JP2000133883A - Nitride semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitride semiconductor light-emitting element which can improve photoelectric conversion efficiency without deteriorating light-emitting output. SOLUTION: An active layer 6 of a multiple quantum well structure, in which a well layer formed of a nitride semiconductor containing. In add a barrier layer formed of a nitride semiconductor whose compositions is different from that of the well layer are stacked between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer is provided. The well layer is undoped, and the single film thickness of the barrier layer is 70-500 angstroms. Then, n-type impurities are doped in the barrier layer in the range of 5×1016-1×1019/cm3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a light emitting diode element, a light-emitting element such as a laser diode element, a solar cell, a light receiving element such as an optical sensor or transistor, a power device such as a nitride semiconductor (an In _X used for electronic devices, A
1 _Y Ga _{1 -XYN} , 0 ≦ X, 0 ≦ Y, X + Y ≦ 1).

[0002]

2. Description of the Related Art Nitride semiconductors have been put into practical use as materials for high-brightness blue LEDs and pure green LEDs in various light sources such as full-color LED displays, traffic signal lights, and image scanner light sources. These LED elements are basically
A buffer layer of GaN on a sapphire substrate;
N-side contact layer made of doped GaN and InGa of a single quantum well structure (SQW: Single-Quantum-Well)
Multiple quantum well structure with N or InGaN (M
QW: Multi-Quantum-Well) active layer and Mg-doped A
a p-side cladding layer of lGaN and Mg-doped GaN
And a p-side contact layer composed of a blue LED with an emission wavelength of 450 nm at 5 mA and an external quantum efficiency of 9.1% and 520 nm at 20 mA.
3mW and an external quantum efficiency of 6.3%, which are very excellent characteristics. The multiple quantum well structure has a structure composed of a plurality of minibands, and can efficiently emit light even with a small current. Therefore, improvement in device characteristics such as higher light emission output than a single quantum well structure is expected. You.

[0003] For example, the present applicant has disclosed in Japanese Patent Application Laid-Open No. 9-3643.
No. 0 discloses a nitride semiconductor light emitting device having a multiple quantum well structure active layer including a well layer of 100 Å or less and a barrier layer of 150 Å or less as a device having a high light output. When the active layer of the device has a quantum well structure as described above, a high output is obtained due to a strained quantum well effect, an Exxington light emission effect, and the like.

[0004]

However, the LED element disclosed by the present applicant has a high output, is sufficiently applicable to practical use, and is applied to various products such as signals. Accordingly, it is desired to further improve the photoelectric conversion efficiency so that the light emission output does not decrease even with low power consumption. If such a good light emission output can be obtained with low power consumption, the life characteristics of the element can be further improved. Therefore, an object of the present invention is to provide a nitride semiconductor light emitting device that can increase photoelectric conversion efficiency without lowering light emission output.

[0005]

That is, the present invention can achieve the object of the present invention by the following constitutions (1) to (4). (1) A well layer made of a nitride semiconductor containing In and a barrier layer made of a nitride semiconductor having a composition different from that of the well layer are stacked between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer. An active layer having a multiple quantum well structure, wherein the well layer is undoped, and the barrier layer has a single thickness of 70 nm.
A nitride semiconductor device having a thickness of up to 500 angstroms and a barrier layer doped with an n-type impurity at a concentration of 5 × 10 ¹⁶ to 1 × 10 ¹⁹ / cm ³ . (2) The nitride semiconductor device according to (1), wherein the active layer is grown on an undoped nitride semiconductor layer. (3) The undoped nitride semiconductor layer has a thickness of 10 to 50.
The nitride semiconductor device according to the above (2), which has a thickness of 0 Å. (4) The above (2) or (3), wherein the undoped nitride semiconductor layer is made of undoped GaN.
3. The nitride semiconductor device according to item 1.

That is, according to the present invention, a barrier layer having a multiple quantum well structure is made to have a specific thickness, that is, a thicker film than a conventional one, and this barrier layer contains an n-type impurity at a specific concentration.
Further, by providing an undoped well layer, it is possible to provide a nitride semiconductor device which enables a low forward voltage (Vf) and a small leak current, and can improve photoelectric conversion efficiency without lowering light emission output. Can be.

