U.S. patent application number 15/683188 was filed with the patent office on 2019-02-28 for semiconductor laser diode with low threshold current.
The applicant listed for this patent is Sharp Kabushiki Kaisha. Invention is credited to Valerie BERRYMAN-BOUSQUET, Shigetoshi ITO, Yoshihiko TANI, Alex YUDIN.
Application Number | 20190067912 15/683188 |
Document ID | / |
Family ID | 65410855 |
Filed Date | 2019-02-28 |
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United States Patent
Application |
20190067912 |
Kind Code |
A1 |
YUDIN; Alex ; et
al. |
February 28, 2019 |
SEMICONDUCTOR LASER DIODE WITH LOW THRESHOLD CURRENT
Abstract
A group III nitride based laser light emitting device includes
an n-side group III nitride based semiconductor region, a p-side
group III nitride based semiconductor region, and a group III
nitride based active region between the p-side group III nitride
based semiconductor region and n-side group III nitride based
semiconductor region. The group III nitride based active region
includes first and second quantum well layers and a barrier layer
between the first and second quantum well layers, the respective
compositions of the first and second quantum well layers comprising
different respective amounts of indium. The first quantum well is
closer to the n-side group III nitride based semiconductor region
than the second quantum well, the second quantum well is closer to
the p-side group III nitride based semiconductor region than the
first quantum well, and the first quantum well has a larger band
gap than the second quantum well.
Inventors: |
YUDIN; Alex; (Oxford,
GB) ; TANI; Yoshihiko; (Osaka, JP) ;
BERRYMAN-BOUSQUET; Valerie; (Oxford, GB) ; ITO;
Shigetoshi; (Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sharp Kabushiki Kaisha |
Osaka |
|
JP |
|
|
Family ID: |
65410855 |
Appl. No.: |
15/683188 |
Filed: |
August 22, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/2031 20130101;
H01S 5/34333 20130101; H01S 5/22 20130101; H01S 5/3211 20130101;
H01S 5/0014 20130101; H01S 5/2009 20130101; H01S 5/3407
20130101 |
International
Class: |
H01S 5/343 20060101
H01S005/343; H01S 5/34 20060101 H01S005/34; H01S 5/22 20060101
H01S005/22 |
Claims
1. A group III nitride based laser diode, comprising: an n-side
group III nitride based semiconductor region, the n-side group III
nitride based semiconductor region comprising an n-cladding layer
and an n-guide layer; a p-side group III nitride based
semiconductor region, the p-side group III nitride based
semiconductor region comprising a p-cladding layer and a p-guide
layer; and a group III nitride based active region between the
p-side group III nitride based semiconductor region and the n-side
group III nitride based semiconductor region, the group III nitride
based active region comprising first and second quantum well layers
and a barrier layer between the first and second quantum well
layers, respective compositions of the first and second quantum
well layers comprising different respective amounts of indium, an
amount of indium of the second quantum well layer being greater
than an amount of indium of the first quantum well layer, an indium
amount ratio of the indium of the second quantum well layer to the
indium of the first quantum well layer being 1.05 to 5; wherein the
first quantum well is closer to the n-side group III nitride based
semiconductor region than the second quantum well, the second
quantum well is closer to the p-side group III nitride based
semiconductor region than the first quantum well, and the first
quantum well has a larger band gap than that of the second quantum
well.
2. The laser diode of claim 1, wherein the respective band gaps of
the first and second quantum wells are determined by the different
indium amounts of the first and second quantum wells.
3. The laser diode of claim 1, wherein: the first quantum well
layer is In.sub.x1Ga.sub.1-x1N, where 0<x1<1; and the second
quantum well layer is In.sub.x2Ga.sub.1-x2N, where
0<x2<1.
4. (canceled)
5. The laser diode of claim 1, wherein the indium amount ratio of
the indium of the second quantum well layer to the indium of the
first quantum well layer is 1.2 to 5.
6. The laser diode of claim 1, wherein the indium amount ratio of
the indium of the second quantum well layer to the indium of the
first quantum well layer is 1.05 to 3.
7. The laser diode of claim 1, wherein the indium amount ratio of
the indium of the second quantum well layer to the indium of the
first quantum well layer is 1.2 to 3.
8. The laser diode of claim 1, wherein a threshold current density
of the laser diode is at least 1000 A/cm.sup.2.
9. The laser diode of claim 1, wherein a thickness of the barrier
layer is at least 5 nm.
10. The laser diode of claim 1, wherein the barrier layer comprises
a larger band gap than the first and second quantum well
layers.
11. The laser diode of claim 1, wherein a thickness of the barrier
layer is 30 nm or less.
12. The laser diode of claim 1, wherein the lasing wavelength is
within a range of at least 450 nm and 550 nm or less.
13. The laser diode of claim 1, wherein the lasing wavelength is
within a range of at least 500 and 550 nm or less.
14. The laser diode of claim 1, wherein the n-guide layer of the
n-side group III nitride based semiconductor region is an InGaN
guide layer.
15. The laser diode of claim 14, wherein a thickness of the InGaN
guide layer is at least 80 nm and 300 nm or less.
16. The laser diode of claim 14, wherein a thickness of the InGaN
guide layer is at least 120 nm and 300 nm or less.
17. The laser diode of claim 14, wherein a thickness of the InGaN
guide layer is at least 160 nm and 300 nm or less.
18. The laser diode of claim 14, wherein the InGaN guide layer is
non-doped.
19. The laser diode of claim 14, wherein the group III nitride
based active region further comprises a cap layer of InGaN, the cap
layer located between the first quantum well layer and the InGaN
guide layer.
20. The laser diode of claim 19, wherein a thickness of the cap
layer is at least 3 nm and 30 nm or less.
21. The laser diode of claim 1, wherein the first and second
quantum well layers are the only quantum well layers of the group
III nitride based active region, and the first and second quantum
well layers are configured such that laser light emission is
achieved only for the second quantum well layer.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to the epitaxial structure of
a semiconductor laser diode, and in particular to an active region
structure of a III-nitride based semiconductor laser diode.
BACKGROUND ART
[0002] Laser Diodes (LDs) based on III-nitride semiconductors are
commercially available with emitting wavelengths in the UV, blue
and green portions of the electromagnetic spectrum. Such devices
are used, for example, in illumination and display applications. It
is particularly important to provide such devices with high
electrical and optical performances.
[0003] A III-nitride (or also nitride) semiconductor structure
based on GaN and its alloys including In and Al (hereafter referred
to as (Al,In,Ga)N alloys) can be made to form high efficiency LD
devices. The semiconductor layer structure for such devices can be
manufactured by forming semiconductor layers sequentially on a
substrate. These layers are physically connected and generally
obtained with high crystal quality using epitaxial deposition or
growth method such as Metal Organic Chemical Vapor Deposition
(MOCVD) or Molecular Beam Epitaxy (MBE). FIG. 1A shows such a
conventional structure of a laser diode device as described by
Nakamura S and Fasol G, The Blue Laser Diode, p. 293, 1997 (Berlin:
Springer). The structure includes an n-side semiconductor region
113, active region 106 and p-side semiconductor region 114 formed
on a substrate 112. The deposition direction or the growth
direction is defined by the direction perpendicular to the
semiconductor layers surface of this structure. The substrate is
made of sapphire.
