6 Devices and materials for next-generation broadcasting

Transcription

1 6 Devices and materials for next-generation broadcasting We are conducting R&D on the next generation of imaging, recording and display devices and materials for broadcast services such as 8K Super Hi-Vision (SHV) and future three-dimensional (3D) television. In our research on imaging technologies, we made progress in developing 3D integrated imaging devices having many pixels and a high frame rate, solid-state image sensors overlaid with multiplier films for 8K cameras with high sensitivity, and organic photoconductive films to realize a compact and high-quality single-sensor 8K camera. In our work on 3D integrated imaging devices, we prototyped a device with pixels (about μm 2 ) and demonstrated that it can achieve characteristics with a wide dynamic range by taking advantage of its capability of pixel-parallel signal processing. Our work on solid-state image sensors overlaid with multiplier films included reducing the dark current by improving the crystallinity of crystalline selenium that constitutes multiplier films and prototyping an 8K CMOS circuit for reading signals from stacked multiplier films. In our work on organic photoconductive films, we improved the quantum efficiency by applying new materials and realized high-efficiency transparent cells for blue and red. In our research on recording technologies, we continued with our work on holographic memory with a large capacity and high data transfer rate for SHV video signals and on a recording device with no moving parts that utilizes the motion of magnetic domains in magnetic nanowires. In holographic memory, we developed a multilevel recording technology that is expected to achieve a large capacity and high speed. As the elemental technologies, we developed a decoding method for reproduced data based on machine learning and an error correction code technology. We also developed a multilevel recording reproduction technology using amplitude modulation as a modulation scheme suited for 4-level multi-modulation. In our research on magnetic nanowires, we investigated suitable magnetic nanowire materials, conducted simulations of magnetic domain formation and a driving domain analysis, and widened the bandwidth of our recording and reproduction evaluation system in order to increase the driving speed of magnetic domains. By incorporating an artificial superlattice structure using cobalt (Co)/terbium (Tb) multilayered films that reduces magnetization, which hinders high-speed driving, and the spin Hall effect produced by platinum (Pt), we achieved magnetic domain driving in excess of 15 m/s, more than 10 times that of conventional devices. In our research on displays, we studied a technology to increase the lifetime and color purity of organic light-emitting diode (OLED, a luminescent diode using organic materials) devices and a technology to increase the image quality and reduce the driving power consumption of displays with the aim of achieving large SHV displays for home use. We also developed elemental technologies for solution-processed devices for future large flexible displays. To increase the lifetime, we fabricated a practical inverted OLED having a luminance half-life of 10,000 hours or more at a low temperature and searched for host materials for the light-emitting layer suited for a longer lifetime. We also realized a green OLED with high color purity through device modification focused on the chemical structure of materials. Our work on higher image quality and lower power consumption included the development of a technology for shortening the channel length of oxide TFTs and proposing a method for controlling the driving power and display luminance for displaying HDR video. We conducted simulations to verify the power-saving effect of the method. For solution-processed devices, we developed a patterning fabrication process for oxide semiconductors that uses a photoreaction and a high-efficiency quantum dot light-emitting diode (QD-LED) using zinc sulfide-silver sulfide indium solid solution. 6.1 Advanced image sensors Three-dimensional integrated imaging devices We are researching imaging devices with a 3D structure in our quest to develop a next-generation image sensor having more pixels and a higher frame rate. These devices are fabricated by stacking a photodetector and a signal processing circuit after each of them is formed. They have a signal processing circuit for each pixel directly beneath the photodetector. Pixel Light Photodetector Bonded surface Pixel-parallel signal processing Buried electrode Support wafer Cross-sectional structure of a pixel Signal processing circuit Figure D integrated imaging device NHK STRL ANNUAL REPORT