Japanese Patent Application Laid-Open No. 9-36430 describes that an n-type impurity may be added to the active layer. However, the thickness of the barrier layer and the concentration of the n-type impurity are not limited. Not listed. Also, Japanese Patent Application Laid-Open No. 10-65
No. 271 discloses that Si, Cd, Zn, M
Doping with an impurity such as g is described, and a barrier layer having a thickness of 30 Å is specifically described in the examples. However, it is difficult to improve the photoelectric conversion efficiency to a sufficiently satisfactory level with such a thickness. . Furthermore, Japanese Patent Application Laid-Open No. 10-1
Japanese Patent No. 35514 discloses that impurities may be contained in the barrier layer.
Since the thickness is set to 0 angstroms, it is difficult to improve the photoelectric conversion efficiency to a sufficiently satisfactory level as in the above publication.

On the other hand, the present inventor has made various studies to improve the photoelectric conversion efficiency. As a result, the present inventors have found that the problem can be solved by increasing the thickness of the barrier layer of the active layer of the multiple quantum well structure. I found it. However, when the thickness of the barrier layer is large, the forward voltage (Vf) tends to increase,
Simply increasing the thickness of the barrier layer makes it difficult to achieve high photoelectric conversion efficiency with low power consumption. As a result of various studies on the problem of the increase in Vf, Vf can be reduced by adding an n-type impurity to a thick barrier layer. It has been found that not only does the efficiency drop, but the leak current tends to increase in the IV characteristics. As described above, when the photoelectric conversion efficiency is improved by using the barrier layer as a thick film, Vf increases, and when an n-type impurity is added to lower Vf, the leak current increases, and all these characteristics cannot be satisfied. . In order to solve such a problem, the present invention examines in detail the above configuration, that is, the relationship between the thickness of the barrier layer and the concentration of the n-type impurity added to the barrier layer. By adding a thick and specific concentration of n-type impurity and undoping the well layer and combining the undoped well layer and the barrier layer, a low V
Since the photoelectric conversion efficiency can be improved with f and a small leak current, a good light emission output can be obtained even with low power consumption.

Further, according to the present invention, an active layer having a multiple quantum well structure is provided on an undoped nitride semiconductor layer, preferably with a thickness of 1 nm.
When the active layer is grown and grown on an undoped nitride semiconductor layer having a thickness of 0 to 500 angstroms, and more preferably on an undoped GaN layer, the crystallinity of the active layer becomes better, so that the photoelectric conversion efficiency can be improved and the Vf can be reduced. preferable.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the active layer has a multi-quantum well structure having a multilayer structure in which a well layer and a barrier layer are sequentially stacked. The minimum stacked structure of the multiple quantum well structure is a three-layer structure including one barrier layer and (two) well layers provided on both sides of the barrier layer, or one well layer and two well layers (2 And three barrier layers. In the multiple quantum well structure, the two outermost layers on both sides are each formed of a well layer or a barrier layer.
Further, one outermost layer may be configured as a well layer and the other outermost layer may be configured as a barrier layer. In the multiple quantum well structure, the side close to the p-side nitride semiconductor layer may end with the barrier layer or the well layer.

In such an active layer having a multiple quantum well structure, both the well layer and the barrier layer can be formed of a nitride semiconductor containing indium and gallium (preferably, InGaN) (however, the composition of both). Is different)
However, the well layer may be formed of a nitride semiconductor containing indium and gallium (preferably, InGaN), and the barrier layer may be formed of another nitride semiconductor, for example, InN or GaN. For example, the well layer of the active layer having the multiple quantum well structure is a nitride semiconductor containing at least In, preferably In _x Ga ₁ -xN (0 <X <1). On the other hand, for the barrier layer, a nitride semiconductor having a band gap energy larger than that of the well layer is selected, and preferably, In _Y Ga _1-Y N (0 ≦
Y <1, X> Y) or _{Al Z Ga 1-Z N (} 0 to <Z <0.5). However, the well layer and the barrier layer can be made of InAlN.

