Interfacial Disorder Research Articles

Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells

We demonstrate that solvent vapor annealing of small molecular weight organic heterojunctions can be used to independently control the interface and bulk thin-film morphologies, thereby modifying charge transport and exciton dissociation in these structures. As an example, we anneal diphenyl-functionalized squaraine (DPSQ)/C(60) heterojunctions before or after the deposition of C(60). Solvent vapor annealing of DPSQ before C(60) deposition results in molecular order at the heterointerface. Organic photovoltaics based on this process have reduced open circuit voltages and power conversion efficiencies relative to as-cast devices. In contrast, annealing following C(60) deposition locks in interface disorder found in unannealed junctions while improving order in the thin-film bulk. This results in an increase in short circuit current by >30% while maintaining the open circuit voltage of the as-cast heterojunction device. These results are analyzed in terms of recombination dynamics at excitonic heterojunctions and demonstrate that the optimal organic photovoltaic morphology is characterized by interfacial disorder to minimize polaron-pair recombination, while improved crystallinity in the bulk increases exciton and charge transport efficiency in the active region.

Crucial role of interfacial alloying on spin-transfer torque in magnetic tunnel junctions

We demonstrate that interfacial disorder in magnetic tunnel junctions (MTJs) has a dramatic effect on the bias behavior of both spin-transfer and field-like spin torques, leading to strong enhancement, sign reversal, and bias asymmetry. The underlying mechanism lies on the impurity-induced resonance energies, which can be selectively controlled with alloying and/or bias. The predictions explain for the first time the experimental results in ``dirty'' junctions and have important implications on exploiting further the alloying effect to optimize the current-induced magnetization switching.

Morphology induced magnetic instabilities on oxide antiferromagnetic surfaces

The role of interfacial disorder and roughness has been pointed out several times in ferromagnetic-antiferromagnetic exchange coupled systems. However, effect of surface morphology on magnetic properties of bare antiferromagnetic material has been mostly ignored. Here we report our experimental observations of magnetic instabilities on high temperature air annealed NiO(100) surfaces through treatment of successive vacuum and oxygen annealing above Neel temperature. Preferential spin-orientations are found to be governed by surface morphology with enhanced roughness and oxygen diffusion process. Stable domain distribution has been observed on smoother surfaces, which are found to be mostly inactive to these annealing treatments.

Effects of Lattice Polarity on Interfacial Space Charges and Defect Disorder in Ionically Conducting AgI Heterostructures

Heterostructured (β/γ)-AgI exhibits a spontaneous lattice polarization not accounted for in standard space-charge models. This polarization field dominates the positional variation of energies of isolated defects, and Ag(+) vacancies and interstitials are stabilized at alternate [β/γ] interfaces. This suggests enhanced Frenkel pair separation, analogous to electronic charge separation in polar semiconductor heterostructures. Stoichiometric systems are, however, characterized by associated Frenkel pairs due to strong V(Ag)-Ag(i) interactions and show no enhancement of defect numbers. In nonstoichiometric systems, lattice polarization does direct the distribution of the excess defect species, and defect-defect interactions enhance local Frenkel pair concentrations at interfaces, suggesting that nonstoichiometry is critical to the extreme room-temperature ionic conductivities observed in heterostructured AgI nanoplates.

Interfacial disorder and magnetic frustration in epitaxial Fe/Cr/Fe trilayers studied by thermoremanent magnetization

In order to investigate the relaxation phenomena in Fe/Cr(001) system with various degrees of interfacial magnetic frustration, we have epitaxially grown Fe/Cr/Fe(001) trilayers on MgO(001) substrate under several thermal conditions by using molecular beam epitaxy (MBE) technique, and measured the time dependence of the thermoremanent magnetization (TRM) in the absence of magnetic field after field cooling process from 300 K to a measuring temperature 250 K under a magnetic field of 100 Oe after a waiting time tw = 0 ∼ 100 min. The reflection high energy electron diffraction (RHEED) was employed for confirming the epitaxial growth and for evaluating degree of the interfacial flatness in each sample. From the experimental results, we found that 1) all the present epitaxially grown Fe/Cr/Fe(001) trilayers are ferromagnetic at 300 K, 2) the TRM shows the logarithmic time dependence with larger magnetic viscosity coefficient for the samples which have more disordered interface relative to the other samples, and 3) the most disordered sample shows clearly tw dependence as observed in spin glasses. These results indicate that the interfacial magnetic frustration in the epitaxial Fe/Cr(001) system increases with increasing the interfacial disorder.

