JPH08213656A - Gallium nitride based compound semiconductor light emitting element - Google Patents

Abstract

PURPOSE: To provide a gallium nitride based compound semiconductor light emitting element wherein gallium nitride based compound semiconductor doped with P-type impurities is made a low resistance P-type, the resistance value is uniform all over the wafer, independently of the film thickness, and an light emitting element can be constituted as double hetero/single hetero structure. CONSTITUTION: The element has at least a PN junction obtained by annealing at a temperature higher than or equal to 400 deg.C, after at least an N-type gallium nitride based compound semiconductor layer and a gallium nitride based compound semiconductor layer doped with P-type impurities are grown on a substrate by a vapor growth method.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device made of a gallium nitride-based compound semiconductor used in a light emitting device such as an ultraviolet light emitting diode, a blue light emitting laser diode, an ultraviolet light emitting diode, and a blue light emitting diode. The present invention relates to a light emitting element.

[0002]

2. Description of the Related Art A blue light emitting device is a ZnSe, I of II-VI group.
Research progressed using V-IV group SiC, III-V group GaN, etc.
Recently, gallium nitride-based compound semiconductors
(In _XAl_YGa_1-XYN (0≤X, 0≤Y, X + Y≤1)
However, it was announced that it emits relatively excellent light at room temperature.
Is being watched. Blue light emitting device having the nitride semiconductor
Is basically a general formula on a sapphire substrate.
InXAlYGa1-X-YN (0≤X, 0≤Y, X + Y≤1)
The epitaxial layers of the nitride semiconductor represented are n-type and
And a structure having a laminated structure of i-type or p-type
Is.

As a method for stacking nitride semiconductors, a metal organic compound vapor phase epitaxy method (hereinafter referred to as MOCVD method),
A vapor phase growth method such as a molecular beam epitaxy method (hereinafter referred to as MBE method) is well known. For example, a method using the MOCVD method will be briefly described. In this method, an organometallic compound gas {trimethylgallium (TMG), trimethylaluminum (TMA), ammonia, etc.) is used as a reaction gas in a reaction container in which a sapphire substrate is installed. To maintain the crystal growth temperature at a high temperature of about 900 ° C. to 1100 ° C. to grow the nitride semiconductor on the substrate, and to supply other impurity gas as necessary, while the nitride semiconductor is n-type , I-type, or p-type.
In addition to sapphire, the substrate includes SiC, Si, etc., but sapphire is generally used. As n-type impurities, Si, Ge, and Sn (however, in the case of a nitride semiconductor, there is a property of becoming n-type without doping with n-type impurities) are well known, and as p-type impurities, Zn, Cd, B
e, Mg, Ca, Ba and the like are mentioned, and among them, M
The best known are g and Zn.

When a nitride semiconductor is directly grown on a sapphire substrate at a high temperature as one of the methods for forming a nitride semiconductor by MOCVD, the surface state and crystallinity of the nitride semiconductor are remarkably deteriorated. Before doing this, first 60
A buffer layer made of AlN is formed at a low temperature around 0 ° C.,
Subsequently, it has been clarified that the crystallinity is remarkably improved by growing it on the buffer layer at a high temperature (JP-A-2-229476). In addition, the present inventor has shown in Japanese Patent Application No. 3-89840 that a crystalline nitride semiconductor having a better GaN layer can be stacked by using a GaN buffer layer than by a conventional method using AlN as a buffer layer.

However, a blue light emitting device having a nitride semiconductor has not yet been put to practical use. This is because the nitride semiconductor cannot be made into a p-type having a low resistance, so that a light emitting element having various structures such as double hetero and single hetero cannot be formed. Even if a nitride semiconductor doped with a p-type impurity is grown by a vapor phase growth method, the obtained nitride semiconductor does not become a p-type and a high-resistance semi-insulating material having a resistivity of 10 ⁸ Ω · cm or more. That is, the actual situation is that it becomes i-type. Therefore, at present, as the structure of the blue light emitting element, only a so-called MIS structure in which a buffer layer, an n-type layer, and an i-type layer are sequentially stacked on a substrate is known.

