Low-confinement high-power semiconductor lasers (1999)

This thesis presents the results of studies related to optimisation of high power semiconductor laser diodes using the low confinement concept. This implies a different approach in designing the transversal layer structure before growth and in processing the wafer after growth, for providing the optimal amount of lateral index-guiding. Basically, for the transverse direction, the maximum of the optical field distribution is shifted away from the active layer, in order to increase the spot size, i.e. to decrease the confinement factor and to correspondingly increase the available output optical power before catastrophic optical degradation. Optical modelling in the transversal direction using the transfer matrix method is in general reliable. The layer structures are designed to have the absorption coefficient lower than 1 cm-1 and the required confinement factor that should correspond to a value of the spot size d / G in the range of 0.8 - 1 mm. Due to the extension of the optical field in the contact layer resonances may occur. These are to be avoided for our laser operation, since they are associated with increased losses and far-field distortions but on the other hand, this effect can be useful for other devices and some suggestions are given for application for DFB laser diodes. Using lower doping levels than usual, laser diode structures having very low values of the absorption coefficient of 1 - 1.5 cm-1 can be reproducibly obtained with both MBE and MOCVD growth techniques. High optical power output of 1.8 W CW per uncoated facet of 50 mm wide stripe, L = 2 mm long devices having an asymmetric transversal layer structure with optical trap layer was demonstrated. This represents an improvement by a factor of 2.5 if compared with conventional structures optimised for low threshold current. The COD level, as expected, increases inversely proportional to the spot size. In the lateral direction, carrier induced antiguiding is decreased proportional to the confinement factor. Weak index guiding allows in principle fundamental mode behaviour for output powers up to 1 W. In practice, thermal and stress effects put a lower limit on the strength of the built-in index-guiding needed to be introduced technologically. Thermal effects, that are studied here using high threshold gain guided devices, can be minimised by lowering the threshold current. Thermal waveguiding is estimated to correspond to a step effective refractive index variation of Dneff » 10-3 for an 8 mm wide stripe device and for an operating current density of 2500 A/cm2. Unexpected effects affect the temporal response of gainguided and weakly index-guided laser diodes. The threshold current density depends on the pulse width and, contrary to what we should normally expect, it decreases when the pulse width increases from 100 ns to 10 ms. The typical decrease may be as large as 50 %, depending on the device. Also, there is a significant delay at threshold between the beginning of the optical and electrical pulse. This delay is in the range of 3 - 5 ms and decreases above threshold. Even when it is no longer noticeable, the optical pulse shows a gradual increase in the first microseconds after the beginning of the pulse. In weakly index guided devices, the delay between the electrical and optical pulses and the dependence of the threshold current on pulse width are present to a lesser extent, and become no longer noticeable for Dneff > 2 x 10-3. However, above threshold the optical pulse shape is strongly affected by the appearance of the first or higher order modes. Corresponding to the peculiar optical shape of the pulse, the far field becomes unstable and the spectrum broadens due to thermal drift. Very often, the "hybrid type" of kink is observed first and very soon after that changes into the "first-order type", when the first order mode is no longer coupled in phase with the fundamental one. At higher injection levels, typically the device shows multimode operation and the optical pulse exhibits oscillations between modes. For 13.5 mm wide stripe, the maximum power available in the fundamental lateral mode for uncoated devices is 200-320 mW/facet and is thermally limited. Measurements were made in pulsed conditions using 10-30 ms pulse width and 10 ms between pulses. The corresponding current density is 2500-3000 A/cm2, which is in agreement with the optical model presented in section 2.3. Stress-induced variations of the effective refractive index by the photoelastic effect can become important for weakly index-guided devices, depending on the stress in the oxide layer and on the ridge shape. They are evaluated here theoretically and experimentally for a profiled ridge waveguide laser diode. An antiguiding of Dneff » 8 x 10-4 may occur below the stripe region, leading to significant perturbations at the stripe edges. Together with thermal effects, it puts a lower limit on the built-in waveguiding to be introduced technologically. Threshold current density and its temperature dependence, apparent internal efficiency above threshold and injected carrier density in barrier and optical trap layers are studied both theoretically and experimentally. If only the classical drift-diffusion model was used for design, lower values of the internal efficiency and higher values of the threshold current density were experimentally obtained if compared with modelling. We attributed these effects to the less efficient carrier capture in the QW region. As a consequence, the carrier population in the barrier layers is significantly larger than predicted by a classical drift-diffusion model. After optimising the active region thickness and the barrier/confinement configuration, the target of our low confinement design was achieved: values of the threshold current density of 300 A/cm2, absorption coefficient of 1 cm-1 and CW operation up to 36 mW/mm for uncoated facets were measured for our devices. The series resistance is about 2 x 10-4 W×cm2, comparable with values typical for common symmetric designs. This is a consequence of the fact that, even if somewhat lower doped, the thickness of the p-confinement layer is smaller and the maximum of the optical field is displaced in the n-type layers. Repeated anodic oxidation was used here for defining the ridge-shaped stripe of laser diodes. Although anodic oxidation is a well known process for GaAs, very few reports are given in literature for AlxGa1-xAs. The etch rate significantly decreases when the Al content x of the layer increases. This work reports the results of studies on the material etch rate as a function of Al content and the etch profile for laser structures grown on n++ substrates. It is found that this method offers an excellent etch depth control, with an accuracy of 20-30 nm for 1 mm total etch depth. Unfortunately, it can only be used for stripes wider than 10 mm, since the profile is strongly underetched for GaAs/Al0.60Ga0.40As configurations used in laser structures. If only one material is etched, for example GaAs, the profile is normal, i.e. the underetch is approximately equal to the etch depth. As soon as the interface Al0.60Ga0.40As is crossed, the profile becomes more underetched. As a final conclusion, the concept of "low confinement" in laser diode structures proves to have definite advantages over the classical design and is worthwhile to be developed further towards commercial CW devices. Mirror coating would improve the output power level by a factor of about 3 if appropriate coatings are used. For the lateral behaviour, an interesting development is the tapered laser design using a low confinement structure. Due to less antiguiding it would allow fundamental lateral mode operation up to higher power output. For single emitters with low threshold current density, stress-induced effects have to be minimised by careful choice of the oxide used as well as the process parameters and heat treatment.