Patterned Sapphire Substrate Research Articles

Growth of lattice-relaxed InGaN thick films on patterned sapphire substrates by tri-halide vapor phase epitaxy

Lattice-relaxed thick InGaN films could be obtained using patterned sapphire substrates (PSSs) by tri-halide vapor phase epitaxy. Furthermore, by inserting the GaN intermediate layer between the InGaN epilayer and the PSS, the crystalline quality of the InGaN epilayer was drastically improved. According to the X-ray diffraction ω-rocking curve measurements, the full width at half maximum of the InGaN epilayer was 850 arcsec around the symmetrical reflection and 967 arcsec around the skew-symmetrical reflection. It was also found that the thickness of the GaN intermediate layer influences the crystalline quality of the InGaN epilayer. As the thickness of the GaN intermediate layer increases, the crystalline quality of the GaN intermediate layer gets higher, resulting in the crystalline quality of the InGaN epilayer was improved.

MXene-GaN van der Waals metal-semiconductor junctions for high performance multiple quantum well photodetectors

A MXene-GaN-MXene based multiple quantum well photodetector was prepared on patterned sapphire substrate by facile drop casting. The use of MXene electrodes improves the responsivity and reduces dark current, compared with traditional Metal-Semiconductor-Metal (MSM) photodetectors using Cr/Au electrodes. Dark current of the device using MXene-GaN van der Waals junctions is reduced by three orders of magnitude and its noise spectral intensity shows distinct improvement compared with the traditional Cr/Au–GaN–Cr/Au MSM photodetector. The improved device performance is attributed to low-defect MXene-GaN van der Waals interfaces. Thanks to the high quality MXene-GaN interfaces, it is possible to verify that the patterned substrate can locally improve both light extraction and photocurrent collection. The measured responsivity and specific detectivity reach as high as 64.6 A/W and 1.93 × 1012 Jones, respectively, making it a potential candidate for underwater optical detection and communication. The simple fabrication of MXene-GaN-MXene photodetectors spearheaded the way to high performance photodetection by combining the advantages of emerging 2D MXene materials with the conventional III-V materials.

Morphological effect of patterned sapphire substrate on efficiency of white-light phosphor LED package

Six-facets patterned sapphire substrates (A-PSS) are manufactured by wet-etching regular semispherical patterned sapphire substrates (r-PSS). The possible crystallographic planes of the etched planes on the semi-sphere pattern is the family of (1102) plane. Metalorganic Chemical Vapor Deposition (MOCVD) GaN epitaxial LED structure was grown on the c-plane sapphire wafer, r-PSS, and A-PSS, which were further processed as horizontal LED chips. Compared to LEDs on the flat c-plane sapphire, the light output power of LEDs on r-PSS and A-PSS is enhanced by 71% and 67%, respectively. The output power of the bare LEDs decreases after the white-light LED package. Remarkably, the output power of the white-light packaged LED on the A-PSS is larger than the white-light packaged LED on the r-PSS. The Monte Carlo ray tracing simulation shows that, before and after silicone-lens encapsulating, the LEDs on r-PSS and A-PSS emit more on the top surface. However, the sidewall emission power ratio of the LED on r-PSS decreases by 3.44% after silicone encapsulating. On the other hand, the sidewall emission power ratio of the LED on A-PSS increases by 1.06% after silicone encapsulating. Note that the emission light of LEDs out of the top surface travels a longer path in the silicone lens than the emission light out of the sidewalls. Hence, more re-extracted light out of the sidewall of LED on A-PSS would be the reason for the LED on A-PSS having a less package loss and the white-light package efficiency than the LED on r-PSS. In conclusion, the light radiation distribution pattern defined by the pattern morphology of A-PSS is the key for the better white-light package efficiency of the LED on A-PSS than that of the LED on r-PSS. • The patterned sapphire substrate morphology can alter the propagation direction of the light emission. • Monte Carlo ray tracing simulation shows the emission power of top surface and sidewall vary before and after lens package. • The emission light of LEDs out of the top surface travels a longer path in the phosphor layer than the sidewalls. • The light radiation distribution defined by the pattern morphology is the key for white-light package efficiency of LED.

