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Porous GaN has been proposed as a novel cladding material for visible light-emitting laser diodes (LDs). Fabrication of nanoporous-GaN bottom-cladding LDs was already realized by selective electrochemical etching (ECE) of the highly n-type doped GaN layer in the LD structure after epitaxy. In this work, we applied a reverse approach: in the first step, locally porous areas in GaN substrate were fabricated, and next, a LD structure was grown on top by plasma-assisted molecular beam epitaxy (PAMBE). We compare the electrical and optical properties of the devices with porous bottom cladding with the devices from the same wafer that was grown on top of a standard GaN layer. Continuous wave (CW) operation is achieved for porous LD at 435.4 nm and slope efficiency of 0.046 W/A. Standard LD was lased in CW mode at 442.6 nm and had a slope efficiency of 0.692 W/A. In porous LD, the internal losses were estimated using the Hakki-Paoli method to be 68 cm−1, while for standard LD, the losses were 25 cm−1. Near-field patterns recorded for the studied devices indicate light scattering on the porous layer to be the possible reason for the increased losses in porous LD.

Typical methods of doping quantification are based on spectroscopy or conductivity measurements. The spatial dopant distribution assessment with nanometer-scale precision is limited usually to one or two dimensions. Here we demonstrate an approach to detect three-dimensional dopant homogeneity in GaN:Si layers using electrochemical etching (ECE). GaN:Si layers are grown by plasma-assisted molecular beam epitaxy. Dopant incorporation is uniform when the growth front morphology is atomically flat. Non-uniform Si incorporation into GaN is observed when step-bunches are present on the surface during epitaxy. In this study we show that local Si concentration in the area of step-bunch is about three times higher than in the area between step-bunches. ECE spatial resolution in our experiment is estimated to be about 50 nm. This makes ECE a simple and quantitative probing tool for local three-dimensional conductivity homogeneity assessment. Our study proves that ECE could be important both for fundamental studies of crystal growth physics and impurity incorporation and for ion-implanted structures and post-processing device control.

We demonstrate electrically pumped III-nitride edge-emitting laser diodes (LDs) with nanoporous bottom cladding. The LD structure was grown by plasma-assisted molecular beam epitaxy. Highly doped 350 nm thick GaN:Si cladding layer with Si concentration of 6 x 1019 cm-3 was electrochemically etched to obtain porosity of 15 +/- 3% with pore size of 20 +/- 9 nm. The devices with nanoporous bottom cladding are compared to the refer-ence structures. The pulse mode operation was obtained at 448.7 nm with a slope efficiency (SE) of 0.2 W/A while the reference device without etched cladding layer was lasing at 457 nm with SE of 0.56 W/A. The de-sign of the LDs with porous bottom cladding was modelled theoretically. Performed calculations allowed to choose the optimum porosity and thickness of the cladding needed for the desired optical mode confinement and reduced the risk of light leakage to the substrate and to the top-metal contact. This demonstration opens new possibilities for the fabrication of III-nitride LDs., 12 pages, 7 figures

Utilizing self-consistent numerical simulation in good agreement with measurements, we analyze internal device physics, performance limitations, and optimization options for a unique laser design with multiple active regions separated by tunnel junctions, featuring surprisingly wide InGaN quantum wells. Contrary to common assumptions, these quantum wells are revealed to allow for perfect screening of the strong built-in polarization field, while optical gain is provided by higher quantum levels. However, internal absorption, low p-cladding conductivity, and self-heating are shown to strongly limit the laser performance.

Worldwide research efforts have been focusing on quantum efficiency enhancements of GaN-based light emitters. A promising approach is the separation of multiple active regions by tunnel junctions, enabling electron-hole pairs to generate more than one photon. Utilizing advanced numerical device simulation, we here analyze internal physics and performance limitations of such InGaN/GaN bipolar cascade laser which recently demonstrated superior slope efficiency.

