WO1996037000A1

WO1996037000A1 - Light-emitting semiconductor component

Info

Publication number: WO1996037000A1
Application number: PCT/DE1996/000761
Authority: WO
Inventors: Jochen Heinen
Original assignee: Siemens Aktiengesellschaft
Priority date: 1995-05-18
Filing date: 1996-05-02
Publication date: 1996-11-21

Abstract

An LED with a cylindrical active region (a) arranged coaxially inside a cylindrical mesa of semiconductor material between covering layers (p, n) of opposite polarity, in which the radii of the cylinders of the active region and the mesa, and the height of the mesa, are such that there is no total reflection of the radiation emitted from the active region on the lateral surfaces of the mesa.

Description

description

Light emitting semiconductor device

The present invention relates to a light-emitting semiconductor diode with short rise and fall times of the radiation generation and high external efficiency of the radiation.

Efficient optocouplers with a high limit frequency require infrared diodes (or light-emitting diodes) with short rise and fall times of the radiation and high external efficiency of the radiation. The possibility of quickly changing the radiation generation is achieved, among other things, by doping the active zone, by making the volume of the active zone as small as possible, by ensuring a high current density of the injected current, and by using low parasitic capacitances and inductors are present. A high external efficiency, i. H. high radiation outward is achieved when the radiation generated in the semiconductor crystal and emitted in all directions strikes the interface between the semiconductor material and the environment at an angle at which no total reflection takes place, so that the radiation emits can leak the crystal.

DE AS 1 297 230 describes a light-emitting semiconductor diode which consists of a cylindrical disk made of n-type GaAs with a diameter of approximately 2 mm and a thickness of approximately 250 μm. A p-type active zone is produced in this disk by the diffusion of zinc atoms. This active zone is arranged circularly and concentrically to the disk and has a diameter of approximately 0.5 mm and a depth of approximately 10 μm. The ratio of the radii of the active zone and the cylindrical disc is chosen so that the emitted light on the jacket of the the disc-formed cylinder does not experience total internal reflection. GB 2 280 061 A describes an LED in which the light emerges on the top and a Bragg reflector is provided for reflection of the radiation in this direction on the side of the active layer opposite the exit surface. EP 0 582 078 A1 describes an LED provided for superluminescence, in which the light incident laterally on oblique end faces, the active layer, is first reflected at right angles to a Bragg reflector arranged parallel to this layer and then is reflected vertically upwards from this reflector. DE 42 31 007 AI describes an LED in which a front side provided for the light exit, including the almost flat side surfaces, with the exception of an area covered with a contact, is covered with a reflection-reducing layer which improves the coupling-out of the radiation.

The object of the present invention is to provide a radiation-generating semiconductor component with a high external radiation efficiency and a rapid possibility of controlling the radiation intensity.

This object is achieved with the semiconductor diode with the features of claim 1. Further configurations result from the dependent claims.

In the light-emitting diode according to the invention, a cylindrical active zone made of semiconductor material and intended for generating radiation is located inside a cylindrical mesa and is arranged coaxially to this mesa. The ratio of the radii of the cladding of the cylinders of this active zone and this mesa is determined in such a way that in all directions of the radiation emitted by the active zone there is no total reflection on the cladding of the cylinder formed by the mesa. In a preferred embodiment of this diode, Bragg reflectors act below and above the active zone. de Layer sequences of layers of alternately high and low refractive index, which reflect the radiation to the lateral boundary surface of the mesa, ie to the jacket of the cylinder formed by the mesa.

A more detailed description of preferred embodiments of this diode follows with the aid of FIGS. 1 to 3.

Figure 1 shows the dimensions of the mesa and the active zone in supervision.

Figures 2 and 3 show embodiments of the diode according to the invention in cross section.

