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Nanocolumns/ Nanowires fabrication literature review

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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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Contents

InGaN based LED and LASER devices.[edit]

The Controlled Growth of GaN Nanowires[1][edit]

Abstract: This paper reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled. The approach is based on conventional metalorganic chemical vapor deposition using regular precursors and requires no additional metal catalyst. The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective growth mask that is patterned by interferometric lithography. It was found that use of a pulsed MOCVD process allowed the nanowire diameter to remain constant after the nanowires had emerged from the selective growth mask. Vertical GaN nanowire growth rates in excess of 2 μm/h were measured, while remarkably the diameter of each nanowire remained constant over the entire (micrometer) length of the nanowires. The paper reports transmission electron microscopy and photoluminescence data.

  • Reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled based on conventional metal-organic chemical vapor deposition (MOCVD) using trimethygallium (TMGa) and ammonia (NH3) and requires no additional metal catalyst.
  • The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective-growth mask that is patterned by interferometric lithography.
  • This method enables to fabricate high quality symmetrical hexagonal sidewall facets over the entire micrometer length of nanowires.
  • This method was carried out in two phases, first phase used continous MOCVD till nanowires emerged out of mask and second phase was pulse MOCVD to fabricate nanowires emerged out of mask at the end of phase one, using pulsed MOCVD in second phase maintained the quality of nanowires and were highly alligned and ordered as desired with respect to the mask.
  • diameter of the nanowire remains constant as it emerges from the growth mask, confirming from XTEM images that the nanowire diameter is indeed controlled by the diameter of the growth mask aperture.
  • XTEM images show no threading dislocations (TDs) in the GaN nanowires, even though TDs were observed in the planar GaN film beneath the growth mask. This was found to be the case for GaN nanowire growth on all substrates, including growth on silicon (111).
  • recorded band-edge PL peak intensity for the nanowire sample was 100 times greater than that measured for a 5 micron planar GaN film and more than 200 times greater than that measured for a 0.6 micron planar GaN film. Much of this intensity increase is undoubtedly due to the geometry of a nonplanar nanowire sample, where the input coupling of the PL pump beam and the out-coupling of the resulting PL will both be increased significantly.

InGaN nanorod arrays grown by molecular beam epitaxy: Growth mechanism structural and optical properties[2][edit]

Abstract: Vertically c-axis-aligned InGaN nanorod arrays were synthesized on c-plane sapphire substrates by radio-frequency molecular beam epitaxy. In situ reflection high-energy electron diffraction was used to monitor the growth process. X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscope, and photoluminescence were used to investigate the structural and optical properties of the nanorods. The growth mechanism was studied and a growth model was proposed based on the experimental data. A red shift of photoluminescence spectrum of InGaN nanorods with increasing growth time was found and attributed to the partial release of stress in the InGaN nanorods.

  • Initially RHEED pattern was streaky, indicating a two-dimensional (2D) growth with a smooth surface at the initial stage. As growth proceeded, one observed mixed RHEED pattern including spots and lines, but the lines were still streaky. After deposition for 13 min, circular spotty RHEED pattern was observed; indicating a three-dimensional (3D) island growth, circular spotty RHEED pattern lasted for 30 min.
  • Afterwards the circular spotty RHEED pattern broadens along the direction parallel to the surface; this is typically the RHEED pattern of nanorods. Such a pattern suggested that the nanorods nucleated on the rough InGaN layer, grew up while narrowed, nanorods were single crystalline and collectively grew along the c-axis.
  • misfit accommodation can take place through the onset of islanding and generating surface roughness in strained epitaxial systems.
  • growth-mode transition from layer-by-layer (2D) growth to 3D islanding occurs quite abruptly when a certain critical strain was reached. In mismatched InGaN-sapphire system, according to RHEED observations, strained InGaN layer started growing nearly pseudomorphical to sapphire and presented relatively smooth surface, when layer thickness exceeded a certain critical layer thickness (CLT) of about 30 nm, strained InGaN layer started to relax and 3D islands began to form.
  • For layer thicknesses well above CLT, the surface became very rough and was dominated by large 3D islands.
  • formation of InGaN nanorod array can be roughly divided into following stages: Firstly, InGaN thin film grew pseudomorphically on c-plane sapphire, and then 3D islands formed when layer exceeded the CLT. InGaN nanorods started to nucleate on rough InGaN layer and then grew up. At last, the InGaN nanorods coalescenced.
  • increased crystal growth times improves crystal quality.
  • band gap of strained InGaN films is larger than that of the relaxed due to the compressive stresses
  • Such a stress in InGaN nanorods could not be eliminated until length exceeded several micrometers

