Chia-Lung Tsai

Optical and electrical properties of undoped ZnO films

Optical and electrical properties of undoped ZnO films were investigated in this study. We find that the conductivity increased with increasing the ratio of (blue luminescence+green luminescence) to ultraviolet luminescence [i.e., (BL+GL)∕UVL] defect emission intensities, due to the increase of the singly ionized oxygen vacancy. We deduce that the BL and GL are related to the oxygen-vacancy deep level and the yellow luminescence is attributed to the band-acceptor transition. We also find that poorer crystal quality may lead to the reduction of the optical transmittance. This suggests that these defects and the crystal quality interact to change the conductivity and optical transmittance of ZnO. The spectroscopic correlations may be used as a predictive tool to identify the quality of ZnO.

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)

Changes in surface band bending, surface work function, and sheet resistance of undoped ZnO films due to (NH4)2Sx treatment

In this study, the interaction of undoped ZnO films with (NH4)2Sx treatment have been investigated by x-ray photoelectron spectroscopy, photoluminescence, optical transmittance, and four-point probe. According to the experimental results, we find that the formation of Zn–S bonds and the reduction of oxygen vacancies (i.e., the S occupation of oxygen vacancies) near the ZnO surface might lead to an increase in the upward band bending, resulting in an increase in the sheet resistance and work function of ZnO.

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.

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.

Discrepancy in mobility extracted from transfer and output characteristics of organic thin film transistors

The discrepancy in mobility extracted from transfer and output characteristics of organic thin film transistors was studied. The extraction from transfer characteristics demonstrates higher mobility, compared to the extraction from output characteristics. It is shown that the contribution of capacitance variation may lead to an increased drain current, thus overestimating mobility.

Low-resistance nonalloyed ohmic contacts on undoped ZnO films grown by pulsed-laser deposition

We report on the formation of nonalloyed Ti and Ni ohmic contacts to ZnO films grown by pulsed-laser deposition. The experimental results show a lower barrier height of the Ti/ZnO samples than that of the Ni/ZnO samples (due to the lower work function of Ti than Ni), suggesting the Fermi-level unpinning at the interfaces. Based on the thermionic-emission or the thermionic-field-emission model, we found weak barrier-height dependence of the contact resistivity, implying that the presence of hydroxide in ZnO (i.e. the formation of the narrow depletion region at the metal/ZnO interface) resulted in the excess current component related to tunnelling, which led to the formation of the low-resistance nonalloyed metal/ZnO contact. The measurement temperature dependence of the contact resistivity revealed that the dominant current transport mechanism is field emission.

Mechanisms of enhancing band-edge luminescence of Zn1-xMgxO prepared by the sol-gel method

Zn1?xMgxO films prepared with different x were deposited on substrates by the sol?gel technique. X-ray photoelectron spectroscopy, x-ray diffraction, atomic force microscopy, photoluminescence and conductivity measurements were used to characterize the Zn1?xMgxO films. The yield in the intensity of the band-edge luminescence (BEL) is seen to increase up to 21-fold for Zn0.94Mg0.06O and up to 4-fold for Zn0.958Mg0.042O compared with the Zn0.973Mg0.027O film. The enhanced BEL intensity has been attributed to an increase in the nonradiative recombination lifetime, a reduction in the oxygen-vacancy related defects and a reduction in the refractive index of the Zn1?xMgxO film.

Effects of ultraviolet treatment on the contact resistivity and electronic transport at the Ti/ZnO interfaces

We report on the effect of ultraviolet (UV) treatment on the specific contact resistance (ρ) and electronic transport at the Ti/ZnO interfaces. The experimental results show the same barrier height of Ti/ZnO samples without UV treatment as Ti/ZnO samples with UV treatment and the higher ρ of Ti/ZnO samples with UV treatment than Ti/ZnO samples without UV treatment, suggesting the barrier-height independence of ρ. Based on the thermionic-emission model and x-ray photoelectron spectroscopy results, we found that the induced decrease in the number of the hydroxides at the surface region of ZnO by UV treatment resulted in decreases in the electron concentration near the surface region and the excess current component related to tunneling, increasing in ρ of Ti/ZnO samples.

Induced increase in surface work function and surface energy of indium tin oxide-doped ZnO films by (NH4)2Sx treatment

The effects of (NH4)2Sx treatment on the surface electronic properties of the thin indium tin oxide (ITO)-doped ZnO films have been examined in this study. According to the experimental results, we found that the formation of S-metal bonds and the removal of oxygen vacancies near the (NH4)2Sx-treated ITO-doped ZnO surface could lead to an increase in the surface energy and the work function, meaning that (NH4)2Sx treatment might be more helpful to form the uniform deposition of the organic semiconductor on ITO-doped ZnO surfaces and improve the efficiency of ZnO-based organic devices.