Patent Publication Number: US-2015069433-A1

Title: Array of luminescent elements

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/528,503, filed Aug. 25, 2009, which is a 371 of PCT/US08/55004, filed Feb. 26, 2008, which claims the benefit of U.S. Provisional Application No. 60/893,804, filed Mar. 8, 2007, the disclosures of which are incorporated by reference in their entireties herein. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to light emitting systems. The invention is particularly applicable to light emitting systems having two or more light emitting regions. 
     BACKGROUND 
     Illumination systems are used in many different applications, including projection display systems, backlights for liquid crystal displays and the like. Projection systems typically use one or more white light sources, such as high pressure mercury lamps. The white light beam is usually split into three primary colors, red, green and blue, and is directed to respective image forming spatial light modulators to produce an image for each primary color. The resulting primary-color image beams are combined and projected onto a projection screen for viewing. 
     More recently, light emitting diodes (LEDs) have been considered as an alternative to white light sources. LEDs have the potential to provide the brightness and operational lifetime that would compete with conventional light sources. Current LEDs, however, especially green emitting LEDs, are relatively inefficient. 
     Conventional light sources are generally bulky, inefficient in emitting one or primary colors, difficult to integrate, and tend to result in increased size and power consumption in optical systems that employ them. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention relates to light emitting systems. In one embodiment, a light emitting system includes two or more monolithically integrated luminescent elements. Each luminescent element includes an electroluminescent device and a dedicated switching circuit for driving the electroluminescent device. At least one luminescent element includes a potential well for down converting light emitted by the electroluminescent device in the luminescent element. 
     In another embodiment, a light emitting system includes two or more monolithically integrated luminescent elements. Each luminescent element includes an electroluminescent device. At least one luminescent element includes a potential well for down converting light emitted by the electroluminescent device in the luminescent element. 
     In another embodiment, a light emitting system includes a first luminescent element capable of outputting light at a first wavelength; a second luminescent element capable of outputting light at a second wavelength different than the first wavelength; and a third luminescent element capable of outputting light at a third wavelength different than the first and second wavelengths. The third luminescent element includes an electroluminescent device capable of emitting light at the first wavelength; a first photoluminescent element for converting at least a portion of light emitted by the electroluminescent device from the first wavelength to the second wavelength; and a second photoluminescent element for converting at least a portion of light emitted by the electroluminescent device or light converted by the first photoluminescent element to the third wavelength. 
     In another embodiment, a light emitting system includes a first luminescent element capable of outputting light at a first wavelength; and a second luminescent element capable of outputting light at a second wavelength different than the first wavelength. The second luminescent element includes an electroluminescent device capable of emitting light at the first wavelength and a potential well for converting at least a portion of light emitted by the electroluminescent device from the first wavelength to the second wavelength. 
     In another embodiment, an optical system includes a pixelated light emitting system capable of emitting light. Each pixel includes an electroluminescent device. At least one pixel includes one or more potential wells for down converting light emitted by the electroluminescent device in the pixel; and a pixelated spatial light modulator for receiving light emitted by the pixelated light emitting system. 
     In another embodiment, a pixelated light emitting system is capable of forming an image and emitting light. Each pixel in the light emitting system includes an electroluminescent device. At least one pixel includes one or more potential wells for down converting light emitted by the electroluminescent device in the pixel. 
     In another embodiment, a display system includes a light emitting system that includes a first plurality of pixels. Each pixel has a dedicated switching circuit for controlling the output light from the pixel. At least one pixel includes an electroluminescent device and a potential well for down converting light emitted by the electroluminescent device. The display system further includes a spatial light modulator (SLM) that receives light from the light emitting system and includes a second plurality of pixels. Each pixel in the first plurality of pixels illuminates a different subset of the second plurality of pixels. 
     In another embodiment, a light emitting system includes two or more monolithically integrated luminescent elements. Each luminescent element is capable of outputting white light. At least one luminescent element includes a light emitting diode (LED) capable of emitting UV or blue light; a first potential well for converting at least a portion of the UV or blue light to green light; and a second potential well for converting at least a portion of the green, UV or blue light to red light. 
     In another embodiment, a light emitting system includes a plurality of electroluminescent devices capable of emitting light. A first light converting element covers two or more electroluminescent devices for down converting at least a portion of light emitted by the two or more electroluminescent devices. 
     In another embodiment, an article includes a plurality of electroluminescent devices fabricated on a first substrate; a potential well for converting light emitted by an electroluminescent device in the plurality of electroluminescent devices; and a plurality of switching circuits fabricated on a second substrate. Each switching circuit on the second substrate is designed to drive a corresponding electroluminescent device on the first substrate. The first substrate is attached to the second substrate with each switching circuit on the second substrate facing a corresponding electroluminescent device on the first substrate. 
     In another embodiment, a light emitting system includes a first luminescent element capable of outputting light at a first wavelength; and a second luminescent element capable of outputting light at a second wavelength different from the first wavelength. The second luminescent element includes an electroluminescent device capable of emitting light at the first wavelength; a first light converting element for converting at least a portion of light emitted by the electroluminescent device from the first wavelength to a third wavelength different from the first and second wavelengths; and a second light converting element for converting at least a portion of light converted by the first light converting element from the third wavelength to the second wavelength. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention may be more completely understood and appreciated in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic top-view of a light emitting system; 
         FIG. 2  is a schematic side-view of a light emitting system; 
         FIG. 3  is a schematic side-view of a light converting element and an electroluminescent device; 
         FIG. 4  is a schematic view of a circuit diagram; 
         FIG. 5  is a schematic side-view of a projection system; 
         FIG. 6  is a schematic side-view of another projection system; 
         FIG. 7  is a schematic representation of a correspondence between the pixels of a light emitting system an a spatial light modulator; 
         FIG. 8  is a schematic side-view of a light emitting system; 
         FIG. 9  is a schematic side-view of a light emitting system; 
         FIGS. 10A-10F  are schematic representations of devices at intermediate stages or steps in a process for fabricating a light emitting system; 
         FIG. 11  is a schematic side-view of a light emitting system; 
         FIG. 12  is a schematic side-view of a luminescent element; 
         FIGS. 13A-13H  are schematic representations of devices at intermediate stages or steps in a process for fabricating a light emitting system; and 
         FIGS. 14A-14F  are schematic representations of exemplary conduction band profiles for a potential well. 
     
    
    
     The same reference numeral used in multiple figures refers to the same or similar elements having the same or similar properties and functionalities. 
     DETAILED DESCRIPTION 
     This application teaches light sources that include an array of light emitting regions. The disclosed light sources can efficiently output light at any wavelength in, for example, the visible region of the spectrum. The light sources can be designed to output, for example, one or more primary colors or white light. The light sources can be compact with reduced weight because, for example, the array of light emitting regions can be compactly integrated onto a substrate. The emission efficiency and compactness of the disclosed light sources can lead to new and improved optical systems, such as portable projection systems, with reduced weight, size and power consumption. 
     The disclosed light sources can have larger and smaller light emitting regions where the output light of each region can be actively and independently controlled. The light sources can be used in, for example, a projection system to illuminate one or more pixelated image forming devices. Each light emitting region of the light source can illuminate a different portion or zone of the image forming device. Such a capability allows for efficient adaptive illumination systems where the output light intensity of a light emitting region of the light source can be actively adjusted to provide the minimum illumination required by a corresponding zone in the image forming device. 
     The disclosed light sources can form monochromatic (for example, green or green on black) or color images. Such disclosed light sources combine the primary functions of light sources and image forming devices resulting in reduced size, power consumption, cost and the number of element or components used in an optical system that incorporates the disclosed light sources. For example, in a display system, the disclosed light sources can function as both the light source and the image forming device, thereby eliminating or reducing the need for a backlight or a spatial light modulator. As another example, incorporating the disclosed light sources in a projection system eliminates or reduces the need for image forming devices and relay optics. 
