U.S. patent application number 15/599288 was filed with the patent office on 2017-09-07 for ii-vi based light emitting semiconductor device.
The applicant listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Kamal Asadi, Patrick John Baesjou, Johannes Franciscus Maria Cillessen, Dagobert Michel De Leeuw, Wilhelmus Cornelis Keur, Cornelis Eustatius Timmering, Frank Verbakel.
Application Number | 20170256676 15/599288 |
Document ID | / |
Family ID | 49165798 |
Filed Date | 2017-09-07 |
United States Patent
Application |
20170256676 |
Kind Code |
A1 |
Asadi; Kamal ; et
al. |
September 7, 2017 |
II-VI BASED LIGHT EMITTING SEMICONDUCTOR DEVICE
Abstract
The invention provides a light emitting semiconductor device
comprising a zinc magnesium oxide based layer as active layer,
wherein the zinc magnesium oxide based layer comprises an aluminum
doped zinc magnesium oxide layer having the nominal composition
Zn.sub.1-xMg.sub.xO with 1-350 ppm Al, wherein x is in the range of
0<x.ltoreq.0.3. The invention further provides a method for the
production of such aluminum doped zinc magnesium oxide, the method
comprising heat treating a composition comprising Zn, Mg and Al
with a predetermined composition at elevated temperatures, and
subsequently annealing the heat treated composition to provide said
aluminum doped zinc magnesium oxide.
Inventors: |
Asadi; Kamal; (Eindhoven,
NL) ; De Leeuw; Dagobert Michel; (Mainz, DE) ;
Cillessen; Johannes Franciscus Maria; (Deurne, NL) ;
Keur; Wilhelmus Cornelis; (Weert, NL) ; Verbakel;
Frank; (Helmond, NL) ; Baesjou; Patrick John;
(Eindhoven, NL) ; Timmering; Cornelis Eustatius;
(Eindhoven, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
Eindhoven |
|
NL |
|
|
Family ID: |
49165798 |
Appl. No.: |
15/599288 |
Filed: |
May 18, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15177542 |
Jun 9, 2016 |
9666758 |
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15599288 |
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14409031 |
Dec 18, 2014 |
9385264 |
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PCT/IB2013/055325 |
Jun 28, 2013 |
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15177542 |
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61739165 |
Dec 19, 2012 |
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61665968 |
Jun 29, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 33/0087 20130101;
H01L 33/0095 20130101; H01L 33/502 20130101; H05B 33/14 20130101;
H01L 33/0037 20130101; C09K 11/643 20130101; H01L 33/36 20130101;
H01L 33/28 20130101; H01L 33/16 20130101; H01L 33/025 20130101;
H01L 33/285 20130101; H01L 2933/0016 20130101; H01L 33/145
20130101 |
International
Class: |
H01L 33/14 20100101
H01L033/14; H01L 33/00 20060101 H01L033/00; H01L 33/28 20060101
H01L033/28 |
Claims
1. A luminescent material comprising zinc magnesium oxide having
the nominal composition Zn.sub.1-xMg.sub.xO with 1-350 ppm Al,
wherein x is in the range of 0<x.ltoreq.0.3.
2. The luminescent material according to claim 1, wherein the zinc
magnesium oxide contains 5-40 ppm Al, and wherein x is in the range
of 0.02<x.ltoreq.0.2.
3. The luminescent material according to claim 1, wherein the zinc
magnesium oxide is polycrystalline.
4. The luminescent material according to claim 1, wherein the zinc
magnesium oxide forms a lattice and the Al is partly present in the
zinc magnesium oxide lattice as dopant.
5. The luminescent material according to claim 1, wherein the zinc
magnesium oxide forms a lattice and the Al at least partially
replaces Zn or Mg lattice positions.
6. The luminescent material according to claim 1, wherein the zinc
magnesium oxide forms a lattice and the Al at least partially
occupies interstitial positions in the lattice.
7. The luminescent material according to claim 1, wherein a content
of sulfur in the luminescent material is lower than 50 ppm.
8. A method for the production of an aluminum doped zinc magnesium
oxide, the method comprising: providing a composition comprising
Zn, Mg and Al having the nominal composition Zn.sub.1-xMg.sub.xO
with 1-350 ppm Al, wherein x is in the range of 0<x.ltoreq.0.3,
and subsequently annealing the composition to provide said aluminum
doped zinc magnesium oxide.
9. The method according to claim 14, further comprising heat
treating the composition at elevated temperatures prior to said
annealing the composition.
10. The method according to claim 15, wherein heat treating the
composition comprises heat treating under oxidative conditions.
11. The method according to claim 14, wherein the composition is
polycrystalline.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of U.S. patent
application Ser. No. 15/177,542 filed on Jun. 9, 2016, titled
"METHOD FOR PRODUCING LIGHT EMITTING SEMICONDUCTOR DEVICE", issuing
as U.S. Pat. No. 9,666,758 on May 30, 2017, which is a divisional
of U.S. patent application Ser. No. 14/409,031 filed on Dec. 18,
2014, issued as U.S. Pat. No. 9,385,264 on Jul. 5, 2016, which is a
.sctn.371 application of International Application No.
PCT/IB2013/055235 filed on Jun. 28, 2013, which claims priority to
U.S. Provisional Patent Application No. 61/739,165 filed on Dec.
19, 2012 and U.S. Provisional Patent Application No. 61/665,968
filed on Jun. 29, 2012. U.S. patent application Ser. No.
15/177,542, U.S. patent application Ser. No. 14/409,031,
International Application No. PCT/IB2013/055325, U.S. Provisional
Patent Application No. 61/739,165, and U.S. Provisional Patent
Application No. 61/665,968 are incorporated herein.
FIELD OF THE INVENTION
[0002] The invention relates to a II-VI based light emitting
semiconductor device, a luminescent material, a method for the
production of such luminescent material, as well as to a method for
the production of a II-VI semiconductor layer for such
semiconductor device.
BACKGROUND OF THE INVENTION
[0003] Wide-band gap II-VI compounds are expected to be one of the
most vital materials for high-performance optoelectronics devices
such as light-emitting diodes (LEDs) and laser diodes (LDs)
operating in the blue or ultraviolet spectral range. Thin films
were commonly grown using the conventional vapor-phase epitaxy
(VPE) method. With the development of science and technology, new
and higher requirements arose for material preparation. For this
reason, novel epitaxial growth techniques were developed, including
hot-wall epitaxy (HWE), metal organic chemical vapor deposition
(MOCVD), molecular-beam epitaxy (MBE), metal organic molecular-beam
epitaxy (MOMBE) and atomic layer epitaxy (ALE). Using these growth
methods, film thickness can be controlled and quality can be
improved. Examples of II-VI semiconductors are ZnS, ZnO, ZnSe,
ZnTe, and CdTe.
[0004] Zinc oxide semiconductor materials comprising zinc and
oxygen as constituent elements have attracted considerable
attention since they can emit not only blue light but also near
ultraviolet rays of 400 nanometers or less because of their wide
band gap similarly to semiconductor materials such as gallium
nitride and the like. Further, their applications to photo
detector, piezoelectric device, transparent conductive electrode,
active device and the like have also been expected without being
limited to light emitting device. To form such a zinc oxide
semiconductor material, various methods such as MBE method using
ultra-high vacuum, sputtering, vacuum evaporation, sol-gel process,
MO-CVD method, and the like have been conventionally examined. With
respect to the light emitting device, the MBE method using
ultra-high vacuum is widely used from the viewpoint of
crystallinity.
[0005] Further, U.S. Pat. No. 4,278,913 describes a zinc
oxide-based phosphor emits yellow light of high luminance under
excitation of low-velocity electrons: xMIIF2.yMIIIF3.ZnO wherein
MII is at least one divalent metal selected from the group
consisting of beryllium, magnesium, calcium, strontium, barium,
zinc and tin, MIII is at least one trivalent metal selected from
the group consisting of aluminum, gallium, indium, thallium,
yttrium and antimony, and x and y are numbers satisfying the
conditions of 0.0001.ltoreq.x+y.ltoreq.0.1, 0.ltoreq.x and 0<y.
The zinc oxide-based phosphor is used as a fluorescent screen of a
low-velocity electron excited fluorescent display device.
SUMMARY OF THE INVENTION
[0006] Currently, the lighting world is in the middle of a
transition from the incandescent bulbs and (compact) fluorescent
lamps towards solid-state lighting, mostly provided by light
emitting diodes (LEDs). The majority of the LEDs in market are
based on gallium nitride (GaN). While GaN is an excellent emitter,
it does suffer from several drawbacks. The main issue is the
susceptibility to defects in the crystal lattice that are generally
detrimental for the emissive properties of GaN layers. Yet, GaN is
the most suitable III-V material for LED fabrication because is
actually one of the more defect-tolerant III-V materials. In order
to prevent emission loss, the defect concentration has to be kept
low by growing the GaN layers epitaxially. Epitaxial growth however
prevents fabrication of large area devices. Additionally, the GaN
covered wafers are generally cut up into small parts (typically
1.times.1 mm) to ensure an acceptable yield in functional devices,
and to ensure optimum use of materials, because of the fact that
gallium is a relatively scarce and expensive element.
[0007] The requirement for small areas has several disadvantages.
In order to have enough light output GaN LEDs are generally
operated at high power densities leading to heating of the devices
which decreases their efficiency and requires the use of heat
dissipation mechanisms such as heat sinks. The high light output
from a small area effectively makes them point sources, which makes
it uncomfortable to look directly into when used for general
lighting applications. In fact, high power LEDs are classified on
par with lasers with respect to eye safety. Therefore, for lighting
applications, some kind of light spreading and glare reduction is
generally required. An approach to solve these issues is to have
large light emitting surfaces that can be driven at much lower
power densities. As mentioned above, a large-area GaN light source
is currently impossible and does not exist.
