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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. 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-a-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 epilayers 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.

Germanium is considered an important electronic material which can serve both as a photo-voltaic material as well as a substrate for low cost III-V solar cells on silicon. However, growth of high quality germanium on silicon is challenging due to 4% lattice mismatch. In this work, we present growth of epitaxial, large grain polycrystalline germanium on silicon using liquid phase crystallization. The method involves re-crystallization of PECVD deposited amorphous Ge on Si using a two-step annealing process by melting (937oC) amorphous thin film and cooling it right below the melting point leading to high quality and large grains. SEM XRD and hall measurements were done to evaluate the material and electronic quality of the films. The films cooled near melting point were found to have largest grains of size ~14 μm and XRD rocking curve FWHM of as low as ~300 arc-sec. Films also demonstrated record high electron mobility of 500-1000 cm2Vs-1.The process is scalable and orientation agnostic with comparable film quality for all orientations.

This work demonstrates UV assisted probing of deep level traps in dielectric/GaN interface. The deep level donor traps lead to threshold voltage and gate leakage instabilities in GaN MISHEMTs. While UV exposure excites the deep traps, gate bias can sweep the trap energy state and trigger de-trapping. The recovery transient is evaluated to study the trap time constant. Besides, this work reveals a non-destructive technique to probe intrinsic traps as the UV exposure does not change the trap density across the device.

A prolonged operation of Graphene FET and interconnects mandate the assessment of temporal evolution of degradation of the material. Contrary to bulk semiconductors, which break only above a critical field, the time-dependent degradation and consequent failure of graphene has been discovered, which precludes the existence of failure threshold and manifests as a potential defect-assisted aging issue for graphene and other 2D material-based devices. Unlike catastrophic failures, which are triggered during redistribution of excess energy, the role of reaction kinetics (time) in inflicting defect-by -defect damage to graphene channel is revealed. Time-evolution of defects in graphene channel while exploring possible pathways through which heat dissipates across the device has been studied on-the-fly using an integrated micro-Raman setup. The role of metal-graphene interface and the substrate in removing excess heat from graphene channel is discussed, while emphasizing the need for sp-hybridized carbon atoms at metal-graphene interface for reliable, long-term operation of graphene-based devices.

Flexible and transparent moisture barrier films have been climacteric importance for the further development of the polymer solar cells (PSCs). Herein, a monolayer graphene (2D atomic crystal) embedded in a flexible polymer (GEPs) was used to encapsulate the solar cells. The improvement of long-term stability of PSCs still needs to be overcome for their commercialization to be feasible.Degradation mechanisms studied in ITO/ZnO/PTB7:PC 70 BM/MoO 3 /Ag inverted PSCs by Capacitance-voltage, Capacitance-frequency and light intensity dependent measurements is reported here. Measurements was performed on encapsulated and nonencapsulated (controlled and ambient environment) cells under AM1.5 illumination. However, the degradation studies performed with the ISOS D1 protocol suggested the increased lifetimes of the PSCs encapsulated with GEPs barrier films. A promising lifetime of more than 20000 hours was demonstrated with less than 22 % degradation for encapsulated devices.

This experimental study reports first observations of (i) SOA boundary shift in GaN HEMTs and (ii) early time to fail of vertical AIGaN/GaN Epi stack under fast changing (sub-10ns risetime) cyclic transient stress conditions for a 600V qualified commercial grade HEMT stack. It is shown that a stack qualified for 10 years lifetime under DC stress, fails faster under cyclic transient stress. Integrated electrical and mechanical stress characterization routine involving Raman/PL mapping and CL spectroscopy reveals material limited unique failure physics under transient stress condition. Failure analysis using cross-sectional TEM investigations reveal signature of different degradation and failure mechanism under transient and DC stress conditions. A failure model is proposed for failure under cyclic transient stress.

