U.S. patent application number 15/600569 was filed with the patent office on 2018-11-22 for electron beam pumped non-c-plane uv emitters.
The applicant listed for this patent is Palo Alto Research Center Incorporated. Invention is credited to Noble M. Johnson, Thomas Wunderer.
Application Number | 20180337515 15/600569 |
Document ID | / |
Family ID | 62110870 |
Filed Date | 2018-11-22 |
United States Patent
Application |
20180337515 |
Kind Code |
A1 |
Wunderer; Thomas ; et
al. |
November 22, 2018 |
ELECTRON BEAM PUMPED NON-C-PLANE UV EMITTERS
Abstract
An ultraviolet (UV) radiation emitting device includes an
epitaxial heterostructure comprising an AlGaInN active region. The
AlGaInN active region includes one or more quantum well structures
with Al content greater than about 50% and having a non-c-plane
crystallographic growth orientation. The AlGaInN active region is
configured to generate UV radiation in response to excitation by an
electron beam generated by an electron beam pump source.
Inventors: |
Wunderer; Thomas; (Palo
Alto, CA) ; Johnson; Noble M.; (Menlo Park,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Palo Alto Research Center Incorporated |
Palo Alto |
CA |
US |
|
|
Family ID: |
62110870 |
Appl. No.: |
15/600569 |
Filed: |
May 19, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/320275 20190801;
H01L 33/10 20130101; H01S 5/305 20130101; H01S 5/34333 20130101;
H01S 5/3013 20130101; H01S 5/32341 20130101; H01S 5/343 20130101;
H01S 2304/04 20130101; H01S 5/04 20130101; H01L 33/32 20130101;
H01S 5/32025 20190801; H01L 33/16 20130101; H01S 5/18369 20130101;
H01S 5/0205 20130101; H01S 5/125 20130101 |
International
Class: |
H01S 5/343 20060101
H01S005/343; H01S 5/04 20060101 H01S005/04; H01S 5/125 20060101
H01S005/125; H01S 5/02 20060101 H01S005/02 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND
DEVELOPMENT
[0001] This invention was made with Government support under
Contract No. HR0011-15-C-0025 DARPA-LUSTER Program awarded by
DARPA. The Government has certain rights in this invention.
Claims
1. An ultraviolet (UV) radiation emitting device comprising: an
epitaxial heterostructure comprising an AlGaInN active region
including one or more quantum well structures with Al content
greater than about 50% having a non-c-plane crystallographic growth
orientation, the AlGaInN active region configured to generate UV
radiation in response to excitation by an electron beam generated
by an electron beam pump source.
2. The device of claim 1, wherein the quantum well structures have
a semi-polar crystallographic growth orientation.
3. The device of claim 2, wherein the semi-polar crystallographic
growth orientation is a (20-21) or (20-2-1) orientation.
4. The device of claim 1, wherein the active region is configured
to provide spontaneous or stimulated emission at wavelengths less
than 250 nm.
5. The device of claim 1, wherein the active region includes about
1 quantum wells to about 50 quantum wells having Al composition of
about 70%.
6. The device of claim 1, wherein the quantum wells of the active
region are separated by barrier layers having Al composition
between about 80% to about 99%.
7. The device of claim 1, wherein a thickness of each quantum well
is in a range of about 0.5 nm to about 5 nm.
8. The device of claim 1, wherein a thickness of the active region
is in a range of about 200 nm to 1000 nm.
9. The device of claim 1, wherein at least a portion of the
heterostructure is n-doped.
10. The device of claim 1, wherein a degree of polarization of UV
radiation emitted by the active region is at least about 35%.
11. The device of claim 1, further comprising a bulk AlN substrate
or substrate remnant having the non-c-plane orientation.
12. A UV radiation emitting device comprising: an epitaxial
heterostructure comprising an AlGaInN active region including one
or more quantum well structures with Al content greater than about
50% having a non-c-plane crystallographic growth orientation, the
AlGaInN active region configured to generate UV radiation in
response to pumping by an electron beam; and a first reflector and
a second reflector, the active region disposed between the first
reflector and the second reflector.
13. The device of claim 12, wherein: the first reflector comprises
an epitaxial distributed Bragg reflector (DBR) having a non-c-plane
orientation; and the epitaxial heterostructure is disposed on the
first reflector.
14. The device of claim 12, further comprising at least one contact
electrically coupled to the heterostructure and configured to
provide a current path for discharging electrons arising from
electron beam pumping of the heterostructure.
15. The device of claim 14, wherein the contact includes an
aperture and the second reflector is disposed within the
aperture.
16. The device of claim 12, wherein the second reflector is an
epitaxial DBR disposed on the heterostructure.
17. A method comprising: electron beam pumping an active region of
an epitaxial III-N heterostructure having a non-c-plane
crystallographic orientation; and generating UV radiation in the
active region in response to pumping by the electron beam, the UV
radiation having a wavelength less than about 250 nm.
18. A method, comprising: slicing a bulk AlN boule to provide a
growth surface having a non c-plane crystallographic orientation;
epitaxially growing an epitaxial AlN layer having the non-c-plane
crystallographic orientation on the growth surface; and epitaxially
growing a III-N heterostructure comprising an UV emitting active
region, the heterostructure having the non-c-plane crystallographic
orientation on the epitaxial AlN layer, the UV emitting active
region configured to emit UV radiation in response to electron beam
pumping of the active region.
19. The method of claim 18, wherein epitaxially growing the
heterostructure comprising the UV emitting active region comprises
epitaxially growing multiple AlGaInN quantum wells having Al
content greater than about 50%.
20. The method of claim 18, wherein the non-c-plane
crystallographic orientation is a (20-21) crystallographic
orientation.
21. The method of claim 18, wherein epitaxially growing the
heterostructure comprises growing a heterostructure having a
surface roughness less than about 1 nm for a 5 .mu.m.times.5 .mu.m
scan as measured by atomic force microscopy.
