JP3823781B2 - Nitride semiconductor substrate and method for manufacturing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nitride semiconductor (In_xAl_yGa_1-xyThe present invention relates to a nitride semiconductor substrate having N, 0 ≦ x, 0 ≦ y, x + y ≦ 1) on the surface.
[0002]
[Prior art]
In recent years, in order to manufacture a nitride semiconductor substrate having a low dislocation density, a method of growing a nitride semiconductor laterally with respect to the substrate on a different substrate such as sapphire, spinel, or silicon carbide is different. (Hereinafter, various studies have been made on the ELOG growth method (Epitaxially Lateral OverGrowth)).
In the region where the nitride semiconductor grows in the lateral direction, the dislocations generated at the nitride semiconductor growth starting point proceed only in the lateral direction along with the growth of the nitride semiconductor, so that a nitride semiconductor having a low dislocation density can be grown. it can.
[0003]
For example, in Jpn. J. Appl. Phys. Vol. 37 (1998) pp. L309-L312, as an ELOG growth method, SiO2 is grown on gallium nitride grown on the C-plane of sapphire.₂It is disclosed that a nitride semiconductor with few dislocations is obtained by partially forming a protective film such as the above and growing a nitride semiconductor on the protective film under a reduced pressure of 100 Torr.
In such an ELOG growth method, by forming a protective film and intentionally growing the nitride semiconductor in the lateral direction, when the dislocation proceeds with the growth of the nitride semiconductor, the dislocation is only on a portion having no protective film. Therefore, a nitride semiconductor with few dislocation defects can be formed on the protective film.
[0004]
JP-A-11-145516 discloses SiO.₂Instead of forming a protective film, a method is disclosed in which an AlGaN layer grown on a silicon substrate is etched in stripes to partially expose the silicon substrate and gallium nitride is grown thereon. Since gallium nitride does not grow epitaxially on the silicon substrate, gallium nitride grows epitaxially in the lateral direction using the AlGaN layer as a seed crystal. Therefore, gallium nitride having a low dislocation density can be grown on the exposed portion of the silicon substrate.
[0005]
According to these ELOG growth methods, the crystal defect density can be reduced by two orders of magnitude or more compared to a nitride semiconductor layer grown using a conventional buffer layer. Accordingly, by forming various nitride semiconductor elements such as LED elements, LD elements, and light receiving elements on the nitride semiconductor substrate manufactured by these ELOG growth methods, the lifetime characteristics of the nitride semiconductor elements are dramatically improved. Can be improved. For example, a gallium nitride-based compound semiconductor laser manufactured using a gallium nitride substrate grown by ELOG can achieve continuous oscillation for 10,000 hours or more.
[0006]
[Problems to be solved by the invention]
However, the nitride semiconductor obtained by forming the protective film as described above and growing the nitride semiconductor in the lateral direction can obtain a nitride semiconductor with few dislocation defects above the protective film.₂In the case where the protective film such as a wide stripe width is formed, the lateral growth of the nitride semiconductor does not proceed on the protective film, which may cause abnormal growth.
[0007]
Also, nitride semiconductors grown laterally starting from the nitride semiconductor exposed on both sides of the protective film are joined together at the central part of the protective film, but the nitride semiconductor is grown laterally by vapor deposition In this case, dislocations are concentrated locally at the junction. This is partly due to SiO₂This is because the growth surface of the nitride semiconductor that grows laterally on the protective film or the like tilts. When an element layer is formed on such a nitride semiconductor substrate by epitaxial growth, fine pits are generated due to nitrogen desorption at the junction where dislocations are concentrated in the substrate heating process for element layer growth. In addition, the pit grows greatly by continuing the epitaxial growth.
[0008]
For this reason, even if a single nitride semiconductor substrate is formed by lateral growth on a protective film using a vapor phase growth method, it cannot be handled in the same manner as a general single crystal substrate, and a semiconductor laser Therefore, it is difficult to secure a sufficient region for device formation, and the device lifetime is not sufficient. Moreover, since the nitride semiconductor substrate that is bonded to form a single substrate is uniform in appearance, it is not easy to recognize the bonding portion from the upper surface of the substrate and perform subsequent element pattern formation with high accuracy. .
[0009]
Further, a method for producing a gallium nitride semiconductor substrate with few crystal defects is disclosed in Mat. Res. Soc. Symp. Proc. Vol. 535, pp. 91-99. According to this method, a GaN seed crystal having a silicon nitride protective film formed thereon is formed in a stripe shape on a 6H-SiC substrate, and the end surface of the GaN seed crystal is used as a growth starting point on the silicon nitride protective film. GaN is grown to Thereby, since GaN grows only in the lateral direction, a nitride semiconductor substrate with few crystal defects can be obtained.
[0010]
However, when a single nitride semiconductor substrate is produced by laterally growing a nitride semiconductor using a protective film on a different substrate different from the nitride semiconductor, the substrate and the protective film having different thermal expansion coefficients are produced. Since the nitride semiconductor layer has a laminated structure, the manufactured nitride semiconductor substrate is likely to be warped.
[0011]
Further, in some cases, a nitride semiconductor substrate may be manufactured by finally removing a dissimilar substrate such as a sapphire substrate. In this case, as a method for removing the dissimilar substrate, a method of polishing the dissimilar substrate or nitriding with the dissimilar substrate A technique is used in which an excimer laser is irradiated to the interface of a physical semiconductor to break the chemical bond at the interface. However, removal by polishing or excimer laser has a problem that processing time is required, and removal of different substrates such as sapphire is not easy.
