Patterned Sapphire Substrate Research Articles

43‐2: Low Efficiency Attenuation and Stable Monochromaticity for Non‐Polar m‐Plane Micro‐Ligh‐Emitting‐Diodes (Micro‐LEDs)

In this paper, the gallium nitride (GaN) free‐standing non‐polar m‐plane micro‐LEDs and polar c‐plane micro‐LEDs on patterned sapphire substrate with a 550‐nm emission light were fabricated and characterized. Compared with c‐plane micro‐LEDs suffering from strong quantum confined stark effect (QCSE), the absence of polarization effect in m‐plane devices contributed to a lower efficiency attenuation and narrower peak and color shift with elevating current density. Specifically, the peak wavelength shift for c‐plane and m‐plane micro‐LEDs was 11.81 nm and 8.12 nm respectively from 1 to 500 A/cm2, and the efficiency droop ratio was 50.2% for c‐plane and 27.2% for mplane devices from 0.1 to 300 A/cm2. Importantly, our m‐plane micro‐LEDs achieved a stable monochromaticity and wider color gamut under different injection current density (97.9% and 98.2% of Rec. 2020 at 1 and 500 A/cm2).

Mechanism of low Ohmic contact resistance to p-type GaN by suppressed edge dislocations

In this paper, an excellent Ohmic contact to p-GaN with a low specific contact resistance (ρc) of 2.0 × 10−5 Ω·cm2 is demonstrated using a patterned sapphire substrate (PSS) and oxidized Ni/Au contacts. GaN epitaxy with high crystal quality on the PSS, confirmed by high-resolution x-ray diffraction, played a key role in the improved Ohmic contact to p-GaN. The edge dislocations were annihilated during the epitaxial process on the PSS to afford a low surface dislocation density, which was in accordance with the results of transmission electron microscopy and cathodoluminescence spectroscopy. Furthermore, a reduced Fermi level and enhanced activation efficiency of Mg with suppressed segregation around the dislocations were demonstrated by Kelvin probe force microscopy and contact Hall measurements, respectively. A GaN p-channel metal oxide semiconductor device fabricated on the PSS displayed a twofold higher forward current density and superior gate controllability compared with that fabricated on a conventional sapphire substrate.

GaN-based green resonant-cavity light-emitting diodes with Al mirror and copper plate.

In this Letter, GaN-based green resonant-cavity light-emitting diodes (RCLEDs) with a low-cost aluminum (Al) metal bottom mirror, a dielectric top mirror, and a copper (Cu) supporting plate were fabricated. The green-emitting epitaxial wafer was grown on a patterned sapphire substrate (PSS) to ensure high crystal quality (CQ). Laser lift-off (LLO) of the PSS and electrical plating of a Cu supporting plate were then carried out to realize the vertical device structure. The emission wavelength and full width at half maximum (FWHM) of the main emission peak of the device are ∼518 nm and 14 nm, respectively. Under the current density of 50 A/cm2, a relatively high light output power (LOP) of 11.1 mW can be obtained from the green RCLED. Moreover, when the current injection is 20 mA (8 A/cm2), the corresponding forward bias voltage is as low as ∼2.46 V. The reasons for the low operating voltage and high LOP can be attributed to the improvement of CQ, the release of residual compressive stress of the GaN-based epilayer due to the removal of PSS, and better heat dissipation properties of the Cu supporting plate.

High Luminous Efficacy Phosphor-Converted Mass-Produced White LEDs Achieved by AlN Prebuffer and Transitional-Refraction-Index Patterned Sapphire Substrate.

Constant advance in improving the luminous efficacy (ηL) of nitride-based light-emitting diodes (LEDs) plays a critical role for saving measurable amounts of energy. Further development is motivated to approach the efficiency limit for this material system while reducing the costs. In this work, strategies of using thin AlN prebuffer and transitional-refraction-index patterned sapphire substrate (TPSS) were proposed, which pushed up the efficiency of white LEDs (WLEDs). The AlN prebuffer was obtained through physical vapor deposition (PVD) method and TPSS was fabricated by dry-etched periodic silica arrays covered on sapphire. Devices in mass production confirmed that PVD AlN prebuffer was able to improve the light output power (φe) of blue LEDs (BLEDs) by 2.53% while increasing the productivity by ~8% through shortening the growth time. Additionally, BLEDs on TPSS exhibited an enhanced top ηext of 5.65% in contrast to BLEDs on the conventional PSS through Monte Carlo ray-tracing simulation. Consequently, φe of BLEDs was experimentally enhanced by 10% at an injected current density (Jin) of 40 A/cm2. A peak ηL of 295.2 lm/W at a Jin of 0.9 A/cm2 and the representative ηL of 282.4 lm/W at a Jin of 5.6 A/cm2 for phosphor-converted WLEDs were achieved at a correlated color temperature of 4592 K.

