The complex interfacial structural issues associated with hetero-polar epitaxial integration of GaP on Si(001) are resolved by improving the kinetics of gallium and phosphorous adatoms during the initial nucleation process. This is achieved through the use of Raman spectroscopy as a feedback mechanism. The peak intensity ratio of forbidden (transverse optical)-to-allowed (longitudinal optical) phonons of GaP under unpolarized Raman scattering is contemplated as an initial guide to ascertain the relative density of defect-mediated non-(001) GaP facets. The azimuthal-angle dependent polarized Raman spectroscopy is then applied to identify and quantify the nucleation of higher-index non-(001) ({111} and {112}) faceted islands in the nucleation layer and their propagation in the overgrown structures. The nucleation of GaP adatoms on the Silicon surface at similar to 525 degrees C leads to a smooth surface and substantially suppresses the defect-mediated facets. The optimal mobility and controlled adsorption of adatoms of Ga and P on the substrate surface under intermediate nucleation kinetics foster the charge-neutral interface, and hence the defect-suppressed two-dimensional layer growth of GaP. The proposed Raman spectroscopy methodology emerges as a rapid and economical alternative to existing in situ reflection anisotropy spectroscopy and high-resolution transmission electron microscopy techniques for structural identification. This method of growth optimization can also be applied to other III-V polar semiconductors to achieve high-quality and efficient monolithic integration of devices on existing Si technology.

Structural complexities evolving at hetero-polar interface of III-V/Si have been critical obstacles to high-quality and cost-effective epitaxial integration of wide-bandgap GaP on closely lattice-matched Si. Unveiling the nature of interface-originated defect structures is quintessential for efficient integration of III-V semiconductors on Si. In this work, investigation of surface and interface of technologically important GaP/Si(001) hetero-structures is presented, through unique implementation of combining the information on variation in spatially resolved polarized Raman spectra and surface topography of grown epilayer. The mechanisms responsible for variation in ratio of Raman scattering cross-section of symmetry forbidden to allowed optical phonons of GaP are delineated using azimuthal angle-resolved polarized Raman measurements by allowing for the contribution of energetically favorable higher-index {111} and {112} crystallographic facets, nucleating near the hetero-interface, to light scattering. This is reasserted from polarized Raman spectroscopy performed on cross-sectional surface of GaP/Si(001) hetero-structures. To the best of our knowledge, this is first of it's kind work wherein angle-dependent polarized Raman measurements are employed for ascertaining the nature and orientations of interfacial defect facets in advanced hetero-structures. Non-invasive and expeditious nature of this optical technique open pathways for fabrication of efficient device structures of III-V semiconductors on Si and Ge platforms.

•Strong presence of symmetry forbidden TO phonon in Raman spectra of GaP/Si(001).•Polar Raman measurements reveal presence of higher-index {111} and {112} defect facets.•Coalescence of faceted islands formed during nucleation led to high-index defect faceting.•Crystal phase co-existence occurs along < 0001>wurtzite/〈111〉zinc-blende directions.•Proposed methodology can provide guidelines for controlling interfacial defects.

Conversion of Wurtzite to Zincblende phase of InAs NWs at high simulated temperature (> 1300 K) is investigated using resonance Raman spectroscopy with specially designed two step heating-cooling experiment. For smaller nanowires (diameter < 700 nm), the certainty of determining the polytypism using polarized Raman spectroscopy, reduces significantly. Low signal to noise ratio for smaller diameter NWs is improved by employing the wavelength dependent unpolarized and polarized Raman spectroscopy to examine the sensitivity of resonance Raman spectroscopy as an alternate for probing polytypism. Innovative use of resonance Raman spectroscopy to probe phase purity of the III-V individual nanostructures, where, conventional methods may not be applicable is realized in this work.This work paves the way for online monitoring of the conversion of mixed phase to phase pure III--V nanowires using laser irradiation for their fast electronic device applications. Further, this kind of surface modification study provides basis for the futuristic crystal structure engineering, which can facilitate the controlled tailoring of material properties in order to enhance their functionality.