Growth of ZnO Nanoflower Arrays on a Patterned Sapphire Substrate

In this study, we investigated a novel technology to grow the ZnO nanoflower arrays on a patterned sapphire substrate using hydrothermal method. The process to prepare the substrate and grow ZnO nanoflower arrays were that the patterned concave sapphire substrates were cleaned using acetone, isopropyl alcohol, and D.I. water in an ultrasonic cleaner. Al sacrificial layer with the thickness of 600 nm was deposited using a thermal evaporation technology. The sol-gel process was used to deposit the ZnO seed layer on the patterned concave sapphire substrates, and then the ZnO seed layer was annealed at 500°C in Ar ambient for 1 h. The optical grade silicone A-B glue was coated on the ZnO seed layer and then the mixed solution of K3Fe(CN)6 : KOH : H2O = 10 g : 1 g : 100 ml was used to etch the sacrificial Al layer. (f) A lift-off technology was used to move the ZnO seed layer/silicone A-B glue to silicon substrate. Finally, a hydrothermal method was used to grow the ZnO nanorods on the ZnO seed layer/silicone A-B glue/silicon substrate at 90oC with the different durations of 10 to 60 min. Focused ion beam system (FIB) was used to observe the cross sectional morphology of patterned ZnO seed layer. The crystal structural of the flower-like ZnO nanostructures was analyzed using X-ray diffraction (XRD) pattern, the morphological of the flower-like ZnO nanostructures was observed using FIB and field emission scanning electron microscope (FESEM). Also, the optical properties of the flower-like ZnO nanostructures were investigated.

Effects of three-step magnesium doping in p-GaN layer on the properties of InGaN-based light-emitting diode

Purpose The purpose of this paper is to enhance the efficiency of the LED by introducing three-step magnesium (Mg) doping profile. Attention was paid to the effects of the Mg doping concentration of the first p-GaN layer (i.e. layer close to the active region). Attention was paid to the effects of the Mg doping concentration of the first p-GaN layer (i.e. layer close to the active region). Design/methodology/approach Indium gallium nitride (InGaN)–based light-emitting diode (LED) was grown on a 4-inch c-plane patterned sapphire substrate using metal organic chemical vapor deposition. The Cp2Mg flow rates for the second and third p-GaN layers were set at 50 sccm and 325 sccm, respectively. For the first p-GaN layer, the Cp2Mg flow rate varied from 150 sccm to 300 sccm to achieve different Mg dopant concentrations. Findings The full width at half maximum (FWHM) for the GaN (102) plane increases with increasing Cp2Mg flow rate. FWHM for the sample with 150, 250 and 300 sccm Cp2Mg flow rates was 233 arcsec, 236 arcsec and 245 arcsec, respectively. This result indicates that the edge and mixed dislocations in the p-GaN layer were increased with increasing Cp2Mg flow rate. Atomic force microscopy (AFM) results reveal that the sample grown with 300 sccm exhibits the highest surface roughness, followed by 150 sccm and 250 sccm. The surface roughness of these samples is 2.40 nm, 2.12 nm and 2.08 nm, respectively. Simultaneously, the photoluminescence (PL) spectrum of the 250 sccm sample shows the highest band edge intensity over the yellow band ratio compared to that of other samples. The light output power measurements found that the sample with 250 sccm exhibits high output power because of sufficient hole injection toward the active region. Originality/value Through this study, the three steps of the Mg profile on the p-GaN layer were proposed to show high-efficiency InGaN-based LED. The optimal Mg concentration was studied on the first p-GaN layer (i.e. layer close to active region) to improve the LED performance by varying the Cp2Mg flow rate. This finding was in line with the result of PL and AFM results when the samples with 250 sccm have the highest Mg acceptor and good surface quality of the p-GaN layer. It can be deduced that the first p-GaN layer doping has a significant effect on the crystalline quality, surface roughness and light emission properties of the LED epi structure.

