Patent Application
20080284424
1. Field of the Invention
The present invention is related to methods and devices for electrical spin detection.
2. Description of the Related Technology
Traditional semiconductor transistors operate through the movement of electrical charge carriers (electrons or holes) in the device. For a long time it has been ignored that these carriers also possess a property called “spin”. Recently a number of spin transistors and other spintronics devices have been proposed [Zutic, Fabian, and Das Sarma, Rev. Modern Physics 76, 323-410 (2004)] that make use of the spin properties or of spin-polarized transport inside the semiconductor.
In general the operation of such a spintronics device may comprise a combination of the following operations:
In order to be able to build circuits consisting of many of these spintronics devices, these operations should be performed in a way that allows large-scale integration. As an example, injection and detection of spin-polarized carriers using an electrical contact leads to much easier integration than optical injection and detection using circularly polarized light. In addition electrical injection and detection are not limited to a subset of semiconductors as is the case for optical injection and detection.
Several of these steps have already been successfully demonstrated, and electrical spin injection is now well established. Electrical spin detection appears to be the reverse operation from injection, and one would have expected that experimental demonstrations would have followed soon after the demonstration of electrical spin injection. However, no large effects have been demonstrated so far: the signals were either unclear, limited to spikes in a very small bias range, or much smaller than expected.
Some methods of spin detection use a single spin-selective contact that allows a current of charge carriers to flow into the contact. Various methods have been used to create a flux of charge carriers with a certain spin-polarization. Some use a beam of circularly polarized light to generate spin-polarized electrons in a GaAs/AlGaAs transport structure, while many proposed spin transistor concepts use an electrical contact to inject spin-polarized carriers into semiconductors such as Si or GaAs.
A common factor between all these experiments is that the amplitude of the current I of charge carriers flowing into the contact is proportional to the number of charge carriers n↑+n↓ (the notations “↑” and “↓” refer to spin-up and spin down states respectively) that are presented to the contact, and with different proportionality factors c↑ and c↓ for both types of spin: I=c↑*n↑+c↓*n↓. The aim of the electrical spin detection contact is then to obtain different currents when the spin-polarization of the incoming flux is changed. One common way of performing the measurement consists of varying the spin-polarization of the incoming flux without changing the total number of carriers, and looking for a change in current into the contact. Another common way is to leave the spin polarization of the incoming flux constant, but to change the proportionality factors of the detector contact. This can e.g. be realized by reversing the magnetization direction of a magnetic contact.
This type of device has shown some spin selectivity, but much less than expected. A problem is that the carriers that are initially rejected by a certain contact will be accepted sometime later, after their spin has been flipped. Let's take an example with a perfectly selective contact that accepts only spin up carriers, i.e. c↓=0. If a spin down electron arrives, it will not be accepted and thus accumulate underneath the contact. After a random time the spin of the carrier will be disturbed, and may flip from spin-down to spin-up. This is a random process where the rate of disturbance is given in terms of a spin-relaxation time.
All the prior art measurements are steady-state measurements, i.e. measurements that are slow compared to the spin-flip time of the carriers in the medium under study. As spin-down carriers keep coming in and are rejected by the contact, more and more of them accumulate underneath the contact. Some of these carriers will see their spin flipped to spin-up, and will be accepted by the contact. At a certain moment (this is after several times the spin-flip time) the number of accumulated spin-down carriers will become so large that the number of carriers that flip from spin-down to spin-up becomes equal to the number of incoming spin-down carriers. When this happens the number of accumulated carriers does not change any longer (this is the so-called steady state condition). The number of carriers that experience a spin-flip event and are accepted by the contact is now equal to the flux of incoming spin-down carriers. In other words, the current into the contact is now identical to the current that would flow if all the incoming carriers had had the right spin to start with.
In other words, in the presence of spin-flip scattering in the medium, a single spin-selective contact loses its spin-selectivity if the time-frame of the experiment is much slower than the characteristic spin-flip time in the medium.
Other methods of spin detection utilize a device with electrical injection and detection contacts that operate under very low bias conditions. When rejected carriers start to accumulate and their density increases, their Fermi-level Ef increases. If this increase ΔEf is larger than the applied bias V (|ΔEf|>|qV| where q=1.602×10−19 C is the elementary charge), this will suppress the further injection of “wrong” carriers, and thus lower the current compared to the situation where carriers with the “right” spin are injected. The bias regime where this feed back mechanism functions is called “linear regime”. Under these conditions the device does discriminate between both types of spins. However since this bias range is very small (not substantially larger than 10 mV), these operating conditions are of limited practical use.