Formation of InGaN nanorods with indium mole fractions by hydride vapor phase epitaxy[3][edit]

Abstract: This work demonstrates the formation of InGaN nanorod arrays with indium mole fractions by hydride vapor phase epitaxy. The nanorods grown on (0001) sapphire substrates are preferentially oriented in the c-axis direction. We found that the In mole fractions in the nanorods were linearly increased at x < 0.1. However, In mole fractions were slightly increased at x ≥ 0.1 and then were gradually saturated at x = 0.2. CL spectra show strong emissions from 380 nm (x = 0.04, 3.26 eV) to 470 nm (x = 0.2, 2.64 eV) at room temperature.

  • nanorods have the potential for negligible non-radiative recombination loss, and thus efficiency of down-conversion is much higher than bulk InGaN/GaN.
  • average diameter and length were 70 nm and 2 μm, respectively. Control of InGaN nanorod’s diameter and length were achieved by adjusting growth temperature and growth time, respectively.
  • Increasing HCl gas flow rate for reacting In metal, that is In precursor, from 10 sccm to 100 sccm, In mole fractions (x) were increased from 0.04 to 0.20 in the nanorods.
  • When HCl reactant gas flow rate was small (≤25 sccm), In mole fractions were linearly increased with HCl gas flow rate. As HCl reactant gas flow rate was increased, however, In mole fractions in InGaN nanorods were slightly increased and then these were saturated.

Selective area metalorganic molecular-beam epitaxy of GaN and the growth of luminescent microcolumns on Si/SiO2[4][edit]

Abstract: Author(s) demonstrate the selective area growth of gallium nitride on patterned Si(111)/GaN/SiO2 wafers by metalorganic molecular beam epitaxy using triethyl gallium as a Ga source. Author(s) show that such selective area deposition may be used to grow isolated microcolumns of GaN with lateral dimensions of tens of nanometers on Si/SiO2 wafers. Via high resolution cathodoluminescence imaging Author(s) show that such microcolumn structures are highly luminescent inspite of a large surface to volume ratio, indicating that nonradiative recombination at free surfaces is not a significant issue in this system.

  • While selectivity is observed, porous growth morphology is clear as well. This is a consequence of high temperatures required for selective area growth by MOMBE, where significant thermal etching can occur. Such porous morphology is not observed for layers grown at ~ 750 °C.
  • temperature requirements for a dense microstructure over a large ( > 1 um) length scale and that for selective area growth of GaN are in conflict with one another for MOMBE growth. high temperatures required for selective area growth result in a loss of integrity of structure and author(s) conclusion therefore is that such selective area growth of GaN by MOMBE is unsuitable for large area device growth.
  • it’s an attractive technique for growing small ( < 500 nm) structures of GaN, where lateral length scales are smaller than length scale at which porosity is observed, since it does offer benefit of a relatively low temperature at which selective area growth occurs compared to MOCVD growth.
  • This ensures compatibility with substrates such as Si where interfacial reactions between the Si and nitride limit usable temperatures to about 1000–1050 °C.
  • The CL lateral resolution is limited by excitation volume of optically active part of specimen and minority carrier diffusion length, depending upon the specimen and measurement details.
  • qualitatively computed upper-bound of non-radiative recombination centers.

Rapid growth and characterization of InN nanocolumns on InGaN buffer layers at a low ratio of N/In [5][edit]

Abstract: c-Axis-aligned InN nanocolumn arrays were vertically grown on 3 μm GaN epilayers with InGaN buffer layers by radio-frequency molecular beam epitaxy without any catalysts. X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscope were used to study the structural properties of the nanocolumns. It has been found that without InGaN buffers, InN films, rather than nanocolumns, were grown even at the same N/In ratio. In addition, high-quality InN nanocolumns can grow faster on InGaN buffers. The growth mechanism was discussed and the joint actions of the gas–solid and Volmer–Weber modes promote the nucleation and the growth of InN nanocolumns.

