Nanopatterned Sapphire Substrates Research Articles

Enhancing light extraction efficiency of the inclined-sidewall-shaped DUV micro-LED array by hybridizing a nanopatterned sapphire substrate and an air-cavity reflector

In this work, we hybridize an air cavity reflector and a nanopatterned sapphire substrate (NPSS) for making an inclined-sidewall-shaped deep ultraviolet micro light-emitting diode (DUV micro-LED) array to enhance the light extraction efficiency (LEE). A cost-effective hybrid photolithography process involving positive and negative photoresist (PR) is explored to fabricate air-cavity reflectors. The experimental results demonstrate a 9.88% increase in the optical power for the DUV micro-LED array with a bottom air-cavity reflector when compared with the conventional DUV micro-LED array with only a sidewall metal reflector. The bottom air-cavity reflector significantly contributes to the reduction of the light absorption and provides more escape paths for light, which in turn increases the LEE. Our investigations also report that such a designed air-cavity reflector exhibits a more pronounced impact on small-size micro-LED arrays, because more photons can propagate into escape cones by experiencing fewer scattering events from the air-cavity structure. Furthermore, the NPSS can enlarge the escape cone and serve as scattering centers to eliminate the waveguiding effect, which further enables the improved LEE for the DUV micro-LED array with an air-cavity reflector.

Study of AlN growth using AMEC Prismo HiT3 MOCVD reactor

Effect and mechanism of carrier gas velocity, V/III ratio, and carrier gas velocity match of group III to group V (MO VM) on AlN growth rate (GR) were investigated in depth by experiment and computational fluid dynamics (CFD) simulation in Prismo HiT3 MOCVD platform. It is found that AlN growth rate increases with hydrogen flow rate at first, reaches saturation and then shows a monotonic decrease trend. The specific value of turning point depends on the equipment and process. At constant total flow rate of 160 slm, GR increases with MO VM by suppressing parasitic reaction, but uniformity show a deteriorate trend due to the occurrence of turbulence and loss of uniform boundary layer. High quality 4-μm-thick AlN films with improved crystalline quality and atomic smooth surfaces were achieved on nano-patterned sapphire substrates with a growth rate of 0.82 um/hr. The full width at half maximum of the X-ray rocking curve was 162/305 arcsec for (002)/(102) planes with total dislocation density ∼ 109 cm−2.

Surface‐enhanced Raman spectroscopy investigation of PO43− to Ag/Al2O3 nanopatterned substrates binding and its dynamics for sensitive detection of phosphate in water

AbstractWe provide fast and sensitive detection of phosphate dissolved in water using Raman spectroscopy. To this end, phosphate‐scavenging planar core/shell surface‐enhanced Raman spectroscopy (SERS) probes consisting of Al2O3‐capped Ag nanopatterned sapphire substrate were prepared and optimized. Thanks to scavenging, the probes provide enhancement sufficient to readily detect phosphate at levels occurring in water sources. In particular, the SERS probes were successfully tested for selective detection of all phosphate ions and down to 1 M phosphate concentration (30 g/L of phosphorus in the solution). The novelty of our contribution lies in real‐time SERS monitoring of the phosphate Raman markers, which allowed to study the dynamics of phosphate interaction with the nano‐structured surface of the probes. Under natural conditions (neutral to basic environment), the phosphates adsorb on the probes in form of PO in two steps.

Suppressing optical crosstalk for GaN/InGaN based flip-chip micro light-emitting diodes by using an air-cavity patterned sapphire substrate as a light filter.

