Structural modification of laser annealed a-Si1−xCx:H films

Hydrogenated amorphous silicon carbon films, with relatively low hydrogen content and carbon fraction x, C/(C + Si), ranging from 0.20 to 0.57 have been deposited by RF-plasma enhanced chemical vapor deposition (PECVD) for excimer laser annealing experiments. After the laser treatments all the films show structural modifications. It has been obtained that with increasing x the crystallinity degree of the Si phase decreases, while that of the SiC phase increases and becomes predominant for x = 0.39. In the overstoichiometric samples only the c-SiC phase has been observed. In all the treated samples 3C-SiC crystallites have been detected.     

Comparative study of porous amorphous a-Si1−xCx films and a-Si1−xCx membranes on structural and luminescence properties

Electrochemical etching of amorphous SiC in fluoride solution was studied. Anodic dissolution and passivation are observed for p-type electrodes under dark illumination. The dissolution of p-type a-Si_1−xC_x is found to be under mixed transport/kinetic control; the diffusion current is of first order in fluoride concentration. Porous etching was not observed in this case. The surface finish of 6H-SiC depends on the experimental conditions; both uniform and porous etching is observed. In this paper, we report the formation of porous p-type amorphous SiC (a-Si_1−xC_x) films, elaborated previously by DC magnetron sputtering and analyze the porous layers (PSC) using scanning electron microscopy, spectrophotometer and photoluminescence. The crystal structures and the preparation conditions of porous SiC are shown to have an effect on the structural and electrical properties of the material obtained. SEM observation indicates that the porous a-Si_1−xC_x layers have shown some specific feature; a semi-cylindrical structure of the porous network has been observed.     

NMR and ESR studies on a-Si1−xGex:H prepared by magnetron sputtering

NMR, IR and ESR measurements have been performed for hydrogenated amorphous Si-Ge alloy films prepared by magnetron sputtering. NMR signals similar to those for a-Si:H are observed with no motional narrowing effect. The content of dispersed hydrogen atoms estimated from the narrow component of the NMR signal is independent of the Ge content, although the total hydrogen content decreases with increasing the Ge content. Such NMR results are discussed in conjunction with the results of ESR.     

Hydrogen depth profile measurement in a-Si1-xCx:H films by elastic recoil detection

The hydrogen content and depth profile in a-Si_1-xC_x:H films were measured by the elastic recoil detection (ERD) technique. It is shown that the hydrogen content changes from 15 to 50 at% with increasing carbon content x. For x<0.4 the hydrogen content increases mainly due to the increase of the Si-CH₃ contribution and for x > 0.4 due to C-H bonds. By combining the ERD and IR results, the proportionality between the number of Si-CH₃ bonds and the intensity of IR absorption due to the Si-CH₃ rocking mode vibration is ascertained. The proportionality constant is found to be A^roc_Si-CH3 = 5 × 10¹⁹ cm^-2.     

Structural and optical properties of a-Si1−xCx:H films synthesized by dc magnetron sputtering technique

Hydrogenated amorphous SiC films (a-Si_1−xC_x:H) were prepared by dc magnetron sputtering technique on p-type Si(1 0 0) and corning 9075 substrates at low temperature, by using 32 sprigs of silicon carbide (6H-SiC). The deposited a-Si_1−xC_x:H film was realized under a mixture of argon and hydrogen gases. The a-Si_1−xC_x:H films have been investigated by scanning electronic microscopy equipped with an EDS system (SEM-EDS), X-ray diffraction (XRD), secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), UV-vis-IR spectrophotometry, and photoluminescence (PL). XRD results showed that the deposited film was amorphous with a structure as a-Si_0.80C_0.20:H corresponding to 20 at.% carbon. The photoluminescence response of the samples was observed in the visible range at room temperature with two peaks centred at 463 nm (2.68 eV) and 542 nm (2.29 eV). In addition, the dependence of photoluminescence behaviour on film thickness for a certain carbon composition in hydrogenated amorphous SiC films (a-Si_1−xC_x:H) has been investigated.     

