Temperature-Dependent Properties of nc-Si Thin Films Synthesized in

Jul 2, 2009 - Plasma Nanoscience Centre Australia (PNCA), CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, New South Wales 2070, Aust...
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J. Phys. Chem. C 2009, 113, 14759–14764

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Temperature-Dependent Properties of nc-Si Thin Films Synthesized in Low-Pressure, Thermally Nonequilibrium, High-Density Inductively Coupled Plasmas Qijin Cheng,† Shuyan Xu,‡ and Kostya (Ken) Ostrikov*,†,§ Plasma Nanoscience Centre Australia (PNCA), CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, New South Wales 2070, Australia, Plasma Sources and Applications Centre, NIE, Nanyang Technological UniVersity, 1 Nanyang Walk, 637616, Singapore, and School of Physics, The UniVersity of Sydney, Sydney NSW 2006, Australia ReceiVed: May 20, 2009; ReVised Manuscript ReceiVed: June 15, 2009

Silicon thin films were synthesized simultaneously on single-crystal silicon and glass substrates by lowpressure, thermally nonequilibrium, high-density inductively coupled plasma-assisted chemical vapor deposition from the silane precursor gas without any additional hydrogen dilution in a broad range of substrate temperatures from 100 to 500 °C. The effect of the substrate temperature on the morphological, structural and optical properties of the synthesized silicon thin films is systematically studied by X-ray diffractometry, Raman spectroscopy, UV-vis spectroscopy, and scanning electron microscopy. It is shown that the formation of nanocrystalline silicon (nc-Si) occurs when the substrate temperature is higher than 200 °C and that all the deposited nc-Si films have a preferential growth along the (111) direction. However, the mean grain size of the (111) orientation slightly and gradually decreases while the mean grain size of the (220) orientation shows a monotonous increase with the increased substrate temperature from 200 to 500 °C. It is also found that the crystal volume fraction of the synthesized nc-Si thin films has a maximum value of ∼69.1% at a substrate temperature of 300 rather than 500 °C. This rather unexpected result is interpreted through the interplay of thermokinetic surface diffusion and hydrogen termination effects. Furthermore, we have also shown that with the increased substrate temperature from 100 to 500 °C, the optical bandgap is reduced while the growth rates tend to increase. The maximum rates of change of the optical bandgap and the growth rates occur when the substrate temperature is increased from 400 to 500 °C. These results are highly relevant to the development of photovoltaic thin-film solar cells, thin-film transistors, and flat-panel displays. 1. Introduction Nanocrystalline silicon (nc-Si) has received considerable interest in recent years, owing to its superior electron mobility, higher doping efficiency, an increased absorption in the red and infrared wavelength ranges, and a reduced hydrogen content compared with amorphous silicon (a-Si).1-5 These unique properties make it have immensely promising applications in solar cells, thin-film transistors, flat-panel displays, and several others.6,7 In particular, in the case of solar cell applications, ncSi thin films combine the advantages of a low-temperature deposition similar to the preparation of a-Si films with the excellent long-term stability against light-induced degradation which is more common for bulk crystalline silicon solar cells.6-8 Very recently, several research groups have reported on the achievement of ∼10% energy conversion efficiency based on single-junction nc-Si solar cells.9,10 This value is much higher compared to the a-Si-based solar cells with a typical energy conversion efficiency of ∼5%.9,10 Recently, low-pressure, thermally nonequilibrium, highdensity inductively coupled plasmas (ICPs) have been demonstrated as an effective and versatile process environment to prepare nc-Si thin films.11,12 This is because the ICPs offer several benefits of a very high electron number density (up to * To whom correspondence should be addressed. Phone: 61-2-94137634. Fax: 61-2-94137200. E-mail: [email protected]. † CSIRO Materials Science and Engineering. ‡ Nanyang Technological University. § School of Physics.

