# Patent

## Title
Vertical light emitting diode (VLED) die having electrode frame and method of fabrication

## Abstract

A vertical light emitting diode (VLED) die includes a metal base; a mirror on the metal base; a p-type semiconductor layer on the reflector layer; a multiple quantum well (MQW) layer on the p-type semiconductor layer configured to emit light; and an n-type semiconductor layer on the multiple quantum well (MQW) layer. The vertical light emitting diode (VLED) die also includes an electrode and an electrode frame on the n-type semiconductor layer, and an organic or inorganic material contained within the electrode frame. The electrode and the electrode frame are configured to provide a high current capacity and to spread current from the outer periphery to the center of the n-type semiconductor layer. The vertical light emitting diode (VLED) die can also include a passivation layer formed on the metal base surrounding and electrically insulating the electrode frame, the edges of the mirror, the edges of the p-type semiconductor layer, the edges of the multiple quantum well (MQW) layer and the edges of the n-type semiconductor layer.

## Background
BACKGROUND
This disclosure relates generally to optoelectronic components and more particularly to a vertical light emitting diode (VLED) die, and method of fabrication.
An optoelectronic system, such as a light emitting diode (LED), can include one or more light emitting diode (LED) dice mounted to a substrate. One type of light emitting diode (LED) die, known as a vertical light emitting diode (VLED) die, includes a multi-layer semiconductor substrate made of a compound semiconductor material, such as GaN. The semiconductor substrate can include a p-type confinement layer having p-type dopants, an n-type confinement layer having n-type dopants, and a multiple quantum well (MQW) layer located between the confinement layers configured to emit light.
The vertical light emitting diode (VLED) die can also include an electrode on the n-type confinement layer, and a mirror in contact with the p-type confinement layer. The electrode provides current to the n-type confinement layer for activating the multiple quantum well (MQW) layer to emit light. The mirror reflects the emitted light outward. One problem with this type of vertical light emitting diode (VLED) die is that current can accumulate on specific areas of the n-type confinement layer, particularly near the electrode, limiting the efficiency of the multiple quantum well (MQW) layer. This problem, which is sometimes referred to as “current crowding”, requires higher forward bias (Vf), and also increases power consumption and heat output. One prior art approach to this problem is to form the electrode with a number of legs that spread out the current. However, this approach tends to cover a large area of the multiple quantum well (MQW) layer limiting brightness.
The present disclosure is directed to a vertical light emitting diode (VLED) die having an electrode frame and to a method for fabricating the vertical light emitting diode (VLED) die. The vertical light emitting diode (VLED) can be used to construct light emitting diodes (LED) having improved thermal and electrical characteristics.
SUMMARY
A vertical light emitting diode (VLED) die includes a metal base, a mirror on the metal base, a p-type semiconductor layer on the mirror, a multiple quantum well (MQW) layer on the p-type semiconductor layer configured to emit light, and an n-type semiconductor layer on the multiple quantum well (MQW) layer. The vertical light emitting diode (VLED) die also includes an electrode on the n-type semiconductor layer, an electrode frame on the n-type semiconductor layer in electrical contact with the electrode, and an organic or inorganic material contained within the electrode frame having selected optical characteristics. The electrode frame has a four-sided picture frame outline contained within a peripheral outline of the n-type semiconductor layer, and is configured to provide a high current capacity and to spread current from the outer periphery to the center of the n-type semiconductor layer. The vertical light emitting diode (VLED) die can also include a passivation layer formed on the metal base surrounding and electrically insulating the electrode frame, the edges of the mirror, the edges of the p-type semiconductor layer, the edges of the multiple quantum well (MQW) layer and the edges of the n-type semiconductor layer.
A method for fabricating the vertical light emitting diode (VLED) die includes the step of providing a metal base having mirror, a p-type semiconductor layer on the mirror, a multiple quantum well (MQW) layer on the p-type semiconductor layer configured to emit light, and an n-type semiconductor layer on the multiple quantum well (MQW) layer. The method also includes the step of forming an electrode on the n-type semiconductor layer, and an electrode frame on the n-type semiconductor layer in electrical contact with the electrode configured to spread current from the outer periphery to the center of the n-type semiconductor layer. The method can also include the step of forming a passivation layer configured to electrically insulate the electrode frame, the edges of the mirror, the edges of the p-type semiconductor layer, the edges of the multiple quantum well (MQW) layer and the edges of the n-type semiconductor layer. The method can also include the step of depositing an organic or inorganic material within the electrode frame having selected optical characteristics.



BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments are illustrated in the referenced figures of the drawings. It is intended that the embodiments and the figures disclosed herein are to be considered illustrative rather than limiting.
FIG. 1 is a schematic perspective view of a vertical light emitting diode (VLED) die having an electrode frame;
FIG. 2A is a schematic side elevation view of the vertical light emitting diode (VLED) die taken along the width side;
FIG. 2B is a schematic side elevation view equivalent to FIG. 2A of the vertical light emitting diode (VLED) die with a passivation layer;
FIG. 3A is a schematic side elevation view of the vertical light emitting diode (VLED) die taken along the length side;
FIG. 3B is a schematic side elevation view equivalent to FIG. 3A of the vertical light emitting diode (VLED) die with a passivation layer;
FIG. 4A is a plan view of the vertical light emitting diode (VLED) die illustrating the electrode frame and electrode;
FIG. 4B is a plan view of the vertical light emitting diode (VLED) die illustrating an alternate embodiment with the electrode extending across the electrode frame;
FIG. 4C is a plan view of the vertical light emitting diode (VLED) die illustrating the mirror;
FIG. 5 is a schematic side elevation view of a vertical light emitting diode having a passivation layer and an organic or inorganic material within the electrode frame; and
FIG. 6 is a schematic side elevation view of a vertical light emitting diode having a passivation layer configured as a dam and an organic or inorganic material contained within the dam.



