Submonolayer Insertions Research Articles

Increasing spin–orbit torque efficiency by doping Pt: sub-monolayer insertions versus alloys

Increasing spin–orbit torque efficiency by doping Pt: sub-monolayer insertions versus alloys

Modulating Spin Polarization and Spin-Orbit Interaction by Submonolayer Engineering at LaAlO3/SrTiO3 Interfaces

Recently, the ${\mathrm{La}\mathrm{Al}\mathrm{O}}_{3}/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ (LAO/STO) interface has been highlighted as a major platform for spintronics, and its fundamental control of spin properties, therefore, becomes a key issue for application of this system. Here, we present a study showing the modulation of magnetic two-dimensional electron gases (2DEGs) with simultaneously enhanced spin-orbit interaction at the interface of LAO/STO by inserting ${\mathrm{La}\mathrm{Co}\mathrm{O}}_{3}$ submonolayers. At first, transport experiments provide evidence that Kondo behavior can be well controlled below about 13 K with the interlayer $\mathrm{Co}$ ions contributing as scattering centers. In addition, the systematic variation of the anomalous Hall effect obtained with increasing fraction of interfacial $\mathrm{Co}$ concentration below 10 K reveals that the spin polarization of 2DEGs is enhanced via submonolayer insertion. Simultaneously, we also observe an enlarged spin-orbit interaction at the buffered LAO/STO interface, resulting in a remarkably strong field-orientation-dependent magnetoresistance. Such tailored LAO/STO interfaces could potentially contribute to stronger spin-to-charge conversion responses and spin-torque measurements. Our observations indicate the subunit-cell insertion of functional layers to be a suitable route to tailor spin, orbital, and lattice interactions of the interfacial 2DEG, which makes the LAO/STO system an intriguing platform for spintronic applications.

Enhancing Spin-Orbit Torque by Strong Interfacial Scattering From Ultrathin Insertion Layers

Increasing dampinglike spin-orbit torque (SOT) is both of fundamental importance for enabling new research into spintronics phenomena and also technologically urgent for advancing low-power spin-torque memory, logic, and oscillator devices. Here, we demonstrate that enhancing interfacial scattering by inserting ultra-thin layers within a spin Hall metals with intrinsic or side-jump mechanisms can significantly enhance the spin Hall ratio. The dampinglike SOT was enhanced by a factor of 2 via sub-monolayer Hf insertion, as evidenced by both harmonic response measurements and current-induced switching of in-plane magnetized magnetic memory devices with the record low critical switching current of ~73 uA (switching current density of 3.6x10^6 A/cm^2). This work demonstrates a very effective strategy for maximizing dampinglike SOT for low-power spin-torque devices.

Peculiarities of the energy spectrum of InSb/InAs/InGaAs/InAlAs/GaAs nanoheterostructures revealed by room temperature photomodulation FTIR spectroscopy

Fourier-transform infrared photoreflectance was used to optimize design and growth temperature of complex III–V nanoheterostructures proposed for efficient mid-IR emitters. They comprised an InAs/InGaAs quantum well (QW) with InSb sub-monolayer insertions and were grown by molecular beam epitaxy on GaAs substrates via a convex-graded InAlAs metamorphic buffer layer. The room temperature photoreflectance spectra supported by the full 8 × 8 Kane model calculations showed that the triple InSb insertions enhance the probability of the main optical transitions in the QW, while lowering the growth temperature results in a red shift of the transitions and further increase in their probability.

Tuning the perpendicular magnetic anisotropy, spin Hall switching current density, and domain wall velocity by submonolayer insertion in Ta/CoFeB/MgO heterostructures

By submonolayer insertion of Au, Pt, or Pd into Ta/CoFeB/MgO/Ta heterostructures, we tune the perpendicular magnetic anisotropy and the coercive field of the ferromagnetic layer. We demonstrate that this has a major influence on the spin Hall switching current density and its dependence on the external magnetic field. Despite a rather small effective spin Hall angle of θSH≈−0.07, we obtain switching current densities as low as 2×1010 A/m2 with a 2 Å Au interlayer. We find that the Dzyaloshinskii-Moriya interaction parameter D is reduced with Au or Pd interlayers, and the perpendicular anisotropy field is reduced by an order of magnitude with the Pd interlayer. The dependence of the switching current density on the current pulse width is quantitatively explained with a domain wall nucleation and propagation model. Interface engineering is thus found to be a suitable route to tailor the current-induced magnetization switching properties of magnetic heterostructures.

