NL1005570C2 - Vertical contact surface emitting laser manufacture - Google Patents

Abstract

Vertical cavity surface emitting laser (VCSEL) semiconductor arrangement consists of, sequentially, first mirror, contact, active, contact and second mirror layers on a semiconductor substrate. Parts of the contact layers are accessible. At least one pair of right angle contact layers connect beside and above the sides of the active layer.

Description

Short designation: VCSEL semiconductor device.

The invention relates to a VCSEL semiconductor device, comprising a semiconductor substrate, a number of layers applied to the substrate, including a first stacked mirror layer, a first contact layer, an active layer, a second contact layer and a second stacked mirror layer, said sequence is applied to the substrate, that portions of the surfaces of the contact layers remote from the substrate remain free, the layers applied to the first contact layer having at least two pairs of substantially parallel sides, respectively, being circular-cylindrical, and the contactless parts of the contact layers.

Such an arrangement can be considered to be known from US patent 5 245 622. (See in particular figure 2 thereof.)

This known device uses layered contact layers and a current blocking region in the active layer, the current blocking region and the contacts arranged on the layered contact layers 20 being annular. By these measures, low series resistance without increasing optical absorption is achieved, as well as suppression of higher mode radiation, to obtain TEM00 mode radiation and high power efficiencies.

Semiconductor lasers are widely used in applications such as fiber optic or free space communication systems, optical data storage systems and printing or scanning systems. The VCSEL semiconductor device (where VCSEL stands for "Vertical Cavity Surface Emitting Laser", that is, a laser with a vertical cavity emitting from a surface parallel to the underlying substrate) is a relatively new type of semiconductor laser that emits radiation in at least a direction is substantially perpendicular to the plane of the PIN junction or the substrate rather than parallel to the plane of the PIN junction as with conventional side-emitting semiconductor lasers. The low divergence circular output beam of VCSELs improves coupling efficiency with single mode fibers and broadcasting at least substantially perpendicular to the substrate allows the fabrication of two-dimensional VCSEL matrices.

A typical semiconductor laser consists of a vibrating cavity included between two parallel mirrors and provided with an active area. In a VCSEL, the thickness of the active region is typically only one micrometer or a fraction thereof. Since this results in a single-pass optical gain of about 1% or less, the reflectivity of the mirrors must be greater than 99% to achieve laser operation. Such high reflectivity can be achieved by using DBR mirrors (where DBR stands for "Distributed Bragg Reflector", ie, a distributed Bragg reflector). A DBR mirror is a stack of layers with alternating high and low refractive index. The number of layers required depends on the difference in the refractive index of the layers and the desired reflectivity.

For such a thin cavity, the wavelength distance between the various longitudinal modes is greater than the gain spectrum of the active region, resulting in single longitudinal mode operation. Operation with a single transverse mode is achieved only with weak current. At high current, prior art VCSELs emit radiation in higher transverse modes. Longitudinal (axial) modes are formed by planar waves running axially along the cavity of the VCSEL. If the VCSEL contains only the fundamental transverse mode TEM00, the laser beam intensity profile has a Gaussian divided section, which is the most intense in the center and less intense at the periphery of the device. Contributions from higher transverse modes (eg TEM01, TEM10, TEMn etc.) lead to bright and dark dots visible on the top surface of the device and a multi-beam radiation field. High transverse mode radiation is more difficult to couple into optical fibers and to focus for free space beam formation. In addition, contributions from high transverse modes lead to mode spreading in optical fibers, since the higher order transverse modes run at a slightly different speed, resulting in broadening of an optical pulse as it travels in an optical fiber.

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When an electric current is applied, optical radiation is generated in the active region, which is reflected back and forth between the mirrors. One of the stacks of mirrors has a lower reflectivity to couple out some of the coherent light built up in the tri-1-5 cavity. The current can be injected through the mirrors (vertical injection) into the active region, or laterally using contact layers within the cavity (radial injection). The prior art electrode configuration is symmetrical for both injection mechanisms (e.g.