     Arrays of luminescent elements, such as arrays of pixels in a display system, are disclosed in which at least some of the luminescent elements include an electroluminescent device, such an LED, capable of emitting light in response to an electric signal. Some of the luminescent elements include one or more light converting elements, such as one or potential wells and/or quantum wells, for down converting light that is emitted by the electroluminescent devices. As used herein, down converting means that the wavelength of the converted light is greater than the wavelength of the unconverted light. 
     Arrays of luminescent elements disclosed in this application can be used in illumination systems, such as adaptive illumination systems, for use in, for example, projection systems or other optical systems. 
       FIG. 11  is a schematic side-view of a light emitting system  1100  that includes an array of luminescent elements, such as luminescent elements  1110 ,  1111 , and  1113 , where each element is capable of independently outputting light. Each luminescent element includes an electroluminescent device that is capable of emitting light in response to an electric signal. For example, luminescent elements  1110 ,  1111 , and  1113  include respective electroluminescent devices  1120 ,  1121 , and  1123  disposed on a substrate  1105 . 
     In some cases, the luminescent elements are configured as an active matrix, meaning that each luminescent element includes a dedicated switching circuit for driving the electroluminescent device(s) in the element. In such cases, a luminescent element includes any switching circuit(s) dedicated to the element. For example, luminescent element  1113  includes a dedicated switching circuit  1130  for driving electroluminescent device  1123 , where the switching circuit includes a transistor  1131 . 
     In some cases, the luminescent elements are configured as a passive matrix, meaning that the luminescent elements are not configured as an active matrix. In a passive matrix configuration, no luminescent element has a dedicated switching circuit for driving the electroluminescent device(s) in the luminescent element. 
     Typically, in a passive matrix configuration, the electroluminescent devices in the light emitting system are energized one row at a time. In contrast, in an active matrix configuration, although the rows are typically addressed one at a time, the switching circuits typically allow the electroluminescent devices to be energized continuously. 
     In some cases, at least some, for example all, of the electroluminescent devices in light emitting system  1100  are monolithically integrated. As used herein, monolithic integration includes, but is not necessarily limited to, two or more electronic devices that are manufactured on the same substrate (a common substrate) and used in an end application on that same substrate. Monolithically integrated devices that are transferred to another substrate as a unit remain monolithically integrated. Exemplary electronic devices include LEDs, transistors, and capacitors. 
     Where portions of each of two or more elements are monolithically integrated, the two elements are considered to be monolithically integrated. For example, two luminescent elements are monolithically integrated if, for example, the electroluminescent devices in the two elements are monolithically integrated. This is so, even if, for example, the light converting element in each element is adhesively bonded to the corresponding electroluminescent device. 
     In cases where the electroluminescent devices include semiconductor layers, the electroluminescent devices are monolithically integrated if the devices are manufactured on the same substrate and/or if they include a common semiconductor layer. For example, where each electroluminescent device includes an n-type semiconductor layer, the devices are monolithically integrated if the n-type semiconductor layer extends across the electroluminescent devices. In such a case, the n-type semiconductor layers in the electroluminescent devices form a continuous layer across the electroluminescent devices. 
     At least one luminescent element in light emitting system  1100  includes one or more light converting elements for converting light emitted by the electroluminescent device(s) in the luminescent element. For example, luminescent element  1110  includes light converting elements  1140  and  1141 , and luminescent element  1111  includes light converting element  1142 . In some cases, a light converting element can be or include a potential well or a quantum well. 
     As used herein, potential well means semiconductor layer(s) in a multilayer semiconductor structure designed to confine a carrier in one dimension only, where the semiconductor layer(s) has a lower conduction band energy than surrounding layers and/or a higher valence band energy than surrounding layers. Quantum well generally means a potential well which is sufficiently thin that quantization effects increase the energy for electron-hole pair recombination in the well. A quantum well typically has a thickness of about 100 nm or less, or about 10 nm or less. 
     Some luminescent elements in light emitting system  1100  do not include a light converting element. For example, luminescent element  1113  includes electroluminescent device  1123  but does not include a light converting element. In such cases, the light output of the luminescent element and the electroluminescent device in the luminescent element have the same wavelength or spectrum. 
     In the context of a display system, a luminescent element can be a pixel or a sub-pixel in the light emitting system. The pixelated light emitting system can emit light at different wavelengths, for example, in the visible region of the spectrum. For example, the electroluminescent devices in light emitting system  1100  can emit blue light. Light converting element  1140  can include a blue-to-green light converting potential well absorbing the blue light emitted by electroluminescent device  1120  and emitting green light. Light converting element  1141  can include a green-to-red light converting potential well absorbing the green light emitted by potential well  1140  and emitting red light. Light converting element  1142  can include a blue-to-green light converting potential well absorbing the blue light emitted by electroluminescent device  1121  and emitting green light. In such cases, luminescent elements  1110 ,  1111 , and  1113  output red, green and blue lights, respectively. 
     Light emitting system  1100  can efficiently output light at any wavelength in, for example, the visible region of the spectrum. For example, light emitting system  1100  can efficiently emit green light since the blue emitting electroluminescent devices and blue-to-green light converting potential or quantum wells can be highly efficient. Improved efficiency can result in reduced power consumption in an optical system that incorporates a light emitting system similar to device  1100 . 
     Light emitting system  1100  can be more compact than conventional light sources. Accordingly, optical systems utilizing light emitting system  1100  can be more compact, for example thinner, and have reduced weight. 
     In some applications, such as in a projection system or a backlight system, light emitting system  1100  can function as a light source for illuminating one or more image forming devices. The light emitting system can be designed to efficiently emit, for example, a primary color or white light. The improved efficiency and the compactness of light emitting system  1100  allows for improved and/or novel system designs. For example, portable battery-powered optical systems can be designed with reduced size, power consumption, and weight. 
     In some applications, such as in a projection system, light emitting system  1100  can function as a light source and an image forming device. In such applications, conventional image forming devices such as liquid crystal image forming devices (LCDs) or digital micro-mirror image forming devices (DMDs) can be eliminated from the projection system. Conventional projection systems include one or more relay optics for transferring light from light sources to image forming devices. The relay optics can be eliminated in a projection system that incorporates light emitting image forming device  1100 , thereby reducing the number of elements, size, power consumption, weight and overall cost. 
     In general, the array of luminescent elements in light emitting system  1100  can be any type array desirable in an application. In some cases, the array can be a row or column, such as a 1×n array where n is 2 or greater. In some cases, the array can be a square array, such as an m×m array, or a rectangular array, such as an m×n array where n and m are both 2 or greater and m is different than n. In some cases, the array can be a trapezoidal array, a hexagonal array, or any other type array, such as any regular type or irregular type array. 
     In some cases, the luminescent elements in the array (or pixels in the array in the context of a display system) can be of equal size, or of different sizes, for example, to account for differences in efficiency of different colors. 
     A luminescent element in an array of luminescent elements can have any shape such as, square, oval, rectangular, or more complex shapes to accommodate, for example, optical and electrical functions of a device incorporating the array. The luminescent elements in an array can be placed in any arrangement that may be desirable in an application. For example, the elements can be uniformly spaced, for example, in a rectangular or hexagonal arrangement. In some cases, the elements may be placed non-uniformly, for example, to improve device performance by, for example, reducing or correcting optical aberrations such as pincushion or barrel distortions. 
       FIG. 1  is a schematic top-view of a light emitting system  100  that includes two or more luminescent elements, such as luminescent elements  110 - 114 . Each luminescent element includes an electroluminescent device that when electrically driven, is capable of emitting light. Each luminescent element further includes a switching circuit for driving the electroluminescent device in the luminescent element. For example, luminescent element  110  includes an electroluminescent device  120  and a switching circuit  130  for driving electroluminescent device  120 . In some cases, a luminescent element may include more than one electroluminescent device. 
