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Density functional theory simulations are used to identify the structural factors that define the material properties of ovonic threshold switches (OTS). They show that the nature of mobility-gap trap states in amorphous Ge-rich Ge50Se50 is related to Ge-Ge bonds, whereas in Se-rich Ge30Se70 the Ge valence-alternating-pairs and Se lone-pairs dominate. To obtain a faithful description of the electronic structure and delocalization of states, it is required to combine hybrid exchange-correlation functionals with large unit-cell models. The extent of localization of electronic states depends on the applied external electric field. Hence, OTS materials undergo structural changes during electrical cycling of the device, with a decrease in the population of less exothermic Ge-Ge bonds in favor of more exothermic Ge-Se. This reduces the amount of charge traps, which translates into coordination changes, an increase in mobility-gap, and subsequently changes in the selector-device electrical parameters. The threshold voltage drift process can be explained by natural evolution of the nonpreferred Ge-Ge bonds (or "chains"/clusters thereof) in Ge-rich GexSe1-x. The effect of extrinsic doping is shown for Si and N, which introduce strong covalent bonds into the system, increase both mobility-gap and crystallization temperature, and decrease the leakage current.

In this work, for the first time, a set of two-and three-dimensional (3D) analysis techniques are combined to clarify the nature of resistive switching (RS) in state-of-the-art TiO2-based vacancy modulated conductive oxide (VMCO) memory. (1) A non-filamentary switching mechanism is observed. (2) The role of oxygen incorporation and motion in the TiO2 is demonstrated. (3) The oxygen profile inside scaled cells is measured and a RS-model based on the modulation of oxygen inside the stack is proposed. In addition, we perform the tomographic analysis of fully-fabricated devices with Scalpel SPM, thus probing in 3D the entire stack and the contribution of TiO2 grain boundaries (GBs) to the switching operations. Finally, devices failed by breakdown (BD) during cycling are characterized, identifying the formation of parasitic filaments as root-cause of the failure.

The influence of Ni thickness on the formation of Nickel silicides was systematically investigated between 0 and 15nm. Annealing thickness gradients distinguishes films that agglomerate (>5nm) and films that are morphologically stable (

A way to lower the manufacturing cost of Cu(In,Ga)Se2 (CIGS) thin film solar cells is to use flexible polymer substrates instead of glass substrates. Because such substrates require a low temperature during absorber deposition, the efficiency of the cells remains slightly lower (18.7%) compared to CIGS on glass substrates (20.3%). Partial compensation of this efficiency loss might be accomplished by Sb doping of the absorber, which is reported to have a positive effect on the morphology of this layer. In this work the defect structure of Sb doped CIGS solar cells is investigated using optical and electrical spectroscopic techniques. Experiments were performed on cells deposited on soda lime glass substrate, adding a thin Sb layer (8, 12 nm) onto the Mo back contact prior to the CIGS absorber deposition. The results are compared with those for cells without Sb doping using the same process. Fourier-Transform near infrared photocurrent measurements in the 10–300K range demonstrate that the band gap of Sb-doped samples is larger than for undoped samples. Photoluminescence spectra in the 5–100K region provide information on shallow-level defects. Deep-Level Transient Spectroscopy spectra of Sb-doped cells exhibit two features not encountered for non-doped cells: 1) a peak at lower temperature than the N1 signal and 2) incomplete charge carrier freeze-out down to 8 K. While the first result appears to be the fingerprint of an extra non-Ohmic contact in the solar cell structure, the second suggests the introduction of a very shallow acceptor by Sb doping. As a salient feature one can accurately monitor the partial hole freeze-out in the 40-60 K range and determine the signature of the intrinsic defects that provide the p-type conductivity of the CIGS absorber using Admittance Spectroscopy.

Vanadium dioxide (VO2) is an intriguing material due to its semiconductor-metal transition (SMT). During this transition, which occurs near 67°C, electrical as well as optical properties change drastically. Possible applications include thermochromic windows, and memories or switches in micro- and optoelectronics. Although atomic layer deposition (ALD) is gaining importance for some of these applications, the growth of VO2 with this technique is not obvious, since in most cases V2O5 is obtained. In our previous work we presented ALD growth of VO2 by using Tetrakis[EthylMethylAmino]Vanadium and ozone at a temperature of 150°C [1]. XPS revealed the 4+ oxidation state of vanadium, indicating growth of VO2. Post-ALD thermal processing proved essential to crystallize the VO2 in the desired tetragonal phase (R). In this work we present the influence of the oxygen partial pressure on phase formation during such thermal processes. Additionally the influence of film thickness and annealing temperature on the post-annealing properties were studied, including morphology and SMT characteristics. During thermal processing a minimum oxygen partial pressure of approximately 1 Pa is indispensable to form crystalline VO2 (R) (figure 1). Oxygen partial pressures above 2 Pa show an intermediate monoclinic phase (B), which transforms to VO2 (R) at higher temperatures. At a value of 35 Pa this VO2 (B) phase finally transforms to V6O13 instead of VO2 (R). For very thin films, the thermal post-processing may result in agglomeration of the VO2 layers on the SiO2 substrate. Samples with a film thickness above 20nm show a typical resistivity ratio during the SMT of more than 2 orders of magnitude when annealed in the range 450°C to 500°C. For thinner films or higher annealing temperatures the resistivity ratio is suppressed and an overall increased resistivity is observed due to agglomeration (figure 2).