Thin Ge Layer Research Articles

Computational model of nonequilibrium phase transitions in a Si-Ge system

A computational model of a Si-Ge system melting and solidification induced by pulsed-laser irradiation is presented in the paper. Phase transitions in the system are modeled using the theory of transition states so that undercooling or overheating of the interface and kinetic phase diagrams are taken into account. The computational solution of the mathematical model is performed using the Galerkin finite element method in a one-dimensional approximation and the moving boundary problem is solved by a front-fixing technique. In a practical application of the model, the melting and solidification of both Si-Ge alloys of various composition and thin Ge layers on the Si bulk induced by ArF excimer laser are simulated. The results of numerical simulations are compared with the available experimental data.

Electronic states of thin epitaxial layers of Ge on Si(100)

The valence density of states of Ge grown epitaxially on Si(100) is investigated as a function of thickness by yield spectroscopy and photoemission techniques. A double edge is present in the yield data for thicknesses smaller than the lattice relaxation critical thickness. Furthermore, the line-up of the Ge states to the Si valence band varies with overlayer thickness. Photoemission techniques fail to detect this behavior. The causes for the discrepancy are analyzed.

X-ray diffraction and x-ray photoelectron spectroscopy study of partially strained SiGe layers produced via excimer laser processing

Structural properties of graded Si(1−x)Gex layers obtained on Si(100) by pulsed laser induced epitaxy were investigated by means of conventional powder x-ray diffraction and x-ray photoelectron spectroscopy. The Si(1−x)Gex epitaxial layers were formed by pulsed KrF-laser driven rapid melting and crystallization of thin amorphous Ge layers deposited onto the Si(100). The experimental results showed that, by increasing the number of laser pulses, good quality and partially strained epitaxial layers could be attained. A Monte Carlo data evaluation algorithm is proposed, which is capable to determine, by the simultaneous fit of data obtained by x-ray diffraction and x-ray photoelectron spectroscopy, the strain level as a function of Ge concentration.

Superconductivity in amorphous Ta/Ge multilayers

This paper reports measurements of the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) of Ta/Ge multilayers for a range of individual layer thicknesses. Thick amorphous Ta layers which are isolated by thick insulating (Ge) layers have a transition at 0.9 K, and it is noted that for thinner isolated layers ${\mathit{T}}_{\mathit{c}}$ approaches zero as the resistance per square approaches the quantum resistance h/(2e${)}^{2}$. However, the transition temperature is enhanced in samples with thin Ge layers, and in films with Ta layers thinner than 1.5 nm ${\mathit{T}}_{\mathit{c}}$ rises to near 3 K. The enhancement is consistent with a proximity effect involving layers of a Ta-Ge alloy at the layer boundary. \textcopyright{} 1996 The American Physical Society.

A new structure of In-based ohmic contacts ton-type GaAs

Electrical, structural and reaction characteristics of In-based ohmic contacts ton-GaAs were studied. Attempts were made to form a low-band-gap interfacial phase of InGaAs to reduce the barrier height at the metal/semiconductor junction, thus yielding low-resistance, highly reliable contacts. The contacts were fabricated bye-beam sputtering Ni, NiIn and Ge targets on VPE-grownn+-GaAs film (≈1 μm, 2 × 1018 cm−3) in ultrahigh vacuum as the structure of Ni(200 A)/ NiIn(100 A)/Ge(40 A)/n+-GaAs/SI-GaAs, followed by rapid thermal annealing at various temperatures (500–900°C). In this structure, a very thin layer of Ge was employed to play the role of heavily doping donors and diffusion limiters between In and the GaAs substrate. Indium was deposited by sputtering NiIn alloy instead of pure In in order to ensure In atoms to be distributed uniformly in the substrate; nickel was chosen to consume the excess indium and form a high-temperature alloy of Ni3ln. The lowest specific contact resistivity (ϱc) of (1.5 ± 0.5) × 10−6 cm2 measured by the Transmission Line Method (TLM) was obtained after annealing at 700°C for 10 s. Auger sputtering depth profile and Transmission Electron Microscopy (TEM) were used to analyze the interfacial microstructure. By correlating the interfacial microstructure to the electronical properties, InxGa1−xAs phases with a large fractional area grown epitaxially on GaAs were found to be essential for reduction of the contact resistance.

