High thermal annealing effect on structural and optical properties of ZnO–SiO2 nanocomposite

The effect of annealing temperature on photoluminescence (PL) of ZnO–SiO₂ nanocomposite was investigated. The ZnO–SiO₂ nanocomposite was annealed at different temperatures from 600 °C to 1000 °C with a step of 100 °C. High Resolution Transmission Electron Microscope (HR-TEM) pictures showed ZnO nanoparticles of 5 nm are capped with amorphous SiO₂ matrix. Field Emission Scanning Electron Microscope (FE-SEM) pictures showed that samples exhibit spherical morphology up to 800 °C and dumbbell morphology above 800 °C. The absorption spectrum of ZnO–SiO₂ nanocomposite suffers a blue-shift from 369 nm to 365 nm with increase of temperature from 800 °C to 1000 °C. The PL spectrum of ZnO–SiO₂ nanocomposite exhibited an UV emission positioned at 396 nm. The UV emission intensity increased as the temperature increased from 600 °C to 700 °C and then decreased for samples annealed at and above 800^°C. The XRD results showed that formation of willemite phase starts at 800 °C and pure willemite phase formed at 1000 °C. The decrease of the intensity of 396 nm emission peak at 900 °C and 1000 °C is due to the collapse of the ZnO hexagonal structure. This is due to the dominant diffusion of Zn into SiO₂ at these temperatures. At 1000 °C, an emission peak at 388 nm is observed in addition to UV emission of ZnO at 396 nm and is believed to be originated from the willemite.     

Surfactant free hydrothermally derived ZnO nanowires,nanorods, microrods and their characterization

ZnO nanowires, nanorods and microrods have been prepared by an organic-free hydrothermal process using ZnSO₄ and NaOH/NH₄OH solutions. The powder X-ray diffraction (PXRD) patterns reveal that the ZnO nano/microrods are of hexagonal wurtzite structure. The Fourier transform infrared (FT-IR) spectrum of ZnO powder shows only one significant spectroscopic band at around 417 cm^?1 associated with the characteristic vibrational mode of Zn–O bonding. The thickness 75–300 nm for ZnO nanorods and 0.2–1.8 μm for microrods are identified from SEM/TEM images. UV–visible absorption spectra of ZnO nano/microrods show the blue shift. The UV band and green emission observed in photoluminescence (PL) spectra are due to free exciton emission and singly ionized oxygen vacancy in ZnO. Finally, the mechanism for organic-free hydrothermal synthesis of the ZnO nano/microrods is discussed.     

Structural characterization of Zn1−xCdxO (0≤x≤0.20) microrods grown by spray pyrolysis

Zn_1−xCd_xO (x= 0.00, 0.05, 0.10, 0.15 and 0.20) thin films were obtained by spray pyrolysis and characterized by XRD, SEM, EDAX and optical measurements. The Zn_1−xCd_xO microrods are in the wurtzite crystallographic phase with (0 0 2) preferred orientation. A narrowing of the fundamental band gap from 3.30 to 3.10 eV was observed with the increasing nominal Cd content up to 20 at% due to the direct modulation of the band gap caused by Cd substitution. The undoped ZnO film showed two emission bands in the spectra: one sharp UV luminescence at ∼382 nm and one broad visible emission ranging from 430 to 600 nm. The sharp peak at ∼382 nm is split into two at 376 and 400 nm upon Cd doping at levels of 5 and 10 at%. However this splitting is not observed in the doped ZnO samples containing 15 at% Cd and more. It should also be mentioned that the broad peak at the range of 430–600 nm has almost disappeared in the films containing 5, 10 and 15 at% Cd.     

