U.S. patent application number 15/173500 was filed with the patent office on 2016-12-08 for multi-color light emitting structures with fixed or variable emission color.
The applicant listed for this patent is Ostendo Technologies, Inc.. Invention is credited to Jyh-Chia Chen, Chih-Li Chuang, Hussein S. El-Ghoroury, Mikhail V. Kisin, Yea-Chuan Milton Yeh.
Application Number | 20160359299 15/173500 |
Document ID | / |
Family ID | 57442085 |
Filed Date | 2016-12-08 |
United States Patent
Application |
20160359299 |
Kind Code |
A1 |
El-Ghoroury; Hussein S. ; et
al. |
December 8, 2016 |
Multi-Color Light Emitting Structures with Fixed or Variable
Emission Color
Abstract
Disclosed herein are multi-layered optically active regions for
semiconductor light-emitting devices (LEDs) that incorporate
intermediate carrier blocking layers, the intermediate carrier
blocking layers having design parameters for compositions and
doping levels selected to provide efficient control over the
carrier injection distribution across the active regions to achieve
desired device injection characteristics. Examples of embodiments
discussed herein include, among others: a multiple-quantum-well
variable-color LED operating in visible optical range with full
coverage of RGB gamut, a multiple-quantum-well variable-color LED
operating in visible optical range with an extended color gamut
beyond standard RGB gamut, a multiple-quantum-well light-white
emitting LED with variable color temperature, and a
multiple-quantum-well LED with uniformly populated active
layers.
Inventors: |
El-Ghoroury; Hussein S.;
(Carlsbad, CA) ; Kisin; Mikhail V.; (Carlsbad,
CA) ; Yeh; Yea-Chuan Milton; (Santa Monica, CA)
; Chuang; Chih-Li; (San Diego, CA) ; Chen;
Jyh-Chia; (Yorba Linda, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ostendo Technologies, Inc. |
Carlsbad |
CA |
US |
|
|
Family ID: |
57442085 |
Appl. No.: |
15/173500 |
Filed: |
June 3, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62171536 |
Jun 5, 2015 |
|
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62275650 |
Jan 6, 2016 |
|
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62290607 |
Feb 3, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/2009 20130101;
H01S 5/4093 20130101; H01L 33/32 20130101; H01S 5/0421 20130101;
H01S 5/34333 20130101; H01L 33/08 20130101; H01S 5/1096 20130101;
H01L 33/06 20130101; H01S 5/3407 20130101; H01S 5/4043
20130101 |
International
Class: |
H01S 5/20 20060101
H01S005/20; H01S 5/10 20060101 H01S005/10; H01S 5/343 20060101
H01S005/343; H01L 33/06 20060101 H01L033/06; H01L 33/30 20060101
H01L033/30 |
Claims
1. A multi-layer semiconductor multicolor light emitting structure
comprising: a first quantum confinement structure for emitting
light of a first wavelength; a first intermediate carrier blocking
layer having a first predetermined band-gap and band-offsets; and a
second quantum confinement structure for emitting light of a second
wavelength, wherein the first intermediate carrier blocking layer
is disposed between the first quantum confinement structure and the
second quantum confinement structure.
2. The multi-layer semiconductor multicolor light emitting
structure of claim 1 wherein the first intermediate carrier
blocking layer has a first predetermined band-gap and band-offsets
selected to provide relative light emissions of the first and
second quantum confinement structures that are variably responsive
to current density in the multi-layer semiconductor multicolor
light emitting structure.
3. The multi-layer semiconductor light emitting structure of claim
1 wherein at least one of the first and second quantum confinement
structures comprises a quantum wire.
4. The multi-layer semiconductor light emitting structure of claim
1 wherein at least one of the first and second quantum confinement
structures comprises a quantum dot.
5. The multi-layer semiconductor light emitting structure of claim
1 wherein at least one of the first and second quantum confinement
structures comprises a quantum well.
6. The multi-layer semiconductor multicolor light emitting
structure of claim 1 wherein at least one of the first and second
quantum confinement structures comprises multiple quantum
wells.
7. The multi-layer semiconductor multicolor light emitting
structure of claim 1 wherein the multi-layer semiconductor
multicolor light emitting structure is a diode structure comprising
an LED or a laser diode for emitting multicolor light.
8. The multi-layer semiconductor multicolor light emitting
structure of claim 1 wherein the predetermined band-gap and the
band-offsets of the first intermediate carrier blocking layer are
defined by composition and doping level of the first intermediate
carrier blocking layer.
9. The multi-layer semiconductor multicolor light emitting
structure of claim 1 further comprising: a second intermediate
carrier blocking layer having a second predetermined band-gap and
band-offsets; and a third quantum confinement structure for
emitting light of a third wavelength, wherein the second
intermediate carrier blocking layer is disposed between the second
quantum confinement structure and the third quantum confinement
structure; the predetermined band-gap and the band-offsets of the
second intermediate carrier blocking layer being defined by
composition and doping level of the second intermediate carrier
blocking layer.
10. The multi-layer semiconductor multicolor light emitting
structure of claim 9 wherein the predetermined band-gap and the
band-offsets of the second intermediate carrier blocking layer
being defined by the composition and the doping level of the second
intermediate carrier blocking layer are selected to provide
relative light emissions of the first, second and third quantum
well structures that are variably responsive to current density in
the multi-layer semiconductor multicolor light emitting
structure.
11. The multi-layer semiconductor multicolor light emitting
structure of claim 9 wherein the second intermediate carrier
blocking layer has a band-gap and band-offsets that are greater
than the band-gap and band-offsets of the first intermediate
carrier blocking layer.
12. The multi-layer semiconductor multicolor light emitting
structure of claim 11 wherein the three quantum confinement
structures are selected to cause the multi-layer semiconductor
multicolor light emitting structure to predominately emit red,
green and blue colors responsive to current density in the
multi-layer semiconductor multicolor light emitting structure.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 62/171,536, filed Jun. 5, 2015, U.S.
Provisional Patent Application No. 62/275,650, filed Jan. 6, 2016,
and U.S. Provisional Patent Application No. 62/290,607, filed Feb.
3, 2016, the contents of each of which are hereby incorporated by
reference as if fully stated herein.
FIELD OF THE INVENTION
[0002] The disclosure herein relates to means for tailoring and
controlling the distribution of charge carriers in multi-layer
active regions of electrically pumped solid state light emitters
such as light-emitting diodes and laser diodes.
BACKGROUND OF THE INVENTION
[0003] Semiconductor light emitting diode structures have become
the established leaders among optical light sources in different
emission wavelength ranges. A conventional light-emitting structure
is typically comprised of multiple layers of III-V compound
semiconductor alloys such as GaAsP, AlGaAs, AlGaInP, or AlGaInN,
depending upon the targeted wavelength emission of the diode
structure. III-nitride AlGaInN alloys take special place among the
possible material systems because of the wide range of available
band-gaps. Light emission from AlGaInN covers the entire visible
spectrum; III-nitride based light sources are currently being
developed also for ultraviolet and infrared emission. Multiple
active layer design of an optoelectronic device active region
offsets a high level of optical and electrical losses and small
strain relaxation length typical for III-nitride based
heterostructures.
[0004] Multiple quantum well (MQW) design of the optically active
region is beneficial for light emitter performance. By increasing
the number of active quantum wells (QWs), the injected carriers can
be spread among the MQWs thus decreasing the average QW population
and minimizing the adverse effects of (i) nonradiative Auger
recombination, (ii) QW thermal depopulation, and (iii) QW optical
transition saturation. On the other hand, MQW active regions of
electrically pumped devices typically suffer from inhomogeneous
distribution of charge carriers, both electrons and holes, which
are injected from the opposite sides of the diode structure. As a
result, uneven and imbalanced population of active QWs unfavorably
affects the device performance. In III-nitride light-emitting
diodes (LEDs), the overpopulated active QWs often intensify the
device efficiency droop either by increasing the nonradiative Auger
recombination loss or by elevating the carrier leakage from the
device active region. In laser diodes (LDs), the under-pumped QWs
append their inter-band absorption to the total optical loss thus
increasing the laser threshold.
[0005] In polar III-nitride heterostructures, non-uniform carrier
injection is additionally aggravated by built-in polarization
fields and related potential barriers. This sometimes makes
nonpolar or semipolar technology an attractive alternative to polar
templates. Nonpolar templates, however, do not solve the problem of
inhomogeneous injection entirely. Even in the absence of internal
polarization fields, MQW structures with sufficiently deep QWs and
strong carrier confinement reveal uneven QW populations in a wide
range of injection currents, so that the carrier population
non-uniformity in III-nitride MQWs is a common feature of both
polar and non-polar templates. Carrier injection inhomogeneity
increases with active QW depth and, therefore, becomes more
pronounced in the longer-wavelength emitters thus holding back the
efficiency of III-nitride based light emitters in the so-called
"green emission gap".
