[en] A comprehensive analysis of operating parameters of Addon RF nitrogen plasma source was made in order to determine how a ratio of different active nitrogen species depends on operating parameters of the source such as supplied power and nitrogen flow. We show that output signal of the optical sensor that measures intensity of the light emitted by the plasma is a direct measure of the amount of active nitrogen available for growth. Results of optical emission spectroscopy and measurements of growth kinetics show that nitrogen excited metastable molecules are the species mainly contributing to the growth of GaN under Ga-rich conditions. A procedure is presented allowing to find an optimal conditions of the plasma cell for high-quality GaN growth. Under these conditions the nitrogen flux contains maximum amount of excited metastable molecules and minimal amount of ionic and atomic nitrogen species to minimize GaN lattice damage, even at high growth rates. - Highlights: ► Operating parameters of Addon radio-frequency nitrogen plasma source studied ► Their influence on molecular beam epitaxy (MBE) growth of GaN analyzed ► MBE growth rate of GaN well correlates with output of the plasma emission sensor. ► Optical emission spectroscopy measurements of the nitrogen plasma made ► Nitrogen excited molecules mainly contribute to plasma-assisted MBE growth of GaN

[en] Direct harvesting of solar energy is no doubt an important part of future energy technology. Rapidly growing solar energy market generates huge demand on novel solutions aimed at both increasing the effectiveness of photovoltaic devices and keeping the per kilowatt price of solar installations within realistic boundaries. Newly emerged class of polymer solar cells has recently demonstrated rapid increase in effectiveness at the same time remaining relatively cheap and, more importantly, scalable technology. In this work we present our results on fabrication and characterization of hybrid solar cell combining π-conjugated polymer with GaN nanowires grown on conductive substrate. Superior electrical properties of nanowires serve the purpose to enhance conductivity of light-absorbing layer, which in case of organic photovoltaics is limited by low charge carrier mobility. Structures of poly(3-hexylthiophene-2,5-diyl) (P3HT) and GaN nanowires were fabricated and characterized for this study. Self-assembled GaN nanowires were grown using Plasma-Assisted Molecular Beam Epitaxy technique. Bottom contact to GaN nanowires was provided through nucleation layer which is an amorphous material of metallic electrical conductivity deposited on silicon substrate. P3HT solution was put on the array of nanowires by spin-coating resulting in a layer of p-type polymer with embedded n-type nanowires. The active layer was covered by PEDOT:PSS electron blocking coating. Top contacts to a solar device are made using transparent indium tin oxide (ITO) layer or by depositing thin metal pattern on the surface of the device. Morphology studies show that the space orientation of nanowires remains close to perpendicular to the substrate surface independently from nucleation layer thickness, result consistent with reported for growth of self-assembled nanowires on different amorphous substrates. Quantum yield spectroscopy measurements were made in closed circuit mode. The results show that structure exhibits photogenerated current maxima at 1.9 and 3.4 eV energy light which correspond to bandgap edge of P3HT and GaN respectively. This means that light generates electron-hole pairs in both materials and that there is efficient transfer between the materials. The observed direction of photocurrent confirms that GaN works as n-type layer and P3HT is a p-type layer in this heterostructure. Electrical characteristics of nucleation layer-GaN interface were measured showing good ohmic contact between the components. Dependence of structure charge carrying properties on GaN doping is shown as well as electrical characteristics of polymer-GaN interface. (author)

[en] The self-assembled growth of epitaxial Au(111) islands on a Mo(110) buffer layer has been investigated as a function of growth temperature and amount of deposited material by reflection high energy electron diffraction and atomic force microscopy. At the growth temperature of 385 deg. C the dendrite-shaped islands coexist with the compact ones. The uniform islands formed at 500 deg. C adopt mostly a shape of truncated pyramids with a well developed (111) top plane and {111} and {100} side facets. As the growth temperature reaches 800 deg. C the Au islands take less regular shapes due to occurrence of coalescence. The averaged area and height of the islands increase with the deposition temperature and the amount of deposited material. The surface density of the islands decreases with increasing temperature. The epitaxial relations at the interface between the Au islands and the Mo buffer determined from the angular dependence of the electron diffraction pattern favour the Nishiyama-Wassermann growth mode. Factors responsible for the island-like growth and possible mechanisms of diffusion are discussed in details.

