Photo-Induced Spin Dynamics in Semiconductor Quantum Wells
© to the authors 2009
Received: 14 November 2008
Accepted: 30 December 2008
Published: 17 January 2009
We experimentally investigate the dynamics of spins in GaAs quantum wells under applied electric bias by photoluminescence (PL) measurements excited with circularly polarized light. The bias-dependent circular polarization of PL (PPL) with and without magnetic field is studied. ThePPLwithout magnetic field is found to be decayed with an enhancement of increasing the strength of the negative bias. However,PPLin a transverse magnetic field shows oscillations under an electric bias, indicating that the precession of electron spin occurs in quantum wells. The results are discussed based on the electron–hole exchange interaction in the electric field.
KeywordsPhotoluminescence Spin transport Exchange interaction
Possibility of using information carried by the spin of the electron in electronic devices, in addition to its charge, has gained a lot of attention since the discovery of long spin lifetimes in semiconductor structures , leading to the growth of the field spintronics [1–3]. This may lead to new devices beyond well-established storage or memory applications, already implemented as giant magnetoresistance (GMR) read-heads and nonvolatile magnetic RAM (MRAM) .
One of the major hurdles in the development of spintronic devices has been the problem of efficiently injecting spin-polarized carriers into a semiconductor, transporting them over reasonable distances without spin-flipping and then detecting them. Much effort  has thus been spent in understanding the transport and dynamics of spins and the generation/injection and detection of spin currents in semiconductors. Generation of spin polarization usually means creating a nonequilibrium spin population. This has been achieved in various ways, e.g. by optical techniques, or by magnetic semiconductors or ferromagnetic contacts, with varying degrees of success . Although the detection of spin current in semiconductors was previously been achieved mainly through optical methods , an electrical means of detecting spin current has been obtained recently [4–6]. Despite many efforts and substantial progress, a further major obstacle to the practical implementation of spintronics is the lack of a proper understanding of spin transport and dynamics in semiconductor-based heterostructures .
In this paper, we focus on spin dynamics in GaAs quantum wells (QWs) under applied electric bias by photoluminescence (PL) measurements excited with circularly polarized light [7–9]. We study the bias-dependent circular polarization of PL (PPL) with and without magnetic field. The PPL without magnetic field is found to be decayed with an enhancement of increasing bias. However, PPL in a transverse magnetic field shows oscillations under an applied bias, indicating that the precession of electron spin occurs in QWs. The results are discussed by exploring the possible roles played in the observed phenomena.
Results and Discussion
One more mechanisms of the relaxation of the excitonic spin oriented by the light might be possible. Relaxation can result in the PL depolarization due to a flip of the exciton spin as a whole or independent flips of the electron and hole spins . In the first case, the decay time of the PPL is directly determined by the exciton spin relaxation rate . If the carrier spins relax independently, the fast flip of the hole spin does not affect the PPL. In this case, the decay of the PPL is controlled by the relaxation of the long-lived electron spin. The above analysis reveals that the bias-induced changes in the polarized PL kinetics are related to transition from dynamics of the exciton spin to that of independent electron and hole spins. In the absence of the bias, the exchange coupling exceeds the spin–phonon interaction , and the main relaxation mechanism is given by the exciton spin flips, as mentioned above.
In the presence of the applied bias, the electric field reduces the electron–hole exchange coupling by spatially separating the charges, and as a result, the interaction of the hole spin with phonons becomes stronger than the exchange interaction. This leads to a breakage of the coupling between the electron and hole spins. As a consequence, the hole spin exhibits fast relaxation, while the electron spin holds its orientation, providing the slow component in the PPL decay. The decay time as measured at −2.5 V is 1 ns, which agrees with the literature value .
As can also be seen from Fig. 4, the oscillation amplitude is initially equal to ~0.8 and slowly decays with time. The data can be fitted by the exponentially damping harmonic function which gives ωL = 0.1 THz and the decay time τ = 200 ps for the −2.5 V bias and H = 5 T. The obtained oscillation frequency ωL corresponds to the value g = 22.7 × 10−2. This agrees with the experimental as well as theoretical estimates of the transverse electron g-factor in GaAs QWs [20–22].
In the absence of the bias, there involves only the behaviour of the exciton spin as a whole. The H mixes the excitonic states, and as a result, the right circularly polarized light becomes capable of exciting several states. The kinetics of the polarized PL is controlled here by the interference of the states of the exciton fine structure split by the combined action of the magnetic field and exchange coupling induced by the external electric field [23, 24]. As Fig. 4a demonstrates, the PPL varies with time in a rather complicated fashion of the PL polarization kinetics , reflecting superposition of the beats at several frequencies. From the above discussion, one can conclude that application of the bias to the QWs weakens the exchange interaction between the electron and hole spins.
The dynamics of spins in GaAs QWs under applied electric bias has been experimentally investigated by PL measurements. The bias-dependentPPLwith and without magnetic field was studied. ThePPLwithout magnetic field was found to be decayed with an enhancement of increasing negative bias. However,PPLin a transverse magnetic field showed oscillations under an applied bias. The oscillation amplitude was found to be increased with increasing the strength of the bias. The results were discussed based on the electron–hole exchange interaction in the electric field.
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