Vladimir P. Popov

SOI nanowires as sensors for charge detection

The properties of silicon-on-insulator nanowires (SOI NWs) fabricated by means of electron lithography and gas etching of SOI in XeF2 or SF6:CFCl3 have been investigated. The method used to fabricate the nanowires was found to require no additional anneal to be given to the final structure for defect removal after nanostructuring. The sensitivity of SOI NWs to negative protein BSA molecules in the pH 7.4 buffer solution was shown to be as high as 1 femtomoles. The gate characteristics of SOI NWs were used to determine the charge density of particles adsorbed on the NW surface. A charge density of 4.6 × 1011 cm−2 was estimated for a 1 femtomole protein concentration. The combined use of open-channel structures with top gates was employed for determining the charge state of structure surfaces after different chemical treatments. Chemical treatments giving rise to a density of the negative charges on the surface of NWs ranging in the interval (7–23) × 1011 cm−2 were examined. Treatments in methanol (after removal of the native oxide) were found to provide stabilization of the SOI surface over a 3-h interval after the treatments.

SOI nanowire for the high-sensitive detection of HBsAg and α-fetoprotein

Silicon-on-isolator-nanowires (SOI-NWs) were used for the label-free, real-time biospecific detection of the hepatitis B marker HBsAg and cancer marker α-fetoprotein (AFP). Specific protein-protein recognition was carried out using individual NWs that were functionalized with antibodies. To solve the problem of non-specific binding of target protein molecules to the sensor element the use of a reference NW with immobilized antibodies against non-target proteins was proposed. Using individual SOI-NW surface functionalization allowed the fabrication of a NW array, containing working NWs and reference NWs within one chip. It was shown that this approach allows us to reach a detection limit up to 10(-14) and 10(-15) M for HBsAg and AFP, respectively. Our investigations also allowed us to reveal the influence of the charged state of the target protein molecules and antibodies in solutions with various pH values on the target protein detection limit. A high sensitivity NW-detector is of interest for the creation of diagnosticums for hepatitis B and for the early stages of cancer diseases.

A SOI-nanowire biosensor for the multiple detection of D-NFATc1 protein in the serum

The detection of cancer protein marker D-NFATc1 in the serum with a reusable nanowire (NW) chip based on silicon-on-insulator (SOI) structures was demonstrated. The NW surface was modified with aptamers against D-NFATc1 to attain the biospecific detection of the target protein. Two fabricated NW chip types – with narrow NWs (w = 90 nm) and wide NWs (w = 3 μm) – were compared with respect to their reuse, i.e. the realizability of the repeated detection–regeneration cycles upon D-NFATc1 detection in the serum. The analysis of the serum has shown that the signal obtained with wide NWs was much more stable than that obtained with narrow NWs. This makes the SOI-NW biosensor with wide NWs much more suitable for protein analysis in biological fluids. The signal stability exhibited by the wide NWs allowed for performing repeated detection–regeneration cycles of this chip for multiple detection of D-NFATc1 protein in the serum with 10−14 M sensitivity. Although the narrow NW chip allows for attaining higher sensitivity (with the concentration detection limit DL = 10−15 M), it exhibits much less signal stability upon analysis of multicomponent biological fluids (serum).

Silicon nanowire transistors for electron biosensors

A method of nanostructuring of silicon-on-insulator (SOI) layers on the basis of gas etching in XeF2 or SF6:CFCl3 is developed for the purpose of obtaining SOI nanowire structures. SOI nanowire transistors (SOI NWTs) with free channels, used as sensors in electron detectors, are fabricated and tested. The results of experiments show that the method used to fabricate nanowires requires no high-temperature operations for elimination of defects after nanostructuring of SOI layers. The sensitivity of SOI NWTs to test molecules of bovine serum albumin is 10−15 mole/liter, which is one of the best results for nanowire biosensors.

SOI-nanowire biosensor for detection of D-NFATc1 protein

The nanowire (NW) detection is one of the fast-acting and high-sensitive methods, which can recognize potentially relevant protein molecules. A NW-biosensor based on the silicon-on-insulator (SOI)-structures has been used for biospecific label-free real time detection of the NFATc1 (D-NFATc1) oncomarker. For this purpose, SOI-nanowires (NWs) were modified with aptamers against NFATc1 used as molecular probes. It was shown that using this biosensor it is possible to reach sensitivity of 10−15 M. This sensitivity was comparable to that of the NW-biosensor with immobilized antibodies used as macromolecular probes. The results demonstrate that approaches used in this study are promising for development of sensor elements for high-sensitive diagnostics of diseases.

