Ge Overlayer Research Articles

Ge/Si(001)-2×N:Surface stress and interaction of dimer vacancy lines

We have studied surface-stress anisotropy as a function of the thickness of molecular-beam-epitaxy-grown Ge overlayers on the Si(001) substrate. The surface-stress anisotropy changes sign at a dimer-vacancy-line (DVL) periodicity of $\ensuremath{\sim}15.$ By varying the separation between two neighboring DVL's we have also investigated the DVL-DVL interaction. Based on a linear-dipole-force model for the step-step interaction and the theory of long-range elastic relaxation of orientationally inequivalent domains, we have estimated the DVL formation energy of $\ensuremath{-}0.003\mathrm{eV}/\AA{}.$

On the feasibility of using ultraviolet/ozone grown oxide as an atomic interdiffusion barrier in Ge/GaAs heterojunctions

A 10 Å ultraviolet/ozone grown oxide was used as an atomic diffusion barrier in a Ge/GaAs heterostructure. Good thermal stability of the oxide layer has been demonstrated by the appearance of dendritic crystallization [K. M. Lui, K. P. Chik, and J. B. Xu, J. Appl. Phys. 81, 7757 (1997)], induced by in situ thermal pulse annealing, of the Ge overlayer. In this work, an abrupt heterointerface was revealed by aligned Rutherford backscattering spectroscopy after annealing and compared with a control with no barrier at the interface where considerable diffusion had taken place. Current–voltage measurement indicated good rectifying properties of the oxide barrier heterojunction.

Investigation of Ge on Si(100) quantum wells by photoelectron spectroscopies

In the present work we report on the evolution of the electronic states as a function of Ge overlayer thickness ranging from a fraction of a monolayer to few tens of monolayers. The Ge states above the Si valence band top could be clearly detected at every overlayer thickness d. For d<∼6 Å, the Ge state density exhibits a double linear edge. Upon increasing the overlayer thickness the split between the two edges decreases and the double edge structure disappears for d>∼6 Å, i.e., at the critical thickness for the transition from strained to unstrained overlayers. For these larger thicknesses, the Ge edge becomes broad and featureless. The energy separation between the Ge onset and the Si valence top increases up to 0.70 eV at larger overlayer thicknesses.

Oxidation kinetics of epitaxial Ge-covered Si(100) surfaces

We compared oxidation kinetics on Ge-covered Si(100) surfaces grown at 350 and 600°C for 0.9 and 2.0 ML Ge overlayer thicknesses. The OKLL intensities showed clear oxidation enhancement on the surfaces grown at 600°C. The oxygen interaction for the surface covered with 2 ML Ge formed at 350°C was weaker than for the Ge(100) surface, indicating that the compressive strain due to the lattice mismatch may suppress the oxygen interaction with surface Ge dimers.

Strain-Induced Atomic Rearrangements in Ge Overlayers on Si(001)

Novel features of strain-induced atomic rearrangements in germanium overlayers on Si(001).(2xl) probed by surface-sensitive XAFS are reported. It is found that the Ge adatoms on Si(001) form elongated dimers with the average adatom-adatom distance La = 2.51 ± 0.01 A, in sharp contrast to total energy calculations for an asymmetric dimer configuration. For two ML's, Ge atoms in the second layer are replaced with Si atoms in the third layer. Upon silicon overgrowth on Ge layers, the Ge atoms in the second layer are replaced with the first layer Si atoms and form Ge dimers on top. These rearrangements of surface atoms are interpreted as the strain-induced site-selective atomic migrations.

