STM study of the geometric and electronic structure of ZnO(0001)-Zn, (0001̄)-O, (101̄0), and (112̄0) surfaces

The research on ZnO has a long history but experiences an extremely vivid revival during the last 10 years. We critically discuss in this didactical review old and new results concentrating on optical properties but presenting shortly also a few aspects of other fields like transport or magnetic properties. We start generally with the properties of bulk samples, proceed then to epitaxial layers and nanorods, which have in many respects properties identical to bulk samples and end in several cases with data on quantum wells or nano crystallites. Since it is a didactical review, we present explicitly misconceptions found frequently in submitted or published papers, with the aim to help young scientists entering this field to improve the quality of their submitted manuscripts. We finish with an appendix on quasi two- and one-dimensional exciton cavity polaritons.

65 years of ZnO research – old and very recent results

derived from BaCO3. Small particle size has been shown to suppress a martensitic phase transition, and in this context microwave plasma synthesis appears rapid enough to prevent particle sintering and subsequent transition to the monoclinic phase. Prolonged exposure (1 h) to an O2 or Ar plasma of either the orthorhombic or monoclinic phases did not result in a phase change as judged by powder XRD. In conclusion, we have used a simple microwave plasma reactor for reproducible bulk synthesis of several ternary niobate and titanate phases. MIP synthesis does not require direct interaction between a solid and microwave radiation and is, therefore, amenable to any solid-state reaction. Importantly, for some reactions the plasma can also be used to heat solids into a temperature regime where additional dielectric heating can occur, resulting in a local temperature much greater than the equilibrium MIP temperature. Experimental Precursor oxides (Nb2O5 ,T iO 2, and BaO 99.99 %) and carbonates (Li2CO3 99.99 %, Na2CO3 99.995 %, K2CO3 99.995 %, CaCO3 99.995+ %, BaCO3 99.999 %, and PbCO3 99.99+ %) were purchased from Aldrich and used as received. Plasma gases (Ar and O2) were purchased from BOC. In a typical synthesis, a total mass of 2 g of precursors was ground in a drybox and pressed at 10 tonnes into a 13 mm pellet, and the pellet was transferred to the microwave apparatus via a desiccator. The pellet was placed in an alumina boat and exposed to a plasma at 13 mbar (1mbar = 10 2 Pa), 900 W, and a gas flow rate of 200 cm 3 min –1 for argon and 142 cm 3 min –1 for dioxygen. After microwave plasma irradiation, products were transferred directly into a drybox for storage. Products were characterized by powder X-ray diffraction (Philips 1800 diffractometer PH 03, with Cu Ka radiation (k = 1.54 A) in reflection mode), scanning electron microscopy (FEI Sirion 200), and analytical transmission electron microscopy (FEI CM200 FEGTEM) that included electron energy loss spectroscopy (EELS).

Synthesis of aligned arrays of ultrathin ZnO nanotubes on a Si wafer coated with a thin ZnO film

Electrically pumped ZnO quantum well diode lasers are reported. Sb-doped p-type ZnO/Ga-doped n-type ZnO with an MgZnO/ZnO/MgZnO quantum well embedded in the junction was grown on Si by molecular beam epitaxy. The diodes emit lasing at room temperature with a very low threshold injection current density of 10 A/cm2. The lasing mechanism is exciton-related recombination and the feedback is provided by close-loop scattering from closely packed nanocolumnar ZnO grains formed on Si.

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Electrically pumped ultraviolet ZnO diode lasers on Si

ZnO nanorods (NRs) and nanotubes (NTs) have been synthesized by a hydrothermal method on Si substrates that had been precoated (by pulsed laser deposition (PLD)) with a thin ZnO film. High-resolution transmission electron microscopy and selected area electron diffraction analysis confirm that the NTs are ZnO single crystals and that their growth direction is along [0001] (the c-axis). Scanning electron microscopy points to the early-time formation of two classes of NRs on the PLD ZnO coating, one of which is longer and displays higher length/diameter aspect ratios than the other. The morphologies of NRs belonging to the first of these classes were seen to evolve with time, progressively tapering, and producing volcano-like surface structures that develop into NTs. In contrast, NRs belonging to the other (shorter) class retain their hexagonal cross-section and have flat tops. To explain these emergent structures and, in particular, the selective growth of ZnO NTs, we have undertaken a systematic investigation of the effects of different substrates (e.g., borosilicate glass, Pt-coated glass, and both bare and PLD ZnO-coated Si wafers) and of the reactive solution on the growth properties of ZnO NRs, NTs, and the ZnO nanopowders that precipitate from the reactive mixture. The experimental findings suggest the following ZnO NT growth mechanism. The PLD ZnO film consists of many nanocrystallites, with a preferred c-axis alignment. These serve to nucleate the hydrothermal growth of (c-axis aligned) NRs. The NRs are deduced to be Zn-polar, but can be either Zn-atom or O-atom terminated. It is proposed that the different surface terminations influence (by electrostatic interactions) the cation (Zn(2+) and ZnOH(+)) to anion (OH(-)) concentration ratio in the double layer at the growing polar surface. Zn-atom termination causes a reduction in the local Zn(2+)/OH(-) (and ZnOH(+)/OH(-)) ratios (i.e., the extent of solution supersaturation) relative to those in the bulk solution, thereby encouraging tapered NR growth and, as the zinc concentration falls further, the emergence of volcano-like structures on the polar surface, which seed the subsequent growth of ZnO NTs.

Mechanism of ZnO nanotube growth by hydrothermal methods on ZnO film-coated Si substrates.

Wannier-Mott excitons in the wurzite-type semiconductor material ZnO are stable at room temperature, have an extremely large oscillator strength, and emit blue light. This makes ZnO an excellent potential candidate for the fabrication of room-temperature lasers where the coherent light amplification is ruled by the fascinating mechanism of the Bose condensation of the exciton polaritons. We report the direct optical measurement of the exciton oscillator strength f in ZnO. The longitudinal transverse splitting of the exciton resonances Γ 5 (B) and Γ 1 (C) are found to achieve record values of 5 and 7 meV, respectively, that, is two orders of magnitude larger than in GaAs. Second, we propose a model ZnO-based microcavity structure that is found to be the most adapted structure for the observation of the polariton laser effect. We thus can compute the phase diagram of the lasing regimes. A record value of the threshold power of 2 mW per device (at power density of 3000 W/cm 2 ) at room temperature is found for the model laser structure.

Mikhail Kaliteevski

Papers

ZnO as a material mostly adapted for the realization of room-temperature polariton lasers