*Corresponding author. Tel.: #81-462-40-3254; fax: #81-462-40-4304.

E-mail address: tyamada@aecl.ntt.co.jp (T. Yamada)

Journal of Crystal Growth 209 (2000) 50}54

Poly-crystalline silicon with large grains depositedfrom Al}Si melt

Takumi Yamada, Masami Tachikawa, Takashi Nishioka, Takeshi Yamada*

NTT Telecommunications Energy Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan

Received 14 July 1999; accepted 20 September 1999Communicated by T. Nishinaga

Abstract

Poly-crystalline Si "lm with large grains was successfully deposited on a glass plate at relatively low temperatures of650}7503C from Al melt as the growth solvent by sputtering. Aluminum nitride was used as the wetting agent for the "rsttime. All of the crystal grains are greater than 1 lm in diameter, and the (1 1 1)-oriented plane is dominant. A smallamount of Al}Si precipitates also exist. Hole concentration in the "lm is of the order of 1019 cm~3, which corresponds tothe solid solubility of Al in Si. The process by which the Si "lm consisting of large and uniform crystal grains grows isdiscussed. ( 2000 Published by Elsevier Science B.V. All rights reserved.

PACS: 81.15.Cd; 81.15.Lm

Keywords: Si "lm; Poly-crystalline; Grain size; Solution growth; Sputter; Solar cell

1. Introduction

A poly-crystalline Si "lm is promising for solarcell application because of economical and envir-onmental advantages. Many techniques have beenexamined to deposit thin poly-crystalline Si "lmsthat satisfy solar-cell and mass-production re-quirements, but the deposition methods still haveproblems.

Solution growth using metal and Si melt is oneway to deposit Si "lms at relatively low substratetemperature [1]. A Si "lm with grain size largerthan 20 lm has been precipitated on a substrate by

supplying sputtered Si into an In or Sn melt [2,3].It was pointed out that the initial solvent layerbecame discontinuous at the Si deposition temper-ature without a wetting agent. The wetting agent,such as Ti, improved the wetting and the resultantsurface morphology of the Si "lms, but photocon-ductivity and minority-carrier lifetime was reducedin the In/Ti system.

Aluminum has been reported to enhance Si crys-tallization in solid phase growth. Amorphous Si onAl-coated substrates crystallized by rapid ther-mal annealing within 5 min at 5503C [4], which isin contrast to the higher annealing temperatureand/or a longer term needed for solid phase crystalgrowth. Easy migration of Si atoms through Al "lmhas also been demonstrated. When a sputteredamorphous Si "lm on a 0.6-lm-thick Al "lm on a Sisingle-crystal wafer was annealed at 5003C, a Si "lm

0022-0248/00/$ - see front matter ( 2000 Published by Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 5 2 7 - 8

Fig. 1. Comparison of Al-covered area on substrate amongwetting agents.

grew epitaxially on the wafer [5]. These resultsindicate the advantages of an Al melt as the growthsolvent.

In this paper, we report the successful depositionof a poly-crystalline Si "lm with large grain sizefrom Al melt by sputtering. The Si "lm was depos-ited on a glass substrate around the melting pointof Al, and the grain size, "lm structure, and electri-cal properties were analyzed. The suitable wettingagent for our growth conditions is also studied.

2. Experimental procedure

The substrate was a Corning 1737 glass plate. Siand Al were deposited by RF magnetron sputter-ing. Following the deposition of a wetting agent onthe substrate, an Al}Si alloy initial layer was depos-ited. Then the substrate was heated to 650}7503C.After the temperature stabilized in a few minutes,10-lm-thick Si was deposited. Si target was 1]1018 cm~3 B-doped. The heating and cooling ratewere "xed at 303C/min. Rf power for sputtering was200 W and the Ar #ow rate was 15 sccm. The typi-cal deposition rate of Si was 3.5 lm/h.

Grain size was analyzed by scanning electronmicroscopy (SEM) after etching. Hydrogen-#uor-ide-based etchant [6] was used for selective etchingof grain boundaries. X-ray di!raction and micro-Auger electron spectra (l-AES) were measuredfor structural analyses. Residual Al and the holeconcentration were measured by secondary ionmass spectrometry (SIMS) and the van der Pauwmethod, respectively. The area analyzed by SIMSwas 28 lm in diameter.

