JP4999070B2 - Surface emitting laser - Google Patents

Description

  The present invention relates to a surface emitting laser, and more particularly to a vertical cavity surface emitting semiconductor laser device having a current confinement structure.

  A vertical cavity surface emitting semiconductor laser (VCSEL: Vertical Cavity Surface Emitting Semiconductor Laser, hereinafter simply referred to as a surface emitting laser) has a light resonating direction perpendicular to the substrate surface. It has been attracting attention as a light source for communication, including optical interconnection, or as a device for various applications such as sensor applications.

  The reason why surface emitting lasers are attracting attention is that surface emitting lasers can form a two-dimensional array structure of elements more easily than conventional edge emitting semiconductor lasers, and they do not need to be cleaved before forming a mirror. It is possible to test the wafer level, and because the volume of the active layer is remarkably small, it can oscillate with a very low threshold current and has low power consumption.

  In the surface emitting laser, a current confinement structure is formed in order to realize oscillation with a low threshold current. For producing this current confinement structure, a selective oxidation method for selectively oxidizing an AlAs layer (for example, see Patent Document 1) or an ion implantation method for performing ion implantation while leaving a current opening (for example, see Patent Document 2). ) Etc. are known.

  The selective oxidation method has an advantage that an increase in element resistance due to the constriction structure can be suppressed because the thickness of the current confinement structure is as small as about 20 nm, for example. However, on the other hand, there is a disadvantage that the device reliability is impaired, for example, stress strain is generated by selective oxidation, and the device is broken during oscillation by the stress strain. On the other hand, the ion implantation method has an advantage that such stress strain does not occur.

  FIG. 7 shows a conventional surface emitting laser having a current confinement structure formed by an ion implantation method. The surface emitting laser 100 is a stacked layer made of a semiconductor material, which is formed on a GaAs substrate 11 in sequence, and includes a lower distributed Bragg reflector (DBR mirror) 12, an active layer 13 having a quantum well structure, and an upper DBR mirror 15. It has a structure. An upper electrode 16 is formed on the upper DBR mirror 15, and a lower electrode 17 is formed on the back surface of the GaAs substrate 11. A power supply voltage is applied between the upper electrode 16 and the lower electrode 17.

  The current injected from the upper electrode 16 is guided to the light emitting region 18 at the center of the active layer 13 by the current confinement structure 14, and laser oscillation occurs between the upper DBR mirror 15 and the lower DBR mirror 12. In the case of the figure, the oscillated laser is emitted upward from a region inside the upper electrode 16.

A photonic crystal surface emitting laser having a structure as shown in Non-Patent Document 1 as means for widening the light emitting area and obtaining fundamental transverse mode oscillation separately from the ion-implanted surface emitting laser element of FIG. Has been proposed. An example of this structure is shown in FIG. In this structure, a circular hole array 20 composed of circular holes arranged in a two-dimensional array is formed in the upper DBR mirror 15 stack. The refractive index sensed by light is slightly reduced by the periodic two-dimensional circular hole array 20, and this two-dimensional circular hole array 20 acts as a cladding for a point defect region having no central circular hole. Since the lateral mode control by the weak refractive index confinement effect is performed in this way, the area of the light emitting region for oscillating only the fundamental lateral mode can be increased. Also in this surface emitting laser, for example, an ion implantation type current confinement structure 14 is formed.
JP 2005-317816 A JP 2005-129960 A IEEE Journal of Selected Topics in Quantum Electronics, Vol.9, No.5, pp.1439-1445, September / October 2003

  By the way, in the current confinement structure manufactured by the ion implantation method, it is difficult to control the implantation depth due to the distribution of implantation energy of ions. For this reason, the thickness of the current confinement structure becomes as large as about 1.5 μm, and the current confinement structure is formed across a plurality of layers of the DBR mirror. In the region where the current confinement structure is formed, there is a problem in that the element resistance increases because the injected current passes through a number of hetero barriers that are interfaces of a distributed Bragg reflector (DBR). This problem is particularly remarkable when the aperture diameter of the light emitting region is reduced, that is, when the current confinement region is widened in order to obtain fundamental transverse mode oscillation.

  In view of the above, the present invention improves a conventional surface emitting laser having a current confinement structure manufactured by an ion implantation method, and thereby provides a surface emitting laser device having low element resistance and easily obtaining fundamental transverse mode oscillation. The purpose is to provide.

