JPH06127135A - Optical record medium - Google Patents

Abstract

PURPOSE:To improve chiefly repetition durability by providing in an optical record medium at least a recording layer, a dielectric layer, and a reflective layer, and using for the crystal state of the recording layer a tellurium alloy which is substantially constituted of a single crystal phase and to be expressed by a specific composition formula. CONSTITUTION:In an optical record medium, information is recorded, erased and regenerated by means of light irradiation on a recording layer formed on a base board. Recording and erasing of the information are done by means of phase change between non-crystal phase and crystal phase. In this case, at least a recording layer, a dielectric layer, and a reflective layer are provided on the optical record medium. The crystal state of the recording layer is made of a tellurium alloy which is substantially constituted of a single crystal phase and to be expressed by the following composition formula: Pdz(SbxTe1-x)1-y-z(Ge0.5Te0.5)y. Where 0.35<=x<=0.7, 0.2<=y<=0.5, 0.0005<=z<=0.005. The x, y, z and the numeral characters express atomicity of each element (mole).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium capable of recording, erasing and reproducing information by irradiating light.

In particular, the present invention relates to a rewritable phase-change type optical recording medium such as an optical disk, an optical card, an optical tape having a recording information erasing / rewriting function and capable of recording an information signal at high speed and high density. It is a thing.

[0003]

2. Description of the Related Art The conventional techniques for rewritable phase change type optical recording media are as follows.

These optical recording media have a recording layer containing tellurium as a main component, and at the time of recording, a focused laser light pulse is irradiated to the recording layer in a crystalline state for a short time to partially cover the recording layer. To melt. The melted portion is rapidly cooled by thermal diffusion and solidified to form a recording mark in an amorphous state. The light reflectance of this recording mark is lower than that of the crystalline state, and it can be optically reproduced as a recording signal.

Further, at the time of erasing, the recording mark portion is irradiated with a laser beam and heated to a temperature below the melting point of the recording layer and above the crystallization temperature to crystallize the recording mark in an amorphous state, and the original unrecorded state. Return to the state.

As a material for the recording layer of these rewritable phase change optical recording media, alloys such as Ge ₂ Sb ₂ Te ₅ (N.Ya
mada et al, Proc. Int. Symp.on Optical Memory 1987 p
61-66) is known.

Optical recording media using these Te alloys as recording layers have a high crystallization rate, and high speed overwriting with a circular single beam is possible only by modulating the irradiation power. In an optical recording medium using these recording layers, a dielectric layer having heat resistance and translucency is usually provided on both surfaces of the recording layer to prevent deformation and opening of the recording layer during recording. Further, a metal reflective layer such as a light-reflective metal such as Al is provided on the dielectric layer on the side opposite to the light beam incident direction to improve the signal contrast at the time of reproduction by an optical interference effect, and at the same time, a non-reflecting effect by a cooling effect. There is known a technique for facilitating the formation of recording marks in a crystalline state and improving the erasing characteristic and the repeating characteristic. In particular, in the "quenching structure" in which the recording layer and the dielectric layer between the recording layer and the reflecting layer are each thinned to about 20 nm, compared with the "slow cooling structure" in which the dielectric layer is thickened to about 200 nm, rewriting is repeated. It is excellent in that the recording characteristics are not deteriorated by the recording and the erasing power has a wide power margin (T.Ohota et al, Japanese Jounal of A
pplied Physics, Vol 28 (1989) Suppl. 28-3 pp123-12
8).

Te-G such as Ge ₂ Sb ₂ Te ₅ mentioned above
A problem in a rewritable phase change type optical recording medium having a quenching structure in which a ternary alloy of e-Sb is used as a recording layer is to reduce the film thickness of the recording film by repeating recording, erasing or rewriting, that is, melting and solidification Variations and the generation of minute apertures are likely to occur, and the repeated recording durability is insufficient.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of an optical recording medium having GeSbTe as a recording film and to provide an optical recording medium having improved repeated durability.

Another object of the present invention is to provide an optical recording medium which has a high recording sensitivity and is excellent in oxidation resistance and resistance to moist heat, and which has no defects even in long-term storage.

