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Plasma-enhanced atomic layer deposition (PEALD) enables the epitaxial growth of ultrathin indium nitride (InN) films at significantly reduced process temperatures and with greater control of layer thickness compared to other growth methods. However, the reliance on plasma-surface interactions increases the complexity of the growth process. A detailed understanding of the relationship between the plasma properties and the growth kinetics is therefore required to guide the tuning of growth parameters. We present an in situ investigation of the early-stage PEALD growth kinetics of epitaxial InN within three different plasma regimes using grazing incidence small-angle x-ray scattering (GISAXS). The GISAXS data are supported by diagnostic studies of the plasma species generation in the inductively coupled plasma source as a function of the relative concentrations of the nitrogen/argon gas mixture used in the growth process. The growth mode is found to be correlated to the production of nitrogen species in the plasma, with high concentrations of the atomic N species promoting Volmer–Weber growth (i.e., island growth) and low concentrations promoting Stranski–Krastanov growth (i.e., layer-plus-island growth). The critical thickness for island formation, island center-to-center distance, and island radius are found to increase with ion flux. Furthermore, the island center-to-center distance and areal density are observed to change only during plasma exposure and to continue changing with exposure even after the methylindium adlayer is believed to have fully reacted with the plasma. Our results demonstrate the potential to control the growth kinetics during PEALD of epitaxial films by intentionally accessing specific regimes of plasma species generation.

We used the Coherent Grazing Incidence Small Angle X-Ray Scattering (Co-GISAXS) technique to study the average kinetics and nanoscale dynamics during early-stage a-WSi$_2$ sputter deposition. The kinetic and dynamic properties are examined as a function of pressure, which is known to be a critical factor in determining final surface roughness. Surface growth kinetics and dynamics are characterized by time parameters extracted from the height-height structure factor and correlation functions. The roughness at a given length scale reaches a maximum before relaxing down to a steady state. The lateral length scale dependence and pressure dependence are then compared among measured kinetics and dynamics time parameters. Surfaces grown at lower pressures are smoother, leading to longer correlation times. The time to reach a dynamic steady state and a kinetic steady state show contrasting pressure dependence. A dynamic steady state is reached earlier than the kinetic steady state at high pressure. A more random deposition direction and lower kinetic energy at higher pressures can explain these phenomena, along with the hypothesis that larger nanoclusters form in vapor before arriving at the surface. A continuum model is applied to simulate the overall behavior with mixed success., Comment: 13 pages, 17 figures

The structural and functional maturation of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is essential for application to pharmaceutical testing, disease modeling, and ultimately therapeutic use. Multicellular 3D-tissue platforms have improved functional maturation of hiPSC-CMs, but probing cardiac contractile properties remains challenging in a 3D environment, especially at depth and in live tissues. Using small angle X-ray scattering (SAXS) images, we show that hiPSC-CMs, matured and examined in a 3D environment, exhibit periodic spatial arrangement of the myofilament lattice, which has not been previously detected in hiPSC-CMs. Contractile force is found to correlate with both scattering intensity (R2=0.44) and lattice spacing (R2=0.46). Scattering intensity also correlates with lattice spacing (R2=0.81), suggestive of lower noise in our structural measurement relative to the functional measurement. Notably, we observe decreased myofilament ordering in tissues with a myofilament mutation known to lead to hypertrophic cardiomyopathy (HCM). Our results highlight the progress of human cardiac tissue engineering and enable unprecedented study of structural maturation in hiPSC-CMs., Comment: 12 pages, 6 figures, supplementary material available upon request to the lead author

Normal incidence 1 keV Ar$^+$ ion bombardment leads to amorphization and ultrasmoothing of Ge at room temperature, but at elevated temperatures the Ge surface remains crystalline and is unstable to the formation of self-organized nanoscale patterns of ordered pyramid-shaped pits. The physical phenomenon distinguishing the high temperature patterning from room temperature ultrasmoothing is believed to be a surface instability due to the Ehrlich-Schwoebel barrier for diffusing vacancies and adatoms, which is not present on the amorphous material. This real-time GISAXS study compares smoothing of a pre-patterned Ge sample at room temperature with patterning of an initially flat Ge sample at an elevated temperature. In both experiments, when the nanoscale structures are relatively small in height, the average kinetics can be explained by a linear theory. The linear theory coefficients, indicating surface stability or instability, were extracted for both experiments. A comparison between the two measurements allows estimation of the contribution of the Ehrlich-Schwoebel barrier to the self-organized formation of ordered nanoscale patterns on crystalline Ge surfaces., 5 pages, 5 figures. arXiv admin note: text overlap with arXiv:2007.12294, arXiv:2007.12253

Investigating the relationship between structure and dynamical processes is a central goal in condensed matter physics. Perhaps the most noted relationship between the two is the phenomenon of de Gennes narrowing, in which relaxation times in liquids are proportional to the scattering structure factor. Here a similar relationship is discovered during the self-organized ion beam nanopatterning of silicon using coherent x-ray scattering. However, in contrast to the exponential relaxation of fluctuations in classic de Gennes narrowing, the dynamic surface exhibits a wide range of behaviors as a function of length scale, with a compressed exponential relaxation at lengths corresponding to the dominant structural motif - self-organized nanoscale ripples. These behaviors are reproduced in simulations of a nonlinear model describing the surface evolution. We suggest that the compressed exponential behavior observed here is due to the morphological persistence of the self-organized surface ripple patterns which form and evolve during ion beam nanopatterning., Comment: 4 pages, 4 figures

Strontium-doped lanthanum manganite is widely used as a cathode material for solid oxide fuel cells (SOFCs). In this study, the oxygen diffusivity in A-site-deficient (La0.8Sr0.2)0.95MnO3±δ (LSM-20) was measured as a function of temperature (600°C, 700°C, and 800°C) and oxygen partial pressure (0.03 atm, 0.16 atm, and 0.30 atm). Tracer (O-18) concentration profiles were measured in single-crystal epitaxial thin films of LSM-20 by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS). The profile was fit to a diffusion equation to obtain the diffusivity of oxygen in bulk LSM-20 as a function of temperature and oxygen partial pressure. A defect model was used in conjunction with the experimental data to calculate the activation energies of oxygen migration, Schottky reaction, redox reaction, and charge disproportionation reaction in LSM-20. These calculated data were then used to calculate the Brouwer diagram of (La0.8Sr0.2)0.95MnO3±δ at 800°C, a typical operating temperature of SOFCs.

