JP2007265481A - Ferroelectric thin film optical memory device using photoconductive film and ferroelectric thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To substantially reduce voltage of drive voltage by making ferroelectric ceramics being a memory device a thinner film in a ferroelectric optical memory. <P>SOLUTION: A ferroelectric thin film optical memory device is made a sandwich-structured device in which a PLZT 2 as a memory material, a CdS 3 as a photoconductive film, and an ITO4 as a transparent electrode are laminated on a Pt(111)/TiO<SB>x</SB>/SiO<SB>2</SB>/Si (100) substrate 1. The ferroelectric ceramics requires several kilovolts for the drive voltage, however, the ferroelectric thin film requires only 1.5 volts and more for the drive voltage. A technology for forming the ferroelectric thin layer is variously developed, and existing thin film forming technologies can be used. The ferroelectric thin film optical memory device has a simple structure and can be produced at low cost. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a device structure in which a photoconductive film and a ferroelectric thin film are formed in a sandwich structure.

Ferroelectric optical memory with a structure in which a photoconductive film is synthesized on ferroelectric ceramics and transparent electrodes are applied to both ends has been studied as an analog recording element for image display. A voltage of about several kilovolts is required, which causes difficulties for practical use and commercial use.

This patent is an element that can reduce the driving voltage by thinning the ferroelectric ceramics, and can write data to any place exposed to light by using a photoconductive film. .

Ferroelectric thin film optical memory can solve from the lower electrode to the upper transparent electrode with the existing thin film forming technology, so it does not require complicated production process and can reduce the production cost.

Ferroelectric optical memories have been proposed and mainly intended for image display elements. Compared with liquid crystal and LED, the following advantages can be mentioned.
(1) There is a semi-permanent memory action using the hysteresis phenomenon.
(2) Large screen display.
(3) Local writing and erasing are possible.
(4) The response speed is fast.

Although the ferroelectric optical memory is a device having the above-mentioned excellent characteristics, it requires a driving voltage of several kilovolts because it uses ferroelectric ceramics as a memory element, and it has not been commercialized commercially. It was.

Since a ferroelectric thin film optical memory uses a ferroelectric thin film as a memory element, a drive voltage of about 1.5 volts to several tens of volts is sufficient.

Therefore, the present invention uses the structure of the ferroelectric optical memory, but uses a thin film technique that is different from the ceramic technique described above.

The problem to be solved by the present invention is to reduce the driving voltage by making the ferroelectric ceramic of the ferroelectric optical memory into a ferroelectric thin film.

The materials constituting the ferroelectric thin film optical memory are described below. The memory element is PbLaZrTiO ₃ , lanthanum-added lead titanium zirconate, and is hereinafter abbreviated as PLZT. The photoconductive film is CdS, cadmium sulfide. The transparent electrode is InO—SnO, indium oxide-tin oxide, and is hereinafter abbreviated as ITO.

FIG. 1 shows a schematic diagram of a ferroelectric thin film optical memory. The ferroelectric thin film optical memory has a sandwich structure in which PLZT2 as a memory material, CdS3 as a photoconductive film, and ITO4 as a transparent electrode are laminated on a Pt (111) / TiO _x / SiO ₂ / Si (100) substrate 1. It is an element.

The ferroelectric thin film optical memory can apply a voltage 6 to an arbitrary position irradiated from the laser light source 5. The detailed driving principle will be described with reference to FIG.

The ferroelectric thin film is synthesized by the sol-gel method by the following process.
After coating PT precursor solution at 1000 rpm for 10 seconds and 3000 rpm for 20 seconds for 20 seconds, thermal decomposition and crystallization at 300 ° C for 5 minutes on a hot plate to synthesize the buffer layer To do. Next, the PLZT precursor solution was coated at 1000 revolutions per minute for 10 seconds in the first step, and at 3000 revolutions per minute for 20 seconds in the second step, and dried at 100 ° C. for 5 minutes on a hot plate. And thermal decomposition at 300 ° C. for 3 minutes. Using a high-speed thermal heat treatment apparatus, calcination is performed in an oxygen atmosphere (700 ° C., 2 minutes, 1 kgf / cm ² ) for crystallization. This process is repeated four times to synthesize a PLZT thin film having a thickness of about 300 to 350 nm.

The photoconductive film CdS is synthesized by vacuum deposition.
1.
The substrate temperature is heated to 400 ° C. for degassing the substrate. At the same time, the evaporation source temperature is gradually heated to 500 ° C. to degas the CdS powder.
2.
When the substrate temperature reaches 400 ° C. and the evaporation source temperature reaches 500 ° C., the substrate and the CdS powder are degassed for 10 minutes.
3.
The substrate temperature is lowered to a predetermined temperature, and the evaporation source temperature is kept at 250 ° C.
Four.
When the substrate temperature reaches a predetermined temperature, the evaporation source temperature is gradually heated to a desired temperature.
Five.
When the substrate temperature reaches a predetermined temperature and the evaporation source temperature reaches a desired temperature, vapor deposition is started.
6.
After vapor deposition for 30 minutes, the vapor deposition is terminated.