2 10 5 Light Average output digital value Luminous intensity of incident light (lx) 96dB 16bit Since this structure enables signals from all pixels to be read out simultaneously, it makes it possible to output digitized signals in pixel units, which can achieve a high frame rate in reading signals even when the number of pixels increases (Figure 6-1). We previously devised a noise-canceling circuit that is capable of pixel-parallel operation and stacking into pixels and developed elemental technologies such as a technology for miniaturizing a buried electrode that connects the photodetector with the signal processing circuit and a highly reliable stacking process. In FY 2017, we prototyped an imaging device using an array of pixels (about μm 2 ). The device is designed to have current buffers within pixels and signal-reading paths to ensure a circuit structure that can output digital values stably from pixels. Regarding the fabrication process, we succeeded in connecting a buried electrode 5 μm in diameter with an alignment accuracy of 1 μm or less by using the elemental technologies that we developed in FY The prototype imaging device achieved 16-bit output in a wide dynamic range of 96 db by taking advantage of the feature of pixel-parallel signal processing (1) (Figure 6-2). This research was conducted in cooperation with the University of Tokyo Figure 6-2. Input/output characteristics of prototype device 10 4 Color filter Electric charge Pixel 8K solid-state image sensor overlaid with multiplier film Multiplier film Pixel electrode Insulating layer CMOS circuit Figure 6-3. Structure of solid-state image sensor overlaid with multiplier film In next-generation broadcasting services such as 8K, the amount of light incident on each pixel of the imaging device decreases as the resolution and frame rate of the camera increase. As a drastic solution to this problem, we are developing a solid-state image sensor overlaid with a photoconductive film (multiplier film) on a CMOS circuit (Figure 6-3). The multiplier film can obtain the effect of electric charge multiplication by only applying a low voltage. In FY 2017, we worked to reduce the dark current of crystalline selenium films that constitute multiplier films and prototyped an 8K CMOS circuit on which a multiplier film is overlaid. Since the dark current that occurs with crystalline selenium films is considered to be closely related to the crystallinity of selenium, we evaluated the crystallinity using an X-ray diffraction method. The results showed that the crystalline state of a tellurium layer, which is inserted to prevent films from peeling from the substrate, affects the crystallinity of selenium. We therefore developed a new deposition process for improving the crystalline state of the tellurium layer. This process successfully suppressed the increase in dark current when the applied voltage increases (Figure 6-4) (2). The unevenness between a pixel electrode and its surroundings in a CMOS circuit to be overlaid with a multiplier film needs to be as small as possible to prevent film defects caused by the excessive concentration of electric field. Since our prototype 8K CMOS circuit had a maximum surface unevenness of about 900 nm, we formed an insulating layer around the pixel electrode and applied a planarization process by polishing. We confirmed that this can reduce the surface unevenness to 5 nm or less. Light Transparent electrode Crystalline selenium Tellurium Gallium oxide Transparent electrode Glass substrate (a) Dark-current density (pa/cm 2 ) New tellurium deposition process Conventional tellurium deposition process Room temperature (25 C) Voltage (V) (b) Figure 6-4. Structure (a) and dark-current characteristics (b) of element for evaluating multiplying effect of crystalline selenium film 42 NHK STRL ANNUAL REPORT 2017

