Method for manufacturing crystalline gallium nitride thin film

Abstract

To provide a method for more efficiently manufacturing a GaN film via atomic layer deposition (ALD), wherein a monovalent gallium compound is used to manufacture a highly crystalline GaN film containing very few impurities without performing a high-temperature thermal treatment such as laser annealing.SOLUTION: The method for manufacturing a crystalline gallium nitride thin film via ALD comprises a step 1 in which a monovalent organic gallium complex is fed into a reaction chamber heated to a substrate temperature of 350°C or less, and a step 2 in which a nitriding gas is fed into the reaction chamber.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for forming a crystalline gallium nitride thin film by atomic layer deposition (ALD).

Background Art

[0002] Gallium nitride (GaN) is an important semiconductor material used to emit blue to blue-violet light in light-emitting diodes (LEDs) and blue lasers. Although it is highly desirable to grow a single-crystalline GaN layer on a silicon substrate, due to the mismatch in the lattice plane spacing between GaN and silicon, crystalline GaN has been epitaxially grown on a sapphire substrate with a lattice constant close to that of GaN. However, since single-crystalline sapphire substrates are generally significantly more costly than silicon substrates, a technique for growing a highly crystalline GaN film on a general-purpose and inexpensive substrate such as a silicon substrate is desired.

[0003] For the film formation of GaN, a chemical vapor deposition (CVD) method is generally used. In the atomic layer deposition (ALD) method, which is a type of CVD method, since the raw materials are not deposited continuously but layer by layer, an ultrathin film on the order of several nanometers can be deposited by a highly controlled method.

[0004] Therefore, a method for forming a crystalline GaN film using ALD has been studied. For example, a method has been reported in which hydrogen is used as a carrier gas, and trimethylgallium (TEG) and ammonia are used to form a single-crystalline GaN thin film on a sapphire substrate at 450 to 900°C (Non-Patent Document 1). Sapphire can withstand high temperatures of 450 to 900°C and is also suitable for epitaxially growing GaN in terms of its high heat resistance.

[0005] In Patent Document 1, as a method for forming a GaN-ALD film, a method is disclosed in which a GaN device layer is formed on a silicon substrate coated with an aluminum nitride (AlN) nucleation layer, and GaN is crystallized by laser annealing.

[0006] As a method of crystallization other than laser annealing, when introducing trimethylgallium (TMG), hydrogen radicals, and ammonia in this order and performing ALD at 100 °C, after introducing hydrogen radicals and after introducing ammonia, an electron beam is irradiated to desorb hydrogen, thereby forming a crystalline GaN film (Non-Patent Document 2). However, the GaN film obtained by this method contains as much as 10 to 35 at% of carbon impurities.

[0007] Also, as an example of crystallization using plasma, in Non-Patent Document 3, a crystalline thin film is formed at 200 °C using TMG and plasma of an ammonia plasma, a mixed gas of nitrogen and hydrogen (hereinafter referred to as "nitrogen / hydrogen plasma"), or a nitrogen plasma. However, in the said crystalline thin film, the N / Ga ratio is significantly N-rich, and the film quality is poor with a nitrogen plasma without hydrogen. In Non-Patent Document 4, a crystalline GaN thin film is formed at 200 °C, 285 °C, and 350 °C using TEG and nitrogen / hydrogen plasma. For the said crystalline thin film, in film formation on the Si(100) plane, there is an amorphous layer of about 18 nm from the interface with the silicon substrate. In any film, the N / Ga ratio is significantly N-rich. In Non-Patent Document 5, ALD is performed at 250 °C using TMG and a mixed plasma of nitrogen / hydrogen to form a crystalline film.