Positive secondary Ion emission from Si1−xGex bombarded by O2

The positive secondary ion yields of B⁺ (dopant), Si⁺ and Ge⁺ were measured for Si_1−xGe_x (0 ≤ x ≤ 1) sputtered by 5.5 keV ¹⁶O₂⁺ and ¹⁸O₂⁺. It is found that the useful yields of Ge⁺ and B⁺ suddenly drop by one order of magnitude by varying the elemental composition x from 0.9 to 1 (pure Ge). In order to clarify the role of oxygen located near surface regions, we determined the depth profiles of ¹⁸O by nuclear resonant reaction analysis (NRA: ¹⁸O(p,α)¹⁵N) and medium energy ion scattering (MEIS) spectrometry. Based on the useful yields of B⁺, Si⁺ and Ge⁺ dependent on x together with the elemental depth profiles determined by NRA and MEIS, we propose a probable surface structure formed by 5.5 keV O₂⁺ irradiation.     

Surface roughening in Si1−xGex alloy films by 100 MeV Au: Composition dependency

Using a 100 MeV Au beam, the surface roughening kinetics of relaxed Si_1−xGe_x alloy films for x=0.5 and 0.7 are studied by means of ex-situ atomic force microscopy (AFM). Swift heavy ion (SHI) irradiation induced surface roughening behavior is demonstrated using the trend in variation of β as a function of fluence when the data are analyzed in terms of the Edwards-Wilkinson (EW) model. By employing the EW model, the observed surface roughening is explained on the basis of the competition between SHI induced sputtering and smoothening through redeposition of the sputtered atoms. The composition dependent variation of surface morphology with increasing fluence is discussed in the light of the strain distribution along the sample surface.     

Structural and surface properties of Si1−xGex thin films obtained by reduced pressure CVD

This paper investigates the structure and surface characteristics, and electrical properties of the polycrystalline silicon-germanium (poly-Si_1−xGe_x) alloy thin films, deposited by vertical reduced pressure CVD (RPCVD) in the temperature range between 500 and 750 °C and a total pressure of 5 or 10 Torr. The samples exhibited a very uniform good quality films formation, with smooth surface with rms roughness as low as 7 nm for all temperature range, Ge mole fraction up to 32% (at 600 °C), textures of 〈2 2 0〉 preferred orientation at lower temperatures and strong 〈1 1 1〉 at 750 °C, for both 5 and 10 Torr deposition pressures. The ³¹P⁺ and ¹¹B⁺ doped poly-Si_1−xGe_x films exhibited always lower electrical resistivity values in comparison to similar poly-Si films, regardless of the employed anneal temperature or implantat dose. The results indicated also that poly-Si_1−xGe_x films require much lower temperature and ion implant dose than poly-Si to achieve the same film resistivity. These characteristics indicate a high quality of obtained poly-Si_1−xGe_x films, suitable as a gate electrode material for submicron CMOS devices.     

Composition dependent structural modification in relaxed Si1−xGex films by 100 MeV Au beam

We report the modification of molecular beam epitaxy grown strain-relaxed single crystalline Si_1−xGe_x layers for x=0.5 and 0.7 as a result of irradiation with 100 MeV Au ions at 80 K. The samples were structurally characterized by Rutherford backscattering spectrometry/channeling, transmission electron microscopy (TEM) and high-resolution X-ray diffraction before and after irradiation with fluences of 5×10¹⁰, 1×10¹¹ and 1×10¹² ions/cm², respectively. No track formation was detected in both the samples from TEM studies and finally, the crystalline to amorphous phase transformation at 1×10¹² ions/cm² was examined to be higher for Si_0.3Ge_0.7 layers compared to Si_0.5Ge_0.5 layers.     

Annealing of thin Zr films on Si1−xGex(0≤x≤1): X-ray diffraction and Raman studies

