Low Full Width Research Articles

Influence of the PALE growth temperature on quality of MOVPE grown AlN/Si (111)

In the present study, the PALE-AlN (pulsed atomic layer epitaxy) epilayers were grown on the Si (111) substrates at different growth temperatures by metal organic vapor phase epitaxy (MOVPE) technique. The oxygen (O) and silicon (Si) concentrations of grown PALE-AlN epilayers and interface between epilayer and substrate were investigated by secondary ion mass spectroscopy (SIMS). It was observed that O and Si concentration change with growth temperature of epilayers as well as the interface significantly. HRXRD (high-resolution x-ray diffraction) analyses showed that the highest growth temperature results with the lowest full width at half maximum (FWHM) value for both ɷ scans. Scanning electron microscope (SEM) and atomic force microscopy (AFM) analyses indicated that relatively low growth temperature grown samples gave rise to 2D-like growth mode with openings while increased growth temperature resulted in change the growth mode to a columnar mode with increasing V-shape pits because of the increasing diffusion coefficient of O impurities and Si atoms in AlN epilayers.

Dependence of the Generation Behavior of Charged Nanoparticles and Ag Film Growth on Sputtering Power during DC Magnetron Sputtering

Effects of sputtering power on the deposition rate and microstructure, crystallinity, and electrical properties of Ag films during direct current (DC) magnetron sputtering are investigated. Thin films (~ 100 nm) are deposited at sputtering powers of 10, 20, 50, 100, 200 and 300 W and analyzed by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and a four-point probe. The film deposited at a sputtering power of 10 W has the lowest growth rate, but the highest crystalline quality, with the lowest full width at half maximum (FWHM) and the lowest resistivity. The film deposited at a sputtering power of 200 W has the highest growth rate, and the second best crystalline quality in view of FWHM and resistivity. The film deposited at a sputtering power of 50 W has the moderate growth rate, and the worst crystalline quality in view of FWHM and resistivity. High-resolution TEM observations reveal that films deposited at sputtering powers of 10 and 200 W have far fewer defects, such as grain boundaries, dislocations and stacking faults than those deposited at a sputtering power of 50 W. Such deposition behavior could be explained by sputtering power, which affected the generation of the charged nanoparticles. And the high quality of films could be obtained at a high deposition rate, in which charge plays an important role.Graphic

Performance Analysis of Apodized Fiber Bragg Gratings for Sensing Applications

Sensing performance of fiber Bragg grating in presence of various apodization functions are studied and compared. Using couple modes theory and matching the fields at various boundaries, the equation of reflectivity of fiber Bragg grating sensors are obtained. The sensors are optimized to get maximum reflectivity with narrow full width half maximum of the resonance peak. It is observed that all considered apodization functions suppress the sidelobes. The suppression of dominating sidelobe is maximum −141.29 dB for Blackman apodization function in all considered cases. The larger sensitivity 6.07 AU/RIU and 6.06 AU/RIU is obtained in Bessel apodization function and Barthann apodization function, respectively. Since, Bessel apodization function shows lower (0.1500 nm) fullwidth half maximum value in comparison to the Barthann apodization function, therefore it gives maximum detection accuracy10329 and quality parameter 40.47 AU/nm-RIU. Followed by Bessel apodization function the Gaussian apodization function also shows high detection accuracy10030 and quality parameter 38.18 AU/nm-RIU due to its lower (0.1545) fullwidth half maximum value. Here the considered Bessel apodization function is recommended for sensing applications.

Subwavelength grating waveguide racetrack-based refractive index sensor with improved figure of merit

An all-subwavelength grating waveguide-based sensing structure for figure of merit (FOM) improvement on a silicon-on-insulator platform is proposed and demonstrated. Four racetrack resonators are applied to narrow the spectrum from the drop port of a single racetrack resonator for lower full width at half-maximum, and the FOM is therefore higher through the spectrum-narrowing operation. Numerical simulation and analysis illustrate that the proposed structure is able to raise the FOM more than twice compared to a single racetrack resonator, and a high FOM of 1850.57/refractive index unit is achieved.

