Towards an Atomic Scale Understanding of Spin Polarized Electron Transport Vincent P. LaBella, College of Nanoscale Science and Engineering, University at Albany / SUNY, DMR0349108 Dr. LaBella’s research group is focused on materials and material interface issues of spintronics. Spintronics - Utilizing the spin of the electron as well as its charge to fabricate novel, multi-functional semiconductor devices. As spintronics moves from mostly metal-based devices such as GMR/TMR stacks into semiconductors, multi-function materials are needed that incorporate ferromagnetism, semiconducting, and dielectric properties into one. Ferromagnetic semiconductors or so-called diluted magnetic semiconductors (DMS) are examples of such materials. These materials will ease the manufacturing of new spintronic devices as they allow ferromagnetic properties to be "on chip" and in the "front end"; a first step in a spintronic device. Ion implantation of Mn and growth of Mn-doped semiconductors such as Si and GaAs are currently being explored as candidates. Realizing the potential of spintronics requires advances in our fundamental understanding of spin polarized electron transport through materials and material interfaces at an atomic level. For example, understanding how spin-flip and spin-dependent scattering of electrons is effected by temperature, materials, and material interface properties may help to tailor material interfaces for efficient spin transfer. Two scanning tunneling microscopy (STM) based techniques are employed that allow the quantification and tagging of defect type to scattering mechanism with atomic resolution. In addition, in situ molecular beam epitaxy is utilized for material synthesis and interface preparation. Personnel The group is currently composed of 2 post docs, 2 Ph.D. students and 1 undergraduate student (not shown in picture). During the year high school students often join the group for a semester long intern project. College The college of Nanoscale Science and Engineering at SUNY Albany is the first such college of its kind and devoted to a multidisciplinary approach to nanoscience in both research and education. It has about 40 faculty and 100 students and expected to double in size in the coming years. More info on the college can be found at www.albany.edu/cnse The PI also teaches a first year grad course on quantum mechanics to all incoming nanoscience graduate students. The course uses a classroom response system (aka clickers) to get immediate feedback on the students level of comprehension. In addition to the National Science Foundation, The work is also funded by MARCO/DARPA through the Interconnect Focus Center and NYSTAR (New York State Office of Science, Technology and Academic Research) Group web page: www.alabny.edu/spin Spintronics - Utilizing the spin of the electron as well as its charge to fabricate novel, multi-functional semiconductor devices. Today, the spin of the electron is utilized in data storage devices such as the read heads on hard disk drives. The fundamental understanding of spin dependant transport helped to fuel the dramatic increase in storage capacity over the past decade. Tomorrow, new spintronic devices for logic and computation are dreamt of that will bring about increases in speed and reductions in power consumption. For example, a single multi-functional spin based device that can replace multiple charge based devices, or a way to make quantum computing using electron spins. To realize these goals, spintronics must move from mostly metal-based devices such as GMR/TMR stacks into semiconductors. Ion implantation of Mn and growth of Mn-doped semiconductors such as Si and GaAs are currently being explored as candidates. In addition, advances in our fundamental understanding of spin polarized electron transport through materials and material interfaces at an atomic level are needed. Spin polarized ballistic electron emission microscopy is being developed which can measure spin transport on the nanometer length scale. Group members (L-R) PI: V. LaBella, Ph.D. Student A. Stollenwerk, Post Doc M. Krause, Post Doc M. Bolduc, Ph.D. Student Chaffra Awo-Affouda Recent achievement of room temperature ferromagnetism in Mn ion implanted Si appeared in Science News and Materials Today. Physical Review B 71 033302 (2005)

Towards an Atomic Scale Understanding of Spin Polarized Electron Transport Vincent P. LaBella, College of Nanoscale Science and Engineering, University at Albany / SUNY, DMR0349108 Above Room temperature ferromagnetism in Mn ion implanted Si Aim of the project: To make silicon a diluted magnetic semiconductor through doping with Mn in the level from 1-10% per atom Research results: Current results show that ferromagnetism can be achieved up to 400K when silicon is ion implanted with Mn. Significance of this work: It is now possible to make Si ferromagnetic by ion implantation which is a technique routinely utilized in the industry for device fabrication. This may find use in future devices (not just spintronic devices) where ferromagnetism is desired. Future plans: Crucial to achieving a DMS is the structure of the material and to prove that the ferromagnetism is mediated by the holes within the material. Future work is focused on uncovering the structure of the material and the source of the ferromagnetism A Combined MBE and STM system Aim: To construct a combined molecular beam epitaxy and scanning tunneling microscopy system capable of performing growth of compound semiconductors and metals. In addition, the STM is capable of performing BEEM measurements. Research results: The system is fully functioning and highlighted in this paper M. Krause et al., Journal of Vacuum Science & Technology B 23 1684 (2005) Significance of this work: Probably the only system in the world that can prepare both compound semiconductor surfaces and silicon, deposit metals on top of them and perform BEEM measurements at 4K all in situ. The research highlights on the next slide are two of the many projects ongoing that use the system. Future plans: To carry out the studies highlighted in the rest of this presentation in particular spin polarized BEEM studies. Above Room temperature ferromagnetism in Mn ion implanted Si (left). Integrating