JP4776460B2 - Method for forming AlN single crystal film - Google Patents

Description

  The present invention relates to a technique for forming an AlN single crystal film used as a base for group III nitride crystal growth on a sapphire substrate.

It is well known that an AlN single crystal is suitable as a substrate for growing a group III nitride single crystal film. Note that the use of an AlN single crystal as a base mainly has the following two advantages. First, since AlN has a wide band gap of 6.2 eV, it is transparent to light having a wavelength in the ultraviolet region of 200 nm. Second, since Al _1-xy In _x Ga _y N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1) has the smallest lattice constant among the substances represented by the composition formula, _{When a 1-xy} In _x Ga _y N film is formed, the tensile stress can be suppressed and the occurrence of cracks can be suppressed.

  As a substrate for growing a group III nitride single crystal thin film, a so-called epitaxial substrate (template substrate) in which an AlN layer is heteroepitaxially grown on a different base material such as sapphire is generally used. This is because it is difficult to apply the single crystal growth technique using the melt growth method to the production of an AlN single crystal. In the case of an epitaxial substrate, since the lattice mismatch between the base material serving as a base and AlN is large, various processes have been devised in order to epitaxially grow a high-quality AlN crystal layer on the base material.

  On the other hand, as a method of forming an AlN crystal layer having excellent crystal quality, a method of forming an AlN single crystal layer by heating a sapphire substrate in a predetermined atmosphere has already been proposed (for example, Patent Document 1, Patent). Reference 2 and Non-Patent Document 1).

JP 2004-137142 A JP 2005-104829 A Hiroyuki Fukuyama et al., "Preparation of single crystal aluminum nitride thin film by sapphire nitriding method and its evaluation" Proceedings of the 53rd Joint Conference on Applied Physics p.360

In Patent Document 1 and Patent Document 2, a single crystal sapphire substrate is heated at a high temperature in a N ₂ —CO mixed gas in a reaction tube in which graphite is installed, whereby an aluminum oxynitride layer ( A technique for simultaneously forming an (ALON layer) and an AlN layer is disclosed.

Non-Patent Document 1 reports a case where an AlN layer was grown directly on a sapphire substrate by heating the single crystal sapphire substrate in a N ₂ —CO mixed gas in a graphite reaction tube. Yes.

  However, any of the methods disclosed in these documents is not preferable from the viewpoint of process safety in that a toxic gas called CO gas is used. In addition, when CO gas is not mixed, it has been confirmed by the inventors of the present application that the sapphire surface is excessively nitrided and the surface flatness is greatly deteriorated.

  Moreover, in the AlN layer formed on C-plane sapphire by the methods disclosed in these documents, the uniaxial orientation as in the epitaxial film is not realized.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method of forming an AlN single crystal film suitable as a base for group III nitride crystal growth on a sapphire substrate.

  In order to solve the above-mentioned problems, the invention of claim 1 is a method of forming an AlN single crystal film on a sapphire substrate, comprising a nitrogen element-containing gas having nitriding ability for sapphire and an oxidizing oxygen element-containing gas. An AlN single crystal film is formed on the main surface of the substrate by heating a substrate made of single crystal sapphire at a heating temperature of 1300 ° C. or higher under a gas atmosphere.

  A second aspect of the present invention is the method for forming an AlN single crystal film according to the first aspect, wherein the nitrogen element-containing gas is nitrogen.

  Invention of Claim 3 is a formation method of the AlN single-crystal film | membrane of Claim 1 or Claim 2, Comprising: The said oxygen element containing gas is given in the said gas atmosphere by heating an oxide. It is characterized by.

  A fourth aspect of the present invention is the method for forming an AlN single crystal film according to the third aspect, wherein the oxide is an Al oxide.

  The invention according to claim 5 is the method for forming an AlN single crystal film according to claim 1 or 2, wherein the oxygen element-containing gas is a water vapor gas.

  A sixth aspect of the present invention is the method for forming an AlN single crystal film according to any one of the first to fifth aspects, wherein the main surface of the sapphire substrate is a C plane.

