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We investigated impacts of the Ar and CF4 plasma during reactive ion etching (RIE) on defect formation in the Ge substrates using the deep-level-transient-spectroscopy (DLTS) technique. It was found that the Ar plasma causes the roughening of the Ge surface. Moreover, the Ar plasma induces a defect with an energy level of 0.31 eV from the conduction band minimum in the Ge substrate, confirming by DLTS spectra. On the other hand, the CF4plasma hardly induces the surface roughness of Ge. However, the CF4plasma induces many kinds of electron and hole traps. It should be noted that the defects associated with Sb and interstitials are widely distributed to around 3-µm.

We have investigated the formation and optoelectronic properties of strain relaxed Ge1−x−y Si x Sn y /Ge1−x Sn x /Ge1−x−y Si x Sn y double heterostructures on ion-implanted Ge substrates. The strain relaxation of Ge1−x−y Si x Sn y and Ge1−x Sn x epitaxial layers was achieved using an ion-implanted Ge substrate. The maximal degree of strain relaxation (DSR) of the Ge1−x Sn x layers was evaluated to be 46%. In addition, we obtained a sharp and strong peak in the photoluminescence (PL) spectra from the sample with a DSR of 46%, while no strong peak was detected from a sample with a smaller DSR (22%). From the theoretical calculation of the energy band structure and the measurement temperature dependence of the PL intensity, the sharp and strong peak can be explained by the transition from an indirect to direct bandgap semiconductor due to the increase of the DSR and a concomitant increase of the Γ-valley electron population. Moreover, the PL intensity increases by the improvement of the crystallinity by a post deposition annealing process.

A thin-film thermoelectric generator composed of p- and n-type poly-Ge1−x Sn x (x ∼ 0.02) on a Si(001) covered with SiO2 has been successfully fabricated by low thermal budget processes (under 300 °C) and demonstrated for the first time. Both the crystallization and dopant activation were simultaneously performed using pulsed UV laser irradiation in flowing water. A recorded activation ratio of Sb in the poly-Ge1−x Sn x enabled a relatively high power factor (9.2 μ Wcm−1 K−2 at RT), which is comparable to the counterparts of n-type Ge1−x Sn x layers epitaxially grown on InP(001).

The interface state density of Al2O3/4H-SiC can be decreased by oxygen radical treatment at room temperature, so we have systematically investigated the mechanism of the interfacial reaction at Al2O3/4H-SiC during oxygen radical treatment. In this study we focus on the characteristics of the Si oxide interlayer, namely whether carbon atoms are included in the film or not, because the interfacial reactions induced by oxygen radical treatment for Al2O3/4H-SiC and Al2O3/Si are different. It is revealed that decarbonization of the SiC x O y layer occurs when the SiC x O y surface is directly exposed to oxygen radicals. Then, the difference in the interfacial reaction caused by oxygen radical treatment between Al2O3/4H-SiC and Al2O3/Si is investigated by intentionally forming SiC x O y and SiO2 interlayers at the Al2O3/4H-SiC interface. Finally, the possible reaction mechanism is discussed on the basis of experimental results. All the results are qualitatively (but clearly) understood by considering that decarbonization of SiC x O y occurs as the starting point.

We have investigated the metal-organic chemical vapor deposition (MOCVD) of Ge1−x Sn x epitaxial layers using tetrakis-dimetyl-amino-tin (TDMASn), which is expected to be more easily decomposable than other Sn precursors such as tri-butyl-vinyl-tin (TBVSn). We found that the deposition rate of Sn atoms using TDMASn is higher than that using TBVSn at a growth temperature as low as 300 °C. Also, we examined the MOCVD growth of a high-Sn-content Ge1−x Sn x layer with TDMASn at a low temperature below 300 °C, and we found that the Sn content saturated at about 5%. The volume of precipitated Sn grains on the surface increases with the partial pressure of Sn precursor, the reason why the substitutional Sn content in Ge1−x Sn x is limited despite increasing the partial pressure of a Sn precursor would not be because of the deposition-rate limitation of Sn by surface chemical reaction but the precipitation of Sn in this MOCVD condition with TDMASn.

Heavily doped Si1−xSnx alloys are strongly desired for next-generation group-IV electronics. A polycrystalline growth study has been conducted on heavily Ga-doped amorphous Si0.9Sn0.1 layers deposited on a tensile-strained Si-on-insulator substrate. P-type conduction and a high Hall hole concentration over 1020 cm−3 are observed for grown Si1−xSnx layers with the initial Ga concentration of 1021 cm−3. In addition, we find that the valence-band offset of the Si1−xSnx relative to Si (ΔEv) is determined by the substitutional Sn content regardless of the hole concentration in the grown layers. Even for the substitutional Sn content less than 5%, a relatively high value of ΔEv (~0.5 eV) could be obtained. These results suggest that with heavily Ga-doped Si1−xSnx alloys could become a p+-source material, which would provide benefits for Si-based tunnel field-effect transistors.