The stability of the Nitrogen-polar (000-1) surface of single-crystal bulk GaN substrates is studied for layers grown by plasma-assisted molecular beam epitaxy (PAMBE) in Nitrogen-rich conditions at 730 °C. It is shown that smooth GaN layers with parallel atomic steps are obtained for substrates when the surface crystal miscut angle is larger than 2o, revealing a highly stable epitaxial growth regime on single crystals. A step meandering pattern is observed on layers grown on lower miscut angle substrates. The meandering periodicity is found to have an inverse dependence on growth rate and miscut angle. This is opposite to what is observed for epitaxy on the Ga-polar surface. Combining analytic modeling and kinetic Monte Carlo simulations, it is shown that the existence of an Ehrlich-Schwoebel Barrier (ESB) in the PAMBE growth of GaN in nitrogen-rich conditions on (000-1) GaN reproduces the experimentally observed periodicity of step meandering. Assuming that ESB height depends on interactions between diffusing adatoms, all experimental phenomena are reproduced.

Blue and violet light-emitting devices based on III-nitrides caused an ongoing revolution in general lighting. One of the highly deliberated discussions in this field is devoted to the problem call...

New growth window for N-polar nitride devices is reported for plasma-assisted molecular beam epitaxy (PAMBE). Nitrogen-rich growth conditions were employed to obtain good optical quality N-polar ( 0 0 0 1 ¯ ) InGaN/GaN heterostructures. Atomically flat interfaces were obtained for N-rich growth on ( 0 0 0 1 ¯ ) GaN substrates with high miscut angle of 4° and 7°. Ga-polar samples grown at the same growth conditions were rough and exhibit no indium incorporation. Obtained results point out significantly different growth kinetics on both GaN polarities and offer efficient way for fabricating N-polar devices by PAMBE.

In this paper the reliability of III-nitride blue laser diodes grown by plasma-assisted molecular beam epitaxy on low threading dislocation density Ammono-GaN substrates is studied. It is found that defects formed in the heavily Mg-doped electron blocking layer (EBL) strongly affect the lifetime of the devices. These defects are identified as basal stacking fault domains which create threading dislocations. The effect of the EBL growth conditions on their generation and influence on lifetime of devices is presented. By optimization of the growth conditions of the EBL the lifetime of III-nitride laser diodes has been increased from 2000 up to 100,000 h.

Study of N-polar GaN surface barrier. Corrigendum: Available online 4 February 2020: Łukasz Janicki, Jan Misiewicz, Marcin Siekacz, Henryk Turski, Joanna Moneta, Sandeep Gorantla, Czesław Skierbiszewski, Robert Kudrawiec; Corrigendum to “Sensitivity of N-polar GaN surface barrier to ambient gases” [Sens. Actuators B: Chem. 281(2019) 561–567]. https://doi.org/10.1016/j.snb.2020.127782, Corrigendum: Available online 4 February 2020: Łukasz Janicki, Jan Misiewicz, Marcin Siekacz, Henryk Turski, Joanna Moneta, Sandeep Gorantla, Czesław Skierbiszewski, Robert Kudrawiec; Corrigendum to "Sensitivity of N-polar GaN surface barrier to ambient gases" [Sens. Actuators B: Chem. 281(2019) 561–567]. https://doi.org/10.1016/j.snb.2020.127782

Properties of tunnel junctions (TJs) grown by plasma-assisted molecular beam epitaxy are investigated. Examined TJs consist of a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum well (QW) sandwiched between ${\mathrm{In}}_{0.02}{\mathrm{Ga}}_{0.98}\mathrm{N}$ barriers. The influence of piezoelectric polarization, doping, and width of a TJ QW on resistivity of TJs is studied. A decrease of TJ resistivity is observed for high doping and high ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ composition. Additionally, the crystal quality of TJs as a function of magnesium and silicon doping in a ($\mathrm{In},\mathrm{Ga})\mathrm{N}$ QW is discussed. Generation of defects in highly doped layers is observed in transmission electron microscopy and by defect selective etching. High doping leads to deterioration of surface morphology, which is corroborated by atomic force microscopy imaging. TJs with low resistance and high crystalline quality, without formation of structural defects, are demonstrated. A theoretical model for calculation of tunneling currents is proposed based on Kane tunneling theory, which considers a realistic electric field distribution in a TJ. A good agreement with the experimental data is obtained. The model is used to predict electrical parameters in a wide range of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ compositions (x = 0--0.40), doping (${n}_{\mathrm{Mg},\mathrm{Si}}={10}^{17}\ensuremath{-}5\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$), and TJ QW width (d = 2 -- 12 nm).

