In Situ Reaction Mechanism Studies on Atomic Layer Deposition of Sb

Dec 31, 2009 - separated by purging with carrier gas. The growth me- chanism is ... usually also have excellent uniformity and conformality. Tradition...
0 downloads 0 Views 814KB Size
Download PDF
1386 Chem. Mater. 2010, 22, 1386–1391 DOI:10.1021/cm902180d

In Situ Reaction Mechanism Studies on Atomic Layer Deposition of Sb2Te3 and GeTe from (Et3Si)2Te and Chlorides Kjell Knapas,* Timo Hatanp€a€a, Mikko Ritala, and Markku Leskel€a Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 University of Helsinki, Finland Received July 17, 2009. Revised Manuscript Received November 20, 2009

Reaction mechanisms in the atomic layer deposition (ALD) of Sb2Te3 from SbCl3 and (Et3Si)2Te at 60 °C and GeTe from GeCl2 3 C4H8O2 (1,4-dioxane complex of GeCl2) and (Et3Si)2Te at 90 °C were studied in situ with a quadrupole mass spectrometer (QMS) and a quartz crystal microbalance (QCM). Also some experiments were conducted on reactions in ALD of the phase change material GST (germanium antimony telluride, Ge2Sb2Te5). The byproduct in both the binary telluride processes was found to be Et3SiCl, and about 78% (36%) of it was released during the SbCl3 (GeCl2 3 C4H8O2) pulse. Obviously -Te(SiEt3) surface groups serve as reactive sites for the metal precursors, cf. -OH surface groups in the oxide ALD processes that use water as the oxygen source. The dioxane, on the other hand, was expectedly found to be released entirely during the GeCl2 3 C4H8O2 pulse. When depositing GST the mechanism of the SbCl3-(Et3Si)2Te reaction was found to change so that only about 50% of the byproduct Et3SiCl was released during the SbCl3 pulse. The same effect was experienced when the SbCl3-(Et3Si)2Te process was executed on surfaces of Al2O3 and Au. Introduction Atomic layer deposition (ALD), previously known as atomic layer epitaxy (ALE), is an advanced chemical gas phase method for depositing thin films,1-3 where the precursors are pulsed alternately into the reactor and separated by purging with carrier gas. The growth mechanism is self-limiting because the precursors can adsorb on the surface as monomolecular layers at most. Therefore one can easily control the thickness of the films, that usually also have excellent uniformity and conformality. Traditionally ALD processes are studied with growth experiments, i.e. growing films and examining them afterward ex situ. When doing so one scarcely obtains information on the reactions taking place during the growth. Knowing those reactions is crucial for controlling the processes and developing new ones. A powerful means of getting knowledge of the reactions is studying the processes in situ with e.g. a quadrupole mass spectrometer (QMS) and a quartz crystal microbalance (QCM).4 The QMS indicates the composition of the gas phase of the reactor and the QCM the mass development of the film. When it comes to ALD of chalcogenides (chalcogen, Ch=nonmetallic group 16 element), studies of oxides and sulfides are myriad, but those of selenides and tellurides infrequent.2 This is partly because of the differences in *To whom correspondence should be addressed. E-mail: Kjell.Knapas@ helsinki.fi.

(1) Suntola, T.; Antson, J. U.S. Pat. 4058430, 1977. (2) Ritala, M.; Leskel€a, M. In Handbook of Thin Film Materials; Nalwa, H. S., Ed.; Academic Press: San Diego, 2001, Vol. 1, Chapter 1. (3) Leskel€ a, M.; Ritala, M. Angew. Chem., Int. Ed. 2003, 42, 5548. (4) Knapas, K.; Ritala, M. Chem. Mater. 2008, 20, 5698 and references therein.

pubs.acs.org/cm

toxicity of the dihydrogen chalcogenides that are otherwise nearly ideal ALD precursors being simple and both highly volatile and thermally stable and in addition offering nice ligand exchange reactions with many metal compounds, cf. reaction 1, where M is a metal ion with charge þn and L a ligand with charge -1. H2O and even the quite toxic H2S can be handled following normal laboratory procedures. On the other hand, H2Se and H2Te are much more toxic and require extensive safety measures. 2MLn ðgÞ þ nH2 ChðgÞ f M2 Chn ðsÞ þ 2nHLðgÞ

