Defects in amorphous phase-change materials

Understanding the physical origin of threshold switching and resistance drift phenomena is necessary for making a breakthrough in the performance of low-cost nanoscale technologies related to nonvolatile phase-change memories. Even though both phenomena of threshold switching and resistance drift are often attributed to localized states in the band gap, the distribution of defect states in amorphous phase-change materials (PCMs) has not received so far, the level of attention that it merits. This work presents an experimental study of defects in amorphous PCMs using modulated photocurrent experiments and photothermal deflection spectroscopy. This study of electrically switching alloys involving germanium (Ge), antimony (Sb) and tellurium (Te) such as amorphous germanium telluride (a-GeTe), a-Ge15Te85 and a-Ge2Sb2Te5 demonstrates that those compositions showing a high electrical threshold field also show a high defect density. This result supports a mechanism of recombination and field-induced generation driving threshold switching in amorphous chalcogenides. Furthermore, this work provides strong experimental evidence for complex trap kinetics during resistance drift. This work reports annihilation of deep states and an increase in shallow defect density accompanied by band gap widening in aged a-GeTe thin film

Epitaxy and characterization of InP/InGaAs tandem solar cells grown by MOVPE on InP and Si substrates

The integration of III-V multi-junction solar cells on Si substrates is currently one of the most promising possibilities to combine high photovoltaic performance with a reduction of the manufacturing costs. In this work, we propose a prospective study for the realization of an InP/InGaAs tandem solar cell lattice-matched to InP on a commercially available Si template by direct MOVPE growth. The InP top cell and the InGaAs bottom cell were firstly separately grown and optimized using InP substrates, which exhibited conversion efficiencies of 13.5% and 11.4%, respectively. The two devices were then combined in a tandem device by introducing an intermediate InP/AlInAs lattice-matched tunnel junction, showing an efficiency of 18.4%. As an intermediate step towards the realization of the tandem device on Si, the InP and InGaAs single junction solar cells were grown on top of a commercial InP/GaP/Si template. This transitional stage enabled to isolate and evaluate the effects of the growth of III-V on Si on the photovoltaic performance through the comparison with the aforementioned devices on InP. Each cell was electrically characterized by external quantum efficiency and dark and illuminated current-voltage under solar simulator. The material quality was also analyzed by means of X-ray diffraction, Atomic-Force Microscopy, Transmission Electron and Scanning Electron Microscopy. The III-V on Si devices showed efficiencies of 3.6% and 2.0% for the InP and InGaAs solar cells, respectively

Electronic techniques for the characterization of silicon thin films

LGEP 2011 ID = 928International audienc

Materials for photovoltaic applications

International audienc

Etude des paramètres de transport et des états de défaut dans les pérovskites en couches minces sous différents environnements - air ou vide - et après trempage à la lumière

Traduit par N. BacaerNous présentons quelques techniques de caractérisation avancées développées pour étudier les propriétés optoélectroniques des semi-conducteurs à couches minces et les appliquer aux couches de pérovskite. Ces techniques sont le réseau photoporteur en régime permanent (SSPG) et la spectroscopie de photocourant à transformée de Fourier (FTPS). Le SSPG a été développé pour étudier la longueur de diffusion ambipolaire des porteurs et le FTPS a été imaginé pour mesurer les variations du coefficient d'absorption en dessous du gap avec l'énergie lumineuse, donnant des informations sur les densités de défaut du gap responsable de cette absorption. Les potentialités de ces techniques sont d'abord détaillées puis illustrées par leur application aux pérovskites en couches minces. Pour étudier leur stabilité, ces films ont été exposés à différents environnements, air ou vide, et à l'état déposé ou après trempage à la lumière avec une lumière intense. Nous trouvons que la longueur de diffusion et la densité des états sont assez stables, même après un trempage lumineux, et suggérons que la dégradation des appareils exposés au soleil provient principalement de l'évolution des contacts au lieu de la pérovskite elle-même

Probing defect states in semiconductors by the modulated photocurrent technique

Séminaire à Institut de physique, Rosario, Argentine