J. MATER. CHEM., 1994, 4(8), 1249-1253 Growth of ZnO by MOCVD using Alkylzinc Alkoxides as Single-source Precursors John Auld," David J. Houlton," Anthony C. Jones,*" Simon A. Rushworth," M. Azad Malik,b Paul O'Brien*b and Gary W. Critchlow" " Epichem Limited, Power Road, Bromborough, Wirral, Merseyside, UK L62 3QF Department of Chemistry, Queen Mary & Westfield College, University of London, Mile End Road, London UK El 4NS " Institute of Surface Science and Technology, University of Loughborough, Loughborough, Leicestershire, UK LE77 3TU Thin films of ZnO have been grown by low-pressure MOCVD using methylzinc isopropoxide, MeZn(OPi), and methylzinc tert-butoxide, MeZn(OBu*), in the absence of an added oxygen source. The films were grown on to glass substrates in the temperature range 250-400 "C with growth rates of between 0.2 and 4.4 pm h-'.Thin films of zinc oxide (ZnO) have a number of important applications in heterojunction solar cells, surface acoustic wave devices, optical waveguides, varistors and gas sensors. ZnO films have been grown by a variety of techniques including, radiofrequency or magnetron ~puttering,'~~ spray pyroly~is,~atomic layer epitaxy' and metal-organic chemical vapour deposition (MOCVD).6-1' Of these techniques, MOCVD has considerable potential due to its capability for large area growth, precise control of doping and film thickness, and superior conformal step coverage. Traditional methods for the growth of ZnO by MOCVD have involved the pyrolysis of diethylzinc (Et2Zn)6>8.10or dimethylzinc (Me,Zn)' in the presence of oxygen and/or H,O.However, a severe premature reaction results in the deposition of particulates upstream from the susceptor. Consequently, alternative, less reactive oxygen sources have been investigated, including C02,9 N20,9,12NO," and oxygen-containing heterocycles such as furan.', Unfortunately, these have generally proved unsatisfactory, leading to only very low ZnO growth rates. However, ZnO has been grown successfully, without sig- nificant prereaction, by the use of Et,Zn14 or the tetrahydro- furan adduct of dimethylzinc, Me,Zn( THF)15in combination with alcohols such as methanol, ethanol and tert-butyl alcohol. It was propo~ed'~,'~ that zinc alkoxides or alcohol adducts are formed as intermediates in the gas phase, although no evidence of the exact nature of these species was presented.It has been established16 that the addition of alcohols (R'OH) to R,Zn compounds, even when the alcohol is present in excess, leads to the formation of alkylzinc alkoxides of the type RZn(OR'), without displacement of the second alkyl group. In this paper we have confirmed that the reaction between Me,Zn(THF) and isopropyl alcohol (Pr'OH) or tert-butyl alcohol (Bu'OH) leads to the formation of methylzinc isopropoxide [MeZn(OPr')] and methylzinc tert-butoxide [MeZn(OBu')], respectively. Although such compounds are tetrameric in the solid state,16 both MeZn(0Pr') and MeZn(0Bu') have proved sufficiently volatile for use as single- source precursors for the growth of ZnO by MOCVD.Experimental General Techniques Inductively coupled plasma emission spectroscopy (ICP-ES) was carried out on a Thermoelectron Plasma 300 spectrometer and proton nuclear magnetic resonance ('H NMR) data were obtained on a Bruker WM250 spectrometer operating at 250 MHz. Auger electron spectroscopy (AES) was carried out on a Varian scanning Auger spectrometer and scanning electron microscopy (SEM) data were obtained on a JEOL JS35CM scanning electron microscope. Film thicknesses were estimated by the time taken to sputter through the layer using Ar+-ion bombardment. Sheet resistance measurements were obtained using a voltmeter-based four-point probe, and the X-ray powder patterns were measured on a Philips X-ray powder diffractometer mod$ PW 1050/25 using Cu-Ka radiation of wavelength 1.5418 A.Precursor Synthesis and Characterization MeZn(0Pr') and MeZn(0Bu') were synthesized by modifi- cation of literature procedure^'^.'