JP7576672B2 - GaAs ingot manufacturing method and GaAs ingot - Google Patents

Description

The present invention relates to a method for producing a GaAs ingot and a GaAs ingot.

The known manufacturing methods for GaAs single crystals (hereinafter also referred to as GaAs ingots) to obtain GaAs single crystal wafers (hereinafter also referred to as GaAs wafers) are the LEC method, the horizontal boat (HB) method, the vertical temperature gradient (VGF) method, and the vertical Bridgman (VB) method. Starting from a single crystal seed crystal, a mass with a straight body of single crystal grown by these manufacturing methods is an ingot, and wafers are cut from the straight body of the ingot. Multiple GaAs wafers can be obtained from the same GaAs ingot, and these multiple wafers are also called a wafer group.

Here, the vertical boat method, such as the vertical temperature gradient (VGF) method and the vertical Bridgman (VB) method, is a method in which a seed crystal is placed at the bottom of a crucible, a single crystal raw material melt and a sealant liquid are placed on the upper layer of the seed crystal, and the crucible is cooled from a predetermined temperature distribution to grow a crystal from the bottom of the raw material melt upward. In the vertical temperature gradient (VGF) method, the temperature itself is lowered, and in the vertical Bridgman (VB) method, the crucible is moved relatively within a predetermined temperature distribution. When producing a Si-doped GaAs single crystal ingot, Si is added to the GaAs melt. Boron oxide (B ₂ O ₃ ) is usually used as a sealant to prevent As, a volatile component, from dissociating from the ingot. During crystal growth, it is known that an oxidation-reduction reaction occurs at the interface between the GaAs melt and the liquid B ₂ O ₃ according to the following reaction formula (1).
3Si (in Melt) + 2B ₂ O ₃ = 3SiO ₂ (in B ₂ O ₃ ) + 4B (in Melt)... (1)

In the vertical boat method, the crystal is grown from the bottom to the top of the raw material melt, so impurities that are not incorporated into the crystal are concentrated in the melt. If a large amount of Si dopant (i.e., Si crystals added together with the GaAs raw material) is added to the GaAs melt to increase the carrier concentration, the equilibrium of the above reaction formula (1) advances to the right side, and a large amount of boron (B) moves into the GaAs melt. As the crystal growth progresses and the impurities become more concentrated, the generation of boron arsenide also progresses, making it impossible to obtain high crystallinity. In other words, in the GaAs single crystal obtained by this manufacturing method, there is a trade-off between carrier concentration and crystallinity.

Patent Document 1 proposes a method for producing GaAs single crystals having a high carrier concentration and high crystallinity by disposing a plate-shaped solid silicon dioxide at the interface between GaAs melt and liquid B ₂ O ₃ in the vertical boat method.

JP 2012-246156 A

In recent years, among semiconductor lasers, surface-emitting lasers, including VCSELs (vertical-cavity surface-emitting lasers), have been actively utilized and are applied to optical communications, optical sensing, and the like. In these applications, silicon (Si)-doped n-type GaAs wafers are used as device substrates, and the GaAs wafers are required to have a strictly controlled carrier concentration and a low dislocation density. Specifically, the carrier concentration is preferably in the range of 1.5×10 ¹⁸ cm ⁻³ to 2.9×10 ¹⁸ cm ⁻³ , and the low dislocation density is preferably such that the maximum etch pit density is 1200 cm ⁻² or less for wafers less than 6 inches (e.g., 4 inches or less), and the maximum etch pit density is 1500 cm ⁻² or less for wafers 6 inches or more.

The GaAs ingot disclosed in Patent Document 1 can only produce a limited number of wafers that satisfy these conditions for maximum carrier concentration and etch pit density, leaving issues in terms of productivity.

In crystal growth, the Si concentration in the GaAs melt increases as crystallization progresses due to the segregation of Si, and the carrier concentration increases along with the Si concentration from the seed side to the tail side of the ingot. If the amount of Si raw material added as a dopant to the GaAs melt is increased in order to make the lower limit of the carrier concentration at the seed side of the straight body of the ingot 1.5×10 ¹⁸ cm ⁻³ or more, polycrystallization may occur at the beginning of crystal growth, making it impossible to obtain a single crystal ingot. For example, in the manufacture of a 4-inch GaAs ingot, if a GaAs raw material not containing Si is used and more than 300 wtppm of Si raw material is added as a dopant, polycrystallization is likely to occur. Therefore, the inventors have conducted intensive research to achieve the above-mentioned object, and have found that in a method for producing a GaAs ingot in which a seed crystal, a Si raw material as a dopant, a GaAs raw material, and boron oxide as a sealant are contained in a crucible and crystal growth is performed by a vertical boat method, the Si raw material is used as a dopant, and a predetermined amount of Si is doped into the GaAs raw material as a raw material, and a predetermined amount of SiO ₂ is contained in the sealant, thereby controlling the Si concentration of the GaAs melt during crystal growth, and that even if the total amount of Si in the melt exceeds an amount that may cause polycrystals if the GaAs raw material is not doped with a predetermined amount of Si (for example, 300 wtppm for 4 inches), the ingot is not polycrystallized, and the carrier concentration and etch pit density can be within a desired range in most of the straight body of the ingot, and the present invention has been completed. Hereinafter, the amount obtained by adding the amount of Si in the Si raw material as a dopant to the amount of Si in the GaAs raw material is referred to as the total silicon charge amount.

