Light Emission Measurements on Diodes

The goal of this assignment was the development of a measurement tool for synchronized electrical and optical analysis of opto-electronic devices. This is beneficial for automated characterization of opto-electronic devices and thus, expand our knowledge on the optical behavior of devices. The communication between two basic instruments is used for this purpose: Keithley 4200 SCS for electrical characterization and a camera from XENICS for optical imaging (visible range: XEVA-257, infra-red range: XEVA-320 InGaAs). These two instruments communicate through semaphores. Measurements were carried out on various devices to confirm the operation of the tool. The optical behavior of ultra-shallow vertical p+n junctions with VBR=7V in avalanche-mode was investigated with the tool. In particular, based on the available knowledge of the processing and current-voltage characteristics of the diodes a certain amount of defects in the n-silicon region were expected. Spatially invariant photon-emission spots from the active area with repeated avalanche-mode biasing indicate that the origin of the spots is in fact connected to the defect-density in the material. The spot-count and hence, total intensity increases with increasing bias, which matches results with prior literature. The number of spots increases in circular diodes with bigger diameters (due to higher active area). The intensity, however, increases for diodes with decreasing diameter, at a given bias. The automation also enables us to record the variation in optical behavior of similar structures on multiple locations of the wafer, which provides greater insight about the level of uncertainty in the fabrication process throughout the wafer. The electro-luminescence of the diodes is also observed in forward bias. The infra-red emission shows a peripheral glow around the diode. Diodes with break-down voltage of 14 V also show a peripheral glow around the diode in the visible range in avalanche-mode. Uniform light emission is obtained in forward bias for the same diode. Series resistance and current components (peripheral and areal) are calculated in order to discuss and support the origin of these observations.