Gallium nitride single crystal substrate and method for manufacturing the same

The present invention relates to an oxygen doping method for a gallium nitride (GaN) single crystal substrate crystal used in a light emitting diode made of a group 3-5 nitride compound semiconductor, a light emitting device such as a semiconductor laser, and an electronic device. Impurity doping of the GaN crystal itself in GaN thin film growth and GaN bulk crystal growth that epigrow on the substrate. Nitride-based compound semiconductors are generally expressed as not only GaN thin films, but also ternary mixed crystal films and quaternary films in which components such as In, P, As, ... are added. This is because mixed crystal films may be laminated. The active layer that generates light is GaInN. However, the main component is GaN. However, since there are other components, it is accurately described as a nitride system. Therefore, in the following description, the terms GaN-based device and GaInN-based device are referred to as the same device.

Light emitting devices using nitride semiconductors have already been put into practical use, including blue LEDs. Conventionally, sapphire has been used as a substrate in a light emitting device using a nitride semiconductor. A GaN layer, GaInN layer, etc. are epitaxially grown on a single crystal sapphire substrate to form an epiwafer. Compared to GaN, Si is used as an n-type dopant. A GaInN-LED device is made on an epiwafer by a wafer process. Sapphire is an extremely stable and robust substrate. The GaN layer and the GaInN layer on it grow well on the sapphire substrate. Even today, GaN-based blue LEDs are built on sapphire substrates. Sapphire (α-Al₂O₃) And GaN have different lattice constants (mismatch), but the GaN layer still grows well on the sapphire substrate. Moreover, the GaN layer is robust without deterioration despite the presence of a large number of dislocations.

Since sapphire forms a trigonal single crystal, a GaN thin film is grown on the C-plane. Since sapphire and GaN have different crystal systems, GaN can be epitaxially grown only on the C-plane with three-fold symmetry. Therefore, all GaInN-LEDs that are currently used and have a proven track record consist of a set of thin films grown in the c-axis direction on a C-plane sapphire substrate.

In other words, the epi layers such as GaN and GaInN thin films on the sapphire surface are all grown on the C surface. As long as sapphire is used for the substrate, only C-plane growth is possible. It was impossible to grow epitaxially in other plane orientations. Therefore, the GaInN-LEDs and GaInN-LDs currently manufactured and used are all stacked C-plane grown GaN and GaInN layers, and there are no thin films with other plane orientations. However, for ELO (epitaxial lateral overgrowth) and pendeo-epi, a surface other than the C surface appears at the end in the middle of growth, and this is not the case.

Sapphire and GaN have a large lattice mismatch and many defects, but GaN has a robustness close to that of ceramics, so that defects do not grow and defects do not increase and become brittle. Although the defect density is enormous, GaN-LEDs have a long life and have already been highly evaluated with a strong track record.

However, the sapphire substrate has some drawbacks. The sapphire substrate is extremely hard and has no cleavage plane. Therefore, after the device is formed on the wafer by the wafer process, it cannot be separated by cleavage when it is cut into chips. There is no choice but to cut (dicing) mechanically. The cost is high due to the dicing process.

In the case of LED, this is fine, but in the case of LD (semiconductor laser), mirror surfaces forming a resonator are required on both sides of the active layer. However, since there is no cleavage plane, the mirror plane cannot be formed by natural cleavage. It is necessary to accurately flatten and smooth the end face by vapor phase etching such as RIE (reactive ion etching) to expose the mirror surface. This is not an easy task. In addition, it is a complicated work because it has to be processed for each chip. The work of surface-facing the resonator is the cause of pushing up the manufacturing cost of GaInN-LD.

Furthermore, since sapphire is an insulator, it cannot be said that electrodes are formed on the bottom surface. Both the p electrode and the n electrode must be formed on the upper surface. It is necessary to stack several n-type layers on the sapphire substrate. Since the current flows laterally, the n-type conductive layer must be formed thick. A p-type layer is stacked on top of the laminated n-type layer to form a pn junction. It is natural to attach a p-electrode to the p-type layer on the upper surface, but the complexity of removing the p-type layer on the outer circumference a little to expose the n-type layer and ohmic-bonding the n-electrode to that part is complicated. is there. The number of processes and process time increase, resulting in high cost. Further, since it is necessary to form two electrodes on the same surface, the required chip area becomes large. From that point as well, the cost increased. Although GaN-based LEDs on sapphire substrates have a proven track record, they cannot overcome the above drawbacks.

An ideal substrate that can solve such a problem is a GaN single crystal substrate. Since epi layers such as GaN and GaInN are deposited, there is no problem of crystal lattice mismatch if it is a GaN substrate. If n-type GaN can be made, the n-type electrode can be taken from the bottom surface of the chip. If the p-electrode and n-electrode can be distributed vertically, device manufacturing will be easier, and wire bonding will be easier when mounting on a package. The required chip area can be reduced.

Above all, since GaN has cleavage, the wafer can be cut into chips by natural cleavage. However, the cleavage plane is in the direction of the side of the equilateral triangle, and is not the cleavage plane on the rectangle. Therefore, it is not possible to cut out a rectangular chip just by cleavage. In that respect, it is a disadvantage unlike Si semiconductors and GaAs semiconductors. However, some chips can be separated by cleavage. Therefore, the cutting process by dicing is reduced. In addition, the mirror surface of the resonator, which is indispensable for semiconductor lasers (LD), can be created by cleavage. If a flat and smooth mirror surface is created by cleavage, it should be possible to make a GaInN blue LD more easily.

