Significant changes in the properties of the nanosized materials compared to their bulk form primarily derive from the confinement of electrons, phonons and increased surface-to-volume ratio. Raman spectroscopy being a local probe is extremely sensitive to the material composition, size, structure, stress, electron-phonon coupling, crystalline quality, local environment etc., and has become a go-to technique for may researchers across the world for obtaining new insights into various nanomaterials. Furthermore, the Raman spectroscopy in conjunction with scanning probe microscopy allows materials reserachers to obtain correlation between vibrational attributes and surface topology, thus opening new doors in the understanding of nanostructured materials.

The objective of Raman spectroscopy Lab at RRCAT is to use optical and scanning probe microscopies together to study optical, electronic, vibrational and electrical properties of micro/nanostructured materials and correlate them with their morphology for obtaining important information on them. The understanding developed will open pathways for the optimization of growth process and timely & non-destructive evaluation of technologically important materials for device fabrication.

Figure: Scanning probe microscopy (SPM) integrated Raman spectroscopy system

Recent Activities and achievements

(1) Angle-resolved polarized Raman spectroscopy to ascertain the nature/relative contribution of defects in GaP/Group-IV hetero-structures grown under different nucleation kinetics

High quality epitaxial structures of compound semiconductors on group-IV (Si, Ge) is the key for the development of high efficiency multijunction solar cells, wide range of detectors, lasers and spin-photonic devices. However, it is reported that growth of high quality III–V polar semiconductor on non-polar substrate, inevitably suffers from hetero-polar interface, lattice mismatch, diffusion along the interface, etc. These complexities give rise interfaceoriginated defect structures. Surface nucleation and adatom kinetics play a vital role in controlling the evolution of structural defects at polar/non-polar GaP/Si,Ge interface, and their propagation in the overgrowth layer. Therefore, the systematic investigation of GaP/Si(001) overgrowth layers grown under different nucleation kinetics e.g., T_n1 ~425ºC, T_n2 ~525ºC and T_n3 ~770ºC (under same flux ratio), is performed by azimuthal angledependent polarized Raman spectroscopy. Raman measurements infer that intermediate nucleation (~525ºC) results in high-quality epilayers having substantial suppression of defectexposed facets. However, the nucleation sites created at high-temperature (~770ºC) give rise to high-density of defect-mediated non-(001) facets in overgrown layer. The study demonstrates that the controlled adsorption and optimal mobility of gallium and phosphorous adatoms at intermediate nucleation temperature (~525ºC) results in superior-quality overgrowth layers. The proposed methodology serves as quick feedback to determine the optimized growth kinetics in the given temperature range and hence the efficient integration of III-V compound semiconductors on Si platform.

Fig. 1.1. Polar Raman dependence of longitudinal optical (LO) phonon of (a-b) bulk GaP(001) and (c-d) GaP/Si(001) epilayer grown under intermediate nucleation kinetics Tn2 ~525ºC in parallel (ei||es) and perpendicular (ei⊥ es) polarization configurations. Polar plots of GaP/Si(001) layer grown under intermediate nucleation closely mimic the azimuthal patterns of bulk GaP(001). In the given temperature range, the intermediate nucleation temperature give rise to best-quality overgrowth layer.

(2) Elucidating the interfacial nucleation of higher-index defect facets in technologically important GaP/Si(001) by azimuthal angle-resolved polarized Raman spectroscopy

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 interfacial defects is quintessential for efficient integration of III-V semiconductors on Si. Herein. the influence of substrate crystallographic orientation and therefore the surface energy on the structural properties of grown layer is explored by investigating the epitaxy of non-polar GaP on extensively used (001) oriented Si substrate orientation i.e., closely lattice-matched GaP/Si(001) heterostructures. Polar Raman measurements on GaP/Si(001) hetero-structures reveal that interfacial nucleation and subsequent coalescence of faceted epiltaxial micro/nano-structures give rise to higher-index defect-exposed {111} and {112} facets in the overgrowth layer. 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.

Fig. 2.1 (a) Typical schematic of Raman back-scattering configuration utilized for determining the azimuthal angle dependence of intensity of optical phonons scattered from a given crystal orientation. (e_i, k_i) and (e_s, k_s) are the incident and scattered light polarization and propagation wave vectors respectively. Observed polar dependence of LO phonon of GaP/Si(001) thick layer (TL) hetero-structure in parallel (e_i||e_s) polarization configuration arising from spatial position of (b) type ‘1′ (where peak intensity ratio R = I_TO/I_LO is less than unity in unpolarized spectra) and (c) type ‘2′ (where peak intensity ratio R = ITO/ILO is nearly one in unpolarized spectra). Calculated azimuthal angle dependence of LO phonon for scattering from (d) (001) and (e) (111) plane. Observed polar plots of forbidden TO phonon of thick layer hetero-structure in parallel configuration at position of (f) type ‘1′ and (g) type ‘2′ . Calculated TO phonon azimuthal dependence for scattering from (h) (111) facets and (i) combination of (001) and (111) facets. Observation of symmetry-forbidden phonon/s and their azimuthal-dependence evidences the formation defect-mediated {111} and {112} facets.

