ALUMINUM HYDRIDES (ALANES)
The use of aluminum hydrides (alanes) for thin films deposition applications is reviewed in[[i]]
Alane AlH3
Alane AlH3 is colorless, air- and moisture sensitive compound and very strong reducing agent (3-Center 2-electron bonds are present). The structure of Alane is following: coordination number 6 at Al; 3-c 2-e bonds (H-Al-H); without direct Al-Al bond.
AlH3 is very unstable at ambient temperatures and polymerizes to give to form a solid in which bridging H-Al-H bonds are present( (AlH3)n); colorless solid which decomposes to produce Al and H2 on its heating to 100 °C and therefore practically not suitable for CVD.
Because of its instability, AlH3 is usually synthesized in situ by the following reactions:
3 Li[AlH4] + AlCl3 → 4 AlH3 + 3 LiCl
2 Al + 3 H2(+300 KJ) → 2 AlH3
Only some reports from the late 60th appeared about the electroless deposition from AlH3 from the vapor phase producing Al films for diffusion barriers on a variety of metallic and non-metallic surfaces. The respective films deposited between 100 – 200 °C had very low oxygen and carbon contents.
[i] J.A. Jegier, W.L. Gladfelter, Coord. Chem. Rev., Vol. 206-207, Sep. 2000, p.631-650
[ii]http://www.uio.no/studier/emner/matnat/kjemi/KJM5100/h06/undervisningsmateriale/09KJM5100_2006_Chemical%20vapour%20deposition_d.pdf
[iii] Craig M. B. Marsh, Henry F. III Schaefer, J. Phys. Chem., 1995, 99 (1), pp 195–206
[iv]J. N. Kidder, Jr.,H. K. Yun, J. W. Rogers, Jr., and T. P. Pearsall, Chem. Mater., 1998, 10 (3), pp 777–783 , “Chemical Composition of AlN Thin Films Deposited at 523−723 K Using Dimethylethylamine Alane and Ammonia”
Alane –alkylamine adducts AlH3(R3N)
Alane AlH3 can be stabilized upon addition of Lewis bases to give Lewis acid –Lewis base adducts. Through the donating ability of the Lewis-base a stable donor-acceptor bond is formed. In the appropriate structure the Al atom possesses an octet of valence electrons which increases it stability with respect to polymerization. Among the commonly known basic donors are NR3, py, PR3, SR2, OR2, thf, … (R = alkyl, aryl, …; py = pyridine).
2/n (AlH3)n + 2 NMe3 → [Me3N x AlH3]2
Amine alane precursors are sensitive to atmospheric moisture and oxygen, however, they are less air sensitive than the respective AlR3 species (R = alkyl). This is attributed to the formula shown earlier (Lewis-base – Lewis-acid adducts). In addition, they are not pyrophoric unless exposed to high humidity. Under atmosphere they decompose to give involatile Al-oxides and –hydroxides. In the case of AlR3 volatile Al alkoxides are formed in presence of oxygen!
Methods of synthesis of Lewis-base Stabilized Alanes
1) Direct Synthesis:
2 Al + 2 NR3 + 3 H2 → 2 (R3N)AlH3
2) Reaction of Alane with Alkylamines:
1/n (AlH3)n +2 Me3N → AlH3 (Me3N)2 (20 °C)
1/n (AlH3)n + Me3N → AlH3(Me3N)
(Me3N)AlH3 → 1/n (AlH3)n + NMe3 (100 °C)
For comparison reaction behavior of alane with ammonia NH3:
1/n(AlH3)+ NH3 → (H3N)AlH3 → 1/n[H2AlNH]n + H2 → 1/n[HAlNH]n
→ 1/n[AlN]n + H2
Alane Alkylamine Adducts as precursors for CVD applications
Tertiary amine adducts of AlH3 may be used as CVD precursors for Al-containing films: facile due to easy cleaving of Al-N bond, and desorption of trimethylamine [[ii]]
Dimethylamine alane AlH3·NHMe2
Dimethylaminealane AlH3·NHMe2 were reported as promising CVD precursor for preapration of metal Al films with growth onset temp 100°C.
Trimethylamine alane AlH3·NMe3
Trimethylamine alane [AlH3•NMe3]2 (TMAA) is a white crystalline solid (mp.75°C) with high vapor pressure at room temperature. It is monomeric in the gas phase but a dimer in solid with units weakly associated by unsymmetrical bridging hydrogens.
[AlH3•NMe3]2 possesses 3-Center 2-Electron Bonds both in gas phase and solid state; its structure was calculated by ab initio quantum-mechanical calculations [[iii]]
[AlH3•NMe3]2 is a promising precursor for growth of high-purity (low-carbon incorporation) AlGaAs layers at low-temperature.
