Method for growing B-Ga2O3-based single crystal

A method for growing a β-Ga2O3-based single crystal, can provide a plate-shaped β-Ga2O3-based single crystal having high crystal quality. In one embodiment, a method for growing a β-Ga2O3-based single crystal employing an EFG method is provided, the method including: bringing a plate-shaped seed crystal into contact with a Ga2O3-based melt, wherein the plate-shaped seed crystal includes a β-Ga2O3-based single crystal having a defect density of not more than 5×105 /cm2 in the whole region; and pulling up the seed crystal to grow a β-Ga2O3-based single crystal.

TECHNICAL FIELD

The invention relates to a method for growing a β-Ga2O3-based single crystal.

BACKGROUND ART

A method for growing a Ga2O3single crystal by EFG method is known (see, e.g., PTL 1). According to a method disclosed by PTL 1, wherein the method features growing a Ga2O3single crystal while gradually expanding a width thereof downward from a contact portion with a seed crystal, namely while expanding a shoulder thereof, a plate-shaped crystal having a width larger than the seed crystal can be obtained.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In the method disclosed by PTL 1, however, a problem may occur that in the process of expanding the shoulder, the Ga2O3single crystal is likely to be twinned. In addition, if a plate-shaped seed crystal having a wide width is used so as to omit the expanding process of the shoulder, a problem is more likely to occur that the Ga2O3crystal grown is subject to a partial polycrystallization or deterioration in the crystal quality.

It is an object of the invention to provide a method for growing a β-Ga2O3-based single crystal that can provide a plate-shaped β-Ga2O3-based single crystal having a high crystal quality.

Solution to Problem

According to one embodiment of the invention, a method for growing a β-Ga2O3-based single crystal set forth in [1] to [4] below is provided.[1] A method for growing a β-Ga2O3-based single crystal employing an EFG method comprises:a step of bringing a plate-shaped seed crystal into contact with a Ga2O3-based melt, the plate-shaped seed crystal comprising a β-Ga2O3-based single crystal having a defect density of not more than 5×105/cm2in a whole region thereof; anda step of pulling up the seed crystal to grow the β-Ga2O3-based single crystal.[2] The method for growing a β-Ga2O3-based single crystal according to [1], wherein the seed crystal is pulled up in a b axis direction thereof without carrying out a shoulder expansion in a width direction thereof to grow the β-Ga2O3-based single crystal.[3] The method for growing a β-Ga2O3-based single crystal according to [1] or [2], wherein the β-Ga2O3-based single crystal comprises a plate-shaped single crystal having a (101) plane or a (−201) plane as a principal surface.[4] The method for growing a β-Ga2O3-based single crystal according to [1] or [2], wherein the seed crystal has a width more than a width in a longitudinal direction of an opening part of a slit of a die configured to raise the Ga2O3-based melt in a crucible up to a position wherein the Ga2O3-based melt is brought into contact with the seed crystal.

Advantageous Effects of Invention

According to the invention, a method for growing a β-Ga2O3-based single crystal can be provided that can provide a plate-shaped β-Ga2O3-based single crystal having a high crystal quality.

DESCRIPTION OF EMBODIMENTS

FIG. 1is a vertical cross sectional view showing a part of an EFG crystal manufacturing device according to the embodiment. The EFG crystal manufacturing device10includes a crucible13configured to receive a Ga2O3-based melt12, a die14disposed in the crucible13, the die having a slit14A, a lid15configured to close the upper surface of the crucible13except for an opening part14B of the slit14A, a seed crystal holding tool21configured to hold a β-Ga2O3-based seed crystal (hereinafter, referred to as “seed crystal”)20, and a shaft22configured to liftably support the seed crystal holding tool21.

The crucible13is configured to accommodate the Ga2O3-based melt12obtained by melting β-GaO3-based powder. The crucible13is comprised of a metal material such as iridium that has heat resistance so as to be able to accommodate the Ga2O3-based melt12.

The die14has the slit14A configured to raise the Ga2O3-based melt12by capillarity phenomenon.

The lid15is configured to prevent the Ga2O3-based melt12having a high temperature from evaporating from the crucible13, in addition, to prevent the vapor of the Ga2O3-based melt12from adhering to parts except for the upper surface of the slit14A.

By lowering the seed crystal20so as to be brought into contact with the Ga2O3-based melt12that ascends through the slit14A of the die14to the opening part14B by capillarity phenomenon, and pulling up the seed crystal20brought into contact with the Ga2O3-based melt12, a plate-shaped β-Ga2O3-based single crystal25is grown. The crystal orientation of the β-Ga2O3-based single crystal25is equal to the crystal orientation of the seed crystal20, in order to control the crystal orientation of the β-Ga2O3-based single crystal25, for example, the plane orientation of the bottom surface of the seed crystal20and the angle thereof in the horizontal plane are adjusted.

