xN Samples Research Articles

Alloy splitting of the FeGa acceptor level in dilute AlxGa1−xN

The results of conventional deep-level transient spectroscopy (DLTS) and high-resolution Laplace DLTS measurements of the FeGa(0/−) acceptor level in dilute AlxGa1−xN layers (x ≤ 0.05) grown by MOVPE technique (metal–organic vapor phase epitaxy) on native ammono-GaN substrates are analyzed and discussed. It is shown that the electron emission signal related to the FeGa acceptor level in AlxGa1−xN splits into individual components due to aluminum fluctuations in the second-nearest neighbor (2NN) shell around the FeGa impurity atoms. The calculations of the probability of finding a given number of aluminum atoms in the 2NN shell of the FeGa defect agree well with the experimental concentrations determined from Laplace DLTS peak intensities. This finding shows that in dilute AlxGa1−xN layers grown by MOVPE, aluminum and iron atoms are randomly distributed in the material. Finally, we demonstrate that the energy level of the FeGa acceptor with no Al atoms in the 2NN shell in the AlxGa1−xN samples shifts linearly with the aluminum content and the shifts are 28 and 55 meV relative to that in GaN for the samples with x = 0.025 and 0.05, respectively.

Control of phase purity in high scandium fraction heteroepitaxial ScAlN grown by molecular beam epitaxy

ScAlN is a promising material for applications spanning wide-bandwidth filters, high-electron-mobility transistors, and ferroelectric memory. We investigate conditions influencing wurtzite phase purity for heteroepitaxial ScAlN, and present methods to rapidly identify phase purity degradation. Even for N-rich samples, phase purity is sensitive to the III/V ratio near the N-rich to metal-rich transition. Epitaxial ScxAl1−xN samples can be grown at 700 °C with x = 0.06–0.22, although the phase purity degrades for x = 0.32. By reducing the substrate temperature to 390 °C, we demonstrate 200 nm Sc0.32Al0.68N heteroepitaxial films with a record low rocking curve full-width at half-maximum of 1840 arcsec.

Strain-stress study of AlxGa1\u2212xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1−xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1−xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1−xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1−xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1−xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1−xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain on the optical phonon energies of the epitaxial layers is also discussed.

Growth and composition-dependent optoelectronic properties of AlxGa1−xN alloy nanowires arrays across the entire composition range on carbon cloth

Composition-tunable ternary AlxGa1−xN (0 ≤ x ≤ 1) alloy nanowires have been grown on flexible carbon cloth in large scale through a one-step and catalyst-free chemical vapor deposition method. Field emission scanning electron microscopy, x-ray diffraction analysis, transmission electron microscopy and x-ray energy-dispersive spectroscopy were employed to characterize the morphology, crystal structure, composition properties. All AlxGa1−xN samples exhibit hexagonal wurtzite single-phase structure and a preferred orientation along [0001] direction. The progressively evolutions in structural, Raman and field emission properties of AlxGa1−xN nanowires demonstrate the continuous composition tunability across the entire composition range. The enhancement in the field emission characteristic of AlxGa1−xN nanowires on carbon cloth is ascribed to the combined effect of the nanostructures and the excellent electron transport path of the conductive carbon cloth. The successful synthesis of AlxGa1−xN nanowires on carbon cloth provides the new possibility for the further development of flexible nano- and opto-electronic devices.

Infrared optical responses of wurtzite InxGa1 − xN thin films with porous surface morphology

Room temperature infrared (IR) optical responses of wurtzite indium gallium nitride (InxGa1−xN) in the composition range of 0.174≤x≤0.883 were investigated by the polarized IR reflectance spectroscopy. Analyses of the amplitudes of oscillation fringes in the non-reststrahlen region revealed that the high frequency dielectric constants of the samples were unusually smaller than the values predicted from the Clausius–Mossotti relation. This odd behavior was attributed to the porous surface morphology of the InxGa1−xN samples. The E1 optical phonon modes of the InxGa1−xN were deduced from the composition dependent reststrahlen features. The obtained values were compared to those calculated through the modified random element iso-displacement (MREI) model. The deviation between the measured data and the MREI model prediction were explained in detail from the aspects of strain, thermal expansion and anharmonic phonon-coupling. Finally, it was found that the large discrepancy of the E1(LO) mode is mainly attributed to the effects of the longitudinal phonon–plasmon coupling.

