Low Indium Content Research Articles

Luminescence properties of InGaN‐based dual‐wavelength light‐emitting diodes with different quantum‐well arrangements

Optimized dual‐wavelength InGaN‐based vertical light‐emitting diode (LEDs) structures were investigated by numerical simulations. The results show that different quantum‐well arrangements in the active region play an important role in obtaining dual‐wavelength emission. It is a better way to obtain the dual‐wavelength with uniform intensity by arranging quantum wells (QW) with low indium content near the p‐side and the QW with high indium near the n‐side. This is because the QWs with lower indium near the p‐side layer have higher hole‐injection efficiency. On the other hand, arranging QW with high indium content near the p‐side leads to poor hole‐injection efficiency due to the high polarization fields. The physical and optical mechanisms of these phenomena were explained by the intensity of electrostatic fields, energy‐band diagrams, and carrier‐concentration distribution in the active region of LEDs.

The effects of V/III gas ratios on the catalyst-assisted growths of InGaN nanowires

Single crystalline InGaN nanowires were grown on Si(100) using Au catalysts at 700°C in a plasma-assisted chemical vapor deposition system. Under the low V/III (nitrogen radicals/(Ga+In vapor)) ratio in the gas phase, poor quality InGaN nanowires were grown along [0001] orientation (c-axis) containing a high concentration of stacking faults and a low indium content of 12%. However, under the high V/III gas ratio, very high quality InGaN nanowires could be grown along [1 0 1¯ 0] orientation (m-axis) free of stacking faults containing a high indium content of 24%.The transformation of nanowire orientation was likely due to the decrease of indium and gallium compositions in gold catalysts from >85% to <60% with increasing the V/III gas ratios. Besides, the increase of V/III gas ratios enhanced the incorporation efficiency of indium into the nanowires and significantly improved the crystal quality of nanowires by stabilizing the formation of InN under a high concentration of nitrogen radicals for reversing the fast thermal decomposition reaction of InN at 700°C.

The Influence of InGaN Interlayer on the Performance of InGaN/GaN Quantum-Well-Based LEDs at High Injections

Introducing a thin InGaN interlayer with a relatively lower indium content between the quantum well (QW) and barrier results in a step-like InxGa1−xN/GaN potential barrier on one side of the QW. This change in the active region leads to a significant shift in photoluminescence (PL) and electroluminescence (EL) emissions to a longer wavelength compared with the conventional QW based light-emitting diodes. More importantly, an improvement against efficiency droop and an enhancement in light output power at the high-current injection are observed in the modified light-emitting diode structures. The role of the inserted layer in these improvements is investigated by simulation in detail, which shows that the creation of more sublevels in the valence band and the increase of hole concentration inside QWs are the main reasons for these improvements.

In-situ XRD study of alloyed Cu2ZnSnSe4–CuInSe2 thin films for solar cells

We investigate the growth of Cu2ZnSnSe4–CuInSe2 (CZTISe) thin films using a 2-stage (Cu-rich/Cu-free) co-evaporation process under simultaneous application of in-situ angle dispersive X-ray diffraction (XRD). In-situ XRD allows monitoring the phase formation during preparation. A variation of the content of indium in CZTISe leads to a change in the lattice constant. Single phase CZTISe is formed in a wide range, while at high In contents a phase separation is detected. Because of different thermal expansion coefficients, the X-ray diffraction peaks of ZnSe and CZTISe can be distinguished at elevated substrate temperatures. The formation of ZnSe appears to be inhibited even for low indium content. In-situ XRD shows no detectable sign for the formation of ZnSe. First solar cells of CZTISe have been prepared and show comparable performance to CZTSe.

