JP4468744B2 - Method for producing nitride semiconductor thin film - Google Patents

Description

  The present invention relates to a method for manufacturing a nitride semiconductor thin film capable of forming a high-quality nitride semiconductor thin film on a sapphire substrate or the like.

  A III-V group compound semiconductor (nitride semiconductor) containing nitrogen has a band gap from the far infrared to the ultraviolet region, and is therefore promising as a material for light reception and light emitting elements from the far infrared to the ultraviolet region. Nitride semiconductors are also promising as materials for electronic devices such as high-temperature resistance, high-power, and high-frequency transistors because they have a strong atomic bonding force, a high breakdown electric field, and a high saturation electron velocity. In addition, the nitride semiconductor has attracted attention as a material that hardly harms the environment and is easy to handle.

  In order to form a practical element using the nitride semiconductor, which is an excellent material as described above, it is necessary to form a nitride semiconductor thin film on a predetermined substrate to form a predetermined element structure. . As the substrate, a nitrogen compound substrate capable of directly growing a nitride semiconductor and having the same lattice constant and thermal expansion coefficient is most suitable. However, the nitrogen compound substrate is currently not practical because the substrate size is as small as 2 inches or less in diameter and is very expensive.

For this reason, at present, a material having a large lattice mismatch with a nitride semiconductor, such as sapphire and silicon carbide, is used as the substrate. Among them, a sapphire substrate that is chemically stable and from which a larger substrate can be easily obtained at a lower cost is most commonly used.
Between the sapphire substrate and gallium nitride (GaN) which is a typical nitride semiconductor, there are 10-20% lattice mismatch and thermal expansion coefficient difference. For this reason, if GaN is directly grown on the sapphire substrate, three-dimensional growth is inevitable, and a flat GaN film cannot be obtained.

  Taking a high-electron mobility transistor that is promising as a device using a compound semiconductor such as a nitride semiconductor as an example, if a flat state is not obtained as described above, it is at the atomic level that is indispensable for improving the performance of the device. A steep hetero interface cannot be obtained. Further, when formed directly on the sapphire substrate, there are many defects in the formed film, and an element having desired characteristics cannot be obtained.

  In order to solve the above-described problem, a technique using a low-temperature buffer layer has been proposed (see Non-Patent Document 1). In this technology, when a nitride semiconductor crystal is grown on a sapphire substrate by metal organic vapor phase epitaxy, a thin layer of nitride semiconductor called a low temperature buffer layer is formed in a low temperature region of about 500 ° C. After that, the temperature is raised to about 1000 ° C. to form a nitride semiconductor crystal layer. According to this technique, GaN, AlN, or the like is used as the low-temperature buffer layer, and crystal layers such as GaN, AlN, and AlGaN can be formed.

  The low-temperature buffer layer is three-dimensionally solid-phase grown on the surface of the sapphire substrate during the temperature rising process, and partially aggregates to form a plurality of island portions. After this state, when trying to grow a nitride semiconductor crystal at a high temperature, the island part becomes the nucleus, the nitride semiconductor crystal layer grows, and the crystal growth layer from each island part fuses Then flattening is performed. As a result, according to the above-described technique, a nitride semiconductor crystal layer (film) constituting the element is formed on the sapphire substrate.

The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
H. Amano, N. Sawaki, L. Akasaki, and Y. Yoyoda, "Metal organic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layer", Appl Phys. Lett. 48, 353 (1986).

  However, the above-described conventional technique has a problem that the reproducibility of quality such as defect density in the formed (crystal grown) nitride semiconductor layer is low. Low temperature buffer layer formation conditions (actual substrate temperature, source gas supply rate, supply ratio, growth furnace pressure, etc.) and temperature increase method after low temperature buffer layer formation (temperature increase rate, furnace atmosphere, pressure) Etc.), the size and density of the above-described island portion are different, which causes a decrease in reproducibility.

  In particular, when a low-temperature buffer layer made of AlN is used, trimethylaluminum (TMA) as a raw material gas and ammonia for nitriding easily react in the gas phase. For this reason, when the low temperature buffer layer made of AlN is formed in a wider area, the in-plane uniformity and reproducibility of the film thickness of the low temperature buffer layer are lowered, and as described above, the island portion is non-uniform. It is thought that the reproducibility of the quality of the crystal layer formed above is reduced.