[0011]

According to the present invention, information can be recorded, erased, and reproduced by irradiating a recording layer formed on a substrate with light.
In an optical recording medium performed by a phase change between an amorphous phase and a crystalline phase, the optical recording medium has at least a recording layer, a dielectric layer and a reflective layer, and the recording layer has a substantially crystalline state. The present invention relates to an optical recording medium characterized by being a tellurium alloy having a single crystal phase and represented by the following composition formula.

Composition formula Pd _z (Sb _x Te _1-x ) _1-yz (Ge _0.5 Te _0.5 ) _y 0.35 ≦ x ≦ 0.7 0.2 ≦ y ≦ 0.5 0.0005 ≦ z ≦ 0 0.005 Here, Pd represents palladium, Sb represents antimony, Te represents tellurium, and Ge represents germanium. Further, x, y, z and the numbers represent the number of atoms of each element (the number of moles of each element).

The material of the recording layer of the present invention is a chalcogen compound containing Te as a main component which can be in at least two states of a crystalline state and an amorphous state, and in the crystalline state,
A new non-stoichiometric composition that becomes a substantially single crystal phase 4
It is made of the original alloy. Since the crystal state is a single phase, the crystallization speed is extremely high, and recording can be rewritten at high speed. Further, since the crystalline state is substantially a single phase, it is difficult for the recording characteristics to deteriorate due to composition segregation.

Pd in the composition formula is within the range of the content represented by z in the composition formula, and fluctuations in the film thickness of the recording layer and apertures in the recording layer caused by repeated repetition of recording, erasing or rewriting. Suppress the occurrence of. Although the details of this mechanism have not been sufficiently clarified, elements such as Te and Sb in the recording layer and palladium strongly bond with each other to increase the viscosity in the high temperature melting state of the recording layer, and thus the recording layer at the time of recording. It is estimated that the liquidity of the Also, Pd is
It also has the effect of improving the thermal stability of the recording mark in the recording layer in the amorphous state. In the recording layer of the present invention, by making the amount of Pd relatively small as shown in the composition formula,
It achieves high crystallization speed, thermal stability of recording, and good repeated recording durability at the same time.

When the value of z in the composition formula is larger than 0.005, the crystallization rate of the recording layer becomes slow, the erasing rate at the time of erasing at a high speed decreases, and recording rewriting becomes incomplete. There are cases. Further, when the value of z exceeds about 0.05, the crystalline state of the recording layer is substantially composed of crystals of a plurality of phases, so that the crystallization rate is extremely slow and segregation is likely to occur, so that erasing and rewriting are repeated. Becomes difficult. Further, when the value of z is less than 0.0005, the above-mentioned significant effect is not exhibited, and repetitive rewriting durability and thermal stability of the recording mark in the amorphous state are significantly reduced. The value of z is preferably 0.001 or more and 0.004 or less because the crystallization rate is high, the thermal stability of the recording mark in the amorphous state is good, and the rewriting durability is high. Particularly, 0.001 or more and 0.002 or less are preferable because the crystallization rate is high.

The value of x in the composition formula is 0.35≤x≤0.
In the range of 7, the crystallization rate is fast, rewriting is possible at high speed, and reversibility of rewriting is also good. When it is more than 0.7, the Sb component becomes too much, and when it is less than 0.35, the Te component becomes too much, and in any case, the crystallization speed becomes remarkably slow and segregation easily occurs. Rewriting becomes difficult. The value of x is 0.
It is preferably 4 or more and 0.5 or less because the crystallization rate is high and the repetitive rewriting durability is high.

The value of y in the composition formula is 0.2≤y≤0.5.
In this range, the thermal stability of the recording mark in the amorphous state is high, the crystallization speed is high, and recording can be rewritten at high speed. If it is more than 0.5, the thermal stability in the amorphous state is good, but the repetitive rewriting durability becomes extremely poor. On the other hand, when it is less than 0.2, the thermal stability of the recording mark in the amorphous state is significantly deteriorated. The value of y is preferably 0.3 or more and less than 0.4 because the repetitive rewriting durability is high and the thermal stability in the amorphous state is high.

A typical layer structure of the optical recording medium of the present invention is
It is composed of a laminate of a transparent substrate / first dielectric layer / recording layer / second dielectric layer / reflection layer (where light is incident from the substrate side). However the invention is not limited thereto, SiO ₂ and ZnS does not impair the effects of the present invention on a reflective layer, Z
A protective layer such as nS-SiO _2, a resin layer such as an ultraviolet curable resin, an adhesive layer for bonding with another substrate, or the like may be provided.

