An examination of the surface decomposition chemistry of lithium niobate precursors under high...

qThis work was supported by the NSF Materials ResearchScience & Engineering Center at the University of Wisconsin-Madison, USA.

*Corresponding author. Tel.: #1-608-265-3644; fax: #1-608-262-6143.

E-mail address: [email protected] (D. Saulys).

Journal of Crystal Growth 217 (2000) 287}301

An examination of the surface decomposition chemistry oflithium niobate precursors under high vacuum conditionsq

D. Saulys!,*, V. Joshkin", M. Khoudiakov!, T.F. Kuech#, A.B. Ellis!,S.R. Oktyabrsky$, L. McCaughan%

!Department of Chemistry, 1101 University Avenue, Madison, WI 53706, USA"Materials Research Science & Engineering Center, 1101 University Avenue, Madison, WI 53706, USA

#Department of Chemical Engineering, 1101 University Avenue, Madison, WI 53706, USA$NYS Center for Advanced Thin Film Technology, Albany, NY, USA

%Department of Electrical & Computer Engineering, 1101 University Avenue, Madison, WI 53706, USA

Received 12 June 1999; accepted 6 March 2000Communicated by C.R. Abernathy

Abstract

We employ a combination of high vacuum (chemical beam) epitaxy and in situ mass spectrometry to examineprecursor surface decomposition chemistry during "lm growth of LiNbO

3and its constituent metal oxides on sapphire

and Si (0 0 1). The four most common CVD precursors for LiNbO3* [Li(OBu5)]

6, Li(tmhd), [Nb(OEt)

5]2, and

Nb(tmhd)4* are examined (OBu5"tertiary butoxide, OEt"ethoxide, tmhd"2,2,6,6-tetramethyl-3,5-heptanedion-

ate). The metal diketonates are unstable at low pressures and temperatures, adversely impacting both storageand use. The metal alkoxides, while of greater thermodynamic stability, are subject to autocatalytic processes thatcan inhibit the "lm growth: these processes generate volatile metal-containing moieties that subsequently desorbfrom the surface. The degree of crystallinity of niobium oxide "lms grown from [Nb(OEt)

5]2

depends on the presenceof one of the autocatalytic elements, water. The growth rate of lithium niobate "lms depends strongly on theLi/Nb precursor ratio, suggesting a chemical interaction between the two precursors. ( 2000 Published by ElsevierScience B.V. All rights reserved.

PACS: 68.55.Bd; 77.55; 81.15

Keywords: Lithium niobate; Chemical beam epitaxy; Precursor chemistry; Thin "lm

1. Introduction

ABO3

ferroelectrics can be used to performa rich variety of nonlinear optical functions, includ-ing optical switching and modulation [1,2], fre-quency conversion [3], and, more recently, asa host for rare earth optical ampli"cation and

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 ( 0 0 ) 0 0 4 1 2 - 7

lasing media [4,5]. While some commercial successhas been achieved with bulk LiNbO

3, e.g., traveling

wave electro}optic modulators and frequencydoubled light generators, most experimentallydemonstrated devices lack su$cient performance,stability, and reproducibility to warrant their in-corporation into the communication or sensortechnologies. The principal impediments in over-coming these limitations are insu$cient materialpurity (e.g., Fe2` in LiNbO

3) and a limited set of

surface modi"cation tools (e.g., thermal di!usion)to control the material composition and dopantdistribution in bulk materials. Ferroelectric thin"lms grown by chemical-based deposition tech-niques can overcome these limitations by providingprecise control of material composition and dopantdistribution, and by taking advantage of poten-tially large di!erential volatilities between precur-sors and contaminants. Hybrid integration of thesethin "lms can also introduce new functionality (e.g.,non-volatile RAM memory elements, scan probetransducer arrays).

LiNbO3

thin "lms have been grown by chemicalvapor deposition (CVD) in several laboratories[6}8]. Despite the almost complete reliance onalkoxide and diketonate precursors for this growth,remarkably little is known about the thermal de-composition chemistry of these species as it relatesto "lm growth [9}12]. Likewise, accurate data onthe physical properties relevant to "lm growth, e.g.,vapor pressure vs. temperature, are also lacking. Asa practical consequence of this, "lm thicknessesattained to date (&0.1}0.5lm) [6}8] are insu$-cient for many applications, including low lossguided wave integrated optic devices and beam-optic frequency doublers, despite simple #ux argu-ments that suggest that growth rates of&10 lm/h(see below) should be achievable at pressures as lowas 5]10~5Torr.

Growth at high and ultra-high vacuum, i.e.,chemical beam epitaxy (CBE), o!ers certain ad-vantages over standard CVD. In addition to theabsence of gas-phase reactions, the lower operatingpressure allows the use of in situ diagnostics, and inour case enhances the sensitivity of our principalprobe, mass spectrometry. The absence of a carriergas may also permit higher precursor delivery#uxes. We have, therefore, chosen to investigate the

surface deposition and growth chemistry of the fourmost common LiNbO

3precursors* [Li(OBu5)]

6,

Li(tmhd), [Nb(OEt)5]2, and Nb(tmhd)

4* under

high vacuum conditions (OBu5"tertiary butoxide,OEt"ethoxide, tmhd"2,2,6,6-tetramethyl-3,5-heptanedionate). The precursor decompositionmodes identi"ed here are consistent with the "lmgrowth behaviors observed: low absolute growthrates, as well as sharp dependence of growth rate ontemperature and precursor interaction.

