Polarity-dependent photoemission of in situ cleaved zinc oxide single crystals
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X-ray photoemission spectroscopy using synchrotron radiation from 100 to 1486.6 eV was used to investigate polarity-dependent differences between the Zn-polar (0001) and the O-polar (0001) faces of ultrahigh vacuum cleaved hydrothermally grown ZnO single crystals. The cleaved polar surfaces showed a characteristic polarity effect in that the intensity of emission from the lowest binding energy O 2p related valence band states was significantly stronger on the Zn-polar face, even when the cleaved surfaces were imperfect with irregular nonatomically flat features. A residual submonolayer hydroxyl termination of approximately 0.5 ML was observed on both the Zn-polar and O-polar surfaces immediately after cleaving. The near-surface downward band bending on the O-polar face was removed by the cleaving process leaving almost flat bands, while on the cleaved Zn-polar face, emission from states above the valence band edge was observed.
I. INTRODUCTION
ZnO is a wide band gap metal oxide semiconductor with a direct gap of 3.35 eV at 300 K that is of particular interest for applications in ultraviolet optoelectronics, transparent electronics, environmental sensing, and catalysis. However, many of its electrical and optical properties such as near-band-edge photoluminescence, work function, photoconductivity, and Schottky contact barrier height vary significantly with crystallographic polarity.1–4 These variations have been linked to the wurtzite crystal structure and the high ionicity of the Zn–O bond (0.616 on the Phillips ionicity scale).5 When viewed along the c-axis, ZnO consists of alternating planes of Zn and O atoms, arranged in 3-fold coordinated “double layers” separated by a single Zn–O bond as shown in Fig. 1. Any termination (during growth or cleavage) of the crystal perpendicular to the c-axis results in one of two structurally and electrically different polar surfaces: the Zn-polar (0001) face, which consists of an outer plane of Zn atoms, or the O-polar (0001) face, which consists of an outer plane of O atoms. X-ray photoemission6 and grazing angle surface x-ray diffraction studies7 have shown that these polar surfaces are most likely terminated by a persistent (1 1) coverage of hydroxyl groups. These hydroxyl groups are thought to play an important role in stabilizing the Zn-polar and O-polar surfaces, which otherwise are electrostatically unstable.8,9 Other stabilization mechanisms have also been proposed, such as (i) a fractional
charge transfer of 0.5 e from the O-polar to the Zn-polar face and (ii) surface reconstructions involving the removal of 25% of the surface Zn/O atoms.8–11 The high ionicity of the Zn–O bond within the wurtzite crystal structure also results in a large spontaneous polarization (5.7 106 C/cm2) along the c-axis.12 This in turn gives rise to equal and opposite bound spontaneous polarization charges on the Zn-polar (5.7 106 C/cm2) and O-polar (15.7 106 C/cm2) faces that are present at zero strain. The associated internal electric field is expected to influence the distribut
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