Untersuchung des Einflusses von Hintergrundladungsträgerdichten auf die Effizienz von InGaN-basierten Lichtemittern

Abstract: Since the development of the rst blue light-emitting diodes based on gal lium nitride (GaN) in the early 1990s by Shuji Nakamura and co-workers at Nichia Corporation, huge e orts have been made by many research groups all over the world to simplify the manufacturing process and to improve the eciency of light-emitting diodes made of InGaN. 

Earlier LEDs were only used as indicator lamps due to their low output power and narrow-band emission spectrum. But with the ability to fabricate blue diodes based on InGaN, white light sources can be produced.

This can be done for example by deposition of a phosphor converter on top of the diode which converts part of the blue ouput photons into yellow photons. Depending on the ratio of blue and yellow photons, white light with di erent color temperature can be generated. Therefore LEDs are gaining more and more importance as a light source for general-purpose illumination such as in private homes or company buildings.

By using diodes instead of conventional light bulbs or fluorescent lamps, a huge amount of energy can be saved. Furthermore, the high lifetime reduces aintenance costs signi cantly.

In LEDs, light is created by radiative recombination of electrons and holes in a p-n junction. This refers to a junction of two adjacent semiconductor layers where one is doped with electron acceptors (p-type doping) and the other with electron donators (n-type doping). The active region of advanced LEDs consists of one or more so called quantum wells nestled by material of higher bandgap energy. This leads to carrier con nement within the quantum wells enhancing radiative recombination eciency of the device. Non-radiative recombination processes do not create light by photon emission but convert energy into heat. So they lead to a decreased eciency of the diode in terms of light output and thus the aim is to suppress those processes in optoelectronic devices.

Assuming three di fferent types of recombination that can occur in the diode, 

  1. recombination through trap levels within the bandgap
  2. radiative transitions
  3. Auger like processes

the so called ABC model can be used to describe the eciency of the device, where each of the three parameters (A,B and C) is a coecient associated with one recombination path.

The internal quantum eciency is then given by

 

Here, injection losses are neglected. For high injection currents the Auger-like recombination processes (represented by the C-parameter) lead to a eciency decrease commonly referred to as droop. In high brightness applications it is required to operate the LED in the high excitation regime, therefore it is desired to avoid this e ect. Although many research groups tried to unterstand this drop in eciency, there is still disagreement among experts in the eld of nitride based semiconductors concerning its physical origin. Possible explanations for the eciency droop that are discussed in literature are defect-assisted SRH recombination, reduction of spontaneous emission, carrier leakage or Auger recombination.

This work intends to investigate whether Auger-like recombination processes lead to droop as it is observed in InGaN based light emitters. For this reason, the e ect of strong doping on the eciency of n-i-n and p-i-p structures was investigated. Measurements on samples with varying background doping concentrations were performed and analyzed. Due to the background doping in the vicinity of the quantum wells the total number of available charge carriers in the active region can be varied and thus the di erent recombination processes can be suppressed or enhanced. Carrier generation is achieved by resonant photoluminescence, excluding a possible in uence of transport related issues. By comparison of our experimental results with calculations based on an extended ABC model, we gain knowledge about the in uence of the di erent Auger-like recombinations. These results help us to understand whether Auger like recombinations could be responsible for the droop phenomenon.