The decomposition and/or hydrolysis chemistryof metal alkoxides plays an important role not onlyin the formation of diverse ferroelectrics, e.g., Ba-TiO

3[13] and lead zirconate titanate [13], but also

in that of other technologically important metaloxide systems, e.g., high-temperature superconduc-tors [14,15]. Thus, the results presented here mayprove to be of general applicability.

2. Experimental procedure

The reactor is depicted schematically in Fig. 1.Li- and Nb-tmhd precursors, as well as[Li(OBut5)]

6and Nb

2(OEt)

5, were obtained from

Strem Co. To minimize hydrolysis upon exposureto air, the precursor vessels were "tted with high-vacuum metal valves and were charged under anatmosphere of dry N

2inside a dry box. Precursor

pressure is controlled by way of capillary tubes (L1

and L2

in Fig. 1) connecting the individually heated(¹

1,2"303}2003C) and valved precursor vessels to

a common, heated (¹0"2003C), delivery beam

line. The conductance ratios of the capillary : beamline : turbo pump throat are &1 : 10 : 100 (measuredwith respect to N

2). Capacitance manometers

(labelled CM, Fig. 1) were used to monitor theprecursor pressure on the high-pressure side of thecapillary tubes, and a hot "lament ionization gaugemonitored the pressure in the reactor vessel. Vac-uum was maintained by a turbomolecular pump(500 l/s). The walls of the growth chamber werecooled with 203C circulating water. The base pres-sure of the system was less than 10~8 Torr. Duringgrowth, typical precursor source pressures were2]10~3Torr and reactor vessel pressures, P

3, were

in the range of 5]10~6}5]10~5Torr. Individualmetal oxides were grown from their respective

288 D. Saulys et al. / Journal of Crystal Growth 217 (2000) 287}301

Fig. 1. Schematic diagram of the CBE reactor. QMS"quadrupole mass spectrometer; CM"capacitance manometers; CC"coldcathode gauge; IG"ionization gauge. Precursor #uxes are determined by the precursor vessel temperatures, ¹

1and ¹

2, and the

capillary conductances, ¸1

and ¸2.

precursors alone, or in the presence of either O2

(reactor vessel partial pressure &2.0]10~5Torr),H

2O (&1.5]10~5Torr), or MeOH (&1.5]10~5

Torr).Silicon (0 0 1) substrates were heated directly by

a tantalum "lament. Basal plane sapphire (0 0 0 1)substrates were mounted on a molybdenum holderusing a gallium}indium eutectic. The "lament wasplaced equidistant from the sample holder anda thermocouple connected to a tantalum plate.Temperature calibration was made using pyro-meter measurements ('5003C) and the meltingpoint of indium (1573C) on the sample surface.Before deposition, the substrates were annealed at9003C for 15 min in vacuum to remove surfacecontaminants, then cooled to the deposition tem-perature. Growth rates were determined from steppro"les using a tantalum blade mask placed abovethe substrate. Decomposition products were deter-mined using a quadrupole mass spectrometer(QMS) with resolution &1.5 amu at larger m/eratios. Films were removed from the reactor forfurther characterization. Auger electron spectra(AES) were obtained on a PHI-660 instrument.Transmission electron microscopy (TEM) was per-

formed on a JEOL-2010 FEG instrument. X-raydi!raction (XRD) data was obtained on a SiemensDXRD.3110 double-crystal di!ractometer withCuK

aradiation.

3. Results

3.1. Diketonate precursors

Two diketonate precursors for LiNbO3

thin-"lmgrowth, Li-2,2,6,6-tetramethyl-3,5-heptanedionate(tmhd) and Nb(tmhd)

4were examined individually

under high-vacuum conditions to determine theirstability and identify decomposition products un-der typical CBE conditions. When introduced intothe growth chamber at room temperature and pres-sures of 0.1 Torr and below, both diketonates emit-ted a continual #ow of volatile organic species, asmonitored by mass spectrometry. In a separateexperiment, which was conducted to isolate andcharacterize organic decomposition products,Li(tmhd) was heated to 2153C under high vacuumin a glass reactor and the decomposition productswere fractionated. Highly reactive intermediates

D. Saulys et al. / Journal of Crystal Growth 217 (2000) 287}301 289

Fig. 2. Measured vapor pressure vs. precursor vessel temperature for: (a) [Li(OBu5)]6; and (b) [Nb(OEt)

5]2. Data consists of a series of

measurements (TjhTvL); data markers are representative of the error range in the measurements. Dashed lines delineate the range inthe "tted curves given by equations A and B in the text for (a) and (b), respectively.

such as ketenes and carbanions were identi"ed[16]. Because of this instability under typicalsource delivery conditions, Li(tmhd) andNb(tmhd)

4were not considered further as CBE

precursors for LiNbO3.

3.2. Vapor pressure of alkoxide precursors

Prior to this study, vapor pressure data in therange 25}1003C for the hexamer [Li(OBu5)]

6were

unavailable in the literature, and the published datafor the dimer [Nb(OEt)

5]2

are contradictory[17,18]. Vapor pressure was therefore measured asa function of precursor vessel temperature(25}1303C) and "tted (see Fig. 2) to the followingexpressions for [Li(OBu5)]

6and [Nb(OEt)

5]2, re-

spectively:

log10

P(Torr)"(9.6$1.1)!(4250$350)/Tl(K)

(A)

and

log10

P(Torr)"(9.8$1.2)!(4350$350)/Tl(K).