[0006]

As means for lowering the resistance of a high resistance i-type and making it closer to a p-type, Japanese Patent Laid-Open No. 2525/1990.
In Japanese Patent No. 7679, a high-resistance i-type gallium nitride compound semiconductor doped with Mg as a p-type impurity is formed on the uppermost layer, and then the surface is irradiated with an electron beam with an accelerating voltage of 6 kV to 30 kV. 0.5 μm
A technique for reducing the resistance of the layer is disclosed. However, in this method, only the penetration depth of the electron beam, that is, only the extreme surface can be lowered, and since the entire wafer must be irradiated while scanning the electron beam, there is a problem that the resistance cannot be uniformly lowered in the plane. there were.

Therefore, an object of the present invention is to use a p-type impurity-doped nitride semiconductor as a p-type having a low resistance, a resistance value is uniform over the entire wafer regardless of the film thickness, and a light-emitting element is double-hetero. The present invention provides a light emitting device made of a nitride semiconductor having a pn junction capable of forming a single hetero structure.

[0008]

A nitride semiconductor light emitting device of the present invention is formed on a substrate by vapor deposition at least n.
Having at least one pn junction obtained by growing a p-type nitride semiconductor layer and a p-type impurity-doped nitride semiconductor layer and then annealing the whole at a temperature of 400 ° C. or higher. Is characterized by.

Annealing: Annealing
After the growth of the p-type impurity-doped nitride semiconductor layer, the process may be carried out in the reaction container, or the wafer may be taken out of the reaction container and used in a device dedicated to annealing. Annealing atmosphere is vacuum, N ₂ , He, N
It is carried out in an atmosphere of an inert gas such as e, Ar or a mixed gas thereof, and preferably in a nitrogen atmosphere pressurized at a decomposition pressure of the nitride semiconductor or higher at the annealing temperature. Because by pressurizing as a nitrogen atmosphere,
This is because N has a function of preventing N in the nitride semiconductor from decomposing and flowing out during annealing.

For example, in the case of GaN, the decomposition pressure of GaN is 8
About 0.01 atmosphere at 00 ° C, about 1 atmosphere at 1000 ° C, 11
It is about 10 atm at 00 ° C. For this reason, when the nitride semiconductor is annealed at 400 ° C. or higher, the nitride semiconductor is more or less decomposed, and its crystallinity tends to deteriorate. Therefore, by pressurizing with nitrogen as described above, decomposition can be prevented.

The annealing temperature is 400 ° C. or higher, preferably 700 ° C. or higher, and the annealing is performed for 1 minute or more, preferably 10 minutes or more. Even at 1000 ° C. or higher, decomposition can be prevented by pressurizing with nitrogen as described above, and as will be described later, a stable p-type nitride semiconductor having excellent crystallinity can be obtained.

As a means for suppressing decomposition of the nitride semiconductor during annealing, a cap layer is further formed on the nitride semiconductor layer doped with p-type impurities.
You may anneal. The cap layer is a protective film, which is formed on a nitride semiconductor doped with a p-type impurity and then annealed at 400 ° C. or higher, not to mention under pressure, under reduced pressure and normal pressure. Even under pressure, the nitride semiconductor can be made to have a low resistance without being decomposed.

To form the cap layer, a nitride semiconductor layer doped with a p-type impurity may be formed and then formed in the reaction vessel, or the wafer may be taken out of the reaction vessel and then the other layer may be formed. Crystal growth equipment, eg plasma CVD
It may be formed by a device or the like. As the material of the cap layer,
Any material that can be formed on the nitride semiconductor and is stable at 400 ° C. or higher may be used, and Ga _X Al _1-X N (where 0 ≦ X ≦ 1) and Si ₃ N ₄ are preferable. , Si
O ₂ can be used, and the type of material is appropriately selected depending on the annealing temperature. The thickness of the cap layer is usually 0.01 to 5 μm. If the thickness is less than 0.01 μm, the effect as a protective film cannot be sufficiently obtained, and the thickness is 5 μm.
If it is thicker than m, it takes time to remove the cap layer by etching after the annealing to expose the p-type nitride semiconductor layer, which is not economical.

[0014]

FIG. 1 is a diagram showing that a nitride semiconductor layer doped with a p-type impurity changes to a low-resistance p-type by annealing. In this method, a GaN buffer layer is first formed on a sapphire substrate using the MOCVD method, and a GaN layer of 4 μm is formed on the GaN buffer layer while doping Mg as a p-type impurity.
FIG. 7 is a diagram in which after the wafer having a thickness of m is formed, the wafer is taken out, the temperature is changed, annealing is performed in a nitrogen atmosphere for 10 minutes, hole measurement is performed on the wafer, and the resistivity is plotted as a function of the annealing temperature. .

