US20020084455A1 - Transparent and conductive zinc oxide film with low growth temperature - Google Patents
Transparent and conductive zinc oxide film with low growth temperature Download PDFInfo
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- US20020084455A1 US20020084455A1 US09/281,198 US28119899A US2002084455A1 US 20020084455 A1 US20020084455 A1 US 20020084455A1 US 28119899 A US28119899 A US 28119899A US 2002084455 A1 US2002084455 A1 US 2002084455A1
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- zinc oxide
- transparent
- dopant
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- gallium
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- XLOMVQKBTHCTTD-UHFFFAOYSA-N Zinc monoxide Chemical compound [Zn]=O XLOMVQKBTHCTTD-UHFFFAOYSA-N 0.000 title claims abstract description 254
- 239000011787 zinc oxide Substances 0.000 title claims abstract description 128
- 239000010409 thin film Substances 0.000 claims abstract description 64
- 239000000758 substrate Substances 0.000 claims abstract description 44
- 239000002019 doping agent Substances 0.000 claims abstract description 42
- GYHNNYVSQQEPJS-UHFFFAOYSA-N Gallium Chemical compound [Ga] GYHNNYVSQQEPJS-UHFFFAOYSA-N 0.000 claims abstract description 39
- 229910052733 gallium Inorganic materials 0.000 claims abstract description 36
- 229910052739 hydrogen Inorganic materials 0.000 claims abstract description 32
- 239000001257 hydrogen Substances 0.000 claims abstract description 32
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims abstract description 28
- 230000003287 optical effect Effects 0.000 claims abstract description 14
- 230000004913 activation Effects 0.000 claims abstract description 12
- 239000012535 impurity Substances 0.000 claims abstract description 11
- 238000000034 method Methods 0.000 claims description 42
- 230000008569 process Effects 0.000 claims description 31
- 238000000151 deposition Methods 0.000 claims description 30
- 239000000463 material Substances 0.000 claims description 30
- 230000008021 deposition Effects 0.000 claims description 25
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims description 25
- 239000004065 semiconductor Substances 0.000 claims description 23
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 12
- 239000001301 oxygen Substances 0.000 claims description 12
- 229910052760 oxygen Inorganic materials 0.000 claims description 12
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 claims description 11
- 238000010521 absorption reaction Methods 0.000 claims description 10
- 229930195733 hydrocarbon Natural products 0.000 claims description 10
- 150000002430 hydrocarbons Chemical class 0.000 claims description 10
- 125000004435 hydrogen atom Chemical group [H]* 0.000 claims description 10
- 239000000203 mixture Substances 0.000 claims description 9
- ATUOYWHBWRKTHZ-UHFFFAOYSA-N Propane Chemical compound CCC ATUOYWHBWRKTHZ-UHFFFAOYSA-N 0.000 claims description 8
- 239000004215 Carbon black (E152) Substances 0.000 claims description 7
- 229910003437 indium oxide Inorganic materials 0.000 claims description 7
- PJXISJQVUVHSOJ-UHFFFAOYSA-N indium(iii) oxide Chemical compound [O-2].[O-2].[O-2].[In+3].[In+3] PJXISJQVUVHSOJ-UHFFFAOYSA-N 0.000 claims description 7
- 238000002834 transmittance Methods 0.000 claims description 7
- 238000005137 deposition process Methods 0.000 claims description 6
- 238000004549 pulsed laser deposition Methods 0.000 claims description 5
- 238000000411 transmission spectrum Methods 0.000 claims description 5
- OTMSDBZUPAUEDD-UHFFFAOYSA-N Ethane Chemical compound CC OTMSDBZUPAUEDD-UHFFFAOYSA-N 0.000 claims description 4
- 230000001747 exhibiting effect Effects 0.000 claims description 4
- 239000003574 free electron Substances 0.000 claims description 4
- 239000001294 propane Substances 0.000 claims description 4
- 239000010408 film Substances 0.000 abstract description 36
- 238000012545 processing Methods 0.000 abstract description 15
- 150000002431 hydrogen Chemical class 0.000 abstract description 4
- AMGQUBHHOARCQH-UHFFFAOYSA-N indium;oxotin Chemical compound [In].[Sn]=O AMGQUBHHOARCQH-UHFFFAOYSA-N 0.000 abstract description 4
- XOLBLPGZBRYERU-UHFFFAOYSA-N tin dioxide Chemical compound O=[Sn]=O XOLBLPGZBRYERU-UHFFFAOYSA-N 0.000 abstract description 4
- 229910001887 tin oxide Inorganic materials 0.000 abstract description 4
- 239000013078 crystal Substances 0.000 abstract description 3
- 229910052738 indium Inorganic materials 0.000 abstract description 3
- APFVFJFRJDLVQX-UHFFFAOYSA-N indium atom Chemical compound [In] APFVFJFRJDLVQX-UHFFFAOYSA-N 0.000 abstract description 3
- 229960001296 zinc oxide Drugs 0.000 description 104
- 239000011521 glass Substances 0.000 description 8
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 6
- 238000005468 ion implantation Methods 0.000 description 5
- 239000010453 quartz Substances 0.000 description 5
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicon dioxide Inorganic materials O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 5
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 description 4
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 4
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 4
- 229910052782 aluminium Inorganic materials 0.000 description 4
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 4
- 125000004429 atom Chemical group 0.000 description 4
- 230000005540 biological transmission Effects 0.000 description 4
- 239000010703 silicon Substances 0.000 description 4
- 229910052710 silicon Inorganic materials 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- 238000001429 visible spectrum Methods 0.000 description 4
- 229910052725 zinc Inorganic materials 0.000 description 4
- 229910052796 boron Inorganic materials 0.000 description 3
- -1 boron ions Chemical class 0.000 description 3
- 125000004430 oxygen atom Chemical group O* 0.000 description 3
- 108091006149 Electron carriers Proteins 0.000 description 2
- 238000002679 ablation Methods 0.000 description 2
- 229910052786 argon Inorganic materials 0.000 description 2
- 239000004020 conductor Substances 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 229910001195 gallium oxide Inorganic materials 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 238000000623 plasma-assisted chemical vapour deposition Methods 0.000 description 2
- 238000004544 sputter deposition Methods 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 239000011701 zinc Substances 0.000 description 2
- PXGOKWXKJXAPGV-UHFFFAOYSA-N Fluorine Chemical compound FF PXGOKWXKJXAPGV-UHFFFAOYSA-N 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 238000001505 atmospheric-pressure chemical vapour deposition Methods 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- HQWPLXHWEZZGKY-UHFFFAOYSA-N diethylzinc Chemical compound CC[Zn]CC HQWPLXHWEZZGKY-UHFFFAOYSA-N 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 230000008020 evaporation Effects 0.000 description 1
- 238000001704 evaporation Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 229910052731 fluorine Inorganic materials 0.000 description 1
- 239000011737 fluorine Substances 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 229910021474 group 7 element Inorganic materials 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 125000002887 hydroxy group Chemical group [H]O* 0.000 description 1
- 239000007943 implant Substances 0.000 description 1
- MRNHPUHPBOKKQT-UHFFFAOYSA-N indium;tin;hydrate Chemical compound O.[In].[Sn] MRNHPUHPBOKKQT-UHFFFAOYSA-N 0.000 description 1
- 238000001659 ion-beam spectroscopy Methods 0.000 description 1
- 238000000608 laser ablation Methods 0.000 description 1
- 239000010410 layer Substances 0.000 description 1
- 238000001755 magnetron sputter deposition Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 1
- 229910052753 mercury Inorganic materials 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000005693 optoelectronics Effects 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052594 sapphire Inorganic materials 0.000 description 1
- 239000010980 sapphire Substances 0.000 description 1
- 239000005361 soda-lime glass Substances 0.000 description 1
