Interface states in polyfluorene-based metal–insulator–semiconductor devices

Hybrid metal–insulator–semiconductor structures based on ethyl-hexyl substituted polyfluorene (PF2/6) as the active polymer semiconductor were fabricated on a highly doped p-Si substrate with Al₂O₃ as the insulating oxide layer. We present detailed frequency-dependent capacitance–voltage (C–V) and conductance–voltage characteristics of the semiconductor/insulator interface. PF2/6 undergoes a transition to an ordered crystalline phase upon thermal cycling from its nematic-liquid crystalline phase, confirmed by our atomic force microscope images. Thermal cycling of the PF2/6 films significantly improves the quality of the (PF2/6)/Al₂O₃ interface, which is identified as a reduced hysteresis in the C–V curve and a decreased interface state density (D_it) from ∼3.9 × 10¹² eV⁻¹ cm⁻² to ∼3.3 × 10¹¹ eV⁻¹ cm⁻² at the flat-band voltage. Interface states give rise to energy levels that are confined to the polymer/insulator interface. A conductance loss peak, observed due to the capture and emission of carriers by the interface states, fits very well with a single time constant model from which the D_it values are inferred.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Effect of doping concentration on the properties of bismuth doped tin sulfide thin films prepared by spray pyrolysis

Bismuth doped tin sulfide (SnS:Bi) thin films were deposited onto glass substrates by the spray pyrolysis technique at the substrate temperature of 350 °C. The effect of doping concentration [Bi/Sn] on their structural, optical and electrical properties was investigated as a function of bismuth doping between 0 and 8 at%. The XRD results showed that the films were polycrystalline SnS with orthorhombic structure and the crystallites in the films were oriented along (111) direction. Atomic force microscopy revealed that the particle size and surface roughness of the films increased due to Bi-doping. Optical analysis exhibited the band gap value of 1.40 eV for SnS:Bi (6 at%) which was lower than the band gap value for 0 at% of Bi (1.60 eV). The film has low resistivity of 4.788×10⁻¹ Ω-cm and higher carrier concentration of 3.625×10¹⁸ cm⁻³ was obtained at a doping ratio of 6 at%.     

Mixing Y(RE)BCO (RE=Nd,Sm) superconductors by crystal pulling method

Y_1−xRE_xBa₂Cu₃O_7−δ [Y(RE)BCO or REsBCO] superconductors were prepared by the crystal pulling method. The RE mixing content x in the crystal can be effectively controlled by the processing temperature. With an increase in processing temperature, the RE concentration in the liquid increased, which resulted in the higher RE substitution content in single crystals. The higher critical temperatures T_c above 92 K could be obtained by an optimized oxygenation treatment. The T_c value of Y(RE)BCO tends to be insensitive to the growth atmosphere of the oxygen partial pressure under a certain limit of the RE content. The Y_.727Nd_.273Ba₂Cu₃O_7−δ and the Y_.941Sm_.059Ba₂Cu₃O_7−δ samples show values of the critical current density J_c are about 2×10⁴ Acm⁻² at 1.2 T and 2.8×10⁴ Acm⁻² at 1.1 T for H//c, respectively, indicating that mixing REs 123 have obvious effects on J_c–H curves since RE ions are likely to substitute at both Y²⁺ and Ba²⁺ sites.     

Electronic properties of Si/Si-Ge Alloy/Si(100) heterostructures formed by ECR Ar plasma CVD without substrate heating

By using our low-energy Ar plasma enhanced chemical vapor deposition (CVD) at a substrate temperature below 100 °C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si_0.4Ge_0.6/Si(100) heterostructure. For the strained Si_0.5Ge_0.5 film, Hall mobility at room temperature was confirmed to be as high as 660 cm² V⁻¹ s⁻¹ with a carrier concentration of 1.3×10¹⁸ cm⁻³ for n-type carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for a p⁺Si/nSi_0.5Ge_0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our low-energy Ar plasma enhanced CVD without substrate heating can be applicable effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.     

