JP2008294370A - Method for growing gallium nitride compound semiconductor and surface treatment method for substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To grow a gallium nitride-based compound semiconductor using a substrate composed of a diboride single crystal having a linear step on the whole and a flat terrace portion having no irregularities on the surface, and a diboride single crystal A substrate surface treatment method comprising:
A method for growing a gallium nitride-based compound semiconductor comprises a diboride single crystal represented by a chemical formula XB ₂ (wherein X includes at least one of Ti and Zr) in a film forming apparatus. In addition, the substrate with the natural oxide film formed on the surface is subjected to heat treatment in vacuum to remove the natural oxide film and expose the atomic step structure on the surface of the substrate. And a step of growing a gallium nitride-based compound semiconductor on the surface of the substrate in a state where the surface of the substrate is not exposed to the atmosphere.
[Selection] Figure 2

Description

  The present invention relates to a growth method of a gallium nitride compound semiconductor layer for growing a gallium nitride compound semiconductor using a substrate composed of a diboride single crystal for forming a gallium nitride compound semiconductor, and from the diboride single crystal. The present invention relates to a surface treatment method for a substrate.

Formula _{In x Al y Ga 1-xy} N ( However, 0 ≦ x + y ≦ 1,0 ≦ x ≦ 1,0 ≦ y ≦ 1) The gallium nitride-based compound semiconductor represented by the, AlN, InN, combinations such as GaN Thus, a mixed crystal of AlGaN, InGaN, InGaAlN or the like can be formed. Such a mixed crystal can change the band gap by selecting its constituent elements and composition, and can emit light from the visible light region to the ultraviolet light region according to the band gap. Accordingly, such a gallium nitride compound semiconductor becomes a constituent material of a light emitting element such as a light emitting diode (LED) or a semiconductor laser (LD), and in particular, a blue light emitting diode or a blue (blue-violet) laser has recently been commercialized. It has become like this. In addition, by combining a blue light emitting diode and a phosphor, application as a white illumination lamp is also progressing.

Such a gallium nitride compound semiconductor is formed on a substrate made of a single crystal. Conventionally, a substrate made of sapphire (Al ₂ O ₃ ) has been mainly used as a substrate for growing a gallium nitride compound semiconductor, and in addition, silicon carbide (SiC), zinc oxide (ZnO), silicon (Si ) And the like are also used, but all of them are heteroepitaxially grown and have a relatively large lattice constant difference from the gallium nitride compound semiconductor.

  Recently, a substrate made of gallium nitride (GaN), which is a substrate for homoepitaxial growth, can be produced by a vapor phase method, a high pressure method, a flux method, or the like, and its use is expanding.

  In addition, although it is a substrate for heteroepitaxial growth, a substrate made of a diboride single crystal having a relatively small lattice constant difference is also attracting attention. Since this substrate has a lattice constant close to that of the gallium nitride compound semiconductor, a gallium nitride compound semiconductor having excellent characteristics such as crystallinity can be obtained.

  Usually, such a substrate for growing a gallium nitride compound semiconductor is formed into a wafer shape by slicing a single crystal ingot, and further subjected to mechanical polishing or chemical polishing in order to flatten the cut surface. Thus, the substrate is used for growing a gallium nitride compound semiconductor.

  The surface of the substrate polished as described above is flat at first glance, but is still not flat at the atomic level, and many polishing marks remain on the surface. In addition, the growth surface of the gallium nitride compound semiconductor on the substrate is often inclined with a deviation from the original growth surface, and the inclination angle is referred to as the off-angle, but the atomic step due to the off-angle is unclear. It may be.

  Generally, when thin film growth is performed on a substrate, it is preferable that an atomic step is formed on the surface of the substrate, and in this case, three-dimensional growth of a gallium nitride-based compound semiconductor crystal is suppressed. Therefore, in order to form a high quality and flat thin film, the surface of the substrate needs to have an atomic step having a height of one atomic level and a flat and clean terrace portion formed between the atomic steps. is there.

Actually, when a thin film such as a gallium nitride-based compound semiconductor is grown on a substrate, the surface of the substrate is treated before the growth to control the surface state of the substrate. For example, in a substrate made of SrTiO ₃ (strontium titanate) single crystal having a perovskite structure, which is used as a substrate for the growth of an oxide superconductor thin film, the substrate is obtained by repeatedly performing a solution treatment and a heating step. Has been disclosed (for example, see Patent Document 1 below).

  Similarly, after the substrate is treated with a ph 4.8 BHF solution, it is further heat-treated in oxygen gas at 900 ° C. for 1 hour or more to form a flat terrace portion and a unit crystal lattice length step (atomic step). A forming method is disclosed (for example, see Patent Document 2 below).

