JP2015214441A - Manufacturing method of self-standing substrate and self-standing substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a self-standing substrate which includes a high concentration of n-type impurity and has an excellent crystal quality.SOLUTION: A manufacturing method of a self-standing substrate includes the steps of: preparing a ground substrate 210 consisting of a group III nitride semiconductor; and forming a group III nitride semiconductor layer 250 on the ground substrate 210 using a hydride vapor growth method. The step of forming the group III nitride semiconductor layer 250 includes a step of forming a high concentration region 220 having an n-type impurity concentration of 5×10cmor more and a thickness of 50 μm or more. A growth temperature in the step of forming the high concentration region 220 is 1120°C or more. The self-standing substrate has a threading dislocation density of 5×10cmor less.

Description

  The present invention relates to a method for manufacturing a free-standing substrate and a free-standing substrate.

  As a free-standing substrate made of gallium nitride (GaN), for example, there is one described in Patent Document 1.

JP 2012-162431 A JP 2009-147319 A JP 2002-353503 A

  If a high-concentration n-type impurity can be contained in a free-standing substrate made of GaN, it is expected that the degree of design freedom and application when manufacturing a device using the free-standing substrate will be expanded. However, it is difficult to contain an n-type impurity at a high concentration in the production of a free-standing substrate.

Although not a method for manufacturing a free-standing substrate, there is a prior art in which an n-type impurity is doped at a high concentration when an n-type layer of a device is formed on a substrate such as GaN or sapphire. Patent Document 2 describes that an n-type impurity is doped at a concentration of 5.8 × 10 ¹⁸ cm ⁻³ . However, it is known that when doping is performed at a high concentration, defects such as an increase in dislocations and occurrence of cracks in severe cases occur.

Actually, in Patent Document 3, in the GaN-based compound semiconductor, the electric resistance decreases as the carrier concentration of the n-type layer increases, but if the carrier concentration exceeds 1 × 10 ¹⁸ cm ⁻³ , the n-type layer cracks. It is described that the frequency of occurrence is increased. Therefore, in reality, it is difficult to dope n-type impurities at a high concentration.

  In addition, doping the n-type impurity at a high concentration is relatively easy for a device layer having a small film thickness, but it is difficult to grow a thick film on a free-standing substrate. Patent Document 2 also describes that surface roughness tends to occur when the film thickness exceeds 20 μm. For a free-standing substrate, it is highly necessary that the dislocation density is lower than that of the device layer, and it is necessary to grow a thick film, so it is difficult to balance the high n-type impurity concentration and the low dislocation density. Met.

  In addition, as a result of intensive studies by the inventor, when forming a GaN layer on a different substrate such as sapphire, it is relatively easy to increase the n-type impurity concentration, but the GaN layer is formed on a self-supporting GaN substrate. In this case, it has been found difficult to increase the n-type impurity concentration. The reason is not clear, but for example, in the growth on a different substrate, it is easy to increase the impurity concentration due to the strain caused by the interaction of GaN with the different substrate, and in the growth on the same substrate, a dislocation density crystal is formed. It is conceivable that the impurity incorporation into the periphery is relatively reduced and the addition of impurities becomes more difficult.

  The present invention provides a self-supporting substrate containing an n-type impurity at a high concentration and excellent in crystal quality.

According to the present invention,
Preparing a base substrate made of a group III nitride semiconductor;
Forming a group III nitride semiconductor layer on the base substrate by hydride vapor phase epitaxy,
The step of forming the group III nitride semiconductor layer includes a step of forming a high concentration region in which the concentration of the n-type impurity is 5 × 10 ¹⁸ cm ⁻³ or more and the thickness is 50 μm or more,
In the step of forming the high concentration region, the growth temperature is set to 1120 ° C. or higher,
A method for manufacturing a self-supporting substrate having a threading dislocation density of 5 × 10 ⁶ cm ⁻² or less is provided.

According to the present invention,
A high-concentration region having a concentration of n-type impurities of 5 × 10 ¹⁸ cm ⁻³ or more over a thickness of 50 μm or more;
A free-standing substrate of a group III nitride semiconductor having a threading dislocation density of 5 × 10 ⁶ cm ⁻² or less is provided.

  According to the present invention, it is possible to provide a self-supporting substrate that contains an n-type impurity at a high concentration and is excellent in crystal quality.

It is a figure which shows the structure of the HVPE apparatus used for manufacture of the self-supporting board | substrate which concerns on 1st Embodiment. It is a figure for demonstrating the process of the manufacturing method of the self-supporting board | substrate which concerns on 1st Embodiment. It is a figure for demonstrating the process of the manufacturing method of the self-supporting board | substrate which concerns on 2nd Embodiment. It is a figure for demonstrating the supply flow rate of doping gas in the modification of 2nd Embodiment. It is a figure for demonstrating the process of the manufacturing method of the self-supporting board | substrate which concerns on 3rd Embodiment. It is a figure which shows the result of having observed the growth surface of the GaN layer formed with the method which concerns on a comparative example with the differential interference microscope. The figure which showed the relationship between (0004) rocking curve half width and carrier concentration, and the relationship between carrier density and etch pit density (EPD) when the growth surface of the formed GaN layer was measured by X-ray diffraction measurement It is. It is a figure which shows the result of having observed the growth surface of the GaN layer (high concentration area | region) of the outermost surface grown with the method which concerns on an Example with the differential interference microscope. And supply partial pressure ratio of _SiH 2 Cl ₂ for HCl, which is a diagram showing the relationship between the carrier concentration of the GaN layer.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same reference numerals are given to the same components, and the description will be omitted as appropriate.

(First embodiment)
According to the first embodiment, a method for manufacturing a self-supporting substrate includes a step of preparing a base substrate 210 made of a group III nitride semiconductor, and a group III nitride semiconductor on the base substrate 210 by hydride vapor phase epitaxy. Forming the layer 250. The step of forming group III nitride semiconductor layer 250 includes the step of forming high-concentration region 220 in which the concentration of n-type impurities is 5 × 10 ¹⁸ cm ⁻³ or more and the thickness is 50 μm or more. In the step of forming the high concentration region 220, the growth temperature is set to 1120 ° C. or higher. The threading dislocation density of the free-standing substrate is set to 5 × 10 ⁶ cm ⁻² or less.

  First, a vapor phase growth apparatus used for growing the group III nitride semiconductor layer 250 in this embodiment will be described. The free-standing substrate according to this embodiment is manufactured using a hydride vapor phase epitaxy (HVPE) method. This is because high-speed growth is possible by the HVPE method and manufacturing efficiency is excellent. FIG. 1 is a diagram showing a structure of an HVPE apparatus 100 used for manufacturing a self-supporting substrate according to the present embodiment.

  The HVPE apparatus 100 includes a reaction tube 121, a substrate holder 123, a group III gas supply unit 139, a nitrogen source gas supply unit 137, a doping gas supply tube 125, a gas discharge tube 135, a first heater 129 and a second heater 130. . The substrate holder 123 is provided in the reaction tube 121. The group III gas supply unit 139 supplies the group III source gas to the growth region 122 including the substrate holder 123 in the reaction tube 121. The nitrogen source gas supply unit 137 supplies nitrogen source gas to the growth region 122. The doping gas supply pipe 125 supplies doping gas to the growth region 122. The gas discharge pipe 135 discharges the gas in the reaction pipe 121.

  In the HVPE apparatus 100, a group III nitride semiconductor layer is grown on the substrate 133 held by the substrate holder 123. The substrate holder 123 is attached to the rotating shaft 132 and is rotatable.

