Real-time X-ray Diffraction Measurements Research Articles

In Situ Study of the Interface-Mediated Solid-State Reactions during Growth and Postgrowth Annealing of Pd/a-Ge Bilayers.

Ohmic or Schottky contacts in micro- and nanoelectronic devices are formed by metal-semiconductor bilayer systems, based on elemental metals or thermally more stable metallic compounds (germanides, silicides). The control of their electronic properties remains challenging as their structure formation is not yet fully understood. We have studied the phase and microstructure evolution during sputter deposition and postgrowth annealing of Pd/a-Ge bilayer systems with different Pd/Ge ratios (Pd:Ge, 2Pd:Ge, and 4Pd:Ge). The room-temperature deposition of up to 30 nm Pd was monitored by simultaneous, in situ synchrotron X-ray diffraction, X-ray reflectivity, and optical stress measurements. With this portfolio of complementary real-time methods, we could identify the microstructural origins of the resistivity evolution during contact formation: Real-time X-ray diffraction measurements indicate a coherent, epitaxial growth of Pd(111) on the individual crystallites of the initially forming, polycrystalline Pd2Ge[111] layer. The crystallization of the Pd2Ge interfacial layer causes a characteristic change in the real-time wafer curvature (tensile peak), and a significant drop of the resistivity after 1.5 nm Pd deposition. In addition, we could confirm the isostructural interface formation of Pd/a-Ge and Pd/a-Si. Subtle differences between both interfaces originate from the lattice mismatch at the interface between compound and metal. The solid-state reaction during subsequent annealing was studied by real-time X-ray diffraction and complementary UHV surface analysis. We could establish the link between phase and microstructure formation during deposition and annealing-induced solid-state reaction: The thermally induced reaction between Pd and a-Ge proceeds via diffusion-controlled growth of the Pd2Ge seed crystallites. The second-phase (PdGe) formation is nucleation-controlled and takes place only when a sufficient Ge reservoir exists. The real-time access to structure and electronic properties on the nanoscale opens new paths for the knowledge-based formation of ultrathin metal/semiconductor contacts.

Changes of the molecular structure in organic thin film transistors during operation

Thin films of organic semiconductors have been widely studied at different length scales for improving the electrical response of devices based on them. Hitherto, a lot of knowledge has been gained about how molecular packing, morphology, grain boundaries, and defects affect the charge transport in organic thin film transistors. However, little is known about the impact of an electric field on the organic semiconductor microstructure and the consequent effect on the device performances. To fill this gap, we investigated the evolution of the structure of pentacene thin film transistors during device operation by in situ real time X-ray diffraction measurements and theoretical calculations. We observed for the first time the occurrence of a reversible structural strain taking place during the bias application mainly due to reorientation at the terrace edges of monolayer islands under the effect of electrical field. Strain exhibits the same trend of the threshold voltage hinting to the existence of a direct ...

Changes of the Molecular Structure in Organic Thin Film Transistors during Operation

Thin films of organic semiconductors have been widely studied at different length scales for improving the electrical response of devices based on them. Hitherto, a lot of knowledge has been gained about how molecular packing, morphology, grain boundaries, and defects affect the charge transport in organic thin film transistors. However, little is known about the impact of an electric field on the organic semiconductor microstructure and the consequent effect on the device performances. To fill this gap, we investigated the evolution of the structure of pentacene thin film transistors during device operation by in situ real time X-ray diffraction measurements and theoretical calculations. We observed for the first time the occurrence of a reversible structural strain taking place during the bias application mainly due to reorientation at the terrace edges of monolayer islands under the effect of electrical field. Strain exhibits the same trend of the threshold voltage hinting to the existence of a direct correlation between the phenomenon of bias stress and the structural modification.

Influence of interfacial reaction upon atomic diffusion studied by in situ Auger electron spectroscopy

Pd reaction on Si(001) substrate was studied under ultra-high vacuum using in situ real time Auger electron spectroscopy (AES). Comparison with in situ real time X-ray diffraction measurements performed on the same samples showed that the AES intensity variations observed during isothermal annealing give information concerning the kinetics of three consecutive phenomena: i) Pd2Si growth, ii) Pd dissolution in Pd2Si, and iii) Si surface segregation on Pd2Si. The kinetics related to these three phenomena allowed to determine, in the same samples, the average effective atomic diffusion during growth, and the Si and Pd self-diffusion after growth in Pd2Si. Atomic transport during growth was found to be several orders of magnitude faster than Si and Pd diffusion after growth under the same experimental conditions. This effect is assumed to be related to point-defect injection in the Pd2Si layer during growth, due to Pd–Si reaction at Pd/Pd2Si and Pd2Si/Si interfaces.

