XXII International Workshop on Physics of Semiconductor Devices 2023 @ IIT Madras

XXII International Workshop on Physics of Semiconductor Devices

13 - 17 December, 2023

organised by

Indian Institute of Technology Madras
@ Research Park, IIT Madras

in association with

Society for Semiconductor Devices (SSD)
Semiconductor Society (India)

Important News

Log in to the IWPSD 2023 website and download your Hostel Accommodation / Registration receipt and Participant Certificate

Title : Machine Learning Defect Properties of Semiconductors
Abstract:
Defects and impurities in semiconductors heavily influence their performance in optoelectronic applications. Quick predictions of defect properties are desired in technologically important semiconductors, but complicated by difficulties in assigning measured levels to specific defects and by the expense of large-supercell first principles computations that involve charge corrections and advanced functionals [1]. We address this issue by combining high-throughput density functional theory (HT-DFT) with machine learning (ML) to develop predictive models for defect formation energies (DFE) and charge transition levels (CTL) of native defects and functional impurities in Group IV, III-V, and II-VI zinc blende (ZB) semiconductors. Using an innovative approach of sampling dozens of metastable polymorphs each from defect configurations in thousands of distinct DFT computations, we generate one of the largest known computational defect datasets, containing many types of vacancies, self-interstitials, anti-site substitutions, impurity interstitials and substitutions, and defect complexes [2,3].
Two distinct types of ML methods are applied: (a) random forest, Gaussian process, and neural network regression models based on manual descriptors encoding the defect atom�s elemental properties, coordination environment, and �unit cell� defect data [2,3], and (b) crystal Graph-based Neural Networks (GNNs) trained using entire defective structures as input [4], specifically using three established GNN techniques, namely Crystal Graph Convolutional Neural Network (CGCNN) [5], Materials Graph Network (MEGNET) [6], and Atomistic Line Graph Neural Network (ALIGNN) [7]. Root-mean square errors (RMSE) in predicting DFE are as high as 1 eV with the former, while ALIGNN yields errors of ~ 0.3 eV or less which represents a prediction accuracy of 98% given the range of values within the dataset, improving significantly on the state-of-the-art.
While the first set of models yield only optimized energies based on a smaller dataset of ~ 1500 points, the GNN models are trained on > 15,000 data points and can be applied to predict accurate unoptimized, partially optimized, or fully optimized DFE values corresponding to any defective structure. The best ML-based DFE predictions for defects in multiple charge states are used to predict relevant CTLs with good accuracy compared to DFT and better accuracy compared to ML models trained directly on the CTL data. Ultimately, the best models are applied to perform screening across hundreds of thousands of hypothetical single defects/dopants and defect complexes to find stable defects which may or may not create energy levels within the band gap and affect the semiconductor�s performance in optoelectronic devices. We also demonstrate that GNN models can be used as an effective surrogate for DFT computations to obtain low energy defective structures for any semiconductor-defect combination, which is very promising for screening over large chemical spaces without the need for expensive DFT.
REFERENCES
[1] Freysoldt, C. et al. First-principles calculations for point defects in solids. Rev Mod Phys 86, 253�305 (2014).
[2] Mannodi-Kanakkithodi, A. et al. Machine-learned impurity level prediction for semiconductors: the example of Cd-based chalcogenides. NPJ Comput Mater 6, 39 (2020).
[3] Mannodi-Kanakkithodi, A. et al. Universal machine learning framework for defect predictions in zinc blende semiconductors. Patterns 3, 100450 (2022).
[4] Rahman, H., Gollapalli, P., Manganaris, P. & Mannodi-Kanakkithodi, A. Accelerating Defect Predictions using Graph-based Neural Networks. In Prep. (2023.
[5] Xie, T. & Grossman, J. C. Crystal Graph Convolutional Neural Networks for an Accurate and Interpretable Prediction of Material Properties. Phys Rev Lett 120, 145301 (2018).
[6] Chen, C., Ye, W., Zuo, Y., Zheng, C. & Ong, S. P. Graph Networks as a Universal Machine Learning Framework for Molecules and Crystals. Chemistry of Materials 31, 3564�3572 (2019).
[7] Choudhary, K. & DeCost, B. Atomistic Line Graph Neural Network for improved materials property predictions. NPJ Comput Mater 7, 185 (2021).

Title : Modeling the mobility in ultrathin N-polar GaN and InGaN-GaN channel heterostructures
Abstract:
Gallium Nitride (GaN) metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) find extensive use in high frequency, high power applications. Most commercially available GaN devices are fabricated in the Ga-polar orientation. Recent research has shown the benefits of the N-polar orientation -- reduced contact resistance, improved scalability of devices, enhanced trans-conductance and larger cut-off frequency. These advances arise mainly from the reversal in the direction of polarization charge with respect to the Ga-polar orientation.
Understanding the mechanisms that limit the mobility of carriers in GaN HEMTs is key to improving their performance. Many authors have modeled different aspects of the electron mobility in Ga-polar structures. The relative impacts of physical processes influencing electron mobility in Ga-polar devices are different from those in N-polar devices. For example, while an increase in carrier density causes a decrease in mobility for Ga-polar devices, it causes an increase in mobility for N-polar structures. However, there are fewer reports on modeling the mobility in N-polar HEMT structures.
This work provides an analytical model for mobility in N-polar GaN heterostructures. Unlike previous reports which either use a variational form or numerically obtained wavefunctions, we describe the wave function analytically in the channel and spacer layers as linear combinations of Airy functions. The solution of a determinant equation, which enforces continuity conditions on the wavefunctions, yields very good solutions for the wavefunctions and an initial estimate of the subband energies. The final charge density, surface potential, and band diagram are obtained by applying first-order perturbation theory to incorporate the effect of charge density on the shape of the confining potential. This analytically obtained wavefunction is the used to estimate the scattering rates due to various processes -- acoustic/optical phonon, dipole, interface roughness, charged interface state, alloy and alloy clustering, surface state dipole scattering
The results obtained match well with experimentally measured data for GaN and InGaN-GaN composite channel devices having different channel thicknesses.

Title : Nanoscale device modeling: From atom to circuits
Abstract:
Quantum transport, which stems from an esoteric quantum-physics theory, has evolved into a very advanced device modeling platform and has also morphed itself into atomistic TCAD frameworks [1,2]. Our group has, over the past decade, developed a composite quantum transport modeling framework spanning several applications including advanced logic and memory devices, emerging devices, optoelectronics and quantum technologies. Our efforts have included proposals on advanced magnetoresistive memory devices [3] and novel transistors that employ spin-valley physics in the 2D-material platform [4]. In this presentation, while conveying the salient points of our developments, we wish to share a futuristic outlook with the ultimate ambition of multi-scaling from the atom to the circuit.
Specifically, we will cover the state-of-the-art atomistic modeling of magnetic random-access memory cells for both spin-transfer torque (STT) and spin-orbit torque (SOT) random access memories (STT-RAMs and SOT-RAMs) [4,5]. We then demonstrate our latest advancements in terms of coupling the atomistic device modeling platform with HSPICE simulations [5,6] to realize energy efficient neurmomorphic circuit functionalities.
We finally close the loop by extending our platform to the field of quantum technologies with our latest developments on machine learning based simulation platforms for qubits in the bilayer graphene platform [7].
[1] G. Klimeck and T. Boykin, Springer Handbook of Semiconductor Devices, 1601-1640, (2022).
[2] K. Camsari, S. Chowdhury and S. Datta, Springer Handbook of Semiconductor Devices, 1583-1599, (2022).
[3] A. Sharma, A. Tulapurkar and B. Muralidharan, J. Appl. Phys., 129. 233901, (2021).
[4] S. Banerjee, K. Jana, A. Basak, M. S. Fuhrer, D. Culcer and B. Muralidharan, Phys. Rev. Applied, 18, 054088, (2022).
[5] V. Vadde, B. Muralidharan and A. Sharma, IEEE Trans. Elec. Dev., 70, 7, 3943, (2023).
[6] V. Vadde, B. Muralidharan and A. Sharma, J. Phys. D : Appl. Phys., 56, 415001, (2023).
[7] A. Mukherjee and B. Muralidharan, 2D Materials, 10, 035006, (2023).

