Abstract:
The semiconductor laser diode formed with a window at a cleavage facet and a fabricating method thereof are disclosed, wherein a ridge adjacent to a cleavage facet of the semiconductor laser diode and part of the p-clad layer underneath the ridge are etched to form a window, such that a current is not applied to along the cleavage facet to thereby prevent the cleavage facet from being degraded and to enhance reliability of the diode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on, and claims priority from Korean Patent Application Number 10-2007-0038976 filed Apr. 20, 2007, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND ART 
     The following description generally relates to a semiconductor laser diode and a method for fabricating the same, and more particularly to a semiconductor laser diode formed with window at cleavage facet and fabricating method thereof. 
     A semiconductor laser diode generates a laser beam having frequency of a narrow width and keen directivity and is thus mainly used in a field such as optical communication, a multiple communication and a space communication. Also, the semiconductor laser diode is extensively used for transmission of data or recording and reading of data in a communication field such as an optical communication and an apparatus such as a compact disc player (CDP) and a digital versatile disc player (DVDP). The semiconductor laser diode also may be applicable as devices with improved speed for data transmission or certain read/write functions in players using optical discs. 
     Among semiconductor light emitting devices, semiconductor laser devices are used as light sources for reading and writing of a signal of an optical recording medium such as CD (Compact Disk), DVD (Digital Versatile Disk) or Blue-Ray Disk. When a semiconductor laser device is used as a light source for writing, a higher-power semiconductor laser device is required because of increased speed and increased capacity of multi-layering media. Therefore, nitride semiconductor laser devices adapted to high power, for each wavelength of infrared, red, blue or the like, have been developed and are stilt now under development. 
     The extensive use of the semiconductor laser diode is due to facts that the emission characteristics of a laser beam can be maintained in a limited space and the semiconductor laser diode is a compact device and has a small critical current value for emission. An increase in the number of industrial fields adopting the semiconductor laser diode results in an increase in a need for a semiconductor laser diode having a more reduced critical current value. That is, it is required to manufacture an excellent semiconductor laser diode capable of enabling low-current emission and having longer lifetime. 
     Since a semiconductor laser device may maintain oscillation characteristics of laser beam in a limited space, may be formed to a small scale, and requires a small critical current for laser oscillations, the semiconductor laser device is widely used. As the number of industrial fields to which the semiconductor laser is applied increases, demand for semiconductor laser devices requiring for a smaller critical current increases. In other words, semiconductor laser devices having excellent characteristics, such as oscillating at a low current, and the ability to pass a lifespan test are needed. 
     For example, a nitride semiconductor laser diode, for example, gallium nitride (GaN)-based laser diode may allow using wavelengths from the ultraviolet region to the visible-green region of the electromagnetic spectrum. The GaN-based laser diodes may be applicable in various areas, including storage/reproduction devices using increased density optical information, laser printers with increased resolution, and projection televisions. 
     The nitride semiconductor laser diode has gained a good reception in terms of environment-friendly aspect of view due to the fact that it does not use arsenic (As) for main components. 
     The semiconductor laser diode employed for light source of optical devices must satisfy a single mode and high output power characteristics. To this end, the semiconductor laser diode for light source must be disposed with a ridge waveguide structure that provides a function to confine the current and requires a small critical current for laser oscillations. 
     The GaN-based laser diodes may have a multi-layered structure of an epitaxial layer including an active layer for emitting laser beam to an upper surface of a sapphire substrate, the epitaxial layer being formed thereon with a ridge, the ridge being formed thereon with a p-electrode, and the epitaxial layer is partially mesa-etched and exposed to form an n-electrode. 
     When a current is applied from the p-electrode via the ridge, light is produced from the active layer in response to hole-electron recombination. The light produced by the active layer reciprocates between a front cleavage facet and a back cleavage facet, is amplified and emitted outside if resonance conditions are met. A high reflection film is formed on the back cleavage facet to reflect the laser beam to allow the laser beam to be emitted only through the front cleavage facet of the semiconductor laser diode, and the front cleavage facet is formed with an anti-reflection film to prevent the laser beam from being reflected. 
