Formation of spin light-emitting diodes based on InGaAs/GaAs heterostructures containing ferromagnetic inclusions

The electroluminescence of light-emitting diodes based on heterostructures with InGaAs quantum wells and a delta 〈Mn〉 doped layer in the GaAs barrier is studied. It is shown that the diodes emit circularly polarized light with the degree of polarization depending on the applied magnetic field and on temperature. We assume that the temperature dependences of the degree of polarization are determined by a change in the mutual position of energy levels of Mn ions in the delta layer and of holes in a quantum well.     

Tunneling and injection in ferromagnetic structures InGaAs/GaAs/(Ga,Mn)As and InGaAs/<Emphasis Type="Italic">n</Emphasis> <Superscript>+</Superscript>-GaAs/(Ga,Mn)As

The spin light-emitting diodes based on InGaAs/GaAs heterostructures with a quantum well and an injector in the form of a (Ga,Mn)As ferromagnetic layer have been studied. It has been demonstrated that the efficiency of electron spin injection in the structure with a (Ga,Mn)As/n⁺-GaAs tunneling barrier can be controlled by varying the parameters of n⁺-GaAs. The spin injection control mechanisms associated with the thermal activation and tunneling of carriers have been discussed.     

The effect of the Mn delta layer on the photosensitivity spectra of structures with InxGa1 ? xAs/GaAs quantum wells

The effect of embedding a Mn delta layer in heteronanostructures with an In _x Ga_{1 − x} As/GaAs single quantum well on the photosensitivity spectra of the quantum well in Schottky barrier diode structures has been investigated. It is shown that the embedding of a Mn delta layer results in the broadening of exciton peak and decrease in the photosensitivity of the quantum well until complete disappearance of photosensitivity at the spacer thickness of 1.5 nm. The suppression of the quantum well photosensitivity is mainly due to the increase in the concentration of defects such as recombination centers in the quantum well during embedding of a Mn delta layer.     

Influence of delta〈Mn〉 doping parameters of the GaAs barrier on circularly polarized luminescence of GaAs/InGaAs heterostructures

The circular polarization of low-temperature electroluminescence of diodes based on heterostructures with an undoped quantum well InGaAs/GaAs and a delta〈Mn〉 layer in the GaAs barrier has been investigated. The possibility of changing the degree of circular polarization of the electroluminescence by varying the main structural parameters of diodes (spacer layer thickness, i.e., the distance between the delta〈Mn〉 layer and the quantum well, atomic concentration in the delta〈Mn〉 layer, and introduction of an additional acceptor delta layer) has been analyzed. It has been revealed that the variation in the spacer layer thickness is the most effective method for controlling the degree of circular polarization of the electroluminescence.     

Photoluminescence response of a quantum well to a change in the magnetic field of the Mn δ Layer in InGaAs/GaAs heterostructures

Ferromagnetic ordering of two types (depending on the sample geometry) is found to occur in a thin Ga_{1 − x} Mn _x As alloy layer (Mn δ layer) in heterostructures containing an InGaAs/GaAs quantum well. Singular samples in which the δ Mn layer is parallel to the (001) GaAs plane exhibit the “3/2” Bloch temperature dependence of magnetization, and vicinal samples in which the δ Mn layer deviates from the (001) GaAs plane exhibit a “percolation” ferromagnetic transition. The photoluminescence polarization of the quantum well is shown to follow changes in the magnetization of the Mn δ layer as a function of temperature according to the Bloch law in the singular samples and to a percolation law in the vicinal samples.     

The photomagnetic effect in Mn delta-doped heteronanostructures with an InGaAs/GaAs quantum well

The influence of embedding a Mn delta layer into heteronunostructures with an InGaAs/GaAs quantum well on the photomagnetic spectra was investigated. The recombination parameters of a number of model structures were determined by the photomagnetic effect and planar photoconduction at high illumination.     

Electron spin beats in InGaAs/GaAs quantum dots

Time-resolved picosecond spectroscopy is used for the first time to study optical orientation and spin dynamics of carriers in self-organized In(Ga)As/GaAs quantum-dot (QD) arrays. Optical orientation of carriers created by 1.2 ps light pulses, both in the GaAs matrix and wetting layer, and captured by QDs is found to last a few hundreds of picosecond. The saturation of electron ground state at high-excitation-light intensity leads to electron polarization in excited states close to 100% and to its vanishing in ground state. Electron-spin quantum beats in a transverse magnetic field are observed for the first time in semiconductor QDs. We thus determine the quasi-zero-dimensional electron g factor in In_0.5Ga_0.5As/GaAs QDs to be: |g _⊥|=0.27±0.03. Fiz. Tverd. Tela (St. Petersburg) 41, 871–874 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Investigation of the effects of spin injection of charge carriers from a ferromagnetic Ni(Co)/GaAs Schottky contact in quantum well heterostructures

The circularly polarized electroluminescence of quantum-confined InGaAs/GaAs heterostructures with a ferromagnetic Ni(Co)/GaAs Schottky contact has been investigated. It is shown that the high degree of circular polarization (to 42%) is due to the injection of spin-polarized holes from the ferromagnetic metal. The dependence of the spin injection efficiency on the type of the metal/GaAs interface and the quantum well depth has been analyzed. The spin coherence length of holes was found to be ≈80 nm at 1.5 K.     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.     

