The alkaline-earth metal oxides BaO and SrO as insulators with high relative dielectric constants of 34 and 14.5, and with band gaps of 4.2 and 6.3 eV, respectively, crystallize in the cubic rock salt structure and are miscible. As we have demonstrated previously, the mixed oxide Ba<SUB>0.7</SUB>Sr<SUB>0.3</SUB>O can be grown epitaxially with the average lattice constant of Si (5.43 Å) as a lattice-matched monocrystalline film on Si(001), with a band gap of 4.3 eV and conduction and valence band offsets of approximately 1.0 eV and 2.2 eV to Si.
Oxide films with thicknesses between 5 and 20 nm were prepared by molecular beam epitaxy with the samples at room temperature. Pure BaO and SrO grow only as amorphous layers because of their significant lattice mismatch (+2.4% of BaO, -5.2% for SrO) with respect to Si(001). For the mixture of 70% BaO and 30% SrO, however, perfectly crystalline Ba<SUB>0.7</SUB>Sr<SUB>0.3</SUB>O films were grown, as revealed by X-ray photoelectron spectroscopy (XPS) and LEED. The interface to Si(001) is atomically sharp with no formation of SiO<SUB>2</SUB> or silicide. For the electrical measurements, Au/Ba<SUB>x</SUB>Sr<SUB>1-x</SUB>O/n-Si(001) MOS capacitors were fabricated by growing a 110 nm thermal SiO<SUB>2</SUB> layer, by lithographical patterning of active windows, and by deposition of the Ba<SUB>x</SUB>Sr<SUB>1-x</SUB>O films and Au metal gates in situ through openings in a tungsten mask overlapping the active windows. These diodes were analyzed by C-V and I-V measurements.
They exhibited typical high-frequency C-V curves without frequency dispersion in the frequency range between 50 kHz and 1 MHz. All C-V curves had only negligible hystereses (<1 mV), indicating the absence of significant rechargeable trap densities in the oxides. The flatband voltages are within 0.3 V of the expected Au/n-Si work function differences, suggesting the absence of larger oxide charges. The effective dielectric constants ε<SUB>eff</SUB> = C<SUB>acc,max</SUB>× d<SUB>ox</SUB>/ε<SUB>0</SUB> determined within a 3-element model are only slightly (10-15%) below the bulk values, which indicates the absence of appreciable interface layers. The leakage current density (at V<SUB>G</SUB>= V<SUB>FB</SUB>+1V) is comparatively large in the amorphous diodes, but it is several orders of magnitude lower (2.1× 10<SUP>-5</SUP>A/cm<SUP>2</SUP>) in the crystalline Ba<SUB>0.7</SUB>Sr<SUB>0.3</SUB>O diode. The Ba<SUB>0.7</SUB>Sr<SUB>0.3</SUB>O diode has a minimum interface trap density value, D<SUB>it</SUB>, of only 6.3× 10<SUP>10</SUP> cm<SUP>-2</SUP>eV<SUP>-1</SUP>, two orders of magnitude below the amorphous oxides. These very low D<SUB>it</SUB> values are comparable to those of the SiO<SUB>2</SUB>/Si interface. Both low leakage currents and low D<SUB>it</SUB> seem to be related to the perfectly lattice matched crystalline Ba<SUB>0.7</SUB>Sr<SUB>0.3</SUB>O/Si interface. We conclude that the crystalline gate oxide Ba<SUB>0.7</SUB>Sr<SUB>0.3</SUB>O displays remarkable electrical properties without any (N<SUB>2</SUB>/H<SUB>2</SUB>) postdeposition annealing procedures: high effective dielectric constant, absence of hysteresis, low leakage currents, and very low interface trap densities in the range of SiO<SUB>2</SUB>/Si.