InGaN Ternary Alloys Research Articles

Valence and Conduction Band Offsets for InN and III-Nitride Ternary Alloys on (−201) Bulk β-Ga2O3

Valence and conduction band offsets of the InN/β-Ga2O3 type-I heterojunction have been determined to be −0.55 ± 0.11 eV and −3.35 ± 0.11 eV, respectively, using X-ray photoelectron spectroscopy. The InN layers were grown using atomic layer epitaxy on (−201) oriented commercial β-Ga2O3 substrates. Combining this data with published band offsets for the GaN and AlN heterojunctions to β-Ga2O3 has allowed us to predict the band offsets for the AlGaN, AlInN, and InGaN ternary alloys to β-Ga2O3. The conduction band offsets for InGaN and AlInN to β-Ga2O3 increased for high In concentration and, similarly, the valence band offsets for AlGaN and AlInN to β-Ga2O3 decreased at high Al concentration.

Design and Simulation of InGaN/GaN p–i–n Photodiodes

InGaN ternary alloys with their band gaps varying from 0.7 to 3.4 eV, are very promising for photodetector devices operating from UV to IR wavelength range. Using Silvaco–Atlas software, an In0.1Ga0.9N/GaN based p–i–n photodiode is designed and the J–V characteristics, the spectral responsivity, the frequency response and the cut‐off frequency as a function of InGaN thickness are studied. The photodiode exhibits a high reverse breakdown voltage of 38 V, a peak responsivity of 0.2 A W−1 at 0.343 μm wavelength and a cutoff frequency of 400 MHz under an applied reverse bias voltage of 2 V and for a 0.1 μm i‐InGaN layer, in good agreement with simulated and experimental results found in literature. It is found that an optimum i‐layer thickness of 1.5 μm for the maximum cutoff frequency of 4 GHz attributed to the predominance of the limitation in capacitance effect on the cutoff frequency at low i‐layer thickness and the transit time on the cuttoff frequency for high i‐layer thickness. For this i‐layer thickness, the highest peak responsivity about 0.244 A W−1 at 0.384 μm wavelength is achieved.

III-Nitride Digital Alloy: Electronics and Optoelectronics Properties of the InN/GaN Ultra-Short Period Superlattice Nanostructures

The III-Nitride digital alloy (DA) is comprehensively studied as a short-period superlattice nanostructure consisting of ultra-thin III-Nitride epitaxial layers. By stacking the ultra-thin III-Nitride epitaxial layers periodically, these nanostructures are expected to have comparable optoelectronic properties as the conventional III-Nitride alloys. Here we carried out numerical studies on the InGaN DA showing the tunable optoelectronic properties of the III-Nitride DA. Our study shows that the energy gap of the InGaN DA can be tuned from ~0.63 eV up to ~2.4 eV, where the thicknesses and the thickness ratio of each GaN and InN ultra-thin binary layers within the DA structure are the key factors for tuning bandgap. Correspondingly, the absorption spectra of the InGaN DA yield broad wavelength tunability which is comparable to that of bulk InGaN ternary alloy. In addition, our investigation also reveals that the electron-hole wavefunction overlaps are remarkably large in the InGaN DA structure despite the existence of strain effect and build-in polarization field. Our findings point out the potential of III-Nitride DA as an artificially engineered nanostructure for optoelectronic device applications.

InN/GaN short‐period superlattices as ordered InGaN ternary alloys

AbstractCoherent (InN)1/(GaN)n short‐period superlattices (SPSs) were successfully grown through dynamic atomic layer epitaxy (D‐ALEp) mode by RF‐plasma molecular beam epitaxy (MBE), where GaN layer thicknesses n were thinned down to 4 monolayer (ML). After this achievement, we demonstrated quasi‐ternary InGaN behavior in their photoluminescence (PL) spectra for the first time. It was found interestingly that GaN layer thickness of n = 4 ML was the criterion both for structural control and continuum‐band formation. Although highly lattice‐mismatched InN/GaN interfaces easily introduce relaxation in (InN)1/(GaN)4 SPSs during growth depending on the dynamic surface stoichiometry condition, this problem was overcome by precise control/removal of fluid‐like residual In/Ga metals on the growth front with in‐situ monitoring method. The (InN)1/(GaN)n SPSs with n ≥ 7 ML showed a constant PL peak energy around 3.2 eV at 12 K, reflecting discrete electron/hole wavefunctions. On the other hand, the (InN)1/(GaN)4 SPSs indicated the red‐shifted PL peak at 2.93 eV at 12 K, which was attributed to the continuum‐band state with increasing in the overlap of electrons/hole wavefunctions. This result is concluded that the (InN)1/(GaN)4 SPSs can be considered as ordered InGaN alloys. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of compressive strain on the incorporation of indium in InGaN and InAlN ternary alloys**Project supported by the National Natural Science Foundation of China (Grant Nos. 61404099 and 61306017) and the Fundamental Research Funds for the Central Universities, China (Grant No. JB141101).

