Effect of metal underlayers on low temperature silicon growthK. Xu, A. Pradhan, and S. Ismat Shah Citation: Journal of Applied Physics 94, 5374 (2003); doi: 10.1063/1.1611633 View online: http://dx.doi.org/10.1063/1.1611633 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/94/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in From amorphous to microcrystalline: Phase transition in rapid synthesis of hydrogenated silicon thin film in lowfrequency inductively coupled plasmas J. Appl. Phys. 108, 113520 (2010); 10.1063/1.3514006 Effect of oxygen on growth and properties of diamond thin film deposited at low surface temperature J. Vac. Sci. Technol. A 26, 1487 (2008); 10.1116/1.2998807 Low temperature deposition and characterization of polycrystalline Si films on polymer substrates J. Vac. Sci. Technol. A 19, 1078 (2001); 10.1116/1.1345905 Structural, optical, and electrical properties of nanocrystalline silicon films deposited by hydrogen plasmasputtering J. Vac. Sci. Technol. B 16, 1851 (1998); 10.1116/1.590097 Ion assisted growth and characterization of polycrystalline silicon and silicon-germanium films J. Appl. Phys. 83, 4472 (1998); 10.1063/1.367209

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Effect of metal underlayers on low temperature silicon growthK. Xu and A. PradhanDepartment of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716

S. Ismat Shaha)

Department of Materials Science and Engineering, and Department of Physics and Astronomy,University of Delaware, Newark, Delaware 19716

~Received 22 April 2003; accepted 30 July 2003!

Silicon films were deposited on bare glass, copper- and gold-coated glass substrates at 200 °C.X-ray diffraction ~XRD! showed that the films deposited on substrates with gold underlayer werepolycrystalline while those deposited on bare glass and copper-coated glass had no identifiablecrystalline silicon XRD peak. Raman spectroscopy was used to confirm the film’s crystallineproperties. The Raman spectra indicated that films deposited on gold-coated glass substrates werecomposed of predominantly crystalline silicon with small amounts of amorphous silicon. Atomicforce microscopy~AFM! was used to study the topography and adatom diffusion on the surface.AFM micrographs showed that the polycrystalline silicon films had grain size up to 95 nm.© 2003 American Institute of Physics.@DOI: 10.1063/1.1611633#

I. INTRODUCTION

Low temperature polycrystalline silicon~poly-Si! thinfilm transistors have drawn considerable interest in applica-tions such as active matrix liquid crystal displays and solarcells since they can be fabricated on cheaper substrates, suchas glass and plastics. Widely used hydrogenated amorphoussilicon transistors have limitations arising from the photode-generation and poor electrical properties compared topoly-Si.1 Generally, poly-Si film growth requires high sub-strate temperatures or post-deposition annealing at around600 °C for several hours.2 These methods are not suitable totemperature sensitive substrates. Various techniques havebeen developed to fabricate the poly-Si films at low tempera-tures. One of the commonly used methods involves firstgrowing an amorphous silicon film by low pressure chemicalvapor deposition~LPCVD! followed by laser annealing tocrystallize the films.3 The method is limited by the produc-tivity and cost. Several publications reported deposition ofpoly-Si at temperatures as low as 250 °C using bias assistedmagnetron sputtering,4 plasma enhanced chemical vapordeposition5 and hot wire chemical vapor deposition.6 In ourprevious work, we studied the effect of the addition of kryp-ton to sputtering gas to grow polycrystalline thin silicon film.Addition of 10% krypton to sputtering gas can enhance theion and neutral bombardment of the growing film, andthereby promote the crystallinity of the silicon film duringdeposition by dc magnetron sputtering.7 In the same manner,the effect of hydrogen on the crystallinity of poly-Si thinfilms was studied by Abelsonet al.8

The role of chemical nature and roughness of the sub-strate surface have been found to have significant effect onpoly-Si nucleation and growth. The ion bombardment andhydrogen plasma related etching play important roles inroughening the surface during growth and affect the crystal-

lization and coalescence processes.9–11 In this work, wepresent a simple method of depositing poly-Si thin film ongold-coated glass at 200 °C using dc magnetron sputtering.In order to understand the effect of substrates on the growthof poly-Si films, we also deposited silicon thin films on glassand copper-coated glass substrates. X-ray diffraction wasused to characterize the structure of the silicon films. Ramanspectroscopy was used to verify the crystalline structure andto determine the structural composition of the silicon films.Atomic force microscopy was used to investigate the topog-raphy and roughness of the films.