The surface and micro structure variations in the technologically important GaP/Si hetero-structures are investigated by the spatially resolved polarized Raman spectroscopy. The hetero-structures probed from two different directions <1 1 1> and< 1 <(1)over bar> 0> reveal the presence of strain distributed zinc-blende (ZB) GaP phases and the dominance of wurtzite (WZ) phase near the GaP-Si interface. The azimuthal dependence of the optical phonon intensity of nucleating layer endorses the dominance of WZ phase at the interface. The study illustrates that spatially resolved polarized Raman probe, across the hetero-junction is extremely useful for distinguishing the WZ and ZB stacks. It can positively be utilized for the integration of crystal phase quantum structures of III-V semiconductors on the Si and Ge substrates, without any physical damage to the structures.

GaP/Ge(111) heterostructure with high residual strain is investigated for identification of allotrope, zincblende (ZB) and wurtzite (WZ) phase distribution in the grown GaP layer. Spatially resolved Raman spectroscopy across the cross-sectional surface of GaP/Ge(111) heteostructure show continuous blue shift in optical phonon frequencies from near interface to surface of GaP/Ge. Polarized Raman spectroscopy from cross-sectional surface, at near and away from interface positions, establishes that WZ phase is dominant at GaP-Ge interface. Further, higher strained regions of GaP/Ge heterostructure show larger content of wurtzite phase in GaP film. This work paves the way for probing the spatial dependent existence of allotropes and their relative content in micro/nanostructured thin films using Raman spectroscopy.

GaP/Ge(111) heterostructures are investigated for the observed red-shift and increased full width at half maximum of Ge optical phonon using Raman spectroscopy. Results are discussed in the light of inter diffusion of substrate and layer elements at the hetero-interface. Wavelength dependent Raman spectra show the co-existence of inter diffused layer (similar to 20 nm) below the GaP nucleating layer that is grown at 425 degrees C with high V/III ratio (similar to 1725). However, GaP/Ge(111) thick layer grown at 770 degrees C having same nucleating layer does not show presence of an inter diffused layer. Consistently, nucleating layer thermally treated at 700 degrees C, close to thick layer growth temperature, does not show the presence of such an inter diffused layer. The study paves the way for probing an inter diffused hetero-interface without damaging the sample using Raman spectroscopy. Further, these results are very useful for understanding the role of nucleating layer for the integration of III-V on germanium.

High quality CdS nanowires (NWs) are grown at room temperature by electrochemical deposition into the pores of prepared porous alumina templates (PATs) from alkaline aqueous electrolyte solution (pH = 8.5) containing Cd2+ and S2O3 2−, using high frequency (200 Hz) alternating current (ac, 21 Vrms). PATs with hexagonally ordered nanoporous columns are prepared using relatively simple and faster single step anodizing at 30 Vdc in 0.24 M oxalic acid. The room temperature synthesis of NWs from aqueous solution using ac voltage simplifies the growth process as well as makes it an inexpensive approach for the fabrication of CdS NWs in large density. Atomic force microscopy and scanning electron microscopy (SEM) confirm the formation of spatially regular and well-arranged hexagonal porous structures. SEM further shows that diameter of grown CdS NWs lie in the range ~ 30–50 nm. Raman spectroscopy and X-ray diffraction of as deposited and annealed PAT–CdS nanowires composites confirm the formation of high crystalline quality CdS NWs having hexagonal wurtzite crystal structure. The preparation method and possible mechanism for the synthesis of CdS NWs is discussed. The deposition method can be generalized to a wide range of semiconductors.

