Growth and characterization of nc-Ge prepared by microwave annealing

Ge nanocrystals embedded in Silicon oxide matrix have been synthesized on Si substrate by co-sputtering of SiO₂ and Ge using RF magnetron sputtering technique. The as deposited films were subjected to microwave annealing at 800 and 900 °C. The structural characterization was performed by using X-ray diffraction (XRD) and Raman spectrometry. XRD measurements confirmed the formation of Ge nanocrystals. Raman scattering spectra showed a peak of Ge-Ge vibrational mode around 299 cm⁻¹, which was caused by quantum confinement of phonons in the Ge nanocrystals. Variation of the nanocrystal size with annealing temperature has been discussed. Advantages of microwave annealing are explained in detail.     

Synthesis of strained SiGe on Si(100) by pulsed laser induced epitaxy

Pulsed laser induced epitaxy (PLIE), based on melting/solidification processes induced by nanosecond laser pulses, is used to synthesize pseudomorphic Si_1 − xGe_x epilayers from 20 to 80 nm thick Ge layers evaporated on a Si(100) wafer. Ge concentration and strain are characterized by transient reflectivity, energy dispersive X-ray analysis and X-ray diffraction from symmetric (004) and asymmetric (224) reflections. For a low Ge thickness or a high laser fluence, PLIE builds up only a pseudomorphic strained Si_1 − xGe_x layer with a graded Ge composition reaching x ≈ 19% near the surface. When the Ge amount is in excess to achieve this situation, PLIE forms additionally a relaxed Si_1 − xGe_x layer with x values up to ≈40% over the previous pseudomorphic layer.     

Laser ablation and growth of Si and Ge

In this work, we investigated the laser ablation and deposition of Si and Ge at room temperature in vacuum by employing nanosecond lasers of 248 nm, 355 nm, 532 nm and 1064 nm. Time-integrated optical emission spectra were obtained for neutrals and ionized Ge and Si species in the plasma at laser fluences from 0.5 to 11 J/cm². The deposited films were characterized by using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. Amorphous Si and Ge films, micron-sized crystalline droplets and nano-sized particles were deposited. The results suggested that ionized species in the plasma promote the process of subsurface implantation for both Si and Ge films while large droplets were produced from the superheated and melted layer of the target. The dependence of the properties of the materials on laser wavelength and fluence were discussed.     

Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si?substrate by a (SiO(2)+Ge)/(SiO(2)+GeO(2)) superlattice approach, using an rf magnetron sputtering technique with a Ge+SiO(2) composite target and subsequent thermal annealing in N(2) ambient at 750?°C for 5?min. X-ray diffraction (XRD) measurements indicated the formation of Ge nanocrystals with an average size estimated to be 9.8?nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode shifted downwards to 298.8?cm(-1), which was caused by quantum confinement of phonons in the Ge nanocrystals. X-ray photoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainly Ge(0) in the (SiO(2)+Ge) layer and Ge(4+) in the (SiO(2)+GeO(2)) layer in the superlattice structure. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (SiO(2)+Ge) layers, and had good crystallinity. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the 'Z' growth direction compared with the conventional Ge-ncs fabrication method using a single and thick SiO(2) matrix film.     

Laser assisted formation of binary and ternary Ge/Si/Sn alloys

Binary and ternary Si/Ge/Sn alloys were epitaxially grown on virtual Germanium buffer layers using pulsed laser induced epitaxy with a 193 nm Excimer laser source. The role of the processing parameters on the intermixing of the components (Sn, Ge and Si) has been studied. Characterization of the resulting Ge_1 − xSn_x and Si_{1 − y − x}Ge_ySn_x alloys yield up to 1% Sn concentration in substitutional sites of the Ge or SiGe matrix.     