In this work, by using three-dimensional finite-difference time-domain (3D FDTD) method, the effect of conventional nano-patterned sapphire substrate (NPSS) on the optical crosstalk and the light extraction efficiency (LEE) for InGaN/GaN-based flip-chip micro light-emitting diodes (µ-LEDs) are systematically studied. We find that the conventional NPSS is not suitable for µ-LEDs. It is because the inclined mesa sidewall for µ-LEDs possesses a good scattering effect for µ-LEDs, but the introduced conventional NPSS causes part of the light be off escape cone between sapphire and air and become the guided light. To suppress the guided light and improve the optical crosstalk, a thick air layer between the n-GaN layer and the sapphire substrate can be used as a light filter to prevent the guided light from propagating into the sapphire. However, in reality, it is challenging to make the aforementioned air layer from point of fabrication view. Therefore, we propose the air-cavity patterned sapphire substrate (AC-PSS) as the light filter. Our results show that the crosstalk ratio can be decreased to the value even lower than 10%. The LEE can also be enhanced simultaneously due to combination effects of the filtering effect of the AC-PSS and the scattering effect of the inclined mesa sidewall.

Paving the Way for High‐Performance UVB‐LEDs Through Substrate‐Dominated Strain‐Modulation

AbstractAlGaN‐based ultraviolet‐B light‐emitting diodes (UVB‐LEDs) exhibit great potential in phototherapy, vitamin D3 synthesis promotion, plant growth regulation, and so on. However, subjected to the excess compressive strain induced by the large lattice mismatch between multiple quantum wells (MQWs) and AlN, UVB‐LEDs that simultaneously satisfy the requirements of high light output power (LOP), low working voltage, and excellent stability are rarely reported. Here, a substrate‐dominated strain‐modulation strategy is proposed. By precisely manipulating the strain in AlN grown on nano‐patterned sapphire substrate (NPSS) to a slightly tensile one, the compressive strain in the following Al0.55Ga0.45N underlayer and Al0.28Ga0.72N/Al0.45Ga0.55N MQWs is successfully suppressed. As a result, an outstanding UVB‐LED with a peak wavelength at 303.6 nm is achieved. The 20 × 20 mil2 UVB‐LED chip shows a wall‐plug efficiency (WPE) of 3.27% under a forward current of 20 mA and a high LOP of 57.2 mW with an extremely low voltage of 5.87 V under a forward current of 800 mA. It is more exciting that the LOP degradation is as low as 17% after 1000 h operation under a forward current density of 75 A cm−2, showing excellent stability. The here‐developed UVB‐LED, with a high LOP and excellent reliability, will definitely promote the applications of AlGaN‐based UVB‐LEDs.

Promoting Light Extraction Efficiency of Ultraviolet Light Emitting Diodes by Nanostructure Optimization

Ultraviolet (UV) light-emitting diodes (LEDs), as one of the more promising optoelectronic devices, are intrinsically limited by poor light extraction efficiencies (LEEs). To unlock the full potential of UV-LEDs, we propose a simple and effective strategy to promote the LEEs of UV-LEDs by screening and tailoring suitable optical structures/designs through rigorous numerical simulations. The photonic crystals (PCs) and/or nano-patterned sapphire substrates (NPSSs) equipped with the nano-pillar, nano-cone, nano-oval, and their derivates, are particularly investigated. The simulated results show that individual PC with an average transmittance of 28% is more efficient than that of individual NPSS (24.8%). By coupling PC and NPSS structures, a higher LEE with an average transmittance approaching 29% is obtained, much higher than that of the flat one (23.6%). The involved mechanisms are clarified and confirm that the promotion of optical performance of the nanostructured devices should be attributed to the widened response angles (from 0 to 60°), rather than the enhanced transmittances in the small angles within 30°.

Study of Defects and Nano-patterned Substrate Regulation Mechanism in AlN Epilayers.

The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains of 0.18 × 10-3 cm-2 for flat substrate AlN and 0.11 × 10-3 cm-2 for nano-patterned substrate AlN are obtained by X-ray diffractometer (XRD). The screw and edge dislocation densities of samples are determined by XRD and transmission electron microscope (TEM), and the results indicate that the nano-patterned substrates are effective in reducing the threading dislocation density. The mechanism of the variation of the threading dislocation in AlN films grown on flat and nano-patterned substrates is investigated comparatively. The etch pit density (EPD) determined by preferential chemical etching is about 1.04 × 108 cm-2 for AlN grown on a nano-patterned substrate, which is slightly smaller than the results obtained by XRD and TEM investigation. Three types of etch pits with different sizes are all revealed on the AlN surface using the hot KOH etching method.

