JP2006069814A - Aluminum nitride laminated substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum nitride laminated substrate having an aluminum nitride (AlN) film.
A single crystal α-alumina substrate such as sapphire is heated and nitrided in the presence of nitrogen, carbon and carbon monoxide to form a crystalline aluminum nitride film on the substrate, and simultaneously in the same process. A method for producing a crystalline aluminum nitride multilayer substrate, wherein pores are irregularly scattered at the interface between the α-alumina substrate and the crystalline aluminum nitride film layer, and the multilayer substrate.
[Selection figure] None

Description

  The present invention relates to an aluminum nitride laminated substrate having an aluminum nitride (AlN) film on the surface.

  A crystal layer made of a group III nitride semiconductor such as gallium nitride (GaN), aluminum nitride (AlN), or aluminum nitride gallium (AlGaN) is a light emitting diode (LED: Light Emitting) that emits short wavelength light in a blue band to an ultraviolet band. In recent years, it has attracted much attention as a functional layer constituting a light emitting device such as a diode or a laser diode (LD).

Group III nitride with a high melting point is a chemical vapor deposition method using molecular beam epitaxy (MBE), aluminum chloride and ammonia on a substrate such as α-alumina (Al ₂ O ₃ ) single crystal (hereinafter also referred to as sapphire). It is formed using a vapor phase growth means such as a method (HVPE) or a metal organic vapor phase growth method (MOVPE) using trimethylaluminum and ammonia.

  However, since the lattice mismatch between the substrate material and the group III nitride is large, there is a problem that it is extremely difficult to obtain a group III nitride thin film with few defects. The light emission efficiency of group III nitride semiconductor devices is largely determined by the initial crystal growth on the substrate, so the development of substrate materials with good consistency is the most important issue that will bring a major breakthrough in this field. ing.

  As one of means for solving the above problems, it is desirable to use an AlN crystal for a crystal material excellent in lattice matching with these group III nitride semiconductors, for example, an AlGaN crystal layer containing a large amount of Al. It is said that. Sublimation reprecipitation method in which AlN powder is sublimated into a gas phase in an ultrahigh temperature environment of about 2250 ° C. and then precipitated into a seed crystal as a means for obtaining an AlN freestanding crystal having a thickness of several tens to several hundreds μm that can be used as a substrate Is known (Non-Patent Document 1). Further, a sapphire, silicon carbide (SiC) single crystal, or silicon (Si) single crystal was used as a substrate (hereinafter also referred to as a base substrate), and an AlN thick film having a thickness of about 1 mm was formed on the substrate by HVPE. A technical means for removing the base substrate later to make an AlN free-standing crystal substrate is disclosed (Non-Patent Document 2).

  However, the AlN crystals obtained by the sublimation reprecipitation method are all small pieces of several mm to several tens of mm, and are not useful as a method for manufacturing LEDs and LDs in large quantities at low cost. In addition, since the base substrate of the AlN crystal formed on the sapphire substrate and the SiC single crystal substrate by the HVPE method is chemically stable and hard, the base substrate can be separated by chemical etching or dicing. Have difficulty. When an Si single crystal substrate is used, an AlN free-standing crystal substrate can be obtained relatively easily by chemical etching of the base substrate. However, there is a disadvantage that a large amount of substances with high environmental impact such as hydrazine and hydrofluoric acid are required. Have.

  The present invention has been made to overcome these problems of the prior art, and by using a sapphire substrate, the AlN crystal layer formed on the surface can be easily separated from the substrate, and a large-diameter AlN crystal free-standing substrate. It is intended to provide a laminated substrate that is suitably used for manufacturing a substrate at low cost.

Prior to the present invention, the present inventors formed a target thin film on a conventional sapphire substrate in the course of conducting a thermodynamic study of an AlN formation reaction using alumina, carbon, nitrogen and carbon monoxide as reaction materials. A good highly crystalline AlN film can be formed by converting the alumina component into aluminum oxynitride (alon) and AlN from the surface of the sapphire substrate to the inside by using this equilibrium reaction instead of forming an adhesion. Heading already proposed. (Patent Document 1)
G. A. Slack and T.W. F. McNelly, Journal of Crystal Growth, 1976, Volume 34, 263.