Scanning tunneling microscopy of a disordered Alq3–metal interface

Scanning tunneling microscopy is used to observe the structure of the first monolayer of Alq3 on Cu(1 1 0). Individual molecules can be resolved at very low coverage where there is also evidence of significant surface mobility. As Alq3 surface coverage increases, molecules aggregate by forming pairs and chain-like structures without ever establishing long range order. Interfacial disorder is maintained even after annealing to 590 K. The tendency to form chain-like aggregates is attributed to anisotropic intermolecular interactions. Anisotropies in these interactions are implicated in chain formation by illustrative density functional theory calculations and the role of direct dipolar interactions, anisotropic van der Waals interactions, and substrate-mediated interactions is discussed.

Raman tensor elements for wurtzitic GaN and their application to assess crystallographic orientation at film/substrate interfaces

This study is aimed at establishing a method of polarized/confocal Raman spectroscopy capable of quantitatively assessing crystallographic orientation in wurtzitic GaN with a micron-scale resolution. First, Raman selection rules are explicitly put forward from a theoretical viewpoint in their complete form; then, experimentally retrieved intensities of the Raman signal as a function of Euler angles are fitted to the obtained theoretical dependencies in order to quantify a set of Raman tensor elements using experiments on known crystallographic planes of a wurtzitic GaN single-crystal. According to the above two procedures, a spectroscopic algorithm, incorporating the use of Raman tensor elements and Euler angles in tandem, becomes available for estimating unknown crystallographic orientations. As an application of the developed method, a confocal Raman probe was used to non-destructively unfold the relative orientation of a wurtzitic GaN epilayer with respect to (0001)-oriented sapphire substrate. The microscopic distribution of tilt/twist angles in space for the GaN lattice could be measured in a bulk region of the sample with no cut or other sample manipulations being necessary. Polarized/confocal Raman spectroscopy provides a practical and convenient tool for characterizing on the micron scale the interfacial disorder that affects energy transport phenomena at GaN/substrate interfaces.

Effect of disorder on spin-transfer torque in magnetic tunnel junctions

We have generalized the nonequilibrium Green’s functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, 〈T∥〉, in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average 〈T∥〉 as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average 〈T∥〉 in terms of the interplay of average spin current densities in collinear configurations is valid even in the presence of disorder. This explains the strong enhancement and sign reversal of 〈T∥〉 in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons.

Study of interfaces in organic semiconductor heterojunctions

The defect structure at the organic heterojunctions is studied using slow positron beam. The structural and electronic properties of heterojunctions are of technological and fundamental importance for understanding and optimization of electronic processes in organic devices. Interface trap centres play a significant role in the electrical conduction through the junctions. Depth dependent Doppler broadened annihilation measurements have been carried out in p- and n-type organic semiconductor thin films (30–80 nm) both single as well as multilayers grown on quartz substrate. The objective of the present study is to investigate the defect structure and to understand the behavior of positrons at the charged organic interfaces. Our result shows the sensitivity of positrons to the interfacial disorders that may be a convoluted effect of the presence of defects as well as the influence of the charge dipole in multilayers.

Dual behavior of antiferromagnetic uncompensated spins in NiFe/IrMn exchange biased bilayers

We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias.

Sensitivity of Ag/Al interface specific resistances to interfacial intermixing

We have measured a Ag/Al interface specific resistance, 2ARAg/Al(111)=1.4 fΩ m2, that is twice that predicted for a perfect interface, 50% larger than for a 2 ML 50%–50% alloy, and even larger than our newly predicted 1.3 fΩ m2 for a 4 ML 50%–50% alloy. Such a large value of 2ARAg/Al(111) confirms a predicted sensitivity to interfacial disorder and suggests an interface ≥4 ML thick. From our calculations, a predicted anisotropy ratio, 2ARAg/Al(001)/2ARAg/Al(111), of more than 4 for a perfect interface should be reduced to less than 2 for a 4 ML interface, making it harder to detect any such anisotropy.

