JP4408509B2 - Method for forming group III nitride thin film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a group III nitride thin film, and more particularly to a method for forming a group III nitride thin film (polarity: (0001)) by molecular beam epitaxy (MBE).
[0002]
[Prior art]
When GaN is grown on a hexagonal sapphire C-plane substrate, hexagonal wurtzite GaN is usually grown with the c-axis aligned with the sapphire substrate. However, since wurtzite GaN has polarity in the c-axis direction, normally, as shown in FIGS. 1 (A) and (B), a crystal having two types of polarities, that is, two types of atomic arrangements, is used. It becomes a mixed film. In this way, in the GaN film grown on the sapphire C-plane substrate 1, the case where N atoms are arranged immediately above Ga atoms as shown in FIG. 1A is GaN (0001) (Ga-face), FIG. The case where Ga atoms are arranged immediately above N atoms as shown in (B) is called GaN (000-1) (N-face). These two types of polarities and the characteristics of the grown film are closely related, and the GaN (0001) film is more GaN (000-1) film, or GaN (0001) and GaN (000-1). (Keller et al., Appl. Phys. Lett. 68 (1996) 1525, Fuke) et al., J. Appl. Phys. 83 (1998) 764). Therefore, how to control the polarity of the grown film to Ga-face is an important key point in producing a high-quality group III nitride semiconductor device.
[0003]
In the metal organic chemical vapor deposition (MOCVD) method, the polarity of the film to be grown is controlled by controlling the supply timing of the metal organic gas and the annealing conditions of the low temperature buffer layer grown on the sapphire substrate to increase the nuclear density. Can be controlled to Ga-face. On the other hand, in the molecular beam epitaxy (MBE) method, only a film in which GaN (000-1) is dominant has been obtained so far, and it has been impossible to obtain a GaN (0001) film.
[0004]
[Problems to be solved by the invention]
The present invention solves the problems in forming a GaN-based group III nitride thin film in the conventional MBE method as described above. The polarity of the growing film is controlled to (0001), so that It is an object to provide a method for forming a GaN-based group III nitride thin film having excellent optical and electrical characteristics.
[0005]
[Means for Solving the Problems]
As a result of intensive studies to solve the problems of the prior art, the present inventors succeeded in controlling the polarity of the growing film to (0001) by using the following means: The present invention has been completed.
[0006]
(1) To epitaxially grow a GaN single crystal thin film by MBE using nitrogen plasma as a nitrogen source and metal Ga as a Ga source on a sapphire C-plane substrate, the substrate is first cleaned by heating at about 800 ° C. . Next, a GaN layer having a thickness of about 200 Å is deposited at a low temperature of about 500 ° C. and then annealed to form a low-temperature buffer layer that promotes GaN nucleation. A method of growing a GaN layer on the low-temperature buffer layer formed in this way at a growth temperature of 600 ° C. to 800 ° C. is usually used. In addition, for the purpose of further increasing the nuclear density, before forming the low-temperature buffer layer, there is a method of irradiating the sapphire substrate with nitrogen plasma to nitride the sapphire substrate and forming an AlN layer having a lattice constant close to that of GaN on the substrate surface. Sometimes adopted. When GaN is grown by the above method, the obtained GaN film becomes a film in which GaN (0001) and GaN (000-1) are mixed. Further, when the nitriding process of the sapphire substrate was introduced, only the mixture ratio of GaN (000-1) increased, and a film of only GaN (0001) could not be obtained.
[0007]
(2) As described in (1) above, before forming the low-temperature buffer layer, the sapphire substrate is irradiated with nitrogen plasma to nitride the sapphire substrate to form an AlN layer having a lattice constant close to that of GaN. It has been found that the mixing ratio of GaN (000-1) increases. This is because when the sapphire substrate is exposed to nitrogen plasma, an AlN (000-1) layer is formed on the surface of the substrate, and the GaN film grown on the AlN layer is a mixture of (000-1). This is because the film has a high rate. In order to prevent the substrate from being exposed to nitrogen plasma at the time of forming the low temperature buffer layer after the substrate cleaning process, the present invention provides a metal Ga or metal Al with one to several atomic layers on the substrate before the plasma irradiation. Then, the film is grown by irradiating a metal mainly composed of nitrogen plasma and metal Ga, and the polarity of the grown GaN-based group III nitride thin film is controlled to (0001). Nitride is to be obtained.