Tripling the light extraction efficiency of a deep ultraviolet LED using a nanostructured p-contact

Despite a wide array of applications, deep ultra-violet light emitting diodes offer relatively poor efficiencies compared to their optical counterparts. A contributing factor is the lower light extraction efficiency due to both highly absorbing p-contacts and total internal reflection. Here, we propose a structure consisting of a hexagonal periodic array of cylindrical nanoholes in the multi-layered p-contact which are filled with platinum. This nanostructure reduces the absorption of the p-contact layer, leading to a higher emission into the n-contact compared to a planar reference. An optimum geometry of the nanostructure allows a light extraction efficiency of 15.0%, much higher than the typical 4.6% of a planar reference. While the nanostructure strongly decreases the light absorption in the p-contact, it is still not able to considerably reduce the total internal reflection. Consequently, the nanostructured p-contact should be combined with other optical strategies, such as nanopatterned sapphire substrates to increase the efficiency even further. Despite this, the nanostructure described in this work provides a readily realizable path to enhancing the light extraction efficiency of state-of-the-art deep ultra-violet light emitting diodes.

Evolution of microstructure, stress and dislocation of AlN thick film on nanopatterned sapphire substrates by hydride vapor phase epitaxy

A crack-free AlN film with 4.5 μm thickness was grown on a 2-inch hole-type nano-patterned sapphire substrates (NPSSs) by hydride vapor phase epitaxy (HVPE). The coalescence, stress evolution, and dislocation annihilation mechanisms in the AlN layer have been investigated. The large voids located on the pattern region were caused by the undesirable parasitic crystallites grown on the sidewalls of the nano-pattern in the early growth stage. The coalescence of the c-plane AlN was hindered by these three-fold crystallites and the special triangle void appeared. The cross-sectional Raman line scan was used to characterize the change of stress with film thickness, which corresponds to the characteristics of different growth stages of AlN. Threading dislocations (TDs) mainly originate from the boundary between misaligned crystallites and the c-plane AlN and the coalescence of two adjacent c-plane AlN crystals, rather than the interface between sapphire and AlN.

The performance improvement of AlGaN/GaN heterojunction by using nano-patterned sapphire substrate

AlGaN/GaN heterojunction structures grown on nano-scale patterned sapphire substrates and planar sapphire substrates were investigated. The X-ray diffraction rocking curves indicated an improvement of crystalline quality by using sapphire substrate with concave nano-scale patterns. The surface morphology improvement of samples was characterized using atomic force microscope and scanning electron microscope. The transmission electron microscope results showed a significant dislocation reduction. The Hall-effect measurement confirmed that the mobility of two-dimensional electron gas was obviously improved.

Enhanced optical output of GaN-based near-ultraviolet light-emitting diodes using an ultra-thin air cavity nanopatterned sapphire substrate

A combined methodology is proposed to fabricate nanopatterned sapphire substrate with air cavity, which is used to improve the crystalline quality and light extraction efficiency of near-ultraviolet light-emitting diodes.

Effect of a Lateral Overgrowth Process on the Strain Evolution of AlN Films Grown on a Nanopatterned Sapphire Substrate for Ultraviolet‐C Light‐Emitting Diode Applications

Strain evolution of AlN grown on hole‐type (HT) and pillar‐type (PT) nanopatterned sapphire substrates (NPSSs) during the epitaxial lateral overgrowth process is comparably studied. It is found that, after complete coalescence, the residual strain of AlN on HT‐NPSS is tensile, whereas AlN on PT‐NPSS is almost strain free. The different strain states are associated with different behaviors of crystal merging during the lateral overgrowth process, depending on the type of pattern. Then, ultraviolet‐C (UVC) light‐emitting diode (LED) wafers are grown on two types of AlN templates with the distinctive strain states. It is confirmed that AlN on HT‐NPSS with tensile strain offers the advantage of producing a high‐quality AlGaN layer with a fairly flat surface, which is the key point for fabricating high‐performance UVC LEDs.