Effects of V/III ratio of InGaN quantum well at high growth temperature for near ultraviolet light emitting diodes

Purpose The aims of this paper is to study the effects of the V/III ratio of indium gallium nitride (InGaN) quantum wells (QWs) on the structural, optical and electrical properties of near-ultraviolet light-emitting diode (NUV-LED). Design/methodology/approach InGaN-based NUV-LED is successfully grown on the c-plane patterned sapphire substrate at atmospheric pressure using metal organic chemical vapor deposition. Findings The indium composition and thickness of InGaN QWs increased as the V/III ratio increased from 20871 to 11824, according to high-resolution X-ray diffraction. The V/III ratio was also found to have an important effect on the surface morphology of the InGaN QWs and thus the surface morphology of the subsequent layers. Apart from that, the electroluminescence measurement revealed that the V/III ratio had a major impact on the light output power (LOP) and the emission peak wavelength of the NUV-LED. The LOP increased by up to 53% at 100 mA, and the emission peak wavelength of the NUV-LED changed to a longer wavelength as the V/III ratio decreased from 20871 to 11824. Originality/value This study discovered a relation between the V/III ratio and the properties of QWs, which resulted in the LOP enhancement of the NUV-LED. High TMIn flow rates, which produced a low V/III ratio, contribute to the increased LOP of NUV-LED.

Polarization-insensitive GaN metalenses at visible wavelengths

The growth of wide-bandgap materials on patterned substrates has revolutionized the means with which we can improve the light output power of gallium nitride (GaN) light-emitting diodes (LEDs). Conventional patterned structure inspection usually relies on an expensive vacuum-system-required scanning electron microscope (SEM) or optical microscope (OM) with bulky objectives. On the other hand, ultra-thin metasurfaces have been widely used in widespread applications, especially for converging lenses. In this study, we propose newly developed, highly efficient hexagon-resonated elements (HREs) combined with gingerly selected subwavelength periods of the elements for the construction of polarization-insensitive metalenses of high performance. Also, the well-developed fabrication techniques have been employed to realize the high-aspect-ratio metalenses working at three distinct wavelengths of 405, 532, and 633 nm with respective diffraction-limited focusing efficiencies of 93%, 86%, and 92%. The 1951 United States Air Force (USAF) test chart has been chosen to characterize the imaging capability. All of the images formed by the 405-nm-designed metalens show exceptional clear line features, and the smallest resolvable features are lines with widths of 870 nm. To perform the inspection capacity for patterned substrates, for the proof of concept, a commercially available patterned sapphire substrate (PSS) for the growth of the GaN LEDs has been opted and carefully examined by the high-resolution SEM system. With the appropriately chosen metalenses at the desired wavelength, the summits of structures in the PSS can be clearly observed in the images. The PSS imaging qualities taken by the ultra-thin and light-weight metalenses with a numerical aperture (NA) of 0.3 are comparable to those seen by an objective with the NA of 0.4. This work can pioneer semiconductor manufacturing to choose the polarization-insensitive GaN metalenses to inspect the patterned structures instead of using the SEM or the bulky and heavy conventional objectives.

Effect of Oxygen on the Electrical Conductivity of Pt-Contacted α-Ga₂O₃/ε(κ)-Ga₂O₃ MSM Structures on Patterned Sapphire Substrates

Electrical conductivity and gas sensitivity of α-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /ε ( κ)-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> structures were measured for oxygen concentrations ranging from 2 % to 100 % and temperatures ranging from 25 °C to 220 °C. It was found that the oxygen sensitivity of the structures depended on the donor dopant concentration. The α-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /ε( κ)-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> structures doped with ~ 1.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> of Sn showed high sensitivity to O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the temperature range from 180 °C to 220 °C and at the bias voltage below 7.5 V. This effect can be attributed to the chemisorption of oxygen molecules on the surface of structures, which reduces energy barriers between ε(k)-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> grains.

