Spin Injection and Relaxation in p -Doped (In,Ga)As/GaAs Quantum-Dot Spin Light-Emitting Diodes at Zero Magnetic Field

Abstract

We report on efficient spin injection in p-doped (In, Ga)As/GaAs quantum-dot (QD) spin light-emitting diodes (spin LEDs) under zero applied magnetic field. A high degree of electroluminescence circular polarization (P c) ∼19% is measured in remanence up to 100 K. This result is obtained thanks to the combination of a perpendicularly magnetized Co-Fe-B/MgO spin injector allowing efficient spin injection and an appropriate p-doped (In, Ga)As/GaAs QD layer in the active region. By analyzing the bias and temperature dependence of the electroluminescence circular polarization, we evidence a two-step spin-relaxation process. The first step occurs when electrons tunnel through the MgO barrier and travel across the GaAs depletion layer. The spin relaxation is dominated by the Dyakonov-Perel mechanism related to the kinetic energy of electrons, which is characterized by a bias-dependent P c. The second step occurs when electrons are captured into QDs prior to their radiative recombination with holes. The temperature dependence of P c reflects the temperature-induced modification of the QD doping, together with the variation of the ratio between the charge-carrier lifetime and the spin-relaxation time inside the QDs. The understanding of these spin-relaxation mechanisms is essential to improve the performance of spin LEDs for future spin optoelectronic applications at room temperature under zero applied magnetic field.

Room-Temperature Spin-Transport Properties in an In0.5Ga0.5As Quantum Dot Spin-Polarized Light-Emitting Diode

“…The EL intensity exponentially increases with increasing injection current, which means that the external quantum efficiency (EQE) becomes higher. This behavior is in sharp contrast to a previous report on the (In, Ga)As QD-based spin LED measured at 10 K [21], which shows a decreased EQE with increasing injection current due to the state-filling effect in QDs [41]. Here, notably, state filling in QDs can be strongly suppressed as the temperature increases above 100 K [42].…”

“…Below 4 mA, the CPD value gradually increases with temperature, while the increase in CPD becomes weaker at 6 mA above 200 K. At 8 mA, the CPD value decreases slightly above 250 K. Therefore, the large difference in the EL CPD behavior between injection currents appears above 200 K. To understand these complex CPD behaviors as functions of temperature and injection current (bias voltage), one needs to extract the degree of spin conservation in the three-dimensional transport barriers and conversion efficiency from spin polarization of electrons to circular polarization of photons of the QDs separately. In general, the net EL spin polarization of the QD spin LED at a fixed injection current can be defined as [21] P EL (T) = P FM D tr (T)η QD (T),…”

“…The optical transfer of spin information of carriers in solid-state circuits is important for optical interconnections in future optospintronics integrations in information processing. Spin-polarized light-emitting diodes (spin LEDs) and laser diodes that can directly convert electron spins into circularly polarized light have been extensively studied by employing III-V semiconductor quantum wells (QWs) [2][3][4][5][6][7][8][9][10][11][12][13][14] and quantum dots (QDs) [15][16][17][18][19][20][21] as active layers. QDs are the most promising material in the active layer because of their significant suppression of carrierspin relaxation and temperature-independent luminescence properties that originate from the strong three-dimensional quantum confinement [22][23][24][25][26].…”

“…At high temperatures, spin relaxation in transport barriers is accelerated because of the well-known D'yakonov-Perel' (DP) mechanism [28]. A decrease in electroluminescence (EL) spin polarization with an increasing bias voltage is observed over a wide temperature range of 10-300 K [6,9,21]. Thus, a thorough understanding of the effect of temperature and bias voltage on the spin-transport properties in spin LEDs is required to achieve a better performance to ensure stable operations at room temperature.…”

“…Here, we investigate the temperature and bias-voltage dependence of the spin-transport properties in QD spin LEDs via a combination of circularly polarized EL and time-resolved photoluminescence (TRPL). Although the systematic temperature and bias-voltage dependence of the EL spin polarization for the QD spin LED has been studied recently [21], the previous study was conducted only at low temperatures below 110 K. This study focuses on EL spin-polarization properties at temperatures above 125 K and provides a comprehensive understanding of the effect of temperature and bias voltage on the spin-transport properties in the spin LED that have not been adequately explored. We find that the spin relaxation in the undoped GaAs barrier mainly influences the EL spin-polarization properties at room temperature.…”