The effects of surface treatment on device performance in pentacene-based thin film transistor

We report on the influence of surface treatment using hexamethyldisilazane (HMDS) on device performance of pentacene-based thin film transistor. The samples with surface treatment using HMDS show higher mobility, lower subthreshold slope, and lower off-current compared to the untreated samples. We have also investigated the effect of various coating methods of HMDS on device performance. In the case of post-baked samples after spin coating, the improvements of mobility and on-current are larger than those of spin-coated samples. Especially a dip-coated sample has threshold voltage of −4.11 V and turn-on voltage of −0.03 V, showing the enhancement mode characteristics, which is useful for the operation as a circuit and a switch device for active-matrix displays. However, the mobility of dip-coated sample is one order lower than that of spin-coated sample, and hysteresis is larger than that of spin-coated sample.     

A method to improve mechanical properties of glass plates by surface-coating titania nanofilms with sol-gel technique

The present work aims to improve mechanical properties of glass plates by coating homogeneous Ce⁴⁺-doped titania films on them with the sol-gel technique. The mechanical properties of coated glass plates and the film microstructure were characterized by using advanced equipments such as X-ray diffractometer, atomic force microscope, X-ray photoelectron spectrometer, nano-indenter and mechanical tester. Experimental results show that, the coated films, with a thickness of 20-100 nm and with a main phase of anatase, firmly adhere to the glass plates by sintering at 450 °C for 2 h. Their mechanical properties such as Young's modulus, strength, fracture toughness and microhardness are increased with the increase of film thickness and significantly increased by Ce⁴⁺ doping titania. Moreover, Ce⁴⁺ doping inhibits the crystallization of rutile phase and decreases anatase granularity. The effects of coating titania films and Ce⁴⁺-doping titania on mechanical properties of glasses are correlated to presence of chemical combinations between film and glass interface, decrease of surface roughness and anatase granularity, as well as reduction of the density of microcracks on the glass surface.     

An in situ ESR study on poly(3-methylthiophene): charge transport due to polarons and bipolarons before the evolution of metallic conduction

In situ ESR measurements with ClO₄⁻-doped poly(3-methylthiophene) (PMT) are carefully performed over a wide range of doping level from 0.02% to 23% in order to correlate a drastic increase of carrier mobilities by doping with the nature of charged species in the π-conjugated polymer. Either or both of a Gaussian and Lorentzian components with different g-factors are observed, depending on the doping level. The broad Gaussian signal ascribable to structural defects in the polymer remains almost unchanged at various doping stages while the Lorentzian signal varies greatly with the doping level. From spin concentrations for the Lorentzian signal, molar fractions of polarons and diamagnetic species (bipolarons) are evaluated as a function of doping level. Combination of the molar fractions and the mobility data obtained earlier demonstrates that the conduction process in the lightly doped PMT film (0.02%–1%) can be expressed by independent driftings of polarons and bipolarons under electric field, whose mobilities are 2×10⁻⁵ and 10⁻³ cm² V⁻¹ s⁻¹, respectively. At higher doping levels of 1%–23%, conductivity is much more enhanced than expected from this model and the line width of the Lorentzian signal increases from 1.2 to 7.5 G, suggesting the evolution of a metallic conduction.     

Incorporation of transition metal ions and oxygen generation during anodizing of aluminium alloys

Enrichment of nickel at the alloy/film interface and incorporation of nickel species into the anodic film have been examined for a sputtering-deposited Al-1.2at.%Ni alloy in order to assist understanding of oxygen generation in barrier anodic alumina films. Anodizing of the alloy proceeds in two stages similarly to other dilute aluminium alloys, for example Al-Cr and Al-Cu alloys, where the Gibbs free energies per equivalent for formation of alloying element oxide exceeds the value for alumina. In the first stage, a nickel-free alumina film is formed, with nickel enriching in an alloy layer, 2 nm thick, immediately beneath the anodic oxide film. In the second stage, nickel atoms are oxidized together with aluminium, with oxygen generation forming gas bubbles within the anodic oxide film. This stage commences after accumulation of about 5.4 × 10¹⁵ nickel atoms cm⁻² in the enriched alloy layer. Oxygen generation also occurs when a thin layer of the alloy, containing about 2.0 × 10¹⁹ nickel atoms m⁻², on electropolished aluminium, is completely anodized, contrasting with thin Al-Cr and Al-Cu alloy layers on electropolished aluminium, for which oxygen generation is essentially absent. A mechanism of oxygen generation, based on electron impurity levels of amorphous alumina and local oxide compositions, is discussed in order to explain the observations.     