Epitaxial regrowth of Ge films on (001) GaAs by in situ thermal pulse annealing of evaporated amorphous germanium

Germanium thin films have been epitaxially regrown on (001) GaAs by in situ thermal pulse annealing of evaporated amorphous germanium under <102 Watt/cm2 broad-band irradiation in high vacuum. Epitaxial regrowth was found to occur only when the duration of the thermal pulse (te) was greater than a critical value tc (≃3.20 s). The crystal quality of the resultant film was examined by high resolution x-ray diffraction technique (HRXRD) and grazing-incidence x-ray diffraction technique (GIXRD). All rocking curves were found to have a full width at half-maximum of about 0.02°. Both HRXRD and GIXRD confirmed the Ge overlayer was grown epitaxially as well as pseudomorphically on the substrate. Scanning electron microscopy and atomic force microscopy revealed the very different surface morphologies resulting from different te. For te<tc, columnar germanium grains with a four-fold symmetry and a high uniformity in size were found, while for te⩾tc, epitaxial regrowth was observed. It is suggested that epitaxial regrowth takes place via a temporary formation of liquid phase Ge.

Spectromicroscopic evidence of Ge-GaSe chemical reactions: Not a Schottky system

Photoelectron-spectromicroscopy experiments unexpectedly revealed a reacted phase for Ge overlayers on GaSe. This finding is in sharp contrast with the notion that this interface is an ideal Schottky system, whose band discontinuities, for example, are determined by the Anderson rule-and with the widespread belief that most III-VI-based interfaces behave like Schottky systems. After this result, to the best of our knowledge, no heterojunction is known that matches the requisites of an ideal Schottky system.

Fabrication of buried epitaxial CoSi2 layer through selective diffusion

The incorporation of metallic layers into the bulk of semiconductors is gaining tremendous attention for device applications. This is mainly achieved by ion beam synthesis. In the ultrathin film regime, however, this technique is not practical due to the damage incurred. Here we report a technique by which fabrication of buried epitaxial CoSi2 is achieved by making use of the selective diffusion behavior of Co. Co atoms diffuse through a Ge overlayer on a Si(111) substrate and are terminated by reaction with the Si atoms underneath. Ion scattering as well as high resolution microscopy results confirm that the CoSi2 layer thus formed is in epitaxial form. This method would help in providing functionality to nanostructure based devices.

ArF excimer laser epitaxy of Si xGe 1− x alloys studied by XRD and XPS

The formation of SiGe alloys was obtained by ArF excimer laser induced epitaxy of thin a-Ge films deposited by Laser-CVD on Si(100) substrates. The alloying process was studied as a function of the thickness of the Ge overlayer, which ranged between 10 and 70 nm. In order to investigate the early stage of the Ge and Si intermixing, single pulse irradiation was performed at fluence (0.49 J/cm 2) appropriate to melt the Ge overlayer and a certain thickness of the underlying substrate. XRD and XPS depth profile analysis revealed the formation of graded alloys, showing high Ge content in the near-surface layer and Si rich alloy regions in the proximity of the alloy/substrate interface. Although Ge was redistributed over lengths predictable by heat flow numerical model calculation, other mechanisms (such as Ge segregation and/or strain field effects) besides diffusion, seem to regulate the observed alloy concentration profiles. The altered surface morphology exhibited after laser processing by the sample having the thickest Ge overlayer sets an upper limit for the thickness of the a-Ge film, which in the present irradiation conditions is about 40 nm.

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.

Interface formation of Ge/ZnSe(100) and Ge/ZnS(111) heterojunctions studied by synchrotron radiation photoemission

The microscopic evolution of interface formation between Ge and II-VI compounds such as ZnSe and ZnS single crystals has been studied by synchrotron radiation photoemission spectroscopy and low energy electron diffraction. Core level intensity measurements from the substrate as well as from the overlayer show a nearly ideal two-dimensional growth mode for the deposition of Ge on ZnSe(100) surface. However, there is a certain deviation from the ideal two-dimensional mode in the case of Ge/ZnS(111) due to the diffusion of substrate atoms into Ge overlayer. Surface sensitive core level spectra indicate that the reaction of Ge with S atoms at Ge/ZnS(111) interfaces is much stronger than that of Ge with Se atoms at Ge/ZnSe(100) interfaces.

