Electroluminescent devices with color adjustment based on current crowding

A lighting system provides a system optical output, such as white light, as a function of an applied electrical signal. The system output can be characterized by a color temperature or other measure that represents the color or output spectrum of the output. The system is designed so that the color temperature changes as a function of the applied electrical signal. The changes in color temperature are at least in part a result of a phenomenon known as “current crowding”.

Reference is made to the following pending and/or commonly filed U.S. patent applications, the features of which can be incorporated into the embodiments presently disclosed: U.S. application Ser. No. 61/175,640, “Re-Emitting Semiconductor Construction With Enhanced Extraction Efficiency”, filed May 5, 2009; U.S. application Ser. No. 61/175,632, “Semiconductor Devices Grown on Indium-Containing Substrates Utilizing Indium Depletion Mechanisms”, filed May 5, 2009; U.S. application Ser. No. 61/175,636, “Re-Emitting Semiconductor Carrier Devices For Use With LEDs and Methods of Manufacture”, filed May 5, 2009; and U.S. application Ser. No. 61/221,660, “White Light Electroluminescent Devices With Adjustable Color Temperature”, filed on even date herewith.

FIELD OF THE INVENTION

This invention relates generally to solid state semiconductor light sources.

BACKGROUND

A wide variety of semiconductor devices, and methods of making semiconductor devices, are known. Some of these devices are designed to emit light, such as visible or near-visible (e.g. ultraviolet or near infrared) light. Examples include electroluminescent devices such as light emitting diodes (LEDs) and laser diodes, wherein an electrical drive current or similar electrical signal is applied to the device so that it emits light. Another example of a semiconductor device designed to emit light is a re-emitting semiconductor construction (RSC).

Unlike an LED, an RSC does not require an electrical drive current from an external electronic circuit in order to emit light. Instead, the RSC generates electron-hole pairs by absorption of light at a first wavelength λ1in an active region of the RSC. These electrons and holes then recombine in potential wells in the active region to emit light at a second wavelength λ2different from the first wavelength λ1, and optionally at still other wavelengths λ2, λ3, and so forth depending on the number of potential wells and their design features. The initiating radiation or “pump light” at the first wavelength λ1is typically provided by a blue, violet, or ultraviolet emitting LED coupled to the RSC. Exemplary RSC devices, methods of their construction, and related devices and methods can be found in, e.g., U.S. Pat. No. 7,402,831 (Miller et al.), U.S. Patent Application Publications US 2007/0284565 (Leatherdale et al.) and US 2007/0290190 (Haase et al.), PCT Publication WO 2009/048704 (Kelley et al.), and pending U.S. Application Ser. No. 61/075,918, “Semiconductor Light Converting Construction” (Attorney Docket No. 64395US002), filed Jun. 26, 2008, all of which are incorporated herein by reference.

When reference is made herein to a light at a particular wavelength, the reader will understand that reference is being made to light having a spectrum whose peak wavelength is at the particular wavelength.

Of particular interest to the present application are light sources that are capable of emitting white light. In some cases, known white light sources are constructed by combining an electroluminescent device such as a blue-emitting LED with first and second RSC-based luminescent elements. The first luminescent element may, for example, include a green-emitting potential well that converts some of the blue light to green light, and transmits the remainder of the blue light. The second luminescent element may include a potential well that converts some of the green and/or blue light it receives from the first luminescent element into red light, and transmits the remainder of the blue and green light. The resulting red, green, and blue light components combine to allow such a device, which is described (among other embodiments) in WO 2008/109296 (Haase), to provide substantially white light output.

Other known white light sources are constructed by combining a blue-emitting LED with a layer of yellow phosphor, such as cerium-doped yttrium aluminum garnet (YAG:Ce). Some of the blue light is absorbed by the phosphor and re-emitted as yellow light, and some of the blue light passes through the phosphor layer. The transmitted blue light combines with the re-emitted yellow light to produce an output beam having an overall output spectrum that is perceived as nominally white light.

Device-to-device variations in phosphor layer characteristics and/or other design details give rise to device-to-device differences in the output spectrum and corresponding differences in perceived color, with some LED/phosphor devices providing a “cool” white color and others providing a “warm” white color, for example. A given “shade” of white may be plotted as an (x,y) color coordinate on a conventional CIE chromaticity diagram, and can be characterized by a color temperature as is known by those skilled in the art. U.S. Pat. No. 7,387,405 (Ducharme et al.) discusses some of these aspects of LED/phosphor devices, and reports that some commercial LED/phosphor devices exhibit color temperatures of 20,000 degrees Kelvin (20,000K) while others exhibit color temperatures of 5750K. The '405 patent also reports that a single one of these LED/phosphor devices allows for no control of color temperature, and that a system with a desired range of color temperature cannot be generated with one device alone. The '405 patent goes on to describe an embodiment in which two such LED/phosphor devices are combined with an optical long-pass filter (a transparent piece of glass or plastic tinted so as to enable only longer wavelength light to pass through) that shifts the color temperature of the devices, and then a specific third LED (an Agilent HLMP-EL 18 amber LED) is added to these filtered LED/phosphor devices to provide a 3-LED embodiment with adjustable color temperature.

