Isao Tsunoda

Ge-fraction-dependent metal-induced lateral crystallization of amorphous-Si1−xGex (0≦x≦1) on SiO2

Metal-induced low-temperature (≦550 °C) crystallization of amorphous-Si1−xGex (0≦x≦1) on SiO2 has been investigated. In the case of low Ge fraction (0≦x≦0.2), Ge-doping enhanced plane growth was observed. This achieved strain-free poly-Si0.8Ge0.2 with large grains (18 μm). On the other hand, dendrite growth became dominant in the case of intermediate Ge fractions (0.4≦x≦0.6). By optimizing the growth conditions (x: 0.4, annealing: 450 °C, 20 h), very sharp needle-like crystal regions (width: 0.05 μm, length: 10 μm) were obtained. These polycrystalline SiGe films on SiO2 should be used for the system-in-display, three-dimensional ultralarge scale integrated circuits, and novel one-dimensional wires.

Ion-beam stimulated solid-phase crystallization of amorphous si on SiO2

Abstract Influences of ion-beam irradiation on solid-phase-crystallization of a-Si on SiO 2 were studied in the temperature range between 200 and 700°C. Significant enhancement of crystal nucleation was observed under ion irradiation (25 keV, 1×10 16 Ar + cm −2 ). As a result, nucleation at a temperature lower than that of the softening of soda-lime glass (450°C) becomes possible. In addition, nuclei growth along the [111] and [110] directions was detected using X-ray diffraction methods. These are a big advantage for the fabrication of high-quality and low-cost thin-film transistors on glass substrates.

A pseudo-single-crystalline germanium film for flexible electronics

We demonstrate large-area (∼600 μm), (111)-oriented, and high-crystallinity, i.e., pseudo-single-crystalline, germanium (Ge) films at 275 °C, where the temperature is lower than the softening temperature of a flexible substrate. A modulated gold-induced layer exchange crystallization method with an atomic-layer deposited Al2O3 barrier and amorphous-Ge/Au multilayers is established. From the Raman measurements, we can judge that the crystallinity of the obtained Ge films is higher than those grown by aluminum-induced-crystallization methods. Even on a flexible substrate, the pseudo-single-crystalline Ge films for the circuit with thin-film transistor arrays can be achieved, leading to high-performance flexible electronics based on an inorganic-semiconductor channel.

Highly strain-relaxed ultrathin SiGe-on-insulator structure by Ge condensation process combined with H+ irradiation and postannealing

Strain-relaxation process of SiGe-on-insulator (SGOI) structures in the oxidation induced Ge condensation method has been investigated as a function of the SiGe thickness. Complete relaxation was obtained for thick SGOI layers (>100nm). However, the relaxation rates abruptly decreased with decreasing SiGe thickness below 50nm, i.e., the relaxation rate of 30% at 30nm SiGe thickness. In order to improve this phenomenon, a method combined with H+ irradiation with a medium dose (5×1015cm−2) and postannealing (1200°C) has been developed. This successfully achieved the high relaxation rate (70%) in the ultrathin SGOI (30nm).

Metal-Induced Solid-Phase Crystallization of Amorphous SiGe Films on Insulator

The metal-induced low-temperature (≤550°C) crystallization of a-Si1-xGex (0 ≤x ≤1) on SiO2 has been investigated. A Ge-fraction-dependent crystal growth was observed. In the case of a low-Ge fraction, plane growth dominated, the velocity of which was enhanced by 80% with increasing Ge fraction from 0 to 20%. This produced strain-free poly-SiGe with large grains (18 µm). On the other hand, dendrite growth became dominant in the case of intermediate Ge fractions (40–60%). By optimizing the growth conditions (x: 0.4, annealing: 450°C, 20 h), very sharp needlelike crystals (width: 0.05 µm, length: 10 µm) were obtained. These new polycrystalline SiGe films on insulator should be used for system-in-display, three-dimensional ultra large-scall integrated circuits, and novel one-dimensional wires.