10 donut or annular). However, the current injection profile tends to be non-uniform for these symmetrical electrode configurations, and as the injection current increases, the current at the edges of the current aperture formed by the current blocking layers will densify. Since the desired optical mode TEM00 has a lower amplitude at the edges of the current aperture formed by the current blocking layers, the conversion efficiency is substantially reduced. In addition, the slight overlap between the optical TEM00 and electrical density profiles at high injection currents results in laser operation with higher order transverse modes.

Another problem with prior art VCSEL designs is the uncontrolled polarization directions of the emitted radiation. In many applications (e.g. magneto-optical plates), lasers with controlled polarization directions are very necessary.

The object of the invention is to achieve operation with a single transverse TEM00 mode and optical radiation with a polarization direction which is substantially parallel to the current injection direction.

To this end, the invention provides a device of the type mentioned in the preamble, characterized in that at least two substantially rectangular contacts are arranged next to and above, respectively, and aligned with two opposite sides of the active layer, respectively two opposite sides of an imaginary polygon defining the substrate facing surface of the second stacked mirror layer on the surface of the second contact layer facing away from the substrate has been applied to the first and second contact layers.

The invention also relates to a VCSEL semiconductor device, comprising a semiconductor substrate, a number of layers applied to the substrate, including a first stacked mirror layer, an active layer and a second stacked mirror layer, which are thus applied to the substrate in the order mentioned that a portion of the surface of the substrate on which the plurality of layers is applied remains free, the layers applied to the substrate having at least two pairs of substantially 10 parallel sides, respectively, being circular cylindrical, and contacts, one of which is on the substrate fitted.

Such a device can also be considered to be known from US patent 5 245 622.

In order to achieve the same objects of the invention as above, the device of the above-mentioned type is characterized in that at least two substantially rectangular contacts are arranged next to and above, respectively, and aligned with two opposite sides of the active layer, respectively. aligned with two opposite sides 20 of an imaginary polygon describing the surface of the first stacked mirror layer facing the substrate, on the substrate, the polygon on the non-exposed portion of the substrate and the surface of the second stacked mirror layer facing away from the substrate.

The proposed new contact structure achieves the above-mentioned objectives, and it is moreover possible to control the direction of the polarization of the light by using more than one pair of contacts.

The invention will now be further elucidated with reference to the accompanying drawing, in which drawing: figure 1 shows a first embodiment of the laser according to the invention with radial current injection; figure 2 shows a variant of the laser of figure 1, with which the light polarization can be controlled in two orthogonal directions, and figure 555570 shows a second embodiment of a laser according to the invention with vertical current injection.

In a first embodiment of the invention, the VCSEL comprises an unspecified vibrating cavity disposed between the upper and lower DBR mirrors, i.e. 5 second and first stacked mirror layers 1, 2, respectively. The DBR mirrors 1, 2 include optically transparent layers, such as epitaxially grown aluminum-gallium arsenide or indium phosphide layers, but also dielectric layers can be used in combination with metal layers such as Si02 / Ti02, MgF2 / ZnSe, 10 Si02 / Si3N4 or Al203 / AlxGalxAs . The vibrating cavity comprises upper and lower or second and first spacing layers 3 and 4, between which an active radiation generating layer 5 is incorporated. The active layer 5 is made of optically active material, such as gallium arsenide, indium gallium arsenide, indium gallium arsenide-15 phosphide, indium gallium nitride, aluminum gallium arsenide, aluminum gallium indium phosphide, zinc selenide or the like. Top and bottom or second and first contact layers 6, 7 are included within the optical cavity between the top and bottom stacked mirror layers 1, 2 and the top and bottom 20 spacer layers 3, 4, respectively, for conducting electric current into the active layer . For lateral flow constriction, between the top and bottom spacer layers 3, 4 and the top and bottom contact layers 6, 7, respectively, are aluminum containing top and bottom, i.e., second and first lateral currents layers 8, 9 (eg, of aluminum arsenide, aluminum gallium arsenide, aluminum indium phosphide, aluminum gallium indium phosphide). Using an oxidation technique selective for aluminum containing layers, the lateral flow constriction layers 8, 9 are converted laterally from the sides of the cavity into transparent oxide layers.

