Gallium Nitride Thin Films Research Articles

Structural and mechanical modifications of GaN thin films by swift heavy ion irradiation

The structural modifications and the evolution of mechanical behavior of gallium nitride (GaN) thin films irradiated by 92 MeV 129Xe23+ at different fluences have been investigated. The modifications induced by irradiation in GaN have been studied using a combination of high resolution X-ray diffraction and Transmission Electron Microscopy observations coupled to nanoindentation. The crystalline lattice of the GaN is modified by irradiation, with an extension of the lattice along the c-direction parallel to the ion path, leading to the development of residual stresses. Correlated to the crystallographic disorder, modification of the deformation mechanisms of the material is observed: damaged areas (highly disordered zones near the surface and black dots deeper in the bulk) hinder the dislocation motion, such as after irradiation, dislocation slip occurs only along the basal plane, and no more prismatic or pyramidal slip is observed. This results in increasing the dislocation loop density, with a subsequent increase in hardness of the GaN film. At higher fluence, the overlapping of the latent tracks created by swift heavy ions results in a significant decrease in the mechanical characteristics of the thin film, and an amorphous-like material behavior.

The Effect of Annealing Process on Some Physical Properties of GaN Thin Films with Gr Doping

This study aims to ensure p-type gallium nitride (GaN) thin-film production through graphene (Gr) and to observe the effects of the annealing process on physical properties. A Gr doped GaN thin film on a glass substrate was coated using the thermionic vacuum arc method and an annealing process was applied to this sample at separate temperatures of 300 °C and 400 °C. To investigate the produced samples, elemental composition, electronic states, nanostructural, morphological, and electrical characteristics were determined by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy, field emission scanning electron microscopy, atomic force microscopy, and Hall effect technique. The thickness of the samples was determined as 60 nm by a Filmetrics F20 thin film analyzer. According to grazing incident-angle XRD patterns of samples, GaN peaks were observed to become dominant after the 400 °C annealing process. The results show that the used production method is suitable for p-type GaN thin film.

Effect of Solution Concentration on Optical and Morphological Properties of Gallium Nitride Thin Films

Effect of Solution Concentration on Optical and Morphological Properties of Gallium Nitride Thin Films

Experimental Study of the Effect of Precursor Composition on the Microstructure of Gallium Nitride Thin Films Grown by the MOCVD Process

Abstract Chemical vapor deposition (CVD) is a widely used manufacturing process for obtaining thin films of materials like silicon, silicon carbide, graphene, and gallium nitride that are employed in the fabrication of electronic and optical devices. Gallium nitride (GaN) thin films are attractive materials for manufacturing optoelectronic device applications due to their wide band gap and superb optoelectronic performance. The reliability and durability of the devices depend on the quality of the thin films. The metal-organic chemical vapor deposition (MOCVD) process, which uses compounds that contain metals and organic ligands as precursors in a CVD reactor, is a common technique used to fabricate high-quality GaN thin films. The deposition rate and uniformity of thin films are critical to a successful and useful process. These are determined by the thermal transport processes and chemical reactions occurring in the reactor, and are manipulated by controlling the operating conditions and the reactor geometrical configuration. In this study, the epitaxial growth of GaN thin films on sapphire (Al2O3) substrates is carried out in two commercial MOCVD systems: a vertical rotating disk MOCVD reactor and a close-coupled showerhead MOCVD reactor. The surface morphology and crystal quality of GaN thin films have been examined using atomic force microscopy (AFM) and scanning electron microscope (SEM). This paper focuses on the composition of the precursor and the carrier gases since earlier studies have shown the importance of precursor composition. The results show that the flow rate of trimethylgallium (TMG), which is the main ingredient in the process, has a significant effect on the deposition rate and uniformity of the films. Also, the carrier gas plays an important role in deposition rate and uniformity. Using hydrogen as a carrier gas enhances the quality of the thin film but a lower deposition rate occurs on the wafer surface. On the other hand, a high flow rate of pure nitrogen gas improves the growth rate of the film. However, it decreases the uniformity of the film and promotes carbon contamination on the wafer surface. Thus, the use of an appropriate mixture of hydrogen and nitrogen as the carrier gas can improve the deposition rate and quality of GaN thin films.