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnO_x thin films were deposited by reactive magnetron sputtering from a tin target in O₂ containing working gas. The evolution from Sn-containing SnO to tetravalent SnO₂ films was investigated. The films could be classified into three groups according to their optical band gaps, which are E_g<2.5 eV, E_g=3.0–3.3 eV and E_g>3.7 eV. The electric measurements show that high conductivity can be obtained much easier in SnO₂ than in SnO films. A high electron mobility of 15.7 cm² V⁻¹ s⁻¹, a carrier concentration of 1.43×10²⁰ cm⁻³ and a resistivity of 2.8×10⁻³ Ω cm have been achieved in amorphous SnO₂ films. Films with the optical band gap of 3.0–3.3 eV remain amorphous though the substrate temperature is as high as 300 °C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO₂ films.     

Enhancing charge transport in copper phthalocyanine thin film by elevating pressure of deposition chamber

Copper phthalocyanine (CuPc)-based thin film transistors were fabricated using CuPc films grown under different deposition pressure (P_dep) (ranging from 1.8 × 10⁻⁴ Pa to 1.0 × 10⁻¹ Pa). The transistor performance highly depended on P_dep. A field-effect mobility of 2.1 × 10⁻² cm²/(V s) was achieved under 1.0 × 10⁻¹ Pa. Detailed investigations revealed that P_dep modulates the molecular packing and orientation of the organic films grown on a SiO₂/Si substrate and influences the charge transport. Furthermore, from a device physics point of view, contact resistance of the fabricated transistors decreased when P_dep increased, which was beneficial in reducing energy consumption.     

Growth of high quality YbBa2Cu3O7−δ thin films by pulsed laser deposition

YbBa₂Cu₃O_7−δ (Yb-123) films are deposited for the first time using Pulsed Laser Deposition (PLD) method at three different substrate temperatures, viz. 675°C, 700°C and 725°C. Films are characterized using XRD, dc electrical resistivity, critical current density (J_c) and microstructural study by Atomic Force Microscopy (AFM) techniques. It is found that 700°C is the optimum growth temperature for growing high quality Yb-123 films. The best T_c and J_c values obtained at optimum growth conditions are 88 K and 2.6×10⁶ A cm⁻² at 77 K, respectively. AFM photographs provide evidence in confirming the relation between growth temperature and superconducting properties.     

Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory

We have investigated the contact resistivity of GeCu₂Te₃ (GCT) phase change material to a W electrode using the circular transfer length method (CTLM). The contact resistivity ρ_c of as-deposited amorphous GCT to W was 3.9×10⁻² Ω cm². The value of ρ_c drastically decreased upon crystallization and crystalline GCT that annealed at 300 °C showed a ρ_c of 4.8×10⁻⁶ Ω cm². The ρ_c contrast between amorphous (as-deposited) and crystalline (annealed at 300 °C) states was larger in GCT than in conventional Ge₂Sb₂Te₅ (GST). Consequently, it was suggested from a calculation based on a simple vertical structure memory cell model that a GCT memory cell shows a four times larger resistance contrast than a GST memory cell.     

Effect of the film thickness on the crystal structure and ferroelectric properties of (Hf0.5Zr0.5)O2 thin films deposited on various substrates

In this work, the effect of the film thickness on the crystal structure and ferroelectric properties of (Hf_0.5Zr_0.5)O₂ thin films was investigated. The thin films were deposited on (111) Pt-coated SiO₂, Si, and CaF₂ substrates with thermal expansion coefficients of 0.47, 4.5, and 22×10⁻⁶/°C, respectively. From the X-ray diffraction measurements, it was found that the (Hf_0.5Zr_0.5)O₂ thin films deposited on the SiO₂ and CaF₂ substrates experienced in-plane tensile and compressive strains, respectively, in comparison with the films deposited on the Si substrates. For films deposited on all three substrates, the volume fraction of the monoclinic phase increased with increasing film thickness, with the SiO₂ substrate having the lowest monoclinic phase volume fraction at all film thicknesses tested. The grain size of the films, which is an important factor for the formation of the ferroelectric phase, remained almost constant at about 10 nm in diameter regardless of the film thickness and type of substrate utilized. Ferroelectricity was observed for the 17 nm-thick films deposited on SiO₂ and Si substrates, and the maximum remanent polarization (P_r) value of 9.3 µC/cm² was obtained for films deposited on the SiO₂ substrate. In contrast, ferroelectricity with P_r=4.4 µC/cm² was observed only for film on SiO₂ substrate in case of 55 nm-thick films. These results suggest that the films under in-plane tensile strain results in the larger ferroelectricity for 17 nm-thick films and have a ferroelectricity up to 55 nm-thick films.     