(B)

3.3. Lithium oxide xlm growth

Before attempting to grow LiNbO3"lms, the

growth parameters of single-metal oxides werestudied. For the geometry shown in Fig. 1 (capillaryconductance &1.1 l/s; delivery tube length/dia-meter ratio"10), at a [Li(OBu5)]

6source pressure

PL*"2]10~3Torr, we calculate that &13% of

the 1.2]1016 mol/s cm2 total #ux is intercepted bythe 1 cm2 substrate. Assuming 100% conversione$ciency, this precursor #ux corresponds toa Li

2O "lm growth rate of &10lm/h. We note

that this #ux corresponds to a beam equivalentpressure of 1.5]10~4Torr, which is large com-pared to the reactor vessel pressure (&10~5Torr)during growth.

Fig. 3a depicts the growth rate vs. substrate tem-perature of Li

2O-containing "lms on silicon (0 0 1)

using [Li(OBu5)]6

with a source pressure ofPL*"2]10~3Torr. The two prominent features of

these results are low overall growth rates anda sharp dependence of the growth rate with temper-ature. The maximum growth rate of &200 nm/hcorresponds to a precursor conversion e$ciency of&2% of the calculated precursor #ux. As we showbelow, the low overall growth rate can be explainedin terms of the surface chemistry of lithium t-butox-ide. Also as seen in Fig. 3, the growth rate of boththe lithium oxide and niobium oxide "lms are se-verely attenuated by the presence of H

2O and

MeOH. (No growth was observed for niobium ox-ide deposition in the presence of MeOH.)

Quadrupole mass spectra (QMS) were takenduring deposition and the intensities of some of thefragments are plotted as a function of substratetemperature in Figs. 4 and 5. While the assignmentsof ions due to heavier metal-containing fragmentsare generally unambiguous (Fig. 4), spectra of mix-tures of lower molecular weight organic species(Fig. 5a) are more di$cult to analyze since many of


Fig. 3. Film growth rates versus substrate temperature using: (a)[Li(O-Bu5)]

6alone (e), in the presence of H

2O (j), and with

MeOH (O); and (b) [Nb(OEt)5]2

alone (e), in the presence ofH

2O (j). No observable growth rate was detected for

[Nb(OEt)5]2

in the presence of MeOH.

Fig. 4. Quadrupole mass spectrometry data for some of themetal-containing deposition products of [Li(O-Bu5)]

6as a func-

tion of substrate temperature.

Fig. 5. (a) Quadrupole mass spectra for some of the organicdecomposition products of [Li(OBu5)]

6as a function of substra-

te temperature. Note that the m/e 18 data has been magni"ed20X. (b) Ratios of peak intensities of selected components. (Seetext for details.)

the peaks contain contributions from several di!er-ent compounds. In the latter situation, considera-tion of lower intensity peaks, peak intensity ratios,and comparable data from the mass spectral litera-ture, allow deconvolution of the mass spectruminto component peaks.

Turning to the organic decomposition productsof lithium t-butoxide, Fig. 5a, the most intensepeaks of the mass spectra of isobutylene (m/e"41,39 and 56) are also found in the spectra of t-BuOH(literature mass spectral data are from Refs.[19,20]). However, peaks at m/e 59 and m/e 31 arefound only in the spectrum of t-BuOH, while thepeak at m/e 15, although of low intensity in the

spectrum of isobutylene, is negligible in that of thealcohol [19,20]. Thus the data in Fig. 5a are consis-tent with t-BuOH and isobutylene concentrationsincreasing with temperature.

A peak at m/e 41 is prominent in the QMS ofboth isobutylene and t-BuOH. As m/e 41 is the basepeak (i.e., the most intense peak) in the mass spec-trum of isobutylene, the intensity ratios of m/e59}41 should decrease, while that of m/e 41}31should increase, with a rise in the isobutylene con-centration with temperature. This is just the behav-ior observed in Fig. 5a. As a check of the peakintensity ratio method, the ratios of m/e 39}41 peakintensities are also plotted in Fig. 5b. Since the m/e


Fig. 6. Quadrupole mass spectrometry data for some of themetal-containing deposition products of [Nb(OEt)

5]2

as a func-tion of substrate temperature.

Fig. 7. Quadrupole mass spectra for some of the organic de-composition products of [Nb(OEt)

5]2

as a function of substratetemperature.

39}41 ratio is about the same for each species[19,20] this proportion should remain approxim-ately constant with temperature, as is observed.

X-ray di!raction scans of "lms grown from[Li(OBu5)]

6within the temperature range from

5003C to 7003C, with and without &1.5]10~5Torr H

2O, were all found to be amorphous.

These "lms were characterized by AES (Section3.6).

3.4. Niobium oxide xlm growth

Under the deposition conditions employed here(source pressure P

N""2]10~3Torr), "lm growth

from [Nb(OEt)5]2

is also temperature dependentand slow, with a maximum growth rate of&75nm/h at &5503C (Fig. 3b). The equivalentdeposition rate, &2]1013moles/s cm2, is only&2]10~3 of the calculated incident #ux rate.QMS data of the volatile by-products from "lmsgrown from [Nb(OEt)

5]2

are presented in Figs.6 and 7. Two types of intensity vs. temperaturebehavior are displayed by the metal-containingfragments (Fig. 6). The peak intensities of the "vefragments with m/e from 125 to 189 begin to de-crease at &2503C, and continue to do so for theentire temperature range observed. Based on anearlier mass spectral study [21], these peaks havebeen assigned (see Fig. 6). Conversely, the intensityof the peak at m/e"109, nominally [NbO]`, be-gins to increase at&2003C, and appears to reacha plateau at&7003C.

