Growth related properties of pentacene thin film transistors with different gate dielectrics

We report a study on the influence of dielectric surface properties on the growth, the morphology and ordering of pentacene layers and associated consequences on the performance of organic field effect transistors (OFETs). This work mainly aims at disentangling the respective influences of surface roughness and surface energy on pentacene growth. A range of inorganic high-k oxides as well as polymer dielectrics were compared: Ta₂O₅ deposited by e-beam evaporation or grown by anodic oxidation, polymethylmethacrylate (PMMA) single layer or PMMA/Ta₂O₅ bi-layer. Some complementary results on anodic HfO₂ were also added. Atomic force microscopy, X-ray diffraction and infrared absorption evidenced that hydrophobicity and surface roughness drastically influence pentacene growth mechanisms. Transistors realized with the different dielectrics show characteristics well correlated to pentacene structural properties. In particularly, we point out the relationship between the grain size and the field effect mobility with the surface energy of the dielectric substrate. The general trend is that the bigger the grains, the higher the mobility but that best electrical performances of OFETs are obtained with a dielectric surface energy close to that of pentacene. This work also bears out our former results on the benefit of a polymer/high-k oxide bilayer dielectric configuration to improve the field effect mobility while keeping a low operating voltage.     

Beyond SiO2 technology: Simulation of the impact of high-κ dielectrics on mobility

A critical challenge for the microelectronics industry is the need for higher permittivity dielectrics to replace silicon dioxide. A number of different high-κ materials have been proposed and analyzed as SiO₂ replacements in the next generation of MOSFETs. High-κ materials allow the use of a thicker gate dielectric, maintaining the gate capacitance with reduced gate leakage. However they all lead to mobility degradation due to, among other factors, the coupling of carriers to surface soft-optical (SO) phonons. A severe mobility degradation in the presence of high-κ becomes evident when comparing the vertical field dependence of mobility for a wide range of high-κ materials against SiO₂. As oxides containing Hf presently appears to be the leading high-κ contenders, we have performed a detailed analysis of Hf-based gate stacks, exploiting alternative structures and compositions. The introduction of a SiO₂ interfacial layer between the channel and the HfO₂ reduces the detrimental mobility degradation resulting from the mobility SO phonon scattering, but increases the equivalent oxide thickness (EOT) of the gate dielectric. A possible material of choice for the first commercial introduction of high-κ gate stacks is hafnium silicate (Si_xHf_1−xO₂), being thermally stable and offering a good compromise between small EOT and large electron mobility.     

Inkjet Printed Poly(3-hexylthiophene) Thin-Film Transistors: Effect of Self-Assembled Monolayer

Organic thin film transistors (OTFTs) with bottom gate and top contact structure had been prepared by inkjet printing. It is found that the surface properties of the substrates have a great influence on the morphology of the inkjet printed droplet and film. An appropriate surface was vital to form a uniform semiconducting film by inkjet printing and also strongly improved the electric characteristics. When a bare SiO₂ layer was applied, the best field-effect mobility of inkjet printed OTFT devices was only 2.37 × 10^?3 cm²V^?1s^?1, with an on/off current ratio of 10². When the PETS treatment or the PTS treatment was applied on the SiO₂ dielectric layer, the field-effect performances were substantially improved and the best field-effect mobility was enhanced to 8.07 × 10^?3 cm^?2V^?1s^?1 and 7.95 × 10^?3 cm^?2V^?1s^?1, respectively and with an on/off current ratio of 10³.     