The insertion of the ALD diffusion barriers: An approach to improve the quality of the GaN deposited on Kapton by PEALD

The influence of growth temperature (200–300 ℃) on the properties of gallium nitride (GaN) directly deposited on Kapton by plasma-enhanced atomic layer deposition (PEALD) has been systematically investigated. With increasing growth temperatures, the O content and surface roughness of the GaN thin films increase. Besides, the diffusion behavior of GaN in Kapton was found when the growth temperature exceeds 250 ℃. In this study, ultrathin ALD aluminum oxide (Al2O3) and aluminum nitride (AlN) films were proposed to see if they could alleviate and/or even suppress the negative effects of interface diffusion during high-temperature (≥250 ℃) GaN growth. It was found that oxygen impurities were significantly decreased within the as-deposited GaN. Furthermore, we observed a complete suppression of GaN diffusion into the Kapton throughout the entire growth process. Additionally, an introduction of either the Al2O3 or AlN before the GaN deposition could improve the surface morphology of the GaN deposited on Kapton.

The role of AlN thickness in MOCVD growth of N-polar GaN

The influence of the thickness of aluminum nitride (AlN) on the surface morphology, crystalline quality, and polarity of N-polar gallium nitride (GaN) grown by metal organic chemical vapor deposition (MOCVD) is investigated in this study. With a medium temperature (1000 ℃) AlN buffer layer of 150 nm, a hexagonal-defect-free N-polar GaN thin film with low dislocation densities (4.2 × 108 cm-2 for screw dislocation density and 3.7 × 109 cm-2 for edge dislocation density) is obtained on a 2-in. vicinal sapphire substrate (c off m plane 2°). With the reduction of the AlN thickness to 80 nm, the N-polar GaN surface becomes rough and the crystalline quality is exacerbated. While the thickness of AlN is increased to 200 nm, the polarity of GaN is changed to Ga-polar. The transmission electron microscopy (TEM) results reveal an undulated interface between GaN and AlN with a large density of pyramidal defects which may contribute to the polarity inversion of GaN. The controllable polarity inversion through changing the thickness of AlN provides a promising method for the polarity engineering of nitride-based devices.

Influence of Precursor Concentration on Crystalline Quality of GaN Thin Films Grown on a Sapphire Wafer

Abstract Chemical vapor deposition (CVD), which involves chemical reactions in gases for deposition on a heated surface, is an extensively used manufacturing technique for obtaining thin films of materials like silicon, graphene, silicon carbide, aluminum nitride, and gallium nitride (GaN). The process is driven by heat and mass transfer, fluid flow, and chemical reactions in the gases and at the surface. GaN is one of the most promising materials for manufacturing optical and electronic devices. However, the reliability and durability of the GaN-based devices depend on the crystalline quality of the thin films used. In this study, the epitaxial growth of GaN thin films on sapphire (Al2O3) wafers is carried out in a vertical rotating disk metalorganic chemical vapor deposition (MOCVD) system. Epitaxial growth refers to the process of growing a crystal of a particular orientation on the top of another crystal, with the orientation being determined by the underlying crystal. MOCVD reactors are CVD systems that use metalorganic compounds that consist of metal and organic ligands, leading to materials like GaAs, AlN, InN, and GaN. The quality of the thin films is largely determined by the choice of operating conditions such as the flowrate, surface temperature, and concentration of the metalorganic precursors that decompose due to heat in the reactor, react, and deposit the desired material on the surface of a wafer or a heated susceptor. In this experimental study, the crystalline quality and surface morphology of GaN thin films are evaluated using atomic force microscopy (AFM), X-ray diffraction (XRD), and Raman spectroscopy. The correlation between the crystalline quality of GaN thin films and the flowrate of the precursors is examined in detail on the basis of an evaluation of the dislocation density. The results indicate that a low concentration (V/III) ratio, where V and III refer to elements in the fifth and third groups of the periodic table, is beneficial for obtaining a high deposition rate since a low value of this ratio implies a high precursor concentration. However, it negatively affects the crystalline quality of the thin film. Similarly, high V/III ratios lead to low deposition rates and better crystalline quality, indicating the need to optimize the process.

