Optimization of Deposition Temperature for the Formation of GaN

Study Case Optimization of Deposition Temperature for the Formation of GaN Thin Film Crystal Structures on Silicon Substrates Using the Sol-Gel Method Michael Kurniawan (2018123456) University of Cincinnati, 2600 Clifton Ave, Cincinnati, OH 45221, United States kurniaml@mail.uc.edu Abstract A thin layer of gallium nitride (GaN) semiconductor was successfully deposited on a Si(004) silicon substrate using the sol-gel method. The gel was synthesized from gallium-citrate-amine crystals formed through a solution containing Ga³⁺ ions and citric acid (CA). After the gel was applied to the substrate, spin-coating was performed at a speed of 1100 rpm, then the samples were heated in a programmable furnace in a nitrogen atmosphere at temperatures of 800°C, 900°C, and 1000°C for 2 hours. The crystal quality of the GaN film was characterized using X-ray diffraction (XRD), while the surface morphology and cross-section were observed using a scanning electron microscope (SEM), and the elemental composition was analyzed using EDX spectroscopy. The results showed that all GaN films deposited on silicon substrates had a polycrystalline orientation. The crystal quality improved with increasing deposition temperature. Keywords: Gallium Nitride Thin Film; Spin-Coating Technique; Silicon Substrate INTRODUCTION Gallium nitride (GaN) is a semiconductor material with a wide energy band gap and direct transition, namely Eg = 3.45 eV at room temperature. This material has high mechanical strength, good electrical conductivity, and structural compatibility with compounds such as InGaN and AlGaN, making it an ideal candidate for various electronic and optoelectronic applications. Currently, GaN layers grown on sapphire (Al₂O₃) substrates are widely used as active materials in devices such as ultraviolet MICHAEL KURNIAWAN 1 Study Case photodetectors, laser diodes, and light-emitting diodes (LEDs) operating at visible wavelengths. In addition, GaN is also used in high-power transistor devices, display systems, and data storage media with high electron mobility that can operate at high temperatures, frequencies, and power [1][2]. Previous studies have shown that GaN layers can be grown on various types of substrates, such as Al₂O₃, 6H-SiC, ZnO, and Si, using various deposition methods, including reactive radio-frequency (RF) sputtering, metal-organic vapor phase epitaxy (MOVPE), plasma-assisted molecular beam epitaxy (PA-MBE), metal-organic chemical vapor deposition (MOCVD), and plasma-assisted MOCVD (PA-MOCVD) [3][4][5][6][7][8][9]. In this study, a study was conducted on the growth of GaN layers on Si(004) silicon substrates using the sol-gel spin-coating method with varying deposition temperatures. The gallium precursor was prepared in the form of gallium-citrate-amine crystals, while the nitrogen source was ultra-high purity (UHP) nitrogen gas. Compared to other conventional deposition techniques, the sol-gel method has the advantages of simplicity, low cost, and ease of growth parameter control. This paper discusses the physical characteristics of thin GaN layers grown on silicon substrates using the sol-gel approach, including crystal structure, surface morphology, and chemical composition. Crystal structure analysis was performed through XRD characterization, morphology was observed using SEM imaging, and elemental composition was analyzed using EDX. EXPERIMENTAL PROCEDURE Thin layers of gallium nitride (GaN) semiconductor were grown using the sol-gel method with spin-coating technique, where gallium-citrate-amine crystals were used as gallium (Ga) precursors, while the nitrogen source was obtained from N₂ gas reacted through heating at high temperatures. This white gallium-citrate-amine crystal was synthesized through a gel formation process from a solution containing Ga³⁺ ions and citric acid (CA). The compound has the chemical formula (NH₄)₃[Ga(C₆H₅O₇)₂]·4H₂O. The synthesis process of gallium-citrate-amine crystals was carried out by dissolving 2.16 g of Ga₂O₃ powder into a mixture of hydrochloric acid (HCl) and nitric acid (HNO₃) with a 1:1 ratio. The resulting solution was then neutralized to a pH of 7.5– 8.0 using ammonium hydroxide (NH₄OH). Next, 1.1 g of citric acid (CA) was added to MICHAEL KURNIAWAN 2 Study Case the solution so that the molar ratio of Ga/CA = 1:1. The mixture is then stirred at 80°C for 2 hours to produce white crystals. The crystals are then rinsed with acetone, dried in a vacuum desiccator, and redissolved in ethylenediamine to obtain a clear gel solution. This solution is used as the main material for GaN film deposition on a monocrystalline silicon Si(004) substrate. The silicon substrate is placed on a spin coater, and 1–2 drops of gel are applied to its surface. The spinning process is carried out at a speed of 1100 rpm for 1 minute to obtain a uniform layer. The layer formed is then dried on a hot plate at a temperature of 150°C, followed by a thermal decomposition stage at 350°C in a furnace to remove residual organic components. Next, the layer is heated using a programmable furnace. The furnace temperature is gradually increased from room temperature to 1000°C at a heating rate of 10°C per minute, in an ultra-high purity (UHP, 99.99%) nitrogen atmosphere that is constantly flowing at a rate of 100 sccm (standard cubic centimeters per minute). The deposition process is carried out at three temperature variations, namely 800°C, 900°C, and 1000°C, for 2 hours, then the system is cooled back to room temperature at a cooling rate of 10°C per minute to produce a crystalline GaN layer. The crystallinity of the deposited layer was analyzed using X-ray diffraction (XRD) to identify the crystal phase formed. Meanwhile, the surface morphology and crosssection were observed using a scanning electron microscope (SEM). Based on the SEM cross-section image, the thickness of the GaN film could be accurately determined. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction (XRD) pattern of a thin gallium nitride (GaN) layer deposited on a Si(004) silicon substrate at various deposition temperatures. The characterization results show that the resulting GaN film has a polycrystalline orientation, marked by the appearance of several crystal plane orientation peaks. The resulting crystal structure exhibits a cubic structure pattern that follows the lattice orientation of the silicon substrate. The deposition temperature has been shown to have a significant effect on the crystal quality of the GaN film. At a deposition temperature of 800°C, the XRD pattern shows that the GaN layer is still amorphous. However, when the temperature was increased to 900°C and 1000°C, diffraction peaks appeared on the (102) and (103) MICHAEL KURNIAWAN 3 Study Case planes, indicating the formation of a crystalline structure. The increase in temperature caused the decomposed nitrogen atoms to have sufficient energy to bond more effectively with gallium atoms. However, at a temperature of 1000°C, the crystal quality decreased again, as seen from the broadening of the diffraction peak on the (102) plane. This was due to excessively high thermal energy, which caused the Ga–N bonds to become unstable due to excessive nitrogen atom mobility. Based on these results, the optimum deposition temperature for the formation of a crystalline GaN layer is 900°C. Figure 1. X-ray diffraction (XRD) patterns of thin GaN layers at various deposition temperatures. Figure 2 shows the surface morphology image of the GaN thin film observed using a scanning electron microscope (SEM) at various nitrogen deposition temperatures (800°C, 900°C, and 1000°C). The SEM images show that the surface of the resulting GaN film is generally smooth and homogeneous, indicating that the interatomic nucleation process occurs slowly, resulting in a soft and uniform grain size. With increasing deposition temperature, the grain size increases, although it generally remains below 100 nanometers (nm). These results show that the sol-gel method is capable of producing thin nanocrystalline layers with high homogeneity. However, microscopic cracks are still found on a surface area of 1 × 1 cm², indicating that the layer is not yet completely uniform. Three-dimensional (3D) image analysis with a scanning area of 1 × 1 μm² shows a smooth surface at low temperatures, but at 1000°C porosity begins to appear, indicating the release of excess nitrogen gas or instability of the Ga–N bond due to excessively high decomposition energy. MICHAEL KURNIAWAN 4 Study Case Figure 2. SEM images of the surface morphology of thin GaN ilm sat deposition temperatures of (a) 800°C, (b) 900°C, and (c) 1000°C. Figure 3 shows SEM images of cross-sections of GaN layers deposited using gels with different molarities, namely 0.3 M, 0.4 M, and 0.6 M. The analysis results show that the film thicknesses are 0.35 μm, 0.4 μm, and 0.7 μm, respectively, indicating that an increase in gel concentration produces a thicker layer. In other words, the growth rate increases with increasing solution molarity. The cross-section of the layer shows a relatively smooth and uniform morphology, comparable to the results of deposition using modern methods such as metal-organic chemical vapor deposition (MOCVD). In general, these results prove that the chemical solution deposition (CSD) method is capable of producing high-quality GaN semiconductor films with uniform submicron thickness. MICHAEL KURNIAWAN 5 Study Case Figure 3. SEM image of a cross-section of a thin GaN film at varying molarities of gallium-citrate-amine gel solution. CONCLUSION A thin layer of gallium nitride (GaN) semiconductor was successfully deposited on a silicon substrate using the sol-gel spin-coating method. XRD characterization results show that films deposited at 800°C are amorphous, while those deposited at 900°C and 1000°C exhibit a polycrystalline structure. Increasing the nitrogen decomposition temperature has been shown to improve the crystal quality of GaN, with optimal conditions achieved at 900°C. SEM analysis showed that the surface of the deposited film had a smooth and nearly homogeneous morphology, making it suitable for use in electronic and optoelectronic device applications. In addition, increasing the molarity of the gel solution was found to increase the growth rate of the layer. Overall, the results of this study prove that the spin-coating-based sol-gel method is an efficient, economical, and promising alternative for producing high-quality GaN semiconductor thin films, comparable to the results of modern deposition techniques such as MOCVD, PLD, MBE, and RF sputtering. MICHAEL KURNIAWAN 6 Study Case REFERENCE MICHAEL KURNIAWAN 7