Guido Mula | University of Cagliari (original) (raw)

Papers by Guido Mula

Chemical Engineering Transactions

Rare earth doping of porous silicon is a very promising technique for the fabrication of all-Si l... more Rare earth doping of porous silicon is a very promising technique for the fabrication of all-Si light emitting devices at the 1.5µm wavelength. However, the lack of detailed knowledge of the mechanisms underlying the electrochemical Er doping (ECD) of the porous layers has, till now, been a major limitation for achieving the expected performances. As we will show, a key parameter of the Er ECD is the current density used during the process. We observed that using low current densities (LD), for equal amounts of transferred charge, leads to a significantly lower Er content in the porous layers with respect to using high current densities (HD). The threshold between "high" and "low" current densities depends on the sample characteristics, being related to the effective surface of the sample. The samples have been characterized by galvanostatic electrochemical impedance spectroscopy (GEIS) for various DC current densities. The GEIS results show a significant difference for LD and HD, and an additional semicircle in the Nyquist plot is visible for HD with respect to LD. This change in the ECD behavior is also observed when studying the applied voltage time evolution in constant-current ECD. With LD, a single transient is observed, while for HD a double transient ?? is observed, coherently with the appearance of an additional semicircle in the GEIS plots. Energy Dispersive Spectroscopy by Scanning Electron Microscopy (EDS-SEM) confirmed the significant difference in the Er content for LD and HD samples with equal total transferred charge, HD samples containing more than one order of magnitude additional Er atoms with respect to LD samples.

Materials

Lithography on a sub-100 nm scale is beyond the diffraction limits of standard optical lithograph... more Lithography on a sub-100 nm scale is beyond the diffraction limits of standard optical lithography but is nonetheless a key step in many modern technological applications. At this length scale, there are several possible approaches that require either the preliminary surface deposition of materials or the use of expensive and time-consuming techniques. In our approach, we demonstrate a simple process, easily scalable to large surfaces, where the surface patterning that controls pore formation on highly doped silicon wafers is obtained by an electrochemical process. This method joins the advantages of the low cost of an electrochemical approach with its immediate scalability to large wafers.

ACS Applied Materials & Interfaces

Surface and Interface Analysis

Secondary ion mass spectrometry studies have been made of the removal of the degraded layer forme... more Secondary ion mass spectrometry studies have been made of the removal of the degraded layer formed on polymeric materials when cleaning focused ion beam (FIB)-sectioned samples comprising both organic and inorganic materials with a 30-keV Ga + FIB. The degraded layer requires a higher-than-expected Ar gas cluster ion beam (GCIB) dose for its removal, and it is shown that this arises from a significant reduction in the layer sputtering yield compared with that for the undamaged polymer. Stopping and Range of Ions in Matter calculations for many FIB angles of incidence on flat polymer surfaces show the depth of the damage and of the implantation of the Ga + ions, and these are compared with the measured depth profiles for Ga +-implanted flat polymer surfaces at several angles of incidence using an Ar + GCIB. The Stopping and Range of Ions in Matter depth and the measured dose give the sputtering yield volume for this damaged and Ga +implanted layer. These, and literature yield values for Ga + damaged layers, are combined on a plot showing how the changing sputtering yield is related to the implanted Ga density for several polymer materials. This plot contains data from both the model flat poly(styrene) surfaces and FIB-milled sections showing that these 2 surfaces have the same yield reduction. The results show that the damaged and Ga +-implanted layer's sputtering rate, after FIB sectioning, is 50 to 100 times lower than for undamaged polymers and that it is this reduction in sputtering rate, rather than any development of microtopography, that causes the high Ar + GCIB dose required for cleaning these organic surfaces.

The Journal of Physical Chemistry C

International Journal of Molecular Sciences

Porous Si/eumelanin hybrids are a novel class of organic-inorganic hybrid materials that hold con... more Porous Si/eumelanin hybrids are a novel class of organic-inorganic hybrid materials that hold considerable promise for photovoltaic applications. Current progress toward device setup is, however, hindered by photocurrent stability issues, which require a detailed understanding of the mechanisms underlying the buildup and consolidation of the eumelanin-silicon interface. Herein we report an integrated experimental and computational study aimed at probing interface stability via surface modification and eumelanin manipulation, and at modeling the organic-inorganic interface via formation of a 5,6-dihydroxyindole (DHI) tetramer and its adhesion to silicon. The results indicated that mild silicon oxidation increases photocurrent stability via enhancement of the DHI-surface interaction, and that higher oxidation states in DHI oligomers create more favorable conditions for the efficient adhesion of growing eumelanin.

