4. Measurement and Analysis Techniques
4.1 Transmission Line Model - TLM
Transmission Line Model (TLM) theory was used for accessing the quality of ohmic contacts as well as electrical properties of ITO films used in this study. This technique was proposed by Reeves and Harrison. A detailed analysis is available in their published work [119,120].
A schematic diagram of a semiconductor material with ohmic contact pads prepared for TLM
analysis is shown in Figure 4.1. It can be seen that the sample is first mesa etched usually to a
semi-insulating substrate or to a depth where there is a natural depletion layer such as between
n+ and p+ material. This is done in order to isolate columns of the conductive epitaxial layer
there by restricting current flow within the mesa height, d. Metal pads, of finite width, w, and
length, s, are then deposited on the mesa at a linearly increasing pad spacing, L, such that L1 <
L2 < L3.
Figure 4.1: Schematic diagram of a semiconductor material with ohmic contact pads prepared
for TLM analysis
4.1.1 Theory
Figure 4.2: An example of a plot of total resistance as a function of TLM pad spacing
From Figure 4.1, it is seen that the resistance, r, between two adjacent pads, is given by:
