Sapphire Substrates Research Articles

Modeling of Material Removal Rate for the Fixed-Abrasive Double-Sided Planetary Grinding of a Sapphire Substrate.

Double-sided planetary grinding (DSPG) with a fixed abrasive is widely used in sapphire substrate processing. Compared with conventional free abrasive grinding, it has the advantages of high precision, high efficiency, and environmental protection. In this study, we propose a material removal rate (MRR) model specific to the fixed-abrasive DSPG process for sapphire substrates, grounded in the trajectory length of abrasive particles. In this paper, the material removal rate model is obtained after focusing on the theoretical analysis of the effective number of abrasive grains, the indentation depth of a single abrasive grain, the length of the abrasive grain trajectory, and the groove repetition rate. To validate this model, experiments were conducted on sapphire substrates using a DSPG machine. Theoretical predictions of the material removal rate were then juxtaposed with experimental outcomes across varying grinding pressures and rotational speeds. The trends between theoretical and experimental values showed remarkable consistency, with deviations ranging between 0.2% and 39.2%, thereby substantiating the model's validity. Moreover, leveraging the insights from this model, we optimized the disparity in the material removal rate between two surfaces of the substrate, thereby enhancing the uniformity of the machining process across both surfaces.

Effect of plasma discharge pulse length for GaN film crystallinity on sapphire substrate by high density convergent plasma sputtering device

Effect of plasma discharge pulse length for GaN film crystallinity on sapphire substrate by high density convergent plasma sputtering device

Modulating growth of graphene on sapphire by chemical vapor deposition

The direct growth of graphene on insulating substrates presents significant promise for the advancement of next-generation semiconductor technologies. Here, the synthesis of graphene directly on sapphire substrates via chemical vapor deposition (CVD) is reported. The CVD growth parameters are systematically manipulated to optimize the crystal quality of graphene. Furthermore, the dependency of graphene growth on the crystallographic orientation of the sapphire substrate is explored to elucidate the underlying mechanisms. It is found that robust C-O chemical bonds are established between the oxygen atoms on the sapphire surface and the carbon atoms in the graphene, predominantly influencing the nucleation process. The oxygen-rich nature of the c-plane sapphire surface contributes to a higher graphene nucleation density and the formation of smaller grains comparing to r-plane sapphire. Accordingly, high quality and uniform monolayer graphene is successfully obtained on both c-plane and r-plane sapphire.

Large-area and few-layered 1T′-MoTe2 thin films grown by cold-wall chemical vapor deposition

This study employs cold-wall chemical vapor deposition to achieve the growth of MoTe2 thin films on 4-inch sapphire substrates. A two-step growth process is utilized, incorporating MoO3 and Te powder sources under low-pressure conditions to synthesize MoTe2. The resultant MoTe2 thin films exhibit a dominant 1T′ phase, as evidenced by a prominent Raman peak at 161 cm−1. This preferential 1T′ phase formation is attributed to controlled manipulation of the second-step growth temperature, essentially the reaction stage between Te vapor and the pre-deposited MoO x layer. Under these optimized growth conditions, the thickness of the continuous 1T′-MoTe2 films can be precisely tailored within the range of 3.5–5.7 nm (equivalent to 5–8 layers), as determined by atomic force microscopy depth profiling. Hall-effect measurements unveil a typical hole concentration and mobility of 0.2 cm2 Vs−1 and 7.9 × 1021 cm−3, respectively, for the synthesized few-layered 1T′-MoTe2 films. Furthermore, Ti/Al bilayer metal contacts deposited on the few-layered 1T′-MoTe2 films exhibit low specific contact resistances of approximately 1.0 × 10−4 Ω cm2 estimated by the transfer length model. This finding suggests a viable approach for achieving low ohmic contact resistance using the 1T′-MoTe2 intermediate layer between metallic electrodes and two-dimensional semiconductors.

Microstructure and reflectance of porous GaN distributed Bragg reflectors on silicon substrates

Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage.