Designing an Ultrathin Film Spectrometer Based on III-Nitride Light-Absorbing Nanostructures.

In this paper, a spectrometer design enabling an ultrathin form factor is proposed. Local strain engineering in group III-nitride semiconductor nanostructured light-absorbing elements enables the integration of a large number of photodetectors on the chip exhibiting different absorption cut-off wavelengths. The introduction of a simple cone-shaped back-reflector at the bottom side of the substrate enables a high light-harvesting efficiency design, which also improves the accuracy of spectral reconstruction. The cone-shaped back-reflector can be readily fabricated using mature patterned sapphire substrate processes. Our design was validated via numerical simulations with experimentally measured photodetector responsivities as the input. A light-harvesting efficiency as high as 60% was achieved with five InGaN/GaN multiple quantum wells for the visible wavelengths.

Reduction of threading dislocations in GaN grown on patterned sapphire substrate masked with serpentine channel

Gallium nitride (GaN) is a promising semiconductor for the fabrication of a wide range of electronic and optoelectronic devices. For GaN grown on widely used sapphire substrate, bundles of threading dislocations (TDs) are induced by thermal and lattice mismatch. These TDs intersect the surface of GaN and degrade the internal quantum efficiency (IQE) of GaN-based devices. To address this issue, two innovative techniques are proposed for effective filtration of TDs. First, a novel design of serpentine channel pattern sapphire substrate (SCPSS) is applied for effective filtration of TDs at the interface. Transmission electron microscopy (TEM) indicated that SCPSS plays an effective role in the filtration of TDs; however, cathodoluminescence (CL) revealed that many TDs still intersect the surface of GaN. To further optimize the growth of GaN, the temperature was ramped down to the intermediate level and an interlayer (IL) of InGaN was introduced near the window of SCPSS. CL analysis indicated that additional use of InGaN-IL further reduced the TDs. Raman microscopy showed that incorporation of IL lead to drastic reduction in compressive stress. Atomic force microscopy (AFM) confirmed the achievement of excellent surface topography with InGaN-IL. Moreover, it is observed that growth of GaN by SCPSS is better than conventional growth because of improved surface morphology and low TDs. The reduction of TDs by SCPSS and InGaN-IL is an effective approach for the growth of high-quality GaN towards efficient devices.