Oxygen adsorption on Ge-covered Si(100) surfaces

We studied oxygen chemisorption on Ge-covered Si(100) surfaces by AES, XPS and STM. Epitaxial Ge overlayers, 1 and 2 ML, on Si(100) surfaces were oxidized in situ at room temperature. Initial uptake of oxygen adsorption on the surface terminated by a monolayer Ge was reduced by ∼ 1 5 compared with that on a Si(100) surface measured in O KLL AES intensities. The oxygen interaction for the surface covered by 2 ML Ge was in the same range as that of a Ge(100) surface, indicating that the top two layers, surface dimers and the underneath layer, play critical roles in oxygen interaction. The STM study for the surface partly covered by Ge showed that oxygen reacted with the Si substrate in preference to the Ge islands.

Synchrotron radiation photoemission study of Ge/ZnS(111) heterojunction

The band lineup at the Ge/ZnS(111) interface has been studied by synchroton radiation photoemission spectroscopy. The experimental results indicate that the deposition of Ge films by evaporation is a two-dimensional growth at the initial stage. With the growing of Ge overlayer, surface S atoms diffuse into Ge overlayer and react with Ge atoms at the interface. We derive the valence band offsets of 1.94 ± 0.1 eV and 2.23 ± 0.1 eV for Ge/ZnS heterojunctions grown at 200°C and room temperature, respectively. The results are in good agreement with theoretical predictions.

Valence band offset and interface formation of Ge/ZnSe(100) studied by synchrotron radiation photoemission

The formation and band lineup of the Ge/ZnSe(100) interface have been studied by synchrotron radiation photoemission spectroscopy. Core level intensity measurements from the ZnSe substrate as well as from the Ge overlayer show a two-dimensional deposition of Ge film. Core level spectra indicate that Ge atoms react with Se atoms slightly at the interface. We derive a valence band offset of 1.76±0.1eV for Ge/ZnSe(100).

Electronic, structural, and dynamical properties of the GaAs(110):Ge surface.

We have investigated the structural, electronic, and vibrational properties of Ge overlayers on the GaAs(110) surface in the framework of the density-functional theory within the local-density approximation. By minimizing the total energy with the help of the Hellmann-Feynman forces, we have found two different zero-force configurations for the GaAs(110) surface covered with one monolayer (ML) Ge as well as for two ML's Ge on GaAs(110). For both coverages the atomic equilibrium positions and most of the electronic surface states corresponding to the optimized structure with the lower energy minimum are very similar to those of the clean GaAs(110) surface. A thermodynamical analysis indicates an instability towards cluster formation. In the case of one ML Ge on GaAs(110) we have calculated the phonon-dispersion curves along high-symmetry lines in the surface Brillouin zone using first-order density-functional perturbation theory without any adjustable parameters. As well as the electronic properties, the features of the phonon spectrum mostly resemble those of the clean surface. Beside the appearance of an additional surface acoustic mode the main effect of Ge adsorption is a slight down-shift of the phonon frequencies. The dynamical analysis of the GaAs(110)-(1\ifmmode\times\else\texttimes\fi{}1):Ge surface (1 ML) at T = 0 shows no instability towards reconstructions with periodicity along the high-symmetry directions \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}X\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}X\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{'}. \textcopyright{} 1996 The American Physical Society.

A new apparatus for surface x-ray absorption and diffraction studies using synchrotron radiation

A new apparatus for structural studies of surfaces and buried interfaces using synchrotron radiation was built and tested at the 27-pole wiggler station BL13B of the Photon Factory. The apparatus was designed to combine x-ray absorption fine structure (XAFS), x-ray standing wave (XSW), and surface x-ray diffraction techniques in the same ultrahigh vacuum (UHV) chamber. The apparatus features a seven-element Si(Li) solid-state detector array for a fluorescence yield measurement and a high precision eight-axis goniometer in the UHV chamber with a base pressure of 1×10−10 Torr. For the same sample mounted on the in-vacuum goniometer, vertically or horizontally polarized surface-sensitive XAFS, surface x-ray diffraction, and XSW can be measured. As a performance test, the structure of Ge overlayers on Si(001) was studied by polarized surface-sensitive XAFS. The results show that the apparatus can probe the local structure of adatoms with ∼0.1 monolayer sensitivity.