Effects of confinements on morphology of InxGa1−xAs thin film grown on sub-micron patterned GaAs substrate: Elastoplastic phase field model

An elastoplastic phase field model is developed to investigate the role of lateral confinement on morphology of thin films grown heteroepitaxially on patterned substrates. Parameters of the model are chosen to represent InxGa1−xAs thin films growing on GaAs patterned with SiO2. We determined the effect of misfit strain on morphology of thin films grown in 0.5 μm patterns with non-uniform deposition flux. Growth of islands inside patterns can be controlled by non-uniformity of deposition flux, misfit strain between film and the substrate, and also strain energy relaxation due to plastic deformation. Our results show that the evolution of island morphology depends non-monotonically on indium content and associated misfit strain due to coupling between the plastic relaxation and the confinements effects. Low indium concentration (0%–40%) causes formation of instabilities with relatively long wavelengths across the width of the pattern. Low surface diffusion (due to low indium concentration) and fewer islands across the pattern (due to small misfit strain) lead to formation and growth of islands near the walls driven by overflow flux. Further increase in indium concentration (40%–75%) increases the lattice mismatch and surface diffusivity of the film, and also activates plastic deformation mechanism, which leads to coalescence of islands usually away from the edges. By further increasing the indium concentration (up to 100%), plastic deformation relaxes most of the strain energy density of the film, which prevents formation of instabilities in the film. Hence, in this case, islands are only formed near the walls.

Importance of growth temperature on achieving lattice-matched and strained InAlN/GaN heterostructure by plasma-assisted molecular beam epitaxy

We investigate the role of growth temperature on the optimization of lattice-matched In0.17Al0.83N/GaN heterostructure and its structural evolutions along with electrical transport studies. The indium content gradually reduces with the increase of growth temperature and approaches lattice-matched with GaN having very smooth and high structural quality at 450ºC. The InAlN layers grown at high growth temperature (480ºC) retain very low Indium content of ∼ 4 % in which cracks are mushroomed due to tensile strain while above lattice matched (&amp;gt;17%) layers maintain crack-free compressive strain nature. The near lattice-matched heterostructure demonstrate a strong carrier confinement with very high two-dimensional sheet carrier density of ∼2.9 × 1013 cm−2 with the sheet resistance of ∼450 Ω/□ at room temperature as due to the manifestation of spontaneous polarization charge differences between InAlN and GaN layers.

Lattice parameters and Raman-active phonon modes of (InxGa1–x)2O3 for x &amp;lt; 0.4

We present X-ray diffraction and Raman spectroscopy investigations of (InxGa1–x)2O3 thin films and bulk-like ceramics in dependence of their composition. The thin films grown by pulsed laser deposition have a continuous lateral composition spread allowing the determination of phonon mode properties and lattice parameters with high sensitivity to the composition from a single 2-in. wafer. In the regime of low indium concentration, the phonon energies depend linearly on the composition and show a good agreement between both sample types. We determined the slopes of these dependencies for eight different Raman modes. While the lattice parameters of the ceramics follow Vegard's rule, deviations are observed for the thin films. Further, we found indications of the high-pressure phase InGaO3 II in the thin films above a critical indium concentration, its value depending on the type of substrate.

InGaN/GaN quantum wells for polariton laser diodes: Role of inhomogeneous broadening

Contrary to the case of III-nitride based visible light-emitting diodes for which the inhomogeneous linewidth broadening characteristic of InGaN-based multiple quantum well (MQW) heterostructures does not appear as a detrimental parameter, such a broadening issue can prevent a microcavity (MC) system entering into the strong light-matter coupling regime (SCR). The impact of excitonic disorder in low indium content (x ∼ 0.1) InxGa1–xN/GaN MQW active regions is therefore investigated for the subsequent realization of polariton laser diodes by considering both simulations and optical characterizations. It allows deriving the requirements for such MQWs in terms of absorption, emission linewidth, and Stokes shift. Systematic absorption-like and photoluminescence (PL) spectroscopy experiments are performed on single and multiple In0.1Ga0.9N/GaN quantum wells (QWs). Micro-PL mappings reveal a low temperature PL linewidth of ∼30 meV, compatible with SCR requirements, for single QWs for which the microscopic origin responsible for this broadening is qualitatively discussed. When stacking several InGaN/GaN QWs, a departure from such a narrow linewidth value and an increase in the Stokes shift are observed. Various possible reasons for this degradation such as inhomogeneous built-in field distribution among the QWs are then identified. An alternative solution for the MC design to achieve the SCR with the InGaN alloy is briefly discussed.

