Electro-Optical

Transparent Conductive Oxide Coatings

Transparent Conductive Oxide (TCO) coatings include materials such as ITO, AZO, IGO, ZGO, and have found wide use due to their unique combination of high electrical conductivity and transparency in the visible portion of the spectrum.

Typical applications include electrodes for touch displays and solar cells, heat mirrors, and EMI shielding. In all these forms, the optical performance is measured by its transparency in the visible portion of the spectrum. For some applications its value is also in its ability to reject IR radiation, typically beyond 1 µm. For example low-E window coatings initially employed Indium Tin Oxide (ITO) layers as the broadband IR reflector.

Acree Technologies is a world leader in transparent conductive oxide coatings. We use sophisticated computer simulations to develop a coating with the desired properties, and then employ proprietary technologies to deposit it. Typically TCO coatings are multi-layered, and can include absorbing, insulating, and metallic/conductive layers. The multi-layer structure permits great flexibility in the type of effects we can achieve. Most of these films are put down using highly ionized deposition techniques, which creates dense and highly adherent films with excellent electro-optical performance. High ionization also features increased reactivity in both the vapor phase and the condensation process, which permits low working temperatures. This allows us to coat highly temperature sensitive materials such as plastics. We use in situ feedback and computer control to produce high quality, reproducible films.

Some examples of unique optical coatings we have developed for clients include:

  • Transparent, electrically conductive coatings for aircraft canopies, windscreens, and windows. These coatings fulfill a variety of purposes- improved EMI shielding, improved resistance to wear, and improved transmission properties.
  • Coatings for HID lamps to reduce UV transmission and increase IR reflection. The coating increases their efficiency by raising the internal temperature, which results in more visible light emitted for less electrical power. This also increases their working lifetime.
  • Surface treatment that provides tenacious adhesion of coatings to plastics, even hard to coat plastics such as acrylic.
  • Hard anti-reflective (AR) coatings for safety lenses and visors.

 

Materials deposited include:

  • Dielectrics- DLC, TiO2, Al2O3, SiO2, Si3N4, Ta2O5
  • Metals- Ag, Pt, Au, Al
  • Transparent conductors (TCO)- ITO, AZO, IZO

Technical Background for Transparent Conductive Oxide coatings

Transparent conductive oxide coatings (TCO) have found wide use due to their unique combination of high electrical conductivity and transparency in the visible portion of the spectrum. Typical applications include electrodes for displays and solar cells, electro-magnetic interference (EMI) shielding for aircraft canopies, and heat mirrors. In all these forms, the optical performance of a TCO is measured by transparency in the visible portion of the spectrum. In other applications TCO is valued for its ability to reject infrared radiation (IR), typically beyond 1μ. For example low-E window coatings initially employed Indium Tin Oxide (ITO) layers as the broadband IR reflector. Typical transmission curves of a single ITO coating on glass are shown in Figure 1. Each curve represents an ITO film with a different carrier concentration (discussed below).

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TCO materials are characterized as semiconductors, the conductivity of which originates from two sources: electrons in the conduction band or holes (or lack of electrons) in the valence band. The vast majority of TCO materials are n-type (electron carriers) and therefore this article limits itself to this specific case. The material resistivity, ρ, is expressed as:
 
(1)ρ = 1μnee
 
where µeis the electron mobility, ne is the electron carrier concentrations, and e is the charge (1.6×10-19 C). The resistivity and film thickness d combine to determine the films sheet resistance Rs:
 
(2)Rs = ρd
 
For EMI shielding \strikeout off\uuline off\uwave offRs\uuline default\uwave defaultis typically the specified parameter since it uniquely determines the shielding properties. The extraction of the optical constants is more complicated. Ideally the complex refractive index n needs to be determined for the material:
 
(3)n = N − ik
 
where N is the refractive index (the real portion of the complex index) andk is the extinction coefficient. With this relation the coatings optical properties (Transmittance, Reflectance and Absorptance) can be modeled using commercial software such as TFCalc. The refractive index and extinction coefficient are related to the real and imaginary parts of the film permittivity (ε1 and ε2)1:
 
(4)N = √(ε21 + ε22)122 + ε12
 
(5)k = √(ε21 + ε22)122 − ε12
 
The relative dielectric constant (ϵr) can be expressed as a sum of the various contributing susceptibilities2
 
