Spin-valve Transistor Research Articles

The contribution of hot-electron spin polarization tothe spin-dependent magnetotransport in a spin-valve transistor atfinite temperatures

The effect of spin mixing due to thermal spin waves and thetemperature dependence of the hot-electron spin polarization to thecollector current in a spin-valve transistor (SVT) has beentheoretically explored. We calculate the spin dependence of thecollector current as well as the temperature dependence of themagnetocurrent at finite temperatures to investigate the relativeimportance of the spin mixing and the hot-electron spinpolarization. In this study the inelastic scattering events inferromagnetic layers have been taken into account to exploreour interests. The theoretical calculations suggest that thetemperature dependence of the hot-electron spin polarization makes asubstantial contribution to the spin-dependent magnetotransport atfinite temperatures in the SVT.

Modeling of spin-dependent hot-electron transport in the spin-valve transistor

A phenomenological model is presented that describes spin-dependent hot-electron transport in the spin-valve transistor. The three-dimensional model is based on the Boltzmann equation and takes into account spin-dependent inelastic and elastic scattering within each metal layer of the base and elastic scattering at the interfaces, as well as the injection and collection characteristics of the emitter and collector Schottky barriers. We numerically calculate the attenuation of the hot electrons, as well as their angular distribution of momentum, as a function of the position in the metallic base. We investigate how elastic scattering affects the attenuation lengths via the angular distribution of momentum and show that elastic scattering at an interface leads to an increase of the effective bulk attenuation length of the layers after that interface. We also find that the magnetocurrent is changed by interface scattering even if it is taken to be independent of spin. We find that when elastic scattering is significant, the true attenuation lengths are markedly larger than those predicted in a one-directional model from the scattering parameters for elastic and inelastic scattering. The calculations demonstrate that elastic scattering may be the primary reason for the small collector currents observed in the spin-valve transistor experimentally.

Spin-valve transistor with an Fe/Au/Fe(001) base

A spin-valve transistor with an epitaxial Fe/Au/Fe(001) base was formed on n-GaAs, the characteristics of which were examined under a magnetic field for emitter voltages up to 3 V. The transfer ratio of the transistor, i.e., the ratio of collector-to-emitter current exceeded 10−3 at 3 V, preserving the magnet current ratio, i.e., the ratio of collector current in the parallel-to-antiparallel magnetic configuration well above 100%. It was suggested that the transfer ratio would be further enhanced by improving the flatness of the tunnel junction for injecting electrons from the emitter into the base, as well as by increasing the electron transmittance at the base/collector interface.

Temperature dependence of magnetocurrent in spin-valve transistor: a phenomenological study

The temperature dependence of magnetocurrent in the spin-valve transistor has been theoretically explored based on phenomenological model. We find that temperature dependence of the collector current strongly depends on the relative orientation of magnetic moment of ferromagnetic metals due to spin mixing effect. For example, the parallel collector current is decreasing with increasing temperatures, while the anti-parallel collector current is increasing. We then obtain decreasing magnetocurrent with increasing temperatures. The result accords with the experimental data in qualitative manner. Along with this, we find that temperature dependence of hot electron spin polarization can also contribute to the magnetocurrent at finite temperatures. This phenomenological model calculations suggest that it is essential to understand the relative importance of spin mixing effect and hot electron spin polarization in the spin-valve transistor at finite temperatures.

The vacuum wafer bonding technique as an alternative method for the fabrication of metal/semiconductor heterostructures

The spin-valve transistor is a magneto-electronic semiconductor/ferromagnet/semiconductor heterostructure in which severe demands are put on the quality of materials and their interfaces. In this paper we discuss the vacuum bonding method as an alternative to heteroepitaxy for the fabrication of such complex devices. With this technique, bonding occurs by joining two substrates with a freshly deposited metal layer on top, in a UHV chamber. No high temperature nor high pressure steps are involved and the only requirement for successful vacuum bonding is the surface roughness. Using MBE techniques fairly complicated structures can be grown smooth enough to allow bonding. We show that UHV wafer bonding in combination with MBE grown semiconductor structures is a versatile technique to fabricate high quality hybrid structures with well controlled interfaces.

Hot-electron transport through Ni80Fe20 in a spin-valve transistor

Hot-electron transport in Ni80Fe20 thin films was studied using a spin-valve transistor. By varying the NiFe thickness from 10 to 100 Å we obtain an attenuation length of 43 Å for majority-spin hot electrons at 0.9 eV above the Fermi level. Based on such relatively long bulk attentuation lengths, one would expect a current transfer ratio that is much larger than the measured value. We propose that the discrepancy can be accounted for by considering interfacial scattering. Increasing the growth quality should thus provide a means to improve the current transfer ratio.

