6 Devices for next-generation broadcasting

6.1 Advanced image sensors

6.1.1 Super-high-sensitivity image sensors

■ Low-voltage multiplier film technology

The sensitivity of solid-state image sensors decreases as the number of pixels and frame rate increase. To overcome this problem, we are developing photoconductive films able to multiply electric charges by applying a low voltage. In FY 2013, we improved the sensitivity to visible light of avalanche-type multiplication films using chalcopyrite semiconductors and re-duced the dark current of injection-type multiplication films us-ing crystalline selenium.

The avalanche multiplication film we prototyped in FY 2012 had a problem in that the carrier concentration of gallium oxide is low at the p-n junction formed by the gallium oxide (n-type material) and chalcopyrite material (p-type material) and a car-rier depletion layer forms only on that side. This decreases the film’s sensitivity to visible light. On the basis of experiments conducted in FY 2012, we increased the carrier concentration with a new film formation method that enables the gallium ox-ide to be doped with tin. This increased the film’s sensitivity to visible light (1) (2).

We are researching the next generation of im-age capture, recording, and display devices for new broadcast services such as 8K Super Hi-Vi-sion (SHV).

In our research related to image capture devic-es, we made progress in developing sensors with ultra-high sensitivity, organic image sensors, and 3D-structured imaging devices. Regarding our work on high-sensitivity devices, we inves-tigated a photoconductive film able to multiply electric charges by applying a low voltage in or-der to increase the sensitivity of solid-state im-age sensors. We improved our field-emitter-array image sensor with HARP film that is used in Hi-Vision cameras for reporting at nighttime and in other low-light situations. In our work on high-resolution organic image sensors, we continued developing technology to stack organic photo-conductive films and transparent thin-film tran-sistor circuits for reading electric charges from the photoconductive films on glass substrates. In our work on 3D-structured imaging devices, we made progress on 3D-structured image sensor technologies capable of pixel-parallel signal pro-cessing, for achieving both ultra-high resolution and a high frame rate. We also verified operating principle of these technologies.

In our research on recording devices, we con-tinued with our development of magnetic and ho-lographic recording devices. In particular, regard-ing our work on magnetic recording devices, we investigated the use of magnetic nano-domains that move along magnetic nano-wires to increase the speed of current-driven magnetic domain motion and developed reproduction technology

using a tunneling magnetoresistive magnetic-field sensor. In our work on holographic record-ing devices, we investigated a multiplex record-ing technology to increase the recording density and technology to speed up reproduction. We developed a two-dimensional angle multiplexing method that doubled the multiplexing number to 600. For the high-speed reproduction technol-ogy, we improved the performance of the record-ing media and improved the signal processing to speed up the data reproduction rate to 500 Mbps.

In our work on sheet-type displays for the SHV system, we fabricated an eight-inch VGA display that incorporates a power-efficient, long-lifetime red phosphorescent OLED that we had previously developed. We confirmed that the device did not deteriorate for 250 days after being sealed in a plastic substrate. We also clarified the light-emit-ting mechanism of OLEDs and fabricated an effi-cient, long-lifetime green phosphorescent OLED. We improved the performance of the TFTs used as driving elements for sheet-type displays by in-creasing the mobility of their charge carriers and reducing the size of the oxide semiconductors and organic TFTs.

We also worked on ultra-high-resolution, high-speed spatial light modulator (SLMs) as spatial imaging technology for 3D television with a wide viewing zone. To expand the viewing zone and increase the pixel count, we improved the perfor-mance of an SLM driven by spin-transfer switch-ing using tunnel magneto-resistance, designed a MOS-transistor-array circuit for the device, and developed an active-matrix driving SLM.

