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The aim of this paper is to give an experimental evidence that point defects (most probably gallium vacancies) induce decomposition of InGaN quantum wells (QWs) at high temperatures. In the experiment performed, we implanted GaN:Si/sapphire substrates with helium ions in order to introduce a high density of point defects. Then, we grew InGaN QWs on such substrates at temperature of 730 °C, what caused elimination of most (but not all) of the implantation-induced point defects expanding the crystal lattice. The InGaN QWs were almost identical to those grown on unimplanted GaN substrates. In the next step of the experiment, we annealed samples grown on unimplanted and implanted GaN at temperatures of 900 °C, 920 °C and 940 °C for half an hour. The samples were examined using Photoluminescence, X-ray Diffraction and Transmission Electron Microscopy. We found out that the decomposition of InGaN QWs started at lower temperatures for the samples grown on the implanted GaN substrates what provides a strong experimental support that point defects play important role in InGaN decomposition at high temperatures.

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.

InGaN/GaN quantum wells grown by Metal Organic Chemical Vapor Phase Epitaxy (MOVPE) were initially studied by optical measurements and X-ray diffraction measurements. The comparison of these two techniques indicated that indium is not distributed homogeneously, which was confirmed by transmission electron microscopy in the nanometer scale. The experimental results of Secondary Ion Mass Spectrometry (SIMS) measurements showed that this analytic method can provide specific information on In spatial distributions not accessible by other methods. SIMS data revealed that In fluctuations occur only in the lower part of 2 nm thick InGaN quantum wells, whereas the QW composition is quite uniform in the upper parts. From the experimental data, one may estimate a SIMS depth resolution of about 0.2 nm and of about 1 μm in lateral directions.

Nitride-based light-emitting diodes (LEDs) are well known to suffer from a high built-in electric field in the quantum wells (QWs). In this paper we determined to what extent the electric field is screened by injected current. In our approach we used high pressure to study this evolution. In LEDs with a narrow QW (2.6 nm) we found that even at a high injection current a large portion of built-in field remains. In LEDs with very wide QWs (15 and 25 nm) the electric field is fully screened even at the lowest currents. Furthermore, we examined LEDs with a tunnel junction in two locations – above and below the active region. This allowed us to study the cases of parallel and antiparallel fields in the well and in the barriers.

Detailed comparison of optical quality of GaN layers grown homoepitaxially on bulk Ga-polar and N-polar substrates by plasma-assisted molecular beam epitaxy was performed. One order of magnitude lower photoluminescence (PL) intensity and decay time at room temperature was observed for layers grown on N-polar substrates, realized using gallium-rich conditions. This nonradiative recombination channel was very efficient also at liquid helium temperatures, what was evidenced by no improvement in PL time decay. Optical quality of GaN layer (PL intensity, lifetimes) grown on N-polar substrates was greatly improved by using nitrogen-rich growth conditions. We attribute this improvement to suppressed formation of nitrogen vacancy-related point defects. Finally, linewidth below 1 meV and lifetime above 0.1 ns (DX line, He temperature) were obtained for layers grown on N-polar substrate.

The distance between the showerhead and the sample surface (GAP) is one of the main growth parameters of the commonly used research reactor, Close Coupled Showerhead. We examine its influence on the growth rate of GaN layers deposited under various conditions (growth temperature, carrier gas, V/III ratio and growth pressure). Regardless of other growth parameters, increasing the GAP value leads to a reduction in the growth rate.

InGaN quantum wells were grown using metalorganic chemical vapor phase epitaxy (vertical and horizontal types of reactors) on stripes made on GaN substrate. The stripe width was 5, 10, 20, 50, and 100 &micro, m and their height was 4 and 1 &micro, m. InGaN wells grown on stripes made in the direction perpendicular to the off-cut had a rough morphology and, therefore, this azimuth of stripes was not further explored. InGaN wells grown on the stripes made in the direction parallel to the GaN substrate off-cut had a step-flow-like morphology. For these samples (grown at low temperatures), we found out that the InGaN growth rate was higher for the narrower stripes. The higher growth rate induces a higher indium incorporation and a longer wavelength emission in photoluminescence measurements. This phenomenon is very clear for the 4 &micro, m high stripes and less pronounced for the shallower 1 &micro, m high stripes. The dependence of the emission wavelength on the stripe width paves a way to multicolor emitters.

