U.S. patent application number 14/988474 was filed with the patent office on 2016-08-18 for plasmonic light emitting diode.
The applicant listed for this patent is HEWLETT PACKARD ENTERPRISE DEVELOPMENT LP. Invention is credited to David A. Fattal, Marco Fiorentino, Michael Renne Ty Tan, Shih-Yuan Wang.
Application Number | 20160240729 14/988474 |
Document ID | / |
Family ID | 42395900 |
Filed Date | 2016-08-18 |
United States Patent
Application |
20160240729 |
Kind Code |
A1 |
Tan; Michael Renne Ty ; et
al. |
August 18, 2016 |
PLASMONIC LIGHT EMITTING DIODE
Abstract
A light emitting diode includes a diode structure containing a
quantum well, an enhancement layer, and a barrier layer between the
enhancement layer and the quantum well. The enhancement layer
supports plasmon oscillations at a frequency that couples to
photons produced by combination of electrons and holes in the
quantum well. The barrier layer serves to block diffusion between
the enhancement layer and the diode structure.
Inventors: |
Tan; Michael Renne Ty; (Palo
Alto, CA) ; Fattal; David A.; (Palo Alto, CA)
; Fiorentino; Marco; (Palo Alto, CA) ; Wang;
Shih-Yuan; (Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HEWLETT PACKARD ENTERPRISE DEVELOPMENT LP |
Houston |
TX |
US |
|
|
Family ID: |
42395900 |
Appl. No.: |
14/988474 |
Filed: |
January 5, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13145995 |
Jul 22, 2011 |
9263637 |
|
|
PCT/US09/32698 |
Jan 30, 2009 |
|
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14988474 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 33/40 20130101;
H01L 33/04 20130101; H01L 33/20 20130101; H01L 33/0062 20130101;
H01L 33/06 20130101; H01L 33/02 20130101; H01L 2933/0016 20130101;
H01L 33/387 20130101; H01L 33/30 20130101 |
International
Class: |
H01L 33/06 20060101
H01L033/06; H01L 33/30 20060101 H01L033/30; H01L 33/00 20060101
H01L033/00; H01L 33/40 20060101 H01L033/40 |
Claims
1. A light emitting diode (LED) comprising: a diode structure
containing a quantum well; an enhancement layer that supports
plasmon oscillations at a first frequency, wherein the plasmon
oscillations having the first frequency couple to photons produced
by a combination of electrons and holes in the quantum well; and a
barrier layer and a patterned contact between the enhancement layer
and the quantum well, wherein the barrier layer comprises a layer
of non-diffusive metal, wherein the barrier layer and the patterned
contact block diffusion between the enhancement layer and the diode
structure and wherein at least the barrier layer allows the plasmon
oscillations of the enhancement layer to interact with the quantum
well.
2. The LED of claim 1, wherein the enhancement layer comprises a
material selected from the group consisting of silver and gold.
3. The LED of claim 1, wherein the patterned contact is through the
barrier layer, the patterned contact being electrically
conductive.
4. The LED of claim 3, wherein the patterned contact comprises a
metal.
5. The LED of claim 1, wherein the layer of non-diffusive metal is
less than 5 nanometers (nm) thick.
6. The LED of claim 1, wherein the non-diffusive metal layer
comprises a layer of platinum.
7. The LED of claim 6, wherein the layer of platinum is between
about 2 nm and 5 nm thick.
8. The LED of claim 1, wherein the diode structure comprises: a
p-type structure containing multiple layers having p-type doping;
an intrinsic structure containing multiple layers of undoped
material; and an n-type structure containing multiple layers having
n-type doping.
9. A fabrication method comprising: fabricating a diode structure
including a quantum well; depositing a barrier layer and a
patterned contact on the diode structure; and depositing an
enhancement layer on the barrier layer and the patterned contact,
wherein at least the barrier layer allows plasmon oscillations of
the enhancement layer to interact with the quantum well to increase
spontaneous emission rates from electron-hole combinations in the
quantum well, wherein the barrier layer and the patterned contact
block diffusion between the enhancement layer and the diode
structure.
