Abstract:
A light emitting diode device has a gallium and nitrogen containing substrate with a surface region with an epitaxial layer overlying the surface region. Preferably the device includes a first active region overlying the surface and configured to emit first electromagnetic radiation having a wavelength ranging from about 405 nm to 490 nm; a second active region overlying the surface and configured to emit second electromagnetic radiation having a wavelength ranging from about 491 nm to about 590 nm; and a third region overlying the surface region and configured to emit third electromagnetic radiation having a wavelength ranging from about 591 nm to about 700 nm. A p-type epitaxial layer covers the first, second, and third active regions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 61/242,372, filed Sep. 14, 2009, which is incorporated by reference herein for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to lighting techniques. More specifically, embodiments of the invention include techniques for combining one or more colored LED devices, such as violet, blue, blue and yellow, or blue and green, fabricated on semipolar or nonpolar materials, and optionally, with use of entities such as phosphors, which emit light. The invention can be applied to applications such as white lighting, multi-colored lighting, general illumination, decorative lighting, automotive and aircraft lamps, street lights, lighting for plant growth, indicator lights, lighting for flat panel displays, other optoelectronic devices, and the like. 
     In the late 1800&#39;s, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC power or DC power source. The conventional light bulb can be found commonly in houses, buildings, and outdoor lightings, and other areas requiring light. Unfortunately, one drawback with the conventional Edison light bulb is that it dissipates much of the electricity supplied to it as thermal energy. More than 90% of the energy used for the conventional light bulb dissipates as thermal energy. Additionally, the conventional light bulb routinely fails often due to thermal expansion and contraction of the filament element. 
     Fluorescent lighting overcomes some of the drawbacks of the conventional light bulb. Fluorescent lighting uses an optically clear tube structure filled with a halogen gas and, typically also contains mercury. A pair of electrodes is coupled between the halogen gas and couples to an alternating power source through a ballast. Once the gas has been excited, it discharges to emit light. Typically, the optically clear tube is coated with phosphors, which are excited by the light. Many building structures use fluorescent lighting and, more recently, fluorescent lighting has been fitted onto a base structure, which couples into a standard socket. 
     Solid state lighting techniques have also been developed. Solid state lighting relies upon semiconductor materials to produce light emitting diodes, commonly called LEDs. At first, red LEDs were demonstrated and introduced into commerce. Red LEDs use Aluminum Indium Gallium Phosphide or AlInGaP semiconductor materials. Most recently, Shuji Nakamura pioneered the use of InGaN materials to produce LEDs emitting light in the blue color range for blue LEDs. The blue colored LEDs led to innovations such as solid state white lighting, the blue laser diode, which in turn enabled the Blu-Ray™ (trademark of the Blu-Ray Disc Association) DVD player, and other developments. Other colored LEDs have also been proposed. 
     High intensity UV, blue, and green LEDs based on GaN have been proposed and even demonstrated with some success. Efficiencies have typically been highest in the UV-violet, dropping off as the emission wavelength increases to blue or green. Unfortunately, achieving high intensity, high-efficiency GaN-based green LEDs has been particularly problematic. The performance of optoelectronic devices fabricated on conventional c-plane GaN suffer from strong internal polarization fields, which spatially separate the electron and hole wave functions and lead to poor radiative recombination efficiency. Since this phenomenon becomes more pronounced in InGaN layers with increased indium content for increased wavelength emission, extending the performance of UV or blue GaN-based LEDs to the blue-green or green regime has been difficult. Furthermore, since increased indium content films often require reduced growth temperature, the crystal quality of the InGaN films is degraded. The difficulty of achieving a high intensity green LED has lead scientists and engineers to the term “green gap” to describe the unavailability of such green LED. In addition, the light emission efficiency of typical GaN-based LEDs drops off significantly at higher current densities, as are required for general illumination applications, a phenomenon known as “roll-over.” Other limitations with blue LEDs using c-plane GaN exist. These limitations include poor yields, low efficiencies, and reliability issues. Although highly successful, solid state lighting techniques must be improved for full exploitation of their potential. 
