JPH0983016A - Method for growing nitride semiconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the crystallinity of nitride semiconductor by growing a layer of Al-Ga-N having inclining composition where the composition of Al decreases gradually on an SiC substrate and then growing an In-Al-Ga-N based nitride semiconductor thereon. SOLUTION: AlN 2 is deposited on one side of a 6H-SiC substrate and an Alx Ga1-x N (0<=X<=1) layer 3 is formed thereon while inclining the composition such that X decreased sequentially. A nitride semiconductor represented by a formula Ina Alb Ga1-a-b N (0<=a, 0<=b, a+b<=1), more sepecifically, a nitride semiconductor comprising an n-type In0.05 Ga0.95 N layer 4 and an In0.2 Ga0.8 N layer 5 is the formed. Subsequently, an Mg doped p-type Al0.15 Ga0.85 N layer 6, an Mg doped p-type Al0.3 Ga0.7 N layer 7, and an Mg doped p-type GaN layer 8 are formed thereon. Finally, etching is performed until the inclining composition n-AlGaN layer 3 is exposed and provided with negative and positive electrodes 10, 11.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a nitride semiconductor In _a Al _b Ga _{1 -ab} N (0≤a, 0≤b, a + b≤ by a vapor phase growth method.
The present invention relates to a method of growing a crystal of 1) on a substrate.

[0002]

2. Description of the Related Art Nitride semiconductors are metal organic chemical vapor deposition (M
OVPE), molecular beam vapor phase epitaxy (MBE), halide vapor phase epitaxy (HDVPE) and the like are used to epitaxially grow on the substrate. In general, for epitaxial growth of a compound semiconductor, it is common knowledge that a substrate with a lattice constant that matches that of the compound semiconductor will yield good crystallinity. It was always grown on sapphire substrates with a lattice constant difference of as much as 13%.

In the case of a sapphire substrate, it is known that before growing a nitride semiconductor, a buffer layer made of AlN or GaN is first grown on the sapphire substrate and then the nitride semiconductor is grown on the buffer layer. . For example, Japanese Examined Patent Publication No. 59-48794 and Japanese Examined Patent Publication No. 4-15200 describe a method of using AlN as a buffer layer, and Japanese Laid-Open Patent Publication Nos. 60-173829 and 4-297023 disclose GaN as a buffer layer. How to do is described. Among them, the method disclosed in JP-A-4-297023 is one of the core technologies of the nitride semiconductor LEDs currently put into practical use.

Other substrates for growing nitride semiconductors include ZnS (Japanese Patent Laid-Open No. 4-68579) and MnO (Japanese Patent Laid-Open No. 4-19859).
209577), ZnO (JP-A-4-236477),
Many proposals have been made, such as SiC (Japanese Patent Laid-Open No. 4-223330). In particular, Japanese Patent Laid-Open No. 4-223330 discloses a technique of forming a SiC buffer layer on the surface of a SiC substrate and growing a nitride semiconductor on the buffer layer. It is shown.

[0005]

Currently, blue LEDs, blue-green LEDs, etc. are practically used for nitride semiconductors grown on a sapphire substrate, but in the future, they will have higher brightness and excellent reliability. In order to realize advanced light emitting devices such as LEDs and LDs, it is necessary to further improve the crystallinity of the nitride semiconductor. Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to improve the crystallinity of a nitride semiconductor grown on a substrate and realize a highly reliable LED, LD, or the like. To do.

[0006]

The method for growing a nitride semiconductor of the present invention is a vapor phase epitaxy method of In _a Al _b Ga _{1 -ab} N.
In a method of epitaxially growing a nitride semiconductor represented by (0 ≦ a, 0 ≦ b, a + b ≦ 1) on a substrate, SiC is used for the substrate, and the composition is such that the X value is gradually decreased on the SiC substrate. Inclined Al _X Ga _1-X N (0 ≦ X ≦
1) A layer is grown, and the nitride semiconductor is grown on the Al _x Ga _1-x N layer.

In the growth method of the present invention, the vapor phase growth method is, for example, the MOVPE method or the MBE method as described above.
Method, HDVPE method, etc. can be adopted, but MOV is preferable.
By growing by the PE method, a semiconductor layer with good crystallinity can be obtained.

For the SiC substrate, a single crystal SiC substrate is used. SiC has various crystal structures such as 4H, 6H, and 3C, but is not particularly limited. Preferably 6H
-SiC (0001) plane, 3C-SiC (111) plane
A nitride semiconductor with good crystallinity can be obtained by growing it on the surface.

