TEM STUDY OF GRAIN BOUNDARIES OF POLYCRYSTALLINE SI FOR SOLAR CELLS

GRAIN BOUNDARIES IN POLYCRYSTALLINE SI HAVE BEEN STUDIED BY TEM. CLASSIFICATIONOF THE GBS HAS BEEN DONE ACCORDING TO THE CSL MODEL. MANY GBS WERE EXAMINED BY TWO METHODS. THE PREDOMINANT TYPE WAS THAT CHARACTERIZED BY CSLS WITH Σ=3Ν, WBEING INTEGRAL, AND WAS THE TILT GBS TYPE. THESE ARE FORMED EITHER AS ISOLATED BOUNDARIES, ENDING ON OTHER INTERFACES OR AS TRIPLE JUNCTIONS HAVING THE AS COMMON ROTATION AXIS. THE MOST COMMON GB IS THAT WHICH IS CHARACTERIZED BY THE Σ=3 {111} CSL. THE RIGID BODY TRANSLATION WHICH IS PRESENT IN THE Σ=3{211} TWIN HAS BEEN DETERMINED BY THE A-TYPE FRINGE SYSTEM ANALYSIS. THIS DETERMINATION LEAD TO THE PROPOSAL OF AN ASYMMETRIC ATOMISTIC MODEL FOR THE DESCRIPTION OF THE INTERFACE. THE Σ=3Ν CSLS PLAY A FUNDAMENTAL ROLE IN THE FORMATION OF MULTIPLE BOUNDARIES, SUCH AS THE Σ=3,39 CSLS, ΤΗΕ Σ=3,9,27 CSLS AND THE Σ=3,27,81 CSLS. THE ASYMMETRIC BOUNDARIES HAVE BEEN EXAMINED IN THESE MULTIPLE BOUNDARIES IN ORDER TO BE COMPARED WITH THE ASYMMETRIC ONES. THE OBSERVATIONS OF THIS WORK SUPPORT THE CONCLUSION THAT COVALENT MATERIALS, LIKE SI, TEND TO FAVOUR COINCIDENCE-TYPE INTERFACES CONNECTED BY SYMMETRY OPERATIONS.