Consequences of interfacial Fe-O bonding and disorder in epitaxial Fe/MgO/Fe(001) magnetic tunnel junctions

We report on a thorough experimental investigation of the influence of an FeO layer at the Fe/MgO interface on the magnetotransport properties in epitaxial Fe/MgO/Fe(001) magnetic tunnel junctions. Interfacial oxygen is introduced by adsorbing ${\text{O}}_{2}$ on the bottom Fe(001) layer. The morphology and composition of the FeO film are investigated by means of scanning tunneling microscopy, reflection high-energy electron diffraction, and x-ray photoelectrons spectroscopy. We show that the amount of interfacial oxygen can be precisely tuned and that either ordered or disordered FeO layers can be grown. Both Fe-O bonding and interfacial disorder strongly alter the conductances of junctions. However, the tunnel magnetoresistance remains large $(>85%)$ even for large oxygen exposure. Interfacial Fe-O bonds are found to lower the parallel conductance in agreement with theoretical predictions.

First-principles study of spin-transfer torques in layered systems with noncollinear magnetization

An efficient first-principles method was developed to calculate spin-transfer torques in layered system with noncollinear magnetization. The complete scattering wave function is determined by matching the wave function in the scattering region with the Bloch states in the leads. The spin-transfer torques are obtained with the aid of the scattering wave function. We applied our method to the ferromagnetic spin valve and found that the material (Co, Ni, and ${\text{Ni}}_{80}{\text{Fe}}_{20}$) dependence of the spin-transfer torques could be well understood by the Fermi surfaces. Ni has much longer spin injection penetration length than Co. Interfacial disorder is also considered. It is found that the spin-transfer torques could be enhanced by the interfacial disorder in some system.

Magnetic properties of La0.6Sr0.4MnO3 thin films on SrTiO3 and buffered Si substrates with varying thickness

La 0.60 Sr 0.40 MnO 3 (LSMO) thin films of varying thickness from 12 to 55 nm were deposited using the pulsed-laser deposition technique onto single-crystalline SrTiO3 (STO) and STO-buffered Si substrates. The Tc of LSMO films grown on STO-buffered Si substrates decreases faster than films directly grown on STO with decreasing film thickness. The LSMO/STO film with thickness of 55 nm shows Tc at about 360 K, which is close to the bulk value, whereas Tc LSMO film on STO-buffered Si film of similar thickness is reduced to 320 K. This difference is attributed to the strain and interfacial disorders in LSMO film on STO/Si. The film surface morphology is influenced by the film thickness. Oxygenation of LSMO films on STO-buffered Si affects the Tc minimally but improved the overall magnetization of the films due to better oxygenation, which is also the case for postannealing the sample at elevated temperatures. The thermomagnetic history effects observed in LSMO films of STO-buffered Si indicate the presence of inhomogeneity, mostly at the interface, which influences the magnetic properties significantly.

Theory of interlayer magnetic coupling in nanostructures with disordered interfaces

Multiple scattering theory of interlayer coupling in nanostructures with disordered alloylike interfaces is developed. Interface-localized potential responsible for the specular electron reflection from a perfect interface (potential step) is found in the framework of multiple scattering theory based on Green's functions approach. This allowed obtaining of a general expression relating the interlayer coupling energy to electronic density of states change due to interferences of electronic waves resulting from multiple reflections at disordered interfaces. In the case of a small spacer electron confinement (small scattering), the interlayer exchange coupling between two ferromagnets separated by a nonmagnetic spacer is found in the coherent potential approximation for electrons scattering at the disordered interfaces. The influence of disorder on the oscillating with a metallic spacer thickness exchange coupling is analyzed. The main effect is found to be a phase shift and amplitude change of these oscillations with the change of the parameters describing specular and diffuse spin-dependent electron scattering caused by the interfacial disorder.