In the present invention, the dielectric layer has an effect of protecting the substrate and the recording layer from heat, such as preventing the substrate and the recording layer from being deformed by heat during recording and deteriorating the recording characteristics, and an optical interference effect. This has the effect of improving the signal contrast during reproduction. As the dielectric layer, Zn
Inorganic thin films such as S, SiO ₂ , silicon nitride, and aluminum oxide can be used. Especially ZnS thin film, Si, G
e, Al, Ti, Zr, Ta, and other metal oxide thin films, Si, Al, and other nitride thin films, Ti, Zr, and Hf
A thin film of carbide and a film of a mixture of these compounds are preferable because of high heat resistance. Further, a mixture of carbon and a fluoride such as MgF ₂ is also preferably used because the residual stress of the film is small. Especially Z
A mixed film of nS and SiO ₂ or a mixed film of ZnS, SiO ₂ and carbon is preferable because deterioration of recording sensitivity, C / N, erasing rate, etc. does not easily occur even after repeated recording and erasing. A mixed film of SiO ₂ and carbon is preferable.

The thickness of the first and second dielectric layers is as follows.
Usually, it is about 10 to 500 nm. The first dielectric layer is more preferably 100 to 400 nm, because it is difficult to peel off from the substrate or the recording layer and defects such as cracks are hard to occur. The second dielectric layer has a recording property such as C / N and an erasing rate, and can be stably rewritten many times.
m is more preferred.

The material of the reflective layer is a metal such as Al or Au having a light reflectivity, which contains Ti or C as a main component.
Examples thereof include alloys containing additional elements such as r and Hf, and metals such as Al and Au mixed with metal compounds such as metal nitrides such as Al and Si, metal oxides and metal chalcogenides. Metals such as Al and Au, and alloys containing these as the main components are preferable because they have high light reflectivity and high thermal conductivity. Examples of the above alloys include
Si, Mg, Cu, Pd, Ti, Cr, Hf, T on Al
a total of at least one element such as a, Nb, and Mn is 5
Atomic% or less, 1 atomic% or more added, or Au
In addition, at least one element such as Cr, Ag, Cu, Pd, Pt, and Ni is added in a total amount of 1 atom% or more and 20 atom% or less.

In particular, an alloy containing Al as a main component is preferable because of low material cost, and particularly Al, Ti, Cr, Ta, Hf, Zr, and
An alloy in which at least one metal selected from Mn and Pd is added in a total amount of 0.5 atom% or more and 5 atom% or less is preferable.

Further, Al-Hf-Pd containing an additive element in a total amount of 0.5 atom% or more and less than 3 atom% is preferable because it has good corrosion resistance and is unlikely to generate hillocks.
Alloy, Al-Hf alloy, Al-Ti alloy, Al-Ti-
Hf alloy, Al-Cr alloy, Al-Ta alloy, Al-T
Al of i-Cr alloy or Al-Si-Mn alloy
It is preferable to use an alloy containing as a main component.

The thickness of the reflective layer is about 10 nm.
It is preferably not less than 200 nm and not more than 200 nm.

In particular, since the recording sensitivity is high, the one-beam overwrite can be performed at a high speed, the erasing rate is large, and the erasing characteristics are good, the main part of the optical recording medium can be constructed as follows. preferable.

That is, the thickness of the first dielectric layer is 100 n.
m to 400 nm, and the thickness of the second dielectric layer is 10 nm
˜30 nm, and the thickness of the recording layer is 10 nm to 30 nm.
nm, the thickness of the reflective layer is 30 nm to 200 nm, the dielectric layer is a mixed film of ZnS and SiO ₂ , the mixing ratio of SiO ₂ is 15 to 35 mol%, and the composition of the recording layer is It is preferably within the range shown.

Compositional formula Pd _z (Sb _x Te _1-x ) _1-yz (Ge _0.5 Te _0.5 ) _y 0.35 ≦ x ≦ 0.5 0.2 ≦ y ≦ 0.4 0.0005 ≦ z ≦ 0 0.005 Here, Pd represents palladium, Sb represents antimony, Te represents tellurium, and Ge represents germanium. Further, x, y, z and the numbers represent the number of atoms of each element (the number of moles of each element).

