Method and apparatus for preparing crystalline thin-films for solid-state lasers

The present invention has been achieved by perceiving the fact to the effect that a semiconductor production process-like manner such as CVD method or the like by which materials and film thickness can be controlled in an atomic scale may be utilized in case of preparing thin-film crystal, and employing such semiconductor production process-like manner being quite different from conventional technique. The invention relates to preparation of crystalline thin-films for solid-state lasers wherein a substrate contained in a vessel under a high vacuum condition is heated, materials for forming the laser material are supplied onto the surface of the aforesaid substrate in the form of gas, ion, single metal or metal compound to grow crystal on the surface of the aforesaid substrate, and a material of active ionic species is supplied onto the surface of the aforesaid substrate simultaneously with supply of the aforesaid materials for forming the laser host crystal, thereby controlling valence number of the material of active ionic species so as to be identical with the valence number of the metal ion constituting the crystal of the aforesaid laser host crystal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and an apparatus for preparing 
crystalline thin-films for solid-state lasers, and more particularly to a 
method and an apparatus for preparing crystalline thin-films for 
solid-state lasers by which such crystalline thin-films for solid-state 
lasers which can be used for the development of micro-cavity and the like, 
and allowed solid-state laser to directly grow on a semiconductor laser 
can be prepared. 
2. Description of the Related Art 
Recently, in order to respond a variety of needs for shorter laser 
wavelength, expansion in wavelength band to be oscillated, higher output 
power in laser, higher efficiency in laser and higher quality in beam and 
so on, extensive studies have been made with respect to research for novel 
solid-state laser materials, and higher quality in the solid-state laser 
crystals than have been heretofore known. 
Heretofore, solid-state laser crystals have been prepared by the flux 
method, crystal pulling method or Verneuil's method and the like method. 
These methods will be described in brief hereinbelow. 
Flux Method 
In this method, crystal is deposited and grown from a molten material at a 
high temperature, and a molten inorganic salt or an oxide functions as its 
solvent. 
Crystal Pulling Method 
In this method, a seed crystal is dipped in molten liquid or saturated 
solution to be compatible with each other, and then the seed crystal is 
gradually pulled up to grow a single crystal on the extreme end portion of 
the seed crystal. 
Verneuil's method 
In this method, a raw material which has been finely pulverized is blown 
into high temperature flame such as oxyhydrogen flame and the like to heat 
and melt the raw material, and the molten material is received by a seed 
crystal so as to grow a single crystal thereon. 
As described above, in any of conventional methods, first a material 
intended to be a solid-state laser crystal is molten at high temperatures, 
and then either a crystal is deposited and grown, or a single crystal is 
allowed to grow on the seed crystal. 
While the above described flux method, crystal pulling method or Verneuil's 
method and the like are very effective methods for preparing a large bulk 
crystal, it is difficult to achieve uniform dispersion of active ionic 
species intented to be the emission subject of a laser beam, as a result 
of spatial control in laser host crystal, and prevention of inclusion of 
ionic species intended to be the absorption subject of laser beam into 
laser host crystal is difficult. 
More specifically, since solid-state laser crystal is allowed to grow in a 
rod-like shape in the prior art, temperatures are different in the central 
portion of the rod-like solid-state laser crystal as compared to from the 
peripheral portion thereof, and as a result diffusion conditions of ion in 
both the portions are different. Accordingly, it is difficult to effect 
uniform doping of the active ionic species intended to be the emission 
subject along the axial and diametrical directions of solid-state laser 
crystal. For this reason, uniform dispersion of active ionic species in 
the laser host crystal could not have been attained heretofore. 
Furthermore, since it is difficult to control valence number of the ion to 
be doped as active ion species, there have been problems where in the ion 
which had been doped did not become the emission subject of laser beam, 
but rather became the ion species which is absorption subject of laser 
beam. For example, in such a case where Al.sub.2 O.sub.3 is used as laser 
host crystal, and doping is effected by using Ti as active ionic species, 
when Ti is trivalent, it becomes the emission subject, but when Ti is di- 
or tetravalent, it becomes the absorption subject. As described above, it 
was extremely difficult to control such valence number in the prior art. 
Moreover, as described above, while in the prior art is preferable to 
prepare a large bulk crystal, it is difficult to prepare thin-film 
crystal. Accordingly, it was difficult to control film thickness on an 
atomic scale. 
In the above described prior art, a solid-state laser crystal is prepared 
by the manner quite different from the conventional semiconductor 
production process. Thus, it was impossible to grow a solid-state laser 
crystal on a semiconductor substrate in a high-vacuum vessel containing 
the semiconductor substrate which is used for semiconductor production 
process. For this reason, solid-state laser crystal heretofore could not 
be produced in combination with semiconductor and semiconductor laser. 