BRIEF SUMMARY

The present application discloses, inter alia, lighting systems that provide a system optical output as a function of an applied electrical signal. The system output, which in exemplary embodiments is or comprises white light, can be characterized by a color temperature or by any other suitable measure that represents in some fashion the color or output spectrum of the system optical output. Desirably, the lighting system is designed so that the color temperature changes as a function of the applied electrical signal. In exemplary embodiments, these changes in color temperature are at least in part a result of a phenomenon known as “current crowding”, which is normally considered to be undesirable in a solid state lighting device, and is described further below.

In exemplary embodiments, the system includes an electroluminescent device and a first light modifying material, such as an RSC or phosphor. The electroluminescent device is adapted to emit light in response to the applied electrical signal. The first light modifying material is adapted to modify a first portion of the emitted light to provide a first light component. The lighting system combines the first light component with at least a second light component associated with a second portion of the emitted light, to produce the system optical output. The system may be characterized by a relative proportion of the first to the second light component, and the changes in color temperature may be associated with changes in the relative proportion.

In further exemplary embodiments, the emitted light is emitted from an output surface of the electroluminescent device, and the electroluminescent device is characterized in that a spatial distribution of the emitted light over the output surface changes as a function of the applied electrical signal, the changes in the spatial distribution being at least in part as a result of current crowding. The system may also include a second light modifying material that modifies the second portion of the emitted light to provide the second light component, the second light component having a second spectrum different from the first spectrum and from the emitted light spectrum. The first light modifying material may cover a first portion of the output surface, and the second light modifying material may cover a second portion of the output surface. For example, the electroluminescent device may include an electrical contact disposed on the output surface, and the first portion of the output surface may be disposed proximate the electrical contact, whereas the second portion of the output surface may be spaced apart from the electrical contact, for example.

Also disclosed are lighting systems that include an electroluminescent device and a first light converting material. The electroluminescent device is adapted to emit light from an output surface in response to an applied electrical current, the electroluminescent device being characterized in that a spatial distribution of the emitted light over the output surface changes as a function of the electrical current at least in part as a result of current crowding. The first light converting material covers a first portion of the output surface, and is adapted to convert a first portion of the emitted light to a first light component. The first light component combines with at least a second light component to provide a system output, and the second light component is associated with a second portion of the emitted light. The first light converting material is spatially distributed such that the changes in the spatial distribution of the emitted light over the output surface produce changes in a color of the system optical output.

In some cases, the second light component may not be associated with any light converting material. For example, the second light component may simply be or comprise the second portion of the emitted light, which may be emitted from a second portion of the output surface. Alternatively, the system may include a second light converting material that covers the second portion of the output surface, and that converts the second portion of the emitted light to the second light component. Note that in addition to including the first and second light components, the system optical output may also include other components such as light that is emitted by the electroluminescent device but not converted to other light by any light converting material. The system optical output may be or include white light, for example, and the changes in color may comprise changes in a color temperature of the system optical output.

Related methods, systems, and articles are also discussed.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Optoelectronic device manufacturers consider the phenomenon known as “current crowding” to be a problem that should be avoided, since it is generally associated with reduced quantum efficiency. See e.g. U.S. Pat. No. 7,078,319 (Eliashevich et al.). However, at least some of the lighting systems described herein are intentionally designed to exhibit current crowding, and to take advantage of it in order to produce color changes as a function of an applied electrical signal. Notwithstanding this, we emphasize that we do not wish to be bound by theory, and that any lighting system that is capable of changing a color of its system optical output based on an applied electrical signal, in a manner that is the same as or similar to any of the embodiments described herein, is intended to be encompassed by the present application even if such a lighting system does not in fact exhibit current crowding.

Referring now toFIG. 1a, we see there a schematic sectional view of a semiconductor electroluminescent device110such as an LED, which is reproduced inFIGS. 1band1c. The reader will understand that the layer construction of the device110is depicted only schematically for purposes of illustration, that the constituent elements of the device are not necessarily drawn to scale, and that additional elements may be included or illustrated elements may be omitted or modified as desired. As shown, the device110includes a semiconductor base layer112, a light-emitting layer114, and a current spreading layer116. The base layer112may be or comprise, for example, p-type GaN, or other suitable semiconductor materials. Light-emitting layer114may be or comprise an active layer sandwiched between a p-type cladding layer and an n-type cladding layer (not shown), each of which may comprise, for example, AlGaInN-based materials, or other suitable semiconductor materials. Current spreading layer116may be or comprise, for example, an n-type semiconductor such as GaN-based material that is doped with a concentration of n-type dopant such as Si, or other suitable semiconductor materials.

