Interfacial Intermixing Research Articles

X-ray photoelectron-diffraction study of intermixing and morphology at the Ge/Si(001) and Ge/Sb/Si(001) interface.

We used the XPD (x-ray photoelectron diffraction) and AED (Auger electron diffraction) from Ge core levels to probe the crystalline structure of 3 and 6 ML of Ge epitaxially grown by molecular-beam epitaxy on the Si(001) surface. In order to check the film tetragonal distortion and the pseudomorphic growth morphology, we used two different temperatures of the substrate during the deposition: room temperature and 400 \ifmmode^\circ\else\textdegree\fi{}C. Evidence for an interfacial intermixing has been found by means of the observation of the angular behavior of the intensity of the emitted electrons. We also investigated the effects of Sb as a surfactant on such an interface. In this case indications of a laminar growth of strained Ge overlayer with reduced intermixing is obtained when 1 ML of Sb is predeposited on the substrate. Furthermore making use of a multiple-scattering approach to reproduce the experimental XPD patterns, a higher amount of accessible information on the morphology of the interface, beyond the determination of the strain content, is obtained. \textcopyright{} 1996 The American Physical Society.

XPD study of atomic intermixing at the Ge/Si(001) interface

XPD (X-ray photoelectron diffraction) was used to probe the crystalline structure of 3 ML and 6 ML of Ge epitaxially grown on Si(001) surface at room and 400°C temperatures in order to check the tetragonal distortion associated with the pseudomorphic growth. Strong evidences for an interfacial intermixing and crystalline growth even at the room temperature of the substrate has been found by means of the observation of the angular behavior of the Ge 3d photoelectron peak.

Anomalous perpendicular magnetism in Ni/Cu(001) films and the effects of capping layers

X-ray magnetic circular dichroism (XMCD) measurements on wedges of Ni grown on Cu(001) are used to investigate magnetic properties in Ni/Cu(001) ultrathin films. A sharp transition from in-plane to perpendicular magnetization is found near 7 ML, and a gradual transition back to in-plane magnetization begins near 37 ML. The critical thickness for epitaxial growth, 13 ML, is determined from a rapid rise in the coercive field versus film thickness. Both transitions in the direction of easy axis are well explained by considering the effects of the surface, shape, and the strain-induced magnetoelastic anisotropies. The critical layer thickness of 13 ML plays a critical role in understanding the transition near 37 ML. Capping the Ni wedge with 2 ML of Co increases the magnitude of the surface anisotropy, forcing the magnetization to remain in plane for thickness up to at least 18 ML. Addition of an Fe capping layer has no effect on the direction of magnetization, suggesting the importance of interface anisotropies or intermixing.

Interface intermixing and magnetoresistance in Co/Cu spin valves with uncoupled Co layers

The interpretation of experiments on the effect of interface intermixing on the giant magnetoresistance (GMR) effect in antiferromagnetic-coupled multilayers can be complicated by the fact that interface intermixing also changes the coupling strength; therefore, we have grown an artificially intermixed region in Co/Cu spin valves with uncoupled Co layers. The structure we used was a newly engineered spin valve composed of 100 Å Co+6 Å Ru+25 Å Co+40 Å Cu+100 Å Co. Here the Ru layer provides an antiparallel alignment of the Co layers and the Cu layer decouples the upper two Co layers. An intermixed CoCu region has been grown at the Cu/Co interface and in some cases also at the Co/Cu interface by alternately sputtering 1 Å Co and 1 Å Cu. X-ray measurements confirm the existence of an intermixed region, although no reduction of magnetic moment is observed as is reported for homogeneous sputtered Co0.5Cu0.5 alloys. This indicates the existence of Co clusters in the intermixed regions. There is no difference in GMR between an intermixed layer of thickness t at one Co/Cu interface or two intermixed layers of thickness t/2 at both Co/Cu interfaces. Thus, it seems that the total thickness of the intermixed regions is decisive for the magnitude of the GMR. Because G, ΔG, and ΔG/Gap all show a gradual decrease when the nominal thickness of the intermixed region increases from 0 to 36 Å, this indicates that there is no strong spin-dependent scattering in this region. This is in agreement with calculations on a model bilayer Co/Cu/Co with the Camley–Barnas model.

