15-01-2013, 11:01 AM
Review of TCO Thin Films
[attachment=46920]
Abstract
The present review paper reports on the physical properties, status, prospects for further development, and applications of polycrystalline or amorphous, transparent, and conducting oxides (TCO) semiconductors. The coexistence of electrical conductivity and optical transparency in these materials depends on the nature, number, and atomic arrangements of metal cations in crystalline or amorphous oxide structures, on the resident morphology, and on the presence of intrinsic or intentionally introduced defects. The important TCO semiconductors are impurity-doped ZnO, In2O3, SnO2 and CdO, as well as the ternary compounds Zn2SnO4, ZnSnO3, Zn2In2O5, Zn3In2O6, In2SnO4, CdSnO3, and multi-component oxides consisting of combinations of ZnO, In2O3 and SnO2. Sn doped In2O3 (ITO) and F doped SnO2 TCO thin films are the preferable materials for most present applications. The expanding use of TCO materials, especially for the production of transparent electrodes for optoelectronic device applications, is endangered by the scarcity and high price of In. This situation drives the search for alternative TCO materials to replace ITO. The electrical resistivity of the novel TCO materials should be ~10-5 cm, typical absorption coefficient smaller than 104 cm-1 in the near UV and visible range, with optical band gap ~3 eV. At present, ZnO:Al and ZnO:Ga (AZO and GZO) semiconductors could become good alternatives to ITO for thin-film transparent electrode applications. The best candidates are AZO thin films, which have low resistivity of the order of 10−4.cm, inexpensive source materials, and are non-toxic. However, development of large area deposition techniques are still needed to enable the production of AZO and GZO films on large area substrates with a high deposition rate.
Introduction
Most optically transparent and electrically conducting oxides (TCO) are binary or ternary compounds, containing one or two metallic elements. Their resistivity could be as low as 10-4 cm, and their extinction coefficient k in the optical visible range (VIS) could be lower than 0.0001, owing to their wide optical band gap (Eg) that could be greater than 3 eV. This remarkable combination of conductivity and transparency is usually impossible in intrinsic stoichiometric oxides; however, it is achieved by producing them with a non-stoichiometric composition or by introducing appropriate dopants. Badeker (1907) discovered that thin CdO films possess such characteristics. Later, it was recognized that thin films of ZnO, SnO2, In2O3 and their alloys were also TCOs. Doping these oxides resulted in improved electrical conductivity without degrading their optical transmission. Al doped ZnO (AZO), tin doped In2O3, (ITO) and antimony or fluorine doped SnO2 (ATO and FTO), are among the most utilized TCO thin films in modern technology. In particular, ITO is used extensively.
. Optical Properties
As mentioned above, besides high conductivity (~106 S), effective TCO thin films should have a very low absorption coefficient in the near UV-VIS-NIR region. The transmission in the near UV is limited by Eg, as photons with energy larger than Eg are absorbed. A second transmission edge exists at the NIR region, mainly due to reflection at the plasma frequency. Ideally, a wide band gap TCO should not absorb photons in the transmission “window” in the UV-VIS-NIR region. However, there are no “ideal” TCOs thin films, and even if such films could be deposited, reflection and interference would also affect the transmission. Hence, 100% transparency over a wide region cannot be obtained.
The optical properties of TCOs transmission T, reflection R, and absorption A, are determined by its refraction index n, extinction coefficient k, band gap Eg, and geometry. Geometry includes film thickness, thickness uniformity, and film surface roughness. T, R and, A are intrinsic, depending on the chemical composition and solid structure of the material, whereas the geometry is extrinsic. There is a negative correlation between the carrier density and the position of the IR absorption edge, but positive correlation between the carrier density and the UV absorption edge, as Eg increases at larger carrier density (Moss-Burstein effect). As a result, the TCO transmission boundaries and conductivity are interconnected.
Trends in the development of TCO materials
While the development of new TCO materials is mostly dictated by the requirements of specific applications, low resistivity and low optical absorption are always significant pre-requisites. There are basically two strategies in managing the task of developing advanced TCOs that could satisfy the requirements. The main strategy dopes known binary TCOs with other elements, which can increase the density of conducting electrons. As shown in Table 1, more than 20 different doped binary TCOs were produced and characterized, of which ITO was preferred, while AZO and GZO come close to it in their electrical and optical performance. Doping with low metallic ion concentration generates shallow donor levels, forming a carrier population at room temperature. Doping In2O3 with Sn to form ITO substantially increased conductivity.