Indium out-diffusion in Al2O3/InGaAs stacks during anneal at different ambient conditions

Indium out-diffusion during anneal enhances leakage currents in metal/dielectric/InGaAs gate stacks. In this work, we study the influence of ambient conditions during anneal on indium out-diffusion in Al2O3/InGaAs structures, prior to the gate metal deposition. Using X-ray photoemission spectroscopy and time of flight secondary ions mass spectrometry, we observed much lower indium concentrations in the Al2O3 layer following vacuum and O2 anneals compared to forming gas or nitrogen anneals. The electrical characteristics of the Ni/Al2O3/InGaAs gate stack following these pre-metallization anneals as well as after subsequent post metallization anneals are presented. Possible explanations for the role of the annealing ambient conditions on indium out-diffusion are presented.

Novel attributes in modeling and optimizing of the new graphene based InxGa1−xN Schottky barrier solar cells

Based on the ability of InxGa1−xN materials to optimally span the solar spectrum and their superior radiation resistance, solar cells based on p-type InxGa1−xN with low indium contents and interfacing with graphene film (G/InxGa1−xN), is proposed to exploit the benefit of transparency and work function tunability of graphene. Then, their solar power conversion efficiency modeled and optimized using a new analytical approach taking into account all recombination processes and accurate carrier mobility. Furthermore, their performance was compared with graphene on silicon counterparts and G/p-InxGa1−xN showed relatively smaller short-circuits current (∼7 mA/cm2) and significantly higher open-circuit voltage (∼4 V) and efficiency (∼30%). The thickness, doping concentration, and indium contents of p-InxGa1−xN and graphene work function were found to substantially affect the performance of G/p-InxGa1−xN.

Transparent conducting indium zinc tin oxide thin films with low indium content deposited by radio frequency magnetron sputtering

Indium zinc tin oxide (IZTO) thin films were deposited on glass substrate by radio frequency magnetron sputtering at various substrate temperatures. The indium content of the IZTO targets was reduced from 60at.% to 30at.% and replaced with equal amounts of zinc and tin (20, 25, 30, and 35at.%). The crystallinity of the as-deposited films improved with increasing substrate temperature. For all IZTO films, the onset of crystallization began at above 100°C. The IZTO films deposited at higher substrate temperature showed better optical transmittance and lower electrical resistivity than those deposited at lower temperatures. The minimum electrical resistivity of approximately 8.0×10−4Ω·cm was observed in the IZTO films prepared deposited at 400°C from targets containing 20at.% Zn and 20at.% Sn. The optical transmittance was above 80% for all IZTO films. On the other hand, higher co-doping resulted in amorphous films, which slightly impaired the electrical properties. The deterioration in the electrical properties of IZTO films, however, was compensated for by the higher optical transparency. Therefore, reduced-indium IZTO films are promising alternatives for some transparent conducting oxide applications.

Influence of a low-temperature capping on the crystalline structure and morphology of InGaN quantum dot structures

Abstract The structure and morphology of uncapped and capped InGaN quantum dots formed by spinodal decomposition was studied by AFM, SEM, XRD, and EXAFS. As result of the spinodal decomposition, the uncapped samples show a meander structure with low Indium content which is strained to the GaN template, and large, relaxed Indium-rich islands. The thin meander structure is responsible for the quantum dot emission. A subsequently deposited low-temperature GaN cap layer forms small and nearly unstrained islands on top of the meander structure which is a sharp interface between the GaN template and the cap layer. For an InGaN cap layer deposited with similar growth parameters, a similar morphology but lower crystalline quality was observed. After deposition of a second GaN cap at a slightly higher temperature, the surface of the quantum dot structure is smooth. The large In-rich islands observed for the uncapped samples are relaxed, have a relatively low crystalline quality and a broad size distribution. They are still visible after capping with a low-temperature InGaN or GaN cap at 700 °C but dissolve after deposition of the second cap layer. The low crystalline quality of the large islands does not influence the quantum dot emission but is expected to increase the number of defects in the cap layer. This might reduce the performance of complex devices based on the stacking of several functional units.