However, it has not been possible to grow high-quality, large-area GaN single crystals for a long time. Since GaN substrates are not available, it was impossible to fabricate GaInN-based LEDs and LDs on GaN substrates. Therefore, it was not possible to manufacture LEDs and LDs on a practical GaN substrate.

Since the vapor pressure of nitrogen is high, it is not possible to make GaN crystals by the usual pulling method of attaching seed crystals to a crucible containing a GaN melt and pulling it up. It is possible to synthesize GaN single crystals by applying ultra-high pressure, but only small ones can be produced. It is impossible to grow GaN crystals of very practical size. In addition, a GaN single crystal cannot be produced by a boat method in which a polycrystal is placed in a boat enclosed in a quartz tube, heated and melted, and solidified from the end. It was not possible to manufacture a large GaN substrate by other crystal growth techniques.

However, in recent years, a method of growing a GaN single crystal by a vapor phase growth method has been proposed and various improvements have been made. Since there is no large GaN substrate, a substrate made of different materials is used. A single crystal layer of GaN is deposited on the substrate by a vapor phase synthesis method similar to thin film growth. The vapor phase growth method is originally a method for growing a thin film, but a thick crystal layer can be obtained by sustaining the growth over time. When a thick GaN crystal grows, the substrate is removed by etching or polishing to form a single substrate of GaN. Of course, it is difficult to obtain high-quality GaN crystals by simply gas-phase synthesis. A number of ingenuity is required.

There are several different methods of gas phase synthesis. All of these are methods developed to grow a thin layer of GaN on a sapphire substrate. Organometallic vapor growth method (MOCVD) using organic metal (for example, trimethylgallium TMG) and ammonia as raw materials, HVPE (hydride vapor phase epitaxy) in which gallium alone is put in a boat and oxidized by HCl gas to obtain GaCl, and organic There are the MOC method (organic metal chloride gas phase growth method) in which a metal reacts with HCl to form GaCl and reacts with ammonia, and the sublimation method in which a GaN polycrystal is heated and sublimated and deposited on a substrate. It can be grown on a sapphire substrate and used to manufacture the GaInN-based LEDs mentioned above. Each has its strengths and weaknesses.

(1) Metalorganic Vapourous Growth Method (MOCVD Method) The most commonly used of these is the MOCVD method. In a cold wall reactor, a raw material gas obtained by diluting TMG and ammonia with hydrogen is sprayed onto a heated sapphire substrate to cause an immediate reaction on the substrate to synthesize GaN. This is wasted because a large amount of gas is blown and only a part of it contributes to the formation of the GaN thin film. The yield is low. The growth rate cannot be increased either. Good for forming thin GaN layers that form part of LEDs, but not suitable for stacking thick GaN crystal layers. In addition, carbon contained in the organic metal is mixed as an impurity, which may deteriorate the characteristics.

(2) Organometallic chloride growth method (MOC method) In the MOC method, TMG and HCl are reacted in a hot wall type reactor to once form GaCl, which is then reacted with ammonia near the heated substrate to form GaN. Since this method goes through GaCl, it contains less carbon than the MOCVD method, but it still contains carbon, which may lead to a decrease in electron mobility.

(3) Hydlide vapor deposition method (HVPE method) The HVPE method uses Ga alone as a raw material. This will be described with reference to FIG. A heater 2 is installed around the hot wall type reactor 1. Gas introduction pipes 3 and 4 for introducing two types of raw material gases are provided at the top of the reactor 1. A Ga boat 5 is provided in the space above the inside of the reactor 1. The Ga melt 6 is housed in the Ga boat 5 and heated by the heater 2. The gas inlet 3 above the reactor 1 opens toward the Ga boat 5. This is H₂Introduce + HCl gas. The other gas introduction pipe 4 opens below the Ga boat 5. This is H₂+ NH₃Introduce gas.

Below the internal space of the reactor 1, the susceptor 7 is rotatably supported by the rotating shaft 8. A GaAs substrate is placed on the susceptor 7. Alternatively, if GaN can be made starting from a GaAs substrate, the GaN substrate can be placed on the susceptor 7. The heater 2 heats the susceptor 7 and the substrate 9. HCl (+ H₂) When gas is supplied from the gas introduction pipe 3 and sprayed onto the Ga melt 6, a gaseous intermediate product called GaCl is formed. It falls in the furnace and comes into contact with ammonia in the vicinity of the heated substrate. GaCl and NH on board 9₃Reaction occurs and GaN is synthesized. This method has an advantage that since the raw material does not contain carbon, carbon is not mixed in the GaN thin film and the electrical characteristics are not deteriorated.

(4) Sublimation method GaN cannot be melted unless high pressure is applied. It sublimates when heated at low pressure. In this method, GaN polycrystals are heated, sublimated, transported in space, and deposited on a cooler substrate.

Further, improvement of the method for growing a GaN thin film on a sapphire substrate has been proposed. One of the promising improvement methods is described.

[Lateral Overgrowth] <nplcit num="1"><text>Akira Usui "Growth of thick-film GaN crystals by hydride VPE" Journal of the Institute of Electronics, Information and Communication Engineers vol.J81-C-II, No.1, p58-64 (January 1998)</text></nplcit>

Non-Patent Document 1 and the like have a detailed explanation of GaN growth by the lateral overgrowth method. A mask with striped (or striped) windows is attached on the sapphire substrate, and GaN is grown on it. Separate crystal grains grow from inside the window, go over the window, and coalesce on the mask outside the window. Therefore, the defect density is reduced. This is a device to reduce the defect density when attaching the GaN film on the sapphire substrate.