A first gas supply pipe 124 and a second gas supply pipe 126 are connected to the reaction pipe 121, and between the supply port of the first gas supply pipe 124 and the supply port of the second gas supply pipe 126. A shielding plate 136 is provided. Of the reaction tube 121, the side close to the supply ports of the first gas supply pipe 124, the doping gas supply pipe 125, and the second gas supply pipe 126 is called the upstream side, and the side close to the gas discharge pipe 135 is the downstream side. Call it. The shielding plate 136 separates the space on the upstream side of the reaction tube 121 into two layers, an upper layer and a lower layer. A source boat 128 is provided in the lower region, and the group III raw material 127 is held in the source boat 128. Group III material 127 is, for example, gallium (Ga). The gas supplied from the first gas supply pipe 124 and the second gas supply pipe 126 can be switched to a purge gas for purging the inside of the reaction pipe 121 as necessary. The purge gas is, for example, nitrogen (N ₂ ) gas.

Nitrogen source gas is supplied from the first gas supply pipe 124 into the reaction pipe 121. The nitrogen source gas is, for example, ammonia (NH ₃ ). A halogen-containing gas is supplied from the second gas supply pipe 126 into the reaction pipe 121. The halogen-containing gas is, for example, hydrogen chloride (HCl). A doping gas is supplied from the doping gas supply pipe 125 into the reaction tube 121. Silicon (Si) can be added as an n-type impurity by using, for example, dichlorosilane (SiH ₂ Cl ₂ ) or monosilane (SiH ₄ ) as a doping gas. Alternatively, germanium (Ge) can be added as an n-type impurity by using, for example, germanium tetrachloride (GeCl ₄ ) or germanium hydride (GeH ₄ ) as a doping gas. Alternatively, oxygen (O) can be added as an n-type impurity by using, for example, oxygen (O ₂ ), dinitrogen monoxide (N ₂ O), or water vapor (H ₂ O) as a doping gas.

  The nitrogen source gas supply unit 137 includes a first gas supply pipe 124 and a region above the shielding plate 136 in the reaction pipe 121 (excluding the doping gas supply pipe 125 and the inside thereof). The group III gas supply unit 139 includes a region below the shielding plate 136 among the second gas supply pipe 126, the source boat 128, the group III raw material 127, and the reaction pipe 121. A first heater 129 is disposed around the nitrogen source gas supply unit 137 and the group III gas supply unit 139.

  The nitrogen source gas supplied from the first gas supply pipe 124 passes through the nitrogen source gas supply unit 137 toward the downstream side and is supplied to the surface of the substrate 133. At that time, the inside of the nitrogen source gas supply unit 137 is maintained at a temperature of, for example, 800 ° C. or more and 900 ° C. or less by heat applied from the first heater 129. Due to this heat, the nitrogen source gas supply unit 137 promotes the decomposition of the nitrogen source gas.

  In the group III gas supply unit 139, a group III source gas is generated from the halogen-containing gas supplied from the second gas supply pipe 126 and the group III source 127 held in the source boat 128. The generated group III source gas is supplied to the surface of the substrate 133 held by the substrate holder 123. At this time, the inside of the group III gas supply unit 139 is maintained at a temperature of, for example, 800 ° C. or higher and 900 ° C. or lower by heat applied from the first heater 129. When the halogen-containing gas supplied from the second gas supply pipe 126 passes through the group III gas supply unit 139 downstream, the surface of the group III raw material 127 held in the source boat 128 or the volatilized III Contact with the group raw material 127. Then, a group III source gas is generated. For example, when the halogen-containing gas is HCl and the group III source material 127 is Ga, the group III gas supply unit 139 generates a group III source gas containing gallium chloride (GaCl).

  In the reaction tube 121, a substrate holder 123 holding the substrate 133 is arranged in the growth region 122 located downstream of the nitrogen source gas supply unit 137 and the group III gas supply unit 139. A nitrogen source gas is supplied from the nitrogen source gas supply unit 137 and a group III source gas is supplied from the group III gas supply unit 139 to the growth region 122. Then, a group III nitride semiconductor layer is formed on the substrate 133. A second heater 130 is disposed around the growth region 122, and heat is applied to the growth region 122 as necessary. While the group III nitride semiconductor layer is formed, the substrate holder 123 is rotated about the rotation shaft 132, whereby a uniform layer can be obtained within the surface of the substrate 133.

Next, the process of the manufacturing method of the self-supporting substrate according to this embodiment will be described.
FIG. 2 is a diagram for explaining a process of the method for manufacturing a self-supporting substrate according to the present embodiment. The method for manufacturing a self-supporting substrate according to the present embodiment includes a step of preparing a base substrate 210 made of a group III nitride semiconductor and a step of forming a group III nitride semiconductor layer 250 on the base substrate 210. In the present embodiment, both the base substrate 210 and the group III nitride semiconductor layer 250 are made of gallium nitride (GaN). The group III nitride semiconductor layer 250 is formed by the hydride vapor phase growth method using the HVPE apparatus 100 described above.

In the present embodiment, an example in which the group III nitride semiconductor layer 250 is a layer composed of only the high concentration region 220 will be described. A layered region having an n-type impurity concentration of 5 × 10 ¹⁸ cm ⁻³ or more and a thickness of 50 μm or more is referred to as a high concentration region 220. In the following description, the region refers to a region formed in a layer shape included in the group III nitride semiconductor layer 250.

First, as shown in FIG. 2A, a base substrate 210 made of a group III nitride semiconductor is prepared. The concentration of the n-type impurity in the base substrate 210 is preferably 1 × 10 ¹⁶ cm ⁻³ or more from the viewpoint of preventing generation of cracks when preparing the base substrate 210 or forming the group III nitride semiconductor layer 250. X10 ¹⁷ cm ⁻³ or more is more preferable. From the same viewpoint, the concentration of the n-type impurity in the base substrate 210 is preferably less than 5 × 10 ¹⁸ cm ⁻³ . Next, the base substrate 210 is attached as a substrate 133 to the substrate holder 123 of the HVPE apparatus 100. The thickness of the base substrate 210 is preferably 50 μm or more, and more preferably 300 μm or more, from the viewpoint of ease of handling.

Purge gas is supplied from the first gas supply pipe 124 and the second gas supply pipe 126 to purge the inside of the reaction pipe 121. The purge gas supplied into the reaction tube 121 is discharged from the gas discharge tube 135. After sufficiently purging the inside of the reaction tube 121, the gas supplied from the first gas supply tube 124 and the second gas supply tube 126 is switched to the carrier gas. The carrier gas is, for example, H ₂ (hydrogen) gas.

Next, the temperature of the reaction tube 121 is raised by the first heater 129 and the second heater 130. During the temperature rise, the supply of the nitrogen source gas is started from the first gas supply pipe 124 in addition to the carrier gas. In the present embodiment, NH ₃ is used as the nitrogen source gas. The nitrogen source gas supplied from the first gas supply pipe 124 is decomposed by heat and supplied to the growth region 122. Subsequently, the temperature rise by the first heater 129 is continued until the temperature of the group III raw material 127 reaches 850 ° C., for example. The temperature rise by the second heater 130 is continued until the temperature of the growth region 122 becomes 1120 ° C. or higher, more preferably 1150 ° C. or higher. Here, the high concentration region 220 is formed at a growth temperature of 1120 ° C. or higher, preferably 1150 ° C. or higher, from the viewpoint of forming the high concentration region 220 having a high concentration of n-type impurities and excellent crystal quality. Hereinafter, the growth temperature refers to the temperature of the growth region 122 during layer formation.

  After the temperatures of the group III material 127 and the growth region 122 are stabilized, supply of the halogen-containing gas in addition to the carrier gas is started from the second gas supply pipe 126. In the present embodiment, HCl is used as the halogen-containing gas. The halogen-containing gas supplied from the second gas supply pipe 126 reacts with the group III source material 127 to generate a group III source gas, and the group III source gas is supplied to the growth region 122.

