JPH0676375A - Information recording medium - Google Patents

Abstract

PURPOSE:To prevent a information recording medium having a semiconductive layer, an insulating layer formed on the semiconductive layer and a ferroelectric substance layer further formed and recording information by a polarization direction of the ferroelectric substance from causing recording/reproducing inferiority. CONSTITUTION:An absolute value of flat band voltage VFB<i> evaluated from a structure constituted of two layers of the semiconductor layer 11 and the insulating layer 12 is composed of three layers of the semiconductive layer 11, the insulating layer 12 and the ferroelectric substance layer 13. The value is made equal to or under the half of the DELTAVFB(=¦VFB<->-VFB<+>¦), difference between the flat band voltage VFB<+> at the time of pluspolling the structure and the flat band voltage VFB<-> at the time of minuspolling the structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an information recording medium, and more particularly, it has a semiconductor layer, an insulating layer and a ferroelectric layer,
The present invention relates to an information recording medium for recording information according to the polarization direction of this ferroelectric substance.

[0002]

2. Description of the Related Art Japanese Unexamined Patent Publication (Kokai) No. 57-27447 discloses an information recording medium capable of recording various kinds of information such as image signals and audio signals, and further used as a data memory for computers and capable of high-density recording. A semiconductor layer and a ferroelectric layer formed on the semiconductor layer,
It is known that information is recorded depending on the polarization direction of the ferroelectric substance. Recording on this information recording medium is performed by moving a conductive head on the ferroelectric layer and applying a voltage to the layer so that only a predetermined portion of the ferroelectric layer is selectively polarized in a predetermined direction. Will be performed. Information reproduction from the information recording medium is performed by detecting a capacitance change of the recording medium due to the depletion layer formed in the semiconductor layer due to the polarization of the ferroelectric substance, by the conductive head.

[0003]

In the above information recording medium, an insulating layer is usually provided between the semiconductor layer and the ferroelectric layer to prevent charge injection mainly from the semiconductor layer to the ferroelectric layer. Is formed.

However, in the conventional information recording medium provided with this insulating layer, it has been recognized that information cannot be correctly recorded and reproduced in many cases.

The present invention has been made in view of the above circumstances, and provides an information recording medium that does not cause defective recording and reproduction even if an insulating layer is provided between a semiconductor layer and a ferroelectric layer. It is intended to be provided.

[0006]

An information recording medium according to the present invention comprises a semiconductor layer, an insulating layer formed on the semiconductor layer, and a ferroelectric layer formed on the semiconductor layer, as described above. In the information recording medium which records information according to the polarization direction of the ferroelectric substance, the absolute value of the flat band voltage V _FB ⁱ evaluated by the structure including only two layers of the semiconductor layer and the insulating layer is The difference ΔV _FB (= | V) between the flat band voltage V _FB ⁺ when positive poling and the flat band voltage V _FB ⁻ when negative poling is performed on the structure including three layers, the insulating layer and the ferroelectric layer.
It is characterized in that it is less than 1/2 of _FB ⁻ −V _FB ⁺ |).

As the ferroelectric material used in the present invention, among the inorganic materials, ferroelectrics having a perovskite structure, barium titanate, lead titanate-lead zirconate solid solution, bismuth titanate, and niobium having a tungsten-bronze structure are used. Acid strontium-barium niobate, glycine sulfate, potassium nitrate, potassium phosphate, C (NH ₂ ) ₃ Al
_{(SO 4) 2 6H 2 O} , sodium nitrite, SBSI, and the like. As the organic ferroelectric material, vinylidene fluoride (VDF) polymer or a copolymer containing it, odd-order nylon, vinylidene cyanide or a copolymer containing it, vinyl fluoride (VF)
Examples thereof include polymers and copolymers containing them.

On the other hand, as the semiconductor layer, a substrate made of a semiconductor may be used as it is. Alternatively, a semiconductor layer may be formed on the substrate by using plastic, glass, or metal in which guide grooves or pits or pits indicating sector information are provided in advance as the substrate. As well-known semiconductors in this case, Si, G
e, III-V group compound semiconductors represented by GaAs, and further II-VI group compound semiconductors can be used. Moreover, polypyrrole, polythiophene, etc. can be used as an organic semiconductor. These may be single crystal, polycrystal or amorphous. The semiconductor resistivity is 0.
It is good to set about 01Ωcm to 1000Ωcm, preferably 1Ω
cm to 100 Ωcm.