     At least one luminescent element in light emitting system  100  includes one or more light converting elements (LCE) for converting light emitted by the electroluminescent device in the luminescent element. For example, luminescent element  110  includes a light converting element  140  capable of converting, such as down converting, light that is emitted by electroluminescent device  120 . As another example, luminescent element  112  does not include a light converting element. 
     Light converting element  140  can include any element capable of receiving light at a first wavelength and converting at least a portion of the received light to light at a second wavelength different than the first wavelength. For example, light converting element  140  can include a phosphor, a fluorescent dye, a conjugated light emitting organic material such as a polyfluorene, a potential well, a quantum well, or a quantum dot. Exemplary phosphors that may be used as a light converting element include strontium thiogallates, doped GaN, copper-activated zinc sulfide, and silver-activated zinc sulfide. 
     Inorganic potential and quantum wells, such as inorganic semiconductor potential and quantum wells, typically have increased light conversion efficiencies and are more reliable by being less susceptible to environmental elements such as moisture. Furthermore, inorganic potential and quantum wells tend to have narrower output spectrum resulting in, for example, improved color gamut. 
     Electroluminescent device  120  is capable of emitting light in the presence of an electric signal. For example, in some cases, electroluminescent device  120  can emit light when a strong electric field is applied across the device. As another example, electroluminescent device  120  can emit light in response to an electric current passing through the device. 
     In some cases, electroluminescent device  120  can include a phosphorescent material capable of emitting light when absorbing electrical energy. In some cases, electroluminescent device  120  can include a semiconductor electroluminescent device such as a light emitting diode (LED) or a laser diode. 
     Light emitting system  100  further includes row enable electrodes  150  and column data electrodes  160  for applying electric signals from an external circuit not shown in  FIG. 1  to the switching circuits. In some cases, row enable electrodes  150  are disposed along the rows of the light emitting system for selectively addressing the rows of the light emitting system and column data electrodes  160  are disposed along the columns of the light emitting system for selectively addressing the columns of the light emitting system. In some cases, row enable electrodes  150  and column data electrodes  160  are connected to respective row and column driver circuits not explicitly shown in  FIG. 1 . 
     An electroluminescent device in light emitting system  100  can be any device capable of emitting light in response to an electrical signal. For example, an electroluminescent device can be a light emitting diode (LED) capable of emitting photons in response to an electrical current as discussed in, for example, U.S. Pat. No. 7,402,831, entitled “Adapting Short-Wavelength LED&#39;s for Polychromatic, Broadband, or ‘White’ Emission”, incorporated herein by reference in its entirety. 
     An LED electroluminescent device can emit light at any wavelength that may be desirable in an application. For example, the LED can emit light at a UV wavelength, a visible wavelength, or an IR wavelength. In some cases, the LED can be a short-wavelength LED capable of emitting UV photons. In general, the LED and/or a light converting element (LCE) may be composed of any suitable materials, such as organic semiconductors or inorganic semiconductors, including Group IV elements such as Si or Ge; III-V compounds such as InAs, AlAs, GaAs, InP, AlP, GaP, InSb, AlSb, GaSb, GaN, AlN, InN and alloys of III-V compounds such as AlGaInP and AlGaInN; II-VI compounds such as ZnSe, CdSe, BeSe, MgSe, ZnTe, CdTe, BeTe, MgTe, ZnS, CdS, BeS, MgS and alloys of II-VI compounds, or alloys of any of the compounds listed above. 
     In some cases, the LED can include one or more p-type and/or n-type semiconductor layers, one or more active layers that may include one or more potential and/or quantum wells, buffer layers, substrate layers, and superstrate layers. 
     In some cases, the LED and/or the LCE can include CdMgZnSe alloys having compounds ZnSe, CdSe, and MgSe as the three constituents of the alloy. In some cases, one or more of Cd, Mg, and Zn, especially Mg, may have zero concentration in the alloy and therefore, may be absent from the alloy. For example, the LCE can include a Cd 0.70 Zn 0.30 Se quantum well capable of emitting in the red, or a Cd 0.33 Zn 0.67 Se quantum well capable of emitting in the green. As another example, the LED and/or the LCE can include an alloy of Cd, Zn, Se, and optionally Mg, in which case, the alloy system can be represented by Cd(Mg)ZnSe. As another example, the LED and/or the LCE can include an alloy of Cd, Mg, Se, and optionally Zn. In some cases, a quantum well LCE has a thickness in a range from about 1 nm to about 100 nm, or from about 2 nm to about 35 nm. 
     In some cases, a semiconductor LED or LCE may be n-doped or p-doped where the doping can be accomplished by any suitable method and by inclusion of any suitable dopant. In some cases, the LED and the LCE are from the same semiconductor group. In some cases, the LED and the LCE are from two different semiconductor groups. For example, in some cases, the LED is a III-V semiconductor device and the LCE is a II-VI semiconductor device. In some cases, the LEDs include AlGaInN semiconductor alloys and the LCEs include Cd(Mg)ZnSe semiconductor alloys. 
     An LCE can be disposed on or attached to a corresponding electroluminescent device by any suitable method such as by an adhesive such as a hot melt adhesive, welding, pressure, heat or any combinations of such methods. Examples of suitable hot melt adhesives include semicrystalline polyolefins, thermoplastic polyesters, and acrylic resins. 
     In some cases, an LCE can be attached to a corresponding electroluminescent device by a wafer bonding technique. For example, the uppermost surface of the electroluminescent device and the lowermost surface of the LCE can be coated with a thin layer of silica or other inorganic materials using, for example, a plasma assisted or conventional CVD process. Next, the coated surfaces can be optionally planarized and bonded using a combination of heat, pressure, water, or one or more chemical agents. The bonding can be improved by bombarding at least one of the coated surfaces with hydrogen atoms or by activating the surface using a low energy plasma. Wafer bonding methods are described in, for example, U.S. Pat. Nos. 5,915,193 and 6,563,133, and in chapters 4 and 10 of “Semiconductor Wafer Bonding” by Q.-Y. Tong and U. Gosele (John Wiley &amp; Sons, New York, 1999). 
     In some cases, a quantum or potential well LCE can have one or more light absorbing layers proximate the well to assist in absorbing light emitted from a corresponding electroluminescent device. In some cases, the absorbing layers are composed of materials in which photogenerated carriers can efficiently diffuse to the potential well. In some cases, the light absorbing layers can include a semiconductor, such as an inorganic semiconductor. In some cases, a quantum or potential well LCE can include buffer layers, substrate layers, and superstrate layers. 
     An electroluminescent device or an LCE can be manufactured by any suitable method. For example, a semiconductor electroluminescent device and/or LCE can be manufactured using molecular beam epitaxy (MBE), chemical vapor deposition (CVD), liquid phase epitaxy (LPE) or vapor phase epitaxy (VPE). 
     Light emitting system  100  enables direct creation of images at very small sizes where the brightness of each luminescent element or pixel can be independently controlled. Alternatively, the light emitting system can be used for “zone illumination” of image forming devices, thereby allowing for reduced power consumption by darkening or reducing the brightness of emitting system pixel(s) that correspond to a dark area of an ultimate image. The ability to provide a highly controllable lighting source provides a large advantage in conserving energy as well as minimizing the size of the optical systems, such as projection systems, that utilize the light emitting system. 
       FIG. 2  is a schematic side-view of a light emitting system  200  that includes an array of luminescent elements, such as luminescent elements  210 - 212 , disposed on a common substrate  205 . Each luminescent element includes an electroluminescent device and a switching circuit for driving the electroluminescent device. For example, luminescent element  210  includes an electroluminescent device  220  and a switching circuit  231  for driving electroluminescent device  220 , where the switching circuit can include one or more transistors. Electroluminescent device  220  includes a first electrode  251 , a p-type semiconductor layer  252 , an optional semiconductor active layer  254 , an n-type semiconductor layer  256 , and an optional second electrode  258 . 
