Local Order Around Er3 Ions in thin Silicon Oxide Layers Grown in Si by MBE

Er doped Si represent an interesting class of materials for the realization of light emitting devices based on Si technology. The local structure around Er ions is known to strongly affect the luminescence properties of the Rare Earth. In this work the Er site in Er + O doped silicon samples prepared by Molecular Beam Epitaxy has been investigated by x-ray Absorption Spectroscopy at the Er LIII edge. Samples with different preparation parameters such as the O/Er ratio and thermal treatments were investigated. Data were fitted using a model consisting in a first shell of O atoms and a second shell of Si atoms and accounting for multiple scattering effects. The rare earth is found to be linked to 5–6 O atoms at around 2.23 Å whereas a well defined Er-O-Si bond angle of ≈136 deg is evidenced

Short range chemical order and local lattice distortion in a compositionally complex alloy

This work presents an X ray absorption spectroscopy study on a single phase state of the Al8Cr17Co17Cu8Fe17Ni33 compositionally complex alloy, focused on the local crystal structure around each alloying element. The comparison of 1st shell bond lengths, obtained by the analysis of extended X ray absorption fine structure EXAFS measured at the K edges of each alloying element, indicates that Al8Cr17Co17Cu8Fe17Ni33 crystallizes in a distorted arrangement of an fcc lattice. A modest bond length dependence of the alloying elements with increasing atomic number is observed, with minima and maxima at Cr Co, and Al Cu, respectively. 1st shell bond lengths spread over 0.03A ; consequently, such variations cannot result in lattice distortions greater than 0.04A . EXAFS results clearly indicate short range order in the alloy pairing of Al with Ni and Cu is favored, correlating well with a g precipitate composition Al Ni Cu rich reported in previous work, while Al Cr bonding is unfavored and no Al Al pairs are observed. Electronic structure information was obtained through comparison between near edge regions of alloying elements and corresponding pure metals. Intensity comparison of K edge features agree with a charge variation of p states in Al8Cr17Co17Cu8Fe17Ni33, where Ni and Cu act as p states electron acceptors, suggesting an orbital hybridization with Al, responsible for a shrinkage in Al metallic radius in the alloy by 0.17

Short range versus long range structure in Cu In,Ga Se2, Cu In,Ga 3Se5, and Cu In,Ga 5Se8

The Cu poor phases Cu In,Ga 3Se5 and Cu In,Ga 5Se8 play an important role both for understanding the Cu In,Ga Se material system and for growing high efficiency Cu In,Ga Se2 thin film solar cells. Using extended X ray absorption fine structure spectroscopy, we have studied the element specific short range structure of Cu In,Ga Se2, Cu In,Ga 3Se5, and Cu In,Ga 5Se8 alloys spanning the entire compositional range. The materials feature different local atomic arrangements and the element specific average bond lengths remain nearly constant despite significant changes of the lattice constants with increasing In to Ga ratio and decreasing Cu content. In particular, the average bond lengths of Cu Se and Ga Se are almost identical while the average In Se bond length is significantly longer in all three phases. The distance between lattice sites with mixed site occupation therefore corresponds to the weighted average of different element specific bond lengths rather than to the individual bond lengths themselves. Furthermore, the increasing number of vacancies with decreasing Cu content lead to both a significant unit cell contraction and a slight bond length expansion. The crystallographic long range structure and the element specific short range structure thus describe different structural aspects that are certainly interrelated but obviously not identica