October 2019 Vol. 17 No. 10

Feature Articles: Creating Novel Functional Materials

• Overview of Novel Materials Creation Research at NTT

  

  Abstract
  The activities of NTT Basic Research Laboratories include creating novel materials with fascinating functions through materials design and arrangement control of atoms, molecules, and crystals in materials. The purpose is to make progress in materials science and eventually contribute to the development of information and communication technology. While our research covers a vast range of materials, we introduce some of the latest research accomplishments in inorganic materials, for example, oxides and nitrides, in the Feature Articles in this issue. In particular, we highlight thin films (less than a micrometer thick), atomic layer materials (less than a nanometer thick), and nanowires.

• Creation of Novel Material Sr₃OsO₆ with the Highest Ferromagnetic Transition Temperature among Insulators

  

  Abstract
  We have synthesized a novel material, Sr₃OsO₆ (Sr: strontium, Os: osmium, O: oxygen) using a unique oxide thin-film growth technique that has been developed over many years at NTT Basic Research Laboratories. The Curie temperature (T_C) value of this material, estimated from the magnetic measurements, is above 780°C, surpassing the T_C record among insulators for the first time in 88 years by more than 100°C. As Sr₃OsO₆ has been synthesized in a single-crystalline thin film form, this brand-new material is expected to be readily implemented in high-performance magnetic device applications such as magnetoresistive random access memories and magnetic sensors that work above room temperature.

• Magnetically Purified Erbium-doped Oxide Crystal―Towards Creating a Quantum-information-manipulation Platform

  

  Abstract
  Erbium (Er) is a rare-earth element that is expected to be a platform for manipulation of quantum information by utilizing telecommunication-wavelength photons. However, high-quality epitaxial growth of Er-doped host crystals is difficult, and the memory time of quantum information, which determines the performance of quantum manipulation, is much shorter than theoretically predicted. We investigated a good-compatibility rare-earth oxide as a host crystal for Er. The host oxide crystal, cerium oxide (CeO₂), had its nuclear spin removed (i.e., magnetically purified). The nuclear spin is the main factor that shortens the lifetime of quantum-information retention time. In this article, we describe high-quality epitaxial growth of CeO₂ and its optical properties as a thin film grown on a silicon substrate.

• MBE Growth and Element-distinctive Atomic-resolution Characterization of High Temperature Superconductors

  

  Abstract
  Cuprate superconductors are in the material family that have the highest superconducting transition temperature (T_c) of ~130 K under ambient pressure. The infinite-layer structure is the essential building block of high-T_c cuprates with a T_c of over 100 K and is therefore vital to understand the mechanism of high-T_c superconductivity. While the infinite-layer phase is inaccessible using bulk single-crystal synthesis methods, we synthesized single-crystalline thin films of the infinite-layer cuprates using our unique oxide molecular beam epitaxy setup. To clarify the relationship between their microscopic crystal structures and electronic responses, we performed atomic-resolution electron microscopy measurements of the CuO₂ (copper peroxide) planes in the infinite-layer cuprates, the playground of high-T_c superconductivity.

• High-quality Atomic-layer Materials Fabricated by Chemical Vapor Deposition

  

  Abstract
  Atomic-layer materials such as graphene and hexagonal boron nitride (h-BN) are promising for their use in next-generation optoelectronic devices due to the novel and unique physical phenomena as well as superior material properties being demonstrated in such materials. Scalable fabrication methods for high-quality atomic-layer materials are essential for their industrial applications. We introduce here chemical vapor deposition growth for high-quality graphene and h-BN.

• Development of Next-generation Wide-bandgap Semiconductors

  

  Abstract
  Cubic boron nitride (c-BN) is a wide-bandgap semiconductor with the highest breakdown field among semiconductors. It has the potential to dramatically improve efficiency in power devices. Although c-BN is a metastable material, we have heteroepitaxially grown high-quality c-BN thin films by developing a unique growth technique. We have also used doping to control the electrical conductivity of the c-BN films. These accomplishments are major steps toward the fabrication of c-BN-based power devices.

• Crystal Growth of Wurtzite GaP Nanowires for Solar-water-splitting Devices

  

  Abstract
  Gallium phosphide (GaP) is an environmentally friendly III-V semiconductor, and it has an indirect bandgap as a stable zincblende structure. However, if controlled nanowire growth is carried out, a wurtzite structure with a direct bandgap can be achieved. We successfully grew wurtzite GaP nanowires without stacking faults by using our original method using chlorine etching and repeated gallium supply. We present here our results of solar-water splitting using a p-type nanowire photoelectrode, and we explain the fabrication and properties of a pin-type nanowire solar cell.

Regular Articles

• High Power and High Efficiency Operation of Semiconductor Optical Amplifier Assisted Extended Reach Electroabsorption Modulated DFB Laser (AXEL) for Extension of Transmission Distance

  

  Abstract
  We have designed a semiconductor optical amplifier integrated with an electroabsorption modulator integrated distributed feedback laser (EML). We call this device AXEL; it is designed to enhance the power conversion efficiency and modulated light output power of EMLs. In this study, we investigated AXELs for both the L-band and O-band wavelength ranges. The results of experiments indicated that dramatically increased power conversion efficiency and modulated light output power were obtained with the fabricated AXELs. For the L-band wavelength range AXEL, a high modulated light output power of 9 dBm and an extension of 10-Gbit/s transmission distance to 80 km due to low chirp characteristics of the AXEL were simultaneously demonstrated. In addition, the 25-Gbit/s 80-km transmission achieved using the O-band wavelength range AXEL along with an avalanche photodiode was demonstrated for the first time thanks to the significantly increased modulated light output power of the AXEL.

Global Standardization Activities

• Surviving in the Digital Transformation Era; Technical Trends and Issues from the Perspective of The Telecommunication Technology Committee

  

  Abstract
  As a standards development organization certified by the Ministry of Internal Affairs and Communications of Japan, The Telecommunication Technology Committee (TTC) has been contributing to standardization activities in the field of information and communication technology by developing and disseminating standards for information and communications networks for more than 30 years. The role of TTC is changing as the environment surrounding information and communications undergoes dramatic changes such as the development of digital transformation. This article describes the current status and challenges of Japanese companies in the global market and explains the latest technological trends and TTC’s initiatives to serve as a reference for future business strategies.

Practical Field Information about Telecommunication Technologies

• Introduction to Troubleshooting Cases Related to Telephone Systems in Customer Premises

  

  Abstract
  In this article, we introduce two cases involving problems that occurred in telephone systems. In the first case, a connection could not be established even when the customer dialed the correct number. In the second case, silence suddenly occurred during the connection. We investigated and solved these problems using various measurement tools and analysis methods. This is the fifty-fourth article in a series on telecommunication technologies.

External Awards/Papers Published in Technical Journals and Conference Proceedings

• External Awards/Papers Published in Technical Journals and Conference Proceedings