Sapphire Substrates Research Articles

High-quality GaN thin film deposition at low temperature by ECR plasma-assisted sputter deposition method and its dependence of sapphire substrate misorientation angle

Gallium nitride (GaN) thin films were deposited by electron cyclotron resonance (ECR) plasma-assisted sputtering, which combines GaN-magnetron sputtering with argon and nitrogen plasma assistance using an ECR high-density plasma. GaN films on the misorientation-angle-0.0° (just) sapphire substrate showed very good crystallinity with a GaN(0002) rocking curve (XRC) full width at half maximum (FWHM) of 0.042° and epitaxial growth confirmed by φ-scan measurements at a low heating temperature of 350 °C. However, the GaN thin film had a rough surface with circular grains about 100 nm in diameter and a surface root-mean-square height (Sq) of 1.21 nm. Therefore, the misorientation angle of the sapphire substrate was varied from 0.2° to 10.0°. As a result, the grains observed on the just substrate disappeared at 0.5°. The film had Sq: 0.33 nm, and the FWHM of the XRC of GaN(0002) was 0.066°, indicating improved surface flatness while maintaining crystallinity. This is considered to be due to the step flow, which promotes ECR plasma-assisted diffusion on the terrace even at the low temperature of 350 °C. The polarity of the GaN thin film was analyzed by time-of-flight atomic scattering surface analysis and found to be N-polar on all substrates.

Micro-mechanism of glycine action in tribochemical mechanical polishing of single-crystal sapphire substrate: Experimental and first-principles analysis

With the rapid development of novel optoelectronic devices, an environmentally friendly and highly efficient polishing technology for SC-sapphire substrates is imminent. In this work, the micro-mechanism of Gly action in TCMP of SC-sapphire substrate is systematically studied by TCMP experiments and first-principles calculations. The results of TCMP experiments confirm that the maximum MRR of 5.37 μm/h with Ra of 0.42 nm can be attained at optimal concentration of Gly, without any acid or alkali corrosive solution. The first-principles calculations of the Gly/SC-sapphire heterostructure further reveal the reaction behavior of the -COOH functional group with and without lattice distortion. The results of both TCMP experiments and VASP simulations demonstrate that the incorporation of Gly can facilitate the friction-induced Gly/SC-sapphire reaction, which significantly enhances the processing surface quality and the processing efficiency of SC-sapphire substrates. Finally, based on the above research, a microscopic material removal model for the Gly-based SC-sapphire substrate TCMP is constructed. The above findings are of great significance in promoting the development of green ultra-precision polishing technology for semiconductor substrates.

Epitaxial growth and characterization of copper gallate (CuGa2O4) thin films by pulsed laser deposition

High-quality crystalline copper gallate (CuGa2O4) thin films were grown on c-plane sapphire substrates using the pulsed laser deposition (PLD) technique by optimizing the growth parameters (i.e., temperature, oxygen chamber pressure, and laser energy density). The obtained XRD patterns validated that the preferred orientation of the crystalline CuGa2O4 thin film is along the [111] direction, and the crystallinity of the films improved with increasing substrate temperature while maintaining a constant O2 pressure, as assessed by measuring the full width at half maximum (FWHM) of the prominent (222) plane. Reducing the oxygen pressure leads to the emergence of β-Ga2O3 peaks rather than the CuGa2O4 peaks due to the incomplete oxidation of copper (Cu). Subsequent XRD phi (φ) scans revealed a sixfold rotational symmetry of CuGa2O4 and a 30° epitaxial relationship between the film and the sapphire substrate. X-ray photoelectron spectroscopy (XPS) spectra confirmed the presence of Cu1+, Cu2+, and Ga3+ oxidation states in the films. Increasing laser energy density resulted in the complete oxidation of Cu and an increase in the film thickness from 51.8 nm to 183.4 nm. The direct bandgap of CuGa2O4 films was determined to be approximately 4.50 eV by analyzing the UV–Vis absorbance spectra using the Tauc equation. The refractive index was found to be approximately 2.05 ± 0.01 based on the spectroscopic ellipsometry data. While the impact of increasing laser energy density was negligible on the parameters such as crystal structure, rotational symmetry, oxidation states of gallium (Ga), oxygen (O), bandgap, and refractive index of CuGa2O4 thin film, notable alterations in the oxidation state of Cu and film thickness were observed. The surface roughness (Rrms) measured from AFM images showed a strong correlation with the values derived from ellipsometry analysis.

