In-operando Photoemission under bias: A tool for devices optimization in the semiconductor field

With the continuous shrinking of device sizes, the importance of surface and interface properties has become more and more important for their performances. Photoemission, a material analysis technique based on the photoelectric effect, is widely available and has proven to be extremely useful in the development of semiconductor devices thanks, in part due to its good surface sensitivity. It is able to measure the core levels (CL) and valence bands (VB) binding energies (BE) of different materials within its analysis depth. However, it’s potential in correlating device performances and physico-chemical characterisation still needs to be further developed.

In present technology, electrical parameters like the effective workfunction (WF_eff) or the flat band voltage (V_FB), the effective oxide thickness (EOT), and the various charges and dipoles present in the gate stack are critical for performances and are usually analysed through capacitance voltage (CV) measurements but often require prior knowledge and multiple devices. A more straightforward characterization method, including process conditions, would thus be welcome. This could be met with the use of photoemission. However, as the photoemission process itself involves modification of the charge states in the analysed device it will induce energy shifts that differ from the intrinsic band diagram. This problem can be partially solved by biasing the gate of the devices to ground, together with the Si substrate. But a full analysis will strongly benefit from a non-zero bias allowing measurements at flatband condition, making the task of comparing the band energies of different gate stacks, accurate and much less complicated.

For future technologies, 2D materials have shown novel properties useful for ultrafast electro-optical electronic devices. Moreover, these materials will enable exotic electrical functionality that can be further altered via factor such as strain, and twist angles, resulting in new properties from the superposition and interaction of the individual atomic lattices in the stack. The full band structure of these 2D-based devices can be investigated by Angle-Resolved PhotoEmission (ARPES) that provides access to the position-and momentum-resolved quasiparticle spectral function, which contains information on dispersion and many-body interactions for low energy excitations around the Fermi level. Combining ARPES with a non-invasive charge carrier control technique (in-operando), where a bias is applied to the sample stack, provides a unique method to investigate the charge carrier electrical properties near conduction band edge, relevant for electron transport measurement. The approach even can be extended to measurements where a current is applied between source and drain electrodes contacted to the stack or even FET devices.

During this PhD work, the potential of in-operando photoemission for semiconductor device characterization will be investigated to reveal the link between electrical performances and chemico-physics characteristics for both present and future technologies. It will make use of a large variety of flavor of photoemission such as classical XPS, high energy (HAXPES) and ARPES, including micro-ARPES at synchrotron site and time resolved photoemission.