     Electrode  251  is designed to provide ohmic contact with and spread electrical current through p-type layer  252 . Optional active layer  254  is typically a semiconductor layer, typically a multiple-quantum-well layer, for radiative recombination of electron-hole pairs injected from p-type layer  252  and n-type layer  256 . 
     In some cases, such as when n-type layer  256  is sufficiently conductive to adequately spread an electrical current flowing through the n-type layer, second electrode  258  may be formed, for example, at a periphery of the electroluminescent device or the light emitting system. 
     In the exemplary light emitting system  200 , n-type layer  256  extends across luminescent elements  210 - 212 , meaning that n-type layer  256  forms a continuous layer across luminescent elements  210 - 212 . In general, a semiconductor layer in a luminescent element may or may not extend across other luminescent elements. For example, in some cases, each luminescent element can have a discrete n-type layer  256 . 
     Luminescent element  210  further includes a light converting element for converting light emitted by electroluminescent device  220 . In general, at least one luminescent element in light emitting system  200  includes a light converting element, such as a potential well or a quantum well, for converting, such as down converting, light emitted by the electroluminescent device in the luminescent element. In some cases, each luminescent element in light emitting system  200  includes a light converting element. 
     In the exemplary light emitting system  200 , luminescent element  210  includes a light converting element  240  disposed on electroluminescent device  220 , luminescent element  211  includes a light converting element  241  disposed on an electroluminescent device  221 , and luminescent element  212  includes a light converting element  242  disposed on an electroluminescent device  222 . 
     In some cases, luminescent element  210  is capable of outputting light  290 A at a first wavelength λ 1 , luminescent element  211  is capable of outputting light  291 A at a second wavelength λ 2 , and luminescent element  212  is capable of outputting light  292 A at a third wavelength λ 3 . In some cases, wavelength λ 2  is different from λ 1  and wavelength λ 3  is different from λ 1  and λ 2 . 
     In some cases, electroluminescent device  220  is capable of emitting light  290  at λ 1 ′, electroluminescent device  221  is capable of emitting light  291  at λ 2 ′, and electroluminescent device  222  is capable of emitting light  292  at λ 3 ′. In some cases, wavelength λ 2 ′ is different from λ 1 ′ and wavelength λ 3 ′ is different from λ 1 ′ and λ 2   ′ . In some cases, wavelength λ 1 ′ is different from wavelength λ 1 , wavelength λ 2 ′ is different from wavelength λ 2 , and wavelength λ 3 ′ is different from wavelength λ 3 . In such cases, light converting element  240  converts at least a portion of light  290  at wavelength λ 1 ′ to light  290 A at wavelength λ 1 , light converting element  241  converts at least a portion of light  291  at wavelength λ 2 ′ to light  291 A at wavelength λ 2 , and light converting element  242  converts at least a portion of light  292  at wavelength λ 3 ′ to light  292 A at wavelength λ 3 . 
     In some cases, the light outputted by luminescent element  210  may simply be the light emitted by electroluminescent device  220 . In such cases, wavelengths λ 1  and λ 1 ′ are substantially the same. In such cases, LCE  240  is eliminated from electroluminescent element  210  and may, for example, be replaced with an equally thick transparent element, for example, to assist in planarizing the light emitting system. 
     In general, light converting element  240  can be any element capable of converting at least a portion of light from a first wavelength to a second wavelength different from the first wavelength. In some cases, light converting element  240  can be a photoluminescent element capable of converting light by absorption and photoluminescence. In some cases, a photoluminescent element can include one or more potential and/or quantum wells. 
     In some cases, the light converting element can include a potential well. In general, the potential well can have any conduction and/or valence band profile. Some exemplary conduction band profiles for a potential well are shown schematically in  FIGS. 14A-14F  where E C  denotes the conduction band energy. In particular, a potential well  1410  shown in  FIG. 14A  has a square or rectangular profile; a potential well  1420  shown in  FIG. 14B  has a first rectangular profile  1421  combined with a second rectangular profile  1422  and a third rectangular profile  1423 ; a potential well  1430  shown in  FIG. 14C  has a linearly graded profile; a potential well  1440  shown in  FIG. 14D  has a linearly graded profile  1441  combined with a rectangular profile  1442 ; a potential well  1450  shown in  FIG. 14E  has a curved, such as a parabolic, profile; and a potential well  1460  shown in  FIG. 14F  has a parabolic profile  1461  combined with a rectangular profile  1462 . 
     Referring back to  FIG. 2 , in some cases, wavelengths λ 1 ′, λ 2 ′, and λ 3 ′ may be in the same region of the spectrum, such as the blue, violet, or UV region of the spectrum. In some cases, wavelengths λ 1 ′, λ 2 ′, and λ 3 ′ may be substantially the same. For example, wavelengths λ 1 ′, λ 2 ′, and λ 3 ′ may be substantially the same wavelength in the blue, violet, or UV region of the spectrum. 
     In some cases, λ 1 ′, λ 2 ′, and λ 3 ′ are substantially the same wavelength, wavelength λ 1  is substantially the same as λ 1 ′, wavelength λ 2  is different from λ 2 ′, and wavelength λ 3  is different from λ 3 ′. For example, wavelengths λ 1 , λ 1 ′, λ 2 ′, and λ 3 ′ can all be about 460 nm (blue), λ 2  can be about 540 nm (green), and λ 3  can be about 630 nm (red). 
     In some cases, λ 1 , λ 1 ′, λ 2 ′, and λ 3 ′ are in the same first region of the spectrum, such as the blue region of the spectrum; wavelength λ 2  is in a second region of the spectrum different from the first region, such as the green region of the spectrum; and wavelength λ 3  is in a third region of the spectrum different from the first and second regions, such as the red region of the spectrum. 
     In some cases, light converting element  240  may convert light at wavelength λ 1 ′ to light at wavelength λ 1  by first converting light at wavelength λ 1 ′ to a third wavelength. For example,  FIG. 3  is a schematic side-view of a light converting element  340  that includes a first photoluminescent element  305  and a second photoluminescent element  310 . Photoluminescent element  305  receives light  390  at wavelength λ 1 ′ from an electroluminescent device  320  similar to device  220 , and converts at least a portion of light  390  to light  390 B at wavelength λ 1 ″. Second photoluminescent element  310  converts at least a portion of light  390 B to light  390 A at wavelength λ 1 . 
     In some cases, electroluminescent device  320  may be capable of emitting light in the blue region of the spectrum, photoluminescent element  305  may convert a portion of the blue light to light in the green region of the spectrum, and photoluminescent element  310  may convert a portion of the green light exiting element  305  to light in the red region of the spectrum. 
     In some cases, each photoluminescent layer converts only a portion of light it receives and transmits the rest. For example, electroluminescent device  320  may emit blue light, photoluminescent element  305  may convert a portion of the blue light to green light and transmit the rest of the blue light, and photoluminescent element  310  may convert a portion of the green and/or blue light to red light and transmit the rest of the blue and green light it receives from photoluminescent element  305 . In such cases, light outputted by photoluminescent element  310  may be substantially white light. 
     In some cases, different luminescent elements in light emitting system  200  of  FIG. 2  may output light at more than three different regions in the visible spectrum. For example, the luminescent elements may output light at five different regions in the visible spectrum, for example, to improve color properties of the overall light outputted by the light emitting system. For example, some luminescent elements may output blue light; some luminescent elements may output cyan light, for example, at about 500 nm; some luminescent elements may output green light; some luminescent elements may output yellow or orange light; and some luminescent elements may output red light. 
     In some cases, a cyan output light can be achieved by using a potential well capable of re-emitting cyan light, or by combining the output of two potential wells where the first potential well is capable of re-emitting, for example, at about 460 nm and the second potential well is capable of re-emitting, for example, at about 540 nm. 