DETAILED DESCRIPTION
Referring to FIG. 1, a vertical light emitting diode (VLED) die 10 (FIG. 1A) includes a metal base 12; a mirror 14 on the metal base 12; a p-type semiconductor layer 16 on the first metal base; a multiple quantum well (MQW) layer 18 (active layer) on the p-type semiconductor layer 16; and an n-type semiconductor layer 20 on the multiple quantum well (MQW) layer 18. The vertical light emitting diode die (VLED) 10 also includes a electrode 22, and an electrode frame 24 on the n-type semiconductor layer 20 in electrical contact with the electrode 22.
The metal base 12, the mirror 14 on the metal base 12, the p-type semiconductor layer 16, the multiple quantum well (MQW) layer 18, and the n-type semiconductor layer 20 are all generally rectangular in peripheral shape. In addition, the electrode frame 24 has a four-sided picture frame shape comprised of two parallel spaced width side walls, and two parallel spaced length side walls orthogonal to the width side walls, contained within the peripheral outline of the n-type semiconductor layer 20. In other words, the electrode frame 24 has a peripheral length, a peripheral width and a peripheral area that are less than the length, the width and the area of the n-type semiconductor layer 20. In addition, as will be further explained, the electrode frame 24 has a peripheral length, a peripheral width and a peripheral area that are greater than (or equal to) the length, the width and the area of the mirror 14, which reduces light blocking by the electrode frame 24.
The metal base 12 (FIG. 1) can have an area greater than that of the a p-type semiconductor layer 16 and the n-type semiconductor layer 20. The metal base 12 can comprise a single metal layer or a stack of two or more metal layers, formed using a suitable deposition process. In addition, the material for the metal base 12 can be selected to provide a high electrical conductivity and a high thermal conductivity. Suitable materials for the metal base 12 include Cu, Ni, Ag, Au, Co, Cu—Co, Ni—Co, Cu—Mo, Ni/Cu, Ni/Cu—Mo and alloys of these metals. Suitable deposition processes for forming the metal base 12 include electro-deposition, electroless-deposition, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), evaporation, and plasma spray.
The mirror 14 (FIG. 1) can be generally rectangular in shape with an area less than that of the p-type semiconductor layer 16. In addition to acting as a reflector, the mirror 14 functions as a anode electrode to provide a current path from the anode (p-type semiconductor layer 16) to the cathode (n-type semiconductor layer 20). The mirror 14 can comprise a single metal layer or stack of metals, such as Ag, Ni/Ag, Ni/Ag/Ni/Au, Ag/Ni/Au, Ti/Ag/Ni/Au, Ag/Pt or Ag/Pd or Ag/Cr, formed by depositing an alloy containing Ag, Au, Cr, Pt, Pd, or Al. A thickness of the mirror 14 can be less than about 1.0 μm. High temperature annealing or alloying can also be used to improve the contact resistance and adhesion of the mirror 14.
The p-type semiconductor layer 16 (FIG. 1) can be generally rectangular in shape with an area greater than that of the mirror 14. A preferred material for the p-type semiconductor layer 16 comprises p-GaN. Other suitable materials for the p-type layer include AlGaN, InGaN and AlInGaN. The multiple quantum well (MQW) layer 18 can comprise a semiconductor material, such as GaAs. As shown in FIG. 1, the multiple quantum well (MQW) layer 18 and the n-type semiconductor layer 20 can have a same rectangular shape as the p-type semiconductor layer 16 but with an decreasing area, such that an epitaxial stack having sloped sidewalls is formed by the semiconductor layers 16, 20 and the multiple quantum well (MQW) layer 18. Stated differently, an epitaxial stack formed by the n-type semiconductor layer 20, the multiple quantum well (MQW) layer 18 and the p-type semiconductor layer 16 is generally pyramidal in shape with the n-type semiconductor layer 20 forming a base and the p-type semiconductor layer forming a flat top. The upper surface of the n-type semiconductor layer 20 can also be flat and planar to provide a base for the electrode frame 24. A preferred material for the n-type semiconductor layer 20 comprises n-GaN. Other suitable materials for the n-type layer include AlGaN, InGaN and AlInGaN.
As will become more apparent as the description proceeds, the geometry of the elements of the vertical light emitting diode (VLED) die 10 (FIG. 1) provide improved performance. For example, with the area of the n-type semiconductor layer 20 greater than the areas of the multiple quantum well (MQW) layer 18, the p-type semiconductor layer 16 and the mirror 14, light blocking is reduced. In particular, the larger area of the n-type semiconductor layer 20 allows the electrode frame 24 to be formed on the n-type semiconductor layer 20 with less light blockage. In addition, as the mirror 14 has the smallest area, current blocking by the mirror 14 is reduced.
The electrode 22 (FIG. 1) can comprise a pad in contact with a side of the electrode frame 24 and having a rectangular or square peripheral outline contained within the electrode frame 24. In addition, the electrode 22 can be configured as a bond pad for wiring bonding to the vertical light emitting diode (VLED) die 10 and to a corresponding support substrate (not shown). Further, the electrode 22 and the electrode frame 24 can comprise a single layer, or a stack of materials, formed using a suitable deposition process such as PVD, E-Gun evaporation, thermal evaporation, electro less chemical deposition, or electrical chemical deposition. Suitable materials for the electrode 22 and the electrode frame 24 can include Ti/Al/Ni/Au, Al/Ni/Au, Al/Ni/Cu/Au and Ti/Al/Ni/Cu/Ni/Au.
In FIG. 1 the following geometrical features are identified. In addition representative dimensions are listed below.
W: The width of the outer periphery of the electrode frame 24 (1 μm ̃1000 μm)
L: The length of the outer periphery of the electrode frame 24 (1 μm ̃25 mm (250000 μm))
E: The wall width of the electrode frame 24 (1 μm ̃100 μm)
d: The distance from the outer periphery of the electrode frame 24 to the edge of the n-type semiconductor layer 20 (1 μm ̃100 μm)
dE: The thickness of the electrode frame 24 (1 μm ̃100 μm)
Bw: The width of the electrode 22 (10 μm ̃1000 μm)
BL: The length of the electrode 22 (10 μm ̃300 μm)
