Pulsed Growth Mode Research Articles

At the Limit of Interfacial Sharpness in Nanowire Axial Heterostructures.

As semiconductor devices approach dimensions at the atomic scale, controlling the compositional grading across heterointerfaces becomes paramount. Particularly in nanowire axial heterostructures, which are promising for a broad spectrum of nanotechnology applications, the achievement of sharp heterointerfaces has been challenging owing to peculiarities of the commonly used vapor-liquid-solid growth mode. Here, the grading of Al across GaAs/AlxGa1-xAs/GaAs heterostructures in self-catalyzed nanowires is studied, aiming at finding the limits of the interfacial sharpness for this technologically versatile material system. A pulsed growth mode ensures precise control of the growth mechanisms even at low temperatures, while a semiempirical thermodynamic model is derived to fit the experimental Al-content profiles and quantitatively describe the dependences of the interfacial sharpness on the growth temperature, the nanowire radius, and the Al content. Finally, symmetrical Al profiles with interfacial widths of 2-3 atomic planes, at the limit of the measurement accuracy, are obtained, outperforming even equivalent thin-film heterostructures. The proposed method enables the development of advanced heterostructure schemes for a more effective utilization of the nanowire platform; moreover, it is considered expandable to other material systems and nanostructure types.

Selective area growth of AlGaN nanopyramids by conventional and pulsed MOVPE

Planar UV-C light emitting diodes still suffer from low efficiency, mainly due to substrate crystalline quality, p doped conductivity and extraction efficiency. One possible way to overcome partly these issues is to realize the whole UV structure on AlGaN pyramids by selective area growth in order to benefit from the advantages of such structures, i.e. the dislocation filtering and the semi polar planes. We present here a detailed study about the epitaxy of AlGaN nano-sized pyramids by metal organic vapor phase epitaxy on patterned templates presenting different holes apertures and pitches as 1.5 μm and 4 μm or 100 nm and 250 nm respectively. While increasing the Al content, their height decreases while the thickness of the deposition on the mask increases whatever the design of the mask. Those changes of the pyramid shapes and deposition are directly linked to the properties of Al adatoms, i.e. low Al diffusion length. Using the conventional growth mode for the epitaxy of those pyramids did not permit the incorporation of Al from the base of the pyramids to their truncated apex. Its presence was concentrated on the edges and top of the pyramids. On the contrary, a pulsed growth mode, coupled with a strongly reduced pitch, allowed an incorporation of Al since the base of the nanopyramid, and a decrease of the deposition height on the mask. These results can be explained by the desorption of Ga species, due to the presence of H2 in the reactor chamber during the step without the metal precursors, leading to a higher Al/Ga ratio. It is even enhanced inside the holes by the reduced pitch.

Role of Readsorption in the Formation of Vertical AIIIBV Nanowires with Self-Catalytic Growth

Using Monte Carlo simulations, we studied the effect of readsorption on the growth rate and morphology of AIIIBVnanowires. It is demonstrated that readsorption of arsenic leads to an increase in the growth rate and premature consumption of a seed drop. The reabsorption of metal atoms leads to a slowdown in the axial growth but prolongs the growth, since it prevents the droplet from disappearing after the length of the nanowire reaches the diffusion length of the metal atoms along the side walls of the wire. It is demonstrated that it is possible to increase the diffusion length of group III elements along the side walls of the nanowire using the pulsed growth mode.

Precursor Duration and Thermal Annealing Effects in InGaN/GaN Multiple Quantum Wells Grown on Nitrogen-Polar GaN Templates by a Pulsed Metallorganic Chemical Vapor Deposition

In this study, the effects of precursor duration and thermal annealing on the material and optical properties of InGaN/GaN multiple quantum wells (MQWs) grown on nitrogen (N)-polar GaN templates by a pulsed metallorganic chemical vapor deposition are investigated. With a 2-sec NH3 precursor duration, an apparent indium aggregation leads to a higher density and larger size of InGaN mounds for more exciton accumulation, enhancing the radiative recombination and luminescence efficiency. In addition, the more, larger, and brighter light spots in the cathodoluminescence (CL) images and a stronger CL intensity in the annealed sample show that a smaller size and higher density of InGaN mounds enhance the radiative recombination and luminescence efficiency. Both a 2-sec NH3 precursor duration and 60-sec thermal annealing are beneficial to the growth conditions of InGaN/GaN MQWs grown on N-polar GaN templates. The research results of the pulsed growth mode provide important information to optimize growth conditions of InGaN/GaN MQWs grown on N-polar GaN templates.