     Meanwhile, degradation and local segregation may often become severe in the facets of high output semiconductor laser diodes. Poor reliability that shortens the service life of the semiconductor laser diode resulting from degradation of the semiconductor laser diodes on the facet at the light emitting side is known as a big problem in achieving higher power of the semiconductor laser device. This is commonly known as COD (Catastrophic Optical Damage) which is a phenomenon in which the light emitting portion is thermally melted thereby causing emission stop. The reason the COD occurs is that the light emitting portion becomes an absorption region in which laser light is absorbed to generate heat and result in a reduced energy band gap. It is said that non-radiative recombination level is attributable to the absorption region. The light emitting portion of the nitride semiconductor laser device in which laser light emission stops is found to have a hole which may be created as the light emitting portion is melted, and it is understood that the degradation of the light emitting portion causes emission stop. 
     The conventional nitride semiconductor laser diode is configured in such a manner that p-contact layer and p-pad electrode are formed up to the cleavage facets of the semiconductor laser diode. 
     If a voltage is applied to the p-pad electrode, holes are introduced along the cleavage facets of the semiconductor laser diode through the p-contact layer, and when the holes are recombined with the electrons to generate light, light absorption becomes high at the cleavage facets to aggravate the degradation on the facets. 
     SUMMARY 
     The object of the instant novel disclosure is to provide a semiconductor laser diode formed with window at cleavage facet and fabricating method thereof. 
     In one general aspect, a semiconductor laser diode formed with window may comprise: a stacked structure sequentially comprised of, on an upper surface of a substrate, an n-contact layer, an n-clad layer, an n-waveguide layer, an active layer, an electron blocking layer (EBL), a p-waveguide layer; a ridge layer comprised of a p-clad layer protrusively formed at a central portion thereof on an upper surface of the p-waveguide layer and a p-contact layer formed on an upper surface of the protrusive p-clad layer; and a window formed at a predetermined depth by etching a ridge adjacent to a cleavage facet. 
     In another general aspect, a semiconductor laser diode formed window may comprise: a substrate sequentially formed thereon with an n-contact layer, an n-clad layer, an n-waveguide layer, an active layer, an electron blocking layer (EBL), a p-waveguide layer; the n-contact layer being partially exposed by being mesa-etched from the p-waveguide layer to part of the n-contact layer; a ridge formed with a p-clad layer protrusively formed at a central portion thereof on an upper surface of the p-waveguide layer and a p-contact layer formed on an upper surface of the protrusive p-clad layer; a protective film formed on a lateral surface of the ridge and the upper surface of the p-clad layer; a window formed by etching a ridge adjacent to a cleavage facet and part of the p-clad layer underneath the ridge; and a p-pad electrode formed to cover the p-contact layer and the part of the protective film, and an n-pad electrode formed on the exposed n-contact layer. 
     In still another general aspect, a semiconductor laser diode formed with window may comprise: a substrate sequentially formed thereon with an n-contact layer, an n-clad layer, an n-waveguide layer, an active layer, an electron blocking layer (EBL), a p-waveguide layer; the p-waveguide layer formed thereon with a central portion protruded p-clad layer and the protruded p-clad layer being formed thereon with the p-contact layer to form a ridge; a protective film formed on a lateral surface of the ridge and the upper surface of the p-clad layer; a window formed by etching a ridge adjacent to a cleavage facet and part of the p-clad layer underneath the ridge; and a p-pad electrode formed to cover the p-contact layer and the part of the protective film, and an n-pad electrode formed at a bottom surface of the substrate, wherein the window is formed at least either adjacent to a front cleavage facet or a back cleavage facet, and a length of the window is in the range of 1˜300 μm, and a distance between the active layer and the window is in the range of 1000˜5000 Å. 
