Rare earth β-diketonate and carboxylate metal complexes as precursors for MOCVD of oxide films

Volatile and thermostable complexs of lanthanide acetylacetonates and pivalates were obtained and investigated by different methods. These compounds were used for lanthanide oxide containing film producing and for fabrication of silica optical fibers doped by lanthanide oxide. The properties of these and already known volatile precursors are compared. It is well recognized that rare earth containing films have many applications in microelectronics. Metal Organic Chemical Vapour Deposition (MOCVD) has become one of the most effective methods for producing of thin films. This technique needs volatile and thermostable lanthanide compounds. There are two groups of volatile lanthanide compounds which can be sublimated at relatively low temperature (120-400~~) chelate complexes with organic ligands (with M-0 or/and M-N bonds) and organometallics (with M-C bonds). The organolanthanides are airand moisture-sensitive that is a specific property of M-C bond. These sensitive compounds have been used only for doping semiconductors of rare earth by MOCVD[11. The volatile lanthanide chelate complexes with organic ligands such as 1,3-diketonates have been successfully used to produce lanthanide containing oxide films, f.e. yttrium aluminium garnet and high temperature superconductors[2,31. Lanthanide complexes with organic 0-donor ligands are compounds with mainly ionic bonds and with high value of central rare earth ion coordination number. These properties are attributable to associated structure of lanthanide 6-diketonates. Only bulky ligands are capable Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993353 386 JOURNAL DE PHYSIQUE IV to inhibit association and promote volatility. Bulky 2,2,6,6,tetramethyl-3,5-heptanedione ligand (Hthd, also termed dipivaloylmethane) is the most useful for preparing volatile lanthanide 6-diketonates. The main idea of our work is search for new volatile lanthanide compounds and the comparison of their behavior with that of known volatile compounds in CVD processes. It was found that such accessible and inexpensive lanthanide 6-diketonates as acetylacetonates (acac, also termed penthane2,4-dionate) and pivalates (Piv, also termed trimethylacetates) can be sublimated and used for film producing. 1.Volatile lanthanide acetylacetonates. It is known that lanthanide acetylacetonates Ln(acacI3 are of oligomeric structure and due to this are practically nonvolatile. Usually these compounds are synthesized as hydrates Ln(acacI3 3H20: LnX3 + 3Hacac + 3NH3.aq ----+ L n ( a c a ~ ) ~ 3 H ~ O (H20, pH 7 ) Hydrolysis and decomposition of these compounds occur when heated: L n ( a c a ~ ) ~ * 3 H ~ O -----+ Ln,(acac) (OH)Z t H20 + Hacac ( 1 5 0 2 0 0 ~ ~ ) Y As an example, thermal decomposition of Y(acacI3 3H20 was shown to give Y4(acac)10(OH)2[41. The mixed ligand complex formation have been used for modification acetylacetonate volatility. The volatile and thermostable forms of lanthanide acetylacetonates were prepared by reactions of nonvolatile acetylacetonate hydrates and 0or N-donor neutral ligands (Q): L n ( a c a ~ ) ~ 3 H ~ O + nQ -------+ ~!.n(acac)~-nQ t 3H20 , where Q is o-phenantroline(Phen)[51, HMPAL61, acetylacetonimine (HAcim)[71, n = 1, 2. The adducts Ln(acacI3.nQ were investigated in solid state and in gaseous phase by IR and thermal analysis, mass spectrometry, photoelectron spectroscopy. It was found that addition of the neutral donor ligands Q was accompanied by formation of adducts which did not hydroluse when heated. Three different modes of adducts MAA,*nQ sublimation were demonstrated: The volatility of MAA3.nQ depends on nature of the lanthanide ion and the donor ligand and can be varied. Data of table 1 illustrate these dependencies for typical elements of cerium and yttrium groups of lanthanides. Table 1. Results of mixed ligand complexes LnAA3*nQ sublimation ( O.Oltorr, 220°c, 60 min, 0.2g) The enthalpies of NdAA3, HoAA3, ErAA3 