ð1Þ

CdSe,5 ZnSe,5-13 ZnTe,11-17 and CdTe16-20 may be deposited with elements as precursors for both ions (reaction 2). In this elemental source ALD process the metal becomes oxidized and the chalcogen reduced and there are no byproducts. A growth rate of exactly one monolayer per cycle is often accomplished. A thermodynamic model for the deposition process has been established with reflected high energy electron diffraction (5) Faschinger, W.; Juza, P.; Ferreira, S.; Zajicek, H.; Pesek, A.; Sitter, H.; Lischka, K. Thin Solid Films 1993, 225, 270. (6) Yao, T.; Takeda, T.; Watanuki, R. Appl. Phys. Lett. 1986, 48, 1615. (7) Takemura, Y.; Konagai, M.; Yamasaki, K.; Lee, C. H.; Takahashi, K. J. Electron. Mater. 1993, 22, 437. (8) Konagai, M.; Ohtake, Y.; Okamoto, T. Mater. Res. Soc. Symp. Proc. 1996, 426, 153. (9) Szczerbakow, A.; Dynowska, E.; Swiatek, K.; Godlewski, M. J. Cryst. Growth 1999, 207, 148. (10) Godlewski, M.; Guziewicz, E.; Kopalko, K.; Lusakowska, E.; Dynowska, E.; Godlewski, M.; Goldys, E.; Philips, M. J. Lumin. 2003, 102-103, 455. (11) Yao, T.; Takeda, T. Appl. Phys. Lett. 1986, 48, 160. (12) Dosho, S.; Takemura, Y.; Konagai, M.; Takahashi, K. J. Appl. Phys. 1989, 66, 2597. (13) Konagai, M.; Takemura, Y.; Yamasaki, K.; Takahashi, K. Thin Solid Films 1993, 225, 256.

Published on Web 12/31/2009

r 2009 American Chemical Society

Article

Chem. Mater., Vol. 22, No. 4, 2010

(RHEED) and Monte Carlo simulation.20 The lower temperature limit of the so-called ALD window is determined by the incompleteness of re-evaporation of weakly bound atoms. The higher temperature limit, on the other hand, is determined by the onset of evaporation or evaporative decomposition of the compound being deposited. MðgÞ þ ChðgÞ f MChðsÞ ð2Þ The same chalcogenides may also be deposited from chalcogen compounds with mechanisms that go through metallic elements. The Zn-H2Se process21 could proceed according to reactions 3a and 3b, where the Zn atoms adsorbed on the surface reduce the incoming H2Se molecules to H2. Something similar could happen in the ZnEt2Se process,22 where butane, ethene, and hydrogen are possible byproducts. Also in the Et2Zn-Et2Se2,23,24 Me2Zn-R2Te,25 Me2Cd-R2Te,25,26 and Me2Hg-R2Te26 processes where alkyl compounds are used as precursors for both ions, the metal precursor apparently pyrolyzes forming the corresponding element on the surface, which then reacts with the chalcogen precursor in a suchlike manner, reactions 4a and 4b, where the notation {R2(g)} means either R2(g) or the β-elimination alkene and H2. Also the chalcogen precursor could pyrolyze (reaction 5). Of course also the ligand exchange reaction 6 would be theoretically possible, but it does not seem to proceed since high temperatures are needed for deposition to occur. The pathway 4a-b was confirmed for the Me2CdR2Te process with separate experiments that verified that Me2Cd pyrolyzed but R2Te did not.25 ZnðgÞ f ZnðadsÞ

