Structural characterization of boron doped hydrogenated nanocrystalline silicon films

Structural properties of boron doped hydrogenated nanocrystalline silicon films deposited by plasma enhanced chemical vapor deposition method were mainly characterized with Raman and X-ray diffraction methods. The experimental Raman data were fitted better by Fano effect profiles than those by phonon confinement effect line shapes chiefly due to high efficiency doping in grown films. The measured Raman spectra were deconvoluted into three-Gaussian profile components: around the peak positions 520 and 480 cm⁻¹ which contribute from crystalline and amorphous tissues separately, as well as a curve centered at about 500 cm⁻¹, which is attributed to the presence of grain boundaries. The average crystalline grain size and crystalline volume fraction were valued with Raman and X-ray diffraction techniques, respectively, while the error derived from different methods was elucidated. Accordingly, the structural changes including crystallites, grain boundaries and amorphous matrices in doped films with boron doping level were analyzed.     

Structural effect on intrinsic stress in nanocrystalline Si:H films

Intrinsic stress in nanocrystalline Si:H films which prepared by the plasma enhanced chemical vapor deposition (PECVD) technique, was illustrated as a compressive stress by means of Raman scattering and radius of curvature measurement. The Raman signals can be well fitted by a model of strain-calibrated phonon confinement, where the sole effect of phonon confinement and Fano interference on Raman scattering was excluded, respectively. The ion bombardment effect on the origination of intrinsic stress in the PECVD films was discussed. The formation of nc-Si:H was explained by etching model in present experimental parameters’ range. The results infer that the intrinsic compressive stress shows intensive correlation to amorphous Si:H, grain boundaries and hydrogen incorporation in the as-deposited materials.     

nc-Si:H薄膜的内应力特性研究

测试了采用PECVD生长的氢化纳米硅(nc-Si：H)薄膜的内应力。利用XRD、Raman、AFM、HRTEM研究了nc-Si：H薄膜的微结构，用全场薄膜应力测试仪测量了nc-Si：H薄膜的内应力。结果表明：nc-Si：H薄膜的内应力与薄膜的微结构密切相关，强烈依赖于制备工艺。压应力随掺杂浓度的提高而增加；在一定功率密度范围内掺磷nc-Si：H薄膜的压应力随功率密度增加而减少，并过渡为张应力；在373-523K之间，掺硼nc-si：H薄膜的压应力随衬底温度升高而增加；nc-Si：H薄膜的压应力随氢气对硅烷稀释比的变化而变化。     

Si-rich a-SiC:H thin films: Structural and optical transformations during thermal annealing

Amorphous hydrogenated silicon-rich silicon carbide (a-Si_0.8C_0.2:H) thin films were prepared by plasma enhanced chemical vapour deposition and were thermally annealed in a conventional resistance heated furnace at annealing temperatures up to 1100 °C. The annealing temperatures were varied and the samples were characterised with Auger electron spectroscopy, glancing incidence X-ray diffraction, Raman spectroscopy, Fourier transformed infrared spectroscopy, transmission electron microscopy and photoluminescence (PL) spectroscopy. As-deposited a-Si_0.8C_0.2:H thin films contain a large amount of hydrogen and are amorphous. When annealing the films, the onset of Si crystallisation appears at 700 °C. For higher annealing temperatures, we observed SiC crystallites in addition to the Si nanocrystals (NCs). The crystallisation of SiC correlates with the occurrence of a strong PL band, which is strongly reduced after hydrogen passivation. Thus PL signal originates from the SiC matrix. Si NCs exhibit no PL yield due to their inhomogeneous size distribution.     

Characterization of Si:O:C:H films fabricated using electron emission enhanced chemical vapour deposition

Silicon-based polymers and oxides may be formed when vapours of oxygen-containing organosilicone compounds are exposed to energetic electrons drawn from a hot filament by a bias potential applied to a second electrode in a controlled atmosphere in a vacuum chamber. As little deposition occurs in the absence of the bias potential, electron impact fragmentation is the key mechanism in film fabrication using electron-emission enhanced chemical vapour deposition (EEECVD). The feasibility of depositing amorphous hydrogenated carbon films also containing silicon from plasmas of tetramethylsilane or hexamethyldisiloxane has already been shown. In this work, we report the deposition of diverse films from plasmas of tetraethoxysilane (TEOS)-argon mixtures and the characterization of the materials obtained. The effects of changes in the substrate holder bias (V_S) and of the proportion of TEOS in the mixture (X_T) on the chemical structure of the films are examined by infrared-reflection absorption spectroscopy (IRRAS) at near-normal and oblique incidence using unpolarised and p-polarised, light, respectively. The latter is particularly useful in detecting vibrational modes not observed when using conventional near-normal incidence. Elemental analyses of the film were carried out by X-ray photoelectron spectroscopy (XPS), which was also useful in complementary structural investigations. In addition, the dependencies of the deposition rate on V_S and X_T are presented.     