[0008]
(3) As described in (1) above, conventionally, a low temperature buffer that promotes nucleation of GaN by depositing and annealing a GaN layer of about 200 angstroms at a low temperature of about 500 ° C. after substrate cleaning. In some cases, a layer is formed. In the case where the AlN buffer layer is grown using nitrogen plasma and metal Al at a high temperature of 650 ° C. to 800 ° C. instead of the low temperature buffer layer, the inventors of the present invention have the polarity of the grown AlN layer ( 0001). The present invention utilizes the above, and after the substrate cleaning process, an AlN buffer layer is grown on the substrate at a high temperature of 650 ° C. to 800 ° C. using nitrogen plasma and metal Al, and then a growth temperature of 600 The film is grown at a temperature of from 800 ° C. to 800 ° C., and the polarity of the resulting GaN-based group III nitride thin film is controlled to (0001) to obtain the desired GaN-based group III nitride.
[0009]
(4) As described in (1) above, there is a conventional method of nitriding a sapphire substrate before forming a low-temperature buffer layer, forming an AlN layer having a lattice constant close to that of GaN on the substrate surface, and further increasing the nuclear density. Sometimes adopted. In the nitriding process of the sapphire substrate, the present inventors use a nitrogen source as ammonia, and when this ammonia is irradiated onto the sapphire substrate at 800 ° C. to 950 ° C., the polarity of the AlN layer formed on the substrate surface is ( 0001). The present invention utilizes the above, and after cleaning the substrate, irradiates ammonia at a high temperature of 800 ° C. to 950 ° C. to form an AlN layer, then uses ammonia or nitrogen plasma as a nitrogen source, and III Irradiating a metal mainly containing Ga as a group source to grow a film at a growth temperature of 600 ° C. to 800 ° C., and controlling the polarity of the resulting GaN-based group III nitride thin film to (0001) A group III nitride is to be obtained.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference examples .
[0011]
According to the reference example , using a normal molecular beam epitaxial apparatus, the raw material is evaporated under a normal vacuum, and this gaseous substance is supplied onto a sapphire C-plane substrate heated to 600 to 800 ° C. Growing to form the desired GaN-based group III nitride thin film. When growing a GaN-based Group III nitride thin film by irradiating a substrate with a nitrogen plasma as a nitrogen source and a metal mainly containing Ga as a Group III source at a growth temperature of 600 ° C. to 800 ° C., the nitrogen source and Simultaneously with the irradiation of the group III source, the metal In is irradiated only during the initial growth. Thereby, it is possible to obtain a thin film excellent in optical and electrical characteristics whose polarity is controlled to (0001). One example of a substrate having a thin film formed in this way is shown in FIGS. 2A and 2B, a GaN-based (0001) film, a GaN-based (000-1) film, or (0001) and (000-1) are formed on the sapphire substrate 21. A mixed film 22, 22 ′ is provided as a base, and a GaN group III nitride thin film (In irradiation layer) 23 formed during In irradiation is formed thereon, and further a GaN group III nitride is formed thereon. A thin film 24 is formed. Here, the GaN-based group III nitride thin film 24 may include In, Al, or the like as a group III metal element in addition to Ga, or may include Be, Mg, Si, or the like as a dopant. In this reference example : As a base of the group III nitride thin film to be grown, any film can be used as long as it is a GaN-based (0001) film, a GaN-based (000-1) film, or a film in which (0001) and (000-1) are mixed. But you can use it. For example, after the sapphire C-plane substrate 21 is heated (for example, about 800 ° C.) to be cleaned, a GaN layer having a predetermined thickness is deposited at a low temperature (for example, about 500 ° C.) and then annealed. The crystal orientation of the low temperature buffer layer 22 (FIG. 2A) or the film 22 ′ grown by another growth method (for example, sputtering, laser deposition, etc.) (FIG. 2B) If it has, it can be used. This low-temperature buffer layer promotes GaN nucleation.
[0012]
2. The nitrogen plasma may be generated by RF or generated by ECR.
[0013]
3. As the group III metal (Ga), one having a strength (flux) of 1 × 10 ¹³ co / cm ^{2 s to} 1 × 10 ¹⁵ co / cm ² s is used. If the strength is less than 1 × 10 ¹³ co / cm ² s, a practical growth rate (0.1 μm / hr) cannot be obtained, and if it exceeds 1 × 10 ¹⁵ co / cm ² s, the crystallinity deteriorates. Because it does.