Identification of spatial localization and energetic position of electrically active defects in amorphous high-k dielectrics for advanced devices

In this work, we report the use of the conductance transient technique (GTT) to evaluate disordered-induced gap states (DIGS) in gate dielectrics of metal-insulator–semiconductor (MIS) structures. These states are electrically active defects inside the dielectric bulk which are preferentially located at regions near the dielectric/semiconductor interface. Conductance transients occur when the MIS structure is driven from deep to weak inversion, at various frequencies and temperatures, allowing us to obtain contour line maps of defects spatially and energetically distributed inside the dielectric. This method has been applied to evaluate DIGS densities in advanced high-k gate dielectrics, such as HfO₂, Al₂O₃, TiO₂, silicates and other mixtures grown on silicon substrates by atomic layer deposition under different process conditions. Commonly, high DIGS densities involve low interface state densities D_it and vice versa, indicating that there is some kind of interaction or evolution between these two types of defects or traps. An explanation for the dynamics dictating the transformation of interface states to DIGS states is a key point in determining the quality of the dielectric films.     

Synthesis and characterization of hafnium oxide and hafnium aluminate ultra-thin films by a sol–gel spin coating process for microelectronic applications

Currently there are intense industry-wide efforts in searching for new high dielectric constant (high-k) materials for use in future generations of ultra-large scale integrated circuits (ULSI). There are number of requirements for the new high-k materials, such as high dielectric constant, thermal stability (400 °C or higher), high mechanical strength, and good adhesion to neighboring layers. Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological significance. Spinels exhibit a wide range of electronic and magnetic properties in particular nickel, hafnium, cobalt, containing spinels. In the present investigation, crack free, dense polycrystalline monoclinic structure of pure HfO₂, and Al₂HfO₅ ultra-thin films have been prepared by a simple and cost effective sol–gel spin coating method. The formation of the monoclinic HfO₂ phase at 600 °C and complete formation of the single phase Al₂HfO₅ at 800 °C has been reported. The composition of the annealed films has been measured and found to be 70 at.% of O, 30 at.% of Hf for HfO₂ and 22 at.% of Al, 12 at.% of Hf and 66 at.% of O for Al₂HfO₅ films, which are close to the stoichiometry of the HfO₂ and Al₂HfO₅ thin films.     

Impact of high-permittivity dielectrics on speed performances and power consumption in double-gate-based CMOS circuits

The performances of double-gate (DG)-based CMOS circuits with high-κ dielectrics are analyzed in terms of inverter delay and static power consumption. We show that the use of a high-κ layer as gate dielectric degrades the short-channel immunity of DG devices and increases the power consumption, but for a gate dielectric relative permittivity κ lower than 50, the circuit performances still fill the ITRS requirements. Moreover, the use of a double gate dielectric layer (thin SiO₂ oxide and high-κ layer) not only does not degrade the circuit performances, but even ameliorates the inverter speed. Finally, the analysis of back gate misalignment in DG circuits with double gate dielectric layer illustrates that the variation of the inverter performances induced by the back gate misalignment in these high-κ-based devices is comparable with that of the conventional (SiO₂ oxide layer) structure.     

Fabrication of two-dimensional MoS2 thin-film transistors using a reactive thermal evaporation method combined with an annealing step

ABSTRACT

Two-dimensional (2-D) MoS₂ films were fabricated by reactive thermal evaporation combined with thermal annealing. The 2-D nature of the MoS₂ films is demonstrated by observation of direct transition and a Van Hove singularity in the absorbance curve. The 1T phase MoS₂ is confirmed by X-ray photoelectron spectroscopy. MoS₂ thin-film transistors (TFTs) are fabricated using the MoS₂ active layer transferred onto an oxidized Si wafer from a sapphire wafer. The MoS₂ TFT demonstrates a threshold voltage of 37.6 V, a field-effect mobility of 6.94 cm²V^?1s^?1 a sub-threshold swing of 29.2 V/dec and a switching ratio of 10³.     

