Light-emitting device of gallium nitride compound semiconductor

Abstract

A light-emitting diode of GaN compound semiconductor emits a blue light from a plane rather than dots for improved luminous intensity. This diode includes a first electrode associated with a high-carrier density n^{+ layer and a second electrode associated with a high-impurity density i_H -layer H-layer. These electrodes are made up of a first ni layer (110 Å thick), a second ni layer (1000 Å thick), an al layer (1500 Å thick), a ti layer (1000 Å thick), and a third ni layer (2500 Å thick). The ni layers of dual structure permit a buffer layer to be formed between them. This buffer layer prevents the ni layer from peeling. The direct contact of the ni layer with GaN lowers a drive voltage for light emission and increases luminous intensity.}

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting device of gallium nitride compound semiconductor which emits a blue light.

2. Description of the Prior Art

Among the conventional light-emitting diodes which emit a blue light is the gallium nitride compound semiconductor. It attracts attention because of its high luminous efficiency resulting from the direct transition and its ability to emit a blue light, one of the three primary colors of light.

The light-emitting diode of gallium nitride compound semiconductor is made up of a sapphire substrate, an n-layer grown on the substrate from a GaN compound semiconductor of n-type conduction, with or without a buffer layer of aluminum nitride interposed between them, and i-layer a p-type impurity doped layer 5 of GaN (0.1 μm thick), an electrode 7 of aluminum, and an electrode 8 of aluminum (in contact with the high-carrier density n+ -layer 3).

This light-emitting diode 10 is produced by the steps which are explained below with reference to FIGS. 2(A) to 4(C).

The entire process was carried out using NH₃, H₂ (carrier gas), trimethyl gallium Ga(CH₃)₃ (TMG for short), trimethyl aluminum Al(CH₃)₃ (TMA for short), silane SiH₄ and diethyl zinc (DEZ for short).

Firstly, sapphire substrate 1 of single crystal (with the a-plane (i.e., {1120} as the principal plane) was cleaned by washing with an organic solvent and by subsequent heat treatment. Then, it was placed on the susceptor in the reaction chamber for metal-organic vaporphase epitaxy (MOVPE). H₂ was fed to the reaction chamber under normal pressure at a flow rate of 2 L/min to perform vapor phase etching on the sapphire substrate 1 at 1100°C

With the temperature lowered to 400°C, the reaction chamber was supplied with H₂, NH₃, and TMA at a flow rate of 20 L/min, 10 L/min, and 1.8×10-5 mol/min, respectively, to form the buffer layer 2 of AlN (500 Å thick).

With the temperature of the sapphire substrate 1 kept at 1150°C, the reaction chamber was supplied with H₂, NH₃, TMG, and SiH₄ (diluted to 0.86 ppm with H₂) at a flow rate of 20 L/min, 10 L/min, 1.7×10-4 mol/min, and 200 mL/min, respectively, for 30 minutes to form the high-carrier density n+ -layer 3 of GaN (2.2 μm thick), with a carrier density of 1.5×10¹8 /cm³.

With the temperature of the sapphire substrate 1 kept at 1150°C, the reaction chamber was supplied with H₂, NH₃, and TMG at a flow rate of 20 L/min, 10 L/min, and 1.7×10-4 mol/min, respectively, for 20 minutes to form the low-carrier density n-layer 4 of GaN (1.5 μm thick), with a carrier density of 1×10¹5 /cm³.

With the temperature of the sapphire substrate 1 kept at 900°C, the reaction chamber was supplied with H₂, NH₃, TMG, and DEZ at a flow rate of 20 L/min, 10 L/min, 1.7×10-4 mol/min, and 1.5×10-4 mol/min, respectively, for 1 minute to form the i-layer p-type impurity doped layer 5 of GaN (0.1 μm thick).

In this way there was obtained the multi-layer structure as shown in FIG. 2(a).

On the i-layer p-type impurity doped layer 5 was formed the SiO₂ layer 11 (2000 Å thick) by sputtering, as shown in FIG. 2(b). The SiO₂ layer 11 was coated with a photoresist 12, which was subsequently patterned by photolithography after the configuration of the electrode for the high-carrier density n+ -layer 3. The exposed part of the SiO₂ layer 11 was removed by etching with hydrofluoric acid, as shown in FIG. 2(c). The exposed part of the i-layer p-type impurity doped layer 5, the underlying part of the low-carrier density n-layer 4, and the underlying upper part of the high-carrier density n+ -layer 3 were removed by dry etching with BCl₃ gas fed at a flow rate of 10 mL/min at 0.04 Torr in conjunction with a high-frequency power of 0.44 W/cm², followed by Ar dry etching, as shown in FIG. 3(A). The SiO₂ layer 11 remaining on the i-layer p-type impurity doped layer 5 was removed with the aid of hydrofluoric acid, as shown in FIG. 3(B).

