Picosecond electric-field-induced threshold switching in phase-change materials

Many chalcogenide glasses undergo a breakdown in electronic resistance above a critical field strength. Known as threshold switching, this mechanism enables field-induced crystallization in emerging phase-change memory. Purely electronic as well as crystal nucleation assisted models have been employed to explain the electronic breakdown. Here, picosecond electric pulses are used to excite amorphous Ag$_4$In$_3$Sb$_{67}$Te$_{26}$. Field-dependent reversible changes in conductivity and pulse-driven crystallization are observed. The present results show that threshold switching can take place within the electric pulse on sub-picosecond time-scales - faster than crystals can nucleate. This supports purely electronic models of threshold switching and reveals potential applications as an ultrafast electronic switch.Comment: 6 pages manuscript with 3 figures and 8 pages supplementary materia

Synthesis and coordination chemistry of 2-(di-2-pyridylamino)pyrimidine; structural aspects of spin crossover in an Fe(II) complex

This paper was accepted on February 26 20122-(Di-2-pyridylamino)pyrimidine (L), a potentially ditopic tetradentate ligand, was synthesized from commercially available di-2-pyridylamine and 2-chloropyrimidine. Despite being capable of bridging two metal atoms with bidentate chelation of both metal centres, L prefers to chelate or bridge through the more basic pyridyl donors of the di-2-pyridylamine moiety. Mononuclear trans-[Fe(NCS)2(L)2] and [Cu(L)2(H2O)](BF4)2•H2O complexes, and a discrete [Ag2(L)4](PF6)2 metallomacrocycle were isolated and structurally characterized by X-ray crystallography. A mononuclear palladium complex [PdCl2(L)]•(solvate), where solvate = ½H2O or CH2Cl2, was also readily obtained in 71% yield. One example of the ligand acting as a bis(bidentate) bridging ligand was observed in a dinuclear [(PdCl2)2(L)]•¾H2O complex that was obtained only in very low yield (ca. 3%) from the reaction that produced [PdCl2(L)]•½H2O. trans-[Fe(NCS)2(L)2] undergoes a temperature dependent HS-LS (HS = high spin; LS = low spin) crossover at ca. 205 K that was 2 observed by X-ray crystallography and magnetic measurements and attempts were made to understand the structural basis of this process. Despite efforts to isolate examples of L bridging two iron(II) centres, only the mononuclear trans-[Fe(NCS)2(L)2] species could be obtained.Rachel S. Crees, Boujemma Moubaraki, Keith S. Murray, and Christopher J. Sumb

Progress in hydrogenation of multicrystalline silicon solar cells

We focus on hydrogenation by firing a double sided PECVD SiN on EFG and block cast mc wafers. Compared to our standard high-efficiency process with a PECVD SiN on the front side and a full area Al-BSF on the rear side we replaced the full rear side Al-BSF by a SiO/SiN stack with a locally alloyed Al-BSF. In contrast to the local firing at the laser fired contacts approach, the whole wafer is heated to release hydrogen from the SiN into the bulk from both sides alloying the Al locally at the rear side. This double sided hydrogenation gives a better bulk passivation compared to the standard single sided hydrogenation. Especially areas of lower quality improve by the double sided hydrogenation, made visible by LBIC measurements. With this contribution we try to give new insights to the wide field of hydrogenation.publishe

Bulk passivation in silicon ribbons : a lifetime study for an enhanced high efficiency process

The effectiveness of hydrogenation either by deposition plus firing of a PECVD SiN layer in a conventional belt furnace or by remote H-plasma was compared quantitatively using spatially resolved lifetime measurements for EFG and string ribbon. Additionally the effect of a preceding phosphorous gettering on the hydrogenation and the presence of a screen printed rear side aluminum during firing was analyzed. Wafer areas with the presence of a rear side aluminum show additional lifetime improvements for both hydrogenation methods probably due to a gettering effect. With preceding P-gettering hydrogenation by SiN deposition plus firing is superior to remote H-plasma. A synergetic effect of a rear side aluminum as described elsewhere is not obtained. First high efficiency EFG solar cells using a PECVD SiN fired in a conventional belt furnace were processed with efficiencies in the 17-18% range

Hydrogen passivation of extended defects in multicrystalline silicon solar cells

Two methods of hydrogen bulk passivation in multicrystalline ingot silicon solar cells were compared. After POCl3-emitter diffusion, four different solar cell types were made out of neighbouring wafers. The first wafer was not hydrogen passivated. The second wafer was passivated by firing of hydrogenated silicon nitride (SiN:H). On the third wafer, a microwave induced remote hydrogen plasma (MIRHP) was applied. The fourth wafer was passivated by both techniques. On all wafers, the recombination activity of dislocations was measured by the correlation of light beam induced current (LBIC) with dislocation density topography. Recombination at grain boundaries was examined using LBIC line scans. The spatial resolution of the measurements was 12.5 μm.publishe