AlInGaN Layers Research Articles

Thin-Film LED based on AlInGaN Layers Grown on Hybrid SiC/Si Substrates

Thin-Film LED based on AlInGaN Layers Grown on Hybrid SiC/Si Substrates

Enhanced Performance of N-Polar AlGaN-Based Ultraviolet Light-Emitting Diodes With Lattice- Matched AlInGaN Insertion in n-AlGaN Layer

AlGaN-based ultraviolet-A light-emitting diodes (UVA LEDs) inevitably suffer from current crowding effects at high injection levels due to their lateral device structure, resulting in non-uniform light emission and device overheating. In N-polar UV LEDs, the problem is further exacerbated by increased hole injection efficiency, leading to current crowding and aggravated hole leakage, which limits the device performance. An n-AlGaN/AlInGaN/AlGaN structure is adopted in this study, through modulation of the Al and In compositions in the AlInGaN quaternary alloy, lattice matching and greater bandgap of AlInGaN to AlGaN template is designed. The numerical results prove that the n-AlGaN/AlInGaN/AlGaN structure can promote current spreading and thus mitigate hole leakage, resulting in the significantly enhanced performance of N-polar UVA LEDs. Furthermore, the use of lattice-matched AlInGaN layers in practical epitaxy is feasible, which can avoid the defect introduction resulting from the lattice mismatch.

Improved Ultraviolet-B Light-Emitting Diodes with Graded All Quaternary Layers in the Active Region

We employ quaternary graded AlInGaN layers numerically in the active region to study the effect on the optoelectronic properties of AlGaN-based ultraviolet light-emitting diodes (UV LEDs). We evaluated the device performance by analyzing carriers concentrations, carrier flux, radiative recombination rate, energy band diagrams and internal quantum efficiency (IQE). We compare the results with the reference device structure and found that the device with quaternary graded AlInGaN layers has high peak efficiency as well as low efficiency droop.

Trap state characterization of Al2O3/AlInGaN/GaN metal-insulator-semiconductor heterostructures

Frequency-dependent conductance measurements were carried out to investigate the trap states in Al2O3/AlInGaN/GaN metal-insulator-semiconductor (MIS) heterostructures. From the electrical measurements, an improvement in trap state characteristics was observed for the device fabricated with lower In composition in quaternary nitride layers. In the presence of higher In composition, segregation of In atoms was observed in AlInGaN layers. Because of this In-segregation, surface quality of the quaternary-N deteriorated that led to the formation of trap states at and near the oxide/semiconductor interface. Therefore, in the presence of lower In composition, an improvement in trap characteristics was observed for the AlInGaN/GaN-based MIS devices.

Electric field dynamics in nitride structures containing quaternary alloy (Al, In, Ga)N

Molecular beam epitaxy growth and basic physical properties of quaternary AlInGaN layers, sufficiently thick for construction of electron blocking layers (EBL), embedded in ternary InGaN layers are presented. Transmission electron microscopy (TEM) measurement revealed good crystallographic structure and compositional uniformity of the quaternary layers contained in other nitride layers, which are typical for construction of nitride based devices. The AlInGaN layer was epitaxially compatible to InGaN matrix, strained, and no strain related dislocation creation was observed. The strain penetrated for limited depth, below 3 nm, even for relatively high content of indium (7%). For lower indium content (0.6%), the strain was below the detection limit by TEM strain analysis. The structures containing quaternary AlInGaN layers were studied by time dependent photoluminescence (PL) at different temperatures and excitation powers. It was shown that PL spectra contain three peaks: high energy donor bound exciton peak from the bulk GaN (DX GaN) and the two peaks (A and B) from InGaN layers. No emission from quaternary AlInGaN layers was observed. An accumulation of electrons on the EBL interface in high-In sample and formation of 2D electron gas (2DEG) was detected. The dynamics of 2DEG was studied by time resolved luminescence revealing strong dependence of emission energy on the 2DEG concentration. Theoretical calculations as well as power-dependence and temperature-dependence analysis showed the importance of electric field inside the structure. At the interface, the field was screened by carriers and could be changed by illumination. From these measurements, the dynamics of electric field was described as the discharge of carriers accumulated on the EBL.

Tuning of the tetragonal distortion in AlInGaN thin films by different contents of Al and In

Systematic investigations were performed on a set of AlInGaN epilayers which were grown by metalorganic chemical vapor deposition on sapphire with a thick (3μm) GaN intermediate layer. The chemical compositions (both In and Al contents) of AlInGaN layers were directly determined by Rutherford backscattering spectrometry and elastic recoil detection analysis. Using high resolution X-ray diffraction and the channeling scan around an off-normal 〈12¯13〉 axis in {101¯0} plane of the AlInGaN layers, the tetragonal distortion eT is determined. The results show that eT, ranging from negative to positive, is drastically influenced by the chemical composition in AlInGaN epilayers.

