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Howling [ Plasma Sources Sci. Molinas-Mata, Ph. Belenguer [ J. Boeuf, Ph. Stutzin, R. Ostrom, A. Gallagher, D. Tanenbaum [ J. Roca i Cabarrocas, P. Gay, A. Hadjadj [ J. A USA vol. Roca i Cabarrocas, S. Hamma, S. Sharma, J. Costa, E. Bertran [ 17th Int. Solids ] B.

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Properties of Amorphous Silicon and Its Alloys E M I S Datareviews Series

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اثر اعمال نانو فیلم کربن شبه الماسی بر بازدهی سلول‌های خورشیدی سیلیکونی

Doping of amorphous semiconductors was unexpected as it is against Mott's 8-N rule [1] stating that atoms in an amorphous solid should have a coordination of min[N, 8-N], where N is the number of valence electrons. Thus boron and phosphorus should be threefold coordinated, i. However in Spear and Le Comber reported on the substitutional doping of amorphous silicon films, produced by the decomposition of silane [2]. Of course, the question as to why it is possible to dope a-Si:H drew a large amount of attention raising the possibility of various applications. Review articles on the doping of a-Si:H can be found in [].

Even though the predictions of non-dopability of a-Si:H were demonstrated to be wrong [2], it appeared that most of the dopants in a-Si:H are threefold coordinated, as expected from the 8-N rule, i. Different models have been developed to explain doping in a-Si. In particular, the formation of valence-alternation-pairs has been suggested as the mechanism responsible for doping [11,12]. However, it was the extension of the 8-N rule to charged impurity states by Street that allowed for a better understanding of doping of a-Si:H [10].

According to Street, doping by phosphorus can be described by a solid state chemical equilibrium reaction: 2 where the equilibrium between threefold and fourfold coordinated phosphorus atoms is balanced by defects silicon dangling bonds. This reaction, first proposed to take place at the. As a direct consequence of this model doping of a-Si:H is accompanied by an increase of the defect density. Moreover the application of the law of mass action to EQN 2 leads to the square root dependence of the doping efficiency EQN I and is in agreement with most of the experimental results concerning doping in a-Si:H and a-Ge:H [3].

Surprisingly, the deposition conditions are not taken into account in EQN 2 ; i. While the importance of solid-phase reactions is supported by a large number of studies concerning thermal equilibrium and metastability in a-Si:H thin films [6], some experimental results cannot be explained, in particular the presence of neutral defects and the fact that the dopant efficiency is dependent on the gas phase concentration and not on the solid phase concentration [14].

This has led some authors to extend Street's approach to specific chemical reactions taking place at the a-Si:H surface during growth [15,16]. Further progress in understanding doping of a-Si:H will be obtained by a better understanding of a-Si:H growth processes [14]. Interestingly, with the growth of a-Si:H by PECVD dopants can be mixed with silane in a controlled way to achieve the desired doping level. Indeed, even though other techniques such as ion implantation or diffusion from a solid source have been used [17,18], the direct addition of the dopant gases to the silane remains the easiest way to achieve doping.

While phosphine and diborane are the most common dopants, other gases have been used to reduce the toxicity or to improve the doping efficiency. In particular, doping with diborane is difficult to control because of the thermal CVD at low temperature [19]. Trimethylboron has attracted much attention because of its higher stability and lower toxicity [20]. Moreover, doping with trimethylboron is accompanied by the incorporation of carbon in the a-Si:H network [21] which can be exploited for the growth of low absorption layers.

Boron trifluoride [22], trimethylgallium [20,23], trimethylaluminium, and triethylboron [24] have been studied as p-type dopants, while liquid organic sources have been used for n-type doping [25]. Impurities at low concentration have also been reported to act as dopants in a-Si:H, with particular reports of the donor-like effects of oxygen [26,27] and nitrogen [28]. In the following we will focus on the doping of a-Si:H by phosphine or diborane.

Few studies have been devoted to the effect of dopants on the plasma chemistry. However, both B2H6 and PH3 have ionisation potentials below 11 eV and may modify the plasma composition in a similar way to Ar and Kr dilution [29]. Mass spectrometry studies have shown that while silane and silane-phosphine discharges are similar, the addition of diborane results in a modification of the discharge because of the higher dissociation of diborane and the formation of diboron-type ions [30].