SOI nanowire transistor for detection of D-NFATc1 molecules

Nanowire (NW) detection is one of the fast and highly sensitive methods. An NW biosensor based on silicon-on-insulator (SOI) structures are used in the reported study for real-time label-free biospecific detection of the NFATc1 (D-NFATc1) cancer marker. For this purpose, the SOI NWs are functionalized with NFATc1 aptamers used as macromolecular probes. It is demonstrated that such a biosensor can ensure a detection limits up to 10−15 M, which is comparable with the sensitivity ensured by an NW biosensor with immobilized antibodies used as macromolecular probes. The results of this study demonstrate that such approaches to the development of sensor elements for highly sensitive diagnostics of diseases are really promising.

Charge state of SOI nanowire sensors

The initial charge state of SOI nanowire sensors after their long storage in air ambient has been identified by measuring the drain-gate characteristics of SOI nanowire transistors. In the examined structures, the density of the surface charge was found to be ranging within the interval 2×1012–4×1012 cm−2. Two surface treatments were given to the samples to clean the surface of SOI nanowire sensors from contaminations; among these treatments an optimal one was chosen enabling stabilization of the charge state of the sensor surface. The value of the surface charge in sensors immersed in buffer solutions with different pH values was measured.

Diamond – Graphite Heterostructures Formed by Nitrogen and Hydrogen Implantation and Annealing

Graphitic-diamond heterostructure may be very helpful not only for high frequency or power devices but also for new generation of electronic devices like single electron transistors or quantum computers operated at room temperature. The goal of our work was a formation of nanothin amorphous carbon or graphite layers with sp3 or sp2 hybridization inside the nitrogen doped synthetic monocristalline diamond by high dose hydrogen implantation. It was found that there is a “critical” dose of 50 keV hydrogen molecular ions equal to 4x1016 cm-2 above which an irreversible drop of the sheet resistivity in implanted layer occurs after annealing above 1000 oC. The nature of this conductivity was investigated and it was shown that variable range hopping mechanism of 3D conductivity dominates in investigated temperature interval. Four times higher value for the onset of this conductivity in comparison with “critical” dose for graphitization is explained by interaction of implantation induced defects with nitrogen atoms and surface defects.

Ultra-thin SOI Layer Nanostructuring and Nanowire Transistor Formation for FemtoMole Electronic Biosensors

In this work nanostructuring of SOI layers with tenth nanometer thickness was made at last stage of electronic biochip producing using fluorine gas-plasma content etching. Electrical characterization verities that used fabrication approach produces high-quality devices operating up to temperatures close to liquid helium. The sensitivity of SOI NWs to negative ions Cl- in aqua solution (pH 6) was shown to be as high as 10 femtoMoles. The sensitivity of SOI NWs to negative protein BSA molecules in pH 7.4 buffer solution was shown to be as high as 1 femtoMoles.

Charge accumulation in the buried oxide of SOI structures with the bonded Si/SiO2 interface under γ-irradiation: effect of preliminary ion implantation

In this study, we examined the effect of preliminary boron or phosphorous implantation on charge accumulation in the buried oxide of SOI-MOSFETs irradiated with ?-rays in the total dose range (D) of 105?5???107?rad. The buried oxide was obtained by high-temperature thermal oxidation of Si, and it was not subjected to any implantation during the fabrication process of SOI structures. It was found that implantation with boron or phosphorous ions, used in fabrication technologies of SOI-MOSFETs, increases the concentration of precursor traps in the buried oxide of SOI structures. Unlike in the case of boron implantation, phosphorous implantation leads to an increased density of states at the Si/buried SiO2?interface during subsequent ?-irradiation. In the ?-irradiated SOI-MOSFETs, the accumulated charge density and the density of surface states in the Si/buried oxide layer systems both vary in proportion to kiln?D. The coefficients ki for as-fabricated and ion-implanted Si/buried SiO2?systems were evaluated. From the data obtained, it was concluded that a low density of precursor hole traps was a factor limiting the positive charge accumulation in the buried oxide of as-fabricated (non-implanted) SOI structures with the bonded Si/buried SiO2?interface.