Andrew E Nikolaev and five others, MRS Internet of Nitride Semiconductor Research References, 2000, Volume 5S1, W6.5. .

JP 2004-137142 A

  The inventors of the present invention have further earnestly researched a method for converting the alumina component to the alon and AlN using the above equilibrium reaction, and as a result, an AlN layer having a completely new orientation relationship can be obtained by changing the cut surface of the sapphire substrate. By selecting the appropriate cut surface of the sapphire substrate, there is no alon layer, the AlN layer is directly stacked on the sapphire substrate, and there are pores between the sapphire substrate and the AlN layer. Succeeded in developing a new multilayer substrate.

  That is, the present invention is characterized in that an aluminum nitride film is laminated on a single crystal α-alumina substrate, and pores are scattered at the interface between the α-alumina substrate and the aluminum nitride film layer. It is an aluminum laminated substrate.

  The laminated substrate obtained by the present invention forms an AlN film having an orientation relationship corresponding to the cut surface of the sapphire substrate directly on the substrate by nitriding the sapphire substrate. In addition, since there are pores at the interface between the two layers, the separation of the two layers proceeds with a slight mechanical impact.

  A laminated substrate having this special structure is formed on an underlying substrate (sapphire substrate) after forming an AlN film having a sufficient strength as a free-standing film on the substrate by using a film forming method such as HVPE having a high epitaxial growth rate. ) Can be easily obtained as a pure AlN free-standing substrate. In addition, the AlN film obtained in the present invention has a structure in which a plurality of symmetric domains are mixed, and in the process of thickening the AlN film on the substrate by a method such as HVPE, each domain The dislocations from the nuclei collide and disappear, and as a result, an AlN free-standing substrate with few dislocations can be obtained.

  Hereinafter, the present invention will be described in detail.

  The present invention is greatly characterized in that the AlN film is formed by direct nitridation of a sapphire substrate. The direct nitridation is a method of nitriding the surface of the sapphire substrate by heat treatment using carbon, nitrogen and carbon monoxide as reaction raw materials.

  FIG. 1 shows an aluminum-oxygen-nitrogen-carbon chemical potential diagram. Nitriding proceeds directly by selecting temperature conditions and atmospheric composition corresponding to the AlN + C stable region in the figure.

  When the cut surface of the sapphire substrate is an A crystal plane: {1 1 −2 0} plane, the c-axis of AlN is substantially perpendicular to the cut surface (hereinafter referred to as the c-axis orientation of AlN), A structure without pores at the interface has been confirmed and has already been proposed.

On the other hand, a cut-out surface generated without the c-axis in the AlN film of the present invention being perpendicular to the cut-out surface, for example, an R crystal plane (hereinafter referred to as R plane): {1 −1 0 2} plane When the sapphire substrate is used as a raw material, crystals having different orientations form grain boundaries, and as a result, pores are generated at the AlN / Al ₂ O ₃ interface. As described above, the laminated substrate having pores of the present invention can be obtained by directly nitriding the R surface of sapphire. The crystal surface of sapphire is 2 ° in any direction from the R surface in addition to the R surface. The same can be obtained by using a surface inclined within the following range and a surface inclined within a range of 2 ° or less in an arbitrary direction from the {1 −1 0 3} plane.

  In order to explain the structure of the obtained laminated substrate, FIG. 2 shows a TEM cross-sectional bright field of an AlN film prepared by placing the R plane: (1 −1 0 2) plane of sapphire under the corresponding conditions.