The present invention has been made to achieve the above object. That is, the surface emitting laser according to the present invention is formed by an ion implantation method , an upper distributed Bragg reflector (DBR mirror), a lower DBR mirror, an active layer disposed between the upper DBR mirror and the lower DBR mirror. In a surface emitting laser including a semiconductor material stack structure including a current confinement structure formed on a substrate, the current confinement structure is disposed in the vicinity of the active layer in the stack constituting the upper or lower DBR mirror. that is formed of a single specific layer, the specific layer, an m 1 or more natural number, the medium in the oscillation wavelength lambda, the refractive index of air is n, and has a thickness of (2m + 1) λ / 4n , defines m as the specific layer is substantially equal to the thickness of the insulating region layer formed by ion implantation, selective current defines an acceleration energy as the ions are implanted in the specific layer Wherein the 窄 structure is formed.

In the surface emitting laser according to the present invention, ion implantation is mainly performed in one layer which is a specific layer near the active layer, and the thickness of the specific layer is (2m + 1) λ / 4n , where m is a natural number of 1 or more. The number of layers constituting the ion implantation region is smaller than that of the conventional surface emitting laser, and the number of hetero barriers through which current passes in the current confinement region is reduced. Therefore, it is possible to realize a surface emitting laser having a low element resistance.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, in order to understand easily, the same code | symbol is attached | subjected to the same element in each drawing.

First Embodiment FIG. 1 is a sectional perspective view of a surface emitting laser device according to the present embodiment. This surface emitting laser element 10 is designed to have an oscillation wavelength of 850 nm. The surface emitting laser element 10 is manufactured by the following manufacturing process. The manufacturing process is shown in FIGS. First, n-type Al _0.8 Ga _0.2 As and n-type Al _0.2 Ga _0.8 As having a thickness of λ / 4n (λ is an oscillation wavelength in air and n is a refractive index) are formed on an n-type GaAs substrate 11 by MOCVD. Are alternately stacked to form a lower distributed Bragg reflector structure (DBR mirror) 12 composed of 35 pairs of multilayer films. The carrier concentration of each layer is 1 × 10 ¹⁸ cm ⁻³ .

Next, a lower cladding layer (not shown), an active layer 13 having a GaAs / AlGaAs quantum well structure (not shown), and an upper cladding layer (not shown) are sequentially stacked on the lower DBR mirror 12. Further grown thereon is a p-type Al _0.8 Ga _0.2 As having a thickness of λ / 4n and a p-type Al _0.2 Ga _0.8 As having a thickness of 25λ / 4n (corresponding to m = 12, the film thickness is 1520 nm). To do. Further, p-type Al _0.8 Ga _0.2 As and λ Al _0.2 Ga _0.8 As each having a thickness of λ / 4n are alternately laminated thereon to form an upper DBR mirror 15 consisting of 29 pairs. The carrier concentration of each layer is 1 × 10 ¹⁸ cm ⁻³ . The thickness of the second p-type Al _0.2 Ga _0.8 As of the upper DBR mirror 15 having a thickness of 25λ / 4n is substantially equal to the thickness of about 1500 nm of an insulating region layer formed by ion implantation described later. Designed. A p-type GaAs layer is formed on the surface of the uppermost layer of the upper DBR mirror 15 to constitute the entire layer structure. The stack in this state is shown in FIG. A p-type Al _0.2 Ga _0.8 As layer having a thickness of 25λ / 4n is denoted by reference numeral 14A.

Next, after a SiNx film 21 is formed on the surface of the layer structure of FIG. 2 by plasma CVD, the photoresist 19 is processed into a cylindrical shape having a diameter of 10 μm by photolithography using a normal photoresist thereon. To do. Here, Au (gold) may be used in place of the photoresist 19 used as an ion implantation mask. Thereafter, hydrogen ions (protons) are implanted by an ion implantation apparatus at an acceleration energy of 400 keV and a dose of 4 × 10 ¹⁴ cm ⁻² , and the outer peripheral portion of the 25λ / 4n-thick p-type AlGaAs layer 14A is implanted into the implantation region 14B. Change. As a result, the current confinement structure 14 is formed in which the non-implanted portion 14A is the light emitting region 18. This state is shown in FIG. The ions to be implanted are not limited to hydrogen, but may be any material that can form a high-resistance insulating layer, such as oxygen. After removing the photoresist 19, annealing is performed at a temperature of 450 ° C., and then the SiNx film 21 is removed.

