Photo-curable epoxy functionalized cyclotetrasiloxane as a gate dielectric for organic thin film transistors

We synthesized a new photo-curable organic/inorganic hybrid material, cyclotetrasiloxane (CTS) derivative containing cyclohexene-1,2-epoxide functional groups (CTS-EPOXY), and its characteristics are compared with a prototypical organic gate insulator of poly(4-vinylphenol) (PVP) in the organic thin film transistors (OTFTs) using pentacene as an active p-type organic semiconductor. Compared with PVP, CTS-EPOXY shows better insulating characteristics and surface smoothness. A metal/insulator/metal (MIM) device with the 300-nm-thick CTS-EPOXY film shows more than two orders of magnitude lower current (less than 40 nA/cm² over the voltage range up to 60 V) compared with PVP. In addition, the pentacene TFT with CTS-EPOXY as a gate dielectric layer shows slightly higher field-effect mobility of μ_FET = 0.20 cm²/V s compared to that with PVP.     

Surface modified polymeric gate insulators for pentacene organic thin-film transistors

This paper demonstrates effects of a surface modification of polymeric gate insulators on a performance of organic thin-film transistor (OTFT). Pentacene OTFTs were fabricated with three types of polymer gate insulators—poly(4-vinylphenol) (PVP, G1) with comparably high dielectric constant, polyimide (PI, G2) with n-octadecyl (C₁₈) side chain, which resulted in hydrophobicity and low dielectric constant, and surface modified PVP(G3). The G3 was prepared by a spin-coating the solution of G2 onto the G1 film. We found that the n-octadecyl group of the G3 protruded from the surface and made the PVP surface more hydrophobic. The less polar surface strongly improved the device performance. Subthreshold slope (s.s.) of the OTFT with G3 as the gate insulator decreased significantly to 2.7 V/dec, which was much smaller than that of OTFTs fabricated with G1 (4.0 V/dec). That is, thin layer with fewer C₁₈ group in contact with pentacene induced a good electrical property like lower s.s. Further the higher dielectric constant of the underlying layer resulted in higher mobility of the device. The mobility (0.50 cm² V⁻¹ s⁻¹) of the OTFT with G3 as the gate insulator showed a higher value compared to that (0.25 cm² V⁻¹ s⁻¹) of the OTFT with G2.     

Mid-infrared photoconductive properties of heavily Bi-doped PbTe p–n homojunction diode grown by liquid-phase epitaxy

We fabricated a heavily Bi-doped (x_Bi ≈ 2 × 10¹⁹ cm⁻³) PbTe p–n homojunction diode that detects mid-infrared wavelengths by the temperature difference method (TDM) under controlled vapor pressure (CVP) liquid phase epitaxy (LPE). The photocurrent density produced by the heavily Bi-doped diode sample is approximately 20 times and 3 times greater than that produced by an undoped and heavily In-doped sample, respectively. By varying the ambient temperature from 15 K to 225 K, the detectable wavelength is tunable from 6.18 μm to 4.20 μm. The peak shift of the detectable wavelength is shorter in the heavily Bi-doped sample than in the undoped sample, consistent with our previously proposed model, in which Bi–Bi nearest donor–acceptor pairs are formed in the heavily Bi-doped PbTe liquid phase epitaxial layer. Current–voltage (I–V) measurements of the heavily Bi-doped diode sample under infrared exposure at 77 K indicated a likely leak in the dark current, arising from the deeper levels. From the dark I–V measurements, the activation energy of the deep level was estimated as 0.067 eV, close to the energy of the deep Tl-doped PbTe acceptor layer. We conclude that the deep level originates from the Tl-doped p-type epitaxial layer.     