Voltage (V)0 5 10 15 2010-11

10-10

10 -9

10 -8

10 -7

10 -6

Dar

k cu

rrent

den

sity

(A/c

m2 )

Transparent electrode

p-type layern-type layer

Crystalline selenium (1μm)

Zinc oxide(50nm)

Glass electrode

Crystalline selenium (1μm)

With zinc oxide

Without zinc oxide

Figure 1. Structure of prototype sandwich cell and dark current comparison

NHK STRL ANNUAL REPORT 2013　｜　33

Regarding the injection multiplication film, our sandwich cell with crystalline selenium between a transparent electrode and a metal electrode (FY 2012 prototype) achieved a quantum ef-ficiency of over 100 at an applied voltage of 10 V by exploiting the electron injection multiplication phenomenon. However, it also had a high dark current. In FY 2013, we found that the dark current can be reduced by forming a p-n junction combining crystalline selenium, which is a p-type material, and zinc ox-ide, which is an n-type material and also a hole blocking mate-rial (Figure 1) (3).

■ Compact super-high sensitivity imaging device for Hi-VisionWe are developing field emitter array image sensors with

HARP film that combine field emitters that emit electrons by simply applying a voltage to them with a sensitive High-gain Avalanche Rushing amorphous Photoconductor (HARP) film (Figure 2). This effort is part of our work on compact, super-high-sensitivity Hi-Vision cameras for reporting at nighttime and in other low-light situations. In FY 2013, we developed an electrostatic focusing field emitter to improve the image quality and designed an active-matrix drive circuit for Hi-Vision image sensors.

The image sensor with the electrostatic-focusing field-emit-ter array we prototyped in FY 2012 had many pixels that could not read the signal charges of the HARP film. This problem was due to the dispersion of the electron beam for each pixel, and it significantly degraded the image quality. In FY 2013, we found that the efficiency of getting electrons (as a beam) out of the cathode is about 30% and that increasing the amount of elec-trons in the beam can improve the image quality. Based on this finding, we developed a new electrostatic focusing field emit-ter that can efficiently produce an electron beam. In particular, we identified a structure that could double the amount of elec-trons over that of the previous method. For the active-matrix drive circuit, we designed a circuit structure with a lower drive voltage for the transistor that would enable the pixel size to

be reduced to one-quarter that (11μm×11μm) of the previous structure. We demonstrated the feasibility of high-speed driv-ing required for Hi-Vision (4).

[References](1) K. Kikuchi, S. Imura, K. Miyakawa, M. Kubota, E. Ohta: “Electrical

and optical properties of Ga2O3/CuGaSe2 heterojunction photocon-ductors,” Thin Solid Films, vol. 550, pp. 635-637 (2014)

(2) K. Kikuchi, S. Imura, K. Miyakawa, H. Ohtake, M. Kubota, E. Ohta: “Improved Electrical Properties of Ga2O3:Sn/CIGS Hetero-Junction Photoconductor,” MRS Proceedings, vol. 1635 (2014)

(3) S. Imura, K. Kikuchi, K. Miyakawa, H. Ootake, M. Kubota: “Photo-electric conversion properties of c-Se based p-n heterojunction pho-todiode,” ITE Winter Annual Convention, 11-4 (2013) (in Japanese)

(4) Y. Honda, M. Nanba, K. Miyakawa, M. Kubota, N. Egami: “Active-matrix drive circuit for image sensor consisting of field emitter array and avalanche photoconductor,” Proceedings of the 20th Interna-tional Display Workshops (IDW ‘13), FED1-2, pp. 806-809 (2013)

6.1.2 Organic image sensors

■ Continuously stacked organic image sensorsWe are developing organic image sensors with an image

quality comparable to that of three-chip color broadcast cam-eras for use in a compact single-chip color camera. These de-vices 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 film on glass substrate (Figure 1).

Organic films are susceptible to high temperatures, but TFT circuits are formed at high temperatures (300°C or higher). Thus, to make this device, we had to increase the heat resis-tance of the organic films and decrease the temperature for forming the TFT circuits. In FY 2013, we improved the TFT char-acteristics by taking advantage of the increased heat resistance of the organic films we had previously developed and a tech-nology to form TFT at lower temperatures. We also prototyped “continuously stacked” devices.