Morphological and chemical instabilities of nitrogen delta-doped GaAs/(Al, Ga)As quantum wells

The microstructure and the element distribution across tensile-strained nitrogen δ-doped GaAs/(Al,Ga)As quantum wells (QW) are investigated by transmission electron microscopy. We find that the nitrogen sub-monolayer insertion results in a several monolayer thick Ga(As,N) layer with thickness and lateral composition fluctuations. The thickness and composition fluctuations are not arbitrary, but they are anticorrelated, i.e., the Ga(As,N) layer is thinner in areas of higher nitrogen content and vice versa. Thus, regardless of the specific position along the QW, the amount of incorporated nitrogen remains constant and close to its nominal value. The increase in the nitrogen content at the insertion promotes an anisotropic shape transition towards highly faceted three-dimensional structures. Our experimental observations indicate that the two-dimensional to three-dimensional morphological transition is determined by intrinsic factors associated with the different Ga-N and Ga-As bonds and hence occurs regardless of the epitaxial strain state of the layers.

Enhancement of electric-field-induced change of magnetic anisotropy by interface engineering of MgO magnetic tunnel junctions

Electric-field-induced modification of magnetic anisotropy is studied using tunnel magnetoresistance of the Co40Fe40B20/ MgO/ Co40Fe40B20 and Co40Fe40B20/ Hf (0.08 nm)/ MgO/ Co40Fe40B20 magnetic tunnel junctions. In both systems, the interfacial perpendicular magnetic anisotropy is increased with increasing electron density at the MgO interface. A quantitative comparison between the two systems reveals that the change of magnetic anisotropy energy with electric field is significantly enhanced in Co40Fe40B20/ Hf/ MgO/ Co40Fe40B20 compared to Co40Fe40B20/ MgO/ Co40Fe40B20. The sub-monolayer Hf insertion at the Co40Fe40B20/MgO interface turns out to be critical to the enhanced electric field control of the magnetic anisotropy, indicating the interface sensitive nature of the effect.

Tuning Between Quantum-Dot- and Quantum-Well-Like Behaviors in Type II ZnTe Submonolayer Quantum Dots by Controlling Tellurium Flux During MBE Growth

We report tuning of properties of type II nanostructures between quantum dot (QD)-like and quantum well (QW)-like behaviors in ZnSe layers with ZnTe submonolayer insertions, grown by migration-enhanced epitaxy. The sizes of QDs are estimated from magneto-photoluminescence (PL) measurements, which showed no significant change in the QD lateral size with increasing Te flux, indicating increase in QD density instead. The area density of QDs is estimated from the results of secondary-ion mass spectrometry measurements. It is determined that, in the sample grown using the highest Te flux, the electronic wavefunctions begin to overlap, leading to QW-like behavior before the formation of a full QW layer. This is also confirmed via temperature-dependent time-resolved PL, which showed significant change of excitonic lifetimes and binding energies of type II excitons.

Distinguishability of stacks in ZnTe/ZnSe quantum dots via spectral analysis of Aharonov-Bohm oscillations

A spectral analysis of the Aharonov-Bohm (AB) oscillations in photoluminescence intensity was performed for stacked type-II ZnTe/ZnSe quantum dots (QDs) fabricated within multilayered Zn-Se-Te system with sub-monolayer insertions of Te. Robust AB oscillations allowed for fine probing of distinguishable QDs stacks within the ensemble of QDs. The AB transition magnetic field, BAB, changed from the lower energy side to the higher energy side of the PL spectra revealing the presence of different sets of QDs stacks. The change occurs within the spectral range, where the contributing green and blue bands of the spectra overlapped. “Bundling” in lifetime measurements is seen at transition spectral regions confirming the results.

Strain-induced composition limitation in nitrogen δ-doped (In,Ga)As/GaAs quantum wells

The local element distribution across tensile-strained N δ-doped (In,Ga)As/GaAs quantum wells (QWs) is investigated by transmission electron microscopy. The sub-monolayer (ML) insertion results in a several monolayers thick (In,Ga)(As,N) layer with lateral composition fluctuations. We also find an inhomogeneous In incorporation across the QW, with a minimum In content, [In]min, exactly at the position of the N-insertion, where N content is maximum, [N]max. Regardless of the position along the QW, [N]max corresponds to [In]min so that an (In,Ga)(As,N) layer of this composition has a lattice parameter close to aGaAs. The impact of tensile strain on this complex chemical configuration is discussed.