[0006] Several conventional methods employ an MQW active region
design in attempt to achieve multi-color emission with fixed or
variable emission colors and/or to increase the injection
efficiency of the device active region. For example, U.S. Pat. No.
7,323,721 describes a monolithic multi-color MQW structure designed
to emit white light by including a sufficient number of QWs with
different emission wavelengths, while U.S. Pat. No. 8,314,429
describes a multi-junction light emitting structure with the MQWs
of each junction being designed to emit a specific wavelength that
combines into white-light emission depending on the designed
emission intensity of each of the multiple junctions comprising the
structure. U.S. Pat. Nos. 7,058,105 and 6,434,178 describe
approaches to achieve high carrier injection efficiency by
incorporating means for increased optical and, respectively,
electrical confinements of MQW active region. U.S. Patent
Publication No. 2011/0188528 describes a MQW III-nitride
light-emitting diode structure that achieves high carrier injection
efficiency by using shallow QWs designed to avoid excessive carrier
confinement and attain uniform MQW carrier population. U.S. Patent
Publication No. 2010/0066921 describes a MQW III-nitride light
emitting structure epitaxially grown on micro rods in which the
epitaxial growth plane of the micro rods promotes higher indium
incorporation in the semi-polar and non-polar orientation which can
lead to multi-color emission form the MQW structure. Thus, the
foregoing conventional approaches use particular ad hoc approaches
relevant to their specific goals.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The embodiments herein are illustrated by way of example and
not by way of limitation in the figures of the accompanying
drawings in which like references indicate similar elements. It
should be noted that references to "an" or "one" embodiment of the
disclosure are not necessarily to the same embodiment, and they
mean at least one. Also, in the interest of conciseness and
reducing the total number of figures, a given figure may be used to
illustrate the features of more than one embodiment of the
disclosure, and not all elements in the figure may be required for
a given embodiment.
[0008] FIG. 1 illustrates a typical energy-band profile of an
active region for explaining a conventional hetero-junction
multi-layer quantum-confinement-based light-emitting diode
structure.
[0009] FIG. 2 illustrates an active region energy-band profile for
explaining the layout of a hetero-junction multi-layer
quantum-confinement-based light-emitting diode structure according
to an embodiment disclosed herein.
[0010] FIG. 3 illustrates a simplified schematic cross-sectional
view for explaining a hetero-junction multi-layer
quantum-confinement-based light-emitting diode structure according
to an embodiment disclosed herein.
[0011] FIGS. 4A-4D illustrate results of active region band profile
simulations in three-color Red-Green-Blue (RGB) LED structures
according to example embodiments disclosed herein. In FIGS. 4A and
4B, the three-color Red-Green-Blue (RGB) LED structure is designed
according to the conventional arrangement of FIG. 1 without
intermediate carrier blocking layers (IBLs). In FIGS. 4C and 4D,
the three-color Red-Green-Blue (RGB) LED structure is designed
according to an embodiment disclosed herein using IBLs. In FIGS.
4A-4D, band profiles are calculated at an LED injection level of 10
A/cm.sup.2 and the band gap vertical scale is reduced by 1 eV for
illustrative purposes. FIG. 4C illustrates the active region band
profile for the RGB-IBL-LED structure of FIG. 3 with IBLs designed
for the purpose of emission color control.
[0012] FIGS. 5A-5D illustrates a comparison of CIE chromaticity
diagrams calculated in a range of injection currents for
three-color RGB LED structures explained using FIGS. 4A-4D
according to example embodiments disclosed herein. The structures
for FIGS. 5A-5D correspond to the structures for FIGS. 4A-4D,
respectively. In FIGS. 5A-5D, a circle marker indicates starting
point of low injection current, a square marker indicates the end
point of high injection current, and a triangle marker corresponds
to nominal injection level of 10 A/cm.sup.2 used in FIGS. 4A-4D.
The various crosses ("X") indicate standard RGB color gamut
primaries of different colors. FIG. 5C illustrates the emission
color control achieved in the RGB-IBL-LED structure illustrated by
FIG. 3 and FIG. 4C.
[0013] FIGS. 6A-6B illustrate the details of a color control
process in the RGB-IBL LED structure illustrated by FIG. 3 and FIG.
4C according to example embodiments disclosed herein. FIG. 6A
illustrates the injection dependence of the CIE chromaticity
coordinates for controlled RGB emission. FIG. 6B illustrates the
injection dependence of relative optical emission power from each
QW. For comparison, dashed lines in FIGS. 6A-6B indicate
corresponding dependencies for the RGB(3)-IBL LED illustrated by
FIG. 4D.
[0014] FIGS. 7A-7F illustrate sensitivity of MQW carrier injection
and output color control to the design of IBL elements of the LED
structure according to example embodiments disclosed herein. Each
of FIGS. 7A-7F illustrates the CIE chromaticity diagram of an
RGB-IBL LED similar to 300 of FIG. 3 with compositions of the two
intermediate carrier blocking layers (IBL1 and IBL2) having
modified design parameters.
[0015] FIGS. 8A-8D illustrate processes of designing RGB-IBL LED
structures according to example embodiments disclosed herein. Each
of FIGS. 8A-8D illustrates the CIE chromaticity diagram of an
RGB-IBL LED such as structure 300 FIG. 3 with designed IBL1 and
IBL2 compositions and varied doping of the first intermediate
carrier blocking layers (IBL1).
[0016] FIGS. 9A-9D illustrate the injection dependence of the CIE
chromaticity coordinates for the RGB-IBL LED structures of FIGS.
8A-8D. FIGS. 9A-9D illustrate CIE chromaticity coordinates for an
RGB-IBL LED such as structure 300 of FIG. 3 with designed IBL1 and
IBL2 compositions and varied doping of IBL1.
[0017] FIGS. 10A-10D illustrate a process of designing an RGB-IBL
LED structure according to example embodiments disclosed herein.
FIGS. 10A-10D illustrate CIE diagrams for an RGB-IBL LED such as
structure 300 of FIG. 3 with designed IBL1 and IBL2 compositions
and varied p-doping level in IBL2.
[0018] FIG. 11A illustrates an embodiment in which color emission
is extended beyond the standard RGB palette by including an
additional aquamarine-emitting QW and using intermediate blocking
layers.
[0019] FIG. 11B-11C relate to an embodiment in which an additional
intermediate carrier blocking layer is incorporated to reduce the
full color control current injection range. FIG. 11B illustrates
the band profile of an RGB-IBL LED structure in which an additional
intermediate carrier blocking layer is incorporated to reduce the
full color control current injection range. FIG. 11C illustrates
the injection dependence of the light emission gamut CIE RGB
chromaticity coordinates of the RGB-IBL LED structure in which an
additional intermediate carrier blocking layer is incorporated.
[0020] FIG. 12A illustrates a comparison of a calculated
variable-color emission spectra with experimental
electroluminescence spectra obtained from a monolithic
color-tunable nitride-based three-color RGB-IBL LED epitaxially
grown according to example embodiments disclosed herein.
[0021] FIG. 12B is a view for explaining output emission colors at
different injection currents obtained from a monolithic
color-tunable nitride-based three-color RGB-IBL LED epitaxially
grown according to example embodiments disclosed herein.
[0022] FIG. 12C illustrates emission colors gamut coverage at
different injection currents obtained from a monolithic
color-tunable nitride-based three-color RGB-IBL LED epitaxially
grown according to example embodiments disclosed herein.
[0023] FIG. 13 illustrates experimental electroluminescence spectra
of an monolithic color-tunable nitride-based three-color RGB-IBL
LED such as structure 300 of FIG. 3 grown according to example
embodiments disclosed herein and illustrates a detailed comparison
with simulation results.
[0024] FIG. 14 illustrates a CIE diagram and injection dependence
of emission chromaticity coordinates calculated for an RGB-IBL LED
such as structure 300 of FIG. 3 designed to produce white-light
emission at maximum LED efficiency point according to example
embodiments disclosed herein.
[0025] FIG. 15 illustrates the experimental electroluminescence
spectra of a monolithic broadband visible light-emitting
nitride-based RGB-IBL LED such as structure 300 of FIG. 3 designed
and grown according to example embodiments disclosed herein.