As can be seen from this figure, the resistivity of the Mg-doped GaN layer sharply decreases from above 400 ° C., and from 700 ° C. or above, it exhibits a substantially constant low-resistance p-type characteristic, and the annealing The effect is appearing. In addition,
The hole measurement results of the non-annealed GaN layer and the GaN layer annealed at 700 ° C. or higher showed that the GaN layer before annealing had a resistivity of 2 × 10 ⁵ Ω · cm and a hole carrier concentration of 8 × 10 ¹⁰ / cm ^3. On the other hand, the GaN layer after annealing has a resistivity of 2 Ω · cm and a hole carrier concentration of 2 ×.
It was 10 ¹⁷ / cm ³ . In addition, although this figure shows GaN, In similarly doped with p-type impurities
It was confirmed that similar results were obtained also in XAlYGa1-X-YN (0≤X, 0≤Y, X + Y≤1).

Further, the 4 μm GaN layer annealed at 700 ° C. is etched to a thickness of 2 μm,
As a result of hole measurement, hole carrier concentration is 2 × 10
^{The value was 17} / cm ³ and the resistivity was ³ Ω · cm, which were almost the same values as before etching. That is, the GaN layer doped with the p-type impurity was p-type with low resistance throughout the entire region uniformly in the depth direction by annealing.

In addition, FIG. 2 also shows a wafer in which a GaN buffer layer and a Mg-doped 4 μm GaN layer are formed on a sapphire substrate by the same MOCVD method.
Annealing was performed for 20 minutes in a nitrogen atmosphere at 000 ° C., and H was applied to the p-type GaN layer of the wafer (a) performed under a pressure of 20 atm and the wafer (b) performed at atmospheric pressure.
It is a figure which irradiates e-Cd laser as an excitation light source, and compares and shows crystallinity by the photoluminescence intensity,
The stronger the blue emission intensity of the photoluminescence at 450 nm, the better the crystallinity can be evaluated.

As shown in FIG. 2, when annealing is performed at a high temperature of 1000 ° C. or higher, the GaN layer tends to be thermally decomposed and its crystallinity tends to deteriorate. However, pressurization prevents the thermal decomposition. P-type G with excellent crystallinity
An aN layer is obtained.

Further, FIG. 3 shows a wafer (c) having a GaN buffer layer and a Mg-doped 4 μm GaN layer formed on a sapphire substrate, and an AlN layer of 0.5 μm as a cap layer on the wafer (c). A wafer (d) grown to a thickness of 1000
After annealing at ℃ for 20 minutes in nitrogen atmosphere,
P exposed by removing the cap layer by etching
It is a figure which compares and shows the crystallinity of the type GaN layer by photoluminescence intensity similarly.

As shown in FIG. 3, the p-type GaN layer (c) that has been annealed without growing the cap layer undergoes decomposition at a high temperature when the p-type GaN layer is annealed at high temperature, so that the emission intensity at 450 nm is increased. Becomes weaker. However, by growing the cap layer (AlN in this case), the AlN of the cap layer is decomposed but the p-type GaN layer is not decomposed, and therefore the emission intensity still remains strong.

The reason why a p-type nitride semiconductor having a low resistance can be obtained by annealing is presumed to be as follows.

That is, in the growth of the nitride semiconductor layer, N
NH ₃ is generally used as a source, and it is considered that during the growth, this NH ₃ is decomposed to form atomic hydrogen.
It is considered that the atomic hydrogen prevents the p-type impurities such as Mg and Zn from functioning as acceptors by binding to Mg, Zn and the like doped as acceptor impurities. Therefore, the nitride semiconductor doped with the p-type impurity after the reaction has high resistance.

However, by performing post-growth annealing, hydrogen bonded in the form of Mg-H, Zn-H, etc. is thermally dissociated, and it is released from the p-type impurity-doped nitride semiconductor layer. Then, since the p-type impurity normally functions as an acceptor, a low-resistance p-type nitride semiconductor can be obtained. Therefore, it is not preferable to use a gas containing hydrogen atoms such as NH ₃ and H ₂ in the annealing atmosphere. Further, it is not preferable to use a material containing hydrogen atoms also in the cap layer for the above reason.