[0004] Under application of a low electrical excitation to this
structure, charge carriers such as electrons and holes will move
across the structure and recombine radiatively in the active region
106 resulting in emission of photons. The wavelength of the emitted
photons and thus of the light emission from the LD device is
determined by the bandgap of the active region.
[0005] Semiconductor laser diode devices as described above require
confinement of the emitted photon and carrier recombination to an
active region and a higher electrical excitation. Additionally, an
optical cavity established by reflecting mirror surfaces at both
ends of the device induces an optical amplification. Once these
requirements are satisfied lasing operation can be achieved.
[0006] Confinement of the emitted photons can be achieved by the
utilization of a separate confinement heterostructure. With
continued reference to FIG. 1A, this may be achieved by arranging a
n-guide layer 105 below and a p-guide layer 108 above the active
region 106, each of the n-guide layer and the p-guide layer having
a refractive index higher than the effective index of the guided
light. Further confinement is achieved by arranging a n-cladding
layer 104 below and a p-cladding layer 109 above the active region
106, each of the n-cladding layer and the p-cladding layer having a
refractive index lower than the effective index of the guided
light. The upper p-guide layer 108 and upper p-cladding layer 109
form part of the p-side semiconductor region 114. The lower n-guide
layer 105 and lower n-cladding layer 104 form part of the n-side
semiconductor region 113. The n-side semiconductor region 113
additionally includes buffer layer 101, n-contact layer 102, and a
buffer layer 103.
[0007] This structure may lead to strong confinement of the emitted
photons in a transverse direction, therefore propagating parallel
to the active region. Between the active region 106 and upper
p-guide layer 108, a carrier blocking layer 107 is provided, which
has a wider band gap compared to adjacent layers.
[0008] The p-cladding layer 109 has a shaped upper surface that may
be formed by post-growth processing. In a typical layout, the
shaped upper surface of the p-cladding layer 109 is a ridge formed
in the plane of growth, which enhances light confinement to the
direction of the longest dimension of the ridge.
[0009] A p-contact layer 110 is formed on top of the upper
p-cladding layer 109. Electrical contact is made to the p-contact
layer 110 by a metal electrode layer 111a formed thereon, which
allows the device to be electrically activated. Injection of charge
carriers to the p-side semiconductor 114 only occurs in the area
defined by the metal electrode layer 111a and the p-contact layer
110 which coincide with the shaped ridge. An electrode metal layer
111b is also formed on the surface of the n-contact layer 102,
which can be achieved by etching through the semiconductor layer
structure described herein by post growth processing and depositing
a metal electrode layer on the n-contact layer 102.
[0010] At either end of the ridge, a surface perpendicular to the
growth direction and the direction of the longest dimension of the
ridge is defined by a process such as cleaving or etching. Light
escaping by transmission through these surfaces is used for the
intended applications. In this described arrangement, the device is
said to be edge emitting.
[0011] FIG. 1B shows the active region 106 of the semiconductor
laser emitting device of FIG. 1A. The active region 106 is designed
to confine recombination of charge carriers (electrons, holes),
formed of a triple quantum well structure where semiconductor
layers of low bandgap (quantum well layers 115) are arranged
between layers of higher band gap (quantum barrier layers 116. The
layers of the active region are formed of InGaN alloy semiconductor
material. The band gap of InGaN material decreases as the amount of
In increases. FIG. 1C shows the amount of In x in the composition
for each quantum well layer 115 and the amount of In y in the
composition for each quantum barrier layer 116. FIG. 1D shows the
corresponding energy levels for electrons and holes. The energy E0
indicates the band gap of the quantum well layers 115.
[0012] Such quantum well structures are highly desirable for
localizing recombination of carriers. However, in III-nitride
material systems there is an issue of non-uniformity in carrier
concentration across the quantum wells and this has an impact on
the laser diode performance. For example, quantum wells 115 close
to the n-side semiconductor region 113 have generally a much higher
concentration of electrons than quantum wells 115 close to the
p-side semiconductor region 114 under a given operating condition.
Analogously, quantum wells closer to the p-side semiconductor
region have a higher hole concentration than quantum wells close to
the n-side semiconductor region. Non-uniformity in hole
concentration is particularly pronounced due to their lower
mobility as compared with electrons in this material system.
[0013] In addition, for achieving a laser diode device with
emission wavelength longer than UV (>405 nm), it is typically
necessary to decrease the bandgap of the quantum wells 115 in the
active region 106. This has a consequence of increasing the
confinement of electrons in the quantum wells 115 closer to the
n-side semiconductor region 113 and holes in the quantum wells 115
closer to the p-side semiconductor region 114. This further
contributes to increased non-uniformity of carriers across the
quantum wells.
[0014] Lasing light emission is achieved when radiative
recombination rate from carrier recombination in the active region
reaches a level which is able to compensate for optical losses.
This corresponds to a carrier concentration under a high electric
excitation also known as threshold current. At this threshold,
lasing will be achieved and the quantum wells contributing to this
lasing process will be the one(s) with the highest radiative
recombination rate. If non-uniformity exists among the quantum
wells 115, the lasing process will be achieved at higher electric
excitation and therefore at a higher threshold current.
[0015] In Nakamura S and Fasol G, The Blue Laser Diode, p. 201-221,
1997 (Berlin: Springer), non-uniformity in carrier distribution may
be reduced by reducing the number of quantum well layers to, for
example, one. However, in such an arrangement, a single quantum
well layer is insufficient to localize all electrons injected to
the active region 106 and significant flow of electrons is not
confined in the active region and electrons are injected into the
p-side semiconductor region 114. This leads to recombination of
electrons and holes in the p-side semiconductor 114, in particular
in the upper guide layer 108, thereby reducing injection of holes
to the active region 106 and causing increase in threshold current.
To reduce this overflow effect, a single quantum well structure
with increased quantum well layer thickness may be used to increase
the confinement of electrons in the quantum well layer. However, it
is difficult to achieve InGaN based quantum well layers with high
crystal quality when the thickness of these layers is increased.
This has a consequence to increase defects in the quantum well
layers which may be responsible for increasing non-radiative
recombination rate and further increasing the threshold current of
the laser emitting device.
[0016] In U.S. Pat. No. 9,123,851B2 (Goda et al, Sep. 1, 2015), the
non-uniformity in carrier concentration of a multi quantum well
structure may be addressed by reducing the thickness of quantum
barrier layers 116 which are arranged between quantum well layers
115. However, in such structure, because the mobility of electrons
is higher than that of the holes, the non-uniformity of electron
concentration is enhanced and results in increased overflow of
electrons to the p-side semiconductor region 114, in particular in
the upper p-guide layer 108. This has a consequence to increase the
recombination of electrons and holes in the p-guide layer and
reduce the injection of holes to the active region. The laser light
emitting device achieved with such structure may exhibit an
increased threshold current.
[0017] In JP4622466B2 (Koji, Mar. 3, 2005), non-uniformity of
carrier concentration across the quantum wells may be reduced by
lowering the band gap energy of the barrier layers 116 compared to
quantum well layers 115 to enhance transport of carriers through
the barrier layers 116. However, in this case, transport of
electrons past the barrier layers is improved to a greater extent
than the transport of holes past the barrier layers, thus overall
increasing overflow of electrons to the p-side semiconductor region
114, in particular in the upper p-guide layer 108.