In this work, role of temperature simulation is investigated for predicting surface modification of InAs nanowire (NW) using laser irradiation. The surface modification is monitored by Raman spectroscopy. We first establish correlation between simulated temperatures at the surface of an InAs nanowire (NW) with time evolution of Raman spectra on laser irradiation. Transient thermal simulations are performed with ANSYS software using finite element method, considering 3D geometry of the irradiation setup. In the systematic study of laser irradiation (laser power densities similar to 30-636 kW/cm(2)) over time duration of similar to 8 min, the simulated temperature is found to corroborate well with the corresponding oxidation processes e.g. weak (WP), intermediate (IP) and strong (SP), occurring on the surface of InAs nanowire under various laser irradiation conditions. The predictability of the methodology was then investigated by applying it to random conditions of NW like diameter, aspect ratio and laser power density etc. for i) NWs found in the same sample and ii) InAs NWs grown elsewhere and transferred on other Si substrate. The study establishes predictability of various oxidation processes for given NWdimensions, laser power density and irradiation time, thereby ascertaining importance and applicability of the temperature simulation in controlling surface modification of randomly chosen InAs NWs, as per the requirement for device applications. (C) 2017 Elsevier B.V. All rights reserved.

We investigate differences observed in the time evolution of Raman spectra for differently oriented (in plane) InAs nanowires (NWs), using polarized Raman spectroscopy. Specially designed polarized Raman spectroscopy experiments elucidate that laser irradiation leads to the formation of an oriented crystalline oxide film on the InAs NW surface. Both the formation of oriented crystalline oxides and Raman selection rules leading to the presence/absence of oxide peaks in the unpolarized Raman spectra are uncommon occurrences and can lead to incorrect interpretations of the oxidation process, if not looked into carefully. Further, the specially designed heating and cooling experiments for a mixed phase (wurtzite + zinc blende) InAs NW revealed the formation of specific allotropes of elemental As, i.e. gray-As (rhombohedral) and black-As (orthorhombic: metastable) at low (700-950 K) and high simulated temperatures (1000-1300 K) on the InAs NW surface, respectively. Both have high electrical conductivity due to a layered structure and control over the growth of only a few layers using laser irradiation envisages properties similar to graphene. This kind of surface of InAs NWs has the potential for novel device applications, where a semiconductor-insulator-metal heterostructure is required.

The structure of tapered InAs nanowire (diameter similar to 400 to 200 nm from base to tip) along the length (10 mu m) is studied using spatially resolved Raman spectroscopy. The observed Raman spectra suggest presence of both wurtzite (WZ) and zincblende (ZB) structure for InAs with increasing content of wurtzite structure towards tip. Polarized Raman measurements shows dominance of E-2h (WZ) similar to 213 cm(-1) in x(y, y)(x) over bar and dominance of TO (ZB) similar to 215.5 cm(-1) x(z, z)(x) over bar configuration for light polarization perpendicular and parallel to nanowire axis, respectively. The blue and red shift in WZ and ZB phonons (base and center) from their bulk value is attributed to changed near neighbor interaction due to presence of the other structure. However, the blue shift of both E-2h (WZ) and TO (ZB) near tip is indicative of additional mechanical compressive stress.

CdS-PVP nanocomposites grown with various concentrations of Cd+/S− and PVP are studied using Raman and atomic force microscopy mapping on same selected areas. The study shows that CdS nanocrystals are embedded in either a thin-film or spheres of PVP, formation of which depends on relative ratio of Cd+/S− concentration (density of nanocrystals) to PVP. Collapse of PVP from planer matrix to the sphere morphology is correlated with high density of CdS nanocrystals and the attractive interaction between CdS nanocrystals and PVP polymer leading to a co-operative growth mechanism. The understanding developed can be used to control the growth of CdS-PVP nanocomposites with desired properties for optoelectronic device applications.