Investigation of interface intermixing and roughening in low-temperature-grown AlAs/GaAs multiple quantum wells during thermal annealing by chemical lattice imaging and x-ray diffraction

The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum well (MQW) structures grown at a low substrate temperature (310 °C) by molecular beam epitaxy has been investigated using chemical lattice imaging and high resolution x-ray diffraction. The low-temperature-grown MQW is of high crystalline quality comparable to the standard-temperature-grown MQW. However, significant interface roughening and intermixing occurs at the quantum well heterointerface when the structures are annealed beyond 700 °C. The effective activation energy for interdiffusion is estimated as 0.24±0.07 eV. The structural properties observed here suggest that the excess arsenic associated with the low-temperature growth substantially enhances the diffusion of column III vacancies across an interface, which leads directly to intermixing of Al and Ga.

Theory of electronic and optical properties of [formula omitted] superlattices

The theory of electronic and optical properties of ( Si) n ( Ge) m superlattices (SLSs), within the framework of the tight-binding approximation, is presented. Results for the band structure, the refractive index, and the absorption coefficient are given. The important role of interface intermixing is studied. Finally a comparative study between the properties of Si Ge SLSs and the corresponding alloys is presented.

Effect of energetic particle bombardment during sputter deposition on the properties of exchange-biased spin-valve multilayers

The effect of energetic particle bombardment during sputter deposition on the physical (magnetoresistance and interlayer coupling) and microstructural (roughness and interface thickness) properties of exchange-biased spin-valve multilayers is investigated. An increasing pressure leads, through enhanced stopping of energetic particles by collisions with the background gas, to a decrease of the interfacial intermixing by collisions during growth, and to a more rough, void-rich structure. These microstructural changes lead to an increase of the transmissivity of the interfaces for conduction electrons, as well as to an increase of the ferromagnetic interlayer coupling with increasing pressure.

Interface intermixing influence on the electronic and optical properties of Si/Ge strained-layer superlattices.

The influence of interface intermixing on the electronic and optical properties of strain-symmetrized Si/Ge superlattices grown on a ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$(001) alloy surface is studied. We find that interface intermixing plays an important role in the determination of the properties near the gap. In particular, it increases the fundamental gap of the superlattice and produces a variation of the absorption coefficient near the gap of the form \ensuremath{\alpha}(E)\ensuremath{\propto}(E-${\mathit{E}}_{\mathit{g}}$${)}^{2}$. A simple alloy layer model with approximately four intermixed Si-Ge atomic layers at the interface can account for the experimental observations.

Effects of Interface Intermixing on the Magnetoresistance of Spin Valves with Uncoupled Co-Layers

ABSTRACTWe have studied the effect of an artificially intermixed region grown at the interfaces of Co/Cu spin valves with uncoupled layers. Two different structures are used: exchange-biased spin valves and engineered spin valves in which two layers are antiferromagnetically coupled and a third layer, on top of this system, is not coupled to the other two. It is shown that structural effects, induced by variation of the deposition parameters and by the intermixing can play an important role. Since the present study uses sputtered layers an intrinsic initial intermixing of 4-5Å is already present. For both types of spin valves Gp, ΔG and ΔR/R all show a gradual decrease when the nominal thickness of the total intermixed region is enlarged from 0 to 36Å. Also when the initial degree of intermixing is decreased by sputtering at higher Arpressure, Gp, ΔG and ΔR/R still show a gradual decrease as a function of intermixed layer thickness. Combined with the fact that there is no difference between an intermixed region of thickness t at one Co/Cu interface or intermixed regions of thickness t/2 at two interfaces, this indicates that the electron scattering in the intermixed region is predominantly spin independent, although this region preserves a magnetic moment.

Vibrational properties of Si/Ge superlattices: Theory and in-plane Raman scattering experiments

Phonons in short-period (001)-Si n Ge n Superlattices (SL's) have been studied both theoretically, by a first principles approach including strain and interface intermixing, and experimentally by micro-Raman spectroscopy where in-plane scattering geometries allow the observation of both longitudinal (L) and transverse (T) modes. Experimental data are found to deviate considerably from theoretical predictions for SL's with ideally sharp interfaces, both in frequency of higher-order confined Si-like modes and in the polarization dependence of the SiGe-like “interface” peak (lineshape and L-T splitting). Supercell calculations representing interface intermixing within a simple model by 2–3 monolayers of SiGe alloy at the interfaces are found to reproduce these major experimental findings. Measurements of Raman resonance profiles of various SL phonon modes strongly confirm their calculated different spatial localization.