ð3aÞ

ZnðadsÞ þ H2 SeðgÞ f ZnSeðsÞ þ H2 ðgÞ

ð3bÞ

R2 MðgÞ f MðadsÞ þ fR2 ðgÞg

ð4aÞ

MðadsÞ þ R2 ChðgÞ f MChðsÞ þ fR2 ðgÞg

ð4bÞ

R2 ChðgÞ f ChðadsÞ þ fR2 ðgÞg

ð5Þ

R2 MðgÞ þ R02 ChðgÞ f MChðsÞ þ 2RR0 ðgÞ

ð6Þ

(14) Ahonen, M.; Pessa, M.; Suntola, T. Thin Solid Films 1980, 65, 301. (15) Takemura, Y.; Nakanishi, H.; Konagai, M.; Takahashi, K. Jpn. J. Appl. Phys. 1991, 30, L246. (16) Hauzenberger, F.; Fashinger, W.; Juza, P.; Pesek, A.; Lischka, K.; Sitter, H. Thin Solid Films 1993, 225, 265. (17) Sadowski, J.; Herman, M. Appl. Surf. Sci. 1997, 112, 148. (18) Pessa, M.; Huttunen, P.; Herman, M. A. J. Appl. Phys. 1983, 54, 6047. (19) Kyt€ okivi, A.; Koskinen, Y.; Rautiainen, A.; Skarp, J. Mater. Res. Soc. Symp. Proc. 1991, 222, 269. (20) Sitter, H.; Fashinger, W. Thin Solid Films 1993, 225, 250. (21) Koukitu, A.; Saegusa, A.; Kitho, M.; Ikeda, H.; Seki, H. Jpn. J. Appl. Phys. 1990, 29, L2165. (22) Kimura, R.; Konagai, M.; Takahashi, K. J. Cryst. Growth 1992, 116, 283. (23) Fujiwara, H.; Nabeta, T.; Shimizu, I. Jpn. J. Appl. Phys. 1994, 33, 2474. (24) Fujiwara, H.; Kiryu, H.; Shimizu, I. J. Appl. Phys. 1995, 77, 3927. (25) Wang, W.-S.; Ehsani, H.; Bhat, I. J. Electron. Mater. 1993, 22, 873. (26) Karam, N.; Wolfson, R.; Bhat, I.; Ehsani, H.; Gandhi, S. Thin Solid Films 1993, 225, 261.

1387

Besides processes that either use elements as precursors or have elements as intermediates, i.e. processes that constitute redox reactions, a few demonstrations of ligand exchange reactions using H2Se have been reported. These include the ZnCl2-H2Se,27,28 Me2Zn-H2Se,29-33 and Me2CdH2Se32,33 processes. Interestingly enough, in situ studies using surface photointerference showed that the latter two processes proceed with opposite mechanisms when it comes to the exchange reaction point, reactions 7a and 7b and 8a and 8b.32,33 In the case of ZnSe, the exchange reaction takes place during the Me2Zn pulse, while in the case of CdSe it takes place during the H2Se pulse. H2 SeðadsÞ þ Me2 ZnðgÞ f ZnSeðsÞ þ 2MeHðgÞ

ð7aÞ

H2 SeðgÞ f H2 SeðadsÞ

ð7bÞ

Me2 CdðgÞ f Me2 CdðadsÞ

ð8aÞ

Me2 CdðadsÞ þ H2 SeðgÞ f CdSeðsÞ þ 2MeHðgÞ

ð8bÞ

Replacing the extremely toxic H2Se and H2Te with less dangerous exchange reactions precursors and thus opening the door for practical ALD of a greater selection of selenides and tellurides has turned out to be no easy task. As already pointed out, alkyl compounds apparently do not work efficiently. Only recently our group managed to tackle this problem with alkylsilyl chalcogenides, (R3Si)2Ch, and successfully performed ALD of Sb2Te3, GeTe, ZnTe, Bi2Te3, ZnSe, Bi2Se3, In2Se3, CuSe, and Cu2Se.34 Especially ALD of the key material for phase change random access memories (PCRAM), GST (germanium antimony telluride, Ge2Sb2Te5), was thoroughly investigated.35 Earlier only plasma assisted ALD process for GST had been studied.36-39 The GST films deposited with the new alkylsilyl tellurium precursor at 90 °C were highly conformal and reasonably pure containing 2.4 at.% oxygen and less than 1 at.% hydrogen, carbon and chlorine.35 The silicon content was less than 2 at.%, probably even below 1 at.%. The ALD GST films (27) Lee, C.; Kim, B.; Kim, J.; Park, H.; Chung, C.; Chang, S.; Lee, J.; Noh, S. J. Cryst. Growth 1994, 138, 136. (28) Lee, C.; Kim, B.; Kim, J.; Chang, S.; Suh, S. J. Appl. Phys. 1994, 76, 928. (29) Hsu, C. Jpn. J. Appl. Phys. 1996, 35, 4476. (30) Hsu, C. Thin Solid Films 1998, 335, 284. (31) Yokoyama, M.; Chen, N.; Ueng, H. J. Cryst. Growth 2000, 212, 97. (32) Yoshikawa, A.; Kobayashi, M.; Tokita, S. Appl. Surf. Sci. 1994, 82/ 83, 316. (33) Yoshikawa, A.; Kobayashi, M.; Tokita, S. Phys. Status Solidi B 1995, 187, 315. (34) Pore, V.; Hatanp€a€a, T.; Ritala, M.; Leskel€a, M. J. Am. Chem. Soc. 2009, 131, 3478. (35) Ritala, M.; Pore, V.; Hatanp€a€a, T; Heikkil€a, M.; Leskel€a, M.; Mizohata, K.; Schrott, A.; Raoux, S.; Rossnagel, S. Microelectron. Eng. 2009, 86, 1946. (36) Lee, J.; Choi, S.; Lee, C.; Kang, Y.; Kim, D. Appl. Surf. Sci. 2007, 253, 3969. (37) Choi, B.; Choi, S.; Shin, Y.; Kim, K.; Hwang, C.; Kim, Y.; Son, Y.; Hong, S. Chem. Mater. 2007, 19, 4387. (38) Choi, B.; Oh, S.; Choi, S.; Eom, T.; Shin, Y.; Kim, K.; Yi, K.; Hwang, C.; Kim, Y.; Park, H.; Baek, T.; Hong, S. J. Electrochem. Soc. 2009, 156, H59.

