U.S. patent application number 13/294984 was filed with the patent office on 2012-03-08 for multi-junction led.
Invention is credited to Ghulam Hasnain, Syn-Yem Hu, Steven D. Lester, Jeff Ramer.
Application Number | 20120058584 13/294984 |
Document ID | / |
Family ID | 42666643 |
Filed Date | 2012-03-08 |
United States Patent
Application |
20120058584 |
Kind Code |
A1 |
Hasnain; Ghulam ; et
al. |
March 8, 2012 |
Multi-Junction LED
Abstract
A light source and method for making the same are disclosed. The
light source includes a substrate and a light emitting structure
that is deposited on the substrate. A barrier divides the light
emitting structure into first and second segments that are
electrically isolated from one another. A serial connection
electrode connects the first segment in series with the second
segment. A first blocking diode between the light emitting
structure and the substrate prevents current from flowing between
the light emitting structure and the substrate when the light
emitting structure is emitting light. The barrier extends through
the light emitting structure into the first blocking diode.
Inventors: |
Hasnain; Ghulam; (Livermore,
CA) ; Lester; Steven D.; (Livermore, CA) ; Hu;
Syn-Yem; (Livermore, CA) ; Ramer; Jeff;
(Livermore, CA) |
Family ID: |
42666643 |
Appl. No.: |
13/294984 |
Filed: |
November 11, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12725424 |
Mar 16, 2010 |
8084775 |
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13294984 |
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Current U.S.
Class: |
438/42 ;
257/E33.066 |
Current CPC
Class: |
H01L 2924/0002 20130101;
H01L 33/44 20130101; H01L 33/62 20130101; H01L 27/153 20130101;
H01L 2924/0002 20130101; H01L 33/145 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
438/42 ;
257/E33.066 |
International
Class: |
H01L 33/62 20100101
H01L033/62 |
Claims
1. A method for fabricating a light source, said method comprising
depositing a transition layer comprising a semiconductor material
of a first conductivity type on a substrate; depositing a blocking
diode layer of an opposite conductivity type on said transition
layer; depositing a light emitting structure on said blocking diode
layer generating a barrier that divides said light emitting
structure into first and second segments; and depositing a serial
connection electrode that connects said first and second substrates
in series, wherein said barrier extends through said light emitting
structure into said first blocking diode layer, said blocking diode
layer preventing current from flowing from said light emitting
structure to said substrate when said light emitting structure is
generating light.
2. The method of claim 1 wherein said barrier is generated by
etching a trench extending through said light emitting structure to
said transition layer, but not to said substrate.
3. The method of claim 2 wherein depositing said serial connection
electrode comprises depositing an insulating layer in said trench,
and depositing a layer of electrically conducting material in said
trench over said insulating layer, said insulating layer preventing
said layer of electrically conducting material from making direct
contact with walls of said light emitting structure that are
exposed in said trench.
4. The method of claim 2 wherein said insulating layer underlies a
portion of said serial connection electrode that overlies said
light emitting structure.
5. The method of claim 1 wherein said substrate semiconductor layer
comprises a plurality of layers of said opposite conductivity type
arranged such that said plurality of layers comprise a plurality of
serially connected reverse-biased diodes when said light emitting
structure generates light.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of co-pending application Ser. No.
12/725,424 filed on 26 Mar. 2010
BACKGROUND OF THE INVENTION
[0002] Light emitting diodes (LEDs) are an important class of
solid-state devices that convert electric energy to light.
Improvements in these devices have resulted in their use in light
fixtures designed to replace conventional incandescent and
fluorescent light sources. The LEDs have significantly longer
lifetimes and, in some cases, significantly higher efficiency for
converting electric energy to light. LED-based white light sources
are typically made by packaging one or more blue LED chips with
suitable yellow and red phosphors.
[0003] For the purposes of this discussion, an LED chip can be
viewed as a semiconductor having three layers, the active layer
sandwiched between two other layers. The active layer emits light
when holes and electrons from the outer layers combine in the
active layer. The holes and electrons are generated by passing a
current through the LED chip. The LED chip is powered through an
electrode that overlies the top layer and a contact that provides
an electrical connection to the bottom layer.
