ALUMINUM ALKYLS
Aluminum alkyls AlR3 are very air- and moisture sensitive and must be strictly handled in inert gas atmosphere. AlR3 spontaneously decompose in air to give aluminum oxide in a very exothermic reaction :
AlR3 + O2 → Al2O3 + organic decomposition products
Synthesis of aluminum alkyls:
1) Methathesis of aluminum chloride with alkali metal alkyls
2/n [AlCl3]n + 6 RX → (AlR3)2 + 6 XCl¯
X = Li, Na, BrMg, …, R = Me, Et
2) Hydroalumination
(AlH3)2 + 6 H2C=CH2 → (AlEt3)2
3) trans-Metallation: 2 Al + 3 HgR2 → 2 R3Al + 3 Hg
4) Industrial Scale Synthesis: Al + 3/2 H2 + 3 R2C=CH2 → Al(CH2CHR2)3
[i] Gärtner, G., Jamiel, P., and Lydtin, H., “Plasma-Activated Metalorganic CVD of IIIb Oxides/Tungsten Layer Structures,” Proc. of 11th Int. Conf. on CVD, (K. Spear and G. Cullen, eds.), pp. 589–595, Electrochem. Soc., Pennington, NJ08534 (1990)
[ii] Sugai, K., et al., “Aluminum CVD with New Gas-Phase Pretreatment using TDMAT for ULSI Metallization,” J. Vac. Sci. Tech., 13(5):2115 (Sept/Oct 1995)
Trimethylaluminum Al2Me6
Trimethylaluminum (TMA) is a clear pyrophoric liquid at 25 °C with vapor Pressure 11 Torr/ 20 °C. It is dimeric (Al2Me6) in gas phase (Al-C-Al unit: 3-Center 2-Electron Bonds; terminal Al-C units: 2-c 2-e- bonds), monomeric at ca. 200 °C.
TMA burns spontaneously upon contact with air and reacts explosively with water.
In the Si semiconductor industry, TMA is used essentially for the MOVPE growth of AlGaAs, AlInP and other Al-contaning III-V layers, and CVD deposition of Al2O3, either pure or as a mixed oxide of other elements such as Silicon, Hafnium or Zirconium.
It has been also extensively used in conjunction with NH3 to deposit AlN films.
For high-k oxide deposition TMA is mostly used in ALD mode, rather than by MOCVD Because of its extreme reactivity to oxygen sources,. In case of MOCVD growth Temp. is around 300 °C.[ 568]
(AlMe3)2 for the CVD applications.
The advantage of trimethylaluminum (Al2Me6) as CVD precursors is that it is liquid and inexpensive. However, as Al2Me6 cannot decompose by β-elimination mechanism, therefore addition of hydrogen is needed for the CVD growth, otherwise grown layers will contain carbon
AlMe3 for Al metal films by MOCVD
The deposition temperatures for metal Al using TMA are usually 350 – 550 °C, . Complicated decomposition mechanisms for the gas phase and surface deposition of TMA as result of competing reactions. This yields to the incorporation of relatively high concentration of carbon into the films. TMA decomposition invokes both the liberation of radical species as CH3 and stable species such as CH4.
Carrier gas N2: Al2Me6 → ½ Al4C3 + 9/2 CH4
Carrier gas H2: Al2Me6 + 3 H2 → 2 Al + 6 CH4
AlMe3 for Al metal films by RF PACVD
AlMe3 has been applied as precursor for the Radio Frequency Plasma-Assisted CVD Deposition of Al thin films on Si substrates at surface temperature 230 – 260 °C. Reflective Al (100) oriented films free from carbon have been grown, growth rates up to 30 nm/min were achieved, films resistivities were 2.7 μWcm.
The precursor decomposition pathway is following:
Al2(CH3)6 → Al(CH3)2 + Al(CH3) + H3C-CH3 + CH4
At the substrate surface:
Al(CH3)2 + H2 → Al + 2 CH4
Al(CH3) + ½ H2 → Al + CH4
AlMe3 for Al metal films by laser-assisted CVD
Al films were deposited from TMA by Laser-Assisted CVD deposition, with laser operating in the UV-Vis region 190 – 270 nm, which induces TMA fotodecomposition in the gas phase to Al and AlMe species, with ratio of these two species being wavelength dependent.
