Zn-polar Face Research Articles

Schottky barrier height behavior of Pt–Ru alloy contacts on single-crystal n-ZnO

We investigated the Schottky barrier height (SBH) behavior of binary alloy Schottky contacts on n-type zinc oxide (n-ZnO) single crystals. Pt–Ru alloy electrodes were deposited on the Zn-polar and O-polar faces of ZnO substrates by combinatorial ion-beam deposition under identical conditions. The crystal structures of the Pt–Ru alloy film changed from the Pt phase (cubic structure) to the Ru phase (hexagonal structure) in the Pt–Ru alloy phase diagram with decreasing Pt content. The SBH, determined from current–voltage measurements, decreased with decreasing Pt content, indicating that the SBH behavior also followed the Pt–Ru alloy phase diagram. The alloy electrodes on the Zn-polar face showed better Schottky properties than those on the O-polar face. Hard x-ray photoelectron spectroscopy revealed a difference in the interface oxidization of the Pt–Ru alloy: the interface of the O-polar face and Pt–Ru mixed phase with poor crystallinity had a more oxidized layer than that of the Zn-polar face. As a result of this oxidization, the O-polar face, Pt–Ru mixed, and Ru phases showed poor Schottky properties.

Bulk transport measurements in ZnO: The effect of surface electron layers

Magnetotransport measurements and x-ray photoemission spectroscopy were used to investigate the surface conductivity of ZnO. Near-surface downward band bending, consistent with electron accumulation, was found on the polar and nonpolar faces of bulk ZnO single crystals. A significant polarity effect was observed in that the downward band bending was consistently stronger on the Zn-polar face and weaker on the O-polar face. The surface electron accumulation layer was found to significantly influence the electrical properties of high resistivity, hydrothermally grown bulk ZnO crystals at temperatures below 200 K, and is largely responsible for the anomalously low electron mobility reported for this material.

Temperature-Dependent Properties of Nearly Ideal ZnO Schottky Diodes

The current-voltage characteristics of low-ideality factor (1.09 at 300 K) ZnO Schottky diodes were investigated over the temperature range of 40-423 K. Planar geometry devices were fabricated on the Zn-polar face of a low carrier concentration (n = 1 times 1014 cm-3) hydrothermally grown bulk ZnO single crystal wafer. The current transport was dominated by thermionic emission between 293 and 423 K, provided the ZnO surface was exposed to air. The nearly ideal characteristics of the diodes yielded an experimental Richardson constant of 10 plusmn 6 Amiddotcm-2middotK-2, close to the theoretical value of 32 Amiddotcm-2middotK-2, and two orders of magnitude larger than previously reported values.

Oxidized noble metal Schottky contacts to n-type ZnO

Deliberately oxidized iridium, platinum, and palladium Schottky contacts were fabricated on the Zn-polar and O-polar faces of hydrothermal bulk ZnO by eclipse pulsed laser deposition in an oxygen ambient. The barrier heights of these oxidized contacts were significantly higher than their plain metal counterparts, with ideality factors approaching the image-force-controlled limit for laterally homogeneous interfaces. The key aspects of this technique are a low deposition energy and the use of an oxidizing environment which reduces interfacial defects, particularly oxygen vacancies. In each case, the barriers on the Zn-polar face were 210–260 meV higher than those on the O-polar face.

Comparative study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks

The authors have an extensive study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks. In the photoluminescence (PL) spectra at 10 K, Zn-polar and O-polar faces show a common emission feature: neutral donor-bound excitons and their longitudinal-optical (LO) phonon replicas are strong and free excitons are very weak. However, in the PL spectra at room temperature (RT), Zn-polar and O-polar faces show extremely different emission characteristics: the emission intensity of Zn-polar face is 30 times larger than that of O-polar face and the band edge of Zn-polar face is 33 meV redshifted from that of O-polar face. The temperature dependence of photoluminescence indicates that the PL spectra at RT are closely associated with free excitons and their phonon-assisted annihilation processes. As a result, it is found that the RT PL spectra of Zn-polar face are dominated by the first-order LO phonon replica of A free excitons, while that of O-polar face is determined by A free excitons. This is ascribed to the difference in exciton-phonon coupling strengths that Zn-polar phase has larger coupling strength than O-polar face.

Influence of oxygen vacancies on Schottky contacts to ZnO

Ni, Ir, Pd, Pt, and silver oxide Schottky contacts were fabricated on the Zn-polar face of hydrothermally grown, bulk ZnO. A relationship was found between the barrier height of the contact and the free energy of formation of its “metal” oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately 0.7eV below the conduction band minimum. Therefore, a key goal in the fabrication of high quality Schottky contacts should be the minimization of oxygen vacancies near the metal-ZnO interface.

