Abstract:
A surface emitting semiconductor laser of a laminated structure having at least a light emitting active layer sandwiched between a dielectric film multi-layer mirror and a p-type semiconductor multi-layer mirror on a semiconductor substrate. The energy ΔEc of conduction band discontinuity is higher than the energy ΔEv of valence band discontinuity between at least two kinds of semiconductor layers with different refractive indices constituting the p-type semiconductor multi-layer mirror. On the other hand, the energy ΔEv of valence band discontinuity is higher than the energy ΔEc of conduction band discontinuity between at least two kinds of semiconductor layers with different refractive indices constituting the n-type semiconductor multi-layer mirror.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a surface emitting semiconductor laser. 
     Conventional surface emitting semiconductor lasers are disclosed in detail and systematically in the article by Jack L. Jewell et al in IEEE Journal of Quantum Electronics, vol. 27, No. 6, pp. 1332-1346 (1991). 
     The conventional surface emitting semiconductor lasers have the following problems to be solved. 
     First, as described in pages 1343-1344 of the above article, a surface emitting semiconductor laser has a large device resistance of several hundreds to several thousands Ω compared with several Ω in a general semiconductor laser. Particularly, its structure in which an active region is reduced to about 10 μn in diameter to define a low threshold current has a very large device resistance. As a result, it is difficult to achieve the CW (continuous wave) operation of the laser due to heat generation, and further the large CR time constant (C: device capacitance, R: device resistance) and the large device resistance which is larger than 50 Ω in the driving circuit also hindered the high speed operation. The large device resistance is ascribed to the fact that the hereto barrier between the semiconductor layers constituting a semiconductor multi-layer mirror is high and its number of periods is as large as 10-30 periods so that a sufficient amount of carriers cannot be injected into the active layer through the multi-layer mirror (FIG. 1A). Particularly, the holes having an effective mass cannot climb over this high hetero barrier, thus causing a great increase in the device resistance in a p-type semiconductor multi-layer mirror region. In order to obviate such a disadvantage, pages 1343-1344 and FIG. 15 in page 1344 make a proposal in which a region with a gradually changing composition or a superlattice layer is inserted between two kinds of semiconductor layers constituting the semiconductor multi-layer mirror. The proposal permits the device resistance to be reduced to about 1000 Ω, but the high hetero barrier still remains. This proposal cannot provide a decisive solution. 
     Another problem to be solved is that in the surface emitting semiconductor laser in a 0.98 μm band having a InGaAs quantum well active layer, its semiconductor multi-layer mirror, which is made of AlAs and GaAs, contains Al atoms so that the reliability of the device is deteriorated. 
     SUMMARY OF THE INVENTION 
     A first embodiment of the present invention provides a surface emitting semiconductor laser having a small device resistance. 
     A second embodiment of the present invention also provides a surface emitting semiconductor laser in a 0.98 μm band having high reliability. 
     The first embodiment of the present invention can be attained by making p-type and n-type semiconductor multi-layer mirrors of a surface emitting semiconductor laser of different material systems. Particularly, as shown in FIG. 1B the p-type semiconductor multi-layer mirror is made of a material system in which the energy ΔEc of conduction band discontinuity is higher than the energy ΔEv of valence band discontinuity while the n-type semiconductor multi-layer mirror is made of a material system in which ΔEv is higher than ΔEc. This embodiment can also be attained by the surface emitting semiconductor laser having a p-type semiconductor multi-layer mirror made of at least two kinds of III-V compound semiconductor layers with only group III atoms replaced and with different refractive index, and an n-type semiconductor multi-layer mirror made of at least two kinds of III-V compound semiconductor layers with only group V atoms replaced and with different refractive index. Further, in addition to the above means, the surface emitting semiconductor laser in which p-type doping concentration of the semiconductor layer having the largest energy gap is higher than that of other semiconductor layers in p-type multi-layer, and n-type doping concentration of the semiconductor layer having the largest energy gap is higher than that of other semiconductor layers in n-type multi-layer. The above means is particularly efficient in the surface emitting semiconductor laser with a substrate of InP which oscillates in long wavelength bands (1.3 μm band or 1.55 μm band). 