Effect of AlN content on the lattice site location of terbium ions in AlxGa1−xN compounds

Terbium lattice site location and optical emission in Tb implanted AlxGa1−xN (0 ≤ x ≤ 1) samples grown by halide vapour phase epitaxy on (0001) sapphire substrates are investigated as a function of AlN content. The samples were implanted with a fluence of 5 × 1014 cm−2 of terbium ions and an energy of 150 keV. Lattice implantation damage is reduced using channelled ion implantation performed along the 〈0001〉 axis, normal to the sample surface. Afterwards, thermal annealing treatments at 1400 °C for GaN and 1200 °C for samples with x > 0 were performed to reduce the damage and to activate the optical emission of Tb3+ ions. The study of lattice site location is achieved measuring detailed angular ion channelling scans across the 〈0001〉, and axial directions. The precise location of the implanted Tb ions is obtained by combining the information of these angular scans with simulations using the Monte Carlo code FLUX. In addition to a Ga/Al substitutional fraction and a random fraction, a fraction of Tb ions occupying a site displaced by 0.2 Å along c-axis from the Ga/Al substitutional site was considered, giving a good agreement between the experimental results and the simulation. Photoluminescence studies proved the optical activation of Tb3+ after thermal annealing and the enhancement of the 5D4 to 7F6 transition intensity with increasing AlN content.

Structural, morphology and optical properties of nanocrystalline GaN and AlGaN alloys prepared by EDTA complex route

GaN and AlGaN alloys were prepared by ethylene diamine tetra acetic acid (EDTA) complex route with five different Al molar ratios. The structure, morphology and luminescence properties of GaN and AlxGa1−xN were investigated by PXRD, SEM, EDX, HRTEM, PL and Raman techniques. The PXRD pattern confirms the formation of hexagonal wurtzite structure of GaN and AlxGa1−xN alloys. When Al is alloy with GaN hexagonal shaped particles change into agglomerated spherical and plate-like morphology of AlxGa1−xN as confirmed by SEM analysis. The results also indicate that with increasing Al composition, the morphology of AlxGa1−xN changes. HRTEM analysis shows that particle size decreases from 24 to 10nm, with increasing Al concentration. Raman analysis confirms the presence of E2(high), A1(TO), E1(LO) and A1(LO) phonon frequencies in AlGaN alloy. PL spectra confirms that the energy of the near band edge emission changes from 383 to 337nm with change in Al concentration in AlxGa1−xN samples.

Molecular beam epitaxy of free-standing wurtzite AlxGa1−xN layers

Recent developments with group III nitrides present AlxGa1−xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1−xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1−xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1−xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1−xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1−xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1−xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8µm/h on 2-in. diameter GaAs and sapphire wafers.

Raman and photoluminescence mapping of InxGa1−xN (x ∼ 0.4) at high pressure: Optical determination of composition and stress

The pressure response of a polar wurtzite InxGa1−xN (x = 0.37) film epitaxially grown on a GaN/sapphire template was studied by means of combined Raman and photoluminescence (PL) mappings. The pressure slopes of the Raman peaks (∂ω/∂P ∼ 4.7 cm−1·GPa−1) of the studied alloy are indicative of its intermediate stiffness between the end members of the InxGa1−xN system. The data analysis suggests that in our experiments the obtained slopes have marginal contribution, if any, from the substrate. Furthermore, the similarity of the ambient pressure value of the PL peak energy (∼1.97 eV) and its pressure slope (∂EPL/∂P ∼ 30 meV·GPa−1) with those obtained by absorption measurements implies that PL can be used to follow the pressure evolution of the energy bandgap. Finally, we demonstrate that all-optical characterization of the composition and residual stress of InxGa1−xN samples is feasible.

InxGa1−xN performance as a band-gap-tunable photo-electrode in acidic and basic solutions

Performance of InxGa1−xN photo-electrodes at concentrations of In ranging from 0 to 100 % was investigated in basic and acidic solutions under 1 Sun illumination. Photocorrosive effects of InxGa1−xN samples in aqueous solutions are revealed and strategies for a more efficient use of these electrodes are discussed. Formation of Ga2O3 phase and N2 under photoanodic conditions can explain the photo-corrosive effect. It is shown that the product of charge carrier density and mobility, n×μ, scales with the photo-current density in GaN.

Structural and electrical characterization of InN, InGaN, and p-InGaN grown by metal-modulated epitaxy

InN, high indium content InGaN, and Mg-doped InGaN were grown by metal modulated epitaxy (MME). Transient reflection high-energy electron diffraction intensities were analyzed during the growth of InN and found to be similar to that previously reported for GaN and AlN. The x-ray diffraction rocking curve and background electron concentration of InN grown by MME were found to be respectable in comparison to recent reports in literature. InGaN alloys grown by MME were also investigated, and a method for detecting indium surface segregation was demonstrated. It was found that the shutter modulation scheme could be modified to prevent phase separation by indium surface segregation, and a range of single-phase InGaN samples with indium contents throughout the miscibility gap were grown. Using the discovered method of suppressing phase separation, several p-InxGa1 − xN samples were grown with indium contents from x = 0 to 0.22. A maximum hole concentration of 2.4 × 1019 cm−3 was detected by Hall effect characterization, demonstrating feasibility of these p-InGaN layers for use in several device applications.

