(Invited) Effect of Individual Dopants in Nano-SOI-MOSFETs and Nano-pn-Diodes
In recent years, novel MOSFET characteristics dominated by a single dopant-atom were reported by several groups [1-4], where carrier transport mechanism is tunneling through individual dopant atoms. In this background, we have proposed and demonstrated a variety of dopant-atom devices, i.e., dopant-atom (DA) transistors , DA memories , DA turnstiles [6-8] and DA photonic devices [9-11]. In those devices, only one or a few dopants are intentionally used and one dopant works as a quantum well for electron (or hole) tunneling transport. Here, we present a brief overview of such devices from experimental and theoretical point of view.
P-donor in nano-Si
When we focus on a phosphorous (P) donor atom, it is known that the ionization energy, or the binding energy, with respect to the Si conduction band minimum, is ~45 meV. Therefore, single-electron tunneling P-atom devices can operate only at low temperatures, below ~20 K, since at high temperatures electrons are thermally excited and tunneling transport mechanism does not work. However, when a dopant is embedded in sufficiently small Si structures, its ionization energy is enhanced due to dielectric and quantum size confinement effects , leading to high-temperature operation.
Recently, we have demonstrated operation of donor-atom SOI-MOSFETs at around 100 K in specifically-designed nano-stub-channels . A good correlation is found between the operation temperature and binding energy (Ea), with the highest SET-operation temperature being achieved for a P donor with Ea≅ 100 meV .
A few coupling donors
So far, in most investigated dopant-atom devices, the dopants were introduced in random positions. Only a few works addressed directly the control of dopants, either in number, using single ion implantation , or in position, with atomic manipulation using scanning tunnelling microscope tips . Most recently, we have challenged a more practical and simpler doping technique using nanoscale doping masks . The channel of SOI-MOSFET was selectively doped within an area of ~30 nm in width by the conventional diffusion process. Even in such a classical doping process, the number of dopants in the doped area may be controlled to around 5. I-V characteristics measured for these selectively-doped FETs are consistent with the model .
Individuality of dopants appears not only in nano-FETs, but also in nano-pn diodes. In fact, we have reported dopant-induced random telegraph signals (RTS) in nanoscale pn diodes , and attributed the RTS to charging and discharging of a single dopant near the pn junction.
Observation of dopant potential
In research of dopant-atom devices, it is extremely important to establish an observation tool to monitor dopants’ positions and their individual potentials. For this purpose, we have developed low-temperature Kelvin probe force microscopy (LT-KFM) which allows measurements of devices under regular operation. Using this technique, we succeeded in detection of potential profiles due to individual dopants and potential changes due to single-electron injection effects [18-21].
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