JP2005243743A - Long wavelength band surface emission semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a long wavelength band surface emission semiconductor laser that has a drastically large optical output and low consumption electric power while satisfying a single transverse mode condition. <P>SOLUTION: The surface emission semiconductor laser is formed by stacking on a substrate (11) a first reflection mirror (12) of a semiconductor multilayer film, a second reflection mirror (13) of an n-type semiconductor multuilayer film, an n-type spacer layer (14), an active layer (15), a p-type spacer layer (16), tunnel joints (17, 18) formed of a p-type semiconductor layer and an n-type semiconductor layer, a third reflection mirror (19) of an n-type semiconductor multilayer film, and a fourth reflection mirror (26) of a semiconductor multilayer film in sequence. The p-type semiconductor layer (17) forming the tunnel joint is made of an In<SB>x</SB>Ga<SB>1-x</SB>As material that is doped with carbon (C) or beryllium (Be), establishing an In-composition ratio of 0≤x≤0.53. The side wall of the active layer (15) has an embedding structure covered with a semi-insulating material (20). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface emitting semiconductor laser used in the field of optical communication industry, and more particularly to a long wavelength band surface emitting semiconductor laser.

  The general structure of a long-wavelength (1.2 to 1.6 μm) surface-emitting semiconductor laser (VCSEL) is roughly classified into the following two types. One type is selective oxidation to form a current and light confinement layer after laminating a GaAs / AlGaAs semiconductor multilayer reflector above and below a GaInNAs active layer, as seen in Non-Patent Document 1, etc. It is the structure of performing. In other types, as seen in Non-Patent Document 2, etc., an active layer is formed of a GaAs / AlGaAs semiconductor multilayer reflector or a dielectric multilayer film on an AlInGaAs or InGaAsP system lattice-matched to an InP substrate, and a tunnel is formed. In this structure, current confinement is performed while leaving a part of the junction.

  However, as described above, the surface light emitting semiconductor laser formed by selectively oxidizing the lateral light and current confinement or partially forming the tunnel junction has the lateral confinement of light and current. Since both of the currents are too small, the threshold current increases, and the condition for satisfying the single transverse mode, which is essential in the long wavelength band, is relatively small with an emission diameter of 3 to 5 μm. It is difficult to obtain an output, and a satisfactory optical output is not always obtained.

  On the other hand, the surface-emitting semiconductor lasers found in Non-Patent Document 3 and the like have an embedded structure, which eliminates the above-mentioned difficulties. However, light absorption and low resistance of the p-type semiconductor layer, which is a reflecting mirror, are eliminated. There is another difficulty that it is difficult to achieve both, and characteristics that have been put to practical use have not been obtained in terms of element resistance, threshold current, and light output. In addition, since a high regrowth temperature is required when performing the buried growth of a semi-insulating semiconductor using semi-insulating iron (Fe) as a dopant in the fabrication of the buried layer, the p-type doping species Zn is changed to Fe. Interdiffusion, resulting in an increase in leakage current and an increase in threshold current.

  On the other hand, a surface emitting semiconductor laser manufactured using a tunnel junction needs to be highly doped with a p-type semiconductor in order to increase the tunnel probability for the purpose of reducing element resistance. Therefore, instead of Zn, carbon (C) or beryllium (Be) that can be doped at a relatively high concentration is used as a material that satisfies the three conditions of lattice matching with InP and transmission with respect to the oscillation wavelength. As seen in Non-Patent Document 2, etc., it is common to use AlInGaAs as a p-type semiconductor. However, in the case of this structure, as a result of changing the dopant species to C or Be, when forming the buried structure, the diffusion of the dopant can be suppressed and the desired doping profile can be easily obtained. In the material system containing the material, it is difficult to remove the natural oxide film, and there is a problem that abnormal growth tends to occur when the buried layer is formed.

