Dynamics of Space Charge Polarization and Electrical Conduction in Low Alkali Boroaluminosilicate Glasses

Low alkali boroaluminosilicate glasses are of tremendous interest for high temperature electronics primarily due to their superior high temperature dielectric properties and extraordinary energy densities. Therefore, evaluating factors causing electrical conduction in these materials is of great importance since it has direct correlation with the device reliability and performance. This research focuses on understanding dynamics of space charge polarization and mechanisms controlling electrical conduction in these glasses. Both DC and AC characterization techniques were developed to elucidate electronic and ionic conduction mechanisms under a variety of temperatures, electric field and frequency conditions. Ionic conduction and space charge polarization have been studied in low alkali glasses as a function of electric field and temperature by thermally stimulated depolarization and low frequency impedance spectroscopy. Moreover, due to the low alkali content in these glasses, it was possible to study the transport properties of alkaline earth ions in multicomponent silicate glasses. It was observed that the potential energy barrier height for ionic hopping was reduced at high electric field. Impedance spectroscopy and second harmonic generation microscopy techniques were applied to determine the thickness and electric field distribution across the cation depleted layer that was generated during the thermoelectric poling. Both of these measurements show that the depletion layer thickness depends on the poling conditions and the intrinsic breakdown strength of the material. In addition, a relationship between the charge and electric field distribution in the depletion layer was determined for a number of glasses with different alkali content. The high breakdown strength of these glasses facilitated the study of electronic conduction under fields greater than 108 V/m. Conduction under these high fields was investigated using high field thermally stimulated depolarization current measurements. The electrons participating in the high field conduction were generated in the depletion layer through Poole-Frenkel emission. This involves field-enhanced excitation of electrons from the trapped states to the conduction band of the glass. It is suggested that high field intrinsic breakdown in thin alkali free boroaluminosilicate glasses may occur when the conduction band gets populated by electrons emitted through Poole-Frenkel emission. Consequently breakdown can occur through an avalanche effect.