Unifying semiconductors to speed up communication

Optoelectronic devices consist of different semiconductor alloys lying on substrates. During the growth of the MQW laser-active region, different layers of semiconductors are sequentially deposited on the substrate, alternating well and barrier regions. In well regions, electrons and holes recombine to provide the laser light, while barrier regions are important for the electrons and holes confinement in the wells. The parameters that are able to modulate the laser wavelength needed to match the minimum adsorption of the optical fibres are the chemical composition and the width of both well and barrier regions.

For low-speed communications, the sequence of “0” and “1” containing the information is produced by directly modulating the MQW laser emission by a variable current. Such devices can be fully characterised using laboratory X-ray diffraction (XRD) and photoluminescence (PL) techniques. For high-speed communications, device instabilities prevent this simple solution and the MQW lasers are fed by a constant current that generates a constant emission, which is not carrying any information. To create the information, chips containing MQW lasers need to be modulated externally. This modulation is achieved with electroabsorption-modulator (EAM) devices, which are normally connected externally to the MQW laser. EAMs are also MQW heterostructures with an energy gap that can be modulated at high frequency applying an external potential (Stark effect). In such a way the EAM can switch from opaque to transparent for the light emitted by the MQW laser.

Scientists are trying to integrate both MQW laser and EAM, occupying a small area (typically 30 × 700 µm², so that in a single 2 inch InP substrate about 2 × 10⁴ devices may be potentially processed). The optimisation of these EML devices has, until now, been carried out by empirical approaches because of the impossibility to carry out a micron-resolved XRD study with laboratory sources. A team from the University of Turin (Italy), Avago Technologies and the ESRF has, for the first time, managed to directly measure the structure of these semiconductors thanks to the micrometre X-ray beam of ID22.

The scientists investigated the EML devices by µ-XRD and µ-X-ray fluorescence at 35 different spatial points with a spatial resolution of 2 µm, moving from the MQW laser to the EAM region. With such data, it is possible to obtain the fundamental structural parameters (width along the growth direction and lattice parameter on the growth plane) of both well and barrier parts of the heterostructure. The result is achieved by fitting the observed pattern using a model based on the dynamic theory of X-ray diffraction. Finally, the combination of synchrotron µ-XRD with laboratory µ-PL allowed the team to obtain the space-resolved chemical composition from the space-resolved lattice parameter.

This unprecedented characterisation gave the team the opportunity to determine the structure of the grown heterostructure with a monolayer resolution (3  Å) along the growth axis and with a micrometre resolution in the growth plane (i.e. to find out what has been empirically grown). This study, requiring a high flux of hard X-rays combined with a micrometre resolution can be achieved in very few beamlines worldwide, such as ID22. “These results show us the way to improve the growth process, which was previously based only on a trial/error approach,” explains Carlo Lamberti, leader of the team and professor at the University of Turin. 

References
L Mino et al. 2010 J. Anal. At. Spectrom. 25 831–836. L Mino et al. 2010 Adv. Mater. 22 2050–2054.

M Capellas

This article appeared in ESRFnews, December 2010. 

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Top image: Selection of some of the 35 full XRD patterns collected moving from the MQW laser (SAG region) to the EAM (FIELD region) that allowed to obtain the fundamental structural parameters of the system.