C. P. Dietrich

Strain distribution in bent ZnO microwires

ZnO microwires grown by carbothermal reduction were mechanically bent and the uniaxial stress state was studied with spatially resolved low-temperature photoluminescence. Inhomogeneous (tensile and compressive) stress (up to ±1 GPa) is visualized by the redshift and blueshift of the wire luminescence. Experimentally determined tensile and compressive strain along the c-axis (wire axis) of up to 1.5 %, symmetrically distributed with respect to the central axis (neutral fiber), is achieved, resulting in maximum energetic shifts of ±30 meV. Within these experiments, we are able to precisely determine the direct relation between energetic shift of the free A-exciton energy and strain to (−2.04±0.02) eV.

Lineshape theory of photoluminescence from semiconductor alloys

The photoluminescence from semiconductor alloys is inhomogeneously broadened due to alloy disorder. We present a model to explain the so-called “S-shape” temperature dependence of peak position, taking into account recombination of free excitons and excitons bound to impurities. We find the following effects to contribute with increasing temperature: exciton localization on impurities at low temperatures, exciton transfer between impurities, exciton ionization from impurities, transfer of excitons between potential minima in the disorder potential, and shrinkage of band gap. We extend the common theory of ionization of excitons from impurities to take into account impurity ionization. We find this effect essential for our lineshape theory. The lineshape theory describes quantitatively the temperature dependent peak position in MgxZn1−xO alloys.

One- and two-dimensional cavity modes in ZnO microwires

We demonstrate that one-dimensional (1D) Fabry–Perot modes (FPM) and 2D whispering gallery modes (WGM) occur separately or simultaneously within a single, tapered ZnO microwire. Their observation is strongly correlated with the size and shape of the microwire cross-section. FPM can preferentially be observed in thick wires with an elongated wire cross-section without hexagonal symmetry. The formation of WGM otherwise requires hexagonal symmetry—they can be observed in thin wires having a regular hexagonal cross-section. All optical eigenmodes were unambiguously identified by their in- and out-of- (cross-sectional) plane photonic mode dispersion.

Defect properties of ZnO and ZnO:P microwires

We report on the defect properties of nominally undoped and phosphorus-doped ZnO microwires grown by carbothermal vapor phase transport. Cathodoluminescence measurements show very narrow (≈300 μeV), donorlike transitions in the UV spectral range. A recombination-line at 3.356 eV, previously assigned to phosphorus acceptors, is observed in our undoped ZnO. Thus the correlation of this recombination process and possible acceptor doping can be excluded. Hall effect measurements confirmed these findings and revealed n-type conductivity in both ZnO and high quality ZnO:P microwires.

Origin of the near-band-edge luminescence in MgxZn1−xO alloys

The carrier dynamics of donor-bound and free excitons, localized in the alloy disorder potential, were investigated for MgxZn1−xO (0.08≤x≤0.33) thin films. The measured transients show a fast decrease in the luminescence intensity within the first nanoseconds, followed by a slow, strongly nonexponential decay. Shortly after the excitation pulse, the time-delayed spectra are dominated by the (D0,X) recombination. With increasing time, the free exciton recombination becomes visible on the high-energy side, dominating the spectra at large times after the excitation pulse. By fitting the transients with nonexponential model decay functions, we can deconvolve the luminescence spectra. As expected, the mean decay time of the excitons localized in the alloy disorder potential significantly increases with increasing Mg content.

Luminescence properties of ZnO/Zn1-xCdxO/ZnO double heterostructures

We report on luminescence properties from T=2 K up to room temperature of ZnO/Zn1−xCdxO/ZnO double heterostructures grown by pulsed-laser deposition on a-plane sapphire substrates. Depending on the growth conditions, the spectral position of the Zn1−xCdxO related maximum has been tuned from 3.19 to 3.056 eV, corresponding approximately to Cd contents between 2.1% and 5.6%. Independent of x we observe intense phonon replicas of the photoluminescence (PL) maximum. The quenching of the luminescence intensity indicates the presence of two thermal activation energies, one of them being assigned to the delocalization of excitons from donors. The temperature-dependent PL spectra exhibit the so-called “S-shape” behavior as function of temperature for the Zn1−xCdxO due to the superposition of the usual S-shape, caused by the alloy, and a change in the peak character from donor-bound exciton to free exciton.

(Zn,Cd)O thin films for the application in heterostructures: Structural and optical properties

We report on (Zn,Cd)O thin films, grown by pulsed laser deposition on a-plane sapphire substrates with high Cd-contents up to 0.25. By incorporating Cd in ZnO and by applying a low growth temperature of about 300°C, the (Zn,Cd)O related luminescence redshifts to an energy of 2.46 eV as a result of the large Cd-content of 0.25. The redshift of the bandgap energy was additionally proven by transmission measurements. By fitting the transmission curves, the spectra of the absorption coefficient and the index of refraction are calculated. The (Zn,Cd)O thin films are single phase and exhibit the wurtzite crystal structure. An increasing a- and c-lattice constant is observed with increasing Cd-content.

Microwire (Mg,Zn)O/ZnO and (Mg,Zn)O/(Cd,Zn)O non-polar quantum well heterostructures for cavity applications

Pulsed-laser deposited, non-polar MgxZn1−xO/ZnO and MgxZn1−xO/Zn1−yCdyO quantum well heterostructures were fabricated in radial direction on ZnO microwires with well-defined hexagonal cross section. Optical resonances modulate room-temperature luminescence spectra for all fabricated heterostructures demonstrating their applicability as microcavities. Quantum confinement was proven by time-integrated and time-resolved luminescence. The ZnO quantum well emission was tuned between 3.76 and 3.35 eV by adjusting the well thickness and barrier composition. In order to further reduce the QW emission energy, active Zn1−yCdyO quantum wells in MgxZn1−xO barriers were grown emitting between 3.07 and 2.70 eV for different well thicknesses but fixed barrier composition.

The corner effect in hexagonal whispering gallery microresonators

Hexagonal microresonators were investigated regarding the influence of the nanometer scale geometry of their corners on light out-coupling, mode quality factor, and lasing threshold of whispering gallery modes (WGMs). For this purpose, ZnO microwires with hexagonal cross section grown by carbothermal vapor-phase transport were fabricated. It turned out that their corners can either be sharp (curvature radius rC<10 nm) or be blunt (rC≈400 nm). Blunt corners enhance light out-coupling of WGM in the low-excitation regime but otherwise increase mode linewidth. Sharp corners hinder light out-coupling at low excitations leading to long cavity photon lifetimes and facilitating room-temperature WGM lasing at lowest reported lasing thresholds of 73 kW/cm2.

MgZnO/ZnO quantum well nanowire heterostructures with large confinement energies

Mg0.25Zn0.75O/ZnO-quantum well nanowire heterostructures were grown with a three-step pulsed laser deposition process. To avoid shadowing effects during the coating, the ZnO nanowires were grown with a low area density on a ZnO buffer layer deposited on an a-plane sapphire substrate. By using spatially resolved cathodoluminescence measurements, the luminescence of axial and radial quantum wells were clearly distinguished. The large bandgap energy of the Mg0.25Zn0.75O barrier material (≈3.85 eV) made it possible to tune the energy of quantum wells from 3.4 to 3.7 eV. The homogeneity of the radial quantum well along the wire axis was probed, revealing that only small fluctuations of about 4 meV are found in the main part of the nanowire. Near the tip of the nanowire, the energy of the radial quantum well increases due to locally modified growth conditions reducing the growth rate by up to 15%. Furthermore, the growth rates of the axial and radial quantum wells were determined, showing that the one in axial ...