G. Steinle, et al. “Data transmission up to 10Gblt / s with 1.3 μm wavelength InGaAsN VCSELs” Electronics Letters, 10th May 2001, Vol.37 No.10 pp.632-634 N. Nishiyama, et. Al. “High efficiency long wavelength VCSEL on InP grown by MOCVD” Electronics Letters, 8th March 2003, Vol.39 No.5, pp.437-439 Y. Ohiso, et. Al. “Single Transverse Mode Operation of 1.55-μm Buried Heterostructure Vertical-Cavity Surface-Emitting Lasers” IEEE Photonics Technology Letters, June 2002, Vol.14, No.6, pp. 738-740 Yoshitaka Ohiso, et. Al. “1.55-μm Buried-Heterostructure VCSELs with InGaAsP / InP-GaAs / AlAs DBRs on a GaAs Substrate” IEEE Journal of Quantum Electronics, September 2001, Vol. 37, No.9 p.1194-1202

  An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a long-wavelength surface emitting semiconductor laser capable of dramatically improving the light output while satisfying the single transverse mode condition.

In order to achieve the above object, the long-wavelength surface emitting semiconductor laser of the present invention comprises two types of different refractive indexes which are alternately stacked on a substrate (11) with a film thickness corresponding to ¼ of the optical wavelength. A first reflecting mirror (12) made of a material, and an n-type second reflecting mirror (13) made of two kinds of materials having different refractive indexes, which are alternately laminated with a film thickness corresponding to ¼ of the optical wavelength, , An n-type first spacer layer (14), an active layer (15), a p-type second spacer layer (16), and a tunnel junction portion (17, 18) comprising a p-type semiconductor layer and an n-type semiconductor layer, The n-type third reflecting mirror (19) made of two kinds of materials alternately laminated with a film thickness corresponding to ¼ of the optical wavelength, and alternately with a film thickness corresponding to ¼ of the optical wavelength. A surface emitting semiconductor laser comprising a fourth reflecting mirror (26) made of two kinds of laminated materials in sequence. I, the p-type semiconductor layer constituting the tunnel junction (17), doped with carbon (C), an In composition ratio x is _{0 ≦ x <0.53, In x} Ga 1-x It is made of an As material and has a buried structure in which the side surface of the active layer (15) is covered with a semi-insulating material (20).

Further, the p-type semiconductor layer (17) constituting the tunnel junction portion is made of a material of In _x Ga _1-x As doped with Be and having an In composition ratio x of 0 ≦ x <0.53. It can also be characterized.

Further, in the case where the optical wavelength is in the 1.55 μm band, the p-type semiconductor layer (17) constituting the tunnel junction portion is formed of In _x Ga having an In composition ratio x of 0.15 <x <0.25. It can be characterized by comprising _1-x As material.

  As described above, the long-wavelength surface emitting semiconductor laser of the present invention uses the n-type semiconductor multilayer film (19) as the third reflecting mirror without using the p-type semiconductor layer, and the p-type spacer layer (16). By introducing the tunnel junction portion (17, 18) between the n-type semiconductor multilayer film (19) and having a buried structure, the light output is dramatically increased while satisfying the single transverse mode condition. Has characteristics.

  Furthermore, the long-wavelength surface emitting semiconductor laser of the present invention has a high concentration of p-type that forms a tunnel junction (17, 18) in order to prevent mutual diffusion of Fe and Zn during buried regrowth and to eliminate leakage current. The semiconductor layer (17) is characterized by using InGaAs, which is an Al-free material, and using carbon (C) or beryllium (Be) as a dopant species.

(Function)
Next, the operation of the present invention will be described in detail.
As described above, since the structure of the present invention uses a semiconductor buried structure, the lateral light and current confinement of the surface emitting semiconductor laser becomes strong, and a stable single transverse mode laser oscillation can be obtained. On the other hand, the structure of the present invention uses an n-type reflecting mirror instead of a p-type reflecting mirror, which is the main cause of electrical resistance and internal loss, which is a problem with the conventional buried structure. Reflective mirrors can be fabricated, and at the same time a buried structure is formed, so that a refractive index waveguide structure is formed, the emission diameter satisfying the single transverse mode oscillation condition is increased, and the light output can be significantly increased. .