The dynamic field effect in Mn delta-doped quantum wells and In(Ga)As/GaAs quantum dot heteronanostructures

The dynamic field effect in Mn delta-doped epitaxial layers and quantum dimensional heteronanostructures of the p type with quantum wells and quantum dots was investigated. It was shown that an embedded Mn delta layer leads to the considerable capture of injected carriers on traps associated with the delta layer.     

Electron spin resonance in InGaAs/GaAs heterostructures with a manganese δ layer

The static and high-frequency dynamic magnetic properties and photoluminescence of two-dimensional semiconductor GaAs heterostructures containing an InGaAs quantum well and a thin manganese layer (δ layer) are studied. It is found that the Curie temperature is T _C ≈ 35 K and the magnetic anisotropy field of the ferromagnetic manganese δ layer is H _a ≈ 600 Oe. The spin resonance spectrum exhibits a line in weak fields (from −50 to 100 Oe), which is observed in the same temperature interval T < 40 K where the ferromagnetic ordering of the manganese δ layer occurs. This line is probably caused by the nonresonance contribution of the spin-dependent scattering of charge carriers to the negative magnetic resistance. The dependence of the degree of polarization of photoluminescence on the magnetic field also points to the ferromagnetic behavior of the manganese δ layer.     

Influence of the cap layer thickness on photoluminescence properties in strained InGaAs/GaAs single quantum wells

The influence of the GaAs cap layer thickness on the luminescence properties in strained In_0.20Ga_0.80As/GaAs single quantum well (SQW) structures has been investigated using temperature-dependent photoluminescence (PL) spectroscopy. The luminescence peak is shifted to lower energy as the GaAs cap layer thickness decreases, which demonstrates the effect of the GaAs cap layer thickness on the strain of InGaAs/GaAs single quantum wells (SQW). We find the PL quenching mechanism is the thermal activation of electron hole pairs from the wells into the GaAs cap layer for the samples with thicker GaAs cap layer, while in sample with thinner GaAs cap layer exciton trapping on misfit dislocations is dominated.     

Anomalous hall effect in Mn δ-doped GaAs/In0.17Ga0.83As/GaAs quantum wells with high hole mobility

Magnetic and magnetotransport properties of GaAs(δ〈Mn〉)/In_0.17Ga_0.83As/GaAs quantum wells with different Mn concentrations are studied. The delta-doped manganese layer has been separated from the GaAs quantum well with a spacer with an optimal thickness (3 nm), which has provided a sufficiently high hole mobility (≥10³ cm²V^?1 s^?1) in the quantum wells and their effective exchange with Mn atoms. It is found that the anomalous Hall effect (AHE) is exhibited only in a restricted temperature range above and below the Curie temperature, while the AHE is not observed in quantum wells with quasi-metallic conductivity. Thus, it is shown that the use of the AHE is inefficient in studying magnetic ordering in semiconductor systems with high-mobility carriers. The features observed in the behavior of the resistance, magnetoresistance, and Hall effect are discussed in terms of the interaction of holes with magnetic Mn ions with regard to fluctuations of their potential, hole transport on the percolation level, and hopping conduction.     

Thermal,transport, and magnetotransport properties of free charge carriers in Mn-doped structures with a GaAs/InGaAs/GaAs quantum well

The results of investigation of the magnetic and transport properties of a GaAs/InGaAs/GaAs quantum well delta-doped with carbon and manganese from different sides and containing a ferromagnetic phase are analyzed. A thermodynamic model is formulated and the composition of a system consisting of neutral Mn atoms, Mn ions, and holes in the quantum well is calculated for determining the concentration of free charge carriers. The contributions to the resistance from different mechanisms of hole scattering are calculated, and good agreement with the experimental temperature dependences of the resistance is attained. The calculated and experimental values of the negative magnetoresistance associated with variation in the contribution of scattering from magnetic ions of the spin-polarized system of charge carriers are found to be in quantitative agreement.     