3 Quantum efficiency (%) For blue Wavelength (nm) For red Figure 6-5. Quantum efficiency of transparent cells for blue and red Organic photoconductive film for single-chip cameras with high S/N We are conducting research on organic image sensors with the goal of realizing a compact single-chip color camera that is small, lightweight and highly mobile. These sensors consist of stacked layers of three different organic photoconductive films (organic films) sensitive to each of the three primary colors of light. The electrodes of organic image sensors that sandwich each organic film must be transparent in order to transmit incident light into lower layers of the stacked organic films. We previously improved the performance of organic films for each color and achieved a quantum efficiency of 80% with a transparent cell in which an organic film for green is sandwiched between transparent electrodes. In FY 2017, we increased the efficiency of transparent cells for blue and red. For the transparent cell for blue, we selected a new hole-transport material having high durability as a photoelectric conversion material and added 5% of the electron-transport fullerene C 60 to promote the separation of electron-hole pairs, which are produced by photoabsorption. We also applied electron beam evaporation, which can suppress damage on organic films, as a method for forming a transparent electrode on an organic film. Our prototype transparent cell achieved a maximum quantum efficiency of 77% when a voltage of about 6 V was applied (Figure 6-5) (3). The research on the transparent cell for blue was conducted in cooperation with Nippon Kayaku Co., Ltd. We also re-examined the materials for the cell for red and developed a transparent cell using boron subnaphthalocyanine, which exhibits both electron and hole transport, as a photoelectric conversion material. The transparent cell achieved a maximum quantum efficiency of 80% in the red region when a voltage of about 11 V was applied (Figure 6-5) (4). This means that we have achieved high efficiency for transparent cells for all three primary colors of light, including the one for green that we previously developed. [References] (1) M. Goto, Y. Honda, T. Watabe, K. Hagiwara, M. Nanba, Y. Iguchi, T. Saraya, M. Kobayashi, E. Higurashi, H. Toshiyoshi and T. Hiramoto: Fabrication of Three-Dimensional Integrated CMOS Image Sensors with Quarter VGA Resolution by Pixel-Wise Direct Bonding Technology, 30th International Microprocesses and Nanotechnology Conference (MNC 2017), 9A-9-2 (2017) (2) S. Imura, K. Mineo, K. Miyakawa, H. Ohtake and M. Kubota: High-efficiency and low dark current crystalline selenium-based heterojunction photodiode with a high-quality tellurium nucleation layer, Proc. of the IEEE Sensors 2017, B3L-C, pp (2017) (3) T. Takagi, Y. Hori, T. Sakai, T. Shimizu, H. Ohtake and S. Aihara: Characteristic improvement in blue-sensitive organic photoconductive film sandwiched between transparent electrodes, ITE Winter Annual Conference, 22C-2 (2017) (in Japanese) (4) T. Takagi, Y. Hori, T. Sakai, T. Shimizu, H. Ohtake and S. Aihara: Fabrication of Transparent-Type Red Sensitive Organic Photoconductive Cell with High Quantum Efficiency, Ext. Abstr. of the 65th JSAP Spring Meet., 20p-A204-6 (2018) (in Japanese) 6.2 Advanced storage technologies Multilevel holographic memory Storing 8K video for a long time requires a storage system for video archiving that has a very high transfer rate and large capacity. We have been researching holographic memory to meet these requirements. In FY 2017, in our work on elemental technologies for multilevel recording, we developed a decoding method for reproduced data based on machine learning and an error correction code technology. We also developed a multilevel recording reproduction technology using amplitude modulation as a modulation scheme suited for four-level modulation. Holographic memory uses laser beams to record and reproduce a data page in which symbol pixels consisting of dark pixels and bright pixels are arranged in two dimensions. Focusing on the fact that a data page is a kind of image, we developed a decoding technology using convolutional neural network (Figure 6-6). This technology reduces the number of bit errors in reproduced data by using preliminary machine learning of the rules of modulation schemes and the causes of optical system errors such as lens aberration (1). We demonstrated that this method reduces the number of bit errors by 60% compared with a conventional method of determining data of a two-level modulation scheme using a threshold value. Since an error correction code for four-level modulation requires a higher correction capability than one for two-level modulation, we developed a spatially coupled low-density parity check (LDPC) code