Nanoscale Patterned Sapphire Substrates Research Articles

Effect of grain coalescence on dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates

The effect of grain coalescence on the dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates with low-temperature grown GaN and physical vapour deposition AlN nucleation layers is comparably investigated.

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.

Improved Light Extraction Efficiency of GaN Based Blue Light Emitting Diode Using Nano-Scaled Patterned Sapphire Substrate

In this study, two different conical-shape nano-patterns were fabricated on the 2-inch diameter sapphire wafer in order to improve the efficiency of light-emitting diodes. The conical-shape nano-patterns were fabricated on the sapphire wafer using a nanoimprint technique and dry etching method. A blue LED structure was grown on the nanoscale-patterned sapphire substrates. The photoluminescence and electroluminescence were measured to confirm the effectiveness of the nano-scale patterns. An improvement in the luminescence efficiency was observed in case that nano-patterned sapphire substrate was used; a 1.44 times-higher photoluminescence intensity and a 1.5 times-higher electroluminescence intensity were observed, compared to those of the light-emitting diodes structure grown on a conventional flat sapphire wafer.

Comparative study of AlGaN/GaN heterostructures grown on different sapphire substrates

Abstract AlGaN/GaN heterostructures were grown on the conventional planar sapphire substrate (CSS), nano-scale patterned sapphire substrate (NPSS) and micro-scale patterned sapphire substrates (MPSS) by metal organic chemical vapor deposition respectively. The heterostructure grown on the MPSS has significantly lower threading dislocation density (TDD) than those grown on the NPSS and CSS. The bending and quenching of threading dislocations induced by lateral growth are the main factors for the reduction of TDD. Correspondingly, the carrier mobility of heterostructure grown on the MPSS is also the highest.

InGaN-based light-emitting diodes grown on various aspect ratios of concave nanopattern sapphire substrate

This study presented the optoelectronic properties of InGaN-based light-emitting diodes (LEDs) grown on nanoscale-patterned sapphire substrates (NPSSs) with various aspect ratio (AR) concave nanopatterns. Concave nanopatterns with low ARs of ∼0.5 (LAR-NPSS) and high ARs of 2, 3 and 4 (HAR-NPSS) were achieved using an anodic aluminum oxide template as the dry etching mask. The diffuse transmission (DT) rate of the incident light wavelength at 450 nm increased linearly, from 2.2% for LAR-NPSS to 12.7% for HAR-NPSS with an AR of 4. However, the diffuse reflection (DR) rate for HAR-NPSS did not increase linearly with the increasing AR values. The limit of the DR rate for HAR-NPSS could be attributed to the guiding effect when the incident light entered into the high AR concave nanopattern; most of the incident light could not be reflected (returned to the air), therefore there was no significant contribution to the DR rate. The luminous intensities of LEDs grown on bare sapphire (B-LED), HAR-NPSS (HAR-LED) and LAR-NPSS (LAR-LED) were 39, 38 and 74 mcd, respectively. Interestingly, the lowest brightness of any LED was the HAR-LED, which had higher DT and DR rates. This significant discrepancy could be attributed to a large number of voids occurring at the initial growth stage of the LED epitaxial layer on the HAR-NPSS that caused the emitted photons to remain trapped inside the LED. In addition, increasing the LED epilayer coverage area (i.e. 1.6-fold) with the inclined facets of low AR concave nanopatterns effectively scattered the emitted photons, leading to an increased light emitting intensity of the LED. The experimental results indicated that the specific AR concave nanopattern of an NPSS could increase the LED epitaxial layer coverage area and prevent void formation, which plays a crucial role in achieving high-brightness in InGaN-based LEDs.

A study of GaN nucleation and coalescence in the initial growth stages on nanoscale patterned sapphire substrates via MOCVD

A study of GaN nucleation and coalescence behaviors in the initial growth stages on nanoscale patterned sapphire substrates (NPSS) is presented.

Reduction of Defects in AlGaN Grown on Nanoscale-Patterned Sapphire Substrates by Hydride Vapor Phase Epitaxy.