Chemical bonding of a-Ge1−xCx:H films grown by RF reactive sputtering

Amorphous hydrogenated germanium-carbon (a-Ge_1−xC_x:H) films were deposited by RF reactive sputtering pure Ge (1 1 1) target at different flow rate ratios of CH₄/(CH₄+Ar) in a discharge Ar/CH₄, and their composition and chemical bonding were investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). XPS and FTIR results showed the content of germanium in the films decreased with the increase of the flow rate ratio CH₄/(Ar+CH₄), and the Ge-C, Ge-H, C-H bonds were formed in the films. The fraction of Ge-C, Ge-H, and C-H bonds was strongly dependent on the flow rate ratio. Raman results indicated that the films also contain both Ge-Ge and C-C bonding. Based on the change of the chemical bonding of a-Ge_1−xC_x:H films with the flow rate ratio CH₄/(CH₄+Ar), an optimal experimental condition for the application of infrared windows was obtained.     

Fabrication of high quality Ge virtual substrates by selective epitaxial growth in shallow trench isolated Si (001) trenches

To further boost the CMOS device performance, Ge has been successfully integrated on shallow trench isolated Si substrates for pMOSFET fabrication. However, the high threading dislocation densities (TDDs) in epitaxial Ge layers on Si cause mobility degradation and increase in junction leakage. In this work, we studied the fabrication of Ge virtual substrates with low TDDs by Ge selective growth and high temperature anneal followed by chemical mechanical polishing (CMP). With this approach, the TDDs in both submicron and wider trenches were simultaneously reduced below 1 × 10⁷ cm^− 2 for 300 nm thick Ge layers. The resulting surface root-mean-square (RMS) roughness is about 0.15 nm. This fabrication scheme provides high quality Ge virtual substrates for pMOSFET devices as well as for III-V selective epitaxial growth in nMOSFET areas. A confined dislocation network was observed at about 50 nm above the Ge/Si interface. This dislocation network was generated as a result of effective threading dislocation glide and annihilation. The separation between the confined threading dislocations was found in the order of 100 nm.     

Zn/ZnO core/shell nanoparticles synthesized by laser ablation in aqueous environment: Optical and structural characterizations

Zn/ZnO core/shell nanoparticles are synthesized by pulsed laser ablation (PLA) of Zn metal plate in the aqueous environment of sodium dodacyl sulfate (SDS). Solution of nanoparticles is found stable in the colloidal form for a long time, and is characterized by UV-visible absorption, transmission electron microscopy (TEM), photoluminescence (PL) and Raman spectroscopic techniques. UV-visible absorption spectrum has four peaks at 231, 275, 356, and 520 nm, which provides primary information about the synthesis of core-shell and elongated nanoparticles. TEM micrographs reveal that synthesized nanoparticles are monodispersed with three different average sizes and size distributions. Colloidal solution of nanoparticles has significant absorption in the green region, therefore, it absorbs 514·7 nm light of Ar⁺ laser and emits in the blue region centred at 350 and 375 nm, violet at 457 nm and green at 550 nm regions. Raman shift is observed at 300 cm⁻¹ with PL spectrum, which corresponds to ³E_2N and E_3L mode of vibrations of ZnO shell layer. Synthesis mechanism of Zn/ZnO core/shell nanoparticles is discussed.     

Residual stresses and Raman shift relation in anatase TiO2 thin film

Anatase TiO₂ film (100-1000 nm thick) grown on glass, sapphire (0001), and Si (100) substrates by pulsed dc-magnetron reactive sputtering were evaluated for stress and strain analysis using Raman spectroscopy and curvature measurement techniques. The X-ray analysis revealed that films prepared for this study were purely anatase, and the measurements indicate that the film exhibit that (101) is the preferred growth orientation of the crystallites, especially for the film thicker than 100 nm. Curvature measurements and Raman spectroscopy, with 514.5 nm excitation wavelength, phonon line shift were used for stress analysis. A comparison between Raman lineshapes and peak shifts yields information on the strain distribution as a function of film thickness. The measurements of residual stresses for crystalline anatase TiO₂ thin film showed that all thin film were under compressive stress. A correlation between Raman shifts and the measured stress from the curvature measurements was established. The behavior of the anatase film on three different substrates shows that the strain in film on glass has a higher value compared to the strain on sapphire and on silicon substrates. The dominant 144 cm^− 1E_g mode in anatase TiO₂ clearly shifts to a higher value by 0.45-5.7 cm^− 1 depending on the type of substrate and film thickness. The measurement of the full width at half maximum values of 0.59-0.80 (2θ°) for the anatase (101) peaks revealed that these values are greater than anatase powder 0.119 (2θ°) and this exhibits strong crystal anisotropy with thermal expansion.     