[0018] Reducing the bandgap energy of the barrier layers 116 can be
achieved by increasing the amount of In in the barrier layers 116
but this degrades the crystal quality of the active region, leading
to higher levels of non-radiative recombination and increasing the
threshold current of the laser diode device.
[0019] In Zhang et al. (Journal of Applied Physics 2009 105:2),
improved performance may be achieved by reducing the absorption
loss of photons in upper p-guide layer 108, lower n-guide layer
105, upper p-cladding layer 109, and lower n-cladding layer 104.
This can be achieved by reducing the concentration of dopant
species in these layers, which however negatively impacts hole
injection from A-side semiconductor region and n-side semiconductor
region into the active region.
[0020] There remains a problem of reducing threshold current of a
III-nitride based semiconductor laser diode by improving carrier
injection and carrier uniformity to the active region while
maintaining strong light confinement and high crystal quality, and
without degrading the characteristics of emitted laser light.
CITATION LIST
Patent Literature
[0021] U.S. Pat. No. 9,123,851B2 (Goda et al, Sep. 1, 2015). [0022]
JP4622466B2 (Koji, Mar. 3, 2005).
Non-Patent Literature
[0022] [0023] Nakamura S and Fasol G, The Blue Laser Diode, p.
201-221, 1997 (Berlin: Springer). [0024] Zhang et al., Confinement
factor and absorption loss of AlInGaN based lasr diodes emitting
from ultraviolet to green, Journal of Applied Physics 105, 023104
(2009).
SUMMARY OF INVENTION
[0025] The present disclosure provides a semiconductor laser diode
that may address the problems of the prior art and produce a laser
light emitting device with low current threshold. The present
disclosure provides a multiple quantum well structure for an active
region of a laser light emitting device. The active region may emit
visible light, and may overcome impracticalities found in previous
conventional devices. For example, the active region of a device of
an embodiment of the present disclosure includes two quantum well
regions, in which the quantum well closest to n-side semiconductor
has an increased band gap compared to the quantum well region
closest to the p-side semiconductor. The device including this
quantum well structure for the active region may provide low
threshold current, long lifetime, and high
wall-plug-efficiency.
[0026] In an aspect of the present disclosure, a group III nitride
based laser diode includes: an n-side group III nitride based
semiconductor region; a p-side group III nitride based
semiconductor region; and a group III nitride based active region
between the p-side group III nitride based semiconductor region and
the n-side group III nitride based semiconductor region, the group
III nitride based active region including first and second quantum
well layers and a barrier layer between the first and second
quantum well layers, respective compositions of the first and
second quantum well layers including different respective amounts
of indium; wherein the first quantum well is closer to the n-side
group III nitride based semiconductor region than the second
quantum well, the second quantum well is closer to the p-side group
III nitride based semiconductor region than the first quantum well,
and the first quantum well has a larger band gap than that of the
second quantum well.
[0027] In some embodiments, the respective band gaps of the first
and second quantum wells are determined by the different indium
amounts of the first and second quantum wells.
[0028] In some embodiments, the first quantum well layer is
In.sub.x1Ga.sub.1-x1N, where 0<x1<1; and the second quantum
well layer is In.sub.x2Ga.sub.1-x2N, where 0<x2<1. In some
embodiments, an indium amount ratio x2/x1 is 1.05 to 5. In some
embodiments, an indium amount ratio x2/x1 is 1.2 to 5. In some
embodiments, an indium amount ratio x2/x1 is 1.05 to 3. In some
embodiments, an indium amount ratio x2/x1 is 1.2 to 3.
[0029] In some embodiments, a threshold current density of the
laser diode is at least 1000 A/cm.sup.2.
[0030] In some embodiments, a thickness of the barrier layer is at
least 5 nm.
[0031] In some embodiments, the barrier layer includes a larger
band gap than the first and second quantum well layers.
[0032] In some embodiments, a thickness of the barrier layer is 30
nm or less.
[0033] In some embodiments, the lasing wavelength is at least 450
nm and 550 nm or less.
[0034] In some embodiments, the lasing wavelength is at least 500
and 550 nm or less.
[0035] In some embodiments, the n-side group III nitride based
semiconductor region includes an InGaN guide layer. In some
embodiments, a thickness of the InGaN guide layer is at least 80 nm
and 300 nm or less. In some embodiments, a thickness of the InGaN
guide layer is at least 120 nm and 300 nm or less. In some
embodiments, a thickness of the InGaN guide layer is at least 160
nm and 300 nm or less.
[0036] In some embodiments, the InGaN guide layer is non-doped.
[0037] In some embodiments, the group III nitride based active
region further includes a cap layer of InGaN, the cap layer located
between the first quantum well layer and the InGaN guide layer. In
some embodiments, a thickness of the cap layer is at least 3 nm and
30 nm or less.
BRIEF DESCRIPTION OF DRAWINGS
[0038] FIG. 1A shows a schematic representation of the layer
structure of a conventional semiconductor laser diode device.
[0039] FIG. 1B shows a schematic representation of the layer
structure of the active region of a conventional semiconductor
laser diode device.
[0040] FIG. 1C shows a graph illustrating the respective amounts of
indium in the layers of the active region of a conventional
semiconductor laser diode device.
[0041] FIG. 1D shows a graph illustrating an energy band structure
of layers in the active region of a conventional semiconductor
laser diode device.
[0042] FIG. 2A shows a schematic representation of a layer
structure of a device according to an exemplary embodiment of the
present disclosure.
[0043] FIG. 2B shows a schematic representation of a layer
structure of the active region of a device according to an
exemplary embodiment of the present disclosure.
[0044] FIG. 2C shows a graph illustrating the respective amounts of
indium in the quantum well layers of the active region of a device
according to an exemplary embodiment of the present disclosure.
[0045] FIG. 2D shows a graph illustrating an energy band structure
of layers in the active region of a device according to an
exemplary embodiment of the present disclosure.
[0046] FIG. 3A shows a graph illustrating a concentration of charge
carriers in quantum well layers of a conventional nitride laser
diode device emitting with an emission wavelength around 405 nm,
obtained from a calibrated laser simulation.
[0047] FIG. 3B shows a graph illustrating a concentration of charge
carriers in quantum well layers of a conventional nitride laser
diode device emitting with an emission wavelength >450 nm,
obtained from a calibrated laser simulation.
[0048] FIG. 4 shows a graph illustrating a change in threshold
current density of a laser light emitting device dependent on
composition of quantum well layers according to an embodiment of
the present disclosure.
[0049] FIG. 5 shows a graph illustrating a change in wall plug
efficiency of a laser light emitting device dependent on
composition of quantum well layers according to an embodiment of
the present disclosure.
[0050] FIG. 6 shows the measured current-voltage characteristic of
a reference laser light emitting device and a laser light emitting
device according to the present disclosure, with 500 nm-510 nm
emission.
[0051] FIG. 7 shows the measured light output power-current
characteristic of a reference laser light emitting device and a
laser light emitting device according to the present disclosure,
with 500 nm-510 nm emission.