Advanced Substrates consist of a thin GaN layer bonded to a carrier wafer. The layer is separated from starting material by Smart Cut™ technology. GaN on sapphire Advanced Substrates were successfully used as seeds for HVPE-GaN growth. Unintentionally doped and silicon-doped thick GaN layers were crystallized. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates. Free-standing HVPE-GaN with high free carrier concentration was obtained. A laser diode was built on the n-type doped HVPE-GaN grown on an Advanced Substrate.

We describe surface patterning as a method to obtain discrete regions of different local miscut angles, δ, in semipolar ( 20 2 ¯ 1 ) GaN substrates. During patterning the region angle δ was varied between 0° and ±2° with respect to the growth plane. We chose two nonequivalent miscut directions: a 〈 1 ¯ 2 1 ¯ 0 〉 and c ׳ 〈 1 ¯ 01 4 ¯ 〉. On such patterned substrates In x Ga 1− x N/GaN quantum wells were grown by a Metalorganic Vapor Phase Epitaxy method. Angled regions were not planarized during growth and their initial miscut angles remained. We studied structural, morphological and optical properties of In x Ga 1− x N/GaN quantum wells as a function of δ . The intended In concentration, x , was about 13% on exact oriented ( 20 2 ¯ 1 ) planes. For miscut towards the a -direction, indium content decreases and luminescence energy increases with δ . For miscut towards c ׳, we observe quite different behaviors, indium content and quantum well width are constant and luminescence energy increases with δ . We propose the explanation of these effects on the basis of sample morphology, In-content and built-in electric field. The obtained results are important for understanding the role of miscut (intentional and unintentional) in basic properties of semipolar InGaN/GaN quantum well structures. It is also worth to note that, the spectral width of luminescence was the smallest for regions miscut 2° towards c ׳ direction, which can be important for semipolar optoelectronic devices.

In the case of nearly perfect semiconductor layers (as AlGaAs on GaAs), the experimental data of X-ray Diffraction are well simulated using the Dynamical Theory of X-ray Diffraction. In the case of AlGaN and InGaN layers, intensity of their XRD peaks depend very strongly on the crystallographic quality of not only the layers, but also the substrates. In the paper, we show differences in XRD peak intensities for the AlGaN layers on GaN of the same thickness and Al-content, but of different dislocation density.

InGaN/GaN multiple quantum well structures were grown on bulk GaN and on sapphire substrates using the metalorganic vapor-phase epitaxy in order to study the influence of hydrogen during the growth of the GaN barriers. This hydrogen flow had the following effects on the structures: (i) the thickness of the QWs was reduced, (ii) the indium concentration in the QWs was decreased and (iii) the growth rate of the quantum barriers was increased.

In this work, the electrical and structural properties of Ti/Al contacts on AlGaN/GaN heterostructures with a different crystalline quality were investigated. In particular, the sample with a lower defects density required a higher temperature (800 °C) to obtain Ohmic contacts, while the more defective sample showed Ohmic behaviour at lower annealing temperature (500 °C). The susceptibility to oxidation of the metal stack upon annealing has been monitored by chemical analyses. Moreover, the temperature dependence of the contact resistance of the Ti/Al annealed contacts revealed that the interface current transport mechanism depends on the material quality. These results were attributed to the presence of V-shaped defect close to the surface of the AlGaN barrier layer. A nanoscale electrical analysis demonstrated a preferential current conduction through these defects, which indeed plays a relevant role for the formation of the Ohmic contact.

The XPS technique combined with low energy Ar + ion sputter depth profiling was used to obtain information about chemical composition and reproducibility of the growth procedure by the MOVPE method of the InGaN multi quantum wells (MQWs). A good in-depth resolution of the XPS depth profiling technique, allows observation of InGaN quantum wells with thickness down to 2 nm. However quantitative characterization of very narrow QWs is limited by the elemental detection range of the XPS analysis. Both the InGaN QWs and GaN barriers parameters were well characterized using the model MQWs samples with various QW thickness. Inter-diffusion of indium within the GaN barrier layers, induced by high-temperature annealing, has been detected. This work evidences the successful application of the XPS depth profiling analysis as a very useful tool to optimize the growth parameters and thermal stability of InGaN/GaN MQWs.