The gist and configuration of the present invention are as follows.
[1] A method for producing a GaAs ingot, in which a seed crystal, a Si raw material as a dopant, a GaAs raw material, and boron oxide as a sealant are placed in a crucible and crystal growth is performed by a vertical boat method,
The GaAs raw material is Si-doped GaAs having a Si concentration of 20 to 200 wtppm relative to GaAs,
The charge amount of the Si raw material is 200 to 300 wtppm relative to GaAs,
The boron oxide contains more than 5 mol% SiO ₂ calculated as Si,
the Si raw material, the GaAs raw material, and the boron oxide are melted by heating, and then crystal growth is carried out while stirring the liquid boron oxide.
[2] A method for producing a GaAs ingot having a diameter of 140 mm or less, wherein the total silicon charge amount, which is the sum of the Si in the GaAs raw material and the Si raw material, exceeds 300 wtppm relative to GaAs; or
A method for producing a GaAs ingot having a diameter of a straight body portion exceeding 140 mm, wherein the charge amount of the Si raw material is 200 to 250 wtppm relative to GaAs, and the total silicon charge amount, which is the sum of the Si in the GaAs raw material and the Si raw material, exceeds 250 wtppm relative to GaAs.
[3] A GaAs ingot having a straight body diameter of 140 mm or less, in which wafers obtained from 70% or more of the straight body have a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1200 cm ^-2 or less, or a GaAs ingot having a straight body diameter of more than 140 mm, in which wafers obtained from 70% or more of the straight body have a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1500 cm ^-2 or less.

The present invention provides a GaAs ingot and a method for producing the same that can efficiently produce GaAs wafers with a strictly controlled carrier concentration and low dislocation density.

1 is a schematic diagram of a GaAs ingot according to the present invention. FIG. 1 is a schematic diagram showing 69 areas set on a 6-inch wafer for measuring etch pit density (EPD). 1 is a schematic cross-sectional view of a manufacturing apparatus used for manufacturing a GaAs ingot according to the present invention. 1 is a schematic cross-sectional view of a crucible 3 used in producing a GaAs ingot according to the present invention, showing the state in which the crucible is filled with raw materials and the like before the start of crystal growth.

The following describes an embodiment of the present invention with reference to the drawings.

<GaAs ingot>
(Seed side, center and tail side of GaAs ingot)
FIG. 1 is a schematic diagram of a GaAs ingot according to the present invention, showing the position 15 where the cone part changes to the straight body part, the position (middle) 16 which is half the length from the position 15 to the position 17, and the position 17 where the straight body part ends. The GaAs ingot has a straight body part 18 with a substantially uniform diameter through a region 19 (also called the cone part) where the diameter increases from the seed crystal 6 (also called the seed). The part from the position 17 where the straight body part ends to the completion of growth is called the tail. If the length from the position 15 where the cone part changes to the straight body part to the position 17 where the straight body part used for wafer processing ends is taken as 100%, the former position 15 is taken as 0%, and the latter position 17 is taken as 100%, the range of 0 to 5% is taken as the seed side of the straight body part (hereinafter also referred to as the seed side), the range of 40 to 60% is taken as the center part of the straight body part (hereinafter also referred to as the center part), and the range of 88 to 100% is taken as the tail side of the straight body part (hereinafter also referred to as the tail side). The end position 17 of the straight body portion (the 100% position) is the position of 90% of the volume (the position of 90% crystallization rate) when the volume of the entire ingot is 100%.
1 shows position 16, which is intermediate (50%) between positions 15 and 17. Hereinafter, position 16 will also be referred to simply as "intermediate."
The position of each wafer in the straight body portion as a percentage from the end of the straight body portion on the seed side is also referred to as the "position from the seed of the straight body." In the GaAs ingot, the solid-liquid interface moves from the seed side 15 to the tail side 17 to perform crystal growth.

The average and maximum carrier concentration and etch pit density (EPD) for GaAs ingots according to the present invention can be obtained by performing measurements on wafers obtained from the body portion 18 of the GaAs ingot.

The diameter of the GaAs ingot 18 is not particularly limited and may be less than 140 mm or may be equal to or greater than 140 mm. The diameter of the GaAs ingot 18 may be, for example, equal to or greater than 50 mm and less than 140 mm, or equal to or greater than 140 mm and equal to or less than 162 mm.
The size of the wafer can be appropriately selected depending on the diameter of the body portion 18 of the GaAs ingot, and can be, for example, 2 to 8 inches. When the diameter of the GaAs ingot 18 is less than 140 mm, the size of the wafer is usually less than 6 inches, and when the diameter of the GaAs ingot 18 is 140 mm or more, the size of the wafer is usually 6 inches or more.

<Method of manufacturing GaAs ingot>
The method for producing a GaAs ingot according to the present invention is a method for growing a crystal by placing a seed crystal, a Si source as a dopant, a GaAs source, and boron oxide as a sealant in a crucible and using a vertical boat method, which may be either a vertical temperature gradient (VGF) method or a vertical Bridgman (VB) method.

The manufacturing method will be described in more detail below with reference to Figures 3 and 4.