Scientific reports, Jan 20, 2017

Er clustering plays a major role in hindering sufficient optical gain in Er-doped Si materials. F... more Er clustering plays a major role in hindering sufficient optical gain in Er-doped Si materials. For porous Si, the long-standing failure to govern the clustering has been attributed to insufficient knowledge of the several, concomitant and complex processes occurring during the electrochemical Er-doping. We propose here an alternative road to solve the issue: instead of looking for an equilibrium between Er content and light emission using 1-2% Er, we propose to significantly increase the electrochemical doping level to reach the filling the porous silicon pores with luminescent Er-rich material. To better understand the intricate and superposing phenomena of this process, we exploit an original approach based on needle electron tomography, EXAFS and photoluminescence. Needle electron tomography surprisingly shows a heterogeneous distribution of Er content in the silicon thin pores that until now couldn't be revealed by the sole use of scanning electron microscopy compositional ...

RSC Adv., 2016

Porous Si–polyaniline composites were realized by electropolymerization with an underlayer of phe... more Porous Si–polyaniline composites were realized by electropolymerization with an underlayer of phenylamine. The composite showed photocurrent properties higher than those of porous Si or Si–polyaniline composites realized without underlayer.

Nanoscale research letters, 2016

In this work, we report a method to process porous silicon to improve its chemical resistance to ... more In this work, we report a method to process porous silicon to improve its chemical resistance to alkaline solution attacks based on the functionalization of the pore surface by the electrochemical reduction of 4-nitrobenzendiazonium salt. This method provides porous silicon with strong resistance to the etching solutions used in optical lithography and allows the fabrication of tailored metallic contacts on its surface. The samples were studied by chemical, electrochemical, and morphological methods. We demonstrate that the grafted samples show a resistance to harsh alkaline solution more than three orders of magnitude larger than that of pristine porous silicon, being mostly unmodified after about 40 min. The samples maintained open pores after the grafting, making them suitable for further treatments like filling by polymers. Optical lithography was performed on the functionalized samples, and electrochemical characterization results are shown.

Journal of Crystal Growth, May 1, 1999

Molecular beam epitaxy (MBE) is now a well established technology for the fabrication of nanomete... more Molecular beam epitaxy (MBE) is now a well established technology for the fabrication of nanometer-scale structures [1]. A precise control of such structures requires knowledge of the atomic mechanisms involved during the MBE growth process. In particular, the ...

Journal of microscopy, Jan 28, 2016

Electron tomography is a key technique that enables the visualization of an object in three dimen... more Electron tomography is a key technique that enables the visualization of an object in three dimensions with a resolution of about a nanometre. High-quality 3D reconstruction is possible thanks to the latest compressed sensing algorithms and/or better alignment and preprocessing of the 2D projections. Rigid alignment of 2D projections is routine in electron tomography. However, it cannot correct misalignments induced by (i) deformations of the sample due to radiation damage or (ii) drifting of the sample during the acquisition of an image in scanning transmission electron microscope mode. In both cases, those misalignments can give rise to artefacts in the reconstruction. We propose a simple-to-implement non-rigid alignment technique to correct those artefacts. This technique is particularly suited for needle-shaped samples in materials science. It is initiated by a rigid alignment of the projections and it is then followed by several rigid alignments of different parts of the projec...

Ultramicroscopy, 2016

An automatic procedure for electron tomography is presented. This procedure is adapted for specim... more An automatic procedure for electron tomography is presented. This procedure is adapted for specimens that can be fashioned into a needle-shaped sample and has been evaluated on inorganic samples. It consists of self-adapting denoising, automatic and accurate alignment including detection and correction of tilt axis, and 3D reconstruction. We propose the exploitation of a large amount of information of an electron tomography acquisition to achieve robust and automatic mixed Poisson-Gaussian noise parameter estimation and denoising using undecimated wavelet transforms. The alignment is made by mixing three techniques, namely (i) cross-correlations between neighboring projections, (ii) common line algorithm to get a precise shift correction in the direction of the tilt axis and (iii) intermediate reconstructions to precisely determine the tilt axis and shift correction in the direction perpendicular to that axis. Mixing alignment techniques turns out to be very efficient and fast. Significant improvements are highlighted in both simulations and real data reconstructions of porous silicon in high angle annular dark field mode and agglomerated silver nanoparticles in incoherent bright field mode. 3D reconstructions obtained with minimal user-intervention present fewer artefacts and less noise, which permits easier and more reliable segmentation and quantitative analysis. After careful sample preparation and data acquisition, the denoising procedure, alignment and reconstruction can be achieved within an hour for a 3D volume of about a hundred million voxels, which is a step toward a more routine use of electron tomography.