[0014]
4). As the metal In, it is preferable to use a metal in a range from a strength two digits lower than the strength of the group III metal (Ga) to be irradiated to a strength one digit higher. This is because there is no effect outside this range of strength.
[0015]
5). The metal In may be irradiated only at the initial stage of growth of the group III nitride thin film, or may be continuously irradiated during the growth.
[0016]
According to the first embodiment of the present invention, after cleaning the sapphire C-plane substrate as described in the reference example , one atomic layer of metal Ga or metal Al is formed on the substrate, and then nitrogen plasma is used as a nitrogen source. Then, a GaN-based low-temperature buffer layer is formed by irradiation with a metal containing Ga as a main component as a group III source, and then a desired GaN-based (0001) thin film is grown on the buffer layer. One example of a substrate having a thin film formed in this way is shown in FIG. As shown in FIG. 3, an atomic layer 32 of metal Ga or metal Al is formed on a sapphire substrate 31, a GaN-based low-temperature buffer layer 33 is formed on the atomic layer, and a desired GaN-based III group is formed thereon. A nitride thin film 34 is formed. Here, the GaN-based group III nitride thin film 34 may contain In, Al, etc. as a group III metal element in addition to Ga, and may contain Be, Mg, Si, etc. as dopants. In this embodiment:
1. The irradiation temperature of metal Ga or metal Al should just be room temperature-600 degreeC.
[0017]
According to the second embodiment of the present invention, after the sapphire C-plane substrate cleaning process as described in the reference example , the substrate is irradiated with nitrogen plasma and metal Al at a growth temperature of 650 ° C. to 800 ° C. An AlN (0001) film is grown, and then a desired GaN-based (0001) thin film is grown on the AlN (0001) film. One example of a substrate having a thin film formed in this way is shown in FIG. As shown in FIG. 4, an AlN (0001) film 42 is formed on a sapphire substrate 41, and a desired GaN-based group III nitride thin film 43 is formed thereon. Here, the GaN-based group III nitride thin film 43 may include In, Al, or the like as a group III metal element in addition to Ga, or may include Be, Mg, Si, or the like as a dopant. In this embodiment:
1. The nitrogen plasma may be generated by RF or generated by ECR.
[0018]
2. As the metal Al, one having a strength of 1 × 10 ¹³ co / cm ^{2 s to} 1 × 10 ¹⁵ co / cm ² s is used.
[0019]
3. Any thickness of the AlN film may be used.
[0020]
4). The GaN-based group III nitride thin film grown on the AlN film may be grown using any growth method such as sputtering, CVD, or laser deposition.
[0021]
According to the third embodiment of the present invention, after cleaning the sapphire C-plane substrate as described in the reference example , the substrate is irradiated with ammonia at 800 ° C. to 900 ° C. to form an AlN (0001) layer. Then, the desired GaN-based (0001) layer is grown. One example of a substrate having a thin film formed in this manner is shown in FIG. As shown in FIG. 5, an AlN (0001) layer 52 is formed on a sapphire substrate 51, and a desired GaN-based group III nitride thin film 53 is formed thereon. Here, the GaN-based group III nitride thin film 53 may include In, Al, or the like as a group III metal element in addition to Ga, or may include Be, Mg, Si, or the like as a dopant. In this embodiment:
1. Ammonia may or may not be thermally decomposed.
[0022]
2. As the flow rate of ammonia, the lower limit may be an amount sufficient to form an AlN (0001) layer, and the upper limit is not particularly limited and may be appropriately selected from an economical viewpoint. Further, the ammonia irradiation time may be any time as long as it is 5 minutes to 2 hours.
[0023]
3. The GaN-based (0001) thin film grown on the AlN (0001) layer may be grown using any growth method such as sputtering, CVD, or laser deposition.
[0024]
【Example】
Embodiments and reference examples of the present invention will be described below with reference to the drawings.
( Reference Example 1 )
Using a normal molecular beam epitaxial apparatus, a GaN-based thin film as shown in FIGS. 2A and 2B was formed as follows.