[0012]
Therefore, an object of the present invention is to provide a nitride semiconductor device having fewer crystal defects than a nitride semiconductor substrate manufactured by a conventional ELOG growth method. Another object of the present invention is to suppress warpage in a nitride semiconductor substrate. Still another object of the present invention is to facilitate the removal of a heterogeneous substrate from a nitride semiconductor substrate.
Thus, the present invention provides a nitride semiconductor device with improved lifetime characteristics, in particular, a gallium nitride compound semiconductor laser capable of emitting blue light, and a method for manufacturing the same with a high yield.
[Means for Solving the Problems]
[0013]
In order to solve the above problems, the nitride semiconductor substrate according to the first embodiment of the present invention is formed on a different substrate different from the nitride semiconductor.
A seed crystal made of a periodically arranged nitride semiconductor;
T-shaped cross-sections periodically arranged so as to cover the seed crystal by laterally growing using the side surface of the seed crystal as a growth starting point and stopping the lateral growth before joining each other above the seed crystal A first nitride semiconductor layer having;
An end face formed by stopping lateral growth before the first nitride semiconductor layers are joined to each other, or the end face and the top face of the first nitride semiconductor layer are grown as growth starting points, A second nitride semiconductor layer covering the entire surface of one nitride semiconductor layer,
A space is formed between the upper surface of the seed crystal and the first nitride semiconductor layer.
[0014]
That is, in the nitride semiconductor substrate of the present invention, after the first nitride semiconductor layer is grown in the lateral direction and bonded, the nitride semiconductor layer is further grown in the lateral direction using the bonded nitride semiconductor layer as a growth starting point and bonded to each other. By stopping the growth, the formation of a junction where dislocations are concentrated is avoided, so that fine pits due to nitrogen desorption do not occur in the substrate heating process for element layer growth. In addition, since the second nitride semiconductor layer grows from the upper surface and the end surface of the first nitride semiconductor layer grown in the lateral direction, the second nitride semiconductor layer has few crystal defects.
Further, since a space is formed between the upper surface of the seed crystal and the first nitride semiconductor layer, that is, below the portion where the second nitride semiconductor layer is joined to each other, Warpage can be suppressed.
[0015]
In the nitride semiconductor substrate of the present invention, since the nitride semiconductor layer is supported on the dissimilar substrate by a discontinuous columnar structure, the bonding strength between the nitride semiconductor layer and the dissimilar substrate is lowered. Therefore, in addition to the conventional excimer laser or polishing method for removing a different substrate, the different substrate can be removed by a mechanical peeling method using vibration or thermal shock. For example, when the nitride semiconductor substrate according to the present invention is polished from the back surface of the heterogeneous substrate, the entire heterogeneous substrate is peeled off by mechanical vibration during polishing. According to such a mechanical peeling method, it is possible to remove the dissimilar substrate in a short time. Although the peeling interface may vary somewhat with the mechanical method, a uniform nitride semiconductor substrate can be obtained by polishing the back surface of the substrate after peeling.
[0016]
In the nitride semiconductor substrate according to the second embodiment of the present invention, in the first nitride semiconductor substrate, a decomposition preventing layer is further formed on the entire upper surface of the seed crystal, and the upper surface of the decomposition preventing layer and the A space is formed between the first nitride semiconductor layer and the first nitride semiconductor layer. In particular, the decomposition preventing layer is made of SiN, SiON or TiO.₂It is characterized by being.
Here, when the reaction element is grown on the substrate after the surface of the seed crystal is exposed, the surface of the seed crystal is decomposed at a temperature of 1000 ° C. or more during the process, so that the surface of the seed crystal is V. A letter-shaped groove is easily formed, which may cause contamination of the first nitride semiconductor layer and the second nitride semiconductor layer. Therefore, the “decomposition prevention layer” means a layer provided on the surface of the seed crystal to prevent thermal decomposition of the seed crystal, and the decomposition prevention layer is a material that does not decompose even at 1000 ° C. For example, SiN, SiON or TiO₂Etc. can be used.
[0017]
That is, in the nitride semiconductor substrate of the present invention, since a decomposition preventing layer such as SiN is formed on the entire upper surface of the seed crystal, the seed crystal decomposes at a temperature of 1000 ° C. or higher when the reaction element is grown on the substrate. By diffusing, it is possible to solve the problem of entering the first and second nitride semiconductor layers to reduce crystallinity and causing abnormal growth or the like.
[0018]
The nitride semiconductor substrate of the present invention is characterized in that the seed crystal is formed in a lattice shape, a periodic stripe shape, or a periodic island shape.
That is, according to the present invention, by forming the seed crystal in the above-described shape, the nitride semiconductor layer can be grown in the lateral direction with the side surface of the seed crystal as a growth starting point.
[0019]
In order to produce the nitride semiconductor substrate of the first embodiment, the present invention provides an entire surface on a different substrate different from the nitride semiconductor.
Forming a seed crystal made of a nitride semiconductor;
Forming a protective film on the entire surface of the seed crystal;
Etching the seed crystal and the protective film;
A first nitride semiconductor layer is laterally grown from the side surface of the etched seed crystal as a growth starting point, and the first nitride semiconductor layer laterally grown on the protective film does not completely cover the protective film. Stopping the lateral growth before joining together,
Removing the protective film by etching;
An end face formed by stopping lateral growth before the first nitride semiconductor layers are bonded to each other, or a second nitride starting from the end face and the upper surface of the first nitride semiconductor layer. And a method of manufacturing a nitride semiconductor substrate including a step of growing a semiconductor layer.
