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Optical interconnects using vcsels based on as/n compounds

Objective


Through a co-ordinated effort, the Optivan consortium has succeeded in controlling the growth and in characterising essential properties of the novel GaInNAs/GaAs material system. This culminated in realising the first 1300nm VCSELs worldwide that are suitable for use in data communications systems. Three epitaxial techniques (MOVPE, Solid-Source-MBE and Gas-Source-MBE) have been investigated in order to get a basic understanding of the growth mechanisms of the GaInNAs alloy and the application of advanced characterisation methods has led to a better understanding of its material properties:
- LETI, for instance has discovered the presence of an anomalous behaviour of the GaInNAs quantum wells. The variation of the photoluminescence peak energy with the temperature shows an inverted S-shape. This effect is explained by a strong localisation of carriers below 100 K.
- Experimental evidence for an increased electron effective mass of (~0.8m(sub0)) was found from PLE investigations of electronic states in GaInNAs/GaAs quantum wells, as performed at the University of Strathclyde. From a theoretical fit to the PLE transitions, a Type I band alignment for the heavy holes was confirmed.
-Measurements of the modal gain performed at IMEC confirmed that the material gain of GaInNAs is broad and high enough to expect superior laser properties.
-By Solid-Source MBE, edge-emitting lasers were realised, which for the first time demonstrated that GaInNAs can rival the established InGaAsP as a laser material. A number of excellent devices have been elaborated within the project: High performance ridge waveguide lasers emitting at 1.29µm with threshold currents as low as 20mA, slope efficiencies of 0.27 W/A, improved high temperature performance and CW operation up to 100°C. Such values represent the very best ridge waveguide results reported in this material system at the time of realisation. 100µm broad stripe lasers exhibited high power CW operation up to 8W, which can be taken as a proof for the excellent material quality.
-Within the third year of the project, the ultimate objective of the project was realised: Using an intra-cavity contact configuration VCSEL structures have been grown by Infineon’s solid source MBE technology. These VCSELs show output-powers of more than 1mW at room-temperature and cw-operation up to 80°C. The threshold-current at room-temperature is 2mA, the side-mode suppression-ratio at a typical drive-current of 5mA (for 2.5GBit/s) is better than 30dB. Data transmission experiments up to 10Gbit/s could be demonstrated.
-At the end of the project Infineon additionally could demonstrate high quality VCSELs from its MOVPE technology. Later, IQE also reported on the MOVPE growth of 1.28µm VCSEL device layer structures.
-With respect to the MOVPE production processes another achievement of the OPTIVAN project is the assessment of several high purity metalorganic precursors for the growth of GaInNAs epilayers. Finally, 1,1-dimethylhydrazine (UDMHy) was confirmed as the best suitable source for the incorporation of nitrogen and a new purification technology has been developed at Epichem.
-Apart from this, the results published by the Optivan partners have stimulated a number of other research groups in their work. The world-wide interest in the work of Optivan
Objectives and content
Vertical Cavity Surface Emitting lasers (VCSELs) are a
rapidly emerging class of lasers diodes which overcomes
the main limitations of the existing devices based on
edge-emitting structures. A relative mature technology
exists for VCSELs operating in the 0.6 to 1.0mm
wavelength range. Their active region is based on III-V
semiconducting materials lattice-matched on GaAs which
are all compatible with the growth of AlGaAs-based DBRs
achieving >99% reflectivity with typically only 20-30
mirror pairs due to the large refractive index difference
(0.5) possible with AlGaAs alloys.
However, an important part of the laser market deals with
diodes used in the 1.3 to 1.55mm wavelength range for
compatibility with optical fibre-based
telecommunications. Furthermore VCSELs in the 1.2 to 1.3 mm wavelength range will be required for the next
generation high performance optical data links where they
offer significant advantages over conventional 850nm
devices. Long Wavelength (LW) VCSELs, which in
laboratory demonstration have traditionally relied on
InGaAsP/InP and AlGaInAs/InP materials technology have
been much more difficult to produce. The primary
difficulty is that the refractive index difference
available in InGaAsP-based DBRs is only 0.2-0.3 which
means that 50 layer pairs would be necessary to give the
required >99% reflectivity. This increases the growth
time dramatically and places stringent requirements on
layer-to-layer reproducibility. This also induces other
negative effects such as high diffractive losses and an
increase of electrical resistance.
The recent appearance of InGaAsN semiconducting material
structures offers a very attractive alternative. InGAAsN
can be grown on GasAs substrates and therefore the entire
LW-VCSEL structure can be produced in a single growth run
using conventional AlGaAs based Bragg mirrors. Since
AlGaAs-based DBRs are retained, we also retained the
advantages of compositional grading, large indexcontrast, and suitability for selective oxidation, and
the GaAs/AlGaAs barrier and confinement regions around
the InGaAsN quantum well active regions provide enhanced
electron confinement to reduce temperature sensitivity.
The objectives of this project are to:
grow high quality InGaAsN/GaAs layers and (SQWs) by
MOCVD and GSMBE epitaxial techniques,
improve the N incorporation to achieve a 1.3 mm
absorption edge combined with the excellent layer
stability and reproducibility,
achieve photoluminescence and electroluminescence from
the DH and SQW structures at 1.2 and 1.3mm and,
achieve edge-emitting lasers and CW LW-VCSELs emitting
at 1.2 and 1.3,um based on InGaAsN/AlGaAs QWs in the
active region.
The consortium comprise major manufacturers of electronic
devices, epitaxial layers, starting chemicals together
with experts in device design, material characterisation
and device processing.

Call for proposal

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Coordinator

INFINEON TECHNOLOGIES AG
EU contribution
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Address
ST. MARTIN STRASSE 53
81609 MUENCHEN
Germany

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Total cost
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Participants (5)