GaN Microrods Research Articles

Novel type of whisker-tip cantilever based on GaN microrods for atomic force microscopy

High-resolution scanning probe microscopy (SPM) is a fundamental and efficient technology for surface characterization of modern materials at the subnanometre scale. The bottleneck of SPM is the probe and scanning tip. Materials with stable electrical, thermal, and mechanical properties for high-aspect-ratio (AR) tips are continuously being developed to improve their accuracy. Among these, GaN is emerging as a significant contender that serves as a replacement for standard Si probes. In this paper, for the first time, we present an approach that demonstrates the application of GaN microrods (MRs) as high-AR SPM probes. GaN MRs were grown using molecular beam epitaxy, transferred and mounted on a cantilever using focused electron beam-induced deposition and milled in a whisker tip using a focused ion beam in a scanning electron/ion microscope. The presence of a native oxide layer covering the GaN MR surface was confirmed by X-ray photoelectron spectroscopy. Current-voltage map measurements are also presented to indicate the elimination of the native oxide layer from the tip surface. The utility of the designed probes was tested using conductive atomic force microscopy and a 24-hour durability test in contact mode atomic force microscopy. Subsequently, the graphene stacks were imaged.

Deterministically-grown GaN microrods on a mask-free plateau patterned substrate

In this study, a polished plateau-patterned sapphire substrate (PP-PSS) was developed to grow epitaxial GaN microrods using an AlN buffer layer via pulsed metal–organic chemical vapor deposition. The diameter of the plateau region could be easily adjusted in the range of 600–1600 nm by altering the polishing time, allowing the selective growth of the GaN microrods in only the plateau region and subsequent deposition on the AlN layer. The GaN microrod growth mechanism on the PP-PSS was explained using the morphology of the grown GaN microrods. Transmission electron microscopy revealed that the single-crystalline GaN microrods had a GaN (0002) d spacing of 0.263 nm, and a measured lattice constant ratio of the (0001) and (10 1‾ 0) planes of 0.535, which is similar to the theoretical value of 0.532 for a fully relaxed GaN. Through Raman spectroscopy, the GaN microrod E2 (high) mode was 566.70 cm−1, which was shifted by only 0.50 cm−1 compared to that of the strain-free reference. These results suggest that stress-free GaN microrod epitaxial growth is possible on PP-PSS.

Role of Temperature in Arsenic-Induced Antisurfactant Growth of GaN Microrods

Due to the antisurfactant properties of arsenic atoms, the self-induced dodecagonal GaN microrods can be grown by molecular beam epitaxy (MBE) in Ga-rich conditions. Since temperature is a key parameter in MBE growth, the role of temperature in the growth of GaN microrods is investigated. The optimal growth temperature window for the formation of GaN microrods is observed to be between 760 and 800 °C. Lowering the temperature to 720 °C did not change the growth mechanism, but the population of irregular and amorphous microrods increased. On the other hand, increasing the growth temperature up to 880 °C interrupts the growth of GaN microrods, due to the re-evaporation of the gallium from the surface. The incorporation of As in GaN microrods is negligible, which is confirmed by X-ray diffraction and transmission electron microscopy. Moreover, the photoluminescence and cathodoluminescence characteristics typical for GaN are observed for individual GaN microrods, which additionally confirms that arsenic is not incorporated inside microrods. When the growth temperature is increased, the emission related to the band gap decreases in favor of the defect-related emission. This is typical for bulk GaN and attributed to an increase in the point defect concentration for GaN microrods grown at lower temperatures.

Room-temperature polariton lasing in GaN microrods with large Rabi splitting.

Room-temperature polariton lasing is achieved in GaN microrods grown by metal-organic vapor phase epitaxy. We demonstrate a large Rabi splitting (Ω = 2g0) up to 162 meV, exceeding the results from both the state-of-the-art nitride-based planar microcavities and previously reported GaN microrods. An ultra-low threshold of 1.8 kW/cm2 is observed by power-dependent photoluminescence spectra, with the linewidth down to 1.31 meV and the blue shift up to 17.8 meV. This large Rabi splitting distinguishes our coherent light emission from a conventional photon lasing, which strongly supports the preparation of coherent light sources in integrated optical circuits and the study of exciting phenomena in macroscopic quantum states.

Two-round quasi-whispering gallery mode exciton polaritons with large Rabi splitting in a GaN microrod.