  Specifically, in the image observed from the direction of the schematic diagram of FIG. 3, the horizontal direction of the image is the [1 1 −2 0] direction of sapphire. As the structure of the AlN film, as the electron diffraction image shows, there are mixed domains in which the c-axis of AlN grows with an inclination of 24 ° with respect to the normal of the raw material substrate surface. The size of each domain is about φ200 to 300 nm, and the part of the grain boundary generated between different domains is etched by heat, resulting in a step of about 20 nm.

  The thickness of the AlN film is not limited at all, and can be arbitrarily controlled by extending the reaction time.

  In the laminated substrate of the present invention, pores exist between the AlN film and the single crystal alumina substrate. The pores are cut so as to form a chemically stable surface such as an R plane or an n plane: {1 1 −2 3} plane of sapphire, and has a regular shape.

The depth of the pores and the ratio of the area where both layers of AlN / Al ₂ O ₃ are separated by the pores (hereinafter also referred to as pore area ratio) can be arbitrarily controlled by manipulating the reaction conditions. For example, when the reaction is performed at 1600 ° C., CO partial pressure 0.25 bar, N ₂ partial pressure 0.75 bar, and reaction time 12 hours, the pore depth is 80 nm and the pore area ratio is 50 to 60%. . The depth and area ratio of these pores increase as the CO partial pressure decreases, and in extreme cases, the film may be peeled off during the reaction and the laminated substrate of the present invention may not be obtained.

  Next, a method for forming an AlN film on a sapphire substrate will be specifically described.

In the present invention, the AlN film is formed directly on the sapphire substrate by directly nitriding the surface of the sapphire substrate. Specifically, a sapphire substrate whose surface crystal plane is, for example, an R plane, and graphite are inserted into the soaking part of the heat treatment apparatus, and the composition of the N ₂ —CO mixed gas is adjusted, so that the oxygen potential and nitrogen The substrate is nitrided in an atmosphere with a controlled potential.

  In the present invention, it is important to perform nitriding (reaction) in the AlN stable reaction condition region of the aluminum-oxygen-nitrogen-carbon chemical potential diagram shown in FIG.

The solid line in FIG. 1 is a phase stability diagram of Al ₂ O ₃ , alon, and AlN under conditions where the total pressure P _CO + P _N2 is 1 bar and the carbon activity a _c is 1. _PCO is the carbon monoxide partial pressure in the reaction system, and _PN2 is the nitrogen partial pressure. In the figure, the region where _PCO is large and the temperature is low is a stable region of Al ₂ O ₃ , and the region where _PCO is small and the temperature is high is a stable region of AlN. Above 1630 ° C., there is an alon stable region at the boundary between the two.

  A general nitriding reaction method will be described below.

  There is no restriction | limiting in particular in the heating apparatus used for this invention, The thing of arbitrary structures can be used. However, it must be capable of exposing the sapphire substrate to the temperature conditions shown in FIG. 1 in a mixed gas composed of nitrogen and carbon monoxide. Moreover, it is desirable that the temperature difference in the sapphire substrate can be kept within 5 ° C.

  The heating furnace material is preferably composed only of graphite, α-alumina, AlN and alon, which are substances involved in the reaction of the present invention. When a material other than graphite is used for the furnace material, since the atmosphere to be introduced is reducing, measures are taken to minimize release of oxygen and metal vapor within the reaction conditions of the present invention. For example, in the case of an α-alumina furnace material, it is effective to place the heating part in the AlN + C stable region of FIG.

  As an α-alumina substrate used in the present invention, specifically, a sapphire substrate, the surface thereof is preferably smooth in order to obtain AlN having good quality and controlled orientation. For this reason, a general sapphire substrate for epitaxial growth is preferably used. When the AlN film having the characteristics of the present invention is formed on the surface of the substrate, an R plane or {1 -1 0 3} plane is used as the crystal plane of the substrate, and the R plane is particularly preferably used. .

  As the carbon, various commercial products can be used. The purity of carbon is preferably 99.9% or more, and more preferably 99.999% or more. The amount of carbon used is preferably 0.1 or more by weight with respect to α-alumina in the reaction system.