[0009] The Al _X Ga _1-X N layer composition gradient is Al _X Ga _1-X N layer configured to be less according to Al ratio is separated from the SiC substrate, the Al _X Ga _1-X N
The layer may be composed of a single layer having a composition gradient, or may be composed of a multilayer film in which a plurality of Al _X Ga _{1 -X} N layers are laminated as described later, and the composition of each layer is separated from SiC as it is separated from SiC. It is also possible to use Al _X Ga _1-X N with a reduced Al mixed crystal ratio.

The Al _X Ga _1-X N layer is preferably grown to a thickness of 5 nm to 5 μm, more preferably 5 nm to 5 μm.
Adjust to 3 μm. If it is thinner than 5 nm, it is difficult to form a compositionally graded layer, and if it is thicker than 2 μm, Al _x Ga _{1 -x.}
This is because the N layer itself is likely to be cracked. Further, it is more preferable that the outermost surface of the compositionally graded Al _X Ga _1-X N layer is GaN. When GaN is used, the crystallinity of the nitride semiconductor layer grown thereon is particularly good.

Next, the growth method of the present invention uses the above-mentioned Al._XGa_1-X
Characterized in that an AlN layer is grown between the N layer and the substrate.
I do. By growing this AlN layer,
Al _XGa_1-XThe crystallinity of the N layer is further improved. Therefore A
l_XGa_1-XCrystallinity of nitride semiconductor layer grown on N layer
Also gets better. The film thickness of the AlN layer is 1 nm to 0.1 μm
It is desirable to form it with a large thickness. If it is thicker than 0.1 μm
Since cracks are likely to occur in the AlN layer itself,
Al with good crystallinity_XGa_1-XIt is difficult for the N layer to grow. Al
The N layer can be grown under the conditions of ordinary vapor phase growth method.
You. For example, in the case of MOVPE method, 400 ° C to 1200
The surface of the SiC substrate heated within the range of ° C contains Al.
To supply organometallic gas and nitrogen hydride
You can grow more. In this case, it was grown below 900 ° C
AlN becomes a crystal containing amorphous AlN,
AlN grown at more than 00 ° C becomes a crystal close to a single crystal.
However, in either case, the bonding is performed on the AlN layer.
Al with good crystallinity_XGa_1-XN layers can be grown.

Next, the Al _X Ga _1-X N layer is characterized by being formed of a multilayer film in which layers having different X values are laminated. That is, an AlGaN layer having a large Al mixed crystal ratio is formed on the SiC substrate side, an AlGaN layer having a small Al mixed crystal ratio is formed thereon, and an AlGa layer having a gradually smaller Al composition ratio is laminated to form a multilayer film. There is no particular problem even if the number of layers of the multilayer film is laminated, but as described above, the total film thickness of the AlGaN layer is 5 nm to 5 μm.
It is desirable to adjust to the range of m.

[0013]

When the compositionally graded AlGaN layer is formed on the SiC substrate, the AlGaN layer can reduce dislocations, strains and the like due to lattice mismatch with the substrate. It can be inferred that this is because the AlGaN layer having a large Al mixed crystal ratio is close to the lattice constant of SiC. Therefore, the compositionally graded Al
If the AlN layer is grown first before the GaN layer is grown, the crystallinity of AlGaN is improved. Moreover, Al
By reducing the mixed crystal ratio, the lattice defects of the AlGaN layer having a large Al mixed crystal ratio formed first are gradually relaxed, and the AlGaN layer having good crystallinity is gradually grown. If an AlGaN layer with good crystallinity can be grown, the nitride semiconductor grown on it will be the AlGaN layer previously formed.
Since the layer serves as a lattice-matched substrate, the crystallinity of the nitride semiconductor is dramatically improved.

[0014]

EXAMPLES The growth method of the present invention by the MOVPE method will be described below.

Hydrogen gas was used as a carrier gas on the (0001) plane of the 6H-SiC substrate heated to 1050 ° C.
TMA (trimethylaluminum) and ammonia gas are supplied to grow a thin film of AlN to a thickness of 50 nm. This AlN thin film can be grown in the range of 400 ° C. to 1200 ° C., and when it is grown at about 900 ° C. or lower, a crystal containing amorphous AlN grows,
Although a single crystal AlN thin film tends to grow when grown at 900 ° C. or higher, either an amorphous AlN thin film or a single crystal AlN thin film may be grown.