In the present invention, the total thickness of the active layer is not particularly limited, but is the total thickness of the stacked layers of the well layer and the barrier layer, for example, specifically, 500 to 5000 Å. Preferably it is 1000-3000 angstroms. When the total thickness of the active layer is within the above range, it is preferable in terms of the light emission output and the time required for crystal growth of the active layer. The single thickness of the barrier layer constituting the multiple quantum well structure of the active layer is 70 to 500 Å,
Preferably it is 100 to 300 angstroms. When the single thickness of the barrier layer is in the above range, the photoelectric conversion efficiency is improved, and the Vf and the leakage current are preferably low. The single thickness of the well layer of the active layer is 100 Å or less, preferably 70 Å or less, more preferably 50 Å or less. The lower limit of the single thickness of the well layer is not particularly limited, but is preferably 10 Å or more. When the single thickness of the well layer is within the above range, it is preferable in terms of emission output and emission spectrum half width.

In the present invention, the well layer of the active layer is undoped (in a state where impurities are not intentionally doped), and only the barrier layer is doped with n-type impurities. The n-type impurities to be doped into the barrier layer include Si, Ge, Sn, S, O, Ti,
A group IV or group VI element such as Zr can be used,
Preferably, Si, Ge, and Sn are used. The doping amount of the n-type impurity into the barrier layer is 5 × 10 ¹⁶ / cm ³ to 1 × 10 ¹⁹ / c
m ³ , preferably 5 × 10 ¹⁶ / cm ³ to 1 × 10 ¹⁸ / cm ³ , more preferably 5 × 10 ¹⁶ / cm ^{3 to} 3 × 10 ¹⁷ / cm ³ . When the doping amount of the n-type impurity is within the above range, the photoelectric conversion efficiency is not reduced, and the leakage current is not increased in the IV characteristics, and Vf is preferably reduced.

Hereinafter, the single film thickness of the barrier layer of the active layer having the multiple quantum well structure and the value of the doping amount of the n-type impurity into the barrier layer will be described in more detail. However, FIGS. 2 and 3
The data are obtained when an active layer having a single thickness of 300 Å and a barrier layer doped with 9 × 10 ¹⁶ / cm ^{3 of} Si of Example 1 described later is used as a barrier layer of the active layer.

The impurity concentration doped into the barrier layer is shown in FIG.
As shown in (a), at an applied voltage of -5 V, the rate of increase of the leak current is small when the concentration is low, but 1 × 1
If it exceeds 0 ¹⁹ / cm ³ , the rate of increase in leak current tends to increase. On the other hand, as shown in FIG. 2B, as the impurity concentration is gradually decreased, the forward voltage increases in inverse proportion to the doping amount, but the impurity doping amount is 5 × 10
^{When it is 16} / cm ³ or more, the increase in the forward voltage tends to increase. Further, as shown in FIG. 3A, as the single thickness of the barrier layer increases, the forward voltage tends to increase in proportion to the increase in the thickness, but the durability and the standard of the nitride semiconductor element are not so high. From the viewpoint, the forward voltage is preferably 3.8 V or less. For this reason, the upper limit of the single thickness of the barrier layer is set to 500 Å or less. As shown in FIG. 3 (b), in the case of a thin film, the luminous output is small, and when the single film thickness is gradually increased to 70 Å, the luminous output rises with a sharp rise, but is increased to 70 Å or more. Then, the light emission output tends to increase slowly. For this reason, the lower limit of the single thickness of the barrier layer is set to 70 Å or more.

From the above, as a result of various investigations to solve various problems caused by increasing the single film thickness of the barrier layer in order to increase the photoelectric conversion efficiency and simultaneously improve the photoelectric conversion efficiency, The single thickness of the barrier layer is 70 to 5
The solution of various problems and the photoelectric conversion efficiency caused by making the barrier layer thick by setting the thickness of the barrier layer to 5 × 10 ¹⁶ / cm ³ to 1 × 10 ¹⁹ / cm ³ by setting the barrier layer to an impurity doping of 5 × 10 ¹⁶ / cm ³ to 1 × 10 ¹⁹ / cm ³ Can be improved.