[0020]
Furthermore, in order to produce the nitride semiconductor substrate of the second embodiment, the present invention provides an entire surface on a different substrate different from the nitride semiconductor.
Forming a seed crystal made of a nitride semiconductor;
Forming a decomposition preventing layer on the entire surface of the seed crystal;
Forming a protective film on the entire surface of the growth prevention layer;
Etching the seed crystal, the decomposition preventing layer and the protective film;
A first nitride semiconductor layer is laterally grown from the side surface of the etched seed crystal as a growth starting point, and the first nitride semiconductor layer laterally grown on the protective film does not completely cover the protective film. Stopping the lateral growth before joining together,
Removing the protective film by etching;
An end face formed by stopping lateral growth before the first nitride semiconductor layers are bonded to each other, or a second nitride starting from the end face and the upper surface of the first nitride semiconductor layer. And a method of manufacturing a nitride semiconductor substrate including a step of growing a semiconductor layer.
[0021]
The method for manufacturing a nitride semiconductor substrate according to the second embodiment of the present invention is characterized in that an etching rate of the decomposition preventing layer is smaller than the etching rate of the protective film. For example, in the case of etching by reactive ion etching, the decomposition preventing layer is formed of SiN, and the protective film is formed of SiON.₂Form with.
[0022]
In the method for manufacturing a nitride semiconductor substrate according to the first and second embodiments of the present invention, the protective film may be etched using either anisotropic etching or isotropic etching.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to the drawings.
First embodiment
The 1st Embodiment of this invention is shown to Fig.1 (a)-(e). In this embodiment, as shown in FIG. 1E, a nitride semiconductor formed on a heterogeneous substrate 1 different from a nitride semiconductor via a buffer layer (not shown) and etched in a stripe shape is used. The seed crystal 2 and the side surface of the seed crystal 2 are grown in the lateral direction, and the seed crystal 2 is periodically arranged so as to cover the seed crystal 2 by stopping the lateral growth before joining the seed crystal 2 above the seed crystal 2. A first nitride semiconductor layer 5 having a T-shaped cross section is formed. Here, a space is formed above the seed crystal 2 and is not in contact with the first nitride semiconductor layer 5.
Further, in the nitride semiconductor substrate of the first embodiment, the end face formed by stopping the lateral growth before the first nitride semiconductor layers 5 are joined to each other and / or the first nitride semiconductor. A second nitride semiconductor layer 6 is formed by growing from the upper surface of the layer 5 as a growth starting point and covering the entire surface of the first nitride semiconductor layer 5.
All of the seed crystal 2, the first nitride semiconductor layer, and the second nitride semiconductor layer 6 have the general formula In_xAl_yGa_1-xyN (0 ≦ x, 0 ≦ y, x + y ≦ 1). However, these may have different compositions.
[0024]
Next, a method for manufacturing the nitride semiconductor substrate of this embodiment and a suitable material will be described.
First, a heterogeneous substrate 1 different from a nitride semiconductor is prepared. The heterogeneous substrate 1 includes a SiC (6H, 4H, 3C) substrate, spinel (MgAl), in addition to a sapphire substrate whose main surface is any of the C-plane, R-plane, and A-plane₂O₄) A substrate, a silicon substrate, Zn, ZnO, GaAs, a nitride semiconductor, an oxide substrate lattice-matched with a nitride semiconductor, or the like can be used. In particular, it is preferable to use a sapphire substrate or a SiC substrate from the viewpoint of the crystallinity of the nitride semiconductor layer grown thereon. Note that the number of crystal defects can be reduced by using a substrate in which the principal surface of these substrate materials is off-angle, more preferably a substrate in which the main surface is off-angled in a stepped manner.
[0025]
Next, as shown in FIG. 1A, a nitride semiconductor seed crystal 2 is grown through a buffer layer (not shown). The buffer layer is formed, for example, by growing AlN, GaN, AlGaN, InGaN or the like to a thickness of about 10 to 0.5 μm at a low temperature of about 300 to 900 ° C. This is for alleviating the lattice constant irregularity between the heterogeneous substrate 1 and the seed crystal 2, and is preferable in terms of reducing crystal defects. The buffer layer may be omitted depending on the nitride semiconductor growth method and the type of substrate.
The seed crystal 2 is a gallium nitride compound semiconductor doped with or without impurities, preferably In_xAl_yGa_1-xyGrow at high temperature using N (0 ≦ x, 0 ≦ y, x + y ≦ 1). For example, undoped GaN and Si-doped n-type GaN are grown at 900 to 1100 ° C., preferably 1050 ° C. The film thickness of the seed crystal 2 is at least 500 mm, preferably 1 μm or more, more preferably 2 μm to 10 μm. When the film thickness is about 2 μm to 10 μm, the crystallinity is good, and the warpage of the substrate due to the difference in expansion coefficient from a different type of substrate is small.
[0026]
Further, a protective film 4 is formed on the entire upper surface of the seed crystal 2. A material for the protective film 4 is selected so that the nitride semiconductor is difficult to grow on the surface or does not grow at all. For example, silicon oxide (SiO_x), Silicon nitride (Si_xN_y), Titanium oxide (TiO_x), An oxide such as zirconium oxide, a nitride, or a multilayer film thereof, or a metal having a melting point of 1200 ° C. or higher can be used. These materials can withstand temperatures of 600 to 1100 ° C., which is the growth temperature of nitride semiconductors, and have the property that nitride does not grow on the surface or is difficult to grow.
The protective film 4 can be formed using, for example, an appropriate vapor deposition method such as vapor deposition, sputtering, or CVD. The thickness of the protective film 4 can be determined as appropriate, but is preferably about 0.3 to 5 μm because it has a function as a protective film and less warpage occurs.