Evaporated iron disulfide thin films with sulfurated annealing treatments

FeS₂ thin films were grown on a glass substrate using a physical vapor deposition technique at room temperature. Subsequently, the thin films were annealed in two different atmospheres: vacuum and vacuum-sulfur. In the vacuum-sulfur atmosphere a graphite box was used as sulfur container and the films were sulfurated successfully at 200–350 ºC. It was found that annealing in a vacuum-sulfur atmosphere was indispensable in order to obtain polycrystalline FeS₂ thin films. The polycrystalline nature and pure phase were determined by XRD and Raman techniques and the electrical properties by the Hall effect. Using the sulfurating technique, the n-type semiconductor was prepared at 200–350 °C and a p-type at 500 °C. The carrier concentrations were between 1.19×10²⁰ and 2.1×10²⁰ cm⁻³. The mobility was 9.96–5.25 cm² V⁻¹ s⁻¹ and the resistivity was 6.31×10⁻² to 1.089×10⁻² Ω cm. The results obtained from EDS showed that the films prepared in the vacuum-sulfur atmosphere were close to stoichiometric and that the indirect band gap varied between 1.03 and 0.945 eV.     

Properties of fluorine-doped SnO2 thin films by a green sol–gel method

Fluorine doped tin oxide (FTO) films were fabricated on a glass substrate by a green sol–gel dip-coating process. Non-toxic SnF₂ was used as fluorine source to replace toxic HF or NH₄F. Effect of SnF₂ content, 0–10 mol%, on structure, electrical resistivity, and optical transmittance of the films were investigated using X-ray diffraction, Hall effect measurements, and UV–vis spectra. Structural analysis revealed that the films are polycrystalline with a tetragonal crystal structure. Grain size varies from 43 to 21 nm with increasing fluorine concentration, which in fact critically impacts resultant electrical and optical properties. The 500 °C-annealed FTO film containing 6 mol% SnF₂ shows the lowest electrical resistivity 7.0×10⁻⁴ Ω cm, carrier concentration 1.1×10²¹ cm⁻³, Hall mobility 8.1 cm²V⁻¹ s⁻¹, optical transmittance 90.1% and optical band-gap 3.91 eV. The 6 mol% SnF₂ added film has the highest figure of merit 2.43×10⁻² Ω⁻¹ which is four times higher than that of un-doped FTO films. Because of the promising electrical and optical properties, F-doped thin films prepared by this green process are well-suited for use in all aspects of transparent conducting oxide.     

Planar copper-tin inter-metallic film formation on strained substrates

This paper presents the results of four-point bending tests investigating the effects of substrate strain on the growth ɛ of interfacial Cu–Sn inter-metallic compounds (IMCs). Test specimens were cut into strips, 27.5 mm in length and 5 mm in width, from 4 in. double polished silicon wafers. A very thin adhesion layer (Ta) was deposited on the silicon substrate by sputtering followed by a 10 μm thick layer of copper using electroplating. Finally, a 30 μm tin layer was deposited over the copper film also by electroplating. Samples were then placed in a furnace at 200 °C to undergo bending in order to introduce in-plane strain under tension or compression. Control samples also underwent the same treatment without applied strain. Our aim was to investigate the influence of substrate strain and aging time on the formation of IMCs (1.54 × 10⁻⁴, 2.3 × 10⁻⁴ and 3.46 × 10⁻⁴). The thickness and separation of each phase (Cu₃Sn) and η (Cu₆Sn₅) are clearly visible in scanning electron microscope images. Compressive strain and tensile strain both increased the thickness of the IMC layer during the aging process; however, the effects of compressive strain were more pronounced than those of tensile strain. We hypothesize that the increase in IMC thickness is related to the strain enhanced out-diffusion of Cu towards the solder as well as strain in the underlying lattice at the diffusion interface.     