We have investigated the thermal stability of heavily Sb-doped n+-Ge and n+-Ge0.94Sn0.06 epitaxial layers grown on a Ge substrate by annealing up to a temperature of 400 °C. The Ge0.94Sn0.06 sample with a Sb concentration higher than 1020 cm−3 retained superior crystallinity and a smooth surface even after annealing. It also maintained an electron concentration higher than the solid solubility of Sb and the highest substitutional Sb concentration after 400 °C annealing. We found the possibility of a low-contact-resistivity device employing Ge0.94Sn0.06 with a high electron concentration and a metal with an electrode fabrication temperature lower than 400 °C.

We have investigated the crystalline and electrical properties of the epitaxial Hf digermanide (HfGe2)/Ge contact prepared through the rapid thermal annealing (RTA) of a TiN/Hf/Ge(001) sample. Detailed X-ray diffraction measurements demonstrate the formation of an epitaxial HfGe2 layer on a Ge substrate and clarified the orientation relationship between epitaxial HfGe2 and Ge. The surface morphology and interface structure of Hf germanide layers were observed using atomic force microscopy and scanning electron microscopy. The sheet resistance of the Hf germanide layer decreases with increasing RTA temperature and is low owing to the formation of an epitaxial HfGe2 layer.

The impact of a silicon germanium tin (Si x Ge1− x − y Sn y ) ternary alloy interlayer on the Schottky barrier height (SBH) of metal/Ge contacts with various metal work functions has been investigated. Lattice matching at the Si x Ge1− x − y Sn y /Ge heterointerface is a key factor for controlling Fermi level pinning (FLP) at the metal/Ge interface. The Si x Ge1− x − y Sn y ternary alloy interlayer having a small lattice mismatch with the Ge substrate can alleviate FLP at the metal/Ge interface significantly. A Si0.11Ge0.86Sn0.03 interlayer increases the slope parameter for the work function dependence of the SBH to 0.4. An ohmic behavior with an SBH below 0.15 eV can be obtained with Zr and Al/Si0.11Ge0.86Sn0.03/n-Ge contacts at room temperature.

A low-temperature process for the formation of heavily doped polycrystalline Ge (poly-Ge) layers on insulators is required to realize next-generation electronic devices. In this study, we have systematically investigated pulsed laser annealing (PLA) in flowing water for heavily doped amorphous Ge1− x Sn x layers (x ≈ 0.02) with various dopants such as B, Al, Ga, In, P, As, and Sb on SiO2. It is found that the dopant density after PLA with a high laser energy is reduced when the oxidized dopant has a lower oxygen chemical potential than H2O. As a result, for the p-type doping of B, Al, Ga, and In, we obtained a high Hall hole density of 5 × 1019 cm−3 for PLA with a low energy. Consequently, the Hall hole mobility is limited to as low as 10 cm2 V−1 s−1. In contrast, for As and Sb doping, because the density of substitutional dopants does not decrease even after PLA with a high energy, we achieved a high Hall electron density of 6 × 1019 cm−3 and a high Hall electron mobility simultaneously. These results indicate that preventing the oxidation of dopant atoms by water is an important factor for achieving heavy doping using PLA in water.

The effect of Ni to Si ratio on effective work function (Φeff) modulation with phosphorus (P) segregation at the Ni silicide/SiO2 interface was systematically investigated. To discriminate the P segregation effect from other parasitic possibilities of Φeff modulation, which are changes in Ni–Si ratio and crystallinity including crystal grain orientation near the interface, we applied a P post-doping process for Ni2Si (Ni3Si2)/SiO2, NiSi/SiO2, and NiSi2/SiO2 systems. In the post-doping process, P atoms were segregated to the Ni silicide/SiO2 interface after Ni silicide gate formation; hence, independent control of a desired interface P density can be realized. In addition, we newly developed a thin Ti-inserted NiSi2 formation process, by which NiSi2 can be formed at a temperature as low as 450 °C, thus revealing the P-segregated Φeff modulation effect at the NiSi2/SiO2 interface for the first time. As a result, the largest Φeff modulation (ΔΦeff = -0.45 eV) occurs at the P-segregated NiSi2/SiO2 interface, and a Φeff of near the Si conduction band edge (4.13 ±0.04 eV) was realized, although the obtained Φeff values with P segregation at Ni2Si (Ni3Si2)/SiO2 and NiSi/SiO2 were 4.38 ±0.01 eV (ΔΦeff = -0.32 eV) and 4.37 ±0.01 eV (ΔΦeff = -0.29 eV) for the same P dose, respectively. Moreover, we found that ΔΦeff at the NiSi2/SiO2 interface is larger than those at the NiSi/SiO2 and the Ni2Si (Ni3Si2)/SiO2 interfaces, even when the segregated P densities are the same. These results can be explained by an increase in the density of effective interface dipole for Φeff modulation at a high-Si-content interface.