Epitaxy N-polar GaN on vicinal Sapphire substrate by MOCVD

A 1-μm thick N-polar gallium nitride (GaN) thin film has been grown on 2-inch vicinal sapphire substrate (c off 2° toward m plane) by metal organic chemical vapor deposition (MOCVD). The smooth surface without any inversion domains is demonstrated by the optical microscopy, atomic force microscopy, and scanning electron microscopy. The N-polar property is confirmed by potassium hydroxide etching and the atomic layout through high-resolution transmission electron microscopy (HRTEM). The good crystalline quality is verified by X-ray rocking curves and photoluminescence.

Real-time in situ process monitoring and characterization of GaN films grown on Si (100) by low-temperature hollow-cathode plasma-atomic layer deposition using trimethylgallium and N2/H2 plasma

In this work, we report on the in situ process monitoring and materials characterization of low-temperature self-limiting grown gallium nitride (GaN) thin films. GaN samples were synthesized on Si (100) substrates via remote hollow-cathode plasma-atomic layer deposition (HCP-ALD) using trimethylgallium and N2/H2 plasma as a metal precursor and a nitrogen coreactant, respectively. A multiwavelength in situ ellipsometer was employed to monitor the saturating surface reactions and determine the self-limiting growth conditions. The subangstrom thickness resolution of ellipsometry enabled the real-time observation of single chemical adsorption and plasma-induced ligand removal/exchange events. Taking advantage of this in situ capability, saturation experiments have been carried out within the 120–240 °C temperature range without interruption featuring 10-cycle subruns for each parameter change. Plasma power, plasma exposure duration, and plasma chemistry (gas composition) are the main process parameters that have been investigated. Ex situ optical, structural, and chemical characterization is carried out on 600-cycle HCP-ALD-grown GaN films as a function of substrate temperature. Hexagonal single-phase polycrystalline GaN films with (002) preferred orientation was obtained at substrate temperatures higher than 200 °C. The crystalline GaN films exhibited below-detection-limit carbon content and slightly gallium rich stoichiometry. Substrate temperature and plasma power played a critical role on GaN film properties with 200 °C and 150 W as threshold values for crystallization. Moreover, we observed that Ar-free N2/H2 plasma gas composition led to a slightly stronger (002) dominant crystal orientation.

Disilane doping of semi-polar (11-22) n-GaN: The impact of terrace-like evolution toward the enhancement of the electrical properties

Semi-polar (11-22) n-type gallium nitride thin films with various disilane doping levels were deposited via metal-organic chemical vapor deposition. The impact of the dissimilar disilane doping levels on the crystal, morphological, strain, and electrical properties was extensively studied. An X-ray rocking curve analysis demonstrated an improved crystal quality with enhanced terrace-like features using 15 standard cubic centimeters per minute of disilane doping. A closely packed step terrace-like feature was exhibited, reducing the density of arrowhead-like features. This facilitated a smoother surface with a root mean square surface roughness of 5.7 nm. Raman spectroscopy revealed that using an intermediate disilane flux reduced the compressive strain. An electrical analysis also showed that a moderate disilane doping level improved the carrier concentration and mobility to 1.3 × 1018 cm−3 and 99.3 cm2. V−1. s−1, respectively.

Direct Synthesis of Oxynitride Nanowires through Atmospheric Pressure Chemical Vapor Deposition.