     In still another general aspect, a method for fabricating a semiconductor laser diode formed with window may comprise: sequentially stacking on an upper surface of a substrate an n-contact layer, an n-clad layer, an n-waveguide layer, an active layer, an electron blocking layer (EBL) and a p-waveguide layer; mesa-etching from the p-waveguide layer to part of the n-contact layer to expose part of the n-contact layer; forming on the p-waveguide layer a central portion protruded p-clad layer and forming a p-contact layer on the protruded p-clad layer to form a ridge; forming a protective film on a lateral surface of the ridge and an upper surface of the p-clad layer; etching the ridge adjacent to a cleavage facet and part of the p-clad layer underneath the ridge to form a window; and covering the p-contact layer and part of the protective layer to form a p-pad electrode and forming an n-pad electrode at an upper surface of the exposed n-contact layer. 
     In still another general aspect, a method for fabricating a semiconductor laser diode formed with window may comprise: sequentially stacking on an upper surface of a substrate an n-contact layer, an n-clad layer, an n-waveguide layer, an active layer, an electron blocking layer (EBL) and a p-waveguide layer; forming on the p-waveguide layer a central portion protruded p-clad layer and forming a p-contact layer on the protruded p-clad layer to form a ridge; forming a protective film on a lateral surface of the ridge and an upper surface of the p-clad layer; etching the ridge adjacent to a cleavage facet and part of the p-clad layer underneath the ridge to form a window; and covering the p-contact layer and part of the protective layer to form a p-pad electrode and forming an n-pad electrode at a bottom surface of the substrate. 
     Implementations of this aspect may include one or more of the following features. 
     The step of forming the window may comprise: sequentially forming a hard mask and a photosensitive polymer on an upper surface of the p-contact layer and the protective film; patterning the photosensitive polymer in order to expose a region adjacent to the cleavage facet and then etching the hard mask; etching by using the hard mask as the etch mask from the p-contact layer adjacent to the cleavage facet to part of the p-clad layer. 
     The hard mask may include any one material selected from a group consisting of Ni, Cr, Pt, Cu, Ti, Al, SiO 2 , SiN, Al 2 O 3  and TiO 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the implementations, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a partial perspective view of a semiconductor laser diode formed with window at a cleavage facet; 
         FIG. 2  is a perspective view of a first exemplary implementation of a semiconductor laser diode formed with a window on a cleavage facet; 
         FIG. 3  is a plan of windows each formed at a front cleavage facet and a back cleavage facet of a semiconductor laser diode; 
         FIGS. 4   a  to  4   f  illustrate a first exemplary implementation of a method for fabricating a semiconductor laser diode formed with a window at a cleavage facet; 
         FIGS. 5   a ,  5   b  and  5   c  illustrate a process of forming a window on a cleavage facet; 
         FIG. 6  is a perspective view of a second exemplary implementation of a semiconductor laser diode formed with a window on a cleavage facet; and 
         FIGS. 7   a  to  7   e  illustrate a second exemplary implementation of a method for fabricating a semiconductor laser diode formed with a window on a cleavage facet. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to easily provide a general concept and principle of the various implementations of the present teachings. 
     Referring to  FIG. 1 , a ridge at a front cleavage facet  100  and part of a p-clad layer  130  may be etched to be exposed upwards. In other words, the front cleavage facet  100  may be etched from a p-contact layer  110  to part ÅA length (d) of the window  160  may be within half the length of a resonator in a semiconductor laser diode but preferably within 1 μm˜300 μm. If the length (d) of the window is 1 μm or less, there is a high probability of holes being doped along the front cleavage facet  100 , making it difficult to prevent the degradation of the front cleavage facet  100 . If the length (d) of the window  160  is longer than 300 μm, there occurs a difficulty in holes being doped into an active layer  150  due to high resistance. Furthermore, a distance (d) of the window  160  detached the active layer  150  is preferably in the range of 1000 Å˜5000 Å. It is understood that the formation of window  160  at the front cleavage facet  100  which is a light emitting portion may prevent a current from being doped along the front cleavage facet  100 , thereby minimizing the degradation of the cleavage facet. In doing do, the problem of reduced reliability due to degradation of the light emitting portion in the semiconductor laser diode can be obviated. 