The effect of Ag buffer layers epitaxially grown on Si(111) substrates on the growth of carbon nanotubes (CNTs) has been investigated. We have examined the morphology of Co particles deposited on the epitaxial Ag layer and SiO2, and that of CNTs grown on the epitaxial Ag layer and SiO2. In this study, the epitaxial Ag buffer layer was stable during CNT growth. The density of Co particles on the epitaxial Ag layer was higher than that on SiO2 and the mean diameter of Co particles on the epitaxial Ag layer was smaller than that on SiO2. Furthermore, the diameter distribution of Co particles on the epitaxial Ag layer was narrower than that on SiO2. According to these results of diameter distribution of Co, the density of CNTs grown on the epitaxial Ag layer was higher than that on SiO2. From these results, the epitaxial Ag layer is suitable as an electrode of a CNT device.

The crystal structures and electrical characteristics of Ti–Si–N metal–oxide–semiconductor (MOS) gate electrodes in the mictamict state, which is a comprehensive term referring to amorphous structures both with and without nanocrystals, were investigated. By increasing the N2 concentration of the sputtering ambient, the nitrogen (N) content of the Ti–Si–N films increased and consequently the Ti–Si–N films did not crystallize. At a N2 concentration of more than 3.0%, the N content of the films was almost constant at about 53%, which indicates that all Ti and Si atoms deposited in the form of TiN and Si3N4, respectively. In such Ti–Si–N films with a saturated N content, only 2–3-nm-grain-size nanocrystallites formed, which were embedded in amorphous layers even after post-deposition annealing (PDA) above 900 °C. At the same time, with increasing N2 concentration of the sputtering ambient, the change in the film resistivity after the PDA became smaller and the capacitance equivalent thickness (CET) fluctuation gradually ceased to occur. The work function of the mictamict Ti–Si–N gate electrodes, which were deposited in 5.0% N2 ambient and annealed at 500 °C, was determined to be 4.6 eV.

The dependence of electrical characteristics on the interfacial structure of epitaxial NiSi2/Si contacts formed from a Ni/Ti/Si system has been investigated. The interfacial structure can be successfully controlled by controlling the annealing conditions of the Ni/Ti/Si system. Annealing a Ni/Ti/Si sample at a temperature as low as 350 °C results in the formation of a large number of NiSi2 domains surrounded by {111} facets, and rapid thermal annealing at a temperature higher than 750 °C leads to an atomically flat NiSi2(001)/Si(001) interface. We also found a strong dependence of the Schottky barrier height on the interfacial structure. A measurable current crowding effect at the contact edge region occurs due to the much lower Schottky barrier height of the (111) facet than the (001) interface in a p-type contact.

We have investigated the behavior of local charge-trapping sites in La2O3–Al2O3 composite films under electrical stress by observing local leakage current by conductive atomic force microscopy (C-AFM). The total area of leakage spots was observed to increase in current images obtained when repeating C-AFM observation at a constant substrate voltage. This phenomenon can be explained by the local trapping of holes in the film in which the Fowler–Nordheim tunneling current becomes predominant. On the other hand, in the area without leakage spots, average leakage current gradually decreases, due to the electron trapping in the film. The present C-AFM observation clearly reveals different behaviors of trapping sites for electrons and holes; the trapped holes are more easily detrapped than the electrons, and the number of electron-trapping sites is more effectively reduced by additional thermal oxidation than the number of holes.

The composition ratio dependence of electrical (work function and resistivity) and structural properties in metal (Ni,Pt)–germanide gate electrodes was investigated for metal–oxide–semiconductor (MOS) devices. X-ray diffraction and cross-sectional transmission electron microscopy clearly revealed that metal–germanide MOS gate electrodes with a single Ni–germanide (Ni–Ge) or Pt–germanide (Pt–Ge) phase can be formed by controlling the thicknesses of the deposited metal and Ge. The resistivities of both Ni–Ge and Pt–Ge were lower than that of conventional polycrystalline silicon (poly-Si) gate electrodes. From the capacitance-voltage characteristics, the work functions of the Ni–Ge gate electrodes were measured to be from 4.6 to 4.9 eV, and those of the Pt–Ge gate electrodes were from 4.9 to 5.3 eV. With increasing metal (Ni,Pt) content, the work function of Ni–Ge decreased, while that of Pt–Ge increased. This composition dependence of the work function of the metal–germanide can be explained by considering the electronegativity of the pure metals that are often used.