Single-stack multilayer infrared mirrors with selectable higher-order interference peaks

A concept for the design of single-stack multilayer mirrors with multiple reflection peaks is presented. Its realization in the infrared is demonstrated by the sputter deposition of TiO2/Al2O3 bilayers on sapphire and HastelloyX substrates and corresponding reflection measurements. Even or odd higher-order interference peaks are selected by adjusting the optical thicknesses of the individual layers. The peak positions are very well reproduced by multiple scattering calculations. With the HastelloyX substrate, a potential application for high temperatures and aggressive atmospheres, e.g., in aircraft turbines, is addressed. A design for matching up to three turbine gas emission bands (CO2, H2O) is proposed.

Impact of c- and m- sapphire plane orientations on the structural and electrical properties of β-Ga2O3 thin films grown by metal-organic chemical vapor deposition

Abstract This work presents a comprehensive investigation into the structural and electrical properties of Ga2O3 thin films grown via metal-organic chemical vapor deposition on both c- and m-plane sapphire substrates. Structural characterization revealed the β-Ga2O3 phase formation in both substrate orientations, with strong epitaxial ( 2 ¯ 01 ) preferential growth on c-plane substrates and polycrystalline films on m-plane substrates. Results show that Ga2O3/m-sapphire exhibits the lower electrical resistivity than its counterpart grown on c-sapphire. Activation energies of acceptor levels were estimated at ~1.4 eV and ~0.7 eV , for Ga2O3 films grown on c- and m-plane, respectively. This result shows that growing Ga2O3 on m-plane sapphire is beneficial to reach a weakly compensated sample. Cathodoluminescence analysis suggests that the additional low activation energy of ~0.18 eV observed in Ga2O3 grown with the highest oxygen flow on m-plane sapphire can be associated to thermally-induced migration of self-trapped hole states.

Low turn-on voltage AlGaN/GaN Schottky barrier diode with a low work function anode and high work function field plate

AlGaN/GaN Schottky barrier diodes (SBDs) with a low work function W anode and a high work function Ni field plate on sapphire substrates are investigated in this Letter. The low work function metal W leads to the low turn-on voltage, while the high work function metal Ni maintains the leakage current and increases the breakdown voltage. An ultralow turn-on voltage of 0.23 V determined by the positive fixed charge at the AlGaN/Si3N4 interface is achieved in this W/Ni SBD. More importantly, benefitting from the dual-metal anode/field plate structure, the on-resistance decreases from 7.99 to 5.53 Ω mm, and the breakdown voltage increases from 493 to 1219 V. In addition, the turn-on voltage decreases less than 57%, and the on-resistance increases less than 186% at a high temperature up to 120 °C. The W/Ni SBD with a cathode–anode length of 18 μm and a field plate length of 4 μm shows a low turn-on voltage of 0.23 V, a low on-resistance of 5.53 Ω mm, a leakage current of 0.65 mA/mm, and a high breakdown voltage of 1.21 kV, indicating a great potential for power electronic applications.

High responsivity and detectivity β-Ga2O3 solar-blind photodetectors optimized by oxygen vacancy engineering

Solar-blind photodetectors (SBPDs) are core essential components for many critical applications such as precision guidance, fire warning, and space communications. Ultra-wide bandgap semiconductor β-Ga2O3 is considered to be an ideal material for the fabrication of SBPDs. However, synthetizing β-Ga2O3 with high quality factor while simultaneously in situ modulation of electronic and optoelectronic properties to enhance performance has been challenging. Here, pulsed laser deposition (PLD) technology is used to synthesize high-quality β-Ga2O3 thin films on a sapphire substrate. The oxygen vacancy engineered β-Ga2O3 films can achieve in situ precise control of their surface morphology, optical parameters, and optoelectronic properties by simply adjusting the oxygen pressure. Meanwhile, the optimal thickness of the β-Ga2O3 film for the developing high-performance SBPD is ∼221 nm, determined by fitting and analyzing the optical parameters measured by the ellipsometry. Subsequently, the influence of oxygen pressure on the performance of β-Ga2O3 SBPD is thoroughly explored, considering the optimization of electrode size and deposition time. When the oxygen pressure is set to 15 Pa, the β-Ga2O3-based SBPD achieves highly competitive responsivity (R) and detectivity (D*) at 250 nm, with values of 1080 A·W−1 and 1.4 × 1016 cm·W−1·Hz1/2, respectively. Additionally, the noise component of the β-Ga2O3 SBPD is further studied to calibrated the traditional device performance results. This work introduces a simple and straightforward approach to in situ tuning of the optoelectronic properties of β-Ga2O3, which is important for advancing β-Ga2O3 film growth technology and fabricating high-performance photodetectors.

