Patent Description:
A technique for growing a gallium oxide single crystal by the vertical Bridgman method is known (see, e.g., Patent Literature <NUM> and Patent Literature <NUM>). In the vertical Bridgman method, in general, a crucible filled with a raw material is inserted into a crystal growth furnace having a vertical temperature gradient and is then pulled down, thereby growing a single crystal. Then, to control crystal orientation to obtain a high-quality crystal, it is necessary to perform a seeding process in which a portion of a seed crystal placed in the crucible is kept in a non-melted state and is brought into contact with a raw material melt. For example, Patent Literature <NUM> disclose in <FIG> a single crystal growth apparatus for growing a single crystal of a gallium oxide-based semiconductor by the vertical Bridgman method, comprising a crucible <NUM> that comprises a seed crystal section to accommodate a seed crystal and a growing crystal section which is located on the upper side of the seed crystal section, and crucible support <NUM> with a cylindrical portion with a uniform inner diameter.

To stably perform the seeding process, there must be a sufficient temperature difference between upper and lower portions of the seed crystal. When the temperature difference between the upper and lower portions of the seed crystal is not large enough, it leads to failure of the seeding process, such as melting the entire seed crystal or leaving the raw material partially not melted, which leads to a decrease in a yield of single crystal growth.

Usually, with use of a heater capable of heating to a temperature well above a melting point of a crystal to be grown, a sufficient temperature difference between the upper and lower portions of the seed crystals can be provided by increasing the vertical temperature gradient in the crystal growth furnace.

However, in growing β-Ga<NUM>O<NUM> single crystal, it is necessary to maintain an oxidizing atmosphere in the crystal growth furnace to prevent decomposition of the raw material melt at high temperature. For this reason, a molybdenum disilicide heating element, which is resistant to oxidation and capable of melting Ga<NUM>O<NUM>, is used as a heater.

The maximum operating temperature of the molybdenum disilicide heating element is about <NUM> and is close to the melting point of Ga<NUM>O<NUM> which is about <NUM>. Therefore, the vertical temperature gradient in the crystal growth furnace is inevitably small, making it difficult to provide a sufficient temperature difference between the upper and lower portions of the seed crystal.

In this regard, there is a method in which a sufficient temperature difference between the upper and lower portions of the seed crystal is provided by using a seed crystal which is longer than usual seed crystals. However, in this method, it is necessary to increase a length of a portion of the crucible accommodating the seed crystal. In growing β-Ga<NUM>O<NUM> single crystal, a crucible formed of an expensive material such as Pt-Rh alloy is used. Therefore, increasing the length of the portion of the crucible accommodating the seed crystal significantly increases the manufacturing cost of the crucible.

It is an object of the invention to provide a single crystal growth apparatus which is capable of providing a sufficient temperature difference between upper and lower portions of a seed crystal in growing a β-Ga<NUM>O<NUM> single crystal without using a particularly long seed crystal while using a unidirectional solidification crystal growth method such as the vertical Bridgeman method.

The invention provides a single crystal growth apparatus as defined in the appended claims.

According to an embodiment of the invention, a single crystal growth apparatus can be provided which is capable of providing a sufficient temperature difference between upper and lower portions of a seed crystal in growing a β-Ga<NUM>O<NUM> single crystal without using a particularly long seed crystal while using the unidirectional solidification crystal growth method such as the vertical Bridgeman method.

<FIG> is a schematic vertical cross-sectional view showing a configuration of a single crystal growth apparatus <NUM> in an embodiment of the invention. The single crystal growth apparatus <NUM> is a vertical Bridgman-type single crystal growth apparatus (a vertical Bridgman furnace) and is capable of growing a single crystal of gallium oxide-based semiconductor. The gallium oxide-based semiconductor here refers to β-Ga<NUM>O<NUM>, or refers to β-Ga<NUM>O<NUM> including a substitutional impurity such as Al, In, or a dopant such as Sn, Si.

The single crystal growth apparatus <NUM> includes a crucible <NUM>, a susceptor <NUM> that supports the crucible <NUM> from below and is movable vertically, a tubular furnace core tube <NUM> that surrounds the crucible <NUM>, the susceptor <NUM> and a crucible support shaft <NUM>, a molybdenum disilicide heating element <NUM> placed outside the furnace core tube <NUM>, and a housing <NUM> that is formed of a thermal insulating material and accommodates the components of the single crystal growth apparatus <NUM> described in the appended claims. The susceptor <NUM> is a tubular member that surrounds the seed crystal section <NUM> of the crucible <NUM> and also supports the crucible <NUM> from below.

