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MOCVD growth regions

Kuang-Hui Li

February 23, 2017 - Posted in Discussion
There are three growth regions for MOCVD. They are kinetic limit, mass transport limit, and thermodynamic limit, as shown in Fig. 1. The kinetic limit happens at low temperature; the mass transport limit happens at medium temperature; the thermodynamic limit happens at high temperature.

AlN growth region.JPG
Fig. 1 Growth regions of AlN.

The kinetic limit comes from the pyrolysis of the metalorganic compound (MO) and other precursors (Ammonia, Arsine, etc.). Take Aluminum Nitride (AlN) as an example; the precursors are Trimethyl-Aluminum (TMA) and Ammonia (NH3). Several radicals (reactants) are produced after TMA and NH3 are pyrolyzed, e.g. Dimethyl-Aluminum (DMA), Monomethyl-Aluminum (MMA), and amino radicals (NH2, NH). The pyrolysis of the precursors are temperature dependent, and the efficiency of MOCVD process depends on how many percentages of the precursors are pyrolyzed. The growth rate increases rapidly when the precursors start pyrolyzing, and the growth will reach maximum growth rate when 100% of the precursors are pyrolyzed.
The mass transport limit comes from how much precursors are injected into the reactor. If all the precursors are pyrolyzed, the growth rate depends on the concentration of reactants. Intuitively, increase the flow rate of the precursors can increase the growth rate. However, if the flow rate is too high, the precursors will react in gas phase generating plenty of particles (explain below) and reduce the growth rate.  The ratio of precursors also plays a role. For AlN as an example, the ratio of NH3 and TMA is called V/III ratio, and in practice V/III ratio is around 100 for AlN. It means one unit volume of NH3 corresponding to a hundredth unit volume of TMA. Gas phase reaction increases when the temperature is high enough; the precursors gain enough kinetic energy reacting with each other generating particles. These particles are tiny AlN crystal; these particles either fall onto the substrate or be evacuated into exhaust by the pump. In the first case, particles deposit on the substrate induce defects of AlN which reduce the quality of AlN. In the second case, a portion of the precursors become waste. MO is an expensive chemical compound, and it costs like gold with the sample weight. In the industries, the engineer and scientist try to find a suitable temperature, pressure, and V/III ratio as the receipt for mass production.
The thermodynamic limit comes from thermal decomposition. Materials will decompose and desorb from the substrate at low pressure even the temperature haven't reach the melting temperature yet. It also applies to MOCVD process. At high temperature, the growth rate would be zero if thermal decomposition and desorption rate is larger than the growth rate.
Acknowledgment: The data in Figure 1 was extracted from modeling done by VR NE 7.5. VR NE is a commercial software developed by STR, Saint Petersburg, Russia. Commercial software developed by STR are widely used in academic and industrial giant including TNS and Aixtron.     
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