The present inventor is advancing the improvement of the method for producing a GaN crystal substrate using the HVPE method among the gas phase synthesis methods. Since we are trying to make a GaN substrate, we use different materials as the substrate, but if we use sapphire as the substrate, we cannot remove only the sapphire. It is chemically and physically robust, and only sapphire cannot be removed by polishing or etching.

On the other hand, there is a method of using GaAs as the substrate. Ga metal and hydrogen gas diluted HCl, hydrogen gas diluted NH on a GaAs substrate with three-fold symmetry₃GaN is grown from the raw material. Naturally, it grows in the c-axis direction and the growth plane is the C plane. As it is, dislocations grow linearly. The dislocations do not disappear and grow forever.

Therefore, the present inventor has devised a method in which a mask having a large number of regularly arranged holes is placed on a GaAs substrate or after the GaN layer has grown to some extent, and GaN continues to grow through the mask holes. This is an application of the Lateral Overgrowth method to GaN growth on a GaAs substrate rather than a sapphire substrate. For example, by the applicant

<patcit num="1"><text>Japanese Patent Application No. 10-183446 (Japanese Patent Laid-Open No. 2000-22212)</text></patcit>

There is an explanation in Patent Document 1 and the like. In this method, the GaAs (111) plane is used as a substrate, the substrate is covered with a mask having dots and stripe windows, and a GaN film is vapor-deposited on the substrate. The number of defects can be reduced because the crystal nuclei grow independently from the isolated window and coalesce on the mask. Crystals with few defects can be grown by cutting off the stretching of dislocations.

By such a method, a GaN layer is vapor-grown on a GaAs (111) plane having three-fold symmetry, and the GaAs substrate is removed by etching (aqua regia) and polishing to produce a self-supporting film consisting only of GaN. It becomes possible. The surface of the GaN crystal thus formed is the C-plane (0001). That is, it is a (0001) plane GaN crystal.

Further, Patent Document 1 proposes a GaN free-standing single crystal substrate having a diameter of 20 mm or more and a thickness of 0.07 mm or more produced by such a manufacturing method. This is also a GaN (0001) crystal with a C-plane. Further, the invention of the present inventor

<patcit num="2"><text>Japanese Patent Application No. 10-171276 (Japanese Patent Laid-Open No. 2000-12900)</text></patcit>

Patent Document 2 also proposes a GaN free-standing single crystal substrate manufactured by such a manufacturing method. This is also a (0001) plane GaN crystal. In these inventions, since GaN is thickly vapor-deposited on a GaAs substrate, warpage is inevitably generated, and the question is how to reduce the warp. In addition, the growth surface (C surface) may be a flat surface or a rough surface with jagged edges, but the conditions are required. The conductive type is hardly a problem.

<nplcit num="2"><text>Kensaku Motoki, TakujiOkahisa, Naoki Matsumoto, Masato Matsushima, Hiroya Kimura, Hitoshi Kasai, Kikurou Takemoto, Koji Uematsu, Tetsuya Hirano, Masahiro Nakayama, SeijiNakahata, Masaki Ueno, Daijirou Hara, Yoshinao Kumagai, Akinori Koukitu and Hisashi Seki GaN Substrates by HydrideVapor Phase Epitaxy Using GaAs as a Starting Substrate ", Jpn. J.Appl.Phys. Vol.40 (2001) pp.L140-143</text></nplcit>

Non-Patent Document 2 manufactures a GaN single crystal free-standing film by a lateral overgrowth method using a GaAs (111) crystal as a substrate. This is also a (0001) GaN crystal. It is a crystal with a thickness of 500 μm and a diameter of 2 inches. It states that it was an n-type conductive type. Dislocation density is 2 × 10⁵cm^-2And the carrier concentration is n = 5 × 10¹⁸cm^-3And the mobility is 170 cm²/ Vs, resistivity is 8.5 × 10^-3The theory that it is Ωcm It is clear. The n-type dopant is not described.

<patcit num="3"><text>Japanese Patent Application No. 11-144151 (Japanese Patent Laid-Open No. 2000-44400)</text></patcit>

Patent Document 3 is the inventor of the present invention, but has found for the first time that oxygen is effective as an n-type dopant. We are also proposing an n-type GaN free-standing film using oxygen as an n-type dopant. We have also found that oxygen has a high activation rate in GaN and is close to 1. Since carbon (C) is also an n-type impurity in GaN, it is necessary to eliminate carbon as much as possible. Therefore, the MOCVD method, which is currently the mainstream, is not preferable. He claims that the HVPE method is good.

GaN is a hexagonal with three-fold symmetry Because it is a crystal of symmetry) The notation of the crystal plane is cubic such as GaAs (zinc ore type) It is different from the case of symmetry). The hexagonal crystal notation will be briefly described. There is a method of expressing with three parameters and a method of using four parameters. Here, an expression using four parameters is used. The first three main axes are a-axis, b-axis, and d-axis. These spindles are on one plane and form a central angle of 120 degrees. Moreover, a = b = d.

Each of a, b, and d has an orthogonal axis. This is called the c-axis. The c-axis is unique to the a-axis, b-axis, and d-axis. There are many parallel crystal planes. The first crystal plane from the origin of the crystal plane is the a / h, b / k, d / m, c / n distance from the origin that cuts the a-axis, b-axis, d-axis, and c-axis. Suppose there is. If the positive part of these axes cannot be cut, consider the intersection with the extended -a, -b, -d. h, k, m, n are always integers. I promise to write the surface index in that case as (hkmn).