The crucible <NUM> has a seed crystal section <NUM> to accommodate a seed crystal <NUM>, and a growing crystal section <NUM> which is located on the upper side of the seed crystal section <NUM> and in which a single crystal <NUM> of gallium oxide-based semiconductor is grown by crystallizing a raw material melt <NUM> accommodated therein.

The growing crystal section <NUM> typically includes a constant diameter portion having a constant inner diameter larger than an inner diameter of the seed crystal section <NUM>, and a diameter-increasing portion that is located between the constant diameter portion and the seed crystal section <NUM> and has an inner diameter increasing from the seed crystal section <NUM> side toward the constant diameter portion, as shown in <FIG>.

The crucible <NUM> has a shape and size corresponding to a shape and size of the single crystal <NUM> to be grown. When growing, e.g., the single crystal <NUM> having a columnar-shaped constant diameter portion with a diameter of <NUM> inches, the crucible <NUM> provided with the growing crystal section <NUM> having a columnar-shaped constant diameter portion with an inner diameter of <NUM> inches is used. Meanwhile, when growing the single crystal <NUM> provided with the constant diameter portion having a shape other than the columnar shape, e.g., a quadrangular prism shape or a hexagonal prism shape, the crucible <NUM> provided with the growing crystal section <NUM> having a quadrangular prism-shaped or hexagonal prism-shaped growing crystal section <NUM> is used. A lid may be used to cover an opening of the crucible <NUM>.

The crucible <NUM> is formed of a heat-resistant material which is capable of withstanding temperature of a gallium oxide-based semiconductor melt as the raw material melt <NUM> (temperature of not less than a melting point of the gallium oxide-based semiconductor) and is less likely to react with the gallium oxide-based semiconductor melt, and the crucible <NUM> is formed of, e.g., a Pt-Rh alloy.

The susceptor <NUM> is a tubular member that surrounds the seed crystal section <NUM> of the crucible <NUM> and also supports the crucible <NUM> from below. The susceptor <NUM> has a thick portion <NUM> at a portion in a height direction that is thicker than the other portions and has a shorter horizontal distance Dt from the seed crystal section <NUM> (has a smaller inner diameter) than the other portions. The thick portion <NUM> surrounds (is located at the same height as) at least a portion of the seed crystal section <NUM> in the height direction.

<FIG> is an enlarged cross-sectional view showing a portion around the susceptor <NUM> in the single crystal growth apparatus <NUM>. The susceptor <NUM> may be composed of plural blocks (blocks 11a to 11c in the example shown in <FIG>) that are coupled vertically, as shown in <FIG>. In this case, a block which is thicker and has a shorter horizontal distance Dt from the seed crystal section <NUM> than the other blocks (the block 11b in the example shown in <FIG>) can be provided as the thick portion <NUM>.

The susceptor <NUM> is formed of a heat-resistant material capable of withstanding growth temperature of gallium oxide-based semiconductor single crystal and not reacting with the crucible <NUM> at the growth temperature, and the susceptor <NUM> is formed of, e.g., zirconia or alumina.

The crucible support shaft <NUM> is connected to the susceptor <NUM> on the lower side, and the susceptor <NUM> and the crucible <NUM> supported by the susceptor <NUM> can be moved vertically by vertically moving the crucible support shaft <NUM> using a drive mechanism (not shown). The crucible support shaft <NUM> may also be able to be rotated about the vertical direction by the above-mentioned drive mechanism. In this case, the crucible <NUM> supported by the susceptor <NUM> can be rotated inside the furnace core tube <NUM>.

The crucible support shaft <NUM> is formed of a heat-resistant material capable of withstanding growth temperature of gallium oxide-based semiconductor single crystal, and is formed of, e.g., zirconia or alumina.

The crucible support shaft <NUM> is typically a tubular member, in the same manner as the susceptor <NUM>. In this case, a thermocouple to measure temperature in the crucible <NUM> can be inserted inside the susceptor <NUM> and the crucible support shaft <NUM>.

