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Residual stress in amorphous and nanocrystalline Si films prepared by PECVD with hydrogen dilution
Abstract
Amorphous and nanocrystalline Si films were prepared using radio-frequency (13.56 MHz) plasma enhanced chemical vapour deposition with different SiH4/H2 ratios. The film residual stress was measured using curvature method as a function of SiH4 gas flow ratio (SiH4/(SiH4 + H2)). Results from high resolution scanning electron microscopy, X-ray diffraction and Raman scattering studies revealed the appearance of nanocrystallites with SiH4 gas ratio less than 3%. With a gradual decrease of SiH4 gas ratio, the film intrinsic stress increased significantly until SiH4 gas ratio reached 2%. Below that critical value, the film intrinsic stress decreased sharply. Different mechanisms of stress formation and relaxation during film growth were discussed, including ion bombardment effect, hydrogen and hydrogen induced bond-reconstruction, nanocomposite effects (nanocrystal embedded in an amorphous Si matrix). Results indicated that nanocomposite effect is the dominant factor in this maximum stress condition.
... Material characterizations. It has been generally accepted that hydrogen dilution plays an important role in removing disorder and reconfiguring the Si-Si network with improved structural ordering 41,42 , although the exact growth mechanism of nc-Si is still debated 43 . The ratio of flowing hydrogen (H 2 ) to silane (SiH 4 ) gases, expressed as R = [H 2 ]/[SiH 4 ], is the most important film deposition parameter that determines the grain size, the crystalline content, and the porosity of the nc-Si in this work. ...
... As R continues to increase >30, the crystalline content increases rapidly and the nanograins begin to contact each other, forming heat conduction percolation paths 44 . The structure transforms from mostly amorphous to mostly nanocrystalline followed by a large reduction of hydrogen concentration to~6 at.% or less 41,42 . This is the point where thermal conductivity becomes interesting and relevant to thermoelectric applications; we thus start our thermal conductivity study from R = 25 and up. ...
... The crystalline content increases rapidly with R reaching a maximum of 73% at R = 100 and drops again for R > 100. Similar R dependence has previously been observed in PECVD-and HWCVD-grown Si films 42,44 . Upon closer inspection of the crystalline peak, one finds it is shifted to the right Porosity for R = 100 is estimated from R = 80 and R = 80/100. ...
Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm−1K−1 of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials. Thermoelectric materials convert heat into electricity and their performance is determined by their figure of merit ZT, which is generally too small in many materials for practical applications. Here, the authors demonstrate that a reduction in grain size for nanocrystalline Si can reduce thermal conductivity and potentially be used as a method to engineer greater ZT in Si for thermoelectric applications.
... VW model describes the evolution of thin film from the initial stage of island formation through island coalescence to the final stage where continuous film is formed. In addition, presence of hydrogen and hydrogen induced reconstruction, ion bombardment (or peening) and defects contribute significantly to the magnitude of the intrinsic stress [10,11]. ...
... It is worthy of note that although a large volume of published data on the intrinsic stress in a-Si:H are available in literature, majority of them do not fulfill the requirements for ANN modeling. In Ref. [11] for instance, intrinsic stresses were determined against variation in plasma power but no information on substrate temperature and chamber pressure was provided. Such types of data are considered incomplete, thus they reduce the volume of relevant data available for the model training and testing. ...
... Three distinct features can be obtained from this figure: (i) a significant compressive stress exists for unhydrogenated a-Si which is about 200 MPa, (ii) a monotonically increasing dependence on hydrogen dilution up to about 15% hydrogen and (iii) a sharp drop in stress up to about À 200 MPa (which is close to its unhydrogenated state) for larger hydrogen concentrations. The reason for this behavior has been extensively discussed in literature [11]. A significant increase in molecular hydrogen in plasma substantially increases the hydrogen content of a-Si [42]. ...
a b s t r a c t An integration of Taguchi method and artificial neural network (ANN) technique for the prediction of intrinsic stresses induced during plasma enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) thin films is presented. Inputs to the ANN model are plasma power, hydrogen dilution ratio, chamber pressure and substrate temperature. Ninety-two data points were used for the network training, model validation and testing in a 2:1:1 relative proportion. An optimized model with a network architecture of 4-5-3-1, a Levenberg-Marquardt training algorithm and a learning rate of 0.1 was obtained from L 9 (3 4) orthogonal array based on Taguchi approach. By using the optimized network, parametric studies were conducted to show how the intrinsic stresses are influenced by the deposition parameters. Analysis of variance (ANOVA) of the ANN variables indicates that the first hidden layer is the most significant parameter contributing about 39% to the changes in the network mean square error (MSE) while the second hidden layer contributes about 15%. Accuracies of the predictive model are within 7 2.5% and 7 13% error bound for compressive and tensile stress regimes, respectively. Also, results of the parametric study show a clear trend between the deposition parameters and the resulting intrinsic stresses, and are found to agree with published data. The results are discussed in the light of physics of PECVD process. & 2012 Elsevier B.V. All rights reserved.
... Therefore, the target range of NG is between 1 and 2. Within this range, it is predicted that a Ge grain will emerge from the grain each time. In addition, when 48 Channel Height = 50 nm 4 -w =50 nm 3.5 -w =100 nm w =200 nm 3 w =300 nm -w =400 nm 2.5 w =500 nm The channel width is increased from 50 nm, which is the ideal value, to 500 nm, which is easier to fabricate in a real system. The purpose of this figure is to determine the channel length that yields an NG value between 1 and 2. ...
... The problem with hydrogen incorporation is that when heated to 450 C, the H that is trapped within the film becomes mobile [47], which leads to H coalescence and and the formation of bubbles. The hydrogen bubbles can eventually burst, destroying the planar Si film [48]. Therefore, the a-Si film must be kept thin. ...
Silicon photonics has emerged as a leading technology to overcome the bandwidth and energy efficiency bottlenecks of standard metal interconnects. Integration of photonics in the back-end-of-line (BEOL) of a standard CMOS process enables the advantages of optical interconnects while benefiting from the low cost of monolithic integration. However, processing in the BEOL requires device fabrication on amorphous substrates, and constrains processing to <450C. In this thesis, a germanium photodetector is fabricated while adhering to these processing constraints in order to demonstrate a proof of concept for BEOL integration. In order to obtain high quality active material, crystalline Ge was grown on Si0 2 by implementing selective deposition in geometrically confined channels. The emerging Ge grains were coalesced to fill a lithographically defined trench, forming the active area of a photodetector. The Ge was measured to have a significant tensile strain of 0.5 %, which was caused by thermal expansion mismatch with the substrate, and concentrated by small voids from imperfect coalescence. The associated resolved shear stress was determined to be below the critical resolved shear stress, verifying that dislocation generation does not occur in this material. The strain was shown to increase the absorption of Ge at long wavelengths, allowing for implementation along the entire telecom window. A Schottky barrier to p-type Ge was developed by the addition of a 1 nm tunneling A120 3 layer between an Al/Ge metal contact. This successfully de-pinned the Fermi level, creating a barrier height of 0.46 eV. The Schottky contacts enabled the fabrication of metal-semiconductor-metal (MSM) photodetectors on standard epitaxial Ge with state-of-the-art dark current densities of 2.1 x 10-2 A cm-2. Gain was observed in these photodetectors, with internal quantum efficiencies (IQE) of 405 %. MSM detectors were also made using Ge on Si0 2, exhibiting an IQE of 370 %. This is the first demonstration of IQE >100% in a Ge MSM or pin photodetector and proves the feasibility of making high performance active photonic devices while adhering to BEOL processing constraints.