In this work, we present a GaN-based blue LED construction utilizing bottom tunnel junction (TJ) grown by plasma-assisted molecular beam epitaxy. The setup allows for N-polar-like built-in field alignment while being grown on a Ga-polar substrate. In this study, we present an efficient bottom TJ LED in which the distance between the quantum well and device surface is only 25 nm. This is achieved by the utilization of an n-type current spreading layer consisting of 20 nm thick In0.02Ga0.98N with a Ge doping concentration of 7 × 1019 cm−3. Heavily Ge and Mg doped bottom TJs allowed achieving a low LED turn-on voltage of 2.75 V at 20 A/cm2 and a differential resistivity of 4.7 × 10−4 Ω cm2 at 1 kA/cm2. The device maintained high crystal quality and smooth morphology, which allows for its use as a light emitting platform for further integration. Although the p-up reference LED exhibits lower resistivity at high current, its luminous efficiency is lower than for bottom TJ LEDs.

A novel approach to fabricate efficient nitride light-emitting diodes (LEDs) grown on gallium polar surface operating at cryogenic temperatures is presented. We investigate and compare LEDs with standard construction with structures where p-n junction field is inverted through the use of bottom tunnel junction (BTJ). BTJ LEDs show improved turn on voltage, reduced parasitic recombination and increased quantum efficiency at cryogenic temperatures. This is achieved by moving to low resistivity n-type contacts and nitrogen polar-like built-in field with respect to current flow. It inhibits the electron overflow past quantum wells and improves hole injection even at T=12K. Therefore, as cryogenic light sources, BTJ LEDs offer significantly enhanced performance over standard LEDs.

The incorporation of indium in GaN (0001) surfaces in dependence of strain is investigated by combining molecular-beam epitaxy (MBE) growth, quantitative transmission electron microscopy, and density-functional theory (DFT) calculations. Growth experiments were conducted on GaN, as well as on $30%\ifmmode\pm\else\textpm\fi{}2%$ partially relaxed $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer layers, serving as substrates. Despite the only $0.6%$ larger in-plane lattice constant of GaN provided by the buffer layer, our experiments reveal that the In incorporation increases by more than a factor of two for growth on the $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer, as compared to growth on GaN. DFT calculations reveal that the decreasing chemical potential due to the reduced lattice mismatch stabilizes the In--N bond at the surface. Depending on the growth conditions (metal rich or N rich), this promotes the incorporation of higher In contents into a coherently strained layer. Nevertheless, the effect of strain is highly nonlinear. As a consequence of the different surface reconstructions, growth on relaxed $\mathrm{I}{\mathrm{n}}_{x}\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{N}$ buffers appears more suitable for metal-rich MBE growth conditions with regard to achieving higher In compositions.

We show that in a wide In0.17Ga0.83N quantum well, placed within an undoped region of the pin diode, a photocurrent in the forward direction is observed. The photocurrent switches to reverse direction when the light intensity is increased and/or photon energy is above the bandgap of the quantum barrier. We propose a model showing that the anomalous photocurrent is due to the fact that when the carriers are pumped into the wide quantum well they cannot recombine until the built-in field is screened. For low-intensity light it takes a long time (milliseconds) for the screening to occur and during that time we observe current flowing in the forward direction. This current originates from the reorganization of carriers forming the depletion regions, rather than directly from the photogenerated carriers. The observed effects lead to the dependence of PC spectra on chopper frequency and on light power. They may also affect the operation of laser diodes and solar cells with wide InGaN quantum wells.

Recently, the use of bottom-TJ geometry in LEDs, which achieves N-polar-like alignment of polarization fields in conventional metal-polar orientations, has enabled enhancements in LED performance due to improved injection efficiency. Here, we elucidate the root causes behind the enhanced injection efficiency by employing mature laser diode structures with optimized heterojunction GaN/In0.17Ga0.83N/GaN TJs and UID GaN spacers to separate the optical mode from the heavily doped absorbing p-cladding regions. In such laser structures, polarization offsets at the electron blocking layer, spacer, and quantum barrier interfaces play discernable roles in carrier transport. By comparing a top-TJ structure to a bottom-TJ structure, and correlating features in the electroluminescence, capacitance-voltage, and current-voltage characteristics to unique signatures of the N- and Ga-polar polarization heterointerfaces in energy band diagram simulations, we identify that improved hole injection at low currents, and improved electron blocking at high currents, leads to higher injection efficiency and higher output power for the bottom-TJ device throughout 5 orders of current density (0.015–1000 A/cm2). Moreover, even with the addition of a UID GaN spacer, differential resistances are state-of-the-art, below 7 × 10−4 Ωcm2. These results highlight the virtues of the bottom-TJ geometry for use in high-efficiency laser diodes.