Study of the surface state density and potential in MIS diode Schottky using the surface photovoltage method

ABSTRACT

The effects of surface preparation and illumination on electric parameters of Au/GaN/GaAs Schottky diode were investigated. The thin GaN film is realized by nitridation of GaAs substrates with different thicknesses of GaN layers (0.7 – 2.2 nm). In order to study the electrical characteristics under illumination, we use an He-Ne laser of 632 nm wavelength. The I(V) current- voltage, the surface photovltage SPV measurement were plotted and analysed taking into consideration the influence of charge exchange between a continuum of the surface states and the semiconductor. The barrier height Ф_Bn, the serial resistance R_s and the ideality factor n are respectively equal to 0.66 eV, 1980 Ω, 2.75 under dark and to 0.65 eV, 1160 Ω, 2.74 under illumination for simple 1 (GaN theckness of 0.7 nm). The interface states density N_ss in the gap and the excess of concentration δn are determined by fitting the experimental curves of the surface photovltage SPV with the theoretical ones and are equal to 4.5×10¹² eV^?1 cm^?2, 5×10⁷ cm^?3, respectively, for sample 1 and 3.5×10¹² eV^?1 cm^?2, 7×10⁸ cm^?3 for sample 2 (GaN theckness of 2 nm). The results confirm that the surface photovoltage is an efficient method for optical and electrical characterizations.     

Growth and characterization of zirconium oxide thin films on silicon substrate

The growth and characterization of zirconium oxide (ZrO₂) thin films prepared by thermal oxidation of a deposited Zr metal layer on SiO₂/Si were investigated. Uniform ZrO₂ thin film with smooth surface morphology was obtained. The thermal ZrO₂ films showed a polycrystalline structure. The dielectric constant of the ZrO₂ film has been shown to be 23, and the equivalent oxide thickness (EOT) of the ZrO₂ stacked oxide is in the range of 3.38–5.43 nm. MOS capacitors with ZrO₂ dielectric stack show extremely low leakage current density, less than 10^?6 A/cm² at ?4 V. Consequently, using this method, high-quality ZrO₂ films could be fabricated at oxidation temperature as low as 600 °C.     

Effect of hydrogen peroxide on hydrofluoric acid etching of high-k materials: ESR investigations

Oxides and silicates of zirconium and hafnium are being actively considered for use as gate dielectrics in MOS devices. Because of their higher dielectric constant (k ∼ 16-22), they permit the use of thicker layers without sacrificing the capacitance value. Wet chemical etching is the method of choice for patterning these oxides. The etching process has to be selective to zirconium/hafnium oxides over silicon dioxide, which may be present in other areas. In this paper, work done on HF etching of ZrO₂ and HfO₂ in presence of hydrogen peroxide is presented and discussed. It was found that addition of hydrogen peroxide to HF solutions lower the etch rate of ZrO₂ and HfO₂ films. Electron spin resonance (ESR) spectroscopy was used to probe the reasons for the decrease in etch rates in this solution.     

Crystal structure of the tetranickel complex with Ipropanethiolate

The title complex [Ni₄(SPrⁱ)₈] was confirmed by single crystal X-ray crystal diffraction analysis. The crystals are monoclinic, space group P2/n with a = 12.760(3), b = 10.059(2), c = 14.484(3) Å, α = 90, β = 93.70(3), γ = 90°, V = 1855.3(6) ų, Z = 4, F(000) = 880, D_c = 1.496 g/cm³, μ = 2.463 mm^?1, the final R = 0.0352 and wR = 0.0580. A total of 18,588 reflections were collected, of which 4404 were independent (R_int = 0.0631). In the crystal packing diagram, intermolecular C—H···Ni hydrogen bonds stabilize the solid state of the title complex.     

Vibrational spectra of vapor-deposited binary phosphosilicate glasses

Room temperature infrared transmission spectra in the range 4000-250 cm^?1 of binary phosphosilicate glass (PSG) films deposited by reacting argon- or nitrogen-diluted PH₃SiH₄O₂ mixtures on heated silicon substrates at 300–400° C have been obtained across the whole composition range. In all the as-deposited binary films, an absorption at ≈1300 cm^?1 characteristics of the P=O vibration was found to persist, together with a couple of broad absorptions in the regions 1200-900 cm^?1 and 500 cm^?1. Using a differential infrared technique the broad feature in the higher frequency region has been resolved into two well-defined bands at ≈1100 and 970 cm^?1. A detailed analysis shows that the intensity variation of the differential band at ≈1100 cm^?1 conforms well, at least to 50 mol% P₂O₅, to a simple structural model that yields an analytic distribution of POSi linkages as a function of composition by assuming chemical mixing in the vapor-deposited P₂O₅SiO₂ system. Furthermore, the system may be written as (P=O)₂ O₃SiO₂ in order to emphasize the similarity of its coordination scheme with that of the B₂O₃SiO₂ system studied earlier. The nature of these CVD films has also been elucidated by thermal and water treatments.     

