Well-known perovskite titanates such as SrTiO₃ and BaTiO₃ generally exhibit n-type semiconductor behavior due to the prevalence of oxygen vacancies that can form under common synthesis conditions. Each oxygen vacancy donates two electrons and in SrTiO₃, for example, a metallic state appears at a relatively low carrier density of about 10¹⁷ cm^-3. Besides oxygen vacancies, cation vacancies may also form in titanates. In particular, formation of A-site cation (Sr, Ba) vacancies is energetically favorable and leads to effective acceptor doping. It is known experimentally that the Fermi level in intrinsic SrTiO₃ is close to the conduction band bottom, i.e., the material is effectively an n-type wide-gap semiconductor, which means that compensation by acceptor-type cation vacancies is not observed and the cation defect densities must be much lower than 10¹⁷ cm^-3, which can be ignored in conventional transport analysis.

Fig. 1. Photocurrent in SrTiO₃ as a function of temperature under ultraviolet (UV) or simultaneous ultraviolet and infrared (UV+IR) excitation for (a) bulk crystal and (b) thin film. Infrared quenching of photocurrent can be seen below 35 K. (c) Energy level diagram illustrating the localization of photogenerated holes. Photocurrent quenching occurs when the trapped holes are released by infrared light illumination.

The situation is very different when optoelectronic applications are considered, where photogenerated carrier mobility, trapping, and lifetime are important parameters. The presence of cation defects needs to be considered because the presence of acceptor states close to the top of the valence band can influence hole trapping and thus the recombination rate of photogenerated non-equilibrium carriers. Unfortunately, typical optical absorption or photoelectron emission spectroscopic techniques cannot detect such low-density vacancy states. We have therefore developed a technique based on measuring the infrared quenching effect on photocurrent, which is illustrated in Fig. 1. Carriers are generated by ultraviolet light across the band gap and the photocurrent is measured as a function of temperature. The magnitude of the photocurrent is determined by the recombination rate of photocarriers, which depends on whether holes are trapped or delocalized. Hole trapping normally occurs at low temperatures in the presence of acceptor defects, reducing the recombination rate and increasing the photocurrent, as can be seen at around 35 K in Figs. 1(a,b). If a crystal is illuminated with ultraviolet and infrared light at the same time, the holes trapped at so-called sensitizing centers can be detrapped and the photocurrent drops due to the increased recombination rate. The energy level diagram illustrating this mechanism is shown in Fig. 1(c). The delocalization of holes depends on the depth of the acceptor state, E₁, and the temperature. For Sr vacancy defects in SrTiO₃, thermal detrapping occurs between 30 K and 90 K, depending on how the crystal was grown. The value of E1 can be obtained by fitting the photocurrent temperature dependence close to the infrared quenching transition temperature. For the SrTiO₃ samples used in this study, E₁ is about 60 meV.

Bulk SrTiO₃ crystals are grown by the Verneuil method and always contain Sr vacancies due to the high crystal growth temperature. Thin films are usually grown at much lower temperatures and it is thus not clear if the Sr vacancy density would be similar to bulk crystals or not. As shown in Fig. 1(b), the photocurrent behavior of a thin film grown at 1200°C is indeed similar to the bulk behavior and the infrared quenching effect can be clearly observed. No infrared quenching is observed in films grown at lower temperatures, but the photocurrent is also much lower due to higher density of lattice defects and strong carrier trapping.

The work shows that attempts to improve titanate lattice quality by increasing the crystal growth temperature can be counter-productive in optoelectronic applications due to the spontaneous formation of cation vacancies at higher temperatures. We demonstrate that photocurrent measurement with multiple light sources can be a simple technique for verifying the presence of cation defects that can be difficult or impossible to detect by other spectroscopic techniques [1].

References

• [1] N. Osawa, R. Takahashi, and M. Lippmaa, Appl. Phys. Lett. 110, 263902 (2017).

Authors

• N. Osawa, R. Takahashi, and M. Lippmaa