- HUAUNKAZQWMVFY-UHFFFAOYSA-M sodium;oxocalcium;hydroxide Chemical compound [OH-].[Na+].[Ca]=O HUAUNKAZQWMVFY-UHFFFAOYSA-M 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 230000003595 spectral effect Effects 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 239000002344 surface layer Substances 0.000 description 1
- RGGPNXQUMRMPRA-UHFFFAOYSA-N triethylgallium Chemical compound CC[Ga](CC)CC RGGPNXQUMRMPRA-UHFFFAOYSA-N 0.000 description 1
- 238000009834 vaporization Methods 0.000 description 1
- 230000008016 vaporization Effects 0.000 description 1
- RNWHGQJWIACOKP-UHFFFAOYSA-N zinc;oxygen(2-) Chemical class [O-2].[Zn+2] RNWHGQJWIACOKP-UHFFFAOYSA-N 0.000 description 1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
- H01L31/1884—Manufacture of transparent electrodes, e.g. TCO, ITO
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D29/00—Independent underground or underwater structures; Retaining walls
- E02D29/12—Manhole shafts; Other inspection or access chambers; Accessories therefor
- E02D29/128—Repairs of manhole shafts
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
- C08G59/00—Polycondensates containing more than one epoxy group per molecule; Macromolecules obtained by polymerising compounds containing more than one epoxy group per molecule using curing agents or catalysts which react with the epoxy groups
- C08G59/18—Macromolecules obtained by polymerising compounds containing more than one epoxy group per molecule using curing agents or catalysts which react with the epoxy groups ; e.g. general methods of curing
- C08G59/40—Macromolecules obtained by polymerising compounds containing more than one epoxy group per molecule using curing agents or catalysts which react with the epoxy groups ; e.g. general methods of curing characterised by the curing agents used
- C08G59/50—Amines
- C08G59/56—Amines together with other curing agents
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L63/00—Compositions of epoxy resins; Compositions of derivatives of epoxy resins
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D29/00—Independent underground or underwater structures; Retaining walls
- E02D29/12—Manhole shafts; Other inspection or access chambers; Accessories therefor
- E02D29/14—Covers for manholes or the like; Frames for covers
- E02D29/1409—Covers for manholes or the like; Frames for covers adjustable in height or inclination
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/02—Details
- H01L31/0224—Electrodes
- H01L31/022466—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers
- H01L31/022483—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers composed of zinc oxide [ZnO]
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D2200/00—Geometrical or physical properties
- E02D2200/11—Height being adjustable
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D2250/00—Production methods
- E02D2250/0023—Cast, i.e. in situ or in a mold or other formwork
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L33/36—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the electrodes
- H01L33/40—Materials therefor
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
Definitions
- the present invention relates to conductive transparent thin films and in particular to a novel chemically stable co-doped zinc oxide material that has a low work function and exhibiting an increase of free electrons in the conduction band.
- the present invention relates to conductive transparent thin films such as zinc oxide. It is well known that pure zinc oxide has low conductivity due to low electron density in the film and makes a poor conductor but, when doped, can exhibit improved electrical and optical properties. Zinc oxide is a common transparent n-type semiconductor material with a wide band gap of about 3.3 eV. Zinc oxide has been used as a thin film electrode in a variety of electrical applications such as solar cells or opto-electronics.
- It has a resistivity in the range of about 1 ⁇ 10 ⁇ 4 to 1 ⁇ 10 ⁇ 2 ohm-cm when doped, may be applied to a substrate using well-known semiconductor processing techniques and is relatively inexpensive when compared to other transparent oxide films such as indium tin oxide (ITO) or tin oxide.
- ITO indium tin oxide
- tin oxide tin oxide
- zinc is a semiconductor material
- researchers have investigated many methods for doping zinc oxide with a wide range of dopants.
- non-stoichiometric zinc oxide compounds are known to have high conductivity and transmittance but, unfortunately, such compounds are electrically and optically unstable.
- zinc oxide may be doped with Group VII elements such as fluorine or with Group III elements such as boron, aluminum, gallium or indium to obtain improved and stable electrical and optical properties. See for example, Jianhua Hu and Roy G. Gordon, Atmospheric pressure chemical vapor deposition of gallium doped zinc oxide thin film from diethyl zinc, water, and triethyl gallium, J. Appl. Phys., Vol. 72, No. 11, Dec. 1, 1992.
- One processing technique that has been used in the past is the ion implantation technique for implanting hydrogen, gallium, aluminum or boron ions at high energy into a zinc oxide film deposited on a glass substrate. It is well known, however, that ion implantation has a tendency to introduce damage to the lattice and create localized dislocations. Thus, an anneal step is also required after ion implantation to minimize the effect of the damage. Although the substrate temperature may be maintained at about 100° C. during the implant process, the zinc oxide is changed from transparent clear to a yellowish-brown color even after annealing.
- Recent literature has also reported a number of attempts to impart electrical and optical properties to zinc oxide that would be comparable to ITO.
- aluminum-doped zinc oxide thin film grown on sapphire sputtering in argon (with hydrogen gas) added onto substrates of Corning 7059 glass, boron doped zinc oxide deposited on soda lime and quartz substrates, fluorine-doped zinc films deposited on soda lime glass substrates, argon ion beam sputtering in a hydrogen atmosphere, and ion implantation of gallium, aluminum and boron atoms in an attempt to improve conductivity of zinc oxide thin film.
- Growth, or deposition, of the thin film described in the prior art references is typically at a high temperature (between 100° C. to 500° C.) to activate the dopant and make the film conductive. Accordingly, the selection of an appropriate substrate is limited to a glass, quartz or silicon substrate able to withstand such temperatures. Further, subsequent processing may be required to achieve activation of the dopant by heating the film at high temperature after deposition. These high processing temperatures prohibit the use of many inexpensive flexible substrates such as plastic. Further still, while it is possible to obtain high conductivity with an appropriate dopant, the clarity of thin film is often not both clear and transparent. Alternatively, even if crystal clear and transparent, the zinc oxide film is not chemically stable.
- ITO indium tin oxide
- zinc oxide a common transparent conductive material used in many electrical and optical applications
- ITO indium tin oxide
- ITO is considerably more expensive than zinc oxide but it has better electrical properties.
- ITO also requires high processing temperatures to achieve the desired properties. What is needed is an improved ITO material that may be activated at lower processing temperatures so that plastic or flexible substrates may be used instead of glass, quartz or silicon substrates.
- the present invention relates to a novel zinc oxide material that is highly conductive, transparent, chemically stable and easily deposited on a variety of substrates, including flexible or plastic substrates, and is well suited for electrical or optical applications.
- the novel zinc oxide material is co-doped with two dopants, gallium (Ga) and hydrogen (H).
- Ga gallium
- H hydrogen
- a thin film is advantageously grown on a variety of substrates by ablation of a gallium-zinc oxide target in a gaseous hydrocarbon atmosphere.
- the co-doped zinc oxide thin film is transparent in the visible spectrum, conductive (0.9 to 3 ⁇ 10 ⁇ 4 ohm-cm), has chemical and thermal stability and has a work function between 2.0 eV to 3.0 eV.
