JP2004112002A - Nitride semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor light emitting element having a low leakage current and a high electrostatic withstand voltage. <P>SOLUTION: The nitride semiconductor element having an active layer made of a nitride semiconductor between a p-side layer and an n-side layer, both being made of two or more nitride semiconductor layers, respectively, wherein the p-side layer includes a p-type contact layer as a layer for forming an ohmic electrode and the p-type contact layer is made up of the p-type nitride semiconductor layers and the n-type nitride semiconductor layers stacked alternately. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a light emitting element such as a light emitting diode (LED), a laser diode (LD), a solar cell, a light sensor, a light receiving element, or a nitride semiconductor (for example, InaAlbGa1-) used for an electronic device such as a transistor or a power device. a-bN, 0 ≦ a, 0 ≦ b, a + b ≦ 1).

Nitride semiconductors are attracting attention as semiconductor materials capable of forming blue light-emitting devices (LEDs, LDs) and pure green light-emitting devices, and are being actively researched and developed. At present, high-intensity blue LEDs, pure green LEDs, etc. have been put to practical use as light sources for full-color LED displays, traffic lights, image scanner light sources, etc. as devices using this nitride semiconductor. It is expected to be. These LED elements basically include a buffer layer made of GaN on a sapphire substrate, an n-side contact layer made of Si-doped GaN, InGaN having a single quantum well structure (SQW: Single-Quantum-Well), or InGaN. And a p-side cladding layer made of Mg-doped AlGaN and a p-side contact layer made of Mg-doped GaN are sequentially stacked. For example, at 20 mA, a blue LED with an emission wavelength of 450 nm has an excellent characteristic of 5 mW, an external quantum efficiency of 9.1%, and a green LED of 520 nm has an excellent external quantum efficiency of 3 mW and 6.3%.
JP 2000-101142 A

However, as the use of nitride semiconductor devices expands in the future, it is expected that further reduction in leakage current and improvement in electrostatic withstand voltage in addition to emission intensity and emission efficiency are desired.

Therefore, an object of the present invention is to provide a nitride semiconductor light emitting device having a low leakage current and a high electrostatic withstand voltage.

To achieve the above object, a nitride semiconductor device according to the present invention has a nitride semiconductor having an active layer made of a nitride semiconductor between a p-side layer and an n-side layer each made of a plurality of nitride semiconductor layers. An element, wherein the p-side layer includes a p-type contact layer as a layer forming a p-ohmic electrode, and the p-type contact layer is formed by alternately stacking p-type nitride semiconductor layers and n-type nitride semiconductor layers. And a superlattice p-type layer formed by alternately forming a first layer made of p-AlGaN and a second layer made of p-InGaN or p-GaN between the p-type contact layer and the active layer. Is further provided.
In the nitride semiconductor light emitting device according to the present invention thus configured, since the p-type contact layer is formed by alternately stacking the p-type nitride semiconductor layer and the n-type nitride semiconductor layer, When a voltage is applied in the negative direction on the p-side and positive in the n-side, the electrostatic breakdown voltage (electrostatic breakdown voltage) can be increased and the leak current can be reduced. It is considered that this is mainly because a reverse bias voltage is applied to the pn junction in the p-type contact layer.
In the nitride semiconductor device according to the present invention, the p-type nitride semiconductor layer may be doped with Mg.
Further, in the nitride semiconductor device according to the present invention, it is preferable that each of the first layer and the second layer of the superlattice p-type layer is doped with more Mg than the p-type nitride semiconductor layer.
In the nitride semiconductor device according to the present invention, the superlattice p-type layer may be in contact with the p-type contact layer.
Further, in the nitride semiconductor device according to the present invention, the thickness of the first layer and the thickness of the second layer are larger than the thickness of the n-type nitride semiconductor layer and the thickness of the p-type nitride semiconductor layer. Preferably, it is thin.
Furthermore, in the nitride semiconductor device according to the present invention, the p-type ohmic electrode may be formed on substantially the entire surface of the p-type contact layer, and a p-pad electrode may be formed on a part of the p-type ohmic electrode. Good.

In the nitride semiconductor device according to the present invention, the thickness ratio of the p-type nitride semiconductor layer to the n-type nitride semiconductor layer (the thickness of the p-type nitride semiconductor layer / the thickness of the n-type nitride semiconductor layer) is It is preferable that the number is set to 1 or more and 9 or less.

{In the nitride semiconductor device according to the present invention, it is preferable that the thickness of the n-type nitride semiconductor layer is 60 ° or less so as not to increase a forward voltage.