     In some cases, a magenta output light can be achieved by combining the output of two potential wells where the first potential well is capable of re-emitting, for example, at about 460 nm and the second potential well is capable of re-emitting, for example, at about 630 nm. Luminescent element  210  in  FIG. 2  further includes a light extractor  270  for extracting light from one or layers, such as layer  240 , disposed below the light extractor. In general, light can be extracted by any means suitable in an application. For example, light can be extracted by encapsulation where the encapsulating element can, for example, have a hemispherical profile for partially collimating the extracted light. Light can also be extracted by patterning or texturing, for example roughening, the top and/or lower surfaces of one or more layers in the luminescent element. As another example, light can be extracted by forming a photonic crystal on the exterior surface of a light converting element and/or an electroluminescent device and/or other layers in the luminescent element. Exemplary photonic crystals are described in, for example, U.S. Pat. Nos. 6,987,288 and 7,161,188. In some cases, light can be extracted by forming an optical element, such as light extractor  270 , on the output surface. Light extractor  270  can be any element and can have any shape capable of extracting at least a portion of light that would otherwise not exit the luminescent element due to, for example, total internal reflection. Exemplary light extractors are described in, for example, commonly-owned U.S. Patent Publication Nos. 2007/0284565, 2010/0051970, and 2011/0121262, the entireties of which are incorporated herein by reference. 
     In some cases, a luminescent element can have a dedicated light extractor. In some cases, a light extractor may extend beyond a luminescent element. For example, in some cases, a light extractor may extend across two or more luminescent elements. 
     In general, light extractor  270  is optically transparent and, in some cases, has a relatively high refractive index. Exemplary materials for the extractor include inorganic materials such as high index glasses (e.g., Schott glass type LASF35, available from Schott North America, Inc., Elmsford, N.Y. under a trade name LASF35) and ceramics (e.g., sapphire, zinc oxide, zirconia, diamond, and silicon carbide). Exemplary useful glasses are described in U.S. Pat. No. 7,423,297 incorporated herein by reference. Sapphire, zinc oxide, diamond, and silicon carbide are particularly useful ceramic materials since these materials also have a relatively high thermal conductivity (0.2-5.0 W/cm K). In some cases, light extractor  270  includes high index polymers or nano-particle filled polymers, where the polymers can be, for example, thermoplastic and/or thermosetting. In some cases, thermoplastic polymers can include polycarbonate and cyclic olefin copolymers. In some cases, thermosetting polymers can be, for example, acrylics, epoxy, silicones, or others known in the art. Exemplary ceramic nano-particles include zirconia, titania, zinc oxide, and zinc sulfide. 
     Extractor  270  can be manufactured by conventional techniques, such as machining or molding, or by using precision abrasive techniques disclosed in commonly assigned U.S. Pat. No. 7,404,756, and U.S. Patent Publication No. 2006/0094322A1; and U.S. Patent Publication No. 2007/0116423 the entireties of which are incorporated herein by reference. Other exemplary manufacturing techniques are described in commonly assigned U.S. Pat. No. 8,141,384 incorporated herein by reference. 
     In some cases, the luminescent elements in light emitting system  200  of  FIG. 2  are configured as an active matrix array. In such cases, each luminescent element in the light emitting system includes a dedicated switching circuit for driving the electroluminescent device within the luminescent element. For example, luminescent element  210  includes switching circuit  231  that may include one or more transistors not shown in  FIG. 2 . 
     In some cases, the luminescent elements in light emitting system  200  are configured as a passive matrix array. In such cases, no luminescent element in the light emitting system has a dedicated switching circuit. In some cases, the p-type electrodes are connected to form rows and the n-type electrodes are connected to form columns. 
     Substrate  205  can include any material that may be suitable in an application. For example, substrate  205  may include or be made of Si, Ge, GaAs, GaN, InP, sapphire, SiC and ZnSe. In some cases, substrate  205  may be n-doped, p-doped, insulating, or semi-insulating, where the doping may be achieved by any suitable method and/or by inclusion of any suitable dopant. 
     In some cases, light emitting system  200  does not include a substrate  205 . For example, various elements of light emitting system  200  may be formed on substrate  205  and then separated from the substrate by, for example, etching or ablation. 
       FIG. 4  is a schematic circuit diagram of three luminescent elements  410 - 412 , similar to luminescent elements  210 - 212 , in a light emitting system  400 , similar to light emitting system  200 , where the luminescent elements are configured as an active matrix array. Light emitting system  400  further includes row enable lines  425  and  426  and column data lines  430 - 432 . Each luminescent element includes a dedicated switching circuit for driving the electroluminescent device within the luminescent element. For example, luminescent element  410  includes a switching circuit  460 , similar to switching circuit  231 , that includes transistors  440  and  441  and capacitor  450 . Each luminescent element further includes an electroluminescent device, similar to electroluminescent device  220 , identified by a diode symbol in  FIG. 4 . In particular, luminescent element  410  includes an electroluminescent device  420  that is connected to a power supply source  405  capable of applying a voltage V s  to electroluminescent device  420 , luminescent element  411  includes an electroluminescent device  421  that is connected to power supply source  405 , and luminescent element  412  includes an electroluminescent device  422  that is connected to power supply source  405 . 
     Transistor  440  includes a gate electrode  440 G, a source electrode  440 S connected to a ground, and a drain electrode  440 D connected to electroluminescent device  420 . Transistor  441  includes a gate electrode  441 G connected to row enable line  425 , a drain electrode  441 D connected to column data line  430 , and a source electrode connected to the gate electrode of transistor  440 . 
     Transistor  441  is primarily designed as a switch transistor for controlling the voltage V G  at gate  440 G of transistor  440 . Voltage V G  is also the voltage across capacitor  450 . Capacitor  450  is primarily designed to maintain the voltage V G  at the gate of transistor  440  even when transistor  441  is off due to, for example, a sufficiently low signal applied to gate  441 G by row enable line  425 . 
     Transistor  440  is primarily designed as a drive transistor for controlling current flow through electroluminescent device  420 . The current flow through electroluminescent device  420  can control the intensity of light emitted by the device. 
     In some cases, light emitting system  400  can be a display device capable of forming an image in a plane of the device. In such cases, each electroluminescent element can be a pixel in the display device. In some cases, electroluminescent elements  410 - 412  can be three sub-pixels in a pixel. For example, electroluminescent element  410  can be a red sub-pixel capable of outputting red light, electroluminescent element  411  can be a green sub-pixel capable of outputting green light, and electroluminescent element  412  can be a blue sub-pixel capable of outputting blue light where, in some cases, the combination of the three outputted lights can result in white or other color light. 
       FIG. 5  is a schematic side-view of a projection system  500  that includes a light emitting system  510 , projection optics  560 , and an optional projection screen  590 . Light emitting system  510  is similar to light emitting system  200 , is capable of emitting light and forming an image in a plane  515  of the emitting system, and is pixelated and includes a plurality of pixels such as pixels  520 - 522 . Each pixel is capable of outputting white light and includes three sub-pixels where each sub-pixel is capable of outputting a different primary color light. For example, pixel  520  includes three sub-pixels  570 - 572  where each sub-pixel includes an electroluminescent device. In particular, sub-pixel  570  includes an electroluminescent device  501 , sub-pixel  571  includes an electroluminescent device  502 , and sub-pixel  572  includes an electroluminescent device  503 , where electroluminescent devices  501 - 503  can be monolithically integrated, for example, on a substrate  505 . Each sub-pixel includes a dedicated switching circuit, not shown in  FIG. 5  for ease of illustration, for driving the electroluminescent device in the sub-pixel. 
