InGaN Thin Films Research Articles

Influence of substrate and substrate temperature on the structural, optical and surface properties of InGaN thin films prepared by RFMS method

In this work, the pure InGaN thin films were grown using n-type and p-type silicon substrates at varying substrate temperatures using the sputtering method. The effects of substrate and substrate temperature on the structural, morphological and optical properties of the thin films grown were investigated. X-ray diffraction (XRD) analyzes of the obtained films illustrates crystal structures at 500̊ C substrate temperature, the films were found to be hexagonal. Scanning electron microscopy (SEM) was used to investigate the shape, size and surface distribution of the particles formed on film surfaces. The reflection and optical band gap (Eg) of the films were investigated from the optical analyzes taken with the UV-VIS spectrophotometer. As a result of these analyzes, it has been reached that the substrate and substrate temperature have a great influence on the structural, morphological and optical properties of the films. The experimental findings obtained in the study are compared with the studies given in the literature and the similarities and differences are discussed.

P-Si(100)/InGaN thin film structure and investigation of its physical properties: N2 gas flow effect

Abstract In this study, effect of N2 gas flow rates on structural, optical and morphological properties of InGaN thin films grown by Radio Frequency Magnetron Sputtering (RFMS) method on p-Si (100) substrate have been investigated. X-ray diffraction (XRD) measurements confirmed that InGaN thin films have been successfully deposited and exhibited diffraction peak belong to (002) plane for each film. The surface roughness of InGaN thin films has been determined by using Atomic Force Microscopy (AFM) and the roughness value is about 13 nm with a maximum height of 36 nm and a maximum depth of 37 nm. The surface roughness value Rq\RMS value is 15 nm, which is almost consistent with the linear roughness value for 0 sccm. The energy band gap of the film was determined by using the optical reflectance spectra and the value of the band gap energy is found to be 2.01, 1.77 eV and 2.26 eV for 0 sccm, 1 sccm and 2 sccm N2 gas flow rates, respectively

(S)TEM methods contributions to improve the fabrication of InGaN thin films on Si, and InN nanostructures on flat Si and rough InGaN

The main results of a complete study by Transmission and Scanning-Transmission Electron Microscopies ((S)TEM) are described for (i) InGaN/Si (111) heterostructures in the whole compositional range of the alloys, and (ii) InN quantum dots (InN QDs) directly grown on Si wafers or on relatively rough InGaN/Si (111) templates. The combination of many (S)TEM-based techniques allowed to evaluate different characteristics of the systems under study: (InN QD/) InGaN/Si and InN QD/Si interfaces and crystal qualities, structural and chemical imperfections and other important features. InxGa1-xN thin films are often identified as single-crystalline, very homogeneous in composition, and mostly wurtzite-type, remarkably at any value of x. Also, (S)TEM techniques revealed that the InN nanostructures were hexagonal single crystals, mostly epitaxial to the supporting lattice. The InN crystals also exhibited partially cubic arrangements when allocated onto In-rich InxGa1-xN (i.e. x > 0.7).

Investigation of GaN/InGaN thin film growth on ITO substrate by thermionic vacuum arc (TVA)

Group-III nitride semiconductors covering infrared, visible and ultraviolet spectral range have direct bandgaps changing from 0.7 eV (InN) to 3.4 eV (GaN). With this feature, optoelectronic devices such as light emitting diodes, laser diodes and ultraviolet (visible rays–UV) photodetectors are made. It is possible to grow high-quality InGaN epitaxial layers by modern crystal growth techniques such as molecular beam epitaxy, radio frequency sputtering method and metal organic chemical vapor deposition. Compared with these methods, the Thermionic Vacuum Arc, which is promising thin film growth technique, is relatively inexpensive and quite effective approach for preparing InGaN thin films. The purpose of this research is to investigate the physical properties of the film. The XRD patterns of the InGaN thin films deposited on the ITO substrate exhibited polycrystal structure. The larger crystallite size and smaller FWHM indicate better crystallization. Microstrain values also exhibit good crystallite films respect to the low dislocation density. This film has a potential for photovoltaic devices based on the absorbance graph. It was observed that the compound is homogeneously dispersed on the surface and that there is a nanoporous structure.

