Noriyuki Uchida

Phosphorus doping and hydrogen passivation of donors and defects in silicon nanowires synthesized by laser ablation

Phosphorus (P) doping was performed during the synthesis of silicon nanowires (SiNWs) by laser ablation. At least three types of signals were observed by electron spin resonance (ESR) at 4.2K. Phosphorus doping into substitutional sites of crystalline Si in SiNWs was demonstrated by the detection of an ESR signal with a g value of 1.998, which corresponds to conduction electrons in crystalline Si, and by an energy-dispersive x-ray spectroscopy spectrum of the PKα line. The ESR results also revealed the presence of defects. These defects were partially passivated by hydrogen and oxygen atoms.

Tensile-Strained GeSn Metal–Oxide–Semiconductor Field-Effect Transistor Devices on Si(111) Using Solid Phase Epitaxy

We demonstrate tensile-strained GeSn metal–oxide–semiconductor field-effect transistor (MOSFET) devices on Si(111) substrates using solid phase epitaxy of amorphous GeSn layers. Amorphous GeSn layers are obtained by limiting the adatom surface mobility during deposition. Subsequent annealing transforms the amorphous layer into single-crystalline GeSn by solid phase epitaxy. Single-crystalline GeSn layers with 4.5% Sn and 0.33% tensile strain are fabricated on Si(111) substrates. To verify the structural quality of thin-film GeSn as a channel material, we fabricate ultrathin GeSn p-channel MOSFETs (pMOSFETs) on Si(111). We demonstrate junctionless depletion-mode operation of tensile-strained GeSn(111) pMOSFETs on Si substrates.

Heavily doped silicon and nickel silicide nanocrystal composite films with enhanced thermoelectric efficiency

In this study, we have developed highly efficient thermoelectric materials based on p-type and n-type composite films of Si nanocrystals and Ni silicide nanocrystals. The heavy doping of the films with boron or phosphorus and thermal annealing of the films caused formation of nanocrystals with high electrical conductivities, low thermal conductivities, and high Seebeck coefficients, consequently leading to the high dimensionless figures of merit (ZT). For the p-type (B-doped) and n-type (p-doped) films, ZT is 0.13 and 0.06, respectively, which were much higher than that of bulk Si (<0.01) at RT and nanostructured bulk Si.

Phosphorus ion implantation in silicon nanocrystals embedded in SiO2

We have investigated phosphorus ion (P+) implantation in Si nanocrystals (SiNCs) embedded in SiO2, in order to clarify the P donor doping effects for photoluminescence (PL) of SiNCs in wide P concentrations ranging in three orders. Some types of defects such as Pb centers were found to remain significantly at the interfaces between SiNCs and the surrounding SiO2 even by high-temperature (1000 °C) annealing of all the samples. Hydrogen atom treatment (HAT) method can efficiently passivate remaining interface defects, leading to significant increase in the intensity of PL arising from the recombination of electron-hole pairs confined in SiNCs, in addition to significant decrease in interface defects with dangling bonds detected by electron spin resonance. From both the results of the P dose dependence before and after HAT, it is found that the amount of remaining defects is higher for samples with SiNCs damaged by implantation with relatively lower P+ doses and then annealed, and that through HAT the observ...

Formation of hydrogenated boron clusters in an external quadrupole static attraction ion trap

We report the formation of icosahedral B(12)H(8) (+) through ion-molecule reactions of the decaborane ion [B(10)H(x)(+) (x=6-14)] with diborane (B(2)H(6)) molecules in an external quadrupole static attraction ion trap. The hydrogen content n of B(12)H(n)(+) is determined by the analysis of the mass spectrum. The result reveals that B(12)H(8)(+) is the main product. Ab initio calculations indicate that B(12)H(8)(+) preferentially forms an icosahedral structure rather than a quasiplanar structure. The energies of the formation reactions of B(12)H(14)(+) and B(12)H(12)(+) between B(10)H(x)(+) (x=6,8) ions, which are considered to be involved in the formation of B(12)H(n)(+), and a B(2)H(6) molecule are calculated. The calculations of the detachment pathway of H(2) molecules and H atoms from the product ions, B(12)H(14)(+) and B(12)H(12) (+), indicate that the intermediate state has a relatively low energy, enabling the detachment reaction to proceed owing to the sufficient reaction energy. This autodetachment of H(2) accounts for the experimental result that B(12)H(8)(+) is the most abundant product, even though it does not have the lowest energy among B(12)H(n)(+).