(Production equipment and temperature control)
FIG. 3 is a schematic cross-sectional view of an example of a manufacturing apparatus for use in the method for manufacturing a GaAs ingot according to the present invention.
The manufacturing apparatus shown in FIG. 3 includes an airtight container 7 that can be evacuated to a vacuum and filled with an atmospheric gas from the outside, a crucible 3 arranged in the center of the airtight container 7, a crucible storage container (susceptor) 2 that houses and holds the crucible 3, a mechanism 14 (only the lifting and rotating rods are shown) that raises and lowers and/or rotates the crucible storage container (susceptor) 2, and a heater 1 that is provided to surround the crucible storage container (susceptor) 2 within the airtight container 7.
An upper rod 21 equipped with a stirring blade 20 that can rotate and move up and down is disposed above the crucible 3. The stirring blade 20 and the upper rod 21 form a stirring means, but the stirring blade 20 is removable from the upper rod 21, and a sealant container or other members can be attached to the upper rod 21.
A crucible made of pyrolytic boron nitride (PBN) can be used as the crucible 3. In Fig. 3, the crucible 3 is filled with a seed crystal 6, a compound semiconductor raw material 5 (also called a raw material melt), and boron oxide ( _B2O3 ) 4 as a sealant, and the airtight container 7 is filled with an inert gas 8 _, for example.

Figure 4 is a schematic cross-sectional view of an example of a crucible for use in the method for producing a GaAs ingot according to the present invention, showing the state in which the raw materials are filled before the start of crystal growth. The crucible 3 shown in Figure 4 is filled with a seed crystal 6, a GaAs raw material 9, a Si raw material 10 as a dopant, and boron oxide 4 as a sealant. In Figure 4, the GaAs raw material 9 is prepared by preparing crushed GaAs polycrystals (9A), cylindrical GaAs polycrystals (9B), and disk-shaped GaAs polycrystals (9C), and the Si raw material is placed in a container made of the cylindrical GaAs polycrystals (9B) and disk-shaped GaAs polycrystals (9C). However, this is not limited to this embodiment, and for example, only crushed GaAs polycrystals may be filled.

After these fillings are completed, in a growth furnace filled with inert gas, the GaAs raw material 9 is heated to 1238°C or higher, the melting point of GaAs, while a PID-controlled heater is used to apply a temperature gradient so that the temperature on the seed crystal 6 side is lowered so as not to melt the seed crystal 6. This causes the GaAs raw material 9 and boron oxide 4 to melt, and the Si raw material 10 to dissolve in the GaAs raw material 9. Next, the temperature near the seed crystal 6 is raised until the upper part of the seed crystal 6 melts, and the entire temperature is gradually lowered while applying a temperature gradient to obtain a GaAs ingot. At this time, it is preferable to set the temperature drop rate to 10°C/h or less.

(Mixing)
In the GaAs manufacturing method according to the present invention, the liquid boron oxide 4 is stirred during crystal growth. Stirring is preferably started between the start of crystal growth on the seed side of the straight body and the middle (1-50% from the seed of the straight body). Stirring is preferably continued until crystal growth on the tail side of the straight body is completed, and may be continued until crystal growth is completed.

The stirring is preferably performed by rotating a stirring means disposed in the sealant (boron oxide 4). In the present invention, it is desirable to maintain the Si concentration in the raw material melt at a relatively high level by preventing the sealant from absorbing Si as much as possible from the start of crystal growth until the start of stirring, and stirring is used to a minimum in order to suppress an extreme increase in the Si concentration due to segregation. For this reason, the rotation speed may be changed stepwise or continuously from the seed side of the straight body to the tail side, but the maximum rotation speed reached is preferably 3 rpm or less.
The stirring means can have a shape in which stirring blades 20 are attached around a rotating shaft.
The shape of the stirring blade 20 is not particularly limited and may be made of a plate-like member of a predetermined shape, for example, 2 to 8 substantially rectangular plate-like members, and examples of the material include carbon, BN, etc. The plate-like members are preferably attached at an inclination angle of 45° or more and 135° or less with respect to the interface formed between the raw material melt and boron oxide 4 in a stationary state.
The stirring blade 20 is preferably sized so that the area formed by the rotational trajectory when the stirring means is rotated is 30% or more of the area of the interface formed between the raw material melt and boron oxide 4 in a stationary state, and more preferably 70% or more.
The distance between the lower end of the stirring blade 20 and the interface formed between the raw material melt and the boron oxide 4 in a stationary state is preferably less than 2 mm, and more preferably 1 mm or less. However, it is preferable that the lower end of the stirring blade 20 does not come into contact with the interface.

The stirring is preferably performed at a rotation speed of 0.5 rpm or more when the crystal growth has progressed to the middle, and more preferably at a rotation speed of 2.5 rpm or more. Since there is a tendency for the segregation of Si in the melt to increase and the carrier concentration to exceed 2.5×10 ¹⁸ cm ^-3 after the middle stage is reached, strong stirring at this time promotes the absorption of Si in the melt into the sealant and suppresses a sudden rise in the Si concentration due to segregation, thereby suppressing the occurrence of dislocations and a sudden rise in the carrier concentration.