Title : Innovative MEMS Sensors for Biological Applications Researched and Developed at The City College of New York (CCNY)
Abstract:
In this talk two devices fabricated with microelectromechanical systems (MEMS) technology and their applications will be presented: (1) MEMS Biosensor with Integrated Impedance Spectroscopy and Gravimetric Measurements for Water Toxicity Testing This presentation describes the design, fabrication and characterization of a multiparametric biosensor based on live mammalian cells. This biosensor combines two biosensing techniques; resonant frequency measurements and electric cell-substrate impedance sensing (ECIS) on a single chip. The biosensor can simultaneously perform in real-time two different types of electric measurements on the same mammalian cell monolayer: (1) monitoring the resonant frequency values of a quartz crystal microbalance (QCM) resonator that will give information about the progression of cells adhesion and cell viscoelasticity and, (2) recording the impedance spectra of the cells, that will report on cell adhesion progress, shape, growth, motility and viability. The sensor is based on the innovative placement of the working microelectrode for ECIS technique as the upper electrode of a QCM resonator. Bovine aortic endothelial cells (BAECs) with different densities were used as sensitive cells. This lab on chip was demonstrated to indicate low concentration of toxicants. The responses of BAECs to toxic samples occurred 5 to 20 minutes after the cell contamination, depending on the type of chemicals and concentrations. A highly linear correlation between signal shifts and chemical concentrations was demonstrated for each toxicant. (2) Wearable and Stretchable Piezoelectric Powergenerator for Skin Applications The presentation describes a stretchable piezoelectric power generator that is intended to harvest energy. The powergenerator was fabricated from zinc oxide (ZnO) piezoelectric thin film embedded in polymer materials. The microfabricated powergenerator will be attached on the skin and stretched by the natural movements of arms, legs or neck. We expect that energy harvested by this device will be able to power wearable skin sensors. In the presentation this device fabrication will be described, and the testing method will be also discussed along with the output power from this device. The device could be also used as smart skin sensitive to touching or a pressure sensor.

Title : Advanced measurement techniques for GaN-based HEMTs: Trap investigation and innovative linearity characterization
Abstract:
The device modeling of a transistor for the RF and Microwave circuits design is a challenging task. The developed model must be capable of predicting the non-linear characteristics of the active transistor, which has a significant role in designing low-noise amplifiers, power amplifiers, switches and oscillators. My talk will be divided into two parts: The first part will be devoted to trapping investigation in GaN-based HEMT, and the second section will focus on large signal characterization.
The first part describes different approaches to characterize the charge trapping effects in GaN-based high electron mobility transistors (HEMTs). The experimental techniques such as dedicated pulsed I-V measurements, Low-Frequency output-admittance (Y22) measurement, Low-frequency noise (LFN) measurement, and Drain Current Transient (DCT) spectroscopy have been used for characterizing the traps in GaN-based HEMTs. The Y22, LFN, and DCT measurements reveal the presence of an electron trap located at (0.45 � 0.5) eV below the conduction band (CB) in the AlGaN/GaN HEMT device structure with iron (Fe)-doped buffer, while the frequency dispersion behavior of the Y21 parameters allows to extract both surface and buffer traps, located at EC- 0.2 eV and EC - 0.45 eV. The Y-parameter simulations are performed in Sentaurus TCAD in order to identify the spatial location of the traps. Moreover, electrically active traps in AlN/GaN HEMTs with C-doped are identified by DCT spectroscopy. In AlN/GaN HEMT with C-doped buffer, a trap signature at EC � 0.33 eV is observed with a lower capture cross-section value of 2�10-20 cm2. Thus, a slower trapping signature is observed in the C-doped AlN/GaN HEMTs, compared to the Fe-related trap in the GaN-based HEMTs.
The second part of the talk reports on the impact of trapping effects on large signal performances and on an innovative linearity measurement technique based on an Unequally Spaced Multi-Tone (USMT) Load-Pull Characterization Technique. The USMT test signal is a multi-tone composed of N pilot tones, arranged in a specific way, in order to ensure no overlapping of frequencies between the pilot tones and the intermodulation products generated by the Device Under Test (DUT) non-linearities. This signal has demonstrated interesting statistical properties to mimic OFDM and complex digital modulation. Our USMT-Load Pull (USMT-LP) set-up is con?gured to make measurements with USMT signals up to eight frequencies and allows measuring simultaneously the output power, gain, Power Added Ef?ciency (PAE), Carrier to Intermodulation (C/I) ratio or Noise Power Ratio (NPR) in order to derive the Error Vector Magnitude (EVM) induced by the device. To validate our proposed approach on-wafer measurements are performed on AlGaN/GaN (HEMT) devices.

Title : Device modeling of all-perovskite tandem solar cells and liquid metal batteries
Abstract:
The talk will have two parts: first focusing on modeling tandem solar cells and the insights obtained from the model. Then the comprehensive model for liquid metal batteries (LMBs) will be discussed, including the effects of heat transfer, mass convection, and electro-vortex flow. All-perovskite tandem solar cells pave the way for increasing the efficiency of solar cells beyond the Shockley-Queisser limit imposed on single-junction perovskite cells. While all-perovskite tandem solar cells with high power conversion efficiency (PCE) of 24.8% have been fabricated, the understanding of the underlying device physics during their operation remains unclear. We developed an optoelectronic model for monolithic all-perovskite tandem solar cell that coupled transfer matrix model for optical effects, and drift-diffusion model for electrical effects while accurately describing the photophysical and optoelectronic processes occurring in the all- perovskite tandem cell. The results of this model have been verified with results from a recent published experimental work on all-perovskite tandem cell with 24% PCE and show good agreement. Even though this model has been developed for an all-perovskite tandem solar cell, the principle guiding the model is general and can be applied to any type of tandem photovoltaic technology.
Liquid metal battery (LMB) performance, especially its discharge voltage, depends on the concentration profile of cathode which in turn is influenced by fluid flow. We examined three significant phenomena that impact fluid behavior: solutal buoyancy, internally heated convection, and electro-vortex flow (EVF). Though several attempts have been made to model these processes individually or in a combination of two, a model that simulates all three aspects for resultant fluid flow has not been reported. Therefore, this study developed a comprehensive multiphysics model that is validated with experimental results for a Li||Bi cell operating at 450 ? by coupling all of the aforementioned phenomena. This all-inclusive model leveraging solutal, thermal, and EVF effects not only offers insights into the fluid motion, concentration gradient, and voltage profile for a lab-scale LMB but can accurately simulate the conditions prevalent in commercial large LMBs, thereby catalysing the path to commercialization of LMBs.

Title : DEFECT DYNAMICS IN GROWING CZOCHRALSKI SILICON CRYSTALS
Abstract:
A vast majority of modern microelectronic devices are built on the monocrystalline silicon substrates produced from the crystals grown by the Czochralski (CZ) and the Float- Zone (FZ) process. Silicon crystals inherently contain various crystallographic imperfections known as microdefects that often affect the yield and the performance of many devices. Hence, the quantitative understanding and the control of the microdefect formation and the microdefect distributions in silicon crystals play a central role in determining the quality of silicon substrates. The microdefects are primarily the aggregates of the intrinsic point defects of silicon, vacancies and self-interstitials, and of oxygen (silicon dioxide). The distribution of microdefects in a CZ crystal is determined by the complex dynamics influenced by various reactions involving the intrinsic point defects and oxygen, and their transport. The distribution of these microdefects can also be strongly influenced and controlled by the addition of impurities such as nitrogen to the crystal. Significant developments in the field of the defect dynamics in growing CZ and FZ crystals are reviewed. The breakthrough discovery of the initial point defect incorporation in the vicinity of the melt/crystal interface made in the early 1980s allowed a simplified quantification of the CZ and the FZ defect dynamics. A deeper insight into the formation and the growth of microdefects was provided in the last decade by various treatments of the aggregation of oxygen and the intrinsic point defects of silicon. In particular, a rigorous quantification of the aggregation of the intrinsic point defects using the classical nucleation theory, a recently developed lumped model that captures the microdefect distribution by representing the actual population of microdefects by an equivalent population of identical microdefects, and another rigorous treatment involving the Fokker-Planck equations are discussed in detail.
The industrially significant dynamics of growing CZ crystals free of large microdefects is also reviewed. Under the conditions of large microdefect free growth, a moderate vacancy supersaturation develops in the vicinity of the lateral surface of a growing crystal, leading to the formation of oxygen clusters and small voids, at lower temperatures. The vacancy incorporation near the lateral surface of a crystal, or the lateral incorporation of vacancies, is driven by the interplay among the Frenkel reaction, the diffusion of the intrinsic point defects, and their convection.
A review of CZ defect dynamics with a particular focus on the growth of large microdefect-free crystals is presented and discussed.