Binary and ternary oxynitride solid alloys were studied extensively in the past decade due to their wide spectrum of applications, as well as their peculiar characteristics when compared to their bulk counterparts. Direct bottom-up synthesis of one-dimensional oxynitrides through solution-based routes cannot be realized because nitridation strategies are limited to high-temperature solid-state ammonolysis. Further, the facile fabrication of oxynitride thin films through vapor phase strategies has remained extremely challenging due to the low vapor pressure of gaseous building blocks at atmospheric pressure. Here, we present a direct and scalable catalytic vapor–liquid–solid epitaxy (VLSE) route for the fabrication of oxynitride solid solution nanowires from their oxide precursors through enhancing the local mass transfer flux of vapor deposition. For the model oxynitride material, we investigated the fabrication of gallium nitride and zinc oxide oxynitride solid solution (GaN:ZnO) thin film. GaN:ZnO nanowires were synthesized directly at atmospheric pressure, unlike the methods reported in the literature, which involved multiple-step processing and/or vacuum operating conditions. Moreover, the dimensions (i.e., diameters and length) of the synthesized nanowires were tailored within a wide range.

Raman Analysis of E2 (High) and A1 (LO) Phonon to the Stress-Free GaN Grown on Sputtered AlN/Graphene Buffer Layer

The realization of high-speed and high-power gallium nitride (GaN)-based devices using high-quality GaN/Aluminum nitride (AlN) materials has become a hot topic. Raman spectroscopy has proven to be very useful in analyzing the characteristics of wide band gap materials, which reveals the information interaction of sample and phonon dynamics. Four GaN samples grown on different types of buffer layers were fabricated and the influence of graphene and sputtered AlN on GaN epitaxial layers were analyzed through the E2 (high) and A1 (LO) phonon. The relationship between the frequency shift of E2 (high) phonons and the biaxial stress indicated that the GaN grown on the graphene/sputtered AlN buffer layer was stress-free. Furthermore, the phonon lifetimes of A1 (LO) mode in GaN grown on graphene/sputtered AlN buffer layer suggested that carrier migration of GaN received minimal interference. Finally, the Raman spectra of graphene with the sputtered AlN interlayer has more disorder and the monolayer graphene was also more conducive to nucleation of GaN films. These results will have significant impact on the heteroepitaxy of high-quality thin GaN films embedded with a graphene/sputtered AlN buffer, and will facilitate the preparation of high-speed GaN-based optoelectronic devices.

High-Temperature Atomic Layer Deposition of GaN on 1D Nanostructures.

Silica nanosprings (NS) were coated with gallium nitride (GaN) by high-temperature atomic layer deposition. The deposition temperature was 800 °C using trimethylgallium (TMG) as the Ga source and ammonia (NH3) as the reactive nitrogen source. The growth of GaN on silica nanosprings was compared with deposition of GaN thin films to elucidate the growth properties. The effects of buffer layers of aluminum nitride (AlN) and aluminum oxide (Al2O3) on the stoichiometry, chemical bonding, and morphology of GaN thin films were determined with X-ray photoelectron spectroscopy (XPS), high-resolution x-ray diffraction (HRXRD), and atomic force microscopy (AFM). Scanning and transmission electron microscopy of coated silica nanosprings were compared with corresponding data for the GaN thin films. As grown, GaN on NS is conformal and amorphous. Upon introducing buffer layers of Al2O3 or AlN or combinations thereof, GaN is nanocrystalline with an average crystallite size of 11.5 ± 0.5 nm. The electrical properties of the GaN coated NS depends on whether or not a buffer layer is present and the choice of the buffer layer. In addition, the IV curves of GaN coated NS and the thin films (TF) with corresponding buffer layers, or lack thereof, show similar characteristic features, which supports the conclusion that atomic layer deposition (ALD) of GaN thin films with and without buffer layers translates to 1D nanostructures.