Of these semiconductors, n-type or p-type silicon doped with impurities is preferable, and the impurity concentration in silicon is about 10 ¹⁹ to 10 ²³ m ^-3 , preferably
It is 10 ²⁰ to 10 ²² m ^-3 .

[0010]

According to the research conducted by the present inventor, the reason why recording / reproducing failure easily occurs when the above-mentioned insulating layer is provided is as follows.
It is considered that the flat band voltage of the semiconductor layer largely changes mainly due to the electric charge existing in the insulating layer.

For example, in a two-layer structure of a semiconductor layer made of a p-type semiconductor and an insulating layer, consider the case where positive charges are present in the insulating layer. Here, the C-V curve showing the electrode voltage-capacitance characteristic in the two-layer structure of the p-type semiconductor layer and the insulating layer having no impurities is as shown in (1) of FIG. 2 because of the positive charge of the insulating layer
The C-V curve of the layer structure changes as indicated by a in FIG.
In other words, the flat band voltage of the semiconductor layer is V in the figure (1).
_{From FB} to V _FB ⁱ of (2), it is largely shifted to the negative voltage side. Although the flat band voltage V _FB strictly depends on the acceptor concentration of the semiconductor, here, as a matter of convenience, as shown in (1) of the figure, the electrode that gives a value of 98% of the maximum capacitance Cmax is given. It is defined by voltage.

Then, a ferroelectric layer is further layered on the insulating layer having the two-layer structure to form an information recording medium, on which negative recording (minus poling) is performed, and a depletion layer is formed on the semiconductor layer. Consider the case where the recording state does not occur.

When such recording is performed, the C-V curve originally changes as shown by b in FIG. 2 (2), and at the time of signal reproduction (the electrode voltage at this time is zero), it is static. The maximum capacitance value Cmax should be detected, and the minimum capacitance value Cmin should be detected elsewhere. However, if the CV curve of the above-mentioned two-layer structure is shifted too much to the negative voltage side as indicated by a in FIG. 2 (2), the actual CV curve after minus poling is as shown in FIG. become. If this is the case, during signal reproduction, the minimum capacitance value Cmin will be detected in the polling portion as in the other portions, and it will be impossible to detect the recording state according to the presence or absence of the depletion layer, that is, the electrostatic capacitance. It will be possible.

By avoiding such a situation, b in FIG.
As shown in C, when the electrode voltage is zero, the maximum capacitance Cmax is taken.
In order to obtain the −V curve, the absolute value of the flat band voltage V _FB ⁱ should be the voltage difference ΔV ′ or less. This voltage difference ΔV ′ is the difference ΔV _FB (= | V _FB ⁻ −V _FB ⁺ ) between the flat band voltage V _FB ⁺ when positive polling and the flat band voltage V _FB ⁻ when negative polling is performed.
Since it is 1/2 of |), if | V _FB ⁱ | ≦ ΔV _FB / 2 (1) after all, correct recording and reproduction can be performed.

The case of performing negative recording using a p-type semiconductor has been described above. However, when positive recording is performed using an n-type semiconductor, negative charges exist in the insulating layer, and a two-layer structure is formed. The situation is the same when the C-V curve is largely shifted to the positive voltage side, and correct recording / reproduction can be performed if the above equation (1) is satisfied.

