J. MATER. CHEM., 1994, 4( S), 1245-1247 Investigations into the Growth of AlN by MOCVD using Trimethylsilylazide as Nitrogen Source John Auld," David J. Houltont Anthony C. Jones,*" Simon A. Rushworth" and Gary W. Critchlov@ a Epichem Limited, Power Road, Bromborough, Wirt-a/, Merseyside, UK L62 3QF Institute of Surface Science and Technology, University of Loughborough, Loughborough, Leicestershire, UK LE113TU Thin films of AIN have been deposited by atmospheric-pressure MOCVD using trimethylaluminium (Me,AI) and trimethyl- silylazide (Me,SiN,) as precursors. The films were deposited at 300 or 450"C and had growth rates of up to 3 pm h-' Thin films of aluminium nitride (AlN) have a number of important applications, such as passive barrier layers and substrates in VLSI and ULSI silicon technology, transparent high-temperature windows, and as optical enhancement layers in magneto-optic multilayer structures.In addition, the mixed ternary alloy Al,Ga,-,N has a large potential application in optoelectronic devices operating in the UV-blue spectral region. The development of these various applications is critically dependent on the capability to deposit thin films of AlN at low to moderate substrate temperatures. Metalorganic chemical vapour deposition (MOCVD) is an attractive technique for the controlled deposition of AlN thin films, having the advantages of large area growth capability, excellent conformal step coverage and precise control of layer thickness. The deposition of AlN by MOCVD has tradition- ally been carried out using mixtures of trimethylaluminium (Me,A1) and ammonia (NH,).',, However, the high thermal stability of NH, necessitates the use of high substrate tempera- tures (typically >9OO0C).This leads to the problem of nitrogen loss from the A1N film which is only partially alleviated by the use of excessively high V/IIT ratios (e.g. >2000: 1). In an effort to achieve A1N growth at lower temperatures, a variety of 'single-source' precursors which already contain an (Al-N) bond have been investigated. These include CA1(NR2)312, CHAI(NR,),I, (R =Me,Et),3 (Me2A1NH2)3,4 ( Et2AlN3),,5 and (Me,A1NR2), (R =Pr'),6 from which A1N has been deposited successfully at low to moderate substrate temperatures (400-800 "C). However, these 'single-source' pre- cursors generally have very low vapour pressures (<1 Torr at room temperature), which necessitates the heating of source and reactor inlet lines and the use of vacuum CVD equipment.It is therefore desirable to develop alternative precursors which may be more conveniently utilized in MOCVD, and hydrazine (N2H4) has been used in combination with Me,Al to deposit AlN at 220°C.7 However, N,H4 is highly toxic (TLV,,, =0.01 ppm) and unstable, which is likely to restrict its large-scale application in MOCVD seriously. In contrast, the primary alkylamines tevt-butylamine (Bu'NH,) and iso- propylamine (PriNH2) are of low toxicity and are stable, which permits their manufacture and use on an industrial scale.In addition, Bu'NH, and PtNH, have convenient vapour pressures of 340 Torr (25 "C) and 476 Torr (20 "C), respectively. Both Bu'NH, and Pr'NH, have recently been used in combination with Me,A1 to deposit A1N films by atmospheric-pressure MOCVD in the temperature range 400-600°C.' Although the mechanism of AlN growth from Me,Al-RNH, mixtures has not been established, it was proposed' that directly bonded species such as (Me,AlNHR), are formed in situ in the gas phase prior to layer growth. The recent report' of AIN growth from the single source molecules (Me,AINHR), (R =But, Pr') strongly supports this proposal. The successful growth of AlN by low-pressure C\'D using the trimeric 'single-source' molecule diethylaluminium azide, (Et2A1N3),,5 has encouraged us to investigate methods of forming such species in situ in the vapour phase using volatile Group I11 and Group V components.This approach, which is an extension of our earlier work,* aims to combine the advantages of convenient source temperatures and high growth rates associated with the use of high vapour pressure reagents, with the low growth temperatures associated with single-source precursors. In this paper, we report the successful growth oj A1N by