Direct extraction of the electron tunneling effective mass in ultrathin SiO2

Electron transport in ultrathin (t(ox) < 40 Angstrom) Al/SiO2/n-Si structures is dominated by direct tunneling of electrons across the SiO2 barrier. By analyzing the tunneling currents as a function of the SiO2 layer thickness for a comprehensive set of otherwise identical samples, we are able to extract an effective mass for the tunneling electron in the SiO2 layer. Oxide films 16-35 Angstrom thick were thermally grown in situ in a dry oxygen ambient. The oxide thicknesses were determined by capacitance-voltage measurements and by spectroscopic ellipsometry. The tunneling effective mass was extracted from the thickness dependence of the direct tunneling current between an applied voltage of 0 and 2 V, a bias range that has not been previously explored. Employing both a parabolic and a nonparabolic assumption of the E-kappa relationship in the oxide forbidden gap, we found the SiO2 electron mass to be m(P)* = 0.30 +/- 0.02 m(e), and m(NP)* = 0.41 +/- 0.01 m(e), respectively, independent of bias. Because this method is based on a large sample set, the uncertainty in the mass determination is significantly reduced over prior current-voltage fitting methods. (C) 1996 American Institute of Physics.