1. Field of the Invention
The present invention relates generally to semiconductor laser devices using gallium nitride-based semiconductor and optical information reproduction apparatuses using the semiconductor laser device as a source of light, and particularly to an improvement in the device's end surface mirror.
2. Description of the Related Art
Nitride based semiconductor materials represented by GaN, InN, AlN and mix crystal semiconductor thereof are used to build a prototype of a semiconductor laser device emitting light in a range from blue color through ultraviolet. FIG. 10 shows a nitride semiconductor laser device lasing with a wavelength of 405 nm, as reported by Masaru KURAMOTO et al., Japanese Journal of Applied Physics, vol. 38, pp. L184-L186, 1999. This semiconductor laser device includes a substrate 901 of n-GaN (of 100 μm in thickness) and thereon a lower clad layer 902 of n-Al0.07Ga0.93N (of 1 μm in thickness), a lower guide layer 903 of n-GaN (of 0.1 μm in thickness), a triple quantum well active layer 904 of In0.2Ga0.8N (of 3 nm in thickness) /In0.05Ga0.95N (of 5 nm in thickness), a cap layer 905 of p-Al0.19Ga0.81N (of 20 nm in thickness), an upper guide layer 906 of p-GaN (of 0.1 μm in thickness), an upper clad layer 907 of p-Al0.07Ga0.93N (of 0.5 μm in thickness) and a contact layer 908 of p-GaN (of 0.05 μm in thickness), successively stacked, with an electrode 909 provided thereon and thereunder. Furthermore, an end surface mirror is provided by cleavage. This laser device has active and guide layers posed between clad layers to have a waveguide structure. The active layer emits light, which is in turn confined in this waveguide structure, and the end surface mirror functions as laser resonator mirror to provide a lasing operation.
This conventional semiconductor laser device as described above, however, is disadvantageous, as follows: the present inventors fabricated a semiconductor laser device having the aforementioned structure and have found that in a waveguide portion at an end surface mirror an epitaxially grown layer (each semiconductor layer upper than substrate 901) can have a cross section, a region in a vicinity of a front surface (i.e., a surface opposite to the substrate) in particular, failing to have a satisfactory cleavage plane. Herein an “unsatisfactory cleavage plane” means that an epitaxially grown layer forming a waveguide has a step offset from a vertical cross section or there exists a plane different in angle from a cleavage plane of the GaN substrate or there exists a rolling portion. The present inventors observed this portion in detail and have found that in particular at the active layer, cracking varies, and from the active layer through the front surface, the cracking is often observed different than in the remainder.
A semiconductor laser device has an end surface mirror provided by externally exerting a force along a scratch on a wafer to warp and thus divide the wafer at a predetermined position. As such, the wafer particularly has a surface and therearound receiving a large magnitude of force exerted thereto and thus considered as cracking in a more complicated manner than the interior of the wafer. Possibly for the aforementioned semiconductor based semiconductor laser device this issue was not noted in designing the structure of a layer provided in a vicinity of an active layer formed in a vicinity of the wafer surface and it is thus believed that cracking tends to vary at the active layer of InGaN.
The conventional technology can thus fail to provide a satisfactory end surface mirror. Thus not only does mirror reflectance variation vary a threshold value, differential efficiency and other similar device characteristics but also a light radiation surface roughened prevents a far field pattern (FFP) from being smoothly unimodal, and a split peek, ripple or other impaired optical characteristics would be introduced. Such a FFP abnormality is not preferable as it results in insufficiently condensed light in application for example to an optical pickup in particular and in an extreme case causes stray light.