Third harmonic generation as a structural probe of ion-implanted silicon

Third harmonic generation in samples of crystalline silicon implanted at room temperature with 80 and 100 keV As⁺ ions at fluxes up to 3×10¹⁶ cm^-2 is discussed. 1.06 μm, 35 ps pulses of a Nd:YAG laser have been used in a reflection geometry to determine the magnitude, phase, and anisotropy of χ⁽³⁾. Loss of anisotropy for ion fluxes above ≈10¹⁴ cm^-2 has been identified with amorphization of the near surface region, but little variation in the phase of χ⁽³⁾, which was earlier suggested by others as a possible origin of the minimum in the third harmonic intensity at the amorphization threshold, is observed. The minimum is interpreted as one in the magnitude of χ⁽³⁾ and it is shown that the linear optical properties, through the linear reflectivity, also display an extremum at the amorphization threshold. It is suggested that these variations are probably due to variations in the short range electronic order. Third harmonic generation is therefore seen to be a unique diagnostic of ion-implanted materials in that it is capable of showing variation in electronic response both below and above the amorphization threshold.