Leakage current control of SrTiO${}_3$ thin films through Al doping at the interface between dielectric and electrode layers via atomic layer deposition

Al is doped to a SrTiO${}_3$ (STO) thin film grown by atomic layer deposition (ALD) to decrease the leakage current. One ALD cycle of Al${}_2$O${}_3$ is processed either at the top or at the bottom of the STO films, and the electrical properties are compared with those of the undoped STO films. The ≈3.5 nm-thick seed layer is first deposited and crystallized through rapid thermal annealing, and from 5 to 20 nm-thick main layers are subsequently grown for in situ crystallization of the main layer. When an Al${}_2$O${}_3$ deposition cycle is inserted below the first seed layer, the crystallization of the STO films is disturbed, and the bulk dielectric constant degrades from 149 (undoped STO) to 71. When the Al${}_2$O${}_3$ deposition cycle is performed after the STO main layer growth, however, the dielectric constant degradation is minimized (120). In both cases, the leakage current decreases 20 times compared with the undoped STO case because of the conduction band offset increase. Consequently, the equivalent oxide thickness and physical oxide thickness decrease from 0.71 to 0.63 nm and from 10.3 to 8.6 nm, respectively, through Al doping on the top of the STO films with a sufficiently low leakage current (<10${}^{-7}$ A cm${}^{-2}$). (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)