In order to investigate the influence of compressive strain on indium incorporation in InAlN and InGaN ternary nitrides, InAlN/GaN heterostructures and InGaN films were grown by metal–organic chemical vapor deposition. For the heterostructures, different compressive strains are produced by GaN buffer layers grown on unpatterned and patterned sapphire substrates thanks to the distinct growth mode; while for the InGaN films, compressive strains are changed by employing AlGaN templates with different aluminum compositions. By various characterization methods, we find that the compressive strain will hamper the indium incorporation in both InAlN and InGaN. Furthermore, compressive strain is conducive to suppress the non-uniform distribution of indium in InGaN ternary alloys.

Characterization of InGaN Solar Cells

The III-V materials are extensively studied for optoelectronic applications in the blue and UV spectral regions. InGaN ternary alloy is considered for its wide spectral coverage, good electrical characteristics and appreciable resistance to high electrical currents. For this purpose, the operation of InGaN photovoltaic cells was studied by 2D numerical simulation under AM1.5 spectrum illumination, using the software Silvaco and the two environments Athena/Atlas.

Phase separation in InxGa1‐xN (0.10 < x < 0.40)

AbstractWe have performed micro‐photoluminescence (µ‐PL) mapping of InGaN films with thickness up to 300 nm with a large gradient of In concentration in a lateral direction (from 10% to 40%), grown by plasma‐assisted molecular beam epitaxy atop a 1.8‐µm‐thick GaN($000 \bar {\rm I}$) buffer deposited on a c‐Al2O3 substrate. The µ‐PL spectra measured at various points with different average composition show almost identical emission bands with fixed wavelength ranges: 490‐540 nm (green band), 550‐590 nm (orange band), and 600‐660 nm (red band). This behavior implies emergence of at least three phases with fixed In contents, which are nearly independent of the average film composition and the variation of the growth temperature. The variation of the average composition of the InGaN ternary alloy results in redistribution of the band intensities that can be explained by redistribution of the relative volumes of the spatially separated phases. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of Droplet Elimination Process by Radical-Beam Irradiation to InGaN Growth and Fabrication of InN/InGaN Periodic Structure

A growth method, named droplet elimination by radical-beam irradiation (DERI), consisting of a metal-rich growth process (MRGP) and a droplet elimination process (DEP), was developed for the growth of InGaN ternary alloys. In the MRGP using the growth of InGaN under a metal-rich condition [N*/(In+Ga)<1], Ga was preferentially incorporated into the growing InGaN and In was forced out to the surface. This In was transformed to epitaxial InN on top of the InGaN underlayer in the DEP using a subsequent nitrogen radical-beam irradiation. By repeating the MRGP and DEP, an InN/InGaN periodic structure was successfully fabricated.

Structural Phase Variation of InGaN Micro-Structures Grown by Using Mixed Source HVPE

In this paper, we report structural changes in InGaN micro-structures grown by using mixed-source hydride vapor phase epitaxy (HVPE). Hexagonal nanorods, bunched with many submicron-sized legs and tetrapod-shaped InGaN micro-structures, are grown on r-plane sapphire, c-plane sapphire and Si (111) substrates. As the growth temperature is increased, the overall shape of the InGaN structures changes from clusters of some needle-like legs to bunch shapes of many legs with thicker and hexagonal edges. The grown InGaN structures were analyzed by using X-ray photoelectron spectroscopy (XPS) to characterize the InGaN ternary crystal alloy. The indium mole fractions of the InGaN structures grown at 650 C, 700 C and 750 C were 44 %, 37 % and 27 %, respectively.

Bowing of the band gap pressure coefficient in InxGa1−xN alloys

The hydrostatic pressure dependence of photoluminescence, dEPL/dp, of InxGa1−xN epilayers has been measured in the full composition range 0&amp;lt;x&amp;lt;1. Furthermore, ab initio calculations of the band gap pressure coefficient dEG/dp were performed. Both the experimental dEPL/dp values and calculated dEG/dp results show pronounced bowing and we find that the pressure coefficients have a nearly constant value of about 25 meV/GPa for epilayers with x&amp;gt;0.4 and a relatively steep dependence for x&amp;lt;0.4. On the basis of the agreement of the observed PL pressure coefficient with our calculations, we confirm that band-to-band recombination processes are responsible for PL emission and that no localized states are involved. Moreover, the good agreement between the experimentally determined dEPL/dp and the theoretical curve of dEG/dp indicates that the hydrostatic pressure dependence of PL measurements can be used to quantify changes of the band gap of the InGaN ternary alloy under pressure, demonstrating that the disorder-related Stokes shift in InGaN does not induce a significant difference between dEPL/dp and dEG/dp. This information is highly relevant for the correct analysis of pressure measurements.

