Nanocolumns/ Nanowires fabrication literature review/InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy

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This is in a series of literature reviews on InGaN solar cells, which supported the comprehensive review by D.V.P. McLaughlin & J.M. Pearce, "Progress in Indium Gallium Nitride Materials for Solar Photovoltaic Energy Conversion"Metallurgical and Materials Transactions A 44(4) pp. 1947-1954 (2013). open access Others: InGaN solar cells| InGaN PV| InGaN materials| InGan LEDs| Nanocolumns and nanowires| Optical modeling of thin film microstructure| Misc.

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InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy[edit | edit source]

Abstract: Single crystalline and single phase InxGa1−xN nanopillars were grown spontaneously on (111) silicon substrate by plasma assisted molecular beam epitaxy. The surface morphology, structural quality, and optoelectronic properties of InGaN nanopillars were analyzed using scanning electron microscopy (SEM), energy dispersive x-ray (EDXA) analysis, high resolution x-ray diffraction (HR-XRD), and both room and low temperature photoluminescence spectra. The EDXA results showed that these nanopillars were composed of InGaN and the amount of indium incorporation in InxGa1−xN NPs could be controlled by changing the growth temperature. The room temperature and low temperature PL spectra revealed that the emission wavelength could be tuned from a blue to green luminescent region depending on the growth temperature. The wavelength tuning was attributed to a higher amount of In incorporation at a lower growth temperature which was consistent with the EDXA and HR-XRD results.

Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations[1][1][1][edit | edit source]

Abstract: The large-scale assembly of nanowires with controlled orientation on surfaces remains one challenge preventing their integration into practical devices. Author(s) report the vapor-liquid-solid growth of aligned, millimeter-long, horizontal GaN nanowires with controlled crystallographic orientations on different planes of sapphire. The growth directions, crystallographic orientation, and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. This paves the way to highly controlled nanowire structures with potential applications not available by other means.

Luminescence properties and defects in GaN nanocolumns grown by molecular beam epitaxy[2][2][2][edit | edit source]

Abstract: Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si(111) and c-sapphire substrates. The nanocolumns density and diameter (600–1500 Å) are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) low-temperature photoluminescence excitonic lines at 3.472–3.478 eV. In addition, the spectra reveal a doublet at 3.452–3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to GaI defects, given the column growth mode (Ga balling).

Two-dimensional exciton behavior in GaN nanocolumns grown by molecular-beam epitaxy[3][3][3][edit | edit source]

Abstract: Author(S) have investigated the behavior of excitons in GaN nanocolumns using time-integrated and time-resolved micro-photoluminescence measurements. In the weak confinement limit, the model of fractional-dimensional space gives an intermediate dimensionality of 2.14 for GaN nanocolumns, with an average diameter of 80 nm. Enhanced exciton and donor binding energies are deduced from a fractional-dimensional model and a phenomenological description. Time-integrated photoluminescence spectra as a function of temperature show a curved emission shift. Recombination dynamics are deduced from the temperature dependence of the PL efficiency and decay times.

Structural and optical characterization of intrinsic GaN nanocolumns[4][4][4][edit | edit source]

Abstract: The morphology and optical properties of GaN nanocolumns grown on Si(1 1 1) and sapphire substrates by plasma-assisted molecular beam epitaxy, are studied by photoluminescence, Raman scattering, scanning electron microscopy and cathodoluminescence. The nanocolumns grow along the [0 0 0 1] direction and exhibit high-crystalline quality. Their section is hexagonal with diameters between 450 and View the MathML source. The photoluminescence spectrum is composed by two excitonic peaks at 3.471 and View the MathML source, and three broad emissions at lower energy. Cathodoluminescence images show that the excitonic emissions originate at the upper nanocolumns body while the low-energy peaks are originated at the 'bulk' material at the bottom of the nanocolumns. The low-energy emissions are related to defects at the interface between the columns and the substrate.

