Patent Application
20070273282
The term refraction is defined herein as the bending of light as it passes between materials of different optical density. The term Refractive Index (n) of a material is defined as the ratio of the speed of light in vacuum (c) to the speed of light in that material (v).
It is to be understood herein, that if a “range” or “group” is mentioned with respect to a particular characteristic of the present disclosure, for example, percentage, chemical species, and temperature etc., it relates to and explicitly incorporates herein each and every specific member and combination of sub-ranges or sub-groups therein whatsoever. Thus, any specified range or group is to be understood as a shorthand way of referring to each and every member of a range or group individually as well as each and every possible sub-range or sub-group encompassed therein; and similarly with respect to any sub-ranges or sub-groups therein.
The present invention provides an optoelectronic device that comprises a light source, an encapsulant with refractive index n1, and a phosphor with refractive index n2. Generally, n2 is within the range of from about 0.85n1 to about 1.15n1. Specifically, n2 can be within the range of from about 0.90n1 to about 1.10n1. More specifically, n2 can be within the range of from about 0.92n1, to about 1.08n1. Most specifically, n2 can be within the range of from about 0.95n1 to about 1.05n1. Generally, n1 is within the range of from about 1.3 to about 1.7. Specifically, n1 is within the range of from about 1.5 to about 1.7; and more specifically, n1 is within the range of from about 1.6 to about 1.7.
The present invention further provides a method of preparing an optoelectronic device, which comprises (i) providing a light source, and (ii) encapsulating the light source with an encapsulant with refractive index n1 combined with a phosphor with refractive index n2. Generally, n2 is within the range of from about 0.85n1 to about 1.15n1. Specifically, n2 can be within the range of from about 0.90n1 to about 1.10n1. More specifically, n2 can be within the range of from about 0.92n1 to about 1.08n1. Most specifically, n2 can be within the range of from about 0.95n1 to about 1.05n1, such as from about 0.98n1 to about 1.02n1. Generally, n1 is within the range of from about 1.3 to about 1.7. Specifically, n1 is within the range of from about 1.5 to about 1.7; and more specifically, n1 is within the range of from about 1.6 to about 1.7.
The present invention also provides a method of adjusting the refractive Index of a phosphor nx which is more than 1.1 times higher than a predetermined value of the refractive index of an encapsulant, n1. The method comprises (i) partially or completely replacing one or more first element(s) in the phosphor with one or more second element(s); and (ii) adjusting refractive index of the phosphor from nx to n2. For example, the method may comprise (i) partially or completely replacing a first element in the phosphor with a second element having lower atomic weight than the first element; and (ii) adjusting refractive Index of the phosphor from nx to from about 0.9n1 to about 1.1n1.
In various embodiments, n2 is within the range of from about 1.3 to about 1.7. Specifically, n2 is within the range of from about 1.5 to about 1.7; and more specifically, n2 is within the range of from about 1.6 to about 1.7.
In an embodiment, the present invention can provide LED phosphors with refractive index matching that of the LED encapsulant material, such as suitable epoxy resin, silicone, polycarbonate, polyvinyl chloride, polyetherimide, or any combination thereof. The refractive index matching may be achieved by varying the ratio of a heavier element (the first element) to a lighter element (the second element) in the host lattice of the phosphor.
Although any heavier element(s) in the phosphor may be partially or completely replaced with any suitable lighter element(s), typically the heavier element and the lighter element are in the same Group of the Periodic Table. For example, a phosphor may comprise an element in the alkaline earth metal group such as beryllium (Be) with an atomic weight of about 9, magnesium (Mg) with an atomic weight of about 24, calcium (Ca) with an atomic weight of about 40, strontium (Sr) with an atomic weight of about 88, barium (Ba) with atomic weight of about 137, or any mixture thereof. According to the invention, Mg may be partially or completely replaced with Be in the phosphor; Ca may be partially or completely replaced with Mg, Be, or any combination thereof; Sr may be partially or completely replaced with Ca, Mg, Be, or any combination thereof; and Ba may be partially or completely replaced with Sr Ca, Mg, Be, or any combination thereof. In a preferred embodiment, Ca is partially or completely replaced with Mg. In another preferred embodiment, Sr is partially or completely replaced with Ca.
