Improved light extraction using feature size and shape control in led surface roughening

IMPROVED LIGHT EXTRACTION USING FEATURE SIZE AND SHAPE CONTROL IN LED SURFACE ROUGHENING

FIELD OF THE INVENTION The present invention relates to the field of semiconductor light emitting devices (LEDs), and in particular to methods for increasing the efficiency of light extraction from the surface of LEDs.

Light emitting devices generally include an active light emitting layer sandwiched between n-type and p-type semiconductor layers, and the generated light is emitted through one of these semiconductor layers. However, when light strikes the light-exit surface of the semiconductor surface, part of the light is returned toward the active layer due to the refractive index difference between the light-exit surface and air or other medium through which the light is emitted. It is reflected ('internal reflection'). Some of the reflected light may then exit the light exit surface, but some will be absorbed by the material within the light emitting device.

The light exit surface is intentionally roughened to reduce the amount of internally reflected light and thereby increase the amount of light extracted from the light emitting device. Non-planar surfaces increase the likelihood that light from the active layer propagating in various directions from the active layer will impinge on some features of the roughened surface that allow the light to exit the surface.

1A-1D illustrate a conventional process for creating a light emitting device 100 with increased light extraction efficiency.

1A illustrates the creation of a semiconductor device on a growth substrate 110 . After the n-type layer 120 is grown on the growth substrate 110 , the active layer 130 and the p-type layer 140 are grown. contact pads 150 are provided for external contact to the n-type and p-type layers; Insulation and internal components for providing this connection are not shown for simplicity of illustration. In a similar manner, layers 120 , 130 , 140 may include multiple layers of material, and other layers or vias may also be present. In another arrangement, p-type layer 140 may be grown on substrate 110 , followed by growth of active layer 130 and n-type layer 120 .

Since the contact pads 150 are typically opaque, light emitted from the active layer 130 is extracted from the surface opposite the contact pads 150 . When the growth substrate 110 is transparent, it may remain intact. Otherwise, to prevent absorption of the emitted light or to add additional scattering to the structure, the growth substrate 110 is removed to form a thin film device and light is emitted from the n-type layer 120 . 1B depicts a "top" of device 100 with contact pads 150 on the "bottom" of device 100 , commonly known in the art as a "flip-chip" embodiment. Illustrated is an exemplary conventional orientation of the light emitting device 100 after removal of the substrate 110 , with the light emitting layer 120 thereon.

As mentioned above, in order to increase the amount of light that can escape from the light exit surface 125 of the layer 120 ('light extraction efficiency') relative to the amount of internally reflected and absorbed light, a light emitting surface is The emitting surface 125 is roughened. A number of techniques can be used to roughen the surface 125 , two common techniques being photo-electrochemical (PEC) wet etching and photochemical (PC) wet etching.

As described in MICROMACHINING OF GaN USING PHOTOELECTROCHEMICAL ETCHING, A PhD Dissertation submitted to the Graduate School of the University of Notre Dame, by Bo Yang, Patrick Fay, Director, Graduate Program in Electrical Engineering, April 2005, Light from the semiconductor-electrolyte interface is absorbed by the semiconductor layers near or at the semiconductor-electrolyte interface. The generated holes drift under the influence of valence band bending towards the interface. There the holes represent broken crystal bonds and enable an etch that would not occur without illumination. The roughness of the etch is due to the uneven distribution of holes on the surface resulting in a non-uniform local etch rate. Material properties significantly affect the etch result. For example, as described in section 2.3.5 and references therein, the density of topographical features is directly related to the dislocation density of the material. Layers with higher defect potentials compared to GaN, such as AlGaN, will have higher feature densities. As a second example, by filtering the spectrum of a high intensity source, the relative etch rates of two materials with different band gaps can be modified. Finally, by adjusting the light intensity and molar concentration, the relative etch rate of layers with different defect densities can be affected.