[0027]
Next, a resist is applied to the protective film 4 and patterned by ordinary photolithography, and the seed crystal 2 and the protective film 4 are etched into a periodic stripe shape, a periodic island shape, or a lattice shape. The island shape may be, for example, a polygon (triangle, square, hexagon, etc.) or a circle. It is preferable that the islands be arranged as densely as possible with a constant interval. Further, the width and interval of the stripes and lattices are selected so that the first nitride semiconductor layer 5 can be grown.
The etching can be stopped while leaving a part of the seed crystal 2, but is preferably performed to a depth at which a part of the heterogeneous substrate 1 is removed. The depth at which the heterogeneous substrate 1 is cut is preferably 500 to 3000 mm, but may be 10 μm or less. By etching to a depth at which a portion of the heterogeneous substrate 1 is cut away, a cavity is formed under the first nitride semiconductor layer 5 grown from the side surface of the seed crystal 2, and the first nitride semiconductor layer 5 is formed on the heterogeneous substrate. 1 is prevented, the warp of the substrate can be reduced, and the crystallinity of the first nitride semiconductor layer 5 can be improved. Furthermore, since the cavity is formed in the heterogeneous substrate 1, the cavity can be recognized by the reflection of light, and using this as a guide, a device is selectively manufactured on a portion of the nitride semiconductor substrate having good crystallinity. Is possible.
In addition, when etching is performed, the etching surface is not limited to a shape in which the end surface is substantially perpendicular to the dissimilar substrate as shown in FIG. 1B, and may be a forward mesa shape, a reverse mesa shape, or a step shape. May be.
[0028]
Next, as shown in FIG. 1C, when the first nitride semiconductor layer 5 is grown in the lateral direction starting from the side surface of the seed crystal 2 and further grown laterally on the protective film 4, The growth is stopped before completely covering the protective film 4. The cross-sectional shape of the first nitride semiconductor layer 5 grown in this manner is a T-shape arranged periodically. That is, the first nitride semiconductor layer 5 includes a first laterally grown portion 5a formed by joining nitride semiconductor layers that are laterally grown with the side surface of the adjacent seed crystal 2 as a growth starting point, The second lateral growth portion 5b is formed by lateral growth on the protective film 4 with the lateral growth portion 5a as a growth starting point. The second lateral growth portion 5b of the adjacent first nitride semiconductor layer 5 is composed of the second lateral growth portion 5b. The lateral growth portions 5b are not joined to each other, and the end surfaces of the second lateral growth portions 5b are separated by about 2 to 20 μm. If the distance between the end faces is about 2 to 20 μm, a sufficient amount of raw material can be supplied to the end faces when forming the second nitride semiconductor layer 6, and the time until bonding is short. The film thickness of the nitride semiconductor layer 6 does not increase. The interval between the end faces is preferably about 2 to 10 μm, and more preferably about 2 to 4 μm. If it is within the above range, the second nitride semiconductor layer 6 does not grow downward when bonded, and therefore is formed above the seed crystal 2 in order to avoid contact with the upper surface of the seed crystal 2. Since it is not necessary to increase the space, the entire substrate can be made thin.
The preferred film thickness of the first nitride semiconductor layer 5 also varies depending on the film thickness and size of the protective film 4. Since the second laterally grown portion 5b needs to have a portion with good crystallinity, it is preferably at least 1.5 times the thickness of the protective film. Therefore, considering the thickness of the seed crystal 2 and the protective film 4, it is preferable to grow the first lateral growth portion 5a with a thickness of 1 to 15 μm.
Here, the first nitride semiconductor layer 5 is not particularly limited, but includes a nitride semiconductor made of GaN doped with p-type impurities, n-type impurities, or non-doped GaN.
[0029]
Next, as shown in FIG. 1D, the first nitride semiconductor layer 5 is laterally grown on the protective film 4, and the protective film 4 is removed while the growth is stopped halfway. Etching can be used as a method for removing the protective film 4, and the etching means is not particularly limited, and examples thereof include dry etching or wet etching, and either isotropic or anisotropic etching may be used. Isotropic dry etching is preferable because etching control is easy.
[0030]
Here, the portion of the first nitride semiconductor 5 that is laterally grown on the protective film (that is, the second laterally grown portion 5b) is a portion with few crystal defects. By removing the protective film, A space can be formed below the portion. For this reason, when growing the 2nd nitride semiconductor layer 6 from the end surface of the 2nd horizontal direction growth part 5b, the stress generate | occur | produced between protective films can be suppressed.
[0031]
Next, as shown in FIG. 1E, the second nitride is formed on the first nitride semiconductor layer 5 from which the protective film 4 has been removed, from the end face and / or the upper face of the first nitride semiconductor layer 5. The semiconductor layer 6 is grown.
The second nitride semiconductor layer 6 is a gallium nitride compound semiconductor doped with or without impurities, preferably In._xAl_yGa_1-xyGrow using N (0 ≦ x, 0 ≦ y, x + y ≦ 1). For example, undoped GaN and GaN doped with n-type impurities such as Si, Ge, Sn, and S, or GaN doped with p-type impurities such as Mg can be used. It is preferable to grow Mg by doping the second nitride semiconductor layer 6 because it tends to extend in the lateral direction and easily fill the gaps in the first nitride semiconductor layer 5. On the other hand, undoped is preferable because the electrical characteristics are stabilized. Since the second nitride semiconductor layer 6 grows in space, Al cannot be used due to low selectivity in the growth on the protective film._xGa_1-xN (0 <x <1) can also be used.