  Further, the variation in defect density in the formed nitride semiconductor crystal layer also affects the insulation for suppressing the leakage current in the operation of the element formed thereon. Due to the variation in defect density, the resistance at the interface between the substrate and the crystal layer varies, and a region with low resistance exists at the interface. For example, in a high electron mobility transistor, a field effect transistor, or the like, a current flowing between a source and a drain in the planar direction of the substrate is applied to the gate electrode and controlled by the voltage. When such an element is formed, if a leakage current occurs near the interface between the substrate and the crystal layer, the current also flows through the interface, and desired transistor characteristics can be obtained even if the gate voltage is changed. Disappear.

  The present invention has been made in order to solve the above-described problems, and has a reproducible quality on a substrate made of a material that is lattice-mismatched with a nitride semiconductor such as a sapphire substrate or a silicon carbide substrate. It is an object of the present invention to form a nitride semiconductor crystal layer having a high thickness.

In the method for producing a nitride semiconductor thin film according to the present invention, an AlO _x N _y layer made of aluminum oxynitride having a predetermined oxygen composition ratio x and a predetermined nitrogen composition ratio y is formed on a substrate. And at least a step of forming a buffer layer made of a nitride semiconductor into which a p-type impurity is introduced on the AlO _x N _y layer.
The difference in lattice constant is reduced by the AlO _x N _y layer, and the p-type level is formed by the introduced p-type impurity in the buffer layer, which is generated at the substrate side interface of the buffer layer. The n-type defect level is compensated.

In the nitride semiconductor thin film manufacturing method, the oxygen composition ratio x of the AlO _x N _y layer is decreased from the substrate side to the buffer layer side, and the nitrogen composition ratio y of the AlO _x N _y layer is decreased from the substrate side. Increase toward the buffer layer. In addition, the formation of the buffer layer, introducing a quantity of p-type impurities compensating the AlO _x N _y layer and the n-type defect level to generate a field surface of the buffer layer.
In the method for producing a nitride semiconductor thin film, the substrate is, for example, a sapphire substrate.

  In the method for manufacturing a nitride semiconductor thin film, the p-type impurity may contain zinc, magnesium, iron, or carbon, and may contain at least two of these.

As described above, according to the present invention, the difference in lattice constant is reduced by the AlO _x N _y layer, and the p-type level is formed in the buffer layer by the introduced p-type impurity. Since the n-type defect level generated at the interface of the buffer layer on the substrate side is compensated, it is reproduced on a substrate made of a material that is lattice-mismatched with a nitride semiconductor such as a sapphire substrate or a silicon carbide substrate. An excellent effect is obtained in that a high-quality nitride semiconductor crystal layer can be formed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Hereinafter, a case where a GaN-based material is used as a nitride semiconductor and a crystal layer of a GaN-based material is formed on a sapphire-based substrate will be described as an example.
FIG. 1 is a process diagram showing an example of a method for producing a nitride semiconductor thin film according to an embodiment of the present invention. First, as shown in FIG. 1A, for example, an Al ₂ O ₃ layer 102, an AlO _x N _y layer 103, and an AlN layer 104 are formed on a disk-shaped sapphire substrate 101 having a diameter of 3 inches. State.

The AlO _x N _y layer 103 is a layer in which the oxygen composition ratio x decreases and the nitrogen composition ratio y increases from the Al ₂ O ₃ layer 102 side to the AlN layer 104 side. This change may be continuous or stepwise (non-continuous). The AlO _x N _y layer 103 has the same configuration as the Al ₂ O ₃ layer 102 when the nitrogen composition ratio y is 0, and the same configuration as the AlN layer 104 when the oxygen composition ratio x is 0. The Al ₂ O ₃ layer 102 is made of aluminum oxide having a stoichiometric composition, and the AlN layer 104 is made of aluminum nitride having a stoichiometric composition.

