(Invited) Characteristics of Si Single-Electron Transistor under Illumination

Abstract

Current of single-electron transistors (SETs) flows by means of one-by-one electron transfer. In a SET, the number of charges in the SET dot is determined by the gate voltage and capacitance between the gate and the dot. If the SETs are made of a semiconductor, the dot accumulates electrons and holes according to the positive and negative gate voltages, respectively. However, since the source and drain electrodes of SETs are made of n-type semiconductor, holes cannot be injected to the dot. A single-hole transistor (SHT) usually needs a p-type semiconductor for source and drain [1]. Here, we measured the SHT characteristics by the use of SET structure device under illumination. Holes excited in the semiconductor is thought to flow through the SET island according to the negative gate voltages. We made silicon SETs, which have n-type source and drain electrodes on thin SOI (silicon on insulator) wafer, by the use of pattern dependent oxidation (PADOX) method [2]. Clear current oscillations and Coulomb diamonds as a function of positive gate voltage were achieved. When the gate voltage was scanned to negative direction, almost no current was observed in the dark or under week photo-excitation. However, current oscillations were observed under the strong photo- excitation of 330 μW/cm2 from a halogen-lamp, though the peak current was limited by the photo-excited currents. Coulomb diamonds were also detected at negative gate voltage region. Photo-excited carriers in Si quantum dots also show another interesting features such as single-electron CCD [3] where electrons and holes exist in the same dot at the same time. In a SET, when a few holes generated by photo-excitation are trapped in the one side of the SET island under the vertical electric field, the number of trapped holes is detected by the electron current flowing through the other side of the island [4]. In the few electron regime, not only the number of net charges but also the each number of electrons and holes plays an important role to decide the electrical characteristics of SETs. However, these characteristics appeared only when the electrons and holes are divided by strong electric field. In this report, we show the characteristics of photo-excited SETs without strong vertical electric fields. When the gate voltage (V g) of a SET was scanned from low to high, drain current (I d) shows interesting features depending on the photo-excited conditions. Figure 1 shows I d-V g characteristics under three different conditions. In any conditions, Coulomb oscillations were observed. The characteristics measured in the dark are fundamental ones, in which the first peak corresponds to the first electron transferred to the SET island. When the SET was continuously irradiated, a new current peak emerges at the lower V g region, which is thought to be an effect of a positive charge (a hole) generated by photo excitation. The facts that the new current peak suddenly dropped and recovered again can be understood that the generated hole recombined with electron flowing through the SET island and another hole was generated again. In the one-shot excitation condition, the SET was illuminated only once at the starting V g, but scan was performed in the dark. Although the new peak was detected first, the current characteristics return to those of in the dark after the sudden current drop. These phenomena were thought to be caused by a photo-excited hole trapped in SiO2 near the SET island. Acknowledgement This work is supported by JSPS KAKENHI (15H01706, 16H0433906 and 16K18073) and by the Cooperative Research Project of the Research Center for Biomedical Engineering with Research Institute of Electronics, Shizuoka University.