1012-1013 cm-3 in the typical pressure range of a few to several tens of Pa), excellent uniformity in both the radical and the axial directions without using any external magnetic confinement, independent control of the ion energy and fluxes as well as the densities of reactive species, substantially reduced ion bombardment-induced damage of the growth surface, and some others.13,14 These unique characteristics are beneficial for promoting the crystallization of the a-Si at low substrate temperatures and low hydrogen dilution.11,12,14 In the previous work, we have studied the structural evolution of nc-Si thin films synthesized by ICP-assisted chemical vapor deposition (CVD) using a silane precursor without any additional hydrogen dilution in the narrow substrate temperature (Ts) range from 100 to 300 °C. This study has revealed that the structural and optical properties of nc-Si (such as the crystal volume fraction, hydrogen content, etc.) are gradually improved with the increased substrate temperature.15 However, there still exists an open question “what is the optimum process temperature to achieve the best crystallinity?” Is it always best to keep increasing Ts? To resolve this issue one should note that there is a competition between two thermokinetic factors, the higher temperature which leads to a better rearrangement of atoms into crystalline lattices and hydrogen passivation which facilitates bond breaking and reforming reactions and thus eliminates weak or strained near-surface Si-Si bonds.2,8,15 However, increasing the process temperature often leads to rapid and uncontrollable hydrogen desorption from the silicon surface. Therefore, these two effects act in the same direction under some threshold

10.1021/jp9047083 CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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temperature and become counteracting above the hydrogen desorption temperature. This delicate balance may lead to intricate refinements in the material’s microstructure and other properties, which is aimed to investigate in this work. To implement this, we have prepared a series of thin film silicon samples in a broad range of substrate temperatures from 100 to 500 °C. 2. Experimental Methods The nc-Si thin films were prepared using a low-pressure, thermally nonequilibrium ICP deposition system from the chemically active silane precursor gas without any additional hydrogen dilution. The details of the plasma reactor (diameter ) 45 cm, height ) 30 cm) and its operation can be found elsewhere.13,16 Briefly, the inductive radio frequency (rf) power (operating at 460 kHz) is delivered via a quartz window (50 cm in diameter and 1.2 cm thick) on the top of the chamber. The reflected power of the ICP system is maintained within the range of ∼5% of the input rf power, to enable the maximum power transfer from the rf generator to the plasma. An externally heated and temperature-controlled substrate stage provides a suitable temperature for the film growth. Prior to the deposition, a base pressure of ∼1 × 10-3 Pa was routinely achieved through the use of a combination of rotary and turbo-molecular pumps. Thereafter, a high-purity (99.999%) silane gas with a flow rate of 30 sccm (sccm denotes cubic centimeter per minute at standard temperature and pressure) was let into the chamber and then the total working gas pressure was maintained at 1.5 Pa through the adjustment of a throttle valve. In this work, a series of samples was deposited by changing the substrate temperature from 100 to 500 °C while the inductive rf power, silane gas flow rate, and total gas working pressure were maintained at 1200 W, 30 sccm, and 1.5 Pa, respectively. The thickness of the deposited films ranged from 1.84 to 2.29 µm. The specimens were deposited on (100)-oriented single-crystal Si substrates for X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) analyses, while those on quartz substrates were intended for the optical transmittance analysis. The crystal structure of the films was analyzed through the use of a Siemens D5005 X-ray diffractometer, operating in a grazing angle mode, wherein the incident X-ray wavelength was 1.54 Å (Cu KR line) at 40 kV and 40 mA. The angle between the incident X-ray and the surface of the film was fixed at 1°, and the diffraction pattern was obtained by changing the position of the counter. Raman measurements were conducted by a Renishaw 1000 micro-Raman system using a 514.5 nm Ar+ laser for excitation. The surface morphology and crosssection of the deposited films were observed using a JEOL JSM6700F field emission scanning electron microscope. The film growth rate Rd was derived from the film thickness (measured by cross-sectional SEM) and deposition time. The optical transmission measurements were performed in the 500-1100 nm spectral range with a Cary 510 Bio spectrometer. 3. Results In this section, we systematically present the morphological, structural, and optical properties of the deposited silicon thin films using a wide range of advanced analytical tools. The crystal structure of the deposited silicon thin films under variable substrate temperatures was analyzed by XRD. Figure 1 displays the XRD spectra of the samples deposited at different substrate temperatures from 100 to 500 °C. One can observe that there are no clearly resolved diffraction peaks at a low substrate temperature of 100 °C, suggesting that the deposited

