Thick InGaN Films Research Articles

Strain-induced formation of self-assembled InGaN/GaN superlattices in nominal InGaN films grown by plasma-assisted molecular beam epitaxy

In this work, we report the spontaneous formation of superlattice structures in nominal InGaN films grown by plasma-assisted molecular beam epitaxy. A 700-nm-thick self-assembled ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}/\mathrm{GaN}$ superlattice with excellent structural quality was achieved. Strain was studied as a possible driving force for the formation of self-assembled superlattice (SASL) structure by growth of InGaN on ZnO substrate using similar growth conditions. The SASL structures were optically characterized using photoluminescence spectroscopy. Structural characterization was conducted via transmission electron microscopy and atom probe tomography. High-resolution x-ray diffraction (XRD) and XRD reciprocal space map were utilized to determine the average composition and the degree of relaxation of InGaN films. We propose that the vertical phase separation observed in the SASL structure is caused by high-temperature growth and intensified by strain. This work provides a method for engineering strain and growth of thick InGaN films for a variety of applications including solar cells and photodetectors.

Observation of self-assembled InGaN/GaN superlattice structure grown on N-polar GaN by plasma-assisted molecular beam epitaxy

In this paper, we report on the observation of self-assembled InGaN/(In)GaN superlattice (SL) structure in a nominal “InGaN” film grown on N-polar GaN substrate. 350 nm thick InGaN films were grown at different temperatures ranging from 600 to 690 °C. Structural characterization was conducted via atomic force microscopy, scanning transmission electron microscopy, high-resolution x-ray diffraction, and XRD reciprocal space map. A SL structure was unexpectedly observed on all samples. However, the In content in each layer varied depending on growth temperature. By increasing the substrate temperature to 670 °C, a periodic structure composed of 3 nm In0.26Ga0.74N and 3 nm of GaN with a surface roughness of ∼0.7 nm was achieved. This work establishes a method for the growth of InGaN films with high structural quality on N-polar GaN and opens a new pathway for the design and fabrication of various electronic and optoelectronic devices with enhanced performance.

Growth of lattice-relaxed InGaN thick films on patterned sapphire substrates by tri-halide vapor phase epitaxy

Lattice-relaxed thick InGaN films could be obtained using patterned sapphire substrates (PSSs) by tri-halide vapor phase epitaxy. Furthermore, by inserting the GaN intermediate layer between the InGaN epilayer and the PSS, the crystalline quality of the InGaN epilayer was drastically improved. According to the X-ray diffraction ω-rocking curve measurements, the full width at half maximum of the InGaN epilayer was 850 arcsec around the symmetrical reflection and 967 arcsec around the skew-symmetrical reflection. It was also found that the thickness of the GaN intermediate layer influences the crystalline quality of the InGaN epilayer. As the thickness of the GaN intermediate layer increases, the crystalline quality of the GaN intermediate layer gets higher, resulting in the crystalline quality of the InGaN epilayer was improved.

Cathodoluminescence spatially resolves optical transitions in thick group-III and N-polar InGaN films

The growth of thick group-III polar InGaN films beyond the critical thickness remains a challenge due to the large miscibility gap and lattice mismatch between InN and GaN leading to phase separation and inhomogeneous distribution of indium that impacts the luminescence properties across the film. The growth of N-polar InGaN can circumvent these challenges due to the increased stability of InN, thus improving film quality. However, overall luminescence from N-polar InGaN films is consistently lower than that of group-III polar InGaN films grown under identical conditions. In this study, spatially resolved cathodoluminescence (CL) measurements are used to reveal the optical properties of group-III and N-polar thick InGaN films. In the case of group-III polar films, predominant CL luminescence arises from the periphery of V-pits which are regions with a high accumulation of indium, while negligible CL luminescence is observed at the apex of the V-pit, indicative of centers for non-radiative recombination. Large differences in the CL luminescence intensity between the group-III polar and N-polar InGaN are a result of significant differences in the carrier lifetimes of the respective polarities (∼50–200 ps: N-polar, ∼500–700 ps: group-III polar InGaN). Since the decay behavior of the transient in N-polar InGaN is bi-exponential, it is suspected that oxygen impurities play a dominant role in the overall luminescence quenching in N-polar InGaN films.

Optimization of Digital Growth of Thick N-Polar InGaN by MOCVD

Smooth 200 nm thick N-polar InGaN films were grown by metal–organic chemical vapor deposition (MOCVD) on sapphire using a digital approach consisting of a constant In, Ga, and N precursor flow with pulsed injection of H2 into the N2 carrier gas. Using this growth scheme, the H2 injection time was altered and the effect on the morphology and indium incorporation in the films observed. The effect of periodic insertion of additional GaN inter-layers on the surface morphology of the InGaN layers was also studied.