The molybdenum disilicide heating element <NUM> is a resistive heating element formed of molybdenum disilicide (MoSi<NUM>), and is a heater to melt a raw material of gallium oxide-based semiconductor accommodated in the growing crystal section <NUM> to obtain the raw material melt <NUM>.

The molybdenum disilicide heating element <NUM> is inserted into the housing <NUM> from a hole provided on the housing <NUM> and is connected, outside of the housing <NUM>, to an external device (not shown) to supply a current to the molybdenum disilicide heating element <NUM>.

The furnace core tube <NUM> is used to regulate heat flow around the crucible <NUM> or to suppress contamination with impurities such as Si, Mo from the molybdenum disilicide heating element <NUM>. The furnace core tube <NUM> typically has a circular tube shape. The furnace core tube <NUM> may alternatively be composed of plural stacked annular members.

In addition, a lid <NUM> may be placed on an upper opening of the furnace core tube <NUM>, as shown in <FIG>. Upward escape of heat around the crucible <NUM> can be suppressed by using the lid <NUM>.

The furnace core tube <NUM> and the lid <NUM> are formed of a heat-resistant material capable of withstanding growth temperature of gallium oxide-based semiconductor single crystal, and are formed of, e.g., zirconia or alumina.

As described above, the maximum operating temperature of the molybdenum disilicide heating element <NUM> is about <NUM> and is close to the melting point of Ga<NUM>O<NUM> which is about <NUM>. Therefore, the vertical temperature gradient in the single crystal growth apparatus <NUM> is inevitably small and it is difficult to provide a sufficient temperature difference between the upper and lower portions of the seed crystal <NUM> by controlling the temperature of the molybdenum disilicide heating element <NUM>. Therefore, in the single crystal growth apparatus <NUM>, the temperature difference between the upper and lower portions of the seed crystal <NUM> is increased by providing the thick portion <NUM> on the susceptor <NUM>.

Since the thick portion <NUM> is thicker than the other portions and has a shorter horizontal distance Dt from the seed crystal section <NUM> than the other portions, heat from the surroundings is less likely to be transferred to a portion of the seed crystal section <NUM> surrounded by the thick portion <NUM>, hence, temperature rise is suppressed. In addition, the thick portion <NUM> intercepts the radiant heat transferred from top to bottom on the outer side of the crucible <NUM>, thereby suppressing the temperature rise of a portion of the seed crystal section <NUM> surrounded by the thick portion <NUM> and of a portion thereunder.

To further increase the temperature difference between the upper and lower portions of the seed crystal <NUM>, it is preferable that a value of a ratio of an inner diameter of the thick portion <NUM> to an outer diameter of the seed crystal section <NUM> be not more than <NUM>.

In addition, to further increase the temperature difference between the upper and lower portions of the seed crystal <NUM>, it is preferable that a lower end of the thick portion <NUM> be located higher than a lower end of the seed crystal section <NUM>.

In addition, to further increase the temperature difference between the upper and lower portions of the seed crystal <NUM>, it is preferable that a width of the thick portion <NUM> in the height direction be not less than <NUM>.

In addition, to further increase the temperature difference between the upper and lower portions of the seed crystal <NUM>, it is preferable that an upper end of the seed crystal section <NUM> be not surrounded by the thick portion <NUM> and a portion below the upper end be surrounded by the thick portion <NUM>, i.e., an upper end of the thick portion <NUM> be located lower than the upper end of the seed crystal section <NUM>.

Firstly, the seed crystal <NUM> of gallium oxide-based semiconductor is placed in the seed crystal section <NUM> of the crucible <NUM>, and a raw material of gallium oxide-based semiconductor, such as sintered β-Ga<NUM>O<NUM>, is placed in the growing crystal section <NUM>.

Next, the inside the single crystal growth apparatus <NUM> (the inner side of the housing <NUM>) is heated by the molybdenum disilicide heating element <NUM> so as to form such a temperature gradient that temperature on the upper side is higher and temperature on the lower side is lower, thereby melting the raw material of gallium oxide in the crucible <NUM> and obtaining the raw material melt <NUM>.

In a typical method, firstly, the height of the crucible <NUM> is adjusted by vertically moving the crucible support shaft <NUM> so that temperature in an upper region in the growing crystal section <NUM> is increased to not less than the melting point of gallium oxide. An upper portion of the raw material inside the growing crystal section <NUM> is thereby melted. Next, the raw material is melted to the bottom while raising the crucible <NUM> at a predetermined speed by moving the crucible support shaft <NUM> upward at a predetermined speed, thereby eventually melting the entire raw material and a portion of the seed crystal.

