6 Devices and materials for next-generation broadcasting

Transcription

1 E G We are researching the next generation of imaging, recording, and display devices and materials for new broadcast services such as 8K Super Hi-Vision (SHV). In our research on imaging devices, we made progress in developing 3D integrated imaging devices, low-voltage multiplier films for solid-state image sensors, and organic image sensors. In our work on 3D integrated imaging devices capable of pixel-parallel signal processing, we reduced the pixel size to 50 μm square by halving the diameter of connection electrodes to 5 μm and modifying the circuit layout. We also developed a circuit for eliminating noise and improved the fabrication process. Our work on lowvoltage multiplier films for solid-state image sensors with high sensitivity included reducing the dark current by changing the fabrication process and reducing the noise of signal-reading circuits. In our work on single-chip organic image sensors with an image quality comparable to that of a three-chip camera, we improved the efficiency of organic photoconductive films and prototyped a transparent cell for green having a maximum quantum efficiency of 80%. In our research on recording devices, we continued with our work on holographic memory with a large capacity and high data transfer rate for SHV video signals, and on a high-speed magnetic recording device with no moving parts that utilizes the motion of magnetic domains in magnetic nanowires. In holographic memory, we developed a prototype drive that has a recording density of 2.4 Tbit/inch 2 and a data transfer rate of 520 Mbps and verified its operation by recording and reproducing compressed SHV video signal. We also began studying multi-value recording to increase the recording density and data transfer rate. In magnetic nanowires, we investigated for suitable magnetic nanowire materials, conducted simulations of magnetic domain formation and driving domain analysis, and widened the bandwidth of our recording and reproduction evaluation system in order to increase the driving speed of magnetic domains. This led to magnetic domain driving in excess of 1 m/s, more than 10 times that of conventional devices. In our research on displays, we studied an organic light-emitting diode (OLED) with a longer lifetime and solution-processed devices for large SHV displays for home use. We also developed elemental technologies for a next-generation display with higher image quality and lower power consumption. For an OLED with longer lifetime, we researched a device structure and materials that achieve both high efficiency and long lifetime and developed a red OLED device with an internal quantum efficiency of 100% and a lifetime of beyond 10,000 hours. For solution-processed devices, we developed a technology for increasing the mobility of solution-processed oxide TFTs and a technology for improving the efficiency of quantum-dot light-emitting diodes (QD-LEDs). In our work on displays with higher image quality and lower power consumption, we investigated oxide semiconductor materials suited for high-mobility TFTs. We also prototyped driving equipment that controls the temporal aperture in line units to suppress motion blur on hold-type displays such as OLED displays and demonstrated its effectiveness. 6.1 Advanced image sensors Three-dimensional integrated imaging devices We are researching imaging devices with a 3D structure in our quest to develop an image sensor having a large number of pixels that can be used as part of a future three-dimensional imaging system. These devices have a signal processing circuit for each pixel directly beneath the photodetector. This enables signals from all pixels to be read out simultaneously so that both a large number of pixels and a high frame rate can be Pixel Pixel-parallel signal processing Light Photodetector Figure 1. Concept diagram of 3D integrated imaging device B Signal processing circuit achieved at the same time (Figure 1). We previously prototyped a two-layered device with pixels (80 μm square each) Variance of (%) With correlated double sampling circuits Without correlated double sampling circuits Variance of standard voltage (%) Figure 2. Variance of relative to variance of standard voltage NHK STRL ANNUAL REPORT

