Search Results (63)

A super-polishing procedure was performed on the Al₂O₃ (0001) surface for epitaxial purposes. The roughness of the final polished surface was measured to be 0.16 nm using atomic force microscopy and X-ray reflectivity techniques. After heat treatment at 130 °C, results from low-energy electron diffraction and Auger energy spectroscopy indicated that the top surface was well ordered and clean, rendering it suitable for epitaxial growth. The successful growth of a GaN thin film on an Al₂O₃ (0001) substrate was confirmed by the hk-circle scan in XRD and the presence of a sharp peak in the rocking curve of the GaN (0002) Bragg peak. These findings indicate that the top surface of the substrate is conducive to epitaxial growth. Full article

Aluminum nitride (AlN)-based ferroelectric films offer significant advantages, including compatibility with CMOS back-end processes, potential for sustainable miniaturization, and intrinsic stability in the ferroelectric phase. As promising emerging materials, they have attracted considerable attention for their broad application potential in nonvolatile ferroelectric memories. However, several key scientific and technological challenges remain, including the preparation of single-crystal materials, epitaxial growth, and doping, which hinder their progress in critical ferroelectric devices. To accelerate their development, further research is needed to elucidate the underlying physical mechanisms, such as growth dynamics and ferroelectric properties. This paper provides a comprehensive review of the preparation methods of AlN-based ferroelectric films, covering AlN, Al_1−xSc_xN, Al_1−xB_xN, Y_xAl_1−xN, and Sc_xAl_yGa_1−x−yN. We systematically analyze the similarities, differences, advantages, and limitations of various growth techniques. Furthermore, the ferroelectric properties of AlN and its doped variants are summarized and compared. Strategies for enhancing the ferroelectric performance of AlN-based films are discussed, with a focus on coercive field regulation under stress, suppression of leakage current, fatigue mechanism, and long-term stability. Then, a brief overview of the wide range of applications of AlN-based thin films in electronic and photonic devices is presented. Finally, the challenges associated with the commercialization of AlN-based ferroelectrics are presented, and critical issues for future research are outlined. By synthesizing insights on growth methods, ferroelectric properties, enhancement strategies, and applications, this review aims to facilitate the advancement and practical application of AlN-based ferroelectric materials and devices. Full article

Atomic layer deposition (ALD) of Y₂O₃ thin films was investigated using Y(MeCp)₂(iPr-nPrAMD) precursor and H₂O reactant. The self-limiting reaction mechanism of ALD-Y₂O₃ thin films was confirmed at a growth temperature of 260 °C. And, the saturated growth rate was confirmed to be ~0.11 nm/cycle. Also, it was demonstrated that a wide ALD temperature window from 150 °C to 290 °C maintains a consistent growth rate. ALD-Y₂O₃ thin films were found to have a typical cubic polycrystalline structure, independent of growth temperature, which can be attributed to their stoichiometric composition of Y₂O₃, negligible carbon impurity, and high film density, analogous to the Y₂O₃ bulk. Even at a low growth temperature of 150 °C, ALD-Y₂O₃ exhibited a markedly lower plasma etching rate (~0.77 nm/min) than that (~4.6 nm/min) of ALD-Al₂O₃ when using RIE at a plasma power of 400 W with a mixed gas of Ar/CF₄/O₂. Furthermore, the growth temperature of Y₂O₃ thin films had minimal impact on the etching rate. Full article

Uncrystallized indium-gallium-zinc-oxide (InGaZnO) thin-film transistors (TFTs) combined with an aluminum nitride (AlN) dielectric have been used to promote performance and steadiness. However, the high deposition temperature of AlN films limits their application in InGaZnO flexible TFTs. In this work, AlN layers were deposited via low-temperature plasma-enhanced atomic layer deposition (PEALD), and InGaZnO films were fabricated via high-power impulse magnetron sputtering (HIPIMS). The band alignment of the AlN/InGaZnO heterojunction was studied using the X-ray photoemission spectrum and ultraviolet visible transmittance spectrum. It was found that the AlN/InGaZnO system exhibited a staggered band alignment with a valence band offset ΔE_v of −1.25 ± 0.05 eV and a conduction band offset ΔE_c of 4.01 ± 0.05 eV. The results imply that PEALD AlN could be more useful for surface passivation than a gate dielectric to promote InGaZnO device reliability under atmospheric exposure. Full article