An estimation of the magnetic disorder at Fe/Cr interfaces in coupled and non-coupled Fe/Cr multilayers

The antiferromagnetic coupling at the Fe/Cr interfaces, inferred from the orientation of the Cr magnetic moments, is used to estimate the magnetic disorder resulting from the interfacial roughness in Fe/Cr multilayers. A crossover from in-plane to out-of-plane orientation of Cr moments depends on the energy cost in either case: (i) to break the interfacial Fe–Cr antiferromagnetic coupling or (ii) having sites with frustrated Cr–Cr antiferromagnetic coupling in the Cr interlayers. A quantitative model of the magnetic frustration due to interfacial disorder in Fe/Cr multilayer systems is described. The step edge density, or terrace size, required to break the interfacial Fe–Cr coupling and destroy the Fe–Fe interlayer exchange coupling is estimated.

In-plane magnetic anisotropies of sputtered Co0.7Fe0.3 films on AlGaAs(001) spin light emitting diode heterostructures

The in-plane magnetic properties of Co0.7Fe0.3 films sputtered onto Al0.1Ga0.9As and Al0.1Ga0.9As∕GaAs epilayers are consistent with a strongly oriented bcc crystal structure with clean metal-semiconductor interfaces. However, the interface induced uniaxial magnetic anisotropy is oriented along one of the in-plane ⟨100⟩, rather than ⟨110⟩, as is the case in molecular beam epitaxy—grown films. Resonant x-ray measurements show interfacial magnetic disorder in films on Al0.1Ga0.9As, which accounts for the difference in magnetic anisotropy behavior between samples on Al0.1Ga0.9As and Al0.1Ga0.9As∕GaAs epilayer substrates, and which may have significant consequences for the spin-injection efficiency across such interfaces.

Controlling interfacial disorder and strain of W-structured type-II superlattices using As 2 flux

We have investigated how the photoluminescence varies with strain induced by interfacial disorder in W-structured type-II superlattices (WSLs). WSLs with compressive, tensile, and very low strain were grown using varying As 2-to-In flux ratios during InAs layer growth. The strain was determined using X-ray diffraction, and the atomic-scale WSL structure was characterized by cross-sectional scanning tunneling microscopy. The changes in the interfacial structures with flux ratio and their effects on the optical properties can be qualitatively understood in terms of competing changes in growth surface morphology and anion cross-incorporation and their net effects on SL strain.

High Resolution XRD Studies of Ion Beam Irradiated InGaAs/InP Multi Quantum Wells

AbstractMulti quantum wells of InGaAs/InP grown by metal organic chemical vapor deposition have been irradiated using swift heavy ions. Irradiation has been performed using 150MeV Ag and 200MeV Au ions. Both as-grown and irradiated samples were subjected to rapid thermal annealing at 500 and 7000C for 60s. As-grown, irradiated and annealed samples were subjected to high resolution x-ray diffraction studies. Both symmetric and asymmetric scans were analyzed. The as-grown and Ag ion irradiated samples show sharp and highly ordered satellite peaks whereas, the Au ion irradiated samples show broad and low intense peaks. The higher order satellite peaks of the annealed samples vanished with increase of annealing temperature from 500 to 7000C, indicating mixing induced interfacial disorder. Annealing of irradiated samples show higher mixing and disorder and no higher order satellite peaks were observed. Negligible strain was observed after high temperature annealing of as grown samples. Strain values calculated from the X-ray studies indicate that the irradiated samples have higher strain which has been reduced upon annealing. This indicates that the annealing induced mixing occurs maintaining the lattice parameter close to that of the substrate. The effect of electronic energy loss for interface mixing has been discussed in detail. The role of incident ion fluence in combination with the electronic energy loss will also be discussed in detail. The results have been compared with the literature and discussed in detail.

Granular Co/Ag multilayers with crystalline coherence

We have made Co/Ag multilayers with ten repeats on MgO (1 0 0) single-crystal substrates at room temperature by magnetron sputtering from independent sources and keeping the Ag layers as thick as 40 Å. The analysis of the X-ray diffraction indicates a coherent structure of Ag layers with a lattice parameter 2.355 Å close to the bulk FCC (1 1 1) spacing and Co layers with 2.13 Å spacing corresponding to HCP (1 0 0) planes. Other experiments show the granular in-plane morphology and a large interfacial disorder. These Co layers develop in-plane magnetization and electrical magneto-transport whose properties are thickness dependent.