The recent development of surface growth studies using X-ray photon correlation spectroscopy in a grazing-incidence small-angle X-ray scattering (Co-GISAXS) geometry enables the investigation of dynamical processes during kinetic roughening in greater detail than was previously possible. In order to investigate the Co-GISAXS behavior expected from existing growth models, calculations and (2+1)-dimension simulations of linear Kuramoto–Sivashinsky and non-linear Kardar–Parisi–Zhang surface growth equations are presented which analyze the temporal correlation functions of the height–height structure factor. Calculations of the GISAXS intensity auto-correlation functions are also performed within the Born/distorted-wave Born approximation for comparison with the scaling behavior of the height–height structure factor and its correlation functions.

Despite extensive study, fundamental understanding of self-organized patterning by broad-beam ion bombardment is still incomplete and controversial. Understanding the nanopatterning of elemental semiconductors, particularly silicon, is both foundational for the broader field and of intrinsic scientific and technological interest itself. This is the second component of a two-part investigation of the kinetics and fluctuation dynamics of self-organized nanoscale ripple development on silicon during 1 keV Ar$^+$ (Part I) and Kr$^+$ bombardment. Here, it's found that the ion-enhanced viscous flow relaxation is essentially equal for Kr$^+$-induced patterning as previously found for Ar$^+$ patterning. The magnitude of the surface curvature dependent roughening rate in the early stage kinetics is larger for Kr$^+$ than for Ar$^+$, qualitatively consistent with expectations for erosive and mass redistributive contributions to the initial surface instability. As with the Ar$^+$ case, fluctuation dynamics in the late stage show a peak in correlation time at the length scale corresponding to the dominant structural feature on the surface -- the ripples. Analogy is made to the phenomenon of de Gennes narrowing in liquids, but significant differences are also pointed out. Finally, it's shown that speckle motion during the surface evolution can be analyzed to determine spatial inhomogeneities in erosion rate and ripple velocity on the surface. This allows the direction and speed of ripple motion to be measured in real time, a unique capability for measuring these fundamental parameters outside the specialized environment of FIB/SEM systems., Comment: 11 pages, 13 figures

Coherent grazing-incidence small-angle X-ray scattering is used to investigate the average kinetics and the fluctuation dynamics during self-organized nanopatterning of silicon by Ar$^+$ bombardment at 65$^{\circ}$ polar angle. At early times, the surface behavior can be understood within the framework of linear theory. The transition away from the linear theory behavior is observed in the dynamics through the intensity correlation function. It quickly evolves to exhibit stretched exponential decay on short length scales and compressed exponential decay on length scales corresponding the dominant structural length scale - the ripple wavelength. The correlation times also peak strongly at the ripple length scale. This behavior has notable similarities but also significant differences with the phenomenon of de Gennes narrowing. Overall, this dynamics behavior is found to be consistent with simulations of a nonlinear growth model., Comment: 10 pages, 12 figures

In this study, a strontium doped lanthanum cobalt ferrite thin film with 30% Sr on A-site, denoted as La 0.7 Sr 0.3 Co 0.2 Fe 0.8 O 3-δ or LSCF-7328, is investigated before and after annealing at 800 °C under CO 2 containing atmosphere for 9 h. The formation of secondary phases on surface of post-annealed LSCF-7328 is observed using atomic force microscope (AFM) and scanning electron microscope (SEM). The extent of Sr segregation at the film surface is monitored using the synchrotron-based total reflection X-ray fluorescence (TXRF) technique. The bonding environment of the secondary phases formed on the surface is investigated by synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Scanning transmission electron microscopy (STEM) and related spectroscopy techniques are used for microstructural and quantitative elemental analyses of the secondary phases on surface. These studies reveal that the secondary phases on surface consist of SrO covered with a capping layer of SrCO 3 . The formation of Co-rich phases is observed on the surface of post-annealed LSCF-7328.

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1–x SrxCo0.2Fe0.8O3−δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO3, SrTiO3, and GdScO3 substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on t...

We study the encapsulation of monolayers of CdSe/CdS/ZnS core/shell/shell quantum dots (QDs) in a ZnO matrix by atomic layer deposition (ALD) in order to gain insight in the interaction between quantum dots and ALD precursors and the resulting metal oxide coating. Using in situ XRF and GISAXS, we show the inhibition of ZnO growth on as-deposited QDs. Growth can, however, be triggered by exposing the QDs to a single pulse of trimethylaluminum (TMA) vapor. Such a TMA pretreatment results in the substitution of 35–40% of the surface Zn by Al. Whereas this drops by half the photoluminescence quantum yield of the QDs, we argue that this replacement primes the QD monolayer for ZnO growth by ALD. Finally, the evolution of the GISAXS pattern during subsequent ALD growth attests the preservation of the ordering of the QDs in the monolayer. These results illustrate the important interplay between highly reactive ALD precursors and the QD surface.

The properties of artificially grown thin films are often strongly affected by the dynamic relationships between surface growth processes and subsurface structure. Coherent mixing of X-ray signals promises to provide an approach to better understand such processes. Here, we demonstrate the continuously variable mixing of surface and bulk scattering signals during real-time studies of sputter deposition of a-Si and a-WSi2 films by controlling the X-ray penetration and escape depths in coherent grazing-incidence small-angle X-ray scattering. Under conditions where the X-ray signal comes from both the growth surface and the thin film bulk, oscillations in temporal correlations arise from coherent interference between scattering from stationary bulk features and from the advancing surface. We also observe evidence that elongated bulk features propagate upwards at the same velocity as the surface. Furthermore, a highly surface-sensitive mode is demonstrated that can access the surface dynamics independently of the subsurface structure. Defects affect materials’ properties. A method is now presented for studying dynamic processes during the growth of thin films — specifically, the evolution of defects — based on the coherent mixing of bulk and surface X-ray scattering signals.