II. EXPERIMENT

Poly-Si thin films were deposited on gold-coated glasssubstrates at 200 °C by dc magnetron sputtering in a tur-bopumped stainless steel chamber. The typical thickness ofsilicon films was about 300–400 nm. The base pressure ofthe system was 531027 Torr and sputtering was carried outusing a 5 cmboron doped silicon target. Three mass flowcontrollers were used to control Ar, Kr, and H2 flow rates. Atotal flow rate of 30 sccm was maintained. Hydrogen isknown to passivate silicon dangling bonds at the film surfaceand grain boundaries, enhance adatom diffusion, and help incrystalline silicon film formation.12–14 The film depositionwas performed in a 5% H2, 10% Kr, and 85% Ar atmo-sphere. Sputtering pressure was controlled by throttling themain gate valve. All deposition experiments were carried outat a pressure of 7 mTorr and a target power between 50 and140 W. Bare glass and copper-coated glass substrates werealso used in order to study the influence of substrate surfaceon the deposited silicon films. The distance between targetand substrate was fixed at 7 cm. The substrates were cleanedin acetone, methanol and de-ionized water prior to beingloaded in the deposition chamber. The substrates were heatedto 200 °C at the base pressure and kept at the same tempera-ture during deposition. Before the start of deposition, thea!Electronic mail: ismat@udel.edu

JOURNAL OF APPLIED PHYSICS VOLUME 94, NUMBER 8 15 OCTOBER 2003

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substrates were cleaned by exposure to hydrogen plasma for10 min at a power of 100 W and a pressure of 0.2 Torr. Thisstep was used to remove the native oxide and carbon con-taminations from the substrates. Immediately prior to thedeposition, the target was presputtered for 10 min with ashutter in place to block any deposition on the substrates.

Raman spectroscopy~0.5 W! was carried out using an Arlaser (l5532 nm) to verify the crystalline structure and todetermine the composition of the silicon films. XRD wasalso used for structural characterization and grain size deter-mination. A Rigaku D-Max B horizontal diffractometer wasused, employing CuKa radiation. The atomic force micro-scope ~AFM! used was a Digital Instrument model SPMIIIA. AFM was used to determine surface morphology,roughness, and grain size. Four-point probe was used to mea-sure the sheet resistivity of the deposited silicon thin films.

III. RESULTS

XRD patterns from the silicon films deposited at 200 °Con glass~a!, gold-coated glass~b! and copper-coated glass~c! are shown in Fig. 1. Films grown on bare glass substratedoes not show any identifiable XRD peak. A very broad peakcentered around 2u527° is observed, as seen in the diffrac-tion pattern~a!. This could be from the amorphous siliconfilm or the glass substrate. Films grown on the gold-coatedglass substrate shows poly-Si XRD reflections with a slight~111! preferred orientation. Typically, films grown on glasssubstrates at low temperatures show a~110! preferred orien-tation. The~111! preferred orientation is most probably dueto the gold underlayer which itself has~111! preferred orien-tation, as can be seen from the diffraction pattern~b!. Theaverage crystal size, calculated from the full width at halfmaximum ~FWHM! of the Si ~111! peak using Scherrer’sformula,15 is 90 nm. The XRD pattern~c! is from a siliconfilm deposited on copper-coated glass substrate showingthree peaks. In addition to the Cu~111! and Cu~200! reflec-tions, there is an additional peak at around 44.5°. This re-flection does not belong to either copper or silicon. Instead, itemanates from a copper–silicon alloy, as also observed byKim et al.16 This suggests that with copper as the underlayerthere is extensive intermixing between the copper and thesilicon, and pure silicon film is difficult to form, at least up toa thickness of 400 nm.