FIG. 2is a perspective view showing a state of the β-Ga2O3-based single crystal during growth. A surface26inFIG. 2is a principal surface of the β-Ga2O3-based single crystal25parallel to the slit direction of the slit14A. In case of forming β-Ga2O3-based substrate by cutting out the β-Ga2O3-based single crystal25grown, the plane orientation of the surface26of the β-Ga2O3-based single crystal25is made coincident with the plane orientation of desired principal surface of the β-Ga2O3-based substrate. For example, in case of forming a β-Ga2O3-based substrate having a (101) plane as a principal surface, the plane orientation of the surface26is configured to be (101).

The β-Ga2O3-based single crystal25and the seed crystal20are formed of a β-Ga2O3single crystal or a β-Ga2O3single crystal with an element such as Cu, Ag, Zn, Cd, Al, In, Si, Ge and Sn added thereto. The β-Ga2O3crystal has a β-gallia structure that belongs to monoclinic system, the lattice constant is typically a0=12.23 angstroms, b0=3.04 angstroms, c0=5.80 angstroms, α=γ=90 degrees, β=103.8 degrees.

The defect density in the whole region of the β-Ga2O3-based single crystal constituting the seed crystal20is not more than 5×105/cm2. By satisfying this condition, even if the seed crystal20is a plate shaped crystal having a wide width, partial polycrystallization and deterioration in crystal quality of the Ga2O3crystal to be grown can be prevented.

The seed crystal20is obtained, for example, by carrying out an evaluation of the defect density for a plurality of the β-Ga2O3-based single crystals grown so as to select single crystals in which the defect density in the whole region is not more than 5×105/cm2. One example of a method for evaluating the defect density is shown below.

First, a part of the β-Ga2O3-based single crystal is cut perpendicularly to the growing direction so as to cut out thin plate-shaped single crystals. Next, a surface of the thin plate-shaped single crystal perpendicular to the growing direction is mirror-polished so as to be subjected to a chemical etching with hot phosphoric acid or the like. At this time, since the etching rate is increased in the defect parts, depressions (etch pits) are generated. By counting the number per unit area of the etch pit, the defect density is examined Further, the defects propagate in the growing direction of the β-Ga2O3-based single crystal, thus by examining the defect density of one thin plate-shaped single crystal, the defect density of the whole of the β-Ga2O3-based single crystal can be examined.

In addition, by using the plate-shaped seed crystal20having a wide width, as shown inFIG. 2, the plate-shaped β-Ga2O3-based single crystal25having a wide width can be obtained without carrying out shoulder expansion. Consequently, a problem associated with the shoulder expansion of single crystals, e.g. twin crystal or twinning at the time of the shoulder expansion in the width direction w can be avoided.

Specifically, in case of growing the plate-shaped β-Ga2O3-based single crystal25by pulling up it in the b axis direction, due to carrying out the shoulder expansion in the width direction w, there is a risk that the β-Ga2O3-based single crystal25causes the twin crystal or twinning. According to the embodiment, for example, the plate-shaped β-Ga2O3-based single crystal25having a (101) plane or a (−201) plane as a principal surface can be grown by pulling up it in the b axis direction without carrying out the shoulder expansion in the width direction w, and without causing the twin crystal or twinning

In addition, for the purpose of growing the plate-shaped β-Ga2O3-based single crystal25having a wide width without carrying out the shoulder expansion, it is preferable that the seed crystal20has a width larger than a width in the longitudinal direction of the opening part14B of the slit14A of the die14.

FIG. 3is a graph showing a relationship between the defect density of the seed crystal and the polycrystallization yield of the β-Ga2O3-based single crystal. The horizontal axis inFIG. 3represents the maximum value of the defect density (/cm2) (a value of the defect density (/cm2) in the region having the largest number of the defect) of the seed crystal20, and the vertical axis represents the polycrystallization yield (%) of the β-Ga2O3-based single crystal25grown by using the seed crystal20. Here, the polycrystallization yield means a yield at the time of judging polycrystallized crystals as a defective product.

FIG. 3shows that the polycrystallization yield becomes not less than 80% when the maximum value of the defect density of the seed crystal20is not more than 5.0×105/cm2. Generally, 80% is often used as the basis of the yield of the oxide single crystal. Thus, in order to suppress the polycrystallization of the β-Ga2O3-based single crystal25, it is preferable that the maximum value of the defect density of the seed crystal20is not more than 5.0×105/cm2, namely the defect density in the whole region is not more than 5×105/cm2.

