WO1996007208A1

WO1996007208A1 - Current conducting structure with at least one potential barrier and method of manufcturing such

Info

Publication number: WO1996007208A1
Application number: PCT/NL1995/000036
Authority: WO
Inventors: Douwe Johannes Monsma
Original assignee: Douwe Johannes Monsma
Priority date: 1994-08-31
Filing date: 1995-01-25
Publication date: 1996-03-07

Abstract

The invention comprises a current conducting structure with at least one potential barrier, for example, a transistor or diode, comprising a region in which electron transport can take place, characterised by the fact that said region consists at least of one material from the group to which belong: metals, such as Au, Ag, Pt, Al; semimetal such as Bi and superconducting materials such as Nb or copper oxides, and also magnetoresistive materials and/or structures. The invention also comprise a method of manufacturing such a structure. This manufacturing method according to the invention is characterised by the fact that the bodies which are to be contacted consist of at least one flat, optically smooth surface and are brought together in a dust arm atmosphere in order to get a permanent electrical and mechanical connection by means of spontaneous bonding.

Description

Current conducting structure with at least one potential barrier afnd method of manufacturing such.

De invention relates to materials and structures with magnetoresistant properties, such as a NiFe film in which the well known anisotropic magnetoresistance occurs, or for example a magnetic sandwich, multilayer or granular system in which the so-called giant magnetoresistance effect or spin-valve effect occurs, and also to (ferro)magnetic tunnel junctions in which the so-called magnetic tunnel effect or spin polarised tunnelling takes place, and also to a method of manufacturing hot-electron transistors, including metal base transistors and structures based on these transistors.

Giant magnetoresistance or the spin-valve effect is a new kind of magnetoresistance, discovered in 1988 in magnetic multilayers consisting of 30 to 60 stacked layers of Fe and Cr with a thickness of about 1 nm, and shows usually a much larger effect than the classical anisotropic magnetoresistance, occurring in e.g. Ni-Fe films (see for example R. Coehoorn in Europhys. News 24(1993), pp. 43-44).

The new development of giant magnetoresistant structures is of great importance especially for the magnetic recording industry because it may lead to very sensitive reading heads. Many new combinations of layers are being examined, both experimentally and theoretically. Generally, the resistance of these films is measured using a four point measuring technique, with the current oriented parallel to the magnetic layers. The disadvantages of measuring with the current in this direction are amongst others that a decrease of the magnetoresistance takes place because not all electrons cross several layers (shunting) and because a non-magnetoresistive diffuse surface scattering occurs, that an antiferromagnetic coupling is needed which limits the sensitivity (a strong magnetic saturation field is required), and that it is difficult to describe the electrical behaviour. These problems can be eliminated if the current is directed perpendicularly to the layers. The problem though is that the resistance will be very small, which is caused by the extremely thin layers in the multilayer, which are of the order of lnm. Two solutions have been worked out experimentally: 1. Measuring the very low resistance of the structure by connecting superconducting leads to the upper and lower side of the multilayer, by which the contact resistance is virtually eliminated [W.P. Pratt et al. in J. Magn. Magn. Mat. 126(1993), pp.406-409]. A disadvantage is that one can measure only at cryogenic temperatures. 2. Etching the multilayer in pillars with a very small diameter (about 1 μm) by which the perpendicular resistance of the of the multilayer is increased [M.A.M. Gijs et al. in Phys Rev. Lett. 70(1993), pp. 3343-3346]. This way allows measurement at room temperature. Disadvantages of both method are the large number of layers to reach a sufficiently large resistance and the general complexity of the realisation and measurement techniques. Confirmed though are the larger effect and the easier interpretation and modelling of the measurement results when measuring with the perpendicular current.

One aspect of the invention provides a method to measure the perpendicular electrical resistance of magnetoresistive materials and structures, amongst others magnetic multilayers and even sandwiches displaying giant magnetoresistant properties, by providing the base of a metal base transistor structure (in some cases called ballistic- or tunnelling hot-electron transfer amplifier, tunnel transistor, tunnel-emission triode or hot-electron triode, but here labelled metal base transistor) with at least a magnetoresistive material or magnetoresistive structure. This solution may lead to new physical insights by the occurrence of some fundamentally new physical processes and to e.g. more sensitive magnetic field sensors or magnetic solid state memory systems.

