U.S. patent application number 11/005505 was filed with the patent office on 2005-07-28 for efficient light emitting diodes and lasers.
This patent application is currently assigned to Kopin Corporation. Invention is credited to Chen, Jyh Chia, Choi, Hong K., Fan, John C. C., Fox, Ken, Hon, Schang-Jing, Narayan, Jagdish, Ye, Jinlin.
Application Number | 20050161689 11/005505 |
Document ID | / |
Family ID | 32396522 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050161689 |
Kind Code |
A1 |
Narayan, Jagdish ; et
al. |
July 28, 2005 |
Efficient light emitting diodes and lasers
Abstract
An optoelectronic device such as an LED or laser which produces
spontaneous emission by recombination of carriers (electrons and
holes) trapped in Quantum Confinement Regions formed by transverse
thickness variations in Quantum Well layers of group III
nitrides.
Inventors: |
Narayan, Jagdish; (Raleigh,
NC) ; Ye, Jinlin; (South Easton, MA) ; Hon,
Schang-Jing; (Taipei, TW) ; Fox, Ken;
(Clinton, MA) ; Chen, Jyh Chia; (Ellicott City,
MD) ; Choi, Hong K.; (Sharon, MA) ; Fan, John
C. C.; (Brookline, MA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
Kopin Corporation
Taunton
MA
|
Family ID: |
32396522 |
Appl. No.: |
11/005505 |
Filed: |
December 6, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11005505 |
Dec 6, 2004 |
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10207649 |
Jul 26, 2002 |
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6881983 |
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10207649 |
Jul 26, 2002 |
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10083703 |
Feb 25, 2002 |
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Current U.S.
Class: |
257/97 ;
257/E33.008 |
Current CPC
Class: |
H01S 5/3425 20130101;
H01L 33/06 20130101; H01S 5/106 20130101; H01S 5/34333 20130101;
B82Y 20/00 20130101; H01L 33/32 20130101 |
Class at
Publication: |
257/097 |
International
Class: |
H01L 029/225; H01L
033/00 |
Claims
What is claimed is:
1. An optoelectronic device comprising: a substrate; and multiple
quantum well (MQW) layers formed of Group III nitrides in which
carriers recombine to emit photons, the layers being formed over
the substrate and wherein the layers periodically vary in thickness
along a continuous length thereof.
2. The device of claim 1, wherein the layers have stress induced
dislocations and the thickness variations result in the formation
of quantum confinement regions which are smaller than separations
between the stress induced dislocations.
3. The device of claim 2, wherein the quantum confinement regions
trap the carriers, which recombine to produce the photons for
efficient spontaneous emissions.
4. The device of claim 1, including contacts formed on the device
and a voltage source coupled to the contacts to enable the device
to operate as a high efficiency LED.
5. The device of claim 1, including a feedback mirror to produce
coherent light.
6. The device of claim 1 in which the MQWs are formed of layers of
In.sub.xGa.sub.(1-x)N and GaN.
7. The device of claim 1 in which the MQWs are formed of layers of
Al.sub.yIn.sub.xGa.sub.(1-x-y)N and Al.sub.zGa.sub.(1-z)N.
8. The device of claim 1 in which the substrate is formed of a
compound from the class of Al.sub.2O.sub.3, Si, SiC, GaN or AIN or
alloys thereof.
9. The device of claim 1 in which the thickness variation is a
relatively short longitudinal range on the order of 2 to 10
nanometers (nm).
10. The device of claim 9 having an additional long-range thickness
variation in the order of 50 to 200 nm and thickness variation more
than 10%.
11. The device of claim 9 having an additional long-range thickness
variation more than 10%.
12. The device of claim 9 having a long-range thickness variation
period less than the separation of dislocations.
13. An LED comprising: a substrate; and multiple quantum well (MQW)
layers formed of Group III nitrides in which carriers recombine to
emit photons, the layers being formed over the substrate and
wherein the layers periodically vary in thickness along a
continuous length thereof.
