Ge Multilayers Research Articles

Self-formation of SiGe oxide, Ge, and void multilayers via thermal oxidation of hydrogenated epitaxial SiGe films

In this work, hydrogenated SiGe thin films epitaxially grown on crystalline Si (c-Si) at 175 °C are annealed at 800 °C in a low vacuum system. The influence of H from the thin film and residual O atoms in the vacuum system on the structural changes of SiGe is presented. Multilayers consisting of c-Si/SiGe/void/Ge/SiOx/SiGeOx are obtained by taking advantage of the hydrogen present in the epitaxial SiGe layer. Various annealing conditions are applied to study the evolution of SiGe layers. The surface morphology, crystallinity, composition and micro structure of the multilayer stack formed during the annealing process are investigated in detail. A discussion based on the diffusion of H and O atoms is presented to account for this extraordinary process.

Fabrication and selective wet etching of Si0.2Ge0.8/Ge multilayer for Si0.2Ge0.8 channel gate-all-around MOSFETs

The fabrication and selective wet etching of Si0.2Ge0.8/Ge stacked multilayer for Si0.2Ge0.8 channel gate-all-around metal–oxide–semiconductor field-effect transistors (MOSFETs) has been demonstrated in detail in this research. First, a high crystal quality of Si0.2Ge0.8/Ge multilayer is realized using a Ge strain relaxed buffer. Subsequently, selective wet etching of Ge versus Si0.2Ge0.8 is investigated in detail. It has been observed that common acid or alkali solutions cannot etch Ge. However, the use of strong oxidizing solutions are necessitated to achieve the selective etching of Ge to Si0.2Ge0.8, such as H2O2 or HNO3. Moreover, it is noted that the selectivity of Ge to Si0.2Ge0.8 increases as H2O2 or HNO3 concentrations rise. However, the use of 30% H2O2 solution, which is the highest concentration in our lab, still gets an obvious erosion of Si0.2Ge0.8 with a rounded profile. Therefore, a stronger oxidation solution (HNO3) is employed to check the selective etching of Ge versus Si0.8Ge0.2. Consequently, a rectangular shape of Si0.2Ge0.8 is attained as the HNO3 concentration increases to more than 60% due to its stronger oxidizing capacity. Finally, 65% HNO3 solution is chosen as the optimal condition for the Si0.2Ge0.8 nanowire release process due to its high selectivity to Si0.2Ge0.8 and low surface roughness.

Dependence of magnetic properties on the growth temperature of Mn0.04Ge0.96 grown on Si (001)

The structural and magnetic properties of Mn 0.04 Ge 0.96 thin films grown on Si (001) substrates at different growth temperatures were investigated. The films were grown by sequential deposition of MnGe and Ge multilayers using molecular beam epitaxy . It was found that the magnetic ordering and Curie temperature could be controlled by adjusting the distance between the Mn ions in the Ge spacer layer depending on growth temperature. We found that the samples grown below 130 °C contained highly disordered ferromagnetic Mn rich domains. Both structural and magnetic characterizations revealed that Mn 5 Ge 3 precipitates were formed in Mn clusters free Ge matrix when the samples were grown at 150 °C. Both the Mn rich domains and Mn 5 Ge 3 precipitates showed out of the plane magnetic anisotropy and similar ESR line shapes. • We investigated the effects of growth temperature on spin systems in Mn 0.04 Ge 0.96 . • The degree of ferromagnetic ordering increases with temperature. • Ferromagnetism occurs due to interaction of Mn 2+ ions in Mn rich clusters. • ESR reveals that the formation of magnetic phases depends on growth temperature.

