JP3832313B2 - Nitride semiconductor growth method and nitride semiconductor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nitride semiconductor having a light emitting element such as a light emitting diode or a laser diode, and a method for growing the nitride semiconductor.
[0002]
[Prior art]
In recent years, attention has been paid to light emitting diodes (LEDs) and laser diodes (LDs), which have a short wavelength from blue to ultraviolet using a nitride semiconductor substrate. Laser diodes are increasingly demanded for use in disk systems capable of recording / reproducing large capacity and high density information such as DVDs. Therefore, various methods for obtaining a nitride semiconductor substrate with good crystallinity, which is considered as an essential condition, have been studied.
[0003]
In general, when a semiconductor is grown on a substrate, it is known that if a substrate lattice-matched with the semiconductor to be grown is used, dislocations in the semiconductor are reduced and crystallinity is improved. Is currently not present in the world, it is generally grown on dissimilar substrates that do not lattice match with nitride semiconductors such as sapphire, spinel, and silicon carbide.
[0004]
On the other hand, attempts to fabricate GaN bulk single crystals that are lattice-matched with nitride semiconductors have been made by various research institutions, but only a few millimeters have been reported yet, and they have been put to practical use. It's far away.
[0005]
As a technique for producing a GaN substrate, for example, in JP-A-7-202265 and JP-A-7-165498, a buffer layer made of ZnO is formed on a sapphire substrate, and a nitride semiconductor is grown on the buffer layer. A technique for dissolving and removing the buffer layer after the treatment is described. However, the crystallinity of the ZnO buffer layer grown on the sapphire substrate is poor, and it is difficult to obtain a good quality nitride semiconductor crystal even if a nitride semiconductor is grown on the buffer layer. Furthermore, it is difficult to continuously grow a nitride semiconductor having a thick film as a substrate on a buffer layer made of thin ZnO.
[0006]
When manufacturing a nitride semiconductor element used in various optical and electronic devices such as LED elements, LD elements, light receiving elements, etc., if a substrate made of a nitride semiconductor with few dislocations can be manufactured, a new one will be formed on the substrate. Since a nitride semiconductor with few lattice defects can be grown by growing a simple nitride semiconductor, the crystallinity of the device is remarkably improved, and a device that has not been realized conventionally can be realized.
[0007]
However, generally, in a nitride semiconductor formed on a different substrate, threading dislocations are generated at the interface between the two due to lattice mismatch with the different substrate and a difference in thermal expansion coefficient. This threading dislocation continues to propagate to the nitride semiconductor and does not disappear at the stage of growth of the nitride semiconductor.
[0008]
Thus, several methods have been reported for controlling the growth direction of nitride semiconductors and suppressing dislocation propagation. For example, after a nitride semiconductor layer is grown on a heterogeneous substrate, in order to suppress the vertical growth of the nitride semiconductor, a plane on which the nitride semiconductor can grow in the vertical direction (for example, a plane of the nitride semiconductor) After forming a protective film having an opening on the surface of a different substrate and controlling the growth direction of the nitride semiconductor, the nitride semiconductor layer is formed again from the opening of the protective film, thereby propagating to the nitride semiconductor layer. Thus, a nitride semiconductor with good crystallinity with reduced dislocations can be obtained.
[0009]
This is because the nitride semiconductor is difficult to grow on the protective film, so that the nitride semiconductor selectively grows from the opening of the protective film and further starts lateral growth on the protective film. At this time, dislocations existing under the protective film are prevented from propagating upward by the protective film, and dislocations are reduced above the protective film. Further, when the nitride semiconductor grows in the lateral direction on the protective film, some of the dislocations in the opening are bent in the lateral direction, and the dislocations are prevented from propagating upward by forming a loop.
[0010]
[Problems to be solved by the invention]
However, when the protective film is formed by the above-described method, dislocations continue to propagate in the portion where the protective film is not formed, and the nitride semiconductors adjacent to each other grow in the lateral direction on the protective film to form a flat layer. Even in this case, dislocations remain on the upper surface of the nitride semiconductor layer where there is a bonding surface. For this reason, it is not possible to expect a uniform reduction in dislocations as a whole, and it is difficult to use the entire surface of the substrate because many dislocations remain on the opening of the protective film. Cannot be expected.