Other methods of spin detection use two contacts with a different sensitivity in such a way that they do not accept any current but measure the Fermi-levels of both spin types (or equivalently the Fermi-level of one type of spin together with an average of the two types of spins—the same information can be gained from both configurations). Again this know-how is limited to operating conditions that are of limited practical use, with small signal levels (not substantially larger than 10 mV), and with little drive capability, i.e., where the output can not be used as an input for a second device.
Aspects of the present invention concern a method for spin detection of charged carriers having a spin and forming a flux in a transport medium, of the method including:
In one embodiment, the sum of the measuring time period and the first travel time period is smaller than the average spin flip time and removing the charge carriers includes providing a waiting time in which no flux is present in the transport medium, wherein the waiting time is higher than the average spin flip time. The method can further comprise the step of determining a current integral over time of the current of charge carriers in the first collection contact over the measuring time period.
In another embodiment, removing the charge carriers may include measuring a second current in a second collection contact in electrical contact with the transport medium, the second collection contact having a second spin selectivity different from the first spin selectivity, wherein the charge carriers travel from the starting point to the second collection contact in a second travel time period. The method can further comprise comparing the first current and the second current. The measuring can be started when the spin of the flux of charge carriers is in a steady state. The method according to other aspects of this embodiment may further comprise:
The first time period preferably starts after the sum of the starting time and the first travel time and the second time period preferably starts after the sum of starting time and second travel time. The duration of the first time period and second time period are preferably smaller than the average spin flip time minus the first travel time and smaller than the average spin flip time minus the second travel time, respectively.
Another embodiment provides for a device for performing spin detection, that includes:
wherein the first selectivity is different from the second selectivity and whereby the flux of charge carriers is guided towards the first and the second collection contacts from the predetermined moment in time. The first collection contact or second collection contact is preferably one of a magnetic metal; a ferromagnetic metal; a ferrimagnetic metal; a tunnel injector selected from the group consisting of a magnetic metal with a tunnel barrier, a non-magnetic metal with a magnetic tunnel barrier, a magnetic metal with a magnetic tunnel barrier, a ferromagnetic metal with a tunnel barrier, a non-ferromagnetic metal with a magnetic tunnel barrier, a ferromagnetic metal with a magnetic tunnel barrier, a ferrimagnetic metal with a tunnel barrier, a non-ferrimagnetic metal with a magnetic tunnel barrier, a ferrimagnetic metal with a magnetic tunnel barrier; a half-metallic ferromagnet, a half-metallic ferrimagnet, a half-metallic antiferromagnet, a magnetic semiconductor, and a ferromagnetic semiconductor. In one aspect of this embodiment, the charge carrier source comprises an electrical injection contact. Advantageously, the injection contact has a predetermined magnetic polarization and the injection contact functions as the means for statistically controlling the spin of the charge carriers. The means for statistically controlling the spin of the charge carriers is may be one of a electromagnetic field localized at a specific position along the path of the charge carriers in the transport medium, an optical injection means, the result of a spintronic action, manipulation, operation, filter or calculation, or the result of a quantum computing or quantum cryptography operation.
In one aspect of this embodiment, the charge carrier source is preferably arranged to provide a potential difference between the injection contact and the first and the second collection contacts. In certain embodiments, the first collection contact and the second collection contact have a different spin polarization. The first collection contact and the second collection contact preferably have different magnetization directions, forming an angle different from zero or having a different amplitude or magnitude.
The injection contact and the first and the second collection contact may be positioned on the same side or on different sides of the transport medium.
In another embodiment the transport medium comprises a semiconductor material or a metal.
Another embodiment provides for a device for performing spin detection, including:
The device may be characterized in that all elements or components present in the detector or detector chain are ultrafast elements.
Another embodiment provides for a method for information transfer, including:
Embodiments of the present invention provide for methods and devices for electrical spin detection, which alleviates or avoids the problems of the prior art.
Various embodiments provide for methods and devices that lead to an improved electrical spin detection. Embodiments of the invention may be used in any device that combines electrical spin detection with other operations such as those listed below or others:
The time after which the spin of an individual particle is disturbed varies stochastically. When considering an ensemble of particles the average rate at which spins are disturbed is characterized by the spin relaxation time. More exactly, the spin relaxation time is the time after which the spin of about 63.21% of the particles has been disturbed (where 63.21%=(1−1/e) with e the base of the natural logarithm). This spin relaxation time is also the parameter which characterizes the speed with which a non-equilibrium ensemble of particles evolves towards the equilibrium state.