[0008] One of the reasons for the vulnerability for defects in the
GaN crystal lattice stems from the low exciton binding energy,
which is below kT. This low value means that at room temperature,
excitons are likely to split up in separate electrons and holes
before they have a chance to radiatively recombine. The separated
charge carriers are then trapped at defect sites, leading to
non-radiative decay. Obviously, this process intensifies at the
elevated temperatures that GaN LEDs are commonly operated at.
[0009] On the other side of the spectrum are organic LEDs (OLEDs)
with an exciton binding energy of 0.5 eV, far larger than kT,
enabling light emission from an essentially amorphous medium which
makes large area lighting applications possible. OLEDs however
require (expensive) encapsulation due to the reactive nature of the
electrode materials used.
[0010] Zinc oxide has long been studied as a material that may have
the best of both worlds. Like GaN, it is a wide band gap
semiconductor (.about.3.3 eV), but ZnO has a high exciton binding
energy of 60 meV (2.4 times kT at room temperature). This value
means that defects should be less detrimental to light emission,
thereby enabling a switch from epitaxial growth methods towards
cheaper, large area deposition techniques like sputtering that
generally result in polycrystalline layers. Furthermore, ZnO is
cheap, abundant and highly stable making it an attractive choice as
a potential light emitting material in large area devices.
[0011] Zinc oxide can be applied using large area deposition
techniques like sputtering, which generally results in
polycrystalline layers. Furthermore, ZnO is cheap, abundant and
highly stable, making it an attractive choice as a potential light
emitting material in large area devices. However, despite these
promises, ZnO has a few issues as a potential phosphor that have
not been solved yet. Firstly, the main (near band gap) emission is
in the UV (.about.385 nm) and secondly the quantum efficiency of
this emission is very low. Up to 3% efficiency has been reported
from powder at room temperature, but generally lower values are
observed.
[0012] A well-known additive is sulfur, which results in a strong,
broad band emission from ZnO centered around 500 nm with a quantum
efficiency of .about.50%. Although the preparation of highly
luminescent ZnO:S powder is rather straightforward, the deposition
of thin films of a similar composition is troublesome.
[0013] Therefore, there is an interest in additives for ZnO that
improve the visible emission and quantum efficiency of the
material, while being compatible with large area deposition
techniques like sputtering. Hence, it is an aspect of the invention
to provide an alternative (light emitting) semiconductor device,
which preferably further at least partly obviates one or more of
above-described drawbacks. It is further an aspect of the invention
to provide an alternative luminescent material, which preferably
further at least partly obviates one or more of above-described
drawbacks. Further, it is an aspect to provide a method for the
production of such luminescent material, especially in the form of
a layer on a substrate that can be used as active layer in such
alternative semiconductor device.
[0014] In a first aspect, the invention provides a light emitting
semiconductor device comprising a stack, the stack comprising a
cathode (which may especially be a cathode layer), a semiconductor
layer comprising an emissive (oxidic) material having an emission
in the range of 300-900 nm, an (oxidic) insulating layer, and an
anode (which may especially be an anode layer), wherein the cathode
is in electrical contact with the semiconductor layer, wherein the
anode is in electrical contact with the insulating layer, such as a
metal oxide layer, and wherein the insulating layer has a thickness
in the range of up to 50 nm (i.e. >0 nm and .ltoreq.50 nm).
[0015] This approach is a realization of the diode by incorporation
of an insulating layer in the device stack, i.e.
metal-insulator-semiconductor-metal (MISM) diode. The cathode or
anode can in principle be of any material that is suitable as
cathode or anode material, respectively. Especially, at least one
of cathode or anode is transmissive. In an embodiment, the cathode
comprises ZnO doped with aluminium or indium tin oxide (ITO).
Hence, in an embodiment, the cathode is transmissive. Herein, the
term "transmissive" indicates that the layer is transmissive for
emission of the active layer. In a further embodiment, the anode
may be a noble metal, such as gold or platinum, or a combination
thereof.
[0016] Suitable materials for the semiconductor layer may
especially be an emissive material selected from the group
consisting of oxides, sulfides or selenides of zinc or cadmium,
such as zinc oxide, zinc magnesium oxide, zinc sulfide, zinc
selenide, cadmium oxide, cadmium sulfide, and cadmium selenide,
especially ZnO, (Zn,Mg)O, ZnS, ZnSe, CdO, CdS, CdSe. Further, also
oxysulfides may be applied, like gadolinium oxy sulfide. Further,
also doped version of these materials may be applied, like ZnO:Al,
(Zn,Mg)O:Al, ZnO:Mn, (Zn,Mg)O:Mn, etc. Hence, in an embodiment, the
emissive material is selected from the group consisting of ZnO,
(Zn,Mg)O, ZnS, ZnSe, CdO, CdS, CdSe, and doped variants of any of
these, (like ZnO:Al, (Zn,Mg)O:Al, ZnO:Mn, (Zn,Mg)O:Mn, etc.). Also
other semi-conducting materials may be applied that show emission
in the visible. Optionally, also tellurides, like ZnTe, might be
applied.
[0017] Hence, in a specific embodiment, the semiconductor layer
comprises an emissive material selected from the group consisting
of zinc (magnesium) oxide and cadmium oxide. The semiconductor
layer is herein also indicated as "active layer". In an embodiment,
the layer is a non-granular layer, such as a layer obtainable by
CVD or sputtering (and annealing), or other techniques known in the
art, such as described herein. However, in another embodiment, the
layer comprises a particulate layer, such as a layer comprising
semiconducting nanoparticles. In another embodiment, the layer
comprises (semi conducting) quantum dots. The layer is especially a
continuous layer, with a porosity of at maximum 5%.
[0018] The term "(oxidic) emissive material" indicates that the
emissive material may be a metal oxide material, such as ZnO.
However, the emissive layer may also comprise a sulphide or
selenide emissive material, etc., see also above.
[0019] The semiconductor layer comprises an emissive material
having an emission in the range of 300-900 nm, such as in the range
of 300-800 nm, like 400-700 nm. Especially, the semiconductor layer
has at least part of its emission in the visible part of the
optical spectrum. Likewise, this may apply to the luminescent
material as described below.
[0020] The term white light herein, is known to the person skilled
in the art. It especially relates to light having a correlated
color temperature (CCT) between about 2000 and 20.000 K, especially
2700-20.000 K, for general lighting especially in the range of
about 2700 K and 6500 K, and for backlighting purposes especially
in the range of about 7000 K and 20.000 K, and especially within
about 15 SDCM (standard deviation of color matching) from the BBL
(black body locus), especially within about 10 SDCM from the BBL,
even more especially within about 5 SDCM from the BBL. The terms
"violet light" or "violet emission" especially relates to light
having a wavelength in the range of about 380-440 nm. The terms
"blue light" or "blue emission" especially relates to light having
a wavelength in the range of about 440-490 nm (including some
violet and cyan hues). The terms "green light" or "green emission"
especially relate to light having a wavelength in the range of
about 490-560 nm. The terms "yellow light" or "yellow emission"
especially relate to light having a wavelength in the range of
about 560-590 nm. The terms "orange light" or "orange emission"
especially relate to light having a wavelength in the range of
about 590-620. The terms "red light" or "red emission" especially
relate to light having a wavelength in the range of about 620-750
nm. The terms "visible" light or "visible emission" refer to light
having a wavelength in the range of about 380-750 nm.
[0021] Specific embodiments of the active layer (material) are
elucidated below, but first the insulating layer is discussed.
[0022] As indicated above, especially good results are obtained due
to the presence of the (oxidic) insulating layer, which may also be
indicated as barrier layer. The term "oxidic insulating layer"
indicates that the barrier layer is a metal oxide layer. This layer
may also comprise a plurality of layers, optionally of different
metal oxides. The term "metal oxide" may also refer to a mixed
metal oxide. This insulating layer should preferably not influence
the optical properties of the active layer. In other words, the
insulating layer should preferably not influence the emission
position of the emission band of the active layer. Especially, the
insulating layer or barrier layer does not substantially react with
the active layer, also not during application of the insulating
layer on the active layer or during application of the insulating
layer on the active layer ("inverted structure"). Hence, it is
highly desirable to have a blocking layer that is stable in air and
does not intermix with the underlying active layer, such as ZnO
phosphor layer (see below) upon annealing (see also below). A good
candidate for such layer is ZrO, which is a stoichiometric oxide
with very limited solubility in ZnO. Especially, the oxidic
insulating layer is selected from the group consisting of SiO2,
MgO, SrTiO3, ZrO2, HfO2, and Y2O3. In a further variant, the
insulating layer is a high bandgap dielectrical material, such as
with a bandgap of at least 5 eV, especially at least 5.5 eV. The
insulating layer may also comprise a non-oxidic material.
[0023] It is further desired that the position of the valence band
and conduction band of the insulating layer is positioned such that
conduction band of the (material of the) insulating layer is higher
than of the conduction band of the (material of the) active layer.
Further, the position of the valence band of the (material of the)
insulating layer may be in the vicinity of the valence band of the
(material of the) active layer.
[0024] Especially, the emissive material has a conduction band at
CBp eV and a valence band at VBp eV from the vacuum level, with
CBp>VBp, wherein the barrier layer has a conduction band at CBb
eV and a valence band at VBb eV from the vacuum level, with
CBb>VBb, wherein CBb>CBp, especially wherein
CBb.gtoreq.CBp+0.25 eV. Further, in an embodiment especially
VBb.ltoreq.VBp+1.5 eV, even more especially VBb.ltoreq.VBp+1 eV.
The vacuum level at 0 V is taken as reference.
