Semipolar Facets Research Articles

Investigating the exciton dynamics in InGaN/GaN core-shell nanorods using time-resolved cathodoluminescence.

This study examines the exciton dynamics in InGaN/GaN core-shell nanorods using time-resolved cathodoluminescence (TRCL), which provides nanometer-scale lateral spatial and tens of picoseconds temporal resolutions. The focus is on thick (>20 nm) InGaN layers on the non-polar, semi-polar and polar InGaN facets, which are accessible for study due to the unique nanorod geometry. Spectrally integrated TRCL decay transients reveal distinct recombination behaviours across these facets, indicating varied exciton lifetimes. By extracting fast and slow lifetime components and observing their temperature trends along with those of the integrated and peak intensity, the differences in behaviour were linked to variations in point defect density and the degree and density of localisation centres in the different regions. Further analysis shows that the non-polar and polar regions demonstrate increasing lifetimes with decreasing emission energy, attributed to an increase in the depth of localisation. The semi-polar facet instead showed a spectral independence of the lifetime which could not be rationalised by an absence of exciton localisation. This investigation provides insights into the intricate exciton dynamics in InGaN/GaN nanorods, offering valuable information for the design and development of optoelectronic devices.

Control of GaN inverted pyramids growth on c-plane patterned sapphire substrates

Growth of gallium nitride (GaN) inverted pyramids on c-plane sapphire substrates is benefit for fabricating novel devices as it forms the semipolar facets. In this work, GaN inverted pyramids are directly grown on c-plane patterned sapphire substrates (PSS) by metal organic vapor phase epitaxy (MOVPE). The influences of growth conditions on the surface morphology are experimentally studied and explained by Wulff constructions. The competition of growth rate among {0001}, { }, and { } facets results in the various surface morphologies of GaN. A higher growth temperature of 985 °C and a lower Ⅴ/Ⅲ ratio of 25 can expand the area of { } facets in GaN inverted pyramids. On the other hand, GaN inverted pyramids with almost pure { } facets are obtained by using a lower growth temperature of 930 °C, a higher Ⅴ/Ⅲ ratio of 100, and PSS with pattern arrangement perpendicular to the substrate primary flat.

Multi-color emission based on InGaN/GaN micro-truncated pyramid arrays

3D micro-nano devices are expected to become the mainstay of multi-color solid-state lighting in the future because of their broad-band characteristic and the advantage of integrating the monolithic light-emitting diode on a single chip. In this work, InGaN/GaN micro-truncated pyramid arrays with six equivalent (101̄1) semi-polar facets and one (0001) polar facet were successfully prepared by the metal-organic chemical vapor deposition technology. The average diameter of the obtained uniform micro-truncated pyramids was 6.8 µm with a height of 2.4 µm. According to the results of micro-photoluminescence performed, the InGaN/GaN micro-truncated pyramid arrays can achieve multi-color emission from blue to red. The luminescent positions corresponding to different wavelengths were detected by the cathode luminescence spectrum. The multi-color emission was related to the quantum hybrid structures apart from the discrepancy of In composition in different positions. The developed microstructure can create multi-color emission by combining distinct luminescence modes, which can aid in the design of future optoelectronic devices.

Surface Morphology Evolution of AlGaN Microhoneycomb Structures during Epitaxial Overgrowth

The surface morphological development during epitaxial overgrowth of AlGaN on microhoneycomb (μ‐HC) patterned relaxed AlGaN/AlN/sapphire templates is studied. Optimization of μ‐pattern design and influence of various growth parameters are carried out in search for conditions resulting in coalescence of μ‐HC structures into planar layers or ordered arrays of V‐pits with well‐defined crystallographic facets. It is found that in a wide range of growth conditions, formation of V‐pits with semipolar facets occurs where their crystallographic orientation is defined by the geometry of the initial μ‐HC patterns and growth conditions. High‐temperature and high V/III ratio conditions are found to favor c‐plane expansion and allow recovery of planarity on the patterned templates. Systematic study of surface morphology evolution in conditions favoring 1) semipolar facets formation and 2) coalescence is carried out and possible mechanisms underpinning the observed behaviors have been proposed.