The supply of the halogen-containing gas in addition to the carrier gas is started from the second gas supply pipe 126, and at the same time, the supply of the doping gas from the doping gas supply pipe 125 to the growth region 122 is started. In the growth region 122, the nitrogen source gas, the group III source gas, and the doping gas react to form a group III nitride semiconductor layer 250 on the base substrate 210. At this time, the supply flow rate of the doping gas is set so that the concentration of the n-type impurity is 5 × 10 ¹⁸ cm ⁻³ or more.

  The growth surface of the group III nitride semiconductor layer 250 according to this embodiment is a c-plane (0001). However, the growth surface may be a nonpolar surface or a nonpolar surface.

The ratio of the supply flow rate of the doping gas to the supply flow rate of the halogen-containing gas when forming the high concentration region 220 is preferably 1 × 10 ⁻⁵ or more, more preferably 3 × 10 ⁻⁵ or more, and 8 × 10. ⁻⁵ or less is preferable, and 5 × 10 ⁻⁵ or less is more preferable. Under these conditions, in the step of forming the high concentration region 220, it is possible to form the high concentration region 220 containing n-type impurities at a high concentration and having excellent crystal quality.

In this embodiment, SiH ₂ Cl ₂ is used as the doping gas, and the n-type impurity is Si. Note that, by using, for example, SiH ₂ Cl ₂ or SiH ₄ as a doping gas, the n-type impurity can be Si. For example, GeCl ₄ or GeH ₄ can be used instead of SiH ₂ Cl ₂ or SiH ₄ as a doping gas, so that the n-type impurity can be Ge. Alternatively, for example, O ₂ , N ₂ O, or H ₂ O The n-type impurity can also be changed to O by using.

  Then, the formation is continued until the thickness of the high concentration region 220 becomes 50 μm or more, more preferably 300 μm or more. By optimizing the supply flow rate of nitrogen source gas, group III source gas (halogen-containing gas), and doping gas, the growth temperature, etc., n-type impurities are increased on the base substrate 210 made of a group III nitride semiconductor. It is possible to form a high concentration region 220 that is contained in a concentration and excellent in crystal quality. Here, the half-width of the rocking curve (0004) measured by the X-ray diffraction measurement of the high concentration region 220 can be 60 arcsec or less.

  As shown in FIG. 2B, when the thickness of the high concentration region 220 reaches a predetermined thickness, the doping gas is supplied from the doping gas supply pipe 125 and the halogen-containing gas is supplied from the second gas supply pipe 126. Is stopped, the first heater 129 and the second heater 130 are stopped, and the temperature of the reaction tube 121 is lowered. The supply of the nitrogen source gas from the first gas supply pipe 124 is continued even during the temperature drop, and when the temperature of the growth region 122 becomes 600 ° C. or less, for example, the gas supplied from the first gas supply pipe 124 is Switch from carrier gas and nitrogen source gas to purge gas. Then, after the temperature is sufficiently lowered, the substrate 133 having the group III nitride semiconductor layer 250 formed on the base substrate 210 is taken out from the reaction tube 121.

Here, the threading dislocation density of the multilayer body including the base substrate 210 and the group III nitride semiconductor layer 250 is 5 × 10 ⁶ cm ⁻² or less. The threading dislocation density is 1 × 10 ³ cm ⁻² or more. The threading dislocation density can be measured by forming etch pits on the upper surface of the group III nitride semiconductor layer 250 and measuring the density of the formed etch pits. In the high concentration region 220, the ratio of the carrier concentration to the n-type impurity concentration is 0.7 or more at 25 ° C. Therefore, the added n-type impurity works efficiently and a device with good characteristics can be manufactured.

  The method for manufacturing a self-supporting substrate according to the present embodiment further includes a step of removing the base substrate 210 after the step of forming the group III nitride semiconductor layer 250. As shown in FIG. 2C, the III-nitride semiconductor from which the base substrate 210 is removed by polishing the surface of the laminate of the base substrate 210 and the group III nitride semiconductor layer 250 where the base substrate 210 is exposed. A free-standing substrate consisting of layer 250 can be obtained. Since group III nitride semiconductor layer 250 includes only high-concentration region 220, a self-supporting substrate in which high-concentration region 220 is exposed on both surfaces is obtained.

However, a structure including the base substrate 210 and the group III nitride semiconductor layer 250 may be a self-supporting substrate without removing the base substrate 210. At this time, a high-concentration region 220 is exposed on one surface, and a free-standing substrate is obtained in which the n-type impurity concentration is less than 5 × 10 ¹⁸ cm ⁻³ in the vicinity of the surface of the other surface. In the following description, the vicinity of the surface indicates a depth region in which the influence of the adhered matter or adsorbed material on the surface does not appear in the measured n-type impurity concentration. The resulting free-standing substrate has a high concentration region 220 having an n-type impurity concentration of 5 × 10 ¹⁸ cm ⁻³ or more over a thickness of 50 μm or more. Further, the threading dislocation density of the free-standing substrate is 5 × 10 ⁶ cm ⁻² or less. Further, the threading dislocation density is 1 × 10 ³ cm ⁻² or more.

  A method for measuring threading dislocations in a free-standing substrate will be described below. In the growth surface of the group III nitride semiconductor layer 250, that is, in the stacked body of the base substrate 210 and the group III nitride semiconductor layer 250 shown in FIG. 2B, the group III nitride far from the base substrate 210 in the thickness direction. The main surface of the nitride semiconductor layer 250 is called a main surface S1, while the main surface of the group III nitride semiconductor layer 250 close to the base substrate 210 in the thickness direction is called a main surface S2. Here, a part of dislocations on the main surface S2 inherited from the base substrate 210 penetrates to the main surface S1 to form threading dislocations. Therefore, the dislocation density on the main surface S1 is lower than the dislocation density on the main surface S2, and the dislocation density on the main surface S1 can be regarded as the threading dislocation density. For example, when base substrate 210 and group III nitride semiconductor layer 250 are c-plane grown GaN, main surface S1 is a Ga surface, and etch pits are formed on main surface S1 by etching to measure the density of etch pits. Thus, the threading dislocation density of the laminate of the base substrate 210 and the group III nitride semiconductor layer 250 can be measured.

  When the base substrate 210 is removed to form a self-supporting substrate as shown in FIG. 2C, the surface that is the main surface S1 in the laminate is the main surface S1 ′ of the self-supporting substrate, and is the side that is the main surface S2. This surface becomes the main surface S2 ′ of the self-standing substrate. Therefore, the dislocation density at the main surface S1 ′ can be obtained by regarding the threading dislocation density of the free-standing substrate. For example, when base substrate 210 and group III nitride semiconductor layer 250 are c-plane grown GaN, main surface S1 ′ is a Ga surface and main surface S2 ′ is an N surface. The N surface does not form etch pits by etching and has a higher etching rate than the Ga surface, so that the main surface S1 ′ and the main surface S2 ′ can be distinguished. Then, the threading dislocation density of the free-standing substrate can be measured by forming and measuring the etch pits on the main surface S1 ′ as described above.

  Alternatively, the dislocation density on both surfaces of the free-standing substrate can be measured, and the smaller one can be obtained as the threading dislocation density. Note that the dislocation density on the N face of the GaN substrate is obtained by cutting from the Ga face side so that the self-supporting substrate becomes thin, and the dislocation density near the N face is obtained by forming and measuring etch pits from the Ga face side.

  Note that, after the group III nitride semiconductor layer 250, a step of slicing the group III nitride semiconductor layer 250 may be included instead of the step of removing the base substrate 210. By slicing the group III nitride semiconductor layer 250 grown in a thick film along a plane parallel to the growth surface, a plurality of free-standing substrates composed of the high concentration region 220 can be obtained.

  The base substrate 210 and the group III nitride semiconductor layer 250 are preferably made of the same group III nitride semiconductor. This is because a group III nitride semiconductor layer 250 having a crystal lattice constant and a thermal expansion coefficient close to each other and good crystallinity can be obtained.

