Spin-Polarized Light Emitting Self-Assembled InAs/GaAs Quantum-Dot Molecular Structures: The Dominant Mechanism for Spin Loss during Spin Injection
Authors: Huang Y Q , Puttisong Y , Buyanova Irina A , Chen Weimin M 
Published: February 2020

While conventional electronics only relies on manipulation of electronic charge, a promising direction for next-generation electronic and photonic devices is expected to benefit additionally from exploiting the spin degree of freedom leading to e.g. spin-FET, spin lasers and spin-LEDs. Among vital ingredients in achieving spin functionalities are efficient injection of spin polarized carriers or excitons, prolonged spin dephasing/relaxation time as well as our ability to manipulate spin in a desirable way. Semiconductor quantum dots (QDs) have emerged as an excellent materials system in this context, as motion-induced spin relaxation that is dominant in bulk and 2D semiconductor structures is quenched due to the three-dimensional confinement. Combining with their high radiative recombination efficiency, QDs are good candidates for spin light-emitting devices and strongly polarized light sources. Indeed, appreciably long spin lifetimes have been reported in semiconductor QDs and a high degree of electron spin and optical polarization has also been demonstrated through optical orientation under resonant excitation of QD excitons. However, upon spin injection from adjacent layers such as a wetting layer (WL) and a barrier layer, that is a necessary step required for operation of most envisaged spintronic devices based on QDs, reported values of electron spin and optical polarization degree of QDs remain very low so far. The exact physical mechanism for the observed low spin injection efficiency is still not understood, unfortunately, though it was generally believed to be associated with accelerated spin relaxation in WL and barrier layers when carrier and exciton motions (i.e. with a non-zero momentum
k
) promote spin-orbit mediated spin relaxation. Up to now, WL and barrier layers surrounding QDs are commonly regarded as being of “ideal” 2D and 3D characters, respectively.

In this work we examine the exact path of spin injection from WL and barriers into QDs in self-assembled InAs/GaAs QDs and lateral QD molecular structures (QMSs), by employing optical orientation, single-dot photoluminescence (PL) and PL excitation spectroscopy. The studied QDs and QMSs include single quantum dots (SQDs), laterally-aligned double QDs (DQDs), quantum rings (QRs), and quantum clusters (QCs). We find, surprisingly, that exciton spin injection in these QDs and QMSs is dominated by localized excitons confined within the QD-like regions of the WL and GaAs barrier layer immediately surrounding QDs and QMSs that in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We identify the microscopic origin of the observed severe spin loss during spin injection as being due to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We find that the AEI of the injected excitons and thus the efficiency of the spin injection processes are directly correlated with the overall geometric symmetry of the QMSs, namely, spin injection is more efficient in the QMSs with a higher symmetry in geometric arrangement such as QRs and QCs as compared with the lower-symmetry DQDs. This correlation is attributed to the fact it is the symmetry of the QMSs that largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier, which governs the AEI. This work represents a significant advance in our understanding of the spin injection processes and spin loss mechanism in QDs and QMSs, and it also provides a useful guideline in designing strategies to improve spin injection efficiency by optimizing the lateral arrangement of the QMSs, thereby overcoming a major obstacle in utilizing semiconductor QDs and QMSs for device applications in spintronics and spin photonics.