23-05-2012, 05:12 PM
Transparent electronics
[attachment=22939]
ABSTRACT
Transparent electronics is an emerging science and technology field focused on producing ‘invisible’ electronic circuitry and opto-electronic devices. Applications include consumer electronics, new energy sources, and transportation; for example, automobile windshields could transmit visual information to the driver. Glass in almost any setting could also double as an electronic device, possibly improving security systems or offering transparent displays. In a similar vein, windows could be used to produce electrical power. Other civilian and military applications in this research field include realtime wearable displays. As for conventional Si/III–V-based electronics, the basic device structure is based on semiconductor junctions and transistors. However, the device building block materials, the semiconductor, the electric contacts, and the dielectric/passivation layers, must now be transparent in the visible –a true challenge! Therefore, the first scientific goal of this technology must be to discover, understand, and implement transparent high-performance electronic materials. The second goal is their implementation and evaluation in transistor and circuit structures. The third goal relates to achieving application-specific properties since transistor performance and materials property requirements vary, depending on the final product device specifications. Consequently, to enable this revolutionary technology requires bringing together expertise from various pure and applied sciences, including materials science, chemistry, physics, electrical/electronic/circuit engineering, and display science.
INTRODUCTION
Transparent electronics is an emerging science and technology field focused on producing ‘invisible’ electronic circuitry and opto-electronic devices. Applications include consumer electronics, new energy sources, and transportation; for example, automobile windshields could transmit visual information to the driver. Glass in almost any setting could also double as an electronic device, possibly improving security systems or offering transparent displays. In a similar vein, windows could be used to produce electrical power. Other civilian and military applications in this research field include real-time wearable displays. As for conventional Si/III–V-based electronics, the basic device structure is based on semiconductor junctions and transistors. However, the device building block materials, the semiconductor, the electric contacts, and the dielectric/passivation layers, must now be transparent in the visible –a true challenge! Therefore, the first scientific goal of this technology must be to discover, understand, and implement transparent high-performance electronic materials. The second goal is their implementation and evaluation in transistor and circuit structures. The third goal relates to achieving application-specific properties since transistor performance and materials property requirements vary, depending on the final product device specifications. Consequently, to enable this revolutionary technology requires bringing together expertise from various pure and applied sciences, including materials science, chemistry, physics, electrical /electronic/ circuit engineering, and display science.
During the past 10 years, the classes of materials available for transparent electronics applications have grown dramatically. Historically, this area was dominated by transparent conducting oxides (oxide materials that are both electrically conductive and optically transparent) because of their wide use in antistatic coatings, touch display panels, solar cells, flat panel displays, heaters, defrosters, ‘smart windows’ and optical coatings. All these applications use transparent conductive oxides as passive electrical or optical coatings. The field of transparent conducting oxide (TCO) materials has been reviewed and many treatises on the topic are available. However, more recently there have been tremendous efforts to develop new active materials for functional transparent electronics. These new technologies will require new materials sets, in addition to the TCO component, including conducting, dielectric and semiconducting materials, as well as passive components for full device fabrication.
COMBINING OPTICAL TRANSPARENCY WITH ELECTRICAL CONDUCTIVITY
Transparent conductors are neither 100% optically transparent nor metallically conductive. From the band structure point of view, the combination of the two properties in the same material is contradictory: a transparent material is an insulator which possesses completely filled valence and empty conduction bands; whereas metallic conductivity appears when the Fermi level lies within a band with a large density of states to provide high carrier concentration.