- a sintered cylindrical-shaped target comprising a mixture of gallium and zinc oxide is positioned in a deposition chamber.
- the chamber atmosphere contains a mixture of oxygen-enriched atmosphere and a gaseous hydrocarbon such as methane, propane or ethane.
- the co-doped zinc oxide thin film is deposited using a pulsed laser deposition process although it is possible to use other deposition processes such as, by way of example, sputtering.
- the co-doping of the zinc oxide with both gallium and hydrogen has significant advantages. Specifically, activation of the dopants at low growth temperature achieves a conductive and transparent zinc oxide film that can be deposited on a variety of substrates.
- the activation temperature is important since it is the minimum temperature to correctly position dopant atoms at the correct location in the lattice.
- a low deposition temperature typically leaves a very high percentage of the dopant atoms (i.e., gallium atoms) at interstitial locations of the lattice where such atoms do not contribute to the electron concentration or mobility. In such instances (e.g.
- a high temperature anneal is required to replace the zinc atoms in the lattice with the gallium atoms.
- no such high temperature anneal is necessary to activate the dopant in the present invention since activation occurs even at low deposition temperatures.
- the hydrogen atoms are incorporated into the zinc oxide most likely attached to the oxygen atoms. Every gallium atom in the lattice can stabilize at least one hydrogen atoms in the lattice.
- the high concentration of hydrogen does not disturb the lattice structure because of the small size of the hydrogen.
- a dopant can replace either a zinc atom or an oxygen atom.
- gallium and hydrogen a gallium atom will replace a percentage of the zinc atoms in the lattice and the hydrogen appears to be absorbed on the oxygen side.
- both sides of the lattice can contribute to the film conductivity. The result is a co-doped zinc oxide thin film with high electron concentration and high mobility compared to zinc oxide doped with a single dopant.
- the high electron concentration of the co-doped zinc oxide thin film lifts the Fermi level to above the conduction band edge by an energy amount known as the Burnstein-Moss shift.
- the co-doped zinc oxide has a reduced work function that is less than 3 eV (more specifically, between 2 eV and 3 eV).
- the co-doped zinc oxide is crystal clear and transparent in the visible spectrum even when grown at relatively low processing temperatures (i.e., between about 30° C. to 170° C.).
- a novel indium tin oxide (ITO) thin film has the necessary electrical and optical properties that are obtained at low processing temperatures sufficient to permit the use of plastic or flexible substrates.
- the novel ITO comprises tin oxide co-doped with indium (In) and hydrogen.
- FIG. 1 is a process flow diagram illustrating a method for manufacturing the co-doped zinc oxide of the present invention
- FIG. 2 illustrates typical transmittance of a thin film of the novel co-doped zinc oxide of the present invention
- FIG. 3 illustrates the chemical stability of the novel co-doped zinc oxide of the present invention
- FIGS. 4A and 4B illustrate the transmission spectra of novel co-doped zinc oxide films grown at selected temperatures
- FIGS. 4 C- 4 D illustrate the transmission spectra of four zinc oxide films having various dopants.
- FIGS. 5 A- 5 B graphically illustrate the Burnstein-Moss shift of the novel zinc oxide thin film of the present invention.
- FIG. 1 one illustrative process for growing the novel zinc-oxide thin film is shown.
- the zinc oxide film is grown using readily available semiconductor processing techniques. Specifically, as indicated at steps 12 and 14 , a gallium-zinc oxide target is positioned proximate to a substrate (not shown) in a deposition chamber (not shown). The chamber is evacuated and an oxygen rich atmosphere of gaseous hydrocarbon is introduced as indicated at step 16 . Using a selected deposition technique, a thin film of co-doped zinc oxide is deposited on the substrate as indicated at step 18 .
- the essential components of pulsed laser deposition methods are disclosed in U.S. Pat. No. 5,411,772 which issued May 2, 1995 to Jeffery T. Cheung the present inventor, the disclosure of which is incorporated herein by reference.
- the target is preferably selected from targets having a gallium concentration of between 0.2 atomic wt % to 2.0 atomic wt %. It should be understood that the actual concentration depends on each particular application of the thin film. Initial experimentation indicates that a concentration of gallium in the resulting thin film of between about 0.5 atomic wt % and 1.5 atomic wt % is preferred for most applications. However, it is believed that higher concentrations of between about 1.5 atomic wt % (preferred) to about 2.0 atomic wt % gallium will maximize the concentration of mobile electrons.
- One preferred target, a cylindrical target is disclosed in U.S. Pat. No. 4,740,386, which issued Apr. 26, 1986 to Jeffery T. Cheung the present inventor.
- this reference teaches the use of a cylindrical target that is rotated and simultaneously translated along a longitudinal axis in the presence of a laser beam to provide a controlled and uniform film deposition over a substrate.
- a sintered disk-shaped target or a powdered mixture of gallium and zinc oxide may be used in place of the cylindrical target.
- methane (CH 4 ) gas is added to an oxygen-rich atmosphere in the deposition chamber as a source of hydrogen atoms.
- the ratio of oxygen to methane is between about 4:1 and 10:1 at 30 milliTorr.
- Other gaseous hydrocarbons, such as propane or ethane, may be selected as the source of hydrogen atoms.
- other sources of hydrogen, other than water (H 2 O), which is difficult to implement in a vacuum may be selected, it only being required that free hydrogen is available during the deposition process.
- a pulsed laser controllably vaporizes the target during the deposition process.
- the laser ablation induced plasma decomposes the methane to form hydrogen atoms.
- Magnetron Sputtering, Plasma Assisted CVD, and Activated Reactive Evaporation are listed as examples.
- Plasma Assisted CVD is preferred if the size of the substrate would otherwise preclude the use of a vacuum chamber.
- the substrate temperature for growth of the zinc oxide thin film is maintained in the range of between 30° C. and 250° C. but preferably below 180° C.
- Table 1 shows representative sheet resistance and light transmission measurements for samples of a 2,000-Angstrom thick thin film of co-doped zinc oxide film grown at selected temperatures.
- the resistivity of the co-doped zinc oxide thin film can be in the range of 1.0 to 4.0 ⁇ 10 ⁇ 4 ohm-cm.
- Those skilled in the art familiar with prior art zinc oxide film will appreciate that with just hydrogen or a Group III element as a dopant, the film tends to have a yellowish hue unless grown at a very high (500°) temperatures.
- the co-doped zinc oxide thin film is remarkably clear and transparent regardless of growth temperature.
- the electrical parameters of the thin film of the co-doped zinc oxide is correlated to deposition temperature but even at a very low deposition temperature the sheet resistance is sufficient for many electronic applications.
- a thin film of gallium doped zinc oxide (but with no hydrogen doping) has a sheet resistance that ranges from about 11,000 ⁇ / ⁇ for a deposition temperature of 200° C. and a film thickness is 5,600 Angstroms—to about 3.6 ⁇ / ⁇ fpr a deposition temperature of 470° C. and a film thickness of 6,600 Angstroms.
- the co-doped zinc oxide thin film is conductive (1.0 to 4.0 ⁇ 10 ⁇ 4 ohm-cm).
- the zinc oxide thin film may be deposited on a wide range of inexpensive low-temperature substrates such as plastic or other flexible substrates.
- the deposition of zinc oxide thin film is no longer restricted to deposition on substrate materials such as glass, quartz, silicon or expensive high-temperature plastic substrates (that is, able to withstand deposition temperatures in excess of 100° C.).
- the zinc oxide film has a high concentration of electrons in the range of 10 16 to about 5.2 ⁇ 10 20 cm ⁇ 3 , exhibiting high mobility.
