W. M. Hlaing Oo

Infrared spectroscopy of ZnO nanoparticles containing CO2 impurities

Impurities play a major role in determining the optical and electrical properties of semiconductor nanoparticles. In this work, the presence and source of CO2 impurities in ZnO nanoparticles were studied by IR absorption spectroscopy. Isotopic substitution was used to verify the vibrational frequency assignment. Isochronal annealing experiments were performed to study the formation and stability of the molecular impurities. Our results indicate that the molecules are much more stable than CO2 adsorbed on bulk ZnO surfaces. By comparing our observations with similar results from IR spectroscopy of CO2 trapped in carbon nanotubes [C. Matranga, L. Chen, M. Smith, E. Bittner, J. K. Johnson, and B. Bockrath, J. Phys. Chem. B 107, 12930 (2003)], we conclude that the molecules are trapped in the ZnO nanoparticles.

Suppression of conductivity in Mn-doped ZnO thin films

We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition. The ion beam, surface, and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was observed for Mn doping up to ∼4.5 at. %. The presence of Mn2+, confirmed by x-ray photoelectron spectroscopy, is correlated with the reduction in conductivity. Variable-temperature Hall effect measurements yield an activation energy of 170 meV, consistent with deep donors in the bulk or at the interface. The results suggest that the incorporation of substitutional Mn suppresses the formation of native defects such as oxygen vacancies.

Acceptors in ZnO nanocrystals

While zinc oxide (ZnO) has potential for optoelectronic applications, the lack of reliable p-type doping remains a major challenge. We provide evidence that ZnO nanocrystals contain uncompensated acceptors. IR absorption peaks at liquid-helium temperatures suggest a hydrogenic acceptor with a hole binding energy of 0.4–0.5 eV. Electron paramagnetic resonance (EPR) measurements in the dark showed a resonance at g=2.003, characteristic of acceptors that involve a zinc vacancy. An EPR resonance due to vacancy hydrogen complexes was observed after exposure to light. Given the lack of alternatives, vacancy complexes may provide a feasible route toward p-type conductivity.

Infrared and Raman spectroscopy of ZnO nanoparticles annealed in hydrogen

The effect of hydrogen on the conductivity of ZnO nanoparticles has implications for nanoscale optoelectronic devices. In this study, infrared reflectance spectra of as-grown and hydrogen-annealed ZnO nanoparticles were measured at near-normal incidence. The as-grown particles were electrically semi-insulating and show reflectance spectra characteristic of insulating ionic crystals. Samples annealed in hydrogen showed a significant increase in electrical conductivity and free-carrier absorption. A difference was observed in the reststrahlen line shape of the conductive sample compared to that of the as-grown sample. The effective medium approximation was applied to model the reflectance and absorption spectra. The agreement between experimental results and the model suggests that the nanoparticles have inhomogeneous carrier concentrations. Exposure to oxygen for several hours led to a significant decrease in carrier concentration, possibly due to the adsorption of negative oxygen molecules on the nanopart...

Optical transitions and multiphonon Raman scattering of Cu doped ZnO and MgZnO ceramics

Cu doped ZnO and MgZnO ceramics were created via a process of cold pressing and annealing, and their optical properties and phonon dynamics were studied. It was found that the ceramics exhibit infrared absorption peak energies at 5783 and 5822 cm−1, indicative of intraband transitions in a substitutional Cu ion of oxidation state +2. The UV photoluminescence (PL) intensity of the ceramics was found to weaken significantly relative to an undoped sample. The low PL intensity is discussed in terms of the CuxZn1−xO alloy system and the indirect bandgap of the CuO end member, as well as in terms of the nonradiative Cu centers. Due to the weak PL, up to ten LO multiphonons were observed in the Raman spectra, pointing to a strong polaron coupling. The resonance behavior of the highest intensity mode was found to exhibit outgoing resonance characteristics.

X-ray diffraction of MgxZn1−xO and ZnO nanocrystals under high pressure

MgxZn1−xO (x=0.15) and ZnO nanocrystals of about 40 nm in diameter were studied using x-ray diffraction and diamond-anvil cells. The equation of state (EOS) for MgZnO is reported for the first time. Between pressures of 9.45 and 10.7 GPa, MgZnO transforms into the rocksalt (NaCl) structure, which persisted to 1.1 GPa upon decompression. The EOS parameters for ZnO are close to their bulk values and in good agreement with values obtained previously. The bulk modulus for MgZnO was slightly lower than that of ZnO. The pressure-induced decrease in c/a ratio was greater for MgZnO, consistent with the tendency for MgZnO to move toward a cubic structure. From previous photoluminescence measurements [J. Huso et al., Appl. Phys. Lett. 89, 171909 (2006)], the band-gap volume deformation potentials for ZnO and Mg0.15Zn0.85O were determined to be −3.6 and −4.0 eV, respectively.