Morphology, growth mode and indium incorporation of MOVPE grown InGaN and AlInGaN: A comparison

We compared InGaN- and AlInGaN-layers grown by metal-organic vapor phase epitaxy (MOVPE) in terms of morphology, growth mode and indium incorporation. The growth parameters of the AlInGaN layers only differed from InGaN growth by an additional trimethylaluminum (TMAl) flow. Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) measurements showed that the indium incorporation in AlInGaN was significantly increased compared to InGaN. Atomic force microscopy (AFM) was used to analyze the morphology and the growth mode. The additional TMAl flow changed the growth mode from a step-flow mode to a 2-dimensional (2D) island nucleation mode, yielding a smoother layer morphology. This behavior can be explained by the low surface mobility of the Al adatoms and their nucleation on terraces between adjacent steps. Step bunching – as observed for InGaN – was avoided during AlInGaN growth. This reduced the AFM root mean square roughness by 40% compared to InGaN. Possible impacts on charge carrier localization in QWs are discussed.

Optical absorption of Mg-doped layers and InGaN quantum wells on c-plane and semipolar GaN structures

We studied optical absorption of Mg-doped AlInGaN layers using excitation-position dependent and polarization resolved photoluminescence from the slab-waveguide edge of a laser structure. The major absorption in the Mg-doped layers was found only when p-doping is activated. It increases with the removal of residual hydrogen, which in case of Mg doping is a p-type passivation impurity, and reversibly disappears after passivation by hydrogen. This absorption is weakly wavelength and temperature dependent, and isotropic. This can be attributed to acceptor-bound hole absorption, because those holes concentration is nearly equal to that of activated acceptors and weakly temperature dependent (unlike the free hole concentration, which is much lower and is an exponential function of temperature due to high ionization energy). The cross section of photon absorption on such activated acceptor was quantified to be in the order of 10−17 cm−2. The absorption cross section of free electrons was found to be at least one order of magnitude lower and below detection limit. The same technique was used to experimentally quantify band structure polarization components along basis directions for green InGaN quantum wells (QWs) grown on c- and semipolar planes. The A1 and B1 valence subbands of c-plane QW were found to comprise mostly |X⟩ and |Y⟩ states. There was rather minor amount of |Z⟩ states with average square fraction of only 0.02. In (20-21) plane, due to small band anticrossing near gamma-point, we observed highly polarized absorption edges of A1- and B1-subbands consisting mainly of |Y⟩ and |X⟩ states, respectively, and found their energy splitting to be ∼40 meV. For (11-22) plane with smaller band splitting and polarization, we observed polarization switching with indium (In) concentration greater than 30% in the QW (or photon energy less than 2.3 eV). We confirmed our study of valence band structures by optical gain measurements.

Polarization-Engineered Enhancement-Mode High-Electron-Mobility Transistors Using Quaternary AlInGaN Barrier Layers

Group III nitride heterostructures with low polarization difference recently moved into the focus of research for realization of enhancement-mode (e-mode) transistors. Quaternary AlInGaN layers as barriers in GaN-based high-electron-mobility transistors (HEMTs) offer the possibility to perform polarization engineering, which allows control of the threshold voltage over a wide range from negative to positive values by changing the composition and strain state of the barrier. Tensile-strained AlInGaN layers with high Al contents generate high two-dimensional electron gas (2DEG) densities, due to the large spontaneous polarization and the contributing piezoelectric polarization. To lower the 2DEG density for e-mode HEMT operation, the polarization difference between the barrier and the GaN buffer has to be reduced. Here, two different concepts are discussed. The first is to generate compressive strain with layers having high In contents in order to induce a positive piezoelectric polarization compensating the large negative spontaneous polarization. Another novel approach is a lattice-matched Ga-rich AlInGaN/GaN heterostructure with low spontaneous polarization and improved crystal quality as strain-related effects are eliminated. Both concepts for e-mode HEMTs are presented and compared in terms of electrical performance and structural properties.

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.

