Gan-based radiation-emitting thin-layered semiconductor component

The present invention relates to radiation-emitting thin-film semiconductor devices based on GaN according to the preamble of the independent patent claim.

Conventional radiation-emitting semiconductor components often have a right-angled geometry for production-related reasons. The semiconductor devices generally consist of a multilayer structure epitaxially deposited on a carrier substrate with an active, radiation-generating layer. The carrier substrate is preferably electrically conductive to allow vertical current flow; Moreover, it is advantageous in many cases if the carrier substrate is transparent to the radiation generated in the active layer of the multilayer structure. However, a high transparency often contradicts a high electrical conductivity of the material for the carrier substrate. For example, the sapphire used for GaN-based light-emitting diodes is transparent to blue light but not electrically conductive.

One possibility for reducing the absorption losses and thus for increasing the external efficiency is therefore the removal of the carrier substrate in conjunction with suitable mirror layers (thin film concept). However, a semiconductor thin film is essentially a plane-parallel plate, the decoupling efficiency of which is not increased due to the geometry compared to a standard diode. In particular, if a low-absorbing carrier substrate (for example GaN on SiC) has already been used for the semiconductor component, the increase in the external efficiency of the thin-film semiconductor component is too small to justify the increased technical complexity of removing the carrier substrate.

In order to explain the problem of the radiation decoupling, FIG <figref idrefs="f0005">FIG</figref> Schematically a semiconductor component with the cones of the radiation coupling-out. Radiation can be produced from the semiconductor component only from a cone with an opening angle of θ = sin^-1(N_Ext/ N_Int), Whereby n_Int The refractive index of the semiconductor material, and n_Ext The refractive index of the environment. For a GaN semiconductor (a_Int = 2.5), the coupling-out angle θ to air (n_Ext = 1) 23 ° and against a plastic encapsulation (a_Ext = 1, 5) 37 °. Radiation generated in the semiconductor device, which does not hit the interfaces within a cone, is ultimately reabsorbed and converted into heat. The decoupling cone for GaN systems in comparison to GaAs systems (n_Int = 3.5) is large, but nevertheless leads to undesirably large radiation losses.

These ratios also do not change significantly when layer thicknesses change. However, the thin film geometry is favorable for the beam coupled out over the top, since the absorption is low due to the short path in the semiconductor; For the side - coupled. Ray, the efficiency due to the multiple reflections in the semiconductor can be even lower.

There are therefore already various approaches to increasing the extral efficiency of semiconductor devices by changing geometries. Here, in particular, a so-called microstructuring of the entire multilayer structure is to be mentioned, which leads to an amplified lateral radiation decoupling due to the larger overall area of ​​the side faces of the multilayer structure. In addition, the side faces of the individual multilayer structures thus produced can be chamfered. Examples of such semiconductor devices are shown in FIG<patcit id="pcit0001" dnum="DE19807758A"><text>DE-A-198 07 758</text></patcit>, <patcit id="pcit0002" dnum="EP0905797A"><text>EP-A-0 905 797</text></patcit> or <patcit id="pcit0003" dnum="JP8288543A"><text>JP-A-08-288543</text></patcit> disclosed.

A further possibility for increasing the radiation decoupling is described in the <figref idrefs="f0002 f0003 f0004">FIGS</figref> and <figref idrefs="f0005">5</figref> of the <patcit id="pcit0004" dnum="DE19911717A"><text>DE-A-199 11 717</text></patcit> shown. In this case, individual radiation decoupling elements in the form of spherical segments or truncated cones are provided for the multilayer structure with the active, radiation-generating layer, which are formed, for example, by appropriate etching of grown-up layers.

However, all of the above-mentioned prior art references are not concerned with GaN-based thin-film semiconductor devices. GaN-based semiconductor devices are predominantly used for the generation of radiation in the blue-green spectral range and have a plurality of layers made of a GaN-based material. In the context of this invention, a GaN-based material is understood to be, in addition to GaN, GaN-derived or GaN-related materials, as well as ternary or quaternary mixed crystals based thereon. In particular, the materials GaN, AlM, InN, Al belong to this_1-xGa_XN, In_1-xGa_XN, In_1-xAl_XN and Al_1x-yIn_XGa_YN with 0 &lt;x &lt;1, 0 &lt;y &lt;1 and x + y ≤ 1.