  Next, after the SiNx film 22 is again formed on the surface of the layer structure by the plasma CVD method, the SiNx film 22 is formed thereon by photolithography using a normal photoresist and RIE (reactive ion etching). A ring-shaped (annular) opening is formed by selective etching. In the formed opening, for example, AuZn is evaporated to form the ring-shaped upper electrode 16. Thereafter, the substrate 11 is polished to a thickness of about 200 μm, and for example, Cr / Au is vapor-deposited on the back surface thereof to form the lower electrode 17. This state is shown in FIG.

When the characteristics of the surface emitting laser element were evaluated by calculation simulation, the element resistance was estimated to be about 80Ω. On the other hand, in the conventional element in which the thickness of the p-type Al _0.2 Ga _0.8 As of the second layer of the upper DBR mirror is λ / 4n, the element resistance is as high as about 140Ω. As described above, it has been found that in the ion implantation type surface emitting laser element, the element resistance can be significantly reduced by increasing the thickness of the ion implantation layer forming the current confinement structure. The present invention is not limited to the ion implantation type, and can be applied to all surface emitting laser elements.

  In the above embodiment, the ion implantation is performed only for one layer in the upper DBR mirror having a large thickness. However, the layer to be ion-implanted is another one in contact with or close to the one layer having the large thickness. Ions may be implanted into the above layers. Such an example is shown in FIG.

Second Embodiment The present embodiment is an example in which the present invention is applied to a photonic crystal surface emitting laser described in Non-Patent Document 1. FIG. 6 shows a cross-sectional perspective view of the surface emitting laser according to the present embodiment. The surface emitting laser is designed to have an oscillation wavelength of 1300 nm. The manufacturing method can be performed in substantially the same manner as in the first embodiment except that a two-dimensional circular hole array is formed.

First, n-type Al _0.9 Ga _0.1 As and n-type GaAs having a thickness of λ / 4n (where λ is an oscillation wavelength and n is a refractive index) are alternately stacked on the n-type GaAs substrate 11 by MOCVD. The lower DBR mirror 12 composed of 35 pairs of multilayer films is formed. The carrier concentration of each layer is 1 × 10 ¹⁸ cm ⁻³ . Next, a lower clad layer (not shown), an active layer 13 having a GaInNAsSb / GaNAs quantum well structure, and an upper clad layer (not shown) are sequentially stacked thereon by a gas source MBE method.

Next, a p-type Al _0.9 Ga _0.1 As layer having a thickness of λ / 4n and a p-type GaAs layer 24A having a thickness of 17λ / 4n (corresponding to m = 8, film thickness is 1540 nm) are formed thereon by MOCVD. Laminate. Further, a p-type Al _0.9 Ga _0.1 As layer and a p-type GaAs layer each having a thickness of λ / 4n are alternately laminated thereon to form an upper DBR mirror 15 having 20 pairs. The carrier concentration of each layer is 1 × 10 ¹⁸ cm ⁻³ .

Next, after a SiNx film is formed on the surface of the laminated structure by a plasma CVD method, a photoresist used as an ion implantation mask is formed into a cylindrical shape having a diameter of 10 μm by photolithography using a normal photoresist on the SiNx film. To process. Note that Au (gold) may be used instead of the photoresist. Thereafter, hydrogen ions (protons) are implanted by an ion implantation apparatus at an acceleration energy of 400 keV and a dose of 4 × 10 ¹⁴ cm ⁻² , and the outer peripheral portion of the 17λ / 4n p-type GaAs layer 24A is changed to an implantation region 24B. . As a result, a current confinement structure 24 is formed in which the non-implanted portion 24A is the light emitting region 18. After removing the photoresist, annealing is performed at a temperature of 550 ° C., and then the SiNx film is removed.

Next, after a SiNx film 22 is formed on the surface of the laminated structure by a plasma CVD method, a period of 5 μm is formed on the SiNx film by photolithography using a normal photoresist and RIE (reactive ion etching). A triangular lattice-shaped circular hole array structure having a circular hole diameter of 2 μm is formed. Using the circular hole array structure formed in the SiNx film as a mask, a two-dimensional circular hole array 20 having a depth of about 4 μm is formed in the stacked structure by ICP (inductively coupled plasma) dry etching using Cl ₂ . Further, the SiNx mask is removed by RIE.

