Patent Application
20050014033
1. Field of the Invention
This invention relates to transparent conductive thin films for use in electro-optical devices.
2. Description of the Related Art
Many electro-optical devices operating in the infrared (generally, wavelengths of ˜0.7 μm to ˜14 μm) require conductive thin films that are extremely transparent. Such devices include liquid crystal cells for beam steering, spatial light modulators, and optical switches for fiber optic communications, switchable and/or tunable polarization modification components, and long wavelength vertical cavity surface emitting lasers (VCSELs). Conventional transparent conductive oxides such as indium tin oxide (ITO), zinc oxide (ZnO), and tin oxide (SnO2) cannot fulfill the requirements of these applications. These materials are mostly transmissive in the visible; however, transmission in the infrared drops due to free carrier absorption. The transmission values versus wavelength for an ITO film on a glass substrate are shown in
U.S. Pat. No. 6,458,637, issued Oct. 1, 2002 to Jeffrey T. Cheung, directed to transparent and conductive zinc oxide films, describes the introduction of hydrogen or a hydrocarbon gas (in addition to oxygen) into the vacuum chamber during growth of the film using pulsed laser deposition from a 2.0 atomic wt. % gallium-doped ZnO target. This approach incorporated hydrogen atoms into the ZnO lattice during growth and activates them to behave as electron donors so that a low electrical resistivity film (0.9-3×10−4 Ω-cm) could be grown at low temperature. The process was found to work only when gallium was also present in the lattice. The resultant films, with or without doping, had a transmissivity of no more then about 90% at wavelengths of 0.35 to 2.0 μm.
Cadmium oxide (CdO) films doped with indium (CdO:In) have been prepared in the past for flat-panel displays and solar cells which require high transparency to visible light. However, these films are well known for their toxicity, and therefore the prior art has steered away from developing such films. Minami et al. reports that CdO:In films have been prepared with a resistivity of the order of 10−5 Ω-cm for flat-panel displays and solar cells, but he states that they are of no practical use because of Cd toxicity. (Minami, Tadatsugu, “New n-Type Transparent Conducting Oxides”, Transparent Conducting Oxides, Volume 25, No. 8, August 2000, p.38). Further, because these films have a yellow color, which differentiates them from prior art transparent films that are clear in appearance, one skilled in the art would be steered away from using these films for any applications requiring transparent films.
Undoped CdO has been prepared by sputtering, MOCVD, and spray pyrolysis (Murthy, L. C. S. & Rao, K. S. R. K., “Thickness Dependent Electric Properties of CdO Thin Films Prepared by Spray Pyrolysis Method,” Bulletin of Material Science, Vol. 22, No. 6, pp.953-957 (October 1999); Subramarnyam, T. K et al., “Preparation and Characterization of CdO Films Deposited by DC Magnetron Reactive Sputtering”, Materials Letter, Vol. 35, pp.214-220, (May 1998); Baranov, A M et al., “Investigation of the Properties of CdO Films”, Tech. Phys. Ltr, 23, (10) pp.805-806 (October 1997)). Transmission no greater then about 85% has been reported in the wavelength range of 0.6-1.6 μm.
Representative data, doping and method of fabrication for these references, along with additional references reporting on CdO films, are listed in Table 1 below.