[0025]
First, the sapphire C-plane substrate 21 is heated to 800 ° C. and cleaned, and the substrate is irradiated with nitrogen plasma and a metal containing Ga as a main component to form a GaN layer having a predetermined thickness at about 500 ° C. Then, annealing was performed at about 600 ° C. to form a low-temperature buffer layer (polarity: a mixture of (0001) and (000-1)) 22. Thereafter, in a vacuum of about 10 ⁻² to 10 ⁻⁴ Pa, a metal raw material (strength: 2 × 10 ¹³ co / cm ² s) mainly composed of Ga as a group III source is evaporated, and this gaseous state The material was supplied to and irradiated on a sapphire C-plane substrate heated to 800 ° C. with nitrogen plasma generated by RF. When a GaN-based thin film is grown at a crystal growth temperature of 730 ° C., gaseous metal In (strength: 7 × 10 ¹² co / cm ² s) is applied at the initial stage of crystal growth simultaneously with the irradiation of the gaseous substance and nitrogen plasma. Irradiated. In this manner, a thin film crystal was grown on the low temperature buffer layer 22 to form a target GaN-based thin film (FIG. 2A). In this case, the metal In was not taken into the GaN-based thin film 23 formed during the metal In irradiation at the initial stage of crystal growth, and the polarity was (0001). The polarity of the GaN-based thin film 24 grown after the metal In irradiation was stopped was also (0001).
[0026]
When the above method was repeated using a film (polarity: mixture of (0001) and (000-1)) 22 ′ grown by sputtering instead of the low-temperature buffer layer 22 as the base, the obtained GaN The polarity of the system thin film was also (0001) as in the above case (FIG. 2 (B)). In addition, a GaN-based (0001) film, a GaN-based (000-1) film, or a mixed film of (0001) and (000-1) was formed as a base, and the above method was repeated using this. However, the polarity of the obtained GaN-based thin film was all (0001) regardless of the polarity of the base. This means that by irradiating metal In, a GaN-based thin film having a desired polarity can be obtained regardless of the polarity of the base.
[0027]
Similar results can be obtained by using nitrogen plasma generated by ECR instead of RF plasma. Further, when the metal In is continuously irradiated during crystal growth, the metal In is not taken into the GaN-based thin film as described above, and the polarity of the obtained thin film is desired.
[0028]
The GaN-based (0001) thin film obtained as described above is excellent in optical and electrical characteristics.
( Example 1 )
In this embodiment, the GaN-based thin film is formed on the sapphire substrate 31 as shown in FIG. 3 by suppressing the substrate from being exposed to nitrogen plasma when the low-temperature buffer layer is formed on the cleaned sapphire C-plane substrate. 34 was formed.
[0029]
As in Reference Example 1 , after the sapphire C-plane substrate cleaning treatment, the substrate 31 was irradiated with metal Ga or metal Al at 500 ° C. to form a single atomic layer 32 of Ga or Al. Using this layer as a base, a GaN-based low-temperature buffer layer (polarity: ((0001) is formed by irradiating nitrogen plasma and a metal mainly containing Ga as a group III source under the same conditions as in Reference Example 1 on this layer. ) And (000-1))) 33 is formed, and a GaN-based thin film 34 is grown at 600 ° C. (FIG. 3). The polarity of the obtained thin film was (0001).
( Example 2 )
After the sapphire C-plane substrate cleaning treatment as in Reference Example 1 , as shown in FIG. 4, an AlN film 42 was grown on the substrate 41 by irradiation with nitrogen plasma and metal Al at a crystal growth temperature of 750 ° C. The polarity of this AlN film was (0001). Next, a GaN-based thin film 43 was grown on the obtained AlN (0001) film by irradiation with nitrogen plasma and a metal mainly composed of Ga as a group III source under the same conditions as in Reference Example 1 . . The polarity of the obtained thin film was (0001).
[0030]
Similar results can be obtained by using nitrogen plasma generated by ECR instead of RF plasma. Further, the thickness of the AlN film as the buffer layer is not particularly limited. The growth method of the GaN-based group III nitride thin film to be grown on the AlN film is not particularly limited. For example, if the growth is performed by sputtering, CVD, laser deposition, the same result as above can be obtained. .