Delta-doped β -Ga2O3 films with narrow FWHM grown by metalorganic vapor-phase epitaxy

We report on the low-temperature metalorganic vapor-phase epitaxy (MOVPE) growth of silicon delta-doped β-Ga2O3 films with a low full width at half maximum (FWHM). The as-grown films are characterized using secondary-ion mass spectroscopy, capacitance–voltage, and Hall techniques. Secondary ion mass spectroscopy measurements show that surface segregation is the chief cause of a large FWHM in MOVPE-grown films. The surface segregation coefficient (R) is observed to reduce with reduction in the growth temperature. Films grown at 600 °C show an electron concentration of 9.7 × 1012 cm−2 and a FWHM of 3.2 nm. High resolution scanning/transmission electron microscopy of the epitaxial film did not reveal any observable degradation in the crystal quality of the delta sheet and surrounding regions. Hall measurements of the delta-doped film on the Fe-doped substrate showed a sheet charge density of 6.1 × 1012 cm−2 and a carrier mobility of 83 cm2/V s. Realization of sharp delta doping profiles in MOVPE-grown β-Ga2O3 is promising for high performance device applications.

Development of a Gd2O2S:Tb Phosphor Screen with an Anti-reflective Layer Fabricated Using Poly Chloro-para-xylylene to Improve Optical Properties in Radiography

In this study, we developed a Gd2O2S:Tb screen with a parylene-C-based antireflective (AR) layer and evaluated its optical and X-ray response properties; in particular, we focused on developing an AR coating layer to improve transmission efficiency. The phosphor screen with the parylene-Cbased AR layer can be fabricated via screen printing and chemical vapor deposition. In particular, experimental results obtained using the AR layer showed improvements of 1.47% in luminescence intensity, as well as a lower full width at half maximum of 9.40, compared to the case without using the AR layer. We observed that the roughness of the AR layer was higher than that of the phosphor screen; the image resolution improved owing to the narrow distribution of the secondary photons resulting from forward scattering attributable mainly to Mie scattering. These results indicate that parylene-C is suitable for developing AR coating materials to improve exposure efficiency while maintaining resolution in radiography systems. Therefore, we believe that the number of photons would be further increased if the AR layer were utilized in commercial products.

Influences of thickness and temperature of low temperature GaAs buffer layer on two-step MOVPE grown GaAs/Ge heterostructures

We investigate influence of GaAs buffer layer (BL) growth parameters such as temperature and thickness on the structural, morphological, crystalline and optical quality of metal organic vapor phase epitaxy (MOVPE) grown heterostructures of GaAs on Ge. It was found that the optimal BL conditions significantly decrease the effects of anti-phase boundaries (APBs) even when grown on offcut Ge substrate by two-step growth technique with AsH3 pre-flow to promote double atomic step formation. It is observed that as the growth temperature increases, the growth rate of the GaAs BL increases, too. Improvement on the structural quality is observed up to BLs temperature of 535 °C, then it decreases. On the other hand, as the different thick BLs, 12, 25, 75 nm are considered, the epilayer grown on the 25 nm thick BL has shown the lowest full width at half maximum (FWHM) value, large photoluminescence peak intensity and internal quantum efficiency (IQE).

Heteroepitaxy of GaAsP and GaP on GaAs and Si by low pressure hydride vapor phase epitaxy

Direct heteroepitaxy of GaAsP and GaP on GaAs and on Si by low-pressure hydride vapor phase epitaxy (HVPE) is investigated as prior studies for photovoltaics and non-linear optics applications. When growing GaAsP on GaAs, it is found that the ambient gas during substrate pre-heating influences the ternary composition as well as the crystalline quality of the subsequent growth. GaAs0.72P0.28 with bandgap energy of 1.76 eV has been achieved, which would be suitable for a top cell in Si tandem solar cell structures. Growth of GaP was investigated on planar GaAs as a prior study for realizing orientation patterned (OP) GaP on OP-GaAs. Threading dislocations caused by the 3.6% lattice mismatch between GaP and GaAs are suppressed by adjusting the GaCl flow, achieving a low full width at half maximum of 146 arcsec for the X-ray diffraction omega scan. Direct heteroepitaxy of GaAsP on Si aiming for achieving a GaAsP/Si dual junction solar cell is demonstrated. The inherent problem of initiating nucleation during the direct heteroepitaxy of III-V on Si by HVPE is overcome by utilizing the vapor mixing approach to grow a low-temperature GaP buffer layer on Si, followed by a GaAsP layer grown by conventional HVPE.