[0011]
Accordingly, the present invention solves the above problems, efficiently stops dislocations generated in the nitride semiconductor layer, and reduces dislocations evenly over the entire growth surface.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a first step of forming pits having a diameter in a range of 0.5 μm or more and 10 μm or less by etching on an upper surface of a first nitride semiconductor layer stacked on a substrate. And a second step of forming a protective film inside the pit, and a third step of growing the second nitride semiconductor layer on the first nitride semiconductor layer as a nucleus. And a method for growing a nitride semiconductor.
[0013]
The nitride semiconductor growth method includes a fourth step of growing a third nitride semiconductor layer without filling pits on the first nitride semiconductor layer after the first step. This is a method for growing a nitride semiconductor.
[0014]
The nitride semiconductor growth method is characterized in that the third nitride semiconductor layer has a molar ratio (V / III ratio) of a group V source to a group III source of 500 or more and 3000 or less.
[0015]
The nitride semiconductor growth method is characterized in that the etching is performed at 50 Å / sec or more and 200 Å / sec or less.
[0016]
The nitride semiconductor growth method is characterized in that the protective film includes at least one of SiO ₂ , SiN, W, and Mo.
[0017]
The nitride semiconductor growth method is characterized in that the protective film is a multilayer film composed of two or more layers.
[0018]
The second nitride semiconductor layer is characterized in that the molar ratio (V / III ratio) of the group V material to the group III material is 6000 or more.
[0019]
The present invention is a nitride semiconductor having a substrate, and a first nitride semiconductor layer and a second nitride semiconductor layer in order on the substrate, the upper surface of the first nitride semiconductor layer being A pit having a diameter in the range of 0.5 μm or more and 10 μm or less is formed, and a protective film is provided inside the pit, and a second nitride is formed on the first nitride semiconductor layer and the protective film. A nitride semiconductor comprising a semiconductor layer.
[0020]
As described above, the present invention forms etch pits in the dislocations generated on the first nitride semiconductor layer stacked on the substrate by etching, or if the etch pits are small, the etch pits are formed. By increasing the size and forming the protective film described above inside the etch pit, it is possible to selectively form the protective film on the dislocations and the dislocation concentration part.
[0021]
This is because the crystallinity of a portion where a plurality of dislocations are concentrated and propagated to the first nitride semiconductor layer is poor, so that an etch pit is selectively formed by etching, and thus a protective film is formed inside the etch pit. Thus, when the second nitride semiconductor layer is stacked on the first nitride semiconductor layer, a plurality of dislocations existing in the first nitride semiconductor layer propagate to the second nitride semiconductor layer. Therefore, the dislocation of the second nitride semiconductor layer can be reduced, and a nitride semiconductor layer having better crystallinity can be obtained more effectively and uniformly.
[0022]
When a nitride semiconductor device is produced using the second nitride semiconductor layer having good crystallinity obtained by the growth method of the present invention as a substrate, the nitride semiconductor device grown on the same is also dislocations. Thus, an element with excellent crystallinity is significantly reduced, and the deterioration of the nitride semiconductor caused by dislocation can be remarkably prevented. In addition, the electrostatic breakdown voltage of the LED is remarkably increased, and a nitride semiconductor with good lifetime characteristics can be provided, and can be applied to an LD.
[0023]
Further, after the nitride semiconductor is thickly stacked on the sapphire substrate by HVPE (Hydride Vapor Phase Epitaxy) or the like, the present invention can be used to make a thick film substrate, and the layer grown on the sapphire substrate It is also possible to use the invention, and then to make a thick layer by HVPE or the like. By removing the obtained nitride semiconductor layer by peeling off the sapphire substrate, a single nitride semiconductor is obtained. It can also be applied to other fields.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
In the method for growing a nitride semiconductor according to the present invention, the first embodiment grows a second nitride semiconductor layer by forming an SiO ₂ film inside the etch pit after etching the first nitride semiconductor layer. It is. In the second embodiment, after etching the first nitride semiconductor layer, a third nitride semiconductor layer is grown on the first nitride semiconductor layer in order to further increase the etch pits generated here. Then, a SiO ₂ film is formed inside the etch pit, and a second nitride semiconductor layer is grown on the third nitride semiconductor layer. The first and second embodiments are the third nitride It differs in whether or not a semiconductor layer is provided. Hereinafter, embodiments of the present invention will be described in detail.