14. The device of claim 13, wherein the layers have stress induced
dislocations and the thickness variations result in the formation
of quantum confinement regions which are smaller than separations
between the stress induced dislocations.
15. The device of claim 13, wherein the quantum confinement regions
trap the carriers, which recombine to produce the photons for
efficient spontaneous emission.
16. The LED of claim 13, wherein the substrate is formed of
A1.sub.2O.sub.3, the quantum well layers are formed of InGaN/GaN,
an n GaN is formed between the A1.sub.2O.sub.3 and quantum well
layers, and a p GaN is formed over the quantum well layers.
17. An optoelectronic device comprising: a substrate; and multiple
quantum well (MQW) layers formed of Group III nitrides in which
carriers recombine to emit photons, the layers being formed over
the substrate and wherein the layers periodically vary in thickness
along a length thereof, where the layers have stress induced
dislocations and the thickness variations result in the formation
of quantum confinement regions which are smaller than separations
between the stress induced dislocations.
18. The device of claim 17, wherein the quantum confinement regions
trap the carriers, which recombine to produce the photons for
efficient spontaneous emissions.
19. An optoelectronic device comprising: a substrate; and multiple
quantum well (MQW) layers formed of Group III nitrides in which
carriers recombine to emit photons, the layers being formed over
the substrate and wherein the layers periodically vary in thickness
along a length thereof, where the thickness variation is a
relatively short longitudinal range on the order of 2 to 10
nanometers (nm).
20. The device of claim 19, having an additional long-range
thickness variation in the order of 50 to 200 nm and thickness
variation more than 10%.
21. The device of claim 19, having an additional long-range
thickness variation more than 10%.
22. The device of claim 19, having a long-range thickness variation
period less than the separation of dislocations.
23. An LED comprising: a substrate; and multiple quantum well (MQW)
layers formed of Group III nitrides in which carriers recombine to
emit photons, the layers being formed over the substrate and
wherein the layers periodically vary in thickness along a length
thereof, where the layers have stress induced dislocations and the
thickness variations result in the formation of quantum confinement
regions which are smaller than separations between the stress
induced dislocations.
24. The device of claim 23, wherein the quantum confinement regions
trap the carriers, which recombine to produce the photons for
efficient spontaneous emission.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Ser. No.
10/207,649, filed Jul. 26, 2002, which is a continuation-in-part of
U.S. application Ser. No. 10/083,703, filed Feb. 25, 2002, the
entire teachings of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] Light emitting diodes ("LEDs") are p-n junction devices that
have been found to be useful in various roles as the field of
optoelectronics has grown and expanded over the years. Devices that
emit in the visible portion of the electromagnetic spectrum have
been used as simple status indicators, dynamic power level bar
graphs, and alphanumeric displays in many applications, such as
audio systems, automobiles, household electronics, and computer
systems, among many others. Infrared devices have been used in
conjunction with spectrally matched phototransistors in
optoisolators, hand-held remote controllers, and interruptive,
reflective, and fiber-optic sensing applications.
[0003] An LED operates based on the recombination of carriers
(electrons and holes) in a semiconductor. When an electron in the
conduction band combines with a hole in the valence band, it loses
energy equal to the bandgap of the semiconductor in the form of an
emitted photon; i.e., light. The number of recombination events
under equilibrium conditions is insufficient for practical
applications but can be enhanced by increasing the minority carrier
density.
[0004] The minority carrier density is conventionally increased by
forward biasing the diode. The injected minority carriers recombine
with the majority carriers within a few diffusion lengths of the
junction edge, generating photons at a wavelength corresponding to
the bandgap energy of the semiconductor.
[0005] As with other electronic devices, there exists both the
desire and need for more efficient LEDs, and in particular, LEDs
that will operate at higher intensity while using less power.