     From the above, it is seen improved optical devices are desired. 
     BRIEF SUMMARY OF THE INVENTION 
     In a specific embodiment, this invention provides a light emitting diode device. The device includes a gallium and nitrogen containing substrate having a surface region. The device also includes an n-type epitaxial layer overlying the surface region. In a specific embodiment, the device has one or more first active regions overlying one or more portions of the surface region and being configured to first emit electromagnetic radiation having a wavelength ranging from about 405 nm to 490 nm; and one or more second active regions overlying one or more portions of the surface region and being configured to second emit electromagnetic radiation having a wavelength ranging from about 520 nm to about 620 nm. In a specific embodiment, the optical device also has a p-type epitaxial layer overlying the one or more first and second active regions. 
     In an alternative specific embodiment, the present invention provides a light emitting diode device. The device has a gallium and nitrogen containing substrate including a surface region. The device also has an n-type epitaxial layer overlying the surface region. In a specific embodiment, the device includes at least one or more second active regions overlying one or more portions of the surface region and being configured to emit first electromagnetic radiation having a wavelength ranging from about 405 nm to 490 nm; one or more second active regions overlying one or more portions of the surface region and being configured to emit second electromagnetic radiation having a wavelength ranging from about 491 nm to about 590 nm; and one or more third regions overlying one or more portions of the surface region and being configured to emit third electromagnetic radiation having a wavelength ranging from about 591 nm to about 700 nm. In a specific embodiment, the optical device has a p-type epitaxial layer overlying the one or more first, second, and third active regions. 
     In yet another alternative embodiment, the present invention provides a light emitting diode device. The device has a gallium and nitrogen containing substrate including a surface region. The device also includes an n-type epitaxial layer overlying the surface region. The device further includes one or more first active regions overlying one or more portions of the surface region. The first active regions are configured to first emit electromagnetic radiation having a wavelength ranging from about 405 nm to 490 nm. The device also includes one or more second active regions overlying one or more portions of the surface region. The second active regions are configured to second emit electromagnetic radiation having a wavelength ranging from about 520 nm to about 620 nm. The one or more second active regions are positioned above the one or more first active regions. The device also includes a p-type epitaxial layer overlying the one or more first and second active regions. In a preferred embodiment, the device further includes a barrier layer positioned between the first active regions and the second active regions. For example, the barrier layer comprises magnesium material. 
     One or more benefits may be achieved using one or more of the specific embodiments. As an example, the present device and method provides for an improved lighting technique with improved efficiencies. In other embodiments, the present method and resulting structure are easier to implement using conventional technologies. In some embodiments, the present device and method provide light at two or more wavelengths that are useful in displays. In a specific embodiment, the blue LED device is capable of emitting electromagnetic radiation at a wavelength range from about 450 nanometers to about 495 nanometers and the yellow-green LED device is capable of emitting electromagnetic radiation at a wavelength range from about 495 nanometers to about 590 nanometers, although there can also be some variations. Depending upon the embodiment, one or more of these benefits can be achieved. These and other benefits are further described throughout the present specification and more particularly below. 
     A further understanding of the nature and advantages of the present invention may be realized by reference to the specification and attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a simplified diagram of a conduction band of an RGB active region in phosphorless white LED on semipolar or nonpolar bulk GaN substrates according to an embodiment of the present invention. 
         FIG. 1B  is a simplified diagram of a conduction band of a blue and yellow active region in phosphorless white LED on semipolar or nonpolar bulk GaN substrates according to an embodiment of the present invention. 
         FIG. 1C  is a simplified diagram of a conduction band of an RGB tunnel junction based active region in phosphorless white LED on semipolar or nonpolar bulk GaN substrates according to an embodiment of the present invention. 
         FIG. 2A  illustrates experimental results showing electroluminescence from multi-color active regions according to an embodiment of the present invention. 
         FIG. 2B  illustrates experimental results showing electroluminescence from multi-color active regions according to an embodiment of  FIG. 1B  of the present invention. 