... The stress observed at room temperature in inductively coupled plasma enhanced chemical vapour deposition (ICPCVD) silicon-based thin films has been reported previously [2]- [5]. ...
... A polynomial approximation of the temperature dependence of stress, σ = f (T ), was best fitted using the least-squares method for the data shown in Fig. 2. The polynomials, dσ 1 /dT for the Si/Ge(100) sample and dσ 2 /dT for the Si/Si(100), as shown in Fig. 2, were then used in (5) to calculate the temperature dependence of CTE, α f (T ), for the ICPCVD Si films on two different substrates. Fig. 3 shows the α f (T ) for the ICPCVD Si films. ...
This paper presents a study of the effects of stress and thermal expansion of inductively coupled plasma enhanced chemical vapor deposited (ICPCVD) amorphous Si thin films on low-temperature microelectromechanical systems test structures. Experimental data were used in conjunction with finite-element modeling (FEM) to predict deformation in simple microstructures across a wide temperature range from 85 to 300 K. Temperature dependence of residual stress and the coefficient of thermal expansion (CTE) of ICPCVD Si thin films was investigated by characterizing the curvature of bilayer thin-film samples through the use of optical profilometry at low temperature. Extracted parameters were used in an FEM package to confirm the experimental results by correlating with observed deformation of fabricated test structures. It is demonstrated that the experimentally determined CTE enables accurate modeling of the mechanical behavior of thin-film microstructures across a wide range of temperatures. [2015-0175]
... Plasma processes used for thin film depositions equally lead to non-negligible residual strains caused by differences in thermal expansion coefficients and ion bombardment. Several experimental works have shown that certain plasma conditions can lead to compressive stresses up to 1000 MPa (corresponding to a strain of roughly 0.7%) [8,9]. From the 1990s on, the strain effect on silicon has been beneficially used in transistors in form of strained silicon technology where epitaxially grown SiGe layers and silicon nitride capping layers are used to introduce strain into the channel of silicon transistors. ...
... For devices deposited by plasma methods like PECVD, intrinsic stresses due to the impinging ions and the growth process itself are present depending on the deposition conditions. Based on the measurements of several groups [9,8], we can expect residual strains up to 0.7%. It is basically with such processes that the desired strain is realized in strained transistor structures, either with embedded SiGe or with silicon nitride capping layers. ...
The influence of mechanical strain on the conductivity (piezoresistivity) of intrinsic and doped hydrogenated amorphous and microcrystalline silicon (a-Si:H and μ-Si:H) thin films as well as indium tin oxide and aluminum doped zinc oxide is examined under uniaxial tension and compression. The aim of this work is to characterize and model the influence of stress on thin film solar cells. The resistivity of intrinsic a-Si:H and μ-Si:H as well as that of n-type a-Si:H and μ-Si:H decreases with increasing tensile strain whereas it is increasing for both p-type materials. Both ITO and ZnO:Al show no significant change in resistivity with tensile strain until a critical strain value of roughly 0.5% that initiates fracture and introduces a non-reversible resistivity increase. Such irreversible changes occur for silicon layers at higher strains (1%). Silicon nitride buffer layers decrease the value of this critical strain. Tensile tests inside a scanning electron microscope demonstrate that such irreversible changes are related to crack formation when a certain tensile strain is exceeded. Analytical and numerical calculations are performed to estimate the influence of strain on the efficiency of p-i-n solar cells, which is roughly +/-0.3% for a biaxial strain of +/-1%.
... In addition, since poly-Si is embedded in the a-Si matrix, it does not relax fully and should also be in a state of tensile stress as well. This conclusion can be validated by the results of Fu et al. obtained for nanocrystalline Si films[38], where the stress in the a-Si films became more tensile with the inclusion of more silicon in the a-Si matrix and the stress was reported to be mainly due to nanocomposite effects. Furthermore, the Raman peak position of the poly-Si phase in this work was found to be relatively constant at 518.8 ±0.5 cm −1 , which is lower compared to its unstressed condition which has a Raman peak position of 520.5 cm −1. ...
... Even though the stress in our Si films on glass is hard to quantify, the stress state was determined earlier to be tensile with poly-Si inclusions. Fu et al.[38]showed that with the inclusion of nanocrystalline Si (up to a crystallinity of 80%), tensile stresses of up to 1000 MPa were introduced. In the case of SPC the crystallinity is in the same range, hence we expect similar stress levels in our films during SPC. ...
By analyzing the solid phase crystallization (SPC) of amorphous silicon (a-Si) with the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, it was found that the crystallization kinetics was non-ideal, with indications of a non-constant nucleation rate. Detailed structural analysis of the a-Si material during SPC revealed an increasing bond angle distortion from the ideal crystalline silicon (c-Si) bond angle of 109.5°. In addition, infrared measurements indicated a change in SiSi bond polarizability, however its role in the SPC behavior is not clear. The increase in tensile stress, bond angle deviation and bond polarizability together with the increase in crystal fraction may suggest that these parameters could be correlated.
... We justify the exclusive examination of ion-bombardmentinduced stresses through the elimination of all other plausible causes of stress in the material: given the growth parameters used in this paper, we exclude surface stresses 27 and coalescence stresses, 6,15 which dominate at film thicknesses of single nanometers. Hydrogen and hydrogen-induced bond reconstruction 28 models are precluded due to the observation of a strong tensile stress regime, as well as the lack of causal analysis present in these theories (see Subsec. ...
... C). Nanocomposite effects 15 can also be reasonably concluded to play a negligible role as the growth temperature and hydrogen dilution ratios are too low to induce a partial phase transition to microcrystalline silicon. 29 Furthermore, we observe no evidence of embedded nanocrystals in the amorphous Si matrix via grazing-incidence x-ray diffraction. ...
Low hole mobility severely limits the conversion efficiencies of amorphous silicon (a-Si) solar cells. Previously it has been proposed that carrier mobility can be improved by introducing certain types of stress into thin films. In this work we explore a range of deposition conditions allowing the formation of intrinsic stresses varying from −924 MPa compressive to 386 MPa tensile. We then discuss the origins of these stresses due to ion bombardment, presenting a model correlating our deposition parameters with our observed stress measurements. In doing so we elucidate the non-linear relationship between deposition pressure and the films intrinsic stress.
... The negative nature of this stress is likely acquired during film growth due to the deposition technique employed. Ion bombardment was found to be a typical process occurring in PECVD where energetic ions strike the silicon atoms in the subsurface layer with an energy higher than the atomic displacement energy of silicon [25][26][27][28]. As a result, the bonding between silicon atoms is compressed producing a force in the plane of the film as shown in box a) in Table 2; however, as the film is constrained by the substrate, it cannot expand, but instead the substrate can bend. ...