^ from the reaction between Me,Zn(THF) adduct (1mol equiv.) and the respective alcohols, isopropyl or tert-butyl alcohol (2 mol equiv.). Residual volatiles were removed in uucuo to leave. in each case, white crystalline solids which could be sublimed in uucuo (0.1 Torr) at temperatures >80 "C. The methylzinc dkoxides were characterized using 'H NMR and ICP-ES. MeZn(0Pr'): 'H NMR (C6D6) 6: 0.21, 3 H, S, CH,Zn [OCH(CH,),]; 1.2, 6 H, d, CH,Zn[OCH(CH,),]; 3.98, 1 H, multiplet, CH,Zn[OCH(CH,),].ICP-ES Zn conlent (YO): found, 46.0; calc., 46.9. MeZn(OBut): 'H NMR (C6D6) 6: 0.12, 3 H, S, CH,Zn [OC(CH,),]; 1.38, 9 H, s, CH,Zn[OC(CH,),]. ICP-ES Zn content (%): found, 41.9; calc., 42.6. The ICP-ES and 'H NMR data are fully consistent with the proposed formulation MeZn(0R) and the 'H NMR data for both compounds agree well with published ZnO Film Growth ZnO films were deposited from MeZn(0Pr') at low pressure (15 Torr) in a cold-wall horizontal quartz reactor (Electro Gas Systems Ltd.) using radiant substrate heating. Soda lime glass substrates were used and these were cleaned (20% nitric aciddeionized water), degreased with acetone arid dried before use. The MeZn(0Pr') source was contained in a stainless-steel bubbler heated to 70°C using N, carrier gas (10 SCCMT).This led to ZnO growth rates of between 0.2 and 1.1 pm h-'. Under these conditions 'H NMR analysis of the precursor before and after use showed no evidence of decomposition. t 1 SCCM = 1 standard cm3 min-' Growth experiments using MeZn(0Bu') were carried out in a home-made cold-wall low-pressure reactor (lop2Torr) described in detail elsewhere.18 Soda lime glass substrates were used and these were heated by the action of a quartz halogen lamp on a graphite susceptor. The MeZn(0Bu') precursor was held in a tube furnace17 heated at either 80 or 150 "C, and growth rates as high as 4.4 pm-' were obtained at a source temperature of 80°C. 'H NMR analysis of the precursor showed no evidence of any decomposition at 80 "C, but indicated that significant decomposition had occurred at 150°C.A full summary of growth conditions is given in Table 1. Results and Discussion ZnO films were grown successfully from both MeZn(0Pr') and MeZn(0Bu') in the temperature range 250-350 "C, with-out the addition of a separate oxygen source. No growth was observed at 400 "C using MeZn(0Pr'). The films showed strong interference colours, characteristic of a high-refractive-index material and were generally slightly dark and absorbing. The films were hard and scratch-resistant with good adhesive properties and in the 'Scotch Tape Test''' the films remained intact as the tape was peeled away from the film. The atomic composition of the films was determined by AES and the data are summarized in Tables 2 and 3.The values quoted are from the bulk of the film and were obtained by combining AES with sequential Ar +-ion bombardment until comparable compositions were obtained for consecutive data points. Compositions were determined using experimen- tally derived relative sensitivity factors based on a ZnO reference material. These data showed that the ZnO films are non-stoichiometric with a slight excess of Zn present, demon- strating Zn :0 ratios in the range 1.01-1.09. The excess of Zn leads to the dark/absorbing character of the films, which were Table 1 