Structural analysis of 1,1′-bis(diphenylphosphino)ferrocene dioxide

The title compound 1,1′-bis(diphenylphosphino)ferrocene dioxide has been analyzed by X-ray crystal diffraction analysis. The crystals are monoclinic, space group P2(1)/c with a = 22.461(5), b = 10.515(2), c = 12.024(2) Å, α = 90, β = 96.28(3), γ = 90°, V = 2822.7(10) ų, Z = 4, F(000) = 1256, D_c = 1.422 g/cm³, μ = 0.683 mm^?1, the final R = 0.0514 and wR = 0.1369. A total of 22218 reflections were collected, of which 4957 were independent (R_int = 0.0422). In the crystal packing diagram, intermolecular O?H···O hydrogen bonds between the P=O and H₂O stabilize the solid state of the title compound.     

X-ray crystal structural analysis of dicyclohexylthiocarbamide

The title compound dicyclohexylthiocarbamide has been determined by single crystal X-ray crystal diffraction analysis. The crystals are monoclinic, space group P2(1)/c with a = 12.5908(9), b = 11.2158(9), c = 10.4255(8) Å, α = 90, β = 110.7360(10), γ = 90°, V = 1376.88(18) ų, Z = 4, F(000) = 528, D_c = 1.160 g/cm³, μ = 0.214 mm^?1, the final R = 0.0381 and wR = 0.1030. A total of 6836 reflections were collected, of which 2423 were independent (R_int = 0.0154). In the crystal packing diagram, intermolecular N?H···S hydrogen bonds stabilize the solid state of the title compound.     

Selection of post-growth treatment parameters for atomic layer deposition of structurally disordered TiO2 thin films

Routes to atomic layer-deposited TiO₂ films with decreased leakage have been studied by using electrical characterization techniques. The combination of post-deposition annealing parameters, time and temperature, which provides measurable aluminum–titanium oxide–silicon structures – i.e., having capacitance–voltage curves which show accumulation behavior – are 625 °C, 10 min for p-type substrates, and 550 °C, 10 min for n-type substrates. The best annealing conditions for p-type substrates are 625 °C with the length extended to 30 min, which produces an interfacial state density of about 5–6 × 10¹¹ cm^?2 eV^?1, and disordered-induced gap state density below our experimental limits. We have also proved that a post-deposition annealing must be applied to TiO₂/HfO₂ and HfO₂/TiO₂/HfO₂ stacked structures to obtain adequate measurability conditions.     

Improvement charge injection in P-type polymer field-effect transistors with low-cost molybdenum electrodes through dodecanoic acid self assembled monolayer

Most of the merit of organic thin film transistors (OTFTs) is that they can be manufactured using cost effective processes. However, expensive gold (Au) electrodes have usually been used as a source/drain (S/D), due to their benign energy level matching, high air stability, and easy patternability. In this article, we report a simple method for improving the charge injection from a low cost molybdenum (Mo) electrode to organic semiconductors in OTFTs by incorporating a dodecanoic acid (DA)–based self-assembled monolayer (SAM). The OTFT performance is remarkably improved when compared to the devices with a pristine Au electrode. The hole carrier mobilities (hole μ_FET) were ～0.13 (rr-P3HT), ～0.55 (PC12TV12T) and ～0.72 (P2100) cm²/V·sec.     