The print <patcit id="pcit0005" dnum="US6258618B1"><text>US 6,258,618 B1</text></patcit> Discloses an LED having a p-type contact which is made of a highly reflective material with closely adjacent apertures that enhance the light extraction efficiency.

The print <patcit id="pcit0006" dnum="US6346771B1"><text>US 6,346,771 B1</text></patcit> Discloses a high-power LED lamp with an LED chip which may have triangular light-emitting elements that improve light extraction from the semiconductor.

The print <patcit id="pcit0007" dnum="WO0141225A2"><text>WO 01/41225 A2</text></patcit> Discloses LEDs with light extraction structures on or in the LED which improve efficiency.

In the <figref idrefs="f0005">FIG</figref> In this document, a radiation-emitting thin-film semiconductor device with a GaN-based multilayer structure is shown, wherein the multilayer structure contains an active, radiation-generating layer and has a first main surface and a second main surface facing away from the first main surface for decoupling the radiation generated in the active radiation-generating layer , Wherein the first main surface of the multilayer structure is coupled to a reflecting surface (the surface of the substrate), and a transparent layer is provided between the first main surface of the multilayer structure and the reflecting boundary surface,And wherein the transparent layer between the first main surface of the multilayer structure and the reflecting boundary surface has convex elevations as light extraction structures.

The radiation-emitting thin-film semiconductor device according to a modification has a GaN-based multi-layer structure which contains an active, radiation-generating layer and has a first main surface and a second main surface facing away from the first main surface for coupling out the radiation generated in the active radiation-emitting layer. Further, the first major surface of the multilayer structure is coupled to a reflective layer, and the region of the multilayer structure adjoining the second main surface of the multilayer structure is one-dimensional or two-dimensionally structured.

The increase in the external efficiency of the radiation decoupling is based on the refraction of the right-angled geometry of the thin-film semiconductor component by structuring the semiconductor thin film itself. The increase in the efficiency is demonstrated by means of simulations in the following detailed description.

The region of the multilayer structure adjoining the second main surface of the multilayer structure has convex elevations, preferably in the form of pyramid stumps, truncated cones, cones or spherical segments (two-dimensional structuring) or with a trapezoidal, triangular or circular segment cross-sectional shape (one-dimensional structuring).

The opening angle of the elevations is between about 30 ° and about 70 °, more preferably between about 40 ° and about 50 °. In addition, the height of the elevations is at least as great, preferably about twice as high, as the height of a planar region of the multilayer structure between the active, radiation-producing layer and the elevations. The grid dimension of the elevations is selected at most about five times, preferably at most about three times as large as the height of the elevations.

The layer or boundary surface coupled to the first main surface of the multilayer structure advantageously has a reflection degree of at least 70%, more preferably at least 85%.

The multilayer structure can either be applied with its first main surface directly or via a reflective layer on a carrier substrate, wherein the reflective layer or the carrier substrate simultaneously serves as a contact surface of the semiconductor component.

In order to compensate for a limited transverse conductivity of the thin semiconductor layer, a conductive, transparent layer can be applied to the second main surface of the multilayer structure.

For protection against external influences, a transparent protective layer can be applied to the second main surface of the multilayer structure.

The object of the present invention is to provide a radiation-emitting thin-film semiconductor device based on GaN, which has an improved external efficiency of the radiation decoupling.

According to the present invention, this object is achieved by a semiconductor component having the features of claim 1. Advantageous refinements and refinements of this semiconductor component are defined in the dependent claims 2 to 11.

This radiation-emitting thin-film semiconductor device according to the invention has a GaN-based multilayer structure which contains an active radiation-generating layer and has a first main surface and a second main surface facing away from the first main surface for decoupling the radiation generated in the active radiation-generating layer. The first main surface of the multilayer structure is in turn coupled to a reflecting layer. In contrast to the semiconductor component described above, a transparent layer is provided between the first main surface of the multilayer structure and the reflecting layer, which is structured one-dimensionally or two-dimensionally.

The structuring of this transparent layer between the multilayer structure and the reflective layer has the same effect as the structuring of the multilayer structure itself and likewise increases the external efficiency of the radiation decoupling.

Preferably, the transparent layer is conductive to compensate for the limited transverse conductivity of a thin multilayer structure.

The transparent layer between the first main surface of the multilayer structure and the reflecting layer or boundary surface has convex elevations, preferably in the form of pyramid stumps or truncated cones (two-dimensional structuring) or a trapezoidal cross-sectional shape (one-dimensional structuring).