The surface profiles and energy bandgap of sub-nanometer-thick silicon nitride layers, which were formed by radical nitridation, were analyzed by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). According to the STM results, the surface roughness of the nitride layers depends only on substrate temperature regardless of radio frequency (RF) power, which indicates that the growth mode of the silicon nitride layer in radical nitridation is attributed to the surface silicon atom migration but does not depend on the types of nitrogen radical. In contrast, STS spectra show that the energy bandgap of the silicon nitride layer is significantly changed with not only substrate temperature but also RF power. The densities of nitrogen radicals were analyzed by absorption and emission spectroscopy, which suggests that the contribution of the excited-state nitrogen atoms to radical nitridation increases as RF power increases. The monolayer-thick nitride layer with both an atomically flat surface and a wide energy bandgap can be formed under appropriate conditions, because they are limited by different thermaly activated features.

We have investigated the structural and electrical properties of n-type doped Si1− x Ge x epitaxial layers (x = 24–26%) grown by chemical vapor deposition with conventional [SiH2Cl2 (DCS)/GeH4] and high-order (Si2H6/Ge2H6) precursor combinations. X-ray diffraction, atomic force microscopy, and deep-level transient spectroscopy (DLTS) measurements were performed for characterization. The crystalline properties and surface morphology of the Si1− x Ge x layer with Si2H6/Ge2H6 grown at temperatures as low as 550 °C show good structural quality similar to that with DCS/GeH4 grown at 615 °C. On the other hand, in terms of electrical properties, the DLTS measurement reveals the existence of vacancy-related complexes in the as-grown layer with Si2H6/Ge2H6. We found that post-deposition annealing at 200 °C for the Si1− x Ge x epitaxial layer is effective for annihilating vacancy-related defects with densities down to as low as that of conventional precursors.

Pr silicate films without interfacial layers were formed by pulsed laser deposition and high-temperature postdeposition annealing (PDA) and were evaluated in terms of structures and electrical properties. In the fabrication processes, film thickness and ambient gases during deposition were controlled. In some films, crystallization of Pr silicate was observed by transmission electron microscopy and it was found that tetragonal Pr2Si2O7 was epitaxially grown on Si(001) substrates. By deposition in Ar, Pr silicate with a small capacitance equivalent oxide thickness (CET) and a low leakage current density (J) was obtained (CET=1.1 nm and J=6 ×10-4 A/cm2 at a gate voltage of +1 V relative to the flat band voltage).

We have observed local current leakage in La2O3–Al2O3 composite films using conductive atomic force microscopy (C-AFM). The La2O3–Al2O3 composite films were synthesized by pulsed laser deposition and post deposition annealing. Current–voltage characteristics obtained from the average current detection from C-AFM current images agreed well with these from MOS capacitors made from the film, and the effect of the local current leakage sites on general current leakage properties of the capacitor was demonstrated. Appearance of current leakage spots is clearly dependent on the voltage measured during C-AFM observations. We confirmed that there are at least three types of leakage spots which have different voltage dependence and discussed the origin of these leakage spots. Breakdown and degradation in this film were also demonstrated.

We have investigated an initial stage of titanium nitride (TiN) growth on SiO2 substrates by ultrahigh-vacuum chemical vapor deposition with TiCl4 and NH3 as source materials. The behaviors of nucleation and grain growth of TiN have been clarified by atomic force microscopy and transmission electron microscopy. It was found that TiN film formation at an initial stage consists of three stages, which are characteristic of the lateral and subsequent vertical growth processes of grains. Deposition time dependence of the lateral growth of TiN grains clearly indicates that a process at 550°C is limited by the surface reaction, irrespective of the TiCl4 flow rate and a pretreatment for the substrates before the deposition. The pretreatment affects in the generation of additional nucleation sites on the SiO2 surface but does not affect the mechanisms of nucleation and grain growth.

A HfON/SiN gate dielectric was prepared using a radical nitridation method. Nitrogen incorporation into HfO2 was confirmed by X-ray photoelectron spectroscopy analysis. The HfON/SiN gate dielectric showed a smaller change in capacitance equivalent oxide thickness (CET) after rapid thermal oxidation (RTO) and a lower leakage current density (J) than those of HfO2 and HfO2/SiN with the same physical thickness. Even after RTO at 700°C for 30 s, a CET of 1.5 nm with J of 2×10-5 A/cm2 at (VFB-1) V was obtained, where VFB is the flat-band voltage. This improvement of the HfON/SiN gate dielectric can be explained by the presence of Si-N bonds at the HfO2/Si-substrate interface and Hf-N bonds in the HfO2 film. These nitrogen bonds strengthen the immunity to oxygen diffusion and improve leakage current characteristics.

