US20050152417A1

US20050152417A1 - Light emitting device with an omnidirectional photonic crystal

Info

Publication number: US20050152417A1
Application number: US10/834,619
Authority: US
Inventors: Chung-Hsiang Lin
Original assignee: Han Shin Co Ltd
Current assignee: Han Shin Co Ltd
Priority date: 2004-01-08
Filing date: 2004-04-29
Publication date: 2005-07-14
Also published as: US20050151145A1; US7078736B2

Abstract

A light emitting device includes a light emitting diode and an omnidirectional photonic crystal formed on one of an upper outer surface and a lower outer surface of the light emitting diode and exhibiting a periodic variation in dielectric constant in such a manner so as to introduce an omnidirectional photonic band gap in a given frequency range such that the radiation generated by the light emitting diode in the frequency range for all incident angles and polarizations can be totally reflected by the omnidirectional photonic crystal, and that at least a portion of the radiation with frequencies outside the frequency range can pass through the omnidirectional photonic crystal.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Application No. 093100473, filed on Jan. 8, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a light emitting device including a light emitting diode and an omnidirectional photonic crystal formed on one of an upper outer surface and a lower outer surface of the light emitting diode.
2. Description of the Related Art
FIG. 1 illustrates a conventional light emitting device 3 that includes a light emitting diode (LED) 12, and a Distributed Bragg Reflector (DBR) 14 embedded in the LED 12. The LED 12 includes a substrate 11, an n-type semiconductor layer 121, a p-type semiconductor layer 124, an active layer 123 sandwiched between the n-type and p- type semiconductor layers 121, 124, and n- and p- electrodes 122, 125 formed respectively on the n-type and p- type semiconductor layers 121, 124. The DBR 14 is sandwiched between the substrate 11 and the n-type semiconductor layer 121, and includes periodically stacked dielectric layers 141, 142 so as to impart a periodic variation in dielectric constant to the DBR 14 and so as to reflect radiation resulting from the LED 12, thereby enhancing light emission efficiency of the LED 12.
In order to avoid large lattice mismatch, which has an adverse effect on emission efficiency of the LED 12, during epitaxial growth of the DBR 14 on the substrate 11, the material used for the DBR 14 has to be compatible with the semiconductor material of the LED 12. As a consequence, variation in dielectric constant for the DBR 14 is undesirably limited to one such that the DBR 14 can only provide total reflection for the radiation with a certain incident angle, such as normal incident angle.
FIG. 2 illustrates another conventional light emitting device 4 that includes an LED 12 having a substrate 11, and a metal reflector 15 formed on an outer surface of the substrate 11 so as to completely reflect radiation resulting from the LED 12.
The LED 12 of the light emitting device 4 is manufactured by cutting a wafer having LED dies thereon. Each LED die is subsequently coated with a metal paste, which forms the metal reflector 15, so as to form the light emitting device 4. The wafer is required to be accurately aligned prior to the wafer cutting operation by using a light source and a detector such that light projecting from the light source through the wafer can be detected by the detector to ensure alignment of the wafer. Since the metal reflector 15 will completely reflect the light from the light source, coating of the metal paste onto the wafer prior to the wafer cutting operation would make the cutting operation inoperable. As a consequence, a light feedback apparatus is required to be used for transmitting the light to the detector, which results in a significant increase in the manufacturing cost. Note that the cutting normally begins from the substrate side of the wafer, i.e., the back side of the wafer, in order to avoid damage to the semiconductor layers of the wafer (the LED dies tend to damage due to accumulated heat during cutting if the cutting begins from the semiconductor side of the wafer), which can result in a significant decrease in the production yield.
U.S. Pat. No. 6,130,780 discloses a high omnidirectional reflector that includes a periodic photonic structure with a surface and a refractive index variation along a direction perpendicular to the surface and that exhibits complete reflection of radiation in a given frequency range for all incident angles and polarizations. The entire disclosure of U.S. Pat. No. 6,130,780 is hereby incorporated herein by reference.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a light emitting device including an LED and an omnidirectional photonic crystal formed on one of an upper outer surface and a lower outer surface of the LED so as to overcome the aforesaid drawbacks associated with the prior art.
According to the present invention, there is provided a light emitting device that comprises: a light emitting diode that defines an upper outer surface and a lower outer surface opposite to the upper outer surface; and an omnidirectional photonic crystal formed on one of the upper outer surface and the lower outer surface of the light emitting diode and exhibiting a periodic variation in dielectric constant in such a manner so as to introduce an omnidirectional photonic band gap in a given frequency range such that the radiation generated by the light emitting diode in the frequency range for all incident angles and polarizations can be totally reflected by the omnidirectional photonic crystal, thereby enhancing radiation extraction from the other of the upper outer surface and the lower outer surface of the emitting diode, and that at least a portion of the radiation with frequencies outside the frequency range can pass through the omnidirectional photonic crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention,
FIG. 1 is a schematic view of a conventional light emitting device;
FIG. 2 is a schematic view of another conventional light emitting device;
FIG. 3 is a schematic view of the first preferred embodiment of a light emitting device according to this invention;
FIG. 4 is a schematic fragmentary view of an omnidirectional photonic crystal of the light emitting device of the first preferred embodiment;
FIG. 5 is a plot showing the relation among the band gap size, the refractive index difference, and the thickness of a dielectric layer of a periodic dielectric structure;
FIG. 6 is a plot showing the presence of an omnidirectional photonic band gap in the dispersion relation of the guided modes in a photonic band structure of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment for Example 1;
FIGS. 7 a and 7 b are plots showing the relation between the band gap size and the thickness of a dielectric layer of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment and the relation between the band gap size and the frequency for Example 1;