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.

The influence of waveguide design on performance of nitride based laser diodes (LDs) grown by plasma assisted molecular beam epitaxy is studied. A large improvement in threshold current density and slope efficiency of LDs is observed when an InGaN interlayer with 8% In content is introduced between multi-quantum-well region and electron blocking layer. This dependence is attributed to reduction of internal losses due to lower optical mode overlap with highly absorptive Mg-doped layers. This led to demonstration of blue LD operating at λ =450 nm with high optical power of 500 mW per facet.

The first part of the paper shows the advantages and disadvantages of using GaN and SiC substrates with an off-cut up to 2 degrees. The following experimental observations were made: i) higher efficiency in p-doping with magnesium, ii) higher critical conditions for AlGaN cracking, iii) trigonal deformation of AlGaN and InGaN unit cells. A successful growth of AlGaN/GaN HEMT epi structure on 2 degree-off SiC substrates will be shown, what paves the way to a monolithic integration of GaN-based and SiC based devices on a common SiC substrate. In the second part of the paper, we show the experimental results on AlGaN/GaN degradation upon prolonged annealing at 600oC. In the case of epi structures on Si and sapphire, the parasitic parallel conductivity appeared, whereas in the case of SiC substrates, annealing introduced no distinct changes.

Advanced Substrates consist of a 200-nm-thick GaN layer bonded to a handler wafer. The thin layer is separated from source material by Smart CutTM technology. GaN on Sapphire Advanced Substrates were used as seeds in HVPE-GaN growth. Unintentionally doped and silicon-doped GaN layers were crystallized. Free-standing HVPE-GaN was characterized by X-ray diffraction, defect selective etching, photo-etching, Hall method, Raman spectroscopy, and secondary ion mass spectrometry. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates.

In this review work we discuss applicability of Bragg scattering to examination of nanocrystals. We approximate the structure of nanograins by a commonly accepted core-shell model. We show that, for principal reasons, the Bragg equation is not applicable directly to nanocrystals. We use the Bragg relation through application of the apparent lattice parameter (alp) concept which we use to evaluate quantitatively the core-shell model. We also introduce a new parameter of the structure, Equivalent Cubic Lattice Parameter (EClp), which quantifies deviation of the real (trigonal) lattice from its parent fcc structure due to the lattice deformation (e.g. by the stacking faults). We show examples of an analysis of experimental X-ray and neutron diffraction data based on the alp methodology and on the theoretical patterns calculated for various core-shell models.

In this article homoepitaxial HVPE-GaN growth in directions other than [0001] is described. Three crystallization runs on (11-20), (10-10), (20-21), and (20-2-1) seeds were performed. In each experiment a different carrier gas was used: N2, H2, and a 50% mixture of N2 and H2. Other conditions remained constant. An influence of the growth direction and carrier gas on growth rate and properties (morphology, structural quality, and free carrier concentration determined by Raman spectroscopy) of obtained crystals was investigated and discussed in details. For all crystallographic directions a lower growth rate was determined with hydrogen used as the carrier gas. Also, the highest level of dopants was observed for crystals grown under hydrogen. A possibility to obtain highly conductive GaN layers of high quality without an intentional doping is demonstrated.

We demonstrate the influence of the operation current density and temperature on the degradation rate of InGaN laser diodes grown via metalorganic vapor-phase epitaxy (MOVPE) and plasma-assisted molecular beam epitaxy (PAMBE). The degradation rate of the MOVPE devices shows an exponential dependence on the temperature, with an activation energy of 0.38–0.43 eV, and a linear dependence on the operating current density. In comparison, the MBE-grown lasers exhibit a higher activation energy, on the order of 1 eV, and typically a lower degradation rate, resulting in a service time exceeding 50,000 h. We suggest that this difference may be related to the lower concentration of H in the Mg-doped MBE-grown GaN.

