Suitability of MO precursors for hybrid MBE using TGA

Thin film materials have impacted many areas of research such as electronics, optics, energy generation, and storage. The phenomenal rise in the development of thin films is likely to continue as they have become the key elements of technological advancements. There are several deposition techniques producing thin films such as CVD, MOCVD, ALD, and MBE all of which require the starting materials to be in the gas form during growth. An alternative technique, hybrid molecular beam epitaxy (hMBE), which has been demonstrated to provide superior control over cation stoichiometry by accessing self-regulated growth kinetics, relies on metal-organic (MO) precursors to supply the element of interest to the sample on which an epitaxial film is grown.¹ In contrast to the deposition techniques such as CVD, MOCVD, and ALD where the MO precursor is supplied to the deposition reactor using a carrier gas, MBE reactor pressures are not compatible with the supply of the MOs using a carrier gas to grow functional thin films. Instead, a heated gas supply is used, and MO precursor selection becomes limited to MO precursors with sufficiently high vapor pressures.² This makes it particularly important to know the vapor pressure curves of MO precursors of interest to determine their suitability for the hMBE approach.

The procedure on how to accurately utilize MO precursors depends exceedingly on the temperature at which a precursor should be held for sufficient vapor pressure. An excellent source for this is vapor pressure versus temperature curves which can be determined via thermogravimetric analysis (TGA) using the Langmuir equation. Due to its thermal stability and very well-known pressure values at temperatures most MO substances evaporate and decompose, benzoic acid was used as the calibration standard. Later, the vapor pressures of the MO precursors such as TTIP, VTIP, Ta(V)n-butoxide, Hf(IV)tert-butoxide, Zr(IV)tert-butoxide were calculated based on this calibration. The suitability of these precursors and their tendency to generate nonvolatile residues at high temperatures will be discussed along with their prospects to utilize them in hMBE systems.



1. Jalan, B., et al., J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 27, 461–464 (2009).

2. Brahlek, M. et al. Adv. Funct. Mater. 28, 1–41 (2018).