New approach towards efficient light emission with bottom-tunnel junctions is developed. The bottom-tunnel junction design aligns the polarization fields in a desired direction in the vicinity of quantum well, while simultaneously eliminating the need for p-type contacts, and allowing efficient current spreading. By preventing electron overshoot past quantum wells, it disables carrier recombination in undesired regions of the heterostructures, increasing injection efficiency and opening new possibilities in heterostructure design. InGaN-based buried-tunnel junction is used to construct first monolithically grown p-type-down laser diode on n-type, Ga-polar bulk GaN substrate. Unique advantages of such construction that enables to separate design of carrier injection and optical mode confinement for such laser diode structures is discussed.

We investigated the influence of the In0.17Ga0.83N:Mg contact layer grown by plasma assisted molecular beam epitaxy on the resistivity of p-type Ni/Au contacts. We demonstrate that the Schottky barrier width for p-type contact is less than 5 nm. We compare circular transmission line measurements with a p-n diode current-voltage characteristics and show that discrepancies between these two methods can occur if surface quality is deteriorated. It is found that the most efficient contacts to p-type material consist of In0.17Ga0.83N:Mg contact layer with Mg doping level as high as 2 × 1020 cm–3.

Doping of Ga(Al)N layers by plasma-assisted molecular beam epitaxy in Ga-rich conditions on c-plane bulk GaN substrates was studied. Ga(Al)N samples, doped with Mg or Si, grown using different growth conditions were compared. In contrast to Si doped layers, no change in the Mg concentration was observed for layers grown using different growth rates for a constant Mg flux and constant growth temperature. This effect enables the growth of Ga(Al)N:Mg layers at higher growth rates, leading to shorter growth time and lower residual background doping, without the need of increasing Mg flux. Enhancement of Mg incorporation for Al containing layers was also observed. Change of Al content from 0% to 17% resulted in more than two times higher Mg concentration.

In this paper we review the recent studies on wide InGaN quantum wells (QWs). InGaN QWs are known to suffer from an extremely high built-in piezoelectric polarization, which separates the electron and hole wavefunctions and causes the quantum-confined Stark effect. We show both by means of modeling and experimentally, that wide InGaN QWs can have quantum efficiency superior to commonly used thin QWs. The high efficiency is explained by initial screening of the piezoelectric field and subsequent emergence of optical transitions involving the excited states of electrons and holes, which have a high oscillator strength. A high pressure spectroscopy and photocurrent measurements are used to verify the mechanism of recombination through excited states. Furthermore, the influence of QW width on the properties of optoelectronic devices is studied. In particular, it is shown how the optical gain forms in laser diodes with wide InGaN QWs.

We examined electroluminescence from In0.17Ga0.83N/GaN quantum wells (QW) of light-emitting diodes (LEDs) and laser diodes (LDs). For increasing QW width we observe transition from electron and hole ground-states recombination to excited states recombination. The effect is accompanied by partial (2.6 nm, 5.2 nm, 7.8 nm QW) or practically complete (10.4 nm QW) screening of the built-in electric field with increasing driving current for both types of emitters. The electric field magnitude was studied using an original high pressure method. The investigations are supported by simulations of the variation with driving current of i) electron and hole wavefunctions overlap affecting the recombination channel, ii) built-in electric field.