Residual stress on nanocrystalline silicon thin films deposited under energetic ion bombardment by using internal ICP-CVD

Nano/microcrystalline silicon thin films were deposited using an internal-type, inductively coupled, plasma-chemical vapor deposition (ICP-CVD) at room temperature by varying the bias power to the substrate. The structural characteristics of the deposited thin film were investigated. The deposition rate was increased by the application of a small RF bias power of 30 W (12.56 MHz), but was then decreased as the bias power was increased above 30 W. In addition, the application of bias power generally increased the residual compressive stress, which was attributed to the increased defect formation in the thin film due to the formation of interstitial atoms. The crystalline volume fraction was also decreased with increasing bias power. However, in the low bias power range of 0-60 W, the compressive stress in the deposited thin film was in the range of − 34 to − 77 MPa, which was lower than the residual stress in the range of − 150 to − 1050 MPa that is observed for the nano/microcrystalline silicon thin films deposited by capacitively coupled plasma.     

Hot-wire chemical vapor deposition and characterization of p-type nanocrystalline Si films for thin film photovoltaic applications

P-type nanocrystalline Si (p-nc-Si) films were deposited by hot-wire chemical vapor deposition (HWCVD) system using SiH₄, B₂H₆, and H₂ as reactants. The effect of H₂ flow rate on the material properties of p-nc-Si films were investigated using Raman spectroscopy, X-ray diffractormeter, ultraviolet-visible-near infrared spectrophotometer, Fourier transform infrared spectroscopy, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Moreover, the electrical properties, such as carrier concentration, activation energy, dark conductivity, and Hall mobility, of p-nc-Si films were also measured. It was found that H₂ flow rate played an important role in forming of p-nc-Si, decreasing the deposition rate, and increasing the crystallinity of p-nc-Si films. FESEM and TEM micrographs also showed the enhancement of crystallinity with adding H₂ flow rate. Furthermore, the change of microstructure at various H₂ flow rates was found to affect the electrical properties of p-nc-Si films. Details of the growth mechanism in p-nc-Si films will be discussed also. Moreover, the optimum p-nc-Si film was used as window layer in n-type crystalline Si heterojunction (HJ) solar cell. After the deposition parameters were optimized, the Si HJ solar cell with the open-circuit voltage of 0.58 V, short-circuit current density of 33.46 mA/cm², fill factor of 64.44%, and the conversion efficiency of 12.5% could be obtained.     

Structural changes in tungsten wire and their effect on the properties of hydrogenated nanocrystalline cubic silicon carbide thin films

We investigated the structural changes in tungsten wire heated to 1800 °C in SiH₄/CH₄/H₂/N₂ atmosphere and the effect of the aging tungsten wire on the properties of N-doped hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) thin films. The aged tungsten wire had two parts: hot parts of the middle of the wire and relatively cold parts connected to clamps. Tungsten carbide (W₂C) layer formed in the wire of the hot parts, while crystalline silicon and cubic silicon carbide (c-Si/3C-SiC) layer deposited on the wire of the cold parts. N-doped nc-3C-SiC:H thin films were deposited for 5 min (thickness: ~ 30 nm) after the tungsten wire was heated under the same condition as during the film deposition for given times (exposure time). No changes in the structural, electrical and optical properties of the nc-3C-SiC:H thin films were observed for the exposure time up to 450 min.     

Effects of hydrogen atmosphere on pulsed-DC sputtered nanocrystalline Si:H films

Hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared by a pulsed-DC magnetron sputtering method under an atmosphere of hydrogen/argon mixture. The effects of hydrogen concentration on the structural and electrical properties of the films were systematically investigated using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and conductivity measurement. A threshold hydrogen concentration of about 70% was found necessary before any crystallinity was detectable. The deposition rate decreased monotonically with increasing hydrogen concentration, while the conductivity varied with crystallite size. The abnormally low conductivity level of these nc-Si:H films was due to the extraordinarily high defect density, which was attributed both to the enhanced ion bombardment from the pulsed-DC plasma and to the oxygen contamination from the target.     

Structural investigations on nanocrystalline Ni-W alloy films by transmission electron microscopy

Electrodeposited Ni-W alloys have been investigated in the as-deposited state by transmission electron microscopy in order to investigate the microstructural features in dependence of the tungsten content. Within the tungsten content range from 7 at.% up to 12 at.%, the microstructure is nanocrystalline characterized by a bimodal grain size distribution, consisting out of 20 to 200 nm sized grains and also larger grains with several 100 nm characteristic dimension. No clear trend in microstructure formation is visible with W content or deposition conditions in the investigated W content range. Only solid solution phase characteristics were observed. The lattice constant is 0.360 nm for 12 at.% W as derived from electron diffraction for the solid solution face centered cubic structure. Larger grains show twinning and stacking faults. Voids with diameter of a few nm were detected along with some multiple twinned particles, indicating high stress level during growth. About 2 at.% difference in the alloy composition from grain to grain was measured.     