The effects of dopants on a-Si:H growth have been widely studied. It is well established that the addition of diborane results in an increased deposition rate for deposition from silane precursors [31], while the addition of phosphine has a small effect or produces a reduction in. The enhancement of the deposition rate by diborane has been discussed in terms of a catalytic effect of diborane [33]. However, when a-Si:H is obtained from halogenated silicon reactants SiCU and SiF4 , the addition of diborane decreases the deposition rate, while phosphine increases it [34].

This opposite effect of dopants on the deposition rate has been explained by the effect of the surface band-bending on the chemisorption of silicon radicals. In-situ ellipsometry studies have been used to monitor the growth of a-Si:H films, the effects of doping with diborane on the initial stages of deposition [35], and the thermal CVD of diborane [19].

Moreover, the in-situ Kelvin probe has been proved an excellent technique to control changes in the Fermi-level position during a-Si:H doping with either phosphine or diborane [36]. The studies of a-Si:H growth clearly indicate that the addition of a small amount of dopant to the discharge results in dramatic changes in the growth processes, especially in the case of diborane. The main effect of doping is the change of the Fermi level position within the gap of the semiconductor.

In a-Si. H, p-type doping allows the Fermi level to be moved down to 0. These changes in the Fermi level position are accompanied by a change of more than eight orders of magnitude in the dark conductivity [2]. However, contrary to crystalline silicon, degenerate doping is not observed in a-Si:H. This is due to the existence of band-tails and deep defects in the gap of a-Si:H, and to the creation of midgap defects along with doping. Besides doping, the effects of dopants on the growth processes are also reflected by changes in the structure and properties of a-Si:H films. Those changes have been found to strongly depend on the doping level.

These effects have been mostly studied in the case of boron doping []. In particular, an optimum doping level in the range OfB2H6ZSiH4 flow rates 10"4 10"3 has been observed and correlated with an improvement in the open circuit voltage of solar cells [43]. Structural changes are also often reported, in particular for boron doped a-Si. H films wherein an inhomogeneous morphology has been observed [44]. The presence of microstructural inhomogeneities in boron doped films, supported by the observation of boron clustering [45], is also inferred from hydrogen evolution measurements which show that boron, contrary to phosphorus, results in a low temperature hydrogen evolution [].

This effect is also supported by annealing studies which show large changes of the optical and electrical properties. As annealing temperature is increased the hydrogen content decreases, dark conductivity increases, and hydrogen accumulates in internal voids as H2 molecules, even for annealing temperatures below the deposition temperature [49]. This evolution of hydrogen at. The addition of dopants in a-Si:H results in an increase of the defect density of the films as shown by the decrease of photoluminescence [6] and measured by a wide range of techniques [3,50,51]. Moreover, it is found that the density of defects is about a factor often higher than the concentration of fourfold coordinated dopant atoms [52].

The changes in defect density are also reflected in strong changes of the photoconductivity, transport and deep trapping []. Overall, doped a-Si:H films have high defect density and poor transport properties, which limits their use as active layers in a device. However they are widely used as contacts with intrinsic a-Si:H films. As discussed above, the doping efficiency is inversely proportional to the square root of the concentration of dopants in the gas phase. However, this refers to films deposited under the same plasma conditions [14].

Change of the discharge conditions affects a-Si:H deposition and the chemical reactions at the film surface, and should therefore affect doping. The substrate temperature has been one of the most studied parameters as it strongly affects the properties of intrinsic a-Si:H. For both phosphorus and boron doped films, the decrease of the substrate temperature results in a decrease of the dark conductivity [5,58,59]. Nevertheless, highly conductive boron and phosphorus doped films have also been obtained at C [60]. Moreover, changing substrate temperature also affects the incorporation of hydrogen and of dopants in the film [59,61], which makes it difficult to correlate process conditions and doping efficiency.

The effect of ion bombardment has attracted particular attention in the study of anode versus cathode deposited samples [39,46] or through the effect of a bias voltage applied to the anode. In the case of intrinsic a-Si:H, ion bombardment improves the quality of the material [62].

However, quite different effects have been reported on doped films [63,64]. As for the substrate temperature, the plasma parameters such as pressure, gas flow rate, power etc. As a matter of fact, the simultaneous changes in hydrogen and dopant concentration suggest that hydrogen incorporation is governed by the Fermi level position [65]. Moreover, hydrogen plays a crucial role in determining the doping efficiency in a-Si:H, particularly in boron doped films where the formation of boronhydrogen complexes, as observed in crystalline silicon [66], may result in the decrease of the doping efficiency [67].

