Abstract:
A guide layer is formed to have a superlattice structure comprising five pairs of layers of AlGaN and InN, each having a thickness of about 10 nm. The guide layer has a total thickness of about 0.1 μm. The guide layer so structured has a reduced elastic constant such that the guide layer acts as a stress relieving layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a laser diode using group III nitride group compound semiconductor. Here a group III nitride group compound semiconductor is represented by a general formula Al x Ga y In 1−x−y N (0≦x≦1, 0≦y≦1, 0≦x+y≦1), which includes binary compounds such as AlN, GaN and InN, ternary compounds such as Al x Ga 1−x N, Al x In 1−x N and Ga x In 1−x N (0&lt;x&lt;1), and quaternary compounds such as Al x Ga y In 1−x−y N (0&lt;x&lt;1, 0&lt;y&lt;1, 0&lt;x+y&lt;1). In this specification, a group III nitride group compound semiconductor includes a group III nitride group compound semiconductor which is doped with impurities to have p-type or n-type conductivity. 
     2. Description of the Related Art 
     A group III nitride group compound semiconductor is a direct-transition-type semiconductor having a wide emission spectrum range from ultraviolet to red, and is applied to light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs). The group III nitride group compound semiconductor is, in general, formed on a sapphire substrate. A laser diode, in general, comprises a guide layer and a cladding layer, which are formed on an n-type and a p-type semiconductor side of an active layer, respectively, sandwiching the same. The cladding layer is formed to have a large band gap and is generally made of Al x Ga 1−x N (0&lt;x&lt;1) including aluminum (Al), in order that electrons and holes injected from the negative and the positive electrode, respectively, form electron-hole pairs in the active layer. The guide layer preferably has a wider band gap than the active layer. The guide layer is preferably made of, e.g., gallium nitride (GaN) in order that laser light can be confined in the active layer by total internal reflection due to the difference in refractive indices. The active layer preferably has a multiple quantum well (MQW) structure. 
     FIG. 5 illustrates the structure of a laser diode (LD)  900  as a conventional group III nitride group compound semiconductor light-emitting device. The laser diode (LD)  900  comprises a substrate  901 , and an AlN buffer layer  902  is formed thereon. 
     On the buffer layer  902 , the following four layers are formed successively: an n-layer  903  made of silicon (Si) doped GaN; an n-cladding layer  904  made of silicon (Si) doped Al x Ga 1−x N; an n-guide layer  905  made of silicon (Si) doped GaN; and an active layer  906  having a multiple quantum well (MQW) structure in which a barrier layer made of GaN and a well layer made of Ga 1−y In y N are laminated alternately. On the active layer  906 , a p-guide layer  907  made of magnesium (Mg) doped GaN, a p-cladding layer  908  made of magnesium (Mg) doped Al x Ga 1−x N, and a p-contact layer  909  made of magnesium (Mg) doped GaN are formed. An electrode  910 A is formed on the p-contact layer  909  and another electrode  910 B is formed on a portion of the n-layer  903 . 
     In the above-described conventional technique, however, stress between the sapphire substrate  901  and the n-layer  903  or the n-cladding layer  904  is applied to the active layer  906  through the n-guide layer  905 . As a result, luminous efficiency of the active layer  906  decreases and oscillation threshold current of the laser increases. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is, therefore, to provide a laser diode using group III nitride group compound semiconductor, which has a guide layer with decreased elastic constant. 
     To achieve the above object, and others, a first aspect of the present invention provides a laser diode using a group III nitride group compound semiconductor diode. The diode comprises a guide layer having a multiple layer structure including an indium nitride (InN) layer and is formed on a substrate side of an active layer. 
     A second aspect of the present invention provides a diode having a guide layer which has a multiple layer structure including an indium nitride (InN) layer which is disposed on the side of the active layer opposite to the substrate. 
     A laser diode using a group III nitride group compound semiconductor comprises a substrate and group III nitride group compound semiconductor layers laminated thereon. By forming a guide layer which has a multiple layer structure including an indium nitride (InN) layer and has a sufficiently lowered elastic constant beneath an active layer, the elastic constant of the guide layer becomes comparatively smaller. As a result, transmission of stress, which is generated by temperature variation during manufacture or use, can be prevented. When a guide layer which comprises a multiple layers including indium nitride (InN) is also formed on an active layer opposite to the substrate side, further improvement can be obtained. Because of the structure of the emission layer, the laser diode of the present invention can be used to produce ultraviolet light. The guide layers are preferably the multiple layers including InN and a group III nitride group compound semiconductor including no indium (In), for example, Al x Ga 1−x N (0≦x≦1). For example, the guide layers preferably comprise the multiple layers including InN and Al x Ga 1−x N (0&lt;x&lt;1) or, InN and GaN. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of the specification, and wherein reference numerals designate corresponding parts in the various figures, wherein: 
     FIG. 1 is a sectional view of a laser diode  100  according to the embodiment of the present invention; 
     FIG. 2 is a sectional view of an n-guide layer; 
     FIG. 3 is a sectional view of an active layer; 
     FIG. 4 is a sectional view of a p-guide layer; and 
     FIG. 5 is a sectional view of a conventional laser diode  900 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be more fully understood by reference to the following embodiment, but not limited thereto. 