Counterbalancing effects of bowing in gallium nitride templates by epitaxial growth on pre-strained sapphire substrates

To minimize bowing in a gallium nitride (GaN) template, which is caused by differences in the properties of the GaN film and the sapphire substrate, we studied how to offset bowing by directly growing GaN using the hydride vapor phase epitaxy (HVPE) method on the frontside of a pre-strained sapphire substrate prepared using a backside grinding process. The relationships between the respective thicknesses and stresses of the undamaged sapphire, damaged sapphire, and GaN layers were analyzed by deriving the bow formula, which is a modification of Freund's equation, and the theoretical formula-derived results were compared with experimentally determined values. The bow of the GaN template was measured using a micrometer, and the residual stresses in the GaN and sapphire, carrier mobilities, and GaN crystal qualities were measured using a micro-Raman spectrophotometer. The results confirmed the annealing effect on the damaged sapphire layer, and the introduction of pre-strained GaN templates facilitated easier bow control compared to that afforded by a conventional GaN template. Additionally, we identified trends and optimal conditions for changes in residual stress, carrier mobility, and crystal quality in GaN and the damaged sapphire layer based on layer thickness and stress variations. We explain the mechanisms responsible for changes in bow, stress, carrier mobility, and crystal quality of the GaN template.

Micro magnetic field sensor based on bifunctional diodes

Multiple-quantum well (MQW) diodes can be used as bifunctional diodes due to the emission-detection spectral overlap. When integrated with magnetic fluids (MFs) that have tunable refractive index, they can be designed as micro magnetic field sensors. The sapphire substrate of the MQW diode chip that consists of an MQW transmitter and receiver that is directly exposed to the MF, and the external magnetic field strength is used to change the refractive index at the boundary between the sapphire and the MF, thus modulating the reflected light and realizing external magnetic field sensing. Verified by experimental measurements, the micromagnetic field sensor has a detection range of 0.001-0.05 T, a sensitivity of 127.3 µA/T, and a resolution of 4.5×10−5 T, with excellent stability and repeatability. Additionally, the sensor demonstrates good velocity resolution under dynamic magnetic fields and can detect the direction of magnetic field motion, providing significant application value.

Enhanced crystallinity of (Sr,Ba)Nb2O6 films on sapphire and alumina substrates

Barium-strontium niobate in thin film form can be considered as a prospective ferroelectric material for microwave applications due to high dielectric nonlinearity. Its performance is linked to film structure quality, and one method to achieve high-quality films on microwave substrates goes through the process of annealing. (Sr,Ba)Nb2O6 films of good crystallinity were formed on sapphire and alumina substrates by magnetron sputtering. The processes of recrystallization of films on single- and polycrystalline substrates under the conditions of ex situ annealing at temperatures of 1100–1200 °C were studied. Highly (00l) oriented films on sapphire substrate were obtained by the deposition in oxygen atmosphere at a substrate temperature of 900 °C with post-growth annealing in air at an annealing temperature of 1150 °C.

Lateral NiO/AlN Heterojunction Rectifiers with Breakdown Voltage >11 kV

Lateral NiO/AIN heterojunction diodes (HJDs) with breakdown voltage up to 11.6 kV and Ni/Au/AlN Schottky barrier diodes (SBDs) with VB of 8.6 kV were fabricated on layers grown on sapphire substrates by metalorganic chemical vapor phase deposition. The power figure-of-merits VB 2/RON where RON is the on-resistance were 0.31 MW·cm−2 for HJD and 0.16 MW·cm−2 for SBD. The lowest turn-on voltages were ∼2.03 and 1.91 V for HJDs and SBDs, respectively, with ON/OFF ratios up to 102. The maximum field before breakdown was 0.45 MV·cm−1 in HJDs and 0.31 MV·cm−1 in SBDs. These correspond to <3% of the critical field in AlN of ∼15 MV·cm−1. This work demonstrates there is still significant optimization to be done in the overall quality of the AlN, including purity, crystal perfection, and defect density to realize the potential of this material as an ultra-wide bandgap semiconductor for efficient multi-kV power switching applications. Our results also demonstrate the promise of NiO as a p-type conducting oxide for forming heterojunctions with AlN.

Developing a robust sensor for infrared imaging bolometers.