DESCRIPTION OF REFERENCE NUMERALS
[0052] 100: light emitting diode device [0053] 101: buffer layer
[0054] 102: n-contact layer [0055] 103: buffer layer [0056] 104:
n-cladding layer [0057] 105: n-guide layer [0058] 106: active
region [0059] 107: carrier blocking layer [0060] 108: n-guide layer
[0061] 109: p-cladding layer [0062] 110: p-contact layer [0063]
111a: metal electrode layer [0064] 111b: metal electrode layer
[0065] 112: substrate [0066] 113: n-side semiconductor region
[0067] 114: p-side semiconductor region [0068] 115: quantum well
layer [0069] 116: quantum barrier layer [0070] 200: light emitting
diode device [0071] 201: n-cladding layer [0072] 202: n-guide layer
[0073] 203: guide layer [0074] 204: active region [0075] 205: guide
layer [0076] 206: carrier blocking layer [0077] 207: p-cladding
layer [0078] 208: contact layer [0079] 209a: metal layer electrode
[0080] 209b: metal layer electrode [0081] 210: substrate [0082]
211: insulating layer [0083] 212: n-side semiconductor region
[0084] 213: p-side semiconductor region [0085] 214: barrier layer
(cap layer) [0086] 215: well layer [0087] 216: barrier layer [0088]
217: well layer [0089] 218: barrier layer (cap layer)
DETAILED DESCRIPTION OF INVENTION
[0090] The present disclosure provides an active region of a
semiconductor laser diode (e.g., a group III nitride-based laser
light emitting diode). The active region may include two quantum
well regions in which the quantum well closest to n-side
semiconductor region has an increased band gap compared to the band
gap of the quantum well region closest to the p-side semiconductor
region. The device including the multiple quantum well structure
for the active region may provide low threshold current, long
lifetime, and high wall-plug-efficiency.
[0091] Turning now to FIGS. 2A and 2B, an exemplary embodiment of a
semiconductor laser diode (e.g., a group III nitride-based laser
light emitting diode) device including the active region is shown
at 200. The group III nitride-based laser light emitting diode
device includes an n-side (group III nitride-based) semiconductor
region 212, a p-side (group III nitride-based) semiconductor region
213, and a (group III nitride-based) active region 204 between the
p-side semiconductor region 213 and the n-side semiconductor region
212. The active region 204 includes a plurality of (e.g., first and
second) stacked group III nitride-based quantum well layers 215,
217 interspersed with barrier layers 214, 216, 218 as shown in FIG.
2B.
[0092] In some embodiments, one or more of the layers of the n-side
semiconductor region 212 are n-type doped layers, while other
layers of the n-side semiconductor region are not n-type doped
layers. In other embodiments, all of the layers of the n-side
semiconductor region 212 may be n-type doped.
[0093] In some embodiments, one or more of the layers of the p-side
semiconductor region 213 are p-type doped layers, while other
layers of the p-side semiconductor region are not p-type doped
layers. In other embodiments, all of the layers of the p-side
semiconductor region 213 may be p-type doped.
[0094] A device of the present disclosure, such as that shown in
FIGS. 2A and 2B, may be obtained by any suitable means such as
Metal-Organic Chemical Vapour Deposition (MOCVD), Molecular Beam
Epitaxy (MBE), or any other suitable deposition method(s). The
device layers may be deposited on any suitable substrate 210 which
may include but is not limited to: GaN, Silicon, Sapphire or SiC.
To produce the n-type semiconductor, Si, Ge, O, S, Se may be used
as the dopant. To produce the p-type semiconductor, Be, Cd, Mg may
be used as the dopant.
[0095] Referring to FIG. 2B, the quantum well layers 215 and 217
may have respective compositions and may each include a first
element. In some embodiments, the first element may be indium. In
an example, the two nitride-based quantum well layers (215,217) are
(Al, In, Ga)N semiconductor alloys and each include In. In some
embodiments, the composition of one or both of the first and second
quantum well layers is InGaN. The composition of the respective
quantum well layers may differ among one another with respect to
the amount of the first element (e.g., In). The variation in the
respective compositions of the quantum well layers 215, 217 may be
dependent on the position of the quantum well layer within the
active region 204. In some embodiments, the first element may be
indium and the composition for a quantum well layer 215 closer to
n-side semiconductor region 212 may include a lower amount of
indium as compared with the amount of indium in the composition of
the quantum well layer closer to p-side semiconductor region 213.
Hence, the quantum well layers (215, 217) may have a composition
including a first element (In) which is dependent on the position
of the quantum well layer within the active region 204. In such
embodiments, and as illustrated by FIG. 2C, the amount of In x1 of
the first quantum well layer 215, located relatively closer to the
n-side semiconductor region 212, is lower compared to the amount of
In x2 of the second quantum well layer 217 which is closer to the
p-side semiconductor region 213.
[0096] FIG. 2D depicts the corresponding energy levels for charge
carriers (i.e. electrons and holes) within the active region 204.
Electron energy is indicated by the energy of the conduction band,
and hole energy is indicated by the energy of the valence band. The
energy band gap E1 of the first quantum well layer 215 and energy
band gap E2 of the second quantum well layer 217 are indicated. In
the example where the composition of each of the first and second
quantum well layers is InGaN, the energy band gap of the InGaN
alloy material is dependent on the amount of the first element In
in the alloy. The energy band gap of InN (.about.0.7 eV) is lower
than that of GaN (.about.3.4 eV).I It therefore follows that
increasing amount of the first element In in an InGaN alloy layer
leads to reduced energy band gap in said layer. In some
embodiments, the composition of one or more of the barrier layers
is InGaN, where the amount of In is less than the In amount in
either of the first and second quantum well layers. In some
embodiments, the composition of one or more of the barrier layers
is GaN.
[0097] As depicted in FIGS. 2C and 2D, the indium amount in the
composition is lower for the first quantum well layer 215 compared
to the second quantum well layer 217. This results in band gap E1
of the first quantum well layer 215 being comparatively larger than
the band gap E2 for the second quantum well layer 217.
[0098] As described above, an active region 204 fabricated in the
(Al, In, Ga)N material system includes a multiple quantum well 215,
217 and quantum barrier layers 214, 216, 218 structure. The active
region 204 is surrounded by a p-side semiconductor region 213 and
an n-side semiconductor region 212. This provides the structure of
a semiconductor laser diode device.
[0099] In n-side semiconductor regions, the majority of charge
carriers are electrons; and in p-side semiconductor regions, the
majority charge carriers are holes. Under electrical excitation,
holes may be injected to the active region 204 from the p-side
semiconductor region 213 and electrons may be injected to the
active region 204 from the n-side semiconductor region 212. The
term "injected" is used herein to denote transport of charge
carriers from origination in the p-side semiconductor region or the
n-side semiconductor region to another layer within the device,
under some electrical excitation applied across the device. These
electron and holes (charge carriers) may accumulate in the quantum
wells (215, 217) due to the lower energy levels compared to the
adjacent barrier layer (216). Further transport of charge carriers
to quantum wells furthest from the injection layer (p-side or
n-side semiconductor region 213 and 212) is limited by the quantum
barrier layers. Therefore, the quantum well 215 close to the n-side
semiconductor region 212 may have a higher concentration of
electrons than the quantum well 217 close to the p-side
semiconductor region 213 under a given operating condition.
