~200 nm have been obtained on h-BN. Temperature-dependent Raman spectroscopic characterization demonstrated the transition from the q1/8 phase to the HT phase at ~250 K. Electrical measurement has also shown the phase transition from the HT phase to the q1/5 phase at around 220 K.
Here in this work, we have successfully synthesized the IrTe 2 thin films using the CVD method. Different Ir precursors (including Ir(acac) 3 , IrCl 3 and Ir) and different substrates (including h-BN, SiO 2 /Si, mica and sapphire) have been investigated. At optimized conditions, polycrystalline IrTe 2 films with thickness of~20 nm and domain size of~200 nm have been obtained on h-BN. Temperature-dependent Raman spectroscopic characterization demonstrated the transition from the q 1/8 phase to the HT phase at~250 K. Electrical measurement has also shown the phase transition from the HT phase to the q 1/5 phase at around 220 K.

CVD Growth of IrTe 2 Films
Hexagonal-BN (h-BN) crystalline powders (PT110, Momentive) were mechanically exfoliated on SiO 2 /Si wafers to prepare h-BN flakes. The h-BN flakes on SiO 2 /Si were annealed at 500 • C for 1-2 h in air to remove the possible polymer residues. Sapphire flakes were annealed at 1000 • C for 1-2 h. Mica flakes were freshly peeled before usage.

IrTe 2 Growth Using Ir(acac) 3 Precursor
An alumina boat containing Ir(acac) 3 precursors (10-20 mg) was put outside of the furnace and heated to 170 • C. Another alumina boat containing Te (~20 g) powder was placed in the upstream of the furnace at a temperature of~500 • C. Substrates were placed in the center of the furnace with a temperature of 700 • C. 20 SCCM Ar and 20 SCCM H 2 was used as the carrier gas and the total pressure was maintained at~5 kPa. The experimental conditions including reaction time, reaction temperature, Ir precursor heating temperature, flux rate and proportion of carrier gas were optimized for growth.

IrTe 2 Growth Using IrCl 3 Precursor
An alumina boat containing Te (about 4 g) was put in the first temperature zone of the two-temperature-zone furnace at 500 • C. Another alumina boat containing SiO 2 /Si substrates with IrCI 3 was placed in the second temperature zone at a temperature of 700 • C ( Figure S3a). 100 SCCM Ar and 20 SCCM H 2 was used as the carrier gas and the total pressure was maintained at atmospheric pressure.

IrTe 2 Growth Using Element Ir as the Precursor
Metal iridium films of about 20 nm thickness on SiO 2 /Si wafers were deposited by the vacuum-evaporating method. An alumina boat containing Te (about 4 g) was put in the first temperature zone of the two-temperature-zone furnace at 500 • C. Another alumina boat containing substrates was placed in the second temperature zone at a temperature of 800 • C ( Figure S3b). 100 SCCM Ar and 20 SCCM H 2 was used as the carrier gas and the total pressure was maintained at atmospheric pressure.

Characterization
Scanning Electron Microscopic (SEM) images were taken on a Hitachi S4800. AFM images were captured on a veeco M-Pico microscope. Raman spectra were measured on a home-built vacuum, variable temperature, low-wavenumber Raman system with 532 nm excitation. An attoDry 800 optical stat (attocube systems AG, Germany) was used for sample cooling. A NA = 0.82 low temperature objective (LT-APO/VIS/0.82, attocube systems AG, Germany) was used for laser focusing and signal collection. In the electrical measurement, IrTe 2 /h-BN flakes were transferred on a new SiO 2 /Si substrate using a PDMS stamp and then a copper TEM grid was used as a shadow mask for the metal contact evaporation. A total of 5 nm Ti and 50 nm Au was electron-beam evaporated for contacts. Low temperature resistance measurements were performed in a physical property measurement system (PPMS, Quantum Design, Inc., San Diego, CA, USA) under He-purged conditions.