[0008] In Non-Patent Document 5, even when using plasma, ALD film formation cannot be performed at less than 210 °C, and the N / Ga ratio of the ALD film obtained at 250 °C is slightly Ga-rich but contains about 9% of C impurities. Crystallization using plasma is also exemplified in Patent Document 2. In Patent Document 2, a GaN thin film is formed using TMG or TEG and nitrogen / hydrogen plasma, and gallium halides such as GaCl 3 、GaCl、GaI 3 etc. are also exemplified.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

[0010] [Non-Patent Document 1] "Atomic layer epitaxy of GaN over sapphire using switched metalorganic chemical vapor deposition", M. Asif Khan et al., Appl. Phys. Lett. 60,1366 (1992) [Non-Patent Document 2] "Electron enhanced growth of crystalline gallium nitride thin films at room temperature and 100 °C using sequential surface reactions", Jaclyn K. Sprenger et al., Chem. Mater. 2016, 28, 15, 5282-5294 [Non-Patent Document 3] "Hollow cathode plasma-assisted atomic layer deposition of crystalline AlN, GaN and AlxGa1-xNthin films at low temperatures", Cagla Ozgit-Akgun et al., J. Mater. Chem. C, 2014, 2, 2123 [Non-Patent Document 4] "PEALD-deposited crystalline GaN films on Si (100) substrates with sharp interfaces", San-JieLiu et al., Chin. Phys. B Vol. 28, No. 2 (2019) 026801 [Non-Patent Document 5] "Low temperature depositions of GaN thin films by plasma-enhanced atomic layer deposition", Tang Wen-Hui et al., Acta Physica Sinica, 66, 098101 (2017)

Summary of the Invention

Problems to be Solved by the Invention

[0011] As Ga sources for forming GaN films by ALD method, TMG and TEG are widely used. However, since they are very unstable in air and pyrophoric, they are not easy to handle. Furthermore, in order to form a highly crystalline GaN thin film, ALD needs to be performed at a high temperature, or a high-temperature heat treatment needs to be combined with an annealing technique by laser or electron beam irradiation simultaneously or immediately after ALD.

[0012] The GaN film formed on the substrate is a monovalent gallium compound. Therefore, if a trivalent gallium complex such as TMG or TEG is used as a raw material, it is necessary to reduce gallium, which may cause contamination of C, and ultimately result in a GaN film with poor crystallinity. Also, even for a monovalent gallium complex, in the case of an inorganic gallium complex, there is a risk that inorganic elements may be incorporated into GaN. Therefore, an object of the present invention is to provide a method for manufacturing a GaN film by more efficient ALD, which can produce a GaN film with extremely few impurities and high crystallinity.

[0013] In addition, conventionally, in order to improve the crystallinity of GaN thin films, substrates with lattice constants close to GaN, such as GaN, sapphire, or AlN, have been used. However, an object of the present invention is also to obtain highly crystalline GaN even on a substrate that does not contain any of nitrogen, gallium, and aluminum, which are constituent elements of GaN or sapphire, as main components.

Means for Solving the Problems

[0014] The method for manufacturing a crystalline gallium nitride thin film of the present invention includes step 1 of supplying a monovalent organic gallium complex into a reaction chamber with a substrate temperature of 350°C or lower using atomic layer deposition (ALD), and step 2 of supplying a nitriding gas into the reaction chamber. The nitriding gas is preferably a nitrogen plasma gas. The organic gallium complex is preferably a cyclopentadienyl complex. It is preferable to further include step 3 of supplying a reducing gas containing no oxygen between step 1 and step 2. In a preferred embodiment of the present invention, the surface of the substrate does not contain any of nitrogen, gallium, and aluminum as a main component. Before step 1, it is preferable to include a step of depositing gallium oxide of 5 nm or less using the raw material and an oxidizing agent as a pretreatment of the substrate.

Advantages of the Invention

[0015] According to the ALD method of the present invention, a highly crystalline GaN film can be formed from an organic gallium complex without performing high-temperature heat treatment such as laser annealing.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0017] Hereinafter, the present invention will be described in detail. The method for manufacturing a gallium nitride (GaN) thin film of the present invention has step 1 of supplying a monovalent organic gallium complex into a reaction chamber with a substrate temperature of 350°C or lower using ALD, and step 2 of supplying a nitriding gas into the reaction chamber.

