Optical information-recording medium

An optical information-recording medium includes a substrate, and a lower protective layer, a recording layer, an upper protective layer, and a reflective layer, formed on the substrate. The recording layer exhibits different optical properties dependent on different thermal histories of a rise and a fall in temperature caused by irradiation of a laser beam on the substrate. The optical information-recording medium has a layer structure constructed such that reflectivity of the optical information-recording medium and the phase of light reflected therefrom vary with the change in the optical properties of the recording layer, with absorptivity of light of the recording layer being larger when the recording layer is in a crystalline state than when the recording layer is in an amorphous state, and at the same time, reflectivity exhibited when the recording layer in the crystalline state being 10% or higher. The reflective layer has a thickness set such that a value of reflectivity of the reflective layer exhibited in the form of a single layer is 90% or more of a value of reflectivity of a bulk form of a substance of the reflective layer, or to a value of 40 nm or larger.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an optical information-recording medium for 
recording information by utilizing changes in optical properties thereof 
induced by different thermal histories of a rise and a fall of temperature 
caused by irradiation of a laser beam thereon, i.e. a phase change optical 
disk. 
2. Description of the Related Art 
An optical disk for recording and reproducing information by the use of a 
laser beam now can be used as a large-capacity portable file memory, and a 
ROM-type, a direct read after write-type, and a writable-type 
magneto-optical disks have already been put into practical use. As a type 
of optical disk which permits overwriting, a phase change optical disk, an 
exchange coupled magneto-optical disk, and a magnetic field modulation 
magneto-optical disk are known. Among them, the phase change optical disk 
is adapted to perform recording and erasing by a change in the optical 
properties thereof induced by a thermal history of a rise and a fall of 
temperature caused by irradiation of a laser beam thereon. That is, this 
type of optical disk is adapted to record information or overwrite new 
information on old one, by modulating optical energy irradiated thereon 
between two intensity levels correspondent to two states thereof which can 
be optically discriminated from each other. Hereinafter, of the two energy 
(intensity) levels, the higher level will be referred to as a recording 
power level, while the lower level as an erasing power level. Reproduction 
of information is performed normally by detecting changes in reflectivity 
or transmittance between the two states. 
To improve the recording density of the optical disks, a mark edge 
recording method in which information is imparted to both ends of each 
recording mark is now under study or development. A recording medium used 
in the phase change optical disk is generally higher in absorptivity when 
it is in the amorphous state than when it is in the crystalline state, and 
therefore, in performing overwriting by the mark edge recording method, 
the length or position of each recording mark recorded varies depending on 
whether the recording mark is recorded on the amorphous state or on the 
crystalline state, and this results in degraded overwriting 
characteristics, such as increased recording jitter, and a lower 
erasability resulting from modulation of an overwriting signal, which 
occurs in dependence on the existing or recorded information. Further, 
when taking into consideration the fact that more latent heat is consumed 
in fusion and the thermal conductivity is larger in the crystalline state 
than in the amorphous state, the layer structure of a phase change optical 
disk should be designed such that absorptivity in the crystalline state 
becomes higher than one in the amorphous state. 
As means for providing such a recording medium, an invention proposed by 
Japanese Unexamined Patent Publication (Kokai) No. 1-149238 is known. In 
this invention, absorptivity in the crystalline state is increased by 
reducing the thickness of a reflective layer formed of a metal to make it 
transparent, thereby reducing the reflectivity and absorptivity of the 
reflective layer. However, when the thickness of the metal reflective 
layer is set to a reduced value, optical characteristics of the resulting 
medium largely depends on the layer thickness, and hence there arises a 
problem of a strict or close manufacturing tolerance of thickness of the 
reflective layer. FIG. 1 shows the relationship between the thickness of a 
single Au layer formed on a glass substrate and reflectivity of the layer 
exhibited in reflecting light irradiated on the substrate. The invention 
disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-149238 
obtains desired effects by reducing the thickness of the Au layer as the 
reflective layer to 20 nm, thereby lowering the reflectivity of the 
reflective layer. However, as can be understood from FIG. 1, the resulting 
reflectivity largely depends on the layer thickness, when the layer 
thickness is small, for example 20 nm or less. Therefore, if the layer 
thickness is largely deviated from a specified value, this causes a large 
undesirable change in the reflectivity of the reflective layer, resulting 
in a large amount of deviation from the desired optical properties of the 
recording medium. In short, the manufacturing tolerance of the thickness 
of the reflective layer is so strict or close. Further, when the thickness 
of the reflective layer is small, the cooling rate of the medium becomes 
low, and hence thermal load thereon becomes large, which can cause a 
problem of degraded characteristics occurring with repetition of recording 
and erasing. 
