GaN-based Optoelectronics Research Articles

Threading dislocations reduction of GaN-on-Si by introducing AlN/3D-GaN with SiN interlayer for photodetectors

Gallium nitride-on-Silicon epitaxy technology has enabled a high level of large-scale GaN integrated manufacturing. However, the poor crystalline quality of GaN on Si substrates dramatically deteriorates device performance. This work demonstrates the application of a buffer layer structure of AlN/3D-GaN with a SiN interlayer to obtain GaN films with high crystalline quality on Si substrates. Transmission electron microscopy and X-ray diffraction reveal reductions in GaN dislocation density. The full width at half maximum values of the X-ray rocking curve for the GaN(0002) and (10–12) planes have decreased from 499 to 710 arcsec to 339 and 350 arcsec, respectively. Ultraviolet photodetectors based on the optimized GaN-on-Si substrates exhibit low dark currents (34 pA@10 V) and high responsivity (6.8 A/W@10 V). Our reported novel buffer architecture provides an important reference for improving the performance of other GaN-based optoelectronics and power electronic devices on Si substrates.

Influence of gallium surface saturation on GaN nanowire polytype selection during molecular-beam epitaxy

We have examined the influence of Ga surface saturation on gallium nitride (GaN) nanowire (NW) polytype selection during molecular-beam epitaxy. The Ga surface saturation in the absence and presence of nitrogen determines the GaN polytype and morphology (i.e., films vs NW) selection, respectively. We discuss the interplay between surface and step-edge diffusion barriers governing the NW-to-film-transition and the influence of SixNy interlayer formation on zinc blende (ZB) vs wurtzite (WZ) polytype selection of GaN. In addition, distinct exciton emissions associated with ZB and WZ GaN are observed, suggesting a type-I WZ/ZB GaN band-offset. This work provides a crucial step toward the realization of polarization-free, CMOS-compatible GaN-based optoelectronics.

Facile Enhancement of Optical Sensitivity in GaN Ultraviolet Photodetectors by UsingIn-SituPlano-Convex Polymer Lens

The facile enhancement of the optical photocurrent in gallium nitride (GaN) ultraviolet (UV) photodetectors was realized by applying an in-situ curing technique to polymer lenses on a GaN surface. To fabricate plano-convex polymer lenses that focus incident UV light, a prepolymer was dropped directly onto the GaN surface and cured at room temperature to induce UV light refraction. When the incident light passed through the polymer lens, the light concentrated on the sensing area of the GaN photodetector, thus generating more photocurrent than GaN photodetectors without the lens. Consequently, photodetectors fabricated with the plano-convex polymer lenses exhibited higher optical sensitivity (up to ~18.16% increase in photocurrent at room temperature) than those fabricated without polymer lenses. This study supports the use of direct and in-situ curing of polymer lenses on GaN surfaces for the facile and reliable enhancement of optical sensitivity of various GaN-based optoelectronics.

Wafer-scale crack-free 10 µm-thick GaN with a dislocation density of 5.8 × 107 cm−2 grown on Si

The increasing demand for complementary metal-oxide-semiconductor integrated GaN-based optoelectronics and power electronics calls for wafer-scale crack-free and high quality GaN thick film grown on Si. However, the huge mismatch in both lattice constant and coefficient of thermal expansion between GaN and Si often causes a high density of threading dislocations (TDD, typically 109–1010 cm−2) and the formation of micro-crack networks, producing a poor-quality GaN film with a limited thickness (<6 µm). By virtue of a well-designed Al-composition step-graded AlN/AlGaN buffer, a wafer-scale crack-free 10 µm-thick successive GaN with a low TDD of 5.8 × 107 cm−2 grown on Si was achieved for the first time. Detailed study on the depth-dependent strain and dislocation evolution shed light on the complex interplay between strain relaxation and defect reduction, as well as their influences on the GaN optical properties along the growth direction. This work paves the way for building high-performance GaN-based optoelectronics and power electronics on the Si platform.

Fabrication of full-color GaN-based light-emitting diodes on nearly lattice-matched flexible metal foils

GaN-based light-emitting diodes (LEDs) have been widely accepted as highly efficient solid-state light sources capable of replacing conventional incandescent and fluorescent lamps. However, their applications are limited to small devices because their fabrication process is expensive as it involves epitaxial growth of GaN by metal-organic chemical vapor deposition (MOCVD) on single crystalline sapphire wafers. If a low-cost epitaxial growth process such as sputtering on a metal foil can be used, it will be possible to fabricate large-area and flexible GaN-based light-emitting displays. Here we report preparation of GaN films on nearly lattice-matched flexible Hf foils using pulsed sputtering deposition (PSD) and demonstrate feasibility of fabricating full-color GaN-based LEDs. It was found that introduction of low-temperature (LT) grown layers suppressed the interfacial reaction between GaN and Hf, allowing the growth of high-quality GaN films on Hf foils. We fabricated blue, green, and red LEDs on Hf foils and confirmed their normal operation. The present results indicate that GaN films on Hf foils have potential applications in fabrication of future large-area flexible GaN-based optoelectronics.

Analysis of GaN Damage Induced by Cl2/SiCl4/Ar Plasma

GaN-based optical devices are fabricated using a GaN/InGaN/GaN sandwiched structure. The effect of radicals, ions, and UV light on the GaN optical properties during Cl2/SiCl4/Ar plasma etching was evaluated using photoluminescence (PL) analysis. The samples were exposed to plasma (radicals, ions, and UV light) using an inductively coupled plasma (ICP) etching system and a plasma ion beam apparatus that can separate the effects of UV and ions both with and without covering the SiO2 window on the surface. Etching damage in an InGaN single quantum well (SQW) was formed by exposing the sample to plasma. The damage, which decreases PL emission intensity, was generated not only by ion beam irradiation but also by UV light irradiation. PL intensity decreased when the thickness of the upper GaN layer was etched to less than 60 nm. In addition, simultaneous irradiation of UV light and ions slightly increased the degree of damage. There seems to be a synergistic effect between the UV light and the ions. For high-quality GaN-based optoelectronics and power devices, UV light must be controlled during etching processes in addition to the etching profile, selectivity, and ion bombardment damage.

Analysis of GaN Damage Induced by Cl2/SiCl4/Ar Plasma

GaN-based optical devices are fabricated using a GaN/InGaN/GaN sandwiched structure. The effect of radicals, ions, and UV light on the GaN optical properties during Cl2/SiCl4/Ar plasma etching was evaluated using photoluminescence (PL) analysis. The samples were exposed to plasma (radicals, ions, and UV light) using an inductively coupled plasma (ICP) etching system and a plasma ion beam apparatus that can separate the effects of UV and ions both with and without covering the SiO2 window on the surface. Etching damage in an InGaN single quantum well (SQW) was formed by exposing the sample to plasma. The damage, which decreases PL emission intensity, was generated not only by ion beam irradiation but also by UV light irradiation. PL intensity decreased when the thickness of the upper GaN layer was etched to less than 60 nm. In addition, simultaneous irradiation of UV light and ions slightly increased the degree of damage. There seems to be a synergistic effect between the UV light and the ions. For high-quality GaN-based optoelectronics and power devices, UV light must be controlled during etching processes in addition to the etching profile, selectivity, and ion bombardment damage.

High-quality AlInN for high index contrast Bragg mirrors lattice matched to GaN

We report on the growth by metalorganic vapor phase epitaxy of high-quality Al1−xInxN layers and AlInN/GaN Bragg mirrors near lattice matched to GaN. Layers are grown on a GaN buffer layer with no cracks over full 2 in. sapphire wafers. The index contrast relative to GaN is around 7% for wavelengths ranging from 950 to 450 nm. We demonstrate the growth of a crack-free, 20 pairs Al0.84In0.16N/GaN distributed Bragg reflector centered at 515 nm with an over 90% reflectivity and a 35 nm stop band. The growth of high quality AlInN lattice matched to GaN may represent a breakthrough in GaN-based optoelectronics which is presently limited by the lack of a high-index-contrast and high-band gap lattice-matched material.

GaN-based optoelectronics on silicon substrates

Cracking of GaN on Si usually occurs due to the large thermal mismatch of GaN and Si when layer thicknesses exceed approximately 1 μm in metalorganic chemical vapor deposition (MOCVD) preventing the realization of device-quality material. The thermal stress can be reduced significantly by a combination of different concepts such as the insertion of low-temperature AlN interlayers, introducing multiple AlGaN/GaN interlayers, and growing on prepatterned substrates. The growth of crack-free GaN-based light emitting diodes (LEDs) on silicon on patterned Si(111) with areas of 100 μm×100 μm is reported

Low Resistance Non-Transparent ohmic Pt-contacts on p-GaN

AbstractThe metal – p-GaN junction for low resistance ohmic contacts is still a challenge to be applied in GaN-based opto electronics as well as in power and high frequency devices. Currently, we try to improve the performance of our blue laser diodes. In order to decrease heat generation during device operation it is necessary to ensure as small contact resistances as possible.In this work, we achieved a specific contact resistance value of RC = 1.8 ± 1.7. 10-5 Ωcm2 for Pt-contacts on MOVPE-grown p-GaN. The Pt-layers were deposited by e-beam and thermally assisted vacuum evaporation after a standard cleaning process. For evaluation of Rc we used optimised circular TLM test patterns defined by photolithography. Best contacts were formed by annealing in Nitrogen athmosphere at 500°C.We also investigated the dependence of the contact resistance on the Mg doping concentration. Therefore p-GaN layers with different Mg-concentrations were grown on SiC-substrates and Pt-contacts were processed. For those samples, we investigated the Mg-concentrations, verified by secondary ion mass spectroscopy (SIMS), the hole concentrations and mobilities in dependence of C(Mg), which we obtained from HALL-measurements, and the contact and sheet resistances, measured by circular TLM measurements.The experiments showed that the optimum Mg-concentration for low contact resistances is higher than 2 1019 cm-3 which was found to provide a maximum hole concentration near 7 1017 cm-3. The influence of self-compensation in p-GaN in bulk and near interfaces will be discussed.