     In some cases, light emitted from each pixel in light emitting system  510  has substantially the same emission spectrum. In some cases, the pixels in light emitting system  510  are configured as an active matrix. In some other cases, the pixels in light emitting system  510  are configured as a passive matrix. In some cases, all the electroluminescent devices in light emitting system  510  are capable of emitting the same color, for example blue, light. In some cases, sub-pixel  570  includes no light converting elements and is capable of outputting blue light, sub-pixel  571  includes a light converting element  530 , that can include a potential or quantum well, for converting blue light to green light resulting in the sub-pixel being capable of outputting green light, and sub-pixel  572  includes a first light converting element  531  for converting blue light to green light and a second light converting element  532  for converting green light to red light resulting in sub-pixel  572  being capable of outputting red light. In some cases, one or more or all of the light converting elements in the light emitting system can be or include potential wells or quantum wells. 
     Projection optics  560  magnifies an image formed by light emitting system  510  and projects the magnified image onto a projection screen  590  for viewing by a viewing audience. In some cases, the projected image may be a virtual image in which case the projection system may not require a projection screen. Projection optics  560  typically includes one or more optical lenses, such as lens  561 . 
     In some cases, projection system  500  may be a rear projection system, in which case, projection screen  590  is preferably a rear projection screen. In some cases, projection system  500  may be a front projection system, in which case, projection screen  590  is preferably a front projection screen. 
     Light emitting system  510  functions as a light source in projection system  500 . Light emitting system  510  also functions as an image forming device in the projection system. Projection system  500  can be considered an emissive projection system because an image is directly produced by modulating light emitting system  510 . 
     The exemplary projection system  500  includes one light emitting system. In general, projection system  500  can include one or more light emitting systems. For example, the projection system can have three light emitting systems each capable of forming the same image in a different primary color. In such cases, projection optics  560  can combine the three images and magnify and project the combined image onto projection screen  590 . 
       FIG. 6  is a schematic side-view of a projection system  600  that includes light emitting system  510 , relay optics  610 , a pixelated spatial light modulator (SLM)  620  for receiving light from system  510  and having a plurality of pixels such as pixel  621 , projection optics  630 , and an optional projection screen  690 . SLM  620  can be any conventional image forming device such as a liquid crystal image forming device (LCD) or a digital micro-mirror image forming device (DMD). Light emitting system  510  functions as a light source in the projection system. Relay optics  610  directs light emitted by light emitting system  510  towards the spatial light modulator for illumination of the SLM. In some cases, each pixel in light emitting system  510  is capable of outputting white light and can include, for example, three sub-pixels where each sub-pixel is capable of outputting a different primary color light. 
     Projection optics  630  magnifies an image formed by SLM  620  and projects the magnified image onto projection screen  690  for viewing by a viewing audience. In some cases, the projected image may be a virtual image in which case the projection system may not require a projection screen. Projection optics  630  typically includes one or more optical lenses. Projection system  600  can be considered a passive projection system because a spatial light modulator is used to form an image. 
     In some cases, projection system  600  may be a rear projection system, in which case, projection screen  690  is preferably a rear projection screen. In some cases, projection system  600  may be a front projection system, in which case, projection screen  690  is preferably a front projection screen. 
     The exemplary projection system  600  includes one light emitting system and one SLM. In general, projection system  600  can include one or more light emitting systems and one or more SLMs. For example, the projection system can have three SLMs and one light emitting system. In such cases, white light from the light emitting system can be broken down into three primary colors. Each SLM is illuminated by a different primary color. The three images formed by the three SLMs are combined by, for example, an optical combiner. The resulting image is magnified and projected by projection optics  630  onto projection screen  690 . 
     In some cases, light emitting system  510  has fewer pixels than spatial light modulator  620 . In some cases, a pixel in light emitting system  510  may correspond to a group of pixels in SLM  620 , meaning that the light emitting system pixel&#39;s illumination of the SLM is substantially limited to the corresponding group of pixels in the SLM.  FIG. 7  shows an exemplary correspondence between the pixels of light emitting system  510  and SLM  620 . In particular, a first group of pixels  720  of SLM  620 , including pixels  621 - 623 , correspond to pixel  520  of the light emitting system and a second group of pixels  721  of the SLM correspond to a pixel  711  of the light emitting system. In some cases, each pixel in light emitting system  510  illuminates a different subset of pixels in SLM  620 . In such cases, each pixel in light emitting system  510  corresponds to a different subset of pixels in SLM  620 . In some cases, there is substantially no pixel overlap between these subsets in SLM  620 . In such cases, at least one pixel in SLM  620  receives light from a single pixel in the light emitting system. Such an arrangement provides for reduced power consumption. For example, pixel  520  need only emit as much light as required by the brightest pixel in pixel group  720 . In some cases, such as when a pixel in the light emitting system corresponds to and illuminates a subset of pixels in the SLM, the light emitting system may be used to illuminate the SLM as an adaptive illuminator, meaning that the output light intensity of a pixel in the light emitting system can be actively adjusted to provide the minimum illumination required by the corresponding subset of the pixels in the SLM, where the minimum required illumination is determined, at least in part, by the brightest pixel in the subset of pixels. In general, adaptive illumination can result in reduced power consumption by, for example, a display that utilizes the adaptive illumination. 
       FIG. 8  is a schematic side-view of a light emitting system  800  that includes luminescent elements  210 - 212 . A light blocking element is disposed between adjacent luminescent elements to reduce or eliminate optical cross talk between, for example, adjacent luminescent elements. In particular, light emitting system  800  includes a light blocking element  810  disposed between luminescent elements  210  and  211  and a light blocking element  811  disposed between luminescent elements  211  and  212 . 
     In some cases, a light blocking element can be an optically absorbing element absorbing some, most, or essentially all light that would otherwise propagate from one luminescent element to a neighboring luminescent element. In some cases, a light blocking element can include a reflective material, such as a metal coating, for reflecting light. In some cases, a light blocking element can include a low index region, such as an air gap, for reflecting light by total internal reflection. 
     Light emitting system  800  further includes a light extractor  820  that extends across multiple luminescent elements. In particular, light extractor  820  extends across luminescent elements  210 - 212 . Light extractor  820  extracts light by optically coupling to the luminescent elements. In particular, light extractor  820  is optically coupled to light converting elements  240 - 242 . 
     In some cases, light extractor  820  can redirect light emitted by luminescent elements  210 - 212 . For example, light extractor  820  can collimate, at least to some extent, light that is extracted from the luminescent elements. In some cases, light that is extracted by the light extractor has a first angular spread and light that exits the light extractor has a second angular spread. In such cases, light extractor  820  can have a collimating effect if the second angular spread is less than the first angular spread. In some cases, light extractor  820  can have a hemispherical profile. 
     The luminescent elements in light emitting system  800  define outermost edges  830  and  831  of the light emitting region. In some cases, light extractor  820  can extend beyond the outermost edges of the light emitting region to increase light extraction efficiency. 
       FIG. 9  is a schematic side-view of a light emitting system  900  that includes a plurality of electroluminescent devices, such as electroluminescent devices  920 - 922 , formed on substrate  205 . In some cases, each electroluminescent device corresponds to a different luminescent element in light emitting system  900 . In some cases, each of the electroluminescent devices is capable of emitting light at a first wavelength, such as a blue wavelength. 
     Light emitting system  900  further includes a light converting element  912  that extends across multiple electroluminescent devices. In particular, light converting element  912  covers electroluminescent devices  920  and  921 . In some cases, light converting element  912  forms a continuous layer across multiple, such as two or three, luminescent elements. 
     Light converting element  912  is primarily designed to convert light from the first wavelength to a second wavelength, where the second wavelength can, for example, be a green wavelength. In some cases, a luminescent element corresponding to electroluminescent device  921  is capable of outputting light at the second wavelength. 
     Light emitting system  900  further includes a light converting element  913  that extends across a single electroluminescent device. In particular, light converting element  913  covers electroluminescent device  920  but does not cover any neighboring electroluminescent devices, such as device  921 . In the exemplary light emitting system  900 , no light converting element is disposed on electroluminescent device  922 . 
