Makarov, Yu.N.

Talalaev, R.A.

Wang, W.N.

Yakovlev, E.V.

Yavich, B.S.

English

Università del Salento: ESE - Salento University Publishing

Modeling and experimental analysis of GaN MOVPE in AIX 200 RF reactorR.A. Talalaev 1) ,  E.V. Yakovlev 1) ,  Yu.N. Makarov 2) ,  B.S. Yavich 3) ,  W.N. Wang 3)1) Soft−Impact Ltd., P.O.Box 83, 194156, St.Petersburg, Russia ,  2) STR GmbH, P.O. Box 1207, D−91002 Erlangen, Germany,  3) Department of Physics, University of Bath, BA2 7AY, Bath, United KingdomIntroductionMOVPE of group III−nitride heterostructures for optoelectronic applications represents the complex processwhose features differ significantly from conventional III−V growth procedure. High deposition temperatures andV/III ratios influence directly flow pattern, heat transfer and transport properties of the carrier gas/precursormixture determining the overall deposition behavior. In this paper, we report on the combined modeling andexperimental study of GaN MOVPE in AIX 200/4 RF−S horizontal reactor with separated supply of group−III andgroup−V precursors.Detailed 3D simulations accounting for all relevant physical processes such as flow dynamics, heat transfer andprecursor mixing and transport are performed in order to evaluate the influence of process parameter variation onthe growth rate and uniformity. Gas−phase chemistry model includes the unimolecular gas−phase decompositionof trimethylgallium. Surface chemistry model [1] covers the whole range of deposition conditions that may beimportant for correct predictions: i.e., low temperature kinetically limited growth (parasitic deposition on reactorwalls), mass transport limited growth at moderate temperatures, and growth limited by gallium desorption atelevated temperatures.Effects of trimethylgallium and hydride flow rates and substitution of ammonia with nitrogen on the growth rateare studied and discussed by comparison of theoretical and experimental results.ExperimentalGaN epitaxial layers were grown on (0001) oriented 2” sapphire rotated substrates, using TMGa and NH3 asprecursors. A quartz plate separated the main reactant flows; as a result, TMGa with H2 carrier gas wereintroduced into the upper stream, and NH3 with H2 or with a mixture of H2 and N2 carrier gases were supplied intothe lower stream. Experimental results were obtained from growth runs that were performed at 200 mbar and 1140or 1160 oC. The growth temperature was measured as a reference from the graphite susceptor bottom. To studygrowth rate variations in a single run by changing the input parameters, in− situ monitoring measurements ofreflectivity were carried out.Results and discussionThe design of the horizontal AIX 200/4 RF−S reactor includes some important features that should be properlyincorporated into the global reactor model. Among these features, one can note the RF heated graphite susceptorconsisting of two disks separated by a thin gap filled with hydrogen that provides the rotation of the uppersusceptor disk and may lead to a temperature drop between the two disks up to 50 °C. Also, there is a coolingchannel around the susceptor for the purpose of localization of heating area. Accurate prediction of the temperaturedistributions over the internal surfaces of the reactor liner (such as side walls and ceiling), heated mainly byradiation from the hot susceptor, is of great importance due to the following reasons. The overall efficiency of thegrowth process is determined by the degree of the utilization of TMGa and the products of its decomposition(primarily monomethylgallium (MMGa)), which, in turn, depends on the intensity of parasitic deposition on theside walls and ceiling. The parasitic deposition intensity is highest on the ceiling and is largely governed by twomain factors: ceiling temperature and transport of trimethylgallium coming from the upper inlet and its mixingwith ammonia and hydrogen supplied through the lower inlet. The presented computational model of the reactorincludes the above geometrical details and physical−chemical mechanisms along with commonly considered flowdynamics, heat and mass transfer, and gas−phase chemical reactions.Typical temperature distributions for the central cross−section of the reactor and over the ceiling are presented inFigs. 