In a preferred embodiment, these elevations have an opening angle between about 30 ° and about 70 °, preferably between about 40 ° and about 50 °. In this case, the height of the elevations is chosen at least as large, preferably approximately twice, as the height of a planar region of the multilayer structure between the active, radiation-generating layer and the elevations, and the grid dimension is at most five times the height Of the elevations.

The layer coupled to the first main surface of the multilayer structure has a reflectance of at least 85%.

The reflective layer can be applied to a carrier substrate or the reflective boundary surface is formed by a carrier substrate, the reflective layer or the carrier substrate at the same time serving as a contact surface of the semiconductor component.

The above and other features and advantages of the present invention will be more fully described with reference to the following detailed description with reference to the accompanying drawings. Therein:<dl id="dl0001"><dt>FIG</dt><dd>4 is a schematic illustration of a first modification of a semiconductor component in section;</dd><dt>FIGS. 2a) to c)</dt><dd>Schematic diagrams for explaining the optimum opening angle of the elevations of the semiconductor component of FIG <figref idrefs="f0001">FIG</figref>;</dd><dt>FIGS. 3a) to e)</dt><dd>Results of different simulations for the explanation of different optimal parameters of the elevations of the semiconductor component of <figref idrefs="f0001">FIG</figref>;</dd><dt>FIG</dt><dd>4 is a schematic representation of a modification of the semiconductor component of FIG <figref idrefs="f0001">FIG</figref>;</dd><dt>FIG</dt><dd>10 is a schematic illustration of an exemplary embodiment of a semiconductor component according to the present invention in section; </dd><dt>FIG</dt><dd>7 is a schematic representation of a further modification of the semiconductor component of FIG <figref idrefs="f0001">FIG</figref>;</dd><dt>FIG</dt><dd>12 is a schematic representation of a still further modification of the semiconductor component of FIG <figref idrefs="f0001">FIG</figref>; and</dd><dt>FIG</dt><dd>A highly schematic representation with respect to the radiation decoupling of conventional semiconductor components.</dd></dl>

In <figref idrefs="f0001">FIG</figref> A first modification of a thin-film semiconductor device is shown. A main component of the semiconductor device 10 is a GaN-based multi-layer structure 12 which contains an active, radiation-generating layer 14. The multilayer structure 12 is conventionally epitaxially grown and contains, as is known, a plurality of GaN-based layers.

The multilayer structure 12 has a first main surface 16 and a second main surface 18 facing away from the first main surface, the radiation generated in the active radiation-producing layer 14 being ultimately coupled out of the semiconductor component 10 through the second main surface 18. In the variation shown, the active layer 14 is positioned closer to the first main surface 16 than to the second main surface 18 of the multilayer structure 12. In the<figref idrefs="f0001">FIG</figref> However, is advantageous for the structuring of the multilayer structure described below since a thicker portion of the multilayer structure 12 is available for the structuring.

The multilayer structure 12 is applied to a carrier substrate 30, for example made of sapphire, Si or SiC, via a reflective layer 28, which preferably consists of an electrically conductive material. The reflective layer 28 can be designed, for example, as a metallic contact surface made of Ag, Al or an Ag or Al alloy or, alternatively, as a dielectric mirror coating of a plurality of dielectric layers. In an alternative modification, the multilayer structure 12 may also be applied directly to the substrate substrate 30, in which case the material of the substrate substrate 30 is selected such that the boundary surface between the multilayer structure 12 and the substrate substrate 30 is reflective.

The region of the multilayer structure 12 above the active layer 12 can, as shown in FIG <figref idrefs="f0001">FIG</figref> , Essentially into a planar region 20 which adjoins the active layer 14 and a structured region 22 which adjoins the second main surface 18. The structuring of the multilayer structure 12 takes place, for example, by means of conventional lithography and / or etching methods on the epitaxially grown semiconductor layers, by means of which groove-like recesses or depressions 24 are formed between which corresponding elevations 26 remain.

The structuring of the multilayer structure 12 can be formed either one-dimensionally, ie with depressions 24 in only one coordinate direction of the plane of the second main surface 18, or two-dimensionally, ie, with depressions 24 in two coordinate directions of the plane of the second main surface 18, The elevations 26 arising between the depressions 24 are usually convex. One-dimensional structuring, for example, with a trapezoidal (see FIG<figref idrefs="f0001">FIG</figref>), Triangular, circular segment or hemispherical cross-sectional shape and two-dimensional structures correspondingly in the form of pyramid stumps, truncated cones, cones, spherical segments or hemispheres.