10. The method of claim 9, wherein the depositing the barrier layer
comprises depositing an insulating material less than about 10 nm
thick.
11. The method of claim 10, comprising forming a contact through
the barrier layer, the contact making an electrical connection
between the enhancement layer and the diode structure.
12. The method of claim 9, wherein the depositing the barrier layer
comprises depositing platinum between 2 nm and 5 nm thick.
13. A light emitting diode (LED) comprising: a substrate; an n-type
structure on the substrate; an intrinsic structure containing a
quantum well on the n-type structure; a p-type structure on the
intrinsic structure; a barrier layer and a patterned contact on the
p-type structure; and an enhancement layer that supports plasmon
oscillations at a desired frequency, wherein the barrier layer and
the patterned contact block diffusion between the enhancement layer
and the p-type structure, wherein the barrier layer allows the
plasmon oscillations of the enhancement layer to interact with the
quantum well.
14. The LED of claim 13, wherein the substrate comprises gallium
arsenide (GaAs).
15. The LED of claim 13, wherein the n-type structure comprises a
multi-layer structure that is approximately 20 nanometers
thick.
16. The LED of claim 13, wherein the intrinsic structure comprises
a multi-layer structure that is approximately 100 nanometers
thick.
17. The LED of claim 16, wherein the intrinsic structure produces
photons with a desired wavelength of approximately 800
nanometers.
18. The LED of claim 13, wherein the p-type structure comprises a
multi-layer structure that is approximately 50 nanometers
thick.
19. The LED of claim 13, wherein the enhancement layer comprises a
material selected from the group consisting of silver and gold.
20. The LED of claim 13, wherein the patterned contact is through
the barrier layer, the patterned contact being electrically
conductive.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of recently allowed U.S.
patent application Ser. No. 13/145,995, filed on Jul. 22, 2011,
which is herein incorporated by reference in its entirety.
BACKGROUND
[0002] Light emitting diodes (LEDs) can convert electrical energy
into optical energy for lighting and optical signaling. In general,
LEDs are semiconductor diodes, typically containing a p-i-n
junction. When an LED is forward biased, a current of electrons
from the n-type material of the diode and holes from the p-type
material of the diode combine. LEDs generally employ materials that
create a suitable energy difference between the conduction band of
electrons and the valence band of holes, so that the combination of
an electron and a hole can spontaneously emit a photon. The energy
difference is generally limited by the available materials but can
otherwise be tuned or chosen to produce a desired frequency of
light. Additionally, an LED can employ multiple layers of materials
with conduction bands of different energies to create a quantum
well that tends to confine electrons or holes and enhance the rate
of spontaneous emissions, thereby improving energy efficiency of
light production.
[0003] The spontaneous emission rate of a quantum well in an LED is
not an intrinsic property of the quantum well, but instead depends
on the electromagnetic environment of the quantum well. A plasmonic
LED can exploit this phenomenon by positioning a quantum well close
to a metal that supports the formation of surface plasmon polariton
with electron-plasma oscillations extending into the quantum well.
These electron-plasma oscillations or plasmons increase the
electron-hole pair recombination rate within the quantum well via
the Purcell effect and decrease the delay between a change in the
current driving the LED and the corresponding change in the light
emitted from the LED. Plasmonic LEDs can emit light with a
modulation speed of about 10 GHz or faster while maintaining a
radiative efficiency above about 20%, which compares well with the
modulation speeds and efficiencies of VCSELs and other
semiconductor lasers. International App. No. US/2008/001319,
entitled "PLASMON ENHANCED LIGHT-EMITTING DIODES" describes some
prior plasmonic LEDs that are fast enough for use in high data rate
signaling.
[0004] One concern in manufacture of plasmonic LEDs is the
materials available that are able to support surface plasmons of
the proper frequencies for a plasmonic LED. Considering the
limitations on the frequency of the emitted light placed by the
available materials suitable for LEDs, silver and gold have been
found to have surface plasmons with a desirable coupling for
improving the response of an LED. Unfortunately, silver and gold,
which must be close to a quantum well to provide the desired
enhancement, have a tendency to migrate or diffuse in the
semiconductor materials used in LEDs, and this diffusion can cause
rapid degradation and shorting of the LED.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIGS. 1A and 1B schematically illustrate cross-sectional
views of plasmonic LEDs in accordance with embodiments of the
invention using alternative barrier structures to prevent unwanted
diffusion but permit plasmon interactions with quantum wells.
