Atomic Layer Epitaxy: modeling Of Growth Parameters for Device Quality GaAs
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ATOMIC LAYER EPITAXY: MODELING OF GROWTH PARAMETERS FOR DEVICE QUALITY GaAs.
E. Colas, R. Bhat, and G. C. Nihous* BELLCORE, Red Bank, NJ 07701-7040 (*): P.I.C.H.T.R., Honolulu, Hawaii 96826 ABSTRACT Device quality GaAs was grown in a conventional Organometallic Chemical Vapor Deposition (OMCVD) reactor, using sequential group Ill and V reactant gas exposures typical of Atomic Layer Epitaxy (ALE). The importance of gas phase concentration transients during the ALE cycles was revealed by systematic investigations of the effect of the sequences used, for the cycles, on impurity incorporation as well as on the growth rates. In this study, we attempt to quantify the effects of such transients by solving the diffusion equation for the reactant gases, with initial conditions specific to ALE. We used this model to calculate the time dependence of the reactant gas concentration at the growing surface. This quantitative study gives us new insights into the ALE technique and confirms that the V/Il ratio at the substrate surface can be controlled by the choice of the gas sequence. INTRODUCTION Atomic Layer Epitaxy (ALE) has received considerable attention in recent years as a novel approach to crystal growth 1,2. In ALE, group III and V sources are sent in cycles of sequential pulses over the substrate, often separated by halt or purge steps. In a certain temperature regime, dependent upon the particular source used, this results in self-limitation of the growth rate to one monolayer (ML) per ALE cycle 2.Moreover, the layer by layer approach of ALE results in films of excellent morphologies, which is a unique advantage for a low temperature epitaxial growth technique. Fig. 1: Sketch of flow rate sequence in an
IATIE
ALE cycle.
1 H1
F
I
T
Sketch of the Ha
Pur
reactor geometry
TM 2.51 Cm
1.0cm
.4Cm
............... . .1...........
30 on 120C
~i~Z SUSCEPTrOR 1astd ZomtF CON) 16.0cam
Mat. Res. Soc. Symp. Proc. Vol. 145. 01989 Materials Research Society
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S
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Recently, we have been successful in obtaining device quality ALE-grown GaAs using a conventional Organometallic Chemical Vapor Deposition (OMCVD) reactor with trimethylgallium (TMG) and arsine (AsH 3 ) as group IlI and V sources respectively 3. This was obtained by minimizing impurity -in particular carbon- incorporation in the material by an adequate choice of the sequence used for the ALE cycles 3.Introducing a halt of the order of
one second between the exposures of group III and V sources resulted in a dramatic decrease of several orders of magnitude in carbon incorporation down from 1018 cm-3 levels. It was further found that, when growth is not ML-limited (e.g. at 550*C), introducing a halt resulted in increased growth rates 3. The interpretation of these results relied on qualitative statements concerning the effective TMG concentration at the surface 3 and the resulting V/Ill ratio upon AsH 3 exposure. In this work, we quantify the time dependence of the TMG concentration at the substrate during the halt step of the sequence, by solving the dif
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