Raman scattering studies of ultrashallow Sb implants in strained Si

Sheet resistance (R _s) reductions are presented for antimony doped layers in strained Si. We use micro-Raman spectroscopy to characterise the impact of a low energy (2 keV) Sb implantation into a thin strained Si layer on the crystalline quality and resultant stress in the strained Si. The use of 325 nm UV laser light enables us to extract information from the top ∼9 nm of the strained Si layer. Prior to implantation the Si layer is fully strained with a tensile stress value ∼1.41 GPa, in agreement with the calculated theoretical maximum on a strain relaxed buffer with 17% Ge content. There is a clear decrease in the intensity of the Si Raman signal following Sb implantation. The lattice damage and lattice recovery achieved by subsequent rapid thermal anneal (RTA) is quantified using the amplitude and full width at half maximum (FWHM) of the crystalline Si peak. The shift of the Raman Si peak is a key parameter in the interpretation of the spectra. The ion-implanted sample is studied in terms of a phonon coherence length confinement model. Carrier concentration effects are seen to play a role in the Raman shift following electrical activation of the Sb atoms by RTA.     

Influence of Ge addition on the morphology and properties of TiN thin films deposited by magnetron sputtering

Thin films of TM-X-N (TM stands for early transition metal and X = Si, Al, etc.) are used as protective coatings. The most investigated among the ternary composite systems is Ti-Si-N. The system Ti-Ge-N has been chosen to extend the knowledge about the formation of nanocomposite films. Ti-Ge-N thin films were deposited by reactive magnetron sputtering on Si and WC-Co substrates at T_s = 240 °C, from confocal Ti and Ge targets in mixed Ar/N₂ atmosphere. The nitrogen partial pressure and the power on the Ti target were kept constant, while the power on the Ge target was varied in order to obtain various Ge concentrations in the films. No presence of Ge-N bonds was detected, while X-ray photoelectron spectroscopy measurements revealed the presence of Ti-Ge bonds. Transmission Electron Microscopy investigations have shown important changes induced by Ge addition in the morphology and structure of Ti-Ge-N films. Electron Energy-Loss Spectrometry study revealed a significant increase of Ge content at the grain boundaries. The segregation of Ge atoms to the TiN crystallite surface appears to be responsible for limitation of crystal growth and formation of a TiGe_y amorphous phase.     

Effect of carbon on lattice strain and hole mobility in Si1-x Ge x alloys

Pseudomorphic Si_1-x Ge _x and partially strain compensated layers with different Ge and C fractions have been grown at 500 °C by ultra high vacuum chemical vapor deposition on Si (100) substrates. The degree of strain compensation of the layers has been investigated by high resolution X-ray diffraction and simple application of the linear elasticity theory. The surface morphology of the layers has been characterized by atomic force microscopy. The dependence of Si–Si Raman mode vibrations on strain and composition of binary and ternary alloys have been explained with experimental and theoretically calculated results. The Hall hole mobility is found to increase with decreasing compressive strain or effective Ge content in the layer throughout the temperature range of 120–300 K.     

