3D semipolar InGaN/GaN based light emitters
new ANKA seminar room, building 348, upper floor
Tilo Baumbach, KIT-ANKA, IPS, LAS
High power InGaN/GaN based green light emitting diodes (LEDs) attract worldwide interest nowadays. However, the efficiency of the InGaN/GaN quantum wells (QWs) decreases drastically with increasing emission wavelength. On one hand, this is attributed to the degraded material quality with increasing indium content . On the other hand, owing to the polar character of the nitride semiconductors, the increasing strain in such QWs leads to huge internal electrical fields resulting in a local separation of electrons and holes and hence reducing their recombination probability. Consequently, many groups are making efforts to fabricate semipolar/nonpolar InGaN/GaN QW structures following different approaches [2, 3]. In our work, three-dimensional (3D) GaN structure is achieved by selective area epitaxy offering the tilted semipolar facet as the surface where the semipolar InGaN/GaN QWs and subsequent p-(Al)GaN layers are deposited to form a complete LED structure (Fig. 1).
We investigated p-doping and optimization of the InGaN epitaxial condition to enhance the output power of the semipolar stripe LEDs.
p-type conductivity of metalorganic vapour phase epitaxy (MOVPE) grown GaN is achieved by Mg doping following low energy electron irradiation  or thermal annealing . Little is known about the p-doping on 3D structures since the Hall measurement is not applicable. We employed imaging secondary ion mass spectrometry to determine the locally-resolved Mg concentration with a resolution of about 150nm for the 3D semipolar LEDs.
As mentioned, the InGaN crystal quality degrades with increasing indium content. The InGaN QWs are typically grown at a low temperature below 800°C in order to obtain high indium content required by the green emission. During the InGaN growth, we varied other epitaxial parameters, e.g. the TMIn molar flow, the TEGa molar flow, the reactor pressure and the InGaN QW thickness, to achieve the same indium content at elevated temperatures. The optimized InGaN epitaxial condition contributes to a factor of 2 higher electroluminescence output power of the semipolar stripe LEDs.
Two LED analysis methods were developed: electron beam-induced current (EBIC) and ABC model fitting. EBIC allows the precise characterization of the location and the width of the depletion zone in pn-junctions. Via the fit of the EL and PL data according to the famous ABC model , carrier injection efficiency and internal quantum efficiency are splitted indicating that low CIE is the main reason for still not satisfying performance of the semipolar LEDs.
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Institute of Optoelectronics, Ulm University, Germany