The passivation of boron by hydrogen has also been observed in a-Si:H through in-situ studies [68] and is supported by nuclear magnetic resonance measurements which indicate that about half of the boron and phosphorus atoms in a-Si:H form H-dopant complexes [69]. Moreover, the importance of hydrogen in the metastability of phosphorus and boron doped a-Si:H films has also been studied [70,71]. However, while light-soaking of phosphorus doped a-Si:H films produces a decrease of the fourfold coordinated P atoms [72,73], light-soaking in boron doped a-Si:H results in an increase of the film conductivity [74], which suggests that in this case light favours the formation of fourfold coordinated boron atoms.

This difference is in agreement with recent models which show that, contrary to. H is limited by H passivation [75]. Light-induced dopant activation has also been observed in the p-layer of p-i-n solar cells, which results in an increase of the open circuit voltage of the devices [76]. Nevertheless, a small fraction of dopants in a-Si:H IO"2 - 10"3 violates this rule and is fourfold coordinated. However, doping is accompanied by the creation of defects and results in highly defective films.

Models based on chemical equilibrium reactions taking place at the growing surface or in the bulk of the material account for most of the observed effects of doping. This contrasts with the poor knowledge of the effects of dopants in the plasma chemistry and growth processes. Incorporation of phosphorus or boron in a-Si:H has quite different effects on the material properties.

While phosphorus doping is quite well understood, doping with diborane still needs further research as it results in large structural changes of the a-Si:H matrix, which are correlated to the changes in the incorporation of hydrogen forming boron-hydrogen complexes. Mott [ Adv. Stutzmann, DK. Biegelsen, R. Street [Phys.

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Properties of Amorphous Carbon (EMIS Datareviews)

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Biegelsen, WB. Jackson, N. Johnson, M. Anderson, W. Yang, A. Catalano, R. Arya, M. Bennet, I. Balberg [ Mater. Kakimuna, Y. Kasuya, M. Sakamoto, S. Shibata [ J. Hoheisel, W. Fuhs [ Philos. Zesch, MJ. Thompson [ Appl. Longeaud, J. Kleider [ J. Pruppers, KHM. Maessen, J. Bezemer, F. M Habraken, WF. Puigdollers, J. Asensi, J. Bertomeu, J. Delgado [ Mater. Carlson, R. Smith, CW. Magee, PJ. Zanzucchi [Philos. Bezemer, MJM. Pruppers, F. Habraken, CW. Hotta, Y. Tawada, H. Okamoto, Y. Hamakawa [ J. C4 France supplement 10 vol. Miiller et al [ Philos. Pankove, R. Wance, J. Berkeyheiser [ Appl.

Margariiio, D. Kaplan, A. Friederich, A. Zongyan, B. Equer, A. Loret, R. Amokrane [ Physica B Netherlands vol. Ready [ Physica B Netherlands vol. Pierz, W. Fuhs, H. MeIl [ Philos. Nebel, R. Street, W. Johnson [ Philos. McCarthy, J. Reimer [ Phys. Okushi, R. Banerjee, K. Fritzche World Scientific Publishing Company, p. Tsuda [ J. Fedders, D. Drabold [ Proc. Solids Netherlands ] P. St'ahel, A. Sladek, P. Roca i Cabarrocas [ Proc. Barcelona Kluwer, ]. Finger and W. Beyer September This makes the material an interesting candidate for meeting the requirements of, for example, stacked solar cells and optoelectronic devices where a certain bandgap, a variety of different bandgaps or other material properties are needed.

Research into preparation techniques for a-Si:Ge:H alloys is necessary because together with alloying and the required shift of the optical gap other material properties are also influenced - often in a way which is detrimental for technical applications. Research activities have thus concentrated on investigating the growth process and proposing and testing alternative deposition techniques in order to control the material properties according to specified optimisation parameters, believed to be responsible for or correlated with desirable performance for applications.

The understanding and interpretation of deposition processes of a-Si:Ge:H is not conclusive. An obvious complication is that optimised growth conditions on the Si- and the Ge-rich side, respectively, might be quite different. While most studies approach from the Si-side, some reports start on the Ge-side. Several seemingly partly contradictory requirements for optimised growth conditions have been postulated or concluded including among others : 1. The reader is referred to review articles and data collections on a-Si:Ge:H preparation techniques and properties.

These review articles have summarised in the past certain stages of development or special preparation techniques and related material issues of a-Si:Ge:H. Most of these review articles contain a large amount of additional literature. The authors represent some of the important research groups involved with a-Si:Ge:H. Early work is summarised in [20] and [21]. An important stage in optimisation with hydrogen dilution and triode systems is reported by Matsuda and Tanaka [22].