[0025] Vc and Vb usually have negative values. Therefore, when
Vc>Vb this implies that that |Vc| is smaller than |Vb|. Such
conditions may give best results in terms of efficiency of the
device. For instance, a conduction band of the barrier layer that
is too close to, or even below the conduction band of the active
layer may lead to an inefficient light emission in comparison with
a barrier layers as indicated above, because the barrier required
for blocking electron transport in the active layer has been
disappeared. Especially, CBb.gtoreq.CBp+0.35 eV, even more
especially, CBb.gtoreq.CBp+0.5 eV. Further, as indicated above
especially VBb.ltoreq.VBp+1.5 eV, even more especially
VBb.ltoreq.VBp+1 eV.
[0026] To give an example, the emissive material (of the active
layer) may have a conduction band at -4 eV and a valence band at -7
eV; hence CBp>VBp. Further, the barrier layer may e.g. have a
conduction band at -3 eV and a valence band at e.g. -6 eV, or -8
eV. Hence, CBb>VBb. Further, also CBb.gtoreq.CBp+0.25 eV and
VBb.ltoreq.VBp+1 eV apply.
[0027] Especially, the thickness of the insulating layer is within
the tunneling limit. Hence, the insulating layer has a thickness
which is especially equal to or smaller than 50 nm, such as equal
to or smaller 30 nm, like especially in the range of 2-30 nm, like
at least 4 nm.
[0028] Here, we present also a novel class of zinc oxide based
phosphors with enhanced quantum efficiency and emission in the
visible part of the spectrum, that are also amenable to robust,
large area thin layer deposition techniques such as sputtering, and
which may also be used as material for an active layer in above
described device. The enhanced emission is achieved by
incorporating both magnesium and a trace amount of aluminum,
followed by annealing in a non-reducing atmosphere, especially in
air. The enhanced emission does not seem to stem from either the Al
or Mg themselves, but is attributed to radiating defects in the
(modified) ZnO lattice, the nature and number of which are thought
to be influenced by the additives. The presence of both Al and Mg
seem to have a synergistic effect. These ZnO based materials are
prospective candidates for the emissive layer in large area LEDs.
The emissive layer is herein also indicated as "active layer". The
term "active layer" indicates that this layer in the semiconductor
device will show the desired luminescence (emission), when the
semiconductor device is driven under the right conditions. The
layer is especially a thin film, such as having a thickness in the
range of 50 nm-1000 nm (1 .mu.m). The layer is especially a
continuous layer, with a porosity of at maximum 5%.
[0029] In a further aspect, the invention provides a (light
emitting) semiconductor device (herein also indicated as "device")
comprising a zinc oxide or zinc magnesium oxide based layer,
especially a zinc magnesium oxide based layer, as active layer,
wherein the zinc magnesium oxide based layer comprises (or
especially consists of) an aluminum doped zinc magnesium oxide
layer having 1-350 ppm Al. The zinc magnesium oxide of the aluminum
doped zinc magnesium oxide layer is of the type ZnO; thus more
precisely (Zn,Mg)O; i.e. especially a (Zn,Mg)O:Al layer is
provided. Instead of, or in addition to the Al dopant, also other
dopants may be applied, like Mn (manganese).
[0030] Especially, the invention provides a semiconductor device
comprising a zinc magnesium oxide based layer as active layer,
wherein the zinc magnesium oxide based layer comprises (even more
especially consists of) an aluminum doped zinc magnesium oxide
layer having the nominal composition Zn1-xMgxO with 1-350 ppm Al,
wherein x is in the range of 0<x.ltoreq.0.3. The phrase
"Zn1-xMgxO with 1-350 ppm Al" may also be, as known in the art,
indicated as Zn1-xMgxO:Al (1-350 ppm). Here, the term "nominal
composition" is applied, as the composition herein indicated
relates to the composition as weighed in. Hence, the nominal
composition might also be indicated as "(1-x)ZnO*xMgO with 1-350
ppm Al".
[0031] It appears that a relative highly efficient active layer is
provided, that has the desired properties in respect of efficiency
and electrical resistance. Further, such layer may be produced
relatively easy. Layers without Mg or without Al are less
efficient. Further, layers having a higher Al content may have
undesired conductive properties.
[0032] It seems that Mg (magnesium) may at least partly be built in
the ZnO lattice (alternatively, one may say that MgO dissolves in
the ZnO lattice). The amount of Mg in the nominal composition is
indicated with x, which is especially in the range of
0<x.ltoreq.0.3, and even more at maximum 0.2. In the range of
0.02<x.ltoreq.0.2 best optical properties may be obtained. The
intrinsic value for x may especially be 0.1-0.2, like about 0.15
for a layer, whereas for a poly crystalline material, the value for
x may especially be 0.04 or lower. The intrinsic value refers to
the x-value of the mixed oxide. The presence of Mg in the zinc
oxide can be determined from XRD (x-ray diffraction), or SIMS, RBS
or ICP/MS, see also below.
[0033] With respect to Al (aluminum), it seems that 1-350 ppm
(parts per million) Al, especially 1-200 ppm, even more especially
1-100 ppm, give good optical properties and also does not lead to a
high conductivity, which is not desired, and which may occur when
high Al amounts are used. An amount of Al in the range of 2-100
ppm, such as 5-100 may be especially suitable, even more a range of
2-80 ppm, such as 2-70, such as 10-60 ppm, like 20-60 ppm,
especially like 30-50 ppm can be used. In an embodiment, the
aluminum content is at least 10 ppm. Aluminum may partly be present
in the zinc magnesium oxide lattice as dopant. Al may replace Zn or
Mg lattice positions or may form or occupy interstitial positions
in the lattice. The presence of Al can be reflected in SIMS
(Secondary Ion Mass Spectrometry) or RBS (Rutherford
backscattering) of the material. Optionally also laser ablation
with ICP/MS (Inductively Coupled Plasma Mass Spectrometry) can be
used to detect the presence of Al. The ppm value of the dopant
relates to the total molar amount of the system. Hence, 10 .mu.mol
Al in 1 mole Zn1-xMgxO:Al will lead to a value of 10 ppm Al, i.e.
Zn1-xMgxO:Al (10 ppm).
[0034] Hence, in a specific embodiment the zinc magnesium oxide
contains 5-100 ppm Al, wherein x is in the range of
0.02<x.ltoreq.0.2 (nominal composition). Further, especially the
sulfur content in the zinc magnesium oxide (based layer) is lower
than 50 ppm. Higher sulfur contents may lead to systems that cannot
easily form the desired composition of the layer. For semiconductor
applications, the layer thickness of the aluminum doped zinc
magnesium oxide layer may be in the range of 50-1000 nm, such as at
least 100 nm. The way in which such active layers may be formed is
further elucidated below.
[0035] A semiconductor device, with such aluminum doped zinc
magnesium oxide layer active layer can advantageously be used to
generate visible light, especially having a dominant wavelength in
the wavelength range of 500-650 nm. The term dominant wavelength
indicates that the emission intensity maximum is found within the
indicated spectral region. Further, it appears that the aluminum
doped zinc magnesium oxide layer having the nominal composition
Zn1-xMgxO with 1-350 ppm Al, wherein x is in the range of
0<x.ltoreq.0.3, can advantageously be used as active layer in a
large area LED, the large area LED at least having a die area of at
least 1 cm2.
[0036] The premise of ZnO is application in large area lighting due
to the exciton binding energy of 60 meV being larger than kT. In
III-V LEDs (such as GaN), the binding energy is smaller than kT.
Hence, for a high photoluminescence efficiency non-radiative
defects have to be avoided. Epitaxially grown thin films are
required; the technology cannot be scaled up to large areas. In
OLEDs however, the exciton binding energy is about 0.5 eV. Light
can be generated in amorphous films that are fabricated by
roll-to-roll processing. The challenge for OLEDs is cost price and
encapsulation.
[0037] The large binding energy makes ZnO a defect tolerant host
material. Epitaxial thin films are not needed; a high efficiency
might be obtained with polycrystalline thin films deposited over
large area. Numerous papers report light emission from
polycrystalline oxide LEDs fabricated by various deposition
methods. The present efficiency is low, but there is not
necessarily a fundamental limitation. When the efficiency can be
optimized it will pave the way for large area solid state lighting.
The advantages are low-cost and environmentally stable diodes that
can be fabricated over a large area with industrially established
deposition techniques.
[0038] As may be known in the art, the ZnO-based layer may be
sandwiched between electrodes of the semiconductor device. Further
modification of the ZnO-based layer to provide the semiconductor
device may also be included. For instance, optionally one or more
electron or hole blocking layers may be applied. This may improve
efficiency. One or more electron or hole blocking layers may be
arranged at different positions within the stack.
[0039] Hence, in an embodiment, the invention provides a light
emitting semiconductor device, wherein the semiconductor layer
comprises aluminum doped zinc magnesium oxide layer having 1-350
ppm Al. Especially, the semiconductor layer has a nominal
composition Zn1-xMgxO with 1-350 ppm Al, wherein x is in the range
of 0<x.ltoreq.0.3.
[0040] The stack especially comprises a cathode, a semiconductor
layer comprising an emissive (oxidic) material, an (oxidic)
insulating layer, and an anode (and a support). Optionally, between
the support and the cathode or anode, one or more other
(functional) layers may be present. Further, optionally between the
cathode and the semiconductor layer one or more other functional
layers may be present. Especially, between the semiconductor layer
and the (oxidic) insulating layer (the barrier layer), and between
the barrier layer and the anode, no further other layers are
present. However, in addition to (or alternative to) the insulating
layer between the semiconductor layer and the anode, one or more
further (or other) other blocking layers may be present in the
device stack that are not necessarily in contact with the anode (or
cathode). However, the one at the anode is especially applied, to
help and facilitate hole injection.