Effect of series resistance on the performance of silicon Schottky diode in the presence of tin oxide layer

The current–voltage (I–V) characteristics of Al/SnO₂/p-Si (MIS) Schottky diodes prepared by means of spray deposition method have been measured at 80, 295 and 350 K. In order to interpret the experimentally observed non-ideal Al/SnO₂/p-Si Schottky diode parameters such as, the series resistance R_s, barrier height Φ_B and ideality factor n, a novel calculation method has been reported by taking into account the applied voltage drop across interfacial oxide layer V_i and ideality factor n in the current transport mechanism. The values obtained for V_i were subtracted from the applied voltage values V and then the values of R_s were recalculated. The parameters obtained by accounting for the voltage drop V_i have been compared with those obtained without considering the above voltage drop. It is shown that the values of R_s estimated from Cheung’s method were strongly temperature-dependent and decreased with increasing temperature. It is shown that the voltage drop across the interfacial layer will increase the ideality factor and the voltage dependence of the I–V characteristics. The interface state density N_ss of the diodes has an exponential growth with bias towards the top of the valance band for each temperature; for example, from 2.37 × 10¹³ eV⁻¹ cm⁻² in 0.70−E_v eV to 7.47 × 10¹³ eV⁻¹ cm⁻² in 0.62−E_v eV for 295 K. The mean N_ss estimated from the I–V measurements decreased with increasing the temperature from 8.29 × 10¹³ to 2.20 × 10¹³ eV⁻¹ cm⁻².     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

Thickness dependence of Ic and Jc of LTG-SmBCO coated-conductor on IBAD-MgO tapes

We have reported SmBa₂Cu₃O_y (SmBCO) films on single crystalline substrates prepared by low-temperature growth (LTG) technique. The LTG-SmBCO films showed high critical current densities in magnetic fields compared with conventional SmBCO films prepared by pulsed laser deposition (PLD) method. In this study, to enhance critical current (I_c) in magnetic field, we fabricated thick LTG-SmBCO films on metal substrates with ion-beam assisted deposition (IBAD)-MgO buffer and estimated the I_c and J_c in magnetic fields.All the SmBCO films showed c-axis orientation and cube-on-cube in-plane texture. T_c of the LTG-SmBCO films were 93.1–93.4 K. J_c and I_c of a 0.5 μm-thick SmBCO film were 3.0 MA/cm² and 150 A/cm-width at 77 K in self-field, respectively. Those of a 2.0 μm-thick film were 1.6 MA/cm² and 284 A/cm-width respectively. Although I_c increased with the film thickness increasing up to 2 μm, the I_c tended to be saturated in 300 A/cm-width. From a cross sectional TEM image of the SmBCO film, we recognized a-axis oriented grains and 45° grains and Cu–O precipitates. Because these undesired grains form dead layers, I_c saturated above a certain thickness. We achieved that I_c in magnetic fields of the LTG-SmBCO films with a thickness of 2.0 μm were 88 A/cm-width at 1 T and 28 A/cm-width at 3 T.     

I–V characteristics of ZnO/Cu2O thin film n–i–p heterojunction

ZnO/Cu₂O thin film n–i–p heterojunctions were fabricated by magnetron sputtering. The microstructure, optical, and electrical properties of n-type (n‐) ZnO, insulating (i‐) ZnO, and p-type (p‐) Cu₂O films deposited on glass substrates were characterized by X-Ray diffraction (XRD), spectrophotometer, and the van der Pauw method, respectively. XRD results show that the mean grain size of i-ZnO film is much larger than that of n-ZnO film. The optical band gap energies of n-ZnO, i-ZnO, and p-Cu₂O film are 3.27, 3.47, and 2.00 eV, respectively. The carrier concentration of n-ZnO film is two orders of magnitude larger than that of p-Cu₂O film. The current–voltage (I–V) characteristics of ZnO/Cu₂O thin film n–i–p heterojunctions with different i-ZnO film thicknesses were investigated. Results show that ZnO/Cu₂O n–i–p heterojunctions have well-defined rectifying behavior. All ideality factors of these n–i–p heterojunctions are larger than 2.0. The forward bias threshold voltage and ideality factor increase when i-ZnO layer thickness increases from 100 to 200 nm. An energy band diagram was proposed to analyze the I–V characteristics of these n–i–p heterojunctions.     

