Instytut Wysokich Ciśnień

Na 17. Konferencji Laser Optics prof. Michał Leszczyński wygłosi wykład zaproszony pt:

The most important issues in technology of AlGaInN-based laser diodes and in arrays

Abstrakt:

Nitride laser diodes (LDs) of 405 nm are used in Blu-Ray players and recorders. 450 nm and 520 nm devices are being introduced in RGB projectors, from small sizes (cellular phones) to big cinema projectors. The car companies (Audi and BMW) have begun to install new headlights based on blue laser diodes illuminating white phosphor (as in wihite LEDs). Such headlights would be smart in future, i.e., they will change the illumination area accordingly to the road situation. The next large market will be "Last mile" communication, in which 490 LDs will be used for Terabyte information transmission through the plastic fibres installed in cars, ships, airplanes and houses.

Despite the bright commercial future of nitride LDs, there is a number of technological issues which are being solved, and some of them will be discussed in the presentation.

Nitride LEDs and HEMTs are manufactured using the foreign substrates: sapphire, SiC and Si. In the case of LDs, single crystals of GaN of very low dislocation density must be used. Unfortunately, the growth of bulk GaN is extremely difficult because of thermodynamical reasons. The biggest GaN substrate manufacturer is Sumitomo. These crystals are grown on GaAs substrates by HVPE (hydride vapour phase epitaxy) using some technological tricks to eliminate the mismatch dislocations on small areas where the laser stripes can be placed. To construct large-area devices (as LD arrays) it is necessary to use uniform GaN substrates grown, for example, by ammonothermal method and joint ammonothermal/HVPE one.

In AlGaInN epitaxy, three issues will be discussed: i) AlGaN cracking, ii) InGaN ununiformity and degradation, iii) influence of hydrogen used in the carrier gas.

Concerning AlGaN cracking, it is a serious problem because the cladding layers must be thick and must contain the large percent of aluminum content to avoid the optical leakage to the substrate. It will be presented how the problem of cracking can be overcome by the lateral patterning.

Concerning InGaN quantum wells, they are grown at very low temperatures what results in indium fluctuations, oxygen incorporation and poor morphology. Moreover, for high In-content layers, the growth of p-type that must be done at high temperature, causes the InGaN decomposition. It will be shown how this effect can be avoided, however, still it is one of the main problems, especially in fabrication of the green LDs.

In the MOVPE (metalorganic chemical vapour phase epitaxy) technology used for growth of other semiconductors, hydrogen is used as the carrier gas. In the growth of nitrides, hydrogen must be used with care, because itetches indium away. However, on the other hand, hydrogen removes oxygen impurities and makes the surface much smoother. In the presentation, the latest results on how hydrogen must be applied will be presented.

Finally, the present state-of-the-art of our lab in a comparison with the other LD manufacturers will be shown. In particular, the specs of the LD arrays (including the high power of a few watts of optical power and addressable ones) are of a special interest because of many commercial applications.