[0041] The invention also provides the (particulate) luminescent
material per se, and thus not only as active (thin) layer in a
semiconductor device. Hence, as indicated above, the invention also
provides a luminescent material comprising zinc magnesium oxide
doped with Al. The zinc magnesium oxide is of the type ZnO; hence,
especially (Zn,Mg)O:Al is provided. Hence, the invention also
provides a luminescent material comprising zinc magnesium oxide
doped with Al having the nominal composition Zn1-xMgxO with 1-350
ppm Al, wherein x is in the range of 0<x.ltoreq.0.3. This may be
a particulate or granular material. Preferred ranges with respect
to Mg content and Al content are the same as indicated above for
the aluminum doped zinc magnesium oxide layer. For instance, the
zinc magnesium oxide (luminescent material) may contain 5-40 ppm
Al, wherein x is in the range of 0.02<x.ltoreq.0.2.
[0042] The fact that the nominal composition "Zn1-xMgxO" is applied
does not exclude (small) non-stoichiometric variations, such as in
the order of at maximum 5%. Further, this chemical nominal
composition does not exclude the presence of other dopants than
aluminum (and magnesium). For instance, also sulfur might be
present. In an embodiment however, no sulfur is present.
[0043] The invention also provides a method for the production of
the light emitting semiconductor device as described herein. Hence,
in a further aspect the invention provides a method for producing a
light emitting semiconductor device, the method comprising
providing a support and forming a stack on the support, wherein the
stack comprises a cathode, a semiconductor layer comprising an
emissive material having an emission in the range of 300-900 nm, an
(oxidic) insulating layer, and an anode, wherein the cathode is in
electrical contact with the semiconductor layer, wherein the anode
is in electrical contact with the insulating layer, and wherein the
insulating layer has a thickness in the range of up to 50 nm. Such
device may be made with conventional semiconductor production
technologies, though formation of the semiconductor layer, i.e. the
active layer, may especially be done via pulsed laser deposition
(PLD) and radio frequency (RF) sputtering. Hence, in an embodiment
the formation of the layer comprises a deposition technique
selected from the group consisting of pulsed laser deposition (PLD)
and radio frequency (RF) sputtering. Other techniques that may be
used as well are e.g. atomic layer deposition (ALD), chemical vapor
deposition (CVD) and its variants of CVD method such as
metal-organic CVD (MOCVD) or plasma enhanced CVD (PECVD),
hydrothermal growth, spray pyrolysis, etc.; in general, any
physical and chemical evaporation technique may be applied.
Likewise, this may apply for one or more of the other layers in the
stack.
[0044] In an embodiment, the production comprises forming the
cathode on the support, the semiconductor layer on the cathode, the
(oxidic) insulating layer on the semiconductor layer, and the anode
on the (oxidic) insulating layer, followed by annealing the stack,
wherein annealing is performed at a temperature in the range of
400-1100.degree. C. However, inverted building is also
possible.
[0045] As indicated above, for the conduction band and valence band
of the insulating layer especially applies CBb>CBp, especially
CBb.gtoreq.CBp+0.25 and/or VBb.ltoreq.VBp+1.5 eV, even more
especially VBb.ltoreq.VBp+1 eV. CBb refers to the conduction band
of the barrier; CBp refers to the conduction band of the active
layer (phosphorescent layer); likewise, VBb refers to the valence
band of the barrier and VBp refers to the valence band of the
active layer. For instance, the (oxidic) insulating layer is
selected from the group consisting of SiO2, MgO, SrTiO3, ZrO2,
HfO2, and Y2O3. Suitable emissive materials are also defined
above.
[0046] In a specific embodiment, the semiconductor layer (thus
formed) has the nominal composition Zn1-xMgxO with 1-350 ppm Al,
wherein x is in the range of 0<x.ltoreq.0.3. In a further
embodiment, the method comprises (a) providing a composition
comprising Zn, Mg and Al having the nominal composition Zn1-xMgxO
with 1-350 ppm Al, wherein x is in the range of 0<x.ltoreq.0.3,
optionally heat treating this composition at elevated temperatures,
and (b) subsequently annealing the optionally heat treated
composition to provide said aluminum doped zinc magnesium
oxide.
[0047] As indicated above, the invention also provides a method for
the production of an aluminum doped zinc magnesium oxide, such as
described above. Hence, in a further aspect, the invention provides
a method for the production of an aluminum doped zinc magnesium
oxide, the method comprising (a) providing a composition comprising
Zn, Mg and Al with having the nominal composition Zn1-xMgxO with
1-350 ppm Al, wherein x is in the range of 0<x.ltoreq.0.3,
optionally heat treating this composition at elevated temperatures,
and (b) subsequently annealing the optionally heat treated
composition to provide said aluminum doped zinc magnesium
oxide.
[0048] Even more especially, the invention provides a method for
the production of an aluminum doped zinc magnesium oxide having the
nominal composition Zn1-xMgxO with 1-350 ppm Al, wherein x is in
the range of 0<x.ltoreq.0.3. This may include a method to
generate a (particulate) luminescent material, but this may also
include a method to produce a thin layer on a substrate.
Especially, the method comprises heat treating (especially under
oxidative conditions) a composition comprising Zn, Mg and Al with a
predetermined nominal composition at elevated temperatures, and
subsequently annealing the heat treated composition to provide said
aluminum doped zinc magnesium oxide. The phrase "a composition
comprising Zn, Mg and Al" may especially refer to one or more
compounds comprising Zn, Mg and/or Al, respectively. These may also
be indicated as precursor(s), see below.
[0049] The term composition may in an embodiment relate to a
combination of one or more precursors of the luminescent material,
such as metal oxides, or metal salts, like nitrates, sulfates,
chlorides, fluorides, bromides, hydroxides, carboxylates such as
oxalates, etc. etc. Optionally also a sulfide (or even optionally a
selenide and/or a telluride), such as zinc sulfide might be applied
as precursor. Especially, one or more of a metal oxide, a nitrate,
a chloride, a hydroxide, and a carboxylate (such as an oxalate) are
applied. Combinations of two or more of such precursor types may
also be applied. Due to the heat treatment, the aluminum doped zinc
magnesium oxide may be formed, but especially the material may be
formed during annealing. The heat treatment and annealing may in an
embodiment be performed until at least a poly crystalline material
is formed.
[0050] In another embodiment, the composition may be formed on a
substrate. This may be done at elevated temperatures. For instance,
the substrate may be heated. Hence, in a specific embodiment of the
method, the method comprises forming an aluminum doped zinc
magnesium oxide based layer having the nominal composition
Zn1-xMgxO with 1-350 ppm Al, wherein x is in the range of
0<x.ltoreq.0.3, the method comprising forming a layer comprising
the composition comprising Zn, Mg and Al with the predetermined
nominal composition on a substrate at elevated temperatures, and
annealing the thus formed layer to provide the aluminum doped zinc
magnesium oxide based layer. For such an embodiment, the formation
of the layer comprises a deposition technique selected from the
group consisting of pulsed laser deposition (PLD) and radio
frequency (RF) sputtering. However, also other deposition
techniques may be applied (see also above).
[0051] As target material, the oxides may be applied, or mixed
oxides may be applied. Especially, as target material (crystalline)
aluminum doped zinc magnesium oxide is applied. Hence, the method
may especially comprise depositing of the layer of said zinc
magnesium oxide on the substrate by pulsed laser deposition or RF
sputtering of an aluminum doped zinc magnesium oxide having the
nominal composition Zn1-xMgxO with 1-350 ppm Al (i.e. here target
material), wherein x is in the range of 0<x.ltoreq.0.3 (nominal
composition). With pulsed laser deposition (PLD) and radio
frequency (RF) sputtering, a layer may be deposited on the
substrate, having the desired composition. In this way, a II-VI
semiconductor layer for a semiconductor device may be produced.
[0052] The term "the predetermined nominal composition" especially
relates to the fact that starting components or a composition are
composed in such a way, that the ratio of the elements may lead to
the desired composition of the end product, i.e. aluminum doped
zinc magnesium oxide having the nominal composition Zn1-xMgxO with
1-350 ppm Al, wherein x is in the range of 0<x.ltoreq.0.3. As
indicated above, the values that can be derived from the formula
"Zn1-xMgxO with 1-350 ppm Al" refers to the nominal composition
that is weighed out, and which forms the zinc magnesium oxide. The
formed material may in addition to the zinc magnesium oxide, also
optionally comprise (remaining) MgO.
[0053] With respect to deposition (of the semiconductor layer; i.e.
the active layer), the deposition is especially performed during a
deposition time, wherein during at least part of the deposition
time the substrate is maintained at a temperature of at least 450
C..degree. for RF sputtering or at least 500.degree. C. for pulsed
laser deposition. With the indicated techniques, layers may be
grown at a rate of about 0.3-1 nm/s, such as 04-0.8 nm/s, like
about 0.6 nm/s.
[0054] The (first) heat treatment is especially at a temperature of
at least 450.degree. C., although for the synthesis of the
luminescent material, even a temperature of at least 900.degree.
C., such as at least 1100.degree. C. may be chosen. For instance,
in the case of the heat treatment to provide the luminescent
material, the temperature may be in the range of 1000-1800.degree.
C. Thereafter, the material may be cooled down, ground (in case of
a particulate material), and be subject to the annealing. In case
of making the semiconductor layer, the (first) heat treatment will
in general be at least 450.degree. C., but not higher than
800.degree. C. However, in yet another embodiment, deposition is
done at a substrate at a lower temperature than 450.degree. C.
Optionally, the substrate may even be at room temperature.
Especially, however, the substrate is at elevated temperatures,
such as indicated above.
[0055] It appears that annealing in a reducing atmosphere does not
give entirely desired results. Especially, annealing is performed
in a neutral or oxidizing atmosphere. Especially, the method
includes annealing in an oxidizing atmosphere. Further, the method
may especially comprise annealing at a temperature of at least
900.degree. C. for at least 30 min. For instance, the temperature
may be in the range of 900-1800.degree. C. Note that heat treatment
and annealing are two different actions, which are in general
separated by one or more other steps, such as including a cooling
step. For the synthesis of a layer, the temperature maximum for the
(first) heat treatment and the annealing may be limited to the
temperature stability of the substrate and/or the reactivity of the
substrate. In general, the temperature should not be higher than
1200.degree. C., such as not higher than 1100.degree. C. For powder
synthesis, the annealing temperature may be above 1000.degree. C.,
such as at least 1200.degree. C.