ROQUESITE AND ASSOCIATED INDIUM-BEARING SULFIDES FROM A PALEOPROTEROZOIC CARBONATE-HOSTED MINERALIZATION: LINDBOM’S PROSPECT, BERGSLAGEN, SWEDEN

The present study describes a new discovery of the copper-indium sulfide mineral roquesite (nominally CuInS2) together with indium-bearing sulfides associated with magnetite in a carbonate-hosted, polymetallic sulfide mineralization. This occurrence, Lindbom’s Prospect, is located in the western part of the Paleoproterozoic Bergslagen ore province, Sweden. Here, roquesite occurs in indium-bearing bornite, characteristically associated with indium and copper-bearing sphalerite, as well as chalcopyrite, cuproan galena, late-stage chalcocite-digenite and covellite, variable amounts of bismuth minerals, abundant magnetite, and locally cassiterite. The indium-bearing ore mineral assemblages were studied by a combination of optical and field emission scanning electron microscopy and field emission electron probe microanalyzer (FE-EPMA) techniques. FE-EPMA analyses of roquesite yield an average composition corresponding to Cu0.93Fe0.02Zn0.06In1.00S2. It occurs as ca . 4–30 micron-sized subhedral to anhedral, often angular crystals in bornite, typically in direct contact, or in close association with, indium-bearing sphalerite. The latter has variable indium content, ranging from below detection limit to at least 1.5 wt.% In, and exhibits an average of 0.03 wt.% In. Notably, sphalerite grains show a slight enrichment of indium towards the rims and the adjacent bornite, where roquesite occurs. The associated bornite, as well as minor chalcopyrite, mostly exhibit low to very low indium contents. These are mostly below detection limit, but locally higher in bornite; average 0.01 wt.% In and maximum 0.2 wt.% In. The highest indium content observed in chalcopyrite is 0.02 wt.% In. Combining textural evidence and high-resolution mineral chemical data, we suggest that roquesite formed as a consequence of reactions between diffusion-driven indium from sphalerite, and the surrounding bornite, during regional metamorphism. Based on available evidence, it is most likely that the studied assemblages represent part of a metamorphically overprinted, in part remobilized ca . 1.89–1.88 Ga volcanic-subvolcanic hydrothermal mineralization.

Wavelength limits for InGaN quantum wells on GaN

The emission wavelength of coherently strained InGaN quantum wells (QW) is limited by the maximum thickness before relaxation starts. For high indium contents x&amp;gt;40% the resulting wavelength decreases because quantum confinement dominates. For low indium content x&amp;lt;40% the electron hole wave function overlap (and hence radiative emission) is strongly reduced with increasing QW thickness due to the quantum confined Stark effect and imposes another limit. This results in a maximum usable emission wavelength at around 600 nm for QWs with 40%-50% indium content. Relaxed InGaN buffer layers could help to push this further, especially on non- and semi-polar orientations.

Impact of wetting-layer density of states on the carrier relaxation process in low indium content self-assembled (In,Ga)As/GaAs quantum dots

Carrier relaxation processes have been studied in low indium content self-assembled (In,Ga)As/GaAs quantum dots (QDs). Temperature-dependent photoluminescence of the wetting layer (WL) and QDs, and QD photoluminescence rise time elongation from $\ensuremath{\sim}$100 to $\ensuremath{\sim}$200 ps in the range of 10--45 K, indicated a complex carrier relaxation scheme. It involves localization of carriers/excitons in the WL, their temperature-mediated release, and subsequent transfer between the states of the WL and QD ensemble. These observations are explained by a thermal hopping model, in which electron-hole pairs are redistributed within two separate sets of zero-dimensional states of considerably different densities connected by a two-dimensional mobility channel.

