Gas Flow Patterns in Horizontal Epitaxial Reactor Cells Observed by Interference Holography

Abstract

Interference holography is used to visualize gas flow patterns and temperature profiles in epitaxial systems. It is demonstrated that in water‐cooled horizontal reactor cells the carrier gases and He give dynamically stable laminar flow profiles throughout the reactor. There is no indication of a stagnant or boundary layer for flow velocities up to 80 cm/sec in this type of cell. In air‐cooled cells, and He also give stable laminar flow profiles, but beyond velocities of 40 cm/sec a cold gas finger appears in these flows due to undeveloped flow and temperature profiles. In contrast to the stable flow characteristics of and He, the flows of and Ar always are unstable due to convective motions. Besides this intrinsic instability, these flows are accompanied by severe entrance effects (especially undeveloped flow profiles), which dominate the flows for flow rates higher than 4 cm/sec. This is observed for both air‐ and water‐cooled cells. Another phenomenom which is observed for and Ar is that beyond 4 cm/sec the convective gas breaks up into a thin (8 mm) laminar layer close to the susceptor across which the entire temperature gradient is present and a highly turbulent/convective part above this laminar layer. Analysis shows that for Ar and about eight times longer entrance lengths in the reactor cell are needed to achieve fully developed velocity and temperature profiles as compared with and He. This explains the dominant influence of this effect on the flows of Ar and . When the influence of the entrance effect on the profiles is taken into account, all the observed flow patterns and temperature gradients are in agreement with the theoretical flows which can be predicted on basis of the respective Reynolds and Rayleigh (or Grashof) numbers.