Molecular beam epitaxy synthesis of Si1−yCy and Si1−x−yGexCy alloys and Ge islands using an electron cyclotron resonance argon/methane plasma

We report the growth of Si_1−yC_y and Si_1−x−yGe_xC_y alloys on Si(001) by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy using an argon/methane gas mixture. Various Si/Si_1−yC_y and Si/Si_1−x−yGe_xC_y multilayers have been grown and characterized principally by X-ray diffraction and Raman spectroscopy. The influence of growth parameters and electron cyclotron resonance plasma source operating conditions on the C substitutional incorporation was studied. Under optimum growth conditions the structures show good structural properties and sharp interfaces with carbon being essentially substitutionally incorporated up to concentrations of 1%. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation. Si_1−x−yGe_xC_y layers grown with this technique exhibit the strain compensation and enhanced thermal stability expected for these ternary alloys. Carbon pre-deposition of Si through surface exposure to the argon/methane plasma is shown to act as an antisurfactant on the growth of Ge islands by suppressing the formation of a Ge wetting layer on the surface.     

Bulk Si1−xGex single- and poly-crystals: a new prospective material for electronics

Si_1−xGe_x is a prospective material for electronics. This is mostly because Si_1−xGe_x-based technology is close to silicon-based technology, which is advanced, widely applicable, and cheap. The majority of work on this material is devoted to Si_1−xGe_x-based heteroepitaxy, and in particular to the Si_1−xGe_x/Si system; few publications are devoted to bulk single-crystal. Here we focus on some interesting properties of bulk Si_1−xGe_x solid solutions. First, under heat treatment and alpha- and beta-irradiation the efficiency of defect introduction decreases with the increase of Ge composition of the Si_1−xGe_x single-crystal. This is because Ge atoms in a crystal lattice are annihilation centers for primary defects. Hence, this material is more resistant to temperature and radiation than silicon. Second, it is known that, since Z(Ge)Z(Si), the sensitivity of the material to irradiation should increase with the concentration of Ge. We show that Si_1−xGe_x nuclear detectors have efficiency three times higher than silicon detectors. Finally, we note that one of the major problems in materials based on solid solutions is the composition uniformity. Our investigations on the influence of composition fluctuations on material properties have shown that the material has a sufficient uniformity at x<0.1. Such an alloy is a prospective material for electronics.     

Self-assembled growth and magnetotransport investigations on strained Si/SiGe multilayers on vicinal (113)-Si surfaces

We have reported on the growth and magnetotransport properties of modulation p-doped Si_1−xGe_x quantum wells on strained multilayers of 2.5 nm Si_1−xGe_x/10 nm Si on vicinal (113) Si surfaces. Owing to the strong step-bunching properties of the (113) Si surface, both the Si_1−xGe_x and the Si layers exhibited a regular pattern of large steps. Low-temperature magnetotransport measurements revealed a hole density (6–9×10¹¹ cm⁻²) independent of direction, whereas a pronounced mobility anisotropy was found. The mobility (1000–2000 cm²/Vs) was approximately two times higher along the [33-2] direction compared to a perpendicular [−110] direction. This is attributed to anisotropic hole scattering caused by anisotropic shear strain which is always present in strained layers on (113) Si. No influence of the large regular steps, whose direction is given by the direction of the substrate miscut, on the mobility was found.     

Doping and electrical characteristics of in-situ heavily B-doped Si1−x−yGexCy films epitaxially grown using ultraclean LPCVD

Doping and electrical characteristics of in-situ heavily B-doped Si_1−x−yGe_xC_y (0.22<x<0.6, 0<y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550°C in a SiH₄–GeH₄–CH₃SiH₃–B₂H₆–H₂ gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH₄ partial pressure, and only at high GeH₄ partial pressure did it decrease with increasing B₂H₆ as well as CH₃SiH₃ partial pressures. With the B₂H₆ addition, the Ge and C fractions scarcely changed and the B concentration (C_B) increased proportionally. The C fraction increased proportionally with increasing CH₃SiH₃ partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si_1−x−yGe_xC_y with y=0.0054 or below, the carrier concentration was nearly equal to C_B up to approximately 2×10²⁰ cm⁻³ and was saturated at approximately 5×10²⁰ cm⁻³, regardless of the Ge fraction. The B-doped Si_1−x−yGe_xC_y with high Ge and C fractions contained some electrically inactive B even at the lower C_B region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially.     