OBJECTS AND SUMMARY OF THE INVENTION 
In view of various problems involved in the prior art, the present 
invention has been made by perceiving the fact to the effect that a 
semiconductor production process-like manner such as CVD (Chemical Vapor 
Deposition) method, wherein a material in gaseous form is used, and the 
gas is thermally decomposed on a substrate, whereby the material is 
deposited on the substrate, by which the material and the film thickness 
can be controlled on an atomic scale, or the like method can be used in 
preparation of thin-film crystal. An object of the present invention is to 
provide a method and an apparatus for preparing crystalline thin-films for 
solid-state lasers in which a semiconductor production process-like 
manner, quite different from the prior art, is utilized and by which the 
above described problems involved in the prior art can be overcome. 
More specifically, an object of the present invention is to provide a 
method and an apparatus for preparing crystalline thin-films for 
solid-state lasers by which an active ionic species can be uniformly 
distributed into its laser host crystal. 
Furthermore, another object of the present invention is to provide a method 
and an apparatus for preparing crystalline thin-films for solid-state 
lasers by which the valence number of the active ionic species can be 
controlled. 
Still further, another object of the present invention is to provide a 
method and an apparatus for preparing crystalline thin-films for 
solid-state lasers by which film thickness can be controlled on an atomic 
scale. 
Yet further, another object of the present invention is to provide a method 
and an apparatus for preparing crystalline thin-films for solid-state 
lasers which can produce solid-state laser crystals in combination with 
semiconductor or semiconductor laser. 
It is to be noted that the semiconductor production process-like manner 
involves the above described CVD method used for growth of gallium 
arsenide and the like, MBE (Molecular Beam Epitaxy) method, ALE (Atomic 
Layer Epitaxy) method, Ablation Deposition Method and the like, and such 
manners wherein crystal is allowed to grow on a heated substrate as a 
result of supplying a material gas, or gaseous or beam-like molecule or 
ion onto the substrate are named generically as "semiconductor production 
process-like manner". 
In order to attain the above described objects, the method for preparing 
crystalline thin-films for solid-state lasers according to the present 
invention is characterized by the steps of heating a substrate in a vessel 
under a high vacuum condition, supplying materials for forming the laser 
material onto the surface of said substrate in the form of a gas, an ion, 
and a single metal or a metal compound to prow crystal on the surface of 
said substrate, and supplying a material of active ionic species onto the 
surface of said substrate simultaneously with supply of said materials for 
forming the laser host crystal, thereby controlling the valence number of 
the material of active ionic species so as to be identical with the 
valence number of the metal ion constituting the crystal of said laser 
host crystal. 
Furthermore, the apparatus for preparing crystalline thin-films for 
solid-state lasers according to the present invention is characterized by 
being provided with a vessel, the inside of which has been highly 
evacuated and for containing a substrate onto the surface of which is 
subjected crystal growth of crystalline thin-films for solid-state lasers, 
a heating device for heating said substrate, a laser host crystal 
supplying device for supplying materials for forming the laser host 
crystal onto the surface of said substrate contained inside said vessel in 
the form of a gas, an ion, a single metal or a metal compound, and a 
material of active ionic species supplying device for supplying a material 
of active ionic species onto the surface of said substrate contained 
inside said vessel simultaneously with supply of said materials for 
forming the laser host crystal. 
According to the above described invention, a substrate is heated in a 
vessel under a high vacuum condition, and materials for forming the laser 
host crystal are supplied in the form of a gas, an ion, a single metal or 
a metal compound onto the surface of the aforesaid substrate to grow a 
crystal thereon. Accordingly, solid-state laser crystal can be grown on 
even a semiconductor substrate such as Si and the like, other than seed 
crystal as in the case of a semiconductor production process-like manner. 
Moreover, since a material of active ionic species is supplied onto the 
surface of a substrate simultaneously with supply of materials for forming 
the laser host crystal, it is possible to control the valence number of 
the material of active ionic species so as to be identical with valence 
number of the metal ion constituting crystal of the laser host crystal. 
Thus, few defective solid-state laser crystals, in which no absorption 
subject or a controlled, small amount of absorption subject exists can be 
prepared. 
