Szymon Grzanka

Visible light communications using a directly modulated 422 nm GaN laser diode

Visible light communications using a Gallium-nitride (GaN) laser diode is reported. Devices, which are cased in TO packages, show modulation bandwidths of up to 1.4 GHz. We demonstrate error-free data transmission, defined as transmission of 1×10(-9) bits without any errors, at 2.5 Gbit/s with a sensitivity of 11.5 dBm.

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.

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.

Growth mechanism of InGaN by plasma assisted molecular beam epitaxy

In this article, the authors discuss the mechanism of InGaN growth by plasma assisted molecular beam epitaxy. They present the evidence of the influence of substrate miscut on indium incorporation for the growths with different gallium fluxes. They propose and discuss the phenomenological model which describes the incorporation of indium into InGaN layers grown under the indium-rich conditions that takes into account following parameters: gallium and nitrogen fluxes, miscut angle, and the growth temperature.

InGaN laser diodes operating at 450–460 nm grown by rf-plasma MBEa)

This work demonstrates the first true blue laser diodes (LDs) grown by plasma assisted molecular beam epitaxy that operate at the region of 450–460 nm. The single quantum well LDs were grown on several types of c-plane bulk GaN substrates, with threading dislocation densities varying from 104 to 108cm−2. The key factors that allowed the authors to achieve lasing in true-blue wavelengths are improvements in the growth technology of the InGaN quantum wells attributed to the high nitrogen flux used and the design of the LD structure, which reduced the light losses in the cavity. The authors discuss the influence of the diodes’ design on the parameters of LDs.

Simulation of trap-assisted tunneling effect on characteristics of gallium nitride diodes

In this paper, simulations of I-V characteristics and band structures of magnesium and silicon doped gallium nitride diodes are presented. The numerical algorithm is based on the drift-diffusion semi-classic model, with the van Roosbroeck differential equation system involved. The model accounts for trap-assisted tunneling, which provides better agreement between the predicted and experimental I-V characteristics of p-n junctions in the low-bias range. We have performed one-dimensional simulations of devices. We compare the results with the results obtained from the standard drift-diffusion model. It is shown that taking the trap-assisted tunneling into account leads to good agreement with experimental data. We also demonstrate that a high doping of the p-n junctions may significantly increase the nonradiative recombination rate due to the prescribed effect.

Hole carrier concentration and photoluminescence in magnesium doped InGaN and GaN grown on sapphire and GaN misoriented substrates

Systematic studies of InxGa1−xN layers (0≤x<0.13) doped with Mg were performed. Samples were grown by metal organic vapor phase epitaxy. Intermediate Mg doping in the range of 2×1019 cm−3 was chosen to achieve a maximum hole carrier concentration, pH (as measured by Hall effect) of 4×1018 cm−3 in samples with high x. We confirmed reports on decreasing resistivity in InxGa1−xN:Mg epitaxial layers observed with increasing x. This finding is very important for applications. In the performed research we attempted to separate contributions to pH increase resulting from increase in In-content and an associated decrease in growth temperature, Tgr (necessary to obtain high x). For this purpose In-content increase was achieved by means of either (i) lowering the growth temperature (from 1020 to 830 °C) or by (ii) varying an intended GaN substrate miscut. We demonstrated that the increase in pH in InxGa1−xN:Mg is caused by higher In concentration while a drop in Tgr plays a secondary role. Studies of photoluminesce...

Enhancement of optical confinement factor by InGaN waveguide in blue laser diodes grown by plasma-assisted molecular beam epitaxy

The influence of InxGa1−xN waveguide on the properties of blue (λ = 450 nm) laser diodes grown by plasma-assisted molecular beam epitaxy was studied. The threshold current density was reduced by 50% (to 3.6 kA/cm2) when the indium content was increased from x = 0.04 to 0.08. This is explained by a substantial enhancement of the optical confinement factor for a high-In-content waveguide, which increases the differential modal gain. Furthermore, we observed a decrease in optical losses when Mg-doped layers were separated from the active region by a thicker waveguide. This is attributed to the lower overlap between the optical mode and absorptive Mg-doped layers.

AlGaN-Free Laser Diodes by Plasma-Assisted Molecular Beam Epitaxy

Room-temperature continuous wave lasing at 432 nm with a threshold current of 7.6 kA/cm2 for nitride-based laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy is reported. The diodes were grown on c-plane GaN substrates with a threading dislocation density of 5×107 cm-2. We used a simplified laser structure design with GaN claddings where the optical modes were confined by the thick 120 nm In0.08Ga0.92N waveguide. Our LDs show a high optical output power of 130 mW, a differential gain of 0.5 W/A, and a lifetime of 50 h.