X-ray diffraction experiments have been combined with Raman scattering and transmission electron microscopy data to analyze the result of rapid thermal annealing (RTA) applied to Zr films, 16 or 80 nm thick, sputtered on Si_1−xGe_x epilayers (0≤x≤1). The C49 Zr(Si_1−xGe_x)₂ is the unique phase obtained after complete reaction. ZrSi_1−xGe_x is formed as an intermediate phase. The C49 formation temperature T_f is lowered by the addition of Ge in the structure. Above a critical Ge composition close to x=0.33, a film microstructure change was observed. Films annealed at temperatures close to T_f are continuous and relaxed. Annealing at T>T_f leads to discontinuous films: surface roughening resulting from SiGe diffusion at film grain boundaries occurred. Grains are ultimately partially embedded in a SiGe matrix. A reduction in the lattice parameters as well as a shift of Raman lines are observed as T exceeds T_f. Both Ge non-stoichiometry and residual stress have been considered as possible origins for these changes. However, as Ge segregation has never been detected, even by using very efficient techniques, it is thought that the changes originate merely from residual stress. The C49 grains are expected to be strained under the SiGe matrix effect and shift of the Raman lines would indicate the stress is compressive. Some simple evaluations of the stress values indicate that it varies between −0.3 and −3.5 GPa for 0≤x≤1 which corresponds to a strain in the range (−0.11, −1.15%). X-ray and Raman determinations are in good agreement.     

A quantitative understanding of pressure dependent conductivity of FeSi1−xGex

A new density of states model, referred to as the Gaussian density of states, is proposed for the quantitative understanding of the electrical conductivity behaviour of FeSi Kondo insulating system. The effects of electron correlation and disorder, responsible for the physical properties of this system, are judiciously incorporated in this model. Within the framework of this model, a detailed quantitative analysis of the temperature and pressure dependent electrical conductivity data of FeSi_1−xGe_x (x=0.0, 0.05 and 0.20) reported by Awadhesh Mani et al. [Phys. Rev. B 63 (2001) 115103] has been carried out. From these analyses the complicated pressure dependence of energy gap seen experimentally in these samples could be satisfactorily rationalized.     

Electronic structure and elastic constants of TiCxN1−x, ZrxNb1−xC and HfCxN1−x alloys: A first-principles study

The structural, electronic and elastic properties of TiC_xN_1−x, Zr_xNb_1−xC and HfC_xN_1−x alloys have been investigated by using the plane-wave pseudopotential method within the density-functional theory. The calculations indicate that the variations of the equilibrium lattice constants and bulk modulus with the composition are found to be linear. The calculated elastic constants C₄₄ and shear constants as a function of alloy concentration reveal the anisotropic hardness of these compounds. The partial and total density of states (DOS) for the binary and ternary compounds had been obtained, and the metallic behavior of these alloys had been confirmed by the analysis of DOS.     

Structure and electrical properties of Bi5GexSe95−x thin films

Bi₅Ge_xSe_95−x (30, 35, 40 and 45 at.%) thin films of thickness 200 nm were prepared on glass substrates by the thermal evaporation technique. The influence of composition and annealing temperature, on the structural and electrical properties of Bi₅Ge_xSe_95−x films was investigated systematically using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX). The XRD patterns showed that the as-prepared films were amorphous in nature with few tiny crystalline peaks of relatively low intensity for 30 and 45 at.% and the Bi₅Ge₄₀Se₅₅ annealed film was polycrystalline. The chemical composition of the Bi₅Ge₃₀Se₆₅ film has been checked using energy dispersive X-ray spectroscopy (EDX). The electrical conductivity was measured in the temperature range 300-430 K for the studied compositions. The effect of composition on the activation energy (ΔE) and the density of localized states at the Fermi level N(E_F) were studied, moreover the electrical conductivity was found to increase with increasing the annealing temperature and the activation energy was found to decrease with increasing the annealing temperature. The results were discussed on the basis of amorphous-crystalline transformations.     

The effect of Co ion implantation on Ge1−xMnx films

Ge_1−xMn_x (x = 0, 0.013, 0.0226, 0.0339, 0.0565, 0.0678, 0.0904, 0.113) films prepared by magnetron sputtering at 773 K had a Ge cubic structure except for x = 0.1130. Co ion implantation into these films can effectively prevent the formation of a second phase. Both single-doped and co-doped samples were ferromagnetic at room temperature. The d-d exchange interaction between the interstitial Mn (MnT) and the substituted Mn (Mn_Ge) resulted in ferromagnetism in the sputtered films. Since Co ion implantation destroyed the Mn_T-Mn_Ge-Mn_T complex, the saturated magnetization decreased. Hall measurements revealed that the Co ion implanted films were n-type semiconductors, and the anomalous Hall Effect (AHE) suggested the ferromagnetism was carrier-mediated in the implanted films.     