ΜΕ ΗΛΕΚΤΡΟΝΙΚΗ ΜΙΚΡΟΣΚΟΠΙΑ ΔΙΕΡΧΟΜΕΝΗΣ ΔΕΣΜΗΣ ΜΕΛΕΤΗΘΗΚΑΝ ΟΙ ΔΙΑΧΩΡΙΣΤΙΚΕΣ ΕΠΙΦΑΝΕΙΕΣ ΤΟΥ ΠΟΛΥΚΡΥΣΤΑΛΛΙΚΟΥ SI, ΤΟ ΟΠΟΙΟ ΧΡΗΣΙΜΟΠΟΙΕΙΤΑΙ ΓΙΑ ΤΗΝ ΚΑΤΑΣΚΕΥΗ ΗΛΙΑΚΩΝ ΣΤΟΙΧΕΙΩΝ. Ο ΧΑΡΑΚΤΗΡΙΣΜΟΣ ΚΑΙ Η ΤΑΞΙΝΟΜΗΣΗ ΤΟΥΣ ΕΓΙΝΕ ΜΕ ΒΑΣΗ ΤΟ ΠΡΟΤΥΠΟ ΤΟΥ CSL. ΓΙΑ ΤΟΝ ΠΡΟΣΔΙΟΡΙΣΜΟ ΤΗΣ ΣΧΕΣΗΣ ΣΤΡΟΦΗΣ ΤΩΝ ΚΡΥΣΤΑΛΛΩΝ ΧΡΗΣΙΜΟΠΟΙΗΘΗΚΑΝΔΥΟ ΜΕΘΟΔΟΙ. ΑΠΟ ΤΙΣ ΠΑΡΑΤΗΡΗΣΕΙΣ ΠΡΟΕΚΥΨΕ ΟΤΙ ΟΙ ΣΥΧΝΟΤΕΡΕΣ ΠΑΡΑΤΗΡΟΥΜΕΝΕΣ ΔΙΑΧΩΡΙΣΤΙΚΕΣ ΕΠΙΦΑΝΕΙΕΣ ΣΤΟ ΠΟΛΥΚΡΥΣΤΑΛΛΙΚΟ SI, ΤΥΠΟΥ SILSO, ΑΝΗΚΟΥΝ ΣΤΗΝ ΟΙΚΟΓΕΝΕΙΑ ΤΩΝ Σ=3NTILT ΔΙΑΧΩΡΙΣΤΙΚΩΝ ΕΠΙΦΑΝΕΙΩΝ, ΟΠΟΥ Ν ΑΚΕΡΑΙΟΣ ΑΡΙΘΜΟΣ. ΑΥΤΕΣΕΜΦΑΝΙΖΟΝΤΑΙ ΕΙΤΕ ΩΣ ΜΕΜΟΝΩΜΕΝΕΣ, ΠΕΡΑΤΟΥΜΕΝΕΣ ΣΕ ΑΛΛΕΣ ΤΥΧΑΙΕΣ ΔΙΕΠΙΦΑΝΕΙΕΣ, ΕΙΤΕ ΩΣ ΣΥΜΠΛΕΓΜΑΤΑ ΜΕ ΚΟΙΝΟ ΑΞΟΝΑ ΤΟΝ . ΕΓΙΝΕ ΑΝΑΛΥΤΙΚΗ ΜΕΛΕΤΗ ΤΗΣ ΜΗ ΣΥΝΑΦΟΥΣ Σ=3 ΔΙΔΥΜΙΑΣ ΜΕ ΕΠΙΠΕΔΟ ΣΥΝΕΠΑΦΗΣ ΤΟ {211} ΚΑΙ ΠΡΟΣΔΙΟΡΙΣΤΗΚΕ ΤΟ ΔΙΑΝΥΣΜΑΤΗΣ ΜΕΤΑΤΟΠΙΣΗΣ ΜΕΤΑΞΥ ΤΩΝ ΔΥΟ ΚΡΥΣΤΑΛΛΙΤΩΝ ΤΟΥ ΔΙΚΡΥΣΤΑΛΛΟΥ ΜΕ ΤΗΝ ΑΝΑΛΥΣΗ ΤΩΝ ΚΡΟΣΣΩΝ ΤΥΠΟΥ-Α. ΠΡΟΤΑΘΗΚΕ ΕΝΑ ΑΣΥΜΜΕΤΡΟ ΑΤΟΜΙΣΤΙΚΟ ΠΡΟΤΥΠΟ ΓΙΑ ΤΗΝ ΠΕΡΙΓΡΑΦΗΤΗΣ ΕΝΔΟΕΠΙΦΑΝΕΙΑΣ ΟΙ ΑΣΥΜΜΕΤΡΕΣ ΕΝΔΟΕΠΙΦΑΝΕΙΕΣ ΕΞΕΤΑΣΤΗΚΑΝ ΜΕΣΑ ΣΤΙΣ ΣΥΜΦΥΣΕΙΣ ΠΟΛΛΑΠΛΩΝ ΚΡΥΣΤΑΛΛΙΤΩΝ ΩΣΤΕ, ΜΕ ΤΗ ΣΥΓΚΡΙΣΗ ΤΟΥΣ, ΝΑ ΣΧΗΜΑΤΙΣΤΕΙ ΜΙΑ ΠΛΗΡΕΣΤΕΡΗ ΕΙΚΟΝΑ ΤΗΣ ΔΟΜΗΣ ΚΑΙ ΤΗΣ ΚΑΤΑΣΤΑΣΗΣ ΤΟΥΣ