The second nitride semiconductor layer 6 is grown at 900 to 1100 ° C. When GaN is used, the film thickness is 3 to 20 μm, preferably 5 to 20 μm._xGa_1-xWhen N is used, the film thickness is preferably 2 to 15 μm.
[0032]
Second embodiment
The 2nd Embodiment of this invention is shown to Fig.2 (a)-(e). In this embodiment, as shown in FIG. 2E, a nitride semiconductor formed on a heterogeneous substrate 1 different from a nitride semiconductor via a buffer layer (not shown) and etched in a stripe shape is used. The seed crystal 2, the decomposition preventing layer 3 formed on the upper surface of the seed crystal 2, and lateral growth starting from the side surface of the seed crystal 2 as a growth starting point, are bonded to each other above the seed crystal 2 and the decomposition preventing layer 3. A first nitride semiconductor layer 5 having a T-shaped cross section periodically arranged so as to cover seed crystal 2 and decomposition preventing layer 3 is formed by previously stopping lateral growth. Here, a space is formed above the seed crystal 2 and the decomposition preventing layer 3 and is not in contact with the first nitride semiconductor layer 5.
Further, in the nitride semiconductor substrate of the first embodiment, the end face formed by stopping the lateral growth before the first nitride semiconductor layers 5 are joined to each other and / or the first nitride semiconductor. A second nitride semiconductor layer 6 is formed by growing from the upper surface of the layer 5 as a growth starting point and covering the entire surface of the first nitride semiconductor layer 5.
All of the seed crystal 2, the first nitride semiconductor layer, and the second nitride semiconductor layer 6 have the general formula In_xAl_yGa_1-xyN (0 ≦ x, 0 ≦ y, x + y ≦ 1). However, these may have different compositions.
[0033]
Here, when the reaction element is grown on the substrate with the surface of the seed crystal 2 exposed, the surface of the seed crystal 2 is decomposed at a temperature of 1000 ° C. or higher during the process. It is easy to form a V-shaped groove. Formation of the V-shaped groove by the decomposition of the seed crystal 2 may cause contamination of the first nitride semiconductor layer 5 and the second nitride semiconductor layer 6.
Therefore, in the second embodiment of the present invention, in addition to the first embodiment of the present invention, a decomposition preventing layer is provided on the surface of the seed crystal 2 in order to prevent thermal decomposition of the seed crystal 2.
[0034]
Next, a method for manufacturing the nitride semiconductor substrate of this embodiment and a suitable material will be described.
First, a heterogeneous substrate 1 different from a nitride semiconductor is prepared. The heterogeneous substrate 1 includes a SiC (6H, 4H, 3C) substrate, spinel (MgAl), in addition to a sapphire substrate whose main surface is any of the C-plane, R-plane, and A-plane₂O₄) A substrate, a silicon substrate, Zn, ZnO, GaAs, a nitride semiconductor, an oxide substrate lattice-matched with a nitride semiconductor, or the like can be used. In particular, it is preferable to use a sapphire substrate or a SiC substrate from the viewpoint of the crystallinity of the nitride semiconductor layer grown thereon. Note that the number of crystal defects can be reduced by using a substrate in which the principal surface of these substrate materials is off-angle, more preferably a substrate in which the main surface is off-angled in a stepped manner.
[0035]
Next, as shown in FIG. 2A, a nitride semiconductor seed crystal 2 is grown through a buffer layer (not shown). The buffer layer is formed, for example, by growing AlN, GaN, AlGaN, InGaN or the like to a thickness of about 10 to 0.5 μm at a low temperature of about 300 to 900 ° C. This is for alleviating the lattice constant irregularity between the heterogeneous substrate 1 and the seed crystal 2, and is preferable in terms of reducing crystal defects. The buffer layer may be omitted depending on the nitride semiconductor growth method and the type of substrate.
The seed crystal 2 is a gallium nitride compound semiconductor doped with or without impurities, preferably In_xAl_yGa_1-xyGrow at high temperature using N (0 ≦ x, 0 ≦ y, x + y ≦ 1). For example, undoped GaN and Si-doped n-type GaN are grown at 900 to 1100 ° C., preferably 1050 ° C. The film thickness of the seed crystal 2 is at least 500 mm, preferably 1 μm or more, more preferably 2 μm to 10 μm. When the film thickness is about 2 μm to 10 μm, the crystallinity is good, and the warpage of the substrate due to the difference in expansion coefficient from a different type of substrate is small.
[0036]
Next, the decomposition preventing layer 3 is formed on the entire upper surface of the seed crystal 2. The material of the decomposition preventing layer 3 may be any material that does not decompose even at a temperature of 1000 ° C. or higher. For example, SiN, SiON, TiO₂Etc. The film thickness of the decomposition preventing layer 3 can be determined as appropriate, but it is preferably about 0.3 to 5 μm because it has a function as a decomposition preventing layer and less warpage occurs.
[0037]
Further, a protective film 4 is formed on the entire upper surface of the decomposition preventing layer 3. A material for the protective film 4 is selected so that the nitride semiconductor is difficult to grow on the surface or does not grow at all. For example, silicon oxide (SiO_x), Silicon nitride (Si_xN_y), Titanium oxide (TiO_x), An oxide such as zirconium oxide, a nitride, or a multilayer film thereof, or a metal having a melting point of 1200 ° C. or higher can be used. These materials can withstand temperatures of 600 to 1100 ° C., which is the growth temperature of nitride semiconductors, and have the property that nitride does not grow on the surface or is difficult to grow.
