GaN Nanocolumns Research Articles

Self-Organized Eu-Doped GaN Nanocolumn Light-Emitting Diode Grown by RF-Molecular-Beam Epitaxy

High-concentration doping of Eu ions working as luminescent centers in GaN is required to improve light output of optical devices. GaN nanocolumns can be used to achieve high crystallinity in high-concentration Eu-doped GaN (GaN:Eu). In this study, optical properties of a GaN:Eu nanocolumn grown by RF-plasma-assisted molecular-beam epitaxy (RF-MBE) are investigated and GaN:Eu nanocolumn light-emitting diodes (LEDs) are demonstrated. Suppression of yellow luminescence and concentration quenching of PL intensity are observed at RT for GaN:Eu nanocolumns. When the effect of Eu concentration on optical properties is analyzed using rate equation, an increase in the Eu concentration is observed to affect 5D0 lifetime corresponding to back transfer, and not the effective cross-section corresponding to transfer efficiency. Based on these results, GaN:Eu nanocolumn LEDs are fabricated on an n-type (111) Si substrate. Clear rectification characteristics and sharp red luminescence are observed. The light output increases with increasing Eu concentration up to a concentration of 6 × 1020 cm−3. These results suggested that GaN:Eu nanocolumns provide feasible crystal to realize novel optical devices.

Laser molecular beam epitaxy growth of porous GaN nanocolumn and nanowall network on sapphire (0001) for high responsivity ultraviolet photodetectors

We report on high-responsivity metal-semiconductor-metal (MSM) structure based ultraviolet (UV) GaN photodetectors fabricated on various GaN nanostructures such as porous nanocolumn network (PNCN), nanowall networks (NWNs), and granular and compact thin films. Different GaN nanostructures were hetero-epitaxially grown on c-sapphire using laser molecular beam epitaxy by tuning the AlN buffer layer growth parameters. High resolution x-ray rocking curve measurements indicate that the crystalline quality of GaN critically depends on the selection of AlN buffer layer growth conditions such as type of ablation target and growth temperature. The porous GaN nanostructures revealed a nearly stress-free wurtzite structure as deduced by Raman spectroscopy measurements. Room temperature photoluminescence spectroscopy showed that the GaN films possess a near band emission (NBE) peak at ∼ 3.39 eV along with a broad yellow luminescence (YL) with maxima at 2.25 eV. For GaN PNCN and NWN structures, the NBE-to-YL emission ratio is more than an order higher compared to the GaN films. The fabricated MSM based UV-detector on GaN PNCN and NWN exhibited a high photo-responsivity of ∼27.72 and 24.8 A/W, respectively, under 2 V applied bias at room temperature. The GaN PNCN and NWN nanostructures with excellent photo-responsivity and optical quality prove to be promising candidates for the fabrication of efficient UV photodetectors due to their continuity in lateral direction, high surface area-to-volume ratio and tailored surfaces.

The effects of GaN nanocolumn arrays and thin SixNy buffer layers on the morphology of GaN layers grown by plasma-assisted molecular beam epitaxy on Si(111) substrates

The effects of GaN nanocolumn arrays and a thin SixNy layer, used as buffer layers, on the morphology of GaN epitaxial layers are investigated. Two types of samples with different buffer layers were synthesized by PA-MBE. The morphology of the samples was characterized by SEM. The crystalline quality of the samples was assessed by XRD. The possibility of synthesis of continuous crystalline GaN layers on Si(111) substrates without the addition of other materials such as aluminum nitride was demonstrated.

Selective area growth of InGaN-based nanocolumn LED crystals on AlN/Si substrates useful for integrated μ-LED fabrication

Triangular-lattice nanopillar-array templates with a lattice constant of 280 nm and with AlN disks on top of the underlying Si pillars were prepared on 2-in. AlN/Si substrates through nanoimprint lithography and dry etching. Regularly arranged GaN nanocolumn arrays with a 220-nm diameter were grown on the templates to fabricate wide-area emission InGaN/GaN nanocolumn light emitting diodes (LEDs). An LED chip with an Indium Tin Oxide (ITO) electrode with an area of 3 × 4 mm2 operated at a current of 100 mA emitted blue-green light (504 nm in wavelength) from the entire surface of the large emission area.