Further, in the nitride semiconductor device according to the present invention, the n-type nitride semiconductor layer is doped with Si in order to obtain good n-type conductivity, and the p-type nitride semiconductor layer is obtained in order to obtain good p-type conductivity. It is preferable that the material semiconductor layer is doped with Mg.

In the nitride semiconductor device according to the present invention, preferably, the n-type nitride semiconductor layer is made of GaN doped with Si, and the p-type nitride semiconductor layer is made of GaN doped with Mg. Thereby, the resistivity of the p-type contact layer can be further reduced.
In the nitride semiconductor device according to the present invention, the n-type nitride semiconductor layer may be an undoped layer, and the p-type nitride semiconductor layer may be doped with Mg. In this case, it is preferable that the n-type nitride semiconductor layer is made of undoped GaN, and the p-type nitride semiconductor layer is made of GaN doped with Mg.

As described above in detail, in the nitride semiconductor device according to the present invention, since the pn junction is formed in the p-type contact layer, electrostatic breakdown when a voltage is applied in the positive and reverse directions. Voltage (electrostatic withstand voltage) can be increased and leak current can be reduced.
This makes it possible to apply the present invention to applications requiring higher electrostatic withstand voltage.

Hereinafter, a nitride semiconductor device according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic cross-sectional view showing the structure of a nitride semiconductor device (LED device) according to an embodiment of the present invention. The nitride semiconductor device according to this embodiment has a structure on a sapphire substrate 1.
(1) AlGaN buffer layer 2,
(2) undoped GaN layer 3,
(3) n-type contact layer 4 made of Si-doped GaN,
(4) undoped GaN layer 5,
(5) Si-doped GaN layer 6,
(6) undoped GaN layer 7,
(7) GaN / InGaN superlattice n-type layer 8,
(8) an active layer 9 having a multiple quantum well structure in which an InGaN layer is a well layer and a GaN layer is a barrier layer;
(9) p-AlGaN / p-InGaN superlattice p-type layer 10,
(10) Mg-doped GaN / Si-doped GaN modulation-doped p-side contact layer 11,
Are sequentially laminated, and are formed by forming p-side and n-side electrodes as follows.

The n-ohmic electrode 21 is formed, for example, by removing the portion from the p-side contact layer 11 to the undoped GaN layer 5 at the corner of the element by etching, exposing a part of the n-type contact layer 4, and exposing the exposed n-type contact layer. 4 is formed.
As the p-side electrode, a p-ohmic electrode 22 is formed on almost the entire surface of the p-side contact layer 11, and a p-pad electrode 23 is formed on a part of the p-ohmic electrode 22.
Here, in particular, the nitride semiconductor device of the present embodiment is characterized in that the p-side contact layer 11 is constituted by a modulation doping layer in which an Mg-doped GaN layer 11a and a Si-doped GaN layer 11b are alternately stacked. This reduces the leakage current and improves the electrostatic withstand voltage.
In the present embodiment, the preferable doping amount of Si in the p-side contact layer 11 (Si-doped GaN layer 11b) is 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²¹ / cm ³ , and more preferably 1 × 10 ¹⁸ / cm ³ . Adjust to a range of cm ^{3 to} 5 × 10 ¹⁹ / cm ³ . When the concentration is 1 × 10 ¹⁷ / cm ³ or more, the effect of reducing the leakage current becomes remarkable, and when it is larger than 1 × 10 ²¹ / cm ³ , the crystallinity is deteriorated and the luminous efficiency tends to decrease. is there.

Further, the preferable doping amount of Mg in the p-side contact layer 11 (Mg-doped GaN layer 11a) is 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ , and more preferably 1 × 10 ¹⁹ / cm ³ . 3 × 10 ²⁰ / cm ³ . By setting it to 1 × 10 ¹⁸ / cm ³ or more, a better ohmic contact with the p-ohmic electrode can be obtained, and if it is larger than 1 × 10 ²¹ / cm ³ , as in the case where a large amount of Si is doped, the crystal becomes This is because the sex becomes worse.

In the present invention, the p-AlGaN / p-InGaN superlattice p-type layer 10 functions as a cladding layer and serves as a layer for confining light and injecting holes into the active layer.
The p-AlGaN / p-InGaN superlattice p-type layer 10 is doped with a p-type impurity, for example, Mg in order to be p-type. The doping amount of Mg may be the same or different, but is preferably set to an amount smaller than the doping amount of Mg in the Mg-doped GaN layer 11a of the p-side contact layer, thereby setting Vf (forward direction). Voltage).
Further, the p-InGaN layer of the p-AlGaN / p-InGaN superlattice p-type layer 10 can also be constituted by a Mg-doped GaN layer.