     FIG. 1 illustrates a sectional view of a laser diode  100 . The laser diode  100  has a sapphire substrate  101  on which about 50 nm in thickness of buffer layer  102  comprising, for example, AlN is formed. 
     About 5 μm in thickness of silicon (Si) doped GaN n-layer  103 , having an electron concentration of 2×10 18 /cm 3 , is formed on the buffer layer  102 . About 1 μm in thickness of silicon (Si) doped Al 0.08 Ga 0.92 N n-cladding layer  104 , having an electron concentration of 2×10 18 /cm 3 , is formed on the n-layer  103 . About 100 nm in thickness of n-guide layer  105 , having a multiple layer structure, is formed on the n-cladding layer  104 . As shown in FIG. 2 the n-guide layer  105  comprises five pairs of layers. Each pair comprises a silicon (Si) doped Al 0.01 Ga 0.99 N layer having a thickness of 10 nm and an electron concentration of 2×10 18 /cm 3  and a silicon (Si) doped InN layer having a thickness of 10 nm and an electron concentration of 2×10 18 /cm 3 , laminated alternately. 
     An active layer  106  having a multiple quantum well (MQW) structure is formed on the n-guide layer  105 . In the active layer  106 , 4 well layers made of Ga 0.85 In 0.15 N, each having a thickness of about 3 nm, and three barrier layers made of GaN, each having a thickness of about 5 nm, are laminated alternately as shown in FIG.  3 . About 100 nm in thickness of p-guide layer  107 , having a multiple layer structure, is formed on the active layer  106 . As shown in FIG. 4 the p-guide layer  107  comprises five pairs of layers. Each pair includes a magnesium (Mg) doped Al 0.01 Ga 0.99 N layer, having a hole concentration of 5×10 17 /cm 3  and a thickness of about 10 nm, and a magnesium (Mg) doped InN layer, having a hole concentration of 5×10 17 /cm 3  and a thickness of about 10 nm. 
     About 1 μm in thickness of magnesium (Mg) doped Al 0.08 Ga 0.92 N p-cladding layer  108 , having a hole concentration of 5×10 17 /cm 3 , is formed on the p-guide layer  107 . A magnesium (Mg) doped GaN p-contact layer  109 , having a thickness of 300 nm and a hole concentration of 5×10 17 /cm 3 , is formed on the p-contact layer  109 . An electrode layer  110 A made of nickel (Ni) is formed on some portion of the p-contact layer  109 . Another electrode  110 B made of aluminum (Al) is formed on some portion of the n-layer  103 . 
     One exemplary method for manufacturing this light-emitting device (semiconductor laser) is explained hereinafter. Each of the semiconductor layers of the light-emitting device  100  is formed by gaseous phase epitaxial growth, called metal organic vapor phase deposition (hereinafter MOVPE). The gases employed in this process are ammonia (NH 3 ), a carrier gas (H 2  or N 2 ), trimethyl gallium (Ga(CH 3 ) 3 , hereinafter TMG), trimethyl aluminum (Al(CH 3 ) 3 , hereinafter TMA), trimethyl indium (In(CH 3 ) 3 , hereinafter TMI), silane (SiH 4 ), and biscyclopentadienyl magnesium (Mg(C 5 H 5 ) 2 , hereinafter CP 2 Mg). 
     The single crystalline sapphire substrate  101  is placed on a susceptor in a reaction chamber for the MOVPE treatment after its “a”-surface is cleaned, for example, by an organic washing solvent and heat treatment. Then the sapphire substrate  101  is baked for about 30 min. at 1100° C. by H 2  vapor fed into the chamber at a flow rate of 2 L/min. under normal pressure. 
     About 50 nm in thickness of an AlN buffer layer  102  is formed on the cleaned “a”-surface of the baked sapphire substrate  101  under conditions controlled by lowering the temperature in the chamber to 400° C., keeping the temperature constant, and concurrently supplying H 2  at a flow rate of 10 L/min., NH 3  at 10 L/min., and TMA at 20 μmol/min. for about 90 seconds. 