FIG. 8 is a plot of the average reflectance of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment for Example 1;
FIG. 9 is a plot of the average reflectance and transmittance of a substrate of a light emitting diode of the light emitting device of the first preferred embodiment for Example 1;
FIG. 10 is a plot of the average reflectance of a light emitting structure of the light emitting diode of the light emitting device of the first preferred embodiment for Example 1;
FIG. 11 is a plot showing the presence of an omnidirectional photonic band gap in the dispersion relation of the guided modes in a photonic band structure of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment for Example 2;
FIGS. 12 a and 12 b are plots showing the relation between the band gap size and the thickness of a dielectric layer of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment and the relation between the band gap size and the frequency for Example 2;
FIG. 13 is a plot of the average reflectance of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment for Example 2;
FIG. 14 is a plot of the average reflectance and transmittance of the substrate of the light emitting diode of the light emitting device of the first preferred embodiment for Example 2;
FIG. 15 is a plot of the average reflectance of the light emitting structure of the light emitting diode of the light emitting device of the first preferred embodiment for Example 2;
FIG. 16 is a plot showing the presence of an omnidirectional photonic band gap in the dispersion relation of the guided modes in a photonic band structure of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment for Example 3;
FIGS. 17 a and 17 b are plots showing the relation between the band gap size and the thickness of a dielectric layer of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment and the relation between the band gap size and the frequency for Example 3;
FIG. 18 is a plot of the average reflectance of the omnidirectional photonic crystal of the light emitting device of the first preferred embodiment for Example 3;
FIG. 19 is a plot of the average reflectance and transmittance of the substrate of the light emitting diode of the light emitting device of the first preferred embodiment for Example 3;
FIG. 20 is a plot of the average reflectance of the light emitting structure of the light emitting dode of the light emitting device of the first preferred embodiment for Example 3; and
FIG. 21 is a schematic view of the second preferred embodiment of the light emitting device according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the sake of brevity, like elements are denoted by the same reference numerals throughout the disclosure.
FIGS. 3 and 4 illustrate the first preferred embodiment of a light emitting device according to the present invention. The light emitting device includes: a light emitting diode (LED) 6 that defines an upper outer surface 601 and a lower outer surface 602 opposite to the upper outer surface 601; and an omnidirectional photonic crystal 7 (in this embodiment, the photonic crystal 7 is a one-dimensional periodic dielectric structure) formed on one of the upper outer surface 601 and the lower outer surface 602 of the light emitting diode 6 (in this embodiment, the photonic crystal 7 is formed on the lower outer surface 602) and exhibiting a periodic variation in dielectric constant in such a manner so as to introduce an omnidirectional photonic band gap in a given frequency range such that the radiation generated by the light emitting diode 6 in the frequency range for all incident angles and polarizations can be totally reflected by the omnidirectional photonic crystal 7, thereby enhancing radiation extraction from the other of the upper outer surface 601 and the lower outer surface 602 of the light emitting diode 6, and that at least a portion of the radiation with frequencies outside the frequency range can pass through the omnidirectional photonic crystal 7.
In this embodiment, the light emitting diode 6 includes a light emitting structure and a substrate 61. The light emitting structure includes first and second semiconductor layers 622, 625 and an active layer 624 sandwiched between the first and second semiconductor layers 622, 625. The first and second semiconductor layers 622, 625 are respectively n-type and p-type semiconductor layers, and cooperatively define a p-n junction therebetween. The first semiconductor layer 622 is formed on the substrate 61. The substrate 61 has an outer surface that is opposite to the first semiconductor layer 622 and that defines said one of the upper outer surface 601 and the lower outer surface 602 of the light emitting diode 6, i.e., the lower outer surface 602. The light emitting device further includes n- and p- electrodes 81, 82 that are respectively formed on the first and second semiconductor layers 622, 625.
The omnidirectional photonic crystal 7 includes periodically stacked dielectric units 71, each of which includes first and second dielectric layers 711, 712 (see FIG. 4). By adjusting the thickness (d₁) of the first dielectric layer 711 and setting a refractive index difference between the first and second dielectric layers 711, 712 to be greater than 0.58, a band gap size larger than 3% for the omnidirectional photonic crystal 7 can be achieved. FIG. 5 illustrates the relation among the band gap size, the refractive index difference, n₁-n₂, (n₁, n₂respectively represent the refractive indices of the first and second dielectric layers 711, 712) between the first and second dielectric layers 711, 712, and the thickness (d₁) of the first dielectric layer 711. The thickness (d₁) of the first dielectric layer 711 is related to a lattice constant a of the omnidirectional photonic crystal 7. The lattice constant a is defined as the total thickness of each dielectric unit 71.
Preferably, the thickness (d₁) of the first dielectric layer 711 ranges from 0.24a to 0.69a, and the refrative index difference (n₁-n₂) ranges from 0.9 to 1.2 so as to obtain the omnidirectional photonic band gap with a band gap size larger than 3%, between the frequency range ranging from 0.27c/a to 0.31c/a, wherein c is the speed of light.
Preferably, the first dielectric layer 711 is made from a compound selected from the group consisting of TiO₂, Ta₂O₅, ZrO₂, ZnO, Nd₂O₃, Nb₂O₅, In₂O₃, SnO₂, Sb₂O₃, HfO₂, CeO₂, and ZnS, whereas the second dielectric layer 712 is made from a compound selected from the group consisting of SiO₂, Al₂O₃, MgO, La₂O₃, Yb₂O₃, Y₂O₃, Sc₂O₃, WO₃, LiF, NaF, MgF₂, CaF₂, SrF₂, BaF₂, AlF₃, LaF₃, NdF₃, YF₃, and CeF₃. More preferably, the first dielectric layer 711 is made from TiO₂, and the second dielectric layer 712 is made from SiO₂.
The present invention will be described in more detail with reference to the following Examples.