Each of the above layers is formed by sputtering as described below. First, if the Al ₂ O ₃ layer 102 is formed to a thickness of about 10 nm by using reactive sputtering using ECR (Electro Cyclotron Resonance) plasma, using aluminum as a target and oxygen as a reactive gas, for example. Good. Note that Ar may be used as the sputtering gas.

Further, the AlO _x N _y layer 103 is formed on the Al ₂ O ₃ layer 102 by changing the amount of oxygen gas to be supplied by using oxygen and N as reactive gases by the same reactive sputtering method. What is necessary is just to form in thickness about 10 nm.
The AlN layer 104 may be formed on the Al ₂ O ₃ layer 102 to a thickness of about 10 nm by using the same reactive sputtering method using nitrogen gas as the reactive gas.

  The formation of each layer by the sputtering method described above may be performed continuously in the same apparatus. For example, each layer can be continuously formed by the same apparatus by gradually changing the supplied reactive gas from oxygen to nitrogen. It goes without saying that each layer may be formed by an individual apparatus.

Here, the lattice constant of each layer is Al ₂ O ₃ layer 102> AlO _x N _y layer 103> AlN layer 104. In particular, the AlO _x N _y layer 103 in which the oxygen composition ratio x is decreased and the nitrogen composition ratio y is increased is almost in the state of aluminum oxide in the vicinity of the interface with the Al ₂ O ₃ layer 102. The interface with the AlN layer 104 is almost in the state of aluminum nitride. Therefore, there is almost no difference in lattice constant between the layers from the Al ₂ O ₃ layer 102 to the AlN layer 104.

Thus, by using a layer made of AlO _x N _y from the sapphire substrate 101 to the AlN layer 104, the difference in lattice constant between the layers is reduced, and the dislocation density of the compound semiconductor layer formed thereon is reduced. Can be very small. The AlO _x N _y layer 103 may have a predetermined oxygen composition ratio x and a predetermined nitrogen composition ratio y. However, the oxygen composition ratio x decreases toward the AlN layer 104, and the nitrogen composition ratio y is reduced. By increasing the state toward the AlN layer 104, a state in which there is almost no difference in lattice constant between the layers can be obtained.

By forming as described above, after forming a layer for reducing the difference in lattice constant composed of the Al ₂ O ₃ layer 102, the AlO _x N _y layer 103, and the AlN layer 104, as shown in FIG. In addition, a buffer layer 105 made of p-type GaN (crystal) doped with zinc (2 × 10 ¹⁸ cm ⁻³ ) is formed on the AlN layer 104. The buffer layer 105 may be grown by metal organic vapor phase epitaxy by using triethylgallium as a source gas and diethyl zinc for doping zinc and setting the substrate temperature to about 1000 ° C.

Next, as shown in FIG. 1C, an active layer 106 made of undoped GaN is formed on the buffer layer 105. The active layer 106 may be grown by metal organic vapor phase epitaxy using triethylgallium as a source gas and a substrate temperature of about 1000 ° C.
Next, as shown in FIG. 1D, on the active layer 106, a spacer layer 107 made of undoped AlGaN, an electron supply layer 108 made of silicon-doped n-type AlGaN, and made of undoped AlGaN. The barrier layer 109 is formed.

By stacking the layers in this manner, electrons supplied from the donor impurity doped in the electron supply layer 108 move to the active layer 106 and accumulate near the heterojunction interface between the spacer layer 107 and the active layer 106. Thus, the channel 115 is formed. The channel 115 is very thin and is referred to as a two-dimensional electron channel.
Each of the nitride semiconductor layers after the buffer layer 105 described above may be formed by continuously growing crystals in the same apparatus by metal organic vapor phase epitaxy with a substrate temperature of 1000 ° C.

  As described above, after the formation up to the barrier layer 109, as shown in FIG. 1E, the gate electrode 110 and the source electrode 111 and the drain disposed so as to sandwich the gate electrode 110 are formed on the barrier layer 109. If the electrode 112 is formed, a high electron mobility transistor can be obtained. For example, the gate electrode 110 may have a laminated structure including a nickel layer and a gold layer formed thereon. The source electrode 111 and the drain electrode 112 may be formed by stacking a titanium layer / aluminum layer / titanium layer / gold layer in this order, and an ohmic connection with the channel 115 may be obtained by sintering by heat treatment.

