V. Augelli

Melanin Layer on Silicon: an Attractive Structure for a Possible Exploitation in Bio-Polymer Based Metal- Insulator-Silicon Devices

Dr. M. Ambrico , Dr. P. F. Ambrico CNR-Istituto di Metodologie Inorganiche e dei Plasmi-UOS di Bari70125 Bari, Italy E-mail: marianna.ambrico@ba.imip.cnr.it Dr. A. Cardone , Dr. S. R. Cicco CNR-Istituto di Chimica dei Composti OrganoMetallici-UOS di Bari70125 Bari, Italy Ligonzo , Dr. . T Prof. Augelli . V Dipartimento Interateneo di FisicaUniversita degli Studi di Bari “Aldo Moro”70125 Bari, Italy Dr. R. Di Mundo , Prof. G. M. arinola F Dipartimento di ChimicaUniversita degli Studi di Bari “Aldo Moro”70125 Bari, Italy

Engineering polydopamine films with tailored behaviour for next-generation eumelanin-related hybrid devices

Eumelanin-type biopolymers have attracted growing interest in the quest for soft bioinspired functional materials for application in organoelectronics. Recently, a metal-insulator-semiconductor device with a good quality interface was produced by spin coating of a commercial synthetic eumelanin-like material on a dry plasma-modified silicon surface. As a proof-of-concept step toward the design and implementation of next-generation eumelanin-inspired devices, we report herein an expedient chemical strategy to bestow n-type performance to polydopamine, a highly popular eumelanin-related biopolymer with intrinsic semiconductor behaviour, and to tune its electrical properties. The strategy relies on aerial co-oxidation of dopamine with suitable aromatic amines, e.g. 3-aminotyrosine or p-phenylenediamine, leading to good quality black polymeric films. Capacitance–voltage experiments on poly(dopamine/3-aminotyrosine) and poly(dopamine/p-phenylenediamine)-based metal insulator semiconductor devices on p-Si indicated a significant increase in flat band voltage with respect to polydopamine and previous synthetic eumelanin-based diodes. Variations of the flat band voltage under vacuum were observed for each device. These results point to polydopamine as a versatile eumelanin-type water-dependent semiconductor platform amenable to fine tuning of its electronic properties through incorporation of π-conjugating aromatic amines to tailor functionality.

Physical characterization of halogenated and hydrogenated amorphous silicon films

Abstract Preliminary results on optical and electrical properties of a-Si films produced under plasma conditions starting from an SiCl4H2 gaseous mixture are reported. The optical gap was evaluated for two samples deposited under different conditions; Hall mobility and dark conductivity were measured and a density of states at the Fermi level equal to about 2×1020 eV−1 cm−3 was evaluated with Motts analysis. Finally, photocurrent measurements as a function of the temperature were carried out.

Optical constants of silicon films deposited by the r.f. glow discharge of SiCl4

Abstract The absorption coefficients and the refractive indices of silicon films deposited by using a glow discharge in an SiCl 4 -H 2 mixture were measured. Specimens were deposited with one deposition parameter being varied and the others kept constant. The dependence of the optical constants on the various deposition parameters was determined. The optical gap was calculated by applying Taucs analysis to the absorption data. The results are interpreted in terms of the hydrogen and chlorine contents and of the morphological changes in the silicon network.

Dark conductivity in amorphous undoped silicon films

Abstract The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl4H2 mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.

Charge trapping induced by plasma in alumina electrode surface investigated by thermoluminescence and optically stimulated luminescence

The plasma of a dielectric barrier discharge can fill traps in the alumina that cover the electrode. Trap energies and lifetimes are estimated by thermoluminescence and optically stimulated luminescence. Comparison with similar results for traps created by other radiation sources clarifies the mechanisms regulating this effect. Alumina’s trap energies are approximately 1 eV, and the traps remain active for several days after plasma exposure. These results could be important to keep dielectric barrier discharge plasmas uniform since a trapped charge can be an electron reservoir.

Memory-like behavior as a feature of electrical signal transmission in melanin-like bio-polymers

The memory-like behavior of melanin biopolymer under electrical stimuli is shown through electrical transport characterization performed on melanin based metal insulator semiconductor structures on silicon. The presence of a memory window and retention behavior is verified by capacitance-voltage read outs before and after the application of voltage pulses. Interestingly, these phenomena occur without the presence of metallic nanoclusters enclosed in the melanin matrix. Charge trapping is considered the main mechanism responsible for the melanin memory-like character. The inability to erase the memory window has been ascribed to the permanent polarization effect during the application of the voltage pulse.

Photoconductivity in amorphous Si:H:Cl films

Amorphous silicon films were deposited in a capacitively coupled glow discharge in SiCl4‐H2 mixture. The photoconductivity has been correlated to the discharge pressure p, the substrate temperature Ts , and to the ratio R of SiCl4 flow to the total flow. Moreover, the dependence of the photoconductivity on the wavelength and light intensity has been investigated. These measurements give useful information about the recombination mechanisms and allow one to estimate the position of the Fermi level. Some features of the gap state distribution are obtained by correlating dark conductivity and luminescence results to the photoconductivity ones.

Nonlinear behavior of the short circuit current of a solar cell with minority carrier lifetime dependent on the light intensity

The charge carrier transport in a solar cell is studied taking into account the dependence of the minority carrier lifetime on the injection level. The nonlinear continuity equation so obtained is solved by using a perturbative method. The spectral response of a silicon solar cell at high light intensity is evaluated on the basis of the proposed model. Theoretical results agree very well with some recently reported experimental data, for silicon solar cells at high intensity. However, there is evidence that in the light intensity range where our perturbative approach can be applied, the effect of the nonconstant minority carrier lifetime on the total short circuit current of the solar cell is quite small.

Characterization and luminescence of a-Si:H:Cl films☆

Abstract a-Si:H:Cl films have been deposited by glow-discharge and characterized by infrared transmission, optical absorption and photoluminescence. The influence of growth parameters on the H and Cl content has been investigated. The luminescence spectra show that three different radiative transitions can occur, at 0.75, 0.95 and ∼1.3 eV. These bands have been interpreted respectively in terms of the following recombinations: defect to defect, defect to band tail, band tail to band tail.