Datta-Das Transistor

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Datta-Das Transistor

EE453 Project Report Submitted

By

Kris Buchanan,

,

Fall 2008

In 1989, Data and Das introduced a new concept for a semiconductor spintronic device that has since become the motivating factor for many experiments.1 The device today is commonly referred as a spin field effect transistor(spinFET) and is often used as an important inspiration for semiconductor spintronics research. Dr. Das is still a proud concept designer of the transistor and uses the design to inspire upcoming students in the field of nanotechnology at the University of Nevada Las Vegas.

Figure 1. Schematic examples of several possible semiconductor spintronic devices.

The Datta-Das spin-field-effect transistor (spin-FET), stimulates plenty of theoretical and experimental works in semiconductor spintronics,but has not yet been realized. Concludeddifficulties are basically: (i) effective controllabilityof the Rashba spin-orbit (SO) coupling strength α, (ii) longspin-relaxation time in two-dimensional electron gas (2DEG) systems, (iii) uniformity of α, and (iv) more efficient spin injection rate. So far, the former two conditions have been basically satisfied in experiments, while the latter two remain to be solved.2

Beginning with the question of what is special about electron spin it must be noted that, there are four important points. The first point to consider is the connection between spin and magnetism, which is useful for informationstorage. Second is an intrinsic connection between spin and quantummechanics, which may be useful for quantum information. Third is the shortrange of spin-dependent exchange interactions, which implies that the role ofspin will continue to grow as the size of nanostructures continues to shrink. Fourth are the issues of speed and power dissipation, which are becomingincreasingly important for electronics at the nanoscale.3

The pioneering spin-transistor proposal of Datta and Das best exemplifies the relevance of electrical control of magnetic degrees of freedom as a means of spin modulating.1 The structure inversion asymmetry (the Rashba SO term) is required to dominate over the bulk inversion asymmetry (the Dresselhaus SO term,7 with coupling strength β) therein. However, the coupling strengths of the Rashba and Dresselhaus terms have been, in fact, found to be of the same order in certain types of quantum wells. Therefore, the influence due to the Dresselhaus term has become another issue in spintronics. For example, Łusakowski et al. had shown that the conductance of the Datta-Das spin-FET depends significantly on the crystallographic direction of the channel in the presence of the Dresselhaus term. A more complete work done by Winkler is the investigation of the spin-splitting due to the effective magnetic field generated by the structure inversion asymmetry and the bulk inversion asymmetry.2

An interesting feature of the spin transistors operation relies on controlling the strength of the gate of the Rashba interaction which has the formin a strictly one-dimensional channel. Uponcrossing the Rashba-active region of length L, a spin-up incomingelectron emerges in the spin-rotated state

where is the rotation angle and m*is the electrons effective mass. The corresponding spinresolvedconductance is found to be .1

Anotherrealization of this device can be used InAs nanowires with a ferromagnetic source and drain electrodes, since InAs has a high spin-orbit coupling. However, one of the difficulties is an efficient spin-injection from the ferromagnet into the nanowire. At the same time there is progress in realizing tunable quantum dots in InAs nanowires defined by topgates.5

Other important characteristics of the device are involved with the reliance on spin properties to switch the device between “on” and “off” states which requires less power and leads to faster device operation compared to standard MOSFET technology.

The spinFET essentially has the generic design shown in Fig. 1(a): a spinpolarized current injected from a ferromagnetic contact into an n-doped semiconductor channel where it is subject to an externally applied electrical field (via a Schottky gate) and then finally the spin-resolved current is detected by another ferromagnetic contact. In narrow gap semiconductors such as InAs, the electrons experience a strong spin-orbit coupling that in the presence of an asymmetric confining potential- results in a Rashba interaction of the form

Figure 2. Spin Control of Data Das Transitor.

where the carriers are confined to the x-y plane by an asymmetric confinement potential along the z-axis. The parameter alpha is proportional to the z-component of the interfacial electric field, sigma represents the Pauli matrices and k is the crystal momentum of the carriers. As a direct consequence of this Hamiltonian, the eigenstates for the electron spin along the z-axis undergo a momentum-dependent spin splitting even in the absence of an external magnetic field. (This spin splitting may be viewed in terms of the relativistic transformation of the electric field into a magnetic field in the rest frame of the electron.) The application of an external electric filed via the Schottky gate modulates the interfacial field, hence resulting in a precession of the electron spin as the current propagates under the gate. If a modulation of the gate voltage results in a complete reversal of the spin polarization of the current, the device can be effectively switched off.5

Figure 3. Datta Das Transistor using Spin Techniques to Turn On and Off.