To make the absorptivity in the crystalline state higher than that in the 
amorphous state, there is proposed another method which employs optical 
phase encoding (i.e. optical phase difference reproduction). In the case 
of the phase change optical disk, in general, the difference in 
reflectivity between the amorphous state and the crystalline state is 
detected for reproduction of information, and to increase the signal 
strength, it is required to increase the difference in reflectivity 
between the amorphous state and the crystalline state. Normally, the 
recording medium is more often used after being crystallized. Therefore, 
the reflectivity in the crystalline state is set to a higher value, and 
the absorptivity of the crystalline state is set to a lower value when the 
disk is arranged to include a reflective layer. However, if the method of 
optical phase encoding is used, in which the optical phase difference 
between light reflected from the amorphous state and light reflected from 
the crystalline state is utilized, it is not necessarily required to 
increase the difference in reflectivity to enhance the signal strength, 
which facilitates increasing the absorptivity in the crystalline state. 
Examples of such phase difference reproduction are disclosed in Japanese 
Unexamined Patent Publication (Kokai) No. 2-73537, Japanese Unexamined 
Patent Publication (Kokai) No. 2-113451, Japanese Unexamined Patent 
Publication (Kokai) No. 3-41638, etc. These inventions, however, are 
constructed such that there is little difference in reflectivity between 
the amorphous state and the crystalline state, with a large phase 
difference between reflected lights, and hence, optical characteristics of 
the recording medium largely depend on the layer thickness, which results 
in a small manufacturing tolerance of layers. Further, according to the 
layer arrangement disclosed in preferred embodiments of the known 
publications, reflectivity is set to a value as low as approximately 8% or 
lower, which results in a problem of unstable servo-mechanism. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an optical 
information-recording medium for recording information by utilizing 
changes in optical properties thereof induced by different thermal 
histories of a rise and a fall of temperature caused by irradiation of a 
laser beam thereon, i.e. a phase change optical disk, which has a large 
manufacturing tolerance, excellent overwriting characteristics in mark 
edge recording, and an excellent reliability in repetition of overwriting. 
To attain the above object, the present invention provides an optical 
information-recording medium including a substrate, and a lower protective 
layer, a recording layer, an upper protective layer and a reflective 
layer, formed on the substrate, the recording layer exhibiting different 
optical properties dependent on different thermal histories of a rise and 
a fall in temperature caused by irradiation of a laser beam on the 
substrate, 
wherein the optical information-recording medium has a layer structure 
constructed such that reflectivity of the optical information-recording 
medium and a phase of light reflected therefrom vary with the change in 
the optical properties of the recording layer, with absorptivity of light 
of the recording layer being larger when the recording layer is in a 
crystalline state than when the recording layer is in an amorphous state, 
and at the same time, reflectivity exhibited when the recording layer is 
in the crystalline state being 10% or higher; and 
wherein the reflective layer has a thickness set such that a value of 
reflectivity of the reflective layer exhibited in the form of a single 
layer is 90% or more of a value of reflectivity of a bulk form of a 
substance of the reflective layer, or to a value of 40 nm or larger. 
The optical information-recording medium of the present invention is 
constructed such that the reflectivity of the medium and the phase of 
reflected light change with a change in the optical properties of the 
recording layer, and therefore, it is possible to obtain a high-quality 
signal without increasing the difference in reflectivity between the 
crystalline state and the amorphous state. Therefore, the absorptivity of 
the recording layer exhibited when the recording layer is in the 
crystalline state can be enhanced more easily, which provides excellent 
overwriting characteristics in mark edge recording. Further, the 
reflectivity exhibited in the crystalline state of the recording layer is 
high, and hence the servo-mechanism is stabilized. Further, since it is 
possible to enhance the absorptivity of the recording layer in the 
crystalline state thereof without reducing the thickness of the reflective 
layer, thermal load on the medium is reduced to thereby achieve a high 
reliability in repetition of recording and erasing of information. The 
thickness of the reflective layer is set to such a range as indicated by 
the arrows in FIG. 1, in which the reflectivity of the reflective layer 
exhibits a value of 90% or higher of the reflectivity of a bulk form of 
the substance used for the reflective layer. Therefore, the reflectivity 
of the reflective layer is not easily changed, while permitting a large 
manufacturing tolerance of the reflective layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Next, the invention will be described in detail with reference to drawings. 