A new type of large area sensor for infrared imaging bolometers has been developed. It replaces the thin and fragile free-standing metal foils, which typically have been used, with a multi-layer coated sapphire (or diamond) substrate. Sapphire is transparent to mid-infrared wavelengths, is robust against transients, and can be thick enough to even be the vacuum window. The primary radiation absorber is still a thin deposited metal layer, but now it is partially insulated from the supporting sapphire substrate by a black (carbon-based) layer, which also acts as a blackbody remitter. Test results indicate 6× more noise equivalent power density (estimated NEPD = 23 W/m2 at 5ms camera exposure time, foil temperature decay time 60ms) for a 2μm gold-coated sapphire disk compared to estimated NEP = 4 W/m2 at 1.8ms exposure time, with foil decay time 420ms, for a nominal 2.5μm thick platinum-free-standing foil.

Effects of interlayer SiO2 or Au on the performances of LNOS-SAW

Abstract Surface acoustic wave (SAW) resonators and filters based on lithium niobate (LN) or lithium tantalate crystals have been widely used in modern wireless communication systems. However, acoustic energy in these kinds of devices is lost in the substrate due to longitudinal attenuation. The enhancements of quality factor and coupling coefficient are still in demand for further applications. This work proposes a SAW based on LN thin film on a sapphire substrate (LNOS). Besides, we explored the effects of using different bonding materials on the LNOS-SAW performances. The presence of the interlayer not only plays a role in bonding but also has an impact on the performance of the resonator. Considering the compatibility of the process in this method, 2 μm-thick silicon dioxide (SiO2) or 1 μm-thick gold (Au) is designed as the interlayer. As the displacement diagrams show, the acoustic energy of the SAW with interlayer is reflected at the interface, and the energy leakage is suppressed. The simulation results show that the presence of SiO2 or Au causes a decrease in resonant frequencies of the resonators, and the effect is more pronounced with Au. Significantly, the presence of SiO2 enhances the coupling efficiency of the resonator while the Au interlayer brings a more obvious improvement in the quality factor.

A high integration side-fed rectangular microstrip antenna based on the sapphire substrate

A high integration side-fed rectangular microstrip antenna based on the sapphire substrate

High-performance solar-blind photodetector of β-Ga2O3 grown on sapphire with embedding an ultra-thin AlN buffer layer

The authors reported the MOCVD growth of high-quality β-Ga2O3 on c-plane sapphire substrate, resulting in the achievement of high performance solar blind deep UV photodetectors by introducing an ultra-thin AlN buffer seed layer (BSL). Compared with the direct growth of Ga2O3 on the sapphire, the oxygen vacancy of β-Ga2O3 is reduced after introducing an AlN BSL and the full width at half maximum (FWHM) of (-201) diffraction peak becomes narrower. The dark current is reduced by 1000 times and from 10−10 A down to 10−13 A for the metal-semiconductor-metal planner photodetector of β-Ga2O3 thin film with the AlN BSL. And the detectivity reaches 4.65×1015 cm·Hz1/2·W at 5 V bias. This improvement in performance is derived from the improvement of the growth quality of β-Ga2O3 mediated by the AlN BSL due to the decrease of lattice mismatch. The defects in β-Ga2O3 are reduced, the carrier concentration is reduced, the Fermi level is shifted, the depletion region is widened, the probability of electron tunneling in the dark state is reduced, and the dark current is effectively reduced.

Elimination of V‐Shaped Pits in Thick InGaN Layers via Ammonia‐Assisted Face‐to‐Face Annealing

InGaN, a group‐III nitride semiconductor, is expected to be widely used in the field of optoelectronics, owing to its excellent physical properties. However, InGaN has various limitations. This study reports face‐to‐face annealing (FFA) using vapor‐phase and in‐plane mass transport to improve the surface flatness of an InGaN template. InGaN layers are grown on a GaN template that is grown on a c‐plane sapphire substrate using metal–organic vapor‐phase epitaxy. NH3‐assisted FFA is performed at 1050 °C for 20 min, causing V‐pits to vanish from the InGaN template despite their initial density of 3.3 × 108 cm−2. The surface condition of the lower InGaN layer is worse than that of the upper InGaN layer due to the FFA‐induced upward mass transport from the lower layer, thereby eliminating the V‐pits. Compositional analysis of the upper layer through Auger electron spectroscopy and energy‐dispersive X‐ray spectroscopy reveals In peaks despite high‐temperature annealing, thus confirming the presence of InGaN. The results of this study offer possibilities for future InGaN crystal growth and InGaN‐based device fabrication.

