Day-Shan Liu

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.

Carrier transport mechanism of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) films by incorporating ZnO nanoparticles

The carrier transport mechanism of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) having ZnOnanoparticles, denoted ZnO-doped PEDOT:PSS, is investigated. Conductivity proportional to ZnOdoping was observed at 300 K. The improved electrical conductivity is considered to mainly come from the mobility enhancement. The carrier mobility in ZnO-doped PEDOT:PSS samples exhibits unexpectedly strong temperature dependence, implying the domination of tunneling in the whole range of temperatures. An exhibition of high mobility of ZnO-doped PEDOT:PSS samples is attributed to a combined effect of the weak electron-phonon couplings and changes in the chemical structure of PEDOT:PSS.

Aluminum-nitride codoped zinc oxide films prepared using a radio-frequency magnetron cosputtering system

Al–N codoped zinc oxide films were prepared using a radio-frequency magnetron cosputtering system at room temperature. AlN and ZnO materials were employed as the cosputtered targets. The as-deposited cosputtered films at various theoretical atomic ratios [Al∕(Al+Zn)at.%] showed n-type conductive behavior in spite of the N atoms exceeding that of the Al dopants, indicating that the N-related acceptors were still inactive. The crystalline structure was obviously correlated with the cosputtered AlN contents and eventually evolved into an amorphous structure for the Al–N codoped ZnO film at a theoretical Al doping level reaching 60%. With an adequate postannealing treatment, the N-related acceptors were effectively activated and the p-type ZnO conductive behavior achieved. The appearance of the Zn3N2 phase in the x-ray diffraction pattern of the annealed Al–N codoped ZnO film provided evidence of the nitrification of zinc ions. The redshift of the shallow level transition and the apparent suppression of the o...

A Transparent and Conductive Film Prepared by RF Magnetron Cosputtering System at Room Temperature

The electrical and optical properties of transparent and conductive oxide films obtained from a cosputtering system with indium–tin oxide (ITO) and zinc oxide (ZnO) targets have been investigated. The resistivity of these ITO films deposited at an rf power of 150 W was markedly improved with additional simultaneous ZnO target cosputtering at rf powers ranging from 25 to 200 W. A transparent film of lowest resistivity (3.69×10-4 Ω cm) was achieved at room temperature where the rf powers of the cosputtering for the ITO and ZnO targets were 150 and 75 W, respectively. The average transmittance obtained from these cosputtered films in the visible spectrum was over 80%.

Electrical, Optical and Material Properties of ZnO-Doped Indium?Tin Oxide Films Prepared Using Radio Frequency Magnetron Cosputtering System at Room Temperature

The relationship between the electrical, optical and material properties of transparent and conductive oxide films prepared by rf cosputtering indium–tin oxide (ITO) and zinc oxide (ZnO) targets has been investigated. The evolution from polycrystalline structure of an undoped ITO film to an amorphous-like ZnkIn2O3+k structure obtained from ZnO-doped ITO films is found to be responsible for the marked improvement in the electrical properties. A low surface roughness is also achieved from this amorphous structure. However, both electrical property and surface uniformity begin to degrade with increasing rf cosputtering power on the ZnO target that corresponds to a high atomic ratio of Zn/(Zn + In). The degradation mechanism is attributed to the appearance of a microcrystalline ZnO structure that is detrimental to the film resistivity. Furthermore, optical band gap calculated from the absorption edge of such cosputtered films also decreases with increasing ZnO impurities.

Transparent cosputtered ITO–ZnO electrode ohmic contact to n-type ZnO for ZnO/GaN heterojunction light-emitting diode

A transparent cosputtered ITO–ZnO film was used as an ohmic contact electrode to the n-type ZnO layer. The contact resistance of the ITO–ZnO/n-ZnO contact system was optimized by a rapid thermal annealing (RTA) treatment. Through x-ray diffraction and Auger electron spectroscopy depth profile measurements, the achievement on the ohmic contact of the ITO–ZnO/n-ZnO contact system was attributed to the outdiffusion of the oxygen atoms at the n-type ZnO layer surface, which was favourable for the increase of the electron carriers. By contrast, the mechanism responsible for the degradation of the contact resistance was due to the significant outdiffusion of the zinc atoms at the n-type ZnO layer surface, which also resulted in the marked peak shift of the Zn2In2O5 phase. The resulting n-ZnO/p-GaN heterojunction light-emitting diode fabrication, which used the transparent ITO–ZnO/n-ZnO ohmic contact system, exhibited rectifying behaviour and emitted a near-ultraviolet radiation at approximately 410 nm.