Dynamic properties of AlGaAs vertical cavity surface emitting lasers with active region based on submonolayer InAs insertions

It has been shown that the use of submonolayer InAs insertions as an active region of AlGaAs vertical cavity surface emitting lasers make it possible to attain resonant frequencies as high as 17 GHz. In this case, single-mode devices with a smaller diameter of the current aperture make it possible to attain higher frequencies at lower current densities than those of multimode devices with a larger aperture diameter. The maximum error-free data transmission rate in the direct modulation mode in NRZ format is 20 Gb/s and is limited by the parasitic cutoff frequency. The high resonant frequency suggests that further optimization of the device design, directed to decreasing the electrical capacitance and resistances, the data transmission rate in lasers based on submonolayer insertions can be increased to 40 Gb/s.

Band gap engineering with sub-monolayer nitrogen insertion into InGaAs/GaAs quantum well

We investigate the band gap engineering with sub-monolayer nitrogen introduction (: N δ-doping) at the middle of InGaAs/GaAs quantum well. Using plasma-assisted molecular beam epitaxy, we prepare samples varying the introduced nitrogen between 0 and 0.4 monolayer. The growths are carried out at substrate temperature of 520∘C, comparatively higher than conventional substrate temperature for dilute nitride compounds. This induces the roughening of the growth front, slightly enhancing the nitrogen incorporation within the crystal. The as-grown and annealed samples show clear room temperature photoluminescence, indicating the band gap shrinkage depending on the amount of nitrogen.

The effect of AlAs submonolayer insertion on the oscillator strength of excitons in GaAs/AlGaAs quantum wells

Non-conventional GaAs/GaAlAs structures, with an AlAs monolayer inserted at different positions in wells, have been investigated to observe the behaviour of excitons in quantum wells. The energy and the oscillator strength for different transitions are calculated as a function of the probe position. The differential reflectivity of the spectrum for some samples is measured in order to test our theoretical simulations. We confirmed the observation of certain parity-forbidden transitions only in the most asymmetric samples.

Structure of Zn–Se–Te system with submonolayer insertion of ZnTe grown by migration enhanced epitaxy

We here report results of high resolution x-ray diffraction, x-ray reflectivity (XRR), as well as optical absorption and reflection measurements on ZnSe samples grown by molecular beam epitaxy, with insertion of planar (δ-) regions of both N as an acceptor dopant and Te as a “co-dopant” to facilitate a p-type doping. We note that to enhance the surface diffusion of Te, migration enhanced epitaxy was adopted in the growth of the “δ-layers;” i.e., Te is deposited in the absence of Zn flux. Structural parameters were extracted by simulating the experimental x-ray diffraction curves using a dynamical model. The results show that only the “δ-layers” (with submonolayer thickness) are rich in ZnTe, while the nominally undoped “spacers” have only a low Te concentration. Moreover, the morphology of the surface and interfaces are studied by XRR. Furthermore, the optical absorption and reflection results show that our samples largely preserve the optical properties of the host material (ZnSe). We note that our results, in particular those on the Te concentration, explain the observed good p-type doping of such samples.

Molecular beam epitaxy of type II InSb/InAs nanostructures with InSb sub-monolayers

Molecular beam epitaxial growth of InSb sub-monolayer insertions in an InAs matrix, exhibiting intense mid-IR photoluminescence (PL) up to room temperature, is reported. The InSb insertions are fabricated by an exposure of the InAs surface to an antimony Sb 4 flux. The nominal thickness of the insertions grown at different temperatures (400–485 °C) changes in the 0.6–1 monolayer range, resulting in the emission wavelength variation from 3.9 to 4.3 μm at 300 K. An integral PL intensity drop from 80 to 300 K does not exceed 20 times. The laser emission at a wavelength of 3.08 μm ( T = 60 K ) with the threshold current density of 3–4 kA/cm 2 under pulse injection pumping has been demonstrated in a hybrid p-AlGaAsSb/InAs/n-CdMgSe double heterostructure with the multiple type II InSb/InAs nanostructures in the active region.

Room-temperature 3.9–4.3 μm photoluminescence from InSb submonolayers grown by molecular beam epitaxy in an InAs matrix

We report on molecular beam epitaxial growth of InSb submonolayer insertions in an InAs matrix, exhibiting intense mid-IR photoluminescence (PL) up to room temperature (RT). The InSb insertions are fabricated by an exposure of InAs surface to an antimony Sb4 flux. The nominal thickness of insertions grown at different temperatures (TS=400–485°C) ranges from 0.6 to 1.4 monolayer, as estimated from x-ray diffraction measurements of InSb∕InAs multiple submonolayer structures. This gives rise to the variation of the emission wavelength within the 3.9–4.3 μm range at RT. An integral PL intensity drop from 77 K to RT does not exceed 20 times.

