U.S. patent application number 12/822658 was filed with the patent office on 2010-10-14 for nanostructured thin-film formed by utilizing oblique-angle deposition and method of the same.
This patent application is currently assigned to National Chiao Tung University. Invention is credited to Chia-Hua Chang, Ching-Hua Chiu, Hao-Chung Kuo, Chin-Sheng Yang, Pei-Chen Yu.
Application Number | 20100261001 12/822658 |
Document ID | / |
Family ID | 41681456 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100261001 |
Kind Code |
A1 |
Chang; Chia-Hua ; et
al. |
October 14, 2010 |
NANOSTRUCTURED THIN-FILM FORMED BY UTILIZING OBLIQUE-ANGLE
DEPOSITION AND METHOD OF THE SAME
Abstract
The present invention discloses a transparent conductive
nanostructured thin-film by oblique-angle deposition and method of
the same. An electron beam system is utilized to evaporate the
target source. Evaporation substrate is disposed on a plurality of
adjustable sample stage. Multiple gas control valve and heat source
is provided to control the gas flow and temperature within the
process chamber. An annealing process is performed after the
evaporation to improve the thin-film structure and optoelectronic
properties.
Inventors: |
Chang; Chia-Hua; (Taipei
County, TW) ; Yang; Chin-Sheng; (Taipei City, TW)
; Chiu; Ching-Hua; (Taipei City, TW) ; Yu;
Pei-Chen; (Hsinchu City, TW) ; Kuo; Hao-Chung;
(Hsinchu City, TW) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE, FOURTH FLOOR
ALEXANDRIA
VA
22314-1176
US
|
Assignee: |
National Chiao Tung
University
Hsinchu City
TW
|
Family ID: |
41681456 |
Appl. No.: |
12/822658 |
Filed: |
June 24, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12289816 |
Nov 5, 2008 |
|
|
|
12822658 |
|
|
|
|
Current U.S.
Class: |
428/315.5 ;
977/700 |
Current CPC
Class: |
H01L 31/022466 20130101;
Y02E 10/50 20130101; Y10T 428/249953 20150401; H01L 31/022475
20130101; C23C 14/226 20130101; H01L 31/022483 20130101; Y10T
428/249978 20150401; C23C 14/086 20130101; H01L 31/1884
20130101 |
Class at
Publication: |
428/315.5 ;
977/700 |
International
Class: |
B32B 3/26 20060101
B32B003/26 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 14, 2008 |
TW |
097131039 |
Claims
1. A film formed by electron bean oblique-angle deposition, wherein
said film comprises porous nanorods with narrow end to form a
single layer structure with graded refractive index and better
transmissivity.
2. The film of claim 1, wherein said target source comprises indium
tin oxide (ITO), aluminum zinc oxide (AZO), or ZnO.
3. The film of claim 1, wherein said evaporation substrate
comprises Si substrate, GaAs substrate, glass substrate, or
flexible substrate.
Description
[0001] This application is a Divisional of application Ser. No.
12/289,816 filed on Nov. 5, 2008, the entire disclosure of which is
herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention generally pertains to an evaporating
method, more particularly, relates to a oblique-angle depositing
method to form nanostructured thin-film used as functional
electrode of optoelectronic device.
DESCRIPTION OF THE PRIOR ART
[0003] Due to the influence of sustainable energy and energy saving
policy, solar cell and light-emitting diode have become two major
technologies with revolutionary development and prospect in the
research of optoelectronic device. Solar cell is used for
transducing the light energy directly into electric energy.
Light-based energy source not only prevents the contamination issue
resulted from the thermal electric generation, but also provides
inexhaustible energy supply. In the illuminating application of
light-emitting diode (LED), light-emitting diode device has longer
lifetime or duration, compact size, and its luminous efficiency is
far higher than the conventional bulb illumination. Therefore LED
is quite adaptive to the energy saving tendency in 21.sup.st
century.
[0004] In the research of two major optoelectronic technologies,
the manufacture of device and the material development are always
the bottleneck and the research direction for the industry. As far
as the manufacture of solar cell device is concerned, if we want to
increase the light-receiving surface of the device, the area of
metal surface electrodes is inevitably reduced. The decrease of
metal electrode number will reduce the amount of collected
photocurrent. On the other hand, if we want to collect more
photocurrent, the disposition area of metal electrode is
necessarily expanded, this approach also shrinks the
light-receiving surface. This problem also occurs in the
application of light-emitting diode. Accordingly, the tradeoff
relation between light-receiving surface and metal electrode area
limits the conversion efficiency of optoelectronic device.