Effects of Introduction of InGaN Quantum Structures on Structural and Optical Properties of InGaN Nanocolumns

We systematically investigate and compare the structural and optical properties of InGaN nanocolumns (NCs) and InGaN/GaN single quantum structures (SQSs) fabricated on top of GaN NCs as functions of the GaN column diameter. Each InGaN/GaN SQS comprises an In‐rich InGaN single quantum dot at the center of a semipolar Ga‐rich InGaN single quantum well. The InGaN NCs and InGaN/GaN SQSs qualitatively exhibit similar optical properties. However, the specific diameters at which the photoluminescence spectra change from single‐peak emission patterns to double‐peak patterns are different. Based on scanning transmission electron microscopy observations of the InGaN layers formed on the GaN NCs with different growth times, a model for the growth of InGaN on GaN NCs is proposed and the origin of the difference in the specific diameters is elucidated. Finally, when QSs are introduced to the InGaN‐based NCs, the radiative recombination probability increases while the nonradiative and surface recombination probabilities decrease, thus leading to high emission efficiency.

Site-controlled GaN nanocolumns with InGaN insertions grown by MBE

The site-controlled plasma-assisted molecular beam epitaxy (PA MBE) has been developed to fabricate the regular array of GaN nanocolumns (NCs) with InGaN insertions on micro-cone patterned sapphire substrates (μ-CPSSs). Two-stage growth of GaN NCs, including a nucleation layer grown at metal-rich conditions and high temperature GaN growth in strong N-rich condition, has been developed to achieve the selective growth of the NCs. Microcathodoluminescence measurements have demonstrated pronounced emission from the InGaN insertions in 450-600 nm spectral range. The optically isolated NCs can be used as effective nano-emitters operating in the visible range.

Growth study of self-assembled GaN nanocolumns on silica glass by plasma assisted molecular beam epitaxy

We demonstrate GaN nanocolumn growth on fused silica glass by plasma-assisted molecular beam epitaxy. The effect of the substrate temperature, Ga flux and N2 flow rate on the structural and optical properties are studied. At optimum growth conditions, GaN nanocolumns are vertically aligned and well separated with an average diameter, height and density of 72nm, 1.2μm and 1.6×109cm−2, respectively. The nanocolumns exhibit wurtzite crystal structure with no threading dislocations, stacking faults or twinning and grow in the [0001] direction. At the interface adjacent to the glass, there is a few atom layers thick intermediate phase with ABC stacking order (zinc blende). Photoluminescence measurements evidence intense and narrow excitonic emissions, along with the absence of any defect-related zinc blende and yellow luminescence emission.

Surface Phonons Studied by Raman Scattering in GaN Nanostructures

We study Raman scattering from surface phonon polaritons in regularly arrayed GaN nanowalls and nanocolumns. The surface phonon mode around 705 cm−1 decreases in intensity with increasing wall width w under a constant interval period p = 355 nm in the nanowalls, and it disappears in the film. This suggests that the interaction between them on the adjacent walls works so as to satisfy the selection rule in the film. The Raman peak from the surface phonon polariton in the nanocolumns is much broader than that in the nanowalls. It decreases in intensity and increases in width with decreasing column diameter d, and it is not observed when d = 75 nm. This behavior is discussed in terms of the quantization of the azimuthal wave vector of a surface phonon polariton on the circumference of the columnar structure. The Raman spectrum of the surface phonon polariton in the regularly arrayed GaN nanocolumns is compared with that of the Frohlich mode in irregularly arrayed ones.

Stable‐wavelength operation of europium‐doped GaN nanocolumn light‐emitting diodes grown by rf‐plasma‐assisted molecular beam epitaxy

For the first time, a rare-earth-doped GaN nanocolumn light-emitting diode is fabricated on an n-type (111) Si substrate, grown by rf-plasma-assisted molecular beam epitaxy. The nanocolumn structure remained in the n-GaN and europium-doped GaN active regions, but the lateral growth was enhanced in the p-GaN region such that a continuous film was obtained. Clear diode characteristics and a sharp red luminescence with two dominant peaks at 620.3 and 633.8 nm were observed at room temperature. The shift in the wavelength with an increase in the injected current was <0.2 nm. Thus, this device has the potential for application as a stable-wavelength optical device.

Influence of GaN column diameter on structural properties for InGaN nanocolumns grown on top of GaN nanocolumns

The influence of GaN column diameter DGaN on structural properties was systematically investigated for InGaN nanocolumns (NCs) grown on top of GaN NCs. We demonstrated a large critical layer thickness of above 400 nm for In0.3Ga0.7N/GaN NCs. The structural properties were changed at the boundary of DGaN=D0 (∼120 nm). Homogeneous InGaN NCs grew axially on the GaN NCs with DGaN≤D0, while InGaN-InGaN core-shell structures were spontaneously formed on the GaN NCs with DGaN&amp;gt;D0. These results can be explained by a growth system that minimizes the total strain energy of the NCs.

