O. Sipr, F. Rocca, G. Dalba
Real-space multiple-scattering analysis of Ag L₁- and L₃ edge XANES spectra of Ag₂0.
J. Synchrotron Rad. 6, 770 (1999).

Light emitting Silicon nanoparticles X-ray absorption spectroscopy

This research proposes  to clarify  the functional  relationship between mictostructural properties and optical properties in  porous silicon and in silicon nanoparticles embedded in silica by the X-ray spectroscopic techniques EXAFS and XANES. The ability of EXAFS and XANES to characterise the short and medium range order has been already used to study porous Si at the Synchrotron Radiation Facility LURE (Orsay-Paris). It is clear from all previous measurements that any direct correlation between structure and optical properties must be better proved, with tailored experiments.

A new apparatus for measuring the X-ray emitted optical luminescence (XEOL) has been realised in our Laboratory and successfully installed on the SA-32 Beamline of Super-ACO at LURE (Orsay-Paris). The apparatus have allowed us to characterise the optical luminescence excited by X-rays at a given X-ray energy, and to recover at the same time many EXAFS-XEOL spectra by integrating the emitted light intensity over different user-defined energy ranges.

In porous silicon, the main results of X-ray Absorption Spectroscopy (XAS) studies can be summarised as follows: a) a well defined crystalline order is clearly present in por-Si, because the FT filtered EXAFS shows at least three coordination shells. b) the intensity of the peaks related to the first three coordination shells is lower for por-Si than for c-Si: this can be due to the low size of the nano-crystals resulting in a very low coordination number, and/or to static disorder induced by the etching procedure. c) comparative measurements of XANES by TEY and XEOL confirm that the optical luminescence of por-Si originates from the inner of nanocrystals, not from surface-related processes. In particular, while TEY measurements are influenced by the presence on surface of oxidized states, the XEOL-XANES spectra show only the typical features of the crystalline silicon.

Light-emitting silicon nanocrystals embedded in SiO2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered transmission electron microscopy and by ab initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO2 is not sharp: an intermediate region of amorphous nature and variable composition links the crystalline Si with the amorphous stoichiometric SiO2. This region plays an active role in the light-emission process.

Bibliografia

G. Dalba, N. Daldosso, P. Fornasini, Grisenti, L. Pavesi, F. Rocca, G. Franzò, F. Priolo, F. Iacona
Chemical composition and local structure of plasma enhanced chemical vapor-deposited Si nanodots and their embedding silica matrix
App. Phys. Lett. 82 (2003) 889

N. Daldosso, M. Luppi, S. Ossicini, E. Degoli, R. Magri, G. Dalba, P. Fornasini, R. Grisenti, F. Rocca, L. Pavesi, S. Boninelli, F. Priolo, C. Spinella, and F. Iacona
Role of the interface region on the optoelectronic properties of silicon nanocrystals embedded in SiO2
Phys. Rev. B 68, 85327 (2003)

G. Vijaya Prakash, N. Daldosso, E. Degoli, F. Iacona, M. Cazzanelli, Z. Gaburro, G. Puker, G. Dalba, F. Rocca, E. Ceretta Moreira, G. Franzò, D. Paci, F. Priolo, C. Arcangeli, A.B. Filonov, S.Ossicini, L. Pavesi Structural and Optical Properties of Silicon Nanocrystals Grown by Plasma-Enhanced Chemical Vapor Deposition
J. Nanosci. Nanotech., 1, 1, 2001

G. Dalba, N. Daldosso, P. Fornasini, M. Grimaldi, R. Grisenti and F. Rocca
Evidence of X-ray absorption edge shift as a function of luminescence wavelength in porous silicon
Phys. Rev. B, 62, 9911, 2000

G. Dalba, P. Fornasini, R. Grisenti, N. Daldosso and F. Rocca
On the sensitivity of the x-ray excited optical luminescence to the local structure of the luminescent Si sites of porous silicon
Appl. Phys. Lett. 74, 10 1454 (1999)

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Local environment of  III - V group dopant impurities in a-Ge:H semiconductors

Chemical doping of amorphous semiconductors continues to be a major challenge to the understanding of these materials. The lack of long range order in the amorphous network raises questions about the validity of the effective mass and other concepts used to explain doping in crystalline semiconductors. The effect of increasing deep-state defect (dangling bond) density with increasing doping level observed in amorphous semiconductors is a further point of continuing controversy. Attempts to explain this behaviour in hydrogenated amorphous silicon (a-Si:H) have invoked equilibrium thermodynamic concepts. However, in view of the non-equilibrium character of amorphous film deposition processes, the validity of thermodynamic concepts in this case is questionable.

The microscopic state of the dopant impurity in the host network constitutes an additional important open question. Since in the amorphous network translational symmetry is absent, it is not clear what will be the coordination state adopted by a dopant impurity atom. In crystalline semiconductors, the periodic lattice of atoms constrains the impurity to have the same coordination as the host atom. However, in the amorphous network such constrain does not exist, and therefore it is likely that the coordination of the impurity will be determined by local energy minimization. It may be expected, therefore, that different impurities will have different distributions of coordination states, according to their atomic properties. The particular coordination of each impurity will, in turn, play an essential role in the determination of the observed doping-induced changes in the electrical conductivity, network structure and optical gap of the material.

EXAFS experiments aim at characterizing the local electronic and geometrical structure of III and V dopant elements in amorphous hydrogenated semiconductors in order to clarify the doping mechanism.The possibility of using a local structural probe like EXAFS gives the opportunity to test the microscopic model in a direct way, and to gain a deeper understanding of the doping process.

Up to now, by means of several experiments carried out at the synchrotron radiation facilities ESRF (Grenoble) and LURE (Paris), we have investigated the local environment of P, Ga, In,  Sb, Bi at several atomic concentrations.

We have found that the hydrogenated amorphous germanium films doped with column III metals Al, Ga, and In do not follow the predictions of the thermal equilibrium models of doping, which are supported by the modified 8-N Mott’s rule. In particular, we have shown that at low-concentration Ga and In atoms are coordinated as the host network, lowering the coordination at higher doping levels. This is due to the relaxation of the impurity-induced internal compressive stress caused by the difference between the atomic size of the four-fold coordinated impurities and the Ge atoms.

As for the investigation of the local order and coordination of Sb and Bi impurities in hydrogenated amorphous germanium thin films the evidence is that the thermal equilibrium model is not applicable even in this case. EXAFS results of Sb qualitatively follow the behavior of Ga and In impurities;  Bi shows a different behaviour. These findings have been found to be consistent with data on the transport properties of Sb- andBi-doped a-Ge:H films.

Bibliografia