JAPAN NANONET BULLETIN - 15th Issue - April 1, 2004

Control and applications of novel spin properties found in semiconducting nano structures

Electrons have spin degree of freedom in addition to charge degree of freedom. It is the charge degree of freedom that various conventional electronic devices to date have been based on. The ultimate purpose in the new research area of semiconductor spintronics is the development of electronic devices that actively utilize the spin degree of freedom of electrons in order to realize functionalities that have never been realized in conventional electronic devices.

What I specially focus on in my PRESTO research is the gate-control of the spin properties in semiconductor heterostructures. Electron spins, which are exemplified by small magnets, have conventionally been controlled by externally applied magnetic fields. Although it had also been proposed that electron spins could be controlled by an electric field (via so-called Rashba spin-orbit interaction) instead of a magnetic field, the main accomplishment in my PRESTO research includes the quantitative clarification of the gate-controlled Rashba spin-orbit coupling using the weak antilocalization analysis as itemized below:

(1) We performed a quantitative analysis on the weak antilocalization phenomena that are observed in a magneto-resistance at low temperatures in the InAlAs/InGaAs/InAlAs quantum well system. We then discovered an existence of zero-field spin-splitting in this system, which should be caused by the asymmetry in the potential shape of the quantum wells. The magnitudes of the spin splitting energies turned out to be consistent with those predicted in theory [1].

(2) We showed, theoretically, that a spin filter device can be realized using a triple barrier resonant tunneling structure. This spin device is composed of InGaAs and InAlAs for the well and barrier layers, respectively. These are both nonmagnetic materials, hence proposing a spin filter device without the use of any magnetic materials [2] (Fig. 1).

(3)We examined a spin interference effect in a square loop array that is nanolithographically fabricated on an InAlAs/InGaAs/InAlAs quantum well heterostructure (Fig. 2).

Regarding the above item (3), recent experimental results showed that the magnitude of the self-interference of the electron wave function varies as a function of the gate voltage. This result indicates that the electron wave function interferes with itself constructively or destructively depending on the value of the applied gate voltage, which supports the fact that spin precession angle is controlled by the magnitude of the spin-orbit interaction (Fig. 2).

For future projects, I would like to make every effort, on the basis of the academic results accumulated to date, in experimental realization of the proposed spin filter device that utilizes resonant tunneling structure, as well as exploration of new research areas such as the examination of the relation between spin-orbit effect and phase relaxation time in a two-dimensional electron gas system.

Fig. 1
(Left) Potential profile of a spin filter device that is realizable without the use of magnetic materials. (Right) Theoretical I-V characteristics of the proposed spin filter device.

Fig. 2
(Left) Scanning electron micrograph of the square loop array that is nano-fabricated on an InAlAs/InGaAs/InAlAs quantum well heterostructure. Electron beam lithography and ECR plasma etching techniques are employed in the nano-fabrication of this structure. (Right) Self-interference effect of the electron wave function in a square loop array. ΔG denotes the contribution of the electron self-interference effect to the electric conductance through the square loop array. In addition to the fact that the magnitude of the electron interference effect goes up and down with the magnetic field B, it can also be varied with the gate voltage Vg that can control the strength of the spin-orbit interaction in turn. For example, ΔG become maximum at B=0 in (a), while ΔG is minimum at B=0 in (b), where (a) and (b) are the measurements performed on a single sample, but with different gate voltages.

Relevant papers