III-Nitride Epitaxy for Electronic and Optoelectronic Devices

High quality AlxGa1-xN material is sought after for a large range of applications including emitters, detectors, and power electronic devices. To realize deep ultraviolet (UV) LEDs and Lasers, highly conductive films with high crystalline quality and high Al content AlGaN are required. Epitaxially grown AlN and AlGaN serve as buffer layers for GaN-based devices on Silicon. Moreover, AlGaN layers form the cornerstone of III-N high electron-mobility transistors. Given the importance of this material, significant research effort has been devoted toward the development of epitaxial techniques for the growth of AlxGa1-xN, with many challenges remaining. For the growth of high quality, higher composition AlGaN, GaN substrates and templates are unsuitable due to the higher lattice constant of GaN compared to AlGaN which causes tensile strain in the overgrown AlGaN leading to cracks and defects. Native substrates for AlGaN are non-existent, and AlGaN-based layers and device structures are commonly heteroepitaxially grown on non-native substrates such as sapphire, SiC, and Si. Due to its lack of centerosymmetry along (0001) direction, III-Nitrides can be grown polar (along 0001 direction for Ga-polar and 000-1 direction for N-polar), non-polar (along a- and m- directions) and in between to allow semi-polar growth. The past two decades have shown tremendous progress in possibilities for growth and device design based on these polarities. Here we present our progress in developing a number of epitaxial growth techniques for devices their function is inspired by specific polarity direction, including AlGaN/GaN When grown along (0001) direction, AlGaN/GaN HEMT structures demonstrate normally-on characteristics with 2DEG density in the 1013 cm-2 range and mobility in excess of 1200 Vsec/cm2. Growth on Si substrate offers opportunity for integration with the rest of the circuitry as well as for development of stretchable electronics where liftoff from substrate is essential. The past decade has seen great progress in managing stress that is developed due to the large thermal expansion coefficient mismatch between the Si substrate and III-Nitride layers. In our research, we employ MOCVD growth technique to develop stress-engineered/high-quality device structure-on-Si. Here we describe two methodologies to obtain thick buffer layers with reduced stress levels which are desirable for obtaining high breakdown voltage structures for power devices: Implementation of in-situ stress-monitoring to monitor stress evolution to achieve sufficient compressive stress in the AlGaN/AlGaN buffer layers; and use of selective area epitaxy to develop as-grown HEMT structures in areas patterned in active device geometric designs (e.g. horseshoe for stretchable geometry). The N-polar orientation has potential for improved and expanded device design space due to the reversal of the built-in and stress-induced polarization fields. Achieving high-conductivity p-type material is crucial for many device applications, but has traditionally been a challenge in III-Nitrides. Previous work has found comparable levels of intentional impurity incorporation and carrier concentration between Ga- and N-polar GaN grown by conventional MOCVD. Recently, pulsed MOCVD growth has emerged as a promising way to achieve higher carrier concentration in Ga-polar p-type GaN, and especially as an effective means of increasing conductivity in p-type AlGaN films . The effect of pulsed growth in the N-polar orientation on dopant incorporation and conductivity is so far unstudied, and has the potential for improved conductivity for p-type N-polar films. Our studies of p-GaN:Mg show a strong effect on incorporation due to polarity when using the pulsed growth mode, with a factor of 3 or more reduction in Mg incorporation for N-polar films grown as compared to traditional continuous growth, from 3x1019 cm-3 to 1x1019 cm-3. Here, we examine the effects of growth mode and polarity on dopant incorporation and activation in single layer and device structures using in-situ stress monitoring, secondary ion mass spectroscopy (SIMS), AFM, XPS, Hall Effect measurements and internal photoemission (IPE).