[0025]
[First Embodiment] As shown in FIG. 1, a first nitride semiconductor layer 2 is grown on a buffer layer grown on a substrate 1. In the present invention, the substrate 1 includes an insulating substrate such as sapphire or spinel (MgAl ₂ O ₄ ) whose main surface is any one of the C-plane, R-plane, and A-plane, SiC (6H, 4H, 3C). ), ZnS, ZnO, GaAs, Si, or other substrate material can be used.
[0026]
As the buffer layer (not shown in the drawing), AlN, GaN, AlGaN, InGaN or the like is used. The buffer layer is grown at a temperature of 300 ° C. or higher and 900 ° C. or lower and a film thickness of 10 to 0.5 μm. Thus, when the buffer layer is formed on the substrate 1 at a temperature of 900 ° C. or lower, the lattice mismatch between the substrate 1 and the first nitride semiconductor layer 2 is relaxed, and the dislocation of the first nitride semiconductor layer 2 is reduced. Less.
[0027]
The first nitride semiconductor layer 2 uses hydrogen or nitrogen as a carrier gas, trimethylgallium (TMG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH ₃ ) or the like as a raw material, and MOCVD (Metal Organic) The growth is performed using an apparatus such as Chemical Vapor Deposition. When the first nitride semiconductor layer 2 is grown, the molar ratio (V / III ratio) of NH ₃ as a group V material to TMG as a group III material is a low V / III ratio of 500 or more and less than 6000. Do. When grown under the conditions described above, a nitride semiconductor layer with reduced dislocations can be obtained, and preferably, the first nitride semiconductor layer 2 having a dislocation density of 5 × 10 ⁸ cm ⁻² or less is formed. Form.
[0028]
Next, as shown in FIG. 2, by etching the upper surface of the first nitride semiconductor layer 2, etch pits are formed in the dislocations that have propagated to the first nitride semiconductor layer. Examples of the method for etching the first nitride semiconductor layer 2 include dry etching, wet etching, polishing, grinding, blasting, and excimer laser irradiation. Examples of dry etching include reactive ion etching (RIE), reactive ion beam etching (RIBE), electron cyclotron etching (ECR), plasma etching, and the like. Preferably, the etching rate is 50 Å / sec to 200 て using RIE. / Sec is performed at a high speed. By etching at a high speed, irregularities can be formed on the surface. This is an etch pit and is formed in the dislocation concentration region. This is presumably because the dislocation region has poor crystallinity. The size of the etch pit is about 0.5 μm to 10 μm in diameter, and the depth is about the same as the diameter. The above is the first step.
[0029]
In the second step shown in FIG. 3, a protective film is formed over the entire area of the first nitride semiconductor layer 2, and the protective film in the flat portion is removed to remove the protective film of the first nitride semiconductor layer 2 using RIE or the like. Etching is performed to the surface, and the protective film 3 is formed only inside the etch pit. The protective film 3 is made of SiO ₂ , SiN, W, Mo, or the like, and is preferably a protective film made of a metal such as W or Mo because heat generation and resistance are reduced when a current flows through the nitride semiconductor. Further, the protective film 3 may be a multilayer film made of the above material, and by using the multilayer film, peeling of the protective film in the pits can be suppressed.
[0030]
As a film forming method, CVD, sputtering, vapor deposition or the like is used. As an etching method for removing the protective film on the flat portion, dry etching such as RIE, wet etching, polishing, grinding, blasting, excimer laser irradiation, or the like is used.
