N. Volbers

Fe-implanted ZnO: Magnetic precipitates versus dilution

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier-mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present article, we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences, temperatures, and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to Fe concentration and process temperature are found: (1) Disperse Fe2+ and Fe3+ at low Fe concentrations and low processing temperatures, (2) FeZn2O4 at very high processing temperatures, and (3) an intermediate regime with a coexistence of metallic Fe (Fe0) and ionic Fe (Fe2+ and Fe3+). Ferromagnetism is only observed in the latter two cases, where inv...

Preparation of ZnO substrates for epitaxy: Structural, surface, and electrical properties

The authors investigate the influence of a high temperature annealing in O2 atmosphere on the structural properties of ZnO substrates. Only at temperatures above 1100°C are atomic step heights and terraces seen by atomic force microscopy. The structural properties of the substrates were determined from the full width at half maximum (FWHM) of the rocking curve of the (0002) reflection. The FWHM is between 28 and 33arcsec for different substrates cut from one ingot but does not change with the annealing. The electrical properties, however, change from highly resistive to n-type conductive, which makes the substrates suitable for top-to-bottom contacting.

Adjustable metal-semiconductor transition of FeS thin films by thermal annealing

FeS polycrystalline thin films were prepared on float glass at 500°C by radio-frequency reactive sputtering. The influence of vacuum annealing on the metal-semiconductor transition of FeS films was investigated. It has been found that with the increase of the annealing temperature from 360to600°C, the metal-semiconductor transition temperature of FeS films first decreases and then increases, associated with first a reduction and then an enhancement of hysteresis width. The thermal stress is considered to give rise to the abnormal change of the metal-semiconductor transition of the FeS film during annealing.

Determination of secondary ion mass spectrometry relative sensitivity factors for polar and non-polar ZnO

Zinc oxide (ZnO) is regarded as a promising material for optoelectronic devices, due to its electronic properties. Solely, the difficulty in obtaining p-type ZnO impedes further progress. In this connection, the identification and quantification of impurities is a major demand. For quantitative information using secondary ion mass spectrometry (SIMS), so-called relative sensitivity factors (RSF) are mandatory. Such conversion factors did not yet exist for ZnO. In this work, we present the determined RSF values for ZnO using primary (ion implanted) as well as secondary (bulk doped) standards. These RSFs have been applied to commercially available ZnO substrates of different surface termination (a-plane, Zn-face, and O-face) to quantify the contained impurities. Although these ZnO substrates originate from the same single-crystal, we observe discrepancies in the impurity concentrations. These results cannot be attributed to surface termination dependent RSF values for ZnO.

Group I elements in ZnO

In this report we focus on the lithium doping of ZnO epitaxial films grown on GaN templates and ZnO substrates. We compare the results with diffusion studies of Li into ZnO single-crystal substrates. The diffused and in-situ doped layers were studied using mass spectroscopy, low temperature photoluminescence and Raman spectroscopy. Li is known to produce a deep acceptor state which takes part in a shallow donor to deep acceptor recombination in the visible spectral region. We will demonstrate that also shallow Li acceptors are introduced depending on the growth/diffusion temperature. The shallow Li-related acceptor has a binding energy around 300 meV. A donor-acceptor pair recombination with its zero phonon line at 3.05 eV followed by LO phonon replica is observed. The intensity ratio of the zero phonon line compared to the replica indicates weak electron phonon coupling, hence small lattice relaxation in contrast to the recombination with the deep Li acceptor.