Figure 1. XRD spectra of the samples deposited at different substrate temperatures from 100 to 500 °C.

film is amorphous. However, at a substrate temperature of 200 °C, one can observe three well-resolved diffraction peaks at 2θ ) 28.4°, 47.3°, and 56.1°, which are attributed to (111), (220), and (311) crystal planes of silicon, respectively.17,18 The occurrence of these three diffraction peaks indicates that the nc-Si thin films can be grown at this low temperature. Moreover, one can notice that for all the XRD spectra at Ts g 200 °C the intensity of the (111) peak is much higher than the intensities of the (220) and (311) peaks. This suggests that (111) is the preferential growth direction of the nc-Si films. The preferential growth along the (111) direction presumably originates from the fact that the surface energy of the (111) crystal facets of Si is the lowest compared with any other orientation.11,19 It has been suggested that the nc-Si-based solar cells have a better performance when the silicon films have a preferential growth along the (220) orientation. This is because the (220)oriented Si films feature electrically inactive (220)-tilt boundary and compact columnar structure, compared with other crystal orientations.20,21 Therefore, it is particularly useful to study the detailed variation trend of the (111) and (220) crystallographic orientations with the increase of the substrate temperature from 200 to 500 °C in the ICP-based process. Figure 2a presents the integrated XRD intensity ratio of (111) to (220) peaks (designated as I(111)/I(220)) as a function of the substrate temperature. Likewise, the mean grain sizes corresponding to the (111) and (220) crystallographic orientations, estimated using the DebyeScherrer formula (d ) 0.9λ/B cos θ, where d is the mean grain size of the silicon crystallites, λ is the incident wavelength of X-ray radiation, B is the full width at half-maximum of the diffraction peak, and θ is the Bragg diffraction angle) are shown in Figure 2b. From Figure 2a, one can see that the I(111)/I(220) ratio monotonously decreases from 6.3 to 1.5 as the substrate temperature is increased from 200 to 500 °C. This means that the increased substrate temperature promotes the multidirectional and more random nucleation and growth of Si crystallites. Furthermore, as shown in Figure 2b, with the increase of the substrate temperature from 200 to 500 °C, the average grain size corresponding to the (111) orientation decreases from 23.4 nm at Ts ) 200 °C to 16.1 nm at Ts ) 500 °C. Meanwhile, the average grain size of the (220) orientation increases from 6.6 nm at Ts ) 200 °C to 15.2 nm at Ts ) 500 °C. The crystallinity of the deposited silicon thin films was investigated by Raman spectroscopy. Figure 3a shows the Raman spectra of the samples deposited at different substrate temperatures. At a substrate temperature of 100 °C, only one broad peak centered at ∼480 cm-1, characteristic of amorphous silicon phase, can be observed. However, at Ts g 200 °C, the dominant peak is located at ∼520 cm-1. This peak is attributed to the transverse optical mode of Si-Si vibrations in the

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Figure 2. (a) Integrated XRD intensity ratios of (111) to (220) (designated as I(111)/I(220)) as a function of the substrate temperature. (b) The mean grain sizes of the (111) and (220) crystallographic orientations of the silicon thin films as a function of the substrate temperature.

crystalline silicon phase, indicating the formation of nc-Si thin films.22,23 The results of the Raman spectroscopy are consistent with the analysis by the XRD. In order to compare the crystal volume fraction Xnc of the deposited silicon films for Ts g 200 °C, we have deconvoluted the predominant peak in the vicinity of 520 cm-1 into three independent Gaussian peaks: (i) amorphous silicon peak at 480 cm-1; (ii) defective silicon phase (due to the grain boundaries or small crystallites less than 10 nm) at ∼505-515 cm-1; and (iii) crystalline silicon phase near 520 cm-1.11,23,24 Thereafter, the crystal volume fraction, Xnc, can be estimated from the deconvoluted curves using the following equation