(Seed crystal)
There is no particular limitation on the seed crystal 6, and a known seed crystal can be used. There is no particular limitation on the size of the seed crystal 6, and it is sufficient that the seed crystal 6 has a cross-sectional area of, for example, 1 to 20% of the cross-sectional area having the inner diameter of the crucible 3 as a diameter, and preferably has a cross-sectional area of 3 to 17%. When the diameter of the crystal to be grown exceeds 100 mm, the cross-sectional area of the seed crystal 6 can be 2 to 10% of the cross-sectional area having the inner diameter of the crucible as a diameter.

(Crucible inner diameter and wafer size)
The inner diameter of the crucible 3 is preferably slightly larger than the target wafer size. The target wafer size can be, for example, 2 inches or more, and is preferably 3 inches or more. There is no particular upper limit to the wafer size, and it can be, for example, 8 inches or less. From an ingot having a straight body diameter of 140 mm or less, wafers having a diameter of, for example, 2 to 4 inches are produced, and from an ingot having a straight body diameter of more than 140 mm, wafers having a diameter of, for example, 6 to 8 inches are produced.

(GaAs raw material)
As the GaAs raw material, Si-doped GaAs with a Si concentration of 20 to 200 wtppm relative to GaAs is used. The GaAs raw material used may be polycrystalline or single crystalline. The Si concentration in the GaAs raw material refers to the Si concentration calculated from the carrier concentration measured by Hall measurement. If the Si concentration calculated from the carrier concentration is within this range, single crystallization is not hindered in crystal growth even if the total amount of silicon charge to GaAs combined with the Si raw material 10 described later is large, and the carrier concentration in the resulting GaAs ingot can be easily controlled to a desired range. Since the GaAs raw material is melted for use, it is sufficient to make the average Si concentration of the entire GaAs raw material used fall within the above range, and GaAs raw materials with different Si concentrations may be used in combination.

Here, the Si concentration of Si-doped polycrystalline GaAs calculated from the carrier concentration can be measured and calculated as follows. The same method can be used for single crystals. The following method of converting carrier concentration to Si concentration is based on the assumption that the dopant of GaAs is Si, and that no other p-type or n-type dopants are intentionally included.
<Carrier concentration by Hall measurement>
The seed side and tail side of the Si-doped GaAs polycrystal are cut into wafers each having a thickness of about 1 mm.
A polycrystalline wafer-like object is scratched with a marking needle at the largest possible crystal grain boundary to a size of 10 mm square, and a sample is cut out at a size of 10 mm square from the center of the wafer-like object.
- Indium electrodes are attached to the four corners of the sample and it is heated to 330-360°C, after which the carrier concentration is measured using the Van der Pauw method.

<Conversion of activation rate to Si concentration>
The activation rate is set to 48%, and the carrier concentration is divided by 0.48 to calculate the Si atom concentration (cm ⁻³ ).
The bulk density of GaAs (5.3161 g/cm ³ ), Avogadro's number, and the atomic weight of Si (28.1) are used to convert the Si atomic concentration value from units (cm ^-3 ) to units (wtppm).

The method of synthesizing Si-doped GaAs as a GaAs raw material is not limited, and can be used in a crucible using the vertical boat method, or in a boat installed in an ampoule using the horizontal boat method, and during synthesis, an amount of Si raw material according to the desired Si concentration for GaAs is charged.

When using the vertical boat method, for example, a crucible can be filled with high-purity Ga, high-purity As, and Si raw material (e.g., high-purity Si shots or crushed high-purity Si substrates) and crystals can be grown to obtain Si-doped GaAs polycrystals. The crucible can be filled in the following order: half of the high-purity As, the entire amount of the Si raw material, the remaining half of the high-purity As, and high-purity Ga, but the filling order is not limited to this. In the synthesis of GaAs polycrystals, it is preferable not to use boron oxide, which is a sealant, in order to promote Si doping and to suppress the amount of boron in the GaAs melt when used as a GaAs raw material.

For the Si-doped GaAs used as the GaAs raw material, recycled products that have been processed or crushed can be used, including parts of other Si-doped GaAs single crystal ingots other than the body part that is not processed into wafers (the cone part or the area after the tail side of the body part), or unnecessary parts generated during wafer processing.

(Dopant)
A silicon source material 10 is used as a dopant. The silicon source material 10 may be high-purity silicon shot or crushed high-purity silicon substrates, and is added to the GaAs source material 9.
The charge amount of the Si source material 10 is set to 200 to 300 wtppm relative to the GaAs in the GaAs source material 9. It is more preferable to set it to 220 to 300 wtppm or more, and further more preferable to set it to 240 to 290 wtppm.

For an ingot with a diameter of the straight body of 140 mm or less, the total silicon charge amount, which is the sum of the Si in the GaAs raw material and the Si raw material 10, is preferably more than 300 wtppm relative to GaAs. In this case, the charge amount of the Si raw material 10 is more preferably 220 to 300 wtppm or less relative to GaAs.
For an ingot having a diameter of the straight body portion exceeding 140 mm, the charge amount of the Si source material 10 is preferably 200 to 250 wtppm relative to the GaAs, more preferably 220 to 250 wtppm, and the total silicon charge amount, which is the sum of the Si in the GaAs source material and the Si source material 10, is preferably more than 250 wtppm relative to the GaAs.
For example, in the manufacture of a 4-inch GaAs ingot, if a GaAs raw material not containing Si is used and more than 300 wtppm of Si raw material is added as a dopant, polycrystallization tends to occur, and in the manufacture of a 6-inch GaAs ingot, if a GaAs raw material not containing Si is used and more than 250 wtppm of Si raw material is added as a dopant, polycrystallization tends to occur. However, in the case where Si is previously contained in the GaAs raw material as in the present invention, even if the total amount of silicon charge is set to such an amount, it is possible to include Si in the GaAs melt without polycrystallization, and the carrier concentration of the entire straight body portion can be increased.
By controlling the total silicon charge amount as described above, single crystallization during crystal growth is not hindered, and the carrier concentration in the resulting GaAs ingot can be easily controlled to a desired range.
Here, the charge amount of the Si source 10 does not include the amount of silicon contained in the Si-doped GaAs source and boron oxide, and the total silicon charge does not include the amount of silicon contained in boron oxide.