Al2(CH3)6 → 2 Al(CH3)3 → Al + AlCH3
Using UV-Vis region 230 – 255 nm, the TMA decomposition pathway is:
Al(CH3)3 + hν → Al(CH3)2 + CH3
Al(CH3)2 → Al + H3C-CH3
For the best results, H2 is used as carrier gas. For Al films deposited on Si(100) at substrate temperature 200 °C, film resistivity is below 10 μWcm, carbon concentration 7 %, oxygen concentration 5 %.
At relatively high laser powers AlHMe2 has been detected in the gas
phase. This can be interpreted by a reaction channel involving an α-hydrogen elimination.
AlMe3 for AlN by MOVPE and MOCVD
The review of MOVPE growth of AlN and other metal nitrides by using various precursors including TMA, has been presented by A. Devi et al. From a technological point of view, there is no real substitute for TMA as Al precursor for actual practical application in this field, however the research in this area indicated that some single-molecule precursors containing the metal and nitrogen atoms in a single molecule, including amide- or azide-based compounds, may be promising as an alternative for the alkyls of Al currently used [[i][PS1] ]
Trimethylaluminum forms adducts with ammonia NH3 in the gas phase, the understanding in which conditions their formation is minimized is important for the high-efficiency MOCVD growth of AlN AlGaN and AlGaInN thin layers. The process of adduct formation was investigated by room-temperature FT_IR experiments, as well as by density functional theory calculations. It was found that at higher partial pressures, a product distinct from the well-known (CH3)3Al:NH3 adduct forms. According to the IR spectra and energy calculations, the second product is the result of hydrogen bonding of a second NH3 molecule to the (CH3)3M:NH3 adduct, i.e. “(CH3)3M:NH3···NH3” having binding energy 26.8 kcal/mol for hydrogen-bond, and is lower in energy (more stable) vs. 1:1 (CH3)3M:NH3 adduct by 7.2 kcal/mol. In contrast, an alternative complex involving the formation of two separate M−N donor−acceptor bonds “H3N:(CH3)3M:NH3”, is calculated to be lower in energy vs. (CH3)3M:NH3 by only 0.1 kcal/mol. Thus, hydrogen bonding plays an important role in the interaction of TMA with NH3 under typical metal organic chemical vapor deposition AlGaInN growth conditions. [[ii][PS2] ]
AlMe3 for AlN by hot-wall MOCVD
TMA has been applied for the growth of AlN thin films at 1200 °C temperature in hot-wall MOCVD system having possibility of straightforward scaling up the process on larger wafer areas to meet the demand of device technologies. Optimized design and process parameters allowed to achieve higher overall growth rate (1-2 μm/h), efficiency, uniformity, which strongly depend on active prevention gas-phase adduct formation consuming growth-limiting species. Uniform epitaxial growth with good thickness uniformity (±1.3% on 2 in. wafers) was achieved by optimized mixing of precursors upstream of the deposition area, as well good uniformity of substrate temperature inherent to the hot-wall reactor as well as rotation of the wafer. The intense near band-gap deep UV emission at ~208 nm was dominating the low-temperature cathodoluminescence spectrum of the AlN epitaxial material. [[iii][PS3] ]
AlMe3 for AlAs and AlGaAs selective area MOVPE
AlMe3 for Al2O3 by remote plasma CVD
TMA has been applied for the growth of Al2O3 films by remote plasma CVD. (see [[v]] and refs therein)
[i]Anjana Devi, Rochus Schmid, Jens Müller and Roland A. Fischer
Topics in Organometallic Chemistry, 2005, Volume 9/2005, 925, DOI: 10.1007/b136142
[ii]G.T. Wang, J. R. Creighton, J. Phys. Chem. A, 2006, 110 (3), pp 1094–1099
Complex Formation of Trimethylaluminum and Trimethylgallium with Ammonia: Evidence for a Hydrogen-Bonded Adduct
[iii] A. Kakanakova-Georgieva, R. R. Ciechonski, U. Forsberg, A. Lundskog and E. Janzn, Cryst. Growth Des., 2009, 9 (2), pp 880–884, DOI: 10.1021/cg8005663
Hot-Wall MOCVD for Highly Efficient and Uniform Growth of AlN
[iv] K. Shimoyama, Y. Inoue, K. Fujii and H. Gotoh, Journal of Crystal Growth, Volume 124, Issues 1-4, 1 November 1992, Pages 235-242, “ Novel selective area growth of AlGaAs and AlAs with HCl gas by MOVPE”
[v] I Volintiru, PhD Thesis UniEindhoven - 2008 - alexandria.tue.nl
“Remote Plasma Deposition of Metal Oxides: Routes for Controlling the Film Growth”
[vi] A.W. Ott, J.W. Klaus, J.M. Johnson and S.M. George. Thin Solid Films 292 (1997), p. 135.