A complementary geometric model for the growth of GaN nanocolumns prepared by plasma-assisted molecular beam epitaxy[5][5][5][edit | edit source]

Abstract: In this article, Author(S) propose a new complementary geometrical growth mechanism, which may partially explain some of the apparent anomalies in our understanding of the growth of GaN nanocolumns by plasma-assisted molecular beam epitaxy (PA-MBE). This geometrical addition to any complete model for nanocolumn growth is based on the fact that most samples are grown using substrate rotation and it predicts an enhanced growth rate in the plane normal to the surface, i.e. vertically compared with the lateral growth rate of the columns. It also suggests a mechanism for the enhanced diffusion of gallium on the sidewalls of the columns even under strongly nitrogen-rich conditions. Finally, geometrical considerations also predict the growth of non-(0 0 0 1) oriented samples from the same mechanism. Some experimental evidence supporting this complementary geometrical model is presented.

Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy[6][6][6][edit | edit source]

Abstract: GaN nanocolumns were grown with AlN buffer layers on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. The AlN buffer layers underneath the nanocolumns were used to nucleate the nanocrystals. The thickness of the AlN buffer layer affected the column configuration (size, shape), the density and the optical properties of the nanocolumns; when the thickness increased from 1.8 to 8.2 nm, the average column diameter gradually decreased from 150 to 52 nm with a small kink, but the column density peaked at a thickness of 3.2 nm at 5×109 cm−2 and finally decreased to 2×108 cm−2. Based on TEM observations, it is suggested that GaN nanocolumns were not grown just on AlN grain but on the edge of AlN grain. Further, the growth behavior of a nanocolumn as a function of AlN buffer layer thickness is suggested. The room-temperature photoluminescence intensity of the nanocolumns was maximized at a buffer thickness of 4.6 nm, where the intensity was 4 times stronger than that of high-quality bulk GaN crystals grown by HVPE with a threading dislocation density of ~8×106 cm−2.

Plasmon effects on infrared spectra of GaN nanocolumns[7][7][7][edit | edit source]

Abstract: Infrared (IR) transmission spectra of GaN nanocolumns were analyzed. In addition to the bulk GaN optical phonon signal, a broad absorption peak was observed from undoped and Mg-doped nanocolumns. The central position and width of the broad peak changed with the growth condition and Mg concentration. Based on the Lorentz–Drude model composed of phonon and plasmon modes associated with depolarization fields in GaN nanocolumns, IR transmission spectra were fitted by adjusting the free-electron concentration and scattering rate. Dependence of these values on the column size and impurity concentration is discussed.

Letter Ultradense, Deep Subwavelength Nanowire Array Photovoltaics As Engineered Optical Thin Films[8][8][8][edit | edit source]

Abstract: A photovoltaic device comprised of an array of 20 nm wide, 32 nm pitch array of silicon nanowires is modeled as an optical material. The nanowire array (NWA) has characteristic device features that are deep in the subwavelength regime for light, which permits a number of simplifying approximations. Using photocurrent measurements as a probe of the absorptance, we show that the NWA optical properties can be accurately modeled with rigorous coupled-wave analysis. The densely structured NWAs behave as homogeneous birefringent materials into the ultraviolet with effective optical properties that are accurately modeled using the dielectric functions of bulk Si and SiO2, coupled with a physical model for the NWA derived from ellipsometry and transmission electron microscopy.

Single nanowire photovoltaics[9][9][9][edit | edit source]

Abstract: This tutorial review focuses on recent work addressing the properties and potential of semiconductor nanowires as building blocks for photovoltaic devices based on investigations at the single nanowire level. Two central nanowire motifs involving p-i-n dopant modulation in axial and coaxial geometries serve as platforms for fundamental studies. Research illustrating the synthesis of these structural motifs will be reviewed first, followed by an examination of recent studies of single axial and coaxial p-i-n silicon nanowire solar cells. Finally, challenges and opportunities for improving efficiency enabled by controlled synthesis of more complex nanowire structures will be discussed, as will their potential applications as power sources for emerging nanoelectronic devices.