Tailoring optoelectronic properties of earth abundant ZnSnN2 by combinatorial RF magnetron sputtering

Zn-IV-N2 semiconductors are often considered as imminent materials alternative to InGaN and requisite thin film photovoltaic absorber materials for light harvesting due to its direct band gap, steep absorption onset and disorder-driven band gap tenability. In view of this, ZnSnN2 thin films were successfully prepared by reactive RF magnetron co-sputtering with different Sn concentrations at 450 °C substrate temperature. The films grown at 450 °C with different Sn target power were found to be orthorhombic crystal structure which has higher grain size (13–28 nm), low carrier density (−4.99 × 1018 to −7.01 × 1017 cm−3) and the maximum mobility (11.1 to 15.5 cm2 V−1 s−1). The phase purity and chemical bonding states of ZnSnN2 were investigated by XPS analysis. Vibrational modes of ZnSnN2 confirmed the presence of N-bonding with Zn and Sn. The optical band gap decreased from 1.96 to 1.77 eV as the Sn power increased. Hence, it is suggested that ZnSnN2 thin film grown under this technique could be utilized as efficient absorber layer in thin film solar cells due to the astonishing optical and electrical properties.

Optical properties of InGaN thin films in the entire composition range

The optical properties of thick InGaN epilayers, with compositions spanning the entire ternary range, are studied in detail. High structural quality, single phase InxGa1-xN (0001) films were grown heteroepitaxially by radio-frequency plasma assisted molecular-beam epitaxy on freestanding GaN substrates. Their emission characteristics were investigated by low temperature photoluminescence spectroscopy, whereas variable angle spectroscopic ellipsometry was applied to determine the complex dielectric function of the films, in the 0.55–4.0 eV photon range. Photoluminescence lines were intense and narrow, in the range of 100 meV for Ga-rich InGaN films (x < 0.3), around 150 meV for mid-range composition films (0.3 < x < 0.6), and in the range of 50 meV for In-rich alloys (x > 0.6). The composition dependence of the strain-free emission energy was expressed by a bowing parameter of b = 2.70 ± 0.12 eV. The films' optical dielectric function dispersion was obtained by the analysis of the ellipsometric data employing a Kramers-Kronig consistent parameterized optical model. The refractive index dispersion was obtained for alloys in the entire composition range, and the corresponding values at the band edge show a parabolic dependence on the InN mole fraction expressed by a bowing parameter of b = 0.81 ± 0.04. The bowing parameter describing the fundamental energy bandgap was deduced to be equal to 1.66 ± 0.07 eV, consistent with the ab initio calculations for statistically random (non-clustered) InGaN alloys.

Impact of embedded voids on thin-films with high thermal expansion coefficients mismatch

Using technology to reduce defects at heterogeneous interfaces of thin-films is at a high-priority for modern semiconductors. The current work utilizes a three-dimensional multiple-slip crystal-plasticity model and specialized finite-element formulations to study the impact of the embedded void approach (EVA) to reduce defects in thin-films deposited on a substrate with a highly mismatched thermal expansion coefficient, in particular, the growth of an InGaN thin-film on a Si substrate, where EVA has shown a remarkable reduction in stresses on the side of the embedded voids.

Effect of indium droplets on growth of InGaN film by molecular beam epitaxy

Effect of indium (In) droplets on InGaN thin films grown by molecular beam epitaxy (MBE) has been investigated. The surface of InGaN covered by In droplets shows a smoother topography than that without droplets, indicating that the presence of In droplets is beneficial to the two dimensional growth. Beneath the In droplets, many ring-like structures are observed. The arrangement of these “ring” shows the movement of the In droplets during the InGaN growth. A qualitative growth model is proposed to explain the evolution of the InGaN surface morphology in In-droplet-induced-epitaxy process, giving an explanation that a local vapor-liquid-solid (VLS) system is preferentially formed at the edge of the droplets, leading to a high growth rate. Furthermore, the energy dispersive X-ray spectroscopy results reveal that the relatively higher In/Ga flux ratio in the region covered by the In droplet results in a locally higher In content.

