MANO, Takaaki / Group Leader

Development of compound semiconductor crystal growth technology and its functional exploration

Overview

III-V compound semiconductor heterostructures are used in various familiar applications, such as light emitting devices, sensing devices, and high-frequency devices. Toward the realization of ioT society in near future, it is necessary to (1) further improve their performance, (2) explore novel functionalities, and (3) realizing price reduction of high quality devices.
Our challenges for solving those issues are developments of novel semiconductor heterostructures by using advanced epitaxial growth techniques and nanophotonics technologies.

Characteristics

Major reserch

• The self-assembled quantum dots normally exhibit asymmetric shapes due the anisotropy of standard (100) surface. By utilizing our original epitaxial growth method of Droplet Epitaxy, we have succeeded in the formation of highly symmetric quantum dots on (111)A surfaces. We have demonstrated the generation of highly entangled photon pair emission from the QDs.
• Narrow bandgap semiconductors-based infrared photodetectors are used in high-quality gas-sensors and night vision cameras. However, they are normally expensive and/or include toxic materials. We have developed highly sensitive InAs-based infrared photodetectors on cheap GaAs substrates by using simple growth method.

Summary

We have developed advanced epitaxial growth technologies of III-V semiconductors and demonstrated novel functionalities of the heterostructures. We will further develop the researches for exploring novel functionalities by integrating various technologies such as theoretical approach, atomic scale analysis, and nanophotonics.

OHTAKE, Akihiro

Development of novel techniques for heteroepitaxy

Overview

We are developing the epitaxial growth techniques for the realization of innovative heterostructures.
By combining advanced growth techniques based on molecular-beam epitaxy and atomic-scale surface/interface characterization techniques, we aim to fabricate atomically-controlled semiconductor heterostructures, mainly focusing on the lattice-mismatched systems.

Characteristics

• Precise control of heteroepitaxy processes based on the atomistic characterization of surface structures.
• Realization of III-V based lattice-mismatched heterostructures by utilizing unique strain relaxation mechanism on (111)-oriented substrates.
• Fabrication of heterostructures consisting of two-dimensional materials such as MoSe₂ and WSe₂ without constraints of lattice matching.
• In-situ observation of formation processes of quantum dots.

Major reserch

Summary

• We are developing the epitaxial growth techniques mainly for III-V semiconductors and transition-metal dichalcogenides, and investigating their possible application for novel devices.
• Precise control of epitaxial growth processes based on the atomistic characterization of surfaces/interfaces is expected to open the way to realize next generation quantum devices, such as quantum light emitters and advanced sensing devices.

KAWAZU, Takuya

Formation technology of Sb-based compound semiconductor quantum dots

Overview

Stranski-Krastanov (SK) mode growth is one of the widely used methods of quantum dot formation. 10 nm scale quantum dots can be fabricated without dislocations by simply stacking materials with different lattice constants on a substrate. Quantum dots fabricated in this way are expected to be applied to high-performance devices such as semiconductor lasers with excellent temperature stability and high-sensitivity infrared detectors.
In this study, we aim to establish a technology to form, control the shape of, and increase the density of Sb-based quantum dots, which cover a wider range of wavelengths, rather than InAs-based materials that have been widely studied in the past.

Characteristics

• Wide range of density and size control of Sb-based quantum dots by droplet epitaxy
• High density of Sb-based quantum dots on highly indexed surface substrates
• Fabrication of array structures of Sb-based quantum dots by tilted substrates

Major reserch

There is a lattice mismatch of about 7% between GaSb, AlSb and GaAs. When GaSb or AlSb is grown on a GaAs substrate, semiconductor island structures (quantum dots) are formed to relax the strain.
We found that by stacking GaSb or AlSb on high-index GaAs substrates at various substrate temperatures and growth rates, quantum dots with significantly different densities and shapes can be formed.

Summary

In this study, we attempted to control the shape of Sb-based quantum dots by selecting parameters such as substrate temperature, growth rate, and substrate orientation. However, optical properties of quantum dots are important for applications in optical devices such as lasers and infrared detectors.
If a method for controlling optical properties can be established based on the knowledge of quantum dot shape control obtained in this study, it is expected to be applied to high-performance optical devices.

HAYASHI, Yusuke

Advanced Fabrication and Evaluation Methods for Compound Semiconductor Optical and Electronic Devices

Overview

III-V compound semiconductors have been widely applied to optical and electronic devices such as infrared wavelength laser diodes and high electron mobility transistors (HEMTs) by utilizing their excellent physical properties such as direct transition band gap and heterostructure interfaces.
This research aims to pioneer innovative technologies to drive next-generation semiconductor devices by promoting integration with new material systems and measurements in extreme environments, based on crystal growth technology.

Characteristics

• By utilizing the III-V/Si wafer bonding technology, we aim to achieve fusion with materials that have very large lattice mismatch with III-V semiconductors and to improve the performance of existing material systems, which has not been achieved by crystal growth of existing material systems.
  By developing this technology, future silicon photonic optical circuits and optoelectronic integrated circuits using III-V/Si platforms are expected to be realized.
• Depth-resolved nanobeam X-ray diffraction measurement using synchrotron radiation can realize strain measurement inside semiconductor devices with high accuracy and high spatial resolution.
  By applying nanosecond fast voltage pulses or kilovolt-class high voltage as external signals to semiconductor devices, we can dynamically observe strain behavior and failures under device operation and elucidate physical phenomena under extreme conditions that have not been revealed before.

