Apparatus for molecular beam deposition sequentially on a plurality of substrates is described. The apparatus includes a growth chamber and an auxiliary (sample-exchange) chamber coupled by an air-lock. The substrates are carried by a rod which can be translated via a bellows mechanism between the two chambers. The auxiliary chamber includes a port which permits access to the samples so that the entire rod-bellows mechanism need not be removed in order to change samples. The auxiliary chamber also includes means for maintaining therein an inert atmosphere at a pressure in excess of atmospheric pressure especially when the port is open. The growth chamber includes a cylindrical liquid nitrogen (LN.sub.2) shroud which has an aperture in its wall to admit molecular beams to only a heated (growth) substrate. The unheated (idle) substrates are thus shaded from the beams. In addition, the shroud surrounds both the growth substrate and idle substrates in the growth chamber. This configuration of the shroud reduces the likelihood of contamination of idle substrates. In addition, the growth chamber includes means for selectively heating the growth substrate, the idle substrates remaining unheated so as to reduce the evaporation of high vapor pressure elements therefrom.Another aspect of the invention is the provision of uniquely designed pyrolytic BN effusion cells for generating the various molecular beams.