Xnc ) (I510 + I520)/(I480 + I510 + I520) wherein I480, I510, and I520 stand for the integrated intensities of the individual deconvoluted peaks located at 480, 510, and 520 cm-1, respectively.11,24 Figure 3b shows a typical deconvoluted spectrum of the sample deposited at a substrate temperature of 400 °C; the crystal volume fraction Xnc, calculated using the above equation, is ≈64.7%. The variation of the crystal volume fraction Xnc as a function of the substrate temperature is displayed in Figure 3c. Interestingly, the Xnc initially increases from 62.6% at Ts ) 200 °C to 69.1% (maximum value) at Ts ) 300 °C, and then is slightly reduced with a further increase of the substrate temperature, reaching the value of 61.3% at Ts ) 500 °C. The optical properties of the deposited silicon films were studied by UV-vis spectroscopy. Figure 4a presents the optical transmittance spectra of the silicon films deposited on quartz substrates under different substrate temperatures. The occurrence of the interference fringes at long wavelengths (photon energies less than the optical bandgap) in the spectra is attributed to the smoothness of the film surface. Furthermore, one can notice the upshift of the absorption edge to longer wavelengths with the increased substrate temperature. In particular, it is evident

Figure 3. (a) Raman spectra of the samples deposited at different substrate temperatures. (b) A typical deconvoluted spectrum of the sample deposited at Ts ) 400 °C. (c) The variation of the crystal volume fraction Xnc as a function of the substrate temperature.

that there exist two jumps in the absorption edge with the increase of the substrate temperature. One of the jumps takes place between 100 and 200 °C, which is closely correlated with the occurrence of the crystallization of the silicon thin films.15 The other jump occurs between 300 and 400 °C, which can possibly be attributed to the strong hydrogen desorption from the hydrogen-terminated film surface.25,26 For the indirect-bandgap semiconductors, the theoretical relationship between the optical bandgap, Egopt, and the absorption coefficient R is given by the Tauc’s equation

√Rhυ ) B(hυ - Egopt) where h is the Planck’s constant, ν is the frequency of the incident photon, and B is the optical density of states.27 Here, R can be determined from the transmission data using the formula IT ) I0e-Rd, where IT is the intensity of the transmitted light, I0 is the intensity of the incident light, and d is the thickness of the silicon thin films; Egopt can be estimated from the plot of Rhυ versus hυ by extrapolating the linear portion of the curve to intercept the energy axis (R ) 0).25,27 Figure 4b shows the optical bandgap of the deposited silicon thin films as a function of the substrate temperature. The optical bandgap varies in the range of 1.29-1.90 eV and exhibits a

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Figure 4. (a) Optical transmittance spectra of the silicon films deposited on quartz substrates under different substrate temperatures. (b) The optical bandgap Egopt of the deposited silicon thin films as a function of the substrate temperature.

