Space Charge Limited Mechanism Research Articles

Improvement of memristive switching of Pb(Zr0.52Ti0.48)O3/Nb:SrTiO3 heterostructures via La doping

The ferroelectric memory with superior stability and high operating speed has broad development prospects in areas such as neural computing and information storage. An effective strategy to improve the performance of the resistive switching (RS) of the ferroelectric memory is to improve the conductivity of ferroelectric thin films while maintaining their ferroelectric properties, because there is a competition between ferroelectric properties and conductivity in the RS behavior. Herein, we prepared La-doped Pb(Zr0.52Ti0.48)O3 (PLZT) thin films on the Nb:SrTiO3 (NSTO) substrate. By controlling La concentration, a compromise between ferroelectricity and conductivity was achieved. The RS on/off ratio reached 2.6 × 104 doped with 2 mol% La. Moreover, the conduction mechanism changes from the Schottky emission controlling PZT/NSTO interface to the coexistence of Schottky emission and space charge-limited mechanism in the PLZT/NSTO heterostructures. The results reveal that the effective matching of the ferroelectricity and conductivity of ferroelectric thin films is necessary for obtaining high RS on/off ratio, which is helpful to the accurate design of high-performance ferroelectric memory devices.

Dielectric and leakage characteristics of MTO/STO bilayer films deposited by R.F. magnetron sputtering technique for optical and electronic applications

Growth of MgTiO3 (MTO) film on SrTiO3 (STO) coated substrate of three different thicknesses (abbreviations: STMT1, STMT2 and STMT3) by R.F. sputtering technique, and their optical, dielectric and electrical responses have been reported. The XRD analysis revealed polycrystallinity and lattice strain. The STMT bilayers showed MTO thickness dependent granular topography. The estimated optical band gaps are found to be in the range of 3.735–3.727 eV. The dielectric constant showed inverse dependency with MTO thickness, and remained almost invariant with temperature (303–350 K). Impedance analysis established a correlation with the microstructures. The leakage current (J = 10−6 A/cm2 for E = 200 kV/cm) analysis revealed: Schottky conduction and space charge limited mechanism conduction in the lower and higher field regions, respectively. The slope of C vs. V is found to be 259 µF/V. The obtained characteristics of STMT bilayers are potential candidates for large band gap optical and electronic integrated circuit applications.

Probing charge transport in manganite film through switching parameters

We have investigated the bipolar resistive switching of Y0.95Ca0.05MnO3 (YCMO) thin film on Si substrate using pulsed laser deposition. Simulation of Mn L3,2 near-edge X-ray absorption fine structure has been executed by CTM4XAS to corroborate the presence of a mixed-valence state of Mn ions and oxygen vacancies. The charge transport in the film is described by the space charge limited mechanism. Murgatroyd and space charge limited mechanism relations are used to calculate the mobility and other switching parameters at high resistance state. With a decrease in the switching layer (near to positively biased electrode) thickness, better resistive switching was observed. This work indicates that the localized switching thickness and temperature strongly affect the resistive switching of the YCMO film.

Extraction of Switching Parameters for Sr‐Doped YMnO3 Thin Film

Herein, the bipolar resistive switching of Y0.95Sr0.05MnO3 (YSMO) film grown on a Si substrate by pulsed laser deposition is reported. The mixed valent state of Mn ions with the presence of oxygen vacancies is confirmed by near‐edge X–ray absorption fine structure. The temperature‐dependent mobility and other switching parameters are extracted using Murgatroyd expression and a space charge–limited mechanism in the high‐resistance state. The YSMO thin film shows better resistive switching as the switching layer (a layer close to a positively biased electrode) thickness decreases. The bipolar resistive switching of the film suggests a strong dependence on localized switching thickness and temperature.

Reconfigurable horizontal–vertical carrier transport in graphene/HfZrO field-effect transistors

We have fabricated at wafer level field-effect-transistors (FETs) having as channel graphene monolayers transferred on a HfZrO ferroelectric, grown by atomic layer deposition on a doped Si (100) substrate. These FETs display either horizontal or vertical carrier transport behavior, depending on the applied gate polarity. In one polarity, the FETs behave as a graphene FET where the transport is horizontal between two contacts (drain and grounded source) and is modulated by a back-gate. Changing the polarity, the transport is vertical between the drain and the back-gate and, irrespective of the metallic contact type, Ti/Au or Cr/Au, the source–drain bias modulates the height of the potential barrier between HfZrO and the doped Si substrate, the carrier transport being described by a Schottky mechanism at high gate voltages and by a space-charge limited mechanism at low gate voltages. Vertical transport is required by three-dimensional integration technologies to increase the density of transistors on chip.