[0005] In the manufacture of optoelectronic device, a transparent
conductive oxides (TCO) is coated on the device surface to increase
the capability of collecting photo charges or current spreading.
Conventional TCO film is provided with transmissivity of
80%.about.90%. In order to increase the transmissivity of
optoelectronic device and achieve better device efficiency,
additional processes are introduced in the manufacturing, such as
the antireflection coating.
[0006] The manufacture of antireflection layer and device electrode
is two different processes. Traditionally, TCO film is coated by
sputtering or depositing, and the antireflection layer is growth by
depositing. Recently, several researches in connection with the
growth of nanostructured antireflection film are proposed in
academy, but the complicated process is still the bottleneck to
overcome. Furthermore, TCO film is always the necessary layer to
optimize and integrate the overall optoelectronic property, which
increase the additional cost. If antireflection layer has electric
conductivity, it can further increase the conversion efficiency of
the optoelectronic device, or totally replace the electrode.
[0007] The prior research investigates the antireflection layer
with graded refractive index and their broadband antireflection
characteristics in air ambient. For the antireflection layer of
optoelectronic device, the reflectivity in the interface is usually
changed with the wavelength of incident light. The incident surface
only allows the incident light with certain wavelength spectrum to
pass, thus the actual light quantity entering into device is
significant reduced. In addition, the incident angle is also a
critical factor for the reflectivity change. Research shows that
the reflectivity will boost when the light incident angle greater
than a specific value, but it also simultaneously decrease the
light transmissivity, causing the degradation of conversion
efficiency in optoelectronic device. Fresnel reflection law shows
that the light will have more possibility to be reflected if the
difference between two refractive indices is larger when light
passes through two different mediums, and devices will loss more
energy when more light is reflected. For this reason, the film
material must have the refractive index which is very close to the
air refractive index of 1 when operating in the air ambient. There
is no solid material with refractive index between 1.about.1.4 in
the nature. Though some artificial porous material can reach this
requirement, their film thicknesses are not thin enough to use as
the optical film. Refractive index not only influence the amount of
light refracted, but also influence the property of light
reflection and diffraction. In general, the depositing is often
used to form high-quality thin-film with low refractive index (i.e.
close to 1). A thin-film with omni-directional, broadband
characteristic is highly desirable to eliminate the Fresnel
reflection. Recently, Many approaches of forming thin-film with
graded-index profile are proposed, especially those used in the air
ambient, including chemical etch-leaching process on glass surface,
recently reported sol-gel process, interference-patterning by two
coherent light beams, and so on. However, Most of these methods
don't have good control over the graded-index profile, and some
method even take more complicated process to realize. A letter in
journal of nature photonics (Vol. 1, March 2007) discloses a
optical film material with broadband, low refractive index to
eliminate the Fresnel reflection. The graded-index profile is
achieved in this research by forming multilayer coating with
different composition, such as SiO.sub.2 and TiO.sub.2. Multilayer
structure implies that multiple processes are required, and their
manufacturing cost is higher.
[0008] On the other hand, a good transparent electrode must be
provided with excellent transmissivity and low resistivity to
enhance external quantum efficiency and conversion efficiency, as
well as restrain the heat production. Transparent electrode with
large disposition area and low resistivity has better current
spreading effect, it's luminescence efficiency may further
increase.
[0009] Accordingly, efficient control over the distribution of
graded refractive index in optical film is very important for
making high-quality thin-film with broadband, low resistivity, high
transmissivity characteristics, which is high desired in many
application.
SUMMARY OF THE INVENTION
[0010] The present invention discloses a method of forming
nanostructured thin-film of transparent conductive oxide by
oblique-angle deposition. The electron-beam oblique-angle
deposition technology is used in embodiment of present invention; a
proper amount of oxygen and nitrogen are introduced during the
process, following a thermal annealing to form thin-film on Si
substrate and glass substrate. Nanostructured film has broadband
characteristics span from visible spectrum to near infra-red
spectrum, as well as the high transmissivity, and excellent
electric conductivity to enhance the charge collection and current
spreading. This process may be widely used in the manufacture of
transparent electrode in semiconductor device, such as solar cell,
light-emitting diode, to increase their conversion efficiency.