[0004] The cost of LED chips and their power conversion efficiency
are important factors in determining the rate at which this new
technology will replace conventional light sources and be utilized
in high power applications. The power conversion efficiency of an
LED chip is defined to be the ratio of optical power emitted by the
LED chip in the desired region of the optical spectrum to the
electrical power dissipated by the light source. Electrical power
that is not converted to light that leaves the LED is converted to
heat that raises the temperature of the LED. Rise in the chip
temperature places a limit on the power level at which an LED
operates. In addition, the conversion efficiency of the LED
generally decreases with increasing current especially at the
higher current densities that enable lowering the cost of light;
hence, while increasing the light output of an LED by increasing
the current increases the total light output, the electrical
conversion efficiency is decreased by this strategy. In addition,
the lifetime of the LED is also decreased by operation at high
currents.
[0005] LED light sources made from a single LED chip even as large
as a square millimeter in size, are not yet capable of generating
sufficient light to replace conventional light sources for many
applications. In general, there is a limit to the light per unit
area of LED that can be practically generated at an acceptable
power conversion efficiency. This limit is imposed by the power
dissipation and the electrical conversion efficiency of the LED
material system. Hence, to provide a higher intensity single LED
source, larger area chips must be utilized; however, the light
extraction efficiency reduces as chip size gets bigger for most
types of LED chips and also there is a limit to the size of a
single LED chip that is imposed by the fabrication process used to
make the LED chips. As the chip size increases, the yield of chips
due to random defects decreases, and hence, the cost per LED chip
increases faster than the increase in light output once the chip
size increases beyond a predetermined size.
[0006] Hence, for many applications, an LED-based light source must
utilize multiple LEDs to provide the desired light output. For
example, to replace a 100-watt incandescent bulb for use in
conventional lighting applications, approximately 25 LED chips of
the order of 1 mm.sup.2 size are required. This number can vary
depending on the color temperature desired and the exact size of
the chips. The drive voltage for a typical GaN LED chip is
typically about 3.2-3.6V. If all of the LED chips are connected in
parallel, the DC power supply must deliver a large current at a low
voltage, which presents challenges in terms of AC to DC power
conversion efficiency and the size of the conductors that must be
used to deliver the high currents without dissipating a significant
fraction of the power in resistive losses.
[0007] One method for reducing these problems involves dividing a
die of more or less optimum size into a plurality of series
connected LED segments. Such a structure is shown in co-pending
application Ser. No. 12/208,502, filed on Sep. 11, 2008, which is
hereby incorporated by reference. The optimum size of a die depends
on the details of the chip design and on the yield of the
semiconductor process used to fabricate the dies. For any given
process there is an optimum size from a cost point of view. If the
die is used as a single LED with a drive voltage of the order of 3
volts, a large current must be provided at the die to maximize the
light output. If the die is divided into N smaller LED segments
that are connected in series, the drive voltage is increased by a
factor of N, and the drive current is decreased by a factor of N,
which provides improvements both in the efficiency of the power
supply that provides the drive current and a reduction in the
resistive losses within the die.
[0008] One prior art method for dividing the die into the component
LED segments involves cutting isolation trenches that extend from
surface of the die to the underlying resistive substrate to isolate
the individual component LEDs from one another. The individual
component LEDs are then connected in series by providing a
conductor that connects the n-layer of each component LED to the
p-layer of an adjacent component LED. These deep trenches increase
the cost of production of the dies and interfere with the
extraction of light from the sides of the die.
SUMMARY OF THE INVENTION
[0009] The present invention includes light source and method for
fabricating the same. The light source includes a substrate and a
light emitting structure that is deposited on the substrate. A
barrier divides the light emitting structure into first and second
segments that are electrically isolated from one another. A serial
connection electrode connects the first segment in series with the
second segment. A first blocking diode between the light emitting
structure and the substrate prevents current from flowing between
the light emitting structure and the substrate when the light
emitting structure is emitting light. The barrier extends through
the light emitting structure into the first blocking diode.
