WO1995031741A1

WO1995031741A1 - Semiconductor component with branched waveguide

Info

Publication number: WO1995031741A1
Application number: PCT/DE1995/000612
Authority: WO
Inventors: Bernhard STEGMÜLLER
Original assignee: Siemens Aktiengesellschaft
Priority date: 1994-05-18
Filing date: 1995-05-09
Publication date: 1995-11-23

Abstract

A semiconducteur component with a strip waveguide (5, 6, 7) on a substrate (1), in which there is a vertical branch point in two superimposed layers (6, 7). A separating layer (3) between said layers is obtained by the suitable setting of the growth conditions when the layers are grown in such a way that there is a continuous increase in the thickness (11) of said separating layer (3) in the branch sections (10). The number of individual layers is the same in the vertical direction.

Description

description

Semiconductor device with branched waveguide

The present invention relates to a semiconductor component with a branched integrated waveguide, in particular a VMZ laser diode.

Forks of waveguides in semiconductor components are said to couple little energy of the guided light waves in radiation modes. To ensure this, the y-shaped branches of the waveguides must form a very small angle and the separating layers between the waveguide branches must be as pointed as possible. Unwanted mode jumps can already occur if the angle between the two branches of a waveguide branching is greater than 1 °. In the publication by J. E. Zucker et al .: "Interferometric Quantum Well Modulators with Gain" in Journ. of Lightwave Techno1. 10, 924 to 932 (1992) describes a Mach-Zehnder interferometer as a branched waveguide in InP. This publication also discusses the problem that the radius of curvature in the case of S-shaped curved branches of the waveguide must be as large as possible in order to prevent excessive radiation loss at the branching point. A VMZ laser diode is e.g. B. in the German patent application P 44 01 444.9 described.

In the publication by E. Colas ea: "In situ definition of semiconduetor struetures by selective area growth and etching" in Appl. Phys. Lett. 5_9_, 2019 to 2021 (1991) describes a method with which layers of semiconductor material that taper in the vertical direction can be produced by means of gas phase epitaxy. The object of the present invention is to provide a structure for a branched waveguide integrated in a semiconductor component, in which the opening angle of two waveguide branches is sufficiently small or increases slowly enough to effectively prevent radiation losses. This integrated waveguide structure should in particular be suitable for the construction of a Mach-Zehnder interferometer. For this purpose, a sufficiently simple manufacturing process should be specified.

This object is achieved with the semiconductor component with the features of claim 1 and with the manufacturing method with the features of claim 7. Further configurations result from the dependent claims.

In the semiconductor component according to the invention, the strip-shaped waveguide is divided in one area into two branches arranged vertically with respect to one another with respect to the layer structure. In a preferred embodiment, the waveguide is formed from a plurality of individual layers grown one above the other. A middle one of these layers is grown substantially thicker in a section of the waveguide than in the sections adjoining in the longitudinal direction of the waveguide. In this way, the layer sequence forming the waveguide is divided into two parts which are separated from one another by this thick layer. In transition areas in which the branching of the waveguide onto the two branches takes place, the thickness of this layer is continuously reduced to the size of the other layers, so that the branching of the waveguide is effected in this way. In the following the areas in which the waveguide is not branched are referred to as single areas and the areas in which the waveguide is divided into two branches are referred to as double areas.

The component according to the invention is described with reference to FIGS. 1 to 4. FIG. 1 shows a component with the wave guide according to the invention in a longitudinal section. Figures 2 and 3 show the cross sections designated in Figure 1. Figure 4 shows an alternative to Figure 1 embodiment.