Hot-wire chemical vapor deposition and characterization of p-type nanocrystalline SiC films and their use in Si heterojunction solar cells

Wide optical bandgap p-type nanocrystalline silicon carbide (p-nc-SiC) films deposited by hot-wire chemical vapor deposition were used as window layers in n-type crystalline Si heterojunction (HJ) solar cells. The effect of H₂ flow rates on the material properties of p-nc-SiC films was investigated by X-ray diffractometer and Raman spectroscopy. Moreover, the optical and electrical properties, such as optical bandgap (E_g), dark conductivity, and activation energy (E_a), of p-nc-SiC films were also measured. It was found that H₂ flow rates played an important role in forming of p-nc-SiC films and increasing the E_g and decreasing the E_a of p-nc-SiC films. Moreover, the effect of hydrogenation process of the amorphous Si buffer layer on solar cell characteristics was investigated. After the deposition and hydrogenation parameters were optimized, the Si HJ solar cells with the open-circuit voltage of 0.59 V, short-circuit current density of 38.06 mA/cm², fill factor of 62.03%, and the conversion efficiency of 14.09% could be obtained.     

Structural study of the initial growth of nanocrystalline CdS thin films in a chemical bath

Nanocrystalline cadmium sulfide CdS thin films with relevance for optical applications were synthesized from aqueous solutions by chemical bath deposition. Grazing incidence X-ray diffraction shows that the films formed on glass or silicon substrates are made up of nanocrystalline particles. About 80% of the particles have a diameter of 5 ± 1 nm. The nanoparticles have either sphalerite or wurtzite structure. The presence of the sphalerite phase is a signature of a highly non-equilibrium state of the nanocrystalline film. Kinetic studies show that the size of the nanocrystals and the relative fraction of the two phases do not depend on the deposition time once it exceeds a duration of 30 min. For longer times, the particle characteristics remain constant while the thickness of the film grows. Thermodynamical analysis of ionic equilibria allows to choose the reaction bath composition for the formation of cadmium hydroxide Cd(OH)₂. The experiments provide strong evidence that the beginning of the deposition of CdS is accompanied by a formation of cadmium hydroxide Cd(OH)₂.     

Role of atomic migration in nanocrystalline stability: Grain size and thin film stress states

As the length scale of materials decreases to the nanometer regime, grain boundaries occupy a relatively larger volume fraction. Consequently, they play an important role in stabilizing nanocrystalline systems. This review looks at the role of solute segregation to grain boundaries in stabilizing such systems. In recent years, grain size stabilization from solute segregation has led to new types of thermodynamic stability maps as a materials design tool. We propose to extend and adapt these concepts of grain boundary solute segregation as a stabilizing effect to thin film stress states. A recent study on Fe–Pt alloy films, where one species enriched the boundaries, was shown to manipulate the stress from tensile-to-compressive as a function of composition. This suggests that intrinsic segregation can be used as a tunable variable to manipulate stress states, analogous to changing film processing parameters, such as deposition rate and pressure. The application of such solute segregation is at the precipice of new opportunities in materials design of thin films.     

Optical properties and crystallinity of hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by rf-PECVD

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared in a home-built radio-frequency (rf) plasma enhanced chemical vapour deposition (PECVD) system have been studied. The rf powers were fixed in the range of 5 W-80 W. The optical properties and crystallinity of the films were studied by X-ray diffraction (XRD), Micro-Raman scattering spectroscopy, high resolution transmission electron microscope (HRTEM), and optical transmission and reflection spectroscopy. The XRD and Micro-Raman scattering spectra were used to investigate the evidence of crystallinity in order to determine the crystallite sizes and crystalline volume fraction in the films. The HRTEM image of the film was used to correlate with the crystallinity that was determined from XRD and Micro-Raman scattering spectra. Optical constants such as refractive index, optical energy gap, Tauc slope, Urbach energy and ionic constants were obtained from the optical transmission and reflectance spectra. From the results, it was interesting to found that the optical constants showed a good correlation with the crystallinity within the variation of rf power. Also, the ionic constants of the films showed an indication of the degree of crystallinity in the films. The variation of the optical energy gap with the rf power based on structure disorder and the quantum confinement effect is discussed.     