Structural,optical and transport properties of SxZnO

In this paper, S-doped ZnO (S_xZnO) was prepared using sol-gel method at different S amounts. The structural, optical and transport properties were investigated. The introduction of S atoms into the ZnO network was found to lower the crystallization level which results in reducing the crystallite size up to x=0.3. The doping process is confirmed by the observed peak at ~610 cm⁻¹ in the ATR spectrum related to the Zn-S linking. EDX mapping shows a homogeneous distribution of S atoms on the particles surface. The best compromise between the band gap (Eg=2.96 eV), the charge carriers (N_A=2.139×10²² cm⁻³), the conductivity (σ=5.56×10⁻⁴ Ω⁻¹ m⁻¹) and the mobility (µ=16.26×10⁻¹⁴ m² V⁻¹ s⁻¹) is obtained for x=0.1. The conduction mechanism is assumed by small hopping polaron. The S-doping has impacted positively the photocatalytic activity of ZnO, with particularly high performance for S_0.2ZnO.     

High-performance diketopyrrolopyrrole-based organic field-effect transistors for flexible gas sensors

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm²/V s on a glass substrate and 2.98 ± 0.19 cm²/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μ_FET, V_Th and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH₃ gas sensors. As the concentration of NH₃ increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼10⁷ to ∼10⁹ Ω at V_SD = −5 V and V_GS = −5 V.     

Influence of Sn content on properties of ZnO:SnO2 thin films deposited by ultrasonic spray pyrolysis

The present work is devoted to the preparation of zinc oxide (ZnO): tin oxide (SnO₂) thin films by ultrasonic spray technique. A set of films are deposited using a solution formed with zinc acetate and tin chloride salts mixture with varied weight ratio R=[Sn/(Zn+Sn)]. The ratio R is varied from 0 to 100% in order to investigate the influence of Sn concentration on the physical properties of ZnO:SnO₂ films. The X rays diffraction (XRD) analysis indicated that films are composed of ZnO and SnO₂ distinct phases without any alloys or spinnel phase formations. The average grain size of crystallites varies with the ratio R from 17 to 20 nm for SnO₂ and from 24 to 40 nm for ZnO. The obtained films are highly transparent with a transmission coefficient equal to 80%. An increase in Sn concentration increases both the effective band gap energy from 3.2 to 4.01 eV and the photoluminescence intensity peak assigned defects to SnO₂. The films electrical characterization indicated that films are resistive. Their resistivities vary between 1.2×10² and 3.3×10⁴ (Ω cm). The higher resistivity is measured in film deposited with a ratio R equal to 50%.     

Resistive switching characteristics of ZnO–graphene quantum dots and their use as an active component of an organic memory cell with one diode-one resistor architecture

We investigated the resistive switching characteristics of a polystyrene:ZnO–graphene quantum dots system and its potential application in a one diode-one resistor architecture of an organic memory cell. The log–log I–V plot and the temperature-variable I–V measurements revealed that the switching mechanism in a low-current state is closely related to thermally activated transport. The turn-on process was induced by a space-charge-limited current mechanism resulted from the ZnO–graphene quantum dots acting as charge trap sites, and charge transfer through filamentary path. The memory device with a diode presented a ∼10³ I_ON/I_OFF ratio, stable endurance cycles (10² cycles) and retention times (10⁴ s), and uniform cell-to-cell switching. The one diode-one resistor architecture can effectively reduce cross-talk issue and realize a cross bar array as large as ∼3 kbit in the readout margin estimation. Furthermore, a specific word was encoded using the standard ASCII character code.     

A detailed comparative study on the main electrical parameters of Au/n-Si and Au/PVA:Zn/n-Si Schottky barrier diodes