[0024]

The present invention will be described in detail with reference to the following examples. [Example 1] First, a well-cleaned sapphire substrate is placed on a susceptor in a reaction vessel. After evacuating the inside of the container, the substrate is heated at 1050 ° C. for 20 minutes while flowing hydrogen gas to remove the oxide on the surface. Then increase the temperature to 5
It was cooled to 10 ° C and T was used as a Ga source at 510 ° C.
The GaN buffer layer has a film thickness of 200 angstrom while flowing MG gas at 27 × 10 ⁻⁶ mol / min, ammonia gas as N source at 4.0 liter / min, and hydrogen gas at 2.0 liter / min as carrier gas. Grow with.

Next, only the TMG gas is stopped and the temperature is set to 103
After raising the temperature to 0 ° C., TMG gas was again added to 54 × 10 ⁻⁶
Cp ₂ Mg (cyclopentadienylmagnesium) gas was newly added at a flow ^rate of 3.6 × 10 ⁻⁶ mol / min for 60 minutes to grow a Mg-doped GaN layer at 4 mol / min.
Grow with a film thickness of μm.

After cooling, the wafer thus grown was taken out of the reaction vessel, placed in an annealing apparatus, and annealed by holding it at 800 ° C. for 20 minutes in a nitrogen atmosphere at normal pressure.

As a result of hole measurement of the p-type GaN layer obtained by annealing, excellent p-type characteristics such as a resistivity of 2 Ω · cm and a hole carrier concentration of 2 × 10 ¹⁷ / cm ³ were shown.

Example 2 In Example 1, after growing the Mg-doped GaN layer, Cp ₂ Mg gas was stopped, and then a GaN layer was grown to a thickness of 0.5 μm as a cap layer.

In the same manner as in Example 1, the annealing apparatus was operated under normal pressure in a mixed gas atmosphere of nitrogen and argon at 80 ° C.
Anneal for 20 minutes at 0 ° C. After that, a layer of 0.5 μm is removed from the surface by dry etching,
The p-type GaN layer was exposed by removing the cap layer, and the hole measurement was performed in the same manner. As a result, excellent p-type characteristics such as a resistivity of 2 Ω · cm and a carrier concentration of 1.5 × 10 ¹⁷ / cm ³ were exhibited. The blue emission intensity of photoluminescence at 450 nm was about 4 times stronger than that in Example 1.

[Example 3] In Example 1, after growing the Mg-doped GaN layer, the wafer was taken out of the reaction vessel and annealed at 20 atm in a nitrogen atmosphere at 800 ° C for 20 minutes. As a result of Hall measurement, it showed excellent p-type characteristics with a resistivity of 2 Ω · cm and a carrier concentration of 2.0 × 10 ¹⁷ / cm ^3, and the photoluminescence intensity of 450 nm was about the same as that of Example 1. It was four times stronger.

Example 4 In Example 1, after the Mg-doped GaN layer was grown, the wafer was taken out of the reaction container and a plasma CVD apparatus was used to deposit a SiO ₂ layer having a thickness of 0.5 μm as a cap layer thereon. It is formed with a film thickness.

In the annealing apparatus, a nitrogen atmosphere,
Anneal for 20 minutes at 1000 ° C. under atmospheric pressure.
After that, the SiO ₂ cap layer was removed with hydrofluoric acid to expose the p-type GaN layer, and the hole measurement was performed in the same manner.
It exhibited excellent p-type characteristics with a resistivity of 2 Ω · cm and a carrier concentration of 2.0 × 10 ¹⁷ / cm ³ . In addition, 4 of photoluminescence
The emission intensity at 50 nm was about 20 times stronger than that of the case where annealing was performed under the same conditions without forming the cap layer.

[Embodiment 5] In Embodiment 1, after the Mg-doped GaN layer is grown, Cp ₂ Mg gas is stopped and new TMA gas is added at 6 × 10 ^-6 mol / min Si.
2.2 × 10 ^-10 mol / min of H ₄ (monosilane) gas to 2
Flowing for 0 minutes, n-type Ga _0.9 Al _0.1 doped with Si
The N layer is grown to a thickness of 0.8 μm.