Description
TECHNICAL FIELD
[0002] This application relates generally to devices that emit
ultraviolet radiation and to systems and methods pertaining to such
devices.
BACKGROUND
[0003] Ultraviolet (UV) emitting devices are of considerable
interest for applications that include water purification,
analytical devices for medical and biotechnology fields, UV curing,
and currency screening, among other applications. Light emitting
devices that emit in spectral ranges suitable for these and other
applications can be fabricated based on a variety of material
systems, including group III-V and II-VI binary, ternary, and
quaternary compounds and alloys and various combinations
thereof.
SUMMARY
[0004] Some embodiments are directed to a device that emits
ultraviolet (UV) radiation through spontaneous emission. The UV
radiation emitting device includes an epitaxial heterostructure
comprising an AlGaInN active region. The active region includes one
or more quantum well structures with Al content greater than about
50% having a non-c-plane crystallographic growth orientation, The
AlGaInN active region is configured to generate UV radiation in
response to excitation by an electron beam from an electron beam
pump source.
[0005] Some embodiments are directed to a device that emits
ultraviolet (UV) radiation through stimulated emission. The UV
radiation emitting device includes an epitaxial heterostructure
comprising an AlGaInN active region. The active region includes one
or more quantum well structures with Al content greater than about
50% and having a non-c-plane crystallographic growth orientation.
The AlGaInN active region is configured to generate UV radiation in
response to pumping by an electron beam. The device includes a
first reflector and a second reflector, wherein the active region
is disposed between the first reflector and the second
reflector.
[0006] Some embodiments are directed to a method of operating a UV
radiation emitting device. The method includes electron beam
pumping an active region of an epitaxial III-N heterostructure
having a non-c-plane crystallographic orientation. UV radiation is
generated in the active region in response to pumping by the
electron beam. The UV radiation has a wavelength less than about
250 nm.
[0007] Some embodiments are directed to a method of making an UV
radiation emitting device. A bulk AlN boule is sliced to provide a
substrate having growth surface with a non c-plane crystallographic
orientation. A heterostructure is epitaxially grown on the AlN
substrate. The heterostructure comprises a UV emitting active
region and has the non-c-plane crystallographic orientation of the
growth surface. In some embodiments, an epitaxial AlN layer may be
epitaxially grown on the grown surface of the bulk AlN boule before
the heterostructure comprising the UV emitting region is
formed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIGS. 1A through 1F show crystallographic planes of
hexagonal materials;
[0009] FIG. 2 shows a cross sectional diagram of a UV emitting
device that can produce spontaneous UV emission in accordance with
some embodiments;
[0010] FIGS. 3A through 3C show cross sectional diagrams of UV
emitting devices that can produce spontaneous UV emission in
accordance with some embodiments;
[0011] FIGS. 4A through 4C show the e-beam penetration into the
heterostructure at e-beam energies of 5 keV, 10 keV, and 15 keV,
respectively;
[0012] FIG. 4D is a graph of simulated electron penetration depth
as a function of electron beam energy in eV for vertical incidence
of the electron beam;
[0013] FIG. 5 shows a cross sectional diagram of a UV emitting
laser device that can produce stimulated UV emission in accordance
with some embodiments;
[0014] FIGS. 6, 7, and 8 show approaches for the formation of a
laser device having a non-c-plane oriented active region in
accordance with some embodiments;
[0015] FIG. 9 is a side-view of a semiconductor laser structure
according to one embodiment of the present disclosure;
[0016] FIG. 10 is a plot of refractive index and field profile
versus thickness for a sample device according to an embodiment of
the present disclosure;
[0017] FIG. 11A is a plot of refractive index and energy deposition
profile versus thickness for the sample device of FIG. 10;
[0018] FIG. 11B is a plot of absorption loss against top cladding
thickness for the sample device of FIG. 10;
[0019] FIG. 12 is a plot of refractive index and field profile
versus thickness for a sample device according to another
embodiment of the present disclosure;
[0020] FIG. 13A is a plot of refractive index and energy deposition
profile versus thickness for the sample device of FIG. 12;
[0021] FIG. 13B is a plot of absorption loss against top cladding
thickness for the sample device of FIG. 12;
[0022] FIG. 14 is a side-view of two semiconductor laser
structures, including conduction path metallization, according to
an embodiment of the present disclosure;
[0023] FIG. 15 is a cut-away side view of a light emitting device
having a substrate and via connection to a discharge structure,
according to an embodiment of the present disclosure;
[0024] FIG. 16 is a top-view of the two semiconductor laser
structures, including conduction path metallization, according to
the embodiment of FIG. 14;
[0025] FIG. 17 is a high-resolution .omega.-2.THETA. x-ray scan of
10.times.AlGaN multiple quantum well heterostructure on semipolar
(20-21) bulk AlN substrate in accordance with some embodiments;
[0026] FIG. 18 is an atomic force microscopy image of a 10.times.
AlGaN multiple quantum well heterostructure grown on (20-21) bulk
AlN substrate in accordance with some embodiments;
[0027] FIG. 19 shows the temperature-dependent photoluminescence
spectra recorded from a semipolar (20-21) AlGaN multiple quantum
well heterostructure in accordance with some embodiments;
[0028] FIG. 20 shows a graph of the normalized
temperature-dependent photoluminescence intensity where values for
the intensities are gained from integrating the spectra of FIG. 10;
and
[0029] FIG. 21 shows polarization-resolved photoluminescence
spectra from semipolar AlGaN multiple quantum wells grown on
(20-21) bulk AlN substrate in accordance with some embodiments.
[0030] The figures are not necessarily to scale. Like numbers used
in the figures refer to like components. However, it will be
understood that the use of a number to refer to a component in a
given figure is not intended to limit the component in another
figure labeled with the same number.
