4.2.2.1 Chemical Vapor Deposition

Metal organic chemical vapour deposition (MOCVD) is a promising technical process for the deüpositipon of high-quality, including epitaxial, thin films and has been shown in the past to produce crystalline binary (Sb₂Te₃ , Bi₂Te₃) and multinary group 15/16 material films.[1] Trialkylstibines SbR₃ (R = Me, Et) and dialkyltellanes TeR'₂ (R = Et, i-Pr, t-Bu) are typically used as molecular precursors,[2] but these often require high pyrolysis temperatures, forcing the incorporation of impurities such as carbon in the material film. Therefore, there is a strong demand for alternative metal organic precursors. Single-source precursors are promising candidates for low-temperature deposition processes.[3]

We demonstrated that (Et₂Sb)₂Te can be used as MOCVD single source precursor for the synthesis of highly stoichiometric Sb₂Te₃ films with very low antisite defect concentrations at temperatures as low as 200 °C.[4] Unfortunately, the Sb₂Te₃ films were rather polycrystalline, consisting of hexagonal Sb₂Te₃ nanoplates. In the case of the (Et₂Sb)₂Te precursor, the low substrate temperatures (200 °C) limited the diffusion mobility of the deposited Sb and Te atoms, hence avoiding the formation of a flat film. In contrast, higher substrate temperatures yielded flat Sb₂Te₃ films, but these were contaminated by elemental antimony.

In contrast, epitaxial Sb₂Te₃ films were deposited using i-Pr₃Sb and Et₂Te₂ between 350 and 400 °C an Al₂O₃(0001) substrates.[5] An almost linear increasing growth rate with increasing substrate temperature was found in the temperature range from 350 °C to 400 °C, reaching 750 nm in 15 min at 400 °C. These findings point to a kinetically-controlled growth process. The film obtained at 400 °C consists of large Sb₂Te₃ flakes, while a film deposited at 350 °C also showed hexagonal Sb₂Te₃ nanocrystals, which adopt perpendicular orientations to the film surface. The higher substrate temperature results in higher decomposition rates and affects the mobility of the adatoms, which increases with increasing substrate temperature, yielding larger Sb₂Te₃ nanoflakes. Moreover, the surface morphology (flatness) of the film improved with increasing substrate temperature.

The AFM micrograph shows regular terraces of 1 nm height, which correspond to one quintuple layer of the layered tetradymite-type structure of Sb₂Te₃. These findings indicate a layered growth as was also observed in MBE-grown Sb₂Te₃ thin films.

The X-ray diffractogram (XRD) of the Sb₂Te₃ film deposited at 400 °C clearly proved the formation of highly crystalline Sb₂Te₃ films. All reflection peaks can be indexed on the basis of the structure of rhombohedral Sb₂Te₃. In addition, a small reflex of the Al₂O₃(0001) substrate is visible at 41.6°. The peaks in the diffractogram correspond to (00l) planes, pointing to a c-axis oriented growth as was previously reported for Sb₂Te₃ films grown by MBE and MOCVD.

The band structure of the surface was also characterized by photoelectron spectroscopy (PES). The overview ARPES spectra of the valence band show intense structures around 0.8 eV binding energy and more faint features near E_F which are well discernible at 16 and 17 eV photon energy but not at 21 eV, where their photoemission intensity is weak due to final state effects. This surface state has previously been studied theoretically and in ARPES experiments at selected photon energies (21.2 eV), 55 eV, 8.44 and 24 eV, respectively.[6]

Very recently, we succeeded in the MOCVD deposition of epitaxial ternary (Sb_xB_1-x)₂Te₃ (0<x<1) thin films at 400 °C using the tailor-made precursors Et₂Te₂, i-Pr₃Sb and Et₃Bi, which were characterized by XRD.[7]

Fig. 6 shows the 006 pole figures of (Bi_0.5Sb_0.5)₂Te₃ films grown on Al₂O₃(0001) (left) and Si(100) (right) at 400 °C. The epitaxial (Bi_0.5Sb_0.5)₂Te₃ film grown on Al₂O₃ only shows a single peak, while the 0006 pole figure of the film grown on Si shows a pattern due to the hexagonal discs standing on the substrate surface. The epitaxial relationship of the film and the Al₂O₃ substrate is (Bi_0.5Sb_0.5)₂Te₃ (001)ǁAl₂O₃(0001).

The films grown on Al₂O₃(0001) substrates show a very smooth surface morphology as was shown by SEM, AFM and TEM, while those grown on Si(100) are rather polycrystalline. The chemical composition of the crystalline films (XRD) was investigated by EDX and XPS and the in-plane transport properties of the MOCVD grown (Bi_xSb_1−x)₂Te₃ films were determined by Hall effect measurements using the van der Pauw geometry in order to determine, how the carrier concentration n, the carrier mobility µ and the conductivity σ are affected by the bismuth content. For pure Sb₂Te₃ films, a high carrier mobility of about 330 cm²V^-1s^-1 was found, comparable to values measured for bulk single crystals and other high quality Sb₂Te₃ films grown by ALD, MOCVD and MBE. With increasing bismuth content x, the carrier mobility decreases to a minimum value of 244 cm²V^-1s^-1 for x = 0.63. With further increasing bismuth content, the mobility increases up to 260 cm²V^-1s^-1 for pure Bi₂Te₃.

References

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