Physics and applications of Si/SiGe/Si resonant interband tunneling diodes

Room temperature (RT) current–voltage characteristics of Si/Si_1−xGe_x/Si p⁺-i-n⁺ interband tunneling diodes are presented. The variation of the structural properties results in a more detailed picture of the tunneling process in these diodes, which allows further improvement of the relevant parameter. Special attention is paid to the peak current density (PCD) and the peak-to-valley current ratio (PVCR) of the devices. For an optimized structure with a 3-nm thick Si_0.54Ge_0.46 layer in the intrinsic zone a record PVCR of 6.0 at a PCD of approximately 1.5 kA/cm² was achieved. By reducing the layer thickness to 2.6 nm and simultaneously increasing the Ge content to 54%, the PCD increases to 30 kA/cm² at a high PVCR of 4.8.     

Nucleation and evolution of Si1−xGex islands on Si(001)

In this study, we report a systematic investigation of the metastable morphologies of Si_1−xGe_x layers obtained by the interplay of kinetics and thermodynamics during growth on Si(001). We show that three main growth regimes can be distinguished as a function of the misfit and of the deposited thickness. They correspond to three equilibrium steady state morphologies that consist of (105)-facetted hut islands, huts and domes in co-existence, and a bimodal size distribution of domes, respectively. The shape transitions between these states are attributed to different levels of relaxation.     

Structural characterization of ion-beam-induced epitaxially crystallized thin layers of III–V and IV–IV semiconductors

Structural properties of ion-beam-induced epitaxial crystallization (IBIEC) for amorphous layers of GaAs on GaAs(100), BP on BP(100) and Si_1−xGe_x and Si_1−x−yGe_xC_y on Si(100) have been investigated. Crystallization was induced by ion bombardment with 400 keV Ne, Ar or Kr at 150 °C for GaAs and at 350 °C for BP. Epitaxial crystallization up to the surface was observed both in GaAs and BP at temperatures much below those required for the solid phase epitaxial growth (SPEG). The growth rate per nuclear energy deposition density has shown a larger dependence on ion dose rate in cases of heavier ion bombardments both for GaAs and BP. Crystallization of a-GaAs with ions whose projected ranges are within the amorphous layer thickness was also observed at 150 °C. Epitaxial crystallization of Si_1−xGe_x and Si_1−x−yGe_xC_y layers (x = 0.13 and y = 0.014 at peak concentration) on Si(100) formed by high-dose implantation of 80 keV Ge and 17 keV C ions has been observed in the IBIEC process with 400 keV Ar ion bombardments at 300–400 °C. Crystalline growth by IBIEC has shown a larger growth rate in Si_1−x−yGe_xC_y/Si} than in Si_1−xGe_x/Si} with the same Ge concentration for all bombardments under investigation. X-ray rocking-curve measurements have shown a strain-compensated growth in Si_1−x−yGe_xC_y/Si}, whereas Si_1−xGe_x/Si} samples have shown a growth with strain accommodation.     

Growth of Si9Ge1 single crystals and their application in high-resolution neutron spectroscopy

We present a new method to make bulk single crystals from Si_1−xGe_x alloys over a wide concentration range, and the potential as a neutron monochromator in particular for high-resolution backscattering spectrometer is discussed. A first monochromator with x = 8.9 at.% has been built and its good performance is demonstrated by a measurement of the tunnel spectrum of a molecular crystal.     

Applications of variable angle spectroscopic ellipsometry to strained SiGe alloy heterostructures

Variable angle spectroscopic ellipsometry (VASE) has been used to characterize several Si_xGe_1−x/Ge heterostructures. First, Si_xGe_1−x/Ge superlattice (SL) structures were characterized in terms of the layer thicknesses, composition, x, of the Si_xGe_{1− x} layer, and oxide thickness. High-resolution X-ray diffraction results are also presented for the Si_xGe_1−x/Ge SL structures and are shown to be in close agreement with the VASE results once strain effects are taken into account. VASE has also been used to study thick, Ge-rich Si_xGe_1−x/Ge heterostructures that have been grown on Si substrates. A stepped buffer has been deposited first in order to minimize the strain in the Si_xGe_1−x/Ge layers. VASE can be used to give a qualitative determination of the residual strain along with the thickness of all layers within the optical penetration depth from the surface.     