The field generated by the DDT’s gate voltagealters the spin-orbit dynamics, shifting each

electron’s spin. Therefore only those electrons with thesame spin orientation as the drain can pass through. Hence, the gate voltage controls thecurrent, just as in an ordinary all-semiconductortransistor, theoretically permitting the deviceto be used for information processing.To model that action, the atomic-beam simulationuses laser light in place of the gate voltage,and the “ground” states (lowest energy conditions)of the atoms in place of spin orientation. When the first laser beam excites the atoms,they then “decay” (that is, shed the energy byemitting a photon) into one of three groundstates. Only one of those states, however, hasthe right quantum properties to be re-excitedby the laser frequencies available, and then reemita photon. This is called a “bright” state,and can be disregarded for the experiment.

The other two ground states cannot absorb(and thus re-emit) the specific frequencies of

any of the laser beams in use, and so they arecalled “dark.” The two different dark states correspondto the two possibilities for electron spinorientation -- “up“ and “down” -- in the DDT. Theproportion of atoms in each of the two darkstates is a function of the relative intensity ofthe second (middle) laser beam, just as the proportionof DDT electrons in a particular spin

state is a function of the device’s gate voltage.An atomic state analyzer at the end of the process

detects which of the two dark states eachatom is in, corresponding to the drain of a DDT,

which filters out electrons with altered spin.Unlike the macroscopic DDT -- with its enormousnumber of atoms and myriad possiblesources of error -- the atom-beam analogue

offers the opportunity to study exceedinglysmall components of the device and to carefully

control the behavior of the system, alteringonly one variable at a time. If eventually constructed,

the atom-beam model should allowphysicists to determine which specific factorsare most critical to the performance of a DDT.5

A number of experimental efforts have been undertaken over the years to measure the characteristic spin-dependent switching behavior expected in a spinFET. However, careful measurements have been observed to show that the spin-dependent switching is originates in a spurious effect: The fringe fields from the patterned ferromagnetic contacts create a local Hall effect that easily leads to misleading results.6 Measurements using a non-local geometry to eliminate contributions from the local Hall effect demonstrate quite conclusively that- in carefully designed structures-there is no evidence for an observable spin switching effect, leading inevitably to the conclusion that spin injection from metallic ferromagnetic contacts into a semiconductor must be very inefficient. This has also lead to a picture wherein the spin injection efficiency is found to be limited, at least in the diffusive regime, by the large mismatch in conductivity between a semiconductor and a metal.6

Figure 4. Left Laser beams as a model of DDT: RIGHT: By “tuning” the Laser beams.

The left picture of Figure 4 illustrates A beam of atoms which cross an area in which the three laser beams overlap. The first beam places each atom in the same excited state. As the atoms pass through the region where the second and third lasers cross, their states may or may not be altered. If no atomic states are changed, the entire beam passes through the analyzer -- the equivalent of maximum current in the drain of a working DDT.6

The picture on the right of Figure 4 shows that changing the relative strengths of the lasers, the state of the atoms in the beam changes in a predictable way, altering the proportion of atoms in each of the three possible ground states, two of which are dark. The quantum difference between the two dark states corresponds to the difference between spin-up and spin-down electrons in a DDT.6

Spintronics is a new paradigm for electronics which utilizes the electron’s spin in addition to its charge for device functionality.6 The primary areasfor applications or potential applications are information storage, computing,and quantum information. In terms of materials, the study of spin insolids now includes metallic multilayers, inorganic semiconductors,transition metal oxides, organic semiconductors, andcarbon nanostructures. The diversityof materials studied for spintronics is a testament to the advances in synthesis,measurement, and interface control that lie at the heart of nano-electronics. In terms of technology, the discoveries of giant magneto resistance(GMR), tunneling magneto resistance (TMR), and spin torquein metallic multilayers have led to significant advances in high-density harddrives and non-volatile random access memory. Advances in semiconductorspintronicsincluding the observation of long spin coherence times and thedemonstration of spin manipulation point toward potential applications in advanced computation such as reconfigurable logic and quantum informationprocessing.6 Once spintronics is fully understood and is applied to modern electronic devices applications incredible advances in speed and storage will be made.

1. SolidState Physics. +the+datta+das+transistor&source=web&ots=b8pVZeD4dE&sig=A60L3w_5f001A4vRJ JcD-9webYQ&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA61,M1

2. Datta-Das transistor: Significance of channel direction, size-dependence of source contacts, and boundary effects. Ming-Hao Liu and Ching-Ray Changs, National Taiwan University, Taipei 106, Taiwan, February 2, 2008.

3. Fundamentals of Spintronics in Metaland Semiconductor Systems

4.Datta–Das transistor with enhanced spin control. J. Carlos Egues, Guido Burkard, and Daniel Loss, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland. 5 February 2003

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5. Nist Spin Control: Modeling the Transistor of the Future

6. Spin Control: Joint Quantum Institute. Modeling the Transistor of the Future, September 2008.