In the case of InGaN alloys grown by metalorganic vapour phase epitaxy on a c-plane GaN, indium content decreases as the substrate miscut is increased. This phenomenon has been previously used to fabricate laser diodes with variable wavelength on one chip [Appl. Phys. Express 5, 021001 (2012)]. In that work, however, wavelength variation was only 5 nm. In the present work we show independent, electrically driven array of light emitting diodes (LED), covering 40 nm emission wavelength range on one chip. This is achieved by a particular patterning technique, which enables the change in the local miscut of the substrate by introducing large enough slopes for practical devices. This technological approach offers a new degree of freedom for InGaN/GaN bandgap modification and device engineering. It can be applied to freestanding GaN as well as to GaN/sapphire templates used for mass production of LEDs. Once optimized, this approach could eventually lead to truly monolithic RGB LEDs.

In this work, we fabricated green-light-emitting laser structures on a (20-21) semipolar GaN substrate. Using cathodoluminescence mapping, x-ray diffraction, and transmission electron microscopy, we revealed the formation of relaxation defects within InGaN waveguides and AlGaN claddings. The observed defects in the AlGaN layers are stripe-like and extend along the a axis, but in the InGaN layers they form a characteristic checkered pattern. We demonstrate that using the selective area growth method we can effectively suppress the formation of both types of defects, thus enabling the fabrication of defect-free green laser structures on semipolar GaN substrates.

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.

Laser diodes (LDs) were grown by plasma-assisted molecular beam epitaxy on semipolar (202¯1) GaN substrates. Metal-rich conditions provided smooth surface morphology and good structural quality as confirmed by atomic force microscopy and transmission electron microscopy studies. No stacking faults or any linear defects were formed during the growth in the InGaN/InGaN multiquantum well active region. Devices were processed with ridge-waveguide along the [1¯21¯0] direction. Mirrors were cleaved or fabricated by focused ion beam (FIB) processing. LDs operated at 388.2 nm with the threshold current density 13.2 kA/cm2 and the threshold voltage 10.8 V. The device with mirrors fabricated by FIB processing reached practically the same threshold current and slope efficiency as the one with cleaved mirrors. The authors present the beneficial role of the InGaN optical confinement layers in semipolar LDs, which can be optimized to improve the threshold current of these devices.

We demonstrate graded-index-separate-confinement-heterostructure InGaN laser diodes (GRINSCH) grown by metal organic vapor phase epitaxy. In this type of structure, the optical mode is confined close to the active region by cladding layers with linearly changing Al content. The virtue of this structure lies in simultaneous improvement of injection efficiency through the funneling effect and mode confinement. After a careful modeling we demonstrate that GRINSCH InGaN laser indeed possess very encouraging properties. Low threshold current density of around 3.5 kA/cm2 accompanied by high differential gain (dG/dJ = 14.93 cm kA−1) make these devices an interesting alternative for the classical step-index structure.

We demonstrate ultra-violet laser diodes emitting at 388 nm grown by plasma-assisted molecular beam epitaxy on semipolar (202¯1)GaN substrates under metal-rich conditions. The threshold current density and voltage of 13.2 kA/cm2 and 10.8 V were measured at room temperature for devices with the laser ridge waveguide oriented along the [1¯21¯0] direction. We show smooth, atomically flat surface morphology after growth. The excellent structural quality of the laser heterostructure was corroborated by transmission electron microscopy.

Multi-quantum well (MQW) structures and light emitting diodes (LEDs) were grown on semipolar (202¯1) and polar (0001) GaN substrates by plasma-assisted molecular beam epitaxy. The In incorporation efficiency was found to be significantly lower for the semipolar plane as compared to the polar one. The semipolar MQWs exhibit a smooth surface morphology, abrupt interfaces, and a high photoluminescence intensity. The electroluminescence of semipolar (202¯1) and polar (0001) LEDs fabricated in the same growth run peaks at 387 and 462 nm, respectively. Semipolar LEDs with additional (Al,Ga)N cladding layers exhibit a higher optical output power but simultaneously a higher turn-on voltage.