[0018] There are two types of ALD, namely thermal ALD and plasma ALD (PEALD). In thermal ALD, a uniform film can be formed along a high aspect ratio surface. On the other hand, although PEALD may be inferior to thermal ALD in terms of the film-forming ability on a high aspect ratio surface, it can be carried out at a low temperature. In the present invention, either method can be used, but from the object of the present invention of efficiently forming a highly crystalline GaN film using monovalent gallium chemical species, PEALD is preferable.

[0019] In PEALD, which is a preferred embodiment of the present invention, GaN is formed by the following deposition cycles (i) to (ii). One form of the deposition cycle is (i) a step of feeding a gaseous precursor into the reaction chamber in order to adsorb the precursor on the surface of the substrate, and (ii) a step of feeding a nitriding gas in which radical species are generated by plasma into the reaction chamber and reacting it with the precursor adsorbed on the surface to form a GaN crystal layer. Then, each cycle is repeated until the formed film reaches the desired thickness.

[0020] In the above (i), a monovalent organic gallium complex as a precursor is supplied in the gas phase into the reaction chamber in which the substrate is installed (Step 1). The substrate temperature is set to any temperature within the range of room temperature to 350 °C, and the monovalent organic gallium complex is evaporated at a temperature lower than the substrate temperature so as not to condense on the substrate.

[0021] Next, in the above (ii), a nitriding gas is supplied into the reaction chamber (Step 2), and the precursor adsorbed on the substrate surface in Step 1 is reacted with the nitriding gas to grow a crystalline GaN thin film on the substrate. At this time, the GaN thin film may be polycrystalline, but is preferably single-crystalline. In the case of a single crystal, the N / Ga ratio of the thin film is 1, but in the case of a polycrystal, a dense film is preferred, and an N / Ga ratio of 1 or less, which is Ga-rich, is preferred. In the GaN thin film, there is almost no amorphous portion, but less than 1% by volume is preferred, less than 0.01% by volume is more preferred, and less than 0.0001% by volume is particularly preferred. Further, the GaN thin film is a high-purity thin film, and carbon in the impurities is preferably 5 atomic% or less, more preferably 1 atomic% or less, still more preferably 0.01 atomic% or less, and particularly preferably 0.001 atomic% or less. Similarly, oxygen in the impurities is preferably 5 atomic% or less, more preferably 1 atomic% or less, still more preferably 0.01 atomic% or less, and particularly preferably 0.001 atomic% or less.

[0022] The nitriding gas supplied in Step 2 is a gas containing nitrogen, and a gas in which radical species are generated by plasma is preferred. The nitriding gas is not particularly limited as long as it can generate nitrogen radicals, but it is preferably carbon-free, more preferably ammonia / hydrogen plasma gas and nitrogen plasma gas, and particularly preferably nitrogen plasma gas that can be used simply. Note that "ammonia / hydrogen plasma gas" refers to a plasma of a mixed gas of ammonia and hydrogen.

[0023] Here, the precursor of the present invention is a monovalent organogallium complex. When the monovalent gallium complex is an inorganic complex such as gallium(I) chloride or gallium(I) bromide, there is a risk of contamination and corrosion, which is not preferable. For example, in the case of GaCl, there is a concern about contamination of Cl and corrosion of the substrate and the chamber due to by-products.

[0024] Examples of the monovalent organogallium complex include cyclopentadienyl complexes represented by the following general formula (1). In general formula (1), R 1 ~R 5 are each independently a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.

[0025] In the general formula (1), it is more preferable that four of R 1 ~R 5 are methyl groups, and the remaining one is a methyl group, an ethyl group, a normal propyl group or an isopropyl group. Specifically, the precursor represented by the general formula (1) is η represented by the following structural formula 5 -pentamethylcyclopentadienylgallium(I) (hereinafter also referred to as "Cp * Ga" or "Ga(C 5 (CH 3 ) 5 )"). It is particularly preferable that it is this. TIFF0007683912000002.tif32168

[0026] As the substrate for growing GaN, for example, a silicon substrate, a sapphire substrate, a silicon carbide substrate, a GaN substrate, etc. are used. Among these, it goes without saying that a GaN substrate with the same material is suitable for forming a highly crystalline GaN film, but a sapphire substrate is also suitable in that its lattice constant is close to that of GaN.

