Thin Film Sandwich Structures Research Articles

Carbazole‐based electroluminescent devices obtained by vacuum evaporation

AbstractTransparent conductive oxide (SnO2)/organic layers/aluminum thin film sandwich structures were obtained by vacuum evaporation. The organic component was either a thin carbazole film or a bilayer. In that case, the carbazole film was deposited onto a thin insulating polymer film. The polymer used was the poly(tetrabromo‐p‐phenyleneselenide) (PBrPDSe). Photoluminescence measurements have shown that the carbazole thin films emit blue light. (I‐V) measurements have shown that the structures exhibit diode characteristics. The forward direction is obtained when the transparent conductive oxide (TCO) is positively biased. However, the reproducibility of the results obtained with a single carbazole layer is poor. It appears that the stability of the sample is improved when a thin PBrPDSe film (40 nm) is introduced between the carbazole and the SnO2. The polymer film avoids the short circuit effect. In that case, the turn‐on voltage of the diode is about 3 V, when the thickness of the carbazole film is around 250 nm and the electroluminescence appears at a voltage of about 5 V. It is shown that the thermionic effect cannot be used to explain the I‐V characteristics, which are interpreted with the help of the Fowler–Nordheim tunnel effect. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2042–2055, 2001

Carbazole based electroluminescent devices obtained by vacuum evaporation

Transparent conductive oxide (SnO2)/organic layers/aluminum thin film sandwich structures have been obtained by vacuum evaporation. The organic component was either a thin carbazole film or a bilayer. In that case, the carbazole film was deposited on a thin insulating polymer film. The polymer used was the poly(tetrabromo-p-phenylenediselenide) (PBrPDSe). Photoluminescence measurements have shown that the carbazole thin films emit blue light. The I–V measurements have shown that the structures exhibit diode characteristics. The forward direction is obtained when the TCO is positively biased. However, the reproducibility of the results obtained with a single carbazole layer is poor. It appears that the stability of the sample is improved when a thin PBrPDSe film (50nm) is introduced between the carbazole and the SnO2. The polymer film avoids short circuit effect. In that case, the turn-on voltage of the diode is about 3V, when the thickness of the carbazole film is around 250nm and the electroluminescence appears at a voltage of about 4V.

Dielectric properties of Al ∣ V 2O 5 ∣ Al thin film sandwich structures

Vanadium pentoxide thin films were prepared by the electron beam evaporation technique onto Corning 7059 glass substrates kept at a temperature of T s=423 K. The dielectric properties of Al ∣ V 2O 5 ∣ Al thin film sandwich structures were studied in the frequency range 0.1–100 kHz and in the temperature range 125–450 K. Both the dielectric constant and the dielectric loss factor were found to depend on frequency and temperature. The activation energy obtained for the dielectric relaxation process was about 0.36 eV.

High-field electrical conduction in thin-film sandwich structures of the metal/organic semiconductor/metal type

The investigated organic compounds are derivatives of the asparagic acid. Thin films of respective compounds were deposited from dimethylformamide solutions. The temperature dependences of the electrical conductivity and Seebeck coefficient are investigated. All compounds have semiconducting properties. The values of some characteristic parameters (activation energy, ratio of carrier mobilities, etc.) have been determined. Using sandwich cells of the type Al/organic compound/Al, the static current–voltage ( J– U) characteristics have been studied. For lower values of intensity of electric field E ( E ≅6×10 2–2×10 3 V cm −1), J– U characteristics are ohmic. At higher values of E , the current density J increases faster than linear. For E ≅10 3–10 4 V cm −1, the obtained experimental data are in good agreement with the Richardson–Schottky law. At fields E >10 5 V cm −1, the experimental points deviate more and more from Schottky characteristics in direction of larger values of J. For explanation of these behaviours, the electron transitions through and over the potential barrier at metal-organic film interface are taken into account.