Subsequently, while maintaining the substrate at 1050 ° C., in addition to TMA gas, TMG (trimethylgallium) gas is gradually flowed to grow a compositionally graded AlGaN layer. The gas flow rates of TMG and TMA are controlled by a mass flow controller, the gas flow rate of TMG gas is gradually increased with the passage of time, and at the same time, the flow rate of TMA gas is gradually decreased to obtain the total gas of TMG gas and TMA gas. The AlGaN layer is grown by always adjusting the amount to be almost the same. Finally, the TMA gas is stopped so that the GaN layer grows. Al having a composition gradient as described above
A GaN layer is grown to a film thickness of 2 μm. Gradient composition A
Although the uppermost layer of the lGaN layer is made of GaN, it is not necessary to make the uppermost layer of GaN as long as it has a graded composition.
Preferably, the uppermost layer is Al _X Ga having an X value smaller than 0.5.
_{A 1-X} N layer, more preferably 0.3 or less, allows a nitride semiconductor layer having good crystallinity to grow on the Al _X Ga _1-X N layer.

Then, the TMA gas was completely stopped, and the TMG was removed.
Gas, ammonia gas at 1050 ℃ 3μ GaN layer
It is grown to a thickness of m.

After growth, the substrate was taken out and the full width at half maximum (FWHM) of the double crystal X-ray rocking curve was used to evaluate the crystallinity of the obtained GaN layer.
It was found that the crystallinity was extremely excellent at 1.5 minutes. Further, when the mobility of the crystal was measured by a Hall measuring device, it showed an excellent value of 900 cm ² / V · sec. Note that the smaller the FWHM, the better the crystallinity, and the larger the mobility, the better the crystallinity. For example, a non-doped GaN single crystal layer grown on a sapphire substrate using GaN as a buffer layer has a mobility of 3 to 5 minutes and a mobility of 500 to 600 cm ² / V · sec.

[Embodiment 2] The same as in Embodiment 1 except that the AlN thin film is not grown on the SiC substrate.
When the N layer was grown, the FWHM was 2 minutes and the mobility was 80.
It was 0 cm ² / V · sec, and the crystallinity was slightly inferior to that of Example 1.

Example 3 In Example 1, after the AlN thin film was grown, the temperature was kept at 1050 ° C.
First, adjust the gas flow rates of A and TMG, and then Al0.9Ga0.1
The N layer is grown to 0.2 μm. Then Al0.8Ga0.2N
Layer is 0.2μm, Al0.7Ga0.3N layer is 0.2μm ...
... Al0.2Ga0.8N layer 0.2 μm, Al0.1Ga
Nine 0.9N layers are laminated in the order of 0.2 μm to grow a compositionally graded AlGaN multilayer film with a thickness of 1.8 μm. Thereafter, in the same manner as in Example 1, Ga was formed on the Al0.1Ga0.9N layer.
When the N layer was grown to 2 μm, the crystallinity of the obtained GaN layer showed almost the same value as in Example 1.

[Embodiment 4] In Embodiment 1, after the AlGaN layer having the graded composition is grown, the same temperature is set to 10.
While maintaining the temperature at 50 ° C., an Al 0.2 Ga 0.8 N layer is grown to 2 μm with TMA, TMG and ammonia gas. This A
FWHM of 0.2Ga0.8N layer is 2 minutes, mobility is 800cm
² / V · sec, which indicates that AlGaN has very good crystallinity.

[Embodiment 5] FIG. 1 is a schematic sectional view showing the structure of a laser device obtained by the method of the present invention.
Embodiment 5 will be described below with reference to this drawing.

An AlN thin film 2 having a thickness of 50 nm is formed on the (0001) plane of a 6 H—SiC substrate 1 having a thickness of 500 μm.
2 μm of n-type AlGaN layer 3 with composition gradient up to GaN
With the same film thickness as in Example 1, the layers are stacked. Note that the compositionally graded AlGaN layer 3 was doped with Si in order to obtain a preferable n-type, and was grown while being doped with silane gas as a Si source while flowing a source gas.

Next, the substrate temperature is set to 800 ° C., and TMI (trimethylindium) gas, TMG, ammonia, and silane gas are used as source gases, and n-type In0.05Ga0.95N is used.
Layer 4 was grown to a film thickness of 0.1 μm.

Subsequently, the flow rate of TMI was increased to form a non-doped In0.2Ga0.8N layer 5 having a thickness of 2 nm as an active layer so that a single quantum well structure was formed.

