Electrical Characterization of Operating PN Junction with

Conductance Mapping and STM


High spatial resolution characterization techniques capable of probing dopant profile, electrostatic potential or carrier charge density, are needed as enabling technologies for the continued increase in density and decrease of size of semiconductor devices.   The highest resolution probe available is scanning tunneling microscopy (STM), which we have previously [1] used to delineate a depletion zone related feature within a Si pn junction.   The coupling of topographical and electronic information in STM measurements complicates interpretation of the images.   Recently, we have implemented the instrumentation necessary for

Fig. 1.  Schematic of instrumentation for conductance/topography mapping.

simultaneous measurement of topography and conductance, using an STM.  Our measured conductance maps from oxide-terminated pn junctions allow delineation of different regions within the junction [2].  The interpretation of these maps is that large conductance corresponds to a tip-gap -semiconductor junction electrically in either accumulation or strong inversion, and smaller conductance to a junction in weak inversion or depletion.  This in turn identifies the type and concentration of majority charge carriers in the area probed by the STM tip.  Maps acquired while ramping of the applied pn-junction bias confirm our earlier interpretation of the electronic nature of “dips” in the STM images [1], i.e. that they are regions of inverted carrier charge: hole dominated in spite of the n-type doping due to their location within the depletion zone.


Fig. 2.  Topography map (left) and conductance map (right) of oxide covered Si device consisting

 of p+ “stripe” within n-type Si substrate, acquired while ramping the pn reverse bias back and

 forth from 0 to 10 volts. p+ stripe is the broad region right of center.  Depletion zone features

are the wedge-shaped regions.

[1] M. L. Hildner, R. J. Phaneuf and E. D. Williams, Appl. Phys. Lett. 72, 3314 (1998).

[2] J. Y. Park, R. J. Phaneuf and E. D. Williams, submitted to J. Appl. Phys. (2001)

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