1. Field of the Invention
The invention relates to a semiconductor luminescent device that includes a luminescent layer between a p-type GaN compound semiconductor material and an n-type GaN compound semiconductor material, and an electrode on the p-type GaN compound semiconductor material, and to a manufacturing method therefor. In particular, the invention relates to a semiconductor luminescent device that has relatively longer life expectancy and higher light output, and a manufacturing method therefor.
2. Description of Related Art
Recently, GaN compound semiconductor materials have been used in most cases as a semiconductor material for semiconductor luminescent devices. The GaN compound semiconductor materials are formed, by an MOCVD method or an MBE method, on a substrate made of Si, SiC, sapphire, oxide, or III-V family compound. When an insulating material such as sapphire as a substrate is used, an electrode cannot be obtained from a back side of the substrate, which is different from a luminescent device made of GaAs or AlGaInP using a conductive substrate. Accordingly, a pair of electrodes is formed on the same surface of a substrate.
Since a p-type GaN compound semiconductor material has a small mobility, current diffusion to a lateral direction is small. Because of this, when a voltage is applied to a semiconductor luminescent device, since a luminescent area is limited to an area directly below an electrode, current easily does not diffuse through an entire surface area of the semiconductor luminescent device. Therefore, an electrode used for this type of semiconductor luminescent device requires transparency and current diffusion to a lateral direction. Since an operating voltage for the semiconductor luminescent device should be small, a contact resistance between the GaN compound semiconductor and the electrode should be small.
The balance between the contact resistance for the transparent electrode and the GaN compound semiconductor material and the resistance (sheet resistance) of the transparent electrode itself should be well adjusted. Since current tends to flow through a low resistance, when the sheet resistance is extremely high compared with the contact resistance, the current does not reach the end of the transparent electrode from a pad for external connection provided on the transparent electrode, and instead flows midway to the GaN compound semiconductor. On account of this, the transparent electrode does not emit light through the entire surface. The current may flow through the luminescent layer in the vicinity of the pad provided on the transparent electrode, and when in an extreme case, only the luminescent layer immediately below the pad, with almost no emitting light.
Ni is well known as a metal that has a p-type GaN compound semiconductor material and ohmic resistance. However, Ni (resistivity 6.84 μΩ·cm) has a higher sheet resistance and a lower current diffusion in a lateral direction, compared with Au (resistivity 2.3 μΩ·cm) or Al (resistivity 2.6 μΩ·cm). Thickening film thickness of Ni decreases a sheet resistance of an electrode to be able to improve current diffusion in a lateral direction within the transparent electrode. However, light transmittance of the transparent electrode decreases, and so does light output of the semiconductor luminescent device.
It is well-known that in order to solve the problems mentioned above, a thin, metallic transparent electrode, which is formed almost all over the surfaces of the semiconductor luminescent device, is disposed as a p-type electrode, and then light is obtained through the transparent electrode. See, for example, Japanese Patent Publication Hei 6-314822. This patent document discloses that Au (the second layer) on Ni (the first layer), Ni (the second layer) on Cr (the first layer), and Ti (the second layer) on Pt (the first layer) are evaporated, by which a transparent electrode of two-layer structure is formed, heat processing is performed below the temperature at which the GaN compound semiconductor material does not resolve, and one portion of the electrode material is diffused inside the GaN compound semiconductor material or scattered outside, which makes the thickness of the transparent electrode 0.001˜1 μm to secure transparency.
An electrode of the prior art to be provided on the semiconductor luminescent device and a method of manufacturing pads will be described, referring to FIGS. 9A to 9D and FIGS. 10A to 10C. The processes for manufacturing an electrode and a pad are generally composed of at least two evaporation processes and one lift-off process. An electrode material and an anti-oxidization film material are evaporated on a p-type GaN compound semiconductor 21 processed beforehand (FIG. 9A) to form a transparent electrode 22 and an anti-oxidization film material 23 (FIG. 9B). Next, resist is coated on the anti-oxidization film material 23 to perform patterning the transparent electrode 22 and the anti-oxidization film material 23 (FIG. 9C).
Then, another resist is coated to form a resist film 24 (FIG. 9D) and make a window by removing a portion of the resist film 24 for forming a pad (FIG. 10A). A pad electrode 26 is formed by evaporating the pad electrode material (FIG. 10B), and is accomplished by stripping the resist film 24 using a lift-off method and performing patterning (FIG. 10C). The above explanation is made of a case in which the anti-oxidization film 23 is formed on the transparent electrode 22. However, the anti-oxidization film 23 may not be formed, which does not reduce the number of the total steps.
The patent document discloses a transparent electrode in which when performing heat process (anneal process), since surface strength of a second layer metal excels adhesion between a first layer metal and a second layer metal, a phenomenon called “ball-up” occurs through which the second metallic layer is condensed like a ball. When the phenomenon occurs, sheet resistance of the transparent electrode gets larger because continuity of the second layer (evaporation film) is damaged. This increases an operation voltage of the semiconductor luminescent device, and degrades current diffusion in a lateral direction, which lowers light output of the semiconductor luminescent device in an area away from the pad. There is a way to thickly form a second metallic layer in order to prevent the “ball-up” phenomenon. When the metal is made thicker, the transmittance of light as a transparent electrode gets lower to reduce the transparency of the electrode.
Al or alloy containing Al or oxide on the surface of a p-type GaN compound semiconductor tends to form a p-type GaN compound semiconductor and a Schottky barrier, which makes direct mounting difficult. Since Ni has a work function of about 5.1 eV, a large value, it is quite often used as an electrode material for forming an ohmic junction with a p-type semiconductor. On the other hand, Al is used as an electrode material for forming an ohmic junction with an n-type semiconductor. However, because Al has a relatively small work function of about 4.1 eV, it readily tends to form a Schottky barrier with a p-type semiconductor.
Since the process for manufacturing an electrode according to the prior art experiences many complicated steps, it has the tendency to have a low yield rate. In addition, because the resist films are stripped off by a lift-off method, one portion of the stripped resist film is apt to remain on the transparent electrode as a foreign object after performing lift-off. On account of this, the foreign object left on the transparent electrode interrupts light from the luminescent layer, which produces a problem that light output from the semiconductor luminescent device gets lower. Moreover, when a LED lamp or a chip LED is made by resin molding, the foreign object damages the adhesion between the lamp resin and the semiconductor luminescent device (chip), and moisture crept in from outside enters the resin, which accelerates deterioration of the chip or electrode and makes the semiconductor luminescent device less reliable. Accordingly, there is a need to solve the problems mentioned above.