[0031]
In the third step shown in FIG. 4, the second nitride semiconductor layer 4 is grown on the first nitride semiconductor layer 2. As the growth conditions for the second nitride semiconductor layer 4, it is preferable to use conditions for accelerating the lateral growth in order to embed the protective film 3, and for this purpose, the V / III ratio is set to 6000 or more. Compared with the growth of the first nitride semiconductor layer 2, the NH ₃ is grown at a higher ratio, and the second film is formed so as to embed the protective film 3 under this condition. The nitride semiconductor layer 4 can be formed. As another condition for promoting the lateral growth, the carrier gas is nitrogen. In addition, the growth pressure is reduced. In addition, doping either or both of Mg and Si can be mentioned.
[0032]
The first nitride semiconductor and the second nitride _{semiconductor} be represented by _{In X Al Y Ga 1-X} -Y N (0 ≦ X ≦ 1,0 ≦ Y ≦ 1,0 ≦ X + Y ≦ 1) it can. Further, in addition to non-doping, an n-type impurity such as Si or Ge or a p-type impurity such as Mg or Zn may be used. For example, the second nitride semiconductor layer can be an n-type contact layer in the LED by doping an n-type impurity. Further, by using the high-quality nitride semiconductor layer having a low dislocation density obtained in the present invention as the n-contact layer of the LED, the electrostatic withstand voltage characteristic can be remarkably improved and the leakage current can be remarkably reduced. Furthermore, the output can be greatly increased particularly for LEDs that emit light in the ultraviolet region.
[0033]
Second Embodiment Similar to the first embodiment, the first nitride semiconductor layer 2 is laminated on the substrate 1 made of sapphire or the like, and the formation of etch pits by etching or the like is the same process as the first embodiment. And under the conditions.
[0034]
Next, in the first step, when an etch pit having a diameter of less than 0.5 μm is formed on the first nitride semiconductor layer as shown in FIG. 5, as the fourth step in FIG. In order to make the diameter 0.5 μm or more and 10 μm or less, the third nitride semiconductor layer 5 is formed on the first nitride semiconductor layer to increase the etch pits.
[0035]
If the formed etch pit has a diameter of less than 0.5 μm, it is difficult or difficult to form a protective film such as SiO ₂ inside the etch pit. This is performed when the size of the etched pit is insufficient.
[0036]
The formation of the third nitride semiconductor layer 5 is performed under the conditions for promoting the vertical growth of the nitride semiconductor or the conditions for growing so as not to fill the etch pits. As the conditions, for example, the V / III ratio is set to 500 or more and 3000 or less similarly to the growth conditions of the first nitride semiconductor layer 2.
[0037]
Next, in the fourth step, as in the first step of Example 1, the protective film 3 is formed inside the etch pit having a diameter of 0.5 μm or more and 10 μm or less, and the protective film 3 is embedded. The second nitride semiconductor layer 4 is first grown in the lateral direction on the protective film 3 and then stacked in the vertical direction under the same conditions as in the first embodiment. A nitride semiconductor in which dislocations in the nitride semiconductor layer 4 are significantly reduced is obtained.
[0038]
As described above, after the first step, the second step and the third step are performed after making the etch pits sufficiently large as described above by the fourth step, and the second embodiment is referred to as the second embodiment.
[0039]
[Example] A sapphire substrate having a C surface as a principal surface and an A surface as an orientation flat surface as a substrate 1 was set in an MOCVD apparatus and subjected to thermal cleaning at a temperature of 1050 ° C. for 10 minutes to remove moisture and deposits on the surface. .
[0040]
Next, the temperature was set to 510 ° C., hydrogen was used as the carrier gas, NH ₃ , TMG, and TMA were used as the source gas, and a buffer layer made of Al _0.1 Ga _0.9 N was grown to a thickness of 200 mm.
[0041]
Next, the first nitride semiconductor layer 2 made of GaN has a temperature of 1150 ° C., a carrier gas of hydrogen, a source gas of NH ₃ and TMG of V / III ratio = 5070, and a growth rate of 2.67 μm / h. The film was grown to a thickness of 4 μm under the conditions described above.