Determination of activation behavior in annealed Al–N codoped ZnO Films

Al–N codoped ZnO films prepared via cosputtering technology were postannealed at 450 °C for 30 min under ambient vacuum and nitrogen. The extrinsic impurities in these annealed samples, resulting in evolutions on the carriers and radiation emissions, were investigated through their photoluminescence spectra and Hall-effect measurements. It was found that the donor-acceptor-pair emission was related to the VZn–AlZn transition at 2.86 eV and predominated over the defect-transition luminescence in the room-temperature photoluminescence (RTPL) spectrum of the vacuum-annealed sample, which possessed a high electron carrier concentration. With the help of temperature-dependent Hall measurements, a shallow donor level corresponding to Al on the Zn site (AlZn) was derived as EC − (51 ± 4) meV. In contrast, the defect-transition luminescence in the RTPL spectrum of the nitrogen-annealed Al–N codoped ZnO film, showing p-type conduction with a hole concentration of 1018 cm−3, was dominated by the VO–NO deep level em...

The Preparation of Piezoelectric ZnO Films by RF Magnetron Sputtering for Layered Surface Acoustic Wave Device Applications

Crystalline structures, intrinsic stress, and surface roughness of piezoelectric ZnO films deposited on sapphire substrates by rf magnetron sputtering and sequential post-annealing treatments were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. The rf power used and O2/(Ar+O2) gas ratio greatly affected the crystallinity and surface roughness of the ZnO film. A c-axis preferred orientation with superior surface roughness was achieved at a specific rf power and O2/(Ar+O2) gas flow ratio. In addition, the original intrinsic stress was also markedly released (stress=0.325 ×1010 dyn/cm2) and a uniform surface (Ra=2.42 nm) was produced by a post-annealing treatment at a temperature of 400 °C under oxygen ambient. An acoustic wave at the center frequency of 240 MHz (hZnO/λ=0.1) was obtained from the ZnO/sapphire-layered surface acoustic wave (SAW) device using this optimized ZnO film.

Hall-effect mobility of pentacene films prepared by the thermal evaporating method with different substrate temperature

We studied the Hall-effect mobility of pentacene films prepared by the thermal evaporating method with different substrate temperature. A crossover from coherent bandlike charge transport with mobilities up to several tens of cm2/V-s at low temperature to an incoherent hopping motion at high temperature is observed. The carrier mobilities of pentacene exhibit a hopping-to-band transition around room temperature. An exhibition of high mobility of pentacene films prepared with substrate temperature of 90 °C is attributed to the increased spacing between molecules.

Microstructure investigations of indium tin oxide films cosputtered with zinc oxide at room temperature

X-ray diffraction coupled with atomic force microscopy measurements were employed to investigate the cosputtered oxide films at various zinc content [Zn∕(Zn+In)at.%] atomic ratios prepared at room temperature using rf cosputtering indium tin oxide (ITO) and zinc oxide (ZnO) targets simultaneously. The crystalline structure of a pure ITO film is polycrystalline with obvious diffraction peaks of (222) and (400). As the atomic ratio reaches 26%, the cosputtered oxide film evolves from a polycrystalline ITO structure into an amorphouslike ZnkIn2Ok+3 structure. This structure also dominates the cosputtered oxide films at the atomic ratios raging from 26% to 54%. The formation of amorphouslike ZnkIn2O3+k structures is found to markedly reduce the associated film resistivity and performs a superior surface uniformity. At an atomic ratio of 60%, a diffraction peak identifies as ZnO (100) phase appears to pile on the original amorphous domain. This phase is attributed to the substitution of In3+ sites in the ITO s...