[0026] FIG. 16 illustrates a comparison of calculated emission
characteristics of monochromatic MQW LEDs designed according to a
conventional arrangement with a MQW-IBL LED designed to sustain
uniformly populated active layers at maximum LED efficiency point
according to example embodiments disclosed herein.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Various embodiments and aspects of the disclosure herein
will be described with reference to details discussed below, and
the accompanying figures will illustrate the various embodiments.
The following description and figures are illustrative of the
disclosure herein and are not to be construed as limiting. Numerous
specific details are described to provide a thorough understanding
of various embodiments. However, it is understood that embodiments
disclosed herein may be practiced without these specific details.
In certain instances, well-known or conventional details, such as
circuits, structures, and techniques, are not described in order to
provide a concise discussion of example embodiments.
[0028] Reference in the specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or
characteristic described in conjunction with the embodiment can be
included in at least one embodiment of the invention. The
appearances of the phrase "in one embodiment" in various places in
the specification do not necessarily all refer to the same
embodiment.
[0029] The inventors herein have recognized the desirability to
provide a systematic approach for tailoring a charge carrier
population distribution and controlling an active layer population
uniformity in multi-layered active regions of semiconductor
light-emitting structures, such that it becomes possible to have
uniform carrier population distribution in the active layer and
emitter efficiency is enhanced. Also, the inventors herein have
recognized that it is desirable to deliberately tailor or directly
control the carrier population distribution among the active layers
of the light-emitting structure, such that it becomes possible to
create monolithic multi-color semiconductor light emitters with
fixed pre-designed emission spectrum (for instance a white-color
emitter) and to develop light emitters with variable emission
color, among many possible other applications. As will be
illustrated by the following descriptions and figures, a systematic
method is disclosed herein for achieving selective and controllable
charge carrier injection into multiple active layers of solid state
light-emitting diode structures. For example, according to an
embodiment herein, multi-layered solid state light emitter
structures are designed and epitaxially grown incorporating means
for tailoring the charge carrier transport in the device
multi-layer active region and controlling the active layer carrier
populations for the numerous applications that benefit from such
capability. Also disclosed herein are examples of numerous possible
applications, for example, display applications and general
lighting applications, high-efficiency solid state light emitters,
multi-color monolithic semiconductor light sources with fixed or
variable emission wavelength, and white-light semiconductor
emitters.
[0030] According to one aspect of the disclosure herein, a
systematic method for tailoring and controlling the charge carrier
population distribution in multi-layered active regions of
semiconductor light emitter structures such as light emitting
diodes and laser diodes is provided by means of incorporating into
the device active region intermediate carrier blocking layers
(IBLs) that are specifically designed in composition and
doping.
[0031] According another aspect of the disclosure herein, targeted
modification of the injection characteristics of multi-layered
light-emitting structures is provided by (i) balancing the
asymmetry in electron and hole transport between the optically
active layers and (ii) balancing the carrier capture rates into the
optically active layers, thus increasing the overall active region
injection efficiency and reducing the active region overflow,
carrier leakage, and carrier recombination loss outside the active
region.
[0032] According yet another aspect of the disclosure herein, a
monolithic semiconductor light-emitting device is provided with
controllable sequential carrier injection in a multi-layered active
region.
[0033] According to an embodiment disclosed herein, a monolithic
semiconductor light-emitting device is provided with uniformly
populated active layers in a multi-layered active region.
[0034] According to another embodiment disclosed herein, a
monolithic semiconductor light-emitting device is provided with
controllable and tunable multi-color emission with fixed or
variable emission wavelength.
[0035] According to yet another embodiment disclosed herein, a
monolithic semiconductor light-emitting device is provided with
controllable and tunable multi-color emission that matches a
specified color emission gamut.
[0036] According to still another embodiment disclosed herein, a
monolithic semiconductor light-emitting device is provided with
controllable and tunable multi-color emission with wide a color
emission gamut.
[0037] According to one embodiment disclosed herein, a monolithic
semiconductor white-light-emitting device is provided with
controllable emission color temperature.
[0038] According to an embodiment disclosed herein, a monolithic
semiconductor light-emitting device is provided with single color
emission that attains high internal quantum efficiency (EQE).
[0039] In the following descriptions, III-nitride semiconductor
alloys are used as an example material system, since III-nitride
semiconductor alloys are poised to make a significant impact in
light emission applications. In addition, multiple-quantum-well
(MQW) active regions are used in this disclosure as an example
active region design, since quantum well (QW) heterostructures are
commonly employed as quantum confinement structures for optically
active layers. It will be understood that the embodiments disclosed
herein are also applicable to other materials systems and optically
active layers incorporating other means of quantum confinement such
as the use of quantum wires and quantum dots.
[0040] Turning to FIG. 1, FIG. 1 illustrates a typical band profile
of an active region of a conventional hetero-junction based
light-emitting diode structure. Light-emitting diode structure 100
is comprised of an n-doped layer 110, an optically active region
130, and a p-doped layer 120. Optically active region 130 of the
diode structure 100 is often completed with the electron blocking
layer (EBL) 140 placed outside of the optically active region 130
at the p-side of the diode structure 100. In quantum-confinement
based LEDs, multi-layered optically active region 130 can be
further comprised of the multiple quantum well (MQW) layers 131
separated by the quantum barrier layers 132. The III-V material
alloy composition in the optically active MQW layers 131 of active
region 130 would typically be selected to set up the desired active
layer band gap and, consequently, the fixed emission wavelength of
the optically active region 130 of the light-emitting diode
structure 100. Electron blocking layer (EBL) 140, which is
incorporated in the diode structure 100 to reduce the electron
leakage at the structure p-side, would typically be a heavily
p-doped wide-band-gap layer having a band gap that is greater than
the band gap of the barrier layers 132 of the active region 130 of
the diode structure 100. Averting carrier leakage has been a
valuable mechanism to increasing light emitter efficiency, and EBL
structures of different kinds are now often standard features of
most III-nitride LED designs. It should be emphasized however that
carrier blocking layers located outside of the optically active
region have little or no effect on the uniformity or carrier
distribution inside the active region and therefore cannot
typically be used as a design element to control or tailor the
active layer carrier populations in multi-layer light-emitting
devices. In fact, such a situation might unfavorably affect the
carrier distribution inside the active region due to possibly
causing an excessive polarization field, especially in active
regions of long wavelength emitters.
[0041] FIG. 2 illustrates an active region band profile for
explaining a hetero-junction multi-layer quantum-confinement-based
light-emitting diode structure according to one embodiment.
Light-emitting diode structure 200 is comprised of an n-doped layer
210, an optically active region 230, and a p-doped layer 220. In
the embodiment of FIG. 2, optically active region 230 of the diode
structure 200 includes electron blocking layer (EBL) 240 placed
outside of the optically active region 230 at the p-side of the
diode structure 200. In other embodiments, EBL 240 is not included.
Multi-layered optically active region 230 can be further comprised
of the multiple quantum well (MQW) layers 231 separated by the
quantum barrier layers 232. In the embodiment of FIG. 2, specially
designed intermediate carrier blocking layers (IBLs) 233 with
selected band-gaps, as defined by their respective compositions,
and band offsets, as affected by their doping levels, are
incorporated directly into the optically active region 230 to
provide means for controlling the carrier transport between the
active layers 231 and balancing the carrier capture rates by active
layers 231. The multiple layers of the light-emitting diode
structure of FIG. 2 can be composed of III-V compound semiconductor
alloys such as GaAsP, AlGaAs, AlGaInP, or AlGaInN, depending upon
the targeted wavelength emission of the diode structure. A diode
structure according to the embodiment of FIG. 2 is applicable to
solid state light emitting diode structures with either polar, or
semi-polar, or non-polar crystalline structures having optically
active layers incorporating means of quantum confinement such as
quantum wells, quantum wires or quantum dots.
[0042] FIG. 3 illustrates a simplified schematic cross-sectional
view for explaining a monolithic three-color hetero-junction
multi-layer quantum-confinement GaN-based LED device structure 300
according one embodiment. In the embodiment of FIG. 3, LED
structure 300 is formed by metal organic chemical vapor deposition
(MOCVD). However, LED structure 300 may be formed by any epitaxial
deposition technique. LED structure 300 comprises a suitable
substrate 311 for epitaxial deposition thereon (e.g., a GaN, Si,
sapphire (Al2O3), or silicon carbide (SiC) wafer with a
nucleation/buffer layer 312 formed thereon) and an overlying
N-doped layer 310 of the active region 330 (e.g., comprising Si as
N-type dopant.) Overlying the N-layer 310 is multi-layered
optically active region 330 (e.g., comprised of combination of GaN,
InGaN, AlGaN, and AlInGaN layers.) Overlying the active region 330
is an optional electron blocking layer (EBL) 340 typically
comprised of AlGaN and followed by P-doped layer 320 (e.g.,
comprising Mg as P-type dopant.) Finally, electrodes 313 and 314
are provided for electrically contacting N- and P-doped layers 310
and 320, respectively.
