1. Conductive epitaxial ZnO layers by ALD Conductive epitaxial ZnO layers by ALD Zs. Baji, Z. Lábadi, Zs. E. Horváth, I. Bársony Research Centre for Natural Sciences, Institute for Technical physics and Material Sciences P.O.Box. 49. H-1525Budapest, Hungary Corresponding author: Zs. Baji, Email: baji.zsofia@ttk.mta.hu, Tel: +36 1 3922225, Fax: +36 1 392 2226 Acknowledgement The authors wish to thank the Hungarian National Science Fund OTKA (Grant No. NK 73424) for the support Experimental The morphology and structure of the epitaxial layers Conclusions Motivation Atomic layer deposition (ALD) is a self limiting layer growth method which consists of consecutive cycles of saturating surface reactions. The operation principle is based on a pulse like introduction of precursor gases into the vacuum chamber, and their subsequent chemisorption on the heated substrate. Between the precursor pulses, the reactor is purged with an inert gas. The growth occurs monolayer by monolayer, and an epitaxial growth can easily be achieved. ALD deposited ZnO layers are transparent, and even the intrinsic layers have a low resistivity ( ~ 10 -2 Ω cm), which can be further reduced by Al doping. High quality epitaxial n-type doped layers with tunable resistivity are very promising materials for next generation UV light emitting diodes or laser diodes. On the one hand they could also be the active layers, or they could serve as template layers as a good alternative for very expensive ZnO single crystals. The possibility of depositing conductive epitaxial layers with ALD has not been examined yet. Growth conditions Precursors: Oxidant: Process pressure: Substrates: Depositiontemperature: Cycle sequence: Reservoire temperature: Injection time: Purge time: Flow rates: ALD parameters Diethyl zinc and Trimethyl aluminium H 2 O 10-15 hPa GaN 120-300°C (i(DEZn+H2O)+TMAl+H2O)j+i(DEZn+H2O), with i between 9 and 60 24°C 0,1 s 3 s Precursors:150 sccm, H 2 O: 300 sccm The optical and electronic properties of the layers The outstandingly low resistivity is due to a high mobility, which is resulted by the excellent crystalline quality of the layers. The carrier concentration is also very high, the reason for which requires further investigations. Epitaxial ZnO layers can be grown on GaN at temperatures above 270°. The conductivity of the epitaxial layers is between 1 and 2*10 -4 Ωcm. Already 12 pulses of Al 2 O 3 deteriorates the epitaxy, although these are still high quality oriented layers. The doping only reduces the resistivity with 30% According to the AFM micrographs the ZnO layer follows the morphology of the GaN substrates exactly. The film is continuous and uniform already after 5 deposition cycles The high resolution TEM images of the samples show epitaxial layers. The XRD results showed that the undoped layers, and the one with only one Al pulse were epitaxial with (001) orientation. The ones with higher doping concentrations showed no (001) epitaxy. layern (10 20 /cm 3 )ρ (10 -4 Ωcm)μ (cm 2 /Vs) 300° 21 AlOx layers4.31.2121 double layer 12 AlOx4.21.691 double layer 17 AlOx3.71.6106 270° 16 AlOx layers4.61.496 300° 12 AlOx layers6.41.0295 270° intrinsic2.91.8122 intrinsic double layer31.9112 300° intrinsic2.91.6131 Intrinsic and doped samples were deposited at 300°C and 270°. Double layers were also prepared with a 15 nm thick bottom layer deposited at 300 °C. This served as a seed layer, to ensure that a top layer deposited at 210°C grows epitaxially. Layers deposited at 210°C do not grow epitaxially otherwise, but these layers have the best conductivity All the layers have excellent transparency and conductivity.