Silver oxide Schottky contacts on n-type ZnO

A method of fabricating highly rectifying Schottky contacts on n-type ZnO using silver oxide has been developed and used to compare diode performance on hydrothermal and melt grown, bulk, single crystals. Silver oxide diodes on hydrothermal ZnO have lower ideality factors, lower reverse current voltage dependence, higher series resistance, and larger surface-polarity related differences in barrier height, compared to those on melt ZnO. These effects are explained by the large difference in resistivity between hydrothermal and melt ZnO. Barrier heights of 1.20eV were achieved on the Zn-polar face of hydrothermal ZnO which are the highest reported for n-type ZnO.

Influence of spontaneous polarization on the electrical and optical properties of bulk, single crystal ZnO

Hall effect, photoluminescence, and Schottky diode measurements were made on the Zn-polar and O-polar faces of undoped, bulk, single crystal, c-axis ZnO wafers. Significant polarity related differences were observed in the PL and Schottky diode characteristics of low carrier concentration, hydrothermally grown wafers. Increased emission from free exciton recombinations and from recombinations between 3.3725 and 3.3750eV was observed on the Zn-polar face. Conversely, emission between 3.3640 and 3.3680eV was more intense on the O-polar face. The barrier heights of silver oxide Schottky diodes were approximately 130meV larger on the Zn-polar face compared to the O-polar face.

Metal Schottky diodes on Zn-polar and O-polar bulk ZnO

Planar Pd, Pt, Au, and Ag Schottky diodes with low ideality factors were fabricated on the Zn-polar (0001) and O-polar (0001¯) faces of bulk, single crystal ZnO wafers. The diodes were characterized by current-voltage and capacitance-voltage measurements. A polarity effect was observed for Pt and Pd diodes with higher quality barriers achieved on the O-polar face. No significant polarity effect was observed for Au or Ag diodes. The highest barriers were achieved with Ag as the Schottky metal with barrier heights varying between 0.77 and 1.02eV. This is possibly due to varying degrees of oxidation of the Ag contacts.

Schottky Contact Behaviour as a Function of Metal and ZnO Surface Polarity

ABSTRACTHall effect, photoluminescence (PL) and Schottky diode measurements were made on the Zn-polar , O-polar and m-plane faces of hydrothermally grown, bulk ZnO. Several polarity related differences were observed in the PL spectra. The most noticeable was increased emission from free excitons and from a triplet of emissions at 3.3725 - 3.3750 eV on the Zn-polar face. Polarity effects were also observed in the properties of highly rectifying, planar, silver oxide diodes fabricated by RF sputtering using an Ag target and an Ar/O2 plasma. The most significant of these was a consistent 130 meV larger barrier height for silver oxide diodes on the Zn-polar face compared to the O-polar face. These polarity effects are thought to result from the internal compensation of bound spontaneous polarization charges at the Zn-polar and O-polar faces. In addition, Au and Ag Schottky diodes with image-force-controlled ideality factors were achieved on the Zn-polar face of bulk ZnO without any special surface treatments.

Structure sensitivity and selectivity of proton abstraction from alkynes on the polar faces of zinc oxide

The terminal alkynes, C/sub 2/H/sub 2/, CH/sub 3/CCH, and C/sub 6/H/sub 5/CCH, decomposed on the (0001)-Zn polar face of ZnO, but not on the (0001)-O polar face. These alkynes were ultimately dehydrogenated on the (0001)-Zn polar face to produce water and to deposit carbon atoms on the surface. Adsorbed carbon was oxidized by lattice oxygen by lattice oxygen during TPD experiments, giving rise to CO and CO/sub 2/ desorption peaks at 780 K. Carbon oxidation was also found to be accompanied by zinc metal evolution. In contrast to the (0001)-Zn face, the (0001)-O face was found to be inactive for dehydrogenation of alkynes, and XPS results indicated that the alkynes did not adsorb dissociatively on this surface. Acetylene and phenylacetylene formed the corresponding acetylide species via dissociative adsorption on the active Zn-polar face. Methylacetylene adsorbed dissociatively on the (0001)-Zn polar face via abstraction of a methyl proton to form stable propargyl species exclusively. This result contrasts with the selectivity of proton abstraction both in the gas phase and on Ag(110), for which abstraction of the acetylenic proton is preferred. These results suggest that covalent contributions to the surface-adsorbate bond on ZnO may play a significant role in determining the selectivitymore » of acid-base reactions on the (0001)-Zn surface.« less

Conversion of methanol, formaldehyde and formic acid on the polar faces of zinc oxide