1388

Chem. Mater., Vol. 22, No. 4, 2010

also exhibited similar phase change properties as sputtered GST films.35 In the paper at hand we present in situ reaction mechanism studies with QMS and QCM of the SbCl3-(Et3Si)2Te and GeCl2 3 C4H8O2-(Et3Si)2Te ALD processes and show that they proceed through clean exchange reactions with no decomposition. Experimental Section The in situ experiments were carried out in a specially modified40 commercial flow-type ALD reactor manufactured by ASM Microchemistry Ltd. The pressure of the reactor was about 3 mbar and the total area of the soda lime glass substrates about 3500 cm2. Nitrogen (Oy AGA Ab, 99.999%) was used as a carrier gas. The gas phase of the reactor was examined with a Hiden HAL/3F 501 RC QMS using a Faraday detector and an ionization energy of 70 eV. The pressure reduction to about 1 3 10-5 mbar in the QMS chamber was accomplished by differential pumping through a 100 μm orifice. The mass development of the film was monitored with a Maxtek TM 400 QCM with a sampling rate of 20 Hz. (Et3Si)2Te was synthesized in good yields by reacting Li2Te with Et3SiCl following literature methods34,41 and held inside the reactor in an open boat at 40 °C. SbCl3 (Sigma-Aldrich, 99þ%) and GeCl2 3 C4H8O2 (germanium(II)chloride 1,4-dioxane complex (1:1), Aldrich) were held similarly at 30 and 65 °C, respectively. Standard procedures were followed to ensure that the air and moisture sensitive precursors were inserted into the reactor without ruining. The precursors were pulsed with inert gas valving.2 All tellurium(-II) waste was allowed to oxidize in air giving elemental tellurium (appears as black deposits on the glassware) which was then oxidized further with concentrated sulfuric acid yielding fairly safe and soluble H2TeO3. The studied processes were SbCl3-(Et3Si)2Te at 60 °C and GeCl2 3 C4H8O2-(Et3Si)2Te at 90 °C. Typical process cycles were 15 s SbCl3 þ 15 s purge þ 35 s (Et3Si)2Te þ 35 s purge and 20 s GeCl2 3 C4H8O2 þ 20 s purge þ 35 s (Et3Si)2Te þ 35 s purge. The reasons and explanations for the extraordinarily long times required are given in the discussion. In addition to the binary processes a few experiments to make GexSbyTez were conducted at 90 °C. GexSbyTez was deposited with the process cycle 15 s SbCl3 þ 15 s purge þ 35 s (Et3Si)2Te þ 35 s purge þ 20 s GeCl2 3 C4H8O2 þ 20 s purge þ 35 s (Et3Si)2Te þ 35 s purge.

Results and Discussion SbCl3-(Et3Si)2Te Process. In the earlier film growth experiments reasonably high purity Sb2Te3 films were obtained with the SbCl3-(Et3Si)2Te process already at 60 °C. The low growth temperature was actually necessary because the growth rate was observed to decrease rapidly with increasing temperature.34 The most simple net reaction for the process would be the exchange reaction 9 forming Et3SiCl as byproduct. It corresponds formally to reaction 1 with the group Et3Si- instead of H-, (39) Choi, B.; Choi, S.; Eom, T.; Ryu, S.; Cho, D.; Heo, J.; Kim, H.; Hwang, C.; Kim, Y.; Hong, S. Chem. Mater. 2009, 21, 2386. (40) Rahtu, A.; Ritala, M. Electrochem. Soc. Proc. 2000, 2000-13, 105. (41) Detty, M.; Seider, M. J. Org. Chem. 1982, 47, 1354.