Characteristics of RF reactive sputter-deposited Pt/SiO2/n-InGaN MOS Schottky diodes

All RF sputtering-deposited Pt/SiO₂/n-type indium gallium nitride (n-InGaN) metal–oxide–semiconductor (MOS) diodes were investigated before and after annealing at 400 °C. By scanning electron microscopy (SEM), the thickness of Pt, SiO_2, n-InGaN layer was measured to be ~250, 70, and 800 nm, respectively. AFM results also show that the grains become a little bigger after annealing, the surface topography of the as-deposited film was smoother with the rms roughness of 1.67 nm and had the slight increase of 1.92 nm for annealed sample. Electrical properties of MOS diodes have been determined by using the current–voltage (I–V) and capacitance–voltage (C–V) measurements. The results showed that Schottky barrier height (SBH) increased slightly to 0.69 eV (I–V) and 0.82 eV (C–V) after annealing at 400 °C for 15 min in N₂ ambient, compared to that of 0.67 eV (I–V) and 0.79 eV (C–V) for the as-deposited sample. There was the considerable improvement in the leakage current, dropped from 6.5×10⁻⁷ A for the as-deposited to 1.4×10⁻⁷ A for the 400 °C-annealed one. The annealed MOS Schottky diode had shown the higher SBH, lower leakage current, smaller ideality factor (n), and denser microstructure. In addition to the SBH, n, and series resistance (R_s) determined by Cheungs׳ and Norde methods, other parameters for MOS diodes tested at room temperature were also calculated by C–V measurement.     

High performance sol–gel spin-coated titanium dioxide dielectric based MOS structures

High-κ TiO₂ thin films have been fabricated using cost effective sol–gel and spin-coating technique on p-Si (100) wafer. Plasma activation process was used for better adhesion between TiO₂ films and Si. The influence of annealing temperature on the structure-electrical properties of titania films were investigated in detail. Both XRD and Raman studies indicate that the anatase phase crystallizes at 400 °C, retaining its structural integrity up to 1000 °C. The thickness of the deposited films did not vary significantly with the annealing temperature, although the refractive index and the RMS roughness enhanced considerably, accompanied by a decrease in porosity. For electrical measurements, the films were integrated in metal-oxide-semiconductor (MOS) structure. The electrical measurements evoke a temperature dependent dielectric constant with low leakage current density. The Capacitance–voltage (C–V) characteristics of the films annealed at 400 °C exhibited a high value of dielectric constant (~34). Further, frequency dependent C–V measurements showed a huge dispersion in accumulation capacitance due to the presence of TiO₂/Si interface states and dielectric polarization, was found to follow power law dependence on frequency (with exponent ‘s’=0.85). A low leakage current density of 3.6×10⁻⁷ A/cm² at 1 V was observed for the films annealed at 600 °C. The results of structure-electrical properties suggest that the deposition of titania by wet chemical method is more attractive and cost-effective for production of high-κ materials compared to other advanced deposition techniques such as sputtering, MBE, MOCVD and ALD. The results also suggest that the high value of dielectric constant ‘κ‘ obtained at low processing temperature expands its scope as a potential dielectric layer in MOS device technology.     

Electrical properties of low-dielectric-constant films prepared by PECVD in O2/CH4/HMDSO

Low-dielectric constant (low-k) films have been prepared by plasma-enhanced chemical vapor deposition (PECVD) from hexamethyldisiloxane (HMDSO) mixed with oxygen or methane. The films are analyzed by ellipsometry, infrared absorption spectroscopy while their electrical properties are deduced from C–V, I–V and R–f measurements performed on Al/insulator/Si structures. For an oxygen and methane fraction equal to 50% and 22%, respectively, the dielectric constant and losses are decreased compared with those of the film prepared in a pure HMDSO plasma. The effect of adding 22% of CH₄ in HMDSO plasma increases the Si–CH₃ bonds containing in the polymer film and as the constant of methyl groups in the film increased the dielectric constant of the film decreases. For this film, the dielectric constant is 2.8, the dielectric losses at 1 kHz are equal to 2×10⁻³, the leakage current density measured for an electric field of 1 MV/cm is 3×10⁻⁹ A/cm² and the breakdown field is close to 5 MV/cm.     

