The present invention relates to a compound semiconductor light-emitting device of gallium nitride series such as purple blue semiconductor laser of gallium nitride series (hereinafter called LD) or blue/green light-emitting diode of gallium nitride series having high luminance (hereinafter called LED).
A conventional shortwave semiconductor laser is improved to an extent that reading/writing of a disk can be executed by a light source of 600 nm zone using InGaAlP and is already put to practical use.
In order to further improve recording density, shortwave blue semiconductor laser has been developed. A laser beam whose oscillation wave is short is useful to reduce a convergent size and to improve recording density.
For this reason, the compound semiconductor device of gallium nitrite series such as GaN, InGaN, GaAlN, InGaAlN has been recently considered as a material of the shortwave semiconductor laser to improve the application to a high density optical disk system.
For example, in the semiconductor laser using GaN series material, a room temperature pulse oscillation having wavelength of 380 to 417 nm is confirmed. However, in the semiconductor laser using GaN series material, a satisfying characteristic cannot be obtained, a threshold voltage for a room temperature pulse oscillation ranges from 10 to 40V, and the variation of the value is large.
This variation is caused by difficulty in a crystal growth of the compound semiconductor layer of gallium nitride series, and large device resistance. More specifically, there cannot be formed the compound semiconductor layer of p-type gallium nitride series having a smooth surface and high carrier concentration. Moreover, since contact resistance of a p-side electrode is high, a large voltage drop is generated, so that the semiconductor layer is deteriorated by a heat generation and a metal reaction even when the pulse oscillation is operated. In consideration of a cheating value, the room temperature continuous oscillation cannot be achieved unless the threshold voltage is reduced to less than 10V.
Moreover, when a current necessary to the laser generation is implanted, the high current flows locally and a carrier cannot be uniformly implanted to an active layer, and an instantaneous breakage of the device occurs. As a result, the continuous generation of the laser cannot be achieved.
Thus, to realize the purple blue semiconductor laser of gallium nitride series having high reliability, which is operated by the low threshold current to be used in the optical disk and the low threshold voltage, the following points are important:
Specifically, the efficient and uniform implantation of the carriers to the active layer and the reduction of the voltage drop by the electrode contact are important.
However, it is difficult to extremely execute such points in the present state.
As mentioned above, in the compound semiconductor laser of gallium nitride series, it is difficult to obtain the compound semiconductor laser having a good p-type gallium nitride series with no fine porous defects. Moreover, since the p-side electrode contact resistance is high, large voltage drop is generated by the electrode contact. Further, since the carriers cannot be uniformly implanted to the active layer, it is difficult to realize the device with the low threshold current and the low operating voltage.
In the light-emitting device of GaN series, since the p-side electrode contract resistance was high, the operating voltage was increased. Moreover, nickel, serving as a p-side electrode metal, and gallium forming the p-type semiconductor layer, were reacted with each other, melted, and deteriorated at an electrical conduction. As a result, it was difficult to continuously generate the laser.