We present a systematic synchrotron radiation photoemission spectroscopy study of the interaction of O2 with defective La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surfaces at low temperatures. First, the surface chemical evolution during low energy Ar+ ion sputtering is investigated and is found to create oxygen vacancies. Subsequently, the interaction of the O2 molecules with La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surface is observed to create an insulating phase. We performed a detailed core-level peak-fitting analysis to identify the chemical nature of this phase and to probe the role of vacancies in the formation of this phase.

The ultra-high vacuum surface preparation of heteroepitaxial lanthanum strontium cobalt ferrite thin films has been studied using soft X-ray photoelectron spectroscopy. Specifically, the effect of annealing the films at low temperatures in low partial pressures of oxygen and argon has been investigated. We find that atmospheric surface carbon contamination of the films can be removed in select anneal temperature regimes in argon, but remains bound to the surface with oxygen annealing at any temperature. Irrespective of the gas used, an insulating phase transition occurs near 300 °C due to strontium segregation at the surface. The surface develops more insulating character if annealed with oxygen. Different species are proposed to be responsible for the discrepancy in insulating character.

The generation of two X-ray pulses with tunable nanosecond scale time separations has recently been demonstrated at the Linac Coherent Light Source using an accelerator based technique. This approach offers the opportunity to extend X-ray Photon Correlation Spectroscopy techniques to the yet unexplored regime of nanosecond timescales by means of X-ray Speckle Visibility Spectroscopy. As the two pulses originate from two independent Spontaneous Amplified Stimulated Emission processes, the beam properties fluctuate from pulse pair to pulse pair, but as well between the individual pulses within a pair. However, two-pulse XSVS experiments require the intensity of the individual pulses to be either identical in the ideal case, or with a accurately known intensity ratio. We present the design and performances of a non-destructive intensity diagnostic based on measurement of scattering from a transparent target using a high-speed photo-detector. Individual pulses within a pulse pair with time delays as short as 0.7 ns can be resolved. Moreover, using small angle coherent scattering, we characterize the averaged spatial overlap of the focused pulse pairs. The multi-shot average-speckle contrasts from individual pulses and pulse pairs are compared.

a b s t r a c t Formation of oxygen vacancies by introducing various mixed-valent cation dopants is a common practice to improve the cathode performance in solid oxide fuel cells. A computational procedure is developed in this work to predict the equilibrium oxygen vacancy non-stoichiometric concentrations at experimen- tally relevant temperatures and oxygen partial pressures for both bulk and surface oxide phases. The calculations are based on the first-principles density functional theory and a constrained free-energy functional. Quantitative agreements are found by direct comparisons to the thermogravimetry and solid electrolyte coulometry measurements for the strontium-doped lanthanum cobalt iron oxides at different compositions. Our results indicate that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and all oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures.

Synchrotron based hard x-ray photoelectron spectroscopy (HAXPES) technique has been utilized to measure the oxidation states of the associated ions on the surface of strontium doped lanthanum cobalt iron oxide (LSCF), as a function of strontium dopant concentration and gas composition. LSCF is a widely used cathode material for solid oxide fuel cells (SOFCs), which are ideally suitable for environmentally benign electricity generation. However, LSCF suffers from long term stability issues by surface segregation phenomena under typical SOFC working conditions. Leveraging the capability of HAXPES to overcome surface contamination issues and obtaining information from tunable penetration depth, depth dependence information about changes in the electronic structure were obtained, which helps better understand the segregation phenomena. Surface chemistry of both as-deposited and post-annealed LSCF thin films with different Sr concentrations are discussed.