*Intrinsic dopant means that donors in the materials are due to defects not impurities
**Measured against glass slide as reference
1Shaganov, II, et al., “Obtaining transparent oxide conducting coatings by pulsed laser sputtering” Sov. J. Opt. Technol. 48(5), p 280-282 (May, 1981)
2Pahtak, Girish et al, “Deposition and properties of cadmium oxide films by activated reactive evaporation”, Thin Solid Films, 245, p17-26 (1994)
3Varkey, AJ et al, “Transparent conducting cadmium oxide thin films prepared by a solution growth technique”, Thin Solid Films, 239, p211-213 (1994)
4Chu, TL et al, “Degenerate Cadmium Oxide films for electronic devices”, J. Electronic Materials , 19, p1003- (1990)
5Tanaka, K et al “Electrical and optical properties of sputtered CdO films”, Japanese J. of Appl. Phys., 8(6), p681-691 (June 1969)
6Minami, Tadatsugu, “New n-Type Transparent Conducting Oxides”, Transparent Conducting Oxides, 25, (8), p38-44 (August 2000)
7Murthy, L. C. S. et al., “Thickness Dependent Electric Properties of CdO Thin Films Prepared by Spray Pyrolysis Method”, Bulletin of Material Science, 22, (6), pp953-7 (Oct. 1999)
8Subramarnyam, T. K et al, “Preparation and Characterization of CdO Films Deposited by DC Magnetron Reactive Sputtering”, Materials Letter, 35, pp 214-220, (May 1998)
9Subramarnyam, T. K et al, “Influence of Oxygen Pressure on the Structural and Optical Properties of DC Magnetron Reactive Sputtered Cadmium Oxide”, Physica Scripta, 57, p317 - (1998)
10Baranov, AM. et al, “Investigation of the Properties of CdO Films”, Tech. Phys. Ltr, 23, (10) pp 805-806 (October 1997)
11Ferro, R et al “F-Doped CdO Thin Films Deposited by Spray Pyrolysis”, PMS. State. Sol. (a) 177, P477-483 (2000)
12Coutts, TJ et al, “Search for improved transparent and conducting oxides: A fundamental investigation of CdO, Cd2SnO4, and Zn2SnO4”, J Vac. Sci. Tech., A18, (6), p2646-2660, (Nov/Dec 2000)
There is a need for films with lower resistivity and low optical absorption at wavelengths that extend from the short-wavelength infrared (SWIR) (1-2 μm) through the mid-wavelength infrared (MWIR) (3-5 μm) and possibly into the long wavelength infrared (LWIR) (8-12 μm). As an example, devices operating in the fiber telecommunication bands at 1.3 or 1.5 μm require an optical transmission of at least 99%.
A doped cadmium oxide (CdO) film with high optical transmission and suitable electrical conductivity has been developed for use in applications requiring conductive, infrared transparent films. Suitable dopants for the film include any of the Group III elements (i.e. boron, aluminum, gallium, indium or thallium), bismuth, fluorine, and hydrogen (as a co-dopant) if properly processed.
The new doped film is substantially transparent to infrared radiation in the range of between about 0.7 μm and 14 μm. Films made according to the invention have a sheet resistance of <600 Ω/□ (typically ≦200 Ω/□), and have exceptionally low optical absorption throughout the IR range: less than 4% (typically <1%) in the SWIR, less than 10% (typically <3%) in the MWIR, and less than 20% (typically <4%) in the LWIR. In addition, a film per the present invention having a sheet resistance of <600 Ω/□ (typically ≦200 Ω/□) has an absorption loss of about ≦0.2% at 1.3 μm and ≦0.5% at 1.5 μm. Films made according to the invention have a resistivity of less than 5×10−4 Ω-cm. Low resistivity films per the present invention can be prepared at much lower temperatures (20° C.) by co-doping with hydrogen.
There are numerous applications for such films functioning as transparent, electrically conductive electrodes, including (but not limited to) liquid crystal cells for beam steering, spatial light modulators, optical switches for fiber optical communications, switchable and/or tunable polarization modification components, top transparent electrodes for SWIR (1.3 and 1.51 μm) VCSELs, Pockels' cells, MEMs devices, and ferroelectric cells. Other applications include radio frequency shielding of windows that can transmit infrared radiation.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which:
a is a plot showing transmission versus wavelength for a CdO:In film per the present invention on a MgO substrate for wavelengths from 2.5 to 10 μm.
b is a plot showing the transmission of the film of
a is a cross-sectional view of a prior art embodiment of a VCSEL device.
b is a cross-sectional view of an embodiment of a VCSEL device incorporating a CdO:In film per the present invention.
a is a cross-sectional view of a prior art embodiment of a ferroelectric device.
b is a cross-sectional view of an embodiment of a ferroelectric device incorporating a CdO:In film per the present invention.
a is a cross-sectional view of an embodiment of a Pockels' cell device incorporating a CdO:In film per the present invention.
b is a cross-sectional view of another embodiment of a Pockels' cell device incorporating a CdO:In film per the present invention.
