Transparent Conductive Oxide Thin Films

Transparent conductive oxides (TCO) have high optical transmission at visible wavelengths and electrical conductivity close to that of metals. They also reflect near infrared and infrared (i.e., heat) wavelengths, and are used in products ranging from energy efficient low-e windows to photovoltaics. One of their most important uses is for transparent electrical contacts. Virtually all applications involve thin films. TCO’s range from simple binary compounds to exotic ternary and quaternary compounds. Examples of TCO’s are indium tin oxide (ITO), zinc oxide (ZnO), tin oxide, aluminum doped zinc oxide (AZO), indium oxide and cadmium oxide. Cost savings using TCO’s is enormous because material usage is low, they are used in a wide range of energy efficiency products and they increase the efficiency of these products. TCO’s are generally n-type wide bandgap semiconductors (although p-type materials are now being developed) with a relatively high concentration of free electrons in the conduction band. The wide bandgap is responsible for high optical transmittance and free electrons increase electrical conductivity. This article will provide a basic understanding of TCO optical and electrical properties, survey TCO applications and types of TCO’s and deposition processes. INTRODUCTION Transparent conductive oxides (TCO) are used in a wide range of applications, including low-e windows, transparent contacts for solar cells, optoelectronic devices, flat panel displays, liquid crystal devices, touch screens, EMI shielding and automobile window deicing and defogging. These materials have been intensely developed since the late 1970’s but have actually been around for a century. Cadmium oxide (CdO) was the first TCO and was used in solar cells in the early 1900’s. Tin oxide (SnO2) was first deposited on glass by pyrolysis and CVD in the 1940s for electroluminescent panels. Since then, applications and deposition processes have mushroomed. Transparent thin films and materials are some of the most commonly used materials that we depend on for a wide range of applications. Of these, transparent conductive oxides (TCOs) head the list. Table 1 lists electrical properties of selected n-type transparent oxides (p-type materials have been developed but . A number of ternary compounds have been developed over the last ten years, including Zn2SnO4, ZnSnO3, MgIn2O4, (GaIn)2O3, Zn2In2O5, and In4Sn3O12 [1].They are critical for energy efficiency applications such as lowe windows, solar cells, and electrochromic windows and are routinely deposited onto plastics and flexible plastics. Transparent conductive thin films are used as the transparent electrical contacts in flat panel displays, sensors, and optical limiters. New developing applications are the charge carrier layers in transparent transistors and solar cells, dye sensitive solar cells and organic solar cells. TCO Lowest Reported Resistivity (μΩ.cm)* ITO 114 [11] In2O3 100 [12] SnO2 400 [12] ZnO 120[12] ZnO:Al 1300 [12] CdSnO2 130 [13] CdO:In 60[12] *Best available literature Table 1. Resistivity’s of selected transparent conductive oxides Transparent Conductive Oxide Thin Films TCO’s are now used in such a wide variety of energy-related applications that the cost savings they provide is difficult to quantify. The energy savings in low-e coatings alone is staggering. These coatings can reduce energy loss by as much as 30% [2]. TCO’s are used as transparent conductive electrodes in virtually every thin solar cell design and several single crystal cells as well as electrochromic windows [3]. We must now look at the electrical and optical properties of transparent metal oxides and electrons in a different way by asking the question “how can a material be both transparent and conductive?” First consider semiconductors. Due to their energy band structure (bandgap ~ 1 – 2 eV), these materials are transparent in near infrared (NIR) and infrared (IR) wavelength ranges and can be reasonably conductive (but not close to the conductivity of metals). Doping also improves their conductivity while optical transmittance is not significantly degraded. To better understand why TCO’s conduct so well it will be instructive to review the band structures of metals and semiconductors. Recall that electrical (and heat) conduction in metals is due to free electrons in the conduction band, as shown in Figure 1 [4]. Because the occupied energy levels fall right below the Fermi Energy (EF), electrons can easily be excited into the empty or partially filled conduction band. From classical Drude theory, the electrical conductivity is simply related to the free electron density by σ = neτ(EF)/m. Where n is the electron density (in the conduction band), e is the charge on the electron, τ is the relaxation time between electron-electron collisions and other scattering events, and m is the mass of the electron. The situation is very different for semiconductors, which have a well defined gap between the valence and conduction bands. Here the conductivity results from electrons and holes and is given by σ = e(nhτh/mh* + neτe/me*). Where the hole and electron contributions have been