         FIG. 3A  is a simplified top-side emitting phosphorless white LED on semipolar or nonpolar bulk GaN substrates according to an embodiment of the present invention. 
         FIG. 3B  is a simplified bottom-side emitting phosphorless white LED on semipolar or nonpolar bulk GaN substrates according to an embodiment of the present invention. 
         FIG. 4  is a chromaticity diagram according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a specific embodiment, the present invention includes a single growth epitaxial structure containing active layers that emit red, green, and blue radiation, or red, green, yellow, and blue radiation, or blue and yellow radiation from the same layer stack resulting in white light emission. The epitaxial structure is fabricated into an LED that emits white light without the need for a phosphor. See, for example,  FIGS. 1A ,  1 B, and  1 C and  FIGS. 2A and 2B . 
     In a specific embodiment, the light emitting layers are formed from InGaN in which the indium content dictates the emission wavelength. The InGaN light emitting layers may be quantum wells separated by quantum barriers or may be double hetereostructures type emitting layers. 
     The emitting layers can be contained within the same p-i-n junction such that adjacent layers can be emitting different colors. Such a configuration would lead to an ideal diode turn-on voltage equal to the largest bandgap of the emitting layers. In order to balance the color characteristics of the integrated emission, careful design of the active region would be necessary. Design aspects would include thickness and number of the emitting layers generating the various colors, the distance the light generating layers are separated from one another, i.e., the barrier thicknesses), the arrangement of the emitting layers, and the addition of doping species to various layers in the active region. For example, in general InGaN layers emitting in the blue region tend to emit more light for a given current than InGaN layers emitting in the green, yellow, or red regions. Thus one strategy would be to increase the number of emitting layers in the green, yellow, and red relative to blue to help balance the color. 
     In certain embodiments, emission layers are positioned based on their bandgap characteristics. For example, the lower bandgap (longer wavelength) light emitting layers are placed below the higher bandgap light emitting layers for better performed compared to other configurations. In various embodiments, barrier layers between the lighting emitting layers are doped with various type of materials, thereby allowing tuning of the relative brightness of the emitting layers. For example, magnesium material is used for doping the barrier layer. The barrier layers allows for better color control. During the tuning process, to achieve the desired color balance, it is possible to make one of the colors relatively brighter than the other. 
     In a separate embodiment regions containing the emitting layers could be coupled together with tunnel junctions, as referenced in  FIG. 1C . Such a configuration would offer an ideal turn-on voltage equal to the sum of the bandgap voltages of the different emitting regions, but may offer better light emission properties since carrier filling of the emitting layers may be more uniform. Design aspects would include thickness and number of the emitting layers generating the various colors, the distance the light generating layers are separated from one another, i.e., the barrier thicknesses, the arrangement of the emitting layers, and the addition of doping species to various layers in the active region. 
     Layers to prevent electron overflow from the light emitting regions such as AlGaN electron blocking layers can be inserted into the structures with various compositions, doping, and thickness. 
     The epitaxial device structure would use a thin (5-200 nm) p-cladding region grown on top of the emitting regions. Ultra thin layers in the range of 5-50 nm grown at temperatures equal to or slightly hotter than the growth temperature used for the light emitting layers would mitigate degradation to the light emitting layers while offering low resistance to current injected into the LED emitting layers. Conducting oxide layers such as indium-tin-oxide (ITO) or zinc oxide (ZnO) would then be deposited directly in contact with the think p-cladding layer. These conducting oxide layers can be deposited at a lower temperature relative to typical p-GaN growth conditions, and may therefore allow for the formation of a p-contact layer that results in ohmic or quasi-ohmic characteristics, at temperatures which would mitigate degradation of the light emitting layers. Additionally, the conducting oxide layers can have optical absorption coefficients at the wavelength ranges of interest which are lower or significantly lower than the optical absorption coefficient of a typical highly doped p-type GaN contact layer, and may therefore help to reduce absorption of emitted light within the device structure. In an alternative embodiment, metallic layers such as silver may be used in place of conducting oxide layers. 