Real-time crystalline orientation measurements during low-density polyethylene blown film extrusion using wide-angle X-ray diffraction

Real-time wide-angle X-ray diffraction studies were successfully used to investigate the effect of film blowing process parameter on the crystalline orientation development during the blown film extrusion of low-density polyethylene. Azimuthal distribution scans showed the evolution of crystalline orientation in the bubble from an isotropic state to an oriented state as inferred from (110) and (200) planes. These real-time X-ray diffraction measurements in a blown film line are consistent with prior observations using polarized Raman spectroscopy (Gururajan and Ogale, J. Raman Spectrosc., 40, 212 (2009)) and small-angle light scattering (Bullwinkel et al., Int. Polym. Proc., 16, 41 (2001)) that significant molecular orientation takes place past the frost-line height, even after the blown film diameter is locked into place. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Real-time X-ray diffraction measurements of structural dynamics and polymorphism in diindenoperylene growth

We investigate the temperature-dependent polymorphs in diindenoperylene (DIP) thin films on sapphire and silicon oxide substrates using in situ X-ray scattering. On both substrates the DIP unit cell is very similar to the high-temperature phase of bulk crystals, with the substrate stabilising this structure well below the temperature where a phase transition to a low-temperature phase is observed in the bulk. Lowering the substrate temperature for DIP growth leads to a change in molecular orientation and an additional polymorph appears, with both these effects being more pronounced on sapphire as compared to silicon oxide. Using real-time reciprocal-space mapping we observe an expansion of the in-plane unit cell during DIP growth, which may be due to changes in molecular orientation as well as strain in the first monolayers.

Real-time x-ray diffraction measurements of shocked polycrystalline tin and aluminum

A new, fast, single-pulse x-ray diffraction (XRD) diagnostic for determining phase transitions in shocked polycrystalline materials has been developed. The diagnostic consists of a 37-stage Marx bank high-voltage pulse generator coupled to a needle-and-washer electron beam diode via coaxial cable, producing line and bremsstrahlung x-ray emission in a 35 ns pulse. The characteristic K(alpha) lines from the selected anodes of silver and molybdenum are used to produce the diffraction patterns, with thin foil filters employed to remove the characteristic K(beta) line emission. The x-ray beam passes through a pinhole collimator and is incident on the sample with an approximately 3 x 6 mm(2) spot and 1 degrees full width half maximum angular divergence in a Bragg-reflecting geometry. For the experiments described in this report, the angle between the incident beam and the sample surface was 8.5 degrees . A Debye-Scherrer diffraction image was produced on a phosphor located 76 mm from the polycrystalline sample surface. The phosphor image was coupled to a charge-coupled device camera through a coherent fiber-optic bundle. Dynamic single-pulse XRD experiments were conducted with thin foil samples of tin, shock loaded with a 1 mm vitreous carbon back window. Detasheet high explosive with a 2-mm-thick aluminum buffer was used to shock the sample. Analysis of the dynamic shock-loaded tin XRD images revealed a phase transformation of the tin beta phase into an amorphous or liquid state. Identical experiments with shock-loaded aluminum indicated compression of the face-centered-cubic aluminum lattice with no phase transformation.

A portable ultrahigh vacuum organic molecular beam deposition system for in situ x-ray diffraction measurements

A portable UHV molecular beam deposition system has been developed for synthesis, in situ, and real-time x-ray diffraction measurements of organic thin films, multilayers, and superlattices. The system has been optimized for small size, while still incorporating full features necessary to achieve thin film growth under molecular beam epitaxy (MBE) conditions. It can be used independently for thin film growth, or it can be transported and mounted on standard diffractometers. Additionally, it can be docked to a stationary multipurpose MBE growth system for sample transfer, thus permitting more extensive growth and characterization. The design and performance of this system are reported, with emphasis on modifications required to deposit organic materials. To demonstrate the capabilities for real-time x-ray scattering experiments, some preliminary results of a study of epitaxial growth of 3,4,9,10-perylene-tetracarboxylic dianhydride on Ag(111) substrates are given.

Real-Time X-Ray Diffraction Measurements of the Phase Transition in KCl Shocked along [100

X-ray diffraction measurements and analyses were developed and used to examine the phase transition in KCl shocked to 7 GPa. Diffraction data were obtained below and above the transition stress, and related quantitatively to macroscopic compression in the two phases. Interplanar spacing measurements revealed isotropic compression of the unit cell. Above the transition stress, a diffraction peak from the (110) planes in phase II was observed consistently and the orientation of the transformed crystal structure was determined with respect to the phase I structure. This determination provides a mechanism for the atomic rearrangement from the rocksalt to the cesium chloride structure in KCl shocked along [100].