Growth conditions used to deposit ZnO from alkylzinc alkoxides (a) MeZn(0Pr') cell pressure substrates 15 Torr soda lime glass N, carrier gas flow 10 SCCM deposition temperature 250-350 "C MeZn(0Pr') source temperature typical growth rates 70 "C 0.2-1.1 pm h-' (b)MeZn(0Bu')" reactor pressure lop2Torr substrates soda lime glass deposition temperature 250-400 "C MeZn (OBu') source temperature typical growth rates 80 or 150 "C 3.7-4.4 pm h-l (source temperature =80 "C) "Carrier gas not used.J. MATER. CHEM., 1994, VOL. 4 Table 3 AES analysis of ZnO films grown on glass from MeZn(OBut) precursor (source temperature 150 "C) substrate etch sample temperature/"C time/s 1 250 50 2 300 90 3 350 90 4 400 150 composition (AES) (atom YO) Zn 0 C 50.3 49.7 0.0 52.2 47.8 0.0 51.5 48.5 0.0 50.2 49.6 0.2 Zn:O 1.01 1.09 1.06 1.01 generally conducting with sheet resistances in the range 2 x lo6 to 20 x lo6R 0-'and with resistivities of between 300 and 900 R cm.The AES data clearly indicate that both MeZn(0Pr') and MeZn(0Bu') undergo an efficient intramolecular decompo- sition process to deposit ZnO, without the need for an external oxygen source. This is in marked contrast to earlier work2' in which it was not found possible to deposit ZnO from the Me,Zn( 1,4-dioxane) adduct in the absence of an added oxygen source, as the oxygen-containing ligand was lost from the adduct on pyrolysis and Zn metal was deposited. Carbon was either not detected in the ZnO films, or was only present close to the estimated AES detection limit of 0.2% and this can be attributed to the formation of a cyclic six-centre transition state (Fig.1) previously proposed as an intermediate in the thermal decomposition of RM (OR') compounds2' (M =Al, Mg, Zn). This involves the abstraction, by an incipient carbanion, of a P-hydrogen from the alkoxy group to eliminate methane, alkene and form ZnO. This decomposition may occur in the hot boundary layer adjacent to the substrate, or more likely, during heterogeneous pyrolysis of the alkylzinc alkoxide on the substrate surface. Scanning electron micrographs of ZnO films deposited at various substrate temperatures from MeZn(0Pr') and MeZn(0Bu') are shown in Figs. 2 and 3. At 250°C the film grown using MeZn(0Pr') shows well ordered columnar crys- tallites of ca.0.1 pm diameter. At 300°C the film is smooth and relatively featureless, whilst at the higher growth tempera- ture of 350°C the film surface becomes disordered with orange-peel-like crystallites of ca. 0.2 pm dimensions. A variety of surface morphologies are also observed in films grown using MeZn(OBu'), see Fig. 3. At 250°C, the film shows an irregular surface with flake-like crystallites of MeZn(0Pi) (R = H) MeZn(0Bd)(R = CH,) Fig. 1 Mechanism proposed for the heterogeneous pyrolysis of alkylzinc alkoxides Table 2 AES analysis of ZnO films grown on glass from MeZn(0Pr') precursor substrate etch composition (AES) (atom YO) sample temperature/"C time/s Zn 0 C S Zn:O 188 250 90 51.0 49.0 0.0 0.0 1.04 187 300 150 51.1 48.8 0.0 0.0 1.05 190 300 150 50.4 49.3 0.4 0.0 1.02 192 350 150 50.7 49.3 0.0 0.1 1.03 186 3 50 90 51.4 48.4 0.0 0.1 1.06 J.MATER. CHEM., 1994, VOL. 4 Fig.2 Scanning electron micrographs of ZnO films grown on glass using MeZn(0Pr') (source temperature =70 "C).