The exponents for the three main axes a, b, and d have a geometric restriction law h + k + m = 0. The exponent n at the intersection with the c-axis is free. Since it is customary not to put a comma in the expression of the surface index, the notation is the one with four positive and negative integers in parentheses. It is a rule of mineralogy that negative numbers are expressed by overlining numbers. Since this is not possible, it is indicated by adding a minus sign in front of it.

What is expressed by (hkmn) is an individual surface expression. What is represented by {hkmn} is a comprehensive surface representation. It is a comprehensive expression that includes all plane orientations that can be transformed by the symmetry operation of the crystal.

Apart from the plane orientation, there is an expression indicating the line direction. The individual direction is expressed by [hkmn]. This means the direction orthogonal to the individual plane orientation (hkmn). The comprehensive directional indication is &lt;hkmn&gt;. This is a set of all individual orientations that can be reached from the individual orientations (hkmn) by the symmetry operation allowed by the crystal.

The C side is the most representative side. It can be said that all the GaN produced by the crystal growth so far is C-plane growth. When a heterogeneous single crystal substrate such as sapphire or GaAs is used, the plane that grows on it is limited to the C plane because there is no choice but to use a three-fold symmetry plane. There are two important planes other than the C plane (0001).

One is the {1-100} plane. This is a cleavage plane. A plane perpendicular to the C plane, which is a set of six individual planes. All of (1-100), (10-10), (01-10), (-1100), (-1010), and (0-110) are called M planes. The cleavage planes form an angle of 60 degrees with each other and are not orthogonal to each other.

Another important aspect is the {11-20} aspect. This also has a common name and is called side A. Side A is not a cleavage plane. Side A is also a general term for six individual sides. All of (11-20), (1-210), (-2110), (2-1-10), (-12-10), and (-1-120) are called side A.

The C side is uniquely determined, but the A side and the M side have three different directions. A certain A plane and a certain M plane are orthogonal to each other. Therefore, the A-plane, M-plane, and C-plane can form a set of orthogonal planes. Of the inventor

<patcit num="4"><text>Japanese Patent Application No. 10-147049 Patent Document 4 proposes a GaN device having a cleavage plane (M plane) as an edge. This is also a GaN crystal with the C plane as the surface. Since it is an invention that makes the cleavage surface a problem, it is listed here. Various devices have been proposed to reduce through-through dislocations. Of the inventor</text></patcit>

<patcit num="5"><text>Japanese Patent Application No. 11-273882 Patent Document 5 does not grow the C-plane in a mirror surface, but grows in the c-axis direction while retaining the faceted plane other than the C-plane to sweep dislocations to the facets and reduce dislocations. .. This is said to maintain the faceted plane, but on average it is C plane growth. Furthermore, of the present inventor</text></patcit>

<patcit num="6"><text>Japanese Patent Application No. 2000-207783 Patent Document 6 has discovered the property that through-dislocations in GaN are stretched orthogonally to the plane. In the case of C-plane growth, penetrating dislocations extend in the c-axis direction. Therefore, this invention uses an elaborate method in which GaN grown on the C-plane is cut out in the A-plane direction and then grown on the A-plane or GaN grown on the C-plane is cut out in the direction of the M-plane and then grown on the M-plane. suggest. After that, it is a clever invention of cutting out at the C plane to obtain a GaN crystal with low dislocations.</text></patcit>

For the first time, it is proposed that only Patent Documents 5 and 6 of this prior art grow in aspects other than the C aspect. Although the purpose is different, this is the first invention that focuses on the plane orientation, so I introduced it here.