Dislocation half-loop control for optimal V-defect density in GaN-based light emitting diodes

V-defects are morphological defects that typically form on threading dislocations during epitaxial growth of (0001)-oriented GaN layers. A V-defect is a hexagonal pyramid-shaped depression with six {101¯1}-oriented sidewalls. These semipolar sidewalls have a lower polarization barrier than the polarization barriers present between the polar c-plane quantum wells and quantum barriers and can laterally inject carriers directly into quantum wells in GaN-based light emitting diodes (LEDs). This is especially important, as the high polarization field in c-plane GaN is a significant factor in the high forward voltage of GaN LEDs. The optimal V-defect density for efficient lateral carrier injection in a GaN LED (∼109 cm−2) is typically an order of magnitude higher than the threading dislocation density of GaN grown on patterned sapphire substrates (∼108 cm−2). Pure-edge dislocation loops have been known to exist in GaN, and their formation into large V-defects via low-temperature growth with high Si-doping has recently been studied. Here, we develop a method for pure-edge threading dislocation half-loop formation and density control via disilane flow, growth temperature, and thickness of the half-loop generation layer. We also develop a method of forming the threading dislocation half-loops into V-defects of comparable size to those originating from substrate threading dislocations.

Double barrier structure of Mo/AlN/Mo/AlN/Mo multilayer and its resonant tunneling effect

We prepare a symmetrical double barrier structure of Mo/AlN/Mo/AlN/Mo by using the reactive magnetron sputtering on a sapphire substrate and examine its resonant tunneling effect. Characterizations of multilayers demonstrate that the AlN (0002) aligns well along the Mo (110). Direct current-voltage (DC I–V) test finds significant negative differential resistance at room temperature around 0.7 and 0.9 V carrier energy and with peak-to-valley current ratios of about 1.1 and 1.2. The simulation using conventional transfer matrix method suggests good consistence between the designed structure, the quantized energy level and the switching voltage. The large signal DC model also fits the experimental I–V properties. Our work provides a new type of resonant tunneling device in room temperature.

High-performance solar-blind photodetector based on Si-doped α-Ga2O3 thin films grown by mist chemical vapor deposition

α-Ga2O3 has gained increasing attention in the field of photodetectors because of its wide bandgap and exceptional chemical and physical properties. Si-doped α-Ga2O3 single-crystal epitaxial films were successfully deposited on sapphire substrates using an affordable and non-vacuum-based mist chemical vapor deposition (Mist-CVD) technique. An interdigital metal-semiconductor-metal (MSM) photodetector with excellent performance was prepared by using the thin film. At 254 nm illumination and a bias voltage of 20 V, the photodetector has a high light-to-dark current ratio of 3.24×106, a responsivity of 3.23×102 A/W, a detectivity of 8.80×1015 Jones, and an external quantum efficiency of 1.58×105%. The results show that Si-doped α-Ga2O3 thin films have great potential for application in high-performance photodetectors.

Growth of α-Ga2O3 from Gallium Acetylacetonate under HCl Support by Mist Chemical Vapor Deposition.

α-Ga2O3 films were grown on a c-plane sapphire substrate by HCl-supported mist chemical vapor deposition with multiple solution chambers, and the effect of HCl support on α-Ga2O3 film quality was investigated. The growth rate monotonically increased with increasing Ga supply rate. However, as the Ga supply rate was higher than 0.1 mmol/min, the growth rate further increased with increasing HCl supply rate. The surface roughness was improved by HCl support when the Ga supply rate was smaller than 0.07 mmol/min. The crystallinity of the α-Ga2O3 films exhibited an improvement with an increase in the film thickness, regardless of the solution preparation conditions, Ga supply rate, and HCl supply rate. These results indicate that there is a low correlation between the improvement of surface roughness and crystallinity in the α-Ga2O3 films grown under the conditions described in this paper.