In the exemplary embodiment shown in FIG. 1, a buffer layer 2, which can also be omitted, is first grown on a substrate 1. On top of this is the waveguide, which is formed by a layer 5 in the single regions and by a lower layer 6 and an upper layer 7 in the double region shown. In the double region, these layers 6, 7 of the waveguide are separated from one another by a separating layer 3, so that here separate waveguiding, but at the same time also coupling of the guided modes, is possible. In the example shown, this separating layer 3 has the indicated thickness 11 in the double region. In branching regions 10, the waveguide is branched continuously from the layer 5 of the individual region into the two layers 6, 7 of the double region. There is a cover layer 4 on the top of this layer structure, which simultaneously planarizes the component. A contact 9 provided for current injection is located on the top side and is electrically insulated from the semiconductor material outside the double region by an insulating layer 8.

The cross section shown in FIG. 2 through a single region shows the strip-shaped waveguide 5, which here consists of several individual layers and has the width 13. The cover layer 4 is applied to the layer 5 of the waveguide and fills the area between the side areas 12 of semiconductor material in a planarizing manner. The metal layer of the contact 9 is electrically insulated from the semiconductor material of this cover layer 4 by the insulating layer 8. A corresponding cross section is shown in FIG shown the double area. The two layers 6, 7 of the waveguide are separated from one another here by the separating layer 3. The strip-shaped layer structure is also laterally bordered by the side regions 12 made of semiconductor material. The planarizing cover layer 4 is applied to the top. The contact 9 is located here in the strip-shaped region of the waveguide directly on the semiconductor material of this cover layer 4 or a highly doped contact layer 14 applied thereon, but is electrically insulated laterally therefrom by the insulating layer 8. In Figure 1, the dashed lines indicate the hidden contours of the side area 12. Other contacts provided for the current injection are z. B. on the underside of the substrate 1 or laterally to the strip-shaped waveguide structure.

In the exemplary embodiment shown here, the waveguide consists of a sequence of several individual layers. The middle of the layers is widened to the thickness 11 in the double region and forms the separating layer 3. The number of layers grown is therefore the same over the length of the waveguide. In the single region, the layer 5 of the waveguide has the same number of individual layers as in the double region, both layers 6, 7 of the waveguide and the separating layer 3, ie the entire area encompassed by the layers 6, 7 of the waveguide. In the double region, in which the waveguide is split into two layers 6, 7, all layers are grown relatively thicker than in the individual regions. The layer thickness in the branching regions 10 increases continuously. In a typical embodiment, the layer 5 of the waveguide has a maximum thickness of 0.2 μm in the individual areas. The lower layer 6 and the upper layer 7 of the waveguide have a thickness of 0.01 to 0.3 μm in each case in the double region. The length of the branch region 10 is typically at least 30 μm. The section of the double region between these branching areas can be shorter than 5 mm his. The width 13 of the waveguide shown in FIG. 2 in the individual areas is z. B. 5 µm maximum. Layers of different thicknesses or continuously increasing thicknesses can, for. B. produce by adapting the conditions when growing to the required thicknesses of these layers. Depending on the epitaxial process, the layer growth is accelerated or slowed down by widening the opening of the mask used. The shape of the mask opening can therefore be used in epitaxy to determine in which sections of the waveguide which layer thicknesses grow. A uniform width of the layer structure of the strip-shaped waveguide can, for. B. can be achieved by subsequent anisotropic etching using a mask with a rectangular opening. The side areas are then evenly filled with semiconductor material. The planarization takes place through the top layer 4, which can optionally consist of several layer layers. Likewise, the buffer layer 2 or an upper portion of the buffer layer 2 in the branching areas can be grown with increasing thickness. The greater thickness of all the individual layers in the double region compared to the individual region and the steady decrease in the layer thicknesses present in the branching regions on the longitudinal outlets of the double region are illustrated in FIG. 1.

In principle, the separating layer 3 can be a layer separated from the waveguide layers. This separating layer 3 is then grown using a mask which completely covers the individual areas. The different layers of the waveguide can each be single-layer or multi-layer layers. With regard to a simplified manufacture of the component, it is recommended if the layer 5 of the waveguide comprises at least three layers in the individual regions, of which the lower layer continues the waveguide into the lower layer 6 of the double region, the middle layer extends to the separating layer 3 of the double area in each case expands and the upper layer the upper Forms layer 7 in the double region. For the sake of clarity, the opening angle of the waveguide in the branches is greatly exaggerated in FIG. 1. With the structure according to the invention, very small opening angles of the waveguides can be produced without the problems occurring at a horizontal realization of the waveguide branching occurring at the branching points.