separated out and mh* and me* are the effective mass of the hole and electron (~ (∂E/∂k)). Figure 1. Metal energy band structure [4] Transparent Conductive Oxide Thin Films Semiconductors are doped with donors (n-type) and acceptors (p-type) to improve their electrical conductivity. Doping introduces states in the band gap just above the valence band or just below the conduction band, as shown in Figure 2 [5]. Defects also create states in the band gap that can increase or decrease the conductivity. Doping and defect sites have the same effect on transparent insulators, such as ZnO, In2O3, CdO, and SnO2. Both doping and oxygen vacancies are used to increase the conductivity of these materials. Figure 3 shows tin (Sn) doping sites in an indium oxide (In2O3) lattice [5]. The Sn atom occupies an interstitial site and contributes an electron (i.e., a donor), making the doped indium oxide into indium tin oxide (ITO). Figure 4 shows the assumed band structure of undoped and Sn doped indium oxide [5]. Note that the occupied states are just below the conduction band, typical of a n-type semiconductor. Oxygen vacancies are mainly responsible for electrons in the conduction band [5]. ITO and other TCOs thus, are highly degenerate n-type semiconductors. Degeneracy simply implies that there are a number of energy bands that overlap and the Fermi energy is close to the edge of the conduction band. Figure 2. Density of states for p and n-type dopants in a semiconductor [2] Figure 3. Sn doping sites in an In2O3 lattice [14] Figure 4. Assumed band structure of In2O3 and ITO [14] Transparent Conductive Oxide Thin Films Defect sites in solids can also scatter electrons, so a balance must be reached of increased conduction by electrons due to defect site and scattering by defect sites. As a result, there is a conductivity maximum or resistivity minimum for TCO when the contributions from doping and scattering (i.e., oxygen vacancies) are optimized. Resistivity is high for low doping concentrations and high again for high doping due to scattering from oxygen vacancies [5]. Figure 5 shows this “resistivity well” [15]. As one would expect, the optical properties of TCOs also depend on doping and oxygen vacancies. Much like metals, TCOs have high NIR and IR reflectance. Low-e windows, solar cell electrical contacts and EMI shielding windows take advantage of this high NIR –IR reflectance and high visible transmittance. Free electrons will absorb EMI radiation, which includes photons. Drude theory can also be used to model the optical properties of metals and transparent conducting materials. The frequency-dependent conductivity can be expressed as σ(ω) = neτ(1 + iωτ)/m(1 + ωτ) [6]. The frequency dependent permittivity is then ε(ω) = εο + iσ(ω)/ω, and from this we obtain the plasma frequency ωp = ne/mεο. The plasma frequency is one of most important concepts in the electrical and optical properties of solids and is the natural frequency of oscillation, or “tuned frequency” of the electron gas. Expressions for the reflectivity of metals and transparent conducting materials are derived from these relations. The plasma wavelength (~ 1/ ωp) for most n-type TCOs is in the NIR. The dielectric constant for a free electron gas can be expressed as ε = 1 ωpτ /(1 + ωpτ) + iωpτ/ω(1 + ωpτ). Figure 5. Resistivity well in a TCO [15] Transparent Conductive Oxide Thin Films Figure 6 shows the behavior of the reflectance (= (√ε − 1)/(√ε + 1)) near the plasma frequency [4]. The TCO is highly reflective at frequencies below the plasma frequency (longer wavelengths) and transmitting at higher frequencies (shorter wavelengths). The free carriers also absorb electromagnetic radiation near the plasma frequency, which is called the Drude tail. Thus far only n-type TCOs have been addressed using a very simplified picture of the conduction mechanisms. Ptype TCOs are now being developed and have very good transmission in the NIR and IR, but higher resistivities. The conduction mechanism for p-type TCOs is very different than n-type materials. Their conductivity is not due to electrons but to small polarons [5]. TCO Thin Film Materials Review Here we address only the most widely used TCO’s, including ITO, ZnO, AZO and p-type films. Indium Tin Oxide (ITO) Indium tin oxide (ITO) is the most widely used and developed transparent conductive oxide material and is still the “gold standard” of TCOs. Applications for thin film ITO include transparent electrodes for a range of display, photovoltaic and sensor applications, EMI shielding, low-e windows, transparent heaters, and transparent electronics (some of which will be addressed shortly). TCOs have a combination of good visible transmittance and electrical conduction, and ITO generally leads the way. High quality ITO has a resistivity in the range of 2 X 10 Ω.cm, compared to 1.8 X 10 Ω.cm for silver. We showed above that TCOs were highly degenerate semiconductors and that the electrical properties depended on carrier density. Figure 6. Reflectivity as a function of frequency normalized to the plasma frequency ωp [16] Transparent Conductive Oxide Thin Films Recall that ITO is actually Sn-doped indium oxide (In2O3). Referring