ECR-CVD方法生长a-SiNx:H薄膜的研究

使用微波电子回旋共振等离子体化学气相沉积(ECR-CVD)方法室温生长了非晶氢化的氮化硅薄膜,通过改变前驱气体(SiH4 80%Ar和NH3)的流量比,研究了薄膜的生长速率、等离子体的发射光谱和薄膜的红外特性.结果表明:随着NH3流量的增加,氮化硅薄膜的生长速率呈下降趋势,这主要是由于等离子体中的气相前驱成分之一硅基团浓度的不断下降所导致的;随着NH3流量的增加,薄膜中键合了较多的具有较高电负性的N原子是Si-N和Si-H伸缩振动发生蓝移的主要原因.红外光谱的定量计算表明所制备的氮化硅薄膜具有相对较低的H浓度,约15%左右.文中对氮化硅薄膜的生长机制也进行了讨论.     

Preferred growth of nanocrystalline silicon in boron-doped nc-Si:H Films

Preferred growth of nanocrystalline silicon (nc-Si) was first found in boron-doped hydrogenated nanocrystalline (nc-Si:H) films prepared using plasma-enhanced chemical vapor deposition system. The films were characterized by high-resolution transmission electron microscope, X-ray diffraction (XRD) spectrum and Raman Scattering spectrum. The results showed that the diffraction peaks in XRD spectrum were at 2θ≈47° and the exponent of crystalline plane of nc-Si in the film was (2 2 0). A considerable reason was electric field derived from dc bias made the bonds of Si-Si array according to a certain orient. The size and crystalline volume fraction of nc-Si in boron-doped films were intensively depended on the deposited parameters: diborane (B₂H₆) doping ratio in silane (SiH₄), silane dilution ratio in hydrogen (H₂), rf power density, substrate's temperature and reactive pressure, respectively. But preferred growth of nc-Si in the boron-doped nc-Si:H films cannot be obtained by changing these parameters.     

Doping effects on the optical properties of evaporated a-Si:H films

Thin films of a-Si:H are deposited on substrates at 300°C by a conventional thermal evaporation technique. The electrical conductivity of these films is modified by the addition of antimony giving n-type films. The optical properties of the films are investigated using spectrophotometric measurements of the transmittance and reflectance in the wavelength range 200–3000 nm. Both the refractive index n and the absorption coefficient increase when the Sb content is increased. The absorption edge shifts to lower energies for doped films. The optical gap E_g is evaluated using three different plots for comparison, namely; (hν)^1/2, (/hν)^1/2 and (hν)^1/3. The value of E_g decreases with doping for the three expressions. The Urbach parameter E₀ is calculated and found to increase with doping from 74 meV for the undoped film to 183 meV for concentrations of 9.4 at.% Sb.     

The influence of the fiber drawing process on intrinsic stress and the resulting birefringence optimization of PM fibers

The propagation properties of optical fibers can be significantly influenced by intrinsic stress. These effects are often undesired but in some cases essential for certain applications, e.g. in polarization maintaining (PM) fibers. In this paper, we present systematic studies on the influence of the fiber drawing process on the generated stress and demonstrate an approach to significantly increase the stress induced birefringence of PM-fibers. It is shown that the thermal stress caused by the material composition is superimposed with the mechanical stress caused by the fiber fabrication process. This intrinsic stress has a strong effect on the optical and mechanical properties of the glass and thus influences the fiber stability and modal behavior. By applying a thermal annealing step, the mechanical stress due to the fiber drawing process can be canceled. It is shown that this annealing step compensates the stress reducing influence of the drawing process on the birefringence of PM-fibers with panda structure. The comparison of the intrinsic stress states after fabrication with the state after the additional high temperature annealing step clearly shows that it is possible to improve the overall birefringence of panda fibers using appropriate preparation steps.     

掺硼a-Si:H薄膜脉冲快速光热退火(PRPTA)的研究

采用射频等离子体增强化学气相沉积（RF-PECVD）法制备掺硼非晶硅（a-Si：H）薄膜,然后用脉冲快速光热退火（PRPTA）法对其进行固相晶化。研究结果表明：掺硼a-Si：H薄膜在550℃恒温条件下退火3h后,其结晶状况无明显变化;而通过加高温热脉冲可以在玻璃衬底上获得晶化较好的P型多晶硅薄膜。另外,非晶硅薄膜的掺硼浓度及脉冲条件对脉冲快速光热退火的效果有一定影响。     

Nanocrystallization in Si:H and quantum size effect on optical gap

Interruption of growth and H-plasma exposure on stacking layers of Si:H film resulted in a remarkable change in material properties. Widening of optical gap and increase in dark conductivity were simultaneous with the reduction in photoconductivity, bonded hydrogen content and optical absorption. An associated change in the network structure from amorphous towards crystalline was observed. Enhanced dose of plasma exposure resulted in the gradual lowering in the size of nanograins and increase in their number density. Systematic widening in optical gap during dehydrogenation of the network appears to be a unique feature related to amorphous semiconductors, which suggests nanocrystallization and quantum size effect in hydrogenated binary alloy. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995