[0011] In one embodiment of present invention, the electron-beam
vapor deposition (i.e. evaporation) is utilized to form electric
conductive thin-film with porous and nanorod structure used as
functional electrode. The electron-beam evaporating system in
present invention is provided with at least one sample stage which
can adjust its tilt angle, a target material is used as the
evaporation source of transparent conductive oxide, and a upper
electrode evaporating substrate. Besides, the temperature and
vacuum pressure are controlled during the process. The apparatus
described herein is a conventional skill, no more unnecessary
detail will be provided in the following description. In the
deposition process, functional electrode with porous or pillar
structure is formed by adjusting the gas flow introduced, chamber
temperature, and angle of deposition substrate, and a following
thermal annealing process is performed to achieve better
optoelectronic property.
[0012] In another embodiment of present invention, a method of
forming porous and pillar conductive thin-film is provided by
electron beam evaporation. In the embodiment, the deposition
temperature is controlled between 100.degree. C. to 450.degree. C.,
and the vacuum pressure is maintain during the process. The
deposition source includes Indium Tin Oxide (ITO), aluminum zinc
oxide (AZO), ZnO, and other material capable of forming transparent
conductive thin-film. The substrate used as upper deposition
substrate includes: Si substrate, GaAs substrate, glass substrate,
flexible substrate, etc. The sample stage can change its tilt angle
between 0.degree..about.90.degree., wherein the angle ranging
between 50.degree..about.90.degree. is preferred. The process gas
includes oxygen, nitrogen, or the combination thereof, wherein the
gas flow is controlled at 0 sccm to 50 sccm (standard cubic
centimeters per minute), and the chamber pressure is controlled
between 10.sup.-3 torr to 10.sup.-6 torr. The following thermal
annealing process lasts 1.about.60 min, the temperature is maintain
around 200.degree. C. to 900.degree. C. Oxygen is also introduced
during the annealing process.
[0013] One aspect of present invention is to provide a
nanostructured conductive thin-film with broadband, high
transmissivity, low reflectivity, as well as low sheet resistance,
to increase the amount of light received and provide excellent
conductive auxiliary electrode characteristic. The nanostructured
thin-film has larger light divergence angle to enhance current
spreading, which is quite adaptive in the optoelectronic device
such as light-emitting diode, and solar cell.
[0014] Another aspect of present invention is to provide a method
of forming nanostructured thin-film. The electron bean oblique
angle deposition is utilized in present invention to form pillar
microstructure in nanometer scale.
[0015] The forgoing forms and other forms, objects, and aspects as
well as features and advantages of the present invention will
become further apparent from the following detailed description of
the presently preferred embodiments, read in conjunction with the
accompanying drawings. The detailed description and drawings are
merely illustrative of the present invention rather than limiting
the scope of the present invention being defined by the appended
claims and equivalents thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1a illustrates the electron beam evaporation system by
oblique angle deposition to form transparent electrode film in
accordance with one embodiment of the present invention;
[0017] FIG. 1b illustrates schematic view of the evaporation
substrate disposed on the incident sample stage in accordance with
one embodiment of the present invention;
[0018] FIG. 2 illustrates the flowchart of forming nanostructure
conductive thin-film by electron bean oblique angle deposition in
accordance with one embodiment of the present invention;
[0019] FIG. 3a illustrates a schematic view of film microstructure
formed by oblique angle deposition in FIG. 2 in accordance with one
embodiment of the present invention;
[0020] FIG. 3b is the SEM image of nanorod structure of FIG.
3a;
[0021] FIG. 3c is the cross-section view of nanorod structure of
FIG. 3a;
[0022] FIG. 4 illustrates the wavelength distribution of nanorod
structure applied on glass substrate in accordance with one
embodiment of the present invention;
[0023] FIG. 5 illustrates the sheet resistance distribution of
nanorod structure applied on glass substrate in accordance with one
embodiment of the present invention; and
[0024] FIG. 6 illustrates the reflectivity distribution of nanorod
structure in accordance with one embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] The invention will now be described in greater detail with
preferred embodiments of the invention and illustrations attached.
Nevertheless, it should be recognized that the preferred
embodiments of the invention is only for illustrating. Besides the
preferred embodiment mentioned here, present invention can be
practiced in a wide range of other embodiments besides those
explicitly described, and the scope of the present invention is
expressly not limited expect as specified in the accompanying
Claims.