  • The different growth rates of InN nano-structures may be due to the different growth modes
  • 100 nm In0.05Ga0.95N and 30 nm In0.25Ga0.75N buffer layers were deposited in sequence at 600 and 530 °C, respectively, on the 3 μm GaN epilayers that were grown by metal organic chemical vapor deposition. After that, InN nanocolumns and films were grown at 470 °C with the N/In ratio equal to 1, 2, and 4, respectively. The N-flow was kept constant to maintain the pressure stable, and the In-cell temperature was varied in order to grow InN nano-structures or films with different N/In ratio.
  • results of the SEM observation suggested that, at a low ratio of N/In, InN nanocolumns could not grow directly on GaN epilayers, but could grow on InGaN buffer layers.
  • varied lattice constants of both a-axis and c-axis indicated indium spontaneous phase separation from InGaN buffer closed to the InGaN surface.
  • it is deduced that formation of InN nanocolumn array may be divided into 3 stages: firstly, phase separation of indium from In0.25Ga0.75N buffer layer took place in the cooling process, forming InGaN with higher indium composition by the end of the growth of the buffer layer. Indium atoms easily gather into clusters. Secondly, InN preferentially nucleated on In-rich regions according to gas–solid (G–S) mode then neighboring InN islands coalescence according to Volmer–Weber (V–W) mode. Finally, the InN nanocolumns grew up on isolated InN islands according to Stranski–Krastanow (S–K) mode. Therefore, it can be presumed that InN nanocolumns preferentially grow on InN, then on indium precipitates, and finally on InGaN.

Growth of InN nanocolumns by RF-MBE[6][edit]

Abstract: InN nanocolumns were grown on (0 0 0 1) sapphire substrates by radio-frequency plasma-assisted molecular-beam epitaxy, simply with substrate nitridation, InN nucleation, and high V/III growth conditions. Here, the InN nucleation was conducted by forming In droplets on the nitridated substrates and subsequently nitriding these In droplets. Author(s) investigated the growth evolution of InN nanocolumns and the growth-temperature dependence of the morphology. X-ray rocking curves (XRC) for the (0 0 0 2) reflection of the samples basically consisted of two components: a broad peak and a very sharp one. Author(s) propose that the broad peak came from the lower part of relatively short nanocolumns and the near-interface region of relatively long nanocolumns. In contrast, the sharp peak came from the (strain-free) upper part of relatively short nanocolumns and the high-crystal-quality region apart from the interface in relatively long InN nanocolumns. The relative intensity of the latter to the former increased with growth time. The shape of the nanocolumns varied with growth temperature: nanocolumns grown at 380 and 420 °C had a taper-like appearance, but the top broadened with increasing growth temperature, becoming broader at the top than the base at 470 and 500 °C. When Author(s) grew InN nanocolumns under the same conditions, but without surface nitridation, the subsequent columns were not all along a c-axis. If In droplets were not used, then columns did not form. Thus, both the surface nitridation and In droplets were needed to form c-axis aligned nanocolumns.

  • As growth time increased from 1 to 10 min, the density of nanocolumns (islands at this early stage) was increased by additional nucleation in areas between original InN islands.
  • GaN nanocolumns were shown to exhibit stronger luminescence than GaN films, explained by nanocolumn's nature containing no or drastically reduced dislocations, which act as non-radiative recombination centers.
  • higher V/III ratios than unity at the growing surface reduced the surface migration of In atoms.
  • InN islands nucleated directly on nitridated substrates during the growth, relatively short nanocolumns also nucleated directly on nitridated substrates during the growth, and relatively long nanocolumns nucleated from the In droplets.
  • InN nucleation with In droplet formation and the subsequent nitriding treatment of In droplets is essential for InN nanocolumn growth.
  • lack of substrate nitridation resulted in a severe deterioration in the c-axis orientation of nanocolumns.
  • InN nanocolumns had a taper-like shape when grown at temperatures below 420 degree C and thickened toward the end with increasing growth temperature.

InGaN nanorods grown on (1 1 1) silicon substrate by hydride vapor phase epitaxy[7][edit]

Abstract: In this study, Author(s) report on the growth of the defect-free (dislocation-free) InGaN nanorods on (1 1 1) silicon substrate by our hydride vapor phase epitaxy (HVPE) system. Morphological and structural characterization of the InGaN nanorods by high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) indicates that the nanorods are straight and preferentially oriented in the c-axis direction. Cathodoluminescence (CL) characteristic of the single InGaN nanorod shows a strong blue emission peaking at around 428 nm at room temperature.