Knapas et al.

Te as Ch and M=Sb3þ, L=Cl- and thus n=3. 2SbCl3 ðgÞ þ 3ðEt3 SiÞ2 TeðgÞ f Sb2 Te3 ðsÞ þ 6Et3 SiClðgÞ ð9Þ Et3SiCl has its molecular peak at m/z = 150. Possible fragments include Et2SiClþ (m/z = 121), EtSiClþ (m/z = 92), SiClþ (m/z = 63), and Et3Siþ (m/z = 115). All the mentioned m/z values including the molecular peak were detected during the ALD process and found to develop similarly and to depict exchange reaction byproducts, i.e. to be released in greater amounts during the ALD process than when the same precursor was pulsed repeatedly. These signals were weak though, the strongest of them being m/z=121. An even stronger signal was found at m/z=93 developing also similarly as the previous ones. This m/z value corresponds to the ion HEtSiClþ. It was specially verified with m/z=95 that one Cl and with m/z= 94 that one Si are included in this ion: these m/z developed similarly to m/z = 93 and constituted about one-third (isotope ratio 37Cl:35Cl=1:3) and one-twentieth (isotope ratio 29Si:28Si=1:20) of it, respectively. For comparison Et3SiCl was loaded also directly into the reactor and the relevant m/z signals were quickly measured at room temperature (this substance evaporated completely in ten minutes). Hereby it could be verified that the signals at m/z=150, 92, 63, and 115 are indeed relatively weak for this compound, while m/z=121 and m/z = 93 are stronger. However, different from the ALD process, m/z=121 was here for some reason about as intense as m/z=93. In light of the above experimental evidence, the conclusion that the byproduct of the SbCl3-(Et3Si)2Te process is Et3SiCl, appears justified. In principle HEt2SiCl, H2EtSiCl..., and butene and/or ethene could form as byproducts too. The molecular peak of butene (m/z = 56) was barely detected, and this small amount may also be formed in the ionizer. What comes to ethene, there is no way to tell whether reaction 10 took place on the substrates or in the ionizer. Therefore nothing points especially to alkenes as byproducts, though the possibility that Et3SiCl does not leave the surface completely intact cannot be excluded either. In the further discussion Et3SiCl is considered as the byproduct anyhow. Et3 SiClðgÞ f HEt2 SiClðgÞ þ C2 H4 ðgÞ

ð10Þ

Because the most intense signal was at m/z = 93, this was chosen for the quantitative measurements. QMS data for the process are shown in Figure 1. As already has been pointed out, this m/z value 93 constitutes HEtSiClþ, which describes the exchange reaction byproduct that is considered to be Et3SiCl. During the SbCl3 pulse in the ALD process a tall and narrow peak is observed. During the measurement it was clearly seen that the peak rises to its full height immediately when the SbCl3 flow is opened. Then the signal returns to its original level in about 5 s, though the SbCl3 flows for 15 s. When SbCl3 is pulsed repeatedly, no peak rises, so there is no background of

Article

Figure 1. QMS results (m/z = 93, HEtSiClþ, fragment of Et3SiCl) of the SbCl3-(Et3Si)2Te ALD process. Arrows in the figure are pointing to the beginnings of the corresponding pulses.

