Spin effects in InAs self-assembled quantum dots
© dos Santos et al; licensee Springer. 2011
Received: 13 August 2010
Accepted: 3 February 2011
Published: 3 February 2011
We have studied the polarized resolved photoluminescence in an n-type resonant tunneling diode (RTD) of GaAs/AlGaAs which incorporates a layer of InAs self-assembled quantum dots (QDs) in the center of a GaAs quantum well (QW). We have observed that the QD circular polarization degree depends on applied voltage and light intensity. Our results are explained in terms of the tunneling of minority carriers into the QW, carrier capture by InAs QDs and bias-controlled density of holes in the QW.
Resonant tunneling diodes (RTDs) are interesting devices for spintronics because the spin character of the carriers can be voltage selected [1–4]. Furthermore, spin properties of semiconductor quantum dots (QDs) are also of high interest because electron spins can be used as a quantum bit  for quantum computing  and quantum communication . In this paper, we have studied spin polarization of carriers in resonant tunneling diodes with self-assembled InAs QD in the quantum well region. The spin-dependent carrier transport along the structure was investigated by measuring the left- and right-circularly polarized photoluminescence (PL) intensities from InAs QD and GaAs contact layers as a function of the applied voltage, laser intensity and magnetic fields up to 15 T. We have observed that the QD polarization degree depends on bias and light intensity. Our experimental results are explained by the tunneling of minority carriers into the quantum well (QW), carrier capture into the InAs QDs, carrier accumulation in the QW region, and partial thermalization of minority carriers.
Our devices were grown by molecular beam epitaxy on a n+ (001) GaAs substrate. The double-barrier structure consists of two 8.3-nm Al0.4 Ga0.6 As barriers and a 12-nm GaAs QW. A layer of InAs dots was grown in the center of the well by depositing 2.3 monolayers of InAs. Undoped GaAs spacer layer of width 50 nm separate the Al0.4 Ga0.6 As barriers from 2 × 1017 cm-3 n-doped GaAs layers of width 50 nm. Finally, 3 × 1018 cm-3 n-doped GaAs layers of width 0.3 nm were used to form contacts. Our samples were processed into circular mesa structures of 400 μm diameter. A ring-shaped electrical contact was used on the top of the mesa for optical access and PL and transport measurements under light excitation. Magneto-transport and polarized resolved PL measurements were performed at 2 K under magnetic fields up to 15 T parallel to the tunnel current by using an Oxford Magnet with optical window in the bottom. The measurements were performed by using a Princeton InGaAs array diode system coupled with a single spectrometer. A linearly polarized line (514 nm) from an Ar+ laser was used for optical excitation. Therefore, photogenerated carriers in the device do not present any preferential spin polarization degree. The right (σ+) and left (σ-) circularly polarized emissions were selected with appropriate optics (quarter wave plate and polarizer).
Results and discussion
Our results indicate that the final polarization from QD emission cannot be solely attributed to the spin-splitting of the QD states under magnetic field and it depends on the spin polarization of the injected carriers into the QW, which are determined by the g-factors and the density of electrons and holes along the RTD structure in a complex way. In fact, a quantitative calculation of the circular polarization degree from the QD emission is a rather complex issue as it depends on various parameters, including the g-factors of the different layers, the resonant and non-resonant tunneling processes, the capture dynamics of the carriers by the QDs, the density of carriers along the structure and the Zeeman and Rashba effects. This suggestion is also supported by previous results obtained for p-i-n and n-type RTDs without QDs [3, 4]. It was observed that the high QW polarization degree observed on those measurements is mostly due to a highly spin polarized carriers from the two dimensional gas formed in the accumulation layer next to the emitter barrier. We also point out that the density of carriers along the RTD structure can be voltage and light controlled, which can be used to vary the circular polarization degree from QDs emission.
In conclusion, we have observed that the QD circular polarization in an n-type RTD can be voltage and light controlled. A maximum value of spin polarization of about -37% was obtained for the hole resonant tunneling condition and for low-laser intensities. We associated this effect to the voltage and light dependence of charge accumulation in the QW region.
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