JP3870259B2 - Nitride semiconductor laminate and growth method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates generally to a method for manufacturing a nitride semiconductor crystal used in an optical device and an electronic device, and in a narrow sense relates to a method for reducing crystal defect density using a buffer layer.
[0002]
[Prior art]
In recent years, nitride semiconductors, which are semiconductor materials with a wide band gap, that is, gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), boron nitride (BN), and mixed crystals thereof (AlGaN, InGaN, etc.) Has attracted much interest because it can be applied to high-temperature, high-power electronic devices and short-wavelength optical devices.
[0003]
Specific application examples include nitride semiconductor devices such as high-power heterojunction field effect transistors, blue light emitting elements, and blue semiconductor lasers. In these nitride semiconductor devices, since a crystal having relatively good quality is relatively obtained from gallium nitride, a device structure is constructed on gallium nitride. That is, gallium nitride is first grown on a substrate, and a device structure is constructed thereon.
[0004]
In order to improve the performance of these devices, a high-quality crystal manufacturing method with few defects is important. As the production method, a crystal growth method such as an organic metal compound vapor phase growth method, a hydride vapor phase growth method, or a molecular beam epitaxy method is used.
[0005]
The organometallic compound vapor phase growth method is a method in which an organic metal and ammonia are used as source gases to react them on a substrate in a high temperature furnace to grow a nitride semiconductor crystal, and the pressure in the growing furnace is Normal pressure or about 1/10 atm. The hydride vapor phase growth method is a method of crystal growth using bromine or the like, and is mainly used when growing thickly. The molecular beam epitaxy method is a method of growing crystals by reacting gallium, aluminum, or the like as a raw material with nitrogen plasma or ammonia on a substrate in a vacuum.
[0006]
In order to grow a high-quality nitride semiconductor material, it is most desirable to use gallium nitride having a thickness of about 300 μm to 500 μm as a substrate, but it is still under development and is difficult to use. In the future, it will be difficult to supply cheaply. Further, since the insulating property is low, the light emitting element structure can be manufactured, but there is a problem that it is not suitable for manufacturing an electronic device structure.
[0007]
Other usable substrates include a (111) plane silicon substrate, a silicon carbide substrate having a crystal structure of 6H or 4H, and a (0001) c plane using α-sapphire c-plane (0001) plane. Although there are sapphire substrates, a (0001) c-plane sapphire substrate is mainly used at present.
[0008]
Since the sapphire substrate and gallium nitride have a lattice constant difference of 10% or more, they grow only locally when grown directly on the sapphire substrate. Alternatively, even if grown on the entire surface, only crystals with a very high lattice defect density close to amorphous can be grown, and cannot be used for semiconductor devices.
[0009]
Therefore, a method of growing a high-quality crystal on a sapphire substrate by first using a layer (buffer layer) for buffering the lattice constant difference is employed (see Patent Documents 1 to 5 below).
[0010]
This is a method for improving crystal quality by using thin AlN, GaN, or the like grown under special growth conditions as a buffer layer and placing it between a substrate and a gallium nitride layer. In addition, since the closer to the substrate, the greater the number of lattice defects, there is a concept of stacking the ones with the largest number of defects in order (see Patent Document 6 below).
[0011]
An important parameter for growing the buffer layer is the growth temperature. When the difference in lattice constant between the material of the buffer layer and the substrate material is large, if the growth is performed under a high temperature condition, a phenomenon occurs in which the growth is locally sparse instead of being uniformly grown on the substrate surface. Therefore, in general, the buffer layer is first grown at a low temperature, and then the temperature is raised to grow the semiconductor device structure.
[0012]
This is because when the growth temperature is low, the speed of movement of atoms such as Ga, which is a supply material, due to thermal motion on the substrate decreases, and a buffer layer is easily formed uniformly on the entire substrate.
[0013]
However, the optimum growth temperature differs depending on the type of buffer layer and the growth method. For example, in a growth method in a vacuum such as a molecular beam epitaxy method, when aluminum nitride is used as a buffer layer, the growth is performed at a slightly high temperature even if it is a low temperature. Since the optimal growth temperature of aluminum nitride is higher than the optimal growth temperature of gallium nitride, the growth temperature of aluminum nitride used as a low-temperature buffer is close to the growth temperature of gallium nitride that is part of the device structure grown on it Or higher than that (see Non-Patent Documents 1 and 2 below).
[0014]
In such crystal growth, the physical properties of nitride semiconductor crystals are closely related. Further, since it is closely related to the description of the present invention, the characteristics of the nitride semiconductor crystal will be described.
[0015]
Nitride semiconductor crystals used for manufacturing devices are mainly hexagonal crystal structures (wurtzite crystals). Although cubic nitride semiconductor crystals exist, they are thermodynamically unstable and are not used in actual applications.
[0016]
Unlike α-sapphire, which is also a hexagonal crystal, a nitride semiconductor crystal, which is a wurtzite crystal, is asymmetric because the orientation of constituent atoms is different in the + c-axis direction and the −c-axis direction. Therefore, when crystal growth is performed, there are those in which the + c axis faces the growth direction and those in which the −c axis faces.
