Above room temperature ferromagnetism in Mn-ion implanted Si

Journal of Applied Physics, 2013

We investigate the possibility of creating an intermediate band semiconductor by supersaturating Si with a range of transition metals (Au, Co, Cr, Cu, Fe, Pd, Pt, W, and Zn) using ion implantation followed by pulsed laser melting (PLM). Structural characterization shows evidence of either surface segregation or cellular breakdown in all transition metals investigated, preventing the formation of high supersaturations. However, concentration-depth profiling reveals that regions of Si supersaturated with Au and Zn are formed below the regions of cellular breakdown. Fits to the concentration-depth profile are used to estimate the diffusive speeds, v D, of Au and Zn, and put lower bounds on v D of the other metals ranging from 10 2 to 10 4 m/s. Knowledge of v D is used to tailor the irradiation conditions and synthesize single-crystal Si supersaturated with 10 19 Au/cm 3 without cellular breakdown. Values of v D are compared to those for other elements in Si. Two independent thermophysical properties, the solute diffusivity at the melting temperature, D s (T m ), and the equilibrium partition coefficient, k e , are shown to simultaneously affect v D . We demonstrate a correlation between v D and the ratio D s (T m )/k e 0.67 , which is exhibited for Group III, IV, and V solutes but not for the transition metals investigated. Nevertheless, comparison with experimental results suggests that D s (T m )/k e 0.67 might serve as a metric for evaluating the potential to supersaturate Si with transition metals by PLM. V C 2013 AIP Publishing LLC.

Applied Physics Letters, 2006

The authors report on ferromagnetism at room temperature in cluster-free, cobalt-and manganese-ion-implanted crystalline silicon. Through magnetic and structural analysis it is shown that the ion-implanted Si consists of two layers of Co-and Mn-containing silicon: ͑1͒ an amorphous Si layer on the surface and ͑2͒ single crystalline Si beneath. The amorphous layer shows very little magnetism by itself but seems to be responsible for partially canceling out or masking the ferromagnetism in the crystalline Si. The authors also observe that etching of the amorphous Si layer dramatically enhances the measured magnetism by as much as 400%.

Journal of Physics: Condensed Matter, 2013

The interaction of Mn with Ge quantum dots (QD), which are bounded by {105} facets, and the strained Ge wetting layer (WL), terminated by a (001) surface, is investigated with scanning tunneling microscopy (STM). These surfaces constitute the growth surfaces in the growth of Mn-doped QDs. Mn is deposited on the Ge QD and WL surface in sub-monolayer concentrations, and subsequently annealed up to a temperature of 400 • C. The changes in bonding and surface topography are measured with STM during the annealing process. Mn forms flat islands on the Ge{105} facet, whose shape and position are guided by the rebonded step reconstruction of the facet. Voltage-dependent STM images reflect the Mn-island interaction with the empty and filled states of the Ge{105} reconstruction. Scanning tunneling spectra (STS) of the Ge{105} facet and as-deposited Mn-islands show a bandgap of 0.8 eV, and the Mn-island spectra are characterized by an additional empty state at about 1.4 eV. A statistical analysis of Mn-island shape and position on the QD yields a slight preference for edge positions, whereas the QD strain field does not impact Mn-island position. However, the formation of ultra-small Mn-clusters dominates on the Ge(001) WL, which is in contrast to Mn interaction with unstrained Ge(001) surfaces. Annealing to T < 160 • C leaves the Mn-clusters on the WL unchanged, while the Mn-islands on the Ge{105} facet undergo first a ripening process, followed by a volume gain which can be attributed to the onset of intermixing with Ge. This development is supported by the statistical analysis of island volume, size and size distribution. Increasing the annealing temperature to 220 • and finally 375 • C leads to a rapid increase in the Mn-surface diffusion, as evidenced by the formation of larger, nanometer size clusters, which are identified as germanide Mn 5 Ge 3 from a mass balance analysis. This reaction is accompanied by the disappearance of the original Mn-surface structures and de-wetting of Mn is complete. This study unravels the details of Mn-Ge interactions, and demonstrates the role of surface diffusion as a determinant in the growth of Mn-doped Ge materials. Surface doping of Ge-nanostructures at lower temperatures could provide a pathway to control magnetism in the Mn-Ge system.