Nano silicon top-layer for composite-induced multiphasic enhancement of thermal stability of hardness of diamond-like carbon nanofilm at 900 °C

The effect of 25-nm silicon top-layer on the hardness and thermal stability of 100-nm diamond-like carbon (DLC) film annealed at 750–900 °C has been investigated. The evolution of surface morphology, microstructure and reaction between C and Si was examined by high resolution scanning/transmission electron microscope, Raman and FTIR spectroscopy. The hardness of films was investigated using nano-indentation. After 750–900 °C annealing, the hardness of single carbon layer greatly decreased at 750 °C and then slightly increased at 900 °C due to the formation of SiC at the interface between the single C film and the Si substrate. In contrast, no significant variation occurred on the hardness of two-layer Si/C film under RTA at 750–900 °C. Although the higher annealing temperature resulted in higher sp²/sp³ bonding ratio as well as more sp² bonding formation in the carbon layer to soften the structure, the added Si top-layer can protect DLC from reaction with environmental oxygen and sustain the hardness of the composite film because of the multiphasic formation with extra SiC on the surface and at the interface between the C layer and Si substrate through great interdiffusion between Si and C for extending DLC high-temperature application.     

The behaviour of chromium during anodizing of Al–Cr alloys

The anodic oxidation of dilute Al–Cr alloys, containing 0.8 and 1.7 at% Cr, has been investigated in order to understand the oxidation behaviour of the alloying element and its influences on the film composition and morphology. The alloys reveal two stages of oxidation: an initial stage, in which only aluminium atoms are oxidized to form a chromium-free anodic alumina film, and a subsequent stage, in which both aluminium and chromium are oxidized, in their approximate alloy proportions, with generation of a chromium-contaminated anodic alumina film. In the first stage, chromium is enriched in a thin layer of alloy, immediately beneath the anodic film, to an amount corresponding to a layer of average thickness 1.5 nm and of average composition, Al–20 at% Cr. Following the oxidation of chromium, oxygen is produced electrochemically within the film at or near the alloy/film interface, probably associated with the development of chromium-rich clusters in the enriched alloy layer and, subsequently, formation of semiconducting chromium-rich oxide. Thus, the film material formed at the alloy/film interface by inward migration of O^2- ions contains many oxygen-filled bubbles with associated high pressures. The chromium species present in the film migrate outward more slowly than Al³⁺ ions. Hence, a layer of chromium-free anodic alumina, which thickens as the film grows, is maintained adjacent to the film/electrolyte interface.     

Dissolution kinetics of small amounts of oxygen in tantalum alloy T-111 and internal oxide displacement reactions during annealing

Oxygen was added to T-111 (Ta-8W-2Hf, wt.%) at 820 and 990°C at an oxygen pressure of about 3×10^–4 Torr (4×10^–2N/m²). The technique employed permitted predetermined and reproducible doping of T-111 up to 3.0 at% oxygen. Based on the temperature dependence of the doping reaction, it is concluded that the initial rates of oxygen pickup are probably controlled by solution of oxygen into the T-111 lattice. Although hafnium oxides are more stable than those of tantalum or tungsten, analyses of extracted residues indicate that the latter oxides predominate in the as-doped specimens, presumably because of the higher concentrations of tantalum and tungsten in the alloy. However, high-temperature annealing promotes gettering of dissolved oxygen and of other oxides to form hafnium oxides. Small amounts of tantalum and tungsten oxides were still present after high-temperature annealing. Tungsten oxide (WO₃) volatilizes slightly from the surface of T-111 at 990°C. The vaporization of WO₃ has no apparent affect on the doping reaction.     