Forbidden energy band gap in diluted a-Ge1 − xSix:N films

By means of electron gun evaporation Ge_1 − xSi_x:N thin films, in the entire range 0 ≤ x ≤ 1, were prepared on Si (100) and glass substrates. The initial vacuum reached was 6.6 × 10^− 4 Pa, then a pressure of 2.7 × 10^− 2 Pa of high purity N₂ was introduced into the chamber. The deposition time was 4 min. Crucible-substrate distance was 18 cm. X-ray diffraction patterns indicate that all the films were amorphous (a-Ge_1 − xSi_x:N). The nitrogen concentration was of the order of 1 at% for all the films. From optical absorption spectra data and by using the Tauc method the energy band gap (E_g) was calculated. The Raman spectra only reveal the presence of SiSi, GeGe, and SiGe bonds. Nevertheless, infrared spectra demonstrate the existence of SiN and GeN bonds. The forbidden energy band gap (E_g) as a function of x in the entire range 0 ≤ x ≤ 1 shows two well defined regions: 0 ≤ x ≤ 0.67 and 0.67 ≤ x ≤ 1, due to two different behaviors of the band gap, where for x > 0.67 exists an abruptly change of E_g(x). In this case E_g(x) versus x is different to the variation of E_g in a-Ge_1 − xSi_x and a-Ge_1 − xSi_x:H. This fact can be related to the formation of Ge₃N₄ and GeSi₂N₄ when x ≤ 0.67, and to the formation of Si₃N₄ and GeSi₂N₄ for 0.67 ≤ x.     

Properties of La0.75Sr0.25MnO3 films grown on Si substrate with Si1−xGex and Si1−yCy buffer layers

The structural and electrical properties of La_0.75Sr_0.25MnO₃ (LSMO) film on Bi₄Ti₃O₁₂ (BTO)/CeO₂/YSZ buffered Si_1−xGe_x/Si (0.05 ≤ x ≤ 0.2 for compressive strain), blank Si, and Si_1−yC_y/Si (y = 0.01 for tensile) were studied. X-ray high resolution reciprocal lattice mapping (HRRLM) and atomic force microscopy (AFM) show that structural properties of LSMO and buffer oxide layers are strongly related to the strain induced by amount of Ge and C contents. The RMS roughness of LSMO on Si_1−xGe_x/Si has a tendency to increase with increasing of Ge content. Electrical properties of LSMO film with Ge content up to 10% are slightly improved compared to blank Si whereas higher resistivity values were obtained for the samples with higher Ge content.     

Raman study of the strain and H<Subscript>2</Subscript> preconditioning effect on self-assembled Ge island on Si (001)

An investigation of the microscopic mechanisms of Ge self-assembling island growth is of great importance for future optoelectronic applications of quantum dot nanostructures. In this study, two sets of self-assembled germanium islands on Si (001) substrate, with and without preconditioning using a high-temperature hydrogenation step on their nucleation and subsequent temporal evolution, were grown by low-pressure chemical vapor deposition (LPCVD). The average germanium concentration, mean diameter of Ge crystalline regions and the strain inside the germanium quantum dots are characterized with high resolution micro-Raman spectroscopy (μRS). Both the intensity and peak position of the Si–Si vibration mode at about 520.07 cm⁻¹ in the Raman spectra have been used as a reference to separate the germanium Raman signal from the overlapping localized Si–Si optical phonon at ∼300 cm⁻¹.In the absence of preconditioning, both the island size and germanium composition increase steadily as a function of deposition time. However, on the H₂ preconditioned surface, the nucleation and growth rates are greatly increased during the first stages and slow down significantly after deposition for 10 s.Our results indicate that the compressive strain inside the islands acts as a barrier for Ge adatoms to diffuse from the wetting layer into the islands. For the growth times used in this study, for both sets of samples with and without H₂ preconditioning, the normalized rate of increase of the Ge concentration (%Δ [Ge]/Δ t) decreases by ∼0.13/s for a 1% compressive strain increase. The H₂ preconditioning can initially increase the density of island nucleation sites, but cannot accelerate the Ge island growth. It tends to lower %Δ [Ge]/Δ t by 0.015/s instead. The decreased strain due to surface roughing is the principal reason why the Ge islands grow so rapidly at the beginning on the H₂ preconditioned samples.     