In the description the following terms are used and are known to those of skill in the art, but an additional definition is given below:
It may be noted that M/P=constant. “Volume” is the physical volume of the element (for instance a collection contact). Ef is the Fermi Energy.
The current polarization is similarly PI=(I↑−I↓)/(I↑+I↓) where I↑ and I↓ are the currents carried by carriers with spin up and spin down, respectively. Depending on how the transport mechanism exactly works, the currents I↑ and I↓ can be proportional with n↑ and n↓ or with the densities weighed with the Fermi velocity VF to a certain power i: n↑ vF̂i and n↓ vF̂i, where the exponent i can be 1 or 2 or other values.
Another embodiment provides for a device for performing spin detection, the device including:
Materials used may include:
In certain embodiments, the means for providing the flux of the charge carriers can be an electrical injection contact.
In preferred embodiments, the injection contact has a predetermined magnetic polarization and the injection contact consequently functions as the means for statistically controlling the spin of the charge carriers.
The means for statistically controlling the spin of the charge carriers may be a electromagnetic field, localized at a certain position along the path of the charge carriers in the transport medium. In certain embodiments, the statistically controlling means may be one of an optical injection means, a result of a spintronic action/manipulation/operation/calculation, and a result of a quantum computing or quantum cryptography operation.
In certain embodiments, the means for providing a flux of charge carriers further comprises a potential difference between the injection contact and the first and the second collection contacts.
In certain embodiments, the first collection contact and the second collection contact can have a different spin polarisation.
The first collection contact and the second collection contact have different magnetization directions, forming an angle different from zero, for example, but not only 180 or 90 degrees.
The injection contact and the first and the second collection contact can be positioned on the same side or on different sides such as, for example, on opposite sides of the transport medium.
The transport medium may comprise a semiconductor material or a metal such as, for example; an illustrative but not extensive list is given below.
Metals may be one of Lithium, Beryllium, Sodium, Magnesium, Aluminium, Potassium, Calcium, Titanium, Vanadium, Chromium, Manganese, Iron, Cobalt, Nickel, Copper, Zinc, Arsenic, Zirconium, Molybdenum, Silver, Gold, Cadmium, Antimony, Barium, Osmium, Platinum, Palladium, Mercury, Thallium, Lead, Uranium, and their alloys and compounds
Semiconductor materials may be one of Group IV elemental semiconductors: Boron (B), Diamond (C),Silicon (Si),Germanium (Ge),Gray Tin (Sn), Phosphorus (P), Selenium (Se), Tellurium (Te); Group IV compound semiconductors: Silicon carbide (SiC), Silicon germanide (SiGe), Boron-Carbon (BC), Boron-Silicon (BSi), Boron-Phosphorus (BP),Silicon-Tin (SiSn), Germanium-Tin (GeSn); Group III-V semiconductors: Boron nitride (BN),Boron Phosphide (BP), Boron Arsenide (BAs), Aluminum antimonide (AlSb),Aluminum arsenide (AlAs), Aluminum nitride (AlN), Aluminum phosphide (AlP), Gallium antimonide (GaSb), Gallium arsenide (GaAs), Gallium nitride (GaN), Gallium phosphide (GaP), Indium antimonide (InSb), Indium arsenide (InAs), Indium nitride (InN), Indium phosphide (InP); Group III-V ternary, quaternary, . . . semiconductor alloys: Aluminum gallium arsenide (AlxGa1−xAs), and other combinations of the above III-V semiconductors; Group II-VI semiconductors: Cadmium selenide (CdSe), Cadmium sulfide (CdS), Cadmium telluride (CdTe), Zinc oxide (ZnO), Zinc selenide (ZnSe), Zinc sulfide (ZnS), Zinc telluride (ZnTe), Mercury selenide (HgSe), Mercury sulfide (HgS), Mercury telluride (HgTe); Group II-VI ternary, quaternary, . . . semiconductor alloys: Cadmium mercury telluride ((Cd,Hg)Te), and other combinations of the above II-VI semiconductors; Group I-VII semiconductors: Cuprous chloride (CuCl); Group IV-VI semiconductors: Lead sulfide (PbS), Lead telluride (PbTe), Tin sulfide (SnS), Layered semiconductors, Lead iodide (PbI2), Molybdenum disulfide (MoS2), Gallium Selenide (GaSe), Miscellaneous oxides, Copper oxide (CuO), Cuprous oxide (Cu2O), Organic semiconductors, Magnetic semiconductors; Ternary Adamantine Semiconductors; Ternary Analogs of the III-V Adamantine Semiconductors; the II-IV-V2 Compounds (Ternary Pnictides); the I-IV2-V3 Compounds; Ternary Analogs of II-VI Adamantine Semiconductors; I-III-VI2 Compounds; I2-IV-VI3 Compounds; I3-V-VI4 Compounds; Defect