Lattice site location and luminescence studies of AlxGa1−xN alloys doped with thulium ions

Rare earth doped AlGaN is being considered for the development of new optoelectronic devices. This wide band gap compound provides lower thermal quenching of rare earth intra-4fn transitions making room temperature operation possible. In this work a detailed study of AlxGa1−xN (x=0.15, 0.77) samples grown by halide vapor phase epitaxy on (0001) sapphire implanted with Tm is presented. The samples were implanted at room temperature with 300keVTm ions to fluences of 1×1014cm−2 and 1×1015cm−2. The damage accumulation and the rare earth lattice site location are investigated by Rutherford Backscattering/Channeling Spectrometry. The structural measurements show a higher resistance of the lattice to irradiation damage with the increase of the AlN content. The analysis of angular scans along different axes using the Monte Carlo code FLUX allows the determination of the rare earth lattice sites. Results for Al0.15Ga0.85N show that Tm ions occupy two preferential sites: the high symmetry substitutional Ga/Al site and a site displaced by ∼0.3Å along the c-axis from this regular site.Rapid thermal annealing treatments at 1200°C under N2 ambient were performed to remove implantation damage and promote the optical activation of rare earth intra-4fn transitions. Photoluminescence measurements were performed to characterize the thulium ions spectroscopic transitions. The effects of thermal annealing and fluence on the fraction of rare earth optically active ions are exploited and discussed.

The stem and roots of Wilms' tumours

AbstractSee related article in EMBO Molecular Medicine http://dx.doi.org/10.1002/emmm.201201516

Raman Scattering Study of InxGa1−xN Alloys with Low Indium Compositions

InxGa1−xN alloys with low indium composition x in the range 0.13 ≤ x ≤ 0.23 are systematically studied mainly based on a Raman scattering technique. Scanning electron microscopy and x-ray diffraction results show that our samples can be divided into two groups: pseudomorphic (0.13 ≤ x ≤ 0.16) and relaxed (0.18 ≤ x ≤ 0.23). The prominent enhancement of A1 longitudinal-optical (LO) mode is found with 325 nm laser excitation. For pseudomorphic samples, the frequencies of A1 (LO) phonons agree well with the theoretical predictions, which verifies that the samples are fully strained. For relaxed InxGa1−xN samples, a linear dependence of the A1 (LO) mode frequency is obtained: Ω0(x) = (740.8 ± 3.3) − (143.1 ± 16.0)x, which is the evidence of one-mode behavior in InxGa1−xN ternary alloys. Residual strains in these partially relaxed samples are also evaluated.

Effect of Al mole fraction on structural and electrical properties of AlxGa1−xN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

The effect of Al mole fractions on the structural and electrical properties of AlxGa1−xN/GaN thin films grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si (111) substrates has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and current–voltage (I–V) measurements. X-ray results revealed that the AlGaN/GaN/AlN was epitaxially grown on Si substrate. By applying Vegard's law, the Al mole fractions of AlxGa1−xN samples were found to be 0.11, 0.24, 0.30 and 0.43, respectively. The structural and morphology results indicated that there is a relatively larger tensile strain for the sample with the smallest Al mole fraction; while a smaller compressive strain and larger grain size appear with Al mole fraction equal to 0.30. The strain gets relaxed with the highest Al mole fraction sample. Finally, the linear relationship between the barrier height and Al mole fraction was obtained.

Electrical Activation Studies of Silicon-Implanted Al x Ga1−x N with Aluminum Mole Fraction of 11% to 51%

Electrical activation studies of Si-implanted AlxGa1−xN with an Al mole fraction of 11% to 51% have been carried out as a function of ion dose and annealing temperature. The AlxGa1−xN samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 1014 cm−2 to 1 × 1015 cm−2, and subsequently annealed from 1100°C to 1350°C for 20 min in a nitrogen environment. The maximum electrical activation efficiencies for the AlxGa1−xN samples with an Al mole fraction less than 40% were obtained for samples implanted with the highest Si dose of 1 × 1015 cm−2. On the other hand, for the AlxGa1−xN samples with an Al mole fraction more than 40%, nearly perfect activation efficiencies of 99% and 100% were obtained for the samples implanted with the lowest Si dose of 1 × 1014 cm−2. The mobility of the Si-implanted AlxGa1−xN samples increased with increasing annealing temperature in spite of the increased number of ionized donors and thus increased impurity scattering, indicating that a greater amount of lattice damage is being repaired with each successive increase in annealing temperature. These results provide suitable annealing conditions for Si-implanted AlxGa1−xN-based devices with an Al mole fraction from 11% to 51%.