Development of porous anodic films on 2014-T4 aluminium alloy in tetraborate electrolyte

Anodic film growth on 2014-T4 aluminium alloy at 60 V in 50 g l⁻¹ di-sodium tetraborate at 60 °C has been examined by transmission electron microscopy and Rutherford backscattering spectroscopy. Initial film growth proceeds at relatively high efficiency on the initially etched and desmutted alloy. During the subsequent period of current decline, the reactive electrolyte species penetrate the outer film at preferred regions, establishing conditions for pore development by field-assisted dissolution. In the alkaline electrolyte, such field-assisted dissolution also appears to proceed locally, probably through mechanical disruption of the film, giving rise to a feathered film morphology. The oxidation of copper from the alloy, in the presence of an enriched layer of copper, developed largely by initial etching, also influences film morphology through parallel oxygen gas generation, creating oxygen-filled voids. Such gas-filled voids may rupture or be removed from the alumina film material through field-assisted dissolution at the pore base. In the former case, cracking allows access of the anodizing electrolyte to the enriched alloy/film interface, with subsequent dissolution of the enriched layer and local film growth; these give rise to lateral porosity in addition to that from pores passing perpendicularly to the alloy surface. The efficiency of anodizing is about 12%, with losses from Al³⁺ ion ejection, field-assisted dissolution, oxygen gas generation, film rupture, interface dissolution and local film repair.     

Industrial News

Abstract

Observationson the oxidation behaviour of zirconium-niobium alloys in pressurised water at 300° indicate a sensitivity to oxygen content of the water and niobium content of the alloy which can be reconciled with the concept of an oxide film of duplex type, the inner portion being of the normal hypostoicheiometric n-type but changing unnder the relatively oxidising conditions in the outer layers to a hyperstoicheiometric p-type oxide contaning Interstitial oxygen ions by virtue of the presence of some Nb^v ions.

There are also preliminary indications of a similar type of sensitivity of corrosion rate to oxygen content of the water wIth a 0·75 wt.-% zIrcomum-ehromium alloy. Experiments carried out with Zircaloy-2 at 290° in water dosed with chromium trioxide and with a 1500 lb/in² overpressure of oxygen to produce chromate ions in solution have shown an increase in corrosion rate suggesting that doping of the zirconia film with higher valent chromium ions is occurring during the test.

There are analogies between the out-of-pile corrosion behaviour of zirconium-niobium alloys under oxidisng conditions and the behaviour of Zircaloy-2 under oxidising conditions in-pile. It is suggested that, owmg to different trapping efficiencies in the longer term there is an imbalance between the concentrations of oxygen vacancies and interstitials under irradiation, and that further, there will be an interaction with the external oxygen pressure such that the concentration of oxygen interstitials will increase at the expense of the vacancy concentration as the oxygen pressure increases. The converse appears to be the case when the 2·5 wt.-% niobium-zironium alloy is subjected to high fast neutron flux under oxidising conditions; a lower rate of oXld~~I~nISobserved compared with the out-of-pIle value under comparable conditions, and the lack of sensitivity to oxygen potential and the appearance of the oxide suggests predominant oxygen transport via vacancies, i.e n-type behaviour.     

Doping competition of anions during the electropolymerization of pyrrole in aqueous solutions

The doping competition of anions in the electropolymerization of pyrrole was studied in aqueous solutions containing two kinds of anions with the same concentration of 0.1 mol dm^-3. The composition analysis of the polypyrrole (PPy) films prepared shows that the doping amount of p-toluene sulfonate (TsO^-) prevails over that of perchlorate, chloride and nitrate anions. The conductivity of the PPy film is close to the higher value of the two PPy films doped with a single anion. When polymerization is performed in the solution containing TsO^- and NO₃^-, TsO^-is mainly doped at Structure I, while NO₃^- is mainly at Structure II of the PPy film.     

