We report on an ultra-broadband tunable mid-IR source delivering 110 nJ, 64 fs-long pulses at 250 kHz repetition rate. The experiment starts with a Yb-doped fiber amplifier system delivering 200 μJ, 300 fs-long pulses, followed by a 70% high-efficiency dual-stage nonlinear compression based on a multipass cell and a 1 m-long capillary filled with argon. The 9.5 fs-long resulting pulses drive intrapulse difference-frequency generation (iDFG) in a 1 mm-thick LiGaS2 crystal and a specially designed waveplate is used prior to this crystal to further enhance the iDFG yield. This source paves the way for new experiments in 2D ultrafast spectroscopy.
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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