[0006] FIG. 2 shows a more detailed cross-sectional view of a
plasmonic LED in accordance with another embodiment of the
invention.
[0007] FIG. 3 shows a cross-sectional view of a plasmonic LED in
accordance with another embodiment of the invention.
[0008] Use of the same reference symbols in different figures
indicates similar or identical items.
DETAILED DESCRIPTION
[0009] In accordance with an aspect of the invention, a plasmonic
LED can include a barrier between the semiconductor structures and
a metal layer (e.g., a silver or gold layer) that supports plasmon
oscillations at a frequency that enhances LED performance. In one
embodiment, the barrier can be thin (e.g., about 10 nm or less) and
include an insulating material such as an oxide and a contact
structure of a conductive material such as a non-diffusive metal
(e.g., platinum). The barrier being relatively thin and mostly made
of a dielectric material allows the surface plasmon oscillations of
the metal layer to interact with the quantum well in the LED, but
the barrier can still block diffusion or spiking of metals such as
silver or gold from the enhancement layer into the semiconductor
layers. The patterned contact is an ohmic contact for injection of
current into the LED and can be made of a non-diffusive metal such
as platinum. Further, the contact can be patterned to improve light
extraction, and the contact area can be minimized to ensure
light-plasmon interaction between the quantum well and the
enhancement layer while still guaranteeing good current injection.
In an alternative embodiment, the barrier can be even thinner
(e.g., about 2 nm) and made of non-diffusive conductive material
such as platinum that blocks diffusion or spiking a metal such as
silver or gold from the metal layer. While the barrier metal may
have poor plasmon characteristics for enhancement of spontaneous
emissions in the quantum well, the barrier being sufficiently thin
still allows interactions of the desired surface plasmons in the
metal layer with the quantum well.
[0010] FIG. 1A shows a schematic representation of a cross-section
of a plasmonic LED 100 in accordance with an embodiment of the
invention. LED 100 has a p-i-n structure, which broadly includes a
p-type structure 110, an intrinsic structure 120, and an n-type
structure 130. Intrinsic structure 120 is generally a multi-layer
structure that includes a quantum well, which is a source of the
light (i.e.., photons) produced by spontaneous emissions when
electrons injected from n-type structure 130 combine with holes
injected from p-type structure 110. An enhancement structure 140
contains a layer 142 of material that supports surface plasmon
oscillations having a frequency that enhances the rate of
spontaneous emissions from the quantum well. Layer 142 may be a
blanket layer or may be patterned or roughened if desired to alter
properties of the plasmons in layer 142. In general, greater
enhancements can be achieved by placing enhancement structure 140
(particularly layer 142 since contacts 146 may have poor plasmonic
properties) nearer to the quantum well so that the effects of
plasmon oscillations extend into the quantum well. Enhancement
layer 142 typically needs to be less than about 50 nm from the
quantum well for significant enhancement of spontaneous emissions
at photon wavelengths around 800 nm. The separation may be greater
in LEDs producing longer wavelength light. In FIG. 1A, enhancement
structure 140 is adjacent to n-type structure but may be better
placed adjacent to p-type structure 110 in embodiments where p-type
structure 110 is thinner than n-type structure 130.
[0011] Layer 142 in enhancement structure 140 may be made of a
metal such as pure or alloyed silver or gold but other metals might
be suitable. Diffusion or spiking of metal atoms from layer 142
into the semiconductor structure is an issue, particularly because
layer 142 needs to be close to the quantum well to enhance
spontaneous emissions. For example, it has been observed that GaAs
dissolves readily into gold and gold based alloys. This dissolution
results in equal amounts of galliumn (Ga) and arsenic (As) entering
into the gold lattice. Arsenic has been shown to be able to pass
easily through the gold lattice and evaporate from the free surface
of the gold. It is likely that atoms of such materials enter the
metallization along grain boundaries or other such imperfections,
although it is possible that the diffusion may enter as a very low
concentration of highly mobile interstitial atoms. This phenomenon
is also observed for the InGaP contact layer used in other
LEDs.
