226 of a controller or amplifier 230 which provides an output signal at an output terminal 232 the latter output signal becomes an input signal to the source power supply 240 via lead 234 and input terminal 236 thus if the vapor stream within the vapor deposition zone 13 v is temporally stable the mass build up or growth of the layer 13 f will proceed at a constant rate the rate monitor 220 will provide a constant signal at output terminal 222 and the source power supply 240 will provide a constant current to the heating elements 135 of the source 134 via the leads 245 and 247 thereby maintaining the temporally stable vapor stream within the deposition zone under stable vapor deposition conditions i e conditions of a constant deposition rate a desired final thickness of an organic hole transporting layer 13 see fig1 is achieved on the structure and on the crystal mass sensor 200 during a fixed deposition duration at which time the vapor deposition is terminated by terminating the heating of the source 134 or by positioning a shutter not shown over the source while a relatively simple source 134 is shown in fig4 for illustrative purposes it will be appreciated that numerous other source configurations can be effectively used.