The remaining non-oxidized portion of the aluminum-containing layers 8, 9 form a flow aperture leading to efficient charge carrier injection. Instead of an annular electrode configuration with a first electrode arranged around the base of the second stacked mirror layer 1 as in the prior art, a strip contact 13 on one side of the second stacked mirror layer 1 is used. Also the annular second electrode of the prior art 1005570 6 has been replaced by a strip contact 14 on the other side of the second stacked mirror layer 1.

More generally, the present invention proposes a VCSEL semiconductor device comprising a semiconductor substrate 5 on which a plurality of layers (1-9) are deposited, including a first stacked mirror layer 2, a first contact layer 7, an active layer 5, a second contact layer 6 and a second stacked mirror layer 1, which are arranged on the substrate 10 in the above-mentioned order, so that parts of the surfaces 10, 71, 61 facing away from the substrate remain of the contact layers 7 and 6, respectively, with the layers 4, 5, 3, 6 and 1 applied in first contact layer 7 have at least two pairs of substantially parallel sides AA, A'A ', BB and B'B', respectively, which are circular cylindrical, and on the non-binding parts of the contact layers 6, 7 arranged contacts 13 and 14. In accordance with measures according to the present invention, at least one pair of substantially rectangular contacts 13, 14 re respectively adjacent and above and aligned with two opposite sides A, A of the active layer 5, respectively aligned with two opposite sides of an imaginary surface of the second stacked mirror layer facing the substrate and of the substrate facing surface of the second contact layer describing polygon applied to the first and second contact layers 6, 7. In plan view, the layers 9, 25, 4, 5, 3, 8, 6, 1 applied to the first contact layer 7 are radially symmetrical, i.e. circular or polygonal, including hexagonal, rectangular and square.

In this way, the current injection from one electrode 13 will be directed to the other 14, and vice versa. The current path through the periphery of the active layer 5, which results in the undesired current densification on the active region sides for the prior art VCSELs, is now no longer preferred for any other current path through the active region. active layer 5. Since the probability of all current paths through the active layer 5 is now the same, the homogeneity of the current injection will increase substantially. As a result, laser operation with higher order transverse modes is suppressed. In addition, as a result of the 1005570 7 direct current injection, the laser will emit optimal radiation with a polarization direction that is substantially parallel to the current injection direction.

Figure 2 shows an embodiment of the device 5 according to the invention, which corresponds to that shown in Figure 1, but which instead of two strip contacts 13, 14 comprises two orthogonal pairs of strip contacts 13, 14 and 15, 16. In Figures 1 and 2, the same reference numerals are used for the same or corresponding parts.

As shown in figure 2, just as in figure 1, two substantially rectangular contacts 13, 14 are next to and above, respectively, and aligned with two opposite sides A, A of the active layer 5 on the first and second contact layers 6 and 7, and furthermore two substantially rectangular contacts 15, 16 are arranged next to and above, respectively, and aligned with two opposite sides A'A 'of the active layer 5 on the first and second contact layers 6, 7.

With the VCSEL of Figure 2, the direction of the polarization of the light can be controlled in two orthogonal directions. With a hexagonal structure, the direction of the light could be directed in three directions. Modulation of the polarization direction of the laser light can be used, for example, in telecommunications networks.

Another embodiment of the device according to the present invention is shown in figure 3. Here, too, the same reference numerals have been used for the same or similar parts as in Figure 1.

The embodiment of the VCSEL according to the present invention shown in figure 3 comprises an upper and lower spacing layers 3, 4, arranged between upper and lower stacked mirror layers 1, 2, upper and lower flow necking layers 8, 9 and active layer 5 the active layer 5 generating optical radiation. In this embodiment, instead of by lateral flow injection using two contact layers in the vibrating cavity, the flow is injected into the active layer 5 by means of the stacked mirror layers 1, 2, more in particular DBR mirrors. As with the embodiment of figure 1, and also the variant thereof in figure 2, instead of annular electrodes or contacts according to the prior art, strip-shaped contacts 13 and 14 have again been applied, but are now not on the respective upper and lower contact layers 6, 7 (in figures 1 and 2), but in this case respectively on the upper or second stacked mirror layer 1 and the top surface of the substrate 10. When the structure is between the two stacked 10 mirror layers 1 and 2 namely the layers 3-5 and 8-9, and these stacked mirror layers 1, 2 is called a mesa, it can simply be said that the two strip-shaped contacts 13, 14 are placed on opposite sides of the mesa. Also in this case, current compaction will be reduced and higher order transverse mode laser operation suppressed. In this embodiment too, several pairs of electrodes can be used, analogous to the variant of the embodiment according to Figure 1 shown in Figure 2, so that the current can also be controlled in two or more directions in this embodiment.