This study reports on the behaviour of the thermoelectric properties of n- and p-type hydrogenated microcrystalline silicon thin films (µc-Si: H) as a function of applied uniaxial stress up to ±1.7%. µc-Si: H thin films were deposited via plasma enhanced chemical vapour deposition and thermoelectric properties were obtained through annealing at 200 °C (350 °C) for n-(p-) type samples, before the bending experiments. Tensile (compressive) stress was effective to increase the electrical conductivity of n-(p-) type samples. Likewise, stress induced changes in the Seebeck coefficient, however, showing an improvement only in electron-doped films under compressive stress. Overall, the addition of elevated temperature to the bending experiments resulted in a decrease in the mechanical stability of the films. These trends did not produce a significant enhancement of the overall thermoelectric power factor, rather it was largely preserved in all cases.
... To evaluate the effect of Ar pressure on the magnetic properties of the amorphous thin films investigated in this study, we determined the internal stresses generated in these films using the Stoney equation [14,16,18]. This technique is more suitable for amorphous materials [22] compared to the classical X-ray diffraction (XRD) technique, which is typically used to determine the residual stress of crystalline and nanocrystalline materials from their diffraction peaks [23]. ...
This study aimed to understand the magnetic behavior of amorphous Fe–Si–B thin films in the presence of residual stress. Residual stress appeared in the thin films during the sputtering process, which was controlled by changing the Ar flow rate during sputtering. Consequently, some thin films developed compressive stress and others developed tensile stress. The residual stress generated in the films ranged from –100 to 400 MPa. The magnetization curves of the samples were measured to extract their magnetic parameters, namely, the magnetic permeability, coercive field, and saturation magnetization. For the films of thickness 500 nm, the relative permeability was higher under tensile stress (µr ∼ 1000) than under compressive stress (µr ∼ 100). A transition between the high and low permeability values occurred for residual stress in the range 0–100 MPa. The relative permeability decreased from 1100 to 800 as the tensile stress increased from 0 to 400 MPa. In addition, the relative permeability decreased from 150 to 100 as the compressive stress increased from 0 to –100 MPa. Moreover, a transcritical state was observed in the samples under compressive stress. For the films of thickness 200 nm, the tensile stress lowered the relative permeability from 800 to 400; however, the compressive stress did not change the permeability significantly.
... Amorphous and polycrystalline hydrogenated silicon (Si: H) thin films have been and are still attractive due to their potential applications in solar cells, Si-based optoelectronic devices and radiation detectors [1][2][3][4]. Recently nanocrystalline (nc:Si) and microcrystalline (μc:Si) silicon films showed more interesting properties such as lower degradation in light, improved transport properties and higher electron mobility compared to amorphous Si films [5,6]. They have potential applications in thin film transistors, non volatile memories and in the enhancement of solar cell efficiency. ...
We prepared hydrogenated thick silicon film by plasma enhanced chemical vapor deposition (PECVD) method using SiH4 and H2 gas mixture and we investigated the effect of the hydrogen dilution ratio defined as \( R=\frac{\left[{H}_2\right]}{\left[{SiH}_4\right]} \) on the as-deposited and annealed films. With increase in hydrogen dilution ratio, amorphous to microcrystalline transition has been observed. The crystallization has been confirmed from Raman spectroscopy, UV reflectance, low angle X-ray diffraction (XRD), spectroscopic ellipsometry and atomic force microscopy (AFM) analysis. Tauc band gap shows a decreasing trend with increasing H2 dilution of silane. It decreases from 1.8 to 1.57 eV. It has been concluded that H2 dilution of silane in PECVD enhances the crystallinity of the film and affects its optical and structural properties.
... The second problem was with its morphology, as the film contained a large density of raised hallow 'micro-bubbles' ranging in diameter from 10 μm to 70 μm. The formation of such bubbles has been previously reported in the literature and has been attributed to trapped hydrogen gas unable to escape [29][30][31]. It is logical to think that both problems are somewhat related, as poor adhesion can provide regions on the surface for hydrogen to accumulate. ...
While stress-free and tensile films are well-suited for released in-plane MEMS designs, compressive films are needed for released out-of-plane MEMS structures such as buckled beams and diaphragms. This study presents a characterization of stress on a variety of sputtered and plasma-enhanced chemical vapour deposition (PECVD)-deposited films, including titanium tungsten, invar, silicon nitride and amorphous silicon, appropriate for the field of bistable MEMS. Techniques and strategies are presented (including varying substrate bias, pressure, temperature, and frequency multiplexing) for tuning internal stress across the spectrum from highly compressive (−2300 MPa) to highly tensile (1500 MPa). Conditions for obtaining stress-free films are also presented in this work. Under certain conditions during the PECVD deposition of amorphous silicon, interesting 'micro-bubbles' formed within the deposited films. Strategies to mitigate their formation are presented, resulting in a dramatic improvement in surface roughness quality from 667 nm root mean square (RMS) to 16 nm RMS. All final deposited films successfully passed the traditional 'tape test' for adhesion.
... The increased gas pressure in the microvoids, along with the H inserted into the a-Si:H network, 10,11 enhances the compressive stress inside the a-Si:H layer. 10,20,21,23 The buildup of compressive stress and the subsequent generation of more strained Si-Si bonds further enhance the formation of a SiH n complex, a precursor for c-Si nucleation at the subsurface layer, through H insertion. 10,11 Due to the high diffusibility of atomic H in a-Si:H films, 24 an ultrathin a-Si:H layer, compared with a thick one, is more favorable to the generation of high compressive stress and high-concentration strained Si-Si bonds in the subsurface layer during plasma treatment, resulting in the formation of more SiH n complexes. An absorption peak of ϳ1954 cm −1 attributed to the SiH n complex is manifested in the ATR-FTIR spectrum of the H-treated 7-nm-thick a-Si:H layer, 10,11 as shown in the inset of Fig. 4͑c͒; such a peak is absent in a thick layer under the same plasma treatment. ...
Microstructures of phosphorus-doped hydrogenated microcrystalline silicon (muc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition are certainly dependent on the thickness of the H2 plasma-treated amorphous silicon (a-Si:H) layers. An ultrathin H-treated a-Si:H layer is beneficial in obtaining a very thin muc-Si:H film with high conductivity. Experimental results indicate that H2 plasma treatment induces the occurrence of high-pressure H2 in microvoids and causes compressive stress inside the ultrathin a-Si:H layers, thereby enhancing the generation of strained Si-Si bonds and nucleation sites and consequently accelerating the nucleation of muc-Si:H films.
... The Raman peak was deconvoluted using a Lorenztian multi-peak function. The crystal fraction was calculated using the formula (I c þ I gb )/(I c þ I gb þ I a ) [11], where I c , I gb and I a denote the peak areas of the crystalline, grain boundary and amorphous peaks, respectively. The crystalline fraction of the mc-Si:H film on glass and PI substrate are 71.1% and 70.6%, respectively. ...