In the present invention, the undoped nitride semiconductor layer formed immediately before the growth of the active layer refers to a layer formed without intentionally doping impurities, the diffusion of impurities from an adjacent layer, a material or a device. Even if a layer is mixed with an impurity due to contamination from the substrate, if the impurity is not intentionally doped, the layer is an undoped layer. When an active layer is grown after forming such an undoped nitride semiconductor layer, the crystallinity of the active layer is improved,
The photoelectric conversion efficiency and the half width of the emission spectrum become better.

In the present invention, the undoped nitride semiconductor layer has a larger band gap energy than that of the active layer, and has undoped GaN and a general formula In _a Ga _{1 -a.}
Undoped InGaN represented by N (0 <a <0.5, preferably a <0.2), and a general formula Al _b Ga _1-b N (0 <
undoped A represented by b <1, preferably b <0.2)
Preferably, it is made of any one of lGaN.
In addition, the general formula In _x Al _Y Ga _{1 -XYN} (0 <X, 0 <
Y, X + Y <1). When undoped InGaN is used as the undoped nitride semiconductor layer, the ratio of In to Ga in the undoped InGaN is set to be smaller than that in the well layer.
The thickness of the undoped nitride semiconductor layer is 10 to 500 Å, preferably 10 to 300 Å, more preferably 20 to 100 Å. It is preferable that the film thickness is in the above range in terms of photoelectric conversion efficiency and light emission output.

In the present invention, the n-type and p-type nitride semiconductor layers are not particularly limited, and various types of nitride semiconductor layers can be used in combination. Further, in addition to the undoped nitride semiconductor layer formed immediately before the active layer, various undoped nitride semiconductors having the same composition or different compositions may be used in any part of the nitride semiconductor device. For example, 200 to 9 on a heterogeneous substrate such as sapphire.
It is preferable to grow a buffer layer made of GaN at 00 ° C. because the nitride semiconductor grown on the buffer layer at a high temperature (higher than about 900 ° C.) has good crystallinity. It is preferable to form a layer of Mg-doped AlGaN or the like on the active layer in terms of light emission output. Thus, an element can be formed by combining various nitride semiconductor layers. As a specific example of the nitride semiconductor element, a nitride semiconductor constituting an LED element described in an example described later can be used. Further, as the electrodes, electrodes having various ohmic contacts can be appropriately selected and used.

The undoped nitride semiconductor layer grown just before the growth of the active layer having the multiple quantum well structure is grown just before the growth of the active layer having the single quantum well structure. The photoelectric conversion efficiency can be improved as in the case of immediately preceding growth.

[0021]

EXAMPLES Examples which are embodiments of the present invention will be shown below, but the present invention is not limited to these. Embodiment 1 In Embodiment 1, a light emitting diode (LED) shown in FIG. 1 is formed. A GaN buffer layer 2 having a thickness of 300 Å is grown at 550 ° C. on the C-plane of the sapphire substrate 1.
An undoped GaN layer 3 having a thickness of 7 μm is grown at 1050 ° C. Next, at the same temperature, the Si-doped n-type G
The aN layer 4 is grown to a thickness of 3.3 μm.

Then, before growing the active layer, 105
At 0 ° C., an undoped GaN layer 5 is grown to a thickness of 400 Å.

Next, a barrier layer made of GaN doped with 9 × 10 ¹⁶ / cm ^{3 of} Si is grown to a thickness of 300 angstroms.
Undoped In _0.3 Ga using G, TMI and ammonia
_{A 0.6} N well layer is grown to a thickness of 30 Å. And barrier + well + barrier + well ... +
The active layer 7 having a multiple quantum well structure having a total thickness of 1950 angstroms is grown by alternately stacking six barrier layers and five well layers in the order of the barriers.

Next, Mg is made of p-type Al _0.2 Ga _{0 .8} N having a thickness of 300 angstroms doped p-type A
An lGaN layer 7 is grown, and a Mg-doped p-type GaN layer 8 having a thickness of 0.2 μm is grown.

