Kouta Takahashi

Self-organized lattice-matched epitaxy of Si1−xSnx alloys on (001)-oriented Si, Ge, and InP substrates

The crystal growth of single-crystalline Si1−xSnx layers with various Sn contents and analytical comparisons of their fundamental physical properties are strongly desired for next-generation group-IV electronics. In the present study, Si1−xSnx layers with varying Sn contents (1%−40%) were grown on various substrates [(001)-oriented Si, Ge, or InP] by solid-phase epitaxy. Crystallographic and composition analyses indicated that the grown Si1−xSnx layers were nearly lattice-matched to the substrates. When grown on Si, Ge, and InP substrates, the substitutional Sn contents were ∼1%, ∼20%, and ∼40%, respectively. Hard X-ray photoelectron spectroscopy revealed a valence-band offset resulting from the Sn substitution. The offset exhibited an upward-bowing tendency when plotted against the Sn content. The Si0.78Sn0.22/n-type Ge junction displayed rectifying diode characteristics with the ideality factor of 1.2.

Low thermal budget n-type doping into Ge(001) surface using ultraviolet laser irradiation in phosphoric acid solution

We have investigated phosphorus (P) doping into Ge(001) surfaces by using ultraviolet laser irradiation in phosphoric acid solution at room temperature. We demonstrated that the diffusion depth of P in Ge and the concentration of electrically activated P can be controlled by the number of laser shots. Indeed, a high concentration of electrically activated P of 2.4 × 1019 cm−3 was realized by 1000-times laser shots at a laser energy of 1.0 J/cm2, which is comparable or better than the counterparts of conventional n-type doping using a high thermal budget over 600 °C. The generation current is dominant in the reverse bias condition for the laser-doped pn-junction diodes independent on the number of laser shots, thus indicating low-damage during the pn-junction formation. These results open up the possibility for applicable low thermal budget doping process for Ge-based devices fabricated on flexible substrates as well as Si electronics.

Dopant behavior in heavily doped polycrystalline Ge1− x Sn x layers prepared with pulsed laser annealing in water

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.

High n-type Sb dopant activation in Ge-rich poly-Ge1- xSnx layers on SiO2 using pulsed laser annealing in flowing water

Heavy n-type doping in polycrystalline Ge (poly-Ge) is still under development owing to the low solid solubility and the low activation ratio of group-V dopants in Ge. To solve this problem, we have investigated ultra-short (55 ns) laser pulse annealing in flowing water for Sb-doped amorphous Ge1−xSnx layers (x ≈ 0.02) on SiO2. It is found that fully melting a Ge1−xSnx layer down to the Ge1−xSnx/SiO2 interface leads to a large grained (∼0.8 μmϕ) growth, resulting in not only a high electrical activation ratio (∼60%) of Sb atoms in the polycrystals but also a high electron density around 1020 cm−3. As a result, the electron mobility in the Ge-rich poly-Ge1−xSnx layers exceeds that in single-crystalline Si even in the region of a high electron density around 1020 cm−3. The low thermal budget process opens up the possibility for developing Ge1−xSnx based devices fabricated on 3D integrated circuits as well as flexible substrates.

Development of in-situ Sb-Doped Ge 1−x Sn x Epitaxial Layers for Source/Drain Stressor of Strained Ge Transistors

We have investigated the crystalline and electrical characteristics of heavily doped n-type Ge<inf>1−x</inf>Sn<inf>x</inf> epitaxial layers with various Sb concentrations up to 10²⁰ cm⁻³. In this study, we focus the thermal stability of Sb doped Ge<inf>0.94</inf>Sn<inf>0.06</inf> and Ge epitaxial layers and clarify the relationship between the crystalline and electrical characteristics. At the as-grown condition, the substitutional Sb concentration was achieved at higher than 2.6×10²⁰ cm⁻³. After the post-deposition annealing, Sb-doped Ge<inf>1−x</inf>Sn<inf>x</inf> maintained their superior crystallinity up to 400 °C, while the sheet resistance increases with Sb segregation. We also found that Sb atoms doped in the Ge<inf>0.94</inf>Sn<inf>0.06</inf> layer show a higher thermal robustness at 300 °C than those in the Ge layer.

Growth and applications of GeSn-related group-IV semiconductor materials

We have developed the epitaxial growth technology of Ge<inf>1−x</inf>Sn<inf>x</inf> and related group-IV materials. The crystalline properties and energy band structure have been investigated for integrating group-IV semiconductors into Si ULSI platform.