Adamantine Semiconductors; IV3-V4 Compounds; III2-VI3 Compounds; Defect Ternary Compounds, III2-IV-VI Compounds; Non-Adamantine Semiconductors and Variable-Composition Semiconductor Phases, IA-IB Semiconductors, Semiconductors in the I-V Binary Systems, IA-VB Compounds, (IA)3-VB Compounds, I-VI Compounds, Copper and Silver Oxides, Copper and Silver Chalcogenides, II2-IV Compounds with Antifluoride Structure, IV-VI Galenite Type Compounds, V2-VI3 Compounds, Binary Compounds of the Group VIIIA Elements, Semiconductor Solid Solutions, Alloys of Elemental Semiconductors, III-V/III-V Semiconductor Alloys, II-VI/II-VI Solid Solutions, Solid Solutions of II-VI and III-V Compounds, and Organic Semiconductors.
A corresponding method for spin detection of charged carriers having a spin and forming a flux in a medium, comprising the steps of:
In certain embodiment the measurements can start when the spin of the flux of charge carriers is saturated.
In a certain further embodiment the method further comprises the steps of:
In certain embodiments, the first current integral and the second current integral are determined over a collection contact dependent period, respectively, starting at starting time+delay time 1 and starting time+delay time 2.
Advantageously, delay time 1 and delay time 2 may be the times necessary for the charge carrier flux to bridge the distance between the starting place and the first and the second collection contact respectively added to the time necessary for a charge carrier to diffuse between the first contact and the second contact.
In certain embodiments this duration in time is smaller than the average spin flip time−delay 1 and smaller than the average spin flip time−delay 2.
In another aspect of the present invention a device for performing spin detection is disclosed, comprising:
In certain embodiments, all elements or components present in the detector or detector chain are ultrafast elements.
A corresponding method for spin detection of charged carriers having a spin and forming a flux in a medium, the flux having active and inactive periods [the inactive periods should last longer than N*spin relaxation time, N, e.g. =about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, 100] comprising:
This method may further comprise determining a current integral over time of the current over a certain range in time.
Advantageously the range can defined by [starting time+delay time, starting time+delay time+duration], whereby delay time+duration<average spin flip time.
Here the delay time is preferably the time necessary for the charge carrier flux to bridge the distance between the starting place and the collection contact.
In another aspect of the present invention, a method for information transfer is disclosed based on the methods and devices disclosed above comprising:
The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.
Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.
An object of certain embodiments of the present invention is a device that comprise two electrical current contacts 10 (used as spin detectors) that have a different spin sensitivity.
The different spin sensitivity of the two detector contacts 10 may be the result of using two materials with different magnetic properties (e.g. two different magnetic materials, or a magnetic and a non-magnetic material), or using two spin-polarized contacts comprising magnetic materials with magnetizations pointing in different directions (the angle between the two magnetizations can e.g. be 180°, or 90°, or any other angle different from 0°; the two magnetic materials can be but need not be identical). The best discrimination will be obtained when the contacts are fully spin-polarized (+100% or −100% polarized, i.e., the current in them is carried by carriers with one type of spin only), and when their polarizations are opposite (i.e., 180° apart). For brevity the following paragraph(s) will be written under this assumption, with the understanding that devices with other configurations will operate with an accordingly reduced discrimination between both spins, but still with the same relative improvement over existing devices with only one detection contact.
The two spin-detector contacts 10 are placed within a small distance from each other, where small is in comparison with the spin diffusion length for the relevant carriers (electrons or holes) inside the transport medium 20, such that spins that are rejected by a certain contact have a large chance to be absorbed/extracted/removed by the other contact with the opposite spin-sensitivity, compared to the chance that their spin will be flipped and that the charge is extracted by the contact that originally rejected it.