The factors that cause a large shift in the flat band voltage V _FB ⁱ in the insulating layer are impurity ions mixed into the insulating layer during the manufacturing process of the insulating layer and ionized traps formed in the insulating layer. Can be mentioned. The sum of these densities is 1 x 10 when a general material (remanent polarization of the order of 0.1 μC) is used as a ferroelectric.
²⁵ pieces / m ³ or less, more preferably 1 × 10 ²⁴ pieces / m ³ or less is preferable in order to satisfy the above-mentioned formula (1). For example, when all of the above ions are Na ions that are mixed in during the manufacturing process, good results are obtained when the concentration is 100 ppm or less, more preferably 10 ppm or less.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. FIG. 1 conceptually shows a side sectional shape of an information recording medium 10 according to an embodiment of the present invention. This information recording medium 10 comprises a semiconductor layer 11 and an organic ferroelectric layer 13 formed thereon with an insulating layer 12 interposed therebetween. In this embodiment, the semiconductor substrate is used as it is as the semiconductor layer 11, and the substrate has a resistivity of 5 Ωcm and an impurity concentration of 5 × 10 ²¹ m 2.
^-3 p-type silicon wafer is used. The insulating layer 12 is formed by layering silicon oxide (SiO ₂ ) in a thickness of 50 nm on the silicon wafer by a thermal oxidation method. And on top of that, VDF which is an organic ferroelectric
An organic ferroelectric layer 13 is formed by forming a thin film of a / TrFE copolymer (VDF is 65 mol%).

This thin film formation is performed as follows by way of example. First, VDF / TrFE copolymer was added to methyl ethyl ketone (MEK) at 10 wt. %, And this solution is spun at 5000 rpm using a commercially available spin coater.
Apply on a silicon wafer at 10 rpm for 10 seconds. Next, this coating film is annealed in an atmosphere of an atmosphere of 145 ° C. for 2 hours in an oven to obtain a film thickness of 1
A μm VDF / TrFE copolymer thin film is formed.

In this example, the above steps were performed in a class 100 clean room. At this time, the concentration of impurity ions (Na ions in this example) in the insulating layer 12 was 10 ppm as analyzed by the ICP method.

The results of recording and reproducing information on the information recording medium 10 having the above structure will be described below. In this case, the information recording medium 10 is fixed to the turntable 20 by an air chuck or the like as shown in FIG. 3, and the turntable 20 is used as one electrode for voltage application during recording. As the other electrode for applying this voltage, a movable needle-shaped electrode
Use 21. In this example, as the needle electrode 21, a tungsten needle having a bottom surface diameter of 50 μm and plated with gold is used.

With the above turntable 20 fixed,
By applying a voltage from the pulse power supply 25 to the organic ferroelectric layer 13 via the needle electrode 21 and the turntable 20 while the needle electrode 21 is brought into contact with the organic ferroelectric layer 13 side of the information recording medium 10. The portion of the organic ferroelectric material facing the needle electrode 21 is polarized in a predetermined direction. Thereby, information can be recorded with this polarization direction.

When electrical polarization is generated in the ferroelectric layer 13 in this way, as shown in FIG. 1, the polarization direction of the ferroelectric layer 13 is downward (toward the semiconductor layer 11 side). Since a depletion layer 14 is generated in the semiconductor layer 11, this depletion layer 14
The recorded information can be read by detecting a change in the electrostatic capacitance due to a known pickup circuit.

Next, the result of evaluation as to whether or not information recording / reproduction can be normally performed will be shown. A two-layer structure sample (referred to as sample A) consisting of the semiconductor layer 11 and the insulating layer 12 before the formation of the ferroelectric layer 13 described above, and the ferroelectric layer 13 further formed thereon. An information recording medium 10 (referred to as a sample B) was prepared, each of which was fixed to the turntable 20, and the CV curves of the samples A and B were measured while the turntable 20 was stopped.

With respect to the sample B, positive poling (recording) for applying a positive voltage and negative poling for applying a negative voltage were performed, and thereafter, the electrostatic capacitance C ⁺ of the positive poling portion and the electrostatic charge of the negative poling portion were performed. The capacitance C ^- and the electrode voltage were measured with zero bias.

For the measurement of the CV curve, a commercially available LCR
Meter (Yokogawa / Hewlett-Packard Co .:
HP4284A) and the frequency was 1 MHz and the voltage range was ± 30 V. On the other hand, the pulse width is 10 for polling (recording).
A pulse voltage of msec was used, and a voltage of +100 V was applied to the sample A at the time of plus poling and −100 V at the time of minus poling.

