To-Sing Li

Thermal effect on the structural, electrical, and optical properties of in-line sputtered aluminum doped zinc oxide films explored with thermal desorption spectroscopy

This work investigates the thermal effect on the structural, electrical, and optical properties of aluminum doped zinc oxide (AZO) films. The AZO films deposited at different temperatures were measured using a thermal desorption system to obtain their corresponding thermal desorption spectroscopy (TDS). In addition to obtaining information of thermal desorption, the measurement of TDS also has the effect of vacuum annealing on the AZO films. The results of measuring TDS imply part of the doped aluminum atoms do not stay at substituted zinc sites in AZO films. The (002) preferential direction of the AZO films in X-ray diffraction spectra shifts to a lower angle after measurement of TDS. The grain size grows and surface becomes denser for all AZO films after measurement of TDS. The carrier concentration, mobility, and average optical transmittance increase while the electrical resistivity decreases for AZO films after measurement of TDS. These results indicate that the AZO films deposited at 200°C are appropriate selections if the AZO films are applied in device fabrication of heat-produced process.

Modulation of curved graphene nanoribbon optical absorption spectra by an electric field

The optical absorption spectra of curved graphene nanoribbons exhibit rich dependence on the magnitude and direction of the electric field. The wave functions have spatial symmetry originating from the equivalence of the two sublattices. There exists an optical selection rule caused by the special structure of the Hamiltonian matrix and the wave function spatial symmetry. An electric field may or may not disrupt such spatial symmetry depending on its direction and magnitude. Therefore, the optical selection rule can be controlled. In addition, the two-fold degeneracy of the optical absorption peaks is lifted by the electric field, and the variations of the absorption peak energies with the field are explored.

Fluorine thermal stability of ZnO:F films investigated by thermal desorption spectroscopy

To estimate the fluorine thermal stability of fluorine doped zinc oxide (ZnO:F) films, ZnO:F films were prepared by co-sputtering zinc oxide(ZnO) and magnesium fluoride (MgF2) targets under different sputtering powers. The prepared films were tested with a thermal desorption system heated from room temperature to 500degC. Four elements: zinc (Zn), oxygen (O), magnesium (Mg) and fluorine (F) from ZnO:F films exhibit clear and similar thermal desorption behavior. The result indicates prepared ZnO:F films will desorb corresponding to low bound strength under 100degC heat treatment, and exhibit multilayer instead of layer by layer desorption behavior based on Polanyi-Wigner model. Fluorine is thermally unstable for ZnO:F films evidenced by thermal desorption test. Electrical and optical properties of ZnO:F films after thermal desorption test become negative. This work contributes to display or solar industry if ZnO:F films are considered to be the transparent conductive oxide films.

Thermal stability of hydrogen annealed aluminum doped zinc oxide films investigated by thermal desorption spectroscopy

To investigate thermal stability of hydrogen annealed aluminum doped zinc oxide (AZO) films, hydrogen annealed AZO films were measured with thermal desorption system and their thermal desorption spectroscopy (TDS) were analyzed. The measured TDS results indicate all three elements in AZO films: oxygen, aluminum and zinc are observed. The result indicates the aluminum impurity in hydrogen annealed AZO films is thermally unstable, and implies the doped aluminum does not stay in the substituted zinc site in AZO films to some extent. The hydrogen annealed AZO films prepared at 200°C substrate is recommended to apply since it shows relatively good thermal stability considering total atomic desorption amount, electric resistivity and average optic transmittances percentage variation.

Absorption spectra of unequal width bilayer graphene nanoribbons in a spatially modulated electric field

The optical absorption spectra of unequal width bilayer graphene nanoribbons can be effectively tuned by a spatially modulated electric field. The absorption spectra exhibit many prominent peaks’ structure owing to the one-dimensional subbands. The number, spectral intensity, and frequency of the absorption peaks depend sensitively on the magnitude, period and phase of the modulated electric potential. The relative displacement between the top and bottom nanoribbons also has strong influence on the spectra. For unequal width bilayer graphene nanoribbons without the interlayer hoppings, there exists an optical selection rule originating from the spatial symmetry of the electron wave functions. Most importantly, such a selection rule can be disrupted by the presence of the interlayer atomic interactions or a spatially modulated electric field. These theoretical predictions can be validated by absorption spectroscopy experiments.

Heating rate of post-annealing on microstructure/electrical/optical properties of gallium and aluminum co-doped zinc oxide films

To investigate whether heating rate of post-annealing can affect the microstructure, electrical and optical properties of gallium and aluminum co-doped zinc oxide (GAZO) films or not, GAZO films were post-annealed with 9°C/min and 30°C/min heating rate. The GAZO films post-annealed with 9°C/min heating rate appears smaller full width at half maxima of (002) diffraction peak, more uniform and bigger grain size, higher mobility, lower resistivity, higher average optical transmittance and higher figure of merit than those with 30°C/min heating rate. Low heating rate like 9°C/min in post-annealing can provide more time in thermal reaction for GAZO films at a certain heating temperature. This scenario may explain the above phenomena.

Microwave hydrogen plasma annealing to improve electrical and optical properties of aluminum doped zinc oxide films

To investigate whether microwave hydrogen plasma can improve the electrical and optical properties of aluminum doped zinc oxide (AZO) films or not, AZO films prepared with different substrate temperature were treated with microwave hydrogen plasma. The AZO films were post-treated by one 2.45 GHz microwave hydrogen plasma system with 25 torr, 700 W and 10 minutes. It was observed the electrical and optical properties of all AZO films prepared with different substrate temperatures were apparently improved with microwave hydrogen plasma. Taking 200°C produced AZO films as an example, the electrical resistivity decreases 43% and the average optical transmittance increases 7% respectively. Microwave hydrogen plasma makes AZO films recrystallization like process from measured results of X-ray diffraction, scanning electron micrograph and transmission electron micrograph. The oxygen adsorption on grain boundary of AZO films after microwave hydrogen plasma apparently decreases observed from results of X-ray photoelectron spectroscopy. The above two phenomena may be the mechanism why microwave hydrogen plasma cam improve the electrical and optical properties of AZO films. This work contributes to industries like touch panel, solar cells. etc. which transparent conductive films belong to the industry products component.

Carbon nano tubes grown on glass substrate with different interface layer

Microwave plasma enhanced chemical vapor deposition (MPECVD) was applied in growing carbon nano tubes (CNTs) on sodium free glass with different interface layer materials. Surface morphology and field emission characteristics of as grown CNTs were measured. Three different materials: titanium(Ti), gold(Au) and indium tin oxide (ITO) thin films were prepared on glass first as the interface role between CNTs and glass. Nickel(Ni) films were sputtered on three different interface films and also direct on glass. After hydrogen plasma pretreatment on nickel films, CNTs were tried to grow on four kinds of glass combination: Ni/glass, Ni/ITO/glass, Ni/Au/glass and Ni/Ti/glass, three substrate temperatures: unheated, 300°C and 500 °C, with the mixture of methane and hydrogen microwave plasma. It was found CNTs can be grown with high CNTs density, high adhesion and 2.5V/ µ m turn on electric field corresponding to Ni/Ti/glass and 500 °C process condition. The same MPECVD system with same pretreatment and process gas can be used to grow CNTs on silicon substrate without extra substrate heating. It is proposed the electrical conductivity of substrate has strong influence on CNTs growth. The interface material like Ti can modify the electrical conductivity of the substrate surface.