User:Chem4061sp13/sandbox

=Transparent Amorphous Oxide Semiconductors (TAOS)=

Introduction
A Transparent Amorphous Oxide Semiconductor (TAOS) is a double oxide composed of heavy metal cations with an electron configuration of (n-1)d10s0, which are -used to conduct electricity across transparent surfaces such as an LCD display. TAOSs were found following the discovery of Transparent Conductive Oxides (TCO's)_by Rupperecht in 1954. After the discovery of the first TCO In2O3Sn, scientists became interested in transparent oxide semiconductors (TOSs), which have a broad use in Ultraviolet light emitting diodes, organic light emitting diodes, solar cells, and transparent film transistors. These TOS carry electrons most effectively in their amorphous state- hence the creation of TAOS from TOS. In 1996, Hideo Hosono, Masahiro Yasukawa, and Hiroshi Kawazoe developed a method to predict the conductive properties of a TAOS by its electronic configuration and its heavy metal cations.

Characteristics
A major characteristic found in all TAOS is a very high electron mobility where the average TAOS has a mobility around 10cm2/V*s. This makes sense as electrical conductivity are the product of electron mobility and carrier concentration. So, in order to increase the conductivity of an amorphous material, the mobility must be increased. For TAOS, this high mobility is obtained through the large overlap of the ns orbitals on the bottom of the molecule’s conduction band. The spatial spreading and overlap of these ns orbitals is insensitive to any variation in the metal-oxygen bonds, an additional feature that is required to effectively increase the mobility of an amorphous material. To ensure this overlap and consistency in the overlap, the electron configuration of (n-1)d10s0 is necessary. In addition to this configuration, TAOSs are selective for the heavy metal cations with which they pair. The image to the right lists the common metal cations used in synthesis of these compounds. These cations can also be classified as "heavy metals" based on their molecular masses. Another important characteristic of a TAOS is that it must be an oxide and will generally prefer to be a double oxide as it is in the amorphous state. The reason TAOS require Oxygen is that the Oxygen 2p orbitals are extremely low in energy, allowing for a uniquely large band gap, as compared to other elements. Because this large band gap is proportional to the amount of overlap between the ns orbitals, the larger the band gap, the larger the electron mobility, which ultimately leads to an increase in conductivity. Also, the amorphous form of the double oxide layer contains Oxygen vacancies, which allow for the generation of conduction electrons. TAOSs must also be formed far below its crystallization temperature, as this is the temperature regime where its optimal carrier electrons are formed, which will most efficiently transport electricity. This means that the amorphous material is an insulator as a solid unless transformed into a semiconductor via carrier doping with ions. This doping process will effectively transform the material from an insulator to a semiconductor Without this process, a TAOS would never be formed. The final characteristic of a TAOS is that it will exhibit a positive to negative anomaly in the Hall Effect. The Hall Effect in a semiconductor is experienced when a semiconductor is kept in a magnetic field. Because of this the charge carriers will experience a Lorentz Force perpendicular to the motion of the charge carriers, and perpendicular to the flow of current. The forces applied will cause the positive charges to move in one direction, and the negative charges to move in the other direction. This change of direction of the charges can lead to an anomaly of the Hall Coefficent. This specific anomaly occurs with the Hall Coefficient, RH, which can be easily seen by looking at this formula for the Hall Effect in semi conductors :$$R_H=\frac{p\mu_h^2 - n\mu_e^2}{e(p\mu_h + n\mu_e)^2}$$ which changes sign to become opposite the sign used in thermopower applications. .

Effect of ion implantation on TAOS conductivity


Ion implantation is a technique used to modify electrical properties of solid materials. This process induces structural changes by bombarding compounds with various ion species. In 2002, Hideo Hosono and his collaborators published the results of proton implantation on the amorphous TCO, 2 CdO•GeO2. They found that the conductivity of this otherwise insulating compound can be raised by implanting it with protons. The protons, when they bombard the compound, cause a shift in structure which raises the crystallization temperature of the compound, making the release of carrier electrons favorable. Prior to implanting ions in this material, its conductivity is ca. 10-9 S cm-. After doping the compound, its conductivity increases to 101 - 102 S cm- without any need for an annealing process.