The QMS data due to organic decompositionproducts of [Nb(OEt)

5]2

are displayed in Fig. 7.Organic species were monitored using the most

intense mass spectrometer peaks [19,20]. Ethanol(m/e"31, 45 and 27), acetaldehyde (m/e"29),ethylene (m/e"28 and 27), and water (m/e"18,molecular ion) were the major decompositionproducts, their amounts increasing with temper-ature over most of the range measured.

The deposition from [Nb(OEt)5]2

was accom-panied by the continuous generation of H

2, as

monitored by mass spectrometry; the variation inintensity of this #ow with temperature was notmeasured. We note that the decomposition of[Li(OBu5)]

6did not produce signi"cant H

2.

Niobium oxide "lms were characterized by AES(Section 3.6), TEM, and XRD. No signi"cant pre-cipitation is observed in the TEM micrograph ofNb

2O

5grown from [Nb(OEt)

5]2

on Si (0 0 1)shown in Fig. 8, consistent with the absence ofgross levels of impurities. The selective area di!rac-tion pattern (inset) con"rms the presence of single-phase Nb

2O

5. Niobium oxide "lms grown at

di!erent temperatures, with and without water, werealso characterized by XRD (Fig. 9). The crystal-linity of niobium oxide "lms increases with growthtemperature and the addition of water at lowertemperatures. Films were grown at temperaturescorresponding to the maximum "lm growth rate,&5003C (Fig. 3b), and at&8003C. The presence of&1.5]10~5Torr H

2O, while decreasing the


Fig. 8. Representative plan view TEM image of a Nb2O

5"lm

grown by CBE from [Nb(OEt)5]2

on Si(0 0 1). Inset: selectivearea di!raction pattern.

Fig. 9. X-ray di!raction scans of niobium oxide "lms grown at&5003C and at &8003C on Si(0 0 1) using [Nb(OEt)

5]2. Dif-

fraction peaks at 2h&233 and 28.53 are consistent with anNb

2O

5phase [44]. Lower two scans were taken of "lms grown

with 1.5]10~5Torr water.

Fig. 10. Measured growth rate of lithium niobate "lms asa function of substrate temperature. Growth conditions: (O) 1 : 1[Li(OBu5)]

6: [Nb(OEt)

5]2; (j) 1 : 1 [Li(OBu5)]

6: [Nb(OEt)

5]2

with &2]10~5Torr O2; (m) 2.3 : 1.0 Li(OBu5)]

6: [Nb(OEt)

5]2

with &2]10~5Torr O2.

growth rate (Fig. 3b), appears to improve the cry-stallinity of the oxide: at 5003C, the "lm is amorph-ous when grown without H

2O, but shows

polycrystallinity in the presence of H2O. Films

grown at higher temperatures,&8003C, are poly-crystalline regardless of the presence or absence ofwater; however, water does enhance crystallinity, aswas observed at 5003C.

3.5. Lithium niobate xlm growth

The growth rate of lithium niobate "lms from[Li(OBu5)]

6and [Nb(OEt)

5]2

was measured asa function of substrate temperature for two precur-sor source pressures, P

L*"P

N""2]10~3Torr

and PL*"2.3, P

N""4.6]10~3Torr, with and

without O2

(&2]10~5 Torr partial pressure asmeasured in the reactor vessel). As seen in Fig. 10addition of O

2increases the growth rate of the

bimetallic oxide approximately two-fold. Withhigher Li/Nb precursor ratio, the growth rate isagain highly temperature dependent (cf. Figs. 3aand b), increasing to lower temperatures.

XRD indicates that while lithium niobate "lmsgrown on Si (0 0 1) were polycrystalline, thosegrown on sapphire (0 0 0 1) were epitaxial (Fig. 11).


Fig. 11. XRD data for LiNbO3"lms grown by CBE from

[Li(OBu5)]6

and [Nb(OEt)5]2

on: (a) Si(0 0 1), incident X-raybeam angle"153; and (b) Al

2O(00 0 1), incident X-ray beam

angle"123.

Fig. 12. AES spectra of (a) Nb2O

5; (b) LiNbO

3; and (c) Li

2O

"lms grown by CBE on Si substrates. Transitions: (j) oxygenKLL; (v) niobium MNN; (m)"niobium NVV; (O) silicon LVVtransitions; (T) lithium KLL; (.) carbon KLL.

The lithium niobate "lms grown on Si were alsocharacterized by AES (Section 3.6).

3.6. AES characterization

The chemical composition of metal oxide "lmswas determined by ex situ AES. The di!erential-mode low-energy AES spectra of the single metaloxides and lithium niobate, recorded after argonetching to remove surface contaminants, are shownin Fig. 12. The most prominent peaks are those dueto oxygen, the relevant metals, and silicon. Theintensity of the Si LVV peaks depends strongly on

the etching time and falls to zero at shallow etching,indicating that these peaks originate at the substra-te}"lm interface. The spectra indicate that the "lmsdo not contain signi"cant amounts of impurities,with the exception of the lithium oxide "lm, whichcontains measurable amounts of carbon even afterlong ion etch times.