[0026] Referring now to FIG. 1a, it illustrates an electron bean
evaporating system 100 by oblique angle deposition to form
transparent electrode film in accordance with one embodiment of the
present invention. The system comprises: an external chamber 101 to
receive the components in electron bean evaporating system 100 and
define a process space. The pressure and temperature inside the
chamber are stably controlled to provide good process quality. A
central disc 102 disposed on the surface center inside the chamber
with a plurality of cantilever hanged therearound; A supporting
shaft 103 is connected on the center of central disc 102 and can
axially rotate with the central disc 102 to achieve better process
uniformity. Each cantilever 106 is provided with a rotating part
105 on the lower end to connect with a sample stage 104. The
evaporation substrate (114 in FIG. 1b) can be fixed on sample stage
104, and rotating part 105 is used to adjust the oblique angle of
sample stage 104. The oblique angle of sample stage 104 directly
changes the incident angle of evaporating particle to the
substrate, thus the property of final product is changed. A
crucible 110 is disposed right below the central disc 102 for
placing the target source to be evaporated; Crucible 110 is made of
high temperature material that would not react with the target
source placed therein during the process. A plurality of gas
control valve (ex. valve 107 and 108) is disposed in the chamber
101 to control the flow of various process gases. Furthermore, a
heat source 111 is disposed to control and maintain the process
temperature. A thermo couple 112 may also be optionally disposed in
the chamber 101 to measure the temperature. The dash line L in FIG.
1a shows the directions of evaporating particles incident to
evaporation substrates, the detail will be described in following
embodiment in FIG. 1b.
[0027] Referring now to FIG. 1b, which illustrates the schematic
view of evaporation substrate 114 placed on the sample stage 104.
The oblique angle deposition apparatus in present invention
comprises a electron beam evaporation system 100 to evaporate the
target source; Gas control valves 107 and 108 are disposed in the
chamber to control the gas flow; A heat source 111 is used to
maintain the temperature; Angle controlling device, i.e. rotating
part 105 in FIG. 1, is used to control the tilt angle of substrate
in oblique deposition process. As illustrated in FIG. 1b, the
incident angle is defined by the included angle 117 between the
incident direction 115 of target particle 118 and the normal
direction of deposition substrate 114. The incident angle of target
particle has significant influence on the physical property of
thin-film.
[0028] In another embodiment of present invention, a method of
forming electric conductive thin-film with porous and nanorod
structure by electron bean oblique angle deposition is disclosed.
An electron bean evaporation system with adjustable sample stage is
used to achieve the oblique deposition effect. As showed in FIG. 2,
the step comprises: 1. adjust and stabilize every parameter in the
process (201), such as the oblique angle of evaporation substrate,
the evaporation temperature in chamber, the flow of introducing
gas. For example, the oblique angle in the process is between
0.degree..about.90.degree., where in the angle of
50.degree..about.90.degree. is preferred; 2. After all parameter in
the process are stabilized, using electron beam evaporation system
to deposit the target material on the substrate (202). High
electric potential is applied in this step to accelerate the
electron and a magnetic field is established to control the
electron trajectory. The accelerated electron will collide with
target material (i.e. evaporation source), producing heat energy
for target material to be evaporated into gas. The target as will
apply on the substrate in the high-vacuum environment, as the
evaporation substrate 114 in the embodiment of present invention.
The target material or evaporation source used in present invention
includes Indium Tin Oxide (ITO), aluminum zinc oxide (AZO), ZnO,
and other material capable of forming transparent conductive
thin-film. The evaporation substrate in present invention includes
Si substrate, GaAs substrate, glass substrate, flexible substrate,
etc. During the process, evaporation substrate will be adjusted to
particular angle, the included angle between normal direction of
substrate and incident direction of target particle is controlled
within 0.degree..about.90.degree. to obtain the desired surface
transparent electrode structure, wherein the angle of
50.degree..about.90.degree. is preferred, as showed in the included
angle 117 in FIG. 1b. Generally, the gas type and flow both
influence the electrical property and optical property of the
thin-film to be deposited if process gas is introduced during the
evaporation, such as nitrogen, oxygen, or acetylene. In the
embodiment of present invention, the chamber is introduced with
oxygen, nitrogen, or the combination thereof during the evaporation
process. The gas flow is configured between 0 sccm to 50 sccm. The
evaporation system simultaneously maintains the chamber pressure
between 10.sup.-3 torr to 10.sup.-6 torr to achieve desired
electrical and optical property. The result will be described in
the following embodiment. Before evaporation process, the process
chamber will be preheated to process temperature. In the embodiment
of present invention, the temperature in electron beam evaporation
system is configured at about 100.degree. C. to 450.degree. C. When
deposition finished, a transparent conductive thin-film is formed
on the evaporation substrate. To obtain the desired thin-film
electrode structure, a thermal annealing process is applied after
the evaporation (203). The oxygen is introduced during the
annealing process, and the temperature is configured at 200.degree.