Analogously, the quantum well 217 closer to p-side semiconductor
region 213 may have a higher hole concentration than the quantum
well 217 close to n-side semiconductor region 212. It is known in
the art that in a (Al,In,Ga)N material system, hole mobility is
lower than electron mobility.
[0100] The operation of the semiconductor laser diode device of the
present disclosure may be characterised by recombination of charge
carriers in the active region. Recombination processes can be
radiative or non-radiative. Radiative recombination denotes that
energy released by recombination of one electron with one hole is
transferred to a photon with energy equivalent to that lost by the
recombining electron and hole. Non-radiative recombination refers
to all the other mechanisms which release energy from electron and
hole in a non-radiative way such as carrier leakage, phonon
recombination, recombination with defects or impurities in the
material etc. The wavelength of the light emitted by the nitride
semiconductor laser diode device of the present disclosure is
therefore determined by the energy of the photon released when an
electron and hole recombine by a radiative process as described
above.
[0101] FIG. 3A illustrates the carrier concentration in a
conventional double quantum well (QW1 and QW2) active region
structure for conventional laser emitting device (e.g., similar to
that shown in FIGS. 1A-1D) with an emission wavelength around 405
nm. FIG. 3A shows that the non-uniform distribution of carrier
concentration across the quantum wells is particularly pronounced
for hole concentration due to comparatively lower mobility compared
to electrons.
[0102] For the purpose of achieving a nitride laser emitting device
with a longer wavelength emission for the device discussed in FIG.
3A, the energy difference between the electron and the hole which
recombine must be comparatively lower. This implies that the energy
band gap of the layer or layers in which the recombination takes
place must be comparatively lower. The comparatively lower band gap
of the quantum well layers increases the relative potential height
of the adjacent barrier layers, which more strongly confines charge
carriers to the quantum well(s) closest to the layer from which
they have been injected into the active region. Therefore,
transport of carriers to layers beyond the barrier to quantum
well(s) farthest from the layer from which they have been injected
into the active region is comparatively reduced. As exemplified in
FIG. 3B (showing a comparison to that in FIG. 3A of both carrier
types in a double quantum well active region), comparatively lower
band gap for quantum wells emitting in the blue/green spectrum
leads to comparatively greater non-uniformity in distribution of
both carrier types across the two quantum well layers.
[0103] Lasing light emission is achieved when radiative
recombination rate from carrier recombination in the active region
reach a level which is able to compensate for optical losses of the
optical cavity. This corresponds to a threshold carrier
concentration under a high electric excitation also known as
threshold current (current corresponding to the amount of carrier
flow through under the electrical excitation across the device).
For lasing to be achieved, the carrier concentration threshold must
be met for both carrier types in the same quantum well layer.
Threshold current may also be expressed in current density, which
is calculated by dividing threshold current by the area of the
laser diode device parallel to the active region in which
electrical current flows. The area is commonly calculated as the
width of the ridge defined by p-GaN contact layer 208 and the
distance between the mirror surfaces defined by cleavage of the
semiconductor layer sequence.
[0104] If non-uniformity of carrier concentration exists between
the quantum wells, the lasing process will be achieved at different
current density for each quantum well layer. In laser diode devices
formed of (Al,In,Ga)N, the threshold carrier concentration is
higher compared to other material systems, such as (Al,In,Ga)As. As
such, threshold current density of (Al,In,Ga)N laser diode devices
is typically 1000 A/cm.sup.2 or higher.
[0105] Referring to FIG. 3A and FIG. 3B, the minimum concentration
of either electrons or holes is comparatively higher in the quantum
well layer QW2 than the quantum layer QW1 for a given operating
condition. Therefore, the carrier concentration threshold is
achieved at a lower current in the quantum well layer QW2. The
current required to achieve the carrier concentration threshold in
the quantum well layer QW1 may be comparatively significantly
higher than the current required to achieve the carrier
concentration threshold in the quantum well layer QW2.
[0106] In an embodiment of the present disclosure, and with
reference to FIG. 2B, the active region may be configured such that
laser light emission is achieved only for the second quantum well
layer 217. In said embodiment, the limiting process for onset of
laser light emission is electron injection to the second well 217
due to comparatively lower concentration of electrons than holes at
a given operating condition. With further reference to FIG. 2D, the
band gap E1 of the first quantum well layer 215 is increased
compared to the band gap E2 of the second quantum well layer 217,
resulting in improved electron injection to the second quantum well
layer 217 and a lower threshold current for laser light emission
from the device.
[0107] In the art, efforts have been made to reduce the extent of
non-uniformity in active regions including multiple quantum well
layers. By reducing non-uniformity, light emission from more than
one quantum well is expected. However, simulation of such
structures has shown that under a desirable operating condition,
light emission remains comparatively significantly higher in
quantum well(s) closest to the p-side semiconductor.
[0108] In the structure of the present disclosure, the carrier
threshold condition is intentionally met only in the second quantum
well layer 217 and laser light emission is only from the second
quantum well layer 217. Therefore, the energy of photons generated
by laser light emission process may be determined by the energy
band gap E2 of the second quantum well layer 217, and may not
depend on the energy band gap E1 of the first quantum well layer
215. It therefore follows that changes to the properties of the
first quantum well layer 215 such as amount of a first element in
the composition may not affect the characteristic of laser light
emitted by the structure. Changes to the properties of the first
quantum well layer 215 may be made to improve electrical and
efficiency characteristics of the device.
[0109] For a semiconductor laser diode device to achieve lasing
operation, its structure should provide confinement of the carrier
recombination and emitted photons to an active region. With
continued reference to FIG. 2A, light confinement in the direction
perpendicular to the active region direction may be achieved by
arranging guide layers 202, 203 below and p-guide layer 205 above
the active region 204, each of the guide layers 202, 203, 205
having a refractive index higher than the effective index of the
guided light. Further confinement may be achieved by arranging
n-cladding layer 201 below and p-cladding layer 207 above the
active region, each of cladding layers 201, 207 having a refractive
index lower than the effective index of the guided light. The
structure of the present disclosure with the guide layers 202, 203,
205 and the cladding layers 201, 207 as described above may provide
light confinement in the direction perpendicular to the active
region. In an example, the guide layers 202, 203, 205 and/or the
cladding layers 201, 207 may be (Al, In, Ga)N semiconductor
alloys.
[0110] The refractive index of InN is comparatively higher compared
to the refractive index of GaN. It follows that the refractive
index of InGaN material will increase with increasing In amount.
This increase may or may not be linear. Furthermore, at wavelengths
longer than 450 nm the refractive index of GaN and InN both
decrease with increasing wavelength such that the refractive index
contrast between GaN and InN is reduced at longer wavelength. It
follows that refractive index contrast between GaN and InGaN, for a
chosen In amount, is reduced at longer wavelength.
[0111] The refractive index of AlN is comparatively lower compared
to the refractive index of GaN. It follows that the refractive
index of AlGaN material will decrease with increasing Al amount.
This decrease may or may not be linear. Furthermore, at wavelengths
longer than 450 nm the refractive index of AlN increases with
increasing wavelength such that the refractive index contrast
between GaN and AlN is reduced at longer wavelength. It follows
that refractive index contrast between GaN and AlGaN, for a chosen
Al amount, is reduced at longer wavelength.
