1. Field of the Invention
The present invention relates to a semiconductor laser device and a method of fabricating the same, and particularly relates to a semiconductor laser device formed by adhering a semiconductor laser device portion and a supporting substrate, and to a method of fabricating the semiconductor laser device.
2. Description of the Related Art
Conventionally, a semiconductor laser device formed by the following procedure and a method of fabricating the same are known. Specifically, the semiconductor laser device is formed by adhering a semiconductor laser device portion and a supporting substrate, and by then splitting the substrate, on which the semiconductor laser device portion is formed (for instance, refer to Technical Report of IEICE, Vol. 102, LQE2002-85, pp. 55-57, which is referred to as Document 1 below).
Document 1 describes a semiconductor laser device fabricated by laser lift-off (LLO) and a method of fabricating the semiconductor laser device.
FIG. 1 is a cross sectional view illustrating a conventional semiconductor laser device disclosed in Document 1. Referring to FIG. 1, in the conventional semiconductor device, a contact metal layer 302 is formed on a GaAs substrate 301 serving as a supporting substrate. The contact metal layer 302 consists of Ti and Au layers in order from the lower tier to the upper tier. A first fusion layer 303 of Sn is formed on the contact metal layer 302. A second fusion layer 304 of Au is formed on the first fusion layer 303. A semiconductor device layer 306 including a ridge 305, which projects downward, is formed on the second fusion layer 304. A GaN layer 307 is formed on the semiconductor device layer 306. Note that the GaAs substrate 301 and the GaN layer 307 are formed in a way that the cleavage planes of the GaAs substrate 301 and the GaN layer 307 respectively meet each other. An electrode 308, which consists of a Ti, Al, Ti and Au layers, is formed on the GaN layer 307. An electrode 309, which consists of a Ti layer and an Au layer, is formed on the back surface of the GaAs substrate 301.
FIGS. 2 to 4 are cross-sectional views of the conventional semiconductor laser device disclosed in Document 1, each for explaining a method (LLO) of fabricating the device. Referring to FIGS. 1 to 4, the method of fabricating the conventional semiconductor laser device is as follows. First, as illustrated in FIG. 2, the GaN layer 307 is grown on the (0001) plane of a sapphire substrate 310 serving as a growth substrate by use of metal organic chemical deposition (MOCVD). Subsequently, the semiconductor device layer 306, which includes the ridge 305, is grown on the GaN layer 307. Thereafter, the second fusion layer 304 of Au is formed on the semiconductor device layer 306.
Subsequently, as illustrated in FIG. 3, the contact metal layer 302, which consists of the Ti and Au layers, is formed on the GaAs substrate 301. On the contact metal layer 302, the first fusion layer 303 of Sn is formed. Then, the first and second fusion layers 303 and 304 are adhered to each other. In this event, the first and second fusion layers 303 and 304 are adhered to each other so that the cleavage planes of the respective GaAs substrate 301 and the GaN layer 307 meet each other. Thereafter, the semiconductor laser device obtained by the adhesion is held for approximately 10 minutes at approximately 310° C. in a nitrogen atmosphere. Thus, Sn of the first fusion layer 303 and Au of the second fusion layer 304 are alloyed, and hence the GaAs substrate 301 and the GaN layer 307 on the sapphire substrate 310 are attached together as, as shown in FIG. 4.
Thereafter, the sapphire substrate 310 is irradiated with the fourth harmonic Nd-YAG laser having a wavelength of 266 mn. By heat generated through the irradiation, GaN in the interface between the sapphire substrate 310 and the GaN layer 307 is melted and decomposed into Ga metal and N. Thus, the sapphire substrate 310 is removed from the GaN layer 307. Incidentally, Ga metal, which is attached to the surface of the GaN layer 307 after the removal of the sapphire substrate 310, is cleaned off with HCl, and is then removed. Thereafter, as illustrated in FIG. 1, the electrode 308 is formed on the GaN layer 307, and the electrode 309 is formed on the back surface of the GaAs substrate 301. The electrode 308 consists of the Ti, Al, Ti and Au layers in order from the lowermost tier to the uppermost tier. The electrode 309 consists of the Ti and Au layers in order from the lower tier to the upper tier.
Finally, the semiconductor laser device is split by cleaving the semiconductor laser device along the cleavage plane of the GaN layer 307. Thereby, the cavity surfaces of the semiconductor laser device are formed. In this manner, the conventional semiconductor laser device disclosed in Document 1 is fabricated. The fabricating method makes it possible to remove the sapphire substrate 310, which serves as a growth substrate, and which has poor cleavability. Accordingly, the cleavability of the nitride semiconductor laser device can be improved. In a case where a GaN substrate is used as a growth substrate instead of the sapphire substrate 310, the GaN substrate is separated and repeatedly used. Because the GaN substrate is expensive, the separation and repeated use makes it possible to reduce costs. For this reason, it is useful that the technique of adhering a semiconductor device portion and a supporting substrate to each other is applied to the method of fabricating a semiconductor laser device portion.
However, the conventional semiconductor laser device disclosed in Document 1 has the following problems. First, the semiconductor device layer 306 is adhered to the GaAs substrate 301 with the second fusion layer 304 of Au having no cleavability. Although the GaAs substrate 301 has cleavability, the flatness of the cleavage plane of the semiconductor laser device is deteriorated due to the existence of the second fusion layer 304 which is made of Au having no cleavability, and which is formed between the GaAs substrate 301 and the GaN layer 307. Second, in a case where, instead of the GaAs substrate 301 having cleavability, metal having no cleavability, such as Cu—W, is used as a supporting substrate, the problem is that the flatness of cleavage plane of the semiconductor laser device is further deteriorated.