[0010] In one aspect of the invention, the substrate includes a
transition layer of semiconductor material that is transparent to
light generated by the light emitting structure. The barrier
includes a trench extending through the light emitting structure
and terminating in the transition layer. Light from the first
segment can travel through the transition layer between the
segments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a top view of segmented LED 60.
[0012] FIG. 2 is a cross-sectional view of segmented LED 60 through
line 2-2 shown in FIG. 1.
[0013] FIG. 3 is a cross-sectional view of a GaN segmented LED
light source according to one embodiment of the present
invention.
[0014] FIG. 4 is a top view of a segmented LED 70 according to
another embodiment of the present invention.
[0015] FIGS. 5A-5C are cross-sectional views of a light source 80
at various stages in the fabrication process.
[0016] FIG. 6 is a cross-sectional view of another embodiment of a
light source according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE
INVENTION
[0017] The manner in which the present invention provides its
advantages can be more easily understood with reference to FIGS. 1
and 2, which illustrate a die containing a segmented LED. FIG. 1 is
a top view of segmented LED 60, and FIG. 2 is a cross-sectional
view of segmented LED 60 through line 2-2 shown in FIG. 1.
Segmented LED 60 includes two segments 64 and 65; however, it will
be apparent from the following discussion that light sources having
many more segments can be constructed from the teachings of the
present invention. Segmented LED 60 is constructed from the same
three-layer LED structure in which the layers are grown on a
sapphire substrate 51. The n-layer 52 is grown on substrate 51, and
then the active layer 55 and p-layer 53 are grown over n-layer
52.
[0018] The segments 64 and 65 are separated by an isolation trench
66 that extends through layer 52 to substrate 51 thereby
electrically isolating segments 64 and 65. Isolation trench 66
includes a plateau 67 that extends only partially into layer 52.
The walls of isolation trench 66 are covered by an insulating layer
57 that includes an open area 58 for making electrical contact to
the portion of layer 52 associated with each segment. Insulating
layer 57 can be constructed from any material that provides an
insulating layer that is free of pinhole defects. For example,
SiNx, SiOx, or other such dielectric films commonly used in
semiconductor device fabrication can be used as the insulating
material. Other materials can include polyimide, BCB, spin-on-glass
and materials that are routinely used in the semiconductor industry
for device planarization.
[0019] Similar trenches are provided on the ends of segmented LED
60 as shown at 68 and 69. A serial connection electrode 59 is
deposited in isolation trench 66 such that electrode 59 makes
contact with layer 52 through opening 58 in insulating layer 57.
Electrode 59 also makes electrical contact with indium tin oxide
(ITO) layer 56 in the adjacent segment. Hence, when power is
provided via electrodes 61 and 62, segments 64 and 65 are connected
in series. As a result, segmented LED 60 operates at twice the
voltage and half the current as a conventional LED.
[0020] It should be noted that layers 52, 53, and 55 are not shown
to scale in FIG. 2. In general, layer 52 is much thicker than layer
53, since the p-type material has a very high resistivity for the
GaN family of materials, and hence, the thickness of this layer is
kept as thin as possible to avoid resistive losses in the layer. It
should also be noted that a significant fraction of the light
generated in active layer 55 is trapped in layers 52 and 53 due to
the large difference in index of fraction between the GaN material
layers and the surrounding medium. Normally, this light exits the
die through the side surfaces of the die and is directed upward by
a suitable reflector. Since layer 52 is much thicker than layer 53,
most of this horizontally traveling light is in layer 52.
Accordingly, the deep trenches through layer 52 interrupt the
transmission of this trapped light. If the material in the trench
(i.e., the material that insulates the walls of the trench or the
conductor) is opaque, this light will be lost. Even in cases in
which the trench is filled with a transparent material, the
difference in index of refraction between that material and the GaN
material results in reflections that, in turn, lead to light
losses. Finally, as noted above, cutting the deep trenches through
all three layers results in increased fabrication cost.
[0021] Accordingly, it would be advantageous to provide a segmented
LED design in which the deep trenches described above are not
utilized to isolate the individual component LEDs. In principle,
the depth of the trenches could be reduced by reducing the
thickness of layer 52. However, there is a minimum thickness for
this layer that is dictated by the need to compensate for the
difference in lattice constants between the LED material and that
of substrate 51. In addition, reducing the thickness does not solve
the problem of light losses due to the interruption of layer 52 by
the trenches.