In situ, real-time measurement of wing tilt during lateral epitaxial overgrowth of GaN

By performing in situ, real-time x-ray diffraction measurements in the metalorganic chemical-vapor deposition environment, we have directly observed the emergence and evolution of wing tilt that occurs during the lateral overgrowth of GaN from stripes patterned in a SiO2 mask. This was done by repeatedly performing line scans through the 101̄3 peak in the direction perpendicular to the [101̄0]GaN stripe direction. The wing tilt developed as soon as the wings started forming, and increased slightly thereafter to reach a value of ∼1.19° after 3600 s of growth. Upon cooldown to room temperature, the tilt increased to ∼1.36°, indicating that thermally induced stresses during cooldown have only a small effect on wing tilt. However, changes in mask density, composition, and stress state during early lateral overgrowth must be considered as possible origins of wing tilt.

Experimental developments to obtain real-time x-ray diffraction measurements in plate impact experiments

An experimental facility was developed to obtain real-time, quantitative, x-ray diffraction data in laboratory plate impact experiments. A powder gun, to generate plane wave loading in samples, was designed and built specifically to permit flash x-ray diffraction measurements in shock-compression experiments. Spatial resolution and quality of the diffracted signals were improved significantly over past attempts through partial collimation of the incident beam and the use of two-dimensional detectors to record data from shocked crystals. The experimental configuration and synchronization issues are discussed, and relevant details of the x-ray system and the powder gun are described. Representative results are presented from experiments designed to determine unit cell compression in shock-compressed LiF single crystals subjected to both elastic and elastic-plastic deformation, respectively. The developments described here are expected to be useful for examining lattice deformation and structural changes in shock wave compression studies.

Characterisation of clusters of alpha-tocopherol in gel and fluid phases of dipalmitoylglycerophosphocholine.

The effect of alpha-tocopherol on the phase behaviour of aqueous dispersions of dipalmitoylglycerophosphocholine has been examined by differential scanning calorimetry, freeze-fracture electron microscopy, and real-time X-ray diffraction methods. The presence of alpha-tocopherol in proportions 2.5, 5, 7.5, and 10 mol/100 mol results in a progressive decrease in the temperature of the gel to liquid-crystalline phase transition from 41.5 degrees C to 36 degrees C and a reduction in transition enthalpy from 35 kJ.mol-1 to 15 kJ.mol-1 phospholipid. The thermal data indicated that the pretransition of the phospholipid is eliminated even in mixtures containing 2.5 mol/100 mol alpha-tocopherol. Real-time X-ray diffraction measurements using synchrotron radiation performed under identical conditions to the thermal studies showed clear transition sequences of L beta-->P beta-->L alpha for all mixtures. The sequence was reversible with hysteresis of 2-3 degrees C on cooling. Low-angle X-ray scattering from mixtures in the gel phase showed three lamellar repeat spacings of 6.35, 7.5, and 8.4 nm. The spacing at 6.35 nm was assigned to pure phospholipid from which alpha-tocopherol has been phase separated into enriched domains giving lamellar repeat spacings of 7.5 nm and 8.4 nm. Low-angle diffraction patterns of mixtures in the fluid phase were characterised by two lamellar repeat spacings. The longer spacing of about 6.6 nm was assigned to pure phospholipid and the shorter spacing at about 6.1 nm to an alpha-tocopherol-enriched phase. Electron microscopy of freeze-fracture replicas of mixtures of phospholipid containing 10 mol/100 mol alpha-tocopherol thermally quenched from 10 degrees C and 60 degrees C, showed evidence of domain structures within the bilayer plane that appeared to be correlated between successive bilayers in multilamellar dispersions. Calculations of the stoichiometry of phospholipid: alpha-tocopherol in the alpha-tocopherol-enriched domains based on enthalpy data and integrated X-ray scattering intensity gave values of 9.6:1 for the fluid phase and 9.2:1 for the gel phase. This was consistent with a clustering of alpha-tocopherol molecules in both gel and liquid-crystal phases of dipalmitoyl-glycerophospholcholine in approximately the same stoichiometry.