Efficient transparent conductors find their niche in a compromise between a sufficient transmission within the visible spectral range and a moderate but useful in practice electrical conductivity. This combination is achieved in several commonly used oxides – In2O3, SnO2, ZnO and CdO. In the undoped stoichiometric state, these materials are insulators with optical band gap of about 3 eV. To become a transparent conducting oxide (TCO), these TCO hosts must be degenerately doped to displace the Fermi level up into the conduction band. The key attribute of any conventional n-type TCO host is a highly dispersed single freeelectron- like conduction band (Figure 1). Degenerate doping then provides both (i) the high mobility of extra carriers (electrons) due to their small effective mass and (ii) lowoptical absorption due to the lowdensity of states in the conduction band. The high energy dispersion of the conduction band also ensures a pronounced Fermi energy displacement up above the conduction band minimum, the Burstein–Moss (BM) shift. The shift helps to broaden the optical transparency window and to keep the intense optical transitions from the valence band out of the visible range. This is critical in oxides which are not transparent throughout the entire visible spectrum, for example, in CdO where the optical (direct) band gap is 2.3 eV.
. Electronic Properties of Conventional TCO Hosts
Conventional n-type TCO hosts (In2O3, SnO2, CdO and ZnO) share similar chemical, structural and electronic properties. Exclusively oxides of the post transition metals with (n-1)d10ns2 electronic configurations, they have densely packed structures with four- or six-coordinate metal ions. Strong interactions between the oxygen 2p and metal ns orbitals give rise to electronic band
structures qualitatively similar for all these oxides (Figures 1 and 2): the bonding and nonbonding O 2p states form the valence band while the conduction band arises from the antibonding Ms–Op interactions. The empty p states of the metal ion form the following band at a higher energy. The partial density of states plots (Figure 2), reveal that the oxygen 2p and metal ns states make similar contributions to the conduction band. This provides a three dimensional Ms–Op network for charge transport once extra carriers fill the band.
Carrier Generation in Conventional TCO Hosts
The optical and transport properties of a conventional TCO are governed by the efficiency and
the specifics of the carrier generation mechanism employed. Evenin the most favorable situation, i.e. when the effects of dopant solubility, clustering, secondary phase formation and charge compensation can be avoided, large concentrations of electron donors (substitutional dopants and/or native point defects) not only promote the charge scattering but also may significantly alter the electronic band structure of the host oxide, leading to a nonrigid band shift of the Fermi level. A detailed band structure analysis of the doped oxides helps to elucidate the role of different factors involved.
[attachment=22939]
ABSTRACT
Transparent electronics is an emerging science and technology field focused on producing ‘invisible’ electronic circuitry and opto-electronic devices. Applications include consumer electronics, new energy sources, and transportation; for example, automobile windshields could transmit visual information to the driver. Glass in almost any setting could also double as an electronic device, possibly improving security systems or offering transparent displays. In a similar vein, windows could be used to produce electrical power. Other civilian and military applications in this research field include realtime wearable displays. As for conventional Si/III–V-based electronics, the basic device structure is based on semiconductor junctions and transistors. However, the device building block materials, the semiconductor, the electric contacts, and the dielectric/passivation layers, must now be transparent in the visible –a true challenge! Therefore, the first scientific goal of this technology must be to discover, understand, and implement transparent high-performance electronic materials. The second goal is their implementation and evaluation in transistor and circuit structures. The third goal relates to achieving application-specific properties since transistor performance and materials property requirements vary, depending on the final product device specifications. Consequently, to enable this revolutionary technology requires bringing together expertise from various pure and applied sciences, including materials science, chemistry, physics, electrical/electronic/circuit engineering, and display science.
INTRODUCTION
Transparent electronics is an emerging science and technology field focused on producing ‘invisible’ electronic circuitry and opto-electronic devices. Applications include consumer electronics, new energy sources, and transportation; for example, automobile windshields could transmit visual information to the driver. Glass in almost any setting could also double as an electronic device, possibly improving security systems or offering transparent displays. In a similar vein, windows could be used to produce electrical power. Other civilian and military applications in this research field include real-time wearable displays. As for conventional Si/III–V-based electronics, the basic device structure is based on semiconductor junctions and transistors. However, the device building block materials, the semiconductor, the electric contacts, and the dielectric/passivation layers, must now be transparent in the visible –a true challenge! Therefore, the first scientific goal of this technology must be to discover, understand, and implement transparent high-performance electronic materials. The second goal is their implementation and evaluation in transistor and circuit structures. The third goal relates to achieving application-specific properties since transistor performance and materials property requirements vary, depending on the final product device specifications. Consequently, to enable this revolutionary technology requires bringing together expertise from various pure and applied sciences, including materials science, chemistry, physics, electrical /electronic/ circuit engineering, and display science.