Physical Review B, 2007

Structural and ferromagnetic properties in Mn implanted, p-type Si were investigated. High resolution structural analysis techniques like synchrotron X-ray diffraction revealed the formation of MnSi 1.7 nanoparticles already in the as implanted samples. Depending on the Mn-fluence, the size increases from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. The observed ferromagnetism yields a saturation moment of 0.21 μ B per implanted Mn at 10 K, which could be assigned to MnSi 1.7 nanoparticles as revealed by a temperature dependent magnetization measurement.

Physical Review B, 2008

The magnetic and electrical transport properties of Mn-doped amorphous silicon (a-Mn x Si 1−x ) thin films have been measured. The magnetic susceptibility obeys the Curie-Weiss law for a wide range of x (0.005-0.175) and the saturation moment is small. While all Mn atoms contribute to the electrical transport, only a small fraction (interstitial Mn 2+ states with J=S=5/2) contribute to the magnetization. The majority of the Mn atoms do not possess any magnetic moment, contrary to what is predicted by the Ludwig-Woodbury model for Mn in crystalline silicon. Unlike a-Gd x Si 1−x films which have an enormous negative magnetoresistance, a-Mn x Si 1−x films have only a small positive magnetoresistance, which can be understood by this quenching of the Mn moment.

Journal of Applied Physics, 2014

Surface and Interface Analysis, 2008

It has been theoretically predicted that manganese atoms in silicon matrix tend to form dimers which can show both ferro-and antiferromagnetic properties. One might expect that the significant part of such dimers (N-mers) should be spattered as cluster ions while SIMS measurement. We have investigated samples of silicon wafers implanted with manganese ions and annealed. In the solid solution the probability of the situation when N doped atoms are nearest neighbours is proportional to Nth power of concentration. However, if in the solution the process of enhancement of neighbouring impurity atoms number takes place, the cluster ion signal should exceed the power law. We have verified the validity of this statement in ion implanted solutions of B and Mn in silicon. SIMS measurements in annealed samples show that the profile of Mn 2 and Mn 3 ions essentially exceed respectively the square and the cube of manganese concentration. Quantitative estimation shows that in the annealed sample at some depths manganese dimers contain about 10% of total Mn atoms. The connection between the obtained profiles and magnetic measurements is discussed.

Physical Review B, 2010

Transition metals such as Mn generally have large local moments in covalent semiconductors due to their partially filled d shells. However, Mn magnetization in group-IV semiconductors is more complicated than often recognized. Here we report the observation of a striking difference between Mn moments in amorphous Si (a-Si) and Ge (a-Ge) over a wide range of Mn concentrations (0.005-0.20). While Mn moments in aGe are large (3 ȝ B), they are entirely quenched in a-Si (<0.1 ȝ B). Corresponding differences are also observed in d-shell electronic structure, sign of Hall effect and atomic density. Density-functional-theory calculations show distinct local structures, and point to Mn coordination number as the key factor. Despite the amorphous structure, Mn in a-Si is in a relatively well defined high coordination, interstitial type site with broadened d-bands associated with low moment and electron (n-type) carriers, while Mn in aGe is in a low coordination, substitutional type site with large local moment and holes (p-type) carriers. At the advent of the contemporary semiconductor era, transition metals (TMs) were viewed as undesirable impurities in group-IV semiconductors. Ludwig and Woodbury developed a model which successfully explained the electronic structure of TM impurities in crystalline Si (c-Si). 1 It is well established that ~10 16 /cm 3 TM impurities create enough deep level states to significantly trap charge carriers and hinder transport. 2 More recently, exploration of novel ways to manipulate electron spins has made TM dopants of interest for spintronic applications, where both charge and spin are manipulated and utilized for information processing. 3-5 Magnetic semiconductors based on group-IV materials, like Si or Ge, are especially interesting due to their predicted high critical temperature T c and predominance as mainstream semiconductor materials for microelectronics. While Mn-doped c-Si and c-Ge have been made by various techniques, it has proven challenging to achieve robust intrinsic ferromagnetism at ambient conditions in either. First-principles calculations

