LIGHT EMITTING DEVICE
A light emitting device (LED) is provided. The LED comprises a light emitting structure and a mixed-period photonic crystal structure. The light emitting structure comprises a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. The mixed-period photonic crystal structure is on the light emitting structure.
The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2009-0017997 (filed on Mar. 3, 2009), which is hereby incorporated by reference in its entirety.
BACKGROUNDThe present disclosure relates to light emitting devices (LEDs).
Light emitting devices (LEDs) are semiconductor devices that convert a current into light. After red LEDs was commercialized, red LEDs and green LEDs have been used as light sources for electronic devices including information communication devices.
For example, because a nitride semiconductor such as a gallium nitride (GaN) semiconductor has a high thermal stability and a wide band gap, it is being extensively researched in the field of photonic devices and high-power electronic devices. The research on a nitride semiconductor LED is being focused to improve the light emitting efficiency.
In terms a semiconductor thin layer, the implementation of a high-efficiency LED requires a method for improving an internal quantum efficiency by increasing the probability of the radiative combination of electrons and holes injected into a light emitting layer, and a method for improving a light extraction efficiency so that the light formed in a light emitting layer is effectively outputted from the thin layer.
The improvement of the internal quantum efficiency requires a technology for growing a high-quality thin layer, and a technology for optimizing a thin layer lamination structure to maximize the quantum effect. For the improvement of the light extraction efficiency, Extensive research is being conducted to control the geometric shape of a semiconductor thin layer.
SUMMARYEmbodiments provide light emitting devices (LEDs) having a good light extraction efficiency.
In one embodiment, an LED comprises: a light emitting structure including a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer; and a mixed-period photonic crystal structure on the light emitting structure.
In another embodiment, an LED comprises: a light emitting structure including a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer; a conductive substrate under the light emitting structure; and a mixed-period photonic crystal structure on the conductive substrate.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.
Light emitting devices (LEDs) according to embodiments will be described in detail with reference to the accompanying drawings.
In the description of embodiments, it will be understood that when a layer (or film) is referred to as being “on/over” another layer or substrate, it can be directly on/over another layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being “under/below” another layer, it can be directly under/below another layer, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present.
Embodiment 1The mixed-period photonic crystal structure can be a structure where a first photonic crystal structure with a large period is filled with a second photonic crystal structure with a small period. Such a mixed-period photonic crystal structure is very difficult to implement through a lithography process.
The surface roughness serves to extract the light confined in an LED by total reflection.
In the embodiment, when the roughness or hole of a surface has a spatial period, it is referred to as a photonic crystal. Structural factors representing the photonic crystal are closely related to a light extraction efficiency. Examples of the structural factors include a period, an etching depth, the radius of a hole, and the arrangement of lattices.
In particular, an incident angle providing an effective diffraction efficiency is determined according to the period of the photonic crystal. Therefore, a photonic crystal structure with mixed periods maintains a high diffraction efficiency for various incident angles in comparison with a photonic crystal structure with a single period, thus increasing the light extraction efficiency.
Referring to
Referring to
It can be seen from
In order to fabricate a mixed-period photonic crystal structure, the space of a first photonic crystal structure with a first period must be filled with a second photonic crystal structure with a second period smaller than the first period. This is difficult to implement through a lithography process. The embodiment provides a method for fabricating a mixed-period photonic crystal structure by forming a first photonic crystal structure through a lithography process and a dry etching process and forming a second photonic crystal structure through a wet etching process.
Referring to
The first substrate 100 may be a sapphire (Al2O3) substrate, to which the embodiment is not limited. A wet cleaning process may be performed to remove the impurities of the surface of the first substrate 100.