Mg Incorporation Efficiency in Pulsed MOCVD of N-Polar GaN:Mg

We report on the effect of growth polarity and pulsed or $$\delta $$ -doped growth mode on impurity incorporation in metalorganic chemical vapor deposition-grown GaN. In Ga-polar orientation, up to 12× enhancement in Mg concentration for given Mg flow rate is observed, resulting in enhanced p-type conductivity for these samples. In contrast, this enhancement effect is greatly diminished for N-polar samples, falling off with increasing Mg flow and showing maximum enhancement of 2.7× at 30 nmol/min Mg flow. At higher Mg flow rates, Mg incorporation at normal levels did not correspond to p-type conductivity, which may be due to Mg incorporation at nonacceptor sites. Concentrations of C, O, and Si were also investigated, revealing dependence on Mg flow in N-polar pulsed samples. Carbon incorporation was found to decrease with increasing Mg flow, and oxygen incorporation was found to remain high across varied Mg flow. These effects combine to result in N-polar samples that are not p-type when using the pulsed growth mode.

The effects of indium aggregation in InGaN/GaN single and multiple quantum wells grown on nitrogen-polar GaN templates by a pulsed metalorganic chemical vapor deposition

In this study, the effects of indium aggregation in InGaN/GaN single and multiple quantum wells (MQW) grown on nitrogen-polar GaN templates by a pulsed metalorganic chemical vapor deposition are investigated. With the pulsed growth mode, InGaN decomposition and indium aggregation lead to InGaN mounds, which forms localized states for trapping carriers. In the MQW sample, a higher density and larger size of InGaN mounds imply that an enhanced indium aggregation can improve luminescence efficiency.

Significantly improved luminescence properties of nitrogen-polar (0001̅) InGaN multiple quantum wells grown by pulsed metalorganic chemical vapor deposition.

We have demonstrated nitrogen-polar (0001̅) (N-polar) InGaN multiple quantum wells (MQWs) with significantly improved luminescence properties prepared by pulsed metalorganic chemical vapor deposition. During the growth of InGaN quantum wells, Ga and N sources are alternately injected into the reactor to alter the surface stoichiometry. The influence of flow duration in pulsed growth mode on the luminescence properties has been studied. We find that use of pulsed-mode creates a high density of hexagonal mounds with an increased InGaN growth rate and enhanced In composition around screw-type dislocations, resulting in remarkably improved luminescence properties. The mechanism of enhanced luminescence caused by the hexagonal mounds is discussed. Luminescence properties of N-polar InGaN MQWs grown with short pulse durations have been significantly improved in comparison with a sample grown by a conventional continuous growth method.

Cross-sectional sizes and emission wavelengths of regularly patterned GaN and core-shell InGaN/GaN quantum-well nanorod arrays

The cross-sectional sizes of the regularly patterned GaN nanorods (NRs) and InGaN/GaN quantum-well (QW) NRs of different heights and different hexagon orientations, which are grown on the patterned templates of different hole diameters, pitches, and crystal orientations, are compared. It is found that the cross-sectional size of the GaN NR, which is formed with the pulsed growth mode, is mainly controlled by the patterned hole diameter, and the thickness of the sidewall QW structure is mainly determined by the NR height. The cross-sectional size variation of GaN NR is interpreted by the quasi-three-dimensional nature of atom supply amount for precipitating a two-dimensional disk-shaped NR segment. The variation of the sidewall QW structure is explained by the condition of constituent atom supply in the gap volume between the neighboring NRs. Also, we compare the cathodoluminescence emission wavelengths among those samples of different growth conditions. Generally speaking, the QW NR with a smaller height, a larger cross-sectional size, or a larger pitch has a longer emission wavelength.

Influence of bimodal distribution and excited state emission on photoluminescence spectra of InAs self-assembled quantum dots

Optical properties of self-assembled quantum dots (SAQDs) samples were investigated through photoluminescence (PL) with variation of temperature and excitation density. Two types of InAs SAQDs were studied: dots grown directly on GaAs, and dots grown inside an InGaAs quantum well, heterostructure known as dot-in-a-well (DWELL). The QDs structures were produced through conventional growth (with several deposition rates) as well as through pulsed growth mode, which consists in deposition cycles with growth interruption intervals. In the samples of QDs grown on GaAs, two PL peaks were identified and their behavior as a function of temperature can be explained by taking into account a bimodal distribution of dot sizes. For the DWELL sample only one PL peak was identified, which was assigned to the emission of the ground state of QDs belonging to a larger Gaussian distribution of dot sizes. SAQDs samples showed an anomalous behavior of the peak energy, integrated intensity and linewidth (FWHM) of the PL signal, as a function of temperature. To explain the observed results, a model was proposed that takes into account the size distribution of dots and the excited state emission of the family of larger dots. The QDs can be distributed in a bimodal size distribution, and the emission from the excited state can be observed, or not, depending on the growth conditions and on the potential profile of the dot. Such aspects can be combined to explain several peculiarities of the PL spectra observed in this and other published papers on SAQDs optical properties.