[0056] The term "substantially" herein, such as in "substantially
all emission" or in "substantially consists", will be understood by
the person skilled in the art. The term "substantially" may also
include embodiments with "entirely", "completely", "all", etc.
Hence, in embodiments the adjective substantially may also be
removed. Where applicable, the term "substantially" may also relate
to 90% or higher, such as 95% or higher, especially 99% or higher,
even more especially 99.5% or higher, including 100%. The term
"comprise" includes also embodiments wherein the term "comprises"
means "consists of".
[0057] Furthermore, the terms first, second, third and the like in
the description and in the claims, are used for distinguishing
between similar elements and not necessarily for describing a
sequential or chronological order. It is to be understood that the
terms so used are interchangeable under appropriate circumstances
and that the embodiments of the invention described herein are
capable of operation in other sequences than described or
illustrated herein.
[0058] The devices herein are amongst others described during
operation. As will be clear to the person skilled in the art, the
invention is not limited to methods of operation or devices in
operation. Hence, the phrase "II-VI based light emitting
semiconductor device" is also directed to a device which is
switched off, and which will in the switched off state not be light
emitting. The semiconductor layer comprising an emissive material
may especially comprise an n-type emissive material. Hence, the
semiconductor layer may be an n-type semiconductor layer, such as
n-ZnO or n-CdS, etc.
[0059] It should be noted that the above-mentioned embodiments
illustrate rather than limit the invention, and that those skilled
in the art will be able to design many alternative embodiments
without departing from the scope of the appended claims. In the
claims, any reference signs placed between parentheses shall not be
construed as limiting the claim. Use of the verb "to comprise" and
its conjugations does not exclude the presence of elements or steps
other than those stated in a claim. The article "a" or "an"
preceding an element does not exclude the presence of a plurality
of such elements. The invention may be implemented by means of
hardware comprising several distinct elements, and by means of a
suitably programmed computer. In the device claim enumerating
several means, several of these means may be embodied by one and
the same item of hardware. The mere fact that certain measures are
recited in mutually different dependent claims does not indicate
that a combination of these measures cannot be used to
advantage.
[0060] The invention further applies to a device comprising one or
more of the characterizing features described in the description
and/or shown in the attached drawings. The invention further
pertains to a method or process comprising one or more of the
characterizing features described in the description and/or shown
in the attached drawings.
[0061] The various aspects discussed in this patent can be combined
in order to provide additional advantages. Furthermore, some of the
features can form the basis for one or more divisional
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts, and in which:
[0063] FIG. 1 depicts emission spectra of a number of luminescent
materials; Normalized emission spectra of ZnO (a), ZnO:Al (10 ppm)
(b), Zn0.9Mg0.1O (c), and Zn0.9Mg0.1O:Al (10 ppm) (d). All measured
as powders sandwiched between quartz plates, excitation at 325
nm;
[0064] FIG. 2 depicts excitation spectra of a number of luminescent
materials; Normalized excitation spectra of ZnO:Al (10 ppm) (b),
Zn0.9Mg0.1O (c), and Zn0.9Mg0.1O:Al (10 ppm) (d); and
[0065] FIG. 3a-3b depict SEM graphs from a sputtered aluminum doped
zinc magnesium oxide layer before (a) and after (b) annealing at
1100.degree. C.
[0066] FIGS. 4a and 4b show PL (photoluminescence) spectra of
sputtered Zn0.85Mg0.15O doped with 40 ppm Al (ZAM-40) deposited on
sapphire (4a) and ITO-coated sapphire (4b). Films were deposited at
room temperature, and were subsequently post annealed; a plurality
of annealing temperatures were investigated; some temperatures are
indicated to better understand the temperature trends;
[0067] FIG. 5 depicts photoluminescence EQE (external quantum
efficiency) measurements as function of post deposition anneal
temperature for ZAM-40 deposited on sapphire;
[0068] FIG. 6 shows a comparison of the PL spectra of
ZAM-40/sapphire and ZAM-40/ITO/sapphire versus post-deposition
anneals temperature;
[0069] FIGS. 7a (left) and 7b (right) show PL spectra of ZAM layer
on ITO coated sapphire capped with ZrO layer of different
thicknesses. FIG. 7a has on the left axis the absolute irradiance
in photons/cm2.nm); FIG. 7b has on the y-axis the normalize
photoluminescence (in arbitrary units); both have a wavelength
scale (in nm) as x-axis;
[0070] FIG. 8 shows normalized PL spectra (in arbitrary units on
the y-axis) of sapphire/ITO/ZAM layer (dot-dashed) capped with MgO
(line) as function of the wavelength (in nm);
[0071] FIGS. 9a (left), 9b (right) schematically show an embodiment
of the device layout of a (ZnO) diode; References I-VI respectively
refer to the anode (I), such as Au and/or Pt, the barrier layer
(II), such as a metal oxide, having a layer width of larger than 0
nm and e.g. equal or smaller than 30 nm, the active layer (III),
such as (Zn.Mg)O:Al, the cathode (IV), such as ITO, electrode(s)
(V), such as Pt electrodes, and a substrate (VI), such as sapphire,
quartz or glass;
[0072] FIGS. 10a (top), 10b (bottom) show: FIG. 10a (top) I-V
characteristics of ITO/ZAM/MgO/Au diode; FIG. 10b (bottom)
electroluminescent spectra of the diodes driven at 10V and 50 mA.
The peak shown with the arrow shows the presence of near-band edge
emission (NBE) of ZnO indicating hole injection into ZAM layer; in
FIG. 10b, the curve with two peaks is the 10V/50 mA
electroluminescent spectrum; the other curve is the PL spectrum
(see e.g. FIG. 8);
[0073] FIG. 11 shows EL spectra of ITO/ZAM/MoOx/Au; ZnO NBE again
indicates the near-band edge emission (NBE) of ZnO; the curve that
is higher at 500 nm but lower at 900 nm is the PL (thin film)
spectrum; the other curve (with more fluctuation on the signal) is
the 12.5 V EL spectrum;
[0074] FIGS. 12a (top), 12b (bottom) show I-V characteristics (top)
and EL spectra taken at 10 V (bottom) of ZAM devices with a ZrO
blocking layer; in FIG. 12b, PL refers to photoluminescence, EL BA
refers to electroluminescence before annealing and EL AA refers to
electroluminescence after annealing;
[0075] FIG. 13 schematically shows an embodiment of energy band
diagrams of the n-ZnO/SiOx/p-type Si diodes under thermal
equilibrium (left) and positive bias at Si side (right).
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0076] For the compositions Zn(1-x)MgxO desired quantities of zinc
oxide (5N purity, Aldrich) and magnesium oxide (FO Optipur, Merck)
were weighed into a 100 ml beaker and mixed for 4 minutes at 1800
rpm using a speed mixer (Hauschild, type DAC 150 FVZ-K). The
compositions were put into an aluminum oxide crucible and fired
inside a chamber furnace in air for 8 hours at 1100.degree. C.
using a heating and cooling rate of 200.degree. C./hour. After
cooling down the powders were grinded using an agate mortar and
pestle and fired once again at 1100.degree. C.
[0077] For aluminum doped Zn(1-x)MgxO first a desired amount of
aluminum nitrate nonahydrate (p.a., Merck) was dissolved in a small
amount of deionized water and diluted with 200 ml ethanol. Next
desired amounts of zinc oxide (5N purity, Aldrich) and magnesium
oxide (FO Optipur, Merck) were added and the obtained suspension
was dried using a rotary evaporator. The compositions were put into
an aluminum oxide crucible and fired inside a chamber furnace in
air for 8 hours at 1100.degree. C. using a heating and cooling rate
of 200.degree. C./hour. After cooling down the powders were grinded
using an agate mortar and pestle and fired once again at
1100.degree. C. From the Zn0.9Mg0.1O+10 ppm Al powder, targets
suitable for sputtering and pulsed laser deposition (PLD) were
prepared.
[0078] A number of 400 nm thin films were grown on epi-polished
a-cut sapphire substrates by PLD and RF magnetron sputtering. The
base pressure of the PLD system was 2.times.10-7 mbar. During the
deposition the substrate temperature was between 25.degree. C. and
550.degree. C. and the partial oxygen pressure was 0.2 mbar. The RF
magnetron sputtering system had a base pressure of 6.times.10-7
mbar and the used substrate conditions were either 25.degree. C.,
450.degree. C. or 550.degree. C. The gas flows during the
sputtering process were resp. 78 and 2 sccm for Ar and O2, the
total pressure was 0.038 mbar, RF power was 60 W.
[0079] The thin film composition was analyzed using x-ray
fluorescence (XRF) and secondary ion mass spectrometry (SIMS). For
optical analysis of the powders, they were sandwiched between Asahi
quartz substrates (that were found to be non-luminescent with at
the excitation wavelengths used) and the sides were sealed with a
UV-transparent epoxy glue (Epo-Tek 305). UV/Vis spectra were
measured on a Perkin Elmer Lambda-950 spectrometer, emission and
excitation spectra on an Edinburgh FLS920 fluorescence
spectrometer. Photoluminescence (PL) emission spectra were measured
on a home-built setup consisting of a Ocean Optics QE65000
spectrometer operating at -20.degree. C., with either a 25 mW 325
nm CW He--Cd laser or a Spectraphysics Explorer 349 nm Nd:YLF
pulsed laser as excitation sources. The latter laser was operated
at 2.5 kHz repetition rate with a pulse length of .about.5 ns. The
power incident on the sample was tuned with a VBA-200 beam splitter
from Jodon Laser combined with a set of neutral density filters.