  Next, the operation and effect of this embodiment will be described. According to this embodiment, it is possible to obtain a self-supporting substrate that contains an n-type impurity at a high concentration and is excellent in crystal quality.

  By manufacturing an optical device or an electronic device using a self-standing substrate having such a high n-type impurity concentration and a low threading dislocation density, device characteristics can be improved. This is because the contact resistance with the electrode, the bulk resistance of the substrate, and the series resistance in the thickness direction of the substrate are reduced.

(Second Embodiment)
FIG. 3 is a diagram for explaining a process of the method for manufacturing a self-supporting substrate according to the second embodiment. The method for manufacturing a self-supporting substrate according to the present embodiment is the self-supporting substrate according to the first embodiment, except that the step of forming the group III nitride semiconductor layer 250 further includes the step of forming the low concentration region 230. This is the same configuration as the manufacturing method. Here, the concentration of the n-type impurity in the low concentration region 230 is less than 5 × 10 ¹⁸ cm ⁻³ . This will be described in detail below.

  In the present embodiment, the step of forming the group III nitride semiconductor layer 250 includes the step of forming the low concentration region 230 before the step of forming the high concentration region 220, and the n-type impurity of the low concentration region 230 is formed. The concentration is equal to or higher than the concentration of n-type impurities in the base substrate 210.

  In the present embodiment, by forming the low concentration region 230 between the base substrate 210 and the high concentration region 220, the n-type impurity concentration differs between the base substrate 210 and the group III nitride semiconductor layer 250. Generation of a three-dimensional growth mode at an interface or an interface between regions can be suppressed, or changes in crystal lattice constant, thermal expansion coefficient, and the like can be reduced. Therefore, a group III nitride semiconductor layer 250 with good crystallinity is obtained.

  In the present embodiment, the low concentration region 230 includes a region 231 and a region 232 having different n-type impurity concentrations. By providing a plurality of regions having different n-type impurity concentrations in the low-concentration region 230 and forming a layer structure in which the n-type impurity concentration gradually increases from the base substrate 210 toward the high-concentration region 220, A change in concentration at each interface can be reduced, and a group III nitride semiconductor layer 250 with better crystallinity can be obtained.

First, as shown in FIG. 3A, a base substrate 210 made of a group III nitride semiconductor is prepared. The concentration of the n-type impurity in the base substrate 210 is preferably 1 × 10 ¹⁶ cm ⁻³ or more from the viewpoint of preventing generation of cracks when preparing the base substrate 210 or forming the group III nitride semiconductor layer 250. X10 ¹⁷ cm ⁻³ or more is more preferable. From the same viewpoint, the concentration of the n-type impurity in the base substrate 210 is preferably less than 5 × 10 ¹⁸ cm ⁻³ . Next, the base substrate 210 is attached as a substrate 133 to the substrate holder 123 of the HVPE apparatus 100. The thickness of the base substrate 210 is preferably 50 μm or more, and more preferably 300 μm or more, from the viewpoint of ease of handling.

Purge gas is supplied from the first gas supply pipe 124 and the second gas supply pipe 126 to purge the inside of the reaction pipe 121. The purge gas supplied into the reaction tube 121 is discharged from the gas discharge tube 135. After sufficiently purging the inside of the reaction tube 121, the gas supplied from the first gas supply tube 124 and the second gas supply tube 126 is switched to the carrier gas. The carrier gas is, for example, H ₂ gas.

Next, the temperature of the reaction tube 121 is raised by the first heater 129 and the second heater 130. During the temperature rise, the supply of the nitrogen source gas is started from the first gas supply pipe 124 in addition to the carrier gas. In the present embodiment, NH ₃ is used as the nitrogen source gas. The nitrogen source gas supplied from the first gas supply pipe 124 is decomposed by heat and supplied to the growth region 122. Subsequently, the temperature rise by the first heater 129 is continued until the temperature of the group III raw material 127 reaches, for example, 850 ° C., and the second heater 130 increases until the temperature of the growth region 122 becomes 1120 ° C. or higher, preferably 1150 ° C. or higher. Continue to raise the temperature.

  After the temperatures of the group III material 127 and the growth region 122 are stabilized, supply of the halogen-containing gas in addition to the carrier gas is started from the second gas supply pipe 126. In the present embodiment, HCl is used as the halogen-containing gas. The halogen-containing gas supplied from the second gas supply pipe 126 reacts with the group III source material 127 to generate a group III source gas, and the group III source gas is supplied to the growth region 122.

  The supply of the halogen-containing gas in addition to the carrier gas is started from the second gas supply pipe 126, and at the same time, the supply of the doping gas from the doping gas supply pipe 125 to the growth region 122 is started. In the growth region 122, the nitrogen source gas, the group III source gas, and the doping gas react to form a layer made of the region 231 on the base substrate 210. At this time, the supply flow rate of the doping gas is, for example, about one fifth of the supply flow rate of the doping gas in the step of forming the high concentration region 220 described later. As a result, the concentration of the n-type impurity in the region 231 is about one fifth of the concentration of the n-type impurity in the high concentration region 220. The thickness of the region 231 is, for example, 10 μm. Note that the n-type impurity concentration of the base substrate 210 according to the present embodiment is lower than the n-type impurity concentration of the region 231. However, it is not necessarily limited to this.

  When the thickness of the region 231 reaches a desired thickness, the supply flow rate of the doping gas from the doping gas supply pipe 125 is adjusted, and the formation of the layer composed of the region 232 is started on the layer composed of the region 231. . The doping gas supply flow rate at the time of forming the region 232 is larger than the doping gas supply flow rate at the time of forming the region 231, and lower than the doping gas supply flow rate in the formation step of the high concentration region 220 described later. . For example, it is set to about two-fifths of the supply flow rate of the doping gas in the step of forming the high concentration region 220 described later. As a result, the n-type impurity concentration in the region 232 is about two-fifths of the n-type impurity concentration in the high concentration region 220. The thickness of the region 232 is, for example, 10 μm.

When the thickness of the region 232 reaches a desired thickness, the supply flow rate of the doping gas from the doping gas supply pipe 125 is adjusted, and the layer composed of the high concentration region 220 is formed on the layer composed of the low concentration region 230. Start forming. At this time, the supply flow rate of the doping gas is adjusted so that the concentration of the n-type impurity in the high concentration region 220 becomes 5 × 10 ¹⁸ cm ⁻³ or more.

  The growth surface of the group III nitride semiconductor layer 250 according to this embodiment is a c-plane (0001). However, the growth surface may be a nonpolar surface or a nonpolar surface.

The high concentration region 220 is formed at a growth temperature of 1120 ° C. or higher, preferably 1150 ° C. or higher. The ratio of the supply flow rate of the doping gas to the supply flow rate of the halogen-containing gas when forming the high concentration region 220 is preferably 1 × 10 ⁻⁵ or more, more preferably 3 × 10 ⁻⁵ or more, and 8 × 10. ⁻⁵ or less is preferable, and 5 × 10 ⁻⁵ or less is more preferable. Under these conditions, in the step of forming the high concentration region 220, it is possible to form the high concentration region 220 containing n-type impurities at a high concentration and having excellent crystal quality.

In this embodiment, SiH ₂ Cl ₂ is used as the doping gas, and the n-type impurity is Si. Note that, by using, for example, SiH ₂ Cl ₂ or SiH ₄ as a doping gas, the n-type impurity can be Si. For example, GeCl ₄ or GeH ₄ can be used instead of SiH ₂ Cl ₂ or SiH ₄ as a doping gas, so that the n-type impurity can be Ge. Alternatively, for example, O ₂ , N ₂ O, or H ₂ O The n-type impurity can also be changed to O by using.