The effects of nitrogen addition to an oxygen plasma in the surface-wave mode with a disk-plate window at a 2.45 GHz microwave in the TM01 mode, and photoresist ashing are investigated in the pressure range from 65.5 Pa to 399 Pa at a microwave power of 1.3 kW. No change in electron density was observed in the experimental range of the amount of nitrogen added (0–50%). On the other hand, atomic oxygen density determined by actinometry reaches the maximum intensity at a nitrogen ratio of 20% and the intensity at 20% is twice as large as that at 0%. Although oxygen plasma generation is different from previous work, similar tendencies of nitrogen addition effects are observed, where the electron density in the surface-wave mode exceed the cutoff density of the microwave. An ashing rate at 20% is twice as large as that at 0%. The Activation energies of the ashing reaction at 0 and 20% accompanied by an aluminum shower head are approximately 0.5 eV, which is equal to the value ashed by atomic oxygen. The nitrogen addition effect on ashing using an oxygen surface-wave mode plasma are clarified.

We have developed a method of microscopically analyzing the degradation of gate SiO2 films in actual metal-oxide-semiconductor (MOS) devices by conductive atomic force microscopy (C-AFM). In C-AFM images of electrically stressed SiO2 films, leakage current spots on a nanometer scale were successfully observed. The observed current spots show characteristic behaviors similar to the transient stress-induced leakage current which can be detected by macroscopic electrical measurements using MOS capacitors. The appearance of the current spots is discussed on the basis of the mechanism by which holes are trapped and detrapped by stress-induced defects in SiO2 films.

We have studied the surfactant effects of adsorbed oxygen layers of 1-2 monolayer thicknesses on the film evolution of CoSi2(001) in reactive deposition epitaxy at 470°C on Si(001). The O atoms segregating to the growth front play essential roles in suppressing the Si outward diffusion from the substrate and Co lateral migration, which promote two-dimensional layer growth of CoSi2(001) films and make the surfaces smooth. A kinetic mechanism is suggested in which Co atoms easily diffuse without interaction through the O layers toward the substrate and react with Si atoms. Nevertheless, {111}-twinned CoSi2 is formed at the very initial growth stage on O-adsorbed Si, similarly to that on clean Si. We have also investigated the morphology and crystallinity of ~20-nm-thick CoSi2(001) films. The O atoms remaining on the surface effectively restrain film surface roughening even at this thickness; however, they cannot lower the interface roughness owing to the formation of {111} facets and {111}-twinned boundaries. The twin boundaries formed in the initial stage grow into the substrate during the epitaxy.

The oxygen-mediated two-step growth of high-quality CoSi2(001) films on Si(001) is demonstrated. In the first-step growth, uniform 6-monolayers (ML)-thick CoSi2(001) films with atomically flat surfaces and interfaces are grown by solid-phase epitaxy (SPE) at 470°C of 3 ML Co on O-adsorbed Si and O-covered 3 ML Co on clean Si. In both cases, O atoms improve the surface and interface flatness in SPE growth. In the second-step growth, 5.6 nm Co is solely deposited at 470°C on the O-covered CoSi2 template. We successfully achieve the growth of 20-nm-thick CoSi2(001) films with smooth surfaces and abrupt interfaces. In the O-mediated two-step growth, O atoms segregating to the film growth front suppress the lateral migration of Co atoms. Furthermore, in the second-step growth, the uniform template promotes homogeneous growth and keeps the surfaces and interfaces of CoSi2 films smooth. This novel technique, applicable to salicide processes, is highly suitable for the formation of epitaxial CoSi2 contacts in future ULSI devices.

We have investigated the initial nitridation processes on oxidized Si(100) with radical nitrogen at a substrate temperature of 850°C using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It is found that the thin oxide layer suppresses the changes of original Si step structures during nitridation, and this effect critically depends on the growth conditions of the oxide layer. Comparison of the nitride island morphology to the case of the clean surface suggests that the migration of the precursor during nitridation is suppressed by the oxygen in the layer.

A novel nonvolatile memory with Si nanocrystals has been developed. It has a floating gate consisting of a thin poly Si film and Si nanocrystals. It could be written or erased when a low voltage of +3 V or -3 V was supplied to the gate electrode. On the other hand, no hysteresis was observed at +1 V or -1 V. These characteristics enable the realization of a random-access memory that operates with extremely low power consumption. In this paper, a memory cell array suitable for novel memory devices is also described. The cell array features well bitline technology and the cell area is reduced to 4 F2 (F: feature size) by winding trench isolation. Applying the novel device to the cell array enables realization of high-density, random-access, nondestructive readout and ultralow-power memories.