m/z=93 during SbCl3 pulse. Therefore Et3SiCl is formed in great amounts as an exchange reaction byproduct and released fast during the SbCl3 pulse in the ALD process. This is to say, that the exchange reactions during the SbCl3 pulse are quite fast indeed in spite of the low reaction temperature of 60 °C. During the (Et3Si)2Te pulse in the ALD process two peaks are seen (Figure 1). The first peak rises to its full height immediately when the (Et3Si)2Te flow is opened, and then the signal descends rather quickly to rise slowly again after a while to form a second broad and flat peak. When the flow of (Et3Si)2Te is closed the signal descends slowly. Also when (Et3Si)2Te is pulsed repeatedly ((Et3Si)2Te background measurement), a broad and flat peak rises, but about 5 s after the flow has been opened. The first sharp peak is missing from this background signal. The background is probably caused by chlorine residues in the filament of the QMS which react with (Et3Si)2Te molecules to give the HEtSiClþ fragment. Therefore the integrated intensity of the first peak was taken as representative for the Et3SiCl released during the (Et3Si)2Te pulse in the ALD prosess. The two peaks are completely explained, and the described integration procedure justified as follows: When the flow of (Et3Si)2Te is opened, it is consumed in the exchange reactions, and Et3SiCl is released as a reaction byproduct. Et3SiCl causes the intensity of m/z = 93 to rise, and no background interference is present. When the exchange reactions get completed, the intensity drops as the release of the byproduct diminishes. At the same time an increasing amount of (Et3Si)2Te goes through the reactor in unreacted form. When the unreacted (Et3Si)2Te finally meets the filament, it reacts with chlorine residues and causes the intensity to rise again as background interference. However, as shall later be seen with QCM, physisorption of (Et3Si)2Te occurs on the surface after the exchange reactions are completed. Therefore the background interference does not rise especially quickly after the exchange reactions come to an end. This is why the intensity visits almost the zero level between the two peaks, and therefore the first peak truly constitutes an excellent approximation for the

Chem. Mater., Vol. 22, No. 4, 2010

1389

Figure 2. QCM results of the SbCl3-(Et3Si)2Te ALD process.

extent of the exchange reactions. Desorption of the physisorbed (Et3Si)2Te molecules explains in turn why the intensity does not drop to zero quickly when the flow of (Et3Si)2Te is closed. QMS results clearly indicate that the byproduct Et3SiCl is released during both precursor pulses. Comparison of the integrated intensities of m/z = 93 during the SbCl3 pulse and in the first peak during the (Et3Si)2Te pulse indicates that (78 ( 3)% (N = 15) of the byproduct Et3SiCl is released during the SbCl3 pulse as compared to a complete ALD cycle. The error limit is the standard deviation and N is the amount of measurements. Figure 2 shows QCM data for the same process. When the SbCl3 flow is opened, the signal descends immediately a bit but ascends then quickly to obtain a plateau. When the flow is closed, the signal descends to a level lower than the starting point. Therefore m1, the mass change during the SbCl3 pulse and purge, is negative. When the flow of (Et3Si)2Te is opened, the signal ascends first rapidly and largely when (Et3Si)2Te reaches the QCM and then continues to increase with a lower rate. Then again the signal descends during the purge, but m0, the mass change during a complete ALD cycle, is quite large. As a result QCM gives m1/m0=-0.36 ( 0.11 (N=10). In addition, it was separately verified that no netto mass increase is observed with the QCM when either of the precursors is pulsed repeatedly. Therefore no decomposition of the precursors occurs. An immediate observation from the QCM pattern is the decendence of the signal during both the purge periods. It was verified that m0 decreases when the purge times are increased. Therefore Sb and Te containing species respectively, i.e. physisorbed precursor molecules, desorb from the surface during the purge periods. Another possibility would be slow desorption of the exchange reaction byproduct Et3SiCl, but this is excluded by QMS. On the other hand, formation of ligand dimers (Cl2, (Et3Si)2) would not lead to decreasing m0 as a function of purge times. As a conclusion, after the exchange reactions complete, additional precursor molecules physisorb on the surface and desorb during the purge periods. These processes reach equilibria slowly due to the low reaction temperature, and therefore long pulse and purge times are necessary in order to obtain

1390

Chem. Mater., Vol. 22, No. 4, 2010

Knapas et al.

stable QCM signals, which are required for the data to be trustworthy. The quite substantial physisorption observed here is thus largely attributed to the long pulse times used. With shorter pulse times, like those used in growth experiments,34 less physisorption occurs, and shorter purge times are sufficient. Continuing with the analogy explained in context with reaction 9, -Te(SiEt3) groups on the surface of Sb2Te3 serve as reactive sites for metal precursors and as such correspond to the hydroxyl group termination of metal oxide surfaces in oxide ALD.2 Therefore the average ALD half-reactions constituting the net reaction 9 are those depicted in reactions 11a and 11b. n -TeðSiEt3 ÞðsÞ þ 2SbCl3 ðgÞ f -Ten Sb2 Cl6 -n ðsÞ ð11aÞ þ nEt3 SiClðgÞ

Figure 3. QMS results (m/z = 93, HEtSiClþ, fragment of Et3SiCl) of the GeCl2 3 C4H8O2-(Et3Si)2Te ALD process.