  Nitrogen and carbon monoxide are usually in a gaseous state, but 99.9999% or more of nitrogen and 99.9% or more of carbon monoxide are preferable.

The total pressure of the reaction system is not particularly limited, but is preferably about 1 bar from the viewpoint of ease of production and operation of the reaction apparatus. During the reaction, a mixed gas having a predetermined partial pressure is supplied at a predetermined flow rate. To maintain the smoothness of the resulting AlN film surface well, if the total pressure of the reaction system was 1 bar, _{P CO} boundary line of AlN stable area in FIG. 1 0~0.15bar low conditions are preferred. For example, since the boundary of the AlN stable region at 1600 ° C. is P _CO = 0.31 bar, the condition where P _CO is 0.16 to 0.31 bar is suitable. The P _CO is 0.15bar or less condition than the boundary of the AlN stable area, the reaction is excessive pores in the interface to proceed rapidly tends to occur delamination in the reaction occurred. Further, defects tend to enter the AlN film and the surface shape and crystallinity tend to be deteriorated.

Since the flow rate of the mixed gas needs to always allow nitrogen atoms to reach the substrate surface, a gas at 25 ° C. and 1 atm is 5 mL / min with respect to a cross-sectional area of 1 cm ² of the reactor in a plane perpendicular to the gas flow. It is preferable to introduce the above. More preferably, an apparatus for preheating the gas introduced before reaching the sapphire substrate is installed. A commercially available flow meter can be used without limitation to control the partial pressure of carbon monoxide and nitrogen to be introduced.

Although the rate of temperature increase can be arbitrarily determined, 5 ° C. or more per minute is preferably employed. The heating time is appropriately determined depending on the desired film thickness. For example, when an R-plane sapphire substrate is nitrided at 1600 ° C. under conditions of P _CO = 0.25 bar and P _N2 = 0.75 bar, the growth rate of the AlN film is 100 to 120 nm for a reaction of 12 hours, The depth is 80-100 nm with a 12 hour reaction.

  In the present invention, nitriding must be performed under specific reaction conditions. In order to perform the nitriding treatment within the reaction conditions, it is necessary to always strictly control the reaction temperature, the state of the heating furnace material, and the partial pressures of carbon monoxide and nitrogen to be introduced.

  The reaction temperature is measured as close to the sapphire substrate as possible. The measuring equipment must be free of impurities in the reaction system. In the present invention, a method of contacting a B thermocouple or a W-5% Re / W-26% Re thermocouple protected by a closed tube of an α-alumina sintered body with a sapphire substrate support, A method of measuring the infrared radiation of the graphite with a radiation thermometer is suitable.

  After completion of the reaction, the supply of carbon monoxide is usually stopped, and the supply of only nitrogen is continued to start cooling.

  By the method described above, an AlN film and pores can be formed on the sapphire substrate in the same process.

  When manufacturing conditions are selected so as to maintain the smoothness of the surface, as a value calculated based on JIS B0601-1994 for height information in the range of 20 μm × 20 μm with a scanning probe microscope (SPM), Ra is 15 nm or less, An AlN film of 20 nm or less in Rms can be obtained. At this time, the crystal orientation represented by the half width of the ω mode rocking curve of the AlN film is 2.0 ° or less on the AlN {0 0 0 2} plane called the tilt direction, and AlN {1 0 −1 called the twist direction. In the 0} plane, it is 1.5 ° or less. Note that the FWHM of the ω mode rocking curve is a diffraction chart obtained by changing the ω by fixing the sum of the X-ray incident angle (ω) and the reflection angle at an angle satisfying the Bragg diffraction condition. The range of ω that represents 50% or more of the maximum value of the X-ray count number, the larger the value, the more disturbed the direction of the crystal and the worse the crystallinity.

Example 1
By nitriding the sapphire substrate cut out on the R plane with a N ₂ —CO mixed gas at 1600 ° C. using a graphite heating element and a support system comprising a graphite heating element and a support base, AlN / Al ₂ An AlN film having pores at the O ₃ interface was produced.