Dynamics of electron–hole plasmas and localized excitons in highly excited GaN-based ternary alloys

We have studied photoluminescence (PL) spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. With an increase of the intensity, the broad PL due to electron–hole plasmas (EHP) appears below the biexciton PL in the GaN sample. On the other hand, the broad EHP PL appears above the localized exciton PL in the InGaN sample. The intensity dependence of PL properties of InGaN crystals is completely different from that of GaN crystals. The effect of alloy disorder on PL processes in ternary alloys is discussed.

Complete composition tunability of InGaN nanowires using a combinatorial approach

The III nitrides have been intensely studied in recent years because of their huge potential for everything from high-efficiency solid-state lighting and photovoltaics to high-power and temperature electronics. In particular, the InGaN ternary alloy is of interest for solid-state lighting and photovoltaics because of the ability to tune the direct bandgap of this material from the near-ultraviolet to the near-infrared region. In an effort to synthesize InGaN nitride, researchers have tried many growth techniques. Nonetheless, there remains considerable difficulty in making high-quality InGaN films and/or freestanding nanowires with tunability across the entire range of compositions. Here we report for the first time the growth of single-crystalline In(x)Ga(1-x)N nanowires across the entire compositional range from x=0 to 1; the nanowires were synthesized by low-temperature halide chemical vapour deposition and were shown to have tunable emission from the near-ultraviolet to the near-infrared region. We propose that the exceptional composition tunability is due to the low process temperature and the ability of the nanowire morphology to accommodate strain-relaxed growth, which suppresses the tendency toward phase separation that plagues the thin-film community.

Polarity dependence of In‐rich InGaN ternary alloys grown by RF‐MBE

AbstractIn‐rich (XIn &gt; 0.5) InGaN ternary alloys were grown on N‐ and Ga‐polarity GaN templates by radio‐frequency plasma assisted molecular beam epitaxy (RF‐MBE) and their polarity dependence was investigated. In‐rich InGaN alloys with In‐content ranging from 0.5 to 1.0 could be grown without phase separation and sharp single diffraction peak was observed for these samples in X‐ray diffraction (XRD) 2θ–ω scan. Compared to N‐polarity samples, In‐polarity InGaN showed better crystalline quality in spite of about 100 °C lower growth temperature. Moreover, the dependence of crystalline quality on V/III ratio was investigated for In‐polarity samples. It was found that the phase separation in the InGaN tends to occur under metal‐rich condition but this can be prevented in N‐rich and around unity stoichiometry conditions as well. Precise control of V/III ratio to around unity stoichiometry was realized by using a shutter control method, which resulted in improvement of crystalline quality and surface morphology of In‐rich InGaN films. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Studies of Stokes shift in InxGa1−xN alloys

InGaN ternary alloys have been studied with photoluminescence, photoluminescence excitation spectroscopy, scanning electron microscopy, and cathodoluminescence spectroscopy. The relatively large Stokes shift observed in the photoluminescence and photoluminescence excitation spectroscopy has been found to be consistent with previous results reported in the literature. By correlating our experimental findings and others reported in the literature with those of scanning electron microscopy and cathodoluminescence spectroscopy, we conclude that the physical origin of the Stokes shift in InGaN ternary alloy system is primarily due to the effects of alloy composition fluctuations. A plausible model responsible for the observed Stokes shift is proposed.

Growth of InGaN layer on GaN templated Al 2 O 3 (0001) and Si (111) substrates by mixed‐source HVPE

The InGaN layers on GaN templated sapphire (0001) and Si (111) substrates are grown by mixed-source hydride vapor phase epitaxy (HVPE) method. As a new attempt in obtaining an InGaN layers, the growth of the thick InGaN layer is performed by putting small amount of Ga into the In source. The InGaN layer is compounded from chemical reaction between a NH3 and an Indium-gallium chloride formed by HCl flown over metallic In mixed with Ga. The InGaN layer is analyzed by X-ray photoelectron spectroscopy (XPS) to characterize the InGaN ternary crystal alloy. The optical property of the selective area growth (SAG) of the InGaN layer is investigated by the photoluminescence (PL) spectrum and the cathodoluminescence (CL) images. Indium compositions are estimated to be in the range 3-10%. In order to obtain the GaN layer on GaN templated Si (111) substrates, an InGaN layer is used as an intermediate layer. The PL and CL of the InGaN intermediate layer are measured at 300K. We find that the InGaN intermediate layer is possible to be one of the growth methods of thick GaN layer on Si substrates by HVPE method. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ab initio studies of indium separated phases in AlGaInN quaternary alloys