1C conceptually illustrates the result of the roughening process on the light exit surface 125 of the light emitting device 100, and FIG. 1D provides an actual surface image of a conventional LED roughened by PEC etching. As illustrated, the roughening process produces a fairly random three-dimensional topology, which includes the composition of the material of the layers 120 being etched, as well as the concentration of the etchant used and the It depends on the parameters of the roughening process, such as type, temperature and duration of etch, applied electrical bias, etc. Conventionally, different sets of etch process parameters are tested with a particular material being etched to determine the set that provides the best light extraction efficiency for that material. Then, the best set determined is used to manufacture LEDs using this material.

<p>It would be advantageous to further increase the light extraction efficiency of the light emitting device. It would also be advantageous to further increase light extraction efficiency via roughening processes conventional in the art.</p>

<p>To better address one or more of the above concerns, in one embodiment of the present invention, structural properties of a light exit surface of a light emitting device are controlled to increase the light extraction efficiency of the surface when the surface is roughened. It has been found that luminescent surfaces comprising layers of materials with different durability to the roughening process exhibit higher light extraction efficiencies than substantially uniform luminescent surfaces exposed to the same roughening process. In a GaN-type light emitting device, a thin layer of AlGaN material on or near the light exit surface produces more distinct features after etching compared to features created by conventionally etching a surface comprising only GaN material .</p>

The invention will be described in more detail by way of example with reference to the accompanying drawings in which: 1A-1D illustrate exemplary conventional processes for creating a light emitting device having a roughened surface that increases light extraction efficiency. 2A-2D illustrate an exemplary process for creating a light emitting device having a composition of a roughened surface that further increases light extraction efficiency. 3 illustrates an exemplary flow diagram for creating a light emitting device having a composition of a roughened surface that further increases light extraction efficiency. Throughout the drawings, like reference numbers indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the present invention.

In the description that follows, for purposes of explanation rather than limitation, specific details are set forth, such as specific architectures, interfaces, techniques, etc., to provide a thorough understanding of the inventive concepts. However, it will be apparent to one skilled in the art that the invention may be practiced in other embodiments that depart from these specific details. In a similar manner, the text of this description is directed to the exemplary embodiments illustrated in the drawings and is not intended to limit the claimed invention beyond the limitations expressly included in the claims. For purposes of brevity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention in unnecessary detail.

For ease of illustration and understanding, although one of ordinary skill in the art, in view of the present disclosure, will clearly recognize that the principles of the present invention are not limited to the use of GaN semiconductors, the present invention relates to light emitting devices comprising GaN semiconductor layers. presented in context. In a similar manner, the present invention is presented in the context of a flip-chip light emitting device, although the present invention is particularly suitable for producing semiconductor light emitting devices with high light extraction efficiency, the principles of the present invention do not rely on flip-chips. It will be apparent to those skilled in the art that it does not necessarily rely on the use of a semiconductor light emitter as a light source. That is, any application that relies on improving light extraction efficiency by roughening the light exit surface may benefit from using the principles presented in this application, the degree of benefit being dependent on the nature of the light emitted by the light source; For example, it depends on the wavelength, the degree of collimation, and the composition of the light exit surface.

2A-2D are exemplary for creating a light emitting device 200 having a composition of a roughened epitaxial surface that further increases the light extraction efficiency compared to the light extraction efficiency of the conventional light emitting device 100 of FIGS. 1A-1D . The process is illustrated.

In embodiments of the present invention, the roughened surface comprises a mixture of materials having material properties that respond differently to the roughening process. Properties may include, for example, material composition (stoichiometry), crystal defect density, type of crystal defects, carrier concentration, epitaxial stress, and the like. These properties can be changed by controlling the deposition conditions. For example, as described in "Organometallic Vapor-Phase Epitaxi, Second Edition: Theory and Practice" by GBStringfellow, defect density increases as the growth temperature decreases below the optimum. The influence of layer properties on the obtained topology will generally depend on the parameters of the etch or other roughening process.