In this study, a 3-μm-thick AlGaN film with an Al mole fraction of 10% was grown on a nanoscale-patterned sapphire substrate (NPSS) using hydride vapor phase epitaxy (HVPE). The growth mechanism, crystallization, and surface morphology of the epilayers were examined using X-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy at various times in the growth process. The screw threading dislocation (TD) density of AlGaN-on-NPSS can improve to 1–2 × 109 cm−2, which is significantly lower than that of the sample grown on a conventional planar sapphire substrate (7 × 109 cm−2). TEM analysis indicated that these TDs do not subsequently propagate to the surface of the overgrown AlGaN layer, but bend or change directions in the region above the voids within the side faces of the patterned substrates, possibly because of the internal stress-relaxed morphologies of the AlGaN film. Hence, the laterally overgrown AlGaN films were obtained by HVPE, which can serve as a template for the growth of ultraviolet III-nitride optoelectronic devices.

Influence of different aspect ratios on the structural and electrical properties of GaN thin films grown on nanoscale-patterned sapphire substrates

This study presents GaN thin films grown on nanoscale-patterned sapphire substrates (NPSSs) with different aspect ratios (ARs) using a homemade metal-organic chemical vapor deposition system. The anodic aluminum oxide (AAO) technique is used to prepare the dry etching mask. The cross-sectional view of the scanning electron microscope image shows that voids exist between the interface of the GaN thin film and the high-AR (i.e. ∼2) NPSS. In contrast, patterns on the low-AR (∼0.7) NPSS are filled full of GaN. The formation of voids on the high-AR NPSS is believed to be due to the enhancement of the lateral growth in the initial growth stage, and the quick-merging GaN thin film blocks the precursors from continuing to supply the bottom of the pattern. The atomic force microscopy images of GaN on bare sapphire show a layer-by-layer surface morphology, which becomes a step-flow surface morphology for GaN on a high-AR NPSS. The edge-type threading dislocation density can be reduced from 7.1×108cm−2 for GaN on bare sapphire to 4.9×108cm−2 for GaN on a high-AR NPSS. In addition, the carrier mobility increases from 85cm2/Vs for GaN on bare sapphire to 199cm2/Vs for GaN on a high-AR NPSS. However, the increased screw-type threading dislocation density for GaN on a low-AR NPSS is due to the competition of lateral growth on the flat-top patterns and vertical growth on the bottom of the patterns that causes the material quality of the GaN thin film to degenerate. Thus, the experimental results indicate that the AR of the particular patterning of a NPSS plays a crucial role in achieving GaN thin film with a high crystalline quality.

Silane controlled three dimensional GaN growth and recovery stages on a cone-shape nanoscale patterned sapphire substrate by MOCVD

The silane-controlled 3D growth led to a high crystalline quality, much strain relaxation and a specular surface for the GaN epilayers on NPSS.

Crystalline quality improvement of GaN coalesced using nano‐PSS with void‐embedded nanostructure by MOVPE

AbstractThe nanoscale patterned sapphire substrates (NPSS) were prepared combining nanoimprint lithography and dry/wet etching technology. The NPSS was patterned with 300 nm diameter, 330 nm depth, 550 nm periodicity hexagonal close packed pillars. Coalescence of GaN has been achieved on the NPSS using MOVPE. The growth mode and conditions were discussed in detail, which was different from that of GaN epitaxial on planar sapphire. The AFM images showed the GaN on NPSS had a smoother surface, and the XRD results showed that the crystal quality had been improved by the NPSS. The threading dislocation density (TDD) suppression was observed in transmission electron microscopy. Depth‐resolved CL measurements were used to quantify the redshift in terms of bandgap renormalization and strain effects. It was supposed that the compressive stress in the GaN layers was changed gradually towards the growth surface. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of large-area nano-scale patterned sapphire substrate with laser interference lithography

Periodic triangle truncated pyramid arrays are successfully fabricated on the sapphire substrate by a low-cost and high-efficiency laser interference lithography (LIL) system. Through the combination of dry etching and wet etching techniques, the nano-scale patterned sapphire substrate (NPSS) with uniform size is prepared. The period of the patterns is 460 nm as designed to match the wavelength of blue light emitting diode (LED). By improving the stability of the LIL system and optimizing the process parameters, well-defined triangle truncated pyramid arrays can be achieved on the sapphire substrate with diameter of 50.8 mm. The deviation of the bottom width of the triangle truncated pyramid arrays is 6.8%, which is close to the industrial production level of 3%.