[0017]
A crystal whose group has a + c axis in the growth direction is called a group III plane crystal. In the case of gallium nitride, it may be called a gallium face crystal, in the case of aluminum nitride, an aluminum face crystal, or indium nitride, an indium face crystal. On the other hand, the crystal with the −c axis facing the growth direction is called a crystal of a group V surface. Or you may call it the crystal | crystallization of a nitrogen surface.
[0018]
Therefore, when a nitride semiconductor crystal is grown on a substrate other than a gallium nitride substrate, for example, a sapphire substrate, a group III surface crystal and a group V surface crystal may be mixed. In fact, in many cases it is mixed. In addition, amorphous nitride semiconductor crystals grown at a low temperature may contain cubic crystals in addition to hexagonal crystals.
[0019]
In terms of thermodynamics, a group III crystal is more stable. In other words, at high temperatures, crystals of group III planes are dominant. However, at low temperatures, the V group surface is also stable and mixed.
[0020]
In addition, the group III crystal is more stable chemically. Although it depends on the density of lattice defects, for example, if the etching is carried out with phosphoric acid kept at 230 ° C. for 5 to 10 minutes, the crystals in which the crystals of the group V plane and the group III plane are mixed are dissolved, but only the group III plane is dissolved. Crystals are not easily etched.
[0021]
Therefore, in the actual device structure manufacturing, the entire surface should be a group III surface. This makes it possible to manufacture devices that can withstand actual applications.
[0022]
In fact, in the metal organic compound vapor phase growth method and the hydride vapor phase growth method, regardless of whether gallium nitride or aluminum nitride is used as the buffer layer, a good group III surface crystal can be obtained uniformly over the entire substrate. Yes.
[0023]
However, in the molecular beam epitaxy method, a good quality crystal has not been obtained yet. Conventionally, in the molecular beam epitaxy method, aluminum nitride is mainly used as a buffer layer. Although it is possible to grow a crystal of a group III plane by this, the defect density is large and it is not yet put into practical use.
[0024]
[Patent Document 1]
JP 10-294492 A
[Patent Document 2]
Shoko 59-48794
[Patent Document 3]
Patent No. 3,026,087
[Patent Document 4]
U.S. Pat.No. 5,290,393
[Patent Document 5]
US Patent 6,270,574
[Patent Document 6]
U.S. Patent 6,261,931
[Non-Patent Document 1]
S. Yoshida, S. Misawa, and S. Gonda: Appl. Phys. Lett. 42 (1983) 427.
[Non-Patent Document 2]
XQ Shin, T. Ide, SH Cho, M. Shimizu, S. Hara, H. Okumura, S. Sonoda and S. Shimizu: Jpn. J. Appl. Phys. 40 (2001) L23.
[0025]
[Problems to be solved by the invention]
The problem to be solved by the present invention is to provide a crystal growth method capable of growing a high-quality crystal even in the molecular beam epitaxy method.
[0026]
The crystal growth method using the low-temperature grown gallium nitride buffer and aluminum nitride buffer described in the prior art was effective when the metal organic compound vapor phase growth method was used, but when the molecular beam epitaxy method was used. Had no effect.
[0027]
First, the method using gallium nitride as the buffer layer does not provide a good quality crystal even when used by molecular beam epitaxy. When the low-temperature grown gallium nitride was actually grown as a buffer layer and observed using a scanning electron microscope (SEM) or atomic force microscope (AFM), the surface was not flat.
[0028]
This is because the buffer layer of gallium nitride grown at a low temperature contains a group III surface crystal and a group V surface crystal, and the molecular beam epitaxy method has a low growth temperature. This is because it will continue to grow stably.
[0029]
Since the molecular beam epitaxy method is a crystal growth method in a vacuum, re-evaporation of gallium metal or the like occurs when the temperature is raised. As a result, it is impossible to make the growth temperature too high, and it is necessary to keep it at about 800 ° C. or lower.
[0030]
This is clear when compared with the case of the organic metal compound vapor phase growth method. In the organic metal compound vapor phase growth method in which the pressure during growth is normal pressure or about 1/10 atm, the growth temperature is about 1000 ° C. or higher, which is 200 ° C. or higher than the growth temperature of the molecular beam epitaxy method. As a result, when the growth is performed by the organometallic compound vapor phase growth method, the group V-plane crystal once formed in the buffer layer is replaced by a thermodynamically stable group III-plane crystal.
[0031]
However, in the molecular beam epitaxy method, the crystal of the group V plane continues to exist relatively stably. As a result, the surface of gallium nitride grown using gallium nitride as a buffer layer by the molecular beam epitaxy method is a very rough surface because the group III surface crystal and the group V surface crystal coexist.
[0032]
As described above, the method of using gallium nitride grown at a low temperature as a buffer layer cannot be used for crystal growth by molecular beam epitaxy.
[0033]
Even when aluminum nitride is used as a buffer layer, a good crystal cannot be obtained by molecular beam epitaxy. The reason is described.