atmospheric-pressure MOCVD using Me,Al in Combination with trimethylsilylazide (Me3SiN3). Experimental General Techniques The reagents used were electronic-grade Me,A1 ( Epichem Limited, vapour pressure =9.7 Torr at 20 "C) and Ue,SiN, (Aldrich; bp =94 "C at 760 Torr).The Me,SiN3 was dcoxygen- ated prior to use by repeated freeze-pump-thaw cycles in a nitrogen atmosphere. Proton nuclear magnetic resonance (lH NMR) data were obtained on a Bruker WM 250 spectrometer operating at 250 MHz, and mass spectral data were obtained using a VG Pegasus, quadrupole mass spectrometer operating at an ionization energy of 70 eV. Auger electron spectroscopy (AES) was carried out on a Varian scanning Auger spectrometer. The atomic c omposi-tions qupted are from the bulk of the film (depth from surface >2000 A) and were obtained by combining AES with sequen- tial Ar +-ion bombardment until comparable compositions were obtained for consecutive data points. Film thicknesses were estimated by the time taken to sputter through the layer using Ar +-ion bombardment.Scanning electron microscopy (SEM) was performed on a JEOL JS 35 CM scanning electron microscope. AlN Film Growth The A1N films were deposited at atmospheric pressure in a simple cold-wall horizontal quartz reactor (Elecrro Gas Systems Ltd.) using radiant substrate heating. The siLbstrates used were Si( 11 1) single-crystal wafers and these were cleaned (20% nitric aciddeionized water), degreased with acetone and dried before use. The Me,Al and Me,SiN, sources were operated at room temperature and were mixed in a 'T-piece' at the reactor inlet. A full summary of growth conditions is given in Table 1. J. MATER. CHEM., 1994, VOL. 4 Table 1 Growth conditions used to deposit A1N films from Me,Al-Me,SiN, mixtures' run no.H2 carrier gas flow H, carrier gas flow Me,SiN, :Me,Al' growth growth through Me,A1 (sccm) through Me,SiN, (sccm) temp./"C rate/pm h -lb 252 35 20.0 1.3 300 253 35 15.6 0.99 450 255 35 81.5 4.99 450 "Cell pressure 760 Torr; Me,Al and Me,SiN, sources at 20 "C; substrates Si( 111) single crystal wafers. bFilm thickness estimated from AES sputter time. 'Based on an estimated Me,SiN, vapour pressure of ca. 4.3 Torr at 20 "C. Table 2 AES analysis of A1N films grown on Si( 11 1) using Me3A1 and Me,SiN, film no. composition (atom Yo) A1 :N ratio A1 N C 0 Si 252 41.6 38.8 11.0 8.6 0.0 1.07 253 44.9 40.9 11.4 2.9 0.0 1.10 255 45.4 43.9 9.8 1.1 0.0 1.04 Table3 AES depth profile through a typical A1N film (255) grown using Me,Al-Me,SiN, mixtures sputter time composition (atom %) /min A1 N C 0 ~ ~~ ~ ~ ~~ ~~~ 1 36.9 0.0 0.0 63.1 21 33.6 8.3 5.1 53.0 31 42.6 36.8 8.4 12.3 60 44.8 43.5 9.0 2.7 68 45.4 43.7 9.8 1.1 Results and Discussion A1N films were successfully deposited using Me,Al and Me3SiN, at substrate temperatures of 300 and 450°C.The atomic composition of the films was determined by AES and the data are summarized in Table 2. These data show that the films have an A1 :N ratio close to unity, although relatively large levels of oxygen and carbon are present. Significantly, silicon was not detected in the films.The high strength of the aluminium-oxygen bond makes it difficult to exclude oxygen from A1N films deposited in relatively unsophisticated MOCVD equipment of the type used in this study. Comparable levels of oxygen contamination (4-5 atom%) were detected by AES in AlN films grown using (Me2A1NH2),, and this was attributed to trace oxygen present in the MOCVD reactor. Support for this proposal is provided by an AES profile through a typical film, see Table 3, which shows that the oxygen content decreases greatly with increas- ing depth, suggesting that oxidation has occured after film deposition and not during film growth. The carbon contamination in the films is likely to be intrinsic to the use of a methyl-based A1 precursor, as pre- viously observed in the deposition of A1 metal films" and AlGaAs layers'' by MOCVD.Comparable levels of carbon (2.7-17 atom%) have also been detected by AES in AlN films grown using Me,A1-RNH, mixtures (R =But, Pr').' The AlN films were extremely hard and scratch resistant and demonstrated specular surface morphology. SEM data for a 1 pm thick film grown at 450°C, see Fig. 1, indicate that the film is amorphous with an average grain size of <0.1 pm. The mechanism of AIN growth from mixtures of Me,A1 and Me,SiN, has not been fully established. However, the very low growth temperatures (300-450°C) and low V/IIT ratios (approx. 