Growth of high-quality relaxed SiGe films with an intermediate Si1−yCy layer for strained Si n-MOSFETs

High quality and thin relaxed SiGe films were grown on Si (0 0 1) using ultra high vacuum chemical vapor deposition (UHV/CVD) by employing an intermediate Si_1−yC_y layer. The Si_1−yC_y/SiGe bilayer was found to change mechanism of relaxation in the SiGe overlayer. Compared with the samples with a Si layer, the equilibrium critical thickness of top SiGe films with rough surface by introducing an intermediate Si_0.986C_0.014 layer are drastically reduced; this result was attributed to larger tensile stress in the inserted Si_0.986C_0.014 layer. With a 210-nm-thick Si_0.8Ge_0.2 overlayer, this Si_0.8Ge_0.2/Si_0.986C_0.014/Si_0.8Ge_0.2 heterostructure has a threading dislocation density (TDs) less than 1 × 10⁵ cm⁻² and a residual strain of 30%. The root mean square (RMS) of surface roughness for this sample was measured to be about 1.8 nm. In this SiGe/Si_1−yC_y/SiGe structure, C atoms in the intermediate Si layer will improve the relaxation of thin SiGe overlayer, however, the relaxation for the 700-nm-thick SiGe overlayer is independent of the addition of C. The point defects rich Si_0.986C_0.014 layer plays the role to confine the misfit dislocations, which formed at the interface of the top Si_0.8Ge_0.2 and the Si_0.986C_0.014 layer, and blocked the propagation of TDs. Strained-Si n-channel metal-oxide-semiconductor transistors (n-MOSFETs) with a 210-nm-thick Si_0.8Ge_0.2 overlayers as buffer were fabricated and examined. Drain current and effective electron mobility for the strained-Si device with this novel substrate technology was found to be 100 and 63% higher than that of control Si device. Our results show that thin relaxed Si_0.8Ge_0.2 films with the intermediate Si_0.986C_0.014 layer serve as good candidates for high-speed strained-Si devices.     

MBE Si regrowth on carbon-induced Si(001)-c(4×4) reconstructions studied by RHEED

Si(001)-c(4×4) surfaces are obtained by exposing Si(001)-2×1 surfaces at 600°C to ethylene doses that determine C coverages in the submonolayer range. This reconstruction reveals a carbon enrichment of the topmost silicon layers. As the c(4×4) reflection high energy electron diffraction pattern can be maintained in spite of rather thick Si regrowth layers, we can conclude that this C derm is able to float at the surface during the Si capping. This segregation process is strongly dependent on the growth mode. As identified by RHEED intensity oscillations, a Si step flow growth is necessary to allow carbon to float in the first four silicon top-layers. An interplay is found between the kinetic growth conditions leading to this C-segregation and those of a self-organization process of C-rich clusters that we have observed in the course of Si_1−yC_y alloy growth obtained by codeposition of silicon and carbon.     

Preparation of B-doped a-Si1−xCx:H films and heterojunction p–i–n solar cells by the Cat-CVD method

B-doped a-Si_1−xC_x:H films for a window layer of Si thin film solar cells have been prepared by the Cat-CVD method. It is found that C is effectively incorporated into the films by using C₂H₂ as a C source gas, where an only little C incorporation is observed from CH₄ and C₂H₆ under similar deposition conditions. Using a-Si_1−xC_x:H films grown from C₂H₂, heterojunction p–i–n solar cells have been prepared by the Cat-CVD method. The cell structure is (SnO₂ Asahi-U)/ZnO/a-Si_1−xC_x:H(p)/a-Si:H(i)/μc-Si:H(n)/Al. The obtained conversion efficiency was 5.4%.     

Catalytic CVD growth of Si–C and Si–C–O alloy films by using alkylsilane and related compounds

Cat-CVD method has been applied to the growth of Si–C and Si–C–O alloy thin films. Growth mechanism has been studied with emphasis on the effects of filament materials. Growth rates and alloy compositions were measured for W, Ta, Mo and Pt filaments at the filament temperatures ranging from 1300 to 2000 °C. Si_1−xC_x films with x ranging from 0.38 to 0.7 could be grown by using single molecule source Si(CH₃)₂H₂ (dimethylsilane). Si–C–O ternary alloy films was successfully prepared by using Si(OC₂H₅)₄ (tetraethoxysilane) and Si(CH₃)₂(OCH₃)₂ (dimethyldimethoxysilane) molecules.     

Chemical vapour deposition of molybdenum carbides: aspects of phase stability

Thin films of different molybdenum carbides (δ-MoC_1−x, γ′-MoC_1−x and Mo₂C) have been deposited from a gas mixture of MoCl₅/H₂/C₂H₄ at 800°C by CVD. The H₂ content in the vapour has a strong influence on the phase composition and microstructure. Typically, high H₂ contents lead to the formation of nanocrystalline δ-MoC_1−x films while coarse-grained γ′-MoC_1−x is formed with an H₂-free gas mixture. This phase has previously only been synthesized by carburization of Mo in a CO atmosphere and it has therefore been considered as an oxycarbide phase stabilized by the presence of oxygen in the lattice. Our results, however, show that γ′-MoC_1−x films containing only trace amounts of oxygen can be deposited by CVD. Stability calculations using a FP-LMTO method confirmed that the γ′-MoC_1−x phase is stabilized by oxygen but that the difference in energy between e.g. δ-MoC_0.75 and oxygen-free γ′-MoC_0.75 is small enough to allow the synthesis of the latter phase in the absence of kinetic constraints. Annealing experiments of metastable δ-MoC_1−x and γ′-MoC_1−x films showed two different reaction products suggesting that kinetic effects play an important role in the decomposition of these phases.     