Relaxation and critical strain for maximum In incorporation in AlInGaN on GaN grown by metal organic vapour phase epitaxy

Quaternary AlInGaN layers were grown on conventional GaN buffer layers on sapphire by metal organic vapour phase epitaxy at different surface temperatures and different reactor pressures with constant precursor flow conditions. A wide range in compositions within 30–62% Al, 5–29% In, and 23–53% Ga was covered, which leads to different strain states from high tensile to high compressive. From high-resolution x-ray diffraction and Rutherford backscattering spectrometry, we determined the compositions, strain states, and crystal quality of the AlInGaN layers. Atomic force microscopy measurements were performed to characterize the surface morphology. A critical strain value for maximum In incorporation near the AlInGaN/GaN interface is presented. For compressively strained layers, In incorporation is limited at the interface as residual strain cannot exceed an empirical critical value of about 1.1%. Relaxation occurs at about 15 nm thickness accompanied by strong In pulling. Tensile strained layers can be grown pseudomorphically up to 70 nm at a strain state of 0.96%. A model for relaxation in compressively strained AlInGaN with virtual discrete sub-layers, which illustrates the gradually changing lattice constant during stress reduction is presented.

Growth Studies on Quaternary AlInGaN Layers for HEMT Application

Quaternary barrier layers for GaN-based high-electron-mobility transistors (HEMT) have recently been a focus of interest because of the possible lattice-matched growth to GaN. This results in a reduction of strain-related defects, while having the option of adjusting the bandgap separately. A further benefit of the quaternary approach is the possibility to achieve high polarization and high carrier mobility simultaneously. This may improve the performance of such devices beyond what is possible with ternary barrier layers. In this work, we report on growth and characterization of AlxInyGa1−x−yN barrier layers within the range of 16% to 56% Al, 2% to 45% In, and 20% to 82% Ga deposited on conventional GaN buffer layers on sapphire. We present an effective way to change the composition of quaternary layers and discuss the influence of tensile and compressive strain on structural and electrical properties. From high-resolution x-ray diffraction (HRXRD), Rutherford backscattering spectroscopy (RBS), and wavelength-dispersive x-ray spectroscopy (WDX), we determined the compositions and strain states of the AlInGaN layers. The bandgaps (Eg) were obtained by spectroscopic ellipsometry (SE). Hall and van der Pauw measurements on thin heterostructure layers yielded high mobilities in excess of 1550 cm2/V s and 5350 cm2/V s at room temperature and 77 K, respectively.

Dielectric function and optical properties of quaternary AlInGaN alloys

The optical properties of quaternary AlxInyGa1-x-yN alloy films with 0.16<x<0.64 and 0.02<y<0.13 are presented. The (0001)-oriented AlInGaN layers were grown by metal-organic vapor phase epitaxy on thick GaN/sapphire templates. High-resolution x-ray diffraction measurements revealed the pseudomorphic growth of the AlInGaN films on the GaN buffer. Rutherford backscattering and wavelength-dispersive x-ray spectroscopy analysis were used in order to determine the composition of the alloys. The ordinary dielectric function (DF) of the AlInGaN samples was determined in the range of 1–10 eV by spectroscopic ellipsometry (SE) at room temperature (synchrotron radiation: BESSY II). The sharp onset of the imaginary part of the DF defines the direct absorption edge of the alloys. At higher photon energies, pronounced peaks are observed in the DF indicating a promising optical quality of the material. These features are correlated to the critical points of the band structure (van Hove singularities). An analytical model, which permits us to accurately describe the dielectric function (or optical constants) in the range of 1–10 eV, is also presented. The band-gap and high-energy interband transition values are obtained by fitting the experimental DF with the analytical model. The strain influence on the bandgap is evaluated by using the k×p formalism. Furthermore, an empirical expression is proposed which allows us to calculate the AlInGaN band-gap and high-energy inter-band transitions in the whole compositional range (x, y). The band-gap values obtained from the empirical expression are in good agreement with both the calculated ab initio and the experimental values determined by SE.

MOVPE grown quaternary AlInGaN layers for polarization matched quantum wells and efficient active regions

AbstractQuaternary AlInGaN quantum wells in GaN barriers with varying thicknesses were deposited in a pulsed metal‐organic precursor mode by vapor‐phase epitaxy. X‐ray diffraction and photoluminescence measurements show the good quality of the deposited material.By adjustment of the quaternary composition we can achieve polarization f eld free conditions at the AlInGaN/GaN interface. By using AlInGaN as barrier material for a ternary InGaN QW we could increase the relative quantum eff ciency to 20% at room temperature. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Composition and luminescence of AlInGaN layers grown by plasma-assisted molecular beam epitaxy