to Figure 7 shows the general dependence of resistivity (curve A) on Sn content. Here the thickness of the magnetron sputtered ITO film is ~ 140 nm. Curves B and C show how the visible transmittance and electron mobility also depend on Sn content. From this Figure, it is evident that these three important properties are interrelated. There is a Sn content ~ 20 wt. % at which the resistivity, visible transmittance, and electron mobility all are optimized. At Sn levels below this value, electrical properties are determined primarily by doping concentration, and by scattering off oxygen vacancies for higher Sn levels. At this doping level, the internal transmittance (does not include substrate transmittance) is 91% and the mobility is 40 cm/Vs. Transmittance is actually optimized by increasing μ. Figure 8 shows the spectral transmittance, reflectance, and absorption of a sputtered film, and Figure 9 shows the spectral transmittance of a sputtered film on a plastic window. From Figure 8 we see that the plasma wavelength (~ 1/ωp) is in the NIR and, as expected, the reflectance increases at longer wavelengths (or lower frequencies). The transmittance of the film is high at visible wavelengths and the reflectance increases in the NIR IR. The structure of the transmittance in the NIR in Figure 9 is due to the transmittance of the plastic window. The optical constants (refractive index and extinction coefficient) of ITO depend to a large degree on the deposition process, Sn content, and oxygen vacancies, and no one set of n and k can represent all films. In general, the refractive index is in the range 1.8 – 1.9 for most films and the extinction coefficient for transmissive ITO is < 0.01 at visible wavelengths. Figure 10 shows the dependence of transmittance on resistivity, again showing the onset of metal-like behavior with decreased resistivity. These spectra demonstrate that conductive films with excellent visible transmittance can be deposited, but visible transmittance generally Figure 7. Dependence of sheet resistance (A), visible transmittance (B), and electron mobility (C) on Sn content for magnetron sputtered ITO. Transparent Conductive Oxide Thin Films decreases and NIR – IR reflectance increases with decreasing resistivity, as predicted by Drude. Finally, good ITO is completely clear. If too much Sn is added the films become tan and dark, and the films turn pale green and yellow as scattering from oxygen vacancies increases. Figure 8. Spectral transmittance, reflectance, and absorption of a magnetron sputtered ITO film Figure 9. Spectral transmittance of a magnetron sputtered ITO film on plastic Figure 10. Dependence of the transmittance of magnetron sputtered ITO at 1000 nm wavelength on sheet resistance Transparent Conductive Oxide Thin Films As with all films that depend on electron conduction, defects and grain boundaries also affect the resistivity. The goal of many deposition processes is to obtain films with the largest possible grain size, which also increases the Hall mobility. This is accomplished by heating the substrate during deposition [8]. If possible, films are often heat treated after deposition. Post deposition heat treatment is generally not an option for films on plastic substrates. ZnO and Related Materials While ITO is the most widely used transparent conductive oxide (TCO), zinc oxide (ZnO) and related compositions are very competitive, and preferred, for many applications. One major reason is the worldwide depletion of indium. Aluminum doped ZnO is now being used extensively as a transparent conductive contact in thin film solar cells and CIGS cells in particular. It is widely used as transparent electrical contacts for solar cells, laser diodes, and LEDs. As we have discussed earlier for TCOs, ZnO is an n-type semiconductor with a direct bandgap of 3.37 eV at room temperature. ZnO thin films are becoming more widely used in thin film piezoelectric devices and medical applications [10]. This high conductivity is believed to be due to oxygen vacancies or Zn interstitials, but the subject remains a bit controversial [11]. An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are the primary conduction mechanism [12]. The most common applications are in laser diodes and light emitting diodes, since they have exciton and biexciton energies of 60 meV and 15 meV, respectively. It is expected that the exciton properties of ZnO will be improved further by epitaxy. ZnO films are usually doped with Al (AZO), Gd (GZO), excess Zn, or In to achieve high conductivity combined with transparency [13.14]. This material is also often alloyed with other TCO materials. Table 2 lists ZnO and numerous related compositions. ZnO thin films are mainly deposited by magnetron sputtering [12] and chemical vapor deposition (CVD) [12]. Planar and cylindrical magnetrons with metal and ceramic targets have been used [13]. The physical properties (electrical, optical, piezoelectric) of ZnO films prepared by dc reactive magnetron sputtering mainly depend on the sputtering parameters such as substrate temperature, oxygen partial pressure and sputtering pressure apart Composition Application Typical resistivity (10 Ω.cm) ZnO Low-e