Ultra-thin single crystal silicon with the (100) surface formed by the local-oxidation-of-silicon (LOCOS) on a silicon-oninsulator (SOI) substrate becomes a quasi-direct band-gap semiconductor due to the quantum mechanical confinement effect. The device is a simple pn diode in a planar structure. Electro-luminescence (EL) has been observed by the lateral carrier injections into the two-dimensional quantum well.

To slow the translocation of single-stranded DNA (ssDNA) through a solid-state nanopore, a nanopore was narrowed, and the effect of the narrowing on the DNA translocation speed was investigated. In order to accurately measure the speed, long (5.3 kb) ssDNA (namely, ss-poly(dA)) with uniform length (±0.4 kb) was synthesized. The diameters of nanopores fabricated by a transmission electron microscope were controlled by atomic-layer deposition. Reducing the nanopore diameter from 4.5 to 2.3 nm slowed down the translocation of ssDNA by more than 16 times (to 0.18 μs base(-1)) when 300 mV was applied across the nanopore. It is speculated that the interaction between the nanopore and the ssDNA dominates the translocation speed. Unexpectedly, the translocation speed of ssDNA through the 4.5 nm nanopore is more than two orders of magnitude higher than that of double-stranded DNA (dsDNA) through a nanopore of almost the same size. The cause of such a faster translocation of ssDNA can be explained by the weaker drag force inside the nanopore. Moreover, the measured translocation speeds of ssDNA and dsDNA agree well with those calculated by molecular-dynamics (MD) simulation. The MD simulation predicted that reducing the nanopore diameter to almost the same as that of ssDNA (i.e. 1.4 nm) decreases the translocation speed (to 1.4 μs base(-1)). Narrowing the nanopore is thus an effective approach for accomplishing nanopore DNA sequencing.

Recently, dielectric breakdown of solid-state membranes in solution has come to be known as a powerful method for fabricating nanopore sensors. This method has enabled a stable fabrication of nanopores down to sub-2 nm in diameter, which can be used to detect the sizes and structures of small molecules. Until now, the behavior of dielectric breakdown for nanopore creation in SiN membranes with thicknesses of less than 10 nm has not been studied, while the thinner nanopore membranes are preferable for nanopore sensors in terms of spatial resolution. In the present study, the thickness dependence of the dielectric breakdown of sub-10-nm-thick SiN membranes in solution was investigated using gradually increased voltage pulses. The increment in leakage current through the membrane at the breakdown was found to become smaller with a decrease in the thickness of the membrane, which resulted in the creation of smaller nanopores. In addition, the electric field for dielectric breakdown drastically decreased when the thickness of the membrane was less than 5 nm. These breakdown behaviors are quite similar to those observed in gate insulators of metal-oxide-semiconductor devices. Finally, stable ionic-current blockades were observed when single-stranded DNA passed through the nanopores created on the membranes with thicknesses of 3–7 nm.

For the nanopore sensing of various large molecules, such as probe-labelled DNA and antigen-antibody complexes, the nanopore size has to be customized for each target molecule. The recently developed nanopore fabrication method utilizing dielectric breakdown of a membrane is simple and quite inexpensive, but it is somewhat unsuitable for the stable fabrication of a single large nanopore due to the risk of generating multiple nanopores. To overcome this bottleneck, we propose a new technique called “two-step breakdown” (TSB). In the first step of TSB, a local conductive thin portion (not a nanopore) is formed in the membrane by dielectric breakdown. In the second step, the created thin portion is penetrated by voltage pulses whose polarity is opposite to the polarity of the voltage used in the first step. By applying TSB to a 20-nm-thick SiN membrane, a single nanopore with a diameter of 21–26 nm could be fabricated with a high yield of 83%.

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