Metal base transistors, belonging to the class of hot electron transistors, were being developed since 1960 to realise transistors for ultra high frequency applications (See for example S.M. Sze, high speed semiconductor devices, New York: Wiley Interscience, 1990, pp.399-461) but lost interest due to basic manufacturing problems and bad prospects on sufficient current gain, characteristics which are not applicable to the present invention.

A second aspect of the invention provides a way to replace a tunnel emitter and/or tunnel collector of a metal base transistor structure by a magnetic tunnel structure, to be able to realise for example sensitive magnetic field sensors. The magnetic tunnel effect takes place in for example ferromagnet / insulator / ferromagnet structures or ferromagnet / semiconductor / ferromagnet structures, in which the tunnel resistance depends on the angle between the magnetisations of the two ferromagnetic layers and on an applied magnetic field, and can thus serve as a magnetic field sensor (See for example T. Yaoi et al. in J. Magn. Magn. Mat. 126(1993) pp. 430-432).

A third aspect of the invention aims at a combination of the first and the second aspect of the invention, resulting in metal base transistor structures in which both the magnetic tunnelling as the magnetoresistance is implemented.

A fourth aspect of the invention relates to photo-excitation of electrons in a magnetoresistive material of magnetoresistive structure wherein said material or structure is placed on a semiconducting body. Electrons within the said material or structure can pass the potential barrier formed by the interface of said material or structure and semiconducting body after stimulation by photo-excitation, leading to a magnetic field dependent leak current of said potential barrier. This aspect of the invention aims at a way to measure perpendicular transport of electrons in for example a magnetic (multilayer) structure, for example for magnetic sensor or magnetic solid state memory applications.

A fifth aspect of the invention relates to a method of manufacturing hot electron transistors, including metal base transistors and structures based on these. De manufacturing of metal base transistor structures is one of the biggest dilemmas for the realisation of properly functioning devices. Deposition of semiconducting materials on a metal, common in fabricating semiconductor / metal / semiconductor metal base transistors, leads to a low quality of the semiconducting material, because deposition on a metal can only take place at low temperatures to avoid diffusion. Low quality semiconductor material leads to poor electron transport which causes serious difficulties to realise metal base transistor with sufficient current gain. Single crystal point contact semiconductors are unpractical from the point of view of Schottky barrier quality, fabrication and reliability. Epitaxial growth techniques in which suicides are being applied as the material of the base region, on which single crystalline silicon can be grown, leads to a limitation of the choice of material of the base region and semiconductors, and is usually restricted to suicides and silicon. These problems have decreased the interest in the use of metal base transistors considerably. The fifth aspect of the invention is characterised by a method of manufacturing hot electron transistors, including metal base transistors and structures base on these, by contacting bodies, provided of at least one clean, flat and optically smooth surface, in a dust-arm atmosphere, by which a spontaneous bonding sets in, a technique known as (direct) bonding. This method of manufacturing may provide important advantages and may be the solution to the realisation and the functioning of hot electron transistors, including metal base transistors and structures based on these. The (direct) bonding is a technique which is being applied to realise single crystalline layers on an insulator, also called SOI or Silicon on Insulator (See for example S. Bengtsson in J. Electronic Mat. 21(1992) pp. 841-862). Bonding techniques in which the bonding is between semiconductors and metals, or between metals is described by J. Haisma et al. in Applied Optics 33(1994) pp. 1154-1169. Not mentioned is the fact that good Schottky barriers can be made using (direct) bonding, and because of this also metal base transistors.

The aspects of the invention are explained by means of schematical drawings which serve only as examples, and sizes are not reproduced on scale. The figures are presented as cross-sections and similar reference is used in the figures to refer to similar or comparable parts.