Nanotechnology, 2010

Here, we speculate that room temperature voltage-controlled ferromagnetic ordering may become a founding phenomenon for the next generation of low-power spintronics nanodevices and discuss the special role of dilute magnetic semiconductors as the most reliable material basis to date. Then, we report on our latest experimental achievements in the voltage manipulation of the ferromagnetism in MnGe quantum dots, experimentally demonstrating the capacity of pushing the Curie temperature further above room temperature for technological applications.

Journal of Applied Physics, 2010

Room temperature ferromagnetic Mn 0.026 B X Si 0.974−X bulk samples with X = 0.001, 0.005, and 0.01 were fabricated by arc melting and followed by thermal annealing at 1000°C. The effects of carrier density on the ferromagnetism were studied. Due to the high carrier densities, all samples showed metallic behavior. Kondo effect was observed when the temperature was below 10 K. Weakly localized carriers monotonously increased with increasing boron concentration, resulting in the enhancement of exchange coupling among Mn ions. The study indicated that the ferromagnetism originates from the hole mediated Ruderman-Kittel-Kasuya-Yosida mechanism.

Journal of Applied Physics, 2008

We present a comprehensive study relating the magnetic properties to structural properties of Mn +-implanted Si 1−x Ge x films as a function of Ge content ͑x =0-0.5͒. Ferromagnetic ordering with three critical temperatures, T B ϳ 10-16 K, T C1 ϳ 650-780 K, and T C2 ϳ 825-860 K, are reported in this material system. Element specific x-ray absorption fine structure results show that the majority of the Mn ions are nonsubstitutional in all samples. The transmission-electron microscopy coupled with z contrast and chemical analysis reveals the presence of Mn-rich nanosized clusters including Mn 4 Si 7 in Si-rich samples and Mn 7 Ge 3 phases in Ge-rich samples. A composition transition occurred at x ϳ 0.2-0.3, where we observe a change in bond lengths and defect structures. Additionally, an enhancement in magnetizations with an increase in both T B and T C1 as well as a conversion from n-type to p-type conduction are also detected.

Journal of Applied Physics, 2008

We present results of the magnetic, structural and chemical characterizations of Mn +-implanted Si displaying n-type semiconducting behavior and ferromagnetic ordering with Curie temperature, well above room temperature. The temperaturedependent magnetization measured by superconducting quantum device interference

Science China Materials

In this work, silicon-germanium (SiGe) thin films are epitaxially grown on Ge substrates by ultra-high vacuum chemical vapor deposition and then doped with Mn element by ion-implantation and subsequent rapid thermal annealing (RTA). The characterizations show that the epitaxial SiGe thin films are single-crystalline with uniform tensile strain and then become polycrystalline after the ion implantation and following RTA. The magnetization measurements indicate that the annealed thin films exhibit Mn concentration-dependent ferromagnetism up to 309 K and the X-ray magnetic circular dichroism characterizations reveal the spin and orbital magnetic moments from the substitutional Mn element. To minimize the influence of anomalous Hall effect, magneto-transport measurements at a high magnetic field up to 31 T at 300 K are performed to obtain the hole mobility, which reaches a record-high value of 1230 cm 2 V −1 s −1 , owing to the crystalline quality and tensile strain-induced energy band modulation of the samples. The first demonstration of Mn-doped SiGe thin films with roomtemperature ferromagnetism and high carrier mobility may pave the way for practical semiconductor spintronic applications.