Abstract:
A protective film is formed on a surface of a semiconductor device corresponding to at least a portion that is not to be disordered, by arranging a heat source on a path through which a precursor of the protective film to be formed passes, to cause a decomposition reaction of the precursor in the presence of the heat source, and by exposing the surface of the semiconductor device to the atmosphere after the decomposition reaction. A portion to be disordered is disordered using a thermal treatment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT/JP2004/005542 filed on Apr. 19, 2004, the entire content of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of fabricating a semiconductor device that includes a portion to be partially disordered, such as a window structure in a semiconductor laser device, by using a thin film deposition method such as the catalytic chemical vapor deposition (CVD) method or the hot wire cell method, which are capable of depositing a thin film of high compactness at low temperatures.  
         [0004]     2. Description of the Related Art  
         [0005]     Conventionally, light-emitting facets of semiconductor laser devices are susceptible to a catastrophic optical damage (COD). The COD is a phenomenon in which a feedback cycle of: increase in temperature of a facet→shrinkage of band gap→light absorption→recombination current flow→increase in temperature of the facet, is created on an emitting-side facet of an active layer, which becomes a positive feedback to cause melting and instantaneous deterioration of the facet.  
         [0006]     A window structure is usually formed at the emitting-side facet of the active layer in order to prevent such COD. The window structure broadens an energy band gap in the emitting-side facet, whereby the facet has less absorption of laser light. As a result, the COD is suppressed and a semiconductor laser device with long lifetime is realized.  
         [0007]     The window structure has been conventionally formed by an independent semiconductor process. For example, a portion where the window structure is to be formed is removed by etching or the like, and thereafter, materials with property corresponding to the window structure are embedded into the portion. The forming of the window structure is also achieved by disordering a portion to be the window structure. When the active layer has a superlattice structure, the process of disordering is performed by introducing impurities by an ion implantation or a thermal treatment into the superlattice structure of a portion where the window structure is to be formed, followed by a thermal treatment with a dielectric film to generate lattice vacancies so that constitutional elements of the respective layers that are spatially separated by hetero interfaces are mixed. The disordered portion exhibits properties different from those before implementing the process of disordering. For example, the portion has different band-gap energy and different refractive index (see Japanese Patent Application Laid-Open No. 2000-208870).  
         [0008]     However, when the window structure is formed by the process of disordering, a thermal treatment must be performed for diffusion of impurities or vacancies. Since the thermal treatment is performed for the entire semiconductor laser device, the thermal treatment adversely affects portions that do not need to be disordered, causing deterioration of performance of the semiconductor laser device.  
         [0009]     When the active layer is made of AlGaAs based materials and a part of the active layer is disordered by depositing a silicon dioxide (SiO 2 ) film, which serves an enhance film for disordering, on an upper surface of the window structure to be disordered, arsenic (As) atoms are desorbed from a device surface corresponding to the active layer that is not to be disordered and the surface is roughened. When an electrode is formed on a contact layer, favorable contact between the electrode and the contact layer is not assured and a laser oscillation performance is deteriorated.  
         [0010]     Particularly, desorbed As atoms from GaAs contact layer of the device surface leaves pits, which are a cause of dislocation defects that grow through non-radiative recombination to reach the active layer, which also deteriorates the laser oscillation performance.  
         [0011]     Conventionally, deposition of silicon nitride (SiN X ) by plasma-enhanced chemical vapor deposition (PECVD) method on the entire upper surface of a semiconductor laser device, so as to function as a heat-resistant protective film, is considered as a method of preventing desorption of As. However, the PECVD method is likely to cause plasma damage on the surface of the semiconductor device, which generates a dislocation defect. Furthermore, the deposited film is coarse and does not sufficiently function as the heat-resistant protective film.  
       SUMMARY OF THE INVENTION  
       [0012]     It is an object of the present invention to solve at least the above problems in the conventional technology.  
         [0013]     A method of fabricating a semiconductor device that includes a disordered portion, according to one aspect of the present invention, includes forming a protective film on a surface of the semiconductor device corresponding to at least a portion that is not to be disordered, by arranging a heat source on a path through which a precursor of the protective film to be formed passes, to cause a decomposition reaction of the precursor in the presence of the heat source, and by exposing the surface of the semiconductor device to the atmosphere after the decomposition reaction; and disordering a portion to be disordered using a thermal treatment.  
         [0014]     A method of fabricating a semiconductor device that includes a disordered portion, according to another aspect of the present invention, includes forming including forming a disordering-enhancing film on a surface of the semiconductor device covering a portion to be disordered; forming including forming a protective film on the surface of the semiconductor device covering at least a second portion not to be disordered, by arranging a heat source on a path through which a precursor of the protective film to be formed passes, to cause a decomposition reaction of the precursor in the presence of the heat source, and by exposing the surface of the semiconductor device to the atmosphere after the decomposition reaction; and disordering the portion to be disordered using a thermal treatment.  