Referring first to FIG. 2, there is shown the basic constitution of an 
optical information-recording medium in cross-section. On a transparent 
substrate 1, there are formed a lower protective layer 2, a recording 
layer 3, an upper protective layer 4 and a reflective layer 5. The 
recording layer 3 is formed of a substance which has its optical 
properties changed depending on different thermal histories of a rise and 
a fall of temperature caused by irradiation of a laser beam thereto at 
different energy levels, e.g. a compound containing a chalcogen-series 
atom, for example, Se and Te. The lower protective layer 2 and the upper 
protective layer 4 are layers for protecting the recording layer from 
heat, and at the same time used for optical interference. Therefore, their 
thickness is set to a desired value such that reflectivity of the medium 
as well as phase of reflected light vary with changes in the optical 
properties of the recording layer 3. It is preferred that the lower 
protective layer 2 and the upper protective layer 4 are formed of a single 
substance selected from nitrides, for example, Si.sub.3 N.sub.4 and AlN, 
which are transparent, oxides, for example, SiO, SiO.sub.2 and Ta.sub.2 
O.sub.5, and chalcogenides, for example, ZnS, ZnSe, and MnS, or a mixture 
thereof. The reflective layer 5 may be formed of a single substance 
selected from metals, for example, Al, Au, Ti, Cr, Mo, W and Ta. 
Alternatively, a mixture of such a substance and an additive, or an alloy 
of such a substance may be used for control of the reflectivity and 
thermal diffusivity of the reflective layer 5, and for enhancing adherence 
of the same to adjacent layers. Further, semiconductors, for example, Si 
and Ge, which are high in refractive index, can be used as well. 
The invention will be described in further detail based on Examples. 
EXAMPLE 1 
On a substrate of polycarbonate, there were formed, one upon another, in 
the order mentioned hereafter, a lower protective layer of 
ZnS-20at%SiO.sub.2 with a thickness ranging from 5 nm to 300 nm, a 
recording layer of Ge.sub.2 Sb.sub.2 Te.sub.5 with a thickness of 20 nm, 
an upper protective layer of ZnS-20at%SiO.sub.2 with a thickness of 20 nm, 
and a reflective layer of Al with a thickness of 60 nm. Then, there were 
calculated a value of reflectivity Rc and a value of absorptivity Ac of a 
recording medium thus arranged, which were exhibited when the recording 
layer 3 was in the crystalline state, and a value of reflectivity Ra and a 
value of absorptivity Aa of the same, which were exhibited when the 
recording layer 3 was in the amorphous state, as well as the optical phase 
difference .DELTA..phi. between reflected light from the amorphous state 
and that from the crystalline state. FIG. 3 shows results of the 
calculations. The wavelength of a laser beam was 830 nm. Reflectivity and 
the optical phase difference between the reflected lights can be 
determined from optical constants of the recording layer, the protective 
layers, and the reflective layer, and thicknesses of the layers, by a 
matrix method (refer, for example, to Masao Tsuruta, "Applied Optics II", 
Applied Physics Engineering Selections Vol. 2, Baifu-kan, 1990, Chapter 
4). The optical constants of the lower protective layer and the upper 
protective layer were 2.2-i0.0, and the optical constant of the recording 
layer was 5.89-i3.47 in the crystalline state, and 4.60-i1.06 in the 
amorphous state, and that of the reflective layer was 2.83-i7.75. 
As indicated by arrows in FIG. 3, when the thickness of the lower 
protective layer was within a range of 43 nm to 137 nm, and a range of 232 
nm to 300 nm, absorptivity was higher in the crystalline state than in the 
amorphous state, with reflectivity in the crystalline state being 10% or 
higher. Further, in this range, the optical phase difference between 
reflected light from the crystalline state and reflected light from the 
amorphous state was large, with the absolute value thereof being equal to 
90.degree. or larger. 
