Silicon Delta Doping in GaAs: An Ongoing Enigma

Infrared (IR) absorption and Raman scattering are reported from the localized vibrational modes (LVM) of Al and Si δ-layer superlattices in MBE (100) GaAs grown at 400°C as a function of the total areal concentrations, [A1] A and [Si] A respectively. The Al superlattices show the expected behavior on passing from sub-monolayer (ML) to thicker layers (thin AlAs) since the impurities still occupy only Ga-sites. The behavior is very different from that found for Si δ-layers. In addition to Si Ga reported previously, we now show that Si As , Si Ga -Si As pairs and the electron trap Si-X are also present in Si δ-layers and superlattices for 0.05 ≤ [Si] A ≤ 0.5 ML. The conductivity of these structures and the concentrations of substitutional Si in GaAs at all sites fall to zero for [Si] A > 0.5 ML but a Raman feature at 470–490 cm −1 , attributed to the vibrations of covalent Si-Si bonds is then detected. This feature is not observed in structures containing very closely spaced dilute (0.01 ML) Si δ-planes. It is inferred that long-range Si diffusion does not occur in the bulk crystal, although there could be surface diffusion during Si deposition. The maximum measured carrier concentrations are always less than 2 × 10 19 cm −3 , the DX limit. The redistribution of Si amongst the various lattice sites is discussed in terms of Si Ga DX-like displacements occurring during growth, followed by local thermally activated diffusion jumps. It is speculated that As Ga antisite defects and Ga-vacancies are produced by this process. The reason why the Si δ-layer is non-conducting remains unclear.