JP4513326B2 - Nitride semiconductor crystal manufacturing method and nitride semiconductor substrate manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a nitride semiconductor crystal and a method for manufacturing a nitride semiconductor substrate.

  Nitride semiconductors such as GaN are attracting attention as materials for ultraviolet to blue light-emitting elements because they have a large forbidden band and a direct transition type.

  As a substrate for manufacturing this nitride semiconductor light emitting device, a heterogeneous substrate such as sapphire has been used, but there are problems such as generation of high-density dislocations due to heteroepitaxial growth and complication of the device manufacturing process. It was.

  In order to solve this, a nitride semiconductor free-standing substrate has been actively developed. As a typical method, a thick GaN layer is formed on a heterogeneous substrate such as sapphire by using an HVPE method (hydride vapor phase epitaxy), and the heterogeneous substrate is removed after growth to form a freestanding GaN substrate. (For example, refer to Patent Document 1). This is because the layer having voids functions as a strain relaxation layer, and the strain caused by the difference in lattice constant and thermal expansion coefficient between the base substrate and the group III nitride semiconductor layer is alleviated. It is said that a group III nitride semiconductor substrate having a low crystallinity and a good crystal quality can be obtained, and the substrate can be easily removed. With this technology, low dislocation GaN substrates are being realized and are beginning to appear on the market.

However, a GaN substrate manufactured by the HVPE method is very expensive. The distribution volume is very small. This is largely due to crystal growth and processing of each substrate one by one. In general, since this process is hetero-growth on a heterogeneous substrate such as sapphire or GaAs, preparation of the base substrate is very laborious in order to prevent the occurrence of dislocations in the growing GaN crystal. To give an example of a method for manufacturing a base substrate, first, a thin GaN layer of about 1 μm is formed on a sapphire substrate by metal organic vapor phase epitaxy (MOVPE method), and then the number of widths is measured using a precise photolithography technique. A stripe mask of about ₂ μm of SiO ₂ is formed. A GaN thick film having a thickness of about 500 μm is formed on the base substrate subjected to such processing using the HVPE method. Thereafter, the GaN thick film is separated from the sapphire substrate using a laser peeling method or the like, and both surfaces are polished to finally complete the GaN substrate. This is performed on all the substrates one by one. Due to such a large number of processes, the production yield of the GaN substrate is very low. In particular, there are many problems in the growth of the underlying GaN layer, the mask formation process, and the removal process of the dissimilar substrate.

Therefore, a method has been proposed in which a GaN substrate obtained by the above method is newly used as a seed crystal, an ingot is produced by growing at a high speed for a long time, and the GaN substrate is cut out therefrom (for example, a patent) Reference 2).
JP 2003-178984 A JP 2003-527296 A

  However, ingot growth fails if the conventional vapor phase growth method is used for simple growth for a long time. One is the occurrence of strain and cracks due to polycrystal adhesion to the side of the ingot and abnormal growth at the outermost periphery of the growth surface, and the other is the poor crystal quality that occurs as a result of changes in the growth position as the thickness increases. It is uniform.

  Accordingly, the present invention solves the above-described problems and proposes an unprecedented high-quality nitride semiconductor substrate and a method for manufacturing the same.

  In order to achieve the above object, the present invention is configured as follows.

According to a first aspect of the present invention, there is provided a method for producing a nitride semiconductor crystal comprising: using a single crystal nitride semiconductor substrate as a seed crystal and producing a nitride single crystal having a thickness of 5 mm or more by hydride vapor phase epitaxy ; With a cover having a region extending radially inward on the end face and having an open upper end face, the growth face edge portion and the side face portion of the growth crystal are separated from the outer periphery of the crystal growth face of the growth crystal by a distance of 1 mm or more. The seed crystal is placed from the lower surface to the upper surface by covering the region from the periphery and making the length of the cover larger than the length of the growing crystal to be grown, and retracting the crystal rotation axis in accordance with the growth rate. The nitride single crystal is grown while correcting and controlling so that the growth position in the furnace of the crystal growth surface of the grown crystal becomes constant.

By providing the cover, it is possible to prevent abnormal growth of the growth surface edge and side surfaces. Further, by making the length of the cover larger than the length of the ingot to be grown, the flow in the furnace can be kept constant even when the crystal grows large. Further, in order to correct the growth position change in the furnace due to the thick growth, the crystal rotation axis is retracted according to the growth rate. This position correction control means can be realized relatively easily by, for example, an optical detection device for the growth surface position and a computer-controlled actuator.

Here, the above distance is, for example, the distance between the crystal and the cover that covers all of the side surface of the seed crystal that is 1 mm or more in the radial direction from the outer periphery of the growth surface and the side surface.