[0022] Refer now to FIG. 3, which is a cross-sectional view of a
GaN segmented LED light source according to one embodiment of the
present invention. Light source 20 includes only two component LEDs
shown at 41 and 42; however, light sources with more component LEDs
can be constructed in an analogous manner. Light source 20 can be
viewed as a segmented LED 44 that is constructed on a compound
substrate 43. Compound substrate 43 is constructed on a sapphire
substrate 21 on which an n-GaN layer 22 is deposited followed by a
p-GaN layer 23. Layer 22 is as thick as the conventional n-GaN
layers used in conventional LEDs, and hence, provides the
advantages associated therewith such as compensating for the
lattice mismatch between the sapphire substrate and the various GaN
layers. A reverse-biased diode is formed by layer 24, which is
discussed below, and layer 23, and hence, current is blocked from
flowing into layer 22 during the operation of the light source.
[0023] Segmented LED 44 is constructed from a thin n-GaN layer 24,
an active layer 25, and a p-GaN layer 26. A current spreading layer
27 is deposited over layer 26. Layer 27 is typically ITO. Light
source 20 is powered by applying a potential difference between
electrodes 32 and 33.
[0024] The segments of segmented LED 44 are isolated from one
another by a trench 36 that extends through layers 24-26 into layer
22. Since no current can flow through the junction of layer 24 and
layer 23, the trench does not need to extend to the sapphire
substrate 21. Accordingly, a much shallower trench can be utilized
to isolate the segments. In addition, light traveling sideways
through layer 22 is no longer interrupted by the trench and any
material deposited in the trench, such as the insulator shown at 39
that provides a structure on which serial connecting electrode 31
is deposited. Hence, the problems associated with interrupting the
flow of light in the horizontal direction are significantly
reduced.
[0025] In principle, the trench only needs to extend to the top
surface of layer 22. However, controlling the etch rate such that
the trench stops on layer 22 presents problems. Accordingly, the
trench is etched slightly into layer 22 as shown in FIG. 3 to
assure that the horizontal transmission of the current is
blocked.
[0026] In the embodiments shown in FIGS. 1 and 2, electrode 59
extends over the entire width of segmented LED 60. The portion of
segment 65 that underlies electrode 59 is non-productive since
light generated below electrode 59 is blocked and absorbed by
electrode 59. This leads to reduced light conversion efficiency as
well as reduced efficiency in utilization of the die surface, and
hence, increased cost for the light source since additional active
die area must be provided to compensate for this lost area. Refer
now to FIG. 4, which is a top view of a segmented LED 70 according
to another embodiment of the present invention.
[0027] Segmented LED 70 differs from segmented LED 60 in that the
wide interconnect electrode 59 has been replaced by a plurality of
serial electrodes such as electrodes 78 and 79. These electrodes
can be only 5-10 microns wide and spaced approximately 150 microns
apart, and thus, cover a much smaller area on segment 65 than
electrode 59. Accordingly, the loss in efficiency discussed above
is substantially reduced. In addition, the n-electrode 72 and
p-electrode 71 have been replaced by narrow electrodes that include
wider pads 71' and 72' for wire bonding to external circuitry. In
one preferred embodiment, the serial electrodes are spaced apart by
a distance that is more than 5 times the width of the electrodes so
that the area covered by the serial electrodes is significantly
less than the width of the segments that are being connected in the
segmented LED.
[0028] The number of serial connection electrodes that are needed
depends on the conductivity of ITO layer 56. There must be
sufficient serial connection electrodes to assure that current is
spread evenly over ITO layer 56. The width of the serial connection
electrodes is set by the amount of current that must be passed
between segments, and hence, depends on the conductor used, the
thickness of the conductor, and the number of serial connection
electrodes. In the regions of segment 65 that are not covered by a
serial connection layer, the isolation trench 77 does not require
an insulating layer, and hence, the underlying LED structure
receives power and generates useful light.