Title : Epitaxial growth of GaN based HEMT heterostructures for high power and high frequency applications
Abstract:
GaN based high electron mobility transistor (HEMT) heterostructures have been demonstrated for high power and high frequency applications. However, the device performances of conventional HEMT heterostructures based on GaN are limited. To enhance device performances (power and frequency), GaN based novel HEMT heterostructures, such as AlN/GaN/AlN HEMT, N-polar HEMT, and multichannel GaN HEMT have been investigated. In order to realize high frequency applications of GaN HEMT devices, the most effective way is to reduce the gate length. However, conventional AlGaN/GaN HEMT heterostructures lead to short channel effects (SCE), which degrade device performance. In order to suppress SCE, it is necessary to reduce the barrier thickness for a higher aspect ratio. The binary alloy, AlN has a higher spontaneous polarization than AlGaN, therefore, the thin barrier AlN/GaN HEMT possesses a higher 2DEG concentration and output current density, especially suitable for high-frequency applications. In addition, AlN/GaN epistructure with AlN buffer as a back-barrier offer the highest thermal conductivity and bandgap (subsequently strongest carrier confinement) among the III-Nitrides, thereby providing better heat dissipation and improved breakdown voltage along with reduced leakage. Together, an AlN/GaN/AlN HEMT heterostructure is promising for developing high frequency operational GaN HEMTs. In addition to ultra-scaling, reduction of contact and access resistances is of pivotal importance in achieving high-frequency operation of GaN HEMTs. N-polar GaN HEMT heterostructures offer unique advantages in this regard. The 2DEG in an N-polar HEMT is induced on top of the barrier, which is opposite to the case of Ga-polar GaN-based HEMTs. This addresses the problem of scaling and improves electron confinement due to the back barrier. The GaN channel on top also reduces contact resistance along with lower access resistance due to the possibility of ultra-scaling. Thus, allowing N-polar HEMT to be operatable at higher frequencies and achieve higher power operations. Reducing contact and access resistances can also be achieved through increasing 2DEG concentration in GaN based HEMT heterostructures with multichannel based HEMT heterostructure design. Moreover, in this design, one can use well studied and reliable AlGaN/GaN technology to achieve higher 2DEG. AlGaN/GaN based multichannel HEMT is one of the approaches that has the potential to achieve higher power operations compared to single channel conventional AlGaN/GaN HEMT. Parallel conducting multi two-dimensional electron gas (2DEG) channels in AlGaN/GaN MC-HEMTs reduces the net effective sheet resistance, leading to lower on-resistance and improved power performance, while maintaining the higher breakdown voltages and other important characteristics of AlGaN/GaN HEMTs. Epitaxial growth and characterizations of GaN-based novel HEMT heterostructures such as AlN/GaN/AlN, N-polar, and multichannel AlGaN/GaN HEMTs will be presented.

Title : Development of Silicon carbide bulk Single crystal growth and wafer fabrication technology
Abstract:
Owing to the superior properties of the wide band gap, high carrier mobility, high thermal conductivity and high stability, 4H silicon carbide (4H-SiC) holds great promise for applications in electric vehicles, 5G communications and new energy systems. Also, Silicon carbide has become front runner for a new generation of high performance, high efficiency RF and power devices. Dramatic growth for SiC based devices and systems are being witnessed by increasing market demand. The success story behind this revolution is availability of improved materials quality after several years of technology and process development efforts.
The 4H-SiC single crystals can be grown by top seeded solution growth (TSSG), high temperature chemical vapor deposition (HTCVD), and physical vapor transport (PVT) approaches, among which the well-developed PVT technology prevails over other growth technologies because of its high maturity, sensitive temperature tunability, and low cost of the solid raw materials. The most used method of growing SiC bulk single crystal is Physical Vapor Transport (PVT) technique usually performed at temperature > 2000oC; this process involves many physical phenomena such as electromagnetic induction, mass and heat transfer, chemical reactions and so on.
Simulation of PVT growth process is a useful tool for optimizing the thermal design and improving the crystal growth process. Growth process is simulated for PVT reactor using Virtual Reactor (VR) software.
Present study focuses on the optimization of process parameters such as temperature, coil position, pressure, source to seed distance and initial SiC source powder grain size. The 4�diameter SiC single crystal were grown using optimized process parameters. Experimental observations such as boule shape, thickness, graphitization degree, porosity, mass flow in the source powder confirms the predictions of numerical simulation study performed by VR software.
As SiC being third hardest materials, wafer fabrication and polishing poses great difficulties compared to traditional semiconductor materials. Processed wafers were characterized by Optical/DIC microscopy, HRXRD and Raman spectroscopy for their structural properties. The analysis confirmed the formation of single crystal with 4H polytype and FWHM value showed good crystalline quality. Types of defects such as micropipes and dislocations were studied by KOH etching. Electrical resistivity of the obtained wafers showed the resistivity > 106ohm-cm confirming semi-insulating nature.
Challenges involved in SiC single crystal growth and wafer fabrication process shall be presented in detail.

Title : Is sub 60 mV/dec subthreshold swing possible with Schottky barrier FETs?
Abstract:
Modeling and simulations have played and continue to play a vital role in the progress of the semiconductor industry as they enable quick exploration of the expansive design space of microelectronic devices. It provides crucial guidelines to the experimentalists and foundries for further refinements. Today, when the industry is trying to continue scaling and reduce the power dissipation by scaling the power supply, it has become increasingly important to thoroughly understand the underlying operating principles of microelectronic devices. Getting a sub-60 mV/dec subthreshold swing (SS) is perhaps the only way to achieve the goals.
Research groups have proposed numerous innovative devices like tunnel FETs, superlattice FETs, Schottky barrier FETs, impact ionization FETs, and ferroelectric FETs for reducing the SS below 60 mV/dec. Among these potential contenders, Schottky barrier FETs (SBFETs) have remained controversial in their ability to provide sub-60 mV/dec SS as different experimental groups have reported varying results [1-3]. Theoretically, a few research groups have shown it is impossible to get sub-60 mV/dec swing with the Schottky barrier FETs [4,5]. In this talk, we will show comprehensively, using simulations, that the electrostatics of the device play a vital role in achieving sub-60 mV/dec SS in Schottky barrier FETs. The sub-60 mV/dec SS in the Schottky barrier FETs results from a change in the barrier (source contact-channel) shape from a rectangular-like to a triangular-like barrier. Our results elucidate that if the transition in the band edge, from rectangular to triangular, is not captured, then SS will be larger than 60 mV/dev as predicted by the theoretical works. The SBFETs provide a unique possibility to transition from tunneling current (in the off-state) to thermionic current (in the on-state), thus delivering the best of both worlds.
References:
1. S. Bhattacharjee, et. al., 'A sub-thermionic MoS2 FET with tunable transport,' in Appl. Phys. Lett.; vol: 111, no. 11, pp. 163501, 2017. https://doi.org/10.1063/1.4996953
2. Q. Li, et. al. 'The large-scale integration of high-performance silicon nanowire field effect transistors,' in Nanotechnology, vol. 20, no. 41, pp. 415202, 2009.
3. J. Knoch, et. al.; ``Tunneling phenomena in carbon nanotube field-effect transistor,'' in Phys. Stat. Sol. (a), vol. 205, pp. 679-694, 2008. https://doi.org/10.1002/pssa.200723528
4. W. G. Vandenberghe, et. al.; 'Figure of merit for and identification of sub-60?mV/decade devices' in Appl. Phys. Lett.; vol. 102, no. 1 pp. 013510, 2013. https://doi.org/10.1063/1.4773521
5. P. M. Solomon, 'Inability of Single Carrier Tunneling Barriers to Give Subthermal Subthreshold Swings in MOSFETs,' in IEEE Electron Device Letters, vol. 31, no. 6, pp. 618-620, June 2010, doi: 10.1109/LED.2010.2046713.