The protective film 4 can be formed using, for example, an appropriate vapor deposition method such as vapor deposition, sputtering, or CVD. The thickness of the protective film 4 can be determined as appropriate, but is preferably about 0.3 to 5 μm because it has a function as a protective film and less warpage occurs.
[0038]
However, as will be described later, after the first nitride layer 5 is formed, only the protective film 4 is etched, and the decomposition preventing layer 3 is left unetched. Therefore, the decomposition preventing layer 3 depends on the etching means used. For this, a material having an etching rate smaller than that of the protective film 4 is used. For example, in the case of reactive ion etching (RIE) using inductively coupled plasma (ICP), although not limited, SiN is used for the decomposition preventing layer 3 and SiO 2 is used for the protective film 4.₂Is used. It is preferable to select the material for the decomposition preventing layer 3 and the protective film 4 in this manner because the etching can be easily controlled.
[0039]
Next, a resist is applied to the protective film 4 and patterned by ordinary photolithography, and the seed crystal 2, the decomposition preventing layer 3, and the protective film 4 are etched into a periodic stripe shape, a periodic island shape, or a lattice shape. To do. The island shape may be, for example, a polygon (triangle, square, hexagon, etc.) or a circle. It is preferable that the islands be arranged as densely as possible with a constant interval. Further, the width and interval of the stripes and lattices are selected so that the first nitride semiconductor layer 5 can be grown.
The etching can be stopped while leaving a part of the seed crystal 2, but is preferably performed to a depth at which a part of the heterogeneous substrate 1 is removed. The depth at which the heterogeneous substrate 1 is cut is preferably 500 to 3000 mm, but may be 10 μm or less. By etching to a depth at which a portion of the heterogeneous substrate 1 is cut away, a cavity is formed under the first nitride semiconductor layer 5 grown from the side surface of the seed crystal 2, and the first nitride semiconductor layer 5 is formed on the heterogeneous substrate. 1 is prevented, the warp of the substrate can be reduced, and the crystallinity of the first nitride semiconductor layer 5 can be improved. Furthermore, since the cavity is formed in the heterogeneous substrate 1, the cavity can be recognized by the reflection of light, and using this as a guide, a device is selectively manufactured on a portion of the nitride semiconductor substrate having good crystallinity. Is possible.
In addition, when etching is performed, the etching surface is not limited to a shape in which the end surface is substantially perpendicular to the dissimilar substrate as shown in FIG. 2B, and may be a forward mesa shape, a reverse mesa shape, or a step shape. May be.
[0040]
Next, as shown in FIG. 2 (c), when the first nitride semiconductor layer 5 is grown in the lateral direction starting from the side surface of the seed crystal 2 and further grown in the lateral direction on the protective film 4, The growth is stopped before completely covering the protective film 4. The cross-sectional shape of the first nitride semiconductor layer 5 grown in this manner is a T-shape arranged periodically. That is, the first nitride semiconductor layer 5 includes a first laterally grown portion 5a formed by joining nitride semiconductor layers that are laterally grown with the side surface of the adjacent seed crystal 2 as a growth starting point, The second lateral growth portion 5b is formed by lateral growth on the protective film 4 with the lateral growth portion 5a as a growth starting point. The second lateral growth portion 5b of the adjacent first nitride semiconductor layer 5 is composed of the second lateral growth portion 5b. The lateral growth portions 5b are not joined to each other, and the end surfaces of the second lateral growth portions 5b are separated by about 2 to 20 μm. If the distance between the end faces is about 2 to 20 μm, a sufficient amount of raw material can be supplied to the end faces when forming the second nitride semiconductor layer 6, and the time until bonding is short. The film thickness of the nitride semiconductor layer 6 does not increase. The interval between the end faces is preferably about 2 to 10 μm, and more preferably about 2 to 4 μm. If it is within the above range, the second nitride semiconductor layer 6 does not grow downward when bonded, and therefore is formed above the seed crystal 2 in order to avoid contact with the upper surface of the seed crystal 2. Since it is not necessary to increase the space, the entire substrate can be made thin.
[0041]
The preferred film thickness of the first nitride semiconductor layer 5 also varies depending on the film thickness and size of the protective film 4. Since the second laterally grown portion 5b needs to have a portion with good crystallinity, it is preferably at least 1.5 times the thickness of the protective film. Therefore, in consideration of the film thickness of the seed crystal 2, the decomposition preventing layer 3, and the protective film 4, it is preferable to grow the first lateral growth portion 5a with a film thickness of 1 to 15 μm.
Here, the first nitride semiconductor layer 5 is not particularly limited, but includes a nitride semiconductor made of GaN doped with p-type impurities, n-type impurities, or non-doped GaN.
[0042]
Next, as shown in FIG. 2D, the first nitride semiconductor layer 5 is grown in the lateral direction on the protective film 4, and the protective film 4 is removed while the growth is stopped halfway. Etching can be used as a method for removing the protective film 4, and the etching means is not particularly limited, and examples thereof include dry etching or wet etching, and either isotropic or anisotropic etching may be used. Isotropic dry etching is preferable because etching control is easy. As described above, depending on the etching conditions used, the decomposition preventing layer 4 has an etching rate lower than that of the protective film 3, so that the protective film 4 is removed by etching, but the decomposition preventing layer 3 is not removed. Remain in. That is, the decomposition preventing layer 3 is formed on the upper surface of the seed crystal 2.
In the second embodiment, since the decomposition preventing layer 3 is formed on the upper surface of the seed crystal 2, the decomposition of the seed crystal 2 can be prevented.