systematic tendency of reduction with the increase of the substrate temperature. This trend of the variation of the optical bandgap is a combined effect of the hydrogen concentration and crystallility of the silicon films deposited under different substrate temperatures.15,28 The morphological and microstructural properties of the deposited silicon films were analyzed by SEM. Figure 5a-c displays the typical surface topography and fracture cross-section of the silicon films deposited at the substrate temperatures of 100, 300, and 500 °C, respectively. As shown in Figure 5a, the morphology of the sample deposited at a substrate temperature of 100 °C features amorphous clusters with a typical size of a few to several tens of nanometers. On the other hand, the topography of the samples deposited at the substrate temperatures of 300 and 500 °C appears to be made up of small nanoparticles without any observable voids between them. However, the roughness of the sample deposited at Ts ) 500 °C is much higher than that of the sample deposited at Ts ) 300 °C. This is because at a high substrate temperature of 500 °C, a large fraction of the Si-H bonds are broken thus leaving a large amount of unsaturated Si dangling bonds. Subsequently, these silicon dangling bonds reconstruct to form silicon nanoparticles, giving rise to the observed fairly rough surface.25 In addition, as shown in Figure 5a-c, the film deposited at Ts ) 100 °C features a homogeneous amorphous structure, while those samples deposited at Ts ) 300 and 500 °C exhibit vertically aligned columnar structures. The growth rate, Rd, of the deposited Si thin films was obtained through dividing the film thickness (measured by the cross-sectional SEM) by the deposition time. Figure 6 presents Rd achieved in this work as a function of the substrate temperature. The growth rate shows a slight and gradual increase from 1.53 nm/s at Ts ) 100 °C to 1.69 nm/s at Ts ) 400 °C, and then sharply increases from 1.69 nm/s at Ts ) 400 °C to 1.91 nm/s at Ts ) 500 °C. The observed rapid increase of the

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Figure 5. (a-c) Typical surface topography and fracture cross-section of the silicon films deposited at Ts ) 100, 300, and 500 °C, respectively.

Figure 6. Variation of the growth rate as a function of the substrate temperature.

growth rate with the increased substrate temperature from 400 to 500 °C is quite possibly attributed to the recrystallization of the unsaturated Si dangling bonds originating from the significant desorption of hydrogen via the process of thermally activated fragmentation of the surface Si-H bonds at a high substrate temperature of 500 °C.25,26 It is noteworthy that in the previous work,11,15 we have demonstrated that the typical growth rates achieved in the ICP-based process are very competitive compared with other techniques (such as conventional 13.56 MHz rf capacitively coupled plasma (CCP) enhanced CVD, hot wire CVD, etc.) of fabrication of nc-Si thin films.23,29,30 4. Discussion We now discuss the influence of the plasma-related effects such as ion fluxes, electric fields, ion-bombarding energy, polarization effects, etc., on the growth of nc-Si thin films in the ICP-based process of our interest and compare this process with other neutral gas-based (thermal CVD) and plasma-based (rf capacitively coupled) processes. It has been demonstrated

Temperature-Dependent Properties of nc-Si Thin Films by numerous experimental works that plasma-based processes enable the crystallization of silicon at a low substrate temperature of ∼200-400 °C.2,6,8 This substrate temperature is significantly lower than Ts used by the neutral gas-based process for crystallization of silicon (neutral gas-based process routinely requires Ts > 600 °C to enable effective crystallization of silicon).31 The synthesis of high-quality nc-Si thin films at significantly lower substrate temperatures in the reactive plasmas is mainly attributed to the strong plasma-surface interactions on the film growth surface. These plasma-surface interactions result in a significant reduction of the surface diffusion activation energy through the transfer of momentum from the impinging species to the growth surface, a higher surface temperature through the ion-bombardment, higher growth rates through the creation of the adsorption sites via the fragmentation and rearrangement of the chemical bonds on the surface by the impinging particles, and some other effects.32-35 Other plasmarelated effects (such as electric field- and polarization-related effects, etc.) can significantly increase the rates of dissociation of silane molecules in the gas phase, promote the fast transport of the reactive species to the growth surface, and further reduce surface diffusion activation barriers.2,36,37 On the other hand, in comparison with many other rf plasma sources, the ICP source features higher electron/ion number densities. The electron number density produced in the ICPs can reach 1012-1013 cm-3. This value is ∼1 or 2 orders of magnitude higher than the density of capacitively coupled plasmas sustained under quite similar process conditions.13,38,39 Low-pressure operation also means that the mean free path of reactive species in ICPs can be significantly increased. This gives rises to two important benefits of the ICP-based process: (i) It becomes possible to reduce the rates of secondary gasphase reactions among the radicals and the source molecules during the radicals’ transport to the substrate and thereby minimize particulate and powder formation, gas-phase polymerization, etc. (ii) Fairly narrow ion energy distribution functions can be achieved. In this way, one can avoid the frequently observed relatively high, uncontrollable and broadly spread energies of the ions impinging on the growth surface common to many other glow discharges. These effects in turn often lead to undesirable entrapments of working gas molecules and dislocations in the films and as such deteriorate the film quality.11,40,41 It is worthwhile to stress that the highest crystal volume fraction and lower surface roughness have been observed for the sample deposited at a substrate temperature of 300 rather than 500 °C. This rather unexpected experimental result can be interpreted by invoking the following thermokinetic arguments. It is generally accepted that a high substrate temperature enhances the mobility of the adsorbed radicals on the growth surface and thereby increases the diffusion length of the adsorbed radicals.15,23 This assists the adsorbed radicals to reach the most energetically favorable sites and gives rise to better crystallization of the synthesized nc-Si thin films. This explains the gradual improvement of the crystal volume fraction of the silicon thin films when the substrate temperature is increased from 100 to 300 °C. However, at higher substrate temperatures (for example, 500 °C in this case), strong desorption of hydrogen from the partially hydrogen-terminated growth surface significantly reduces the surface coverage of atomic hydrogen and consequently enhances the surface reactivity of the adsorbed radicals.25,26 This improves the growth rate significantly, as has been shown in Figure 6, and also reduces the crystalline fraction of the nc-Si film.

