Effects of natural and forced convection in vapor phase growth systems

Abstract Convective temperature oscillations (CTO) introduce growth striae and other inhomogeneities in melt grown crystals, whereas a literature review suggests that the cause of such defects in materials grown from the vapor phase by heterogeneous chemical reaction, either in closed tubes or in flow-through systems, is not satisfactorily understood. Although CTO occur in room temperature gases at values of Rayleigh number ( R ) greater than about 6 × 10 3 , it was not clear whether CTO occur in practical open tube chemical vapor deposition systems at elevated temperature, because numerical calculations indicated that at constant pressure the R of gases depends on temperature as T -n , with n ≈ 4.5. Measurements carried out in the present work in flowing systems similar to those used in open tube chemical vapor deposition have shown that, in general agreement with hydrodynamic predictions, CTO with amplitude up to 20 °C and frequency up to 0.25 cycles/sec occur both in vertical and horizontal apparatus with cold walls, but not in apparatus with hot walls. Hydrogen and helium are found experimentally to be very much more stable than argon or nitrogen under equivalent conditions, also in agreement with Rayleigh number calculations. The observed superiority from a hydrodynamic stability viewpoint, of hydrogen compared to helium, is an important anomaly, since Rayleigh number calculations indicate helium should be slightly more stable than hydrogen over the range from room temperature to 1200 °K.