Hsing-Cheng Chang

Effects of Na content on the luminescence behavior, conduction type, and crystal structure of Na-doped ZnO films

This study investigates the effect of Na content on the structural, optical, and electrical properties of sol-gel Na-doped ZnO films using x-ray diffraction, photoluminescence, and conductivity measurements. It is shown that a p-type conversion of the Na-doped ZnO film might be due to a combined effect of the increased substitutional-Na density and the decreased oxygen-vacancy (VO) density. However, excess Na incorporation into ZnO shows an ambiguous carrier type due to the increase in the donorlike VO density. These results indicate that compensation effects limit the hole concentration in the Na-doped ZnO films. In addition, when more Na is substituted into the ZnO system, the difference in the ionic radii of Zn2+ and Na+ starts playing an increasingly important role, causing the presence of tensile stress in the Na-doped ZnO film.

Capacitance-voltage and current-voltage characteristics of Au Schottky contact on n-type Si with a conducting polymer

Capacitance-voltage and current-voltage characteristics of Au/n-type Si (n-Si) and Au/poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS)/n-Si Schottky diodes were investigated in this study. The interfacial phenomenon was explained in terms of the generation of an interfacial dipole subsequently affecting the electron-injection barrier. The authors found that inserting a PEDOT:PSS layer at the Au/n-Si interface may result in the formation of an interfacial dipole, increase the upward band bending in Si near the interface and reduce the reverse-bias leakage current. In this study, higher quality Schottky junctions were formed on n-Si using a simple technique of spin-coating PEDOT:PSS as the metal electrode. (Some figures in this article are in colour only in the electronic version)

Nitrogen-vacancy-related defects and Fermi level pinning in n-GaN Schottky diodes

The relationship between the surface states related to nitrogen-vacancy defects and surface Fermi level pinning has been investigated using x-ray photoelectron spectroscopy and capacitance–voltage measurements. Barrier heights of 1.09, 0.50, 1.20, and 0.50 eV, respectively, were obtained for Ni/(NH4)2Sx-treated n-GaN, Ni/etched n-GaN, Au/(NH4)2Sx-treated n-GaN and Au/etched n-GaN Schottky diodes. For Schottky diodes treated with (NH4)2Sx, the observed Schottky barrier height is very close to the Schottky limit, due to the reduction of the surface state density. This also suggests that a large number of surface states related to nitrogen-vacancy defects in the etched n-GaN surface would lead to the pinning of the Fermi level at 0.50 eV below the conduction band edge.

Effects of Mg incorporation on the optical properties of ZnO prepared by the sol-gel method

ZnO films with and without Mg doping (Zn1−xMgxO) were deposited on substrates by the sol-gel technique. X-ray photoelectron spectroscopy, x-ray diffraction, photoluminescence, and conductivity measurements were used to characterize the Zn1−xMgxO semiconductors. It is worth noting that the intensity of the band-edge luminescence (BEL) of the Zn0.973Mg0.027O film at room temperature was nearly six times the ZnO film. The enhanced BEL intensity has been attributed to the suppression of capacitance variation related to trapping/detrapping of charges, a decrease in the number of nonradiative recombination defects, and an increase in the nonradiative recombination lifetime.

An image-based LCD positioning system utilizing the modified FHT method

Summary In this paper, a LCD positioning system based on an image processing method is described. This method is effective in finding the angle of the geometric features in the cross registration mask. However, when two marks appear in the same frame, the problem of situation verification becomes more complex. We use an algorithm that assists with calculating the location and rotation angle for two marks precisely and quickly using the Fast Hough Transform (FHT). This method determines the locations of two masks and calculates the rotation angle from a blurred edge to adjust the LCD position and orientation.