[0034]
The characteristics of the aluminum nitride buffer differ between when grown at a low temperature and when grown at a high temperature. When aluminum nitride is grown at a low temperature of about 800 ° C. or lower, crystals of the group V plane are also mixed. As a result, the group V surface enters between the crystals of the group III surface, resulting in a rough crystal surface.
[0035]
On the other hand, when aluminum nitride is grown at a high temperature of about 800 ° C. or higher, the group V plane is thermodynamically unstable and thus excluded from the initial growth stage, but the aluminum nitride layer grown as a buffer layer is A layer composed of grains having a diameter of about 0.1 μm is formed, resulting in a crystal with many defects such as threading transition. For example, a crystal containing defects such as threading transitions of 10 <10> or more per unit square centimeter.
[0036]
Such a crystal surface is flat as long as it is observed with a scanning electron microscope or an optical microscope, but is not flat at the atomic level. When observed with an atomic force microscope (AFM), a high-density threading transition is observed. The surface is composed of fine irregularities of about 0.5 nm to 2 nm. Therefore, it is almost impossible to form a two-dimensional electron gas necessary for the operation of the electronic device.
[0037]
In other words, when aluminum nitride is used, there is no optimum condition and either a rough surface crystal with a mixed group V surface or a flat surface and a crystal with many threading transitions. .
[0038]
As described above, even when a low-temperature GaN buffer is used or when aluminum nitride is used as a buffer layer, no good crystal can be obtained by growth by molecular beam epitaxy.
[0039]
In addition, as described above, in the molecular beam epitaxy method, the reason why there is no optimal buffer layer growth method is not only because there is a problem that the crystal of the V group plane continues to grow as it is because the growth temperature is low, This is because there is a problem that even if they are grains of the same III group and are adjacent to each other, they continue to grow while being separated.
[0040]
For example, when gallium nitride is grown on a buffer layer of aluminum nitride grown at a high temperature by using molecular beam epitaxy, a group III plane crystal is formed over the entire substrate, but the diameter is actually 0.1 μm. A structure completely separated into grains of a certain degree is formed, and even if the film thickness of crystal growth is increased, the grain structure does not completely disappear but remains.
[0041]
The reason why the individual grains exist without being bonded although they are adjacent to each other is that the crystal orientations of the individual grains are different and a crystal defect exists between them. When such grains are present, the crystal growth is far from a good crystal growth mode called step flow growth, and as a result, the crystal has a high defect density.
[0042]
This situation is greatly different from that in the case of the organometallic compound vapor phase growth method. Since the growth temperature is high in the metal organic vapor phase method, grains and grains are likely to be combined, and the density of defects decreases as the growth proceeds. The density of crystal defects is highest in the vicinity of the interface between the substrate and the nitride semiconductor crystal thereon, and the defect density decreases as the growth thickness of the nitride semiconductor crystal is increased. Although this phenomenon depends on the effect that defects are canceled each other or the number of defects becomes one as the growth film thickness is increased, it is largely due to the effect that crystal grains (grains) are easily combined. .
[0043]
As described above, the molecular beam epitaxy method has disadvantages such as the presence of a group V crystal, a high defect density, and a grain structure because the growth temperature is low. However, because it is grown in vacuum, there are advantages such as high uniformity of film thickness within the substrate surface, the formation of steep hetero interfaces, and control of film thickness on the atomic layer order. It is an important crystal growth method for manufacturing an optoelectronic device structure utilizing the effect.
[0044]
Therefore, the problem to be solved by the present invention is to grow a high-quality crystal in which the crystal surface of the V surface is not mixed, the grain structure does not exist, and the crystal surface is stepped even when the molecular beam epitaxy method is used. It is to let you.
[0045]
[Means for Solving the Problems]
A nitride semiconductor crystal is a wurtzite crystal and is not symmetrical in the + c-axis direction and the −c-axis direction. For this reason, when growing on different substrates such as sapphire substrates and silicon carbide substrates having different lattice constants, if they are grown at a low temperature so as to cover the entire substrate, crystals of group III and group V surfaces are mixed. On the other hand, when the crystal is grown suddenly at a high temperature, the crystal grows only locally on the substrate, and the crystal orientation varies.
[0046]
In addition, it is impossible to obtain high-quality crystals in the molecular beam epitaxy method simply by raising or lowering the growth temperature as in the prior art method (see Patent Documents 1 and 4). For example, a gallium nitride buffer layer grown at a low temperature includes a Group V surface and a Group III surface. Even if an attempt is made to grow it at a high temperature, since there is a limit to the temperature, crystal growth is performed on both the crystal of the group V face and the crystal of the group III face, and a flat crystal cannot be obtained.
[0047]
Therefore, the present invention provides a method of dividing the thickness of the buffer layer and growing it at a low temperature, and applying heat treatment for each growth. FIG. 1 illustrates the method of the present invention.
[0048]
In the molecular beam epitaxy method, it is not realistic to raise the growth temperature because of re-evaporation of the source metal. However, it is possible to increase the temperature of the heat treatment. Although depending on the temperature of the heat treatment, by performing the heat treatment for an appropriate time, when the layer thickness is thin, the crystal of the group V face can be converted into the crystal of the group III face. Further, the buffer layer grown at a low temperature includes cubic crystals, which are also converted into group III plane crystals by heat treatment.