- the novel zinc oxide thin film has low electrical resistance where resistance is a function of electron mobility. It will be appreciated that electrons should be as mobile as possible since the product of mobility times the carrier concentration defines the resistivity of the film.
- Experimental evaluation of the thin film indicates that at a 175° C. deposition temperature, the mobility is about 30 cm 2 V-second for a concentration of about 0.5 atomic wt. % for gallium.
- activation occurs at low temperature.
- activation means that the impurity atoms (gallium and hydrogen) occupy positions in the lattice and become electrically active (that is, rapidly react to the application of a stimulus such as an applied voltage).
- impurity atoms gallium and hydrogen
- the low temperature activation is believed due, at least in part, to substitution of zinc atom with gallium atoms in the zinc oxide lattice and attachment of the hydrogen atoms with oxygen to form O—H bonds so that both sides of the lattice contribute electrons.
- each gallium atom in the lattice is capable of stabilizing at least one hydrogen atom.
- Each hydrogen atom most likely attaches to an oxygen atom to form a hydroxyl group that acts as a donor (n-type) impurity.
- the hydrogen dopant in the zinc oxide lattice is stabilized by the presence of gallium so there are no mobile hydrogen ions in the thin film. With the hydrogen bound in the lattice, the electrical properties of the film are very stable even at elevated operating temperatures. Accordingly, the film does not readily it form an oxidized (insulating) surface layer and does not react with other components of the electrical or optical system in which the film is used.
- FIG. 2 illustrates the transmittance, an important optical property for many applications, of the co-doped zinc oxide film in a portion of the visible spectrum.
- a 5,500-Angstrom thick thin film of co-doped zinc oxide was deposited on a glass substrate and the incident light transmitted through the film is measured and plotted at 20 .
- An interference pattern is due to the thickness of the thin film but an approximate average transmittance, as indicated by line 22 , is shown.
- the average transmission of incident visible radiation ranges from 77% for shorter wavelengths to about 85% for longer wavelengths.
- FIG. 3 illustrates the chemical stability and high electron field emission efficiency of the co-doped zinc oxide in a vacuum.
- the electron-field emission from a co-doped zinc oxide film is relatively independent of vacuum level indicating that the oxide does not readily form an insulating oxide layer.
- a field potential of between 5 and 30 volts per 10 ⁇ 4 cm is applied across a thin film sample of the zinc oxide film electrically connected to a ground potential and an electrode separated from the film by a vacuum gap of about 1.0 ⁇ 10 ⁇ 6 meters.
- the emission current density is then measured for various vacuum levels. As shown in FIG. 3, the current density does not vary substantially even as the vacuum level is varied between 10 ⁇ 9 Torr to 10 ⁇ 6 Torr. In the prior art material, there is substantial emission current density degradation as the vacuum is varied.
- FIGS. 4 A- 4 D the optical transmission spectra of the co-doped zinc oxide is illustrated at selected temperatures.
- a significant monotonic decrease in transmission of light through the thin film of co-doped zinc oxide is shown at longer wavelengths. This decrease is due to free carrier absorption that occurs when electron concentration is high enough to provide electrons in the conduction band.
- the amount of free carrier absorption for a co-doped zinc oxide is higher than zinc oxide with a single dopant of either gallium or hydrogen grown at a temperature of 175° C. Even if the co-doped zinc oxide film of the present invention is grown at near-room temperature, the free carrier concentration is dominant.
- FIG. 4B a shift of absorption edge is shown at shorter wavelengths.
- This shift is known as the Burnstein-Moss shift (the energy difference between the absorption edges of doped and undoped sample) is due to the filling of electrons in the conduction band.
- the Burnstein-Moss shift is due to the filling of electrons in the conduction band.
- the presence of the large Burnstein-Moss shift in the co-doped zinc oxide is critical to achieving the low work function, since the work function is the difference between the electron affinity (typically between 3.4 eV and 2.6 eV for zinc oxide) and the Burnstein-Moss shift.
- Electron affinity is the energy difference between the vacuum energy level and the conduction band edge of the specific material.
- Gallium and hydrogen co-doped zinc oxide shows the larger Burnstein-Moss shift compared to zinc oxide doped with a single dopant of either gallium or hydrogen grown under the similar conditions.
- FIG. 4C illustrates the effect of depositing films having different dopants at a high temperature (for example, at 175° C.).
- the Burnstein-Moss shift for the co-doped zinc oxide of the present invention is significant compared to undoped zinc oxide, gallium-doped zinc oxide and hydrogen-doped zinc oxide.
- the free carrier absorption and the magnitude of the Burnstein-Moss shift shown in FIGS. 4C and 4D clearly illustrates the substantial difference achieved by the co-doping process and the co-doped zinc oxide thin film of the present invention.
- FIGS. 5A and 5B the relationship between the Burnstein-Moss shift and the work function is shown in detail.
- the Burnstein-Moss shift ( ⁇ E BM )is shown, with respect to undoped and co-doped zinc oxide, as being about 0.54 eV.
- the Burnstein-Moss shift is also shown in FIG. 5B.
- work function refers to the minimum energy needed by an electron to escape to the vacuum from the attractive forces that normally bind the electron to the zinc oxide material. More specifically, the work function may be thought of as the energy difference of an electron in a vacuum and the Fermi level, which is the highest energy level of an electron in the solid zinc oxide.
- the work function of the co-doped zinc oxide is estimated to be about 2 eV to 3 eV. This estimate is obtained from theoretical considerations and indirect observation of work function refraction due to Burnstein-Moss shift in co-doped zinc oxide compared to undoped zinc oxide.
- anode 14 comprises a thin film of indium oxide having two dopants. More particularly the indium oxide is doped with both tin (Sn) and hydrogen (H).
- the indium oxide thin film is deposited on a suitable substrate by pulsed laser deposition or other suitable deposition process with a tin-indium-oxide target and a gaseous hydrocarbon (such as methane by way of example) rich atmosphere. During deposition, the methane decomposes to free hydrogen atoms while tin atoms are supplied from vaporization of the target.
- the concentration of tin in the co-doped indium oxide thin film may be readily varied for specific engineering applications. By co-doping, free electron concentration and mobility is increased in the indium oxide thin film.
- the co-doped indium oxide material may be grown at a low temperature with less tin content than used in prior art oxides. That is, the co-doped tin oxide of the present invention is grown with less than 5 atomic wt % tin, and preferably between 0.5 atomic wt % to 2 atomic wt % tin rather than about 9% as is typical in prior art ITO.
Abstract
The present invention relates to a novel zinc oxide thin film having hydrogen (H) and gallium (Ga) dopants. Advantageously, the activation temperature is low. The co-doped zinc oxide is highly conductive, transparent, chemically stable, easily deposited on a variety of substrates, including flexible or plastic substrates, and is well suited for electrical or optical applications. By co-doping with two impurities, both sides of the zinc oxide lattice contribute to the film conductivity resulting in high electron concentration and high mobility. The co-doped zinc oxide thin film has an increased Fermi level and a reduced work function that is less than 3 eV. The co-doped zinc oxide is crystal clear and transparent even when grown at relatively low processing temperatures. In another preferred embodiment of the present invention, a novel low-temperature activation indium tin oxide (ITO) thin film comprising tin oxide co-doped with indium (In) and hydrogen is disclosed.
Description
- 1. Field of the Invention
- The present invention relates to conductive transparent thin films and in particular to a novel chemically stable co-doped zinc oxide material that has a low work function and exhibiting an increase of free electrons in the conduction band.