Strain management and AlN crystal quality improvement with an alternating V/III ratio AlN superlattice

We report the metal-organic chemical vapor deposition growth of high-quality AlN on sapphire enabled by an alternating V/III ratio AlN superlattice. We demonstrated that the insertion of an alternating V/III ratio AlN superlattice facilitates the relaxation of tensile stress during growth and assists the annihilation of threading dislocations. Dislocation inclination was proposed to play a major role in the stress relaxation and dislocation reduction, which not only provides an effective misfit-dislocation component but also increases the dislocation reaction probability. By this method, crack-free 3.25 μm-thick AlN films were grown on flat sapphire substrate (FSS) and nano-patterned sapphire substrate (NPSS) at 1180 °C, and the full width at half maximum of (002)/(102) rocking curves was 204/408 and 152/323 arcsec for such AlN/FSS and AlN/NPSS templates.

Temperature Dependence of Stress and Optical Properties in AlN Films Grown by MOCVD.

AlN epilayers were grown on a 2-inch [0001] conventional flat sapphire substrate (CSS) and a nano-patterned sapphire substrate (NPSS) by metalorganic chemical vapor deposition. In this work, the effect of the substrate template and temperature on stress and optical properties of AlN films has been studied by using Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible spectrophotometer and spectroscopic ellipsometry (SE). The AlN on NPSS exhibits lower compressive stress and strain values. The biaxial stress decreases from 1.59 to 0.60 GPa for AlN on CSS and from 0.90 to 0.38 GPa for AlN on NPSS sample in the temperature range 80–300 K, which shows compressive stress. According to the TEM data, the stress varies from tensile on the interface to compressive on the surface. It can be deduced that the nano-holes provide more channels for stress relaxation. Nano-patterning leads to a lower degree of disorder and stress/strain relaxes by the formation of the nano-hole structure between the interface of AlN epilayers and the substrate. The low crystal disorder and defects in the AlN on NPSS is confirmed by the small Urbach energy values. The variation in bandgap (Eg) and optical constants (n, k) with temperature are discussed in detail. Nano-patterning leads to poor light transmission due to light scattering, coupling, and trapping in nano-holes.

Different scattering effect of nano-patterned sapphire substrate for TM- and TE-polarized light emitted from AlGaN-based deep ultraviolet light-emitting diodes

In this work, the scattering mechanism by nano-patterned sapphire substrate (NPSS) for flip-chip AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) has been investigated systematically via three-dimensional finite-difference time-domain (3D FDTD) method. It is found that for the conventional DUV LED with a thick p-GaN layer, the NPSS structure can enhance the light extraction efficiency (LEE) for the transverse magnetic (TM)-polarized light because the TM-polarized light with large incident angles can be scattered into escape cones. However, the LEE for the transverse electric (TE)-polarized light is suppressed by NPSS structure because NPSS structure scatters some TE-polarized light out of the escape cones. Moreover, the highly absorptive p-GaN layer also seriously restricts the scattering efficiency of NPSS structure. Therefore, to reduce the optical absorption, meshed p-GaN structure is strongly proposed to greatly enhance the LEEs for both TM- and TE-polarized light of DUV LEDs grown on NPSS. Compared to the DUV LED with only NPSS structure and that with only meshed p-GaN layer, the LEE for the TE-polarized (TM-polarized) light for DUV LEDs with the combination of NPSS structure and meshed p-GaN structure can be enhanced by 124% (5 times) and 112% (4 times), respectively.

Realization of high efficiency AlGaN-based multiple quantum wells grown on nano-patterned sapphire substrates

Growth of AlGaN-based multiple quantum wells (MQWs) with an IQE > 80% at room temperature has been realized on nano-patterned sapphire substrates. A DUV-LED device is then fabricated taking such high IQE MQWs as the active region.