Inter-Layer Energy Transfer through Wetting-Layer States in Bi-layer InGaAs/GaAs Quantum-Dot Structures with Thick Barriers

The inter-layer energy transfer in a bi-layer InGaAs/GaAs quantum dot structure with a thick GaAs barrier is studied using temperature-dependent photoluminescence. The abnormal enhancement of the photoluminescence of the QDs in the layer with a larger amount of coverage considering the resonant Forster energy transfer between the at 110K is observed, which can be explained by wetting layer states at elevated temperatures.     

Coherent spin dynamics of carriers in ferromagnetic semiconductor heterostructures with an Mn δ layer

The coherent spin dynamics of carriers in the heterostructures that contain an InGaAs/GaAs quantum well (QW) and an Mn δ layer, which are separated by a narrow GaAs spacer 2–10 nm thick, is comprehensively studied by the magnetooptical Kerr effect method at a picosecond time resolution. The exchange interaction of photoexcited electrons in QW with the ferromagnetic Mn δ layer manifests itself in magnetic-field and temperature dependences of the Larmor precession frequency of electron spins and is found to be very weak (several microelectron volts). Two nonoscillating components related to holes exist apart from an electron contribution to the Kerr signal of polarization plane rotation. At the initial stage, a fast relaxation process, which corresponds to the spin relaxation of free photoexcited holes, is detected in the structures with a wide spacer. The second component is caused by the further spin dephasing of energyrelaxed holes, which are localized at strong QW potential fluctuations in the structures under study. The decay of all contributions to the Kerr signal in time increases substantially when the spacer thickness decreases, which correlates with the enhancement of nonradiative recombination in QW.     

Investigation of Mn incorporation in fifteen-period InGaAs/GaAs quantum well system

A ferromagnetic ordering with a Curie temperature of 50 K of fifteen layer of InGaMnAs/GaAs multi quantum wells (MQWs) structure grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE) was found. It is likely that the ferromagnetic exchange coupling of sample with Curie temperature of 50 K is hole-mediated resulting in Mn substituting In or Ga sites. Temperature and excitation power dependent PL emission spectra of InGaMnAs MQWs sample grown at temperature of 170 °C show that an activation energy of Mn ion on the first quantum confinement level in InGaAs quantum well is 36 meV and impurity Mn is partly ionized. It is found that the activation energy of 36 meV of Mn ion in the QW is lower than the activation energy of 110 meV for a substitutional Mn impurity in GaAs. These measurements provide strong evidence that an impurity band existing in the bandgap due to substitutional Mn ions and it is the location of the Fermi level within the impurity band that determines Curie temperature.     

Emitting heterostructures with a bilayer InGaAs/GaAsSb/GaAs quantum well and a GaMnAs ferromagnetic layer

The radiative and magnetic properties of novel heterostructures with a bilayer InGaAs/GaAsSb/GaAs quantum well and a GaMnAs ferromagnetic layer are studied. The circular polarization of electroluminescent radiation is observed at temperatures from 10 to 160 K. The magnetic field dependences of the degree of circular polarization are nonlinear with a hysteresis loop at temperatures from 10 to 50 K, and they become linear at higher temperatures. The magnitude of polarization at the saturation magnetization of GaMnAs in the 2000 Oe field remains at the level of ~0.2%.     

Reduction in relaxation time due to ionized impurities in GaAs/AlGaAs quantum well structures

Time-resolved photoluminescence measurements in δ -doped GaAs/AlGaAs on the quantum well structures are performed to study effects of ionized impurities relaxation process of photoexcited carriers. It is theoretically shown that a thin quantum well with a δ -doping layer inserted in the barrier layer of double quantum wells enhances the impurity scattering rate significantly. Photoluminescence decay time in the δ -doped samples is found to decrease compared with the undoped samples.     

Epitaxially stabilized AgGaSe2 for high-efficiency spin-polarized electron source

High-quality spin-polarized electron source (SPES) is of fundamental importance in the investigation of spin-dependent phenomena. Generally speaking, an ideal material for SPES application should have both large spin–orbit and positive crystal-field splitting. Currently, almost all sources in use with accelerators are based on photoemission from GaAs and related materials such as strained GaAs grown on GaAsP or InGaAs grown on GaAs. Nevertheless, the reduced critical layer thickness of these strained films leads to poor material quality and, consequently, low quantum efficiency. Besides other ordered ternary semiconductor compounds, tetragonal chalcopyrite ternary compounds have also been considered. However, since all these compounds have zero or negative crystal-field splitting, the achieved polarization and quantum efficiency are rather low. Here we propose a new material, AgGaSe₂ in the CuAu phase, as a high-quality SPES. We show that it is possible to grow epitaxially strain-free AgGaSe₂ in the CuAu phase on ZnSe substrate. Since this material has a direct-band gap, a large spin–orbit splitting, as well as a large positive crystal-field splitting, it is predicted to be a promising material for SPES with 100% spin polarization.