(Photo)conductivity of conjugated oligomer films: mobile charge carrier formation by oxygen

Electrical dark and photo conductivities, (σ_d and σ_p, of vapor deposited films of -sexithiophene, 4,4′-distyrylstilbene, and titaniumoxophthalocyanine were investigated as function of temperature, exposure time to oxygen of variable pressure, irradiation time and intensity. Molecular oxygen increases σ_d and σ_p by orders of magnitude. Careful removal of oxygen at elevated temperatures under high vacuum and assistance of irradiation will reduce both σ_d and σ_p to almost negligible values. From the rise and decay kinetics of conductivity and from luminescence quenching it is concluded that oxygen must be complexed with the film constituting molecules in order to act as separation center for electrical charge. The complex is formed already in the dark, but more efficiently upon irradiation. The interaction with oxygen is essentially reversible but leads in a side reaction also to irreversibly oxidized photoproducts.     

Enhanced conductivity of aluminum doped ZnO films by hydrogen plasma treatment

Aluminum doped zinc oxide (AZO) thin films prepared by radio-frequency (RF) magnetron sputtering at various RF power were treated by hydrogen plasma to enhance the characteristics for transparent electrode applications. The hydrogen plasma treatment was carried out at 300 °C in a plasma enhanced chemical vapor deposition system. X-ray diffraction analysis shows that all AZO films have a (002) preferred orientation and film crystallinity seems no significant change after plasma treatment. The plasma treatment not only significantly decreases film resistivity but enhances electrical stability as aging in air ambient. The improved electrical properties are due to desorption of weakly bonded oxygen species, formation of Zn-H type species and passivation of deep-level defects during plasma treatment.     

Resisting oxygen plasma damage in low-k hydrogen silsesquioxane films by hydrogen plasma treatment

Low-density materials, such as the commercially available hydrogen silsesquioxane (HSQ) offer a low dielectric constant. Thus, HSQ with a low value of k (∼ 2.85) can be spin-coated if the density of Si–H bonding is maintained at a high level and the formation of –OH bonds and absorption of water in the film is minimized. O₂ plasma exposure on HSQ film increases leakage current. Also the dielectric constant shows a significant increase after O₂ plasma exposure. Another consequence of the O₂ plasma exposure is the significant decrease in the contact angle of the HSQ surface, which is not desirable. In this paper, we demonstrate that the surface passivation by hydrogen followed by oxygen plasma treatment of HSQ film for 30 min each leads to a regain of leakage current density and dielectric constant. These results show that the H₂ plasma treatment is a promising technique to prevent the damage in the commercially available and highly applicable low-k materials and it also increases the visibility of its use at the 0.1-μm technology. The more hydrophilic nature of the HSQ surface after O₂ plasma exposure leads to an increased moisture absorption with a subsequent increase in the dielectric constant.     

High density plasma treatment of polyimide substrate to improve structural and electrical properties of Ga-doped ZnO films

We investigated the effects of a high density O₂ plasma treatment on the structural and electrical properties of sputter-deposited GZO films. The GZO films were deposited on polyimide substrate without substrate heating by RF magnetron sputtering from a ZnO target mixed with 5 wt.% Ga₂O₃. Prior to the GZO film growth, we treated a polyimide substrate with highly dense inductively coupled oxygen plasma. The optical transmittance of the GZO film, about 80%, was maintained regardless of the plasma pre-treatment. However, the resistivity of the film was strongly influenced by the plasma pre-treatment. The resistivity of the GZO film decreased from 1.02 × 10^− 2 Ω cm without an O₂ plasma pre-treatment to 1.89 × 10^− 3 Ω cm with an O₂ plasma pre-treatment.     

Charge carrier and spin doping in ZnO thin films

Recent efforts on doping ZnO films for charge and spin functionality are reviewed, focusing on chemical doping for charge and spin device formation. Discussion includes the behavior of phosphorus as an acceptor and magnetism in transition metal-doped ZnO. Evidence for p-type behavior in phosphorus-doped (Zn,Mg)O grown by pulsed laser deposition is presented. The magnetic properties of ZnO co-doped with Mn and Sn are also discussed.     