Higher intensity LEDs, for example, are particularly useful for
displays or status indicators in various high ambient environments.
High efficiency LEDs with lower power consumption, for example, are
particularly useful in various portable electronic equipment
applications. An example of an attempt to meet this need for higher
intensity, lower power, and more efficient LEDs may be seen with
the development of the AlGaAs LED technology for the red portions
of the visible spectrum. A similar continual need has been felt for
LEDs that will emit in the green, blue and ultraviolet regions of
the visible spectrum which ranges from 400 nanometers (nm) (3.10
eV) to 770 nm (1.61 ev). Because red, green, and blue are primary
colors, their presence is necessary to produce full color displays
or pure white light.
[0006] As mentioned above, the wavelength () of photons that can be
produced by a given semiconductor material is a function of the
material's bandgap (E.sub.g). This relationship can be expressed as
(nm)=1240/E.sub.g(eV). Thus smaller bandgap materials produce lower
energy, longer wavelength photons, while wider bandgap materials
are required to produce higher energy, shorter wavelength photons.
For example, one semiconductor commonly used for lasers is indium
gallium aluminum phosphide (InGaAlP). This material's bandgap
depends upon the mole or atomic fraction of each element present,
and the light that InGaAlP can produce is limited to the yellow to
red portion of the visible spectrum, i.e., about 560 to 700 nm.
[0007] In order to produce photons that have wavelengths in the
green, blue or ultraviolet (UV) portions of the spectrum,
semiconductor materials with relatively large bandgaps are
required. Typical candidate materials include silicon carbide
(6H--SiC with a bandgap of 2.5 eV) and alloys of indium nitride
(InN with a bandgap of 1.9 eV), gallium nitride (GaN with a bandgap
of 3.4 eV) and aluminum nitride (AlN with a bandgap of 6.2 eV).
Since these nitrides can form solid solutions, the bandgap of these
alloys (AlInGaN) can be tuned potentially from 1.9 eV to 6.2 eV
with a corresponding wavelength varying from 653 nm to 200 nm at
room temperature.
SUMMARY OF THE INVENTION
[0008] Aluminum indium gallium nitride (AlInGaN) is a very
attractive LED candidate material for green, blue and UV
wavelengths because of its relatively large bandgap at room
temperature and because it is a direct bandgap material rather than
an indirect bandgap material. Generally speaking, an LED formed in
a direct bandgap material is more efficient than one formed in an
indirect bandgap material because the recombination of carriers
occurs directly without the help of phonons (lattice vibration) and
the photon from the direct transition retains more energy than one
from an indirect transition.
[0009] Because the bulk gallium nitride (hexagonal GaN; a=0.3189
nm, c=0.5185 nm) substrates are not readily available, AlInGaN
layers are typically grown on a sapphire (hexagonal
alpha-A1.sub.2O.sub.3; a=0.4578 nm, c=1.299 nm) or on a silicon
carbide substrate (hexagonal 6H--SiC; a=0.308 nm, c=1.512 nm). The
AlInGaN films grow by lattice matching epitaxy on 6H--SiC, and by
domain matching epitaxy on sapphire. The AlInGaN growth on sapphire
involves a 30/90 degree rotation in the basal plane, and 6/7 domain
matching of the major planes between the film and the substrate.
The differences in lattice constants and coefficients of thermal
expansion between the film and the substrate cause misfit strains
which result in high dislocation densities in AlInGaN layers,
typically around 10.sup.10 cm.sup.-2. When carriers (electrons and
holes) are trapped by dislocations, they recombine
non-radioactively without generating light.
