R.D. Gould

A.c. electrical properties of thermally evaporated thin films of copper phthalocyanine

Abstract The a.c. electrical properties of copper phthalocyanine (CuPc) thin films have been studied in the frequency range 10 2 −2 × 10 4 Hz and in the temperature range 173–360 K. The a.c. conductivity σ(ω) was found to vary as ω s with the index s ⩽ 1, indicating a dominant hopping process at low temperatures and high frequencies. At higher temperatures and lower frequencies, free carrier conduction with mean activation energy of 0.3 eV was observed. Capacitance and loss tangent decreased with frequency and increased with temperature. Such characteristics were found to be in good qualitative agreement with the existing model of Goswami and Goswami. Both the conductance and capacitance were lowered over the entire frequency spectrum after heat treatment at 370 K. This reduction was ascribed to the desorption of oxygen by the heat treatment, as previously observed in d.c. conduction measurements.

On the Van der Pauw method of resistivity measurements

Abstract The Van der Pauw method is one of the most effective and widely used methods of the four-probe mode of determining the resistivity of materials in the form of thin films. The right version of the suggested equations for the correction function that fit the given graph was confirmed by numerical calculations. Then, the equation was experimentally verified by designing samples of different shapes and contact configurations. For easy, fast and accurate estimation of the correction factor, tables of the numerical values are given with steps of 0.1 and 1.0 for the range of R ′/ R ″ ratio from 1 to 200. It is recommended that these tables be used rather than an inaccurate estimation from the graph or recalculation of the factor.

D.c. properties of ZnO thin films prepared by r.f. magnetron sputtering

In the development of ZnO-based varistors the electrical properties of ZnOBi2O3 junctions and of the two individual oxides are being investigated. Following our recent work on a.c. conductivity in AlZnOAl sandwich structures we currently report d.c. measurements. The structures were prepared by r.f. magnetron sputtering in an argon/oxygen mixture in the ratio 4:1. Capacitance-voltage data confirm that the AlZnO interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 9.7. With increasing voltage the current density changed from an ohmic to a power-law dependence with exponent n≈3. Furthermore measurements of current density as a function of reciprocal temperature showed a linear dependence above about 240 K, with a very low activation energy below this temperature consistent with a hopping process. The higher temperature results may be explained assuming a room-temperature electron concentration n0 and space-charge-limited conductivity, dominated by traps exponentially distributed with energy E below the conduction band edge according to N = N0exp(−EkTt), where k is Boltzmanns constant. Typical derived values of these parameters are: n0 = 7.2 × 1016 m−3, N0 = 1.31 × 1045 J−1 m−3 and Tt = 623 K. The total trap concentration and the electron mobility were estimated to be 1.13 × 1025 m−3 and (5.7−13.1) ×10−3m2V−1s−1 respectively.

D.C. electrical properties of evaporated thin films of CdTe

Abstract High field d.c. current-voltage J-V measurements have been performed on evaporated thin films of CdTe, with thicknesses in the range 0.4–1.3 μm, at temperatures between 200 and 300 K. Samples showed a log J ∝ V 1 2 dependence which was indicative of the Poole-Frenkel effect. However, values of the field-lowering coefficient β when calculated using the traditional Poole-Frenkel equation, were a factor of α = 1.5–2.75 times the predicted value of 2.83 × 10 −5 eV m 1 2 V 1 2 . The results were thus re-interpreted in terms of a modified Poole-Frenkel model which includes the effects of electric field variations within the CdTe films. The effective Poole-Frenkel coefficient then becomes αβ, where β is the normal theoretical value. The model predicts localized electric fields of up to α2 times the mean electric field F, which was up to a factor of approximately 8 times in the present case.