[en] Solar energy is seen by many as one of the most prospective alternative energy sources, especially if one considers significant progress made lately in development of new classes of solar cells. Among others, bulk heterojunction (BHJ) based organic solar cells, utilizing π- conjugated polymer mixture with fullerene; have attracted significant attention despite relatively modest power conversion efficiency. In this work, authors report on application of self-assembled GaN nanowires grown on silicon substrate by plasma-assisted molecular beam epitaxy technique as an ordered inorganic acceptor in bulk heterojunction solar cells with active layer fabricated with poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). Prepared organic mixture was spin-coated on top of inorganic acceptor layer, resulting structure was annealed in 150 ℃ for 15 min and metal contacts were deposited afterwards. In order to investigate processes occurring on the interfaces between organic active layer, GaN nanowire and silicon, comparisons of photovoltaic devices with different inorganic acceptor layer were made. J - V characteristics were measured in dark and under AM1.5 light conditions, short circuit current vs. incident light wavelength measurements were performed in the range from 300 to 1400 nm using a tungsten lamp combined with a monochromator as a light source. Results indicate that addition of ordered GaN nanowire acceptor increases power conversion efficiency of a device compared with planar silicon acceptor, although planar GaN acceptor shows similar result. Comparison of external quantum efficiencies calculated form photocurrent shows, that efficiency of carrier collection by GaN nanowire is the highest between all measured samples. We assume existence of energy barrier on P3HT:PCBM/Si interface which explains low rate of charge carrier transfer between organic active layer and silicon substrate. (author)

[en] High quality Schottky contacts are formed on GaN nanowires (NWs) structures grown by the molecular beam epitaxy technique on Si(111) substrate. The current-voltage characteristics show the rectification ratio of about 10³ and the leakage current of about 10⁻⁴ A/cm² at room temperature. From the capacitance-voltage measurements the free carrier concentration in GaN NWs is determined as about 10¹⁶ cm⁻³. Two deep levels (H200 and E280) are found in the structures containing GaN NWs. H200 is attributed to an extended defect located at the interface between the substrate and SiN_x or near the sidewalls at the bottom of the NWs whereas E280 is tentatively assigned to a gallium-vacancy- or nitrogen interstitials-related defect

[en] Nucleation kinetics of GaN nanowires (NWs) by molecular beam epitaxy on amorphous Al_xO_y buffers deposited at low temperature by atomic layer deposition is analyzed. We found that the growth processes on a-Al_xO_y are very similar to those observed on standard Si(111) substrates, although the presence of the buffer significantly enhances nucleation rate of GaN NWs, which we attribute to a microstructure of the buffer. The nucleation rate was studied vs. the growth temperature in the range of 720–790 °C, which allowed determination of nucleation energy of the NWs on a-Al_xO_y equal to 6 eV. This value is smaller than 10.2 eV we found under the same conditions on nitridized Si(111) substrates. Optical properties of GaN NWs on a-Al_xO_y are analyzed as a function of the growth temperature and compared with those on Si(111) substrates. A significant increase of photoluminescence intensity and much longer PL decay times, close to those on silicon substrates, are found for NWs grown at the highest temperature proving their high quality. The samples grown at high temperature have very narrow PL lines. This allowed observation that positions of donor-bound exciton PL line in the NWs grown on a-Al_xO_y are regularly lower than in samples grown directly on silicon suggesting that oxygen, instead of silicon, is the dominant donor. Moreover, PL spectra suggest that total concentration of donors in GaN NWs grown on a-Al_xO_y is lower than in those grown under similar conditions on bare Si. This shows that the a-Al_xO_y buffer efficiently acts as a barrier preventing uptake of silicon from the substrate to GaN

[en] An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (∼150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure. (paper)