- 2. Description of Related Art
- The present invention relates to conductive transparent thin films such as zinc oxide. It is well known that pure zinc oxide has low conductivity due to low electron density in the film and makes a poor conductor but, when doped, can exhibit improved electrical and optical properties. Zinc oxide is a common transparent n-type semiconductor material with a wide band gap of about 3.3 eV. Zinc oxide has been used as a thin film electrode in a variety of electrical applications such as solar cells or opto-electronics. It has a resistivity in the range of about 1×10−4 to 1×10−2 ohm-cm when doped, may be applied to a substrate using well-known semiconductor processing techniques and is relatively inexpensive when compared to other transparent oxide films such as indium tin oxide (ITO) or tin oxide.
- Since zinc is a semiconductor material, researchers have investigated many methods for doping zinc oxide with a wide range of dopants. For example, non-stoichiometric zinc oxide compounds are known to have high conductivity and transmittance but, unfortunately, such compounds are electrically and optically unstable. It is also known that zinc oxide may be doped with Group VII elements such as fluorine or with Group III elements such as boron, aluminum, gallium or indium to obtain improved and stable electrical and optical properties. See for example, Jianhua Hu and Roy G. Gordon, Atmospheric pressure chemical vapor deposition of gallium doped zinc oxide thin film from diethyl zinc, water, and triethyl gallium, J. Appl. Phys., Vol. 72, No. 11, Dec. 1, 1992.
- Dopants are introduced to the zinc oxide using readily known semiconductor techniques. Such techniques typically require the high processing temperatures associated with semiconductor processing. This high temperature limits the selection of suitable substrates to substrates, such as glass, quartz or silicon, capable of withstanding the processing temperatures. Unfortunately, such substrates are expensive. Accordingly, it is desirable to develop a process that minimizes the processing temperature so that inexpensive plastic or other flexible substrates may be used.
- One processing technique that has been used in the past is the ion implantation technique for implanting hydrogen, gallium, aluminum or boron ions at high energy into a zinc oxide film deposited on a glass substrate. It is well known, however, that ion implantation has a tendency to introduce damage to the lattice and create localized dislocations. Thus, an anneal step is also required after ion implantation to minimize the effect of the damage. Although the substrate temperature may be maintained at about 100° C. during the implant process, the zinc oxide is changed from transparent clear to a yellowish-brown color even after annealing. See for example Shigemi Kohiki, Mikihiko Hishitani and Takahiro Wada, Enhanced electrical conductivity of zinc oxide thin films by ion implantation of gallium, aluminum, and boron atoms, J. Appl. Phys., Vol. 75, No. 4, Feb. 15, 1994.
- Recent literature has also reported a number of attempts to impart electrical and optical properties to zinc oxide that would be comparable to ITO. By way of example, aluminum-doped zinc oxide thin film grown on sapphire, sputtering in argon (with hydrogen gas) added onto substrates of Corning 7059 glass, boron doped zinc oxide deposited on soda lime and quartz substrates, fluorine-doped zinc films deposited on soda lime glass substrates, argon ion beam sputtering in a hydrogen atmosphere, and ion implantation of gallium, aluminum and boron atoms in an attempt to improve conductivity of zinc oxide thin film. Other research has focused on undoped zinc oxide films having reduced resistivity brought about by treating the film surface with hydrogen in a mercury-sensitive photo-CVD process. Such zinc oxide thin films suffer from a variety of problems such as lacking chemical stability, having a hue other than transparent clear (i.e., yellowish), having poor transmissity and requiring high deposition or activation temperatures.
- Growth, or deposition, of the thin film described in the prior art references is typically at a high temperature (between 100° C. to 500° C.) to activate the dopant and make the film conductive. Accordingly, the selection of an appropriate substrate is limited to a glass, quartz or silicon substrate able to withstand such temperatures. Further, subsequent processing may be required to achieve activation of the dopant by heating the film at high temperature after deposition. These high processing temperatures prohibit the use of many inexpensive flexible substrates such as plastic. Further still, while it is possible to obtain high conductivity with an appropriate dopant, the clarity of thin film is often not both clear and transparent. Alternatively, even if crystal clear and transparent, the zinc oxide film is not chemically stable.
- Notwithstanding the improvements obtained by the above noted efforts, the processing techniques and the resulting zinc oxide thin film remain less than ideal for many electrical and optical applications. There exists great need for a zinc oxide material that is highly conductive, transparent, chemically stable and easily deposited on a variety of substrates, including flexible or plastic substrates.
- As is well known to those skilled in the art, a common transparent conductive material used in many electrical and optical applications is indium tin oxide (ITO). ITO is considerably more expensive than zinc oxide but it has better electrical properties. Unfortunately, ITO also requires high processing temperatures to achieve the desired properties. What is needed is an improved ITO material that may be activated at lower processing temperatures so that plastic or flexible substrates may be used instead of glass, quartz or silicon substrates.
- The present invention relates to a novel zinc oxide material that is highly conductive, transparent, chemically stable and easily deposited on a variety of substrates, including flexible or plastic substrates, and is well suited for electrical or optical applications. The novel zinc oxide material is co-doped with two dopants, gallium (Ga) and hydrogen (H). A thin film is advantageously grown on a variety of substrates by ablation of a gallium-zinc oxide target in a gaseous hydrocarbon atmosphere. The co-doped zinc oxide thin film is transparent in the visible spectrum, conductive (0.9 to 3×10−4 ohm-cm), has chemical and thermal stability and has a work function between 2.0 eV to 3.0 eV.
- In one preferred embodiment, a sintered cylindrical-shaped target comprising a mixture of gallium and zinc oxide is positioned in a deposition chamber. The chamber atmosphere contains a mixture of oxygen-enriched atmosphere and a gaseous hydrocarbon such as methane, propane or ethane. The co-doped zinc oxide thin film is deposited using a pulsed laser deposition process although it is possible to use other deposition processes such as, by way of example, sputtering.
- The co-doping of the zinc oxide with both gallium and hydrogen has significant advantages. Specifically, activation of the dopants at low growth temperature achieves a conductive and transparent zinc oxide film that can be deposited on a variety of substrates. The activation temperature is important since it is the minimum temperature to correctly position dopant atoms at the correct location in the lattice. As will be appreciated, a low deposition temperature typically leaves a very high percentage of the dopant atoms (i.e., gallium atoms) at interstitial locations of the lattice where such atoms do not contribute to the electron concentration or mobility. In such instances (e.g. prior art zinc oxide films), a high temperature anneal is required to replace the zinc atoms in the lattice with the gallium atoms. However, no such high temperature anneal is necessary to activate the dopant in the present invention since activation occurs even at low deposition temperatures. Further, the hydrogen atoms are incorporated into the zinc oxide most likely attached to the oxygen atoms. Every gallium atom in the lattice can stabilize at least one hydrogen atoms in the lattice. Advantageously, the high concentration of hydrogen does not disturb the lattice structure because of the small size of the hydrogen.
- In the lattice of zinc oxide, a dopant can replace either a zinc atom or an oxygen atom. However, by co-doping with gallium and hydrogen, a gallium atom will replace a percentage of the zinc atoms in the lattice and the hydrogen appears to be absorbed on the oxygen side. Thus, by co-doping with two impurities, both sides of the lattice can contribute to the film conductivity. The result is a co-doped zinc oxide thin film with high electron concentration and high mobility compared to zinc oxide doped with a single dopant. Further, the high electron concentration of the co-doped zinc oxide thin film lifts the Fermi level to above the conduction band edge by an energy amount known as the Burnstein-Moss shift. Further still, the co-doped zinc oxide has a reduced work function that is less than 3 eV (more specifically, between 2 eV and 3 eV). Further still, the co-doped zinc oxide is crystal clear and transparent in the visible spectrum even when grown at relatively low processing temperatures (i.e., between about 30° C. to 170° C.).