A phosphor-in-glass film prepared with a novel orange Lu2GdMg2AlSi2O12:Ce3+ phosphor and a cyan BaSi2O2N2:Eu2+ phosphor

The preparation of phosphor-in-glass (PiG) films containing orange and cyan phosphors is one of the possible ways to improve the color rendering index (CRI) and correlated color temperature (CCT) of white LEDs packaged with PiG films. Recently, by matrix ion substitution, we have synthesized a novel orange Lu2GdMg2AlSi2O12:Ce3+ phosphor. Then, we used a “coating and low-temperature sintering” method to prepare PiG films containing this novel orange phosphor and the commercial cyan BaSi2O2N2:Eu2+ phosphor on sapphire substrates. Combining these PiG films with blue LED chips, warm white LEDs with CRI above 90 and CCT below 4000 K can be obtained. This suggests that the PiG films containing cyan and orange phosphors are promising to improve the CCT and CRI of white LEDs packaged with PiG films.

Modeling Study of Si3N4 Waveguides on a Sapphire Platform for Photonic Integration Applications.

Sapphire has various applications in photonics due to its broadband transparency, high-contrast index, and chemical and physical stability. Photonics integration on the sapphire platform has been proposed, along with potentially high-performance lasers made of group III-V materials. In parallel with developing active devices for photonics integration applications, in this work, silicon nitride optical waveguides on a sapphire substrate were analyzed using the commercial software Comsol Multiphysics in a spectral window of 800~2400 nm, covering the operating wavelengths of III-V lasers, which could be monolithically or hybridly integrated on the same substrate. A high confinement factor of ~90% near the single-mode limit was obtained, and a low bending loss of ~0.01 dB was effectively achieved with the bending radius reaching 90 μm, 70 μm, and 40 μm for wavelengths of 2000 nm, 1550 nm, and 850 nm, respectively. Furthermore, the use of a pedestal structure or a SiO2 bottom cladding layer has shown potential to further reduce bending losses. The introduction of a SiO2 bottom cladding layer effectively eliminates the influence of the substrate's larger refractive index, resulting in further improvement in waveguide performance. The platform enables tightly built waveguides and small bending radii with high field confinement and low propagation losses, showcasing silicon nitride waveguides on sapphire as promising passive components for the development of high-performance and cost-effective PICs.

Feasibility and Structural Transformation of Electrodeposited Copper Foils for Graphene Synthesis by Plasma‐Enhanced Chemical Vapor Deposition: Implications for High‐Frequency Applications

AbstractLarge‐area graphene is typically synthesized on rolled‐annealed copper foils, which require transferring to other substrates for applications. This study examines large‐area graphene growth on electrodeposited (ED) copper foils—used in lithium‐ion batteries and printed circuit boards—via plasma‐enhanced chemical vapor deposition (PECVD). It reveals that, for a set plasma power, a minimum growth time ensures full graphene coverage, leading to monolayer and then multilayer graphene, showing PECVD growth on ED copper is not self‐limited. The process also beneficially modifies the ED copper substrate, like removing the surface zinc layer and changing copper grain size and orientation, thus improving graphene growth. Additionally, the study includes high‐frequency scattering parameter (S‐parameter) measurements in a coplanar waveguide (CPW) system. This involves graphene on a sapphire substrate with a silver electrode. The S‐parameter data indicate that the CPW with graphene shows reduced insertion losses in high‐frequency circuits compared to those without graphene. This underscores graphene's role in reducing insertion losses between metallic and dielectric layers in high‐frequency settings, offering valuable insights for industrial and technological applications.

Dissociation of [formula omitted] misfit dislocation at the interface of α-Ga2O3 thin film deposited on m-plane sapphire

In this study, α-Ga2O3 films of thickness about 1 μm were epitaxially deposited on m-plane sapphire substrate by halide vapor phase epitaxy. The lattice mismatch between α-Ga2O3 and sapphire substrate is about 3.5 % along c-axis and 4.8 % along a-axis, resulting in the formation of misfit dislocations. High-angle annular dark-field scanning transmission electron microscopy was employed to study the misfit dislocations at the α-Ga2O3/sapphire interface. The study was carried out along the [112‾0] zone axis and focused on the dislocations responsible for a misfit strain relaxation along c-axis. The resulting magnitude of the projected Burgers vector onto (112‾0) plane has been compared with the estimated values for 13<1‾101> and 13<2‾021> dislocations. It has been revealed, that misfit dislocations tend to dissociate into partial dislocations at the interface.