     Meanwhile, the window  160  may be formed on a front cleavage facet  100  and on a back cleavage facet  180  as well. Each length of the window  160  formed on the front and back cleavage facets is preferably in the range of 1 μm˜150 μm. 
     Now, referring to  FIG. 2 , in a first exemplary implementation of a semiconductor laser diode formed with a window on a cleavage facet, a substrate  200  may be sequentially stacked thereon with an n-contact layer  201 , an n-clad layer  202 , an n-waveguide layer  203 , an active layer  204 , an electron blocking layer (EBL,  205 ) and a p-waveguide layer  206 . 
     A mesa-etching may be performed from the p-waveguide layer  206  to part of the n-contact layer  201  to expose part of part of the n-contact layer, an upper surface of the p-waveguide layer  206  may be formed with a central portion protruded p-clad layer  207 , and the central portion protruded p-clad layer  207  may be formed thereon with the p-contact layer  208  to form a ridge. 
     A lateral surface of the ridge and an upper surface of the p-clad layer  207  may be formed with a protective film  209 . A ridge adjacent to a front cleavage facet  250  from which light generated by the active layer  204  is emitted and part of the p-clad layer  207  may be etched to form a window  260 , a p-pad electrode  210  may be formed to cover the p-contact layer  208  and part of the protective film  209 , and an n-pad electrode  211  may be formed on an upper surface of the exposed n-contact layer  201 . 
     A structure of forming a p-pad electrode  210  and an n-pad electrode  211  on the same planar surface in a semiconductor laser diode is called a top-top stacking configuration of a semiconductor laser diode. 
     In a top-top stacking configuration, a sapphire which is an insulation material may be employed for the substrate  200 , such that a mesa-etching is performed from the p-waveguide layer  206  to part of the n-contact layer  201  to expose part of the n-contact layer  201 , and the n-pad electrode  211  is formed on the exposed n-contact layer  201 . The substrate  200  may be a sapphire (Al 2 O 3 ) substrate, a silicon carbide (SiC) substrate, a silicon (Si) substrate, a gallium arsenide (GaAs) substrate, but a sapphire is preferably used for the substrate  200 . 
     The n-contact layer  201  may be generally composed of an n-GaN layer, and the n-clad layer  202  may be represented by the Formula In x Al y Ga 1−x−y N (0≦x&lt;1, 0≦y&lt;1, 0≦x+y&lt;1). 
     The n-waveguide layer  203  may be made of material having a refractive index lower than that of the active layer  204 . The active layer  204  may be comprised of a single quantum well structure of a barrier layer represented by the Formula In x Ga 1 —N (0≦x&lt;1) and a well layer, or a multiple quantum well structure sequentially and repeatedly stacked with the barrier layers and the well layers. 
     The electron blocking layer (EBL,  205 ) may be composed of an AlGaN layer for preventing an overflow of electrons caused by low carrier concentration and mobility of p-type nitride semiconductor. Preferably, the EBL  205  is composed of AlGaN having a high Al composition (more than 20%) for an effective energy barrier function. 
     The p-waveguide layer  206  may be composed of material having a refractive index lower than that of the active layer  204  and may be mainly composed of p-GaN layer. 
     The p-clad layer  207  is composed of the same material layer as that of the n-clad layer  202 , except that doped conductive impurities are different. In other words, the p-clad layer  207  is represented by the Formula p-In x Al y Ga 1−x−y N (0≦x&lt;1, 0≦y&lt;1, 0≦x+y&lt;1). 