As described above, in the present embodiment, the difference in lattice constant between the substrate side and the nitride semiconductor thin film side is first reduced by the AlO _x N _y layer 103. In addition, according to the manufacturing method shown in FIG. 1, the p-type level exists due to the buffer layer 105 doped with the p-type impurity. When a nitride semiconductor thin film is grown on a substrate having a different lattice constant and different thermal expansion coefficient, an n-type defect is caused by a high dislocation density generated at the interface of the nitride semiconductor thin film (buffer layer 105). It is considered that a level occurs. This n-type defect level is compensated by the p-type level in the buffer layer 105 described above.
As a result, according to the present embodiment, a higher quality nitride semiconductor thin film can be produced on the sapphire substrate.

Next, the addition of p-type impurities will be described in more detail. In FIG. 1, the buffer layer 105 to the barrier layer 109 are nitride semiconductor thin films.
FIG. 2 is a characteristic diagram showing the state of leakage current at the interface between the nitride semiconductor thin film (buffer layer 105) and the substrate (AlN layer 104), and shows the results evaluated from the current-voltage (IV) characteristics. ing. Similar evaluation can be performed by Hall effect measurement or eddy current measurement.

FIG. 2 shows a change in resistance when the thickness of the buffer layer 105 is changed, and shows the highest resistance at a thickness of about 200 nm, and the leakage current is reduced most.
When the buffer layer 105 is thin, the low resistance state is such that the p-type impurity concentration (p-type level density) in the buffer layer 105 is sufficient to compensate for the above-described n-type defect level density. Occurs because there is not enough.

On the other hand, when the buffer layer 105 is thick, the low resistance state occurs because the p-type impurity concentration (p-type density) in the buffer layer 105 is higher than the n-type defect level density described above. To do.
When the added amount of zinc is 2 × 10 ¹⁸ cm ⁻³ and the film thickness of the buffer layer 105 is 200 nm, the p-type level density by the buffer layer 105 and the n-type defect level density substantially coincide. It is considered a thing. In the case of this example, it can be seen that desired characteristics can be obtained if the thickness of the buffer layer 105 is 150 nm or more.

FIG. 3 is a characteristic diagram showing an in-plane resistance distribution serving as a reference for the above-described leakage current in a sample in which up to the barrier layer 109 shown in FIG. 1 is formed on a circular sapphire substrate having a diameter of 3 inches. The distribution in the direction perpendicular to the orientation flat of the sapphire substrate is shown. In FIG. 3, A shows the characteristics of the sample in which the buffer layer 105 having a thickness of 200 nm doped with zinc is formed, and B shows the active layer 106 to the barrier layer 109 using a low-temperature buffer layer as in the prior art. The characteristic of the sample produced similarly is shown.
As is clear from FIG. 3, the sample A has a uniform in-plane distribution of the substrate, but the sample B has a distribution in the in-plane resistance.

In the above description, zinc is used as an example of the p-type impurity, but magnesium, iron, carbon, or the like may be used as the p-type impurity. Even when cyclopentadienylmagnesium (Cp ₂ Mg) is simultaneously introduced together with the source gas when the nitride semiconductor layer is crystal-grown, the amount of magnesium added can be reduced even when the nitride semiconductor layer is doped with magnesium. In the range up to 5 × 10 ¹⁸ cm ⁻³ , the same tendency as in FIG. 2 is shown.

Further, when crystal growth of the nitride semiconductor layer is performed, when ferrocene is simultaneously introduced together with the source gas so that the nitride semiconductor layer is doped with iron, the amount of iron added is up to 5 × 10 ¹⁹ cm ^−3. In this range, the same tendency as in FIG. 2 is shown. Similarly, when crystal growth of the nitride semiconductor layer is performed, when carbon tetrabromide is simultaneously introduced together with the source gas so that the nitride semiconductor layer is doped with carbon, the amount of carbon added is 5 × 10 ^19. In the range up to cm ^−3, the same tendency as in FIG. 2 is shown.