Emission was detected at 90.degree. angle to the incident laser
beam by collection with a collimating lens, passed through a
long-pass filter to remove residual laser light and then focused
into an optical fiber connected to the spectrometer. The sample was
oriented at a 120.degree. angle with respect to the incident beam
to prevent the specular reflection of the laser beam from entering
the collimating lens. Absolute external quantum efficiencies were
determined using a 6'' integrating sphere from Labsphere (model
RTC-060-SF) which was equipped with a center mount. The laser only
spectrum was taken with the center mount rotated parallel to the
beam, so that the beam did not touch the sample mount directly. For
the sample measurement, the beam hit the sample at 10.degree. C.
rotated with respect to the normal of the sample surface, so that
the specular reflection of the laser beam was kept inside the
sphere. Spectrometer, optical fibers and integrating sphere were
all calibrated with a LS-1-CAL calibration lamp from Ocean Optics,
to enable absolute irradiance measurements. Cathodoluminescence
(CL) was measured on a modified SEM. All optical characterizations
were conducted at room temperature.
[0080] Normalized PL emission spectra of ZnO+10% Mg (curve c),
ZnO:Al (10 ppm) (curve b) and Zn0.9Mg0.1+10 ppm Al (curve d) are
shown in FIG. 1. The PL from pure ZnO (curve a) is the typical
near-band gap emission (NBE) at .about.385 nm, with a very shallow,
broad emission in the visible that is generally attributed to
(oxygen related) defects. For all the other samples the situation
is reversed and the primary PL signal is a broad emission in the
visible, centered around 500-600 nm, again attributed to defect
emission. This visible emission is specifically not originating
from direct luminescence of the dopants themselves. Some minor NBE
signal is visible in the UV. It is especially noteworthy that the
addition of only a small amount of Al (10 ppm) (curve b) changes
the PL output completely from almost entirely NBE-emission to
almost entirely defect emission, with the peak maxima remaining
virtually unchanged from the host ZnO.
[0081] Some differences in the wavelength of maximum visible
emission between the different powders are observed: 520 and 585 nm
for Zn0.9Mg0.1O (curve c) and ZnO:Al (curve b), respectively, with
the sample according to the invention being in the middle at 555 nm
(curve d).
[0082] The excitation spectra of ZnO+10% MgO (curve c), ZnO:Al (10
ppm) (curve b) and Zn0.9Mg0.1+10 ppm Al (curve d) as measured with
an Edinburgh fluorescence spectrometer are shown in FIG. 2. The
excitation spectrum of ZnO could not be measured due to the very
low emission. It can be seen that the optimal excitation wavelength
is about 385 nm for the non-Mg containing powder which coincides
with the NBE emission of ZnO. The Mg containing samples have their
optimal excitation wavelength at 350 nm, but in both cases a
secondary peak is observed at 385 nm, which is especially high in
the ZnO/Mg case. This secondary maximum is again indicative that
the ZnO and MgO have not fully mixed.
[0083] Table I shows the results from absolute (external) quantum
efficiency (EQE) measurements on ZnO powders with various amounts
of Mg and/or Al, measured at 349 nm excitation. Absorption at this
wavelength is typically about 85% of the incident light. The power
of the laser was tuned so as to be in a regime where the emission
varied linearly with the intensity. It is immediately clear that
having none or only one of Mg and Al present in the powder results
in only limited quantum efficiency. When both are present, a large
increase in EQE is observed.
TABLE-US-00001 TABLE I External quantum efficiencies (%) of Zn(1 -
x)MgxO:Al powders as a function of composition. Excitation with 349
nm laser. % Mg ppm Al 0 1 5 7.5 10 15 20 0 0.8 2.1 3.0 2.5 10 2.5
5.6 13.7.sup.a 8.7 20 14.7 40 15.3 23.7 70 10.6 100 2.0 9.4 1000
1.1 8.0 .sup.aan earlier batch of powder, that was used to prepare
the target for PLD and sputtering, was found to have an EQE of
9.8%.
[0084] The dependence on the Al content is intriguing. Only a tiny
amount (.about.10 ppm) is needed to increase the EQE of the ZnO/Mg
powder, and adding (much) more has no substantial effect or may
lead to other undesired properties, like a too large electric
conductivity. Hence, an amount of at maximum 200 ppm, especially at
maximum 100 ppm seems beneficial.
[0085] Normally for a phosphor at low activator content, the PL
output increases linearly with doping content as the emission
competes with non-radiative processes in the host lattice. This
linearity generally remains until concentration quenching sets in,
typically above a few percent dopant, as at such higher
concentrations the dopant centers start to interact by processes
like Auger recombination. The dependence on Mg content is also
found to be non-linear.
[0086] From the Zn0.9Mg0.1O+10 ppm Al 400 nm thin layers were
deposited on sapphire substrates by PLD and RF sputtering. Analysis
of the sputtered layers by XRF and SIMS showed the Mg and Al
content to be 9.6% and 14 ppm respectively, so the concentration of
both dopants is more or less preserved during the deposition
process. X-ray analysis showed both deposition techniques to afford
essentially epitaxial layers.
[0087] While the layers were deposited at elevated substrate
temperatures (500.degree. C. for PLD, 450.degree. C. for
sputtering), the PL of the as deposited layers was low. It was
found that annealing of the samples was required to achieve maximum
luminescence, as is shown for both types of deposition. The minimum
temperature for maximum PL appears to be 900.degree. C. for both
samples, although there is a marked difference in the evolution of
the PL as a function of anneal temperature for the two deposition
techniques.
[0088] For the PLD sample, at 700.degree. C. there appears to be an
intermediate stage where 2 peaks are visible in the PL spectrum.
After anneal at 900.degree. C., the spectrum is more or less
identical to that of the parent powder. Above 900.degree. a slight
apparent increase in PL output could still be observed. The sample
itself however exhibited formation of a haze in the formerly
transparent sample according to the invention layer. SEM showed
this haze to be due to the presence of slightly larger
`crystallites` that have grown at elevated temperatures. Cracks
were not observed. This haze affect is likely to lead to a more
efficient outcoupling of the light normal to the plane of the
sample according to the invention layer (where the PL emission is
measured). The sputtered layers were found to remain clear upon
annealing up to 1100.degree. C. SEM pictures from a sputtered
aluminum doped zinc magnesium oxide layer before (a) and after (b)
annealing at 1100.degree. C. are shown in FIG. 3 (a and b,
respectively).
[0089] In order to answer the question if annealing at higher than
900.degree. really results in higher output or if the hazing effect
clouds the issue, for both types of deposition techniques the
absolute EQE as a function of anneal temperature was also
determined. The results are listed in Table II, and indeed the EQE
at 1000.degree. C. anneal is slightly lower than at 900.degree. C.
(although the values are close to the detection limit). A similar
anneal experiment was performed for Zn0.85Mg0.15O+40 ppm Al where a
similar trend was observed, as well as higher EQE values. The
optimum temperature was found to be 950.degree. C., in line with
the data for Zn0.9Mg0.1O+10 ppm Al.
[0090] Table II reflects systems wherein the layers have the
nominal composition Zn0.9Mg0.1O:Al (10 ppm) and Zn0.85Mg0.15O:Al
(40 ppm).
TABLE-US-00002 TABLE II Absolute EQE (at 349 nm excitation) for
samples according to the invention layer deposited on sapphire,
versus anneal temperature. Anneal performed in air for 30 minutes.
Absolute QE (%) Absolute QE (%) Zn.sub.0.85Mg.sub.0.15O: Al Anneal
Zn.sub.0.9Mg.sub.0.1O: Al (10 ppm) (40 ppm Al) Temperature
(.degree. C.) (PLD) (sputter) 500 (as deposited) 0.26 700 0.55 900
1.10 1.64 950 7.23 1000 0.97 6.13 1050 4.32 1100 0.9 1150 0.46
[0091] In the case of the sputtered layer, two things become
apparent. Firstly, the wavelength of maximum emission is red
shifted some 50 nm with respect to the parent powder emission.
Secondly, upon annealing at increased temperatures, a second peak
starts to appear at 480 nm. Upon further annealing, the 480 nm peak
starts to disappear again and a slight blue shift of the main peak
is observed. At the highest anneal temperature (1100.degree. C.)
the 480 nm peak is completely gone and the main peak has shifted to
550 nm. The resulting PL spectrum is completely identical to a
powder sample according to the invention. It appears that
sputtering results in different phases in the layer, and annealing
at 1100.degree. C. is gives best results.
[0092] Apart from the temperature, the effect of the annealing
atmosphere was also checked. Identical samples of sample according
to the invention on sapphire, deposited by deposition, were
annealed in different atmospheres (neutral, reducing and oxidizing)
for 1 hour at 650.degree. C. Note that this lower temperature was
dictated by the requirements of one of the electrode materials
(ZnO+2% Al). The PL output was measured using the qualitative part
of the setup as the EQE's were generally below the detection limit
of the quantitative setup. As the outcoupling characteristics of
the samples were similar, this still affords a good comparison of
the emission. For most atmospheres, the maximum emission was
observed at 610 nm. In several samples a shallow shoulder was
observed at 790 nm that was especially visible in the vacuum
annealed sample. The relative results of the PL output are listed
in Table III, with the sample annealed for 1 hour in air set at
100%. The conductivities of the layers were also determined.