4. Discussion

4.1. Lithium oxide

Mass spectral studies indicate that lithium t-butoxide exists as a hexamer that is highly stable inthe gas phase up to at least 2003C [22,23]. Whilea high-quality solid state structure of lithium tert-butoxide is not available, it seems reasonable that itis analogous to related lithium alkoxide species ofknown structure [24]: a hexagonal prismatic core


Fig. 13. Possible gas-phase structure of [Li(OBu5)]6.

of alternating oxygen and lithium atoms, sur-rounded by a bulky organic periphery of tertiarybutyl groups, as shown in Fig. 13.

In terms of chemisorption on either the initialsubstrate or the subsequent lithium oxide-contain-ing surface, the organic periphery would be ex-pected to be relatively inert, and chemisorption of[Li(OBu5)]

6would require dissociation of the hexa-

mer to expose oxygen or Li atoms. Given the largevolume of the tertiary butyl group (indicated bya van der Waals radius parallel to the group axis of0.228nm [25]) relative to the ionic radii of four-coordinate Li` and eight co-ordinate O2~ (0.059and 0.142 nm, respectively, [26]) the site-blockingcapability of the organic group may also play a rolein the low growth rates. The temperature at whichappreciable "lm growth from [Li(OBu5)]

6com-

mences,&3503C (Fig. 3a), coincides with the sharpdrop in the QMS peak intensities due toLi

3(OBu5)`

2, m/e"167, and Li

2(OBu5)`, m/e"87

(Fig. 4). As m/e"87 is the base peak in the gas-

phase mass spectrum of lithium tert-butoxide [22],this behavior is indicative of the hexamer's dis-sociation and subsequent adsorption on the sub-strate surface. Thus the low-temperature portion ofFig. 3a is consistent with growth limited by dis-sociation of the lithium alkoxide hexamer. Sim-ilarly, the maximum in growth rate occurs in thesame temperature regime where the QMS peakintensities of the lithium butoxide hexamer frag-ments are at a minimum,&5003C (Fig. 4).

Lithium alkoxides, including [Li(OBu5)]6, have

been studied in the gas phase by means of time-of-#ight and magnetic sector mass spectrometry[22,23]. Ions of the formula [Li

/(OR)

/~1]` are

found to comprise the majority of the ion current,along with much weaker peaks corresponding toloss of CH

3and CH

4. While we also observe

intense peaks corresponding to loss of neutralLi(OBu5) fragments from [Li

4(OBu5)

3]` and

[Li3(OBu5)

2]`, the data in Fig. 4 also exhibit

a series of peaks corresponding to successive elim-ination of MeLi (*m/e"22), a fragmentation be-havior not observed in the gas-phase mass spectra[22,23]. Thus the series m/e"167, 145, 123, andm/e"87, 65 are consistent with the formation ofspecies containing the carbonylic ligand O"CMe

2by loss of methyl lithium (e.g., [Li

2(OBu5)

(O"CMe2)]` at m/e"145). This interpretation is

supported by QMS data indicating the presence ofacetone (O"CMe

2) among the volatile decompo-

sition products (not shown). That we do not ob-serve surface-generated MeLi directly by massspectrometry is consistent with it being a highlyreactive surface-generated monomer, resulting ina relatively small capture cross section by the spec-trometer, as discussed below for LiOH.

The decrease in the "lm growth rate at temper-atures above &6003C (Fig. 3a) is expected to co-incide with an increase in intensity of QMS peaksdue to Li-containing fragments. While no such sig-ni"cant increase is observed (Fig. 4), it may be thatthis increase is masked by dynamic range e!ects,i.e., competition for ion current with the more high-ly volatile organic species at higher temperatures,and/or by limitations inherent in observing the veryreactive organic Li monomers by QMS (see below).

The formation of the volatile organic decomposi-tion products (Fig. 5a) can be explained in terms of


1 It should be noted that the use of mass spectrometry as ourprimary spectroscopic probe allows us to infer, but not conclus-ively identify the speci"c surface species involved in the reactionsdescribed. For example, reaction with water might certainlyinvolve the elements of water, i.e., surface-bound protons andhydroxyl groups, as opposed to molecular water. As neither thenet stoichiometry nor our ultimate conclusions would be a!ec-ted by the intermediacy of particular surface species, we gener-ally discuss reaction sequences in terms of the known molecularchemistry of the metal alkoxides.

Scheme 1. Model for the coupled hydrolysis and dehydration oflithium t-butoxide.

the known chemistry of metal alkoxides, allowingthe formulation of several parallel decompositionpathways for lithium t-butoxide on a hot surface:hydrolysis, alcoholysis, and, as discussed above, theelimination of MeLi. For example, tertiary metalalkoxides are known to eliminate the parent alco-hol and an alkene upon sublimation [27]. TheQMS data displayed in Fig. 5a indicate the forma-tion of analogous products, t-BuOH (m/e 31, 41, 59)and isobutylene (m/e 15, 41). The intensities ofQMS peaks associated with t-BuOH and isobuty-lene (Fig. 5a) begin to increase sharply at&350}4003C, temperatures which mark the onsetof "lm growth in Fig. 3a.

The most salient feature of the chemistry of metalalkoxides is their high susceptibility to hydrolysis[28]. This susceptibility increases with the elec-tropositivity of the metal, and is extreme for GroupI metals. Hydrolysis of a Li(OBu5) monomer wouldyield tert-butanol and lithium hydroxide1 .

