F. Reuss

ZnMgO epilayers and ZnO-ZnMgO quantum wells for optoelectronic applications in the blue and UV spectral region

We have investigated the properties of ZnMgO epilayers and ZnO–ZnMgO quantum well structures grown by metalorganic vapor-phase epitaxy. A well-controlled incorporation of magnesium, x⩽0.10, could be confirmed resulting in a blueshift of the photoluminescence emission wavelength of the Zn1−xMgxO layers up to 200meV. Using ZnMgO as barrier material, ZnO–ZnMgO quantum well structures with different well widths have then been fabricated. The confinement effect in the ZnO quantum wells leads to the expected increase of the corresponding quantum well emission energy with decreasing well width. A comparison to calculations also suggests a further enhancement of the exciton binding energy in the quantum wells of up to 90meV.

Optical and structural analysis of ZnCdO layers grown by metalorganic vapor-phase epitaxy

The development of ZnO-based semiconductor devices requires band gap engineering. Ternary Zn1−xCdxO allows reduction of the band gap relative to ZnO, which would be necessary for devices emitting visible light. We have analyzed the structural and optical properties of Zn1−xCdxO layers grown by metalorganic vapor-phase epitaxy. A narrowing of the fundamental band gap of up to 300 meV has been observed, while introducing a lattice mismatch of only 0.5% with respect to binary ZnO. Photoluminescence, high-resolution x-ray diffraction, and spatially resolved cathodoluminescence measurements revealed a lateral distribution of two different cadmium concentrations within the Zn1−xCdxO layers.

Optical investigations on the annealing behavior of gallium- and nitrogen-implanted ZnO

Gallium and nitrogen ions have been implanted into ZnO crystals and metal organic vapor phase epitaxy grown ZnO layers. Postimplantation annealing behavior in the temperature range between 200 and 900 °C has been studied by means of Raman scattering and low-temperature photoluminescence. The temperature for healing of the implantation-induced defects was found to be 800 °C. Implanted gallium acts as donor with a donor binding energy ED of 53 meV, thus allowing the control of n-type doping in ZnO. From photoluminescence measurements of the donor-acceptor pair transition of a series of nitrogen-implanted ZnO samples we estimate the binding energy EA of the nitrogen acceptor between 163 and 196 meV. Electrical characterization of nitrogen-implanted samples shows a behavior ranging from low n-type to highly compensated. But no unambiguous and reproducible type conversion could be achieved.

Influences of biaxial strains on the vibrational and exciton energies in Zno

We have investigated the structural, optical, and vibrational properties of strained heteroepitaxial ZnO layers by high resolution x-ray diffraction, reflectivity, and Raman measurements. The ZnO layers were grown by metalorganic vapor phase epitaxy on sapphire substrates under varying growth conditions. A Poisson number of μ=0.303 and phonon deformation-potential parameters of a=−690 cm−1, b=−940 cm−1 for the high-energy E2 optical phonon mode have been determined. The shift of the excitonic resonances due to the strain in the layers agrees well with the experimentally determined Poisson ratio using the deformation-potentials D1–D4 determined by Wrzesinski and Frohlich [Phys. Rev. B 56, 13087 (1997)].

Analysis of ZnO and ZnMgO nanopillars grown by self-organization

In this contribution we analyse the structural and optical properties of ZnO as well as ZnMgO nanopillars grown catalyst-free by metalorganic vapour-phase epitaxy. The nanostructures were grown directly onto different substrate materials with various orientations. The nanopillars deposited on a-plane sapphire show the best vertical c-axis alignment and have a typical diameter of about 50 nm and a height of several micrometres, depending on growth time. We achieved well ordered, almost completely c-axis oriented pillars, as confirmed by scanning electron microscopy and high resolution x-ray diffraction. Photoluminescence measurements revealed very narrow donor-bound exciton emission lines with half widths as small as 0.5 meV. In order to investigate the possibility of a combination of band gap engineering and nanopillar growth, ZnMgO nanopillars were also grown. The Mg incorporation was confirmed by photoluminescence measurements and a blue shift of the band gap of up to 170 meV could be achieved for the nanopillars with the highest Mg concentration.

Magnetoresistance in epitaxially grown degenerate ZnO thin films

The magnetoresistance of high-quality epitaxial doped ZnO(:Ga) thin films with various electron concentrations ranging from 3.2×1018 to 1.3×1020cm−3 has been measured. All samples investigated exhibit a negative magnetoresistance at low magnetic fields. Its magnitude systematically depends on carrier concentration and temperature. Low-doped samples switch the sign of the magnetoresistance and the conventional positive component dominates at high fields, whereas highly doped degenerate samples only show a negative component up to fields of 14.5 T. Therefore, the data are analyzed as the sum of a positive and negative contribution to the magnetoresistance applying a semiempirical expression to describe the observed behavior. The model takes into account third-order s–d exchange Hamiltonians describing the negative part and a two-band model for the positive contribution. Least-squares fits to the data are presented. Theory and experiment are in excellent agreement.

Excitonic Properties of ZnO Films and Nanorods

We report on the comparative studies of linearly polarized photoluminescence (PL) in a ZnO epitaxial film and ZnO nanorods. At low temperatures the PL spectrum of both samples included a number of narrow lines attributed to donor‐bound excitons and a peak of free A excitons. An additional line observed in the nanorod sample was assigned to the excitons bound to some defects introduced during the sample post‐growth history and located near the nanorod tips. The emission of mixed longitudinal — transverse exciton polariton modes was observed at elevated temperatures in both samples.

Temperature dependent dynamics of the excitonic photoluminescence in zinc oxide nanorods

The temporal dynamics of the exciton photoluminescence (PL) in ZnO nanorod samples was investigated experimentally as a function of temperature and excitation intensity. Excitonic photoluminescence is observed up to room temperature. The excitation dependence of the PL dynamics reveals a saturable non‐radiative recombination center. Under high excitation conditions the time‐resolved photoluminescence shows two components: the ZnO M‐band which decays with a temperature independent sub‐100 ps time constant, and the intrinsic exciton PL with a time constant of several 100 ps increasing with temperature. Exciton‐exciton scattering effects are notably absent, which is attributed to the reduced polariton phase space resulting from the small nanorod diameter of 50 nm.