We have fabricated Au/n-Si and Au/PVA:Zn/n-Si Schottky barrier diodes (SBDs) to investigate the effect of organic interfacial layer on the main electrical characteristics. Zn doped poly(vinyl alcohol) (PVA:Zn) was successfully deposited on n-Si substrate by using the electrospinning system and surface morphology of PVA:Zn was presented by SEM images. The current–voltage (I–V) characteristics of these SBDs have been investigated at room temperature. The experimental results show that interfacial layer enhances the device performance in terms of ideality factor (n), zero-bias barrier height (Φ_B0), series resistance (R_s), and shunt resistance (R_sh) with values of 1.38, 0.75 eV, 97.64 Ω, and 203 MΩ whereas those of Au/n-Si SBD are found as 1.65, 0.62 eV, 164.15 Ω and 0.597 MΩ, respectively. Also, this interfacial layer at metal/semiconductor (M/S) interface leads to a decrease in the magnitude of leakage current and density of interface states (N_ss). The values of N_ss range from 1.36×10¹² at E_c—0.569 eV to 1.35×10¹³ eV^?1 cm^?2 at E_c—0.387 eV for Au/PVA:Zn/n-Si SBD and 3.34×10¹² at E_c—0.560 eV to 1.35×10¹³ eV^?1 cm^?2 at E_c—0.424 eV for Au/n-Si SBD. The analysis of experimental results reveals that the existence of PVA:Zn interfacial layer improves the performance of such devices.     

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.     

Polymer films with multilayer low-E coatings

The paper presents the experimental results on depositing a multilayer low-emissivity (low-E) coating with oxide–metal–oxide structure on polyethylene terephthalate (PET) and polyethylene (PE) films by magnetron sputtering. The TiO₂/ZnO:Ga/Ag/ZnO:Ga/TiO₂ coating on the PET film with high water-resistance and capability to be used outside of sealed double-glazed panes was proposed. The optimal thickness of coating layers was experimentally determined. The coating with the optimal structure has 82% transmittance over the visible spectrum and 91% reflection over the infrared spectrum. The window with a PET film and low-E coating was investigated in terms of heat engineering. It was revealed that heat transfer resistance increased up to 0.73 m² °C W⁻¹ for the windows with a PET film and low-E coating. Heat transfer resistance of the windows without a polymer film was 0.38 m² °C W⁻¹. The water-resistant ZnO:Ga/Ag/ZnO:Ga/SiO₂ coating on a PE film with 77% transmittance and 91–92% reflection in the IR range was proposed to be used as greenhouse covering material. The possibility of using the PE film with a low-E coating to reduce heat loss in greenhouses and enhance yielding capacity was demonstrated.     

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.     

The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350–470 °C under an atmosphere of compressed air or nitrogen (N₂) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350–430 °C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 °C had a preferred (100) orientation while those deposited above 400 °C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N₂ atmosphere in the range of 370–410 °C showed dense surface morphologies and resistivity values of 0.6–1.1 Ω-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N₂ gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 °C under the N₂ atmosphere was n-type with a carrier density of 8.9–9.2×10¹⁶ cm^-3 and mobility of ～70 cm²/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70–80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N₂ compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier–carrier interactions.     

Multiple exciton generation regions in phosphorescent white organic light emitting devices

We demonstrate a white electrophosphorescent organic light emitting device (WOLED) with a three-section emission layer (EML) where excitons are formed in the multiple emission regions. The EML consists of a stepped progression of highest occupied and lowest unoccupied molecular orbital energies of the ambipolar hosts. Analysis shows that (36 ± 6)% of the excitons form in the blue emitting region, while (64 ± 6)% form in the green emitting region at 100 mA/cm². The doping of the red, green and blue phosphors, each in its own host, allows for efficient utilization of excitons formed in these multiple regions. Based on this architecture, the WOLED has an internal quantum efficiency close to unity. The WOLED has total external quantum and power efficiencies of η_ext,t = (26 ± 1)% and η_p,t = (63 ± 3) lm/W at 12 cd/m², decreasing to η_ext,t = (23 ± 1)% and η_p,t = (37 ± 2) lm/W at 500 cd/m². When an undoped electron transport layer is used, the peak efficiency is η_ext,t = (28 ± 1)%. Due to the distributed exciton formation in the EML, the WOLED exhibits higher total efficiency than monochromatic devices employing the same red, green and blue dopant–host combinations.     