Bottom-gate microcrystalline silicon thin film transistors (μc-Si:H TFTs) were fabricated by conventional 13.56 MHz RF plasma-enhanced chemical vapor deposition at 200 °C. In the high pressure depletion regime, the deposition rate of the μc-Si:H film is 24 nm/min and the amorphous incubation layer near the μc-Si:H/silicon nitride interface is unobvious. The crystalline fraction of μc-Si:H film with the thickness of 50 nm is 71%. From nano beam electron diffraction, the μc-Si:H film has a better crystalline order within a short-range lattice structure. The lattice parameter was measured to be 3.1 Å, which reflecting the lattice plane has a (111) direction. Finally, the μc-Si:H film was used as the active layer in TFTs structure. The field effect mobility, subthreshold swing and the threshold voltage are 0.95 cm2/V, 0.85 V/dec. and 2.05 V, respectively. The output characteristic also shows no evidence of current crowing at low drain-source voltage (Vds), implying good contact properties achieved with the n+ a-Si:H source-drain ohmic contact layer. After 70 h 1 μA constant current stress, the threshold voltage shifts are 4.44 V and 0.42 V for the a-Si:H TFT and μc-Si:H TFT, respectively. The μc-Si:H thin film transistors show a better electrical stability than the amorphous silicon thin film transistors because of the lower defect density in the μc-Si:H film.
... Moreover, the existence of residual stresses have been evidenced in the amorphous silicon film by plasma enhanced chemical vapor deposition (PECVD) method. [5] The results reveal that the ion bombardment enhanced with the implantation energy results in the residual stresses reaching values up to hundreds MPa. The above works also indicate that the compressive stress induced by ion implantation would restrict the crack propagation and would be a benefit for the reliability of products in service. ...
Surface strengthening and residual stress in Ti+ implanted p-Si (100) wafers are investigated by scanning electron microscopy, X-ray diffraction, nano-indentation and micro-Raman spectroscopy. The experimental results revealed that the crystallinity decreases gradually in transition area, whose structure varies continuously from the crystalline Si to amorphous phase which appears in ion implanted area. Moreover, the hardness and elastic modulus increase gradually in the transition area. Compressive intrinsic-stress that comes from lattice mismatch between the implanted layer and Si substrate is one factor giving rise to residual stresses.
A digital variable capacitor has been designed and fabricated based on multi-cantilevers of doped nanocrystalline silicon with variable lengths, suspended over a bottom electrode on top of a high-k material, HfO2, to increase the tuning range of the capacitance. By applying a voltage between the electrodes, the electrostatic force pulls the beams in one-by-one, realizing a digital increase in capacitance. These devices were fabricated using a 4-mask process, and electrical tests confirmed the stepwise increase of the capacitance with voltage from the multi-cantilever digital capacitors.
In this study, nanocrystalline silicon thin films and amorphous silicon thin films are grown on both the single crystalline silicon and glass substrates by plasma-enhanced chemical vapor deposition (PEVCO). Raman scattering is used on those films by three different excitation wavelengths, from red to near ultraviolet region. It shows that the Raman spectrum is different by varying the incident wavelength based on analyses of the incident light intensity energy, the photo absorption coefficient of silicon and the penetration depth. The experimental results are well explained by the theoretical analysis of thin film inter atomic number and bond energy. It shows that the optimum laser should be chosen in order to get further information of material microstructure. The results are advantageous to characterize the composite microstructures and the interface between those different materials. Different Raman spectra excited by deferent wavelengths are also valuable to explain the material damage and failure with service.
The structure of hydrogenated nanocrystalline and amorphous silicon thin films deposited by conventional radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) through decomposition of silane diluted with argon has been studied by X-ray diffraction (XRD) Fourier transform infrared (FTIR) and Raman spectroscopy as well as transmission electron microscopy (TEM), respectively. It has been observed that argon as dilution gas and RF power both play significant roles in the growth of nano-crystal grains and the amorphous network in a-Si:H thin films. The nanocrystallization process initiating at a relatively low dilution ratio has been observed. Moreover, a positive effect of argon dilution and a more complicated nonlinear effect of RF power on the nanocrystallization also have been observed. The observed structural changes are explained by a proposed model based on the energy exchange between argon plasma constituted with Ar* and Ar+ radicals and growth region of the testing thin films.
Inductively coupled plasma enhanced chemical vapor deposited (ICPCVD) a-Si is used as a structural material in many microelectromechanical systems (MEMS). For a-Si to function as a sound structural component, the material must display long term mechanical stability. This paper evaluates the Young's modulus, hardness, and residual stress of a-Si under prolonged heat treatment. It is found that Young's modulus and hardness are not impacted by heat treatment, while the residual stress becomes more tensile with increased annealing time. Increased tensile stress is a result of hydrogen offgassing which can lead to improved film stability [1].
This paper describes the development of a cryogenic optical profilometry system (COPS) for the measurement of material properties of thin films across a wide temperature range. A cryostat was machined and integrated with a Zygo NewView 600K optical profilometer. Curvature data were taken for a SiNx thin film on a GaAs substrate from 300 K down to 95 K. From the curvature data, the coefficient of thermal expansion was calculated. The COPS was validated against a three-beam curvature technique, and demonstrated good agreement across the full temperature range from 300 K to 95K
Femtosecond laser processing of hydrogenated amorphous silicon is a perspective method for thin film solar cells production. It allows to make local crystallization and surface texturing of the films which results in the enhancement of their light absorption and stability of parameters. Thickness of modified material depends strongly on a laser wavelength. However laser wavelength affects also other properties of the film. Therefore here we study structure, surface morphology and photoelectric properties of hydrogenated amorphous silicon films treated by femtosecond laser pulses of different photon energies, namely above, around and below the mobility gap of amorphous silicon.
Catalytic chemical vapor deposition (Cat-CVD) can produce amorphous silicon (a-Si) films with low film stress, in general, compared to plasma-enhanced CVD, and is thus suited for the preparation of precursor a-Si films for thick poly-Si films applied for solar cells. The stress of a-Si films is known to sometimes play an important role for the crystallization of a-Si films and resulting grain size of polycrystalline Si (poly-Si) films formed. I investigate the impact of the stress of Cat-CVD a-Si films on the mechanism of explosive crystallization (EC) induced by flash lamp annealing (FLA). The stress of Cat-CVD a-Si films can be controlled by changing the temperatures of substrates and/or a catalyzing wire during film deposition. Cat-CVD a-Si films with tensile stress (~ 200 MPa) can be deposited as well as films with compressive stress. The enlargement of grain size is observed in a part of flash-lamp-crystallized (FLC) poly-Si films formed from Cat-CVD films with tensile stress compared to those with compressive stress, which might be an indication of a certain degree of impact of film stress on poly-Si formation. The grain size is, however, much smaller than that of FLC poly-Si films formed from electron-beam- (EB-) evaporated a-Si films with similar tensile stress. This fact may indicate the existence of other critical determinants of EC mechanism.
The fabrication and characterization of thin-film silicon bulk resonators processed on glass substrates is described. The microelectromechanical (MEMS) structures consist of surface micromachined disk resonators of phosphorous-doped hydrogenated amorphous silicon (n+-a-Si:H) deposited by radiofrequency plasma enhanced chemical vapour deposition (RF-PECVD). The devices are driven into resonance by electrostatic actuation and the vibrational displacement is detected optically. Resonance frequencies up to 30 MHz and quality factors in the 103-104 range in vacuum were measured. A high density of modes that increases with resonator diameter was observed. Membrane-like vibrational modes show good agreement with finite element simulations. The effect of geometrical dimensions of the disks on the resonance frequency was also studied. When operated in air higher harmonic modes show increasing quality factors.