AFM micrographs of silicon thin films deposited onglass, copper- and gold-coated glass substrates, correspond-ing to XRD samples in Fig. 1, are shown in Fig. 2. Figure2~a! is an AFM micrograph of the silicon film grown on glasssubstrate at 200 °C. It reveals a very fine grain structure ofthe film. The corresponding XRD pattern in Fig. 1~a! indi-cated that the film is amorphous. The average grain size mea-sured for the AFM micrograph is less than 20 nm. The AFMimage of a silicon film deposited on gold-coated glass at200 °C is shown in Fig. 2~b!. The micrograph shows that thesurface is composed of larger grains with no substructure.XRD also showed that the film is polycrystalline. The aver-age grain size calculated from the AFM image is about 95nm which is consistent with the results obtained from XRD.Figure 2~c! shows an AFM micrograph of the silicon filmgrown on copper-coated glass substrate at 200 °C. Eventhough this film has a much larger grain size, there is asubstructure in the grains, unlike the film deposited on gold-coated glass substrate shown in Fig. 2~b!. The average size ofthe substructure is 50 nm. The tapping mode AFM showsonly a single phase film. The copper reflections in the XRDpattern, therefore, are from the copper underlayer and thefilm is predominantly a copper–silicon alloy.

Raman spectroscopy was used to verify and evaluate thecrystallinity of the deposited silicon films. Raman spectra ofsilicon films grown on glass and gold-coated glass substratesat 200 °C are shown in Fig. 3. The Raman spectrum of thesilicon film deposited on glass, curve~a!, displayed a broadpeak centered around 480 cm21, which is typical of anamorphous silicon film. The Raman spectrum of the siliconfilm deposited on gold-coated glass, curve~b!, shows a sharppeak around 520 cm21 which is the characteristic of thecrystalline silicon TO line.17 There is an additional peak at alower wavelength, centered around 480 cm21. This peak isgenerally considered to be from amorphous silicon. The sili-con film, therefore, is actually a mixture of amorphous andcrystalline phases. We used the ratio of the amorphous sili-con and crystalline silicon Raman peak areas to estimate thefraction of the crystalline phase in the film. Using a commer-cial peak fitting software, we calculated the percentage ofcrystalline phase to be 80%.

The sheet resistivity of the deposited film is an indirectindication of the crystallinity and the purity of the film. Allother parameters being equal, a larger grain size film shows alower sheet resistivity due to the decreased grain boundaryscattering of the charge carriers. We have measured the sheetresitivity of the silicon films deposited on various substrates.The results are summarized in Table I. The lowest sheet re-sistivity measured is from the film deposited on the copper-coated glass substrate. As described earlier, these films areessentially silicon–copper alloy films. As such, a low sheetresistivity is as expected. Films deposited on gold-coatedglass substrates also show low sheet resistivity owing to thelarge grain size of the film. Silicon films deposited on bareglass substrates have a large sheet resistivity. This is due tothe fact that these films are amorphous. Additionally, ourintial results on the study of the nucleation and growth ofsilicon films via in situ XPS analyses indicate that theremight also be an oxide component in the films deposited on

FIG. 1. XRD patterns of silicon films deposited at 200 °C on~a! bare glass,~b! gold-coated glass, and~c! copper-coated glass substrates.

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bare glass substrates which can also contribute to the highsheet resistivity of the films.

IV. DISCUSSION

The work of Kondoet al.18 suggested that conductiveand oxygen free substrates not only affect the initial stages ofthe growth of silicon films, but also are responsible for the

eventual crystalline nature of the thick silicon film. The ori-gin of the substrate dependence was explained in terms ofthe ion bombardment from the plasma, which is related tothe surface potential and, therefore, the conductivity of thesurface. Bray and Parsons19 characterized the surface trans-port kinetics during low-temperature silicon thin film depo-sition by using time dependent surface topography and dy-namic scaling models in which surface diffusion lengthswere associated with the root mean square~rms! roughnessand the correlation lengths. The mechanism of diffusion iscontroversial. Bray and Parsons measured an activation bar-rier of 0.2 eV related to hydrogen-mediated adspecies diffu-sion, whereas Smetset al.20 measured an activation energyof 1 eV, consistent with the diffusion of chemisorbed hydro-gen. In both these studies, the system was essentially achemical vapor deposition system with very little or no ion

FIG. 2. AFM micrographs of siliconfilms grown on ~a! bare glass,~b!gold-coated glass, and~c! copper-coated glass substrates.

FIG. 3. Raman spectra from silicon films grown at 200 °C on~a! bare glassand ~b! gold-coated glass substrates.

TABLE I. Sheet resistivity of silicon films deposited on various substrates.

Silicon thin film Sheet resistivity~V/h!

