In this contribution, substrate modes in edge-emitting lasers in the material system Gallium-Nitride are analyzed by
means of comprehensive measurements and simulations. The simulations are complex vectorial optical mode
calculations using a finite-element method. The simulation domain comprises the ridge waveguide and the full substrate
with open boundary conditions on the sides. Therefore, the coupling mechanisms of the waveguides formed by the ridge
and the substrate can be analyzed in a realistic setup. The characterization data include the optical loss spectrum obtained
from Hakki-Paoli measurements, optical near field, and farfield measurements. The devices used for characterization are
ridge waveguide quantum well lasers grown on GaN substrates. A comparison of the measurement data with the
simulations explains the characteristics of the substrate modes in a consistent way, and shows very good agreement for
the optical loss oscillations, farfield angle, and nearfield pattern. It is shown that material losses, material dispersion and
optical diffraction are key ingredients for the analysis of substrate modes.
We demonstrate the potential of Kelvin Probe Force Microscopy (KPFM) for analyzing degradation effects in GaN-based
laser diodes (LDs). Thereby, the surface potential at the mirror facet was measured locally for both, unbiased LDs
and LDs exposed to a well-defined current. In the unbiased case, our KPFM measurements demonstrate the impact of
aging on the mirror facet, which we attribute to a photon enhanced facet oxidation. In case of an externally applied
voltage, the local variation of the Kelvin voltage across the heterostructure layer sequence is analyzed. A clear
correlation between macroscopic <i>I-V</i>-characteristics and the microscopic data obtained with the KPFM is found.
We combine a scanning near-field microscope (SNOM) with a time-resolved detection scheme to measure the mode dynamics of InGaN laser diodes emitting at 405 nm. Observed phenomena are filaments, mode competition, near-field phase dynamics, near-field to far-field propagation, and substrate modes. In this article we describe in detail the self-built SNOM, specialized for these studies. We also provide our recipe for SNOM tip preparation using tube etching. Then we compare the mode dynamics for a 3 μm narrow and a 10 μm wide ridge waveguide laser diode.
In this paper we present a combined current-voltage, capacitance-voltage, Deep Level Transient Spectroscopy and electroluminescence study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. In the early stages of the aging tests at low forward current levels (15-20 mA), LEDs present a decrease in optical power, which stabilizes within the first 50 hours and never exceeds 10% (measured at 20 mA). The spectral distribution of the electroluminescence intensity does not change with stress, while <i>C-V</i> profiles detect changes consisting in apparent doping and/or charge concentration increase within quantum wells. This increase is correlated with the decrease in optical power. Capacitance Deep Level Transient Spectroscopy has been carried out to clarify the DC aging induced generation/modification of the energy levels present in the devices. Remarkable changes occur after the stress, which can be related to the doping/charge variation and thus to the efficiency loss.
The realization of group III--nitride laser diodes with a vertical current path on a n-conducting SiC substrate is described. The vertical current path and the possibility of cleaved laser facets result in a simpler process technology. Gain spectra measured by the Hakki-Paoli method show a modulation of the modal gain due to parasitic modes in the SiC substrate. As up to now no defect reduction technique was successfully transfered to GaN on SiC,
degradation is the major issue. We discuss the impact of degradation on the gain spectra, facet degradation, and rule out formation of cracks during degradation. We show that the high heat conductivity of SiC may give an advantage with respect to degradation as it results in a only moderate temperature rise of the active region.
Solid state lighting has seen a rapid development over the last decade. They compete and even outperform light sources like incandescent bulbs and halogen lamps. LEDs are used in applications where brightness, power consumption, reliability and costs are key parameters as automotive, mobile and display applications. In the future LEDs will also enter the market of general lighting. For all of these new applications highly efficient, scalable and cost efficient technologies are required. These targets can be matched by SiC based flip chip LEDs which enable the design of high current chips with efficiencies of up to 28 lm/W in white solderable packages. An alternative approach is the implementation of thinfilm technology for GaInN. The LED is fabricated by transferring the epilayers with laser lift off from sapphire to a GaAs host substrate. In combination with efficient surface roughening and highly reflective p-mirror metalization an extraction efficiency of 70% and wall plug efficiency of 24% at 460 nm have been shown. The chips showed 16 mW @ 20 mA with a Voltage of 3.2 V. The technology is scalable from small size LEDs to high current Chips and is being transferred to mass production.
Data are presented for an GaInN based thinfilm LED. The LED is fabricated by transferring the epilayers with laser lift off from sapphire to a GaAs host substrate. In combination with efficient surface roughening and highly reflective p-mirror metallisation an extraction efficiency of 70% and wall plug efficiency of 24% at 460nm have been shown. The chips showed 12mW @ 20mA with a Voltage of 3.2V. The technology is scalable from small size LEDs to high current Chips and is being transferred to mass production.
Nanostructures based on III-V semiconductor materials have reached a status which enables basic physical studies on size effects in device and in nanostructures. The expected benefits of high modulation bandwidth, low laser threshold, and improved linewidth enhancement factor in DFB lasers, to say only a few, which are believed to be based mainly on the changed density of states (DOS) function in low dimensions might be counterbalanced by altered carrier energy relaxation and k-space filling in those structures. To investigate systematically size effects and device aspects, a continuous change of structure and active device size is needed from 2D to 0D dimensions. This requirement can be met by high resolution electron beam lithography in conjunction with low damage etch processes and epitaxial overgrowth. In this presentation we discuss the technology and design considerations of lasers with low dimensional active regions as well as DOS effects and device relevant carrier relaxation effects. The technology part will focus especially on low damage etch processes such as RIE- ECR. Nearly damage free structuring processes can be demonstrated. Based on this low damage dry etch process we obtained electrically pumped wire DFB lasers with relatively high output power (up to 6 mW) and operation temperature (60 degrees C). Time resolved optical ps-spectroscopy as well as high excitation spectroscopy on wire and dot nanostructures demonstrate strongly changed k-space filling and carrier relaxation mechanisms in low dimensions and represent a serious limitation of device speed. Results obtained from electrically pumped wire DFB lasers confirm the carrier relaxation and k-space filling effects in device structures which have been observed by optical pump experiments in nanostructures. Despite the band filling effects in low dimensional structures, the wire DFB lasers show clearly the expected feature of gain coupling and enhanced differential gain which might demonstrate the applicability of mesoscopic laser devices in common data communication approaches.