Manfred Madel

Enforced c-axis growth of ZnO epitaxial chemical vapor deposition films on a-plane sapphire

To enforce perfect c-axis orientation of ZnO epitaxial films grown on a-plane sapphire, we first grew perfectly perpendicularly aligned (i.e., c axis oriented) ZnO nanopillars on such sapphire substrates, and then over-grew these by a closed epitaxial film using a modified chemical vapor deposition process at atmospheric pressure. X-ray diffraction and low temperature photoluminescence measurements confirm the desired epitaxial relationship and very high crystalline quality. This growth scheme is an efficient method to suppress dislocations and polycrystalline growth of ZnO and could work equally well for other heteroepitaxial epilayer/substrate systems.

Composition dependent valence band order in c-oriented wurtzite AlGaN layers

The valence band order of polar wurtzite aluminum gallium nitride (AlGaN) layers is analyzed for a dense series of samples, grown heteroepitaxially on sapphire substrates, covering the complete composition range. The excitonic transition energies, found by temperature dependent photoluminescence (PL) spectroscopy, were corrected to the unstrained state using input from X-ray diffraction. k⋅p theory yields a critical relative aluminum concentration xc=(0.09±0.05) for the crossing of the uppermost two valence bands for strain free material, shifting to higher values for compressively strained samples, as supported by polarization dependent PL. The analysis of the strain dependent valence band crossing reconciles the findings of other research groups, where sample strain was neglected. We found a bowing for the energy band gap to the valence band with Γ9 symmetry of bΓ9=0.85eV, and propose a possible bowing for the crystal field energy of bcf=−0.12eV. A comparison of the light extraction efficiency perpendic...

Persistent photoconductivity in ZnO nanowires: Influence of oxygen and argon ambient

ZnO nanowires typically show persistent photoconductivity (PPC), which depends in their temporal behaviour on the ambient. We investigate ZnO nanowires in oxygen and argon ambient and analyze the PPC both on the short and on the long time scale to sort out the underlying mechanisms. Wavelength dependent excitation shows the energy barrier for the PPC to be around 150 meV below the band gap of ZnO, independent of the ambient atmosphere. In photocurrent measurements at constant wavelength, a log-logistic dependence of the conductivity on the partial oxygen pressure is observed. The experimental results are compared to a model of Bonasewicz et al. [J. Electrochem. Soc. 133, 2270 (1986)] and can be explained by oxygen adsorption processes occurring on the surface of the ZnO nanowires. From temperature dependent measurements of the decay times in oxygen and argon ambient, the related activation energies for the fast and slow decay processes are determined. Comparing our results to theoretical calculations of e...

Polarity-controlled ultraviolet/visible light ZnO nanorods/p-Si photodetector

Vertically aligned ZnO nanorods of high quality were grown on p-type silicon substrate by a modified chemical vapor phase process. Low temperature photoluminescence measurements show a near band gap donor bound exciton full width at half maximum of less than 500 μeV, and only weak green defect luminescence proofing the superior optical quality of the ZnO structures. n-ZnO/p-Si photodetector devices were processed from these upright standing nanorods. The devices show bias dependent selective photon response: under forward bias, an ultraviolet to visible sensitivity ratio of three orders of magnitude was achieved, while under reverse bias ultraviolet and visible light contributed similarly to the electrical readout current. Thus, the characteristics of this detector element can be switched between “solar blind” and “broadband.”

Oxygen vacancies induced DX center and persistent photoconductivity properties of high quality ZnO nanorods

Ultraviolet sensors based on homoepitaxially grown ZnO nanorods were fabricated using clean room technology. We study the spectral dependence and frequency dependence of the photoresponse of these rods at different temperatures and ambient conditions. Whereas the response for above-bandgap light is fast, we find a slow response to light below band gap and clear signatures of persistent photoconductivity. These findings are explained by switching oxygen vacancies by light from nonconductive to conductive state, whereas the oxygen vacancies undergo a large lattice relaxation. The threshold photon energy for this process is found to be 2.6 eV at room temperature.