Exploration on nano-composite fumed silica-based composite polymer electrolytes with doping of ionic liquid

Fumed silica (SiO₂)-based composite polymer electrolytes were prepared by means of solution casing technique. Horizontal attenuated total reflectance-Fourier Transform Infrared (HATR-FTIR) study shows the complexation between polymer matrix and SiO₂. The highest ionic conductivity of (4.11 ± 0.01) × 10^? 3 Scm^? 1 is achieved upon inclusion of 8 wt.% of SiO₂. Three different regions have been observed in the frequency dependence–ionic conductivity study. The conductivity rises sharply with frequency at low frequency regime. It is followed by a frequency independent plateau region and sharp decrease in the conductivity at high frequency range. The dielectric permittivity (ε^') and dielectric loss (ε^") are decreased with increasing the frequency. This phenomenon is mainly attributed to the electrode polarization effect. The formation of electrical double layer has been proven in these dielectric permittivity studies. This indicates the non-Debye properties of the nano-composite polymer electrolytes.     

Synthesis of the Copolymer Based on Diketopyrrolopyrrole with Didecyl Chain for OPVs

A new accepter unit, diphenylpyrrolo[3,2-b]pyrrole-2,5-dione with didecyl chain, was prepared and utilized for the synthesis of the conjugated polymer containing electron donor-acceptor pair for OPVs. The iDPP, part of the structure of a natural dye found in lichens, is the regioisomer of the known DPP with switched position of the carbonyl group and nitrogen atom. At the 4-positions of the N-substituted phenyl groups of 1,4-bis(4-butylphenyl)-pyrrolo[3,2-b]-pyrrole-2,5-dione unit in P-butyl, the butyl group was substituted with decyl group to increase solubility. The absorption spectrum of polymer with diphenylpyrrolo[3,2-b]pyrrole-2,5-dione unit exhibit two maximum peaks at about 365 and 542 nm. The spectrum of the P1 as the solid thin film shows absorption band with maximum peaks at 370 and 536 nm, and the absorption onset at 703 nm, corresponding to band gap of 1.76 eV. The oxidation and reduction potential onset of the synthesized polymer were estimated to be 0.84 and ?1.22 V, which correspond to HOMO and LUMO energy levels of ?5.64 and ?3.58 eV, respectively. The devices comprising P1 with PC₆₁BM annealed at 100°C showed a V_OC of 0.79 V, a J_SC of 1.75 mA/cm², and a FF of 0.31, leading to the power conversion efficiency of 0.43% under white light illumination (AM 1.5 G, 100 mW/cm²).     

Relationship between structure and optical properties in rare earth-doped hafnium and silicon oxides: Modeling and spectroscopic measurements

The SiO₂–HfO₂ binary system is recognized as a promising candidate for Erbium-doped waveguides amplifiers fabrication. Recently, it was demonstrated that Er³⁺-activated 70SiO₂–30HfO₂ planar waveguides with valuable optical and structural properties can be prepared by sol–gel technique with dip-coating processing. The important role played by hafnium in the silica network was evidenced by the particular spectroscopic properties presented by Er³⁺-ions in the silica–hafnia planar waveguides. In this work we present preliminary results on HfO₂–SiO₂ bulk xerogels doped with Eu³⁺ ions, with the aim to go inside the role of hafnium on the rare earth ions local environment. Spectroscopic measurements of the Eu³⁺ photoluminescence emission are given. Numerical simulations by the molecular dynamics method have been performed showing clearly a phase separation for the HfO₂ richer samples. Moreover it is found than the rare earth-doping ions stay preferentially in hafnium rich domains, thus explaining why the rare earth spectroscopic properties are strongly modified.     

Oxygen-conducting crystals of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>: Growth and main properties

Single crystals of the anionic conductor La₂Mo₂O₉ are grown by crystallization from a nonstoichiometric melt. Their polymorphism and domain structure, as well as the temperature dependences of conductivity and dielectric permittivity, are studied. In the temperature range 750–600°C, the conductivity of these crystals is as high as 10^?1–10^?2 Ω^?1 cm^?1.