C. to 900.degree. C., the process duration is about 1.about.60
minutes; The thermal annealing process in present invention may
further increase the transmissivity of thin-film microstructure and
lower their resistivity.
[0029] Referring now to FIG. 3a, it illustrates the schematic view
of thin-film microstructure by oblique angle deposition. As showed
in figure, the thin-film grown by above-mentioned method has
nanorod structure 119, which is distributed uniformly on the
evaporation substrate 114. FIG. 3b and FIG. 3c illustrates the SEM
top image and cross-section image of the nanorod structure 119,
respectively. The nanorod structure 119 is the aggregation of
tread-like pillar, similar to the fiber in the cloth. The nanorod
structure is tightly distributed on evaporation substrate to form
compact, porous thin-film microstructure. Further, as showed in
FIG. 3a, the nanorod in present embodiment has narrow end. The
level closer to evaporation substrate has higher film density, and
the level near film surface has lower density. This kind of
structure can achieve the graded refractive index effect in one
single layer. On the other hand, the approach in prior art is by
performing multiple evaporation process on a single substrate to
form multi-layer thin-film structure, wherein the target material
of respective layer may be different. Comparatively, the advantage
of present invention is to achieve the graded refractive index
effect in one single layer by tuning the parameter of evaporation
process. Thus the required process material and cost is reduced,
and the excellent electrical property of transparent thin-film
electrode is achieved.
[0030] The nanostructure thin-film formed by the method of present
invention is provided with excellent transparent electrode
property. FIG. 4 shows the transmissivity distribution of
nanostructure thin-film applied on glass substrate. As shows in the
figure, the transparent thin-film electrode formed in present
invention has the transmissivity up to 98% under incident light
with wavelength ranging from 450 nm to 800 nm, showing the
effective wavelength of thin-film in present invention spans from
visible spectrum to near infra red spectrum. In the application of
transparent electrode in optoelectronic device, high transmissivity
means more incident light is received. Because a single incident
light has specific wavelength range except of a wavelength value,
the transparent electrode with broadband characteristic
substantially receives more incident light. Further, as showed in
FIG. 5, the sheet resistance of transparent electrode in present
invention is change with the flow of introducing gas. As far as the
application of LED is concerned, the rising resistance between
transparent electrode and substrate not only degrade the device's
conversion efficiency, but also produce unnecessary heat that
lowers the lifetime of device. In the embodiment of present
invention, the sheet resistance may lower to 50
.OMEGA./.quadrature. by introducing more nitrogen, the transparent
electrode has good current spreading and excellent electric
conductivity. In the application of Si substrate, FIG. 6 shows the
reflectivity distribution of transparent electrode structure in
present invention. As showed in the figure, the reflectivity
distribution is dependent on the used target source (ex. ITO or
SiO.sub.2) and introduced gas type (N.sub.2 or O.sub.2). For
nanorod thin-film formed in present invention, the incident light
with wavelength ranging from 550 nm to 800 nm has reflectivity
performance below 10%. With lower reflectivity, the energy loss of
incident light is fewer, thus the electric efficiency of solar cell
is significantly improved.
[0031] Accordingly, the method of present invention can form
nanostructure conductive thin-film with the characteristic like
broadband, high transmissivity (over 98%), low reflectivity (under
10%), and low sheet resistance (50 .OMEGA./.quadrature.), to
increase the amount of received light and provides excellent light
absorption for electric conductive auxiliary electrode, which is
adaptive in the application of surface thin-film electrode on solar
cell. Furthermore, the thin-film formed by nanorod structure has
the surface roughness to some extent for expanding the light
divergence angle as well as enhancing the current spreading.
Therefore, the present invention can improve the optoelectronic
property of LED device.
[0032] While the embodiments of the present invention disclosed
herein are presently considered to be preferred embodiments,
various changes and modifications can be made without departing
from the spirit and scope of the present invention. The scope of
the invention is indicated in the appended claims, and all changes
that come within the meaning and range of equivalents are intended
to be embraced therein.
* * * * *