(3) Investigations on the origin of strain variation in the zinc-blende phase along the depth of GaP/Si(111) using spatially resolved polarized and wavelength dependent Raman spectroscopy

The surface and micro structure variations in the technologically important GaP/Si heterostructures are investigated by the spatially resolved polarized and wavelength dependent Raman spectroscopy. The hetero-structures probed from two different directions <1 1 1> and <1 1 0> reveal the presence of strain distributed zinc-blende (ZB) GaP phases along the depth 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. The study bestows with new opportunities for engineering of the GaP/Si based crystal phase quantum structures with atomically sharp interfaces. The findings illuminate that the Spatially resolved Raman spectroscopy can serve as a quick and impactful alternative to time consuming and cumbersome high-resolution transmission electron microscopy / in situ reflection anisotropy spectroscopy measurements for probing the surface and interface properties of such hetero-structures.

Fig. 3.1. (a) Optical image of cross-sectional surface of GaP/Si(111) thick layer (b) Raman spectra at seven spatial positions (separated by ~100nm) across the cross-sectional surface marked in (a), and (c) Deconvolution of contributions to Raman spectra at positions ‘1’, ‘4’ and ‘7’ using minimum number of Lorentzian and Gaussian lineshapes. E2H (wurtzite), SO and TO/LO (zinc-blende) phonons contributions are shown in orange, magenta and blue colors, respectively. Red colour is total fit to data (+). Evolution of Raman spectra from near-interface (position 1) to near surface (position 2) showcases the strain-gradient zinc-blende phases and wurtzite phase GaP.

(4) Predicting surface modification of InAs nanowires on laser irradiation using transient thermal simulation and time evolution of Raman spectra

For fine tuning the properties of semiconductor nanowires(NWs), modifications of the surface and dimensions of these nanowires using laser irradiation are being explored as one of the methods. In order to control the oxidation processes leading to surface modifications, one needs to obtain understanding of temperature rise at the surface of NW under laser irradiation. In this work, we first establish correlation between simulated temperatures at the surface of a 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 ~ 30-636 kW/cm2) over time duration of ~8 minutes, the simulated temperature is found to corroborate well with the corresponding oxidation processes e.g. weak (WP), intermediate (IP) and strong process (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 NW dimensions, laser power density and irradiation time, thus validating the importance of temperature simulation in predicting/ controlling the required surface modification for other III-V NWs useful in nanotechnology.

Fig. 4.1. (a) 3D temperature profile (staedy state) (b) Time evolution of simulated temperarture for InAs nanowire (NW: d~800 nm, L~36 μm) on Si substrate for laser power density (LPD)~300 kW/cm2, (c) Variation of simulated temperature at 100s as a function of LPDs and (d) Raman spectra with 488 nm at 100s for LPDs. Simulated temperature and oxidation processes for each spectra are noted.

Raman spectroscopy monitored laser irradiation experiment in corroboration with DFT/TDDFT calculations and X-ray photoelectron spectroscopy suggests: (i) Si phonons: 495 - 510 cm-1 originate from the smaller size (< ~ 4 nm) Si nanocrystals and it’s frequency is governed by dominant surface/interface effect. (ii) Si phonons: 515 - 519 cm-1 originate from larger size Si nanocrystals (> ~ 6 nm) and it’s frequency is governed by the confinement effect. (iii) Si phonons: 511 - 514 cm-1 originate from intermediate size Si nanocrystals (~ 4 to 6 nm) and it’s frequency is governed by simultaneous contribution of core and surface.

Fig. 1.1. (a) Optical (b) Raman and (c) AFM images of the type I and II Si nanocrystals as depicted in the pictures, (d) Raman spectra of type I and type II nanocrystals. 3D (e) Raman and (f) AFM images of the same type I and type II nanocrystals.

The sensitivity of Raman spectroscopy to surface/interface and confinement effects in nanocrystals is effectively used to correlate Raman mapping with AFM to understand that Si NCs are clustered in i) smaller clusters (~ 100 nm) organized closely in two dimensions and ii) big (~ 2 μm) three dimensional isolated clusters.

Fig. 2.1. Left side Image: (a) Raman map (b) AFM scan (c) Raman spectra and (d) PL spectra of CdS nanoscrystals embedded thin film of PVP at three spatial positions marked by ‘1’, ‘2’ and ‘3’. Right side image: (a) Raman map (b) 3D intensity profile (c) Raman spectra at three spatial positions of CdS nanoscrystal embedded in PVP sphere, (d) AFM map of the region containing CdS nanocrystals embedded in PVP spheres.

• 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 the interaction between nanocrystals and polymer leading to a co-operative growth mechanism.
  
  
• The stoke-antiStoke ratio, line shape, line width of phonons observed in Raman spectra gives information of growth mechanism, crystalline quality and confirms presence of CdS in PVP balls: imaged using CdS: LO phonon peak.

