In previous work, the researchers had demonstrated the high temperature stability of such Ge-DEQDs when employed as a CMOS-compatible gain material in optically pumped lasers. Their recent paper “Room-Temperature Group-IV LED Based on Defect-Enhanced Ge Quantum Dots” published in the ACS Photonics Journal reveals that embedded layers of Ge-DEQDs could also stably emit light at room-temperature when electrically pumped (at high current densities and at device temperatures of at least 100°C, with limited quenching).
Operating at near-infrared (NIR) telecom wavelengths of 1.3 to 1.5μm, their devices consisted of vertically stacked layers of dots (ranging from 3 to 7) into the 200nm thick intrinsic region of a p-i-n Si diode. From their experiment, the researchers noted that the emission intensity of their devices scaled with the number of quantum dot layers embedded, demonstrating that larger gain material volumes could potentially be grown to reach lasing thresholds, eventually leading to CMOS-compatible electrically pumped room-temperature lasers.
The Ge-DEQDs were grown through chemical vapor deposition, with subsequent ion bombardment to create the lattice defects within the Ge dots. These processes can be run in standard CMOS production lines. The 100×100μm2 DEQDs-based LEDs operated well at current densities up to 20kA/cm2 and heat-sink temperatures up to 100°C under pulsed driving conditions at a repetition frequency of 10kHz and a current pulse length of 5μs (duty-cycles up to 5%).
But testing was largely limited by the experimental setup (current source, Peltier temperature controller and the bonding wires) note the researchers, arguing that their devices could certainly operate at far higher current densities and duty cycles.
Even for a large driving current density of 10kA/cm2, significant temperature quenching of the Ge-DEQDs LED emission only took place above 240K, much later than what literature reports for direct-bandgap GeSn or SiGeSn materials. The paper also highlights that the output intensity of the DEQD LEDs only drops slowly with temperature and still remains at 28% of the global maximum value for a heat-sink temperature of 100°C (373K).
The researchers attribute the temperature stability of Ge-DEQD emission to the strong confinement of holes in the quantum dot potential, with associated activation energies for thermal quenching above 300 meV. Though even under quenching, the spectral characteristics remained the same up to the maximum tested temperature of 100°C.
As future research, the authors point to stacking more layers of quantum dots (to create a larger gain material volume) and optimizing the diode structure employed for pumping the Ge quantum dots, further investigating the doping parameters and intrinsic region thickness to reach the best optical recombination current in the DEQDs.
Johannes Kepler University Linz – www.jku.at