     The second embodiment of the present invention can be attained by the surface emitting semiconductor laser in a 0.98 μm band having an InGaAs quantum well active layer in which its semiconductor multi-layer mirror is made of two kinds of semiconductor layers with no Al and different refractive index, particularly a combination of InGaP and GaAs, or InGaAsP and GaAs. 
     Referring to FIG. 1B, the first operation of the present invention will be explained below. First, if the p-type semiconductor multi-layer mirror is made of a material system (ΔEc&gt;ΔEv) in which the energy ΔEC of conduction band discontinuity is higher than the energy ΔEv of valence band discontinuity, the energy ΔEv of valence band discontinuity becomes small. As a result, as shown in FIG. 1B, the height of the hereto barrier for holes is reduced so that the carriers can be smoothly injected into an active layer through the p-type multi-layer mirror and the resistance of the p-type multi-layer mirror regions can be greatly reduced. Further, the high hetero barrier existing in the conduction band assures a sufficient difference in the refractive index so that the reflectivity in the p-type semiconductor multi-mirror can be set for a large value using the periodical structure of the semiconductor layers having a thickness of λ/4n (λ: laser wavelength, n: refractive index in each layer). Thus, the p-type semiconductor multi-layer can be operated as a resonator for laser oscillation. The condition of ΔEc&gt;ΔEv can be assured by the combination of InGaAsP and InAlAs (ΔEv*=0.0-0.15), InGaAsP and InGaAlAs (ΔEv*=0.0-0.15), InGaAsP and InAlAsP (ΔEv*=0.2-0.35), InAlAs and InGaAlAs (ΔEv*=0.2-0.35) or InGaAlAs and InGaAlAs having different Al compositions (ΔEv*=0.2-0.35). Here, the ΔEv* represents the ratio of valence band discontinuity defined by ΔEv*=ΔEv/(ΔEc+ΔEv). In particular, in the combinations of InGaAsP and InAlAs, and InGaAsP and InGaAlAs, ΔEv can be brought close to nearly zero. There is almost no hetero barrier for the holes so that the resistance of the p-type semiconductor multi-layer mirror can be substantially determined by that of the material and can therefore be greatly reduced. Further, a system of III-V compound semiconductor materials with only group III atoms replaced satisfies the condition of ΔEc&gt;ΔEv so that the p-type semiconductor multi-layer mirror made of such a material system contributes to resistance reduction as described above. Examples of such a material system are the combinations of InAlAs and InGaAlAs, and InGaAlAs and InGaAlAs having different Al compositions. Additionally, even if the n-type multi-layer mirror comprises dielectric multi-layer mirror, the resistance of the p-type semiconductor multi-layer mirror can be reduced in the described manner. 
     On the other hand, if the n-type semiconductor multi-layer mirror is made of a material system of ΔEv&gt; ΔEc, as shown in FIG. 1B, the height of the hetero barrier for electrons is reduced so that the electrons can be smoothly injected into an active layer through the n-type multi-layer mirror and the resistance of the n-type multi-layer mirror regions can be greatly reduced. The condition of ΔEv&gt;ΔEc can be assured by the combination of InP and InGaAsP (ΔEc*=0.20-0.40) or InGaAsP and InGaAsP having different compositions (ΔEc*=0.20-0.35). Now, the ΔEc* represents the ratio of conduction band discontinuity defined by ΔEc* =ΔEc/(ΔEc+ΔEv). Further, a material system of III-V compound semiconductor materials with only group V atoms replaced satisfies the condition of ΔEv&gt;ΔEc so that the n-type semiconductor multi-layer mirror made of such a material system contributes to resistance reduction as described above. 