To improve the TFT characteristics, we optimized the condi-tions of sputtering amorphous In-Ga-Zn-O (IGZO) film, reduced the resistance of transparent wiring for reading signals, and devised a process to reduce defects in IGZO film. This increased the mobility and achieved one-digit better on-off characteris-tics. We also prototyped a two-tiered continuously stacked device. The low-temperature formation technology is used as follows: a stacked film consisting of aluminum oxide and ti-tanium oxide and an interlayer insulator made of epoxy resin are formed on the first layer of the TFT reading circuit with an organic photoconductive film for red; a second layer consist-

HARP filmPixel

Field emitter array (incorporating drive circuit)

Insulating layer

Focusing electrode

Electron beam

Gate electrode Emitter

Electrostatic focusing field emitter (FY 2013)

Figure 2. Schematic diagram of image sensor and electrostatic focusing field emitter

Interlayer insulator

Organic film

Glass substrateZoom

10μm Transparent TFT circuit

Light

Figure 1. Cross section of continuously stacked device

2nd layer of TFT circuit +organic film for green

1st layer of TFT circuit+organic film for red

2層目TFT回路+緑色用有機膜

Interlayer insulatorapprox. 10 μm

2nd layer of TFT circuit +organic film for green

1st layer of TFT circuit+organic film for red

Figure 2. Prototype continuously stacked organic imaging device and im-aging example

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6　Devices for next-generation broadcasting　｜　6.1 Advanced image sensors

ing of a TFT reading circuit with an organic fi lm for green is then formed. We verifi ed that the device could produce images (Figure 2) (1) and showed the feasibility of a high-resolution or-ganic image sensor. The organic photoconductive fi lms were developed in cooperation with Saitama University, and the TFT circuits were developed in cooperation with the Kochi Univer-sity of Technology.

[References](1) T. Sakai, H. Seo, S. Aihara, H. Ohtake, M. Kubota, M. Furuta: “Color

Image Sensor Using Stacked Organic Photoconductive Films with Transparent Readout Circuits Separated by Thin Interlayer Insula-tor,” To be published in Proc. SPIE (2014)

6.1.3 Core technology for 3D-structured imaging devices

We are researching imaging devices with a 3D structure to implement pixel-parallel signal processing as a way of improv-ing the resolution and increasing the frame rate. These devices have signal processing circuits for each pixel directly beneath the photoelectric conversion element. This enables the signals from all pixels to be read in parallel so that a high frame rate can be maintained even if the pixel count increases (Figure 1).

In FY 2013, we designed signal processing circuits, developed a technology to stack a photoelectric conversion elements and signal processing circuits, and verifi ed the operation principle of devices incorporating this technology. The signal process-ing circuits we designed have a wide dynamic range, and they convert the incident light intensity into electrical pulses (Figure 2). We also devised a simulation method considering the 3D wiring path (1) and prototyped a device with a signal process-ing circuit beneath the photoelectric conversion element by combining circuit wafer thinning technology we developed in FY2013 with technology to directly bond substrates with metal electrodes and insulators on them that we had developed in FY 2012. We confi rmed that the device outputs a number of pulse signals in proportion to the incident light intensity and demon-strated the basic operating principle of 3D-structured imaging devices capable of converting the light into digital signals in the pixels and reading them out in the depth direction of the device. This research was conducted in cooperation with the University of Tokyo.

[References](1) M. Goto, K. Hagiwara, Y. Iguchi, H. Ohtake, T. Saraya, E. Higurashi,

H. Toshiyoshi, T. Hiramoto: “Design of an In-pixel A/D Converter for 3D-structured Image Sensors,” The 30th Sensor Symposium on Sensors, Micromachines and Applied Systems, 6PM3-PSS-44 (2013) (in Japanese)

6.2 Advanced storage technology

6.2.1 Magnetic recording technology

■ Magnetic recording devices utilizing magnetic nano-domains

With the goal of realizing a high-speed magnetic recording device with no moving parts, we are developing recording de-vices that utilize the motion of nano-sized magnetic domains on magnetic nanowires. In addition to technology from FY 2012 that uses pulse currents to control the magnetic nano-domains, we worked on technology to detect the magnetization direc-tions (facing up or down) of the driven magnetic nano-domains by using a magnetic fi eld sensor attached to the nanowires. In other words, it is a reproduction technology to read out binary data corresponding to magnetic nano-domains from magnetic nanowires.