     About 5 μm in thickness of Si-doped GaN is formed on the buffer layer  102 , as an n-layer  103  with an electron concentration of 2×10 18 /cm 3 , under conditions controlled by keeping the temperature of the sapphire substrate  101  at 1150° C. and concurrently supplying H 2  at a flow rate of 10 L/min., NH 3  at 10 L/min., TMG at 200 μmol/min., and silane (SiH 4 ) diluted to 0.86 ppm by H 2  at 100 mol/min. 
     About 1 μm in thickness of Al 0.08 Ga 0.92 N is formed on the n-layer  103 , as an n-cladding layer  104 , under conditions concurrently supplying N 2  or H 2 , NH 3 , TMA, TMG, and silane. (SiH 4 ). 
     About 10 nm in thickness of Al 0.01 Ga 0.99 N is formed on the n-cladding layer  104 , under conditions concurrently supplying N 2  or H 2 , NH 3 , TMA, TMG, and silane (SiH 4 ). About 10 nm in thickness of InN is formed under conditions lowering the temperature of the sapphire substrate  101  at 450° C. and concurrently supplying N 2  or H 2 , NH 3 , TMI, and silane (SiH 4 ). By repeating these processes, five pairs of Al 0.01 Ga 0.99 N and InN layers are formed under the same conditions. As a result, an n-guide layer  105  with a multiple layer structure having a total of about 100 nm in thickness is obtained. 
     A Ga 0.85 In 0.15 N layer about 3 nm thick is formed on the n-guide layer  105 , as a well layer, while concurrently supplying N 2  or H 2 , NH 3 , TMG, and TMI. A layer of about 5 nm in thickness of GaN is formed on the well layer, as a barrier layer, under conditions concurrently supplying N 2  or H 2 , NH 3 , and TMG. Another two pairs of layers, each including a well layer and a barrier layer are laminated under the same conditions described above, and then about 3 nm in thickness of a Ga 0.85 In 0.15 N well layer is formed thereon. Accordingly, an active layer  106  with a multiple quantum well (MQW) structure, having four repeated well layers with intervening barrier layers, is obtained. 
     An Al 0.01 Ga 0.99 N layer about 10 nm thick is formed while concurrently supplying N 2  or, H 2 , NH 3 , TMG, TMI, and CP 2 Mg. An InN layer, also about 10 nm thick, is formed under conditions lowering the temperature of the substrate to 45° C. and concurrently supplying N 2  or H 2 , TMI, and CP 2 Mg. By repeating these processes, five pairs in total of Al 0.01 Ga 0.99 N layers and the TnN layers are formed on the active layer. As a result, about 100 nm in thickness of a p-guide layer  107  of a multiple layer structure is obtained. Likewise, about 1 μm in thickness of Al 0.08 Ga 0.92 N layer, as a p-cladding layer  108 , is formed under conditions concurrently supplying N 2  or H 2 , NH 3 , TMA, TMG, and CP 2 Mg. 
     About 300 nm in thickness of magnesium (Mg) doped GaN is formed on the p-cladding layer  108 , as a p-contact layer  109 , under conditions controlled by keeping the temperature of the sapphire substrate  101  at 1100° C. and concurrently supplying N 2  or H 2  at a flow rate of 10 L/min., NH 3  at 10 L/min., TMG at 100 μmol/min., and CP 2 Mg at 2 μmol/min. 
     The three layers are substantially uniformly irradiated using an electron beam, preferably using a reflective electron beam diffraction device. The irradiating electrons are, for example, accelerated to 10 kV at a sample current of 1 μA. In this example, the beam is scanned at 0.2 mm/s and has a beam aperture of 60 μmφ. The irradiation is preferably performed at a pressure of about 50 μTorr. By this irradiation the p-contact layer  109 , the p-cladding layer  108 , and the p-guide layer  107 , have respective hole concentrations of 5×10 17 /cm 3 , 5×10 17 /cm 3 , and 5×10 17 /cm 3 . As a result, a wafer with a multiple layer structure is obtained. 
     An SiO 2  layer may be formed on the p-contact layer  109  by sputtering, and a photoresist layer may-be laminated on the SiO 2  layer prior to execution of the photolithography process. The photoresist layer of the electrode forming part on the n-layer  103  is removed and the SiO 2  layer, which is not covered by the photoresist layer, is removed, for example by using a hydrofluoric acid system etching solution. 