(Invited) Synthesis of Photoactive Hexagonal-Shape GaN Nanoplate Using Solid Nitrogen Source in Molten Salt

Gallium nitride (GaN), is a group III nitride semiconductor. Thin films of GaN have beenwidely adopted for both commercial and fundamental research, such as in light-emitting diodes (LEDs) and UV light water-splitting. Much interest has also been paid to the creation of nanostructures of GaN including the morphology of wires, rods, sheets, seeds and hollows. The nitridation reactions are often conducted using gas flow techniques for growing GaN crystals. In this process, a nitrogen-rich gas, such as ammonia, is usually used as a source of nitrogen. We have reported a new technique to synthesize low cost GaN powder under moderate condition. Li3N or LiNH2 were used as a high reactive solid nitrogen source to improve the reaction rate and lowering the reaction temperature. Crystalline wurtzite GaN powder with several hundred nanometer size was successfully synthesized. But the reaction yield was not so high because its solid phase reaction. Molten salts are often used as reaction media to increase the reaction rate and the crystallinity of ceramics. In this study, the synthesis of GaN nanoplate has been established by using LiCl as the molten salt at relatively low temperatures. Also, we anticipated that the formation of hexagonal-shape GaN nanoplates instead random-shaped crystals could enhance the charge transport in the photoelectrochemical applications. Here, we present the effect of the molten salt on the crystal growth of the GaN microstructure and the photoelectrochemical properties of the GaN electrodes.GaCl3 was reacted with LiNH2 at temperature of 400 - 650oC, LiCl was used as molten salts. LiCl has a melting point of 614°C. The graphite crucible with the starting materials (GaCl3, LiNH2) and molten salts (LiCl) was put into a sealed stainless reaction vessel. The reaction temperature was at 400 - 650°C. The products were washed with water and HCl solution after cooling.Figure 1 shows SEM photographs of GaN samples synthesized at (a) 550oC, (b) 600oC, (c) 650oC, (d) 700oC and (e) 800oC. The hexagonal-plate morphologies were observed at 600°C, 650°C, and 700°C, GaN formed with the particle size smaller than 500 nm. These GaN nanoplates are termed as 2D HS-GaN. Notably, by increasing temperature up to 700°C, particle sizes increased to be more than 300 nm and the crystal shape changed from granular to hexagonal. However, at 800°C, the crystal size decrease and the particles adopted a random plate-like shape. It is consider that thermal decomposition occurred at 800°C. The thickness of the 2D HS-GaN nanoplates varied from 50 to 80 nm by increasing the temperature from 600°C to 700°C. It is most likely that the LiCl assisted synthesis facilitated a more homogeneous reaction than that of the solid phase synthesis at 550°C. In the molten salt, the mobility of the reactants GaCl3 and LiNH2 is higher than that in the solid phase. The increased mobility causes the reactants to be uniformly diffused within a short period. Thus, at temperatures, more than 600°C, GaCl3 and LiNH2 in the molten salt allowed the reaction to proceed at an atomic level. The photoelectrochemical characterization revealed that the hexagonal structure gave a higher anodic photocurrent by a factor of up to 2 compared to the random-shaped GaN obtained at a higher temperature. To our belief, the photo-electrochemical properties of 2D HS-GaN electrode indicate that it has significant potential for photo-catalytic water splitting. Figure 1

Electrical Characterization of Gallium Nitride Thin Films Synthesized By Electrochemical Deposition