[0010] In accordance with a first embodiment of the invention, an
efficient optoelectronic device of the type which produces
spontaneous emission by radiative recombination of carriers
(electrons and holes) is formed of a layered quantum well (QW)
structure in which the thickness of the QW layers varies
periodically. It is believed that the thickness variations result
in the formation of Quantum Confinement (QC) regions, which trap
the carriers. If the QC regions are smaller than the separation
between dislocations, the trapped carriers recombine radioactively
and efficiently produce photons. In another embodiment, Al is added
to InGaN to increase the wavelength to produce a
Al.sub.yIn.sub.xGa.sub.(1-x-y)N Laser Device (LD) or multiquantum
well (MQW) LED.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0012] FIG. 1 is a schematic of Short-Range Thickness Variation
(SR-TV) and Long-Range Thickness Variation (LR-TV) of a portion of
the active GaN/InGaN quantum well superlatice layers in a high
efficiency light emitting diode structure.
[0013] FIG. 2 is a schematic of showing details of the LED
structure. FIG. 3A is a scanning transmission electron
microscopy-atomic number (STEM-Z) contrast image in cross-section
showing short-range thickness variation in the active InGaN layer
in a high efficiency LED structure.
[0014] FIG. 3B is a enlargement of FIG. 3A.
[0015] FIG. 4 is a STEM-Z contrast image in cross-section showing
long-range thickness variation in all ten InGaN layers of a
multiple-quantum-well (MQW) structure.
[0016] FIG. 5A is a STEM-Z contrast image of cross-section showing
characteristic long-range thickness variation in a InGaN/GaN MQW
structure from another high-efficiency LED wafer.
[0017] FIG. 5B is a STEM-Z contrast image of the cross-section
showing the short-range thickness variation in a InGaN/GaN MQW
structure (same wafer as FIG. 5A).
[0018] FIG. 6A is a STEM-Z contrast image showing uniform InGaN
layers in a relative low efficiency LED structure.
[0019] FIG. 6B is a enlargement if FIG. 6A.
[0020] FIG. 7 is a comparison of output power from an LED with
periodic thickness variation (A) and an LED with uniform thickness
(C).
[0021] FIG. 8A is a schematic of the laser diode (LD) structure
where the LD is grown on sapphire.
[0022] FIG. 8B is a schematic of the laser diode (LD) structure
where the LD is grown on n GaN or SiC.
DETAILED DESCRIPTION OF THE INVENTION
[0023] A description of preferred embodiments of the invention
follows.
[0024] In accordance with the invention, In.sub.xGa.sub.(1-x)N
based multiquantum well (MQW) light emitting diodes (LEDs) and
laser devices having high optical efficiency, are fabricated in
which the efficiency is related to the Thickness Variation (TV) of
the In.sub.xGa.sub.(1-x)N active layers. The thickness variation of
active layers is found to be more important than the In composition
fluctuation in quantum confinement (QC) of excitons (carriers) in
these devices. In the invention, we have produced MQW
In.sub.xGa.sub.(1-x)N layers with a periodic thickness variation,
which results in periodic fluctuation of bandgap for the quantum
confinement of carriers. Detailed STEM-Z contrast analysis (where
image contrast is proportional to Z.sup.2 (atomic number).sup.2)
was carried out to investigate the spatial distribution of In. We
discovered that there is a longitudinal periodic variation in the
thickness of In.sub.xGa.sub.(1-x)N layers with two periods, one
short-range (SR-TV, 3 to 4 nm) and other long-range thickness
variations of (LR-TV, 50 to 100 nm). It was also found that the
effect of variation in In concentration is considerably less than
the effect of thickness variation in the LED structures which
exhibit high optical efficiency. A comparative microstructural
study between high and low optical efficiency MQW structures
indicates that thickness variation of In.sub.xGa.sub.(1-x)N active
layers is the key to their enhancement in optical efficiency.