Nanoindentation responses of Si–Ge multilayers

Abstract In this study, we employed the nanoindentation technique to evaluate the pop-in events of Si–Ge multilayers under extra-low forces. X-ray diffraction revealed a shift of the peaks of the Ge atoms from 68.70 to 68.50°, due to gradual mixing of previously isolated Si and Ge atoms into an SiGe compound, upon increasing the annealing temperature. Atomic force microscopy images of the vicinity near the triangular indentation mark revealed that the primarily plastic deformation, the pop-in event observed in the load–displacement curve, was based on slightly active dislocation nucleation and propagation during treatment with the artificial indenter. The samples annealed at RT, 400, 500, and 600°C exhibited hardnesses (H) of 18.6 ± 1.2, 17.9 ± 1.1, 18.9 ± 1.2, and 15.0 ± 0.8 GPa, respectively, and elastic moduli (E) of 220.0 ± 5.2, 224.9 ± 5.4, 220.7 ± 4.5, and 186.7 ± 3.8 GPa, respectively. These values reveal that elastic/plastic contact translation of the Si–Ge multilayer occurred to various extents depending upon the annealing conditions; in addition, the values of h f/h max for the samples annealed at RT, 400, 500, and 600°C were 0.449, 0.416, 0.412, and 0.470, respectively. In a crystal structure, release of the indentation load reflects the directly compressed volume; the total penetration depth into the film was approximately 30 nm with a peak load of 500 μN. Accordingly, the annealed samples can exhibit pop-in after indentation earlier than samples treated merely at RT.

Stacked Ge nanocrystals with ultrathinSiO2 separation layers

The aim of this work is the tailored growth of Ge nanocrystals (NCs) in (GeOx/SiO2) multilayers (ML) for photovoltaic applications. For this purposethe fabrication of regularly stacked Ge NCs separated by ultrathinSiO2 layers isessential to enable charge carrier transport by direct tunnelling. In this paper we report on the fabricationof (GeOx/SiO2)50 multilayer stacks via reactive dc magnetron sputtering and Ge NCs formationafter subsequent annealing. It is shown that magnetron sputtering allows usto deposit very regular ML stacks with a total thickness of about 300 nm,characterized by ultrathin (down to 1 nm) and very smooth (roughness ∼ 0.6 nm)SiO2 separation layers. A main challenge is to keep these properties for a thermal budgetnecessary to form Ge NCs. For this reason, the temperature dependence of phaseseparation. Ge crystallization and ML morphology was investigated by Rutherfordbackscattering, x-ray scattering, Raman spectroscopy and electron microscopy.The formation of size confined Ge NCs of about 5 nm after annealing of only550 °C is confirmed. This low thermal budget ensures the suppression of GeO emanation and multilayerstability. Spectroscopic ellipsometry was applied to determine the optical Ge NC bandgap to(1.65 ± 0.5) eV.

Magnetic and electric properties of (Fe, Co)/(Si, Ge) multilayers

Abstract We review selected results concerning the interlayer exchange coupling in Fe/SixFe1−x , Fe/Ge and Co/Si layered structures. Among the ferromagnet/semiconductor systems, Fe/Si structures are the most popular owing to their strong antiferromagnetic interlayer coupling. We show that such interaction depends not only on semiconducting sublayer thickness, but also on deposition techniques and on the chemical composition of the sublayer as well. In similar heterostructures e.g. Fe/Ge, antiferromagnetic coupling was observed only in ion-beam deposited trilayers at low temperatures. In contrast, in Fe/Ge multilayers deposited by sputtering, no such coupling was found. However, when the Ge is partially substituted by Si, antiferromagnetic interlayer coupling appears. For Co/Si multilayers, we observed a very weak exchange coupling and its oscillatory behavior. The growth of Co on Si occurs in an island growth mode. The evolution of magnetic loop shapes can be successfully explained by the interplay between interlayer coupling and anisotropy terms.

Structure, magnetic, and transport properties of sputtered Fe∕Ge multilayers

The structure, magnetic, and transport properties, especially the Hall effect, of Fe∕Ge multilayers fabricated by magnetron sputtering were investigated. Structure characterization indicates a periodic modulated structure with alternately deposited polycrystalline Fe and amorphous Ge layers. The room-temperature magnetic measurements reveal that the uniaxial magnetic anisotropy Ku of the Fe∕Ge multilayers with a period of 5.2nm is 2.27×103J∕m3. The temperature coefficient of resistivity of all the films is positive at room temperature but turns to be negative at low temperatures due to the weak localization effect. The Fe∕Ge multilayers show anomalous Hall effect and the Hall sensitivity KH is independent of the temperature, showing that Fe∕Ge multilayers have the potential applications in the field of magnetic sensors. When the period Λ is 5.2nm, the anomalous Hall coefficient Rs reaches its largest value of 1.8×10−7Ωm∕T, which is three orders of magnitude larger than that of the bulk Fe material.

