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This section details the initial experiments performed using hard x-ray photoelec- tron spectroscopy (HAXPES) in order to understand the capabilities and limits of both the technique and the systems used. As the DCU research group was only the second user of a new HAXPES end station on beamline X24a at the NSLS, it was necessary to undertake a number of baseline experiments in order to determine the measurement capabilities of this system.

Firstly, the increase in sampling depth and energy resolution over conventional x-ray photoelectron spectroscopy (XPS) is explored. The sampling depths avail- able at these higher photon energies was investigated in order to understand the layer thicknesses which are measurable with this technique. These experiments were performed at a range of photon energies and compared with conventional

nesses. The relationship between energy resolution and photon energy is also demonstrated. This shows the advantage of conventional XPS in the character- isation of the sample surface and thin deposited films, while HAXPES is better suited to the study of bulk material properties or chemical changes in multilayer structures and at buried interfaces.

The next study involved the extension of the conventional application of XPS, in the chemical characterisation of surfaces, to the study of buried interfaces in Al/SiO2/Si structures. The motivation behind this study was the investigation of

Al diffusion and Al-silicide formation at the buried SiO2/Si interface. The advan-

tage of HAXPES in this particular study is that the silicide formation was driven by the thermal annealing of pre-formed Al/SiO2/Si MOS structures. Conventional

XPS studies of this system would involve the measurement of samples following each step in the fabrication process and subsequent thermal anneal. The lim- ited sampling depth would also necessitate the fabrication of multilayer structures with sufficiently thin layers. HAXPES, however, can measure pre-fabricated and annealed structures, thus simplifying the experiment.

A novel application of HAXPES in the evaluation of the electronic properties of metal-oxide-semiconductor (MOS) structures is demonstrated. The capability of HAXPES in providing chemical and electronic information on much larger depth scales than conventional XPS1–3 has potential application in the study of buried

interfaces, as found at the oxide/semiconductor interface, particularly if changes at this interface are of interest following the subsequent deposition of a metal cap- ping layer in the fabrication of MOS structures. Photoemission sampling depths in excess of 10 nm4 allow for direct comparison between results obtained from elec-

trical characterisation techniques and photoemission experiments, as these mea- surements can be made on identical structures, thereby bridging the gap between interface chemistry and electrical properties at buried interfaces. Both n and p- doped substrates were investigated before and after the deposition of a metal gate in order to study any difference in Fermi level behaviour between the different doping types. The use of n and p-doped substrates also ensured samples with both a small and large difference between the substrate and metal workfunction for both Al (whose workfunction is close to the flatband position of n-InGaAs)

and Ni (whose workfunction is close to the flatband position of p-InGaAs). This enables the Fermi level movement through the entire bandgap to be investigated. Initial experiments were performed on HfO2/InGaAs based MOS structures, where

high (Pt) and low (Al) workfunction metals are deposited as the gate metal in or- der to induce band-bending in the valence and conduction bands of the InGaAs, thus resulting in the Fermi level moving towards the valence or conduction bands respectively. These results were compared with surface potential measurements de- rived from capacitance-voltage profiling (C-V), the conventional method by which to determine the electrical performance of a MOS structure, on near identical samples in order to evaluate the accuracy of the HAXPES method. While Fermi level differences are detected for both n and p-InGaAs samples, and the samples capped with high and low workfunction metals, an issue with photon energy drift was encountered and this issue along with a working solution will be discussed.

Finally, the complementary HAXPES and C-V method of characterising the electrical structure of MOS devices was employed to characterise SiO2/Si based

structures. For this study, exploring the combination of HAXPES and electri- cal characterisation techniques, the experiments were performed on SiO2/Si MOS

structures. This system was selected as the interpretation of the multi-frequency C-V response is well developed,5 the interface is representative of an unpinned surface Fermi level, and the structure allows accurate determination (±50 meV6) of surface potential for a given gate voltage based on C-V measurements. The samples were formed over n and p-doped silicon substrates and were capped with high (Ni) and low (Al) work function metals to induce surface potential shifts at the SiO2/Si interface for examination by the HAXPES and C-V methods. Using a

photon energy of 4150 eV in these investigations allowed for the simultaneous de- tection of photoemission signals from metal, oxide and substrate core levels. The band gap of Si (1.1 eV) is wide enough to allow differences in the binding energy (BE) of the n and p-doped substrate core levels to be detected, which directly reflect different Fermi level positions in the band gap.7 Attempts were made to perform similar experiments on forming gas (H2N2) annealed SiO2/Si based struc-

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