[0012] To prevent diffusion from layer 142 into adjacent
semiconductor layers, LED 100 includes an insulating barrier layer
144 containing a patterned conductive contact 146 that electrically
connects layer 142 and n-type structure 130. Barrier layer 144 and
patterned contact 146 can be less than about 10 nm thick and are
preferably about 5 nm. In general, barrier layer 144 and contact
146 can be as thin as possible provided that barrier layer 144 and
contact 146 sufficiently block diffusion from layer 142.
[0013] LED 100 can be operated by applying an appropriate voltage
in a forward bias direction across LED 100. For example, for the
p-i-n architecture of FIG. 1A, an electrical signal having positive
polarity can be applied to layer 142 of LED 100 while layer 110 is
connected to a reference voltage or ground. Electrical signals
would generally be applied to LED 100 through a contact structure
(not shown in FIG. 1A). The relatively negative voltage on n-type
structure 110 can be thought of as driving electrons toward the
quantum well in intrinsic structure 120, and the relatively
positive voltage on layer 142 can be thought of as driving holes
toward the quantum well. The quantum well may be made of a direct
bandgap semiconductor material having an electronic bandgap energy
that is smaller than the electronic bandgaps of the remaining
layers of the LED 100. When the applied voltage difference is large
enough to inject electrons from n-type structure 130 and holes from
p-type structure 110 into the quantum well, spontaneous emissions
resulting from combination of electrons and holes in the quantum
well generate light that can be output from the LED 100 through
p-type structure 110, that is opposite from enhancement structure
140.
[0014] The enhancement that structure 140 achieves can be
understood by treating the combining of electrons and holes as
decays of electron-hole dipoles. In general, the spontaneous
emission rate of a decaying dipole depends not only on the strength
of the dipole, but also on the electromagnetic environment of the
dipole. By changing the electromagnetic environment near a dipole,
the spontaneous decay rate of the dipole can be tuned (i.e.,
suppressed or enhanced), which is called the "Purcell effect." In
the present case, introducing enhancement structure 140, which
supports plasmon oscillations that couple to desired frequencies of
light, enhances the rate at which the electron-hole dipoles decay
into the desired electromagnetic mode or frequency. The Purcell
factor F.sub.P quantifies the enhancement and is given by:
F p = Spontaneous Emission rate in complex environment Spontaneous
Emission rate in bulk material ##EQU00001##
where the complex environment refers to the quantum well with
adjacent enhancement structure 140, and the bulk material refers to
the surrounding material, such as n-type and p-type structures 130
and 110, without enhancement structure 140. The larger the Purcell
factor, the faster the spontaneous emission rate.
[0015] FIG. 1B illustrates an LED 150 using an enhancement
structure 145 with a barrier layer 148 in accordance with an
alternative embodiment of the invention. LED 150 includes a p-type
structure 110, an intrinsic structure 120, an n-type structure 130,
and a metal layer 142, which can be identical to the corresponding
structures in LED 100 of FIG. 1A. LED 150 differs from LED 100 in
that barrier 148 is a very thin (less than 5 nm) layer of a
non-diffusive metal such as platinum between the thicker metal
(e.g., Ag or Au) layer 142 and the underlying semiconductor
structure. Barrier layer 148 may have poor plasmon properties for
enhancement of spontaneous emission, but layer 148 is thin enough
that the surface plasmon enhancement of the combination of layers
148 and 142 can still be as efficient as that of a single thick
layer 142. In particular, a combination Pt/Au layer when the Pt
portion is thin enough (e.g., less than 5 nm) can still be as
efficient as a single Au layer at enhancing spontaneous emissions.
Further, if barrier layer 148 is a platinum layer as thin as 2 to 3
nm, barrier layer 148 can still prevent unwanted diffusion between
layer metal layer 142 and underlying semiconductor structures.
Barrier layer 148 being conductive also has the advantage of
providing a low resistance connection between layer 142 and the
underlying semiconductor structure.