Hydrogenated intrinsic Si (Si:H) thin films were deposited by a plasma-enhanced chemical vapor deposition technique, and the effect of the SiH4 flow rate ([SiH4]) and hydrogen dilution ratio (R) on the crystalline phase transition was studied using Raman spectroscopy and high-resolution transmission electron microscopy. The crystalline volume fraction (X-c) was strongly affected by [SiH4] as well as R, and higher X-c could be achieved by using a lower [SiH4] at the same R. The dependence of phase transition on the crystallinity of the under-lying layer was also investigated by depositing Si:H films on various substrates. To evaluate the effects of [SiH4] and R on the quality of amorphous-Si:H (a-Si:H) films, etch rates and absorption coefficients were obtained by an H-2-plasma etching process and UV-Vis spectrophotometry, respectively. Even under the deposition condition resulting in a-Si:H films, the deposition rate decreased by a factor of 3 to 4.6, and the film density considerably increased as R increased from 4 to 19. On the other hand, the absorption coefficient very slightly decreased with R. The presence of microcrystalline Si: H could be observed only from the samples deposited with R > 19 when the film was deposited on amorphous substrate.
Microstructure evolution in the surface layer of hydrogenated amorphous silicon (a-Si:H) film exposed to H2 plasma is investigated using grazing-incidence small-angle x-ray scattering and attenuated total reflection-Fourier transform infrared spectroscopy. Molecular hydrogen generated in the microvoids through H-abstraction reaction drives the evolution of the void shape from spherical to ellipsoidal as well as increases the average void volume and total void volume fraction. High-pressure H2 in the microvoid promotes the formation of a strained structure with high compressive stress within the a-Si:H film, which favours the generation of the SiHn complex in the subsurface layer of the a-Si:H film by H insertion into strained Si-Si bonds.
A mixed-phase of amorphous (a-Si:H) and ultra nanocrystalline silicon thin films (ultra nc-Si:H) were grown in a low deposition pressure regime (0.10–0.62 Torr) using a very high frequency (60 MHz) plasma enhanced chemical vapor deposition (VHF PECVD) technique. The deposition rate, stress, hydrogen configuration and morphology varying with the deposition pressure were systematically studied. The maximum deposition rate was found to be 8.3 Å/s at a pressure of 0.47 Torr. The stress of these films decreases from 669.7 MPa to 285 MPa with the increase of deposition pressure from 0.10 to 0.62 Torr. The change in deposition pressure showed the variation of the microstructure and hydrogen bonding configuration of the nc-Si:H films. The ultra small crystallites (size ~ 2 nm) were formed in the films which were confirmed by the X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements. An attempt was made to understand their growth by analysis of the bonding environment.
Effects of substrate surface defects and deposition parameters on deposition of silicon coatings onto silicon carbide ceramics by plasma source ion plating for surface modification were studied in this paper. Substrate surface defects like holes and protuberances were not duplicated during the coating process. The main discontinuities on silicon coatings turned out to be notches which were probably generated by combined impact of excessive internal stress and persistent ion bombardment during the coating process. Continuous, homogenous and well-bonded 2 μm thick amorphous silicon coatings with smooth surface were obtained by improving plasma power, slowing down deposition rate and rising substrate temperature. The total reflectance of silicon coatings within 400–750 nm wavelength was also studied. The higher the residual compressive stress, the higher is the total reflectance. Rising substrate temperature would also significantly improve the total reflectance. The enhancement might be attributed to shortened SiSi bonding under high compressive stress and higher compactness at higher substrate temperature, respectively.
The structure of hydrogenated amorphous silicon (a-Si:H) thin films embedded with nano-crystal grains deposited by conventional radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) through decomposition of silane diluted with argon has been studied by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy. It is observed that argon as dilution gas plays a significant role in the growth of nano-crystal grains and amorphous network in a-Si:H thin films. The structural variation of the thin films with different dilution ratios suggests that changing of plasma conditions in the chamber leads to the nanocrystallizing of the thin films. The nanocrystallization process initiating at a relatively low dilution ratio has been observed. Moreover, a positive effect of argon dilution on the nanocrystallization has also been observed. The structural changes studied under argon dilution are explained by a proposed model based on the energy exchange between argon plasma constituted of Ar* and A+ radicals and the growth region of the thin films.
Boron phosphide films were grown on silicon substrate by radio frequency reactive magnetron sputtering using boron target and hydrogen phosphine at different gas flow ratios (PH3/Ar) at lower temperature. The chemical composition, microstructure and mechanical properties were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectrum, FTIR spectrum, surface profilometer and nano-indenter. The results indicate that the atomic ratio (P/B) rises from 1.06 up to 1.52 with the gas flow ratio increasing from 3/50 to 15/50. Simultaneously, the hardness and Young's modulus decrease from 25.4GPa to 22.5GPa, and 250.4GPa to 238.4GPa, respectively. Microstructure transforms from microcrystalline state to amorphous state along with the gas flow ratio increasing. Furthermore higher gas flow ratio leads to lower stress. The BP film prepared at the gas flow ratio of 3/50 can be contributed with the best properties.
A 4.1-inch flexible QVGA AMOLED display with microcrystalline silicon (μc-Si:H) TFTs backplane on colorless polyamide (PI) substrate is demonstrated. The PI substrate has the features of high Tg (∼350°C) and high light transmittance (∼90%). The bottom-gate μc-Si:H TFTs backplane is fabricated at 200°C by a conventional (13.56 MHz) plasma-enhanced chemical vapor deposition (PECVD) system. The flexible μc-Si:H TFTs backplane shows better electrical stability, flexibility, and uniformity.
A 32-inch microcrystalline silicon thin film transistor liquid crystal display was manufactured on 1500×1850 mm2 (G6) glass substrate. We have successfully deposited non-porous and highly crystalline microcrystalline silicon film with interfacial treatment as the active channel layer for TFTs in such large size display. Microcrystalline silicon was deposited by a conventional (13.56 MHz) plasma-enhanced chemical vapor deposition (PECVD) system, and the crystalline fraction is 60%. Crystalline fraction was successfully improved with optimized gas treatment on SiNx layer before μc-Si:H film deposition. Microcrystalline silicon TFTs with interfacial modification have about 2.8 times and 4 times more on-current than μc-Si:H TFT without interfacial modification measured at room temperature and at −10°C, respectively. The μc-Si:H film with good crystallinity used as the active channel layer will significantly improve the reliability of the microcrystalline silicon TFTs. Furthermore, during the bias-temperature-stress test (BTS), VTH shift for μc-Si:H TFT with treatment after 3hr BTS test is only 1.3V while μc-Si:H TFT without treatment has a VTH shift of 13V.