     Light converting element  913  is primarily designed to convert light from the second wavelength to a third wavelength, where the third wavelength can, for example, be a red wavelength. In some cases, a luminescent element corresponding to electroluminescent device  920  is capable of outputting light at the third wavelength. 
     Light emitting system  900  further includes an optically transparent (for example, transparent to light incident from layers below) or clear layer  940  that transmits light without any wavelength conversion. In some cases, layer  940  is primarily designed to planarize the output surface of light emitting system  900 . 
     In some cases, the optical transmission of transparent layer  940  in a desired region(s) of the spectrum (for example, the blue region, the green region, the red region, or the visible region) can be, for example, greater than 50%, or greater than 70%, or greater than 80%. 
     In some cases, the first wavelength can correspond to blue light, the second wavelength can correspond to green light, and the third wavelength can correspond to red light. 
     In the exemplary light emitting system  1100  of  FIG. 11 , light converting element  1140  fully covers the emitting output surface of electroluminescent device  1120  and light converting element  1141  fully covers the output surface of light converting element  1140 . In general, an upper layer may or may not fully cover the output surface of a lower layer. For example, in some cases, an upper layer can cover only a portion of a lower layer. 
     For example, as schematically shown in  FIG. 12 , light converting element  1140  partially covers output surface of electroluminescent device  1120  and light converting element  1141  covers only a portion of the output surface of light converting element  1140 . In such cases, luminescent element  1110  is capable of outputting light that includes light at the first, second, and third wavelengths. In some cases, the first wavelength λ B  is a blue wavelength, the second wavelength λ G  is a green wavelength, and the third wavelength λ R  is a red wavelength. In such cases, the output light of luminescent element  1110  includes blue, green, and red light that can, for example, combine to produce white or any other color light. 
     Light emitting systems disclosed in this application can be fabricated using methods commonly used in, for example, fabrication of microelectronic and semiconductor devices and other wafer-based devices. Known methods include molecular-beam epitaxy (MBE), metal-organic vapor-phase epitaxy (MOVPE), photolithography, wafer bonding, deposition methods and etching methods. An exemplary fabrication process for fabricating an active matrix light emitting system is schematically outlined in  FIGS. 13A-13H . The process includes fabricating the various components of the light emitting system onto four different wafers, each designated a component wafer, and combining the four component wafers to construct a light emitting system. In particular, the electroluminescent devices are fabricated on a first substrate; the switching circuits for driving the electroluminescent devices are fabricated on a second substrate different than the first substrate; the light converting elements, such as potential or quantum wells, for converting light emitted by the electroluminescent devices are fabricated on a third substrate different from the first and second substrates; and the light extracting elements are fabricated on a fourth substrate different from the first, second, and third substrates. Next, the four substrates are attached to form a light emitting system. 
       FIG. 13A  is a schematic side-view of a first component wafer  1350  and a second component wafer  1360 . First component wafer  1350  includes an array of switching circuits  1315  fabricated on a wafer or substrate  1302 . In general, each switching circuit can include one or more transistors and one or more capacitors. In some cases, the switching circuits can be first fabricated individually and then integrated onto substrate  1302 . In some other cases, the switching circuits can be fabricated directly onto substrate  1302  resulting in a monolithically integrated array of switching circuits. The switching circuits can be fabricated using, for example, conventional methods for fabrication of thin film microelectronic circuits that employ, for example, additive and/or subtractive fabrication processes. An additive process typically includes the steps of photolithography, deposition, and lift-off. A subtractive processing typically includes the steps of deposition, photolithography, and etching. 
     A switching circuit typically includes active layers, electrically conductive electrode layers such as metal electrode layers, and electrically insulative layers such as metal oxide layers. Typical materials used for an active region of a transistor in a switching circuit include single crystal silicon, amorphous or polycrystalline silicon, or other materials that may be suitable as a transistor active layer. Exemplary materials for use as an electrically conductive electrode include Al, Cu, Au, Ni or any other metal that may be suitable in an application. Exemplary material compositions for use as an electrically insulative layer include SiO x  such as SiO 2 , Al 2 O 3 , Si 3 N 4  or any other electrically insulative material that may be suitable in an application. Exemplary deposition methods include physical vapor deposition such as thermal evaporation, electron beam deposition or sputtering; chemical vapor deposition such as MOCVD, PECVD, LPCVD, MBE or reactive sputtering; or any other method that can suitably be used in an application. 
     First component wafer  1350  may include other layers, such as passivation layers, protective layers, and planarization layers for planarizing the wafer. In some cases, one or more planarization layers may be included to planarize the top surface of the first component wafer for improved subsequent bonding to another wafer. 
     In some cases, substrate  1302  can be a Si substrate, a GaN substrate, or a SiC substrate. In general, substrate  1302  can be any substrate that may be suitable in an application. 
     Second component wafer  1360  includes an array of electroluminescent devices  1310  disposed on a wafer or substrate  1301 . In some cases, the electroluminescent devices can include light emitting diodes (LEDs) that can be fabricated utilizing an array of known methods and materials. 
     In some cases, electroluminescent devices  1310  can be monolithically integrated by virtue of, for example, being formed directly onto substrate  1301  using photolithography, etching methods, and epitaxial or quasi-epitaxial deposition methods in, for example, an MOCVD system. In some cases, substrate  1301  can be a sapphire wafer, or any other material compatible with, for example, growth of LED materials. 
     In some cases, electroluminescent devices  1310  can include transparent electrically conductive layers such as indium tin oxide (ITO) layers, planarization layers, passivation layers, bonding layers for a subsequent bonding to another wafer, vias, and light blocking elements similar to, for example, light blocking element  810 . 
     In some cases, after first component wafer  1350  and second component wafer  1360  are fabricated, the two wafers are bonded to each other with the active components facing each other as shown schematically in  FIG. 13B . The bonding can be accomplished by, for example, bringing the top surfaces of the first and second component wafers into intimate contact or by applying one or more bonding layers during the bonding process. In some cases, the bonding layers can assist in providing electrical connection between electroluminescent devices  1310  and corresponding switching circuits  1315 . 
     In some cases, electrical connection between corresponding features in wafers  1350  and  1360  can be accomplished by forming solder bumps on one or both wafers. After the solder bumps are formed, the two wafers are aligned and bonded to each other. In some cases, the bonding process may include one or more solder re-flow steps and one or more via patterning and/or filling. In some cases, wafers  1350  and  1360  can be aligned using IR illumination through one of the wafers. In cases where one or more of the wafers is optically transmissive in the visible region of the spectrum, alignment can be accomplished by using visible light illumination. 
     After wafers  1350  and  1360  are bonded, at least a portion of substrate  1301  is removed to expose electroluminescent devices  1310  as shown schematically in  FIG. 13C . The removal of substrate  1301  can be carried out using, for example, an etching process or laser ablation. 
       FIG. 13D  is a schematic side-view of a third component wafer  1370  and a fourth component wafer  1380  for fabrication of a light emitting system. Third component wafer  1370  includes an array of light converting elements  1335  disposed on a substrate  1322 . 
     In some cases, light converting elements  1335  can include potential or quantum wells and one or more light absorbing layers for absorbing light that is emitted by a corresponding electroluminescent device. In some cases, the potential wells of light converting elements  1335  can include II-VI potential wells constructed on an indium phosphide (InP) wafer  1322 . 
     In some cases, light converting elements  1335  can be directly fabricated on substrate  1322  by one or more deposition methods such as molecular beam epitaxy (MBE). Light converting elements  1335  may include one or more vias, planarization layers, passivation layers, optical blocking elements such as light blocking elements  810 , and bonding layers for a subsequent bonding to another component wafer. 
