Square Root Of Electric Field Research Articles

On the possible nature of deep centers in Ga2O3

The electric field dependence of emission rate of the deep traps with level near Ec−0.6 eV, so-called E1 traps, was studied by means of deep level transient spectroscopy measurements over a wide range of applied voltages. The traps were initially introduced by 900 °C ampoule annealing in molecular hydrogen. The results indicate the activation energy of the centers and the ratio of high-field to low-field electron emission rates at a fixed temperature scale as the square root of electric field, suggesting that the centers behave as deep donors. The possible microscopic nature of the centers in view of recent theoretical calculations is discussed. The most likely candidates for the E1 centers are SiGa1–H or SnGa2–H complexes.

Internal Photoemission Spectroscopy Measurements of the Energy Barrier Heights between ALD Dielectrics and Ta-Based Amorphous Metals

High speed metal/insulator/metal (MIM) diodes show promise for rectenna based energy harvesting of IR radiation, for IR sensing, and as building blocks for beyond CMOS hot-electron (MIMIM) transistors. Operation of these devices is ideally based on Fowler-Nordheim tunneling, which is exponentially dependent both on electric field and the height of the energy barriers between the metal electrodes and the insulator. Smooth interfaces are thus necessary to control electric fields. The amorphous metal ZrCuAlNi (ZCAN) has been shown to function well as an ultra-smooth bottom electrode for MIM devices,1,2 but suffers from an interfacial oxide layer and thermal instability above ~300 °C. Recently, Ta based amorphous metals such as TaWSi and TaNiSi have been shown to have larger work functions and stability approaching 900 °C and above.3 Precise knowledge of metal/insulator barrier heights is critical for predicting, understanding, and optimizing MIM diode device operation. Although the Schottky-Mott model may be used to predict ideal barrier heights (φBn = ΦM - χi) from the metal vacuum work function (ΦM) and insulator electron affinity (χi), actual barrier heights depend strongly on deposition method and interfacial properties and often deviate substantially. Internal photoemission (IPE) spectroscopy may be used to directly measure barrier heights at buried interfaces. To date, there have been few reports of IPE measurements of MIM structures and none on these amorphous metals. In this work, IPE is utilized to measure the metal/insulator barrier heights in MIM stacks with Ta-based amorphous metal bottom electrodes with TaN as a reference. Results are compared with current-voltage measurements. TaWSi and TaNiSi bottom electrodes were sputtered onto 100 nm of thermally grown SiO2 on silicon followed by deposition of various high-k insulators via atomic layer deposition. Al and Au dots were evaporated through a shadow mask to serve as semitransparent (~10 nm thick) top gates. For IPE, current was monitored under a fixed applied field while photon energy was swept from 2 to 5 eV. A range of negative and positive applied fields were used to characterize both bottom and top interfaces. Barrier heights will be reported for all electrode/insulator combinations. Schottky plots of bias dependent barrier heights vs. square root of electric field for Al2O3 are shown in Fig. 1 for (a) positive bias (Au interface) and (b) negative bias (TaXN interface). A number of conclusions may be drawn from the results of this measurement array. First, metal/Al2O3 barrier heights are qualitatively consistent with the expected TaXN metal work function as determined as a weighted average of the constituent metals. Second, Au/insulator barrier heights are not affected by the bottom electrode. Third, the asymmetry in the current-voltage response (I(-V)/I(V)) is qualitatively consistent with the IPE determined barrier heights whereas ideal theory predictions are not always. TaWSi and TaNiSi electrodes showed consistently higher barrier heights than ZCAN electrodes and comparable performance to high-quality TaN, indicating promise as a thermally stable bottom electrode for MIM tunnel diodes. Support from NSF Center for Sustainable Materials Chemistry, CHE-1606982. 1 N. Alimardani, S.W. King, B.L. French, C. Tan, B.P. Lampert, and J.F. Conley Jr., J. Appl. Phys. 116, 24508 (2014). 2 N. Alimardani, E.W. Cowell, J.F. Wager, J.F. Conley Jr., D.R. Evans, M. Chin, S.J. Kilpatrick, and M. Dubey, J. Vac. Sci. Technol. A 30, 01A113 (2012). 3 J.M. McGlone, K.R. Olsen, W.F. Stickle, J.E. Abbott, R.A. Pugliese, G.S. Long, D.A. Keszler, and J.F. Wager, MRS Commun. 7, 715 (2017). Figure 1