     The p-contact layer  208  is composed of the same material layer as that of the n-contact layer  201 , except that doped conductive impurities are different. In other words, the p-contact layer  208  is composed of p-GaN and has a high doping concentration than that of the p-clad layer  207  in order to reduce a contact resistance with the p-pad electrode  210 . 
     The protective film  209  may include any one material selected from a group consisting of SiO 2 , Si 3 N 4 , Al 2 O 3 , HfO and TiO 2 . The p-pad electrode  210  and the n-pad electrode  211  may be formed by any one metal selected from a group consisting of Cr, Ni, Au, Al, Ti and Pt, or a metal alloy of laminated structure selected from the group consisting of Cr, Ni, Au, Al, Ti and Pt. 
     Meanwhile, the window  260  may be formed on a front cleavage facet  250  of the semiconductor laser diode or on a back cleavage facet  280  as well.  FIG. 3  shows a planar view where a window is formed on the front cleavage facet  250  and the back cleavage facet  280  as well. 
     Now, referring to  FIGS. 4   a  to  4   f , a first exemplary implementation of a method for fabricating a semiconductor laser diode formed with a window at a cleavage facet may comprise: sequentially stacking on an upper surface of a substrate  200  an n-contact layer  201 , an n-clad layer  202 , an n-waveguide layer  203 , an active layer  204 , an electron blocking layer ( 205 , EBL) and a p-waveguide layer  206  ( FIG. 4   a ). 
     Successively, a mesa-etching may be performed from the p-waveguide layer  206  to part of the n-contact layer  201  to expose part of the n-contact layer  201  upwards ( FIG. 4   b ). 
     Next, the p-clad layer  207  whose central portion is protruded may be formed on an upper surface of the p-waveguide layer  206 , and the p-contact layer  208  may be formed on an upper surface of the protruded p-clad layer  207  to form a ridge ( FIG. 4   c ). 
     Thereafter, a protective film  209  may be formed on the lateral surface of the ridge and the upper surface of the p-clad layer  207  ( FIG. 4   d ). 
     Then, the ridge adjacent to the front cleavage facet  250  and part of the p-clad layer  207  may be etched to form a window  260  ( FIG. 4   e ). 
     Finally, the p-clad electrode  210  may be formed to cover the p-contact layer  208  and part of the protective film  209 , and the n-pad electrode  211  may be formed on an upper surface of the exposed n-contact layer  201  ( FIG. 4   f ). 
     Now, a process of forming the window  260  will be illustrated in detail with reference to  FIGS. 5   a  to  5   c.    
     First, referring to  FIG. 4   d , the protective film  209  is formed on the lateral surface of the ridge and on the upper surface of the p-clad layer  207  (see  FIG. 4   d ), and then, the p-contact layer  208  and the protective film  209  are sequentially formed thereon with hard mask  291  and the photosensitive polymer  292  (see  FIG. 5   a ). 
     The hard mask may include any one material selected from a group consisting of Ni, Cr, Pt, Cu, Ti, Al, SiO 2 , SiN, Al 2 O 3  and TiO 2  and may be a single layer or a multiple layer composed of the above material. 
     Successively, light exposure or light development may be performed to pattern the photosensitive polymer  292  so that a partial region adjacent to the cleavage facet can be exposed, and the patterned photosensitive polymer  292  may be used as the etch-mask to etch the hard mask  291  and to remove the remaining photosensitive polymer  292 . As a result of this process, the hard mask  291  adjacent to the cleavage facet is removed to expose the p-contact layer  208  as shown in  FIG. 5   b.    
     Next, the hard mask  291  may be used as the etch-mask to etch from the exposed p-contact layer  208  to the part of the p-clad layer  207  and to form a window adjacent to the cleavage facet (see  FIG. 5   c ). 
     It should be apparent that the depth of etching be so adjusted as to allow the upper surface of the p-clad layer  207  to maintain a distance of 1000 Å˜5000 Å from the active layer  204 . In other words, the depth of etching must be adjusted lest the active layer  204  should be damaged. 