3. Results and discussion

Before Si "lm deposition on the substrate, a bareglass substrate was compared to AlN, TiN, SiN,and W-coated glass substrates to improve the wet-ting of the Al melt. A 100-nm-thick Al layer washeated at 7003C for 10 min on various substrates.After cooling to room temperature, the Al-coveredarea on the substrates was measured. Fig. 1 showsthe areas covered with Al droplets. Wetting on AlNwas improved at least three times compared to that

on the bare glass. Note that there were a lot ofsmall, thin Al droplets among large Al droplets ononly the AlN-coated substrate. In counting thecovered area, those small droplets were ignored.On W- and TiN-coated substrates, there were no Aldroplets, because the Al reacted with the W andTiN. The area covered on the SiN could be mea-sured, though the SiN reacted with the melting Alpartly.

The bare glass substrates were eroded by the Almelt during Si deposition. However, no evidence ofreaction between Al melt and AlN was found in thecross-sectional SEM observation. All these resultsindicate that AlN is suitable as the wetting agentfor Si deposition from Al}Si melt. The samples inthe following results were deposited on AlN-coatedsubstrates.

Plan view and cross-sectional SEM photographsof a sample deposited at 7003C are shown inFigs. 2a and b, respectively. Many large crystalgrains several micrometers in diameter are evidentin Fig. 2a. Fig. 2b shows that the crystal grains arebulky and grew continuously from the substrate tothe "lm surface. According to transmission electronmicroscopy, these crystal grains are closely packed.The photograph also shows the stability of AlNlayer during Si deposition, as mentioned before.The dark parts with the bright rims in Fig. 2a willbe discussed later. The grain size was analyzed fromplan-view SEM photographs. Fig. 3 shows the sub-strate temperature dependence of the areas coveredwith crystal grains having diameters greater than 1,

T. Yamada et al. / Journal of Crystal Growth 209 (2000) 50}54 51

Fig. 2. Plan view (a) and cross-sectional view (b) of poly-crystal-line Si "lm after etching. The sample was deposited at 7003C.

Fig. 3. Substrate temperature dependence of the area coveredwith Si crystals with grain diameters greater than 1, 5, 10 lm.

Fig. 4. X-ray di!raction pattern of poly-crystalline Si "lm.

5 and 10 lm. Grain size increases as the substratetemperature rises. About 80% of the surface arecovered with crystal grains greater than 5 lm indiameter at substrate temperature of 7503C. Notethat all of the crystal grains are greater than 1 lm indiameter, and there are neither microcrystalline noramorphous areas according to Raman spectra.

Fig. 4 shows the X-ray di!raction pattern ofa poly-crystalline Si "lm deposited at 7503C.The thickness of the Al}Si alloy initial layer was200 nm. A strong Si(1 1 1) peak and weak Si(2 2 0)and Si(3 1 1) peaks are evident. The spectra of "lmsgrown at lower temperature are similar to those inFig. 4, and the intensity became weak as the growthtemperature decreased. The poly-crystalline "lmobtained here proved to be (1 1 1) oriented. Domi-nant (1 1 1) orientation in Si "lms was reported in

crystallization using the thermal equilibrium condi-tion, such as solid phase crystallization or solutiongrowth [7,8]. In Fig. 4, there is a relatively strongpeak at around 363. This peak corresponds to theAl}Si alloy (2 2 0) peak [9]. At the same position,we also observed a peak from the AlN layer. The Si"lm was so thick (10 lm) that the contribution ofAlN to the di!raction pattern was small. Therefore,the peak indicates the existence of Al}Si alloy in the"lm.

Surface mapping of constituent element analysisfor Al was carried out by l-AES analysis. Themapping indicated the existence of Al precipitateson very small areas of the "lm surface. l-AESspectra also revealed a small amount of Si at thesame areas. The estimated composition of Si fromthe spectra is 10}20 at%, which is near the

52 T. Yamada et al. / Journal of Crystal Growth 209 (2000) 50}54

Fig. 5. Depth pro"le of Al measured by SIMS.

Fig. 6. Relationship between hole concentration of "lms anddoped B concentration in Si targets.

Fig. 7. Schematic diagram of polycrystalline Si "lm depositedfrom Al}Si melt.

composition of the eutectic point in the phase dia-gram (11 at% of Si). Therefore, these Al-rich pre-cipitates are the Al}Si alloys observed in X-raydi!raction. The dark parts with bright rims inFig. 2a correspond to holes, where Al}Si alloyprecipitates were removed by etching. In otherareas, no Al peak over the l-AES detection limitwas observed. To investigate the inner part of the"lm, cross-sectional mapping by l-AES analysiswas also carried out. The mapping was similar tothat on the "lm surface; that is, only small Al}Siareas were observed on the "lm cross-section. Thus,Al}Si alloy precipitates exist not only on the sur-face of the "lm but also in the "lm.