In the alternative exemplary embodiment shown in FIG. 4, the buffer layer 2, which can in principle also be omitted, grew thinner as an example in the double region than in the individual regions. The waveguide forks are curved in an S-shape here. The opening angle between the (tangential) directions of the branches of the waveguide in the waveguide forks gradually increases from 0 to a maximum and then back to 0, so that the layers 6, 7 of the waveguide are straight in the middle section of the double region and are guided parallel to each other. In this exemplary embodiment too, the separating layer 3 is shown as the middle layer of the layer 5 of the waveguide in the individual regions.

Several individual layers, which form the layers of the waveguide, are particularly necessary if the waveguide is to be designed as an MQW layer (Multiple Quantum Well). For use as a vertical Mach-Zehnder interferometer (eg VMZ laser diode), an active layer for radiation generation and a tuning layer can be arranged one above the other in the layer structure of the waveguide in the double region. This tuning layer and this active layer can be separated from one another by a separating layer both in the lower layer 6 or the upper layer 7 of the waveguide in the double region. Instead, the tuning layer and the active layer can be distributed over the lower layer 6 of the waveguide and the upper layer 7. Separate current injection into these two layers takes place above and in between located doped layers of semiconductor material. When arranging the active layer z. B. in the lower layer 6 of the waveguide and the tuning layer in the upper layer 7 of the waveguide, the separating layer 3 consists of doped semiconductor material. The portion of the top layer 4 located on the upper side of the upper layer 7 and at least the portion of the buffer layer 2 located under the lower layer 6 are also doped. The signs of the doping are selected so that current is separated into the active layer and contacts via the top surface of the component via contacts connected to the regions in an electrically conductive manner (eg via lateral doped semiconductor regions and / or the doped substrate 1) the tuning layer can be injected. In this way, with the waveguide according to the invention, a tunable laser diode with integrated machin

Zehnder interferometer are manufactured, which avoids the disadvantages of previously tunable VMZ laser diodes at the abrupt transitions between single and double areas.

Preferred semiconductor materials for the described exemplary embodiment are e.g. B. InP or GaAs for the substrate 1, a sequence of alternating InGaAsP and InGaAlAs for the MQW individual layers (this may include the separating layer 3), InP for the buffer layer 2 which may be present, the cover layer 4 and possibly the separating layer 3 and conventional dielectric materials such as Al 2 O 3 or SiO _x for the insulating layer 8. A sufficiently low-resistance transition between the semiconductor material and the metal of the contact 9 is achieved by at least an upper layer portion of the covering layer 4 being highly doped. However, a contact layer 14 (see FIGS. 1 and 3) can additionally be present at least in the area of the metal-semiconductor contact between the cover layer 4 and the contact 9. Depending on the exemplary embodiment, the side regions 12 can be semi-insulating Fe: InP or electrically conductive doped InP. All

Layers 5, 6, 7 of the waveguide can be undoped. In the case of a VMZ laser diode with a controlled double range the separating layer 3 or a single layer, which functions as a separating layer between the active layer and the tuning layer, in one of the layers 6, 7 of the waveguide together with at least a portion of the side regions 12 for the one conduction type, while the buffer layer 2 and the Cover layer 4 and possibly the contact layer 14 are doped for the opposite conductivity type. When electrically connected via the substrate 1, the substrate for the conductivity type of the buffer layer 2 is also doped. The doped regions in the side regions 12 are preferably delimited laterally by insulation regions. The contact is made by contacts on the upper side, which are each connected in an electrically conductive manner to the areas and layers to be connected by doped areas of the appropriate sign.