Abstract The decomposition reactions of methanol, formaldehyde, and formic acid on the (0001)-O and (0001 )-Zn polar faces of ZnO were examined by temperature programmed desorption and X-ray photoelectron spectroscopy. The O-polar crystal face exhibited no reactivity, and these reactants desorbed intact. In contrast, on the Zn-polar crystal face HCOOH and CH2O were oxidized primarily to CO2 via surface formates, while CH3OH was dehydrogenated to CO and CH2O via surface methoxy species and oxidized to CO2 via surface formates. The selectivity of oxidation and dehydrogenation reactions on the (0001)-Zn polar face appeared to be dependent on the extent of surface reduction in the vicinity of adsorbed intermediates. XPS spectra of adsorbed methoxy and formate intermediates were obtained, demonstrating the utility of XPS in monitoring the interconversion of intermediates on oxide surfaces. XPS spectra of adsorbed molecular species suggest that the lack of reactivity of the O-polar crystal face is due to the absence of accessible acid-base site pairs necessary for dissociative adsorption on this surface, and not due to differences in the interaction of molecularly chemisorbed species with the two polar surfaces. The evolution of oxidation products on the (0001)-Zn surface was accompanied by Zn metal desorption. The results of this investigation are consistent with previous studies on polycrystalline ZnO and clarify some of the discrepancies between these studies and previous investigations on single crystal surfaces.

Conversion of methanol, formaldehyde and formic acid on the polar faces of zinc oxide

The decomposition reactions of methanol, formaldehyde, and formic acid on the (0001̄)-O and (0001 )-Zn polar faces of ZnO were examined by temperature programmed desorption and X-ray photoelectron spectroscopy. The O-polar crystal face exhibited no reactivity, and these reactants desorbed intact. In contrast, on the Zn-polar crystal face HCOOH and CH 2O were oxidized primarily to CO 2 via surface formates, while CH 3OH was dehydrogenated to CO and CH 2O via surface methoxy species and oxidized to CO 2 via surface formates. The selectivity of oxidation and dehydrogenation reactions on the (0001)-Zn polar face appeared to be dependent on the extent of surface reduction in the vicinity of adsorbed intermediates. XPS spectra of adsorbed methoxy and formate intermediates were obtained, demonstrating the utility of XPS in monitoring the interconversion of intermediates on oxide surfaces. XPS spectra of adsorbed molecular species suggest that the lack of reactivity of the O-polar crystal face is due to the absence of accessible acid-base site pairs necessary for dissociative adsorption on this surface, and not due to differences in the interaction of molecularly chemisorbed species with the two polar surfaces. The evolution of oxidation products on the (0001)-Zn surface was accompanied by Zn metal desorption. The results of this investigation are consistent with previous studies on polycrystalline ZnO and clarify some of the discrepancies between these studies and previous investigations on single crystal surfaces.

Comparison of the chemical properties of the zinc-polar, the oxygen-polar, and the nonpolar surfaces of zinc oxide

The temperature programmed decomposition results of methanol, formaldehyde, and formic acid on an O-polar (0001) surface of ZnO are presented. From these results, and similar ones for the Zn-polar (0001) and the nonpolar (1010) and (5051) surfaces, a comparison of the properties of the different ZnO surfaces are made. Oxygen-containing molecules that possess strong dipole moments, such as alcohols, formaldehyde, acetone, and formic acid, interact most strongly with the Zn-polar face, less strongly with the nonpolar face, and least strongly with the O-polar face. This leads to the observed differences in the desorption temperature of molecular desorption, temperature of evolution of the decomposition products, and the fraction of methanol decomposed. These differences are explained by the different magnitudes and orientations of surface dipole moments, a different degree of ionicity of the surface Zn ions, and a different degree of steric hindrance of surface Zn ions for the different surfaces. Desorption of other molecules, such as propene and CO, whose adsorption is dominated by different types of interaction than the alcohols and aldehydes, shows different dependence on the surface structure. 37 references, 5 figures, 3 tables.

A study of the optically produced hexagonal pattern on ZnO single crystals

A hexagonal spot pattern (or light figure) was observed on ZnO single crystals during an Auger electron spectroscopy (AES) study. It is known that blue-green luminescence excitation can be produced in ZnO by electron bombardment. Thus, the electron bombardment provides an alternative internal luminescence light source to produce the light figure on the etched ZnO crystals. A main feature of the electron-induced luminescence light figure is characterized by a missing central spot. A detailed comparison of the similarities and differences between the light figure produced by electron bombardment and by illuminating with an external collimated light beam is given. An overall consistent picture between these two types of light figures can be drawn and understood on the basis of ray tracing. Though both the O polar face and Zn polar face show distinguishable etched structures, the result shows that the forming of the light pattern is mainly due to the existence of the hexagonal pyramidal etched structure on the O polar face.