In an embodiment, a phosphor may comprise an element in the halogen group such as fluorine (F) with an atomic weight of about 19, chlorine (Cl) with an atomic weight of about 35, bromine (Br) with an atomic weight of about 80, iodine (I) with an atomic weight of about 127, or any mixture thereof. According to the invention, Cl may be partially or completely replaced with F in the phosphor; Br may be partially or completely replaced with Cl, F, or any combination thereof; and I may be partially or completely replaced with Br, Cl, F, or any combination thereof. In a preferred embodiment, Cl is partially or completely replaced with F.
In an embodiment, a phosphor may comprise an element in the alkali metal group such as lithium (Li) with an atomic weight of about 7, sodium (Na) with an atomic weight of about 23, potassium (K) with an atomic weight of about 39, rubidium (Rb) with an atomic weight of about 85, cesium (Cs) with an atomic weight of about 133, or any mixture thereof. According to the invention, Na may be partially or completely replaced with Li in the phosphor; K may be partially or completely replaced with Li, Na, or any combination thereof; Rb may be partially or completely replaced with Li, Na, K, or any combination thereof; and Cs may be partially or completely replaced with Li, Na, K, Rb, or any combination thereof.
In an embodiment, Group 3 element in a phosphor such as scandium (Sc) with an atomic weight of about 45 may be used to replace any suitable trivalent element with an atomic weight greater than 45, such as Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or Lu. In a preferred embodiment, the suitable trivalent element(s) with an atomic weight greater than 45 may be host lattice constituent(s) of a garnet phosphor, e.g. cerium activated yttrium aluminum garnet (YAG:Ce), cerium activated terbium aluminum garnet (TAG:Ce), and the like.
In an embodiment, Group 14 element in a phosphor such as silicon (Si) with an atomic weight of about 28 may be used to replace any suitable tetravalent element with an atomic weight greater than 28, such as Ge or Sn.
In preferred embodiments, when heavier element(s) in the phosphor is partially or completely replaced with lighter element(s), the particular host lattice of the phosphor remains unchanged, or the phosphor still has substantially the same crystallographic structure.
Examples of phosphors that may be subject to the method of refractive index adjustment include, but are not limited to, cerium activated garnet phosphors, divalent europium activated alkaline earth metal silicate phosphors, rare earth activated alkaline earth metal fluorohalide phosphors; divalent europium activated alkaline earth metal fluorohalide phosphors; rare earth element activated oxyhalide phosphors; cerium activated trivalent metal oxyhalide phosphors; bismuth activated alkaline metal halide phosphors; divalent europium activated alkaline earth metal halophosphate phosphors; divalent europium activated alkaline earth metal haloborate phosphors; divalent europium activated alkaline earth metal hydrogenated halide phosphors; cerium activated rare earth complex halide phosphors; cerium activated rare earth halophosphate phosphors; divalent europium activated cesium rubidium halide phosphors; divalent europium activated cerium halide rubidium phosphors; divalent europium activated composite halide phosphors; and tetravalent manganese activated alkaline earth metal fluorogermanate phosphors.
In an embodiment, the phosphor may comprise a europium and manganese doped alkaline earth pyrophosphate phosphor, for example, Sr2P2O7:Eu2+,Mn2+, Ca2P2O7:Eu2+,Mn2+, Mg2P2O7:Eu2+,Mn2+, Be2P2O7:Eu2+,Mn2+, or any mixture thereof. The phosphors may be represented as (Sr1-x-yEuxMny)P2O7, (Ca1-x-yEuxMny)P2O7, (Mg1-x-yEuxMny)P2O7, (Be1-x-yEuxMny)P2O7, or any mixture thereof, wherein 0<x≦0.2 and 0<y≦0.2.
When the refractive index of Ca2P2O7:Eu2+,Mn2+ is to be adjusted, Mg may be used to completely or partially replace Ca. The product will have a formula such as (MgnCa1-n)2P2O7:Eu2+,Mn2+, wherein 0<x≦1.