Effect of initial condition on the quality of GaN film and AlGaN/GaN heterojunction grown on flat sapphire substrate with ex-situ sputtered AlN by MOCVD

The growth condition of initial medium (870 °C–920 °C) temperature (MT) layer is crucial to the quality of high temperature (HT) GaN layer and AlGaN/GaN heterojunction grown on flat sapphire substrate (FSS) with ex-situ sputtered AlN. The superior wetting of GaN and AlN makes the MT layer tend to directly two-dimensional (2D) growth mode at the initial stage. In this study, it was found that high pressure was effective in lowering Ga adatom diffusion and promoting early three-dimensional (3D) growth, which resulted in reducing (102) X-ray full-width-at-half-maximum as well as edge dislocation density significantly. The lowest X-ray rocking curve full-width-at-half-maximum of (002) and (102) were 35 arcsec and 220 arcsec respectively, which were comparable with GaN layer grown on patterned sapphire substrate (PSS). High quality AlGaN/GaN heterojunction was achieved with sheet electron density of 1.18×1013cm−2, Hall mobility of 1909 cm2/V⋅s and sheet resistance of 336 Ω/□. Controlled growth interruption was carried out to study the evolution mechanism of MT layer growth.

Mechanisms and Performance Analysis of GaN-Based Micro-LED Grown on Pattern Sapphire Substrate by Laser Lift-Off Process

The laser lift-off (LLO) process was employed to fabricate a monochromatic GaN-based green micro-light-emitting-diode (Micro-LED) 14 × 14 arrays grown on pattern sapphire substrate (PSS) in this research. A new experimental phenomenon and internal mechanism of some epitaxial residues on the top and sidewalls of PSS protrusion after laser stripping were deeply discussed. In addition, the reasons for the formation of micro lattice dislocation of epitaxial layer crystal caused by laser were also analyzed. The leakage current of the array devices after LLO was 200 times higher that before LLO under the same voltage, but the light output current efficiency was discovered to be 109.7% higher under the 500 cd m−2 brightness. The results indicate peak wavelength was blue shift 3 nm once the LLO process had been performed. Furthermore, the optical pattern from a “heart-shaped” distribution before LLO was changed into the maximum intensity directly above the array devices. The luminous angle was also reduced from ±75° to ±65°, which indicates that the collimation of the light after LLO process was increased. The results are high important for understanding the optical properties of Micro-LED array devices after LLO process.

Effects of InGaN-interlayer on closed stripes of GaN grown by serpentine channel patterned sapphire substrate

Gallium nitride (GaN) is a widely investigated semiconductor owing to its fascinating features suitable for a plethora of optoelectronic applications; nevertheless, high-quality growth of this material remains a challenge. In this work, the crystal quality of GaN grown by serpentine channel patterned sapphire substrates (SCPSS) is improved via introducing indium gallium nitride (InGaN) as an interlayer (IL). The closed stripes of GaN are grown by the modified design of SCPSS. Our results show that the crystal quality of GaN grown in the form of closed stripes is superior than conventionally grown counterpart. Transmission electron microscopy (TEM) revealed that the design of SCPSS controls all threading dislocations (TDs) while cathodoluminescence (CL) analysis showed that the closed stripes are relaxed and produce many TDs. Therefore, further control over the TDs was achieved by InGaN-IL. During the growth process, the temperature of GaN was reduced and 20 nm thick InGaN-IL was added at 800 The detailed characterization of our developed samples indicated that the additional use of InGaN-IL in SCPSS is very effective for improving the crystal quality of GaN. We anticipate that our findings will improve the current understanding about the growth of high-quality GaN towards efficient optoelectronic devices.

