Prof. Dr. B. Witzigmann (University of Kassel, Germany)
Monday, September 3, 2018 - 14:00
Weierstraß-Institut
Mohrenstr. 39, 10117 Berlin, Erhard-Schmidt-Hörsaal, Erdgeschoss
INSTITUTSKOLLOQUIUM
III-nitride based multiple quantum well structures are the active region of choice for LEDs and lasers from the ultraviolet to the visible spectral regime. For maximum efficiency, the electron and hole carrier densities need to be distributed uniformly in the quantum wells at all relevant operating conditions. We analyze the carrier injection and distribution in polar and semi-polar LEDs using a microscopic carrier transport model. It is based on drift-diffusion currents, and quantum wells are treated as scattering centers, with self-consistent kp-band structure calculation for the quantum well carriers. Tunneling currents at hetero interfaces are included via an effective potential model. Impurities such as Si, Mg and O are activated according to a Schottky-type model. Their distribution is found from experimental SIMS data. The activation mode includes both the impurity density as well as the local electric field (Poole-Frenkel effect). Comparison to experimental data reveals the following findings: the hole injection efficiency depends critically on the proximity of the Mg dopants to the p-side quantum well. At the same time, electron leakage is suppressed by ionized Mg atoms forming an electrostatic barrier, which is reduced with rising current due to saturation of the acceptor levels. Impurities that form shallow donors (e.g. oxygen) also hamper the hole injection efficiency as they reduce the Mg ionization rate. In UV LEDs, special care has to be taken for electron injection into the deep quantum wells. Therefore a high performance LED requires nanoscale control of intentional and unintentional impurity distribution. Finally, it is shown that the ideality factor of the pin-diode contains information on the injection homogeneity of the LED.
submitted by koehler (torsten.koehler@wias-berlin.de, 030 20372582)