Electrical conduction processes in silicon nitride thin films prepared by r.f. magnetron sputtering using nitrogen gas

Abstract Silicon nitride (Si 3 N 4 ) is an important VLSI material owing to its high resistivity and breakdown strength and its use in surface encapsulation during ion implantation and annealing. Previous work has focused on films prepared by low-pressure and plasma-enhanced chemical vapour deposition (LPCVD and PECVD), but in the present work the DC electrical properties of films prepared by r.f. magnetron sputtering were investigated. Al-Si 3 N 4 -Al sandwich structures were fabricated from a Si 3 N 4 target at a discharge power of 100 W using N 2 as the sputtering gas at a pressure of approximately 0.5 Pa. Capacitance was independent of voltage, indicating the absence of a Schottky barrier at the Al/Si 3 N 4 interface. Measurements of the capacitance as a function of inverse dielectric thickness implied a relative permittivity value of 6.3. However, although films prepared using PECVD exhibited Poole–Frenkel conductivity and tunnelling at higher voltages, the present sputtered films showed space-charge-limited conductivity (SCLC). Conductivity was dominated by an exponential distribution of trap levels, as indicated by a power-law dependence of current density J on applied voltage V , with a typical exponent value of 3.2. Measurements of J as a function of temperature confirmed the appearance of SCLC and indicated that the bulk trap density was of the order of 2×10 24 m −3 as observed in LPCVD and PECVD films, with the appearance of hopping conductivity at low temperatures.

A study of the response rate to nitrogen dioxide exposure in metal phthalocyanine thin film sensors

Abstract The present study has been concerned with the response of gas sensors based on sublimed films of copper phthalocyanine (CuPc) and iron phthalocyanine (FePc) to nitrogen dioxide (NO 2 ). An alternative strategy of deriving information on the concentration of NO 2 from data of the response rate within the initial few minutes was accomplished, rather than from electrical signals obtained under conditions of saturation. The response to NO 2 for the CuPc sensor was found to obey the Elovich equation, d θ /d t = a exp(− bθ ), where d θ /d t represents the rate of change of the surface coverage θ , and a and b are constants. A detailed investigation of the dependence of the constants in the equation on the NO 2 concentration revealed that the constant a was directly proportional to the concentration in the range 0.8–12.8 ppm. It was therefore possible to derive the NO 2 concentration after a period of only 1 min exposure. However, the response of the FePc sensors did not follow the Elovich equation in the first 2 min exposure, and therefore, longer time periods were required to determine concentrations than for the case of CuPc. It is thought that the nature of the central metal ion in the phthalocyanine molecule might be responsible for this difference.

Frequency dependence of electronic conduction parameters in evaporated thin films of cobalt phthalocyanine

Abstract A.c. dark current measurements were taken on purified cobalt phthalocyanine (CoPc) thin films in the frequency range 10 2 −2 × 10 4 Hz and in the temperature range 163–433 K. Measurements revealed that the a.c. conductivity σ(ω) varied as ω s , where the index s was variable and always less than unity, generally increasing with increasing frequency and decreasing with increasing temperature. A.c. activation energy measurements at fixed frequencies showed that at temperatures below approximately 230 K the conductivity is frequency dependent with very low activation energy, while at higher temperatures a frequency independent activation energy of approximately 0.54 eV was observed. These results were interpreted in terms of hopping through a band of localized states at lower temperatures and by free band conduction at higher temperatures. Measurements of capacitance and loss tangent showed a well defined decrease with increasing frequency and an increase with increasing temperature. An interpretation is presented, based on existing theory, for the case of a thermally activated process when using ohmic contacts. A comparison of samples before and after heat treatment at 450 K showed a decrease in both conductivity and capacitance. This reduction was ascribed to desorption of oxygen, which acts as an acceptor level, as previously inferred from d.c. measurements.

