JP2007311632A - Surface-emitting laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain the low resistance and the high output power of a surface-emitting laser (VCSEL) device having tunnel junction. <P>SOLUTION: The VCSEL device 10 has a tunnel junction 17 between an n-type lower DBR mirror 12 and an n-type upper DBR mirror 19, and the tunnel junction 17 has a quantum dot layer 32 of 2.5 mono layer thickness between a p<SP>+</SP>-GaAs layer 31 and an n<SP>+</SP>-InGaAs layer 33. The quantum dot layer 32 has a small bandgap energy, and by raising the tunnel probability in the tunnel junction 17, the low resistivity and the high output power of the VCSEL device 10 are made to be possible. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a surface emitting laser device, and more particularly to a long wavelength band surface emitting laser (VCSEL) device having an oscillation wavelength of 1.0 μm to 2.5 μm.

  Long wavelength surface emitting laser (VCSEL) devices are expected to be required in large quantities in the future in the field of optical interconnection and optical communication because they enable transmission speeds of 10 Gbps or higher. Currently used long wavelength band VCSEL devices do not satisfy the specifications required in the field of optical interconnection and optical communication in terms of higher optical output and higher speed.

One of the factors is that the absorption loss due to the DBR mirror layer is large in the long wavelength band. In a VCSEL element with a short wavelength in the 780-980 nm band, a p-type semiconductor DBR layer and an n-type semiconductor DBR layer are used as the DBR mirror layer. However, in the long wavelength band having a wavelength longer than 1000 nm, the valence band absorption by the p-type layer is increased, and there is a demand for not using the p-type semiconductor DBR layer as much as possible. One solution is to replace the p-type semiconductor layer with an n-type semiconductor layer using a tunnel junction (T / J). In this method, a quantum thin film of about 10 nm is used for the tunnel junction layer.

  3A and 3B show details of the laminated structure of the 1300 nm band VCSEL device using the conventional tunnel junction layer. The VCSEL device 10A includes an n-type lower DBR mirror 12, an n-GaAs cladding layer 13, and three well layers formed sequentially on an n-type GaAs substrate 11 as shown in FIG. The p-GaAs layer 16, the T / J layer 23, the n-GaAs cladding layer 18, and the n-type upper DBR mirror 19 including the well (MQW) active layer 14, the oxide constriction layer 15 therein. A first electrode (power supply electrode) 20 is formed on the n-type DBR mirror 19, and a second electrode (ground electrode) 21 is formed on the back surface of the n-type GaAs substrate 11.

  The 2λ resonator structure 30 is composed of the n-GaAs cladding layer 13 to the n-GaAs cladding layer 18. Although not shown, the upper layer from the upper part of the n-type lower DBR mirror 12 is formed in a mesa post structure, and the mesa post is generally embedded with a polyimide layer. Laser light is emitted upward in the drawing from the window 22 opened in the first electrode 20.

As shown in FIG. 3B, the T / J layer 23 includes a p ⁺ -GaAs layer 31 having a carrier concentration of 1.5 × 10 ²⁰ cm ⁻³ and a thickness of 5 nm, and a carrier concentration of 5 × 10 ¹⁹ cm. ^-3 and a 5 nm thick n ⁺ -In _0.13 GaAs layer 33 and a carrier concentration of 1 × 10 ¹⁹ cm ⁻³ and a 20 nm thick n ⁺ -GaAs layer 34. A VCSEL element using the T / J layer 23 is described in Non-Patent Document 1.
“1.1μm InGaAs VCSEL for optical interconnection” Naofumi Suzuki et al., LQE2005-113 K. Otsubo N. Hatori, M. Ishida, S. Okumura, T. Akiyama, Y. Nakata, H. Ebe, M. Sugawara and Y. Arakawa, Jpn. J. Appl. Phys. 43, L1124 (2004).

A VCSEL element using a tunnel junction has a problem of high element resistance. For example, Non-Patent Document 1 reports that the resistivity is 6.4 × 10 ⁻⁶ ohm · cm ² using a p-GaAs / n-In _0.15 GaAs tunnel junction. In a VCSEL element having an oxide confinement structure formed by selective oxidation of Al, when the current opening diameter is 6 μm, this tunnel junction layer has a resistance value corresponding to a series resistance of 20Ω. An increase in resistance value causes an increase in CR time constant and self-heating, and a decrease in modulation band.

  In order to further increase the speed of the VCSEL element using the tunnel junction, it is essential to reduce the resistance of the tunnel junction. This is because when the resistance is high, especially in a VCSEL element used in a long wavelength band, the gain is greatly reduced due to self-heating, and the optical output and the modulation band are reduced.

  Incidentally, lasers and semiconductor optical amplifiers (SOA) using semiconductor quantum dots having a three-dimensional structure as active layers have been researched and developed. Non-Patent Document 2 reports that the use of quantum dots in the active layer of a laser has a low threshold value. However, there have been no examples of using quantum dots for tunnel junctions.

