G. Leroy

ANALYTICAL EXPRESSIONS FOR CORRECTION FACTORS FOR NOISE MEASUREMENTS WITH A FOUR-POINT PROBE

The linear four-point probe method is useful to measure the resistivity, by passing a current I14 through the outer probes and by measuring the voltage V23 between the inner probes. The contacts are on a line and denoted by 1, 2, 3, 4, respectively. The sheet resistance for thin layers with thickness t is Rsh = ρ/t (Ω). The sheet resistance is measured as Rsh = (V23/I14). C23 where the correction factor is C23 = π/ln2 = 4.53 for t smaller than s, the distance between probe points. In order to characterize the 1/f noise of a film with a four-point probe we need noise correction factors for the calculation of the conductance noise from the voltage noise between the pair of points, where the current is not passed through. Our calculations of the noise correction factors are based on the general theory for conductance noise investigations with four arbitrarily shaped and placed electrodes. Analytical expressions for noise correction factors Fij and fij are derived and compared with experimental results for the three cases. The subscripts of F and f denote the voltage sensor contacts. The following cases are investigated: 1) I14 and V23, 2) I13 and V24 and 3) I12 and V34. The subscripts in I and V denote the current driver contacts and voltage sensor contacts, respectively. From the calculations and experimental results it follows that: i) case1 (I14 and V23) is the best choice in order to suppress a non-intentional noise contribution from the interface between probe tip and layer, ii) the reciprocity relation is applicable to voltage noise due to conductance fluctuations. The voltage noise SV23 that is observed by passing a constant current I14 through the pair of probes 1, 4 is equal to the voltage noise SV14 if we pass the same current through the contacts 2, 3. The four-point probe method with the noise correction factors can be recommended to investigate e.g., the conductance noise of conductive polymer layers even with an insulating top layer and without the preparation of rectangular samples with perfect noise-free line contacts. The four-point probe can punch an insulating top layer and allows the measurement of sheet resistance and noise. Several tests show that the noise contributions from the interface between probe tip and layer are negligible. The ratio of the conductance (1/f) noise, Cus, normalized for frequency and area and the sheet resistance gives an indication of the degree of percolation in composite conductive layers.

Analytical expressions for the conductance noise measured with four circular contacts placed in a square array

In the ideal case, noise measurements with four contacts minimize the contribution of the contact interface. There is a need to characterize conductance noise and noise correction factors for the different geometries provided with four contacts, as already is the case for resistivity measurements with van der Pauw structures. Here, we calculate the noise correction factors for two geometries with a pair of sensors and a pair of current driver electrodes placed in a square array. The first geometry investigated is a very large film compared to the distance L between four circular electrodes, which are placed in a square array far away from the borders of the film. The second is a square-shaped conductive film with side length L and provided with four quarter-circle corner contacts with radius l. The effect of the conductance noise in the film can be observed between current free sensors in a four-point measurement or between current carrying drivers in a two-point measurement. Our analytical expressions ar...

Correlations between 1/f noise and thermal treatment of Al-doped ZnO thin films deposited by direct current sputtering

Al-doped ZnO thin films (AZO) have been deposited on amorphous glass substrates by DC sputtering at different substrate temperatures Ts. X-Ray diffraction results reveal that AZO thin films have a hexagonal wurtzite structure with (002) preferred orientation. (002) peaks indicate that the crystalline structure of the films is oriented with c-axis perpendicular to the substrate. Three-dimensional (3D) atomic force microscopy images of AZO thin films deposited on glass substrate at 200 °C, 300 °C, and 400 °C, respectively, shows the improvement of the crystallinity and the homogeneity of AZO thin films with Ts which is in agreement with the noise measurements. The noise was characterized between 1 Hz and 100 kHz and we have obtained 1/f spectra. The noise is very sensitive to the crystal structure especially to the orientation of the crystallites which is perpendicular to the substrate and to the grain boundaries which generate a high current flow and a sharp increase in noise. Through time, Rsh and [αμ]eff...

NOISE MEASUREMENTS ON NbN THIN FILMS WITH A NEGATIVE TEMPERATURE RESISTANCE COEFFICIENT DEPOSITED ON SAPPHIRE AND ON SiO2

We characterize granular NbNx thin cermet films deposited on either sapphire substrate or on SiO2 and compare the 1/f noise at 300 K and 80 K. The films were characterized with an impedance analyzer from 20 Hz to 1 MHz and analyzed as a resistor R in parallel with a capacitor C. The calculated noise voltage spectral density SvTh of the sample is in agreement with the experimentally observed noise for unbiased samples. The noise measurements on biased samples show 1/f noise. We checked that contact noise does not contribute to our measurements. The 1/f noise was compared for the films deposited on the silicon substrate and on the sapphire. Finally a model is proposed to explain the observed trends in the temperature resistance coefficient (TRC < 0 for all samples) and the relative 1/f noise at 300 K and 80 K with the composition x = N/Nb of the layers.

