My research

Topics of interest:

  • MBE epitaxy of III-Nitride for photonics and electronics
  • Structural and optical characterization of MBE grown crystals
  • Optical (LEDs and Lasers) and electronic (Diodes and FETs) device fabrication
  • Simulation and modeling for optical devices
  • Novel application of nano-devices in medicine and health

Specialization with MBE:

  • Operation and maintenance of 3 generations of Veeco MBEs: Gen-930, Gen-Xplor (high temperature and high growth rate system) and Gen-10 (a semi-automated industry grade system)
  • Epitaxial growth of thick InN, GaN and AlN films
  • Linearly graded AlxGa1-xN and InxGa1-xN for polarization doping
  • Use of various substrates: high-quality bulk GaN and AlN, Si, and SiC
  • Ultra-thin GaN/AlN quantum structures for deep UV photonics
  • Resonant tunnel diodes (RTDs) and ISB transitions
  • AlGaN/GaN and GaN quantum well n-FET and p-FETs

Collaborators in research:

Project: Deep-UV photonics

Sub-280 nm deep ultraviolet optical devices are demanding to be used for disinfection and purification. Unlike visible LEDs and Lasers, the current challenge for deep-UV counterparts is very low wall-plug efficiency due to a fundamental material challenge. We have proposed novel design schemes to improve all the factors that limit the wall-plug efficiency of such optical emitters and demonstrated that the efficiency can be improved significantly leading to the promising route for deep-UV commercial products. Our deep-UV related papers have been covered by more than 40 times in the media highlighting the impact.

Project: AlN based power electronics

AlGaN/GaN FETs have matured since its inception more than 25 years ago. Currently, the challenge for such AlGaN/GaN based design lies in the use for power electronics and is limited by breakdown voltage and power dissipation. The choice of GaN as the buffer layer limits the upper limit for power handling capacity because of the bandgap of 3.4 eV. By using the novel AlN/SiC platform based FETs can overcome the limits with GaN. Such GaN quantum-well FET has also the capability of realizing both n-and p-channel configurations to realize CMOS circuits. Our current research is focused on achieving higher mobility for both n- and p-channel devices.

Project: Ultra-thin quantum heterostructures

Sub-nm ultra-thin III-Nitride quantum heterostructures are attractive for visible and deep-UV photonic applications and tunneling based electronic devices. MBE is a great tool to realize such atomically thin quantum structures due to the virtue of ultra-high vacuum, source purity, and slow growth rates. We use delta-GaN layers in AlN matrix to realize sub-250 nm deep UV photonic devices. Such design benefits from higher internal quantum efficiency compared to conventional AlGaN based active regions due to better carrier overlap. Use of GaN also ensures better light extraction efficiency.

Project: III-Nitride p-FETs for CMOS

Realization of p-FETs are necessary to make CMOS-based devices. Though n-channel III-Nitride devices like AlGaN/GaN HEMTs have matured to be used in industrial applications, the p-channel FET counterpart is way inferior in terms of carrier transport properties. Our aim is to find the root cause of the limited transport for the III-Nitride p-FET devices and overcome them to come up with design that will help to make industry standard devices. At present we are investigating both InGaN/GaN and GaN/AlN quantum well based designs structures to achieve the high mobility p-FET structures.

Project: Resonant tunnel diodes with III-Nitrides

Resonant tunnel diodes have promising applications in terahertz communication. Though RTDs have matured for Arsenides, room temperature negative differential resistance could not be achieved consistently until recently and we are one of the frontliners in this effort. This was possible due to advancement in the epitaxy method and our in-depth knowledge about RTD physics. The current focus is to enhance the operating current and peak-to-valley ratio of NDR for the AlN/GaN RTD structures.

Project: Polarization induced doping

P-type hole injection for wide bandgap semiconductors like AlGaN is a challenge due to high p-dopant activation energies. Traditional thermally activated doping scheme is not very effective for this reason. Use of compositionally graded AlGaN heterostructures paves a way to solve the p-doping problem by the virtue of inherent field ionized polarization charges. The temperature independence of field ionized charges also enables cryogenic operation of photonic devices which is very useful for disinfection applications like food preservation.

Project: InN and InGaN for photovoltaics

Indium gallium nitride is very attractive material for applications including photovoltaics, electronics, and visible light emitters. The crystal growth of InGaN has been challenging due to lack of suitable native substrates. A conventional way of dealing with lattice mismatched generation of threading dislocation during epitaxy has been to find suitable buffer layers. Our research has been devoted to finding such novel buffer schemes that help to achieve an ultra-smooth epitaxial surface with good crystal quality.

Collaborative projects related to MBE epitaxy:

I have worked with collaborators on various other topics where my role was the MBE epitaxy of the crystal structures with desired properties. The polarization induced Zener tunnel diodes to be used as steep transistors were one of the challenging projects so far. By insertion of ultra-thin InGaN layers within a GaN p-n junction, the depletion width of the junction can be shrunk significantly with the help of built-in polarization charges. Such a design is being used to realize ultra-low power electronic switches and the current challenge is to overcome the crystal growth-related issues for lattice mismatch between InGaN and GaN. I have also collaborated with a group who investigated the optical modes in polar GaN films epitaxially grown on SiC. The aim was to excite the surface phonon polaritons in an array of sub-diffraction pucks. Such structures have application to support surface modes in mid- and far-infrared ranges.