Effect of Oxygen on the Gas-Sensitive Properties of α-Ga2O3/ε-Ga2O3 Structures

Miniature O2 sensors with low energy consumption are of practical interest for the chemical and metallurgical industries, development of systems for analyzing the performance of internal combustion engines and as functional elements of artificial lung ventilation devices. The requirements for miniaturization, high sensitivity, speed and relative cheapness are satisfied by O2 sensors based on β-Ga2O3. The chemical and thermal stability of β-Ga2O3 allows developing gas sensors with extremely high operating temperatures of 400-1100 °C ensuring high reproducibility of their characteristics and high speed of operation. In turn, the high operating temperatures of O2 β-Ga2O3 sensors are their drawback causing high energy consumption.Previously, we studied the effect of H2 on the gas-sensitive properties of the α-Ga2O3/ε-Ga2O3 structure with Pt contacts grown by the halide vapor phase epitaxy (HVPE) on the patterned sapphire substrates (PSS) [1]. Low operating temperatures, weak dependence of properties on the humidity and control of selectivity by means of voltage changes indicate that the research of α-Ga2O3/ε-Ga2O3 structures as sensitive elements of gas sensors is promising. We presume that in addition to increasing the applied voltage to the structures, the sensitivity of α-Ga2O3/ε-Ga2O3 to certain gases, in particular O2 can be increased by reducing the level of doping with a donor impurity. Thus this work is devoted to research the effect of O2 on gas-sensitive properties of α-Ga2O3/ε-Ga2O3 structures doped with Sn.Ga2O3 films were grown by the HVPE in Perfect Crystals LLC. PSS of (0001) orientation and 430 microns in thickness were used as substrates. During the synthesis, the films were doped with Sn. To measure the gas-sensitive properties of the α-Ga2O3/ε-Ga2O3 structures Pt contacts were formed on their surface.The samples consisted of a mixture of α and ε phases with the orientation (0001). Using scanning and transmission electron microscopy, it was found that the α-phase forms columnar structures at the top of the sapphire cone, and the ε-phase fills the gaps between the columns. The characteristic triangular shape of columnar structures indicates that they consist of α-Ga2O3 with trigonal symmetry. ɛ-Ga2O3 has a grain but not polycrystalline structure taking into account the XRD results.Figure 1 shows the I-U characteristics of α-Ga2O3/ε-Ga2O3 structures at the exposure of O2 in a wide range of concentrations C O2 from 2 to 100 % at temperature T = 200 °C. The current through the samples decreases with C O2 increasing. In the range of the T = 180 - 200 °C, at T = 200 °C, the highest response to O2 are observed. It is worth noting that the operating temperatures of the α-Ga2O3/ε-Ga2O3 structures in the reaction to O2 are significantly lower than the operating temperatures of O2 β-Ga2O3 sensors. These low values of T are an advantage in developing gas-analytical systems with low energy consumption. The concentration dependences of the response at the T = 180 - 200 °C are approximated fairly accurately by a power function: response ~ C O2 b , where b is the exponent that depends on the temperature.Figure 1-Effect of O2 on I-U characteristics of α-Ga2O3/ε-Ga2O3.A decrease in the Sn impurity concentration in α-Ga2O3/ε-Ga2O3 from ~4×1018 cm-3 to ~1.5×1017 cm-3 leads to a significant sensitivity to O2 in the temperature range from 180 to 220 °C at low values of the applied voltage U ≤ 7.5 V. The I-U characteristics of α-Ga2O3/ε-Ga2O3 structures in a dry gas mixture N2 + O2 are described by the model of MSM structures based on the theory of thermoelectronic emission in diode approximation with the resistance of the semiconductor layer exceeding the resistance of the space charge regions at the metal/semiconductor interface. The sensory effect consists in the chemisorption of oxygen molecules on the surface of ε-Ga2O3 that according to SEM has a grain structure. As a result, the energy barrier at the grain boundaries of ε-Ga2O3 increases leading to a decrease in the current. The studied structures showed high sensitivity to relatively low concentrations (0.745 %) of H2 and CO at the T = 180-220 °C and practically did not react to the effects of NO2 and CH4.This research was financially supported by the Russian Scientific Foundation, grant No 20-79-10043.[1] A V Almaev, V I Nikolaev, S I Stepanov, A I Pechnikov, A V Chikiryaka, N N Yakovlev, V M Kalygina, V V Kopyev and E V Chernikov. J. Phys. D: Appl. Phys. 53 (2020) 414004 (9pp) https://doi.org/10.1088/1361-6463/ab9c69 Figure 1

Rational construction of staggered InGaN quantum wells for efficient yellow light-emitting diodes

High-efficiency InGaN-based yellow light-emitting diodes (LEDs) with high brightness are desirable for future high-resolution displays and lighting products. Here, we demonstrate efficient InGaN-based yellow (∼570 nm) LEDs with optimized three-layer staggered quantum wells (QWs) that are grown on patterned sapphire substrates. Numerical simulations show that the electron–hole wavefunction overlap of staggered InGaN QWs with high In content exhibits a 1.7-fold improvement over that of square InGaN QWs. At the same injection current, LEDs with staggered QWs exhibit lower forward voltages and narrower full widths at half maximum than LEDs with square QWs. The light output power and external quantum efficiency of a staggered QW LED are 10.2 mW and 30.8%, respectively, at 15 mA. We combine atomic probe tomography (APT), time-resolved photoluminescence (TRPL), and transmission electron microscopy (TEM) with energy-dispersive x-ray (EDX) mapping spectroscopy to shed light on the origin of enhanced device performance. APT results confirm the staggered In profile of our designed staggered QWs structure, and TRPL results reveal decreased defect-state carrier trapping in staggered QWs. Furthermore, TEM with EDX mapping spectroscopy supports the viewpoint that staggered QWs exhibit uniform elemental distribution and improved crystal quality. Together, these factors above contribute to enhanced LED performance. Our study shows that staggered InGaN QWs provide a promising strategy for the development of LEDs that are efficient in the long-wavelength region.