2 Dark current density (pa/cm 2 ) Room temperature (25 ) Tellurium layer thickness : 1.0nm Tellurium layer thickness : 0.1nm Voltage (V) Figure 3. Dark-current characteristics of crystalline selenium film and confirmed that it can a signal of 16 bits with a wide dynamic range by utilizing a circuit structure taking advantage of pixel-parallel signal processing. In FY 2016, we reduced the pixel size, developed a circuit that can eliminate noise, and improved the fabrication process. We successfully miniaturized pixels to 50 μm square each by reducing the diameter of electrodes that connect the upper and lower circuits from 10 μm to 5 μm and modifying the circuit layout. For noise reduction, we devised a correlated double sampling (CDS) circuit that is capable of pixel-parallel operation and stacking into pixels (1). We conducted measurements by simulating changes in the standard voltage of a photodetector due to noise and confirmed that this circuit can reduce the variance of the (Figure 2). Regarding the improvement of the fabrication process, we found that impurities that adhered to the bonding surface during the wafer bonding process decrease the bond strength, and we developed a pretreatment technique for preventing the adherence of impurities. This research was conducted in cooperation with the University of Tokyo. Low-voltage multiplier films for solid-state image sensors The sensitivity of cameras incorporating solid-state image sensors decreases as the number of pixels and the frame rate increase because the amount of light incident on each pixel decreases. To address this problem, we are developing a solidstate image sensor overlaid with a photoconductive film (lowvoltage multiplier film) on a CMOS circuit. A photoconductive film is able to multiply the electric charge by only applying a low voltage. In FY 2016, we improved the characteristics of crystalline selenium films and chalcopyrite CIGS films, which are two candidate materials for low-voltage multiplier films. We also studied the reduction of the noise of CMOS imaging devices on which a multiplier film is overlaid. We considered that the dark current of crystalline selenium films is caused by tellurium, which is added to prevent films from separating from the substrate. We therefore developed a new deposition method for thinning the tellurium layer. The method achieved a tellurium layer with a thickness only onetenth that of a conventional film while maintaining the original effectiveness that reduced the dark current at room temperatures by almost half (Figure 3) (2). We also improved the crystallinity of chalcopyrite CIGS films by changing the film deposition process. A CMOS imaging device to be overlaid with a multiplier film has a different pixel circuit structure and signal-reading operation from those of an ordinary CMOS imaging device and thus requires a new process for reducing noise that occurs at the time of the reset operation. We investigated a method of performing digital correlated double sampling over two frames by using a prototype device. Evaluation results showed that Organic film for blue Organic film for green Organic film for red Transparent TFT circuit Figure 4. Structure of organic image sensor Transparent counter electrode Buffer layer Organic film for green Glass substrate Transparent electrode (a) Quantum efficiency (%) Light Green Blue Red Figure 5. Cross section (a) and spectral sensitivity (b) of prototype transparent cell for green performing this process halved the noise to about 15 electrons (at a 60Hz frame frequency) and clarified the relationship between noise and the dark current of circuits (3). Organic photoconductive film for single-chip cameras with high S/N 0 Applied voltage 15V Blue Green Red Wavelength (nm) (b) We are conducting research on organic image sensors with an image quality comparable to that of three-chip color broadcast cameras. These sensors consist of alternating layers of three different organic photoconductive films (organic films) sensitive to each of the three primary colors of light and transparent thin-film-transistor (TFT) circuits for reading the signals from the photoconductive films (Figure 4). The electrodes of organic image sensors that sandwich each organic film must be transparent in order to transmit light into the lower layers of the stacked organic films. In FY 2016, we developed a technology for improving the quantum efficiency of a transparent cell in which an organic film for green is sandwiched between transparent electrodes. The quantum efficiency of the organic films of transparent cells is reduced to about 10% because the films are damaged by the high energy of material particles when transparent counter electrodes are formed on the films. To solve this problem, we adopted an electron beam evaporation technique that can theoretically reduce the energy of material particles as a new way of forming transparent electrodes. We also inserted a transparent buffer layer with a robust molecular frame between the organic film and transparent counter electrode in order to suppress the damage on the organic films (Figure 5(a)). Evaluation results of the spectral sensitivity of the prototype transparent cell for green demonstrated that a quantum efficiency of 80% was achieved when green light with 500-nm wavelength was irradiated (Figure 5(b)) and that a transparent counter electrode was successfully formed without damage on the organic films (4). (1) M. Goto, Y. Honda, T. Watabe, K. Hagiwara, M. Nanba, Y. Iguchi, T. Saraya, M. Kobayashi, E. Higurashi, H. Toshiyoshi, T. Hiramoto: In- Pixel A/D Converters with 120-dB Dynamic Range Using Event- Driven Correlated Double Sampling for Stacked SOI Image Sensors, 42 NHK STRL ANNUAL REPORT 2016