Optical and electrical properties of ZnO nanoparticle thin films deposited on quartz by sparking process

ZnO nanoparticle thin films were deposited on quartz substrates by a novel sparking deposition which is a simple and cost-effective technique. The sparking off two zinc tips above the substrate was done repeatedly 50-200 times through a high voltage of 10 kV in air at atmospheric pressure. The film deposition rate by sparking process was approximately 1.0 nm/spark. The ZnO thin films were characterized by X-ray diffraction, Raman spectroscopy, UV-vis spectrophotometry, and ionoluminescence at room temperature. The two broad emission peaks centered at 483 nm (green emission) and 650 nm (orange-red emission) were varied after two-step annealing treatments at 400-800 °C. Moreover, the electrical resistivity of the films was likely to be proportional to the peak intensity of the orange-red emission.     

Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment

This study examined the effects of an oxygen plasma treatment on the properties of ZnO nanowires with diameters of 80 nm using a single nanowire field effect transistor. After the oxygen plasma treatment, the carrier concentration and mobility of individual ZnO nanowires decreased with a substantial positive shift in the threshold voltage. The shifting was accounted to the surface modification, resulted to the improved gas sensitivity under hydrogen gas exposure and an enhanced photocurrent response time in ultraviolet illumination. The plausible surface mechanisms responsible for these significant changes after the surface modification were suggested by considering the surface analysis and electrical transport mechanism.     

高致密度FeNi纳米颗粒薄膜制备与性能研究

传统磁控溅射装置制备的纳米颗粒薄膜粒径不均一并且实现粒径大小调控比较困难.本研究采用电场辅助沉积技术,在沉积平台施加5～30 kV的电场,以Si(100)为衬底制备了一系列纳米颗粒粒径均一的高致密度FeNi纳米颗粒薄膜材料.通过XRD、SEM以及VSM测量,研究了不同沉积电场下FeNi纳米颗粒薄膜的结构、形貌和磁性能....     

Effects of thermal annealing in oxygen plasma for buffer layers on properties of ZnO thin films

ZnO thin films with ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si(100) substrates. Before the growth of the ZnO thin films, the ZnO buffer layers were deposited on the Si substrates for 20 minutes and then annealed at the different substrate temperature ranging from 600 to 800 degrees C in oxygen plasma. The structural and optical properties of the ZnO thin films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and room-temperature (RT) photoluminescence (PL). A narrower full width at half maximum (FWHM) of the XRD spectra for ZnO(002) and a larger grain are observed in the samples with the thermal annealed buffer layers in oxygen plasma, compared to those of the as-grown sample. The surface morphology of the samples is changed from rugged to flat surface. In the PL spectra, near-band edge emission (NBEE) at 3.2 eV (380 nm) and deep-level emission (DLE) around 1.77 to 2.75 eV (700 to 450 nm) are observed. By increasing the annealing temperatures up to 800 degrees C, the PL intensity of the NBEE peak is higher than that of the as-grown sample. These results imply that the structural and optical properties of ZnO thin films are improved by the annealing process.     

Transparent conducting ZnO thin films deposited by vacuum arc plasma evaporation

A high rate deposition of co-doped ZnO:Ga,F and ZnO-In₂O₃ multicomponent oxide thin films on large area substrates has been attained by a vacuum arc plasma evaporation method using oxide fragments as a low-cost source material. Highly transparent and conductive ZnO:Ga,F and ZnO-In₂O₃ thin films were prepared on low temperature substrates at a deposition rate of approximately 375 nm/min with a cathode plasma power of 10 kW. A resistivity of 4.5×10⁻⁴ Ω cm was obtained in ZnO:Ga,F films deposited at 100 °C using ZnO fragments co-doped with 1 wt.% ZnF₂ and 1 wt.% Ga₂O₃ as the source material. In addition, the stability in acid solution of ZnO films was improved by co-doping. It was found that the Zn/(In+Zn) atomic ratio in the deposited ZnO-In₂O₃ thin films was approximately the same as that in the fragments used. The ZnO-In₂O₃ thin films with a Zn/(In+Zn) atomic ratio of approximately 10-30 at.% deposited on substrates at 100 °C exhibited an amorphous and smooth surface as well as a low resistivity of 3-4×10⁻⁴ Ω cm.     