The diode is preferably made up of epitaxially grown layers. The active zone a drawn in FIG. 2 preferably consists of MQW layers (multiple quantum well), which guarantee a small volume of the active regions producing the radiation with short lifetimes of the charge carriers and thus great speed and also a low absorption for them penetrating radiation. The layer or MQW layer structure containing the active zone is arranged between cladding layers (p, n) arranged vertically with respect to the layer plane and transparent to the radiation. These cladding layers can have a thickness of up to a few μm and are doped on different sides of the active zone for electrical conduction to opposite conductivity types. These cladding layers p, n can each comprise layers or layer sequences which cause reflection of the incident radiation to the lateral boundary surface of the mesa at an angle suitable for the exit of the radiation from the mesa. If the diode z. B. is realized in the material system of GaAs, the cladding layers can be made essentially of AlGaAs with an Al content of z. B. 10% to 15%, and for total reflection of the radiation guided in the cladding layers to the side surface of the mesa down layers of AlGaAs with an Al content of z. B. 70% to 100% available his. Instead, layer sequences acting as Bragg reflectors can be present, which consist of a sequence of layers with alternately higher and lower refractive index. The thickness of these layers is such that these layer sequences act as Bragg reflectors B for the radiation impinging on them from the active zone, which reflect the radiation to the lateral boundary surfaces of the mesa. The radiation is therefore essentially in the region held between these Bragg reflectors B, so that a contact K provided for current injection can be applied to the top of the cylindrical mesa containing the active zone and the cladding layers, without thereby covering an area provided for light exit with a material that absorbs or reflects radiation. If the active zone a is provided in a layer which consists of a semiconductor material suitable for generating radiation and comprises the entire lateral extent of the mesa, the area in which radiation is generated can be directed onto the cylindrical active zone provided in the middle of the Mesa can be limited by only injecting electricity into this area. For this purpose, as shown in FIG. 2, the contact K is only applied in the area F of the projection of the active zone perpendicular to the layer plane on the upper cladding layer p and in the outer ring-shaped area by an insulator layer I from the semi-conductor Term material electrically insulated. In this exemplary embodiment, the mesa is located on a substrate which is electrically conductively doped and which is provided with a second connection contact K on its rear side opposite the mesa. For the layer sequence of the semiconductor materials of the mesa z. B. the material system of GaAs can be used. Ternary layers are then e.g. B. from AlGaAs, quaternary layers from AlInGaAs. The material system from InP can also be used for this diode.

In the embodiment of FIG. 3, the contact provided for current injection is on the top of the mesa only in the ring-shaped outer area around the projection of the active zone perpendicular to the layer plane. The contact has a round opening in this example, which is provided as a window for the exit of the radiation, above the active zone. In this embodiment, e.g. B. only the upper Bragg reflector can be omitted. In order to concentrate the current injection onto the active zone in the middle, there is a layer sequence with pnp transitions or npn transitions below the contact. This layer sequence is z. B. formed in that a layer doped for electrical conduction of the opposite conductivity type is grown in the upper cladding layer. Current injection into the active zone is made possible in that in the area of the projection of the active zone perpendicular to the layer planes, for. B. by implantation or diffusion of dopant, this oppositely doped layer is redoped in the upper cladding layer, so that there are no pn or np junctions in the area above the active zone. The region D provided with diffused dopant is indicated in FIG. 3 by the dashed line. An anti-reflective layer AR is applied to the jacket of the cylinder formed by the mesa in order to reduce the partial reflection.

The mesa forms a cylinder with the radius R (see FIG. 1). The active zone in the interior of this mesa is preferably also cylindrical with the radius r. The active zone can either be formed by a cylindrical layer of semiconductor material suitable for generating radiation or by the described arrangement of the contact provided for current injection in the form provided. According to the invention, the ratio of the radii of these cylinders is chosen so that all laterally emitted rays from radiation sources in the active zone hit the surface of the shell of the cylinder formed by the mesa at an angle α to the vertical (surface normal) that is smaller than that Critical angle for the total flexion between crystal semiconductor ^'and the surrounding Medi¬ order. In Figure 1, the beam path for three different exit directions of the radiation is shown with arrows as an example. Starting from the center of the active zone, the radiation strikes the outer surface of the mesa vertically and leaves the mesa in a straight line. The radiation emitted by the edge areas of the active zone (eg from point A) strikes the lateral surface at an angle α to the normal and is refracted in the direction of the arrow drawn in a continuous line or the arrow drawn in dashed lines. The thickness of the cladding layers between the Bragg reflectors or the height of the mesa is such that, together with the ratio of the radii R, r shown in FIG Radiation takes place. In the case of semiconductor material with a refractive index of approximately 3.4 and air as the medium surrounding the mesa, there is a limit of 0.3 for the quotient r / R for very flat mesas. This limit value must not be exceeded if total reflection is to be avoided. The active zone can therefore be smaller for a given dimension of the mesa than would correspond to this limit. If the diode is embedded in an amorphous solid body that is transparent to the radiation, larger values result for the quotient r / R. If the diode z. B. is embedded in casting resin with a refractive index of about 1.5, then the limit for the quotient r / R is about 0.44 at the specified refractive index for the semiconductor material. Because of the required rapid response of the diode to changes in the applied voltage, the volume of the active zone, that is to say the radius r, is chosen to be quite small. A value for r between 20 μm and 40 μm is favorable.