EXAMPLE 2 
Specimens of the recording medium were prepared in the same manner as in 
Example 1 with the exception that the thickness of the recording layer was 
set to 15 nm. Results of calculations are shown in FIG. 4. As indicated by 
arrows in FIG. 4, when the thickness of the lower protective layer was 
within a range of 5 nm to 67 nm, a range of 102 nm to 256 nm, and a range 
of 291 nm to 300 nm, the absorptivity was higher in the crystalline state 
than in the amorphous state, with reflectivity in the crystalline state 
being 10% or higher. 
EXAMPLE 3 
Specimens of the recording medium were prepared in the same manner as in 
Example 1 with the exception that the thickness of the recording layer was 
set to 25 nm. Results of calculations are shown in FIG. 5. As indicated by 
arrows in FIG. 5, when the thickness of the lower protective layer was 
within a range of 56 nm to 107 nm and a range of 244 nm to 296 nm, 
absorptivity was higher in the crystalline state than in the amorphous 
state, with reflectivity in the crystalline state being 10% or higher. 
EXAMPLE 4 
Specimens of the recording medium were prepared in the same manner as in 
Example 1 with the exception that the thickness of the recording layer was 
set to 30 nm. Results of calculations are shown in FIG. 6. As indicated by 
arrows in FIG. 6, when the thickness of the lower protective layer was 
within a range of 42 nm to 122 nm and a range of 230 nm to 300 nm, 
absorptivity was higher in the crystalline state than in the amorphous 
state, with reflectivity in the crystalline state being 10% or higher. 
EXAMPLE 5 
Specimens of the recording medium were prepared in the same manner as in 
Example 1 with the exception that the thickness of the recording layer was 
set to 40 nm. Results of calculations are shown in FIG. 7. As indicated by 
arrows in FIG. 7, when the thickness of the lower protective layer was 
within a range of 35 nm to 118 nm and a range of 222 nm to 300 nm, 
absorptivity was higher in the crystalline state than in the amorphous 
state, with reflectivity in the crystalline state being 10% or higher. 
EXAMPLE 6 
On a substrate of polycarbonate with a diameter of 130 nun and a thickness 
of 1.2 mm, which was formed with a V-shaped guide groove, there were 
formed, by a sputtering method, one upon another, in the order mentioned 
hereafter, a lower protective layer of ZnS-20at%SiO.sub.2 with a thickness 
of 100 nm, a recording layer of Ge.sub.2 Sb.sub.2 Te.sub.5 with a 
thickness of 20 nm, an upper protective layer of ZnS-20at%SiO.sub.2 with a 
thickness of 20 nm, and a reflective layer of Al with a thickness of 60 
nm. Further, there was additionally formed thereon a layer of an 
ultraviolet ray-cured resin with a thickness of 9.2 .mu.m to thereby 
obtain a specimen. Reflectivity of light irradiated on the substrate was 
11.6% in the amorphous state, and 13.5% in the crystalline state. Optical 
calculations by the matrix method show that absorptivity was 67.7% in the 
amorphous state, and 80.5% in the crystalline state, and the optical phase 
difference between reflected light from the crystalline and reflected 
light from the amorphous state was -151.8.degree.. 
EXAMPLE 7 
A specimen of the recording medium was prepared in the same manner as in 
Example 6 with the exception that the thickness of the lower protective 
layer of ZnS-20at%SiO.sub.2 was set to 130 nm. Reflectivity of light 
irradiated on the substrate was 9.2% in the amorphous state, and 22.6% in 
the crystalline state. Optical calculations by the matrix method show that 
absorptivity was 69.5% in the amorphous state, and 72.1% in the 
crystalline state, and the optical phase difference between reflected 
light from the crystalline and reflected light from the amorphous state 
was -107.6.degree.. 
COMATIVE EXAMPLE 1 
A specimen of a recording medium as Comparative Example 1 was prepared, 
which had a large difference in reflectivity between the crystalline state 
and the amorphous state, and a small optical phase difference between 
reflected light from the crystalline state and reflected light from the 
amorphous state, and exhibited a higher absorptivity in the amorphous 
state than in the crystalline state, in the same manner as in Example 6 
with the exception that the thickness of a lower protective layer of 
ZnS-20at%SiO.sub.2 was set to 180 nm. Reflectivity of light irradiated on 
the substrate was 4.2% in the amorphous state, and 33.9% in the 
crystalline state. Optical calculations by the matrix method show that 
absorptivity was 73.3% in the amorphous state, and 61.7% in the 
crystalline state, and the optical phase difference between reflected 
light from the crystalline state and reflected light from the amorphous 
state was small, assuming a value of -48.1.degree.. 