[0043] Still referring to FIG. 3, multi-layer (MQW) active region
330 comprises a stacked plurality of quantum barrier layers 332
with respective quantum well optically active layers 331, for
blue-green- and red-light emission. Each of the barrier layers 332
is about 5-20 nm-thick and comprised mainly of GaN; optically
active layers 331 are each about 2-3 nm-thick and comprised of
InGaN. As shown in FIG. 3, the optically active layers 331 may
comprise a red-emitting layer, a green-emitting layer and a
blue-emitting layer. It will be appreciated that in other
embodiments, other optically active layers may be used, including
additional red-green- and blue-emitting layers, as well as
aquamarine-emitting layers.
[0044] The multi-layer active region 330 also comprises specially
designed intermediate carrier blocking layers (IBLs) 333 which, in
this embodiment, include an additional amount of aluminum (Al) and
doping to adjust their respective band-gap and band offsets.
Selection of design parameters for intermediate blocking layers
(IBLs) 333 functions to selectively control the population of
injected carriers inside the respective quantum wells in order to
enhance their spontaneous light emission at a given carrier
injection level. In the embodiment of FIG. 3, the band-gap of an
intermediate carrier blocking layer (IBL) is greater than that of
the associated quantum well layer and its thickness is selected to
avoid high forward voltage and excessive heating which can lead to
performance degradation. However, other band-gaps and thicknesses
may be selected.
[0045] As will be explained in further detail below, optically
active layers 331 in the structure can each be comprised of a
multiplicity of QW layers separated by additional barrier layers
332 as required to achieve full coverage of the device emission
color gamut. In this regard, some of the following descriptions
describe modifications to the structure illustrated in FIG. 3, such
that FIG. 3 is merely one example of the arrangement and number of
quantum barrier layers, active layers and intermediate blocking
layers included in the LED structure.
[0046] Moreover, multiple-quantum-well (MQW) active regions are
used as quantum confinement structures in the embodiment of FIG. 3.
However, other quantum confinement structures may be used, such as
a quantum well or wells, quantum wires and quantum dots. In this
regard, in embodiments involving quantum wires and quantum dots,
the combination of a quantum barrier layer, an active layer and the
quantum barrier layer on the other side of the active layer as
shown in FIG. 3 is replaced by quantum wires or quantum dots as the
quantum confinement structures.
[0047] FIGS. 4A and 4B illustrate results of an active region band
profile simulation for multiple-color Red-Green-Blue (RGB) LED
structures designed according to the embodiment of FIG. 1 (FIG. 4A
relates to an undoped RGB structure; FIG. 4B relates to a doped RGB
structure). FIGS. 4C AND 4D illustrate results of an active region
band profile simulation for multiple-color Red-Green-Blue (RGB) LED
structures designed according to the embodiment of FIG. 2 in which
IBLs 233 have been incorporated into the active region (FIG. 4C
relates to an RGB-IBL structure; FIG. 4D relates to an RGB(3)-IBL
structure). The simulation results shown in FIGS. 4A-4D were
obtained by using a simulation program, such as Optical Device
Modeling & Simulation (ODMS) software. For clarity of results,
the EBL layer has been excluded from these simulations.
[0048] FIGS. 4A and 4C illustrate the active region band profiles
of the structures of FIG. 1 and FIG. 2, respectively. FIG. 4C
illustrates the simulated active region energy band profiles of an
RGB-IBL LED device structure with the active layer band-gaps and
the IBLs compositions and doping levels selected for variable (or
tunable) color control over a desired light emission gamut. All LED
energy band profiles illustrated in FIGS. 4A-4D are simulated at
approximately the same LED injection level of 10 A/cm.sup.2. The
dashed lines in FIGS. 4A-4D show the quasi-Fermi levels for
electrons (lines 401, 404, 407, 410) and holes (lines 403, 406,
409, 412). The dashed-dotted lines (lines 402, 405, 408, 411) in
FIGS. 4A-4D indicate the internal potential distribution inside the
active region.
[0049] Table 1 provides examples of active region layout design
parameters of the designed RGB-IBL LED structure of FIG. 4C,
including the semiconductors material composition of each of the
structure's layers and their respective doping levels. Actual
emission results of LED structures epitaxially grown in accordance
with the disclosure herein are discussed below and confirm the
described simulation results for an RGB-IBL LED structure for FIG.
4C.
TABLE-US-00001 TABLE 1 Donors Acceptors Width (10.sup.17 (10.sup.17
Layer nm Composition cm.sup.-3) cm.sup.-3) P-Layer 250 GaN 0.0
100.0 EBL 15.0 Al.sub.0.20Ga.sub.0.80N 0.0 500 Barrier 10.0 GaN 0.0
5.0 Red QWs 3.0 Ga.sub.0.65In.sub.0.35N 0.0 0.0 Barrier 5.0 GaN 0.0
0.0 IBL 2 (Green-Red) 10.0 Al.sub.0.20Ga.sub.0.80N 0.0 15.0 Barrier
5.0 GaN 0.0 0.0 Green QWs 3.0 Ga.sub.0.75In.sub.0.25N 0.0 0.0
Barrier 5.0 GaN 0.0 0.0 IBL1 (Blue-Green) 10.0
Al.sub.0.07Ga.sub.0.93N 0.0 0.0 Barrier 5.0 GaN 0.0 0.0 Blue QWs
3.0 Ga.sub.0.85In.sub.0.15N 0.0 0.0 Barrier 10.0 GaN 1.0 0.0
N-Layer 500 GaN 10.0 0.0
[0050] In another embodiment, which is illustrated by FIG. 4D,
additional active layers are incorporated into the diode structure
active region in order to balance the device light emission gamut.
According to this embodiment, the RGB-IBL device active region
incorporates additional Blue-emitting QWs introduced to improve RGB
light emission gamut coverage. Adding more Blue-emitting QWs
increases the total carrier capture by the Blue-emitting QWs, thus
extending the emission characteristics of the RGB-IBL LED device to
include a desired color emission primary (blue, in the embodiment
of FIG. 4D) of the light emission gamut.
[0051] FIGS. 5A-5D compare light emission gamut (CIE chromaticity
diagrams) for the multi-color LED structures of FIGS. 4A-4D over a
range of injection currents. The embodiments of FIGS. 5A and 5C
correspond to the embodiments of FIGS. 4A and 4C, respectively. In
the FIGS. 5A-5D, the circle markers indicate the starting point at
the lowest injection (simulated value of 1 .mu.A/cm.sup.2), the
square markers correspond to the highest injection level (simulated
value of 10 kA/cm.sup.2), the triangles mark the injection level
(simulated value of 10 A/cm.sup.2) used in FIGS. 4A-4D, and the
crosses ("X") indicate the primary colors of a standard RGB gamut
(red is indicated by crosses 502, 506, 510, 514; green is indicated
by crosses 501, 505, 509, 513; blue is indicated by crosses 503,
507, 511, 515), such as an HD color gamut, for example. FIGS. 5C
and 5D illustrate embodiments in which RGB-IBL devices are designed
with selected IBLs band-gaps and band-offsets to provide a
controllable carrier injection distribution within a device active
region and tunable multi-color emission covering the standard RGB
gamut. FIG. 5D, in particular, shows an embodiment in which the
emission of the RGB-IBL LED 300 of FIG. 3 (and also of FIG. 4C) is
further improved to achieve full coverage of the standard RGB gamut
by adding two Blue-emitting QWs before growth of the associated
barrier layer and the first IBL (herein referred to as, IBL1).
[0052] FIGS. 6A-B detail the process of color control in the
RGB-IBL LED 300 of FIG. 3 (and also of subplot C of FIG. 4). In
FIGS. 6A-B, lines 606 and 611 indicate red; lines 604 and 609
indicate green, lines 602 and 608 indicate blue. FIG. 6A
illustrates the injection dependence of the light emission gamut
CIE RGB chromaticity coordinates. FIG. 6B illustrates the relative
optical emission power in each QW color group,
P.sub.i/.SIGMA.P.sub.i where index i relates to a Red-, Green, or
Blue-emitting QW, respectively. Dashed lines (lines 601 and 607
indicate blue; lines 603 and 610 indicating green; lines 605 and
612 indicating red) show the corresponding injection dependencies
for RGB-IBL(3) light emitting structure incorporating three
Blue-emitting QWs illustrated in plot 470 of FIG. 4. It should be
noted that additional optically active QW layers 331 separated by
additional barrier layers 332 are added in the device structure of
FIG. 3 as required to achieve full coverage of the device emission
color gamut and/or a desired P.sub.i/.SIGMA.P.sub.i relative
optical emission power in each QWs color group. Accordingly, the
structure of FIG. 3 may include fewer or additional optically
active QW layers 331 and, correspondingly, fewer or additional
barrier layers 332 to separate them.