Surface photo-absorption (SPA) studies revealed that TMAA starts to decompose at about 150°C on an As-stabilized surface - much lower than aluminum trialkyls. A comparison of photoluminescence of AlGaAs layers grown with TMAA/triethylgallium and AlEt3/triethylgallium shows that the band-to-carbon acceptor transition is greatly reduced by using TMAA.[348]
AlH3·NMe3 for Al metal films by CVD
Pure Al films were deposited from solid colorless TMAA at high deposition rates and temperatures below 100 °C (vapor pressure 25 °C of ca. 1 Torr). The Al-N bond in the precursor cleaves easily cleaved, and films with practically no carbon impurities (less than 10-3 %) were obtained.
The pyrolysis pathway of (Me3N)AlH3 is described below - it produces Al films below 100 °C with liberation of H2 and NMe3: (Me3N)AlH3 → Al + NMe3 + 3/2 H2
Surface Reaction Mechanism of (Me3N)AlH3 on Al(111), Al(100) and Si(111)
pMAA Precursor is highly efficient at > 30°C/ 0.5 Torr - it forms Al, H2, NMe3, the grown Al films are highly reflective, without C, N, and O impurities. The activation energy for gas-phase dissociation of the Al-N bond: 28 kcal mol-1, while the activation energy for the elimination of hydrogen from AlH3 is 42 kcal mol-1; the barrier activation energy for the desorption of H2 from a metallic surface is16 kcal mol-1.
Decomposition of (Me3N)2AlH3 on Al surfaces follows the reactions below:
(Me3N)2AlH3(g) → (Me3N)AlH3(ads) + NMe3(g)
(Me3N)AlH3(ads) → AlH3(ads) + 2 NMe3(ads)
AlH3(ads) → Al(ads) + H2(g) + H(ads)
2 H(ads) → H2(g)
The decomposition Mechanism of (Me3N)AlH3 adsorbed on Al(111) and Al(100) surfaces in the cold-wall CVD reactor is following: (Me3N)2AlH3 → (Me3N)AlH3 + NMe3 (Al-N strength: 18 kcal mol-1)
(Me3N)AlH3 → AlH3 + NMe3 (dissociation enthalpy for 2nd NMe3: 26 kcal mol-1)
The decomposition conditions on various subsrtates are:
on Si(100): 200 – 300 °C, growth rate 140 nm/min, pressure 0.075 Torr.
on Cu: < 200 °C, growth rate 1000 nm/min.
In the hot-Wall Reactor, depositions more likely contain a contribution from chemistry occuring in the gas-phase.
AlH3·NMe3 for AlGaN thin films by LPCVD
TMAA has been applied for the growth of AlxGa1−xN epilayers by low pressure (76 Torr) MOCVD. The grown layers exhibited excellent surface morphology and very strong room temperature photoluminescence.
AlH3·NMe3 for AlN thin films by LPCVD
AlN layers grown with TMAA had carbon contamination as low as 1017 cm−3. [349]
Triethylamine alane AlH3·NEt3
Triethylamine alane AlH3·NEt3 (Triethyl Amine Alane = TEAA) is a liquid precursor with vapor pressure 0.5 Torr.
TEAA was reported as promising CVD precursor with growth onset temp 100°C.
The decomposition pathway during CVD deposition of Aluminum Films from Triethyl Amine Alane; (Et3N)AlH3), at 250 °C on Ti are following:
(Et3N)AlH3(g) → (Et3N)AlH3(ads)
(Et3N)AlH3(ads) → AlH3(ads) + NEt3(ads)
Et3N(ads) → Et3N(g)
AlH3(ads) → Al(s) + 3 H(ads)
3 H(ads) → 3/2 H2(g)
Kinetics controlled by the ratio of Al-N bond cleavage as well as the rate on H2 desorption.
AlH3·NMe3 for metal Al thin films by CVD
Pure Al films were deposited from TEAA at high deposition rates and temperatures below 100 °C (vapor pressure 25 °C of ca. 2 Torr). The Al-N bond is easily cleaved and the films did not contain carbon impurities (less than 10-3 %). TEAA, as liquid precursor, is more desirable because it provides a constant, more reproducible molar flow of precursors into the CVD reactor.