Also provided are first and second electrodes118a,118bthat make ohmic contact with the respective outer semiconductor layers so that an electrical drive current or similar electrical signal supplied by an external electronic circuit can be applied to the device such that it emits light. The device110may be made with an asymmetrical design, as shown, in order to emit light preferentially from one of its major surfaces, e.g. outer surface116aof layer116. For this reason, first electrode118acan be made to cover only a portion of surface116a, whereas second electrode118bcan be made to cover substantially all of the opposite major surface112a. If the second electrode118bis at least partially reflective, e.g. metallic, then some light generated within the device110that would otherwise escape via major surface112acan be reflected by the electrode118bso that it escapes via the side surfaces or the major surface116aof the device. Of course, semiconductor electroluminescent devices that may be used in the disclosed embodiments need not be asymmetric in the forward/backward characteristic of their light emission, or in the arrangement of their electrodes.

FIGS. 1band1cshow in a qualitative, schematic fashion the behavior of the device110when an electrical signal is applied across the electrodes. InFIG. 1b, a first signal having a first magnitude is applied. InFIG. 1c, a second signal having a second magnitude is applied. The signal magnitudes may be characterized as desired, but, in view of the I-V (current-voltage) characteristics of most semiconductor diode devices, it is most logical to characterize the signal magnitude in terms of electric current. Alternatively, one can characterize the signal magnitude in terms of electric potential (voltage). In any case, the electric current distribution from electrode118athrough the interior of the device110to the other electrode118bis depicted by arrows120for the first signal (FIG. 1b), and by arrows122for the second signal (FIG. 1c).

Recalling that the respective signals are being applied to the very same device110, the difference in current distributions exemplified by the different width or lateral dimension of the pattern of arrows120compared to that of arrows122is a result of the different magnitudes of the respective signals and the “current crowding” phenomenon. As is apparent from the figures, current crowding results in a narrower or more concentrated current distribution for the second signal (FIG. 1c) compared to the first signal (FIG. 1b). This current crowding phenomenon typically occurs at high current densities. That is, the second signal would typically have a greater signal magnitude, measured in electric current, than the first signal. In the case of device110, the current crowding may be the result of the p-n junction having a decreased electrical resistance at the higher current levels. The decreased resistance tends to cause the current to flow in a more direct path from electrode118ato electrode118b, producing current “bunching” under the electrode118acompared to the current distribution ofFIG. 1b.

The change in current distribution within the device for different signal magnitudes also has an effect on the spatial distribution of light generated by the device110. This is because the light emitting layer114will generate light only to the extent electric current is flowing through it. Portions of the light emitting layer in which the current density is low will generate less light than portions in which the current density is high, within limits. By specifying a cutoff value of generated light per unit volume, or per unit projected area (e.g., when viewing the device along an axis perpendicular to the major surface116a), one can define a high-intensity region of the light emitting layer114. The cutoff value may be, for example, a fraction of a reference light emission level, where the reference level may be a maximum light emission, or a spatially averaged light generation over the entire light emitting layer, and where the fraction may be selected as desired such as ½, 1/10th, or 1/e of the reference value. Whichever of these parameters one selects, one can use the resulting cutoff value to provide a consistent way to characterize the spatial extent of the higher-intensity region of the light emitting layer. The higher-intensity region is identified by label124inFIG. 1band by label126inFIG. 1c. As one would expect, the region124has a greater width or lateral dimension than region126. This may be the case even though region124may have greatly reduced overall light emission levels compared to region126, in view of the (typically) substantially greater electric current level used to produce region126compared to region124.

The differences in the lateral dimension or other spatial extent of the regions124,126give rise to corresponding differences in the spatial distribution of light emitted from the outer surface116aof the device. Thus, the relatively wide region124gives rise to a relatively wide spatial distribution of light emitted from the surface116a, compared to the relatively narrow region126, which gives rise to a narrower spatial distribution of light emitted from the surface116a. Stated differently, the spatial distribution of light emitted from surface116ausing the second signal is more concentrated towards the electrode118athan that of the first signal. Just as with the regions124,126, the spatial distribution of light emitted from the surface116amay be characterized or measured in relative terms, e.g., as a fraction or percentage of a maximum or average value, for example. Thus, assuming the second signal has a greater magnitude than the first signal, differences in their respective spatial distributions of emitted light from the surface116aare independent of the fact that the overall amount of light emitted from the surface116amay be greater for the second signal than for the first signal.

In large part, the degree of current crowding may be controlled by the sheet resistivity of the current-spreading layer. Mathematical models of such current crowding effects may be found in Chapter 8 ofLight-Emitting Diodes, Second Edition, by E. Fred Schubert (Cambridge University Press).

Having now described the current crowding phenomenon, or certain aspects thereof, and its effect on the spatial distribution of light emitted from the surface of an electroluminescent device, we now go on to describe how this can be used to provide lighting systems whose optical output, e.g. white light, can be made to change color, e.g. color temperature, based on an applied electrical signal.