MOVPE growth and characterization of a ‐plane AlGaN over the entire composition range

AbstractWe report the metal organic vapor phase epitaxy (MOVPE) growth and characterization of non‐polar (11$ \bar 2 $0) a ‐plane Alx Ga1–xN on (1$ \bar 1 $02) r ‐plane sapphire substrates over the entire composition range. Alx Ga1–xN samples with ∼0.8 μm thick layers and with x = 0, 0.18, 0.38, 0.46, 0.66, and 1.0 have been grown on r ‐plane sapphire substrates. The layer quality can be improved by using a 3‐stage AlN nucleation layer and appropriate V/III ratio switching following nucleation. All a ‐plane AlGaN epilayers show an anisotropic in‐plane mosaicity, strongly influenced by Al incorporation and growth conditions. Careful lattice parameter measurements show anisotropic in‐plane strain that results in an orthorhombic distortion of the hexagonal unit cell, making Al composition determination from X‐ray diffraction difficult. In general lower Al incorporation is seen in a ‐plane epilayers compared to c ‐plane samples grown under the same conditions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitation pathways and efficiency of Eu ions in GaN by site-selective spectroscopy

Using combined excitation emission spectroscopy, we performed a comparative study of europium ions in GaN in samples that have been in situ doped during interrupted growth epitaxy (IGE) or conventional molecular beam epitaxy (MBE) as well as samples that were grown using organometallic vapor phase epitaxy (OMVPE) and subsequently ion implanted with Eu ions. Through site-selective resonant excitation, we are able to unambiguously assign all major observed transitions to a combination of different incorporation sites and electron–phonon coupled transitions. We identified at least nine different incorporation sites of Eu ions in GaN and studied how these sites behave under different excitation conditions and how their relative number is modified by different growth and doping conditions. The coupling to phonons has also been studied for a series of AlxGa1−xN samples with x=0…1. We find that a main site most resembling an unperturbed Eu ion on Ga site is always dominant, while the minority sites are changing substantially in relative numbers and can occur in some samples fairly close in emission intensity to the main site. In terms of the excitation pathway after the creation of electron-hole pairs, we found three types of centers: (1) sites that are dominantly excited through shallow defect traps; (2) sites that are excited through a deep defect trap; (3) sites that cannot be excited at all including the majority of the main sites. We interpret this finding to indicate that the ion in this environment is not very efficient in trapping excitation and that the indirect excitation involving other traps depends on the ion/trap distance. Many of the main sites are far away from these traps and cannot be excited through this channel at all. The efficiency of excitation is highest for the deep traps, indicating that it would be desirable to enrich the respective site, as has been done with some success in the IGE grown samples.

Three-dimensional atom probe analysis of green- and blue-emitting InxGa1−xN∕GaN multiple quantum well structures

The three-dimensional atom probe has been used to characterize green- and blue-emitting InxGa1−xN∕GaN multiple quantum well structures with subnanometer resolution over a 100nm field of view. The distribution of indium in InxGa1−xN samples with different compositions is analyzed. No evidence is found wherein the indium distribution deviates from that of a random alloy, which appears to preclude indium clustering as the cause of the reported carrier localization in these structures. The upper interface of each quantum well layer is shown to be rougher and more diffuse than the lower interface, and the existence of monolayer steps in the upper interfaces is revealed. These steps could effectively localize carriers at room temperature. Indium is shown to be present in the GaN barrier layers despite the absence of indium precursor flux during barrier layer growth. A strong evidence is produced to support a mechanism for the presence of indium in these layers, namely, that a layer of indium forms on the surface of the growing InxGa1−xN quantum well, and this layer then acts as a source of indium during GaN barrier layer growth.

Traps in Si-doped AlxGa1-xN Grown by Molecular Beam Epitaxy on Sapphire Characterized by Deep Level Transient Spectroscopy

ABSTRACTDeep levels in Si doped AlxGa1−xN samples, with Al mole fraction in the range of x = 0 to 0.30, grown by radio-frequency plasma activated molecular beam epitaxy on sapphire substrates were characterized by deep level transient spectroscopy (DLTS). DLTS measurements show two significant electron traps, P1 and P2, in AlGaN at all aluminum mole fractions. The electron trap, P2, appears to be a superposition of traps A and B , both of which are observed in GaN grown by various growth techniques and are thought to be related to VGa-shallow donor complexes. Trap P1 is related to line defects and N-related point defects. Both of these traps are distributed throughout the bulk of the epitaxial layer. An additional trap P0 which was observed in Al0.20Ga0.80N and Al0.30Ga0.70N is of unknown origin, but like P1 and P2, it exhibits dislocation-related capture kinetics. The activation energy measured from the conduction band of the defects is found to increase with Al mole content, a behavior consistent with other III-V semiconductors.