     In general, a light control element in a luminescent element may or may not extend across neighboring luminescent elements. For example, in the exemplary third component wafer  1370  shown in  FIG. 13D , each light converting element  1335  is dedicated to a single luminescent element and does not extend across other luminescent elements. In some cases, light converting elements  1335  may form a continuous layer across two or more luminescent elements or electroluminescent devices. 
     Fourth component wafer  1380  includes a plurality of light extractors  1330  disposed on a substrate  1321 . In some cases, light extractors  1330  include light extracting elements and light management elements such as lenses for collimating the extracted light or steering the extracted light in one or more particular directions. In general, light extractors  1330  may be constructed in a variety of ways using a variety of materials. In some cases, light extractors  1330  can be fabricated in a mold and transferred onto substrate  1321 , where the substrate can be a temporary substrate. 
     In some cases, after third component wafer  1370  and fourth component wafer  1380  are fabricated, the two wafers are bonded to each other with the extractors and LCEs facing each other as shown schematically in  FIG. 13E . The bonding can be accomplished using any existing bonding layers in one or both wafers and/or by applying one or more additional bonding layers during the bonding process. After the completion of the bonding process, substrate  1322  is removed resulting in a construction shown schematically in  FIG. 13F . 
     In some cases, after the two constructions of  FIGS. 13C and 13F  are fabricated, the two constructions are bonded to each other as shown schematically in  FIG. 13G . Next, temporary substrate  1321  may be removed resulting in light emitting system  1300  shown schematically in  FIG. 13H . 
     An exemplary fabrication process for fabricating an active matrix light emitting system capable of outputting white or any other color light is schematically outlined in  FIGS. 10A-10F . The process includes fabricating the various components of the light emitting system onto two different wafers, each designated a component wafer, and combining the two component wafers to construct a light emitting system. In particular, the process includes forming two or more light converting elements and selectively removing one or more of the light converting elements to obtain a desired output spectrum. 
     The selective removal of the light converting elements may be accomplished by a variety of known methods such as wet or dry chemical etching or any combinations of the two. Exemplary dry chemical etching methods include reactive ion etching and focused ion beam etching. Exemplary patterning methods include photolithography. 
       FIG. 10A  is a schematic side-view of a first component wafer  1060  and a second component wafer  1070 . First component wafer  1060  includes a first LCE  1520  disposed on a substrate  1510  and a second LCE  1530  disposed on LCE  1520 . For ease of illustration and discussion and without loss of generality, it is assumed that LCE  1520  is capable of down converting green light to red light, and light converting element  1530  is capable of down converting blue light to green light. 
     First component wafer  1060  may be fabricated using known fabrication methods, such as epitaxial deposition methods, carried out on a wafer substrate, such as an InP substrate. For example, a molecular-beam epitaxy (MBE) process may be used to deposit alloys of II-VI semiconductor materials on an InP substrate  1510  to form layers of potential or quantum wells as light converting elements  1520  and  1530 . 
     Second component wafer  1070  includes a plurality of electroluminescent devices  1540  disposed on a substrate  1511 . In some cases, the electroluminescent devices may be light emitting diodes (LEDs). In such cases, the LEDs may be constructed from III-V semiconductor materials including, for example, GaN using known fabrication methods such as vapor phase epitaxy (VPE) on a sapphire substrate  1511 . In some cases, the LEDs can include such layers and/or components as electrodes, transparent electrical contacts, vias, and bonding layers. In general, electroluminescent devices  1540  can be fabricated using conventional methods used in the semiconductor micro-fabrication industry, such as by conventional photolithography methods and conventional etching and/or deposition methods. 
     After first component wafer  1060  and second component wafer  1070  are fabricated, the two wafers are bonded to each other with the active components facing each other as shown schematically in  FIG. 10B . The bonding may be carried out by, for example, direct wafer bonding or by disposing one or more bonding layers between the two wafers during the bonding process. A bonding layer may, for example, include one or more thin metal layers, one or more thin metal oxide layers, or one or more layers of other materials such as adhesives, encapsulants, high index glasses, or sol-gel materials such as low temperature sol-gel materials, or any combinations thereof. 
     In some cases, the thickness of a bonding layer may be in a range from about 5 nm to about 200 nm, or from about 10 nm to about 100 nm, or from about 50 nm to about 100 nm. The bonding between the two wafers may be accomplished by, for example, lamination or an application of temperature and/or pressure. 
     After first component wafer  1060  is bonded to second component wafer  1070 , substrate  1510  of the first component wafer is removed resulting in the structure shown schematically in  FIG. 10C . Substrate  1510  can be removed using known methods such as wet chemical etching. In the case of an InP substrate  1510 , the removal of the substrate can be accomplished by, for example, etching the substrate using, for example, a solution of hydrochloric acid and water. 
     In some cases, first component wafer  1050  may include a buffer layer, not shown in  FIG. 10A , disposed on substrate  1510  between the substrate and LCE  1520 . In such cases, the buffer layer may also be removed when removing substrate  1510 . In the case of an InP substrate  1510 , the buffer layer may include GaInAs. A GaInAs buffer layer can be removed by, for example, using an etching solution of adipic acid, ammonium hydroxide and hydrogen peroxide. The etching solution can be prepared by, for example, adding 30 mL of ammonium hydroxide (˜30%) and 5 mL of hydrogen peroxide to 40 grams of adipic acid in 200 mL of water. 
     After substrate  1510  is removed, the light converting elements are selectively removed. For example, as shown in  FIG. 10D , LCE  1520  is removed from areas covering electroluminescent devices  1541  and  1542  but not from the area covering electroluminescent device  1543 . The selective removal of LCE  1520  results in LCE  1521  covering electroluminescent device  1543 . 
     As part of the selective removal of the light converting elements, LCE  1530  is removed, as shown in  FIG. 10E , from the area covering electroluminescent device  1541  but not from the areas covering electroluminescent devices  1542  and  1543 . The selective removal of LCE  1530  results in LCE  1531  covering electroluminescent devices  1542  and  1543 . 
     In the exemplary construction of  FIG. 10E , LCE  1531  extends across electroluminescent devices  1542  and  1543 . In some cases, a portion of LCE  1531  between the two corresponding electroluminescent devices may be removed, as shown schematically in  FIG. 10F , resulting in LCE  1533  covering electroluminescent device  1542  and LCE  1532  covering electroluminescent device  1543 . In such cases, LCE  1533  and LCE  1532  are from a same layer. In such cases, a light blocking element such as light blocking element  810  of  FIG. 8 , may be formed between electroluminescent devices  1542  and  1543  as well as, for example, between electroluminescent devices  1541  and  1542 . 
     The removal of a light converting element can be accomplished by, for example, using known patterning and etching methods. Exemplary patterning methods include photolithography. Exemplary etching methods include wet etching. For example, a II-VI semiconductor light converting element can be etched using a solution that contains methanol and bromine. 
     Light emitting systems disclosed herein may be used in any application that light sources or image forming devices are currently being used or are anticipated to be used in the future. Exemplary applications include, but are not limited to, display systems, graphic display systems, signage systems, projection systems, liquid crystal displays, automotive headlamps, traffic signals, interior lighting, architectural or artistic lighting, general illumination, inspection and/or measurement systems, and any other application where the disclosed light emitting systems may be used. 
     As used herein, terms such as “vertical”, “horizontal”, “above”, “below”, “left”, “right”, “upper” and “lower”, “top” and “bottom” and other similar terms, refer to relative positions as shown in the figures. In general, a physical embodiment can have a different orientation, and in that case, the terms are intended to refer to relative positions modified to the actual orientation of the device. For example, even if the construction in  FIG. 2  is inverted as compared to the orientation in the figure, light extractor  270  is still considered to be on “top” of light converting element  240 . 
     While specific examples of the invention are described in detail above to facilitate explanation of various aspects of the invention, it should be understood that the intention is not to limit the invention to the specifics of the examples. Rather, the intention is to cover all modifications, embodiments, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.