L. C. Feldman

GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy

Ge x Si1−x films are grown on Si by molecular beam epitaxy and analyzed by Nomarski optical interferencemicroscopy, Rutherford ion backscattering and channeling, x‐ray diffraction, and transmission electron microscopy. The full range of alloy compositions will grow smoothly on silicon. Ge x Si1−x films with x≤0.5 can be grown free of dislocations by means of strained‐layer epitaxy where lattice mismatch is accommodated by tetragonal strain. Critical thickness and composition values are tabulated for strained‐layer growth. Multiple strained layers are combined to form a Ge x Si1−x /Si strained‐layer superlattice.

Clustering on surfaces

Abstract In this review we summarize the current theoretical and experimental understanding of clustering phenomena on surfaces, with an emphasis on dynamical properties. The basic theoretical concepts to predict evolving cluster size distributions are presented, with extensions to less restrictive assumptions, such as including the influence of non-zero deposition rates. The discussion of experimental results to test these concepts is preceded by a brief introduction of the experimental techniques used in morphological cluster studies. Finally, two important physical surface quantities, surface diffusion coefficients and adatom binding energies, are used to demonstrate the impact of clustering in their understanding.

Improved inversion channel mobility for 4H-SiC MOSFETs following high temperature anneals in nitric oxide

Results presented in this letter demonstrate that the effective channel mobility of lateral, inversion-mode 4H-SiC MOSFETs is increased significantly after passivation of SiC/SiO/sub 2/ interface states near the conduction band edge by high temperature anneals in nitric oxide. Hi-lo capacitance-voltage (C-V) and ac conductance measurements indicate that, at 0.1 eV below the conduction band edge, the interface trap density decreases from approximately 2/spl times/10/sup 13/ to 2/spl times/10/sup 12/ eV/sup -1/ cm/sup -2/ following anneals in nitric oxide at 1175/spl deg/C for 2 h. The effective channel mobility for MOSFETs fabricated with either wet or dry oxides increases by an order of magnitude to approximately 30-35 cm/sup 2//V-s following the passivation anneals.

Effect of nitric oxide annealing on the interface trap densities near the band edges in the 4H polytype of silicon carbide

Results of capacitance–voltage measurements are reported for metal–oxide–semiconductor capacitors fabricated using the 4H polytype of silicon carbide doped with either nitrogen (n) or aluminum (p). Annealing in nitric oxide after a standard oxidation/reoxidation process results in a slight increase in the defect state density in the lower portion of the band gap for p-SiC and a significant decrease in the density of states in the upper half of the gap for n-SiC. Theoretical calculations provide an explanation for these results in terms of N passivating C and C clusters at the oxide–semiconductor interface.

Observation of long-range exciton diffusion in highly ordered organic semiconductors

Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10-50 nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2-8 μm), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.

Extremely high electron mobility in Si/GexSi1−x structures grown by molecular beam epitaxy

A modulation‐doped Si/GexSi1−x structure was fabricated in which a thin Si layer is employed as the conduction channel for the two‐dimensional electron gas. The strained heterostructure is fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The mobility of the two‐dimensional electron gas as determined from Hall measurements was 1600 cm2/V s at 300 K and 96 000 cm2/V s at 4.2 K. Recently, a 4.2 K mobility of 125 000 cm2/V s was observed from a similar sample.

SiO2 film thickness metrology by x-ray photoelectron spectroscopy

Silicon dioxide films grown by industrial thermal furnace, rapid thermal, and low-pressure thermal methods were measured by x-ray photoelectron spectroscopy, transmission electron microscopy (TEM), spectroscopic ellipsometry, and capacitance–voltage analysis. Based on TEM measurements, the photoelectron effective attenuation lengths in the SiO2 and Si are found to be 2.96±0.19 and 2.11±0.13 nm, respectively. The oxide physical thicknesses (range from 1.5 to 12.5 nm) as measured by all above techniques are in good agreement. The electrical thickness is noted to be slightly thicker than the physical thickness.

Semiconductor to metal phase transition in the nucleation and growth of VO2 nanoparticles and thin films

The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.

Pseudomorphic growth of GexSi1−x on silicon by molecular beam epitaxy

GexSi1−x layers are grown on Si substrates over the full range of alloy compositions at temperatures from 400–750 °C by means of molecular beam epitaxy. At a given growth temperature films grow in a smooth, two‐dimensional manner up to a critical germanium fraction xc. Beyond xc growth is rough. xc increases from 0.1 at 750 °C to 1.0 at ∼550 °C. Rutherford ion backscattering measurements indicate good crystallinity over a wide range of growth conditions. Transmission electron microscopy reveals that in thin films, the lattice mismatch between the GexSi1−x and Si layers can be accommodated by lattice distortion rather than by misfit dislocation formation. This pseudomorphic growth condition can persist to alloy thicknesses as large as l/4 μm.