Controlled signal transduction across interfaces of “intelligent” molecular systems

The atomistic understanding and control of information transfer across interfaces is of primary importance for any future molecular or bioelectronic device. In a systematic approach to reach this long term goal we investigate “model systems”, i.e. thin film sandwich structures with geometrically and chemically controlled compositions, and we relate their transport properties to their spectroscopically and microscopically determined structures. Silicon is by far the most important substrate for such structures because its processing is well known from current micro- and nanotechnologies. Simple “model systems” therefore consist of organic overlayers on the substrate Si(100), but also on other substrates such as SiO 2 or Au(111). More complex “model systems” include receptor-ligand structures, transport and channel proteins, enzymes, biomolecular function units, whole cells, and tissues. Finally we discuss briefly the two alternative approaches in the development of future biomolecular devices, i.e. the use of natural biological structures or of completely synthesised biomimetic structures. We chose the “electronic nose” as a specific example to point out the importance of hybrid devices.

Electrical conductivity in oxygen-doped thin films of cobalt phthalocyanine using gold and aluminium electrodes

The electrical conductivity of cobalt phthalocyanine thin film sandwich structures using ohmic gold and blocking aluminium electrodes has been measured both after exposure to oxygen for 30 days and after annealing at temperatures up to 423 K. Current density-voltage characteristics under forward bias (gold electrode positive) were found to be due to space-charge-limited conduction controlled by an exponential distribution of traps at lower voltage sections and by a discrete trap level at higher voltage regions. Under reverse bias, the conduction process was interpreted in terms of an electrode-limited to a bulk-limited transition, from Schottky emission to the Poole-Frenkel effect. The effect of annealing resulted in similar behaviour and was found to reduce current density due to oxygen desorption.

On the current-voltage characteristics of some thin-film sandwich structures of the metal/organic semiconductor/metal type

The investigated organic compounds are derivatives of the thiosemicarbazide complexed with cadmium cations. Thin films of these compounds were deposited from pyridine solutions. Using sandwich cells of the type Cu-organic compound-Al/Cu, the static curr ent-voltage ( J-U) characteristics have been studied. For lower values of intensity of electric field E( E = 6 × 10 2-2 × 10 4 V/cm) J-U characteristics are ohmic. At hi gher fields the current density J increased faster than linear. In order to establish the nature of the electrical conduction mechanism the different representations of the dependences J = f( U) are investigated. The experiments showed that for E ≌ 10 5-10 7 V/cm the thermionic emission is predominante and consequently the experimental data are in good agreement with Richardson-Schottky law. At fields E > 10 7 V/cm the experimental points deviate more and more from Schottky characteristics in direction of larger values of J. For explanation of these behaviours the presence of electronic transitions also through the upper part of the potential barrier at the metal-organic film interface are taken into account.

Energy loss straggling of MeV ions in thin solid films

Abstract High resolution measurements of the energy loss distribution for 0.5 MeV/u 15 N ions in solids are obtained using the 1 H( 15 N, αγ) 12 C resonant nuclear reaction at 6.4 MeV and thin film sandwich structures composed of a target material on a silicon substrate with an intermediate hydrocarbon layer. Energy loss and energy loss straggling are determined for targets with 6 ⩽ Z 2 ⩽ 79. It is shown that measurements of the variance of the energy loss distribution as a function of film thickness can be used to determine the contribution from thickness variations such as surface roughness. Excellent agreement is found with the results of atomic force profilometry. In this way, the most significant experimental error in the determination of straggling cross sections in solids can be eliminated. Calculations based on the plane-wave Born approximation are performed for representative cases to determine straggling cross sections and charge exchange cross sections near equilibrium. These are used to estimate the contribution of fluctuations in the charge state distribution (CSDF) of the traversing ions to the total straggling. Experimental results are also compared with various other theoretical estimates such as the formalisms of Bohr and Bethe-Livingston. It is found that the data is very well described by the sum of the CSDF estimates and the Bethe-Livingston approximation for fully stripped ions.