Effect of growth temperature on InGaN/GaN heterostructures grown by MOCVD

InGaN/GaN heterostructured samples were grown at different temperatures varying from 680°C to 760°C on c-plane sapphire substrates using a horizontal flow Metal Organic Chemical Vapor Deposition (MOCVD) reactor. Systematic investigation of structural, optical, morphological and electrical properties has been carried out using High Resolution X-Ray Diffraction (HRXRD), Photoluminescence (PL), Atomic Force Microscope (AFM) and Hall Measurement respectively. From the HRXRD results, the respective Indium (In) composition in the InGaN/GaN heterostructures can be calculated using InGaN (0002) peaks. In incorporation is very much sensitive to the temperature and incorporation of In in InGaN epilayer was found to decrease with increase in growth temperature. PL spectra reveal a shift towards higher energy side because of In incorporation with increasing reactor temperature. AFM images of the sample grown at 680°C shows the In rich clusters due to compositional inhomogeneity. With further increase in the growth temperature, decomposition of InN starts and hence decreases the surface roughness. Hall Effect studies confirm that with increasing growth temperature bulk electron concentration of the InGaN thin films decrease and mobility increases. Maximum bulk electron concentration of ~4.5×1019cm−3 and a mobility of ~295cm2/V-s was obtained for 19% of In. Efforts are being made to optimize the incorporation of In content in InGaN/GaN heterostructures for photovoltaic application.

Investigation of phase separation in InGaN alloys by plasmon loss spectroscopy in a TEM

ABSTRACTPhase separation of InxGa1-xN alloys into Ga-rich and In-rich regions was observed by a number of research groups for samples grown with high indium content, x. Due to the radiation sensitivity of InGaN to beam damage by fast electrons, high-resolution imaging in transmission electron microscopy (TEM) or core-loss electron energy-loss spectroscopy (EELS) may lead to erroneous results. Low-loss EELS can yield spectra of the plasmon loss regions at much lower electron fluxes. Unfortunately, due to their delayed edge onset, the low energetic core losses of Ga and In partially overlap with the plasmon peaks, all of which shift with indium content.Here we demonstrate a method to quantify phase separation in InGaN thin films from the low-loss region in EELS by simultaneously fitting both plasmon and core losses over the energy range of 13-30eV. Phase separation is shown to lead to a broadening of the plasmon peak and the overlapping core losses, resulting in an unreliable determination of the indium concentration from analyzing the plasmon peak position alone if phase separation is present. For x=0.3 and x=0.59, the relative contributions of the binary compounds are negligibly small and indicate random alloys. For xnom.=0.62 we observed strong broadening, indicating phase separation.

Effect of rapid thermal annealing on polycrystalline InGaN thin films deposited on fused silica substrates

In this work, we report on the effects of Rapid Thermal Annealing (RTA) on the structural, electrical and optical properties of polycrystalline InGaN thin films deposited on amorphous fused silica substrates by molecular beam deposition. Films with 20%, 35% and 50% indium content were grown and subjected to post-deposition RTA treatments. Annealing promoted crystallization in the case of the film with 0.5 InN mole fraction while in the lower indium content cases no apparent effect on the improvement of crystallinity was observed. For RTA temperature above 550°C, film resistivity was reduced by at least two orders of magnitude due to annealing-induced increased carrier concentration. The optical properties of the films were systematically studied by variable angle spectroscopic ellipsometry. In the highest indium content films, a monotonic optical band gap widening was observed upon annealing, explained by the Burstein–Moss effect. In contrast, photoluminescence peak position was not affected by the resulting Fermi level changes. This is attributed to the different mechanisms between optical absorption and emission in such highly doped semiconductors.

