Design and characterization of thick InxGa1-xAs metamorphic buffer layers grown by hydride vapor phase epitaxy

Thick InxGa1-xAs metamorphic buffer layers (MBLs) grown by hydride vapor phase epitaxy (HVPE) were studied. Relationships between MBL properties and growth parameters such as grading rate, cap layer thickness, final x(InAs), and deposition temperature (T-D) were explored. The MBLs were characterized by measurement of in-plane residual strain (epsilon(parallel to)), surface etch pit density (EPD), and surface roughness. Capping layer thickness had a strong effect on strain relaxation, with thickly capped samples exhibiting the lowest epsilon(parallel to). EPD was higher in samples with thicker caps, reflecting their increased relaxation through dislocation generation. epsilon(parallel to) and EPD were weakly affected by the grading rate, making capping layer thickness the primary structural parameter which controls these properties. MBLs graded in discrete steps had similar properties to MBLs with continuous grading. In samples with identical thickness and 10-step grading style, epsilon(parallel to) increased almost linearly with final x(InAs), while total relaxation stayed relatively constant. Relaxation as a function of x(InAs) could be described by an equilibrium model in which dislocation nucleation is impeded by the energy of the existing dislocation array. EPD was constant from x(InAs) = 0 to 0.24 then increased exponentially, which is related to the increased dislocation interaction and blocking seen at higher dislocation densities. RMS roughness increased with x(InAs) above a certain strain rate (0.15%/mu m); samples grown below this level possessed large surface hillocks and high roughness values. The elimination of hillocks at higher values of x(InAs) is attributed to increased density of surface steps and is related to the out-of-plane component of the burgers vector of the dominant type of 60 degrees dislocation. T-D did not affect epsilon(parallel to) for samples with a given xInAs. EPD tended to increase with T-D, indicating dislocation glide likely is impeded at higher temperatures.