  A seventh aspect of the present invention is the method for forming an AlN single crystal film according to any one of the first to sixth aspects, wherein the heating of the substrate is determined according to the gas atmosphere and the heating temperature. The AlN single crystal film is formed within a time range in which the formed film thickness is 0.1 μm or less.

  According to the first to seventh aspects of the present invention, an AlN single crystal film suitable as a substrate for growing a group III nitride crystal can be formed without using a toxic gas such as CO gas.

  In particular, according to the invention of claim 2, by using a nitrogen gas having an appropriate nitriding action, controllability of the nitriding treatment can be improved as compared with the case of using other gases.

  In particular, according to the invention of claim 3 and claim 4, it is possible to add a small amount of the oxygen element-containing gas as compared with the case where the oxidizing oxygen element-containing gas is directly mixed with the nitrogen element-containing gas. It becomes possible. In addition, the amount of the oxygen element-containing gas added can be defined by the vapor pressure in the heat treatment furnace.

  In particular, according to the invention of claim 4, it is possible to realize a nitrogen element gas-containing atmosphere to which an oxygen element-containing gas is added without generating impurities.

  In particular, according to the invention of claim 5, it is possible to perform the treatment more safely by using the water vapor gas which is not toxic and easy to handle.

  In particular, according to the invention of claim 6, it becomes easy to ensure the surface flatness of the AlN single crystal film.

  In particular, according to the invention of claim 7, it is possible to form an AlN single crystal film having no deterioration in surface flatness, no peeling, and good crystal quality.

<Nitriding treatment>
In the method for forming an AlN single crystal film according to the present embodiment, a single crystal sapphire substrate (hereinafter also simply referred to as a sapphire substrate) is heated in a predetermined processing apparatus under a gas atmosphere containing a nitrogen element-containing gas. In this way, a reaction between the surface layer portion of the sapphire substrate and the nitrogen element-containing gas is caused to generate an AlN single crystal film on the sapphire substrate. That is, this is a method of generating an AlN single crystal film by so-called nitriding treatment in which AlN that is an Al nitride is generated by a reaction between sapphire that is an Al oxide and a nitrogen element-containing gas.

  Specifically, the sapphire substrate is at least 1300 ° C. or higher in a mixed gas atmosphere in which a nitrogen element-containing gas having nitriding ability (nitriding action) for sapphire is a main component and an oxidizing oxygen element-containing gas is added thereto. In addition, preferably, AlN which is a nitride is formed on the main surface of the sapphire substrate by heating to 1500 ° C. or higher. Note that the oxidizing oxygen element-containing gas does not include reducing CO gas. That is, the method for forming an AlN single crystal film according to the present embodiment is a method for forming an AlN single crystal film without using toxic CO gas. In the following description, the oxidizing oxygen element-containing gas may be simply referred to as an oxygen element-containing gas. However, this description is for the sake of simplicity of description and includes a reducing oxygen element-containing gas. Not the purpose.

  The reason why the temperature of the heat treatment is set to at least 1300 ° C. or higher in the nitriding treatment is that the reaction in AlN occurs due to nitriding of the sapphire substrate by heating in such a temperature range. Further, when the heating temperature is set to 1500 ° C. or higher, an effect of reducing dislocations in the formed AlN single crystal film can be obtained. However, the heating temperature needs to be lower than the melting temperature of sapphire. Preferably, it is 1700 ° C. or lower. This is because the formation of AlN polycrystals is suppressed.

  The reason why the oxygen element-containing gas is added to the atmospheric gas for performing the nitriding treatment is to form an AlN single crystal film with good surface flatness by appropriately suppressing the generation rate of AlN. In an atmosphere containing only nitrogen element-containing gas, it has been confirmed that the generation rate of AlN is too high, and it may be difficult to control the surface shape of the AlN single crystal film. As the nitrogen element-containing gas, for example, nitrogen gas or ammonia gas can be used. In particular, it is preferable to use high purity nitrogen gas. This is because the nitrogen gas has an appropriate nitriding action, so that the controllability of the nitriding treatment is enhanced as compared with the case where other gases are used.

