ZnO(0001) surfaces probed by scanning tunneling spectroscopy: Evidence for an inhomogeneous electronic structure

Abstract

The stability of the polar Zn-terminated ZnO surface is probed by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Surface states in the bandgap of ZnO are evidenced by STS and their presence is correlated with the local surface corrugation. Very defective surface regions are characterized by a bulk electronic structure showing a wide bandgap while nanometer-scale defect free regions exhibit a narrower bandgap and surface states. We also image atomically resolved (3×3)R30° reconstructions on the defect-free areas.

Atomic resolution non-contact atomic force microscopy of clean metal oxide surfaces

“…An atomic (1 × 1) pattern on the surface was, however, never successfully resolved by STM, and some controversy still remains concerning the surface structure, as discussed in recent reports on different reconstructions [163,164] and effects of temperature [158]. The apparent difficulties in achieving atomic resolution by STM may be related to the rather low conductivity of ZnO.…”

Physical Chemistry of the Mn/ZnO (0001̅) Interface Probed by Hard X-ray Photoelectron Spectroscopy

“…The confinement is explained by the very localized nature of VIGS in real space leading to the formation of accumulation layers at the ZnO (0001) surface, which can even show quantized electron sub-band states. 36,44 Upon annealing, a thin MnO film forms on top of the surface of ZnO. As seen in Figure 5a, the valence band edge appears at a binding energy roughly 1 eV above that of ZnO.…”

X-ray standing waves reveal lack of OH termination at hydroxylated ZnO(0001) surfaces

“…Indeed, stoichiometric surfaces have not been observed by scanning probe microscopy (SPM). Hitherto resolved structures can be roughly separated in microscopically reconstructed but macroscopically flat surface areas [15][16][17] and areas that are reconstructed into triangular islands and holes (and thus * jens.niederhausen@helmholtz-berlin.de macroscopically rough) and additionally exhibit high densities of vacancies and pits [13,15,[17][18][19][20][21]. The presence of OH at the surfaces investigated in these SPM measurements is expected even if the samples were not intentionally exposed to water or hydrogen [5], because under typical UHV conditions, residual H 2 O and H 2 , as well as atomic H created by the pressure gauge filament and/or ion getter pump, [7,18,22] amount to nonintentional doses of 1 langmuir (L) on the timescale of a few hours.…”

“…A problem with SPM of Zn-ZnO is that only a part of the surface area can be unambiguously determined [15,16,19]. This experimental uncertainty and results from earlier calculations [5,14] can explain why predominately OH-terminated Zn-ZnO surfaces are assumed when rationalizing results from spectroscopy and diffraction experiments [10,11,14,23,[25][26][27].…”

“…Therefore, approach (II) is not affected by d H variations in the Zn-ZnO surface crystal structure. The Zn-ZnO surfaces were prepared according to three different recipes (labeled A-C) that represent the range of annealing temperatures employed in most previous Zn-ZnO surface studies [7,13,15,17,18,20,22,36,37] and also include H 2 O exposure [37]. All crystals were hydrothermally grown (CrysTec, Berlin), annealed under atmospheric conditions (1000°C, 2 h) and in-situ (420°C, 10 min), and Ar + sputtercleaned (0.5 keV, 15 min) [29].…”