In situ real-time Rutherford Backscattering Spectrometry study of Ni/Ge Interaction

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Pondo, Kanyembo Justin
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Silicon-based microelectronic technology is approaching its performance limits due to the rapid downscaling of electronic devices to ever smaller dimensions. Germanium, with its high electron and hole mobility, high carrier injection velocity, very low carrier free-out temperature, etc. is generally considered to be a possible replacement for silicon as the semiconductor of choice for high-performance devices. Before germanium can be adopted a suitable material for making electrical contact to the active areas of the germanium based devices must be identified. In analogy with the current Si-based technology where metal-silicides are used as contacts for the active areas of the devices, the use of metal-germanides is proposed for this purpose. Of all possible metal-germanides, nickel monogermanide (NiGe) appears to be the most promising. In order to successfully make use of nickel germanides in Ge-based devices it is essential to thoroughly understand all processes and mechanisms involved in germanium metallization as well as the properties of the phases formed. This is necessary in order to avoid or control problems that can be encountered during integration. Some of the important information required during fabrication includes the identification of the phase formation sequence, dominant diffusing species and growth kinetics during the germanidation process. Rutherford backscattering spectrometry (RBS) is one of the most powerful and frequently used analytical tools for analysis of composition, thickness and depth profiles of thin solid films or surface and near-surface regions of solids. It has proved to be a very valuable technique in studying the growth characteristics during phase formation. In conventional RBS a number of samples are prepared and annealed in sequence for various durations or at different temperatures to induce differing amounts of phase formation. These specimens are subsequently analyzed to unravel the phase formation characteristics. In contrast, the in situ real-time (i.e. during annealing) RBS used in this study allows all the phase formation information to be conveniently obtained from one sample in a single experimental run. This has the advantages of decreasing the workload, reducing material consumption and eliminating the influence of small differences in the sample preparation procedure and experimental conditions. Additionally, it enables one to follow the reaction at all stages of compound growth which limits the risk of overlooking important steps during the phase formation process. Two samples prepared in high vacuum by electron beam evaporation have been used in this investigation. In both samples thin layers of tantalum approximately 5-6 Å thick have been used as inert markers to monitor the direction of atomic mobility. The first sample had a configuration of Ge<100>/Ta(5 Å)/Ni(800 Å) while the configuration of the second sample was Ge<100>/Ta(6 Å)/Ge(490 Å)/Ni(470 Å). The solid state reactions of nickel with germanium were induced by in situ ramped thermal annealling in a scattering chamber. Using a 2 MeV 4He+ beam the formation characteristics of the nickel germanides were captured in real-time. The phase formation sequence has been identified to begin with the formation of Ni5Ge3 followed by NiGe. The RBS data acquired from the first sample revealed the formation of the Ni5Ge3 phase to commence at around 145 oC and also showed the simultaneous growths of Ni5Ge3 and NiGe. It was found from the first sample that Ni was the sole diffusing species during Ni5Ge3 formation. The formation of Ni5Ge3 in the second sample was observed at room temperature. In this sample it was found that both Ni and Ge diffused during the formation of NiGe. However, Ni was the dominant diffusing species during the growth of the NiGe phase. The activation energy of diffusional growth of Ni5Ge3 was found to be 0.75 ± 0.01 eV. Keywords: Nickel germanides, In situ real-time, Ramped thermal anneal, Dominant diffusing species, RBS, simultaneous growth.
Back scattering Physics , Spectrometry