[0016] FIG. 2 shows an LED 200 in accordance with a specific
embodiment of the invention that produces light having a wavelength
of about 800 nm. LED 200 includes a gallium arsenide (GaAs)
substrate 250, a multi-layer n-type structure 130 on substrate 250,
a multi-layer intrinsic structure 120 on n-type structure 130, a
multi-layer p-type structure 110 on intrinsic structure 120, and an
enhancement structure 140 on p-type structure 110. The description
of LED 200 below provides details of one specific embodiment of the
invention. However, as will be understood by those in the art, the
details regarding specific structural parameters such as materials,
dopants, doping concentrations, the number of layers, the order of
layers, and layer thicknesses are subject to variations in
different embodiments of LEDs.
[0017] The n-type structure 130, which can be deposited or grown on
substrate 250, includes five layers 232, 234, 235, 236, and 238 in
the illustrated embodiment of FIG. 2. The bottom layer 232 is an
n-type layer of indium-gallium-phosphorus (InGaP) about 20 nm thick
and is doped with silicon (Si) to a concentration of about
2.times.10.sup.18 cm.sup.-3. The next three layers 234, 235, and
236 are mixtures of aluminum (Al), gallium (Ga), and arsenic (As).
Layer 234, which is on layer 232, is Al.sub..35Ga.sub..65As about
300 nm thick and doped with silicon to a concentration of about
2.times.10.sup.18 cm.sup.-3. Layer 236, which is on layer 235, is
Al.sub..65Ga.sub..35As about 500 nm thick and doped with silicon to
a concentration of about 5.times.10.sup.17 cm.sup.-3. Layer 235,
which is between layers 234 and 236, is a graded layer that is an
Al.sub.xGa.sub.1-xAs mixture in which x ranges from 0.35 to 0.65 so
that the composition of layer 235 transitions smoothly from the
composition of layer 234 to the composition of layer 236. Graded
layer 235 is about 15 nm thick and doped with silicon to a
concentration of about 2.times.10.sup.18 cm.sup.-3. The top n-type
layer 238 is another graded layer of Al.sub.xGa.sub.1-xAs about 15
nm thick, where x ranges from 0.65 to 0.35 so that layer 238
transitions smoothly from the composition of layer 236 to the
composition of an overlying layer 222. The compositionally graded
semiconductor layers 235 and 238 have the electronic bandgaps that
vary with position and can be produced by changing the composition
or ratios of the constituents used during a deposition process. The
graded layers are used to improve the current flow by minimizing
junction discontinuities and thereby reducing the series resistance
between the semiconductor layers.
[0018] Intrinsic structure 120 includes three layers 222, 225, and
228 to create a quantum well with a bandgap structure that produces
photons with the desired wavelength of about 800 nm. In the
illustrated embodiment, bottom layer 222 is an undoped or intrinsic
mixture of Al.sub..35Ga.sub..65As and about 80 nm thick. Layer 225
is a mixture GaAs.sub..885P.sub..115 that is about 10 nm thick, and
layer 228 is another layer of undoped Al.sub..35Ga.sub..65As but is
about 10 nm thick. The bandgaps of layer 222, 225, and 228 are such
that layer 225 corresponds to a quantum well. Further, quantum well
layer 225 has tensile strain of about +0.42% which results because
of the thickness of layer 225 and the difference in the lattice
constant of quantum well layer 225 and layers 222 and 228.
[0019] The p-type structure 110 includes three layers 212, 214, and
216 in the embodiment of FIG. 2. Layer 212 is the same mixture
Al.sub..35Ga.sub..8As as intrinsic layer 228 but is about 40 nm
thick and is p-type with a doping of carbon at a concentration of
about 1.times.10.sup.18 cm.sup.-3. Layer 214 is
Al.sub..2Ga.sub..8As that is about 7 nm thick and doped with a
dopant such as carbon to a concentration of about 1.times.10.sup.18
cm.sup.-3. Layer 216 is p-type InGaP that is about 3 nm thick and
doped with zinc to a concentration of about 1.times.10.sup.18
cm.sup.-3. In general, to maximize the Purcell factor, p-type
structure 110 is as thin as possible to minimize the separation
between overlying enhancement structure 140 and the quantum well in
intrinsic structure 120.
