Sapphire Films Research Articles

Growth optimization, optical, and dielectric properties of heteroepitaxially grown ultrawide-bandgap ZnGa2O4 (111) thin film

Ultrawide bandgap ZnGa2O4 (ZGO) thin films were grown on sapphire (0001) substrates at various growth temperatures with a perspective to investigate the electrical and optical characteristics required for high-power electronic applications. Due to the variation in the vapor pressure of Zn and Ga, severe loss of Zn was observed during pulsed laser deposition, which was solved by using a zinc-rich Zn0.98Ga0.02O target. A pure phase single-crystalline ZGO thin film was obtained at a deposition temperature of 750 °C and an oxygen pressure of 1 × 10−2 Torr. The out-of-plane epitaxial relationship between the sapphire and ZGO thin film was obtained from φ-scan. The x-ray rocking curve of the ZGO thin film grown at 750 °C exhibits a full width at half maximum of ∼0.098°, which indicates a good crystalline phase and quality of the thin film. Core-level x-ray photoelectron spectroscopy of ZGO grown at 750 °C indicated that Zn and Ga were in the 2+ and 3+ oxidation states, respectively, and the atomic ratio of Zn/Ga was estimated to be ∼0.48 from the fitted values of Zn-2p3/2 and Ga-2p3/2. The high-resolution transmission electron microscopy images revealed a sharp interface with the thickness of the ZGO film of ∼265 nm, and the signature of minor secondary phases was observed. The bandgap of the ZGO film at different growth temperatures was calculated from the ultraviolet-diffuse reflectance spectroscopy spectra, and its value was obtained to be ∼5.08 eV for the 750 °C grown sample. The refractive index (n) and the extinction coefficient (k) were determined to be ∼1.94 and 0.023 from the ellipsometric data, respectively, and the real dielectric function (ɛr) was estimated to be ∼6.8 at energy 5 eV. The ultrawide bandgap and dielectric function of ZGO recommend its possible potential applications in deep-ultraviolet optoelectronic devices and high-power electronics.

Revealing the influence of aerosol particle state on the high-quality α-Ga2O3 single crystal thin film growth by Mist CVD epitaxy

There is little research on α-Ga2O3 single crystal film because it is a metastable phase of Ga2O3 crystal and difficult for controllable fabrication. In this paper, highly crystallized Ga2O3 thin films of poly-crystalline, α-Ga2O3 and β-Ga2O3 were prepared by modulating the growth temperature from 400 °C to 700 °C by mist-assisted CVD (Mist CVD) method. The results show that films grown at 525 °C owes the best crystalline quality of α-Ga2O3 film. To further improve the crystallinity of α-Ga2O3 thin film, a systematic simulation revealing the distribution changes of aerosol particle size, flow field, and temperature gradient in the reaction chamber were explored. The optimal growth region C with uniform particle size, well-distribute temperature gradient and stable flow field is determined. Meanwhile, the experimental results of α-Ga2O3 thin films grown at different positions in the reaction chamber was proved to consistent with the simulation results. Furthermore, the optimal single crystallized α-Ga2O3 thin films with smooth surface morphology and optical property were obtained. The PL spectrum operated at room temperature show the α-Ga2O3 film with low defect density and high crystalline quality. It shows that the single crystallized α-Ga2O3 is orientated grown on sapphire substrate with epitaxial relationship of α-Ga2O3 (0006)//c-Al2O3 (0006). Although, slight lattice mismatch is observed on the selected area electron diffraction pattern of the interface between sapphire and α-Ga2O3 film, it is ideal for further application in devices. This work reveals the key point for α-Ga2O3 single crystal film growth by mist CVD is to construct a steady and orderly environment through controlling aerosol particle state.

