Atomic layer deposition (ALD) is a special case of chemical vapour deposition (CVD) in which the total deposition reaction is split in two or more self-limiting reactions in a sequential way [1]. A special feature of ALD is that the reactant molecules can only react with a limited number of reaction sites at the surface and that the reaction stops once all the reaction sites have reacted. The remaining reactant molecules will not react and will just be pumped from the ALD reactor. Consequently, ALD enables precise and subtle material engineering and the film thickness can be controlled by the number of ALD cycles. Atomic Layer Deposition (ALD) was originally termed atomic layer epitaxy (ALE). The best studied ALD chemistry is the deposition of aluminium oxide using trimethyl aluminium [TMA, Al(CH3)3] and water as shown in Figure 1. In steady state conditions, the c-Si surface is covered with hydroxyl (OH) groups. The TMA molecule reacts with the hydroxyl groups by splitting off one methyl (CH3) group forming gaseous methane (CH4). Once all hydroxyl groups have reacted, the reaction stops as the TMA molecule cannot react with methyl groups. Subsequently, the remaining TMA molecules, reaction products, and inert carrier gas is pumped from the system. The surface is then exposed to water (H2O) molecules which react with the methyl groups on the surface and form gaseous methane. When all methyl groups have reacted, the surface is covered with hydroxyl groups again and the remaining water, reaction products, and inert carrier gas is pumped from the ALD system. Ideally, one atomic layer is grown per ALD cycle.
Fig.1 A typical ALD cycle showing how Al2O3 is deposited from TMA and water.
There are types of ALD process: thermal ALD and plasma or radical-enhanced ALD. The main benefit of plasma- or radical-assisted ALD is that their significantly higher reactivity which allows for processes to take place at significantly lower temperatures or allows for the growth of materials which are not feasible using only CVD processes. Examples of that are pure metals and some semiconductor materials which need the high reactivity of the radicals (e.g. oxygen or hydrogen) to reduce the parent molecules. ALD processes that can be done using only reactant molecules are referred to as thermal ALD processes.
ALD has only recently been introduced in the silicon photovoltaic industry. This was mainly due to the developments in ALD Al2O3 for surface passivation of silicon solar cells [2, 3] as ALD films can yield a better performance for significantly thinner films which is beneficial from a cost point of view. More recently, ALD is also intensively investigated for the deposition of carrier-selective contacts as in this case typically <10 nm films are required with a high conformality. For example, titanium oxide (TiOx) has shown an outstanding performance as surface passivation film [4] and electron selective contact for silicon solar cells [5]. Hence, ALD could become an even more important technology for silicon solar cell production.
An animation of the atomic layer deposition process at UNSW Sydney is shown below.
[1] George, S.M., Chemical Reviews 110, 111-131 (2010)
[2] Hoex, B., J. Schmidt, P. Pohl, M.C.M. van de Sanden, and W.M.M. Kessels, Journal of Applied Physics 104, 12 (2008)
[3] Dingemans, G. and W.M.M. Kessels, Journal of Vacuum Science & Technology A 30, 40802 (2012)
[4] Liao, B.C., B. Hoex, A.G. Aberle, D.Z. Chi, and C.S. Bhatia, Applied Physics Letters 104, 253903 (2014)
[5] Yang, X.B., Q.Y. Bi, H. Ali, K. Davis, W.V. Schoenfeld, and K. Weber, Advanced Materials 28, 5891 (2016)