[0112] Between the upper guide layer 205 and upper cladding layer
207, a carrier blocking layer 206 is provided, which has a higher
band gap compared to adjacent layers 205 and 207. This layer may
reduce transport of electrons from the guide layer 205 to the
cladding layer 207 and may not strongly impact transport of holes
from cladding layer 207 to guide layer 205.
[0113] As exemplified in FIG. 2A, the upper cladding layer 207 may
have a shaped upper surface, for example, formed by post-growth
processing. In a typical layout, the shaped upper surface may be a
ridge formed in the plane of growth, which additionally confines
light propagation to the direction of the longest dimension of the
ridge. In the example shown, a p-type GaN contact layer 208 is
formed on top of the upper cladding layer 207 prior to post-growth
processing, to aid electrical contact. Electrical contact is made
to the substrate 210 and electrical contact is made to p-type GaN
208 by metal layer electrodes 209a and 209b, which allows the
device to be electrically activated. An insulating layer 211 is
formed between the upper cladding layer 207 and metal layer
electrode 209a such that injection of charge carriers to the device
only occurs in the area defined by the p-type GaN contact layer 208
that coincides with the shaped ridge.
[0114] In an embodiment of the present disclosure, two mirror
surfaces are provided by cleavage (or other methods) of the
semiconductor layer structure perpendicular to the longest
dimension of the ridge and to the active region layer structure to
confine the light in the direction parallel to the active region
204. The two mirror surfaces together may define an optical cavity.
The mirror surfaces can be treated for example by deposition of a
further other material (e.g. SiNx, SiO.sub.2, Al.sub.2O.sub.3) to
alter the refractive index property in order to change their mirror
reflectivity to the laser light emission. Light may escape by
transmission through these surfaces and this light may be used for
the intended application. In this described arrangement, the device
is said to be edge emitting, the edge being defined as
perpendicular to active region layer structure and the longest
dimension of the ridge. The arrangement of the optical cavity of
the laser emitting device may also be defined parallel to the
active region layer structure, resulting in a vertically emitting
laser device.
[0115] Quantum wells 215, 217 may be comprised of InGaN material
and may include In in respective amounts x1 and x2 in the range
0<x1.ltoreq.1 and 0<x2.ltoreq.1. The quantum barrier layer
216 may comprise In.sub.yGa.sub.1-yN material and may include In in
an amount y in the range 0.ltoreq.y<1 and satisfying y<x1 and
y<x2. But in some embodiments, the quantum barrier layer 216 may
not include In. This may improve crystal quality and/or improve
growth quality of the second quantum well 217. The barrier layer
216 may have a larger band gap than the first and second quantum
well layers.
[0116] Quantum barrier layers 214, 218 (also referred to herein as
cap layers) may include In.sub.zGa.sub.1-zN and may include In in
an amount z in the range 0.ltoreq.z<1 and satisfying z<x1 and
z<x2. But in some embodiments, one or both of the quantum
barrier layers 214, 218 may not include In. This may improve
crystal quality of these layers and/or provide a smooth surface for
growth of subsequent layers.
[0117] In some embodiments, a thickness of each of the quantum well
layers 215, 217 may be at least 1 nm and less than 10 nm. In some
embodiments, a thickness of each of the quantum barrier layers 214,
216 may be at least 5 nm and less than 30 nm. In some embodiments,
a thickness of the quantum barrier layer 218 may be at least 1 nm
and less than 30 nm.
[0118] In some embodiments, the active region 204, comprised of
quantum well layers 215 and 217, quantum barrier layer 216, and cap
layers 214 and 218, is non-doped. Introduction of dopants, such as
but not limited to, Si or Mg (commonly used in (Al,In,Ga)N material
system) can in some embodiments reduce the crystal quality which
would then impact the device performance. However non-doped layers
may have the effect of reducing the injection of carriers to the
quantum wells and increasing threshold current. Accordingly, in
some embodiments, one or more of the layers within the active
region 204 may be doped (e.g., by one or more dopants such as but
not limited to Si, Ge, O, S, Se, Be, Cd, Mg).
[0119] In embodiments wherein the bandgap E1 of the first quantum
well layer 215 is larger than the bandgap E2 of the second quantum
well layer 217, the amount of Indium x1 of the first quantum well
layer 215 of In.sub.x1Ga.sub.1-x1N material is different from the
amount of Indium x2 of the second quantum well layer 217 of
In.sub.x2Ga.sub.1-x2N material. This difference may be defined by
the Indium amount ratio x2/x1. In some examples, the ratio x2/x1 is
equal to 1.05 or greater. In other examples, the In amount ratio
x2/x1 of first and second quantum wells is equal to 1.2 or greater.
In other examples, the In amount ratio x2/x1 of first and second
quantum wells is equal to 2 or greater.
[0120] In some embodiments, the wavelength of the laser light
emitted by the device of the present disclosure, as determined by
the energy gap E2 of the second quantum well layer 215, is at least
450 nm. In this case it has been found that the use of a lower
amount of indium x1 in the composition of the first quantum well
layer 215 is preferable to reduce threshold current of the laser
emitting device.
[0121] In some embodiments, the wavelength of the laser light
emitted by the device of the present disclosure is at least 500 nm.
Under these conditions, the threshold current of such laser device
according to this embodiment can be reduced by lowering the amount
of In x1 of the first quantum well layer 215 according to the
present disclosure in comparison to a laser device prepared without
lowering the amount of In x1 of quantum well layer 215.
[0122] Decreased threshold current may have a positive impact on
wall-plug-efficiency (WPE) as calculated by the ratio of useful
light energy output to the electrical energy input expressed as a
percentage. In some embodiments, the wavelength of the laser light
emitted by the device of the present disclosure is 550 nm or
less.
[0123] In an exemplary embodiment of the present disclosure,
wavelength emission of the semiconductor laser diode device is at
least 450 nm. The cladding layers 201, 207 may be AlGaN and may not
have all equal amounts of Al. The guide layers 202,203, 205 may be
InGaN and may not have all equal amounts of In. As it is been
described, the refractive index of the cladding layers 201,207 and
the guide layers 202,203,205 will reduce when the wavelength light
emission of the active region is increased, and therefore the
refractive index difference between guide layers 202,203,205 and
cladding layers 201,207 will decrease. This may result in a reduced
confinement of light of the laser light emitting device and may
impact the device performance. Therefore, in some embodiments, it
may be preferable to set the In amount in the InGaN guide layer 205
to 2% or more by atom fraction. In this case, a difference in
conduction band energy is created between the second quantum well
cap layer 218 and the InGaN guide layer 205. This may reduce hole
injection and create an accumulation of holes at the interface
between these layers. As the effect of the present disclosure is to
increase the transport of electrons toward the second quantum well
layer 217, it also increases the transport into the InGaN guide
layer 205. It therefore may also be preferable that the In amount
ratio x2/x1 differs by a factor of 5 or less as otherwise
significant recombination of electrons and holes occurs in
In.sub.xGa.sub.1-xN guide layer 205, reducing injection of holes to
the active region 204. In some embodiments, the In amount ratio
differs by a factor of 3 or less.
[0124] In some embodiments, the InGaN guide layer 205 is p-type
doped. This may improve the electrical performance of the laser
device.