Figure 1 gives a cross section of a possible arrangement of the first aspect of the invention, namely a semiconductor / metal or metallic structure / semiconductor metal base transistor structure. In this example layer 1 is a semiconducting body, for example single crystalline n-silicon, and serves in this example as a collector. Placed on it is an ohmic contact 4 which serves as a collector connection. The base 2 of the transistor structure is for example a metal film or metallic structure, for example Au with a thickness of 10 run, and can be deposited amongst others by means of standard deposition techniques such as evaporation, sputtering, molecular beam epitaxy and so on. A contact 5 is placed on base 2 and serves as a base connection. Layer 3 is a semiconducting body, for example single crystalline n-gallium arsenide, which serves as an emitter in this example and on which is placed an ohmic contact 6, serving as an emitter connection. Two Schottky barriers can be distinguished. On the interface of bodies 1 and 2 (collector Schottky barrier)and on the interface of bodies 2 and 3 (emitter Schottky barrier). The electrical behaviour can roughly be explained by means of a common base configuration: Bias Ve provides injection of electrons from emitter 3 to base 2 via an emitter barrier which is fixed in forward. Bias Vc causes a reversely biased collector barrier. The electrons which are being injected from the emitter to the base gain an extra energy (comparable to the barrier height of the emitter Schottky barrier, of the order of 1 eV) and are therefore called hot electrons. These electrons can pass the collector barrier if this energy is larger that the barrier height of the collector barrier, in contrast to the electrons in the base region which are not injected and have an energy around the Fermi level. The injected electrons thus form a current in the collector: transistor operation. The size of this current is strongly dependent on the electron mean free path in the base: in case of a small mean free path a lot of electrons loose their energy in the base and can therefore not pass the collector barrier anymore, resulting in a small collector current. Experimentally (neglecting the collector leak current) the following relationship has been found: Ic=Ie exp (-W/λ), in which Ic is the collected current, Ie the injected current, α a transport factor, W the thickness of the base layer (of the order of 10 nm) and λ the electron mean free path in the base. Magnetoresistive materials are characterised by the variation of the electron mean free path under influence of a magnetic field (mean free path is related to resistivity). Implementation of a magnetoresistant material of magnetoresistive structure in base region 2 results in a magnetic field dependent transfer of the transistor, and thus in a collector current which is a measure of an applied magnetic field. Because the transfer depends exponentially on the (inverse of the) mean free path of the injected electrons in the base (λ) an amplification of the magnetoresistance will be possible, when a multilayer or a sandwich (for example a 1.5 nm Co / 2 nm Cu multilayer with a total thickness of 140 nm or a 6 nm NiFe / 0.96 nm Co / 2 nm Cu / 2.4 nm Co sandwich) characterised by the giant magnetoresistance effect is being implemented in the base (2) is it also possible to utilise the advantages of perpendicular electron transport in such structures.

A second example of a possible arrangement of the first aspect of the invention is illustrated in the second figure. In this case the emitter barrier is not formed by a Schottky barrier as in figure 1, but by a tunnel barrier. An insulator or semiconductor 7 is placed in this case between base 2 and emitter metal film 8.

A third example of a possible arrangement of the first aspect of the invention is illustrated in the third figure. In this case the collector barrier is not formed by a Schottky barrier as in figure 2, but by a tunnel barrier. An insulator or semiconductor 9 is in this case placed between the base region 2 and the collector metal film 10.

The second aspect of the invention can be illustrated with figure 2, in which layers 8 and 2 consist at least of a (ferro)magnetic material, e.g. Co, Ni or Fe. By this a (ferro)magnetic tunnel barrier is formed, which may serve as an emitter barrier. By this the magnetic field dependent tunnel resistance and possible magnetic field dependent mean free path in the base 2 can be utilised to create a magnetic field dependent transfer of the transistor. The arrangement can be extended to a structure in which not only the emitter barrier is characterised by a (ferro)magnetic tunnel junction, but also the collector barrier. This can be elucidated by means of figure 3, in which layers 2, 8 and 10 should then at least consist of a (ferro)magnetic material.

The third aspect of the invention aims at combining the first and the second aspect of the invention, resulting in metal base transistor structures in which both the magnetic tunnelling as the magnetoresistance are implemented. This can be illustrated by means of figures 2 and 3, where in this case the base region 2 consists of at least a magnetoresistive material or magnetoresistive structure. Layers 8 and 10 are then (ferro)magnetic materials or structures. The fourth aspect of the invention can be illustrated by means of the schematic drawings in figure 4. Layer 11 is a semiconducting body with on it deposited a layer 12 consisting of at least a magnetoresistive material or magnetoresistive structure. Connection 13 and 14 are connected to a voltage source which cause a reverse state of the (Schottky)barrier, formed by the interface of layers 11 and 12. Photo-excitation (referred to as "hv") of electrons on the surface of body 12 causes an increased leak current of the reverse biased Schottky barrier. Variation of the resistivity and thus the mean free path of the electrons in layers 12 influenced by a magnetic field causes a change in the leak current of the Schottky barrier, causing the leak current to be a measure for the applied magnetic field. In this way for example a magnetic field sensor can be realised, and in case of the application of a magnetic multilayer or sandwich characterised by the giant magnetoresistance effect, the perpendicular electron transport through such structures can be measured and employed.