Ease-of-use analytics at scale is the holy grail of industrial strength machine learning. While there have been advances in APIs, algorithms, and user interfaces, building an end to end work flow involving data ingestion, data preparation, model training, model scoring, and visualization received limited investment and effort producing only marginal innovation. This paper outlines a proof of concept that demonstrates an analytical workflow that integrates multiple analytical tools and techniques for image recognition. The solution combines Relational Databases and Machine Learning (teradata), Deep Learning (TensorFlow), Distributed File System (HDFS), Graphical Processing Units, and user interface tools over a communication fabric (teradata QueryGrid). In particular, we demonstrate hand written word recognition through an application of Convolutional Neural Networks in TensorFlow and teradata’s custom analytical functions to recognize first names and last names in the payee field section of financial negotiable instruments such as in a cheque. We hope that this paper will serve as a guide to successful implementation of analytical workflows in production.

A FinFET device architecture is effective in realising normally off operation in Al x Ga (1-x) N/GaN HEMTs with low on resistance and increased gate control. This paper discuses simulation and experimental study of normally off Al x Ga (1-x) N/GaN FinFET devices. Atlas silvaco simulator is used to carry out electrostatic simulations to demonstrate normally off operation in Al x Ga (1-x) N/GaN FinFETs, by considering the tri-gate induced depletion effect on 2DEG. FinFET devices are fabricated with the understanding gained from simulation studies. The fin width, height and length of the fabricated device were 80 nm, 70 nm abd 1.5 µm respectively. The fabricated device exhibited a threshold voltage of 2.5 V with a maximum drain current of 260 mA/mm at a gate overdrive voltage of 2.5 V. A maximum field effect mobility of 130 cm2/Vs is achieved for the fabricated FinFET device.

Microstructure evolution of GaN grown on c-sapphire by MOCVD has been systematically studied using transmission electron microscopy based techniques. Individual samples have been derived by interrupting the GaN growth at various steps, starting from nitridation at 530° C, followed by the standard two step growth comprising of first deposition of low temperature GaN nucleation layer (LT-GaN NL) and then ramping up the temperature followed by high temperature GaN epilayer growth. Effect of nitridation, and the microstructure of the nitride layer for various nitridation temperatures has been recently reported by our group [1], where we have shown that the nitride layer formed at this nitridation temperature is cubic spinel Al x O y N z . In this paper it will be shown, that the LT-GaN grown on this Al x O y N z (after nitridation at 530° C) is primarily cubic zinc blende (zb) in structure, with multiple twin variants existing about various {111} planes. Its crystallographic orientation relationships with the underlying nitride layer and the sapphire substrate will be shown. The transformation of LT-GaN and the regions of transformation from cubic zb phase to the wurtzite (w) phase during the annealing step will be presented. Subsequent effects on the GaN epilayer growth due to the microstructural evolution of these underlying layers along with the evolution of defects will also be discussed.

Germanium integration on silicon is required for fabricating high efficiency III-V low-cost solar cells on silicon. In this work, we present a single step thermal annealing of amorphous germanium films as a method of monolithic integration of epitaxial germanium on crystalline silicon (110) wafers. The re-crystallization progresses via liquid phase epitaxy by heating the germanium layer in inert ambient to right above its melting temperature of $937 ^{0}C$. On slow cooling; the Ge layer gets oriented by the underlying Si (100) wafer, yielding an epitaxial Ge film on Si. SEM show that the films are continuous and crack free whereas XRD measurements indicated highly oriented films. 1 micron thick film had rocking curve FWHM of 0.15 degree corresponding to dislocation densities of low 109 cm-2. Also, AFM measurements indicate that 1 micron films have RMS roughness of ˜ 80nm for scan area of 25 μm2.

Low cost high-power electronics era has begun with the fabrication of Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMTs) on Silicon substrates. Silicon is however not an optimum material from the viewpoint of the final substrate. Having GaN based devices on other substrates would enable better performing RF and high-power devices. Here we report the process development of the transfer technology, of a fabricated GaN HEMT transistors to a glass wafer using temporary bonding material. Epoxy has been used to finally bond the GaN stack and the glass wafer. Transfer and output characteristics of the HEMT and the capacitance measurements of the capacitors were performed on the identical devices before and after the transfer process. Also, to understand the stress before and after the transfer process Raman measurements were carried out.