Characteristics of PdIn ohmic contacts to n-GaAs with a thin Ge layer

Effect of a thin (2 nm) Ge layer in PdIn ohmic contacts with different deposition structure were studied. An appropriate deposition structure may extend the optimum annealing temperature window down to 400°C, whereas an inappropriate one lets the effect of a thin Ge layer be negligible. In the former case, specific contact resistances ( ϱ c) were reduced significantly in low temperature range because a regrown GaAs layer doped heavily with Ge formed. Special attention was paid to confirm the mechanism of ohmic contact formation and a possible reaction sequence was derived. After rapid thermal annealing at 600°C for 10 s, a ϱ c value of 2 × 10 −6 Ω- cm 2 was obtained. Also, a thin (4 nm) In x Ga 1− x As layer with an average x value of 0.7 was observed in the metal/GaAs interface. Increment in ϱ c values was observed after sequential aging at 400°C.

Raman study of strain relaxation in Ge on Si

Strain in thin Ge layers grown by molecular beam epitaxy on (100) Si is measured by a Raman technique. When the average Ge thickness is 7 monoatomic layers (ML), Raman results show that the layer is almost coherent to the Si lattice. The strain begins to decrease at an average thickness of 10 ML, i.e., the critical thickness of dislocation generation is 10 ML. On the other hand, the relaxation begins at a thickness of 5 ML, according to reflection high-energy electron diffraction observation during the growth. This initial stage relaxation is due to deformation of islands and not due to dislocation formation. Raman results for thicker layers show that with increasing layer thickness, the misfit strain decreases gradually but more rapidly than predicted by the theory of Matthews and Blakeslee .

Atom-rearrangement in Ge layer grown on Si substrate during anneal observed in real time by coaxial impact collision ion scattering spectroscopy

The atom-rearrangement in thin Ge layers grown on Si(001) substrates during post-growth anneal has been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). CAICISS measurements were carried out in real time during the anneal. A Ge layer was grown at 130°C by molecular beam epitaxy, and after growth the sample temperature was raised to 600°C at a rate of 1°C/s. The intensity of the back-scattered ion beam with incidence along the [01 1 ] direction was measured during the anneal. A gradual decrease in the scattering intensity was observed while the temperature was raised from 300 to 520°C. The decrease in the scattering intensity observed here can be understood in terms of a reduction in the number of interstitial atoms due to atom-rearrangement along the [01 1 ] direction. Plan-view transmission electron microscope (TEM) images showed clear moiré fringes for the sample after anneal, while such moiré fringes were not observed for an as-grown sample. The result of a TEM observation also shows atom-rearrangement in the Ge layer due to the anneal, which is in good agreement with the results of CAICISS measurements.

Very high mobility two-dimensional hole gas in Si/GexSi1−x/Ge structures grown by molecular beam epitaxy

Modulation-doped Si/GexSi1−x/Ge/GexSi1−x structures were fabricated in which a thin Ge layer was employed as the conduction channel for the two-dimensional hole gas. The strained heterostructure was fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The best mobility of the two-dimensional hole gas is 55 000 cm2/V s at 4.2 K with a concentration-dependent hole effective mass of ≤0.10m0. The effect of the Ge/GeSi interface roughness on the 2DHG mobility was studied.

Phase transitions during solid-state formation of cobalt germanide by rapid thermal annealing

Phase transitions that involve solid-state reactions between cobalt and thin films of germanium have been investigated. Germanides are formed by reacting Co (300 Å thick) with thin layers of Ge (∼2000 Å thick) deposited on silicon substrates. Germanium was deposited on Si by rapid thermal chemical-vapor deposition and cobalt was deposited onto Ge by evaporation. The Co/Ge/Si stacked structure samples were then rapid thermally annealed at atmospheric pressure in an inert ambient consisting of Ar. Using x-ray-diffraction spectroscopy, Co5Ge7 and CoGe2 are identified as the phases which form at 300 and 425 °C respectively. The sheet resistance was found to be a strong function of the annealing temperature and a minimum resistivity of approximately 35 μΩ cm is obtained after annealing at 425 °C. The minimum resistivity material corresponds to the CoGe2 phase with an orthorhombic crystal structure. Above 600 °C, the resistivity increases due to an instability of the solid-phase reaction between Co and thin Ge layers deposited on Si. This instability is attributed to rapid Co diffusion at the temperatures which are required to form CoGe2 along with structural defects in the Ge layer.