- In another preferred embodiment of the present invention, a novel indium tin oxide (ITO) thin film has the necessary electrical and optical properties that are obtained at low processing temperatures sufficient to permit the use of plastic or flexible substrates. The novel ITO comprises tin oxide co-doped with indium (In) and hydrogen.
- Additional advantages to the present invention become apparent upon reading and understanding this specification.
- For a more complete understanding of the present invention, the following Detailed Description of the Preferred Embodiments makes reference to the accompanying Drawings in which:
- FIG. 1 is a process flow diagram illustrating a method for manufacturing the co-doped zinc oxide of the present invention;
- FIG. 2 illustrates typical transmittance of a thin film of the novel co-doped zinc oxide of the present invention;
- FIG. 3 illustrates the chemical stability of the novel co-doped zinc oxide of the present invention;
- FIGS. 4A and 4B illustrate the transmission spectra of novel co-doped zinc oxide films grown at selected temperatures;
- FIGS.4C-4D illustrate the transmission spectra of four zinc oxide films having various dopants; and
- FIGS.5A-5B graphically illustrate the Burnstein-Moss shift of the novel zinc oxide thin film of the present invention.
- In the following description of the preferred embodiment, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and techniques are not shown or discussed in detail in order to avoid unnecessarily obscuring the present invention.
- Referring now to FIG. 1, one illustrative process for growing the novel zinc-oxide thin film is shown. The zinc oxide film is grown using readily available semiconductor processing techniques. Specifically, as indicated at
steps - The target is preferably selected from targets having a gallium concentration of between 0.2 atomic wt % to 2.0 atomic wt %. It should be understood that the actual concentration depends on each particular application of the thin film. Initial experimentation indicates that a concentration of gallium in the resulting thin film of between about 0.5 atomic wt % and 1.5 atomic wt % is preferred for most applications. However, it is believed that higher concentrations of between about 1.5 atomic wt % (preferred) to about 2.0 atomic wt % gallium will maximize the concentration of mobile electrons. One preferred target, a cylindrical target, is disclosed in U.S. Pat. No. 4,740,386, which issued Apr. 26, 1986 to Jeffery T. Cheung the present inventor. Specifically, this reference teaches the use of a cylindrical target that is rotated and simultaneously translated along a longitudinal axis in the presence of a laser beam to provide a controlled and uniform film deposition over a substrate. Alternatively, a sintered disk-shaped target or a powdered mixture of gallium and zinc oxide may be used in place of the cylindrical target.
- During deposition of the zinc oxide thin film, methane (CH4) gas is added to an oxygen-rich atmosphere in the deposition chamber as a source of hydrogen atoms. In one preferred embodiment, the ratio of oxygen to methane is between about 4:1 and 10:1 at 30 milliTorr. Other gaseous hydrocarbons, such as propane or ethane, may be selected as the source of hydrogen atoms. Alternatively, other sources of hydrogen, other than water (H2O), which is difficult to implement in a vacuum, may be selected, it only being required that free hydrogen is available during the deposition process.
- In one preferred ablation process, a pulsed laser controllably vaporizes the target during the deposition process. The laser ablation induced plasma decomposes the methane to form hydrogen atoms. It will be appreciated, however, that other known deposition techniques may be readily employed to obtain the co-doped zinc oxide thin film. Magnetron Sputtering, Plasma Assisted CVD, and Activated Reactive Evaporation are listed as examples. Plasma Assisted CVD is preferred if the size of the substrate would otherwise preclude the use of a vacuum chamber. Advantageously, the substrate temperature for growth of the zinc oxide thin film is maintained in the range of between 30° C. and 250° C. but preferably below 180° C.
- Table 1 shows representative sheet resistance and light transmission measurements for samples of a 2,000-Angstrom thick thin film of co-doped zinc oxide film grown at selected temperatures. The resistivity of the co-doped zinc oxide thin film can be in the range of 1.0 to 4.0×10−4 ohm-cm. Those skilled in the art familiar with prior art zinc oxide film will appreciate that with just hydrogen or a Group III element as a dopant, the film tends to have a yellowish hue unless grown at a very high (500°) temperatures. In contrast, the co-doped zinc oxide thin film is remarkably clear and transparent regardless of growth temperature.
TABLE 1 Typical Light Transmittance Thin Film Growth Sheet Resistance (visible spectral region Temperature (° C.) (Ω/□) incident on glass substrate) 175 5 88% to 90% 108 11 84% 78 15 82% 37 20 79% - As indicated in Table 1, the electrical parameters of the thin film of the co-doped zinc oxide is correlated to deposition temperature but even at a very low deposition temperature the sheet resistance is sufficient for many electronic applications. By way of comparison to prior art zinc oxide thin films, a thin film of gallium doped zinc oxide (but with no hydrogen doping) has a sheet resistance that ranges from about 11,000 Ω/□ for a deposition temperature of 200° C. and a film thickness is 5,600 Angstroms—to about 3.6 Ω/□ fpr a deposition temperature of 470° C. and a film thickness of 6,600 Angstroms. The co-doped zinc oxide thin film is conductive (1.0 to 4.0×10−4 ohm-cm). When the deposition temperature of the present zinc oxide is minimized (i.e., in a range of about 30° C. to 100° C.), the zinc oxide thin film may be deposited on a wide range of inexpensive low-temperature substrates such as plastic or other flexible substrates. Thus, the deposition of zinc oxide thin film is no longer restricted to deposition on substrate materials such as glass, quartz, silicon or expensive high-temperature plastic substrates (that is, able to withstand deposition temperatures in excess of 100° C.).
- With the present invention, the zinc oxide film has a high concentration of electrons in the range of 1016 to about 5.2×1020 cm−3, exhibiting high mobility. The novel zinc oxide thin film has low electrical resistance where resistance is a function of electron mobility. It will be appreciated that electrons should be as mobile as possible since the product of mobility times the carrier concentration defines the resistivity of the film. Experimental evaluation of the thin film indicates that at a 175° C. deposition temperature, the mobility is about 30 cm2V-second for a concentration of about 0.5 atomic wt. % for gallium.
- In the co-doped zinc oxide, activation occurs at low temperature. The term “activation” means that the impurity atoms (gallium and hydrogen) occupy positions in the lattice and become electrically active (that is, rapidly react to the application of a stimulus such as an applied voltage). In comparison, it is not uncommon of a large percentage of the dopant in the prior art film to remain inactive (that is they do not occupy a lattice site) even with high process or activation temperatures. In the zinc oxide film of the present invention, the low temperature activation is believed due, at least in part, to substitution of zinc atom with gallium atoms in the zinc oxide lattice and attachment of the hydrogen atoms with oxygen to form O—H bonds so that both sides of the lattice contribute electrons. Advantageously, each gallium atom in the lattice is capable of stabilizing at least one hydrogen atom.
- Each hydrogen atom most likely attaches to an oxygen atom to form a hydroxyl group that acts as a donor (n-type) impurity. The hydrogen dopant in the zinc oxide lattice is stabilized by the presence of gallium so there are no mobile hydrogen ions in the thin film. With the hydrogen bound in the lattice, the electrical properties of the film are very stable even at elevated operating temperatures. Accordingly, the film does not readily it form an oxidized (insulating) surface layer and does not react with other components of the electrical or optical system in which the film is used.
