Process temperature-dependent interface quality and Maxwell-Wagner interfacial polarization in atomic layer deposited Al2O3/TiO2 nanolaminates for energy storage applications.

Considering the excellent tunability of electrical and dielectric properties in binary metal oxide based multi-layered nanolaminate structures, a thermal atomic layer deposition system is carefully optimized for the synthesis of device grade Al2O3/TiO2 nanolaminates with well-defined artificial periodicity and distinct interfaces, and the role of process temperature in the structural, interfacial, dielectric and electrical properties is systematically investigated. A marginal increase in interfacial interdiffusion in these nanolaminates, at elevated temperatures, is validated using X-ray reflectivity and secondary ion mass spectrometry studies. With an increase in deposition temperature from 150 to 300 °C, the impedance spectroscopy measurements of these nanolaminates exhibited a monotonic increment in dielectric constant from ∼95 to 186, and a decrement in dielectric loss from ∼0.48 to 0.21, while the current-voltage measurements revealed a subsequent reduction in leakage current density from ∼2.24 × 10-5 to 3.45 × 10-7 A cm-2 at 1 V applied bias and an improvement in nanobattery polarization voltage from 100 mV to 700 mV, respectively. This improvement in dielectric and electrical properties at elevated processing temperature is attributed to the reduction in impurity content along with the significant enhancement in sublayer densities and the conductivity contrast driven Maxwell-Wagner interfacial polarisation. Additionally, the devices fabricated at 300 °C exhibited a higher capacitance density of ∼22.87 fF µm-2, a low equivalent oxide thickness of ∼1.51 nm, and a low leakage current density of ∼10-7 A cm-2 (at 1 V bias), making this nanolaminate a promising material for high-density energy storage applications. These findings highlight the ALD process temperature assisted growth chemistry of Al2O3/TiO2 nanolaminates for superior dielectric performance and multifaceted applications.

Maxwell-Wagner Relaxation-Driven High Dielectric Constant in Al2O3/TiO2 Nanolaminates Grown by Pulsed Laser Deposition.

Multilayer nanolaminates (NLs) of alternate ultrathin sublayers of Al2O3 and TiO2 (ATA) with the thickness ranging ∼2 to 0.5 nm were fabricated by optimized pulsed laser deposition (PLD). Maxwell-Wagner (M-W) relaxation-induced interfacial polarization was realized and engineered by precisely controlling the sublayer thicknesses and the number of interfaces. X-ray reflectivity and cross-sectional transmission electron microscopy measurements of ATA NLs revealed an artificial periodicity with well-defined uniformly thick amorphous sublayers with chemically and physically distinct interfaces down to a sublayer thickness of ∼0.8 nm. The dielectric constants and loss of ATA NLs were found to increase from ∼60 to 670 and decrease from ∼0.9 to 0.16, respectively, as sublayer thicknesses reduced from ∼2 to 0.8 nm. However, for a sublayer thickness below 0.8 nm, the trend was reversed. Furthermore, temperature-dependent impedance spectroscopy studies revealed two distinct thermally activated relaxation processes, corresponding to TiO2 and Al2O3 sublayers, corroborating the M-W relaxation. The conductivity contrast between the sublayers of ATA NLs enhanced with reducing sublayer thickness and plateaued at a sublayer thickness of ∼0.8 nm, resulting in dominant M-W interfacial polarization and a high cut-off frequency of ∼50 kHz. These results demonstrate that ATA NLs grown by PLD may find application as potential high-k materials for next-generation nanoelectronic devices.