(6)εr = ε1 + iε2 = 1 + χBG + χFE + χPH
 
where χBG is the semiconductor band edge, χFE is the free electron gas, and χPH is the optical phonons contributions respectively. The band edge contribution dominates in the visible spectrum and the optical phonon contribution is significant in the far infrared (FIR, 15-1000μm). In the near (0.75-1.4μm) and mid-wave (3-8μm) IR the free electron susceptibility is the most significant contributing factor. Therefore, in the mid-wave IR the relative dielectric constant can be simplified as:
 
(7)εr = ε∞ + χFE
 
where ε∞ represents the macroscopic static electronic dielectric constant – essentially a fitting parameter. Using the Drude model of collective free electron behavior to describe χFEFE the two components of the permittivity (ε1and ε2) can be extracted:1
 
(8)ε1 = ε∞⎛⎝1 − ω2pω2⎞⎠
 
 
(9)ε2 = ⎛⎝ε∞ω2pω3τ⎞⎠
 
where ω is the angular frequency and ωp is the plasma frequency, given by:
 
(10)ωp = ⎛⎝nee2ε0ε∞m*e⎞⎠12
 
with m*e the electron free mass, ε0 the vacuum permittivity, and ne the carrier concentration. In Equation (9) above, the damping coefficient, τ, is given by:
 
(11)τ = μm*ee
 
Therefore using the above relations the refractive index, N, and the extinction coefficient,k, can be determined as a function of the mobility μ and the electron concentration (ne).
The total optical loss is an additional important consideration. The incident light on the surface of the film must be transmitted, reflected or absorbed. If the reflectance,R, is minimized (or accounted for) the transmittance is approximated by the Beer-Lambert law:
 
(12)T = e − αd
 
where d is the film thickness and α is the absorption coefficient, which is related to the extinction coefficient, k by:
 
(13)α = 4πk/λ
 
where λ is the wavelength. Therefore, when all of the above equations (1-13) are used together, the optical and electrical performance of the coating can be predicted from the values of ne, μ, and d alone. This is very significant as it allows the detailed examination of the potential trade-offs between the carrier mobility, concentration and thickness.
 
Acree Technologies Inc. has created a model using the relations described above which calculates the complex refractive index from a specified carrier concentration and mobility. The complex refractive index is then used in a commercial thin film modeling program to calculate the transmittance, reflectance and absorptance which takes into account the film thickness. This is also used to create models for antireflective (AR) coating designs to minimize the reflectance, thereby maximizing the transmittance.
 
It is worthwhile to use this model to compare the performance of two different TCO coatings. The complex refractive index for two different TCO films with equal resistivity is compared, with the difference between the coatings being how the resistivity is achieved. These values are based on films created at Acree Technologies Inc. The first film is a typical AZO coating and has a moderate mobility of 30 cm2/Vs and carrier concentration of 10×1020 cm-3;  while the second film is an Acree proprietary coating and has a high mobility of 300 cm2/Vs and carrier concentration of 1×1020 cm-3.  The calculated complex indexes are shown in Figure 2.
The calculation ignores the band gap contribution at low wavelengths, which does not affect the region of interest for this work. The dip in the real part of the index (N) is indicative of the plasma frequency location. As expected the lower carrier concentration decreases the plasma frequency shifting it to longer wavelengths. This same transition is clearly evident in the rise of the extinction coefficient. However, the increased mobility has the added benefit of producing and maintaining a lower value of N. This increases the ratio of k ⁄ N, which leads to an increase in the achievable transmittance, as first detailed by Berning and Turner.3
 
A pertinent question is to know when the highest possible transmittance for a given combination of substrate and Transparent Conductive Oxide coating has been achieved. This is answered through the concept of the potential transmittance, as described by Macleod.4 The potential transmission, ψPot, is defined as the ratio of the intensity leaving the rear surface to that actually entering the surface of a layer or assembly of layers:
 
(14)ψPot = IexitIent
 
where Iexit is the intensity at the exit of the assembly and Ient is the intensity entering the assembly. Therefore ψPot represents the maximum theoretical transmittance that can be achieved if the reflectance of the assembly were zero. It is worth noting the distinction between Ient and the total incident intensityIo. The total incident intensity is the intensity arriving at a surface; i.e. the intensity that would be measured if the film assembly were not present. This is contrasted to Ient which is the intensity actually entering the assembly; i.e. the total incident intensity minus any reflective loss. The potential transmittance can be more conveniently expressed as:
 