Transmission electron microscopy of annealed a-Si1−xCx:H/Al films with different Al grain sizes

The crystallization behavior of a-Si_1−xC_x:H/Al films after annealing has been investigated by transmission electron microscopy and Raman scattering. It is found that the crystallization process is complex and non-uniform, and that both equiaxial and branching Si grains with many twins and stacking faults arise at annealing temperatures as low as 250 °C. Both fine polycrystalline β-SiC grains and fractal-like -SiC aggregates are first observed in a few regions in a-Si_1−xC_x:H/Al films annealed at 350 °C. The increase of the Al grain size can cause a decrease in the crystallization temperature and a rise in the grain growth rate of Si. At higher annealing temperatures, the reaction process SiC+Al→Al₄C₃+Si is predominant.     

Epitaxial growth of strained Si1−yCy films by the hot-wire cell method and its application to metal oxide semiconductor devices

We succeeded in obtaining strained Si_1−yC_y films at a substrate temperature of 200 °C by the hot-wire cell method. The substitutional carbon concentration in films annealed at 700 °C was 0.9%, while it was limited to 0.13% for a sample grown by gas-source molecular beam epitaxy (MBE) at a substrate temperature of 700 °C. We investigated the thermal stability of strained Si_1−yC_y films for device application. Annealing at over 900 °C caused the formation of 3C-SiC and relaxation of the strain occurred. From this result, we found that the process temperature should be lower than 800 °C. A low-temperature MOSFET process, in which all process temperatures after deposition of Si_1−yC_y were lower than 800 °C, was developed and a strained Si_1−yC_y MOSFET was fabricated.     

Highly conductive microcrystalline silicon carbide films deposited by the hot wire cell method and its application to amorphous silicon solar cells

Microcrystalline silicon carbide (μc-Si_1−xC_x) films were successfully deposited by the hot wire cell method using a gas mixture of SiH₄, H₂ and C₂H₂. It was confirmed by Fourier transform infrared and X-ray diffraction analyses that the films consisted of μc-Si grains embedded in a-Si_1−xC_x tissue. The p-type μc-Si_1−xC_x films were deposited using B₂H₆ as a doping gas. A dark conductivity of 0.2 S/cm and an activation energy of 0.067 eV were obtained. The p-type μc-Si_1−xC_x was used as a window layer of a-Si solar cells, in which the intrinsic layer was deposited by photo-chemical vapor deposition, and an initial conversion efficiency of 10.2% was obtained.     

Heteroepitaxial growth of Zn1−xCdxSe on cleavage-induced GaAs (110) surface in ultra high vacuum

Zn_1−xCd_xSe epitaxial growth by molecular beam epitaxy (MBE) on the GaAs (110) surface cleaved in ultra high vacuum (UHV) was investigated. The growth mode of Zn_1−x Cd_xSe on GaAs (110) was not a simple Stranski–Krastanow type. At initial growth stage, growth mode was two-dimensional type. However, as the growth proceeds three-dimensional island growth and two-dimensional growth modes compete. As a result, two kinds of structures were spontaneously formed on the surface, pyramidal-shaped islands and ridge structures aligned to the [1 0] direction. Anisotropic in-plane strain relaxation on (110) is suggested as the formation mechanism of such structures.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Contribution to the comprehension of dissipation phenomena in lead zirconate titanate: aliovalent doping effect

By the solid reaction method, undoped, potassium doped and niobium doped lead zirconate titanate (PZT) are elaborated. The mechanical losses measured in the range of the Hz as a function of temperature shows two peaks R₁ and R₂, and a ferroelectric transition peak P₁ between ferroelectric and para-electric states on the undoped PZT—Pb(Zr_0.54Ti_0.46)O₃—noted PZT54/46. Potassium doped PZT—Pb_1−xK_x(Zr_0.54Ti_0.46)O₃—shortly called PKZT 100x/54/46 shows an increase in the height of both the peaks at a doping content, x, less than 0.5 at.% but an opposite effect is observed above this value. Niobium doped PZT—Pb[(Zr_0.54Ti_0.46)_1−yNb_y]O₃—shortly called PNZT 100y/54/46, shows the vanish of the R₂ peak and the decrease of the height of the R₁ peak when the doping content increases.