  • many threading dislocations (TDs) are produced in bulk GaN and InGaN due to lattice mismatch with the substrate and difference of thermal expansion coefficient between film and substrate, and thus they affect significantly the device performance as non-radiative recombination centers.
  • growth mechanism for nanorods is completely different, and threading dislocations can be all non-existent in nanorods.
  • nanorods have potential for negligible non-radiative recombination loss, and thus the efficiency of down-conversion is much higher than in bulk InGaN/GaN layer.
  • Control of the InGaN nanorod’s diameter and length were achieved by adjusting the growth temperature and growth time, respectively.
  • A strong blue emission was observed from a single nanorod peaking at around 428 nm (≈2.9 eV) with electron beam excitation, no GaN-related emission from this nanorod was observed implying that InGaN nanorods grown by using In metal was a ternary single crystal without mixing GaN.

InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy[8][edit]

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.

  • lower substrate temperature promotes higher indium incorporation in InGaN.
  • driving force for 3-D growth mode of III-nitrides on silicon is lattice mismatch strain energy and the high surface energy of nitrogen stabilized (0001) III-N surfaces. The major difference between GaN and InN nanopillars growth is substrate temperature used.
  • growth of InN nanopillars, however, can be sustained at higher substrate temperature without the observation of such metallic accumulation due to the nitrogen rich condition employed. Hence InxGa1−xNNPswere grown at Tsub that was varied from 500 to 650 degree C.
  • InxGa1−xN nanopillars were grown at Tsub varied from 500 to 650 ◦C. At lower Tsub of 500 and 550 ◦C, the formation of InxGa1−xN nanopillars was not observed but, on the contrary, InxGa1−xN was formed as a porous film. However, InxGa1−xN nanopillars were formed at Tsub equal to or higher than 575 ◦C.
  • The InxGa1−xN nanopillars have a faceted structure, which is consistent with being a hexagonal pyramid bounded by {1-101} facets. This structure is energetically stable and has been observed in microscale feature growth.
  • As the Tsub increases, the diameter of the InGaN NPs decreases while the density increases as a result of lesser coalescence at higher Tsub.
  • EDX spectra confirmed that the Nanopillars were composed of InGaN.
  • samples with higher density, improved crystalline quality, smaller size and different composition of nanopillars, resulted in higher PL intensity.
  • author(s) observed that higher In incorporation degraded the optical quality (lower PL intensity and broadening of the spectra) of the InxGa1−xN NPs.
  • optical quality of InxGa1−xN NPs improved at higher growth temperature.

High-quality In0.47Ga0.53N/GaN heterostructure on Si(111) and its application to MSM detector[edit]

Abstract: Purpose – This paper aims to report on the use of radio frequency nitrogen plasma-assisted molecular beam epitaxy (RF-MBE) to grow high-quality n-type In0.47Ga0.53N/GaN on Si(111) substrate using AlN as a buffer layer. Design/methodology/approach – Structural analyses of the InGaN films were performed by using X-ray diffraction, atomic force microscopy, and Hall measurement. Metal-semiconductor-metal (MSM) photodiode was fabricated on the In0.47Ga0.53N/Si(111) films. Electrical analysis of the MSM photodiodes was carried out by using current-voltage (I-V) measurements. Ideality factors and Schottky barrier heights for Ni/In0.47Ga0.53N, was deduced to be 1.01 and 0.60 eV, respectively. Findings – The In0.47Ga0.53N MSM photodiode shows a sharp cut-off wavelength at 840 nm. A maximum responsivity of 0.28 A/W was achieved at 839 nm. The detector shows a little decrease in responsivity from 840 to 200 nm. The responsivity of the MSM drops by nearly two orders of magnitude across the cut-off wavelength. Originality/value – Focuses on III-nitride semiconductors, which are of interest for applications in high temperature/power electronic devices.

InGaN/GaN quantum-well nanocolumn crystals on pillared Si substrate with InN as interlayer[edit]

Abstract: Nanocolumn InGaN/GaN crystals were deposited on micropillared Si substrate by molecular beam epitaxy. Low-temperature InN was used as interlayer. With enough free space, the column crystals grew around all the surface plane of the Si pillars and formed InGaN/GaN quantum-well flower structure. The QW crystals are about 100 nm in diameter and 1.1-1.4 μm in length. Raman spectra measurement of the fower structure shows that E2 mode peak line observed at 567.28 cm–1. Photoluminescence measurement indicates a room temperature PL peak position of 620 nm and two peak positions of 404 nm and 519 nm at temperature 15 K. Hg lamp excited photoluminescence demonstrated a clear fluorescence distribution in the flower structure and much stronger emission compared with the quantum-well crystals on the flat Si substrate.