TABLE-US-00003 TABLE III relative PL output and conductivity of PLD
samples according to the invention-10 layers on sapphire, as a
function of the anneal atmosphere. Anneal done for 1 hour (unless
stated otherwise) at 650.degree. C. and atmospheric pressure. Sheet
resistance Anneal atmosphere Relative photon flux (%)
(M.OMEGA./square) As deposited (500.degree. C.) 0.6 1E+5 Air (1
hour) 100.0 <1E+4 Air (64 hour) 96.8 3E+4 Argon 97.5 4E+1 Oxygen
64.1 3E+4 5% hydrogen in argon 1.1 8E+4 vacuum 42.3 1E+1 NH.sub.3
6.7 1E+4 Nitrogen (dry) 83.8 1E+1
[0093] From Table III it is clear that ambient air affords the best
performing samples for PL output. Upon annealing for prolonged
periods of time in air, a slight decrease in performance is
observed as well as a small redshift of the emission to about 630
nm. The `neutral` atmospheres argon and nitrogen provided results
similar to air. Vacuum and pure oxygen, had roughly half the output
of the air sample, presumably by both influencing the (number of)
oxygen vacancies in a non-ideal way. The reducing atmospheres
(H2/Ar and NH3) had severely diminished output, presumably by
removal of oxygen from the sample according to the invention
layer.
[0094] The sheet resistance of the layers was generally high
(10-100 G.OMEGA./square) for all atmospheres barring the `neutral`,
non-oxygen containing ones (vacuum, argon, nitrogen) where it was 3
orders lower.
[0095] Hence, a new type of zinc oxide based phosphors has been
prepared by incorporating both MgO (e.g. up to 15%) and a trace
(e.g. 10-40 ppm) of Al as dopants. These phosphor powders showed
visible emission and an order of magnitude increase in quantum
efficiency compared to ZnO with no or only one of Mg and Al
present. The phosphors proved robust to thin layer deposition
techniques such as PLD and RF sputtering. Annealing in air at
elevated temperatures (up to 900-1100.degree. C. depending on the
deposition technique) was found to be very beneficial for
integration of all the substituent materials in the thin layers and
increase the photoluminescence. The enhanced emission in both
powder and thin layer could not be attributed to direct emission of
the additives, but is thought to stem from radiating defects in the
ZnO lattice, most likely oxygen-related. Only band edge excitation
was observed, which was further corroborated by CL, showing that
these phosphors operate through energy absorption by the host
material, followed by energy transfer to the radiant defect and
subsequent emission, making these materials potential candidates
for the emissive layer in large area LEDs.
[0096] Herein, we further present a generic solution toward
achieving light-emission from devices that are made of thin-films
of ZnMgO:Al phosphor sandwiched between two/or more layers.
Functional ZnO LEDs are demonstrated, with EL spectra that match
that of the ZnO phosphor.
[0097] For detailed preparation of emissive material, we refer to
the above. Here a short explanation of the phosphor preparation is
given. For aluminium doped Zn(1-x)MgxO first a desired amount of
aluminium nitrate nonahydrate (p.a., Merck) was dissolved in a
small amount of deionised water and diluted with 200 ml ethanol.
Next desired amounts of zinc oxide (5N purity, Aldrich) and
magnesium oxide (FO Optipur, Merck) were added and the obtained
suspension was dried using a rotary evaporator. The compositions
were put into an aluminium oxide crucible and fired inside a
chamber furnace in air for 8 hours at 1100.degree. C. using a
heating and cooling rate of 200.degree. C./hour. After cooling down
the powders were grinded. After firing once again at 1100.degree.
C., targets suitable for sputtering and pulsed laser deposition
(PLD) were prepared.
[0098] Thin films of ZnO phosphor were RF magnetron sputtered on a
variety of substrates. Thin films of other metal oxides were either
sputtered of physical vapor deposition. First 400 nm thin films of
ZnO phosphor was grown on ITO coated epi-polished a-cut or c-cut
sapphire substrates by PLD or RF magnetron sputtering. The base
pressure of the PLD system was 2.times.10-7 mbar. During the
deposition the substrate temperature was between 25.degree. C. and
550.degree. C. and the partial oxygen pressure was 0.2 mbar. The RF
magnetron sputtering system had a base pressure of 6.times.10-7
mbar and the used substrate conditions were either 25.degree. C.,
450.degree. C. or 550.degree. C. The gas flows during the
sputtering process were resp. 78 and 2 sccm for Ar and O2,
respectively. The total pressure was 0.038 mbar, and the RF power
was 60 W, and the bias voltage was around 250V. Next a layer of
metal-oxide was deposited on to the phosphor layer and then metal
contacts were deposited. Devices were annealed and then
measured.
[0099] Photoluminescence (PL) emission spectra were measured as
defined above.
[0100] Electrical measurements were conducted in a dark chamber at
ambient. Light emission from the devices was recorded using a
photo-diode. Current-voltage characteristics of the diodes were
recorded using HP semiconductor analyzer. To record the EL spectrum
of the LED, the Ocean Optics QE65000 spectrometer operating at
-20.degree. C. was used. The emitted light from the LED was fed
into an optical fiber that was mounted on top of the emissive area
and connected to the spectrometer.
Sputtered Thin Layers
[0101] The RF magnetron sputtering was used to sputter thin films
of different variation of Zn0.90Mg0.10O (ZAM-10) and Zn0.85Mg0.15O.
The phosphors used here were doped with Al in range of 0 to 100
ppm. The range of Al doping can be higher. The substrate
temperature could be controlled during the deposition. Many
phosphor compositions were made, measured and used in devices.
Thin-film deposition conditions were varied, e.g. substrate
temperature, from RT, to .about.500.degree. C. Here we only present
the results on the Zn0.85Mg0.15O doped with 40 ppm Al (ZAM-40)
deposited at RT.
[0102] Thin film sputtering was conducted at a base pressure of
6.times.10-7 mbar. The substrate temperature during deposition was
kept at room temperature. The RT substrate temperature was
justified by our investigation that showed samples having different
substrate deposition temperature have similar PL after annealing at
T>550.degree. C. Hence the choice of low substrate temperature
is justified.
[0103] Sputtered films were prepared on Sapphire and ITO-coated
Sapphire substrates. After deposition each substrate was subjected
to annealing at one particular temperature. Thus no thermal
histories were present for the samples. The annealing temperature
was varied between RT up to 1150.degree. C. for 30 min in ambient.
After annealing samples were cooled down relatively slowly for
10-15 min in ambient air. Subsequently PL and EQE were measured.
Later XRD and AFM were performed.
[0104] Primary results of the PL measurements are given in FIG.
4a-4b, where PL is measured as a function of post-annealing for RT
sputtered thin film of Zn0.85Mg0.15O doped with 40 ppm Al, on the
sapphire and ITO-coated sapphire substrates. Deposition of the
phosphor layer at room temperature results in low PL emission as
shown in the insets of FIG. 4a-4b. It is clear that post-annealing
of the films have a profound influence on the PL spectra, as the
emission enhances with increasing annealing temperature. However
there is an optimum for the anneal temperature. It seems that there
is an optimum annealing temperature is between 900-1000.degree. C.,
where the PL response maximizes.
[0105] The optimum of post-anneal temperature for ZAM/sapphire was
determined by EQE measurement of the different samples. The results
of the EQE measurements as function of temperature, is given in
FIG. 5. It seems that the best annealing temperature for
ZAM-40/sapphire is 950.degree. C., where EQE exceeds 7.2%. EQE of
the sputtered thin-film of ZAM-40 is almost a factor of 2 larger
than that of the epitaxially grown GaN (4%).
[0106] In fabrication of the LED however the ZAM layer is deposited
on to another layer of either metal or metal-oxide which acts as
the electrode for charge injection into the device. Therefore PL
response of the ZAM layer could be different. To this point ZAM-40
was deposited onto ITO-coated sapphire. PL spectra is given in FIG.
4a. The only effect of the ITO seems to be red-shifting the defect
emission peak of the ZAM-40 from 550 nm to >600 nm. The initial
red-shift gradually decreases toward the original defect emission
of ZAM-40 (FIG. 4a) as the annealing temperature rises. At
900.degree. C. however the shift of the defect emission peak toward
lower wavelengths stops and PL abruptly changes. This abrupt change
in the PL spectra has to do with the fact that ZAM-40 at
temperatures higher than 900.degree. C. start to form alloy with
ITO hence changes the PL spectra. It is however of interest to see
whether presence of ITO hampers the light emission from the ZAM-40
layer. To do so, we calculated photon flux emitted from ZAM-40
deposited on both sapphire and ITO-coated sapphire and compared
both.
[0107] In FIG. 6 a comparison has been made for the photon flux
(PF) of the ZAM on sapphire, and ITO/sapphire. Presence of the ITO
does not compromise on the optical performance of the ZAM layer up
to 800.degree. C. At the same time this figure shows that annealing
temperatures in the range of 400.degree. C. to 800.degree. C. have
a very negligible influence on the PL emission of the ZAM. At
400.degree. C. the phosphor is already activated. The lower
performance of the ZAM/ITO/sapphire in compare to ZAM/sapphire, at
temperatures higher than 800.degree. C. is due to the degradation
of the ITO and possibly formation of ZAM:ITO alloy. For
ZAM/sapphire substrates, there is a rise in photon flux with a
maximum at around 950.degree. C.-1000.degree. C., indicating the
optimum annealing temperature. Surprisingly, light emission from
both samples is the same and the best of phosphor activation is
reached when ZAM is annealed up to 950.degree. C.-1000.degree. C.
ITO however cannot withstand these high temperatures. Application
of metals or conducting metal-oxide which can stand high annealing
temperature would be advantageous in this respect, as it allows
full activation of phosphor in real devices.
PL Spectra of ITO/ZAM/Insulating-Oxide Stack
[0108] The first question to be addressed here is whether
deposition of an extra oxide layer would change the emission
spectra of the ZAM layer. To do so, we sputtered ZAM onto the
ITO-coated substrate. As a test model, we deposited 5 nm and 10 nm
of ZrO onto the ZAM layer. The substrates were annealed at
600.degree. C. for 30 min and slowly cooled down. The respective PL
spectra of the samples are shown in FIG. 7a-7b. Clearly insertion
of the ZrO layer does not change the PL spectra. The intensity
however seemingly drops slightly in the presence of ZrO layer.
