School of Physics and Astronomy

Tailor made dots

Friday 19th October 2012

One potential application of this work is a micro-fluidic coupled cavity laser array for cell analysis being developed at Cardiff.

Cardiff based researchers have produced InP self assembled quantum dot lasers with record low threshold current density of 130 Acm^-2 and reduced threshold temperature sensitivity for bio-photonic applications.

Lower power consumption is required for today's compact and portable biophotonic applications such as OCT (Optical Coherence Tomography) and "lab-on-a-chip" applications. The Cardiff structures, with their low threshold, reduced temperature sensitivity, extended wavelength range and broad gain spectrum have the potential to fulfil these requirements. To achieve these benefits the group improved the design of the epitaxial layers.

Fig.1: Lower threshold current density for Ga=0.56 sample than Ga=0.52 sample, with scaled Ga=0.52 threshold showing improvement in temperature sensitivity.

The composition of the confining layers immediately above and below InP self assembled quantum dots affects the composition and strain of the dots themselves, strongly influencing their properties. The researchers grew a series of laser structures with an active region consisting of five layers of InP dots confined below with lattice matched Al_0.3GaInP and above with Ga_xInP, where x, the Ga fraction, was varied from 0.43 to 0.58. The active region was enclosed in AlGaInP optical waveguides. Spectroscopic measurements showed dot state energies ranging from 1.67 to 1.77 eV as the fraction of Ga increased. An additional shift in energy was produced by adjusting the growth temperature of the p-cladding layer.

Laser measurements gave a room temperature threshold current density below 130 Acm^-2 in 2 mm long, broad area oxide stripe lasers with uncoated facets at a Ga fraction of 0.54, rising to only 220 Acm^-2 at a temperature of 80°C, with a range of laser wavelengths of 696 – 725 nm available across the composition and growth temperature range.

Fig2.: Transitions between energy states in the structures. Measured dot ground and excited states (black). Measured upper confining layer states (red and blue points) with calculated values (lines). The grey dashed line is the measured energy of the lower confining layer. The grey solid line is the energy of bulk unstrained Ga_xInP.

Further work from the group looks to exploit these advances in novel device structures and in integrated format using our sophisticated fabrication facilities.

More details of this work are published in Semiconductor Science and Technology (DOI: 10.1088/0268-1242/27/9/094008).

The research work was carried out by Prof Peter Smowton and Dr Stella Elliott in the Condensed Matter and Photonics group at Cardiff University on structures grown by Dr Andrey Krysa at the EPSRC National Centre for III-V Technologies with the TEM carried out by Dr Richard Beanland at the Materials and Analytical Science Centre at the University of Warwick.