DESCRIPTION
[0031] The efficiency of UV emitting devices that are grown on
c-plane substrates or templates drops by several orders of
magnitude as the emission wavelength becomes shorter than about 250
nm. The drop in efficiency occurs because the radiation is mainly
transverse-magnetic (TM) polarized at the shorter wavelengths and
cannot be easily extracted out of the wafer plane. Embodiments
described herein involve structures in which the crystal
orientations of the substrate, and thus the orientation of the
light emitting layers, e.g., quantum wells (QWs) grown on the
substrate, are rotated to a non-c-plane orientation. Rotating the
crystal orientation of the substrate and QWs can enhance extraction
of UV radiation, particularly for UVC radiation between 200 nm and
290 nm.
[0032] Some embodiments discussed herein involve approaches that
enhance the efficiency of light extraction from vertically emitting
or edge emitting UV sources based on the AlGaN material system. The
approaches discussed below involve the use of non-c-plane oriented
AlGaN quantum wells that may be epitaxially grown on a non-c-plane
oriented surface of an AlN substrate.
[0033] Another major challenge for UV wavelength emitters is the
p-type conductivity of these high band gap materials needed for UV
radiation. Today's device efficiencies suffer from either high
light absorption losses when p-GaN is used as hole injection layer
or high voltages and reduced carrier injection when p-AlGaN
materials are implemented to gain at least partially UV
transparency. Some embodiments discussed herein use electron beam
(e-beam) excitation for the creation of electron-hole pairs and
decreased light absorption losses.
[0034] FIGS. 1A through 1F show crystallographic planes of
hexagonal materials such as GaN and AlN. FIG. 1A illustrates the
polar plane (0001), referred to as the c-plane; FIG. 1B illustrates
the non-polar (1100) or (1-100) plane, referred to as the m-plane;
and FIG. 1C illustrates the non-polar (1120) or (-1-120) plane,
referred to as the a-plane. FIGS. 1D through 1F show semi-polar
planes (1122) or (11-22), (2021) or (20-21), and (2021) or
(20-2-1). Other common semi-polar crystal planes that could be used
for embodiments disclosed herein include (10-11), (10-1-1),
(11-2-2), (10-13), (10-1-3), (30-31), and (30-3-1), and others.
High quality c-plane AlGaN structures can be grown on c-plane
substrate surfaces, but the UV emission is only TE polarized for
longer emission wavelength and the device is plagued by extraction
issues discussed above. AlGaN structures can be grown on non-polar
m- and a-plane substrate surfaces. However, epitaxial growth
conditions are more demanding for producing materials with
featureless surfaces that are free of structural defects. Some
embodiments described herein are directed to devices with AlGaN or
AlGaInN QWs grown on surfaces with semi-polar orientations. As
discussed below, growth on semi-polar orientations, such as (20-21)
and (20-2-1) planes, were shown to produce smooth surfaces and
sharp heterostructure interfaces needed for high quality UV
emitters.
[0035] FIG. 2 shows a cross sectional diagram of a UV emitting
device 200 that can be used to produce spontaneous UV emission 221
in accordance with some embodiments. The device includes an
epitaxial heterostructure 220 comprising a UV emitting active
region. For example, the UV emitting active region may comprise one
or more quantum well (QW) structures that are grown to have a
non-c-plane crystallographic orientation. The active region is
configured to emit radiation in response to pumping by an electron
beam 231 produced by an electron beam source 230. In some
embodiments, the UV active region includes AlGaN or AlGaInN with Al
content greater than 50% such that the UV radiation emitted by the
active region is less than about 250 nm. Portions of the device 200
may be disposed within a vacuum chamber 280 in some
embodiments.
[0036] In some embodiments, the active region may comprise several
quantum wells, e.g., 1 to 50 quantum wells, having Al composition
of about 70%. The thickness of the quantum wells can be in a range
of about 0.5 nm to about 5 nm, or in a range of about 1 nm to about
3 nm, for example. Each quantum well is disposed between barrier
layers. In some embodiments, the Al composition of the barrier
layers is in a range of about 80% to about 99%. The active region
including the quantum wells and barrier layers may have a thickness
of between about 200 nm to about 1000 nm or between about 400 nm to
about 800 nm in some embodiments.
[0037] To further facilitate current flow through the
heterostructure 220, one or more heterostructure layers may be
doped with n-type dopants, increasing the electrical conductivity
of the layers. The heterostructure layers may be doped n-type by
incorporating impurities such as Si, Ge, and Sn. For example, one
or more quantum wells and/or barrier layers may be doped with an
n-type dopant at a level of about 10.sup.17 to 10.sup.20/cm.sup.3
to achieve a conductivity in the doped layers between about 0.01
(.OMEGA. cm).sup.-1 and 10.sup.4 (.OMEGA. cm).sup.-1. For example,
at least a portion of the heterostructure layers may be n-doped
with Si at a level greater than about 10.sup.17/cm.sup.3. The
impurity concentration is typically less than 3% of the total
atomic concentration in the material.
[0038] In some embodiments, the heterostructure 220 may have a
non-polar crystallographic orientation, e.g., m-plane or a-plane
orientation. In some embodiments, the heterostructure may have a
semi-polar crystallographic orientation. For example, suitable
semi-polar orientations include the (10-11), (11-22), (20-21),
(20-2-1), (10-11), (10-1-1), (11-2-2), (10-13), (10-1-3), (30-31),
and (30-3-1), crystallographic orientations.