Overwriting characteristics of Examples 6 and 7, and Comparative Example 1 
were evaluated. Each of specimens in the form of a disk was rotated at a 
speed of 11.3 m/s. An optical head used in recording and erasing had an 
numerical aperture (NA) of 0.55 for the objective lens, through which a 
laser beam having a wavelength of 830 nm was emitted. The reproducing 
power was set to 1.0 mW. Mark edges were detected using zero cross points 
of a second differential signal. Jitter was detected by measuring a time 
interval from a leading edge of a pulse to that of a subsequent one, and a 
trailing edge of the former to that of the latter, independently of each 
other by the use of a time interval analyzer. The specimens were exposed 
to the laser beam in advance before recording to thereby change the 
recording layer from the amorphous state, in which it had been immediately 
after preparing each laminate of layers, to the crystalline state. Even in 
Example 6, in which reflectivity in the crystalline state was the 
smallest, reflectivity was equal to 13.5%, i.e. a value higher than 10%, 
so that there did not arise a problem of unstable servo-mechanism. FIG. 8 
shows results of measurement of jitter in cases where after a signal of 
3.7 MHz was once recorded, and then a signal of 2.12 MHz was overwritten 
thereon. It is recognized that compared with Comparative Example 1, the 
jitter became smaller in the order of Example 7, and Example 6. This is 
because by virtue of a layer arrangement with a high absorptivity in the 
crystalline state, the rate of temperature rise in the crystalline state 
was made equal to that of temperature rise in the amorphous state, which 
made an overwrite signal less liable to be deformed by a signal recorded 
in advance. Further, it is understood that the difference between the 
leading edge jitter and the trailing edge jitter became smaller also in 
the order of Comparative Example 1, Example 7, and Example 6. The leading 
edge jitter is more readily increased than the trailing edge jitter 
because the temperature of the recording layer at a leading edge rises 
more gently than that at a trailing edge, and hence the leading edge is 
more susceptible to variation in recording conditions. In Example 6 and 
Example 7, however, the rate of temperature rise in the crystalline sate 
and that of temperature rise in the amorphous state were made equal to 
each other to thereby reduce variation in the recording conditions, which 
contributed to reduction of the difference between the leading edge jitter 
and the trailing edge jitter. 
COMATIVE EXAMPLE 2 
A specimen of a disk as disclosed in Japanese Unexamined Patent Publication 
(Kokai) No. 1-149238, was prepared as Comparative Example 2, in which a 
reflective layer of a metal was made thinner to make it transparent, 
whereby the reflectivity and absorptivity of the reflective layer were 
reduced for the purpose of enhancing the absorptivity in the crystalline 
state. On a substrate of polycarbonate, there were formed, by the 
sputtering method, one upon another, in the order mentioned hereafter, a 
lower protective layer of ZnS-20at%SiO.sub.2 with a thickness of 100 nm, a 
recording layer of Ge.sub.2 Sb.sub.2 Te.sub.5 with a thickness of 20 nm, 
an upper protective layer of ZnS-20at%SiO.sub.2 with a thickness of 20 nm, 
and a reflective layer of Au with a thickness of 20 nm. Further, there was 
additionally formed thereon a layer of an ultraviolet ray-cured resin with 
a thickness of 9.2 .mu.m. Reflectivity of light irradiated on the 
substrate was 8.0% in the amorphous state, and 12.8% in the crystalline 
state. Optical calculations by the matrix method show that absorptivity 
was 71.7% in the amorphous state, and 81.7% in the crystalline state, that 
is, absorptivity being higher in the crystalline state, and the optical 
phase difference between reflected light from the crystalline state and 
reflected light from the amorphous state was large, assuming a value of 
-134.2.degree.. 
As to Comparative Example 2, jitter was measured, after a signal of 3.7 MHz 
was recorded and then a signal of 2.12 MHz was overwritten thereon. Very 
excellent results equivalent to those of Example 6 were obtained. 