Comparative study of bulk and interface transport in disordered fullerene films

The characterization of the charge carrier transport in disordered fullerene films, grown by physical vapor deposition, is important for organic electronics in order to improve carrier mobility and understand transport processes. In this contribution, the electron mobility in the bulk of the fullerene film and at the interface with dielectrics are compared. The bulk mobility is measured in diode structures using the Charge Extraction by Linearly Increasing Voltage (CELIV) technique, which allows a simultaneous study of the electric field, concentration and temperature dependence. The interface mobility is determined using organic field effect transistor (OFET) geometry. The electron mobility values are lower and the dependence on carrier density, field and temperature is stronger in diodes compared to OFETs. In both structures different temperature dependence of the mobility on the carrier concentration and on the electric field is obtained. The dependence shows Meyer–Neldel rule (MN-rule) behavior with similar MN temperatures and MN energies. Activation energy for electron transport plotted as a function of the square root of electric field is linear (Gill's law behavior), in accordance with Poole–Frenkel-type charge carrier transport.

Field-induced crossover from phonon to field assisted hopping conductivity in organic materials

Recently the change in temperature dependence of conductivity observed in polythiophene field-effect transistors has been attributed to field-induced metal-insulator transition. Under a high source-drain voltage, the conductivity becomes independent from temperature and depends exponentially on the inverse of the square root of electric field. In this paper we present a theoretical interpretation of this behavior in the framework of hopping mechanism. The ingredient of this model, is the energy gained from the electrical field is always larger than the energy difference between the localized states involved into the charge transitions, which induces a crossover from phonon-assisted hopping to field-assisted hopping.

Effect of dipoles on carrier drift and diffusion of molecularly doped polymers

Drift mobilities and diffusion coefficients of molecularly doped polymers, with dipole moment as a parameter, have been measured by fitting a theoretical equation to time-of-flight photocurrent transients. The slope of the logarithm of mobility vs the square root of electric field is proportional to the dipole moment squared. A similar relationship is observed for diffusion coefficients. We analyze the results using disorder formalism and estimate the dipolar contribution ${\ensuremath{\sigma}}_{d}$ to the width of the density of states. The constant of proportionality in the relation between ${\ensuremath{\sigma}}_{d}$ and the dipole moment squared coincides with the theoretical value given by the dipole trap model. This result provides an important demonstration of the validity of both the disorder formalism and the dipole trap model.

Drift mobility of holes in vacuum-deposited films of zinc tetraphenylporphyrin

Drift mobilities (μ) of holes in zinc tetraphenylporphyrin (ZnTPP) films were measured using the time-of-flight method. The plot of log μ vs. a square root of electric field ( E 1 2 ) fit a straight line and its extrapolation to E = 0 yielded an extremely small μ value of the order 10 −10 cm 2 V −1 s −1 at 20°C. The field dependencies of the hole mobilities at different temperatures were analyzed on the basis of the disorder formalism developed by Bässler and co-workers. As a result, the slow hole transport in the ZnTPP films was ascribed to a greater disorder parameter ( σ = 0.15 eV) resulting from a nearly amorphous structure of the ZnTPP films.

Electron-Beam-Deposited Thin Polymer Films: Electrical Properties vs Bombarding Current

Polymer films about 150 Å thick were deposited on glass substrates by electron bombardment of tetramethyltetraphenyltrisiloxane (DC704). They were sandwiched between evaporated aluminum electrodes, the top one semitransparent. The capacitance, conductance, and photoconductance of the sandwiches were measured at room temperature as a function of the electron bombarding current which formed the polymer. The polymer thickness was obtained independently from Christy's formula for the rate of formation. The dielectric constant, calculated from the capacitance and thickness, increased with bombarding current up to our maximum current density of 4 mA/cm2, starting from the value 2.6 measured for the bulk liquid DC704. After exposure of the film to air, the values fell to 2.6 regardless of bombarding current. The conductance, which showed an exponential dependence on square root of electric field, was interpreted as field-assisted thermal ionization of donors heavily compensated by acceptors. It increased as the cube of bombarding current, and fell several orders of magnitude on exposure to air. The photocurrent, which increased with photon energy at fixed voltage bias, showed a threshold which fell from 3.5 to 2 eV with increasing bombarding current and a proportionality constant which increased. It was not measured before air exposure. Our conclusion from these results is that with increasing bombarding current, the polymer undergoes an increase in both crosslinking bonds (which contribute to the carrier mobility) and dangling bonds (which function as donors or acceptors). Exposure to air drastically reduces the density of dangling bonds, with donor radicals more susceptible to oxidation than acceptors, but does not much affect the crosslinking.