3 Proc. of IEEE SOI-3D-Subthreshold Microelectronics Technology Unified Conference (IEEE S3S), 6b.3 (2016) (2) S. Imura, K. Mineo, Y. Honda, K. Hagiwara, T. Watabe, K. Miyakawa, M. Namba, H. Ohtake, M. Kubota: Improvement of Dark Current in c-se-based Photodiode by Reducing the Thickness of Adhesion Layer, Ext. Abstr. of 77th JSAP Autumn Meet., 16a-A35-5 (2016) (in Japanese) (3) T. Watabe, Y. Honda, M. Namba, T. Kosugi, H. Ohtake, M. Kubota: Random Noise and Dark Current of Readout Circuit for Stacked CMOS Image Sensor, Proc. of IEICE Society Conference, C (2016) (in Japanese) (4) T. Takagi, Y. Hori, T. Sakai, T. Shimizu, H. Ohtake, S. Aihara: Characteristic Improvement in Color-Selective Photodetector with Organic Photoconductive Film Sandwiched between Transparent Electrodes, Ext. Abstr. of 64th JSAP Spring Meet., 17a-P4-20 (2017) (in Japanese) 6.2 Advanced storage technologies Multi-level recording holographic memory An archive system for the long-term storage of 8K Super Hi- Vision (SHV) video will need a very high data transfer rate and large capacity. We have been researching holographic memory to meet these needs. In FY 2016, we developed a practical prototype drive and improved the signal-to-noise ratio (SNR) for multi-level recording. The prototype drive has a laser light source wavelength of 405 nm, a recording density of 2.4 Tbit/inch 2, and a data transfer rate of 520 Mbps. It can record 2 terabytes worth of data into a 130-mm disk medium (Figure 1). We confirmed that SHV video signals compressed to 85 Mbps can be recorded in a disk medium and reproduced in real time using the prototype drive. We exhibited the drive and reproduced video at the NHK STRL Open House 2016 (1). This drive is equipped with a wavefront compensation technology for suppressing the deterioration of reproduced signals due to hologram distortion. We confirmed that this technology is effective for reducing the error rate by more than 50% when reproduced signals have poor quality. In our work on elemental technologies for holographic memory, we developed the dual-page reproduction technology and achieved a reproduction transfer rate of 1 Gbps in FY 2015 (2). In FY 2016, we began studying multi-level recording to further increase the recording density and data transfer rate. For multi-level recording, it is necessary to develop technologies for improving the SNR. We investigated the use of a roll-off filter to reduce intersymbol interference, and confirmed using numerical simulation that it can improve the SNR by 1.9 db. We also improved the SNR by 1.5 db by applying a method that divides reproduced signals by a fixed-pattern noise element. This means that we can improve the SNR by more than 3 db by combining these methods. The prototype drive was developed in cooperation with Hitachi, Ltd., and Hitachi-LG Data Storage, Inc. Magnetic high-speed recording devices utilizing magnetic nanodomains With the goal of realizing a high-speed magnetic recording device with no moving parts, we are developing a recording device that utilizes the motion of nano-sized magnetic domains Figure 1. Prototype drive [Co/Pd] Multilayered film Ru interlayer [Co/Pd] Multilayered film Under layer Surface oxidized Si wafer substrate (a) Before driving After driving 500nm (b) Figure 2. Cross section of the prototype artificial ferrimagnetic nanowire and results of magnetic domain driving in magnetic nanowires. We previously verified the operation principle for this recording device, i.e., the formation (recording), detection (reproduction), and current driving of magnetic nanodomains by adopting a magnetic recording head used in hard disk drives (3). In FY 2016, we developed fundamental technologies for stable recording, reproduction and high-speed driving. On the basis of our finding that the misalignment of magnetic nanowires and the magnetic recording head deteriorates the recording efficiency and SNR, we added a mechanical system for precisely adjusting the contact angle and position of the head in the experimental setup for evaluating the recording and reproduction. In our quest for magnetic materials with low magnetization that would enable the high-speed driving of magnetic domains, we fabricated an artificial