Efficiency improvement analysis of nano-patterned sapphire substrates and semi-transparent superlattice contact layer in UVC light-emitting diodes

UVC light-emitting diodes (LEDs, λ = 275 nm) with different types of contact layers and sapphire substrates were demonstrated on high-quality AlN templates. For LEDs on flat sapphire substrates (FSSs), replacing the absorbing p-GaN contact with p-AlGaN short-period superlattices (p-SPSLs) strongly enhanced the emission along the substrate normal. The integrated external quantum efficiency (EQE) increased from 2.4% to 3.9% under I = 350 mA. For LEDs with a p-SPSL contact, replacing the FSS with nano-patterned sapphire substrates slightly deteriorated the quality of epitaxy, but the overall EQE is still enhanced to 4.4% under I > 350 mA without lens encapsulation. According to the far-field intensity measurement, the light extraction is better improved along the high emission angle to the substrate normal. The interplay among substrates, dipole polarization, and EQE enhancement factors was further analyzed and discussed in the context.

Quasi-2D Growth of Aluminum Nitride Film on Graphene for Boosting Deep Ultraviolet Light-Emitting Diodes.

Efficient and low‐cost production of high‐quality aluminum nitride (AlN) films during heteroepitaxy is the key for the development of deep ultraviolet light‐emitting diodes (DUV‐LEDs). Here, the quasi‐2D growth of high‐quality AlN film with low strain and low dislocation density on graphene (Gr) is presented and a high‐performance 272 nm DUV‐LED is demonstrated. Guided by first‐principles calculations, it is found that AlN grown on Gr prefers lateral growth both energetically and kinetically, thereby resulting in a Gr‐driven quasi‐2D growth mode. The strong lateral growth mode enables most of dislocations to annihilate each other at the AlN/Gr interface, and therefore the AlN epilayer can quickly coalesce and flatten the nanopatterned sapphire substrate. Based on the high quality and low strain of AlN film grown on Gr, the as‐fabricated 272 nm DUV‐LED shows a 22% enhancement of output power than that with low‐temperature AlN buffer, following a negligible wavelength shift under high current. This facile strategy opens a pathway to drastically improve the performance of DUV‐LEDs.

Direct Growth of Nanopatterned Graphene on Sapphire and Its Application in Light Emitting Diodes

AbstractDirect growth of graphene films on functional substrates is immensely beneficial for the large‐scale applications of graphene by avoiding the transfer‐induced issues. Notably, the selective growth of patterned graphene will further boost the development of graphene‐based devices. Here, the direct growth of patterned graphene on the c‐plane of nanopatterned sapphire substrate (NPSS) is realized and the superiority of the patterned graphene for high‐performance ultraviolet light‐emitting diodes (UV‐LED) is demonstrated. As confirmed by density functional theory calculations and analog simulations, compared to the concave r‐plane the flat c‐plane of NPSS is characterized by a lower active barrier for methane decomposition and carbon species diffusion, as well as a greater supply of carbon precursor for graphene growth. The synthesized patterned graphene on the c‐plane of NPSS is verified to be monolayer and high quality. The patterned graphene enables the selective and well‐aligned nucleation of aluminium nitride (AlN) to promote rapid epitaxial lateral overgrowth of single‐crystal AlN films with low dislocation density. Consequently, the fabricated UV‐LED demonstrates high luminescence intensity and stability. The method is suitable for obtaining various patterned graphene by substrate design, which will allow for greater progress in the cutting‐edge applications of graphene.

Improved Light Extraction Efficiency of Light-Emitting Diode Grown on Nanoscale-Silicon-Dioxide-Patterned Sapphire Substrate

A conical-shaped Si dioxide nano-pattern was employed to sapphire substrate in order to improve the light extraction efficiency of light-emitting diodes. The conical-shaped Si dioxide nano-patterns were fabricated on a 2-inch sapphire wafer using direct imprinting of hydrogen silsesquioxane material. A blue-LED structure was grown on conical-shaped silicon-dioxide nano-patterned sapphire substrates. Photoluminescence and electroluminescence measurements were used to confirm the effectiveness of the nanoscale Si oxide patterned sapphire. An improvement in the luminescence efficiency was observed when nanoscale Si oxide patterned sapphire substrate was used. 1.5 times higher PL intensity and 1.6 times higher EL intensity were observed for GaN LED structure grown on nanoscale Si oxide patterned sapphire, compared to LED structure grown on conventional flat sapphire wafer.