In order to improve the electrical operation of very thin gate oxide metal-oxide-semiconductor (MOS) devices, it is necessary to understand generated defects in the non-stoichiometric SiO x area and at the Si–SiO 2 interface. For this purpose, we extended our new measurement technique, which is a temperature dependent method, to MOSFETs and Al-gate MOS devices to investigate slow-state traps and their relationship with fast-state traps (influence of technology, temperature and field dependence). With this method, activation energies for slow-state traps can be determined. Hydrogen species have been characterized. These defects are created when the MOS capacity is under a strong electric field that induces injection of electrons by tunnel effect through the silicon dioxide, SiO 2 .

We investigated the structural and electrical characteristics of HfO2 films deposited by a pulsed laser deposition (PLD) method. It was found that the formation of interlayers at the HfO2/Si interface depends strongly on the oxygen pressure rather than the deposition temperature during PLD. Furthermore, we determined the electrical properties which are closely related to the structure and orientation of grains in the polycrystalline HfO2 films. It was noted that randomly oriented HfO2 films have capacitance-voltage curves with no hysteresis and low leakage current.

We have investigated nitridation processes on clean Si(100)-2×1 surfaces and the electrical characteristics of metal-insulator-semiconductor (MIS) capacitors with nitride films formed by radical nitrogen. The radical nitrogen exposure to Si surfaces at 500°C initially causes both nitridation of Si and local detachment reactions on the surface. It is found from scanning tunneling microscopy and scanning tunneling spectroscopy that the nitridation proceeds in a layer-by-layer manner in which a continuous film is formed by the coalescence of two-dimensional islands. At nitride thicknesses above 0.6 nm, the nitride film is homogeneously formed over the entire surface. The MIS capacitor with a 5.4-nm-thick nitride film formed by radical nitrogen shows low hysteresis of less than 0.1 V and low leakage current density. These electrical properties indicate that the nitride films formed by radical nitrogen are applicable to gate insulators.

We have investigated the selective growth of a Ge1− x Sn x epitaxial layer on a line/space-patterned SiO2/Si substrate by metal–organic chemical vapor deposition. We examined the behavior of a Sn precursor of tributyl(vinyl)tin (TBVSn) during the growth on Si and SiO2 substrates and investigated the effect of the Sn precursor on the selective growth. The selective growth of the Ge1− x Sn x epitaxial layer was performed under various total pressures and growth temperatures of 300 and 350 °C. The selective growth of the Ge1− x Sn x epitaxial layer on the patterned Si region is achieved at a low total pressure without Ge1− x Sn x growth on the SiO2 region. In addition, we found that the Sn content in the Ge1− x Sn x epitaxial layer increases with width of the SiO2 region for a fixed Si width even with low total pressure. To control the Sn content in the selective growth of the Ge1− x Sn x epitaxial layer, it is important to suppress the decomposition and migration of Sn and Ge precursors.

We have examined the formation of SiC thin films by chemical vapor deposition (CVD) using vinylsilane and investigated the chemical bonding state and crystallinity of the prepared SiC thin films. We achieved the formation of a Si–H–less SiC film at growth temperatures as low as 600 °C. Also, we investigated the in situ doping effect of N by the incorporation of NH3 gas in the SiC growth and demonstrated that the chemical composition of N in SiC thin films was controlled by adjusting the NH3 flow rate. In addition, we examined the growth of SiC thin films on a Cu substrate and achieved the formation of a SiC thin film while avoiding any significant reaction between SiC and Cu at a growth temperature of 700 °C.

We examined the numerical calculation for evaluating the energy band offset of Si1− x Sn x semiconductors and compared our calculation results with the results of previous theoretical calculation and experimental estimation. By estimating the charge neutrality level of Si1− x Sn x as a mutual basis level for comparison of the first-principles calculations for different Si1− x Sn x contents, the calculated valence band offset of Si1− x Sn x to Si was found to sensitively shift with upward bowing with increasing Sn content compared with that obtained using a conventional linear interpolation model. This is in good agreements with the experimental result.

The effect of hydrogen-surfactant-mediated molecular beam epitaxy (MBE) growth of Ge1− x Sn x layer on Ge(001) substrate on crystalline quality and photoluminescence (PL) property has been investigated. The effect of irradiation of atomic hydrogen (H) generated by dissociating molecular hydrogen (H2) were examined during the MBE growth. H irradiation significantly improves the surface morphology with the enhancement of the two-dimensional growth of the Ge1− x Sn x epitaxial layer. Enhanced diffuse scattering is observed in the X-ray diffraction profile, indicating a high density of point defects. In the PL spectrum of the H2-irradiated Ge1− x Sn x layer, two components are observed, suggesting the radiative recombination with both indirect and direct transitions, while one component related to the direct transition is observable in the H-irradiated sample. The postdeposition annealing in nitrogen ambient at as low as 220 °C decreases the PL intensity of the H-irradiated Ge1− x Sn x layer, although the intensity is recovered after annealing at 300 °C, suggesting the annihilation of point defects in the Ge1− x Sn x layer.