[0039] FIGS. 3A through 3C show cross sectional diagrams of UV
emitting devices 300A, 300B, 300C that can be used to produce
spontaneous UV emission in accordance with some embodiments. The
epitaxial heterostructure 320 includes a non-c-plane oriented
active region that is grown on and takes the orientation of a
substrate 310 with the non-c-plane orientation. In some
embodiments, the substrate 310 comprises a bulk AlN substrate that
has been sliced from a bulk AlN boule or otherwise formed to
provide a non-c-plane epitaxial growth surface. The substrate 310
may include an epitaxial AlN layer grown, e.g., by metal-organic
vapor phase epitaxy (MOVPE) on the bulk AlN substrate. In some
embodiments, the active region includes AlGaN with Al content
greater than 50% such that the UV radiation emitted by the active
region is less than about 250 nm. The surface roughness of the
heterostructure 320 may be less than about 0.35 nm as measured by
atomic force microscopy.
[0040] As illustrated by device 300A of FIG. 3A, the substrate 310
may be removed entirely, as indicated by arrow 399, or may be
mechanically and/or chemically thinned to a substrate remnant 311
having a thickness less than about 100 .mu.m or less than about 50
.mu.m or even less than 20 .mu.m, for example. After removal or
thinning of the substrate, the backside of the heterostructure (or
the remaining surface of the substrate remnant) may be roughened
for enhanced light extraction.
[0041] In some embodiments, the AlGaN QWs may have a thickness
between 0.5 nm and 5 nm, or between 1 nm and 2 nm. Barrier layers
that separate the QWs can have barrier thicknesses between 5 and 50
nm. The optimal active zone thickness depends on e-beam energy, for
example between 5 to 30 keV. FIGS. 4A through 4C show the e-beam
penetration into the heterostructure at e-beam energies of 5 keV,
10 keV, and 15 keV, respectively. FIG. 4D is a graph of simulated
electron penetration depth in .mu.m as a function of electron beam
energy in eV for vertical incidence of the electron beam in a
sample comprising 47 Al.sub.0.7Ga.sub.0.3N/AlN QWs/barriers on bulk
AlN.
[0042] As shown in FIG. 3B, the UV emitting device 300B can include
a heat sink 340 disposed on the heterostructure 320. Suitable
heatsink materials include diamond, copper, copper-tungsten,
aluminum, AlSiC, and/or other materials or material composites. In
the case of non-transparent heat sink or heat spreader materials
and bottom light emission a (beveled) hole should be included in
the heat sink. If the semiconductor film is rather thin it needs to
be mounted on a transparent heat spreader material first (e.g.,
diamond, sapphire, quartz, etc.)
[0043] As shown in FIG. 3C, the UV emitting device 300C may include
one or more contacts 350 electrically coupled to the
heterostructure 320 and configured to provide a current path that
discharges electrons arising from the electron beam pumping of the
heterostructure 320. In some embodiments, the contact 350 is a
single contact 350 that can include an aperture 351 which exposes
the heterostructure to the electron beam 231. In some embodiments,
the UV emitting device can include multiple contacts coupled to the
same electrical potential to discharge the electrons arising from
the electron beam pumping of the heterostructure 320. In some
embodiments the contact can be a continuous metal film that is thin
enough to be penetrated by the e-beam and also acts as an optical
reflector. For example, the contact 350 may comprise Al which shows
a high reflectivity in the deep-UV.
[0044] In some embodiments, a UV emitting device may be a
vertically emitting laser device having a non-c-plane oriented
active region which is configured for generation of UV stimulated
emission. FIG. 5 shows a cross sectional diagram of a UV vertically
emitting laser device 500 that can produce stimulated UV emission
521 in accordance with some embodiments. The device 500 includes an
epitaxial heterostructure 520 comprising a UV emitting active
region having a non-c-plane crystallographic orientation. The UV
emitting active region may comprise one or more quantum well
structures that are grown to have the non-c-plane crystallographic
orientation. The heterostructure 520 is disposed between. The
active region is disposed between first and second reflectors 561,
562 and is configured to emit laser radiation in response to
pumping by an electron beam 231. In some embodiments, the UV active
region includes AlGaN with Al content greater than 50% such that
the UV radiation emitted by the active region is less than about
250 nm. Additional information regarding structures for electron
beam pumped vertical cavity surface emitting lasers is described in
commonly owned U.S. Pat. No. 9,112,332 which is incorporated herein
by reference.
[0045] As previously discussed, the heterostructure 520 may have a
non-polar crystallographic orientation, e.g., either an m-plane or
an a-plane orientation. In some embodiments, the heterostructure
520 may have a semi-polar crystallographic orientation such as the
(10-11), (11-22), (20-21), (20-2-1), or other crystallographic
orientations.
[0046] FIGS. 6 and 7 show approaches for the formation of a laser
device having a non-c-plane oriented active region. In some
embodiments, shown in the top cross sectional diagram of FIG. 6,
reflector 661 is a distributed Bragg reflector (epi-DBR) that is
epitaxially grown on the non-c-plane oriented substrate 610. The
epi-DBR 661 comprises a series of alternating semiconductor layers
disposed on the substrate 610 that take on the non-c-plane
orientation of the substrate 610. For example, if non-c-plane
oriented AlN is used as the substrate 610, then an epi-DBR may
comprise alternating layers of non-c-plane oriented AlGaN and AlN
grown on the non-c-plane oriented AlN substrate 610. Using an
epi-DBR helps to promote heat dissipation from the active region
because the semiconductor layers of the epi-DBR can provide some
heat sinking. A non-c-plane oriented heterostructure 620, including
the active region is epitaxially grown on the non-c-plane oriented
epi-DBR 661.
[0047] As shown in the middle cross sectional diagram of FIG. 6,
after the non-c-plane oriented epi-DBR 661+heterostructure
subassembly 605 is formed, the substrate 610 may optionally be
fully or partially removed, for example, using a laser liftoff
process (LLO), mechanical polishing and/or dry/wet chemical
etching. In some configurations, a thin remnant of the substrate
may remain after the removal.