Study of geometry effect on the performance of thin-film transistors fabricated with ZnGa2O4 epilayer grown by metalorganic chemical vapor deposition for high voltage applications

Enhancement mode thin film transistors (TFTs) having different geometries were fabricated on the Zinc gallium oxide (ZnGa2O4) epilayer grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). A field emission scanning electron microscope, atomic force microscopy, and X-ray diffraction were employed to conduct a detailed analysis of the film composition, morphology, and crystal structure. The distance between the source-to-gate and gate length was kept to be 7 μm and 3 μm in all the devices with varying gate-to-drain lengths of 10, 20, and 30 μm. Moreover, neutral beam etching technology was employed to isolate the individual devices and reduce the defects induced by plasma etching. It was found that the threshold voltage changes from 6 V to 7.6 V with the increase in source to drain length from 20 μm to 40 μm. The On/Off ratio of the devices was found to be 105, with a maximum current around 1 μA/mm. Additionally, the device breakdown voltage increased from 240 V to about 656 V with the increase in the device length. The results demonstrate the fabrication of the high breakdown voltage ZnGa2O4-based TFT and establish the co-relation of the source to drain length with the device characteristics.

Growth of Monolayer MoS2 Flakes via Close Proximity Re-Evaporation.

We report a two-step growth process of MoS2 nanoflakes using a low-pressure chemical vapor deposition technique. In the first step, a MoS2 layer was synthesized on a c-plane sapphire substrate. This layer was subsequently re-evaporated at a higher temperature to form mono- or few-layer MoS2 flakes. As a result, the close proximity re-evaporation enabled the growth of pristine MoS2 nanoflakes. Atomic force microscopy analysis confirmed the synthesis of nanoclusters/nanoflakes with lateral dimensions of over 10 μm and a flake height of approximately 1.3 nm, demonstrating bi-layer MoS2, whereas transmission electron microscopy analysis revealed triangular MoS2 nanoflakes, with a diffraction pattern proving the presence of single crystalline hexagonal MoS2. Raman data revealed the typical modes of high-quality MoS2 nanoflakes. Finally, we presented the photocurrent dependence of a MoS2-based photoresist under illumination with light-emitting diode of 405 nm wavelength. The measured current-voltage dependence across various luminous flux outlined the sensitivity of MoS2 to polarized light and thus opens further opportunities for applications in high-performance photodetectors with polarization sensitivity.

Influence of quantum well thickness on the Cathodoluminescent properties of AlGaN-based UVC light source tube with CNT field emitters

Far ultraviolet C (UVC) light sources work well for a variety of applications, such as water purification, bacteria and virus sterilization, and sensing, because of their short wavelength. Herein, the recognized problems with a conventional UVC-LED are solved by creating a triode structure UVC light source tube (UVC-LST) operating at ∼268 nm employing a carbon nanotube (CNT) electron emitter as an excitation source. The UVC light emission of an aluminum reflector-capped AlGaN multiple quantum well (MQW) heterostructures synthesized on a nanopatterned sapphire substrate based on a cathodoluminescence (CL) layer is assessed. For luminescence results comparison, the heterostructure of AlGaN-based MQWs was fabricated with various QW layer thicknesses of 1, 1.5, and 2 nm. The utilization of a thinner QW layer with a thickness of 1 nm significantly improved the UVC light emission efficiency compared to the UVC-LSTs fabricated with thicker QW layers of 1.5 nm and 2 nm. The UVC-LST with the 1 nm QW layer exhibited approximately a 30 % improvement in light output power compared to the UVC-LST with a 1.5 nm QW layer, and closely a 33 % improvement compared to the UVC-LST with a 2 nm QW layer. Moreover, the UVC-LST with the thinner QW layer demonstrated a high power efficiency of 5.24 % when operating on a 2-inch light-emitting area, while consuming a low power of 7 W. This high power efficiency indicates that a significant portion of the input electrical power is effectively converted into UVC light output, highlighting the superior performance of the UVC-LST based on the 1 nm QW layer design.

Structural characterization of threading dislocation in α-Ga2O3 thin films on c- and m-plane sapphire substrates

We discuss the structure of threading dislocations in α-Ga2O3 thin films grown on c- and m-plane sapphire substrates. The thickness-dependent threading dislocation density in both films directly affects the electrical properties of the films including carrier concentration and mobility. Two distinct types of threading dislocations are identified for each of the c- and m-plane α-Ga2O3 thin films. The c-plane α-Ga2O3 thin film shows Burgers vectors of 1/3[11¯01] and 1/3[112¯0], while the m-plane α-Ga2O3 thin film displays Burgers vectors of 1/3[21¯1¯0] and 1/3[11¯01]. This paper presents a detailed structure of the threading dislocations in α-Ga2O3, which has been little disclosed thus far mainly due to the difficulty in synthesizing the metastable α-Ga2O3.

