Universite Paris - Sud 11 École

Phase change materials combine a pronounced contrast in resistivity and reflectivity between their disordered amorphous and ordered crystalline state with very fast crystallization kinetics. Typical phase-change alloys are composed out of the elements germanium, antimony and tellurium. Due to the exceptional combination of properties phase-change materials find already broad application in non-volatile optical memories such as CD, DVD or Bluray Disc. Furthermore, this class of materials demonstrates remarkable electrical transport phenomena in their disordered state, which have shown to be crucial for their application in electronic storage devices. The threshold switching phenomenon denotes the sudden decrease in resistivity beyond a critical electrical threshold field. The threshold switching phenomenon facilitates the phase transitions at practical small voltages. Below this threshold the amorphous state resistivity is thermally activated and is observed to increase with time. This effect known as resistance drift seriously hampers the development of multi-level storage applications desired to increase the storage capacity of phase-change memory devices. Hence, understanding the physical origins of threshold switching and resistance drift phenomena is crucial to improve non-volatile phase-change memories. Even though both phenomena are often attributed to localized defect states in the band gap, the defect state density in amorphous phase-change materials has remained little studied. Starting from a brief introduction of the physics of phase-change materials this thesis summarizes the most important models behind electrical switching and resistance drift with the aim to discuss the role of localized defect states. The centerpiece of this thesis is the investigation of defects state densities in different amorphous phase-change materials and electrical switching chalcogenides. On the basis of Modulated Photo Current (MPC) Experiments and Photothermal Deflection Spectroscopy, a sophisticated band model for the disordered phase of the binary phase-change alloy GeTe has been developed. By this direct experimental approach the band-model for a-GeTe is shown to consist of a shallow and deep defect band in addition to the band tail states. Furthermore, this study on a-GeTe has shown that the data analysis withinMPC experiments can be drastically improved. This thesis illustrates on the example of a-GeTe and a-Ge2Sb2Te5, that the problem of unphysical low values for the attempt-to-escape frequency of the order 108s−1 can be resolved by taking the temperature dependence of the band gap into account. iii te l-0 07 43 47 4, v er si on 1 19 O ct 2 01 2 In this work the defect state density of three different electrical switching alloys a-GeTe, a-Ge2Sb2Te5 and a-Ge15Te85 has been investigated. In a generation-recombination model the threshold field is expected to increase with increasing optical band gap and increasing mid gap-state density. However, a-GeTe, a-Ge2Sb2Te5 and a-Ge15Te85 show a large variation in their corresponding electrical threshold fields, which can be not understood by their change in optical band gap value alone. This observation raises the question of the role of localized defect states. MPC experiments reveal that the measured density of mid gap states is observed to decrease with decreasing threshold field known from literature. This result favors a generationrecombination model behind electrical switching in amorphous chalcogenides as originally proposed by Adler. To get a better understanding of resistance drift phenomena this study focuses on the evolution of resistivity on heating and ageing, activation energy of electronic conduction, optical band gap, defect state density, mechanical stress and nearest neighbor ordering in a-GeTe thin films. After heating the samples one hour at 140°C the activation energy for electric conduction increases by 30 meV, while the optical band gap increases by 60 meV. In addition to band gap opening MPC experiments on the same a-GeTe thin film at different sample ages have revealed complex trap kinetics. Ageing results in an increasing concentration of shallow defect states, whereas the detected density of deep mid gap states decreases. In contrast to both defect levels the conduction and valence band tail remained unaffected during resistance drift of a-GeTe thin films. These findings of the thesis clearly demonstrate the impact of band gap opening and defect annihilation on resistance drift in a-GeTe. Furthermore, drift phenomena observed in a-GeTe are compared to drift phenomena of covalent glasses known from literature. This comparison reveals wide differences between drift phenomena in chalcogenide and covalent glasses. Furthermore this thesis discusses the stoichiometric dependence of resistance drift phenomena in a-GeSnTe phase-change alloys. A systematic decrease in the amorphous state resistivity, activation energy for electric conduction, optical band gap and defect density is observed with increasing tin content resulting in a low resistance drift for tin rich compositions such as a-Ge2Sn2Te4. This study on GeSnTe systems demonstrates, that phase change alloys showing a more stable amorphous state resistivity are characterized by a low activation energy of electronic conduction. This finding found in GeSnTe alloys holds also true for GeSbTe and AgInSbTe systems. On the example of a-Ge2Sn2Te4 and a-GeTe exhibiting a strong resistance drift, the evolution of the amorphous state resistivity is shown to be closely linked to the relaxation of internal mechanical stresses resulting in an improving structural ordering of the amorphous phase. iv te l-0 07 43 47 4, v er si on 1 19 O ct 2 01 2