Browsing by Author "Dash, Saroj Prasad"

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Towards spin injection into silicon
(2007) Dash, Saroj Prasad; Carstanjen, Heinz Dieter (Prof. Dr.)
The efficient spin injection into semiconductors could pave the way to a new generation of electronics devices such as spin memories, spin transistors, and spin quantum computers. The most important semiconductor for industrial application, Si has been studied for the purpose of spin injection extensively in this thesis. Three different concepts for spin injection into Si have been addressed: (1) spin injection through a ferromagnet-Si Schottky contact, (2) spin injection using MgO tunnel barriers in between the ferromagnet and Si, and (3) spin injection from Mn-doped Si (DMS) as spin aligner. (1) FM-Si Schottky contact for spin injection: In a heterostructure of a ferromagnetic thin film on a Si substrate, any structural disorder at the interface would drastically reduce the spin polarization at the interface and, hence, the spin injection efficiency. To be able to improve the interface qualities one needs to understand the atomic processes involved in the formation of such silicide phases. In order to obtain more detailed insight into the formation of such silicide phases the initial stages of growth of Co and Fe were studied in situ by HRBS with monolayer depth resolution. As understood, it was important to prohibit the in-diffusion of Co into interstitial sites at the initial stages of growth and the out-diffusion of Si atoms in the latter stages. So in order to control and improve the interface, equilibrium growth conditions were followed (i) by lowering the growth temperature and (ii) by surfactant-mediated growth. Low temperature growth of Co on Si (100): Already at very low coverage Co diffusion into the bulk Si has been observed. The amount of in-diffused Co is, however, less than at room temperature. In contradiction to room temperature growth, Co atoms form layers of pure Co on top of the Si surface already at very low coverage. Every second Si layer, starting with the first Si layer, is Co depleted. This leads to an oscillatory Co distribution in the Si lattice which is preserved up to higher coverages (1.3 ML). Surfactant-mediated growth of Co on Si (100) : The lower surface free energy of Sb in comparison to Co and Si, makes it a potential candidate for surfactant mediated growth. By the use of one monolayer of Sb adsorbed on a Si (100) surface, Co-Si intermixing at the interface is strongly reduced in comparison to the interface without Sb as surfactant. The improved interface quality with Sb-mediated growth is also reflected in magnetic measurements. Co with Sb-mediated growth shows a higher magnetic moment. It was shown that simple solutions can reduce the FM-Si inter diffusion at the interface and improve the interface quality. However these non-equilibrium growth conditions could not stop the silicide formation completely. (2) MgO tunnel barrier for spin injection into Si: On the other hand, using an ultra-thin tunnel barrier between FM and Si will have three advantages: (i) form a chemical barrier between the FM and Si, (ii) circumvent the conductivity mismatch problem, and (iii) in addition act as a spin filter. The fabrication and characterization of ultra-thin crystalline MgO tunnel barriers on Si (100) was presented. Some of the important properties required for tunnel barriers on Si have been addressed. Ultra-thin stoichiometric MgO tunnel barriers with sharp interface with Si (100), very homogeneous, without pin-holes, and crystalline in structure could be fabricated by reactive molecular beam epitaxy. Co and Fe on an ultra thin MgO tunnel barrier were found to have island-like growth with a rough surface. Ultra-thin Co and Fe films are found to be thermally quite stable up to 450 °C. (3) Mn doped Si for spin injection: For spin injection purpose, instead of contacting the Si with a ferromagnetic metal, the contact could be made with another semiconductor, one with ferromagnetic properties. This solves the conductivity mismatch problem by ensuring that the resistivities of the materials on both side of the interface are comparable in magnitude. Si-based diluted magnetic semiconductor samples were prepared by doping Si with Mn by two different methods i) by Mn ion implantation and ii) by in-diffusion of Mn atoms (solid state growth). In the case of implanted samples, Mn atoms do not substitute Si sites. The implanted samples show room temperature ferromagnetism as measured by a SQUID magnetometer. The magnetic moment per Mn atom is found to decrease with increasing implantation dose. It has been observed that the implanted samples show carrier mediated ferromagnetism and, more importantly, mediated by both holes and electrons in contrast to statements in the literature. Solid state growth of Mn doped Si : For evaporation of Mn on Si (100), Mn atoms diffuse deep into the Si bulk already at room temperature, even for very low coverage (0.25 ML) with an oscillatory concentration depth profile as observed by HRBS with monolayer depth resolution. This results in natural MnxSi1-x/Si digital layers on the surface. Surprisingly, the samples prepared by this solid state diffusion process show room-temperature ferromagnetism having a magnetic moment of 1.8 µB per Mn atom, which is much higher than that of the ion-implanted samples. In contrast to ion-implanted samples the ferromagnetism in these samples does not show any carrier mediation.