[0049]
This is a phenomenon in which heat treatment causes rearrangement or conversion to the most stable crystal coordination at high temperature, that is, to a group III crystal. This phenomenon is thought to be related to a phenomenon such as mass transport, but the details are not yet known.
[0050]
In order to maximize the effect of this heat treatment, the thickness of the divided buffer layer is particularly important. When the film thickness is large, the crystal of the group V face hardly changes to the crystal of the group III face. Usually, it is necessary to select a film thickness that changes within a few minutes.
[0051]
Therefore, first, a thin film is grown at low temperature so that the crystal of the group V plane is sufficiently changed to the group III plane crystal by heat treatment. Thereafter, the substrate temperature is raised and heat treatment is performed. If the temperature and film thickness of the heat treatment are appropriate, the group V face crystal changes to the group III face crystal within a practically acceptable time, for example, within 2 to 3 minutes.
[0052]
When the crystal film thickness is small, there is a possibility that the crystal grows locally three-dimensionally. For this reason, it is not suitable to be referred to as a film, but it is considered that it grows at a low temperature and is sufficiently evenly distributed on the substrate surface.
[0053]
After the heat treatment, the temperature is lowered again to grow a divided thin buffer layer, and the heat treatment is performed again. Thus, by repeating the growth of the thin buffer layer at a low temperature and the heat treatment at a high temperature, the entire substrate is sufficiently covered with the buffer layer of the group III surface. For example, the total buffer layer thickness is set to about 20 nm or more.
[0054]
Further, the thicknesses of the buffer layers to be divided and grown need not be the same. When growing on a different kind of substrate, since the crystal of the V group plane enters most during the first growth, it is necessary to reduce the thickness of the buffer layer to be grown first. However, after repeating the growth and heat treatment of the thin buffer layer several times, a considerable portion of the substrate is covered with the group III surface buffer layer, so the thickness of the buffer layer grown at low temperature is slightly thicker. You may do it.
[0055]
By growing by such a method, it is possible to properly match the c-axis direction of the substrate used for the growth with the c-axis direction of the nitride semiconductor crystal grown thereon. This is because if the buffer layer grown in a divided manner is thin, the heat treatment tends to be the most stable form, that is, the form with the c-axis aligned.
[0056]
As a result, the orientations of the crystal axes of the individual grains can be very well aligned, and the grains are easily bonded even when grown by the molecular beam epitaxy method having a low growth temperature. That is, the grains having the same c-axis direction are eventually combined to form a large grain, and finally, the grains are bonded over the entire substrate, so that a high-quality crystal having no grains can be obtained.
[0057]
If the above method is used, even if the molecular beam epitaxy method is used, it is possible to grow a high-quality nitride semiconductor material having no grain structure and having no V-group crystal mixed therein.
[0058]
As a matter of course, the growth method in which the buffer layer is divided and grown while performing the heat treatment can be used not only in the molecular beam epitaxy method but also in other organic metal compound vapor phase growth methods and hydride growth methods. is there. Also in these growth methods, since the temperature of the heat treatment can be raised above the growth temperature, it is possible to improve the crystal quality as compared with the conventional method by using the method of the present invention.
[0059]
Next, the material of the buffer layer will be described.
[0060]
Currently, gallium nitride is the easiest to handle as the material of the buffer layer actually used. This is because gallium nitride changes from a group V crystal to a group III crystal by heat treatment relatively easily within the temperature range of the substrate heater provided in the molecular beam epitaxy apparatus.
[0061]
This is thought to be related to the temperature characteristics of gallium nitride mass transport, and a buffer layer composed of small grains mixed with group V and group III surfaces by heat treatment due to a phenomenon such as mass transport has a large III It is thought that it will change to a buffer layer consisting of grains of family faces.
[0062]
Further, when gallium nitride is used as the buffer layer, the effect of heat treatment can be improved by adding or irradiating indium.
[0063]
Indium re-evaporates by heat treatment. However, it is possible to efficiently rearrange gallium nitride on the group V surface to gallium nitride on the group III surface.
[0064]
Further, in the case of using indium nitride as a buffer layer, indium nitride itself is unstable at the growth temperature of gallium nitride, which is currently most frequently used, and should not be used for the buffer layer. However, it is considered possible to use it as a buffer layer if the structure grown thereon is within the stable temperature range of indium nitride.
[0065]
On the other hand, aluminum nitride is stable within the temperature range reached by the substrate heater and does not readily change from the group V surface to the group III surface. Or it doesn't become a big grain even if it changes to the crystal of the group III face. In other words, aluminum nitride is less likely to cause mass transport or the like than gallium nitride.
[0066]
Therefore, although the characteristics are slightly improved by performing the heat treatment, the buffer layer made of small grains is not changed to the buffer layer made of large grains, and no significant improvement is observed.