Ferromagnetism of Mn/Ge Multilayers Grown by Molecular Beam Epitaxy

We report on novel ferromagnetic Mn/Ge multilayers for spintronics applications investigated both experimentally and theoretically. Two Mn/Ge multilayers are grown on GaAs (001) substrates by molecular beam epitaxy. The period of each multilayer consists of an Mn layer of varying thickness (0.6 and 5 A) and a 10 A thick Ge spacer layer. From temperature-dependent magnetization and hysteresis loop measurements, the Mn (0.6 A)/Ge (10 A) multilayer showed very weak ferromagnetic ordering, which is persistent up to 260 K, whereas the Mn (5 A)/Ge (10 A) multilayer exhibited strong ferromagnetism up to 305 K. The coercive field of the Mn (5 A)/Ge (10 A) multilayer was 277 Oe at 200 K. Density functional electronic band structure calculations on a number of Mn/Ge (001) multilayers determined them to be ferromagnetic, and estimates of their critical temperatures are reported.

Ultrathin multilayer of Fe and Ge: structure and magnetic properties

Metal-semiconductor multilayers are interesting, artificial structures as prospective candidates for spin injection devices. A Fe–Ge multilayer sample with very thin individual layers (few crystallographic planes) has been deposited by sputtering on Si[1 0 0] substrate. We have characterized the structure of this multilayer sample using X-ray diffraction, X-ray reflectometry and neutron reflectometry. The magnetic moment density in the ferromagnetic Fe layer has been obtained by polarized neutron reflectometry and the bulk magnetic behavior of the thin film by SQUID magnetometer measurements. We found that the film is a soft ferromagnet at room temperature with a substantially reduced magnetic moment of the Fe atoms.

Antiferromagnetic interlayer exchange coupling across epitaxial, Ge-containing spacers

We give experimental evidence of antiferromagnetic interlayer exchange coupling of Fe(001) layers across epitaxial, Ge-containing spacers consisting of either Ge wedges embedded between two Si boundary layers or Si–Ge-multilayers. The coupling strengths are of the order of 1 mJ/m2 and decay on a length scale below 2 Å as determined from magneto-optic Kerr effect and Brillouin light scattering. The coupling evolves with the spacer thickness from ferromagnetic to prevailing 90° or antiferromagnetic for Ge wedges and Si–Ge multilayers, respectively. The bilinear coupling is comparable in both cases, but the biquadratic contribution is suppressed for Si–Ge-multilayer spacers. Thus, Si–Ge-multilayer spacers give rise to perfect antiparallel alignment of the Fe film magnetizations.

Reconstruction of the 2D hole gas spectrum for selectively doped p-Ge/Ge1−x Six heterostructures

The magnetic field (0≤B≤32 T) and temperature (0.1≤T≤15 K) dependences of longitudinal and Hall resistivities have been investigated for p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 12≤d w ≤20 nm and hole densities p s =(1–5)×1015 m−2. An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level gaps deduced from the activation magnetotransport, etc.) to the quantum well profile is revealed in the cases where the Fermi level reaches the second confinement subband. An unusually high density of localized states between the Landau levels is deduced from the data. Two models for the long-range random impurity potential (the model with randomly distributed charged centers located outside the conducting layer and the model of the system with a spacer) are used to evaluate the impurity potential fluctuation characteristics: the random potential amplitude, the nonlinear screening length in the vicinity of integer filling factors v=1 and v=2, and the background density of states (DOS). The described models are suitable for explanation of the observed DOS values, while the short-range impurity potential models fail. For half-integer filling factors, a linear temperature dependence of the effective quantum Hall effect plateau-plateau (PP) transition widths v 0(T) is observed, contrary to the expected scaling behavior of the systems with short-range disorder. The finite T→0 width of the PP transitions may be due to an effective low-temperature screening of a smooth random potential due to the Coulomb repulsion of electrons.