Furthermore, in an atmosphere where a processing gas containing a different kind of atom different from the constituent atoms of the semiconductor substrate exists, the semiconductor substrate is irradiated with an electron beam to cause a surface excitation reaction, and a different atom part forming step and an atomic step are performed. And a surface treatment method for forming the atomic step in a predetermined shape by controlling the movement of the atomic step with different atoms during the atomic step moving process. For example, see Patent Document 3 below).
JP-A-8-92000 JP 2000-159600 A Japanese Patent Laid-Open No. 9-219395

  However, for a substrate made of a diboride single crystal having a lattice constant close to that of a gallium nitride compound semiconductor and a thermal expansion coefficient, a method for controlling the surface state has not been established, and there has been a problem in surface flatness. It was.

  A substrate made of a single diboride single crystal forms a natural oxide film that is firmly bonded to the surface of the substrate simply by being left in the atmosphere for a short time. The natural oxide film inhibits lattice matching when growing the gallium nitride compound semiconductor layer on the substrate, and cannot take advantage of the original characteristics of the diboride single crystal, and the gallium nitride compound semiconductor crystal. There was a problem in that it was not possible to improve the performance.

  Also, polishing marks remain on the surface of the substrate, and even if the substrate has no polishing marks on the surface and atomic steps and terraces can be confirmed, the shape of the atomic steps is not linear but curved. In addition, deformations such as irregularities in the terrace portion were observed, and it was not possible to obtain a surface state substrate having clear atomic steps and terrace portions.

  Therefore, the above problem has been an impediment to improving the characteristics of the gallium nitride compound semiconductor grown on the substrate.

  Conventionally, the substrate is subjected to thermal cleaning at about 1000 ° C. before the growth of the gallium nitride compound semiconductor in order to remove the natural oxide film on the surface of the substrate. Since the diboride single crystal has a high melting point of about 3200 ° C., the bonding force between atoms constituting the crystal is strong, and it has been considered that the vapor pressure of boron is low at the temperature of such thermal annealing. However, according to the inventor's research, even at a temperature sufficiently lower than the melting point, boron is detached from the surface of the substrate composed of a diboride single crystal in which the atomic steps and terraces are in an incomplete surface state, and the surface It turned out that the flatness of becomes worse.

  Due to the problems of the surface of the substrate composed of the diboride single crystal as described above, the original good lattice matching is not utilized, and the characteristics of the gallium nitride compound semiconductor grown on the substrate are expected. Was not obtained.

  Accordingly, the present invention has been completed in view of the above-mentioned problems in the prior art, and its object is to provide a diboride having a linear step on the whole and a flat terrace portion having no irregularities on the surface. It is an object of the present invention to provide a method for growing a gallium nitride compound semiconductor using a substrate made of a single crystal and a surface treatment method for a substrate made of a diboride single crystal.

The method for growing a gallium nitride-based compound semiconductor according to the present invention comprises a diboride single crystal represented by the chemical formula XB ₂ (wherein X includes at least one of Ti and Zr) in a film forming apparatus. A process of removing the natural oxide film and exposing an atomic step structure on the surface of the substrate by subjecting the substrate having a natural oxide film formed thereon to heat treatment in a vacuum, and subsequently forming the film And a step of growing a gallium nitride-based compound semiconductor on the surface of the substrate in a state where the surface of the substrate is not exposed to the atmosphere in the apparatus.

In the method for growing a gallium nitride-based compound semiconductor on a substrate of the present invention, the periphery of the substrate is preferably a diboride represented by the chemical formula XB ₂ (where X includes at least one of Ti and Zr). The heat treatment is performed in a state covered with the sintered body.

  In the method for growing a gallium nitride-based compound semiconductor according to the present invention, preferably, the heat treatment is performed at a temperature of 1500 ° C. or higher for 30 minutes or longer.

  In the gallium nitride compound semiconductor growth method of the present invention, preferably, the heat treatment is performed at a vacuum degree of 100 Pa or less.

The substrate surface treatment method of the present invention comprises a diboride single crystal represented by the chemical formula XB ₂ (wherein X includes at least one of Ti and Zr), and a natural oxide film is formed on the surface. The substrate is subjected to heat treatment in a vacuum to remove the natural oxide film and expose an atomic step structure on the surface of the substrate.