As the material of the substrate of the present invention, various known transparent synthetic resins, transparent glass and the like can be used.

In order to avoid the effects of dust and scratches on the substrate, it is preferable to use a transparent substrate and perform recording from the substrate side with a focused light beam. Examples of such transparent substrate material include glass, polycarbonate and polymethyl -Methacrylate, polyolefin resin, epoxy resin, polyimide resin, etc. may be mentioned. In particular, a polycarbonate resin and an amorphous polyolefin resin are preferable because they have a small optical birefringence, a small hygroscopicity, and easy molding.

The thickness of the substrate is not particularly limited, but 0.01 mm to 5 mm is practical.
If it is less than 0.01 mm, it is likely to be affected by dust even when recording with an optical beam focused from the substrate side, and it becomes 5 mm.
If it exceeds, it becomes difficult to increase the numerical aperture of the objective lens, and the irradiation light beam spot size increases, so that it becomes difficult to increase the recording density.

The substrate may be flexible or rigid. The flexible substrate is used in the form of tape, sheet, or card. The rigid board is used in the form of a card or disk. In addition, these substrates, after forming the recording layer,
An air sandwich structure, an air incident structure, and a close-bonding structure may be used by using two substrates.

Examples of the light source used for recording on the optical recording medium of the present invention include high-intensity light sources such as laser light and strobe light. Particularly, the semiconductor laser light can be downsized and has low power consumption. It is preferable because it can be easily modulated.

Recording is performed by irradiating a crystalline recording layer with a laser light pulse or the like to form an amorphous recording mark. Alternatively, conversely, a recording mark in a crystalline state may be formed on the recording layer in an amorphous state. Erasure can be performed by irradiating a laser beam to crystallize an amorphous recording mark or to amorphize a crystalline recording mark. A method of forming an amorphous recording mark at the time of recording and crystallizing at the time of erasing is preferable because the recording speed can be increased and the deformation of the recording layer is less likely to occur. Further, the light intensity is high at the time of forming the recording mark and slightly weak at the time of erasing, and rewriting is performed by irradiating the light beam once.
Beam overwrite is preferable because the time required for rewriting is shortened.

Next, a method for manufacturing the optical recording medium of the present invention will be described. Examples of the method for forming the reflective layer, the recording layer and the like on the substrate include known thin film forming methods in vacuum, such as a vacuum vapor deposition method, an ion plating method and a sputtering method. In particular, the sputtering method is preferable because the composition and the film thickness can be easily controlled.

The thickness of the recording layer or the like to be formed can be easily controlled by monitoring the deposition state with a known crystal oscillator film thickness meter.

The recording layer or the like may be formed either with the substrate fixed, or with the substrate moved or rotated. Since the in-plane uniformity of the film thickness is excellent, it is preferable to rotate the substrate, and it is more preferable to combine the revolution.

Further, within a range that does not significantly impair the effects of the present invention, after forming a reflective layer or the like, a dielectric layer such as ZnS or SiO ₂ or a resin such as an ultraviolet curable resin is protected to prevent scratches and deformation. Layers and the like can be provided as needed. Further, after forming the reflective layer or the like, or after further forming the above-mentioned resin protective layer, the two substrates may be opposed to each other and adhered with an adhesive.

The recording layer is preferably preliminarily crystallized by irradiation with light such as a laser beam or a xenon flash lamp before actual recording.

[0039]

EXAMPLES The present invention will be described below based on examples. [Analysis / Measurement Method] The compositions of the reflective layer and the recording layer were confirmed by ICP emission analysis (manufactured by Seiko Instruments Inc.).
The carrier-to-noise ratio and the erasing rate (difference in reproduced carrier signal intensity after recording and after erasing) were measured by a spectrum analyzer.

The film thickness during the formation of the recording layer, the dielectric layer and the reflective layer was monitored by a quartz oscillator film thickness meter. The thickness of each layer was measured by observing the cross section with a scanning electron microscope or a transmission electron microscope.

Example 1 A substrate made of polycarbonate with a spiral groove having a thickness of 1.2 mm, a diameter of 13 cm, and a pitch of 1.6 μm was 3 per minute.
The recording layer, the dielectric layer, and the reflective layer were formed by a high frequency sputtering method while rotating at 0 rotation.