Atomic Layer Epitaxy (ALEp) is a promising subset of atomic layer processes (ALPs) which has the potential to open a new realm of non-equilibrium semiconductor growth. In ALEp, the objective is to grow crystalline, epitaxial layers on a crystalline substrate for active regions of electronic and optoelectronic devices. This requires expansion of the atomic layer deposition (ALD) processing space to higher temperatures and adds constraints of crystallinity and purity; electronic grade requires impurity concentrations less than ppm. While ALEp has been shown to maintain the self-limiting nature of ALD at temperatures up to 500°C, the additional materials quality criterion requires a more complete understanding of the ALEp process if it is to be fully successful. In this regard, it is essential to develop a set of surface science tools that can be employed either in situ or in vacuo to ensure that atmospheric exposure does not influence or interfere with observed process mechanisms. In this work, we highlight the development and early application of a suite of in situ or in vacuo characterization techniques aimed at providing surface and near-surface structure assessments: low energy electron diffraction (LEED) and grazing incidence small angle x-ray scattering (GISAXS); as well as surface chemistry assessments: (x-ray photoelectron spectroscopy (XPS), resonant ion trap mass spectrometry (RIT-MS) and reflection-absorption infrared spectroscopy (RAIRS). We present select results from these methods during efforts to develop plasma-assisted ALPs for GaN surface preparation for epitaxy and for early plasma-assisted ALEp growth of heteroepitaxial AlN, InN and AlInN films on GaN and Al2O3 substrates. In the former example, in situ GISAXS studies have been used to optimize an emulated gallium flash off (GFO) ALP of as-received GaN substrate surfaces as part of a preparation of such surfaces for epitaxy. A combination of an ex situ UV/O3 oxidation and concentrated HF etch followed by 10 cycles of an emulated GFO ALP result in the smoothest, cleanest surfaces – highly suitable for epitaxy [1,2]. In the latter application, in situ GISAXS studies are used to characterize the nature of the growth mode of AlN and InN binaries and select AlInN ternaries achieved through digital alloying on optimally prepared GaN substrate surfaces. For InN binary growth, which ultimately grows in a 3D mode, the duration of the plasma pulse is shown to influence the growth mode between a bimodal distribution of islands for short pulses to a single mode distribution for intermediate pulses to etching for the longest pulses [3]. Further, we found that the transition from early 2D to ultimate 3D growth and the shape and size of the islands that result are quite sensitive to growth temperature even inside the “ALEp window”[4]. We discovered that although pure InN and AlN grew in 3D and 2D modes, respectively, the InAlN growth mode did not follow a simple trend as the nominal In composition was tuned from InN to AlN. Instead, select compositions (50% and 83% In) showed more 3D growth while others (19% and 64% In) showed 2D growth. Changes in plasma chemistry are also found to affect growth mode and film quality and will be highlighted. These changes in film properties with plasma pulse variations are correlated to independent measurements of plasma properties in an effort to establish plasma process – film property relationships. A combination of RAIRS and RIT-MS will be presented to further illustrate the role of plasma chemistry, while combined in vacuo LEED and XPS will be presented to highlight early film growth evolution. [1] S.G. Rosenberg, D.J. Pennachio, C. Wagenbach, S.D. Johnson, N. Nepal, A.C. Kozen, J.M. Woodward, Z.R. Robinson, H. Joress, K.F. Ludwig, C.J. Palmstrom and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 020908 (2019). [2] S.G. Rosenberg, C. Wagenbach, V.R. Anderson, S.D. Johnson, N. Nepal, A.C. Kozen, J.M. Woodward, Z.R. Robinson, M. Munger, H. Joress, K.F. Ludwig and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 020928 (2019). [3] N. Nepal, V. Anderson, S. D. Johnson, B. P. Downey, D. J. Meyer, Z. Robinson, J. M. Woodward, K. F. Ludwig, C. R. Eddy, Jr., Journal of Vacuum Science and Technology A 37, 020910 (2019). [4] J.M. Woodward, S.G. Rosenberg, A.C. Kozen, N. Nepal, S.D. Johnson, C. Wagenbach, Z.R. Robinson, K.F. Ludwig, Jr. and C.R. Eddy, Jr., Journal of Vacuum Science & Technology A 37, 030901 (2019). Figure 1

The surface topological evolution during the growth of indium nitride (InN) by plasma-assisted atomic layer epitaxy (ALEp) on gallium nitride (GaN) (0001) substrates was studied using in situ real-time grazing incidence small-angle x-ray scattering (GISAXS) for 180, 250, and 320 °C growth temperatures. The GISAXS data reveal that the ALEp growth of InN on GaN in this temperature range proceeds in a Stranski–Krastanov mode, in which the 2D–3D transition occurred after 2.3 monolayers for 180 °C, 1 monolayer for 250 °C, and 1.5 monolayers for 320 °C. The corresponding initial island center-to-center distances were 7.4, 11.6, and 11.7 nm. Additionally, island coarsening was observed to increase with temperature. After 200 growth cycles, the mean island diameters were 3.9, 5.6, and 7.0 nm, and the mean island center-to-center distances were 8.6, 13.7, and 17.1 nm for 180, 250, and 320 °C growth temperatures, respectively. For the 320 °C growth, the mean island shape was observed to gradually evolve from relatively mounded to cylindrical. These results are supported by atomic force microscopy and specular x-ray reflectivity.

In situ and in vacuo surface studies of in situ and ex situ GaN substrate preparation were conducted to advance fundamental understanding of GaN surface preparation for low temperature atomic layer epitaxial growth. Grazing incidence small angle x-ray scattering (GISAXS) information is complemented with in vacuo x-ray photoelectron spectroscopy and ex situ atomic force microscopy studies to assess different ex situ sample preparation methods to produce the most suitable GaN surface for epitaxy. The authors have determined that a UV-ozone exposure followed by an HF dip produces the cleanest and smoothest GaN surface. They have further determined with GISAXS that subjecting the optimum surface to the established low temperature emulated gallium flash-off atomic level process (ALP) eliminates the need for any nitridation ALP. These ex situ and in situ cleaning preparations result in clean, highly-ordered surfaces that should provide an ideal substrate for high quality crystalline epitaxial films.In situ and in vacuo surface studies of in situ and ex situ GaN substrate preparation were conducted to advance fundamental understanding of GaN surface preparation for low temperature atomic layer epitaxial growth. Grazing incidence small angle x-ray scattering (GISAXS) information is complemented with in vacuo x-ray photoelectron spectroscopy and ex situ atomic force microscopy studies to assess different ex situ sample preparation methods to produce the most suitable GaN surface for epitaxy. The authors have determined that a UV-ozone exposure followed by an HF dip produces the cleanest and smoothest GaN surface. They have further determined with GISAXS that subjecting the optimum surface to the established low temperature emulated gallium flash-off atomic level process (ALP) eliminates the need for any nitridation ALP. These ex situ and in situ cleaning preparations result in clean, highly-ordered surfaces that should provide an ideal substrate for high quality crystalline epitaxial films.

In situ synchrotron x-ray studies were employed to develop a fundamental understanding of the low temperature atomic level processes (ALPs) for GaN substrates to develop in situ methods for preparation of epitaxy ready surfaces. An emulated gallium flash-off (GFO) ALP, followed by a hydrogen clean ALP, and a subsequent nitridation ALP are studied as a function of temperature and number of cycles. The results demonstrate that ideal GFO ALP results are achieved at a higher temperature, 500 °C, and that only ten GFO ALP cycles are needed to remove the surface oxide and result in an ordered GaN surface. Continued GFO ALP cycles at 500 °C roughen the GaN surface. GFO ALP executed at 400 °C only roughens the surface, while executing the GFO ALP at 250 °C causes uneven surface features presumably due to the incomplete removal of the oxide. The hydrogen clean ALP generally roughens the surface at all three temperatures after 30 cycles of the GFO ALP. Further, the nitridation ALP executed after 30 cycles of the GFO ALP, at any of the above temperatures, has little effect since the surface of the GaN has been roughened beyond recovery. These results provide insight into optimal GaN substrate surface preparation at temperatures consistent with the low temperature atomic layer epitaxy process.