Synthesis of TiO2 nanoparticles and spectroscopic upconversion luminescence of Nd3+-doped TiO2–SiO2 composite glass

Nd³⁺-doped TiO₂–SiO₂ composites were prepared by sol–gel method. Optical properties such as radiative life-time (τ), stimulated emission cross-section (σ_p) and branching ratio (β) were calculated using Judd–Ofelt theory. Violet to blue upconversion emissions at 380 nm (⁴D_3/2→⁴I_11/2), 399 nm (²P_3/2→⁴I_11/2), 420 nm (²D_5/2→⁴I_9/2) and 452 nm (²P_3/2→⁴I_13/2) were obtained under 578 nm xenon-lamp excitation. The choice of 578 nm is justified by the absorption spectra of the same samples, which shows a strong absorption peak at 578 nm. This 578 nm excitation pump produces upconversion in Nd³⁺ by a sequential two-photon absorption process.     

Optical and electrical properties of Ge-implanted SiO2 layers on n-Si and p-Si

Ge ions of 100 keV were implanted into a 120 nm-thick SiO₂ layer on n-Si at room temperature while those of 80 keV were into the same SiO₂ layer on p-Si. Samples were, subsequently, annealed at 500°C for 2 h to effectively induce radiative defects in the SiO₂. Maximum intensities of sharp violet photoluminescence (PL) from the SiO₂/n-Si and the SiO₂/p-Si samples were observed when the samples have been implanted with doses of 1×10¹⁶ and 5×10¹⁵ cm⁻², respectively. According to current–voltage (I–V) characteristics, the defect-related samples exhibit large leakage currents with electroluminescence (EL) at only reverse bias region regardless of the type of substrate. Nanocrystal-related samples obtained by an annealing at 1100°C for 4 h show the leakage at both the reverse and the forward region.     

Chemical bath deposition of CdS channel layer for fabrication of low temperature-processed thin-film-transistors

～66 nm thick CdS film with a hexagonal structure was uniformly generated via a low temperature-processed chemical bath deposition at 80 °C using a complexing agent of ethylenediaminetetraacetic acid and its crystal structure, surface morphology, optical transmittance, and Raman scattering property were measured. Grown CdS film was used as a channel layer for the fabrication of bottom-gate, top-contact thin-film-transistor (TFT). The TFT device with 60 °C-dried channel layer exhibited a poor electrical performance of on-to-off drain current ratio (I_on/I_off) of 5.1 × 10³ and saturated channel mobility (μ_sat) of 0.10 cm²/Vs. However, upon annealing at 350 °C, substantially improved electrical characteristics resulted, showing I_on/I_off of 5.9 × 10⁷ and μ_sat of 5.07 cm²/Vs. Furthermore, CdS channel layer was chemically deposited in an identical way on a transparent substrate of SiN_x/ITO/glass as part of transparent TFT fabrication, resulting in I_on/I_off of 5.8 × 10⁷ and μ_sat of 2.50 cm²/Vs.     

Enhanced photosensitivity of InGaZnO-TFT with a CuPc light absorption layer

A copper phthalocyanine (CuPc) organic semiconductor is capped onto an amorphous indium–gallium–zinc-oxide (InGaZnO) thin film transistor (TFT) to enhance the photosensitivity of InGaZnO-TFT. The CuPc organic semiconductor is served as a light absorption layer and forms a p–n junction with the InGaZnO film. After 60 s white light illumination, light responsivity (R) of InGaZnO-TFT with a CuPc light absorption layer reaches a value of 148.5 A/W at a gate-source voltage (V_GS) of 20 V, which is much larger than that (31.2 A/W) of the conventional InGaZnO-TFT. The results are attributed to the following mechanism. First, a CuPc layer is employed as the light absorption layer. Second, CuPc/InGaZnO p–n junction enables the injection of electron into InGaZnO film. Our results indicate that using CuPc as light absorption layer is an effective approach to improve the photosensitivity of InGaZnO-TFT.     