The method for growing a gallium nitride-based compound semiconductor according to the present invention comprises a diboride single crystal represented by the chemical formula XB ₂ (wherein X includes at least one of Ti and Zr) in a film forming apparatus. In the process of removing the natural oxide film and exposing the atomic step structure on the surface of the substrate by subjecting the substrate with the natural oxide film formed thereon to heat treatment in vacuum, And a step of growing a gallium nitride compound semiconductor on the surface of the substrate in a state where the surface of the substrate is not exposed to the atmosphere, thereby obtaining a substrate having a terrace portion having a clear atomic step and excellent flatness on the surface. And there is no natural oxide film at the interface between the substrate and the gallium nitride compound semiconductor because the substrate is not exposed to the air atmosphere after heat treatment. It made. As a result, the gallium nitride compound semiconductor can be grown by reflecting the original lattice constant and thermal expansion coefficient of the diboride single crystal, which has excellent compatibility with the gallium nitride compound semiconductor. Therefore, it is possible to obtain a gallium nitride compound semiconductor having a low dislocation density and high crystallinity.

In the method for growing a gallium nitride compound semiconductor according to the present invention, preferably, the periphery of the substrate is sintered with diboride represented by the chemical formula XB ₂ (where X includes at least one of Ti and Zr). By performing the heat treatment in a state of being covered with a body, it is possible to lower the heat treatment temperature at which a clearer atomic step and a flatter terrace portion are exposed. Furthermore, by covering the substrate with a sintered body, a phenomenon of bonding of atomic steps (bunching phenomenon) occurs, and by controlling the heat treatment temperature and heat treatment time, the height of the atomic step and the width of the terrace are controlled. It becomes possible to do.

  The bunching phenomenon can be caused by removing the natural oxide film from the surface of the substrate by heat treatment and removing boron and zirconium to form a clear atomic step. It is considered that since the surface of the substrate becomes high temperature, boron and zirconium released from the surrounding sintered body are supplied and adhered to the surface of the substrate, and atomic steps are bonded to each other by migration of the surface of the substrate.

The lattice constants of ZrB ₂ and GaN in the c-axis direction are 0.353 nm and 0.519 nm, respectively, and GaN is about 1.5 times larger. Thus, if the lattice constants in the c-axis direction are different, defects such as formation of polarity inversion regions and stacking faults occur. However, if the height of the atomic step can be controlled by the growth method of the gallium nitride compound semiconductor according to the present invention, for example, the height of the atomic step of ZrB ₂ is controlled to a height of 3 atoms, so that the step becomes 1.059 nm. This is almost the same as 1.038 nm, which is the height of the atomic step of 2 atoms of GaN, and apparently reduces the difference in c-axis length compared to 0.353 nm and 0.519 nm. be able to. Therefore, the dislocation density of the gallium nitride compound semiconductor can be reduced and the crystallinity can be increased.

  In the method for growing a gallium nitride-based compound semiconductor according to the present invention, preferably, the heat treatment is performed at a temperature of 1500 ° C. or higher for 30 minutes or longer to remove a natural oxide film existing on the surface of the substrate and damage due to polishing. The removal of the layer is sufficient, and the surface of the substrate is sufficiently reconstructed by supplying boron and zirconium from the sintered body. As a result, a clear atomic step and a flatter terrace portion can be exposed on the surface of the substrate. Below 1500 ° C., the atomic step becomes clearer than before the heat treatment, but it is not sufficient because a curved portion is seen in the shape of the atomic step or unevenness remains in the terrace portion. Similarly, the formation of atomic steps and terraces is insufficient even at a processing time of less than 30 minutes at 1500 ° C.

  In the method for growing a gallium nitride-based compound semiconductor according to the present invention, the heat treatment is preferably performed at a vacuum degree of 100 Pa or less, and at such a high vacuum degree, boron and zirconium are detached from the damaged layer on the surface of the substrate. Further, the removal of the natural oxide film is sufficient, and a clear atomic step and a flat terrace portion can be exposed. When the degree of vacuum is higher than 100 Pa, boron and zirconium cannot be sufficiently separated from the surface of the substrate, and zirconium is aggregated on the surface of the substrate or the surface of the substrate is roughened.

The substrate surface treatment method of the present invention comprises a diboride single crystal represented by the chemical formula XB ₂ (wherein X includes at least one of Ti and Zr), and a natural oxide film is formed on the surface. By subjecting the substrate to heat treatment in a vacuum, the natural oxide film is removed and an atomic step structure is exposed on the surface of the substrate. When a gallium nitride compound semiconductor is formed on a substrate made of a diboride single crystal whose step structure is exposed, the original lattice constant and thermal expansion coefficient of the diboride single crystal approximate to that of a gallium nitride compound semiconductor crystal are obtained. Reflecting the epitaxial growth, it becomes possible to form a gallium nitride compound semiconductor having a low dislocation density and a small residual strain.