Conductivity and dielectric properties of silicon nitride thin films prepared by RF magnetron sputtering using nitrogen gas

Abstract Silicon nitride is an important material in very-large-scale integration fabrication and processing. Recent work on films prepared by radio frequency magnetron sputtering using nitrogen gas have shown that the relative permittivity is typically 6.3 and that aluminium forms an ohmic contact to this material. Under direct current (DC) bias the films exhibited space-charge-limited conductivity with a bulk trap density of the order of 2×1024 m−3. In the present work alternating current electrical measurements were made on identical samples as a function of frequency and temperature. Conductivity appeared to be by hopping at lower temperatures, giving way to a free-band conduction process with activation energy of typically 0.44 eV at higher temperatures. Over a limited range of frequency and temperature the model of Elliott was applicable, and yielded a value of 2.87×1023 m−3 for the density of localised states, in reasonable agreement with our estimate of the trap density from DC measurements. As in the DC measurements capacitance followed a geometric relationship with relative permittivity 6.3, and showed a moderate decrease with increasing frequency and an increase with increasing temperature, tending towards a constant value at high frequencies and low temperatures. The loss tangent showed a minimum in its frequency dependence, which appeared to shift to higher frequencies with increasing temperature. The measurements are consistent with the model of Goswami and Goswami for samples having ohmic contacts, and are typical of results obtained on other insulating thin film structures.

The effects of oxygen doping and annealing on the surface and bulk electrical conductivity in planar copper phthalocyanine thin films for gas-sensing applications

Abstract In the development of gas-sensing devices based on phthalocyanine materials, it has become evident that the current drawn depends on previous exposure to oxygen, which acts as an acceptor impurity, and on the relative importance of the bulk and surface regions. The present work focuses on the effects of oxygen on α -phase thin films of copper phthalocyanine (CuPc) having a planar configuration. In unannealed samples both ohmic and space-charge-limited conductivity (SCLC) regions were observed in films of thickness 0.1 μ m, whereas for films of thickness 1 μ m, SCLC was not observed below a field strength of 2×10 6 V m −1 . These variations may be explained in the context of the differing effects of oxygen on the surface and bulk regions of the films. Exposure to oxygen has the effect of enhancing the conductivity and increasing the threshold voltage between the ohmic and SCLC regions. In the thinner films, samples lightly doped with oxygen had a trap distribution exponentially distributed in energy, whereas long exposure to oxygen resulted in the establishment of a discrete dominant trap level. While annealing was found to reduce the conductivity due to oxygen desorption, the resulting activation energy was found to be dependent on the concentration of oxygen within the films. Observations of oxygen desorption from films of different thickness revealed that the binding energy of oxygen to the CuPc molecule is lower on the surface than in the bulk of the films. It was concluded that the adsorbed or absorbed oxygen molecules resulted in the establishment of a partial charge-transfer complex, and that the hole concentration on the surface of the films is lower than that in the bulk under vacuum.

A comparison of the DC conduction properties in evaporated cadmium selenide thin films using gold and aluminium electrodes

Abstract As part of a comprehensive investigation into the electrical conduction and interfacial properties of cadmium selenide thin films, electrical measurements have been performed on metal–CdSe–metal sandwich structures with electrodes of aluminium and gold. Such studies are important in the context of inexpensive solar cells based on this material. Samples were deposited using conventional vacuum evaporation. Both dark current density–voltage ( J – V ) and differential capacitance–voltage ( C – V ) measurements were performed. Samples having symmetric gold electrodes showed ohmic conduction at low voltages and space-charge limited conduction (SCLC) at higher voltage levels, with a thermally activated carrier concentration of 4.2×10 20 m −3 . In accordance with earlier work, samples with symmetric aluminium electrodes, showed similar ohmic and SCLC behaviour, but with carrier concentrations of 2.0×10 18 m −3 . Asymmetric samples with one gold and one aluminium electrode showed carrier concentrations of 4.2×10 19 m −3 and 7.3×10 19 m −3 when the electrodes on the substrate side of the films were aluminium and gold, respectively. At higher voltage levels, a field-lowering coefficient β =(2.70–3.61)×10 −5 eV m 1/2 V −1/2 , depending on the electrode configuration, was determined, indicative of Schottky emission or the Poole–Frenkel effect. The results suggest that gold electrodes may be beneficial in providing efficient electron injection into CdSe films.