During the past 10 years, the classes of materials available for transparent electronics applications have grown dramatically. Historically, this area was dominated by transparent conducting oxides (oxide materials that are both electrically conductive and optically transparent) because of their wide use in antistatic coatings, touch display panels, solar cells, flat panel displays, heaters, defrosters, ‘smart windows’ and optical coatings. All these applications use transparent conductive oxides as passive electrical or optical coatings. The field of transparent conducting oxide (TCO) materials has been reviewed and many treatises on the topic are available. However, more recently there have been tremendous efforts to develop new active materials for functional transparent electronics. These new technologies will require new materials sets, in addition to the TCO component, including conducting, dielectric and semiconducting materials, as well as passive components for full device fabrication.
COMBINING OPTICAL TRANSPARENCY WITH ELECTRICAL CONDUCTIVITY
Transparent conductors are neither 100% optically transparent nor metallically conductive. From the band structure point of view, the combination of the two properties in the same material is contradictory: a transparent material is an insulator which possesses completely filled valence and empty conduction bands; whereas metallic conductivity appears when the Fermi level lies within a band with a large density of states to provide high carrier concentration.
Efficient transparent conductors find their niche in a compromise between a sufficient transmission within the visible spectral range and a moderate but useful in practice electrical conductivity. This combination is achieved in several commonly used oxides – In2O3, SnO2, ZnO and CdO. In the undoped stoichiometric state, these materials are insulators with optical band gap of about 3 eV. To become a transparent conducting oxide (TCO), these TCO hosts must be degenerately doped to displace the Fermi level up into the conduction band. The key attribute of any conventional n-type TCO host is a highly dispersed single freeelectron- like conduction band (Figure 1). Degenerate doping then provides both (i) the high mobility of extra carriers (electrons) due to their small effective mass and (ii) lowoptical absorption due to the lowdensity of states in the conduction band. The high energy dispersion of the conduction band also ensures a pronounced Fermi energy displacement up above the conduction band minimum, the Burstein–Moss (BM) shift. The shift helps to broaden the optical transparency window and to keep the intense optical transitions from the valence band out of the visible range. This is critical in oxides which are not transparent throughout the entire visible spectrum, for example, in CdO where the optical (direct) band gap is 2.3 eV.
. Electronic Properties of Conventional TCO Hosts
Conventional n-type TCO hosts (In2O3, SnO2, CdO and ZnO) share similar chemical, structural and electronic properties. Exclusively oxides of the post transition metals with (n-1)d10ns2 electronic configurations, they have densely packed structures with four- or six-coordinate metal ions. Strong interactions between the oxygen 2p and metal ns orbitals give rise to electronic band
structures qualitatively similar for all these oxides (Figures 1 and 2): the bonding and nonbonding O 2p states form the valence band while the conduction band arises from the antibonding Ms–Op interactions. The empty p states of the metal ion form the following band at a higher energy. The partial density of states plots (Figure 2), reveal that the oxygen 2p and metal ns states make similar contributions to the conduction band. This provides a three dimensional Ms–Op network for charge transport once extra carriers fill the band.
Carrier Generation in Conventional TCO Hosts
The optical and transport properties of a conventional TCO are governed by the efficiency and
the specifics of the carrier generation mechanism employed. Evenin the most favorable situation, i.e. when the effects of dopant solubility, clustering, secondary phase formation and charge compensation can be avoided, large concentrations of electron donors (substitutional dopants and/or native point defects) not only promote the charge scattering but also may significantly alter the electronic band structure of the host oxide, leading to a nonrigid band shift of the Fermi level. A detailed band structure analysis of the doped oxides helps to elucidate the role of different factors involved.