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Measurement of free carriers in silicon and silicon carbide using infrared ellipsometry
Abstract
Free carrier concentration profiles in samples of silicon and silicon carbide were determined by Fourier Transform Infrared (FTIR) variable angle spectroscopic ellipsometry. The silicon samples consisted of dopant ion-implanted and CVD homoepitaxial p-/p+ wafers, and the silicon carbide samples consisted of bulk, homoepitaxial n-/n+ and hydrogen-implanted wafers. The technique exploits carrier absorption in the mid-infrared spectral range, and combines the sensitivity of ellipsometry with a simple Drude free carrier absorption model to determine the carrier profile. In this study, the carrier profiles were modeled as graded multilayers that were constrained to a specific functional form (e.g., Gaussian, complementary error function) when appropriate. Homoepitaxial p-/p+ doped silicon samples (before and after annealing) were measured, and the results were compared to theory. Carrier profiles from boron and arsenic ion implanted that had been subjected to furnace or Rapid Thermal Annealing (RTA) annealed silicon wafers were compared to Spreading Resistance Probe and Secondary Ion Mass Spectrometry profiles. The dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates was also measured. Photon absorption by transverse optical phonons produces a strong reststrahlen band between 797 and 1000 cm−1 with the effects of phonon anisotropy being observed in the region of the longitudinal phonon energy (960 to 1000 cm−1). The shape of this band is influenced by plasma oscillations of free electrons, which we describe with a classical Drude equation. For the 6H-SiC samples, we modify the Drude equation to account for the strong effective mass anisotropy. Detailed numerical regression analysis yields the free electron concentrations, which range from 7 × 1017 to 1019 cm −3, in good agreement with electrical and secondary ion mass spectrometry measurements. Finally, we observe the Berreman effect near the longitudinal optical phonon energy in n-/n+ homoepitaxial 4H SiC and hydrogen implanted samples, and we are able to determine the thickness of associated surface layers.
Subject Area
Electrical engineering|Condensation|Optics
Recommended Citation
Tiwald, Thomas Edward, "Measurement of free carriers in silicon and silicon carbide using infrared ellipsometry" (1999). ETD collection for University of Nebraska-Lincoln. AAI9929237.
https://digitalcommons.unl.edu/dissertations/AAI9929237