Effects of the substrate temperature and solution molarity on the structural opto-electric properties of ZnO thin films deposited by spray pyrolysis

Zinc oxide (ZnO) was largely studied in various applications such as photovoltaic conversion, optoelectronics and piezoelectric, because of its interesting physical properties (morphological, structural, optical and electrical). The present work deals with the preparation of zinc oxide thin films (ZnO) deposited by the spray pyrolysis method. The starting solution was zinc chloride (ZnCl₂). Effects of solution molarity and substrate temperature on films properties were investigated. All films deposited were characterized by various techniques such as X-ray diffraction for structural characterizations, profilometry for thickness measurements, UV–vis transmission spectrophotometry for optical properties and the four probes conductivity measurements for electrical characterization. The X-ray diffraction (XRD) patterns show that the films deposited are polycrystalline with (0 0 2) plan as preferential orientation. The UV–vis spectroscopy confirms the possibility of good transparent ZnO thin films deposition with an average transmission of about ∼85% in the visible region. However, the measured electrical resistivities of the deposited films were in the order of 10⁴ Ω cm     

Dielectric surface-polarity tuning and enhanced operation stability of solution-processed organic field-effect transistors

The electrical performance of triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs) was significantly affected by dielectric surface polarity controlled by grafting hexamethyldisilazane and dimethyl chlorosilane-terminated polystyrene (PS-Si(CH₃)₂Cl) to 300-nm-thick SiO₂ dielectrics. On the untreated and treated SiO₂ dielectrics, solvent–vapor annealed TES-ADT films contained millimeter-sized crystals with low grain boundaries (GBs). The operation and bias stability of OFETs containing similar crystalline structures of TES-ADT could be significantly increased with a decrease in dielectric surface polarity. Among dielectrics with similar capacitances (10.5–11 nF cm⁻²) and surface roughnesses (0.40–0.44 nm), the TES-ADT/PS-grafted dielectric interface contained the fewest trap sites and therefore the OFET produced using it had low-voltage operation and a charge-carrier mobility ∼1.32 cm² V⁻¹ s⁻¹, on–off current ratio >10⁶, threshold voltage ∼0 V, and long-term operation stability under negative bias stress.     

Effect of high-temperature annealing on lanthanum aluminate thin films grown by ALD on Si(1 0 0)

Amorphous lanthanum aluminate thin films were deposited by atomic layer deposition on Si(1 0 0) using La(ⁱPrCp)₃, Al(CH₃)₃ and O₃ species. The effects of post-deposition rapid thermal annealing on the physical and electrical properties of the films were investigated. High-temperature annealing at 900 °C in N₂ atmosphere leads to the formation of amorphous La-aluminosilicate due to Si diffusion from the substrate. The annealed oxide exhibits a uniform composition through the film thickness, a large band gap of 7.0 ± 0.1 eV, and relatively high dielectric constant (κ) of 18 ± 1.     

High electron mobility triazine for lower driving voltage and higher efficiency organic light emitting devices

The triazine compound 4,4′-bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1′-biphenyl (BTB) was developed for use as an electron transport material in organic light emitting devices (OLEDs). The material demonstrates an electron mobility of ∼7.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at a field of 8.00 × 10⁵ V cm⁻¹, which is 10-fold greater than that of the widely used material tris(8-hydroxyquinoline) aluminum (AlQ₃). OLEDs with a BTB electron transport layer showed a ∼1.7–2.5 V lower driving voltage and a significantly increased efficiency, compared to those with AlQ₃. These results suggest that BTB has a strong potential for use as an OLED electron transport layer material.     