According to a second aspect of the invention, in the manufacturing method of the nitride semiconductor crystal according to claim 1, wherein the correction control, the laser beam is irradiated to the crystal growth surface, the laser light reflected at said crystal growth surface By reading the position of the crystal growth surface from the detection signal acquired by detecting, and controlling the position of the crystal growth surface in the furnace according to the deviation between the position of the crystal growth surface and a predetermined position The method is characterized in that the crystal position retreats at the same speed as the crystal growth speed, and the growth surface is always kept at the same position.

A third aspect of the present invention, in the manufacturing method according to claim 2 nitride semiconductor crystal according, characterized in that the growth rate is 350 .mu.m / h or more.

A method for manufacturing a nitride semiconductor substrate according to a fourth aspect of the present invention is characterized by being obtained by slicing a nitride semiconductor crystal manufactured by the method according to any one of the first to third aspects.

According to a fifth aspect of the present invention, in the method for manufacturing a nitride semiconductor substrate according to the fourth aspect , after the nitride semiconductor crystal is shaped into a cylindrical shape, an orientation flat is formed and then sliced.

The invention of claim 6 is the method for manufacturing a nitride semiconductor substrate according to claim 5 , wherein two or more orientation flats are formed.

<Key points of the invention>
The main point of the present invention is that, in the growth of a nitride semiconductor single crystal ingot by vapor phase growth, by providing a cover that covers the outer periphery and the side surface of the growing crystal, it prevents adhesion of other crystals to the crystal side surface. At the same time, the crystal rotation axis is retracted according to the growth rate, and the growth surface is always maintained at a constant position in the furnace, thereby solving the above problems and obtaining a uniform and high-quality nitride semiconductor single crystal. There is.

  In detail, the ingot growth by vapor phase growth in the present invention is greatly different from the method by pulling up from a melt such as Si or GaAs in the following two points.

(1) Abnormal growth on side surfaces and edge portions In vapor phase growth, the source gas circulates not only to the growth surface of the growth crystal but also to the side surfaces, causing abnormal growth such as polycrystal adhesion on the crystal side surface. . If there is adhesion of polycrystals, they proliferate toward the center of the crystal as they grow, and most of the grown crystals may become polycrystallized. Further, since the flow of the raw material gas is disturbed at the edge portion of the growth surface, the swell is generated, and further, the gas flow is disturbed and the shape of the grown crystal is significantly impaired. Gradually growing crystals themselves are also a factor in changing the flow.

(2) Growth position change due to increase in thickness Since a normal vapor phase growth apparatus does not assume ingot growth, the position of the crystal support shaft is fixed. Therefore, as the thickness increases, the position of the growth surface advances toward the upstream. Since the vapor phase growth of nitride semiconductor crystals is a system in which a vapor phase reaction is intense, the change in the growth surface position has a great influence. The growth rate is most affected, and a change of several millimeters in the growth position can cause a large fluctuation of several tens of percent. When the growth rate changes greatly, the dopant concentration, defect density, and strain become non-uniform, and when a substrate is made by slicing an ingot, the characteristics vary depending on the position to be cut out.

  Based on the above two considerations, the following method has been devised in a method for producing a nitride single crystal ingot by vapor phase growth.

  First, in order to correct the growth position change in the furnace due to the thick growth, the crystal rotation axis is retracted according to the growth rate. This correction control can be realized relatively easily by using, for example, an optical detection device for the growth surface position and a computer-controlled actuator. At the same time, a cover for covering the growth surface edge portion and the side surface portion of the growth crystal is provided. This prevents abnormal growth of the growth surface edge and side surfaces. Furthermore, the length of the cover is made larger than the length of the ingot to be grown. Thereby, the flow in the furnace can always be kept constant even if the crystal grows greatly.

  In addition to the use of the seed crystal and the growth apparatus as described above, at the time of ingot growth, the growth rate is set to 350 μm / h or more, so that contamination of impurities can be suppressed and a high quality crystal can be obtained.

  The grown ingot is preferably formed with an orientation flat before slicing. As a result, the cost can be significantly reduced as compared with the case of processing one sheet at a time.

  According to the present invention, the nitride single crystal is grown while preventing the abnormal growth of the growth surface edge portion and the side surface by covering the growth surface edge portion and the side surface portion of the growth crystal with a cover from the periphery at a predetermined distance. Can do.

  Further, according to another feature of the present invention, the nitride single crystal is grown while the crystal rotation axis is retracted in accordance with the growth rate, and the growth position in the furnace is controlled to be constant, so that the nitride single crystal is grown thick. can do.

  Therefore, by using the nitride single crystal obtained by the manufacturing method of the present invention, a nitride semiconductor substrate having few defects such as cracks and dislocations and having a high level of characteristics can be obtained at low cost.

  Hereinafter, embodiments of the present invention will be described focusing on examples.

