Measurement of the Femtosecond Optical Absorption of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ Heterostructures: Evidence for an Extremely Slow Electron Relaxation at the Interface

The photocarrier relaxation dynamics of an $n$-type ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterointerface is investigated using femtosecond transient absorption (TA) spectroscopy at low temperatures. In both ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures and electron-doped ${\mathrm{SrTiO}}_3$ bulk crystals, the TA spectrum shows a Drude-like free carrier absorption immediately after excitation. In addition, a broad absorption band gradually appears within 40 ps, which corresponds to the energy relaxation of photoexcited free electrons into self-trapped polaron states. We reveal that the polaron formation time is enhanced considerably at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterointerface as compared to bulk crystals. Further, we discuss the interface effects on the electron relaxation dynamics in conjunction with the splitting of the $t_{2g}$ subbands due to the interface potential.

Figure 1

(a) PL spectrum of LAO/STO heterostructure at 7 K under an excitation of 3.82 eV. The PL intensity above 3.0 eV is expanded. (b) Schematic picture of the penetration depth for excitation light at 3.82 and 3.49 eV. (c), (d) PL spectra of LAO/STO heterostructures and Nb-STO crystals near the band gap energy at 7 K under an excitation of 3.82 and 3.49 eV.

Figure 2

(a) TA dynamics of Nb-STO at 7 K under different probe energies. The broken curves show the exponential fitting results with a relaxation time of 40 ps. (b) Enlarged view of the TA dynamics in the range of 3–13 ps at a probe energy of 1.38 eV. (c) TA spectra monitored at 5 and 300 ps. The broken curves for TA at 5 and 300 ps are the fitted curves for a simple Drude dispersion (${\omega }^{-2}$) and Drude dispersion added with a Gaussian function, respectively. (d) Band diagram and TA processes in ${\mathrm{SrTiO}}_3$. Red arrows correspond to ($X$) free carrier absorption and ($Y$) transition from localized states to the free conduction band state. The blue broken arrow refers to the energy relaxation of electrons into localized states.

Figure 3

TA dynamics of (a) Nb-STO and (b) LAO/STO heterostructures at 7 K under an excitation of 3.88, 3.65, and 3.35 eV. The solid curves correspond to fits of a single exponential function in Nb-STO and a double exponential function in LAO/STO heterostructures.

Figure 4

(a) Electron relaxation times of Nb-STO, Ar-STO, and LAO/STO heterostructures as a function of the penetration depth of the excitation light. (b) Schematic band diagram at the LAO/STO heterointerface and the penetration depth for excitation light at 3.88 and 3.35 eV. (c) Schematic graph of the energy diagram of $t_{2g}$ levels of Ti $3d$ orbitals.