A first conductivity type semiconductor layer 110 can be formed on the first substrate 100. For example, a chemical vapor deposition (CVD) process, a molecular beam epitaxy (MBE) process, a sputtering process, or a hydride vapor phase epitaxy (HVPE) process may be used to form the first conductivity type semiconductor layer 110. Also, the first conductivity type semiconductor layer 110 may be formed by injecting tri-methyl gallium gas (TMGa), ammonia gas (NH3), nitrogen gas (N2), or silane gas (SiH4) containing n-type impurity such as silicon (Si) into a process chamber.
An active layer 120 can be formed on the first conductivity type semiconductor layer 110. The active layer 120 emits a light of energy determined by the specific energy band of the active layer (light emitting layer) material when electrons injected through the first conductivity type semiconductor layer 110 recombine with holes injected through the second conductivity type semiconductor layer 130. The active layer 120 may have a quantum well structure that is formed by alternately laminating nitride semiconductor layers with different energy bands once or several times. For example, the active layer 120 may have a quantum well structure with an InGaN/GaN structure that is formed by injecting tri-methyl gallium gas (TMGa), ammonia gas (NH3), nitrogen gas (N2), or tri-methyl indium gas (TMIn), to which the embodiment is not limited.
A second conductivity type semiconductor layer 130 can be formed on the active layer 120. For example, the second conductivity type semiconductor layer 130 may be formed by injecting tri-methyl gallium gas (TMGa), ammonia gas (NH3), nitrogen gas (N2), or bisethylcyclopentadienyl magnesium (EtCp2Mg){Mg(C2H5C5H4)2} containing p-type impurity such as magnesium (Mg) into a process chamber, to which the embodiment is not limited.
A second electrode layer 140 may be formed on the second conductivity type semiconductor layer 130. The second electrode layer 140 may include an ohmic contact layer, a reflection layer, a coupling layer, and a second substrate.
For example, the second electrode layer 140 may include an ohmic contact layer, and may be formed by laminating a single metal or a metal alloy several times in order to provide efficient hole injection. Also, the ohmic contact layer may include metal oxide material or metal material. For example, the ohmic contact layer may include at least one of ITO, IZO (In—ZnO), GZO (Ga—ZnO), AZO (Al—ZnO), AGZO (Al—Ga ZnO), IGZO (In—Ga ZnO), IrOx, RuOx, RuOx/ITO, Ni/IrOx/Au, and Ni/IrOx/Au/ITO, to which the embodiment is not limited.
Also, the second electrode layer 140 may include a reflection layer and/or a coupling layer. For example, if the second electrode layer 140 includes a reflection layer, the reflection layer may be formed of a metal layer containing aluminum (Al), argentum (Ag), or an Al or Ag-containing metal alloy. The Al or Ag effectively reflects the light generated from the active layer, thus making it possible to greatly improve the light extraction efficiency of the LED.
Also, for example, if the second electrode layer 140 can include a coupling layer, the reflection layer may serve as the coupling layer or the coupling layer may be formed using nickel (Ni) or aurum (Au).
Also, in the embodiment, the second electrode layer 140 may include a second substrate 200. If the first conductivity type semiconductor layer 110 has a sufficient thickness of about 50 μm or more, a process of forming the second substrate 200 may be omitted.
The second substrate 200 may be formed of highly conductive metal, metal alloy, or conductive semiconductor material in order to provide efficient hole injection. For example, the second substrate 200 may be formed of at least of one of copper (Cu), Cu alloy, Mo, carrier wafer such as Si, Ge, SiGe. The second substrate 200 may be formed using an electrochemical metal deposition process or a eutectic metal based bonding process.
Referring to
The first substrate 100 may be removed using a high-power laser lift-off process or a chemical etching process. Also, the first substrate 100 may be removed using a physical grinding process. The removal of the first substrate 100 exposes the first conductivity type semiconductor layer 110. The exposed first conductivity type semiconductor layer 110 may have a surface defect layer generated when the first substrate 100 is removed. The surface defect layer may be removed through a wet etching process or a dry etching process.
