CHARACTERIZATION OF Si NANOSTRUCTURES USING INTERNAL QUANTUM EFFICIENCY MEASUREMENTS

Hemispherical reflectance and internal quantum efficiency measurements have been employed to evaluate the response of Si nanostructured surfaces formed by using random and periodic reactive ion etching techniques. Random RIE-textured surfaces have demonstrated solar weighted reflectance of= 3 ‘?XO over 3001200-nm spectral range even without the benefit of anti-reflection films. Random RIE-texturing has been found to be applicable over large areas (-1 80 cmz) of both single and multicrystalline Si surfaces. Due to the surface contamination and plasma-induced damage, RIE-textured surfaces did not initially provide increased short circuit current as expected from the enhanced absorption. Improved processing combined with wet-chemical damage removal etches resulted in significant improvement in the short circuit current with IQEs comparable to the random, wet-chemical Iy textured surfaces. An interesting feature of the RIE-textured surfaces was their superior performance in the near IR spectral range. The response of RIE-textured periodic surfaces can be broadly classified into three distinct regimes. Onedimensional grating structures with triangular profiles are characterized by exceptionally low, polarizationindependent reflective behavior. The reflectance response of such surfaces is similar to a graded-index antireflection film. The IQE response from these surfaces is severely degraded in the UV-Visible spectral region due to plasma-induced surface damage. One-dimensional grating structures with rectangular profiles exhibit spectral Iy selective absorptive behavior with somewhat similar IQE response. The third type of grating structure combines broadband anti-reflection behavior with significant IQE enhancement in 8001200-nm spectral region. The hemispherical reflectance of these 2D grating structures is comparable to random RIE-textured surfaces. The IQE enhancement in the long wavelength spectral region can be attributed to increased coupling into obliquely propagating transmitted diffracted orders inside the Si substrate. Random RIE texturing techniques are expected to find widespread commercial applicability in low-cost, large-area multicrystalline Si solar cells. Grating-texturing techniques are expected to find applications in thinflm and space solar cells. .. . ..... .,., . .. ,,. . .,. , . ...~-y