Improved detection performance of self-driven InZnO / p-GaN heterojunction UV photodetector by lanthanum doping

By using the sol-gel spin-coating method, La-InZnO films with different La (0 at%, 2 at%, 4 at%, and 6 at%) doping concentrations were grown on p-GaN / sapphire film templates, and self-driven p-GaN / n-ZnO heterojunction ultraviolet (UV) detectors were prepared. The effects of La doping concentration on the microstructure, optical, and electrical properties of synthesized La-InZnO films were studied. In addition, the optical sensing characteristics and photoelectric response performance of four p-n heterojunction UV detectors were compared. La doping can decrease the content of oxygen vacancies. It also reduces the carrier concentration and mobility of the films. At zero bias voltage, the 2 at% doped device has the best performance, with a peak responsivity of 46 mA/W at 366 nm, a high spectral selectivity full width at half maximum (FWHM) of only 6.92 nm, and a fast response with the rise and decay times of 1.15 ms and 2.42 ms, respectively. This study demonstrates a feasible method for improving the performance of self-driven heterojunction UV detectors.

Performance optimization of Ce:YAG sapphire films for high power density white laser-driven lighting applications

To withstand higher power density (PD), exhibit higher luminous efficiency (LE), use in smaller size and lower cost has become the direction of solid-state lighting technology. In the face of high-density laser irradiation and strong thermal shock, improving the performance of color conversion materials related to laser illumination has become a research hotspot. In this study, The Y3Al5O12:Ce3+ phosphor-in-glass film (Ce:YAG-PiF) was obtained by scraping method. The optimized Ce: YAG-PiF external quantum efficiency (EQE) reaches 74.7%, closing to the phosphor (EQE = 78.47%). Excitingly, at 573 K, the thermal conductivity of sample remains at 9.47 W m−1 K−1. Besides, the sample has an ultra-high LE of 256 lm/W @ 32.47 W/mm2. After recombining with the red phosphor CaAlSiN3:Eu2+ (CASN), the color rendering index (CRI) is 84.6, the correlation color temperature (CCT) is 5594 K, and the LE is 192.91 lm/W @ 39.87 W/mm2. More importantly, the relative intensity of laser module packaged by Ce:YAG & CASN-PiF decreased by 15% when tested at 4 W for 24 h. Besides, the packaged laser flashlight has good collimation within a visible illuminance. All the results show that the prepared sample can maintain high emission intensity at high laser PD and have a good heat resistance, which is a promising color conversion material in the field of solid-state laser illumination.

Ultrahigh Power Density LuAG:Ce Green Converters For High‐Luminance Laser‐Driven Solid State Lighting

AbstractLu3Al5O12:Ce (LuAG:Ce) phosphor has been designed and fabricated into various forms of green fluorescent converters and plays a critical role in the field of solid‐state laser lighting. Phosphor in glass (PiG) and phosphor in glass film (PiGF) are expected to be viable strategies for high‐power and high‐brightness laser diode (LD) illumination due to their easy synthesis and low cost. In this study, LuAG:Ce‐PiG with an internal quantum efficiency of 90.5% is prepared by investigating the sintering conditions and combining with a borosilicate glass. It possesses a luminous flux (LF) of 584 lm and luminous efficiency (LE) of 225 lm W−1 @ 20 W mm−2. On this basis, LuAG:Ce‐PiG sapphire film (LuAG:Ce‐PiGSF) with high thermal conduction (10.26 W (m K) −1) is successfully sintered on the sapphire to obtain the LF of 833 lm and LE of 252 lm W−1 @ 36.36 W mm−2. The maximum tolerated power density can reach 45.45 W mm−2. In addition, when the color converters are placed in a commercial LD transmission module and continuously light for 24 h at 16.5 W, the relative light intensity only decreases by 14.15% and 12.82%, respectively. Therefore, it is believed that the work has promising applications in laser illumination and accelerating the development of LD lighting.

Enhanced performance of 275-nm AlGaN-based deep-ultraviolet LEDs via internal-roughed sapphire and SiO2-antireflection film.