Enhanced photoconduction of free-standing ZnO nanowire films by L-lysine treatment

Flexible paper-like ZnO nanowire films are fabricated and the effect of L-lysine passivation of the nanowire surfaces on improving the UV photoresponse is studied. We prepare three types of nanowires with different defect contents, and find that the L-lysine treatment can suppress the oxygen-vacancy-related photoluminescence as well as enhance the UV photoconduction. The nanowires with fewer defects gain larger enhancement of UV photoconduction after L-lysine treatment. Reproducible UV photoresponse of the devices in humid air is obtained due to L-lysine surface passivation, ruling out the influence of water molecules in degrading the UV photocurrent.     

Thermal stability of carrier centers in various types of transparent conductive ZnO and Ga-doped ZnO films

We investigated the thermal stability of the transparent conductive properties of undoped ZnO and Ga-doped ZnO (GZO) films when they were annealed in a high vacuum with stepwise increasing temperature. The ZnO samples included V_O-rich and Zn-rich ZnO films; the primary donors were respectively oxygen vacancies (V_O) or Zn atoms highly unsaturated with oxygen atoms. V_O-rich ZnO was the most unstable against annealing; resistivity initially within the 10⁻³ Ω cm range diverged higher than 10 Ω cm when a critical temperature was exceeded. The critical temperature between 350 and 450 °C depended on the film thickness, which indicated that V_O's were diminished through recombination with migrating interstitial oxygen atoms. In contrast, Zn-rich ZnO films remained highly conductive up to 550 °C. They became more and more transparent and their crystallinity improved at higher annealing temperatures, which was the consequence of metallic-like Zn atoms being removed through desorption from the surface or being accommodated into the crystalline lattice. Comparatively, GZO films were more robust against annealing with their resistivities remaining unchanged up to 350 °C.     

Structural transition and charge density waves in thin films

A special type of low-temperature structural transition occurs in thin, size-quantized films with a strong anisotropy of the Fermi curve (e.g., in Bi films). The lattice appears to be unstable for such films. The structural transition is accompanied by the appearance of charge density waves and is characterized by peculiar temperature and thickness dependences of the electron concentration. Experimental data can be explained by the present theory. The possibility of new experimental investigations is discussed.     

Forming n-p junctions based on p-CdHgTe with low charge carrier density

The first results on the formation of n ⁺-p photodiodes with a long-wavelength boundary at λ_0.5 = 10.6 μm based on p-CdHgTe epilayers with a hole density of 3.3 × 10¹⁴ cm^?3 are presented. The CdHgTe layers were obtained by molecular beam epitaxy and exhibited p-type conduction in the as-grown state. The hole density and mobility were determined by means of the mobility spectrum technique. The photodiode structures were prepared by B⁺ ion implantation into as-grown p-CdHgTe epilayers without additional annealing. The R ₀ A product for the n ⁺-p photodiodes obtained is about 20 Θ cm² at 77 K.     

Charge carrier transport and space charge in thin films of antimony trisulphide

Data on the resistance of thin sputtered zinc films before and after thermal aging suggest the existence of two layers: the lower layer, about 140 Å thick, corresponding to the first critical thickness, is strongly modified by aging, and electronic conduction occurs by reflection at grain boundaries (M-S model); the top layer is slightly modified by aging, and conduction occurs by reflection at grain boundaries and on the top surface. The phenomena are quite consistent with the theory of the nucleation growth of sputtered films.     

