Broadcast Technology No.60, Spring 2015 ● C NHK STRL 21

Low-Voltage-Operation Avalanche Photodiode Based on N-Gallium Oxide/P-Crystalline Sele-nium HeterojunctionApplied Physics Letters, Vol.104, No.24, pp. 242101.1-242101.4 (2014)Shigeyuki Imura, Kenji Kikuchi, Kazunori Miyakawa, Hiroshi Ohtake, Misao Kubota

Because of the strong demand for high-definition and high-frame rate video cameras, highly sensitive imaging devices are significantly desirable. We are developing the high-sensitivity image sensors over-laid with photoconversion layers which use carrier multiplication at low-applied voltage. In this study,

we use crystalline selenium (c-Se), which has an extremely large absorption coefficient over the entire visible region, as a photoconversion layer to fabricate a test structure to measure the photoconversion characteristics. As a result of the measurements, the dark current was significantly reduced by using an n-type wide band gap gallium oxide (Ga2O3) to prevent the number of carriers injected from an external electrode; hence, we demon-strated avalanche multiplication in the c-Se films exhibiting an extremely high external quantum efficiency (EQE) of over 100% for the first time. Furthermore, tin (Sn) doping of the Ga2O3 layer effectively increases the carrier concentration, allowing the depletion layer between Ga2O3 and c-Se to be spread into the c-Se layer, resulting in a low-voltage-operation avalanche photodiode. These results made a giant step toward highly sensitive image sensors, opening the door to the age of next-generation ultra high-definition imaging systems.

Development of a Multilink 10 Gbit/sec Mapping Method and Interface Device for 120 Frames/sec Ultra High-Definition Television SignalsSMPTE Motion Imaging Journal Vol. 123, No. 4, pp. 29-38, MAY/JUNE (2014)Takuji Soeno, Yukihiro Nishida, Takayuki Yamashita, Yuichi Kusakabe, Ryohei Funatsu, and Tomohiro Nakamura

We are researching a next-generation ultra high-definition television (UHDTV) broadcasting system. The video parameters of UHDTV systems with 120 frame/sec signals are specified in Recommendation ITU-R BT.2020. In this study, a new mapping method was developed to transmit various UHDTV signals,

including 120 frame/sec signals. A prototype interface for connecting UHDTV video devices was also developed. The method is to transform UHDTV signals into multilink 10 Gbit/sec streams. The number of 10 Gbit/sec streams differs according to the UHDTV format (frame frequency, pixel count, and sampling lattice). To realize a compact and low-power interface, we implemented the prototype using a parallel fiber-optic transceiver with a capacity of 10 Gbit/sec per channel. Finally, we verified the practicality and feasibility of the multilink 10 Gbit/sec mapping method and the prototype interface.

Low-Current-Density Spin-Transfer Switching in Gd22Fe78-MgO Magnetic Tunnel JunctionJournal of Applied Physics, Vol. 115, 203903.1-203903.3 (2014)Hidekazu Kinjo, Kenji Machida, Koichi Matsui*, Ken-ichi Aoshima, Daisuke Kato, Kiyoshi Kuga, Hiroshi Kikuchi, and Naoki Shimidzu*Tokyo Denki University

We have been investigating a spatial light modulator driven by spin-transfer switching (spin-SLM) for three dimensional holography systems. In the present study, we fabricated perpendicular TMR light modulation devices with a Co-Fe/MgO/Co-Fe tunnel junction and a Gd-Fe light modulation layer.

At about 560 nm × 560 nm in size, although the TMR ratio was 7.0%, which is a considerably low value, the magnetization of the light modulation layers switched at a low current density of 1.0×106 A/cm2. This low-current switching is mainly attributed to thermally assisted spin-transfer switching as a consequence of thermal magnetic behavior arising from Joule heating, because Gd-Fe alloys are temperature-sensitive materials.