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Noble gases ͑Ar, Kr, and Xe͒ were trapped in an amorphous carbon matrix in the 1-11-GPa pressure range. Extended and near-edge x-ray-absorption spectroscopies indicate clustering of noble gases induced by the host matrix internal pressure. Simultaneously, the matrix pressure promotes a shift of the noble-gas core-level binding energy of ϳ1 eV. The Auger parameter reveals that both the initial state and the host relaxation terms contribute to the binding-energy shift. Ab initio calculations performed on an Ar 7 cluster and on Ar atoms clustered in aromatic molecules support the experimental findings.

Time resolved photoluminescence of porous silicon at room temperature was measured for several emission energies under 2 ns nitrogen laser excitation. For each emission energy studied there is a broad distribution of lifetimes extending over a few decades. The mean value of the distribution varies with the emission energy, from 3 (2.77 eV) to 50 μs (1.96 eV). The results can be explained by assuming a tunneling limited recombination mechanism between bands of localized states. We associate this behavior with a superficial disordered Si:O:H compound rather than with quantum confinement effects.

A review of the current status of research o n E r 3+ doped hydrogenated amorphous silicon a-Si:H is presented. Er has been introduced in a-Si:H and a-SiOx:H by ion implantation, co-sputtering and PECVD. In all cases, the characteristic atomic-like i n tra-4f 4 I 13=2 ! 4 I 15=2 photoluminescence emission at s 1.54 m is observed at room temperature. The Er 3+ luminescence probability is determined by the local neighborhood of the ions. Therefore, local probes like EXAFS and M ossbauer spectroscopy h a ve yielded very important information. A discussion of excitation processes, electroluminescence, and electronic doping e ects, is also presented. PACS Number s : 71.23. Cq, 73.61.Jc, 81.05.Gc I IntroductionThe global telecommunications network has experienced an enormous expansion in the last few years. The present bandwidth requirements are beyond the capability of conventional electronic technology. One solution for this problem has been the development o f photonics, the use of light for information processing and distribution. Silica-based ber optics have allowed a bandwidth increase of orders of magnitude relative to microwaves or cabled communications. Commercial systems are already irting with the terabit sec barrier on a single ber 1 . In the laboratory, 2.6 Tbit sec has been achieved over 120 km of ber 2 . Conventional silica optical bers present the minimum attenuation losses at wavelengths near 1.5 m, de ning the so-called third spectral window" the other two are respectively at 0.9 and 1.3 m An important c hallenge to materials and semiconductors scientists nowadays is to develop light sources and detectors operating at these wavelengths at low cost and high reliability. Moreover, it would be advantageous if the new devices would be compatible with the well established silicon microelectronic technology. Currently the only reliable light sources for ber optics are the III-V nearband edge lasers, whose high cost render large scale consumer applications prohibitive. An important alternative consists on the use of rare-earth based light emitters, which operate based on the atomic-like transitions within the incomplete 4f levels of these elements. A great e ort has been devoted to the study of rare-earth doped semiconductors 3, 4 , specially erbium doped silicon that emits light in the third spectral window range. The recent developments are inducing the emergence of a new eld of research, that of siliconbased optoelectronics 5 . The rare earth elements are distinguished by their incomplete internal 4f shell. When diluted in solid hosts, the rare earth atoms become almost always trivalent ions, losing the two 6 s and one 4f electrons. The 4f electrons are shielded from external elds by the two remaining electronic shells with larger radial extension 5s 2 5p 6 . The intra 4f electronic transitions are thus atomic-like, weakly dependent on the details of the local chemistry. H o wever, f , f transitions are electric dipole forbidden l = 0 in free ions. Only when the rare earth ions are incorporated in a soli...