Toward the Lambertian Limit of Light Trapping in Thin Nanostructured Silicon Solar Cells[10][10][10][edit | edit source]

Abstract: Author(s) examine light trapping in thin silicon nanostructures for solar cell applications. Using group theory, Author(s) design surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band at normal incidence. Further, Author(s) demonstrate that the absorptance of nanorod arrays closely follows the Lambertian limit for isotropic incident radiation. These effects correspond to a reduction in silicon mass by 2 orders of magnitude, pointing to the promising future of thin crystalline silicon solar cells.

Crystallographic alignment of high-density gallium nitride nanowire arrays[11][11][11][edit | edit source]

Abstract: Single-crystalline, one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics. Elucidation of the vapour–liquid–solid growth mechanism has already enabled precise control over nanowire position and sizeyet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array. Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and bandgap may be used to tune the physical properties of nanowires made from a given material. Here Author(s) demonstrate the use of metal–organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100) gamma-LiAlO2 and (111) MgO single-crystal substrates resulted in the selective growth of nanowires in the orthogonal [1-10] and [001] directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration.

Self-catalyzed growth of GaAs nanowires on cleaved Si by molecular beam epitaxy[12][12][12][edit | edit source]

Abstract: Self-assembled GaAs nanowires have been grown on Si by molecular beam epitaxy without the use of any outside metal catalyst. The growth occurs on Si facets obtained by the cleavage of Si(100) substrates. The growth has been obtained with or without Ga pre-deposition. In both cases two kinds of nanowires have been obtained. The wires of the first type clearly present a Ga droplet at their free end and have a lattice structure that is wurtzite for wide regions beneath the Ga droplet. The second type, in contrast, ends with pyramidally shaped GaAs and has a crystal lattice that is mainly zincblende with only a few and small wurtzite regions, if any. The Ga-ended nanowires are longer than the others and thinner on average. The experimental findings suggest that the two types of nanowires grow after different growth processes.

Direct Observation of Vapor−Liquid−Solid Nanowire Growth[13][13][13][edit | edit source]

Conclusion: The direct observation of nanowire growth unambiguously confirms the validity of vapor−liquid−solid crystal growth mechanism at the nanometer scale and should allow us to rationally control the nanowire growth which is critical for their potential implementation into the nanoscale electronic and optoelectronic devices.

• Three well-defined stages have been clearly identified during the process:  a) metal alloying, b) crystal nucleation, and c) axial growth.
• Alloying process: Au clusters remain in the solid state up to author(s) maximum experimental temperature 900 °C if there is no Ge vapor condensation. This was confirmed by selected area electron diffraction on the pure Au clusters. With increasing amount of Ge vapor condensation and dissolution, Ge and Au form an alloy and liquefy. volume of the alloy droplets increases, and elemental contrast decreases (due to dilution of the heavy metal Au with the lighter element Ge) while the alloy composition crosses sequentially, a biphasic region (solid Au and Au/Ge liquid alloy) into a single-phase region (liquid).
• Nucleation: Once the composition of the alloy crosses the second liquidus line, it enters another biphasic region (Au/Ge alloy and Ge crystal). This is where nanowire nucleation starts. author(s) suggest that nucleation occurs in a supersaturated alloy liquid.
• Axial growth: Once the Ge nanocrystal nucleates at the liquid/solid interface, further condensation/dissolution of Ge vapor into the system will increase the amount of Ge crystal precipitation from the alloy. The incoming Ge species prefer to diffuse to and condense at the existing solid/liquid interface, primarily due to the fact that less energy will be involved with the crystal step growth as compared with secondary nucleation events in a finite volume. Consequently, secondary nucleation events are efficiently suppressed, and no new solid/liquid interface will be created. The existing interface will then be pushed forward (or backward) to form a nanowire.
• Generally, the diameters of the nanowires are larger than the sizes of initial clusters by several nanometers due to the Au/Ge alloying process. This size correlation clearly points out a simple approach to selectively grow nanowires of different diameters using monodispersed clusters of different sizes as catalysts.