Crystallization of encapsulated very thin amorphous Ge layers

The crystallization processes and morphologies of a-Ge have been studied in the kinckness range 1–30 nm. The thin Ge layers were evaporated under ultrahigh vacuum (5 × 10 -9 Torr) onto SiO x substrates and were also covered by a thin SiO x layer. The samples were annealed in forming gas and examined by transmission electron microscopy. The morphological stability of the layers depends on the film thickness and surface and interface diffusion processes during crystal growth resulting in ribbon shaped crystals in the thinnest layers. The single crystalline, ribbon-shaped crystals are 100 nm wide and can grow to the length of several μm. The morphological instability of the very thin amorphous layer close to the crystalline phase is proved by the formation of empty regions along the ribbons and around crystalline nuclei. The temperature of crystallization increases with decreasing film thickness in the thickness range 1–5 nm.

Characterization of the B/Si surface electronic structures

High-resolution angle-resolved core-level and valence-band photoelectron spectroscopy have been used to characterize the electronic structures of the B/Si (111)-(√3×√3) surfaces. The results have been compared with theoretic calculations and other group-III metals and Si-terminated Si (111) surfaces that share the same type of surface reconstruction. We have observed a structure evolution from B-T4 to B-S5 and finally to Si-T4 as deposited boron atoms diffuse into the substrate with increasing annealing temperature. The chemically shifted component appearing in the Si 2p core-level spectrum is attributed to charge transfer from the top layer Si and Si adatoms to the sublayer B-S5 atoms. For the Si/Si (111)-(√3×√3) surface, a newly discovered chemically shifted component is associated with back bond formation between the Si adatoms and the underneath Si atoms. A new emission feature has been observed in the valence-band spectra unique to the B/Si (111)-(√3×√3) surface with B-S5 configuration. Thin Ge layer growth on this structure has also been performed, and we found that no epitaxial growth could be achieved and the underneath structure was little disturbed.

In situ oxidation of a thin layer of Ge on Si(001): Observation of GeO to SiO2 transition

A thin layer of Ge grown on Si(001) surface is oxidized in situ and investigated using XPS, AES, RHEED, and MEIS. The samples used are, Ge layer formed by deposition at room temperature and SiGe mixed layer formed by deposition at 550 °C. Oxidation at 250 °C of the RT grown layer leads to the formation of GeO and on heating the surface to 360 °C, oxygen bonds with Si forming SiO2, thereby reducing GeO to elemental Ge. In the epitaxially grown layer (grown at 550 °C), after oxidation, SiO2 and a small amount of GeO are formed. Similar reaction takes place on this surface also, forming SiO2 as the final product on the surface. In the RT grown layer, after oxidation, MEIS shows evidence for the diffusion of Si through the Ge layer towards the surface.

ZnSe-GaAs heterojunction parameters

We synthesized ZnSe-GaAs(100) heterostructures by molecular beam epitaxy and characterized in situ the band offsets by means of X-ray photoemission spectroscopy. We measured a valence band offset of 0.78 eV, i.e. 0.3 eV lower than that observed earlier for ZnSe-GaAs(110) heterostructures grown at room temperature. A further reduction in the valence band offset was obtained by fabricating thin pseudomorphic Ge layers in the interface region. The additional Ge-induced interface dipole increases monotically with Ge layer thickness in the 1 to 6 monolayer range, and saturates at higher coverages. The low valence band offset (0.44 eV) observed in the engineered interfaces should improve hole injection in ZnSe-GaAs based n-p laser diodes.

Interface Phenomena in very thin Si-Ge Heterostructures

ABSTRACTInterfacial properties of thin Si/Ge heterostructures are discussed. It is shown that (SimGen) p superlattices with well defined periodicity can be synthesized on both (100) Si and Ge using low temperature (350 °C) Molecular beam epitaxy. Multilayer structures are however intermixed over one or two monolayers and exhibit a roughness with lateral scale > 100 nm and an amplitude of the order of two Monolayers. On Si the roughness is strongly correlated from interface to interface, possibly due to small surface mobility during growth. Segregation of Ge during growth has also been evidenced by x-ray photoelectron spectroscopy and may explain some of the interface Mixing. The strong composition dependence of Ge diffusion in Si1−xGex alloys causes anisotropie diffusion upon annealing whereby a large composition gradient can be preserved at the heterointerfaces due to the very slow diffusion rate of Ge atoms in pure Si. This effect is important in structures which alternate thin Ge and thick Si layers and is thus more prominent in superlattices grown on Si substrates.