  In addition, by subjecting the substrate to heat treatment in vacuum, the natural oxide film present on the surface of the substrate can be removed, and the composition that appears to be a damaged layer due to polishing present on the surface of the substrate has a chemical amount. A layer deviating from the theoretical composition can be removed. Accordingly, it is possible to expose a clear atomic step and a flat terrace portion due to the original off angle on the surface of the substrate.

  By performing the surface treatment of the substrate as described above, it is possible to obtain a substrate having a terrace portion having a clear atomic step and excellent flatness and having no residual oxide. Since the surface of such a substrate has very high chemical stability, it is difficult for surface re-oxidation to occur, and even when the substrate is heated for the growth of a gallium nitride-based compound semiconductor, surface roughness is unlikely to occur. . Therefore, by growing a gallium nitride compound semiconductor on a substrate that has been subjected to the surface treatment of the present invention, a gallium nitride compound semiconductor having good crystallinity and characteristics can be obtained.

  Embodiments of a method for growing a gallium nitride compound semiconductor and a surface treatment method for a substrate according to the present invention will be described below in detail with reference to the accompanying drawings.

The substrate surface treatment method of the present invention comprises a diboride single crystal represented by the chemical formula XB ₂ (wherein X includes at least one of Ti and Zr), and a natural oxide film is formed on the surface. The substrate is heat-treated in a vacuum to remove the natural oxide film and expose the atomic step structure on the surface of the substrate.

The diboride single crystal represented by the chemical formula XB ₂ (where X includes at least one of Ti and Zr) is composed of a ZrB ₂ single crystal, a TiB ₂ single crystal, etc. In view of excellent lattice matching and thermal expansion coefficient matching, those made of a ZrB ₂ single crystal are preferable. In the ZrB ₂ single crystal, a part of Zr may be substituted with Ti or Hf. Further, the ZrB ₂ single crystal may contain Ti, Hf, Mg, Al, etc. as impurities to such an extent that the crystallinity and lattice constant do not change greatly.

For example, for ZrB ₂ single crystal, a single crystal ingot is prepared by a crystal growth method such as a floating zone method, and then processed into a wafer having a desired shape and thickness by a wire cutting machine, and the surface is further processed with a diamond abrasive or colloidal. A substrate having a mirror surface is obtained by mechanical polishing with silica or the like.

FIG. 1 shows a view of the surface of a substrate made of a ZrB ₂ single crystal that has not been subjected to the surface treatment method of the present invention, for example, observed with an atomic force microscope (AFM). In FIG. 1, a stepped portion considered to be an atomic step corresponding to the off-angle of the surface (shift angle from a predetermined crystal plane such as the (0001) plane) can be observed. In FIG. 1, it can be seen that a plurality of step portions extending in the vertical direction are formed. The atomic step in FIG. 1 has many portions that are curved in a plan view, and has an overall disordered shape. Furthermore, unevenness can be observed on the terrace, and the flatness is poor. Further, it can be confirmed that a natural oxide film made of, for example, ZrO ₂ is formed on the surface of the substrate by another analysis method (X-ray photoelectron spectroscopy (XPS) or the like).

Therefore, for example, the following surface treatment is performed on a substrate made of a ZrB ₂ single crystal before the gallium nitride compound semiconductor is formed.

First, a sintered body made of ZrB ₂ powder is prepared. The sintered body is filled with a ZrB ₂ powder together with a resin binder and the like, and is subjected to pressure by a pressing device to produce a molded body (generated mold body). The molded body may be sintered as it is, but CIP (Cold Iso-static Press) or the like may be applied to the molded body in order to improve the density. Further, in addition to the ZrB ₂ powder, boron may be added excessively, and it becomes possible to sufficiently supply boron from the sintered body to the substrate during the heat treatment of the substrate.

  Sintering is performed by setting the molded body in a cylindrical carbon jig and raising the temperature by high-frequency induction heating. The temperature at this time may be about 1900 ° C. and the time may be about 10 hours, and a sintered body having a sufficient density of 90% or more can be obtained.

  The sintering method is not particularly limited, and can be performed by a known method such as a hot press method or a HIP (Hot Iso-static Press) method.

