the magnitude of the change in threshold current with temperature in InP quantum dot lasers caused by the distribution of carriers among dot states is quantified and demonstrated. Samples with differing distributions of allowed states, as assessed using absorption spectra and achieved by varying the composition of the quantum well above each layer of quantum dots, are affected differently by this thermal broadening although the underlying mechanism is the same. This difference is shown to be a result of different optical loss and the different gain magnitude achieved at a similar inversion level in the different samples. Uncoated, cleaved facet Fabry-Perot lasers with 2 mm long cavities are demonstrated with a threshold current density of 138 Acm<sup>-2</sup> at 300 K that increases to 235 Acm<sup>-2</sup> at 350 K (77ºC).
The facets of InP/(Al)GaInP/GaAs quantum dot laser active regions offer superior resistance to catastrophic optical mirror damage at high facet power densities. These structures degrade by bulk damage. We have used a new range of techniques to identify changes occurring during damage in working devices: thermography through windows in the nmetallization, photoluminescence via p-metallization windows and photocurrent studies. Devices were aged with single very high current pulses or pulses of increasing size and monitored during this process with these techniques. Previous investigation with panchromatic cathodoluminescence revealed dark non-radiative spots throughout the plane of the active region. The dark spots, which were present even in unprocessed material, increased in size in the pumped regions only during lasing action. The spots and background regions darkened throughout the pumped stripe area only for the whole duration of the current pulse. Thermography after successive pulses confirmed damage originating from a point in the bulk rather than at the facet. p-windows observations of light and dark regions showed a blue shift in the photoluminescence spectra of the dark regions. Photocurrent studies of more gently aged devices showed a greater decrease in signal in the region associated in previous work with defective very large dots. Identification of such spectral regions, which were previously found to be influenced by changes in structure design and growth conditions offer a route to control degradation mechanisms by this means.
We demonstrate lower temperature sensitivity at high temperature in a strained layer InP/AlGaInP self-assembled
quantum dot design grown by MOVPE. The lasers emit between 700 - 730 nm, finding application in photodynamic
therapies and bio-photonic sensing. We previously achieved a 300 K threshold current density of 150 Acm<sup>-2</sup> in similar
structures for 2mm long lasers with as-cleaved facets, however at elevated temperatures J<sub>th</sub> increases rapidly with
temperature. To address this issue we redesign the layers around the active regions, consisting of five layers of dots, each
grown on a lower confining layer of (Al<sub>0.30</sub>Ga<sub>0.70</sub>)InP lattice matched to GaAs, formed from 3 mono-layers of InP and
with a GaxIn(1-x)P upper confining layer. We grew two series of samples, x=0.43-0.58 with (Al<sub>0.70</sub>Ga<sub>0.30</sub>)<sub>0.51</sub>In<sub>0.49</sub>P
waveguide claddings, and x=0.52-0.58 (AlInP claddings). Dot properties are strongly influenced by the UCL. Properties
varied with Ga fraction. Measured absorption and lasing energies increase with Ga percentage, maintaining a constant
separation from upper confining layer transition energies. A Ga fraction of x=0.54 (lightly tensile strained with respect to
GaAs) gave the strongest and most well defined absorption, the lowest 300K Jth for 2mm long broad area lasers
(uncoated facets) of 180 Acm<sup>-2</sup> and lowest rate of J<sub>th</sub> increase with temperature.
By optimising an InP/AlGaInP quantum dot size distribution a broad and relatively flat topped gain spectra can be
achieved. Using the segmented contact method we measure the optical gain spectra and use this to explain the range of
lasing wavelengths that can obtained by varying the grating structure of deep etched DBR lasers. We describe the
optimisation of a simple single stage ICP etch process suitable for producing anisotropic microstructures in this material
system and the resulting deep-etched DBR lasers. Measurements of emission wavelength made between 220 and 320 K
on a ridge laser, fabricated with cleaved facets, reveals a temperature dependence on of 0.14 nm/K. DBR structures have
been used to improve this behaviour, with a dependence of peak wavelength with temperature of 0.07 nm/K, over the
same temperature range. Measurements on a 4 μm wide DBR ridge laser show they can be operated up to 17 nm from
the peak emission of a ridge laser operating at the same current density.
Quantum dots (QD) offer significant advantages over quantum wells (QW) as the active material in high power lasers.
We have determined power density values at catastrophic optical mirror damage (COMD), a key factor limiting high
power laser diode performance, for various QW and QD red and NIR emitting structures in the in the AlGaInP system.
The devices used were 50 μm oxide stripe lasers mounted p-side up on copper heatsinks operated pulsed. The COMD
power density limit decreases as pulse length increases. At short pulse lengths the limit is higher in QD (19.1±1.1
MW/cm<sup>2</sup>) than in QW devices (11.9±2.8 MW/cm<sup>2</sup> and 14.3±0.4 MW/cm<sup>2</sup> for two different spot sizes). We used the high
energy Boltzmann tail of the spontaneous emission from the front facet to measure temperature rise to investigate the
physical mechanisms (non-radiative recombination of injected carriers and reabsorption of laser light at the facet)
leading to COMD and distinguish between the behaviour at COMD of QW and QD devices. Over the range 1x to 2x
threshold current the temperature rise in the QW structures was higher. Scanning electron microscopy showed a
difference between the QD and QW lasers in the appearance of the damage after COMD.