  The heating element was installed in a horizontal shape in a cylindrical shape, and a graphite block on which a sapphire substrate was placed was placed in the center of a support base having an arc-shaped bottom matching the inner diameter of the cylinder. Infrared light emitted from the graphite block during heating was measured with a radiation thermometer to control the temperature of the raw material substrate.

First, the inside of the furnace was once evacuated by a rotary pump, and replaced with a mixed gas having a carbon monoxide (CO) partial pressure of 0.25 bar and a nitrogen (N ₂ ) partial pressure of 0.75 bar. The composition atmosphere was flowed at a constant flow rate (2 L / min). The total pressure in the system is 1 bar. It heated at 1 degreeC / min to 1600 degreeC, and after reaching | attaining 1600 degreeC, it hold | maintained for 12 hours. After 12 hours, carbon monoxide in the flowing atmosphere was stopped, and the heater was turned off to cool naturally. The cooling rate at this time was 30 minutes from 1600 ° C. to 1100 ° C.

  4 (A) and 4 (B) show the growth of the AlN film and pores on the sapphire substrate cut out on the R-plane: (1 −1 0 2) plane and the R-plane sapphire substrate before reaction as described above. FIG. 5 is a pole figure showing which direction the {0 0 0 2} plane of AlN is facing in an XRD (X-ray diffractometer) using a Cu tube as an X-ray source. Here, the R plane of sapphire was set to ψ = 0 °, and the electron beam incident direction of the TEM cross-sectional image of FIG. 2 was set to φ = 0 °. At this time, crystallographically, the electron beam incident direction and the sapphire <1 1 -2 0> intersect perpendicularly, and the directions of φ = 90 ° and 270 ° are the sapphire [-1 -1 2 0] direction and [1 1 1 -2 0] direction.

  As a result of analyzing the sapphire substrate before the reaction under the same conditions, as shown in FIG. 4A, the {1 0 -1 4} plane of sapphire and the {1 0 -1 4} plane and the {1 Diffraction from the 1 −2 0} plane was detected.

  From FIG. 4 (B) showing the substrate analysis results after the reaction, the AlN {0 0 0 0 1} plane is tilted by the tilt angle ψ It can be seen that the center of the peak is at a position where each is about 25 °, and the c-axis of AlN grows only in these two directions. FIG. 5 shows the result of a ψ scan in the [−1 −1 2 0] direction, and it was found that the tilt angle was 24 °. That is, the following crystal orientation relationship is established.

それぞれのドメインについて、

For each domain,

図６にＡｌＮ膜の結晶性を評価するために測定したωモードロッキングカーブのグラフ図を示す。測定した面はＡｌＮ｛０ ０ ０ ２｝面で、前述の情報を基にφ＝９０°、ψ＝２４°の方向に合わせて実施した。ＡｌＮの半価幅は１．１°であった。

FIG. 6 shows a graph of the ω mode rocking curve measured to evaluate the crystallinity of the AlN film. The measured surface was an AlN {0 0 0 2} surface, which was implemented in the direction of φ = 90 ° and ψ = 24 ° based on the above-mentioned information. The half width of AlN was 1.1 °.

FIG. 7 shows a pole figure of the {1 0 −1 1} plane of the generated AlN film. Here, φ = 0 ^o of the pole figure is the TEM electron beam incident direction of FIG. From FIG. 7, it can be seen that the {1 0 −1 1} plane of AlN shows six diffraction peaks centering on the AlNc axis orientation direction clarified in FIG. 4 as indicated by the dotted line. This indicates that the mixed two-direction AlN-oriented crystals each have only one three-dimensional orientation.