     Further, in addition to the above means, the operation of the present invention can be promoted by the &#34;modulation doped structure&#34; of a surface emitting semiconductor laser in which p-type doping concentration of the semiconductor layer having the largest energy gap is higher than that of other semiconductor layers in p-type multi-layer, and n-type doping concentration of the semiconductor layer having the largest energy gap is higher than that of other semiconductor layers in n-type multi-layer. Namely, this assists in the reduction in the height of the hetero barrier in each semiconductor multi-layer mirror so that the resistance thereof can be further reduced. 
     The second operation of the present invention will be explained below. It is well known that the semiconductor laser of a GaAlAs system in which the region through which laser light propagates includes Al atoms has a much more inferior device reliability than that of the semiconductor laser of an InGaAsP system. It can be supposed that this mainly results from an oxidation of Al atoms which are component atoms. In order to remove such a phenomenon, in accordance with the present invention, the semiconductor multi-layer mirror is made of two kinds of semiconductor layers with no Al and different refractive indices, particularly a combination of InGaP and GaAs, or InGaAsP and GaAs. Thus, the surface emitting semiconductor laser in a 0.98 μm band having an InGaAs quantum well active layer can be provided which is free from the defect of Al oxidation and has high reliability. In this case, although the difference in the refractive index between InGaP and GaAs which is 0.3 is smaller than 0.6 between GaAs and AlAs, increasing the number of periods of semiconductor layers can provide a high reflectivity. Thus, the resultant semiconductor multi-layer mirror can operate as a resonator for laser oscillation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are theory views for explaining the operation of the present invention; and 
     FIGS. 2 to 7 are sectional views showing the semiconductor lasers according to embodiments of the present invention, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 2 shows an application of the present invention to a 1.3 μm surface emitting semiconductor laser. On a p-type InP substrate 1 a p-type semiconductor multi-layer mirror 2 was formed which has a periodic structure of 25 periods composed of p-type InGaAsP (band gap wavelength: 1.2 μm) films and p-type InAlAs films which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. An InGaAsP active layer 3 was formed on the mirror 2. On the active layer 3 an n-type semiconductor multi-layer mirror 5 was formed which has a periodic structure of 20 periods composed of n-type InGaAsP (band gap wavelength: 1.25 μm) films and n-type InP films which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. The mirror 2, active layer 3 and mirror 4 were successively deposited on the substrate 1 by metal organic chemical vapor deposition. An electrode 5 for the n-type semiconductor was formed to have an area having a diameter of 10 μm. Reactive ion etching was performed using the electrode 5 as a mask to form a protruding light-emitting region which reaches the p-type InP substrate 1. Finally, an electrode 6 for the p-type semiconductor was formed. The thickness of an Au film constituting the n-type semiconductor was set for 50  nm which holds the transmission of light of several %s and permits light to taken from above. The fabricated device experimentally oscillated at the threshold current of 5 mA in a CW operation at room temperature. Since both p- and n-type semiconductor multi-layer mirrors have a low resistance structure, the device resistance could be reduced to about 20 Ω which is 1/20 as large as the conventional surface emitting semiconductor laser. As a result, the modulation bandwidth limited by a CR time constant could exceed 50 GHz. 
     Embodiment 2 
     FIG. 3 shows an application of the present invention to a 1.55 μm surface emitting semiconductor laser having a quantum well structure. On an n-type InP substrate 7 an n-type semiconductor multi-layer mirror 8 was formed which has a periodic structure of 30 periods composed of n-type InGaAsP (band gap wavelength: 1.0 μm) films and n-type InGaAsP films (band gap wavelength: 1.45 μm) which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. On the mirror 8 a multiple quantum well active layer is formed which has a periodic structure of 5-20 periods composed of InGaAs quantum well layers each having a thickness of 7 nm and InGaAsP barrier layers each having a thickness of 10 nm. On the active layer 9 a p-type semiconductor multi-layer mirror 10 was formed which has a periodic structure of 25 periods composed of p-type InGaAlAs (band gap wavelength: 1.40 μm) films and p-type InAlAs films which are alter-nately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. The mirror 8, active layer 9 and mirror 10 were successively deposited on the substrate 7 by metal organic chemical vapor deposition. Reactive ion etching was performed to etch the n-type semiconductor multi-layer mirror 8 to its middle, thereby providing a protruding light-emitting region (5 μm=5 μm). An Fe doped InP layer 11 is embedded to grow in the etched area. The n-type InP substrate 7 is selectively etched away to a window region 12. Finally, electrodes 5 and 6 for the n- and p-type semiconductors were formed, respectively. The device fabricated experimentally oscillated at the threshold current of 2 mA in a CW operation at room temperature. Since both p- and n-type semiconductor multi-layer mirrors have a low resistance structure, the device resistance could be reduced to about 30 Ω which is 1/20 as large as the conventional surface emitting semiconductor laser. As a result, the modulation bandwidth limited by a CR time constant could exceed 50 GHz. Further, an increase in the relaxation oscillation frequency of the device which is attributable to the quantum size effect by the quantum well active layer permitted a very high speed operation of 40 Gbit/s to be carried out. 