[0042]
After the growth of the first nitride semiconductor layer 2, the first nitride semiconductor layer 2 was etched from the surface by 2 μm using RIE at an etching rate of 125 ｓｅｃ / sec. After performing RIE, when the surface of the sample is observed with an optical microscope as shown in FIG. 7, etch pits having a diameter of about 2 μm and a density of about 6 × 10 ⁶ cm ⁻² are observed. It was done.
[0043]
Next, in order to embed the SiO ₂ protective film 3 in the etch pit, first, an OCD (liquid SiO ₂ ) film having a thickness of 0.6 μm was formed at a rotational speed of 5000 rpm. Thereafter, annealing was performed twice at 160 ° C. for 20 minutes and 300 ° C. for 30 minutes, and a SiO ₂ film having a thickness of 0.5 μm was formed thereon by CVD at 360 ° C. for 10 minutes.
[0044]
Next, RIE was performed from the surface side of the first nitride semiconductor layer 2 to remove SiO ₂ on the flat portion other than the inside of the etch pit, thereby leaving the SiO ₂ film 3 only in the etch pit portion.
[0045]
Next, a second nitride semiconductor layer 4 made of GaN is formed on the first nitride semiconductor layer 2 at a temperature of 1150 ° C., nitrogen as a carrier gas, NH ₃ and TMG as source gases, and a V / III ratio = 6637. The film was grown to a film thickness of 6 μm under a growth rate of 2.67 μm / h. Growth conditions of the second nitride semiconductor layer 4, in the comparative example shown later, it was not possible to embed the SiO ₂ film on the etch pits at the time of the second nitride semiconductor growth, in order to embed the SiO ₂ film This is a preferable condition for promoting lateral growth.
[0046]
After the growth of the second nitride semiconductor layer 4, the surface of the nitride semiconductor becomes flat, and the SiO ₂ film 3 in the etch pit is almost buried, as shown in SEM (Scanning Electron Microscopy) of FIG. 8 and FIG. It was observable from an optical microscope image, and the pit density on the surface of the second nitride semiconductor layer 4 was about 1 × 10 ⁶ cm ⁻² . Since the etch pit density on the surface of the first nitride semiconductor layer 2 was about 6 × 10 ⁶ cm ⁻² , lateral growth was performed on the SiO ₂ film formed on the etch pits by optimizing the growth conditions. Then, by embedding the SiO ₂ film, the number of pits is reduced to about 1/6. Further, when CL (Cathode Luminescence) measurement was performed on the same sample, a tendency of strong emission intensity was observed at the upper part of the etch pit, and in the cross-sectional TEM (Transmission Electron Microscopy) image of FIG. 10, it was embedded by the second nitride semiconductor. A low dislocation region was formed on the upper part of the etched pit, and a dislocation-free region was confirmed in the cross-sectional TEM observation visual field of 2 μm at the smallest portion. The dislocation density in the low dislocation region is 1 × 10 ⁸ pieces cm ⁻² or less, and the dislocation density of the first nitride semiconductor is 9 × 10 ⁸ pieces cm ^−2. It was confirmed that it was effective in reducing
[0047]
In addition, since the upper part of the etch pit becomes a low dislocation region, if even one etch pit exists in the depth direction in the crystal, a crystal having a low dislocation density can be obtained on the upper part. By repeating the method a plurality of times, it is possible to realize a dislocation density of 1 × 10 ⁸ cm ⁻² or less on the entire surface, so that the dislocation density can be further reduced evenly. Furthermore, since the film thickness required to fill the etch pits in one step is about 4 to 6 μm, compared with the conventional method of reducing dislocations by lateral growth of nitride semiconductor, it is a thin film. It is also effective in achieving the purpose.
[0048]
[Comparative Example] The same method as in Example 1 until the step of laminating the first nitride semiconductor layer 2 on the sapphire substrate 1, etching by RIE, and then forming the SiO ₂ film 3 only inside the etch pit. And performed under conditions.
[0049]
Next, the second nitride semiconductor layer 4 is formed on the first nitride semiconductor layer 2, the temperature is 1150 ° C., the carrier gas is hydrogen, the source gas is NH ₃ and TMG, and the V / III ratio is 5070, and the growth rate is increased. Was grown to a film thickness of 2 μm under the condition of 2.67 μm / h, and the SiO ₂ film was embedded on the etch pit portion.