Intensive luminescence from a thick, indium-rich In0.7Ga0.3N film

An In0.7Ga0.3N layer with a thickness of 300 nm deposited on GaN/sapphire template by molecular beam epitaxy has been investigated by highly spatially resolved cathodoluminescence (CL). High crystal film quality without phase separation has been achieved. The InGaN layer shows intense emission in the IR spectral region. The lateral as well as the vertical luminescence distribution is used to probe the In composition ([In]) homogeneity: the thick InGaN film exhibits laterally a rather homogeneous emission intensity at 1.04 eV (∼1185 nm) with a FWHM of only 63 meV. Carrier localization into regions of enhanced In concentration originating from compositional fluctuations is revealed. The evolution of emission in growth direction has been explored by a cross-sectional CL linescan showing a slight spectral redshift from the bottom to the surface of the InGaN film corresponding to an increase of [In] of only 0.5% within the layer thickness of 300 nm.

Correlation between pit formation and phase separation in thick InGaN film on a Si substrate

We demonstrate improved surface pit and phase separation in thick InGaN grown on a GaN/Si (111) substrate, using plasma-assisted molecular beam epitaxy with an indium modulation technique. The formation of surface pit and compositional inhomogeneity in the InGaN epilayer are investigated using atomic force microscopy, scanning electron microscopy and temperature-dependent photoluminescence. Indium elemental mapping directly reveals that poor compositional homogeneity occurs near the pits. The indium-modulation epitaxy of InGaN minimizes the surface indium segregation, leading to the reduction in pit density and size. The phase separation in InGaN with a higher pit density is significantly suppressed, suggesting that the pit formation and the phase separation are correlated. We propose an indium migration model for the correlation between surface pit and phase separation in InGaN.

Characterisation of InGaN by Photoconductive Atomic Force Microscopy.

Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used to characterise four thick (∼130 nm) InGaN films with x = 5%, 9%, 12%, and 15%. Lower photocurrent was observed on elevated ridges around defects (such as V-pits) in the films with %. Current-voltage curve analysis using the PC-AFM setup showed that this was due to a higher turn-on voltage on these ridges compared to surrounding material. To further understand this phenomenon, V-pit cross sections from the 9% and 15% films were characterised using transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. This identified a subsurface indium-deficient region surrounding the V-pit in the lower indium content film, which was not present in the 15% sample. Although this cannot directly explain the impact of ridges on turn-on voltage, it is likely to be related. Overall, the data presented here demonstrate the potential of PC-AFM in the field of III-nitride semiconductors.

Digital growth of thick N-polar InGaN films on relaxed InGaN pseudosubstrates

Smooth relaxed N-polar InGaN films were grown by metal–organic CVD (MOCVD) on N-polar InGaN pseudosubstrates (PSs) using a novel digital approach consisting of a constant In precursor flow with the pulsed injection of H2 carrier gas. InGaN layers grown on PSs exhibited an In composition of about 50% higher than those of the layers grown on N-polar GaN templates, assuming the in-plane lattice constant of the relaxed PSs, corresponding to In0.11Ga0.89N. Additionally, the luminescence recorded from InGaN layers grown on PSs at 490 nm was twice as intense as that obtained from the layers deposited on coloaded GaN-on-sapphire templates, which emitted at 430 nm.

In situ stress measurements during MOCVD growth of thick N-polar InGaN

The growth of N-polar InGaN films by metalorganic chemical vapor deposition (MOCVD) on N-polar GaN was investigated in order to understand the evolution of growth stress and the dislocation microstructure of relatively thick InGaN films (140 nm) in the absence of hexagonal hillocks and V-pits, which are typically present in III-polar InGaN. During in situ stress measurements of N-polar InGaN, growth initiates under an initial low stress state, which gradually transitions to a constant compressive incremental stress for the remainder of the film thickness. This behavior in the growth stress evolution in N-polar InGaN occurs regardless of the stress state of the GaN base layer (i.e., compression or tension), which was controlled by varying the temperature of the initial low temperature layer in a two-step temperature growth process for N-polar GaN. A blue shift in the PL peak emission of N-polar InGaN was, however, observed with increasing incremental compressive stress in the N-polar GaN base layer. These results provide insight into potential mechanisms of plastic relaxation in high crystal quality thick N-polar InGaN films grown by MOCVD.