Wafer-scale mono-crystalline GeSn alloy on Ge by sputtering and solid phase epitaxy

Germanium-tin (GeSn) alloys are of great interest for electronic as well as photonic applications, and their integration on a group-IV (Si or Ge) platform. This letter describes a low deposition temperature, process integration-friendly, high throughput and low cost approach for growing device-quality GeSn epilayers on Ge substrates. Mono-crystalline, wafer-scale GeSn alloy with 3.4% Sn content was grown on Ge (100) substrates by sputtering an amorphous GeSn layer followed by solid phase epitaxy (SPE) at . Deposition of amorphous layers at room temperature enables incorporation of high bulk Sn content in GeSn epilayers. SPE of the amorphous layer at leads to a strained, high quality GeSn epilayer having a low full width half maximum of , root mean square surface roughness of 0.3 nm and a good GeSn/Ge interface as confirmed through extensive x-ray diffraction measurements, atomic force microscopy and transmission electron microscopy imaging. Additionally, an SiO2 capping layer employed during SPE is shown to prevent Sn out-diffusion and segregation at the surface. To examine electrical properties for potential applications of the GeSn/n-Ge heterostructure, metal/GeSn/n-Ge vertical contact diodes and metal/GeSn circular transfer length method contact structures were fabricated and electrically characterized.

Growth Control of High-Performance InAs/GaSb Type-II Superlattices via Optimizing the In/Ga Beam-Equivalent Pressure Ratio**Supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0402402 and 2016YFB0400601), the National Basic Research Program of China (Grant No. 2015CB351902), the National Science and Technology Major Project (2018ZX01005101-010), the National Natural Science Foundation of China (Grant Nos. 61835011 and U1431231), the

The performance of type-II superlattice (T2SL) long-wavelength infrared devices is limited by crystalline quality of T2SLs. We optimize the process of growing molecular beam epitaxy deposition T2SL epi-layers on GaSb (100) to improve the material properties. Samples with identical structure but diverse In/Ga beam-equivalent pressure (BEP) ratio are studied by various methods, including high-resolution x-ray diffraction, atomic force microscopy and high-resolution transmission electron microscopy. We find that appropriately increasing the In/Ga BEP ratio contributes to improving the quality of T2SLs, but too large In BEP will much more easily cause a local strain, which can lead to more InSb islands in the InSb interfaces. The InSb islands melt in the InSb interfaces caused by the change of chemical potential of In atoms may result in the “nail” defects covering the whole T2SLs, especially the interfaces of GaSb-on-InAs. When the In/Ga BEP ratio is about 1, the T2SL material possesses a lower full width at half maximum of +1 first-order satellite peak, much smoother surface and excellently larger area uniformity.

Crystal structure, phonon modes, and bond characteristics of AgPb2B2V3O12 (B = Mg, Zn) microwave ceramics

AbstractAgPb2B2V3O12 (B = Mg, Zn) ceramics with low sintering temperature were synthesized via the conventional solid‐state reaction route. Rietveld refinements of the X‐ray diffraction patterns confirm cubic symmetry with space group . The number of observed vibrational modes and those predicted by group theoretical calculations also confirm the space group. At the optimum sintering temperature of 750°C/4 hours, AgPb2Mg2V3O12 has a relative permittivity of 23.3 ± 0.2, unloaded quality factor () of 26 900 ± 500 GHz (), and temperature coefficient of resonant frequency of 19.3 ± 1 ppm/°C, while AgPb2Zn2V3O12 has the corresponding values of 26.4 ± 0.2, 28 400 ± 500 GHz () and –18.4 ± 1 ppm/°C at 590°C/4 hours. Microwave dielectric properties of a few reported garnets and Pb2AgB2V3O12 (B = Mg, Zn) ceramics were correlated with their intrinsic characteristics such as the Raman shifts as well as width of A1g Raman bands. Higher quality factor was obtained for lower full width at half‐maxima (FWHMs) values of A1g modes. The increase in B‐site bond valence contributes to high and low |τf| with the substitution of Zn2+ by Mg2+. Furthermore, the high ionic polarizability and unit cell volume with Zn2+substitution contribute to increased relative permittivity.

