Conductive epitaxial ZnO layers by ALDConductive 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 SciencesP.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

AcknowledgementThe 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 (~10-2Ωcm), 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 conditionsPrecursors:Oxidant:Process pressure:Substrates:Depositiontemperature:Cycle sequence:Reservoire temperature:Injection time:Purge time:Flow rates:

ALD parametersDiethyl zinc and Trimethyl aluminiumH2O10-15 hPaGaN120-300°C(i(DEZn+H2O)+TMAl+H2O)j+i(DEZn+H2O), with i between 9 and 6024°C0,1 s3 sPrecursors:150 sccm, H2O: 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 Al2O3 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.

layer n (1020/cm3) ρ (10-4Ωcm) μ (cm2/Vs)

300° 21 AlOx layers 4.3 1.2 121

double layer 12 AlOx 4.2 1.6 91

double layer 17 AlOx 3.7 1.6 106

270° 16 AlOx layers 4.6 1.4 96

300° 12 AlOx layers 6.4 1.02 95

270° intrinsic 2.9 1.8 122

intrinsic double layer 3 1.9 112

300° intrinsic 2.9 1.6 131

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.

30,0 30,5 31,0 31,5 32,0 32,5 33,0 33,5 34,0 34,5 35,0 35,5 36,0 36,5 37,0 37,5 38,01

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intrinsic double layer 270° intrinsic 300° with one AlOx 270°16 AlOx layers 300° 12 AlOx layers 300°15 AlOx layers 300° 21 AlOx layers double layer 17 AlOx double layer12 AlOx

(100) (001) (101)