ferromagnetism into semiconductors is a first step at achieving semiconductor based spintronic devices. We demonstrate this is possible by ion implanting Mn into Si. Our ultimate goal is to make Si a diluted magnetic semiconductor (DMS). DMS materials are semiconductors doped with an impurity which makes in ferromagnetic. Typically, Mn is utilized in the range from 1-10% per atom giving these materials the novel property of electric field control of its ferromagnetism. The figure to the right shows the temperature dependence of the normalized remnant magnetization (top) for Mn-implanted Si both p-type (solid markers) and n-type (open markers). A Bloch-law dependence is fitted (solid line) with 90% confidence. The ferromagnetic hysteresis loops at 10 K, 77 K, and 300 K from Mn-implanted Si after rapid thermal annealing are shown in the bottom plot. The inset shows a ferromagnetic hysteresis loop at 300 K before annealing. These results show that it maybe possible to produce a Si-based DMS. The current research challenges are in the synthesis of the material to avoid cluster formation. Physical Review B 71 033302 (2005) Albany High Seniors Joel Leibo and Josh Hancox give their final presentation on STM and BEEM from summer internship A Combined MBE and STM system has been constructed to characterize both compound and elemental semiconductor surfaces and interfaces. A custom designed sample transfer mechanism connects the two chambers. The MBE chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ± 0.5 C. In addition, the microscope can operate at temperatures as low as 4 K and perform ballistic electron emission microscopy. The system has a separate preparation chamber with an evaporation source for metals. The entire STM chamber rests on an active vibration isolation table, while still maintaining an all UHV connection to the MBE. This state-of-the-art system is used for the research projects highlighted in these slides. In addition, the group studies compound semiconductor surfaces and Dr. LaBella and post-doc M. Krause just submitted a review article to Surface Science Reports cover the past 40 years of research on the technologically important GaAs(001) surface. Scaled drawing of the interconnected III-V MBE and STM system which is also capable of performing BEEM measurements. See M. Krause et al., Journal of Vacuum Science & Technology B 23 1684 (2005) Ph.D. Student Andy Stollenwerk works with Albany High Senior Joanna Belding taking STM and BEEM data.

Towards an Atomic Scale Understanding of Spin Polarized Electron Transport Vincent P. LaBella, College of Nanoscale Science and Engineering, University at Albany / SUNY, DMR0349108 BEEM studies of MnSi Aim of the project: To measure the hot electron transport properties of the metal semiconductor interface MnSi/Si, which is a potential spin injecting contact. Research results: BEEM spectra show evidence of two onset voltages indicating a complex interface band structure that maybe due to the fact the Mn is a transition metal. Significance of this work: Both previous IV and photoemission measurements of the barrier height disagreed by 0.1 eV, which is about the difference in the two onsets we see. This type of double onset is unexpected for Si and maybe common across all transition metal silicides. For example, discrepancies exists for the Schottky heights for CoSi2 as measured by BEEM. Future plans: To perform spin polarized BEEM on ferromagnetic metal overlayers on Si to measure the spin dependant attenuation lengths. Mn Growth on Si(001) Aim of the project: To study the growth of Mn on Si. Research results: Current results show that the MnSi islands that form undergo Ostwald ripening in time and other regions show row like structures composed of Mn forming on the surface. Significance of this work: It has been theoretically predicted that a two layer MnSi structure on Si has ferromagnetic properties. This study will provide the activation energy for diffusion of Mn on Si, which is an important parameter for epitaxial growth. In addition this study will help to determine the lowest energy structures of Mn on Si. Future plans: to extract the activation energy for diffusion and compare STM images with theoretically generated ones. BEEM Spectra of MnSi/Si(001) (left) shows evidence of multiple thresholds indicating a complex interface band structure [1]. This system is a potential spin injection contact as it has been theoretically predicted that a layered Si-Mn structure on Si(001) has ferromagnetic properties [2]. Spin polarized BEEM is currently in progress. With spin polarized BEEM a ferromagnetic STM tip is utilized to inject a polarized tunneling current. This technique will be utilized to measure the spin dependant attenuation lengths of the hot-electrons in ferromagnetic metal over-layers on Si. Spin dependant scattering is utilized in GMR devices and can also be used to increase the polarization state of a current of electrons for future spintronic devices. [1] A. Stollenwerk et al. JVST (submitted) [2] Wu et al. Phys. Rev. Lett. 92 237202 (2004) PI LaBella works with Ph.D. Students Andy Stollenwerk and Chaffra Awo-Affouda on the MBE system. Mn growth on Si(001) is being studied with in situ STM. After Mn deposition and annealing, MnSi islands form. In between the islands evidence of monolayer-high Mn structures which agree with ab inito calculations can be seen (Mn rows) [2]. In addition, the islands undergo Ostwald ripening as annealing time is increased, which is being utilized to extract the activation energy for diffusion of Mn on the Si(001) surface. Part of this work is in collaboration with Peter Kratzer’s theory group at the Fritz Haber Inst. in Berlin. The goal of this research is to learn how to grow ordered MnSi thin films which are ferromagnetic to be utilized as spin injection contacts for silicon. [1] M. Krause et al. PRB (in preparation) [2] Wu et al. Phys. Rev. Lett. 92 237202 (2004) 0.1 ML Mn on Si(001) after annealing at 300 C for 1 min MnSi island Albany high senior Joanna Belding taking STM and BEEM data. 50 nm × 50 nm Mn Rows on Si PI LaBella teaching graduate quantum using classroom response system (clickers). “I love the clickers!” say the students. 680 nm × 680 nm 10 nm × 10 nm