windows, thin film solar cells 1 ZnO:Al (AZO) Thin film solar cells 1 3 ZnO:Gd (GZO) Thin film solar cells < 10 Zn2SnO2 Thin film transistors < 50 ZnSnO3 Thin film solar cells < 50 Zn2In2O3 Thin film solar cells 3 ZnO:As Table 2. ZnO family of thin film compositions Transparent Conductive Oxide Thin Films from the target-substrate distance, sputtering power, and deposition rate. Resistivity and optical transmittance were found also to depend on the crystallinity of the sputtering target [12]. Crystalline ZnO has a hexagonal structure. Films deposited using a c-axis oriented target have decidedly lower resistivity and higher visible transmittance than those deposited with a noncrystalline target. As with other TCOs reported, electrical and optical properties also depended on placement over the sputtering target. Figure 11 compares the dependence of the resistivity of these two types of films on substrate position. Applications Low-e windows and spectrally selective windows The low-e window is now the most widely marketed and installed window treatment. The low-e coating is essentially an induced transmission filter, having one or more semitransparent Ag or Al layer sandwiched between transmissive dielectric layers, usually TiO2, Ta2O5, SnO2, or even a TCO such as ZnO. The low-e coating is deposited onto flexible plastic and sandwiched between two glass pains. A number of low-e window treatments are available: • Double-Glazed with Low-Solar-Gain Low-E Glass (Figure 12a): U-factor = 0.30 • Double-Glazed with Moderate-Solar-Gain Low-E Glass (Figure 12b): U-factor = 0.25 • Double-Glazed with High-Solar-Gain Low-E Glass (Figure 12c): U-factor = 0.24 • Solar control (Figure 13) Low-e windows are graded with respect to the U-factor, which is a measure of building insulation. The lower the better. Table 3 shows U-values for building components. Figure 11. Comparison of resistivity of ZnO films deposited using crystalline and noncrystalline sputtering targets [12] Transparent Conductive Oxide Thin Films Visible transmission and IR (heat) transmission depend on the design of the low-e or solar control coating. Transmission decreases and heat reflection increases with increased number of silver layers. Thus, a low-e window with a U factor of 0.24 has visible transmittance of 0.64 and a heat transmittance of 0.27, while a window with a U factor of 0.30 has a visible transmittance of 0.75 and a heat transmittance of 0.71. Figures 14 and 15 show transmission spectra of a low-e and solar control coatings (Figure 13) marketed by Pilkington at AOI of 0°, 30° and 45°. The low-e coating has SnO2/ITO/Ag/ZnO/TiO2 layers and the solar control coating is a bit more complex with SnO2/ITO/Ag/ITO/Ag/ZnO/TiO2 layers. The color of a window glazing should not change significantly when viewed at various angles. Figure 16 shows the change in color with AOI varying from 0° – 45° for the low-e coating. We see that reflected color changes from white to blue-green with increased AOI. Transmission color does not vary significantly with this change in AOI. Again, because the reflectance does not increase significantly with increased AOI, this coating will keep essentially the same color with change in viewing angle. Figure 12a. Solar light transmitted and heat reflected by double glazed window with low solar gain low-e glass. Figure 12b. Solar light transmitted and heat reflected by double glazed window with low solar gain low-e glass. Figure 12c. Solar light transmitted and heat reflected by double glazed window with low solar gain low-e glass. Transparent Conductive Oxide Thin Films Figure 13. Layer structure of selective solar control coating Table 3. U-values for building components, featuring window glazings Figure 14. Transmission spectra of low-e coating for AOI of 0°, 30°, and 45° Transparent Conductive Oxide Thin Films Figure 15. Transmission spectra of solar control coating for AOI of 0°, 30°, and 45° Figure 16. Change in color with AOI varying from 0° – 45° for the low-e coating Transparent Conductive Oxide Thin Films Organic Light Emitting Diodes In addition to low-e windows, one of the most important applications for ITO films is for transparent electrodes, used in electronic display applications. Figure 17 shows the layer structure of an organic light emitting device (OLED) with an ITO layer. The ITO layer is needed to transmit light out of the device. ITO is also used in thin film heterojunction solar cells [13]. Figure 18 shows the layer structure of a Si-ITO heterojunction cell. Other thin film cells consist of n-CdTe/ITO layers [14] and organic cells have the layer structure shown in Figure 19 [15]. Many of the organic layers were deposited using ink-jet technology. Thin Film Solar Cells ITO is the most important and widely used thin film TCO. However, dwindling world indium supplies is a worry, and as a result, there is an effort to replace ITO due to the indium used in this material. To this end, ZnO and AZO are now being used as transparent electrical contacts in thin film CIGS and silicon solar cells [4,16]. The AZO films obtained at higher powers have good transparency in visible region. The film with lowest resistivity of 6.67 x 10 