In the p-AlGaN / p-InGaN (p-GaN) superlattice p-type layer 10, the thickness of each of the p-AlGaN layer and the p-InGaN (p-GaN) layer is 100 ° or less, more preferably 70 ° or less. , More preferably in the range of 10 to 40 °. In this case, the thickness of the p-AlGaN layer and the thickness of the p-InGaN (p-GaN) layer may be the same or different. The superlattice p-type layer 10 is formed by alternately growing a p-AlGaN layer and a p-InGaN (p-GaN) layer. For example, even if the p-AlGaN layer is stacked from the p-AlGaN layer and ends with the p-AlGaN layer. Often, one may start with a p-InGaN (p-GaN) layer and end with a p-InGaN (p-GaN) layer. However, since the InGaN layer is easily thermally decomposed, it is preferable that the surface of the InGaN layer be terminated with the p-AlGaN layer so as not to be exposed to a high-temperature atmosphere for a long time.

Further, the total thickness of the p-type AlGaN / p-InGaN (p-GaN) superlattice p-type layer 10 is preferably set to 2000 ° or less, more preferably, to increase the light emission output and lower the Vf. Is set at 1000 ° or less, more preferably at 500 ° or less.
In addition, it is preferable that each film thickness of the p-AlGaN / p-InGaN (p-GaN) superlattice p-type layer 10 be smaller than each film thickness of the p-type contact layer. That is, the layer adjacent to the p-type contact layer of the multilayer film is a superlattice layer, and each film thickness is made smaller than the respective film thicknesses of the n-type layer and the p-type layer of the p-type contact layer. A nitride semiconductor device having a high withstand voltage can be configured.

Further, in the present embodiment, the form using the p-AlGaN / p-InGaN superlattice p-type layer 10 has been described, but the present invention is not limited to this, and at least AlGaN Alternatively, a single AlGaN layer may be used. By using a p-AlGaN / p-InGaN superlattice, the crystallinity becomes better than that of an AlGaN single layer, and the resistivity further decreases, and _Vf tends to decrease.

In the above embodiment, in order to reduce Vf, as a preferable mode, the p-type contact layer is preferably formed of an n-type nitride semiconductor layer made of GaN (Si-doped GaN layer) and a p-type nitride semiconductor layer (Mg-doped GaN layer). Layer), but the present invention is not limited to this. In addition, in the case of InGaN containing a small amount of In or AlGaN containing a small amount of Al, substantially the same effect as GaN can be obtained. Further, even if other trace elements (elements other than In and Al) are contained in GaN, the same effect as GaN can be obtained.

Further, in the above embodiment, the Si-doped GaN layer was used as the n-type nitride semiconductor layer constituting the p-type contact layer. However, the present invention is not limited to this. It may be composed of an undoped n-type layer. That is, in the present invention, an undoped nitride semiconductor layer may be used as the n-type nitride semiconductor layer by utilizing the fact that the undoped nitride semiconductor layer exhibits n-type conductivity. When an undoped nitride semiconductor layer is used as the n-type nitride semiconductor layer, it is preferable to use an undoped GaN layer. More preferably, the p-type contact layer is formed by combining the undoped GaN layer and the Mg-doped GaN layer.

Hereinafter, the present invention will be described more specifically with reference to examples.
First, as Example 1, three types of samples in which the thickness ratio of the Mg-doped GaN layer 11a and the Si-doped GaN layer 11b in the p-side contact layer 11 were changed were manufactured, and the electrostatic withstand voltage characteristics in the opposite directions were evaluated. did.
In the first embodiment, the thickness of each semiconductor layer is set as shown in Table 1, and the ratio of the thickness of the Mg-doped GaN layer 11a to the thickness of the Si-doped GaN layer 11b in the p-side contact layer 11 of each sample is shown in Table 2. As shown.

Table 1

　尚、本実施例１において、ＧａＮ層１１ａのＭｇドープ量は１×１０^２０ｃｍ^−３とし、ＧａＮ層１１ｂのＳｉドープ量は５×１０^１８ｃｍ^−３とした。
　また、各サンプルは１つのＧａＮ層１１ａと１つのＧａＮ層１１ｂとを１周期として１０周期とした。 Table 2

In the first embodiment, the Mg doping amount of the GaN layer 11a was 1 × 10 ²⁰ cm ^−3, and the Si doping amount of the GaN layer 11b was 5 × 10 ¹⁸ cm ⁻³ .
In each sample, one cycle of one GaN layer 11a and one GaN layer 11b was set to 10 cycles.