In addition to the above, it is easy to control film thickness on an atomic 
scale, and a material of active ionic species can uniformly be injected 
from a spatial point of view.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the method and apparatus for crystalline thin-films for 
solid-state lasers according to the present invention will be described in 
detail hereinbelow by referring to the accompanying drawings. FIG. 1 
illustrates an example of the apparatus for crystal-line thin-films for 
solid-state lasers according to the present invention and which comprises 
a cylindrical vessel 10 made of stainless steel, a connecting tube 12 
which is connected with the vessel 10 and for supplying a metal material 
for forming a laser host crystal into the vessel 10, a connecting tube 14 
which is connected with the vessel 10 and for supplying an oxidizing agent 
or halogenating agent for forming the laser host crystal into the vessel 
10, a connecting tube 16 which is connected with the vessel 10 and for 
supplying a material of active ionic species into the vessel 10, an 
electromagnetic valve means 18 for opening and closing the connecting tube 
12, an electromagnetic valve means 20 for opening and closing the 
connecting tube 14, an electromagnetic valve means 22 for opening and 
closing the connecting tube 16, a quartz rod 24 disposed vertically in the 
vessel 10 in such that one end 24a of which is positioned outside the 
vessel 10, and a halogen lamp 26 for emitting infrared rays into the end 
24a to heat a substrate 100 rest on the other end 24b of the quartz rod 
24. Furthermore, the interior of the vessel 10 is in high vacuum condition 
as a result of evacuation of an order of 10.sup.-7 Torr by means of a 
turbo-molecular pump (not shown). 
In the above described construction, such a case where titanium-sapphire 
(laser host crystal: Al.sub.2 O.sub.3, active ionic species: Ti) thin 
films are prepared as crystalline thin-films for solid-state lasers on the 
substrate 100 being Si (100) substrate by the use of a material gas 
alternately supplying type CVD method as a semiconductor production 
process-like manner will be described. 
It is to be noted that a material such as Si (100), .alpha.-Al.sub.2 
O.sub.3 or the like lattice constant of which matches with that of the 
laser host crystal may be used as the substrate 100. For example, in the 
case where .alpha.-Al.sub.2 O.sub.3 is used as the substrate 100, there is 
no trouble because .alpha.-Al.sub.2 O.sub.3 is the same material as that 
of the laser host crystal. While Si (100) is a material being different 
from that of the laser host crystal, both the lattice constants of these 
materials match with each other so that Si (100) can be used as the 
substrate 100. 
In this connection, since the present embodiment is one in which Al.sub.2 
O.sub.3 is formed as the laser host crystal and Ti is used as the active 
ionic species, it is to be arranged such that to the vessel 10 are 
delivered trimethylaluminum (TMA) as a metallic material for forming the 
laser host crystal through the connecting tube 12 by opening and closing 
the electromagnetic valve 18 means, N.sub.2 O as an oxidizing agent for 
forming the laser host crystal through the connecting tube 14 by opening 
and closing the electromagnetic valve means 20, and Ti gas as the material 
of active ionic species through the connecting tube 16 by opening and 
closing the electromagnetic valve means 22, respectively. 
In this case, TMA, N.sub.2 O and Ti gas are supplied into the vessel 10 as 
gas pulses in accordance with the following manner. Namely, TMA and 
N.sub.2 O material gases are alternately jetted in every two seconds onto 
the substrate 100 placed on the vessel 10 for a gas jetting period of one 
second. More specifically, the TMA gas (or N.sub.2 O gas) is supplied to 
the interior of the vessel 10 for one second to raise pressure of the TMA 
gas (or N.sub.2 O gas), thereafter supply of the TMA gas and N.sub.2 O gas 
is stopped for a period of two seconds, and the interior of the vessel 10 
is evacuated by a turbo-molecular pump. Then, N.sub.2 O gas (or TMA gas) 
is supplied for one second to elevate pressure of N.sub.2 O gas (or TMA), 
thereafter supply of the TMA gas and N.sub.2 O gas is stopped for a period 
of two seconds, and the interior of the vessel 10 is evacuated by the 
turbo-molecular pump. The treatment described herein is utilized as one 
cycle, and a prescribed number of times of this cycle is repeated. In this 
connection, one cycle requires six seconds. Furthermore, Ti gas being the 
active ionic species is supplied simultaneously with the TMA gas. 
In addition, the substrate 100 is placed on the end 24b of the quartz rod 
24, and is heated from the back thereof with infrared rays irradiated from 
the halogen lamp 26. 
In this case, a base pressure is on an order of 1.times.10.sup.-7 Torr and 
the pressure in the vessel 10 at the time of introducing the gas was set 
at 2.times.10.sup.-4 Torr. 