A doped Cadmium Oxide (CdO) film has been developed for use primarily in applications requiring conductive, infrared transparent films. Suitable dopants for the film include any of the Group III elements (i.e. boron, aluminum, gallium, indium or thallium), bismuth, fluorine, and hydrogen (as a co-dopant). In addition, hydrogen and a group III element such as indium can be used simultaneously as co-dopants to lower the growth temperature of transparent and electrically conductive films. These dopants are believed to increase the electron concentration of CdO, thus making it more conductive.
There are numerous applications for the film, including:
While not requiring the transparent properties of the CdO film, the present films can also be used in electrodes for piezoelectric devices.
The terms “sheet resistance” and “resistivity” are used herein; they are defined as follows. Resistance (R) is electrical resistance measured between two contacts on an electrically conductive material of any shape and thickness. It is not a quantitative description of material property. The unit of resistance is the ohm (Ω).
Resistivity (ρ), also known as specific resistance, is the resistance measured across the opposite faces of a 1 cm×1 cm×1 cm cubic shaped material. The unit of resistivity is the Ω-cm. This quantity depends only on the fundamental property of a thin film material; i.e., it does not depend on its thickness. However, as will be discussed below, sometimes the resistivity of a thin film is different near the initial growth interface.
Sheet resistance (Rs) is a quantity that is often used to describe the conducting thin film or layers in a device (e.g., the RF shield and electrodes for liquid crystal and ferroelectric devices). It is expressed in units of Ω/□ and is dependent on thickness “d”. It is related to resistivity (ρ) by the following relationship: ρ=Rs×d, with d specified in cm.
Films made according to the invention have a sheet resistance of <600 Ω/□(typically ≦200 Ω/□), and have exceptionally low optical absorption throughout the IR range: less than 4% (typically <1%) in the SWIR, less than 10% (typically <3%) in the MWIR, and less than 20% (typically <4%) in the LWIR. In addition, a film per the present invention having a sheet resistance of <600 Ω/□ (typically ≦200 Ω/□) has an absorption loss of about ≦0.2% at 1.3 μm and ≦0.5% at 1.5 μm. Films made according to the invention have a resistivity of less than 5×10−4 W-cm.
The film can be grown on any suitable substrate such as MgO, glass, quartz, sapphire, silicon, AION, spinel, ZnSe, ZnS, GaAs, Ge, etc. Many methods are known for growing Group III element doped CdO films, including metal organic chemical vapor deposition (MOCVD), spray pyrolysis, sol-gel, vapor transport, hot wall epitaxy, close-space vapor transport, plasma-enhanced chemical vapor deposition (PECVD), sputtering, activated reactive evaporation, ion-assisted deposition, and pulsed electron-beam evaporation (see Handbook of Deposition Technologies For Films and Coatings, Second Edition, Noyes Publications, Bunshah, R. F. 1994). However, to obtain the combination of low electrical resistivity and high optical transmissivity described herein, the present invention requires that the film be formed using a method which “activates” the dopants.
A preferred method is pulsed laser deposition (PLD) since it allows for accurate control of the film composition and thickness, and facilitates the activation of dopants. This method is described in great detail in “Pulsed Laser Deposition of Thin Films”, Chrisey, 1994. Specifically, the experimental set up comprises a target holder and a substrate holder housed in a vacuum chamber. A well-blended mixture of the material to be vaporized and condensed on the substrate is placed in the target holder. A high-power laser used as an external energy source is focused on the target to vaporize the mixture in a controlled manner. Various optical components are used to focus and raster the laser beam over the target surface. (see “Pulsed Laser Deposition of Thin Films”, Chrisey, 1994, p. 3). Absorption characteristics of the material to be evaporated determine the laser wavelength to be used. Pulse width, repetition rate, and pulse intensity are selected for specific applications. (see Bunshah, pg. 167). The thickness of the film can be controlled by varying the pulse repetition rate and distance of the target. The PLD process has been used to produce films having a thickness between 100-4000 Å. The composition of the grown film is substantially the same as that of the target composition.