In the Eu2+ and Mn2+ doped alkaline earth pyrophosphate phosphor, the Eu ions generally act as sensitizers and Mn ions generally act as activators. Thus, the Eu ions absorb the incident energy (i.e., photons) and transfer the absorbed energy to the Mn ions. The Mn ions are promoted to an excited state by the absorbed transferred energy and emit a broad radiation band having a peak wavelength that varies from about 575 to 620 nm.
The method of adjusting the Refractive Index of a phosphor may be accomplished during the process of manufacturing the targeted phosphor. A phosphor may be made, for example, by any ceramic powder method, such as a wet chemical method or a solid state method.
In an embodiment, europium and manganese doped strontium pyrophosphate phosphor may be prepared according to the following step. First, the starting compounds are manually blended or mixed in a crucible or mechanically blended or mixed in another suitable container, such as a ball mill, to form a starting powder mixture. The starting compounds may comprise any oxide, phosphate, hydroxide, oxalate, carbonate and/or nitrate starting phosphor compound. The preferred starting phosphor compounds comprise strontium hydrogen phosphate, SrHPO4, manganese carbonate MnCO3, europium oxide, Eu2O3, and ammonium hydrogen phosphate (NH4)HPO4 powders. The (NH4)HPO4 powder may be added in an amount 2% in excess of its targeted stoichiometric ratio. A small excess of the Sr compound may also be added if desired. Under the present invention, calcium, barium and magnesium starting compounds may be added to substitute some or all of the strontium with calcium, barium and/or magnesium. The starting powder mixture may then be heated in air for about 1-5 hours at about 300 to 800° C., preferably at 600° C. The resulting powder may then be re-blended and subsequently fired in a reducing atmosphere at about 1000 to 1250° C., preferably 1000° C., to form a calcined phosphor body or cake. Preferably the starting powder mixture is calcined in a furnace in an atmosphere comprising nitrogen and 0.1 to 10% hydrogen for about four to ten hours, preferably about eight hours, and subsequently cooled in the same atmosphere by turning off the furnace.
In an embodiment, the phosphor may be a divalent europium activated alkaline earth silicate phosphor, ASIO:Eu2+, where A comprises at least one of Ba, Ca, Sr or Mg. The ratio between Ba, Ca, Sr or Mg may be so adjusted according to the present invention, to produce a phosphor product with desirable refractive index. For example, A may comprise at least 30% Ca, 60% or less Sr, and the balance Ba.
In the alkaline earth silicate phosphor, the europium activator substitutes on the alkaline earth lattice site. Other lighter dopants or impurities may be contained in the alkaline earth silicate phosphor. For example, the phosphor may contain an amount of fluorine incorporated during powder processing from a fluorine-containing flux compound, such as CaF2 or EuF3, and partially substituting the oxygen in its host lattice. This may adjust further the refractive index of the phosphor.
An exemplary method of making (Ca,Sr,Ba)2SiO4:Eu2+ phosphor comprises the following steps. First, the starting compounds of the phosphor may be manually blended or mixed in a crucible or mechanically blended or mixed in another suitable container, such as a ball mill, to form a starting powder mixture. The starting compounds may comprise any oxide, hydroxide, oxalate, carbonate and/or nitrate starting phosphor compound. The preferred starting phosphor compounds comprise calcium carbonate CaCO3, strontium carbonate SrCO3, barium carbonate BaCO3, europium oxide, Eu2O3, and silicic acid, SiO2.xH2O. Preferably, a flux, such as NH4Cl is added to the starting materials in an amount of 0.5 to 3 mole percent per mole of the phosphor produced. The starting powder mixture may then be fired in a reducing atmosphere, such as an atmosphere comprising nitrogen and 0.1 to 10% hydrogen at about 1100 to 1400° C. for 5 to 10 hours, to form a calcined phosphor body or cake.
Solid calcined phosphor bodies may be converted to phosphor powder in order to easily coat the phosphor powder on a portion of optoelectronic device. The solid phosphor body may be converted to phosphor powder by any crushing, milling or pulverizing method, such as wet milling, dry milling, jet milling or crushing. Preferably, the solid body is wet milled in propanol, methanol and/or water, and subsequently dried.