With the temperature kept at 225°C and the degree of vacuum kept at 8×10-7 Torr, the sample was entirely coated with the Ni layer 13 (3000 Å thick) by vapor deposition, as shown in FIG. 3(C). The Ni layer 13 was coated with a photoresist 14, which was subsequently patterned by photolithography after the configuration of the electrode for the i-layer p-type impurity doped layer 5.

The unmasked part of the Ni layer 13 was etched off using nitric acid and the photoresist 14 was removed by acetone, so that the Ni layer 13 partly remained on which the electrode for the i-layer p-type impurity doped layer 5 was formed afterward, as shown in FIG. 4(A).

With the temperature kept at 225°C and the degree of vacuum kept at 8×10-7 Torr, the sample was entirely coated with the Al layer 15 (3000 Å thick) by vapor deposition, as shown in FIG. 4(B).

The Al layer 15 was coated with a photoresist 16, which was subsequently patterned by photolithography after the configuration of the respective electrodes for the high-carrier density n+ -layer 3 and the i-layer p-type impurity doped layer 5, as shown in FIG. 4(C).

The exposed part of the Al layer 15 was etched off using nitric acid and the remaining photoresist 16 was removed by acetone, Thus there were formed the electrode 7 for the i-layer p-type impurity doped layer 5 and the electrode 8 for the high-carrier density n+ -layer 3.

In this way there was obtained the GaN light-emitting device of MIS structure as shown in FIG. 1.

Incidentally, the undercoating layer 13 on the i-layer p-type impurity doped layer 5 may be formed from Ag or Ti or an alloy thereof in place of Ni. Also, the electrode 7 for the i-layer p-type impurity doped layer 5 and the electrode 8 for the high-carrier density n+ -layer 3 may be formed from any metal such as Ti, in place of Al, which permits ohmic contact.

The thus prepared light-emitting diode 10 was tested for luminous intensity and drive voltage by applying current (10 mA) across the electrodes. The results were compared with those of the conventional one having the Al layer formed directly on the i-layer p-type impurity doped layer 5, which gave a luminous intensity of 30 mcd. The results vary depending on the metal used for the undercoating of the electrode for the i-layer p-type impurity doped layer as shown in the table below. The data of luminous intensity and drive voltage are given in terms of index values compared with those of a conventional sample.

______________________________________
Undercoating
Luminous   Drive   Light-emitting
metal      intensity  voltage pattern
______________________________________
Ni         1.5        0.82    FIG. 5(b)
Ag         1.4        0.90    FIG. 5(c)
Ti         1.05       0.95    FIG. 5(d)
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It is noted that the light-emitting diode pertaining to the present invention has a higher luminous intensity and a lower drive voltage than conventional diodes.

Example 2

A light-emitting diode was prepared in the same manner as in Example 1. As shown in FIG. 6, it is composed of a sapphire substrate 1, a buffer layer 2 of AlN, a high-carrier density n+ -layer 3 of GaN, a low-carrier density n-layer 4 (1.1 μm thick) having a carrier density of 1×10¹5 /cm³, a low-impurity density i_L -layer L-layer 51 (1.1 μm thick) having a Zn density of 2×10¹8 /cm³, and a high-impurity density i_H -layer H-layer 52 (0.2 μm thick) having a Zn density of 1×10²0 /cm³. It should be noted that the i-layer p-impurity doped layer is of dual structure with 51 and 52.

A hole 60 was formed which penetrates the high-impurity density i_H layer H-layer 52, the low-impurity density i_L layer L-layer 51, and the low-carrier density n-layer 4, reaching the high-carrier density n+ -layer 3. In this hole 60 was formed an electrode 80 for the high-carrier density n+ -layer 3. An electrode 70 was also formed for the high-impurity density i_H -layer H-layer 52.

The electrode 70 is composed of a first Ni layer 71 (100 Å thick), a second Ni layer 72 (1000 Å thick), an Al layer 73 (1500 Å thick), a Ti layer 74 (1000 Å thick), and a third Ni layer 75 (2500 Å thick). The electrode 80 is also composed of a first Ni layer 81 (100 Å thick), a second Ni layer 82 (1000 Å thick), an Al layer 83 (1500 Å thick), a Ti layer 84 (1000 Å thick), and a third Ni layer 85 (2500 Å thick).