Correlation of Pulsed Gas Flow on Si-doped α-Ga2O3 Epilayer Grown by Halide Vapor Phase Epitaxy

Gallium oxide (Ga2O3) is the one of the ultra-wide-band-gap semiconductor material. The Ga2O3 semiconductor has been studied for use in highly energy efficient devices. Among its phases, the β-phase of Ga2O3 has mainly been researched until recently. However, the α-phase, which has superior properties (e.g., wider band-gap and higher symmetry) has been discussed in the electronic-materials field of late. There are problems that need to be overcome in order for the α-phase to be a suitable electronic material; one of them is the production of high-quality thin films. This article reports the fabrication of a high-quality Si-doped α-Ga2O3 epilayer on α-Al2O3 substrate by halide vapor-phase epitaxy with the controlled pulse flow of each precursor. The α-Ga2O3 epilayer grown with a controlled pulse flow of O2 has a lower full width at half maximum (FWHM; 1306 arcsec) for the 10–14 diffraction compared to the FWHM (2011 arcsec) of the as-grown sample. The pulse-flow mode of introducing Ga and O2 strongly influences the crystal quality of α-Ga2O3. In the O2-control mode, we demonstrated Si-doped α-Ga2O3 epilayers with higher electron mobility up to 51.57 cm2 V−1 s−1 and wider carrier concentrations range of 1017 ∼ 1019 cm−3 by improving the crystal quality.

Growth and characterization of Ti:MgAl<sub>2</sub>O<sub>4</sub> laser crystal by Czochralski method

The melting point of Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is as high as 2130 °C, it is a challenge to obtain a large-sized and high-quality laser crystal. By optimizing the crystal growth process, Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal with a size of 30 mm× 70 mm is successfully grown by the Czochralski method under the condition of weak reducing atmosphere. The X-ray diffraction pattern is studied, and the x-ray rocking curve indicates that the grown crystal has a high crystalline quality in terms of the lower full width at half maximum(FWHM) intensity, which provides a material basis for the next laser output experiment. In a range of 100–1000 cm<sup>–1</sup>, there are four Raman vibration peaks located at 312, 410, 675 cm<sup>–1</sup> and 771 cm<sup>–1</sup> respectively. The grown crystal has an absorption cutoff range of 250–318 nm and two wide absorption bands of 395–495 nm and 550–1100 nm. Excited by 271 nm, the grown crystal shows a strong broadband emission ina range of 340–650 nm with a peak centered at 480 nm. After annealing in hydrogen atmosphere, shape of the transmittance spectrum and emission spectrum are both unchanged, but the fluorescent emission intensity is significantly reduced. After annealing in air atmosphere, the original two absorption bands disappear while none of the characteristics of fluorescence emission in a 340–650 nm range changes significantly. In addition, a new fluorescence emission peak near 725 nm is observed. Combining with the ESR spectrum, what we canconfirm is that the Ti:MgAl<sub>2</sub>O<sub>4</sub> as-grown crystal contains Ti<sup>3+</sup> and Ti<sup>4+</sup> ions, and no ESR signal of Ti<sup>3+</sup> is observed after annealing in air atmosphere. Moreover, excitationspectrum is also recorded. The fluorescence lifetime is 14 μs at room temperature, which is 4–5 times that of Ti:Al<sub>2</sub>O<sub>3</sub> crystal and Ti:BeAl<sub>2</sub>O<sub>4</sub> crystal. Furthermore, the emission cross section of the grown Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is calculated from the Füchtbauer-Ladenburg (F-L) formula and its value is 2 × 10<sup>–20</sup> cm<sup>2</sup>, large emission cross section which is beneficial for realizing laser oscillation. All the above results show that the Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is a potential crystal material for realizing broadband tunable blue laser output.