[0048] As shown in the bottom cross sectional diagram of FIG. 6, in
some configurations the DBR+heterostructure subassembly 605 can be
bonded epi-DBR side down to a heat sink 640 to enhance heat
dissipation from the active region. The heat sink 640 may comprise
a metal, metal-alloy or other materials having sufficient thermal
conductivity, e.g., diamond, copper, copper-tungsten, aluminum,
AlSiC, and/or other materials or material composites. One or more
contacts 650 for discharging electrons from the active region can
be deposited over the heterostructure 620.
[0049] Another approach for the formation of a laser structure,
shown in FIG. 7, involves the use of a first DBR 761 comprising a
dielectric layers which are deposited on the backside of a
non-c-plane oriented epitaxially grown heterostructure 720. For
example, the dielectric DBR 761 may comprise pairs of
SiO.sub.2/Al.sub.2O.sub.3, SiO.sub.2/Y.sub.2O.sub.3 or
SiO.sub.2/HfO.sub.2. Dielectric materials used to form a dielectric
DBR can have higher refractive index contrast than semiconductor
materials. Thus, for the same reflectivity, a dielectric DBR can
have fewer layer pairs when compared to the number of semiconductor
layer pairs of an epitaxially grown DBR. However, dielectric
materials have lower thermal conductivity which may be a factor for
devices that need heat dissipation from the active region.
[0050] As shown in the cross sectional diagram at the top of FIG.
7, a non-c-plane oriented heterostructure 720 comprising a
non-c-plane oriented active region configured to emit UV radiation
is epitaxially grown on substrate 710. The substrate 710 is fully
or partially removed from the heterostructure 720 as shown in the
middle cross sectional diagram of FIG. 7. After removal of the
substrate, a dielectric DBR 761 is deposited on one surface of the
heterostructure 720, e.g. the backside surface which is the surface
of initial epitaxial growth of the heterostructure 720. If a
remnant of the substrate remains, the dielectric DBR 761 is
deposited on the substrate remnant.
[0051] One or more contacts 750 are disposed on the heterostructure
surface opposite the dielectric DBR as shown in the bottom cross
sectional diagram of FIG. 7. In the implementation shown in FIG. 7,
the DBR+heterostructure subassembly 705 can be arranged DBR side
down on a heat sink 740 that provides dissipation of heat generated
in the active region.
[0052] Returning now to FIG. 5, according to some embodiments, one
or more of the first and second reflectors 561, 562 may be external
reflectors. A gap separates an external reflector from the
heterostructure 520 and/or other layers disposed on the
heterostructure 520. An external reflector may include an epitaxial
DBR and/or a dielectric DBR as discussed above. In some
embodiments, the second DBR may comprise an epi-DBR or dielectric
DBR disposed on the heterostructure as illustrated by FIG. 7.
[0053] The cross sectional diagram at the top of FIG. 8 shows a
non-c-plane oriented heterostructure 820 comprising a UV emitting
active region. The non-c-plane oriented heterostructure has been
epitaxially grown on a non-c-plane oriented substrate which is
subsequently completely removed or thinned to a remnant and is not
shown in FIG. 8. After removal of the substrate, a dielectric DBR
861 is deposited on one surface of the heterostructure 820, e.g.
the backside surface which is the surface of initial epitaxial
growth of the heterostructure 820. If a remnant of the substrate
remains, the dielectric DBR 861 is deposited on the substrate
remnant. Alternatively, a non-c-plane oriented epi-DBR may be grown
on the substrate prior to growth of the non-c-plane oriented
heterostructure 820.
[0054] One or more contacts 850 are disposed on the heterostructure
surface opposite the DBR 861 as shown in the bottom cross sectional
diagram of FIG. 8. In the implementation shown in FIG. 8, the
DBR+heterostructure subassembly 805 can be arranged DBR side down
on a heat sink 840 to provide for dissipation of heat generated in
the active region.
[0055] A second DBR 862, e.g., an epi-DBR or dielectric DBR, may be
epitaxially grown or deposited such that the second DBR 862 is
disposed within the aperture 851 of the patterned contact 850.
[0056] In some embodiments, a UV emitting device may be an edge
emitting laser device having a non-c-plane oriented active region.
The edge emitting laser device may be configured for generation of
UV stimulated emission. In some embodiments the edge emitting
devices are grown on a substrate, e.g., AlN having a non-polar
orientation along the (1100) or (1-100) plane, referred to as the
m-plane. In some embodiments the surface of the substrate may be
oriented along the non-polar (1120) or (-1-120) plane, referred to
as the a-plane. The surface of the substrate may be oriented along
a semi-polar plane such as (1122) or (11-22), (2021) or (20-21),
and (2021) or (20-2-1) in some embodiments. Other common semi-polar
crystal planes that could be used in embodiments disclosed herein
include (10-11), (10-1-1), (11-2-2), (10-13), (10-1-3), (30-31),
and (30-3-1), and others. The layers above the substrate, including
the cladding and active layers, take on the semi-polar or non-polar
orientation of the substrate.
[0057] FIG. 9 shows a cross sectional diagram of a UV edge emitting
laser 10 in accordance with some embodiments. Device 10 comprises a
semiconductor laser structure 12 and electron-beam pump source 14
according to one embodiment of the present disclosure.
Semiconductor laser 12 comprises a non-polar or semi polar
substrate 16. In some embodiments, the substrate 16 may comprise
bulk AlN. Above substrate 16 is formed a lower cladding layer 18 of
Al.sub.xGa.sub.1-x-yIn.sub.yN where x is between 0.6 and 1 and y is
between 0 and 0.03, for example n-Al.sub.0.74Ga.sub.0.26N.
[0058] A lower waveguide 20 may be formed over lower cladding layer
18 of Al.sub.zGa.sub.1-z-yIn.sub.yN where z is between 0.5 and 1
and y is between 0 and 0.03, and z<x. For example, lower
waveguide 20 may comprise n-Al.sub.0.7Ga.sub.0.3N (e.g., 40 nm
thick).