-Ten Sb2 Cl6 -n ðsÞ þ 3ðEt3 SiÞ2 TeðgÞ f -Te3 Sb2 ½TeðSiEt3 Þn ðsÞ þ 6 -nEt3 SiClðgÞ

ð11bÞ

Now for n in reactions 11a and 11b, the QMS result gives 4.7 ( 0.2 and the QCM result 4.5 ( 0.5. The former is obtained by noting that 78% of Et3SiCl is released during the SbCl3 pulse as compared to a complete ALD cycle and that in reactions 11a and 11b there are six Et3SiCl molecules being released in total, thus n=6 3 0.78. The latter is obtained with molecular masses according to m0 = M(Sb2Te3) = 626.4 and m1 = 2M(SbCl3)-nM(Et3SiCl))=456.3 - 150.72n, which give m1/m0=(456.3150.72n)/626.4 and further n = (456.3-626.4 m1/m0)/ 150.72. Since the results obtained with QMS and QCM are in good agreement, the mechanism 11a-b with n=4.6, reactions 12a and 12b, may be considered experimentally well grounded. 4:6 -TeðSiEt3 ÞðsÞ þ 2SbCl3 ðgÞ f -Te4:6 Sb2 Cl1:4 ðsÞ þ 4:6Et3 SiClðgÞ

ð12aÞ

-Te4:6 Sb2 Cl1:4 ðsÞ þ 3ðEt3 SiÞ2 TeðgÞ f -Te3 Sb2 ½TeðSiEt3 Þ4:6 ðsÞ þ 1:4Et3 SiClðgÞ

ð12bÞ

In reaction 12a -SiEt3 groups of the surface -Te(SiEt3) groups bind to chlorine atoms of the approaching SbCl3 molecules, whereby Et3SiCl is formed and released into the gas phase. The antimony atoms and the remaining chlorine atoms are left on the surface, and there are less than one chlorine atom per each added antimony atom. In reaction 12b (Et3Si)2Te molecules release the remaining chlorine atoms as Et3SiCl and form -Te(SiEt3) groups bonded to the antimony atoms. To ensure the stoichiometric deposition of Sb2Te3 and the same coverage of -Te(SiEt)3 groups as in the beginning of the cycle, a majority of the (Et3Si)2Te molecules must chemisorb dissosiatively on the Sb2Te3 surface rather than undergo exchange reactions. The GeCl2 3 C4H8O2-(Et3Si)2Te Process. In growth experiments appropriate GeTe films were obtained with the GeCl2 3 C4H8O2-(Et3Si)2Te process at 90 °C.34 Practically

Figure 4. QMS results (m/z = 88, molecular peak of C4H8O2) of the GeCl2 3 C4H8O2-(Et3Si)2Te ALD process.

all the remarks made on the byproduct Et3SiCl in the SbCl3-(Et3Si)2Te process apply for this process, reaction 13, as well. QMS data (m/z=93) are shown in Figure 3. The formation and release of Et3SiCl appears a bit slower though, which also leads to that the two peaks during the (Et3Si)2Te pulse are not fully resolved, though a sharp peak is still seen on a broad background. On the other hand, no background peak is seen during the GeCl2 3 C4H8O2 pulse. The same integration procedure that was used for the SbCl3-(Et3Si)2Te process, gives here that (36 ( 10)% (N = 8) of Et3SiCl is released during the GeCl2 3 C4H8O2 pulse as compared to a complete ALD cycle. The other byproduct, 1,4-dioxane (C4H8O2), is monitored well by its molecular peak at m/z = 88 (Figure 4) and found to be released completely during the GeCl2 3 C4H8O2 pulse. This is quite expected because dioxane is a neutral adduct ligand and as such more weakly bonded than the negative chlorides. GeCl2 3 C4 H8 O2 ðgÞ þ ðEt3 SiÞ2 TeðgÞ f GeTeðsÞ þ 2Et3 SiClðgÞ þ C4 H8 O2 ðgÞ

ð13Þ

QCM data (Figure 5) for the GeCl2 3 C4H8O2-(Et3Si)2Te processes may also be interpreted the same way as for the SbCl3-(Et3Si)2Te process. However, m1 is here positive, and the result is m1/m0 = 0.18 ( 0.18 (N=13). In addition, no mass increase and therefore no decomposition of GeCl2 3 C4H8O2 is observed, when it is pulsed repeatedly.

