Research

Epitaxial growth of thin film and nanostructures

Our group is involved in experimentally forming and understanding metal/semiconductor interfaces and semiconductor heterostructures. We are motivated by the need to modify Gp-III nitride materials that can be used to form high performance devices such as full-spectrum solar cells, high brightness light emitting diodes, high mobility transistors etc. We combine surface physics sub-monolayer coverage and Molecular Beam Epitaxy of thicker films, towards the need to form different thin films and structures of III-nitrides by forming both epitaxial 2D films and nanostructures. We use sophisticated Ultra High Vacuum (UHV) growth by Molecular Beam Epitaxy (MBE) and characterization tools such as, Reflection High Energy Electron Diffraction (RHEED), Ellipsometry, X-ray Photoelectron Spectroscopy (XPS), Low Energy Electron Diffraction (LEED), Photoluminescence (PL), High Resolution XRD, etc.

Our work at present is in five directions as follows:

 

1.      Lattice Matching Epitaxy:

We have developed a novel approach to form stable superstructural submonolayer phases by adsorption of metal adatom on Si(111) & Si(100) surfaces and use them as templates to grow high quality GaN and InN 2D films with low defect density and high structural and emission properties. The unit cell of the metal (Ga, In, Al) induced superstructures is chosen to integrally match with a-b plane unit cell of GaN and InN. Our results hold promise to tailor-make III-nitride structures on silicon, with tunable structural, electrical and optical properties. We also study the effect of surface modifications such as nitridation on the morphology and properties of thin films and nanostructures.

 

2.      Formation of GaN nanostructures: (Nano-walls, rods and wires):

Slowing diffusion kinetics of Ga adsorbates on sapphire and silicon surfaces of different orientations and surface modifications, we form high density of self order and non-catalytic spontaneous growth of nanostructures, in a narrow growth parameter window. The GaN adsorption is kinetically driven into a nano-wall network formation by the strain relaxation of nucleation at edge dislocations and eventually at screw dislocations to form nanorods  and  nanotubes. We show by Fast Fourier Transformation (FFT) that the nanorods are crystalline, c-oriented, wurtzite hexagonal facetted and also have a hexagonal supra-structural arrangement. Very interesting properties of these nanostructures such as enhanced photoemission, thermoelectric power, carrier concentration, etc. are measured.

 

3.      Band gap issue in InN:

We have grown InN both on Al2O3 and silicon under different growth conditions, including several surface modifications. We address the issue of Mei-resonance by In-inclusion, oxide and oxy-nitride formation, quantum size effect, etc discussed in literature to explain the band gap variation. We show that structural mis-orientation of wurtzite crystallite is responsible for the high band gap observed.  The non parabolic form of the Moss-Burstian Shift is evoked to explain the variation in the band gap results in the literature.

 

4.      GaN nanowall network properties:

The ordered GaN c-oriented GaN nanowalls network, formed under nitrogen rich conditions has length scale that depends on growth conditions. The high surface area, rigidity, temperature stability, carrier concentration, confinement effects, makes it a versatile and potential candidate for novel materials. We have observed a very strong photoluminescence enhanced due to coupling of band-edge-emission with the surface plasmon of the metal nanoparticles. We also see a novel effect of the frequency shift induced by Ag nanoparticle sizes and spatial distribution. This can be very important for the wavelength conversion and tuning applications. We are also explore the utility of transport thermoelectric power, field emission and water splitting properties of these interesting surfaces.

 

5.      Metal/Semiconductor Interface:

We also perform surface science experiments of sub-monolayer adsorption of Gp III metal on Silicon surfaces of various orientations and reconstructions. We use stable potential energy profiles of these surfaces to form low-dimensional nanostructures of the metal adsorbates. We have explored several aspects of the low index Si(100) and Si(111) surfaces and also on the high index Si(5 5 12) that has interesting 1-D faceted grooves enabling formation of single atom wide nanowire. We form 2D-superstructural phase diagram for these systems which indicate the pathways to be employed to attain desired structural and electronic properties. These submonolayer phases can also be template for growing epitaxial films and nanostructures. 

 

PhD Students

Varun Thakur

Arpan De

Sanjay Nayak

Abhijit Chatterjee

Rajendra Kumar

 

Shivaram Kubbakaddi

 

Int PhD Student


 

 


 

 

 

   Last modified date: 25-04-2017