Bullot [23], and in much detail Tsuo and Luft [24] and Luft and Tsuo [25], compare and describe different preparation techniques for a-Si:Ge:H and collect results obtained from the different techniques. The latter two are the most comprehensive works found to date on preparation techniques for a-Si:Ge:H. Also [27] contains data on the preparation and the properties of a-Si:Ge:H alloys. Among the reports which deal with the end-point of the alloy system a-Ge:H summaries are given in [28,29]. Data on the alloying effect on the density of states and the electronic transport and recombination in a-Si:Ge:H material is reported in [].

Materials from different preparation techniques different source gases are also compared in [36,37], emphasising the importance of microstructure, and the direction for future research is discussed in [37]. Finally, the widest collection of selected properties photosensitivity, slope of the optical absorption edge and defect density as a function of the optical gap in a-Si:Ge:H prepared by different techniques is given by Ichikawa and Sasaki [38].

By far the most data found for photosensitivity, slope of the optical absorption edge and defect density in the literature falls in the envelope spanned by this collection. We want to stress that for the present Datareview more than articles in journals, conference proceedings and books dealing with a-Si:Ge:H dating back to have been searched.

Among them about report on details of the preparation techniques. Thus the data collections given in [24], [25] and [38], together with the individual data points quoted in this Datareview, should give a complete overview. Starting from this basic technique, where results have been described in the early review articles [20,21], the following alternatives to the standard PECVD process have been used and described in the literature:. References on these methods are to be found in [24,25]. Hydrogen dilution was also used in earlier work but not systematically to improve the material properties see early review articles.

The concept is based on the ideas that 1 Ge related radicals might have smaller surface diffusion coefficients on the film growing surface than Si-related radicals, and 2 H atoms bonding with Ge are thermally evolved at lower temperatures compared with H on Si, which causes a higher density of free bonds on the growing surface of Ge-rich a-SiGe alloys [6]. By adding hydrogen in the discharge it was believed that the H radical density increases, leading to a better surface coverage and thus a higher surface diffusion coefficient.

Although it was found later that in fact upon hydrogen dilution the H radical density decreases in an SiH4ZH2 discharge [39], and thus the role of hydrogen dilution had to be reinterpreted [5], the H2-dilution method was very successful in improving the electronic properties at a given optical gap.

H2-dilution is also the method most widely used to date in combination with other methods triode reactor, fluorinated gases, disilane, microwave frequencies etc. Generally the improvements reported by the numerous studies confirm the trends already reported in the original work [6]. Data are collected in [25,38].

An effect considered as a disadvantage for technical applications is the decrease in deposition rate upon dilution. As an offshoot of the hydrogen dilution method one can consider the hydrogen plasma annealing method where a thin deposited a-Si:Ge:H layer is exposed to a hydrogen plasma and this step is repeated many times.

The idea is to relax the SiGe network and to passivate dangling bond states. Marked improvement over other techniques was not achieved []. Most recent reports using hydrogen dilution of process gases are given in [12,15,17,]. On the basis of the preferential attachment effect of hydrogen to silicon in SiH4-GeH4 mixture based a-Si:Ge:H alloys, Paul et al [7,20] proposed in the use of fluorinated gases or the incorporation of oxygen for a-Si:Ge:H deposition. Fluorine and oxygen was considered to saturate Ge dangling bonds more effectively than hydrogen.

The use of fluorine had already been proposed earlier for the improvement of a-Si films [48]. These results were confirmed by other researchers [49]. Oda et al [50] attributed the improvement to plasma chemical effects. Based on the reported improvement of photoconductivity and on the fact that fluorinated gases are much safer and easier to handle, several groups started programmes to investigate SiF4 SiHt -GeF4-H2 based alloys [49,].

They noted in the substrate temperature range between and 35OC a much smaller influence of Ts on hydrogen content and optical gap for fluorinated material than for hydrogenated a-SiGe. The higher photoconductivity about an order of magnitude in fluorinated material was attributed to a changed microstructure rather than to the saturation of dangling bonds by fluorine. A particularly wide range of parameters was investigated by Morin et al [55,56]. For gas mixtures of purely fluorinated gases with H2 they reported some disadvantages: a rather low deposition rate e.

Better results deposition rate of 3 - 5. Using the initial defect density as a figure of merit, best films were grown at a process gas pressure between 0. Chatham and Bhat [58] report that the dissociation rates for germane and monosilane differ by about a factor of two whereas the dissociation rates for germane and disilane differ by a factor of 0.