[en] We present a comprehensive description of the self-assembled nucleation and growth of GaN nanowires (NWs) by plasma-assisted molecular beam epitaxy on amorphous Al_x O_y buffers (a-Al_x O_y) prepared by atomic layer deposition. The results are compared with those obtained on nitridated Si(111). Using line-of-sight quadrupole mass spectrometry, we analyze in situ the incorporation of Ga starting from the incubation and nucleation stages till the formation of the final nanowire ensemble and observe qualitatively the same time dependence for the two types of substrates. However, on a-Al_x O_y the incubation time is shorter and the nucleation faster than on nitridated Si. Moreover, on a-Al_x O_y we observe a novel effect of decrease in incorporated Ga flux for long growth durations which we explain by coalescence of NWs leading to reduction of the GaN surface area where Ga may reside. Dedicated samples are used to analyze the evolution of surface morphology. In particular, no GaN nuclei are detected when growth is interrupted during the incubation stage. Moreover, for a-Al_x O_y, the same shape transition from spherical cap-shaped GaN crystallites to the NW-like geometry is found as it is known for nitridated Si. However, while the critical radius for this transition is only slightly larger for a-Al_x O_y than for nitridated Si, the critical height is more than six times larger for a-Al_x O_y. Finally, we observe that in fully developed NW ensembles, the substrate no longer influences growth kinetics and the same N-limited axial growth rate is measured on both substrates. We conclude that the same nucleation and growth processes take place on a-Al_x O_y as on nitridated Si and that these processes are of a general nature. Quantitatively, nucleation proceeds somewhat differently, which indicates the influence of the substrate, but once shadowing limits growth processes to the upper part of the NW ensemble, they are not affected anymore by the type of substrate. (paper)

[en] The electrical, structural, and optical properties of coalescent p-n GaN nanowires (NWs) grown by molecular beam epitaxy on Si (111) substrate are investigated. From photoluminescence measurements the full width at half maximum of bound exciton peaks AX and DA is found as 1.3 and 1.2 meV, respectively. These values are lower than those reported previously in the literature. The current-voltage characteristics show the rectification ratio of about 10"2 and the leakage current of about 10"−"4 A/cm"2 at room temperature. We demonstrate that the thermionic mechanism is not dominant in these samples and spatial inhomogeneties and tunneling processes through a ∼2 nm thick SiN_x layer between GaN and Si could be responsible for deviation from the ideal diode behavior. The free carrier concentration in GaN NWs determined by capacitance-voltage measurements is about 4 × 10"1"5" cm"−"3. Two deep levels (H190 and E250) are found in the structures. We attribute H190 to an extended defect located at the interface between the substrate and the SiN_x interlayer or near the sidewalls at the bottom of the NWs, whereas E250 is tentatively assigned to a gallium-vacancy- or nitrogen interstitials-related defect

[en] Polarization engineering of GaN-based heterostructures opens a way to develop advanced transistor heterostructures, although measurement of the electric field in such heterostructures is not a simple task. In this work, contactless electroreflectance (CER) spectroscopy has been applied to measure the electric field in GaN-based heterostructures. For a set of GaN( d   =  0, 5, 15, and 30 nm)/AlGaN(20 nm)/GaN(buffer) heterostructures a decrease of electric field in the GaN(cap) layer from 0.66 MV cm⁻¹ to 0.27 MV cm⁻¹ and an increase of the electric field in the AlGaN layer from 0.57 MV cm⁻¹ to 0.99 MV cm⁻¹ have been observed with the increase in the GaN(cap) thickness from 5–30 nm. For a set of GaN(20 nm)/AlGaN( d   =  10, 20, 30, and 40 nm)/GaN(buffer) heterostructures a decrease of the electric field in the AlGaN layer from 1.77 MV cm⁻¹ to 0.64 MV cm⁻¹ and an increase of the electric field in the GaN layer from 0.57 MV cm⁻¹ to 0.99 MV cm⁻¹ were observed with the increase in the AlGaN thickness from 10–40 nm. To determine the distribution of the electric field in these heterostructures the Schrödinger and Poisson equations are solved in a self-consistent manner and matched with experimental data. It is shown that the built-in electric field in the GaN(cap) and AlGaN layers obtained from measurements does not reach values of electric field resulting only from polarization effects. The measured electric fields are smaller due to a screening of polarization effects by free carriers, which are inhomogeneously distributed across the heterostructure and accumulate at interfaces. The results clearly demonstrate that CER measurements supported by theoretical calculations are able to determine the electric field distribution in GaN-based heterostructures quantitatively, which is very important for polarization engineering in this material system. (paper)