The oxygen element-containing gas can be added by adding moisture (water vapor gas), NO _x gas, or O ₂ gas to the nitrogen element-containing gas. For example, such addition is realized by supplying a mixed gas obtained by mixing these additive components to the nitrogen element-containing gas to a treatment apparatus (heat treatment furnace) for nitriding treatment. However, when a carbon member is used in the heat treatment furnace, it should be noted that mixing of O ₂ gas has a high possibility of causing deterioration of the member depending on the amount of addition.

Alternatively, in the case of adding an oxygen-containing gas, as described later, in the heat treatment furnace used for the nitriding treatment, for example, the oxide crystal is placed on the sapphire substrate in the vicinity or directly below or directly above, Alternatively, the furnace body and furnace material of the heat treatment furnace are formed of oxide crystals, and when the sapphire substrate is heated, the oxide crystals are thermally decomposed to release the oxygen element-containing gas into the heat treatment furnace. You may do it. For example, it is a preferable embodiment that a sheath crucible for holding a sapphire substrate is formed of an oxide crystal. Also in this case, as a result, an atmosphere in which the oxygen element-containing gas is added to the heat treatment furnace can be realized. Further, in such a case, a small amount of addition can be realized as compared with the case where O ₂ gas is directly mixed, and there is an advantage that the addition amount can be defined by the vapor pressure in the heat treatment furnace. The oxide crystal is preferably an oxide of Al. This is because the generation of impurities during heating can be suppressed.

  From the viewpoint of safety, it is most preferable to add water which is not toxic and can be easily performed. However, even in the case of moisture gas addition, there is a possibility of reaction with the carbon member. Therefore, if necessary, the gas flow is controlled by supplying a barrier gas (not shown) or blowing a gas to the substrate. It is more desirable to optimize.

  Preferably, a sapphire substrate having a C-plane main surface (hereinafter also referred to as a C-plane sapphire substrate) is used. This is because it is easy to ensure the surface flatness of the formed AlN single crystal film.

  The formation of the AlN single crystal film by the method according to the present embodiment is desirably performed in a range where the film thickness is 0.1 μm or less. Specifically, it is desirable to heat the sapphire substrate within a time range in which the film thickness is 0.1 μm or less under a certain gas atmosphere and the heating temperature. This is because if the film thickness exceeds 0.1 μm, the surface flatness of the AlN single crystal film deteriorates and the film itself may be peeled off. In addition, if an AlN single crystal film having a good crystal quality is formed with a thickness of 0.1 μm or less, that is, about several tens of nm, a film suitable as a base for the growth of a group III nitride crystal is formed. Therefore, the formation of an AlN single crystal film having a thickness larger than that is not essential for the purpose of forming a good base.

  For example, according to the forming method according to the present embodiment, an AlN single crystal having a thickness of about 20 nm and a surface roughness (ra) evaluated by AFM of about 1 nm, which is sufficient as a base for growing a group III nitride crystal. The film can be formed on the main surface of the sapphire substrate.

<Configuration of nitriding apparatus>
FIG. 1 is a diagram illustrating a nitriding apparatus 100 used in the method for forming an AlN single crystal film according to this embodiment. The nitriding apparatus 100 is a so-called heat treatment furnace, and includes an oxide sheath 102 inside a carbon furnace body 101, and one or a plurality of single crystal substrates 1 are placed inside the sheath 102 by a jig (not shown). It can be held (FIG. 1 shows an example in which four single crystal substrates 1 are held). Alumina, sapphire, or the like is preferable as the oxide used as the material of the sheath 102. This is because the AlN single crystal does not contain an element that becomes an impurity other than oxygen. Alternatively, an embodiment using MgO, BeO, CaO, SiO ₂ or the like may be used.

  In the nitriding apparatus 100, nitrogen gas supplied from the nitrogen gas supply source 103 is supplied into the furnace body 101 through the supply pipe 104. The sheath 102 is provided with openings 105 and 106 so that nitrogen gas supplied through the supply pipe 104 also enters and exits the sheath 102. Further, the furnace body 101 is provided with an exhaust port 107. The interior of the furnace body 101 is heated by a heating means (not shown). For example, a resistance heating method, an RF heating method, a lamp heating method, or the like can be used.

