Electric suppression of spin-orbit interaction in GaAs (111) QWs

Two main challenges for an efficient manipulation of the spin vector in semiconductors are the achievement of long electron spin coherence times as well as the development of spin manipulation techniques that do not compromise the spin lifetime. In III-V semiconductor quantum wells (QWs), the spin-orbit (SO) interaction provides a powerful tool to manipulate spins but, at the same time, can lead to spin dephasing within an electron ensemble via mechanisms like the Dyakonov-Perel, the Elliott-Yafet mechanism or via intersubband spin relaxation. The SO-interaction can be controlled by external electric and strain fields, thus providing an interesting mechanism for spin manipulation. Here, we demonstrate the electric control of spin lifetime in GaAs QWs grown along the non-conventional [111] crystallographic direction. We show that an electric field applied across a GaAs (111) QW can efficiently suppress the SO-interaction, thus leading to very long spin lifetimes (exceeding 100 ns) at low temperatures.

Figure 1. Relative orientation of the BIA (BBIA) and SIA (BSIA) contributions to the SO-interaction acting on an electron spin (S) oriented perpendicular to the quantum well (QW) plane. The SIA field is induced by an electric field (EZ) applied across the QW. The lower panel shows the symmetry of the BIA and SIA fields in the GaAs (111) QW plane.

Figure 2. Out-of-plane spin lifetime as a function of the bias voltage (Vb) and its corresponding electric field (Ez) extracted from time- and polarization-dependent photoluminescence under continuous (solid squares) and pulsed (open squares) bias voltages.

The SO-interaction is associated with the fact that an electron moving in the lattice potential of a III-V semiconductor experiences an effective, wave vector (k) dependent magnetic field BSO(k), which acts on its spin. In GaAs QWs, the SO-interaction is governed by two major contributions. The first is related to the bulk inversion asymmetry (BIA) of the zinc-blende lattice. In the case of (111) QWs, the linear in k components of the effective magnetic field associated with this contribution, BBIA(k), lies in the QW plane and has the orientation dependence on k sketched in Figure 1. The second important contribution arises from structural inversion asymmetries (SIA) due to external fields. In the case of an electric field EZ applied across the QW, the associated effective magnetic field BSIA(k, Ez) is proportional to k and E­z and also lies in the QW plane (cf. Figure 1). Since the linear terms of both contributions have exactly the same symmetry, if one applies a compensation electric field, Ec, for which the condition BBIA(k) + BSIA(k, Ec) = 0 is satisfied, then the total SO-interaction will simultaneously vanish for all k vectors. We have provided the experimental verification of this BIA/SIA compensation mechanism by demonstrating the electrically driven transition from a BIA-dominated spin dephasing regime to a SIA-dominated one.

 

The studies were carried out in (111) QWs embedded in the intrinsic region of an n-i-p diode, where bias voltage, Vb, applied between the top n-doped layer and the p-doped substrate generates the vertical electric field EZ. We have studied the electron spin dynamics by exciting out-of-plane spin polarized electrons in the QWs using a circularly polarized, pulsed laser beam, and by detecting the circular polarization of the photoluminescence (PL) emitted when the carriers recombine. Figure 2 shows the dependence of the electron spin lifetime on reverse bias in a sample including 20 QWs: the spin lifetime increases with increasing reverse bias from barely 1 ns to up to 60 ns (black squares). For large reverse voltages, the overlap between the electron and hole wave functions reduces significantly, thereby reducing the PL intensity and hindering the optical detection of the spin dynamics. We could overcome this limitation by carrying out experiments under pulsed reverse bias. Here, laser and bias pulses with the same repetition frequency are synchronized, so that the laser pulse hits the sample at a time shortly after the application of a reverse bias pulse of amplitude Vb. The photoexcited carriers are then driven towards opposite interfaces of the QW by the vertical electric field, where they remain stored until the bias pulse is removed. Then the stored carriers quickly recombine, giving rise to a short PL burst. We extract the spin dynamics from the circular polarization of these PL bursts after voltage pulses of different durations. By repeating this procedure for different Vb, we determined the electric field dependence of the spin lifetime indicated by the open squares in Figure 2. The observation of a maximum spin lifetime maximum in the voltage dependence unambiguously establishes the BIA/SIA compensation as the mechanism for bias-induced spin lifetime enhancement in (111) QWs. Furthermore, the observed peak spin lifetimes exceeding 100 ns are among the longest values reported for undoped GaAs structures.

Publications

3 Autor A. Hernández-Mínguez , K. Biermann , R. Hey , P. V. Santos
Titel

Spin transport and spin manipulation in GaAs (110) and (111) quantum wells

Source Phys. Status Solidi B , 251 , 1736 ( 2014 )
DOI : 10.1002/pssb.201350202 | 2535 Cite : Bibtex RIS
A. Hernández-Mínguez, K. Biermann, R. Hey, and P. V. Santos

2 Autor A. Hernández-Mínguez , K. Biermann , R. Hey , P. V. Santos
Titel

Electrical suppression of spin relaxation in GaAs(111)B quantum wells

Source Phys. Rev. Lett. , 109 , 266602 ( 2012 )
DOI : 10.1103/PhysRevLett.109.266602 | Download: PDF | 2375 Cite : Bibtex RIS
A. Hernández-Mínguez, K. Biermann, R. Hey, and P. V. Santos

1 Autor K. Biermann , A. Hernández-Mínguez , R. Hey , P. V. Santos
Titel

Electrically tunable electron spin lifetimes in GaAs(111)B quantum wells

Source J. Appl. Phys. , 112 , 083913 ( 2012 )
Download: PDF | 2364 Cite : Bibtex RIS
K. Biermann, A. Hernández-Mínguez, R. Hey, and P. V. Santos