Light emitting devices with improved light extraction efficiency

1 LIGHT EMITTING DEVICES WITH IMPROVED LIGHT EXTRACTION EFFICIENCY

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No. 10/938,237, filed on Sep. 10, 2004, which is a Division of U.S. patent application Ser. No. 09/880,204, filed on Jun. 12, 2001, now U.S. Pat. No. 7,064,355, which is a Continuation-In-Part of U.S. patent application Ser. No. 09/660,317 filed on Sep. 12, 2000, now U.S. Pat. No. 7,053, 419. Each of U.S. patent application Ser. No. 10/938,237, U.S. Pat. No. 7,064,355, and U.S. Pat. No. 7,053,419 is incorporated herein by reference.

BACKGROUND

1. Field of Invention

The present invention relates generally to methods offorming light emitting devices with enhanced light extraction efliciency.

2. Description of Related Art

The light extraction efliciency of a light emitting diode (LED) is defined as the ratio of the LED’s extemal quantum efliciency to the LED’s intemal quantum efficiency. Typically, the light extraction efficiency of a packaged LED is substantially less than one, i.e., much of the light generated in the LED’s active region never reaches the external environment.

Light extraction efficiency is reduced by total internal reflection at interfaces between the LED and surrounding material followed by reabsorption of the totally internally reflected light in the LED. For example, for a cubic geometry LED on a transparent substrate encapsulated in epoxy, the refractive index (n) at the emission wavelength changes from a value of, for example, n ~3.5 in the LED semiconductor

to n ~1.5 in the epoxys/,.m",fhe corresponding critical angle

for tfltaI intemal reflection of light incident on the epoxy encapsulant from the LED semiconductor of this example is 0c:arcsin(neP0x}/nsem-)~25°. Neglecting scattering and multiple reflections, light emitted over 475 steradians from a point in the active region of the cubic LED crosses a semiconductor/epoxy encapsulant interface only if it is emitted into one of six narrow light cones, one for each interface, with each light cone having a half angle equal to the critical angle. Additional losses due to total intemal reflection can occur at the epoxy/ air interface. Consequently, an eflicient conventional geometry (for example, rectangular parallelepiped) transparent substrate AlInGaP LED encapsulated in epoxy, for example, may have an extemal quantum efliciency of only ~40%, despite having an intemal quantum efficiency of nearly 100%.

The effect of total internal reflection on the light extraction efliciency of LEDs is further discussed in U.S. Pat. Nos. 5,779,924; 5,793,062; and 6,015,719 incorporated herein by reference.

In one approach to improving light extraction efliciency, LEDs are ground into hemispherical shapes. Light emitted from a point in the active region of a hemispherically shaped LED intersects the hemispherical interface at near normal incidence. Thus, total internal reflection is reduced. However, this technique is tedious and wasteful of material. In addition, defects introduced during the grinding process may compromise the reliability and performance of the LEDs.

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In another approach, LEDs are encapsulated (encased) in a material with a dome or hemispherically shaped surface. For example, the epoxy encapsulant of the above example may be dome shaped to reduce losses due to total internal reflection at the epoxy encapsulant/air interface. However, shaping the surface of a low refractive index encapsulant such as epoxy does not reduce losses due to total intemal reflection at the semiconductor/ low index encapsulant interface. Moreover, epoxy encapsulants typically have coefficients of thermal expansion that poorly match those of the semiconductor materials in the LED. Consequently, the epoxy encapsulant subjects the LED to mechanical stress upon heating or cooling and may damage the LED. LEDs are also encapsulated in dome shaped high index glasses, which increase the critical angle for the semiconductor/encapsulant interface. Unfortunately, optical absorption in high index glasses and mechanical stress typically degrade the performance of an LED encapsulated in such glass.

What is needed is a method for increasing the light extraction efficiency of light emitting diodes which does not suffer from the drawbacks of previous methods.

