Satoshi Sugahara

A spin metal–oxide–semiconductor field-effect transistor using half-metallic-ferromagnet contacts for the source and drain

We propose and theoretically analyze a metal–oxide–semiconductor field-effect-transistor (MOSFET) type of spin transistor (spin MOSFET) consisting of a MOS structure and half-metallic-ferromagnet (HMF) contacts for the source and drain. When the magnetization configuration of the HMF source and drain is parallel (antiparallel), highly spin-polarized carriers injected from the HMF source to the channel are transported into (blocked by) the HMF drain, resulting in the magnetization-configuration-dependent output characteristics. Our two-dimensional numerical analysis indicates that the spin MOSFET exhibits high (low) current drive capability in the parallel (antiparallel) magnetization, and that extremely large magnetocurrent ratios can be obtained. Furthermore, the spin MOSFET satisfies other important requirements for “spintronic integrated circuits,” such as high amplification capability, low power-delay product, and low off-current.

Ferromagnetism and high Curie temperature in semiconductor heterostructures with Mn δ-doped GaAs and p-type selective doping

We have found high ferromagnetic transition temperature in Mn delta-doped GaAs-based heterostructures grown on GaAs(001) substrates by molecular beam epitaxy. A 0.3 ML Mn d-doped GaAs samples showed high resistivity at low temperature and did not show a ferromagnetic behavior. However, in a selectively doped heterostructure (Mn delta-doped GaAs / Be-doped AlGaAs), where holes were supplied from the Be-doped AlGaAs layer, clear ferromagnetic order was observed. The ferromagnetic transition temperature of the selectively doped heterostructure was as high as 172K with suitable low-temperature (LT) annealing treatment.

High temperature ferromagnetism in GaAs-based heterostructures with Mn delta doping.

We show that suitably designed magnetic semiconductor heterostructures consisting of Mn delta (delta)-doped GaAs and p-type AlGaAs layers, in which the locally high concentration of magnetic moments of Mn atoms are controllably overlapped with the two-dimensional hole gas wave function, realized remarkably high ferromagnetic transition temperatures (T(C)). A significant reduction of compensative Mn interstitials by varying the growth sequence of the structures followed by low-temperature annealing led to high T(C) up to 250 K. The heterostructure with high T(C) exhibited peculiar anomalous Hall effect behavior, whose sign depends on temperature.

Tunneling magnetoresistance in fully epitaxial MnAs/AlAs/MnAs ferromagnetic tunnel junctions grown on vicinal GaAs(111)B substrates

We have fabricated fully epitaxial single-crystal MnAs/AlAs/MnAs magnetic tunnel junctions (MTJs) grown by molecular-beam epitaxy on vicinal GaAs(111)B substrates. After the bottom MnAs layer was grown at 250 °C, the successive AlAs tunnel barrier and the top MnAs layer were grown at a lower temperature of 200 °C in order to suppress the surface segregation of Mn atoms. High-resolution transmission electron microscopy revealed that a monocrystalline MnAs/AlAs/MnAs trilayer heterostructure with atomically flat and chemically abrupt interfaces was realized. Tunneling magnetoresistance (TMR) was clearly observed in fully epitaxal MTJs made up of this trilayer heterostructure. The TMR ratio was 1.4% at 10 K and it decreased with increasing the bias voltage and with increasing temperature, but the TMR effect still remained at room temperature.

Spin MOSFETs as a basis for spintronics

This article reviews a recently proposed new class of spin transistors referred to as spin metal-oxide-semiconductor field-effect transistors (spin MOSFETs), and their integrated circuit applications. The fundamental device structures, operating principle, and theoretically predicted device performance are presented. Spin MOSFETs potentially exhibit significant magnetotransport effects, such as large magneto-current, and also satisfy important requirements for integrated circuit applications such as high transconductance, low power-delay product, and low off-current. Since spin MOSFETs can perform signal processing and logic operations and can store digital data using both charge transport and spin degrees of freedom, they are expected to be building blocks for memory cells and logic gates in spin-electronic integrated circuits. Novel spin-electronic integrated circuit architectures for nonvolatile memory and reconfigurable logic employing spin MOSFETs are also presented.