(15)ψPot = T1 − R
 
where T is the transmittance and R the reflectance. Therefore a value of ψPotbelow 100% indicates the potential transmittance is limited by absorption present in the coating layers. For this specific application the TCO layer thickness is determined by the mobility, carrier concentration and the desired sheet resistance. When the TCO layer properties are fixed (in order to achieve the desired electrical properties) the maximum potential transmittance is determined entirely by the refractive index of the TCO. As an example, using the index profiles shown in Figure 2, the transmittance, reflectance, and absorptance were calculated for a film thickness of 100 nm and sapphire thickness of 1 mm. Using this result the maximum potential transmittance of the TCO-substrate combinations was calculated. The results are shown in Figure 3.
 
Uncoated sapphire is also included for reference. The difference in the possible transmittance of these coatings is substantial. Even though the electrical resistivities are equivalent, the increased mobility and decreased carrier concentration substantially increases the transmittance that can be achieved through the midwave IR. The 1 mm sapphire substrate alone decreases the transmittance at 5μ to 91.6%. Applying 100 nm of the high mobility TCO decreases the transmittance to 86.3%, or 94% of that achievable on 1 mm thick sapphire. In contrast, the maximum Transmittance achievable for the conventional TCO with a mobility of 30 cm2/Vs is only 10%. While not meant to be exhaustive, the above analysis highlights the importance of achieving high carrier mobility in order to achieve high transmittance in the midwave IR. The significance of the above relations can be summarized as follows: the electrical conductivity of the TCO is a function of both the carrier concentration and its mobility, while the IR reflectance is a function primarily of the carrier concentration and only weakly on the mobility. Therefore, a low carrier concentration will produce increased IR transmission. With this lower carrier concentration the film conductivity is maintained if the carrier mobility is increased. This is a very significant result as the EMI shielding capability is dependent only upon the sheet resistance of the TCO coating. Therefore the ability to trade off carrier concentration against mobility to increase IR transmission while maintaining high conductivity is a key attribute of Acree Technologies’ Inc.’s approach.
 
The exact combination of mobility and carrier concentration required is not a straightforward question. As the carrier concentration is reduced for a fixed mobility the film thickness must be increased to achieve the desired sheet resistance. This increased thickness counters the benefit of the reduced carrier concentration. From testing to date we know that it is desirable to achieve a carrier mobility greater than 300 cm2/Vs, resistivity below 1×10-4Ω/cm, and carrier concentration below 2×1020 cm-3 in order to meet the project goals. TCO coatings with low carrier concentration and high mobility have been developed at Acree Technologies Inc. using energetic deposition processes (EDP). Using EDP CdO and CdInO films with carrier mobilities of 350 cm2/Vs and carrier concentrations below 2×1020 cm-3 have already been repeatedly fabricated.
References
 
1) Jin, Z., Hamberg, I., & Granqvist, C. (1988). Optical properties of sputter deposited ZnO: Al thin films. Journal of applied physics, 64(July 1988), 5117-5131.
 
2) Coutts, T., Young, D., & Li, X. (2000). Characterization of transparent conducting oxides. MRS bulletin, (August), 58-65.
 
3) Berning, P. H., & Turner, a. F. (1957). Induced Transmission in Absorbing Films Applied to Band Pass Filter Design. Journal of the Optical Society of America, 47(3), 230. doi:10.1364/JOSA.47.000230
 
4) A. Macleod, “Thin-Film Optical Filters”, Institute of Physics Publishing, 2001.
5) Calnan, S., & Tiwari, a. N. (2010). High mobility transparent conducting oxides for thin film solar cells. Thin Solid Films, 518(7), 1839-1849. doi:10.1016/j.tsf.2009.09.044
 
6) A. Fischer-Cripps, “Nanoindentation 2nd Edition”, Springer-Verlag, 2004.
 
7) Sullivan, R., Phelps, A., Kirsch, J., Welsh, E., & Harris, D. (2007). Erosion studies of infrared dome materials. Proceedings of SPIE, 6545(2), 65450G-65450G-11.
 
8) Zabinski, J., Hu, J., Bultman, J., Pierce, N., & Voevodin, A. (2008). Stoichiometry and characterization of aluminum oxynitride thin films grown by ion-beam-assisted pulsed laser deposition. Thin Solid Films, 516(18), 6215-6219..
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