Group III-nitride semiconductor materials show promise in a variety of electronics applications, including diodes, transistors, LEDs and photovoltaic applications. The new generation of semiconductors rely heavily on nanotechnology and advances in nanomaterials. Group III-nitride semiconductor materials such as GaN, AlN and InN are promising materials since their properties are ideal for use in optoelectronic devices, high power and high temperature electronic devices. These materials have wide band gaps between 1.9 and 6.3 eV and, for electronic applications, these materials are usually synthesised by means of chemical vapour deposition or molecular beam epitaxy. These techniques are expensive, therefore we investigated room temperature electrochemical growth to synthesize GaN. The advantages of electrochemical deposition are the controllability of the thickness and surface morphology by varying the deposition parameters, cost-effectiveness, the minimum waste generation, long bath lifetime and easy set-up. In this study, room temperature electrochemical deposition has been used to synthesise GaN nitride on Si substrate. Results: The XRD spectrum of GaN thin films grown under the different conditions are shown in Figure 1. Sample a (1 mA/cm2 for 30 min) and b(3 mA/cm2 for 30 min) were prepared at room temperature. The XRD analysis showed the presence of hexagonal and cubic structure with crystallite sizes calculated from Scherer’s formula were 130 and 140 nm From SEM images, however, the results varied with growth condition. Good quality Schottky diodes were fabricated on the GaN thin films and DLTS measurements were performed. Electron traps with an activation energy of 0.48 and 0.47 eV, were observed in sample a and b, respectively. Figure 1

Multiple epitaxial lateral overgrowth of GaN thin films using a patterned graphene mask by metal organic chemical vapor deposition

Single-crystal gallium nitride (GaN) thin films were grown using a graphene mask via multiple epitaxial lateral overgrowth (multiple-ELOG). During the growth process, the graphene mask self-decomposed to enable the emergence of a GaN film with a thickness of several hundred nanometres. This is in contrast to selective area growth of GaN using an SiO2 mask leading to the well known hexagonal-pyramid shape under the same growth conditions. The multiple-ELOG GaN had a single-crystalline wurtzite structure corresponding to the crystallinity of the GaN template, which was confirmed with electron backscatter diffraction measurements. An X-ray diffraction rocking curve of the asymmetric 102 reflection showed that the FWHM for the multiple-ELOG GaN decreased to 405 from 540′′ for the underlying GaN template. From these results, the self-decomposition of the graphene mask during ELOG was experimentally proven to be affected by the GaN decomposition rather than the high-temperature/H2 growth conditions.

Effect of the lattice mismatch on threading dislocations in heteroepitaxial GaN layers revealed by X-ray diffraction

In this study, structural investigations of the mismatched gallium nitride (GaN) layers grown on SiC, Al2O3 and Si substrate, were performed employing x-ray diffraction techniques. The resulted threading dislocation density evolved from the lattice mismatch, as a consequence of the strain relaxation processes, was quantified in two independent ways: firstly, using the mosaic block model, which takes into account peak width and, secondly, using the diffuse scattering model, which consider only the tails of the rocking curves. Both models indicated the close relationship between the amount of dislocations and the lattice mismatch that plays a fundamental role in TDs’ generation. To gain further insights regarding threading dislocations, grazing incidence x-ray diffraction was employed to obtain the absorption profiles of the x-rays. Although these profiles encompass the contributions of both sample absorption and dislocations, we decoupled these two effects in the framework of the dynamical theory of diffraction, showing that the existence of dislocations governs different power law dependencies for the experimental absorption profiles. Moreover, the absorption profiles bring information regarding the uniformity of the dislocations along the z-axis, revealing a uniform distribution along the z-axis for thin layer, while for the thick layer an annihilation of the dislocations was taking place. In addition, photoluminescence peaks found at 3.41 eV, 3.39 eV and 3.35 eV correspond to near band-edge (NBE) and ultraviolet donor-acceptor pair (DAP) transition. Significant changes of their intensity ratio, as well as the additional broadening of E2 (high) Raman peak confirm the presence of TDs in the investigated GaN thin films.