InGaN thin film deposition on Si(100) and glass substrates by termionic vacuum arc

Group-III nitride semiconductors covering infrared, visible and ultraviolet spectral range has direct band gaps changing from 0,7 eV (InN) to 3,4 eV (GaN). LEDs emit red, blue, green light, ultraviolet (UV) laser diodes (LD), UV light detectors and high power electronic devices are obtained and commercialized based on group-III nitride materials. InGaN semiconductor can be deposited by different techniques such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD). In this study, InGaN thin films were prepared on Si and glass substrates as well as on GaN layer by termionic vacuum arc (TVA) which is a plasma asisted thin film deposition technique. The film was deposited at 10-6 torr working pressure, 18A filament current. Plasma was produced at 200 V with 0,6A plasma current. The purpose of this research is to investigate the properties of InGaN thin films. X-ray diffraction (XRD) spectrophotometer was used to analyze microstructure of the deposited films. Scanning electon microscopy (SEM) were used for surface morphology characterizations. Compositional analysis was done by energy dispersive X-ray spectroscopy (EDAX).

Influence of pH on the Quantum-Size-Controlled Photoelectrochemical Etching of Epitaxial InGaN Quantum Dots

Illumination by a narrow-band laser has been shown to enable photoelectrochemical (PEC) etching of InGaN thin films into quantum dots with sizes controlled by the laser wavelength. Here, we investigate and elucidate the influence of solution pH on such quantum-size-controlled PEC etch process. We find that although a pH above 5 is often used for PEC etching of GaN-based materials, oxides (In2O3 and/or Ga2O3) form which interfere with quantum dot formation. At pH below 3, however, oxide-free QDs with self-terminated sizes can be successfully realized.

Combination of electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy to determine indium concentration in InGaN thin film structures

We demonstrate a method to determine the indium concentration, x, of InxGa1-xN thin films by combining plasmon excitation studies in electron energy-loss spectroscopy (EELS) with a novel way of quantification of the intensity of x-ray lines in energy-dispersive x-ray spectroscopy (EDXS). The plasmon peak in EELS of InGaN is relatively broad. We fitted a Lorentz function to the main plasmon peak to suppress noise and the influence from the neighboring Ga 3d transition in the spectrum, which improves the precision in the evaluation of the plasmon peak position. As the indium concentration of InGaN is difficult to control during high temperature growth due to partial In desorption, the nominal indium concentrations provided by the growers were not considered reliable. The indium concentration obtained from EDXS quantification using Oxford Instrument ISIS 300 x-ray standard quantification software often did not agree with the nominal indium concentration, and quantification using K and L lines was inconsistent. We therefore developed a self-consistent iterative procedure to determine the In content from thickness-dependent k-factors, as described in recent work submitted to Journal of Microscopy. When the plasmon peak position is plotted versus the indium concentration from EDXS we obtain a linear relationship over the whole compositional range, and the standard error from linear least-squares fitting shows that the indium concentration can be determined from the plasmon peak position to within Δx = ± 0.037 standard deviation.

The influence of voltage applied between the electrodes on optical and morphological properties of the InGaN thin films grown by thermionic vacuum arc.

The aim of this research is to investigate the optical and morphological properties of the InGaN thin films deposited onto amorphous glass substrates in two separate experiments with two different voltages applied between the electrodes, i.e. 500 and 600 V by means of the thermionic vacuum arc technique. This technique is original for thin film deposition and it enables thin film production in a very short period of time. The optical and morphological properties of the films were investigated by using field emission scanning electron microscope, atomic force microscope, spectroscopic ellipsometer, reflectometer, spectrophotometer, and optical tensiometer. Optical properties were also supported by empirical relations. The deposition rates were calculated as 3 and 3.3 nm/sec for 500 and 600 V, respectively. The increase in the voltage also increased the refractive index, grain size, root mean square roughness and surface free energy. According to the results of the wetting experiments, InGaN samples were low-wettable, also known as hydrophobic.