  In addition, the structure of the present invention has a low resistance by using carbon (C) or beryllium (Be), which is a dopant capable of high concentration, instead of the dopant Zn conventionally used for forming a p-type semiconductor. Since it is possible to realize a simple element and to perform buried growth without Zn diffusion, it is possible to eliminate the leakage current as much as possible and to realize laser oscillation with a low threshold current.

  Furthermore, the structure of the present invention can suppress abnormal growth during buried growth by making the material forming the tunnel junction Al-free.

In addition, the p-type carrier concentration of C-doped In _0.53 Ga _0.47 As lattice-matched to the conventional InP substrate is the growth temperature, the V / III ratio which is the molar ratio of the V group and the III group during growth, Even if the annealing conditions after the growth were examined, it was about 1-3 × 10 ¹⁹ cm ⁻³ , which was not sufficient to obtain a high tunnel probability. In contrast, in the structure of the present invention, In _x Ga _1-x As (0 ≦ x <0.53) having tensile strain is used for the p-type semiconductor layer at the tunnel junction portion with respect to the InP substrate. Therefore, since the Ga composition ratio is larger than that of the conventional lattice-matched In _0.53 Ga _0.47 As, C incorporation is increased during MOCVD crystal growth, and (5-10) × 10 ¹⁹ The high concentration doping of cm ⁻³ is facilitated, the band gap is increased, and the wavelength at the absorption edge of the semiconductor is on the short wavelength side, so that the absorption coefficient with respect to the oscillation wavelength (1.2 to 1.6 μm) As a result, the optical output of the surface emitting semiconductor laser can be improved.

FIG. 1 shows the In composition ratio x of In _x Ga _1-x As (0 ≦ x <0.53) according to the present invention, the maximum value p of the C-doped p-type carrier concentration, the tunnel current Jp, The relationship between the absorption coefficient α and the laser oscillation wavelength of 1.55 μm is shown in a graph.

The maximum value ρ of the p-type carrier concentration is doped as the In composition ratio x decreases. On the other hand, the tunnel current Jp increases as the doping concentration increases. However, when the In composition ratio x decreases, the band gap increases, so the tunnel current Jp decreases, and eventually the In composition becomes maximum near 0.2. Further, the absorption coefficient α increases as the band gap is closer to the absorption edge of the band gap, but is not significantly affected when the In composition x is near 0.2. Thus, it can be seen that the characteristics of the surface emitting semiconductor laser can be drastically improved if a tunnel junction is made of a material of In _0.2 Ga _0.8 As.

In FIG. 1, the oscillation wavelength is 1.55 μm as an example. However, when the laser oscillation wavelength is different, the optimum value of the In composition x is different. When the oscillation wavelength is in the 1.55 μm band, the In composition range in which the effects of the present invention are exhibited is 0 or more and less than 0.53, more preferably 0 or more and 0 or more when the tunnel current Jp is 10 ² kA / cm ² or more. .43 or less, more preferably 0 or more and 0.35 or less at which the tunnel current Jp is 10 ³ kA / cm ² or more, and 0.15 or more and 0.25 or less is optimal.

Further, a surface emitting semiconductor laser obtained by Be doping the p-type semiconductor layer In _x Ga _1-x As (0 ≦ x <0.53) in the tunnel junction portion and growing the MBE crystal has the same effect.

  As described above, according to the long-wavelength surface emitting semiconductor laser of the present invention, it is possible to dramatically improve the light output while satisfying the single transverse mode condition.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 2 shows a cross-sectional structure of the long wavelength surface emitting semiconductor laser according to the first embodiment of the present invention. The manufacturing process is almost the same as that described in Non-Patent Document 4 except for the surface emitting semiconductor laser structure.

  On the GaAs 11 substrate, as the first reflecting mirror 12, GaAs / AlAs having different refractive indexes, which are alternately laminated with a film thickness corresponding to ¼ of the optical wavelength of 1.55 μm, are epitaxially grown.

