Cathodoluminescence study of the excitons localization in AlGaN/GaN and InGaN/GaN quantum wells grown on sapphire

Al_0.1Ga_0.9N(5 nm)/GaN(2 nm) and In_0.2Ga_0.8N/GaN quantum wells (QWs) grown on GaN/sapphire have been studied by cathodoluminescence (CL) spectroscopy and imaged using an experimental setup especially developed for scanning near-field CL microscopy, which combines a scanning force microscope and a scanning electron microscope. The CL spectra show the characteristic band edge emission peak of GaN at λ= 364 nm and the emission peaks related to the presence of QWs, at λ= 353 and 430 nm for the AlGaN/GaN and the InGaN/GaN samples, respectively. Monochromatic CL images reveal that the emission of the AlGaN/GaN and InGaN/GaN QWs is localized at the level of the grains observed by SFM. A cross sectional analysis of the InGaN/GaN sample gives insight into its growth and an estimation of the exciton diffusion length of about L=180 nm.     

Organometallic vapor phase epitaxy growth of upright metamorphic multijunction solar cells

We demonstrate an integrated metamorphic AlGaInP/AlGaInAs/GaInAs/Ge 4 J solar cell on Ge substrate using organometallic vapor phase epitaxy (OMVPE). A step graded GaInAs buffer was grown right after the Ge subcell was formed to change the lattice constant from that of Ge to that of Ga_0.8In_0.2As lattice constant followed by a 1.14 eV Ga_0.8In_0.2As subcell, a 1.5 eV (AlGa)_0.8In_0.2As subcell, and a 1.85 eV Al_xGa_0.32?xIn_0.68P subcell. Transmission electron microscope (TEM) study shows the threading dislocation density (TDD) is about 6×10⁶ cm^?2. The X-ray diffraction reciprocal space map (RSM) shows that the structure is 100% relaxed. Bandgap dependent (Al_xGa_1?x)_0.32In_0.68P subcell performance is systematically investigated. As the Al_xGa_0.32?xIn_0.68P cell bandgap goes up to 1.9 eV, the external quantum efficiency (EQE) goes down significantly. Theoretical simulation shows that the decrease of diffusion length causes the lower EQE, which indicates the material quality degrades with the increasing Al content. Integrated 4 J solar cells are fabricated and characterized with spectral response and tested under the AM1.5D terrestrial spectrum at both 1 sun and 2000 suns.     

TEM study of defects in AlxGa1−xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al_xGa_1?xN layers with various Al mole fractions (x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al_xGa_1?xN layer (x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed.The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al_xGa_1?xN layer with both polarities.The lattice relaxation of metal-polar Al_0.2Ga_0.8N was observed, while N-polar Al_0.2Ga_0.8N did not relax. The dislocation densities in the N-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers were 5.5×10⁹ cm^?2 and 9×10⁹ cm^?2, respectively, and in metal-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers these were 1×10¹⁰ cm^?2 and 6×10⁹ cm^?2, respectively.Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.     

Growth improvement and characterization of AgGaxIn1−xSe2 chalcopyrite crystals using the horizontal Bridgman technique

AgGa_xIn_1?xSe₂ single crystals with x=0.4 have been grown by the horizontal Bridgman technique for nonlinear optical application requires phase matching. High purity polycrystalline synthesis of AgGa_xIn_1?xSe₂ was carried out at 850 °C, which is a relatively lower temperature compared to those in earlier reports, thus reducing secondary phase formation. An average Ga:In ratio of 62:38 (±3%) was measured using energy dispersive spectroscopy (EDS). As grown, a single crystal shows very high IR transmission of ～65% in the spectral range of 4000–600 cm^?1. There was no significant change in its IR transmission after annealing it at 500 °C for 20 days in vacuum in the presence of AgGa_xIn_1?xSe₂ powder. This indicates a low concentration of defects in the crystal. The results demonstrate that the improved new synthesis method for crystal growth was promising and that the quality of the crystal was good.     