(a)Substrate tempera- ture 250 'C;film thickness 0.19 pm; (b) substrate temperature 300 "C; (c) substrate temperature 350 "C; film thickness 1.65 pm. ca. 0.4 pm diameter [cJ: Fig. 2(c)]. At 300°C the film displays columnar crystallites of ca. 0.1 pm dimension, whilst at a growth temperature of 350 "C, the columnar crystallites have become larger and bulbous with a grain size of ca. 1 pm.The X-ray diffraction (XRD) data for a film grown at 300 "C on soda-lime glass using MeZn(OBut) are illustrated in Fig. 4. The data indicate that the ZnO has crystalliFed in hexagqnal form, wjth a space group P63/mc; a=3.253 A; b= 3.253 A; c =5.213 A; a =90"; fi= 90"; 6 =120". A comparison with standard ASTMS data for ZnO, see Table 4, shows that Fig. 3 Scanning electron micrographs of ZnO films grown on glass using MeZn(OBut) (source temperature =80 "C). (a) SubstTate tem- perature 250 "C; film thickness 5.5 pm; (b) substrate temperature 300°C; film thickness 4.4 pm; (c) substrate temperature 350 C. a number of reflections are absent and indicates that the crystals are oriented in the 110 direction (i.e. c axis parallel to the substrate surface).These data contrast strongly with those obtained for ZnO grown at 300-400°C on Si or glass using Et2Zn/0,10 or Et2Zn/ROH,l4 in which the XRD pattern was dominated by the (002) peak, indicating that the c axis is perpendicular to the substrate. The results presented herein provide a valuable insight into the gas-phase chemistry occurring during the growth of ZnO from R2Zn compounds and alcohols. The present study and previous work16 have shown that the addition of an alcohol (R'OH), even in excess, leads to the elimination of only one alkyl group from R2Zn and to the formation of alkylzinc alkoxides RZn(0R'). Therefore, in the growth of ZnO by MOCVD from mixtures of Et,Zn/RC)H14 or Me,Zn( THF)/ROH" the probable gas-phase intermediates are RZn(OR'), of which the tert-butoxide derivative has been reported2' to exist as an oligomer in the vapour phase.J. MATER. CHEM., 1994, VOL. 4 2@/degrees Fig. 4 X-Ray diffraction data for a 3 pm ZnO film grown at 300 "C on soda-lime glass from MeZn(0Bu') (source temperature 80 "C) Table4 X-Ray diffraction results for ZnO prepared from MeZn(0Bu') compared with standard ASTMS data for ZnO powder experimental ASTMS d/A; intensity (YO) h k 1 d/A; intensity (YO) h k 1 2.80( 5) 2.60(3) 2.46( 11) -1.62( 100) -1.37(3) 1.36(1) 100 002 101 110 112 201 2.82( 55) 2.61(41) 2.48( 100) 1.9 1 (24) 1.63( 36) 1.48 (34) 1.41(5) 1.38( 29) 1.36( 14) 1.30( 2) 10 00 10 10 1 1 10 20 1 1 20 00 0 2 1 2 0 3 0 2 1 4 1.24(5) 1.18( 3) 20 10 2 4 1.09(11) 20 3 1.06( 4) 21 0 1.04( 12) 1.02( 7) 0.99(5) 0.98 (9) 0.96( 2) 21 11 21 1 0 20 1 4 2 5 4 Conversely, associated molecules of the type [MeZn(OR)], (R =Pr', But) can be pre-synthesized, as reported herein, and used directly as precursors to ZnO.It has previously been reported', that trace amounts of water play a crucial role in the deposition of ZnO from Et,Zn/ROH mixtures. The most transparent films were obtained when lop3mol H20 were present in the alcohol and it was proposed that the role of water is not limited to initial nucleation processes, but significantly affects subsequent dis- sociation and reconstruction processes.14 Support for this is provided by the present work in which absorbing films were obtained using the solid crystalline compounds MeZn(0Pr') and MeZn(OBu'), from which trace H20 is clearly absent (see 'H NMR data).Conclusions ZnO films have been grown successfully by MOCVD using the single-source precursors MeZn(0Pr') and MeZn(OBut) without any added oxygen source. Both precursors are stable at source temperatures of 70-80°C. A mechanism for ZnO growth has been proposed which involves the formation, and subsequent decomposition, of a six-centre transition state complex. This work was supported by the DTI under the LINK/ATP initiative and the Teaching Company Scheme. D.J.H. is a Teaching Company Associate (University of Keele). 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