Fabrication and characterization of gallium nitride thin film deposited on a sapphire substrate for photoelectrochemical water splitting applications

Gallium nitride (GaN) is of special interest as photoanode material in the photoelectrochemical (PEC) water splitting applications due to its excellent thermodynamically stable structure, tunable band edge potentials, and chemical stability. In this paper, we deposited a high-quality GaN thin film on a sapphire (Al2O3) substrate using a low-cost chemical vapor deposition technique. The X-ray diffraction (XRD) study confirmed the presence of the hexagonal wurtzite structure of GaN thin film. Raman spectra confirmed the existence of E2 (low), E2 (high) and A1 (LO) phonon mode in GaN thin film which are in agreement with XRD analysis. Field emission scanning electron microscopy (FESEM) and Atomic force microscopy (AFM) measurements revealed a smooth surface morphology and stepped-trace like structure formation of GaN with an average height of 3.3 Å. The functional groups were identified using Fourier transform infrared (FTIR) spectroscopy. The room temperature photoluminescence (PL) studies show near band-edge emission (NBE) of 3.21 eV. The thermogravimetry analysis (TGA) revealed that GaN thin film is thermally stable up to a temperature of 894 °C and a definitive mass loss was observed above 900 °C. Further, the photoelectrochemical (PEC) water splitting measurements showed that the GaN thin film exhibited a photocurrent density of 0. 099 mA/cm2 and a photoconversion efficiency of 0.73% at 0.59 V vs. RHE.

Properties of N-Type GaN Thin Film with Si-Ti Codoping on a Glass Substrate

In this study, n-type gallium nitride (GaN) films were fabricated by a silicon–titanium (Si-Ti) codoping sputtering technique with a zinc oxide (ZnO) buffer layer on amorphous glass substrates with different post-growth annealing temperatures for optimizing the GaN crystal quality. Si-Ti-codoped n-type GaN films that were thermally annealed at 400 °C had a low thin-film resistivity of 2.6 × 10−1 Ω-cm and a high electron concentration of 6.65 × 1019 cm−3, as determined through Hall measurement. X-ray diffraction (XRD) results revealed a high (002) XRD intensity with a narrow spectral line and a full width at half maximum (FWHM) value that indicated the superior crystal growth of a hexagonal structure of the GaN thin films. In addition, photoluminescence measurement results demonstrated a near-band-edge emission at 365 nm, indicating the crystal growth of GaN thin films on glass substrates. The Burstein–Moss effect was observed in the Tauc plot results, indicating that the Fermi level inside the conduction band moves upward and thus improves the n-type properties of the GaN thin film. X-ray photoelectron spectroscopy measurement results revealed that all atoms doped into the GaN film are present and that both Si and Ti atoms bond with N atoms.

Effects of Thermal Annealing on Optical Properties of Be-Implanted GaN Thin Films by Spectroscopic Ellipsometry

Wide bandgap III-V compounds are the key materials for the fabrication of short-wavelength optical devices and have important applications in optical displays, optical storage devices and optical communication systems. Herein, the variable-angle spectroscopic ellipsometry (SE) measurements are performed to investigate the thickness and optical properties of beryllium-implanted gallium nitride thin films that have been deposited on (0001) sapphire substrates by using low-pressure metalorganic chemical vapor deposition (LPMOCVD). The film layer details are described by using Parametric Semiconductor oscillators and Gaussian oscillators in the wavelength range of 200–1600 nm. The thickness, refractive indices and extinction coefficients of the Be-implanted films are determined at room temperature. Analysis of the absorption coefficient shows that the optical absorption edge of Be-implanted films changes from 3.328 eV to 3.083 eV in the temperature range of 300–850 K. With the variable temperature, Eg is demonstrated to follow the formula of Varshni. A dual-beam ultraviolet–visible spectrophotometer (UV–VIS) is used to study the crystal quality of samples, indicating that the quality of rapid thermal annealing (RTA) sample is better than that unannealed sample. By transport of ions in matter (TRIM) simulation and SE fitting the depths of Be implanted gallium nitride (GaN) films are estimated and in good agreement. The surface and cross-section morphologies are characterized by atomic force microscopy (AFM) and scanning electron microscope (SEM), respectively. The surface morphologies and thickness measurements of the samples show that RTA can improve crystal quality, while increasing the thickness of the surface roughness layer due to partial surface decomposition in the process of thermal annealing.