The phosphor may also comprises a divalent europium activated alkaline earth aluminate phosphor, AAIO:Eu2+, where A comprises at least one of Ba, Sr, Ca or mixture thereof. The ratio between these elements of different atomic weight may be so adjusted according to the present invention, to produce a phosphor product with desirable refractive index.
Other europium activated alkaline earth silicate phosphors are described in detail in G. Blasse et al., “Fluorescence of Eu2+ Activated Silicates” 23 Philips Res. Repts. 189-200 (1968), incorporated herein by reference. The europium activated alkaline earth aluminates phosphors are described in detail in G. Blasse et al., “Fluorescence of Eu2+ Activated Alkaline-Earth Aluminates” 23 Philips Res. Repts. 201-206 (1968), incorporated herein by reference.
In an embodiment, the phosphor may comprise a divalent europium activated halophosphate phosphor, DPOCl:Eu2+, where D comprises at least one of Sr, Ba, Ca, Mg, or any mixture thereof. The DPOCl:Eu2+ phosphor may comprise the commercially available “SECA” phosphor, D5(PO4)3Cl:Eu2+. In addition to adjusting the ratio between Sr, Ba, Ca, and Mg, Cl may be partially or completely replaced with F to lower the refractive index of the phosphor. Optionally, a small amount of phosphate may be replaced by a small amount of borate to increase the emission intensity.
Other phosphor examples may comprise a divalent europium activated alkaline earth metal aluminate phosphor, AMgAlO:Eu2+, where A comprises at least one of Ba, Ca, Sr, or any mixture thereof. The aluminate phosphor may have various magnesium, aluminum and oxygen molar ratios and is commercially available under the name “BAM”. Other examples comprise divalent europium activated aluminate phosphors such as EO•AIO:Eu phosphor, EAIO:Eu2+ phosphor and/or a GAIO:Eu2+ phosphor, where E comprises at least one of Ba, Sr or Ca ions and G comprises at least one of K, Li, Na or Rb ions. According to the invention, the ratio between Ba, Sr and Ca, and the ratio between K, Li, Na and Rb ions may be so adjusted, to produce a phosphor product with desirable refractive index. The EO•AIO, EAIO and GAIO phosphors are described in the following references, each incorporated herein by reference in their entirety: A. L. N. Stevels and A. D. M. Schrama-de Pauw, Journal of the Electrochemical Society, 123 (1976) 691; J. M. P. J. Verstegen, Journal of the Electrochemical Society, 121 (1974) 1623; and C. R. Ronda and B. M. J. Smets, Journal of the Electrochemical Society, 136 (1989) 570.
In an embodiment, the method of the invention may be used to modify pre-existing phosphor preparative procedure. For example, the synthesis of BAM and SECA phosphors is described on pages 398-399 and 416-419 of S. Shionoya et al., Phosphor Handbook, CRC Press (1987, 1999), incorporated herein by reference. In general, a method of making a commercial BAM phosphor involves blending starting materials comprising barium carbonate, magnesium carbonate, alumina or aluminum hydroxide, europium oxide and optionally a flux, such as aluminum fluoride or barium chloride. The starting powder mixture is then fired in a reducing atmosphere at about 1200 to 1400° C. to form a calcined phosphor body or cake. The cake may be reground and refired under the same conditions. A method of making a commercial SECA phosphor involves blending starting materials comprising strontium carbonate, strontium orthophosphate, strontium chloride and europium oxide. The starting powder mixture may then be fired in a reducing atmosphere at about 1000 to 1200° C. to form a calcined phosphor body or cake. The cake is then ground into a phosphor powder.
In an embodiment, phosphor particles may be prepared from larger pieces of phosphor material by any grinding or pulverization method, such as ball milling using zirconia-toughened balls or jet milling. In other embodiments, phosphor particles may also be prepared by crystal growth from solution, and their size may be controlled by terminating the crystal growth at an appropriate time.