In the <figref idrefs="f0001">FIG</figref> , In the form of pyramid stumps, have an opening angle α, the definition of which can also be transferred to the other mentioned forms of the elevations 26. The radiation generated in the active layer 14 is reflected several times at the interfaces of the multilayer structure 12 until the radiation finally reaches the second main surface 18 or the bottom of the recesses in the radiation decoupling cone depending on the refractive indices of the materials and the environment 24 and thus can uncouple.

As in the <figref idrefs="f0001">Figures 2a</figref>) To c), the efficiency of the radiation decoupling depends on the opening angle α of the elevations 26. Very steep flanks, as in<figref idrefs="f0001">FIG. 2a</figref>) Increase the surface area of ​​the component and are thus favorable for the radiation decoupling, but a reduction in the number of modes which can not be coupled out due to the total reflection is not achieved in this case. Likewise, the flanks of the elevations 26 should not be too shallow, as shown in FIG<figref idrefs="f0001">FIG. 2c</figref>) Since, in this case, the deviation from the plan-parallel plate is only small, and a large number of multiple reflections must be carried out until the coupling-out, which is negative because of the damping which is unavoidable.

The most favorable is in <figref idrefs="f0001">FIG. 2b</figref>), The radiation, which is totally reflected by a facet of the projection 26, can be coupled out during the impact on the next facet of the projection 26 within the radiation extraction cone, as a result of which the number Of the multiple reflections in the multilayer structure is kept small.

This estimate is also confirmed by a simulation, the result of which is shown in Fig <figref idrefs="f0002">FIG. 3a</figref>). The opening angle α of the pyramid-shaped elevations 26 is plotted on the abscissa, and the external efficiency of the radiation output is plotted on the ordinate. It is clearly seen that the best efficiency is achieved in a range of the opening angle α between about 30 ° and about 70 °, more precisely between about 40 ° and about 50 °. For values ​​of the opening angle .alpha. Above 70.degree. And below 30.degree., The efficiency of the radiation decoupling drops significantly. An opening angle α in the region of approximately 45 ° is thus to be preferred.

A further parameter which influences the external efficiency of the radiation decoupling is the height h1 of the elevations 26. In order to achieve a high efficiency, the height h1 of the elevations 26 should be at least as great as the height h2 of the planar region 20 adjoining the active layer 14 become. The elevations 26 are preferably formed twice as high as the planar region 20; A further increase in the elevations 26 does not bring about any further increase in the radiation output.

This is done by a <figref idrefs="f0002">3b</figref>) Is confirmed. For a planar region 20 with a height h2 of approximately 2 μm, the simulation result shows the external efficiency of the radiation decoupling over the height h1 of the elevations 26. At a height h1 of the elevations 26 below 2 μm, ie less than the height h2 of the planar region 20, there is only insufficient radiation decoupling, whereas at heights h1 of the elevations 26 greater than about 4 μm no significant increase in the efficiency is any longer recognizable.

Furthermore, elevations 26 with relatively small lateral dimensions are also to be preferred. As the simulation result of<figref idrefs="f0003">FIG. 3c</figref>), A grid dimension d of the elevations of at most approximately four to five times the height h1 of the elevations 26, preferably only approximately one to three times the height h1 of the elevations, is advantageous for a good efficiency.

Since the concept of thin-film semiconductor components is also based on multiple reflections, the reflectivity of the component rear side, ie the reflecting layer 28 or the reflecting boundary surface, also has an influence on the external efficiency of the semiconductor component. One can see in the diagram of FIG<figref idrefs="f0003">FIG</figref>) That in a conventional planar thin film the efficiency of the radiation decoupling depends only slightly on the reflectivity of the rear contact surface (lower curve in FIG <figref idrefs="f0003">FIG</figref>)). For a structured multilayer structure 12 as in FIG<figref idrefs="f0001">FIG</figref> The efficiency strongly depends on the reflectivity of the reflecting layer 28 or boundary surface (upper curve in FIG <figref idrefs="f0003">FIG</figref>)) And should preferably be selected above 70%, preferably above 85%.

