Underlining physics and applications of ZnO thin films, hetrostructures and nanostructures:

• With the increasing demand in the field of optoelectronics, transparent electronics, piezoelectrics and spintronics; ZnO becomes a potential oxide counterpart for GaN and other III-V semiconductors. Naturally ZnO grows in a stable wurtzite-hexagonal crystal structure with a wide band gap of 3.35 eV and a room temperature exciton binding energy of 60 meV, which makes the material promising for the fabrication of UV LEDs and lasers. In spite of these intriguing properties, an assortment of unrevealed physics is to be explored in this material. Our central interests in this material are,
  • Growth of high quality and stable p-type ZnO thin films
  • Achieving room temperature ferromagnetism in transition metal ions (Co, Mn, Ni and Fe) doped
  • ZnO thin films and nanostructures
  • Fabrication of highly transparent and flexible ZnO thin films for transparent electronics
  • Growth of ZnO nanostructures and its functional properties
  • Synthesis of white light emitting ZnO based nanocomposites phosphor

Nano-ink based CZTS solar cells:

It has become important to harvest solar energy in order to satisfy our increasing energy demands. In our group, we work on CZTS thin film solar cells. In order to avoid the processing cost of vacuum based deposition methods, we have developed simple chemical methods to prepare and deposit CZTS and CdS on desired substrates. The following schematic explains the process involved in preparation of CZTS solar cell.

Molybdenum layer serves as metal back contact, which we deposit on soda lime glass by sputtering. CZTS acts as absorber layer in the solar cell. We have developed a method to prepare the CZTS nano-ink by hot injection technique. This CZTS nano-ink is deposited on Mo coated soda lime glass by simple drop casting or Doctor’s blade method. The CdS buffer layer is then deposited on it by chemical bath deposition (CBD) method. Transparent conducting oxide is deposited then by Sputtering or Pulsed Laser Deposition (PLD). To get the final device architecture, we deposit metal contacts by sputtering/thermal evaporation. We are optimizing all the layers in order to get efficient solar cell.

We have studied the defect study in the polycrystalline CZTS thin films by Raman spectroscopy. Change in Cu stoichiometry and variation in incident laser beam intensity both induce a disorder in the system giving rise to a disorder phonon mode at 331cm-1 corresponding to partially disordered Kesterite structure (PD-KS). We predict that correlating basic defect physics of material and its influence on phonon spectrum to transport properties may reveal the mechanism behind surge in efficiency under non-stoichiometric conditions. (Sunil Kumar Samji et.al, Appl. Phys. Lett. 104, 152106 (2014)).

Fig: Raman spectra of Cu₂-xZnSnS₄ thin films recorded at three different intensities of incident laser beam I₀, I₁ (10 I0) and I₂ (25 I0) corresponding to different Cu-content, (a) x = 0, (b) x = 0.3, and (c) x = 0.5.

Formation of ordered defect compounds and anomalous grain boundary physics are unique to Cu chalcogenides CuInX₂ (S/Se) and its alloys. We have focused on defect physics i.e., possibility of formation of ordered vacancy compounds as well as grain boundary (GB) physics in quaternary systems. X-ray photoelectron spectroscopy (XPS) studies were carried on Cu₂-xZn1.3SnS₄ (x= 0.0, 0.3, 0.5 and 0.7) thin films to determine the position of valence band edge and explore the formation of ordered vacancy compounds along with absorption studies. To understand GB physics, we have studied CZTS thin films with Conductive atomic force microscopy (C-AFM) (S.K. Samji et al., Scripta Materialia 117 (2016) 11–15).

Fig.4: (i). Difference spectra obtained by subtracting the intensity of valence band edge of Cu₂-xZn1.3SnS₄ (x=0.3, 0.5 and 0.7) from x=0.0 composition, (ii) absorption spectra of Cu₂-xZn1.3SnS4 (x = 0.0, 0.3, 0.5 and 0.7) recorded in the visible region.