Then, reliability in repetition of overwriting was measured on Example 6 
and Comparative Example 2. The overwriting characteristics were measured 
under the following conditions: a linear velocity of 11.3 m/s; a recording 
frequency of 3.7 MHz; duty for writing pulse of 45%; and recording power 
of 12 mW and erasing power of 6.5 mW for Example 6, and recording power of 
11 mW and erasing power of 6 mW for Comparative Example 2. Results of 
measurement are shown in FIG. 9. In the case of Example 6, even after 
overwriting performed 10.sup.6 times, jitter was not increased, whereas in 
the case of Comparative Example 2, jitter started to increase from a time 
point at which overwriting was performed approximately 10.sup.4 times. In 
the case of Comparative Example 2, the thickness of the reflective layer 
was as thin as 20 nm, and hence the cooling rate was low, resulting in an 
increased thermal load on the medium, which more readily caused 
degradation of the characteristics of the recording medium when recording 
and erasing were repeatedly carried out. 
EXAMPLE 8 
Similarly to Comparative Example 2, there were prepared specimens of a 
disk, with exception that the thickness of a reflective layer is varied, 
and then reliability in repetition of overwriting was measured. The 
relationship between the thickness of the reflective layer and the amount 
of increase in jitter detected after overwriting was repeatedly performed 
10.sup.6 times is shown in FIG. 10. It is understood that if the 
reflective layer had a thickness of 40 nm or larger, jitter did not 
increase even after overwriting was performed 10.sup.6 times. 
COMATIVE EXAMPLE 3 
A specimen of a disk was prepared as Comparative Example 3, which had such 
a layer arrangement as exhibits very little difference in reflectivity and 
a large optical phase difference between reflected light from the 
crystalline state and reflected light from the amorphous state, as 
disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-73537, 
Japanese Unexamined Patent Publication (Kokai) No. 2-113451, and Japanese 
Unexamined Patent Publication (Kokai) No. 3-41638. On a substrate of 
polycarbonate, there were formed, by the sputtering method, one upon 
another, in the order mentioned hereafter, a lower protective layer of 
ZnS-20at%SiO.sub.2 with a thickness of 250 nm, a recording layer of 
Ge.sub.2 Sb.sub.2 Te.sub.5 with a thickness of 15 nm, an upper protective 
layer of ZnS-20at%SiO.sub.2 with a thickness of 15 nm, and a reflective 
layer of Al with a thickness of 60 nm. Further, there was additionally 
formed thereon a layer of an ultraviolet ray-cured resin with a thickness 
of 9.2 .mu.m. Reflectivity of light irradiated on the substrate was 8.3% 
in the amorphous state, and 8.5% in the crystalline state. Optical 
calculations by the matrix method show that absorptivity was 59.9% in the 
amorphous state, and 81.5% in the crystalline state, that is, absorptivity 
being higher in the crystalline state, and the optical phase difference 
between reflected light from the crystalline state and reflected light 
from the amorphous state was large, assuming a value of 139.1.degree.. 
Jitter characteristics and reliability in repetition of recording and 
erasing of information were tried to be measured. However, since the 
reflectivity was equal to such a small value lower than 10%, the 
servo-mechanism was unstable to make the measurement very difficult. 
EXAMPLE 9 
On a substrate of polycarbonate with a diameter of 130 mm and a thickness 
of 1.2 mm, which was formed with a V-shaped guide groove, there were 
formed, by the sputtering method, one upon another, in the order mentioned 
hereafter, a lower protective layer of Ta.sub.2 O.sub.5 with a thickness 
of 90 nm, a recording layer of Ge.sub.1 Sb.sub.4 Te.sub.7 with a thickness 
of 20 nm, an upper protective layer of Ta.sub.2 O.sub.5 with a thickness 
of 15 nm, and a reflective layer of Al with a thickness of 60 nm. Further, 
there was additionally formed thereon a layer of an ultraviolet ray-cured 
resin with a thickness of 9.2 .mu.m to thereby obtain a specimen. 
Reflectivity of light irradiated on the substrate was 5.8% in the 
amorphous state, and 13.6% in the crystalline state. Optical calculations 
by the matrix method show that absorptivity was 79.8% in the amorphous 
state, and 78.9% in the crystalline state, and the optical phase 
difference between reflected light from the crystalline state and 
reflected light from the amorphous state was -85.5.degree.. Optical 
constants used in the optical calculations were 1.96-i0.0 for Ta.sub.2 
O.sub.5 of the lower protective layer and the upper protective layer, 
4.60-i1.60 for Ge.sub.1 Sb.sub.4 Te.sub.7 of the recording layer in the 
amorphous state, and 5.70-i3.52 for the same in the crystalline state. 
