Interface Chemistry and Electronic Structure in Voltage-Adjustable Paraelectric Capacitances for 5G Applications

Abstract:

The deployment of 5G technology has raised significant issues of energy consumption. This can be minimized by adjusting the antenna impedance to 50 ohms; therefore, a voltage-controllable impedance matching circuit with highly tunable capacitance (varactor) is needed. Specifically, a tuning ratio of at least 5 and low dielectric losses in the 5G band (2-5 GHz) are essential to preserve energy efficiency (leakage current 1 nA). Voltage-tunable paraelectric (PE) capacitors meet these requirements due to their field-dependent relative permittivity.

The perovskite Ba_0.45Sr_0.55TiO₃ (BST) is widely used in 4G varactors for its excellent tunability/losses compromise, offering superior quality factors compared to other technologies. However, an acoustic resonance frequency f_r of 3 GHz due to electrostriction limits present-day 4G applications. Thus, 5G require improved varactors, specifically with f_r> 5 GHz. A BST thickness below 50 nm, shifting f_r above 6 GHz, can meet these specifications. However, these thin varactors exhibit degraded tunability and higher leakage current, due to reduced dielectric permittivity near electrodes from uncompensated polarization charges and static leakage through bulk-limited transport. Enhancing the Schottky Barrier Height (SBH) at the electrode/BST interface through band alignment can significantly reduce leakage by preventing carrier injection into the dielectric.

Ab initio calculations highlight the importance of incorporating a perovskite Interface Control Layer (ICL) of a few nanometers of La_1-xSr_xMnO₃ between the bottom electrode and the BST in varactors. Factors such as polar discontinuity and interfacial B-site cation environment asymmetry can enhance interface polarizability and the Schottky Barrier Height (SBH).

Understanding the mechanisms controlling electrode/PE interfaces is crucial for 5G applications, revealing chemical and electrostatic modifications of SBH and chemical potential. We propose investigating the electronic and chemical states of these interfaces at the sub-micrometric scale, compared with DFT calculations.

Combinatorial Pulsed Laser Deposition (CPLD) was used to vary chemical compositions and thicknesses orthogonally on a single substrate. Chemical modulation at the BST/ LSMO_0.7 interface was achieved by inserting 1.2 nm thick LSMO_1-x layer, which changes the chemical and electronic structure of the interface and directly influences the SBH. We investigated the interface chemistry and electronic structure relative to BST thickness. Photoemission spectroscopy showed modulation of the work function, interface carrier density at the Fermi level, and interface polarization, demonstrating the impact of the 1.2 nm thick chemically modulated ICL.

Finally, we fabricated voltage-tunable BST varactors using ICL engineering. We investigated the SBH versus polar discontinuity at the interface. Operando HAXPES provided access to both top and bottom interfaces, allowing us to estimate the electronic band structure and quantify the SBH. Inducing a polar discontinuity at the interface resulted in a reduction of leakage current. For 10×10 µm² BST-engineered varactors, the leakage current is expected to be close to 1 nA, an improvement by two orders of magnitude compared to current 4G cellphone varactors.

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