The transparent electrode ITO is synthesized by a sputtering method.
An ITO thin film was synthesized by sputtering as the upper transparent electrode.
The sputtering conditions were applied voltage frequency of 13.6 MHz, anode current of 80 milliA, anode voltage of 0.9 kilovolt, introduced gas was a composite gas in which 90% of argon gas was mixed with 10% of oxygen, and the substrate heating temperature was 350 ° C. The sputtering time is about 20 minutes.
The above is the means for solving the problem.

A method for driving a ferroelectric optical memory will be described.
FIG. 2 shows a driving schematic diagram of a ferroelectric thin film optical memory. Since an electric field is applied to the ferroelectric thin film and the photoconductor, the voltage is divided. At this time, the quantitative relationship between the partial pressures is the equation (1).
E = V _F + V _P (1)
Here, E is the applied electric field, V _F is the electric field applied to the ferroelectric, the V _P is the electric field applied to the photo conductor.

The electric field is applied so as to satisfy the condition of V _F <E _C. Here, E _C is a coercive electric field of the ferroelectric. Next, when the photoconductor is irradiated with light, the resistance R _P1 of the photoconductor changes to R _P2 due to the photoconductive effect. At this time, the resistance value of the photoconductor is R _P2 <R _P1 . As a result, the electric field applied to the ferroelectric becomes V _F > E _C , causing polarization inversion and completing the memory writing.

As described above, the means for solving the problems according to the present invention aims at drastically lowering the driving voltage by making the ferroelectric ceramic in the ferroelectric optical memory into a ferroelectric thin film.

The present invention comprises a process of synthesizing a ferroelectric thin film on a Si substrate, a process of synthesizing a photoconductive film, and a step of synthesizing a transparent electrode.

Ferroelectric ceramics require a single crystal to synthesize a single grain size due to the long firing time. In addition, the manufacturing process becomes precise and the production cost increases. Have. Since the ferroelectric thin film has a short firing time, it can form a fine particle size and control the size of the particle size.

Ferroelectrics have been demonstrated to have a recording density of 1.5 terabits per square inch by using a nonlinear dielectric microscope. This recording density is a high-density recording far exceeding the current magnetic recording density of 6 gigabits per square inch. Therefore, the ferroelectric has an excellent potential as a material having a high recording density.

Monochromatic light such as laser light has been reduced in cost and size by the development of semiconductor lasers.
In optical disk memory devices typified by DVD and CD, Nakamura succeeded in developing a blue semiconductor laser, and a blue-ray DVD (hereinafter referred to as DVD-blue) has been put into practical use. A high-density recording optical memory typified by DVD-blue improves the diffraction limit by using blue light having a shorter wavelength than conventional ones, and achieves high-density recording by reducing the laser light to a smaller value. Similarly, it is considered that a ferroelectric optical memory can achieve high-density recording by using a blue laser.

Therefore, as described above, the ferroelectric thin film optical memory helps the industrial activation.

Schematic diagram of ferroelectric thin film optical memory Driving schematic diagram of ferroelectric thin film optical memory

Explanation of symbols

_{1 ··· Pt (111) / TiO} x / SiO 2 / Si (100) substrate 2 ... memory element 3 ... photoconductive film 4 ... transparent electrode 5 ... laser light source 6 ... applied Voltage

Claims (5)

Sandwich structure based on silicon, with platinum as the lower electrode, ferroelectric thin film and photoconductive film stacked in order from the bottom, and transparent electrodes as the upper electrode. A ferroelectric thin film optical memory device characterized by   The ferroelectric thin film according to claim 1, wherein the ferroelectric thin film is a ferroelectric having a perovskite structure having both high transparency and refractive index and excellent optical characteristics. Thin film optical memory device.   3. The ferroelectric thin film optical memory according to claim 1, wherein the ferroelectric thin film is a ferroelectric thin film using lanthanum-doped lead zirconate titanium zirconate, lithium niobate, or lithium tantalate. element.   2. The ferroelectric thin film optical memory element according to claim 1, wherein cadmium sulfide, cadmium selenide, or cadmium telluride having a photoconductive effect is used as a material for the photoconductive film according to claim 1. The ferroelectric thin film according to claim 1 is formed by a sol-gel method, a photoconductive film is formed by a vacuum deposition method, and a thin film of a transparent electrode is formed by a sputtering method, and each thin film is laminated. 2. The ferroelectric thin film optical memory device according to claim 1.