. Industrial Application of TCOs
TCO’s have diverse industrial applications – some of the more important ones will be described in this section. TCO coatings are applied to transparent materials used for work surfaces and closet doors, particularly in clean rooms used for electronics assembly, in order to prevent harmful static charge buildup. In this application relatively high surface resistances (e.g. k/) can be tolerated.
Transparent heating elements may be constructed from TCO coatings. These are applied as defrosters in aircraft and vehicular windshields. Their advantage over traditional hot air blowers is that they can have a much shorter effective defrosting time, and work uniformly over large areas. This application requires either the use of very low surface resistance coatings (e.g. ~1 /), or a high voltage power source. The application of TCO coatings to passenger vehicles has proven to be technically successful but a commercial failure, due to the high cost of a supplemental alternator to deliver the requisite high voltage. If the automobile industry will adopt a higher bus voltage, as has been widely discussed, then this application may prove to be more commercially feasible in the future.
Conclusions
The expanding use of TCO materials, especially for the production of transparent electrodes for optoelectronic device applications, has developed into a world wide multi-billion $US economy that in general depends on the availability of ITO. This economy is endangered by the scarcity and high price of In. The situation drives the search for alternative TCO materials to replace ITO, and motivates an intensive investigation of the physics and chemistry of TCO materials.
The main significant progress in the research and development of TCO thin films has been made in understanding the physics of TCO semiconductors. The physical processes that make possible the coexistence of electrical conductivity and optical transparency are well clarified and understood. In particular, the role of oxygen vacancies and various dopants in the formation of shallow donor levels is well established. In addition to binary TCOs, progress has also been made in developing new TCO compounds, consisting of combined segregated-binaries, ternary and quaternary oxides. However, the objective of developing new TCOs with conductivity similar or even higher than that of ITO has not been realized. The conductivity of the recently developed ternary, quaternary, and binary-combination TCOs is lower than that of ITO. It is now appreciated that the attainment of higher conductivity is limited by the negative correlation between carrier density and electron mobility.
[attachment=46920]
Abstract
The present review paper reports on the physical properties, status, prospects for further development, and applications of polycrystalline or amorphous, transparent, and conducting oxides (TCO) semiconductors. The coexistence of electrical conductivity and optical transparency in these materials depends on the nature, number, and atomic arrangements of metal cations in crystalline or amorphous oxide structures, on the resident morphology, and on the presence of intrinsic or intentionally introduced defects. The important TCO semiconductors are impurity-doped ZnO, In2O3, SnO2 and CdO, as well as the ternary compounds Zn2SnO4, ZnSnO3, Zn2In2O5, Zn3In2O6, In2SnO4, CdSnO3, and multi-component oxides consisting of combinations of ZnO, In2O3 and SnO2. Sn doped In2O3 (ITO) and F doped SnO2 TCO thin films are the preferable materials for most present applications. The expanding use of TCO materials, especially for the production of transparent electrodes for optoelectronic device applications, is endangered by the scarcity and high price of In. This situation drives the search for alternative TCO materials to replace ITO. The electrical resistivity of the novel TCO materials should be ~10-5 cm, typical absorption coefficient smaller than 104 cm-1 in the near UV and visible range, with optical band gap ~3 eV. At present, ZnO:Al and ZnO:Ga (AZO and GZO) semiconductors could become good alternatives to ITO for thin-film transparent electrode applications. The best candidates are AZO thin films, which have low resistivity of the order of 10−4.cm, inexpensive source materials, and are non-toxic. However, development of large area deposition techniques are still needed to enable the production of AZO and GZO films on large area substrates with a high deposition rate.
Introduction
Most optically transparent and electrically conducting oxides (TCO) are binary or ternary compounds, containing one or two metallic elements. Their resistivity could be as low as 10-4 cm, and their extinction coefficient k in the optical visible range (VIS) could be lower than 0.0001, owing to their wide optical band gap (Eg) that could be greater than 3 eV. This remarkable combination of conductivity and transparency is usually impossible in intrinsic stoichiometric oxides; however, it is achieved by producing them with a non-stoichiometric composition or by introducing appropriate dopants. Badeker (1907) discovered that thin CdO films possess such characteristics. Later, it was recognized that thin films of ZnO, SnO2, In2O3 and their alloys were also TCOs. Doping these oxides resulted in improved electrical conductivity without degrading their optical transmission. Al doped ZnO (AZO), tin doped In2O3, (ITO) and antimony or fluorine doped SnO2 (ATO and FTO), are among the most utilized TCO thin films in modern technology. In particular, ITO is used extensively.