Watching GaN Nanowires Grow[14][14][14][edit | edit source]

Abstract: Author(s) report real-time high temperature transmission electron microscopy observations of the growth of GaN nanowires via a self-catalytic vapor−liquid−solid (VLS) mechanism. High temperature thermal decomposition of GaN in a vacuum yields nanoscale Ga liquid droplets and gallium/nitrogen vapor species for the subsequent GaN nanowire nucleation and growth. This is the first direct observation of self-catalytic growth of nanowires via the VLS mechanism and suggests new strategies for synthesizing electronically pure single-crystalline semiconductor nanowires.

• synthesis of GaN nanowires via the vapor−liquid−solid (VLS) process commonly relies on transition metal clusters such as Ni, Fe, and Co, which inevitably results in undesired contamination within the otherwise single-crystalline nanowires.
• By heating GaN sample in-situ, it is possible to observe this decomposition and the resulting nanostructure evolution in real time and at high spatial resolution.
• current observation of VLS GaN nanowire growth without any foreign transition metal catalysts opens possibility of producing semiconductor nanowires through a self-catalytic process which could effectively avoid undesired contamination from foreign metal atoms.
• Because of the possible nitridation of the Ga liquid droplets during and after the nanowire growth, author(s) have not detected Ga-rich nanoparticles at any of the GaN nanowire tips.
• Unlike those GaN nanowires grown using foreign metal catalysts, the ends of author(s) prototype nanowires are rather smooth without obvious liquid droplet tips.

Periodic Si Nanopillar Arrays Fabricated by Colloidal Lithography and Catalytic Etching for Broadband and Omnidirectional Elimination of Fresnel Reflection[15][15][15][edit | edit source]

Abstract: Periodic Si nanopillar arrays (NPAs) were fabricated by the colloidal lithography combined with catalytic etching. By varying the size of colloidal crystals using oxygen plasma etching, Si NPAs with desirable diameter and fill factor could be obtained. The Fresnel reflection can be eliminated effectively over broadband regions by NPAs; i.e., the wavelength-averaged specular reflectance is decreased to 0.70% at wavelengths of 200−1900 nm. The reflectance is reduced greatly for the incident angles up to 70° for both s- and p-polarized light. These excellent antireflection performances are attributed to light trapping effect and very low effective refractive indices, which can be modified by the fill factor of Si in the NPA layers.

Gallium nitride nanorod arrays as low-refractive-index transparent media in the entire visible spectral region[16][16][16][edit | edit source]

Abstract: Vertically aligned gallium nitride (GaN) nanorod arrays grown by the catalyst-free, self-organized method based on plasma-assisted molecular-beam epitaxy are shown to behave as subwavelength optical media with low effective refractive indices. In the reflection spectra measured in the entire visible spectral region, strong reflectivity modulations are observed for all nanorod arrays, which are attributed to the effects of Fabry-Pérot microcavities formed within the nanorod arrays by the optically flat air/nanorods and nanorods/substrate interfaces. By analyzing the reflectivity interference fringes, Author(s) can quantitatively determine the refractive indices of GaN nanorod arrays as functions of light wavelength. Author(s) also propose a model for understanding the optical properties of GaN nanorod arrays in the transparent region. Using this model, good numerical fitting can be achieved for the reflectivity spectra.

Nucleation and Growth of GaN Nanowires on Si(111) Performed by Molecular Beam Epitaxy[17][17][17][edit | edit source]

Abstract: GaN nanowires (NWs) have been grown on Si(111) substrates by plasma-assisted molecular beam epitaxy (PAMBE). The nucleation process of GaN-NWs has been investigated in terms of nucleation density and wire evolution with time for a given set of growth parameters. The wire density increases rapidly with time and then saturates. The growth period until the nucleation of new nanowires is terminated can be defined as the nucleation stage. Coalescence of closely spaced nanowires reduces the density for long deposition times. The average size of the well-nucleated NWs shows linear time dependence in the nucleation stage. High-resolution transmission electron microscopy measurements of alternating GaN and AlN layers give valuable information about the length and radial growth rates for GaN and AlN in NWs.