Specific examples of phosphors that are efficiently excited by radiation of 300 nm to about 500 nm include yellow-emitting phosphors such as YAG:Ce3+, TAG:Ce3+, (Ca,Sr,Ba)SiO4:Eu2+, (Ba,Ca,Sr) (PO4)10(Cl,F)2:Eu2+, Mn2+, green-emitting phosphors such as Ca8Mg(SiO4)4Cl2:Eu2+, Mn2+; GdBO3:Ce3+,Tb2+; CeMgAl11O19:Tb3+; Y2SiO5:Ce3+, Tb3+; and BaMg2Al16O27:Eu2+, Mn2+ etc.; red-emitting phosphors such as Y2O3:Bi3+,Eu3+; Sr2P2O7:Eu2+,Mn2+; SrMgP2O7:Eu2+,Mn2+; (Y,Gd)(V,B)O4:Eu3+; and 3.5 MgO0.5 MgF2.GeO2:Mn4+ (magnesium fluorogermanate) etc.; and blue-emitting phosphors such as BaMg2Al16O27:Eu2+; Sr5(PO4)10Cl2:Eu2+; (Ba,Ca,Sr)(PO4)10(Cl,F)2:Eu2+; and (Ca,Ba,Sr)(Al,Ga)2S4:Eu2+ etc.
Many phosphors such as halophosphate phosphors typically have a refractive index of about 1.7˜1.8, while the highest refractive index of epoxy resin is about 1.5˜1.7, such as about 1.6˜1.7. In an embodiment of the invention, such halophosphate phosphors may be modified to have a refractive index matching that of the epoxy resin. For example, a LED phosphor with the formula (Mg,Ca,Eu,Mn)5(PO4)3(F,Cl) may be produced with a refractive index of about 1.60˜1.63, through adjustment of the Mg/Ca ratio, adjustment of the F/Cl ratio, lowering Eu concentration, or any combination thereof.
With no necessity of changing the phosphor size or coating the phosphor, the present invention redesigns the phosphor itself to match the refractive index of the encapsulant regardless of phosphor particle size. In an embodiment, when the refractive index of the phosphor matches that of the encapsulant material, the phosphor particles can visually “disappear” in the encapsulant and practically eliminate the light scattering losses.
Substitution of lighter for heavier elements generally tends to decrease the refractive index. It is believed that the index of refraction depends on the polarizability of the compound. Heavier elements have more electrons and tend to be more polarizable. Elements with lower atomic number have fewer electrons and so tend to be less polarizable. The index of refraction increases with increasing polarizability of the atoms. Therefore, one can alter the refractive index by changing the ratio of certain elements in a phosphor host lattice.
In preferred embodiments, the reflectance at phosphor/encapsulant surface, defined as [(n1−n2)/(n1+ n2)] is preferably less than 0.2%, and more preferably less than 0.1%. With very little reflectance at phosphor-encapsulant interface, little light will be trapped within the phosphor, and little light will be scattered by the phosphor particles. Also the light which is refracted or scattered will be primarily scattered in the forward direction out from the encapsulant package.
Suitable encapsulants, such as polyimides, epoxies, silicones and polyurethanes can be found with refractive indices as high as 1.62-1.63. However, these materials typically have aromatic rings which tend to absorb blue and near UV radiation from the chip and degrade with time. Stable polymers suitable as encapsulants generally have only aliphatic organic groups and have refractive indices in the range of 1.45-1.53. In order to have the low reflectance values mentioned above, the refractive index of the phosphor is preferably within 8-9% of that of the encapsulant and most preferably within 5-6%.
By using an element typically above the element commonly used in the periodic table, one can generally lower the phosphor refraction index, making it closer to that of a good encapsulant.
The present invention can provide numerous technical benefits in industrial applications. For example, the phosphor products are chemically stable and highly processable in the manufacturing of optoelectronic devices. Since the phosphors of the invention have minimal or no scattering loss for the light output, efficiency of optoelectronic device such as LED may be significantly improved. In an embodiment, reducing or eliminating the undesirable light scattering can improve the efficiency of the LEDs by up to 20%, depending on the specific design of the LEDs. The LED products manufactured according to the invention can be used, for example, as white and colored LEDs for transportation, signage and general illumination applications.
As described supra, the present invention provides an optoelectronic device that comprises a light source, an encapsulant with refractive Index n1, and a phosphor with refractive Index n2. Generally, n2 is within the range of from about 0.85n1 to about 1.15n1. Specifically, n2 can be within the range of from about 0.90n1 to about 1.10n1; and more specifically, n2 can be within the range of from about 0.92n1 to about 1.08n1.