EXAMPLE 1

In this Example, the first and second semiconductor layers 622, 625 of the light emitting diode 6 are made from GaN material. The substrate 61 is made from sapphire. The light emitting diode 6 is capable of emitting a UV light radiation having a wavelength range ranging from 300 nm to 420 nm. The omnidirectional photonic crystal 7 is formed on the outer surface of the substrate 61 through e-beam evaporation techniques, and includes fourteen dielectric units 71, each of which includes the first and second dielectric layers 711, 712 which are respectively made from TiO₂and SiO₂and which respectively have refractive indices of 2.6 and 1.8, i.e., a refractive index difference of 1.12. The lattice constant a is equal to 110 nm. The thickness (d₁) of the first dielectric layer 711 is equal to 0.42a.
FIG. 6 shows the presence of an omnidirectional photonic band gap in a frequency range between two dash lines in FIG. 6, i.e., between 0.273c/a and 0.3c/a, in the dispersion relation of guided modes in a photonic band structure of the omnidirectional photonic crystal 7 of the light emitting device of Example 1. Definitions of the guided modes, TE, TM, and wave vector or wave number k_yin FIG. 6 can be found in U.S. Pat. No. 6,130,780.
FIGS. 7 a and 7 b illustrate variation of the band gap size of the omnidirectional photonic band gap as a function of the thickness (d₁) of the first dielectric layer 711 (TiO₂) for Example 1. A maximum band gap size of about 10% is obtained when the thickness (d₁) of the first dielectric layer 711 is about 0.42a.
FIG. 8 shows variation of the reflectance of the omnidirectional photonic crystal 7 of the light emitting device of Example 1 as a function of the wavelength. A reflectance greater than 99.5% is obtained for a wavelength range ranging from 369 nm to 401 nm.
FIGS. 9 and 10 respectively show variation of the reflectance and transmittance of the substrate 61 (the refractive index of sapphire is about 1.7 for a wavelength of 385 nm) and the light emitting structure (the refractive index of GaN is about 2.58 for a wavelength of 385 nm) of the light emitting diode 6 of the light emitting device of Example 1 as a function of the wavelength. A reflectance greater than 99.5% is obtained for a wavelength range ranging from 369 nm to 401 nm.
The results show that an overall reflectance greater than 99.5% can be achieved for the light emitting device of Example 1 for a wavelength range ranging from 369 nm to 401 nm.