For preparation of the CdO film incorporating features of the invention, a preferred composition contains from about 0.5%, to about 6% by weight of the dopant. However, films with suitable properties can be obtained with indium concentrations as low as 0.05%(atomic weight). Films with 0.5-6.0% indium exhibit excellent optical properties up to a wavelength of about 2.0 μm, beyond which the free carrier absorption becomes dominant. By reducing the doping concentration to about 0.05%(atomic wt.) of indium, optical transmission is improved at longer wavelengths.
The films can be grown at temperatures from about 20° C. to about 425° C. The substrate temperature affects the quality of the film with higher temperature producing a film with higher electrical conductivity and optical transparency in the infrared and visible range. An exemplary procedure for growth of a doped CdO film per the present invention is set forth in the example below.
A ceramic target comprising In2O3 and CdO was placed in the target holder of the PLD chamber about 10 inches away from an MgO substrate. The target was prepared by mixing 1% by weight In2O3 powder with 99% by weight CdO powder in a jar, filling the jar with a quantity of methanol approximately equal in volume to the CdO/In2O3 mixture to form a slurry, and adding a like volume of ceramic beads (approximately 1 cm in diameter) to the mixture. The mixture was then “rolled” for 24 hours, the process being known as “ball milling”. The methanol was then evaporated from the mixture and the beads were sifted out. The remaining powder was pressed into a cylindrical shape and sintered in a high temperature furnace (about 1100° C.) to form a ceramic target.
The target was then placed in a vacuum chamber having an O2 partial pressure of approximately 5-50 millitorr and the temperature of the substrate was raised to 440° C. The target was irradiated using a Kr:F excimer laser beam having a wavelength of 248 nm with a pulse energy of 390 mJ/pulse at a frequency of 5 pulses/sec. The energy delivered by the laser was 1 J/cm2 with a pulse duration of 32 ns. Other excimer laser types, such as fluorine, ArF, and XeCl, or a Q-switched Nd:YAG laser with frequency quadrupler that delivers high energy pulsed UV output, might also be used.
Electrical conductivity was determined using a four-point probe measurement. The CdO:In film was found to have a sheet resistance of 112 Ω/□ and exceptionally high transmission as shown in
In
An alternative method of contrasting indium-doped CdO films with prior known conductive films is to compare the real refractive index (n) and the imaginary refractive index (k) of both. The complex refractive index ({overscore (n)}) is the sum of the real refractive index, n (the real part of the complex refractive index), and the imaginary part of the complex refractive index, which is the extinction coefficient, k, according to the formula {overscore (n)}=n+ik. The absorption coefficient, α, is obtained from the following relationship:
T=I/I0=e−αd
Where α4=πk/λ, λ=wavelength, d=thickness of the film, and I/Io=light incident/light transmitted.
Hydrogen doping or co-doping, accomplished by adding H2 or CH4 in the chamber atmosphere during film production, allows growth of low resistivity and highly transparent films such as described herein, at lower temperatures (approximately 200° C. and lower).
This data demonstrates that co-doping with hydrogen provides the capability of producing low resistivity films at much lower temperatures. These films may have a slight, but acceptable, optical loss at wavelengths greater than 1.1 microns.
Gallium doped CdO has similar properties to the Indium doped CdO. Other Group III elements should provide similar properties, as should bismuth and fluorine.