Next, the raw material melt <NUM> is crystallized from the lower side (the seed crystal <NUM> side) while lowering the crucible <NUM> at a predetermined speed by moving the crucible support shaft <NUM> downward, thereby growing the single crystal <NUM>. After the entire raw material melt <NUM> is crystallized, the crucible <NUM> is removed and the single crystal <NUM> is taken out.

In the single crystal growth apparatus <NUM> in the embodiment of the invention, the temperature difference between the upper and lower portions of the seed crystal <NUM> is increased by providing the thick portion <NUM> on the susceptor <NUM>. Therefore, a high-quality single crystal <NUM> of gallium oxide-based semiconductor which has a melting point close to the maximum operating temperature of the heating element can be grown by the vertical Bridgeman method.

In addition, the method in which the thick portion <NUM> is provided on the susceptor <NUM> can increase the temperature difference between the upper and lower portions of the seed crystal <NUM> with substantially no increase in the device manufacturing cost unlike the method using a particularly long seed crystal.

In the meantime, the larger the diameter of the single crystal <NUM> to be grown, the greater the amount of heat required to melt the raw material and the greater the amount of heat transferred to the seed crystal <NUM>, hence, the temperature difference between the upper and lower portions of the seed crystal is likely to be small. For this reason, in general, it is difficult to grow a large-diameter single crystal of gallium oxide-based semiconductor by the vertical Bridgeman method. However, in the embodiment of the invention, it is possible to obtain, e.g., a gallium oxide-based semiconductor single crystal (ingot) having a columnar-shaped constant diameter portion with a diameter of <NUM> to <NUM> inches, and wafers with a diameter of <NUM> to <NUM> inches can be cut out from it.

A demonstration experiment was conducted to investigate the effect of providing the thick portion <NUM> on the susceptor <NUM> in the single crystal growth apparatus <NUM>. Next, the details of the demonstration experiment and its results will be described.

<FIG> are vertical cross-sectional views showing configurations of two types of susceptors <NUM> used in the demonstration experiment in Example <NUM>. The susceptor <NUM> shown in <FIG> as background art (hereinafter, referred to as Sample A), which does not include the thick portion <NUM>, has a constant thickness. On the other hand, the susceptor <NUM> shown in <FIG> (hereinafter, referred to as Sample B) includes the thick portion <NUM> at a position surrounding a portion of the seed crystal section <NUM>. In this regard, the susceptor <NUM> and the seed crystal section <NUM> in both Samples A and B have an annular shape in a horizontal cross section.

Here, a thickness T of Sample A is <NUM> and the horizontal distance Dt between Sample A and the seed crystal section <NUM> is <NUM>. Meanwhile, a thickness T1 of a portion of Sample B other than the thick portion <NUM> is <NUM>, a thickness T2 of the thick portion <NUM> is <NUM>, a horizontal distance Dt<NUM> between the portion of Sample B other than the thick portion <NUM> and the seed crystal section <NUM> is <NUM>, and a horizontal distance Dt2 between the thick portion <NUM> and the seed crystal section <NUM> is <NUM>.

<FIG> is a graph showing measurement results of temperature difference between upper and lower portions of the seed crystal sections <NUM> of Samples A and B. The vertical axis in <FIG> indicates TC1-TC2 [°C] which is a difference between a temperature TC1 of a measurement point P1 at the top of the seed crystal section <NUM> and a temperature TC2 of a measurement point P2 at the bottom of the seed crystal section <NUM>. The horizontal axis indicates elapsed time [h] where the reference time is <NUM>. The reference time here is the time at which the height of the crucible <NUM> and output of the molybdenum disilicide heating element <NUM> reach the same predetermined values in each measurement.

According to the results shown in <FIG>, TC1-TC2 of Sample A at the reference time was <NUM> to <NUM>, while TC1-TC2 of Sample B was about <NUM>. These results confirmed that providing the thick portion <NUM> on the susceptor <NUM> is effective to increase the temperature difference between the upper and lower portions of the seed crystal sections <NUM>.

A relationship between the shape of the susceptor <NUM> and the temperature difference between the upper and lower portions of the seed crystal sections <NUM> in the single crystal growth apparatus <NUM> was investigated by simulation. Next, the details of the simulation and its results will be described.