Structural, morphological, optical and opto-thermal properties of Ni-doped ZnO thin films using spray pyrolysis chemical technique

Nickel-doped zinc oxide thin films (ZnO : Ni) at different percentages were deposited on glass substrates using a chemical spray technique. The effect of Ni concentration on the structural, morphological, optical and photoluminescence (PL) properties of the ZnO : Ni thin films were investigated. X-ray diffraction analysis revealed that all films consist of single phase ZnO and was well crystallized in würtzite phase with the crystallites preferentially oriented towards the (002) direction parallel to the c-axis. The optical transmittance measurement was found to be higher than 90%, the optical band gap values of ZnO thin films decreased after doping from 3.29 to 3.21 eV. A noticeable change in optical constants was observed between undoped and Ni-doped ZnO. Room-temperature PL is observed for ZnO, and Ni-doped ZnO thin films.

1/ƒ noise in ZnO films

We have investigated the low-frequency noise of ZnO film deposited by dc sputtering technique on glass substrate. We characterized the noise below 100kHz and obtained classical 1/ƒ spectra. We attempted to verify the validity of Hooges empirical relation and to test its usefulness as a diagnostic tool. The 1/ƒ noise normalized for bias, frequency and unit area, Cus is proportional with the sheet resistance Rsh. We found that the noise results depend strongly on illumination.

Research on the properties of ZnO films by 1/f noise measurement

ZnO films were deposited by dc sputtering technique on glass and Pt/Si substrates. The effect of growth parameters is investigated on sheet resistance and noise. The 1/f noise normalized for bias, frequency and unit area, Cus is proportional with the sheet resistance Rsh. We found that the noise results correlate strongly with the crystalline structure of ZnO. For comparison, we have also studied the ZnO films structural properties.

Dielectric and conductivity properties of composite polyaniline/polyurethane network

In this work, we present the dielectric characterization of polyaniline/polyurethane composite. The samples consisting of 0.5%, 1%, and 5% of polyaniline were deposited on glass fiber, and the measurements were performed in a frequency range of 20 Hz to 20 GHz. The results showed a dielectric relaxation strongly dependent on the concentration of polyaniline. This phenomenon is explained by a theoretical model. In this model, we assume that the alternative conductivity of the polymer network systems is due to conducting clusters whose lengths followed a Gaussian distribution. Depending on their size and the frequency of the excitation signal, the clusters showed a resistive or capacitive effect.

Effect of substrate temperature deposition on the 1/f noise of Al-doped ZnO thin films

We have investigated the low-frequency noise and the sheet resistance of Al-doped ZnO thin films deposited by DC sputtering technique on glass substrate at different temperature. We characterized the noise below 100 kHz and obtained 1/f spectra. The 1/f noise normalized for bias, frequency and unit area, Cus is proportional with the sheet resistance Rsh· Sheet resistance decreases with the increasing of the substrate temperature Ts. The decrease of the normalized noise showed the improvement of the crystallinity and homogeneity of AZO thin films with Ts which indicates the sensitivity of the noise with the crystalline quality of the thin films.

Effects of thickness on 1/f noise of Co and In co-doped ZnO

We have investigated the effects of thickness on the electrical properties of Co2% and In1% co-doped ZnO films (CIZO) grown on glass substrates at room temperature by Radio Frequency (RF) magnetron sputtering technique. The sheet resistance Rsh [Ω], the resistivity ρ [Ω.cm], and the 1/f noise have been analysed as a function of thickness from 50 nm to 450 nm. The electrical properties and 1/f fluctuation noise were carried out by using four-point probe method. In order to reduce the contacts problems, we applied a constant current through the pair of inner electrodes and observed the voltage fluctuations across the pair of outer electrodes. We characterized the noise below 100 kHz and obtained classical 1/f spectra. We attempted to verify the validity of Hooges empirical relation and to test its usefulness as a diagnostic tool. The results show that the noise coefficient K [cm2/03A9] = Cus / Rsh increases with the thickness twhich indicates that mobility and the noise parameter αH shrinks with a shrinking thickness. We have also studied the structural and optical properties of these films.