Influence of crystal orientation and oxygen vacancies on the circular photogalvanic effect in hydrogenated SrTiO3

Abstract

SrTiO3 is a widely studied material with various novel phenomena. This study investigates the circular photogalvanic effect (CPGE) in hydrogenated SrTiO3 (H:STO), which has strong spin–orbit coupling. The CPGE photocurrent intensity in H:STO exhibits a strong dependence on crystal orientation. The largest photocurrent is observed for H:STO (001), followed by (110), while (111) is the weakest. This anisotropy is ascribed to the orientation-dependent hydrogen diffusion rate. The [001] direction provides straight diffusion channels that facilitate hydrogen migration, whereas the [110] and [111] directions involve more tortuous pathways, thereby leading to a weaker CPGE. A nearly linear relationship between the CPGE photocurrent and hydrogenation time is observed. However, a distinct suppression of the CPGE emerges at an “anomalous region” around 55 s. This suppression results from the competition between interstitial hydrogen and hydrogen trapped at oxygen vacancies. During the hydrogenation process, oxygen desorbs from the surface, thereby generating vacancies. Hydrogen preferentially occupies oxygen vacancies to form a more stable configuration. This process reduces the density of interstitial hydrogen, consequently leading to the observed decrease in CPGE photocurrent. This study demonstrates that oxygen vacancies and crystal orientation are critical factors that must be considered during hydrogenation.