The difference ΔV _FB (= | V) between the flat band voltage V _FB ^{i of} the sample A obtained as described above, the flat band voltage V _FB ⁺ when positive poling and the flat band voltage V _FB ⁻ when negative poling is performed. _FB ^-- V _FB ⁺
|) And capacitances C ⁺ and C ⁻ are summarized in Table 1.

As a comparative example, samples A and B similar to the above were prepared by setting the manufacturing process environment to a general laboratory instead of a clean room, and the same measurement as above was performed. The concentration of impurity ions (Na ions in this example) in the insulating layer 12 at this time was 200 pp as determined by ICP analysis.
It was m. The measurement results are also shown in Table 1.

[0028]

[Table 1]

In the embodiment according to the present invention, the absolute value of the flat band voltage V _FB ⁱ of the sample A is 2 V, and the flat band voltage V _FB ⁺ of the sample B when the positive poling and the flat band voltage when the negative poling are performed. V _FB ^-
Difference of ΔV _FB (= | V _FB ^-- V _FB ⁺ |) 1/2 = 25V
Is much smaller than. Therefore, the depletion layer is not formed at the time of minus poling of the sample B, the electrostatic capacitance C ⁻ at the time of reproduction takes a relatively large value, and the depletion layer is formed at the time of positive poling, and the electrostatic capacitance C at the time of reproduction is formed.
⁺ Has a relatively small value. This proves that the sample B, that is, the information recording medium 10 according to the present invention, enables normal recording and reproduction.

On the other hand, in the comparative example, the absolute value of the flat band voltage V _FB ⁱ of the sample A is 28 V, and the flat band voltage V _FB ⁺ of the sample B when the positive poling and the flat band voltage when the negative poling are performed. V _FB ^-
Difference of ΔV _FB (= | V _FB ^-- V _FB ⁺ |) 1/2 = 25V
Is over. Therefore, the depletion layer is formed regardless of the polarization direction of the ferroelectric material during the negative poling or the positive poling of the sample B, and the electrostatic capacitances C ⁺ and C ⁻ have the same value. That is, in the information recording medium of this comparative example, since the polarization direction of the ferroelectric substance and the presence or absence of the depletion layer do not correspond one to one, correct recording / reproducing cannot be performed.

Judging from the Na ion concentrations of the above-described examples and comparative examples, in the comparative example, positively charged ions due to contamination exist mainly in and in the insulating layer of the sample A, and therefore the semiconductor layer It is considered that the flat band voltage V _{FB of 1} is largely shifted to the negative voltage side.

[0032]

As described above in detail, in the information recording medium of the present invention, the absolute value of the flat band voltage V _FB ⁱ evaluated by the structure composed of only two layers of the semiconductor layer and the insulating layer is the flat band voltage V _FB ⁺ when the structure of three layers of the layers and the ferroelectric layer plus polling, flat band voltage when the negative poll V _FB ^- difference between ^{_{ΔV FB (= | V FB -}} -V _FB ⁺ |)
Since the polarization direction of the ferroelectric substance and the presence / absence of the depletion layer always have a one-to-one correspondence with each other, it is possible to always correctly record / reproduce information. Become.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a side sectional shape of an information recording medium according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram for explaining the function and effect of the present invention.

FIG. 3 is a schematic side view showing an apparatus for performing recording and reproduction on the information recording medium.

[Explanation of symbols]

 10 Information recording medium 11 Semiconductor layer 12 Insulating layer 13 Ferroelectric layer 14 Depletion layer 20 Turntable 21 Needle electrode 25 Pulse power supply

Claims (1)

[Claims] 1. A semiconductor layer, an insulating layer formed on the semiconductor layer, and a ferroelectric layer formed on the semiconductor layer, and information is recorded according to the polarization direction of the ferroelectric layer. In the information recording medium, the flat band voltage V evaluated by the structure composed of only two layers of the semiconductor layer and the insulating layer.
The absolute value of _FB ⁱ is the flat band voltage V _FB ⁺ when the structure consisting of the three layers of the semiconductor layer, the insulating layer and the ferroelectric layer is positively polled, and the flat band voltage V _FB ⁻ when the negative poling is performed. the difference ^{_{ΔV FB (= | V FB -}} -
V _FB ⁺ |) 1/2 or less of the information recording medium.