The first Ni layer 71 (81) was formed by vacuum deposition at 225° C. The second Ni layer 72 (82) was also formed by vacuum deposition with heating. (The two steps were separated by an interval in which the vacuum chamber was opened and the water was conditioned at normal pressure and normal temperature.) The Al layer 73 (83), Ti layer 74 (84), and third Ni layer 75 (85) were formed successively by vacuum deposition. The Al layer 73 (83) and Ti layer 74 (84) permit a solder bump to be formed on the third Ni layer 75 (85).

The thus prepared light-emitting diode has a drive voltage for light emission which is 0.8 times that of a conventional diode having an aluminum electrode. In addition, it also exhibits a luminous intensity of 150 mcd at 10 mA current, which is 1.5 times that (100 mcd) of the conventional diode having an aluminum electrode.

It was also found that the same result as mentioned above is obtained even in the case where the electrode 70 for the high-impurity density i_H -layer H-layer 52 is made of Ni in multi-layer structure and the electrode 80 for the high-carrier density n+ -layer 3 is made of aluminum in single-layer structure.

Example 3

The light-emitting diode in this example differs from that in the previous example in that the first Ni layer 71 (81) and second Ni layer 72 (82) are replaced by a Ni layer 710 (810) of single-layer structure, which is 300 Å thick, as shown in FIG. 7. This difference in structure has nothing to do with its performance. The Ni layer 710 (810) should preferably have a thickness in the range of 50 Å to 3000 Å. With a thickness lower than specified, it will be subject to attack by solder when a solder bump is formed. With a thickness greater than specified, it causes the light source to be localized near the electrode rather than the center and it is liable to peeling at the time of soldering in a solder bath.

Claims

1. A light-emitting device of gallium nitride compound semiconductor material comprising:

an n-layer of n-type gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0); and

an i-layer of i-type a p-type impurity doped layer gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0) doped with a p-type impurity;

wherein a first electrode layer including ni is formed in contact with said i-layer p-type impurity doped layer and functions as an electrode therefore therefor; and

wherein said first electrode layer is a multi-layer structure having a first ni layer of predetermined thickness formed over said i-layer p-type impurity doped layer, a second ni layer which is thicker than said first ni layer and formed thereon, an al layer formed over said second ni layer, a ti layer formed over said al layer, and a third ni layer which is thicker than said first ni layer formed over said ti layer.

2. A light-emitting device of gallium nitride compound semiconductor material comprising:

an n-layer of n-type gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0) material; and

an i-layer of i-type a p-type impurity doped layer gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0) material doped with a p-type impurity;

wherein each of said n-layer and said i-layer p-type impurity doped layer include respective electrodes formed on a same relative surface, the electrode for said i-layer p-type impurity doped layer being composed of at least one layer with each said at least one layer being made of one of ni, ag, ti, an alloy including ni, an alloy including ag, and an alloy including ti; and

wherein the electrode for said i-layer p-type impurity doped layer has an over layer formed thereon which is made of one of al and an alloy containing al.

3. A light-emitting device of gallium nitride compound semiconductor, comprising:

at least two layers of gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0);

a first electrode layer for one layer of said at least two layers; and

a second electrode layer for another of said at least two layers;

said first and second electrode layers provide an improved luminous intensity of said light-emitting device;

wherein at least one layer of said first and second electrode layers includes a contact layer made of one of ni, ag, an alloy including ni, an alloy including ag, and an alloy including ti, said contact layer being directly contacted with any of said at least two layers of gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0).

4. A light-emitting device of gallium nitride compound semiconductor according to claim 3, wherein said contact layer of said first electrode layer is uniformly formed on a light emitting surface of said one layer, said one layer being an i-layer of semi-insulation doped with a p-type impurity p-type impurity doped layer and said another layer being an n-layer with n-type conduction.

5. A light-emitting device of gallium nitride compound semiconductor according to claim 3, wherein said second electrode layer includes a contact layer made of one of ni, ag, an alloy including ni, an alloy including ag, and an alloy including ti, said contact layer being directly contacted with said another layer and said another layer being an n-layer with n-type conduction.

6. A light-emitting device of gallium nitride compound semiconductor according to claim 3, wherein said first electrode layer is a multi-layer structure having a first ni layer of predetermined thickness formed over said one layer, a second ni layer which is thicker than said first ni layer and formed thereon, an al layer formed over said second ni layer, a ti layer formed over said al layer, and a third ni layer which is thicker than said first ni layer formed over said ti layer.

7. A light-emitting device of gallium nitride compound semiconductor according to claim 3, wherein at least one layer of said first and second electrode layers has an over layer which is made of one of ni, ag, ti, an alloy including ni, an alloy including ag, and an alloy including ti.