  By heating the inside of the furnace body 101 with a heating means (not shown) while supplying nitrogen gas from the nitrogen gas supply source 103, the oxide forming the sheath 102 is decomposed inside the sheath 102, thereby the oxygen element. Containing gas is generated. Thereby, a nitrogen gas atmosphere to which the oxygen element-containing gas is added is formed. That is, the sapphire substrate 10 is exposed to such an atmosphere. The sapphire substrate 10 is nitrided from the main surface side by heating at a heating temperature of 1300 ° C. or higher while appropriately adjusting the supply flow rate of nitrogen gas, the pressure inside the furnace body 101, the vapor pressure of the oxygen element-containing gas, and the like. Thus, an AlN single crystal film is formed.

  As described above, according to the method of forming an AlN single crystal film according to the present embodiment, it is suitable as a growth base for a group III nitride crystal without using a toxic gas such as CO gas. An AlN single crystal film can be formed.

Example 1
A C-plane sapphire substrate is held inside an alumina sheath 1202 of the nitriding apparatus 100 as shown in FIG. 1, and nitrogen gas is supplied from a nitrogen gas supply source 103, and a heating temperature of 1650 ° C. for 20 hours, The C-plane sapphire substrate was heated. A C-plane AlN single crystal having a thickness of 20 nm and a surface roughness (ra) of about 1 nm was formed on the surface of the heated C-plane sapphire substrate. That is, it was confirmed that an AlN single crystal film suitable as a base for growth of a group III nitride crystal can be formed.

(Comparative Example 1)
The treatment was performed in the same manner as in Example 1 except that a SiC sheath was used instead of the alumina sheath. On the surface of the heated C-plane sapphire substrate, a granular AlN crystal having an average thickness of 200 nm and a surface roughness (ra) of about 20 nm mixed with components having different crystal orientations was formed.

(Contrast between Example 1 and Comparative Example 1)
Unlike Example 1, in Comparative Example 1 using a non-oxide SiC sheath, an AlN single crystal film was not formed, and therefore an oxide furnace material was used as in Example 1. This can be said to be effective in forming the AlN single crystal film on the sapphire substrate.

It is a figure which illustrates the nitriding apparatus 100 used for the formation method of an AlN single crystal film.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Single crystal base material 10 Sapphire substrate 100 Nitriding processing apparatus 101 Furnace body 102 Sheath 103 Nitrogen gas supply source 104 Supply pipe 105, 106 (Sheath) opening 107 Exhaust port

Claims (7)

A method of forming an AlN single crystal film on a sapphire substrate,
By heating a substrate made of single crystal sapphire at a heating temperature of 1300 ° C. or higher under a gas atmosphere containing a nitrogen element-containing gas having nitriding ability for sapphire and an oxidizing oxygen element-containing gas, the main surface of the substrate An AlN single crystal film is formed thereon,
A method for forming an AlN single crystal film. A method for forming an AlN single crystal film according to claim 1,
The nitrogen element-containing gas is nitrogen;
A method for forming an AlN single crystal film. A method for forming an AlN single crystal film according to claim 1 or 2,
Providing the oxygen element-containing gas in the gas atmosphere by heating an oxide;
A method for forming an AlN single crystal film. A method for forming an AlN single crystal film according to claim 3,
The oxide is an Al oxide;
A method for forming an AlN single crystal film. A method for forming an AlN single crystal film according to claim 1 or 2,
The oxygen element-containing gas is water vapor gas;
A method for forming an AlN single crystal film. A method for forming an AlN single crystal film according to any one of claims 1 to 5,
The main surface of the sapphire substrate is a C-plane;
A method for forming an AlN single crystal film. A method for forming an AlN single crystal film according to any one of claims 1 to 6,
The heating of the substrate is performed within a time range in which the formation thickness of the AlN single crystal film is 0.1 μm or less, which is determined according to the gas atmosphere and the heating temperature.
A method for forming an AlN single crystal film.

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