Optical characterization of processed gallium arsenide

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Virginia Tech


Raman scattering and ultraviolet-visible reflectivity have been used to characterize the structural and electronic changes that occur in GaAs during ion implantation and subsequent annealing. In this work, the damaged structure is modelled as an amorphous GaAs matrix embedded with GaAs microcrystals. The longitudinal-optic (LO) Raman-line characteristics were monitored to determine the amorphous volume fraction, the average microcrystal diameter and, for the annealed samples, the carrier concentration. An oscillator analysis of the reflectivity spectra, along with the effective medium approximation, was carried out to determine the linewidths of the interband peaks and the amorphous volume fraction in the damage layer.

To determine damage depth profiles, spectra were taken as a function of the amount of material removed via chemical etch. A new method of interpreting reflectivity spectra was developed to deal with the etchant-induced roughness. This roughness reduced the reflectivity by a constant factor in the region between 4.5 and 5 eV. The ratio between reflectivities at 4.55 and 4.75 eV was monitored to determine qualitatively the amount of damage.

The annealing studies show that structural recovery occurs at a lower temperature than that for which electrical activation occurs. The depth profile of a sample annealed at 400°C reveals that nucleation takes place not only at the boundary between the damaged and undamaged layers (i.e., "epitaxial regrowth") but also at the microcrystal/amorphous interfaces within the damage layer.

The oscillator analysis of the dielectric properties was further developed, and a connection was established between the Strengths, positions, and linewidths of the interband oscillators and the shift in position of the LO Raman line. The results indicate that the static dielectric constant is independent of microcrystal size.

A comparison between (211) and (100) oriented Si-implanted GaAs was done as well, showing greater near surface damage and a shallower total damage layer for the (211) samples.

Finally, a method for characterizing damage, based on the observed shifts of the two-phonon ("2LO") Raman peak as the incident photon energy is varied around the E₁ interband energy (2.9 eV), has been developed. The results suggest that the total mass of the electron-hole pair involved in the scattering process increases even for large (>400 Å) microcrystals. The 525°C annealed sample had little damage, and was studied with this technique.