Exposed-core localized surface plasmon resonance biosensor

We propose and numerically characterize an exposed-core photonic crystal fiber for biosensing applications. Surface plasmons are excited within gold (Au) strips, and titanium dioxide is employed to support adhesion of Au with glass. In consideration of ease of fabrication, only four air holes are used to simplify the sensor structure. Simulation results show an improved wavelength and amplitude sensitivity of 34,000 nm/RIU and 1170 RIU−1, respectively, that comes with a low confinement loss of 0.79 dB/cm. Results also indicate a low full width at half-maximum that contributes to high figure of merit of 310.

AlGaN/AlN MOVPE heteroepitaxy: pulsed co-doping SiH4 and TMIn

We report a new growth approach pulsed co-doping growth of AlxGa1−xN (x > 0.5) epilayers on AlN/Al2O3 templates by metal organic vapor phase epitaxy (MOVPE). Using this approach SiH4 (silane) and TMIn (trimethylindium) supplied to the growth chamber alternately and pulsed during the growth of AlGaN epilayers. Structural and morphological quality of AlGaN epilayers were investigated by high resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), Raman spectroscopy, and scanning electron microscopy (SEM) techniques. It has shown that higher crystalline quality with low full width at half maximum (FWHM) and smoother surface morphology with reduced hexagonal hillock density has been obtained by the pulsed co-doping growth approach. Volcano like hillock structures has been confirmed by Raman mapping.

Electro-Precipitation of Actinides on Boron-Doped Diamond Thin Films for Solid Sources Preparation for High-Resolution Alpha-Particle Spectrometry

In this work, we investigate a novel approach to prepare high-performance alpha-particle solid sources fabricated on diamond thin support layers, offering the properties of diamond such as a low-Z material with corrosion and mechanical hardness. As-prepared solid sources onto boron-doped-diamond (BDD) substrate exhibited high performance of the autoradiography and spectroscopic resolution at the level of other more conventional materials such as stainless steel. A straightforward precipitation process in the Na2SO4 or NaNO3 simple electrolytes under mild experimental conditions with a low current of several mA.cm−2 were successfully developed onto BDD substrates for deposition of single 241Am as well as 239Pu, 241Am, and 244Cm mixed radionuclides. The results demonstrate that solid sources deposited onto such BDD substrates can match the performance of those prepared onto stainless steel substrates with excellent uniformity and high-resolution spectroscopy, together combining the robustness, chemical resilience, and X-ray transparence of the diamond. Alpha-particle spectra exhibiting a low full width at half maximum (FWHM) of 12.5 keV at the energy of 5.485 MeV (241Am) could be practically obtained for BDD substrates.

Ultrahigh-Detectivity Photodetectors with Van der Waals Epitaxial CdTe Single-Crystalline Films.

Van der Waals epitaxy (vdWE) is crucial for heteroepitaxy of covalence-bonded semiconductors on 2D layered materials because it is not subject to strict substrate requirements and the epitaxial materials can be transferred onto various substrates. However, planar film growth in covalence-bonded semiconductors remains a critical challenge of vdWE because of the extremely low surface energy of 2D materials. In this study, direct growth of wafer-scale single-crystalline cadmium telluride (CdTe) films is achieved on 2D layered transparent mica through molecular beam epitaxy. The vdWE CdTe films exhibit a flat surface resulting from the 2D growth regime, and high crystal quality as evidenced by a low full width at half maximum of 0.05° for 120 nm thick films. A perfect lattice fringe appears at the interfaces, implying a fully relaxed state of the epitaxial CdTe films correlated closely to the unique nature of vdWE. Moreover, the vdWE CdTe photodetectors demonstrate not only ultrasensitive optoelectronic response with optimal responsivity of 834 A W-1 and ultrahigh detectivity of 2.4 × 1014 Jones but also excellent mechanical flexibility and durability, indicating great potential in flexible and wearable devices.