Low-temperature formation of self-assembled Ge quantum dots on Si(100) under high carbon mediation via solid-phase epitaxy

CMOS-compatible low-temperature formation of self-assembled Ge quantum dots (QDs) by carbon (C) mediation via a solid-phase epitaxy (SPE) has been demonstrated. The samples were prepared by a solid-source molecular beam epitaxy (MBE) system. C and Ge were successively deposited on Si(100) at 200 °C and Ge/C/Si heterostructure was annealed in the MBE chamber. Sparse Volmer-Weber mode Ge dots without a wetting layer were formed for C coverage (θ_C) of 0.25 and 0.5 ML by lowering SPE temperature (T_S) to 450 °C, but small and dense Stranski-Krastanov (SK)-mode Ge QDs with the wetting layer were obtained with increasing C coverage of 0.75 ML even at 450 °C. From the investigation of SPE temperature effect on Ge QD formation for θ_C of 0.75 ML, SK-mode Ge QDs of about 10 nm in diameter and of about 4.5×10¹¹ cm⁻² in density were formed at T_S≥400 °C. The wetting layer of SK-mode QDs was almost constant 0.2-nm thick at T_S≥450 °C. Measurements of chemical binding states of C in Ge QDs and at Ge/Si interface revealed that a large amount of C–Ge bonds were formed in the wetting layer for high C coverage, and the formation of C–Ge bonds, together with the formation of C–Si bonds, enabled the low-temperature formation of small and dense Ge QDs. These results suggest that the C-mediated solid-phase epitaxy is effective to form small and dense SK-mode QDs at low temperature.     

In0.75Ga0.25As channel layers with record mobility exceeding 12,000 cm2/Vs for use in high-κ dielectric NMOSFETs

In_0.75Ga_0.25As channel layers with a record mobility exceeding 12,000 cm²/Vs for use in high-κ dielectric NMOSFETs have been fabricated. The device structures which have been grown by molecular beam epitaxy on 3″ semi-insulating InP substrate comprise a 10 nm strained In_0.75Ga_0.25As channel layer and a high-κ oxide based dielectric layer (κ ≅ 20). Electron mobilities of 12,033 and 7,042 cm²/Vs have been measured for sheet carrier concentrations n_s of 2.5 × 10¹² and 6 × 10¹² cm⁻², respectively.     

Direct synthesis of quaternary Cd(Zn,S)Se thin films: Effects of composition

In the present communication, the binary CdSe and quaternary Cd_1-xZn_xSe_1-yS_y (0 ≤ x = y ≤ 0.35) thin films were synthesized using a chemical bath deposition. Thin film deposition was carried out at the optimized conditions (pH = 10 ± 0.1, deposition temperature = 70 ± 0.1 °C, deposition time = 100 min and substrate rotation speed = 65 ± 2 rpm). X-ray diffraction studies confirmed hexagonal-wurtzite crystal structure with the formation of quaternary Cd(Zn, S)Se phase along with binary CdSe, CdS, ZnS and ZnSe, phases of the as-grown Cd_1-xZn_xSe_1-yS_y thin films. Elemental analysis showed presence of Cd²⁺, Zn²⁺, S^2- and Se^2- in the deposited films. Fourier transform infrared spectroscopy shown the bands at 911.15 cm⁻¹ – 901.62 cm⁻¹ which are assigned to the stretching frequency of Cd–Se bond. Scanning electron microscopy show transformation of the microstructure from globular crystallites to a rhomboid flake like network. The electrical conductivity was typically ≈ 10⁻⁷ Ω⁻¹ cm⁻¹. At low temperatures, the conduction was by variable range hopping, and this changed to thermally activated grain boundary dominated conduction for T > 350 K.     

Long lengths of silver-clad Bi2223 superconducting tapes with high current-carrying capacity

Long lengths of silver-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) high-T_c multifilamentary tapes were produced using the powder-in-tube (PIT) technique followed by a thermomechanical process. The relationships between microstructure and electrical, magnetic and mechanical properties of the heat treated tape were evaluated from the critical current density measurements, irreversibility magnetic field determination and mechanical bending tests. Emphasis was stressed on the J_c behavior in magnetic fields at different temperatures. A J_c of 10,000 A/cm² at 77 K in a zero field for a 10 m tape and 75,000 A/cm² at 23 K in a field of 3 T for a short tape was achieved. The results obtained showed that Bi2223/Ag high-T_c composite tapes are a potential alternative to conventional low-T_c superconductors in magnetic levitation (MAGLEV) applications.