The asymmetric peak at 212 - 218 cm-1 occurring in InAs micro-nano wires is investigated using spatially resolved Raman spectroscopy (SRRS) of uniform, bent and long tapered MNWs grown on a Si (001) substrate. It is attributed to superposition of E2h phonon (wurtzite : WZ) and TO phonon (zinc blende : ZB) of InAs. Polarized and wavelength dependent SRRS establishes the presence of WZ and ZB phases in these MNWs. However, formation of WZ phase for larger diameter InAs MNWs is not commensurate with existing growth mapping studies, which needs to be understood further. Study of several of these MNWs suggest that the fraction of WZ to ZB in a MNW is decided not only by diameter, but also by local growth seeding/conditions leading to either tapered or uniform MNWs formation, although, external growth conditions are same. Variation of these frequencies that from bulk value are correlated to residual stress generated in ZB and WZ phases due to presence of WZ and ZB phases, respectively. Consistently, temperature dependent Raman data shows that there is a measurable contribution of stress to dw/dT, a positive for ZB and negative for WZ phonons, due to difference in their thermal expansions. Further, effective thermal expansion coefficient of WZ InAs in presence of ZB phase is calculated to vary in the range 10 - 19 x 10-6/K from base to tip of a MNW at 80 K, which is not possible to determine otherwise.

Correlation between size, bonding of Si and O at the surface/interface of Si nanocrystals and frequency of Si phonons in Si-SiO2 nanocomposites is established using Raman mapping and X-ray photoelectron spectroscopy. Corroboration of these results with absorption spectroscopy shows that lowering of direct transition of light electron conduction band to heavy hole valence band similar to 2.8 and 2.95 eV is a result of the interplay between effect of electron confinement (increases band gap) and oxidation bonding environment at the surface/interface (decreases band gap) of a nanocrystal in Si-SiO2 nanocomposites. These studies have generated deeper insight into the fundamental understanding of the nanocomposite, which can allow easy optical microscopy monitoring of Si-SiO2 based device fabrication. (C) 2017 Elsevier B.V. All rights reserved.

Effects of lattice and polar/nonpolar mismatch between the GaP layer and Ge(111) substrate are investigated by spatially resolved Raman spectroscopy. The red shifted transverse optical (TO) and longitudinaloptical (LO) phonons due to residual strain, along with asymmetry to TO phonon similar to 358 cm(-1) are observed in GaP/Ge(111). The peak intensity variation of mode similar to 358 cm(-1) with respect to TO phonon across the crystallographic morphed surface of GaP micro structures is associated with the topographical variations using atomic force microscopy mapping and Raman spectroscopy performed on both in plane and cross-sectional surface. Co-existence of GaP allotropes, i.e. wurtzite phase near heterojunction interface and dominant zinc-blende phase near surface is established using the spatially resolved polarized Raman spectroscopy from the cross sectional surface of heterostructures. This consistently explains effect of surface morphology on Raman spectroscopy from GaP(111). The study shows the way to identify crystalline phases in other advanced semiconductor heterostructures without any specific sample preparation. (C) 2017 Elsevier B.V. All rights reserved.

Raman spectroscopy/mapping is used to investigate the variation of Si phonon wavenumbers, i.e., lower wavenumber (LW ~ 495–510 cm−1) and higher wavenumber (HW ~ 515–519 cm−1) phonons, observed in Si–SiO2 multilayer nanocomposite (NCp) grown using pulsed laser deposition. Sensitivity of Raman spectroscopy as a local probe to surface/interface is effectively used to show that LW and HW phonons originate at surface (Si–SiO2 interface) and core of Si nanocrystals, respectively. The consistent picture of this understanding is developed using Raman spectroscopy monitored laser heating/annealing and cooling experiment at the site of the desired wavenumber, chosen with the help of Raman mapping. Raman spectra calculations for Si41 cluster with oxygen and hydrogen termination show strong mode at 512 cm−1 for oxygen terminated cluster corresponding to the vibration of surface Si atoms. This supports our attribution of LW phonons to be originating at the Si–SiO2 surface/interface. These results along with XPS show that nature of interface (oxygen bonding) in turn depends on the size of nanocrystals and LW phonons originate at the surface of smaller Si nanocrystals. The understanding developed can conclude the ongoing debate on large variation in Si phonon wavenumbers of Si–SiO2 NCps in the literature. Copyright © 2015 John Wiley & Sons, Ltd.

