1. Introduction and overview 2. Film stress and substrate curvature 3. Stress in anisotropic and patterned films 4. Delamination and fracture 5. Film buckling, bulging and peeling 6. Dislocation formation in epitaxial systems 7. Dislocation interactions and strain relaxation 8. Equilibrium and stability of surfaces 9. The role of stress in mass transport.

Thin Film Materials: Stress, Defect Formation and Surface Evolution

Cerium oxidation state in ceria nanoparticles studied with X-ray photoelectron spectroscopy and absorption near edge spectroscopy

Although several types of architecture combining memory cells and transistors have been used to demonstrate artificial synaptic arrays, they usually present limited scalability and high power consumption. Transistor-free analog switching devices may overcome these limitations, yet the typical switching process they rely on—formation of filaments in an amorphous medium—is not easily controlled and hence hampers the spatial and temporal reproducibility of the performance. Here, we demonstrate analog resistive switching devices that possess desired characteristics for neuromorphic computing networks with minimal performance variations using a single-crystalline SiGe layer epitaxially grown on Si as a switching medium. Such epitaxial random access memories utilize threading dislocations in SiGe to confine metal filaments in a defined, one-dimensional channel. This confinement results in drastically enhanced switching uniformity and long retention/high endurance with a high analog on/off ratio. Simulations using the MNIST handwritten recognition data set prove that epitaxial random access memories can operate with an online learning accuracy of 95.1%. Controlled widening of threading dislocations in SiGe layers epitaxially grown on Si allows the realization of resistive switching devices with enhanced uniformity, high on/off ratio and long retention times.

SiGe epitaxial memory for neuromorphic computing with reproducible high performance based on engineered dislocations

The question whether $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ accepts doping with tetravalent cations such as cerium and thus allows the preparation of electron-doped mixed-valent lanthanum manganites has been discussed controversially so far. Against the background of this problem, we present a comparative x-ray photoemission (XPS) study of epitaxial ${\mathrm{La}}_{0.7}{\mathrm{Ce}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCeMO) and ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) thin films. We focus on the exchange splitting of the Mn $3s$ core level peak, which is a direct indicator of the Mn valence and allows us to quantify the Mn valence in the outermost $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ of the films. We demonstrate that, depending on the oxygen content, the Mn valence can be tuned between a mixed ${\mathrm{Mn}}^{3+∕4+}$ state and a mixed ${\mathrm{Mn}}^{2+∕3+}$ state in both the LCeMO and the LCMO film. The oxygen content was varied by heating in ultrahigh vacuum for deoxygenation and in an oxygen atmosphere for reoxidation. In the LCeMO film, the deoxygenation not only changes the Mn valence, but also the Ce valence is driven from the $4+$ towards the $3+$ state.

XPS investigation of Mn valence in lanthanum manganite thin films under variation of oxygen content

The current state of the art of electrostatic force microscopy (EFM) is presented. The principles of EFM operation and the interpretation of the obtained local voltage and capacitance data are discussed. In order to show the capabilities of the EFM method, typical results for semiconducting nanostructures and lasers are presented and discussed. Improvements to EFM and complementary electrical methods using scanning microscopy demonstrate the continuing interest in electrical probing at the nanoscale range.

https://iopscience.iop.org/article/10.1088/0957-4484/12/4/321/pdf

Electrostatic force microscopy: principles and some applications to semiconductors

The silicon-cerium oxide interface is studied using x-ray photoelectron spectroscopy. The oxidation and reduction of species at the interface are examined as a function of annealing temperature both in vacuum and oxygen ambient, in order to determine their relative stabilities. By depositing a very thin CeO2 film (similar to 30 Angstrom), the cerium and silicon core level peaks can be monitored simultaneously. The presence of characteristic chemical shifts of the Si 2p peak gives information about any SiOx, layer that may form at the interface. The oxidation state of the cerium can be probed from three different areas of the spectrum. From this information we can infer the oxidation state of both the silicon and the cerium. For the first time a complete picture of the interface is obtained. The implications of these findings on the utility of CeO2 in device applications are discussed.