. Optical Properties
As mentioned above, besides high conductivity (~106 S), effective TCO thin films should have a very low absorption coefficient in the near UV-VIS-NIR region. The transmission in the near UV is limited by Eg, as photons with energy larger than Eg are absorbed. A second transmission edge exists at the NIR region, mainly due to reflection at the plasma frequency. Ideally, a wide band gap TCO should not absorb photons in the transmission “window” in the UV-VIS-NIR region. However, there are no “ideal” TCOs thin films, and even if such films could be deposited, reflection and interference would also affect the transmission. Hence, 100% transparency over a wide region cannot be obtained.
The optical properties of TCOs transmission T, reflection R, and absorption A, are determined by its refraction index n, extinction coefficient k, band gap Eg, and geometry. Geometry includes film thickness, thickness uniformity, and film surface roughness. T, R and, A are intrinsic, depending on the chemical composition and solid structure of the material, whereas the geometry is extrinsic. There is a negative correlation between the carrier density and the position of the IR absorption edge, but positive correlation between the carrier density and the UV absorption edge, as Eg increases at larger carrier density (Moss-Burstein effect). As a result, the TCO transmission boundaries and conductivity are interconnected.
Trends in the development of TCO materials
While the development of new TCO materials is mostly dictated by the requirements of specific applications, low resistivity and low optical absorption are always significant pre-requisites. There are basically two strategies in managing the task of developing advanced TCOs that could satisfy the requirements. The main strategy dopes known binary TCOs with other elements, which can increase the density of conducting electrons. As shown in Table 1, more than 20 different doped binary TCOs were produced and characterized, of which ITO was preferred, while AZO and GZO come close to it in their electrical and optical performance. Doping with low metallic ion concentration generates shallow donor levels, forming a carrier population at room temperature. Doping In2O3 with Sn to form ITO substantially increased conductivity.
. Industrial Application of TCOs
TCO’s have diverse industrial applications – some of the more important ones will be described in this section. TCO coatings are applied to transparent materials used for work surfaces and closet doors, particularly in clean rooms used for electronics assembly, in order to prevent harmful static charge buildup. In this application relatively high surface resistances (e.g. k/) can be tolerated.
Transparent heating elements may be constructed from TCO coatings. These are applied as defrosters in aircraft and vehicular windshields. Their advantage over traditional hot air blowers is that they can have a much shorter effective defrosting time, and work uniformly over large areas. This application requires either the use of very low surface resistance coatings (e.g. ~1 /), or a high voltage power source. The application of TCO coatings to passenger vehicles has proven to be technically successful but a commercial failure, due to the high cost of a supplemental alternator to deliver the requisite high voltage. If the automobile industry will adopt a higher bus voltage, as has been widely discussed, then this application may prove to be more commercially feasible in the future.
Conclusions
The expanding use of TCO materials, especially for the production of transparent electrodes for optoelectronic device applications, has developed into a world wide multi-billion $US economy that in general depends on the availability of ITO. This economy is endangered by the scarcity and high price of In. The situation drives the search for alternative TCO materials to replace ITO, and motivates an intensive investigation of the physics and chemistry of TCO materials.
The main significant progress in the research and development of TCO thin films has been made in understanding the physics of TCO semiconductors. The physical processes that make possible the coexistence of electrical conductivity and optical transparency are well clarified and understood. In particular, the role of oxygen vacancies and various dopants in the formation of shallow donor levels is well established. In addition to binary TCOs, progress has also been made in developing new TCO compounds, consisting of combined segregated-binaries, ternary and quaternary oxides. However, the objective of developing new TCOs with conductivity similar or even higher than that of ITO has not been realized. The conductivity of the recently developed ternary, quaternary, and binary-combination TCOs is lower than that of ITO. It is now appreciated that the attainment of higher conductivity is limited by the negative correlation between carrier density and electron mobility.