Characterization of SiGe and SiGeGaP thermoelements

Abstract

Polycrystalline p-type (boron-doped) and n-type (phosphorus-doped) SiGe and SiGeGaP alloys were characterized for their grain size morphology, alloy homogeneity, presence of precipitates and dislocation density. The diagnostic techniques included optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive spectroscopy with both SEM and TEM. Both the binary and the ternary alloys exhibit a mottled grain structure consisting of fine grains (0–0.3 μm) and coarse grains (up to 30 μm). This heterogeneous grain structure is most pronounced in the ternary SiGeGaP alloys. Significant variations were observed in the silicon and germanium contents within a grain, particularly in the doped SiGe alloys. The amount of GaP in solution in the p-type boron-doped ternary alloy did not exceed 0.3%. The microstructure of the SiGeGaP alloys showed the presence of very small precipitates at the boundaries of the fine grains. Phonon-grain boundary scattering due to the fine (0.1–0.3 μm) grain structure and enhanced by the presence of precipitates at the boundaries of these grains could account for the reported higher thermal resistivity of the doped SiGeGaP alloys as compared with the doped SiGe alloys as well as the zone-leveled SiGe alloys. The density of dislocations observed by TEM within the grains varied in the range 10 8–10 9 cm −2 for both the doped binary and the doped ternary alloys. Thus, this structural defect is not considered a significant factor as regards differences in the thermal resistivities of these alloys.