We are also interested to study the electrical transport properties of the absorber material CZTS as it plays an important role in the solar cell performance. We are carrying out the temperature dependent resistivity studies with samples of different thickness to explain the mechanism of the electrical conductivity in the absorber material

Fig: ln(σ) vs inversion of temperature of CZTS thin films

Perovskite solar cells

The first efficient state Perovskite cells were reported by mid-2012 and then after extremely rapid progress is reported in the following years where the cell efficiency reached 16.2 % by the end of 2013. Again this value reached 17.9 % in 2014 and presently achieved a value more than 20%. These values showed that this technology is capable of producing solar energy comparable to the conventional solar cells and it can be mass produced with printing techniques from inexpensive and abundant perovskite materials. Solar cell applications feature mostly the organo-metal halide perovskite materials with stoichiometry AMX₃, where M is typically a divalent metal ion such as Pb₂+, Sn₂+, and Ge₂+ while X is a halide anion such as I-, Cl- and Br- and A is an organic cation. Some of the advantages of perovskite structure are ease of manufacturing, strong absorption of solar energy, lower carrier recombination, higher charge carrier mobility and higher carrier lifetime, which confirms the availability of charge carriers to be extracted as electric current. Some of the negative sides of the perovskite solar cells are its instability to the moisture and UV, usage of lead as a major constituent, which is having toxic issues during fabrication, usage, and disposal.

• Our group focus on replacing the standard hole transporting and the electron transporting layers with novel materials which can produce high-efficiency solar cells. Electrical and optical properties of each layer are also evaluated with simulation and corresponding experimental results to optimize each layer thickness and hence to achieve the best solar cells.

Diamond thin films on silicon substrates

Layered materials, most commonly Vander Waals materials with anisotropy in bonding were the common source of 2D materials. Graphene was the first and well-studied candidate in the 2D family, discovered in 2004. However, many other 2D materials, which came later-onand can be prepared in the laboratories by simple exfoliation and various other deposition techniques e.g. Hexagonal boron nitride (h-BN) and Transition Metal Dichalcogenides (TMDs), fall under this category. These materials drawn enormous attention in the material science community due to their supreme properties, which makes them a better and reliable candidate for various electronic, opto-electronic and photonic applications.

Wafer scale growth of 2D materials via Pulsed laser deposition. Especially large area growth of reduced graphene oxide (rGO), MoS₂ and WS₂ were enabled by PLD. Systematic tuning of their thicknesses and stoichiometry helps in achieving the opto electronic tunability of the grown thin films. Moreover, this Physical vapour deposition growth mechanism helps to attain highly uniform, good quality large area thin films with desired opto-electronic properties.

Graphene

Graphene, a wonder material known for its all intriguing properties posses very high potential towards thermal management applications. Due to its extremely high thermal conductivity, even higher than the best bulk heat conductor diamond, it is considered as a heat superconductor which can be readily used for heat management applications in electronic and optoelectronic devices. Hence understanding the heat flow mechanism in graphene as well as the role of material parameters like strain, defect density and rotation angle in bi-layer graphene posses immense importance. So basically we investigate the heat flow mechanism in 2D material systems and the role of different material parameters on the thermal properties.

Ferroelectric FET for Non-volatile Memory: MoS₂ Fe-FET devices with epitaxial PZT as back-gate exhibit low switching voltages ≤ 2 V which is much lower than the reported 2D-FET devices (8 - 20 V), high ON-OFF ratios ≥ 104 which is comparable to state of the art Fe-FET devices and reproducible hysteresis behaviour with good sustainability over 200 cycles switching operation. This study paves the way to the realization of low power and reliable non-volatile memory devices.

Figure: Local nanoscale ferroelectric hysteresis loops of (a) Polycrystalline PZT thin films and (b) Polycrystalline PZT- MoS₂ heterostructure grown on Pt coated Si substrate.

More information: Kolla, Lakshmi Ganapathi, Martando Rath, and MS Ramachandra Rao. "Polarization induced switching in PZT back gated multilayer MoS₂ FETs for low power non-volatile memory." Semiconductor Science and Technology (2018). https://doi.org/10.1088/1361-6641/aaf9e6.

Cavity-less NIR LASER for biological applications: MoS₂ /Au Nanoparticles/ZnOheterostructures were designed to obtain room temperature near infrared region (NIR) random lasing (800-950 nm), with a threshold of nearly 500 μW. This work provides new dimensions towards realization of low power on-chip NIR random lasers made up of bio-compatible materials. The present work opens up new dimensions towards realization of low power on-chip NIR random lasers made up of biocompatible materials.