FIG. 2 is a graph showing the results of evaluating the electrostatic breakdown voltages of the samples 1 to 3 manufactured as described above.
The vertical axis of the graph in FIG. 2 is a value normalized by the electrostatic breakdown voltage of the reference sample (comparative example). This reference sample was configured in the same manner as in Example 1 except that the p-side contact layer was a single layer of GaN doped with Mg at 1 × 10 ²⁰ cm ⁻³ .
As shown in the graph of FIG. 2, it was confirmed that the electrostatic breakdown voltage of each of Samples 1 to 3 of Example 1 was higher than that of Comparative Example.
It was also confirmed that the electrostatic breakdown voltage can be maximized by setting the film thickness ratio to 7: 3.

Modification of First Embodiment In the first embodiment, an AlGaN or GaN layer having a low impurity concentration can be formed between the superlattice p-type layer and the p-type contact layer 11, whereby the electrostatic breakdown voltage can be further reduced. Can be higher. This low concentration AlGaN or GaN layer is preferably formed with a thickness of 0.5 μm or less, for example, 0.2 μm. This layer may be formed undoped, or may be formed while doping with a p-type impurity, for example, Mg. However, in the case where the layer is formed while doping with Mg, the Mg concentration of the adjacent layer is lower than that of the adjacent layer. To By doing so, the electrostatic withstand voltage can be made higher than that of the device of the first embodiment.

Samples 4 in which the thickness of the Mg-doped GaN layer 11a was 36 ° and the thickness of the Si-doped GaN layer 11b was 84 ° were added to the samples 1 to 3 of Example 1, and the Si in the Si-doped GaN layer 11b was changed for each sample. The forward voltage and the light emission output were evaluated for each sample while changing the doping amount from 0 to 1.5 × 10 ¹⁹ cm ⁻³ .
The results are shown in FIGS.
As shown in FIG. 3, it was confirmed that the forward voltage was not increased in Samples 1 to 3, and as shown in FIG. 4, the light emission output of Samples 1 to 4 was equivalent to that of the reference sample. Or more.
In addition, E + 18 and E + 19 in FIG. 4 mean (× 10 ¹⁸ ) and (× 10 ¹⁹ ), respectively, and the unit is cm ⁻³ .

In the third embodiment, the lamination period of the Mg-doped GaN layer 11a and the Si-doped GaN layer 11b is fixed to 10 periods, the thickness of the Mg-doped GaN layer 11a is set to 84 °, and the thickness of the Si-doped GaN layer 11b is set to 36 °. Three types of samples were prepared in which the Si doping amount of the Si-doped GaN layer 11b was 1.0 × 10 ¹⁸ / cm ³ , 2.5 × 10 ¹⁸ / cm ³ , and 5 × 10 ¹⁸ / cm ^3. Then, the electrostatic breakdown voltage was measured.
The result is shown in FIG.
As shown in FIG. 5, it was confirmed that the electrostatic breakdown voltage was improved as the Si doping amount in the Si-doped GaN layer 11b was increased.

　ここで、ＳｉドープＧａＮ層１１ｂのＳｉドープ量は、５×１０^１８／ｃｍ^３とした。
　その結果を、図６、図７及び図８に示す。
　図６及び図７に示すように、順方向電圧及び発光出力は積層周期数にはほぼ依存しないことが確認された。
　また、図８に示すように、静電破壊電圧は、１０周期の場合が最も高くなり、次は１５周期の場合であった。 In Example 4, five types of samples shown in Table 3 below were manufactured by fixing the thickness ratio of the Mg-doped GaN layer 11a and the Si-doped GaN layer 11b to 7: 3 and changing the period. The forward voltage, light emission output, and electrostatic breakdown voltage were measured.
Table 3

Here, the Si doping amount of the Si-doped GaN layer 11b was 5 × 10 ¹⁸ / cm ³ .
The results are shown in FIGS. 6, 7 and 8.
As shown in FIGS. 6 and 7, it was confirmed that the forward voltage and the light emission output were almost independent of the number of lamination cycles.
As shown in FIG. 8, the electrostatic breakdown voltage was highest in the case of 10 periods, and was next in the case of 15 periods.