This experimental study reports ESD behavior of novel designs of GaN Schottky diodes. Impact of electro-thermal transport, device degradation and trap generation on its ESD robustness is analyzed. Role of interface traps in ESD failure of GaN Schottky diode is investigated. Transition from sofit-to-hard failure, which is found to depend on diode area, presence of traps and diode design is discussed. Unique degradation trends, cumulative nature of degradation and trap assisted failure modes are discovered.

High-efficiency III-V solar cells on low-cost substrates such as steel, are limited because a) the overlying III-V semiconductor stack gets contaminated with iron, b) it is not easy to deposit crystalline Ge thin-films on amorphous steel substrates. In this work, we report a TiN-based barrier layer that inhibits Fe diffusion into the overlying semiconductor. We also report a method to deposit crystalline germanium thin-film on steel on which high quality III-V layers can be deposited using laser annealing. PECVD amorphous germanium films were irradiated with a 532 nm laser with a fluence of 4 - 8 x 108 Jcm-2 which melted and re-crystallized the irradiated amorphous film into nano-crystalline germanium film. The recrystallized films were characterized using SEM and Raman spectroscopy. Films irradiated at higher fluence and films with higher thicknesses, had more crystalline order. The work enables future development of lost-cost GaAs solar cells.

All-oxide solar cells are attractive due to their stability, low cost and ease of fabrication. Very few oxide-absorbers are known and the efficiencies are limited by low mobility and lifetimes. In this work, for the first time, photovoltaic properties of a new oxide semiconductor, Mn2V207 (MVO) are described. Optical measurements show that MVO has an indirect bandgap of 1.6 eV and a direct bandgap of 1.75 eV, which suggests that it absorbs efficiently in the visible region of the solar spectrum. The valence and conduction band positions of the intrinsically n-doped MVO film are determined to be at 5.5 eV and 3.9 eV. Schottky solar cells fabricated using Pt/MVO heterojunction show low short circuit current (Jsc) and open circuit voltage (Voc). Annealing in nitrogen ambience results in a Jsc of 0.46 mA/cm2 a 50x increase and of Voc 0.21 V, almost 10x increase compared to un-annealed device. A maximum power of 24.1 μcm2is obtained, which is two orders of magnitude higher than un-annealed devices.

Here we present a novel vapor-annealing technique to improve morphology of pure-iodide perovskite (CH3NH3PbI3) thin-films and get pin-hole-free coverage with > 10µm grain size. X-Ray diffraction (XRD) measurement confirms that the annealed films are phase pure perovskite. The larger grain size leads to a significant increase in the effective recombination lifetime of charge carriers, with the best films showing lifetime of >50 µs, Solar cells fabricated with the large grain perovskite thin film shows significant improvement in short-circuit current density (Jsc) and open-circuit voltage (V oc).

Metal-oxide semiconductors are attractive as solar absorbers because they are abundant, stable, environmentally-safe, and low-cost. This work reports the electronic and optical properties of Zn2M030s (ZMO), a novel oxide-based solar absorber. ZMO has a direct bandgap of 1.9-2.1 eV, so it can efficiently absorb visible photons. As-deposited films are polycrystalline and unintentionally doped n-type with $\sim 10^{17}\mathrm{cm}^{-3}$ carriers. Electron mobility of $0.6-0.7 \mathrm{cm}^{2}/\mathrm{V}-\mathrm{s}$ is comparable to other thin-film absorbers like CdTe. The conduction and valence band edges in ZMO are 4.4 eV and 6.3 eV below vacuum level, respectively. Preliminary devices show photoconductivity with a 1. $5\mathrm{x}$ increase in current upon illumination

Integration of high quality germanium on silicon is prerequisite for fabricating high efficiency low-cost III-V solar cells on silicon. In this work, we present a method of monolithic integration of epitaxial germanium on crystalline silicon wafers where a single-step anneal in hydrogen ambient is used to recrystallize PECVD-deposited amorphous germanium into crystalline germanium. The re-crystallization progresses via liquid phase epitaxy by heating the germanium layer in inert ambient to just above its melting temperature of 937 °C. On cooling, the Ge layer gets oriented by the underlying Si (100) wafer, yielding an epitaxial Ge film on Si. SEM show that the films are continuous and void free. XRD pattern indicates highly oriented films. Symmetric Rocking curve measurements showed a thickness dependence with 1 µm thick film showing the lowest threading dislocation density (TDD) of the order of 108 cm−2. AFM measurements indicate surface measurements ~10 nm. Quality of epitaxy depends on thickness of film.