We investigate the exciton polaritons and their corresponding optical modes in a hexagonal GaN microrod at room temperature. The dispersion curves are measured by the angle-resolved micro-photoluminescence spectrometer, and two types of exciton polaritons are identified with the help of the finite-difference time-domain simulation. By changing the pump position, the photon part of the exciton polaritons is found to switch between the quasi-whispering gallery modes and the two-round quasi-whispering gallery modes. The exciton polaritons formed by the latter are observed and distinguished for the first time, with a giant Rabi splitting as large as 2Ω = 230.3 meV.

Size dependent polaritonic effects in GaN microrod studied through optical property investigation

We have investigated the optical properties of gallium nitride microrods systematically by spatially resolved cathodoluminescence, Raman and photoluminescence measurements. Blueshifting in near band gap energy with the decrease in diameter of studied microrods has been observed from cathodoluminescence and is attributed to the formation of whispering gallery mode polaritons in microrod cavity. Raman spectra along the length of the tapered microrod shows no noticable shifting in Raman mode positions confirm that the band gap energy changes with the decrease in diameter is not due to the stress/strain or different doping concentration. PL was carried out to look at the probable possibilities of polariton formation in microrod cavity. Exciton photon coupling strength depends on the diameter of the microrods and the coupling strength decreases with the decrease in diameter as studied from spatial-resolved photoluminescence spectra. Rabi splitting energy of nearly 300 ± 40 meV and so polariton lasing was observed from the wide end but that gradually decreases with the decrease in diameter.

Arsenic‐Induced Growth of Dodecagonal GaN Microrods with Stable a‐Plane Walls

AbstractNano‐ and micro‐rods of GaN offer many functionalities that are not present in regular flat nanostructures. Therefore, development of new growth methods of such structures is a hot topic. In this work the arsenic‐induced growth of GaN microrods under Ga‐rich conditions in the molecular beam epitaxy is presented. It is a self‐catalyst vapor–liquid–solid process with native Ga droplets. The formation of Ga droplets is induced by antisurfactant properties of arsenic. The presence of As during the epitaxial process promotes the growth of dodecagonal microrods with 12 walls: six m‐planes and six a‐planes. It is possible since As changes the growth rates for the different GaN planes comparing to arsenic‐free conditions, where hexagonal microrods are usually formed. The growth parameters and their influence on the sample morphology are carefully studied in this work. Microrods with an average height and diameter of 3 and 0.7 µm, respectively, and the density of 2.3 × 107 cm−2, are obtained under optimal growth conditions. The observed mechanism of growth of microrods can also be present in other material systems by introducing atoms with antisurfactant properties under metal‐rich conditions, where the surface is covered by a metal monolayer.

Flexible and monolithically integrated multicolor light emitting diodes using morphology-controlled GaN microstructures grown on graphene films

We report flexible and monolithically integrated multicolor light-emitting diode (LED) arrays using morphology-controlled growth of GaN microstructures on chemical-vapor-deposited (CVD) graphene films. As the morphology-controlled growth template of GaN microstructures, we used position-controlled ZnO nanostructure arrays with different spacings grown on graphene substrates. In particular, we investigated the effect of the growth parameters, including micropattern spacings and growth time and temperature, on the morphology of the GaN microstructures when they were coated on ZnO nanostructures on graphene substrates. By optimizing the growth parameters, both GaN microrods and micropyramids formed simultaneously on the graphene substrates. Subsequent depositions of InGaN/GaN quantum well and p-GaN layers and n- and p-type metallization yielded monolithic integration of microstructural LED arrays on the same substrate, which enabled multicolor emission depending on the shape of the microstructures. Furthermore, the CVD graphene substrates beneath the microstructure LEDs facilitated transfer of the LED arrays onto any foreign substrate. In this study, Cu foil was used for flexible LEDs. The flexible devices exhibited stable electroluminescence, even under severe bending conditions. Cyclic bending tests demonstrated the excellent mechanical stability and reliability of the devices.

Raman investigation on the surface carrier concentration of single GaN microrod grown by MOCVD

Abstract Raman measurement was applied to investigate the carrier concentration distribution along the [0001] direction of single GaN microrod. By measuring the Raman shifts of longitudinal phonon-plasmon (LPP) modes at different locations, the carrier concentration distributions along the sidewall of the microrod were obtained in the [0001] direction. It was found that the carrier concentration at the bottom of the microrod was 9.07×1016 cm−3, which was higher than that at the top. Carrier concentration gradient existed along the growth direction of the microrod. This was mainly due to the uneven distribution of some donor impurities (eg: oxygen impurities) during the growth process. Furthermore, based on the E2high mode, the distributions of residual stress and crystal qualities were characterized along the [0001] direction.