CVD growth of InGaN nanowires[9][edit]

Abstract: In this paper, the chemical vapor deposition (CVD) growth of InGaN nanowires was systematically studied. The catalyst was Au and the starting materials were Ga, In and NH3. The samples were characterized with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron spectroscopy (TEM), and X-ray diffraction (XRD), etc. The influence of the growth temperatures, Au thicknesses, gas flowrates and Ga and In amount on the morphology and properties of InGaN nanowires was investigated. It is found that 600 °C is a suitable growth temperature. On the substrate with Au thickness of 150 Å, helical InGaN nanowires are obtained. The change of NH3 partial pressure and Au thickness will result in the morphology change of the samples. An increase of Ga results in shorter InGaN nanowires while an increase of In amount will lead to longer InGaN nanowires. The morphology will also change when both the amount of In and Ga were increased or reduced without changing the ratio of Ga to In.

  • growth temperature, catalyst thickness, gas flow rates, Ga and In amount are all found to have an effect on the morphology.
  • suitable fabrication temperature is 600 °C.
  • thickness of Au has an effect on the morphology of InGaN nanowire and no InGaN nanowires can be obtained when there is no Au on the substrates.
  • On the substrate with Au thickness less than 100 Å, only straight InGaN nanowires were observed while both straight and helical InGaN nanowires were obtained on the substrates with Au thickness of 150 Å.
  • Different NH3 and Ar flow rates were tried and it is found that larger NH3 partial pressure will result in InGaN nanowires with better morphology.
  • Both straight and helical InGaN nanowires were obtained with larger NH3 partial pressure on substrates with Au of 150 Å.
  • Different In and Ga amount were also tried and it is discovered that an increase of Ga amount results in shorter InGaN nanowires and an increase of In amount will lead to longer InGaN nanowires. When both the amount of In and Ga were increased or decreased without changing the ratio, the morphology also changed and no helical InGaN nanowires were observed when Ga and In amount were changed.

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

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[10][edit]

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[11][edit]

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[12][edit]

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[13][edit]

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[14][edit]

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[15][edit]

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[16][edit]

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[17][edit]

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[18][edit]

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[19][edit]

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[20][edit]

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[21][edit]

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[22][edit]

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[23][edit]

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[24][edit]

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[25][edit]

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[26][edit]

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[27][edit]

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[28][edit]

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]