The authors present an in situ study of the effect of nitrogen plasma pulse time on the temporal evolution of the surface morphology of InN growth on a-plane sapphire at 250 °C by plasma assisted atomic layer epitaxy (ALEp). The growth surface evolution was monitored in real-time using grazing incidence small angle x-ray scattering (GISAXS) measurements at an x-ray incidence angle of 0.8°. Nitrogen plasma pulse time (tp) was varied between 15 and 30 s in 5-s steps, and for all tp, the near specular scattering broadens and correlated peaks develop and evolve along the Yoneda Wing (YW). For tp ≥ 20 s, a YW with one correlated length scale evolves. At the end of the growth, the longest correlated length scale is 16.54 nm for tp = 25 s. Porod analysis of GISAXS intensity at high qy for tp = 25 s shows the formation of mounded shapes at the early stage of nucleation that transitioned to cylinders after about 3 unit cells of InN growth. Additionally, at tp = 25 s, the growth rate is highest with root mean square surface roughness and carbon impurity levels at or below atomic force microscopy and x-ray photoelectron spectroscopy sensitivity limits, respectively. At tp 30 s, it appears that trimethylindium precursor molecules start to decompose, resulting in higher carbon content in the film. Thus, the nature of GISAXS correlated length scale directly correlates with the material quality. Additional ex situ characterizations reveal an electron mobility of 6–31 cm2/V s for a 3–5 nm thick InN film on a-plane sapphire, which is similar to the reported value of 30 cm2/V s for a 1300 nm thick InN film grown by molecular beam epitaxy directly on sapphire. Thus, the combination of in situ synchrotron x-ray analysis and ex situ characterization is a powerful approach to develop understanding of the growth mechanisms of ALEp of III-N materials in order to improve the quality by reducing impurities and broaden material applications.The authors present an in situ study of the effect of nitrogen plasma pulse time on the temporal evolution of the surface morphology of InN growth on a-plane sapphire at 250 °C by plasma assisted atomic layer epitaxy (ALEp). The growth surface evolution was monitored in real-time using grazing incidence small angle x-ray scattering (GISAXS) measurements at an x-ray incidence angle of 0.8°. Nitrogen plasma pulse time (tp) was varied between 15 and 30 s in 5-s steps, and for all tp, the near specular scattering broadens and correlated peaks develop and evolve along the Yoneda Wing (YW). For tp ≥ 20 s, a YW with one correlated length scale evolves. At the end of the growth, the longest correlated length scale is 16.54 nm for tp = 25 s. Porod analysis of GISAXS intensity at high qy for tp = 25 s shows the formation of mounded shapes at the early stage of nucleation that transitioned to cylinders after about 3 unit cells of InN growth. Additionally, at tp = 25 s, the growth rate is highest with root mean squar...

This paper explores the effects of different plasma treatments on low dielectric constant (low-k) materials and the consequences for the growth behavior of atomic layer deposition (ALD) on these modified substrates. An O2 and a He/H2 plasma treatment were performed on SiCOH low-k films to modify their chemical surface groups. Transmission FTIR and water contact angle (WCA) analysis showed that the O2 plasma changed the hydrophobic surface completely into a hydrophilic surface, while the He/H2 plasma changed it only partially. In a next step, in situ X-ray fluorescence (XRF), ellipsometric porosimetry (EP), and Rutherford backscattering spectroscopy (RBS) were used to characterize ALD growth of TiO2 on these substrates. The initial growth of TiO2 was found to be inhibited in the original low-k film containing only Si-CH3 surface groups, while immediate growth was observed in the hydrophilic O2 plasma treated film. The latter film was uniformly filled with TiO2 after 8 ALD cycles, while pore filling was delayed to 17 ALD cycles in the hydrophobic film. For the He/H2 plasma treated film, containing both Si-OH and Si-CH3 groups, the in situ XRF data showed that TiO2 could no longer be deposited in the He/H2 plasma treated film after 8 ALD cycles, while EP measurements revealed a remaining porosity. This can be explained by the faster deposition of TiO2 in the hydrophilic top part of the film than in the hydrophobic bulk which leaves the bulk porous, as confirmed by RBS depth profiling. The outcome of this research is not only of interest for the development of advanced interconnects in ULSI technology, but also demonstrates that ALD combined with RBS analysis is a handy approach to analyze the modifications induced by a plasma treatment on a nanoporous thin film.

In this paper, we present an x-ray based approach for in situ characterization during ALD processes. In a first case study the growth of HfO2 on H-terminated and oxidized Si and Ge surfaces is studied. X-ray fluorescence (XRF) is used to monitor the amount of deposited Hf atoms, while grazing incidence small angle x-ray scattering (GISAXS) provides information on the evolution of surface roughness. A second case study concerns the growth of TiO2 in a porous titania film containing ink-bottle mesopores. XRF and GISAXS measurements are used to monitor the Ti uptake and the density of the porous film, respectively.

Successful X-ray photon correlation spectroscopy studies often require that signals be optimized while minimizing power density in the sample to decrease radiation damage and, at free-electron laser sources, thermal impact. This suggests exploration of scattering outside the Fraunhofer far-field diffraction limitd2/λ \llR, wheredis the incident beam size, λ is the photon wavelength andRis the sample-to-detector distance. Here it is shown that, in an intermediate regimed2/λ >R\ggdξ/λ, where ξ is the structural correlation length in the material, the ensemble averages of the scattered intensity and of the structure factor are equal. Similarly, in the regimed2/λ >R\ggdξ(τ)/λ, where ξ(τ) is a time-dependent dynamics length scale of interest, the ensemble-averaged correlation functionsg1(τ) andg2(τ) of the scattered electric field are also equal to their values in the far-field limit. This broadens the parameter space for X-ray photon correlation spectroscopy experiments, but detectors with smaller pixel size and variable focusing are required to more fully exploit the potential for such studies.