Figure: Schematic of MoS₂/Au-NPs/ZnOheterostructure (left) and the Excitation powerdependent NIR emission of from these structures (right). The spectra are plotted in waterfall mode for clarity

More information: Tejendra Dixit, Ankit Arora, Ananth Krishnan, K. Lakshmi Ganapathi, Pramoda K. Nayak, and MS Ramachandra Rao. "Near Infrared Random Lasing in Multilayer MoS₂." ACS Omega 3, no. 10 (2018): 14097-14102. https://doi.org/10.1021/acsomega.8b01287

Piezoelectrics and Multiferroics

Piezoelectricity was discovered by Nobel Laureates, Pierre and Jacques Curie in 1880 while studying the effect of pressure on the generation of electrical charges by some natural crystals such as quartz, tourmaline, Rochelle salt etc. Piezoelectric effect states to development of electrical charges on application of a mechanical stress (direct effect) and vice-versa (indirect effect). After the discovery of piezoelectricity, it procured nearly 3-4 decades to understand the use of these materials for practical applications. During World War I, quartz and Rochelle salt based natural piezoelectric crystals were used for sonar application, but the hygroscopic nature of these crystals was the main drawback for their use. Barium titanate (BaTiO3) developed during World War II was considered as a major breakthrough in the history of piezoelectric material. Soon the number of known ferroelectric materials began to increase rapidly. New representatives of perovskite crystals were discovered: such as potassium niobate (KNbO₃) in 1949, lead titanate (PbTiO3) in 1950, and antiferroelectric lead zirconate (PbZrO₃) in 1951. The most promising piezoelectric material PbZr_0.52Ti_0.48O3 (PZT) was discovered during 1952 with high piezo properties compared to barium titanate. Among all piezoelectric oxide materials PZT has been subject to a great deal of interest for a wide range of applications such as piezoelectric transducers, microelectromechanical-systems (MEMS) and high-density nonvolatile random access memory (NVRAM) etc. Although piezoelectricity and ferroelectricity had been recognized decades ago, the renewed interest stems from modern deposition techniques that can produce high quality materials and attractive proposed device concepts. Piezoelectric thin films have been integrated into microelectromechanical system (MEMS) devices such as micro-scale accelerometers, displacement transducers, and actuators. Moreover, pyroelectric properties of thin films are used for high sensitivity IR detectors. Progress has also occurred in the application of high dielectric constant thin films and electro-optic thin films in the area of high frequency devices and optical switches, respectively, for integrated optical systems. Pulsed laser deposition (PLD) is one of the best and versatile technique among physical vapor techniques to grow high quality thin films of multi-component oxide systems (e.g. YBa₂Cu_3O7 (YBCO), La₁-xCaxMnO₃, Pb(Zr,Ti)O₃ etc). By optimizing laser raster scanning conditions and controlling PbO evaporation, we have grown high quality large area (2x2 inch) PZT thin films using custom-built off axis PLD technique1. The achievement of high quality large area PZT thin films with uniform polarization might be used for under-water SONAR devices and power harvesting. In another study we have grown the PZT thin film on hot filament chemical vapor deposition (HFCVD) grown conducting boron doped monocrystalline diamond (B-NCD) film2. The narrow degradation of remnant polarization (10%) even after 1010 switching cycles of the sample demonstrates the excellent ferroelectric performance of the heterostructures. We also demonstrated polarization induced switching in PZT back gated multilayer MoS₂ ferroelectric field effect transistors (Fe-FETs) for low power non-volatile memory devices3. Due to the environmental issues taken against Pb-based ferroelectrics, our group started work on Pb-free ferroelectrics also. Possessing relatively high Curie temperature, piezoelectric and electromechanical properties, K1-xNaxNbO₃ (KNN) based materials have ensured to be the most promising substitutes for Pb-based compositions for sensor and actuator applications. Our group is also working on multiferroic heterostructures thin films. Multiferroic materials with coexisting ferroelectricity and magnetism have enjoyed flurry of studies in recent years due to fundamental scientific interest and significant technological promises for their potential applications in future generation microelectronic devices. Because of the weak magneto-electric coupling in single-phase multiferroic materials, alternative composite structures, such as multiferroic thin film heterostructures composed of separate ferromagnetic and ferroelectric layers, have been engineered to achieve enhanced coupling between magnetic and electric properties for device applications.

References:
• 1. Martando Rath, E. Varadarajan, E Natarajan and M. S. Ramachandra Rao, Ceram. Int., 44, 8749–8755(2018).
• 2. Martando Rath, Dinesh Kumar and M. S. Ramachandra Rao, Appl. Phys. Lett., 113, 066829 (2018 )
• 3. Lakshmi Ganapathi Kolla, Martando Rath and M S Ramachandra Rao, Semicond. Sci. Technol., DOI: https://doi.org/10.1088/1361-6641/aaf9e6

Superconducting Diamond