Incorporation of Cu acceptors in ZnO nanocrystals

Doping of semiconductor nanocrystals is an important problem in materials research. Using infrared and x-ray photoelectron spectroscopy, we have observed Cu acceptor dopants that were intentionally introduced into ZnO nanocrystals during growth. The incorporation of Cu2+ dopants increased as the average diameter of the nanocrystals was increased from ∼3 to 6 nm. Etching the nanocrystals with acetic acid revealed a core-shell structure, where a lightly doped core is surrounded by a heavily doped shell. These observations are consistent with the trapped dopant model, in which dopant atoms stick to the surface of the core and are overgrown by the nanocrystal material.

Hydrogen Donors in ZnO

Zinc oxide (ZnO) has shown great promise as a wide-bandgap semiconductor with a range of optical, electronic, and mechanical applications. The presence of compensating donors, however, is a major roadblock to achieving p-type conductivity. Recent first-principles calculations and experimental studies have shown that hydrogen acts as a shallow donor in ZnO, in contrast to hydrogen’s usual role as a passivating impurity. Given the omnipresence of hydrogen during growth and processing, it is important to determine the structure and stability of hydrogen donors in ZnO. To address these issues, we performed vibrational spectroscopy on bulk, single-crystal ZnO samples annealed in hydrogen (H2) or deuterium (D2) gas. Using infrared (IR) spectroscopy, we observed O-H and O-D stretch modes at 3326.3 cm -1 and 2470.3 cm -1 respectively, at a sample temperature of 10 K. These frequencies indicate that hydrogen forms a bond with a host oxygen atom, consistent with either an antibonding or bond-centered model. In the antibonding configuration, hydrogen attaches to a host oxygen and points away from the Zn-O bond. In the bond-centered configuration, hydrogen sits between the Zn and O. To discriminate between these two models, we measured the shift of the stretch-mode frequency as a function of hydrostatic pressure. By comparing with first-principles calculations, we conclude that the antibonding model is the correct one. Surprisingly, we found that the O-H complex is unstable at room temperature. After a few weeks, the peak intensity decreases substantially. It is possible that the hydrogen forms H2 molecules, which have essentially no IR signature. Electrical measurements show a corresponding decrease in electron concentration, which is consistent with the formation of neutral H2 molecules. The correlation between the electrical and spectroscopic measurements, however, is not perfect. We therefore speculate that there may be a second “hidden” hydrogen donor. One candidate for such a donor is a hydrogen-decorated oxygen vacancy. Table I. Band gaps of several important wide-band-gap semiconductors. Energies are given in eV (nm). The cubic structure for GaN and AlN is zincblende. The hexagonal structure for GaN, AlN, and ZnO is wurtzite. 4H, 6H, and 2H denote the hexagonal polytypes of SiC.

Infrared Spectroscopy of Impurities in ZnO Nanoparticles

Semiconductor nanoparticles have a range of potential applications in electronic, optoelectronic and spintronic devices. Zinc oxide (ZnO), a wide-bandgap semiconductor, has emerged as an important material for such applications. In this work, impurities in ZnO nanoparticles were investigated with infrared (IR) spectroscopy, and the results show the presence of CO2 impurities in ZnO nanoparticles. Isotopic substitution was used to verify the frequency assignment and the results demonstrate conclusively that the impurities originate from the precursors. Isochronal annealing experiments were performed to study the formation and stability of the CO2 molecules. In addition to unintentional CO2 impurities, we intentionally introduced hydrogen into ZnO nanoparticles. Our results show that post-growth annealing in hydrogen dramatically changes IR transmission, reflection and electrical properties of the nanoparticles.

Strong Fano resonance of oxygen-hydrogen bonds on oblique angle deposited Mg nanoblades

Magnesium hydroxide [Mg(OH)2] thin layers were formed by a chemical reaction between Mg nanoblades and water. Infrared (IR) spectroscopy showed a OH bond-stretching vibrational mode at 3699cm−1. The assignment was verified by reactions with heavy water, producing OD bonds with the expected isotopic frequency shift. An asymmetric Fano line shape was observed for the OH layer on metallic Mg, while a symmetric Lorentzian (or Bright–Wigner) profile was observed for the OH layer on insulating MgH2. The results indicate that the OH layer on the Mg nanoblades is so thin that the vibrational mode couples to the free-electron continuum of Mg metal.