Hexagonal boron nitride (h-BN) is recognized as a 2D wide bandgap material with unique properties, such as effective photoluminescence and diverse lattice parameters. Nitride alloys containing h-BN have the potential to revolutionize the electronics and optoelectronics industries. The energy band structures of three boron-containing nitride alloys—B_xAl_1−xN, B_xGa_1−xN, and B_xIn_1−xN—were calculated using standard density functional theory (DFT) with the hybrid Heyd–Scuseria–Ernzerhof (HSE) function to correct lattice parameters and energy gaps. The results for both wurtzite and hexagonal structures reveal several notable characteristics, including a wide range of bandgap values, the presence of both direct and indirect bandgaps, and phase mixing between wurtzite and hexagonal structures. The hexagonal phase in these alloys is observed at very low and very high boron concentrations (x), as well as in specific atomic configurations across the entire composition range. However, cohesive energy calculations show that the hexagonal phase is more stable than the wurtzite phase only when x > 0.5, regardless of atomic arrangement. These findings provide practical guidance for optimizing the epitaxial growth of boron-containing nitride thin films, which could drive future advancements in electronics and optoelectronics applications. Full article

Developing cost-effective methods to synthesize large-size GaN films remains a challenge owing to the high dislocation density during heteroepitaxy. Herein, AlGaN/GaN HEMTs were grown on 6- and 8-inch Si(111) substrates using metal–organic chemical vapor deposition, and their basic properties and dislocation evolution characteristics were investigated thoroughly. With the insertion of a 100 nm thin AlGaN buffer layer, bow–warp analysis of the epitaxial wafers revealed excellent stress control for both the 6- and 8-inch wafers. HR-XRD and AFM analyses validated the high crystal quality and step-flow growth mode of GaN. Further, Hall measurements demonstrated the superior transport performance of AlGaN/GaN heterostructures. It is worth noting that dislocations tended to annihilate in the AlN nucleation layer, the thin AlGaN buffer layer, and the GaN buffer layer in the initial thickness range of 200–300 nm, which was indicated by ADF-STEM. To be specific, the heterointerfaces exhibited a significant effect on the annihilation of c-type (b = <0001>) dislocations, which led to the formation of dislocation loops. The thin inserted layers within the AlGaN buffer layer played a key role in promoting the annihilation of c-type dislocations, while they exerted less influence on a-type (b = 1/3<11$\overline{2}$0>) and (a+c)-type (b = 1/3<11$\overline{2}$3>) dislocations. Within an initial thickness of 200–300 nm in the GaN buffer layer, a-type and (a+c)-type dislocations underwent strong interactions, leading to considerable dislocation annihilation. In addition, the EELS results suggested that the V-shaped pits in the AlN nucleation layer were filled with the AlGaN thin layer with a low Al content. Full article

This work presents highly responsive gate-controlled p-GaN/AlGaN/GaN ultraviolet photodetectors (UVPDs) on Si substrates with a high-transmittance ITO gate. The two-dimensional electron gas (2DEG) in the quantum well of the polarized AlGaN/GaN heterojunction was efficiently depleted by the p-GaN gate, leading to a high photo-to-dark current ratio (PDCR) of 3.2 × 10⁵. The quantum wells of the p-GaN/AlGaN and AlGaN/GaN heterojunctions can trap the holes and electrons excited by the UV illumination, thus efficiently triggering a photovoltaic effect and photoconductive effect, separately. Furthermore, the prepared photodetectors allow flexible adjustment of the static bias point, making it adaptable to different environments. Compared to traditional thin-film semi-transparent Ni/Au gates, indium tin oxide (ITO) exhibits higher transmittance. Under 355 nm illumination, the photodetector exhibited a super-high responsivity exceeding 3.5 × 10⁴ A/W, and it could even exceed 10⁶ A/W under 300 nm illumination. The well-designed UVPD combines both the advantages of the high-transmittance ITO gate and the structure of the commercialized p-GaN/AlGaN/GaN high-electron-mobility transistors (HEMTs), which opens a new possibility of fabricating large-scale, low-cost, and high-performance UVPDs in the future. Full article

