Exploiting kinetics and thermodynamics to grow phase-pure complex oxides by molecular-beam epitaxy under continuous codeposition

We report the growth of ${\mathrm{PbTiO}}_3$ thin films by molecular-beam epitaxy utilizing continuous codeposition. In addition to the requirements from thermodynamics, whether the resulting film is single-phase ${\mathrm{PbTiO}}_3$ or not at a particular temperature depends strongly on the film growth rate and the incident fluxes of all species, including titanium. We develop a simple theory for the kinetics of lead oxidation on the growing film surface and find that it qualitatively explains the manner in which the adsorption-controlled growth window of ${\mathrm{PbTiO}}_3$ depends on lead flux, oxidant flux, and titanium flux. We successfully apply the kinetic theory to the dependence of the growth of ${\mathrm{BiFeO}}_3$ on oxidant type and surmise that the theory may be generally applicable to the adsorption-controlled growth of complex oxides by MBE.

Figure 1

(a) The adsorption-controlled growth window of ${\mathrm{PbTiO}}_3$ as a function of PbO gas pressure and substrate temperature, as calculated from thermodynamics [13, 55]. (b) Indexed x-ray diffraction patterns and RHEED images taken along the substrate $\langle 100\rangle$ azimuth of films grown within each of the three regions of condensed phases: (i) ${\mathrm{PbO}+\mathrm{PbTiO}}_3$; (ii) ${\mathrm{PbTiO}}_3$ only; and (iii) ${\mathrm{TiO}}_2$. Arrows have been added to aid the eye in the ${\mathrm{TiO}}_2$-phase RHEED image. Impurities are labeled as follows: A=anatase ${\mathrm{TiO}}_2$; M=massicot PbO; L=litharge PbO; P=pyrochlore ${\mathrm{Pb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_6$. (See Ref. [56] for a full list of growth conditions and a description of secondary phases appearing in part (b)).

Figure 2

Dependence of observed film phase as a function of titanium flux, lead flux, or oxidant pressure. Only one variable (source flux or oxidant pressure) was changed in addition to temperature at once. Phases observed when (a) oxidant background pressure and lead flux were fixed, (b) oxidant background pressure and titanium flux were fixed, and (c) titanium and lead fluxes were fixed. (See Ref. [56] for a full list of growth conditions.)

Figure 3

(a) Adsorption-controlled growth window of ${\mathrm{BiFeO}}_3$ as a function of molecular oxygen pressure as calculated from thermodynamics, after Ihlefeld and co-workers [21]. (b) Dependence of observed film phase as a function of fraction ozone in the oxidant blend. We note that different phases of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ appear as secondary phases outside the growth window at the two different oxidant mixtures. Ozone is $250\times$ more active in oxidizing bismuth than diatomic oxygen in the process chamber in which we grew ${\mathrm{BiFeO}}_3$. (See Ref. [56] for a full list of growth conditions and for our measurements of ozone activity.)