We demonstrate the fabrication of 100 x 100µm2 nitride-based tricolor micro-LEDs. While the optical properties of the devices are very promising, the electrical characterizations show large penalty voltage due to the introduction of compensating defects in the p-GaN layers upon plasma etching. Therefore, we consider replacing the p-GaN layers by p+/n+ tunnel junctions.
We analyzed the surface morphology and the Mg, Si, H and C concentration of different p/n junctions grown at different temperature. SIMS results show the C concentration drastically increases from 10^16/cm3 at 1100°C to about 10^17/cm3 at 975°C without significantly affecting the electrical properties of both p- and n-type layers. However, the decomposition of tetramethylsilane (TMSi) strongly depends on the growth temperature, and thus the Si/Ga ratio has to be carefully controlled with temperature. The Mg shows a strong memory effect in the n-side since its concentration is slowly decaying of only one decade after 50nm from the p/n interface. The pining of the Fermi level, close to the conduction band in the n-GaN, destabilizes the Mg-H complex and allows the Mg to be activated (Mg/H<<1). In addition, at the high Mg/Ga and Si/Ga ratio typically required in p+/n+ tunnel junction, we observed a change of the GaN surface morphology with the promotion of inversion domains and 3D growth respectively, whatever the growth temperature.
Based on these data, we show that a p+/n+ tunnel junction is a trade off between high structural and electrical quality.
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