Metal-semiconductor contacts: electronic properties

Rectification in metal-semiconductor contacts was first described by Braun in 1874. We owe the explanation of this observation to Schottky. He demonstrated that depletion layers exist on the semiconductor side of such interfaces. The current transport across such contacts is determined by their barrier heights, i.e., the respective energy difference between the Fermi level and the edge of the majority-carrier band. Since Schottky had published his pioneering work in 1938 the mechanisms, which determine the barrier heights of metal-semiconductor contacts, have remained under discussion. In 1947, Bardeen attributed the failure of the early Schottky-Mott rule to the neglect of electronic interface states. The foundations for a microscopic description of interface states in ideal Schottky contacts was laid by Heine in 1965. He demonstrated that a continuum of metal-induced gap states (MIGS), as they were called later, derives from the virtual gap states of the complex semiconductor band-structure. Neither this MIGS model nor any of the many other monocausal approaches, the most prominent is Spicer's Unified Defect Model, can explain the experimental data. In 1987, Mönch concluded that the continuum of MIG states represents the primary mechanism, which determines the barrier heights in ideal, i.e., intimate, abrupt, and homogeneous metal-semiconductor contacts. He attributed deviations from what is predicted by the MIGS model to other and then secondary mechanisms. In this respect, interface defects, structure-related interface dipoles. interface strain, interface compound formation, and interface intermixing, to name a few examples, were considered.

Interface mode in Si/Ge superlattices: Theory and experiments.

The Raman peak associated with Si-Ge vibrations in short-period Si/Ge (001) superlattices is studied both experimentally and theoretically. On the experimental side, we use a microprobe technique which allows us to investigate the longitudinal (L) and transverse (T) spectra, and find an unexpected behavior of the line shape and L-T splitting of this peak. By means of first-principles calculations, taking into account both strain and interface intermixing, we show that such behavior is consistent with the picture of an intermixed alloy layer at the interfaces, and we are able to identify the character and spatial localisation of the individual atomic clusters contributing to the vibrations

Vibrational properties of Si/Ge and alpha -Sn/Ge superlattices with intermixed interfaces.

It is found that Raman spectra of Si/Ge and \ensuremath{\alpha}-Sn/Ge[001] superlattices are drastically modified by the introduction of interface intermixing. Some new peaks appear due to intermixing. The calculations clearly show that the peaks around 400 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in Si/Ge and 230 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in \ensuremath{\alpha}-Sn/Ge superlattices observed in Raman experiments originate from the alloyed layers formed at the interface. It is suggested that relative frequencies and relative intensities of some Raman peaks could be used as a measurement of the composition profiles and the degree of interface perfectness.

Interface and structure characterization of Co/Re multilayers using RBS/channeling and hyperfine interactions

Cobalt/rhenium multilayers were prepared by rf and dc sputtering and studied using RBS and hyperfine interaction techniques. The layer thickness is determined with a precision of 3Å, and the interface intermixing is less than 5Åup to 773 K. Solid state reactions occur for higher temperatures. The hyperfine field value confirms the hcp structure of the Co. A small shift of the hyperfine field measured as function of the cobalt layer thickness is explained by the effect of lattice stresses.

Application of High-Resolution Electron Microscopy to the Study of Magnetic Thin Films and Multilayers

ABSTRACTThe performance of a wide range of modern magnetic thin-film materials for information storage is found to depend dramatically on grain nucleation and coherence at interfaces. Three such systems, Fe/Pt Multilayers, CO/Pt Multilayers, and CoCrTa/Cr bilayers are considered here. High-resolution electron Microscopy (HREM) shows that ordered CoPt and FePt L1o structures can be formed and oriented, using coherence with the substrate, with their magnetically easy c-axes perpendicular to the film plane. This orientation results in the perpendicular magnetization required for Magneto-optic recording Media. Different sputtering gases used in producing CO/Pt Multilayers result in varying degrees of interfacial intermixing, which is shown to strongly affect the perpendicular magnetic anisotropy energy. The required high in-plane coercivity of longitudinal recording media is also correlated with the observed interface coherence and the resulting oriented growth of CoCrTa with the magnetically hard <110> axis perpendicular to the film plane. Many features of the microstructure can be observed directly at the atomic level by HREM, and so the usefulness of this technique is emphasized in this paper.