This work reports the very first systematic study on the physics of avalanche instability and SOA concerns in AlGaN/GaN HEMT using sub-μs pulse characterization, post stress degradation analysis, well calibrated TCAD simulations and failure analysis by SEM and TEM. Impact of electrical, as well as thermal effects on SOA boundary and avalanche instability are investigated. Trap assisted cumulative nature of degradation is studied in detail, which was discovered to be the root cause for avalanche instability in AlGaN/GaN HEMTs. Post failure SEM/TEM analysis reveal distinct failure modes in presence and absence of carrier trapping.

This experimental study reports ESD failure analysis of AlGaN/GaN HEMTs. Effect of MESA isolation, gate and parasitic MESA Schottky diode on ESD robustness is studied. Cause of snapback instability, multiple NDCs and transition from soft-to-hard failure are discussed. Unique leakage trends and cumulative nature of degradation are discovered. Post failure analysis reveals role of inverse piezoelectric effect, fringing electric field, contact resistivity, temperature and field induced contact metal migration in degradation of AlGaN/GaN HEMTs under ESD conditions.

A two-step growth method for high-quality epitaxial germanium films on silicon (100) is presented. During the first step a seed layer of crystalline Ge is deposited on silicon. The seed layer is a 50–100 nm germanium thin-film crystallized by melting amorphous germanium deposited using PECVD. During the second step germanium is epitaxially deposited on top of the re-crystallized seed layer using LPCVD at 600 °C. Scanning electron microscopy (SEM) reveal that the resulting germanium films are continuous, conformal and crack-free. X ray diffraction (XRD) pattern in the θ-2θ configuration shows a strong Bragg's peak corresponding to Ge (400). XRD rocking curve pattern in the vicinity of the Bragg peak for Ge(400) shows a sharp peak with a FWHM of only 512 arc sec, indicating highly-oriented Ge film. XRD phi-scan for (221), (111) and (110) planes of germanium show sharp peaks at 45° interval, proving the in-plane epitaxy of germanium which have a four-fold cubic symmetry. The epitaxial germanium films on silicon are useful for heterogeneous CMOS devices, photonics, IR photodetectors, and low-cost III-V photovoltaics.

Texture in images is used as a cue for various computer vision tasks like segmentation, classification and object detection. In this paper, we explore a variation of texture detection technique using oriented Gaussian derivative filters with multiple scales and orientations. After obtaining the filter responses at each pixel location, K-means clustering is used to determine regions with different textures. The modification to the algorithm is that, we are using only magnitude of the filter responses at each pixel location for determining regions with different textures through K-means clustering. This modification results in drastic changes in texture detection quality compared to original algorithm. In this work, we also propose a novel strategy to determine the optimal range of filter scale that yields semantically meaningful textures. The first and second order statistics of the filter response is used to determine the optimal range of filter scales. The proposed method is illustrated on synthetic images containing only textured regions with different textures.

Nanowires (NWs) are promising building blocks for flexible electronics and sensors and a number of approaches have been used to develop them. Among these, the vapor-liquid-solid (VLS) mechanism has been most appealing as it provides the electronic quality NWs at low fabrication cost. For these reasons, this method plays an important role in many applications including NWs based flexible electronics. The performance of NWs based electronics and sensors depend on their quality and the underlying growth mechanism, which thus far has not attracted much attention. In this paper, we present the physical chemistry model that explains the atomistic aspects of the growth mechanism of silicon nanowires. The mechanistic equations have been derived for various steps involved in a standard VLS growth process. The supersaturation under the steady state conditions has been calculated and utilized to estimate the growth rate of Si-NWs under different temperature conditions. The estimated values are found to be consistent with the reported measured values. The results from our study indicate that the Si-NW growth rate is directly related to the temperature. High-temperatures (~900°C) lead to longer Si-NWs (tens of microns length). This knowledge about growth conditions for Si-NW will enable better control of Si-NW dimensions and hence will have significant positive impact on using Si-NW in flexible electronics - especially the contact printing of NWs based electronic layers on flexible substrates.

We are reporting the 2DEG mobility and sheet carrier concentration behavior of AlGaN/GaN HEMT stacks that are grown on different kinds of transitions from AlN buffer to epi-GaN. The role of each transition in terms of interfacial roughness and epi-GaN crystallinity are discussed. Further, the observed material parameters are correlated with the HEMT 2DEG property. Enhancement in 2DEG mobility was observed from the stacks those exhibit low surface roughness irrespective of the scattering seen in HRXRD FWHMs. This results show that other than the dislocation reduction and crack free film, the low surface roughness is essential for a better confinement of 2DEG and an optimum carrier concentration.

The Micro and Nano Characterization Facility (MNCF) located in Centre for Nano Science and Engineering (CeNSE) building on the Indian Institute of Science (IISc, Bangalore, INDIA) campus is a centralized facility for characterization of Micro and Nano Devices. The National Nanofabrication Centre (NNFC) which is 10,000 sq. ft. of clean room housing state of the art equipment for nanofabrication is located in the same building. The MNCF is 5000 sq. ft precision controlled environment facility housing 4 distinct laboratories for Electrical, Mechanical, Optical and Material Characterization. The MNCF is a national facility with plethora of high end equipment spanning over multiple disciplines of nano science and engineering rarely found under a single roof. The prime funding agencies to establish this facility are NPMAS-DRDO and MCIT. MNCF is a fully-staffed, user research facility at IISc that offers researchers convenient and reasonably priced access to a wide- range of state-of-the-art analytical instrumentation and services. The Mission of this facility is to support research and educational objectives of CeNSE, IISc and offer state of the art characterization facilities and services to nanoscience academia (thorugh INUP), related industries and National Laboratories. The CeNSE witnesses multidesciplinary research activities in nano- scale electronics, nano technologies, smart materials, micro and nanoelectromechanical systems (NEMS, MEMS), bio- electronic interfaces and integrated small-scale systems. Various characterization techniques available in MNCF are RF Characterization, AC/DC Characterization, AC/DC Characterization at different temperatures in electrical characterization lab. Mechanical characterization of MNCF is equipped with techniques like laser dopplar vibrometry, Stroboscopic Video Microscopy, White Light Interferometry, Non-destructive acoustic Investigations, Atomic Force Microscopy, Tensile and compressive strength measurements. Material Characterization Lab houses SEM, FIB, Nano particle analysis techniques and optical characterization lab facilitates STM, PL, RAMAN Spectroscopy, Solar Cell Characterization and quantum efficiency measurements. XPS and XRD facilities are scheduled for commissioning soon. This paper discusses various challenges involved in establishing the infrastructure of the facility, financial and technical operations, staffing, access control and maintenance of user discipline during its' 24/7 operations.

We present the evolution of the state-of-the-art multiuser, open access, Nanofab facility at the Indian Institute of Science Bangalore. The process behind the design and execution of the cleanroom will be described. The challenges faced in moving the equipments from the old facility and the issues in coordinating installation of new equipments and utility hookup will be presented. The methodology used to manage the facility, challenges in staffing to run the facility and balancing the time sharing between internal and external users will be discussed.

In this study, in-situ wafer curvature measurements were used to monitor the evolution of film stress during MOCVD growth of GaN and AlGaN. These studies were carried out using a multi- beam optical stress sensor (MOSS) incorporated onto a custom-designed vertical cold-wall MOCVD reactor. The MOSS system provides real-time information on growth rate and changes in substrate curvature which are related to film stress via a modified version of Stoney's equation. Post-growth structural characterization including atomic force microscopy, X-ray diffraction and transmission electron microscopy was used to correlate measured changes in film stress to film morphology evolution.