Synchrotron-based X-ray fluorescence (XRF) is introduced as a promising in situ technique to monitor atomic layer deposition cycle-per-cycle. It is shown that the technique is greatly suitable to study initial nucleation on planar substrates. The initial growth of TiO2 from tetrakis(dimethylamino)titanium (TDMAT) and H2O is found to be linear on thermally grown SiO2, substrate-inhibited on H-terminated Si and substrateenhanced on atomic layer deposited Al2O3. Furthermore, in situ XRF is employed to monitor the Ti uptake during deposition of TiO2 in nanoporous silica films. In mesoporous films, the Ti content varied quadratically with the number of cycles, a behavior that is attributed to a decreasing surface area with progressing deposition. In microporous films, the XRF data suggest that 1−3 ALD cycles shrunk the pore diameters below the kinetic diameter of the TDMAT molecule.

Strontium doped lanthanum cobalt ferrite (LSCF), a widely used cathode material in solid oxide fuel cells (SOFC), can have long-term stability issues that can adversely affect electrochemical performance. Heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3 with varying Sr content (x = 0.4, 0.3, 0.2) were deposited on single crystal NdGaO3, SrTiO3 and GdScO3 substrates by pulsed laser deposition. The lattice mismatch between the films and the substrate led to different strains in the films. The extent of Sr-rich precipitate formation on the film surface was quantified using total reflection X-ray fluorescence (TXRF), and atomic force microscopy (AFM). The microstructure and the nature of the bonding of the surface Sr-rich phases were investigated by scanning/transmission electron microscopy (S/TEM) and hard x-ray photoelectron spectroscopy (HAXPES), respectively. The strain in the thin films was measured by high-resolution x-ray diffraction (HRXRD). The combined effects of the strontium content and strain on the extent of surface phase formation will be discussed.

Silicon surface evolution during room temperature low-energy (300, 500 and 1000 eV) normal incidence Ar + ion bombardment in the presence of Mo seed atoms has been studied with real-time grazing-incidence small-angle X-ray scattering and ex situ atomic force microscopy. When a small amount of Mo atoms was supplied to the Si surface during ion bombardment, the development of correlated structures with two different characteristic length scales was observed. The shorter length scale features (“dots”) coarsened with time until they reached a constant spatial wavelength. The longer length scale corrugations associated with kinetic roughening, however, continued to grow in amplitude during bombardment. The overall roughness is dominated by different corrugations at different times in the kinetic evolution, showing a complex behavior. The evolution of the kinetic roughening can be described by the Family–Vicsek scaling hypothesis, but measured scaling exponents are not in agreement with those of existing models.

In this paper, we present detailed studies of growth of AlN and GaN films on R-plane sapphire substrates. Contrary to previously reported work that GaN grows with its A-plane (110) parallel to the R-plane (102) of sapphire, our studies indicate that the crystallographic orientation of the III-nitride films grown on such substrates depends strongly on the kinetic conditions of growth of the AlN buffer. Specifically, the group III-rich conditions of growth of AlN buffer lead to nitride films having the (110) planes parallel to the sapphire surface, while growth of the buffer under N rich conditions leads to nitride films with the (116) planes parallel to the sapphire surface. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Detailed quantitative measurement of surface dynamics during thin film growth is a major experimental challenge. Here x-ray photon correlation spectroscopy with coherent hard x rays is used in a grazing-incidence small-angle x-ray scattering (i.e., Co-GISAXS) geometry as a tool to investigate nanoscale surface dynamics during sputter deposition of a-Si and a-$\mathrm{WS}{\mathrm{i}}_{2}$ thin films. For both films, kinetic roughening during surface growth reaches a dynamic steady state at late times in which the intensity autocorrelation function ${g}_{2}(q,\phantom{\rule{0.16em}{0ex}}t)$ becomes stationary. The ${g}_{2}(q,\phantom{\rule{0.16em}{0ex}}t)$ functions exhibit compressed exponential behavior at all wavenumbers studied. The overall dynamics are complex, but the most surface sensitive sections of the structure factor and correlation time exhibit power law behaviors consistent with dynamical scaling.

Several proposed mechanisms and theoretical models exist concerning nanostructure evolution on III-V semiconductors (particularly GaSb) via ion beam irradiation. However, making quantitative contact between experiment on the one hand and model-parameter dependent predictions from different theories on the other is usually difficult. In this study, we take a different approach and provide an experimental investigation with a range of targets (GaSb, GaAs, GaP) and ion species (Ne, Ar, Kr, Xe) to determine new parametric trends regarding nanostructure evolution. Concurrently, atomistic simulations using binary collision approximation over the same ion/target combinations were performed to determine parametric trends on several quantities related to existing model. A comparison of experimental and numerical trends reveals that the two are broadly consistent under the assumption that instabilities are driven by chemical instability based on phase separation. Furthermore, the atomistic simulations and a survey of material thermodynamic properties suggest that a plausible microscopic mechanism for this process is an ion-enhanced mobility associated with energy deposition by collision cascades.

The texture of CoSi2 films on Si(001) substrates has been examined using synchrotron based high-resolution pole figure analysis. We find that axiotaxy, a recently recognized type of texture which was observed in NiSi thin films on Si(001), is also occurring in CoSi2 layers. Typical of axiotaxy texturing, the CoSi2 pole figures show symmetrical patterns of lines. Calculations show that these features on the pole figures are created by three off-normal fiberlike texture components. These off-normal fiber orientations are a consequence of the alignment of CoSi2〈110〉 planes with Si〈110〉 planes because of very similar d spacings, which causes a periodic arrangement along Si〈100〉 directions in the plane of the interface. This one-dimensional periodicity may provide a lower interfacial energy; however, the periodic arrangement along the Si〈100〉 directions may also be indicative of growth anisotropy. The influence of Ti alloying has been investigated by using in situ x-ray diffraction measurements, as well as pol...

