MOCVD MANIFOLD SWITCHING EFFECTS ON GROWTH AND CHARACTERIZATION

The components of metalorganic chemical vapor deposition (MOCVD) supply manifolds have significant effects on the ability to rapidly introduce and exhaust growth materials. This in turn has direct bearing on the achievable heterojunction interface abruptness for a given manifold design. Research to quantify the effects of various manifold components on the switching speed has been performed with a combined modeling and experimental approach. An overview of the experimental investigation is reported. Manifold switching sections were incorporated in a manifold line instrumented with an atmospheric pressure sampling mass spectrometer. A constant flow rate of nitrogen was established in the manifold line to serve as the carrier gas. Into this flow, controlled concentration pulses of argon were introduced from one switching section at a time to model the transport of source gases. A square-wave generator was used to control the on-time of the source for both single and multiple pulse experiments. The results of the single pulse experiments are reported here. The sudden displacement of the valve seats in the air-actuated bellows valves was found to introduce significant pressure and concentration spikes. These spikes demonstrate the need to carefully resolve the effects of pressure and concentration on the measurement technique. For the mass spectrometer system, measurement artifacts occur due to the pressure pulses increasing the rate at which gas enters the sampling orifice. Similar measurement artifacts would be expected in any concentration measurement system which measures number density or which employs a sampling orifice.