FIG. 3 is a graphical representation indicating dependency in number of gas 
supplying cycle for growth of Al.sub.2 O.sub.3 film on Si (100) as the 
substrate 100 thickness (a thickness of the thin film: nm) is plotted as 
ordinate and number of cycles as abscissa. As described above, both the 
pressures of TMA and N.sub.2 O in the vessel 10 were set at 
2.times.10.sup.-4 Torr (pressure ratio is "TMA/N.sub.2 O=1"), and the 
temperature of substrate was 360.degree. C. (T.sub.sub =360.degree. C.). 
It is understood from FIG. 3 that the Al.sub.2 O.sub.3 film thickness 
grows linearly at a rate of about 0.4 nm/cycle. 
More specifically, a thickness of the Al.sub.2 O.sub.3 film was 
proportional to the number of gas supplying cycle, and the growth rate was 
0.4 nm/cycle. Furthermore, it was also found that as a result of XPS 
(X-ray photoelectron spectroscopy), a composition of the thin film had 
been stoichiometrically 2:3. 
Next, both the pressures of TMA and N.sub.2 O in the vessel 10 were set at 
2.times.10.sup.-4 Torr as in the case described above, respectively, and a 
temperature of the substrate (T.sub.sub) was changed. The results in this 
case were measured by a spectroscopic ellipsometer with respect to the 
thin films and refractive indices. 
FIG. 4 is a graphical representation indicating a relationship between 
growth rate of the thin film and temperature changes in the substrate in 
which the growth rate (speed of growth: nm/cycles) obtained by dividing 
film thickness with number of cycle is plotted as ordinate and changes in 
temperatures (.degree.C.) of the substrate (T.sub.sub) as abscissa. 
Furthermore, FIG. 5 is a graphical representation indicating a 
relationship between refractive indices of the thin film and changes in 
temperatures of substrate in which refractive indices (n) at 632.8 nm are 
plotted as ordinate and changes in temperature (.degree.C.) of the 
substrate (T.sub.sub) as abscissa. 
It is found from FIG. 4 that the growth rate is as low as 0.02 nm/cycle 
before the temperature reaches 320.degree. C. In this case, the refractive 
indices were 1.6-1.7 which correspond to equal to or higher than the 
refractive index of amorphous Al.sub.2 O.sub.3. At the higher substrate 
temperature of 360.degree. C., the growth rate was 0.4 nm/cycle and the 
refractive index was 1.77, respectively. Accordingly, the optimum thin 
film growth rate is when the substrate temperature is at least 320.degree. 
C. Moreover, timing at the doping of an active ionic species synchronizes 
with timing of the supply of TMA as shown in FIG. 2. According to this 
arrangement, trivalent A1 is replaced by Ti so that the Ti to be doped may 
be made only trivalent or substantially trivalent. 
FIGS. 6(a) through 8, inclusive, illustrate each example of possible 
combinations of materials of laser host crystal, materials of active ionic 
species, and materials of substrate wherein FIG. 6 shows a case of forming 
Al.sub.2 O.sub.3 as the laser host crystal, FIG. 7 shows a case of forming 
Y.sub.3 Al.sub.5 O.sub.12 as the laser host crystal, and FIG. 8 shows a 
case of forming LiYF.sub.4 as the laser host crystal, respectively. 
In addition, FIG. 9 illustrates another embodiment of the apparatus for 
preparing crystalline thin-films for solid-state lasers according to the 
present invention wherein the corresponding component parts to those of 
FIG. 1 are represented by the same reference characters as those in FIG. 
1, and explanation for the detailed construction and function thereof will 
be omitted. In the case where a single metal or a metal compound being the 
metallic material for forming laser host crystal as well as a material of 
active ionic species are in solid state at normal temperature, the 
apparatus for preparing crystalline thin-films for solid-state lasers as 
shown in FIG. 9 may be used for the sake of effecting evaporation by 
heating, or transpiration by means of laser, electron beam, and ion beam. 
In the apparatus for preparing crystalline thin-films for solid-state 
lasers shown in FIG. 9, the vessel 10 is provided with a window section 30 
for introducing laser, whereby it is possible to irradiate laser into the 
vessel 10 from the outside. Furthermore, the apparatus is constructed in 
such that a holder 23 for supporting a single metal or a metal compound 
being the metallic material which forms a solid-state laser material at 
normal temperature as well as a material of active ionic species is 
located at a position to which is irradiated laser in the vessel 10, and 
this holder 32 is rotatably driven by a motor through a magnetic coupling 
34. 