Major reserch

Summary

Focusing on optical and electronic devices realized with III-V compound semiconductors, we aim to establish next-generation device fabrication and device operation measurement technologies by integrating new technologies such as wafer bonding and synchrotron radiation measurement, in addition to advanced epitaxial growth technology.
By developing this research, we will work to elucidate physical phenomena under extreme conditions in order to enhance the functionality and reliability of semiconductor devices.

MIYAZAKI, Hideki T. / Managing Researcher

High-sensitivity mid-infrared detectors integrating optical antennas and quantum wells

Overview

Mid-infrared wavelength range (5-10 µm) is important for environmental sensing and security, because molecule-specific absorption and thermal emission appear in this range. However, hazardous mercury and cadmium have been used for conventional high-sensitivity mid-infrared detectors.
In this research, by integrating less hazardous but low-sensitivity quantum-well infrared detectors with unique optical antenna structures, sensitivity comparable to that of conventional detectors has been achieved. The center wavelength can be freely engineered by the design of quantum wells and optical antennas. Therefore, the new detectors are suitable for high-sensitivity detection of a specific gas. Their applications to environmental sensing, agriculture, and medicine are expected.

Characteristics

Major reserch

Fundamental configuration of an optical antenna is a structure sandwiching a dielectric layer between a top and a bottom metallic layers. By selecting the dimension of each portion properly, the optical antenna resonates at a specific wavelength. In this study, a GaAs/AlGaAs quantum-well photodetector, which exhibits a sensitivity in the mid-infrared range and is less hazardous, is sandwiched between a top and a bottom gold layers. Such optical antennas integrated with quantum wells selectively collect incident light at a specific wavelength, rotate its electric field to a suitable direction for the absorption by quantum wells, and greatly enhance the sensitivity.
Furthermore, by bending the wires interconnecting the antennas into a zigzag shape to adjust the phase f the current, we succeeded in extracting high photocurrent while maintaining the synchronized resonance of the whole antennas. (Arrays of optical antennas are called metamaterials or metasurfaces.)



• The absorption wavelength of a quantum-well infrared photodetector can be freely engineered by the thicknesses of GaAs/AlGaAs layers and Al composition. By adjusting the resonance wavelength of the optical antennas with that of the quantum wells and by optimizing the zigzag wires, the sensitivity of the original quantum wells is enhanced by 800 times, and a responsivity of 3.3 A/W (at 6.7 µm and 78 K), an external quantum efficiency of 61%, and a detectivity of 3.9×10¹⁰cmHz^1/2/W were achieved. This detectivity is higher than those of HgCdTe photodetectors, conventionally used as high-sensitivity detectors, and nearly approaching the theoretical limit. The responsivity has a sharp peak at a specific wavelength, thus, our detector is suitable for measuring the concentration of a specific gas. We also demonstrated fast measurement of NO₂ gas concentration (absorption wavelength 6.25 µm) at a response time less than 1 ms.

Summary

By integrating optical antennas and quantum wells, we demonstrated high-sensitivity mid-infrared photodetectors with comparable performances of conventional detectors without using hazardous materials. Because the center wavelength can be freely selected by the design of the quantum wells and optical antennas, our detector is suitable for sensors detecting a specific gas with a high sensitivity.
By raising the operation temperature based on the structure optimization and by further integration of the devices, we would like to apply our detectors to environmental sensing, agriculture, and medicine.

IMURA, Masataka

Development of optical, electronic, and magnetic devices using next-generation semiconductors

Overview

In the field of next-generation semiconductor optical, electronic, and magnetic devices, the introduction of wide-gap semiconductors to replace Si is essential. I have focused early on III-nitride semiconductors and diamond semiconductors, which possess excellent applicability, practicality, and high potential, and am developing devices using these materials.
At NIMS, we have established a research system where crystal growth, device processing, and crystal evaluation are integrated, working together with constant feedback.

Characteristics

• Development of radiation and proton detectors using diamond semiconductors
• Development of full-spectrum visible light photonic devices using InGaN-based nitride semiconductor with nanofabrication
• Development of high-efficiency ultraviolet light-emitting diodes using AlGaN-based nitride semiconductors
• Development of high-frequency power devices using AlGaN/GaN and AlInN/GaN heterostructures
• Development of magnetic devices using BAlN and ScAlN-based nitride semiconductors

Major reserch

Summary

Using diamond, we fabricated detectors and evaluated their response characteristics and durability when exposed to vacuum ultraviolet radiation and proton beams. The results demonstrated the device's capability for long-term stable operation.
Recently, we have been conducting research and development on full-spectrum visible light photonic devices, high-efficiency ultraviolet light-emitting diodes, high-frequency power devices, and magnetic devices using novel materials, all based on Group III nitride semiconductors.