The height of electrode 22 is the same as dE,
In the vertical light emitting diode (VLED) die 10 (FIG. 1), the width W of the outer periphery of the electrode frame 24 can be optimized to provide enough current capacity to spread the current on the n-type semiconductor layer 20 from the outer periphery or sides to the center thereof. In addition, the four-sided, picture frame peripheral outline of the electrode frame 24 has an inner spacing d from the peripheral edge of the n-type semiconductor layer 20. The length L of electrode frame 24 can be selected to satisfy a required emission area. For example, the required emission area can be A, such that length L can be selected to be L=A/W. Depending on the brightness requirement, a longer L provides a larger emission area, A which can provide higher brightness.
Also in the vertical light emitting diode (VLED) die 10 (FIG. 1), the width W of the outer periphery of the electrode frame 24 can also be optimized to correspond to the lateral current spreading ability of the n-type semiconductor layer 20. For example, the current spreading ability of the n-type semiconductor layer 20 is dependent on electron conductivity, electron mobility, and thickness. A thicker n-type semiconductor layer 20 has more room to spread the current from the edge to the center. The electron conductivity, and mobility of the n-type semiconductor layer 20 is dependant on the optimization of doping concentration.
Also in the vertical light emitting diode (VLED) die 10 (FIG. 1), the thickness dE of the electrode frame 24 can range from 1 μm to 100 μm. A thicker dE provides the electrode frame 24 with a lower resistance to spread the current faster. In addition, higher current injection can provide higher brightness. Further, the uniform and fast current conduction on the optimized thickness dE of electrode frame 24 can avoid the local non-uniform high current density around the electrode 22. The local high current density around the electrode 22 can generate local high thermal density and damage the vertical light emitting diode (VLED) die 10. In other words, the optimized thickness of electrode frame 24 can help to spread not only current, but any locally generated thermal hot spot can also be dissipated. Still further, a thicker electrode frame 24 can also provide lower resistance such that the operation voltage at high current can be reduced.
Referring to FIGS. 2A and 3A, in the vertical light emitting diode (VLED) die 10, the width Wm of the mirror 14 is shown (where Wm<W+2d or optionally Wm=W or Wm<W). As shown in FIG. 3A, the mirror 14 also has a length Lm and is spaced from the inner periphery of the electrode frame 24 (where Lm<L+2d or optionally Lm=L or Lm<L). The spacing of the mirror 14 from the electrode frame 24 is designed to provide a current blocking area for the electrode frame 24. In addition, the mirror 14 underneath the electrode 22 provides a current blocking area with almost no electron-hole recombination underneath of the electrode 22. This allows a higher current density and a higher brightness for a given emission area A.
Referring to FIGS. 2B and 3B, the vertical light emitting diode (VLED) die 10 can also include a passivation layer 26 formed on the metal base 12 surrounding and electrically insulating the electrode frame 24. In addition, the passivation layer 26 electrically insulates the edges of the mirror 14, the edges of the p-type semiconductor layer 16, the edges of the multiple quantum well (MQW) layer 18 and the edges of the n-type semiconductor layer 20. The passivation layer 26 can comprise an electrically insulating deposited material having a height of dp on the n-type semiconductor layer 20. (Optionally dp<dE, dp=dE, dp>dE) Suitable materials for the passivation layer 20 include photosensitive or non-photosensitive materials, resist, polymers, polyimide, epoxy, thermoplastics, parylene, dry film resist, silicone, SU8 and NR7.
Referring to FIGS. 4A-4C, additional features of the electrode frame 24, the electrode 22 and the mirror 14 are illustrated. As shown in FIG. 4C, the length Lm of the mirror 014 can be smaller than that of the length L+2d (or optionally Lm=L or Lm<L) and the Wm of the mirror 14 can be smaller than that of the width W+2d (or optionally Wm=W or Wm<W). In addition, the electrode current blocking area 30 (FIG. 4C) is dependant on the design of electrode 22. For example, WB has almost the same size as Bw, and WL has almost the same size as BL. Optionally as shown in FIG. 4B, the electrode 22 can extend across the width of the electrode frame 24 and can be partially covered by the electrode frame 24.
Referring to FIG. 5, the vertical light emitting diode (VLED) die 10 can also include an organic or inorganic material 28 contained within the electrode frame 24 having selected optical characteristics. In addition, the electrode frame 24 can be configured as a dam for containing the organic or inorganic material 28. In particular, the organic or inorganic material 28 is contained within the four sidewalls of the electrode frame 24 and by the upper surface of the n-type semiconductor layer 20. Also in this embodiment the height of the passivation layer 26 dp (FIG. 2B) is preferably less than the height dE (FIG. 1) of the electrode frame 24 (dp<dE). The organic or inorganic material 28 can comprise a light transmissive material such as silicone. In addition, the organic or inorganic material 28 can be selected to provide desired optical characteristics such as spectrum conversion or high reflective index.
Referring to FIG. 6, the vertical light emitting diode (VLED) die 10 can also include the organic or inorganic material 28 having selected optical characteristics contained within the passivation layer 26. Also in this embodiment the height of the passivation layer 26 dp (FIG. 2B) is preferably greater than the height dE (FIG. 1) of the electrode frame 24 (dp>dE). In addition, the passivation layer 26 can be configured as a dam for containing the organic or inorganic material 28. Further, the organic or inorganic material 28 can also encapsulate the electrode frame 28.
Thus the disclosure describes an improved vertical light emitting diode (VLED) die and method of fabrication. While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and subcombinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.