J. Phys. Chem. C, Vol. 113, No. 33, 2009 14763 It is noteworthy that the low-pressure, thermally nonequilibrium, high-density plasmas have been extensively applied in the synthesis of a variety of nanostructured materials. For instance, we have reported on the deterministic plasma-enabled growth of ultralong straight, helical, and branched silicon oxide nanowires;42 plasma-aided synthesis of highly uniform, stoichiometric SiC quantum dots;16,43 plasma-enhanced deposition of high-aspect-ratio and vertically aligned carbon nanotips.35,44 Let us now briefly discuss the benefits of low-temperature plasmas on the growth of ultralong silicon oxide nanowires. In the absence of the plasmas, the length of silicon oxide nanowires never exceeded a few hundred nanometers. On the other hand, when the low-temperature plasma was used, it became possible to synthesize ultralong (up to several millimeters in length) straight and twisted nanowires.42 This is because various plasmarelated effects promoted increased rates of production of suitable building units, repositioned and lifted off the metal catalyst nanoparticles from the substrate surface, focused plasmagenerated ion beams toward the tips of the growing nanowires, significantly increased the surface temperature through ion bombardment.42 Further details of the benefits of the lowtemperature plasmas that enabled the growth of ultralong silicon oxide nanowires and other nanostructures can be found elsewhere.42,45-49 5. Conclusion In this work, we have systematically studied the morphological, structural, and optical properties of the Si thin films under a broad range of substrate temperatures from 100 to 500 °C prepared by ICP-assisted CVD. The main findings of this work can be summarized as follows. • Silicon films synthesized at a substrate temperature of 100 °C are amorphous, while the nc-Si films can be formed when the substrate temperature is higher than 200 °C. • All the deposited nc-Si films show a preferential growth along the (111) direction. The mean grain size of the (111) orientation slightly and gradually decreases while the mean grain size of the (220) orientation shows a monotonous increase when the substrate temperature is increased from 200 to 500 °C. • The crystal volume fraction initially increases with the increase of the substrate temperature from 200 to 300 °C and reaches a maximum value of ∼69.1% at a substrate temperature of 300 °C, and then slightly decreases with a further increase of the substrate temperature to Ts ) 500 °C. The occurrence of the highest crystal volume fraction and lower surface roughness at a substrate temperature of 300 rather than 500 °C has been explained from the viewpoint of the interplay of the two counteracting kinetic (surface diffusion and surface termination) effects. • The optical bandgap shows a tendency of reduction while the growth rate tends to increase at higher substrate temperatures. The maximum rates of change of the optical bandgap and the growth rate take place when the substrate temperature is increased from 400 to 500 °C. Finally, we stress that the delicate balance between the two thermokinetic factors (surface diffusion and surface termination), giving rise to intricate refinements in the material’s microstructure and other properties, has been thoroughly studied in this work. The results of this work are particularly important for the development of thin film nc-Si-based solar cells, transistors, and flat-panel displays. Future work will be focused on the fabrication and performance testing of nc-Si-based singlejunction solar cells with the ultimate target of achieving solar cell energy conversion efficiency of more than 10%.