After stopping the TMG gas, TMA gas and SiH ₄ gas and cooling to room temperature while flowing hydrogen gas and ammonia gas, the wafer was taken out and placed in an annealing apparatus and kept at 700 ° C. for 20 minutes in a nitrogen atmosphere. And do annealing.

Thus, the p-type G is formed on the sapphire substrate.
An element having a single hetero structure in which an aN layer and an n-type Ga _0.9 Al _0.1 N layer were sequentially stacked was obtained. In the nitride semiconductor layer of this device, a part of the n-type Ga _0.9 Al _0.1 N layer was etched by a conventional method to expose a part of the p-type GaN layer, and ohmic electrodes were attached to the respective layers, followed by dicing. It was cut into chips with a saw. Electrodes were taken out from the exposed n-type layer and p-type layer and then molded to produce a blue light emitting diode. The characteristics of this light emitting diode showed blue light emission with a light emission output of 90 μW at a forward current of 20 mA and a forward voltage of 5 V, and the peak wavelength was 430 nm. This light emission output is a high value that has never been reported as the output of the blue light emitting diode.

On the other hand, when a light emitting diode having the same single hetero structure was manufactured without annealing, the light emitting diode had a forward voltage of about 60 V at a forward current of 20 mA and emitted light only slightly. It only glowed yellowish and soon broke, and the emission output could not be measured.

[Embodiment 6] Similar to Embodiment 1, a GaN buffer layer is formed on a sapphire substrate to a thickness of 200 Å.

Next, only the TMG gas is stopped and the temperature is set to 103
After raising the temperature to 0 ° C., TMG gas was again 54 × 10 5.
^-6 mol / min and SiH ₄ (monosilane) gas at a flow rate of 2.2 × 10 ⁻¹⁰ mol / min for 60 minutes to grow a Si-doped n-type GaN layer with a thickness of 4 μm. grow up.

Then, the SiH ₄ gas is stopped, and Cp ₂ Mg gas is grown at a flow ^rate of 3.6 × 10 ^-6 mol / min for 30 minutes to grow the Mg-doped GaN layer to a thickness of 2.0 μm.

After stopping the TMG gas and Cp ₂ Mg gas and cooling to room temperature while flowing hydrogen gas and ammonia gas, the gas flowing in the reaction vessel was replaced with nitrogen gas, and nitrogen gas was flowed in the reaction vessel. The temperature is raised to 1000 ° C. and kept in the reaction vessel for 20 minutes for annealing.

The device thus obtained was used as a light emitting diode to emit light in the same manner as in Example 43.
It exhibits blue light emission with an emission peak near 0 nm, the emission output is 50 μW at 20 mA, and the forward voltage is 2 as well.
It was 4 V at 0 mA. Moreover, when an element having the same structure was manufactured as a light emitting diode without performing annealing, 2
It emitted a slight yellow light at 0 mA and immediately the diode broke.

[0042]

As described above, the light emitting device of the present invention is
In contrast to a nitride semiconductor that does not have a low resistance p-type even when doped with a p-type impurity in the past, it has a p-type with a low resistance.
Since n-junction can be realized, it is possible to provide devices having various structures. Further, the conventional method using electron beam irradiation can reduce the resistance of only the uppermost surface, but in the present invention, the entire surface of the nitride semiconductor layer doped with p-type impurities can be made p-type by annealing. The p-type layer can be formed uniformly inside and even in the depth direction, and the p-type layer can be formed on any layer. In addition, since a thick film layer can be formed, a high-luminance blue light emitting element can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an annealing temperature and a resistivity according to an example of the present invention.

FIG. 2 is a diagram showing a comparison of crystallinity of p-type GaN layers according to an example of the present invention by photoluminescence intensity.

FIG. 3 is a diagram showing a comparison of crystallinity of p-type GaN layers according to an example of the present invention by photoluminescence intensity.

Claims (1)

[Claims] 1. After growing at least an n-type gallium nitride compound semiconductor layer and a p-type impurity-doped gallium nitride compound semiconductor layer on a substrate by a vapor phase epitaxy method, the whole is grown at 400 ° C. A gallium nitride-based compound semiconductor light-emitting device having at least one pn junction obtained by annealing at the above temperature.