[0027] In the present invention, in order to form a highly crystalline GaN film, a substrate that does not contain any of nitrogen, gallium, and aluminum as a main component can be used, and as such a substrate, a silicon substrate is suitable. The silicon substrate may be naturally oxidized in the air and its surface may be covered with a very thin silicon dioxide film.

[0028] As a pretreatment for forming a GaN film on a substrate, it is also preferable to form a very thin gallium oxide thin film. The thickness of this gallium oxide thin film is preferably 5 nm or less, more preferably 2 nm or less, and particularly preferably 1.5 nm or less so as not to adversely affect the characteristics of the GaN film. As a method for forming the gallium oxide thin film, atomic layer deposition is preferable, and since film formation can be continuously performed, it is preferable to use the same raw materials as those of the present invention. As the oxidizing agent, any oxidizing agent such as water, oxygen, ozone, oxygen plasma, or a combination thereof can be used as long as it can form a gallium oxide thin film. This gallium oxide thin film may be amorphous or crystalline, but is preferably amorphous which is easy to form. Also, it may be 1 to 5 atomic layers or a single atomic layer.

[0029] The temperature when performing ALD needs to be lower than the temperature at which the monovalent organogallium complex adsorbed on the substrate thermally decomposes and be at a temperature sufficient to react sufficiently with the nitriding gas, and is preferably 50 to 350 °C, more preferably 150 to 250 °C. When using pentamethylcyclopentadienylgallium (Cp * Ga, GaC 5 (CH 3 ) 5 ) as the precursor, 200 °C at which Cp * Ga does not thermally decompose can be said to be a suitable temperature. Note that the substrate temperature and the reaction temperature are the same.

[0030] Between Step 1 and Step 2, a Step 3 of further supplying a reducing gas containing no oxygen may be included. The reducing gas has the effect of desorbing the counter ion of Ga or the cyclopentadienyl group from the monovalent organogallium complex adsorbed on the substrate. That is, the reducing gas is usually supplied to desorb the ligand after supplying the monovalent organogallium complex which is the precursor and adsorbing it on the substrate and before supplying the nitriding gas and reacting it.

[0031] As the reducing gas, ammonia and / or hydrogen is preferable, and a gas in which nitrogen and / or an inert gas (for example, argon) is introduced into these at an appropriate ratio may also be used. At this time, it is also preferable to generate radical species by the plasma of these gases.

[0032] In addition, after the above-described Step 1, Step 2, and Step 3, in order to purge unreacted raw materials and by-products from the reaction space, an inert gas such as nitrogen or argon is usually introduced.

[0033] In a preferred embodiment of the method for producing a crystalline gallium nitride thin film of the present invention, film formation is performed in a cycle of a precursor such as Cp * Ga, an ammonia / hydrogen plasma gas, and a nitrogen plasma gas. When the precursor is irradiated with the ammonia / hydrogen plasma gas, the precursor adsorbed on the substrate reacts with the ammonia / hydrogen plasma gas, and the ligand of the precursor is desorbed. Next, by irradiating with nitrogen plasma, H in the NH group or NH 2 group remaining in the film formed on the substrate is removed, and a crystalline GaN thin film is formed. In this nitridation reaction, while introducing a gas containing nitrogen of 0.1 to 1000 mTorr under vacuum, for example, nitrogen gas is excited, dissociated, and ionized using 400 W of power to generate plasma. At this time, the power is not limited in magnitude as long as plasma can be generated. Also, the power may be applied directly near the substrate to generate plasma, or may be applied at a slightly distant location to generate plasma.

[0034] As described above, according to the ALD method of the present invention, a highly crystalline GaN film can be formed from a monovalent organic gallium complex without performing high-temperature heat treatment such as laser annealing.

