Electrical studies on hybrid MIS structures incorporating CdS nanoparticles in organic films.

This project involves the investigation of a.c. and d.c. electrical characterisations and low-frequency noise properties of Langmuir-Blodgett (LB) films in metal-insulator-semiconductor (MIS) structure. Two types of insulating films based on hybrid organic-inorganic materials sandwiched between metal and semiconductor were fabricated. The original insulating films (untreated) were 40 layers Y-type LB films of Cd-salt stearic acid (CdSt[2]). The second type of insulating films were formed after the treatment of CdSt[2] films with H[2]S gas over a period of 12 hours at room temperature to grow CdS nanoparticles within the stearic acid matrix (treated). The capacitance-voltage (C-V) measurement of CdSt[2] LB films exhibit significant dependence on the measurement frequency in the accumulation region due to high d.c. leakage currents. By embedding CdS nanoparticles into the stearic acid matrix, less frequency dependent C-V curves were obtained. The problem in determining the true insulator capacitance due to frequency dispersion was overcome by using the Yang's model. The corresponding dielectric constant of LB films of CdSt[2] was found to be 2.3 and increased to 5.1 when embedded with CdS nanoparticles. The results from the dielectric loss measurement show that both devices agree well with Goswami and Goswami model. By incorporating CdS nanoparticles in the stearic acid matrix, the dielectric loss was found to increase which could be due to electrons being trapped by the CdS nanoparticles. A large current density was observed in the untreated devices at room temperature giving evidence of a leaky dielectric. The analysis of the temperature dependent I-V characteristics shown that current is independent of temperature, similar to the results published by several researchers which explained the current conduction mechanism in term of electron hopping and tunnelling through each bilayer of the LB films. In contrast, by embedding the CdS nanoparticles in the stearic acid matrix the currents have reduced by one-order of magnitude. The temperature dependence of the I-V characteristics showed the dependence of current on the device temperature at low electric field densities whilst less temperature dependence was observed at higher electric field density. Further investigation into the carrier transport mechanism, has found that the Poole-Frenkel effect was the dominant mechanism in the treated devices.A low frequency noise measurement setup has been designed and validated. The results of low-frequency noise measurement reported here are new. 1 / f noise was the only low-frequency noise observed in treated and untreated devices for frequencies up to 1kHz. The current noise spectral density S[I](f), was found to fit well with themodified Hooge's empirical model; [mathematical formula] where C, /, and f are noisemagnitude, current and frequency respectively. The exponential values of gamma and beta were found to lie within the acceptable ranges of 0.7<gamma<1.4 and 1<beta<3 respectively. The current noise power spectral density (PSD) at several fixed bias current was found to be dependent on the bias current with the PSDs for treated devices found to be approximately two-orders of magnitude higher. These results show that low-frequency noise measurement can be used to probe into the microstructure of the electron devices. It is believed that by embedding the CdS nanoparticles into the stearic acid matrix, electron trapping centres have been created which result in different current conduction behaviour from the untreated LB films of cadmium stearate.