Anri Nakajima

Photoluminescence of porous Si, oxidized then deoxidized chemically

We examined the change in photoluminescence spectra of porous Si when it is oxidized then deoxidized chemically. After both steps, photoluminescence shifted to higher frequencies and increased in intensity. These shifts to higher frequencies indicate the photoluminescence is a result of the quantum size effect. Moreover, the increase in photoluminescence intensity after oxidation suggests that termination by hydrogen on the porous Si surface does not always play a key role in the photoluminescence mechanism.

Microstructure and electrical properties of Sn nanocrystals in thin, thermally grown SiO2 layers formed via low energy ion implantation

We have developed a simple technique for fabricating Sn nanocrystals in thin thermally grown SiO2 layers using low energy ion implantation followed by thermal annealing. The formed Sn nanocrystals have excellent size and depth uniformity. Their average diameter is 4.2 nm with a standard deviation of 1.0 nm. Our experimental results clearly reveal that a stable depth of Sn exists in the SiO2 layer at about 2 nm from the SiO2/Si interface. Most of the Sn nanocrystals reside near this stable depth. The I–V characteristics of the diode structure show a clear Coulomb blockade region of 0.12 V and a Coulomb staircase at 4.2 K. A Coulomb blockade region around 0 V was observed until reaching a temperature of 77 K. The features of these nanocrystals will open up new possibilities for the creation of novel devices.

Si single electron tunneling transistor with nanoscale floating dot stacked on a Coulomb island by self-aligned process

We fabricated a Si single electron tunneling transistor which has a nanoscale floating dot gate stacked on a Coulomb island by a self-aligned process. This device exhibits drain current (Id) oscillations due to the Coulomb blockade effect and quantized threshold voltage (Vth) shifts resulting from a single electron tunneling from the channel to the floating dot gate. The high on/off current ratio of the Id oscillation combined with the quantized Vth shifts leads to the possibility of developing ultralow power consumption memory.

Microstructure and optical absorption properties of Si nanocrystals fabricated with low‐pressure chemical‐vapor deposition

We report a simple technique for fabricating a layer of isolated Si quantum dots on SiO2 glass substrates. This technique uses conventional low‐pressure chemical‐vapor deposition for an extremely short deposition time in the early stage of poly‐Si film growth. The layer after a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. The measurements of optical absorption coefficient α show that the absorption edge for Si nanocrystals shifts to higher energies compared to that of bulk Si, indicating a widening of the energy gap caused by quantum size effects. The linear relationship (αhν)1/2 against hν suggests that the Si nanocrystal, whose diameter is as small as 10 nm, basically maintains the properties of an indirect band‐gap semiconductor. Special attention must be paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots.

Formation of Sn nanocrystals in thin SiO2 film using low-energy ion implantation

This letter reports on a simple technique for fabricating Sn nanocrystals in thin SiO2 film using low-energy ion implantation followed by thermal annealing. These Sn nanocrystals have excellent size uniformity and position controllability. Their average diameter is 4.8 nm with a standard deviation of 1.0 nm. Most of the Sn nanocrystals reside at the same depth. The lateral edge-to-edge spacing between neighboring Sn nanocrystals is fairly constant: about 3 nm. A narrow as-implanted ion distribution profile and the effect of the SiO2–Si interface are considered to contribute to the size uniformity and position controllability. The features of these nanocrystals will open up new possibilities for novel devices.

Coulomb blockade in Sb nanocrystals formed in thin, thermally grown SiO2 layers by low-energy ion implantation

Sb nanocrystals were formed in thin, thermally grown SiO2 layers using low-energy ion implantation followed by thermal annealing. These Sb nanocrystals have good size and position uniformity. Both the narrow as-implanted profile and the compressive strain that exists near the SiO2/Si interface supposedly contribute to the uniformity. The I–V characteristics of the diode structure show a Coulomb blockade region around 0 V and a Coulomb staircase at 4.2 K. The Coulomb blockade region was observed up to a temperature of 100 K. The technique offers the possibility of developing practical Si-based single-electron devices.

Microstructure of porous silicon

We examined the microstructure of luminescent porous silicon by cross‐sectional high‐resolution transmission electron microscopy and found a threadlike structure consisting of Si microcrystals. We also found Si microcrystals with sizes ranging about 3–20 nm randomly distributed throughout the porous silicon.

Room temperature operation of Si single-electron memory with self-aligned floating dot gate

Reports on a new Si single-electron memory device comprised of a narrow channel field effect transistor (FET) having an ultra-small selfaligned floating dot gate and its ability to exhibit clear, single-electron memory effects at room temperature.

Single electron charging of Sn nanocrystals in thin SiO/sub 2/ film formed by low energy ion implantation

We report on a simple technique for fabricating a Sn nanocrystal array in thin SiO/sub 2/ film. This technique uses low energy ion implantation followed by thermal annealing. Isolated Sn nanocrystals 5 nm in diameter are formed in an array with excellent size and position uniformity. Barrier height between a Sn nanocrystal and the substrate was obtained by measuring the temperature and frequency dependence of the capacitance of the diode structure. Single electron charging effects of the Sn nanocrystals were observed from current-voltage characteristics.