High optical quality GaN nanopillar arrays[18][18][18][edit | edit source]

Abstract: GaN nanopillar arrays have been fabricated by inductively coupled plasma etching of GaN films using anodic aluminum oxide film as an etch mask. The average diameter and length of these pillars are 60–65 nm and 350–400 nm, respectively. Ultraviolet microphotoluminescence measurements indicate high photoluminescence intensity and stress relaxation in these GaN nanopillars as compared to the starting epitaxial GaN films. Evidence of good crystalline quality is also observed by micro-Raman measurements, wherein a redshift of the E2high mode from GaN nanopillars suggests partial relaxation of the compressive strain. In addition, breakdown of the polarization selection rules led to the appearance of symmetry-forbidden and quasipolar modes.

Characterization of InGaN/GaN Multiple Quantum Well Nanorods Fabricated by Plasma Etching with Self-Assembled Nickel Metal Nanomasks[19][19][19][edit | edit source]

Abstract: High-density (3.0×1010 cm-2) InGaN/GaN multiple quantum well (MQW) nanorods were fabricated from an as-grown bulk light-emitting diode structure by inductively coupled plasma dry etching with self-assembled nickel metal nanomasks. The self-assembled nickel metal nanomasks were formed by rapid thermal annealing of a nickel metal film at 850°C for 1 min. The influence of the thicknesses of the Ni metal film on the dimensions and density of the nanorods was also investigated. The structural and optical properties of the InGaN/GaN MQW nanorods were established using field emission scanning electron microscopy, transmission electron microscopy and photoluminescence measurements. The diameters and heights of nanorods were estimated to be 60 to 100 nm and more than 0.28 µm, respectively. The peak emission wavelength of the nanorods showed a blue shift of 5.1 nm from that of the as-grown bulk. An enhancement by a factor of 5 in photoluminescence intensity of the nanorods compared with that of the as-grown bulk was observed. The blue shift is attributed to strain relaxation in the wells after dry etching, the quantum confinement effect, or a combination of the two, which results in the enhancement of emission intensity.

• Author(s) found that no isolated Ni nanoislands were formed below an RTA of 800 C. RTA at 850 C formed isolated Ni metal nanoislands with a density of about 3 x 10⁹ cm^-2 observed from morphology measurements.
• thickness of initial metal film can play an important role in determining surface morphology at a given annealing temperature.
• origin of decrease in nanorod density may be increase in dimensions of Ni metal islands as the initial Ni metal layer increases, resulting in an increase in the mean dimension of the nanorods.
• In the radial direction, the nanorods exhibited a circular geometry indicating an isotropic, homogeneous etch.
• nanorod has a larger diameter at top than at bottom, and its sidewalls are sloped and slightly rough. author(s) attribute imperfect sidewalls and lateral dimensions to mask erosion and ion-damage during dry etch step.
• deposition of Ni film on Si3N4 film results in a large degree of compression in the 'a' direction and tensile strain in the 'c' direction. Therefore, formation of isolated Ni nanoislands on Si3N4 film surface was performed during a thermal treatment.
• blue shift in PL can be attributed to strain relaxation in the wells, the quantum confinement effect or a combination of the two.
• It has been proposed that piezoelectric fields arise due to mismatch-induced strain in In1-xGaxN/GaN MQW. In the presence of the piezoelectric field, the electron and hole wave function separate spatially leading to a reduced overlap and hence a reduced recombination rate.
• PL intensity in nanorods is enhanced by a factor of approximately 5 over bulk emission. enhancement could be due to a better overlap of electron and hole wave functions with a reduced piezoelectric field, and an increase in radiative recombination rate.
• Light scattering off the etched sidewalls of nanorods could also increase the PL intensity.

References[edit | edit source]