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Method of producing multi-wavelength semiconductor laser deviceRelated Patent Categories: Coherent Light Generators, Particular Active Media, Semiconductor, Injection, Monolithic Integrated, Laser ArrayMethod of producing multi-wavelength semiconductor laser device description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20050286590, Method of producing multi-wavelength semiconductor laser device. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a multi-wavelength semiconductor laser device, and more particularly to a multi-wavelength semiconductor laser device capable of simultaneously or selectively oscillating laser light of three different wavelengths (e.g., 405 nm, 650 nm and 780 nm), and a method for producing the multi-wavelength semiconductor laser device. [0003] 2. Description of the Related Art [0004] In general, a semiconductor laser device is one that produces light amplified by stimulated emission of radiation. The light produced by the semiconductor laser device has a narrow frequency width (one of short-wavelength characteristics), superior directivity and high output. Due to these advantages, the semiconductor laser device is used as a light source for an optical pick-up apparatus of an optical disc system, such as a CD (compact disc) or DVD (digital video disc) player, as well as, is widely applied to a wide range of fields of optical communications multiplex communications, space communications and the like. [0005] In recent years, a multi-wavelength semiconductor laser device capable of oscillating two or more different wavelengths has been required in the field of optical discs using laser as a light source for writing and reading information. For example, a two-wavelength semiconductor laser device is currently developed as a light source for both CD players having a relatively low data density and DVD players having a relatively high data density. [0006] FIGS. 1a to 1g are cross-sectional views illustrating the overall procedure of a conventional method for producing a two-wavelength semiconductor laser device. [0007] Referring to FIG. 1a, a first semiconductor laser epitaxial layer oscillating light at a wavelength of 780 nm is formed on an n-type GaAs substrate 11. Specifically, the first semiconductor laser epitaxial layer is formed by sequentially growing an n-type AlGaAs clad layer 12a, an AlGaAs active layer 13a and a p-type AlGaAs clad layer 14a on the GaAs substrate 11. [0008] Thereafter, the first semiconductor laser epitaxial layer, including the layers 12a, 13a and 14a, is selectively removed by photolithography and etching to expose a portion of a top surface of the GaAs substrate 11, as shown in FIG. 1b. [0009] Next, as shown in FIG. 1c, a second semiconductor laser epitaxial layer oscillating light at a wavelength of 650 nm is formed on the exposed portion of the GaAs substrate 11 and the unremoved portion of the first semiconductor laser epitaxial layer. Specifically, the second semiconductor laser epitaxial layer is formed by sequentially growing an n-type AlGaInP clad layer 12b, a GaInP/AlGaInP active layer 13b and a p-type AlGaInP clad layer 14b. [0010] Thereafter, the second semiconductor laser epitaxial layer, including the layers 12b, 13b and 14b, formed on the first semiconductor laser epitaxial layer is removed by photolithography and etching, and at the same time, the first epitaxial layer is separated from the second epitaxial layer, as shown in FIG. 1d. [0011] Next, as shown in FIG. 1e, the p-type AlGaAs clad layer 14a and the p-type AlGaInP clad layer 14b are selectively etched by a common process to form ridge-shaped layers 14a' and 14b', which contribute to an improvement in current injection efficiency. Then, as shown in FIG. 1f, n-type GaAs current-limiting layers 16a and 16b and p-type GaAs contact layers 17a and 17b are formed. [0012] Finally, as shown in FIG. 1g, p-side electrodes 19a and 19b formed of Ti, Pt, Au or an alloy thereof are formed on the p-type GaAs contact layers 17a and 17b, respectively, and then an n-side electrode 18 formed of Au/Ge, Au, Ni or an alloy thereof is formed on a bottom surface of the GaAs substrate 11 to produce the two-wavelength semiconductor laser device 10. [0013] In this manner, the semiconductor laser device 10 oscillating light of two different wavelengths is produced on a single substrate, enabling integration into one chip. Accordingly, the conventional method is advantageous compared to a method wherein respective semiconductor laser devices are separately produced, and are then attached to one substrate by die bonding, in terms of the following advantages: i) the separate production and bonding processes are omitted, thus shortening the overall production procedure, and ii) poor alignment caused during die bonding of chip can be solved. [0014] As explained earlier in FIGS. 1a to 1g, the conventional method is limited to the two-wavelength (650 nm and 780 nm) semiconductor laser device, and thus cannot be applied to a three-wavelength (further including light of a short wavelength) semiconductor laser device. A three-wavelength semiconductor laser device commonly required in the art is one which can oscillate light both at wavelengths of 650 nm and 780 nm, and at a shorter