Platform work package 3: Fast Fab and Characterisation

Recent technical and scientific challenges have involved fast fab of vertical cavity surface emitting lasers (VCSELs); scaling up characterisation for optical devices using auto probe wafer mapping reporting degradation mechanisms of III V lasers on Si, testing and expanding existing device degradation studies and developing fabrication and measurement techniques for mode locked lasers.


Dr Craig Allford (


To develop fast fabrication and characterisation techniques to feedback to design and growth, minimising development cycle time. The key work package deliverables include, reliability measurements (III Vs on Silicon), fast fab on large area VCSEL wafers, in line characterisation to support integration.

Progress and challenges

Figure 1. Electro-optical probing on a VCSEL tile.

Using a newly installed Bentham monochromator we have upgraded our capability to for fast characterisation of VCSEL epi wafers, via photovoltage spectroscopy. The technique, which is a much faster than fabricating and testing VCSELs, allows the QW transition energies to be determined and the cavity resonance, which will define the VCSEL lasing wavelength. This is valuable information which can be fed back to design and production teams, to inform wafer spec and determine if growth parameters need to be adjusted. The auto prober (Figure 1) has been used extensively to develop a fast turnaround VCSEL fabrication and characterisation process in order to rapidly assess different VCSEL designs. This work is a collaboration between wafer supplier (IQE) processing facility (ICS) and CS Hub WP3. Real measured results are presented in Figure 2 where the ON resistance of 900 VCSELs are displayed as a heatmap, and example light current voltage (L I V) characteristics and spectra at shown for two VCSEL sizes. Automatic probing at the wafer level reduces cost and speeds up development of material and process characterisation.

We have reported on the degradation mechanism for III-V QD lasers directly grown on Si substrates, which appears in IEEE Journal of Selected Topics in Quantum Electronics (Volume 25 Issue 6 Nov Dec 2019). We have upgraded the reliability and degradation study facilities to achieve semi automated testing. First experiments on 1 3 𝜇m emitting InAs QD lasers were successfully completed in Q 4 2019 with a run time of 1076 hours. These lasers were developed as part of WP 4 and we are working towards longer term studies, at further elevated temperatures in 2020.To support WP 9 we and subjected them to harsh radiation environments in collaboration with our industrial partners. The aim is to simulate environments that will be experienced in the applications targeted in WP 9. Initial results show excellent promise and further radiation experiments are planned for 2020.

We have developed a fabrication process for monolithic passively mode locked lasers utilising indium phosphide (InP) quantum dots (QDs) emitting in the 730 nm waveband. Researchers at the CS Hub, in collaboration with colleagues at Heriot Watt University have characterised the passive mode locking properties of these devices with pulse widths of 6 ps, at a repetition
frequency of 12 55 GHz having been measured. Full details of this work have been submitted for publication, with the manuscript currently under peer review.

Figure 2: Fast-fab VCSEL tile layout and expanded microscope image of VCSEL array (top left);  Map of ON resistance for 900 VCSELs (bottom left). L-I-V & spectra (at two currents) for a single VCSEL (right).
Figure 3: Monolithic integrated two-section passive mode-locked laser, inset SEM image showing the cross-section profile, the scale bar is 2 μm (left); bonded laser (middle); Autocorrelation trace from an InP QD mode‑locked laser. (right).

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