Structures and reactions of missing dimers in epitaxial growth of Ge on Si(100).

We present first-principles total-energy calculations for surface atomic structures which provide a natural explanation for island formation in heteroepitaxial growth of Ge on Si(100) surfaces. We first investigate structures of dimer vacancies of missing dimers (MD's) which appear on Ge overlayers on Si(100) due to lattice mismatch (\ensuremath{\sim}4.3%) between the two materials. It is found that rebonded MD's aligned straight in the direction perpendicular to dimer rows (MD lines) are stable against their meandering. This result is consistent with the recent scanning tunneling microscopy measurements and the subsequent statistical analysis. Next we investigate diffusion of a Ge atom adsorbed on the Ge overlayers which exhibit the straight MD lines. The energy barrier for diffusion along the direction of the dimer rows, along which the fast diffusion of Ge adatoms on the clean Si(100) surfaces occurs increases by 0.8 eV near the rebonded MD. This value is much larger than the corresponding value of 0.36 eV from the previous empirical potential calculations. More importantly, this additional energy barrier is large enough to confine the Ge adatom in the flat region (on terrace) surrounded by the MD lines and thereby enhances the island formation of Ge atoms on the Ge overlayers on Si(100).

Ge overlayers on Si(001) studied by surface-extended x-ray-absorption fine structure.

The local structure of the Ge overlayers on Si(001) has been studied in situ by surface-extended x-ray absorption fine structure on the Ge K-edge, using a grazing-incidence fluorescence excitation and synchrotron radiation from a multipole wiggler. The results for 1 ML Ge on Si(001) indicated that the Ge adatoms form elongated dimers with the average Ge-Ge distance of 2.51\ifmmode\pm\else\textpm\fi{}0.04 \AA{}, in sharp contrast to the shortened adatom-adatom bond length for clean (2\ifmmode\times\else\texttimes\fi{}1) Si(001). The observed adatom-substrate distance 2.40\ifmmode\pm\else\textpm\fi{}0.08 \AA{} indicates a contraction of the substrate Si atom by the same amount (-2.4%). The results suggest that adatoms take the ${\mathit{p}}^{3}$-like configuration due to a substrate-to-adatom charge transfer.

Superconductivity in ultrathin quench-condensed Pb/Sb and Pb/Ge multilayers.

We present the results of in situ transport and tunneling measurements on quench-condensed Pb/Sb and Pb/Ge multilayers. The superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) and the density of states increase concurrently with the number of Pb layers (n) and eventually saturate. Both the variation of ${\mathit{T}}_{\mathit{c}}$ with n and the saturated value of ${\mathit{T}}_{\mathit{c}}$ can be accounted for by electron transfer between Pb layers through localized states in the Sb(Ge). We find that the basic mechanism for superconductivity in these multilayers is not fundamentally altered from that in homogeneous amorphous superconducting films. Our results clarify the role the Ge or Sb underlayer and overlayer plays in the transport and superconductivity of uniform quench-condensed films.

Structural and electronic properties during the initial stages of Ge-GaAs(110) interface formation.

Electronic and structural properties of the Ge and GaAs(110) surfaces, as well as, of the Ge-GaAs(110) interface with Ge overlayers of varying thickness on a semi-infinite GaAs substrate have been studied using the semiempirical tight-binding method combined with a local-charge-neutrality condition. Changes of the electrostatic layer potentials near the surface or interface are analyzed in a self-consistent way by iterative determinations of the optimally relaxed structure and of the related charge transfer. It is shown that the band offset of the Ge-GaAs(110) interface with Ge overlayers of varying thicknesses is independent of the surface relaxation. A band offset is established when the first Ge adlayer is adsorbed and it changes continuously with the number of Ge adlayers towards the final value of 0.52 eV for the Ge-GaAs(110) heterojunction interface formed by two semi-infinite crystals. Convergence to the heterojunction interface value is essentially reached for eight Ge adlayers. The calculated electronic structure of the systems shows that characteristic interface features do occur already for the first Ge adlayer.