Yellow-Green Luminescence Due to Polarity-Dependent Incorporation of Carbon Impurities in Self-Assembled GaN Microdisk.

Yellow-green luminescence (YGL) competes with near-bandgap emission (NBE) for carrier recombination channels, thereby reducing device efficiency; yet uncovering the origin of YGL remains a major challenge. In this paper, nearly stress-free and low dislocation density self-assembled GaN microdisks were synthesized by Na-flux method. The YGL of GaN microdisks highly depend on their polar facets. Variable accelerating voltage/power CL, variable temperature PL, and Raman spectroscopy are further performed to clarify the origin of polarity dependence of GaN microdisk YGL behavior, which indicates its independence of dislocations, surface effects, stress, crystalline quality, and gallium vacancies. It was found that the incorporation ability of carbon impurities in the polar (0001) facet is greater than that in the semipolar (101̅1) facets, producing higher content of CN or CNON defects, resulting in a more pronounced YGL in the polar (0001) facet of GaN.

Three dimensional truncated-hexagonal-pyramid vertical InGaN-based white light emitting diodes based on β-Ga2O3.

In this Letter, we describe the fabrication of three dimensional (3D) truncated-hexagonal-pyramid (THP) vertical light emitting diodes (VLEDs) with white emission grown on β-Ga2O3 substrate. In the 3D n-GaN layer, it is noted that the longitudinal growth rate of the 3D n-GaN layer increases as the flow rate of N2 decreases and H2 increases. Moreover, the 3D THP VLED can effectively suppress the quantum-confined Stark effect (QCSE) compared with planar VLEDs due to the semipolar facets and strain relaxation. Thus, the internal quantum efficiency (IQE) of the 3D THP VLED has been doubled and the V-shaped pits have been greatly reduced. In particular, the 3D THP VLED enables multi-wavelength emission (448.0 nm and 498.5 nm) and also shows better light extraction efficiency (LEE), which presents an effective way for the realization of phosphor-free white LED devices.

(Invited) Selective Area Etching and Doping of GaN for High-Power Applications

Selective-area doping techniques of GaN have been studied intensively for high-power applications. Diffusion or implantation of dopants is a very mature approach in Si and SiC, but it is still under investigation for GaN. Selective area epitaxy and doping is an alternative method to overcome the barrier and is reported by many groups. Two aspects will be discussed, including selective-area etching and non-uniform Mg doping in non-planar regrown trenches.Dry etching has been commonly applied to create high-aspect ratio and smooth trenches, but has also been reported to generate serious optical and electronic defects and impurities on the treated surface. Here, we report using an in-situ etchant, tertiarybutylchloride (TBCl), to replace the dry etching process. In order to demonstrate the effectiveness of TBCl etching, planar PN diodes were grown and fabricated, including continuous, dry-etched, and TBCl-etched and regrown ones. TBCl-etched and regrown device demonstrates electrical properties approach that of the continuous grown diode in the forward and reverse biased conditions.On the other hand, non-uniform doping of Mg was observed in the non-planar regrowth by several groups. With nanoscale characterization technique, atom probe tomography (APT), we identified the origin of the non-uniformity. The Mg concentration is approximately inversely proportional to the growth rate beginning ~20 nm from the regrowth interface. It indicates that the existence of semi-polar facets together with the anisotropy in the growth rate are the origin of non-uniformity. Furthermore, the evolving shape of semi-polar facets could be explained by the kinetic Wulff theorem.