Raman and atomic force microscopy (AFM) mapping on the same selected area are used to get unique information about the morphology of Si nanocrystals (NCs) embedded in SiO2, which is difficult to obtain by any other conventional technique. The sensitivity of Raman spectroscopy to surface/interface and confinement effects in NCs is effectively used to correlate the Raman intensity profile in Raman mapping with the topography obtained from AFM to understand that Si NCs are clustered in i) smaller clusters (similar to 100 nm) organized closely in two dimensions (2D) and ii) big (similar to 2 mu m) three dimensional (3D) isolated clusters, although the growth is carried out to be multilayer (Si/SiO2). Raman mapping performed by varying the focal spot along the depth shows stacking of larger (>similar to 60 angstrom) to smaller sizes (<similar to 40 angstrom) Si NCs from bottom to top for some clusters. To understand the observed morphologies, further study of specially grown Si-SiO2 nanocomposites is performed, which suggest formation of smaller Si NCs at the top due to annealing at 800 degrees C in Si rich SiO2 and possible existence of thermal gradient in an insulating matrix of SiO2. Larger Si NCs are formed in the laser induced plume (plasma) itself. (C) 2016 Elsevier B.V. All rights reserved.

In the present work, important information on crystalline structure and band offset of InAs/Ge heterojunction epitaxially grown directly on Ge (111) substrate is presented. As revealed by high resolution x-ray diffraction (HRXRD) and Raman measurements the InAs nanostructures are highly crystalline and oriented with the substrate having predominantly zinc-blende (ZB) structure. The relaxation ratio of grown layer is of 95%. The lateral and vertical coherence lengths are estimated to be ∼65 nm and ∼14 nm, respectively, which closely match with atomic force microscopy results. As revealed from HRXRD experiments, two domains/sub-lattices of ZB InAs (111) structures having different stacking configurations were found to coexist. A band diagram is constructed for InO/InAs/Ge system. Valence band offset of 300 meV and conduction band offset of 20 meV for InAs/Ge have been determined. Hence, InAs/Ge (111) system can have potential application in low power devices.

Micro-Raman imaging along with other techniques are applied to study the morphology, structure and crystalline quality of various types of InAs nanowires (NWs). The NWs of low and high densities are formed using metal organic vapor phase epitaxy. Raman mapping is effectively used as a local probe to gain information about the structure and crystalline quality of low-density NWs where the conventional characterization techniques are not very useful. However, for high-density NWs, the image and crystalline quality obtained from the LO phonon strongly corroborate with scanning electron microscopy and x-ray diffraction (XRD) results, respectively. These low-density (10 4 cm − 2 ) and high-density (10 8 cm − 2 ) NWs are grown on Si(0 0 1) under various growth conditions such as catalyst-assisted and catalyst-free growth, growth on native oxide-covered and oxide-cleaned Si, grooved Si surfaces and also varying the V / III ratio and growth temperature. NWs (1 μ m long and 50–100 nm wide) with high density and tapered NWs (50–80 μ m long and 200–500 nm wide at the tip) with low density are formed under different growth conditions. The growth of hillock- and wire-like structures is observed under the same growth condition. Raman, XRD, scanning electron microscopy and atomic force microscopy analyses confirm that the hillocks are grown along the (cid:2) 0 0 1 (cid:3) direction, whereas the wires are grown along [1 1 0] directions in the plane of Si(0 0 1). Furthermore, the Raman analysis of these NWs confirms that the smaller NWs have much better crystalline quality (half-width of LO phonon frequency ∼ 6 cm − 1 ) compared to the larger NWs (half-width of LO phonon frequency ∼ 15 cm − 1 ) although both NWs are oriented with the Si(0 0 1) surface. (Some