The spin-valve transistor: Fabrication, characterization, and physics (invited)

An overview is given of the fabrication, basic properties, and physics of the spin-valve transistor. We describe the layout of this three-terminal ferromagnet/semiconductor hybrid device, as well as the operating principle. Fabrication technologies are discussed, including vacuum metal bonding. We characterize properties of the device relevant for possible applications in magneto-electronics, such as relative magnetic response, output current, and noise behavior. Furthermore, we illustrate the unique possibilities of the spin-valve transistor for fundamental studies of the physics of hot-electron spin transport in magnetic thin film structures.

Fabrication technology for miniaturization of the spin-valve transistor

A new fabrication technology that allows miniaturization of the spin-valve transistor is presented. The spin-valve transistor consists of a spin-valve base (Pt 2 nm/NiFe 3 nm/Au 3.5 nm/Co 3 nm/Au 4 nm) sandwiched between a Si emitter and collector. With the use of a silicon-on-insulator wafer and vacuum metal bonding, spin-valve transistors down to a few tens of micron size are realized through conventional photolithography and etching processes. These spin-valve transistors show 275% magnetocurrent at 87 K and 170% at room temperature in small magnetic fields.

Noise properties of the spin-valve transistor

Noise measurements have been performed on a spin-valve transistor. This transistor consists of a Pt/NiFe/Au/Co/Au multilayer sandwiched between two semiconductors. For comparison, we also studied metal base transistors with a Pt/Au or Pt/NiFe/Au base. All samples show full shot noise in the collector current. The inclusion of a spin-valve in the base layer decreases the absolute value of the collector current and with it the noise level but it does not change the nature of the noise in this device. Similarly, the collector current, and therefore, the noise changes as a function of magnetic field for the spin-valve transistor, but no additional noise of magnetic origin is observed.

Improvement of the transfer coefficient of GaAs/Si spin-valve transistors

Nowadays GaAs/Si hot electron spin-valve transistors can be readily made using the vacuum bonding technique. They show sharp variations in collector current in small magnetic fields, good for sensor applications. However, the transfer coefficient of the device, defined as collected current over injected current, is only around 10 −4%. We address the structural properties of GaAs/Si spin-valve transistors that influence the transfer coefficient. An improvement of the transfer coefficient of more than one order of magnitude is obtained by implementing a GaAs/AlAs emitter launcher maintaining 93% of relative collector current change.

Temperature dependence of magnetocurrent of hot electrons in a spin-valve transistor

Spin-dependent transport of hot electrons across a spin valve has been studied as function of temperature using a spin-valve transistor with a soft Ni 80Fe 20/Au/Co spin-valve base. Spin-dependent scattering makes the collector current highly sensitive to small magnetic fields that change the magnetic state of the base. The magnetocurrent approaches 400% at 100 K but decays to about 240% at room temperature. The reduction is attributed to mixing of the two spin channels due to spin-flip scattering of hot electrons by thermal spin waves.

300% magnetocurrent in a room temperature operating spin-valve transistor

Here we present the realization of a room temperature operating spin-valve transistor with huge magnetocurrent (MC=300%) at low fields. This spin-valve transistor employs hot-electron transport across a Ni 81Fe 19/Au/Co spin valve. Hot electrons are injected into the spin valve across a Si–Pt Schottky barrier. After traversing the spin valve, these hot electrons are collected using a second Schottky barrier (Si–Au), which provides energy and momentum selection. The collector current is found to be extremely sensitive to the spin-dependent scattering of hot electrons in the spin valve, and therefore on the applied magnetic field. We also illustrate the role of the collector diode characteristics in determining the magnetocurrent under collector bias.

The spin-valve transistor1

The spin-valve transistor is a magnetoelectronic device that can beused as a magnetic field sensor. It has a ferromagnet-semiconductor hybridstructure. Using a vacuum metal bonding technique, the spin-valve transistorstructure Si/Pt/NiFe/Au/Co/Au/Si is obtained. It employs hot electrontransport across the spin valve (NiFe/Au/Co). The hot electrons are injectedinto the spin valve across the Si/Pt Schottky diode. After traversing acrossthe spin valve these hot electrons are collected across the Au-Si Schottkydiode with energy and momentum selection. The output current is found to beextremely sensitive to the spin-dependent scattering of hot electrons in thespin valve. This gives a magnetocurrent above 200% in a few oersted ofmagnetic field at room temperature. The different physical effects whichgovern the output current of the device are examined by studying differenttypes of spin-valve transistors that have Si/Au, Si/Pt and Si/Co collectorSchottky diodes and Si(100) and Si(111) orientations. It has been observedthat along with the Schottky diodes the vacuum metal bonding also plays animportant role in determining the output current. In addition, it is realizedthat collector diodes with extremely low leakage currents, are essential inorder to observe huge magnetotransport properties at room temperature.