[0029] Refer now to FIGS. 5A-5C, which are cross-sectional views of
a light source 80 at various stages in the fabrication process. The
process starts by growing the various GaN layers on a sapphire
substrate 81. These layers include the layers that form a compound
substrate 82 and the layers 83 that form the LED segments. The
compound substrate layers include an n-GaN layer 82a that mitigates
the problems associated with the lattice mismatch between GaN and
sapphire, and a p-GaN layer 82b that provides the current blocking
layer that prevents current from flowing into layer 82a during the
operation of the light source.
[0030] The LED segments are constructed from the three layers shown
at 83a-83c, namely, an n-GaN layer 83a, an active layer 83b, and a
p-GaN layer 83c. The combination of layer 83c and layer 82b form a
reverse-biased diode that prevents current from flowing into layers
82b and 82a during the operation of the light source.
[0031] It should be noted that each of the layers discussed above
may include a plurality of sub-layers. For example, the active
layer 83b typically includes a plurality of quantum well layers
separated by buffer layers. To simplify the discussion, the various
sub-layers have been omitted, as those layers are conventional in
the art.
[0032] Refer now to FIG. 5B, which illustrates light source 80
after the isolation trench(s) has been etched to isolate the
various component LEDs that are to be serially connected to form
the final light source. The isolation trench is shown at 84b and
extends down to 82a. Additional trenches are cut as shown at 84a
and 84c to provide the anode and cathode contacts. A patterned
layer of an insulator such as SiNx is then deposited to protect the
side surfaces of the LED related layers and to insulate the areas
that do not generate light to prevent power from being wasted in
these areas.
[0033] Refer now to 5C. After the insulating layer 85 is deposited,
the insulating bridges 87 are deposited and a patterned layer of
ITO is deposited as shown at 86. Finally, the anode, serial
connecting electrodes, and cathode are deposited as a patterned
metal layer as shown at 88, 89, and 90, respectively.
[0034] The above-described embodiments utilize a single reverse
biased electrode to insulate the underlying n-GaN layer from the
LED segments. However, embodiments in which a plurality of diodes
are deposited before depositing the LED segment layers can also be
constructed. Refer now to FIG. 6, which is a cross-sectional view
of another embodiment of a light source according to the present
invention. Light source 110 includes two component LEDs that are
constructed in a manner analogous to that discussed above with
respect to the embodiments shown in 5A-5C. Light source 110 differs
from the above-described embodiments in that two reverse-biased
diodes are utilized to insulate n-GaN layer 115 from the LEDs. The
first diode is at the boundary of p-GaN layer 111 and n-GaN layer
112. The second reverse-biased diode is at the boundary between
p-GaN layer 113 and n-GaN layer 114.
[0035] The additional reverse biased diode or diodes provide added
isolation of the component LEDs from the underlying n-GaN substrate
and provide increased protection against electrostatic discharge
damage, since the discharge voltage needed to short the device is
now increased by the sum of the breakdown voltages of the reverse
biased diodes. It should be noted that some leakage current can
flow between the isolated component LEDs so long as the magnitude
of that current is small compared to the current flowing through
the series connected LEDs. For the purposes of this discussion, the
component LEDs will be defined to be electrically isolated by the
reverse biased diodes if the leakage current is less than 2 percent
of the current flowing through the series connected component LEDs
via the connecting bridges.
[0036] The above-described embodiments utilize the GaN family of
materials. For the purposes of this discussion, the GaN family of
materials is defined to be all alloy compositions of GaN, InN and
AlN. However, embodiments that utilize other material systems and
substrates can also be constructed according to the teachings of
the present invention.
[0037] The above-described embodiments utilize a reversed diode
arrangement to block current from passing under the barriers that
separate the component LEDs. However, any form of diode that blocks
the current could be utilized.
[0038] The above-described embodiments of the present invention and
the summary of the invention have been provided to illustrate
various aspects of the invention. However, it is to be understood
that different aspects of the present invention that are shown in
different specific embodiments can be combined to provide other
embodiments of the present invention. In addition, various
modifications to the present invention will become apparent from
the foregoing description and accompanying drawings. Accordingly,
the present invention is to be limited solely by the scope of the
following claims.
* * * * *