Strong exciton–photon coupling and polariton lasing in GaN microrod

We report on the studies of strong exciton photon coupling and polariton lasing at room temperature from single GaN microrods grown by metal-organic vapor phase epitaxy technique. Emission spectra were recorded at room temperature by exciting GaN microrods with pulsed Nd:YAG laser of 355 nm wavelength at different excitation power density. Clear mode formation corresponding to Whispering gallery modes in GaN microrod cavity has been observed. Strong exciton–photon coupling as well as large Rabi splitting energy of maximum 400 ± 40 meV has been observed from the microrods. This is attributed to the formation of new quasi-particles named polaritons with much lighter mass than free electron mass. Among those polariton modes, one of the modes increases super linearly with the increase in excitation power density which is an indication of polariton lasing.

Effect of incident laser power and incorporating polariton effects in GaN microrods

Optical property of GaN microrods was investigated by power dependent photoluminescence (PL). Two PL peak envelops were observed in near band edge region and this feature has been attributed to the exciton-photon coupling. Clear mode formation in PL spectrum was observed and modes were redshifted slightly with the increase in laser power for two microrods. Redshifting of PL peak envelops in near band edge region was huge in one microrod as compare to other microrods with the increase in laser power. This is attributed to both the electron-hole plasma effect and exciton-photon coupling which depends on the morphology. Modes in the defect related region were prominent than the modes in near band edge region at low laser power and with the increase in laser power, modes in the defect region disappears and modes in the near band edge region becomes more prominent. Cavity modes get guided in the cavity at higher power when the polariton formation takes place. Cavity modes are not guided at low power when modes are photonic.

Monolithic multiple colour emission from InGaN grown on patterned non-polar GaN

A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated. Such a multiple-facet structure can be achieved by means of overgrowth on non-polar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates. Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions. Detailed cathodoluminescence measurements and excitation-power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs. It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.

Axial p–n Junctions in GaN Microrods

GaN microrods with an axial p–n junction are grown by metal‐organic vapor phase epitaxy (MOVPE). Scanning electron microscopy in combination with cathodoluminescence measurements have been performed to visualize the microrod sections consisting of n‐ and p‐type GaN and the p–n junction. Current–voltage measurements are carried out between different microrod sections to prove the successful formation of a p–n junction. Photovoltaic and photodetecting properties have been determined by illumination of the p–n junction with a UV laser. The shorter p‐type sections in axial microrods and the presence of large diameter rods suggest a growth mode change from vertical to lateral growth during p‐type deposition.

Tunable single-mode lasing in a single semiconductor microrod.

Developing micro/nanoscale wire lasers with single-mode operation and lasing wavelength modulation is essential for realizing their practical applications such as optical communication and saturated spectroscopy. We demonstrated, to the best of our knowledge, the first tunable single-mode microrod laser without complicated micro/nano-manipulation and without additional environmental requirement. In this letter, we realized the wavelength modulation in a single semiconductor microrod simply and directly by changing the axial location of the active region, owing that the active region position plays a key role in determining the lasing mode of microrod lasers. Based on this feature, we proposed a pair of asymmetrical distributed Bragg reflectors (DBRs) with specific spectral selectivity to be induced in a GaN microrod to realize tunable single-mode lasing in a single semiconductor microrod. By using this method, lasing wavelength can be modulated from 369.5 to 375.7 nm flexibly and repeatedly in a 45 μm GaN microrod with the change of the excitation source position. This approach demonstrates a big application potential in numerous fields consisting of optical telecommunication and environmental monitoring.

Spatially resolved luminescence properties of non- and semi-polar InGaN quantum wells on GaN microrods

Spatially resolved emission properties of InGaN/GaN quantum wells on the facets of microrods are analyzed by means of photoluminescence and cathodoluminescence measurements. We observe strongly localized emissions of the non- and semi-polar InGaN quantum wells from the top part of the microrods, suggesting optical emitters of high efficiency. The quantum wells are characterized by transmission electron microscopy measurements with respect to their indium composition and thickness. Those wells oriented in non- and semi-polar directions possess high In concentrations as well as low internal polarization fields which makes these directions excellent candidates for InGaN quantum well emitters. These investigations show that the applied microrod growth concept is an effective method to realize high quality InGaN quantum wells on GaN facets oriented in non- and semi-polar directions which makes optoelectronic emitters in the green gap region feasible.