Spatial emission distribution and carrier recombination dynamics in regularly arrayed InGaN/GaN quantum structure nanocolumns

Emission mechanisms in regularly arrayed InGaN/GaN quantum structures on GaN nanocolumns were investigated, focusing on the spatial emission distribution at the nanocolumn tops and the carrier recombination dynamics. The double-peak emission originated from the dot- and well-like InGaN areas with different In compositions was observed. From the results regarding the spatial emission distribution, we proposed a simple analytical approach to evaluating the carrier recombination dynamics using the rate equations based on the two energy states. The considerable six lifetimes can be uniquely determined from the experimental results. Carrier transfer from the high- to the low-energy state is dominant at high temperatures, producing the increased total emission efficiency of the inner low-energy area. In addition, the internal quantum efficiency should not be simply discussed using only the integrated intensity ratio between low and room temperatures because of the carrier transfer from high- to low-energy states.

Mode adjustment in hexagonal microresonators with an active region

We present original types of III-nitride monocrystal microresonators with an inserted active quantum-sized region. Modelling of microresonator's modes allows us to select right parameters of a polarized quantum well (width, composition) in the way that excitation of its optical transitions would be selectively amplified by interaction with the resonator modes. Adjustment of a GaN nanocolumn resonator to an InGaN quantum well is performed.

Polarity dependent strongly inhomogeneous In-incorporation in GaN nanocolumns

In this work, GaN/InGaN/GaN nanocolumns (NCs) have been grown by molecular beam epitaxy. Selective area growth (SAG) and self-organized growth (SOG) were performed simultaneously in patterned and unpatterned regions of the same substrate, respectively. The resulting structures show different tip morphologies and structural properties due to the different polarity along the growth direction, namely Ga-polar with r-plane faceted tips for the SAG NCs and N-polar with c-plane top facet for the SOG ones. When growing Ga-polar GaN/InGaN NCs, no indium is incorporated at a substrate temperature of °C. Rather, indium incorporation takes place under the same growth conditions on the N-polar NCs. The In-incorporation is investigated by means of nano x-ray fluorescence and diffraction, high-angle annular dark-field scanning transmission electron microscopy and high-resolution transmission electron microscopy.

Crystal structure and optical properties of a high-density InGaN nanoumbrella array as a white light source without phosphors

AbstractWe demonstrated the fabrication of a peculiar GaN/InGaN-based high-density nanocrystal array on a nitrogen polarity GaN layer using a simple self-assembly process for the first time. The nanocrystals consist of bending InGaN nanoplates and supporting GaN nanocolumns. The nanocrystals are umbrella shaped with diameters of ∼200–700 nm; therefore, they are referred to as InGaN nanoumbrellas. Transmission electron microscopy revealed the crystal structures of the nanoumbrellas and provided information about their growth mechanism. The photoluminescence (PL) properties of the InGaN nanoumbrellas were also characterized, and an extremely wide range of optical emission wavelengths (∼360–800 nm) was obtained from a small excitation diameter of ∼10 μm. Multiple sharp peaks resembling lasing actions were also observed in the PL spectrum; the resonant mode was likely caused by the whispering gallery mode. These results indicate that the high-density GaN/InGaN-based nanoumbrella array can be used as a source of white light without phosphors.

Structural and optical properties of Eu-doped GaN nanocolumns on (111) Si substrates grown by RF-plasma-assisted molecular beam epitaxy

Eu-doped GaN (GaN:Eu) shows a sharp line emission and the thermal stability of the emission wavelength. To improve their optical properties, GaN:Eu nanocolumns were grown on GaN nanocolumn platforms with high crystalline quality by RF-plasma-assisted molecular beam epitaxy. The GaN:Eu growth temperature strongly affected the nancolumn configuration. Although a high growth temperature of more than 700 °C enhanced lateral growth, a low growth temperature of 600 °C led to independent GaN:Eu nanocolumns. Although it was revealed that an increase in Eu concentration brought about polycrystalline growth, the optically active site concentration increased with increasing Eu concentration without concentration quenching, which suggests that the nanocolumn crystal is a valuable material for developing the novel optical devices utilizing GaN:Eu.

Oxygen-related photoluminescence quenching in selectively grown GaN nanocolumns: Dependence on diameter

This work reports on the effects of air exposure on the photoluminescence intensity of GaN nanocolumns, with diameters ranging from below 40nm up to around 230nm, grown selectively on GaN/sapphire and GaN/Si(111). The high control of dimensions provided by selective area growth epitaxy allowed for a better study of the relationship between the observed phenomena, namely the photoluminescence intensity quenching due to oxygen photo-adsorption, and the nanocolumns properties (morphology and dimensions). For nanocolumns with diameters below 120nm and lengths of about 300nm, photoluminescence intensity dropped by more than 90% of the initial value, while for shorter nanocolumns a reduced drop value was found.

