Electronic Materials Research Institute at Northeastern University

Spintronics and Semiconductor Nanostructures

Don Heiman (Physics), Clive Perry (Physics) and Kate Ziemer (Chemical Engineering)

Spin-polarized carriers in a magnetic semiconductor represent a new operational paradigm for developing an entirely new generation of microelectronic devices used in mobile/portable applications requiring low power consumption while maintaining high performance. The spin of an electron can exist in two states—either "up" or "down", rather than on or off in conventional charge current based electronics. This quality will be exploited to build smaller (potentially on the atomic scale) binary heterostructures that use less power than charge-current-based devices. Furthermore, because of its quantum nature, electron spins may exist in infinitely many intermediate states, depending on the energy of the system. Thus, the possibility exists for highly parallel computation, which could make a quantum computer much faster for certain types of calculations. Also, when electron spins are aligned (i.e. all spin up or all spin down) they create a large-scale net magnetic moment as seen in magnetic materials like iron and cobalt and these have lead to the development of giant magnetoresistance materials now found in the read heads of computer hard disks. Ferromagnetic semiconductors are at the forefront of emerging research on spin-based electronics (spintronics) and quantum computation. Research on spintronics in nanofabricated ferromagnetic semiconductors is just beginning but highly promising as indicated by the current interest of DARPA and the NSF.

Three NU researchers have pooled their expertise to address spintronics in nano-structures. Heiman’s research is focused on synthesis and characterization of ferromagnetic semiconductor structures via molecular beam epitaxy (MBE). Perry is an expert in spectroscopic characterization of nanometer–scale sub-surface structures and assemblies in semiconductors. Ziemer specializes in understanding the physics and chemistry of thin film growth and in engineering semiconductor interfaces. All three will collaborate in the investigation and optimization of spin based electronic devices.

Perry is developing an ultra-high resolution characterization method that combines conventional Scanning Tunneling Microscopy (STM) with Light Emission Spectroscopy (LES) and Ballistic Electron Emission Microscopy (BEEM) in the Egan Research Center. The technique is intended for the simultaneous mapping of local nano-scale electronic and optical properties of buried semiconductor morphologies. The methodology is particularly suited for the characterization of defects and defect densities in materials containing quantum dots, wires and quantum wells used in semiconductor devices. Our experimental approach requires the surface science and interface expertise of Ziemer, the high magnetic field, low temperature, and spectroscopic facilities provided by Heiman.

Heiman has been engaged in nanoscale characterization of other spin-related device materials grown at NU. His group would utilize the recently requested Near-field Scanning Optical Microscope (NSOM) and the STM/LES facilities provided by Perry. The NSOM will also be used in conjunction with Heiman’s time-resolved luminescence apparatus. Using the NSOM or the STM/LES we will investigate the luminescence of individual magnetic quantum dots containing dilute magnetic ions. This allows us to observe the narrow intrinsic luminescence of MQDs and isolated Cr ions in the NU grown samples, without complications due to ensemble broadening. We seek to measure the small exchange splitting of excitons confined in the MQD which is afforded by the narrow linewidth. From this measurement, the p-d exchange energy can then be determined. In addition, luminescence from dilute MQDs is expected to show emission due to recombination of the hole bound to the Mn²⁺ ion. Fine structure of the hole-Mn complex has never been observed due to ensemble broadening. The NSOM provides us with the opportunity to observe the exchange-induced fine structure and thus probe the exchange mechanism directly.

Cr in GaAs is a new material which may have superior properties. Recent theories indicate that GaCrAs could have a high Curie temperature, even above room temperature, and that zincblende CrAs is expected to be a half-metallic ferromagnet. These materials are beginning to be studied in Japan and at Northeastern. Initial magnetization measurements on Ga_1-xCr_xAs, x~0.05, samples grown in our MBE machine show a Curie temperature of about T_C~140 K, higher than the highest TC obtained for GaMnAs. Not much is known about the Cr center in GaAs except that it has a deep acceptor level at low concentrations. Samples of low concentration GaAs:Cr will be grown in our MBE machine. Using the nano-scale characterization techniques available we will attempt to observe luminescence associated with isolated Cr ions in order to determine their electronic properties and perhaps learn about the exchange interaction.

Ziemer uses UHV processing techniques and surface science analysis techniques to provide a detailed understanding of the relationships between surface chemistry, film growth or etching, and film or surface characteristics. The surface science tools and heterointerface expertise will be used to study the material growth processes through material chemistry and structure, the engineering of the spin injection interfaces and the interfaces necessary for working device design.