<FIG> is a vertical cross-sectional view showing a configuration of a model used in the simulation in Example <NUM>. Regarding TC1-TC2 which is a difference between the temperature TC1 of the measurement point P1 at the top of the seed crystal section <NUM> and the temperature TC2 of the measurement point P2 at the bottom of the seed crystal section <NUM>, change in TC1-TC2 when changing parameters related to the shape of the susceptor <NUM> of the model shown in <FIG> was simulated.

Firstly, change in TC1-TC2 when changing an inner diameter D1 of the thick portion <NUM> of the susceptor <NUM> was investigated. In this simulation, the model shown in <FIG> was configured as follows: an outer diameter D2 of the seed crystal section <NUM> was <NUM>, an inner diameter D3 of a portion of the susceptor <NUM> other than the thick portion <NUM> was <NUM>, the thickness T1 of the portion of the susceptor <NUM> other than the thick portion <NUM> was <NUM>, a length L of the seed crystal section <NUM> was <NUM>, a distance Dt3 in the height direction between the upper end of the seed crystal section <NUM> and the thick portion <NUM> was <NUM>, and a width W of the thick portion <NUM> in the height direction was <NUM>.

<FIG> is a graph showing a relationship between TC1-TC2 and a value of D1/D2 which is a ratio of the inner diameter D <NUM> of the thick portion <NUM> to the outer diameter D2 of the seed crystal section <NUM>. According to the results shown in <FIG>, TC1-TC2 increases as D1/D2 becomes closer to <NUM>.

Based on the fact that temperature of a solid-liquid interface in the seeding process fluctuates within a range of about ±<NUM> and temperature inside the single crystal growth apparatus <NUM> also fluctuates within a range of about ±<NUM> even in the state in which the interface temperature is maintained, TC1-TC2 is preferably not less than <NUM>. Therefore, from the results shown in <FIG>, D1/D2 is preferably not more than <NUM>. Table <NUM> below shows the numerical values of the plotted points on the graph in <FIG>.

Next, change in TC1-TC2 when changing the width W of the thick portion <NUM> of the susceptor <NUM> in the height direction was investigated. In this simulation, the model shown in <FIG> was configured as follows: the outer diameter D2 of the seed crystal section <NUM> was <NUM>, the inner diameter D1 of the thick portion <NUM> was <NUM>, the inner diameter D3 of the portion of the susceptor <NUM> other than the thick portion <NUM> was <NUM>, the thickness T <NUM> of the portion of the susceptor <NUM> other than the thick portion <NUM> was <NUM>, the length L of the seed crystal section <NUM> was <NUM>, and the distance Dt3 in the height direction between the upper end of the seed crystal section <NUM> and the thick portion <NUM> was <NUM>.

<FIG> is a graph showing a relationship between the width W of the thick portion <NUM> in the height direction and TC1-TC2. According to the results shown in <FIG>, TC1-TC2 increases as the width W increases and the position of the lower end of the thick portion <NUM> in the height direction comes closer to the position of the lower end of the seed crystal section <NUM> in the height direction. And then, when the width W further increases and the position of the lower end of the thick portion <NUM> in the height direction becomes lower than the position of the lower end of the seed crystal section <NUM> in the height direction, TC1-TC2 decreases by about <NUM>.

It is considered that the reason why TC1-TC2 decreases when the position of the lower end of the thick portion <NUM> in the height direction becomes lower than the position of the lower end of the seed crystal section <NUM> in the height direction is because radiant heat from the seed crystal section <NUM> is less likely to escape downward when the thick portion <NUM> surrounds the lower end of the seed crystal section <NUM>. Therefore, the width W of the thick portion <NUM> in the height direction is preferably as large as possible within a range where the thick portion <NUM> does not surround the lower end of the seed crystal section <NUM>.

From the results shown in <FIG>, the lower end of the thick portion <NUM> is preferably located higher than the lower end of the seed crystal section <NUM>, and the width W of the thick portion <NUM> in the height direction is preferably, e.g., not less than <NUM>. Table <NUM> below shows the numerical values of the plotted points on the graph in <FIG>.