(Invited) Selective Area Etching and Doping of GaN for High-Power Applications

Selective area doping (SAD) of gallium nitride (GaN), especially p-type doping, is desirable for high-power applications but yet challenging. The lack of this process greatly limits the power device design flexibility, so the reported device figure-of-merits are well below the theoretical calculation/simulation. Selective-area etching followed by regrowth is the approach we choose to overcome this obstacle. For the etching step, instead of adopting conventional Cl-based plasma etching process, we explored in-situ TBCl etching, studied the etching mechanism, and optimized the process to prepare smooth surface and trenches. Room-temperature PL, XPS, and electrical characterizations indicate that TBCl etching creates a low-defect surface. Moreover, selective area growth of p-GaN in a patterned trench was analyzed by atom probe tomography (APT). We found a reverse proportionality between the local growth rate and Mg doping concentration, and the development of fast-growing semi-polar facet from the sidewall. As a consequence, non-uniform Mg doping in non-planar growth is observed.

Luminescence redshift of thick InGaN/GaN heterostructures induced by the migration of surface adsorbed atoms

Nitride heterostructures with an extremely high number of InGaN/GaN multiple quantum wells (MQWs) were grown and studied by multiple techniques. Large redshift (282 meV) in photoluminescence (PL) spectra was observed with an increasing number of quantum wells (QWs) in the structure from 10 to 60. From the comparison of structures grown on sapphire and GaN substrates, the phenomenon of giant redshift is explained. A theory based on the surface migration of the adsorbed atoms from the semi-polar V-pit facets to the c-plane facets is suggested and proven by several experimental techniques including scanning transmission electron microscopy (STEM), secondary ion-mass spectroscopy (SIMS), secondary electron microscopy (SEM), white light interferometry (WLI), and spectrally and spatially resolved cathodoluminescence (SSRCL). Finally, a change in the surface morphology caused by the growing size of the V-pits and the transformation of the c-plane surface areas is discussed.

Selective Area Regrowth Produces Nonuniform Mg Doping Profiles in Nonplanar GaN p–n Junctions

Nonplanar GaN p–n junctions formed by selective area regrowth were analyzed using pulsed laser atom probe tomography. Dilute Al marker layers were used to map the evolution of the p-GaN growth interface, enabling extraction of time-varying growth rates for nonpolar, semipolar, and polar surfaces from the trench edge to the center, respectively. The Mg dopant concentration is facet-dependent and varies inversely with the growth rate for the semipolar facets that grow rapidly away from the trench sidewalls. The negligible growth on the vertical sidewall of the trench coincides with an order of magnitude higher Mg concentration and substantial clustering of likely inactive dopants. A high Mg concentration is also observed near the regrowth interface of polar and semipolar planes, which we attribute to etching damage. We conclude that device fabrication processes employing selective area regrowth on nonplanar interfaces should consider both the spatial and temporal dependencies of growth rate that lead to nonuniform doping and explore growth conditions that could reduce variations in growth rate when nonuniform doping would adversely affect device performance.

Color‐Tunable 3D InGaN/GaN Multi‐Quantum‐Well Light‐Emitting‐Diode Based on Microfacet Emission and Programmable Driving Power Supply

AbstractColor‐tunable InGaN/GaN multi‐quantum‐well (MQW) light‐emitting diodes (LEDs) are reported based on GaN microfacet structure directly grown on c‐plane patterned sapphire substrate by metal organic vapor phase epitaxy (MOVPE) through promoting 3D growth. By adjusting GaN growth temperature and pattern arrangement, a GaN microfacet with almost pure {101} semipolar facets is obtained. The multifacetted InGaN/GaN MQW LED chip evolves three distinct emission peaks around 630, 530, and 450 nm in electroluminescence (EL) as injection current increases from 1 to 100 mA. The EL behavior originates from locally different facets of the complex 3D structure: MQWs grown on c‐planes and semipolar facets, respectively, which is confirmed by cathodoluminescence characterization in a scanning transmission electron microscope (STEM‐CL). Considering the dependence of emission wavelength and intensity on injection currents, a programmable power supply is designed to drive the LED. The specific color of the LED is tuned by time‐shared driving of the currents based on three channels with controllable magnitudes and duty cycle from the power supply, covering red, yellow, green, cyan, blue, and purple. Furthermore, white LEDs with high color rendering index (CRI) up to 96.1 and correlated color temperature (CCT) between 4000 and 10 000 K are achieved.