Effects of Li content on the structural, optical, and electrical properties of LiZnMgO films

This study investigates the effect of Li content on the structural, optical, and electrical properties of sol-gel LiZnMgO films by x-ray diffraction, scanning electron microscopy, photoluminescence, and conductivity measurements. A dependence of crystallite size and conduction type upon Li content has been found. The abnormal shift of the (002) diffraction peak position determined from x-ray diffraction measurements and conduction type determined by Hall effect measurements are closely related to the defects/different defect types. It is shown that n-type conversion of LiZnMgO films is a result of the increase in the donor density based on the change in stoichiometry.

Electronic Transport and Schottky Barrier Heights of Ni/Au Contacts on n-Type GaN Surface with and without a Thin Native Oxide Layer

Effects of a thin native oxide layer on Au/Ni/n-type GaN Schottky diodes were investigated in this study. The tunneling current was induced in the presence of native oxides on the GaN surface, making the thermionic emission (TE) theory inapplicable in this case. We find that the value of the barrier height (BH) calculated using the thermionic field emission (TFE) model is similar to that obtained by capacitance–voltage characteristics. This suggested that the discrepancy between BH according to the TFE and TE model for Au/Ni/n-type GaN Schottky diodes could be attributed to the presence of a native oxide layer at the Ni/n-type GaN interface and oxygen-induced and nitrogen-vacancy-related states on the GaN surfaces. Further, the characteristics of Schottky diodes improved when the n-type GaN was treated with (NH4)2Sx solution, an effective agent for removing native oxides and reducing surface states.

Surface band bending, nitrogen-vacancy-related defects, and 2.8-eV photoluminescence band of (NH4)2Sx-treated p-GaN

We have employed the photoluminescence and x-ray photoelectron spectroscopy measurements to study the effects of (NH4)2Sx treatment on the p-type GaN (p-GaN). After (NH4)2Sx treatment, we found that the reduction of the surface state, related to nitrogen-vacancy defects on the p-GaN surface, led to a reduction in surface band bending by 0.25 eV. The surface band bending reduction and surface state reduction caused by the (NH4)2Sx surface treatment could be useful for the formation of ohmic and Schottky contacts between the metal and p-GaN layers. In addition, the intensity of the 2.8-eV photoluminescence band depended on the amount of nitrogen vacancy of p-GaN, which was also investigated in this study.

High-barrier rectifying contacts on undoped ZnO films with (NH4)2Sx treatment owing to Fermi-level pinning

We report on the formation of high-barrier Ti and Ni contacts to (NH4)2Sx-treated ZnO films grown by pulsed-laser deposition. The x-ray photoelectron spectroscopy (XPS) results show that the position of the Zn 3d core-level peak at the (NH4)2Sx-treated ZnO surface is the same as that at the Ti/(NH4)2Sx-treated ZnO or Ni/(NH4)2Sx-treated ZnO interfaces, suggesting the occurrence of Fermi-level (EF) pinning and the formation of a barrier height of ∼2.7eV. From the photoluminescence and XPS measurements, it is suggested that a high Zn-vacancy density might cause the ZnO EF to be pinned close to the Zn-vacancy defect level at approximately 0.7eV above the valence band maximum. In addition, the discrepancy in barrier-height values obtained from XPS and current‐voltage measurements suggests the formation of S‐Zn surface dipoles with S atoms on the surfaces. (Some figures in this article are in colour only in the electronic version)

Changes in surface roughness and work function of indium-tin-oxide due to KrF excimer laser irradiation

In this study, from the observed x-ray photoelectron spectroscopy and atomic force microscopy results, and Kelvin probe measurements, it is suggested that the induced indium-tin-oxide (ITO) surface chemical changes by KrF excimer laser irradiation had strong effects on the surface work function (SWF) and surface roughness of ITO. During the laser irradiation, the incorporation of O22− peroxo species or the dissolution of oxygen species near the ITO surface leads to the reduction of the surface roughness and an increase in the SWF. In addition, it is worth noting that the laser-irradiated ITO sample has an excellent stability in the SWF.