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Growth conditions and surface morphology of AlN MOVPE

Here we give a brief summary of combined experimental and modeling studies of AlN Metal-Organic Vapor Phase Epitaxy (MOVPE) aimed at the analysis of surface morphology variation with growth conditions.

Modeling included detailed 3D calculations of fluid dynamics, heat transfer and species transport, gas-phase and surface chemistry, as well as nucleation, growth and transport of AlN nanoparticles. The AlN MOVPE model was previously verified on the data obtained in various reactors [1,2]. Gas-phase reaction mechanisms are shown schematically in Figure 1.

Figure 1 Schematic representation of the gas-phase reaction mechanisms.


Figure 2 AIX200RF LP-MOVPE reactor

Experimental part of the work was conducted by S. B. Thapa and F. Scholz, Institute of Optoelectronics of University Ulm, Germany. About 500 nm thick AlN layers were grown on c-sapphire substrates in a AIX200RF LP-MOVPE system using a separate supply of TMAl and ammonia precursors and a N2/H2 mixture as carrier gas, Figure 2. The bulk AlN layers were grown on thin low-temperature AlN nucleation layers. Samples were grown at V/III ratios of 500-4000, substrate temperatures of 1100-1195 oC, total flow rates of 2.0-4 slm, ammonia flow rates of 150-2000 sccm, and N2/H2 ratios in the carrier gas of 0.4-3. The chamber pressure was fixed at 35 mbar.


Figure 3 Growth terrace width d and diffusion length λ

Although the adverse effect of parasitic reactions accompanied by formation of AlN nanoparticles in the gas phase is well known, its exact mechanisms are still unclear. According to numerous experimental data, the surface is getting rougher at enhanced parasitic reactions in the gas-phase, i.e. at high pressure, long residence time, high V-group and III-group precursor flows. Ammonia flow, growth rate, and temperature are also assumed to affect the surface morphology via the changes of the surface diffusion length λ. While flat surfaces and step-flow growth mode are observed under conditions with λ larger than the terrace width d, 2D growth mode with nucleation on flat terraces or even 3D growth mode with rough surface morphology will appear at λ < d, Figure 3.

We investigated variations of the AlN growth rate and surface morphology with TMAl flow, total flow rate, and N2/H2 ratio. Measurements were made in the center of a half of a 2 wafer placed at the front edge of the susceptor . As expected, the AlN growth rate increased with the TMAl flow rate, Figure 4. Here, the ammonia flow rate was kept constant, while V/III ratio was reduced from 4000 to 500. In was also observed that the surface roughness increased with with TMAl flow rate, see Figure 5. Samples grown at high V/III ratio had 2 to 3 times lower pit density than those grown at low V/III ratios. At low V/III ratios, pits merging led to degradation of the surface quality and 3D growth.

Figure 4 AlN growth rate variation with TMAl flow rate

Figure 5 Rms roughness variation with TMAl flow rate


Figure 6 Effect of the wafer position

Similar degradation of the surface quality was observed when the wafer was shifted downstream even at comparatively low growth rates. According to the computations, the particle density near the surface is quite low since thermophoretic forces push the particles away from the susceptor. Thus, morphology degradation due to a direct interaction of particles with the surface seems unlikely. In accordance with our model, we can estimate the contribution of three main species DMAl:NH2, (DMAl:NH2)2, and AlN to the growth rate under typical MOVPE conditions. As we have found above, the increase of surface roughness correlates with the increase of the JAlN/Jtotal ratio. We assume that adsorbed AlN species may nucleate and form small clusters on the surface, especially, on the defect places such as threading dislocations, resulting in transition from step-flow to 2D or even 3D growth mode.

An increase of the growth temperature from 1100oC to 1190oC resulted in a significant roughness reduction from 21 nm to 0.2 nm at a constant growth rate and V/III ratio of 1000. Increase in diffusion length at high temperatures appears to be stronger than the effect of JAlN/Jtotal ratio, hence, AlN clusters were not forming.

The dependence of the growth rate on the total flow rate is shown in Figure 7. Here, the total flow rate is raised at a constant V/III ratio by simultaneously increasing flow rates of ammonia and TMAl. According to our computations, an enhancement of parasitic reactions is overcompensated by the shorter residence time at higher total flow rates. The growth rate on the wafer decreases due to the change of transport conditions and overall downstream shifting of the growth rate profile. Note that for this experimental series, the surface roughness remained almost the same. The computed JAlN/Jtotal ratio decreases with higher total flow rates. We suggest that negative an influence of the ammonia flow rate that should increase the roughness is compensated by the reduction of the amount of AlN clusters on the surface.

Figure 7 AlN growth rate variation with total flow rate

Figure 8 Growth rate variation with N2/H2 ratio

Higher growth rates at a lower N2/H2 ratios (Figure 8) are caused by higher diffusivity of the Al-containing species in hydrogen atmosphere. Here, the surface roughness remains nearly the same most likely due to compensation of two counteracting effects: lower true substrate temperature at high N2/H2 ratios and reduced growth rate at low N2/H2 ratios.

[1] A. V. Lobanova, K.M. Mazaev, R.A. Talalaev, M. Leys, S.Boeykens, K.Cheng, S. Degroote, J. of Crystal Growth 287 (2006) 601

[2] E.V. Yakovlev, R.A. Talalaev, N. Kaluza, H. Hardtdegen, H.L. Bay, J. of Crystal Growth 298 (2007) 413

[3] Growth conditions and surface morphology of AlN MOVPE,
A.V. Lobanova, E.V. Yakovlev, R.A Talalaev, S.B. Thapa and F. Scholz
Journal of Crystal Growth, Volume 310, Issue 23, 15 November 2008, Pages 4935-4938, The Fourteenth International conference on Metalorganic Vapor Phase Epitax, The 14th International conference on Metalorganic Vapor Phase Epitax


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