[0025] As shown in FIG. 1, QC regions 2 are formed within the
boundaries of either LR-TV or SR-TV as a result of the thickness
variations. The QC regions 2 trap the carriers, which recombine
without being affected by the presence of stress induced
dislocations. A detailed STEM-Z contrast analysis shows that the
thickness variation of In.sub.xGa.sub.(1-x)N layers 12 is more
important then the In composition fluctuation in producing quantum
confined regions for carriers, leading to enhanced optical
efficiency of LEDs and LDs. The bandgap change is dictated by the
thickness:
(L.sub.z) via: .DELTA.E.sub.1=h.sup.2n.sup.2/(8m*L.sub.z.sup.2)
(1),
[0026] where E, is the lowest allowed energy level, h is Planck's
constant, and m* is effective mass.
[0027] A schematic of an LED structure set forth in FIG. 2 is grown
on a sapphire substrate 6 by means ofmetal-organic
chemicalvapordeposition (MOCVD). Sources forthe growth are
trimethylgallium, trimethylaluminum, trimethylindium are used as
sources for group-III elements, ammonia for the nitrogen, disilane
for the n-type doping and biscyclopentadienyl (CP.sub.2Mg) for the
p-type doping. First, a nucleation layer of AlGaInN 5-30 nm thick
is grown at a temperature of .about.500.degree. C. Then Si-doped
n-type GaN layer 4 (3-5 .mu.m thick) is grown at a temperature
between 1000 and 1050.degree. C. Then an InGaN(12)/GaN MQW(14)
structure 8 is grown at a temperature between 700 and 750.degree.
C. for InGaN and 850 and 950.degree. C. for GaN. Then Mg-doped
p-type GaN layer 10 is grown at a temperature between 950 and
1000.degree. C. Optionally, Mg-doped p-type AlGaN layer 10A is
grown between the MQW structure and p-type GaN at a temperature
between 950 and 1000.degree. C.
[0028] In order to create the thickness variation, the growth
temperature of part of the n-type layer (.about.0.1 micron) near
the active region is lowered. The preferred temperature range is
between 880 and 920.degree. C., while conventional growth
temperature is between 1000 and 1050.degree. C. Wafer A was grown
under the preferred growth conditions, while wafer C was grown
under the conventional growth temperature. (See FIG. 7).
[0029] We have analyzed the InGaN/GaN MQW structures using STEM-Z
transmission electron microscopy (TEM), in which the image contrast
is proportional to Z.sup.2 (Z=atomic number). Since the atomic
number of In (49) is much higher than that of Ga (31), the contrast
due to In is enhanced by two and a half times compared to Ga, and
the image contrast is dictated primarily by the In
concentration.
[0030] FIGS. 3-5 show STEM-Z contrast images in cross-section from
two LED structures which exhibited high optical efficiencies. These
specimens show short-range (3 to 4 nm period) and long-range (50 to
100 nm period) thickness variations in InGaN layers. This contrast
analysis reveals that there are variations in In concentration, but
they are not very large. In other embodiments, depending upon the
growth of the structure, short-range thickness variations can range
between 2 to 10 nm and long-range thickness variations can range
between 50 and 200 nm.
[0031] In contrast to the high optical efficiency specimens (FIGS.
3-5), a specimen with relatively low optical efficiencies is shown
in FIGS. 6A and 6B. In these specimens, where optical efficiencies
are lower by a factor of two to three than those for the specimens
in FIGS. 3-5, the superlattice thickness as well as indium
concentration is quite uniform.
[0032] The change in bandgap of In.sub.xGa.sub.(1-x)N alloys can
occur as function of the composition `x` and the thickness
`L.sub.z` of the superlattice. For a typical active layer
composition (x=0.4), the change in bandgap is estimated to be as
follows: x=0.4, bandgap=2.58 eV; x=0.5, bandgap=2.43 eV; x=0.3,
bandgap=2.75 eV. This amounts to a +/-25% change (from x=0.4) in
active layer composition. Experimentally observed composition
fluctuations are less than +/-5%, which should lead to a less than
0.07 eV change in the bandgap. On the other hand, thickness
variation from 3 to 2 nm can change the bandgap by more than 0.2
eV. Experimentally observed LR-TV is in the range of 20 to 50%, and
SR-TV is less than 10%. Based on this analysis, we believe that the
QC regions are defined principally by the thickness in InGaN
layers.
