Electrical properties of high permittivity epitaxial SrCaTiO3 grown on AlGaN/GaN heterostructures

Eric N Jin,Brian P Downey,Matthew T Hardy,Jason A Roussos,Tyler A Growden,D Scott Katzer,Neeraj Nepal,Vikrant J Gokhale,Jeffrey P Calame,David J Meyer

doi:10.1063/5.0063295

Abstract

Epitaxial integration of perovskite oxide materials with GaN has unlocked the potential to improve functionality and performance in high-power RF and power-switching applications. In this work, we demonstrate structural and electrical properties of high dielectric constant Sr1−xCaxTiO3 epitaxial layers grown on AlGaN/GaN/4H-SiC high-electron-mobility transistor structures with compositions ranging from x = 0 to x = 0.53 and oxide film thicknesses ranging from 7 to 126 nm. We show (111) orientation in the SrCaTiO3 (SCTO) thin films using a 1 nm (100) TiO2 buffer layer grown by RF-plasma-assisted oxide molecular beam epitaxy. Current–voltage measurements show up to 5 orders of magnitude reduced leakage with SCTO films when compared to Schottky contacted samples. Capacitance–voltage measurements show minimal hysteresis, an extracted dielectric constant (κ) as high as 290, and a fixed positive interface charge density of 2.38 × 1013 cm−2 at the SCTO/AlGaN interface. The direct integration of the SCTO layer does not significantly affect the two-dimensional electron gas (2DEG) density or the channel mobility with the 2DEG density as a function of SCTO thickness having good agreement with 1D Poisson–Schrödinger simulations. RF characterization of interdigitated capacitors using the SCTO films on unintentionally doped GaN/SiC shows that the films maintain their high κ into microwave frequencies and only exhibit a slight reduction in κ with increased lateral electric fields. These results demonstrate that the integration of a high-κ oxide with GaN can potentially improve electric field management in RF high-electron-mobility transistors and increase the device breakdown voltage without significant degradation to channel transport properties.

Highlights

Advances in epitaxial growth techniques in recent years have enabled the monolithic integration of multifunctional perovskite oxides on a wide variety of conventional semiconductors including Si, Ge, GaAs, and GaN.1–8 The GaN platform is especially attractive due to its wide bandgap, thermal and chemical stability, high saturation electron velocity, and large breakdown electric field.9 Notably, the development of AlGaN/GaN high-electron-mobility transistors (HEMTs)10 brought forth significant improvements in solid-state power-switching and high-power, high-frequency electronics.11 GaN-based HEMTs have demonstrated RF power densities of >40 W/mm,12 but as operation extends to millimeter-wave frequencies, breakdown voltage reduces, lowering output power density
The epitaxial growth of (111)-oriented Sr1−xCaxTiO3 on AlGaN/GaN heterostructures is demonstrated using a thin TiO2 buffer layer, and electrical measurements are performed on vertical capacitor structures to probe the independently varied. Current–voltage (I–V) and C–V characteristics and lateral Interdigitated capacitors (IDCs) structures to investigate microwave frequency response
The “extreme-κ” of the SCTO layer minimizes the reduction in barrier capacitance for a given dielectric thickness, which can result in maintaining frequency performance and device scaling, while improving the electric field management and breakdown

Summary

INTRODUCTION

Advances in epitaxial growth techniques in recent years have enabled the monolithic integration of multifunctional perovskite oxides on a wide variety of conventional semiconductors including Si, Ge, GaAs, and GaN. The GaN platform is especially attractive due to its wide bandgap, thermal and chemical stability, high saturation electron velocity, and large breakdown electric field. Notably, the development of AlGaN/GaN high-electron-mobility transistors (HEMTs) brought forth significant improvements in solid-state power-switching and high-power, high-frequency electronics. GaN-based HEMTs have demonstrated RF power densities of >40 W/mm, but as operation extends to millimeter-wave (mm-wave) frequencies, breakdown voltage reduces, lowering output power density. A transistor design with “extreme-κ” (>100) dielectric layers has been proposed for improved electric field management, which reduces peak electric fields and enhances the breakdown voltage of the device.. A transistor design with “extreme-κ” (>100) dielectric layers has been proposed for improved electric field management, which reduces peak electric fields and enhances the breakdown voltage of the device.16,17 Perovskite oxides such as SrTiO3 (STO) and CaTiO3 (CTO) could be considered “extreme-κ” dielectric materials in this sense, as they have bulk room temperature dielectric constants of 30018 and 180,19 respectively. The alloy Sr1−xCaxTiO3 (SCTO) has a rich phase diagram in which the Ca alloy fraction can both tune the dielectric constant and induce ferroelectric and antiferroelectric phase transitions.20 Integrating these oxides with GaN can both provide further reductions in gate leakage in HEMTs due to their high dielectric constants and provide better field management.. RF characterizations are presented to compare values of κ at microwave frequencies

EXPERIMENTAL DETAILS

SCTO structural characterization

Current–voltage measurements

Capacitance–voltage measurements

RF measurements

CONCLUSIONS