Kristof Dessein

Technology and materials issues in semiconductor-based magnetoelectronics

There has been an increased interest in the introduction of magnetic thin films into semiconductors. This interest is motivated by the benefit found in using the magnetic thin-film properties (giant or tunnelling magnetoresistance and hysteresis) in magnetic memory (MRAM) products. Furthermore, the use of the electron spin in electronic, spintronic devices requires intimate ferromagnetic/semiconductor combinations. We review the technology and materials aspects of both the MRAM and spintronics fields that highlight the challenges that must be overcome in order to make magnetic (multilayer) films a standard ingredient in future electronics.

Technology assessment for the implementation of magnetoresistive elements with semiconductor components in magnetic random access memory (MRAM) architectures

We describe the DRAM-like approach towards a non-volatile magnetoresistive memory integrating magnetic and semiconductor devices into one cell. The speed at which the magnetic memory signal can be read depends on many factors. An important factor is the magnetic element itself, the size, magnetic characteristics and absolute resistance. Secondly, the design of the read-out electronics is a key issue. A third determining factor is the technology in which the electronics are fabricated. Some features are indicated that are essential in optimizing MRAM in future.

Hybrid epitaxial structures for spintronics

The use of electron spin in future spintronic applications requires hybrid ferromagnetic/semiconductor structures with well controlled materials properties. Besides the control of the magnetic properties there are strict requirements for the material aspects of these devices: single phase, single crystal, perfect interfaces, defect control. Molecular beam epitaxy (MBE) has been used very successfully over the past 10 years to produce the most interesting spintronic heterostructures. In this paper we review our effort in using MBE to fabricate spintronic materials and heterostructures. We also discuss some aspects of future spintronic devices with the focus on the material aspects of transferring the electron spin information from the magnetic side of the heterostructure into the semiconductor side.

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 vacuum metal 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 involvedandthe only requirement for successful vacuum bonding is the surface roughness.Using MBE techniques fairly complicatedstructures 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 hybridstructures with well controlledinterfaces.

Technology assessment for MRAM cells with magnet/semiconductor bits

Although MRAM circuits can be fabricated without the need for an integrated semiconductor switch in each bit, most applications will require a DRAM-like floorplan with combined magnetlsemiconductor bits. In a first approach to assess the feasibility of such integration, we have integrated spin-valve structures with semiconductor diodes [I]. Further, in an [MEC I INESC collaboration (ESPRIT #28.229 I TI-MRAM) the integration feasibility of tunnel junctions is assessed. The main issues we are tackling are the fabrication of arrays of magnetic tunneljunction I semiconductor elements and the MTI performance as a function of the parameters of the integration pmcess. In this paper we present results of our assessment of MRAM circuits with magnetlsemiconductor bits.

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.