[0067]
However, if the ultimate temperature of the substrate heater is improved by improving the substrate heating method in the future, mass transport of aluminum nitride will be possible, and the grain structure can be enlarged by heat treatment at high temperature, or all grains are combined. It is considered that a high-quality buffer layer can be formed.
[0068]
For boron nitride, the hexagonal type is more stable than the wurtzite type. And hexagonal boron nitride has no distinction between + c axis direction and -c axis direction. Therefore, when boron nitride is used as the buffer layer, the process of growing amorphous boron nitride at a low temperature and thereafter performing heat treatment is repeated.
[0069]
However, boron nitride is more stable than aluminum nitride and cannot be heated to a temperature that causes rearrangement or mass transport. In the future, it may be possible to modify the heater and improve the heating method.
[0070]
Next, the substrate will be described.
[0071]
Special attention must be paid when using sapphire as a substrate. In the molecular beam epitaxy method, crystal growth is performed after first nitriding a sapphire substrate. Since this nitriding process is performed by supplying a nitrogen source to the substrate surface, this is a process in which only nitrogen is supplied, that is, in a situation where nitrogen is excessive.
[0072]
Thus, in a situation where the supply of nitrogen, that is, a group V atom is excessive, the crystal of the group V plane becomes more stable. Therefore, the surface of the sapphire substrate is covered with the group V plane aluminum nitride, which is one of the major causes of the nitride semiconductor crystal grown thereon becoming a group V plane crystal. This is one of the reasons why good crystals could not be obtained by the molecular beam epitaxy method.
[0073]
Care must be taken especially in low-temperature nitriding. Since a sapphire substrate is not stable in a vacuum at a high temperature, there is a method of performing a process called low-temperature nitridation in order to obtain a flat surface. Low-temperature nitriding is a technique for forming a very thin aluminum nitride layer on the surface of a sapphire substrate by irradiating the surface of the sapphire substrate with nitrogen plasma with a substrate temperature of about 200 ° C. or less.
[0074]
In this case, since the aluminum nitride is formed at a low temperature, the entire surface of the sapphire substrate is a group V plane aluminum nitride, which strongly promotes the crystal growth of the group V plane. Therefore, it is necessary to perform a heat treatment before growing the first thin nitride semiconductor buffer layer at a low temperature. In this case, the heat treatment temperature is preferably 900 ° C. or higher, preferably 1000 ° C. or higher.
[0075]
The method of the present invention is considered effective not only for sapphire substrates but also for silicon carbide substrates, silicon substrates, and the like. This is because even when such a substrate is used, crystals of the group III plane and the group V plane are mixed. Therefore, it is possible to control the crystal to be a group III face crystal by the method of the present invention.
[0076]
The method of the present invention is also effective when growing other nitride semiconductors having different lattice constants on the nitride semiconductor. For example, when gallium nitride is grown on aluminum nitride, the crystal defect density can be reduced by using the method of the present invention.
[0077]
DETAILED DESCRIPTION OF THE INVENTION
Here, a method of growing a gallium nitride buffer layer by dividing the sapphire substrate while performing a heat treatment on a sapphire substrate using a molecular beam epitaxy apparatus, and growing gallium nitride on the gallium nitride will be described. In principle, nitride semiconductor materials other than gallium nitride can also be used as the buffer layer.
[0078]
For the growth of the nitride semiconductor, a molecular beam epitaxy apparatus including a growth chamber, a substrate transfer chamber, and a substrate carry-in chamber is used. If such a three-chamber vacuum chamber is used, only the substrate loading chamber is exposed to the atmosphere during loading. Further, since there is a substrate transfer chamber between the substrate carry-in chamber and the growth chamber, it is possible to minimize the entry of moisture and the like into the growth chamber.
[0079]
In the examples of the present invention, MBE32 manufactured by Liber Corporation was used. The growth chamber was equipped with an ion pump and an oil diffusion pump. During the growth, vacuum was drawn with an oil diffusion pump. Since the growth conditions and the like described in the embodiment of the invention vary depending on the apparatus, it is necessary to pay attention to the conditions when actually performing them.
[0080]
Gallium (Ga), aluminum (Al), indium (In), etc., which are Group III materials, are supplied to the growth substrate using a K cell in a vacuum in the growth chamber. The K cell supplies raw materials such as Ga, Al, In, etc. into a PBN crucible, heated and evaporated. Further, atomic nitrogen (nitrogen plasma) is used as a group V material. Therefore, an rf plasma cell that can generate plasma with a radio frequency (rf, 13.56 MHz) is used. In this method, high-purity nitrogen is converted into plasma in a vacuum in a growth chamber and supplied to the growth substrate.
[0081]
In the case of growing a nitride semiconductor crystal, it is necessary to monitor the state of the crystal surface during the growth. For this purpose, high-speed reflection electron diffraction (RHEED) is used. Information on rearrangement and flatness on the substrate surface can be obtained by the shape of the RHEED pattern and its brightness.