The key role of a smooth impuritypotential in formation of the hole spectrum for p-Ge/Ge1-xSix heterostructures in the quantum Hallregime

We have measured the temperature (0.1⩽T⩽15 K) and magnetic field (0⩽B⩽12 T) dependencesof longitudinal and Hall resistivities for p-Ge0.93Si0.07/Ge multilayers with different Ge layerwidths 10⩽dw⩽38 nm and hole densitiesps = (1-5)×1015 m-2. Two models forthe long-range random impurity potential (the model withrandomly distributed charged centres located outside theconducting layer and the model of the system with a spacer) areused for evaluation of the impurity potential fluctuationcharacteristics: the random potential amplitude, nonlinearscreening length in the vicinity of integer filling factors (FFs) ν = 1and 2 and the background density of states (DOS). Thedescribed models are suitable for an explanation of the unusuallyhigh value of DOS at ν = 1 and 2, in contrast to theshort-range impurity potential models. For half-integer FFs the linear temperature dependence of the effectivequantum Hall effect (QHE) plateau-to-plateau transition width ν0(T) is observed incontrast to scaling behaviour for systems with short-rangedisorder. The finite T→0 width of QHE transitions maybe due to an effective low-temperature screening of smoothrandom potential owing to Coulomb repulsion of electrons.

Structural and optical properties of vertically correlated Ge island layers grown at low temperatures

We studied the vertical correlation of small hut-like Ge islands in Si–Ge multilayers grown by molecular beam epitaxy (MBE) at 510 °C in the modified Stranski–Krastanov growth mode. The fundamental, structural and optical properties of stacked Ge islands embedded in Si are analyzed by in-situ RHEED, TEM, photoluminescence (PL), and photocurrent (PC) spectroscopy. Ge islands of about 20 nm width and 2 nm height reveal a reduction in the critical Ge coverage for correlated growth for Si spacer widths below about 14 nm. This value is much smaller than observed for dome-like islands of about 80 nm size deposited at higher temperatures. Such a scaling of island separation for stacked growth with island diameter is suggested by finite element simulations of elastic strain relaxation in islands. The correlation affects island size, island density, and local strain fields which may have a direct impact on band offsets and the electronic coupling of stacked islands. This is important for possible application in devices like NIR Si–Ge dot photodetectors and tunneling structures.

Characterization of a Quantum Well in an Si/Si1– x Ge x /Si Heterostructure by X-ray Diffractometry

Rocking curves with a pronounced pendulum-solution pattern are obtained for Si1 – xGex/Si specimens with a single quantum well (QW) by double-crystal x-ray diffractometry. These oscillations are typical of the rocking curves for GaAs multilayers with QWs. With Si–Ge multilayers, they are obtained for the first time. The Ge depth profiles of the QW are reconstructed from the rocking curves. As a result, the thicknesses of both QW interfacial layers are estimated.

Probing the p-Ge1-xSix/Ge/p-Ge1-xSix quantum wellby means of the quantum Hall effect

We have measured the temperature (0.1⩽T⩽15 K)and magnetic field (0⩽B⩽32 T) dependences oflongitudinal and Hall resistivities for the p-Ge0.93Si0.07/Ge multilayers with different Ge layer widths 10⩽dw⩽38 nm and hole densities ps = (1-5)×1015 m-2. An extremely high sensitivity ofthe experimental data (the structure of magnetoresistance traces,relative values of the inter-Landau-level (LL) gaps deduced from theactivation magnetotransport etc) to the quantum well (QW)characteristics has been revealed in the cases when the Fermilevel reached the second confinement subband. The backgrounddensity of states (5-10)×1014 m-2 meV-1 deduced from the activation behaviour of themagnetoresistance was too high to be attributed to the LLtails, but may be accounted for within a smooth randompotential model. The hole gas in the Ge QW was foundto separate into two sublayers for dw>~35 nm and ps≈5×1015 m-2. Concomitantly thepositive magnetoresistance emerged in the weakest fields, fromwhich different mobilities in the sublayers were deduced. Amodel is suggested to explain the existence of the plateauxclose to the fundamental values in a system of two parallellayers with different mobilities.

