Variation in the lattice parameter and crystal quality of commercially available Si-doped GaAs substrates

Variations in the lattice parameter and crystal quality of commercially available GaAs (n-type, Si: 1-7 x 10(18) cm(-3)) substrates have been 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 either the same as or slightly larger (similar to 0.0001 Angstrom) than semi-insulating GaAs(SI) while crystals grown by vertical gradient freeze (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 showed a faint cellular dislocation network related to threading dislocations, VGF samples showed almost flawless crystal quality with only a faint swirl pattern commonly associated with small amounts of segregation at the growth front. Topographs of liquid-encapsulated Czochralski (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 GaAs(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) range 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 effects. On the other hand, the larger lattice constant of HE crystals compared to GaAs(SI) crystals, is consistent with the dilation of the lattice driven by the free electron density which outweighs the contraction of the lattice by the Si-dopant size effect.