The internal-roughed sapphire in a 275-nm AlGaN-based deep-ultraviolet (DUV) LED is fabricated using a laser stealth dicing technique to improve the high-angle extraction. Furthermore, the low-angle extraction is enhanced by depositing a SiO2-antireflection film on the internal-roughed sapphire surface. Compared with conventional DUV LEDs with a light output power (LOP) of 33.05 mW at 350 mA, the LOP of DUV LEDs with internal-roughed sapphire and SiO2-antireflection film increases by 20.85% to 39.94 mW. In addition, combined with finite-difference time-domain simulations, the effect of internal-roughed sapphire on the transmission and light extraction efficiency (LEE) of the DUV LEDs is revealed. The combination of the internal-roughed sapphire substrate and SiO2-antireflection film improves the LEEs of transverse electric (TE) and transverse magnetic (TM) polarized light by 1.6% and 108%, respectively. These results offer the potential for large-scale, low-cost industrial production of high-efficiency DUV LEDs.

Unique sandwich design of high-efficiency heat-conducting phosphor-in-glass film for high-quality laser-driven white lighting

Multi-color phosphor-in-glass (PiG) film has been considered as a promising color converter in high-quality laser lighting owing to its outstanding merits of phosphor versatility, tunable luminescence, and simple preparation. However, the opto-thermal properties of PiG film are severely affected by the photon reabsorption and backward scattering of phosphor structure and the heat conduction of substrate. Herein, a unique sandwich design of phosphor structure was introduced in the multi-color PiG film for high-quality laser lighting. By elaborately synthesizing the borosilicate glass with low glass transition temperature (Tg), similar expansion coefficient, and high refractive index (RI), the sandwiched PiGs were prepared by sintering (~600 °C) broadband green and red phosphor glass films on the double sides of sapphire. The green and red PiG films were tightly coated on the sapphire with no delamination and maintained higher luminescence intensity than raw phosphors at high temperatures. By simultaneously coupling photon reabsorption and backward scattering, the sandwiched green PiG film—sapphire—red PiG film (G—S—R PiG) yields a high-quality white light with a high luminous efficacy of 163 lm/W and an excellent color rendering index (CRI) of 85.4 under a laser power of 2.4 W, which are the best comprehensive results yet reported. Benefiting from the ingenious sandwich design with heat-conducting sapphire and thin PiG films, the G—S—R PiG displays low working temperatures (< 200 °C) under high-power laser excitation. This work reveals the role of sandwiched phosphor structure in photon loss and heat dissipation, which provides a new strategy to design PiG films for high-quality laser lighting.

Electrical and optical properties of degenerate and semi-insulating ZnGa2O4: Electron/phonon scattering elucidated by quantum magnetoconductivity

We study the electrical and optical properties of degenerate ZnGa2O4 films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire and semi-insulating films grown by pulsed laser deposition (PLD) on fused silica. After a forming-gas anneal at 700 °C, the MOCVD film is highly conducting, with a room-temperature carrier concentration of 2 × 1020 cm−3, a mobility of 20 cm2/V s, and direct bandgap absorptions at 3.65 eV and 4.60 eV. Under the same annealing conditions, the PLD film is semi-insulating, with a direct bandgap absorption at 5.25 eV. The phonon structure, important for electrical and thermal conduction as well as superconductivity and other quantum phenomena, is very complicated due to the large number of atoms (and, thus, phonon branches) in the unit cell. However, we show that the phonon contributions to electron mobility (μph) can be directly measured by quantum-based magnetoconductivity over the temperature span T = 10–200 K. From an approximate analytical formula, μph = function (Tph, T), we calculate an effective phonon energy kTph(T) that takes account of all phonon contributions at temperature T. For T = 10–200 K, the value of kTph ranges from about 10 to 90 meV, consistent with the energy range of the ZnGa2O4 phonon density of states (at 0 K) calculated by density functional theory. The total measured mobility can then be modeled by μtot−1 = μii−1 + μph−1, where μii is the mobility due to ionized-impurity scattering. With a high bandgap, controllable conductivity, high breakdown voltage, and bulk-growth capability, ZnGa2O4 offers opportunities for high-power electronics and UV detectors.

