AVS2011 Session SS-ThA: Semiconducting & Ferroelectric Surface

BaTiO₃ is currently used in Random Access Memories and is a versatile material with many potential applications. Ferroelectricity at a surface provides unique possibilities to examine mechanisms of molecular adsorption. Experimentally, the polarization can be manipulated in situ by negatively and positively poling the surface. In this manner, the interaction with foreign gaseous species can be controlled. It is necessary to develop fundamental understanding of BaTiO₃ surfaces in order to control the ferroelectric polarization and consequent interactions with adsorbates and in devices. Recent advances have indeed shown that polarization affects molecular adsorption of a variety of molecules; however, the mechanistic processes are not yet understood. Here we present STM of atomically resolved reconstructions, (√5x√5) R26.6°and (3×1) and relate water assoprtion to local polarization. Structural variations and adsorption sites will be discussed in terms of Density Functional Theory predictions. Preliminary results of the interaction of BaTiO₃ (001)’s surface and CO₂ will be presented as well.

The technologically useful properties of semiconductor oxides such as titania and zinc oxide often depend on the concentration and diffusion of point defects. Near-surface effects are particularly important in nanoscale devices because most of the bulk is located in the vicinity of the surface. Past work in our laboratory with silicon and titania has shown that semiconductor surfaces serve as efficient pathways for generation and annihilation of point defects in the underlying bulk. Surfaces can, in addition, support electrically charged defects which create near-surface strong electric fields that can influence the local motion of charged defects resulting in the formation of space-charge layers. The electric field-driven accumulation or depletion of charged oxygen defects in such space-charge regions in metal oxides have direct implications on the performance of nanoscale devices such as gas sensors and memory resistors. Oxygen diffusion behavior was studied by exposing natural-abundance single-crystal rutile to isotopically labeled oxygen gas. The resulting profiles were measured by secondary ion mass spectrometry and subsequently modeled with continuum equations for the reaction, Fickian diffusion and electric field-driven diffusion of the key point defects. The degree of charge build-up at the surface can be quantified by an electric potential (V_i). The profiles calculated show a characteristic steep concentration gradient near the surface followed by a normal bulk diffusion profile. By identifying the charge-mediated field-driven diffusion mechanism as the controlling factor for the near-surface pile-up of oxygen, we demonstrate that this method allows the spatially resolved characterization of space charge regions near the surfaces of crystalline semiconductor metal oxides. The capability to predict oxygen pile-up in nanoscale metal oxide devices may be beneficial in device improvement via defect engineering of surfaces.

GaN is a wide-bandgap semiconductor (3.4 eV) which has approximately 1 eV of upward surface band bending for n-type material, thereby producing a depletion region that can be detrimental to device performance. This band bending can be indirectly measured using the surface photovoltage (SPV) effect. By illuminating the surface with above-bandgap light, electron-hole pairs are created in the depletion region and holes are swept to the surface to reduce the negative charge and band bending. This change in surface charge is measured by a Kelvin probe in an optical cryostat. When the samples are illuminated with a HeCd laser, the SPV signal immediately rises to approximately 0.6, 0.3 and 0.4 eV for the Ga-polar (c-plane), N-polar, and m-plane sample orientations, respectively. The noticeably smaller SPV value for N-polar GaN indicates that this particular orientation has a smaller value of band bending. After this immediate rise, the SPV signal then begins to slowly change due to photo-induced surface processes. In an oxygen environment, the Ga-polar and m-plane orientations demonstrate a slow decrease in SPV of about 0.1 to 0.2 eV, which is attributed to the photo-induced adsorption of oxygen species [1]. There is no observable change for N-polar GaN, indicating that N-polar GaN is less reactive under UV illumination. In vacuum, all three orientations show a slow increase in the SPV signal of 0.1 to 0.2 eV over 1 h, which is due to the photo-induced desorption of charged surface species. When illumination is ceased after 1 h and the surface is restored in vacuum, subsequent illumination results in a constant, steady-state SPV signal, confirming that the photo-induced removal of any surface contamination layers is complete after approximately 1 h. The restoration behavior of the SPV can be fit for all three orientations using a thermionic model with logarithmic time decay [2]. The N-polar GaN is significantly faster, however, and fully restores in only minutes, as opposed to hours or days for the other orientations. It therefore appears that N-polar GaN has the most distinctive SPV behavior among these orientations, with the lowest SPV value, least amount of photo-induced surface reactivity, and fastest restoration behavior.

[1] M. Foussekis, A. A. Baski, and M. A. Reshchikov, Appl. Phys. Lett. 94, 162116 (2009).

[2] M. A. Reshchikov, M. Foussekis, and A. A. Baski, J. Appl. Phys. 107, 113434 (2010).

The different low index surface terminations of lithium tetraborate, Li₂B₄O₇, are dominated by electronic states that fall within the projected band gap of the bulk states. As a pyroelectric material, Li₂B₄O₇ is a wide band gap dielectric, yet the (110) surface has a much smaller band gap because of occupied surface states that fall at binding energies between the valence band maximum and the Fermi level. The (100) surface is dominated, however, by unoccupied surface states that also fall in the gap between the conduction band minimum and valence band maximum, but at binding energies just below the conduction band minimum. These states have been identified in photoemission studies of Li₂B₄O₇(110) and inverse photoemission studies of Li₂B₄O₇(100) [1]. There is good qualitative agreement between these experiments and the electronic band structure calculations showing that the different surface terminations of lithium tetraborate yield very different surface electronic states.