  In view of the problem of high resistance in the conventional VCSEL element having the above-described tunnel junction, the present invention provides a VCSEL element having a tunnel junction particularly suitable for use in a long wavelength band and having a broadband property. For the purpose.

In order to achieve the above object, a surface emitting laser device of the present invention includes a lower DBR mirror having a periodic structure in which a high refractive index layer and a low refractive index layer are alternately formed on a substrate, a high refractive index layer, In a surface emitting laser having an upper DBR mirror having a periodic structure in which low refractive index layers are alternately formed, and a resonator structure including an active layer that is sandwiched between the lower DBR mirror and the upper DBR mirror and generates light ,
A tunnel junction is provided between the lower DBR mirror and the upper DBR mirror or in the lower or upper DBR mirror, and at least a part of the tunnel junction is formed of a quantum dot layer. It is characterized by.

  In the surface emitting laser device according to the present invention, the use of a quantum dot layer having a small band gap energy for the tunnel junction layer increases the tunnel probability as compared with the conventional tunnel junction quantum thin film, and thus a lower resistance can be obtained. .

  As the quantum dot layer, for example, an InAs or InGaAs layer is preferable, and when these layers are employed, it is preferably formed on a GaAsSb underlayer. Lattice matching is easy.

  The thickness of the quantum dot layer is, for example, about 2 to 4 monoatomic layers (monolayer: ML). Thus, by reducing the volume of the quantum dot layer, the amount of light absorption can be kept low, which is advantageous for high light output. By reducing the resistance and increasing the optical output, it is possible to obtain a long wavelength band VCSEL device usable in a wide band.

The maximum value of the impurity doping amount of the tunnel junction layer is preferably suppressed to 5 × 10 ¹⁹ cm ⁻³ . If the doping amount is larger than this, light loss due to intervalence band absorption or free carrier absorption increases.

  In any of the lower DBR mirror, the upper DBR mirror, and the resonator structure, a selective oxidation type current confinement layer may be formed in the vicinity of the active layer to form an oxide confinement structure. Alternatively, the current confinement structure may be formed by an embedded hetero structure.

  The upper DBR mirror may be formed of a semiconductor layer, or at least a part of the layer may be replaced with a dielectric layer.

  The surface emitting laser element of the present invention is particularly preferably applied to a surface emitting laser element used in a long wave band having an oscillation wavelength of 1000 to 2500 nm.

Embodiment 1
FIG. 1 is a perspective view showing the structure of a VCSEL device according to an embodiment of the present invention. 2A and 2B are cross-sectional views showing details of the laminated structure. In these figures, for ease of understanding, the same parts as those of the conventional VCSEL element shown in FIG.

  The VCSEL device of this embodiment is an example in which the present invention is applied to a VCSEL device having a GaInNAsSb active layer that oscillates in the 1300 nm band. The VCSEL element 10 of this embodiment has the same structure as the conventional VCSEL element 10A except for the structure of the tunnel junction 17 shown in FIG.

  Specifically, the VCSEL device 10 according to the present embodiment includes an n-type GaAs substrate 11, an n-type lower semiconductor DBR mirror 12, an n-GaAs cladding layer 13, and three layers that are sequentially formed on the n-type GaAs substrate 11. A multi-quantum well (MQW) active layer 14, a p-GaAs layer 16 including an oxide constriction layer 15 therein, a tunnel junction (T / J) 17, an n-GaAs cladding layer 18, and an n-type upper portion A stacked structure including a semiconductor DBR mirror 19. A first electrode 20 is formed on the n-type upper semiconductor DBR mirror 19, and a second electrode 21 is formed on the back surface of the n-type GaAs substrate 11.

  The 2λ resonator structure 30 is composed of the n-GaAs cladding layer 13 to the n-GaAs cladding layer 18. The resonator structure 30 is sandwiched between the lower DBR mirror 12 and the upper DBR mirror 19. The entire stack from the top to the top of the n-type lower DBR mirror 12 is formed in a mesa post structure, and the mesa post is embedded with a polyimide layer 24. The laser light is generated in the MQW active layer 14, resonated by the action of the resonator structure 30, and emitted upward in the drawing from the window 22 opened on the first electrode side 20.

As shown in FIG. 2B, the tunnel junction 17 includes a p ⁺ -GaAs layer 31 having a carrier concentration of 1.5 × 10 ²⁰ cm ⁻³ and a thickness of 5 nm and a carrier concentration of 5 × 10 ¹⁹ cm ⁻³ sequentially from the lower layer side. And an n ⁺ -InAs quantum dot (QD) layer 32 having a supply amount of 2.5 monolayer (ML), a n ⁺ -In _0.13 GaAs layer 33 having a carrier concentration of 5 × 10 ²⁰ cm ⁻³ and a thickness of 5 nm, and a carrier The n ⁺ -GaAs layer 34 has a concentration of 1 × 10 ¹⁹ cm ⁻³ and a thickness of 20 nm.