Another object of certain embodiments of the present invention is a method to detect the spin of charge carriers in a medium 20, using electrical contacts 10. The method consists of having a medium in which charge carriers are present that can have a spin polarization, and of which one wants to determine this spin polarization.
On the medium 20, two detector contacts 10 are present that have a different spin sensitivity. The distance between these contacts is preferably smaller than N times the spin diffusion length [e.g., N=10, 5, 4, 3, 2, 1, 0.9 to 0.1, 0.01, 0.001], such that there is a significant (theoretically non-zero) probability that a carrier that is rejected by one of the contacts, because it has the wrong spin, will reach the second contact and attempt to enter there before its spin is flipped, thus before it becomes possible for this carrier to enter the contact that originally rejected it.
In a preferred embodiment, the distance is smaller than the spin diffusion length.
In certain embodiments the distance between the (optional) injection contact 45 or the latest place for which statistical control of the particle states in the flux 30 of particles is present (area 40 in Figures), and one or both of the collection contacts 10, is smaller than N′ times the spin diffusion length [e.g., N′=10, 5, 4, 3, 2, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.01, 0.001]. N′ for one of the collection contacts 10 may be different from N for the other collection contact 10.
In one embodiment, the method comprises measuring a first current on the first contact with a first spin selectivity on the medium 20, measuring a second current on the second contact with a second spin selectivity on the medium 20, comparing the first current and the second current, and deriving the average or statistically relevant spin state of the charge carriers present in the medium 20.
Another object of certain embodiments of the present invention is a device for time-resolved measurements of the spin polarization.
All other examples of this paper can be used to illustrate the functioning of the methods and devices according to embodiments of the present invention.
These so called spin flip or spin relaxation times set the lower limit on the speed that separates “fast” from “slow” spin detectors. A spin detector consists of different elements or component, each with its own characteristics. To build a “fast” spin detector, all elements may need to be fast. For instance an embodiment of the detector according to the present invention has the following specification (typical values are given in the following): wire bonds: about 6 GHz (flip-chip connections would be: about 10 GHz), feed throughs and cabling in the cryostate for low T measurements: about 5-6 GHz, signal and delay generators: about 10 GHz, detector (e.g., an oscilloscope): about 20 GHz, coplanar wave guides: about 40 GHz. The lowest frequency of the whole chain determines the performance of the system. At low temperatures this corresponds to about 5 GHz or a spin flip time of about 200 ps. At room temperature, this would correspond to about 10 GHz or a spin flip time of about 100 ps. Current microprocessors and integrated circuits already run at these speeds.
An ultra fast detector may need ultra fast contacts. For embodiment of the present invention related to the time-resolved measurement and device therefore, such an ultra fast contact—and moreover ultra fast chain of other elements or components needs to be present.
In another embodiment, one or more of such ultrafast contacts may be advantageous.
One of the elements in a chain of elements/components, making up a spin detector, is the injection means, which needs to be ultrafast for certain embodiments of the present invention related to the time-resolved measurement and device therefore.
In another embodiment, ultrafast injection contacts may also be advantageous.
A further object of certain embodiments of the present invention is a method for electrical spin detection using one or more ultrafast contacts. The description below is given for an exemplary embodiment comprising one ultrafast contact. It may also be used with two or more ultrafast contacts, where it can e.g. combine the improvements of the differential measurement method of certain embodiments of this invention with the benefits of the ultrafast measurements. The implementation of this kind of combination is straightforward to one of skill in the art and is therefore not detailed here.
The method for electrical spin detection using one or more ultrafast contacts comprises the following steps:
optionally the method may comprise emptying the transport medium 20 such that a new measurement can be performed, e.g., by performing one of the following:
Other embodiments and modifications than those described above are also included within the scope of the invention which is defined below in the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| EP 5447084.4 | Apr 2005 | EP | regional |
This application is a divisional of U.S. patent application Ser. No. 11/282,312, filed Nov. 18, 2005, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application No. 60/630,317 filed Nov. 22, 2004 for “SPINTRONICS DEVICE AND METHODS FOR USE”, and claims the benefit under 35 U.S.C. § 119(a) of European Patent Application No. EP 5447084.4 filed on Apr. 21, 2005 for “SPINTRONICS DEVICE AND METHODS FOR USE,” which are incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60630317 | Nov 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11282312 | Nov 2005 | US |
| Child | 12180398 | US |