Deep-level defects related to the emissive pits in thick InGaN films on GaN template and bulk substrates

We studied the emissive pits in InGaN films grown on compressive and strain-free GaN underlying layers. Pit density decreased with the full width at half maximum of ω(0002) of InGaN. The films grew on compressive and strain-free GaN underlying layers with spiral and step-flow modes, respectively. Carbon impurities accumulated inside the pits. Comparison of cathodoluminescence inside the pits and steady-state photocapacitance spectra showed that the energy level of the carbon impurities appeared at ∼2.8 eV below the conduction band (Ec) for both types of pits. Deep-level defects at Ec −2.4 eV resulting in green fluorescence emission were considered to originate from pits related to screw dislocations.

Molecular beam epitaxy of thick InGaN(0001) films: Effects of substrate temperature on structural and electronic properties

Indium gallium nitride films with compositions close to the middle of the miscibility gap and thickness approximately up to 0.5μm were epitaxially grown on GaN(0001) by plasma-assisted molecular beam epitaxy at growth temperatures spanning a range of 400–590°C. Epilayers were characterized by X-ray diffraction, transmission electron microscopy and Hall effect measurements. The effect of substrate temperature during growth, on the structural and electronic properties of the films, was studied. Single phase films, with record high electron mobilities were obtained at lower temperatures. Increased growth temperatures led to epilayers with higher defect densities and phase separation. Strain relaxation through sequestration layering and introduction of multiple basal stacking faults was observed at such temperatures.

InGaN-based thin film solar cells: Epitaxy, structural design, and photovoltaic properties

InxGa1−xN, with the tunable direct bandgaps from ultraviolet to near infrared region, offers a promising candidate for the high-efficiency next-generation thin-film photovoltaic applications. Although the adoption of thick InGaN film as the active region is desirable to obtain efficient light absorption and carrier collection compared to InGaN/GaN quantum wells structure, the understanding on the effect from structural design is still unclear due to the poor-quality InGaN films with thickness and difficulty of p-type doping. In this paper, we comprehensively investigate the effects from film epitaxy, doping, and device structural design on the performances of the InGaN-based solar cells. The high-quality InGaN thick film is obtained on AlN/sapphire template, and p-In0.08Ga0.92N is achieved with a high hole concentration of more than 1018 cm−3. The dependence of the photovoltaic performances on different structures, such as active regions and p-type regions is analyzed with respect to the carrier transport mechanism in the dark and under illumination. The strategy of improving the p-i interface by using a super-thin AlN interlayer is provided, which successfully enhances the performance of the solar cells.

Optical properties associated with strain relaxations in thick InGaN epitaxial films.

Structural and optical properties of thick InGaN layers with strain and composition inhomogeneities are investigated. High resolution x-ray diffractions (XRD) and reciprocal space mapping (RSM) along an asymmetric axis reveal that the In composition inhomogeneity is accompanied by strain relaxations during the growth of thick InGaN layers. According to the structural analysis, the commonly observed double photoluminescence (PL) peaks have been confirmed to be associated with the strain relaxation in thick InGaN films. Temperature-dependent PL measurements further indicate that the relaxed phase in InGaN films exhibits better emission efficiency than the strained phase. Recombination dynamics reveal that the carrier localization effect is more pronounced in the relaxed phase due to the compositional pulling effect. The correlations between emission efficiency and localization effect in thick InGaN films are discussed.

Electrical Characterization of Thick InGaN Films for Photovoltaic Applications

ABSTRACTWe have electrically characterized a 300 nm-thick unintentionally-doped In0.09Ga0.91N film grown by metal-organic chemical vapor deposition on a GaN template, employing capacitance-voltage (C-V), thermal admittance spectroscopy (TAS), and steady-state photocapacitance spectroscopy (SSPC) techniques on Schottky barrier diodes. TAS measurements revealed a degenerating-like shallow-donor defect with a thermal activation energy of ∼7 meV, which most likely acts as a source of residual carriers with their concentration of ∼1017 cm-3 determined from C-V measurements. Additionally, SSPC measurements revealed two characteristic deep-level defects located at ∼2.07 and ∼3.05 eV below the conduction band, which were densely enhanced near the underlayer. These electronic defects are probably introduced by alloying InN with GaN.