[0059] An active layer 22, such as a multiple quantum well
heterostructure (MQW) or double heterostructure (DH) region may be
formed over lower waveguide 20. Active layer 22, in the case of a
MQW, may comprise pairs of Al.sub.xGa.sub.1-xN/AlyGa.sub.1-yN of
varying thickness (e.g., 5 layer pairs of
Al.sub.0.57Ga.sub.0.43N/Al.sub.0.62Ga.sub.0.38N, 5.4 nm and 9.6 nm,
respectively). In general, active layer 22 may comprise at least
one layer of Al.sub.uGa.sub.1-u-vIn.sub.vN where v is between 0 and
0.03, and 0.4<u<z. In the case of a MQW, in general the
barrier may be Al.sub.sGa.sub.1-s-tIn.sub.tN, where 0.4<u<Z
and s>u+0.04, and t is between 0 and 0.03. In such a case, the
quantum well thickness may be between 1 and 6 nm and the barrier
thickness may be between 3 and 20 nm.
[0060] An upper waveguide 24 may be formed over active layer 22,
comprising, for example, n-doped AlGaN (such as 40 nm of
n-Al.sub.0.7Ga.sub.0.3N), undoped AlN, and so on.
[0061] Optionally, upper cladding layer 26 may be formed over upper
waveguide 24. (Optional layers, elements, and features for various
embodiments may be indicated with dashed outlines.) When fabricated
without an upper cladding layer, upper waveguide 24 is formed to be
relatively thick, on the order of about 200 nm for example. When
fabricated with upper cladding layer 26, layer 26 may comprise
n-doped AlGaN (such as having at least 70% Al, for example
n-Al.sub.0.78Ga.sub.0.22N, 220 nm thick). In some embodiments, such
as at very high Al concentration, the upper cladding layer 26 may
be undoped. In some embodiments, at least one of the lower cladding
layer, lower waveguide layer, light emitting layer, upper waveguide
layer, or upper cladding layer is doped n-type.
[0062] Optionally, in certain embodiments, at least one of the
lower cladding layer, lower waveguide layer, light emitting layer,
upper waveguide layer, and upper cladding layer is a short-period
superlattice. In addition, in certain embodiments, at least one of
the lower cladding layer, lower waveguide layer, upper waveguide
layer, or upper cladding layer may be a monotonically-varying
alloy-compositional gradient, with the lower band gap composition
of the graded layer nearest the light emitting layer.
[0063] Also optionally, a contact layer 28 may be provided over
upper cladding layer 26. Contact layer 28 may comprise n-GaN as one
example. A suitable ohmic metal layer 29 may be formed over contact
layer 28 (such as 30 nm of Ti) to permit conduction of charge from
the laser device, discussed further below.
[0064] It will be appreciated that at relatively high thicknesses
of active layer 22, active layer 22 may itself provide a wave
guiding function, obviating the need for a separate lower waveguide
20 and/or upper waveguide 24, and otherwise providing a light
guiding layer. Thus, in certain embodiments lower and upper
waveguides form a light guiding structure within which a light
emitting layer is disposed. In other embodiments, the thickness and
other characteristics of the light emitting layer provide light
guiding at upper and lower margins, thereby also forming a light
guiding structure within which a light emitting layer is
disposed.
[0065] Electron-beam pump source 14 is disposed to be over, and in
some embodiments, spaced apart from a top surface of semiconductor
laser 12. Electron beam ("e-beam") pump source 14 is connected to a
driving voltage such that it produces a line-pattern (e.g., 12
.mu.m.times.500 .mu.m) electron beam in a direction toward and into
the top surface of semiconductor laser 12.
[0066] The structure described above has been built and modeled.
Emission wavelengths for such a structure may be obtained,
depending upon materials and compositions, for example in the range
of 200 nm to 365 nm. FIG. 10 is an illustration of the modeling of
the structure for a target emission wavelength at k=265 nm. For the
model, the structure includes a 220 nm thick
n-Al.sub.0.78Ga.sub.0.22N top cladding layer, a 5 nm thick n-GaN
contact layer, and a 30 nm thick Ti metal top contact/discharge
layer. As can be seen from FIG. 9, the refractive index for the
various layers is shown, as is the optical mode (i.e., e-field of
the lasing mode) profile. In the ideal case, the carrier generation
peaks at the center of the MQW, in order to most efficiently
generate photons therein. The model of FIG. 10 predicts a
comparatively high .GAMMA., on the order of 25%, and a
comparatively low loss of less than 1 cm.sup.-1.
[0067] FIG. 11A is a graph of the modeling of the structure of FIG.
9, showing calculated energy deposition profile versus depth into
the semiconductor laser structure for two selected e-beam energies.
As can be seen from FIG. 10A, for 10 keV and 12 keV, about 32
percent and 30 percent of the total energy, respectively, is
deposited into the MOW region and waveguide layers (upper and
lower). The rate of electron-hole generation at a given depth is
proportional to the energy deposition at that depth. The more
energy that is deposited into these regions, the higher the
expected laser output power.
[0068] The effect of cladding layer thickness on absorption losses
for this structure is illustrated in FIG. 11B. It can be seen from
FIGS. 11A and 11B that a thicker cladding layer limits the depth of
the peak of the energy deposition profile yet provides lower
absorption loss, and conversely a thinner cladding layer permits a
deeper peak of the energy deposition profile (e.g., closer to the
active region) yet results in higher absorption loss. From data
such as provided in FIGS. 11A, 11B, design parameters may be chosen
as a function of the specific application of the teachings of the
present disclosure to permit balancing and ultimately optimizing
both the energy deposition profile of the electron beam and the
absorption losses induced mainly by the top surface layers (contact
layer, top metallization).
[0069] The results shown in FIGS. 10 and 11A, 11B support the
operability and advantages of a laser structure, such as that
described above, that operates without an upper p-doped layer.