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Acknowledgment. This work was partially supported by the National Research Foundation (Singapore), CSIRO, and the Australian Research Council (Australia). The authors thank J. D. Long from the Solar Energy Research Institute of Singapore and S. Y. Huang from the National Institute of Education, Singapore for fruitful discussions and technical assistance. References and Notes (1) Sriraman, S.; Agrawal, S.; Aydil, E. S.; Maroudas, D. Nature (London) 2002, 418, 62. (2) Denysenko, I. B.; Ostrikov, K.; Xu, S.; Yu, M. Y.; Diong, C. H. J. Appl. Phys. 2003, 94, 6097. (3) Hwang, N. M.; Kim, D. Y. Int. Mater. ReV. 2004, 49, 171. (4) Krawiec, B. S.; Henderson, R. R. C.; Coffer, J. L.; Rho, Y. G.; Pinizzotto, R. F. J. Phys. Chem. 1996, 100, 13776. (5) Cvelbar, U.; Mozetic, M.; Sunkara, M. K.; Vaddiraju, S. AdV. Mater. 2005, 17, 2138. (6) Shah, A. V.; Meier, J.; Sauvain, E. V.; Wyrsch, N.; Kroll, U.; Droz, C.; Graf, U. Sol. Energy Mater. Sol. Cells 2003, 78, 469. (7) Kelzenberg, M. D.; Evans, D. B. T.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Lewis, N. S.; Atwater, H. A. Nano Lett. 2008, 8, 710. (8) Dalal, V. L.; Graves, J.; Leib, J. Appl. Phys. Lett. 2004, 85, 1413. (9) Gordijn, A.; Rath, J. K.; Schropp, R. E. I. Prog. PhotoVolt.: Res. Appl. 2006, 14, 305. (10) Finger, F.; Mai, Y.; Klein, S.; Carius, R. Thin Sol. Films 2008, 516, 728. (11) Cheng, Q. J.; Xu, S.; Huang, S. Y.; Ostrikov, K. Cryst. Growth Des. 2009, 9, 2863. (12) Cheng, Q. J.; Xu, S.; Ostrikov, K. J. Mater. Chem. 2009, XX, XXX (DOI: 10.1039/b904227j). (13) Xu, S.; Ostrikov, K. N.; Li, Y.; Tsakadze, E. L.; Jones, I. R. Phys. Plasmas 2001, 8, 2549. (14) Moon, B. Y.; Youn, J. H.; Won, S. H.; Jang, J. Sol. Energy Mater. Sol. Cells 2001, 69, 139. (15) Cheng, Q. J.; Xu, S.; Ostrikov, K. Nanotechnology 2009, 20, 215606. (16) Cheng, Q. J.; Xu, S.; Long, J. D.; Ostrikov, K. Appl. Phys. Lett. 2007, 90, 173112. (17) Hotta, Y.; Toyoda, H.; Sugai, H. Thin Solid Films 2007, 515, 4983. (18) Cheng, Q. J.; Xu, S.; Long, J. D.; Huang, S. Y.; Guo, J. Nanotechnology 2007, 18, 465601. (19) Kakinuma, H.; Mohri, M.; Sakamato, M.; Tsuruoka, T. J. Appl. Phys. 1991, 70, 7374. (20) Kondo, M.; Matsuda, A. Thin Solid Films 2001, 383, 1.