8. A light-emitting device of gallium nitride compound semiconductor material according to claim 4, wherein at least one layer of said first and second electrode layers has an over layer which is made of one of ni, ag, ti, an alloy including ni, an alloy including ag, and an alloy including ti.

9. A light-emitting device of gallium nitride compound semiconductor according to claim 3, wherein at least one layer of said first and second electrode layers has over layer formed thereon which is made of one of al and an alloy containing al.

10. A light-emitting device of gallium nitride compound semiconductor according to claim 4, wherein at least one layer of said first and second electrode layers has over layer formed thereon which is made of one of al and an alloy containing al.

11. A light-emitting device of gallium nitride compound semiconductor according to claim 7, wherein at least one layer of said first and second electrode layers has over layer formed thereon which is made of one of al and an alloy containing al.

12. A light-emitting device of gallium nitride compound semiconductor according to claim 8, wherein at least one layer of said first and second electrode layers has over layer formed thereon which is made of one of al and an alloy containing al.

13. A light-emitting device of gallium nitride compound semiconductor according to claim 4, wherein said second electrode layer is made of one of al and an alloy containing al, and said first electrode has an over layer which is made of one of ni, ag, ti, an alloy including ni, an alloy including ag, and an alloy including ti.

14. A light-emitting device of gallium nitride compound semiconductor material according to claim 4, wherein said first electrode layer is a multi-layer structure having a first ni layer of predetermined thickness formed over said i-layer, a second ni layer which is thicker than said first ni layer and formed thereon, an al layer formed over said second ni layer, a ti layer formed over said al layer, and a third ni layer which is thicker than said first ni layer formed over said ti layer. 15. A light-emitting device of gallium nitride compound semiconductor, comprising:

a first layer of gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0) doped with p-type impurity;

a second layer of n-type gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0);

a first electrode layer for said first layer;

a second electrode layer for said second layer; and

wherein said first electrode layer is made of at least one of ni, ag, an alloy including ni, an alloy including ag, and an alloy including ti and said second electrode layer is made of at least one of al, ti, an alloy

including al, and an alloy including ti. 16. A light-emitting device of gallium nitride compound semiconductor according to claim 15, wherein said first and second layers are formed on a buffer layer and said buffer layer is formed on a sapphire substrate. 17. A light-emitting device of gallium nitride compound semiconductor according to claim 15, wherein said second layer is gallium nitride (GaN) of low resistivity doped with silicon (Si) for uniform flow

of current through said first layer. 18. A light-emitting device of gallium nitride compound semiconductor according to claim 17, wherein said first electrode layer is made of one of ni and an alloy including ni and second electrode layer is made of one of al and an alloy including al. 19. A light-emitting device of gallium nitride compound semiconductor according to claim 15, wherein said first electrode layer further comprises a multi-layer structure having at least one over layer made of metal different from metal of a layer under said over layer. 20. A light-emitting device of gallium nitride compound semiconductor according to claim 15, wherein said first electrode layer is uniformly formed on a light emitting surface of said first layer.

21. A light-emitting device of gallium nitride compound semiconductor according to claim 15, wherein at least one layer of said first and second electrode layers further comprises at least one over layer made of one of ni, ti, an alloy including ni, and an alloy including ti. 22. A light-emitting device of gallium nitride compound semiconductor according to claim 15, wherein said second electrode layer further comprises at least one over layer made of one of al and an alloy containing al. 23. A light-emitting device of gallium nitride compound semiconductor according to claim 18, wherein said first electrode layer further comprises at least one over layer made of metal

excluding ni. 24. A light-emitting device of gallium nitride compound semiconductor, comprising:

a first layer of gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0) doped with p-type impurity;

a second layer of n-type gallium nitride compound semiconductor material (al_x Ga₁-x N, x≧0);

a first electrode layer for said first layer;

a second electrode layer for said second layer; and

wherein said first electrode layer is made of at least one of ni and an alloy including ni and said second electrode layer is made of al, ti, an

alloy including al, and an alloy including ti. 25. A light-emitting device of gallium nitride compound semiconductor according to claim 24, wherein said first electrode layer has a multi-layer structure comprising at least one over layer made of metal different from metal of a layer under said over layer. 26. A light-emitting device of gallium nitride compound semiconductor according to claim 3, wherein said improved luminous intensity is achieved by reduction of driving voltage for supplying a predetermined current. 27. A light-emitting device of gallium nitride compound semiconductor according to claim 3, wherein said at least one layer of said first and second electrode layers further comprises at least one over layer made of a metal excluding a metal of said contact layer.