Influence of the parameters of plasma enhanced chemical vapor deposition (PECVD) on the surface morphology of hydrogenated amorphous silicon (alpha-Si:H) film was investigated. The root-mean-square (RMS) roughness of the film was measured by atomic force microscope (AFM) and the relevant results were analyzed using the surface smoothing mechanism of film deposition. It is shown that an alpha-Si:H film with smooth surface morphology can be obtained by increasing the PH3/N2 gas flow rate for 10% in a high frequency (HF) mode. For high power, however, the surface morphology of the film will deteriorate when the SiH4 gas flow rate increases. Furthermore, optimized parameters of PECVD for growing the film with smooth surface were obtained to be SiH4: 25 sccm (standard cubic centimeters per minute), Ar: 275 sccm, 10%PH3/N2: 2 sccm, HF power: 15 W, pressure: 0.9 Torr and temperature: 350°C. In addition, for in thick film deposition on silicon substrate, a N2O and NH3 preprocessing method is proposed to suppress the formation of gas bubbles.
A variable RF capacitor with a-Si:H (doped with phosphine) cantilevers as the top electrode were designed and fabricated. Because the top multi-cantilever electrodes have different lengths, increasing the applied voltage pulled down the cantilever beams sequentially, thus realizing a gradual increase of the capacitance with the applied voltage. A high-k material, HfO2, was used as an insulating layer to increase the tuning range of the capacitance. The measured capacitance from the fabricated capacitor was much lower and the pull-in voltage was much higher than those from theoretical analysis because of incomplete contact of the two electrodes, existence of film differential stresses and charge injection effect. Increase of sweeping voltage rate could significantly shift the pull-in voltage to higher values due to the charge injection mechanisms.
Hydrogenated amorphous silicon (a-Si:H) refers to a broad class of atomic configurations, sharing a lack of long-range order, but varying significantly in material properties, including optical constants, porosity, hydrogen content, and intrinsic stress. It has long been known that deposition conditions affect microstructure, but much work remains to uncover the correlation between these parameters and their influence on electrical, mechanical, and optical properties critical for high-performance a-Si:H photovoltaic devices. We synthesize and augment several previous models of deposition phenomena and ion bombardment, developing a refined model correlating plasma-enhanced chemical vapor deposition conditions (pressure and discharge power and frequency) to the development of intrinsic stress in thin films. As predicted by the model presented herein, we observe that film compressive stress varies nearly linearly with bombarding ion momentum and with a (−1/4) power dependence on deposition pressure, that tensile stress is proportional to a reduction in film porosity, and the net film intrinsic stress results from a balance between these two forces. We observe the hydrogen-bonding configuration to evolve with increasing ion momentum, shifting from a void-dominated configuration to a silicon-monohydride configuration. Through this enhanced understanding of the structure-property-process relation of a-Si:H films, improved tunability of optical, mechanical, structural, and electronic properties should be achievable.
The structural evolution and optical characterization of hydrogenated silicon (Si:H) thin films obtained by conventional radio
frequency (RF) plasma enhanced chemical vapor deposition (PECVD) through decomposition of silane diluted with argon were studied
by X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, transmission electron microscopy
(TEM), and ultraviolet and visible (UV-vis) spectroscopy, respectively. The influence of argon dilution on the optical properties
of the thin films was also studied. It is found that argon as dilution gas plays a significant role in the growth of nano-crystal
grains and amorphous network in Si:H thin films. The structural evolution of the thin films with different argon dilution
ratios is observed and it is suggested that argon plasma leads to the nanocrystallization in the thin films during the deposition
process. The nanocrystallization initiating at a relatively low dilution ratio is also observed. With the increase of argon
portion in the mixed precursor gases, nano-crystal grains in the thin films evolve regularly. The structural evolution is
explained by a proposed model based on the energy exchange between the argon plasma constituted with Ar* and Ar+ radicals and the growth regions of the thin films. It is observed that both the absorption of UV-vis light and the optical
gap decrease with the increase of dilution ratio.
Key wordsnanocrystallization–plasma enhanced chemical vapor deposition (PECVD)–hydrogenated silicon (Si:H)
Thick films of polycrystalline silicon for solar cells can be made by crystallization of amorphous silicon. Due to the high internal stress of amorphous silicon films, deposition of such thick films is challenging. In this paper, we investigate the failure behavior of thick amorphous silicon films. Depending on the deposition method of amorphous silicon films, they can be cracked or delaminated. While nonhydrogenated amorphous silicon films deposited by an evaporator are cracked when the accumulated strain in thick films reaches a critical point, hydrogenated amorphous silicon films deposited by plasma enhanced chemical vapor deposition are delaminated from the substrate. Critical cracking/delamination thicknesses of amorphous silicon films are theoretically derived and excellent agreement with experimental results is shown.
Nanocrystalline Si films were prepared with a RF-PECVD system using different SiH4/H2 ratios, plasma powers, substrate temperatures and annealing conditions. The film's intrinsic stress was characterized in relation to the crystallization fraction. Results show that an increasing H2 gas ratio, plasma power or substrate temperature can shift the growth mechanism across a transition point, past which nanocrystalline Si is dominant in the film structure. The film's intrinsic stress normally peaks during this transition region. Different mechanisms of stress formation and relaxation during film growth were discussed, including ion bombardment effects, hydrogen induced bond-reconstruction and nanocomposite effects (nanocrystals embedded in an amorphous Si matrix). A three-parameter schematic plot has been proposed which is based on the results obtained. The film structure and stress are presented in relation to SiH4 gas ratio, plasma power and temperature.
We report the observation of room-temperature and low-temperature visible photoluminescence from nanocrystalline silicon (nc-Si) thin films produced by plasma-enhanced chemical vapor deposition (PECVD) through a gas discharge containing SiH4 diluted in Ar and H2. The nanocrystalline silicon films were characterized using transmission electron microscopy, spectroscopic ellipsometry, infrared and Raman spectroscopy, and were examined for photoluminescence. Luminescent films consisted of dense silicon nanocrystals that grew in a columnar structure with approximately 20%–30% void space dispersed inside the film. Aside from having small crystalline silicon regions, the structure of the nc-Si films is different than that of porous Si, another luminescent Si material generally produced by electrochemical anodization. Yet, the photoluminescence spectra of the thin nc-Si films were found to be similar to those observed from porous silicon. This similarity suggests that the same mechanism responsible for light emission from porous silicon may also be responsible for emission from nc-Si. The photoluminescence spectra are analyzed in terms of a simple quantum confinement model. Although the mechanism of visible luminescence from porous Si is still a point of controversy, our results support the hypothesis that some of the luminescence from porous silicon and nc-Si films is due to quantum confinement of electrons and holes in crystals with dimensions 2–15 nm. © 1997 American Institute of Physics.
Hydrogen incorporation in silicon layers prepared by plasma-enhanced chemical-vapor deposition using silane dilution by hydrogen has been studied by infrared spectroscopy (IR) and elastic recoil detection analysis (ERDA). The large range of silane dilution investigated can be divided into an amorphous and a microcrystalline zone. These two zones are separated by a narrow transition zone at a dilution level of 7.5%; here, the structure of the material cannot be clearly identified. The films in/near the amorphous/microcrystalline transition zone show a considerably enhanced hydrogen incorporation. Moreover, comparison of IR and ERDA and film stress measurements suggests that these layers contain a substantial amount of molecular hydrogen probably trapped in microvoids. In this particular case the determination of the total H content by IR spectroscopy leads to substantial errors. At silane concentrations below 6%, the hydrogen content decreases sharply and the material becomes progressively microcrystalline. The features observed in the IR-absorption modes can be clearly assigned to mono- and/or dihydride bonds on (100) and (111) surfaces in silicon crystallites. The measurements presented here constitute a further indication for the validity of the proportionality constant of Shanks etal. [Phys. Status Solidi B 110, 43 (1980)], generally used to estimate the hydrogen content in ‘‘conventional’’ amorphous silicon films from IR spectroscopy; additionally, they indicate that this proportionality constant is also valid for the microcrystalline samples.