The high breakdown electric field, n-type doping capability, availability of high-quality substrates, and high Baliga’s figure of merit of Ga₂O₃ demonstrate its potential as a next-generation power semiconductor material. However, the thermal conductivity of Ga₂O₃ is lower than that of other wide-bandgap materials, resulting in the degradation of the electrical performance and reduced reliability of devices. The heterostructure formation on substrates with high thermal conductivity has been noted to facilitate heat dissipation in devices. In this work, Ga₂O₃ thin films with an Al₂O₃ interlayer were deposited on SiC substrates by radio frequency sputtering. Post-deposition annealing was performed at 900 °C for 1 h to crystallize the Ga₂O₃ thin films. The Auger electron spectroscopy depth profiles revealed the interdiffusion of the Ga and Al atoms at the Ga₂O₃/Al₂O₃ interface after annealing. The X-ray diffraction (XRD) results displayed improved crystallinity after annealing and adding the Al₂O₃ interlayer. The crystallite size increased from 5.72 to 8.09 nm as calculated by the Scherrer equation using the full width at half maximum (FWHM). The carrier mobility was enhanced from 5.31 to 28.39 cm² V⁻¹ s⁻¹ in the annealed Ga₂O₃ thin films on Al₂O₃/SiC. The transfer and output characteristics of the Ga₂O₃/SiC and Ga₂O₃/Al₂O₃/SiC back-gate transistors reflect the trend of the XRD and Hall measurement results. Therefore, this work demonstrated that the physical and electrical properties of the Ga₂O₃/SiC back-gate transistors can be improved by post-deposition annealing and the introduction of an Al₂O₃ interlayer. Full article

Activation energy, bipolar resistance switching behavior, and the electrical conduction transport properties of ITO_X:SiO₂ thin film resistive random access memory (RRAM) devices were observed and discussed. The ITO_X:SiO₂ thin films were prepared using a co-sputtering deposition method on the TiN/Si substrate. For the RRAM device structure fabrication, an Al/ITO_X:SiO₂/TiN/Si structure was prepared by using aluminum for the top electrode and a TiN material for the bottom electrode. In addition, grain growth, defect reduction, and RRAM device performance of the ITO_X:SiO₂ thin film for the various oxygen gas flow conditions were observed and described. Based on the I-V curve measurements of the RRAM devices, the turn on-off ratio and the bipolar resistance switching properties of the Al/ITO_X:SiO₂/TiN/Si RRAM devices in the set and reset states were also obtained. At low operating voltages and high resistance values, the conductance mechanism exhibits hopping conduction mechanisms for set states. Moreover, at high operating voltages, the conductance mechanism behaves as an ohmic conduction current mechanism. Finally, the Al/ITO_X:SiO₂/TiN/Si RRAM devices demonstrated memory window properties, bipolar resistance switching behavior, and nonvolatile characteristics for next-generation nonvolatile memory applications. Full article

Semipolar AlGaN multiple quantum wells (MQWs) have unique advantages in deep ultraviolet light emitters due to the weak Quantum-Confined Stark Effect. However, their applications are hampered by the poor crystalline quality of semipolar AlGaN thin films. Different treatments were developed to improve the crystal quality of semipolar AlGaN, including a multistep in situ thermal annealing technique proposed by our group. In this work, temperature-dependent and time-resolved photoluminescence characterizations were performed to reveal the carrier localization in the MQW region. The degree of carrier localization in semipolar AlGaN MQWs grown on top of the in situ-annealed AlN is similar to that of conventional ex situ face-to-face annealing, both of which are significantly stronger than that of the c-plane counterpart. Moreover, MQWs on in situ-annealed AlN show drastically reduced dislocation densities, demonstrating its great potential for the future development of high-efficiency optoelectronic devices. Full article

Aluminum gallium nitride (AlGaN) is a nanohybrid semiconductor material with a wide bandgap, high electron mobility, and high thermal stability for various applications including high-power electronics and deep ultraviolet light-emitting diodes. The quality of thin films greatly affects their performance in applications in electronics and optoelectronics, whereas optimizing the growth conditions for high quality is a great challenge. Herein, we have investigated the process parameters for the growth of AlGaN thin films via molecular dynamics simulations. The effects of annealing temperature, the heating and cooling rate, the number of annealing rounds, and high temperature relaxation on the quality of AlGaN thin films have been examined for two annealing modes: constant temperature annealing and laser thermal annealing. Our results reveal that for the mode of constant temperature annealing, the optimum annealing temperature is much higher than the growth temperature in annealing at the picosecond time scale. The lower heating and cooling rates and multiple-round annealing contribute to the increase in the crystallization of the films. For the mode of laser thermal annealing, similar effects have been observed, except that the bonding process is earlier than the potential energy reduction. The optimum AlGaN thin film is achieved at a thermal annealing temperature of 4600 K and six rounds of annealing. Our atomistic investigation provides atomistic insights and fundamental understanding of the annealing process, which could be beneficial for the growth of AlGaN thin films and their broad applications. Full article