Title : 2D Lateral Heterostructures for Optoelectronic Devices
Abstract:
There are enormous possibilities in combining diverse 2D materials for the unique design of ultra-smart and flexible optoelectronic devices, including transistors, and quantum emitters.1 Considerable efforts have been devoted to the van der Waals hetero-integration of different 2D layered transition metal dichalcogenides (TMDs) to form vertical superlattices by transferring their exfoliated or as-grown flakes. On the other hand, 2D lateral heterostructure is possible only via direct growth, which can offer exciting opportunities for engineering the formation, confinement, and transport of electrons, holes, exciton, phonon, and polariton. Furthermore, the performance of most 2D heterostructure-based devices falls far below the predicted values owing to several intrinsic and extrinsic factors. These significant issues will be discussed. We reported the direct fabrication of seamless, high-quality TMDs lateral heterostructures and superlattices in the chemical-vapor-deposition process, only changing the reactive gas environment in the presence of water vapor.2-5 Our novel approach offers greater flexibility for the continuous growth of multi-junction TMDs lateral heterostructures, controlled 1D interfaces, alloying, and layer numbers. The extent of the spatial modulation of individual TMD domains and their optical and electronic transition characteristics across the heterojunctions are studied in detail. Electrical transport measurements revealed diode-like responses across the 2D lateral junctions and showed promising photodetection and electroluminescence properties at room temperature. Temperature-dependent photoluminescence from neutral exciton, trion, and defect-bound exciton provides a better understanding of the optical properties of these as-grown 2D lateral heterostructures. These studies will further supplement the quantitative evaluation of the optical and electronic qualities of various 2D heterostructures to develop more complex and atomically thin superlattices and exotic devices.
References:
[1] S. Chakraborty et al. iScience (2022)
[2] P. K. Sahoo et al., Nature, 553, 63�67 (2018)
[3] P. K. Sahoo et al., ACS Nano 13, 12372-12384 (2019)
[4] F. Nugera et al. Small 2106600, 1 (2022).
[5] S. Berweger et al. ACS Nano 14, 14080 (2020)

Title : Metal-Oxide Semiconductors for Electronic Biosensors
Abstract:
Metal oxide semiconductors have received interest for bioanalytes detection, because of their charge transport characteristics, electrical, and optical properties. When synthesized at the nanoscale and integrated with miniaturize devices, they are very sensitive and have a quick response time. Now a day�s different metal oxide nanostructures are used for different electronics biosensing platform like impedance based electrochemical biosensors and Field effect transistor-based biosensors. Metal oxides like TiO2, In2O3, WO3, SnO2, ZnO, and CuO are the most popular materials used as biosensors. Here we are utilizing thin film of WO3 for impedance and FET based electronic biosensors development. The thin film of WO3 nanostructures were used in EIS biosensor for Gram-positive bacteria detection. A facile, reusable, and highly sensitive label-free impedance-based biosensor is developed for discriminating between Gram-positive and Gram-negative bacteria in liquid media. WO3 thin film surface was modified with vancomycin, a glycopeptide antibiotic known to have a specific interaction with the peptidoglycan layer of Gram-positive bacteria and limit of detection of 102 cfu/ml was achieved for fabricated sensor. Again, the thin film of tungsten oxide was utilized as a FET channel layer for sensitive and selective detection of ammonia in human blood plasma for liver disorder monitoring. The FET biosensor was fabricated on Si/SiO2 wafer by depositing WO3 thin film and followed by lithographic steps. The fabricated sensors show good electrical characteristics with on/off ration of two orders. The limit of detection of 6 �M was achieved with wide dynamic range from 1 �M to 100 �M of ammonia in human blood plasma. The fabricated FET sensor shows stable response and the possibility of batch fabrication. In another work, by taking advantage of excellent electrical and sensing properties of TiO2 semiconducting metal oxide. MoS2/TiO2 hybrid nanostructure thin film FET biosensor was fabricated for the detection of Gram-positive bacteria on real-time basis in liquid media. Field-effect transistor (FET) biosensors based on hybrid nanostructures present unique advantages of high sensitivity, small size, excellent dynamic range, and compatibility with integrated circuits. The fabricated FET biosensors are efficiently capable of discriminating between Gram-positive and Gram-negative bacteria with a limit of detection 50 cfu/ml. The unique hybrid nanostructure-based FET with desirable biosensing properties presents the potential to screen the pathogenic bacteria. Live/dead monitoring of bacteria may help in deciding the course and dosage of the antibiotic treatment.
References:
[1] S. Singh, A. Moudgil, N. Mishra, S. Das, P. Mishra, Biosensors & bioelectronics. 136, 23-30 (2019).
[2] S. Sharma, A. Moudgil, S. Das, P. Mishra, Advance Materials and Interfaces. 9, 19, 2200647 (2022).
[3]. S. Singh, A. Moudgil, N. Mishra, S. Das, P. Mishra, Advance Materials Technologies. 5,1, 1900615 (2020).

Title : Excitons in 2H-MoS2 under electric and magnetic fields
Abstract:
The optical spectrum of transition metal dichalcogenide (TMDC) semiconductors such as MoS2 reveal the presence of multiple Coulomb bound many-body states such excitons, biexcitons, trions etc., due to weak dielectric screening and consequently large binding energies. It is therefore important, even for room temperature device applications, to identify the origins of such many-body transitions and their binding energies. Recent studies on few-layer TMDCs have uncovered exciton species with unusual properties such as the interlayer (IL) exciton which has a very large electric dipole moment. We have investigated such many-body entities in bulk and few layer 2H-MoS2 using reflectance spectroscopy under electric and magnetic fields and tried to understand their properties by comparison with ab-initio electronic structure calculations.
We provide evidence that IL excitons exist even in bulk 2H-MoS2.[1] However we show that in bulk films the excited state exciton A2s undergoes nearly three times as large Stark splitting compared to IL and try to understand it by comparison with the hydrogenic exciton model.[2] The identity and evolution of different exciton species with applied electric Fz || c-axis is found from comparison with DFT + GW-BSE calculations by our collaborator. At high Fz one can identify the states as the ground state A1s, split interlayer IL+, IL� and split excited state A2s� exciton and the latter�s anti-crossing with A1s.[2] Extrapolation to low Fz indicates that the IL and A2s states can be strongly mixed even at low fields. In our study on few layer 2H-MoS2 we observed two strong features in the reflectance spectrum around the ground state A exciton transition, in trilayer films.[3] In an electro-reflectance (ER) spectrum, the corresponding features show opposite phase response to the applied voltage. First principles ER spectral lineshape simulation and also photoluminescence spectroscopy together suggest that the two features are likely to be A1s exciton subspecies proposed earlier, whose energy depends on which layer the electron-hole pair is located in. The excited state A2s exciton transition is also identifiable in ER, enabling approximate estimation of the exciton binding energy and its variation with film thickness. The latter was phenomenologically analyzed assuming a film thickness and capping material dependent effective dielectric constant, which also showed that the A1s exciton is mostly confined to a single S-Mo-S layer, as predicted by theory.
Another way of getting information on excitons is through their Land� g-factor (LGF), which gives a measure of the splitting of an exciton state under a magnetic field. We devised a magneto-modulated reflectance (MR) spectroscopy technique [4] with circularly polarized light to measure the LGF of excitons in semiconductors, which uses a small oscillating magnetic field of 50mT (rms) as compared to dc field strengths of 5T-10T required for conventional LGF measurements. Our study on bulk 2H-MoS2 shows that the LGF for the ground state A1s exciton at 1.92eV (at 5K) and the next higher energy exciton feature at 1.96eV are around -2.8�0.5 and +10�3, respectively. The different magnitude and sign of the LGF indicates that the 1.96eV feature cannot be purely due to the excited state A2s exciton transition, as was previously believed. We show that by identifying this feature as one with a dominant IL exciton contribution and simultaneously invoking the valley angular momentum of carriers in TMDCs, one can explain both the large value and opposite sign of the LGF. In turn it provides additional proof for the existence of the IL exciton in bulk 2H-MoS2.