On-Wire Design of Axial Periodic Halide Perovskite Superlattices for High-Performance Photodetection.

Precise synthesis of all-inorganic lead halide perovskite nanowire heterostructures and superlattices with designable modulation of chemical compositions is essential for tailoring their optoelectronic properties. Nevertheless, controllable synthesis of perovskite nanostructure heterostructures remains challenging and underexplored to date. Here, we report a rational strategy for wafer-scale synthesis of one-dimensional periodic CsPbCl3/CsPbI3 superlattices. We show that the highly parallel array of halide perovskite nanowires can be prepared roughly as horizontally guided growth on an M-plane sapphire. A periodic patterning of the sapphire substrate enables position-selective ion exchange to obtain highly periodic CsPbCl3/CsPbI3 nanowire superlattices. This patterning is further confirmed by micro-photoluminescence investigations, which show that two separate band-edge emission peaks appear at the interface of a CsPbCl3/CsPbI3 heterojunction. Additionally, compared with the pure CsPbCl3 nanowires, photodetectors fabricated using these periodic heterostructure nanowires exhibit superior photoelectric performance, namely, high ION/IOFF ratio (104), higher responsivity (49 A/W), and higher detectivity (1.51 × 1013 Jones). Moreover, a spatially resolved visible image sensor based on periodic nanowire superlattices is demonstrated with good imaging capability, suggesting promising application prospects in future photoelectronic imaging systems. All these results based on the periodic CsPbCl3/CsPbI3 nanowire superlattices provides an attractive material platform for integrated perovskite devices and circuits.

Investigation and Comparison of the Performance for β-Ga2O3 Solar-Blind Photodetectors Grown on Patterned and Flat Sapphire Substrate

Ga2O3, with its large band gap, is a promising material suitable for utilization in solar-blind photodetection. Sapphire with a higher lattice match with Ga2O3 was used as the substrate for epitaxial growth of Ga2O3. Here, the epitaxial layers of Ga2O3 were deposited by MOCVD on patterned sapphire substrates. The structure of epitaxial Ga2O3 layers on patterned substrates has been identified by X-ray diffractometry. To investigate the influence of the patterned substrates on the formation of epitaxial layers, thin Ga2O3 layers were grown on a flat sapphire substrate under the same conditions. Both types of samples were β-phase. However, no improvement in the layers’ crystalline quality was discovered when utilizing patterned sapphire substrates. In addition, the performance of the obtained two types of Ga2O3 photodetectors was compared. The photoelectric properties, such as responsivity, response speed, and detection capability, were different in the case of flat samples.

Diagnostics of phase composition of lithium niobate films on sapphire by principal component analysis using Raman spectra

Thin films of lithium niobate were deposited by RF diode sputtering of a LiNbO3 single-crystal target on a heated sapphire substrate (R-cut, 650 °C). Radial distributions of the phase composition of the films along the substrate were obtained by principal component analysis (PCA) using Raman spectra. It was found that at small target-substrate distances (12 mm is the distance between the target and the substrate, 76 mm is the diameters of the target and the substrate holder, 30 mm is the size of the film deposition area in axial geometry), the effects of resputtering of the deposited film do not lead to a considerable decrease in the lithium content: no phases other than LiNbO3 are formed in the extended central part of the film, the edge of the film contains no more than 2 % of the undesirable LiNb3O8 phase, and the difference in the measured values of deviation of LiNbO3 phase composition from stoichiometry in the film compared to the target is 0.7 mol% Li2O. These results indicate that the technology for depositing of lithium niobate films can be improved since there is no need to produce a target additionally enriched with Li2O.