- FIG. 2 illustrates the transmittance, an important optical property for many applications, of the co-doped zinc oxide film in a portion of the visible spectrum. A 5,500-Angstrom thick thin film of co-doped zinc oxide was deposited on a glass substrate and the incident light transmitted through the film is measured and plotted at20. An interference pattern is due to the thickness of the thin film but an approximate average transmittance, as indicated by
line 22, is shown. For this sample, deposited at a substrate temperature of 150° C., the average transmission of incident visible radiation ranges from 77% for shorter wavelengths to about 85% for longer wavelengths. For purposes of comparison, measurements for an ITO thin film of about 1,000 Angstrom with the same resistivity as the co-doped zinc oxide is shown atline 24. For purpose of further comparison, a 3,000-Angstrom thick thin film of ITO has an average transmittance in the visible spectrum of about 80% as reported in the literature. Clearly, the co-doped zinc oxide thin film is more transparent than a comparable ITO thin film. - FIG. 3 illustrates the chemical stability and high electron field emission efficiency of the co-doped zinc oxide in a vacuum. The electron-field emission from a co-doped zinc oxide film is relatively independent of vacuum level indicating that the oxide does not readily form an insulating oxide layer. In one experiment, a field potential of between 5 and 30 volts per 10−4 cm is applied across a thin film sample of the zinc oxide film electrically connected to a ground potential and an electrode separated from the film by a vacuum gap of about 1.0×10−6 meters. The emission current density is then measured for various vacuum levels. As shown in FIG. 3, the current density does not vary substantially even as the vacuum level is varied between 10−9 Torr to 10−6 Torr. In the prior art material, there is substantial emission current density degradation as the vacuum is varied.
- Referring now to FIGS.4A-4D, the optical transmission spectra of the co-doped zinc oxide is illustrated at selected temperatures. As illustrated in FIG. 4A, a significant monotonic decrease in transmission of light through the thin film of co-doped zinc oxide is shown at longer wavelengths. This decrease is due to free carrier absorption that occurs when electron concentration is high enough to provide electrons in the conduction band. The amount of free carrier absorption for a co-doped zinc oxide is higher than zinc oxide with a single dopant of either gallium or hydrogen grown at a temperature of 175° C. Even if the co-doped zinc oxide film of the present invention is grown at near-room temperature, the free carrier concentration is dominant. In undoped zinc oxide, the electron carrier concentration is relatively low (this is known as the intrinsic level) and there is no free carrier absorption. In contrast to the co-doped zinc oxide, light transmission remains high for the visible to infrared sprecta for undoped zinc oxide, as shown in FIG. 4D. This phenomenon is also shown for zinc oxide doped with either hydrogen or gallium, which also have low electron carrier concentration.
- In FIG. 4B, a shift of absorption edge is shown at shorter wavelengths. This shift is known as the Burnstein-Moss shift (the energy difference between the absorption edges of doped and undoped sample) is due to the filling of electrons in the conduction band. Even at a growth temperature of 37° C. there is a significant Burnstein-Moss shift compared to undoped zinc oxide or zinc oxide with a single dopant. The presence of the large Burnstein-Moss shift in the co-doped zinc oxide is critical to achieving the low work function, since the work function is the difference between the electron affinity (typically between 3.4 eV and 2.6 eV for zinc oxide) and the Burnstein-Moss shift. Electron affinity is the energy difference between the vacuum energy level and the conduction band edge of the specific material. Gallium and hydrogen co-doped zinc oxide shows the larger Burnstein-Moss shift compared to zinc oxide doped with a single dopant of either gallium or hydrogen grown under the similar conditions.
- FIG. 4C illustrates the effect of depositing films having different dopants at a high temperature (for example, at 175° C.). The Burnstein-Moss shift for the co-doped zinc oxide of the present invention is significant compared to undoped zinc oxide, gallium-doped zinc oxide and hydrogen-doped zinc oxide. The free carrier absorption and the magnitude of the Burnstein-Moss shift shown in FIGS. 4C and 4D clearly illustrates the substantial difference achieved by the co-doping process and the co-doped zinc oxide thin film of the present invention.
- Referring now to FIGS. 5A and 5B, the relationship between the Burnstein-Moss shift and the work function is shown in detail. In FIG. 5A, the Burnstein-Moss shift (ΔEBM)is shown, with respect to undoped and co-doped zinc oxide, as being about 0.54 eV. The Burnstein-Moss shift is also shown in FIG. 5B. In the co-doped zinc oxide excess mobile electrons are present in the conduction band so its Fermi level (Ef) is lifted to be above the conduction band edge (EC) by an amount equal to the Burnstein-Moss shift. These mobile electrons require less energy to escape the material by field emission process and accordingly, a lower work function (Φ). The phrase “work function” refers to the minimum energy needed by an electron to escape to the vacuum from the attractive forces that normally bind the electron to the zinc oxide material. More specifically, the work function may be thought of as the energy difference of an electron in a vacuum and the Fermi level, which is the highest energy level of an electron in the solid zinc oxide. The work function of the co-doped zinc oxide is estimated to be about 2 eV to 3 eV. This estimate is obtained from theoretical considerations and indirect observation of work function refraction due to Burnstein-Moss shift in co-doped zinc oxide compared to undoped zinc oxide.
- In yet another preferred embodiment, a novel ITO thin film is disclosed. In this embodiment,
anode 14 comprises a thin film of indium oxide having two dopants. More particularly the indium oxide is doped with both tin (Sn) and hydrogen (H). As in the deposition process described above for the co-doped zinc oxide, the indium oxide thin film is deposited on a suitable substrate by pulsed laser deposition or other suitable deposition process with a tin-indium-oxide target and a gaseous hydrocarbon (such as methane by way of example) rich atmosphere. During deposition, the methane decomposes to free hydrogen atoms while tin atoms are supplied from vaporization of the target. The concentration of tin in the co-doped indium oxide thin film may be readily varied for specific engineering applications. By co-doping, free electron concentration and mobility is increased in the indium oxide thin film. Advantageously, the co-doped indium oxide material may be grown at a low temperature with less tin content than used in prior art oxides. That is, the co-doped tin oxide of the present invention is grown with less than 5 atomic wt % tin, and preferably between 0.5 atomic wt % to 2 atomic wt % tin rather than about 9% as is typical in prior art ITO. - While certain exemplary preferred embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention. Further, it is to be understood that this invention shall not be limited to the specific construction and arrangements shown and described since various modifications or changes may occur to those of ordinary skill in the art without departing from the spirit and scope of the invention as claimed.
Claims (39)
1. A transparent, chemically stable, conductive wide band gap semiconductor material comprising an oxide having a first dopant and a second dopant, said oxide exhibiting free electrons in the conduction band.
2. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 1 wherein said oxide comprises zinc oxide, said first dopant comprises gallium and said second dopant comprises hydrogen.
3. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 1 wherein said first dopant comprises gallium.
4. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 3 wherein said second dopant comprises hydrogen.
5. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 4 wherein said oxide comprises zinc oxide.
6. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 4 wherein the concentration of said first dopant is between about 0.5 atomic wt % and about 2.0 atomic wt %.
7. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 6 wherein the concentration of said second dopant is about four times the concentration of said first dopant in said co-doped zinc oxide thin film.
8. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 4 wherein the concentration of said second dopant is between about one to about four times the concentration of said first dopant in said co-doped zinc oxide thin film.
9. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 1 wherein said oxide comprises indium oxide, said first dopant comprises tin and said second dopant comprises hydrogen.
10. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 9 wherein the concentration of said tin is between 0.5 atomic wt % and about 2.0 atomic wt % and the concentration of said hydrogen is about four times the concentration of said tin.
11. The transparent, chemically stable, conductive wide band gap semiconductor material of claim 9 wherein the concentration of said tin is between 0.5 atomic wt % and about 2.0 atomic wt % and the concentration of said hydrogen is between about one to four times the concentration of said tin.
12. A transparent conductive wide band gap semiconductor material comprising a co-doped oxide having a first dopant and a second dopant, said co-doped oxide exhibiting free electrons in the conduction band and optical transparency sufficient to transmit at least 75% of incident light.
13. The transparent conductive wide band gap semiconductor material of claim 12 wherein said oxide has an electron concentration of at least 5×1020 cm−3.
14. The transparent conductive wide band gap semiconductor material of claim 12 wherein said oxide has a low work function.
15. The transparent conductive wide band gap semiconductor material of claim 12 wherein said co-doped oxide cathode has an optical transmission spectra absorption threshold below 350 nanometers whereby light having a wavelength below said absorption threshold is not transmitted through said transparent cathode.
16. The transparent conductive wide band gap semiconductor material of claim 12 wherein said co-doped oxide has an optical transmission spectra absorption threshold below 375 nanometers whereby light having a wavelength below said absorption threshold is not transmitted through said transparent cathode when said co-doped zinc oxide has been grown at a temperature below 50° C.
17. The transparent conductive wide band gap semiconductor material of claim 12 wherein said co-doped oxide comprises a zinc oxide having a gallium dopant and a hydrogen dopant.
18. An improved deposition process for growing co-doped zinc oxide thin film having a low growth temperature for dopant activation comprising the steps of:
providing a zinc oxide target having at least one impurity;
maintaining a substrate at a selected temperature;
providing a mixture of oxygen and gaseous hydrocarbons between said target and said substrate; and
depositing, by deposition means, a thin film of co-doped zinc oxide on said substrate.
19. The process of claim 18 wherein said step of providing at least one impurity in said zinc oxide target comprises the step of providing about 2 atomic wt % of gallium.
20. The process of claim 18 wherein said at least one impurity in said zinc oxide target comprises about 1.5 atomic wt % of gallium.
21. The process of claim 18 wherein said at least one impurity in said zinc oxide target comprises between 0.2 atomic wt % and 2 atomic wt % of gallium.
22. The process of claim 18 wherein said gaseous hydrocarbon is selected from the following: ethane, propane or methane.
23. The process of claim 18 wherein said mixture comprises about four parts oxygen and one part gaseous hydrocarbon.
24. The process of claim 18 wherein said deposition means comprises a pulsed laser deposition chamber.
25. The process of claim 18 wherein said zinc oxide thin film comprises gallium and hydrogen dopants.
26. The process of claim 18 wherein said gallium dopant comprises about 0.5 atomic wt %.
27. The process of claim 18 wherein each gallium atom stabilizes at least one hydrogen atom.
28. The process of claim 22 wherein said depositing step comprises pulsed laser deposition to decompose said gaseous hydrocarbon and to controllably vaporize said target.
29. The process of claim 18 further comprising the step of maintaining said substrate temperature at less than 200° C.
30. The process of claim 18 further comprising the step of maintaining said substrate temperature in the temperature range of between 35° C. and 200° C. during deposition of said thin film.
31. The process of claim 30 wherein said substrate temperature is maintained between about 170° C. and 175° C. during deposition of said thin film.
32. The process of claim 30 wherein said substrate temperature is maintained below 50° C. during deposition of said thin film.
33. The process of claim 21 wherein said gaseous hydrocarbons is selected from methane, propane or ethane.
34. The process of claim 21 wherein said mixture comprises about four parts oxygen and one part methane.
35. The process of claim 18 wherein said at least one impurity in said zinc oxide target comprises about 1.5 atomic wt % of gallium and said mixture comprises about four parts oxygen and one part methane.
36. The process of claim 18 wherein said at least one impurity in said zinc oxide target comprises between 0.2 atomic wt % and 2 atomic wt % of gallium and said mixture comprises about four parts oxygen and one part methane.
37. The process of claim 29 wherein said at least one impurity in said zinc oxide target comprises between 0.2 atomic wt % and 2 atomic wt % of gallium and said mixture comprises about four parts oxygen and one part methane.
38. The process of claim 35 wherein said co-doped zinc oxide has a transmittance at least 75% of visible light.
39. The process of claim 36 wherein said co-doped zinc oxide has a resistivity of less than 2.5×10−4 ohms per square centimeter and an electron concentration of about 5.2×1020.
Priority Applications (6)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US09/281,198 US20020084455A1 (en) | 1999-03-30 | 1999-03-30 | Transparent and conductive zinc oxide film with low growth temperature |
EP00101090A EP1041644A3 (en) | 1999-03-30 | 2000-01-20 | Transparent and conductive zinc oxide film with low growth temperature |
TW089103787A TW456051B (en) | 1999-03-30 | 2000-03-03 | Transparent and conductive zinc oxide film with low growth temperature |
KR1020000011424A KR100624677B1 (en) | 1999-03-30 | 2000-03-08 | Transparent and conductive zinc oxide film with low growth temperature |
JP2000088851A JP2001007026A (en) | 1999-03-30 | 2000-03-28 | Semiconductor material equipped with wide conductivity band gap of transparency and chemical stability, and deposition method |
US09/784,411 US6458673B1 (en) | 1999-03-30 | 2001-01-16 | Transparent and conductive zinc oxide film with low growth temperature |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US09/281,198 US20020084455A1 (en) | 1999-03-30 | 1999-03-30 | Transparent and conductive zinc oxide film with low growth temperature |
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US09/784,411 Division US6458673B1 (en) | 1999-03-30 | 2001-01-16 | Transparent and conductive zinc oxide film with low growth temperature |
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US20020084455A1 true US20020084455A1 (en) | 2002-07-04 |
Family
ID=23076358
Family Applications (2)
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---|---|---|---|
US09/281,198 Abandoned US20020084455A1 (en) | 1999-03-30 | 1999-03-30 | Transparent and conductive zinc oxide film with low growth temperature |
US09/784,411 Expired - Lifetime US6458673B1 (en) | 1999-03-30 | 2001-01-16 | Transparent and conductive zinc oxide film with low growth temperature |
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US09/784,411 Expired - Lifetime US6458673B1 (en) | 1999-03-30 | 2001-01-16 | Transparent and conductive zinc oxide film with low growth temperature |
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US (2) | US20020084455A1 (en) |
EP (1) | EP1041644A3 (en) |
JP (1) | JP2001007026A (en) |
KR (1) | KR100624677B1 (en) |
TW (1) | TW456051B (en) |
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Also Published As
Publication number | Publication date |
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EP1041644A2 (en) | 2000-10-04 |
KR20000062776A (en) | 2000-10-25 |
US6458673B1 (en) | 2002-10-01 |
TW456051B (en) | 2001-09-21 |
JP2001007026A (en) | 2001-01-12 |
US20020028571A1 (en) | 2002-03-07 |
KR100624677B1 (en) | 2006-09-18 |
EP1041644A3 (en) | 2006-05-17 |
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