Fig. 5

Double enhancement of the thermoelectric power factor in a 2DES. Carrier concentration dependences of a electrical conductivity (σ), b Hall mobility (μ_Hall), c thermopower (−S), and d power factor [PF (S²·σ)] of [1 uc SrTi_1 − xNb _x O₃ | 11 uc SrTiO₃]₁₀ 2DESs (x is ranging from 0.2 to 0.9) at room temperature. Similar to the trends in the bulk values, σ increases almost linearly with n. μ_Hall for lower x samples (x ≤ 0.5) fluctuates around 3–5 cm² V⁻¹ s⁻¹, while that for higher x ones (x ≥ 0.6) is ~6 cm² V⁻¹ s⁻¹. Slope of –S versus log n for bulk SrTi_1 − xNb _x O₃ is –198 μV K⁻¹, which is ∼1.5 times lower than –300 μV K⁻¹ for the 2DESs. Double enhancement of PF is seen in x = 0.6 (5.1 mW m⁻¹ K⁻² at n ∼ 8 × 10²⁰ cm⁻³). Since the PF values are scattered due to the rather large distribution of μ_Hall (3–6 cm² V⁻¹ s⁻¹), we calculated PFs using the relationship between S and n (c) at constant μ_Hall (6 cm² V⁻¹ s⁻¹). The optimized PF of the 2DES should be ~5 mW m⁻¹ K⁻² at n ∼ 8 × 10²⁰ cm⁻³, which doubles that of bulk SrTi_1 − xNb _x O₃ (PF ∼ 2.5 mW m⁻¹ K⁻² at n ∼ 2 × 10²¹ cm⁻³)