Related Publications

1. V. Gupta, A. A. Ingale and R. Aggarwal, Novel use of selectivity of Resonance Raman spectroscopy to study polytypism and mixed to pure phase conversion in individual InAs NWs on laser irradiation, Appl. Surf. Sci. 600, 154091 (2022)
   
   
2. R. Aggarwal, A. A. Ingale and V. K. Dixit, Elucidating the interfacial nucleation of higher-index defect facets in technologically important GaP/Si(001) by azimuthal angleresolved polarized Raman spectroscopy, Appl. Surf. Sci. 554, 149620 (2021)
   
   
3. R. Aggarwal, Alka A. Ingale and V.K. Dixit, Investigations on the origin of strain variation in the zinc-blende phase along the depth of GaP/Si(111) using spatially resolved polarized and wavelength dependent Raman spectroscopy, Appl. Surf. Sci. 514, 145933 (2020)
   
   
4. R. Aggarwal, Alka A. Ingale, V. K. Dixit and V. Sathe, Raman spectroscopy investigation of inter-diffusion in GaP/Ge(111) hetero-structures, Superlatt. and Microstruct. 125 190 (2019)
   
   
5. R. Aggarwal, P. Ram Sankar, A. Sahu, Alka A. Ingale, A. K. Sinha, C. Mukherjee, Template based room temperature growth of high density CdS nanowires from aqueous electrolyte using high frequency alternating current, J. of Mater. Sci.: Mater. Electron. 29, 427 (2018)
   
   
6. V. Gupta, A. A. Ingale, A. Bhattacharya, M. Gokhale, R. Aggarwal, S. Pal, Understanding the effect of nanowire orientation on time evolution of Raman spectra from laser irradiated InAs nanowire surface, Nanotechnology, 29, 425709 (2018)
   
   
7. R. Aggarwal, Alka A.Ingale, V. K. Dixit, Raman spectroscopy and atomic force microscopy study of interfacial polytypism in GaP/Ge(111) heterostructures, Applied Surface Science 427, 754-762 (2018)
   
   
8. Vandna K. Gupta, Alka A. Ingale, Vikas Jain, R. Aggarwal, S. Pal, Predicting surface modification of InAs nanowires on laser irradiation using transient thermal simulation and time evolution of Raman spectra, Journal of Alloys and Compounds 735,1331 (2018).
   
   
9. Ekta Rani, Alka Ingale, D. M. Phase, A. Chaturvedi, C. Mukherjee, M. P. Joshi and L. M. Kukreja, Band gap tuning in Si-SiO2 nanocomposite: Interplay of confinement effect and surface/ interface bonding, Applied Surface Science 425C, 1089 (2017).
   
   
10. Ekta Rani, Alka A Ingale and A K Sinha, Interaction between CdS nanocrystals and PVP leading to co-operative growth of CdS PVP nanocomposites: A Raman and AFM mapping study, Journal of Alloys and Compounds 729C, 597 (2017)
    
    
11. Vandna K. Gupta, Alka A. Ingale, Suparna Pal, R. Aggarwal and V. Sathe, Spatially resolved Raman spectroscopy study of uniform and tapered InAs micro-nano wires: Correlation of strain and polytypism, J. Raman spectroscopy 48, 855 (2017).
    
    
12. Ekta Rani, Alka A. Ingale, A. Chaturvedi, M. P. Joshi and L. M. Kukreja, Corroboration of Raman and AFM mapping to study Si nanocrystals embedded in SiO2, J. of Alloys and Compounds 672, 403 (2016).
    
    
13. Ekta Rani, Rahul Aggarwal, Alka A Ingale, K. Bapna, C. Mukherjee, M. K. Singh, P. Tiwari and A. K. Srivastava, Insight into one-step growth of nearly monodispersive CdS nanocrystals embedded in polyvinyl pyrrolidone spheres, J. of Materials Science 51, 1581 (2016).
    
    
14. Ekta Rani, Alka A. Ingale, A. Chaturvedi, C. Kamal, D. M. Phase, M. P. Joshi, A. Chakrabarti, A. Banerjee and L. M. Kukreja, Correlation of size and oxygen bonding at the interface of Si nanocrystal in Si-SiO2 nanocomposite: A Raman mapping study, J. of Raman Spectroscopy 47, 457 (2016)
    
    
15. Ekta Rani, Alka A. Ingale, A. Chaturvedi, M. P. Joshi and L. M. Kukreja, Resonance Raman mapping as a tool to monitor and manipulate Si nanocrystals in Si-SiO2 nanocomposite, Appl. Phys. Lett. 107, 163112 (2015).
    
    
16. Suparna Pal, R. Aggarwal, Vandna Kumari Gupta and Alka Ingale, Time evolution studies of laser induced chemical changes in InAs nanowire using Raman spectroscopy, Appl. Phys. Lett. 105, 012110 (2014).
    
    
17. R. Aggarwal, Alka A. Ingale, Suparna Pal, V.K. Dixit, T. K. Sharma and S. M. Oak, Intersubband plasmon-phonon coupling in GaAsP/AlGaAs near surface quantum well, Appl. Phys. Lett. 102, 181120 (2013).