Next, the TMI is stopped and the temperature of the substrate is changed to 1050.
℃, the source gas TMG, TMA, ammonia, p
Cp2Mg (cyclopentadienylmagnesium) is used as a type impurity gas, and Mg-doped p-type Al0.15Ga0.
The 85N layer 6 was grown to 0.1 μm.

Subsequently, the flow rate of TMA was increased to grow the Mg-doped p-type Al0.3Ga0.7N layer 7 to a thickness of 0.1 μm.

Finally, TMA was stopped and Mg-doped p-type Ga was used.
The N layer 8 was grown to 0.5 μm.

The wafer on which the nitride semiconductor layers are laminated as described above is taken out of the reaction container, and the composition gradient n-AlGa is formed from the uppermost p-GaN layer 8 by an etching apparatus.
Etching is performed until the N layer 3 is exposed. After etching, the negative electrode 10 is provided on the exposed n-AlGaN layer 3,
Striped positive electrodes 11 were provided on the uppermost p-GaN layer.

After the electrode is installed, the wafer is cleaved in a direction perpendicular to the stripe of the positive electrode, and a reflective film made of a dielectric multilayer film is formed on the cleaved surface by a conventional method to obtain a laser device. FIG. 1 shows a sectional view of an element cleaved in a direction perpendicular to the stripe-shaped positive electrode 11. This laser device showed laser oscillation at room temperature at a threshold current density of 500 mA / cm ² and an output of 5 mW. This is because the nitride semiconductor grown on the compositionally graded AlGaN layer has good crystallinity, and the cleavage plane of the substrate facilitates the formation of a resonance surface.

This laser device has the following advantages. First of all, when SiC is used as the substrate, since the SiC substrate has conductivity, a normal negative electrode is provided in contact with the SiC substrate. That is, the positive electrode and the negative electrode face each other. However, SiC and nitride semiconductor are heteroepitaxial. Therefore, since a barrier caused by heteroepitaxial exists at the interface between the SiC and the nitride semiconductor layer, Vf (forward voltage) of the device increases. On the other hand, the laser device according to the present invention has a structure in which the negative electrode is not provided on the substrate side but the nitride semiconductor is intentionally etched to be provided on the same surface side, although the conductive substrate of SiC is used. Therefore, since the current does not flow at the interface between the SiC and the nitride semiconductor layer, Vf
Can be suppressed. Secondly, compositionally graded AlGaN
By growing the layer 3 to a thickness of 1 μm or more, it becomes a contact layer for forming a negative electrode and a clad layer for confining light emission of the active layer. Thirdly, Si
Unlike the conventional sapphire substrate, C has a cleavability. Therefore, by utilizing the cleavage property of SiC, it is very convenient to use the cleavage surface of the nitride semiconductor as the optical resonance surface of the laser element.

[0032]

As described above, according to the method of the present invention, a nitride semiconductor layer having good crystallinity can be obtained. For example, in hole measurement of crystals, the mobility is 900 cm ² / V · sec.
Is a very excellent value for nitride semiconductors. Further, according to the present invention, a nitride semiconductor having good crystallinity can be obtained. Therefore, when a light emitting device is produced as in Example 5, a device having a high light emission output can be obtained, and its industrial utility value is great.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a nitride semiconductor laser device obtained by a method according to an embodiment of the present invention.

[Explanation of symbols]

 1 ... SiC substrate 2 ... AlN thin film 3 ... Si-doped n-type AlGaN layer 4 ... Si-doped n-type In0.05Ga0.95N layer 5 ... Undoped In0.2Ga0. 8N active layer 6 ... Mg-doped p-type Al0.15Ga0.85N layer 7 ...- Mg-doped p-type Al0.3Ga0.7N layer 8 ... p-type GaN layer 10 ... Negative electrode 11 .... Positive electrode

Claims (3)

[Claims] 1. In _a Al _b Ga.
_In a method of epitaxially growing a nitride semiconductor represented by _1-ab N (0 ≦ a, 0 ≦ b, a + b ≦ 1) on a substrate, SiC is used as the substrate and the X value is gradually reduced on the SiC substrate. Compositionally graded Al _X Ga _1-X N
A method for growing a nitride semiconductor, which comprises growing a (0 ≦ X ≦ 1) layer and growing the nitride semiconductor on the Al _X Ga _{1- X} N layer. 2. A layer between the Al _x Ga _{1 -x} N layer and the substrate.
The method for growing a nitride semiconductor according to claim 1, wherein an IN layer is grown. 3. The growth of the nitride semiconductor according to claim 1, wherein the Al _X Ga _1-X N layer is composed of a multilayer film in which layers having different X values are laminated. Method.