Example

[0035] Hereinafter, the present invention will be described more specifically based on examples, but the present invention is not limited by the following examples. [Example 1] (1) Preparation of Cp * Ga Cp* Ga(pentamethylcyclopentadienylgallium) was synthesized according to the method described in P. Jutzi et al., J. Organomet. Chem. 654, 176 (2002). As a result of differential scanning calorimetry (DSC), an exothermic peak due to decomposition was observed for Cp * Ga at 250 °C.

[0036] (2) Formation of GaN film by ALD A silicon wafer with a native oxide film was placed in an ALD apparatus (FlexAL; manufactured by Oxford Instruments Co., Ltd.), and ALD film formation was performed using Cp * Ga as a precursor, and ammonia / hydrogen plasma gas and nitrogen plasma gas as reducing gas and nitriding gas, respectively. At this time, the temperature for vaporizing Cp * Ga externally was set to 80 °C, and the substrate temperature was set to 200 °C. That is, in order to obtain a GaN film, ALD film formation was performed in the cycle of Cp * Ga → ammonia / hydrogen plasma → nitrogen plasma.

[0037] When the cross-section of the GaN film formed by this method for 300 cycles was observed by a transmission electron microscope (TEM), it was crystallized. This result (XTEM image) is shown in Fig. 1. In addition, as a result of compositional analysis of the GaN film portion of this sample using a high-resolution RBS analyzer (HRBS500; manufactured by Kobe Steel, Ltd.), C and O impurities were below the detection limit (C; about 4 atomic%, O; about 3 atomic%), and the N / Ga ratio was 0.9. Thus, according to the method of the present invention, a GaN film with extremely few impurities and high crystallinity can be manufactured.

[0038] [Example 2] (1) Preparation of Cp * Ga It was prepared in the same manner as in Example 1.

[0039] (2) Formation of GaN film by ALD A silicon wafer with a native oxide film was placed inside an ALD apparatus (FlexAL; manufactured by Oxford Instruments Co., Ltd.), and Cp was used as a precursor. * Using Cp * Ga as a precursor and water and oxygen plasma gas as oxidants in this order, a 1.1 nm-thick gallium oxide thin film was formed on the silicon wafer with a native oxide film. Next, Cp * ALD film formation was performed using Cp * Ga, ammonia / hydrogen plasma gas and nitrogen plasma gas as a reducing gas and a nitriding gas, respectively. At this time, the temperature at which Cp * Ga was vaporized externally was set to 40 °C, and Ar bubbling was performed. The substrate temperature at this time was set to 200 °C. That is, in order to obtain a GaN film, ALD film formation was performed in the cycle of Cp * Ga → ammonia / hydrogen plasma → nitrogen plasma. When the cross-section of the GaN film formed by 500 cycles by this method was observed with a transmission electron microscope (TEM), it was crystallized. This result (XTEM image) is shown in Fig. 2.

Claims

1. A method for manufacturing a crystalline gallium nitride thin film using atomic layer deposition (ALD) method, comprising: Step 1 of supplying a monovalent organic gallium complex into a reaction chamber with a substrate temperature of 350°C or lower; and Step 2 of supplying a nitriding gas into the reaction chamber, wherein the carbon impurity is 5 atomic% or less, the oxygen impurity is 5 atomic% or less, and the N / Ga ratio is 1 or less.

2. The method for manufacturing a crystalline gallium nitride thin film according to Claim 1, wherein the nitriding gas is a nitrogen plasma gas.

3. The method for manufacturing a crystalline gallium nitride thin film according to Claim 1 or 2, wherein the organic gallium complex is a cyclopentadienyl-based complex.

4. The method for manufacturing a crystalline gallium nitride thin film according to any one of Claims 1 to 3, further comprising Step 3 of supplying a reducing gas containing no oxygen between Step 1 and Step 2.

5. The method for manufacturing a crystalline gallium nitride thin film according to any one of Claims 1 to 4, wherein the surface of the substrate does not contain any of nitrogen, gallium, and aluminum as a main component.

6. The method for manufacturing a crystalline gallium nitride thin film according to any one of Claims 1 to 5, further comprising, before Step 1, as a pretreatment of the substrate, a step of depositing gallium oxide of 5 nm or less using the monovalent organic gallium complex and an oxidizing agent.

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