Ge fraction dependent improved thermal stability of in situ doped boron in polycrystalline Si1-xGex (0≤x≤0.5) films on SiON

Postannealing characteristics of in situ doped B atoms in poly-Si1−xGex (x⩽0.5) films on SiON have been investigated. Supersaturated electrically active B (2×1020cm−3) is obtained for as-chemical vapor deposition samples, and their thermal stability is significantly improved by increasing Ge fraction, e.g., the stability for poly-Si0.6Ge0.4 is ten times as high as that for poly-Si at 700–800°C. Such a Ge induced improvement will be a powerful tool to achieve poly-SiGe gate electrode for the next generation ultralarge scale integrated circuits. In addition, the deactivation process of electrically active B has been analyzed. Results indicated that deactivation processes consist of fast and slow processes. The former is due to movement of B atoms from substitutional to interstitial sites, which is enhanced by a local strain induced by the difference in atomic radii between Si and B atoms. The slow process was due to trapping of B at grain boundaries during grain growth. The two-state model based on the loca...

Nucleation Control in Solid-Phase Crystallization of a-Si/SiO2 by Local Ge Insertion

The effects of local Ge insertion on the solid-phase crystallization (SPC) of a-Si films have been investigated. Three types of stacked structures, i.e., (a) a-Si/a-Ge/a-Si/SiO2, (b) a-Si/a-Ge/SiO2, and (c) SiO2/a-Ge/a-Si/SiO2, were annealed at 600°C. For structure (a) with thin (~5 nm) Ge films, Ge atoms completely diffused into both sides of a-Si regions, and SPC was not enhanced. However, when Ge thickness was increased to more than 10 nm, Ge atoms were localized. Such localization became significant for structures (b) and (c) even for samples with thin Ge films. In addition, significant enhancement of SPC of a-Si was observed. These results indicated that crystal nucleation was initiated in Ge layers, and then propagated into a-Si layers. Therefore, interface-nucleation-driven SPC becomes possible using structures (b) and (c). This will be a useful tool in achieving oriented Si growth on SiO2.

A crystalline germanium flexible thin-film transistor

We experimentally demonstrate a flexible thin-film transistor (TFT) with (111)-oriented crystalline germanium (Ge) layers grown by a gold-induced crystallization method. Accumulation-mode metal source/drain p-channel Ge TFTs are fabricated on a polyimide film at ≤ 400 ° C. A field-effect mobility (μFE) of 10.7 cm2/Vs is obtained, meaning the highest μFE in the p-TFTs fabricated at ≤ 400 ° C on flexible plastic substrates. This study will lead to high-performance flexible electronics based on an inorganic-semiconductor channel.

Enhancement of bulk nucleation in a-Si1−xGex on SiO2 for low-temperature solid-phase crystallization

Nucleation phenomena in a-si 1-x Ge x films on SiO 2 were examined in order to achieve low-temperature solid-phase crystallization. First, film thickness dependence of nucleation was investigated. The nucleation frequency per unit area increased with increasing film thickness, which was attributed to that bulk nucleation was dominant compared with interface or surface nucleation. Next, in order to enhance bulk nucleation in thin films, effects of the initial amorphicity modulation induced by Ar+ irradiation (25 keV, 1x10 16 cm -2 ) before annealing were investigated. The incubation time for nucleation in pre-irradiated samples during subsequent annealing at 600 °C was significantly decreased to 1/20 of that without pre-irradiation, which was tentatively assigned to enhancement of atomic arrangement induced by densification of a-Si 1 - x Ge x films. It is expected that optimization of the irradiation conditions will realize low-temperature (<500 °C) formation of poly-Si 1 Ge x films on SiO 2 .

Enhanced solid-phase growth of β-FeSi2 by pre-amorphization

Abstract Effects of Ar + ion-irradiation on solid-phase growth of β-FeSi 2 have been investigated. Fe (10 nm)/Si structures were irradiated with 20 keV Ar + (5.0×10 15 cm −2 ) at room temperature (RT) or 400 °C, and subsequently annealed at 800 °C. The results of X-ray diffraction measurements suggested that the formation of β-FeSi 2 for the sample irradiated at RT was faster than that irradiated at 400 °C. However, Auger electron spectroscopy measurements showed that atomic mixing at the Fe/Si interface before annealing was larger for the sample irradiated at 400 °C. These results suggested that amorphization of the Si substrate, in addition to atomic mixing at the Fe/Si interface, enhanced formation of β-FeSi 2 , which was attributed to atomic rearrangement induced during defect relaxation in annealing process.