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al₂O₃ substrates by using a HT (>1150 °C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4 Å. The full width at half maximum (FWHM) for 800 nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

Broad UHF ferromagnetic resonance of iron rich-aluminum pulsed laser deposited thin films

The magnetic susceptibility of Fe–Al off-normal pulsed laser deposited thin films was measured at ultra high frequencies, UHF. Different Fe_1?x–Al_x films from pure Fe to x = 0.2 Al were prepared. The films were ≈40 nm thick and non-crystalline peaks were detected by the X-ray diffractometry studies. The magnetization of the films remained between 2.0 and 1.8 T for composition less than or equal to 20% Al. A magnetic anisotropy, from H_k ≈ 18 Oe for pure Fe to H_k ≈ 130 Oe for 20% Al was measured. These samples exhibited a well-defined ferromagnetic resonance at frequencies between ≈2.0 GHz and 3.8 GHz depending on composition. The broad resonance peaks had a width, at half maximum, w_h, in the interval from 2.5 GHz to 4.0 GHz depending on Al content. After fitting the magnetic hysteresis loops using a simple distribution of anisotropy values, we used the Landau–Lifshitz–Gilbert equation to fit the UHF magnetic susceptibility. From this last fit we obtained a high damping coefficient value (≈4 times higher than that corresponding to Co or CoFe films), explaining this broad ferromagnetic resonance of these Fe_1?x–Al_x films.     

Calorimetric study and Kissinger analysis of melt-spun DyMn6−xGe6−xFexAlx (1 ⩽ x ⩽ 2.5) alloys

Thermal properties of rapidly quenched alloys from the DyMn_6?xGe_6?xFe_xAl_x (1 ? x ? 2.5) series produced by melt-spinning have been investigated by differential scanning calorimetry (DSC). The DSC curves show two exothermic effects connected with crystallization processes. Crystallization temperatures and enthalpies ΔH have been estimated. The systematic changes in these parameters allow concluding that the crystallization exothermic events are independent. Effective activation energies E have been determined using the Kissinger analysis and relatively high values up to 480 ± 20 kJ/mol for DyMn₄Ge₄Fe₂Al₂ have been found indicating high thermal stability of the amorphous state in this alloy series.     

Thermally reversing window in Ge15Te85 − xInx glasses: Nanoindentation and micro-Raman studies

Nanoindentation studies on Ge₁₅Te_85 ? xIn_x glasses indicate that the hardness and elastic modulus of these glasses increase with indium concentration. While a pronounced plateau is seen in the elastic modulus in the composition range 3 ≤ x ≤ 7, the hardness exhibits a change in slope at compositions x = 3 and x = 7. Also, the density exhibits a broad maximum in this composition range. The observed changes in the mechanical properties and density are clearly associated with the thermally reversing window in Ge₁₅Te_85 ? xIn_x glasses in the composition range 3 ≤ x ≤ 7. In addition, a local minimum is seen in density and hardness around x = 9, the chemical threshold of the system. Further, micro-Raman studies reveal that as-quenched Ge₁₅Te_85 ? xIn_x samples exhibit two prominent peaks, at 123 cm^? 1 and 155 cm^? 1. In thermally annealed samples, the peaks at 120 cm^? 1 and 140 cm^? 1, which are due to crystalline Te, emerge as the strongest peaks. The Raman spectra of polished samples are similar to those of annealed samples, with strong peaks at 123 cm^? 1 and 141 cm^? 1. The spectra of lightly polished samples outside the thermally reversing window resemble those of thermally annealed samples; however, the spectra of glasses with compositions in the thermally reversing window resemble those of as-quenched samples. This observation confirms the earlier idea that compositions in the thermally reversing window are non-aging and are more stable.     

Epitaxial growth of the metastable phase ytterbium monoxide on gallium nitride surfaces

Molecular beam deposition systems allow for unparalleled control of film composition and structure. This article addresses the capacity for controlling metal and oxidant fluxes in the Yb/O₂ system to access the metastable phase ytterbium monoxide (YbO). Experiments exploring the growth of polycrystalline YbO_x films by molecular beam deposition demonstrate that a 2:1 molar ratio of Yb:O₂ fluxes is necessary to achieve preferential growth of the divalent oxide. Applying similar deposition conditions to a (0 0 1) GaN surface leads to the growth of epitaxial (1 1 1) YbO films. Similar to other rocksalt oxides grown on GaN surfaces, YbO films display a 3D growth mechanism that leads to a grainy morphology with crystallites of 50 nm lateral dimensions. Rocking curves in ω and φ have full-width half-maximum values of 1.77° and 4.1°, respectively; further improvements in crystal quality appear to be limited by the thermal stability of the YbO phase.     

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require Al_xGa_1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, Al_xGa_1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of Al_xGa_1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite Al_xGa_1-xN wafers. Zinc-blende and wurtzite Al_xGa_1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and Al_xGa_1-xN wafers. At a thickness of ～30 µm, free-standing GaN and Al_xGa_1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and Al_xGa_1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and Al_xGa_1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride Al_xGa_1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.     