[0033] An LED is fabricated by forming an ohmic contact 20 on the
top p-type GaN surface 10 of FIG. 2 and forming another contact on
the n-type GaN surface 4 after it is exposed by etching the p-type
GaN layer 10 and the active region 8. The light output power of
LEDs from two such wafers is shown in FIG. 7. The output power of
the LED from the specimen with InGaN thickness variation 22 is
about a factor of 2-3 higher than that from the specimen with
uniform InGaN thickness 24. Thus our experimental results on a
comparative study of high- and low-efficiency LEDs demonstrate that
thickness variation coupled with indium concentration variation is
the key to enhancing the optical efficiencies in LEDs. We believe
that the thickness variation is caused by two-dimensional strain in
the In.sub.xGa.sub.(1-x)N layer below its critical thickness. Since
strain energy increases with thickness, the uniform thickness
breaks into a periodic variation by which the free energy of the
system can be lowered. Since the strain also increases with In
concentration, some fluctuation in In concentration is also
expected. This phenomenon of thickness variation has been well
documented for pure germanium thin film growth on (100) silicon
below its critical thickness where no composition fluctuation is
involved references. We have modeled the thickness variation and
derived the following relation for TV period (.lambda.):
.lambda.=.pi..gamma.(1-.nu.)/[2(1+.nu.).sup.2.mu..epsilon..sup.2]
(2),
[0034] where .gamma. is the surface energy, .nu. is the Poisson's
ratio, .mu. is the shear modulus of the film, and .epsilon. is the
strain normal to the film surface. To avoid non-radiative
recombination at the dislocations (density .rho.), we derive the
optimum structure to be:
.rho..sup.1/2>.pi..gamma.(1-.nu.)/[2(1+.nu.).sup.2.mu..epsilon..sub.2]
(3) or
.rho.<{.pi..gamma.(1-.nu.)/[2(1+.nu.).sup.2.mu..epsilon..sub.2]}.sup.-2
(4).
[0035] We have estimated a typical value of .lambda. using the
following parameters for our growth conditions. For
In.sub.0.4Ga.sub.0.6N, shear modulus is estimated to be 82 Gpa,
Poisson's ratio to be 0.3, surface energy 4,000 ergs/cm.sup.2, and
strain 2%. These values result in .lambda. of 80 nm, which is in
good agreement with observed LR-TV. Since the period varies as
.epsilon..sup.-2, the large misfit strain initially could lead to
observed SR-TV.
[0036] Although we have shown the formation of QC regions 2 due to
thickness variation in InGaN, a similar effect can be obtained in
AlInGaN for shorter wavelength LEDs. In this case, superlattice can
be formed between AlInGaN and AlGaN or between AlInGaN layers with
different alloy compositions.
[0037] The QC regions 2 can also be beneficial for laser diode
fabrication. FIGS. 8A and 8B show the schematic of a LD structure
26. In order to form the waveguide, either AlGaN, AlGaN/GaN
superlattice, or AlInGaN layers can be used for the cladding
layers, and InGaN/GaN or AlInGaN/AlGaN MQWs can be used for the
active layers. To facilitate the ohmic contacts, a p-type GaN or
InGaN cap layer 28 is added on top of the p-type cladding layer 30.
If the LD structure 26 is grown on an insulating substrate 6 such
as sapphire, the n contacts 32 are formed after the n-type GaN
layer 4 is exposed by etching the top layers. If it is grown on top
of a conducting substrate 7 such as SiC or GaN, the n contacts 32
are formed on the bottom of the substrate. The fabrication of the
laser is completed by forming feedback mirrors. This can be done
either by cleaving the wafer perpendicular to the contact stripe or
by etching vertical walls using anisotropic etching techniques.
[0038] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *