Nanopatterned Sapphire Substrates Research Articles

GaN-based substrates and optoelectronic materials and devices

In order to solve the problems of GaN heteroepitaxy on sapphire substrate, some techniques were explored. Freestanding GaN substrates have been made by hydride vapor phase epitaxy (HVPE), laser lift-off (LLO), and chemical mechanical polishing techniques. Wafer bending and cracking in the HVPE growth were partly settled by pulsed flow modulation method. High-crystal quality was established for 1.2 mm thick GaN substrate by X-ray diffraction measurement, in which the full width of half maximum values were 72, 110 arcsec for (102), (002) peaks. A novel micro-size patterned sapphire substrate (PSS) and a nano PSS were also fabricated. High-power vertical structure light emitting diodes (VSLEDs) have been developed by Au–Sn eutectic wafer bonding, homemade micro-area LLO, and light extraction structure preparation. The high-injection-level active region with low temperature GaN sandwiched layers was used for low-efficiency droop. The light output power of VSLED was achieved as 400 mW driven at 350 mA, and the dominant wavelength is about 460 nm. The structures and properties of strain modulated superlattices (SLs) and quantum wells as well as advanced simulation of carriers transport across the electron blocking layer were investigated in laser diodes. The hole concentration was achieved as high as 1.6 × 1018 cm−3 in AlGaN/GaN SLs:Mg by inserting an AlN layer. High-quality AlGaN epilayers and structures were grown by MOCVD. Some device structures of UV LEDs and detectors were demonstrated. The emission wavelength of 262 nm UV LED has been successfully fabricated. At last, high-quality InN and InGaN materials for solar cell were grown by boundary-temperature-controlled epitaxy and growth-temperature-controlled epitaxy. Hall-effect measurement showed a recorded electron mobility of 3,280 cm2/(V s) and a residual electron concentration of 1.47 × 1017 cm−3 at 300 K.

Efficiency improvement of InGaN light emitting diodes with embedded self-assembled SiO2 nanosphere arrays

In this study, the periodic SiO2 nanosphere nanopatterned sapphire substrate (SiO2-NPSS) was made using self-assembled SiO2 nanosphere monolayer template and inductively coupled plasma (ICP) etching. And the self-assembled SiO2 nanosphere monolayer was directly embedded into the GaN/sapphire interface by nanoscale epitaxial lateral overgrowth (NELOG). For comparison, a common nanopatterned sapphire substrate (C-NPSS) was also made through dry etching with the SiO2 nanospheres used as the mask. Compared with LEDs grown on C-NPSS and flat sapphire substrate (FSS), the external quantum efficiency of LEDs with SiO2 nanopheres (SiO2-NPSS) was increased by 30.7% and 81.9% under a driving current 350mA. The SiO2-NPSS not only improved the crystalline quality of GaN but also enhanced the light extraction efficiency (LEE) of LED. And the SiO2-NPSS LED also showed more light in vertical direction and more uniform light distribution. By finite-difference time-domain (FDTD) simulation, we confirmed that more light could be reflected from the GaN/SiO2 interface than the GaN/sapphire interface because the refractive index of SiO2 was lower than that of sapphire. Therefore, LED grown on the SiO2-NPSS showed superior light extraction efficiency compared to that on C-NPSS.

The improvement of GaN-based light-emitting diodes using nanopatterned sapphire substrate with small pattern spacing

Self-assembly SiO2 nanosphere monolayer template is utilized to fabricate nanopatterned sapphire substrates (NPSSs) with 0-nm, 50-nm, and 120-nm spacing, receptively. The GaN growth on top of NPSS with 0-nm spacing has the best crystal quality because of laterally epitaxial overgrowth. However, GaN growth from pattern top is more difficult to get smooth surface than from pattern bottom. The rougher surface may result in a higher work voltage. The stimulation results of finite-difference time-domain (FDTD) display that too large or too small spacing lead to the reduced light extracted efficiency (LEE) of LEDs. Under a driving current 350 mA, the external quantum efficiencies (EQE) of GaN-based LEDs grown on NPSSs with 0-nm, 50-nm, and 120-nm spacing increase by 43.3%, 50.6%, and 39.1%, respectively, compared to that on flat sapphire substrate (FSS). The optimized pattern spacing is 50 nm for the NPSS with 600-nm pattern period.

Investigation and Comparison of the GaN-Based Light-Emitting Diodes Grown on High Aspect Ratio Nano-Cone and General Micro-Cone Patterned Sapphire Substrate

In this paper, we demonstrated the high performance GaN-based LEDs by using a high aspect ratio cone-shape nano-patterned sapphire substrate (HAR-NPSS). We utilized nano-imprint lithography (NIL) and dry-etching system to fabricate a high depth HAR-NPSS. The micro-scale patterned sapphire substrate (PSS) was also used for comparison. A great enhancement of light output was observed when GaN-based LEDs were grown on a HAR-NPSS or a PSS. The light output power of LEDs with a HAR-NPSS and LEDs with a PSS were enhanced of 49 and 38% compared to LEDs with a unpatterned sapphire substrate. The high output power of the LED with a HAR-NPSS indicated that the technology of NAR-NPSS not only can improve the crystalline quality of GaN-based LEDs but also a promising development to a NPSS.

Large-scale fabrication of nanopatterned sapphire substrates by annealing of patterned Al thin films by soft UV-nanoimprint lithography

Large-scale nanopatterned sapphire substrates were fabricated by annealing of patterned Al thin films. Patterned Al thin films were obtained by soft UV-nanoimprint lithography and reactive ion etching. The soft mold with 550-nm-wide lines separated by 250-nm space was composed of the toluene-diluted polydimethylsiloxane (PDMS) layer supported by the soft PDMS. Patterned Al thin films were subsequently subjected to dual-stage annealing due to the melting temperature of Al thin films (660°C). The first comprised a low-temperature oxidation anneal at 450°C for 24 h. This was followed by a high-temperature annealing in the range of 1,000°C and 1,200°C for 1 h to induce growth of the underlying sapphire single crystal to consume the oxide layer. The SEM results indicate that the patterns were retained on sapphire substrates after high-temperature annealing at less than 1,200°C. Finally, large-scale nanopatterned sapphire substrates were successfully fabricated by annealing of patterned Al thin films for 24 h at 450°C and 1 h at 1,000°C by soft UV-nanoimprint lithography.

InGaN-Based Light-Emitting Diodes Fabricated on Nano Patterned Sapphire Substrates with Pillar Height of More than 600 nm by Nanoimprint Lithography

GaN-based light-emitting diodes (LEDs) were fabricated on nano patterned sapphire substrates (nano-PSSs) by nanoimprint (NIP) lithography. A nano-PSS with a pillar height of more than 600 nm was achieved. The surface emission of the LEDs was strongly affected by pillar height, and the surface emission intensity was highest at a pillar height of 250 nm. In contrast, the external quantum efficiency of the LEDs on the nano-PSSs with diameters of 100 and 450 nm was approximately 30% higher than that on a flat sapphire substrate, which is similar to that on a conventional PSS.

Effect of exposure parameters and annealing on the structure and morphological properties of nanopatterned sapphire substrates prepared by solid state reaction

Nanopatterned sapphire substrates were prepared by solid state reaction of patterned Al films obtained by E-Beam lithography of a PMMA/copolymer bilayer resist and lift-off. The effect of exposure parameters and annealing on the structure and morphological properties of nanopatterned sapphire substrates prepared by solid state reaction were investigated by a scanning electron microscope (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and Raman, respectively. The circular Al patterns with diameter/spacing being 500nm/1000nm were obtained by optimal exposure diameter/exposure dosage of 400nm/200μC/cm2. Patterned Al films were subsequently subjected to solid state reaction by dual stage annealing due to the melting temperature of Al thin films (660°C). The hillocks formation on Al films was minimized with an oxidation anneal at 450°C. Moreover, the little change in the morphology of Al patterns was observed after annealing at 450°C. The SEM and AFM results show the patterns were retained on sapphire substrates after high temperature annealing at less than 1200°C. The XRD and Raman results reveal that the composition and orientation of island patterns prepared by solid state reaction for 24h at 450°C, and 1h 1000°C were the same as that of the sapphire (0001) substrates.

Fabrication of nanopatterned sapphire substrates by annealing of patterned Al thin films by Laser Interference Lithography

Nanopatterned sapphire substrates were fabricated by annealing of patterned Al thin films. Square-patterned Al thin films with the diagonal length of 600 nm, period of 1 um and height of ∼200 nm were obtained by the Laser Interference Lithography and Reactive Ion Etching. Patterned Al thin films were subsequently subjected to dual stage annealing due to the melting temperature of Al thin films (660 ∘C). The first comprised a low temperature oxidation anneal. The hillocks formation on Al thin films was minimized with an oxidation annealing at 450 ∘C for 24 h. The little change in the morphology of patterned Al thin films was observed at 450 ∘C for 24 h. This was followed by a high temperature annealing to induce growth of the underlying sapphire single crystal to consume the oxide layer. The SEM results show the patterns were retained on sapphire substrates after high temperature annealing at less than 1200 ∘C. The XRD and Raman results reveal that the orientation of island patterns by dual stage annealing of patterned Al thin films for 24 h at 450 ∘C, and 1 h at 1000 ∘C, was the same as that of the sapphire (0001) substrates.

Fabrication of volcano-shaped nano-patterned sapphire substrates using colloidal self-assembly and wet chemical etching

Patterned sapphire substrates (PSS) have been widely used to enhance the light output power in GaN-based light emitting diodes. The shape and feature size of the pattern in a PSS affect its enhancement efficiency to a great degree. In this work we demonstrate the nanoscale fabrication of volcano-shaped PSS using a wet chemical etching approach in combination with a colloidal monolayer templating strategy. Detailed analysis by scanning electron microscopy reveals that the unique pattern shape is a result of the different corrosion-resistant abilities of silica masks of different effective heights during wet chemical etching. The formation of silica etching masks of different effective heights has been ascribed to the silica precursor solution in the interstice of the colloidal monolayer template being distributed unevenly after infiltration. In the subsequent wet chemical etching process, the active reaction sites altered as etching duration was prolonged, resulting in the formation of volcano-shaped nano-patterned sapphire substrates.

Research for Patterned Sapphire Substrates of GaN-based LEDs

Patterned Sapphire Substrate(PSS) which can reduce the density of threading dislocation and enhance the effect of scattering is widely used to fabricate high-power Light-Emitting-Diode(LED) chip.In this paper,the finitedifference time-domain(FDTD) method was used to simulate and analyze the light extraction efficiency(LEE) of GaN-based micro-scale and nano-scale patterned sapphire substrates LED.The results show that the nano-patterned sapphire substrate(NPSS) has a significantly better LEE than that of micro-patterned sapphire substrate(MPSS).And in NPSS,the LEE of the pillar structure improveed 96.6% comparing to other nano-patterned structures.Large areas of table-like nano-sapphire patterned substrates are successfully prepared through soft embossing technology.The photoluminescence(PL) of the LED grown on table-like nano-sapphire patterned substrates is 8 times stronger than that of the LED grown on the unpatterned sapphire wafers.

282-nm AlGaN-based deep ultraviolet light-emitting diodes with improved performance on nano-patterned sapphire substrates

We first report AlGaN-based deep ultraviolet light-emitting diodes (UV-LEDs) grown on nano-patterned sapphire substrates (NPSS) prepared through a nanosphere lithography technique. The AlN coalescence thickness on NPSS is only 3 μm due to AlN's nano-scaled lateral growth, which also leads to low dislocation densities in AlN and epi-layers above. On NPSS, the light-output power of a 282-nm UV-LED reaches 3.03 mW at 20 mA with external quantum efficiency of 3.45%, exhibiting 98% better performance than that on flat sapphire. Temperature-dependent photoluminescence reveals this significant enhancement to be a combination of higher internal quantum efficiency and higher light extraction efficiency.

Light extraction efficiency analysis of GaN-based light-emitting diodes with nanopatterned sapphire substrates

In this paper, we propose a method to analyze the light extraction efficiency (LEE) enhancement of a nanopatterned sapphire substrates (NPSS) light-emitting diode (LED) by comparing wave optics software with ray optics software. Finite-difference time-domain (FDTD) simulations represent the wave optics software and Light Tools (LTs) simulations represent the ray optics software. First, we find the trends of and an optimal solution for the LEE enhancement when the 2D-FDTD simulations are used to save on simulation time and computational memory. The rigorous coupled-wave analysis method is utilized to explain the trend we get from the 2D-FDTD algorithm. The optimal solution is then applied in 3D-FDTD and LTs simulations. The results are similar and the difference in LEE enhancement between the two simulations does not exceed 8.5% in the small LED chip area. More than 10(4) times computational memory is saved during the LTs simulation in comparison to the 3D-FDTD simulation. Moreover, LEE enhancement from the side of the LED can be obtained in the LTs simulation. An actual-size NPSS LED is simulated using the LTs. The results show a more than 307% improvement in the total LEE enhancement of the NPSS LED with the optimal solution compared to the conventional LED.

I-1-3 CHF_3、BCl_3誘導結合プラズマを用いたナノパターンサファイア基板のエッチング(窒化物半導体デバイスの精密加工プロセス-窒化物LEDに関わる先端デバイスプロセシング-,口頭発表)

I-1-3 CHF_3、BCl_3誘導結合プラズマを用いたナノパターンサファイア基板のエッチング(窒化物半導体デバイスの精密加工プロセス-窒化物LEDに関わる先端デバイスプロセシング-,口頭発表)

Improvement of Epitaxy GaN Quality Using Liquid-Phase Deposited Nano-Patterned Sapphire Substrates

A relatively simple and easy and inexpensive liquid-phase deposition (LPD) method is employed to introduce nanoscale silica hemispheres on sapphire substrates for fabricating a nano-patterned sapphire substrate (NPSS). Compared with GaN grown on sapphire without any pattern, the NPSS-GaN film is of much better quality as observed by scanning electron microscopy, transmission electron-microscopy, X-ray diffraction, cathodoluminescence, and photoluminescence. This is because GaN is initiated from the c-plane instead of the LPD-silica surface. In addition, many dislocations within the NPSS-GaN bend toward the patterns, or end at the GaN/void interfaces.

Investigation of nanopatterned c-plane sapphire Substrates for Growths of polar and nonpolar GaN epilayers

The nanopatterned c-plane sapphire substrate with different morphologies can be successfully used to grow polar and nonpolar GaN epilayers. The nanopattern of sapphire substrate was prepared with natural lithography and dry-etching methods and the following GaN epilayer was grown by MOCVD. The GaN crystal orientation and crystalline quality were then characterized by high resolution X-ray diffraction, which reveals the GaN epilayer can be c-plane (0001) or m-plane (101̄0) orientation depending on the surface morphology of nanopatterned sapphire substrate. The corresponding full width at half maximum of the rocking curves for c- and m-plane GaN are 211 and 316arcsec, respectively. The rms surface roughness was measured to be 0.3nm by atomic force microscopy. The atomic structure of the sapphire-GaN heterointerface was studied by high resolution transmission electron microscope to reveal the growth mechanism. Furthermore, photoluminescence, time-resolved photoluminescence, and polarization Raman spectroscopy were employed to realize the optical and structural properties of these GaN epilayers.

Investigation of GaN Films Grown on Liquid-Phase Deposited SiO2 Nanopatterned Sapphire Substrates

A simple, easy and relatively inexpensive liquid phase deposition (LPD) method was used to introduce nano SiO2 on sapphire substrates to fabricate nanoscale patterned sapphire substrates (PSS). Two kinds of nanoscale PSS were used to grow GaN, namely “NPOS” which is nano-pattern oxide on sapphire substrate and “NPSS” which is nano-patterned sapphire substrate. It was found that upper region of NPSS-GaN had the best quality. This is because as the growth time increased, laterally-grown GaN caused the threading dislocations to bend toward the patterns. Besides, voids formed on the NPSS pattern sidewalls caused more threading dislocation bending toward these voids.

GaN-Based LEDs on Nano-Patterned Sapphire Substrates

We have successfully fabricated light emitting diodes (LEDs) based on patterned sapphire substrates (PSSs) fabricated by employing nanoimprint lithography (NIL) technique. The nano-patterns were designed as regular triangles consisting of columns, whose diameters and pitches were 100, 150, 200, 250 nm and 200, 300, 400, 500 nm, respectively. 412 nm wavelength LEDs grown by metal organic chemical vapor deposition (MOCVD) method were also demonstrated. The NIL technique and nano-etching by employing RIE were demonstrated in details. The qualities of all LEDs based on PSSs are superior compared with that non-patterned sapphire substrate LED. The experimental results showed that the light output power was increased by using the PSS structure. At a driving current of 20 mA, the light output powers of LEDs based on PSSs with 200, 300, 400 and 500 nm pitches are enhanced by 59%, 79%, 42% and 48%, compared with the conventional LEDs. These results provide promising potential to increase output powers of commercial light-emitting devices.

The aspect ratio effects on the performances of GaN-based light-emitting diodes with nanopatterned sapphire substrates

The nanopatterned sapphire substrates (NPSSs) with aspect ratio that varied from 2.00 to 2.50 were fabricated by nanoimprint lithography. We could improve the epitaxial film quality and enhance the light extraction efficiency by NPSS technique. In this work, the aspect ratio effects on the performances of GaN-based light-emitting diodes (LEDs) with NPSS were investigated. The light output enhancement of GaN-based LEDs with NPSS was increased from 11% to 27% as the aspect ratio of the NPSS increases from 2.00 to 2.50. Owing to the same improvement of crystalline quality by using various aspect ratios of NPSS, these results indicated that the aspect ratio of the NPSS is strongly related to the light extraction efficiency.

Light‐emitting diodes fabricated on nanopatterned sapphire substrates by thermal lithography

AbstractGaN‐based light‐emitting diodes (LEDs) were fabricated on nanopatterned sapphire substrates (nano‐PSSs) by thermal lithography with laser irradiation. The characteristics of the LEDs were measured by X‐ray diffraction analysis and scanning electron microscopy. The crystalline qualities of the LEDs deteriorated with the shortening of the nanopattern periods. The nucleation of GaN was initiated from the nanopatterns, which increased the number of grain boundaries. The output power of the LEDs on the nano‐PSSs with a diameter of 200 nm and 500 nm periods was 70% higher than that on flat sapphire substrate. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved output power of GaN-based light-emitting diodes grown on a nanopatterned sapphire substrate

This letter describes the improved output power of GaN-based light-emitting diodes (LEDs) formed on a nanopatterned sapphire substrate (NPSS) prepared through etching with a self-assembled monolayer of 750-nm-diameter SiO2 nanospheres used as the mask. The output power of NPSS LEDs was 76% greater than that of LEDs on a flat sapphire substrate. Three-dimensional finite-difference time-domain calculation predicted a 40% enhancement in light extraction efficiency of NPSS LEDs. In addition, the reduction of full widths at half maximum in the ω-scan rocking curves for the (0 0 2) and (1 0 2) planes of GaN on NPSS suggested improved crystal quality.