A lighting system210is shown in a schematic top view inFIG. 2aand in a schematic sectional view inFIG. 2b. The system comprises an electroluminescent device212adapted to emit light in response to an applied electrical signal. The electroluminescent device212includes a main body214to which first and second electrodes216a,216bhave been applied at outer surfaces thereof. The main body214is drawn schematically, and may comprise any suitable stack of semiconductor layers (not shown individually) such as would be found in a semiconductor LED, for example. The main body may be in the form of an individual chip or die, as shown, or in the form of an entire semiconductor wafer prior to dicing. Significantly, the layers that make up the body214are designed so that the device212exhibits the current crowding phenomenon, or so that it otherwise exhibits a spatial distribution of emitted light that substantially changes as a function of a magnitude of the applied electrical signal. The electrical signal is applied, of course, across the electrodes216a,216b, where a thin wire218may connect to the electrode216avia a wire bond to help convey the signal from an external electrical driver or source to the device212. The light generated within the device212may escape or be emitted via the relatively small side surfaces or via the major surface214a, to which the first electrode216ais applied. Such emitted light may be referred to as pump light, and is labeled λpin the figure. The pump light may be or comprise blue, violet, or ultraviolet light, e.g., with a peak wavelength in a range from 350 to 500 nm, but other spectral characteristics of the pump light are also contemplated.

As a result of the design of the electroluminescent device, a spatial distribution of pump light λpthat is emitted from the surface214asubstantially changes as a function of a magnitude of the applied electrical signal. For example, a first electrical signal having a relatively small magnitude may provide pump light whose spatial distribution as emitted from the surface214ais relatively uniform, and a second electrical signal having a larger magnitude may provide pump light whose spatial distribution as emitted from the surface214ais concentrated in a smaller area such as an annulus surrounding the electrode216a.

In addition to the electroluminescent device212, the system210also includes first and second light modifying materials220,222. These materials may convert at least some of the pump light striking them into light components of other wavelengths. For example, first light modifying material220may be or comprise a phosphor that converts at least some of the pump light λpstriking it into a first light component at another wavelength λ1, typically longer than a wavelength of the pump light. The second light modifying material may be or comprise an RSC having at least one potential well that converts at least some of the pump light λpstriking it into a second light component at another wavelength λ2different from λ1, and also typically longer than a wavelength of the pump light. One or both of light modifying materials220,222may also be partially transparent to the pump light, such that light propagating out of first light modifying material220may comprise a combination or mixture of the first light component at wavelength λ1and the pump light λp, and/or light propagating out of second light modifying material222may comprise a combination or mixture of the second light component at wavelength λ2and the pump light λp. Alternatively, both light modifying materials220,222may be substantially opaque to the pump light λp. In any case, light propagating out of the first light modifying material220may be characterized by a spectrum S1, which includes at least the first light component at wavelength λ1and may also include the pump light, whereas light propagating out of the second light modifying material222may be characterized by a different spectrum S2, which includes at least the second light component at wavelength λ2and may also include the pump light.

The light characterized by the spectra S1and S2combine, whether by free space propagation or via mechanisms such as optical diffusers, lenses, minors, or the like, and optionally with other light components, to produce a system optical output of lighting system210represented schematically by arrow230. The system optical output230thus includes some amount of the light of the first spectrum S1and some amount of the light of the second spectrum S2. Stated differently, the system optical output230includes some amount of the first light component at wavelength λ1and some amount of the second light component at wavelength λ2. The different spectra S1, S2are associated with different perceived colors, and their combination (optionally with other light components) produces yet another perceived color for the system optical output230. The perceived color (e.g. a color temperature) of the system optical output230can thus be adjusted or changed by changing the relative amounts of the first and second light components that are included in the system optical output230.

In view of the current-crowding characteristics of the electroluminescent device212described above, such a change in the relative amounts of light provided by the first and second light modifying materials can be achieved by simply changing the magnitude of the applied electrical signal, provided the first and second light modifying materials220,222are spatially distributed in a way that is synergistic with the changes in the spatial distribution of the pump light that is emitted from the surface214a. For example, if a change in the applied electrical signal results in more pump light emitted from a first portion of the surface relative to a second portion, and if the first light modifying material is disposed at the first portion while the second light modifying material is disposed at the second portion, then the change in the applied electrical signal may provide relatively more of the first light component (or of the light of spectrum S1) and relatively less of the second light component (or of the light of spectrum S2) in the system optical output230, thus changing the perceived color of the system optical output.