Thermal spin-wave scattering in hot-electron magnetotransport across a spin valve.

The role of thermal scattering in spin-dependent transport of hot electrons at 0.9 eV is studied using a spin-valve transistor with a soft Ni(80)Fe(20)/Au/Co base. Spin-dependent scattering makes the collected electron current depend sensitively on the magnetic state of the base. The magnetocurrent reaches 560% at 100 K, decays with increasing temperature, and a huge effect of 350% still remains at room temperature. The results demonstrate that thermal spin waves produce quasielastic spin-flip scattering of hot electrons, resulting in mixing of the two spin channels.

Low-field magnetocurrent above 200% in a spin-valve transistor at room temperature

A spin-valve transistor (SVT) that employs hot electrons is shown to exhibit a huge magnetotransport effect at room temperature in small magnetic fields. The SVT is a ferromagnet-semiconductor hybrid structure in which hot electrons are injected into a NiFe/Au/Co spin valve, and collected on the other side with energy and momentum selection. This makes the collector current extremely sensitive to spin-dependent scattering. The hot-electron current output of the device changes by more than a factor of three in magnetic fields of only a few Oe, corresponding to a magnetocurrent above 200% at room temperature.

Evaluation of vacuum bonded GaAs/Si spin-valve transistors

In this article a new type of spin-valve transistor, a hybrid GaAs/Si device, is presented. In this device the Si emitter is replaced by a GaAs emitter launcher structure. The integration of the GaAs with the Si was done by means of a room temperature vacuum bonding technique. By using a soft NiFe/Au/Co spin-valve structure as metal base, a 63% change in collector current is obtained at room temperature for a saturation field of 30 Oe. The corresponding in-plane magnetoresistance is only 1%.

The Spin-Valve Transistor - a New Magnetoelectronic Device

A spin-valve transistor showing high sensitivity at low fields was devel- oped. A large magnetocurrent, above 500% is realized by a magnetic field change of 2 to 4 Oe at 80 K. Hot electrons are injected into the spin-valve layer through a Si—Pt Schottky diode. These hot electrons, while traversing through the spin-valve, are spin-dependently scattered. Those electrons with right energy and momentum are collected by a collector (an Au—Si Schottky diode) constituting a collector current. The relative orientation of the mag- netic layer in the spin-valve is changed by the application of a magnetic field and causes a change in collector current giving a large magnetocurrent.

The spin-valve transistor: technologies and progress

The paper describes the necessary technologies needed for realising a RT operating spin-valve transistor (SVT) which is in fact a magnetic controlled metal base transistor. The preparation of a 350×350 μm 2 SVT consisting of an Si emitter and collector and Co/Cu/Co GMR multilayer are described. The metal bonding technology in vacuum is described, which is essential for preparing small SVTs with photolithography and etching technologies. The quality of the bonding interfaces as well as the interface between GMR layer and semiconductor are important for the electrical properties. In more general terms the SVT research also establishes the feasibility of various hybrid structures combining semiconductor technology and spin electronics.

Room temperature-operating spin-valve transistors formed by vacuum bonding

Functional integration between semiconductors and ferromagnets was demonstrated with the spin-valve transistor. A ferromagnetic multilayer was sandwiched between two device-quality silicon substrates by means of vacuum bonding. The emitter Schottky barrier injected hot electrons into the spin-valve base. The collector Schottky barrier accepts only ballistic electrons, which makes the collector current very sensitive to magnetic fields. Room temperature operation was accomplished by preparing Si-Pt-Co-Cu-Co-Si devices. The vacuum bonding technique allows the realization of many ideas for vertical transport devices and forms a permanent link that is useful in demanding adhesion applications.

Theoretical approach to the spin-dependent hot-electron transport in a spin valve

We analyzed spin-dependent hot-electron transport in a spin-valve transistor, by calculating current-voltage characteristics for a simplified three-terminal structure. The calculation was performed by using the Liouville equation for the Wigner distribution function. We found that the effect of impurity scattering on the current-voltage characteristics is enhanced by the multiple-reflection between the barriers at the emitter/base and at the base/collector interfaces. It was suggested that the large magnetoresistance ratio observed in the spin-valve transistor could be understood by taking into account not only the spin asymmetry in the density of states but also the enhanced impurity scattering and a forward-focusing effect of the tunneling barriers.