  FIG. 1 is a schematic view of a crystal growth apparatus (HVPE furnace) by the HVPE method used in an embodiment of the present invention.

Reference numeral 1 denotes a quartz reaction tube having an exhaust port 10 at the top, which is heated by a heater 2 disposed around it. A Ga source boat 5 is installed below the quartz reaction tube 1, and each gas can be introduced through the NH ₃ introduction tube 3, the HCl introduction tube 4 and the GaCl introduction tube 7. Above the quartz reaction tube 1, a crystal rotation shaft 8 having a seed substrate 9 fixed at the lower end is suspended and can be displaced in the vertical direction by an actuator 15.

  The current position of the surface of the GaN single crystal grown from the seed substrate 9 is grasped by emitting the light 12 of the laser light source 11 obliquely from below and detecting the reflected light by the detector 13. That is, the detection signal of the substrate growth surface position from the detector 13 is read by the computer 14, and the actuator 15 is controlled according to the deviation from the desired position, so that the crystal growth surface is always at the same desired position in the furnace. To do.

  The growth surface edge portion and side surface portion of the growth crystal are covered with a cover 16 from the periphery at a predetermined distance of 1 mm or more from the outer periphery of the growth surface of the seed crystal 9. The structure of the cover 16 is as shown in FIG. The cylindrical main body has an inner diameter of 62 mm, and the lower end surface is a cylindrical graphite cover 16 having a region a extending 3.5 mm radially inward and having a cover opening 17 having a diameter of 55 mm.

<Example>
An embodiment relating to the present invention will be described with reference to FIG.

  First, as a seed substrate, a single crystal GaN substrate having a diameter of 60 mm was manufactured using a void formation peeling method (a manufacturing method disclosed in Japanese Patent Laid-Open No. 2003-178984 of Patent Document 1). The seed substrate is subjected to double-side mirror polishing and has a thickness of 430 μm. The main surface is a c-plane, and the front side is a Ga polar surface.

  This seed substrate was set in the HVPE furnace shown in FIG.

  The edge portion of the seed crystal (the region from the outer periphery to the radially inner side 2.5 mm in the crystal growth surface) and the side surface portion were covered with a cylindrical graphite cover 16 having a length of 50 mm as shown in FIG. The cover opening diameter of the seed crystal growth surface is 55 mm.

When HCI gas is flowed through the HCl introduction tube 4 into the Ga source boat 5 installed in the quartz reaction tube 1 heated by the heater 2, it reacts with the metal Ga6 filled in the boat 5 to become GaCl. This is supplied to the seed substrate 9 fixed to the crystal rotation shaft 8 through the GaCl introduction tube 7. NH ₃ is supplied to the seed substrate 9 through the NH ₃ introduction tube 3 independently of GaCl. GaCl and NH ₃ react on the seed substrate 9 to grow a GaN single crystal. At this time, the position of the substrate growth surface is always monitored by detecting the laser beam 12 emitted from the laser light source 11 and reflected by the crystal growth surface by the detector 13. The signal is read by the computer 14 and the actuator 15 is controlled so that the crystal growth surface is always at the same position in the furnace. Using such an HVPE apparatus, crystal growth was performed on the seed crystal 9 at a growth rate of 400 μm / h for 100 hours.

  As a result, a cylindrical ingot having a diameter of 55 mm and a length of 40 mm was obtained. Due to the effect of the cover 16, no polycrystals adhered to the side surfaces. Moreover, it was confirmed from the moving speed of the crystal rotation axis 8 that the growth speed is always constant.

  The obtained ingot was cylindrically ground to a diameter of 50 mm. The position of the (1-100) plane was determined by X-ray diffraction, and an orientation flat with a length of 15 mm was created. In order to distinguish the front and back after slicing, a second orientation flat having a length of 10 mm was also formed at a position rotated 90 degrees. Then, it sliced using the wire saw and cut out 40 sheets of 500-micrometer-thick GaN substrates. Each substrate was subjected to both mirror polishing processes to form a transparent GaN single crystal substrate having a diameter of 50 mm.

When the dislocation density of the obtained 40 GaN substrates was measured by the EPD method, it was 3 × 10 ⁵ cm ⁻² ± 10%. The measurement result of the carrier concentration by Hall measurement was 1.5 × 10 ¹⁸ cm ⁻³ ± 12%. From these results, it was found that a GaN substrate having high quality and uniform characteristics was obtained.

<Comparative example: Stopping the shaft and no cover>
A seed crystal similar to that in Example 1 was set in the same apparatus and grown under the same conditions. However, the cover 16 as described in Example 1 was removed. The position of the crystal rotation axis 8 was also fixed. As a result, an ingot having a length of 35 mm was obtained, but polycrystals were firmly attached to the side surface. The growth surface was rough as wrinkles approached, and the edges were also raised.