Contamination-Free Graphene Transfer from Cu-Foil and Cu-Thin-Film/Sapphire

The separation of graphene grown on metallic catalyst by chemical vapor deposition (CVD) is essential for device applications. The transfer techniques of graphene from metallic catalyst to target substrate usually use the chemical etching method to dissolve the metallic catalyst. However, this causes not only high material cost but also environmental contamination in large-scale fabrication. We report a bubble transfer method to transfer graphene films to arbitrary substrate, which is nondestructive to both the graphene and the metallic catalyst. In addition, we report a type of metallic catalyst, which is 700 nm of Cu on sapphire substrate, which is hard enough to endure against any procedure in graphene growth and transfer. With the Cr adhesion layer between sapphire and Cu film, electrochemically delaminated graphene shows great quality during several growth cycles. The electrochemical bubble transfer method can offer high cost efficiency, little contamination and environmental advantages.

Nd:sapphire channel waveguide laser

We present continuous wave laser activity in neodymium-doped sapphire ridge waveguides. The ridges were prepared using diamond blade dicing of thin Nd3+:sapphire films grown on sapphire substrates by pulsed laser deposition. Lasing was realized at wavelengths of 1092 nm and 1097 nm for ridge waveguide orientations addressing the σ- and π-polarization, respectively. A maximum slope efficiency with respect to incident pump power of 12% was achieved in σ-polarization with a ridge cross section of 40.8 × 2.6 µm2 and a ridge length of 8 mm. With an available incident pump power of 2.8 W from a Ti:sapphire laser, a maximum output power of 322 mW was realized.

Study of high-temperature Smart Cut™: Application to silicon-on-sapphire films and to thin foils of single crystal silicon

Two novel processes of elaboration of silicon thin films and silicon foils are proposed, based on the knowledge brought from the Smart Cut™. The first relies on the laser-beam annealing of an implanted silicon wafer in order to induce a separation layer within the implanted wafer and a transfer upon a transparent wafer. The second consists in depositing a layer of liquid silicon upon an implanted silicon wafer and to form a silicon foil by liquid phase epitaxial growth that can be separated from the substrate by Smart Cut™. The separation kinetics of an implanted silicon wafer is characterized for temperature between 450°C and 700°C, considering doses of implantation from 3.5∗1016Hcm−2 to 1.0∗1017Hcm−2. The out-diffusion of hydrogen is studied by Energy Recoil Detection Analysis and a model of diffusion of hydrogen in implanted silicon is proposed. Based on this analysis, a model for the kinetics of splitting at high temperature is established. Smart Cut™ separation is demonstrated for temperature up to 1250°C, considering an implanted silicon wafer bonded with a sapphire wafer, through which a laser beam anneals the structure. The kinetics of separation by laser beam annealing is characterized and compared to the kinetics established between 450°C and 700°C. The roughness of the silicon on sapphire film is characterized by Atomic Force Microscopy and a transfer is realized considering an implanted silicon bonded with a glass wafer of 200mm of diameter. Finally, this article presents results of liquid silicon deposition onto an implanted silicon substrate. These results demonstrate the possibility to detach the film grown by liquid phase epitaxy and the upper part of the implanted substrate by Smart Cut™. Electron Backscattering Diffraction Pattern analysis is considered in order to demonstrate the occurrence of epitaxy of the deposited liquid onto the implanted substrate.

Production of VO2 thin films through post-deposition annealing of V2O3 and VOx films

VO2 thin films were produced on sapphire and silicon substrates through post-deposition ex-situ thermal treatment of V2O3 and VOx films. Thin epitaxial films of V2O3 on sapphire and amorphous VOx films on silicon substrates were grown using oxygen assisted molecular beam epitaxy. The post-deposition annealing was performed at different temperatures using an Ar flow. Structural, optical and electrical characterizations were performed to confirm the transformation of the films. The transformed films present a change in resistance across the metal to insulator transition of four orders of magnitude for annealed V2O3 on sapphire and around one order of magnitude in the case of annealed VOx on silicon.

