Marko Stölzel

Control of interface abruptness of polar MgZnO/ZnO quantum wells grown by pulsed laser deposition

A strong quantum confined Stark effect (QCSE) was observed in wedge shaped MgZnO/ZnO quantum wells (QWs) grown by pulsed laser deposition. A reduced laser fluence of 1.8 J/cm2 was used. Reference samples grown at higher standard fluence 2.4 J/cm2 showed only a negligible QCSE. Using off-axis deposition without substrate rotation, a constant composition of the barriers was maintained while varying the well width in a wedge shaped QW. A redshift of the QW luminescence with increasing QW thickness up to 230 meV below the ZnO emission was found, accompanied by an increase in the exciton lifetime from 0.3 ns up to 4.2 μs.

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.

Highly textured fresnoite thin films synthesized in situ by pulsed laser deposition with CO 2 laser direct heating

Fresnoite Ba2TiSi2O8 (BTS) thin films were grown and crystallized in situ using pulsed laser deposition (PLD) with CO2 laser direct heating of the a-plane sapphire (1 1 0) substrates up to 1250 ◦ C. Starting with 775 ◦ C growth temperature, (0 0 1)- and (1 1 0)-textured BTS and BaTiO3 phases, respectively, could be assigned in the films, and the typical fern-like BTS crystallization patterns appear. For higher process temperatures of 1100 to 1250 ◦ C, atomically smooth, terraced surface of the films was found, accompanied by crystalline high-temperature phases of Ba–Ti–Si oxides. HAADF micrographs taken in both scanning transmission electron microscopy and energy-dispersive x-ray spectrometry mode show details of morphology and elemental distribution inside the films and at the interface. To balance the inherent Si deficiency of the BTS films, growth from glassy BTS × 2 SiO2 and BTS × 2.5 SiO2 targets was considered as well. The latter targets are ideal for PLD since the employed glasses possess 100% of the theoretical density and are homogeneous at the atomic scale.

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.

Electronic and optical properties of ZnO/(Mg,Zn)O quantum wells with and without a distinct quantum-confined Stark effect

The luminescence properties of polar ZnO/(Mg,Zn)O quantum wells (QWs) are determined, besides confinement effects, by a redshift caused by the Stokes shift and the quantum-confined Stark effect (QCSE). We present a comprehensive study of photoluminescence and optical transmission measurements to separate these two effects. Single QW structures have been prepared by pulsed laser deposition on a-plane sapphire exhibiting excitons with and without a distinct QCSE. The QCSE leads to a redshift of the QW luminescence maximum beneath the free exciton energy in ZnO as well as a change of the dynamics from a single exponential decay function to a non-exponential one, well described by a stretched exponential decay function. The internal electric field was evaluated to 0.66 MV/cm. In the absence of an internal electric field, the Stokes shift was determined to drop from 64 meV down to 24 meV with increasing well width. Additionally, with increasing temperature, the QCSE vanishes as the internal electric field is s...

Determination of the spontaneous polarization of wurtzite (Mg,Zn)O

We report on the experimental determination of the spontaneous polarization of wurtzite-(Mg,Zn)O by examination of the recombination dynamics of polar ZnO/(Mg,Zn)O quantum wells (QWs). The thickness-dependent decay time of the unscreened single-exciton states inside the QWs was modeled by a self-consistent solution of Schrodinger- and Poisson-equation to deduce the total polarization across the QW for different Mg-contents inside the barriers. By the separation of the piezoelectric components of the polarization, a linear increase in spontaneous polarization with increasing Mg-content x of P/x = (0.151 ± 0.015) C/m2 was determined for Mgx Zn1−x O.

Semiconducting oxide heterostructures

The properties of semiconducting oxide heterostructures are demonstrated using ZnO and its ternary alloy MgxZn1 − xO as a model system. This system is of particular importance, as it shows by far the most detailed research activities among oxide semiconductors. MgxZn1 − xO can be grown pseudomorphically on single crystalline ZnO in the step flow growth mode yielding atomically flat surfaces. In such structures, a two-dimensional electron gas was detected with an areal concentration of up to n2d = 3.9 × 1013 cm−2. The quantum Hall effect was observed in ZnO/MgxZn1 − xO single heterostructures, the first oxide system ever to show the quantum Hall effect. Likewise, quantization plays a dominating role in double heterostructures of ZnO and MgxZn1 − xO. Quantum confinement and the quantum-confined Stark effect were observed in ZnO/MgxZn1 − xO quantum wells grown by MBE and also by PLD. Finally, the ability to induce and control quantized states in ZnO/MgxZn1 − xO heterostructures was combined with the extraordinary properties of one-dimensional nanostructures, thus resulting in nano-sized building blocks.