P. Schmuki et al., IN-SITU CHARACTERIZATION OF ANODIC SILICON-OXIDE FILMS BY AC-IMPEDANCE MEASUREMENTS, Journal of the Electrochemical Society, 142(5), 1995, pp. 1705-1712
Anodic oxide films grown on Si in an aqueous NH4OH solution have been
characterized by in situ. ac impedance techniques and their behavior c
ompared to thermal SiO2 oxides. Mott-Schottky analysis of oxide-free p
- and n-type Si leads to doping concentrations which are in excellent
agreement with independent data of conductivity measurements. The grow
th of anodic oxides can be monitored with in situ impedance measuremen
ts. Anodic oxide films exhibit a nonideal capacitive behavior and henc
e a constant-phase element has to be introduced for the evaluation of
the experimental results in terms of an equivalent circuit. For oxides
thicker than approximate to 50 Angstrom the roughness factor for the
oxide surface can be determined by a comparison of the capacitance res
ults with the film thickness obtained from x-ray photoelectron spectro
scopy (XPS) measurements. For thinner oxides the space-charge capacita
nce in the Si and the effect of the surface roughness have to be consi
dered to obtain a good agreement of impedance and XPS data. A comparis
on of impedance spectra of anodic and thermal oxides shows that anodic
oxides not only exhibit less ideal capacitive behavior but also show
a drastically lower charge-transfer resistance indicating a relatively
high mobility of ions in the anodic oxide film. The nonideality of th
e anodic oxides is also evident from cyclic capacitance-voltage (C-V)
measurements in which a strong hysteresis is observed in contrast to t
hermal oxides for the anodic oxide films. Furthermore, the dielectric
constant of the anodic oxides is higher than for ideal SiO2 films, whi
ch can most probably be attributed to the presence of hydroxides in th
e film. The quality of the anodic oxide films can be significantly imp
roved by annealing. The imperfect nature of the as-grown anodic oxide
films is also reflected in a lower chemical resistance to etching in N
H4OH solutions compared to thermal oxides. By postoxidation annealing
also the chemical resistance of anodic oxides is greatly improved.