The kinetics of the disorder−order transition (ODT) in a triblock of polystyrene (PS) and poly(ethylene-co-butylene) (PEB), in mineral oil, a selective solvent for the middle PEB block, was followed by time-resolved SAXS using both temperature ramp and rapid temperature quench techniques. Data analysis, using the Percus−Yevick interacting hard-sphere model plus Gaussians for the Bragg peaks in the ordered body-centered-cubic (bcc) state, shows that the core radius Rc, the hard-sphere volume fraction φ, and their interaction radius Rhs increased with decreasing temperature in the disordered state. At the ODT, φ reaches a maximum and Rc and φ change on ordering. The kinetics exhibits two stages corresponding to temperature equilibration and supercooling of the micellar fluid followed by nucleation and growth of the ordered state. The induction time for the onset of the second stage shows a minimum around the glass transition of the polystyrene cores.

Atomic ordering in AlGaN films produced by plasma assisted molecular beam epitaxy (PA-MBE) was investigated with X-ray diffraction (XRD) and transmission electron microscopy selected area diffraction (TEM-SAD). Under nitrogen-rich growth conditions, films undergo 1×1 monolayer cation ordering along the [0001] growth direction. This type of ordering is stronger in N-polar films than in Ga-polar films and is sensitive to the growth ratio of group-III to group-V flux. These observations are in qualitative agreement with the first principal calculations by Northrup and co-workers. Under group-III rich growth conditions of Ga-polar films, the XRD data is consistent with the quasi-random stacking of spontaneously formed 14-ML and 12-ML superlattice structures. Reciprocal space mapping shows a small tilt (0.25°) between the crystallographic c-axis of the wurtzite structure and the superlattice axis. This result can be explained by the presence of c-axis oriented superlattice domains with a parallel step translation between neighboring domains. This result is in qualitative agreement with a recently proposed model by Venezuela and co-workers of self-organized superlattice formation during step flow growth of strained alloys on vicinal surfaces.

Structural and optical properties of GaN/AlGaN multiple quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy on ( 1 1 0 0 ) plane free-standing GaN substrates and (0 0 0 1) GaN quasi-substrates have been compared. Atomic force microscopy studies indicate that the films and MQW structures grown on both substrates replicate the surface morphology of the substrates. MQWs with AlGaN barriers grown in the presence of In flux have stronger photoluminescence (PL) intensity than those with AlGaN barriers without In. X-ray diffraction spectra of MQWs grown on the (0 0 0 1) GaN substrates show larger number of superlattices peaks than those grown on ( 1 1 0 0 ) substrates suggesting that the former have smoother interfaces. The PL spectra of MQWs deposited on (0 0 0 1) GaN substrates, where the growth is in a polar direction, exhibit a red-shift as well as a decrease in peak intensity with increase in well widths. Similar MQW structures on the ( 1 1 0 0 ) GaN, on which the growth is in a non-polar direction, do not exhibit this phenomenon, which we attribute to the absence of internal electric fields in these structures. PL intensity of MQWs with a well width of 75 A is 20 times stronger for those grown on the ( 1 1 0 0 ) plane than on the (0 0 0 1) plane GaN substrate.

Atomic layer deposition (ALD) enables the conformal coating of porous materials, making the technique suitable for pore size tuning at the atomic level, e.g., for applications in catalysis, gas separation and sensing. It is, however, not straightforward to obtain information about the conformality of ALD coatings deposited in pores with diameters in the low mesoporous regime (

Texture development in (Ti,Ta)Si2, formed from Ti–Ta alloy (0–6 at. % Ta) thin films sputtered on Si(001) and poly-Si substrates, was studied using both in situ and ex situ x-ray diffraction. The addition of Ta fundamentally changes the C54 texture. At 6 at. % Ta, an effective composition for lowering the C49–C54 transformation temperature, films on both substrates develop a strong (010) texture normal to the film plane. Alloy films on Si(001) also exhibit an in-plane orientational relationship between the final C54 texture and the underlying silicon lattice. A pseudoepitaxial film relationship with the substrate may, therefore, be playing an important role in the C54 transformation process. For 32 nm films on single-crystal substrates, the in situ measurements show that significant evolution of the C54 texture occurs during the growth process, suggesting that those grains well oriented with respect to the Si substrate lattice have higher growth velocities. The possibility of a template effect by the C40 ...

The transformation from the C49 to the C54 phase in TiSi2 thin films has been studied for more than a decade because of its importance to the semiconductor industry. In previous work we identified three dominant C54 orientations in TiSi2 thin films on Si(001) substrates using x-ray pole figure analysis. In the current study the evolution of the three prominent C54 texture components was further examined using synchrotron based time-resolved in situ x-ray diffraction. The kinetics results show significant differences in the nucleation and growth kinetics of the individual orientations. Simulations suggest that this could be explained by an inhomogeneous nucleation of different texture components in the thin film, but not by simple differences in nucleation and/or growth rates. Classical Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis of nucleation and growth was applied to model the C49–C54 phase transformation kinetics for each C54 orientation. These results showed a systematic decrease of the Avrami expon...