EXAMPLE 2

The light emitting device of this Example differs from the previous Example in that the light emitting diode 6 is capable of emitting a blue light radiation having a wavelength range ranging from 420 nm to 480 nm, that the thickness (d₁) of the first dielectric layer 711 is equal to 0.42a, and that the first and second dielectric layers 711, 712 respectively have refractive indices of 2.42 and 1.47, i.e., a refractive index difference of 0.95. The lattice constant a is equal to 134 nm.
FIG. 11 shows the presence of an omnidirectional photonic band gap in a frequency range between two dash lines in FIG. 11, i.e., between 0.291c/a and 0.305c/a, in the dispersion relation of guided modes in a photonic band structure of the omnidirectional photonic crystal 7 of the light emitting device of Example 2.
FIGS. 12 a and 12 b illustrate variation of the band gap size of the omnidirectional photonic band gap as a function of the thickness (d₁) of the first dielectric layer 711 (TiO₂) for Example 2. A maximum band gap size of about 5% is obtained when the thickness (d₁) of the first dielectric layer 711 is about 0.44a.
FIG. 13 shows variation of the reflectance of the omnidirectional photonic crystal 7 of the light emitting device of Example 2 as a function of the wavelength. A reflectance greater than 99.5% is obtained for a wavelength range ranging from 440 nm to 464 nm.
FIGS. 14 and 15 respectively show variation of the reflectance of the substrate 61 and the light emitting structure (the refractive index of GaN is about 2.48 for a wavelength of 450 nm) of the light emitting diode 6 of the light emitting device of Example 2 as a function of the wavelength. A reflectance greater than 99.5% is obtained for a wavelength range ranging from 440 nm to 464 nm.
The results show that an overall reflectance greater than 99.5% can be achieved for the light emitting device of Example 2 for a wavelength range ranging from 440 nm to 464 nm.

EXAMPLE 3

The light emitting device of this Example differs from the first Example in that the light emitting diode 6 is capable of emitting a green light radiation having a wavelength range ranging from 480 nm to 550 nm, that the thickness (d₁) of the first dielectric layer 711 is equal to 0.45a, and that the first and second dielectric layers 711, 712 respectively have refractive indices of 2.36 and 1.46, i.e., a refractive index difference of 0.9. The lattice constant a is equal to 151 nm.
FIG. 16 shows the presence of an omnidirectional photonic band gap in a frequency range between two dash lines in FIG. 16, i.e., between 0.297c/a and 0.308c/a, in the dispersion relation of guided modes in a photonic band structure of the omnidirectional photonic crystal 7 of the light emitting device of Example 3.
FIGS. 17 a and 17 b illustrate variation of the band gap size of the omnidirectional photonic band gap as a function of the thickness (d₁) of the first dielectric layer 711 (TiO₂) for Example 3. A maximum band gap size of about 3.5% is obtained when the thickness (d₁) of the first dielectric layer 711 is about 0.45a.
FIG. 18 shows variation of the reflectance of the omnidirectional photonic crystal 7 of the light emitting device of Example 3 as a function of the wavelength. A reflectance greater than 99.5% is obtained for a wavelength range ranging from 492 nm to 512 nm.
FIGS. 19 and 20 respectively show variation of the reflectance of the substrate 61 and the light emitting structure (the refractive index of GaN is about 2.44 for a wavelength of 500 nm) of the light emitting diode 6 of the light emitting device of Example 3 as a function of the wavelength. A reflectance greater than 99.5% is obtained for a wavelength range ranging from 492 nm to 512 nm.
The results show that an overall reflectance greater than 99.5% can be achieved for the light emitting device of Example 3 for a wavelength range ranging from 492 nm to 512 nm.
FIG. 21 illustrates the second preferred embodiment of the light emitting device according to this invention. The light emitting device of this embodiment is similar to the previous embodiment, except that the omnidirectional photonic crystal 7 is formed on the other of the upper outer surface 601 and the lower outer surface 602 of the light emitting diode 6, i.e., the upper outer surface 601. The substrate 61 is transparent so as to permit radiation resulting from the light emitting structure of the light emitting diode 6 to pass therethrough. The omnidirectional photonic crystal 7 completely reflects the radiation in the frequency range to be extracted from the upper outer surface 601 of the light emitting diode 6.
Moreover, a thermal conductor 9 is connected to the first and second semiconductor layers 622, 625 through two connecting members 83 for heat dissipation.
With the inclusion of the omnidirectional photonic crystal 7 in the light emitting device of this invention, and with the omnidirectional photonic crystal 7 being formed on one of the upper outer surface 601 and the lower outer surface 602 of the light emitting diode 6, the light emitting efficiency is enhanced, the expensive light feedback apparatus as required in the cutting operation of the wafer that includes the conventional light emitting dies thereon is dispensed with, and the cutting operation can be conducted at the substrate side of the wafer, thereby eliminating the aforesaid drawbacks associated with the prior art.
With the invention thus explained, it is apparent that various modifications and variations can be made without departing from the spirit of the present invention.

