Defects in the $\beta$-Ga$_2$O$_3$($\bar201$)/HfO$_2$ MOS system and the effect of thermal treatments

TL;DR

This study investigates defect properties in the $eta$-Ga$_2$O$_3$/HfO$_2$ MOS system after thermal treatments, revealing defect bands, their spatial distributions, and the impact of annealing on defect densities using combined experimental and simulation methods.

Contribution

It introduces a physics-based approach combining impedance measurements and simulations to analyze defect states and their distributions in $eta$-Ga$_2$O$_3$/HfO$_2$ MOS capacitors after annealing.

Findings

Identified two defect bands in HfO$_2$ with specific ionization energies.

Demonstrated that annealing affects defect energy distributions and densities.

Confirmed oxygen vacancy accumulation near the Cr/HfO$_2$ interface via XPS.

Abstract

We have investigated the properties of the $\beta$-Ga$_2$O$_3$($\bar201$)/HfO$_2$/Cr/Au MOS (metal-oxide-semiconductor) system after annealing (450$^\circ$C) in different ambient conditions (forming gas, N$_2$ and O$_2$). Defect properties have been analyzed using an approach combining experimental impedance measurements with physics-based simulations of the capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of $\beta$-Ga$_2$O$_3$/HfO$_2$ MOS capacitors. This approach enabled us to detect two defect bands in HfO$_2$ characterized by thermal ionization energies of ~1.1eV (acceptor-like) and ~2eV (donor-like) attributed to a polaronic self-trapping state and an oxygen vacancy in HfO$_2$, respectively. This study demonstrates how thermal treatments affect the energy distributions and densities of the observed defects. The adopted methodology also enabled the extraction of the spatial distribution of defects across the HfO$_2$ thickness and Cr/HfO$_2$ interface. The high concentration of oxygen vacancies close to the Cr/HfO$_2$ interface extracted from experimental and simulated electrical data is confirmed by in-situ XPS analysis which shows how Cr is scavenging oxygen from the HfO$_2$ and creating the donor band confined near the Cr/HfO$_2$ interface. This donor band density is observed to be reduced after annealing as per simulation and unchanged for different annealing conditions. We speculate this may be due to the formation of dense films and polyforms of HfO$_2$ under different ambient as revealed by high-resolution TEM images.