     One device embodiment would be a top-side emitting LED, as illustrated by way of  FIG. 3A . In this case a transparent conducting material such as indium-tin-oxide (ITO) or zinc oxide (ZnO) would be used as the p-electrode. This contact would offer low voltage and low absorption loss to the emitted light. This device would contain some sort of reflector on the bottom of the chip to reflected downward emitted light back up through the topside to increase light extraction. The device may have a vertical electrical conduction path (one top-side p-contact and one bottom-side n-contact) or a lateral electrical conduction path (two top-side contacts). The reflector layer may be formed on the bottom of the chip, or may be formed on the submount to which the chip is attached. In this latter case, the chip is attached to the submount using a die-attach silicone or epoxy which is optically transparent at the wavelength range of interest. The reflector layer may be metallic, or may be formed of a multi-layer dielectric stack. Further, the top and/or bottom surface of the device as well as the edges of the device may be suitably textured or roughened in order to increase light extraction from the chip. The thickness and lateral dimensions of the chip may be suitably chosen so as to minimize absorption of the emitted light and to enhance extraction. In a specific embodiment, the present optical device and related method can be configured in one or more packages. 
     An alternative device embodiment would be a bottom-side emitting LED in which the LED chip is flipped and mounted with p-side down, as illustrated by way of  FIG. 3B . In this case, a transparent conducting material such as indium-tin-oxide (ITO) or zinc oxide (ZnO) may be used as the p-electrode, and a suitable reflector may be placed adjacent to this layer in order to reflect downward emitted light back up through the topside to increase light extraction. In an alternative embodiment, a metallic reflector layer may be placed in direct contact with the p-type semiconductor layer to form a low voltage contact. The device may have a vertical electrical conduction path (one top-side n-contact and one large-area bottom-side p-contact) or a lateral electrical conduction path (two bottom-side contacts). Further, the top and/or bottom surface of the device as well as the edges of the device may be suitably textured or roughened in order to increase light extraction from the chip. The thickness and lateral dimensions of the chip may be suitably chosen so as to minimize absorption of the emitted light and to enhance extraction. 
     Although the above has been described in terms of an embodiment of a specific package, there can be many variations, alternatives, and modifications. As an example, the LED device can be configured in a variety of packages such as cylindrical, surface mount, power, lamp, flip-chip, star, array, strip, or geometries that rely on lenses (silicone, glass) or sub-mounts (ceramic, silicon, metal, composite). Alternatively, the package can be any variations of these packages. 
     In other embodiments, the packaged device can include one or more other types of optical and/or electronic devices. As an example, the optical devices can be OLED, a laser, a nanoparticle optical device, and others. In other embodiments, the electronic device can include an integrated circuit, a sensor, a micro-electro-mechanical system, or any combination of these, and the like. 
     In a specific embodiment, the packaged device can be coupled to a rectifier to convert alternating current power to direct current, which is suitable for the packaged device. The rectifier can be coupled to a suitable base, such as an Edison screw such as E27 or E14, bipin base such as MR16 or GU5.3, or a bayonet mount such as GU10, or others. In other embodiments, the rectifier can be spatially separated from the packaged device. 
     Additionally, the present packaged device can be provided in a variety of applications. In a preferred embodiment, the application is general lighting, which includes buildings for offices, housing, outdoor lighting, stadium lighting, and others. Alternatively, the applications can be for display, such as those used for computing applications, televisions, projectors, micro-, nano-, or pico-projectors, flat panels, micro-displays, and others. Still further, the applications can include automotive, gaming, and others. 
       FIG. 4  is a chromaticity diagram according to an embodiment of the present invention. As shown, the chromaticity diagram shows tie lines illustrating loci of phosphorous-free white light emitting diode devices, as shown. Referring to line “b,” blue quantum wells emitting at 460 nm coupled with yellow quantum wells emitting at 580 nm yields a warm white light emitting diode device at 2850K. Referring to line “a,” green quantum wells emitting at 500 nm coupled with red quantum wells emitting at 605 nm yields a warm white light emitting diode device at 2850K. 
     While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. As an example, the packaged device can include any combination of elements described above, as well as outside of the present specification. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.