Phase transformations in the La1−xNdxNi3.5Al1.5–H2 (x = 0.1 and 0.2) system

Interactions in the La_1?xNd_xNi_3.5Al_1.5-Н₂ (x = 0.1 and 0.2) system was studied from room temperature up to 950 °C at the initial hydrogen pressure of 5 MPa through differential thermal (DTA) and X-ray phase analyses. Heating two-phase alloys (x = 0.1 and 0.2) in hydrogen results in their disproportionation (at 530 and 560 °С, respectively) and the formation of NiAl and unidentified amorphous products. The single-phase La_0.9Nd_0.1Ni_3.5Al_1.5 alloy decomposes in hydrogen at 900 °С into a hydride of rare-earth metals and an Ni₃Al intermetallic; traces of NiAl and hydride of a phase of the CaCu₅-type structure have also been observed. Heating the disproportionated samples in vacuum to 520–550 °С leads to their recombination into a homogenized phase with a CaCu₅-type structure. In other words, the increase of neodymium content shifts the reaction equilibrium of La_1?xNd_xNi_3.5Al_1.5 alloys with hydrogen towards recombination.     

Investigation of glass forming ability in Ce–Al–Ni alloys

The compositional dependence of the glass forming ability (GFA), the correlation between their GFA and the GFA related parameters, and the thermal stability of the Ce–Al–Ni alloys were investigated. Rapidly quenched Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) ribbons were prepared by melt spinning, and their phase transformations were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The experimental results indicated that the GFA of Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) alloys increased firstly and then decreased with the increasing of the Al content up to 20 at.%, respectively. It was found that only one parameter, F₁, in evaluated currently available empirical GFA parameters searching for metallic glasses with a good GFA, can reflect the GFA of the Ce–Al–Ni alloys. It was indicated that the thermal stability of alloy with fully amorphous maybe lower than that of alloy with partial amorphous.     

Single crystal growth from the melt and magnetic properties of hexaferrites–aluminates

Single crystals of aluminum substituted barium hexaferrite were grown by the floating zone method with optical heating. Single crystals were produced from a melt of stoichiometric composition. The process was carried out under a pressure of 50 atm of oxygen. In the system BaO–(x)Al₂O₃–(6?x)Fe₂O₃ the region of single phase crystal growth from the melt is limited by the value x=3. For higher substitutions single-phase crystallization is not observed. The grown single crystals are cylindrical boules with a diameter of 4–5 mm and with lengths up to 50 mm. To avert cracking the crystals have been annealed during the process of growth at 1100 °C. The content of FeO in the composition of single crystals of barium hexaferrite, grown by zone melting under an oxygen pressure of 50 atm, is approximately 0.3 wt%. In the system of hexaferrite–aluminates the macroscopic magnetic moment of the material disappears at x=3.     

Interplay between spherical periodicity and angular correlation

We report on structural properties and the resistivity of amorphous (In₅₀Sb₅₀)_100−xAu_x (0 < x < 80). Immediately after deposition at T = 4 K the static structure was measured by electron diffraction and the resistivity by a four-probe technique. The structural data can be described as induced by a resonance effect (Hume–Rothery-, Peierls-like) between the electronic system and the forming static structure. If the electronic system is changed, the structure adjusts to the new situation. With increasing Au-content (shrinking the Fermi-sphere diameter), for example, a resonance-induced structural peak at 2k_F shifts to lower scattering values. By analyzing the static structure in even more detail, indications of angular correlations appear, quite similar as has been observed in amorphous precursor alloys of quasicrystals. After deposition the resistivity is quite large at the In₅₀Sb₅₀ – rich side. Annealing alloys with x > 0 gives a sharp decrease by roughly 10% around T = 160 K which is interpreted as a separation into two amorphous phases. One which is In₅₀Sb₅₀ – rich or may even consist of pure In₅₀Sb₅₀, and another one which is enriched by Au. Around T = 300 K there is a second resistivity drop, interpreted as the crystallization of a spherically-periodic ordered Au-rich metallic phase which itself can be described as a so-called amorphous Hume–Rothery phase, stabilized by the electronic system.     

Luminescence of Dy3+-doped GexGa5Se(95−x) glasses

Dysprosium doped Ge_xGa₅Se_(95?x) (x = 15–30) chalcogenide glasses were synthesized in this present work. The Vis–NIR transmission spectra, photoluminescence spectra and lifetime were measured. Glasses (x = 27.5, 29.17 and 30) doped with 0.2 wt% dysprosium ions shows relatively strong emission bands at 1146 and 1343 nm when pumped at 808 nm. The emission lifetime ranged from 440 to 540 μs. The oscillator strengths and intensity parameters Ω_t (t = 2, 4 and 6) were calculated using Judd–Ofelt theory.     