The experimental study of Ba 6pjnk |M| = 0, 1 autoionizing series

The spectra of the Ba 6pnk autoionizing Stark states with |M| = 0, 1, converging to the 6p_1/2⁺ and 6p_3/2⁺ ionization thresholds, are measured as a function of the electric field strength F. Several 6p_jnk Stark manifolds with n = 13–15 have been systematically studied in order to explore their characteristics of configuration interaction. Experimental results are analyzed by fitting them to the Lorentzian profile, from which the positions and widths are determined. Different spectroscopic properties between the Ba 6p_1/2nk and 6p_3/2nk autoionizing Stark states are investigated. Comparison between the Ba 6p_jnk autoionizing Stark states with |M| = 0 and those with |M| = 1 are made.     

Effect of the composition on the superconducting properties of IBAD-MgO SmBCO coated conductors with superconducting film directly deposited on epi-MgO layer

Sm_1+xBa_2?xCu_3+yO_7?δ (SmBCO) films were directly deposited on the epi-MgO/IBAD-MgO/Y₂O₃/Al₂O₃/Hastelloy template by co-evaporation using the evaporation using drum in dual chambers (EDDC) system without the buffer layer in order to investigate the effect of the composition ratios on superconducting property, microstructure and texture of SmBCO film. The films with gradient composition ratios of Sm:Ba:Cu were deposited using a shield with an opening which was placed between the substrate and the boats. The highest I_c of 52 A (corresponding to J_c = 1.6 MA/cm² and a thickness of 800 nm) was observed at 77 K in self field at a composition x = 0.01–0.05 and y = ?0.23 to ?0.46. When the composition ratio is outside this range, the I_c value rapidly decreased. The superconducting critical current was highly dependent on the composition ratio. As the composition ratio is farther away from that of the highest I_c, the SmBCO (1 0 3) peak intensity increased and the amount of a-axis oriented parts increased. A dense microstructure with round-shape grains was observed in the region showing the highest I_c. The optimum composition ratio can be found by analyzing films deposited with variable deposition rates of each depositing element.     

Utilization of surface plasmon resonance of Au/Pt nanoparticles for highly photosensitive ZnO nanorods network based plasmon field effect transistor

Hydrothermally processed highly photosensitive ZnO nanorods based plasmon field effect transistors (PFETs) have been demonstrated utilizing the surface plasmon resonance coupling of Au and Pt nanoparticles at Au/Pt and ZnO interface. A significantly enhanced photocurrent was observed due to the plasmonic effect of the metal nanoparticles (NPs). The Pt coated PFETs showed I_on/I_off ratio more than 3 × 10⁴ under the dark condition, with field-effect mobility of 26 cm² V⁻¹ s⁻¹ and threshold voltage of −2.7 V. Moreover, under the illumination of UV light (λ = 350 nm) the PFET revealed photocurrent gain of 10⁵ under off-state (−5 V) of operation. Additionally, the electrical performance of PFETs was investigated in detail on the basis of charge transfer at metal/ZnO interface. The ZnO nanorods growth temperature was preserved at 110 °C which allowed a low temperature, economical and simple method to develop highly photosensitive ZnO nanorods network based PFETs for large scale production.     