Localized and nonlocalized states in thin Ge layers grown by molecular-beam epitaxy at low temperatures.

Symmetry-allowed and -forbidden resonant and nonresonant Raman scattering from the Ge-Si and the Ge-Ge optical phonons of thin Ge layers grown epitaxially at low temperatures by molecular-beam epitaxy on Si(001) have been studied to characterize the spatial localization of these modes and the perfection of the Ge layers. Both the large wave-vector Ge layer phonons and the Ge-Si interface LO phonons in nanometer thickness films strongly mix with the Si host phonons producing significant perturbations to the simple picture of confined phonon modes in heterostructures. We identify the presence of alloylike regions in these samples from the resonant behavior of the Si-Ge optical phonons, demonstrating that the interfaces between the Si and Ge are not atomicly abrupt. The symmetry properties of the Ge-derived Raman scattering show the layered character of the Ge regions and also indicate that these layers are not perfect.

The Si/Pd(Si,Ge) ohmic contact on n-GaAs

A modified Si/Pd ohmic contact to n-GaAs has been developed based on the solid-phase regrowth mechanism. The Si/Pd contact usually yields a contact resistivity of 2×10−6 Ω cm2. A thin (∼15 Å) layer of additional Ge or Si embedded in the Pd layer of the Si/Pd contact structure is used to reduce the contact resistivity from ∼2×10−6 to 2–4×10−7 Ω cm 2 without suffering from a loss of thermal stability. The reduction in the contact resistivity is explained in terms of the formation of an n+ GaAs surface layer via solid-phase regrowth. The modified contacts show uniform surface and interface morphologies. The contact resistivity of the modified contacts remains in the mid-10−6 Ω cm2 range after annealing at 400 °C for 50 h.

Two-dimensional lattice-mismatched heteroepitaxy of germanium on silicon beyond the critical thickness by introducing a surfactant

Smooth germanium films have been grown on Si(100) surfaces in a two-dimensional fashion by using antimony as a surfactant. Different ways of depositing the surfactant (at the interface between substrate and growing film, after the deposition of a thin Ge layer, and by coevaporation) have been investigated. The grown films, investigated by high-resolution electron microscopy and reflection high-energy electron diffraction, show that the surfactant does not act at the interface. A kinetical approach for the description of surfactant behavior in the growing front is necessary.

Effect of the growth temperature on the RHEED pattern of thin Ge layers on Si(001)−2 × 1 surface

The evolution of RHEED (reflection high-energy electron diffraction) pattern upon deposition of Ge on the Si(001)−2×1 surface substantially depended on the growth temperature. Ge was deposited onto the Si(001)−2×1 surface at 300, 400, 500 and 600°C up to 40 ML (monolayers) by molecular beam epitaxy (MBE). The results indicated that the surface covered by a few ML Ge consists of many steps for low temperature (300, 400°C) deposition, while it is smooth for 600°C deposition. For the low temperature deposition, the islands with {105} facets began to appear at the first stage of the island formation, and changed to the {113} facets with increase in Ge thickness, while for 600°C deposition, the islands with {113} facets grew even at the first stage. For the deposition beyond 20 ML, the RHEED patterns showed the presence of islands with {113} facets for all deposition temperatures.

Characterization of ultrathin Ge epilayers on (100) Si

The understanding of the epitaxy of pure Ge layers on Si is an important step towards the synthesis of SimGen (m, n < 10 monolayers) short-period superlattices. The possibility of a direct band-gap character makes these structures extremely attractive. We have grown thin buried Gen ([Formula: see text] monolayers) films on (100) Si by molecular beam epitaxy and studied their structural properties by a variety of techniques including Raman scattering spectroscopy, glancing incidence X-ray reflection, Rutherford backscattering, transmission electron microscopy, and extended X-ray absorption fine structure analysis. All these techniques allowed detection of the thin Ge layers and provided information about the thickness, morphology, strain distribution, and interface sharpness of these heterostructures. The Ge„ films with [Formula: see text] had a two-dimensional nature and showed no sign of strain relaxation. Intermixing at the Si–Ge interfaces was present in all these films and estimated to be not more than two monolayers. This smearing at the interfaces may have contributed to the maintenance of that pseudomorphicity. A thicker Ge layer (n = 12) showed evidence of strain relaxation and clustering in three-dimensional islands.