Energetics of Interfaces and Strain Partition in GaN/AlN Pseudomorphic Superlattices

We present the results of a twofold experimental and computational study of (0001) GaN/AlN multilayers forming pseudomorphic superlattices. High-Resolution Transmission Electron Microscopy (HRTEM) shows that heterostructures with four c-lattice parameters thick GaN Quantum Wells (QW) are misfit-dislocation free. Accurate structural data are extracted from HRTEM images via a new methodology optimizing the residual elastic energy stored in the samples. Total energy calculations are performed with several models analogous to the experimental QWs with increasing thicknesses of GaN, whereas this of the AlN barrier is kept fixed at n = 8 c-lattice parameters. With vanishing external stresses, minimum energy configurations of the studied systems correspond to different strain states. Linear elasticity accurately yields the corresponding lattice parameters, suppressing the need for on-purpose total energy calculations. Theoretically justified parabolic fits of the excess interfacial energy yield the values of interfacial stress and elastic stiffness as functions of the GaN QW thickness. Total species-projected densities of states and gap values extracted from there allow deciphering the effect of the evolving strain on the electronic structure of the superlattice. It is found that the gap energy decreases linearly with increasing the strain of the QW. These results are briefly discussed in the light shed by previous works from the literature

Effect of edge threading dislocations on the electronic structure of InN

The open issue of the n-type conductivity and its correlation to threading dislocations (TDs) in InN is addressed through first principles calculations on the electronic properties of a-edge TDs. All possible dislocation core models are considered (4-, 5/7-, and 8-atom cores) and are found to modify the band structure of InN in a distinct manner. In particular, nitrogen and indium low coordinated atoms in the eight-atom core induce states near the valence band maximum and above the conduction band minimum, respectively. The formation of a nitrogen–nitrogen “wrong” bond is observed at the 5/7-atom core resulting in a state inside the band gap. The 4- and 5/7-atom cores induce occupied states resonant in the conduction band due to In–In strain induced interactions and wrong bonds, respectively. These occupied states designate TDs as a source of higher electron concentrations in InN and provide direct evidence that TDs contribute to its inherent n-type conductivity

Cu3N/Cu2O core–shell nanowires: growth and properties

Raman, XRD and TEM characterization of Cu3N/Cu2O nanowire

Reconstructions and electronic structure of (11-22) and (11-2-2) semipolar AlN surfaces

The energetics, atomic geometry, and electronic structure of semipolar (11-22) and (11-2-2) AlN surfaces are investigated employing first principles calculations. For metal-rich growth conditions, metallic reconstructions are favoured on both polarity surfaces. For N rich to moderate Al rich conditions, the (11-22) planes promote semiconducting reconstructions having 2 × 2 or c(2 × 2) periodicity. In contrast, under the particular range of the Al chemical potential the (11-2-2) surfaces stabilize reconstructions with excess metal and it is only at the extreme N rich limit that the semiconducting c(2 × 2) N adatom structure prevails. The present study reveals that the reconstructed (11-22) surfaces do not contain steps in contrast to (11-2-2) where surface steps are inherent for N rich to moderate metal rich growth conditions and may result in intrinsic step-flow growth and/or growth of parasitic semipolar orientations

Electronic structure of 1/6⟨20-23⟩ partial dislocations in wurtzite GaN

The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {11-20} (a-plane) and {1-100} (m-plane) grown GaN. Their importance is established by recent experimental results, which correlate the partial dislocations (PDs) bounding I1 BSFs to the luminescence characteristics of GaN. PDs are also found to play a critical role in the alleviation of misfit strain in hetero-epitaxially grown nonpolar and semipolar films. In the present study, the energetics and the electronic structure of twelve edge and mixed 1/6⟨20-23⟩ PD configurations are investigated by first principles calculations. The specific PD cores of the dislocation loop bounding the I1 BSF are identified for III-rich and N-rich growth conditions. The core structures of PDs induce multiple shallow and deep states, attributed to the low coordinated core atoms, indicating that the cores are electrically active. In contrast to edge type threading dislocations no strain induced states are found

Structure, stability and mechanical performance of AlN: ag nanocomposite films

Nanocomposite films consisting of a hard AlN matrix incorporating soft Ag inclusions (AlN:Ag) and which are suitable for protective coatings are presented. The growth has been performed using Pulsed Laser Deposition and the film structural properties, such as nanoparticle size and distribution, were studied in relation to the growth parameters, such as metal content and PLD working pressure and laser power. High resolution transmission electron microscopy and nanoindentation were employed in order to determine the film composition, inclusions' crystal structure and mechanical properties respectively. The employed Ag nanoparticles had average sizes ranging between 3–10 nm and were clearly separated by the matrix material. The critical parameters, which determine the nanoparticle size and distribution, and the decisive role of the latter on the mechanical performance of AlN:Ag nanocomposite films are establishe

Screw threading dislocations in AlN: Structural and electronic properties of In and O doped material

Density functional theory calculations were performed on undoped AlN screw threading dislocations (TDs) as well as TDs doped by indium and oxygen, prompted by integrated experiments through transmission electron microscopy and spectroscopic techniques demonstrating enhanced In and O concentrations in screw dislocation cores. It is revealed that screw TDs act as conduction pathways to charge carriers, introducing multiple levels in the bandgap due to overstrained, dangling, and “wrong” bonds formed even in the undoped cores. The presence of impurities and especially metallic In elevates the metal-like electronic structure of the distorted material and promotes the conductivity along the dislocation line. Hence screw dislocations in AlN are established as highly prominent conductive nanowires in semiconducting thin films and prospects for novel, highly functional nano-device materials through exploitation of screw TDs are attested