     Meanwhile, in the semiconductor laser diode of the present novel concept, the window may be formed by using the photosensitive polymer as the mask to etch a region adjacent to the cleavage facet at a predetermined depth following the formation of the p-pad electrode and the n-pad electrode in the conventional method. 
     Because all the nitride semiconductors are grown on the substrate to form the windows in the instant disclosure, the length, width of the window and etching depth can be easily adjusted and the nitride semiconductors can be grown in an in-situ condition. If, in order to prevent the COD from occurring on the cleavage facet, a substrate is formed with grooves, and the nitride semiconductors are grown on the substrate to allow the grooves to take shapes adjacent to the cleavage facet, it is difficult to meet the growth condition of the nitride semiconductors and to form groove shapes of desired sizes. 
     Now, referring to  FIG. 6 , an upper surface of a substrate  300  is sequentially stacked with an n-contact layer  301 , an n-clad layer  302 , an n-waveguide layer  303 , an active layer  304 , an electron blocking layer ( 305 , EBL) and a p-waveguide layer  306 . 
     An upper surface of the p-waveguide layer  306  is formed with a central portion protruded p-clad layer  307 , and an upper surface of the protruded p-clad layer  307  is formed with a p-contact layer  308  to form a ridge. A protective film  309  is formed on a lateral surface of the ridge and on an upper surface of the p-clad layer  307 . 
     The ridge adjacent to a front cleavage facet  350  from which light generated from the active layer  304  is emitted and part of the p-clad layer  307  are etched to form a window. 
     A p-pad electrode  310  is formed to cover the p-contact layer  308  and part of the protective layer  309  and an n-pad electrode  311  is formed at a bottom surface of the substrate  300 . 
     A structure of forming an n-pad electrode  311  at the bottom surface of a substrate  300  is called a top-down stacking configuration of a semiconductor laser diode, where a conductive substrate, i.e., an n-GaN substrate, is employed for the substrate  300 . In the instant implementation as in the previous one, the window may be formed at the front cleavage facet  350  and the back cleavage facet  380  of the semiconductor laser diode as well. 
     Now, referring to  FIGS. 7   a  to  7   e , a method for fabricating a semiconductor laser diode formed with a window on a cleavage facet according to the second exemplary implementation will be described. 
     First, an upper surface of a substrate  300  is sequentially stacked with an n-contact layer  301 , an n-clad layer  302 , an n-waveguide layer  303 , an active layer  304 , an electron blocking layer (EBL,  305 ) and a p-waveguide layer  306  (see  FIG. 7   a ). 
     Next, the p-waveguide layer  306  is formed thereon with a central portion protruded p-clad layer  307 , and the protruded p-clad layer  307  is formed thereon with the p-contact layer  308  to form a ridge (see  FIG. 7   b ). 
     Successively, a protective film  309  is formed on the lateral surface of the ridge and the upper surface of the p-clad layer  307  (see  FIG. 7   c ), and the ridge adjacent to a front cleavage facet  350  and part of the p-clad layer  307  are etched to form a window  360  (see  FIG. 7   d ), where the method of forming the window is the same as described above. 
     Then, a p-pad electrode  310  is formed to cover the p-contact layer  308  and part of the protective layer  309  and an n-pad electrode  311  is formed at a bottom surface of the substrate  300 . 
     Meanwhile, in the semiconductor laser diode of the present novel concept, the window may be formed by using the photosensitive polymer as the mask to etch a region adjacent to the cleavage facet at a predetermined depth following the formation of the p-pad electrode and the n-pad electrode in the conventional method. 
     While the foregoing has been particularly shown and described with reference to exemplary implementations, it will be understood by those skilled in the art in view of the teachings herein that various modifications and alterations in form and details to the described implementations may be made therein without departing from the spirit and scope of the general inventive concept as defined by the appended claims and their equivalents.