Sulfur embrittlement on γ/γ′ interface of Ni-base single crystal superalloys

The impurity sulfur embrittlement on γ/γ′ interface of Ni-base single crystal superalloys has been investigated by first principles quantum mechanics DMol calculations. Using a local sum of vertical (∑BO^v) and horizontal (∑BO^h)Mayer bond orders, we proposed a new method to evaluate the competition between the shear and cohesive strengths of the interface. Coupled with the phenomenological theory of fracture, we define the ratio of R_BO=∑BO^h/∑BO^v to assess the embrittlement trend of the interface related to sulfur doping or sulfur doping combined with Re substitution. It is shown that sulfur increases R_BO by 121% relative to the sulfur-free γ/γ′ interface, which could induce interface embrittlement from the electron bonding point of view. Calculations of both BO and charge density distribution reveal that it is the strong bonds between sulfur and Ni atoms lying within the interface that contribute to the interface embrittlement. The substitution of Re for Al at the γ/γ′ interface results in reduced R_BO, thus relieving the tendency of interface embrittlement. Furthermore, our model on sulfur embrittlement is compared with previous models.     

Anodic oxidation of Ta/Fe alloys

The behaviour of iron during anodizing of sputter-deposited Ta/Fe alloys in ammonium pentaborate electrolyte has been examined by transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. Anodic films on Ta/1.5 at.% Fe, Ta/3 at.% Fe and Ta/7 at.% Fe alloys are amorphous and featureless and develop at high current efficiency with respective formation ratios of 1.67, 1.60 and 1.55 nm V⁻¹. Anodic oxidation of the alloys proceeds without significant enrichment of iron in the alloy in the vicinity of the alloy/film interface and without oxygen generation during film growth, unlike the behaviour of Al/Fe alloys containing similar concentrations of iron. The higher migration rate of iron species relative to that of tantalum ions leads to the formation of an outer iron-rich layer at the film surface.     

Recent progress in silicon molecular-beam epitaxy

Traditional concepts of epitaxial growth assume that for growth of a film with a finite thickness in finite time there is a temperature (T_epi) separating the regime of epitaxial, single-crystalline growth of the film from the regime of amorphous growth. Recent progress in the understanding of epitaxial growth shows that this concept is not applicable for silicon molecular-beam epitaxy. Instead, growth at a given temperature always proceeds epitaxially for a certain limiting thickness (h_epi) before the film becomes amorphous. Thus, a finite thickness can be grown at any temperature and, in particular, at temperatures low enough to suppress the surface segregation of dopants present at traditional growth temperatures. The growth of arbitrarily complex doping profiles by thermal, coevaporative doping becomes possible. Unity incorporation and activation of antimony with thermal, coevaporative doping up to dopant concentrations of ～10²¹ cm^?3 without any post-growth annealing have been achieved.     

点缺陷模型在2205双相不锈钢中的应用

采用电化学极化曲线和电化学阻抗技术对2205双相不锈钢在0.1%、1.0%及3.5%（质量分数,%）三种不同浓度的NaCl溶液中的腐蚀性能进行测试,采用点缺陷模型(PDM)对测试结果进行建模与分析。研究结果表明,2205双相不锈钢随着溶液浓度的升高抗点蚀能力下降,这是由于在钝化膜的生长过程中,氧离子缺陷产生于金属/膜界面,消耗于膜/溶液界面,而金属离子缺陷产生于膜/溶液界面,消耗于金属/膜界面;氧离子缺陷的迁移导致钝化膜的生长,而金属离子缺陷的迁移使得钝化膜发生溶解。同时,根据PDM模型理论并从金属相角度出发对2205不锈钢建立钝化膜溶解模型,可知2205双相不锈钢奥氏体相γ上的钝化膜可能比铁素体相α优先发生溶解。     

Determination of the Diffusivity of Point Defects in Passive Films on NiTi and NiTiAl Alloys

A point defect model based on the movement of cation and anion defects in an electrostatic field was carried out to explain the growth and dissolution behavior of a passivation layer on NiTi and NiTiAl thin films. The calculated value of diffusivity was in range of 10⁻¹⁶ to 10⁻¹⁷ cm²/s. The defect of oxygen vacancy revealed that the passive film was an n-type semiconductor. Mott-Schottky analysis showed that the doping level within a passive film was rather large and in the order of 10²⁰ to 10²¹ cm⁻³ film, which was considered to be a highly doped structure. The high-resolution transmission electron microscopy (HRTEM) images showed that the highly doped structure consisted of amorphous and crystalline structures of TiO₂ and Al₂O₃, respectively. A thermodynamic evaluation for the difference between crystalline and the fully amorphous oxides was calculated to be 57.57 and 96.87 kJ/mol, respectively. In the amorphous region, the electronic level arises from the presence of an energy band gap in the ideal crystalline structure. Therefore, the smaller donor density and the lower diffusion coefficient retarded the defect movement in the passivation layer, and improved the stability of the passive film during corrosion.     