( Example 3 )
As in Reference Example 1 , after the sapphire C-plane substrate cleaning treatment, as shown in FIG. 5, the substrate 51 was irradiated with ammonia at 900 ° C. to form an AlN layer 52 on the substrate surface. The polarity of the obtained AlN layer was (0001). Next, under the same conditions as in Reference Example 1 , a GaN-based thin film 53 was grown by irradiating a metal mainly composed of nitrogen plasma and Ga as a group III source. The polarity of the obtained thin film was (0001).
[0031]
The same results were obtained whether or not ammonia used as a nitrogen source was thermally decomposed. Further, the flow rate of ammonia is not particularly limited, and the lower limit is sufficient if it is an amount that can form an AlN layer on the substrate surface, and the same result can be obtained even if it is excessive. Further, the same result can be obtained if the irradiation time is 5 minutes to 2 hours.
[0032]
In addition, the growth method of the GaN-based thin film grown on the AlN (0001) layer is not particularly limited. For example, if the growth is performed by sputtering, CVD, laser deposition, etc., the same result as above can be obtained. It is done.
[0033]
As described above, the GaN-based thin film formed in the present invention can be used by being incorporated in a desired type of electronic device or optoelectronic device.
[0034]
【The invention's effect】
According to the present invention, it is possible to control the polarity of the GaN-based group III nitride thin film, which was impossible in the conventional MBE method, to (0001) excellent in optical and electrical characteristics. There is an effect that a quality group III nitride semiconductor device can be manufactured.
[Brief description of the drawings]
FIG. 1A is a model diagram showing an atomic arrangement state for explaining the polarity (0001) of a GaN-based film formed on a substrate.
(B) The model figure which shows the atomic arrangement | sequence state for demonstrating the polarity (000-1) of the GaN-type film | membrane formed on the board | substrate.
2A is a cross-sectional view of a substrate having a GaN-based thin film obtained according to Reference Example 1 of the present invention. FIG.
(B) Sectional drawing of the board | substrate which has the GaN-type thin film obtained by the reference example 1 of this invention.
FIG. 3 is a cross-sectional view of a substrate having a GaN-based thin film obtained according to Example 1 of the present invention.
FIG. 4 is a cross-sectional view of a substrate having a GaN-based thin film obtained according to Example 2 of the present invention.
FIG. 5 is a cross-sectional view of a substrate having a GaN-based thin film obtained according to Example 3 of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Sapphire C surface substrate 21, 31, 41, 51 Sapphire C surface substrate 22, 22 'Underlayer 23 In irradiation layer 24 GaN-type thin film 32 Metal Ga or metal Al atomic layer 33 GaN-type low-temperature buffer layer 34 GaN-type thin film 42 AlN Film 43 GaN-based thin film 52 AlN layer 53 GaN-based thin film

Claims (3)

On the sapphire C-plane substrate, metal Ga or metal Al is irradiated at an irradiation temperature of room temperature to 600 ° C. to deposit one atomic layer, and then a GaN-based low-temperature buffer layer is formed. Irradiating a metal mainly containing Ga as a group III source to epitaxially grow a GaN-based group III nitride thin film by molecular beam epitaxy, and controlling the polarity of the grown film to (0001) A method for forming a nitride thin film. On a sapphire C-plane substrate, nitrogen plasma is used as a nitrogen source on the sapphire C-plane substrate, and its strength (flux) is 1 × 10 ¹³ co / cm ^{2 s to} 1 × 10 ^15. After an AlN (0001) layer is grown using a metal of Al / cm ² s as an Al source, the molecule is irradiated with a metal mainly composed of nitrogen plasma and Ga at a growth temperature of 600 ° C. to 800 ° C. A method for forming a group III nitride thin film, comprising epitaxially growing a GaN-based group III nitride thin film by line epitaxy and controlling the polarity of the grown film to (0001). After irradiating ammonia as a nitrogen source on the sapphire C-plane substrate for 5 minutes to 2 hours at a high temperature of 800 ° C. to 950 ° C. to form an AlN (0001) layer, 600 ° C. to At a growth temperature of 800 ° C., GaN-based group III nitride thin films are epitaxially grown by molecular beam epitaxy by irradiating ammonia or nitrogen plasma as a nitrogen source and a metal mainly containing Ga as a group III source. A method for forming a group III nitride thin film, wherein the polarity of the film is controlled to (0001).

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