Emission characteristics of GaN and AlGaInP based Light Emitting Diodes
Reinhold Hetzel
Institute of Solid State Physics, TU Graz
http://www.if.tugraz.at
11:00 - 11:40 Thursday 24 October 2013 AT02002J The junction temperature of electroluminescence devices is a key parameter which influences optoelectronic properties of Light Emitting Diodes (LEDs). Hence efficiency, light output, reliability and lifetime are strongly linked to this device parameter. Altering the junction temperature leads to variations of the emission spectra i.e. shifts of the peak wavelength as well as broadening or narrowing of the emission peak. Therefore, understanding the behavior of the junction temperature and its monitoring is a prerequisite to design and operate light engines with long lifetimes, high efficacies and excellent and long term color quality and stability.
Due to chip encapsulation, in situ measurement of the junction temperature is basically not possible. There are several measuring methods which attempt to probe the temperature of the device (direct measuring (e.g. sensing elements) as well as indirect measurement (e.g. infrared camera)). However, most of them rather probe the temperature of the device environment than the actual junction temperature of the active region.
We present our studies based on contactless methods for deriving accurately the junction temperature of LEDs in an assembled light engine under operation. Additionally we will show that the junction temperature can be obtained out of several externally accessible parameters (forward voltage, energy gap, etc.) on DC and dynamically driven devices.
We will further present low temperature measurements in liquid Helium (4.2 K) and liquid Nitrogen (77 K) which allows us to gain insight into the spontaneous electron hole recombination process. We will discuss the appearance and the influence of piezoelectric fields in GaN as well as in AlGaInP LEDs. Specifically we distinguish between thermal and electrical effect, especially the appearance of the quantum confined stark effect (QCSE), exciton localization effect (ELE) as well as electron-phonon and many body effects.
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