INTERFERENCE MEASUREMENT OF THIN SURFACE LAYERS 1215 



tion, a change in the photoelectric voltage occurs. For more careful 

 measurements one might plot the photovoltage versus the x-coordinate 

 in units of the micrometer. Such a plot allows an accurate location of 

 the junction in these units. If the micrometer is set for this reading, the 

 needle will rest on the p-n junction. 



4. Potential Prohe 



This is another method for locating the junction where the junction 

 is at least slightly rectifying. One needs two contacts to the sample, one 

 on the p-type side and the other on the n-type side. When a current is 

 passed through the sample in the reverse direction, the voltage between 

 the needle and either contact shows a discontinuity at the junction. The 

 voltage can be plotted in a similar way as described for the previous 

 method, and thus the needle can be set on the p-n junction. 



m 



MONOCHROMATIC 

 LIGHT SOURCE 



HALF-SILVERED 

 MICROSCOPE SLIDE 



BEVELED CRYSTAL 



Fig. 5 — Diagrammatic view of the light path in the interferometer. 