Nanostructural and optical features of amorphous AlxSi0.45−xO0.55 (0 ⩽ x ⩽ 0.05) thin films prepared by RF-magnetron sputter techniques

The nanostructural, chemical, and optical features of Al_xSi_0.45−xO_0.55 (0 ⩽ x ⩽ 0.05) thin films were investigated in terms of Al concentration and post-deposition annealing conditions; the films were prepared by co-sputtering a Si main target and Al-chips, and the annealing was carried out at temperatures of 400–1100 °C. The a-Si_0.45O_0.55 films prepared without Al-chips and annealed at 800 °C contain ∼3.5 nm-sized Si nanocrystallites. The photoluminescence (PL) intensity as well as the volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the intensity of the PL spectra of the Al_0.025Si_0.425O_0.550 films which were annealed at 800 °C increased significantly at wavelengths of ∼580 nm. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume of the ∼3.5 nm-sized nanocrystallites in the films. The addition of Al as well as the post-deposition annealing allow adjustment and control of the nanostructural and light-emission features of the a-SiO_x films.     

Glass composition dependence of luminescence due to Ge2+ center in germanate glasses

Germanate glasses were prepared by the melt-quenching method using an assembled hot-thermocoupler equipped in a sample chamber of a fluorescence spectrometer, and subsequently their luminescence and excitation spectra were measured. In the GeO₂ glass, luminescence bands due to the Ge²⁺ center appeared at the central wavelengths of 300 and 395 nm, their excitation bands being at 250 and 330 nm, respectively. In the (100 − x)GeO₂ − xM_mO_n glasses, for M_mO_n = B₂O₃ (x ≦ 50), SiO₂ (x ≦ 40), and Al₂O₃ (x ≦ 2), the luminescence intensity and therefore the amount of the Ge²⁺ center increased with increasing the content of M_mO_n, where M^(2n/m)+ ions (B³⁺, Si⁴⁺, and Al³⁺) have lower basicities than a Ge⁴⁺ ion. Contrarily, for M_mO_n = Li₂O (x ≦ 30), Na₂O (x ≦ 20), K₂O (x ≦ 20), CaO (x ≦ 20), SrO (x ≦ 3), BaO (x ≦ 15), ZnO (x ≦ 20), Ga₂O₃ (x ≦ 10), Sb₂O₃ (x ≦ 20), Bi₂O₃ (15 ≦ x ≦ 25), TiO₂ (x ≦ 3), and Nb₂O₅ (x ≦ 10), the luminescence intensity and the amount of the Ge²⁺ center rapidly decreased with increasing the amount of additives and disappeared, where M^(2n/m)+ ions (Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺, Ga³⁺, Sb³⁺, Bi³⁺, Ti⁴⁺, and Nb⁵⁺) have higher basicities than a Ge⁴⁺ ion.     

Laser heated pedestal growth of potassium lithium niobate for UV generation

Potassium lithium niobate (KLN) is a nonlinear optical material with a high nonlinearity. It has the potential to improve the performance and reduce the cost of blue and UV lasers. KLN crystals are not commercially viable because growth by traditional techniques is not possible. In an effort to develop commercially viable KLN, single crystals of the material were grown by the laser heated pedestal growth method (LHPG) with compositions of x=0.02, 0.06 and 0.2 following K₃Li_2?xNb_5+xO_15+2x. Noncritical phase matching at 20 °C for previously unreported compositions of x=0.02 and 0.06 was measured at 795 nm and 805 nm, respectively. Overall, the results suggest that single crystal KLN can be used for SHG into the UV region of the spectrum and can be developed into a commercially viable nonlinear optical material.     

The effect of fluoride on the Fe2+/Fe3+-redox equilibrium in glass melts in the system Na2O/NaF/CaO/Al2O3/SiO2

Liquids with the base compositions (16 − x/2)Na₂O · xNaF · 10CaO · 74SiO₂ (x = 0, 1, 3, and 4) and (10 − x/2) · Na₂O · xNaF · 10CaO · yAl₂O₃ · (80 − y)SiO₂ (x = 0, 1, 3, 5 and y = 5 and 15) doped with 0.25 mol% Fe₂O₃ were studied by means of square-wave voltammetry in the temperature range from 1000 to 1500 °C. With increasing temperature, the redox equilibria were shifted to the reduced state. Also while increasing the alumina concentration, the Fe²⁺/Fe³⁺-redox equilibrium is shifted to the reduced state. In the soda-lime–silica melt the addition of fluoride shifts the equilibrium to the oxidized state, while in the aluminosilicate melts with 15 mol% Al₂O₃, the equilibrium is shifted to the reduced state. In the aluminosilicate melts with 5 mol% Al₂O₃, the equilibrium was not affected by the fluoride concentration. This is explained by the structure of the respective glass compositions.