[0125] In another exemplary embodiment of the present disclosure,
wavelength emission of the semiconductor laser diode device is at
least 450 nm. As it has been described, the refractive index of the
cladding layers 201,207 and the guide layers 202,203,205 will
reduce and therefore the refractive index difference between guide
202,203,205 and cladding layers 201,207 will decrease. This may
result in a reduced confinement of light of the laser light
emitting device and may impact the device performance. Therefore,
in some embodiments, it may be preferable to set the thickness of
InGaN guide layer 203 to 80 nm or more, and the In amount to 2% or
more by atom fraction. With these properties, the crystal quality
of the InGaN guide layer 203 may be reduced, which may negatively
impact the crystallinity of the active region, in particular the
quantum wells, leading to an increase in threshold current.
Accordingly, by lowering the In amount x1 of the first quantum well
215, the crystallinity of the second quantum well 217 can be
improved and threshold current density reduced. This structure may
be preferable when the InGaN guide layer 203 thickness exceeds 120
nm. Furthermore, this structure may be particularly preferable when
the InGaN guide layer 203 thickness exceeds 160 nm. In some
embodiments, the InGaN guide layer 203 thickness is 300 nm or
less.
[0126] The semiconductor laser diode device of the present
disclosure may include a quantum barrier layer 216 arranged between
the first quantum well layer 215 and second quantum well layer 217.
In an embodiment, the quantum barrier layer 216 thickness is larger
than 5 nm. As the quantum barrier layer 216 thickness is increased,
defects which exist in the epitaxially grown material originating
from the first quantum well layer 215 can be removed, which is
preferable for improving the crystal quality of the second quantum
well layer 217. However as the quantum barrier layer 216 thickness
is increased, injection of electrons to the second quantum well
layer 217 may also be negatively impacted. Lowering the In amount
x1 of the first quantum well 215 can improve electron injection to
the second quantum well layer 217. In some embodiments, the quantum
barrier layer 216 thickness is 30 nm or less (and larger than 5
nm).
[0127] As shown in FIG. 2A, the device of the present disclosure
may have a cap layer 214 formed below the first quantum well 215.
The cap layer 214 may improve crystal quality of the subsequent
quantum well layer 215. The cap layer 214 may be comprised of
In.sub.z1Ga.sub.1-z1N with an amount of In z in the range
0.ltoreq.z1<0.05, satisfying z1<x1 and z1<x2. In some
embodiments, the cap layer 214 is non-doped to maintain high
crystal quality. When the In amount z1 of the cap layer 214 is
equal to zero, a layer with very high crystal quality can be
fabricated. When the In amount z1 of the cap layer 214 is greater
than zero, electron injection to the active region may be improved,
but crystal quality of the subsequent layer 215 may be reduced. In
some embodiments, the amount of In z1 of the cap layer 214 may be
0.05 or less.
[0128] When a laser diode structure includes a cap layer 214 it was
found that by setting the amount of In x1 of the first quantum well
215 lower than amount of In x2 of the second quantum well 217, the
threshold current of this laser emitting device may be improved in
comparison to a similar structure without features of the first and
second quantum wells in accordance with the present disclosure.
[0129] In some embodiments, to achieve improvement in crystal
quality of the first quantum well layer 215, the thickness of the
cap layer 214 is at least 3 nm. In some embodiments, the thickness
of the cap layer is 30 nm or less and at least 3 nm.
[0130] In some embodiments, the InGaN guide layer 203 is n-type
doped. This may improve the electrical performance of the laser
device.
[0131] In some embodiments, the InGaN guide layer 203 is non-doped.
This may improve the crystallinity of the active region. Under
these conditions, the threshold current of such laser device
according to this embodiment can be reduced by lowering the amount
of In x1 of the first quantum well layer 215 according to the
present disclosure in comparison to a laser device prepared without
lowering the amount of In x1 of quantum well layer 215.
Examples
[0132] Examples of the embodiments of the device of the present
disclosure are now described, although such examples are not
intended to be limiting in any respect.
[0133] A first exemplary embodiment of the semiconductor laser
diode is described with reference to FIGS. 2A and 2B. A
semiconductor laser diode device produces light with a wavelength
in the range 440 nm to 460 nm. The laser diode device includes a
substrate 210 of free-standing GaN. On the substrate 210 an
n-Al.sub.0.04Ga.sub.0.96N cladding layer 201 with thickness 1000 nm
and n-dopant Si concentration 5e18 cm.sup.-3 is formed. On the
cladding layer 201 an n-GaN guide layer 202 with thickness 400 nm
and n-dopant Si concentration 5e18 cm.sup.-3 is formed. On the
n-GaN guide layer 202 an In.sub.0.025Ga.sub.0.975N guide layer 203
with thickness 80 nm and n-dopant Si concentration 1e18 cm.sup.-3
is formed.
[0134] On the guide layer 203, a laser light emitting active region
204 is formed which comprises (as described in FIG. 2B), a lower
cap layer 214 of GaN with thickness 10 nm, a first quantum well
layer 215 of In.sub.0.08Ga.sub.0.92N with thickness 2.5 nm, a
barrier layer 216 of GaN with thickness 10 nm, a second quantum
well 217 of In.sub.0.16Ga.sub.0.84N with thickness 2.5 nm and an
upper cap layer 218 of GaN with thickness 2 nm. The In amount ratio
of the quantum wells of this example is equal to 2. On the active
region 204 an In.sub.0.025Ga.sub.0.975N guide layer 205 with
thickness 100 nm is formed. On the guide layer 205 a
p-Al.sub.0.2Ga.sub.0.8N carrier blocking layer 206 with thickness
10 nm and p-dopant Mg concentration 5e19 cm.sup.-3 is formed. On
the carrier blocking layer 206 a p-Al.sub.0.04Ga.sub.0.96N cladding
layer 207 with thickness 500 nm and p-dopant Mg concentration 1e19
cm.sup.-3 is formed. On the cladding layer 207 a p-GaN contact
layer 208 with thickness 100 nm and p-dopant Mg concentration 1e19
cm.sup.-3 is formed.
[0135] The p-Al.sub.0.04Ga.sub.0.96N cladding layer 207 and p-GaN
contact layer 208 are shaped to form a ridge by some post
deposition method. An insulating layer 211 of SiO.sub.2 of
thickness 50 nm is formed between the p-Al.sub.0.2Ga.sub.0.8N
cladding layer 207 and metal electrode layer 209a. The metal
electrode layers 209a and 209b (for example made of Ni, Cu, Pd, Ad,
Ir, Pt, Au, Sc, Ti, V, Cr, Y, Zr, Nb, Mo, La, Hf, Ta, W, Al, Ti)
are respectively disposed on the surface of the SiO.sub.2
insulating layer 211 and the p-GaN contact layer 208, and the
bottom surface of the substrate 210, such that injection of charge
carriers only occurs in the area defined by the p-GaN contact layer
surface in contact with the metal contact layer.
[0136] Two surfaces are formed by cleavage of the semiconductor
layer sequence 200 at both ends of the ridge, which act as mirror
surfaces, thus forming a laser cavity. One mirror surface has
reflectivity 0.9 or higher to light propagating in the laser
cavity. The second mirror surface has reflectivity 0.7 or lower to
light propagating in the laser cavity, light escaping from this
mirror surface is used for the application. Reflection properties
of mirror surfaces may be altered for example by deposition of
another material layer structure (for example SiO.sub.2, SiN.sub.x
or Al.sub.2O.sub.3) on the surface accordingly to well-known
techniques. Electrical contact is made to the metal electrode
contact layer 209a and 209b to generate electrical excitation to
the laser emitting device.
