In recent years, gallium nitride-based compound semiconductors, with the compound expressed by AlxInyGa1-x-yN (0≦x<1, 0≦y<1, x+y<1), are drawing attention as materials for light-emitting diodes (LEDs) that emit light in the region from ultraviolet to blue or green. Use of compound semiconductors of the above material makes it possible to emit ultraviolet or blue and green light while maintaining a high light-emitting intensity which could not be accomplished before. The gallium nitride-based compound semiconductor is, usually, grown on a sapphire substrate, which is an insulating substrate, and this makes it difficult to provide an electrode on the back surface of the substrate, unlike a GaAs light-emitting device. Therefore, it is necessary to form both the negative electrode and the positive electrode on the side of the grown crystalline semiconductor layer.
In the case of the semiconductor device using the gallium nitride-based compound semiconductor, further, a sapphire substrate permits the emitted light waves to pass through. Therefore, attention has been given to a semiconductor device of the flip chip type that is mounted with the electrode surface on the lower side to allow light to leave from the side of the sapphire substrate.
FIG. 1 is a view which schematically illustrates the structure of a general light-emitting device of this type. The light-emitting device has a buffer layer 2, an n-type semiconductor layer 3, a light-emitting layer 4 and a p-type semiconductor layer 5 which are grown on a substrate 1, wherein the light-emitting layer 4 and the p-type semiconductor layer 5 are partly removed by etching so that the n-type semiconductor layer 3 is exposed. Besides, a positive electrode 10 is formed on the p-type semiconductor layer 5 and a negative electrode 20 is formed on the n-type semiconductor layer. The above light-emitting device is mounted with its electrode-forming surface facing, for example, a lead frame and is, thereafter, bonded thereto. Light emitted from the light-emitting layer 4 is taken out from the side of the substrate 1. To efficiently take out light from the light-emitting device of this type, the positive electrode 10 is made of a reflecting metal and is provided to cover most of the p-type semiconductor layer 5, so that light heading toward the positive electrode side from the light-emitting layer is reflected by the positive electrode 10 and leaves from the side of the substrate 1.
Therefore, the positive electrode material must have a small contact resistance and a high reflection factor. Ag and Al have generally been known as highly reflecting metals, and there has been proposed a reflecting positive electrode having a high reflecting factor formed by directly providing an Ag layer of a thickness of not smaller than 20 nm on the p-type semiconductor layer (see, for example, Japanese Unexamined Patent Publication No. 11-186599). As means for using Ag, Japanese Unexamined Patent Publication No. 11-186599 teaches a positive electrode obtained by forming a silver layer on a p-type nitride semiconductor layer and by over-coating the silver layer with a stabilizer layer. It has been described that the role of the stabilizer layer is to improve mechanical and electrical properties of the silver layer.
When the silver layer is over-coated with the stabilizer layer, the stabilizer layer is formed by using a material having a reflection factor lower than that of Ag or Al, as a matter of course. To achieve a high reflection factor of the positive electrode, the silver layer is designed to be as wide as possible, and the area where the stabilizer layer comes in contact with the p-type semiconductor layer is decreased to as small as possible. For this purpose, the stabilizer layer must be adhered to the p-type semiconductor layer while maintaining a large adhering strength.
As a technology for making the positive electrode metal film exfoliate less from the p-type semiconductor layer, there has been fabricated a gallium nitride semiconductor device having dents of a depth greater than a lattice constant (see, for example, Japanese Unexamined Patent Publication No. 2002-155507). According to this method, however, recessed portions must be formed in the step of growing the semiconductor, impairing the productivity.