Defect formation and recrystallization in the silicon on sapphire films under Si+ irradiation

Silicon-on-sapphire (SOS) is one of the most promising silicon-on-insulator (SOI) technologies. SOS structures are widely used in microelectronics, but to meet modern requirements the silicon layer should be 100nm thick or less. The problem is in amount of damage in the interface layer, which decreases the quality of the produced devices. In order to improve the crystalline structure quality SOS samples with 300nm silicon layers were implanted with Si+ ions with energies in the range from 180 up to 230keV with fluences in the range from 1014 up to 5×1015cm−2 at 0°C. The crystalline structure of the samples was studied with RBS and the interface layer was studied with SIMS after subsequent annealing.It has been found out that to obtain silicon films with high lattice quality it is necessary to damage the sapphire lattice near the silicon–sapphire interface. Complete destruction of the strongly defected area and subsequent recrystallization depends on the energy of implanted ions and the substrate temperature. No significant mixing in the interface layer was observed with the SIMS.

Single crystalline Er2O3:sapphire films as potentially high-gain amplifiers at telecommunication wavelength

Single crystalline thin films of Er2O3, demonstrating efficient 1.5 μm luminescence of Er3+ at room temperature were grown on Al2O3 substrate by molecular beam epitaxy. The absorption coefficient at 1.536 μm was found to reach 270 cm−1 translating in a maximal possible gain of 1390 dBcm−1. In conjunction with the 10% higher refractive index as compared to Al2O3, this opens the possibility to use Er2O3:sapphire films as short-length waveguide amplifiers in telecommunication.

Effects of High Dose Rate Ionizing Radiation on Fused Silica and Sapphire Films

The effects of high dose rate electron beam exposures on the electrical conductivity of fused silica and sapphire films are investigated via modern experimental techniques. Transient measurements have been obtained for dose rates ranging from 3×10 5 rad(Si)/s to 1×10 10 rad(Si)/s and for pulse widths ranging from 50 ns to 500 ns. The data indicate that the radiation-induced conductivity (RIC) has a varying power-law relationship with dose rate (i.e., the power-law relationship for low dose rates is different than for high dose rates) and that the RIC coefficient (k c) increases at lower dose rates, peaks, and then decreases at higher dose rates. It is also shown that the RIC response of each film increases several orders of magnitude during a high-intensity ionizing radiation exposure and is linearly dependent on applied bias. Two-dimensional (2-D) numerical simulations are used to qualitatively capture the observed RIC response and provide insight into the physics of the induced current transients.

Peculiarities of galvanomagnetic effects in GaN epilayers and GaN/InGaN quantum wells

AbstractTemperature dependent Hall effect, magnetoresistivity and time resolved photoconductivity studies have been performed on GaN epilayers and GaN/InGaN multiple quantum wells (MQWs) grown on sapphire films by Metal Organic Chemical Vapor Deposition (MOCVD) method. The MQW structures consisted of different width or different amount of quantum wells. We related the electrical parameters with the quantum structure parameters and found that the geometrical variation of the quantum wells may be substituted by the variation of the amount of the quantum wells. The Hall and the magnetoresistivity measurements showed the complex behavior, which was attributed to the inhomogeneities of the samples, similarly to the case of the crystals with great size defects (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Enhanced growth rates and reduced parasitic deposition by the substitution of Cl 2 for HCl in GaN HVPE