New uses of ion accelerators

1. Ion-Excited X-Ray Analysis of Environmental Samples.- I. Introduction.- II. General Considerations for Ion Beam Analysis of Environmental Samples.- III. Formalism and Optimization.- IV. The UCD/ARB Aerosol Analysis System.- A. The Primary Ion Beam.- B. Detection of X-Rays.- C. Data Acquisition and Reduction.- D. System Calibration.- E. Target Preparation and Matrix Effects.- F. Estimation of Analytical Costs.- G. Validation of System Operations.- V. Ion-Excited X-Ray Analysis Programs.- Appendix (Forward Scattering).- Acknowledgments.- References.- 2: Material Analysis by Nuclear Backscattering.- A. Introduction.- General Comments on Nuclear Backscattering.- Appendix (Numerical Examples).- References.- B. Applications.- I. Introduction.- II. Ion Implantation.- III. Thin Films: Growth and Deposition.- IV. Thin Film Reactions: Interdiffusion and Compound Formation.- V. Bulk Effects: Composition, Diffusion and Solubility.- VI. Concluding Remarks.- Acknowledgments.- References.- Formalism.- 1. Three Basic Concepts in Backscattering.- A. Backscattering Kinematic Factor Mass ? Analysis.- B. Differential Scattering Cross Section ? Quantitative Analysis.- C. Energy Loss ? Depth Analysis.- 2. Depth Scale in Backscattering Analysis [S].- A. Depth Scale in Backscattering Analysis.- B. Surface Approximation.- C. Linear Approximation.- 3. Height of an Energy Spectrum.- A. Surface Approximation for Spectrum Height.- B. Thick Target Yield.- C. Backscattering Yield of a Thin Film.- 4. Applications of Backscattering from Elemental Targets.- A. Surface Contamination and Ion Implantation.- B. Doping Level of a Bulk Sample.- C. Film Thickness Measurement and dE/dx Measurements.- D. Yield Formula and dE/dx Measurements.- E. Differential Scattering Cross Section Measurement.- 5. Application of Backscattering to Compound Targets.- A. Thin Film Analysis.- B. Thick Compound Targets.- C. Analysis on Composition Varying Continuously with Depth.- Appendix 1. Notations.- Appendix 2. Formulae.- Appendix 3. Sources for dE/dx Information.- References.- 3: Material Analysis by Means of Nuclear Reactions.- Charged Particle Activation Analysis.- Charged Particle Activation Analysis - Examples.- Prompt Radiation Analysis.- Nonresonant Nuclear Reactions - Gamma Rays Observed.- Nonresonant Nuclear Reactions - Nuclear Particles Observed.- Resonant Nuclear Reactions.- Summary.- Acknowledgment.- References.- 4: Lattice Location of Impurities in Metals and Semiconductors.- I. Introduction.- II. Impurity Detection.- III. The Channeling Technique.- 1. Channeling Concept.- 2. Experimental Technique.- IV. Lattice Location Analysis.- V. Examples.- 1. Substitutional Impurities.- 2. Nearly Substitutional Impurities.- 3. Interstitial Impurities.- 4. High Impurity Concentrations.- 5. Radiation-Induced Change in Impurity Sites.- VI. Summary of the Literature on Channeling Lattice Location Data.- VII. Limitations.- VIII. Conclusions.- References.- 5: Ion Implantation in Metals.- Historical Perspective.- Friction and Wear.- Corrosion.- 1. Oxides with Anion Defects.- 2. Oxides with Cation Defects.- Ion Backscattering.- Titanium and Stainless Steel.- Zirconium.- Aluminum.- Copper.- Aqueous Corrosion.- Practical Applications in Corrosion.- Electrochemistry and Catalysis.- Implantation Metallurgy.- Equipment for the Ion Implantation of Metals.- Conclusions.- References.- 6: Ion Implantation in Superconductors.- Definition of the Superconducting Parameters.- Influence of Radiation Damage on the Superconducting Properties.- a. Non-Transition Metals.- b. Transition Metals.- c. Transition Metal Alloys.- d. Superconductors with A-15 and NaCl-Structure.- e. Transition Metal Layer Compounds.- f. Quantitative Estimation of Damage in Superconductors.- Influence of Implanted Ions on the Superconducting Transition Temperature.- a. Magnetic Impurities in Non Transition Metals.- b. Pd-, Pd-Noble Metal Alloy, -Hydrogen System.- c. Ion Implanted Transition Metal Systems.- d. Aluminum Based Ion Implanted Systems.- Application to Superconducting Devices.- Conclusions.- References.- 7: Ion-Induced X-Rays from Gas Collisions.- 1. Introduction.- 2. Collision Models.- 2.1. Survey of Models.- 2.2. Coulomb Ionization.- 2.3. The Molecular-Orbital Model.- 3. Measurements of Inner-Shell Excitations.- 3.1. Introduction.- 3.2. Theory of Energy-Loss Measurements.- 3.3. X-Ray and Electron Emission.- 3.4. Typical Apparatus-Ionization and Inelastic Energy Loss.- 3.5. Scattered- Ion-X-Ray/Electron Coincidence Apparatus.- 4. Discussion of Typical Data.- 4.1. Ionization States.- 4.2. Inelastic Energy Loss.- 4.3. Electron Emission Cross Sections.- 4.4. Fluorescence Yield Effects.- 4.5. X-Ray-Scattered-Ion Coincidence Data.- 4.6. X-Rays from Highly Stripped Fast Ion Beams.- 5. Summary.- References.- 8: Ion-Induced X-Rays in Solids.- 1. Introduction.- 2. Accelerators and Target Chambers.- 2.1. Ion Sources.- 2.2. Target Chambers.- 3. The Detection and Analysis of X-Rays.- 3.1. The Gas Flow Proportional Counter.- 3.2. The Si(Li) Detector.- 3.3. The X-Ray Crystal or Grating Spectrometer.- 4. The Use of Protons and Helium Ions to Generate X-Rays from Solid Targets.- 4.1. Current Areas of Fundamental Interest.- 4.2. Applications.- 5. The Use of Heavy Ions to Generate X-Rays from Solid Targets.- 5.1. General Background.- 5.2. Physical Processes.- 5.3. Applications.- 6. Conclusions.- References.- Author Index.