[0082]
As the growth substrate, a c-plane sapphire substrate (0001) is used. As a method for heating the substrate, a sapphire substrate is transparent, and therefore, a method in which molybdenum (Mo) or the like (thickness of 500 nm or more) is deposited on the back surface is used. Since etching using phosphoric acid is performed in the pretreatment, it is necessary to select a metal that does not dissolve in phosphoric acid. Mo is suitable. After the substrate is purchased, Mo or the like is first deposited on the back surface, and then the pretreatment described below is performed.
[0083]
The growth substrate is pretreated before being loaded into the substrate loading chamber. After removing the oil using an organic solvent, the surface is etched using a 1: 3 mixture of commercially available orthophosphoric acid and sulfuric acid. At this time, the temperature of the mixed solution is 110 ° C., and the etching time is 30 minutes.
[0084]
In etching with phosphoric acid, the flatness of the substrate varies greatly depending on the conditions. If the conditions for etching with phosphoric acid are optimized, the surface becomes extremely flat. In this state, only the steps corresponding to the atomic spacing in the c-axis direction of sapphire are aligned substantially in parallel. It is important to have such a stepped surface.
[0085]
After etching, the substrate is attached to the substrate holder attached to the molecular beam epitaxy apparatus. Bring it into the substrate loading chamber of the molecular beam epitaxy system and evacuate it sufficiently. Thereafter, the substrate holder to which the growth substrate is attached is attached to the substrate heater in the growth chamber after passing through the substrate transfer chamber. Since the temperature of heat treatment or the like is high, it is necessary to be able to heat Mo deposited on the back surface directly with a heater.
[0086]
FIG. 2 illustrates the process after loading into the molecular beam epitaxy apparatus.
First, the aluminum oxide of several molecular layers on the surface of the sapphire substrate is changed to aluminum nitride. This treatment is called nitriding, and how flat the surface is nitrided is important. For this purpose, the surface of the sapphire substrate is irradiated with atomic nitrogen generated by the rf plasma cell while keeping the temperature of the sapphire substrate low (as a guideline, approximately 200 ° C. or less with a thermocouple attached to the back side of the substrate). The flow rate of high purity nitrogen to the rf plasma cell is 1 sccm, the rf power to the rf plasma cell is 380 W, and the irradiation time is 2 hours. In the subsequent processes, this condition was used for the flow rate of nitrogen and the power of rf plasma.
[0087]
By irradiating atomic nitrogen (nitrogen plasma) while keeping the sapphire substrate at a low temperature in this way, the surface of the sapphire substrate can be nitrided flat without causing damage due to plasma. Moreover, it can confirm using RHEED that the several atomic layer of the surface changes from aluminum oxide to aluminum nitride. Since aluminum oxide and aluminum nitride have different lattice constants, the RHEED pattern changes when the surface changes to aluminum nitride.
[0088]
Next, rf plasma irradiation is stopped, and the substrate temperature is raised to 1000 ° C. or higher to perform heat treatment. In this process, the aluminum nitride on the group V surface changes to the aluminum nitride on the group III surface. Also, there is a slight change in the brightness of RHEED during this process. Immediately after nitriding at a low temperature, only nitrogen plasma is supplied to the substrate surface, and because the temperature is low, it is considered that group V plane aluminum nitride and group III plane aluminum nitride are mixed. However, when the heat treatment is performed at an elevated temperature, all the aluminum nitride on the substrate is rearranged into crystals of the group III face. Therefore, RHEED becomes slightly brighter.
[0089]
Also, since the sapphire substrate itself is not very stable at high temperatures in a vacuum, the heat treatment can be completed in as short a time as possible. Actually, the thickness of aluminum nitride on the substrate is so thin that it can be sufficiently rearranged by a short heat treatment. In fact, even if the temperature was lowered immediately after raising the temperature to 1000 ° C. or higher at a temperature rising time of 100 ° C./min, there was a sufficient effect.
[0090]
Next, the temperature is lowered to a temperature suitable for the growth of the buffer layer. The temperature is such that the movement of the source metal Ga on the substrate surface is sufficiently suppressed, while gallium nitride can grow. The growth temperature used in the present invention is about 600 ° C. The gallium K cell shutter is opened and nitrogen plasma is supplied for growth. The temperature of the gallium K cell was 900 ° C. The growth rate at this time is about 300 nm / hour.
[0091]
When indium is added to the buffer layer, indium is also supplied to the substrate at the same time. The supply amount of indium may be about 10% of the supply amount of gallium.
[0092]
Here, the growth thickness of the buffer layer for each time and the temperature of the heat treatment are closely related and need attention. This is because whether or not the rearrangement is performed in a practically executable short time depends on the temperature and film thickness of the heat treatment.
[0093]
In the apparatus used in the present invention, the maximum substrate temperature reached was about 1050 ° C. Since it takes time to raise the temperature around the maximum temperature, the temperature and time of the heat treatment were set to about 1000 ° C. for 20 seconds. Under these conditions, when it was grown to 5 nm or more, a portion considered to be a crystal of a group V plane remained. Therefore, the thickness of the gallium nitride grown separately is set to 4 nm or less.
[0094]
On the other hand, when the temperature of the heat treatment can be further increased, it is possible to further increase the thickness of the buffer layer to be divided and grown. Regarding this, it is necessary to determine the conditions because the temperature reached differs from device to device.