The Si raw material 10 is preferably placed below (on the seed crystal side) the center of the crucible (the position corresponding to the center of the ingot). The Si raw material 10 may be placed in a GaAs container so that the dopant in the container does not come out of the container until the temperature at which the GaAs container melts. Since silicon has a lower density than GaAs, the dopant (silicon) floats up in the melt, which may reduce the silicon concentration in the GaAs melt on the seed crystal side, and the silicon concentration on the seed side of the GaAs ingot may be reduced. However, the use of a GaAs container is effective in avoiding such a situation. The GaAs container can be made of processed GaAs raw material, and may be made of processed Si-doped GaAs single crystal or processed Si-doped GaAs polycrystal.

Amphoteric dopants other than the Si raw material 10 can be used as long as they do not impair the effects of the present invention. For example, In raw materials such as high-purity In and indium compounds (e.g., high-purity indium arsenide (InAs)) can be used.

Other elements that may be considered as dopants include beryllium (Be), magnesium (Mg), aluminum (Al), carbon (C), germanium (Ge), tin (Sn), nitrogen (N), sulfur (S), selenium (Se), tellurium (Te), as well as zinc (Zn), cadmium (Cd), chromium (Cr), and antimony (Sb). Although unavoidable amounts of these elements are permitted, it is preferable that they are not added intentionally.

(Sealant)
Boron oxide (B ₂ O ₃ ) 4 is used as a sealant. The boron oxide 4 contains SiO ₂ of more than 5 mol % in terms of Si. Within this range, the amount of Si absorbed by B ₂ O ₃ in the raw material melt is suppressed, and the B of B ₂ O ₃ is prevented from being incorporated into the single crystal, and the Si concentration of the raw material melt can be controlled within an appropriate range even when stirring is performed. The amount of SiO ₂ contained in the boron oxide 4 is preferably 6 mol % or more in terms of Si. Since there is a limit to the amount of Si that can be dissolved in boron oxide, the amount of SiO ₂ contained in the boron oxide 4 can be 10 mol % or less in terms of Si, and is preferably 7 mol % or less.

The amount of boron oxide 4, which is the sealant, relative to the GaAs raw material 9 can be 0.02 wt% or more, preferably 0.03 wt% or more, in order to prevent As from escaping from the raw material melt, and can be 0.10 wt% or less, preferably 0.06 wt% or less, in order to prevent the amount of Si taken in by the stirred sealant from becoming excessive.

The method for producing a GaAs ingot according to the present invention can provide a GaAs ingot having a straight body diameter of 140 mm or less, in which wafers obtained from 70% or more (preferably 90% or more) of the straight body have a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1200 cm ^-2 or less, or a GaAs ingot having a straight body diameter of more than 140 mm, in which wafers obtained from 70% or more of the straight body have a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1500 cm ^-2 or less.

That is, when the diameter of the straight body part of a GaAs ingot is 140 mm or less, 70% or more (preferably 90% or more) of the total number of wafers that can be cut from the straight body part satisfy the conditions that the carrier concentration is 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 or less and the maximum etch pit density is 1200 cm ^-2 or less. It is more preferable that the maximum etch pit density is 1000 cm ^-2 or less. There is no particular restriction on the lower limit of the maximum etch pit density, and it may be 0.
More preferably, 92% or more of the wafers satisfy the above condition, and all the wafers (100% of the wafers) may satisfy the above condition.

When the diameter of the straight body of a GaAs ingot exceeds 140 mm, 70% or more of the wafers that can be cut out from the straight body satisfy the conditions that the carrier concentration is 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and the maximum etch pit density is 1500 cm ^-2 or less. The lower limit of the maximum etch pit density is not particularly limited and may be 0.
More preferably, 75% or more of the wafers satisfy the above condition, and all the wafers (100% of the wafers) may satisfy the above condition.

(Method of measuring carrier concentration)
The carrier concentration is determined by cutting out a 10 mm x 10 mm piece from the center of a wafer extracted from a GaAs ingot, attaching indium electrodes to the four corners, heating to 330 to 360°C, and then measuring the carrier concentration by Hall measurement using the Van der Pauw method.

(Method of measuring etch pit density (EPD))
The etch pit density (EPD) is measured by pretreating the surface of a wafer extracted from a GaAs ingot with a sulfuric acid-based mirror-finish etching solution ( _H2SO4 : _{H2O2 : H2O} =3:1:1 (volume ratio)), then immersing the wafer in a KOH melt at a liquid temperature of 320 _° C for 35 minutes to generate etch pits, and then counting the number of etch pits. Etch pits to be measured are those that have a hexagonal shape specific to zinc blende crystals and have a major axis (the length of the diagonal line passing through the center) of 20 μm or more.