1−2. The distributions are given for the nominal deposition temperature 1160 °C, and one can see that under10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.05R.A. Talalaev et al such operating conditions the maximal temperature on the ceiling may be as high as about 700 °C, which meansthat over some part of the ceiling parasitic deposition of GaN is limited by species transport to the ceiling ratherthan by low−temperature kinetics. The resulting distribution of the parasitic deposition rate over the ceiling isdemonstrated in Fig. 3. The deposition intensity has a rather sharp maximum in the region located opposite thesusceptor frontal edge due to relatively high temperatures and favorable transport conditions (TMGa suppliedtrough the upper inlet readily reaches these region) there. The “hole” on the parasitic rate distribution is related tothe presence of the viewport opposite the wafer center, where hydrogen is supplied. Fig. 4 represents the growthrate mapping over the 2” wafer. The computed growth rate non−uniformity of about 25 % is close to theexperimental observations.Several parametric dependences of the growth rate have been computed. The enhancement of growth rate with theTMGa flow (see Fig. 5) is a well−known effect reproduced by the computations, but with some underestimation ofthe growth rate level. Fig. 6 shows that the growth rate decreases with increasing hydride flow supplied throughthe lower inlet (this increase is made by keeping the ammonia flow constant (3 SLM) and elevating the hydrogenflow), which is due to the effect of dilution of group−III species by the surrounding hydrogen/ammonia mixture.It is interesting to consider how the composition of the mixture coming from the lower inlet influences the growthrate. Three series of experimental runs at fixed lower flow (8 SLM) have been performed, and the obtained resultsare given in Fig. 7. At first, the ammonia flow was increased with the corresponding decrease in the hydrogen flowrate. Then, nitrogen was added into the hydride mixture, and experiments were conducted at two constantammonia flows (1.5 and 3 SLM) with gradual substitution of hydrogen with nitrogen. From the data collected, wecan draw two main conclusions: (1) adding nitrogen into the hydrogen/ammonia mixture (at fixed total flow)affects the growth rate in a way very similar to the increase in the ammonia flow; (2) substitution of ammonia withnitrogen has a weak effect on the growth rate. Thus, substitution of hydrogen coming from the lower inlet withboth ammonia and nitrogen has a practically the same effect on the growth rate. The computations performed forthe ammonia flow of 3 SLM and varied H2/N2 ratio (see Fig. 7) fit well the experimental data. The growth ratedrops by a factor of four as the nitrogen flow is raised from zero to 4.5 SLM, and this effect may be attributed togenerally slower, compared to hydrogen, diffusion of TMGa and, consequently, MMGa in the densernitrogen−containing mixture, as well as to delayed heating of the precursor mixing zone.ConclusionsWe have performed a combined experimental and modeling analysis of GaN MOVPE in the horizontal AIX 200/4RF−S reactor. A computational model of the reactor, including the most important details of the reactor design andaccounting for the physical−chemical mechanisms governing the growth process, in particular, low and moderatetemperature parasitic deposition on the reactor walls, has been developed. It has been demonstrated that theparasitic deposition rate is highest on the ceiling due to the specificities of the heat transfer and mass transport inthe reactor. The model verification has been performed using experimental data on the GaN growth rate underdifferent operating conditions. A reasonable reproduction of the measured dependencies of the growth rate on theTMGa and hydride flow has been obtained without any fitting. In addition, it has been shown experimentally thatthe substitution of the hydrogen coming from the lower inlet with both nitrogen and ammonia has a similar effecton the growth rate that decreases as the hydride mixture composition is changed in favor of ammonia or nitrogen.References[1] R.A. Talalaev, E.V. Yakovlev, S.Yu. Karpov, Yu.N. Makarov, Journal of Crystal Growth, 230 (2001) 232.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.05R.A. Talalaev et al 10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.05R.A. Talalaev et al 10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.VI.05R.A. Talalaev et al 

Modeling and experimental analysis of GaN MOVPE in AIX 200 RF reactor

ESE - Salento University Publishing

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Modeling and experimental analysis of GaN MOVPE in AIX 200 RF reactor

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