VARIATION IN THE LATTICE-PARAMETER AND CRYSTAL QUALITY OF COMMERCIALLY AVAILABLE SI-DOPED GAAS SUBSTRATES

Variations in the lattice parameter and crystal quality of commerciall y available GaAs (n-type, Si: 1-7 x 10(18) cm(-3)) substrates have bee n studied by high-resolution X-ray diffraction (HRXRD) and asymmetric crystal topography (ACT). Crystals grown by vertical Bridgman (VB) and horizontal Bridgman (HE) typically have a lattice parameter that is e ither the same as or slightly larger (similar to 0.0001 Angstrom) than semi-insulating GaAs(SI) while crystals grown by vertical gradient fr eeze (VGF) had a lattice parameter that could be as much as similar to 0.0002 Angstrom smaller than GaAs(SI). ACT topography confirmed also fundamental differences in crystal quality. While VB and HE crystals s howed a faint cellular dislocation network related to threading disloc ations, VGF samples showed almost flawless crystal quality with only a faint swirl pattern commonly associated with small amounts of segrega tion at the growth front. Topographs of liquid-encapsulated Czochralsk i (LEG) samples showed extensive cross-hatching probably caused by the presence of precipitates. The observed lattice parameter differences can be explained by the presence of boron impurities and not by other defects as previously postulated. SIMS measurements found that VGF GaA s(Si, n-type) contained boron in the 3-4 x 10(18) cm(-3) concentration range and HB crystals contained boron only in the low 10(16) cm(-3) r ange while HB crystals contained no detectable boron. In VGF crystals the boron concentration is so high (and the boron atom so small) that the lattice contraction can be entirely attributed to boron size effec ts. On the other hand, the larger lattice constant of HE crystals comp ared to GaAs(SI) crystals, is consistent with the dilation of the latt ice driven by the free electron density which outweighs the contractio n of the lattice by the Si-dopant size effect.