         [0015]     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIGS. 1A  to  1 F are schematics for illustrating a method of fabricating a semiconductor device according to an embodiment of the present invention;  
         [0017]      FIGS. 2A and 2B  are schematics for illustrating the method of fabricating a semiconductor device according to the embodiment;  
         [0018]      FIG. 3  is a schematic of a catalytic CVD equipment, used in the method according to the embodiment;  
         [0019]      FIG. 4  is a schematic for illustrating phenomena to occur in a portion to be disordered and a potion not to be disordered, in the method according to the embodiment;  
         [0020]      FIG. 5  is a schematic for illustrating pits and cracks generated by the pits in a semiconductor device;  
         [0021]      FIG. 6  is a graph of injection current versus light output of a semiconductor laser device with and without a window structure according to the embodiment; and  
         [0022]      FIG. 7  is a schematic for illustrating an example of depositing a protective film that covers at least the potion not to be disordered, in the method according to the embodiment.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Exemplary embodiments of the present invention will be explained in detail below with reference to the accompanying drawings.  
         [0024]      FIGS. 1A  to  2 B are schematics for illustrating a method of fabricating a semiconductor device according to an embodiment of the present invention. The semiconductor device is a semiconductor laser device having multiple quantum well (MQW) structure that outputs laser light within a wavelength band of 0.98 micrometer. The semiconductor laser device includes a lower cladding layer  2 , a lower waveguide layer  3 , an active layer  4 , an upper waveguide layer  5 , an upper cladding layer  6 , and a contact layer  7  grown in this order on a substrate  1 . The active layer  4  is formed by successively growing a lower carrier blocking layer  4   c , a multiple quantum well layer  4   b , and an upper carrier blocking layer  4   a . Since the layers preferably have a superlattice structure, they are grown by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).  
         [0025]     First, a method of fabricating the semiconductor laser device will be described according to the sequence of fabrication. As shown in  FIG. 1A , a Al 0.08 Ga 0.92 As lower cladding layer  2  with a thickness of 2.4 micrometers is grown on a GaAs substrate  1 . A GaAs lower waveguide layer  3  with a thickness of 0.48 micrometer is then grown on the lower cladding layer  2 . An active layer  4  is then formed on the lower waveguide layer  3 .  
         [0026]     The active layer  4  is formed by growing a Al 0.4 Ga 0.6 As lower carrier blocking layer  4   a  with a thickness of 0.035 micrometer, the multiple quantum well layer  4   b  with two stacked In 0.14 Ga 0.86 As layers each with a thickness of 0.01 micrometer on the lower carrier blocking layer  4   a , and a Al 0.4 Ga 0.6 As upper carrier blocking layer  4   c  with a thickness of 0.035 micrometer on the multiple quantum well layer  4   b.    
         [0027]     The GaAs upper waveguide layer  5  with a thickness of 0.45 micrometer is formed on the active layer  4 , the Al 0.32 Ga 0.68 As upper cladding layer  6  with a thickness of 0.8 micrometer is formed on the upper waveguide layer  5 , and a GaAs contact layer  7  with a film thickness of 0.3 micrometer is formed on the upper cladding layer  6 .  
         [0028]     After the contact layer  7  with a superlattice structure is formed, a disordering-enhancing film  8  made of SiO 2  is deposited on the entire upper portion of the contact layer  7  to a film thickness of 20 nanometers by using electron beam evaporation method. Thereafter, the disordering-enhancing film  8  is removed by using a photolithographic technique, except for portions corresponding to a window structure  14 . As a result, as shown in  FIG. 1B , the device substrate  10 A with the disordering-enhancing film  8  deposited only on portions corresponding to the window structure  14  is obtained. The disordering-enhancing film  8  corresponds to the active layer  4  formed in a stripe shape, and is formed in a stripe with a width of 20 micrometers so as to cover the active layer  4 .  
         [0029]     As shown in  FIG. 1C , a protective film  9  made of SiN X  is deposited on the entire upper portion of the substrate including the disordering-enhancing film  8  to a thickness of  50  nanometers by using a catalytic CVD method. As a result, the device substrate  10 B with the protective film  9  deposited thereon is obtained. Thereafter, the rapid thermal anneal (RTA) is performed on the device substrate  10 B to cause disordering of the portion beneath the disordering-enhancing film  8 , thereby to form a window structure  14  in that portion.  
