Method for visualizing the distribution of current in alloyed germanium p +-n junctions

Abstract

In order to study the origins of inferior reverse characteristics of alloyed junctions, a method for visualizing the distribution of current in the junctions has been developed. In principle, this method consists of the following processes: the sample is immersed in an electrolytic solution and a voltage is applied between the sample and a counter electrode; an electrochemical change occurs on the surface of the sample corresponding to the value of current which flows through the junction; the patterns resulting from the electrochemical change are observed under the microscope. In the case of p +-n-junctions prepared by alloying indium into n-type germanium wafer, a copper plating method is most satisfactory, in which copper is deposited in correspondence with the current distribution of the junction. In order to get definite correspondence of the copper deposition pattern with the distribution of junction current, it is important to remove the potential barriers existing between plating solution and the surface of the sample. For this purpose, care must be taken with respect to the components of plating solution and to the method of application of plating voltage. Application of d.c. plating voltage is only effective for the samples having special voltage-current characteristics, whereas the pulse method, in which a short pulse voltage is applied to remove the potential barrier, and subsequently a small current is given to provide copper deposition, gives good results. This method can be utilized not only to detect defective structure in the junction which brings about inferior characteristics, but also to observe the distribution of junction current as a function of applied plating voltage, by the repetition of plating under several values of voltage, which leads to bringing out of non-uniform localized breakdown spots.