Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-03-03
2004-03-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S050121, C372S054000
Reexamination Certificate
active
06711191
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a nitride semiconductor laser device and particularly, to a laser device having an effective refractive index type narrow stripe ridge structure, which is used in an optical information processing field and can realize a continuous-wave operation at a high power without a kink.
BACKGROUND OF THE INVENTION
As the information-oriented society has developed in recent years, a &phgr; device which can store a large amount of information has been required. Particularly, the layer light source having a short wavelength is longed for as a light source for a large amount media such as DVD and as a light source for communication. The applicators reported a nitride semiconductor layer device having a lifetime of ten thousand hours or more under room-temperature continuous-wave operation of in the single mode at the wavelength of 403.7 nm.
As mentioned above, the next problem the nitride semiconductor device which can realize continuous-wave oscillation is to raise the power of the device, in order to put the device to practical use and expand the application fields. And a longer lifetime of stable continuos-wave oscillation must be realized.
Particularly, the semiconductor laser device which is used as a light source for the mass storage optical disk such as DVD requires sufficient optical power to enable recording and regenerating. And in such a semiconductor layer device, the stable lateral mode of oscillation is required. Concretely, the oscillation at 5 mW and 30 mW is required as optical power during recording and regenerating. And under such an oscillation, there is needed no kink in the electric current—optical power characteristics.
However, when the injection current into the laser device is increased and the optical power is increased, there is generally a kink due to the instability of the lateral mode, subsequently to the linear region after the initiation of oscillation, in the current—optical power characteristics. In order to put the laser device to practical use, the stable basic single lateral mode without a kink must be achieved in the optical power region of from the initiation of oscillation to 30 mW and further, a longer lifetime of oscillation must be achieved.
DISCLOSURE OF THE INVENTION
In view of the above-mentioned problems, the first object of the present invention is to provide a semiconductor laser device in which the stable lateral mode of the basic oscillation mode can be achieved in the region of from relatively low optical power to the high optical power and there is no kink in the current-optical power characteristics in such a region. The object can be accomplished by the effective refractive index type nitride semiconductor laser device comprising an active layer and at least a p-side cladding layer and a p-side contact layer which are laminated in turn on the active layer, wherein the waveguide region of a stripe structure is provided by etching from the p-side contact layer to above the active layer, characterized in that the stripe width providing by etching is within the narrow stripe range of 1 to 3 &mgr;m and the etching depth is above the active layer and below the remaining thickness of the p-side cladding layer of 0.1 &mgr;m.
That is, according to the present invention, the waveguide having such stripe width and depth enables the basic mode emission having a stable lateral mode and there is no kink in the wide light output power range, resulting in a semiconductor laser device having a longer lifetime under continuous-wave operation.
Preferably, the insulating film other than Si oxide is formed on the both side surfaces of the stripe which has been exposed by said etching or the flat surface of the nitride semiconductor continuing with said side surfaces, and an electrode is formed on the p-side contact layer which is the uppermost layer of the stripe via said insulating film. Thereby, in such a nitride semiconductor laser device, the insulating property between the positive electrode and p-side cladding layer is improved and particularly, the light output power is enhanced. That is, the effect tends to be remarkable as the driving current increases. Therefore, the device having less leak current and a high reliability can be obtained.
Preferably, the stripe width is 1.2 to 2 &mgr;m. This nitride semiconductor device has a refractive index waveguide structure in which the lateral mode is stable in the high light output power region, for example, in the region exceeding 5 mW, therefore, a basic (single) mode emission is further improved to enable the emission without kink in the wide light output power range.
Concretely, said insulating film other than Si oxide may include at least one among an oxide containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf and Ta, and SiN, BN, SiC and AlN. Said insulating film gives the device a high reliability.
The waveguide region having a stripe structure in this laser device is formed by the following steps, resulting in the nitride semiconductor laser device having a extremely good precision and a good yield. The method comprises a first step of laminating a p-side contact layer containing the second p-type nitride semiconductor on the p-side cladding layer containing the first p-type nitride semiconductor and thereafter, forming a first protective film in the stripe structure on the surface of the p-side contact layer; a second step of etching the nitride semiconductor in the part on which the first protective film is not formed via the protective film so as to form a waveguide region having a stripe structure just under the protective film; a third step of, after the second step, forming a second protective film made of the material other than that of the first protective film and having an insulating property on the side surface of the stripe waveguide and the flat surface of the nitride semiconductor layers which has been exposed by etching; and the fourth step of, after the third step, removing the first protective film. In this case, as mentioned below, the first protective film may be formed in the desired shape using the third protective film.
And in order to apply the laser device to the laser light source as mentioned above, it is needed to improve the properties, particularly optical properties of the laser device, that is, to improve the waveguide of the semiconductor laser, for example, to improve the aspect ratio and the far-field pattern and prevent the leakage of light. Concretely, said laser device having a longer lifetime is an effective refractive index waveguide type and it is needed to realize a high-precision control of the lateral mode. In the ridge waveguide structure, the effective refractive index changes depending on the etching depth, stripe height and the like. Such a structure change effects on the device properties extremely. Then, the second object of the present invention is to improve the beam shape, that is, the aspect ratio in the F.F.P (far-field pattern). If the laser device is applied to the optical disk system or the laser printer, the laser light is corrected and adjusted by each optical system. In this case, if the aspect ratio is large, the correction optical system is a large-scale one, therefore, the design, manufacturing and the loss via the optical system is a large problem. Further, for the nitride semiconductor light emitting device, the measures to control light leakage which has been a problem since a prior time is needed, because the light leakage appears as a ripple in the laser device and causes the problem of noise in the laser device application.
According to the present invention, the above-mentioned second object is accomplished by the effective refractive index type nitride semiconductor laser device comprising an active layer and at least a p-side optical waveguide layer, a p-side cladding layer and a p-side contact layer which are laminated on the active layer, wherein the waveguide region of a stripe structure is provided by etching from the p-side contac
Kozaki Tokuya
Nagahama Shinichi
Nakamura Shuji
Sano Masahiko
Ip Paul
Nichia Corporation
Nixon & Vanderhye P.C.
Rodriguez Armando
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