The main limitation in the application of hydride vapor phase epitaxy for the large scale production of thick free-standing GaN substrates is the so-called parasitic deposition, which limits the growth time and wafer thickness by blocking the gallium precursor inlet. By utilizing Cl 2 instead of the usual HCl gas for the production of the gallium chlorine precursor, we found a rapid increase in growth rate from ∼80 to ∼400 μm/h for an equally large flow of 25 sccm. This allowed us to grow, without any additional optimization, 1.2 mm thick high quality GaN wafers, which spontaneously lifted off from their 0.3° mis-oriented GaN on sapphire HCl-based HVPE templates. These layers exhibited clear transparencies, indicating a high purity, dislocation densities in the order of 10 6 cm −2, and narrow rocking curve XRD FWHMs of 54 and 166 arcsec in for the 0002 and 101−5 directions, respectively. In view of these findings, the possibility of depositing Cl2-based HVPE GaN directly on sapphire via the standard low temperature nucleation-high temperature overgrowth process of classic HCl-based HVPE was investigated. By varying the Cl 2 flow during the 3 min long 615 °C nucleation phase, the nucleation conditions yielding smooth 0.7∼mm thick GaN on sapphire films were determined. The nucleation and coalescence mechanism of Cl 2-based HVPE GaN was found to proceed similar to that of HCl-based HVPE.

Properties of Al2O3: nc-Si nanostructures formed by implantation of silicon ions into sapphire and amorphous films of aluminum oxide

Photoluminescence, infrared Fourier spectroscopy, Raman scattering, transmission electron microscopy, and electron diffraction were used to study the luminescent, optical, and structural properties of aluminum oxide layers (sapphire and films of Al2O3 deposited on silicon) implanted with Si+ to produce silicon nanocrystals. It is established that, in both cases, a high-temperature annealing of heavily implanted samples brings about the formation of silicon nanocrystals. However, the luminescent properties of the nanocrystals are strongly dependent on the type of pristine matrix; namely, nanocrystals in Al2O3 films emit light in the spectral range typical of Si quantum dots (700–850 nm), whereas in sapphire this photoluminescence is not observed. This difference is interpreted as being due to the fact that local stresses arise in the nanocrystal/sapphire system and break chemical bonds at the interface between the phases, whereas in Al2O3 films stresses are relaxed.

Reduction of threading dislocations in ZnO/(0001) sapphire film heterostructure by epitaxial lateral overgrowth of nanorods

Transmission electron microscopy was used to study threading dislocations (TDs) in epitaxial ZnO films on (0001) sapphire substrates produced by a two-step method. First, ZnO was deposited by pulsed laser deposition. It was found that the sample consisted of a continuous buffer layer with a high density, 7×1010/cm2, of TDs, with c-aligned nanorods on its top. The nanorods revealed few, if any, TDs. A further layer of ZnO was then grown under conditions favoring nanorod growth, using either chemical vapor deposition (CVD) or a hydrothermal method. In both cases the nanorods grew laterally and eventually coalesced to form a continuous overgrowth. The nanorods remained mostly free of dislocations until coalescence. New grain boundary dislocations were generated where nanorods coalesced, but many of these dislocations migrated laterally and interacted with other dislocations to form closed loops. The TD density at the top of the continuous film was thereby reduced to 1×109/cm2 and 7×109/cm2 in the cases of hydrothermal and CVD treatments, respectively. The mechanism of growth and the means by which TDs are reduced are explained.

Defect reduction in GaN/(0001)sapphire films grown by molecular beam epitaxy using nanocolumn intermediate layers

Transmission and scanning electron microscopies are used to examine the epitaxial lateral overgrowth of GaN on GaN nanocolumns grown on AlN/(0001)sapphire by molecular beam epitaxy. Initially, N-rich growth gave a bimodal morphology consisting of defect-free Ga-polar nanocolumns emanating from a compact, highly defective N-polar layer. Under subsequent Ga-rich conditions, the nanocolumns grew laterally to produce continuous Ga-polar overlayers. Threading dislocation (TD) densities in the overlayer were in the range of 108–109cm−2, up to two orders of magnitude less than in the N-polar underlayer. It is proposed that the change in polarity is a key factor controlling the reduction in TD density.