In the embodiment ofFIGS. 2a-b, the first light modifying material220is disposed in a circle or an effective annulus proximate the electrode220. The second light modifying material222is disposed on the remainder of the surface214a, i.e., spaced apart from the electrode220. This spatial arrangement of light modifying materials220,222is synergistic with the changes in the spatial distribution of the pump light resulting from the current crowding phenomenon. At low electrical current levels, a relatively uniform distribution of pump light is emitted from surface214a, which produces an initial or baseline proportion of the light of spectrum S1to the light of spectrum S2in the system optical output. At a higher electrical current, at which current crowding confines the spatial distribution of emitted pump light to a smaller area such as an annulus surrounding the electrode216a, the system optical output230will include relatively more of the light of spectrum S1and relatively less of the light of spectrum S2, thus yielding a different proportion of such light components and a different perceived color of the system optical output230. The first and second light modifying materials may be selected as appropriate so that the change in color of the system optical output for an increase in magnitude of the applied electrical signal corresponds to an increase in color temperature of a nominally white light output. Alternatively, the light modifying materials may be selected to produce an opposite effect, wherein the change in color of the system optical output for an increase in magnitude of the applied electrical signal corresponds to a decrease in color temperature of a nominally white light output.

In alternative embodiments to that shown inFIGS. 2a-b, the first light modifying material220may be omitted, or the second light modifying material222may be omitted, such that only one light modifying material is included in the lighting system. Thus, for example, if the first light modifying material220is omitted, then the light of spectrum S1may include only pump light of wavelength λpthat is emitted from the annular region of the surface214asurrounding the electrode216a. Alternatively, if the second light modifying material222is omitted, then the light of spectrum S2may include only pump light of wavelength λpthat is emitted from the portion of surface214aoutside of such annular region.

In still other alternative embodiments, additional light modifying materials such as a third, fourth, etc. light modifying material, may be provided and spatially arranged with the other light modifying materials in patterns that are synergistic with the changes in the spatial distribution of the pump light resulting from the current crowding phenomenon, so that a color of the system optical output changes based on a magnitude of the applied electrical signal. Note that a given light modifying material may convert pump light at wavelength λpto a light component that is characterized by only a single peak (having e.g. a Gaussian or bell-shaped spectral distribution) at one peak wavelength, or to a light component that is characterized by multiple peaks at multiple peak wavelengths, depending on the design of the light modifying material. An RSC, for example, may comprise only one potential well, or may comprise multiple potential wells of the same or similar design, such that the spectrum of the converted light is characterized by only a single peak. Alternatively, an RSC may comprise multiple potential wells of substantially different design, such that the spectrum of the converted light is characterized by a plurality of peaks.

Those skilled in the art will be familiar with a tool or standard used to characterize and quantify perceived colors, in particular, the well-known 1931 CIE chromaticity diagram, promulgated by the Commission International de l'Eclairage (International Commission on Lighting) or “CIE”. The color (or “chromaticity” or “chromaticity coordinates”) of a light source or article can be precisely measured or specified by a point or region expressed in terms of one or more chromaticity coordinates (x,y) on the CIE chromaticity diagram, using the CIE 1931 standard colorimetric system.

Such a chromaticity diagram is shown inFIG. 3. Those skilled in the art will recognize curve310as the “Planckian locus”, i.e., the color of an ideal blackbody source over a range of temperatures measured in degrees Kelvin, which temperature is referred to as “color temperature” Tc.

Points312and314represent the color coordinates of the light of spectrum S1and the light of spectrum S2, respectively, for one embodiment of the lighting system210ofFIGS. 2a-b. The spectra of these points are plotted as a function of optical wavelength inFIGS. 3aand3b, respectively. That is,FIG. 3aplots the spectrum S1corresponding to point312inFIG. 3, andFIG. 3bplots the spectrum S2corresponding to point314inFIG. 3. Spectrum S1has two main components: a relatively narrow spectral peak S1acorresponding to pump light at wavelength λpthat is transmitted through first light modifying material220, and a broader spectral peak S1bcorresponding to the first light component at wavelength λ1produced by the first light modifying material, e.g., YAG:Ce phosphor. Spectrum S2also has two main components: a relatively narrow first spectral peak S2aand a relatively narrow second spectral peak S2b. These two peaks can be produced by a blue-LED-pumped RSC that has at least one potential well capable of converting the blue pump light to light at the first peak S2aand at least another potential well capable of converting the pump light to light at the second peak S2b. Note by the absence of any peak at the pump wavelength λpin spectrum S2that the second light converting material, i.e., the RSC, absorbs or otherwise effectively blocks all pump light of wavelength λpincident upon it in this particular embodiment.

The line segment316inFIG. 3, whose endpoints are points312and314, represents the set of all possible lighting system optical outputs for systems whose outputs are composed of a linear combination of light of spectrum S1and light of spectrum S2. Thus, for example, a lighting system whose optical output is composed of equal parts of the light of spectrum S1and light of spectrum S2is represented by a point that bisects line segment316. If the proportion of the light of spectrum51is increased, the system point moves along line segment316towards point312. If instead the proportion of the light of spectrum S2is increased, the system point moves along line segment316towards point314.