<p> The GaN vapor phase growth method on a sapphire substrate is grown with the C plane as the upper surface without exception. Sapphire board (α-Al₂O₃When GaN is grown on the 3-fold symmetric plane of), the C-plane has 6-fold symmetry and becomes a mirror plane, which is the easiest to grow. Therefore, the GaInN-LED and GaInN-LD on the sapphire substrate currently manufactured and used are a stack of C-plane GaN layers and GaInN layers.</p><p> The same applies when GaAs is used as a substrate, and when GaN is grown on the three-fold symmetric plane (111) of GaAs, the C plane is grown so as to be the surface. The present inventor wants to use oxygen (O) as the n-type dopant for GaN. The present inventor has recently discovered that when trying to dope oxygen, it is difficult to enter the growth surface (C surface).</p><p> This is not an easy-to-understand phenomenon. That's why no one noticed until recently. The present inventor analyzed the surface composition of the C-plane grown GaN sample by SIMS (Secondary Ion Mass Spectroscopy). This is a method of accelerating ions (primary ions) to hit a sample and counting the number of secondary ions knocked out from the sample to determine the abundance ratio of substances present on the sample surface. At the beginning, the resolution was not sufficient and the ion beam had a considerable spread, so secondary ions were emitted from a wide range. Therefore, since the secondary ions of oxygen were also emitted from the sample surface, it seemed that oxygen was also doped on the C surface.</p><p>However, it has become surprising to increase the resolution of SIMS by narrowing the beam. If you look closely at the roughened C surface, there are many irregularities (facets) in addition to the C surface part, and it may grow while holding the inclined surface. Secondary ions are emitted from both the C-plane and the uneven portion. When the oxygen secondary ions were measured by distinguishing between the C-plane and the uneven portion (facet), it was found that almost no oxygen came from the C-plane. When converted, the oxygen concentration other than the C plane is 5 × 10.¹⁸cm^-3Even at this time, the oxygen concentration on the C plane on the same crystal surface is 1 × 10.¹⁷cm^-3Turned out to be less than. In other words, there is a 50-fold difference in the ability to take in oxygen. Oxygen secondary ions are actually released from facets rather than from the C plane.</p><p> In addition, when the growth conditions were changed, a sample was prepared with the C surface as a full mirror surface, and analysis was performed from the surface with SIMS, oxygen was still 1 × 10.¹⁷cm^-3It was less than and the concentration was low.</p><p> This means that the C surface is hardly doped with oxygen. It is C-plane growth, and oxygen does enter naturally even though oxygen does not enter in C-plane because there are faceted surfaces other than C-plane. That's what I've learned for the first time.</p><p> Since the single crystal is growing, the crystal orientation is the same in every part in the plane. The facet part also has a structure with the upper part being the c-axis. That is why it grows in the c-axis direction. That is the case, but the exposed surface is not the C surface. Whether or not oxygen contained in the raw material gas is taken into the crystal depends not on the internal crystal structure of the part but on the microscopic structure of the surface itself at the time of crystal growth. If the surface is tilted and a surface other than the C surface, such as the M surface or the A surface, is exposed, oxygen is taken in by the unique oxygen adsorption capacity of that surface.</p><p> The faceted parts of GaN growth are often continuous along the average growth direction. Facets do not disappear and occur frequently. For example, when a crystal grows in the c-axis direction, the facet plane is maintained and GaN grows in the c-axis direction in the vertical direction. Therefore, when SIMS and etching are combined and the oxygen concentration is measured in the c-axis direction in the crystal depth direction, the oxygen concentration distribution shows similar variations at almost all depths. Therefore, oxygen is doped even in the C-plane growth because there are microscopic faceted planes other than the C-plane.</p><p> Such a thing is still unknown to experts in the field. The reason why the oxygen-doped GaN becomes n-type has been clarified for the first time by the present inventors in Patent Document 3. Oxygen will replace nitrogen sites to become n-type impurities. However, the use of oxygen as an n-type dopant in GaN is not yet mainstream. The inventor only claims. The mainstream n-type dopant in GaN is silicon (Si). It is believed that Si replaces the gallium (Ga) site to become n-type. At present, no one other than the present inventor has found the idea of using oxygen as an n-type dopant. Moreover, it is not yet known that oxygen doping has plane orientation dependence. Since GaN growth grows from dissimilar materials with three-fold symmetry, only C-plane growth is always performed. However, from the above experiment, almost no oxygen enters the C surface. Therefore, if you stick to C-plane mirror growth, you will not be able to dope oxygen at the desired concentration. This means that it is not possible to make an n-type GaN substrate with the desired resistivity. Based on such new discoveries, the present invention proposes a GaN crystal growth method capable of efficiently doping oxygen.</p>

<p> As a result of repeated detailed studies such as changing the growth surface to grow, it was found that the amount of oxygen uptake depends on the plane orientation. It is the result of the experiment of the present inventor that the amount of oxygen doping is dependent on the plane orientation. Besides, it turned out that it does not mean that oxygen does not enter in any plane direction although it does not enter the C plane. There is a surface other than the C surface where oxygen easily enters (dops). It has become clear that there are two main types of plane orientations that are easily oxygen-doped.</p><p> It has the following plane orientation. (1) {kk-2kh} (k and h are integers) In particular, it is known that the {11-20} plane shows a remarkable effect. The efficiency of oxygen uptake is also high on the {11-22} side. It seems that the oxygen uptake efficiency tends to decrease as the surface index increases.</p><p>(2) {k-k0h} (k and h are integers) In particular, the {1-100} plane shows a remarkable effect. Oxygen uptake efficiency is also high on the {1-101} side. It seems that the oxygen uptake efficiency tends to decrease as this plane orientation also becomes a high plane index.</p><p> In other words, the surface {hkmn} has a unique oxygen doping capacity, which can be expressed by a function called OD {hkmn}.</p><p> The details of OD {hkmn} are still unknown, but it can be said that OD {hkmn}&gt; OD {0001} for surfaces {hkmn} other than surface C. In other words, the C side is the most difficult side to do oxygen doping.</p><p> For side A {11-20}, OD {11-20}&gt; 50 OD {0001}. Side A is more than 50 times more likely to be oxygen-doped than side C.</p><p> The M plane {1-100} is also OD {1-100}&gt; 50 OD {0001}. The M-plane is more than 50 times more likely to be oxygen-doped than the C-plane.</p><p> In order to perform oxygen doping, it is possible to efficiently dope by growing crystals so as to have a surface other than the C surface on the upper surface. To dope by this method, oxygen doping is performed in the bulk crystal in a region where the history of crystal growth is a plane other than the C plane.</p><p> The entire surface does not necessarily have to be these crystal planes (other than the C plane). It may only be partially present in the form of faceted surfaces. Of course, when the C-plane growth portion is widely present, the oxygen uptake efficiency in that portion is reduced.</p><p> The present inventors have just discovered the phenomenon of the plane orientation dependence of oxygen uptake. The details of the mechanism are still unknown. It is considered that the way impurities are taken in changes because the way of bonding specific elements is different due to the difference in the state of the bonding of atoms appearing on the surface of the crystal plane. In particular, when the GaN (0001) Ga plane becomes the growth plane, it is presumed that a mechanism that makes it extremely difficult for oxygen to enter the nitrogen site where oxygen should enter as an n-type carrier works. As a matter of course, these phenomena are universal phenomena that can be seen even when any substrate or seed crystal such as sapphire, SiC, or GaN is used.</p>

<p> Oxygen doping has been almost impossible with the C-plane mirror growth of GaN that has been carried out so far. In the present invention, GaN growth is performed while exposing surfaces other than the C surface by growing the surface other than the C surface so that the upper surface is the upper surface, or by growing the C surface while maintaining the facet. According to the present invention, oxygen can be effectively taken into a GaN crystal. The amount of oxygen doping can be accurately controlled by determining the plane orientation. Oxygen can effectively function as an n-type dopant. It is an extremely efficient oxygen doping method.</p>

For oxygen doping, it is most effective to include water in the raw material gas during crystal growth. Ammonia (NH) for HVPE₃), Soak hydrogen chloride gas (HCl) with water. Originally NH₃, HCl often contains water as an impurity, and may be oxygen-doped by the originally contained water without adding water to the raw material gas. However, in order to perform stable oxygen doping, it is desirable to quantitatively add a small amount of water to the raw material gas.