(1)

Furthermore, the heterogeneous dehydration of t-BuOH on metal oxides is facile at temperatures of3003C and above [29], producing isobutylene asthe organic product

(2)

The proclivity of metal alkoxides to hydrolyze,along with the extremely low QMS intensity of thewater peak (m/e 18) in Fig. 5a (note the 20X scalemagni"cation for water) lead us to conclude thatthe water generated in equimolar amounts in theabove reaction is consumed by lithium t-butoxideto generate LiOH, a key Li-containing surface spe-cies, along with equimolar amounts of tert-butylalcohol (reaction (1)). The t-BuOH dehydrates inturn (reaction (2)), the entire reaction sequence con-stituting an autocatalytic cycle, as shown in Scheme1. (In the sense that water is both a reagent anda product in the surface decomposition of[Li(OBu5)]

6, we describe this cycle as being

autocatalytic.) The high exothermicity of reaction1 generates LiOH with a large kinetic energy(&0.87 eV) [30], contributing to its volatility. Asa result, reactions otherwise capable of generatinglithium oxide "lms, e.g.

2LiOHPLi2O#H

2O (3)

are limited by low surface concentrations of LiOH.Thus, the volatility of LiOH monomers can ac-count for the dramatic reduction in growth rateabove 5003C (Fig. 3a).

We note that it is not surprising that we do notobserve LiOH in our mass spectra of decomposi-tion products, since the combination of high reac-tivity of an LiOH monomer and the smallacceptance angle of the spectrometer make the re-action with the chamber walls much more probablethan spectrometric detection. (As a "rst-order esti-mate, assuming an absorption coe$cient of 1 withthe reactor walls and a mass spectrometer witha solid acceptance angle of &2.5]10~3 sr, theprobability of mass spectrometric detection in oursystem is only&2]10~4.) This is consistent withthe observation that LiOH spectra have been


reported only as a result of special experimentalprocedures, e.g., by volatilizing LiOH directly in-side or adjacent to the ionization chamber of themass spectrometer at temperatures over 9003C[31]. Indeed, the inability to obtain mass spectra ofLiOH even on the introduction of neat LiOH intoa conventional high-resolution instrument has beenexplicitly noted [23].

Consistent with our postulate that reaction withwater severely attenuates lithium oxide "lm growthare the data from "lms grown from [Li(OBu5)]

6in the presence of water (Fig. 3a). Likewise, intro-duction of [Li(OBu5)]

6into the growth chamber

was always accompanied by a sharp decreasein the background intensity of the QMS waterpeak.

As the autocatalytic hydrolysis of Li(OBu5) re-quires the stoichiometric consumption of t-BuOHand production of isobutylene, the continual in-crease with temperature of both of the QMS peaksdue to these species (Fig. 5a) implies the following:(1) incomplete dehydration of surface-generated t-BuOH under the conditions employed here (2) theexistence of additional reaction channels for thesurface decomposition of Li(OBu5) that generatesfree t-BuOH, and volatile Li-containing species,and/or (3) spurious sources of t-butanol, not asso-ciated with the surface decomposition of Li(OBu5).The "rst condition operating by itself is notsu$cient to explain our results, since the loss oft-BuOH will take with it the essential species neces-sary for the autocatalytic reaction, water (seescheme), precluding the formation of LiOH. Con-tinual removal of t-BuOH from the surface in lieuof dehydration requires other pathways, e.g.,the scavenging by a butoxy group of a protonfrom (1) incoming Li(OBu5) #ux; (2) its organicdecomposition products; or (3) `backgroundagases, with concomitant formation of volatileLi-containing by-products. Despite precautionstaken to minimize exposure to air and water,precursor delivery vessels containing both[Li(OBu5)]

6and [Nb(OEt)

5]2

generated signi"-cant amounts of the respective parent alcoholsupon standing. Similarly, use of [Nb(OEt)

5]2

demonstrated a memory e!ect in the growth cham-ber: a number of species of varying volatility,ethanol among them, exhibited measurable mass

spectra for several days after an experimental run.(Ethanol was found to be a major decompositionproduct of [Nb(OEt)

5]2

under CBE conditions; seebelow.)

In reactions mechanistically similar to hydroly-sis, metal alkoxides are known to react with freealcohol to generate new or mixed alcoholates whileeliminating the original alkoxy ligand as free alco-hol [32]. The alcoholysis of Li(OBu5) by ethanolresidual from [Nb(OEt)

5]2

experiments wouldform lithium ethoxide and t-butanol

(4)

Films grown from [Li(OBu5)]6

in the presence ofexcess methanol exhibited severely attenuated "lmgrowth (Fig. 3a), similar to the case with addedwater, due, we believe, to the formation and sub-sequent evaporation of the highly reactive LiORmonomer. As in the case of LiOH, the detection byQMS of the lithium-containing alcoholysis productmonomers, Li(OR), R"Me or Et, would be se-verely hampered by the high reactivity of thesemonomers and the small acceptance angle of themass spectrometer.

4.2. Niobium oxide

Nuclear magnetic resonance [33], mass spectral[20] and infra-red [34] studies indicate that atroom temperature, pure liquid or gaseous niobiumpentaethoxide exists predominantly as a six-coor-dinate dimer: each Nb atom is bound to four ter-minal OEt groups, and bridged by two additionalethoxy ligands through their oxygen atoms (seereaction (5), below). As in the case of [Li(OBu5)]

6,

one expects that bonding with the substrate wouldbe facilitated by exposure of the metal and/orO atoms of the precursor to the surface. Initialadsorption might thus be expected to requiredissociation into a species that o!ered a free nio-bium coordination site, e.g., a penta-coordinate


monomer, viz.,

(5)

Gas-phase mass spectra of [Nb(OEt)5]2

do indi-cate an increasing proportion of monomer relativeto dimer with increasing temperature [21]. Morespeci"cally, for the structurally similar speciesTa

2Cl

10, Raman spectra indicate a gradual dis-

sociation to monomeric TaCl5

between 2203C and3503C [35].