By judicious selection of the position (color) of points312and314, the line segment316can be made to closely approximate a portion of the Planckian locus310, e.g., the portion of the locus310over a range of color temperatures from 2500K to 5000K, or from 3000K to 5000K, for example. In such a case, a system point on line segment316that moves towards point312corresponds to a color shift towards higher color temperatures, or a “cooler” (higher blue content) white light source. If instead a system point moves towards point314, it corresponds to a color shift towards lower color temperatures, or a “warmer” (higher red content) light source. Note that the lighting system210ofFIGS. 2a-b, utilizing the light modifying materials described in connection withFIGS. 3,3a, and3b, generates a system optical output that shifts from lower to higher color temperatures (along the line segment316towards point312) as the magnitude of the applied electrical signal is increased.

The particular shapes of the spectra S1and S2, when plotted as a function of wavelength, not only determine the positions of their respective points312,314on the CIE chromaticity diagram, but also determine a characteristic known as the “color rendering index” of the resulting system light. The color rendering index (CRI) is a parameter that may be important to a lighting system designer if the designer is concerned not only with the appearance or color of the system optical output as it is perceived by direct observation with the eye, but also with the appearance of objects or articles that are viewed for example in reflected light using the system optical output. Depending on the reflectivity spectrum of the objects or articles, their appearance may be very different when illuminated with a first nominally white light source than when illuminated with a second nominally white light source, even though the first and second white light sources may have identical color coordinates on the CIE chromaticity diagram. This is a consequence of the fact that a particular color coordinate on the CIE chromaticity diagram may be associated with numerous optical spectra that may differ substantially from each other. A common illustration demonstrating the effect of color rendering is the sometimes very different appearance that colored objects have when illuminated with sunlight as compared to illumination with a fluorescent office lights for example, or as compared to illumination with a gas discharge street lamp, even though all of these illumination sources may appear to be nominally white when viewed directly.

The color rendering index of a given source can be measured using the method described in the CIE publication 13.3-1995, “Method of Measuring and Specifying Colour Rendering Properties of Light Sources”. The color rendering index in general ranges from a low of 0 to a high of 100, with higher values generally being desirable. Furthermore, numerical techniques and software are available from the CIE, that are capable of calculating the color rendering index of a given spectrum representing a given light source, based on the CIE 13.3-1995 publication.

When such software is used to calculate the color rendering index of system optical outputs composed of a linear combination of the spectra S1and S2shown inFIGS. 3aand3b, the result is a color rendering index of at least 80 over a color temperature range (corresponding to different proportions of the spectra S1and S2) from 2500K to 5000K. In exemplary embodiments, the color rendering index is at least 60, or at least 70, or at least 80, over a color temperature range from 2500K to 5000K, or from 3000K to 5000K, for example. In order to achieve high color rendering index values, it is desirable to ensure that each of the constituent spectra (S1, S2) that make up the system optical output is characterized by at least two distinct spectral peaks, e.g. the peaks S1a, S1bofFIG. 3aor the peaks S2a, S2bofFIG. 3b, which peaks may be separated from each other by at least 10 nm, for example. Further reference in this regard is made to commonly filed U.S. Application 61/221,660, “White Light Electroluminescent Devices With Adjustable Color Temperature” (Attorney Docket No. 65330US002), which is incorporated herein by reference.

Turning now toFIG. 4, we see there a schematic sectional view of another solid state lighting system410capable of exhibiting a substantial color shift as a function of a magnitude of an applied electrical signal, at least in part as a result of current crowding. The system410includes a two-terminal semiconductor electroluminescent device412, such as an LED. The device is mounted on a metal header414having a first conductive post416electrically coupled to a base electrode of the device412. A second conductive post418, electrically insulated from the header414, electrically couples to a top electrode of the device via a thin wire420and wire bond422. The posts416,418form the two terminals of the system410, across which the electrical signal is applied to energize the device. The top electrode of the device is smaller than the base electrode, and offset to one side of an output surface412aof the electroluminescent device412.

An RSC424covers a first portion of the output surface412a, which first portion may as shown be spatially arranged to be spaced apart from the top electrode. The RSC424is operable to convert the emitted or pump light generated within the electroluminescent device into a first light component having a spectrum S3, e.g., characteristic of amber light. The spectrum S3may comprise or consist essentially of a distinct first and second spectral peak, e.g., the same as or similar to the spectrum S2ofFIG. 3b. The spectrum S3may optionally include a distinct third spectral peak corresponding to residual pump light transmitted by the RSC424, or it may contain no such third spectral peak in the case that RSC424substantially blocks such pump light.

A phosphor426covers a second portion of the output surface412a, the second portion being different from the first portion, and including areas or zones that are proximate the top electrode. The phosphor426is operable to convert at least some of the pump light into a second light component, e.g., yellow light having a spectral peak similar to peak S1bofFIG. 3a, to result in emitted light having spectrum S4. The light having spectrum S4may include not only yellow light generated by the phosphor426, but also residual pump light transmitted by the phosphor, e.g. as shown in the spectrum S1ofFIG. 3a. The light of spectrum S3and the light of spectrum S4are combined, optionally with other light components, to provide system optical output428, e.g., white light, whose color temperature is dependent upon the relative amounts or proportion of the first and second light components included in the system output.