In alternative embodiments, e.g. B. for passive branches, the electrical connection through the contact 9 can be omitted. It is possible to attach several double regions of the type shown along the waveguide. The

Layers of the waveguide can be braced in one or more layers or be braced. The materials and doping used can in principle correspond to those of waveguide branches produced horizontally in the layer plane. The thickness of the separating layer 3 can also vary over the entire length of the double region. As in FIG. 1, the layers of the waveguide can bend in the branching regions 10 at the longitudinal ends of the double region or, as in FIG. 4, gradually approach one another without a sudden change in direction, so that the

Tips of the separating layer 3 run particularly flat in the waveguide forks. When this component is designed as a VMZ laser diode, the layers of the waveguide in the double region are arranged so closely adjacent to one another and provided with dimensions and material compositions that a coupling between different modes carried in these layers occurs in the double region. The length of the double region is approximately a natural multiple of the coupling length belonging to these modes.

Claims

1. Semiconductor component with a waveguide integrated therein, provided for a direction of propagation, in which there is at least one single region, in which this waveguide is present as a layer (5), in which there is at least one double region, in which this waveguide is applied as two above one another and separate layers (6, 7) are present, and in which these layers (6, 7) of this double region approach each other in the direction of this single region and open into the layer (5) of the waveguide in the single region.

2. The semiconductor component as claimed in claim 1, in which the double region in the direction of propagation is delimited on both sides by single regions and in which the layers (6, 7) of the waveguide in this double region approach each other in the direction of a single region and into The layer (5) of the waveguide opens into this single region.

3. Semiconductor component according to claim 1 or 2, in which all layers (5, 6, 7) of the waveguide are formed by a strip-shaped layer structure of individual layers present one above the other and in which in the single region the layer (5) of the waveguide has as many of these As in the double region, single layers have the region (3, 6, 7) encompassed by the two layers of the waveguide.

4. Semiconductor component according to one of claims 1 to 3, in which an active layer and a tuning layer are present in the double region in the waveguide and in which means are provided which enable a separate current injection into this active layer and this tuning layer.

5. The semiconductor component as claimed in claim 4, in which the means for separate current injection doped semiconductor layers, which delimit the active layer and the tuning layer vertically with respect to the layer planes, and contacts which are electrically conductively connected to one of these semiconductor layers.

6. The semiconductor component as claimed in claim 4 or 5, in which the layers (6, 7) of the waveguide in the double region are arranged so closely adjacent to one another and have dimensions and material compositions such that in the double region there is a coupling between in these layers (6 , 7) occurs in different modes, and in which the length of the double region is a natural multiple of the coupling length belonging to these modes.

7. A method for producing a semiconductor component according to one of claims 1 to 6, in which in a first step onto a substrate (1, 2) using a mask, a strip-shaped layer or layer sequence of semiconductor material to form a lower portion of the layer (5) of the waveguide is grown in the single region and the lower layer (6) of the waveguide in the double region, in which, in a second step, using a mask on this layer or layer sequence, a strip-shaped layer of semiconductor material is grown such that this layer is in one a section of the waveguide provided for a double region is thickened to an extent sufficient for a provided separation of the layers of the waveguide in this double region, in which, in a third step, using a mask on this layer, a strip-shaped layer or layer sequence of semiconductor material for forming an upper portion of the layer (5) of the waveguide in the single region and the upper layer (7) of the waveguide in the Double region is grown and in a fourth step these stripe-shaped layers are overgrown with semiconductor material.

8. The method according to claim 7, in which in a second step, by using a mask with a strip-shaped opening of varying width, it is achieved that the strip-shaped layer in the section provided for the double region has an adjacent section intended for a single region decreasing thickness (11) grows up.

9. The method according to claim 7 or 8, wherein in the second step, the opening of the mask is limited to the section provided for the double region.

10. The method according to claim 9, wherein the layer grown in the second step receives a doping provided for separate current injection into each of the layers of the waveguide in the double region.