Electrical, photovoltaic and photosensitivity characteristics of p-ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for photodiode applications

In this work, heterojunctions of p-ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for photovoltaic and photodiode application were grown epitaxially by using molecular beam system. Current-voltage profile of the heterojunction device was studied in dark and under various illumination intensities. The obtained photocurrent is found to depend on the light intensity. The main heterojunction parameters, such as shunt and series resistances, the barrier height and the ideality factor were extracted from the current-voltage profile using diverse methods at ambient temperature. Values of the barrier height and the series resistance were obtained from Cheung's functions. A large value of the series resistance causes the non-ideal characteristics of current–voltage measurements. The study of the current-voltage characteristics of high voltage region suggests a predominant space charge limited mechanism. Moderate values of short circuit current and open circuit voltage were obtained through a light intensity of 140mW/cm2, a current and voltages of 0.336mA and 370mV, respectively. The high photosensitivity and responsivity for the current under illumination condition suggests that the prepared heterojunction device could be employed as a photodiode sensor.

A detailed analysis of current-voltage characteristics of Au/perylene-monoimide/n-Si Schottky barrier diodes over a wide temperature range

The current-voltage characteristics of Au/perylene-monoimide (PMI)/n-Si Schottky device have been investigated at a wide temperature range between 75 and 300 K in detail. The measured current-voltage (I-V) characteristics of the device show a good rectification behavior at all temperatures. The electronic parameters such as the ideality factor and the barrier height are determined from the experimental data using standard current-voltage analysis method and also temperature dependence of these parameters is analyzed. In addition to the standard analysis, using the Cheung and Cheung method, the series resistance and some other electrical properties are calculated for the device, and a good agreement is obtained between relevant diode parameters. It was observed that Au/PMI/n-Si Schottky diodes exhibit space charge limited (SCL) conduction at all temperatures. Therefore, we have analyzed this SCL current mechanism in more detail. From this analysis, several electronic parameters related with the SCL mechanism are determined, and it is found that Poole-Frenkel effect is dominant in reverse bias.

Extraction of poly (3-hexylthiophene) (P3HT) properties from dark current voltage characteristics in a P3HT/n-crystalline-silicon solar cell

The dark current-voltage characteristics of poly (3-hexylthiophene) (P3HT)/n-type crystalline silicon solar cells were analyzed using an electrical equivalent circuit. We found that without illumination transport occurs due to hopping between localized states at the P3HT/silicon interface not only at low voltages, through multitunneling capture emission, but also at medium voltages, through tunneling-enhanced recombination. At high voltages the current is limited by series resistance and space-charge limited mechanisms. At low reverse voltages the current is limited by shunt resistance. From the temperature dependence of the equivalent circuit’s fitting parameters, we were able to estimate some physical parameters of the P3HT layer, namely the electron affinity, the charge carrier concentration and the characteristic temperature of the exponential trap distribution. The extracted P3HT values are in good agreement with previously reported values obtained using different methods but our approach takes into account that the P3HT layer is in a solar cell.

Direct fabrication and IV characterization of sub-surface conductive channels in diamond with MeV ion implantation

In the present work we report about the investigation of the conduction mechanism of sp2 carbon micro-channels in single crystal diamond. The structures are fabricated with a technique which employs a MeV focused ion-beam to damage diamond in conjunction with variable thickness masks. This process changes significantly the structural properties of the target material, because the ion nuclear energy loss induces carbon conversion from sp3 to sp2 state mainly at the end of range of the ions (few micrometers). Furthermore, placing a mask with increasing thickness on the sample it is possible to modulate the channels depth at their endpoints, allowing their electrical connection with the surface. A single-crystal HPHT diamond sample was implanted with 1.8 MeV He+ ions at room temperature, the implantation fluence was set in the range 2.1×1016-6.3×1017 ions cm-2, determining the formation of micro-channels with a graded level of damage extending down to a depth of about 3 μm. After deposition of metallic contacts at the channels’ endpoints, the electrical characterization was performed measuring the I-V curves at variable temperatures in the 80-690 K range. The Variable Range Hopping model was used to fit the experimental data in the ohmic regime, allowing the estimation of characteristic parameters such as the density of localized states at the Fermi level. A value of 5.5×1017 states cm-3 eV-1 was obtained, in satisfactory agreement with values previously reported in literature. The power-law dependence between current and voltage is consistent with the space charge limited mechanism at moderate electric fields.