dedicated to multilevel holographic memory (2). Verification of the performance of this code through simulations demonstrated that the code can correct errors if the bit-error rate before correction data page is or less. As a way to create a data page of four-level modulation, we developed a 10:9 modulation code that assigns bright pixels of three different luminance levels to three out of nine symbols. This method, which sets a standard luminance pixel to any of the three symbols, has the feature of high tolerance against the brightness non-uniformity that occurs at the time of recording to and reproduction from the recording media. We evaluated the recording and reproduction by a holographic memory optical system using this method and confirmed that data can be reproduced with a bit-error rate that can be corrected by a spatially coupled LDPC code. Magnetic high-speed recording devices utilizing magnetic nanodomains With the goal of realizing a high-speed magnetic recording device with no moving parts and a high reliability, we are developing a recording device that utilizes the high-speed-motion characteristics of nanosize magnetic domains in magnetic nanowires. In FY 2017, we developed fundamental technologies for further increasing the driving speed of recorded magnetic NHK STRL ANNUAL REPORT

4 Reproduced data page Convolutional neural network Select Figure 6-6. Decoding using convolutional neural network domains. In our quest for a device that enables the high-speed driving of magnetic domains, we previously employed an artificial superlattice structure that has an ultrathin ruthenium (Ru) interlayer between cobalt (Co)/palladium (Pd) multilayered films. This structure is effective for reducing magnetization, which hinders high-speed driving, and achieved magnetic domain driving at 1 m/s. In FY 2017, we began fabricating and evaluating a cobalt/terbium (Tb) multilayered film as a new material for even higher-speed driving (3). Since terbium is weakly magnetized in the opposite direction to that of cobalt, the net magnetization of a multilayered film stacking these elements equals the difference in magnetization between cobalt and terbium, which means that a further reduction in magnetization is possible. We also fabricated a magnetic nanowire structure having a thin platinum (Pt) layer stacked on this cobalt/terbium multilayered film and found that this structure can improve the driving speed of magnetic domains significantly by using the spin Hall effect produced by platinum (Figure 6-7(a)). In particular, magnetic nanowires with a structure of Pt (3 nm)/[co (0.3 nm)/tb (0.6 nm)] achieved magnetic domain driving at 15 m/s, more than 10 times that of previous [Co/Pd] nanowire. As elemental technologies for the high-speed driving of magnetic nanodomains, we increased the bandwidth of the signal preamplifier system of a magnetic recording head that is used to detect magnetic domains in magnetic nanowires. When driving magnetic domains at high speed, conventional direct-current amplifiers have difficulty in tracking and detecting changes of their magnetization direction. To address this problem, we prototyped a new recording and reproduction evaluation unit equipped with the precise control of a current-induced magnetic field during recording and the capability to capture radio-frequency signals up to about 1.6 GHz with low noise by series-connecting a head preamplifier for a hard disk drive with the input/output unit of the magnetic recording head. We also developed evaluation equipment that can detect the motion of magnetic domains during driving in real time by using a magneto-optical Kerr effect microscope to enable the evaluation of the shape of driving magnetic domains (Figure 6-7(b)) and we evaluated the driving speed. To further increase the driving speed of magnetic domains in magnetic nanowires, we conducted simulations using the Landau Lifshitz Gilbert (LLG) equation, which describes magnetization dynamics and damping in general magnetic materials, to investigate the effect of applying a magnetic field on assisting magnetic domain driving. The results showed that the driving speed can be almost doubled by applying a local static magnetic field that has only the component of the in-plane direction in magnetic nanowires to the immediate proximity of the magnetic domain wall simultaneously with the application of pulse currents, compared with that in the conventional driving method using only