In the exemplary light emitting device 200 , an epitaxial 'template' layer 240 is formed over which a light emitting stack (n-type layer 120 , active layer 130 and p-type layer 140 ) is grown. provide a surface. As described in further detail below, epitaxial layer 240 is added to provide etch properties different from those of n-type layer 120 . A thin 'starter' layer 230, sometimes referred to as a growth initiation layer, is grown on the substrate 110, depending on the particular technique used to form/grow the device 200. A suitable seeding film for initiating epitaxial growth can be provided.

After the substrate 110 is removed, the device 200 is subjected to a roughening process, which may include a conventional KOH-based PEC etch, as illustrated in FIG. 2B . When conventionally etching a light emitting surface, it is generally necessary to test multiple sets of etch process parameters to determine an etch recipe to achieve optimal roughening. Optimal etch parameters will depend on the particular wavelength of the emitted light and the particular materials used to form the epitaxial layer 240 and the n-type layer 120 . In the example device 200 , the etch process parameters are such that the etch extends through a portion of the epitaxial layer 240 to the n-type layer 120 , as illustrated in FIG. 2C .

In the example of a GaN light emitting device, epitaxial layer 240 is Al_xGa₁_-_xa thin layer of N, where x is the fractional amount of Al in the layer relative to the amount of Ga. Due to the different compositions of the epitaxial layer (AlGaN) 240 and the n-type layer (GaN) 120 , the etching process will have different effects on these layers 240 , 120 .

Again in the example of FIGS. 1C and 1D , conventional etching of the GaN surface results in a topology with relatively flat peaks on most features. That is, as the etch extends downward to form "valleys" between the "hills," the edges and tops of these hills are also worn by the etch.

In contrast, etching of AlGaN will generally produce a topology with sharper edges, due to the presence of Al in the structure. Additionally, once the etch is extended with the less durable GaN layer, the time required to etch the GaN layer may not be long enough to wear the edges and tops of the AlGaN peaks. For purposes of the present invention, if features in a first topology exhibit more points or edges than features in a second topology or exhibit less curvature in profile than features in a second topology, then the first topology is 'Sharper' than the second topology. Additionally, the size and mutual isolation of the rough features can be controlled by controlling the composition and growth conditions for the epitaxial AlGaN layer 240 .

FIG. 2C provides a conceptual illustration of the result of etching a two-layer structure in which the etched first layer 240 is more durable to the etch process than the second layer 120 . When the optional starter layer 230 is used, it may be removed using a wet etching process or another suitable process such as dry etching. More generally, a dry etch step can be used to place the starting point for the PEC etch in the most advantageous position. A timed or endpointed dry etch using industry standard dry etch equipment can be used to accurately create a starting point for a PEC etch. In the exemplary AlN/AlGaN/GaN layer stack produced when using AlN as the starter layer 230, Cl₂, BCl₃ Or a chlorine-comprising plasma (dry) etch chemistry, such as mixtures thereof, can be used to achieve this purpose. Inert gases such as argon or helium can be added to the plasma chemistry to precisely control the etch process and improve etch process performance metrics such as etch rate, selectivity and uniformity. Additional process parameters for controlling the dry etch process include pressure, flow rates and ratios of the constituent gases, microwave or radio frequency power input or inputs to the dry etch chamber, wafer or package temperature, and ceramic or conductive shields and uniformity- chamber hardware components such as trituration rings.

As illustrated, the top tips of the surface 225 of the topological structure of FIG. 2C (and exemplary FIG. 2D ) of device 200 after etching are removed from device 100 of FIG. 1C (and exemplary FIG. 1D ). It shows the roughened geometry of the epitaxial layer 240 , which is significantly 'sharper' than the top tips of the topology of . As also illustrated in FIG. 1C , the topology of features formed in epitaxial layer 240 is substantially sharper than the topology of features formed in N-type layer 120 . That is, in profile, the shape of the upper tips (hills) of the features formed in the epitaxial layer 240 is substantially less curved than the shape of the features (valleys) formed in the N-type layer 120 . The size of these rough features plays an important role in the extraction efficiency, as the extraction efficiency varies with the wavelength of light emitted for a particular feature geometry and size. By adding the epitaxial layer 240, it is possible to optimize the rough surface by feature sizing for high light extraction efficiency over a wide range of wavelengths.