Quick response PZT/P(VDF-TrFE) composite film pyroelectric infrared sensor with patterned polyimide thermal isolation layer

The fabrication method and the pyroelectric response of a single element infrared sensor based lead zirconate titanate (PZT) particles and polyvinylidene fluoride P(VDF-TrFE) copolymer composite thick film is reported in this paper. A special thermal insulation structure, including polyimide (PI) thermal insulation layer and thermal insulation tanks, was used in this device. The thermal insulation tanks were fabricated by laser micro-etching technique. Voltage responsivity (R_V), noise voltage (V_noise), noise equivalent power (NEP), and detectivity (D^*) of the PZT/P(VDF-TrFE) based infrared sensor are 1.2 × 10³ V/W, 1.25 × 10⁻⁶ V Hz^1/2, 1.1 × 10⁻⁹ W and 1.9 × 10⁸ cm Hz^1/2 W⁻¹ at 137.3 Hz modulation frequency, respectively. The thermal time constant of the infrared sensor τ_T was about 15 ms. The results demonstrate that the composite infrared sensor show a high detectivity at high chopper frequency, which is an essential advantage in infrared detectors and some other devices.     

Flow versus permeability weighting in estimating the forward volumetric transfer constant (Ktrans) obtained by DCE-MRI with contrast agents of differing molecular sizes

PurposeTo quantify the differential plasma flow- (F_p-) and permeability surface area product per unit mass of tissue- (PS-) weighting in forward volumetric transfer constant (K^trans) estimates by using a low molecular (Gd-DTPA) versus high molecular (Gadomer) weight contrast agent in dynamic contrast enhanced (DCE) MRI.Materials and methodsDCE MRI was performed using a 7T animal scanner in 14 C57BL/6J mice syngeneic for TRAMP tumors, by administering Gd-DTPA (0.9 kD) in eight mice and Gadomer (35 kD) in the remainder. The acquisition time was 10 min with a sampling rate of one image every 2 s. Pharmacokinetic modeling was performed to obtain K^trans by using Extended Tofts model (ETM). In addition, the adiabatic approximation to the tissue homogeneity (AATH) model was employed to obtain the relative contributions of F_p and PS.ResultsThe K^trans values derived from DCE-MRI with Gd-DTPA showed significant correlations with both PS (r² = 0.64, p = 0.009) and F_p (r² = 0.57, p = 0.016), whereas those with Gadomer were found only significantly correlated with PS (r² = 0.96, p = 0.0003) but not with F_p (r² = 0.34, p = 0.111). A voxel-based analysis showed that K^trans approximated PS (< 30% difference) in 78.3% of perfused tumor volume for Gadomer, but only 37.3% for Gd-DTPA.ConclusionsThe differential contributions of F_p and PS in estimating K^trans values vary with the molecular weight of the contrast agent used. The macromolecular contrast agent resulted in K^trans values that were much less dependent on flow. These findings support the use of macromolecular contrast agents for estimating tumor vessel permeability with DCE-MRI.     

Scanning tunneling spectroscopic evidence of crossover transition in the two-impurity Kondo problem

To explore the crossover transition in the two-impurity Kondo problem, we calculate the differential conductance (dI / dV) corresponding to scanning tunneling spectroscopy (STS) measurements of a magnetic dimer adsorbed on a metal surface covered by a decoupling layer. With the aid of the numerical renormalization group (NRG) technique, we find that the peak structure of the dI / dV spectra near the Fermi level changes gradually as a function of the adatom separation and the coupling between the adatom localized spins and the metal surface conduction band. When the coupling becomes small, the peak disappears and, instead, a dip structure appears near the Fermi level. This dip structure is the manifestation of the strong antiferromagnetic correlation between the localized spins. We conclude that the gradual change of the dI / dV structure from a peak structure to a dip structure is an evidence of the crossover transition in the two-impurity Kondo problem.     