Wafer-Scale LSPR Substrate: Oblique Deposition of Gold on a Patterned Sapphire Substrate.

Label-free detection of biomolecules using localized surface plasmon resonance (LSPR) substrates is a highly attractive method for point-of-care (POC) testing. One of the remaining challenges to developing LSPR-based POC devices is to fabricate the LSPR substrates with large-scale, reproducible, and high-throughput. Herein, a fabrication strategy for wafer-scale LSPR substrates is demonstrated using reproducible, high-throughput techniques, such as nanoimprint lithography, wet-etching, and thin film deposition. A transparent sapphire wafer, on which SiO2-nanodot hard masks were formed via nanoimprint lithography, was anisotropically etched by a mixed solution of H2SO4 and H3PO4, resulting in a patterned sapphire substrate (PSS). An LSPR substrate was finally fabricated by oblique deposition of Au onto the PSS, which was then applied to label-free detection of the binding events of biomolecules. To the best of our knowledge, this paper is the first report on the application of the PSS used as an LSPR template by obliquely depositing a metal.

A Novel Synthesis of ZnO Nanoflower Arrays Using a Lift-Off Technique with Different Thicknesses of Al Sacrificial Layers on a Patterned Sapphire Substrate.

A novel method to synthesize large-scale ZnO nanoflower arrays using a protrusion patterned ZnO seed layer was investigated. Different thicknesses of aluminum (Al) film were deposited on the concave patterned sapphire substrate as a sacrificial layer. ZnO gel was layered onto the Al film as a seed layer and OE-6370HF AB optical glue was used as the adhesive material. A lift-off technique was used to transfer the protrusion patterned ZnO/AB glue seed layer to a P-type Si <100> wafer. The hydrothermal method using Zn(CH3COO)2 and C6H12N4 solutions as liquid precursors was used to synthesize ZnO nanoflower arrays on the patterned seed layer. X-ray diffraction spectra, field-effect scanning electron microscopy, focused ion beam milling (for obtaining cross-sectional views), and photoluminescence (PL) spectrometry were used to analyze the effects that different synthesis times and different thicknesses of Al sacrificial layer had on the properties of ZnO nanoflower arrays. These effects included an increased diameter, and a decreased height, density (i.e., number of nanorods in μm−2), total surface area, total volume, and maximum emission intensity of PL spectrum. We showed that when the synthesis time and the thickness of the Al sacrificial layer were increased, the emission intensities of the ultraviolet light and visible light had different variations.

Cathodoluminescent Analysis of Sapphire Surface Etching Processes in a Medium-Energy Electron Beam.

Sapphire crystals are widely used in optics and optoelectronics. In this regard, it is important to study the stability of crystals under external influence and the possibility of modifying their surfaces by external influence. This work presents the results of studying the processes of the action of an electron beam with an average energy of 70 keV or less under vacuum conditions on the surfaces of sapphire substrates of various orientations. The effect of etching a sapphire surface by an electron beam in vacuum at room temperature was discovered. The highest etching rate was observed for A-plane sapphire (the average pit etching rate was 10−6 µm3/s). It was shown that the rate of etching of a sapphire surface increased many times over when gold is deposited. An in situ method for studying the process of etching a sapphire surface using cathodoluminescence analysis was considered. Possible mechanisms of sapphire etching by a beam of bombarding electrons were considered. The results obtained could be important in solving the problem of the stability of sapphire windows used in various conditions, including outer space. In addition, the proposed method of metal-stimulated etching of a sapphire surface can be widely used in patterned sapphire substrate (PSS) technology and further forming low-dislocation light-emitting structures on them.

Polymorphism and Faceting in Ga2O3 Layers Grown by HVPE at Various Gallium‐to‐Oxygen Ratios

In article number 2100331, Sevastian Shapenkov and co-workers describe phase contents and properties of layers of metastable gallium oxide phases obtained in halide vapor phase epitaxy on different substrates with different oxygen-to-gallium ratio. The cover image demonstrates crystallization of gallium oxide on patterned sapphire substrate (upper figures) in secondary electrons and on gallium nitride layer (bottom figures) in cathodoluminescence. On patterned sapphire, α-Ga2O3 is formed on substrate's cons and κ-Ga2O3 in valleys. On gallium nitride, there are hexagonal κ-Ga2O3 pillars correlating with positions of threading dislocations (black dots) in GaN. – This article belongs to the Special Section “Compound Semiconductors”, comprising 14 articles in this issue: see the Guest Editorial (article number 2200031) by Mattias Hammar, Anders Hallén, and Sebastian Lourdudoss.

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.

Application of patterned sapphire substrate for III-nitride light-emitting diodes.

Recent decades have witnessed flourishing prosperity of III-nitride emitters in solid-state lighting and high-resolution displays. As one of the widely used substrates, sapphire shows superiority for heteroepitaxial growth of III-nitride light-emitting diode (LED) structure, due to the advantages of stability, low cost, high mechanical strength, as well as mature fabrication technology. However, realization of efficient LEDs grown on sapphire substrate is impeded by high density of defects in epilayers and low light extraction efficiency. The emergence of patterned sapphire substrate (PSS) turns out to be a promising and effective technology to overcome these problems and enhance the LED performances. In this review, we first introduce the background and recent advances of PSS applied in III-nitride visible and ultraviolet LEDs are. Then, we summarize the fabrication methods of PSS, together with novel methods to define nanometre-scale patterned structures. We further demonstrate the effect of PSS that contributes to reduce the threading dislocation density (TDD) of epilayers in detail. Meanwhile, mechanism of light extraction efficiency enhancement by adopting PSS is presented based on numerical analysis. Next, we explore the influence of PSS structural parameters (e.g. pattern size, pattern shape and aspect ratio) on LED performances, spanning from visible to deep ultraviolet UV emission region. Finally, challenges and potential prospects in PSS for future LED development are proposed and forecasted as well.

Influence of growth temperature of p-GaN layer on the characteristics of InGaN/GaN blue light emitting diodes

Optimisation for p-GaN epitaxial growth temperature ranges from 920°C to 1020°C was successfully conducted on InGaN/GaN blue light-emitting diodes (LEDs). These InGaN/GaN blue LEDs have been grown on patterned sapphire substrates (PSS) via metal organic chemical vapour deposition (MOCVD). The significance of p-GaN growth temperature was studied in detail according to the structural and optoelectronic performances of InGaN/GaN blue react to depth mechanism of surface evolution to LEDs grown on PSS. From experimental analysis, increment in the epitaxial growth temperature of p-GaN layer caused the crystalline quality of GaN and structural properties slightly improve. Other than that, there are decrement of RMS roughness and PL FWHM due to the increment in the epitaxial growth temperature of p-GaN layer. However, the epitaxial growth temperature of p-GaN increase led to no improvement of optoelectronic properties of InGaN/GaN blue LEDs. The LEDs with higher epitaxial growth temperature of the p-GaN layer show poorer optoelectronic properties. Therefore, the optimised epitaxial growth temperature of the p-GaN layer is of significant importance in achieving highly efficient InGaN/GaN blue LEDs.Besides that, introducing PSS as the substrate did not demonstrate high impact to achieve better performance LEDs. Further improvement of the p-GaN layer crystalline and structural quality might be required for the optimisation of the optoelectronic properties of InGaN/GaN blue LEDs.

Effect of nucleation time on GaN layer grown on different shapes of patterned sapphire substrate

In this work, GaN layer was grown on patterned sapphire substrate (PSS) with the shape of cone-patterned sapphire substrate (CPSS) and dome-patterned sapphire substrate (DPSS) at different nucleation times. The nucleation time was varied at 40 and 80 seconds. The results showed that the nucleation islands were bigger with nucleation time for the growths on CPSS and DPSS. With bigger islands, the GaN layer evenly coalesced, which in turn, reduced the dislocation density as well as surface roughness in and of the overgrown GaN layer. Nonetheless, the results were better for the GaN grown on DPSS in comparison to the GaN layer grown on CPSS. The reason was associated to the curvature of the dome patterns, which was more slanted than cone patterns, and thereby, resulting in smoother surface of nucleation on the sidewalls. Hence, the multiple growth orientations were less promoted in comparison to the growth on CPSS.