Subsequently, the substrate made of a ZrB ₂ single crystal is placed in a carbon jig in a heat treatment furnace in a state where the periphery of the substrate is covered with a ZrB ₂ sintered body. Then, it heats by high frequency induction. At this time, the heat treatment furnace is evacuated by a rotary pump, a turbo molecular pump, or the like. The higher the degree of vacuum, the easier it will be to remove boron and zirconium from the surface of the substrate and to supply boron and zirconium from the sintered body, so the time required to obtain a clear atomic step and a flat terrace will be shortened. The heat treatment temperature can be reduced. Therefore, it is preferable to heat-process with a vacuum degree of 100 Pa or less. More preferably, it is 10 Pa or less, and more preferably 10 ⁻³ Pa or less. Although it is possible to form atomic steps even at a degree of vacuum higher than 100 Pa, it is not preferable because a longer processing time and higher temperature processing are required for the above reasons. The degree of vacuum can be measured using a Pirani gauge or the like.

  The heat treatment is performed after reaching a sufficient degree of vacuum. The heat treatment is performed at 1500 ° C. or higher, preferably about 1700 ° C. The higher the temperature, the shorter the time required for the treatment. The heating method is not particularly limited, and examples thereof include high frequency induction heating, infrared heating, resistance heating, and microwave heating. Even at a temperature lower than 1500 ° C., there is a possibility that a surface state having a clear atomic step and a flat terrace portion can be obtained, but the processing time becomes long.

  The temperature at the time of heating can be measured by a thermocouple, but the temperature obtained by the thermocouple is different from the actual temperature of the substrate surface, and also differs depending on the installation position of the thermocouple. Therefore, the temperature at which a clear atomic step and a flat terrace portion are obtained varies depending on the furnace in which the heat treatment is performed. In the present invention, the surface temperature of the substrate represents a temperature measured by a thermocouple placed in contact with a carbon jig disposed in the vicinity of the substrate.

  When the heat treatment time is 1500 ° C. or higher and the temperature is 30 minutes or longer, a clear atomic step and a flat terrace portion can be exposed on the surface of the substrate. If it is less than 30 minutes, boron and zirconium are not sufficiently detached from the surface of the substrate in the heat treatment, and only the migration occurs on the surface of the substrate, and the elements on the surface of the substrate are sufficiently reconstructed. As a result, the surface becomes extremely rough.

In addition, a hydrofluoric acid aqueous solution treatment can be performed before the heat treatment. As a result, a natural oxide film, for example, a ZrO ₂ film that is firmly bonded to the surface of the substrate can be substantially removed. At this time, the hydrofluoric acid concentration of the hydrofluoric acid aqueous solution is preferably 0.1% by volume or more and less than 50% by volume. If the concentration is less than 0.1% by volume, the natural oxide film cannot be removed. If the concentration is more than 50% by volume, the etching of the substrate itself proceeds to cause surface roughness. More preferably, it is 0.5 volume% or more and 5 volume% or less, and if it is about 1 volume%, the natural oxide film on the surface of the substrate can be removed without causing surface roughness.

  Furthermore, although the etching time varies depending on the concentration of the hydrofluoric acid aqueous solution, if the etching is performed at a concentration of about 1% by volume, the natural oxide film can be sufficiently removed in about 1 minute.

  By appropriately adjusting the degree of vacuum, temperature, and time in the heat treatment as described above, the natural oxide film can be removed and a clear atomic step and a flat terrace portion can be exposed. In addition, the height of the atomic step and the width of the terrace portion can be controlled by the bunching phenomenon between the atomic steps.

  Thereafter, a gallium nitride compound semiconductor is grown. As a method for growing a gallium nitride compound semiconductor, metal organic vapor phase epitaxy (MOVPE) is used. In addition, molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), and pulsed laser deposition (PLD). ) Law.

The gallium nitride-based compound semiconductor of the present invention is represented by the chemical formula In _x Al _y Ga _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). Examples include gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), and the like.

  Further, the gallium nitride compound semiconductor may be p-type, n-type, or semi-insulating, and the conductivity type or conductivity may be controlled. In this case, for example, magnesium (Mg), zinc (Zn), boron (B) can be used as the p-type dopant, and silicon (Si) or germanium (Ge) can be used as the n-type dopant.

  The heat treatment of the substrate and the growth of the gallium nitride compound semiconductor are preferably performed in the same film formation apparatus. By performing in the same film formation apparatus, a substrate having no natural oxide film at the interface between the surface of the substrate and the gallium nitride compound semiconductor and having a clear atomic step and a flat terrace portion on the surface is obtained. be able to. Therefore, it is possible to grow a gallium nitride compound semiconductor reflecting the original lattice constant and thermal expansion coefficient of the diboride single crystal, and to obtain a gallium nitride compound semiconductor having a low dislocation density and high crystallinity. Become.

