Identification of Single Dopant Position in Silicon Nanotransistor


Yukinori Ono, Mohammed A. H. Khalafalla, Katsuhiko Nishiguchi, and Akira Fujiwara
Physical Science Laboratory

 Future nanoscale field-effect transistors are expected to be fatally sensitive to electronic charges of a small number of dopants in the channel. On the other hand, there are reports of emerging devices using dopant atoms as functional part, such as quantum dots, for the manipulation of electronic charges. It is therefore important to establish a technology for detecting and controlling the dopant charges. We have so far reported the detection of single boron atoms in nanoscale silicon transistors [1]. Here, we report the measurements and analysis for identifying the depth position of observed single boron atoms [2].
 Nano transistors whose gate length is 40 nm were fabricated on a silicon-on-insulator substrate [3]. The transistors comprise the channel lightly doped with boron, p-type source/drain, and electrically formed leads inserted between the channel and the source/drain. The insertion of the leads prevents dopant diffusion from the source/drain and enables us to investigate the conductance of the channel containing only a few dopant atoms. The conductance G was measured at 6 K. Figure 1 shows the contour plot of dLogG/dVF as a function of the front-gate voltage (VF) and of the substrate back-gate voltage (VB). The conductance modulation, indicated by the arrows, was observed in transistors containing a single boron atom [Figs. 1(b) and (c)] but not in an undoped one [Fig. 1(a)]. These modulations are due to the trapping of a single hole by the boron atom. From the capacitance analysis of the data, we have identified the depth position of the single boron atom as near the front interface (b) and around the middle of the silicon layer (c).
 By doing similar analysis using drain bias as a parameter, we will also be able to obtain information about the dopant’s lateral position, i.e., the position along the transport channel, which will lead us to a complete identification of dopant locations in a nanotransistor.

[1] Y. Ono et al., Appl. Phys. Lett. 90 (2007) 102106.
[2] M. A. H. Khalafalla et al., Appl. Phys. Lett. 91 (2007) 263513.
[3] Y. Ono et al., Appl. Surf. Sci. 254 (2008) 6252.

Fig. 1. Measurement results for an undoped transistor (a), and for transistors containing a single boron atom near the front-gate interface (b) and around the mid of the silicon layer (c). OX and BOX are the gate oxides of the front and back gates, respectively.

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