AlH3·NMe2Et for AlN thin films by ALD
Dimethylethylamine alane AlH3·NMe2Et in combination with ammonia (NH3) as co-reactant, has been applied for the growth of AlN thin films by atomic layer growth (ALG) process (identical to ALD, with alternating delivery of reactants) on Si(100), Si(111), Al2O3(00.1), and Al2O3(01.2) substrates at 250−450°C. The chemical composition of the films was studied by XPS and Auger spectroscopy. Some carbon and oxygen contamination were detected on the surface, and smaller concentrations of C, O were found in the bulk. In the high-resolution XPS C(1s) data, binding energies for C−H and C−N species were found but no C−Al species were detected. In the N(1s) data, N−O species were not identified but chemically bonded H was present in the films as NH3-x (x = 0−2) species. The composition varied with process conditions, f.e. hydrogen content decreased in AlN films processed above ~327°C. [[iv][PS1] ]
AlH3·NMe2Et for TiAlN thin films by CVD
Dimethylethylamine alane AlH3·NMe2Et in combination with titanium tetrachloride and ammonia has been used for the CVD growth of Ti1-xAlxN film (x= 0.04 - 0.79) at 220- 410°C as Cu diffusion barriers. 20 nm Ti0.76Al0.24N film was better barrier than 50nm CVD-TiN (Cu diffused into Si substrate at 700°C, 30 min vacuum annealing vs. 400°C for TiN). Al content in Ti1-xAlxN films depended linearly on the partial pressure of AlH3·NMe2Et .[350]
AlH3·NMe2Et for Al2O3 thin films by laser CVD
Dimethylethylamine alane [(CH3)2C2H5N·AlH3] (and for comparison trimethylamine alane [(CH3)3N·AlH3] ), mixed with oxygen O2 as co-reactant, was applied as volatile precursor to grow aluminum oxide Al2O3 photonic band-gap structures (PBGs) by laser chemical vapor deposition (LCVD), or laser rapid prototyping (laser-induced direct-write deposition from the gas phase). The grown Al2O3 structures consisted of layers of parallel rods forming a face-centered tetragonal lattice with lattice constants of 66 and 133 micrometers; the preapred structures showed transmission minima centered around 4 terahertz (75 micrometers) and 2 terahertz (150 micrometers), respectively. [[v],[vi],[vii]
[v]Michael C. Wanke, Olaf Lehmann, Kurt Müller, Qingzhe Wen, Michael Stuke
Laser Rapid Prototyping of Photonic Band-Gap Microstructures,
http://www.sciencemag.org z SCIENCE z VOL. 275 z 28 FEBRUARY 1997, p.1284-1286
[vi]O. Lehmann and M. Stuke, Science 270, 1644 (1995) and references therein., )
[vii]O. Lehmann and M. Stuke, Mater. Lett. 21, 131, (1994).
Dimethylethylamine alane (DMEAA) [AlH3·NMe2Et], and for comparison AlMe3, Al(tBu)3, were employed as Al precursors for the growth of (AlGa)Sb and (AlGa)(AsSb) by MOVPE in an industrial multiwafer planetary reactor _(. The thermal activation behavior of the aluminum precursors appeared to be a critical factor, because
the growth is in the kinetically controlled regime. The efficiency of AlH3·NMe2Et, as well as of Al(tBu)3 were dependent on V/III-ratio, what was attributed to prereactions with trimethylantimony. Oxygen and carbon concentrations were determined for
various compositions of (AlGa)Sb; it was found, that in (Al0.19Ga0.81)Sb layer the alane adduct AlH3·NMe2Et results in lower oxygen level (ca. one order of magnitude), compared to Al(tBu3) [[i]]
[i] C. Agert, P. Lanyi, A.W. Bett, J. Cryst. Growth, 2001, Vol.225, Iss.2–4, p.426-430, « MOVPE of GaSb, (AlGa)Sb and (AlGa)(AsSb) in a multiwafer planetary reactor »,https://doi.org/10.1016/S0022-0248(01)00908-3
AlH3·NMe2Et (DMEAAl) (and for comparison AlMe3) was applied as Al precursor for the growth of Al-contaning antimonide laser heterostructures (InGaAsSb/AlGaAsSb) by MOCVD. One of major problems for the growth of antimonide layers by MOVPE (especially Al-contaning waveguides and claddings) is the O and C contamination,
related to high affinity of Al atoms to oxygen and carbon, and difficulty to suppression of the impurities by increasing the V/III ratio (as for antimonides growth the optimal V/III ratio is about unity). The potential solution would be to apply a proper aluminium
precursor (the one containing less or none of carbon atoms bonded to Al atom – f.e. alane-based). The correct choice of Al precursor can improve the properties of the obtained AlGaSb and AlGaAsSb layers. Very good structural quality films were obtained
by using both DMEAAl and AlMe3 precursors, the results suggested a substantial influence of precursors pre-reactions on the epitaxial process. The oxygen contamination measured by SIMS confirmed its dependence on the precursor choice (the lowest O-contamination
was obtained using DMEAAl as precursor). Quaternary InGaAsSb layers were obtained even within the predicted miscibility gap (with As content > 10% values). InGa(As)Sb/AlGa(As)Sb Type-I quantum wells having a fundamental optical transition in the 1.9–2.1
μm range at RT were grown and characterised by PL and photoreflectance spectroscopy; they are potentially applicable for laser devices.[i]
[i] M. Wesołowski, W. Strupiński, M. Motyka, G. Sęk, E. Dumiszewska, P. Caban, A. Jasik, A. Wójcik, K. Pierściński, D. Pierścińska, Opto-Electronics Review, Vol.19, Iss. 2, p.140–144, « Study of MOCVD growth of InGaAsSb/AlGaAsSb/GaSb heterostructures using two different aluminium precursors TMAl and DMEAAl » , doi.org/10.2478/s11772-011-0020-8.