The etch pit density (EPD) is measured by setting areas of 3 mm diameter at 69 or 37 points on the wafer, observing each area with a microscope, and counting the etch pits that have occurred.
The 69-point or 37-point area is to be distributed evenly over the entire surface of the wafer.
In the case of a 6-inch wafer, an area of 69 points is set at positions evenly distributed at 15 mm intervals, and in the case of a 3-inch wafer, an area of 37 points is set at positions evenly distributed at 10 mm intervals.
When the 69 areas are set at equally distributed positions, the areas are spaced 5 mm apart for a 2-inch wafer, 10 mm apart for a 4-inch wafer, and 20 mm apart for an 8-inch wafer.
FIG. 2 shows a schematic diagram of 69 areas set on a 6-inch wafer.

A 10x objective lens with a field of view diameter of 1.73 mm is used to observe each area. For each area, the field of view in which the most pits are observed is found, and the etch pits are counted, and the etch pit counts are converted to per unit area (cm ^-2 ). The maximum of the converted values of the etch pit counts in each area is taken as the maximum etch pit density (EPD). The average of the converted values of the etch pit counts in each area is taken as the average etch pit density (EPD).

The GaAs wafer according to the present invention has a strictly controlled carrier concentration and a low dislocation density, making it suitable as a substrate for surface-emitting lasers, including VCSELs (vertical-cavity surface-emitting lasers).

The above are examples of typical embodiments of the present invention, and the present invention is not limited to these.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

(GaAs raw material)
<GaAs raw material 1>
This is GaAs polycrystal made by synthesizing 6N (purity 99.9999% or more) Ga and 6N As.

<GaAs raw material 2>
This is a Si-doped GaAs polycrystal synthesized by the vertical boat method using 6N (purity 99.9999% or more) Ga and 6N As, 350wtppm of high purity Si, _{and no sealing agent B2O3 . The Si concentration calculated from the Hall measurement value is 80wtppm relative to GaAs. Since B2O3 is} not used, boron is not added to the Si-doped GaAs polycrystal.

<GaAs source material 3>
This is a recycled product made by crushing the parts other than the straight body of other single crystal ingots. The Si concentration calculated from the Hall measurement is 33 wtppm relative to GaAs. Boron is included in the raw material.

<GaAs raw material 4>
This is a Si-doped GaAs polycrystal synthesized by the vertical boat method using 450 wtppm of high purity Si for 6N (purity 99.9999% or more) Ga and _{6N As, without using the sealing agent B2O3 . The Si concentration converted from the measured value of Hall measurement is 175 wtppm for GaAs. Since B2O3 is} not used, boron is not added to the Si-doped GaAs polycrystal.

(Sealant)
<Sealant 1>
It is B ₂ O ₃ containing 7 mol % SiO ₂ calculated as Si.

<Sealant 2>
It is B ₂ O ₃ containing 5.3 mol % SiO ₂ calculated as Si.

(Crystal No. 1)
GaAs ingots were manufactured using a manufacturing apparatus having the configuration shown in Fig. 3. The stirring blades were four rectangular plate-like members made of materials that do not affect the crystal characteristics, such as carbon or BN, attached to a rod, and the area formed by the rotation locus when rotated was 50% or more of the area of the interface formed between the GaAs melt _{and B2O3} in a stationary state.

<Filling the crucible with raw materials>
A PBN crucible with an inner diameter of 112 mm and an inner diameter of 6 to 6.5 mm was prepared as the crucible. As shown in FIG. 4, the crucible was filled with 11,900 g of GaAs raw material 1 and a GaAs seed crystal cut so that the (100) plane was the crystal growth surface. The diameter of the GaAs seed crystal was adjusted by combining mechanical grinding and etching so that it was 0.5 mm smaller than the inner diameter of the seed part of each crucible. In the middle of filling the GaAs raw material 1, a shot of high purity Si was filled as a dopant (Si raw material) at 300 wtppm relative to GaAs. No impurity elements were intentionally added other than Si. The granular dopant (Si raw material) 10 was filled in a GaAs container having a cylindrical GaAs polycrystal 9B sandwiched between disk-shaped GaAs polycrystals 9C from above and below, and the dopant was prevented from escaping from the GaAs container until the GaAs container was heated to a melting temperature.

<Crystal Growth>
After these raw materials were filled, 550 g of the sealant 1 was filled. The filled crucible was set in a crucible storage container (susceptor). The inside of the manufacturing apparatus shown in FIG. 3 was repeatedly evacuated and replaced with Ar gas to create an inert gas atmosphere, and then a single crystal was grown by the VGF method.
In the crystal growth process, the raw material in the crucible was first heated to 1238°C or higher, the melting point of GaAs, to form a melt while applying a temperature gradient using a PID-controlled heater so that the temperature on the seed crystal side was lowered so as not to melt the GaAs seed crystal. Thereafter, the temperature near the seed crystal was raised so that the upper part of the seed crystal melted, and then the temperature in the entire furnace was lowered at a rate of 0.5°C/h or less by controlling the heater while applying a temperature gradient, thereby growing an n-type GaAs ingot with Si as a dopant.
When the ingot had grown to the point where the cone part had grown to the crystal body part, the stirring blade was inserted into the sealant so that the distance between the interface between the sealant (boron oxide) and the raw material melt and the lower end of the stirring blade was 1 mm or less, and the lower end did not come into contact with the interface. The stirring blade rotation speed was kept at 0 rpm until the ingot had grown to the middle, and then rotated at 0.5 rpm from the middle to the tail, and stirring was continued until the tail had grown.
The resulting GaAs ingot has n-type conductivity.