The VCSEL device 10 of the above embodiment is manufactured as follows. First, on the (100) surface of the n-type GaAs substrate 11, the lower DBR mirror 12 and the MQW active layer 14 having 35.5 pairs of n-type Al _0.9 Ga _0.1 As / GaAs layers having a λ / 4 film thickness are formed. A 2λ resonator including the upper DBR mirror 19 having 23 pairs of n-type Al _0.9 Ga _0.1 As / GaAs layers is grown in this order. Thereby, each lamination of the resonator structure 30 having a film thickness of 2λ is formed.

The 2λ resonator structure 30 is formed in an arrangement as described below. An MQW active layer 14 is disposed around a portion separated from the top of the n-type lower DBR mirror 12 by λ / 2. The MQW active layer 14 is composed of, for example, a GaIn _0.37 N _0.012 AsSb _0.016 quantum well layer having a thickness of 7.3 nm and a GaN _0.017 As barrier layer having a thickness of 20 nm, and includes three quantum well layers. The oxide constriction layer 15 made of a p-AlAs selective oxide layer has a thickness of 20 nm, and is disposed around a portion away from the top of the n-type lower DBR mirror 12 by 5λ / 4.

The p / n junction part of the tunnel junction 17 is arranged centering on a part away from the uppermost part of the n-type lower DBR mirror 12 by 7λ / 4. As shown in FIG. 2B, the tunnel junction 17 includes a p ⁺ -GaAs layer 31 having a carrier concentration of 1.5 × 10 ²⁰ cm ⁻³ and a thickness of 5 nm, which is sequentially arranged from the lower layer side, and a carrier concentration of 5 × ^{10 19} cm ^-3 at a feed rate 2.5ML of n ^+-INAS quantum dot layer 32, a carrier concentration of 5 × ^{10 19} cm ^-3 film thickness is 5nm of n ⁺ -In _0.13 GaAs layer 33, the carrier concentration The n ⁺ -GaAs layer 34 is 1 × 10 ¹⁹ cm ⁻³ and has a thickness of 20 nm.

  Each layer of the tunnel junction 17 is grown by any one of molecular beam epitaxy (MBE), gas source MBE, chemical molecular beam epitaxy (CBE), and metal organic chemical vapor deposition (MOCVD). The mesa post has a diameter of 30 μm, and the current opening diameter of the oxidized constriction layer 15 formed by selective oxidation is 6 μm. The portions other than the mesa post are filled with the polyimide layer 24 and flattened.

When the VCSEL device 10 of the above embodiment was produced, the resistivity of the tunnel junction 17 portion was reduced to 3 × 10 ⁻⁶ ohm · cm ² . Since the current opening diameter is 6 μm, this portion corresponds to 10Ω in series resistance. The overall device resistance was reduced to 80Ω compared to the device resistance of a conventional VCSEL device using a tunnel junction of 90Ω. As static characteristics in laser oscillation, the threshold current was 0.5 mA, the slope efficiency was 0.3 W / A, and CW oscillation was obtained at 100 ° C. or higher.

In this embodiment, the quantum dot layer 32 is grown on the p ⁺ -GaAs layer 31, but the quantum dot layer 32 may be grown on the p-GaAsSb layer. The MQW active layer 14 is exemplified by a GaInNAsSb-based active layer, but may be a GaAsSb quantum well or an InGaAs quantum well as long as a GaAs substrate is used. Furthermore, in the present embodiment, the InAs quantum dot layer 32 is grown, but an InGaAs quantum dot layer may be used instead. Furthermore, although an example in which the MQW active layer is three layers has been shown, it may be, for example, two to 15 layers.

Although an example in which all layers are n-type semiconductor layers has been shown as the upper DBR mirror layer, all or a part of the upper DBR mirror layer may be formed of a dielectric layer. Further, although the n-type semiconductor quantum dot layer of the tunnel junction 17 is arranged, a p-type semiconductor quantum dot layer may be arranged. Furthermore, as the doping amount of the semiconductor layer, doping is performed up to a carrier concentration of 1.5 × 10 ²⁰ cm ^−3, but since the tunnel probability is increased using the quantum dot layer, the doping amount is limited accordingly. ^{Alternatively} , the carrier concentration may be reduced to a maximum of 5 × 10 ¹⁹ cm ⁻³ . In this case, there is an advantage that light absorption at the tunnel junction is reduced.