When taking a closer look, it must also be taken into account that the point of radiation generation and the point at which the radiation leaves the mesa do not have to lie in the same plane with respect to the layer sequence. The angle between the radiation direction in the mesa and the perpendicular the lateral surface of the mesa from which the radiation emerges, at the point at which the radiation emerges, is also referred to below as α. This angle assumes a maximum value for a radiation direction which, starting from a point at the edge of the active zone, is directed to a point on the jacket of the cylinder formed by the mesa, which lies on the side facing the radiation generation point. It is therefore only necessary to take into account points of radiation generation on the edge of the active zone. The distance of the point at which the radiation leaves the mesa from the plane which is coplanar with the layer plane and in which the light beam in question is generated is hereinafter referred to as h. In this plane of radiation generation, θ denotes the angle which is included by the radius directed in the plane of the radiation generation from the center of the active zone to this point of the radiation generation and by the projection of the plane perpendicular to the layer planes Center of the active zone to the point at which the radiation emerges from the mesa into the plane of the radiation generation.

When viewed from above, the view shown in FIG. 1 results with the point of radiation generation A on the edge of the active zone and the point of exit of radiation B from the mesa, the points directed at these points from the center of the active zone outgoing rays projected into the plane of radiation generation include the angle θ. In this case, however, point B of the radiation exit is offset upwards or downwards perpendicular to the plane of the drawing; point A lies in the plane of the drawing (level of radiation generation). However, the angle θ is measured in the plane of the drawing. For this purpose, point B must be projected into the drawing plane perpendicular to the drawing plane. However, the angle α is now to be regarded as a spatial angle that lies between the perpendicular to the surface of the cylinder formed by the mesa at point B and the connecting path between the point ten A and B is formed. The following equation is obtained for this angle, in which the distance of point B from the drawing plane perpendicular to the drawing plane is again denoted by h:

cos α = (R - r cos θ) (R ² - 2Rr cos θ + r ² + h ² ) ^-1 / ²

The angle α is maximum if:

cos θ = (r ² + h ²⁾ / (r R), or θ = 0 in the case of h> (Rr - r ^{2) from} ^{^1/2.}

For the value sin α, which may at most be equal to the quotient of the refractive indices of the outer medium and the semiconductor material of the mesa, one obtains:

sin α = (r ² + h ²⁾ ^{^1/2} / R h <= (Rr - r ²⁾ ^{^1/2}

sin α = h ((Rr) ² + h ²⁾ ^-1 / ² for h> = (Rr - r ^{2) from} ^{^1/2.}

The figures given are examples of 0.3 and 0.5 for r / R is (Rr - r ^{²⁾ ^1/2} = 0.458 R = 0.5 and R, ie at a sym¬ metric layer structure of the mesa, the height the mesa is almost equal to half the diameter, which is unlikely to be realized in practical exemplary embodiments. It can therefore be assumed that the maximum value of sin α is given by the term (r ² + h ² ) ¹ / / R. The maximum angle to the standard on the cylinder jacket of the mesa thus results for the radiation which, starting from the edge of the active zone, reaches the upper or lower edge of the mesa, specifically in the radiation direction which is perpendicular to the connection route between point A of the radiation generation and the projection of the center of the active zone perpendicular to the layer plane into the plane of the radiation exit from the mesa, which plane is coplanar with the layer plane (ie approximately the upper or lower base area of the surface of the mesa) ¬ formed cylinders).