         [0030]     As shown in  FIG. 1D , the RTA is performed by placing the device substrate  10 B on a mount  11  made of silicon carbide (SiC), arranging the mount  11  within a quartz tray  12 , and heating a lamp heater  13  below the quartz tray  12  in a nitrogen (N 2 ) gas atmosphere to 930° C. for 30 seconds.  
         [0031]     During the process of RTA, gallium (Ga) atoms are absorbed from the respective layers under the disordering-enhancing film  8  into the disordering-enhancing film  8 , leaving vacancies on the surface. The vacancies are diffused, particularly into the active layer  4  to disorder the active layer. By the process, as shown in  FIG. 1E , a device substrate  10 C with the window structure  14  is obtained.  
         [0032]     After the window structure  14  is formed, as shown in  FIG. 1F , the protective film  9  and the disordering-enhancing film  8  are removed to obtain a device substrate  10 D. Thereafter, as shown in  FIG. 2A , an upper electrode  21  and a lower electrode  22  are then formed on the device substrate  10 D to obtain a device substrate  10 E with the electrode structure. The respective steps explained above are performed on the substrate  1 .  
         [0033]     The device substrate  10 E with the lower electrode  22  is cleaved along a broken line C shown in  FIG. 2A , so that a laser bar containing a plurality of semiconductor laser devices is separated. As shown in  FIG. 2B , on the cleavage surfaces of the separated laser bar, a low reflection film  23  is coated for its emission side and a high reflection film  24  is coated for its reflection side. The laser bar is cut, in parallel with the paper surface of the drawing, into the respective semiconductor laser devices, to complete the fabrication of semiconductor laser devices  10 .  
         [0034]     A method of depositing the protective film  9  is explained next. The protective film  9  made of SiN X  is deposited using a catalytic CVD method. According to the catalytic CVD method, source gases for forming the protective film  9  are catalytically decomposed by a heated catalyzer and chemical reactions occur on a film to be formed. As a result, the protective film  9  is formed.  
         [0035]      FIG. 3  is a schematic of a catalytic CVD equipment  300 . As shown in  FIG. 3 , the catalytic CVD device  300  includes a chamber  31  in which the protective film  9  is deposited, a shower head  32  to introduce source gases for the protective film  9  into the chamber  31 , a filament (tungsten wire)  33  serving as a heated catalyzer, a substrate holder  35  on which the substrate  1  is placed, a substrate heater  36  to heat the device substrate  10 A, a vacuum pump  37  to evacuate the chamber  31 , and a pressure adjustment valve  34  to adjust the pressure within the chamber  31 . The tungsten wire  33  is placed between the shower head  32  and the substrate holder  35 . After the pressure of the chamber  31  is reduced, source gases are introduced from the shower head  32  into the chamber  31 , and contact the tungsten wire  33  to be decomposed thereat. The decomposed source gases flow on the device substrate  10 A placed on the substrate holder  35 .  
         [0036]     Next, a process of depositing the protective film  9  is explained below. The device substrate  10 A is placed on the substrate holder  35 . The vacuum pump  37  is then operated and the pressure adjustment valve  34  is opened so that the pressure within the chamber  31  is reduced to a predetermined pressure. When the pressure within the chamber  31  is reduced to the predetermined pressure, the substrate heater  36  is energized to maintain the temperature of the substrate at about 250° C.  
         [0037]     Ammonia (NH 3 ) gas is introduced via the shower head  32  into the chamber  31  at a flow rate of 200 sccm. The tungsten wire  33  is energized so as to maintain its temperature at 1680° C. Silane gas (SiH 4 ) is introduced via the shower head  32  into the chamber  31  at a flow rate of 2 sccm and the pressure adjustment valve  34  is adjusted so as to maintain the pressure within the chamber  31  at 4.0 Pascals. 360 seconds thereafter, introduction of SiH 4  is stopped, energizing of the tungsten wire  33  is stopped, introduction of NH 3  is stopped, the pressure adjustment valve  34  is closed. Dry nitrogen gas (N 2 ) is introduced to return the pressure of the chamber  31  to atmospheric pressure. Thus, the device substrate  10 B with the 50 nanometer-thick SiN X  protective film  9  formed thereon is obtained.  
         [0038]     During the above process, SiH 4  molecules and NH 3  molecules introduced into the chamber  31  contact the heated tungsten wire  33  to be decomposed into active SiH Y  and NH Z , which reach onto the device substrate  10 A. Since SiH Y  and NH Z  are activated, chemical reactions proceed on the device substrate  10 A at a relatively low temperature of 250° C., and thereby, SiN X  is generated.  
         [0039]     The protective film  9  has a smaller amount of oxygen and hydrogen mixed therein and therefore has a higher compactness than films deposited by a PECVD method. In addition, since the protective film  9  is deposited at a lower temperature as compared to films deposited by thermal CVD method, its internal stress arising from thermal non-equilibrium is small. Thus, the protective film  9  has chemically and physically stable characteristics.  