[0050]
As a result of observing a cross-sectional TEM image of the nitride semiconductor layer obtained by the above process, GaN is refracted in the horizontal (horizontal) direction at the upper part of the etch pit, and the upper part of the etch pit is a low dislocation region. However, as shown in FIG. 11, the etch pits could not be completely embedded, and pits were observed at a density of 5 × 10 ⁶ cm ^{−2 on} the surface of the second nitride semiconductor layer.
[0051]
【The invention's effect】
By the nitride semiconductor growth method of the present invention, dislocations generated in the nitride semiconductor layer can be uniformly reduced over the entire surface of the nitride semiconductor layer, and a nitride semiconductor with good crystallinity can be provided. In addition, according to the present invention, LEDs and LDs having good life characteristics can be expected.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a nitride semiconductor showing an embodiment of the present invention.
FIG. 2 is a schematic cross-sectional view of a nitride semiconductor showing an embodiment of the present invention.
FIG. 3 is a schematic cross-sectional view of a nitride semiconductor showing an embodiment of the present invention.
FIG. 4 is a schematic cross-sectional view of a nitride semiconductor showing one embodiment of the present invention.
FIG. 5 is a schematic cross-sectional view of a nitride semiconductor showing one embodiment of the present invention.
FIG. 6 is a schematic cross-sectional view of a nitride semiconductor showing one embodiment of the present invention.
FIG. 7 is an optical micrograph of a first nitride semiconductor layer in examples and comparative examples of the present invention.
FIG. 8 is an SEM photograph of a second nitride semiconductor layer in an example of the present invention.
FIG. 9 is an optical micrograph of a second nitride semiconductor layer in an example of the present invention.
FIG. 10 is a cross-sectional TEM photograph of first and second nitride semiconductor layers in an example of the present invention.
FIG. 11 is a cross-sectional TEM photograph of first and second nitride semiconductor layers in a comparative example of the present invention.
[Brief description of symbols]
DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... 1st nitride semiconductor layer 3 ... Protective film 4 ... 2nd nitride semiconductor layer 5 ... 3rd nitride semiconductor layer

Claims (8)

A first step of forming pits having a diameter in the range of 0.5 μm or more and 10 μm or less by etching on the upper surface of the first nitride semiconductor layer laminated on the substrate; and a protective film is formed inside the pits. A nitride semiconductor comprising: a second step of forming a film; and a third step of growing the second nitride semiconductor layer on the first nitride semiconductor layer as a nucleus. Growth method.   The nitride semiconductor growth method includes a fourth step of growing a third nitride semiconductor layer without filling pits on the first nitride semiconductor layer after the first step. The method for growing a nitride semiconductor according to claim 1.   3. The nitride semiconductor growth according to claim 2, wherein the third nitride semiconductor layer is grown at a molar ratio of a group V source to a group III source (V / III ratio) of 500 or more and 3000 or less. Method. The etch the nitride semiconductor method of growing according to any one of claims 1 to 3, characterized in that is carried out in the following 50 Å / sec or more 200 Å / sec. The protective layer SiO 2, _SiN, W, a nitride semiconductor method of growing according to any one of claims 1 to 4, characterized in that it consists of at least one Mo. The protective layer, a nitride semiconductor method of growing according to any one of claims 1 to 5, characterized in that a multilayer film composed of two or more layers. The second nitride semiconductor layer, according to any one of claims 1 to 6, characterized in that growing the molar ratio of the group V material for group III raw material (V / III ratio) is more than 6000 Nitride semiconductor growth method. A nitride semiconductor having a substrate, and a first nitride semiconductor layer and a second nitride semiconductor layer in order on the substrate;
On the upper surface of the first nitride semiconductor layer, pits having a diameter in the range of 0.5 μm to 10 μm are formed,
Inside the pit has a protective film,
A nitride semiconductor comprising a second nitride semiconductor layer on the first nitride semiconductor layer and the protective film.

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