First, the inside of the vacuum chamber was evacuated to 1 × 10 ^-5 Pa, and then SiO 2 in an Ar gas atmosphere of 2 × 10 ^-1 Pa.
ZnS containing 20 mol% of ₂ was sputtered to form a first dielectric layer having a film thickness of 300 nm on the substrate. continue,
Pd on a ternary alloy with a composition of about Ge _0.18 Sb _0.26 Te _0.56
The composite target in which the small pieces of Pd are arranged is sputtered to form a film having a composition Pd _0.002 Ge _0.178 Sb _0.26 Te _{0.56 and} a film thickness of 25 nm
Recording layer was formed. Further, the above-mentioned second dielectric layer 20
nm, and on this Pd _0.002 Hf _0.02 Al _0.978
The alloy was sputtered to form a reflective layer having a thickness of 100 nm. After removing this disk from the vacuum container,
An acrylic ultraviolet curable resin was spin-coated on this reflective layer and cured by irradiation with ultraviolet rays to form a resin layer having a film thickness of 10 μm to obtain an optical recording medium of the present invention.

A recording layer on the entire surface of the disk was crystallized and initialized by a beam of a semiconductor laser having a wavelength of 820 nm on this optical recording medium. After that, the numerical aperture of the objective lens is 0.5 and the wavelength of the semiconductor laser is 780 nm under the condition of a linear velocity of 6 m / sec.
After performing overwriting recording 100 times with a semiconductor laser beam modulated under the conditions of a frequency of 3.7 MHz, a pulse width of 60 nsec, a peak power of 9 to 17 mW, and a bottom power of 4 to 9 mW using the optical head of 1., the reproduction power of 1. Three
C / N was measured under the condition of a band width of 30 kHz by irradiating a semiconductor laser beam of mW.

Further, this portion was irradiated with semiconductor laser light modulated at 1.4 MHz in the same manner as above, and one-beam overwriting was performed, and the erasing rate at 3.7 MHz at this time was measured.

With a peak power of 15 mW or more, a C / N of 50 dB or more, which is practically sufficient, is obtained, and a bottom power of 5 to 5 is obtained.
At 8 mW, a practically sufficient erasing rate of 20 dB or more and a maximum of 30 dB was obtained.

Further, under the conditions of peak power of 17 mW, bottom power of 8 mW, and frequency of 3.7 MHz, one beam overwrite was repeated 1000 times and 200,000 times, and the same measurement was performed. The change in erasing rate was within 2 dB, and almost no deterioration was observed.

The optical recording medium was left in an environment of 80 ° C. and 80% relative humidity for 1000 hours, and then the recorded portion was reproduced. The change in C / N was less than 2 dB, and there was almost no change. Recording and erasing are performed again, and C / N,
When the erasure rate was measured, almost no change was observed.

Comparative Example 1 The composition of the recording layer of the optical recording medium of Example 1 was Ge _0.18 Sb.
An optical recording medium having the same configuration as that of Example 1 was prepared _except that it was _0.26 Te _0.56, and the same measurement as that of Example 2 was performed.

The C / N is 53 dB in the initial stage, 52 dB in 150 hours, and 40 dB after 1000 hours, which is 10 dB.
After 00 hours, remarkable deterioration was observed. From this, it became clear that the thermal stability of the recording mark was insufficient and there was a problem in the long-term storage stability of the recording.

Example 2 The composition of the recording layer of Example 1 was changed to Pd _0.001 Ge _0.179 Sb.
_0.26 Te _0.56 and Pd _0.004 Ge _0.178 Sb _0.26 Te
Optical recording media having the same configurations as in Example 1 _except that _0.558 was manufactured. The two optical recording media were used in Example 1.
The recording characteristics were measured under the same recording conditions with the same device as described above. In each case, a peak power of 15 mW or more can obtain a C / N of 50 dB or more, which is practically sufficient, and a bottom power of 5 to 5
At 8 mW, a practically sufficient erasing rate of 20 dB or more and a maximum of 30 dB was obtained.

Further, under the conditions of a peak power of 17 mW, a bottom power of 8 mW, and a frequency of 3.7 MHz, one beam overwrite was repeated 1000 times and 200,000 times, and the same measurement was performed. As for the change of the erasing rate, almost no deterioration was observed within 2 dB.