Thick AlN layers grown on micro-scale patterned sapphire substrates with sputter-deposited annealed AlN films by hydride vapor-phase epitaxy

Misaligned crystallite growth is found on the sidewalls of dome-shaped patterns which hinders the coalescence of the c-plane AlN layer growing on the c-axis-oriented sapphire plane. In this study, thick AlN layers without cracks were grown by hydride vapor-phase epitaxy on micro-scale patterned sapphire substrates using annealed sputter-deposited AlN with a thickness of 200 nm. It was observed that undesired misaligned AlN growth can be prevented by selecting an appropriate growth temperature. A two-step approach was used to promote the lateral growth and coalescence of AlN. A relatively smooth surface without discontinuities and holes was obtained. The influence of the multi-oriented sapphire facets provided by micro-scale patterned sapphire substrates was investigated. According to the geometry of the misaligned crystallites, the grown AlN deviating from the c-axis on the near sapphire r-plane sidewalls has less influence on the coalescence of AlN, compared to the near n-plane sidewalls.

Suspended MoS2 Photodetector Using Patterned Sapphire Substrate

AbstractThe introduction of patterned sapphire substrates (PSS) has been regarded as an effective method to improve the photoelectric performance of 2D layered materials in recent years. Molybdenum disulfide (MoS2), an intriguing transition metal 2D materials with splendid photoresponse owing to a direct‐indirect bandgap transition at monolayer, shows promising optoelectronics applications. Here, a large‐scale, continuous multilayer MoS2 film is prepared on a SiO2/Si substrate and transferred to flat sapphire substrate and PSS, respectively. An enhanced dynamic distribution of local electric field and concentrated photon excitons across the interface between MoS2 and patterned sapphire substrates are revealed by the finite‐difference time‐domain simulation. The photoelectric performance of the MoS2/PSS photodetector is improved under the three lasers of 365, 460, and 660 nm. Under the 365 nm laser, the photocurrent increased by 3 times, noise equivalent power (NEP) decreases to 1.77 × 10−14 W/Hz1/2 and specific detectivity (D*) increases to 1.2 × 1010 Jones. Meanwhile, the responsivity is increased by 7 times at 460 nm, and the response time of the MoS2/PSS photodetector is also shortened under three wavelengths. The work demonstrates an effective method for enhancing the optical properties of photodetectors and enabling simultaneous detection of broad‐spectrum emissions.

Metalorganic chemical vapor deposition-grown tunnel junctions for low forward voltage InGaN light-emitting diodes: epitaxy optimization and light extraction simulation

In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition (MOCVD)-grown tunnel junctions (TJs) utilizing selective area growth (SAG) for regular size (0.1 mm2) and micro-size InGaN light-emitting diodes (LEDs and µLEDs). Finite-difference time-domain simulations show that the SAG apertures result in a more directional light emission of far-field radiation pattern for the SAG TJ LEDs grown on patterned sapphire substrate. Moreover, it is noted that the n-InGaN insertion layer and Si-doped concentration in the n+GaN TJs layer is essential to realize a low forward voltage (V f) in TJs LEDs. For both 0.1 mm2 LEDs and µLEDs, the V f is independent on the SAG aperture space varied from 3 to 8 µm when the Si-doping level in the n+GaN layer is as high as 1.7 × 1020 cm−3. The optimized TJ LEDs exhibit a comparable differential resistance of 1.0 × 10−2 Ω cm2 at 100 A cm−2 and a very small voltage penalty of 0.2–0.3 V compared to the conventional indium tin oxide contact LEDs. The low V f penalty is caused by a higher turn on voltage, which is the smallest one among the MOCVD-grown TJs LEDs and comparable to the best molecular beam epitaxy-grown TJs LEDs.