  Then, as in the first embodiment, the formation is continued until the thickness of the high concentration region 220 becomes 50 μm or more, more preferably 300 μm or more. Here, the half-width of the rocking curve (0004) measured by the X-ray diffraction measurement of the high concentration region 220 can be 60 arcsec or less.

  As shown in FIG. 3B, when the thickness of the high concentration region 220 reaches a predetermined thickness, the doping gas is supplied from the doping gas supply pipe 125 and the halogen-containing gas is supplied from the second gas supply pipe 126. Is stopped, the first heater 129 and the second heater 130 are stopped, and the temperature of the reaction tube 121 is lowered. The supply of the nitrogen source gas from the first gas supply pipe 124 is continued even during the temperature drop, and when the temperature of the growth region 122 becomes 600 ° C. or less, for example, the gas supplied from the first gas supply pipe 124 is Switch from carrier gas and nitrogen source gas to purge gas. Then, after the temperature is sufficiently lowered, the substrate 133 having the group III nitride semiconductor layer 250 formed on the base substrate 210 is taken out from the reaction tube 121.

Here, the threading dislocation density of the multilayer body including the base substrate 210 and the group III nitride semiconductor layer 250 is 5 × 10 ⁶ cm ⁻² or less. The threading dislocation density is 1 × 10 ³ cm ⁻² or more. In the high concentration region 220, the ratio of the carrier concentration to the n-type impurity concentration is 0.7 or more at 25 ° C. Therefore, the added n-type impurity works efficiently and a device with good characteristics can be manufactured.

  The n-type impurity concentration in the regions 231 and 232 is lower than the n-type impurity concentration in the high-concentration region 220, and the n-type impurity concentration in the region 231 is lower than the n-type impurity concentration in the region 232.

  The method for manufacturing a self-supporting substrate according to the present embodiment further includes a step of removing a part of the group III nitride semiconductor layer 250 and the base substrate 210 after the step of forming the group III nitride semiconductor layer 250. For example, the region other than the high concentration region 220 in the group III nitride semiconductor layer 250 and the base substrate 210 are removed.

  As shown in FIG. 3C, the base substrate 210 and the low-concentration region 230 are removed by polishing the surface of the laminate of the base substrate 210 and the group III nitride semiconductor layer 250 where the base substrate 210 is exposed. As a result, a free-standing substrate consisting of only the high concentration region 220 can be obtained.

However, not only the high concentration region 220 but also the whole or part of the low concentration region 230 may be left as a self-supporting substrate. Alternatively, a structure including the base substrate 210, the low concentration region 230, and the high concentration region 220 may be used as a self-supporting substrate without removing the base substrate 210. In any case, at least one surface of the obtained free-standing substrate is an exposed surface of the high concentration region 220. The resulting free-standing substrate has a high concentration region 220 having an n-type impurity concentration of 5 × 10 ¹⁸ cm ⁻³ or more over a thickness of 50 μm or more. Further, the threading dislocation density of the free-standing substrate is 5 × 10 ⁶ cm ⁻² or less. Further, the threading dislocation density is 1 × 10 ³ cm ⁻² or more.

  The base substrate 210, the low concentration region 230, and the high concentration region 220 are preferably made of the same group III nitride semiconductor. This is because a group III nitride semiconductor layer 250 having a crystal lattice constant and a thermal expansion coefficient close to each other and good crystallinity can be obtained. As a modification, for example, the base substrate 210 and the low concentration region 230 may be GaN, and the high concentration region 220 may be aluminum gallium nitride (AlGaN).

  In the present embodiment, an example in which the low concentration region 230 includes two regions of the region 231 and the region 232 has been described. However, the low concentration region 230 may be formed as one region, and the concentration of the n-type impurity is different. You may form in three or more area | regions. In addition, at the interface between regions, the smaller the change in concentration, the more reliable the effect of suppressing the occurrence of three-dimensional growth modes and the reduction of changes in crystal lattice constant and thermal expansion coefficient at the interface. Is obtained. Therefore, group III nitride semiconductor layer 250 with better crystallinity is obtained.

  Next, a modified example of the method for manufacturing a self-standing substrate according to the present embodiment will be described. In this modification, the step of forming the group III nitride semiconductor layer 250 includes a step of forming a concentration gradient region in which the concentration of the n-type impurity continuously increases as the distance from the base substrate 210 increases. This will be described in detail below.

  In this modification, an example in which the low-concentration region 230 includes only the concentration gradient region will be described. However, the present invention is not limited to this example.

  FIG. 4 is a view for explaining the supply flow rate of the doping gas in this modification. In FIG. 4, the horizontal axis represents time, and the vertical axis represents the ratio of the doping gas supply flow rate to the halogen-containing gas supply flow rate. A change in the ratio of the supply flow rate of the doping gas from the time when the doping gas is not supplied to the time when the formation of the high concentration region 220 is started is shown.

  FIG. 4A corresponds to a change in the supply flow rate ratio of the doping gas in the first embodiment. The supply flow rate ratio of the doping gas is changed in one step from the state where the doping gas is not supplied to the supply flow rate ratio necessary for forming the high concentration region 220. The low concentration region 230 is not formed.

  FIG. 4B corresponds to a change in the supply flow rate ratio of the doping gas in the second embodiment. The doping gas supply flow rate ratio is changed stepwise from the state in which the doping gas is not supplied to the supply flow rate ratio necessary for forming the high concentration region 220. From the start of supplying the doping gas to the start of the formation of the high concentration region 220, regions each having a different n-type impurity concentration are formed for each stage of the supply flow rate ratio. In the example of FIG. 4B, two regions 231 and 232 are formed.

  FIG. 4C shows a change in the supply flow rate ratio of the doping gas in this modification. The supply flow rate ratio of the doping gas is continuously changed from the state where the doping gas is not supplied to the supply flow rate ratio necessary for forming the high concentration region 220. A concentration gradient region is formed between the start of supplying the doping gas and the start of formation of the high concentration region 220. In this way, by continuously changing the supply flow ratio of the doping gas, a concentration gradient region in which the concentration of the n-type impurity continuously increases as the distance from the surface of the base substrate 210 increases is formed.

  As described above, by continuously changing the concentration of the n-type impurity, it is possible to prevent a discontinuous change in concentration from occurring at the interface between the base substrate 210 and the group III nitride semiconductor layer 250 or between the regions. be able to. Therefore, the effect of suppressing the occurrence of three-dimensional growth mode at the interface and reducing changes in the lattice constant, thermal expansion coefficient, etc. of the crystal can be obtained more reliably, and the group III nitride semiconductor layer having better crystallinity 250 is obtained.

  In this modification, an example in which the low concentration region 230 is formed as a concentration gradient region has been described. However, the whole or part of the high concentration region 220 may be formed as a concentration gradient region.

  Depending on the structure of the free-standing substrate to be obtained, the step of forming the group III nitride semiconductor layer 250 includes the step of forming a region where the n-type impurity concentration decreases continuously as the distance from the surface of the base substrate 210 increases. May be included.

  In this embodiment, the example in which the doping gas is always supplied in the step of forming the low concentration region 230 has been described. However, the low concentration region 230 may include a region formed without supplying the doping gas. .

  In this embodiment, the example in which the step of forming the low concentration region 230 is provided only before the step of forming the high concentration region 220 has been described, but the present invention is not limited to this. Depending on the structure of the free-standing substrate to be obtained, for example, a step of forming the low concentration region 230 may be provided after the high concentration region 220 is formed, or the low concentration region 230 is formed before and after the high concentration region 220 is formed. You may provide the process to do. At this time, for example, from the viewpoint of simplifying subsequent device manufacturing, in the step of removing the base substrate 210, it is desirable to perform polishing so that at least one surface is an exposed surface of the high concentration region 220.

