Detection of electrical spin injection by light-emitting diodes in top- and side-emission configurations

Abstract

Detection of the degree of circular polarization of the electroluminescence of a light-emitting diode fitted with a spin injecting contact (a spin-LED) allows for a direct determination of the spin polarization of the injected carriers. Here, we compare the detection efficiency of (Al,Ga)As spin-LEDs fitted with a (Zn,Be,Mn)Se spin injector in top-and side-emission configuration. In contrast with top emission, we cannot detect the electrical spin injection in side emission from analysing the degree of circular polarization of the electroluminescence. To reduce resonant optical pumping of quantum-well excitons in the side emission, we have analysed structures with mesa sizes as small as 1 µm.-2 -The efficient electrical injection and detection of a spin polarized current into semiconductor heterostructures is a key element for the development of semiconductor spintronics [1,2]. Originally, much effort was devoted to detecting magnetoresistance changes in ferromagnet / semiconductor / ferromagnet interfaces [3,4], where, because of the very small effects observed, it turned out to be difficult to find evidence for spin injection [5]. An important step forward could only be made when zincblende-based light-emitting diodes (LED) (e.g. in the (Al,Ga)As system) were used as detectors for electrical spin injection. In this scheme, already proposed in the 70's by Aranov and Pikus [6], the spin polarized current is converted into circularly polarized electroluminescence; the degree of circular polarization of the electroluminescence of the spin-LED is directly proportional to the spin polarization of the carriers in the detection quantum well (QW). This approach has yielded evidence for spin injection using semi- [7] or ferro-magnetic [8] semiconductors, as well as ferromagnetic metal [9,10,11] contacts on GaAs based LEDs.Spin-LEDs are typically used in a top-emission measurement-geometry, where the external magnetic field is parallel to the current path, and parallel to the wave vector of the emitted light [7,9,10,12,13 ], as depicted in Fig. 1(a).However, for ferromagnetic materials like (Ga,Mn)As where the magnetic moment of the manganese system is typically perpendicular to the growth axis, side-emitted electroluminescence has been used to infer the spin polarization [8]. In this geometry the magnetic field is also parallel to the wave vector of the emitted light, -3 -but at the same time perpendicular to the growth axis of the structure and the confinement axis of the QW (see Fig. 1(a)). This quasi-Voigt geometry gives rise to different selection rules for optical transitions in GaAs based QWs [14] compared with those applicable for top emission [15]. If the selection rules are strictly valid, they may prohibit the determination of electron spin polarization by circular polarized light. Recent work on (Ga,Mn)As based spin-LEDs reveals a remarkable influence of the measurement geometry on the observed spacer layer thickness dependence of the circular polarization of the electroluminescence [16].In view of this, it se...

Spintronics: Fundamentals and applications

“…The electroluminescence in a quantum well was measured perpendicular to the growth direction [the easy magnetization axis of (Ga,Mn)As and the applied magnetic field were both perpendicular to the growth direction]. The corresponding relation between the P circ and hole density polarization P p is not straightforward; the analysis was performed only on the electroluminescence [for possible difficulties see Fiederling et al (2003)]. A small measured signal (P circ ϳ1% at 5 K), consistent with the expectation for holes as the injected spin-polarized carriers, was also obtained in an additional experiment .…”

Semiconductor spintronics

“…Other spintronic devices include a proposed scheme for reconfigurable logic , a room temperature spin-transference device (Dery et al, 2006), electron spin resonance transistor (Vrijen et al, 2000), a 2d channel spin valve controlled by ferromagnetic gates (Ciuti et al, 2002), spin capacitor (Žutić et al, 2001a;Datta, 2005), spin Esaki diodes (Kohda et al, 2001;Johnston-Halperin et al, 2002), spin lasers (Rudolph et al, 2003), unipolar magnetic diodes (Flatté and Vignale, 2001), spin light-emitting diodes (Fiederling et al, 1999(Fiederling et al, , 2003Ohno et al, 1999), field-effect magnetic switch Matsukura et al, 2002a), or spinflip or single electron spin-valve transistors (Brataas et al, 2006;Wetzels et al, 2005). In the following section we describe in detail spintronic devices based on magnetic resonant tunneling.…”

Spin‐injection in semiconductors: materials challenges and device aspects

“…Optical detection allows us to isolate the injection process, and assess directly the injected electron spin polarization. The preferred way is to use surface-normal emission [2], for which a perpendicular spin-component is required. This can be obtained by applying a strong out-of-plane magnetic field to manipulate the magnetization direction of the magnetic contact, or, in our case, by applying a smaller magnetic field under an angle of 45° and manipulating the spins inside the semiconductor.…”