The reason why the amount of addition in iron or carbon is about an order of magnitude is considered to be that the impurity levels formed by these dopants are deeper than in the case of zinc or magnesium.
Further, it has been found that the same effect as described above can be obtained even when two or more of the above-described dopants are added in combination. In this case, it has also been found that the amount of impurities added can be reduced as compared with the case of using alone.

  As described above, according to the method for manufacturing the nitride semiconductor thin film illustrated in FIG. 1, a high resistance state at the interface between the substrate and the crystal layer of the nitride semiconductor can be obtained with good reproducibility and uniformity. Therefore, it greatly contributes to the yield improvement in device fabrication using nitride semiconductors. Further, according to this manufacturing method, a higher quality nitride semiconductor thin film having a reduced defect state density can be obtained. Therefore, a high electron mobility transistor using a nitride semiconductor as shown in FIG. Other field effect transistors using semiconductors can be operated as designed.

  In the above description, the sapphire substrate is used. However, the present invention is not limited to this, and the silicon carbide substrate is the same as the sapphire substrate. In the above description, the nitride semiconductor layer is formed by metal organic vapor phase epitaxy. However, the present invention is not limited to this, and other crystal growth methods such as MBE, CBE, and MOMBE are used. It goes without saying.

  In the above description, the buffer layer into which the p-type impurity is introduced is made of GaN. However, the buffer layer is not limited to this, and may be made of other nitride semiconductors such as AlN. Moreover, although GaN and AlGaN are taken as examples of the nitride semiconductor thin film to be manufactured, the same applies to InN and InGaN.

It is process drawing which shows the example of the preparation methods of the nitride semiconductor thin film in embodiment of this invention. It is a characteristic view showing the state of leakage current at the interface between the nitride semiconductor thin film (buffer layer 105) and the substrate (AlN layer 104). 2 is a characteristic diagram showing an in-plane resistance distribution serving as a reference for leakage current in a sample in which up to a barrier layer 109 shown in FIG. 1 is formed on a circular sapphire substrate having a diameter of 3 inches.

Explanation of symbols

101 ... sapphire substrate, 102 ... Al ₂ O ₃ layer, 103 ... AlO _x N _y layer, 104 ... AlN layer, 105 ... buffer layer, 106 ... active layer, 107 ... spacer layer, 108 ... electron supply layer, 109 ... barrier Layer 110 gate electrode 111 source electrode 112 drain electrode

Claims (7)

A step of forming an AlO _x N _y layer made of aluminum oxynitride having a predetermined oxygen composition ratio x and a predetermined nitrogen composition ratio y on the substrate;
And a step of forming a buffer layer made of a nitride semiconductor into which a p-type impurity is introduced on the AlO _x N _y layer,
The oxygen composition ratio x of the AlO _x N _y layer is decreased from the substrate side to the buffer layer side;
The nitrogen composition ratio y of the AlO _x N _y layer is increased from the substrate side to the buffer layer side,
Wherein in the formation of the buffer layer, introducing the AlO _x N _y layer with the amount of p-type impurities compensating the defect level of the n-type to produce a field surface with the buffer layer,
A method for producing a nitride semiconductor thin film, comprising: forming a nitride semiconductor thin film on the buffer layer. The method for producing a nitride semiconductor thin film according to claim 1 ,
The method for producing a nitride semiconductor thin film, wherein the substrate is a sapphire substrate. In the method for manufacturing a nitride semiconductor thin film according to any one of claims 1-2,
The p-type impurity includes zinc. A method for manufacturing a nitride semiconductor thin film. In the method for manufacturing a nitride semiconductor thin film according to any one of claims 1-2,
The p-type impurity includes magnesium. A method of manufacturing a nitride semiconductor thin film, wherein: In the method for manufacturing a nitride semiconductor thin film according to any one of claims 1-2,
The p-type impurity contains iron. A method for manufacturing a nitride semiconductor thin film, wherein: In the method for manufacturing a nitride semiconductor thin film according to any one of claims 1-2,
The p-type impurity includes carbon. A method for producing a nitride semiconductor thin film, wherein: In the method for manufacturing a nitride semiconductor thin film as claimed in any one of claims 1-2,
The p-type impurity contains at least two of zinc, magnesium, iron, and carbon.

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