pulse currents (4). [References] (1) Y. Katano, T. Muroi, N. Kinoshita and N. Ishii: Image Recognition Demodulation Using Convolutional Neural Network for Holographic Data Storage, Tech. Dig. ISOM 17, Tu-G-04, pp (2017) (2) N. Ishii, Y. Katano, T. Muroi and N. Kinoshita: Spatially coupled low-density parity-check error correction for holographic data storage, Jpn. J. Appl. Phys., 56, pp. 09NA NA03-4 (2017) (3) M. Okuda, M. Kawana, Y. Miyamoto and N. Ishii: Precise Control of Current Driven Domain Wall Motion by Diphasic Current Pulses, MMM 2017, EC-08, pp (2017) (4) M. Kawana, M. Okuda, Y. Miyamoto and N. Ishii: Estimation on current-driven domain wall motion in magnetic nanowire by use of magnetic field assist, TMRC 2017, DP-11, pp (2017) Pt [Co/Tb] (1) Torque generated by spin is transmitted from Pt to [Co/Tb] layer (2) Magnetic moment rotates to move magnetic domains (a) Concept of spin Hall effect produced by platinum Electrode (1) Initial state (2) After applying 500-ns single pulse once (3) After applying twice (4) After applying three times (5) After applying four times Magnetic nanowire (40 μm long) Downward domain (Dark region) Upward domain (Bright region) Electrodes Multiple domains move to the right (b) State of magnetic domain driven by current Figure 6-7. Concept of magnetic domain driving assisted by spin Hall effect and state of magnetic domain driven by current 44 NHK STRL ANNUAL REPORT 2017

5 6.3 Next-generation display technologies Flexible OLED displays with longer lifetime and higher color purity Organic light-emitting diode (OLED) devices use active materials such as alkali metals for their electron injection layer. Since these materials are sensitive to moisture and oxygen, the devices deteriorate over time when used on a flexible substrate such as a plastic film. This poses the greatest challenge in realizing a flexible OLED display. To address this issue, we are developing an OLED that does not use alkali metals and can better withstand oxygen and moisture, called an inverted OLED. In FY 2017, we developed a technology for fabricating inverted OLEDs at a low temperature to realize a flexible OLED with a longer lifetime. Conventional inverted OLEDs use zinc oxide, which requires heat treatment at 400 C, for their electron injection layer. This makes it difficult to apply an inverted OLED to a versatile plastic film substrate, which does not have sufficient heat resistance. We therefore employed zinc oxide nanoparticles mixed with a tin compound, which can be formed at a low temperature of 120 C, for the electron injection layer and realized a practical inverted OLED with a luminance half-life of 10,000 hours or more. This showed the feasibility of its application to film substrates (1). While it had been known that using phosphorescent materials for an OLED s light-emitting layer can produce a high emitting efficiency, the design criteria for materials for a device with a longer lifetime had not been systematically understood. We therefore used thermally activated delayed fluorescence (TADF) materials having a similar molecular structure for the host materials of the light- emitting layer and compared and analyzed their lifetime characteristics to search for the design criteria for host materials appropriate for a device with a longer lifetime. The results showed that the use of TADF materials having a smaller molecular size can realize a device with a longer lifetime (2). To reproduce a wide color range of SHV on an OLED display, it is necessary to develop a power-saving green OLED with a high color purity. To meet this requirement, we used a platinum complex having a rigid network molecular structure as the luminescent material and employed a top-emission structure that takes light out of the upper electrode. This realized a green OLED having a color purity with x-y chromaticity coordinates of (0.18, 0.74) (3) (Figure 6-8). Technologies for increasing image quality and lowering driving power consumption We are conducting R&D on improving the performance of high-mobility oxide TFTs and driving and signal processing technologies to increase the image quality and lower the power consumption of sheet-type OLED displays. In FY 2017, we developed a technology for shortening the channel length of oxide TFTs. We found that a TFT channel partly changes to a conductor when hydrogen is injected into a oxide semiconductor layer that uses In-Ga-Sn-O (IGTO). By taking advantage of this