Claims

claims

1. Light-emitting semiconductor diode with a sequence of layers of semiconductor material grown one above the other in relation to a layer plane, which has an active zone (a) provided for radiation generation in a layer of semiconductor material suitable for radiation generation, arranged vertically with respect to this layer plane and for electrical conduction mutually opposite conductivity types doped cladding layers (p, n), in which this layer sequence is formed in a cylindrical mesa, in which this active zone is cylindrical and is arranged coaxially to the cylinder formed by this mesa, in which the quotient of the radii of the cladding of the cylinder formed by this active zone and of the cylinder formed by the mesa is at most the value of the quotient of the refractive index of the material surrounding this mesa in the layer plane of the active zone and of the active zone in this this layer level u material of this mesa and in which these cladding layers are provided with means which direct the radiation to the cladding of the cylinder forming the mesa.

2. Diode according to claim 1, in which the dimensions of the mesa and the active zone are selected such that each point on the jacket of the cylinder formed by the mesa, which is provided for the exit of radiation generated in the active zone, of a plane parallel to the layer plane in the active zone is at most such a distance that the root divided by the radius of the jacket of the cylinder formed by the mesa is taken from the sum of the square of this distance and the square of the radius of the jacket of the the cylinder formed by the active zone at most the value of the quotient of the refractive index of this mesa in the layer plane of the active one Zone surrounding material and the material surrounding this active zone in this layer plane of this mesa.

3. Diode according to claim 1 or 2, in which the mesa is surrounded by air and the quotient of the radii of the cladding of the cylinder formed by the active zone and the cylinder formed by the mesa is at most 0.3.

4. Diode according to claim 1 or 2, wherein the mesa is surrounded by an amorphous solid which is transparent to the radiation to be generated in the active zone and the quotient of the radii of the shells of the cylinder formed by the active zone and of the mesa formed cylinder is at most 0.5.

5. Diode according to one of claims 1 to 4, in which the cladding layers each comprise a layer which is arranged at a vertical distance with respect to the layer planes to the layer plane of the active zone and which has a lower refractive index than the material which faces the active zone adjacent to this layer.

6. Diode according to claim 5, in which the cladding layers are essentially Al _x Ga_- _x As with x at least 0.1 and at most 0.15 and in which the layer of lower refractive index is Al _x Ga _ι - _x As with x at least 0 , 7 and at most 1.

7. Diode according to one of claims 1 to 6, in which the cladding layers each comprise a layer sequence of layers of different refractive indices grown one above the other with respect to the layer plane, and in which these layers are arranged alternately with higher and lower refractive indices and are each so thick that this layer sequence each for the wavelengths of the in radiation generated by the active layer forms a Bragg reflector.

8. Diode according to one of claims 1 to 7, wherein the active zone (a) through a region in a

Layer of radiation-generating semiconductor material is formed and in which a contact (K) is provided on a base of the cylinder formed by the mesa, which contact is provided for current injection into the active zone and with a cladding layer in an area that relates to the layer plane vertical projection of the active zone is limited, is electrically conductively connected.

9. Diode according to one of claims 1 to 7, wherein the active zone (a) through a region in a

Layer of radiation-generating semiconductor material is formed, in which there is a contact (K) on a base of the cylinder formed by the mesa, which contact is provided for current injection into the active zone and is electrically conductively connected to a cladding layer (p), in which this contact in a region which comprises at least the projection of the active zone (a) perpendicular to the layer plane, has a window-like recess and in which the jacket layer (p) electrically conductively connected to this contact outside this projection of the active zone perpendicular to the layer plane comprises a layer sequence in which a layer (s) doped for the conductivity type opposite to this cladding layer is arranged vertically with respect to the layer plane between layers doped for the conductivity type of this cladding layer.

10. Diode according to one of claims 1 to 9, in which the radius of the jacket of the cylinder limiting the active zone is between 20 μm and 40 μm.