Changes in color temperature of the system output with a changing magnitude of the applied electrical signal are achieved by ensuring that the electroluminescent device412exhibits current crowding, i.e., that the spatial distribution of the pump light emitted over the output surface412asubstantially changes as a function of such magnitude, and further by ensuring that such changes in the spatial distribution of emitted light are synergistic with the spatial distributions of the RSC424and phosphor426so that the relative amounts or proportion of the first and second light components included in the system output change in a corresponding fashion. In particular, by offsetting the top electrode to one side of the electroluminescent device, opposite the RSC424, the effects of the current crowding phenomenon are promoted.

FIG. 5shows a schematic sectional view of a lighting system510similar to system410, along with an associated graph of current density as a function of position. The system510includes an electroluminescent device512having an output surface512afrom which pump light generated within the device is emitted. The device512also includes a top electrode514, a base electrode516, and constituent semiconductor layers518,520,522, which layers may be or comprise a current spreading layer, a p-n junction layer, and a substrate layer respectively. The current spreading layer may comprise, for example, AlGaInN, or other suitable semiconductor material; the p-n junction may comprise, for example, GaInN, or other suitable semiconductor material; and the substrate layer may comprise, for example, silicon, or other suitable semiconductor material. A first light modifying material524, which may be substantially the same as RSC424ofFIG. 4, covers a first portion of the output surface512aand receives a first portion of the pump light. A second light modifying material526, which may be substantially the same as phosphor426ofFIG. 4, covers a second portion of the output surface512aand receives a second portion of the pump light. Light emitted from the first light modifying material, having a spectrum S5, and light emitted from the second light modifying material, having a spectrum S6different from S5, combine to form a system optical output528.

Due to the geometry or layout of the electrodes514,516, and one or more electrical properties of one or more constituent layers of the electroluminescent device512that change in response to a magnitude of the electrical signal applied across the electrodes, a substantial current crowding phenomenon is observed. For example, the thickness and/or conductivity of the n-GaN layer can be designed to provide a controlled amount of current crowding at high currents. In this regard, a graph of an expected current density through the p-n junction layer520, as a function of lateral position along the p-n junction layer, is provided in the figure. Curve530is representative of a first electrical current applied across electrodes514,516, and curve532is representative of a second electrical current greater than the first current. The curves assume that the resistivity of the p-n junction layer520is lower for the second electrical current than for the first electrical current. Although both curves exhibit a plateau or maximum at positions corresponding to the top electrode514, and taper off with increasing distance from that electrode, curve532is more heavily weighted or concentrated at positions close to the electrode, while curve528more nearly approximates a uniform spatial distribution. These differences in current density result in corresponding differences in the spatial distribution of pump light emitted from output surface512a, which, in combination with the different spatial distributions of the first and second light modifying materials, result in different relative amounts of the light of spectrum S5and light of spectrum S6in the system optical output528, thus producing changes in the color or color temperature of the output528.

The composition of the first and second light modifying materials, and their respective layout or spatial distribution on the output surface512a(e.g., the size of the gap between the electrode514and the first light modifying material524), may if desired be chosen to provide a system optical output having a nominally white light output that at low applied currents exhibits a particular color temperature, e.g., 2500K or 3000K, and which at higher applied currents exhibits a color temperature that increases. The increase in color temperature with increasing current may be designed to approximate the change in color temperature associated with an incandescent light source, for example.

FIGS. 6 and 7show schematic top views of other lighting systems capable of providing a system optical output whose color or color temperature changes as a function of an applied electrical signal. For brevity, details of the design of the electroluminescent device are not shown, but reference is made in that regard to the discussion above. Instead,FIGS. 6 and 7illustrate alternative designs of the top electrode and spatial distributions of the first and second light modifying materials that can be used to produce the desired color-changing effects.

InFIG. 6, a lighting system610includes a top electrode612disposed as shown on an output surface of an electroluminescent device. A first light modifying material614is spatially arranged or disposed to be proximate the electrode612. A second light modifying material616is spatially arranged or disposed to be spaced apart from the electrode612. Current crowding may result in more light associated with the first light modifying material, relative to light associated with the second light modifying material, to be present in the system optical output as a magnitude of the electrical signal is increased.

InFIG. 7, a lighting system710includes a top electrode712disposed as shown on an output surface of an electroluminescent device. A first light modifying material714is spatially arranged or disposed to be proximate the electrode712. A second light modifying material716is spatially arranged or disposed to be spaced apart from the electrode712. A zone or area718of the output surface is not covered with any light modifying material, such that pump light generated in the electroluminescent device is emitted from this area without modification. Current crowding may result in more light associated with the first light modifying material, relative to light associated with the second light modifying material, to be present in the system optical output as a magnitude of the electrical signal is increased.