Cheng et al. (21) Rath, J. K. Sol. Energy Mater. Sol. Cells 2003, 76, 431. (22) Chen, Y.; Peng, B.; Wang, B. J. Phys. Chem. C 2007, 111, 5855. (23) Mukhopadhyay, S.; Das, C.; Ray, S. J. Phys. D: Appl. Phys. 2004, 37, 1736. (24) Han, D.; Wang, K.; Owens, J. M.; Gedvilas, L.; Nelson, B.; Habuchi, H.; Tanaka, M. J. Appl. Phys. 2003, 93, 3776. (25) Song, D. Y.; Cho, E. C.; Conibeer, G.; Cho, Y. H.; Huang, Y. D.; Huang, S. J.; Flynn, C.; Green, M. A. J. Vac. Sci. Technol. B 2007, 25, 1327. (26) Gerbi, J. E.; Abelson, J. R. J. Appl. Phys. 2001, 89, 1463. (27) Cheng, Q. J.; Xu, S.; Long, J. D.; Ni, Z. H.; Rider, A. E.; Ostrikov, K. J. Phys. D: Appl. Phys. 2008, 41, 055406. (28) Bhattacharya, K.; Das, D. Nanotechnology 2007, 18, 415704. (29) Yang, H. D.; Wu, C. Y.; Huang, J. K.; Ding, R. Q.; Zhao, Y.; Geng, X. H.; Xiong, S. Z. Thin Sol. Films 2005, 472, 125. (30) Jadkar, S. R.; Sali, J. V.; Takwale, M. G.; Musale, D. V.; Kshirsagar, S. T. Thin Sol. Films 2001, 395, 206. (31) Aoyama, T.; Kawachi, G.; Konishi, N.; Suzuki, T.; Okajima, Y.; Miyata, K. J. Electrochem. Soc. 1998, 136, 1169. (32) Ostrikov, K. Vacuum 2008, 83, 4. (33) Levchenko, I.; Rider, A. E.; Ostrikov, K. Appl. Phys. Lett. 2007, 90, 193110. (34) Ostrikov, K. Rev. Mod. Phys. 2005, 77, 489. (35) Rukevych, P. P.; Ostrikov, K.; Xu, S.; Vladimirov, S. V. J. Appl. Phys. 2004, 96, 4421. (36) Arulsamy, A. D.; Ostrikov, K. Phys. Lett. A 2009, 373, 2267. (37) Ostrikov, K.; Levchenko, I.; Xu, S. Pure Appl. Chem. 2008, 80, 1909. (38) Ostrikov, K. N.; Xu, S.; Yu, M. Y. J. Appl. Phys. 2000, 88, 2268. (39) Tsakadze, Z. L.; Levchenko, I.; Ostrikov, K.; Xu, S. Carbon 2007, 45, 2022. (40) Cabarrocas, P. R. i. Phys. Stat. Sol. C 2004, 1, 1115. (41) Ren, Y. P.; Ostrikov, K.; Xu, S. Phys. Plasmas 2008, 15, 023502. (42) Huang, S. Y.; Ostrikov, K.; Xu, S. J. Appl. Phys. 2008, 104, 033301. (43) Cheng, Q. J.; Long, J. D.; Xu, S. J. Appl. Phys. 2007, 101, 094304. (44) Levchenko, I.; Ostrikov, K. J. Phys. D: Appl. Phys. 2007, 40, 2308. (45) Han, Z. J.; Tay, B. K.; Shakerzadeh, M.; Ostrikov, K. Appl. Phys. Lett. 2009, 94, 223106. (46) Ostrikov, K.; Murphy, A. B. J. Phys. D: Appl. Phys. 2007, 40, 2223. (47) Levchenko, I.; Ostrikov, K.; Mariotti, D. Carbon 2009, 47, 344. (48) Tam, E.; Levchenko, I.; Ostrikov, K. J. Appl. Phys. 2006, 100, 036104. (49) Long, J. D.; Xu, S.; Cai, J. W.; Jiang, N.; Lu, J. H.; Ostrikov, K. N.; Diong, C. H. Mater. Sci. Eng., C 2002, 20, 175.

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