[0020] Enhancement structure 140 can have substantially the same
structure as described above in regard to FIG. 1A. In particular,
enhancement structure 140 includes a barrier layer 144 made of an
insulating material such as silicon dioxide or more preferably a
high refractive index insulator such as titanium dioxide, which
more closely matches the refractive indices of adjacent
semiconductor structures. Barrier layer 144 is preferably less than
about 10 nm thick. Contact 146 is made of a conductive material
such as a non-diffusive metal and has a pattern with openings that
permit interaction of optical modes of the quantum well with
surface plasmons at the interface between layer 142 and barrier
144. Contact 146 can be made of a material having poor plasmon
properties for enhancement of spontaneous emissions from the
quantum well and accordingly may block the desired plasmon
interactions in the areas of contacts 146. Ideally, the area
occupied by contact 146 is kept to a minimum since contact 146
contributes little to the surface plasmon enhancement. Making
contacts smaller may therefore improve enhancement of spontaneous
emissions but may also increase the resistance to currents driven
through LED 200. The area of contact 146 can be chosen to balance
concerns for enhancement of spontaneous emissions and diode
resistance. Alternatively, enhancement structure 140 can be
replaced with the enhancement structure 145 of FIG. 1B, which
provides a low resistance contact between metal layer 142 and
underlying semiconductor structures.
[0021] FIG. 3 illustrates a plasmonic LED 300 that includes
external electrodes 310 and 360. LED 300 includes a p-type
structure 110 and an intrinsic structure 120, which can be of the
type described above. An n-type structure 330 of LED 300 can
include layers 234, 235, 236, and 238 of FIG. 2. Layer 232 acts as
an etch stop layer for a process that etches through substrate 250
(FIG. 2) to leave a region 350 (FIG. 3) surrounding the light
emitting area of LED 300. Electrode 360 is on the remaining region
350 of the substrate and can be made of any suitable composition
and may, for example, include a titanium adhesion layer and a gold
contact layer. A transparent conductor such as indium tin oxide
could alternatively or additionally be employed over the light
emitting area of LED 300.
[0022] An enhancement structure of LED 300 includes a layer 142 of
material such as AgZn, or Pt/AgZn with a very thin (<5 nm) Pt
diffusion barrier, which can support surface plasmons with a strong
coupling to photons produced by spontaneous emissions in the
quantum well. This layer 142 can be deposited by standard
techniques such as e-beam deposition or sputtering. Layer 142 is
electrically connected to contact 310. Barrier layer 144 and
contact 146 are between layer 142 and p-type structure 110 in the
embodiment of FIG. 3. The active area of LED 300 can be defined by
an oxygen implant into an outer portion of the semiconductor
structure to create insulating oxide regions 340 that surround the
active region through which drive current is channeled.
Alternatively, a mesa structure can be formed by etching past the
quantum well into the bottom n-type AlGaAs layer. The active region
of LED 300 for high data rate signaling would typically have a
width or diameter of about 10 to 50 .mu.m because larger areas tend
to increase capacitance and cause signal delays. An insulating
layer 320 of a material such as polyimide can also be deposited to
better confine the drive current through electrode 310 to the
active area of LED 300.
[0023] LED 300 can be operated by applying a positive polarity
electrical signal, which may have a high frequency modulation for
data transmissions, to electrode 310. Electrical current then flows
from electrode 310, through layer 142 and contacts 146 into p-type
structure 110, and p-type structure 110 injects holes into (i.e.,
empties electron valence states in) intrinsic structure 120. The
drive current also corresponds to electrons flowing from electrode
360, through region 350, layer 232, and n-type structure 330 into
intrinsic structure 120, where conduction electrons fall into
emptied valence states, causing spontaneous emission of photons.
The availability of plasmon oscillations in layer 142 enhances
spontaneous emissions into the desired electromagnetic mode.
[0024] Although the invention has been described with reference to
particular embodiments, the description is only an example of the
invention's application and should not be taken as a limitation.
Various other adaptations and combinations of features of the
embodiments disclosed are within the scope of the invention as
defined by the following claims.
* * * * *