We re-examined the pulse current application system used to drive the magnetic nano-domains and succeeded in driving the nano-domains with pulses having temporal widths of only 50 ns, ten times faster than the 500 ns of our experiments in

FY 2012. Regarding the magnetic nano-domain reproduction technol-

ogy, we prototyped a magnetic domain scope that uses a tun-neling magnetoresistive (TMR) magnetic fi eld sensor to cover a wide area with nano-scale resolution (nano-MDS). We also developed a way to directly detect the density and spatial distri-bution of the magnetic fl ux generated from the magnetic nano-domains. The scope can quantitatively evaluate the magnetic domains, unlike magnetic force microscopy (Figure 1). We used the data we obtained to develop a principle for designing the materials and structures of magnetic nanowires.

We also modifi ed the electrode structure and method of pro-ducing magnetic nanowire elements to verify the motion and reproduction of current-driven magnetic domains simultane-ously on a single sample. We developed a process to make ultra-fl at nanowires with fewer trap sites in their magnetic do-mains by combining electron beam lithography using a nega-tive-type electron-beam resist and ion-beam milling.

Pixel

Pixel-parallel signal processing

Incident light Photoelectric conversion element

Signal processing circuit

Figure 1. 3D-structured imaging devices

Photoelectric conversion element (upper layer)

Signal processing circuit (lower layer)

Incident lightOutput

Pulse

Photoelectric conversion element (upper layer)

Figure 2. Circuit structure

NHK STRL ANNUAL REPORT 2013　｜　35

6　Devices for next-generation broadcasting　｜　6.1 Advanced image sensors　｜　6.2 Advanced storage technology

The combination of these technologies of high-speed driv-ing of magnetic nano-domains, the magnetic domain scope covering a wide area with nano-order resolution (nano-MDS), and ultra-flat magnetic nanowires has enabled multiple mag-netic nano-domains to be driven on magnetic nanowires with a pulse current width of 50 ns and at the same time to detect the motion of these nano-domains in real time as continuous changes in the magnetization direction by using a magnetic field sensor directly attached to the magnetic nanowires. We demonstrated this method can be used as a magnetic-record-ing reproduction technology.

[References](1) M. Okuda, Y. Miyamoto, E. Miyashita, N. Hayashi: “Evaluation of

Magnetic Flux Distribution from Magnetic Domains in [Co/Pd] Nanowires by Magnetic Domain Scope Method using Contact-scan-ning of Tunneling Magnetoresistive Sensor,” J. Appl. Phys., Vol. 115, No. 17, pp. 17D113.1-17D113.3 (2014)

(2) Y. Miyamoto, M. Okuda, E. Miyashita: “Ultra-fast Recording Device Utilizing Current-driven Domain Wall Motion in Magnetic Nanow-ires,” ITE Journal, Vol. 68, No. 1, pp. J34-J40 (2014) (in Japanese)

6.2.2 Holographic memory

■ High-speed and high-density holographic memoryArchives for 8K Super Hi-Vision will need to be very large

capacity and have high transfer rates. We are researching holo-graphic memory to meet these needs, because it can reproduce and record a whole page of two-dimensional data at once and is capable of high-density storage with multiplexed recording. In FY 2013, we worked on multiplex recording technology to in-crease the recording density and further accelerated the high-speed reproduction technology.

Our previous multiplex recording technology overwrites multiple data on the same part of a recording device by varying the incident angle of light in one dimension. With this method, the multiplexing number is limited by the available range of the incident angle. The maximum multiplexing number that we previously achieved with this method was 300. To increase that number, we expanded the one-dimensional angle multiplexing to a two dimensions, which records data by changing the angle of recording media in the in-plane direction as well as the inci-dent angle. We confirmed that the average bit error rate could be limited to a practical level of 10-4 even when the multiplexing number was 600.

Regarding the high-speed reproduction technology, we im-plemented the reproduction signal processing algorithms we had developed for graphics processing units (GPUs) in a field programmable gate array (FPGA) and confirmed a throughput of 500 Mbps. We refined the algorithms to verify the required number of bits and reduce the number of multipliers by using GPUs before applying them to the FPGA. The resulting FPGA

could reproduce ultra-high resolution video with more pixels than Hi-Vision in real-time(1) (Figure 1).