Semi-polar (11–22) GaN epitaxial films with significantly reduced defect densities grown on m-plane sapphire using a sequence of two in situ SiNx interlayers

This letter reports an approach for growing semipolar (11–22) GaN films with significantly reduced defect densities on m-plane sapphire substrates by incorporating a sequence of two in situ SiNx layers. The first SiNx layer is deposited on an initial epitaxial GaN layer and acts as a nanomask for preventing the propagation of extended defects. The second SiNx layer is deposited just after subsequent epitaxial GaN overgrowth begins to form self-organized GaN islands, which encourages further GaN growth to initiate from the exposed island sidewalls while blocking the penetration of the remaining defects. X-ray rocking curve measurements show that our semipolar GaN films provide relatively low full width at half maximum values at 0.119° along both the [11–23] and [10–10] directions. Additionally, transmission electron microscopy analyses confirm that our semipolar GaN films provide a significantly reduced density of threading dislocations down to ∼6 × 108 cm−2, which is two orders of magnitude less than those of conventionally deposited films.

Nearly lattice-matched molybdenum disulfide/gallium nitride heterostructure enabling high-performance phototransistors

Molybdenum disulfide (MoS2)-based phototransistors are attractive for optical electronics in a large-scale size, such as transparent touch screens. However, most of the work done over the past decade has been on an opaque SiO2/Si wafer with a small size (micrometer to millimeter). In this work, a large-scale multilayer MoS2-based phototransistor has been fabricated on a transparent freestanding gallium nitride (GaN) wafer using a scalable chemical vapor deposition method. Due to the near lattice match and small thermal expansion mismatch between GaN and MoS2, the as-grown multilayer MoS2-on-GaN film shows high material quality in terms of low full width at half-maximum (∼5.16 cm−1) for the E2g1 Raman mode and a high absorption coefficient (∼106 cm−1) in the wavelength range of 405–638 nm. Under a wavelength of 405 nm at an incident power of 2 mW and applied voltage of 9 V, the fabricated MoS2-on-GaN phototransistor achieved a maximum responsivity of 17.2 A/W, a photocurrent gain of 53.6, and an external quantum efficiency of 5289%, with specific detectivity (∼1010–1012 Jones) and low noise equivalent power (10−12–10−14 W/Hz1/2) in the visible range of 405–638 nm. A typical response time of 0.1–4 s in the ambient air has also been recorded for the demonstrated MoS2-on-GaN phototransistor. Our work paves a technologic stepping stone for MoS2-based phototransistors for multifunctional transparent and touch-based optoelectronics in the future.

The contribution of AsH3: Pre-flow and V/III ratio effects on heteroepitaxially grown GaAs/Ge

We report the effects of AsH3 (arsine) on morphological, structural, optical and crystalline quality of MOVPE (metal organic vapor phase epitaxy) grown GaAs/Ge heterostructures. AsH3 pre-flow supplied on Ge substrate to have As atoms as a first atomic layer on the surface of Ge substrate. Additionally, the V/III ratio effects of LT-GaAs buffer layer, grown on arsenized Ge substrate, have been analyzed to have reduced APBs (anti-phase boundaries) in the interface between epilayer and Ge substrate. It has been considered that the optimal AsH3 pre-flow duration and V/III ratio of GaAs buffer layer extremely influence the effects of APBs even we have used miss-cut Ge substrate and grown by two-step growth technique to obtain double atomic step. It is shown that without AsH3 pre-flow the surface of GaAs epilayer becomes rougher while it is optically smooth under the longer AsH3 pre-flow. On the other hand, even the surface situation does not change with longer AsH3 pre-flow the structural, optical and crystalline qualities become worse because of the possible presence vacancies of created during the growth. Similar behavior has been observed for the V/III effects of GaAs buffer layer and it has resulted in low full width at half maximum of high-resolution X-ray diffraction and high photoluminescence peak intensity for the optimal V/III ratio.