FIG. 4is a graph showing a relationship between the defect density of the seed crystal and the defect density of the β-Ga2O3-based single crystal. The horizontal axis inFIG. 4represents the maximum value of the defect density (/cm2) of the seed crystal20and the vertical axis represents the maximum value of the defect density (/cm2) of the β-Ga2O3-based single crystal25grown by using the seed crystal20.

FIG. 4shows that the more the maximum value of the defect density of the seed crystal20becomes large, the more the maximum value of the defect density of the β-Ga2O3-based single crystal25becomes large, in particular, when the maximum value of the defect density of the seed crystal20is not less than 3.0×105/cm2, the more the maximum value of the defect density of the seed crystal20becomes large, the more the difference between the maximum value of the defect density of the seed crystal20and the maximum value of the defect density of the β-Ga2O3-based single crystal25becomes large. According toFIG. 4, when the maximum value of the defect density of the seed crystal20is not more than 5.0×105/cm2, the maximum value of the defect density of the β-Ga2O3-based single crystal25becomes not more than 1.0×106/cm2.

Generally, in case of using the β-Ga2O3-based single crystal as an LED substrate, the defect density of not more than 1.0×106/cm2is often utilized as an index of crystal quality. Thus, in order to grow the β-Ga2O3-based single crystal25having high quality for the LED, it is preferable that the maximum value of the defect density of the seed crystal20is not more than 5.0×105/cm2, namely the defect density in the whole region is not more than 5×105/cm2.

Further, the seed crystal20and the β-Ga2O3-based single crystal25according toFIGS. 3, 4are a plate-shaped β-Ga2O3single crystal having a (101) plane or a (−201) plane as a principal surface, and the β-Ga2O3-based single crystal25is a crystal grown in the b axis direction by pulling up the seed crystal20in the b axis direction. However, if the β-Ga2O3-based single crystal25is grown in the b axis direction by pulling up the seed crystal20in the b axis direction, the plane orientation of the principal surface of the seed crystal20and the β-Ga2O3-based single crystal25is not limited, but a crystal of which plane orientation is any one without limitation has also the same characteristics as the crystal of which plane orientation is a (101) plane or a (−201) plane.

Hereinafter, one example of the growth condition of the β-Ga2O3-based single crystal25will be explained.

For example, the growth of the βGa2O3-based single crystal25is carried out under a nitrogen atmosphere or under a mixed atmosphere of nitrogen and oxygen.

As shown inFIG. 2, in case of using the plate-shaped seed crystal20having a wide width, the seed20is larger than a seed crystal used for the usual crystal growth, thus it is susceptible to thermal impact. Consequently, it is preferable that the lowering speed of the seed crystal20before being brought into contact with the Ga2O3-based melt12is low to some extent, for example, not less than 5 mm/min is preferable.

It is preferable that the waiting time until the seed crystal20is pulled up after being brought into contact with the Ga2O3-based melt12is long to some extent so as to prevent thermal impact by further stabilizing the temperature, for example, not less than 1 min is preferable.

It is preferable that the temperature ascending speed when raw materials in the crucible13are melted is low to some extent so as to prevent that the temperature around the crucible13rapidly ascends so that thermal impact is applied to the seed crystal20, for example, it is preferable that the raw materials are melted while taking a time of not less than 5 hours

(Effects of the Embodiment)

According to the embodiment, by using the plate-shaped seed crystal20in which the defect density in the whole region is not more than 5×105/cm2, the plate-shaped β-Ga2O3-based single crystal25that suppresses polycrystallization and has high crystal quality can be grown.

In addition, even if the seed crystal20is a plate-shaped crystal having a wide width, the β-Ga2O3-based single crystal25that suppresses polycrystallization and has high crystal quality can be also grown. Thus, the plate-shaped β-Ga2O3-based single crystal25having a wide width can be grown without carrying out the shoulder expansion so that a problem associated with the shoulder expansion, e.g. twin crystal or twinning of the β-Ga2O3-based single crystal25can be avoided.

Although the embodiment of the invention has been described above, the invention according to claims is not to be limited to the above-mentioned embodiment. Further, it should be noted that all combinations of the features described in the embodiment are not necessary to solve the problem of the invention.

INDUSTRIAL APPLICABILITY

A method for growing a β-Ga2O3-based single crystal is provided which can provide a plate-shaped β-Ga2O3-based single crystal having a high crystal quality.

REFERENCE SIGNS LIST