  Next, after a SiNx film 22 is formed by plasma CVD on the surface of the laminated structure in which the two-dimensional circular array 20 is formed, photolithography using a normal photoresist and RIE (reactive ion etching) are formed thereon. The SiNx film 22 is selectively etched to form a ring-shaped opening. For example, AuZn is vapor-deposited in the opening to form the ring-shaped upper electrode 16. Thereafter, the substrate 11 is polished from the back surface so as to have a thickness of about 200 μm, and for example, Cr / Au is vapor-deposited on the back surface to form the lower electrode 17.

When the characteristics of the surface emitting laser having the photonic crystal structure were evaluated by calculation simulation, the element resistance was estimated to be about 90Ω. In the conventional photonic crystal surface emitting laser in which the thickness of the p-type Al _0.2 Ga _0.8 As of the second layer of the upper reflector structure is λ / 4n, the element resistance is as high as about 150Ω. Therefore, it was found that the element resistance can be greatly reduced in the ion implantation type photonic crystal surface emitting laser element of this embodiment.

In the conventional photonic crystal surface emitting laser, a current confinement structure using an AlAs selective oxide layer has been adopted. In this conventional structure, since the refractive index difference between AlAs and AlOx oxidized by AlAs is large, the mode may become unstable during high current injection or high-speed modulation. On the other hand, according to the structure of the present invention, in forming a current confinement structure, an ion implantation method that does not cause a difference in refractive index is used, and ion implantation is performed mainly on one layer portion in a DBR mirror of a photonic crystal surface emitting laser. Since the thickness of this one layer is (2m + 1) / 4n times the oscillation wavelength in the medium, a photonic crystal surface emitting laser element having a stable mode and low element resistance can be obtained.

  Although the present invention has been described based on the preferred embodiment, the surface emitting laser of the present invention is not limited to the configuration of the above embodiment, and various modifications and changes can be made from the configuration of the above embodiment. Those subjected to are also included in the scope of the present invention.

1 is a perspective view showing a part of a surface emitting laser element according to a first embodiment of the present invention. The perspective view which shows the surface emitting laser of FIG. 1 in the one process step in the manufacturing process. The perspective view which shows the surface emitting laser of FIG. 1 in the process step following FIG. The perspective view which shows the surface emitting laser of FIG. 1 in the process step following FIG. The perspective view which cuts off and shows a part of surface emitting laser element which concerns on the modification of 1st Embodiment. The perspective view which notches and shows a part of surface emitting laser element which concerns on 2nd Embodiment. The perspective view which cuts off and shows a part of conventional surface emitting laser element of an ion implantation type. FIG. 5 is a perspective view showing a photonic crystal type conventional surface emitting laser element with a part cut away.

Explanation of symbols

10: surface emitting laser 11: substrate 12: lower DBR mirror 13: active layer 14: current confinement structure 14A: p-type Al _0.2 Ga _0.8 As layer 14B: injection region 15: upper DBR mirror 16: upper electrode 17: lower electrode 18 : Light emitting region 19: Photoresist 21: SiNx film 22: SiNx film 24: Current confinement structure 24A: p-type GaAs layer 24B: Injection region

Claims (6)

An upper distributed Bragg reflector (DBR mirror);
A lower DBR mirror,
An active layer disposed between the upper DBR mirror and the lower DBR mirror ;
In a surface emitting laser comprising a laminated structure of a semiconductor material including a current confinement structure formed by an ion implantation method on a substrate,
The current confinement structure is formed of a single specific layer disposed in the vicinity of the active layer in the stack constituting the upper or lower DBR mirror ,
The specific layer has a thickness of (2m + 1) λ / 4n , where m is a natural number greater than or equal to 1, λ is the oscillation wavelength in the medium, and n is the refractive index in the air, and the specific layer is formed by ion implantation. A surface-emitting laser characterized in that a current confinement structure is formed by determining m so as to be substantially equal to the thickness of the insulating region layer and by determining acceleration energy so that ions are selectively implanted into the specific layer. . The surface emitting laser according to claim 1 , wherein the specific layer has a thickness of 1 µm or more and 2 µm or less. 3. The surface emitting laser according to claim 1 , wherein the implanted ions are hydrogen ions. 3. The surface emitting laser according to claim 1 , wherein the implanted ions are oxygen ions. The upper distribution DBR mirror, two-dimensional pore arrangement having a vacancy defects in the central portion, characterized in that it is formed so as to realize the fundamental transverse mode lasing, any of claims 1 to 4 A surface-emitting laser according to claim 1. 6. The surface emitting laser according to claim 1 , wherein the oscillation wavelength is 1 μm or more and 1.6 μm or less.

Images