Electrolyte effects on charge, polarization, and conduction in thin anodic Al2O3 films. I. Initial charge and temperature-dependent polarization

Abstract

A sequence of three current-voltage (I-V) curves of Al–Al2O3–Au capacitors has been used to study the effect of anodizing electrolyte on Qin, the initial charge introduced into Al2O3 during anodization and the temperature dependence of polarization of anodic Al2O3. The first I-V curve, with VA<0V, polarizes the Al2O3 film; the difference between the next two I-V curves is a measure of polarization induced by the negative voltage. Two aqueous electrolytes, 0.1M ammonium pentaborate (bor-H2O) and 0.1M ammonium citrate (citrate), and one nonaqueous electrolyte, 0.1M ammonium pentaborate per liter of ethylene glycol (bor-gly), are used for anodizing; Al2O3 thicknesses are between 12 and 54nm. The galvanostatic anodizing rate is the same for anodizing in bor-H2O and bor-gly electrolytes after initial transients. The anodizing rate in citrate electrolyte is constant but is 0.7 times the rate in the other two electrolytes. Qin for Al2O3 films formed in bor-H2O and bor-gly is proportional to Al2O3 thickness; it is distributed throughout the film. The magnitude of Qin is ∼100 times greater for Al2O3 films formed in the aqueous electrolyte. Qin for Al2O3 films formed in citrate is nearly constant with thickness. Qin anneals out when I-V curves are measured. Two quantities are measured that depend on polarization of anodic Al2O3, QPK, and Qmax. QPK is due solely to polarization produced by negative bias. Both polarization and conduction currents contribute to Qmax. Both QPK and Qmax depend exponentially on the polarizing field. Maximum values of QPK for samples anodized in bor-H2O or citrate are ∼13 times larger than for Al2O3 formed in bor-gly; maximum values of Qmax are more than 100 times larger for anodic Al2O3 formed in aqueous electrolytes. The temperature dependence of Qmax is also less for Al2O3 films formed in bor-gly than for Al2O3 formed in bor-H2O or citrate.