     Embodiment 3 
     FIG. 4 shows an application of the present invention to a 1.55 μm surface emitting semiconductor laser having a strained quantum well type active layer. On a p-type InP substrate 1 a p-type semiconductor multi-layer mirror 13 was formed which has a periodic structure of 30 periods composed of p-type InGaAsP (band gap wavelength: 1.45 μm) films and p-type InGaAlAs films (band gap wavelength: 0.86 μm) which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. On the mirror 13 a strained quantum well type active layer 14 was formed which has a periodic structure of 5-30 periods composed of InGaAs strained quantum well layers each having a thickness of 4 nm and a strain of Δa/a of +0.9% and InGaAsP barrier layers each having a thickness of 10 nm. On the active layer 14 an n-type InP layer 15 was formed. The mirror 13, active layer 14 and mirror 15 were successively deposited on the substrate 1 by metal organic chemical vapor deposition. By etching reaching the strained quantum well type active layer 14 and undercut etching, the strained quantum well active layer 14 is defined to an area having a diameter of 8 μm. A polyimide film 16 which is an organic insulator is embedded in the etched area so as to provide a planar structure. On the n-type InP layer 15, a dielectric multi-layer mirror 17 was formed which has a periodic structure of 5 periods composed of TiO 2  films and SiO 2  films which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. Finally, ring-shaped electrodes 5 and 6 for the p- and n-type semiconductors were formed. The device fabricated experimentally showed the feature of the strained quantum well type active layer that it oscillated at the very low threshold current of 1 mA in a CW operation at room temperature. Since the p-type semiconductor multi-layer mirrors have a low resistance structure, the device resistance could be reduced to about 25 Ω which is 1/20 as large as the conventional surface emitting semiconductor laser. Further, the effect of reducing a CR time constant and an increase in the relaxation oscillation frequency of the device which is attributable to the quantum size effect by the strained quantum well active layer permitted a very high speed operation of 50 Gbit/s to be carried out. 
     Embodiment 4 
     FIG. 5 shows an application of the present invention to a 1.55 μm band surface emitting semiconductor laser. On a p-type InP substrate 1 a p-type semiconductor multi-layer mirror 18 was formed which has a periodic structure of 30 periods composed of p-type InGaAlAs (p-doping concentration: 2×10 18  cm -3 , band gap wavelength: 1.40 μm ) films and p-type InGaAlAs films (p-doping concentration: 4×10 18  cm -3 , band gap wavelength: 0.89 μm which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. A 1.55 μm band InGaAsP active layer 19 was formed on the mirror 18. On the active layer 19 an n-type semiconductor multi-layer mirror 20 was formed which has a periodic structure of 25 periods composed of n-type InGaAsP (doping concentration: 1×10 18  cm -3 , band gap wavelength: 1.45 μm) films and n-type InP films (doping concentration: 3×10 18  cm -3 ) which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. The mirror 18, active layer 19 and mirror 20 were successively deposited on the substrate 1 by metal organic chemical vapor deposition. Etching is made to remain an area of 7 μm×7 μm. An n-type embedded layer 21 and an n-type embedded layer 22 were formed. An n-type InP layer 14 is formed to provide a planar structure. An electrode 5 having an opening window for the n-type semiconductor and an electrode 6 for the p-type semiconductor were formed. The device fabricated experimentally, which has a modulation doped structure in both p- and n-type semiconductor multi-layer mirrors, greatly enjoys the effect of resistance reduction. Namely, the device resistance could be reduced to 8 Ω which is substantially equal to that of the conventional edge emitting semiconductor laser. 