  Note that, after the group III nitride semiconductor layer 250, a step of slicing the group III nitride semiconductor layer 250 may be included instead of the step of removing the base substrate 210. A plurality of free-standing substrates can be obtained by slicing the group III nitride semiconductor layer 250 grown in a thick film along a plane parallel to the growth surface. At this time, for example, from the viewpoint of simplifying subsequent device manufacturing, it is desirable to perform slicing so that at least one surface is an exposed surface of the high-concentration region 220.

Note that in the step of forming the group III nitride semiconductor layer 250, the group III nitride semiconductor layer is set such that the concentration of the n-type impurity in the region other than the high concentration region 220 is less than 5 × 10 ¹⁸ cm ^−3. In addition to the high-concentration region 220, the n-type impurity concentration is 5 × 10 ¹⁸ cm ⁻³ or more and the thickness is less than 50 μm. Such a high concentration thin layer region may be formed. For example, by forming a high-concentration thin layer region having a lower n-type impurity concentration than the high-concentration region 220 between the low-concentration region 230 and the high-concentration region 220, the change in concentration at the interface between the regions is reduced. Thus, the effects of suppressing the occurrence of the three-dimensional growth mode and reducing the change in the lattice constant and the thermal expansion coefficient of the crystal at the interface can be obtained more reliably. Therefore, group III nitride semiconductor layer 250 with better crystallinity is obtained. From the same viewpoint, the high concentration thin layer region may be formed as a concentration gradient region.

  Next, the operation and effect of this embodiment will be described. In this embodiment, the same operation and effect as in the first embodiment can be obtained.

(Third embodiment)
FIG. 5 is a diagram for explaining the steps of the method for manufacturing a self-supporting substrate according to the third embodiment. The method for manufacturing a self-supporting substrate according to the present embodiment is characterized in that the step of forming the group III nitride semiconductor layer 250 includes a temperature increase forming step of forming the group III nitride semiconductor layer 250 while increasing the growth temperature. Except for this, the configuration is the same as that of the method for manufacturing a self-supporting substrate according to the second embodiment. This will be described in detail below.

  This embodiment includes a step of forming the low concentration region 230 after the step of preparing the base substrate 210 and before the step of forming the high concentration region 220. The low concentration region 230 includes a region 241 and a region 242 in addition to the regions 231 and 232 similar to those in the second embodiment. In the step of forming the low concentration region 230, the region 241 is formed at a growth temperature lower than that of the high concentration region 220. Further, the region 242 is formed while increasing the growth temperature.

First, as shown in FIG. 5A, a base substrate 210 made of a group III nitride semiconductor is prepared. The concentration of the n-type impurity in the base substrate 210 and when a substrate is prepared 210, from the viewpoint of crack prevention in forming a high-concentration region 220, 1 × ¹⁰ 16 ^{cm -3} or more preferably, 5 × ^{10 17} More preferably, it is cm ⁻³ or more. From the same viewpoint, the concentration of the n-type impurity in the base substrate 210 is preferably less than 5 × 10 ¹⁸ cm ⁻³ . Next, the base substrate 210 is attached as a substrate 133 to the substrate holder 123 of the HVPE apparatus 100. The thickness of the base substrate 210 is preferably 50 μm or more, and more preferably 300 μm or more, from the viewpoint of ease of handling.

Purge gas is supplied from the first gas supply pipe 124 and the second gas supply pipe 126 to purge the inside of the reaction pipe 121. The purge gas supplied into the reaction tube 121 is discharged from the gas discharge tube 135. After sufficiently purging the inside of the reaction tube 121, the gas supplied from the first gas supply tube 124 and the second gas supply tube 126 is switched to the carrier gas. The carrier gas is, for example, H ₂ gas.

Next, the temperature of the reaction tube 121 is raised by the first heater 129 and the second heater 130. During the temperature rise, the supply of the nitrogen source gas is started from the first gas supply pipe 124 in addition to the carrier gas. In the present embodiment, NH ₃ is used as the nitrogen source gas. The nitrogen source gas supplied from the first gas supply pipe 124 is decomposed by heat and supplied to the growth region 122. Subsequently, the temperature rise by the first heater 129 is continued until the temperature of the group III raw material 127 reaches 850 ° C., for example, and the temperature rise by the second heater 130 continues until the temperature of the growth region 122 becomes 1040 ° C., for example.

  After the temperatures of the group III material 127 and the growth region 122 are stabilized, supply of the halogen-containing gas in addition to the carrier gas is started from the second gas supply pipe 126. In the present embodiment, HCl is used as the halogen-containing gas. The halogen-containing gas supplied from the second gas supply pipe 126 reacts with the group III source material 127 to generate a group III source gas, and the group III source gas is supplied to the growth region 122.

  The supply of the halogen-containing gas in addition to the carrier gas is started from the second gas supply pipe 126, and at the same time, the supply of the doping gas from the doping gas supply pipe 125 to the growth region 122 is started. In the growth region 122, the nitrogen source gas, the group III source gas, and the doping gas react to form a layer made of the region 241 on the base substrate 210. At this time, the supply flow rate of the doping gas is, for example, about one fifth of the supply flow rate of the doping gas in the step of forming the high concentration region 220 described later. The thickness of the region 241 is, for example, 50 μm.

  When the thickness of the region 241 reaches a desired thickness, the growth region 122 is kept at 1120 ° C. or higher, preferably while the supply of the nitrogen source gas, the group III source gas, and the doping gas to the growth region 122 is continued. The temperature is raised by the second heater 130 until 1150 ° C. or higher. A region 242 is formed during the temperature rise (temperature rise formation step). The thickness of the region 242 is, for example, 100 μm.

  When the temperature of the growth region 122 is stabilized, the region 231 is formed on the region 242 at a steady temperature. Thereafter, the formation method of the regions 231 and 232 and the high concentration region 220 is the same as the method according to the second embodiment.

  As shown in FIG. 5B, when the thickness of the high concentration region 220 reaches a predetermined thickness, the doping gas is supplied from the doping gas supply pipe 125 and the halogen-containing gas is supplied from the second gas supply pipe 126. Is stopped, the first heater 129 and the second heater 130 are stopped, and the temperature of the reaction tube 121 is lowered. During the temperature drop, the supply of the nitrogen source gas from the first gas supply pipe 124 is continued, and when the temperature of the growth region 122 becomes 600 ° C. or less, for example, the gas supplied from the first gas supply pipe 124 is Switch from carrier gas and nitrogen source gas to purge gas. Then, after the temperature is sufficiently lowered, the substrate 133 having the group III nitride semiconductor layer 250 formed on the base substrate 210 is taken out from the reaction tube 121.

  The method for manufacturing a self-supporting substrate according to the present embodiment further includes a step of removing a part of the group III nitride semiconductor layer 250 and the base substrate 210 after the step of forming the group III nitride semiconductor layer 250. For example, the region other than the high concentration region 220 in the group III nitride semiconductor layer 250 and the base substrate 210 are removed.

  As shown in FIG. 5C, the base substrate 210 and the low-concentration region 230 are removed by polishing the surface of the laminate of the base substrate 210 and the group III nitride semiconductor layer 250 where the base substrate 210 is exposed. As a result, a free-standing substrate consisting of only the high concentration region 220 can be obtained.

However, not only the high concentration region 220 but also the whole or part of the low concentration region 230 may be left as a self-supporting substrate. Alternatively, a structure including the base substrate 210, the low concentration region 230, and the high concentration region 220 may be used as a self-supporting substrate without removing the base substrate 210. In any case, at least one surface of the obtained free-standing substrate is an exposed surface of the high concentration region 220. The resulting free-standing substrate has a high concentration region 220 having an n-type impurity concentration of 5 × 10 ¹⁸ cm ⁻³ or more over a thickness of 50 μm or more. Further, the threading dislocation density of the free-standing substrate is 5 × 10 ⁶ cm ⁻² or less. Further, the threading dislocation density is 1 × 10 ³ cm ⁻² or more.