1005570

Claims (10)

1. VCSEL semiconductor device, comprising a semiconductor substrate, a plurality of layers applied to the substrate, including a first stacked mirror layer, a first contact layer, an active layer, a second contact layer and a second stacked mirror layer, said sequence is provided on the substrate, that portions of the surfaces facing away from the substrate remain free of the contact layers, the layers applied to the first contact layer having at least two pairs of substantially parallel sides, respectively, being circular-cylindrical, and on the non-binding portions contacts arranged on the contact layers, characterized in that at least two substantially rectangular contacts are arranged next to and above, respectively, and aligned with two opposite sides of the active layer, respectively aligned with two opposite sides of an imaginary , the surface facing the substrate varnish of the second stacked mirror layer is applied to the surface of the second contact layer facing away from the substrate, polygon describing the first and second contact layers. 2. VCSEL semiconductor device, comprising a semiconductor substrate, a plurality of layers deposited on the substrate, including a first stacked mirror layer, an active layer and a second stacked mirror layer, so arranged on the substrate in said order that a portion of the surface of the substrate on which the plurality of layers is applied remains free, the layers applied to the substrate having at least two pairs of substantially parallel sides, respectively, being circular cylindrical, and contacts, one of which is applied to the substrate, characterized in at least two substantially rectangular contacts adjacent and above and aligned with two opposite sides of the active layer, respectively, aligned with two opposite sides of an imaginary surface facing the substrate of the first 35 mirror layer on the substrate trailing polygon is applied to the non-committal portion of the substrate and the surface of the second stacked mirror layer remote from the substrate 1005570 «. The VCSEL semiconductor device according to claim 1, wherein the layers applied to the first contact layer are rectangular or 5, characterized in that a first pair of substantially rectangular contacts are located next to and above, respectively, and aligned with two opposite sides of the active layer is provided on the first and second contact layers and a second pair of substantially rectangular contacts adjacent to and above 10 respectively and aligned with two opposite sides of the active layer perpendicular to said opposed sides of the active layer and second contact layers are applied. VCSEL semiconductor device according to claim 2, wherein the layers applied to the substrate are rectangular or square, characterized in that a first pair of substantially rectangular contacts are arranged next to and above, respectively, and aligned with two opposite sides of the active layer. is applied to the non-committal portion of the substrate and the surface of the second stacked mirror facing away from the substrate 20, and a second pair of substantially rectangular contacts next to and above, respectively, and aligned with two perpendicular to said two opposite sides of the active layer is disposed on opposite sides of the active layer on the non-binding part of the substrate and the surface of the second mirror layer remote from the substrate. VCSEL semiconductor device according to any of the preceding claims, characterized in that the active layer is contained between first and second spacing layers. VCSEL semiconductor device according to any one of the preceding claims, characterized in that first and second lateral current constriction layers are arranged between the first and second spacer layers and the first and second contact layers, respectively. VCSEL semiconductor device according to claim 6, 35, characterized in that the lateral current constriction layers are aluminum-selectively oxidized layers of a material selected from 1005570 aluminum arsenide, aluminum gallium arsenide, aluminum indium phosphide, aluminum gallium indium phosphide and the like. VCSEL semiconductor device according to any one of the preceding claims, characterized in that the optically active material of the active layer is selected from gallium arsenide, indium gallium arsenide, indium gallium arsenide phosphide, indium gallium nitride, aluminum gallium arsenide, aluminum gallium indium phosphide or the like, zinc selenide. VCSEL semiconductor device according to any one of the preceding claims, characterized in that the stacked mirror layers comprise optically transparent layers, such as epitaxially grown aluminum gallium arsenide or indium phosphide layers or dielectric layers in combination with metal layers. 10. An optical modulator, characterized in that it comprises a VCSEL semiconductor device according to any one of the preceding claims with at least two pairs of contacts, as well as switching means for switching power supply between pairs of contacts. 20 10U5570