phenomenon, we successfully shortened the channel length of TFTs (4). We confirmed that this method can shorten the channel length to 1.4 μm. This channel-shortening technology was developed in cooperation with Kobe Steel, Ltd. As a signal processing technology for increasing the image quality of OLED displays, we studied a method for controlling the driving power and display luminance for displaying HDR video and conducted simulations to verify the effects. When displaying HDR video with a high average luminance on an OLED display, conventional technologies had a problem of tone degradation in dark regions because they suppress the luminance of signal levels uniformly to limit the power. To address this problem, we devised a method for suppressing the driving power while maintaining the tone representation of dark and light regions and confirmed its effectiveness using evaluation images (5). Solution-processed devices for large flexible displays With the goal of realizing a large flexible display that is thin, Platinum complex + top-emission structure (x:0.18, y:0.74) High-color-purity green Platinum complex + conventional structure Conventional device 4K/8K color standard 1 y Conventional HDTV color standard Luminescence intensity Narrow = High-color-purity green Wavelength (nm) x Figure 6-8. Chromaticity diagram and spectrum of developed OLED device NHK STRL ANNUAL REPORT

6 lightweight and rollable, we are conducting R&D on oxide TFTs that can be fabricated by solution process without using a large vacuum chamber, which is necessary for panel production, and on electroluminescent devices using quantum dots (QDs) called quantum dot light-emitting diodes (QD-LEDs). In our work on solution-processed oxide TFTs, we developed a method for fabricating TFTs more easily and simply by taking advantage of a solution process. Previously, TFTs were fabricated by a complicated technique such as photolithography using photoreactive organic materials. We successfully simplified this fabrication process by developing a method for patterning oxide semiconductors by a direct photoreaction. The fabricated TFTs by this new method showed electrical performance comparable to that of TFTs fabiricated by a conventional method (6). This result indicates that this method is effective for realizing an inexpensive large display. A QD-LED, which is a luminescent device using QDs as luminescent materials. QDs consist of semiconductor nanocrystals with a size of about 10 nm, can control the wavelength and full width at half maximum of the emission spectrum by using the capability of grain size control. In FY 2017, we prototyped a QD-LED that uses a zinc sulfide-silver sulfide indium solid solution (ZAIS) as low-toxicity QD material. ZAIS QDs can change the luminescence wavelength by controlling the elemental composition ratio as well as the grain size. Our QD-LED using ZAIS QDs emitted red light with an external quantum efficiency of 1.9% (7). The research on the QD-LED using ZAIS was conducted in cooperation with Nagoya University and Osaka University. [References] (1) T. Sasaki, H. Fukagawa, T. Shimizu, Y. Fujisaki and T. Yamamoto: Improved operational stability of inverted organic light-emitting diodes using Sn-doped zinc oxide nanoparticles as an electron injection layer, Proceedings of EuroDisplay 2017 (2) H. Fukagawa, T. Shimizu, Y. Iwasaki and T. Yamamoto: Operational lifetimes of organic light-emitting diodes dominated by Forster resonance energy transfer, Scientific Reports, DOI: /s (2017) (3) T. Oono, Y. Iwasaki, T. Hatakeyama, T. Shimizu and H. Fukagawa: Demonstration of Efficient Green OLEDs with High Color Purity, SID Digest, pp (2017) (4) M. Nakata, M. Ochi, H. Tsuji, T. Takei, M. Miyakawa, Y. Fujisaki, H. Goto, T. Kugimiya and T. Yamamoto: Fabrication of a Short-Channel Oxide TFT Utilizing the Resistance-Reduction Phenomenon in In-Ga- Sn-O, SID 2017 Digest, pp (2017) (5) T. Yamamoto, T. Okada, T. Usui and Y. Fujisaki: Picture Level Control Method for Super Large-Area Display, IDW 17 VHF6-1, pp (2017) (6) M. Miyakawa, M. Nakata, H. Tsuji and Y. Fujisaki: Direct Photoreactive Patterning Method for Fabricating Aqueous Solution-Processed IGZO TFTs, IDW 17 AMD3-2, pp (2017) (7) G. Motomura, T. Tsuzuki, T. Kameyama, T. Torimoto, T. Uematsu, S. Kuwabata and T. Yamamoto: Fabrication of Low Toxic Quantum Dot Light-Emitting Diode using ZnS-AgInS2, ITE Annual Convention 2017, 32C-1 (2017) (in Japanese) 46 NHK STRL ANNUAL REPORT 2017