[References](1) T. Muroi, N. Kinoshita, N. Ishii, K. Kamijo, H. Kikuchi: “Holographic

Data Storage with Wavefront Compensation and Parallel Signal Processing for Readout of Beyond HD Video Signal,” International Workshop on Holography and Related Technologies (IWH) 2013 Technical Digest, 16c-5 (2013)

6.3 Next-generation display technologies

6.3.1 Flexible displays

We are researching large, lightweight, and flexible sheet-type displays that can be rolled up and used in the home for showing 8K Super Hi-Vision. In FY 2013, we developed fabrication tech-nologies, thin-film transistors (TFTs) for driving active matrix displays, and organic materials and devices.

■ Display panel fabrication technologies

We fabricated an active-matrix driving organic light-emit-ting diode (OLED) display panel using plastic film substrates. It is necessary to reduce the panel’s power consumption, in addition to developing processing technology suited for large screens. In FY 2013, we fabricated an eight-inch VGA (640×480 pixels) panel at a temperature below 200℃ by using red phos-phorescent OLEDs featuring low power consumption and long

Figure 1. Real-time reproduction of ultra-high resolution video

Up-facing magnetic domains

Down-facing magnetic domains

Up-facing magnetic domains(+30mT)

Down-facing magnetic domains(−30mT) Magnetic

flux density＋30 (mT)

0

−302μm 2μm

(a) Result of conventional evaluation using magnetic force microscopy shows the directions of magnetic domains only.

(b) Result of evaluation using magnetic domain scope with nano-MDS shows the magnetic flux density as well as the directions of magnetic domains.

Figure 1. Comparison of evaluation methods of magnetic nano-domain structure

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6　Devices for next-generation broadcasting　｜　6.2 Advanced storage technology　｜　6.3 Next-generation display technologies

lifetime. The red OLEDs use a platinum complex as an emitting mate-

rial and a beryllium complex as a host material to transfer en-ergy to the emitting material. We found that these OLEDs had excellent light-emitting characteristics, wherein almost 100% of the injected charge was emitted as photons (1).

The fabricated eight-inch panel was very thin (about 0.3 mm thick), very light (about 15 g), and bendable. The oxide TFT ar-ray, which is the driving device, had a mobility of approximately 8cm2/Vs, and the panel maintained the same level of driving performance over its whole area. The red, green and blue pixels emitted light equally with a peak luminance of around 100cd/m2, and the panel could display moving images with almost uniform quality over the entire screen.

We developed a panel fabrication technology based on print-ing technologies to make large flexible displays and increase productivity. In FY 2013, we formed patterns of OLED materials by using ink-jet printing, developed a coatable gate insulator, and formed OLED electrodes by screen printing. This effort re-sulted in a prototype panel built using printing technologies for not only the OLEDs but also the wiring and electrodes.

■ TFT materials and devicesWe are researching oxide semiconductor TFTs (IGZO-TFT)

with high field-effect mobility that can be used for large screens and organic TFTs with a flexible structure that can be formed at low temperatures as driving elements of large-sized sheet-type OLED displays. We developed a method to produce self-aligned IGZO-TFTs that have less parasitic capacitance and a shorter channel length. We fabricated a self-aligned IGZO-TFTs with channel lengths of 9 μm and lowered the IGZO’s resistance by irradiating it with excimer laser light. We showed that this technology has a sufficient process-margin for the irradiation intensity of the excimer laser (2). As it can suppress the influ-ences of uneven laser intensity and the intensity dispersion of each irradiation, it can uniformly form self-aligned IGZO-TFTs on a large area. We also developed a new high-speed method of evaluating the trap density, to analyze the TFT characteris-tics. A combination of capacity measurements and high-speed simulations enabled rapid measurement of the trap density in one tenth the time required for conventional methods using a two-dimensional simulator. This advance will improve the pro-duction process.

For organic TFTs, we are studying a fine patterning method for solution-processable semiconductors using selective modi-fication of the solvent wettability on the gate insulator. We de-veloped a technology to improve the uniformity of crystalline film by controlling the direction of solvent evaporation. We also fabricated and evaluated a TFT array with transparent paper substrates as a technology for future displays with a high level of flexibility. We experimentally showed that its electrical char-acteristics are equivalent to those of conventional plastic sub-

strates and that it can operate even when bent with a curvature radius of 2 mm.