Characterization of PbSe1−xTex thin films

The lead salts and their alloys are extremely interesting semiconductors due to their technological importance. The fabrication of devices with alloys of these compounds possessing detecting and lasing capabilities has been an important recent technological development. The high quality polycrystalline thin films of PbSe_1−xTe_x with variable composition (0≤x≤1) have been deposited onto ultra clean glass substrates by vacuum evaporation technique. As deposited films were annealed in vacuum at 350 K. The optical, electrical and structural properties of PbSe_1−xTe_x thin films have been examined. The optical constants (absorption coefficient and bandgap) of the films were determined by absorbance measurements in the wavelength range 2500-5000 nm using Fourier transform infrared spectrophotometer. The dc conductivity and activation energy of the films were measured in the temperature range 300-380 K. The X-ray diffraction patterns were used to determine the sample quality, crystal structure and lattice parameter of the films.     

Growth dynamics of C-induced Ge dots on Si1−xGex strained layers

We address the growth mechanism of Ge quantum dots (QDs) on C-alloyed strained Si_1−xGe_x layers by in situ reflection high-energy electron-diffraction (RHEED). We show that C-induced growth on a Si-rich surface leads to a high density (about 10¹¹ cm⁻²) of small dome-shaped islands. On surfaces up to ≈65% richer in Ge we observe a decrease of the dot density by two orders of magnitude, which is associated to the increase of the adatom diffusion. Based on quantitative RHEED analysis, the islands are believed to grow in a Volmer-Weber mode even though their spotty electron transmission pattern is not detectable in the initial stages of growth due to the reduced size of the three-dimensional nucleation islands.     

Analysis of the Ge1−xCx films deposited by MFMST

Ge_1−xC_x films deposited by using a medium frequency magnetron sputtering technique (MFMST) were analyzed with X-ray photoelectron and Raman spectroscopy. The deposited Ge_1−xC_x films consist of C, Ge, GeC and GeO_y. The GeC content in the Ge_1−xC_x films linearly decreases, and the C content linearly increases with increasing deposition temperature from 150 to 350 °C. The GeC content decreases from 11.6% at a substrate bias of 250 V to a lowest value of 9.6% at 350 V, then increases again to 10.4% at 450 V. While the C content increases from 49.0% at the bias of 250 V to a largest value of 58.0% at 350 V and then maintains this level at 450 V. It is found that selecting a bias parameter seems more effective than deposition temperature if we want to obtain a higher content of GeC in the deposited films. In addition, a new method is presented in this paper to estimate the changes of GeC content in the Ge_1−xC_x films by observing the shifts of Ge-Ge LO phonon peak in Raman spectra for the Ge_1−xC_x films. The related mechanism is also discussed in this paper.     

Magnetic transitions in Ni1−xMox and Ni1−xWx disordered alloys

We present a mean-field study of the magnetic phase diagram of Ni_1−xMo_x and Ni_1−xW_x alloys. The pair energies that enter the internal energy part of the free energy are obtained from a first-principles calculation. We try to understand why spin-glass phase is not observed in these alloys.     

SIMS direct surface imaging of Cu1−xCrx formation

Cu-Cr alloys, irradiated with a low-energy, high-current electron beam, are analyzed by high-resolution secondary ion mass spectrometry. Mass spectra and images of Cu⁺ and Cr⁺ surface distributions finely reveal the regions enriched in Cu and Cr. For electron beam energies above a threshold value, the formation of a non-equilibrium Cu_1−xCr_x solid solution, extending over sub-micrometer areas is highlighted for the first time. A discussion of the process leading to Cu_1−xCr_x formation is given.     

Combinatorial ion beam synthesis of CdSxSe1−x nanocrystals

In this presentation we focus on the synthesis of buried multielemental semiconductor nanoparticles by sequential high dose ion implantation and post-implantation annealing. Nanocluster formation and alloying was studied by Raman-, Rutherford Backscattering Spectroscopy (RBS) and X-ray diffraction analysis (XRD) on a materials library of CdS_xSe_1−x nanoclusters buried in thermally grown SiO₂ on silicon. Characteristic peak shifts of the LO-Raman signal and XRD-peaks due to varying S- and Se-fraction indicate that the ion beam synthesized clusters consist of a solid solution of Cd, S and Se. In addition the influence of the implanted dose ratios on the structural evolution of the nanocluster-SiO₂ system will be discussed.