C. J. Tsai

Strain modification in coherent Ge and SixGe1-x epitaxial films by ion-assisted molecular beam epitaxy

We have observed large changes in Ge and SixGe1−x layer strain during concurrent molecular beam epitaxial growth and low‐energy bombardment. Layers are uniformly strained, coherent with the substrate, and contain no dislocations, suggesting that misfit strain is accommodated by free volume changes associated with injection of ion bombardment induced point defects. The dependence of layer strain on ion energy, ion‐atom flux ratio, and temperature is consistent with the presence of a uniform dispersion of point defects at high concentration. Implications for distinguishing ion‐surface interactions from ion‐bulk interactions are discussed.

X-Ray Diffraction Determination of Stresses in Thin Films

This paper presents the methodology employed in the determination of the stress tensor for thin crystalline films using x-ray rocking curves. Use of the same equipment for the determination of the average stress in poly- or non-crystalline thin films attached to a crystalline substrate is also discussed. In this case the lattice curvature of the substrate is determined by measurement of the shift In the Bragg peak with lateral position in the substrate. Strains in single crystal layers may be measured using Bragg diffraction from the layers and from the substrate or a reference crystal, with the highest strain sensitivity of any known technique. The difference in Bragg angles for a strained and an unstrained crystal is related to the change in d spacing of the Bragg planes, and the elastic strain is related to this angular difference. The separation of two peaks on an x-ray rocking curve is generally not equal to the difference in Bragg angles of two diffracting crystals, so diffractometer measurements must be carefully Interpreted in order to obtain x-ray strains in crystalline films (x-ray strains are strains relative to the reference crystal). The unstrained d spacings of the film and the d spacings of the reference crystal must be known to obtain the elastic strains in the film, from which the stress tensor is determined.

Strain modification by ion-assisted molecular beam epitaxy in the SixGe1−x alloy system: a kinetic analysis

Significant changes in strain are produced in SixGe1−x epitaxial films grown on Si and Ge (001) substrates as a result of low energy ion beam assisted molecular beam epitaxy (IAMBE). Films grown in the temperature range of 200 to 450°C with concurrent Ar+ or Xe+ ion bombardment are coherently and uniformly strained in the growth direction by up to 1.5% in Ge films and 0.5% in Si films and contain no dislocations. Analysis of the strain modification kinetics suggests that defect complexes produced by the injection of ion beam induced native point defects and trapped gas are responsible for the strain changes. The dependence of film strain on ion/atom flux ratio can be explained by a steady state analysis in which beam-generated defects are annihilated at the growth surface. The dependence of strain modification on ion energy and ion species indicates the relative importance of surface defects and bulk defects in the free volume expansion of the films. Evolution of strain upon post-growth isochronal annealing suggests the complex nature of as-grown defects and rearrangement of defects during annealing. The relative roles of native defects and trapped gas in strain modification and thermal stability are discussed.

Application of x‐ray interference method for residual strain measurement in low energy Ar ion‐bombarded Si (001)

We have employed the x‐ray interference method for measurement of the residual strains in Si (001) surfaces bombarded by low energy Ar+. This method, combined with transmission electron microscopy, permits determination of the average residual strain in very thin bombarded layers. The residual strain in the bombarded layers was found to monotonically increase as the density of pointlike defects increases and saturates upon the formation of extended defects. Annealing data also suggest that defects formed by low energy ion bombardment at moderate temperature (≊450 °C) require high temperature (≳800 °C) annealing for their removal.

Strain Modification and Thermal Stability of SixGe1−x Films Grown by Ion-Assisted Molecular Beam Epitaxy

Significant changes in strain are produced in Si x Ge 1−x epitaxial films grown on Si and Ge (001) substrates as a result of low energy ion beam assisted molecular beam epitaxy (IAMBE). Films grown with concurrent Ar + or Xe + ion bombardment are coherent and uniformly strained in the growth direction by up to 1.5% in Ge films and 0.5% in Si films and contain no dislocations. The dependence of the films strain perpendicular to the growth surface on ion-atom flux ratio, and ion energy can be explained by the injection of uniformly distributed point defects. Post-growth isochronal annealing of Si x Ge 1−x films suggests that the existing defects in the IAMBE films are defect complexes and that the strain relaxation path is determined by the overall thermodynamic driving force toward the strain-relieved state.

Lattice Disordering, Phase Transition, and Substrate Temperature Effects in MeV-ION-Implanted III-V Compound Semiconductors

An experimental study of lattice disordering, the crystalline-to-amorphous (c-a) phase transition, and substrate temperature effects in MeV-ion-implanted III-V compound semiconductor crystals is presented. A comparison has been made between the GaAs and InP systems, which have been implanted with 2 MeV oxygen ions at either room temperature (RT) or near liquid nitrogen temperature (LT). A strong in situ dynamic annealing has been found in the RT implanted GaAs, and the LT implanted GaAs exhibits heterogeneous (at the end-of-range of the ions) and homogeneous (at the subsurface region) c-a phase transitions. In InP crystals, in situ annealing is much less pronounced in RT implantation, and dose-dependent damage nucleation and layer-by-layer amorphization take place. LT implantation results in lattice disordering and phase transition with a critical dose at least one order lower than that for GaAs. The mechanisms and kinetics of lattice disordering by ion irradiation are also discussed.

Dynamics of Microstructure in the Early Stages of Ion Beam Assisted Film Growth

Recent progress in low energy ion-surface interactions, and the early stages of ion-assisted epitaxy of semiconductor thin films is described. Advances in three areas are discussed: dynamics of displacements and defect incorporation, nucleation mechanisms, and the use of ion bombardment to modify epitaxial growth kinetics in a truly surface-selective manner.