         [0040]     Effects of the protective film  9  are explained next.  FIG. 4  is a cross section of the device substrate  10 B. As shown in  FIG. 4 , the protective film  9  is deposited on an upper portion of an area  14   b  where the window structure  14  is not formed. When RTA is performed for the purpose of disordering, the above characteristics of the protective film  9  is helpful in preventing desorption of As atoms from a part of the active layer  4  corresponding to the area  14   b . Therefore, during the process of RTA, a part of the active layer  4  corresponding to the area  14   a  is disordered while a part of the active layer  4  corresponding to the area  14   b  is not disordered. With the protective film  9 , the process of disordering is selectively performed in a successful manner, with no damage on the multiple quantum well (MQW) structure of the active layer  4  and with no deterioration in performance of a laser light output.  
         [0041]     Moreover, as shown in  FIG. 5 , since desorption of As atoms in the area  14   b  is prevented, the surface of the upper contact layer  7  is smooth and free from pits  41  due to the desorption of As atoms. Consequently, the device is free from cracks generated due to the pits  41  and favorable contact between the contact layer  7  and the upper electrode  21  is assured.  
         [0042]      FIG. 6  is a graph showing the difference in light-to-injection current characteristics between the semiconductor laser device with the window structure  14  and the semiconductor laser device without the window structure  14 . As shown in  FIG. 6 , in the semiconductor laser device without the window structure  14 , an increase in injection current causes an increase in light output, which lead to an increase in the amount of heat generated. The increase in the amount of heat generated causes a thermal saturation of light output at the injection current of around 2000 to 2500 milliamperes, where COD occurs at the facet (see L 2  in  FIG. 6 ). On the other hand, in the semiconductor laser device with the window structure  14 , although the light output decreases due to thermal saturation, COD does not occur (see L 1  in  FIG. 6 ). Since the laser light output facet is strengthened by the window structure  14 , the semiconductor laser device  10  can emit higher output power and enjoy longer lifetime. Particularly, the process of disordering to form the window structure in the fabricating method according to the embodiment does not affect the active layer  4 , causing no deterioration in the light output performance of the semiconductor laser device  10 .  
         [0043]     While the protective film  9  is deposited so as to cover the entire surface of the device substrate in this embodiment, a device substrate  11 B with a protective film  9   a  deposited thereon so as to cover at least the area  14   b  may be used, as shown in  FIG. 7 . Namely, the protective film  9  may be deposited so as to cover at least the portion corresponding to the area  14   b  that is not to be subjected to a process of disordering.  
         [0044]     While in this embodiment is described an example in which the process of disordering is performed with the disordering-enhancing film  8  deposited, the present invention is not limited to such example but can be applied to performing the process of disordering by diffusing impurities such as Zn and Si by a thermal treatment.  
         [0045]     While SiH 4  and NH 3  are used as materials (precursors) for depositing a SiN X  film serving as the protective film  9  in this embodiment, the present invention is not limited to such materials. Precursors containing Si and N can be used or precursors prepared by combining Si with NH 3  can be used.  
         [0046]     While a process of disordering to form a window structure of the semiconductor laser device  10  is described in this embodiment, the present invention is not limited to forming the window structure. The present invention can be generally applied to disordering of local areas.  
         [0047]     While a heating temperature of the tungsten wire  33  is set to 1680° C. in this embodiment, the heating temperature of the tungsten wire  33  may be higher than a temperature above which the tungsten wire  33  is not silicided and lower than a temperature below which a vapor pressure of the tungsten wire  33  does not affect deposition of the protective film  9 . Therefore, the heating temperature of the tungsten wire  33  can be set to between 1600° C. and 1900° C., for example.  
         [0048]     A flow rate of NH 3  gas to SiH 4 , a pressure within the chamber  31 , and the like can be set optimally to realize high compactness of the protective film  9 .  
         [0049]     The semiconductor laser device  10  is not limited to one of a particular laser structure or of a particular composition, but may generally be a semiconductor laser device of any structure. While a composition of an active area used for the semiconductor laser device  10  can be selected from GaAs, AlGaAs, InGaAs, InAlGaAs, InGaAsP, and the like depending on oscillation wavelengths, other compositions can be used.  
         [0050]     As described above, according to the present invention, a method of fabricating a semiconductor device such as a semiconductor laser device, in which only a portion to be partially disordered, such as a portion to be a window structure, is disordered with no adverse effect on the other portions not to be disordered, and thereby to provide a method of fabricating a semiconductor device of high output power, long lifetime, and high reliability.  
         [0051]     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.