Improved mobility of the copper phthalocyanine thin-film transistor

Copper phthalocyanine (CuPc) organic thin-film transistor (OTFT) was fabricated by thermal evaporation deposition on p-SiO₂ dielectric layer. Organic thin-film transistors used in large display areas need the enhancement of transistor performances by increasing the I_on/I_off ratio and the mobility. The output and transfer characteristics of CuPc-OTFT having source/drain interdigitated-finger geometry were investigated. The mobility, I_on/I_off ratio and inverse sub-threshold slope for the CuPc-OTFT were found to be 5.32 × 10^?3 cm² V^?1 s^?1, 1.94 × 10⁴ and 2.5 V/decade, respectively. The interface state density of the transistor was found to be 3.73 × 10¹¹ eV^?1 cm^?2 using the conductance-frequency method. The CuPc film indicated a homogeneous surface having 3.878 nm small roughness values as observed by atomic force microscope (AFM) measurements. The obtained results indicate that we have improved a CuPc-OTFT transistor with high mobility without being of any substrate treatment.     

Influence of indium doping on the properties of spray deposited CdS0.2Se0.8 thin films

Polycrystalline indium doped CdS_0.2Se_0.8 thin films with varying concentrations of indium have been prepared by spray pyrolysis at 300 °C. The as deposited films have been characterized by XRD, AFM, EDAX, optical and electrical resistivity measurement techniques. The XRD patterns show that the films are polycrystalline with hexagonal crystal structure irrespective of indium doping concentration. AFM studies reveal that the RMS surface roughness of film decreases from 34.68 to 17.76 with increase in indium doping concentration up to 0.15 mol% in CdS_0.2Se_0.8 thin films and further it increases for higher indium doping concentrations. Traces of indium in CdS_0.2Se_0.8 thin films have been observed from EDAX studies. The optical band gap energy of CdS_0.2Se_0.8 thin film is found to decrease from 1.91 eV to 1.67 eV with indium doping up to 0.15 mol% and increase after 0.15 mol%. The electrical resistivity measurement shows that the films are semiconducting with minimum resistivity of 3.71 × 10⁴ Ω cm observed at 0.15 mol% indium doping. Thermoelectric power measurements show that films exhibit n-type conductivity.     

Thermal properties of zirconia co-doped with trivalent and pentavalent oxides

Zirconia doped with 6–8 wt% (3.2–4.2 mol%) yttria (6–8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb–Y and Ta–Y co-doped zirconia samples containing only substitutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6–8YSZ. In these samples, Nb⁵⁺ and Td⁵⁺ are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta⁵⁺ doping than Nb⁵⁺ doping. Finally, Nb⁵⁺/Ta⁵⁺–Y³⁺ doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal+cubic mixtures are again as low as that of the conventional 6–8YSZ.     

A fundamental study of Fe-Cr binary alloy-oxide film interfaces at 288 °C by computational chemistry calculations

A quantum chemical molecular dynamics method was used in order to understand the oxide film degradation mechanism at metal/metal-oxide interfaces. The present study shows that oxygen diffusivity in the metal is significantly higher at a Fe-Cr/Fe₂O₃ interface compared to a Fe-Cr/Cr₂O₃ interface. This indicates that Cr₂O₃ enables protection of the surface for a longer period of time than Fe₂O₃ in a high temperature environment. Applied tensile strain enhances the oxygen mobility towards the metal surface. This process helps to increase the oxidation of the metal surface by forming metal oxygen bonds. Atomic charge analysis reveals that the oxygen atoms are negatively charged and the chromium atoms are more highly positively charged than iron ones. The negatively charged oxygen atoms are able to make covalent bonds with the positive metal atoms. This charge transfer process facilitates the formation of metal-oxygen bonds and weakens the metallic bonds.