Si on copper-coated glass 10.7Si on gold-coated glass 20.3Si on bare glass .1000

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bombardment. In the magnetron process, ion bombardmentis omnipresent and critical for the crystalline growth of sili-con films. Similar to Kondo’s work, it is the enhanced diffu-sion of the adatoms which plays the most important role indetermining the crystallinity of the films. Additionally, thereis very little hydrogen in the system to either make the pro-cess SiH3 or H diffusion dependent. Previously, we havecalculated the energy of the energetic species arriving at thesubstrate surface to be greater than 1 eV.7 At such energies,significant diffusion is possible. The difference in the crys-talline nature of the films grown on the glass substrate andgold-coated glass substrate is most likely due to the energeticspecies bombardment. For the bare glass substrate which isat a floating potential, there is essentially no initial ion bom-bardment, as the nonconductive substrate charges up veryquickly. Only the neutral species are allowed to bombard thegrowing film. On the gold-coated substrates, the ion bom-bardment remains effective throughout the growth process.In order to quantify the effect of energetic particle bombard-ment, we calculated rms roughness of silicon films based onAFM images. These rms roughness values are shown in Figs.4~a! and 4~b! for the glass and gold-coated glass substrates,respectively. From the graphs, it can be seen that silicondeposited on glass has much smaller rms roughness thansilicon deposited on gold-coated glass. Silicon deposited oncopper-coated glass has similar rms roughness compared tosilicon deposited on gold-coated glass. It means that the sili-con atoms on glass surface at 200 °C have no significant

diffusion. They essentially condense wherever they land onthe surface of the substrate. This is generally known as arandom deposition~RD! process.21 Silicon films depositedon gold-coated glass or copper-coated glass, however, havelarger rms roughness. It suggests that there is a significantamount of diffusion involved in the film growth. Some re-ports suggest that the correlation length, which correspondsto the critical point where the rms roughness becomes con-stant, can be used to estimate the surface diffusion length.19

In our case, the calculated correlation lengths for siliconfilms grown on glass and gold-coated glass substrates are;300 and;700 nm, respectively. These numbers are obvi-ously too large for a surface diffusion length. We believe thatthe longer correlation length is related to a longer diffusionlength. However, a linear relationship between the two is, asyet, unconfirmed.

The initial nucleation plays an important role in theeventual film crystal size. The initial nucleation, however,depends on surface roughness and ion bombardment. Arougher surface provides more nucleation sites. With AFM,we have confirmed that the glass substrates have a muchflatter surface compared to the gold surface. The rms rough-ness for glass and gold-coated glass substrate is measured tobe 0.3 and 2 nm, respectively. This relatively higher rough-ness is partly responsible for easier nucleation on gold-coated substrates and results in eventually a more crystallinefilm. Additionally, the template effect of the crystalline goldfilm also helps to enhance the crystallinity of the siliconfilms, whereas the amorphous glass surface has no such ef-fect. As seen in the XRD measurements, the gold films~111!orientation induces a slight~111! preferred orientation in thesilicon film. The copper underlayer has the same effect onthe growing silicon film except that copper has a muchhigher diffusion coefficient in silicon,22 which results in asilicon–copper alloy formation rather than the growth ofpoly-Si.

V. CONCLUSIONS

A simple method is used to deposit poly-Si thin film atlow temperature by dc magnetron sputtering. The crystallinestructure of silicon film depends on the nature of the sub-strate. The substrate affects the initial nucleation and growthof the silicon films. Films grown on bare glass substrates at200 °C were found to be amorphous by XRD and Ramanspectroscopy. Films grown under similar conditions withgold underlayer on the glass substrate were polycrystallinewith small amounts of amorphous silicon. The average grainsize was measured to be about 95 nm. Films grown on cop-per were polycrystalline, had subgrain structures and weremainly composed of silicon–copper alloy. Compared to filmsdeposited on glass, films deposited on substrates with goldunderlayer had larger surface diffusion length, which resultsin the poly-Si thin film deposition. Sheet resistivity of thefilms also changes with the crystalline quality of the films.Films deposited on the gold-coated glass substrates show thelow sheet resistivity due to their large grain size. Filmsgrown on bare glass substrates have very high sheet resistiv-ity since the films are typically amorphous. Films on copper-

FIG. 4. rms roughness vs length scale for silicon on~a! glass substrate and~b! gold-coated glass substrates.

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coated glass substrates also have low resistivity owing to thefact that these films are actually silicon–copper alloys.

ACKNOWLEDGMENT

The authors gratefully acknowledge support from ACS/PRF Grant No. 4-5-24-3106-06.

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