The above-described heat treatment of the substrate in vacuum and subsequent growth of the gallium nitride compound semiconductor are preferably performed continuously in the same chamber from the viewpoint of preventing reoxidation of the substrate surface. It is only necessary that the heat treatment and the growth chamber for the gallium nitride compound semiconductor are connected separately by a load lock or the like. Also, the growth chamber and the vacuum heat treatment is a completely different heat treatment furnace need only be placed in a glove box, for example, N ₂ atmosphere gas, also be a N ₂ atmosphere as taken out of the furnace The substrate after the heat treatment can be placed in the growth chamber through the glove box without being exposed to the atmosphere.

  As described above, the process of removing the natural oxide film and exposing the atomic step structure on the surface of the substrate by subjecting the substrate made of the diboride single crystal to the heat treatment in vacuum in the film forming apparatus. Then, in the film-forming apparatus, a step of growing a gallium nitride compound semiconductor in a state where the surface of the substrate is not exposed to the atmosphere, thereby having a lattice constant with the gallium nitride compound semiconductor that the diboride single crystal has A gallium nitride compound semiconductor can be formed by sufficiently reflecting the characteristic that the difference and the difference in thermal expansion are small. As a result, a gallium nitride compound semiconductor having a low dislocation density, a small residual stress, and excellent characteristics can be obtained.

  Also, the vacuum heat treatment and the growth apparatus are installed separately. Even when the substrate is exposed to the atmosphere after the heat treatment of the substrate and the substrate is installed in the growth apparatus, a clear atomic step and a flat surface are formed on the surface of the substrate. A terrace portion is exposed, and the composition of the surface of the substrate is a stoichiometric composition, which is chemically stable. Therefore, the oxidation does not easily proceed as compared with the surface of the substrate that has not been subjected to the heat treatment. Therefore, the gallium nitride compound semiconductor can be grown in a state where the heat-treated substrate is less affected by the natural oxide film on the surface of the substrate.

  Further, a light emitting layer or a two-dimensional electron gas layer is formed on the gallium nitride-based compound semiconductor, whereby a light emitting element or an electronic element can be obtained.

  When the gallium nitride compound semiconductor constitutes a light emitting element, as shown in FIG. 4, the gallium nitride compound semiconductor 3 has the following configuration.

That is, a gallium nitride compound semiconductor layer 3 is formed on a substrate 1 made of a diboride single crystal through a buffer layer 2 made of GaN or the like. The buffer layer 2 can be omitted. For example, the gallium nitride compound semiconductor 3 is a gallium nitride compound semiconductor represented by the chemical formula Ga _1-x1-y1 In _y1 Al _x1 N (where 0 ≦ x1 + y1 ≦ 1, x1 ≧ 0, y1 ≧ 0). a first conductive type gallium nitride-based compound semiconductor layer 3a made of, represented by the chemical formula _{Ga 1-x2-y2 in y2} Al x2 N ( provided, however, that 0 ≦ x2 + y2 ≦ 1, x2 ≧ 0, y2 ≧ 0.) Ga _{2 -x 3 -y 3} In _{y 3} Al _{x 3} N (where 0 ≦ x3 + y3 ≦ 1, x3 ≧ 0, y3) A structure in which a light emitting layer 3b made of a gallium nitride-based compound semiconductor represented by ≧ 0 is sandwiched and joined (where (x3, y1, y2) <(x1, x2), (x3, y1) , Y2) ≦ y3)).

  For example, the first conductive gallium nitride compound semiconductor layer 3a and the second conductive gallium nitride compound semiconductor layer 3c are a p-type gallium nitride compound semiconductor layer and an n-type gallium nitride compound semiconductor layer, respectively. In order to make a gallium nitride compound semiconductor a p-type semiconductor, magnesium (Mg), which is an element of Group 2 in the periodic table, may be mixed into the gallium nitride compound semiconductor as a dopant. In order to make the gallium nitride compound semiconductor an n-type semiconductor, silicon (Si), which is a group 4 element in the periodic table, may be mixed into the gallium nitride compound semiconductor as a dopant.

  The first conductivity type gallium nitride compound semiconductor layer 3a and the second conductivity type gallium nitride compound semiconductor layer 3c are both made of a gallium nitride compound semiconductor containing aluminum (Al), both of which are light emitting layers 3b. It is better to increase the content than aluminum contained in. In this case, the forbidden band widths of the first and second conductivity type gallium nitride compound semiconductor layers 3a and 3c are both larger than the forbidden band width of the light emitting layer 3b. , And these electrons and holes can be efficiently recombined to emit light with strong emission intensity. The first and second conductivity type gallium nitride compound semiconductor layers 3a and 3c are made of a gallium nitride compound semiconductor containing aluminum, so that the first and second conductivity type gallium nitride compound semiconductor layers 3a and 3c are formed. The forbidden band width of the first and second conductive type gallium nitride compound semiconductor layers 3a and 3c can be reduced in the absorption of light on the short wavelength side such as ultraviolet light. The first and second conductivity type gallium nitride compound semiconductor layers 3a and 3c may be made of a gallium nitride compound semiconductor containing aluminum.