wavelength of 405 nm. In this connection, there is a problem that since a GaN-based epitaxial layer is required to produce a semiconductor laser structure oscillating light at a wavelength of 405 nm, the three semiconductor laser structures of the three-wavelength semiconductor laser device cannot be formed on the same substrate. [0015] More specifically, since the lattice constant of an AlGaAs epitaxial layer for the semiconductor laser structure oscillating light at a wavelength of 650 nm is similar to that (about 5.6 .ANG.) of an AlGaInP epitaxial layer for the semiconductor laser structure oscillating light at a wavelength of 750 nm, they can be grown on the same substrate, such as a GaAs substrate. However, since there is a large difference in the lattice constant between the AlGaInP epitaxial layer and an GaN epitaxial layer (about 3.2 .ANG.) for the semiconductor laser structure oscillating light at 405 nm, many crystal defects occur while the epitaxial layers are grown on a GaAs substrate, which makes practical application difficult. In other words, since substrates inherent in the growth of a nitride semiconductor, such as GaN, sapphire and SiC substrates, are required in order to grow the GaN epitaxial layer thereon, a multi-wavelength semiconductor laser device oscillating light, for example, at wavelengths of 650 nm, 780 nm and 405 nm, cannot be substantially produced by the conventional method for producing a two-wavelength semiconductor laser device. SUMMARY OF THE INVENTION [0016] Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for producing a multi-wavelength semiconductor laser device oscillating light of three different wavelengths by growing GaN epitaxial layers on a separate substrate, followed by separation and attachment. [0017] It is another object of the present invention to provide a multi-wavelength semiconductor laser device having a novel structure which is produced by the method. [0018] In order to accomplish the above objects of the present invention, there is provided a method for producing a multi-wavelength semiconductor laser device, comprising the steps of: preparing a substrate for growth of a nitride single crystal thereon; sequentially growing a first conductivity-type first clad layer, a first active layer and a second conductivity-type first clad layer on the substrate, to form a nitride epitaxial layer; separating the nitride epitaxial layer from the substrate; attaching the separated nitride epitaxial layer to a first conductivity-type substrate; selectively removing the nitride epitaxial layer such that a portion of the substrate is exposed, to form a first semiconductor laser structure; sequentially growing a first conductivity-type second clad layer, a second active layer and a second conductivity-type second clad layer on the exposed portion of the first conductivity-type substrate, to form a second semiconductor laser structure separated from the first semiconductor laser structure; sequentially growing a first conductivity-type third clad layer, a third active layer and a second conductivity-type third clad layer on the remaining portion of the top surface of the first conductivity-type substrate, to form a third nitride epitaxial layer separated from the first and second semiconductor laser structures; and forming a first electrode connected to a bottom surface of the first conductivity-type substrate and forming second electrodes connected to the respective second conductivity-type clad layers of the first, second and third semiconductor laser structures. [0019] In a preferred embodiment of the present invention, the method of the present invention further comprises the steps of: selectively etching the respective second conductivity-type clad layers of the first, second and third semiconductor laser structures, after the formation of the third semiconductor laser structure and before the formation of the first electrode and the second electrodes, to form ridge-shaped layers, and forming an insulating layer on top surfaces of the second conductivity-type clad layers except for top ends of the ridge-shaped layers. In this case, the second electrodes can be connected to the respective second conductivity-type clad layers through the top ends of the ridge-shaped layers. [0020] The insulating layer may be formed in such a manner that it is extended to side faces of the first, second and third semiconductor laser structures. The insulating layer may be formed of SiO.sub.2 or Si.sub.3N.sub.4. [0021] Further, the separation of the nitride epitaxial layer from the substrate can be performed by irradiating the bottom surface of the substrate with laser light to lift-off the nitride epitaxial layer. In this case, the method of the present invention may further comprise the step of lapping the bottom surface of the substrate, before the laser irradiation, to decrease the thickness of the substrate. [0022] Further, the attachment of the nitride epitaxial layer to the first conductivity-type substrate can be performed by applying pressure to the separated nitride epitaxial layer on a top surface of the first conductivity-type substrate at high temperature. Continue reading about Method of producing multi-wavelength semiconductor laser device... 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