An oxidation process was applied to the fabrication of fluoride resonant tunneling diodes (RTDs) on Si substrates. The oxidation process was carried out after the growth of an ultrathin CaF2 layer on Si, and expected to passivate the Si surface in pinholes generated in the CaF2 layer. Leakage currents of Au/Al/CaF2/Si(111) metal insulator semiconductor (MIS) diodes were extremely reduced by introducing this process. Au/Al/CaF2/CdF2/CaF2/Si(111) double-barrier RTDs were also fabricated by introducing this process, the leakage currents of which were extremely reduced and a very large peak to valley current ratio (PVCR) of 1500 was obtained. It was also shown that the chemical reaction between a CdF2 well layer and the Si substrate was suppressed by the oxidation process. Owing to this effect, high-temperature growth (up to 300°C) of the CdF2 well layer was realized for the first time in the fluoride RTDs, and resulted in the reduction of undesirable current drift.

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We have studied the electronic structure of SnSe and Na-doped SnSe by means of angle-resolved photoemission spectroscopy. The valence band top reaches the Fermi level by the Na doping, indicating that Na-doped SnSe can be viewed as a degenerate semiconductor. However, in the Na-doped system, the chemical potential shift with temperature is unexpectedly large and is apparently inconsistent with the degenerate semiconductor picture. The large chemical potential shift and anomalous spectral shape are key ingredients for understanding of the novel metallic state with the large thermoelectric performance in Na-doped SnSe.

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