[vii] V.E. Drozd, A.P. Baraban and I.O. Nikiforova. Appl. Surf. Sci. 82–83 (1994), p. 583.
[viii]Y. Kim, S.M. Lee, C.S.Park, S.I. Lee and M.Y. Lee. Appl. Phys. Lett. 71 (1997), p. 3604.
[ix] L. Hiltunen, H. Kattelus, M. Leskelä, M. Mäkelä, L. Niinistö, E. Nykänen, P. Soininen and M. Tiitta. Mater. Chem. Phys. 28 (1991), p. 379.
Triethylaluminum (AlEt3)2
Triethylaluminum (Al2Et6, TEAl) is an alternative to TMA aluminum alkyl MOCVD precursor for the growth of Al-containing layers.
TEAl is liquid at 25 °C having relatively low vapor pressure 0.1 Torr at 36 °C, which requires the precursor bubbler to be heated (> 40 °C) to achieve reasonable molar flow. At temperatures below ca. 150 °C the precursor starts to partially dimerize.
MOCVD deposition of Aluminum Using AlEt3 as Precursor
During Al deposition from TEAl, the following primary reactions occur:
(H3C-H2C)3Al → (H3C-H2C)2AlH + H2C=CH2 (β -hydrogen elimination)
2 (H3C-H2C)2AlH → (H3C-H2C)3Al + (H3C-H2C)AlH2
2 (H3C-H2C)AlH2 → (H3C-H2C)2AlH + AlH3
AlH3 → Al + 3/2 H2
The activation energy for the β-hydrogen transfer is 29 kcal/mol.
The deposited aluminum films obtained contain Al, as well as Al-carbide and free carbon.
Triisobutylaluminum AliBu3
Tris(tert-butyl)aluminum AltBu3
Tris(tert-butyl)aluminum AltBu3 (M=198.33) has vapor pressure 2Torr/22°C; thus it is much more volatile than tris-isobutylaluminum having vapor pressure 0,1 Torr/20°C and 0,2Torr/30°C.
However, other sources give different vapor pressure dat: 0.001 Torr(?)/20°C. [[i]]
The synthesis of Al(tBu)3 is described in [[ii]]:
Tri-tert-butylaluminum can be prepared free from isomers and in high yield (60–90%) yield by reaction of AlCl3 or di-tert-butylaluminum fluoride with alkyllithium. Al(tBu)2F can be obtained by reaction of the BF3*Et2O with the etherates of Al(tBu)2Cl, which is produced from AlCl3 and tBuMgCl.
AltBu3 has been applied as precursor for the growth of AlN films by atnospheric pressure MOCVD, using tert-butylamine as nitrogen source.