Role of surface defect states in visible luminescence from oxidized hydrogenated amorphous Si/hydrogenated amorphous Ge multilayers

Nanocrystalline Ge embedded in amorphous silicon dioxide matrix was fabricated by oxidizing hydrogenated amorphous Si/hydrogenated amorphous Ge (a-Si:H/a-Ge:H) multilayers. The structures before and after oxidation were systematically investigated. The orange-green light emission was observed at room temperature and the luminescence peak was located at 2.2 eV. The size dependence in the photoluminescence peak energy was not observed and the luminescence intensity was increased gradually with oxidation time. The origin for this visible light emission is discussed. In contrast to the simple quantum effect model, the surface defect states of nanocrystalline Ge are believed to play an important role in radiative recombination process.

Fabrication of luminescent ge nanocrystals started from unlayered hydrogenated amorphous SiGe films or hydrogenated amorphous Si/hydrogenated amorphous Ge multilayers

Nanocrystalline Ge embedded in SiOx matrix is fabricated by oxidizing hydrogenated amorphous SiGe alloys or hydrogenated amorphous Si/hydrogenated amorphous Ge multilayers. The structures before and after oxidation are systematically investigated. Visible light emission was observed from both samples. The luminescence peak is located at 2.2 eV which is independent of the starting materials. Compared to the luminescence from unlayered samples, the photoluminescence spectrum from multilayered samples has a narrower band width, which can be attributed to the uniform size distribution. The light emission origin is also discussed briefly and a mechanism different from the quantum size effect is suggested.

Realization of a delayed choice experiment using a multilayer cold neutron pulser

A delayed choice experiment was done using the cold neutron spin interferometer of Jamin-type arrangement which is installed at the JRR-3M of JAERI. A pair of identical composite neutron mirrors composes the pair of the wave splitters of the interferometer. The composite neutron mirror consists of a Ni/Ti multilayer ((Ni : 85 Å/Ti : 85 Å) × 5 (layers)), a Ge gap layer (4000 Å), and a Permalloy (Fe 55Ni 45)/Ge multilayer ((Permalloy : 85 Å/Ge : 85 Å)×5 (layers)) (PGM), evaporated in series on the single Si wafer in a magnetic field of 130 G. The second composite mirror was placed in the pulsed magnetic field for producing the conditions of whether the second wave splitter is introduced or not when a polarized neutron reaches the second wave splitter. It was shown that whether the interference fringe is obtained or not depends only on whether the second wave splitter is introduced or not, after the neutron passed through the first wave splitter. Delaying choice has no effect on the interference. We could deduce that the neutron propagates both ways with equal probability after being split by the wave splitter and does not choose one of two ways, as explained by quantum mechanics.

Photoepitaxy of Si and Ge by synchrotron radiation

The correlating growth kinetics and crystal structures are described for synchrotron-radiation-excited Si and Ge epitaxy from Si 2H 6 and GeH 4. The characteristics of growth mediated by photon-stimulated hydrogen desorption from irreversibly chemisorbed surface hydrides are distinguished from those due to photolysis of the reactants in the gas-phase and at the surface. For Si homoepitaxy on Si(100), it is shown how the morphology of the Si layer changes with the migration length of adatoms. For Ge homoepitaxy on Ge(100), a novel p(2×2) surface structure is observed when low-temperature deposition is followed by solid-phase epitaxy. The surfactant effect of hydrogen atoms results in uniform heteroepitaxy of Si on Ge(100) and Ge on Si(100), which enables nonthermal growth of Si Ge multilayers.

Effect of low hydrostatic pressures on the solid state reactions in multilayers

In amorphous SiGe multilayers annealing at 858 K under 180 bar led to the depression of porosity formation. In crystalline Ni-Zr multilayers a small enhancement of the growth of amorphous phase was observed under 200 bar. In CoSn bilayers the kinetics of electrical resistivity at 298 K showed a characteristic cross-over as the pressure increased above 150 bar.