SUMMARY

Light emitting devices with improved light extraction efliciency are provided. The light emitting devices have a stack of layers including semiconductor layers comprising an active region. The stack is bonded to a transparent optical element.

In some embodiments, the optical element is a lens, for example a hemispheric lens or a Fresnel lens. In other embodiments, the optical element is an optical concentrator using, for example, a total internal reflector (TIR). The optical element is formed, for example, from optical glass, III-V semiconductors, II-VI semiconductors, group IV semiconductors and compounds, metal oxides, metal fluorides, diamond, sapphire, zirconium oxide, yttrium aluminum gamet, or combinations thereof. The refractive index of the optical element for light emitted from the active region is preferably greater than about 1 .5, more preferably greater than about 1 .8.

In one embodiment, the transparent optical element is directly bonded to at least one of the semiconductor layers of the stack. In another embodiment, the transparent optical element is directly bonded to a transparent superstrate disposed above the semiconductor layers. The transparent superstrate preferably has a refractive index for light emitted from the active region greater than about 1.8.

In other embodiments, the light emitting device includes a transparent bonding layer disposed between the optical element and a surface of the stack. The transparent bonding layer bonds the optical element to the surface of the stack. In one embodiment, the surface includes a surface of one of the semiconductor layers. In another embodiment, the surface includes a surface of a transparent superstrate layer disposed above the semiconductor layers. The transparent bonding layer is formed, for example, from metals, phosphide compounds, arsemde compounds, antimonide compounds, nitride compounds, or any of the materials listed above for the transparent optical element. In one embodiment, the transparent bonding material has an index of refraction for light emitted from the active region greater than about 1.5, preferably greater than about 1.8.

A method of bonding a transparent optical element to a light emitting device having a stack of layers including semiconductor layers comprising an active region is provided. The method includes elevating a temperature of the optical element and the stack and applying a pressure to press the optical element and the stack together. In one embodiment, the

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method also includes disposing a layer of a transparent bonding material between the stack and the optical element. The bonding method can be applied to a premade optical element or to a block of optical element material which is later formed or shaped into an optical element such as a lens or an optical concentrator.

Bonding a high refractive index optical element to a light emitting device improves the light extraction efliciency of the light emitting device by reducing loss due to total internal reflection. Advantageously, this improvement can be achieved without the use of an encapsulant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of an optical element and a light emitting diode to be bonded to each other in accordance with an embodiment of the present invention.

FIG. 1B is a schematic diagram of an optical element bonded with a bonding layer to a light emitting diode in accordance with an embodiment of the present invention.

FIG. 1C is a schematic diagram of an optical element bonded to a light emitting diode in accordance with another embodiment of the present invention.

FIG. 1D is a schematic diagram of an optical concentrator bonded to a light emitting diode in accordance with another embodiment of the present invention.

FIG. 2 is a schematic diagram of an optical element directly bonded to a light emitting diode in accordance with an embodiment of the present invention.

FIG. 3 is a schematic diagram of an optical element bonded with a bonding layer to a light emitting diode having beveled sides in accordance with an embodiment of the present invention.

FIG. 4 is a schematic diagram of an optical element bonded with a bonding layer to a light emitting diode having substrate and superstrate layers in accordance with an embodiment of the present invention

FIG. 5 is a schematic diagram of an optical element directly bonded to a light emitting diode having substrate and superstrate layers in accordance with an embodiment of the present invention

FIG. 6 is a schematic diagram of an optical element bonded with a bonding layer to a light emitting diode having a “flip chip” geometry in accordance with an embodiment of the present invention.

FIG. 7 is a schematic diagram of an optical element directly bonded to a light emitting diode having a “flip chip” geometry in accordance with an embodiment of the present invention.

FIG. 8 is a schematic diagram of an optical element bonded with a bonding layer to a light emitting diode having an active region substantially perpendicular to the optical element.

FIG. 9 is a schematic diagram of an optical element bonded directly to a light emitting diode having an active region substantially perpendicular to the optical element.