[0095]
Further, when indium is irradiated during the heat treatment, the heat treatment can be performed efficiently. The amount of irradiation may be about the same as when adding indium during growth. The added or irradiated indium is considered to have re-evaporated because the temperature of the heat treatment is high.
[0096]
As described above, the total thickness of all the buffer layers is set to 20 nm by repeating the growth of the divided thin buffer layer and the heat treatment thereof. Therefore, in the case of growing by 4 nm, it is necessary to grow in 5 times.
[0097]
But they don't have to all be the same thickness. Although it is necessary to reduce the thickness for the first several times, there is no significant change in the result even if the film thickness is gradually increased thereafter. For example, good results can be obtained even when the film thickness of the first layer is increased to 2 nm, 3 nm, 4 nm, 5 nm, or 6 nm.
[0098]
After the division and finishing the heat treatment of the last buffer layer, gallium nitride is grown. Therefore, the substrate temperature is reset to the growth temperature of gallium nitride. The substrate temperature during the growth of gallium nitride was 800 ° C.
[0099]
At this temperature, gallium as the source metal re-evaporates, so it is necessary to increase the supply amount of gallium. The temperature of the gallium cell was 970 ° C. Further, when the pressure of the gallium vapor was measured using a vacuum gauge located at the same position as the substrate, that is, a vacuum gauge called a flux monitor, about 7 × 10 ^-7 It was about Torr. The growth rate is still about 300 nm / hour. Under this condition, it is considered that 60% to 70% of the supply amount of gallium is reevaporated.
[0100]
In this way, when gallium nitride is grown to 0.6 to 1 μm or more, a gallium nitride crystal with few defects is obtained. In a high-quality crystal, the crystal surface has a structure in which steps corresponding to atomic intervals in the c-axis direction of gallium nitride are arranged. The growth with such a step covering the surface is called step flow type growth. In such a state, the defect density decreases as the crystal film thickness increases. However, since the sapphire substrate and the gallium nitride substrate have different thermal expansion coefficients, the growth film thickness is set to 4 μm or less.
[0101]
In order to grow a semiconductor device structure such as a heterojunction field effect transistor, a heterostructure is grown thereon. For example, aluminum gallium nitride (AlxGa1-xN) having an aluminum composition ratio x of about 0.2 may be grown to a thickness of about 20 nm. Thereby, a two-dimensional electron gas can be formed at the heterojunction portion, that is, at the interface between gallium nitride and aluminum gallium nitride. Details of the device structure such as doping concentration are not described in detail because they are outside the scope of the present invention.
[0102]
In the embodiment of the present invention, the case where the molecular beam epitaxy apparatus is used has been described. However, the same principle can be applied even when another growth apparatus such as an organic metal compound vapor phase growth method is used. However, it is necessary to change the conditions appropriately. In particular, the growth conditions such as the nitrogen flow rate, the substrate temperature, and the growth film thickness change as the crystal growth apparatus used changes.
[0103]
In addition, when ammonia is used as a nitrogen source, care must be taken because ammonia contains hydrogen atoms. During the heat treatment or the like, crystals may be re-evaporated by ammonia, so it is expected that the supply of ammonia needs to be stopped or reduced. On the other hand, the effect of heat treatment may be promoted. Since this differs depending on the crystal growth method, it is necessary to determine the conditions.
[0104]
In the case of growing by mixing hydrogen, the conditions are different because the effect of heat treatment is promoted by hydrogen.
[0105]
The present invention is also effective when a silicon carbide substrate or silicon substrate other than a sapphire substrate is used. However, as in the case of using a sapphire substrate, it is necessary to flatten the substrate surface on the atomic layer order before growth. With respect to these substrate pretreatment methods, the substrate may be pretreated by a known method.
[0106]
Further, even in the case of crystal growth including As, P, etc. other than nitrogen as a group V atom, it is possible to grow using the buffer layer forming method according to the present invention as long as it is hexagonal crystal growth.
[0107]
【The invention's effect】
The effect of the invention has also been described in the above description. The most important point is that even if the molecular beam epitaxy method is used, if the method of the present invention in which the buffer layer is formed by repeating the low-temperature thin film growth and the heat treatment, the crystal orientation in the direction perpendicular to the substrate, that is, the c-axis direction is strictly It is oriented, and the grain structure disappears, and a growth mode called step flow growth is achieved. As a result, crystal defects such as helical transition and surface defects can be greatly reduced as compared with the case of using a conventional aluminum nitride single layer buffer layer.
[0108]
In addition, since the orientation of the c-axis of the nitride semiconductor crystal grown in this way exactly matches the orientation of the crystal axis perpendicular to the substrate of the substrate, the step interval and step direction of the crystal surface can be set according to the surface of the substrate. It can be controlled by the state.
[0109]
For example, when a sapphire substrate is used, if the surface of the sapphire substrate is the (0001) c plane, the step interval of the grown nitride semiconductor crystal becomes wide. On the other hand, if a substrate whose surface of the sapphire substrate is inclined by about 0.5 ° from the (0001) c plane, that is, a substrate called an off substrate, the step interval of the grown nitride semiconductor crystal becomes narrow. By using such a substrate, it is also possible to control the step interval, step direction, and the like.