Again, one significant advantage provided thereby is elimination of
a p-doped layer with high electrical conductivity and low
absorption losses that is very challenging to fabricate for high
band gap materials.
[0070] Using an electron beam as an excitation source takes
advantage of the fact that generation of carriers by electron beam
means that carrier generation and injection does not rely on a
p-/n-junction. This obviates the challenge of forming a highly
conductive (p-type) material in high band gap semiconductors that
is able to carry the current densities necessary for laser
emission. Thus, no p-type doping of an upper cladding layer is
required. In addition, carrier injection by e-beam is comparatively
deep, extending beyond the top most MQW layers. This homogeneous
carrier injection supports an increased number of quantum wells in
the MQW layer than typically is achieved in a laser diode. Higher
gain and improved device performance are thereby provided.
[0071] E-beam pumping produces a net charge within the laser
structure. Thus, the laser heterostructures and the device
architectures disclosed herein include features to allow effective
discharge of the device, e.g., it includes conductive layers that
are n-doped (and may include p-doped layers), metal films and/or
contacts and connections to ground or the anode of the e-beam
source. In the example above, an n-type upper cladding and metal
ohmic contact are provided over the MQW, solely for this discharge
function. However, it will be understood that this is simply one
example of a family of structural configurations in which an upper
p-type cladding is obviated bye-beam pumping. Many variations are
contemplated herein.
[0072] For example, FIG. 12 shows an alternative semiconductor
laser design for electron beam pumped operation at a target
emission wavelength of A=265 nm, and FIG. 13A shows calculated
e-beam energy deposition profile for that design. FIG. 13B
illustrates the effect of cladding layer thickness on absorption
losses for this structure. The structure modeling in FIGS. 12 and
13A, 13B is similar to that of FIGS. 10 and 11A, 11B, with the
exception that the structure of FIGS. 12 and 13A, 13B has an
undoped 120 nm thick AlN top cladding layer and a lack of any
contact layer and upper ohmic metal layer. Noteworthy is the result
that lower absorption losses may be obtained for a thinner top
cladding layer as compared to the structure of FIGS. 10 and 11A,
11B. The energy profile peak may therefor occur closer to the
active region. The lack of upper contact layers means that
discharge of the device is provided not via the top surface, but
instead through the n-AlGaN lower cladding layer and/or laterally
via mesa side contacts, as an option.
[0073] The laser heterostructure in the prior example was designed
to allow discharge via its top surface. Therefore the AlGaN
composition of the cladding layer was limited to provide
sufficiently high conductivity (e.g., the AI-composition is 78%).
To avoid high absorption losses through the GaN cap layer and/or
metal contact, the cladding layer in that example was chosen to be
relatively thick, on the order of 220 nm. In the present example,
however, there is no concern with regard to conductivity of the
upper cladding layer, so it may be entirely non-conductive AlN.
Furthermore, by reducing the thickness of the AlN upper cladding
later, carrier generation and confinement within the MQW active
region is improved. Thus, the upper cladding layer was selected, as
an example here, to be on the order to 120 nm thick.
[0074] In order to provide a discharge path for the device
illustrated in FIG. 12, etching is performed to form a mesa
structure, with the lower cladding layer exposed for electrical
contact. Metal may be deposited on selected portions of the exposed
surfaces, either at the bottom of the etched region adjacent the
device or the sidewall of the device, or both, for the requisite
conduction path. Again, no GaN contact layer or top metal is
needed. The waveguide simulations produce a comparatively high
.GAMMA., on the order of 25%, and a comparatively low loss of less
than 1 cm.sup.-1.
[0075] The energy deposition profiles of the excitation source are
shown in FIG. 13A for excitation energies of 8, 10, and 12 keV.
This produced 52, 41, and 28% of the total energy is deposited into
the active zone and waveguide layers for 8, 10, and 12 keV,
respectively. The absolute deposited energy in the active zone and
waveguide layers, with the same current for the different
excitation energies, is very similar for 8 and 10 keV and about 20%
less for 12 keV.
[0076] An individual device or array of devices may be formed on a
substrate having a non-polar or semi-polar orientation and having
the layer composition illustrated in FIG. 14. A structure may be
formed with such a layer composition, and then etched to form
individual devices. Two such devices 30, 32 are shown in FIG. 14,
separated by an etched region 34. A dry etch, for example using
chemically assisted ion beam etching (CAME) or inductively-coupled
plasma reactive ion etching (ICP-RIE), facet cleaving, or other
processes known in the art may be used to form both the etched
region 34 as well as the laser facets. Etching may proceed into the
lower cladding layer 18, exposing that layer. A contact metal 40
(for example, a layered structure of Ti(20 nm)/Al(1 00 nm)/Ni(45
nm)/Au(60 nm) annealed at about 900.degree. C. for roughly one
minute, which forms a functional ohmic contact to the n-type AlGaN)
may then be formed over portions of devices 30, 32, and within
etched region 34 to form a conduction path for discharging the
devices (e.g., tied to ground). Optionally, sidewall passivation 42
may be formed on the sidewalls of devices 30, 32 prior to forming
contact metal 40.
[0077] While the previous embodiment forms a discharge path using a
deposited metal in etched regions between devices, in other
embodiments, direct contact to the ohmic metal layer 29 may be made
to each device, or groups of devices, for discharging. As
mentioned, contact layer 29 is relatively thin, on the order of 30
nm for a material such as Ti, such that the electron beam may
easily penetrate into the active region of the device. Similarly, a
backside metal layer may be provided for electrical and thermal
discharge. One such device 70 is illustrated in FIG. 15. Device 70
comprises a substrate 72 having a via 74 formed therein, and a
conductive discharge structure 76 disposed on a first surface. A
light emitting device 78 is disposed on a second surface of
substrate 72 opposite the surface on which the light emitting
device 78 is disposed. Via 74, or more precisely, conductive
material disposed in via 74, electrically interconnects conductive
discharge structure 76 and device 78 such that charge that may
accumulate in device 78 due to electron beam pumping from electron
beam pump 80 may be discharged through structure 76, such as to
ground potential. Many other discharge structures are contemplated
hereby, including one or more conductive intermediate layers,
sidewalls, and so on.