PLD‐Grown Semi‐Insulating Ga2O3 Thin Film‐Based Optoelectronic Artificial Synaptic Devices for Neuromorphic Computing Applications

AbstractDevelopment of artificial opto‐electronic synaptic devices plays a crucial role for the practical implementation of energy‐efficient, parallel processing of human brain. In this article, two terminal inter‐digitated devices are fabricated on Gallium oxide (Ga2O3) thin films grown on sapphire substrates by pulsed laser deposition (PLD) technique to study its ability to mimic biological synaptic behaviors. The layers are found to exhibit long persistent photo‐conductivity (PPC) effect, which is identified to be the key parameter to replicate the behavior of biological synapses. Channel resistance and PPC time constants should also be optimized to improve the efficiency of response and energy consumption of synaptic devices. It has been observed that both conductivity and the PPC decay time of Ga2O3‐films can be controlled by varying oxygen pressure and growth temperature (TG). These devices demonstrate their ability to perform paired pulse facilitation (PPF) at very low applied bias in mV‐range. They can mimic biological synapses showing short‐to‐long‐term memory transition (STM‐to‐LTM) and learning‐forgetting behavior. One of these devices is found to show synaptic behavior with the energy consumption of as low as 71fJ electrical and 21nJ optical per synaptic event. These findings thus strengthen the candidature of Ga2O3 films for the development of next‐generation opto‐electronic neuromorphic devices and systems.

High performance composite phosphor-in-glass film for laser-driven warm white light on patterned sapphire substrate

Phosphor-in-glass film (PiF) based on sapphire substrate (SS) is a promising laser-driven color converter. Herein, by using an optimized silicon borate glass component, high-performance Y3Al5O12:Ce3+ PiFs are successfully prepared on both flat sapphire substrate (FSS) and patterned sapphire substrate (PSS) surfaces. Compared with planar structure of FSS, PSS has a periodic arrangement of conical structures on its surface, which effectively increases the contact area between the PiF and the substrate by approximately 56%, thereby forming a more robust film composite structure. The blue laser light is affected by the structure of PSS and produces a typical diffraction effect, which realizes the dispersion of the convergent beam. This significantly expands the laser spot on PSS-PiF and improves the uniformity of light. The saturation power density of PiF-PSS (6.29 W/mm2) is found to be 91% higher than that of PiF-FSS, while maintaining a high luminous efficiency (220 lm/W) and a low correlated color temperature (4500 K). Finally, the thermal quenching mechanism of PiF with different substrate surface morphologies is compared and analyzed. The present results provide important support for designing the interface structure between SSs and PiF to achieve higher efficiency and higher saturation threshold for warm white light.

Highly-crystalline Vanadium Dioxide by Hydrothermal for Infrared Detectors

Abstract Infrared detectors are utilized across a diverse range of applications, yet they currently face challenges such as high power consumption and sensitivity to environmental. Vanadium dioxide, a popular material for infrared detectors, boasts a superior temperature coefficient of resistance and rapid response speed. However, it suffers from a substantial thermal hysteresis width. To overcome this limitation, the film structure is meticulously designed to exhibit a uniform, dense texture with a high degree of orientation and crystallinity on a sapphire substrate. This enhancement significantly boosts the infrared responsiveness of the detector. Under optimal conditions, specifically a bias voltage of 50 mV and infrared irradiation of 150 mW, the photocurrent can achieve a remarkable value of 5.55 μA, while the response speed reaches an impressive 0.23 μA/s. These improvements hold promising implications for infrared detection technology, particularly in areas such as night vision and electrical maintenance.

High-Power and High-Efficiency 221 nm AlGaN Far Ultraviolet Laser Diodes

The optical features of far ultraviolet laser diodes (UV LDs) with peak wavelength emission of 221 nm have been numerically analyzed. Global research teams are developing aluminum gallium nitride (AlGaN)-based farUV LDs on Sapphire and AlN substrates as an alternative to Mercury lamps for air-water purification, polymer curing, and bio-medical devices. In this study, the light output power, internal quantum efficiency, stimulated recombination rate curve, and optical gain curve of the compositionally graded p-cladding layer (p-CL) were studied and show significant improvements. Therefore, the optimized structure can reduce the overflow of electrons and increase the injection of holes. This approach proves to be an efficient method for enhancing farUV LDs’ overall performance when compared to the reference structure.