Electrical properties of dysprosium phthalocyanine films

Forty-four nanometers thick films of alkyl-substituted dysprosium phthalocyanine molecules were prepared on carefully cleaned indium tin oxide (ITO) substrates by the spin casting method. UV–vis absorption spectra and dc conduction characteristics were obtained in order to investigate the structure–conduction property relationship. The conduction is Ohmic at low applied voltages but the space charge limited mechanism is prevalent at relatively high voltages. The conductivities of both as-deposited and heat treated films are found to show directionally anisotropic behaviour. The columnar conductivity was found to be significantly large in the liquid crystalline state.

EFFECT OF MANGANESE DOPING ON THE ELECTRICAL CHARACTERISTICS OF SOL-GEL DERIVED LEAD ZIRCONATE TITANATE THIN FILMS

ABSTRACT Manganese (Mn) doped Pb(Zr,Ti)O3 thin films were prepared by sol-gel technique and their dielectric, ferroelectric and dc leakage current characteristics were studied in detail. There was a decrease in the dielectric constant, phase transition temperature and increase in the leakage current with the increase in the concentrations of the Mn ions. The decrease in the transition temperature is due to the breaking down of translation symmetry of the polarization, thereby establishing the local dipole moments on suppressing the long-range ferroelectric order. The dielectric dispersion was large for the higher concentration of Mn ions in PZT thin films at lower frequencies and is attributed to the space charge accumulation. The loss tangent initially increases and then decreases with the increase in the Mn ions concentrations indicating the disappearance of the conducting boundary layers. The undoped and Mn doped PZT thin films exhibit the hysteresis loops which confirms the ferroelectric behavior. The dc leakage current conduction mechanism obeyed to space charge limited mechanism and the activation energy calculated from the dc leakage current is attributed to shallow traps.

Structural and electrical studies on sol–gel derived spun TiO2 thin films

Titanium dioxide thin films were prepared by spin coating of sol precursor onto microscopic glass slides, silicon and indium tin oxide (ITO) coated glass substrates. Spin speed was varied between 1000 and 6000 rpm. From the morphological analysis, it is found that thin films spun with speed ω⩽4000 rpm assume higher ordered structure than those spun at a speed higher than 4000 rpm. Conduction at low voltages is believed to be variable range hopping at temperatures T<220 K while the ionization of donors situated at Ea = 0.46 eV below the conduction band edge becomes dominant at temperatures higher than 220 K. At high field charge transport is due to trap-controlled space charge limited mechanism. Traps with a density Nt≈1×1022 m−3 are thought to be situated at energy level Et = 0.3 eV below the conduction band and are associated with film nonstoichiometry and interface states.

Effect of excess bismuth on the microstructures and electrical properties of strontium bismuth tantalate (SBT) thin films

Strontium bismuth tantalate (SBT) films with excess Bi contents were prepared on Pt/TiO 2/SiO 2/Si substrates by a metallorganic decomposition technique. Effect of excess Bi contents on the microstructure and electrical properties were investigated. A predominant layered perovskite structure could be formed when an excess Bi less than 30% was added. For films above 30% excess of Bi, secondary phases occurred. The remnant polarization and dielectric constant decreased with excess Bi content. This was attributed to a smaller grain size and the presence of secondary phases. The leakage current characteristics were also examined. Space charge limited mechanism was observed in SBT films. In summary, 10% excess Bi was found to be the optimum composition with respect to grain size, morphology, and electrical properties.

Dependence of the mobility and trap concentration in evaporated copper phthalocyanine thin films on background pressure and evaporation rate

Current density-temperature measurements performed at constant applied voltage have been made on evaporated copper phthalocyanine thin films, with chamber pressure to deposition rate ratio P/R varying in the range 2.6*104-1.3*109 Pa m-1s. In common with previous measurements these results were interpreted in terms of a space-charge limited mechanism dominated by an exponential distribution of trapping levels. There was a systematic decrease in mobility from 10-6 to 10-8 m2 V-1 s-1 and an increase in trapping concentration from 5*1022 to 6*1024 m-3 as P/R was increased in the stated range, but the temperature parameter characterising the trapping distribution remained constant at approximately 750K. The variations in mobility and in trapping concentration were ascribed to the scattering and trapping effects of incorporated ambient gas molecules, thus indicating that these parameters are particularly sensitive to deposition conditions.