Investigation of enhancement mode HfO2 insulated N-polarity GaN/InN/GaN/In0.9Al0.1N heterostructure MISHEMT for high-frequency applications

In this paper, we examined normally-OFF N-polar InN-channel Metal insulated semiconductor high-electron mobility transistors (MISHEMTs) device with a relaxed In_0.9Al_0.1N buffer layer. In addition, the enhancement-mode operation of the N-polar structure was investigated. The effect of scaling in N-polar MISHEMT, such as the dielectric and the channel thickness, alter the electrical behavior of the device. We have achieved a maximum drain current of 1.17 A/mm, threshold voltage (V_T) =0.728 V, transconductance (g_m) of 2.9 S mm⁻¹, high I_ON/I_OFF current ratio of 3.23×10³, lowest ON-state resistance (R_ON) of 0.41 Ω mm and an intrinsic delay time (τ) of 1.456 Fs along with high-frequency performance with f_t/ f_max of 90 GHz/109 GHz and 180 GHz/260 GHz for T_CH =0.5 nm at V_ds =0.5 V and 1.0 V. The numerically simulated results of highly confined GaN/InN/GaN/In_0.9Al_0.1N heterostructure MISHEMT exhibits outstanding potential as one of the possibility to replace presently used N-polar MISHEMTs for delivering high power density and frequency at RF/power amplifier applications.     

Enhanced performance of bi-layer Nb2O5 coated TiO2 nanoparticles/nanowires composite photoanode in dye-sensitized solar cells

Dye sensitized solar cells (DSSCs) were fabricated based on coumarin NKX-2700 dye sensitized bi-layer photoanode and quasi-solid state electrolyte sandwiched together with cobalt sulfide coated counter electrode. A novel bi-layer photoanode has been prepared using composite mixtures of 90 wt.% TiO₂ nanoparticles + 10 wt.% TiO₂ nanowires (TNPWs) as active layer and Nb₂O₅ is coated on the active layer, which acts as scattering layer. Hafnium oxide (HfO₂) was applied over the TNPWs/Nb₂O₅ photoanode film, as a blocking layer. TiO₂ nanoparticles (TNPs), TiO₂ nanowires (TNWs) and TNPWs/Nb₂O₅ were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sensitizing organic dye coumarin NKX-2700 displayed maximum absorption wavelength (λ_max) at 525 nm, which could be observed from the UV–vis spectrum. DSSC-1 fabricated with composite bi-layer photoanode revealed enhanced photo-current efficiency (PCE) as compared to other DSSCs and illustrated photovoltaic parameters; short-circuit current J_SC = 18 mA/cm², open circuit voltage (V_OC) = 700 mV, fill factor (FF) = 64% and PCE (η) = 8.06%. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSC-1 showed the lowest charge transport resistance (R_tr) and the longest electron lifetime (τ_eff). Therefore, in the present investigation, it could be concluded that the novel bi-layer photoanode with blocking layer increased the short circuit current, electron transport and suppressed the recombination of charge carriers at the photoanode/dye/electrolyte interface in DSSC-1.     

Electrical properties of carbon-nanotube-network transistors in air after gamma irradiation

We experimentally evaluate the electrical properties of carbon nanotube (CNT)-network transistors before and after ⁶⁰Co gamma-ray irradiation up to 50 kGy in an air environment. When the total dose is increased, the degree of the threshold voltage (V_th) shift towards positive gate voltages in the drain current–gate voltage (I_D–V_GS) characteristics decreases for total irradiation doses above 30 kGy, although it is constant below 30 kGy. From our analysis of the I_D–V_GS characteristics along with micro-Raman spectroscopy, the gamma-ray irradiation does not change the structure of the CNT network channel for total doses up to 50 kGy; it instead generates charge traps near the CNT/SiO₂ gate insulator interfaces. These traps are located within the SiO₂ layer and/or the adsorbate on the device surface. The positively charged traps near the CNT/SiO₂ interface contribute less to the V_th shift than the interface dipoles at the CNT/metal electrode interfaces and the segment of the CNT network channel below doses of 30 kGy, while the contribution of the charge traps increases for total doses above 30 kGy. Our findings indicate the possibility of the application of CNT-network transistors as radiation detectors suitable for use in air for radiation doses above 30 kGy.