Anisotropy and Mechanistic Elucidation of Wet‐Chemical Gallium Nitride Etching at the Atomic Level

Etching of gallium nitride is a key step in the production of blue and white light‐emitting diodes (LEDs). Etching in aqueous KOH solution creates a rough surface on the LED chip to facilitate outcoupling of the photons generated, drastically increasing the resulting LED's efficiency. Compared with the common technique of dry etching, wet‐chemical etching using aqueous KOH solution has significant advantages, e.g., lower complexity and cost and less remaining surface damage. An in‐depth analysis of the molecular etch reaction by characterization of the reaction products is reported. The mechanism identified explains the cause of anisotropic etching, which leads to the formation of hexagonal pyramids. The concept of hydroxide repulsion by protruding NH and NH2 groups established in the literature is adapted and further developed. The susceptibility of several polar, semipolar, and nonpolar crystal facets may also be explained, as well as the commonly observed increase in average pyramid size over etch time.

A discrete core-shell-like micro-light-emitting diode array grown on sapphire nano-membranes

A discrete core-shell-like micro-light-emitting diode (micro-LED) array was grown on a 100 nm-thick sapphire nano-membrane array without harmful plasma etching for chip singulation. Due to proper design for the sapphire nano-membrane array, an array of multi-faceted micro-LEDs with size of 4 μm × 16 μm was grown. Threading dislocation density in the micro-LED formed on sapphire nano-membrane was reduced by 59.6% due to the sapphire nano-membranes, which serve as compliant substrates, compared to GaN formed on a planar substrate. Enhancements in internal quantum efficiency by 44% and 3.3 times higher photoluminescence intensity were also observed from it. Cathodoluminescence emission at 435 nm was measured from c-plane multiple quantum wells (MQWs), whereas negligible emissions were detected from semi-polar sidewall facets. A core-shell-like MQWs were formed on all facets, hopefully lowering concentration of non-radiative surface recombination centers and reducing leakage current paths. This study provides an attractive platform for micro-LEDs by using sapphire nano-membrane.

Growth of a-axial GaN core nanowires, semi-polar ([formula omitted]) GaN/InGaN multiple quantum well co-axial nanowires on Si substrate, and their carrier dynamics

The quantum-confined Stark effect (QCSE) reduces the quantum efficiency of optical devices due to the reduced overlap of electron and hole wave functions because of the growth of GaN/InGaN multiple quantum wells (MQWs) on the polar facet. Here, we report the growth of non-polar [112‾0] GaN core nanowires on Si substrate by MOCVD. Subsequently, the active region of GaN/InGaN MQWs is grown on (11‾01) semipolar growth facet. The morphology of GaN core and GaN/InGaN MQWs is examined by SEM and TEM. The growth direction of GaN core and GaN/InGaN MQWs is confirmed by SAED patterns. The bandgap is tuned from 3.11 eV to 2.45 eV by increasing the InGaN QW thickness. Finally, time-resolved photoluminescence is conducted to evaluate the carrier dynamics. An ultrashort time constant between 51 ps and 74 ps is measured with increased QW thickness. The underlying reason for the very short carrier lifetime is the suppressed QCSE because the piezoelectric and polarization fields are reduced due to the semi-polar growth facet of QWs. The semi-polar growth of GaN/InGaN MQW co-axial nanowires on Si substrate underlines the potential of our fabrication technique for a variety of optical applications.