According to the idea of the present invention, there are roughly two methods for efficient oxygen doping. One is to grow in a direction other than the C-axis (non-C-axis growth), and the other is to grow facets in the C-axis direction. That is, non-C-axis growth and faceted C-axis growth.

(A) [Non-C-axis growth] Using a seed crystal whose surface (upper surface) has a plane {hkmn} other than the C-plane, crystal growth is performed on the plane other than the C-plane to manufacture a single crystal ingot extending in that plane orientation. how to. In the method of A, when the crystal plane {hkmn} of the seed crystal is maintained as it is and the crystal grows, oxygen doping is efficiently performed on the entire surface.

For example, efficient oxygen doping is performed on the entire surface of the seed crystal when it is the {1-100} plane (M plane), or generally the {k-k0h} plane (k, h is an integer).

The same applies to the {11-20} (A plane) or generally the {kk-2kh} plane (k and h are integers). The oxygen doping efficiency in this case is simply

OD = OD {hkmn}

Symbolically represented by. This method is simple in principle but has some problems to implement. A GaN single crystal having a surface other than the C-plane does not exist naturally, and cannot be produced by vapor deposition from a dissimilar substrate. The GaN and GaInN thin films currently used for LEDs and LDs grown on the three-fold symmetric plane of sapphire are C-plane crystals. When grown on a sapphire substrate as described above, the sapphire cannot be removed and a single GaN crystal cannot be obtained.

When vapor phase is grown on the GaAs (111) plane, a GaN crystal with a C plane grows. Since the GaAs substrate can be removed with aqua regia, a single crystal of GaN can be obtained. However, the surface of the crystal is also C-plane. A thick GaN crystal is formed, for example, cut in the A-plane direction to form a single crystal having the A-plane on the surface, and this is used as a seed crystal. In this way, a pre-process of producing a seed crystal having a plane other than the C plane is required.

(B) [Facet C-axis growth] A method of growing a crystal whose upper surface is the C-plane, but from a microscopic point of view, it grows so as to have a facet-plane other than the C-plane.

In the method B, even if the average crystal plane of the seed crystal surface is the C plane, if it grows microscopically with faceted planes other than the C plane, the effect of doping oxygen through the faceted plane can be obtained.

Specific facet planes include {k-k0h} planes (k and h are integers) such as {1-101} planes. These are planes with an inclined M plane. Since the M plane itself is perpendicular to the C plane, it does not become a facet plane.

Alternatively, there is a {kk-2kh} surface such as a {11-22} surface. These are planes with the A plane tilted. Since the A plane itself is perpendicular to the C plane, it does not become a faceted plane in the C plane growth. This is the case when a single facet plane is included in the C plane.

Since a GaN crystal has 6-fold symmetry around the c-axis even if it is single, these planes are a set of 6 individual planes. Hexagonal pyramid-shaped holes (pits) and hexagonal pyramid-shaped protrusions can be formed on the C-plane even on a single surface. Although not all surfaces may appear, they still form triangular pyramidal holes, protrusions or irregular pentagonal pyramidal holes and protrusions.

This is the case when it contains a single faceted surface, but oxygen doping can be enabled by allowing C-plane growth to include multiple faceted surfaces. For example, oxygen doping can be performed by crystal growth including a plurality of faceted planes consisting of {kk-2kh} plane and {k-k0h} plane. For example, six {11-21} planes and six {1-101} planes can form a regular dodecagonal cone. The combination of the two surfaces can form such holes or protrusions. If three or more faces are gathered together, it is possible to create holes and protrusions of pyramids with more complicated shapes.

An inverted hexagonal pyramid (hexagonal pyramid hole) consisting of a set of {kk-2kh} planes and {k-k0h} planes (k and h are integers), and a pit-shaped facet surface in the shape of an inverted dodecagonal pyramid (twelve pyramid hole). When growing the C-plane while holding it, oxygen doping can be performed on this pit-shaped facet plane. This method is complex, and if the probability of existence of the {hkmn} plane in the C plane is written as ρ {hkmn}, the oxygen doping efficiency is

OD = Σρ {hkmn} OD {hkmn} It can be expressed symbolically like this.

As the GaN growth method, all of the HVPE method, MOC method, MOCVD method, sublimation method, etc., which are effective as the growth method on the conventional sapphire substrate, can be used.

[Example 1 (Crystal growth with M-plane (1-100) as the upper surface; Fig. 2)] A GaN seed crystal having an M-plane (1-100) surface cut out from a GaN single crystal ingot was prepared (Fig. 2 (a)). A GaN single crystal is obtained by growing GaN on the C-plane by a lateral overgrowth method on a GaAs substrate and dissolving and removing the GaAs substrate with aqua regia. Since it is the M-plane, it is cut by one plane parallel to the growth direction of this crystal.