Impact of light polarization on photoluminescence intensity and quantum efficiency in AlGaN and AlInGaN layers

We analyzed emission intensity, quantum efficiency, and emitted light polarization of c-plane AlGaN and AlInGaN layers (λ = 320–350 nm) by temperature dependent photoluminescence. Low indium content in AlInGaN structures causes a significant intensity increase by change of the polarization of the emitted light. Polarization changes from E ⊥ c to E ‖ c with increasing aluminum content. It switches back to E ⊥ c with the incorporation of indium. The polarization degree decreases with temperature. This temperature dependence can corrupt internal quantum efficiency determination by temperature dependent photoluminescence.

Ternary mixed crystal effects on electron-interface optical phonon interactions in InxGa1−xN/GaN quantum wells

Based on the modified random-element isodisplacement model and dielectric continuum model, the dispersions of interface optical phonons, electron-interface phonon interaction and ternary mixed crystal effect on interface optical phonons in InxGa1−xN/GaN quantum wells are studied in a fully numerical manner. The results indicate that there are two indium concentration intervals that interface optical phonons exist. The indium concentration has important effects on the dispersions and electron–phonon interactions of interface optical phonons. The electron–IO phonon interactions in higher indium concentration are more important than that in lower indium concentration.

Properties of low indium content Al incorporated IZO (indium zinc oxide) deposited at room temperature

A very low indium content (35 cation % In) a-IZO film, denoted as IZO35/65 or a-ZIO, was fabricated at room temperature. The effect of aluminum (Al) incorporation in IZO35/65 was studied systematically, and reasonably high electrical conductivity and optical transmittance were obtained. After Al doping to an optimum extent, the conductivity was interestingly increased 3 times higher than that of a-ZIO, and this effect could not be attributed to the formation of Al clusters as a further increase in Al led to a significant drop in conductivity. Nonlinear and U-shape resistivity-temperature relationship was observed for some Al doped samples. In addition, an obvious conductivity increase accompanying by the Al2O3 nano-crystallites formation was observed, which provided new evidence to the insulator-metal transition model reported by Nagarajan et al. [Nature Mater. 7, 391 (2008)] recently.

Phase separations in graded-indium content InGaN/GaN multiple quantum wells and its function to high quantum efficiency

Phase separations have been studied for graded-indium content InxGa1−xN/GaN multiple quantum wells (MQWs) with different indium contents by means of photoluminescence (PL), cathodeluminescence (CL) and time-resolved PL (TRPL) techniques. Besides the main emission peaks, all samples show another 2 peaks at the high and low energy parts of the main peaks in PL when excited at 10 K. CL images show a clear contrast for 3 samples, which indicates an increasing phase separation with increasing indium content. TRPL spectra at 15 K of the main emissions show an increasing delay of rising time with indium content, which means a carrier transferring from low indium content structures to high indium content structures.

Radio-frequency thermal plasma synthesis of nano-sized indium zinc tin oxide powders with reduced indium content

A route to obtain nano-sized IZTO (Indium Zinc Tin Oxide) materials with relatively low indium content is investigated using an RF (Radio-Frequency) thermal plasma system. For this purpose, In2O3, SnO2 and ZnO with the sizes of 0.1–1μm were mixed at the cation ratios of 6:2:2 and 2:1:1, and then, injected into RF thermal plasma with a plate power level of ~26.6kVA. From the field emission scanning electron microscopy images and the transmission electron microscopy–electron energy loss spectroscopy mapping images of the as-synthesized powders, it was found that the three oxides were unified into nano-sized IZTO particles (<50nm), with each element of In, Zn and Sn uniformly distributed. In addition, X-ray diffraction data and inductively coupled plasma-optical emission spectrometry analysis results showed that the IZTO powders with a single phase of bixbyite In2O3 structure can be achieved even with an indium content of 60at.%.