A mixed-period photonic crystal structure can be formed in a partial region R of the exposed first conductivity type semiconductor layer 110. The partial region R of the first conductivity type semiconductor layer 110 may be formed around a first electrode to be formed later.
If a rough surface is applied to an electrode in fabrication of a device based on a mixed-period photonic crystal structure (i.e., a surface roughness), it causes an additional optical loss. According to the embodiment, a mixed-period photonic crystal structure may be formed in a certain region R and an electrode region may be maintained to be planar, as shown in
Hereinafter, with reference to
Referring to
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When the wet etching process is performed after the dry etching without removing the first pattern mask, a roughness can be formed on a surface not covered with the first pattern mask and the height of a hole (or pillar) is further increased.
In the embodiment, the second photonic crystal structure may be formed to have a non-uniform second period as shown in
Referring to
Unlike the embodiment 1, the embodiment 2 can remove a first pattern 310 and performs a wet etching process. Referring to
The second period b may be smaller than the first period. For example, if the first period is about 400 nm to about 3,000 nm, the second period may be about 100 nm to about 800 nm.
Even if the mask of the first pattern 310 is removed and a wet etching process is performed as shown in
A mixed-period photonic crystal structure 115 may be completed by the method of the embodiment 1 or 2. The GaN surface supporting the mask of the first pattern 310 delays the wet etching process, thereby forming a roughness of a fine second period with a size of about 100 nm to about 300 nm. Therefore, through the wet etching process, the surface roughness covers the entire photonic crystal region, thus improving the light extraction efficiency.
For example, in the case of a vertical type GaN LED structure as in the embodiments 1 and 2, a wet etching process can be performed on an n-type GaN layer 110 or an undoped-GaN layer 112 from which a sapphire substrate is removed. If a wet etching process performed after removal of a substrate as shown in
On the other hand, if a wet etching process is performed after Ga ions on the GaN surface are removed through a dry etching process as shown in
According to the embodiment, a fine surface roughness fills the entire photonic crystal space as shown in
According to the embodiment, a first photonic crystal is formed through a lithography process and a dry etching process and a second photonic crystal is formed through a wet etching process to fabricate a mixed-period photonic crystal structure.
Also, the mixed-period photonic crystal structure can increase the light extraction efficiency. That is, the mixed-period photonic crystal structure according to the embodiment can provide better light extraction efficiency characteristics than a single-period photonic crystal structure or a light extraction structure with surface roughness.
Embodiment 3Referring to
The embodiment 3 may use the technical features of the embodiments 1 and 2. The embodiment 3 can be a vertical type LED structure where a light emitting structure including a first conductivity type semiconductor layer 110, an active layer 120, and a second conductivity type semiconductor layer 140 is formed on a conductive substrate 110a, wherein the substrate need not be removed during the fabrication process.
That is, the embodiment 3 can use a conductive substrate and forms a mixed-period photonic crystal structure 115 on a portion of the conductive substrate.
A method for fabricating an LED according to the embodiment 3 will be described with reference to
Referring to
Like the embodiments 1 and 2, a light emitting structure including a first conductivity type semiconductor layer 110, an active layer 120, and a second conductivity type semiconductor layer 140 is formed on the conductive substrate 100a.
A portion of the bottom of the conductive substrate 100a can be removed. For example, a polishing process may be performed to reduce the thickness of the bottom layer of the conductive substrate 100a. The thickness of the conductive substrate 100a after the polishing process may vary according to the application product of a desired device. For example, the conductive substrate 100a with a thickness of about 400 μm to about 500 μm can be polished to a thickness of about 70 μm to about 100 μm, to which the embodiment is not limited.
When a nitride semiconductor thin layer can be formed on the conductive substrate 100a at high temperatures by means of a thin layer growth equipment, the surface crystal quality of the bottom surface of the conductive substrate 100a may degrade due to high thin layer growth temperatures and reactive gases. Thus, polishing the bottom layer of the conductive substrate 100a can improve the electrical characteristics of the device.