Thick InGaN Growth by Metal Organic Vapor Phase Epitaxy with Sputtered InGaN Buffer Layer

A thick InGaN film was grown on a sapphire substrate by metal organic vapor phase epitaxy using a thin buffer layer of sputter-deposited InGaN. A thick film of highly luminescent In0.2Ga0.8N can be successfully grown at a rate as high as 2 µm/h. The crystal quality of InGaN grown on the sputter-deposited InGaN buffer layer was better than that of InGaN grown directly on the sapphire substrate. The sample showed n-type conduction with a carrier concentration and mobility of 5×1018 cm-3 and 3 cm2/(V·s), respectively.

Growth of thick InGaN films with entire alloy composition using droplet elimination by radical-beam irradiation

Droplet elimination by radical-beam irradiation (DERI), which is suitable for the growth of InN and periodic InN/InGaN structures, was developed for the growth of thick InGaN films by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). In this method, Ga-to-nitrogen radical (N) beam flux ratio was fixed in the entire growth process, and In beam supply was modulated under the condition in which excess In of more than 2–3 monolayer wetting layers always existed on the surface, that is, no precise control of In beam flux during growth was needed. Thick and uniform InGaN films were successfully grown by the developed method. In addition, InGaN films with an entire alloy composition could be simply grown by controlling Ga-to-N beam flux ratio.

Study of Defect Levels in the Band Gap for a Thick InGaN Film

Deep-level transient spectroscopy, thermal admittance spectroscopy (TAS), and deep-level optical spectroscopy techniques have been used to investigate the defect levels in the band gap of a non-doped In0.09Ga0.91N film with a thickness of 300 nm. At temperatures ranging from 10 to 450 K, TAS analysis revealed defect levels at ∼7 meV under the conduction band (Ec) which were homogenously distributed in the InGaN film. The defect levels are considered to be attributed to the residual carrier density, probably originating from indium fluctuations and/or nitrogen vacancies (VN). Compared with the defect density of GaN, the defects of the InGaN film corresponding to the cation vacancies (VIII) and/or the VIII–O complex at 2.07 eV, and the shallow acceptor carbon (CN) defect at 3.05 eV, respectively, were remarkably enhanced. The possible increase of these defects is discussed in terms of VN and its complex with VIII that have been induced in the InGaN film.

Study of Defect Levels in the Band Gap for a Thick InGaN Film

Deep-level transient spectroscopy, thermal admittance spectroscopy (TAS), and deep-level optical spectroscopy techniques have been used to investigate the defect levels in the band gap of a non-doped In0.09Ga0.91N film with a thickness of 300 nm. At temperatures ranging from 10 to 450 K, TAS analysis revealed defect levels at ∼7 meV under the conduction band (Ec) which were homogenously distributed in the InGaN film. The defect levels are considered to be attributed to the residual carrier density, probably originating from indium fluctuations and/or nitrogen vacancies (VN). Compared with the defect density of GaN, the defects of the InGaN film corresponding to the cation vacancies (VIII) and/or the VIII–O complex at 2.07 eV, and the shallow acceptor carbon (CN) defect at 3.05 eV, respectively, were remarkably enhanced. The possible increase of these defects is discussed in terms of VN and its complex with VIII that have been induced in the InGaN film.

Molecular beam epitaxy growth of InGaN-GaN superlattices for optoelectronic devices

In the absence of native substrates for InGaN films, the achievement of thick InGaN films of high structural quality remains a challenge. The investigation of InGaN-GaN superlattice (SL) structures is one potential way to increase optical absorption at energies below the GaN bandgap while reducing the formation of detrimental defects. In this article the authors evaluate the structural and optical properties of InGaN-GaN superlattices grown by plasma assisted molecular beam epitaxy with indium compositions of up to 38% and periods from 8 to 20 nm. Of primary concern was the degree of film relaxation as determined by x-ray diffraction (XRD) reciprocal space mapping as a function of indium content and thickness of the InGaN layers. Indium well fractions of up to 0.15 were found to exhibit little or no relaxation for the structures tested by x-ray diffraction. For indium well fractions near ∼0.2, relaxations of the superlattices were in the range of 35% depending on total layer thickness. The samples with indium fractions of 0.33 and 0.38 had relaxations near 30%. For all of the superlattice layers, the onset of absorption began at significantly lower energy if one compares the average indium fraction in the SL to a uniform InGaN film of the same fraction. p-n photodiode structures based on superlattice layers were fabricated and tested for I-V characteristics and spectral response. The high indium content superlattice devices exhibited a substantial spectral response extension down to ∼2.3 eV. However, the I-V behavior was leaky at both forward and reverse biases, which is in a good agreement with the XRD analysis showing that material relaxation and defect generation occur in the SL layers with higher indium content.