The taper angle and feature size of the roughened surface 225 is Al_xGa₁_-_xIt will depend on the fractional ratio of Al in the N layer 240 as well as the material quality of the epitaxial layer 240 . To provide sharper and more densely packed features and correspondingly increase light extraction efficiency, a fractional ratio (x) of Al between 0.3 and 0.8, and in some embodiments between 0.5 and 0.7, will be effective.

The refractive index of the AlGaN layer 240 is lower than that of the GaN layer 120 ; To prevent internal reflection, the thickness of AlGaN may be less than 50 nm. Alternatively, all of the AlGaN layer 240 except 50 nm or less may be removed by using a dry etching step prior to the PEC etching.

2d shows Al over a conventional GaN n-type layer.₀_.6Ga₀_.4The image of the actual surface after KOH-PEC etching of the surface of the device containing the N epitaxial layer is illustrated. In contrast to the image of FIG. 1d , it is evident that the peaks of each of the features/hills are sharper and more pronounced. The presence of Al in the epitaxial layer (AlGaN) makes the taper angle of the rough features sharper, and the flatness of the rough surface is reduced compared to the conventional GaN roughness illustrated in FIG. 1D . By sharpening these rough features, the light from the LED chip is more efficiently coupled, increasing the light extraction efficiency.

3 illustrates an exemplary flow diagram for manufacturing a light emitting device having a roughened surface that provides increased light extraction efficiency. In this example, it is assumed that the semiconductor material of the light emitting device is GaN, although those skilled in the art will recognize that the principles presented herein will apply to a variety of materials.

Optionally, in step 310, a thin starter layer of GaN or AlGaN or AlN may be formed on the growth substrate. For example, if Si is a growth substrate, an AlN layer will be formed to separate the substrate and control the strain.

In step 320, an AlGaN layer is formed, preferably with a thickness of less than 500 nm.

In steps 330-350, a light emitting device is formed using conventional techniques. In step 330, an N-type GaN layer is formed; In step 340, an active layer is formed; In step 340, a P-type GaN layer, if formed, is formed.

In step 360, contact pads are formed during a device manufacturing process commonly known in the art as a "wafer fab" to facilitate external connections to the N and P type layers. Insulating vias can be used to connect the N-type layer through the active layer and the P-type layer so that in this example there are two contact pads on the same surface of the device, opposite the light emitting surface, which will be the surface of the AlGaN layer. . These contact pads facilitate mounting the device onto a printed circuit board or lighting fixture in a 'flip-chip' configuration.

The growth substrate is removed in step 370 , and the exposed surface is roughened in step 380 . The roughening can consist of a one-step process of PEC etching, the optional starter layer can be removed and extended through the AlGaN layer to the N-type GaN layer. Alternatively, a multi-step process combining dry and wet etching processes is used to remove undesirable layers such as the initial AlN, and the second etching step, PET etching, provides the most distinct features. can stop on the layers of the structure when

In step 390, the device comprises, for example, forming a wavelength converting (phosphor) layer on the roughened surface and/or encapsulating the light emitting device with a transparent material such as glass or an epoxy dome. can be further processed.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be regarded as illustrative or exemplary and not restrictive; The invention is not limited to the disclosed embodiments. For example, it is possible to carry out the invention in embodiments known in the art as "vertical thin film" LEDs made on the side where the n-contact to the LED device is roughened. The n-contact can be made before or after the roughening step in the process flow.

Additionally, since the increased light extraction efficiency is a result of different roughening topologies, caused by using layers with different durability to the roughening process, the principles of the present invention are limited to using only two layers with different durability. and is not limited to a specific arrangement of different layers.

Other modifications to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the term "comprising" does not exclude other elements or steps, and "a, an" does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.