[i] Physical Properties of Inorganic Materials Precursors, www.crcnetbase.com/doi/abs/10.1201/9781420041422.ch3
[ii]Lehmkuhl, H.; Olbrysch, O.; Nehl, H. Liebigs Ann. Chem. 1973, 708,
MSc thesis of A. Das, University of Toledo, 2007
http://etd.ohiolink.edu/send-pdf.cgi/Das%20Anirban.pdf?toledo1185478138
Tris(tert-butyl)aluminum AltBu3
Tris(tert-butyl)aluminum AltBu3 (M=198.33) has vapor pressure 2Torr/22°C; thus it is much more volatile than tris-isobutylaluminum having vapor pressure 0,1 Torr/20°C and 0,2Torr/30°C However, other sources give different vapor pressure dat: 0.001 Torr(?)/20°C. [[i]], or 2Torr/22°C.
The synthesis of Al(tBu)3 is described in [[ii]]: Tri-tert-butylaluminum can be prepared free from isomers and in high yield (60–90%) yield by reaction of AlCl3 or di-tert-butylaluminum fluoride with alkyllithium. Al(tBu)2F can be obtained by reaction of the BF3*Et2O with the etherates of Al(tBu)2Cl, which is produced from AlCl3 and tBuMgCl.
AltBu3 for AlN films by atmospheric pressure MOCVD using tBuNH2
AltBu3 has been applied as precursor for the growth of AlN films by atmospheric pressure MOCVD, using tert-butylamine as nitrogen source. Thin films of AIN have been deposited at 500 and 600 °C on Si(111) by atmospheric-pressure MOCVD , with growth rates of 0.5 µm /h obtained at 500 °C. Post-growth oxidation of the AIN films was prevented by the deposition of a protective Al overlayer using AltBu3 [iii]
AltBu3 for AlN films by low-pressure MOCVD using tBuNH2
Tris(tert-butyl)aluminium (AltBu3) with ammonia (NH3) as co-reactant has been applied for the growth of thin films of AlN on sapphire (c-Al2O3) by low-pressure MOCVD. Growth temperature was 1050°C, growth rate 0.35 μm/h (resulting in film thicknesses 200- 3600 Å), FWHM of rocking curve < 320 arcsec. Thin AlN films were applied as buffer layers for the deposition of gallium nitride (GaN) at 950°C using GaEt3. The improvement of Structural and optical properties of GaN on AlN buffer layers were measured by XRD and Raman, photothermal deflection and PL spectroscopies. The variation of the layer optical properties (sub-bandgap absorption, the Raman-active phonons and photoluminescence) vs. AlN buffer layer thickness was investigated.[iv]
AltBu3 for AlN films by low-pressure MOCVD using NH3
Tris(tertiarybutyl)aluminum
Al(tBu)3 with NH3 as co-reactant, was used as precursor for the deposition of amorphous, polycrystalline, or epitaxial (depending on the growth conditions like temperature 400 °C-1100 °C.) AIN films on (0001) Al2O3 substrates at by low‐pressure CVD. The thermal stability as well as thermally induced effusion of H, CHx, and N-H was studied. The elemental composition vs. growth temperature was investigated by elastic recoil detection
analysis (ERDA). The combination of ERDA and thermal desorption measurements was used for study of the influence of growth rate and crystallite size on the concentration of surface adsorbed hydrocarbons and carbon oxides. The stability of and the nitrogen
flux from the AIN surfaces was determined by XRD and thermal decomposition experiments. [v]
[i] Physical Properties of Inorganic Materials Precursors, www.crcnetbase.com/doi/abs/10.1201/9781420041422.ch3
[ii]Lehmkuhl, H.; Olbrysch, O.; Nehl, H. Liebigs Ann. Chem. 1973, 708,
MSc thesis of A. Das, University of Toledo, 2007
http://etd.ohiolink.edu/send-pdf.cgi/Das%20Anirban.pdf?toledo1185478138
[iii]A.C. Jones, John Auld, Simon A. Rushworth, David J. Houlton, Gary W. Critchlow, J. Mater. Chem., 1994,4, 1591-1594, DOI: 10.1039/JM9940401591