Title : Exploring Optical Properties in Perovskites-Based Solar Cells from Many-body Perturbation Theory
Abstract:
Materials for optoelectronic applications have received a lot of attention worldwide in a variety of fields over the past several decades. Understanding these materials at the theoretical perspective is never been easy because of the exchange-correlation functional that needs to be carefully analyzed in the light of electron's self-interaction error and spinorbit coupling (SOC). Conventional first principles-based simulation under the framework of Density Functional Theory (DFT) with local/semi-local functionals (viz. LDA or GGA), is not sufficient to determine the excited state properties. Therefore, to study the excited state properties of potential photovoltaic materials, we employ a general framework of the first-principles calculation under manybody perturbation theory approach (GW/BSE) [1]. Some of the findings of different class of materials are discussed here. For example, to reduce Pb-toxicity from lead halide perovskites, we find significant degradation of optical properties and solar cell efficiency on more than 50% substitution of Sn in place of Pb [2,3]. Thus, as an alternative to lead halide perovskites, we have explored environment friendly double perovskites and their derivatives obtained by doing sublattice mixing via high-throughput screening [4-5]. Unfortunately, we find apart from a few promising ones, most of the conformers obtained via sublattice mixing suffer from stability issues [4-5]. This motivates us to explore a different class of materials viz. chalcogenide perovskites as they are earth abundant, environment friendly, stable and exhibit optimal optical response [6, 7]. We have studied the optical and excitonic properties of bulk as well as RuddlesdenPopper (RP) phases of chalcogenide perovskites [8] and vacancy ordered double perovskites [9] that show enough promise as an evolving class of upcoming optoelectronic materials, especially in solar cells. More recently, we have also found some of these class of materials (viz. CsPbF3 perovskite) to be useful as quantum materials, showing reversible spin textures by switching the ferro-electric polarization, making it potent for perovskite-based spintronic applications [10].
1. P. Basera, A. Singh, D. Gill, SB J. Mater. Chem. C 9, 17113 (2021)
2. P. Basera, M. Kumar, S. Saini, SB Phys. Rev. B 101, 054108 (2020)
3. M. Jain, A. Singh, P. Basera, M. Kumar, SB J. Mater. Chem. C 8, 10362 (2020)
4. M. Kumar, M. Jain, A. Singh, SB Appl. Phys. Lett. 118, 021901 (2021)
5. D. Gill, P. Bhumla, M. Kumar, SB J. Phys. Mater. 4, 025005 (2021)
6. M. Kumar, A. Singh, D. Gill, SB J. Phys. Chem. Lett. 12, 5301 (2021)
7. P. Basera, SB J. Phys. Chem. Lett. 13, 6439 (2022)
8. D. Gill, A. Singh, M. Jain, SB J. Phys. Chem. Lett. 12, 6698 (2021)
9. P. Bhumla, M. Jain, S. Sheoran, SB J. Phys. Chem. Lett. 13, 11655 (2022)
10. P. Bhumla, D. Gill, S. Sheoran, SB J. Phys. Chem. Lett. 12, 9539 (2021)

Title : Single Crystal Growth of pure and doped ?-Ga2O3 by OFZ technique and its characterization
Abstract:
?-Ga2O3 is one of the most promising wide band gap materials for optoelectronic applications as well as a conducting substrate for GaN based device technologies. Single crystals of ?-Ga2O3 exhibit a wide bandgap (~ 4.8 eV) and can be used in optoelectronic devices which are operating in shorter wavelength. Due to the ultra-wide bandgap of this material, it behaves like an insulator. But under reduced conditions, it becomes an n-type semiconductor. The conductivity is mainly due to the oxygen deficiencies in the crystal lattice. Single crystals of undoped and doped (Sn, Si, Nb, Ta, Eu, Zn and Ce) ?-Ga2O3 were grown by Optical Floating Zone technique under various growth atmospheres like O2, Ar, O2/Ar and compressed dry air. Optimizing the growth atmosphere is the key to avoid flaws in the grown crystals[2,6,7]. During growth with optical heaters, ?-Ga2O3 decomposes slowly, oozing out gaseous components like Ga and �O2 from the feed rod when it reaches the temperature above 1200 ?. Oxygen atmosphere can be used to avoid decomposition of the molten feed, but, bubbles will be created inside the molten zone in the presence of 100% oxygen atmosphere. The bubbles formed may lead to total collapse of the molten zone. Instead, Ar/O2 partial pressure has been attempted with less oxygen to diffuse out the bubbles formed in the melt. Crystals grown by these methods were bluish in color due to oxygen vacancies[2,7]. Compressed air atmosphere has been successfully used with optimized flow rate towards the development of completely stoichiometric and furrows free transparent Ga2O3 crystals. The properties of ?-Ga2O3 are highly anisotropic. Optimization of wafers processing along different orientation suitable for device development has been carried out. Structural, optical and electrical properties of the wafers were determined. The results are discussed in detail.
References:
[1] Y.Tomm, J.M.Ko, A.Yoshikawa and T.Fukuda, Floating Zone growth of ?-Ga2O3 : A new window material for optoelectronic device applications, Sol. Energy Mater. Sol. Cells, 2001, 66, 369-374
[2] S. M. Koohpayeh, D. Fort, and J. S. Abell, �The optical floating zone technique: A review of experimental procedures with special reference to oxides,� Prog. Cryst. Growth Charact. Mater., 2008, 54, 121�137
[3] S. I. Stepanov, V. I. Nikolaev, V. E. Bougrov, and A. E. Romanov, �Gallium oxide: Properties and applications - A review,� Rev. Adv. Mater. Sci., 2016, 44. 63�86
[4] S. J. Pearton, J. Yang, P. H. Cary, F. Ren, J. Kim, M. J. Tadjer and M. A. Mastro, A review of Ga2O3 materials, processing, and devices, Applied Physics Reviews, 2018, 5, 011301
[5] H. A. Dabkowska and A. B. Dabkowski, �Crystal Growth of Oxides by Optical Floating Zone Technique,� Springer Handbook on Cryst. Growth, 2010, pp. 367�391,
[6] K. V. Akshita, Dhandapani Dhanabalan, Rajendran Hariharan, Sridharan Moorthy Babu, �Effect of gamma-irradiation on structural, morphological, and optical properties of ? -Ga2O3 single crystals�, J. Mater Sci: Mater Electron, 34, (2023) 841
[7] V L Ananthu, D.Dhanabalan, K.V. Akshita, S.Moorthy Babu, �Investigation of Sn Incorporation in ?-Ga2O3 Single Crystals and its Effect on Structural and Optical Properties.� ECS Journal of Solid State Science and Technology, 11, (2022) 10
[8] D.Dhanabalan, V.Ananthu, K.V.Akshita, Sudipto Bhattacharya, E.Varadarajan, S.Ganesamoorthy, S.Moorthy Babu, V.Natarajan, Sudeep Verma, Meenakshi Srivatsava, S.Lourdudoss, �Studies on Schottky Barrier Diodes fabricated using single crystal wafers of ?-Ga2O3 grown by OFZ technique�, Physica Status Solidi B, 259 (2022) 2100496
Acknowledgements :
This work was funded by DRDO (Defence Research and Development Organisation) and carried out under the project titled as Development of Single crystal Ga2O3 growth technology for Power Device applications � (ERIP/ER/201808007/M/01/1740)

Title : Growth-Microstructure-Device Performance Correlations for III-nitride Optoelectronic and Power Devices on Sapphire and Silicon
Abstract:
The talk will describe the defect-to-device performance correlations for optoelectronic and power devices on hetero-epitaxially grown III-nitride epi-layers on silicon and sapphire. While UV detectors and power electronic devices might seem far apart, we see similarities between the defect origins of dark current in the former and leakage currents in the latter. Both have their origins in crystal growth and in particular the first few seconds of growth that impact the formation of crystals at the epi-layer substrate interface. The first part of the work focuses on the correlation between performance parameters of vertical deep-ultraviolet photodetectors on c-plane sapphire and the density of extended defects vis-�-vis screw and edge dislocations. Through a careful optimization of nucleation density on the growth surface, state-of-the-art crystalline quality AlN epi-layers were grown. This led to the realization of record performance 289 nm p-i-n photodiodes with zero-bias quantum efficiency of 92 %, leakage current below 1 nA at 100 V and breakdown field in excess of 6 MV/cm. Performance of III-nitride power devices on silicon, on the other hand, was found to be more sensitive to structural defects such as surface pits as compared to dislocations. By utilizing a two-temperature growth technique which involved an AlN layer grown at high temperatures, a significant reduction in the pit density could be achieved. This resulted in three-orders of magnitude reduction in lateral as well as vertical leakage in AlGaN/GaN high electron mobility transistors (HEMTs) on silicon. To achieve a further reduction in the reverse leakage current and an enhancement in the device breakdown voltage, point defect control also becomes important. Carbon doping to reduce the background n-carrier concentration in the GaN epi-layers is discussed as one of the possible approaches. Reduction of pit density and C-incorporation cumulatively led to the realization of 2 MV/cm vertical breakdown field in AlGaN/GaN HEMTs on silicon. The talk will end with our latest results in which we see that, in addition to the impact of the beginning of growth, interfaces within stack also play a key role.

