Light-emitting device of gallium nitride compound semiconductor

Abstract: Disclosed herein is a light-emitting diode of GaN compound semiconductor which emits a blue light from a plane rather than dots for improved luminous intensity. It has an electrode 80 for the high-carrier density n⁺-layer 3 and an electrode 70 for the high-impurity density i H -layer 52. These electrodes 70 (80) are made up a first Ni layer 71 (81) (100Å thick), a second Ni layer 72 (82) (1000Å thick), an Al layer 73 (83) (1500Å thick), a Ti layer 74 (84) (1000Å thick), and a third Ni layer 75 (85) (2500Å 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 the drive voltage for light emission and increases the 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 emits a blue light is that of 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 an i-layer grown on the n-layer from a GaN compound semiconductor which is made i-type by doping with a p-type impurity. (Japanese Patent Laid-open Nos. 119196/1987 and 188977/1988) It is known that the above-mentioned light-emitting diode will improve in luminous intensity when the i-layer is provided with an electrode of large area because light emission takes place directly under or near the i-layer. Much has been reported on the study of crystal growth for light-emitting diodes of GaN compound semiconductor. However, only a little has been reported on the process of producing such light-emitting diodes. This is true particularly of the electrode for the i-layer in light-emitting diode of MIS (metal insulator semiconductor) structure. It has been disclosed only in Japanese Patent Laid-open No. 46669/1982, and nothing has so far been discussed about how the electrode for the i-layer is associated with light emission. The electrode for the i-layer has the layer structure as shown in vertical section in Fig. 8 which is a reproduction from the Japanese patent just given above. Referring to Fig. 8, there is shown a light-emitting diode 60, which has an electrode 67 for the i-layer and an electrode 68 for the n-layer. The electrode 67 is formed from nickel deposited on an aluminum substrate deposited directly on the i-layer. The electrode 68 is also formed from nickel deposited on an aluminum substrate deposited in a hole penetrating the i-layer. A disadvantage of forming the electrode on aluminum in direct contact with the i-layer is that light is emitted from coarse dots rather than a uniform plane, as shown in Fig. 5(a). The resulting light-emitting diode does not increase in luminous intensity notwithstanding its large light-emitting area. SUMMARY OF THE INVENTION The present invention was completed to address the above-mentioned problem. It is an object of the present invention to provide a light-emitting diode of GaN compound semiconductor which emits a blue light from a plane rather than dots for the improved luminous intensity. The present invention is embodied in a light-emitting device of gallium nitride compound semiconductor as specified in appended claim 1. Preferable embodiments are set out in dependent claims 2 to 5. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a vertical sectional view showing the structure of the light-emitting diode pertaining to an embodiment of the present invention. Figs. 2 to 4 are vertical sectional views showing the steps of producing the light-emitting diode pertaining to said embodiment of the present invention. Fig. 5 is a photomicrograph showing the metal surface structure (as the light-emitting pattern) of each substrate metal of the electrode for the i-layer. Fig. 6 is a vertical sectional view showing the structure of the light-emitting diode pertaining to another embodiment of the present invention. Fig. 7 is a vertical sectional view showing the structure of the light-emitting diode pertaining to further another embodiment of the present invention. Fig. 8 is a vertical sectional view showing the structure of the conventional light-emitting diode. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 The invention will be described in more detail with reference to an embodiment. Fig. 1 shows in vertical section a light-emitting diode 10 pertaining to the present invention. It has a sapphire substrate 1, on which are successively formed a buffer layer 2 of AlN (50 nm thick), a high-carrier density n + -layer 3 of GaN (2.2 µm thick), a low-carrier density n-layer 4 of GaN (1.5 µm thick), an i-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 to 4. The entire process was carried out using NH 3 , H 2 (carrier gas), trimethyl gallium Ga(CH 3 ) 3 (TMG for short), trimethyl aluminum Al(CH 3 ) 3 (TMA for short), silane SiH 4 and diethyl zinc (DEZ for short). In the first place, the sapphire substrate 1 of single crystal (with the a-plane (i.e., {11 2 0} ) 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). H 2 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 H 2 , NH 3 , and TMA at a flow rate of 20 L/min, 10 L/min, and 1.8 x 10 -5 mol/min, respectively, to form the buffer layer 2 of AlN (50 nm thick). With the temperature of the sapphire substrate 1 kept at 1150°C, the reaction chamber was supplied with H 2 , NH 3 , TMG, and SiH 4 (diluted to 0.86 ppm with H 2 ) at a flow rate of 20 L/min, 10 L/min, 1.7 x 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 x 10 18 /cm 3 . With the temperature of the sapphire substrate 1 kept at 1150°C, the reaction chamber was supplied with H 2 , NH 3 , and TMG at a flow rate of 20 L/min, 10 L/min, and 1.7 x 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 x 10 15 /cm 3 . With the temperature of the sapphire substrate 1 kept at 900°C, the reaction chamber was supplied with H 2 , NH 3 , TMG, and DEZ at a flow rate of 20 L/min, 10 L/min, 1.7 x 10 -4 mol/min, and 1.5 x 10 -4 mol/min, respectively, for 1 minute to form the i-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 5 was formed the SiO 2 layer 11 (2000Å thick) by sputtering, as shown in Fig. 2(b). The SiO 2 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 SiO 2 layer 11 was removed by etching with hydrofluoric acid, as shown in Fig. 2(c). The exposed part of the i-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 BCl 3 gas fed at a flow rate of 10 mL/min at 5.33 Pa (0.04 Torr) in conjunction with a high-frequency power of 0.44 W/cm 2 , followed by Ar dry etching, as shown in Fig. 3(d). The SiO 2 layer 11 remaining on the i-layer 5 was removed with the aid of hydrofluoric acid, as shown in Fig. 3(e). With the temperature kept at 225°C and the degree of vacuum kept at 1.07 x 10 -4 Pa (8 x 10 -7 Torr), the sample was entirely coated with the Ni layer 13 (300 nm thick) by vapor deposition, as shown in Fig. 3(f). 