Results and Discussion
The CVD setup for IrTe 2 growth using Ir(acac) 3 as precursor is shown in Figure 2a. Figure 2b-e show the SEM images of as-grown IrTe 2 films on h-BN, SiO 2 /Si, sapphire and mica substrates. The domain size of IrTe 2 films grown in h-BN substrate is about~200 nm, which is larger than that on other substrates. This is because the atomic smoothness and free dangling of bonds of the h-BN surface minimized the nucleation sites and promoted in-plane epitaxy of IrTe 2 flakes [21,25,26]. However, SiO 2 /Si, sapphire and mica lack this advantage, and only smaller crystal domains were obtained on these substrates. Moreover, the as-grown IrTe 2 films using Ir(acac) 3 as precursor showed a larger grain size and a thinner thickness (~20 nm, inset of Figure 2b) than using inorganic precursors (IrCI 3 , Ir) ( Figure 3). The E g mode at~128 cm −1 and A 1g mode at~165 cm −1 are observed from all as-grown IrTe 2 films (Figure 3c).    Figure S4), respectively, which is consistent with the trigonal structure of IrTe2 [27]. The Energy Dispersive Spectrum (EDS) measurement revealed a Te/Ir ratio of 2.08 in the as-grown IrTe2 film (Figure 2g), close to the ideal ratio of 2.    Figure S4), respectively, which is consistent with the trigonal structure of IrTe2 [27]. The Energy Dispersive Spectrum (EDS) measurement revealed a Te/Ir ratio of 2.08 in the as-grown IrTe2 film (Figure 2g), close to the ideal ratio of 2.  Figure S4), respectively, which is consistent with the trigonal structure of IrTe 2 [27]. The Energy Dispersive Spectrum (EDS) measurement revealed a Te/Ir ratio of 2.08 in the as-grown IrTe 2 film (Figure 2g), close to the ideal ratio of 2.
Temperature-dependent Raman spectroscopic characterization was carried out to study the phase transitions in the as-grown IrTe 2 films on h-BN. At room temperature, IrTe 2 has a trigonal symmetry, and there are two characteristic Raman peaks: E g (~128 cm −1 ) and A 1g (~165 cm −1 ) [28]. As shown in Figure S5, E g and A 1g modes refer to the intralayer and interlayer vibration of Te atoms, respectively. Figure 4b shows the 2D Raman image of as-grown IrTe 2 film on h-BN in the temperature range of 4~300 K. The fitted peak position and intensity of E g and A 1g modes are shown in Figure 4c,d. At low temperatures, the crystal structure of IrTe 2 is monoclinic, and E g splits to two peaks: E 1 g (~124 cm −1 ), E 2 g (~132 cm −1 ) and A 1g splits to A 1 1g (~161 cm −1 ), A 2 1g (~172 cm −1 ) (Figure 4a) due to the symmetry breaking. As the temperature increased up to 250 K, both E g and A 1g modes showed a single Lorentzian peak (Figure 4a). Moreover, the splitting of E g and A 1g modes can only be observed when T <~250 K, so the phase transition temperature should be at about 250 K ( Figure S6), which is consistent to that in previous literature [29]. In addition, the Raman peak positions of E g and A 1g shifted to lower frequency at~123 cm −1 and~161 cm −1 , respectively. In addition, the intensity of A 1g and E g modes increased significantly (Figure 4d). This is because of the strengthening of Te-Te and hence increase of electronic polarizability at high temperatures [28]. We did not observe the Raman feature change in the warming process at about 180 K for the phase transition from q 1/8 to q 1/5 phase. This phase transition is also not expected in the warming process because of the high pinning energy of the q 1/8 phase [10].
Temperature-dependent Raman spectroscopic characterization was carried out to study the phase transitions in the as-grown IrTe2 films on h-BN. At room temperature, IrTe2 has a trigonal symmetry, and there are two characteristic Raman peaks: Eg (~128 cm −1 ) and A1g (~165 cm −1 ) [28]. As shown in Figure S5, Eg and A1g modes refer to the intralayer and interlayer vibration of Te atoms, respectively. Figure 4b shows the 2D Raman image of as-grown IrTe2 film on h-BN in the temperature range of 4~300 K. The fitted peak position and intensity of Eg and A1g modes are shown in Figure 4c, d. At low temperatures, the crystal structure of IrTe2 is monoclinic, and Eg splits to two peaks: (~124 cm −1 ), (~132 cm −1 ) and splits to (~161 cm −1 ), (~172 cm −1 ) (Figure 4a) due to the symmetry breaking. As the temperature increased up to 250 K, both Eg and A1g modes showed a single Lorentzian peak (Figure 4a). Moreover, the splitting of Eg and A1g modes can only be observed when T < ~250 K, so the phase transition temperature should be at about 250 K ( Figure S6), which is consistent to that in previous literature [29]. In addition, the Raman peak positions of Eg and A1g shifted to lower frequency at ~123 cm −1 and ~161 cm −1 , respectively. In addition, the intensity of A1g and Eg modes increased significantly (Figure 4d). This is because of the strengthening of Te-Te and hence increase of electronic polarizability at high temperatures [28]. We did not observe the Raman feature change in the warming process at about 180 K for the phase transition from q1/8 to q1/5 phase. This phase transition is also not expected in the warming process because of the high pinning energy of the q1/8 phase [10]. The q1/8 to HT phase-transition temperature in the as-grown IrTe2 film (~250 K) is lower than that in the literature (280 K) [28]. This may be due to the fact that the crystal domain in our samples is smaller than that in the literature, which affects the phase transition from the q1/8 phase to the HT phase. We also collected the temperature-dependent Raman Spectra of as-grown IrTe2 film on The q 1/8 to HT phase-transition temperature in the as-grown IrTe 2 film (~250 K) is lower than that in the literature (280 K) [28]. This may be due to the fact that the crystal domain in our samples is smaller than that in the literature, which affects the phase transition from the q 1/8 phase to the HT phase. We also collected the temperature-dependent Raman Spectra of as-grown IrTe 2 film on SiO 2 /Si, and no Raman change was observed as temperature increased from 4 K to 300 K, which may be due to the much lower quality of the IrTe 2 thin film grown on SiO 2 /Si ( Figure S7).
A two-terminal device was fabricated to measure the electric property of as-grown IrTe 2 films on h-BN ( Figure 5a). As shown in Figure 5b, the electrical measurement result shows that the resistance of the as-grown IrTe 2 thin film decreased in the cooling process from 300 K to 225 K, and was stable at 225 K. Then as the temperature lowers further, the R increased from 210 K to 150 K. When the temperature reached 150 K, the resistance reached a peak value, and then decreased for T < 150 K. There was no obvious phase transition from temperature resistance measurement for the as-grown IrTe 2 thin film on SiO 2 /Si ( Figure S8), which may due to the much lower quality of IrTe 2 crystal grown on SiO 2 /Si. SiO2/Si, and no Raman change was observed as temperature increased from 4 K to 300 K, which may be due to the much lower quality of the IrTe2 thin film grown on SiO2/Si ( Figure S7).
A two-terminal device was fabricated to measure the electric property of as-grown IrTe2 films on h-BN ( Figure 5a). As shown in Figure 5b, the electrical measurement result shows that the resistance of the as-grown IrTe2 thin film decreased in the cooling process from 300 K to 225 K, and was stable at 225 K. Then as the temperature lowers further, the R increased from 210 K to 150 K. When the temperature reached 150 K, the resistance reached a peak value, and then decreased for T < 150 K. There was no obvious phase transition from temperature resistance measurement for the as-grown IrTe2 thin film on SiO2/Si ( Figure S8), which may due to the much lower quality of IrTe2 crystal grown on SiO2/Si. The resistance increase from 210 K to 150 K is correlated to the HT phase to q1/5 phase transition [12]. In the q1/5 phase, Ir 4+ forms the local spin-orbit Mott state, increasing the gap of the Hubbard gap. The DOS (Density of States) at EF of the q1/5 phase is lower than that of the HT phase due to the break of the Te2 3− polymeric bond [30]. Then, the resistance of the q1/5 phase is higher than that of the HT phase. The q1/5 to q1/8 phase transition expected at around 180 K was not observed. This may be due to defects and impurities in polycrystalline IrTe2 film, which can inhibit the phase transition.