A study of AlInGaN epilayers, grown by plasma-assisted molecular beam epitaxy, was performed using spatially resolved x-ray microanalysis and luminescence spectroscopy in order to investigate competition between the incorporation of In, Al, and Ga as a function of the growth temperature in the 565–660 °C range and the nominal AlN mole fraction. The samples studied have AlN and InN mole fractions in the ranges of 4%–30% and 0%–16%, respectively. Composition measurements show the effect of decreasing temperature to be an increase in the incorporation of InN, accompanied by a small but discernible decrease in the ratio of GaN to AlN mole fractions. The incorporation of In is also shown to be significantly increased by decreasing the Al mole fraction. Optical emission peaks, observed by cathodoluminescence mapping and by photoluminescence, provide further information on the epilayer compositions as a function of substrate temperature, and the dependencies of peak energy and linewidth are plotted.

Growth behavior of AlInGaN films

The structural and surface properties of AlInGaN quaternary films grown at different temperatures on GaN templates by metalorganic chemical vapor deposition are investigated. Formation of two quaternary layers is confirmed and the difference between them is pronounced when the growth temperature is increased. The surface is featured with V-shaped pits and cracks, whose characteristics are further found to be strongly dependent on the growth temperature of AlInGaN layers. The two-layer structure is interpreted by taking into account of the strain status in AlInGaN layers.

Quaternary AlInGaN-based photodetectors

The growth of quaternary AlInGaN epitaxial layer on GaN/sapphire substrates by metalorganic chemical vapour deposition is reported. It was found that AlInGaN layers were grown three dimensionally with rough surface at low temperatures and transferred to smooth two-dimensional growth at 860°C. It was also found that In mole fraction in the layers decreased significantly as the AlInGaN growth temperature was increased while Al composition ratio was much less temperature dependent. Furthermore, it was found that solar-blind metal-insulator-semiconductor photodetectors with AlInGaN layer prepared at 860°C could provide us a photocurrent-to-dark-current contrast ratio of 1.93×104 and a UV-to-visible rejection ratio of 38.0.

Indium incorporation effects on luminescence mechanisms in quaternary AlInGaN layers

The optical and structural properties of quaternary AlInGaN layers grown by a pulsed metalorganic chemical vapor deposition (PMOCVD) have been investigated by means of photoluminescence (PL) and time-resolved PL (TRPL) measurements and X-ray diffraction. The AlInGaN layers show strong blueshift and linewidth broadening of the PL emission band with increasing excitation power. With increasing indium mole fraction, the AlInGaN layer exhibited a stronger PL intensity, a faster PL decay, and a smaller lattice mismatch between the AlInGaN layer and the GaN buffer layer. Based on both the PL and TRPL data, we suggest that the incorporation of indium creates more band-tail states and enhances the luminescence efficiency, indicating that the PMOCVD-grown AlInGaN is better suited as the active region material for ultraviolet light-emitting diodes than conventional MOCVD-grown AlInGaN.

Influence of V/III flux ratio on electrical properties of AlInGaN/Gan heterostructures grown by MOCVD

The electrical properties of AlInGaN/GaN heterostructures grown on sapphire substrate by metal-organic chemical vapor deposition under different V/III flux ratios from 3880 to 9082 were investigated. Rutherford backscattering and channeling were used to determine the composition and to evaluate crystalline quality of AlInGaN quaternary epilayers. Hall measurements at room temperature and 77 K showed that less sheet concentration and higher electron mobility were in lower V/III ratio samples. An intense photoluminescence at 351 nm with a narrow FWHM about 48.3 meV was assigned to be from Al 0.08In 0.015Ga 0.905N layer grown under 3880 V/III flux ratio. With a comparison of Raman spectra of wafers of different V/III flux ratios and membranes on Si, local strain in AlInGaN layers were also discussed. Point defects were considered to be related with large local stress in GaN template layer under AlInGaN of high V/III flux ratio, which may decrease the electron mobility in AlInGaN/GaN heterostructures.

Chemical composition and elastic strain in AlInGaN quaternary films

High-quality AlInGaN quaternary layers were grown on c-Al2O3 using a thick GaN template. A full width at half maximum of 0.075° from AlInGaN(0004) rocking curve and a minimum yield of 5.6% from Rutherford backscattering/channelling spectrometry (RBS) prove the AlInGaN layer of a comparable crystalline quality with GaN layers. The chemical compositions (both of Al and In contents) of AlInGaN layers are directly obtained from RBS and elastic recoil detection analysis. The lattice parameters both in perpendicular and parallel directions are deduced from X-ray diffraction. The AlInGaN layer is found to process a compressive strain in parallel direction and a tensile strain in perpendicular direction.