Electrical spin injection into InGa(N)As quantum structures and single InGaAs quantum dots

Abstract

We investigate electrical spin injection from a semi-magnetic n-type ZnMnSe spin aligner into III -V p -i -n diodes with InGaAs quantum dots (QDs) in the active layer. Quantitative transmission electron microscopy techniques are applied to characterize the different dot types used. Analysis of the circular polarization degree (CPD) of the device emission indicates the spin polarization of the injected electrons. Values of more than 70% are obtained for the wetting layer and high-energy QD states. However, the CPD shows a strong spectral dependence due to spin relaxation at the stage, before the electrons are finally captured in the dots. This is, e.g., evidenced by an initial increase of the polarization degree with rising temperature, attributed to motional narrowing effects. As a prerequisite for more detailed studies, we also demonstrate electrical spin injection into single InGaAs QDs, which should provide the basis for future single spin manipulation experiments. Finally, we suggest GaInNAs as optically active material for the realization of spin-polarized light-emitting diodes with long-wavelength emission. First results indicate CPD values of up to 80% for λ = 1130 nm, suggesting this approach to be very promising.

Pulsed Electrical Spin Injection into InGaAs Quantum Dots: Studies of the Electroluminescence Polarization Dynamics

“…A crucial factor may be the potential barrier at the n-ZnMnSe/i-GaAs interface, which the carriers have to pass by a (phonon-assisted) tunnelling process [3,4]. When the carrier density at the beginning of the pulse is still low, spin-polarized electrons can easily tunnel into the empty QD states and retain their polarization.…”

“…As has been shown, the band bending and the position of the electronic quasi-Fermi level give rise to a tunnel barrier and often the formation of a twodimensional electron gas (2DEG) at the III-V / II-VI interface. Further details may be found in [3,4] and references therein.…”

“…We then excited the device at 5 K with current pulses of several nanoseconds width. From the circular polarization degree (CPD) of the emit- ted light at different wavelengths we directly determined the degree of spin polarization [3] in different parts of the spin-LEDs (QDs, wetting layer, GaAs). For the detection of the emitted light, an avalanche photodiode and a time-correlated single photon counting (TCSPC) setup were employed.…”

“…As n-type material a diluted magnetic semiconductor (DMS) is used, which provides the possibility to electrically initialize the spin states by spin-polarized electrical currents. We already succeeded in proving that initialization of spin states with very high fidelity close to 100% can be achieved for single quantum dots under a constant electrical current in the spin-LEDs [2,3]. However, in the QD ensemble the spin injection fidelity is limited to significantly lower values by relaxation processes of the spin [2,3].…”

“…We already succeeded in proving that initialization of spin states with very high fidelity close to 100% can be achieved for single quantum dots under a constant electrical current in the spin-LEDs [2,3]. However, in the QD ensemble the spin injection fidelity is limited to significantly lower values by relaxation processes of the spin [2,3]. In the experiments presented in this contribution, we analyze the initialization of spin states utilizing pulsed electrical excitation of the spin-LED and time-resolved detection of the subsequent electroluminescence.…”