[0043]
Here, the portion of the first nitride semiconductor 5 that is laterally grown on the protective film (that is, the second laterally grown portion 5b) is a portion with few crystal defects. By removing the protective film, A space can be formed below the portion. For this reason, when growing the 2nd nitride semiconductor layer 6 from the end surface of the 2nd horizontal direction growth part 5b, the stress generate | occur | produced between protective films can be suppressed.
[0044]
Next, as shown in FIG. 2E, the second nitride is formed on the first nitride semiconductor layer 5 from which the protective film 4 has been removed, from the end face and / or the upper face of the first nitride semiconductor layer 5. The semiconductor layer 6 is grown.
The second nitride semiconductor layer 6 is a gallium nitride compound semiconductor doped with or without impurities, preferably In._xAl_yGa_1-xyGrow using N (0 ≦ x, 0 ≦ y, x + y ≦ 1). For example, undoped GaN and GaN doped with n-type impurities such as Si, Ge, Sn, and S, or GaN doped with p-type impurities such as Mg can be used. It is preferable to grow Mg by doping the second nitride semiconductor layer 6 because it tends to extend in the lateral direction and easily fill the gaps in the first nitride semiconductor layer 5. On the other hand, undoped is preferable because the electrical characteristics are stabilized. Since the second nitride semiconductor layer 6 grows in space, Al cannot be used due to low selectivity in the growth on the protective film._xGa_1-xN (0 <x <1) can also be used.
The second nitride semiconductor layer 6 is grown at 900 to 1100 ° C. When GaN is used, the film thickness is 3 to 20 μm, preferably 5 to 20 μm._xGa_1-xWhen N is used, the film thickness is preferably 2 to 15 μm.
[0045]
【Example】
Examples of the present invention are shown below, but the present invention is not limited thereto.Example 1
Using a sapphire substrate 1 with the C-plane as the principal plane and the orientation flat plane as the A-plane, by MOCVD, the temperature is 510 ° C., the carrier gas is hydrogen, the source gas is ammonia and trimethylgallium (TMG), and the sapphire A buffer layer made of GaN was grown on the substrate 1 to a thickness of 200 mm (not shown).
[0046]
After growth of the buffer layer, only TMG is stopped, the temperature is raised to 1050 ° C., and when it reaches 1050 ° C., TMG and ammonia are used as the source gas, and the seed crystal 2 made of undoped GaN is formed to a thickness of 2.5 μm. Grown up.
[0047]
The top surface of the seed crystal 2 is SiO by CVD.₂A protective film 4 having a thickness of 0.5 μm was formed.
[0048]
Next, a striped photomask was formed using a general photolithography technique, and the protective film 4 and the seed crystal 2 were etched by RIE using ICP. The stripe direction is a direction perpendicular to the A surface of the sapphire substrate.
[0049]
Next, the first nitride semiconductor layer 5 made of GaN was grown by MOCVD at a temperature of 1050 ° C. under reduced pressure and using TMG and ammonia as source gases. At this time, the first nitride semiconductor layer 5 grows laterally starting from the side surface of the seed crystal 2 as a growth starting point and further grows laterally on the protective film 4. Here, the growth was stopped before the first nitride semiconductor layer 5 completely covered the protective film 4. The interval between adjacent first nitride semiconductor layers was about 2 μm. The film thickness of the second lateral growth portion 5b was set to 2 μm.
[0050]
Next, oxygen and CF are used as an etching gas by isotropic etching, which is dry etching.₄Then, the protective film 4 was removed at a temperature of 120 ° C.
[0051]
Further, from the end face and the upper face of the first nitride semiconductor layer 5 grown in the lateral direction, the temperature is raised to 1050 ° C. by MOCVD at normal pressure, TMG and ammonia are used as source gases, and the second nitride made of GaN. The physical semiconductor layer 6 was grown to a thickness of 15 μm. Note that the second nitride semiconductor layer 6 may be grown not under normal pressure but under reduced pressure.
[0052]
When the surface of the obtained second nitride semiconductor layer 6 was observed by cathodoluminescence (CL), almost no crystal defects were observed and the crystallinity was good. The number of crystal defects is about 6 × 10⁶cm^-2Met.
[0053]
Example 2
In this example, a nitride semiconductor substrate was fabricated in the same manner as in Example 1 except that the decomposition preventing layer 3 was formed before the protective film 4 was formed on the upper surface of the seed crystal 2.
[0054]
After the seed crystal 2 was formed in the same manner as in Example 1, a decomposition preventing layer 3 made of SiN was grown to a thickness of 0.3 μm on the top surface of the seed crystal 2 by CVD, and then SiO 2 was formed thereon.₂A protective film 4 made of a film having a thickness of 0.3 μm was formed.
Since the subsequent steps are the same as those in the first embodiment, they are omitted here.
[0055]
When the surface of the obtained second nitride semiconductor layer 6 was observed by cathodoluminescence (CL), almost no crystal defects were observed and the crystallinity was good. The number of crystal defects is about 6 × 10⁶cm^-2Met.
[0056]
【The invention's effect】
According to the method for manufacturing a nitride semiconductor substrate of the present invention, since the nitride semiconductor layer is formed only by lateral growth, a nitride semiconductor substrate with few crystal defects can be obtained. When grown from different substrates in the vertical direction, the number of crystal defects is 10⁶-10⁷10 which is the number of practical crystal defects for device formation.⁵-10⁶cm^-2In order to reduce the thickness, the ELOG technique needs to be repeated many times, so the thickness of the obtained substrate is about 20-30 μm, but the nitride semiconductor substrate of the present invention has a thickness of about 10-20 μm, Number of crystal defects in CL measurement: 6 × 10⁵cm^-2Degree can be achieved.