Semipolar ( 20 2 1 ) , nonpolar m-plane ( 10 1 0 ) and polar c-plane (0001) GaN and InGaN layers were grown by plasma-assisted molecular beam epitaxy. The surface of semipolar and nonpolar GaN grown under Ga-rich conditions is very smooth. The indium incorporation efficiency in InGaN layers grown under In-rich growth conditions is studied on three surface orientations (i) as a function of temperature from 570 to 650 °C and (ii) for varied active nitrogen flux from 0.41 to 2.03 µm/h. The In content follows the relation ( 10 1 0 ) ( 20 2 1 )

The tunnel junctions (TJs) in the III-nitrides optoelectronic structures open possibilities for new applications such as vertically integrated multicolor light emitting diodes [1], [2] or laser diodes [3], [4]. In plasma-assisted molecular beam epitaxy (PAMBE) the hydrogen-free growth process is used that allows to obtain p-type conductivity in buried layers without post growth activation. This approach allows to integrate vertically optoelectronic structures with low series resistance TJs. Therefore PAMBE seems to be better suited than metal organic vapor-phase epitaxy (MOVPE) for practical realization of the vertical devices with buried p-type region - especially for laser diodes (LD) containing interband TJs [3]. It was shown by S. Krishnamoorthy et al. that by adding an undoped InGaN quantum well (QW) between heavily doped p-type and n-type GaN layers tunneling probability increases by orders of magnitude [5]. This effect is caused by strong polarization field inside GaN:Mg/InGaN/GaN:Si heterostructure.

Most commercially available nitride devices are obtained along [0001] direction. That is why the internal polarization-induced electric fields in violet to green nitride light emitting diode (LED) and laser diode (LD) structures point in a direction opposite to what is desired for efficient flow of electrons and holes. This arrangement persist because of the need to have p-type layers on top of the structure to activate it and the lack of efficient structures grown along [0001] direction. To go around these problems one can use plasma-assisted molecular beam epitaxy (PAMBE) to grow buried tunnel junction (TJ) and invert current flow direction with respect to the built-in polarization [1], [2]. Recent progress in PAMBE resulted in obtaining LDs in wide spectral range (Fig 1) and enabled combining these structures with TJs [3] and even stacking two LDs together [4].

We demonstrate a stack of two III-nitride laser diodes (LDs) interconnected by a tunnel junction grown by plasma-assisted molecular beam epitaxy. Hydrogen-free growth is used to obtain as-grown p-type conductivity essential for buried tunnel junctions (TJ). We show the impact of the design of tunnel junction. In particular, we show that, apart from the beneficial piezoelectric polarization inside the TJ, heavy doping reduces the differential resistivity even further. The device starts to lase at a wavelength of 459 nm with a slope efficiency (SE) of 0.7 W/A followed by lasing at 456 nm from the second active region doubling the total SE to 1.4 W/A. This demonstration opens new possibilities for the fabrication of stacks of ultraviolet and visible high power pulsed III-nitride LD.

In this paper, we demonstrate a novel approach utilizing tunnel junction (TJ) to realize GaN-based distributed feedback (DFB) laser diodes (LDs). Thanks to the use of the TJ the top metal contact is moved to the side of the ridge and the DFB grating is placed directly on top of the ridge. The high refractive index contrast between air and GaN, together with the high overlap of optical mode with the grating, provides a high coupling coefficient. The demonstrated DFB LD operates at λ=450.15 nm with a side mode suppression ratio higher than 35dB. The results are compared to a standard Fabry-Perot LD.

We demonstrate the applications of tunnel junctions (TJs) for new concepts of monolithic nitride-based multicolor light emitting diode (LED) and laser diode (LD) stacks. The presented structures were grown by plasma-assisted molecular beam epitaxy (PAMBE) on GaN bulk crystals. We demonstrate a stack of four LDs operated at pulse mode with emission wavelength of 453 nm. The output power of 1.1 W and high slope efficiency of 2.3 W/A is achieved for devices without dielectric mirrors. Atomically flat surface after the epitaxy of four LD stack and low dislocation density is measured as a result of proper TJ design with optimized doping level. The strain compensation design with InGaN waveguides and AlGaN claddings is shown to be crucial to avoid cracking and lattice relaxation of the 5 &micro, m thick structure. Vertical connection of n-LDs allows for cascade emission of photons and increases the quantum efficiency n-times. The two-color (blue and green) LEDs are demonstrated. Application of TJs simplifies device processing, reducing the need for applications of p-type contact. The key factor enabling demonstration of such devices is hydrogen-free PAMBE technology, in which activation of buried p-type layers is not necessary.