GaN nucleation on patterned sapphire substrate with different shapes for improved GaN overgrowth

GaN was grown on cone- and dome-patterned sapphire (CPSS and DPSS) at different nucleation times of 40, 80 and 160 s. The GaN was also grown on flat sapphire substrate (FSS) for comparison. The results showed that the GaN growth improved by increasing the nucleation time, except for the case of FSS. Compared to CPSS, DPSS led to better GaN growth. This was due to the geometry of the dome patterns, which had more slanted curvature than cone patterns. Hence, more nucleation was promoted on the trenches and this was desirable to improve the GaN overgrowth. With 160 s of nucleation, the GaN coalescence was better for the growth on DPSS than on CPSS. This resulted in an improved surface of the overgrown GaN layer and lower full-width at half-maximum of XRD (FWHM-XRD) peaks. Strain study showed that the GaN layers grown on DPSS exhibited larger residual in-plane strain than the layers on CPSS due to the achievement of better overgrown GaN. Nonetheless, the strain reduced with nucleation time due to further dislocations inclination. Generally, the GaN layers on FSS showed better crystalline quality than on CPSS and DPSS. However, the layers suffered from larger strain.

Polymorphism and Faceting in Ga2O3 Layers Grown by HVPE at Various Gallium‐to‐Oxygen Ratios

Herein, the influence of the oxygen flow on the formation of metastable polymorphs of gallium oxide (Ga2O3) grown by halide vapor phase epitaxy (HVPE) on c‐plane patterned sapphire substrates (PSS), on gallium nitride (GaN) templates, and on m‐plane smooth sapphire substrates is reported. X‐ray diffraction, scanning electron microscopy, and cathodoluminescence are used to identify different polymorphic phases. The samples deposited on bare PSS exhibit faceted growth of the α‐Ga2O3 on the cones of the sapphire substrate and the formation of the κ‐Ga2O3 between the cones. In contrast, growth on GaN templates results in hexagonal columns of κ‐Ga2O3 which produce a continuous smooth layer upon coalescence. The growth of Ga2O3 on m‐plane sapphire substrates results in overgrown pyramids of the α‐phase. For all types of substrates, the variation of the oxygen flow affects only the thickness and coalescence of Ga2O3 layers. Thus, the growth of Ga2O3 metastable polymorphs is mainly influenced by the form, orientation, and symmetry of the substrate and shows a weak dependence on stoichiometry.

Development of highly efficient ultraviolet LEDs on hybrid patterned sapphire substrates.

A hybrid patterned sapphire substrate (HPSS) aiming to achieve high-quality Al(Ga)N epilayers for the development of GaN-based ultraviolet light-emitting diodes (UV LEDs) has been prepared. The high-resolution X-ray diffraction measurements reveal that the Al(Ga)N epilayers grown on a HPSS and conventional patterned sapphire substrate (CPSS) have similar structural quality. More importantly, benefiting from the larger refractive index contrast between the patterned silica array and sapphire, the photons can escape from the hybrid substrate with an improved transmittance in the UV band. As a result, in comparison with the UV LEDs grown on the CPSS, the LEDs grown on the HPSS exhibit a significantly enhanced light output power by 14.5% and more than 22.9% higher peak external quantum efficiency, owing to the boost of the light extraction efficiency from the adoption of the HPSS which can be used as a promising substrate to realize high-efficiency and high-power UV LEDs of the future.

Special Issue on the 40th Anniversary of University of Macau

Special Issue on the 40th Anniversary of University of Macau

Stacked GaN/AlN last quantum barrier for high-efficiency InGaN-based green light-emitting diodes.

Realization of efficient InGaN-based green light-emitting diodes (LEDs) is highly desirable in solid-state lighting industry. Here, we propose a stacked last quantum barrier (SLQB) GaN/AlN layer for green (∼550nm) LEDs grown on patterned sapphire substrate to improve the device performance. A green LED with a SLQB achieves a light output power (LOP) of 13 mW at 15 mA, which is 35% higher than that of LEDs with a conventional last quantum barrier (CLQB) GaN layer. In addition, the forward voltage of the green LED with the SLQB is reduced by 0.16 V compared with green LEDs with the CLQB. Simulation results demonstrate that the AlN layer in the SLQB increases the effective barrier height for electrons and alleviates the electron leakage effect. It also enables an intraband tunneling process for holes to inject into the active region, promoting the hole injection efficiency. As a result, we successfully obtain InGaN-based green LEDs with remarkably improved carrier injection efficiency by adopting the SLQB structure.