The surface of this M-plane seed crystal has been polished, and the processed alteration layer has been removed on the surface so that it does not exist at all.

A GaN crystal was grown on this seed crystal by the HVPE method (Fig. 2 (b)). The growth conditions are as follows. NH, which is a raw material gas for nitrogen₃For, a raw material gas containing about 2 ppm of water was used. Water is included as an oxygen source.

Growth temperature 1020 ° C NH₃Partial pressure 0.2atm (2 × 10)⁴Pa) HCl partial pressure 1 × 10^-2atm (10³Pa) Growth time 6 hours

The growth film thickness was about 500 μm. Then, the seed crystal part of the base was ground and removed (Fig. 2 (c)). The surface was further polished (Fig. 2 (d)). The thickness of the crystal layer excluding the seed crystal and leaving only the growth part was about 400 μm.

When the electrical characteristics of this sample are determined by Hall measurement, the average at 4 points is Carrier concentration = 6 × 10¹⁸cm^-3 Carrier mobility = 160Vs / cm²It was almost uniform in the crystal.

Furthermore, SIMS (Secondary) near the surface of the same sample Ion Mass Spectroscopy) analysis was performed. As a result of the measurement, the following was found.

Hydrogen (H) 2 × 10¹⁷cm^-3Carbon (C) 3 × 10¹⁶cm^-3Oxygen (O) 8 × 10¹⁸cm^-3Silicon (Si) 3 × 10¹⁷cm^-3

Carrier concentration is 6x10¹⁸cm^-3And the oxygen concentration is 8 × 10¹⁸cm^-3Is. Carbon (10) that can be an n-type impurity in GaN¹⁶cm^-3Order), Silicon (10)¹⁷cm^-3(Order of) is the carrier concentration (10)¹⁸cm^-3(Order) Good but much lower. This means that these carriers (electrons) are derived from oxygen. It is suggested that oxygen acts as an n-type impurity and its activation rate is quite high.

When the resistivity was measured, it was 7 × 10.^-3It has a fairly high conductivity of about Ωcm. It can be used as an n-type conductive GaN substrate. In other words, unlike sapphire, it is not necessary to take n electrodes on the upper surface, and it is possible to take n electrodes from the bottom surface of an n-type GaN substrate. The sample produced by this example is a simple substance GaN substrate having a flat surface and a thickness of 400 μm. The shape is such that a device can be manufactured by epitaxially growing it on a GaN substrate.

[Comparative Example 1 (Crystal Growth with C-plane (0001) as the upper surface; Fig. 3)] A GaN seed crystal having a C-plane (0001) surface as a surface cut out from a GaN single crystal ingot was prepared (Fig. 3 (a)). The polarity of the surface is the Ga surface. The surface of this C-plane seed crystal is polished, and there is no processed alteration layer on the surface.

GaN was grown on this seed crystal by the HVPE method. The growth conditions are as follows. NH, which is a raw material gas containing nitrogen, as in Example 1.₃For, a raw material gas containing about 2 ppm of water was used.

Growth temperature 1050 ° C NH₃Partial pressure 0.15 atm (1.5 x 10)⁴Pa) HCl partial pressure 5 × 10^-3atm (5 × 10²Pa) Growth time 10 hours

The growth film thickness was about 500 μm (Fig. 3 (b)). The surface was a flat mirror surface consisting of (0001) planes. It can be seen that the surface after growth also maintains the C surface. Then, the seed crystal part of the base was ground and removed (Fig. 3 (c)). The thickness of the crystal layer obtained by polishing the surface to remove the seed crystal and leaving only the growth part was about 400 μm (Fig. 3 (d)).

An attempt was made to determine the electrical properties of this sample by Hall measurement, but it was unmeasurable. The reason is that the GaN crystal is a high-resistance film and its electrical conductivity is extremely low, so it cannot be measured with the measuring equipment currently owned. It was impossible to measure at any point on the substrate surface. In other words, there are few electrons that can move freely and the carrier concentration is too low, so sufficient current does not flow and measurement is not possible.

Furthermore, SIMS (Secondary Ion) near the surface of this sample Mass Spectroscopy) analysis was performed. As a result of the measurement, the following was found.

Hydrogen (H) 1 × 10¹⁸cm^-3Carbon (C) 7 × 10¹⁶cm^-3Oxygen (O) 1 × 10¹⁷cm^-3Silicon (Si) 2 × 10¹⁶cm^-3the following

Thus, the oxygen concentration is much lower than in Example 1. It can be seen that it has decreased to about 1/100. This is due only to the difference in plane orientation (C plane and M plane). In other words, there is a significant plane orientation dependence on oxygen uptake. Si is also reduced to about 1/10, and plane orientation dependence is also seen for Si. Rather, carbon and hydrogen seem to enter the crystal more in the C-plane growth. However, there are few dependencies. Oxygen shows the most remarkable plane orientation dependence.

In this comparative example, since the amount of oxygen taken in as an n-type impurity is small, it is considered that the n-type carrier (electrons) is not emitted and the insulator becomes an insulator. Such a high-resistance substrate cannot be used as a conductive substrate for a GaN device because the n electrode cannot be taken out from the bottom surface.

[Example 2 (Crystal growth with the C-plane (0001) as the upper surface and maintaining the pit-shaped faceted plane; Fig. 4)] A GaN seed crystal having a C-plane (0001) surface cut out from a GaN single crystal ingot was prepared (Fig. 4 (a)). The polarity of this surface is the Ga surface. This C-plane seed crystal has been surface-polished, and the processed alteration layer on the surface has been removed and is completely absent.