  A cross-sectional TEM image of the AlN film and pores obtained in this example is shown in FIG. In the AlN film, there are periodically thin portions at intervals of 200 to 400 nm, and the contrast of the crystal phase changes at the portions. When electron beam diffraction images of adjacent crystal phases with different contrasts were taken, AlN diffraction patterns with different c-axis orientation directions were obtained according to the information obtained by XRD. That is, AlN crystal phases having different orientation directions form a mosaic structure with a size of 200 to 400 nm. Irregular pores are generated under the AlN film in both width and density. However, the pore depth and bottom shape are very regular. Judging from the direction of observation, the bottom surface of the pores was composed of the (1 −1 0 2) plane, (1 1 −2 3) plane of sapphire, and the like.

  From the height information on the surface of the AlN film obtained in this example, Ra = 13.3 nm and RMS = 17.3 nm were obtained for the surface roughness in the range of 20 × 20 μm.

Example 2
An AlN film having pores at the AlN / Al ₂ O ₃ interface by nitriding a sapphire substrate cut out on the R plane using the same apparatus as in Example 1 and changing the temperature and CO partial pressure to the following conditions: Was made.

The atmosphere in the furnace is a mixed gas having a carbon monoxide (CO) partial pressure of 0.41 bar and a nitrogen (N ₂ ) partial pressure of 0.59 bar, and the atmosphere of the same composition flows at a constant flow rate (2 L / min). did. The total pressure in the system is 1 bar. Heated to 1075 ° C. at 10 ° C./min, and held for 12 hours after reaching 1675 ° C.

It was found by X-ray diffraction analysis that AlN was produced in the obtained reacted substrate with the same orientation relationship as in Example 1. From the TEM cross-sectional image, it was found that the ratio of the area where both layers of AlN / Al ₂ O ₃ were separated by pores was approximately 40%. Moreover, the alon layer was not confirmed (FIG. 8).

Comparative Example 1
By nitriding a sapphire substrate cut out on the A crystal plane: {1 1 −2 0} plane under the same apparatus and manufacturing conditions as in Example 1, an AlN film having no pores at the AlN / Al ₂ O ₃ interface is obtained. Produced.

  FIG. 9 shows a cross-sectional TEM image of the AlN film and pores obtained in this example. The entire surface of the sapphire substrate is covered with an AlN film having a thickness of 5 to 20 nm. However, the contrast considered to be a grain boundary was not confirmed in the AlN film, and no pores were observed at the sapphire / AlN film interface.

An aluminum-oxygen-nitrogen-carbon chemical potential diagram is shown. The TEM cross-section bright field of the AlN film | membrane produced by nitriding the sapphire substrate cut out by R surface is shown. The schematic diagram which showed the observation direction of the visual field of FIG. (A) Sapphire substrate cut out by R plane (B) XRD pole figure about the AlN {0 0 0 2} plane of the invention which nitrided the sapphire substrate cut out by R plane is shown. 2 shows a ψ scan chart showing the inclination angle of the AlNc axis with respect to the substrate surface in Example 1. The omega mode rocking curve about AlN {0 0 0 2} surface of Example 1 is shown. The pole figure about the AlN {1 0 -1 1} surface of Example 1 is shown. The TEM cross-sectional image of the board | substrate obtained in Example 2 is shown. The TEM cross-sectional image of the board | substrate obtained by the comparative example 1 is shown.

Claims (4)

An aluminum nitride laminated substrate, wherein an aluminum nitride film is laminated on a single crystal α-alumina substrate, and pores are scattered at the interface between the α-alumina substrate and the aluminum nitride film layer. 2. The aluminum nitride laminated substrate according to claim 1, wherein the crystal plane of the single crystal α-alumina substrate is an R plane. 3. The nitriding according to claim 1, wherein the R crystal face of the single crystal α-alumina is nitrided in the presence of carbon, nitrogen and carbon monoxide to form an aluminum nitride film on the R crystal face. A method for producing an aluminum laminated substrate. A method for producing an aluminum nitride film, comprising nitriding an R crystal plane of single crystal α-alumina in the presence of carbon, nitrogen and carbon monoxide to form an aluminum nitride film on the R crystal plane.

Images