Article

Chem. Mater., Vol. 22, No. 4, 2010

Figure 5. QCM results of the GeCl2 3 C4H8O2-(Et3Si)2Te ALD process.

The general ALD half-reactions for the GeCl2 3 C4H8O2-(Et3Si)2Te process are given in reactions 14a and 14b. n -TeðSiEt3 ÞðsÞ þ GeCl2 3 C4 H8 O2 ðgÞ f

1391

Figure 6. QMS results (m/z = 93, HEtSiClþ, fragment of Et3SiCl) of the SbCl3-(Et3Si)2Te-GeCl2 3 C4H8O2-(Et3Si)2Te ALD process.

-Ten GeCl2 -n ðsÞ þ nEt3 SiClðgÞ þ C4 H8 O2 ðgÞ ð14aÞ -Ten GeCl2 -n ðsÞ þ ðEt3 SiÞ2 TeðgÞ f -TeGe½TeðSiEt3 Þn ðsÞ þ 2 -nEt3 SiClðgÞ

ð14bÞ

Now, the QMS result gives n=0.7 ( 0.2 (=2 3 0.36) and the QCM result n = 0.7 ( 0.2 (m0 = M(GeTe), m1 = M(GeCl2)-nM(Et3SiCl)). Once again these are in good agreement, so the mechanism 14a and 14b with n=0.7 is well grounded and given singly in reactions 15a and 15b. 0:7 -TeðSiEt3 ÞðsÞ þ GeCl2 3 C4 H8 O2 ðgÞ f -Te0:7 GeCl1:3 ðsÞ þ 0:7Et3 SiClðgÞ þ C4 H8 O2 ðgÞ

the

SbCl3-(Et3Si)2Te-GeCl2 3

ð15aÞ

large enough amount of -Te(SiEt3) groups to provide an n value as high as 4.6 of 6.

ð15bÞ

Conclusion

-Te0:7 GeCl1:3 ðsÞ þ ðEt3 SiÞ2 TeðgÞ f -TeGe½TeðSiEt3 Þ0:7 ðsÞ þ 1:3Et3 SiClðgÞ

Figure 7. QCM results of C4H8O2-(Et3Si)2Te ALD process.

In reaction 15a more than one chlorine atom remains on the surface per one surface germanium atom; in other words some adsorbing GeCl2 molecules keep both their chlorine atoms. On the other hand, all adsorbing (Et3Si)2Te molecules undergo exchange reactions, and some even release both their -SiEt3 groups. Deposition of GST. GST films were earlier grown by mixing the SbCl3-(Et3Si)2Te and GeCl2 3 C4H8O2(Et3Si)2Te processes.34,35 Now, QMS (m/z = 93) and QCM data for deposition of GexSbyTez are shown in Figures 6 and 7, respectively. Replacing every second SbCl3 pulse with a GeCl2 3 C4H8O2 pulse decreases the n of the SbCl3-(Et3Si)2Te process significantly, from 4.6 closer to 3. The mechanism of the GeCl2 3 C4H8O2-(Et3Si)2Te process, on the other hand, did not change significantly. A similar decrease of n was observed with the SbCl3-(Et3Si)2Te process on Al2O3 and Au surfaces as well as on the surface of GeTe present in the GST process. Apparently only a surface of Sb2Te3 can accommodate a

In this paper the chemistry of some recently introduced alkylsilyl telluride based ALD processes was studied. The SbCl3-(Et3Si)2Te and GeCl2 3 C4H8O2-(Et3Si)2Te processes constitute clean exchange reactions to telluride films producing Et3SiCl as byproduct. The mechanisms in the processes are concluded to be the ones depicted in reactions 12a and 12b and 15a and 15b, respectively. They differ quite much in that how large fraction of Et3SiCl is released during the different pulses. This must be due to differences between Sb and Ge. It also is no surprise, when one keeps in mind that even the mechanisms for the Me2Zn-H2Se and Me2Cd-H2Se processes were entirely opposite, reactions 7aand 7b and 8a and 8b. Certainly if these processes which differ only with the Zn-Cd pair, where the elements are much alike, have such different mechanisms, the mechanisms with Sb and Ge may be different as well. Acknowledgment. Mr. Viljami Pore is acknowledged for consultation.