  1. Hersee, S.D., Sun, X. & Wang, X., 2006. The Controlled Growth of GaN Nanowires. Nano Lett., 6(8), pp.1808-1811
  2. Wu, K.M., Pan, Y. & Liu, C., 2009. InGaN nanorod arrays grown by molecular beam epitaxy: Growth mechanism structural and optical properties. Applied Surface Science, 255(13-14), pp.6705-6709
  3. Kim, H. et al., 2004. Formation of InGaN nanorods with indium mole fractions by hydride vapor phase epitaxy. physica status solidi (b), 241(12), pp.2802-2805
  4. Guha, S. et al., 1999. Selective area metalorganic molecular-beam epitaxy of GaN and the growth of luminescent microcolumns on Si/SiO2. Applied Physics Letters, 75(4), p.463
  5. Pan, Y. et al., 2010. Rapid growth and characterization of InN nanocolumns on InGaN buffer layers at a low ratio of N/In. Journal of Crystal Growth, 313(1), pp.16-19
  6. Nishikawa, S. et al., 2007. Growth of InN nanocolumns by RF-MBE. Journal of Crystal Growth, 301-302(0), pp.490-495
  7. Kim, H.-M. et al., 2003. InGaN nanorods grown on (1 1 1) silicon substrate by hydride vapor phase epitaxy. Chemical Physics Letters, 380(1-2), pp.181-184
  8. Vajpeyi, A.P. et al., 2009. InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy. Nanotechnology, 20(32), p.325605
  9. X.M. Cai, F. Ye, S.Y. Jing, D.P. Zhang, P. Fan, and E.Q. Xie, "CVD growth of InGaN nanowires," Journal of Alloys and Compounds, Volume 467, Issues 1–2, 7 January 2009, Pages 472-476
  10. D. Tsivion, M. Schvartzman, R. Popovitz-Biro, P. von Huth, and E. Joselevich, “Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations,” Science, vol. 333, no. 6045, pp. 1003 -1007, 2011
  11. E. Calleja et al., “Luminescence properties and defects in GaN nanocolumns grown by molecular beam epitaxy,” Physical Review B, vol. 62, no. 24, pp. 16826-16834, Dec. 2000
  12. J. H. Na et al., “Two-dimensional exciton behavior in GaN nanocolumns grown by molecular-beam epitaxy,” Applied Physics Letters, vol. 86, no. 12, pp. 123102-123102-3, Mar. 2005
  13. J. Sánchez-Páramo, J. M. Calleja, M. A. Sánchez-Garcı́a, E. Calleja, and U. Jahn, “Structural and optical characterization of intrinsic GaN nanocolumns,” Physica E: Low-dimensional Systems and Nanostructures, vol. 13, no. 2-4, pp. 1070-1073, Mar. 2002
  14. C. T. Foxon et al., “A complementary geometric model for the growth of GaN nanocolumns prepared by plasma-assisted molecular beam epitaxy,” Journal of Crystal Growth, vol. 311, no. 13, pp. 3423-3427, Jun. 2009
  15. H. Sekiguchi, T. Nakazato, A. Kikuchi, and K. Kishino, “Structural and optical properties of GaN nanocolumns grown on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy,” Journal of Crystal Growth, vol. 300, no. 1, pp. 259-262, Mar. 2007
  16. T. Iwanaga, T. Suzuki, S. Yagi, and T. Motooka, “Plasmon effects on infrared spectra of GaN nanocolumns,” Applied Physics Letters, vol. 86, no. 26, pp. 263102-263102-3, Jun. 2005
  17. D. Tham and J. R. Heath, “Ultradense, Deep Subwavelength Nanowire Array Photovoltaics As Engineered Optical Thin Films,” Nano Lett., vol. 10, no. 11, pp. 4429-4434, 2010
  18. B. Tian, T. J. Kempa, and C. M. Lieber, “Single nanowire photovoltaics,” Chem. Soc. Rev., vol. 38, no. 1, pp. 16-24, Nov. 2008
  19. S. E. Han and G. Chen, “Toward the Lambertian Limit of Light Trapping in Thin Nanostructured Silicon Solar Cells,” Nano Lett., vol. 10, no. 11, pp. 4692-4696, 2010
  20. T. Kuykendall et al., “Crystallographic alignment of high-density gallium nitride nanowire arrays,” Nat Mater, vol. 3, no. 8, pp. 524-528, 2004
  21. F. Jabeen, V. Grillo, S. Rubini, and F. Martelli, “Self-catalyzed growth of GaAs nanowires on cleaved Si by molecular beam epitaxy,” Nanotechnology, vol. 19, no. 27, p. 275711, Jul. 2008
  22. Y. Wu and P. Yang, “Direct Observation of Vapor−Liquid−Solid Nanowire Growth,” J. Am. Chem. Soc., vol. 123, no. 13, pp. 3165-3166, 2001
  23. E. A. Stach, P. J. Pauzauskie, T. Kuykendall, J. Goldberger, R. He, and P. Yang, “Watching GaN Nanowires Grow,” Nano Lett., vol. 3, no. 6, pp. 867-869, 2003
  24. H.-P. Wang, K.-Y. Lai, Y.-R. Lin, C.-A. Lin, and J.-H. He, “Periodic Si Nanopillar Arrays Fabricated by Colloidal Lithography and Catalytic Etching for Broadband and Omnidirectional Elimination of Fresnel Reflection,” Langmuir, vol. 26, no. 15, pp. 12855-12858, 2010
  25. H.-Y. Chen, H.-W. Lin, C.-Y. Wu, W.-C. Chen, J.-S. Chen, and S. Gwo, “Gallium nitride nanorod arrays as low-refractive-indextransparent media in the entire visiblespectral region,” Optics Express, vol. 16, no. 11, pp. 8106-8116, May 2008
  26. R. Calarco, R. J. Meijers, R. K. Debnath, T. Stoica, E. Sutter, and H. Lüth, “Nucleation and Growth of GaN Nanowires on Si(111) Performed by Molecular Beam Epitaxy,” Nano Lett., vol. 7, no. 8, pp. 2248-2251, 2007
  27. Y. D. Wang, S. J. Chua, S. Tripathy, M. S. Sander, P. Chen, and C. G. Fonstad, “High optical quality GaN nanopillar arrays,” Applied Physics Letters, vol. 86, no. 7, pp. 071917-071917-3, Feb. 2005
  28. T.-H. Hsueh et al., “Characterization of InGaN/GaN Multiple Quantum Well Nanorods Fabricated by Plasma Etching with Self-Assembled Nickel Metal Nanomasks,” Japanese Journal of Applied Physics, vol. 44, no. 4, pp. 2661-2663, Apr. 2005