  The base substrate 210 and the group III nitride semiconductor layer 250 are preferably made of the same group III nitride semiconductor. This is because a group III nitride semiconductor layer 250 having a crystal lattice constant and a thermal expansion coefficient close to each other and good crystallinity can be obtained. As a modification, for example, the base substrate 210 and the low concentration region 230 may be GaN, and the high concentration region 220 may be aluminum gallium nitride (AlGaN).

  In the present embodiment, the step of forming the group III nitride semiconductor layer 250 includes a temperature increase formation step of forming the group III nitride semiconductor layer 250 while increasing the growth temperature. It is possible to suppress the thermal decomposition of the substrate, to suppress the generation of a three-dimensional growth mode at the interface between regions having different formation temperatures, and to reduce changes in the crystal lattice constant, the coefficient of thermal expansion, and the like. Therefore, a group III nitride semiconductor layer 250 with good crystallinity is obtained.

  In this embodiment, the example in which the low concentration region 230 includes the region 231, the region 232, the region 241, and the region 242 has been described. However, the region structure of the low concentration region 230 is not limited to this example. For example, the low-concentration region 230 may be composed of three or less regions, or five or more regions. The low-concentration region 230 is continuously formed as the concentration gradient region or the concentration of the n-type impurity moves away from the surface of the base substrate 210. It may include a region that becomes lower.

  In this embodiment, the example in which the doping gas is always supplied in the step of forming the low concentration region 230 has been described. However, the low concentration region 230 may include a region formed without supplying the doping gas. .

  In this embodiment, the example in which the step of forming the low concentration region 230 is provided only before the step of forming the high concentration region 220 has been described, but the present invention is not limited to this. Depending on the structure of the free-standing substrate to be obtained, for example, a step of forming the low concentration region 230 may be provided after the high concentration region 220 is formed, or the low concentration region 230 is formed before and after the high concentration region 220 is formed. You may provide the process to do. At this time, for example, from the viewpoint of simplifying subsequent device manufacturing, in the step of removing the base substrate 210, it is desirable to perform polishing so that at least one surface is an exposed surface of the high concentration region 220.

  Note that, after the group III nitride semiconductor layer 250, a step of slicing the group III nitride semiconductor layer 250 may be included instead of the step of removing the base substrate 210. A plurality of free-standing substrates can be obtained by slicing the group III nitride semiconductor layer 250 grown in a thick film along a plane parallel to the growth surface. At this time, for example, from the viewpoint of simplifying subsequent device manufacturing, it is desirable to perform slicing so that at least one surface is an exposed surface of the high-concentration region 220.

Note that in the step of forming the group III nitride semiconductor layer 250, the group III nitride semiconductor layer is set such that the concentration of the n-type impurity in the region other than the high concentration region 220 is less than 5 × 10 ¹⁸ cm ^−3. In addition to the high-concentration region 220, the n-type impurity concentration is 5 × 10 ¹⁸ cm ⁻³ or more and the thickness is less than 50 μm. Such a high concentration thin layer region may be formed. Then, the high-concentration thin layer region may be formed as a concentration gradient region, or may be formed in a temperature rising formation process.

  Next, the operation and effect of this embodiment will be described. In this embodiment, the same operation and effect as in the first embodiment can be obtained.

Hereinafter, comparative examples and examples of the present invention will be described.

(Comparative example)
A GaN free-standing substrate was used as a base substrate 210, and a GaN layer was formed on the base substrate 210 by the HVPE method. NH ₃ was used as a nitrogen source gas, HCl was used as a halogen-containing gas, Ga was used as a group III source 127, and SiH ₂ Cl ₂ diluted with hydrogen to a concentration of 3000 ppm was used as a doping gas. The gas supply flow rate during the formation of the GaN layer was 200 cc / min for HCl, 3000 cc / min for NH ₃ , and the growth temperature during the formation of the GaN layer was 1040 ° C. Here, as a result of setting the supply flow rate of HCl to 200 cc / min, GaCl was supplied to the growth region 122 at 200 cc / min. The growth rate was about 180 μm / h, and the film thickness of the formed GaN layer was 180 μm. The surface on which the GaN layer on the surface of the base substrate 210 is grown and the growth surface of the GaN layer were c-plane (0001).

FIG. 6 is a diagram showing a result of observing the growth surface of the GaN layer formed by the method according to this comparative example with a differential interference microscope. FIGS. 6A and 6B are images of the surface of the GaN layer formed on the underlying substrate without supplying a doping gas. When the carrier concentration of the GaN layer was measured by terahertz (THz) wave, it was 7.2 × 10 ¹⁶ cm ⁻³ . FIGS. 6C and 6D are images of the surface of the GaN layer formed by supplying the doping gas at a supply flow rate of 1 cc / min on the base substrate. When the carrier concentration of this GaN layer was measured by THz wave, it was 8.3 × 10 ¹⁷ cm ⁻³ . FIGS. 6E and 6F are images of the surface of a GaN layer formed by supplying a doping gas at a supply flow rate of 3 cc / min on a base substrate. When the carrier concentration of this GaN layer was measured by THz wave, it was 2.3 × 10 ¹⁸ cm ⁻³ . FIGS. 6G and 6H are images of the surface of the GaN layer formed on the base substrate by supplying the doping gas at a supply flow rate of 5 cc / min. When the carrier concentration of the GaN layer was measured by THz wave, it was 3.7 × 10 ¹⁸ cm ⁻³ . As shown in FIG. 6, when the growth temperature was 1040 ° C., the surface morphology deteriorated as the doping concentration of Si as the n-type impurity was increased.

FIG. 7 shows the relationship between the (0004) rocking curve half width and the carrier concentration, and the relationship between the carrier density and the etch pit density (EPD) when the growth surface of the formed GaN layer is measured by X-ray diffraction measurement. FIG. Under the conditions of this comparative example (1040 ° C.), the (0004) rocking curve half-width and etch pit density increased for crystals with a high Si doping concentration and a carrier concentration of 3.7 × 10 ¹⁸ cm ^−3. The deterioration of crystallinity was confirmed. The etch pit density was measured by etching the surface of the formed GaN layer with a mixed solution of sulfuric acid and phosphoric acid heated to 240 ° C. for 2 hours, and observing the exposed etch pits with a differential interference microscope.

  The GaN layer formed under the conditions shown in this comparative example was not sufficiently doped with n-type impurities and had poor crystallinity. One of the reasons is that since the growth temperature is low, the migration of Si is suppressed and segregation as silicon nitride is performed, thereby inhibiting growth and adversely affecting the crystallinity.

(Example)
A GaN free-standing substrate was used as a base substrate 210, and a GaN layer was formed on the base substrate 210 by the HVPE method. NH ₃ was used as a nitrogen source gas, HCl was used as a halogen-containing gas, Ga was used as a group III source 127, and SiH ₂ Cl ₂ diluted with hydrogen to a concentration of 3000 ppm was used as a doping gas. The gas supply flow rate during the formation of the GaN layer was 400 cc / min for HCl and 4000 cc / min for NH ₃ . As a result of the supply flow rate of HCl being 400 cc / min, GaCl was supplied to the growth region 122 at 400 cc / min. The growth rate was about 240 μm / h.

  In this example, as described in the third embodiment, after the low concentration region 230 is grown on the base substrate 210, the high concentration region 220 is grown. However, the doping gas was not supplied when the regions corresponding to the region 241 and the region 242 according to the third embodiment were formed. First, a GaN layer (region 241) was formed on the base substrate 210 at a growth temperature of 1040 ° C. for 5 minutes without supplying a doping gas. Subsequently, a GaN layer (region 242) was formed for 30 minutes without supplying a doping gas. In this 30 minutes, the growth temperature was raised from 1040 ° C. to 1160 ° C. (temperature rise formation step). Then, a GaN layer (region 231) was formed at a growth temperature of 1160 ° C. for 10 minutes while supplying a doping gas at a supply flow rate of 1 cc / min. Subsequently, the GaN layer (region 232) was formed at a growth temperature of 1160 ° C. for 10 minutes at a doping gas supply flow rate of 2 cc / min. Finally, a GaN layer (high concentration region 220) was formed for 45 minutes with a doping gas supply flow rate of 5 cc / min. The film thickness of the formed GaN layer (high concentration region 220) was 180 μm. The surface on which the GaN layer on the surface of the base substrate 210 is grown and the growth surface of the GaN layer were c-plane (0001).