In addition, the nitride semiconductor substrate of the present invention is manufactured by laterally growing the second nitride semiconductor layer using the end face of the first nitride semiconductor layer formed only by lateral growth as a growth starting point. Therefore, the region without crystal defects is wide.
Furthermore, the nitride semiconductor substrate of the present invention is obtained by laterally growing the second nitride semiconductor layer described above in a space without forming a laminated structure with a material having a different thermal expansion coefficient. Few.
Thus, the semiconductor device fabricated on the nitride semiconductor substrate of the present invention has high electrical characteristics and improved lifetime characteristics.
[Brief description of the drawings]
FIG. 1 is a schematic view showing steps of a manufacturing process of a nitride semiconductor substrate according to a first embodiment of the present invention.
FIG. 2 is a schematic view showing stepwise the manufacturing process of a nitride semiconductor substrate according to a second embodiment of the present invention.
FIG. 3 is a schematic cross-sectional view of a conventional nitride semiconductor substrate.
[Explanation of symbols]
1 ... Different substrates
2 ... Seed crystal,
3 ... Decomposition prevention layer,
4 ... Protective film,
5 ... 1st nitride semiconductor layer,
5a ... 1st lateral growth part,
5b ... second lateral growth part,
6 ... Second nitride semiconductor layer,
101 ... SiC substrate,
102 ... GaN seed crystal,
103 ... SiN protective mask,
104: AlGaN layer.

Claims (12)

On a heterogeneous substrate different from a nitride semiconductor,
A seed crystal made of a periodically arranged nitride semiconductor;
A first nitride semiconductor layer having a T-shaped cross section periodically arranged to cover the seed crystal;
A second nitride semiconductor layer covering the entire surface of the first nitride semiconductor layer,
Here, the first nitride semiconductor layer protrudes laterally from the first laterally grown portion and the upper side surface of the first laterally grown portion, and is formed continuously with the first laterally grown portion. A second lateral growth portion formed;
A lower side surface of the first lateral growth portion is joined to a side surface of the seed crystal;
A cavity is formed in a portion of the heterogeneous substrate below the bottom surface of the first lateral growth portion, whereby the bottom surface of the first lateral growth portion does not contact the heterogeneous substrate;
The second laterally grown portions of adjacent first nitride semiconductor layers are not joined together;
A space is formed between the upper surface of the seed crystal and the lower surface of the second lateral growth portion,
Here, the second nitride semiconductor layer is bonded to the entire upper surface of the first nitride semiconductor layer and the end face of the second lateral growth portion,
Here, the seed crystal, the first nitride semiconductor layer, and the second nitride semiconductor layer may be the same or different and have a general formula of In _x Al _y Ga _1-xy N (0 ≦ x, 0 A nitride semiconductor substrate having a composition represented by ≦ y, x + y ≦ 1).   2. The decomposition preventing layer is formed on the entire upper surface of the seed crystal, and a space is formed between the upper surface of the decomposition preventing layer and the first nitride semiconductor layer. Nitride semiconductor substrate. 3. The nitride semiconductor substrate according to claim ₂ , wherein the decomposition preventing layer is made of SiN, SiON, or TiO2.   4. The nitride semiconductor substrate according to claim 1, wherein the seed crystal is formed in a periodic stripe shape.   4. The nitride semiconductor substrate according to claim 1, wherein the seed crystal is formed in a periodic island shape. 5.   The nitride semiconductor substrate according to claim 1, wherein the seed crystal is formed in a lattice shape. On the entire surface of a different substrate different from the nitride semiconductor,
Forming a seed crystal made of a nitride semiconductor;
Forming a protective film on the entire surface of the seed crystal;
Etching the seed crystal and the protective film;
A first nitride semiconductor layer is laterally grown from the side surface of the etched seed crystal as a growth starting point, and the first nitride semiconductor layer laterally grown on the protective film does not completely cover the protective film. Stopping the lateral growth before joining together,
Removing the protective film by etching;
An end face formed by stopping lateral growth before the first nitride semiconductor layers are bonded to each other, or a second nitride starting from the end face and the upper surface of the first nitride semiconductor layer. A method of manufacturing a nitride semiconductor substrate, comprising a step of growing a semiconductor layer. On the entire surface of a different substrate different from the nitride semiconductor,
Forming a seed crystal made of a nitride semiconductor;
Forming a decomposition preventing layer on the entire surface of the seed crystal;
Forming a protective film on the entire surface of the growth prevention layer;
Etching the seed crystal, the decomposition preventing layer and the protective film;
A first nitride semiconductor layer is laterally grown from the side surface of the etched seed crystal as a growth starting point, and the first nitride semiconductor layer laterally grown on the protective film does not completely cover the protective film. Stopping the lateral growth before joining together,
Removing the protective film by etching;
An end face formed by stopping lateral growth before the first nitride semiconductor layers are bonded to each other, or a second nitride starting from the end face and the upper surface of the first nitride semiconductor layer. A method of manufacturing a nitride semiconductor substrate, comprising a step of growing a semiconductor layer.   The manufacturing method according to claim 8, wherein an etching rate of the decomposition preventing layer is smaller than an etching rate of the protective film. The manufacturing method according to claim 9, wherein the decomposition prevention layer is SiN, SiON, or TiO ₂ , and the protective film is SiO ₂ .   The manufacturing method according to claim 7, wherein the etching of the protective film is anisotropic etching.   11. The manufacturing method according to claim 7, wherein etching of the protective film is isotropic etching.

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