Thermal balance for measuring transport during vapour phase crystal growth

Journal of Crystal Growth 2 (1968) 113—114 © North-Holland Co.,Amsterdam

LETFER TO THE EDITORS THERMAL BALANCE FOR MEASURING TRANSPORT DURING VAPOUR PHASE CRYSTAL GROWTH D. A. GEARY* and J. M. HOUGH Physics Department, University of Hull, Hull, England Received 28 July 1967; revised manuscript received 4 December 1967

A new method for determining the rate of material transport during vapour phase crystal growth is described.

A need for data on the transport of material during vapour phase crystal growth arose whilst studying the growth of large crystals of cadmium sulphide in a sealed system. The measurement of total mass transported in a given time was considered undesirable since the assumption of a constant growth rate is necessary -~

*

Now at P. 0. Eng. Res. Station, Dollis Hill, London, England.

arm (the support comprised two alumina rods strapped together to provide a cradle for the capsule). If it is assumed that 1) the capsule is of constant cross-section, 2) the faces of the charge and crystal are parallel, and

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3) the faces are perpendicular to the axis of the capsule, then the separation of the faces b will remain constant during the vapour transport. The transport of a quantity of material q will produce a couple of moment qb. Using the quantities indicated in fig. 2 elementary mechanics shows that A’fdL

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and no account of the initial nucleation period can be made. Thus a system of continuous weight monitoring during the transport process was devised employing a chemical balance. Fig. I shows schematically the experimental system used and fig. 2 gives the essential elements. The sealed specimen capsule lies on a horizontal support whose ends are attached to the ends of the extended balance

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114

D. A. GEARY AND J. M. HOUGH

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reduces errors due to 1) the faces not being perpendicular to the axis of the capsule, 2) non-uniformity of the

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cross-section of the capsule, and 3) uncertainty as to

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10 cm and the charge face moves a few mm errors due to the above causes should not exceed 1

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ing the transport of 10 mg of material. For specimen capsules of 1 cm diameter measurements have been

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theThe temperature experimental gradient. apparatus an was experiment, capable of if detectb is made of rates asInlow as 0.04 Fig. 3 shows thetransport mass transport—time curve for g/hr. a sample of cadmium sulphide; the transport of material appears to be a linear function of time. The thermal balance may be employed satisfactorily for kinetics measurements provided accurate temperature control is available; it is also desirable to use continuous automatic recording.

Fig. 3. Mass transport—timecurve for cadmiumsulphide. Charge temperature = 1100 °C; crystal temperature = 1050 °C.

Acknowledgements where Mis the mass required to balance the couple qb. The sensitivity of the balance is dependent on both b and the geometry of the balance; a large value of b

One of us (D.A.G.) thanks the Ministry of Aviation for a research grant. Thanks are due to Mr. B. Lunn for stimulating discussions.