[0110]
If the step interval is further narrowed, for example, even if small grains remain, it can be eliminated from the relationship of thermodynamic surface energy, leading to reduction of crystal defects.
[0111]
The results of actual growth using molecular beam epitaxy are described below.
[0112]
As a method capable of measuring the crystal orientation in the c-axis direction using X-ray diffraction, there is a method of reflecting X-rays symmetrically in the direction perpendicular to the substrate surface, that is, in the c-axis. This is a method called ω scan. When a large number of lattice defects are present, the crystal orientation is locally dispersed due to helical transitions, surface defects, and the like, and the orientation of X-ray diffraction is shifted. And the width of the X-ray peak becomes wide. Therefore, not only the orientation in the c-axis direction but also the defect density can be examined.
[0113]
By using the method of the present invention, gallium nitride having a thickness of about 0.8 μm grown on a sapphire substrate was measured using X-ray diffraction using gallium nitride as a buffer layer. As a result, the full width at half maximum was 50 arcsec to 60 arcsec. When grown by the MOCVD method, it is usually 100 to 200 arcsec or more and very narrow.
[0114]
50 arcsec is 0.00025 radians, and when 10,000 atoms are lined up, it is only about 2 to 3 sides. Therefore, since the directions of the individual grains are aligned, the grains are bonded to each other and the crystal surface has a step structure. Furthermore, it becomes a growth mode called step flow growth, and defects are reduced.
[0115]
In order to investigate the surface structure of the actually grown crystal, the surface of a crystal having a thickness of about 0.8 μm grown by the method of the present invention was measured using an atomic force microscope. As a result of measuring about 5 μm square, a step structure consisting of steps was observed on the entire measured surface. This is a step corresponding to the distance between atoms in the direction perpendicular to the substrate, that is, in the c-axis direction, and it was found that high-quality crystal growth was performed. Similar measurement results were obtained for all parts of the 2-inch substrate, and it was found that good quality crystals were obtained over the entire substrate.
[0116]
The surface of gallium nitride grown by a conventional method was also measured using an atomic force microscope. The growth temperature of the measured crystal aluminum nitride buffer layer is 750 ° C., and the film thickness is 200 nm. The growth temperature of gallium nitride thereon is 800 ° C., and the film thickness is 0.5 μm. The measurement was performed by enlarging to 1 μm square, but no proper steps were seen. This is because crystals and defects such as threading transition enter at a high density when grown by a conventional method using aluminum nitride as a buffer layer.
[0117]
Similarly, the electrical characteristics of gallium nitride grown by the method of the present invention were measured. In the case of non-doping, it is N-type and the background carrier density is 5 × 10. ¹⁶ cm ^-3 The mobility was 100 cm / Vs or higher. When rf plasma was grown using a conventional method using nitrogen as a nitrogen source, the defect density was high, so the resistance was extremely high and measurement was impossible.
[0118]
As can be seen from the above, it is understood that a high-quality nitride semiconductor crystal can be obtained using the molecular beam epitaxy method by using the method of the present invention.
[Brief description of the drawings]
FIG. 1 Growth method of the present invention
FIG. 2 shows a growth method of the present invention when gallium nitride is used as a buffer layer.

Claims (6)

A method for growing a nitride semiconductor stacked body, comprising:
Crystal growth of a nitride semiconductor including a crystal of a group V surface and a crystal of a group III surface;
By performing a heat treatment to rearrange the crystal of the group V face to the crystal of the group III face,
Grow a buffer layer covering the whole substrate,
A method for growing a nitride semiconductor laminate, comprising growing a nitride semiconductor crystal on the buffer layer. 2. The method for growing a nitride semiconductor multilayer structure according to claim 1, wherein crystal growth of a nitride semiconductor including a crystal of a group V surface and a crystal of a group III surface, and conversion of the crystal of the group V surface to a crystal of a group III surface. A method for growing a nitride semiconductor stacked body, wherein the rearrangement heat treatment is repeated twice or more. 2. The method for growing a nitride semiconductor stacked body according to claim 1, wherein the buffer layer is gallium nitride. 2. The method for growing a nitride semiconductor laminate according to claim 1, wherein crystal growth of a nitride semiconductor including a crystal of a group V surface and a crystal of a group III surface, and conversion of the crystal of the group V surface to a group III surface. A method for growing a nitride semiconductor laminate, comprising adding or irradiating indium in the heat treatment for rearrangement. 2. The method for growing a nitride semiconductor multilayer body according to claim 1, wherein the growth method is performed using a molecular beam epitaxy method. 6. The method for growing a nitride semiconductor stacked body according to claim 5, wherein said molecular beam epitaxy method uses plasma nitrogen as a nitrogen source, nitriding the surface of the sapphire substrate with plasma nitrogen, A method for growing a nitride semiconductor stacked body, wherein the buffer layer is grown after heat treatment.

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