Investigations into the growth of AIN by MOCVD using tri-tert butylaluminium as an alternative aluminium source
[iv]O. Ambacher, R. Dimitrov, D. Lentz, T. Metzger, W. Rieger, M. Stutzmann Journal of Crystal Growth, Volume 167, Issues 1–2, 2 September 1996, Pages 1–7, “ Growth of GaN/ AlN by low-pressure MOCVD using triethylgallium and tritertbutylaluminium”
[v]Ambacher, O., Brandt, M. S. ; Dimitrov, R. ; Metzger, T. ; Stutzmann, M. ; Fischer, R. A. ; Miehr, A. ; Bergmaier, A. ; Dollinger, G.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures 1996, volume: 14 , Issue: 6, Page(s): 3532 – 3542, Ther”mal stability and desorption of Group III nitrides prepared by metal organic chemical vapor deposition”
Tritertiarybutylaluminum Al(tBu)3 (TTBAl), combined with GaEt3, As(tBu)3 and SbEt3, was applied as MOCVD precursor for the growth of AlxGa1−xAsySb1−y
on InAs substrates using MOVPE. Substrate temperatures >610°C were needed to avoid phase separation in the material. Reproducible solid composition, high structural quality and mirror-like surface morphology were achieved by optimization of the substrate
stabilization, annealing time and purging time between stabilization and growth. V/III-ratios 1.8 to 4 combined with 625°C-645°C growth temperatures resulted in the best electrical properties of of AlxGa1−xAsySb1−y. The p-type background
doping level increased from 4×1017 cm−3 to 2×1018 cm−3 and mobility of the holes decreased from 150 to 50 cm2/Vs when Al content increased from 0.20 to 0.48, the increased hole concentration
was due to carbon incorporation, according to SIMS measurements. The electrical and optical properties of A1GaAsSb were improved by using “EpiPure” grade TEGa and TESb synthesized using a completely oxygen-free route (Epichem Ltd.).[i]
[i] Ch Giesen, A Szymakowski, S Rushworth, M Heuken, K Heime, J. Cryst. Growth, 2000, Vol. 221, Iss. 1–4, p.450-455, « MOVPE of AlGaAsSb using TTBAl as an alternative aluminum precursor », https://doi.org/10.1016/S0022-0248(00)00739-9
https://www.sciencedirect.com/science/article/pii/S0022024800007399
Tritertiarybutylaluminum Al(tBu)3 (TTBAl) (and for comparison Trimethylaluminum,AlMe3 and dimethylethylamine alane AlH3*NMe2Et), combined with SbMe3) was
applied as Al source for the growth of (AlGa)Sb and (AlGa)(AsSb) layers by MOVPE in an industrial multiwafer planetary reactor. The thermal activation behavior of the Al precursors appeared to be a critical factor in the kinetically controlled regime. The
efficiency of Al(tBu)3 (as well as that of AlH3*NMe2Et) depended on the V/III-ratio which was attributed to prereactions with SbMe3. Al(tBu)3 resulted in one order of magntude higher O contamination in (Al0.19Ga0.81)Sb
layers, compared to AlH3*NMe2Et precursor.[i]
[i] C. Agert, P. Lanyi, A.W. Bett , 2001, vol. 225, Iss. 2–4, p. 426-430, J. Cryst. Growth ?, « MOVPE of GaSb, (AlGa)Sb and (AlGa)(AsSb) in a multiwafer planetary reactor », doi.org/10.1016/S0022-0248(01)00908-3
Tritertiarybutylaluminum Al(tBu)3 (and for comparison dimethylethylamine alane AlH3*NMe2Et) was applied as Al precursor for the MOVPE growth of Sb-based III–V materials (like (AlGa)Sb) at 575-625°C temperatures. High O background impurity levels were attributed to the insufficient
quality of the novel precursors; also memory effects and prereactions in the gas phase were observed.[i]
[i] F Dimroth, C Agert, A.W Bett, J. Cryst. Growth, 2003, vol. 248, p.265-273, “Growth of Sb-based materials by MOVPE”, https://www.sciencedirect.com/science/article/pii/S0022024802018183,