Conclusion
In this paper, we report CVD growth of IrTe2 thin films. At optimized conditions, poly crystalline IrTe2 films with a thickness of ~20 nm and domain size of ~200 nm have been obtained on h-BN using Ir(acac)3 as the precursor. Temperature-dependent Raman spectroscopic characterization has shown the q1/8 phase to HT phase at ~250 K (warming process), and electrical measurement has shown the HT phase to q1/5 phase at around 220 K (cooling process). The phase transition temperature of as-grown IrTe2 thin films is slightly lower than that in the literature for the bulk materials (280 K) [28].
Supplementary Materials: The following are available online at www.mdpi.com/xxx/SI, Figure S1: Vapor pressure of iridium (III) acetylacetonate in 140-210 °C, Figure S2: Vapor pressure of Te in 360-540 °C, Figure S3: Illustration of growth setups for the growth of IrTe2 films using IrCI3 and Ir precursors. Figure S4: Crystal plane of IrTe2 corresponding to diffraction peaks. Figure S5: Eg and A1g vibration modes of IrTe2. Figure S6: Raman spectra of as-grown IrTe2 film on h-BN using Ir(acac)3 precursor from 170 K to 260 K (warming process). Figure  S7: Variable-temperature Raman spectra of as-grown IrTe2 film on SiO2/Si using Ir precursor. Figure S8: R-T curves of as-grown IrTe2 films on SiO2/Si using Ir as precursor.
Author Contributions: L.X. conceived the project; Z.Z. and R.Z. performed the experiments; R.Z., J.W. and L.X. wrote the paper. All authors commented on the manuscript. The resistance increase from 210 K to 150 K is correlated to the HT phase to q 1/5 phase transition [12]. In the q 1/5 phase, Ir 4+ forms the local spin-orbit Mott state, increasing the gap of the Hubbard gap. The DOS (Density of States) at E F of the q 1/5 phase is lower than that of the HT phase due to the break of the Te 2 3− polymeric bond [30]. Then, the resistance of the q 1/5 phase is higher than that of the HT phase. The q 1/5 to q 1/8 phase transition expected at around 180 K was not observed. This may be due to defects and impurities in polycrystalline IrTe 2 film, which can inhibit the phase transition.

Conclusions
In this paper, we report CVD growth of IrTe 2 thin films. At optimized conditions, poly crystalline IrTe 2 films with a thickness of~20 nm and domain size of~200 nm have been obtained on h-BN using Ir(acac) 3 as the precursor. Temperature-dependent Raman spectroscopic characterization has shown the q 1/8 phase to HT phase at~250 K (warming process), and electrical measurement has shown the HT phase to q 1/5 phase at around 220 K (cooling process). The phase transition temperature of as-grown IrTe 2 thin films is slightly lower than that in the literature for the bulk materials (280 K) [28].
Author Contributions: L.X. conceived the project; Z.Z. and R.Z. performed the experiments; R.Z., J.W. and L.X. wrote the paper. All authors have read and agreed to the published version of the manuscript.