http://authors.library.caltech.edu/2020/01/PREjvstb01.pdf

Stability of cerium oxide on silicon studied by x-ray photoelectron spectroscopy

We have investigated undoped, semi‐insulating GaAs, grown by the liquid encapsulated Czochralski technique in pyrolytic BN crucibles. The concentration of carbon, the principal shallow acceptor, depends on the water content of the B2O3 encapsulant. Dry B2O3 (100–150 ppm H2O) results in carbon concentrations as high as 8×1015 cm−3, while wet B2O3 (500 ppm H2O) results in carbon concentrations below the detection limit of 3–5×1014 cm−3. In samples with carbon below 5×1014 cm−3, the compensation (acceptors minus donors shallower than EL2) is also low, and the room‐temperature carrier concentration can exceed 1011 cm−3. For samples with carbon above 5×1014 cm−3, the room‐temperature carrier concentration is below 5×107 cm−3 for all samples measured. The compensation is less than the carbon concentration in high carbon samples, indicating that donors shallower than EL2 are significant in determining the value of the compensation in these samples.

Carbon in semi‐insulating, liquid encapsulated Czochralski GaAs

Using electron beam evaporation, a Si/CeO2/Si(111) structure has been grown in a molecular beam epitaxy machine In situ low energy electron diffraction, cross sectional transmission electron microscopy, selected area diffraction, and atomic force microscopy have been used to structurally characterize the overlying silicon layer and show it to be single crystalline and epitaxially oriented Rutherford backscattering and energy dispersive x-ray analysis have been used to confirm the presence of a continuous 23 A CeO2 layer at the interface Rutherford backscattering and x-ray photoemission spectroscopy show an additional presence of cerium both at the exposed silicon surface and incorporated in low levels (~ 1%) within the silicon film, suggesting a growth mechanism with cerium riding atop the silicon growth front leaving behind small amounts of cerium incorporated in the growing silicon crystal

Epitaxial silicon grown on CeO2/Si(111) structure by molecular beam epitaxy

Strain relaxation in Si1–xGex/Si superlattices and alloy films is studied as a function of ex situ anneal treatment with the use of x-ray diffraction and Raman spectroscopy. Samples are grown by molecular-beam epitaxy at an unusually low temperature (≈365 °C). This results in metastably strained alloy and superlattice films significantly in excess of critical thicknesses previously reported for such structures. Significant strain relaxation is observed upon anneal at temperatures as low as 390 °C. After a 700 °C, 2 h anneal, superlattices are observed to relax less fully (~43% of coherent strain) than corresponding alloys (~84% of coherent strain). Also, the strain relaxation kinetics of a Si1–xGex alloy layer is studied quantitatively. Alloy strain relaxation is approximately described by a single, thermally activated, first order kinetic process having activation energy Ea=2.0 eV. The relevance of our results to the microscopic mechanisms responsible for strain relaxation in lattice-mismatched semiconductor heterostructures is discussed.

/pdf/strain-relaxation-kinetics-in-si1-xgex-si-heterostructures-3ryvyzt0vq.pdf

Strain relaxation kinetics in Si1–xGex/Si heterostructures

An electrostatic force microscope was used to write and image localized dots of charge in a double barrier CeO2/Si/CeO2/Si(111) structure By applying a relatively large tip voltage and reducing the tip to sample separation to 3–5 nm, charge dots 60–200 nm full width at half maximum of both positive and negative charge have been written The total stored charge is found to be Q = ±(20–200)e per charge dot These dots of charge are shown to be stable over periods of time greater than 24 h, with an initial charge decay time constant of tau ~ 95 h followed by a period of much slower decay with tau > 24 h The dependence of dot size and total stored charge on various writing parameters such as tip writing bias, tip to sample separation, and write time is examined

/pdf/charge-storage-in-ceo2-si-ceo2-si-111-structures-by-3bo36srm61.pdf

O. J. Marsh

Papers

Stability of cerium oxide on silicon studied by x-ray photoelectron spectroscopy

Carbon in semi‐insulating, liquid encapsulated Czochralski GaAs

Epitaxial silicon grown on CeO2/Si(111) structure by molecular beam epitaxy

Strain relaxation kinetics in Si1–xGex/Si heterostructures

Charge storage in CeO2/Si/CeO2/Si(111) structures by electrostatic force microscopy