[0078] Following the formation of devices 30, 32, individual
electron beam sources may be formed or disposed over each device.
According to one embodiment, prefabricated electron beam sources
are selected to have a beam length comparable to the length of the
resonance cavity (between facets 50, 52 of device 30 and facets 54,
56 of device 32), for example on the order of 300-500 .mu.m.
Indeed, the length of the resonator is primarily limited by the
output format of the electron beam source. FIG. 16 illustrates a
top-view of devices 30, 32, with the position of an electron beam
on each device indicated by lines 44, 46, respectively.
[0079] The resulting device may be operated in vacuum. According to
certain embodiments, the output power density may be in the range
of 50 kW/cm.sup.2 to about 1 to 2 MW/cm.sup.2. The acceleration
voltage may be on the order of 10 kV. The spot size may be on the
order of 12.times.500 microns. That is, current is on the order of
0.6 mA to 6-12 mA, with a higher current producing a higher the
output power. Again, these ranges are merely one set of examples,
and other ranges are also contemplated by the present disclosure.
Additional information regarding e-beam pumped edge-emitting
devices is described in U.S. Pat. No. 8,964,796 which is
incorporated herein by reference.
Example
[0080] AlGaN multiple quantum wells (MQWs) were grown on semipolar
(20-21) AlN to mitigate the issues related to the polarization
switching for short-wavelength deep-UV optical emitters. Structural
properties were accessed by x-ray diffraction and atomic force
microscopy (AFM) while the emission behavior was monitored by
photoluminescence (PL) studies.
[0081] The single crystal AlN substrates used in these studies were
fabricated from AlN boules grown by physical vapor transport and
were nominally (20-21) oriented with less than 1.degree. in miscut.
After slicing, the substrates were chemo-mechanically planarized
(CMP) and have a typical root mean square (rms) surface roughness
of about 0.1 nm as measured by AFM. The structural quality of the
substrates were confirmed by x-ray rocking curve measurements which
reveal full width at half maximum (FWHM) values as low as 37 arcsec
for both the (002) and (102) reflections. The dislocation density
was less than 10.sup.5 cm.sup.-2.
[0082] The AlGaN heterostructures were grown by metal-organic vapor
phase epitaxy (MOVPE) with conventional pre-cursor sources. The
multiple quantum wells (MQWs) active zone consisted of 10 pairs of
Al.sub.xGa.sub.1-xN/Al.sub.yGa.sub.1-yN QWs with a thickness of
about 3 and 10 nm, and x=0.69, y=0.90, respectively. FIG. 17
compares simulated 1701 and measured 1702 plots of .omega.-2.THETA.
x-ray diffraction scan of the (20-21) semipolar AlGaN MQW stack. As
can be seen, higher-order Pendelloesung fringes can be resolved,
which imply sharp and abrupt interfaces between the individual
layers of the heterostructure. Good agreement between measurement
and simulation is also evident.
[0083] To further assess the structural properties atomic force
microscopy (AFM) images were taken from the sample surface. FIG. 18
is an AFM image of the sample surface. As shown in FIG. 18
relatively smooth layers could be realized with an rms surface
roughness of less than 1 nm or even less than 0.35 nm for a 5
.mu.m.times.5 .mu.m scan.
[0084] The optical properties were assessed by temperature- and
polarization-dependent photoluminescence measurements by using a
pulsed 193 nm ArF excimer laser as excitation source. The emission
was recorded perpendicular to the sample surface without a
polarizer in the optical path for the temperature-dependent
measurements. FIG. 19 shows a series of photoluminescence (PL)
spectra recorded between 8K and room temperature 295K. A clear QW
emission was observed at about 237 nm at low temperature which
shifts to longer wavelength with increasing temperature. The rather
pronounced decrease in emission intensity with increasing
temperature can be ascribed to the presence of non-radiative
recombination channels that dominate the recombination process at
elevated temperatures. An upper value for the internal quantum
efficiency (IQE) of about 7.5% at room temperature is estimated
taking into consideration the ratio of the PL intensity at room
temperature and low temperature as indicated in FIG. 20.
[0085] The polarization characteristics of the semipolar QWs
emission were assessed by inserting a polarizer into the optical
detection path of the PL setup. In contrast to the case for
conventional c-plane material, polarized light emission is expected
for group III-Nitrides grown on non- and semipolar crystal
orientations. As can be seen in FIG. 12 strong light polarization
was determined for the measured MQW sample emitting at .lamda.=237
nm. The integrated polarization is at least 35%, with the spectral
dependence shown in FIG. 21. As theoretically predicted the
stronger emission was recorded for light that is linearly polarized
parallel to the c'-axis of the wurtzite crystal as shown in plot
2101, whereas the weaker emission originates from polarized light
perpendicular to the c-axis as shown in plot 2102.
[0086] A number of values and ranges are provided in various
aspects of the implementations described. These values and ranges
are to be treated as examples only, and are not intended to limit
the scope of the claims. For example, embodiments described in this
disclosure can be practiced throughout the disclosed numerical
ranges. In addition, a number of materials are identified as
suitable for various facets of the implementations. These materials
are to be treated as exemplary, and are not intended to limit the
scope of the claims.
[0087] The foregoing description of various embodiments has been
presented for the purposes of illustration and description and not
limitation. The embodiments disclosed are not intended to be
exhaustive or to limit the possible implementations to the
embodiments disclosed. Many modifications and variations are
possible in radiation of the above teaching.
* * * * *