We have developed a new method of growing Si or Ge ultrathin films on a Ag(111) surface by using a Ag-induced layer exchange (ALEX) process toward the creation of 2D honeycomb sheets of Si and Ge, known as silicene and germanene, respectively. In the present paper, we clarify ALEX features, specifically the surface segregation of Si (or Ge) atoms from the underlying substrate, focusing on the annealing temperature and time. Hard X-ray photoelectron spectroscopy analyses demonstrate that surface-segregated Si (or Ge) exists on the Ag surfaces after the epitaxial growth of the Ag layer on Si(111) [or Ge(111)] substrates; the amount of segregated Si (or Ge) can be controlled by a subsequent annealing. Also, we find that the segregation of an ultrathin Si or Ge layer proceeds at an interface between Ag and the AlO x capping layer., Article Published 5 July 2016

We have carried out density functional theory (DFT) calculation for Si1− x Sn x alloy and investigated the effect of the displacement of Si and Sn atoms with strain relaxation on the lattice constant and E– k dispersion. We calculated the formation probabilities for all atomic configurations of Si1− x Sn x according to the Boltzmann distribution. The average lattice constant and E– k dispersion were weighted by the formation probability of each configuration of Si1− x Sn x . We estimated the displacement of Si and Sn atoms from the initial tetrahedral site in the Si1− x Sn x unit cell considering structural relaxation under hydrostatic pressure, and we found that the breaking of the degenerated electronic levels of the valence band edge could be caused by the breaking of the tetrahedral symmetry. We also calculated the E– k dispersion of the Si1− x Sn x alloy by the DFT+U method and found that a Sn content above 50% would be required for the indirect–direct transition.

We investigated the effect of GeO2 deposition temperature (T depo) on electronic properties of Al/Al2O3/GeO2/Ge MOS capacitors. Capacitance–voltage characteristics show frequency dispersions under depletion and strong inversion conditions, which can be attributed from the interface states at the atomic layer deposition (ALD)-GeO2/Ge interface and from the defect states in the quasi-neutral region in the Ge substrate, respectively. We found that the interface state density (D it) shows similar values and energy distributions as T depo decreases to 200 from 300 °C, while a higher D it is observed at a T depo of 150 °C. Also, from the temperature dependence of conductance, the frequency dispersion under the strong inversion condition can be related to the minority carrier diffusion to the quasi-neutral region of the Ge substrate. The frequency dependence of conductance reveals that the undesirable increment of the bulk defect density can be suppressed by decreasing T depo. In this study, the bulk defect density in a MOS capacitor prepared at a T depo of 200 °C decreases one tenth compared with that at a T depo of 300 °C. The ALD of GeO2 at a low temperature of around 200 °C is effective for both obtaining a low D it and preventing the undesirable introduction of bulk defect density.

We have investigated the atomic-scale reaction between a Ti thin layer and highly oriented pyrolytic graphite (HOPG) mainly by scanning tunneling microscopy. A deposited Ti layer shows an epitaxial orientation structure on a HOPG substrate even in room-temperature deposition, while the bonding between Ti and HOPG is very weak. The chemical reaction between Ti and HOPG takes place, and epitaxial TiC domains on HOPG are formed for annealing at above 600 °C. The TiC domains shows a smooth surface corresponding to the TiC(111) plane after annealing at 700 °C. The formation of TiC(001) facets and significant surface roughening of not only TiC but also HOPG substrates are observed for annealing at as high as 900 °C.

We examined the molecular beam epitaxy of Ge1− x Sn x with in situ Sb doping on Ge substrates. The effects of Sb doping on the crystalline and electrical characteristics of Ge1− x Sn x epitaxial layer were investigated in detail. We found that Sb doping with a concentration of 1020 cm−3 remarkably improves the crystallinity, and surface uniformity of the Ge1− x Sn x epitaxial layer by changing the growth mode by the surfactant effect of Sb atoms. Low-temperature Ge1− x Sn x growth with in situ Sb doping realizes a very high electron concentration of 1020 cm−3, which is above the thermal equilibrium solid solubility, as a result of suppressing Sb segregation and precipitation.

