Marianne C. Tarun

Nitrogen is a deep acceptor in ZnO

Zinc oxide is a promising material for blue and UV solid-state lighting devices, among other applications. Nitrogen has been regarded as a potential p-type dopant for ZnO. However, recent calculations [Lyons, Janotti, and Van de Walle, Appl. Phys. Lett. 95, 252105 (2009)] indicate that nitrogen is a deep acceptor. This paper presents experimental evidence that nitrogen is, in fact, a deep acceptor and therefore cannot produce p-type ZnO. A broad photoluminescence(PL) emission band near 1.7 eV, with an excitation onset of ∼2.2 eV, was observed, in agreement with the deep-acceptor model of the nitrogen defect. The deep-acceptor behavior can be explained by the low energy of the ZnOvalence band relative to the vacuum level.

Infrared absorption of hydrogen-related defects in strontium titanate

Hydrogen has a significant impact on the structural and electronic properties of metals, insulators, and semiconductors. Despite the prevalence of hydrogen, little is known about its behavior in oxides such as strontium titanate (SrTiO3). In this work, hydrogen-related defects in SrTiO3 were investigated using infrared absorption spectroscopy. Two local vibrational modes (LVMs) are observed at 3355 and 3384 cm−1 after hydrogenation at 800 °C. Isotope substitution experiments revealed that the defect consists of two hydrogen atoms bound to host oxygen atoms. From the temperature dependence of the LVMs, we ascribe the complex to a strontium vacancy passivated by two hydrogen atoms. The thermal stability of the defect was determined through a series of isochronal annealing experiments on the hydrogenated SrTiO3 sample. These measurements provided evidence of “hidden hydrogen,” possibly H2 molecules, in the crystal.

Acceptors in ZnO

Zinc oxide (ZnO) has potential for a range of applications in the area of optoelectronics. The quest for p-type ZnO has focused much attention on acceptors. In this paper, Cu, N, and Li acceptor impurities are discussed. Experimental evidence indicates these point defects have acceptor levels 3.2, 1.4, and 0.8 eV above the valence-band maximum, respectively. The levels are deep because the ZnO valence band is quite low compared to conventional, non-oxide semiconductors. Using MoO2 contacts, the electrical resistivity of ZnO:Li was measured and showed behavior consistent with bulk hole conduction for temperatures above 400 K. A photoluminescence peak in ZnO nanocrystals is attributed to an acceptor, which may involve a Zn vacancy. High field (W-band) electron paramagnetic resonance measurements on the nanocrystals revealed an axial center with g⊥ = 2.0015 and g// = 2.0056, along with an isotropic center at g = 2.0035.

Cu-Doping of ZnO by Nuclear Transmutation

Zinc oxide single crystals were doped with copper acceptors by means of the nuclear transmutation doping method, which gives highly uniform dopant distributions and has a much higher probability of controlling the dopant locations in the lattice. The Cu doping was confirmed by the infrared absorption signature of Cu2+ at 5780 cm−1. Hall-effect measurements were performed to study the effect of CuZn on the electrical properties of ZnO. These measurements indicated that the Cu acceptor level lies 0.160 eV below the conduction-band minimum.

Recharging behavior of nitrogen-centers in ZnO

Electron Paramagnetic Resonance was used to study N2-centers in ZnO, which show a 5-line spectrum described by the hyperfine interaction of two nitrogen nuclei (nuclear spin I = 1, 99.6% abundance). The recharging of this center exhibits two steps, a weak onset at about 1.4 eV and a strongly increasing signal for photon energies above 1.9 eV. The latter energy coincides with the recharging energy of NO centers (substitutional nitrogen atoms on oxygen sites). The results indicate that the N2-centers are deep level defects and therefore not suitable to cause significant hole-conductivity at room temperature.

Defects and persistent conductivity in SrTiO3

Strontium titanate (SrTiO3) is often used as a substrate for oxide thin films such as high-temperature superconductors. It has the perovskite structure and an indirect band gap of 3.25 eV. Our prior work showed that hydrogen impurities form a defect complex that contains two hydrogen atoms. The complex was tentatively attributed to a passivated strontium vacancy. Alternatively, it could be a partially passivated titanium vacancy. In order to create titanium vacancies, we annealed samples in an evacuated ampoule with SrO powder. These samples show unexpected behavior. After illuminating with sub-gap light, the free-electron concentration increases significantly. After the light is turned off, the high conductivity persists at room temperature. We attribute persistent photoconductivity (PPC) to the excitation of an electron from a vacancy into the conduction band, with a low recapture rate.

Hydrogen-related defects in bulk ZnO

Zinc oxide (ZnO) has attracted resurgent interest as an active material for energy-efficient lighting applications. An optically transparent crystal, ZnO emits light in the blue-to-UV region of the spectrum. The efficiency of the emission is higher than more “conventional” materials such as GaN, making ZnO a strong candidate for solid-state white lighting. Despite its advantages, however, ZnO suffers from a major drawback: as grown, it contains a relatively high level of donors. These unwanted defects compensate acceptors or donate free electrons to the conduction band, thereby keeping the Fermi level in the upper half of the band gap. This paper reviews recent work on hydrogen donors and nitrogen-hydrogen complexes in ZnO.