Carrier gas and VI/II ratio effects on carbon clusters incorporation into ZnO films grown by MOCVD

Undoped ZnO films were deposited by atmospheric metal-organic chemical vapor deposition (MOCVD) on (0001) ZnO substrate. The films were grown at various partial pressure ratios of oxygen and zinc precursors (VI/II) using either N₂ or H₂ as carrier gas. Micro-Raman scattering was employed to study the effects of carrier gas, VI/II ratio and annealing on carbon impurity incorporation into the ZnO films. Besides the well known phonon modes of ZnO, Raman spectra of the samples grown with N₂ carrier gas show two additional broad peaks, which are ascribed to carbon sp² clusters related modes, spreading in the frequency range 1300–1600 cm^?1 and dominate the Raman spectrum of the sample grown under oxygen deficiency (VI/II=0.25). In addition, a band centered at ～520 cm^?1, considered as some defects related local vibrations, appears in the samples grown with N₂ as carrier gas and its intensity increases when the VI/II ratio decreases. The average cluster size, estimated from the intensity ratio of D over G bands of the carbon sp² clusters, ranges from 16.5 to 19.4 Å. However, in all the samples grown with H₂ as carrier gas, the bands related to carbon sp² clusters and defects, are largely suppressed and the second-order-Raman scattering band (1050–1200 cm^?1) is clearly observed in addition to the bulk ZnO lattice modes. After annealing the samples at 900 °C in oxygen ambient, the crystal quality has been improved for all the samples but the carbon related bands, formed in the as-deposited films grown with the N₂ carrier gas, were only weakened.     

Magnetron-sputtered high performance Y-doped ZnO thin film transistors fabricated at room temperature

In this report, sputtered-grown undoped ZnO and Y-doped ZnO (ZnO:Y) thin film transistors (TFTs) are presented. Both undoped ZnO and ZnO:Y thin films exhibited highly preferred c-axis oriented (002) diffraction peaks. The ZnO:Y thin film crystallinity was improved with an increase of (002) peak intensity and grain size. The electrical properties of ZnO:Y TFTs were significantly enhanced relative to undoped ZnO TFTs. ZnO:Y TFTs exhibited excellent performance with high mobility of 38.79 cm² V⁻¹ s⁻¹, small subthreshold swing of 0.15 V/decade, and high I_on/I_off current ratio of the order of 8.17 × 10⁷. The O_1s X-ray photoelectron spectra (XPS) showed oxygen vacancy-related defects present in the ZnO:Y TFTs, which contributed to enhancing the mobility of the TFTs.     

A high-yield vacuum-evaporation-based R2R-compatible fabrication route for organic electronic circuits

Advances are described in a vacuum-evaporation-based approach for the roll-to-roll (R2R) production of organic thin film transistors (TFTs) and circuits. Results from 90-transistor arrays formed directly onto a plasma-polymerised diacrylate gate dielectric are compared with those formed on polystyrene-buffered diacrylate. The latter approach resulted in stable, reproducible transistors with yields in excess of 90%. The resulting TFTs had low turn-on voltage, on–off ratios ∼10⁶ and mobility ∼1 cm²/V s in the linear regime, as expected for dinaphtho[2,3-b:2′,3′-f] thieno[3,2-b]thiophene the air stable small molecule used as the active semiconductor. We show that when device design is constrained by the generally poor registration ability of R2R processes, parasitic source–drain currents can lead to a >50% increase in the mobility extracted from the resulting TFTs, the increases being especially marked in low channel width devices. Batches of 27 saturated-load inverters were fabricated with 100% yield and their behaviour successfully reproduced using TFT parameters extracted with Silvaco’s UOTFT Model. 5- and 7-stage ring oscillator (RO) outputs ranged from ∼120 Hz to >2 kHz with rail voltages, V_DD, increasing from −15 V to −90 V. From simulations an order of magnitude increase in frequency could be expected by reducing parasitic gate capacitances. During 8 h of continuous operation at V_DD = −60 V, the frequency of a 7-stage RO remained almost constant at ∼1.4 kHz albeit that the output signal amplitude decreased from ∼22 V to ∼10 V. Over the next 30 days of intermittent operation further degradation in performance occurred although an unused RO showed no deterioration over the same period.