Fig. 5 shows the depth pro"le of the Al compon-ent measured by SIMS. The "gure shows that Alwith the concentration of around 1020 cm~3 wasincorporated into the "lm. The area analyzed bySIMS was 28 lm in diameter. Therefore, the mea-sured Al concentration is the average of Si crystalgrains and Al}Si precipitates. Then, the Al concen-tration in Si crystals must be lower than 1020 cm~3.The hole concentration of the Si "lms was mea-sured by the van der Pauw method, and is shown inFig. 6. The samples were prepared using Si targetswith B doped to three di!erent concentrations. Thehole concentrations were 0.5}2]1019 cm~3 anddid not depend on the B concentrations in thetargets. Thus, Al was the major dopant in theresultant Si "lm. These measured hole concentra-tions correspond to the reported solid solubility ofAl in Si of 1019 cm~3 at 700}7503C [10].

Here, we summarize the results using schematicdiagram of the Si "lm deposited from Al}Si melt inFig. 7. The "lm consists of large Si crystal grainsabove 1 lm in diameter. The grains have (1 1 1)orientation and are closely packed. The averagesize of the Si crystal grains is 5 lm. Si crystal grainsare continuous from the substrate to the "lm sur-face. The hole concentration of polycrystallineSi "lm is 1]1019 cm~3. A small number of Al}Sialloy precipitates exists sporadically on the surfaceof the "lm and at the boundary of Si crystal grainsin the "lm. The Al}Si precipitates are what remainof the initial Al}Si layer, a part of which was in-corporated into the Si crystals as a dopant.

The Si-"lm deposition process was also investi-gated. Fig. 8 is an optical microscope photographof a 1-lm-thick Si polycrystalline "lm obtained bystopping in the course of the ordinary deposition.Area (a) of Fig. 8 consists of white, round materialsand large faceted grains. A comparison to thel-AES measurements of 10-lm-thick Si "lmsrevealed that the white, round materials are Al}Si

T. Yamada et al. / Journal of Crystal Growth 209 (2000) 50}54 53

Fig. 8. Optical microscope picture of 1-lm-thick Si poly-cry-stalline "lm obtained by stopping in the course of the ordinarydeposition.

alloys and the large faceted grains are Si crystalgrains that precipitated from the Al}Si melt. On theother hand, there are only small Si crystal grains inarea (b); neither large Si crystal grains nor Al}Sialloys can be seen. It was mentioned above thatthe heated Al layer became discontinuous and Aldroplets formed at initial stage of deposition. Thesolution growth mainly proceeds at large Al}Sidroplets, because the supplied Si easily melts intothe Al}Si droplets, migrates, and recrystallizes. Asa result, large crystal grains grow in area (a). On theother hand, solution growth stops at the early stageat the small and thin Al}Si droplets in area (b),which results in the formation of micro-Si grains.However, recall that the resultant 10-lm-thick Si"lm consist of large Si crystal grains, as shown inFig. 2a. Comparing Figs. 2a and 8, one can see thatlarge Al}Si alloy droplets resolve neighboringmicro-Si grains and the area of the droplets spreadsas deposition proceeds. A "lm with grains of similarsize may be "nally obtained, after Al}Si dropletscombine with other droplets.

4. Summary

Poly-crystalline Si "lm with large grains has beendeposited on a glass plate at a relatively low tem-perature of 650}7503C using Al melt as the growthsolvent. Si was supplied by magnetron sputter. AlN

was chosen as a wetting agent for the Al melt, andwetting of Al was improved by three-times betterthan that on a glass substrate. There was no erosionof an AlN-bu!ered glass substrate. All of the grainsin the "lms deposited at 650}7503C were greaterthan 1 lm in diameter and microcrystalline oramorphous areas were not observed. Obtained "lmwas strongly Si(1 1 1) oriented. A l-AES analysisrevealed the presence of a small number of Al}Siprecipitates on the "lm surface and in the "lm. Holeconcentration of 0.5}2]1019 cm~3 was obtained,and corresponds to the solid solubility of Al in Si.Poly-crystalline Si "lm obtained by our method isuseful for the back surface "eld layer and/or seedlayer for thin polycrystalline Si solar cells.

Acknowledgements

The authors wish to thank K. Takeya, I.Yamada, and H. Iwamura for their encouragementand also thank T. Ohara for his assistance in SEMmeasurements.

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54 T. Yamada et al. / Journal of Crystal Growth 209 (2000) 50}54