<Evaluation>
The straight body of the grown GaAs ingot was sliced with a wire saw into wafers. The wafer size was equivalent to 4 inches. The length of the straight body was calculated based on the total number of wafers obtained from the straight body, which was 297, and the position of each wafer from the straight body seed was calculated. The carrier concentration was measured by the method described above for the wafers cut from the positions from the straight body seed shown in Table 1. In addition, the average and maximum EPDs were measured by the method described above for the wafers cut from the positions from the straight body seed shown in Table 1.
Furthermore, for the carrier concentration and EPD from the seed side to the tail side, which are the measured wafer positions (1st to 287th) listed in Table 1, the intermediate values between each measured value were calculated by connecting each measured value with a straight line, and the percentage of wafers in the measurement range from the seed side to the tail side having a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1200 cm ^-2 or less (the percentage of wafers satisfying the specified conditions) was calculated relative to the total number of wafers (297 wafers) cut out from the GaAs ingot, and is shown in Table 1. In calculating the percentage of wafers satisfying the specified conditions, wafers located outside the wafer furthest from the tail side and closer to 100% from the straight body seed were deemed not to satisfy the specified conditions.

(Crystal No. 2-3)
GaAs ingots of crystal numbers 2 and 3 were produced and evaluated in the same manner as for crystal number 1, except that the conditions were changed as shown in Table 1. The obtained GaAs ingots had an n-type conductivity.

(Crystal No. 4)
A GaAs ingot of crystal number 4 was produced and evaluated in the same manner as crystal number 1, except that after the GaAs seed crystal, GaAs raw material 1, and high-purity Si shot were filled, a quartz plate (disk-shaped with a diameter of 51 mm and a thickness of 5 mm) was placed thereon, and then the sealing material 1 was filled. The obtained GaAs ingot had an n-type conductivity.

Table 1 shows GaAs ingots with a diameter of the straight body of 140 mm or less, and for crystal numbers 2 and 3, 96.6% of the total number of wafers (297 wafers) in the measurement range from the seed side to the tail side in Table 1 had a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and satisfied the condition that the maximum etch pit density was 1,200 cm ^-2 or less.
Crystal No. 1 is an example in which a GaAs ingot was grown using a high-purity GaAs polycrystalline raw material that was not doped with Si, but the percentage of wafers that satisfied the above conditions was only 60.9%.
Crystal No. 4 is an example in which a GaAs ingot was produced using a quartz plate, and corresponds to Patent Document 1. In this example, the quartz plate reduces the effect of suppressing the increase in the Si concentration due to stirring, so the increase in the carrier concentration from the center onwards could not be suppressed, and the percentage of wafers that satisfied the above conditions was only 67.6%.
For crystal numbers 1 and 4, even if the wafers located closer to 100% from the seed than the wafer closest to the tail among the wafers being measured satisfy the specified conditions, the proportion of wafers that satisfy the conditions is less than 70%.

(Crystal No. 5)
<Filling the crucible with raw materials>
A PBN crucible with an inner diameter of 159.9 mm and an inner diameter of 6.0 to 6.5 mm was prepared as the crucible. The crucible was filled with 20,000 g of GaAs raw material 1 and GaAs seed crystals cut so that the (100) plane was the crystal growth surface. The diameter of the GaAs seed crystals was adjusted by combining mechanical grinding and etching so that it was about 0.5 mm smaller than the inner diameter of the seed part of each crucible. In the middle of filling the GaAs polycrystals, high-purity Si shots were filled as a dopant (Si raw material) at 210 wtppm relative to the GaAs. No impurity elements were intentionally added other than Si.

<Crystal Growth>
After these raw materials were filled, 965±10 g of sealant 2 was filled. The filled crucible was set in a crucible storage container (susceptor). The inside of the manufacturing apparatus shown in FIG. 3 was repeatedly evacuated and replaced with Ar gas to create an inert gas atmosphere, and then a single crystal was grown by the VGF method.
In the crystal growth process, the raw material in the crucible was first heated to 1238°C or higher, the melting point of GaAs, to form a melt while applying a temperature gradient using a PID-controlled heater so that the temperature on the seed crystal side was lowered so as not to melt the GaAs seed crystal. After that, the temperature near the seed crystal was raised so that the upper part of the seed crystal melted, and then the temperature in the entire furnace was lowered at a rate of 0.5°C/h or less by controlling the heater while applying a temperature gradient, thereby growing an n-type GaAs ingot with Si as a dopant.
When the ingot had grown to the point where the cone part had grown to the crystal body part, the stirring blade was inserted into the sealant so that the distance between the interface between the boron oxide sealant and the raw material melt and the lower end of the stirring blade was 1 mm or less, and the lower end did not come into contact with the interface. The rotation speed of the stirring blade 20 was kept at 0 rpm until the ingot had grown to the middle, and then rotated at 0.5 rpm from the middle to the tail, and stirring was continued until the tail had grown.
The resulting GaAs ingot has n-type conductivity.