The fourth aspect of the invention, relating to the method of manufacturing of hot electron transistor structures, including metal base transistor and structures based on these, and which is base on the technique known as (direct) bonding, can be illustrated as an example by means of a semiconductor / metal or metallic structure / semiconductor structure as in figure 1. Semiconducting bodies 1 and 3 have to be provided of a flat and optically smooth surface and need to be clean prior to the deposition of at least the metal or metallic structure 2 on at least on of said semiconducting bodies. If the metal or metallic structure is placed on only body 1, then body 3 can be contacted to body 2 in a dust-arm atmosphere, after cleaning and possible removal of an oxide layer to get a better electrical connection. A spontaneous bonding will take place, known by the name (direct) bonding. The formed body 3 / body 2 interface turns out to show good Schottky barrier characteristics. By subjecting it to an elevated temperature, the adhering effect is even increased and the electrical properties can be improved. The bonding technique can also take place by depositing a part of body 2 on body 3 and another part of body 2 on body 1, after which bodies 1 and 3, both with a part of body 2, are being brought together and contacted, after which spontaneous bonding sets in between both parts of body 2. Base region 2 can for example consist of a single metal such as Au, Ag, Pt or Al, a magnetoresistive material or magnetoresistive structure such as a NiFe film or a Co/Cu or Fe/Cr multilayers, a superconducting material such as Nb or a copper oxide such as YBaCuO, or a material with a long free electron mean free path such as for example Bi. The contacting of both bodies can also take place in an atmosphere with low pressure, for example a vacuum chamber of a sputtering or evaporation system, during or after deposition of body 2, amongst others to prevent forming of oxides on the contacting parts.

Claims

Claims.

1. Current conducting structure with at least one potential barrier, for example a transistor or diode, comprising a region in which electron transport can take place, characterised by the fact that said region consists at least of one material from the group to which belong: metals, such as Au, Ag, Pt, Al; semimetals such as Bi and superconducting materials such as Nb or copper oxides, and also magnetoresistive materials and/or structures.

2. Structure according to claim 1, wherein the magnetoresistive material or magnetoresistive structure is characterised by the giant magnetoresistance effect or spin-valve effect.

3. Structure according to claim 1, wherein the magnetoresistive material or magnetoresistive structure is characterised by the anisotropic magnetoresistance effect.

4. Structure according to claim 1, wherein said region forms the base region of a hot electron transistor structure or particularly of a metal base transistor structure.

5. Structure according to claim 4, wherein the emitter region and/or collector region of said hot electron transistor structure or metal base transistor structure consists of a structure characterised by (ferro)magnetic tunnelling or spin polarised tunnelling

6 Structure according to claim 5, wherein the (ferro)magnetic tunnelling or spin polarised tunnelling is caused by a ferromagnet / insulator / ferromagnet structure and/or a ferromagnet / semiconductor /ferromagnet structure.

7 Structure according to claim 2 or 3, characterised by the fact that at least said region is placed on a semiconducting body and further characterised by a potential barrier formed at the interface of said region and semiconducting body which displays a magnetic field dependent leak current under the influence of photo-excitation of electrons in said region.

8. Method of manufacturing structures according to one of the claims 1-6, characterised by the fact that the bodies which are to be contacted consist of at least one flat, optically smooth surface and are brought together in a dust arm atmosphere in order to get a permanent electrical and mechanical connection by means of spontaneous bonding.

9. Method of manufacturing according to claim 8, characterised by the fact that at least one of the parts which are to be contacted, consist of at least the region mentioned in claim 1, consisting of at least one flat, optically smooth surface.

10. Method of manufacturing according to one of claims 8-9, characterised by the fact that during or after contacting said bodies, bodies are subjected to a temperature of at least 30°C, to obtain better electrical and/or mechanical properties.

11. Method of manufacturing according to one of the claims 8-10, characterised by the fact that before contacting said bodies, oxide is removed from at least one of said bodies, by means of an etching procedure 12. Method of manufacturing according to one of the claims 8-11, characterised by the fact that contacting said bodies takes place in an atmosphere with a pressure lower than lOOPa.