Room temperature spin diffusion in (110) GaAs/AlGaAs quantum wells
© Hu et al; licensee Springer. 2011
Received: 14 September 2010
Accepted: 16 February 2011
Published: 16 February 2011
Transient spin grating experiments are used to investigate the electron spin diffusion in intrinsic (110) GaAs/AlGaAs multiple quantum well at room temperature. The measured spin diffusion length of optically excited electrons is about 4 μm at low spin density. Increasing the carrier density yields both a decrease of the spin relaxation time and the spin diffusion coefficient D s.
The interest in the spin properties of carriers in semiconductors has increased dramatically in the past 10 years due to potential application in the field of spintronics [1, 2]. The design of practical spintronic devices usually requires efficient spin injection in the semiconductor, long carrier spin lifetimes, and long spin transport/diffusion lengths [3–7].
One of the key parameters describing the properties of carrier spin transport in semiconductors is the spin diffusion coefficient D s, which is often assumed to be the same as charge diffusion coefficient D c. A direct optical measurement of the electron spin diffusion coefficient can be performed by creating electron spin grating in time-resolved four-wave mixing experiments . This powerful transient spin grating (TSG) technique was used recently to study the spin transport properties and determine the spin diffusion coefficient D s[9–11]. In particular it was demonstrated theoretically and experimentally that the spin diffusion coefficient D s in n-doped (100)-grown GaAs quantum wells can be smaller than the charge diffusion coefficient D c due to Coulomb interaction among the electrons (the so-called Spin Coulomb Drag effect) [10, 12]. In these (100)-grown GaAs quantum wells, the electron spin lifetime is of the order of 100 ps at room temperature (RT) due to very efficient D'yakonov-Perel (DP) spin relaxation mechanism . In the classical two-component drift-diffusion model , the spin diffusion length L s is determined by the spin lifetime and the spin diffusion coefficient D s through . As a consequence, the spin diffusion length L s at RT is smaller than 1 μm, limited by the short spin lifetime . In (110)-grown GaAs/AlGaAs QW, the DP spin relaxation mechanism is not efficient for electron spins parallel to the growth direction because the spin orientation of electrons is parallel to the direction of effective magnetic field induced by spin-orbit coupling . Spin relaxation times longer than 1 ns at RT in (110) GaAs QW have indeed been measured . Long electron spin diffusion lengths can thus be expected at high temperature in these structures. In this report, the electron spin diffusion is measured by the TSG technique with heterodyne detection in (110) GaAs/AlGaAs QWs at RT. We find that the spin diffusion length L s is about 4 μm at low carrier density. We also demonstrate that the spin diffusion coefficient D s decreases when the carrier density increases.
where A is a constant, Γs is the decay rate of the spin grating, and Δt is the delay time between pump and probe beams.
Results and discussion
Figure 1b presents the signal of TSGs as a function of the time delay for two typical pump powers, 2 and 18 mW, respectively. The wave vector q of the spin grating is equal to . It is clear that both curves exhibit different mono-exponential decays. Using equation (1), we find Γs = 0.063 and 0.044 ps-1 for the pump powers 2 and 18 mW, respectively.
In order to obtain the spin diffusion length L s, the spin lifetime is measured independently by time-resolved Kerr rotation . The excitation powers are the same as the ones used in the measurement of TSG. Figure 2b presents the Kerr rotation dynamics for two excitation powers. The spin lifetimes are extracted by mono-exponential fits, which yield ~1220 ps and ~880 ps with excitation powers of 2 and 18 mW, respectively. As expected for (110)-grown QWs, the spin lifetimes for both excitation powers are much longer than the ones ( ~ 50-100 ps) measured in (100)-grown GaAs/AlGaAs QWs at RT . By combining the D s measurement obtained with the spin grating technique and the electron spin lifetime probed by the Kerr rotation experiment, we find that the spin diffusion length decreases from L s ~ 3.5 μm down to 2.2 μm when the excitation power increases from 2 to 18 mW. To the best of our knowledge, these values are the longest electron spin diffusion lengths reported at room temperature in inorganic semiconductors.
In order to get further insights on this power dependence, we also measured the charge diffusion coefficient D c with a concentration grating technique for different pump powers. We find that D c remains constant with a typical value D c ~ 12.5 cm2/s (data not shown here). This value is in good agreement with previous studies performed in non-intentionally doped (100)-grown GaAs QWs which demonstrate that the concentration grating experiments are governed by the hole diffusion .
where <v 2> is the mean square velocity of electrons and τp is the momentum relaxation time. In a very simple approach, <v 2> in a QW can be approximated by . The momentum relaxation τp is strongly dependent on the density of photogenerated electrons n e, with a typical power law . In the low density regime below 2.5 × 1010 cm-2, which corresponds to a pump power of 10 mW, the experimental data are well fitted by this power law as shown by the blue line in Figure 3a. In the high density regime above 2.5 × 1010 cm-2, the spin diffusion coefficient is almost constant and the density dependence can no more be interpreted by the simple power law. In this density range, the above discussion is clearly oversimplified and we hope that these experimental results will stimulate theoretical investigations to elucidate the origin of the carrier density dependence of the spin diffusion coefficient.
We have measured optically the spin diffusion coefficient D s in non-intentionally doped GaAs/AlGaAs (110) QWs at room temperature for different excitation powers. Under low excitation, the electron spin diffusion length L s is around 4 μm; to the best of our knowledge, this is the largest reported value at T = 300 K in III-V semiconductors. We also show that the spin diffusion coefficient of optically excited electrons decreases when the excitation density increases. These results could be useful to understand the spin transport properties in semiconductor structures, and possibly control/manipulate the spin transport by varying the excitation condition.
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transient spin grating.
We thank Ming-Wei WU for useful discussions. We acknowledge the financial support of this study from National Science Foundation of China, Grant number: 10534030, 10774183, 10911130356, 10874212; also supported by Ministry of Finance and Chinese Academy of Sciences, National Basic Research Program of China (2006CB921300, 2009CB930500), the ANR project SpinMan.
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