For a gas flow ratio of. Using SCLC measurements, the latter authors reported a density of states near the Fermi level of 3 x cm"3 eV"1 for a bandgap of 1. Improvement of a-Si:Ge:H alloys by using low power disilane-germane discharges without hydrogen dilution was recently reported by Matsuda et al [5]. Under their deposition conditions they find no influence of hydrogen dilution on Urbach tail width and defect absorption. In another recent report [46] use of disilane together with strong hydrogen dilution is compared with various other preparation concepts and the lowest absorption tail slopes, defect densities and microstructure factors see Datareview 1.

The aim was to reduce or eliminate the impingement of ionic species on the growing surface so that only neutral radicals with long reaction lifetimes contribute to the growth of the film. An improved film microstructure was expected. Indeed, an increased photoconductivity as well as less dihydride, according to IR absorption and hydrogen effusion peaks shifted to higher temperature, was observed for SiH4-GeH4 based a-Si:Ge:H films.

Matsuda's results were confirmed by Ichimura et al [59] who reported a better photoconductivity, a lower density of dangling bonds and a very sharp Urbach tail as compared with films produced in a conventional diode system. These improved electronic properties were associated with a more compact structure according to IR absorption, hydrogen effusion and transmission electron microscopy. The effect of ion bombardment during growth on structural and electronic properties of triode RF plasma deposited a-Si:Ge:H was studied by Perrin et al [H].

While the Ge and Si composition of the material did not change when the ion bombardment was changed, structural and optoelectronic properties as well as the deposition rate changed drastically at low deposition temperatures. Ion bombardment at a moderate energy of about 50 eV was found to decrease hydrogen concentration and optical gap, reduce the preferential attachment of hydrogen to silicon in favour of germanium and to improve the film microstructure as well as the photoconductivity.

Results for diode and triode reactors using DC plasma were compared by Weller et al [60,61]. Both reactor types gave even better results in the whole mixture range when employing hydrogen dilution. Beyer et al [62] found this latter material to be compact according to hydrogen effusion and diffusion. Deposition on the cathode, i.

Wickboldt et al [15] applied this method, which had previously been used for a-Ge:H deposition [13,28], to the preparation of Ge-rich a-Si:Ge:H films. An improvement of the steady state photoconductivity and the ambipolar diffusion length compared to a-SiGe:H alloys grown on the grounded electrode anode is reported. This improvement is primarily attributed to a reduction of the long range structural heterogeneity and of the midgap state density. In another approach the ion energies have been varied in an ion-gun chemical vapour deposition system [14] which delivers high ion currents in the range 6 0 - 1 0 0 eV.

While considerable influence of ion energies is found, the optoelectronic properties could not be improved. Similar to hydrogen dilution, helium dilution was used in some earlier work, not with the particular aim of improving material properties but for technical reasons like reducing hazard through pre-mixed process gases or controlling the process pressure. It was found that dilution leads to higher defect densities in all cases and no distinctive improvement upon dilution is reported.

More recently Tsuo et al [63,64] compare H2, He and Ar dilution and find an improved mobility-lifetime product and ambipolar diffusion length with He dilution but unaltered photoconductivity and sensitivity. Ar dilution is reported to lead to poor material [46,63]. No improvement of properties is found. Middya et al [17] compare effects OfH 2 and He dilution and different deposition rates at different temperatures. They find similar electronic properties but different structure. H2 dilution results in less polyhydride i. Hazra et al [19] propose the use of He dilution for operation in a high power regime where structural relaxation is achieved through de-excitation of ionic He species.

They report high quality low defect density low gap 1. Microwave and, in particular, ECR plasmas allow high plasma densities. Usually, remote plasma arrangements are applied, separating the growth process from the plasma generation process. This allows independent control of ion bombardment and hydrogen radical density [24]. In a modification, hydrogen is excited in a microwave plasma and is fed into an RF plasma of SiF4ZGeF4 or reacts with fluorinated radicals from a thermal source [65].

The improved quality of these a-SiGe:H samples was attributed by Perrin et al [11] to ion bombardment during film growth. Guha et al [68] reported an Application of low pressure 10 mtorr remote electron-cyclotron-resonance for a-SiGe:H films was recently reported by Kaushal et al [69]. IVA: - C. Abbiamo fatto il possibile per renderlo accessibile secondo le linee guida WCAG 1. Signoriello, M. Beghi, Exploitation of Brillouin spectroscopy for the characterization of a silica film, Sensor Letters, 6, F.

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