We have used small-angle x-ray scattering (SAXS), and small-angle neutron scattering (SANS) to study the micelle structure of a polystyrene-block-poly(ethene-co-butene)-block-polystyrene triblock copolymer in dilute - semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradecane) Measurements of equilibrium structure factors showed that micelles were formed in both types of selective solvents. In the case of dioxane, the micelles are isolated whereas in the case of heptane or tetradecane, a bridged micellar structure may be formed at higher copolymer concentrations. In both cases we observed an ordered cubic structure of insoluble domains (micellar cores) at high concentrations (> 8 %). The micellar scattering function was fit to the Percus-Yevick interacting hard-sphere model The temperature dependence of the core radius, the hard-sphere interaction radius and the volume fraction of hard spheres were obtained We also used synchrotron-based time-resolved SAXS to examine the kinetics of ordering of the micelles on a cubic lattice for many different temperature jumps into the ordered cubic phase starting from the disordered micellar fluid phase in different solvents at different concentrations. The time evolution of the structure changes was determined by fitting the data with Gaussians to describe the structure factor of the ordered Bragg peaks and the Percus-Yevick structure factor was used to describe the micellar fluid. The time dependence of the peak intensities and widths as well as of the micellar parameters will be presented The results showing the kinetics of the transformation from the fluid to the ordered phase were analyzed using the Mehl-Johnson-Avrami theory of nucleation.

The texture of blanket C49 and C54 phase TiSi2 films on Si (001) substrates was examined with x-ray pole figure analysis. Textures were studied both in films with initial Ti deposited by sputtering and by low-temperature, plasma assisted chemical vapor deposition. Detailed differences were observed between silicide films grown by subsequently annealing films deposited with the two different methods. However, in both cases, the dominant C49 phase orientational relationships develop with respect to the substrate (001) planes. In contrast, we find that the dominant crystallographic relationships of the C54 phases in both cases are not with the surface (001) plane of the substrate, but instead with the Si (111) planes. Texturing in the case of the sputtered films is particularly complex with one prominent orientation having C54 (11¯0)‖(1¯11) Si with C54 (331)‖(011) Si and a second having C54 (1¯03¯)‖(111) Si with C54 (040)‖(11¯0) Si. These orientations, which suggest local epitaxy of the disilicide with...

We present a detailed structural investigation of self-assembled indium gallium nitride nanodots grown on c-plane aluminum nitride templates by the droplet heteroepitaxy technique in a plasma-assisted molecular beam epitaxy reactor. Various growth parameters, including the total coverage of the metal species, relative and total metal effusion fluxes, and nitridation temperature were investigated. Analyses of in situ reflection high-energy electron diffraction patterns and comparison with simulations showed that the resulting crystal structure was a mixture of wurtzite and twinned zinc blende phases, with the zinc blende phase increasingly dominant for lower metal coverages and lower nitridation temperatures, and the wurtzite phase increasingly dominant for higher nitridation temperature. Studies by field emission scanning electron microscopy and atomic force microscopy revealed that the nanodots exhibit trimodal size distributions, with the dot morphologies of the intermediate size mode often resembling a...

The temporal evolution of high quality indium nitride (InN) growth by plasma-assisted atomic layer epitaxy (ALEp) on a-plane sapphire at 200 and 248 °C was probed by synchrotron x-ray methods. The growth was carried out in a thin film growth facility installed at beamline X21 of the National Synchrotron Light Source at Brookhaven National Laboratory and at beamline G3 of the Cornell High Energy Synchrotron Source, Cornell University. Measurements of grazing incidence small angle x-ray scattering (GISAXS) during the initial cycles of growth revealed a broadening and scattering near the diffuse specular rod and the development of scattering intensities due to half unit cell thick nucleation islands in the Yoneda wing with correlation length scale of 7.1 and 8.2 nm, at growth temperatures (Tg) of 200 and 248 °C, respectively. At about 1.1 nm (two unit cells) of growth thickness nucleation islands coarsen, grow, and the intensity of correlated scattering peak increased at the correlation length scale of 8.0 a...

Wide bandgap semiconducting nitrides have found wide-spread application as light emitting and laser diodes and are under investigation for further application in optoelectronics, photovoltaics, and efficient power switching technologies. Alloys of the binary semiconductors allow adjustments of the band gap, an important semiconductor material characteristic, which is 6.2 eV for aluminum nitride (AlN), 3.4 eV for gallium nitride, and 0.7 eV for (InN). Currently, the highest quality III-nitride films are deposited by metalorganic chemical vapor deposition and molecular beam epitaxy. Temperatures of 900 °C and higher are required to deposit high quality AlN. Research into depositing III-nitrides with atomic layer epitaxy (ALEp) is ongoing because it is a fabrication friendly technique allowing lower growth temperatures. Because it is a relatively new technique, there is insufficient understanding of the ALEp growth mechanism which will be essential to development of the process. Here, grazing incidence small...

Synchrotron based x-ray fluorescence (XRF) and grazing incidence small angle scattering (GISAXS) are demonstrated to be excellent in situ methods for monitoring atomic layer deposition (ALD) processes. XRF allows to identify and to quantify the amount of material deposited, whereas GISAXS is a powerful technique for monitoring nanoscale morphology. Three case studies are discussed where these in situ techniques are used to investigate specific aspects of ALD processes that are of relevance for applications in micro-electronics: the initial growth of gate oxides, the initial nucleation during metal ALD processes, and the penetration of ALD deposited materials into nanoporous low-k oxides.

A broad class of ``local response'' theories seeks to predict morphology evolution during energetic particle irradiation in terms of average surface height response to individual impacts---an approach that has been generalized by the crater function formalism of Norris et al. [J. Phys.: Condens. Matter 21, 224017 (2009); Nat. Commun. 2, 276 (2011)]. Keeping only the terms in the crater function formalism associated with the response of a flat surface has facilitated the use of molecular dynamics simulations of individual ion impacts to predict the stability or instability of a flat surface to ion bombardment. Here we report a sensitive experimental test of this truncated crater function theory that is independent of any a priori knowledge of the crater function itself. Existing measurements for 1 keV ${\mathrm{Ar}}^{+}/\mathrm{Si}$ and ${\mathrm{Kr}}^{+}/\mathrm{Ge}$ are inconsistent with the predictions of truncated crater function theory, for any conceivable crater function, at high bombardment angles. The failure of the theory suggests that the prediction of surface evolution from simulations of single-ion impacts will be more challenging than had been assumed.