InGaN thin films grown by RF sputtering for HIT solar cells

ABSTRACTThe wide range of optical band gaps of InGaN thin films led to considerable research in increasing the efficiency of solar cells. In this study, we present electrical, optical and structural properties of InGaN thin films grown by in situ rf sputtering method. Several samples of InGaN were deposited on aluminosilicate glass and silicon(111) substrates at different temperatures and varying N2 flow ratio. Growth temperatures are 35 °C, 150 °C, 200 °C. Aluminum metal contacts are deposited using DC sputtering method. Resistivity, mobility, conductivity values and their changes with respect to temperature are recorded using hall-effect measurement system. Band gap values and their changes with respect to N2 flow ratio in ( Ar+N2) mixture were calculated using UV spectrophotometer and tauc plots. Atomic composition was calculated using Energy-dispersive X-ray spectroscopy (EDX) analysis. Amorphous/crystalline nature of the samples are verified using XRD analysis.

Quantum-size-controlled photoelectrochemical fabrication of epitaxial InGaN quantum dots.

We demonstrate a new route to the precision fabrication of epitaxial semiconductor nanostructures in the sub-10 nm size regime: quantum-size-controlled photoelectrochemical (QSC-PEC) etching. We show that quantum dots (QDs) can be QSC-PEC-etched from epitaxial InGaN thin films using narrowband laser photoexcitation, and that the QD sizes (and hence bandgaps and photoluminescence wavelengths) are determined by the photoexcitation wavelength. Low-temperature photoluminescence from ensembles of such QDs have peak wavelengths that can be tunably blue shifted by 35 nm (from 440 to 405 nm) and have line widths that narrow by 3 times (from 19 to 6 nm).

Indium droplet formation in InGaN thin films with single and double heterojunctions prepared by MOCVD.

Indium gallium nitride (InGaN) samples with single heterojunction (SH) and double heterojunction (DH) were prepared using metal-organic chemical vapor deposition. SH has a layer of InGaN thin film (thicknesses, 25, 50, 100, and 200 nm) grown on an uGaN film (thickness, 2 μm). The DH samples are distinguished by DH uGaN film (thickness, 120 nm) grown on the InGaN layer. Reciprocal space mapping measurements reveal that the DH samples are fully strained with different thicknesses, whereas the strain in the SH samples are significantly relaxed with the increasing thickness of the InGaN film. Scanning electron microscopy results show that the surface roughness of the sample increases when the sample is relaxed. High-resolution transmission electron microscopy images of the structure of indium droplets in the DH sample indicate that the thickness of the InGaN layer decreases with the density of indium droplets. The formation of these droplets is attributed to the insufficient kinetic energy of indium atom to react with the elements of group V, resulting to aggregation. The gallium atoms in the GaN thin film will not be uniformly replaced by indium atoms; the InGaN thin film has an uneven distribution of indium atoms and the quality of the epitaxial layer is degraded.

Band Engineered Epitaxial 3D GaN-InGaN Core–Shell Rod Arrays as an Advanced Photoanode for Visible-Light-Driven Water Splitting

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Photoluminescence Spectroscopy as a Tool for Quality Control of GaN Thin Film to Be Used in Solar Cell Devices

The use of III-V and semiconductor nitrides in solar cells has been of interest in the PV-community due to the wide variation range of the band gap in these materials. Particularly, the processing of hetero-junction structures of InGaN/GaN and Si(p)/GaN(n) has been of great interest recently. In this work, the quality of GaN and InGaN thin films grown by Molecular Beam Epitaxy (MBE) on different substrate and buffer layers has been studied by photoluminescence spectroscopy (PL). The PL measurements were processed as function of sample temperature and pump power. In the PL spectra it is possible to observe a strong near band-gap-edge emission and a broad blue, green and yellow luminescence (BL, GL, YL), which can be assigned to the presence of Ga and N vacancies, amorphous phases, deep level impurities and structural defects. The relative intensity between the different peaks of the bands related to defects or impurities was studied as a tool for quality control of the films.