Nitrides of aluminum (Al) and titanium (Ti) mixtures have long been studied and used as commercial coatings because of their high hardness and high oxidation resistance due to the formation of an alumina layer on the coating surface. To fully understand the contribution of Al and Ti to the properties of the film, a combinatorial deposition approach was employed using half-disk targets. Film growth was carried out using a magnetron sputtering system powered by a 13.56 MHz radio frequency power supply with varying argon (Ar) and nitrogen (N₂) gas ratios. Depending on the location of the substrate relative to the target, atomic percent gradients of 0.60–0.70 Al and 0.30–0.40 Ti across the substrate surface were obtained from energy dispersive X-ray spectral analysis. X-ray diffraction peaks at 43.59°, 74.71° (face-centered cubic), and 50.60° (wurtzite) confirmed the presence of aluminum titanium nitride (AlTiN) mixtures, with an increasing amount of wurtzite phase at higher Al concentrations. For all samples, cauliflower-like nanograins were obtained and samples of the 80:20 Ar:N₂ gas pressure ratio showed the smallest grain size among the three gas ratio combinations. The 80:20 Ar:N₂ films revealed a relatively high hardness compared to the other gas ratios. All thin films exhibited good adhesion to 304 stainless steel substrates. Full article

We utilized a plasma-enhanced atomic layer deposition (PEALD) process to deposit an AlN passivation layer on AlGaN/GaN surface to enhance the polarization effects, which enabled the fabrication of an enhancement-mode (E-mode) AlGaN/GaN metal-insulator-semiconductor heterojunction field-effect transistor (MIS-HFET) without the need for a gate recess process. The AlN film deposited by PEALD exhibited a crystalline structure, not an amorphous one. The enhanced polarization effect of introducing the PEALD AlN film on a thin AlGaN barrier was confirmed through electrical analysis. To fabricate the E-mode AlGaN/GaN MIS-HFET, the PEALD AlN film was deposited on a 4.5 nm AlGaN barrier layer and then a damage-free wet etching process was used to open the gate region. The MIS-gate structure was formed by depositing a 15 nm plasma-enhanced chemical vapor deposition (PECVD) silicon dioxide (SiO₂) film. The fabricated thin-AlGaN/GaN MIS-HFET demonstrated successful E-mode operation, with a threshold voltage of 0.45 V, an on/off ratio of approximately 10⁹, a specific on-resistance of 7.1 mΩ·cm², and an off-state breakdown voltage exceeding 1100 V. Full article

We report on the epitaxial lateral overgrowth (ELO) of high-quality AlN on stripe-patterned Si(111) substrates with various trench widths. By narrowing down the trench and ridge widths of patterned Si substrates, crack-free, 6-micrometer-thick, high-quality AlN films on Si substrates were produced. The full-width-at-half-maximum values of the X-ray-diffraction rocking curves for the AlN (0002) and (10$\overline{1}2$) planes were as low as 260 and 374 arcsec, respectively, corresponding to a record low dislocation density of 1.3 × 10⁹ cm⁻². Through the combination of a micro-Raman study and the X-ray diffraction analysis, it was found that narrowing the stripe width from 5 μm to 3 μm can reduce the vertical growth thickness before coalescence, resulting in a large decrease in the internal tensile stress and tilt angle, and, therefore, better suppression in the cracks and dislocations of the ELO–AlN. This work paves the way for the fabrication of high-performance Al(Ga)N-based thin-film devices such as ultraviolet light-emitting diodes and AlN bulk acoustic resonators grown on Si. Full article

Environmental monitoring of pollutants, such as NO_x and CO_x, which can be facilitated by a range of gas sensors, is of considerable fundamental and practical importance. This work has been focused on the synthesis and evaluation of zeolite β with tin (Sn-BEA) and dealuminated β (DeAl-BEA) zeolites. The zeolite samples have been extensively investigated by IR, UV-VIS and NMR spectroscopy, XRD, TGA, and N₂ adsorption-desorption. The prepared Sn-BEA sample is characterised by the submicron particle size, an almost defect-free structure, and high hydrophobicity. Sensors containing selective microporous layers based on Sn-BEA and DeAl-BEA zeolites have been prepared and extensively tested. Both the Sn-BEA and DeAl-BEA zeolites have been deposited in thin films and evaluated as gas sensors for CO, CO₂, NO, and NO₂ in the presence of water vapour at room temperature. The Sn-BEA zeolite-based sensor showed high selectivity towards NO₂, while the DeAl-BEA is selective towards CO₂ and NO₂. Full article