Ω cm and transparency greater than 90% was obtained at deposition power of 175 W and substrate temperature of 200 °C after vacuum annealing at 400 C. Figure 17. Layer structure of an OLED showing placement of the ITO film Figure 18. ITO heterojunction solar cell [13] Figure 19. Organic solar cell containing ITO layer [15] Figure 20. Layer structure of thin film CIGS solar cell Transparent Conductive Oxide Thin Films Deposition of TCO’s High quality TCO films are deposited on glass, semiconductors, and plastics by planar magnetron sputtering [17], closed field magnetron sputtering [18], ion beam sputtering [19], rotatable magnetron sputtering [20,21,22,23], reactive thermal and electron beam evaporation [24], and CVD and PECVD processes [25]. The sputtering process for ITO films can be tricky, particularly if a metal InSn target is used. The density and optical constants of sputtered films are near the values of the corresponding bulk materials, and deposition rates are high using a metal target (but not as high as evaporation processes). The trick with magnetron sputtering using an alloy target is to avoid poisoning the surface of the target. This is accomplished by mapping out the dependence of the target voltage on oxygen partial pressure and working just at the metal-oxide knee of the hysteresis curve. The two main techniques of process control involve keeping the target voltage constant and monitoring the change in transmittance of an optical monitor trace. The process can also be controlled by keeping the target voltage constant by adjusting the oxygen flow. Both these processes depend on the composition of the target and must be very finely calibrated and monitored to achieve the desired combination of transmittance and electrical resistance. The easiest and best way to control the magnetron sputtering process is to use a ceramic target with the desired composition and Sn doping. The target consists of a mixture of SnO2 and In2O3 powders. Since the target is conductive, DC magnetron sputtering can still be used with just a very small partial pressure of oxygen to compensate for small changes in stoichiometry due to preferential sputtering. Deposition rates for this method are lower than for the metal target case. RF sputtering can also be used to deposit from a nonconductive ceramic target. As always, not to reinvent the wheel, the recent literature should be checked before any deposition processes are developed. Because ITO applications are so extensive, the specific application or coating properties have already been described in the literature. Because ITO films can be deposited at low substrate temperature by magnetron sputtering, it is very amenable to vacuum web coating of ITO onto flexible plastics and to plastics in general [26]. In-line processes have also been developed to coat large glass sheets [26]. Figure 21 shows the placement of the magnetron cathodes in the in-line coater, but the geometry is much the same in a web coater. Additionally, ITO deposition can be combined with other web coating processes to construct an entire thin film structure, such as thin film solar cells [27]. Figure 21. Placement of magnetron cathodes in an in-line ITO process [8] Transparent Conductive Oxide Thin Films Deposition of AZO AZO films are mainly deposited using rotatable magnetron sputtering [20-23]. The layer structure of a CIGS thin film solar cell is shown in Figure 20. As expected, visible transmission and resistivity of AZO films depend on deposition conditions. For RF sputtered films there is a significant correlation between the oxygen deficiency and the electrical and optical properties [23]. Figure 22 shows the decrease in resistivity with annealing temperature and Figure 23 shows visible transmission for substrate temperature of 250 °C for three target powers. Increase in deposition power increases the Al incorporation in the film. Oxygen deficiency also increases at higher substrate temperatures, which is the main reason for the decrease in resistivity of the AZO films. Annealing film also significantly decreased resistivity. This decrease in resistivity is due to increase in oxygen deficiency in the film after annealing. Films were found to be polycrystalline with the prominent orientation (002) perpendicular to the substrate surface and crystallinity increases with the increase in substrate temperature and decreased with the increase in deposition power. Materion offers a wide range of services and products related to transparent conductive oxides, including sputtering targets, deposition sources, and coating services. Their web site includes sputtering target materials, thin film deposition materials, solar materials applications for TCO’s and solar energy materials, indium tin oxide, zinc oxide and reports on TCO a range of applications. Materion offers large area deposition technology needed for cost effective deposition of transparent oxide thin films. Figure 22. Decrease in resistivity of AZO films with annealing temperature [23] Figure 23. Visible transmission of sputtered AZO films for substrate temperatures of 200 °C and 250 °C for three target powers [23] Transparent Conductive Oxide Thin Films