Although mass spectrometer instrumental lim-itations do not allow us to distinguish between therelative proportions of dimer and monomer speciespresent in our system, the decrease in intensity ofQMS peaks (m/e"125}189) due to Nb(OEt)

5fragments with increasing temperature (Fig. 6)correlates with the temperature dependence of"lm growth rates plotted in Fig. 3b: both thedecline in QMS peak intensities and the onsetof measurable "lm growth occur at approxim-ately 2003C. Thus, it appears that the "lm growthin the low temperature region is limited by thedissociation of the pentaethoxide dimer. The onsetof appreciable "lm growth also correlates withthe increasing intensity of a peak at m/e 109,nominally NbO` (Fig. 6). This is consistentwith either niobium monoxide itself, or a largermoiety of which NbO is a decomposition product,as the primary surface species participating in thegrowth of niobium oxide "lms. Alternatively,a chemically plausible intermediate (see below),Nb(OEt)

2(OH)

2, m/2e"108.5 amu, must be

considered, given the low resolution of the massspectrometer.

Models for the decomposition of "lm precursorsare generally deduced from the mass spectra ofvolatile decomposition products and the chemicalidentity of deposited materials. When available,vibrational spectroscopy of surface species can pro-vide additional insight. While the decomposition of

Nb(OEt)5

has not been previously studied, chemic-ally relevant species, i.e., niobium ethoxide surfacemoieties, have been generated and characterized inthe course of catalysis studies. Speci"cally, Iwasawaet al. [36] have attached Nb(V) oxide catalysts toSiO

2surfaces by the reaction of Nb(g3-C

3H

5)4

with silica, followed by high-temperature treatmentwith H

2and O

2. When dosed with ethanol, inter-

mediates of the typeMSiO2NNb("O)(OH)(OEt) were

identi"ed by IR spectroscopy (braces denote sur-face species). Two modes of reactivity were ob-served for the adsorbed ethanol: (1) at lowtemperatures (150}2503C) and in the presence ofethanol vapor, dehydrogenation to acetaldehydeand dihydrogen took place; and (2) under highvacuum at higher temperatures (greater than3273C), in the absence of excess ethanol, dehydra-tion yielded ethylene and water. Under CBE condi-tions [Nb(OEt)

5]2

yields the same volatile organicproducts as observed by Iwasawa: acetaldehyde,water, ethylene, and dihydrogen. In addition, how-ever, large amounts of ethanol are also observed(Fig. 7).

The coincidence of volatile organic decomposi-tion products under both catalytic and CBE condi-tions suggests that chemically similar intermediatesmay be involved in both systems. In particular, itlends credence to the identi"cation of the volatileNb-containing decomposition product withm/e"&109 (Fig. 6) as the doubly-charged ion[Nb(OEt)

2(OH)

2]``, m/2e"108.5. (Doubly

charged ions of high valent early transition metalcomplexes have been previously observed [37].)The formation and facile desorption ofNb(OEt)

2(OH)

2could account for the decreasing

"lm growth rates at higher temperatures. That thisspecies is also a likely hydrolysis product ofNb(OEt)

/or its fragments is likewise consistent

with the attenuated growth rate observed underwet conditions (Fig. 3b).

As demonstrated by Iwasawa [36], the surfacedecomposition of niobium pentaethoxide can gen-erate water both directly, as a by-product in theproduction of ethylene, or indirectly, from the de-hydration of liberated ethanol. This water can thenreact in turn with extant Nb-(OEt) linkages inwell-known hydrolysis reactions generating Nb"Oand/or Nb(OH) linkages (M"transition metal)


2Condensation reactions of this general type have been usedto describe the formation of heterometallic alkoxides fromLiOH and metal alkoxides via sol}gel techniques.

[38,39]

M(OR)/#H

2OPM(OR)

/~1(OH)#ROH, (6)

M(OR)/#H

2OPM("O)(OR)

/~2#2ROH. (7)

Thus, as was the case for lithium t-butoxide, theniobium ethoxide system is auto-catalytic in thesense that water is both a reactant and a product inthe surface decomposition chemistry. As the actionof water is detrimental to "lm growth in the case ofboth the lithium and niobium alkoxides, this chem-ical characteristic is of prime importance. Alcoholsplay a similar, detrimental role. Given the highervalence of transition metals, however, the hydroly-sis chemistry of the niobium alkoxide is inherentlymore complex than that of lithium t-butoxide.

4.3. X-ray diwraction examinations of lithium oxideand niobium oxide

Films of lithium oxide grown from [Li(OBu5)]6

under various conditions of temperature andmoisture were shown to be amorphous by XRD.Di!raction patterns from niobium pentoxide "lmson Si (0 0 1) are shown in Fig. 9. The crystallinity ofthese "lms improves with both temperature and thepartial pressure of water. Not unexpectedly, thequality of the "lms is a function of surface mobility,which is dictated by temperature, as well as chem-ical factors. Regarding the latter point, hydrolysisimpacts not only the rate of "lm growth (by meansof the autocatalytic cycle discussed above) but alsothe quality of the niobium oxide "lms.