In an embodiment, the light source may be a light emitting diode (LED) or a laser diode; and the encapsulant may be any suitable epoxy resin, silicone, polycarbonate, polyvinyl chloride, polyetherimide, or any combination thereof.
Optionally, the encapsulant may be combined with one or more refractive index modifiers to further narrow the gap between the encapsulant's refractive index and the phosphor's refractive index. Non-limiting examples of suitable refractive index modifiers are compounds of Groups II, III, IV, V, and VI of the Periodic Table. Non-limiting examples are titanium oxide, hafnium oxide, aluminum oxide, gallium oxide, indium oxide, yttrium oxide, zirconium oxide, cerium oxide, lead oxide, or gallium nitride.
Optoelectronic device of the invention may be any solid-state and other electronic device for generating, modulating, transmitting, and sensing electromagnetic radiation in the ultraviolet, visible, and infrared portions of the spectrum. Optoelectronic devices, sometimes referred to as semiconductor devices or solid state devices, include, but are not limited to, light emitting diodes (LEDs), charge coupled-LED devices (CCDs), photodiodes, vertical cavity surface emitting lasers (VCSELs), phototransistors, photocouplers, opto-electronic couplers, and the like. However, it should be understood that the encapsulant formulation can also be used in devices other than an optoelectronic device, for example, logic and memory devices, such as microprocessors, ASICs, DRAMs and SRAMs, as well as electronic components, such as capacitors, inductors and resistors, among others.
Several non-limiting examples of optoelectronic devices of the present invention are illustrated in the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating, and are, therefore, not intended to indicate relative size and dimensions of the optoelectronic devices or components thereof.
Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the invention. In the drawings and the following description below, it is to be understood that like numeric designations refer to component of like function.
With reference to
The LED chip 104 emits radiation from the radiation emitting surface 109. The LED may emit visible, ultraviolet or infrared radiation. The LED chip 104 may be any LED chip containing a p-n junction of any semiconductor layers capable of emitting the desired radiation. For example, the LED chip 104 may contain any desired Group III-V compound semiconductor layers, such as GaAs, GaAlAs, GaN, InGaN, GaP, etc., or Group II-VI compound semiconductor layers such as ZnO, ZnSe, ZnSSe, CdTe, etc., or Group IV-IV semiconductor layers, such as SiC. The LED chip 104 may also contain other layers, such as cladding layers, waveguide layers and contact layers.
The LED is packaged with an encapsulant 111 prepared according to the present invention. In one embodiment, the encapsulant 111 is used with a shell 114. The shell 114 may be any plastic or other material, such as polycarbonate, which is transparent to the LED radiation. However, the shell 114 may be omitted to simplify processing if encapsulant 111 has sufficient toughness and rigidity to be used without a shell. Thus, the outer surface of encapsulant 111 would act in some embodiments as a shell 114 or package. The shell 114 contains a light or radiation emitting surface 115 above the LED chip 104 and a non-emitting surface 116 adjacent to the lead frame 105. The radiation emitting surface 115 may be curved to act as a lens and/or may be colored to act as a filter. In various embodiments the non-emitting surface 116 may be opaque to the LED radiation, and may be made of opaque materials such as metal. The shell 114 may also contain a reflector around the LED chip 104, or other components, such as resistors, etc., if desired.
According to the present invention, the phosphor with encapsulant-matching refractive index may be coated as a thin film on the LED chip 104; or coated on the inner surface of the shell 114; or interspersed or mixed as a phosphor powder with encapsulant 111. Any suitable phosphor material according to this invention may be used with the LED chip.
While the packaged LED chip 104 is supported by the lead frame 105 according to one embodiment as illustrated in
With reference to
A vertical cavity surface emitting laser (VCSEL) is illustrated in
While the invention has been illustrated and described in typical embodiments, it is not intended to be limited to the details shown, since various modifications and substitutions can be made without departing in any way from the spirit of the present invention. As such, further modifications and equivalents of the invention herein disclosed may occur to persons skilled in the art using no more than routine experimentation, and all such modifications and equivalents are believed to be within the spirit and scope of the invention as defined by the following claims. All patents and publications cited herein are incorporated herein by reference.