  When the obtained ingot was cylindrically ground to a diameter of 50 mm, it was found that a large number of minute cracks were contained inside the crystal. This is presumably because cracks were generated during growth due to the stress caused by the adhesion of polycrystals on the side surfaces and the rise of the growth surface edge. The above-mentioned roughening of the growth surface is considered to have occurred because the growth progressed while embedding cracks. The position of the (1-100) plane was determined by X-ray diffraction to produce an orientation flat having a length of 15 mm. In order to distinguish the front and back after slicing, a second orientation flat having a length of 10 mm was also formed at a position rotated 90 degrees. Then, it sliced using the wire saw and cut out 25 pieces of GaN substrates with a thickness of 500 μm. Each substrate was subjected to both mirror polishing processes to form a transparent GaN single crystal substrate having a diameter of 50 mm.

When the dislocation density of the obtained 25 GaN substrates was measured by the EPD method, it showed a high value of 5 × 10 ⁷ cm ⁻² ± 50%, and it varied greatly. This is considered to be caused by growing while filling the generated cracks. The measurement result of the carrier concentration by Hall measurement was 1.5 × 10 ¹⁸ cm ⁻³ near the root of the ingot, increased as it approached the tip, and 3 × 10 ¹⁸ cm ⁻³ near the tip. This indicates that the growth surface position changed with increasing thickness, and at the same time the ambient flow changed, thereby gradually reducing the growth rate. Eventually, only those with poor crystallinity and large variations were obtained.

<Other application examples and modifications>
In the embodiment described above, the example in which the present invention is applied to the method for manufacturing a GaN substrate has been described. However, the manufacture of a single crystal free-standing substrate of a ternary mixed crystal such as aluminum gallium nitride or gallium indium nitride, Mg, etc. It can also be applied to the manufacture of a doped p-type GaN substrate.

  It can also be applied to the sublimation method and other growth methods.

  It is also conceivable to repeatedly use the crystal produced by this method as a seed substrate.

  The group III nitride semiconductor substrate obtained by the present invention can be widely used as a substrate for a GaN-based device. In particular, when used as a substrate for a laser diode, a high-quality GaN-based crystal with a low defect density can be obtained, so that a highly reliable high-performance laser diode can be manufactured.

It is a schematic diagram of the crystal growth apparatus by HVPE method which shows one Example of this invention. It is the figure on which the shape of the cover in FIG. 1 was drawn.

Explanation of symbols

1 Quartz reaction tube 3 NH ₃ introduction tube 4 HCl introduction tube 5 Ga source boat 6 Metal Ga
7 GaCl introduction tube 8 Crystal rotation axis 9 Seed substrate 11 Laser light source 12 Laser light 13 Detector 14 Computer 15 Actuator 16 Cover 17 Cover opening

Claims (6)

In a method of manufacturing a nitride single crystal having a thickness of 5 mm or more by a hydride vapor phase growth method using a single crystal nitride semiconductor substrate as a seed crystal,
The growth surface edge portion and the side surface portion of the growth crystal are separated from the outer periphery of the crystal growth surface of the growth crystal by a distance of 1 mm or more by a cover having a region extending radially inward at the lower end surface and having an open upper end surface. The cover is covered with the region from the periphery, and the length of the cover is made larger than the length of the growing crystal to be grown, and the seed is moved from the lower end surface to the upper end surface by retracting the crystal rotation axis in accordance with the growth rate. A method for producing a nitride semiconductor crystal, comprising: retreating a crystal and growing a nitride single crystal while correcting and controlling so that a growth position in a furnace of a crystal growth surface of the grown crystal is constant. In the manufacturing method of the nitride semiconductor crystal according to claim 1 ,
The correction control reads the position of the crystal growth surface from a detection signal obtained by irradiating the crystal growth surface with laser light and detecting the laser light reflected on the crystal growth surface, By controlling the position of the crystal growth surface in the furnace according to the deviation between the position of the surface and the predetermined position, the position of the crystal is retreated at the same speed as the crystal growth speed, and the growth surface is always at the same position. A method for producing a nitride semiconductor crystal, wherein In the manufacturing method of the nitride semiconductor crystal according to claim 2 ,
A method for producing a nitride semiconductor crystal, wherein the growth rate is 350 μm / h or more. Nitride semiconductor substrate manufacturing method, characterized in that obtained by slicing the fabricated nitride semiconductor crystal by the method according to any one of claims 1-3. In the manufacturing method of the nitride semiconductor substrate according to claim 4 ,
A method of manufacturing a nitride semiconductor substrate, comprising: shaping a nitride semiconductor crystal into a cylindrical shape, forming an orientation flat, and then slicing the orientation flat. In the manufacturing method of the nitride semiconductor substrate according to claim 5 ,
A method of manufacturing a nitride semiconductor substrate, wherein two or more orientation flats are formed.

Images