Resistivity Measurement of Semiconducting Epitaxial Layers by the Reflection of a Hyperfrequency Electromagnetic Wave

The aim of this work is to measure nondestructively the resistivity of a semiconductor in the form of an epitaxial layer. The method involves the measurements of the attenuation suffered by an electromagnetic wave reflected by a semiconducting surface. A mathematical discussion leads in the first instance to the establishment of a theoretical curve of attenuation vs. resistivity for homogeneous samples and later, to the family of curves of attenuation vs. resistivity for various thicknesses of epitaxial layer grown on highly doped substrate. The apparatus consists of a 70-Gc/s microwave bridge. The measurement is made by comparing the amplitude of the wave reflected by the semiconductor with that reflected by a short circuit. The reflection of the wave takes place at the end of the waveguide thus avoiding the necessity of cutting the sample for its insertion into the waveguide. The experimental results are obtained 1) by using a large number of samples of knowni resistivity and with readings of attenuation accurate down to the hundredth of a decibel, and 2) by the repeated reading on each sample; the small scatter on these readings indicates a high degree of reading precision. The theoretical curves are then compared with the experimental results. A discussion follows on possible sources of errors and the precautions taken to avoid them.