Excluding all the experimental and instrumental errors, one
possible reason will be less light out-coupling when an extra ZrO
oxide layer is incorporated onto the stack.
[0109] To further investigate whether the top insulating layer
influences the PL of the ZAM layer, we deposited MgO layer onto the
ZAM layer and subsequently annealed the stack at 800.degree. C.
FIG. 8 shows the PL spectra of the ZAM layer capped with MgO layer
in comparison with a bare ZAM. Clearly there is no influence of the
insulating MgO layer on the PL of the phosphor even after annealing
at 800. Incorporation of an insulating layer into the diode stack
therefore has no influence on the PL spectra of the emissive ZAM
layer. In fabrication of the diodes we therefore tried different
insulating metal oxides such as, MgO, MoOx, V2O5, NiOx and ZrO.
Experiments with SiO2 (SiOx) and other oxides were also conducted,
and similar results were obtained.
Fabrication of ZnO LEDs
[0110] Here, a diode is realized by incorporation of an insulating
layer in the device stack, i.e. metal-insulator-semiconductor-metal
(MISM) diode. Typical diode layout is shown in FIGS. 9a-9b.
However, other configurations may also be possible (including an
inverted structure).
[0111] In the following we present the data obtained for MISM ZnO
diode fabricated with the sputtered thin films of Zn0.75Mg0.15O
doped with 40 ppm Al. We used different substrates, e.g. sapphire,
quartz and glass. Here only the results of devices fabricated on
sapphire substrate are presented. The operation mechanism of the
diode is discussed in the later section.
[0112] As cathode we used both Al doped ZnO and ITO both sputtered
onto the substrate. We note that any metal, or transparent
conductive metal-oxide can be used as cathode. ZnO:Al however is
advantageous as it provides a good template for ZAM growth. In most
of our experiments we used ITO as cathode. Thermal annealing at
temperatures .about.600.degree. C. was performed to activate the
phosphor. Sputtered ITO on glass did show very little degradation
in sheet conductivity upon annealing up to 750.degree. C.
Conductivity varied from 30 at RT to 75 .OMEGA./square for ITO
annealed at 750.degree. C. Glass however, is not stable at
T>700.degree. C. Therefore we used either ITO coated sapphire or
ITO coated quartz as substrate for ZAM growth and device
fabrication.
[0113] In the next step we introduced the Pt pad on the ITO-coated
sapphire with shadow mask evaporation followed by ZAM deposition.
We note that it in our experiments the Pt-cathode pads were masked
from the ZAM layer. We do not expect however significant
differences if the Pt-contact pads are in touch with the ZAM layer.
In the next step either a combination of metal contacts e.g. Ni/Au,
or a combination of metal-oxide/metal contacts were introduced as
anode. Later annealing of the device was performed to activate the
phosphor and to form the contact.
[0114] We note that annealing of the devices is another crucial
step in device fabrication. In order to fabricate reproducible
device, first the contacts were deposited and then annealed at the
desired temperature. Subsequent slow cooling down process of the
substrate to RT is vital. Rapid cooling of the sample or deposition
of contact after annealing, both resulted in devices with symmetric
I-V characteristics and no light emission.
[0115] Here we present the results obtained with magnesium oxide
(MgO), molybdenum oxide (MoOx), vanadium oxide (V2O5) and zirconium
oxide (ZrO). We note that the same results were obtained with other
insulating blocking layers in combination with different anodes.
Moreover ZnO LEDs with the MISM layout can also be fabricated in an
inverted structure. An example would be ZAM deposited onto p-type
Si with a few nm thick SiOx oxide layer.
Electrical Characterization of ZnO LEDs
[0116] In this section we present electrical characteristics of
MISM ZnO diodes. Current-voltage characteristics and
electroluminescent spectra for sapphire/ITO/ZAM/MgO/Au diode are
given in FIGS. 10a-10b. The I-V characteristic of the device,
measured in dark, shows that the diode is rectifying. The
rectification ratio however is not large due to the leakage
current. The primary target here is demonstration of a functional
diode and electroluminescence. A photo-detector (photo-diode) was
placed over the ZnO diode to record the light emission of the
device. In dark we measured light with the photo diode. The power
dissipated in the ZnO LED was less than 0.5 W, hence just not
enough to record a measurable black body radiation. We measured the
electroluminescent of the ZnO LED in forward bias of 10 V. The
current running through device was 50 mA. An EL spectrum of the
device is given in FIG. 8. The PL spectrum of the ZAM is also
presented as a reference.
[0117] FIGS. 10a-10b show nice matching of the EL spectrum of the
ZnO LED with the PL of the ZAM thin film. It is really intriguing
to note that the EL shows a peak at 358-360 nm. This EL peak is
exactly at the position where near-band edge emission of the ZnO in
thin films of ZAM takes place. Moreover the peak at 670 nm also
nicely matches with the emission of the ZAM phosphor. Presence of
358-360 peak unambiguously demonstrate that hole injection is
achieved with the MISM structure. The MISM device layout is
therefore viable to overcome the challenge that has impeding
arriving at ZnO LED for more than 60 years. To further prove that
the MISM concept is generic for ZnO LEDs, in the next step we used
MoOx as a blocking layer. The device layout therefore was
sapphire/ITO/ZAM/MoOx/Au.
[0118] In FIG. 11, we have only presented the EL spectra of the
device. El was measured at 12.5 V and a good spectral match between
the PL of ZnO thin film and EL is achieved. Once again, presence of
the near-band edge emission in the EL spectra indicates successful
achievement of hole injection into the valance band of the ZnO.
[0119] It is highly desirable to have a blocking layer that first,
is stable in air, and second, does not intermixes with the
underlying ZnO phosphor layer upon annealing. A good candidate for
such layer is ZrO, which is a stoichiometric oxide with very
limited solubility in ZnO. ZnO diodes were fabricated with ZrO
blocking layer. The device stack was sapphire/ITO/ZAM/ZrO/Au. ZrO
layer was sputtered from Zr target in an oxidizing atmosphere.
FIGS. 12a-12b, show the I-V characteristics and EL spectra of the
respective ZnO diodes. The diodes show an excellent rectification
behavior as well as decent light emission. The power dissipated in
the diode was .about.50 mW. The emitted light therefore cannot be
infra-red emission due to heat dissipation, as shown by the EL
spectra of the device. EL spectra were recorded before and after
annealing of the diode. Before annealing, EL shows a peak at 900
nm, and it is not due to heat dissipation. Upon annealing, several
peaks appear in the EL spectra of the device, with the first peak
being at .about.650 nm. In comparison with the PL spectra of the
thin-film with ZrO layer on top, it seems that the main emission
peak at 600 nm is red shifted. Moreover the near band edge emission
peak of the ZAM layer is not present in the EL spectra.
Additionally few other peaks are present in the EL. We believe that
sputtering of the ZrO layer on to ZAM has caused damages at the
ZAM/ZrO interface. Due to soft bombardments of the ZAM interface,
shallow diffusion of the Zr or ZrO into the ZAM layer could
potentially change the EL spectrum by causing more surface
recombination, which is manifested by appearance of the new peaks.
Physical vapour deposition of ZrO (and potentially all the blocking
layers) onto the ZAM layer is therefore recommended for having good
spectral match.
[0120] The EL-spectra presented here are among the first EL spectra
reported for ZnO LEDs. The I-V characteristics and EL spectra
achieved for the ZnO LEDs demonstrate the viability of the MISM
device layout.
Light-Emission Mechanism of ZnO LEDs
[0121] A tentative mechanism is presented in FIG. 13 for the MISM
stack with highly p-type doped Si as the anode and SiOx as the
blocking contact. Similar mechanism is at work when p-type Si is
replaced with a metal electrode as anode.
[0122] The energy band diagrams of the diode at equilibrium and
under bias are shown. When positive forward bias is applied on the
anode, here p-type Si, the bands of Si near the Si/SiOx interface
will bend upward. The band bending at the Si/SiOx interface will
gradually induce an inversion layer for n-ZnO/SiOx/p-Si diodes,
which is responsible for the hole injection. As a result,
accumulated holes in the inversion layer could tunnel through the
barrier into the valence band of ZnO and recombine with the
electrons in ZnO conduction band that are blocked by the SiOx
interface layer, resulting in UV emission of 359 nm as well as the
visible emission at 600 nm.
[0123] A zinc oxide light emitting diode based on a newly developed
zinc oxide phosphors has been demonstrated. These phosphor thin
film showed visible emission. The phosphors proved robust to thin
layer deposition techniques such as PLD and RF sputtering.
Annealing in air at elevated temperatures (400-1100.degree. C.) was
favorable to increase the photoluminescence and initiate the
electroluminescence. To fabricate ZnO LED we used a blocking layer
between the anode and the emissive layer. The blocking layer
impedes the electron to arrive at the anode from the ZnO layer.
Accumulation of the electron enhances hole injection and hence the
LED begin the light emission.
[0124] The recorded electroluminescence and the photoluminescence
spectra of the ZnO thin film and ZnO LED match nicely.
Interestingly even band gap emission of the ZnO is present in the
EL spectra, which indicates that hole injection has been
successfully achieved by incorporation of the blocking layer. The
enhanced emission in ZnO thin layer could not be attributed to
direct emission of the additives, but is thought to stem from
radiating defects in the ZnO lattice, most likely oxygen-related.
Only band edge excitation was observed, which was further
corroborated by CL, showing that these phosphors operate through
energy absorption by the host material, followed by energy transfer
to the radiant defect and subsequent emission. The combination of
the material and device presented here makes ZnO phosphors an
attractive potential candidate for the large area LEDs.
[0125] As insulating layers, SiO2, MgO and ZrO were tried, and they
all worked.
* * * * *