After completion of the reaction, the temperature is lowered to room temperature, and the wafer is placed in a reaction vessel in a nitrogen atmosphere.
Anneal at ℃ to reduce the resistance of the p-type layer.

After annealing, the wafer is taken out of the reaction vessel, a mask of a predetermined shape is formed on the surface of the uppermost p-side GaN layer 8, and RIE (reactive ion etching) is performed.
In the apparatus, the stacked p-type GaN layer 8, p-type AlGaN layer 7, active layer 6, and undoped GaN layer 5 are partially removed by etching to form an n-type Ga for forming an n-electrode.
The surface of the N layer 4 is exposed.

Next, a p-electrode made of Ni / Au is formed on the p-type GaN layer 8, and an n-electrode 11 made of Ti / Al is formed on the surface exposed on the n-type GaN layer 4, thereby forming an LED.
A device is manufactured.

This LED device emits blue light of 470 nm at a forward current of 20 mA, and Vf is 3.5 V
The light emission output was 6.5 mW, and the Vf was reduced by nearly 0.3 V at Vf, and the output was improved by 1.5 times or more, as compared with the conventional multiple quantum well structure LED element. As a conventional LED element having a multiple quantum well structure, an LED was fabricated in the same manner as in Example 1 except that the barrier layer constituting the active layer was changed to an undoped film thickness of 20 angstroms. As described above, in the device of the present invention, the light emission output is increased at the same forward current value as compared with the comparative device, and the photoelectric conversion efficiency can be improved. As a result, power consumption can be reduced.

[Example 2] An LED element is manufactured in the same manner as in Example 1, except that the undoped GaN layer 5 grown immediately before growing the active layer is not formed. Although the obtained LED element has a slightly lower photoelectric conversion efficiency and a slightly wider half-width of the emission spectrum as compared with Example 1,
It has almost the same good characteristics.

[Comparative Example 1] In the same manner as in Example 1, except that the well layer was doped with 2 × 10 ¹⁹ / cm ³ of Si,
An ED element is manufactured. The obtained LED element had an applied voltage of -5 V, a leakage current of 800 mA, and a light emission output of 0.5 mW.
, And good results could not be obtained.

[0031]

The present invention solves the conventional problems by undoping the well layer of the active layer of the multiple quantum well structure, and further defining the thickness and impurity concentration of the barrier layer, thereby achieving low power consumption. It is possible to provide a nitride semiconductor light emitting device having good light emission output and high photoelectric conversion efficiency.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a structure of an LED element according to an embodiment of the present invention.

FIG. 2 is a graph showing a leakage current when the impurity concentration of a barrier layer of an active layer in Example 1 is changed stepwise [FIG.
(A)] and a graph plotting changes in forward voltage [FIG. 2 (b)].

FIG. 3 is a graph showing the relationship between the forward voltage [FIG. 3 (a)] and the emission output [FIG. 3 (b)] when the thickness of the barrier layer of the active layer in Example 1 is changed stepwise. It is the graph which plotted the change.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 sapphire substrate 2 buffer layer 3 undoped GaN layer 4 undoped GaN layer 5 undoped GaN layer 6 active layer 7 Mg doped AlGaN layer 8 ..Mg-doped GaN layer 9 ... p electrode 11 ... n electrode

Claims (4)

[Claims] A well layer made of a nitride semiconductor containing In and a barrier layer made of a nitride semiconductor having a composition different from that of the well layer between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. Having an active layer of a multiple quantum well structure in which
The well layer is undoped, the barrier layer has a single thickness of 70 to 500 angstroms, and the barrier layer has n
A nitride semiconductor element, wherein a type impurity is doped at 5 × 10 ¹⁶ to 1 × 10 ¹⁹ / cm ³ . 2. The nitride semiconductor device according to claim 1, wherein said active layer is grown on an undoped nitride semiconductor layer. 3. An undoped nitride semiconductor layer comprising:
3. The nitride semiconductor device according to claim 2, wherein the nitride semiconductor device has a thickness of about 500 Å. 4. The undoped nitride semiconductor layer is made of undoped GaN.
3. The nitride semiconductor device according to item 1.