UV Raman studies on carbon nitride structures

Visible (514 nm) and deep UV (257 nm) Raman spectra of monoclinic tetracyanoethylene (tcne) are recorded at ambient conditions and also after laser heating at ambient pressure and at 40 GPa. Tetracyanoethylene (C₂(CN)₄) is a convenient precursor to synthesize hard C₃N₄ materials. At low incident laser powers the UV Raman spectra of virgin tcne resemble visible Raman spectra, and at higher powers there appear new, broad modes that increase in intensity as a function of laser power. When tcne is laser-heated at ambient pressure, there are two broad UV Raman peaks about 1,405 cm⁻¹ and 1,604 cm⁻¹ whereas visible laser Raman excitation results in too high a fluorescent background to show up any Raman modes. Raman spectrum of tcne laser heated at 40 GPa show broad peaks indicative of multiphase formation. The spectrum has additional modes at lower frequencies, and comparison with calculated Raman frequencies points to possible formation of α-C₃N₄.     

Effect of a pulsed Nd:YAG laser irradiation on multi-walled carbon nanotubes film

Multi-walled carbon nanotubes (MWCNTs) film have been analyzed by Raman spectroscopy to clarify the effect of a pulsed Nd:YAG laser heating. The MWCNTs film surface was flashed with the fundamental harmonic (λ = 1064 nm) or the second harmonic (λ = 532 nm) of a single pulse of Nd:YAG laser in the air. The dynamics of pulsed nanosecond laser heating process was simulated by the solution of the one-dimensional heat conduction equation. At the laser fluence of 500 mJ/cm² with Nd:YAG laser (λ = 1064 nm), the surface reached the maximum temperature 1395 °C at 12 ns. Moreover, the Raman spectroscopy of MWCNTs films before and after irradiation were measured. The intensity of the two characteristic Raman shifts I_D (defect-mode) and I_G (graphite-mode) was measured by the Raman spectroscopy. The maximum surface temperature was calculated and compared with the I_G/I_D ratio of MWCNTs film. The graphitization occurred on the sample after irradiation.     

Effect of the orientation on the ferroelectric-antiferroelectric behavior of sol-gel deposited (Pb,Nb)(Zr,Sn,Ti)O3 thin films

The (Pb,Nb)(Zr,Sn,Ti)O₃ (PNZST) antiferroelectric thin films were prepared on two different substrates by sol-gel methods. Films derived on the LNO/Pt/Ti/SiO₂/Si substrates showed a strong (100) preferred orientation. The dependence of electrical properties derived on the Pt/Ti/SiO₂/Si and LNO/Pt/Ti/SiO₂/Si substrates have been studied, with the emphasis placed on field-induced phase switching from the antiferroectric to the ferroelectric state. The PNZST thin films deposition on two kinds of substrates show different phase transition behavior and associated properties such as antiferroelectric (AFE) to ferrroelectric (FE) switching field E_AFE-FE, FE to AFE switching field E_FE-AFE and the hysteresis ΔE=E_AFE-FE−E_FE-AFE.     

Inspection of intermediate stress-induced electronic traps in Si/Al2O3 system

The physical characteristics of device-grade thin silicon film at (1 0 0) grown on α-Al₂O₃ substrate using the chemical vapour deposition (CVD) technique has been studied in this paper. Its thickness, crystalline structure, elemental inter-diffusion in the interface region and the quality were characterized by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), core level X-ray photoelectron spectroscopy (XPS) and nuclear resonance reaction ²⁷Al(p,γ)²⁸Si, respectively. The results of stoichiometric defect profile and individual silicon suboxide (such as SiO, and Si₂O₃ components with respect to the metallic Si element) formation in the intermediate region were observed. The deep traps located around E_c=0.26eV, in ∼500 nm thick n-type Si films, were attributed to the defects caused by the strain of the silicon lattice. Raman spectroscopy was used to evaluate the compressive stress in the Si film.     

Optical properties of CulnSe2 thin films

The optical constants of vacuum-deposited CulnSe₂ films were determined from the measured transmittance and reflectance at normal incidence of light in the wavelength range 500 to 2000 nm. The analysis of the experimental points of the absorption coefficient revealed the existence of two optical transition processes: an allowed direct transition withE _g=1.03±0.01 eV and a forbidden direct transition withE _f=1.254±0.001 eV. The optical constants of the films were independent of the substrate temperature.On leave to the Kingdom of Saudi Arabia.