Precipitation of amorphous ferromagnetic semiconductor phase in epitaxially grown Mn-doped Ge thin films

We investigated the origin of ferromagnetism in epitaxially grown Mn-doped Ge thin films. Using low-temperature molecular beam epitaxy, Mn-doped Ge films were successfully grown without precipitation of ferromagnetic Ge–Mn intermetallic compounds, such as Mn5Ge3. Magnetic circular dichroism measurements revealed that the epitaxially grown Mn-doped Ge films exhibited clear ferromagnetic behavior, but the Zeeman splitting observed at the critical points was not induced by the s,p–d exchange interactions. High-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy analyses show phase separation of amorphous Ge1-xMnx clusters with high Mn content from a Mn-free monocrystalline Ge matrix. Since amorphous Ge1-xMnx was characterized as a homogeneous ferromagnetic semiconductor, the precipitation of the amorphous Ge1-xMnx clusters is the origin of the ferromagnetic semiconductor behavior of the epitaxially grown Mn-doped Ge films.

MBE growth, structural, and transport properties of Mn δ-doped GaAs Layers

We have grown Mn δ-doped GaAs layers by molecular beam epitaxy with various Mn sheet concentrations (δMn defined in the monolayer (ML) unit. Structural analyses using cross-sectional transmission electron microscopy and secondary ion mass spectroscopy revealed that when δMnis below I ML, Mn dopants are abruptly confined within 2-3 ML without lattice dislocations and clusters, and also without diffusion and surface segregation. Characterizations on the thermal stability of the doping profiles showed that the profiles retained abruptness even at elevated growth temperature T, = 400°C without diffusion and significant segregation. Strong surface segregation of Mn atoms, lattice dislocations, and MnAs clusters appeared when θMn exceeds 1ML. The transport properties of Mn δ-doped GaAs layers without and with selectively p-type doping are also presented.

A spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) with a ferromagnetic semiconductor for the channel

We propose and theoretically analyze a metal-oxide-semiconductor field-effect-transistor type of spin transistor (spin MOSFET) employing a ferromagnetic semiconductor (FS) for the channel. A ferromagnetic Schottky junction between the FS channel and a ferromagnetic metal (FM) is used for the source and drain. The output characteristics of the spin MOSFET depend on the relative magnetization configuration of the FS channel and FM source/drain. A large magnetocurrent ratio can be obtained and it is insensitive to the drain-source bias conditions, owing to the spin-filter effect of the FS/FM Schottky junction. Furthermore, excellent transistor performance, such as high transconductance and small subthreshold swing, is predicted. A new nonvolatile memory architecture using a single spin MOSFET cell is also presented, in which the programming current can be drastically reduced using the electrical manipulation of magnetization reversal of the FS channel.

Novel Reconfigurable Logic Gates Using Spin Metal?Oxide?Semiconductor Field-Effect Transistors

We propose and numerically simulate novel reconfigurable logic gates employing spin metal-oxide-semiconductor field-effect transistors (spin MOSFETs). The output characteristics of the spin MOSFETs depend on the relative magnetization configuration of the ferromagnetic contacts for the source and drain, that is, high current-drive capability in parallel magnetization and low current-drive capability in antiparallel magnetization [S. Sugahara and M. Tanaka: Appl. Phys. Lett. 84 (2004) 2307]. A reconfigurable NAND/NOR logic gate can be realized by using a spin MOSFET as a driver or an active load of a complimentary MOS (CMOS) inverter with a neuron MOS input stage. Its logic function can be switched by changing the relative magnetization configuration of the ferromagnetic source and drain of the spin MOSFET. A reconfigurable logic gate for all symmetric Boolean functions can be configured using only five CMOS inverters including four spin MOSFETs. The operation of these reconfigurable logic gates was confirmed by numerical simulations using a simple device model for the spin MOSFETs.

Electrical and Optical Control of Ferromagnetism in III-V Semiconductor Heterostructures at High Temperature (?100 K)

We demonstrate the electrical and optical control of ferromagnetism in semiconductor heterostructures at high temperatures of 100–117 K. The heterostructures consist of Mn delta (δ)-doped GaAs and p-type AlGaAs. We are able to isothermally change the paramagnetic state to the ferromagnetic state and vice versa, by applying a gate electric-field or by light irradiation. The large modulation of TC (ΔTC~15 K) at high temperatures (>~100 K) demonstrated here may pave the way for functional device applications compatible with the present semiconductor technology.