1:1 to 5: 1) used successfully in the present study strongly suggest that a 'directly bonded' (Al- N) species is the active precursor to A1N deposition.It is significant that Si is not detected by AES in the films and this indicates that the (Me,Si) fragment is efficiently removed from the growth zone during film growth. Information concerning the possible growth mechanism has been obtained by the ex situ addition of Me,SiN, (9.2 g, 0.08 mol) to Me,Al (5.9 g, 0.08 mol) at room temperature. This resulted in a volatile liquid product which was isolated by distillation in uucuo, and which had a mass spectrum identical with an authentic sample of tetramethylsilane (Me,Si) [rnlz; 88 (Me,%+), 74 (Me,SiH+), 73 (Me,Si+), 45 (MeSiH,+)] and which showed no evidence of any residual Me,SiN, [rnlz; 115 (Me,SiN,+), 100 (Me2SiN3+) 73 (Me,%+), 45 (MeSiH,+), 28 (N2+)].The lower volatility liquid residue had an identical 'H NMR spectrum to that reported" for Me,AlN, (6 Al-CH, =0.53) which is signifi- cantly different from the 'H NMR spectrum of Me,Al Fig. 1 Scanning electron micrograph of an AlN film (no. 255) grown at 450 "C on Si( 111) (magnification x 10000) J. MATER. CHEM., 1994, VOL. 4 (6 A1-CH, =0.3). A similar displacement of an alkyl group from an organometallic A1 compound has been reported', in the room-temperature reaction between Me,AlI and Me,SiN,, which leads to methylaluminiumiodo azide, MeAlIN, . These data strongly suggest that Me3A1 and Me,SiN, will react similarly in the gas phase, either at room temperature, or in the hot boundary layer, to form Me,AlN, and the volatile, relatively stable species tetramethylsilane (Me&).Tetramethylsilane is unlikely to be significantly pyrolysed at the low growth temperatures used in the present study and thus Si will be transported effectively away from the growth zone. Subsequent pyrolysis of Me,AlN, leads to the deposition of AlN, although in the absence of any added gettering agent, the presence of methyl radicals strongly bound to an A1 atom on or near the growth surface, followed by their subsequent decomposition, will lead to the significant carbon contami- nation observed in the A1N films. Further support for this proposed mechanism is provided by the observation that pyrolysis of the Me,SiN, precursor alone failed to deposit a film at substrate temperatures below 550 "C, indicating that a more active species which already contains a direct (Al-N) bond is the precursor to A1N deposition.At 600 "C a film containing both nitrogen (49.9 atom%) and silicon (22.5 atom%) was deposited, which indicates that Me,SiN, is unlikely to be suitable for AlN growth at more elevated temperatures. It was thought that mixtures of trimethylgallium (Me,Ga) and Me3SiN, might prove suitable for the deposition of GaN by MOCVD. However, AES data indicate that high levels of silicon (18-35 atomyo), in addition to oxygen and carbon, are present in films deposited at 450°C. This suggests that a different growth mechanism is in operation, and that the (Me,Si) group is no longer eliminated efficiently from the growth surface.Further studies are in progress aimed at elucidating these various growth mechanisms. Conclusions AlN films have been deposited by atmospheric-pressure MOCVD using Me,Al in combination with Me,SiN,. High growth rates of 3 pm h-' were obtained at low substrate temperatures (300-450 "C).The films contained relatively high levels of carbon and oxygen impurities, although silicon was notably absent from the films. A growth mechanism involving the formation of intermediate gas-phase species such as Me,A1N3 has been proposed and is supported by tdie ex situ addition of Me,SiN, to Me,Al. This work was supported by the Department of Trade and Industry under the LINK/ATP initiative and the Teaching Company Scheme. David Houlton is a Teaching Company Associate (University of Keele).References 1 M. Morita, S. Isogai, N. Shimizu, K. 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