Soft photomask lithography and droplet spreading nano-imprinting for manufacturing patterned sapphire substrates

This paper proposes an innovative method for fabricating soft photomasks which can be used for contact type of photolithography. It utilizes a droplet spreading nano-imprinting technology to imprint a droplet of carbon-black photoresist (PR) on top of a glass substrate with a soft polydimethylsiloxane (PDMS) mold which contains patterned surface micro-cavities. By proper surface treatments on both surfaces of the PDMS mold and the glass substrate, it is possible to reversely fill the carbon-black resist into the surface micro-cavities of the mold and therefore result in a soft PDMS photomask. The experimental investigation is carried out with the application on photolithographic patterning on a 4″ sapphire substrate to achieve hexagonally arrayed PR micro-pillars with a feature size of around 1 to 2 μm. The soft PDMS photomask embedded with carbon-black resist is successfully produced. Excellent photolithography patterning results are also achieved on the 4″ sapphire wafer, which demonstrates the potential for manufacturing patterned sapphire substrates in the light-emitting-diode (LED) industry. Further developments and other possible applications of the proposed soft photomask lithography will be addressed.

Etching Behaviors of Sapphire's C- Plane Cavity

Techniques to fabricate patterned sapphire substrates (PSSs) have attracted much attention in recent decades. Wet etching behaviors and crystalline sapphire processes are critical for PSSs applications to improve the performance of light-emitting diodes. This study investigated the shape evolution behaviors and associated kinetics of cavities on the c-{0001} plane in crystalline sapphire during wet etching. It was revealed that wet etching reduces the cavity aspect ratio, and the cavity shape is a complicated structure constructed by 15 faceted planes of c-{0001}, r-{11¯02}, p-{112¯3}, m-{101¯0}, and s-{11¯01} families. A constant etching rate was demonstrated, suggesting the step flow mechanism of etching. The etching activation energy of crystalline sapphire is reduced by preformation of cavities as elucidated by the Arrhenius kinetic model followed during the etching process. This study provides new insight into wet etching behaviors of crystalline sapphire and might open up a way for fabricating sapphire substrate with large aspect ratio patterns.

Demonstration of GaN-based white LED grown on 4-inch patterned sapphire substrate by MOCVD

A blue light emitting diode (LED) chip has been facilely fabricated on a 4-inch patterned sapphire substrate based on an InGaN/GaN multiple quantum well structure, composed of a Ce3+ doped Yttrium Aluminum Garnet (YAG) yellow phosphor packaged into a white LED chip. The epitaxial growth, fabrication, and package process of the device have been briefly described; the structural properties and device performance have also been systematically studied. The surface morphology of the sample in a whole wafer has a good uniformity. And the curvature distribution with a maximum value of ~70 μm, it gradually becomes smaller from the middle to the periphery, which is mainly resulting from the strain state evolution during the film growth. When increasing the injection current, it is significant interestingly observed in the electroluminescence (EL) spectrum that the blue light peak is blue-shifted, it is due to the influence reducing of the polarization field and/or the energy band filling, while the blue light peak and the yellow-green light peak are red-shifted due to the thermal effect of the chip. In addition, the light output power exhibits a linearly incremental relationship with a little saturation when the injection current varies from 0 to 100 mA. Therefore, it is firmly believed that the corresponding chip fabrication and characterization technology will play a certain role in promoting the development of solid-state lighting field.

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Improving the Light Extraction Efficiency of GaN-Based Light-Emitting Diode

The light extraction efficiency caused by total internal reflection is low. Based on the analysis of the existing technology, a new design scheme is proposed in this paper to improve the light extraction efficiency. The air gap photonic crystal is embedded on the GaN-based patterned sapphire substrate, which can reduce line misalignment and improve light extraction efficiency. The internal structure of the GaN-based LED epitaxial layer is composed of an electron emission layer, a quantum well in the light-emitting recombination region, and an electron blocking layer. Experimental results show that this method significantly improves the extraction efficiency of LED light.