Growth of self-organized quantum dots for optoelectronics applications: nanostructures, nanoepitaxy, defect engineering

We report on the most recent results in novel nanoepitaxy approaches for applications in optoelectronics. The first class of self-organized semiconductor structures is related to spontaneous ordered surface nanofaceting. The most recent data on reflection of high-energy electron diffraction and high-resolution electron microscopy studies prove formation of corrugated superlattices and individual GaAs clusters (quantum dots) on (3 1 1)A AlAs surface with characteristic 3.2 nm lateral periodicity. Three-dimensional islands may also result in well-ordered surface structures and represent the second class. Recent results indicate that Volmer-Weber and Stranski-Krastanow growth can be realized in the same material system just by tuning the composition of the alloy substrate or cap layer. The other classes are related to two-dimensionally shaped quantum dots (QDs) formed by submonolayer insertions in the InAs–GaAs and other III–V or II–VI systems, to ordered nanostructures formed by spinodal and/or activated spinodal decomposition, for example, in the InGaAs–GaAs and InGaAsN–GaAs materials systems and to arbitrary combinations of different approaches. Uniform in size and in shape QDs may be used in a variety of novel devices. A different class of growth effects is related to dislocations. The elastic interaction of deposited material with dislocations may result in deposit repulsion from the dislocated regions. This effect allows selective elimination of dislocations using in situ annealing. In some cases, dislocation networks may be used for direct fabrication of coherent pedestals having nanoscale sizes for QD fabrication or for use as templates for further nanoepitaxy. We illustrate the application of defect-free self-organized QDs for GaAs-based edge- and surface-emitting lasers operating in the 1.3 μm range.

Self-Organized InGaAs Quantum Dots for Advanced Applications in Optoelectronics

We report on the fabrication of quantum dot (QDs) heterostructures for applications in optoelectronics. Different kinds of QDs are currently used: (i) three-dimensional quantum dots obtained by Stranski-Krastanow or Volmer-Weber growth in the InAs–GaAs material system, (ii) two-dimensionally-shaped QDs formed by submonolayer insertions in the InAs–GaAs and similar systems, (iii) GaAs QDs formed on a corrugated (311)A AlAs surface, (iv) and QDs obtained by spinodal decomposition and activated spinodal decomposition in InGaAs–GaAs and InGaAsN–GaAs material systems. Formation of uniformly sized and shaped QDs is possible in all of these approaches and is mostly governed by thermodynamics. Ultrahigh modal gain and giant optical nonlinearity can be achieved in dense arrays of very small QDs. Long wavelength (1.3–1.6 µm) emission can be achieved using large InAs QDs. Recent advances in growth have made possible the realization of GaAs 1.3 µm continuous wave (CW) vertical-cavity surface-emitting lasers (VCSELs) with ∼ 0.8 mW output power and long operation lifetime.

Fine Structure and Spin Relaxation of Excitons Localized at CdSe Sub-Monolayer Insertions in a ZnSe Matrix

Resonant spin-flip Raman scattering in tilted magnetic fields up to 14 T has been used to investigate excitons localised by the random disorder created by sub-monolayer insertions of CdSe in a ZnSe matrix. The field dependences of the Raman shifts, of the linewidths of the signals and of their intensities have been measured and fitted by a simple theoretical model and the g-factors and exchange parameters have been determined. It is concluded that spin-flip of the total exciton spin is one of the important mechanisms of spin relaxation.

Arrays of Two-Dimensional Islands Formed by Submonolayer Insertions: Growth, Properties, Devices

Ultrathin insertions of a narrow band-gap material in wide band-gap matrices represent a challenging medium in view of aspects of growth phenomena, unique optical properties, and non-trivial approaches for structural characterization. In a very general case ultrathin submonolayer insertions may form arrays of islands due to the principally discrete nature of the growth front. If the islands are large enough, these islands may act as locally formed quantum well (QW) insertions. If, however, the islands' size is comparable to the Bohr radius and the band-gap difference between the insert and the matrix material is large enough, quantum dots (QD) are formed. Realization of the first or the second regime depends on the surface properties of the substrate and the deposit, particularly, on the tensors of the intrinsic surface stress of both materials and on the lattice mismatch. In this work we consider in detail the case of ultrathin CdSe insertions in wide gap ZnMgSSe matrices: that the nominal thickness is chosen below the critical thickness for three-dimensional (3D) island formation. We give an overview of the experimental results available for these structures obtained by submonolayer or about-one monolayer CdSe depositions. A comparison with similar phenomena observed in conventional III-V and III-N systems is given and possible growth scenarios are discussed. We also discuss practical device applications of the structures based on ultrathin insertions for non-traditional devices. Examples of resonant waveguiding and lasing in edge geometry, of surface emitting lasers with low finesse cavities, and of broad-miniband high-frequency Esaki-Tsu anti-dot superlattices are given.