Geometry and composition comparisons between c-plane disc-like and m-plane core-shell InGaN/GaN quantum wells in a nitride nanorod

With the nano-imprint lithography and the pulsed growth mode of metalorganic chemical vapor deposition, a regularly-patterned, c-axis nitride nanorod (NR) array of quite uniform geometry with simultaneous depositions of top-face, c-plane disc-like and sidewall, m-plane core-shell InGaN/GaN quantum well (QW) structures is formed. The differences of geometry and composition between these two groups of QW are studied with scanning electron microscopy, cathodoluminescence, and transmission electron microscopy (TEM). In particular, the strain state analysis results in TEM observations provide us with the information about the QW width and composition. It is found that the QW widths are narrower and the indium contents are higher in the sidewall m-plane QWs, when compared with the top-face c-plane QWs. Also, in the sidewall m-plane QWs, the QW width (indium content) decreases (increases) with the height on the sidewall. The observed results can be interpreted with the migration behaviors of the constituent atoms along the NR sidewall from the bottom.

MOVPE and photoluminescence of narrow band gap (0.77 eV) InN on GaN/sapphire by pulsed growth mode

This paper reports the metalorganic vapor phase epitaxy (MOVPE) of InN on GaN/sapphire templates by pulsed growth mode. Photoluminescence analysis of the films has been performed both at room temperature (T = 300 K) and low temperature (T = 77 K). At room temperature, the band gap of InN is measured as 0.77 eV which is observed to have strong dependence on the growth temperature of the film. The appearance of otherwise less dominant transitions is associated to defects related luminescence from the films grown at relatively low growth temperatures. The electron mobility of 681 cm2/(V s) is obtained for InN alloy grown under optimal condition. The X-ray diffraction (XRD) analysis, structural quality and the evolution of the surface morphology of thin InN films have also been studied in the context of the given growth conditions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of growth temperature and V/III ratio on the optical characteristics of narrow band gap (0.77 eV) InN grown on GaN/sapphire using pulsed MOVPE

In this study, we report on the pulsed metalorganic vapor phase epitaxy (MOVPE) of InN as well as the optical and electronic properties of the films as a function of V/III ratio and growth temperatures. The growth of InN films was conducted utilizing a vertical reactor with TMIn and NH 3 as the In- and N-precursors, respectively. Metallic droplet-free InN films were achieved on GaN/sapphire template in a pulsed MOVPE mode with low V/III ratio condition. In the pulsed growth mode, NH 3 was constantly flowing while the TMIn was sent into the reactor chamber for a 36-s pulse and then it bypassed the reactor chamber for an 18-s pulse for a total cycle time of 54 s. At a growth pressure of 200 Torr, the effects of growth temperature (510–575 °C) and V/III ratio (12,460–17,100) on the photoluminescence (PL) transitions were investigated. Morphological evolution as well as the electrical quality of the overgrown films have also been studied for the given growth conditions.

Laser-assisted synthesis of carbon nanofibers: From arrays to thin films and coatings

Carbon nanofiber assemblies in the form of non-aligned films, arrays of vertically aligned nanofibers, aligned nanofiber mats and composite coatings were produced by laser-assisted catalytic chemical vapor deposition. A visible argon ion laser was used to thermally decompose pure ethylene over alumina supported nickel catalysts. Straight, vermicular, beaded, branched and coiled individual nanofibers were observed. The effects of the laser irradiation time on individual nanofiber characteristics, thus on overall nanofiber assembly characteristics were investigated. The arrays, nanostructured films and coatings were examined by scanning electron microscopy. The individual nanofibers were examined by transmission electron microscopy. Nanofiber texture and nanotexture were assessed by lattice fringe analysis of high resolution transmission electron microscopy images. The observed variation in the interfringe distance along the nanofiber wall suggests a pulsed growth mode. This growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure are provided.