Further, five pairs of n-type InGaAsP / InP second reflecting mirrors 13 having different refractive indexes, which are alternately stacked on an InP substrate (not shown) with a film thickness corresponding to ¼ of the optical wavelength of 1.55 μm. An n-type first spacer layer 14, an active layer 15, a p-type second spacer layer 16, a C-doped p ++ In _0.2 Ga _0.8 As layer 17 and an n ++ InP layer 18. A tunnel junction portion and a pair of n-type InGaAsP / InP third reflecting mirrors 19 alternately stacked with a film thickness corresponding to ¼ of an optical wavelength of 1.55 μm are sequentially formed by MOCVD (organometallic vapor phase). Epitaxial growth is performed by an epitaxial growth method. Thereafter, methane-based dry etching is performed using a SiO ₂ mask to form a mesa (plateau type).

Next, an Fe-doped semi-insulating InP layer 20 as a buried layer is grown on the InGaAsP / InP second reflecting mirror 13, and after removing the SiO ₂ mask, the n-type InP layer 21 and the n ++ The InGaAs contact layer 22 is grown and the InGaAs portion immediately above the mesa is removed by chemical selective etching to complete the embedding process.

  Thereafter, the embedded surface (19) and the Si substrate (not shown) are directly bonded and fixed with wax (brazing), and the InP substrate (not shown) is completely removed, and the GaAs / AlAs first reflecting mirror is fixed. The surface of No. 12 is brought into contact with the surface of the InGaAsP / InP, which is the second reflecting mirror 13, and annealed in a hydrogen atmosphere to form a covalent bond between GaAs and InP. Perform fusion (Wafer fusion).

Thereafter, the Fe-doped buried layer is partially removed by etching, and a lower electrode 23, an upper electrode and an electrode pad 24, and an AR (antireflection) coat 25 are sequentially deposited, and then 1.55 μm as a fourth reflecting mirror. SiO ₂ / TiO ₂ dielectric multilayer films 26 alternately laminated with a film thickness corresponding to ¼ of the optical wavelength of the above are formed on the buried surface (19), and then the dielectric on the upper part of the electrode pad portion 24. By removing the multilayer film 26, a long-wavelength surface emitting semiconductor laser having a cross-sectional structure shown in FIG. 2 is obtained.

FIG. 3 shows the current-voltage-optical characteristics of the surface-emitting semiconductor laser according to the first embodiment of the present invention during continuous operation at room temperature, and the current-voltage-optical characteristics of the surface-emitting semiconductor laser manufactured according to the prior art. . That is, in FIG. 3, a solid characteristic curve (a) shows a current-light output characteristic of the surface emitting semiconductor laser of the first embodiment manufactured by applying the present invention, and a solid characteristic curve (b) shows a conventional technique. 2 shows current-light output characteristics of a surface emitting semiconductor laser when the p-type semiconductor layer of the tunnel junction portion is made of In _0.53 Ga _0.47 As. A broken line characteristic curve (a ′) shows a current-voltage characteristic of the surface emitting semiconductor laser according to the first embodiment manufactured by applying the present invention, and a broken line characteristic curve (b ′) shows a tunnel junction according to the prior art. The current-voltage characteristic of a surface emitting semiconductor laser in which a partial p-type semiconductor layer is made of In _0.53 Ga _0.47 As is shown.

  In the surface emitting semiconductor laser according to the present invention, the In composition ratio of the p-type semiconductor layer in the tunnel junction portion is that of the characteristic curve (a ′) of the present invention compared to that of the prior art (b ′). It can be seen that since x is small, high-concentration doping is possible and the electrical resistance is small.

  Also, the characteristic curve (a) of the present invention has a significantly increased light output and a lower threshold current than the characteristic curve (b) of the prior art, which is also a p-type semiconductor. Since the In composition ratio x is smaller in the characteristic curve (a) of the present invention than in the characteristic curve (b) of the prior art, the internal loss in the laser cavity is greatly reduced. It is.

