R. Grötzschel

Room temperature ferromagnetism in carbon-implanted ZnO

Unexpected ferromagnetism has been observed in carbon doped ZnO films grown by pulsed laser deposition [H. Pan et al., Phys. Rev. Lett. 99, 127201 (2007)]. In this letter, we introduce carbon into ZnO films by ion implantation. Room temperature ferromagnetism has been observed. Our analysis demonstrates that (1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e., ion implantation, and (2) the chemical involvement of carbon in the ferromagnetism is indirectly proven.

Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

In the last decade, transition-metal-doped ZnO has been intensively investigated as a route to room-temperature diluted magnetic semiconductors (DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g., the anisotropy and the superparamagnetic blocking temperature, can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.

Precipitation, ripening and chemical effects during annealing of Ge+ implanted SiO2 layers

Abstract The Ge redistribution and nanocrystal formation in Ge + implanted SiO 2 layers have been systematically studied by RBS, TEM and XPS. Annealing in N 2 or Ar leads to a dramatic change of the as-implanted Gaussian-like Ge depth distribution to a bimodal profile and, additionally, an accumulation of Ge at the Si/SiO 2 interface. XTEM images show no Ge nanocrystals in the region of the sub-surface peak, whereas the deeper peak can be clearly related to Ge nanocrystals. A similar Ge redistribution occurs after annealing in pure O 2 , but in this case amorphous GeO x clusters have been found too. The annealing behaviour is explained by the in-diffusion of an oxidant (O 2 or a few ppm of moisture) from the annealing ambient and its subsequent reaction with dissolved and/or clustered Ge. Kinetic 3D lattice Monte-Carlo simulations have been performed to prove this model of the Ge redistribution and nanocluster evolution.

Multimodal impurity redistribution and nanocluster formation in Ge implanted silicon dioxide films

The depth distribution of Ge implanted into thermally grown SiO2 films has been studied after annealing using transmission electron microscopy, Rutherford backscattering spectrometry, and x-ray diffraction. At annealing temperatures above 900 °C a significant redistribution of the as-implanted Ge profile was found. Crystalline Ge nanoclusters embedded in the SiO2 matrix are formed within a cluster band with well defined boundaries. The evolution of nanoclusters can be explained qualitatively by a model based on nucleation, growth and Ostwald ripening of Ge precipitates. Besides, chemical and interface reactions lead to the formation of additional Ge peaks near the surface and at the Si/SiO2 interface.

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...

The formation of narrow nanocluster bands in Ge-implanted SiO2-layers

Abstract The paper describes the formation of Ge nanocrystals in thin thermally grown SiO 2 -layers ( d ox ⩽100 nm) using implantation of 10 15 –2×10 16 Ge + /cm 2 and subsequent annealing. Although the implanted Ge depth profile is distributed over almost the whole SiO 2 layer, a very narrow band (typical width 5 nm) of Ge nanoclusters very close but well-separated to the Si/SiO 2 -interface is formed by self-organization under specified annealing conditions. A possible mechanisms for this self-organization process is discussed including nucleation phenomena, Ostwald ripening and defect-stimulated interface processes. Simple MOS-structures were prepared and the effect of charge storage inside the clusters has been derived from C – V characteristics.

Microstructural investigation of ion beam synthesised germanium nanoclusters embedded in SiO2 layers

Abstract Ion beam synthesised Ge nanoclusters in amorphous SiO2 layers were produced by ion implantation and subsequent annealing and investigated by cross section transmission electron microscopy (TEM). After annealing in inert atmosphere at appropriate temperatures, a Ge cluster band with sharp transitions regions appear around the projected range of the implantation profile. The size of the Ge nanoclusters in the band varies from a few nanometers at the transition regions up to about 10 nm in the middle of the band and depends strongly on the annealing temperature and time. The Ge clusters are crystalline with a lattice constant slightly higher than that of Ge bulk material. Much larger Ge clusters line up far from the maximum of the implantation profile (position of the mean projected range) have diameters up to 40 nm if the annealing conditions were changed to an H2 containing atmosphere. The results of cross section TEM are supported by scanning TEM analysis combined with an energy dispersive X-ray detection system and RBS investigations.

THE APPLICATION OF HIGH ENERGY ION IMPLANTATION FOR SILICON RADIATION DETECTORS

Abstract High energy ion implantation of phosphorous and boron in the MeV-range has been applied to modify the vertical electric field distribution of silicon pn-junction radiation detectors. Low dose phosphorous implantation (10 MeV, 10 11 –10 12 cm −2 ) was used to realize detectors with local high field regions characterized by an internal electric field strength up to 150 kV/cm. A field related decrease of the effective window down to 35 nm and a corresponding reduction of the pulse height defect (PHD) for the spectroscopy of low energy heavy ions have been obtained. Additional subjects for the application of the MeV-ion implantation technique are briefly summarized.

INFLUENCE OF SURFACE ROUGHNESS ON MEASURING DEPTH PROFILES AND THE TOTAL AMOUNT OF IMPLANTED IONS BY RBS AND ERDA

For rough surfaces depth profiling of surface layers becomes questionalble if the layers to be analysed have a thickness comparable to or larger than the dimension of the surface roughness. In this work nitrogen ions have been implanted at normal incidence into carbon samples with smooth and rough surfaces, respectively. These implanted layers have been measured by Rutherford Backscattering Spectrometry (RBS) using 1.7 MeV 4He ions and by Elastic Recoil Detection Analysis (ERDA) using 35 MeV 35Cl ions at different angles of incidence. For smooth surfaces the N depth profiles obtained from both RBS and ERDA are in accordance with TRIM simulations. This is also the case for RBS measurements at normal incidence and 10° emergence relative to the surface normal of a sample displaying few 100 nm surface roughness. For this rough sample, ERDA measurements under oblique angles of incidence and emergence yield long distribution tails towards larger depths, at reduced peak N concentrations, compared to smooth surfaces, while the total N contents are roughly identical. Computer simulations using an idealised surface topography to model surface-roughness effects give qualitative agreement with the ERDA depth profiles obtained from the rough sample.

MICROSTRUCTURAL INVESTIGATION OF SN NANOCLUSTERS IN DOUBLE-ENERGY IMPLANTED AND ANNEALED SIO2 LAYERS WITH CROSS-SECTIONAL TEM

Abstract 500 nm SiO 2 layers were implanted with 350 and 700 keV Sn ions at room temperature to study the formation of nanoclustering within the insulating layer with cross-sectional TEM. Ion energy and fluence were adjusted to achieve an almost constant Sn concentration of about 1.25 and 2.5 at.% in the SiO 2 layer, respectively, as measured with RBS. After a subsequent annealing treatment for 30 min in dry N 2 ambient, three different kinds of regions of nanoclusters appear in the insulating layer at temperatures above 600°C. Close to the surface of the specimens a nanocluster band is observed, which is related to in-diffusion of moisture from the surrounding ambient during annealing. That results in the formation of crystalline SnO x clusters (typical size 2–5 nm). Similar effects occurred for comparable Ge implantations. Following the cluster band close to the surface another region of clusters appears in the micrographs. However, these crystalline Sn clusters are significantly larger up to 60 nm and, in addition, their mean distance is relatively far from each other. In a distance of less than 10 nm away from the SiO 2 /Si interface, a third but very narrow Sn nanocluster band appears, well separated from the interface. This band is directly related to the irradiation damage formed by recoils in that region during implantation. In this contribution, the influence of the fluence and annealing temperature on the nanoclustering is presented.