The substrate grown by the method according to this example grew flat.
FIG. 8 is a differential interference microscope image of the growth surface of the outermost GaN layer (high-concentration region 220) grown by the method according to this example. From this figure, it can be seen that a layer having a flat surface was obtained. Moreover, the (0004) rocking curve half value width measured by X-ray diffraction measurement was 51 arcsec. Therefore, it was confirmed that the crystallinity was good.

When the GaN layer (high concentration region 220) on the outermost surface of the substrate grown by the method according to this example was analyzed using secondary ion mass spectrometry (SIMS), the Si concentration was 9 × 10 ^18. cm ⁻³ . In addition, the hole measurement showed that the carrier concentration was 7 × 10 ¹⁸ cm ⁻³ and the mobility was 186 cm ² V ⁻¹ S ⁻¹ at room temperature (25 ° C.).

Furthermore, when the etch pit density (threading dislocation density) was measured on the surface of the formed GaN layer (high concentration region 220) in the same manner as in the comparative example, it was 1.1 × 10 ⁶ cm ⁻² .

  As described above, from this embodiment, the supply flow rates of the nitrogen source gas, the group III source gas (halogen-containing gas), and the doping gas are optimized, the growth temperature is optimized, and the low concentration region 230 is formed. As a result of providing the step of forming and the temperature rising forming step, it was confirmed that the high-concentration region 220 containing n-type impurities at a high concentration and excellent in crystal quality can be formed on the base substrate 210 made of GaN.

FIG. 9 is a diagram showing the relationship between the supply partial pressure ratio of SiH ₂ Cl _{2 to} HCl and the carrier concentration of the GaN layer. It shows about the result by a comparative example (1040 degreeC) and an Example (1160 degreeC). The carrier concentration was determined by THz wave measurement. FIG. 9 shows that the doping efficiency is improved by optimizing conditions such as the growth temperature.

  In the comparative example, when the growth of a layer having a high n-type impurity concentration was continued, the crystallinity deteriorated. In contrast, for example, by optimizing the conditions as in this embodiment, a thick GaN layer having a high n-type impurity concentration and good crystallinity can be formed.

  As mentioned above, although embodiment of this invention was described with reference to drawings, these are the illustrations of this invention, Various structures other than the above are also employable.

100 HVPE apparatus 121 Reaction tube 122 Growth region 123 Substrate holder 124 First gas supply tube 125 Doping gas supply tube 126 Second gas supply tube 127 Group III raw material 128 Source boat 129 First heater 130 Second heater 132 Rotation Shaft 133 Substrate 135 Gas exhaust pipe 136 Shielding plate 137 Nitrogen source gas supply unit 139 Group III gas supply unit 210 Base substrate 220 High concentration region 230 Low concentration regions 231, 232, 241, 242 Region 250 Group III nitride semiconductor layer

Claims (21)

Preparing a base substrate made of a group III nitride semiconductor;
Forming a group III nitride semiconductor layer on the base substrate by hydride vapor phase epitaxy,
The step of forming the group III nitride semiconductor layer includes a step of forming a high concentration region in which the concentration of the n-type impurity is 5 × 10 ¹⁸ cm ⁻³ or more and the thickness is 50 μm or more,
In the step of forming the high concentration region, the growth temperature is set to 1120 ° C. or higher,
A method for manufacturing a self-supporting substrate having a threading dislocation density of 5 × 10 ⁶ cm ⁻² or less. In the manufacturing method of the self-supporting substrate according to claim 1,
The step of forming the group III nitride semiconductor layer further includes a step of forming a low concentration region,
The method for manufacturing a self-supporting substrate, wherein the concentration of the n-type impurity in the low concentration region is less than 5 × 10 ¹⁸ cm ⁻³ . In the manufacturing method of the self-supporting substrate according to claim 2,
The step of forming the group III nitride semiconductor layer includes the step of forming the low concentration region before the step of forming the high concentration region,
The method for manufacturing a freestanding substrate, wherein the concentration of the n-type impurity in the low concentration region is equal to or higher than the concentration of the n-type impurity in the base substrate. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 3,
In the step of forming the group III nitride semiconductor layer, the group III nitride semiconductor layer is set such that the concentration of the n-type impurity in the region other than the high concentration region is less than 5 × 10 ¹⁸ cm ^−3. A method for manufacturing a self-supporting substrate. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 4,
The step of forming the group III nitride semiconductor layer includes a step of forming a concentration gradient region in which the concentration of the n-type impurity continuously increases as the distance from the base substrate increases. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 5,
The step of forming the group III nitride semiconductor layer includes a temperature rising formation step of forming the group III nitride semiconductor layer while increasing a growth temperature. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 6,
A method for manufacturing a self-supporting substrate, further comprising a step of removing the base substrate after the step of forming the group III nitride semiconductor layer. In the manufacturing method of the self-supporting substrate according to claim 7,
In the step of removing the base substrate, a method for manufacturing a self-supporting substrate in which the base substrate and a part of the group III nitride semiconductor layer are removed. In the manufacturing method of the self-supporting substrate according to claim 7 or 8,
In the step of removing the base substrate, a method of manufacturing a self-supporting substrate in which at least one surface is an exposed surface of the high concentration region. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 6,
A method for manufacturing a self-supporting substrate, further comprising a step of slicing the group III nitride semiconductor layer after the step of forming the group III nitride semiconductor layer, wherein at least one surface is an exposed surface of the high concentration region. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 10,
The base substrate and the group III nitride semiconductor layer are each a method for producing a self-supporting substrate made of the same group III nitride semiconductor. In the manufacturing method of the self-supporting substrate according to claim 11,
The base substrate and the group III nitride semiconductor layer are each a method for manufacturing a self-supporting substrate made of gallium nitride. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 12,
A method for manufacturing a free-standing substrate, wherein the n-type impurity is Si. In the manufacturing method of the self-supporting substrate according to any one of claims 1 to 13,
A method for manufacturing a self-supporting substrate, wherein a ratio of a carrier concentration to a concentration of the n-type impurity in the high concentration region is 0.7 or more at 25 ° C. A high-concentration region having a concentration of n-type impurities of 5 × 10 ¹⁸ cm ⁻³ or more over a thickness of 50 μm or more;
A free-standing substrate of a group III nitride semiconductor having a threading dislocation density of 5 × 10 ⁶ cm ⁻² or less. The self-supporting substrate according to claim 15,
A self-supporting substrate in which the high-concentration region is exposed on at least one surface. The self-standing substrate according to claim 16,
A self-supporting substrate in which the concentration of the n-type impurity is less than 5 × 10 ¹⁸ cm ^{−3 in the} vicinity of the surface of the other surface. The self-standing substrate according to claim 16,
A self-supporting substrate in which the high concentration region is exposed on both sides. The self-supporting substrate according to any one of claims 15 to 18,
The freestanding substrate, wherein the group III nitride semiconductor is gallium nitride. In the self-supporting substrate according to any one of claims 15 to 19,
The free-standing substrate, wherein the n-type impurity is Si. In the self-supporting substrate according to any one of claims 15 to 20,
A self-supporting substrate in which a ratio of a carrier concentration to a concentration of the n-type impurity in the high concentration region is 0.7 or more at 25 ° C.