We examined the epitaxial growth of an ultrahigh-Sn-content Ge1− x Sn x layer on a Ge substrate and investigated the impact of a Ge1− x Sn x interlayer on the Schottky barrier height (SBH) of the metal/Ge contact. In this study, we considered guidelines of the strain energy and growth temperature to realize a high-Sn-content Ge1− x Sn x layer while keeping the epitaxial growth and suppressing the Sn precipitation. By reducing the film thickness and keeping a low growth temperature, we formed an atomically flat and uniform Ge1− x Sn x epitaxial layer with a Sn content up to 46% on a Ge(001) substrate. We also performed the current density–voltage measurement for Al/Ge1− x Sn x /n-Ge Schottky diodes to estimate the SBH. We found that the SBH of Al/Ge1− x Sn x /n-Ge contact decreases with increasing Sn content in the Ge1− x Sn x interlayer. The shift of the pinning position towards the conduction band edge of Ge is one of the reasons for the SBH reduction of Al/Ge1− x Sn x /n-Ge contact because the valence band edge of Ge1− x Sn x would rise as the Sn content increases.

We have investigated the effects of NO annealing on the electrical properties of a SiO2/4H-SiC interface. The electrical properties of the NO-annealed sample are different from those of the wet-annealed sample. NO or wet annealing generates positive or negative charges, respectively, in the insulator. The interface trap density (D it) near the conduction band edge (E c − 0.1 eV, where E c is the conduction band edge) increases with NO annealing. In contrast, the D it of the NO-annealed sample at around E c − 0.2 eV is lower than that of the wet-annealed sample. The interface state near the conduction band edge in the NO-annealed sample is identified to be of the donor type. Thus, it is considered that the higher D it near the conduction band edge and/or Coulomb scattering due to positive charges causes a decrease in the n-channel mobility in the nitrided SiO2/SiC interface by NO annealing.

We have demonstrated the crystal growth of Si1−x−ySnxCy ternary alloy layers on Si(001) substrates by radio-frequency magnetron sputtering. We have investigated the crystalline properties of the ternary alloy layers and clarified the influence of Sn and C on the crystalline structure of Si1−x−ySnxCy layers. We found that the epitaxial growth temperature of Si1−x−ySnxCy decreased and the island growth was suppressed by the introduction of Sn. The Sn precipitation was also effectively suppressed by C introduction. The substitutional C content of Si1−x−ySnxCy was estimated to be 2.7% and exceeded the thermal equilibrium solid solubility of C into the Si matrix.

We have investigated the electrical properties of NiGe/n-type Ge contacts with various crystalline structures. We found that the Schottky barrier height (SBH) of the NiGe/Ge contact could be modulated by controlling the crystalline structures of the NiGe layer. An epitaxial NiGe(100)/n-type Ge(110) contact shows an SBH 0.15 eV lower than that of the polycrystalline NiGe/n-type Ge(110) contact, whereas the SBH of the epitaxial NiGe(111)/n-type Ge(001) contact is not so low compared with that of the polycrystalline NiGe/n-type Ge(001) contact with Fermi level pinning, even though an epitaxial NiGe layer with good crystalline quality is formed. This distinctive modulation of SBH could not be explained by considering only the metal-induced gap states model. We consider that this modulation of SBHs is a consequence of the variation in the density of the interface states induced by the extrinsic factors, such as the dangling bonds and/or defects. Our results give us a hint that the modulation of the SBH by controlling the crystalline structures is possible.

We have investigated the crystalline and optical properties of epitaxial layers of the ternary alloy Ge1−x−ySnxCy grown on a Si substrate. We achieved the formation of epitaxial Ge1−x−ySnxCy layers with a C content as high as 2% even with a high C incorporation efficiency. X-ray photoemission spectra and Raman scattering spectroscopy measurements revealed that C atoms preferentially bond with Sn atoms in the Ge matrix, which is considered to enhance C introduction into substitutional sites in Ge with local strain compensation. We also demonstrated the control of the energy bandgaps of epitaxial Ge1−x−ySnxCy layers by controlling Sn and C contents.

A floating-gate memory with surface-nitrided Si nanocrystals buried in a SiO2 matrix has been fabricated employing radical nitridation. Si nanocrystals with a number density higher than 1×1012 cm-2 and an average grain size smaller than 6 nm have been grown using an ultrathin amorphous Si layer predeposited on the SiO2 surface. Since the radical nitridation of the formed Si nanocrystals effectively suppresses excess oxidation of nanocrystals during the control oxide formation, the Si nanocrystals have been successfully buried in the SiO2 matrix without losing their number density, grain size and fine spherical shape. Electrical properties of the floating-gate memory were also examined. A flat band voltage shift larger than 1 V, which is consistent with the number density of Si nanocrystals, was observed in capacitance–voltage measurements.

A Ge CMOS inverter and a ring oscillator composed of Si-passivated ultrathin-body (UTB, 10 nm) depletion-type poly-Ge p- and n-MISFETs with a single metal gate have been successfully fabricated and demonstrated for the first time. A low Ioff coming from a depletion-type thin channel, a thermally stable gate stack interface against interlayer deposition for interconnection due to Si passivation, and a large Vth difference between p- and n-MISFETs originating from Si passivation enabled poly-Ge CMOS IC operations.