Phase behaviour of membrane lipids containing polyenoic acyl chains

The low-temperature thermal behaviour of di-18:2 phosphatidylethanolamine (di-18:2 PE) is shown to be characterised by similar broad low-enthalpy transitions to those previously reported for polyenoic samples of phosphatidylcholines (Keough and Kariel (1987) Biochim. Biophys. Acta 902, 11–18), and monogalactosyldiacylglycerol (Sanderson and Williams (1992) Biochim. Biophys. Acta. 1107, 77–85). Real-time X-ray diffraction measurements indicate that these transitions correspond to transitions between the gel (L β) and liquid-crystal (L α) phases of the lipids. The gel phase of these lipids is, however, much more loosely packed than the corresponding phases of membrane lipids containing monoenoic or fully-saturated acyl chains. The low enthalpy and reduced co-operativity of the L α a ̊ L β phase transitions of the polyenoic lipids is attributed to the reduced contribution of van der Waals interactions between their acyl chains in the gel-state of these lipids. Comparison with the earlier results obtained for MGDG suggest that the acyl chains of polyenoic lipids can form well-ordered lattices but require the additional energy input associated with the formation of a hydrogen bond network between the lipid headgroups in order to do so.

The effect of ice on membrane lipid phase behaviour

The effects of ice on the lipid phase behaviour of di-18:1 PE and di-18:2 PE were studied by comparing the behaviour of these lipids in supercooled and frozen dispersions. The presence of ice raised the onset temperature of the L α → L β phase transition of di-18:1 PE from −10°C to −6.5°C and increased its molar enthalpy from 6.1 to 8.5 kcal/mol but had little effect on the co-operativity of the transition. Real-time X-ray diffraction measurements of the H II → L α phase transition of di-18:2 PE suggested that this transition could take place in the presence of ice but that the corresponding L α → H II phase transition could not take place until the ice melted. Measurements of the temperature dependence of the d-spacing of di-18:1 PE and di-18:2 PC in frozen dispersions indicated that the amounts of unfrozen water in such dispersions changes significantly with temperature. It was concluded that the increases in onset temperature and molar enthalpy seen for the L α →L β transition of di-18:1 PE probably reflected the effects of osmotic dehydration. The main effect of ice in the case of the H II a ̊ L α phase transition, however, appeared to be to limit the ability of the lipid to undergo structural reorganisation.

Phase transition characterization in low hydration phases of dilauroylphosphatidylcholine

Phase transitions in slightly hydrated samples of dilauroylphosphatidylcholine (DLPC) were studied using fixed temperature and real time X-ray diffraction. Only a single transition was observed involving, in all cases, the crystalline ( L c ) and liquid crystalline ( L α ) bilayer phases. Real time X-ray diffraction measurements indicated that this transition proceeded via a two-state or first-order transition mechanism involving both mesophases and acyl chain packings. Transition temperatures, structural parameters, and transition mechanisms obtained from real time X-ray diffraction measurements of DLPC bilayers in limited water samples were the same as those derived from fixed temperature X-ray experiments comparing samples of the same lipid/water concentration using stepwise temperature changes. These results differ from previous reports on slightly hydrated phospholipids, which show different transition mechanisms for the gel to liquid crystalline phase transition, which depended on whether real time or static X-ray measurements were used.

X-radiation damage of hydrated lecithin membranes detected by real-time X-ray diffraction using wiggler-enhanced synchrotron radiation as the ionizing radiation source

Radiation damage of hydrated lecithin membranes brought about by exposure to wiggler-derived synchrotron radiation at 8.3 keV (1.5 Å) is reported. Considerable damage was observed with exposures under 1 h at an incident flux density of 3 × 10 10 photon s −1 mm −2, corresponding to a cumulative radiation dose of ≦10 MRad. Damage was so dramatic as to be initially observed while making real-time X-ray diffraction measurements on the sample. The damaging effects of 8.3 keV X-rays on dispersions of dipalmitoyllecithin and lecithin derived from hen egg yolk are as follows: (1) marked changes were noted in the X-ray diffraction behaviour, indicating disruption of membrane stacking. (2) Chemical breakdown of lecithin was observed. (3) The X-ray beam visibly damaged the sample and changed the appearance of the lipid dispersion, when viewed under the light microscope. Considering the importance of X-ray diffraction as a structural probe and the anticipated use of synchrotron radiation in studies involving membranes, the problem of radiation damage must be duly recognized. Furthermore, since dipalmitoyllecithin, the major lipid used in the present study, is a relatively stable compound, it is not unreasonable to expect that X-ray damage may be a problem with other less stable biological and non-biological materials. These results serve to emphasize that whenever a high intensity X-ray source is used, radiation damage can be a problem and that the sensitivity of the sample must always be evaluated under the conditions of measurement.