As to the specimen of Example 9, a signal of 3.7 MHz was recorded, and then 
a signal of 2.12 MHz was overwritten thereon. Measurement of jitter was 
performed, obtaining more excellent results than those obtained with 
Comparative Example 1, similarly to Example 3. Jitter did not increase 
even after overwriting was performed 10.sup.6 times. 
As described heretofore, the present invention is very effective in 
reducing overwriting jitter occurring with a phase change optical disk. 
For recording in an even higher density, the adaptation of a light source 
such that it emits a laser beam having a reduced wavelength, is effective, 
since it permits reduction of the diameter of the laser beam. 
EXAMPLE 10 
Therefore, to demonstrate that the present invention is also effective for 
a short wavelength of 690 nm, which is shorter than that used in the above 
examples, specimens of Example 10 were prepared by forming, on a substrate 
of polycarbonate, one upon another in the order mentioned hereafter, a 
lower protective layer of ZnS-20at%SiO.sub.2 with a thickness in a range 
of 100 nm to 300 nm, a recording layer of Ge.sub.2 Sb.sub.2 Te.sub.5 with 
a thickness of 15 nm, an upper protective layer of ZnS-20at%SiO.sub.2 with 
a thickness of 20 nm, and a reflective layer of Al with a thickness of 60 
nm. 
Then, there were calculated a value of reflectivity Rc and a value of 
absorptivity Ac of a recording medium thus arranged and Rc exhibited when 
the recoding layer was in the crystalline state, and a value of 
reflectivity Ra and a value of absorptivity Aa of same when the recording 
layer was in the amorphous state, as well as the optical phase difference 
.DELTA..phi. between reflected light from the amorphous state and that 
from the crystalline state. FIG. 11 shows results of the calculations. The 
wavelength of a laser beam was 690 nm. Optical constants of the lower 
protective layer and the upper protective layer were 2.1-i0.0 for the 
wavelength of 690 nm, and an optical constant of the recording layer 
4.03-i3.87 in the crystalline state, and 3.79-i1.36 in the amorphous 
state, and an optical constant of the reflective layer 1.73-i7.96. As 
indicated by arrows in FIG. 11, when the thickness of the lower protective 
layer was within a range of 210 nm to 277 nm, absorptivity was higher in 
the crystalline state than in the amorphous state, with reflectivity in 
the crystalline state being 10% or higher. 
EXAMPLE 11 
Specimens of the recording medium were prepared in the same manner as in 
Example 10 with the exception that the thickness of the upper protective 
layer was set to 30 nm. Results of calculations are shown in FIG. 12. As 
indicated by arrows in FIG. 12, when the thickness of the lower protective 
layer was within a range of 210 nm to 265 nm, absorptivity was higher in 
the crystalline state than in the amorphous state, with reflectivity in 
the crystalline state being 10% or higher. Further, in this range, the 
optical phase difference between reflected light from the crystalline 
state and reflected light from the amorphous state is large, assuming a 
value of 90.degree.. 
EXAMPLE 12 
On a substrate of polycarbonate with a diameter of 130 mm and a thickness 
of 1.2 mm, which was formed with a V-shaped guide groove, there were 
formed, by the sputtering method, one upon another, in the order mentioned 
hereafter, a lower protective layer of ZnS-20at%SiO.sub.2 with a thickness 
of 230 nm, a recording layer of Ge.sub.2 Sb.sub.2 Te.sub.5 with a 
thickness of 15 nm, an upper protective layer of ZnS-20at%SiO.sub.2 with a 
thickness of 20 nm, and a reflective layer of Al with a thickness of 60 
nm. Further, there was additionally formed thereon a layer of an 
ultraviolet ray-cured resin with a thickness of 9.2 .mu.m to thereby 
obtain a specimen. Reflectivity of light irradiated on the substrate was 
13.6% in the amorphous state, and 14.9% in the crystalline state. Optical 
calculations by the matrix method show that absorptivity was 74.2% in the 
amorphous state, and 81.0% in the crystalline state, and the optical phase 
difference between reflected light from the crystalline state and 
reflected light from the amorphous state was -105.2.degree.. 