Title : Advances in III-V Compound Semiconductor/Si Tandem Photovoltaics and Role of Innovative Materials Science and Defect Engineering
Abstract:
Silicon-based tandem solar cells are rapidly opening pathways to advance Si solar cell Silicon-based tandem solar cells are rapidly opening pathways to advance Si solar cell performance that greatly exceed the single-junction limit of standard Si solar cells. High efficiency without large increase in cost at the manufacturing scale, combined with outstanding long-term device reliability and compatibility with existing Si manufacturing infrastructure are critical to increase PV penetration into traditional and non-traditional energy markets. The ideal bandgap profiles for highest efficiency enabled by III-V/Si integration, coupled with the proven and outstanding long-term reliability of the constituent technologies therefore make III-V/Si tandems very attractive. This presentation focuses on the critical role and impact of defect and interface engineering at the materials science level for advancing monolithically-integrated III- V/Si tandem solar cells. While a brief review of the status of III-V/Si tandem solar cells will be provided for context, the primary focus is on GaAsP/Si tandem solar cells produced by metalorganic chemical vapor deposition (MOCVD) with ideal 1.7 eV/1.1 eV bandgap profiles for maximum efficiency. Device designs that account for the presence of elevated dislocation densities on transport properties in early stage monolithic GaAsP/Si tandem solar cells have already resulted in the current certified record efficiency of 23.4%, for this cell type even at dislocation densities that are greater than 1x107 cm-2 . While this dislocation density value has appeared to have generally reached a plateau for the past several years, we have made a recent breakthrough in the growth and design of metamorphic GaAsP/GaP buffers on Si that have resulted in TDD values 10x lower through the insertion of novel dislocation glide enhancing structures during GaP growth. At these improved dislocation densities in the low 106 cm-2 range, a near term pathway to achieve stable Si tandem efficiencies >28% and up to 31% for two junction tandems is presented.
performance that greatly exceed the single-junction limit of standard Si solar cells. High efficiency without large increase in cost at the manufacturing scale, combined with outstanding long-term device reliability and compatibility with existing Si manufacturing infrastructure are critical to increase PV penetration into traditional and non-traditional energy markets. The ideal bandgap profiles for highest efficiency enabled by III-V/Si integration, coupled with the proven and outstanding long-term reliability of the constituent technologies therefore make III-V/Si tandems very attractive. This presentation focuses on the critical role and impact of defect and interface engineering at the materials science level for advancing monolithically-integrated III- V/Si tandem solar cells. While a brief review of the status of III-V/Si tandem solar cells will be provided for context, the primary focus is on GaAsP/Si tandem solar cells produced by metalorganic chemical vapor deposition (MOCVD) with ideal 1.7 eV/1.1 eV bandgap profiles for maximum efficiency. Device designs that account for the presence of elevated dislocation densities on transport properties in early stage monolithic GaAsP/Si tandem solar cells have already resulted in the current certified record efficiency of 23.4%, for this cell type even at dislocation densities that are greater than 1x107 cm-2. While this dislocation density value has appeared to have generally reached a plateau for the past several years, we have made a recent breakthrough in the growth and design of metamorphic GaAsP/GaP buffers on Si that have resulted in TDD values 10x lower through the insertion of novel dislocation glide enhancing structures during GaP growth. At these improved dislocation densities in the low 106 cm-2 range, a near term pathway to achieve stable Si tandem efficiencies >28% and up to 31% for two junction tandems is presented.

Title : Ferromagnetism in two dimensional Fe 3 GeTe 2
Abstract:
The van der Waals ferromagnets, such as CrI 3 , CrTe 2 , Cr 2 X 2 Te 6 (X = Si and Ge) and Fe 3 GeTe 2 provide an excellent platform to investigate ferromagnetism in two dimensional (2D) systems which are being considered as the potential material for future spintronic devices. In particular, Fe 3 GeTe 2 and Fe 5 GeTe 2 are being investigated vigorously due to exceptional tunability of their magnetic properties and high Curie temperature. Ferromagnetism in 2D is forbidden according to Hohenberg-Mermin- Wagner (HMW) theorem[1,2]. Short ranged exchange interaction, such as 2D Ising and 2D XY driven phase transition has been observed in Cr 2 X 2 Te 6 and CrI 3 respectively. However, 2D-Heisenberg ferromagnet has not yet been observed in accordance with HMW theorem. The presence of long range (LR) interaction changes the scenario completely [4]. LR interactions have been shown to be responsible for (i) interesting finite temperature transitions, (ii) new critical phenomena, and (iii) continuously varying critical exponents. Though it is realized that LR driven ferromagnetism is essential for circumventing HMW theorem in 2D, but experimental realization remains elusive in magnetic materials. Here, we show that Fe 3 GeTe 2 is a 2D Heisenberg ferromagnet by breaking the limit of HMW theorem. This has been possible due to the presence of both itinerant and local-moment magnetism in Fe 3 GeTe 2 [5]. Temperature dependence of magnetization with signature of 2D magnons, Rhodes-Wohlfarth (RW) and generalized RW based analysis [6] established that Fe 3 GeTe 2 is a 2d ferromagnet with itinerant magnetism which can be tuned by magnetic field. So far critical phenomena and second order phase transition have been dealt with renormalization group theory. However, the same in the presence of long range interaction is nontrivial [4,7]. Here we present an improvised method to deal with phase transition and critical phenomena mediated by long Heisenberg interaction in 2D in the presence of long range Ruderman-Kittel-Kasuya-Yusida (RKKY). We show that critical exponents in this case can vary with external parameter[8]. The critical exponents cross from mean field universality class to 2d Heisenberg universality class as field increases. We argue that variable critical exponents is the key signature for 2d Heisenberg ferromagnetism, as emphasized by recent theoretical calculations [9]. Fe in Fe 3 GeTe 2 has two inequivalent charge states i.e. Fe 2+ and Fe 3+ which lead to multi-orbital path for with differing localization responsible for underlying long range ordering for both itinerant and local- moment ferromagnetism.

Title : Inkjet-printed super-flexible oxide electronics and its application at the sensor interfaces
Abstract:
Printed electronics is a rapidly maturing field of research, where, high throughput fabrication of electronic devices are aimed at. Over the last two decades, printing of oxide semiconductors has attracted particular attention, however, challenges, such as limited flexibility, absence of performance-matched p-type semiconductors, inadequate reliability and large variability in the performance of printed thin film transistors (TFTs) etc. are certain showstoppers for which fully-printed circuits based on oxide semiconductors have rarely been reported in the literature.
Here, we will try to address some of these challenges. In the beginning, an organic-inorganic hybrid semiconductor will be demonstrated that can combine the electronic transport of inorganic oxides with the ultra-flexibility of organic materials. Next, all-NMOS unipolar inverters will be shown that operates at deep-subthreshold (near off-state) regime, and thereby can demonstrate very high signal gain (?) coupled with low dynamic power dissipation. A set of benchmark circuit elements, ring oscillators, SRAMs etc., will be demonstrated. Next, the inverters will be further optimized to improve their noise immunity to be used to fabricate read-out electronics at the printed sensor interfaces. Common source amplifiers, differential amplifiers and analog-to-digital converters (ADCs) will be presented that operate at 1 kHz frequency and at a battery compatible operation voltage of ?2 V. Finally, an effort towards further improvement in the magnifying capacity of weak signals will be demonstrated using so-called negative capacitance field-effect transistors (nc-FETs). Here, for the first time, fully-printed nc-FETs will be shown with subthreshold slope as low as 2 mV/dec and signal gain of the printed inverters >2000 when measured at a 2 mV step size. Interestingly, these nc-FETs can be achieved with both organic/inorganic and all-organic insulator stacks. Unsurprisingly, the printed nc-FETs will show orders of magnitude low power dissipation, compared to the standard MOSFETs and would also exhibit memory behavior with capacity of holding the charged-state for days.