4.4. Films grown from [¸i(OBut)]6 and[Nb(OEt)5]2

The growth rate of lithium niobate "lms grownfrom [Li(OBu5)]

6and [Nb(OEt)

5]2

is highly de-pendent on temperature, precursor ratio, and thepresence of O

2(Fig. 10). The sensitivity to precur-

sor ratio may re#ect a competition between pre-cursor interaction and desorption of precursorfragments. Interaction between the Li and Nbprecursors would be consistent with the metalalkoxide reactivity patterns discussed above(scheme, reactions (6) and (7)). That is, higher con-

centrations of the lithium precursor would favorthe occurrence of a reaction such as

LiOH#MSNNb(OR)3(OEt)PMSNNb(OR)

3(OLi)

#EtOH, R"Et or H, (8)

a plausible "rst step in the formation of a lithiumniobate precursor [40].2 As previously discussed,both [Li(OBu5)]

6and [Nb(OEt)

5]2

are highly sus-ceptible to hydrolysis and alcoholysis. In "lmsgrown from both precursors (reaction (8)), the re-active hydroxide is presumably LiOH, which reactsin a manner analogous to water or alcohol. In thiscontext, reaction (8) is a speci"c case of a generalsensitivity to hydroxyl-bearing species. (The gen-eration of ethanol in reaction (8) is likewise consis-tent with the low absolute growth (&320nm/h) rateof LiNbO

3.)

The lithium niobate "lms grown on Si (0 0 1)werepolycrystalline, as seen by X-ray di!raction. Onsapphire (0 0 0 1), growth at 6500C using the 2.3 : 1Li/Nb precursor ratio produced epitaxial LiNbO

3"lms (Fig. 11). TEM and XRD results indicate thatthis substrate-dependent di!erence in "lm quality isat least partially due to substrate}"lm interdi!u-sion processes [41].

5. Conclusions

Despite their heavy use in metal oxide CVD,little is known about the decomposition behaviorof metal alkoxides and diketonates under condi-tions relevant to "lm growth. While the less thanideal performance of metal diketonates and alkox-ides as CVD precursors in general [42,43], andniobium pentaethoxide, in particular [45], has beenfrequently noted, the connection between surfacedecomposition modes and low growth rates hasnot been previously made. We have studied thedecomposition chemistry of the most frequentlyused precursors to Li, Nb and Li/Nb oxide thin"lms with the ultimate goal of enhancing LiNbO

3


epitaxy, and identi"ed surface decomposition mod-els that are consistent with the low growth ratesfrom these precursors.

Both Li(tmhd) and Nb(tmhd)4

decompose undervacuum at room temperature. Lithium tertiary-butoxide and [Nb(OEt)

5]2, though more thermally

stable than the diketonates, generate the parentalcohols upon prolonged heating under vacuum.

The surface decomposition modes of lithium t-butoxide include elimination of MeLi, hydrolysis,and, under our conditions, alcoholysis. Niobiumpentaethoxide decomposes by dehydrogenation,dehydration, hydrolysis, ethanolysis, and forma-tion of ethanol. Film growth rates at low temper-atures appear to be limited by dissociation of thealkoxide precursor oligomers. The decrease ingrowth rates observed at higher temperatures islinked to the formation of volatile metal hydroxidemonomers: LiOH in the case of [Li(OBu5)]

6, and

possibly Nb(OEt)2(OH)

2in the case of

[Nb(OEt)5]2. Water plays a crucial role, as both

reactant and product, in the decomposition cycle ofboth alkoxides: precursor hydrolysis precludes highgrowth rates of metal oxides under the CBE condi-tions employed here. Alcoholysis reactions playa similar, detrimental role.

Thus, high growth rate CBE of lithium or nio-bium oxide "lms from lithium or niobium alkox-ides requires rigorous exclusion (or expeditiousremoval from the surface) of H

2O. Alternatively,

chemical modi"cation of precursors to reduce hy-drolytic sensitivity could circumvent this restric-tion. The presence of chelating organic ligands, e.g.,diketonates, aminoalcohols, etc., renders metal pre-cursors less susceptible to hydrolysis [46]. Whilethe most common metal diketonate precursors areunstable under CBE conditions, mixed-ligandmetal complexes, i.e., those containing both chelat-ing and alkoxide ligands, may exhibit an intermedi-ate thermal and hydrolytic stability. Preparation ofsuch hybrid species is currently underway in ourlaboratories.

Another approach to increased growth rates of"lms deposited from the metal alkoxide precursorsstudied here is to block the type of back reactions(depicted in the Scheme) that generate volatile spe-cies that desorb from the surface. This may beachieved by engineering surface reactions to re-

move the small molecules (e.g., H2O and EtOH)

that participate in such reactions. This may bepossible by the co-deposition of chemical reagentsselected for their reactivity to particular species(e.g., organosilanes to produce ethers from ethanolor ethoxides).

While sensitivity to reaction with hydroxyl func-tionalities appears to play a key role in the lowgrowth rates of oxide "lms from individual alkox-ide precursors, this same reactivity may be respon-sible for the condensation reaction leading toheterometallic Li/Nb species in "lms grown fromboth precursors. Thus, it is not only the thermaland chemical stability of the individual precursorsunder development that is of interest, the natureand degree of their chemical interaction must alsobe probed.

While we have elucidated surface chemical pro-cesses consistent with the low growth rates of monoand bimetallic oxide "lms from Li and Nb alkox-ides, other factors, such as the molecular structureof the growing "lm surface and substrate}precursorinteractions (di!usion) contribute heavily to "lmquality [41].

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16.


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