COMATIVE EXAMPLE 4 
A specimen of a recording medium as Comparative Example 4 was prepared, 
which had a large difference in reflectivity, and a small optical phase 
difference between reflected light from the crystalline state and 
reflected light from the amorphous state, and exhibited a higher 
absorptivity in the amorphous state than in the crystalline state, in the 
same manner as in Example 12 with the exception that the thickness of the 
lower protective layer of ZnS-20at%SiO.sub.2 was set to 160 nm. 
Reflectivity of light irradiated on the substrate was 11.9% in the 
amorphous state, and 32.5% in the crystalline state. Optical calculations 
by the matrix method show that absorptivity was 75.6% in the amorphous 
state, and 64.3% in the crystalline state, and the optical phase 
difference between reflected light from the crystalline state and 
reflected light from the amorphous state was small, assuming a value of 
-59.0.degree.. 
Overwriting characteristics of Example 12 and Comparative Example 4 were 
evaluated. Each of specimens in the form of a disk was rotated at a speed 
of 9.42 m/s. An optical head used in recording and erasing had an 
numerical aperture (NA) for the objective lens of 0.55, through which a 
laser beam having a wavelength of 690 nm was emitted. The reproducing 
power was set to 1.0 mW. The specimens were exposed to the laser beam in 
advance before recording to thereby change the recording layer from the 
amorphous state, in which it had been immediately after preparation of the 
laminate of layers, to the crystalline state. Even in Example 12, in which 
reflectivity in the crystalline state was relatively small, the 
reflectivity was equal to 14.9%, which is a value higher than 10%, so that 
there did not arise a problem of unstable servo-mechanism. Jitter was 
measured, after a signal of 2.90 MHz was once recorded and then a signal 
of 1.81 MHz was overwritten thereon. In Example 12, leading edge jitter 
detected for first recording was 1.3 ns, and trailing edge jitter detected 
for the same was 1.2 ns, whereas leading edge jitter detected for 
overwriting was 2.0 ns and trailing edge jitter for the same was 1.90 ns, 
showing that amounts of increase in jitter caused by overwriting were 
small. Further, in spite of the small difference in reflectivity, there 
was obtained an excellent carrier-to-noise (C/N) ratio of 60.3 dB. On the 
other hand, with Comparative Example 4, leading edge jitter detected for 
first recording was 1.8 ns, and trailing edge jitter for the same was 1.6 
ns, showing that they were small enough. However, leading edge jitter 
detected for overwriting was 3.8 ns, and trailing edge jitter for the same 
was 2.6 ns, proving that amounts of increase in jitter caused by 
overwriting were large. The carrier-to-noise (C/N) ratio was excellent, 
assuming a value of 61.5 dB. As described above, it is confirmed that the 
present invention is also effective for a laser beam having a wavelength 
of 690 nm. 
The optical information-recording medium of the present invention is 
constructed such that reflectivity and phase of reflected light change 
with a change in the optical properties of a recording layer, which 
enables a highquality signal to be obtained without increasing the 
difference in reflectivity between the crystalline state and the amorphous 
state. Therefore, the absorptivity of the recording layer exhibited when 
the recording layer is in the crystalline state can be enhanced more 
easily, which provides excellent overwriting characteristics in mark edge 
recording. Further, since reflectivity exhibited in the crystalline state 
of the recording layer is high, the servo-mechanism can be stabilized. 
Further, since it is possible to enhance the absorptivity of the recording 
layer exhibited in the crystalline state thereof without reducing the 
thickness of the reflective layer, thermal load on the medium is reduced 
to realize a high reliability in repetition of recording and erasing. The 
thickness of the reflective layer is set to such a range as indicated by 
the arrows in FIG. 1, in which the reflectivity of the reflective layer is 
90% or higher of the reflectivity of a bulk form of the substance used for 
the reflective layer. Therefore, the reflectivity of the reflective layer 
is not readily varied, permitting a large manufacturing tolerance of the 
reflective layer. 
As describe heretofore, according to the optical information-recording 
medium of the present invention, in forming a phase change optical disk, 
the manufacturing tolerance of layers in respect of thickness thereof 
becomes large, and at the same time excellent overwriting characteristics 
in mark edge recording can be obtained, with a high reliability in 
repetition of recording and erasing. In short, it is possible to obtain a 
highly reliable phase change optical disk with a larger storage capacity, 
which is comprised of layers which can easily be formed.