sublimation were calculated from mass spectrometric data (table 2) 181. The values of AHsub for known lanthanide dipivaloylmethanates [91 are given in table 2 in order to compare with the ones for lanthanide acetylacetonates. The comparison shows that AHsub of monomeric lanthanide acetylacetonates are quite close to that of dipivaloylmethanates. However the mass spectrometric data demonstrate that Ref. [51 [ 5 1 151 [ 5 1 161 t 6 1 [ 6 1 [ 6 1 [ 7 1 [ 7 1 [71 t71 Compound NdAA3*Phen GdAA3.Phen ErAA3-Phen YAA3 -Phen NdAA3*HMPA GdAA3 HMPA ErAA3.HMPA YAA3 . HMPA NdAA3*2HAcim GdAA3-2HAcim ErAAg.HAcim YAA3.HAcim Mass loss (mol.% of RE,+5%) 100 100 100 100 40 6 0 9 5 9 0 nonvo 1. nonvo 1. 80 70 Mode of sublimation 3 3 3 3 2 2 2 2 1 1 1 1 388 JOURNAL DE PHYSIQUE IV Lnthd3 decompose in gaseous phase at more high temperature than acetylacetonates. The thermal stability of Ythd3 have been investigated in vapour[lOl. Its stability temperature range extends up to 500-550'~. Lanthanide acetylacetonates decompose in vapour at 400°C. Table 2 The enthalpy of sublimation of lanthanide acetylacetonates[81 and dipivaloylmethanates 191. 2.Volatile lanthanide pivalates. Lanthanide pivalates belong to the class of complexes with carboxylate organic ligands. It is thought to be unexpected result that lanthnide carboxylates are volatile. These lanthanide compounds are ionic and polymeric and as rule they are non volatile [Ill. However pivalates contain bulky alkyl group t-butyl which promotes decreasing of intermolecular forces and increasing of volatility. Besides that lanthanide pivalates were obtained in form of coordination saturated compounds LnPiv3-HPiv (n = 3, 3.5 with coordination number equal 8. The adducts LnPiv3 -nHPiv have been investigated by different methods: IR, TA, x-ray analysis, mass spectrometry and sublimation. A comparison of the IR spectra of LnPiv3.nHPiv with those of HPiv indicates an coordination between the carboxylic C=O and metal ion: band characteristic of the CO H poup of Hpiv (1710cm-'1 shifts to lower frequen-I ce (1700-1690cm in the spectra of LnPiv3.nHPiv. By determining the crystal structure of NdPiv3-3HPiv was showed that it was a dimer. It is shown that two processes take place when heating LnPiv3.nHPiv at low pressure (40 torr, NZ): formation of MPiv3 Temperature range,K 392-408 385-417 365-417 430-491 420-458 410-454 Compound NdAA3 HoAA3 ErAA3 Ndt hd3 Hot hd3 Er t hd3 AH sub,kcal/mol 34.6k1.6 33.2f1.8 31.0f1.8 37.9f1.2 31.4k1.1 31.8k1.2 below 200'~ and sublimation of MPiv3 in temperature range 330400'~. The compounds MPiv3 decompose at 450-550'~ with lanthanide oxide formation. Unlike MPiv3.nHPiv carboxylates MPiv3 are airand moisture-sensitive. Adducts MPiv3.nHPiv have excellent storage characteristics and are used for sublimation study (tabl. 3). Table 3. The results of lanthanide pivalates sublimation ( 0.01 torr, 90 min, 0.2g) The sublimation experiments have shown that the difference in volatilities of MPiv3.nHPiv formed by lanthanides of yttrium and cerium groups has been attributed to differences in sublimation kinetics. The kinetic factors strongly effectlanthanide pivalate sublimation because these compounds have oligomeric structures. Mass spectroscopy data demonstrate their trimeric or dimeric + + + structures in vapour: Ln3Piv5 , Ln2Piv4 , LnPiv2 at 654 K. Partial thermal decomposition (3-5%) of LnPiv3 was found under the conditions of mass spectrometric experiment. For usual sublimation experiments (40 torr, N2) partial thermal decomposition of LnPiv3 appears only in presence of moisture traces in carrier gas. The vapor densities of NdPiv3 and ErPiv3 were determined by flow methods in the temperature range of 330-370'~. The obtained values of density are about 3.10-4mol/ml what is close to ones for lanthanide dipivaloylmethanates but the latters sublimate at Compound LaPiv3 -3.5HPiv PrPiv3*3HPiv NdPiv3 ~3HPiv GdPiv3.3HPiv HoPiv3.3HPiv ErPiv3.2. 5HPiv YbPiv3.2. 5Piv YPiv3 ~2.5HPiv Mass loss, mol. % RE f 2% 9 5 95 95 90 4 5 3 5 3 0 2 0 T, OC