[0053] Referring again to FIGS. 5A-5D, a comparison of the light
emission gamut of FIGS. 5A and 5C (which also correspond to FIGS.
4A and 4C, respectively) illustrates the effect of incorporating
the IBLs with selected band-gaps and band-offsets into the active
region of a semiconductor light emitting diode structure that
incorporates multiple active layers with band-gaps corresponding to
a multi-color light emission. As can be seen from FIG. 5A, without
the IBLs, the color emission trajectory with the increase in the
carriers injection level (rate) is limited mostly to the p-side
(Red) and n-side (Blue) active layers (QWs) with corresponding
colors emission; respectively, and totally missing the dominance of
the Green color primary of the gamut at any injection level, thus
unable to achieve full coverage of the standard RGB color gamut. As
can be seen from FIG. 5B, doping the barriers between the active
layers of the semiconductor light emitting diode structure
according to conventional approaches without IBLs causes the light
emission to be dominated by the Green and Blue emission active
layers (QWs) at the low and high injection levels, respectively,
and causes the dominance of the Red color primary of the gamut to
be missing at any injection level. It is therefore not possible to
achieve full coverage of the standard RGB color gamut. As can be
seen from FIGS. 5C and 5D, with IBLs incorporated into the active
region of the LED structure, the color emission trajectory with the
increase in the injection level (rate) fully covers the standard
RGB color gamut.
[0054] Selection of the composition of intermediate carrier
blocking layers (IBLs) will now be described with reference to
FIGS. 7A-7F. In this regard, FIGS. 7A-7F illustrate the sensitivity
of MQW injection to the design of the IBLs. FIGS. 7A-7F show the
CIE chromaticity diagram of an RGB-IBL LED similar to that of
RGB-IBL LED 300 in FIG. 3, except with design parameters of one of
the IBLs modified. In FIGS. 7A-7F, similar to FIGS. 5A-5D, the
circle markers indicate the starting point at the lowest injection,
the square markers correspond to the highest injection level, the
triangles mark the injection level, and the crosses ("X") indicate
the primary colors of a standard RGB gamut (crosses 702, 706, 710,
714, 718, 722 indicate red; crosses 701, 705, 709, 713, 717, 721
indicate green; crosses 703, 707, 711, 715, 719, 723 indicate
blue.)
[0055] FIGS. 7A-7C illustrate sensitivity of color control to the
choice of an intermediate blocking layer placed between Green and
Red active layers (QWs), herein referred to as IBL2, as affected by
its material composition. In FIGS. 7A-7C, an intermediate blocking
layer placed between Blue and Green QWs herein referred to as IBL1,
is absent and the band-gap of IBL2 located between Green and Red
QWs is increased as affected by increasing the Aluminum
concentration within its composition systematically from that of
FIG. 7A to that of FIG. 7C. FIGS. 7A-7C thus show how the IBL2
band-gap as affected by its material composition is used to control
Red-Green color balance.
[0056] FIGS. 7D-7F illustrate how the band-gap of the IBL1 as
affected by its material composition is used to adjust the LED
output Green-Blue color balance at a specific injection level. In
FIGS. 7D-7F, IBL2 design is fixed (as in RGB-IBL LED 300 of FIG. 3)
while Aluminum concentration in an undoped IBL1 is increased
systematically from that of subplot D to that of subplot F. FIGS.
7D-7F thus show how IBL1 band-gap as affected by its material
composition can be used to control Green-Blue color balance at a
specified nominal injection of 10 A/cm.sup.2 indicated by triangle
marker in subplots D-F. In an embodiment described below, this
feature will be used to adjust the white-light LED emission to the
maximum efficiency point.
[0057] Selection of the doping amount of an intermediate blocking
layer (IBL1) placed between Blue and Green active layers will now
be discussed with reference to FIGS. 8A-8D and FIGS. 9A-9D. In this
regard, FIGS. 8A-8D and FIGS. 9A-9D further illustrate the
sensitivity of MQW injection to the design of IBLs using the
process of RGB-IBL LED structure design as an illustrative example.
FIGS. 8A-8D illustrate CIE diagrams for an RGB-IBL LED with
selected and fixed IBL1 and IBL2 compositions and with varying
level of IBL1 doping. FIGS. 9A-9D present the corresponding
injection dependencies of emission chromaticity coordinates. In
this regard, the embodiments of FIGS. 9A-9D correspond,
respectively, to the embodiments of subplots FIGS. 8A-8D. As can be
seen from FIGS. 8A-8D, either n or p doping of IBL1 would affect
the color balance at the Red-Green or the Green-Blue sides,
respectively. In the structure of an RGB-IBL designed according to
the embodiment of FIG. 8C (also the embodiment in FIG. 5C), IBL1 is
undoped in order to achieve improved coverage of a target color
gamut represented by the cross marks in FIGS. 8A-8D, where crosses
802, 806, 810, 814 indicate red, crosses 801, 805, 809, 813
indicate green and crosses 806, 807, 811, 815 indicate Blue. In
FIGS. 8A-8D, similar to the plots in FIGS. 5A-5D, the circle
markers indicate the starting point at the lowest injection, the
square markers correspond to the highest injection level, the
triangles mark the injection level. In FIG. 9, lines 903, 906, 909,
912 indicate red; lines 902, 905, 908, 911 indicate green; and
lines 901, 904, 907, 910 indicate blue.
[0058] Selection of the doping amount of an intermediate blocking
layer (IBL2) placed between Green and Red active layers will now be
discussed with reference to FIGS. 10A-10D. FIGS. 10A-10D further
illustrate the sensitivity of MQW injection to the design of IBLs
using the process of RGB-IBL LED structure design as an
illustrative example. In this regard, FIGS. 10A-10D present CIE
diagrams for RGB-IBL LED with selected and fixed IBL1 and IBL2
compositions and with varying level of p-doping in IBL2. As can be
seen from FIGS. 10A-10D, the most extended coverage of the desired
color gamut through varying the carrier injection level (rate) is
established when IBL2 p-doping is set at the level shown in FIG.
10C, below or above which the gamut coverage does not sufficiently
extend across the desired RGB standard color gamut. In the
structure presented in FIG. 4C, the doping level of IBL2 is
Na=1.5.times.1018 cm.sup.-3 in order to achieve the most extended
coverage of a target color gamut represented by the Red, Green and
Blue "X"-marks in FIGS. 10A-10D in which crosses 1002, 1006, 1010,
1014 indicate red; crosses 1001, 1005, 1009, 1013 indicate green;
and crosses 1003, 1007, 1011, 1015 indicate blue.
[0059] An embodiment will now be discussed with reference to FIG.
11A in which additional multi-color QWs are included in the active
region. As illustrated in FIG. 11A, the RGB gamut is controlled by
including additional QWs and re-designing the intermediate carrier
blocking layers (IBLs). In this embodiment, indicated by the
RG(Aqua)B-IBL column in FIG. 11A (plots 1160 and 1165), instead of
using one green-emitting QW with nominal emission wavelength 525
nm, two QWs including a green-emitting QW with nominal emission
wavelength 531 nm and an aquamarine-emitting QW with nominal
emission wavelength 512 nm are used, separated by an additional
intermediate carrier blocking layer (IBL3). The RGB-IBL column of
FIG. 11A (plots 1150 and 1155) corresponds to the embodiment of
FIG. 3, in which the structure does not include an
aquamarine-emitting QW.
[0060] Table 2A compares the designed IBLs in an LED structure
without an aquamarine-emitting QW (RGB-IBL LED) and in an LED
structure according to this embodiment including an
aquamarine-emitting QW (RGAB-IBL LED). FIG. 11A also shows the CIE
characteristics of each active QW indicated by corresponding color.
In this regard, lines 1118 and 1123, crosses 1103 and 1111 and dots
1104 and 1112 indicate red; lines 1117 and 1122, arrow 1119,
crosses 1102 and 1110, and areas 1101 and 1109 indicate green;
lines 1116 and 1121, arrow 1120, crosses 1105 and 1113, dots 1106
and 1114, and area 1108 indicate blue. In FIG. 11A, the RGAB-IBL
structure is designed with strong blue-shift in aquamarine QW
emission to extend the green-emission range beyond the nominal
injection level of 10 A/cm.sup.2.
