Tadashi Okumura

A Light Source Using 1.3-

As a solution to provide small-footprint and low-cost light sources for silicon platforms, light sources using a lens-integrated surface-emitting laser (LISEL), and a grating coupler fabricated on a silicon-on-insulator substrate is proposed. Light emitted from the LISEL (based on an InP distributed feedback laser) has a narrow far-field pattern, so the LISEL can be directly mounted on the silicon platform with no additional external optical components. The alignment tolerance between the LISEL and grating coupler was experimentally measured. This measurement confirmed large alignment tolerance. The excess loss due to misalignment of ±5 μm is only 1.5 dB.

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We demonstrated electroluminescence (EL) from germanium (Ge) waveguides on lateral silicon-on-insulator (SOI) diodes with silicon nitride (SiN) stressors. Tensile strain of 0.09 % on side walls of the Ge waveguides was predicted by finite element modeling of the strain field. The EL spectra showed enhanced light emission efficiency with a red-shift due to direct band gap shrinkage of 0.014 eV.

Lens-Integrated Surface-Emitting Laser for Silicon Platforms

We propose lateral carrier injections into a Ge waveguide on a silicon-on-insulator diode for monolithic light sources. We confirmed enhanced electroluminescence from the diode under high current densities of 1 MAcm-2.

Enhanced Electroluminescence from Germanium Waveguides by Local Tensile Strain with Silicon Nitride Stressors

Silicon (Si) monolithic lasers are key devices in large-scale, high-density photonic integrated circuits. Germanium (Ge) is promising as an active layer due to the complementary metal-oxide semiconductor process compatibility with Si. A net optical gain from Ge is essential to demonstrate lasing operation. We fabricated Ge waveguides and investigated the n-type doping effect on the net optical gain. The estimated net gain of the n-Ge waveguide increased from -2200 to -500/cm, namely reducing loss, under optically pumped condition.

Lateral carrier injection to germanium for monolithic light sources

Ge waveguides (WGs) were successfully fabricated on an SiO2 layer by combining epitaxial lateral overgrowth, chemical mechanical polishing (CMP), and reactive ion etching (RIE) of a Ge layer selectively grown on SiO2 patterns using low-pressure chemical vapor deposition. Selectivity was promoted by increasing the growth temperature; the length of the epitaxial lateral overgrown Ge layer reached 5 µm on the SiO2 layer under conditions of optimal selective growth at a temperature of 750 °C. The Ge layers were planarized using CMP down to a thickness of 1 µm, and then Ge WGs as active regions for light emitting devices were formed by using RIE on the planarized Ge layers. After defective regions around the Ge/Si interface were removed, 4-times-higher photoluminescence was obtained from the Ge WGs compared with one that contained the Ge/Si interface. These results indicate that this combined technique efficiently improved the performance of Ge light-emitting devices.

Optical net gain measurement in n-type doped germanium waveguides under optical pumping for silicon monolithic laser

We report on the photoluminescence dynamics of highly n-doped tensile-strained germanium on silicon. The instantaneous decay photoluminescence lifetime changes with intensity, which is explained by the interplay of Shockley-Read Hall and Auger processes.

Crystallinity improvements of Ge waveguides fabricated by epitaxial lateral overgrowth

For a multi mode fiber optical link, a high speed silicon photonics receiver based on a highly alignment tolerant vertically illuminated germanium photodiode was developed. The germanium photodiode has 20 GHz bandwidth and responsivity of 0.5 A/W with highly alignment tolerance for passive optical assembly. The receiver achieves 25 Gbps error free operation after 100 m multi mode fiber transmission.

Time-resolved photoluminescence study of highly n-doped germanium grown on silicon

The photoluminescence from Ge stripes increased due to the large amount of tensile strain created by Si₃N₄/SiO₂ stressors. The low temperature deposition of Si₃N₄ was also effective at improving the optical properties of the Ge layer.

25 Gbps silicon photonics multi-mode fiber link with highly alignment tolerant vertically illuminated germanium photodiode

Germanium (Ge) (111) fins of 320 nm in height were successfully fabricated using a combination of flattening sidewalls of a silicon (Si) fin structure by anisotropic wet etching with tetramethylammonium hydroxide, formation of thin Ge fins by selective Si oxidation in SiGe layers, and enlargement of Ge fins by Ge homogeneous epitaxial growth. The excellent electrical characteristics of Ge(111) fin light-emitting diodes, such as an ideality factor of 1.1 and low dark current density of 7.1 × 10−5 A cm−2 at reverse bias of −2 V, indicate their good crystalline quality. A tensile strain of 0.2% in the Ge fins, which originated from the mismatch of the thermal expansion coefficients between Ge and the covering SiO2 layers, was expected from the room-temperature photoluminescence spectra, and room-temperature electroluminescence corresponding to the direct band-gap transition was observed from the Ge fins.

Improvement of photoluminescence from Ge Layers with Si 3 N 4 /SiO 2 Stressors

Strain of Ge waveguide were analysed in view of polarization dependent waveguide characteristics in addition to μ-Raman spectroscopy. Compressive strain of dry etched waveguide was confirmed despite tensile strain after Ge growth. The compressive strained state was relaxed or changed to tensile strained state by adapting external stressor.