Gain spectroscopy in violet and ultraviolet InAlGaN laser structures
Even though GaN-based laser diodes emitting blue-violet have already reached high levels of performance a lot of the fundamental issues (optical confinement, origin of optical loss) in these laser structures are not fully understood. A better understanding of these issues is pivotal for development of laser devices at even shorter wavelength. The target of our investigation was to study the optical gain and loss mechanism in optically pumped laser heterostructures emitting in the wavelength range from 400nm to 340nm. We report on gain spectroscopy using the variable-stripe-length-method performed on InAlGaN-heterostructures grown on c-plane sapphire substrates by MOCVD. Modal gain of ternary (InGaN, AlGaN ) and quaternary (InAlGaN) multi quantum wells (MQW) was studied as well as modal losses due to leakage or subbandgap absorption in Mg-doped waveguide layers. The primary benefit of the optical pumping technique is the ability to evaluate the optical properties of the active region and waveguide layers separately from the electronic properties of the device. Independent of the emission wavelength we consistently observed enhanced photoluminescence and optical gain due to incorporation of Indium within the active region. For laser heterostructures with InAlGaN MQW active region emitting at 342nm, we achieved optical gain of 60cm-1 and demonstrate optically pumped lasing at threshold power densities of 0.8MW/cm2 on laser cavities with dry-etched mirror facets without additional reflection coatings. The growth of high aluminum containing layers without the formation of cracks remains a challenge limiting the thickness of AlGaN cladding layers. Gain spectroscopy on ultraviolet (UV) lasers revealed absorption of leakage modes within the underlying GaN buffer layer as major loss mechanism. Since the photon energy in UV lasers is considerably larger then GaN bandgap already very small overlap of the optical mode (<0.1%) with the GaN buffer layer results in significant modal losses (¡Ö100cm-1). We will show, that this can be overcome by appropriate design of the waveguide in combination with an increased thickness of the cladding layer. Samples grown on a 3mm thick AlGaN cladding layer show significant reduction of leakage losses. The subbandgap absorption of Mg-doped waveguide layers was identified as second loss mechanism. We fond that the placement and the doping concentration of the Mg doped layers are of great importance for the performance of nitride laser structures. As-grown InGaN lasers emitting at 388 nm with Mg-doped GaN-waveguide layer required significantly higher (about 50kW/cm2) excitation powers to achieve the same net gain compared to undoped reference structures. Activation of the Mg-doped layer through rapid thermal annealing (8500C, 5 min) resulted in an even more pronounced decrease of the net gain. We find that the impact of Mg-doping on the net gain seemed to be less prominent with increasing Al-content in the waveguide layers.
Wolst, O.; Kneissl, M. A. ; Teepe, M. R. ; Miyashita, N. ; Yang, Z.; Johnson, N. M. ; Schmidt, O. ; Kiesel, P. ; Gain spectroscopy in violet and ultraviolet InAlGaN laser structures. International Workshop on Nitride Semiconductors, Pittsburgh (2004).