TABLE-US-00002 TABLE 2A IBL-LED structures IBL IBL and IBL
positions donors acceptors RGB-IBL RGAB-IBL IBL (10.sup.17
(10.sup.18 LED LED composition cm.sup.-3) cm.sup.-3) IBL1
Al.sub.0.05Ga.sub.0.95N 8.0 0.0 (Blue-Aqua) IBL1
Al.sub.0.07Ga.sub.0.93N 0.0 0.0 (Blue-Green) IBL2
Al.sub.0.20Ga.sub.0.80N 0.0 1.0 (Aqua-Green) IBL2
Al.sub.0.20Ga.sub.0.80N 0.0 1.5 (Green-Red) IBL3
Al.sub.0.02Ga.sub.0.98N 0.0 0.7 (Green-Red)
[0061] Selection of carrier injection current will now be described
with reference to FIGS. 11B-C. In the embodiment illustrated by
FIGS. 11B-C, an additional intermediate carrier blocking layer
(IBL0) in front (n-side) of the blue-emitting QW has been
incorporated into an RGB-IBL LED structure disclosed herein, and
the other intermediate blocking layers (IBL1 and IBL2) and barrier
layers of the structure's active region are re-designed.
Accordingly, it is possible to improve the full color current
injection range of the RGB-IBL LED structure. FIG. 11B illustrates
the band profile the RGB-IBL LED structure of this embodiment.
Similar to FIG. 4, the dashed lines in FIG. 11B show the
quasi-Fermi levels for electrons (line 1124) and holes (line 1125).
The dashed-dotted line (line 1126) in FIG. 11B indicates the
internal potential distribution inside the active region.
[0062] Table 2B provides example parameters for design of the
active region according to the embodiment of FIGS. 11B-C. As shown
in Table 2B, besides the addition of IBL0 at the n-side of the
structure, re-designing the active region of the light emitting
layout structure includes: (i) adding IBL0 with an Aluminum content
at 22% accompanied by light Si doping at 7.times.1017 cm.sup.-3,
(ii) increasing the barrier separation between B-QW and G-QW layers
accompanied by Mg doping at 1.times.1017 cm.sup.-3, (iii)
increasing the Aluminum content in IBL1 up to 23% accompanied by
light Mg doping at 0.3.times.1017 cm.sup.-3, and (iv) increasing
the Aluminum content in IBL2 up to 25% accompanied by Mg doping at
5.times.1017 cm.sup.3.
[0063] FIG. 11C shows the injection dependence of the light
emission gamut CIE RGB chromaticity coordinates of the RGB-IBL LED
structure example of this embodiment. In FIG. 11C, triangle 1126
indicates green, square 1127 indicates green and cross 1128
indicates green. Also, triangle 1129 indicates red, square 1130
indicates red and the cross 1131 indicates red. Square 1132
indicates blue, triangle 1133 indicates blue and cross 1134
indicates blue. The dotted lines (line 1136 indicating Red; line
1138 indicating Green; line 1140 indicating Blue) in FIG. 11C show
the emission gamut of the RGB-IBL LED structure with the additional
IBL0 but without re-designing of the remaining structure. The solid
lines (line 1135 indicating Red; line 1137 indicating Green; line
1139 indicating Blue) in FIG. 11C show the emission gamut of the
RGB-IBL LED structure with the additional IBL0 and re-designing of
the remaining structure. As can be seen from the solid lines FIG.
11C, the RGB-IBL LED structure with the additional IBL0 and
re-designed structure of this embodiment fully covers the standard
RGB gamut within an injection current ranging from 20 mA/cm.sup.2
to 50 A/cm.sup.2, which is substantially narrower than the examples
of the previous embodiments.
[0064] An additional benefit of incorporating the intermediate
carrier blocking layers (IBLs) into the active region of a
multi-color light-emitting diode structure (such as active region
330 of the structure 300 in FIG. 3) is that the IBLs also act as
intermediate strain-compensation layers. As such, in addition to
balancing the carrier transport across the active region,
incorporation of the IBLs also minimizes the crystalline strain
across a multiple band gap active region (such as active region 330
of light-emitting diode structure 300). As a result, incorporation
of the IBLs also promotes higher indium intake into nitride-based
(InGaN) optically active layers (such as optically active layers
331 of the multi-color light-emitting diode structure 300), thus
enabling the epitaxial growth of optically active layers having
long wavelength light emission ranging from Amber (615-nm) to Red
(625-nm) within the active region of the multi-color light-emitting
diode structure. Thus, incorporating the intermediate carrier
blocking layers (IBLs) also enables production of a color-tunable
nitride-based light-emitting diode structure that has current
injection controllable light emission across the visible light
spectrum.
TABLE-US-00003 TABLE 2B Donors Acceptors Width (10.sup.17
(10.sup.17 Layer nm Composition cm.sup.-3) cm.sup.-3) P-Layer 250
GaN 0.0 100.0 EBL 15.0 Al.sub.0.20Ga.sub.0.80N 0.0 500 Barrier 10.0
GaN 0.0 5.0 Red QW 3.0 Ga.sub.0.65In.sub.0.35N 0.0 0.0 Barrier 10
GaN 0.0 0.0 IBL2 (Green-Red) 10.0 Al.sub.0.25Ga.sub.0.75N 0.0 5.0
Barrier 10 GaN 0.0 0.0 Green QW 3.0 Ga.sub.0.75In.sub.0.25N 0.0 0.0
Barrier 10.0 GaN 0.0 0.0 IBL1 (Blue-Green) 10.0
Al.sub.0.23Ga.sub.0.77N 0.0 0.3 Barrier 25.0 GaN 0.0 1.0 Blue QW
3.0 Ga.sub.0.85In.sub.0.15N 0.0 0.0 Barrier 10.0 GaN 0.0 0.0 IBL0
(Blue) 10 Al.sub.0.22Ga.sub.0.78N 7.0 0.0 Barrier 10.0 GaN 1.0 0.0
N-Layer 500 GaN 10.0 0.0
[0065] FIG. 12A provides a comparison of the variable-color
emission spectra obtained from simulation using an RGB-IBL LED (the
RGB-IBL column of FIG. 11A) with experimental electroluminescence
spectra obtained from three-color Amber-Green-Blue-IBL LED
epitaxially grown according to the RG(Aqua)B-IBL column of FIG.
11A. In the simulated spectra (plot 1200) the voltage drop across
the active region changes from 3.0V to 3.7V with 20 mV step. In
plot 1200, moving from the line 1201 towards line 1202 on the plot,
line 1201 indicates purple, the next several lines indicate blue,
then green, then yellow, then orange, then red, then repeats again
starting at purple until line 1202 indicates red. In the
experimental data set (plot 1220) the legend indicates total LED
injection current. In the legend of plot 1220, 4 mA indicates red,
7 mA indicates light green, 11 mA indicates dark blue, 31 mA
indicates light blue, 61 mA indicates purple, 101 mA indicates dark
blue, 151 mA indicates light blue, 201 mA indicates dark blue, 251
mA indicates light blue, 301 mA indicates red, 340 mA indicates
dark green, 350 mA indicates light green, and 353 mA indicates
orange. Referring to plot 1220, line 1222 corresponds to 4 mA
indicating red, and line 1221 corresponds to 353 mA indicating
orange. Moving from line 1222 up towards line 1221, the
intermediate lines correspond, in order, to the legend of plot
1220. Accordingly, the line above line 1222 corresponds to 7 mA
indicating light green, and the next line up corresponds to 11 mA
indicating dark blue, and so on.
[0066] FIGS. 12B and 12C provide the variable-color emission
spectra obtained from an RGB-IBL LED designed in accordance with
the disclosure herein and epitaxially grown using III-nitride. In
order to achieve the full color gamut demonstrated in FIGS. 12B and
12C the epitaxially grown RGB-IBL LED structure includes the
incorporation of an IBL in between QWs with the same emission
wavelength. Specifically, the variable-color emission spectra
obtained from the epitaxially grown RGB-IBL LED provided in FIGS.
12B and 12C incorporated three Blue emission QWs and the Blue-Green
IBL1 then two Green emission QWs separated by an additional IBL,
designated as IBL1.5, followed by the Red emission QW which is
separated from the second Green emission QW by IBL2. The
composition and doping of the three IBLs in this epitaxially grown
RGB-IBL LED was selected as described above to achieve the color
gamut coverage at the injection range provided in FIGS. 12B and
12C. In FIG. 12B, (a) at 5 mA demonstrates emission of red light,
(b) at 20 mA demonstrates emission of orange light, (c) at 30 mA
demonstrates emission of yellow light, (d) at 100 mA demonstrates
emission of light green light, (e) at 200 mA demonstrates emission
of light blue light, and (f) at 350 mA demonstrates emission of
blue light. In FIG. 12C, 700 nm and 640 nm indicate red; 620 nm
indicates red-orange; 600 nm indicates orange; 590 nm indicates
light orange; 580 nm indicates light orange; 570 nm indicates
yellow; 560 nm and 540 nm indicate light green; 520 nm, 510 nm and
500 nm indicate green; 496 nm indicates blue; 480 nm indicates
light blue; and 480 nm and 460 nm indicate dark blue.