White spot syndrome virus (WSSV) is widely distributed in most of the Asian countries where penaeid shrimp are cultured, as well as in some regions of the USA. Six geographic isolates of WSSV-1 each from penaeid shrimp from China, India, Thailand, and the US states of Texas and South Carolina, and 1 isolated from crayfish at the National Zoological Park in Washington, DC-were compared by combining the methods of restriction analysis and Southern blot hybridization. DNA was extracted from purified viruses and then digested with selected endonucleases: AccI, BglII, ClaI, BamHI, EcoRI, HindII, HaeI, SacI and XhoI. The blots were detected with digoxigenin-11-dUTP-labeled WSSV genomic probes: LN4, C42 and A6. No distinctive differences among the 5 WSSV isolates from penaeid shrimp were detected; however, differences in the WSSV isolate from crayfish were observed. A 2.8 kb DNA fragment originating from the crayfish isolate and encompassing the LN4 region was subcloned into pBluescript and sequenced for comparison with the LN4 fragment from the Thailand WSSV isolate. The results indicate that some genomic components of WSSV from different geographic regions share a high degree of homology. This method can be used to distinguish between the WSSV isolate from crayfish and the WSSV isolates from penaeid shrimp.
X-ray Diffraction: A Practical Approach
High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 mum in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (muc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or muc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5-2.5 mum) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.
The qualitative and quantitative structural distribution of HCl- and/or (NH4)2Sx-treated GaAs surface was investigated by angle-resolved X-ray photoelectron spectroscopy. Carbon contamination and elemental As layers were observed in HCl-treated GaAs surface. The (NH4)2Sx treatment following HCl led to the elemental As replacing the As-S bond. Compared with HCl treatment, the more rugged oscillation of photoelectron intensity of Ga and As was observed according to take-off angle in (NH4)2Sx-treated GaAs due to the periodic arrangement of S-passivation layer. The quantitative depth distribution of surface constituents was derived by processing the experimental angular profile and comparing with a simplified layer model. Our results indicate that the distributing order of surface layer on GaAs substrate is a C layer of 5.81 ± 0.54 Å and elemental As layer of 4.57± 0.40 Å with HCl treatment, and a C layer of 5.71 ± 0.52 Å and S-passivation layer (As-S) of 4.73 ± 0.40 Å with (NH4)2Sx treatment from the surface. Through the analysis of low-energy electron diffraction, the elemental As after HCl treatment was revealed to be randomly distributed and the passivated GaAs surface showed (2 × 1) -reconstructed structure at room temperature with regular distribution of As-S bonds.
The surface composition of silicon films deposited from SiH4, Ar, and H2 plasmas was studied using in situ attenuated total reflection Fourier transform infrared spectroscopy with emphasis on the effects of H2 dilution. In the absence of H2, the surface is primarily covered with SiH3 and SiH2. With heavy H2 dilution, the surface is predominantly monohydride terminated with infrared absorption frequencies consistent with the presence of SiH on Si (100) and Si (111) surfaces.
Thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition with 10% SiH4 in hydrogen have been studied concerning the effect of film thickness on the hydrogen concentration, interconnected void network and mechanical stress. The hydrogen concentration was determined by nuclear reaction analysis. The interconnected void network was studied by the method of ion exchange in glass substrate. The films were prepared at a substrate temperature in the range of 150–270 °C. The results show that at the substrate temperature of 150 °C the film starts to grow with an extensive void network, and its structural improvement with thickness is manifested by an increase of the film density. In contrast, at 270 °C the film starts to grow with a dense structure, and its improvement is manifested by an increase of the intrinsic compressive stress. The hydrogen concentration does not depend on the film thickness at any substrate temperature. © 2002 American Institute of Physics.
Intrinsic and n-type microcrystalline silicon thin films were deposited on intrinsic hydrogenated amorphous silicon by the layer-by-layer technique. The growth of the samples has been analyzed in situ by kinetic ellipsometry, spectroscopic ellipsometry, and dark conductivity measurements. This in situ analysis has shown that the process of deposition can be divided into four phases: incubation, nucleation, growth, and steady state. Moreover we have found striking differences between the growth of undoped and n-type samples in both the kinetics of the formation of crystallites and the zone where the nucleation of crystallites takes place. According to our in situ conductivity measurements, the percolation threshold occurs for a crystalline volume fraction higher than 20% in both cases. Moreover, we can produce very thin (6 nm) and highly conductive (σd ≈ 0.2 S cm−1) n-type microcrystalline silicon films on intrinsic amorphous silicon. © 1997 American Institute of Physics.
In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The validity and application ranges of the different models that can be used to calculate these parameters are critically discussed. © 2001 American Institute of Physics.
The linear thermal expansion coefficient, β, of hydrogenated amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon films has been investigated. The value of β of a-Si:H and μc-Si:H films was ∼1×10−6 K−1 at room temperature. It is also found that the β values for thermally annealed a-Si and polycrystalline Si were ∼4×10−6 K−1 at room temperature, coefficients similar to that of single crystalline Si. The linear thermal expansion coefficients in Si films seem to be strongly influenced by the hydrogen incorporation.
The early stages of hydrogenated amorphous Si (a-Si:H) growth from a hydrogen-diluted silane plasma at 250°C and structural change induced by a H2 plasma treatment subsequent to the film growth have been investigated by Raman scattering under a surface sensitive condition. For films thinner than 2 nm a Raman band due to the wagging mode of silicon hydrides is observed, while Raman bands originating from amorphous silicon network increase with progressive film growth after 2 nm deposition. The result indicates that there exists a hydrogen-rich a-Si network within 2 nm from the top surface terminated mainly with SiH3 and SiH2 bonds. Also spectral changes caused by exposing as-grown 2 and 3 nm-thick films to a H2 plasma at 250°C have shown that the H2 plasma treatment promotes the chemical reactions for the desorption of bonded hydrogen on and near the surface region and resulting propagation and structural relaxation of a-Si network.
In hydrogenated amorphous silicon it is demonstrated that the maximum compressive intrinsic stress correlates with the optimum electronic properties. Undoped films were deposited over a range of temperatures in a remote hydrogen plasma (RHP) reactor and, for comparison, in a rf glow discharge (GD) system. The dependence of the stress on deposition temperature is qualitatively identical for the two reactors. Quantitatively, both the maximum compressive stress and the optimized electronic properties (e.g., minimum defect density) are obtained at 400 °C for the RHP films and near 250 °C for the GD films. Additionally, it is demonstrated that the transition from amorphous to microcrystalline silicon, induced by high hydrogen dilution, is accompanied by a reduction in compressive stress. Formation of compressive stress during RHP growth is ascribed to the insertion of hydrogen into the rigid silicon network immediately beneath the growing surface.