In this work, ab initio total energy electronic structure calculations are combined with Monte Carlo simulations to study microscopically the indium separated phases taking place in AlxGayIn1–x–yN quaternary alloys. The presence of aluminum in the InGaN alloy is shown to enhance the phase separation process, compared to the InGaN ternary alloy with the same In compositions. We also observe that even in the stable region of the quaternay alloy there are composition fluctuations towards InGaN- and AlGaN-like alloys formation. From our findings the origin of the emissions which have been observed from AlGaInN quaternary is discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal redistribution of localized excitons and its effect on the luminescence band in InGaN ternary alloys

Temperature-dependent photoluminescence measurements have been carried out in zinc-blende InGaN epilayers grown on GaAs substrates by metalorganic vapor-phase epitaxy. An anomalous temperature dependence of the peak position of the luminescence band was observed. Considering thermal activation and the transfer of excitons localized at different potential minima, we employed a model to explain the observed behavior. A good agreement between the theory and the experiment is achieved. At high temperatures, the model can be approximated to the band-tail-state emission model proposed by Eliseev et al. [Appl. Phys. Lett. 71, 569 (1997)].

Growth and optical properties of InxAlyGa1−x−yN quaternary alloys

In x Al y Ga 1−x N quaternary alloys with different In and Al compositions were grown by metalorganic chemical vapor deposition. Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. It was observed that the dominant optical transition at low temperatures in InxAlyGa1−xN quaternary alloys was due to localized exciton recombination, while the localization effects in InxAlyGa1−xN quaternary alloys were combined from those of InGaN and AlGaN ternary alloys with comparable In and Al compositions. Our studies have revealed that InxAlyGa1−xN quaternary alloys with lattice matched with GaN epilayers (y≈4.8x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensity (or quantum efficiency) in InxAlyGa1−x−yN quaternary alloys than that of GaN. The quantum efficiency of InxAlyGa1−xN quaternary alloys was also enhanced significantly over AlGaN alloys with a comparable Al content. These results strongly suggested that InxAlyGa1−x−yN quaternary alloys open an avenue for the fabrication of many optoelectronic devices such as high efficient light emitters and detectors, particularly in the ultraviolet region.

Chemical ordering in AlGaN layers grown by MOCVD

ABSTRACTOne of the applications of wurtzite gallium based nitride compounds will be to provide optoelectronic devices from 6.2eV (AlN) to 1.89eV (InN). This will depend on the possibility to grow wurtzite AlGaN and InGaN ternary alloys. As expected, the most difficult region is InGaN due to the large misfit between GaN and InN (= 10%), in this case, ordering, phase separation and growth instabilities have been reported. In the case of AlN and GaN, the misfit is smaller (∼ 2.5%) and one would expect more stable growth. However, it was in this system that ordering along the c axis between AlN and GaN was reported for the first time.In this work, we have found that the growth of AlGaN may be more complicated. Not only the wurtzite lattice can be decreased to simple hexagonal by AlN/GaN ordering along the c axis, but the growth can lead to other types of stackings. Even in the low Al composition range, 10 - 15%, we have found that three processes can operate:1. Ordering into AlN/GaN as two simple hexagonal sublattices.2. The 3:1 ordering which has been recently reported to occur in InGaN.3. A new type of ordering where diffraction experiments (XR and electron diffraction) detect superlattice reflection with a period close to 3 nm. The most adequate model which was found to take this into account shows that, in these growth conditions, the system has preferred to form one AlN cell in between 5 GaN cells, leading to a 5:1 ordering.

Indium incorporation above 800 °C during metalorganic vapor phase epitaxy of InGaN

We have studied the indium incorporation into InGaN ternary alloys during low-pressure metalorganic vapor-phase epitaxy as a function of the trimethylindium flow and the growth temperature in the 800–860 °C range. Partially relaxed InxGa1−xN bulk films with indium compositions 0.02≲x≲0.14 have been grown. In relation to the band-gap energy at room temperature, determined by photothermal deflection spectroscopy, we find a downward band-gap bowing of 2.65±0.15 eV. The required change of the trimethylindium flow as a function of the growth temperature, necessary to obtain isocomposition InGaN films, can be described by an Arrhenius law. We find an indium desorption energy of 0.8±0.3 eV.