Example 3 The composition of the recording layer of Example 1 was changed to Pd _0.001 Ge _0.179 Sb.
Optical recording media having the same configurations as in Example 1 were prepared, except that the thickness of the first dielectric layer was _0.26 Te _0.56 and the thickness of the first dielectric layer was 200 nm. This optical recording medium was used in the same apparatus as in Example 1 with a linear velocity of 15 m / sec and a recording frequency of 7.35 MH.
z, the frequency of overwriting at the time of erasing rate measurement is 2.7
Recording characteristics were measured by the same method as in Example 1 except that the recording pulse width was 6 MHz and the recording pulse width was 50 nsec. With a peak power of 15 mW or higher, a practically sufficient C / N of 50 dB or higher was obtained, and with a bottom power of 7 to 10 mW, a practically sufficient 20 dB or higher, a maximum erasing rate of 27 dB was obtained.

Example 4 An optical recording medium similar to that of Example 1 was prepared, except that the substrate of the optical recording medium of Example 1 was replaced with another substrate having a format and the thickness of the reflective layer was 120 nm. .

Thereafter, under the condition of linear velocity of 8.5 m / sec, the numerical aperture of the objective lens is 0.5 and the wavelength of the semiconductor laser is 830.
Using an optical head of nm, pulse width 50 nsec,
A semiconductor laser light modulated under conditions of 20 mW peak power and 9 mW bottom power overwrites a random data pattern of 2-7 code (1.5T frequency 5.3 MHz) on the same track 100 times and 100,000 times more.・ Recorded in mode. After that, reproduction was performed and the reproduced waveform was observed. As a result, a good reproduced waveform was obtained with almost no deterioration as compared with after recording 100 times in the initial stage. Furthermore, when the bit error rate (BER) of this track was measured, it was 5 × 10.
It was a good value of ^-5 .

Further, the recording sector by moving the recording material,
Almost no collapse of the reproduced waveform was observed at the leading and trailing end portions, and there was almost no disturbance in the reproduced waveform of the intermediate data portion.

Comparative Example 2 A conventional optical recording medium having the same structure as that of Example 4 except that the composition of the recording layer was Ge _0.22 Sb _0.23 Te _0.55 was outside the scope of the present invention. When the recording sensitivity of this optical recording medium was measured, it was almost the same as in Example 1. This optical recording medium was repeatedly overwritten 100 times and 100,000 times as in Example 1, and the reproduced waveform was observed.
After 0,000 times, the fluctuation in the film thickness of the recording layer was larger than that at the 100th time, and a large number of parts in which the amplitude of the signal in the data part was significantly decreased were seen. When the bit error rate (BER) was measured, it was found to be 3 × 10 ⁻¹ or more, and even when error correction was performed, it was deteriorated to a level at which data reproduction was extremely difficult.

Further, the reproduction waveform collapse at the front end and the rear end of the recording sector due to the movement of the recording material was noticeable.

[0058]

According to the present invention, the composition of the recording layer of the optical recording medium is set to a specific composition, so that the following effects are obtained. (1) High sensitivity, high erasing rate and C / N. (2) Even if recording and erasing are repeated a large number of times, the operation is stable and there is almost no deterioration of characteristics or occurrence of defects. (3) Excellent resistance to moist heat and oxidation, and long life. (4) The recording mark has high thermal stability and excellent long-term storage stability of the record. (5) It can be easily manufactured by the sputtering method. (6) Good overwrite characteristics can be obtained even at high linear velocity.

Claims (1)

[Claims] 1. Information can be recorded, erased, and reproduced by irradiating a recording layer formed on a substrate with light, and the recording and erasing of information is performed in a phase between an amorphous phase and a crystalline phase. In the optical recording medium performed by the change, the optical recording medium has at least a recording layer, a dielectric layer, and a reflective layer, and the recording layer has a crystalline state consisting of a substantially single crystal phase as follows: An optical recording medium, which is a tellurium alloy represented by the formula. Composition formula Pd _z (Sb _x Te _1-x ) _1-yz (Ge _0.5 Te _0.5 ) _y 0.35 ≦ x ≦ 0.7 0.2 ≦ y ≦ 0.5 0.0005 ≦ z ≦ 0.005 here Pd is palladium, Sb is antimony, Te is tellurium, and Ge is germanium. Further, x, y, z and the numbers represent the number of atoms of each element (the number of moles of each element).