<Evaluation>
The straight body of the grown GaAs ingot was sliced with a wire saw into a wafer shape. The wafer size was equivalent to 6 inches. The length of the straight body was calculated by assuming that the total number of wafers obtained from the straight body was 161, and the position of each wafer from the straight body seed was calculated. The final cut surface was located 20 mm from the end of the ingot opposite to the seed crystal side (towards the seed side). The carrier concentration was measured by the above-mentioned method for the wafers cut from the positions from the straight body seed shown in Table 2. In addition, the average and maximum EPDs were measured by the above-mentioned method for the wafers cut from the positions from the straight body seed shown in Table 2.
Furthermore, by calculating the carrier concentration and EPD from the seed side to the tail side, which are the measured wafer positions (2nd to 148th) listed in Table 2, assuming that the intermediate value between each measured value is taken as a straight line connecting each measured value, the percentage of wafers in the measurement range from the seed side to the tail side having a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1500 cm ^-2 or less (the percentage of wafers satisfying the specified conditions) out of the total number of wafers (161 wafers) cut out from the GaAs ingot is shown in Table 2. In calculating the percentage of wafers satisfying the specified conditions, wafers located outside the wafers to be measured were considered not to satisfy the specified conditions.

(Crystal No. 6)
A GaAs ingot of Crystal No. 6 was produced and evaluated in the same manner as for Crystal No. 5, except that the conditions were changed as shown in Table 2. The resulting GaAs ingot had an n-type conductivity.

(Crystal No. 7)
A GaAs ingot of crystal number 7 was produced and evaluated in the same manner as crystal number 6, except that GaAs source material 4 (Si concentration 175 wtppm) was used as shown in Table 2, and the rotation speed of the stirring blade was kept at 0 rpm until the crystal grew to the middle, and then rotated at 2.5 rpm from the middle to the tail. The resulting GaAs ingot had an n-type conductivity.

Table 2 shows GaAs ingots with a straight body diameter of more than 140 mm. For crystal number 6, 79.1% of the total number of wafers (161 wafers) in the measurement range from the seed side to the tail side obtained from the straight body had a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and satisfied the condition that the maximum etch pit density was 1,500 cm ^-2 or less.
For crystal number 7, 77.1% of the total number of wafers (161) in the measurement range from the seed side to the tail side obtained from the straight body portion had a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and satisfied the condition that the maximum etch pit density was 1,500 cm ^-2 or less.
On the other hand, crystal No. 5 is an example in which a GaAs ingot was grown using high-purity GaAs polycrystalline raw material that was not doped with Si, but the proportion of wafers that satisfied the above conditions was only 37.7%.
In the case of crystal number 5, even if the wafers positioned outside the wafer to be measured satisfy the predetermined condition, the ratio of wafers satisfying the condition is less than 70%.

The present invention provides a GaAs ingot and a manufacturing method thereof that can efficiently produce GaAs wafers with a strictly controlled carrier concentration and low dislocation density, and is highly useful in industry.

1 Heater 2 Crucible storage container (susceptor)
3 Crucible 4 Boron oxide 5 Compound semiconductor raw material (raw material melt)
6: seed crystal 7: airtight container 8: inert gas 9: GaAs polycrystalline raw material 10: Si raw material 14: crucible lifting and rotating mechanism 15: position where the cone portion changes to the body portion 16: halfway between position 15 and position 17 (middle)
17 Position where the straight body portion ends 18 Straight body portion of GaAs ingot 19 Cone portion of GaAs ingot 20 Stirring blade 21 Upper rod 30 6-inch wafer 31 Area

Claims (3)

A method for producing a GaAs ingot, comprising: placing a seed crystal, a silicon source material as a dopant, a GaAs source material, and boron oxide as a sealant in a crucible and growing the crystal by a vertical boat method;
The GaAs raw material is Si-doped GaAs having a Si concentration of 20 to 200 wtppm relative to GaAs,
The charge amount of the Si raw material is 200 to 300 wtppm relative to GaAs,
The boron oxide contains more than 5 mol% SiO ₂ calculated as Si,
the Si raw material, the GaAs raw material, and the boron oxide are melted by heating, and then crystal growth is carried out while stirring the liquid boron oxide. A method for producing a GaAs ingot having a diameter of 140 mm or less, the method comprising the steps of: a total silicon charge amount, which is the sum of the Si in the GaAs raw material and the Si raw material, exceeds 300 wtppm relative to GaAs;
2. A method for producing a GaAs ingot having a diameter of a straight body portion exceeding 140 mm, comprising the steps of: a charge amount of the Si raw material is 200 to 250 wtppm relative to GaAs; and a total silicon charge amount, which is the sum of the Si in the GaAs raw material and the Si raw material, exceeds 250 wtppm relative to GaAs. A 4-inch Si-doped GaAs ingot, in which 4-inch wafers obtained from 70% or more of the straight body have a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1200 cm ^-2 or less, or a Si-doped GaAs ingot with a straight body diameter of more than 140 mm, in which wafers obtained from 70% or more of the straight body have a carrier concentration of 1.5×10 ¹⁸ to 2.9×10 ¹⁸ cm ^-3 and a maximum etch pit density of 1500 cm ^-2 or less.

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