Next, change in TC1-TC2, when the susceptor <NUM> does not have the thick portion <NUM> and the thickness T1 of the susceptor <NUM> is changed, was investigated as comparative example. The thickness T <NUM> was changed by changing an outer diameter D4 of the susceptor <NUM> while fixing the inner diameter D3. In this simulation, the model shown in <FIG> was configured as follows: the outer diameter D2 of the seed crystal section <NUM> was <NUM>, the inner diameter D3 of the susceptor <NUM> was <NUM>, and the length L of the seed crystal section <NUM> was <NUM>.

<FIG> is a graph showing a relationship between the thickness T1 of the susceptor <NUM> not provided with the thick portion <NUM> as comparative example and TC1-TC2. According to the results shown in <FIG>, TC1-TC2 increases as the thickness T1 increases.

Next, change in TC1-TC2, when the susceptor <NUM> has the thick portion <NUM> and the thickness T1 of the portion of the susceptor <NUM> other than the thick portion <NUM> is changed, was investigated. The thickness T1 was changed by changing the outer diameter D4 of the susceptor <NUM> while fixing the inner diameter D3. In this regard, since the outer diameter D4 of the susceptor <NUM> is the same at the thick portion <NUM> and at the portion other than the thick portion <NUM> and each of the inner diameters D1 and D3 is constant, the thickness of the thick portion <NUM> excluding the thickness of the inwardly-protruding portion is the same as the thickness T1 of the portion other than the thick portion <NUM>. In this simulation, the model shown in <FIG> was configured as follows: the outer diameter D2 of the seed crystal section <NUM> was <NUM>, the inner diameter D3 of the portion of the susceptor <NUM> other than the thick portion <NUM> was <NUM>, the length L of the seed crystal section <NUM> was <NUM>, the distance Dt3 in the height direction between the upper end of the seed crystal section <NUM> and the thick portion <NUM> was <NUM>, and the width W of the thick portion <NUM> in the height direction was <NUM>.

<FIG> is a graph showing a relationship between the thickness T1 of the portion of the susceptor <NUM> other than the thick portion <NUM> and TC1-TC2 in case that the susceptor <NUM> is not provided with the thick portion <NUM>. In <FIG>, the data shown in <FIG>, which is the case where the thick portion <NUM> is not provided on the susceptor <NUM>, is also shown as Comparative Example.

According to the results shown in <FIG>, TC1-TC2 increases as the thickness T1 of the portion of the susceptor <NUM> other than the thick portion <NUM> (the thickness of the thick portion <NUM> excluding the thickness of the protruding portion) increases, and its degree (the slope of the straight line in the graph) is greater than when the thick portion <NUM> is not provided on the susceptor <NUM>.

TC1-TC2 in the seeding process can range up to <NUM> depending on deterioration of the surrounding thermal insulating material or heater, even if the susceptors having the same structure are used. Therefore, TC1-TC2 is preferably not less than <NUM> as described above, but is more preferably not less than <NUM> to stably perform the seeding without being affected by deterioration of the surrounding members. Therefore, from the results shown in <FIG>, the thickness T1 of the portion of the susceptor <NUM> other than the thick portion <NUM> (the thickness of the thick portion <NUM> excluding the thickness of the protruding portion) is preferably not less than <NUM>. Table <NUM> below shows the numerical values of the plotted points on the graph in <FIG>.

Claim 1:
A single crystal growth apparatus (<NUM>) to grow a single crystal (<NUM>) of a gallium oxide-based semiconductor, the single crystal growth apparatus (<NUM>) comprising:
a crucible (<NUM>) that comprises a seed crystal section (<NUM>) to accommodate a seed crystal (<NUM>), and a growing crystal section (<NUM>) which is located on the upper side of the seed crystal section (<NUM>) and in which the single crystal is (<NUM>) grown by crystallizing a raw material melt (<NUM>) accommodated therein;
a tubular member (<NUM>) surrounding the seed crystal section (<NUM>) and also supporting the crucible (<NUM>) from below; and
a molybdenum disilicide heating element (<NUM>) to melt a raw material in the growing crystal section (<NUM>) to obtain the raw material melt (<NUM>),
characterized in thatthe tubular member (<NUM>) comprises a thick portion (<NUM>) at a portion in a height direction that is thicker and has a shorter horizontal distance from the seed crystal section (<NUM>) than other portions, and
wherein the thick portion (<NUM>) surrounds at least a portion of the seed crystal section (<NUM>) in the height direction.