  The first conductive gallium nitride compound semiconductor layer 3a and the second conductive gallium nitride compound semiconductor layer 3b may be an n-type gallium nitride compound semiconductor layer and a p-type gallium nitride compound semiconductor layer, respectively.

  Conductive layers (electrodes) 4 and 5 for injecting current into the light emitting layer 3b are formed in the first conductive type gallium nitride compound semiconductor layer 3a and the second conductive type gallium nitride compound semiconductor layer 3c, respectively. . Thereby, a light emitting element such as an LED or a semiconductor laser (LD) is formed.

  Further, the composition of the gallium nitride compound semiconductor forming the light emitting layer 3b may be set to an appropriate one that can obtain a desired light emission wavelength. For example, if the light emitting layer 3b is made of GaN containing neither aluminum nor indium, the forbidden band width is about 3.4 electron volts (eV), and ultraviolet light having an emission wavelength of about 365 nanometers (nm). The light emitting layer 3b can emit light. When the emission wavelength is shorter than this, the light emitting layer 3b is made of a gallium nitride compound semiconductor containing aluminum, which is an element that increases the forbidden band width, in an amount set according to the emission wavelength. It should be.

  In addition, the light emitting layer 3b may contain indium (In), which is an element for reducing the forbidden band width, and more aluminum may be contained so as to obtain a desired emission wavelength. What is necessary is just to set a composition ratio suitably. The light emitting layer 3b has a multi-layer quantum well structure (MQW: Multi-layer) that is a superlattice in which a quantum well structure composed of a barrier layer having a wide forbidden band and a well layer having a narrow forbidden band is regularly stacked a plurality of times. Quantum Well).

  Such a light emitting device operates as follows. That is, by applying a bias current to the gallium nitride compound semiconductor 3 including the light emitting layer 3b, ultraviolet light to near ultraviolet light having a wavelength of about 350 to 400 nm is generated in the light emitting layer 3b, and the ultraviolet light to near ultraviolet light is emitted outside the light emitting element. Operates to extract ultraviolet light.

  The light-emitting element can be applied to a semiconductor laser as a light source for an optical pickup of an optical recording medium such as a CD and a DVD, and high recording is achieved by using ultraviolet light to near ultraviolet light having a wavelength of about 350 to 400 nm or purple light. An optical recording medium capable of recording / reproducing for a long time with a density can be manufactured and used. Such an optical pickup may have a well-known configuration, for example, a light emitting element and a beam splitter, a polarizing beam splitter, a prism, a reflecting mirror, a diffraction grating, and the like installed on the optical axis of light emitted from the light emitting element It can be easily configured by combining a slit, a condensing lens, and the like.

  The light-emitting element can be used for a lighting device, and the lighting device includes a light-emitting element and at least one of a phosphor and a phosphor that emit light by receiving light emitted from the light-emitting element. is there. With this configuration, a lighting device with high luminance and illuminance can be obtained. The lighting device may be configured so that the light emitting element is covered or encapsulated with a transparent resin or the like, and a phosphor or phosphor is mixed in the transparent resin or the like. ~ Near ultraviolet light can be converted into white light or the like. In addition, a light reflecting member such as a concave mirror can be provided in a transparent resin or the like in order to improve the light collecting property. Such an illuminating device consumes less power than a conventional fluorescent lamp or the like, and is small in size. Therefore, the illuminating device is effective as a small and high-luminance lighting device.

  Note that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the present invention.

  Examples of the substrate surface treatment method of the present invention will be described below.

In Example 1, a ZrB ₂ substrate having a (0001) plane with the C-plane of the crystal exposed on the surface as a growth surface of the gallium nitride compound semiconductor was used as the substrate made of a diboride single crystal. It was confirmed by X-ray photoelectron spectroscopy (XPS) that a natural oxide film made of ZrO ₂ was formed on the ZrB ₂ substrate.

  Further, when the surface of the substrate was observed with an atomic force microscope (AFM), the atomic step can be confirmed as in FIG. 1, but the shape of the atomic step is curved in plan view, It was. In addition, irregularities were seen on the terrace.

  This substrate was placed on a carbon table and placed in a cylindrical carbon jig. At that time, the thermocouple was placed in contact with the carbon jig. These are installed in a chamber made of quartz and heated by high frequency induction from the outside.