Title : P-Channel oxide thin-films transistors with world-record hole mobility
Abstract:
Transition metal oxide semiconductors have many uses, e.g. active matrix displays, sensors, thin-film solar cells, and transparent electrodes. Most oxide semiconductors are N-type, e.g., indium gallium zinc oxide (IGZO) TFTs routinely achieve electron field effect mobility of 60 cm2/Vs [1]. However, there are no good options for p-type oxides because most oxides do not conduct holes. Cu+1 oxides are a notable exception [2]. Cu2O has the highest hole mobility of any known oxide [2]. This is a long-standing and fundamental limitation that prevents the use of oxides for interesting applications like back-end-of-line (BEOL)-compatible field effect transistors (FETs) for three-dimensional (3D) monolithic integration. In this talk, we will present our latest results on Cu2O p-type films and devices. We hold the record for the highest hole mobility in Cu2O of 262 cm2/Vs. Using the Cu2O we will demonstrate three types of devices, P-channel thin-film transistor, Cu2O/CuO memristor and NH3 gas sensor. The most important innovation of our work is the pioneering use of chemical vapour deposition (MOCVD), not sputtering or pulse-laser deposition. CVD provides excellent control of nucleation and grain growth, leading to larger grains, smoother morphology, and superior mobility in Cu-oxide films. Precursor selection is also very important to obtain superior electronic properties.
We will report p-type TFTs with champion field-effect mobility of 0.99 cm2/Vs, the third-highest number ever reported. We will discuss the reasons for the 100x difference in bulk and field-effect mobility. We shall also discuss ways to improve the performance further. Finally, we shall present data on stress tests, interfacial modifications and the impact of gate dielectric.
We will report a very novel type of memristor using a heterojunction of cupric oxide (CuO) and cuprous oxide (Cu2O). Electrical and material characterisations illustrate that grains or corresponding grain boundaries are vital in controlling the switching behaviour. Compared to competing approaches we demonstrate good consistency in device parameters such as reproducibility, and endurance.
Finally, we report a NH3 gas sensor. The device's response time (

Title : Non volatile and volatile memristors on unconventional semiconductors for neuromorphic computing
Abstract:
Designing materials and devices for energy efficient hardware, are actively researched for computing tasks beyond von Neumann. This necessitates materials whose characteristics can be controllably changed by an external stimulus such as temperature, voltage, current, electric field, magnetic field, spin orbit torque etc. Resistance states of the devices, typically exhibits multistate behavior for the same control stimulus and in this way information can be stored as conductance. This yields computing primitives relevant for unconventional computing such as those used in neuromorphic, probabilistic and quantum computing schemes. In this context, we have exploited the rich phase space, intrinsic to complex oxides, for tunability of multistate resistance by electric field and Joule heating and relying on the interplay between the spin, charge and lattice degrees of freedom in these systems. Devices on complex oxide semiconductors, such as doped SrTiO3 (STO), bring in functionalities that are largely derived from the temperature and electric field dependence of its dielectric permittivity. I will discuss a couple of memristive device types, where tailoring of the energy landscape enables multistate resistance states. The first work that I will discuss is based on interface based memristive devices on an unconventional oxide semiconductor: Nb doped SrTiO3. We have demonstrated that using magnetic electrodes, tailored energy landscape at the Schottky interface exhibits multi-level switching between highly resistive states by tuning the interface electric field. We show the ability to achieve analogue switching with remarkably high resistive windows by downscaling, high endurance and low device and cycle-to-cycle variation integrated directly on a semiconducting platform. The second work will discuss a volatile memristor where phase transitions in a strained manganite La0.67Sr0.33MnO3 (LSMO) film is used to stabilize competing magnetic states in a network. Our results show Joule heating induced hysteretic resistive switching and a multilevel resistive switching in such structures. We found that these distinct resistive states are highly stable over time and possess minimal cycle-to-cycle variability. With rapid advances made in the palette of materials and devices available for neuromorphic hardware, our work highlights the importance of complex oxide interfaces in this direction. References:
1. M. Ahmadi, D. Gupta, I. Bhaduri, B. J. Kooi and T. Banerjee, Advanced Electronic Materials 9, 2201111 (2023)
2. A. Jaman, A. S. Goossens, J. van Rijn and T. Banerjee, Front. Nanotechnol. 5:1121492 (2023)

Title : Kilovolt-class and highly scaled gallium oxide devices
Abstract:
Within a decade of the first demonstration of a MESFE device, monoclinic beta-Ga2O3 (Ga2O3) devices have made incredible progress in breakdown voltages, power device figure of merit and high-speed performance. It has emerged as a promising ultra-widebandgap semiconductor for next generation power, GHz switching and RF applications. In this talk, we will present an overview of our studies on fundamental transport properties, kilovolt class power MOSFETs and aggressively scaled RF devices. The large bandgap of Ga2O3 leads to a high critical field strength. This high field strength in combination with demonstrated room temperature mobility and calculated electron velocity leads to a higher Baliga�s Figure of Merit (BFoM) and Johnston Figure of Merit (JFoM) than current commercially available widebandgap technologies. Additionally, the large bandgap also enables high temperature operation and radiation hardness making it attractive for space applications such as Mars and Venus missions.
Next, I will describe our work on the lateral MOSFETs with improved field plate design and beyond-kV breakdown. Temperature dependent analysis and device simulation suggest an extrinsic breakdown mechanism outside the channel. A simple and yet effective SU-8 polymer passivation technology was developed that results in significant improvement in breakdown voltages. The higher field strength of the SU-8 polymer enables a significant increase in breakdown voltage to 8 kVs in lateral MOSFETs. However, these devices show a high Ron, which is due to the depletion caused by RIE of the channel. We will present the use of ultra-high vacuum annealing techniques to improve the on-resistance of the devices still maintaining the multi-kilo-volt rating of the devices.
Due to its high JFoM, Ga2O3 is a potential candidate for high power density RF amplifiers. We will also present the performance advancement that can be obtained in RF power performance using Ga2O3 technology. We will present the challenges in the technology and potential solutions. We will present the experimental DC and pulsed measurements on laterally scaled MOSFETs with 100 nm gate length. Severe DC-RF dispersion was observed which is attributed to the RIE damage and surface states on the oxide. Passivation of the devices shows reduced current collapse. Using aggressive gate lengths and gate-source spacing scaling, (AlxGa1-x)2O3/Ga2O3 heterostructure FETs were fabricated that show record high RF performance. The device transconductance was limited by the regrowth interface resistance. Ex-situ silicon nitride passivation was shown to be effective to reduce DC-RF dispersion.

Title: Solution processible materials and printing techniques for 2D/3D microelectronic devices
Abstract: Printed and flexible electronics enable interesting novel applications in the fields of sensors, bioelectronics and security applications. An interesting class of materials to realize this are metal oxides that are known for a variety of useful characteristics such as high electron mobility in thin films, printability, non-toxicity including environmentally friendly solvents, transparency, to name but a few. In addition, metal oxide based nanoparticle inks allow to print semiconducting thin films at low temperatures (around 100 °C). Equally important to the question of materials are the structuring techniques. A widespread digital printing approach is inkjet printing, which can be further combined with other techniques such as laser printing to yield even 3D device architectures.
In the talk, the plethora of inorganic materials and printing techniques used in the fabrication of multilayer devices such as transistors, memristors and even fully integrated circuits, will be discussed. A memristor is an electronic device in a simple sandwich-like structure containing two metals and a thin insulating layer. It exhibits a huge potential as a non-volatile data storage device and plays a vital role in novel paradigms of computing, such as neuromorphic computing. The fabrication and electrical performance of inkjet and laser printed ZnO digital memristors as well as WO analog memristors, which show “learning” behavior, will be presented. The conductance of analog type devices can be gradually manipulated to emulate different behaviors of the biological synapse, such as short time plasticity, long term potentiation, and long-term depression.
In addition, progress towards transistor architectures using nanoparticle-based In-oxide as the semiconductor and Pt or ITO as the electrode material will be presented. Along these lines, a transistor with In-oxide-semiconducting nanoparticles printed into a 3D laser written polymer reservoir with 10 μm height is presented. Electrical characteristics are discussed including transfer and output curves as well as FIB cuts and cross-sectional SEM images. Our devices show high ion currents in the range of ~mA for typical widths of few hundreds of micrometers. Last but not least, systems of integrated logic devices (transistor-resistor based inverters) are shown as applications for physical unclonable functions and basic building blocks to perform multiply and add functions.

