ELECTRON MIKKOK MICROSCOPY 



ror type image prevails, therefore, despite pictures" of thin films are obtained by pass- 

 positive applied bias potentials. This is the ing an electric current across the specimen, 

 case around the electrical center because a thus creating in the direction of the current 

 voltage drop occurs across, the thickness of flow, on the specimen and in front of it, a 

 the selenium film caused by a small percent- potential pattern related to the conductivity 

 age of the electrons impinging there. It is also pattern. A high potential gradient in the di- 

 the case outside this area because there the rection of the current flow will correspond to 

 normal component of the velocity of the elec- a low conductivity and a low potential gradi- 

 tron is not sufficient to cause electron im- ent to a high conductivity. Where the gradi- 

 pingcment on the mirror-specimen. The inner ent of the potential changes, i.e., where the 

 area, increasing in diameter with increasing area conductivity changes, areas of convex 

 positive bias potential, is filled with random, or concave cuivature of the equipotentials 

 noiselike fluctuations which are more pro- on the specimen and in front of it will be 

 nounced in the center of that area than near transformed by the particular kind of image 

 the border (see Fig. 3). formation of electron mirror microscopy 



A single frame from the motion picture into dark or bright areas on the viewing 

 cannot, of course, adequately portray the screen. Areas of lower conductivity will 

 movements observable on the screen. For therefore appear as areas with dark borders 

 example, one has to imagine the granular on the one side and bright border areas on 

 structure close to the center area in constant the opposite side. Because of this configura- 

 random, noise-like fluctuations. At a certain tion of dark and bright areas, conductivity 

 positive applied bias potential a pronounced pictures appear as elevated areas, corre- 

 wave-like motion begins which originates sponding to areas of higher conductivity, il- 

 most often in parts of the white-rimmed luminated by a light source from the side 

 border (see upper part of Fig. 3), and moves where the negative voltage is applied. By 

 in a general direction toward the center of taking two consecutive electron mirror 

 the area. Long before reaching this center, micrographs distinguished solely by the 

 the intensity of the waves diminishes and amount of current passing through the speci- 

 they disappear in random fluctuations. The men, it is even possible in certain cases to 

 velocity of the wave-hke motion described obtain pairs of stereomicrographs which are 

 could be changed from practically zero to a three-dimensional pictorial representations 

 few centimeters per second on the screen, of the electrical conductivity distribution in 

 corresponding, at an average magnification thin films (3). With electrical current ap- 

 of about 50 X, to velocities from zero to plied, potential gradient distributions across 

 about one millimeter per second on the speci- p-n junctions (6) or low angle grain bound- 

 men. At present, neither the random fluctua- aries or dislocation lines in semiconductors 

 tions nor the waves moving toward the elec- can also be obser\'ed by electron mirror mi- 

 trical center are fully understood. There is scropy. Figure 4 shows as an example an 

 no doubt, however, that these phenomena electron mirror stereomicrograph pair of the 

 are of electrical origin. That means electron electrical potential gradient across a low 

 mirror microscopy reveals here for direct angle grain boundarj^ in germanium, 

 visual observation the movements of elec- Finally, there is the broad field of magne- 

 trical charges. tism which may profit considerably from the 



Electron mirror microscopy is also capa- potentialities inherent in electron mirror mi- 



ble of depicting the conductivity distribution scropy. Whenever the task arises to observe 



in thin films of semiconductors deposited on visually the distribution of magnetic field 



insulating substrates. Such "conductivity intensity above a surface, electron mirror 



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