[0137] Laser diode devices according to the first exemplary
embodiment have a threshold current of 25 mA and a wall plug
efficiency of 15% measured when emitting 30 mW of light output
power.
[0138] A second exemplary embodiment of the semiconductor laser
diode is described with reference to FIGS. 2A and 2B. A
semiconductor laser diode device produces light with a wavelength
in the range 490 nm to 510 nm.
[0139] The laser light emitting device includes a substrate 210 of
free-standing GaN. On the substrate 210 an
n-Al.sub.0.05Ga.sub.0.95N cladding layer 201 with thickness 1000 nm
and n-dopant Si concentration 5e18 cm.sup.-3 is formed. On the
cladding layer 201 an n-GaN guide layer 202 with thickness 400 nm
and n-dopant Si concentration 5e18 cm.sup.-3 is formed. On the
guide layer 202 an In.sub.0.03Ga.sub.0.97N guide layer 203 with
thickness 160 nm and n-dopant Si concentration 1e18 cm.sup.-3 is
formed.
[0140] On the guide layer 203 a laser light emitting active region
204 is formed which comprises, a lower cap layer 214 of GaN with
thickness 10 nm, a first quantum well layer 215 of
In.sub.0.1Ga.sub.0.9N with thickness 2.5 nm, a barrier layer 216 of
GaN with thickness 10 nm, a second quantum well 217 of
In.sub.0.24Ga.sub.0.76N with thickness 2.5 nm and an upper cap
layer 218 of GaN with thickness 2 nm. The Indium amount ratio of
the two quantum wells 215 and 217 is equal to 2.4. On the active
region 204 an In.sub.0.03Ga.sub.0.97N guide layer 205 with
thickness 150 nm is formed. On the guide layer 205 a
p-Al.sub.0.2Ga.sub.0.8N carrier blocking layer 206 with thickness
10 nm and p-dopant Mg concentration 5e19 cm.sup.-3 is formed. On
the carrier blocking layer 206 a p-Al.sub.0.05Ga.sub.0.95N cladding
layer 207 with thickness 500 nm and p-dopant Mg concentration 1e19
cm.sup.-3 is formed. On the cladding layer 207 a p-GaN contact
layer 208 with thickness 100 nm and p-dopant Mg concentration 1e19
cm.sup.-3 is formed.
[0141] The p-Al.sub.0.05Ga.sub.0.95N cladding layer 207 and p-GaN
contact layer 208 are shaped to form a ridge by some post
deposition method. An insulating layer 211 of SiO.sub.2 of
thickness 50 nm is formed between the p-Al.sub.0.2Ga.sub.0.8N
cladding layer 207 and metal electrode layer 209a. The metal
electrode layers 209a and 209b (for example made of Ni, Cu, Pd, Ad,
Ir, Pt, Au, Sc, Ti, V, Cr, Y, Zr, Nb, Mo, La, Hf, Ta, W, Al, Ti)
are respectively disposed on the surface of the SiO.sub.2
insulating layer 211 and the p-GaN contact layer 208 and on the
bottom surface of the substrate 210, such that injection of charge
carriers only occurs in the area defined by the p-GaN contact layer
surface in contact with the metal contact layer.
[0142] Two surfaces are formed by cleavage of the semiconductor
layer sequence at both ends of the ridge, which act as mirror
surfaces, thus forming a laser cavity. One mirror surface has
reflectivity 0.9 or higher to light propagating in the laser
cavity. The second mirror surface has reflectivity 0.7 or lower to
light propagating in the laser cavity, light escaping from this
mirror surface is used for the application. Reflection properties
of mirror surfaces may be altered for example by deposition of
another material layer structure (for example SiO.sub.2, SiNx or
Al.sub.2O.sub.3) on the surface accordingly to well-known
techniques. Electrical contact is made to the metal electrode
contact layer 209a and 209b to generate electrical excitation to
the laser emitting device.
[0143] Laser diode devices according to the second exemplary
embodiment have a threshold current of 50 mA and a wall plug
efficiency of 5% measured when emitting 30 mW of light output
power.
[0144] The exemplary embodiments described above include the
structure of the active region shown in FIG. 2B, in particular that
the first quantum well layer 215 located relatively closer to the
n-side semiconductor 212 has lower indium amount x1 compared to the
second quantum well layer 217 located relatively closer to the
p-side semiconductor 213. The structures of the exemplary
embodiments provide reduced threshold current density compared to
structures where indium amount x1 of the first quantum well layer
215 and x2 of the second quantum well layer 217 are equal. FIGS. 4
and 5 show the threshold current density of the exemplary
semiconductor laser diodes.
[0145] Referring to FIG. 4 and FIG. 5 the exemplary structure in
accordance with the present disclosure for 440 nm-460 nm emission
has 7% lower threshold current density (J.sub.th) and 1.1% higher
wall plug efficiency (WPE) at 30 mW light output power compared to
a conventional structure that does not utilize the active region of
the present disclosure.
[0146] Improvements in threshold current and WPE measured on
fabricated devices are higher than predicted by simulation
experiment, this is because additional benefits of high crystal
quality when In amount of the first quantum well is reduced are not
accounted for by simulation.
[0147] Referring to FIG. 4 and FIG. 5, the exemplary structure in
accordance with the present disclosure for 490 nm-510 nm emission
has 17% lower threshold current density (J.sub.th) and 1.1% higher
wall plug efficiency at 30 mW light output power compared to a
conventional structure that does not utilize the active region of
the present disclosure.
[0148] FIG. 6 shows the measured current-voltage characteristic of
a reference laser light emitting device and a laser light emitting
device according to the present disclosure, with 500 nm-510 nm
emission. FIG. 7 shows the measured light output power-current
characteristic of these devices. In the reference structure, the
quantum wells have the same In amount and in the device according
to the present disclosure the first quantum well layer has lower In
amount compared to the quantum second well layer. The In amount
ratio is 1.1
[0149] Although the invention has been shown and described with
respect to a certain embodiment or embodiments, it is obvious that
equivalent alterations and modifications will occur to others
skilled in the art upon the reading and understanding of this
specification and the annexed drawings. In particular regard to the
various functions performed by the above described elements
(components, assemblies, devices, compositions, etc.), the terms
(including a reference to a "means") used to describe such elements
are intended to correspond, unless otherwise indicated, to any
element which performs the specified function of the described
element (i.e., that is functionally equivalent), even though not
structurally equivalent to the disclosed structure which performs
the function in the herein illustrated exemplary embodiment or
embodiments of the invention. In addition, while a particular
feature of the invention may have been described above with respect
to only one or more of several illustrated embodiments, such
feature may be combined with one or more other features of the
other embodiments, as may be desired and advantageous for any given
or particular application.
INDUSTRIAL APPLICABILITY
[0150] A laser diode device in accordance with an embodiment of the
present disclosure may be used as a visible light source. Said
light sources may be used in illumination or display applications.
In particular a laser diode device with low threshold current and
high wall plug efficiency is particularly suitable for portable
applications where power is delivered from a battery or other
limited power source.
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