Hillock assisted p-type enhancement in N-polar GaN:Mg films grown by MOCVD

We report on the enhanced incorporation efficiency of magnesium dopants into facets of hexagonal hillock structures in N-polar GaN, studied by comparative analysis of GaN:Mg films grown by MOCVD on high and low hillock density GaN template layers. Total magnesium concentration in planar regions surrounding a hillock structure is comparable to that within hillock sidewall facets measured at 1.3 × 1019 cm−3 by atom probe tomography, and clustering of Mg atoms is seen in all regions of the film. Within individual hillock structures a decreased Mg cluster density is observed within hillock structures as opposed to the planar regions surrounding a hillock. Additionally, the Mg cluster radius is decreased within the hillock sidewall. The favorable incorporation of Mg is attributed to Mg dopants incorporating substitutionally for Ga during growth of semi-polar facets of the hillock structures. Enhanced p-type conductivity of GaN:Mg films grown on high hillock density template layers is verified by optical and electrical measurement.

Study of the morphology evolution of AlN grown on nano-patterned sapphire substrate

This study focused on the evolution of growth front about AlN growth on nano-patterned sapphire substrate by metal-organic chemical vapor deposition. The substrate with concave cones was fabricated by nano-imprint lithography and wet etching. Two samples with different epitaxy procedures were fabricated, manifesting as two-dimensional growth mode and three-dimensional growth mode, respectively. The results showed that growth temperature deeply influenced the growth modes and thus played a critical role in the coalescence of AlN. At a relatively high temperature, the AlN epilayer was progressively coalescence and the growth mode was two-dimensional. In this case, we found that the inclined semi-polar facets arising in the process of coalescence were type. But when decreasing the temperature, the semi-polar facets arose, leading to inverse pyramid morphology and obtaining the three-dimensional growth mode. The 3D inverse pyramid AlN structure could be used for realizing 3D semi-polar UV-LED or facet-controlled epitaxial lateral overgrowth of AlN.

Orthogonally Polarized, Dual-Wavelength Quantum Wire Network Emitters Embedded in Single Microrod.

Semiconductor nanowires are attractive building blocks of optoelectronics due to high efficiency and optical controllability. In particular, the mutual controllability of wavelength and polarization of light is essential for versatile applications such as displays, precise metrology, and bioimaging. We present quantum wire network emitters embedded in a single microrod capable of exhibiting orthogonally polarized dual-wavelength visible light at room temperature. The InGaN/GaN shell layers were grown on a single hexagonal GaN core microrod, spontaneously forming site-selective In-rich InGaN quantum wires on each edge between the nonpolar facets as well as each boundary between the nonpolar and semipolar facets. The orthogonally self-arranged, two sets of six quantum wires formed on the edges and the boundaries showed efficient violet and blue-green color emissions with strong linear polarization parallel and perpendicular to the c-axis at room temperature, respectively. This intriguing emission from a single microrod allows us to mutually manipulate the color and the polarization of light, which would be beneficial for photonic applications with unprecedented controllability and functionality.

Optical and electrical characterizations of the V-shaped pits in Fe-doped bulk GaN

The leakage current path in the vertical-type metal–semiconductor–metal structured device fabricated on a Fe-doped bulk GaN substrate with a V-shaped pit is investigated. Micro-Raman, micro-photoluminescence, and atomic force microscope show nonuniform distributions of the optical and electrical properties in the V-shaped pit. The distribution of carrier concentration is determined by the distributions of Si, O, and Fe impurities, which are further influenced by the incorporation efficiency of the corresponding impurities in the semi-polar growth facets. The mechanism of current leakage in a V-shaped pit is explained by the big difference between the resistance inside and outside the pit.

Homoepitaxial growth by halide vapor phase epitaxy of semi-polar GaN on ammonothermal seeds

During crystallization of GaN in the c-direction by halide vapor phase epitaxy the formation of semi-polar facets as well as growth in semi-polar directions are observed. Crystallization process on these semi-polar facets is described in this paper in detail. Ammonothermal GaN crystals were used as seeds. Morphology, structural quality, growth rate, impurities concentration and free carrier concentration are analyzed and compared to data obtained for crystals grown in the c-direction.

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.