Impact of temperature-induced coalescence on SERS properties of Au nanoparticles deposited on GaN nano-columns

Nanostructured GaN surfaces sputtered with Au provide very promising and reproducible platforms for surface enhanced Raman scattering (SERS). The enhancement factor (EF) in SERS is expected to depend strongly on the local metal (Au) surface structure (size, distribution and morphology). Herein we show how temperature-induced coalescence followed by recrystallization of Au on GaN nano-columns occurs well below the melting point of gold. This process is reflected in SEM, TEM images and SERS spectra of Au/GaN – bound p‐mercaptobenzoic acid (4-MBA), a model Raman scatterer. SERS signals of 4-MBA bound to Au/GaN reach minimum for platforms exposed to t=350°C before regaining the intensity when annealed in the range of 450–900°C. The results have been discussed in the light of the nature of SERS active sites – so called “hot spots” and structure of Au nanoparticles.

Material characteristics of self-assembled mushroom-like InGaN nanocolumns

The material characteristics of self-assembled mushroom-like N-polar InGaN/GaN nanowire heterostructure have been clarified, which were achieved by different In content self-assembled InGaN nanocolumns grown on self-assembled GaN nanocolumns template on (111)-silicon-substrate under N-rich condition by plasma-assist molecular beam epitaxy (PA-MBE). The In component of the InGaN nanocolumns was determined by XRD (2θ-ω scans). SEM has been used to study the morphology which demonstrated that the diameter of the nanocolumns became larger with higher In content. The structural properties of the individual InGaN nanocolumn were further analyzed by HAADF image, EDX and TEM. The high-In-content (85%) single mushroom-like InGaN nanocolumn showed some cracks on the sidewall, however the GaN nanocolumns showed dislocation free. The (0002) facet of the nanocolumn show very clearly hexagonal structure. It is quite clear that the formation of the mushroom-like InGaN nanocolumns comes from that the lateral epitaxy is dominating with the high In content embedded.

Effect of N2* and N on GaN nanocolumns grown on Si(111) by molecular beam epitaxy

The self-induced growth of GaN nanocolumns (NCs) on SixN1−x/Si (111) is investigated as a function of the ratio of molecular to atomic nitrogen species generated via plasma assisted molecular beam epitaxy. Relative concentrations of the molecular and atomic species are calculated using optical emission spectroscopy. The growth rate (GR), diameter, and density of NCs are found to vary with the molecular to atomic nitrogen species relative abundance ratio within the plasma cavity. With increasing ratio, the GR and diameter of NCs increase while the density of NCs seems to be decreasing. The morphologies and the coalescence of GaN NCs exhibit a trend for molecular/atomic ratios up to 11, beyond which they still change but at a lower rate. The detrimental effect of taperedness of the NCs decreases with increasing molecular/atomic ratios. This is possibly because of reduction in radial growth in NCs due to increase in diffusivity of nitrogen species with increasing ratios.

Self-organization of dislocation-free, high-density, vertically aligned GaN nanocolumns involving InGaN quantum wells on graphene/SiO2 covered with a thin AlN buffer layer

We demonstrated the self-organization of high-density GaN nanocolumns on multilayer graphene (MLG)/SiO2 covered with a thin AlN buffer layer by RF-plasma-assisted molecular beam epitaxy. MLG/SiO2 substrates were prepared by the transfer of CVD graphene onto thermally oxidized SiO2/Si [100] substrates. Employing the MLG with an AlN buffer layer enabled the self-organization of high-density and vertically aligned nanocolumns. Transmission electron microscopy observation revealed that no threading dislocations, stacking faults, or twinning defects were included in the self-organized nanocolumns. The photoluminescence (PL) peak intensities of the self-organized GaN nanocolumns were 2.0–2.6 times higher than those of a GaN substrate grown by hydride vapor phase epitaxy. Moreover, no yellow luminescence or ZB-phase GaN emission was observed from the nanocolumns. An InGaN/GaN MQW and p-type GaN were integrated into GaN nanocolumns grown on MLG, displaying a single-peak PL emission at a wavelength of 533 nm. Thus, high-density nitride p–i–n nanocolumns were fabricated on SiO2/Si using the transferred MLG interlayer, indicating the possibility of developing visible nanocolumn LEDs on graphene/SiO2.