The heating furnace containing the chamber was heated with a rotary pump and a turbo molecular pump at a vacuum of 7 × 10 ⁻⁴ Pa. After heating at 1700 ° C. for 3 hours, the furnace was cooled.

  About the board | substrate after heat processing, after taking out from the inside of a heating furnace, surface observation was performed by AFM. FIG. 2 shows the observed surface state of the substrate, and it can be seen that it exhibits clear atomic steps and a flat terrace. The shape of the atomic step is a straight line and is not disturbed as a whole, and the terrace portion is flat with no irregularities. The height of the atomic step corresponds to the height of one atom.

  From the above, it was confirmed that a clear atomic step and a flat terrace portion were exposed by performing a heat treatment in a vacuum on the substrate.

Also in Example 2, a ZrB ₂ substrate having a (0001) plane with the C-plane of the crystal exposed on the surface as the growth surface of the gallium nitride compound semiconductor was used as the substrate made of a diboride single crystal. It was confirmed by X-ray photoelectron spectroscopy (XPS) that a natural oxide film made of ZrO ₂ was formed on the ZrB ₂ substrate.

The substrate was first covered with a ZrB ₂ sintered body, the substrate and the sintered body were placed in a carbon jig, and heat treatment was performed under the same conditions as in Example 1 except that heat treatment was performed in a vacuum. went.

  About the board | substrate after heat processing, after taking out from the inside of a furnace, surface observation was performed by AFM. FIG. 3 shows an observation of the surface of a substrate heated at 1700 ° C. for 3 hours. A clear atomic step and a flat terrace portion were formed on the surface of the substrate. FIG. 3 is different from FIG. 2 and shows the surface state of a 10 μm square region. Unlike Example 1, the width of the terrace portion was widened, the height of the atomic step was equivalent to the height of 4 atoms, and the atomic step was bunched. By covering the periphery of the substrate with the sintered body and performing heat treatment in a vacuum, there is a supply of boron and zirconium from the sintered body, which promotes the reconstruction of the surface of the substrate and seems to have caused bunching. .

  Experiments as in Examples 1 and 2 were conducted with various changes in heating temperature, time, and degree of vacuum. When the heating temperature was 1500 ° C. or higher, the heating time was 30 minutes or longer, and the vacuum range was 100 Pa or lower. Thus, it was possible to obtain a surface state having a clear atomic step and a flat terrace as shown in FIGS. Under the conditions other than the above, the atomic step structure was not observed, the surface was roughened, the atomic step was inferior in linearity even if the atomic step could be observed, and the meandering portion was observed, and the surface state was incomplete. .

The surface of the substrate made of ZrB ₂ single crystal after polishing the surface of the substrate is a photograph observed by an atomic force microscope. 2 is a photograph of the surface of a substrate obtained by the surface treatment of Example 1 observed with an atomic force microscope. 2 is a photograph of the surface of a substrate obtained by the surface treatment of Example 2 observed with an atomic force microscope. It is sectional drawing of the light emitting element produced by the growth method of the gallium nitride type compound semiconductor of this invention.

Explanation of symbols

1: Substrate 2: Buffer layer 3: Gallium nitride compound semiconductor 3a: First conductivity type gallium nitride compound semiconductor layer 3b: Light emitting layer 3c: Second conductivity type gallium nitride compound semiconductor layer 4: Conductive layer 5: Conductive layer

Claims (5)

In the film forming apparatus, a vacuum is applied to a substrate made of a diboride single crystal represented by the chemical formula XB ₂ (where X includes at least one of Ti and Zr) and having a natural oxide film formed on the surface. A step of removing the natural oxide film and exposing an atomic step structure on the surface of the substrate by performing a heat treatment in the inside, and the surface of the substrate is not exposed to the atmosphere in the film forming apparatus. And a step of growing a gallium nitride compound semiconductor on the surface of the substrate in a state. The heat treatment is performed in a state where the periphery of the substrate is covered with a diboride sintered body represented by the chemical formula XB ₂ (wherein X includes at least one of Ti and Zr). The method for growing a gallium nitride-based compound semiconductor according to claim 1.   The method for growing a gallium nitride-based compound semiconductor according to claim 1, wherein the heat treatment is performed at a temperature of 1500 ° C. or more for 30 minutes or more.   4. The method for growing a gallium nitride-based compound semiconductor according to claim 1, wherein the heat treatment is performed at a vacuum degree of 100 Pa or less. A substrate made of a diboride single crystal represented by the chemical formula XB ₂ (wherein X includes at least one of Ti and Zr) and having a natural oxide film formed on the surface is subjected to heat treatment in vacuum. And applying the step to remove the natural oxide film and expose an atomic step structure on the surface of the substrate.

Images