Optical Properties of ZnO/GaN/InGaN Core–Shell Nanorods

Upright ZnO/GaN/InGaN core–shell nanorods arrayed in a well defined pattern are very good candidates for sensing applications. In our approach, we grew single ZnO nanopillars on top of ordered GaN pyramids, which were subsequently overgrown with GaN and a single InGaN quantum well, followed by a final GaN barrier layer. Spatially and spectrally resolved low temperature cathodoluminescence was used to investigate the optical properties of the rods. We found the dominant quantum well luminescence to be well defined and homogeneously distributed, with a maximum intensity at the edges of the pillars. Although the hydrogen atmosphere during the nitride growth together with the elevated growth temperature should lead to complete desorption of the initial ZnO pillar template, we found evidence for ZnO relicts on the pillar surface, and for incorporation of Zn in GaN at the tips of the rods. Furthermore, we were able to distinguish between the luminescence contributions from the quantum well, Zn-doped GaN, and possible structural defects.

Silicon-on-insulator based ZnO nanowire photodetector

The authors present here a technologically easy process to electrically contact ensembles of ZnO nanowires on silicon-on-insulator substrates for multiple sensor applications. Microtrenches defined by standard photolithography and etching processes were bridged by ZnO nanowires. The ZnO nanowires grown by a high temperature process show excellent crystalline quality as confirmed by photoluminescence. These wires connecting the opposite sides of the microtrenches were investigated here as solar-blind photosensitive detectors, but may equally well serve for any other sensing application, where the ZnO conductivity is altered by the attachment of specific gases, proteins, etc. The straightforward design allows for an easy integration into CMOS processes.The authors present here a technologically easy process to electrically contact ensembles of ZnO nanowires on silicon-on-insulator substrates for multiple sensor applications. Microtrenches defined by standard photolithography and etching processes were bridged by ZnO nanowires. The ZnO nanowires grown by a high temperature process show excellent crystalline quality as confirmed by photoluminescence. These wires connecting the opposite sides of the microtrenches were investigated here as solar-blind photosensitive detectors, but may equally well serve for any other sensing application, where the ZnO conductivity is altered by the attachment of specific gases, proteins, etc. The straightforward design allows for an easy integration into CMOS processes.

Composition analysis of coaxially grown InGaN multi quantum wells using scanning transmission electron microscopy

GaN nanotubes with coaxial InGaN quantum wells were analyzed by scanning transmission electron microscopy in order to determine their structural properties as well as the indium distribution across the InGaN quantum wells. For the latter, two process steps are necessary. First, a technique to prepare cross-sectional slices out of the nanotubes has been developed. Second, an existing scanning transmission electron microscopy analysis technique has been extended with respect to the special crystallographic orientation of this type of specimen. In particular, the shape of the nanotubes, their defect structure, and the incorporation of indium on different facets were investigated. The quantum wells preferentially grow on m-planes of the dodecagonally shaped nanotubes and on semipolar top facets while no significant indium signal was found on a-planes. An averaged indium concentration of 6% to 7% was found by scanning transmission electron microscopy analysis and could be confirmed by cathodoluminescence measu...

Co-existence of harmonic generation and two-photon luminescence in selectively grown coaxial InGaN/GaN quantum wells on GaN pyramids

Nonlinear optical properties of coaxial InGaN/GaN multiple quantum well (MQWs) submicron hetero-structures were investigated using a tunable femtosecond laser at room temperature. Co-axial InGaN/GaN MQW hetero-structures were fabricated by depositing InGaN/GaN layers on the side walls of GaN submicron tubes on top of GaN micro-pyramids. Excitation and signal collection from a single micro-structure was achieved using multi-photon spectroscopy. Two photon excited photoluminescence (TPEL) was observed at around 390 nm independent of excitation wavelength. In addition to TPEL, observation of second harmonic signal of the excitation laser is also presented.