Title: GaN RF HEMT: Some new approaches for Ohmic contacts and dopant free buffer layer
Abstract: GaN based High Electron Mobility transistors are used as RF power transistors in T/R modules and as switch in power converters. Even though these devices are commercially available, there is still scope for improvement in the process of fabrication which leads to better performance of the device. In this talk I will present some of the results of studies conducted on GaN based RF HEMTs. Epitaxial GaN HEMT stacks grown on silicon substrates were chosen for these studies. Optimization of ohmic contact resistance and improvement of surface morphology in highly scaled RF HEMTs is necessary to obtain high values of f t .L g product (f t is the cut off frequency and L g is the gate length). GaN buffer in the HEMT epitaxial stack is either Carbon or Iron doped to compensate the unintentional background carrier concentration to make the buffer insulating. This brings with it additional issues in growth of the epistack and buffer related current collapse. Our studies are an attempt to address both these issues.
InAlN/GaN hetero structures possess a very high two-dimensional electron concentration > 2E13 cm -2 and are attractive candidates for high power RF applications. Sc/Al/Ni/Au metal stack deposited on InAlN/GaN HEMT and annealed at 775C resulted in a contact resistance of 0.39.Ωmm. Detailed TEM investigations and EDS analysis were conducted to determine the mechanism of ohmic contact formation with low contact resistance. It was found that Scandium diffused through dislocations and there were ScGaN inclusions at the interface. In another study, we found that chemical surface treatment with TMAH before the deposition of the ohmic metal stack on InAlN/GaN HEMT, resulted in lower contact resistance. Here again TEM investigations were performed to determine the phenomena underlying the process of lower contact resistance.
Highly scaled InAlN/GaN HEMTs were fabricated with gate lengths ranging from 40nm to 200nm. Record values of f t -L g were demonstrated. Back gating experiments were conducted to investigate the role of buffer in the kink in the I D -V D characteristics.
CMOS like processing for fabrication of device is a path for reducing the cost of production of devices. Ti/Al/Ti/W was Sputter deposited as ohmic metal on recess etched AlGaN/GaN epitaxial stack and annealed at low temperature (550C). These resulted in low contact resistance with improved surface morphology. Detailed studies revealed the formation of TiN inclusions even at low annealing temperatures.
As an alternative to Carbon or Iron doped buffer, we investigated polarization graded undoped AlGaN layer as the buffer. This showed low leakage current and low buffer related current collapse. RF performance of HEMTs fabricated using these stacks were also studied.

Tutorial Program

Please click on the tutorial title for talk details.
Tutorial Program - 13^th December 2023( IST ) @ Research Park, IIT Madras

Tutorial Program Flyer

09:00 AM - 10:15 AM

Pradeep R Nair
Indian Institute of Technology, Mumbai, India

Title: Fundamentals of Solar Cells

10:15 AM - 11:00 AM

Break

11:00 AM - 12:30 PM

Muhammad A. Alam
Purdue University, USA

Title: Physics of Solar Farms

12:30 PM - 01:30 PM

Lunch

01:30 PM - 02:45 PM

Muhammad A. Alam
Purdue University, USA

Title: Reliability Physics of Photovoltaic Technology

02:45 PM - 03:15 PM

Break

03:15 PM - 04:30 PM

Roopmati Meena
PV Module QAQC

Title: Electrical and Physical Characterization of Solar Modules

09:00 AM - 10:15 AM

Anant Agarwal
Ohio State University

Title: Silicon Carbide (SiC) Devices & Technology

10:15 AM - 11:00 AM

Break

11:00 AM - 12:30 PM

Davide Bisi
Transphorm Inc.

Title: Gallium Nitride (GaN) Devices & Technology

12:30 PM - 01:30 PM

Lunch

01:30 PM - 02:45 PM

Vijay Bolloju
GyanTrek

Title: E-Vehicles Applications and Device Requirements

02:45 PM - 03:15 PM

Break

03:15 PM - 04:30 PM

Shiladri Chakraborty
IIT Bombay

Title: Packaging Technology for Wide Bandgap Power Devices

09:00 AM - 10:00 AM

Supravat Karak
IIT Delhi, India

Title: Introduction to the Fundamentals of Organic Semiconductors: Physics and Applications
Abstract: Organic semiconductors are a novel class of materials that cartel the electronic advantages of semiconductors with the chemical and mechanical flexibility of organic compounds. Thus, the ability to absorb and emit light, and conduct electricity is combined with a tunable material structure that can be easily modified by facile chemical routes for specific applications, for example, tailoring electronic properties, and emission wavelength, manipulating its solubility, or allowing for mechanically robust, lightweight, and flexible thin films. These properties imply that almost every semiconductor application such as displays, lighting panels, solar cells, detectors, wearable sensors and IoT-based devices can be manufactured with a low-cost, solution-processable, flexible and scalable technique based on organic semiconductors. However, the fundamental properties of this new class of material are vastly different from the well understood inorganic semiconductor such as Si, particularly for their optoelectronic applications where both the electronic transport and photon interactions are involved. Therefore understanding the photophysics of organic semiconductors is crucial for their successful applications. In this tutorial, firstly, a brief overview of organic electronics and the evolution of this class of material will be discussed. In the next phase, the fundamental photophysics of organic semiconductors, such as band formation, various optical and electrical properties, charge carrier transport, exciton generation, recombination etc. will be discussed in detail. In the last phase, a short overview of the various possible applications of organic semiconductors such as OLED, OPV, OFET and wearable devices will be introduced.

10:00 AM - 10:15 AM

Break

10:15 AM - 11:15 PM

Shree Prakash Tiwari
IIT Jodhpur, India

Title: : Crucial Aspects for Designing High Performance Organic Thin Film Transistors
Abstract : Organic electronics has emerged as a promising technology for demonstration of smart and wearable products due to inherent capabilities for low temperature, low cost, flexibility, and large area processing, along with multi-functional capabilities in devices. Organic field-effect/thin-film transistors (FETs/TFTs) are widely explored active devices for organic and flexible electronics due to their suitability for circuits such as backplane drivers and many sensing applications. These devices can act as a crucial block for smart wearable applications such as real-time health monitoring, electronic textiles, and electronic skin due to flexibility and conformability. However, in real systems, these devices face various mechanical, thermal, electrical, and other environmental stimulations during operation. Hence, these devices should be highly stable and reliable under these internal or external influences. For integration of the flexible devices in real systems, various contact resistances have to be minimized. Moreover, parasitic effects arising from overlaps and interconnects have to be carefully looked into. In this tutorial, firstly, a historical perspective along with potential applications of TFTs will be discussed. Further, crucial aspects for designing high performance organic TFTs, and strategies to achieve high quality dielectric semiconductor interface and low contact resistance will be discussed. Finally, options for alternative natural and biodegradable materials and processes aiming for eco-sustainable, ingestible, and green electronics will be highlighted.

11:30 PM - 12:30 PM

Moritz Riede
Oxford University, UK

Title: Organic Photovoltaic Devices
Abstract: Organic Photovoltaics, an emerging solar cell technology based on organic semiconductors, is opening up many new markets in addition to silicon photovoltaics and has the potential to become the cheapest form of electricity, beating even silicon photovoltaics. However, having the potential is not sufficient, unless it's realised. Organic photovoltaics have come a far way, from a lab curiosity and power conversion efficiencies of less than 1% in the early days, to now reaching nearly 20% in the laboratory. Similarly, lifetimes have significantly improved with reports exceeding 20y. Current and future applications include such as building integrated photovoltaics or portable electronics, and already now organic photovoltaics have the smallest environmental footprint of all solar technologies. However, the challenges on the road to large scale commercialisation of organic solar cells is not small. This tutorial will start with the organic photovoltaics fundamentals, highlight some of the key advances and sketch out the main remaining challenges on the way to large scale commercialisation.

12:30 PM - 01:30 PM

Lunch

1:30 PM - 02:45 PM

G. Rajeswaran
IIT Madras, India

Title: Organic Light Emitting Diodes

02:45 PM - 03:15 PM

Break

03:15 PM - 04:30 PM

G. Rajeswaran
IIT Madras, India

Title: AMOLED Display

Tutorial Program

Please click on the tutorial title for talk details.
Tutorial Program - 13^th December 2023( IST ) @ Research Park, IIT Madras

Tutorial Program Flyer

1. An Atom-to-Farm Introduction to the Solar Cell Technology
Venue: EG-01 (Saha)

2. Power Semiconductor devices and applications
Venue: EG-11 (Ramanujan)

3. Organic semiconductor devices and applications
Venue: EG-02 (Sarabhai)

Pradeep R Nair
Indian Institute of Technology, Mumbai, India

Break

Muhammad A. Alam
Purdue University, USA

Lunch

Muhammad A. Alam
Purdue University, USA

Break

Roopmati Meena
PV Module QAQC

Anant Agarwal
Ohio State University

Break

Davide Bisi
Transphorm Inc.

Lunch

Vijay Bolloju
GyanTrek

Break

Shiladri Chakraborty
IIT Bombay

Supravat Karak
IIT Delhi, India

Break

Shree Prakash Tiwari
IIT Jodhpur, India

Moritz Riede
Oxford University, UK

Lunch

G. Rajeswaran
IIT Madras, India

Break

G. Rajeswaran
IIT Madras, India

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