Electrical spin injection into p-doped quantum dots through a tunnel barrier

Abstract

We have demonstrated by electroluminescence the injection of spin polarized electrons through Co/Al 2 O 3 /GaAs tunnel barrier into p-doped InAs/GaAs quantum dots embedded in a PIN GaAs light emitting diode. The spin relaxation processes in the p-doped quantum dots are characterized independently by optical measurements (time and polarization resolved photoluminescence). The measured electroluminescence circular polarization is about 15 % at low temperature in a 2T magnetic field, proving an efficient electrical spin injection yield in the quantum dots. Moreover, this electroluminescence circular polarization is stable up to 70 K.The achievement of spintronic devices necessitates on the one hand the ability to inject spin polarized currents into semiconductors 1 , and on the other hand sufficiently long spin relaxation times to manipulate or store the spin orientation of the injected carriers. The fundamental problem of conductivity mismatch between a ferromagnetic metal and a semiconductor pointed by Schmidt et al. 2 can be solved by the insertion of a tunnel barrier 3,4 which may be achieved either by a thin Schottky barrier 5,6 , or by an insulating layer as aluminium oxide [6][7][8] or Manganese Oxide 9 . In addition to efficient injection, long electron spin relaxation time is a requirement for efficient storage and manipulation. This is indeed the case for semiconductor quantum dots 10 . Electrical spin injection into InAs/GaAs intrinsic QDs has been reported with spin aligner systems based on magnetic semiconductor 11-13 or ferromagnetic metal/semiconductor Schottky barrier. [14][15] In this letter, we demonstrate the efficient electrical spin injection of polarized electrons into p-doped InAs/GaAs quantum dots through an insulating tunnel barrier of Al 2 0 3 . These QDs are very promising for several reasons: (i) the polarization of p-doped QD luminescence is directly related to electron spin polarization even for zero applied magnetic field, because of the cancellation of the anisotropic exchange interaction in the positively charged exciton 16 ; ii) very long electron spin relaxation times were reported recently 16 in these QDs under weak external magnetic field (about 100mT), which screens the effects of hyperfine interaction between electron and nuclear spins; (iii) long polarization decays (under excitation in the GaAs barrier) were measured even at high temperature. 17,18 The Spin-LED (SLED) structure (Fig. 1) was grown by molecular beam epitaxy for the semiconductor part and by sputtering for the tunnel barrier/ferromagnetic spin injector part. It consists in a P-I-N GaAs structure grown on a p-doped GaAs:Zn substrate (p = 2. 10 18 cm -3 ). The p-doped GaAs:Be layer (p = 1. 10 18 cm -3 ) is 2 µm thick. The doping of the n-GaAs layer (thickness: 50 nm) is 10 16 cm -3 to minimize the spin relaxation

Growth-temperature dependence of optical spin-injection dynamics in self-assembled InGaAs quantum dots

“…[5][6][7][8][9][10] Such spin injection has the potential to provide the key to applying the spin states of QDs to spin-functional optical devices, as one would need to inject spin-polarized carriers from electrodes into QDs to achieve a practical device structure. Indeed, spin-polarized light emitting diodes and lasers based on QDs have been discussed; [11][12][13][14] however, spin injection is recognized as being much more difficult than spin-independent carrier injection due to the relative instability of the spin states in layered semiconductors, which allows them to easily relax during the injection process.…”

Spin injection at remanence into III-V spin light-emitting diodes using (Co/Pt) ferromagnetic injectors

“…3 This explains the large amount of work on spintronics with hybrid metal/semiconductor heterostructures for the past ten years, once it was proposed to solve the problem of impedance mismatch 4 by the use of an interface resistance, typically a tunneling barrier. [5][6][7] Various experiments have been performed to detect by electrical (e.g., the electrical Hanle effect) or by optical means a spin-polarized current injected from a ferromagnetic reservoir into a III-V semiconductor through an Al 2 O 3 barrier, [8][9][10][11][12] through MgO, [12][13][14][15][16] and through GaO, 17 or into Si through Al 2 O 3 (Refs. [18][19][20][21][22] and SiO 2 (Refs.…”

“…[23][24][25] as well as into Ge through MgO. [26][27][28][29][30][31][32][33][34] Among the latest experiments, the transformation of a spin-polarized electron current into left-or righthanded circularly polarized light in a spin light-emitting diode (spin LED) integrating a III-V semiconductor heterostructure 8,11,[13][14][15][16][17]19,23,34,35 is one of the most striking physical phenomena. The electric dipolar selection rules involved in a quantum well 36 (QW) embedded in a spin LED during electron-hole recombination require spin injection with an out-of-plane magnetization.…”

Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots

“…By fitting the polar diagram of the emission with simple analytical expressions obtained from k·p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. A large variety of promising applications for self assembled semiconductor quantum dots in photonics and spin electronics are based on the discreteness of the interband transitions [1] and on long carrier spin relaxation times [2,3]. Combining the two characteristics allows electrical tuning of the light polarization of single photon emitters based on quantum dots [4].…”