Effects of annealing on the trap distribution of cobalt phthalocyanine thin films†

Current density-voltage characteristics have been obtained from evaporated cobalt phthalocyanine thin Film sandwich structures using ohmic gold contacts, both before and after annealing. Results show that at low voltages the conduction process is ohmic, while at high voltages space-charge limited conduction is present. These results are consistent with current density-voltage characteristics obtained with copper phthalocyanine thin films. Annealing resulted in little change in current density within the space-charge limited region but a significant reduction within the ohmic region. Subsequent exposure to air gave an increase in current density tending towards levels obtained with fresh samples. Analysis of the results yields a typical room temperature hole concentration of p 0 = 8 × 1017 m−3 for unannealed samples. An exponential trap distribution P(E) = P 0 exp (−E/kT t) was consistent with the higher-voltage data, where typically P 0 = 8 × 1043J−1 m−3 and T t = 885 K. Variations of parameters with heating conditions are discussed, with particular reference to the desorption of oxygen during annealing and its absorption during exposure to air.

New interpretation of the I–V characteristics of electroformed thin film sandwich structures

The dc I–V characteristics of an Al/CuSiO x/Al thin film sandwich structure, determined using a computerised measurement technique, shows a fine structure superimposed on the usual I–V dependence, with voltage-controlled negative resistance (VCNR). The ac I–V characteristic (for frequencies > 50 Hz) is non-linear. The results are interpreted by considering the Cu particles to be Poole-Frenkel dipole centres which develop needle-like protrusions at sufficiently high electric fields. A combination of electric field, local temperature and size of centre influences the electron emissive properties of the centre.

A computerised technique for recording the I-V characteristics of thin-film sandwich structures

A fully automated technique is described which uses a BBC microcomputer to obtain the I-V characteristics of a thin-film sandwich device. The potential difference across the sample can be accurately controlled in a series of 256 linear steps up to a maximum value of 25 V and the circulating current recorded and displayed.

Structural and electronic properties of evaporated thin films of cadmium telluride

An investigation is made into the structure and dc electrical properties of evaporated CdTe thin film sandwich structures with thickness d of up to 1 μm. X-ray diffraction studies show that the films are microcrystalline with typical grain size 74 nm and with a preferred orientation in the [111] direction. Capacitance measurements indicate that the films have a relative permittivity of approximately 9.6. Room temperature current density-voltage (J–U) characteristics show ohmic conduction below a threshold voltage Ut and a power-law dependence with exponent 5.7 to 6.1 at higher voltages. This behaviour is interpreted in terms of space-charge limited conductivity controlled by an exponential distribution of traps below the conduction band edge. Further evidence for this conduction process is provided by a linear dependence of Ut on d2 and of log J on log d with approximate slope − 15. Analysis of the results yields a room temperature electron concentration n0 ≈ (1.2 to 4.8) × 1011 m−3 and an exponential trap distribution of concentration N(E) = N0 exp (− E/kTt) at an energy E below the conduction band edge, where N0 ≈ (1.63 to 2.08) × 1043 J−1 m−3 and Tt ≈ 1402 to 1493 K.

Electrical properties of thin films of SiO/In203

Electrical measurements on thin films of SiO/In2O3 are reported. The current Ic for DC conduction in thin film sandwich structures (MIM, metal-insulator-metal) is found to obey a relation of the form Ic αVc 1/2 , where Vc is the applied voltage. The behaviour of the AC conductivity σ(ω) is similar to the earlier observation for many other types of amorphous semiconductors, which obey an equation of the form σ(ω)=Aω3,ω being the radial frequency and the index s having different values at different temperatures.

Electrical conduction in thin films based on In2O3and In2O3/CeO2

Electrical measurements on thin films of In2O3 and In2O3/CeO2 before and after electroforming are reported. The high field conduction (expressed in terms of circulating current I c and base voltage V b) in thin film sandwich structures (MIM, Metal-Insulator-Metal) of Cu-In2O3-Cu and Cu-In2O3/CeO2-Cu is found to obey a relation of the form log I cαV b ½. The electroformed samples show voltage controlled negative resistance (VCNR). Voltage memory, thermal voltage memory and pressure voltage memory effects are observed and the results are explained in terms of the filamentory model of electrical conduction.