Like the embodiment 1 or 2, a mixed-period photonic crystal structure 115 may be formed in a partial region of the conductive substrate 100a.
A first electrode 150 may be formed in the remaining region of the conductive substrate 100a, except the partial region having the mixed-period photonic crystal structure 115 formed therein.
Embodiment 4Referring to
The embodiment 4 may use the technical features of the embodiments 1 to 3. The embodiment 4 can have a horizontal type LED structure and may include a mixed-period photonic crystal structure 115 on the second conductivity type semiconductor layer 140.
Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments.
Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.
Claims
1-20. (canceled)
21. A light emitting device (LED), comprising:
- a light emitting structure comprising a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer; and
- a mixed-period photonic crystal structure on the light emitting structure.
22. The LED of claim 21, wherein the mixed-period photonic crystal structure comprises:
- a first photonic crystal structure having a first period in a partial region of the first conductivity type semiconductor layer; and
- a second photonic crystal structure having a second period in the partial region of the first conductivity type semiconductor layer that includes the first photonic crystal structure.
23. The LED of claim 22, wherein the second period is smaller than the first period.
24. The LED of claim 23, wherein the second period is non-uniform.
25. The LED of claim 23, wherein the second period is uniform.
26. The LED of claim 23, wherein the second period is about 100 nm to about 800 nm.
27. The LED of claim 21, further comprising an undoped semiconductor layer on the first conductivity type semiconductor layer.
28. The LED of claim 27, wherein the mixed-period photonic crystal structure comprises:
- a first photonic crystal structure having a first period in a partial region of the undoped semiconductor layer; and
- a second photonic crystal structure having a second period in the partial region of the undoped semiconductor layer that includes the first photonic crystal structure.
29. The LED of claim 21, further comprising a nonconductive substrate on a side of the light emitting structure that is opposite to a side of the light emitting structure with the mixed-period photonic crystal structure.
30. The LED of claim 29, wherein the mixed-period photonic crystal structure is on the second conductivity type semiconductor layer.
31. A light emitting device (LED), comprising:
- a light emitting structure comprising a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer;
- a conductive substrate on the light emitting structure; and
- a mixed-period photonic crystal structure on the conductive substrate.
32. The LED of claim 31, wherein the mixed-period photonic crystal structure comprises:
- a first photonic crystal structure having a first period in a partial region of the conductive substrate; and
- a second photonic crystal structure having a second period in the partial region of the conductive substrate that includes the first photonic crystal structure.
33. The LED of claim 32, wherein the second period is smaller than the first period.
34. The LED of claim 32, wherein the second period is non-uniform.
35. The LED of claim 32, wherein the second period is uniform.
36. The LED of claim 33, wherein the first period is about 400 nm to about 3,000 nm and the second period is about 100 nm to about 800 nm.
37. The LED of claim 31, further comprising a first electrode on a portion of the conductive substrate without the mixed-period photonic crystal structure.
38. The LED of claim 37, wherein the first electrode comprises at least one of an ohmic contact layer, a reflection layer, and a coupling layer.
39. The LED of claim 31, further comprising a second electrode layer on the second conductivity type semiconductor layer.
40. The LED of claim 39, wherein the second electrode layer comprises at least one of an ohmic contact layer, a reflection layer, a coupling layer, and a second substrate.
41. The LED of claim 31, wherein the conductive substrate comprises at least one of gallium nitride, gallium oxide, zinc oxide, silicon carbide, and a metal oxide.
42. The LED of claim 31, wherein the conductive substrate is polished to a thickness of about 70 μm to about 100 μm.
Type: Application
Filed: Sep 29, 2009
Publication Date: Sep 9, 2010
Inventors: Jin Wook LEE (Seoul), Sun Kyung Kim (Yongin-si)
Application Number: 12/569,435
International Classification: H01L 33/00 (20060101);