## Claims

1. A vertical light emitting diode (VLED) die comprising:
a metal base;
a mirror on the metal base;
a p-type semiconductor layer on the mirror;
a multiple quantum well (MQW) layer on the p-type semiconductor layer configured to emit light;
an n-type semiconductor layer on the multiple quantum well (MQW) layer having a peripheral outline and a center;
an electrode on the n-type semiconductor layer; and
an electrode frame on the n-type semiconductor layer in electrical contact with the electrode, the electrode frame contained within the peripheral outline of the n-type semiconductor layer configured to spread current from an outer periphery to a center of the n-type semiconductor layer,
wherein the electrode frame has a generally rectangular shape with an outer periphery width W and an outer periphery length L, and the mirror has a width Wm less than or equal to the width W of the electrode frame and a length Lm less than or equal to the length L of the electrode frame.

2. The vertical light emitting diode (VLED) die of claim 1 wherein the electrode comprises a pad in contact with a side of the electrode frame.

3. The vertical light emitting diode (VLED) die of claim 1 further comprising a passivation layer on the metal base surrounding and electrically insulating the electrode frame, the edges of the mirror, the edges of the p-type semiconductor layer, the edges of the multiple quantum well (MQW) layer and the edges of the n-type semiconductor layer.

4. The vertical light emitting diode (VLED) die of claim 3 wherein the passivation layer is configured as a dam and further comprising an organic or inorganic material contained within the dam having selected optical characteristics.

5. The vertical light emitting diode (VLED) die of claim 1 further comprising an organic or inorganic material contained within the electrode frame having selected optical characteristics.

6. The vertical light emitting diode (VLED) die of claim 1 wherein the p-type semiconductor layer and the n-type semiconductor layer comprises a material selected from the group consisting of GaN, AlGaN, InGaN and AlInGaN.

7. The vertical light emitting diode (VLED) die of claim 1 wherein the electrode comprises a bond pad contained within the electrode frame in contact with a side of the electrode frame.

8. The vertical light emitting diode (VLED) die of claim 1 wherein the length L of the electrode frame is selected to satisfy a required emission area A where L=A/W.

9. The vertical light emitting diode (VLED) die of claim 1 wherein the n-type semiconductor layer has a first area greater than the a second area of the mirror.