  As a result, according to the long wavelength surface emitting semiconductor laser of the present invention, it is possible to dramatically improve the light output while satisfying the single transverse mode condition.

(Other embodiments)
In the first embodiment of the present invention described above, the p-type semiconductor layer constituting the tunnel junction portion is C-doped using the MOCVD method. However, the present invention is not limited to this, and for example, MBE Needless to say, even if the crystal is grown by the method (molecular beam epitaxial growth method), the same effect is obtained. Further, it goes without saying that the same effect as that of the above-described embodiment of the present invention can be obtained even with a Be-doped surface emitting semiconductor laser regardless of the crystallinity length method such as the MOCVD method and the MBE method.

  Further, the present invention is not limited to the above-described embodiment, and it is needless to say that changes, substitutions, etc. can be made within the scope of the claims.

In _x Ga _1-x As (0 ≦ x <0.53) In composition ratio x, C doped p-type carrier concentration maximum value ρ, tunnel current Jp, and laser oscillation according to the present invention It is a graph which shows the relationship with the absorption coefficient (alpha) with respect to wavelength 1.55micrometer. It is sectional drawing which shows the structural example of the long wavelength band surface emitting semiconductor laser in 1st Embodiment of this invention. The current-voltage-light output characteristics of the long-wavelength surface emitting semiconductor laser according to the first embodiment of the present invention and the length of the conventional technique in which the p-type semiconductor layer of the tunnel junction is made of In _0.53 Ga _0.47 As. It is a graph shown in comparison with the current-voltage-light output characteristics of the wavelength band surface emitting semiconductor laser.

Explanation of symbols

11 GaAs substrate 12 GaAs / AlAs first reflecting mirror 13 n-type InGaAsP / InP second reflecting mirror 14 n-type first spacer layer 15 active layer 16 p-type second spacer layer 17 p ++-InGaAs layer (tunnel junction portion)
18 n ++ -InP layer (tunnel junction)
19 n-type InGaAsP / InP third reflector 20 semi-insulating InP
21 n-type InP layer 22 n ++ InGaAs layer 23 electrode 24 electrode 25 AR coat 26 SiO ₂ / TiO ₂ fourth reflecting mirror

Claims (3)

First reflective mirrors made of two kinds of materials having different refractive indexes, which are alternately stacked on the substrate with a film thickness corresponding to ¼ of the optical wavelength, and alternately with a film thickness corresponding to ¼ of the optical wavelength. An n-type second reflecting mirror made of two kinds of materials with different refractive indexes, an n-type first spacer layer, an active layer, a p-type second spacer layer, a p-type semiconductor layer, and an n-type A tunnel junction portion made of a semiconductor layer, an n-type third reflecting mirror made of two kinds of materials stacked alternately with a thickness corresponding to ¼ of the optical wavelength, and ¼ of the optical wavelength. A surface emitting semiconductor laser configured by sequentially laminating a fourth reflecting mirror made of two kinds of materials alternately laminated with a film thickness,
The p-type semiconductor layer constituting the tunnel junction portion is made of a material of In _x Ga _1-x As doped with carbon (C) and having an In composition ratio x of 0 ≦ x <0.53. ,
A long-wavelength surface emitting semiconductor laser having a buried structure in which a side surface of the active layer is covered with a semi-insulating material. The p-type semiconductor layer constituting the tunnel junction portion is composed of a material of In _x Ga _1-x As doped with Be and having an In composition ratio x of 0 ≦ x <0.53. The long-wavelength surface emitting semiconductor laser according to claim 1. In the case where the optical wavelength is in the 1.55 μm band, the p-type semiconductor layer constituting the tunnel junction portion is formed of In _x Ga _1-x As with the In composition ratio x being 0.15 <x <0.25. The long-wavelength surface emitting semiconductor laser according to claim 1, wherein the long-wavelength surface emitting semiconductor laser is formed of the following material.

Images