[0067] FIG. 13 further specifies the experimental
electroluminescence spectra of the monolithic color-tunable
nitride-based three-color RGB-IBL LED 300 of FIG. 3 designed and
grown according to the disclosure herein and provides a more
detailed comparison with simulation results. Plots 1321-1323 in
FIG. 13 show the EL spectra measured at room temperature at a low,
moderate, and high injection current density. For low current
density of about 0.5 A/cm.sup.2 (plot 1323), the light emission is
mainly in red color with wavelength ranging roughly from 560 to 650
nm and with a full width at half maximum (FWHM) around 50 nm. For
moderate current density of about 10 A/cm.sup.2 (plot 1322), the
light emission is mainly in green color with wavelength ranging
roughly from 480 to 540 nm and with a full width at half maximum
(FWHM) around 45 nm. For current density of about 50 A/cm.sup.2
(plot 1321), the light emission is mainly in blue color with
wavelength ranging roughly from 420 to 475 nm and with a full width
at half maximum (FWHM) around 35 nm. Plots 1334-1335 show
corresponding results for spectral emission power at three voltage
bias values (in Volts). In plot 1334, line 1301 indicates a voltage
bias value of 3.46 Volts, line 1302 indicates a voltage bias value
of 3.16 Volts, and line 1303 indicates a voltage bias value of 2.98
Volts. Plot 1335 presents the results on a normalized scale. In
plot 1305, line 1305 indicates red, line 1306 indicates green and
line 1304 indicates blue.
[0068] In the preceding embodiments multiplicity of intermediate
carrier blocking layers (IBLs) are incorporated within the
optically active region of the RGB-IBL LED. As described, in some
of these embodiments the IBLs separate regions of active layers
incorporating one or more QWs having a specific wavelength
emission. Also in some of these embodiments an IBL separates active
layers incorporating QWs having the same wavelength emission. Yet
in other embodiments of this invention an IBL is incorporated at
n-side of the optically active region of the RGB-IBL LED. In all of
these embodiments the IBLs composition and doping are selected and
designed as described in the preceding discussion to enable
controllable (or tunable) color emission covering wide color gamut
over a given carrier injection (I,V) range. The general design
criteria of the IBLs composition and doping in these embodiments
include one or more of the following: (1) the conduction band (CB)
energy level of the IBLs should be higher than the CB edge of the
optically active region and progressively increasing from the
n-side toward the p-side of the structure; and (2) the valence band
(VB) energy level of the IBLs should be lower than the VB edge of
the optically active region and progressively decreasing from the
n-side toward the p-side of the structure.
[0069] According to the embodiments disclosed above, multi-color
semiconductor light emitting structures (for example, a
color-tunable RGB-IBL LED device) are designed by selection of one
or more of the following design parameters of the semiconductor
light-emitting structure: (1) the band-gaps of the active layers
QWs being selected to achieve light emission over a desired color
emission gamut; (2) the number of active layers QWs with a band-gap
corresponding to the light emission at a given wavelength being
selected to include a desired color emission primaries into the
device light emission gamut at a desired relative optical emission
power for each color; (3) the multiplicity of IBLs with optimized
material composition and doping levels being incorporated into the
active region of the light emitting device in order to enable
carrier injection rate control of the device light emission color
within the desired gamut; (4) the multiplicity of IBLs with
optimized material composition and doping levels being incorporated
into the active region of the light emitting device in order to
enable control of the device light emission color within a given
carrier injection range; and (5) the band-gap of the multiplicity
of active layers QWs corresponding to the light emission at a given
wavelength being selected to provide a wide gamut color
emission.
[0070] Several examples will now be described with reference to
FIGS. 14-16 in which the design parameters of the semiconductor
light-emitting structure are selected to provide for various
applications. FIG. 14 illustrates an embodiment in which the design
parameters of the IBLs incorporated within a multi-color LED
structure are selected to create a monolithic white LED with
variable color temperature. FIG. 14 shows CIE chromaticity diagram
1400 and injection dependence of emission chromaticity coordinates
1410. In the embodiment of FIG. 14, the design parameters of an
RGB-IBL-White LED structure have been selected to produce
white-light emission at the maximum LED efficiency point at an
injection current density of 50 A/cm.sup.2 (indicated by the
inverted triangle in 1400 of FIG. 14) at the white point (marked by
the cross 1303 in plot 1400 of FIG. 14) of the color gamut formed
by emission color primaries of the RGB-IBL structure active layers
QWs. In plot 1400 of FIG. 14, cross 1303 and dot 1304 indicate red;
cross 1302 and area 1301 indicate green; cross 1305 and dot 1306
indicate blue. In plot 1410 of FIG. 14, line 1308 indicates red,
line 1309 indicates green and line 1307 indicates blue.
[0071] Table 3 compares the IBLs designed for the white-light
emitting RGB-IBL-White LED with IBLs designed for the RGB-IBL LED
300 of FIG. 3 (also presented in FIG. 4C and in FIG. 5C.)
TABLE-US-00004 TABLE 3 IBL-LED structures IBL IBL and IBL positions
donors acceptors RGB-IBL RGB-IBL- IBL (10.sup.17 (10.sup.18 LED
White LED composition cm.sup.-3) cm.sup.-3) IBL
Al.sub.0.24Ga.sub.0.76N 3.8 0.0 (Blue-Green) IBL
Al.sub.0.07Ga.sub.0.93N 0.0 0.0 (Blue-Green) IBL
Al.sub.0.06Ga.sub.0.94N 2.8 0.0 (Green-Red) IBL
Al.sub.0.20Ga.sub.0.80N 0.0 1.5 (Green-Red)
[0072] Plot 1410 of FIG. 14 shows the injection dependence of
emission chromaticity coordinates of the white-light emitting
RGB-IBL-White LED. In this embodiment, the design parameters of the
RGB-IBL-White LED structure are selected to create white light
emission of a given color temperature, for example 6500.degree. K,
at the nominal injection current density of 50 A/cm.sup.2. As shown
in plot 1410 of FIG. 14, when the injection current is increased to
a value higher than the nominal injection current density of 50
A/cm.sup.2, the emitted white light will include higher level of
relative intensity contribution from the Blue active layer QW, thus
causing the emitted white light color temperature to
correspondingly increase to a value T.sub.+ higher than the
temperature of the white light emitted at the nominal injection
current density of 50 A/cm.sup.2. Similarly, as shown in plot 1410
of FIG. 14, when the injection current is decreased to a value
lower than the nominal injection current density of 50 A/cm.sup.2,
the emitted white light will include higher level of relative
intensity contribution from the Green and Red active layers QWs,
thus causing the emitted white light color temperature to
correspondingly decrease to a value T.sub.- lower than the
temperature of the white light emitted at the nominal injection
current of 50 A/cm.sup.2. The RGB-IBL-white LED structure design
parameters are selected to create an injection current control
range above and below the nominal injection current density that
correspond to a desired white light emission temperature range
around a nominal emitted white light temperature. For example, when
a white light temperature of 6500.degree. K is selected at the
nominal injection current density of 50 A/cm.sup.2, the
RGB-IBL-white LED structure design parameters could also be
designed to allow a range of white color temperature ranging from
2500.degree. K to 8000.degree. K, for example, when the device
injection current is decreased or increased over a given range
below and above, respectively, the nominal injection current
density of 50 A/cm.sup.2.
[0073] As can be seen from FIG. 14, design parameters are selected
for IBLs that are incorporated within the active region of a
multi-color LED structure to create a monolithic white LED device
whose emitted white light temperature is changed by changing its
current injection level. In the general lighting field of
application, the controllable (or adjustable) white emission
temperature monolithic white LED device can be used to create a
solid state light bulb having a the controllable (adjustable) white
emission temperature. In the display field of application, the
controllable (adjustable) white emission temperature monolithic
white-light emitting LED device can be used to create a solid state
back light unit (BLU) for liquid crystal displays (LCD) or any
other types of displays that require back illumination light such
as quantum dots displays. One advantage of using such a monolithic
white LED as a back light for displays is that its brightness as
well as its white color temperature can be adjusted to match the
display requirements without sacrificing the dynamic range of the
displayed images. In that regard, in conventional white back lit
displays, the backlight color temperature, and often its intensity,
are typically fixed and the displayed image brightness or hue are
adjusted by adjusting the displayed image pixels RGB values; an
approach that typically results in a reduction of the displayed
image dynamic range since a portion is being used to adjust the
displayed image brightness and hue rather than being used to
express the grayscale value of each pixel color. A display BLU that
uses the controllable (adjustable) white emission temperature
monolithic white-light emitting LED device can be used to alleviate
these limitations and thus enable high dynamic range (HDR)
displays.
