Thermal relaxation in strained InGaAs/GaAs heterostructures

The kinetics of strain relaxation during annealing of epitaxial InGaAs films grown on GaAs by MOCVD at atmospheric pressure was studied by optical microscopy and transmission electron microscopy. The density of misfit dislocations and crosshatching was measured as a function of annealing temperature and annealing time. These experiments show that the generation of misfit dislocations during the annealing process is thermally activated. The kinetic rate constant increases as thickness increases. After long annealing times, the sample reaches its steady-state condition in which a residual strain apparently still exists. Apparently this residual strain is not accommodated by misfit dislocations and does not change with annealing temperature nor sample thickness.     

Role of interface chemistry and growing surface stoichiometry on the generation of stacking faults in ZnSe/GaAs

The existence of Zn-As and vacancy-contained Ga-Se interfacial layers are suggested by transmission electron microscopy of Zn-and Se-exposed (or - reacted) ZnSe/GaAs interfaces, respectively. A very low density of faulted defects in the range of ∼10⁴cm² was obtained in samples with Zn passivation on an Asstabilized GaAs-(2 × 4). However, the density of As precipitates increases as the surface coverage of c(4 × 4) reconstruction increased on the Zn-exposed Asstabilized GaAs-(2 × 4) surface and this is associated with an increase of the density of extrinsic-type stacking faults bound by partial edge dislocations with a core structure terminated on additional cations. On the other hand, densities of extrinsic Shockley-and intrinsic Frank-type stacking faults are of ∼5 × 10⁷/cm² in samples grown on Se-exposed Ga-rich GaAs-(4 × 6) surfaces. Annealing on this Se-exposed Ga-rich GaAs-(4 × 6) generated a high density of vacancy loops (1 × 10⁹/cm²) and an increase of the densities of both Shockley-and Frank-type stacking faults (>5 × 10⁸/cm²) after the growth of the films. Furthermore, we have studied the dependence of the generation and structure of Shockley-type stacking faults on the beam flux ratios in samples grown on Zn-exposed As-stabilized GaAs-(2 × 4) surfaces. Cation-and anion-terminated extrinsic-type partial edge dislocations were generated in samples grown under Zn-and Se-rich conditions, respectively. However, an asymmetric distribution on defect density under varied beam flux ratios (0.3 ≤ P_Se/P_Zn ≤ 10) is obtained.     

Preferential etching of dislocations and stacking faults in gallium phosphide

A combination of optical microscopy and transmission electron microscopy has been used to provide direct evidence that pits formed by the action of a solution of 3 HF:1 H₂O₂ on epitaxial and substrate GaP material close to the (001 ) orientation are associated with dislocations. The formation of different pit shapes can be correlated with previous work on etch pits in GaP, namely the formation of D and S pits. Grooves have been produced on certain crystallographic orientations which are associated with certain types of stacking faults. It is now possible to use this etch for a direct count of the dislocation density in (001) GaP and to indicate the extent of stacking faults.     

The isolation and nucleation of misfit dislocations in strained epitaxial layers grown on patterned,ion-damaged GaAs

As shown previously, the misfit dislocation density of strained epitaxial III–V layers can be significantly reduced by isolating sections (via patterned etching) of a GaAs substrate before epitaxial growth. A disadvantage of this technique is that the wafer surface is no longer planar, which can complicate subsequent device fabrication. As an alternative, we have investigated growth of 350 nm of In_0.5Ga{0.95}As by molecular beam epitaxy at two temperatures on substrates which were patterned and selectively damaged by Xe ion implantation (300 keV, 10¹⁵ cm²). Selectively etched substrates were prepared as reference samples as well. The propagation of the misfit dislocations was stopped by the ion-implanted regions of the low growth temperature (400° C) material, but the damaged portions also acted as copious nucleation sources. The resulting dislocation structure was highly anisotropic, with dislocation lines occurring in virtually only one direction. At the higher growth temperature (500° C) the defect density fell, but the ion damaged sections no longer blocked dislocation glide. Images from cathodoluminescence and transmission electron microscopy show thatthe low growth temperature material has a dislocation density of 70,000 cm^-1 in the 110 direction and less than 10,000 cm^-1 in the 110 direction. Ion channeling and x-ray diffraction show that strain is relieved in only one direction. The strain relief is consistent with the relief derived from TEM dislocation counts and Burgers vector determination. However, even this high dislocation count is not sufficient to reach the expected equilibrium strain. Reasons for the anisotropy are discussed.     

Structural defects in laser crystallized silicon ribbons and their influence on photovoltaic behaviour

Analysis of structural defects in laser crystallized silicon sheets has been carried out. It is found that (non-equilibrium) defect structure strongly depends on growth parameters such as thermal profiles, ambient gases and the operational laser parameters. Laser crystallization with intense beams can lead to surface defects which are formed by convection flow in the melt (Marangoni effect). Characteristic defects due to various growth ambients are outlined. Influences of ribbon defects on photovoltaic performance are also described. By properly controlling growth conditions, high quality sheets have been grown which yield solar cell efficiency ∼ 13%, and better performance can be predicted based on projected improvements in growth conditions.