D.c. conduction in thin films of SiO/In2O3 before and after electroforming

Electrical measurements on thin co-evaporated SiO/In2O3 films before and after electroforming are reported. The high-field conduction (expressed in terms of circulating currentl c and bias voltageV b) in thin film sandwich structures of Al−SiO/In2O3-Al and Cu-SiO/In2O3-Cu is found to obey a relation of the form logI c ∞V b 1/2 . The electroformed samples show voltage-controlled negative resistance, voltage memory, thermal-voltage memory and pressure-voltage memory effects and the results are explained in terms of the filamentary model of electrical conduction.

Some electrical properties of mixed amorphous thin films of germanium and silicon monoxide before electroforming

The co-evaporated SiO x -Ge system was studied. Thin-film MIM sandwich structures were deposited by vacuum evaporation at a pressure of ≈ 10−4 Pa and were measured at a pressure of 10−3 Pa. The conductivity at low temperature and under d.c. fields has been found to be governed by a combination of an electronic hopping process and free-band conduction. At fields greater than 2 × 106 V m−1, it is concluded that the conduction process is governed by the Poole-Frenkel effect. Comparison with earlier results on SiO x -GeO2 films showed small differences in activation energy for conduction for samples of broadly similar overall composition.

Spin-polarised Critical-Angle Neutron Scattering from Ultrathin Magnetic Layers

We discuss the application of a beam of spin-polarized neutrons at grazing incidence to study ferromagnetism in ultrathin films of Fe(001) and Co(001) epitaxially grown in the metastable f.c.c. phase. Orders of magnitude increase in signal sensitivity are obtained by covering the magnetic layer with an epitaxial coating of Cu(001), thereby creating neutron interference enhancement in the thin-film sandwich structure. The ferromagnetism in cobalt films down to a single monolayer thickness can be measured with this technique.

Time, temperature and pressure dependence of circulating and emission currents in sandwich structures of Cu-SiO/GeO-Cu

Abstract Thin-film sandwich structures of Cu-SiO/GeO-Cu, in which the dielectric film was prepared by a co-evaporation technique, have been examined. The circulating currents for a fixed bias voltage in the electroformed samples were studied for periods of up to two hours. The relaxation of the high-resistance memory state ( Vb= 50 V) was studied for different loading voltages and temperatures. The thermal-voltage memory effects were found to be independent of temperature in the range from room temperature up to approximately 100°C. Pressure-voltage memory effects were also observed. On increasing the ambient pressure to 760 torr, the electron emission ceased. The emission current stayed at an essentially constant value from — 95°C up to room temperature under a normal vacuum of the testing ambient atmosphere. The experimental results are compatible with a theory based on the development of conducting filaments during the forming process

A study of thermal-voltage memory effects in M-I-M structures with co-evaporated SiO/B2O3as the insulator material

Thin-film sandwich structures of Cu-SiO/B2O3-Cu with dielectric thicknesses of the order of 2000 A were prepared by vacuum evaporation, and circulating and emission currents were measured as functions of the applied voltage. A thermal-voltage memory effect was recorded and the relaxation of the high-impedance memory state was studied. The results are interpreted in terms of the filamentary growth theory.

Improved sputter-depth profiles using two ion guns

A method is described which leads to a substantial improvement in the depth resolution of sputter-depth profiles obtained using Auger electron spectroscopy. Two ion guns are used to develop the profile, each symmetrically inclined to the surface normal, whose effect is to suppress the formation of ion induced topography. The effectiveness of the method is illustrated by comparing profiles obtained using one and two ion guns through the same Ag/Cu multi-layer thin film sandwich structure.