In the fifth embodiment, the ratio of the film thickness (84 °) of the Mg-doped GaN layer 11a to the film thickness (36 °) of the Si-doped GaN layer 11b is set to 7: 3, and this is repeated for 10 periods, and the contact layer is repeated. The forward voltage and the light emission output were evaluated by variously changing the Si doping amount of the Si-doped GaN layer 11b in the range of 0 to 1.5 × 10 ¹⁹ / cm ³ .
The results are shown in FIGS.
As shown in FIG. 9, it was confirmed that the light emission output and the forward voltage hardly depended on the Si doping amount of the Si-doped GaN layer 11b.

In the sixth embodiment, the ratio of the film thickness (84 °) of the Mg-doped GaN layer 11a to the film thickness (36 °) of the Si-doped GaN layer 11b is set to 7: 3, and this is repeated for 10 periods. In each of the samples in which the Si doping amount of the Si-doped GaN layer 11b was variously changed within the range of 0 to 1.5 × 10 ¹⁹ / cm ³ , the hole measurement was performed before and after the thermal annealing.
The heat treatment was performed at 650 ° C. for 0.5 hour.
The result is shown in FIG.
As a result, it was confirmed that the thermal annealing significantly reduced the resistivity of the sample with a relatively large doping amount of 5 × 10 ¹⁸ / cm ³ and 1 × 10 ¹⁹ / cm ^{3 in} the Si-doped GaN layer 11b. Was done.
These resistivity values are lower than 10 Ω · cm, which is the resistivity when the p-contact layer is formed of a single layer film of p-GaN, and the Mg-doped GaN layer 11 a and the Si-doped GaN layer It has been confirmed that a contact layer formed by alternately stacking 11b and 11b is also effective for lowering the resistance.

FIG. 1 is a schematic sectional view of a nitride semiconductor device according to an embodiment of the present invention. 4 is a graph showing the electrostatic breakdown voltage of each sample of Example 1 according to the present invention. 9 is a graph showing the forward voltage of each sample of Example 2 according to the present invention. 7 is a graph showing the light emission output of each sample of Example 2. 9 is a graph showing the electrostatic breakdown voltage of each sample of Example 3 according to the present invention. 14 is a graph showing the forward voltage of each sample of Example 4 according to the present invention. 14 is a graph showing the light emission output of each sample of Example 4. 13 is a graph showing the electrostatic breakdown voltage of each sample of Example 4. 14 is a graph showing the forward voltage of each sample of Example 5 according to the present invention. 15 is a graph showing the light emission output of each sample of Example 5. It is a graph which shows the resistivity before and after the heat treatment of each sample of Example 6 concerning this invention.

Explanation of reference numerals

1: Sapphire substrate,
2 ... buffer layer,
3 ... undoped GaN layer,
4 ... n-type contact layer,
5 ... undoped GaN layer
6 ... Si-doped GaN layer,
7 ... undoped GaN layer,
8 ... GaN / InGaN superlattice n-type layer,
9: an active layer having a multiple quantum well structure,
10 ... p-AlGaN / p-InGaN superlattice p-type layer,
11: Mg-doped GaN / Si-doped GaN modulation-doped p-side contact layer,
11a: Mg-doped GaN layer,
11b: Si-doped GaN layer,
21 ... n ohmic electrode,
22 ... p ohmic electrode,
23 ... p pad electrode.

Claims (6)

A nitride semiconductor device having an active layer made of a nitride semiconductor between a p-side layer and a n-side layer each made of a plurality of nitride semiconductor layers,
The p-side layer includes a p-type contact layer as a layer forming a p-ohmic electrode, and the p-type contact layer is formed by alternately stacking a p-type nitride semiconductor layer and an n-type nitride semiconductor layer,
A superlattice p-type layer formed by alternately forming a first layer made of p-AlGaN and a second layer made of p-InGaN or p-GaN between the p-type contact layer and the active layer; A nitride semiconductor device characterized by the above-mentioned. The nitride semiconductor device according to claim 1, wherein the p-type nitride semiconductor layer is doped with Mg. 3. The nitride semiconductor device according to claim 2, wherein the first layer and the second layer of the superlattice p-type layer are each doped with a smaller amount of Mg than the p-type nitride semiconductor layer. 4. The nitride semiconductor device according to claim 1, wherein the superlattice p-type layer is in contact with the p-type contact layer. 5. The film thickness of the first layer and the film thickness of the second layer are smaller than the film thickness of the n-type nitride semiconductor layer and the film thickness of the p-type nitride semiconductor layer. 6. A nitride semiconductor device according to any one of the above. 6. The p-type ohmic electrode according to claim 1, wherein the p-type ohmic electrode is formed on substantially the entire surface of the p-type contact layer, and a p-pad electrode is formed on a part of the p-type ohmic electrode. The nitride semiconductor device as described in the above.

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