AVS2013 Session SA+AS+MG+SS-TuA: HAXPES Studies on Interfaces and Buried Layers

Synchrotron radiation has proven to be a very important tool for the study of materials in applications including earth sciences, energy and semiconductors. Most of the early applications focused on the study of surface phenomena using valence and core level spectroscopies using soft x-rays. Although one of the first studies at a multi-GeV synchrotron used hard x-rays, its practical use was limited by low counting rates. With the advent of high brightness synchrotron sources along with highly efficient electron energy analyzers, x-ray photoelectron spectroscopy using multi-keV x-rays has seen a rebirth for the study of buried interfaces and bulk materials properties. This talk will discuss the evolution of synchrotron radiation photoelectron spectroscopy from the early experiments to the present day.

Over the past 15 years, due to intrinsic limitations of laboratory X-ray sources, photoemission using synchrotron radiation has played an increasing role in solving issues of technologically-relevant materials and systems. With narrow spectral widths, synchrotron sources enable photoemission at uncomparable effective energy resolutions for refined assessment of chemical states at interfaces [1]. The broad energy range offers ultimate surface sensitivities with soft x-rays [2], and also much deeper photoelectron escape depths in the hard x-ray range which is crucial to investigate buried interfaces intrinsically found in devices. This contribution will highlight the advantages and achievements of hard x-ray photoemission (HAXPES) compared to soft x-ray photoemission, through a selection of recent studies performed on nanotechnological materials and devices (CMOS high-k/metal gate stacks, memory devices) [3]. Finally, we will briefly mention new developments: first, the extension of photoelectron spectromicroscopy (XPEEM) from chemical state and band structure imaging, to hard x-rays excitation (HAXPEEM) [4]. Second, the application of inelastic background analysis of HAXPES spectra.

[1] O. Renault et al., Appl. Phys. Lett. 81 (2002), 3627; 90 (2007), 052112 ; J. Appl. Phys. 96 (2004) 6362 ; E. Martinez et al., Appl. Surf. Sci. 285 (2012), 2107.

[2] K. Huang, P. Reiss, O. Renault et al., ACS Nano 4 (2010), 4799; O. Renault et al., Appl. Phys. Lett. 87 (2005), 163119.

[3] R. Boujama et al., J. Appl. Phys. 111 (2012), 054110; P. Calka et al., J. Appl. Phys. 109 (2011) 124507.

[4]C. Wiemann et al., Appl. Phys. Lett. 100 (2012), 223106.

In this talk, we will discuss developments of the HAXPES technique at the NIST beamline X24A at the National Synchrotron Light Source for the study of electronic materials. Examples will include nitrogen treatment of HfO2 gate stacks on Si, depth profiling of the HfO2/SiO2 interfaces, Ga and As “out-diffusion” at semiconductor/oxide interfaces, band offsets and Schottky barrier heights at semiconductor/oxide and diamond/metal interfaces, and oxygen vacancies in N doped TiO2 and solid-oxide fuel cells. In all cases, the increased probing depth of HAXPES over traditional lab based XPS is crucial to study the electronic structure of entire overlayers and/or buried interfaces with thicknesses of industrial significance.

The introduction of Ge in CMOS devices beyond the 14 nm technology node requires effective passivation of the Ge gate stack. Besides the interface passivation, a highly scaled gate stack is needed for the next generation of CMOS devices. Scaling of the gate stack can be achieved by several means, for instance by changing process conditions. In this work, we investigate the influence of both the high‑κ stack and the metal gate on the properties of a GeO_xS_y interfacial passivation layer by Hard X-ray Photoelectron Spectroscopy (HAXPES). Using high energy x-rays (4 to 8 keV), we are able to probe the buried interface between the high-κ layer and the Ge substrate, and hence to reveal direct information on the chemistry and the thickness of the GeO_xS_y passivation layer. Note that such a buried interface is not accessible using a “standard” XPS tool relying on Al K_α x-ray radiation.

In this study, we considered three high-κ materials (Al₂O₃, HfO₂ and an Al₂O₃/HfO₂ bi-layer) and three metal gates (TiN, TiW and Pt). Samples have been measured both directly after atomic layer deposition and after forming gas anneal, to investigate the effect of a thermal treatment on the interfacial properties. To disentangle the impact of the particular metal gate, comparison is made to a high-κ stack sample without metal gate.

We first demonstrate the importance of analyzing full stacks, as no effect of annealing was observed on the stacks without metal gates, while clear modification of the Ge/high-κ interfacial layer thickness is observed when a gate is present. We than show that this effect on interfacial layer thickness depends on both the high‑κ and the metal gate material used. This can lead for example to an increase (i.e. HfO₂/TiN) or to a decrease (i.e. Al₂O₃/TiN) of the interfacial layer thickness after annealing. As a global trend, the thinnest interfacial layers are obtained for pure Al₂O₃. However, the interfacial layer thickness appears to be more sensitive to variations in the metal gate rather than the high-κ material. We also show that the introduction of a very thin Al₂O₃ layer (~2 Å) between the Ge substrate and the HfO₂ layer strongly influences this observed sensitivity of the interfacial layer properties to the metal gate and forming gas anneal. Aside from quantifying the interfacial layer thickness, we will also analyze changes in the interfacial layer from a chemical point of view. As a conclusion, the final layer structure (hence, the Equivalent Oxide Thickness of the gate stack) is thus a complex interplay between the initial GeO_xS_y thickness before forming gas anneal and the chemistry of the high-κ and metal gate materials.

Magnetoelectric coupling (the electrical control of magnetic properties or vice versa) has promising applications in computer memory and logic, magnetic sensing and energy scavenging. Magnetoelectric interfaces are a potential new method to improving magnetoelectric coupling strength and controllability. We utilize x-ray absorption spectroscopy, photoemission electron microscopy, and second harmonic generation to understand both the order parameters of the individual layers and the resulting interface. This talk will focus on bilayers composed of ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) and ferroelectric PbZr_0.2Ti_0.8O₃ (PZT). Through photoemission electron microscopy imagining, ME coupling was confirmed at the interface. X-ray absorption spectroscopy of Mn was taken across wedged samples of varying ferroelectric and ferromagnetic thicknesses. The change of Mn valences at different thicknesses of LSMO and PZT helps to understand ME coupling and impact of thickness on the ME properties. This work suggests a strategy for improving not only magnetoelectric devices, but also magnetic systems. This work is supported by West Virginia's Higher Education Policy Commission.