ferrimagnetic material that has an ultrathin ( nm) ruthenium interlayer between cobalt/palladium multilayered films. The new material reduced the net magnetization by 80% (4). We prototyped a nanowire structure using this material and drove magnetic domains with our experimental setup. The results demonstrated the successful high-speed driving of magnetic domains with 1/12 of the driving current density and more than ten times the driving speed of conventional materials (Figure 2). As elemental technologies for further increasing the driving speed, we modified the signal preamplifier system of a magnetic recording head that is used to detect magnetic domains in magnetic nanowires. In particular, we increased the bandwidth of the reproduction system from the conventional value of 100 MHz to about 1 GHz. We also conducted simulations using the Landau Lifshitz Gilbert (LLG) equation, which describes magnetization dynamics and damping in general magnetic materials, to investigate the directional dependence of a recording magnetic field on the magnetization reversal time. We found that domain nucleation for magnetization reversal starts at 10 picoseconds by applying a magnetic field obliquely to magnetic nanowires and demonstrated that the speed of magnetic domain formation can be increased by changing the direction of a recording magnetic field Magnetization (arbitrary unit) NHK STRL ANNUAL REPORT

4 (1) Y. Katano, T. Muroi, N. Kinoshita, N. Ishii: Prototype Holographic Drive with Wavefront Compensation for Playback of 8K Video Data, Proc. IEEE ICCE, pp (2017) (2)Y. Katano, T. Muroi, N. Kinoshita and N. Ishii: Efficient High-Speed Readout in Holographic Memory by Reusing Transmitted Reference Beam, MOC, 14B-2 (2016). (3) M. Okuda, Y. Miyamoto, M. Kawana, E. Miyashita, N. Saito, S. Nakagawa: Operation of [Co/Pd] nanowire sequential memory utilizing bit-shift of current-driven magnetic domains recorded and reproduced by magnetic head, IEEE Trans. Magn. Vol.52, No.7, pp (2016) (4) M. Okuda, M. Kawana, Y. Miyamoto: Current-Driven Magnetic Domains Motion in [Co/Pd] Nanowire with Ru Interlayer, Proc. ITE Annual Convention, 34D-5 (Aug. 2016) (in Japanese) 6.3 Next-generation display technologies We are researching a flexible organic light-emitting diode (OLED) display with a longer lifetime, solution-processed devices for large rollable displays, and technologies for a nextgeneration display with higher image quality and lower power consumption. Flexible OLED displays with longer lifetime We are researching organic device structures and materials that extend the operating/storage lifetime and reduce the power consumption of flexible OLED displays. OLED devices use active materials such as alkali metals for their electron injection layer. These materials are sensitive to moisture and oxygen and deteriorate on the substrate. 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 2016, we studied a power-saving inverted OLED with a long lifetime with the aim of realizing a flexible OLED display. We developed a new doping technique for improving the electron injection performance of the inverted OLED. This led to the development of a practical red device that requires a driving voltage of only 3.4 V at a luminance of 200 cd/m 2, 2 V less than that for a conventional device, and has an internal quantum efficiency of about 100% and a lifetime in excess of 10,000 hours (1). The device was developed in cooperation with Nippon Shokubai Co., Ltd. We also worked toward the development of new materials for an OLED with longer lifetime and higher efficiency. Increasing the performance of OLEDs requires the development of a peripheral material that transports positive holes and electrons as well as the development of a luminescent layer material. Until now, there have been few reports on holetransporting materials designed for high-performance green OLEDs. We therefore analyzed the device characteristics of several hole-transporting layer materials through molecular orbital calculations and identified the molecular structure of a hole-transporting layer material suitable for long-life and highly efficient OLEDs (2). This finding is a major step toward the development of a practical OLED with longer lifetime and higher efficiency. Solution-processed devices for large rollable displays With the goal of realizing a very flexible, large rollable display, we are conducting R&D on solution-processed oxide TFTs that can be fabricated easily without using vacuum apparatus. Solution-processed oxide materials often contain solvent-related defects and have low mobility. In FY 2016, we developed a technique that can improve mobility by adding fluorine to solution-processed oxide materials (3). We found that solution-processed IGZO (In-Ga-Zn-O) with fluorine added improved the mobility of TFTs from 1.8 cm 2 /Vs to 4.7 cm 2 /Vs. By additionally applying a film quality improvement technique using hydrogen injection and oxidation that we are developing, we achieved a maximum mobility of 7.0 cm 2 /Vs. We are also researching electroluminescent devices using quantum dots (QDs), called quantum dot light-emitting diodes (QD-LEDs), as a luminescent material that is solutionprocessable and capable of light emission with high color purity. A QD, which is a semiconductor nanocrystal 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 size control. However, most of the high-color-purity quantum dot materials that have been reported use toxic cadmium and there is a need for the development of a cadmium-free material. In FY 2016, we fabricated a QD-LED using specific cadmium-free materials in cooperation with external organizations. What is also important for increasing the efficiency of QD-LEDs is the development of a carrier transport material that can be used with QDs. In contrast to the island growth of a conventional carrier transport material in deposition on a QD film, the material we developed forms a uniform amorphous film, demonstrating that it is useful for increasing the efficiency of QD-LEDs (4). Technologies for increasing image quality and lowering power consumption We are conducting R&D on high-mobility TFTs to increase the image quality and lower the power consumption of sheettype displays. In FY 2016, we developed a high-mobility TFT that uses zinc oxynitride (ZnON) as the semiconductor material. Although ZnON exhibits high mobility, it causes significant time degradation of device characteristics. As a way of suppressing the deterioration of device characteristics, we developed a technique for adding a small quantity of silicon to ZnON (5). Our prototype TFT achieved a mobility of 54 cm 2 /Vs, about five times as high as that of the conventional IGZO (In- Ga-Zn-O) TFTs. We continued with our research on adaptive temporal aperture control for suppressing motion blur on hold-type displays such as OLED displays and extending the lifetime of OLEDs. In FY 2016, we derived a driving method to control the temporal aperture in line units, insert a transition area between a dynamic area and static area, and change the temporal aperture gradually when shifting between the dynamic and static areas to suppress image quality degradation by blinking artifacts and screen flickering. We prototyped driving equipment that controls the aperture time line by line and demonstrated the effectiveness of this method with actual equipment, which was evaluated through simulations (6). In our research on the driving technology of multipledivision-scanning-drive displays with a higher frame rate and larger screen, we proposed a driving method of suppressing image distortion, which is an issue with multiple-divisionscanning driving, by changing the time from data writing to light emitting for each horizontal line to minimize the difference in the light-emitting timing (7). We verified its effectiveness through simulations. 44 NHK STRL ANNUAL REPORT 2016

5 (1) H. Fukagawa, K. Morii, M. Hasegawa, T. Oono, T. Sasaki, T. Shimizu, T. Yamamoto: Demonstration of Highly Efficient and Air-Stable OLED Utilizing Novel Heavy-Doping Technique, SID Digest, pp (2016) (2) H. Fukagawa, T. Shimizu, H. Kawano, S. Yui, T. Shinnai, A. Iwai, K. Tsuchiya, T. Yamamoto: Novel Hole-Transporting Materials with High Triplet Energy for Highly Efficient and Stable Organic Light- Emitting Diodes, Journal of Physical Chemistry C, Vol.120, pp (2016) (3) M. Miyakawa, M. Nakata, H. Tsuji, Y. Fujisaki, T. Yamamoto: Improvement of TFT characteristics by fluorine additive on solutionprocessed oxide semiconductor, Abstract of 64th Japan Society of Applied Physics Spring Meeting, 16a (2017) (in Japanese) (4) T. Tsuzuki, G. Motomura, T. Yamamoto: Quantum dot light-emitting diode using 2,2 -bis(n-carbazolyl)-9,9 -spirobifluorene as a morphologically and thermally stable hole-transporting material, Physica Staus Solidi A,Vol.213, No.12, pp (2016) (5) H. Tsuji, T. Takei, M. Nakata, M. Miyakawa, Y. Fujisaki, T. Yamamoto: Suppression of degradation of electrical characteristics in ZnON- TFTs by silicon doping, Abstract of 64th Japan Society of Applied Physics Spring Meeting, 16a (2017) (in Japanese) (6) T. Usui, Y. Takano, T. Yamamoto: Development of OLED Display using Adaptive Temporal Aperture Control Driving Method with Transition Area Insertion, IDW 16, DES2-2, pp (2016) (7) T. Usui, T. Okada, Y. Takano, T. Yamamoto: A study of a driving method for suppressing image distortion of tiled OLED displays, ITE Technical Report, Vol.41, No.2, pp (2017) (in Japanese) NHK STRL ANNUAL REPORT