In <figref idrefs="f0004">FIG</figref> Is a modification of the semiconductor device of FIG <figref idrefs="f0001">FIG</figref> Respectively. The difference between the two modifications is that a protective layer 32 is provided on the structured second main surface 18 of the multilayer structure 12. This protective layer 32 is intended, on the one hand, to protect the semiconductor from external influences; on the other hand, the protective layer 32 can act as an antireflective layer if the refractive index and thickness are suitably selected.

As a further variant of the semiconductor component, a transparent, conductive layer with the smallest possible transition resistance to the semiconductor can be provided on the structured second main surface 18 of the multilayer structure 12. Such a transparent, conductive layer can compensate for the disadvantage that the structuring of the multilayer structure at the same time causes a reduction in the transverse conductivity thereof to increase the efficiency of the radiation decoupling. Optimum current supply to all regions of the semiconductor device is achieved without impairing their radiation decoupling by metal contacts on the multilayer structure.

The transparent conductive layer consists, for example, of ZnO, SnO, InO, CdO, GaO or a combination thereof. These materials exhibit n- or p-conductivity and can be deposited by sputtering, CVD or evaporation.

One embodiment of a radiation-emitting semiconductor device according to the invention is shown in FIG <figref idrefs="f0005">FIG</figref> Respectively.

The thin-film semiconductor device 10 has a GaN-based multilayer structure 12 with an active, radiation-generating layer 14. In contrast to the first modification described above, however, the second main surface 18 of the multilayer structure 12, through which the radiation generated in the active layer 14 is ultimately decoupled, is not structured here, but rather, between the first main surface 16 and the reflective layer or boundary surface on the Carrier substrate 30, a transparent layer 34 is provided, which is structured to increase the radiation decoupling. This structure is particularly preferred when the metals which are well-connected to the semiconductor 12 are not particularly highly reflective and therefore better reflecting metals such as Ag are to be used,

In order to compensate for a lower transverse conductivity of the thin-film semiconductor, it is advantageous to form the transparent layer 34 of a conductive material.

The structuring essentially corresponds to that described above with reference to the first modification. However, convex elevations 26 'here are primarily those in the form of pyramid stumps or truncated cones or those with a trapezoidal cross-sectional shape. The above on the basis of it<figref idrefs="f0002 f0003 f0004">FIG</figref> Can be transferred to the elevations 26 'of this exemplary embodiment. The plane layer 35 between the active layer 14 of the multilayer structure 12 and the transparent layer 34 is to be used as the reference value.

A further alternative modification of the semiconductor device of FIG <figref idrefs="f0001">FIG</figref> Is in <figref idrefs="f0005">FIG</figref> shown. In this semiconductor device 10, the multilayer structure 12 itself is not structured, but a compensation layer 32 provided on the second main surface 18 of the multilayer structure 12 is provided with corresponding convex elevations 36.

Typical coating layers 32, for example of SiO₂ Or SiN_X, Have a refractive index of less than 2, so that the radiation is partially totally reflected at the interface between the semiconductor 12 and the compensation layer 32. As the diagram of<figref idrefs="f0004">3e</figref>), The effectiveness of the patterned coating layer 32 decreases significantly with increasing deviation of the refractive index from that of the semiconductor with 2.5. However, a structured compensation layer with a low refractive index can nevertheless be advantageous since a total reflected wave penetrates into the material with a lower refractive index up to a depth of half the wavelength, but exponentially decays. The height of the structured coating layer should therefore not be more than a few 100 nm and the lateral dimensions are in the range of micrometers.

If the lateral dimensions of the structures 36 of the compensation layer 32 are reduced to the range of the wavelength of the radiation to be coupled out, an incident wave is scattered on such a microstructure 36, thereby fanning the beam into a larger angular range.

Finally, in <figref idrefs="f0005">FIG</figref> A further modification of the semiconductor device of FIG <figref idrefs="f0001">FIG</figref> shown. A transparent, conductive layer 38 of, for example, ZnO, SnO, InO, CdO, GaO or a combination thereof is applied to the multilayer structure 12, which is not structured in this case. This transparent, conductive layer 38 is analogous to the first modification of FIG<figref idrefs="f0001">FIG</figref> Structured <figref idrefs="f0005">FIG</figref> A one-dimensional structuring with elevations with a trapezoidal cross-sectional shape are shown.

The transition resistance between the transparent, conductive layer 38 and the semiconductor 12 should be as low as possible. If this is not the case, a metal layer (not shown), which is preferably very thin and thus semitransparent or interrupted, may be required between the layer 38 and the multilayer structure 12.