X-Ray Investigation of Strain Relaxation in Short-Period SimGen Superlattices using Reciprocal Space Mapping

AbstractThe optoelectronic properties of SimGen strained layer superlattices (SLS's) depend strongly on the structural perfection. We used double crystal and triple axis x-ray diffractometry to characterize the structural properties of short period Si9Ge6 SLS's grown on about lμm thick step-graded SiGe alloy buffers. As grown SLS's and samples annealed subsequently at 550°C, 650°C and 780°C for 60 mmn were investigated. Precise strain data were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points. These data were used as refined input parameters for the dynamical simulation of the integrated intensity along the qll[004] direction. Annealing causes interdiffusion as indicated by the decreasing superlattice (SL)-satellite peak intensities and by the change of the Si/Ge thickness ratio. However, the full width at half maximum of the SL satellite peaks does not change significantly with annealing up to 650°C. The in-plane SL lattice constant in both samples is increased only slighty by annealing (< 9×10−3 Å). Consequently the interface intermixing due to interdiffusion is the main cause for the shift of the luminescence energy to higher values in these annealed samples.

Molecular-beam epitaxy of strained silicon germanium/silicon structures

The growth of strained layer heterostructures is influenced by intentionally undesired phenomena as surface undulations, alloy clustering, defect generation, interface intermixing. The origin of these effects is explained and strategies to overcome them are discussed. Recently, in the silicon germanium (SiGe) system exciting progress toward layers controlled on a monolayer scale was obtained. The status will be demonstrated by doped and undoped heterostructures, multiple heterostructures, and superlattices. Special emphasis will be given to structures relevant for devices as heterobipolar transistors (HBT), modulation doped field effect transistors (MODFET), and novel optoelectronic devices. For all these structures the strain is explicitly utilized for band structure engineering. In several areas the progress in material preparation pushed an tremendous performance increase. Examples given are the low base sheet resistivity (1.6 kΩ/⧠) for high speed (42 GHz) SiGe–HBTs, the 4.2 K mobility increase in n-MODFET channels to 160 000 cm2/V s, and the luminescence from various strained SiGe structures.

Rapid isothermal processing of Pt/Ti contacts to p-type III-V binary and related ternary materials

Results of an extensive study of the interfacial intermixing and interaction of the Pt/Ti bilayer metallization to seven III-V binary and related ternary compound semiconductor systems, as well as the contact electrical properties, are given. Pt/Ti contact to InAs and In/sub 0.53/Ga/sub 0.47/As were ohmic as deposited, while the same metallization scheme on GaAs, GaP, InP, In/sub 0.52/Al/sub 0.48/As, and Ga/sub 0.7/Al/sub 0.3/As provided a rectifying contact as-deposited. The latter group of contacts, with the exception of InP and GaP, were transformed to an ohmic contact as a result of rapid thermal processing (RTP) at the temperature range of 300 to 450 degrees C. A linear correlation between the semiconductor bandgap value and the Schottky-barrier height, measured in the Pt/Ti contacts, was observed. >

Vibrational and electronic properties of strained α-Sn/Ge superlattices

Phonon spectra of strained α-Sn/Ge superlattices are studied using a generalized Keating model. General features of phonons are discussed. Effects of interface intermixing on Raman spectra are investigated by a supercell calculation. In addition, electronic band structures are calculated by a tight-binding method.

Vibrational properties of isolated AlAs monolayers embedded in GaAs: a theoretical study of the effects of disorder

We present ab-initio calculations of Raman spectra of a prototype superlattice (SL) formed by the periodic repetition of an AlAs single monolayer embedded between two thicker GaAs layers. By comparing results for the ideal structure and for disordered configurations characterized by cationic intermixing, we demonstrate that AlAs-like longitudinal optical modes are extremely sensitive to disorder and may allow a quantitative characterization of the actual composition profiles obtained in real samples. The general implications of this conclusion for the study of interface intermixing in GaAs/AlAs SL's are discussed.