     Embodiment 5 
     FIG. 6 shows an application of the present invention to a 0.98 μm band surface emitting semiconductor laser. On a n-type GaAs substrate 23 an n-type semiconductor multi-layer mirror 24 was formed which has a periodic structure of 25 periods composed of n-type InGaP films and n-type GaAs films which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. On the mirror 24, an n-type InGaAs spacer layer 25, an InGaAs strained well type active layer 26 and a p-type InGaAs Spacer layer were successively formed on the mirror 23. On the spacer layer 27 an p-type semiconductor multi-layer mirror 28 was formed which have a periodic structure of 30 periods composed of p-type InGaP films and p-type GaAs films which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. The mirror 24, spacer layer 25, active layer 26, spacer layer 27 and mirror 28 were successively deposited on the substrate 23 by metal organic chemical vapor deposition. An electrode 6 for the p-type semiconductor was formed to have an area having a diameter of 3-10 μm. Reactive ion etching was performed using the electrode 6 as a mask to form a protruding light-emitting region. Finally, an electrode 5 having a window region was formed for the p-type semiconductor. The device fabricated experimentally oscillated at the threshold current of 0.5 mA in a CW operation at room temperature. Since the region through which laser light propagates does not contain Al atoms, the resultant device has very high reliability. Namely, no deterioration was observed after a life test with 5000 hours under a 60° C. 2 mW constant optical output operation. 
     Embodiment 6 
     FIG. 7 shows an application of the present invention to a 0.98 μm band surface emitting semiconductor laser. On a p-type GaAs substrate 29 a p-type semiconductor multi-layer mirror 30 was formed which has a periodic structure of 30 periods composed of p-type InGaAsP (band gap wavelength: 0.75 μm) films and p-type GaAs films which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. On the mirror 30, an p-type InGaAs spacer layer 27, an InGaAs strained well type active layer 26 and an n-type InGaAs spacer layer 25 were successively formed. On the spacer layer 25 an n-type semiconductor multi-layer mirror 31 was formed which has a periodic structure of 35 periods composed of n-type InGaAsP films (band gap wavelength: 0.75 μm) and n-type GaAs films which are alternately stacked and have a thickness 1/4 times as large as the lasing wavelength in the corresponding medium. The mirror 30, spacer layer 27, active layer 26, spacer layer 25 and mirror 28 were successively deposited on the substrate 29 by metal organic chemical vapor deposition. An electrode 5 for the n-type semiconductor was formed to have an area having a diameter of 3-10 μm. Reactive ion etching was performed using the electrode 5 as a mask to form a protruding light-emitting region. Finally, an electrode 6 having a window region was formed for the p-type semiconductor. The device fabricated experimentally oscillated at the threshold current of 0.5 mA in a CW operation at room temperature. Since the region through which laser light propagates does not contain Al atoms, the resultant device has very high reliability. Namely, no deterioration was observed after elapse of 10,000 hours in a life test under a 60° C. 2 mW constant optical output operation. 
     In accordance with the present invention, the height of the hetero barrier for electrons and holes is reduced so that a surface emitting semiconductor laser having a low device resistance can be provided. In accordance with the present invention, a 0.98 μm surface emitting semiconductor laser having a semiconductor multi-layer mirror including no Al atoms can also be provided. As a result, the present invention is efficient for CW oscillation at room temperature and high speed modulating operation of a long wavelength band surface emitting semiconductor laser. Further, the present invention is efficient to improve the reliability of the 0.98 μm band surface emitting semiconductor laser.