10. The vertical light emitting diode (VLED) die of claim 9 further comprising an organic or inorganic material contained within the electrode frame having selected optical characteristics.

11. The vertical light emitting diode (VLED) die of claim 9 wherein the p-type semiconductor layer and the n-type semiconductor layer comprises a material selected from the group consisting of GaN, AlGaN, InGaN and AlInGaN.

12. The method of claim 11 wherein the p-type semiconductor layer and the n-type semiconductor layer comprise a material selected from the group consisting of GaN, AlGaN, InGaN and AlInGaN.

13. The vertical light emitting diode (VLED) die of claim 9 wherein the electrode comprises a bond pad contained within the electrode frame in contact with a side of the electrode frame.

14. A vertical light emitting diode (VLED) die comprising:
a metal base;
a mirror on the metal base having a width Wm and a length Lm;
a p-type semiconductor layer on the mirror;
a multiple quantum well (MQW) layer on the p-type semiconductor layer configured to emit light;
an n-type semiconductor layer on the multiple quantum well (MQW) layer having a peripheral outline and a center;
an electrode on the n-type semiconductor layer; and
an electrode frame on the n-type semiconductor layer in electrical contact with the electrode configured to spread current from an outer periphery to a center of the n-type semiconductor layer, the electrode frame having a four-sided picture frame shape spaced from the peripheral outline of the n-type semiconductor layer by a distance d, the electrode frame having a peripheral width W greater than or equal to Wm, a peripheral length L greater than or equal to Lm, a wall width E and a thickness of dE.

15. The vertical light emitting diode (VLED) die of claim 14 wherein the length L is selected to satisfy a required emission area A where L=A/W.

16. The vertical light emitting diode (VLED) die of claim 14 wherein the thickness dE is from 1 μm to 100 μm.

17. The vertical light emitting diode (VLED) die of claim 14 wherein the p-type semiconductor layer, the multiple quantum well (MQW) layer and the n-type semiconductor layer form a generally pyramidal shaped epitaxial stack with the n-type semiconductor layer forming a base and the p-type semiconductor layer forming a flat top.

18. The vertical light emitting diode (VLED) die of claim 14 further comprising a passivation layer on the metal base surrounding and electrically insulating the electrode frame, the edges of the mirror, the edges of the p-type semiconductor layer, the edges of the multiple quantum well (MQW) layer and the edges of the n-type semiconductor layer.

19. The vertical light emitting diode (VLED) die of claim 18 wherein the passivation layer is configured as a dam and further comprising an organic or inorganic material contained within the dam having selected optical characteristics.

20. A method for fabricating a vertical light emitting diode (VLED) die comprising:
providing a metal base comprising a mirror having a width Wm and a length Lm, a p-type semiconductor layer on the mirror, a multiple quantum well (MQW) layer on the p-type semiconductor layer configured to emit light, and an n-type semiconductor layer on the multiple quantum well (MQW) layer having a peripheral outline;
forming an electrode on the n-type semiconductor layer; and
forming an electrode frame on the n-type confinement layer in electrical contact with the electrode, the electrode frame contained within the peripheral outline of the n-type semiconductor layer configured to spread current from an outer periphery of the n-type semiconductor layer to a center of the n-type semiconductor layer, the electrode frame having a four sided shape with an outer periphery width W and an outer periphery length L, and with the width Wm of the mirror less than or equal to the width W of the electrode frame and the length Lm of the mirror less than or equal to the length L of the electrode frame.

21. The method of claim 20 further comprising forming a passivation layer on the metal base configured to electrically insulate the electrode frame, edges of the mirror, edges of the p-type semiconductor layer, edges of the multiple quantum well (MQW) layer and edges of the n-type semiconductor layer.

22. The method of claim 21 further comprising forming the passivation layer as a dam and depositing an organic or inorganic material within the dam having selected optical characteristics.

23. The method of claim 20 further comprising depositing an organic or inorganic material within the electrode frame having selected optical characteristics.