Structural characterization of highly boron doped SiGe/Si heterostructures

Changes in lattice parameters of SiGe layers were studied when the epilayers were highly doped with boron. SiGe base layers of heterojunction bipolar transistor (HBT) structures, grown by means of chemical vapor deposition, were used for the evaluation. In these device structures a thin highly boron doped SiGe layer is sandwiched in between thin undoped SiGe layers of equal Ge content. High-resolution X-ray diffraction (XRD), secondary ion mass spectroscopy and ellipsometry were employed to investigate the effect of boron doping (>1×1019 cm−3) on the structural properties. The results of full pattern simulations of X-ray rocking curves clearly revealed that the lattice of the doped SiGe layers is contracted with respect to the undoped layers. The observed differences of the SiGe lattice parameters are directly related to the boron concentration. Taking the doping effect not into account for the determination of the Ge content by XRD leads to a significant deviation from the true value. For a doping level of 6×1019 cm−3 this results in an apparent difference in Ge concentration of about 2.5% between the doped SiGe and the undoped SiGe cladding layers.