As indicated above, a second important property of the infrared transparent doped CdO films is the resistivity of the films. While an undoped CdO film prepared by this method will have a resistivity of about 1.45×10−3 Ω-cm, a significant reduction of resistivity to about 5×10−4 Ω-cm can be obtained by adding as little as 0.05% indium, and this can be further reduced to about 0.5×10−4 Ω-cm by increasing the dopant concentration to 2.0%. Another basis for comparison of various optical materials is the absorption coefficient, α, which is defined above. The absorption coefficient is a measure of how well a material absorbs light of a certain wavelength. A graph plotting absorption coefficient vs. wavelength for films with six different resistivities is shown in
Indium-doped CdO films deposited by PLD show electrical and optical properties superior to those grown by other techniques previously reported. The following list tabulates the properties of films made in accordance with the present invention, as might be used in various types of devices. Absorption loss (best value) of other transparent conductive materials and CdO films grown by other techniques with the same sheet resistance are listed in the parenthesis for comparison.
PLD not only produces CdO films with high activation of the dopants (i.e. indium or other elements) to increase its electrical conductivity, it also significantly improves the film's structural quality. One example is the present film's exceptionally thin transition layer (sometimes referred to as a “dead layer”) at the interface between the grown film and substrate. A dead layer has very poor quality, with low electron mobility and high electrical resistivity. A thick dead layer will therefore increase a film's optical loss without contributing to its electrical conducting characteristics.
However, for the present film, the combination of PLD and doped CdO results in a dead layer that is only approximately 100-200 Å thick (see
One application of the doped CdO films is for transparent conductive electrodes for liquid crystal cells used in optical beam steering, spatial light modulators, and optical switches operating in the SWIR and MWIR ranges. A first embodiment of a device incorporating features of the invention, shown in
In certain applications, conductive transparent films of doped CdO as described herein can be used to prevent the transmission of radio frequency interference from optical systems. In this application, the film is coated onto a transparent window surface. The infrared signal is transmitted while longer wavelength radio frequency signals (>10 MHz, e.g.) are reflected as the result of the conductivity of the film.
VCSELs (Vertical Cavity Surface Emitting Lasers) are a class of solid-state laser that can be fabricated on a semiconductor wafer by batch processing to produce a dense two-dimensional array of lasers whose optical outputs are perpendicular to the substrate. These devices are small with low power consumption, fast, and vital to optical communication systems.
Schematic cross sectional views of a prior art “top emitting VCSEL” and a top emitting VCSEL which employs a film in accordance with the present invention are shown in
Ferroelectric devices, including, for example, ferroelectric memory devices, electro-optical devices, and piezoelectric devices, represent another class of devices which may benefit from the use of a doped CdO film per the present invention; a prior art electro-optical ferroelectric device and an electro-optical ferroelectric device in accordance with the present invention are shown in
Another advantage offered by using the doped C dO electrode in an electro-optical device fabricated from a single crystal ferroelectric material is that light can now transmit through the electrodes 546, 548 in the direction along the polarization of the crystal, as well as in the direction perpendicular to the direction of polarization inside the crystal. In the case of using metal electrodes 542, 544 (
Another type of device which may benefit from the use of a doped CdO film per the present invention is a Pockels' cell, which is an electro-optic device in which birefringence is modified under the influence of an applied voltage. If the birefringent medium is a solid, then the applied voltage is normally applied using transparent, electrically-conductive films applied on both sides of the solid. If the birefringent medium is a liquid, then transparent, electrically-conductive films are applied on the inside of the window used to confine the liquid.
a illustrates an arrangement for a Pockels' cell in which the birefringent medium 550 is a solid. The light must pass through the two doped CdO film electrodes 552, 554 that modify the birefringence.
Yet another possible application of a doped CdO film per the present invention is with optical micro-electromechanical devices (MEM devices), in which a transparent electrode is used as part of the activation mechanism for the device's electrostatic actuator. There could be many different MEM device embodiments, including, for example, a multi-faceted focusing mirror, or a variable-wavelength filter or reflector.
While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
This application claims the benefit of U.S. Provisional Application Ser. No. 60/282,337 filed Apr. 6, 2001, and is a continuation-in-part of Ser. No. 10/112,465 filed Mar. 29, 2002.
| Number | Date | Country | |
|---|---|---|---|
| 60282337 | Apr 2001 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10112465 | Mar 2002 | US |
| Child | 10889686 | Jul 2004 | US |