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 5. The unmasked part of the Ni layer 13 was etched off by 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 5 was formed afterward, as shown in Fig. 4(g). With the temperature kept at 225°C and the degree of vacuum kept at 1.07 x 10 -4 Pa (8 x 10 -7 Torr), the sample was entirely coated with the Al layer 15 (300 nm thick) by vapor deposition, as shown in Fig. 4(h). 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 5, as shown in Fig. 4 (i). The exposed part of the Al layer 15 was etched off by nitric acid and the remaining photoresist 16 was removed by acetone. Thus there were formed the electrode 7 for the i-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 5 may be formed from Ag or an alloy of Ni and Ag in place of Ni. Also, the electrode 7 for the i-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 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 as shown in the table below. The data of luminous intensity and drive voltage are given in terms of index compared with those of the conventional sample. The Ti undercoating does not form part of the present invention. Undercoating metal Luminous intensity Drive voltage Light-emitting 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) It is noted that the light-emitting diode pertaining to the present invention has a higher luminous intensity and a lower drive voltage than the conventional one. 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 x 10 15 /cm 3 , a low-impurity density i L -layer 51 (1.1 µm thick) having a Zn density of 2 x 10 18 /cm 3 , and a high-impurity density i H -layer 52 (0.2 µm thick) having a Zn density of 1 x 10 20 /cm 3 . It should be noted that the i-layer is of dual structure with 51 and 52. A hole 60 was formed which penetrates the high-impurity density i H layer 52, the low-impurity density i 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 52. The electrode 70 is composed of a first Ni layer 71 (10 nm thick), a second Ni layer 72 (100 nm thick), an Al layer 73 (150 nm thick), a Ti layer 74 (100 nm thick), and a third Ni layer 75 (250 nm thick). The electrode 80 is also composed of a first Ni layer 81 (10 nm thick), a second Ni layer 82 (100 nm thick), an Al layer 83 (150 nm thick), a Ti layer 84 (100 nm thick), and a third Ni layer 85 (250 nm 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 wafer 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 the conventional one having an aluminum electrode. In addition, it exhibits a luminous intensity of 150 mcd at 10 mA current, which is 1.5 times that (100 mcd) of the conventional one 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 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 30 nm 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 5 nm to 300 nm. 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: A light-emitting device of gallium nitride compound semiconductor material having an i-layer (5, 52) of i-type Al x Ga 1-x N, x ≥ 0, doped with a p-type impurity, an n-layer (3, 4) of n-type Al x Ga 1-x N, x ≥ 0, thereunder, an i-layer electrode (7, 13, 70) for said i-layer (5, 52), and an n-layer electrode (8, 80) for said n-layer (3, 4), characterized in that said n-layer (3, 4) and said i-layer (5) have their respective electrodes formed on the same surface and said i-layer electrode (7, 13, 70) comprises a layer (13, 71, 710) formed in contact with said i-layer (5, 52) and being made of Ni, or of Ag or an alloy thereof. A light-emitting device of gallium nitride compound semiconductor material according to claim 1, wherein said electrode for said i-layer (7, 13, 70) has an overlayer (15; 7) made of Al or an alloy containing Al on said layer (13) formed directly on said i-layer (5, 52), and wherein the electrode for said n-layer (8) is made of Al or an alloy containing Al. A light-emitting device of gallium nitride compound semiconductor material according to claim 1, wherein said electrode for said i-layer (70) has a multi-layer structure composed of said layer (710) formed directly on said i-layer (5, 52), whereby said layer (710) formed directly on said i-layer is a Ni layer, an Al layer (73) formed on said Ni layer (710), a Ti layer (74) formed on said Al layer (73), and a further Ni layer (75) formed on said Ti layer (74). A light-emitting device of gallium nitride compound semiconductor material according to claim 3, wherein said layer formed directly on said i-layer comprises a first Ni layer (71) formed directly on said i-layer and a second Ni layer (72) thicker than said first Ni layer formed directly on said first Ni layer (71) and below said Al layer (73). A light emitting device of gallium nitride compound semiconductor material according to claim 3 or 4, wherein said electrode for said n-layer (80) has a multi-layer structure composed of the same layers in the same order as the respective i-layer electrode (70).

Inventors: HASHIMOTO MASAFUMI, KOTAKI MASAHIRO, MANABE KATSUHIDE, TAMAKI MAKOTO, HASHIMOTO, MASAFUMI, KOTAKI, MASAHIRO, MANABE, KATSUHIDE, TAMAKI, MAKOTO, Kotaki, Masahiro, c/o TOYODA GOSEI Technical Ctr, Manabe, Katsuhide, c/o TOYODA GOSEI Technical Ctr, Tamaki, Makoto, c/o TOYODA GOSEI Technical Ctr
Applicants: TOYODA GOSEI KK, TOYODA GOSEI CO., LTD.
Application year: 1993
Main classification: H01L33
IPC codes: H01L33