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EXAMPLE 3

Influence of the reactor inlet configuration on the AlGaN growth efficiency

AlGaN MOCVD

Figure 1 Conventional and inverted inlet configurations

In this example we will discuss the results of a combined modeling and experimental analysis of AlGaN deposition in the horizontal two-flow AIX 200/4 RF-S reactor. The purpose of this study was to examine conventional and inverted supply of the precursors into the reactor with respect to the growth reproducibility and efficiency of the aluminum (Al) incorporation.

Effective growth of AlGaN layers with high Al content still remains a critical issue. Difficulties with Al incorporation are related to parasitic gas-phase reactions and formation of nano-size particles in the reactor volume. The reduction of particle formation intensity and enhancement of the Al incorporation can be achieved via lowering of the reactor pressure, decrease of the gas residence time, reduction of the TMAl flow rate, and lowering V/III ratios. The intensity of parasitic chemical processes and the range of the optimal operating conditions also depend on the reactor design and schematics of the precursor supply. In this work, we investigated the growth of AlGaN layers at relatively low pressure of 50 mbar for both conventional and inverted supply of the precursors, Figure 1.

Modeling approach

The following features of CVDSim software package were employed to build efficient and predictive model of MOVPE processes:

  • 2D/3D reactor models including flow dynamics and detailed heat transfer calculations;
  • multicomponent diffusion and thermodiffusion;
  • gas-phase chemistry model;
  • gas-phase nucleation, growth and transport of the AlN nanoparticles;
  • themophoretic effect on nanoparticle transport.

We assumed that the gas-phase reaction has a multi-step mechanism. Nameley, the reaction between TMAl and ammonia produces TMAl:NH3 adduct that produces DMAl:NH2 via methane elimination reaction and interactions with ammonia. Subsequently formed (DMAl:NH2)2 and (DMAl:NH2)3 species may produce AlN in the gas phase, initiating AlN particle nucleation. Further growth of the solid particles takes place at the expense of interactions between the AlN nuclei and Alcontaining species such as AlN, DMAlNH2, and [DMAlNH2]2. Another way of Al losses is the formation of oligomers ((DMAl:NH2)n, n≥3) that do not contribute to the layer deposition, Figure 2.


Figure 2 Schematic representation of the gas-phase reaction mechanisms.


Results

For both conventional and inverted inlets, modeling results agreed well with the experiment, see Fig. 2. Increase of the Al content in the gas-phase (the ratio QTMAl/(QTMAl+QTMGa)) from 9% to 55% was achieved via gradual increase of the TMAl flow rate and simultaneous decrease of the TMGa flow. The higher contents (from 55 to 79.5%) corresponds to an increase of the TMAl flow rate only, keeping the TMGa flow rate constant. Both inlet configurations allow the growth of AlGaN with Al content up to about 70% and exhibit a non-linear dependence of the Al incorporation on the Al gas-phase content: Al incorporation efficiency is fairly high up to the Al content of about 60%, after which Al losses rapidly intensify. So, to get the Al composition of about 70%, the total flow in the reactor was increased up to 10 slm.


Figure 3 AlGaN solid-vapor relationship for the conventional (left) and inverted (right) inlet configurations. Symbols indicate experimental results, solid lines are for the modeling predictions. Blue dashed lines correspond to the results of computations performed for the constant total flow of 7.5 slm when the Al gas-phase composition is raised from 55% to 79.5%. Dashed line shows for reference the ideal (no aluminum losses) solid-vapor relationship.


To find out which Al content could be achieved if the TMAl flow was raised without increasing total flow in the reactor, a series of computations has been done with varying only the TMAl flow rate. As one can see in Fig 3, when the total flow is kept constant at 7.5 slm, the Al content in the growing layer reaches saturation point at some TMAl flow rate and eventually stars to decrease. This behavior is related to intensification of particle formation and depletion of the gaseous mixture with Al in flow direction. The effect is somewhat stronger for the inverted supply of the precursors: the Al percentage corresponding to the highest Al gas-phase composition of 79.5% is even lower than that at the Al content in the gas phase of 55%.

A promising way to get higher Al contents is to increase the total flow in the reactor in order to suppress the formation of particles by lowering of the Al-containing species partial pressures and the residence times. We have performed computations for the highest Al gasphase composition of 79.5% and varied total flow rate. At the total flow of 11 slm, the Al content in the solid is practically the same as the Al gas-phase composition for the conventional inlet, but still remains somewhat lower for the inverted one, See Fig. 4. For both inlet configurations, the Al incorporation efficiency increases significantly with the total flow due to gradual suppression of particle formation, Fig. 5.


Figure 4 AlGaN layer composition as a function of the total flow for Al content in the gas phase of 79.5%. susceptor


Figure 5 Particle density distributions for the inverted inlet at three different total flow rates


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