Summary form only given. In the present paper we report on room temperature transmission electrom... more Summary form only given. In the present paper we report on room temperature transmission electromodulation in the wavelength range of 1.5 /spl mu/m of multiple quantum well (MQW) CdHgTe heterostructure diodes.

Zinc-blende AlGaN epilayers were grown by plasma-assisted MBE on thick (001) SiC deposited by CVD... more Zinc-blende AlGaN epilayers were grown by plasma-assisted MBE on thick (001) SiC deposited by CVD on Si substrates. Alloy compositions were controlled in situ by reflection high energy diffraction (RHEED) oscillations, and confirmed by Rutherford backscattering (RBS) post growth analysis. The zinc-blende nature of our samples was assessed by RHEED analysis. From reflectivity measurements at room temperature, we deduce the variation of the lowest direct absorption edge of zinc-blende AlGaN as a function of the Al content x, E 0 (x) = 3.25 (1-x) + 6.05x-1.4x (1-x), E 0 in eV. The dispersion of the refractive index n of cubic AlN was also extracted from our reflectivity and RBS data, and fitted by a Sellmeier-type relation, n 2 = 3.05 + 1.38 l 2 /(l 2-180 2), l being the wavelength in nm.

Journal of Physics: Conference Series, 2014

Bismuth sulfide is a promising n-type semiconductor for solar energy conversion. We have explored... more Bismuth sulfide is a promising n-type semiconductor for solar energy conversion. We have explored the colloidal synthesis of Bi2S3 nanocrystals, with the aim of employing them in the fabrication of solution-processable solar cells and to replace toxic heavy metals chalcogenides like PbS or CdS, that are currently employed in such devices. We compare different methods to obtain Bi2S3 colloidal quantum dots, including the use of environmentally benign reactants, through organometallic synthesis. Different sizes and shapes were obtained according to the synthesis parameters and the growth process has been rationalized by comparing the predicted morphology with systematic physical-chemistry characterization of nanocrystals by X-ray diffraction, FT-IR spectroscopy, Transmission Electron Microscopy (TEM).

MRS Proceedings, 2002

ABSTRACTRecent experimental and theoretical studies highlighted the importance of the growing sur... more ABSTRACTRecent experimental and theoretical studies highlighted the importance of the growing surface structure on the final morphology of GaN. Actually, optimum morphology is achieved by growth in presence of a Ga bilayer adsorbed on the GaN surface. The threshold fluxes limiting the region of the Ga bilayer adsorption have been measured as a function of the GaN substrate temperature, giving rise to a Ga adsorption phase diagram. The Ga flux limiting the regions in the adsorption phase diagram exhibit a linear behavior in an Arrhenius plot. However, both energy activation (about 5 eV) and prefactor (in the 1025 range) are surprinsingly high. These questions were adressed by studying the adsorption/desorption of Ga adatoms and small islands (consisting of 2 and 3 Ga adatoms) on the Ga bilayer surface employing first principle density functional theory calculations. We find a desorption barrier of 2.1 eV and a binding energy between two Ga atoms of approximately 0.3 eV. Using these n...

CLEO/Europe Conference on Lasers and Electro-Optics, 1998

ABSTRACT Not Available

physica status solidi (a), 2001

We demonstrate that, depending on substrate temperature, exposure of cubic or hexagonal GaN, AlN ... more We demonstrate that, depending on substrate temperature, exposure of cubic or hexagonal GaN, AlN or AlGaN surface to Ga leads to the formation of a self-regulated film. Subsequent exposure to N flux leads to the growth of a discrete quantity of GaN. This atomic layer epitaxy process can be repeated several times, allowing for the controlled growth of QWs or QDs.

physica status solidi (a), 1999

We present a study on the growth of hexagonal and cubic GaN. In particular, the effect of the pre... more We present a study on the growth of hexagonal and cubic GaN. In particular, the effect of the presence of Mg atoms on the surface has been analyzed. The samples have been studied by means of Reflection High Energy Electron Diffraction, Photoluminescence and optical microscopy. A remarkable change in the growth kinetics is observed for the hexagonal phase, where the presence of Mg atoms on the growing surface induces a deep hollow in the growth rate as a function of the Ga flux. The cubic phase seems to be less sensitive to the presence of Mg atoms, and we observed only an increase of the growth rate in the N-rich regime. A model for the behavior observed in the hexagonal phase is proposed.