Claims

1. A light emitting device comprising:

a light emitting diode that defines an upper outer surface and a lower outer surface opposite to said upper outer surface; and

an omnidirectional photonic crystal formed on one of said upper outer surface and said lower outer surface of said light emitting diode and exhibiting a periodic variation in dielectric constant in such a manner so as to introduce an omnidirectional photonic band gap in a given frequency range such that the radiation generated by said light emitting diode in said frequency range for all incident angles and polarizations can be totally reflected by said omnidirectional photonic crystal, thereby enhancing radiation extraction from the other of said upper outer surface and said lower outer surface of said emitting diode, and that at least a portion of the radiation with frequencies outside said frequency range can pass through said omnidirectional photonic crystal.

2. The light emitting device of claim 1, wherein said light emitting diode includes first and second semiconductor layers, an active layer sandwiched between said first and second semiconductor layers, and a substrate, said first and second semiconductor layers cooperatively defining a p-n junction therebetween, said first semiconductor layer being formed on said substrate, said substrate having an outer surface that is opposite to said first semiconductor layer and that defines said one of said upper outer surface and said lower outer surface of said light emitting diode.

3. The light emitting device of claim 2, wherein said omnidirectional photonic crystal includes periodically stacked dielectric units, each of which includes first and second dielectric layers that have a refractive index difference greater than 0.58.

4. The light emitting device of claim 3, wherein said omnidirectional photonic crystal defines a lattice constant a that is equal the total thickness of each of said dielectric units, said first dielectric layer having a thickness ranging from 0.24a to 0.69a and said refractive index difference ranging from 0.9 to 1.2 so as to obtain said omnidirectional photonic band gap between said frequency range ranging from 0.27c/a to 0.31c/a, wherein c is the speed of light.

5. The light emitting device of claim 4, wherein said first dielectric layer is made from a compound selected from the group consisting of TiO₂, Ta₂O₅, ZrO₂, ZnO, Nd₂O₃, Nb₂O₅, In₂O₃, SnO₂, Sb₂O₃, HfO₂, CeO₂, and ZnS, and said second dielectric layer is made from a compound selected from the group consisting of SiO₂, Al₂O₃, MgO, La₂O₃, Yb₂O₃, Y₂O₃, Sc₂O₃, WO₃, LiF, NaF, MgF₂, CaF₂, SrF₂, BaF₂, AlF₃, LaF₃, NdF₃, YF₃, and CeF₃.

6. The light emitting device of claim 5, wherein said first dielectric layer is made from TiO₂, and said second dielectric layer is made from SiO₂.

7. The light emitting device of claim 1, wherein said light emitting diode includes first and second semiconductor layers, an active layer sandwiched between said first and second semiconductor layers, and a transparent substrate, said first and second semiconductor layers cooperatively defining a p-n junction therebetween, said first semiconductor layer being formed on said substrate, said second semiconductor layer having an outer surface that is opposite to said active layer and that defines said one of said upper outer surface and said lower outer surface of said light emitting diode.

8. The light emitting device of claim 7, further comprising a thermal conductor that is connected to said first and second semiconductor layers for heat dissipation.