[0074] FIG. 15 shows the electroluminescence spectra of the
monolithic broadband visible light-emitting nitride-based RGB-IBL
LED 300 of FIG. 3 epitaxially grown and designed according to the
disclosure herein. Plot 1500 of FIG. 15 illustrates an EL spectrum
at a driving current density of about 1 A/cm.sup.2. The light
emission is mainly in red band with wavelength range roughly from
560 to 680 nm and a full width at half maximum (FWHM) of 70 nm.
Plot 1510 of FIG. 15 illustrates the EL spectrum at a medium
driving current density of about 10 A/cm.sup.2. The light emission
is mainly in a combined red and green band with wavelength range
roughly from 500 to 690 nm and a full width at half maximum (FWHM)
of 120 nm. Plot 1520 of FIG. 15 illustrates the EL spectrum at
injection current density of about 40 A/cm.sup.2. The light is
emitted in a combined red, green and blue bands with wavelength
spanning roughly from 440 to 700 nm and a full width at half
maximum (FWHM) of 190 nm. As can be seen from FIG. 15, design
parameters are selected for IBLs that are incorporated within the
active region of a multi-color LED structure to create a monolithic
white LED device having a broadband white light emission.
[0075] FIG. 16 illustrates an embodiment in which the design
parameters of the IBLs incorporated within a monochromatic LED
structure are selected to create uniformly populated active layers
within the LED structure, thus alleviating the uneven and
imbalanced population of active QWs that would unfavorably affect
the device performance (for example, in terms of efficiency). FIG.
16 compares monochromatic blue-emitting (plots 1620 and 1640) and
green-emitting (plots 1625 and 1645) MQW LEDs of the conventional
structure of FIG. 1 (plots 1620 and 1625) with MQW-IBL LEDs
designed according to the disclosure herein to sustain uniformly
populated active layers in the MQW active region (plots 1640 and
1645). In each row, the subplots of FIG. 16 present distributions
of optical emission power among active QWs (P.sub.i/.SIGMA.P.sub.i)
in 3-QW LEDs according to the conventional structure of FIG. 1
(plots 1620 and 1625) and 3-QW IBL-LEDs with IBLs selected for
uniform emission distribution at LED injection level corresponding
to maximum LED internal quantum efficiency (IQE) achieved at
injection current density of 50 A/cm.sup.2 (plots 1640 and 1645).
Conventional LED structures (such as FIG. 1) in plots 1620 and 1625
include structures without EBL (solid line 1602 indicating blue,
solid line 1604 indicating light blue, solid line 1606 indicating
purple, solid line 1608 indicating green, solid line 1610
indicating light green, and solid line 1612 indicating dark green)
and with p-side EBL (dashed line 1601 indicating blue, dashed line
1603 indicating light blue, dashed line 1605 indicating purple,
dashed line 1609 indicating green, dashed line 1611 indicating
light green, dashed line 1613 indicating dark green) thus
illustrating the insignificant effect of the EBL on MQW emission
uniformity except at the highest injection levels where the whole
LED internal quantum efficiency (IQE) already deteriorates. On the
contrary, MQW-IBL LEDs reveal relatively uniform emission
distribution in a wide range of practically important injection
currents (marked by the circles in plots 1640 and 1645) well
exceeding the initial optimization injection current of 50
A/cm.sup.2. In plots 1640 and 1645 of FIG. 16, line 1605 indicates
blue, line 1606 indicates light blue, line 1607 indicates purple,
line 1614 indicates dark green, line 1615 indicates light green and
line 1616 indicates green.
[0076] Table 4 compares the emission uniformity characteristics of
conventional LEDs (without IBLs) and IBL-LEDs at nominal injection
level of 50 A/cm.sup.2 and shows IBL's Aluminum compositions and
p-doping acceptor concentrations selected according to this
embodiment.
TABLE-US-00005 TABLE 4 IBL Aluminum IBL1/IBL2, IBL acceptors
P.sub.min/P.sub.max LED structures % IBL1/IBL2, 10.sup.18 cm.sup.-3
at 50 A/cm.sup.2 BBB (prior art) 0.15 BBB-IBL 20/10 0.5/1.0 0.59
GGG (prior art) 0.06 GGG-IBL 20/15 1.2/1.5 0.63
[0077] As can be seen from FIG. 16, the LED structure can be
designed to have an IQE at a desired current injection density
within a low to intermediate range by selected design parameters
for IBLs that are incorporated within the active region of a
monochromatic LED structure. The embodiment of FIG. 16 is
particularly advantageous in applications in which an array of
micro-scale solid state light emitters is used as emissive pixels
of a display, herein referred to as emissive micro-scale solid
state light display. In such an application, the entire emissive
aperture of the device is also its optical aperture, and the device
can therefore operate at a rather high optical coupling efficiency
and thus would not typically require the use of high current
injection density. In contrast, typical solid state light
applications generally require the use of rather high current
injection density. Emissive micro-scale solid state light displays
would typically operate a low current injection regime in
comparison with typical solid state light applications which
generally operate a high current injection regime. For such low
injection regime, the solid state micro-scale emitters IQE can be
designed at their typical current injection operating point in
order to preserve and further increase their operational
efficiency. In another embodiment, the embodiment of FIG. 16 is
used to design the IQE of emissive micro-scale solid state light
displays in order to increase their operational efficiency.
[0078] According to the methods and structures described herein,
and particularly by incorporating IBLs within the active region of
a multi-color solid state light emitting structure, it is possible
to provide a variable-color emission (or tunable) solid state light
emitting material and variable-color emission (or tunable) solid
state light emitting devices. As discussed above, there are
numerous fields of application for such variable-color emission (or
tunable) solid state light emitting materials and devices,
including general lighting and display applications. As one
example, the variable-color emission (or tunable) solid state light
emitting materials and devices disclosed herein is in the field of
emissive micro-scale solid state light display described in U.S.
Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231 and 8,098,265
and U.S. Patent Publication Nos. 2010/0066921 and 2012/0033113, the
contents of each of which are incorporated herein by reference. In
this type of display, micro-scale pixel multi-color emission is
realized by stacking multiple layers of state light emitting
structures on top of a controlling silicon based complementary
metal oxide semiconductor (CMOS) structure to form an individually
addressable (in color and intensity) emissive micro-pixel array
device. One advantage of such emissive micro-scale solid state
light display technology is that it can be used to realize small
pixel size in the range of few microns. How small the pixel pitch
of such an emissive micro-scale solid state light display can be
made is dependent upon the number electrical contacts needed to
control emission color and intensity of each micro-scale pixel. In
the case when three color primaries are used to create the display
gamut, beside the one common contact for the entire micro-pixel
array, at least three contacts are required per each emissive
micro-scale pixel, which based on current semiconductor equipment
capabilities allows the realization of a micro-scale pixel pitch in
the range of approximately 10 microns. When the variable-color
emission (or tunable) solid state light emitting material disclosed
herein is used in the context of this type of emissive micro-scale
solid state light display, beside the one common contact for the
entire micro-pixel array, only one contact per micro-scale pixel is
needed to control each pixel light emission. This reduction in the
number of required contacts per micro-pixel makes it advantageously
possible to realize an emissive multi-color micro-scale pixel pitch
of 5 microns or less. In addition, when the variable-color emission
(or tunable) solid state light emitting material disclosed herein
is used in the context of this type of emissive micro-scale solid
state light display, only one solid state light emitting layer is
needed to realize a multi-color emissive micro-scale pixel display
(instead of the typical three layers), which in turn will
substantially reduce the manufacturing cost of the display.
[0079] While the invention has been described in terms of several
embodiments, those of ordinary skill in the art will recognize that
the invention is not limited to the embodiments described, but can
be practiced with modification and alteration within the spirit and
scope of the appended claims. The description is thus to be
regarded as illustrative instead of limiting. There are numerous
other variations to different aspects of the invention described
above, which in the interest of conciseness have not been provided
in detail. Accordingly, other embodiments are within the scope of
the disclosure herein, aspects of which are defined in the
claims.
* * * * *