Synthesis of New Amorphous Semiconductors Assembled from Transition-Metal-Encapsulating Si Clusters

We synthesized amorphous films composed of transition-metal-encapsulating Si clusters (MSin: M=Mo or Nb) by deposition of hydrogenated MSinHx clusters onto silica substrates followed by annealing at 500 ?C for dehydrogenation. The MoSin (n=7?16) cluster films are semiconductors with an optical gap >0.6 eV and resistivity >1 ? cm. In particular, the MoSi12 cluster film has a large gap of 1.1 eV and resistivity of 120 ? cm with high hole mobility of 32 cm2/(V s). In these films, Si atoms form amorphous networks similar to those in hydrogenated amorphous Si but the electronic disorder is reduced by the use of MSin clusters as the building blocks.

Initial Growth of Polycrystalline Silicon Films on Substrates Subjected to Different Plasma Treatments

Undoped 0.2-µm-thick polycrystalline Si (poly-Si) films were deposited on fused quartz substrates by plasma-enhanced chemical vapor deposition from a SiH4–H2 mixture. All films were prepared under the same deposition conditions, just after the substrates were exposed for 1 min to CF4–He plasma excited with various rf powers. Poly-Si films with improved crystallinity and large grains were obtained when the films were deposited on substrates with the proper degree of surface roughness of uniform size and shape. These films were also found to have lower values of stress and higher values of g, as compared with those of poly-Si films on substrates with a flat surface or an extremely rough surface. The X-ray diffraction (XRD) spectra exhibited only the texture, and the intensity was proportional to the third power of the average grain size estimated from the width values of the XRD spectra. These results suggested that the growth of grains is three-dimensionally controlled and depends on the surface morphology of the substrates, while the concentration of grains per unit area is roughly independent of the morphology of the substrates.

Fabrication of high-k/metal-gate MoS 2 field-effect transistor by device isolation process utilizing Ar-plasma etching

We investigated a device isolation process for MoS2-based devices and fabricated high-k/metal-gate MoS2 MOSFETs. An Ar-ion etching process was utilized for the device isolation process. It circumvents damage in the device channel, as confirmed by Raman spectroscopy. A top-gate MoS2 MOSFET was fabricated with a HfO2 thin film 16 nm thick as the gate insulator. Utilizing capacitance–voltage (C–V) measurements, the capacitance equivalent thickness (CET) was estimated to be 5.36 nm, which indicates that a gate stack with the sufficiently thin insulator was successfully realized. The device exhibited a mobility of 25.3 cm2/(Vs), a subthreshold swing (SS) of 86.0 mV/decade, and an ON/OFF ratio of 107. This satisfactory device performance demonstrates the feasibility of the proposed device isolation process.

Thermoelectric properties of heavily boron- and phosphorus-doped silicon

In recent years, nanostructured thermoelectric materials have attracted much attention. However, despite this increasing attention, available information on the thermoelectric properties of single-crystal Si is quite limited, especially for high doping concentrations at high temperatures. In this study, the thermoelectric properties of heavily doped (1018–1020 cm−3) n- and p-type single-crystal Si were studied from room temperature to above 1000 K. The figures of merit, ZT, were calculated from the measured data of electrical conductivity, Seebeck coefficient, and thermal conductivity. The maximum ZT values were 0.015 for n-type and 0.008 for p-type Si at room temperature. To better understand the carrier and phonon transport and to predict the thermoelectric properties of Si, we have developed a simple theoretical model based on the Boltzmann transport equation with the relaxation-time approximation.

Synthesis and formation mechanism of hydrogenated boron clusters B12Hn with controlled hydrogen content

We present the formation of hydrogen-content-controlled B(12)H(n) (+) clusters through the decomposition and ion-molecule reactions of the decaborane (B(10)H(14)) and diborane (B(2)H(6)) molecules in an external quadrupole static attraction ion trap. The hydrogen- and boron-contents of the B(10-y)H(x) (+) cluster are controlled by charge transfer from ambient gas ions. In the process of ionization, a certain number of hydrogen and boron atoms are detached from decaborane ions by the energy caused by charge transfer. The energy caused by the ion-molecule reactions also induces H atom detachment. Ambient gas of Ar leads to the selective generation of B(10)H(6) (+). The B(10)H(6) (+) clusters react with B(2)H(6) molecules, resulting in the selective formation of B(12)H(8) (+) clusters. Ambient gas of Ne (He) leads to the generation of B(10-y)H(x) (+) clusters with x=4-10 and y=0-1 (with x=2-10 and y=0-2), resulting in the formation of B(12)H(n) (+) clusters with n=4-8 (n=2,4-8). The introduction of ambient gas also increases the production of clusters. PBE0/6-311+G(d)//B3LYP/6-31G(d)-level density functional theory calculations are conducted to investigate the structure and the mechanism of formation of B(10-y)H(x) (+) and B(12)H(n) (+) clusters.