Abstract
Efficient electric-luminescence (EL) is the major purpose and function of light emitting devices. Much engineering effort and technical breakthrough lie in the developing of new materials and structures for LEDs. Therefore, mapping the EL with high spatial resolution from the LEDs is an important method in characterizing various properties of the LEDs, for instance, in failure analysis. In this study, we are reporting a new development in EL mapping that allows functional imaging in probing the dynamical behavior of the LEDs. Traditionally, the response time of a semiconductor device is measured as the whole response of the device.
principle
The principle of OBIC is simple: a tightly focused pulsed laser light is shone on the surface of a photosensitive semiconductor device. The relative position of the illumination and the specimen is changed in a raster scanning fashion. The signal, corresponding to a given spatial location, is obtained via the electrical contact and a bias voltage applied on the device.
When the specimen is illuminated by continuous wave (CW), light electron–hole pairs form. These carriers can be detected by applying an appropriate bias voltage on the specimen in order to drive the generated carriers out. When, instead of CW light, a pulsed laser is used one can observe transient states of the semiconductor device. Pulsed laser light in this sense merely acts as illumination of extremely high modulation frequency. Optical illumination means that carrier injection occurs via optical excitation so no additional physical contacts are needed. Because we use a spatially strongly confined beam to excite the sample, RF OBIC offers high spatial resolution at exceptionally wide bandwidth.
 Electric-luminescence diagram
Experimental setup
A commercially available laser scanning module (FV-300, Olympus) is adapted for the EL mapping. The confocal mapping optics of the FV-300 allows point-to-point characterization of the device. The time-resolved measurement with frequency domain method is achieved with the use of a RF lock-in amplifier (SR844, Stanford Research). Its working range is from 25 KHz to 200 MHz. To synchronize the phase sensitive lock-in loop (with millisecond response time) with the rather rapid galvano-mirror based scanning imaging mechanism, special modification is made to the control
module of the scanning microscope to enable external trigger
 Electric-luminescence configuration
Application
The green and red image are the EL signal output from Lock-in Amplifier. The color coded by Olympus fluoview software.
 This poster shows the effectiveness of time-resolved measurements in monitoring the electroluminescence signal from the LEDs. The observed time-delay is likely caused by the finite drift velocity of the carriers within the device, which levies the ultimate limits on the response of the devices.
This study shows the effectiveness of time-resolved measurements in monitoring the electroluminescence and OBIC signal from the LEDs. The observed time-delay is likely caused by the finite drift velocity of the carriers within the device, which levies the ultimate limits on the response of the devices.
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