Measurements of the intrinsic stress in hydrogenated amorphous silicon (a‐Si : H) films grown by rf glow discharge decomposition of silane diluted to varying degrees in argon are presented. Films are found to grow under exceedingly high compressive stress. Low values of macroscopic film density and low stress values are found to correlate with high growth rate. An abrupt drop in stress occurs between 2 and 3% silane at precisely the point where columnar growth morphology appears. No corresponding abrupt change is noted in density, growth rate, or plasma species concentrations as determined by optical emissioin spectroscopy. Finally a model of diffusive incorporation of hydrogen or some gaseous impurity during growth into the bulk of the film behind the growing interface is proposed to explain the results.
The role of ions on the growth of microcrystalline silicon films produced by the standard hydrogen dilution of silane in a radio frequency glow discharge is studied through the analysis of the structural properties of thick and thin films. Spectroscopic ellipsometry is shown to be a powerful technique to probe their in-depth structure. It allows to evidence a complex morphology consisting of an interface layer, a bulk layer, and a subsurface layer. The ion energy has been tuned by codepositing series of samples on the grounded electrode and on the powered electrode, as functions of pressure and power. On the one hand, reducing the ion energy through the increase of the total pressure and depositing on the grounded electrode, favors the formation of large grains and results in improved bulk transport properties, but leaves an amorphous interface layer with the substrate. On the other hand, we achieve fully crystallized films on glass substrates under conditions of high energy ion bombardment. We suggest that ion bombardment, and particularly the implantation of hydrogen ions, favors the formation of a porous layer where the nucleation of crystallites takes place. These results are further supported by in situ spectroscopic ellipsometry measurements of the film morphology as a function of the ion energy. © 2003 American Institute of Physics.
The plasma processes and growth reactions involved in the deposition of amorphous, polymorphous and microcrystalline silicon thin films are reviewed. The reference being a-Si:H deposition through surface reactions of SiH3 radicals, we study the growth of microcrystalline silicon films produced by the layer-by-layer and standard hydrogen dilution techniques. We show that subsurface reactions play a key role, particularly during the incubation phase where hydrogen is responsible for the formation of a porous layer in which nucleation takes place. The evolution of the film properties is related to the long range effects of hydrogen. Coming back to a-Si:H deposition, we further consider the deposition at low substrate temperature (<200°C) and pressure (<5 Pa) where the role of ions is dominant and at deposition rates where powder formation takes place. We propose that rather than a drawback, nanoparticle formation in silane plasmas might be considered as a potential for obtaining new silicon films. We address in particular the deposition of polymorphous silicon consisting of an a-Si:H matrix with silicon nanocrystallites produced in the gas phase. Despite their heterogeneity polymorphous silicon films have improved transport properties and stability with respect to a-Si:H.
The properties of silicon films prepared by plasma-enhanced chemical vapor deposition (PECVD) at low temperatures (<400 °C) are controlled by the physical and chemical processes that occur in the gas phase, at the top-most film surface, within the first several monolayers of the surface, and even well into the film bulk. Recent advances in understanding these processes have led to several new developments in silicon PECVD. The advances include: (i) novel concepts for depositing high-rate, device-quality silicon films, and (ii) deposition phase diagrams for optimizing silicon films for high-stability, high-performance devices. In situ and real time probes of the gas phase, the film surface, and its sub-surface have played key roles in these advances.
Structural changes in silicon films with variations of hydrogen dilution and substrate temperature have been studied. At a critical value of hydrogen dilution in silane (φ) a sharp transition from amorphous to microcrystalline phase has been observed. An increase of substrate temperature from 170 to 340 °C shifts the transition point from φ=95.5% to 93.5%. The change in dark conductivity and absorption corroborate with the change in crystalline volume fraction in the films. The grain size varies from 45 to 360 Å depending on deposition conditions. Optical absorption and hydrogen content in the film decreases drastically with formation of microcrystalline structure. Silicon films developed at 340 °C show moderate photosensitivity together with low light induced degradation at low crystallinity, which might be suitable properties for solar cell application.
Hydrogenated silicon (Si:H) films near the threshold of crystallinity were prepared by very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a wide range of hydrogen dilution RH=[H2]/[SiH4] values of 2–100. The effects of H dilution RH on the structural properties of the films were investigated using micro-Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy. The obtained Raman spectra show that the H dilution leads to improvements in the short-range order and the medium-range order of the amorphous network and then to the morphological transition from amorphous to crystalline states. The onset of this transition locates between RH=30 and 40 in our case, and with further increasing RH from 40 to 100, the nanocrystalline volume fraction increases from ∼23% to 43%, and correspondingly the crystallite size enlarges from ∼2.8 to 4.4 nm. The FTIR spectra exhibit that with RH increasing, the relative intensities of both the SiH stretching mode component at 2100 cm−1and wagging mode component at 620 cm−1 increase in the same manner. We assert that these variations in IR spectra should be associated with the formation of paracrystalline structures in the low H dilution films and nanocrystalline structures in the high H dilution films.
A technique to grow a number of materials on a single substrate using a physical mask in a hot wire chemical vapor deposition (HWCVD) system is developed. Using this technique, we examine materials continuously varying from amorphous to microcrystalline silicon as we vary the hydrogen-to-silane gas ratio from 0 to 20 at a substrate temperature of about 250°C. Raman and reflectance spectra clearly show that the material structure changes rapidly at a ratio of 2 to 4. The results indicate that the near-transition materials still retain optoelectronic properties similar to amorphous silicon.
The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous silicon films (a-Si:H) near and above the threshold of microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a-Si:H films deposited near the threshold of microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability.
The intensity of the Raman-active Gamma25' mode of nanometer-sized crystalline silicon, nc-Si, normalized to that of calcium fluoride, CaF2, at 322 cm-1 was measured for samples deposited under controllably varied conditions. Changes of the intensity by a factor of up to approximately 6.7 were found. These are correlated with the lattice expansion and with the compressive stress in thin films of the material. It is suggested that the enhancement of the scattering cross section, which scales with the observed optical-absorption coefficient and diffuse elastic light scattering, is due to enhanced coupling of the electromagnetic field of the incident light to the charge-density fluctuations at the grain boundaries of the quasi-isolated crystallites.
We have examined the nucleation and growth of hydrogen clusters using pseudopotential total-energy calculations. A double-layer {111} H platelet, resulting from clustering of diatomic H2* complexes, is lower in energy by 0.15 eV per H pair than interstitial H2 molecules. This result explains two experimental observations: (1) predominance of Si-H bonding observed in Si containing high H concentrations and (2) the formation of H {111} platelets. The H vibrational frequencies, H-H separations, and other properties of the platelet are in general agreement with experiment.
X-ray Diffraction: A Practical Approach , C. Suryanarayana and M. Grant Norton, 1998. Plenum Press, New York and London. xiii + 273 pages. (hardback, $49.50, U.S. and Canada; $59.40, elsewhere).
It is the aim of this text to teach undergraduates majoring in materials science the use of powder X-ray diffraction for materials characterization. Since it does not treat X-ray diffraction and crystallography in a general way, it would have been better if it were given a more specific title, such as X-Ray Powder Diffraction for Metallurgical Characterization. A Primer and Workbook . As a laboratory course with work pages to be filled out by the student, it might have been spiral-bound to facilitate such use.
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