Yow-Jon Lin

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.

Increasing the work function of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) by ultraviolet irradiation

The effects of ultraviolet (UV) irradiation on the work function of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) have been investigated in this study. Spectroscopic methods [x-ray photoelectron spectroscopy and Raman spectroscopy (532nm excitation)] and electrical conductivity measurements were used to characterize the electrical conducting polymer PEDOT:PSS prepared by UV irradiation. The authors found that UV irradiation could lead to an increase in the work function and the enhancement of electrical conductivity of PEDOT:PSS, resulting from a decrease in the number of the charge-trapping-related defects and the formation of linear or expanded-coil conformation.

Application of the thermionic field emission model in the study of a Schottky barrier of Ni on p-GaN from current–voltage measurements

Barrier height values of Ni contacts to Mg-doped p-type GaN (p-GaN) were obtained from current–voltage measurements in this study. The induced deep level defect band through high Mg doping led to a reduction of the depletion layer width in the p-GaN near the interface and an increase in the probability of thermionic field emission. It also resulted in an increase in current flow under forward bias condition, which was not analyzed using the thermionic emission model. Further, the calculated barrier height value of Ni contacts to p-GaN using the thermionic field emission model is in good agreement with the value of 1.9eV obtained from x-ray photoelectron spectroscopy measurements.

Schottky barrier height and surface state density of Ni/Au contacts to (NH4)2Sx-treated n-type GaN

By using capacitance–voltage and photoluminescence measurements, we have investigated the Schottky barrier height and surface state density of Ni/Au contacts to n-type GaN with, and without, (NH4)2Sx treatment. The Schottky barrier height of 1.099 eV is very close to the Schottky limit of 1.10 eV for Au/Ni/n-type GaN treated with (NH4)2Sx. This result indicates that there is no severe Fermi level pinning induced by surface states. The reduction of the surface state density for the (NH4)2Sx-treated n-type GaN is attributed to the decrease of dangling bonds and occupation of nitrogen-related vacancies due to the formation of Ga–S bonds.

Investigation of surface treatments for nonalloyed ohmic contact formation in Ti/Al contacts to n-type GaN

To investigate the possibility of preparing a nonalloyed ohmic contact to n-type GaN, the effects of GaN surface treatments on the metal/GaN interface were studied using x-ray photoelectron spectroscopy. A specific contact resistance of 5.0×10−5 Ω cm2 for the Ti/Al nonalloyed ohmic contacts to (NH4)2Sx-treated n-type GaN can be obtained. The nonalloyed ohmic contact formation would be impeded by the native oxide and the hydroxyl induced from the surface treatment of chemical solutions.

Current transport mechanism of heterojunction diodes based on the reduced graphene oxide-based polymer composite and n-type Si

The present work reports the fabrication and detailed electrical properties of heterojunction diodes based on n-type Si and poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) having the reduced graphene oxide (RGO). This heterojunction diode showed a good rectifying behavior with an ideality factor of 1.2. A photocurrent decay model is presented that addresses the charge trapping effect and doping mechanisms for composite PEDOT:PSS films having RGO sheets. The enhanced dark conductivity was observed by incorporating RGO into PEDOT:PSS. For heterojunction diodes, the high photocurrent density originates from efficient hole transport combined with electron trapping with long-second lifetime.

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.

Investigation of oxidation mechanism for ohmic formation in Ni/Au contacts to p-type GaN layers

To investigate the function and mechanisms of oxidation, we present the ohmic performances for Ni/Au contacts to p-type GaN treated with various conditions. When the p-type GaN sample was preoxidized at 750 °C for 30 min in air ambient and then treated with (NH4)2Sx solution, we routinely obtained a specific contact resistance of 4.5×10−6 Ω cm2 for the Ni/Au contacts to samples alloyed at 500 °C for 10 min in air ambient. The fact that, in this configuration, ohmic performance improved one order of magnitude [compared with (NH4)2Sx surface treatment], is attributable to the strengthened formation of GaOx (aided by the preoxidation process), as well as the fact that more holes were induced on the oxidation-free p-type GaN surface.

X-ray photoelectron spectroscopy study of (NH4)2Sx-treated Mg-doped GaN layers

We present here the passivation mechanism and the chemistry of an (NH4)2Sx-treated Mg-doped GaN surface examined by using x-ray photoelectron spectroscopy. The native oxide on the GaN surface can be removed by the (NH4)2Sx treatment process. The S atoms not only bond as elemental sulfur and disulfides, but occupy nitrogen-related vacancies.

Modification of the electrical properties of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) upon doping of ZnO nanoparticles of different content

A photocurrent decay model is presented that addresses the charge trapping and doping mechanisms for composite poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) films having ZnOnanoparticles of different contents. It is shown that ZnOdoping introduced changes in the chemical structure of PEDOT:PSS. Dark current proportional to ZnOdoping was observed. For n-type Si/ZnO-doped PEDOT:PSS devices, the high photocurrent density originates from efficient hole transport combined with long-lifetime electron trapping.