FIG. 10 is a schematic diagram of a light emitting diode located in a recess of a surface of an optical element to which it is directly bonded.

FIG. 11 is a schematic diagram of a light emitting diode located in a recess of a surface of an optical element to which it is bonded with a bonding layer.

DETAILED DESCRIPTION

FIG. 1A depicts a transparent optical element 2 and a light emitting diode (LED) die 4 to be bonded to each other in accordance with an embodiment of the present invention. In FIG. 1B, in accordance with one embodiment of the present

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invention, transparent optical element 2 is bonded to LED die 4 with a transparent bonding layer 6.

The term “transparent” is used herein to indicate that an optical element so described, such as a “transparent optical element,” a “transparent bonding layer,” “transparent substrate,” or a “transparent superstrate” transmits light at the emission wavelengths of the LED with less than about 50%, preferably less than about 10%, single pass loss due to absorption or scattering. The emission wavelengths of the LED may lie in the infrared, visible, or ultraviolet regions of the electromagnetic spectrum. One of ordinary skill in the art will recognize that the conditions “less than 50% single pass loss” and “less than 10% single pass loss” may be met by various combinations of transmission path length and absorption constant. As used herein, “optical concentrator” includes but is not limited to total intemal reflectors, and includes optical elements having a wall coated with a reflective metal or a dielectric material to reflect or redirect incident light.

LED die 4 illustrated in FIGS. 1A and 1B includes a first semiconductor layer 8 of n-type conductivity (n-layer) and a second semiconductor layer 10 of p-type conductivity p-layer). Semiconductor layers 8 and 10 are electrically coupled to active region 12. Active region 12 is, for example, a p-n diode junction associated with the interface of layers 8 and 10. Altematively, active region 12 includes one or more semiconductor layers that are doped n-type or p-type or are undoped. N-contact 14 and p-contact 16 are electrically coupled to semiconductor layers 8 and 10, respectively. Active region 12 emits light upon application of a suitable voltage across contacts 14 and 16. In alternative implementations, the conductivity types of layers 8 and 9, together with contacts 14 and 16, are reversed. That is, layer 8 is a p-type layer, contact 14 is a p-contact, layer 10 is an n-type layer, and contact 16 is an n-contact.

Semiconductor layers 8 and 10 and active region 12 are formed from III-V semiconductors including but not limited to AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, II-VI semiconductors including but not limited to ZnS, ZnSe, CdSe, CdTe, group IV semiconductors including but not limited to Ge, Si, SiC, and mixtures or alloys thereof These semiconductors have indices of refraction ranging from about 2.4 to about 4.1 at the typical emission wavelengths of LEDs in which they are present. For example, III-Nitride semiconductors such as GaN have refractive indices of about 2.4 at 500 nm, and III-Phosphide semiconductors such as InGaP have refractive indices of about 3 .7 at 600 nm.

Contacts 14 and 16 are, in one implementation, metal contacts formed from metals including but not limited to gold, silver, nickel, aluminum, titanium, chromium, platinum, palladium, rhodium, rhenium, ruthenium, tungsten, and mixtures or alloys thereof . In another implementation, one or both of contacts 14 and 16 are formed from transparent conductors such as indium tin oxide.

Although FIGS. 1A and 1B illustrate a particular LED structure, the present invention is independent of the number of semiconductor layers in LED die 4, and independent of the detailed structure of active region 12. Also, LED die 4 may include, for example, transparent substrates and superstrates not illustrated in FIGS. 1A and 1B. It should be noted that dimensions of the various elements of LED die 4 illustrated in the various figures are not to scale.

In one embodiment, a layer of bonding material is applied to a top surface 18 of LED die 4 (FIG. 1A) to form transparent bonding layer 6 (FIG. 1B) with which to bond optical element 2 to LED die 4. Transparent bonding layer 6 is, for example, about 10Angstroms (A) to about 100 microns (um) thick. The bonding material is applied, for example, by conventional