We study the impact of increased active nitrogen flux (N-flux) on the indium content and structural properties of InAlN layers grown by plasma-assisted molecular beam epitaxy. It is shown that high N-flux can stabilize In-N bonds, so that In0.18Al0.82N is grown at 605 °C, which is the highest reported temperature so far for the composition lattice-matched (LM) to GaN. A diagram of InAlN indium content is shown as a function of growth temperature and N-flux. The InAlN layers grown using low and high N-flux had grainy surface morphology typical for N-rich conditions. Inhomogeneity in indium distribution on nanometer scale, i.e. typical honeycomb microstructure, is found for InAlN layers grown using both: low and high N-flux. An increase of average cell size is observed for LM-InAlN when the N-flux and growth temperature are increased.

One of the most challenging issues to address in nitride based devices like light emitting diodes (LEDs) and laser diodes (LDs) is poor conductivity of p-type region. Additionally, high resistance of the p-type contact, that is very difficult to reduce, determines performance and influences design of InGaN LEDs and LDs. Recently, there has been increasing attention given to the interband tunnel junctions (TJ) for effective carrier conversion between n-type and p-type material region in nitride LEDs and LDs. It was clearly demonstrated that TJ resistance for wide bandgap semiconductors can be effectively suppressed by making use of the piezoelectric fields in the region of the junction [1]. Low resistance TJ were investigated and realized using GaN/InGaN/GaN heterostructures. The MBE technology seems to be more efficient than MOVPE for practical realization of the devices with TJ. For MOVPE it is difficult to activate the p-type conductivity in the (In)GaN:Mg layers which are deeply buried below n-type layers due to the fact that diffusion of hydrogen is completely blocked through n-type layers. Therefore several attempts for hybrid MOVPE/MBE growth was performed and TJ LEDs [2], TJ LDs [3] and TJ VCSELs [4] were shown where active p-i-n region was grown by MOVPE and p-n TJ by MBE. However, the presence of the impurities in the MOVPE p-type surface used for MBE regrowth can deteriorate/limit performance of devices grown by hybrid method.

In this paper, we study the plastic relaxation of InGaN layers deposited on (0001) GaN bulk substrates and (0001) GaN/sapphire templates by molecular beam epitaxy. We demonstrate that the InGaN layers relax by the formation of (a+c)-type misfit dislocations gliding on pyramidal planes in the slip system ⟨11 2 ¯3⟩{11 2 ¯2} down to the interface where they form a trigonal dislocation network. Combining diffraction contrast and large angle convergent beam electron diffraction analyses performed using a transmission electron microscope, we reveal that all (a+c)-type dislocations belonging to one subset of the network exhibit Burgers vectors with the same c-component. This relaxation mechanism leads to partially relaxed InGaN layers with smooth surfaces and threading dislocation densities below 109 cm−2. Such layers are of potential interest as pseudo-substrates for the growth of InGaN heterostructures.

We present an innovative approach to integrate arrays of isolated, strain-free GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donor-bound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that a reduction of threading dislocations is achieved in their top part.

We present nitride based laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy. In this work we investigate influence of epitaxial structure on LDs pa-rameters. In particular we compare LDs with and without AlGaN cladding layers operating at 430 to 460 nm. We show results of waveguide simulations which explain the increasing threshold current density of longer wavelength LDs. The PAMBE LD with InGaN waveguides and AlGaN bottom cladding has threshold current density of 4.3 kA/cm2 and slope efficiency of 0.56 W/A. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we investigate InGaN/GaN short period superlattices (SPSLs) grown on (0001) GaN and on relaxed In0.2Ga0.8N buffers with varying degree of plastic relaxation. The SPSLs were fabricated by plasma assisted molecular beam epitaxy (PAMBE). Despite the nominal SPSL structure consisted of InN monolayers (MLs) embedded in GaN barriers, we observed single monolayer InGaN layers instead of InN. We study the photoluminescence (PL) of such SPSLs as a function of the substrates lattice constant. Increase of the substrate lattice constant from a = 3.183 A (for GaN grown on sapphire) to a = 3.216 A resulted in a peak emission shift from 379 nm to 419 nm. We attribute the PL red-shift to the increased In incorporation in the ML-thick InGaN on relaxed buffers as a consequence of the reduced lattice mismatch between the substrate and the film. Our experiment confirms that coherent growth of InN is prohibited on GaN and points out the important role of substrate lattice constant engineering for realization of InN/GaN SPSLs.