■ Display materials and devicesWe are researching new device structures, light emitting ma-

terials, and charge transport materials to reduce power con-sumption and extend the operating/storage lifetime of flexible OLEDs. The greatest challenge in applying OLEDs to flexible displays is that OLEDs are sensitive to oxygen and moisture and deteriorate on a flexible plastic substrate. In FY 2013, we devel-oped an inverted OLED (iOLED) whose electrode structure is opposite to that of conventional OLEDs, and we confirmed that it did not deteriorate after being sealed in plastic substrate for 250 days (Figure 2). The inverted structure increased the selec-tivity of materials for the cathode and electron injection layers, which are the main factors of deterioration. The development of a new electron injection layer especially helped to make the OLED resistant to oxygen and moisture. We fabricated a five-inch monochromatic display using the iOLED (3). The iOLED was developed in cooperation with Nippon Shokubai Co., Ltd.

We elucidated the light-emitting mechanism in phosphores-cent OLEDs. It had been considered difficult to clarify the light-emitting mechanism of OLED devices because it has many processes. By simplifying the processes related to light emis-sion, we identified the excited state behaviors that contribute to the efficiency and lifetime of these devices. We also investi-gated how the device performance depends on charge trans-port materials and elucidated the molecular structure of charge transport materials suitable for efficient and stable green phos-phorescent OLEDs. Part of this research was conducted as a government-commissioned project from the Ministry of Inter-nal Affairs and Communications, titled “R&D on highly efficient and stable organic light-emitting device toward the realization of ultimate power-saving display.”

■ Thermoacoustic device

A flexible sound generator based on the thermoacoustic ef-fect was developed for use with flexible devices. The device was composed of a very thin electrode and several flexible films. It generates sound with no mechanical moving parts. We measured various acoustic characteristics of this device and found that it had a wide frequency band, from the audible range up to 100 kHz.

[References](1) G. Motomura, Y. Nakajima, T. Takei, Y. Fujisaki, H. Fukagawa, H.

Tsuji, M. Nakata, T. Shimizu, T. Yamamoto: “Oxide-TFT-Driven Flex-ible Display Using Highly Efficient Phosphorescent OLED,” Proceed-ings of 33rd International Display Research Conference, 10.4 (2013)

(2) M. Nakata, H. Tsuji, Y. Fujisaki, H. Sato, Y. Nakajima, T. Takei, T. Yamamoto, T. Kurita: “Fabrication method for self-aligned bot-tom-gate oxide thin-film transistors by utilizing backside excimer-laser irradiation through substrate,” Appl. Phys. Lett., Vol. 103, pp. 142111.1-142111.4 (2013)

OLED structure

OLED

Preservation period

1 day 15 days 51 days 103 days

1 day 8 days 58 days 104 days 250 daysiOLED

Figure 2. extending the lifetime of inverted OLEDs (iOLEDs)

Figure 1. Fabricated eight-inch VGA flexible OLED display

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6　Devices for next-generation broadcasting　｜　6.3 Next-generation display technologies

(3) H. Fukagawa, K. Morii, Y. Arimoto, M. Nakata, Y. Nakajima, T. Shi-mizu, T. Yamamoto: “Highly Efficient Inverted OLED with Air Stable Electron Injection Layer,” Technical Digest Paper of SID Sympo-sium, P-140L, pp. 1466-1469 (2013)

6.3.2 Advanced display devices

■ Spatial light modulator driven by spin-transfer switching

We are researching electronic holography to realize a spa-tial imaging three-dimensional television that can provide fully natural 3D images. Displaying the 3D images with a wide view-ing zone requires a spatial light modulator (SLM) having a very small pixel pitch, a large number of pixels, and high driving speed. We are developing a spin-transfer SLM (spin-SLM) with a pixel pitch under 1 μm (1). This spin-SLM uses the magneto-optical Kerr effect, in which the polarization plane of reflected light rotates according to the magnetization direction of the magnetic materials in the pixel. The magnetization direction is controlled by the direction of the current in the pixel, and this is called spin-transfer switching. We previously developed a one-dimensional SLM with giant magneto resistance (GMR) structure, and confirmed its successful operation with passive-matrix driving by directional voltage application.

