R. Czernecki

Degradation mechanisms in InGaN laser diodes grown on bulk GaN crystals

We have investigated the aging processes in InGaN laser diodes fabricated by metal organic vapor phase epitaxy on low-dislocation-density, high-pressure-grown bulk gallium nitride crystals. The measured threshold current turned out to be a square root function of aging time, indicating the importance of diffusion for device degradation. The differential efficiency, in contrast, was roughly constant during these experiments. From these two observations we can conclude that the main reason for degradation is the diffusion-enhanced increase of nonradiative recombination within the active layer of the laser diode. Additionally, microscopic studies of the degraded structures did not reveal any new dislocations within the active area of the aged diodes, thus identifying point defects as a source of nonradiative processes.

Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes

The low-temperature breakdown of the electroluminescence intensity (ELI) of blue/violet InGaN-based light-emitting diodes (LEDs) is shown to be independent of the structural details of the LED active region. Instead, the presence of an electron blocking layer (EBL) plays a decisive role. The authors attribute the ELI collapse to the low-temperature hole-blocking properties of the EBL. However, removing the EBL leads to a much reduced ELI because of a disproportional increase of electron overflow processes, which shows that the presence of an EBL in blue/violet InGaN-based LEDs is still essential. Optimization of the EBL by means of Mg doping is discussed.

Application of a composite plasmonic substrate for the suppression of an electromagnetic mode leakage in InGaN laser diodes

We demonstrate an InGaN laser diode, in which the waveguiding quality of the device is improved by the introduction of highly doped (plasmonic) layer constituting an upper part of the GaN substrate. Thanks to this, we were able to suppress the electromagnetic mode leakage into the substrate without generating additional strain in the structure, in contrast to the typical design relying on thick AlGaN claddings. The plasmonic substrate is built as a stack of gallium nitride layers of various electron concentrations deposited by a combination of hydride epitaxy and high-pressure solution method. The mentioned improvements led to the reduction of the threshold current density of our devices down to 2 kA/cm2 and to the optimization of the near and far field pattern.

Correlation between luminescence and compositional striations in InGaN layers grown on miscut GaN substrates

The influence of the miscut angle of GaN substrate on compositional and optical properties of InxGa1−xN epilayers (0.05<x<0.1) was examined using x-ray diffraction, photoluminescence (PL), cathodoluminescence, and Z-contrast scanning electron microscopy. We show that single atomic steps bunch during growth of InGaN and form macrosteps. Indium is incorporated differently at treads and risers of these macrosteps, which causes the layer to decompose and induces the formation of compositional growth striations. Since the growth step density increases with growing miscut angle of the substrate, the average indium concentration decreases and the average PL peak energy blueshifts and broadens with increasing miscut angle. The presented work enables understanding on microscopic scale effects related to the inhomogeneous distribution of indium in InGaN layers on miscut substrates, which is significant from the point of view of optoelectronic applications.

Search for free holes in InN:Mg-interplay between surface layer and Mg-acceptor doped interior

We measured lateral ac transport (up to 20 MHz), thermopower, as well as resistivity and Hall effect in InN:Mg samples with various Mg content. The sign of the Hall effect for all the samples was negative (electrons), however, the thermopower (α) measurements have shown the p-type sign of α for moderate Mg content—in the window centered around 1×1019 cm−3. Further overdoping with Mg yields donor type of defects and the change of thermoelectric power sign. The ac measurements performed as a function of frequency revealed that in both samples exhibiting and nonexhibiting p-type sign of thermopower, the n-type inversion layer at the surface does not prevent the electric contact to the bulk layer. Therefore we conclude that the n-type Hall effect invariably reported for all the Mg-doped samples originates from electron domination in mobility-weighted contributions of both types of carriers.

Optimization of InGaN–GaN MQW Photodetector Structures for High-Responsivity Performance

InGaN-GaN multiple-quantum-well (MQW)-based photodetectors, with a detection edge at 450 nm and a high responsivity, have been fabricated and characterized. We show that the performance of MQW-based photodetectors strongly depends on a proper device design, i.e., number of QWs, and barrier and blocking layer thickness and doping level. Namely, the responsivity can be varied in the ~ 1 to ~ 100 mA/W range in similar structures and with the same number of QWs. These results support a model where the photocurrent increase is due to the improvement of collection efficiency caused by a change in transport mechanism for carriers photogenerated in the QWs. The transport mechanism depends on the location of the QWs in relation to the depletion region.

Elimination of AlGaN epilayer cracking by spatially patterned AlN mask

The inherent problem in III-nitride technology is the cracking of AlGaN layers that results from lattice mismatch between AlGaN and GaN. In case of thin substrates (30–90μm), such as, bulk GaN grown by the high-pressure/high-temperature method, the bowing of AlGaN∕GaN strained structures becomes an additional problem. To eliminate cracking and bowing, AlGaN layers were grown on GaN substrates with an AlN mask patterned to form 3–15μm wide windows. In the 3μm window, the AlGaN layer was not cracked, although its thickness and Al composition exceeded critical values for growth on nonpatterned substrates. Dislocation density in the windows was of 5×106∕cm2.

Efficient radiative recombination and potential profile fluctuations in low-dislocation InGaN∕GaN multiple quantum wells on bulk GaN substrates

We investigated the relation between structural properties and carrier recombination processes in InGaN∕GaN multiple quantum well (MQW) structures with quantum well widths of 3 and 9nm, grown by metal-organic chemical-vapor deposition on bulk GaN crystals. Quantum barriers of the samples are heavily n-type doped in order to effectively screen the large polarization-induced electric fields which commonly occur in hexagonal InGaN∕GaN quantum structures. High thermal stability in these structures, reflected by strong photoluminescence (PL) even above 400K, is attributed to a combination of low-dislocation densities and potential profile fluctuations in the InGaN∕GaN quantum wells. The role of potential profile fluctuations is further investigated by time-resolved photoluminescence and cathodoluminescence (CL) mapping. Comparison of both samples shows that the sample with 3‐nm-wide QWs exhibits (i) a larger width of the PL peak in the temperature range of 8–420K, (ii) a higher amplitude of potential profile f...

Cavity suppression in nitride based superluminescent diodes

We have fabricated two types of InGaN superluminescent diodes applying two different concepts of cavity suppression: a tilted waveguide geometry and passive absorber solution. Both types of devices showed superluminescence behavior, but both eventually lased under the application of high enough current. The lasing threshold turned out to be higher for tilted waveguide devices. By using long (2 mm) waveguides, we managed to demonstrate the power in superluminescent mode exceeding 100 mW in blue/violet part of the spectrum.

Effect of efficiency “droop” in violet and blue InGaN laser diodes

We have studied two types of InGaN laser diodes emitting at 410 and 440 nm. Each device was characterized by measuring light-current characteristics in two geometries for which the light was collected: along the resonator and perpendicularly to the cavity. In the first configuration, the 410 nm device displays no reduction of differential efficiency while 440 nm laser shows evidence of droop. In the perpendicular configuration both devices show the pronounced droop. We associate the suppression of the droop for 410 nm laser in the “along cavity” configuration with the appearance of the stimulated recombination.