  In the above embodiment, an example in which a VCSEL element is formed on a GaAs substrate has been described. However, a VCSEL element may be formed on an InP substrate. In that case, the active layer is selected from among GaInAsP quantum wells, AlGaInAs quantum wells, GaInNAsSb quantum wells, and In (Ga) As quantum dots.

The material of the tunnel junction 17 is p ⁺ -InP / n + -In (Ga) As quantum dots / n + -InGaAs or p ⁺ -InP / p + Al (In) As / n ⁺ -In (Ga) As quantum dots. It is composed of / n ⁺ -InGaAs or the like. Furthermore, in the above-described embodiment, an example in which the oscillation wavelength is 1300 nm is shown, but the oscillation wavelength is selected from, for example, 1000 to 2500 nm.

  A GaAsSb buffer layer was used as the quantum dot buffer layer, and a VCSEL device was fabricated for a structure having an Sb composition of 2% and a film thickness of 7 nm. In this VCSEL device, it was confirmed that the dot density can be increased while maintaining the high photoluminescence (PL) intensity. Increasing the density of quantum dots while maintaining a high PL intensity increases the tunnel probability and further decreases the resistance value. The Sb composition of this GaAsSb buffer layer can be arbitrarily selected within the range of 0.2% to 100%.

  Moreover, in the said embodiment, although the tunnel junction part was provided in the resonator, you may provide outside a resonator. For example, the n-GaAs cladding layer 18 may not be provided, and the thickness of the p-GaAs cladding layer 16 may be adjusted to match the resonator length, and the tunnel junction 17 may be provided in the upper DBR mirror. In that case, when providing a part of the laminated structure constituting the upper DBR mirror between the clad layer 16 and the tunnel junction 17, the part is constituted by a p-type semiconductor laminate, and the upper DBR on the tunnel junction 17 is formed. The mirror part is composed of a stack of n-type semiconductors.

  Although the present invention has been described based on the preferred embodiments, the surface emitting laser element of the present invention is not limited to the configuration of the above embodiments, and various modifications and changes can be made to the configurations of the above embodiments. Changes are also included in the scope of the present invention. In addition, each configuration described as a preferred aspect of the present invention or each configuration described in the embodiment is preferably used together with the essential configuration of the present invention, but about a configuration that exhibits a beneficial effect even when used alone. However, it is not always necessary to use all the configurations described as the essential configurations of the present invention.

1 is a perspective view of a surface emitting laser element according to an embodiment of the present invention. Sectional drawing which shows the laminated structure of the surface emitting laser element of FIG. Sectional drawing which shows the laminated structure of the conventional surface emitting laser element.

Explanation of symbols

10, 10A: Surface emitting laser (VCSEL) element 11: n-GaAs substrate 12: n-type lower semiconductor DBR mirror 13: n-GaAs cladding layer 14: MQW active layer 15: oxidized constriction layer 16: p-GaAs layer 17: Tunnel junction (T / J)
18: n-GaAs cladding layer 19: n-type upper semiconductor DBR mirror 20: first electrode (power supply electrode)
21: Second electrode (ground electrode)
22: Window 23: Tunnel junction (T / J)
24: polyimide layer 30: 2λ resonator structure 31: p ⁺ -GaAs layer 32: quantum dot layer 33: n ⁺ -InGaAs layer 34: n ⁺ -GaAs layer

Claims (9)

A lower DBR mirror having a periodic structure in which a high refractive index layer and a low refractive index layer are alternately formed on a substrate, and an upper DBR mirror having a periodic structure in which a high refractive index layer and a low refractive index layer are alternately formed And a surface emitting laser having a resonator structure including an active layer that is sandwiched between the lower DBR mirror and the upper DBR mirror and generates light,
A tunnel junction is provided between the lower DBR mirror and the upper DBR mirror or in the lower or upper DBR mirror, and at least a part of the tunnel junction is formed of a quantum dot layer. A surface emitting laser element characterized by the above.   The surface emitting laser element according to claim 1, wherein the quantum dot layer is an In (Ga) As layer.   The surface emitting laser element according to claim 2, wherein the quantum dot layer is formed on a GaAsSb underlayer.   The surface emitting laser element according to claim 1, wherein the quantum dot layer has a thickness in the range of 2 to 4 monoatomic layers. The surface emitting laser element according to claim 1, wherein a maximum value of an impurity doping amount of the tunnel junction layer is 5 × 10 ¹⁹ cm ⁻³ .   The selective oxidation type current confinement layer is formed in the vicinity of the active layer in any of the lower DBR mirror and the upper DBR mirror, or the resonator structure. Surface emitting laser element.   The surface emitting laser element according to claim 1, wherein a current confinement structure is formed by a buried type hetero structure.   The surface emitting laser element according to claim 1, wherein at least a part of the upper DBR mirror is formed of a dielectric layer.   The surface emitting laser element according to claim 1, wherein the oscillation wavelength is in the range of 1000 to 2500 nm.

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