In the above construction, the apparatus is arranged in accordance with the 
similar manner as that of the embodiment shown in FIG. 1 in such that into 
the vessel 10 is delivered TMA as a metallic material for forming the 
laser host crystal through a connecting tube 12 by opening and closing an 
electromagnetic valve means 18, and N.sub.2 O as an oxidizing agent for 
forming the laser host crystal through the connecting tube 14 by opening 
and closing the electromagnetic valve means 20, besides Ti is attached to 
the holder 32, and the laser introduced from the window section 30 for 
introducing laser is irradiated to the Ti to heat the same while driving 
rotatively the holder 32 by means of the motor 36 through the magnetic 
coupling 34. Thus, the Ti attached to the holder 32 is transpired so that 
titanium-sapphire (laser host crystal: Al.sub.2 O.sub.3, active ionic 
species: Ti) thin films can be formed on the substrate 100. 
Moreover, in also the case where a solid material is used as one for 
forming the laser host crystal, it may be arranged in such that a similar 
apparatus as in the above described case of Ti is further provided, and 
laser irradiation is effected to ablate the solid material. 
Although not specifically illustrated, electron beam or ion beam may be 
irradiated in place of the laser in the embodiment shown in FIG. 9, and in 
this case a source for electron beam or ion beam may be disposed directly 
on the interior of the vessel 10. 
While the metallic material for forming laser host crystal and the 
oxidizing agent or halogenating agent have been alternately supplied in 
the above described respective embodiments, the invention is not limited 
thereto, but the metallic material for forming the laser host crystal may 
be supplied simultaneously with the oxidizing agent or halogenating agent 
without supplying them alternately. 
Furthermore, heating for the substrate 100 is not limited to heating by 
means of infrared rays, but resistance heating may be used and in this 
case, a heating coil may be formed on the substrate 100 to heat 
electrically the same. 
Moreover, it may be arranged in such that a laser irradiates the substrate 
100 from the obliquely upper direction in the vessel 10 to heat the 
substrate 100. 
Since the present invention has been constructed as described above, the 
following advantages are obtained. 
The method for preparing crystalline thin-films for solid-state lasers 
according to the present invention is characterized by the steps of 
heating a substrate in a vessel under a high vacuum condition, supplying 
materials for forming the laser material onto the surface of said 
substrate in the form of a gas, an ion, a singled metal or a metal 
compound to grow crystal on the surface of said substrate, and supplying a 
material of active ionic species onto the surface of said substrate 
simultaneously with supply of said materials for forming the laser host 
crystal, thereby controlling the valence number of the material of active 
ionic species so as to be identical with the valence number of the metal 
ion constituting the crystal of said laser host crystal in the laser 
crystal. Furthermore, the apparatus for preparing crystalline thin-films 
for solid-state lasers according to the present invention is characterized 
by being provided with a vessel the inside of which has been highly 
evacuated and for containing a substrate onto the surface of which is 
subjected crystal growth of crystalline thin-films for solid-state lasers, 
a heating device for heating said substrate, a laser host crystal 
supplying device for supplying materials for forming the laser host 
crystal onto the surface of said substrate contained inside said vessel in 
the form of a gas, an ion, a single metal or a metal compound, and a 
material of active ionic species supplying device for supplying a material 
of active ionic species onto the surface of said substrate contained 
inside said vessel simultaneously with supply of said materials for 
forming the laser host crystal. Because of the above described 
construction of the present invention, it is possible that a substrate is 
heated in a vessel under a high vacuum condition, materials for forming 
the laser host crystal are supplied onto the surface of the substrate in 
the form of a gas, an ion, a single metal or a metal compound to effect 
crystal growth on the surface of the substrate. Thus, it becomes possible 
to grow solid-state laser crystal on a semiconductor substrate made of Si 
and the like other than seed crystal as in the case of semiconductor 
production process-like manner. 
Furthermore, since a material of active ionic species is supplied onto the 
surface of the substrate simultaneously with supply of the materials for 
forming the laser host crystal, the valence number of the material of 
active ionic species can be controlled so as to be identical with valence 
number of the metal ion constituting crystal of the laser host crystal, 
whereby a solid-state laser crystal having few defects in which no 
absorption subject or a controlled small amount of absorption subject 
exists can be prepared. 
Still further, it is easy to control a film thickness on the atomic level, 
and a material of active ionic species can also be uniformly injected from 
a spatial point of view. 
It will be appreciated by those of ordinary skill in the art that the 
present invention can be embodied in other specific forms without 
departing from the spirit or essential characteristics thereof. 
The presently disclosed embodiments are therefore considered in all 
respects to be illustrative and not restrictive. The scope of the 
invention is indicated by the appended claims rather than the foregoing 
description, and all changes that come within the meaning and range of 
equivalents thereof are intended to be embraced therein.