Observation of Anisotropic Growth and Compositional Discontinuity in AlGaN Electron-Blocking Layers on GaN Microrods

In GaN microrods, phase separation of the AlGaN electron-blocking layer is an enormous obstacle for achieving high-efficiency light-emitting diodes, as this phenomenon negatively affects the device efficiency by inducing unwanted band-energy modulations. Here, we found that the AlGaN electron-blocking layer on each m-plane of the GaN microrod appears to be phase separated, and each electron-blocking layer has a different thickness and length. Our careful analysis based on atom probe tomography reveals that the Al distribution in AlGaN is not uniform and that Al-rich and Al-deficient regions are clearly present. In addition, the longer surface diffusion length of Ga adatoms, as compared to Al adatoms, and the different initial strain state of each m-plane in the GaN rods are deeply associated with the different growth rates and inhomogeneous Al composition of AlGaN, resulting in phase separation of the AlGaN electron-blocking layer. These atomic-scale observations in the structural and chemical composition of AlGaN grown on GaN microrods could provide expanded opportunities for building a wide range of high-quality AlGaN electron-blocking layers.

Understanding GaN/InGaN core–shell growth towards high quality factor whispering gallery modes from non-polar InGaN quantum wells on GaN rods

GaN microrods are used as a basis for subsequent InGaN quantum well (QW) and quantum dot deposition by metal-organic vapor phase epitaxy. The coverage of the shell along the sidewall of rods is dependent on the rod growth time and a complete coverage is obtained for shorter rod growth times. Transmission electron microscopy measurements are performed to reveal the structural properties of the InGaN layer on the sidewall facet and on the top facet. The presence of layers in the microrod and on the microrod surface will be discussed with respect to GaN and InGaN growth. A detailed model will be presented explaining the formation of multiple SiN layers and the partial and full coverage of the shell around the core. Cathodoluminescence measurements are performed to analyze the InGaN emission properties along the microrod and to study the microresonator properties of such hexagonal core–shell structures. High quality factor whispering gallery modes with are reported for the first time in a GaN microrod/InGaN non-polar QW core–shell geometry. The GaN/InGaN core–shell microrods are expected to be promising building blocks for low-threshold laser diodes and ultra-sensitive optical sensors.

Spatially-resolved optical and structural properties of semi-polar mathrm{(11}bar{2}mathrm{2)} AlxGa1\u2212xN with x up to 0.56

Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar mathrm{(11}bar{2}mathrm{2)} AlxGa1−xN epilayers with AlN contents up to x = 0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.

The influence of MOVPE growth conditions on the shell of core-shell GaN microrod structures

A core-shell geometry is employed for most next-generation, three-dimensional opto-electric devices based on III–V semiconductors and grown by metal organic vapor phase epitaxy (MOVPE). Controlling the shape of the shell layers is fundamental for device optimization, however no detailed analysis of the influence of growth conditions has been published to date.We study homogeneous arrays of gallium nitride core-shell microrods with height and diameter in the micrometer range and grown in a two-step selective area MOVPE process. Changes in shell shape and homogeneity effected by deliberately altered shell growth conditions were accurately assessed by digital analysis of high-resolution scanning electron microscope images.Most notably, two temperature regimes could be established, which show a significantly different behavior with regard to material distribution. Above 900°C of wafer carrier temperature, the shell thickness along the growth axis of the rods was very homogeneous, however variations between vicinal rods increase. In contrast, below 830°C the shell thickness is higher close to the microrod tip than at the base of the rods, while the lateral homogeneity between neighboring microrods is very uniform. This temperature effect could be either amplified or attenuated by changing the remaining growth parameters such as reactor pressure, structure distance, gallium precursor, carrier gas composition and dopant materials. Possible reasons for these findings are discussed with respect to GaN decomposition as well as the surface and gas phase diffusion of growth species, leading to an improved control of the functional layers in next-generation 3D V–III devices.

Optical properties and internal quantum efficiency of InGaN/GaN core-shell microrods for solid state lighting

We investigate, via temperature and excitation density dependent quasi-resonant confocal micro-photoluminescence, the optical properties and internal quantum efficiency (IQE) of InGaN/GaN single quantum wells (QWs) on Ga-polar GaN microrods selectively grown by continuous flow metal organic vapor phase epitaxy on patterned SiO2/n-GaN/sapphire template. Seven samples were grown with different growth parameters for the InGaN/GaN QW. The homogeneity of their optical properties is analyzed by mappings along the m-plane facet of the microrods in order to get insight on the growth mechanisms of the shell. Excitation density dependent measurements show that the IQE is affected by the high doping level of the core, which is required to grow such high aspect-ratio structures. Local IQEs between 15±1 % near the tip and 44±5 % near the base of microrods are estimated from measurements at room and low temperature. By comparison with results reported on planar c-plane QWs, we conclude that the radiative recombination rate is the main limitation for the emission efficiency.