GaN was grown on this seed crystal by the HVPE method. The growth conditions are as follows. NH, which is a raw material gas for nitrogen₃For, a raw material gas containing about 2 ppm of water was used.

Growth temperature 1030 ° C NH₃Partial pressure 0.2atm (2 × 10)⁴Pa) HCl partial pressure 1 × 10^-2atm (10³Pa) Growth time 5 hours

The growth film thickness was about 500 μm (4 (b)). The surface condition was not a flat C-plane mirror surface as in Comparative Example 1. The surface of the grown crystal has a large number of faceted faces consisting of facets other than the C-plane. The facet surface reflects light and appears to shine. In particular, a pit-like morphology consisting of inverted hexagonal pyramid-shaped and inverted dodecagonal pyramid-shaped faceted surfaces can be seen. In other words, it is a set of pyramid pits. These weight surfaces are faceted surfaces. Almost no C-plane is seen in this sample.

Those with various plane orientations are mixed. Most of them are {1-101} planes, {11-22} planes, {1-102} planes, and {11-24} planes. These can be collectively expressed as {k-k0h} (k and h are integers) and {kk-2kh} planes (k and h are integers).

Then, the seed crystal part of the base was ground and removed (Fig. 4 (c)). The thickness of the crystal layer excluding the seed crystal and leaving only the growth part was about 400 μm. The surface of this substrate is not flat because it is a faceted surface. Therefore, both sides were polished to obtain a substrate with a thickness of 350 μm (Fig. 4 (d)).

When the electrical characteristics of this sample are determined by Hall measurement, the average at 4 points is Carrier concentration = 5 × 10¹⁸cm^-3 Carrier mobility = 170Vs / cm²

It was almost uniform in the crystal. Furthermore, SIMS (Secondary Ion) near the surface of the same sample Mass Spectroscopy) analysis was performed. As a result of the measurement, the following was found.

Hydrogen (H) 2 × 10¹⁷cm^-3Carbon (C) 3 × 10¹⁶cm^-3Oxygen (O) 5 × 10¹⁸cm^-3Silicon (Si) 4 × 10¹⁶cm^-3 the following

Carrier concentration is 5x10¹⁸cm^-3And the oxygen concentration is 5 × 10¹⁸cm^-3Is. Carbon (10) that can be an n-type impurity in GaN¹⁶cm^-3Order), Silicon (10)¹⁶cm^-3(Order of) is the carrier concentration (10)¹⁸cm^-3(Order) Good but much lower. This means that these carriers (electrons) are derived from oxygen. The fact that the oxygen concentration and the carrier concentration are similar suggests that oxygen acts as an n-type impurity and its activation rate is considerably high.

When the resistivity was measured, it was 6 × 10.^-3It has a fairly high conductivity of about Ωcm. It can be used as an n-type conductive GaN substrate. In other words, unlike sapphire, it is not necessary to take n electrodes on the upper surface, and it is possible to take n electrodes from the bottom surface of an n-type GaN substrate. This embodiment means that oxygen can enter from the faceted surface and a low-resistance n-type GaN crystal can be produced by growing while maintaining the faceted surface other than the C surface even when growing in the c-axis direction. The sample piece according to this example was a single n-type GaN substrate having a flat surface and a thickness of 350 μm. After that, it was further epitaxially grown on the surface of the GaN substrate, and the shape was such that a device could be manufactured.

<figref num="1">Schematic cross-sectional view of a GaN crystal growth apparatus by the HVPE method.</figref>

<figref num="2">FIG. 5 is a cross-sectional view of a GaN crystal showing the process of Example 1 in which a GaN layer is grown on a GaN seed crystal having an M plane (1-100) by a vapor phase growth method. (a) is a cross-sectional view of a GaN seed crystal having an M plane (1-100), and (b) is a cross-sectional view of a GaN crystal in a state where a (1-100) crystal is grown on a GaN seed crystal. (c) is a cross-sectional view of a GaN crystal of only the grown portion from which the seed crystal has been removed. (d) is a cross-sectional view of the M-plane GaN crystal in a further polished state.</figref>

<figref num="3">FIG. 5 is a cross-sectional view of a GaN crystal showing the process of Comparative Example 1 in which a GaN layer is grown on a GaN seed crystal having a C plane (0001) by a vapor phase growth method. (a) is a cross-sectional view of a GaN seed crystal having a C-plane (0001), (b) is a cross-sectional view of a GaN crystal in a state where a (0001) crystal is grown on a GaN seed crystal. (c) is a cross-sectional view of a GaN crystal of only the grown portion from which the seed crystal has been removed. (d) is a cross-sectional view of the C-plane GaN crystal in a further polished state.</figref>

<figref num="4">FIG. 5 is a cross-sectional view of a GaN crystal showing the process of Example 2 in which a GaN layer is grown by a vapor phase growth method while maintaining a faceted surface on a GaN seed crystal having a C plane (0001). (a) is a cross-sectional view of a GaN seed crystal having a C-plane (0001), and (b) is a cross-sectional view of a GaN crystal in a state where a (0001) crystal having many facets is grown on a GaN seed crystal. (c) is a cross-sectional view of a GaN crystal of only the grown portion from which the seed crystal has been removed. (d) is a cross-sectional view of the C-plane GaN crystal in a further polished state.</figref>

Code description

1 HVPE reactor 2 Heater 3 Raw material gas introduction pipe 4 Raw material gas introduction pipe 5 Ga Reservoir (Ga Boat) 6 Ga melt 7 susceptor 8 axis of rotation 9 board 10 Gas outlet