In FY 2013, we investigated tunnel magneto-resistance (TMR) for large-scale spin-SLMs. We fabricated an active-matrix (AM) driving TMR spin-SLM that can accurately turn the pixel on and off by using a transistor built in each pixel. The AM driv-ing TMR spin-SLM employs a structure in which a TMR light modulation element is stacked on the transistor that drives each pixel (Figure 1). The TMR light modulation element con-tains a pinned layer made of multi-layer film of terbium-iron-cobalt (Tb-Fe-Co) and cobalt-iron (Co-Fe) alloys, an insulating layer of magnesium oxide (MgO) film, and a light modulation layer of gadolinium-iron (Gd-Fe) alloy film with gadolinium and Co-Fe buffer layers. The magnetic tunnel junction formed with ultrathin (1-nm thick) MgO film between the Co-Fe films en-ables low-current operation. The gadolinium buffer layer inside the light modulation layer increased the Gd-Fe film’s tendency to magnetize in the vertical direction (a phenomenon called perpendicular magnetic anisotropy) and improved the light modulation performance. For the AM driving device, we built a circuit board with a two-dimensional array (10×5 pixels, 5-μm pixel pitch) of MOS transistors formed on a single crystal sili-con substrate. To make a two-dimensional array of submicron-sized TMR elements, we developed a nano-fabrication process-ing technology with a position adjustment accuracy of 20 nm or less, prototyped a two-dimensional spin-SLM, and verified that the magnetization of the Gd-Fe light modulation layer could be switched by controlling the external magnetic field.

We fabricated a pattern with a GMR structure (GMR holo-gram with a 1 μm pixel pitch and 3840×2160 pixels) and verified that the 3D image could be reproduced with a viewing-zone angle of 37 degrees. We also confirmed that the reproduced im-age could be turned on and off by applying an external mag-netic field to the GMR hologram (2). This research was supported by the National Institute of Information and Communications Technology (NICT) as part of the project titled “R&D on Ultra-Realistic Communication Technology with Innovative 3D Video

technology.”

■ Light beam directionality control deviceWe investigated optical devices for future integral 3D displays

without a micro-lens array. We fabricated a device consisting sub-micron-sized dielectric structures that has capability of deflecting a light beam in a designated direction. In FY 2013, we devised a device structure to reduce sidelobe (unintended light beam emission) and stray light (light leaking on unintend-ed paths) and verified its operation. Simulations showed that sidelobe can be reduced to 20% by increasing the number of structures from three to six (3). We designed a device with sub-micron sized structures, and we fabricated a device with light-shielding metal film to prevent stray light and confirmed that it had reduced sidelobe and stray light.

[References](1) K. Aoshima, N. Funabashi, K. Machida, Y. Miyamoto, K. Kuga, T.

Ishibashi, N. Shimidzu, F. Sato: “Submicron Magneto-Optical Spatial Light Modulation Device for Holographic Displays Driven by Spin-Polarized Electrons,”J. Display Tech., Vol . 6, No. 9, pp. 374-380 (2010)

(2) K. Machida, D. Kato, T. Mishina, H. Kinjo, K. Aoshima, A. Kuga, H. Kikuchi, N. Shimidzu, “Three-dimensional image reconstruction with a wide viewing-zone-angle using a GMR- based hologram,” OSA Topical Meeting Digital Holography and 3-D Imaging, DTh2A.5 (2013)

(3) Y. Hirano, K. Tanaka, Y. Motoyama, N. Saito, H. Kikuchi, N. Shimid-zu, “Directionality Control of Light-emitting Devices Through Sub-micron Dielectric Structures,” 2nd Int. Conf. on PHOTOPICS 2014, pp. 163-169 (2014)

Driving element (Si backplane)

TMR light modulation

element

Transparent electrode

Insulating layer 1

Gate

MOS transistorSi substrate

Source Drain

Electrode

Light modulation layerInsulating layer

Pinned layer

Insulating layer 2

Figure 1. Schematic illustration of AM driving TMR spin-SLM structure

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