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.

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.

Epitaxy of m-plane GaN on nanoscale patterned c-plane sapphire substrates

Nonpolar ( 10 1 ¯ 0 ) m-plane GaN epilayer was grown on nanoscale patterned c-plane sapphire substrates by MOCVD. The nanoscale patterned sapphire substrates were fabricated by natural lithography and dry etching methods. The nanopatterns are defined particularly with no c-plane surfaces exposed. The ( 10 1 ¯ 0 ) m-plane GaN epilayer was characterized by X-ray diffraction and atomic force microscope, which indicated the FWHM of the rocking curve is 316 arcsec and the rms surface roughness is 0.3 nm, respectively. The SEM and TEM studies reveal that the m-plane GaN grains nucleate on the exposed n-plane facets of the etched c-plane sapphire substrates.

Wafer-level fabrication and optical characterization of nanoscale patterned sapphire substrates

The wafer-level aperiodic nanostructures were fabricated atop the sapphire substrates in order to increase the transmittance over broadband spectra. The fabrication was presented along with characterization of their optical properties. The nanostructures were patterned using natural lithography with nickel silicide as a hard mask, and the subsequent etching was performed using inductively coupled plasma dry-etching method. The sapphire substrates with nanostructures compared to conventional sapphire substrates, which exhibit antireflective characteristics over broadband spectra at a wide range of incident angles. The nanostructures reduce the reflection down to 5% in the visible spectrum for normal incidence. The transmittance of visible to near-IR spectra was found to be 94% at normal incidence and over 90% at an incident angle of 45°. In the mid-IR spectrum, the transmittance exceeds 88% until the reflection is no longer suppressed by nanostructures. The polarization properties have also been investigated. The nanostructures can enhance the reflectivity ratio 90% for wavelengths shorter than 400 nm. As the amplitude ratio, enhanced from 50% to 80% over the whole visible spectrum.

High extraction efficiency GaN-based light-emitting diodes on embedded SiO2 nanorod array and nanoscale patterned sapphire substrate

In this paper, GaN-based LEDs with a nanoscale patterned sapphire substrate (NPSS) and a SiO2 photonic quasicrystal (PQC) structure on an n-GaN layer using nanoimprint lithography are fabricated and investigated. The light output power of LED with a NPSS and a SiO2 PQC structure on an n-GaN layer was 48% greater than that of conventional LED. Strong enhancement in output power is attributed to better epitaxial quality and higher reflectance resulted from NPSS and PQC structures. Transmission electron microscopy images reveal that threading dislocations are blocked or bended in the vicinities of NPSS layer. These results provide promising potential to increase output power for commercial light emitting devices.

Pattern-size dependence of characteristics of nitride-based LEDs grown on patterned sapphire substrates

Micro- and nano-scale patterned sapphire substrates (PSS) were fabricated by conventional photolithography and nanosphere lithography, respectively. Nitride-based light-emitting diodes (LEDs) were grown on different pattern sizes of patterned sapphire substrates, and the structural, electrical, optical properties of these LEDs were investigated throughout. The structural properties clearly indicate that the crystalline quality of epitaxial GaN film could be effectively improved by using the PSS technique. The leakage current is related to the crystalline quality of epitaxial GaN film, and it was also improved by using the PSS technique. The forward voltages of the LEDs grown on different pattern sizes of the PSS were similar. The light output power and external quantum efficiency of the LED grown on nano-scale of the PSS was the largest in all the samples. It indicates that the pattern size of the PSS is related to the capability of light extraction.