ZnO纳米线阵列修饰对材料表面浸润性调控

采用溶胶-凝胶法制备了ZnO薄膜,利用溶剂热沉积法获得大面积均匀ZnO纳米线阵列。通过对水在ZnO材料表面的浸润性研究,发现薄膜材料表面的粗糙度对ZnO膜亲水性有增强作用,而周期性ZnO阵列微结构表面可以实现其疏水性质增强效果。同时从理论上分析了这两种现象的物理机制,讨论了空气填隙对ZnO纳米线阵列表面的浸润性质的敏感性。制备出ZnO纳米线阵列的表观接触角约为103°,具有较强的疏水性质,可为进一步的ZnO光流控研究提供实验基础。     

Stability of hydrogen incorporated in ZnO nanowires by plasma treatment

The stability of hydrogen in ZnO is studied using hydrogenated nanowires by plasma treatment. Enhanced near band edge UV emission and reduced defect level green emission is observed after hydrogen plasma treatment. Through thermal stability tests, this effect is found to be stable at room temperature and nearly stable up to ~500 K, but begins to deteriorate at higher temperature. The study of the irradiation stability of the hydrogen in ZnO nanowires shows that the hydrogen is stable under an electron beam with an accelerating voltage lower than 5 kV, but is not stable under 10 kV or under an intensive laser beam. The results could benefit the further understanding of the role of hydrogen in ZnO and light-emitting devices based on hydrogenated ZnO.     

Activation of p-type conduction in ZnO:N films by annealing in atomic oxygen

ZnO:N epitaxial films have been grown by reactive magnetron sputtering. The effect of annealing in atomic oxygen on the structural and electrical properties of the ZnO:N films has been studied by X-ray diffraction, atomic force microscopy, Hall effect measurements, and X-ray photoelectron spectroscopy. By annealing at temperatures from 500 to 700°C, we have obtained p-type ZnO:N films with a resistivity of ～57 Ω cm, hole mobility of ～2.7 cm²/(V s), and hole concentration of ～6.8 × 10¹⁷ cm^?3. X-ray photoelectron spectroscopy results suggest that the p-type conductivity of the films is due to a decrease in the concentration of (N₂)_O and V _O donors.     

Texture control of ZnO films by inductively coupled plasma assisted chemical vapor deposition

ZnO films were grown on Si (100) and quartz substrates by inductively coupled plasma-assisted chemical vapor deposition using diethylzinc, O₂, and Ar. ZnO films with the (002) preferred orientation (PO) were formed at substrate temperatures > 250 °C regardless of any other changes made to process variables, since the (002) plane has the lowest formation energy with the highest number of unsaturated Zn-ZnO or O-ZnO bonds. At temperatures < 250 °C, the a-axis plane PO such as (100), (110), and (101) as well as the c-axis (002) plane PO were able to form by varying the temperature, plasma power, and deposition rate. The a-axis PO was formed when the radio frequency power was high enough to produce a crystalline ZnO film but was insufficient to form a (002) PO. The a-axis PO was also formed at higher deposition rates ≥ 20 nm/min when the radio frequency power was high enough to produce crystalline ZnO film. Since the (002) plane grew slowly, the grain exposing (002) plane was overgrown by the grains of the a-axis plane at higher deposition rates.     

Oxygen atom density in microwave oxygen plasma

We determined the density of neutral oxygen atoms in microwave plasma using a fiber-optics catalytic probe (FOCP). Plasma was created within a quartz tube with an outer diameter of 5 cm by a 2.45 GHz microwave generator with an output power up to 1000 W. The oxygen flow was varied between 4 and 20 l/h. The O-atom density was found to increase monotonically with the increasing discharge power, and it decreased with the increasing flow rate. The degree of dissociation of oxygen molecules in the plasma column depended largely on the flow rate. At the oxygen flow of 4 l/h, it was about 18% but it decreased to about 6% at the flow of 20 l/h.