FIG. 8shows an illustrative device800that combines an RSC808and an LED802. The LED has a stack of LED semiconductor layers804, sometimes referred to as epilayers, on an LED substrate806. The layers804may include p- and n-type junction layers, light emitting layers (typically containing quantum wells), buffer layers, and superstrate layers. The layers804may be attached to the LED substrate806via an optional bonding layer816. The LED has an upper surface812and a lower surface, and the upper surface is textured to increase extraction of light from the LED compared to the case where the upper surface is flat. Electrodes818,820may be provided on these upper and lower surfaces, as shown. When connected to a suitable power source through these electrodes, the LED emits light at a first wavelength λ1, which may correspond to blue or ultraviolet (UV) light. Some of this LED light enters the RSC808and is absorbed there.

The RSC808is attached to the upper surface812of the LED via a bonding layer810. The RSC has upper and lower surfaces822,824, with pump light from the LED entering through the lower surface824. The RSC also includes a quantum well structure814engineered so that the band gap in portions of the structure is selected so that at least some of the pump light emitted by the LED802is absorbed. The charge carriers generated by absorption of the pump light diffuse into other portions of the structure having a smaller band gap, the quantum well layers, where the carriers recombine and generate light at the longer wavelength. This is depicted inFIG. 8by the re-emitted light at the second wavelength λ2originating from within the RSC808and exiting the RSC to provide output light.

FIG. 9shows an illustrative semiconductor layer stack910comprising an RSC. The stack was grown using molecular beam epitaxy (MBE) on an indium phosphide (InP) wafer. A GaInAs buffer layer was first grown by MBE on the InP substrate to prepare the surface for II-VI growth. The wafer was then moved through an ultra-high vacuum transfer system to another MBE chamber for growth of II-VI epitaxial layers used in the RSC. Details of the as-grown RSC are shown inFIG. 9and summarized in Table 1. The table lists the thickness, material composition, band gap, and layer description for the different layers associated with the RSC. The RSC included eight CdZnSe quantum wells930, each having a transition energy of 2.15 eV. Each quantum well930was sandwiched between CdMgZnSe absorber layers932having a band gap energy of 2.48 eV that could absorb blue light emitted by an LED. The RSC also included various window, buffer, and grading layers.

An exemplary semiconductor stack comprising an RSC capable of simultaneously emitting light having a spectrum that includes two peak wavelengths, similar to the spectrum shown inFIG. 3b, is set forth below in Table 2. The stack includes one green-emitting (555 nm) quantum well, producing a green spectral peak, and one red-emitting (620 nm) quantum well, producing a red spectral peak. The relative intensities of the green and red peaks are principally controlled by the thicknesses of the absorber layers associated with the respective quantum wells. By using relatively thin absorber layers adjacent the green-emitting quantum well, more of the pump light will pass through these layers and be absorbed in the absorbing layers adjacent the red-emitting quantum well. This can result in the emission of more red light than green light. The ratio of green light to red light may also be somewhat influenced by the presence of any light-extraction features, e.g., where such features are etched into or attached to the outer surface of the cyan blocker.

TABLE 2Band gap/Band gap/emissionemissionThicknessenergywavelengthLayer typeMaterial(nm)(eV)(nm)cyan blockerCd0.38Mg0.21Zn0.41Se10002.48500barrierCd0.23Mg0.43Zn0.34Se202.88430absorberCd0.34Mg0.27Zn0.39Se1502.58480quantum wellCd0.72Zn0.28Se~42.00620absorberCd0.34Mg0.27Zn0.39Se1502.58480barrierCd0.23Mg0.43Zn0.34Se202.88430absorberCd0.34Mg0.27Zn0.39Se302.58480quantum wellCd0.47Zn0.53Se~32.23555absorberCd0.34Mg0.27Zn0.39Se302.58480windowCd0.23Mg0.43Zn0.34Se5002.88430
The person skilled in the art will understand how to tailor the composition of the CdMgZnSe alloys to achieve the listed band gap energies for the various layers. For example, the band gap energies of the CdMgZnSe alloys are primarily controlled by the Mg content. Emission wavelengths (or energies) of the quantum wells are controlled both by the Cd/Zn ratio, and the precise thickness of the quantum well.

Unless otherwise indicated, all numbers expressing quantities, measurement of properties, and so forth used in the specification and claims are to be understood as being modified by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and claims are approximations that can vary depending on the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the present application. Not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, to the extent any numerical values are set forth in specific examples described herein, they are reported as precisely as reasonably possible. Any numerical value, however, may well contain errors associated with testing or measurement limitations.

Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the spirit and scope of this invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. For example, the reader should assume that features of one disclosed embodiment can also be applied to all other disclosed embodiments unless otherwise indicated. It should also be understood that all U.S. patents, patent application publications, and other patent and non-patent documents referred to herein are incorporated by reference, to the extent they do not contradict the foregoing disclosure.