     Then, the p-contact layer  109 , the p-cladding layer  108 , the p-guide layer  107 , the active layer  106 , the n-guide layer  105 , the n-cladding layer  104 , and a portion of the n-layer  103 , which are not covered by the photoresist layer and the SiO 2  layer, are dry-etched under conditions set at 0.04 Torr vacuum and at 0.44 W/cm 2  for a high-frequency power, concurrently supplying BCl 3  gas at a flow rate of 10 ml/min., and then dry-etched by using argon (Ar). In this manner, an electrode region is formed on the n-layer  103 . 
     Nickel (Ni) is deposited on the p-contact layer  109 , and an electrode  110 A is formed thereon. Aluminum (Al) is deposited on the n-layer  103 , and an electrode  110 B is formed thereon. 
     Dry-etching is carried out in order to form a resonator facet. A scribing groove is formed in a scribing process. Then strips are obtained by dicing the in x-axis direction, which is parallel to the resonator facet. The thus-obtained laser-diode  100  is found to have an output power of 10 mW and an oscillation wavelength of 380 nm when driving voltage current supplied to the device is 1000 mA. 
     For comparison, a conventional laser diode  900  is formed as shown in FIG.  5 . The laser diode  900  comprises a guide layer without a multiple layer structure, and the same layers each having the same thickness as those formed in the laser diode  100 . That is, each composition ratio, materials, and thickness of a sapphire substrate  901 , a buffer layer  902 , an n-layer  903 , an emission layer  906  having a multiple quantum well (MQW) structure, a p-contact layer  909 , electrodes  910 A and  910 B formed in the conventional laser diode  900 , and etching or other treatment of the laser diode  900  almost correspond to those of the laser diode  100 . The n-guide layer  905  of the laser diode  900  is a silicon (Si) doped GaN having an electron concentration of 2×10 18 /cm 3  and a thickness of 100 nm. The p-guide layer  907  of the laser diode  900  is a magnesium (Mg) doped GaN having a hole concentration of 5×10 17 /cm 3  and a thickness of 100 nm. A conventional laser diode  900 , manufactured according to the foregoing method, tends to be subject to stresses between layers and may, in fact, break when a driving voltage is applied. 
     In the above embodiments, the light-emitting device is manufactured by using metal organic chemical vapor deposition (MOCVD). Alternatively, a semiconductor layer can be formed by using such as molecular beam epitaxy (MBE), Halide vapor phase epitaxy, liquid phase epitaxy, or any other appropriate manufacturing method as understood by those skilled in the art. 
     In the above embodiments, a laser diode which has a multiple quantum well (MQW) structure emission layer is disclosed as an example. Alternatively, the light-emitting device can have a homojunction structure, a heterojunction structure, or a double heterojunction structure. These, structures can be formed through formation of, for example, a PIN junction or a p-n junction. Also, the emission layer can have a single quantum well (SQW) structure. 
     A sapphire substrate is employed in the above described embodiments. However, materials such as Si, SiC, MgAl 2 O 4 , ZnO, MgO, GaN, and other group III nitride group compound semiconductors may also be employed as a substrate for crystal growth. In the various embodiments, in order to grow the group III nitride group compound semiconductor having excellent crystallinity, a buffer layer is formed on the substrate for compensating the lattice mismatch between the substrate and the group III nitride group compound semiconductor. Even when the substrate is made of other materials, a buffer layer is preferably formed on the substrate. The buffer layer may be made of a group III nitride group compound semiconductor satisfying the formula Al x Ga y In 1−x−y N (0≦x≦1, 0≦y≦1, 0≦x+y≦1), which is formed at a low temperature. Preferably, the buffer layer is made of Al x Ga 1−x N (0≦x≦1). 
     The group III nitride group compound semiconductor can be also made of a group III nitride group compound in which a part of the group III element is changed to boron (B) or thallium (Tl), and a part of the nitrogen (N) is changed to phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), and so on. When the group III nitride group compound semiconductor functions as a light-emitting device, a binary or a ternary group III nitride group compound semiconductor may be preferably employed. 
     Group III nitride group compound semiconductor composition ratios of the layers in the guide layers  105  and  108  having a multiple layer structure and well and barrier layers in the active layer having multiple quantum well (MQW) structure are not limited to the above embodiments. Alternatively, a group III nitride group compound semiconductor satisfying the formula Al x Ga y In 1−x−y N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) can be employed to form each sublayer layer in the guide layers  105  and  108  and the active layer  106 . The layers need not have identical compositions and may each have different aluminum composition x, gallium composition y, and indium composition 1−x−y. 
     While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.