Dhrubes Biswas

High-resolution X-ray diffraction analysis of AlxGa1−xN/InxGa1−xN/GaN on sapphire multilayer structures: Theoretical, simulations, and experimental observations

The work presents a comparative study on the effects of In incorporation in the channel layer of AlGaN/GaN type-II heterostructures grown on c-plane sapphire by Plasma Assisted Molecular Beam Epitaxy. The structural characterizations of these samples were performed by High-Resolution X-Ray Diffraction (HRXRD), X-ray Reflectivity (XRR), Field Emission Scanning Electron Microscopy, and High Resolution Transmission Electron Microscopy. The two-dimensional electron gas in the AlGaN/GaN and AlGaN/InGaN interface was analyzed by electrochemical capacitance voltage and compared with theoretical results based on self-consistent solution of Schordinger–Poisson equations. The carrier profile shows enhanced confinement in InGaN channel (1.4393 × 1013 cm−2 compared to 1.096 × 1013 cm−2 in GaN). On the basis of HRXRD measurements, the stress-strain of the layers was examined. The c- and a-lattice parameters of the epilayers as well as in-plane and out-of plane strains were determined from the ω-2θ for symmetric scan a...

Role of ultra thin pseudomorphic InP layer to improve the high-k dielectric/GaAs interface in realizing metal-oxide-semiconductor capacitor

In this article, we report GaAs metal-oxide-semiconductor (MOS) capacitors with a metal organic chemical vapor deposited ultrathin (1.5 nm) pseudomorphic InP interface passivation layer (IPL) and a thin (5 nm) ZrO2 high-k dielectric. Reduction of the surface states on InP passivated GaAs surfaces was observed from the photoluminescence study. The x-ray photoelectron spectra confirmed the dramatic reduction of GaAs native oxides (Ga-O and As-O) from the interface of ZrO2 and p-GaAs, implying that the Fermi level at the high-k/GaAs interface can be unpinned with good interface quality. As a result, very low values of interface trap density (1.1 × 1011 cm−2 eV−1) and hysteresis (8.21 mV) were observed. The same was done for directly deposited ZrO2 on GaAs surface to understand the efficacy of InP interface passivation layer on GaAs MOS devices. A systematic capacitance-voltage and current density-voltage studies were performed on bothAl/ZrO2/InP/p-GaAs and Al/ZrO2/p-GaAs structures. It was found that inserti...

Dependence of structural and electrical properties of AlGaN/GaN HEMT on Si(111) on buffer growth conditions by MBE

Multiple AlGaN/GaN heterostructure has been grown on Si(111) substrate by molecular beam epitaxy with different buffer growth conditions. Its influence on physical and electrical properties of two-dimensional electron gas (2DEG) has been investigated. Correlation between growth temperature variation in AlN intermediate layer and thick GaN buffer layer on 2DEG transport property has been observed. Besides the variation in growth temperatures, dissimilar partial doping in the thick GaN buffer has also been studied. Impact of different silicon substrate doping has been examined to inspect the electrical properties of high electron mobility transistors (HEMT). DC characteristics of large area fabricated HEMT have been compared in terms of current, transconductance, and linearity for power amplifier applications.

Gold-Free Fully Cu-Metallized InGaP/GaAs Heterojunction Bipolar Transistor

A gold-free, fully Cu-metallized InGaP/GaAs heterojunction bipolar transistor using platinum as the diffusion barrier has been successfully fabricated. The HBT uses Pd/Ge and Pt/Ti/Pt/Cu for n-type and p+-type ohmic contacts, respectively, and Ti/Pt/Cu for interconnect metals with platinum as the diffusion barrier. The Ti/Pt/Cu structure was stable during annealing up to 350°C judging from the X-ray diffraction (XRD) data and sheet resistance. A current-accelerated stress test was conducted on the device with a current density JC=140 kA/cm2 for 24 h, and the current gain showed no degradation. The devices were also thermally annealed at 250°C for 24 h and showed little change. We have successfully demonstrated that an Au-free, fully Cu-metallized HBT can be realized using Pt as the diffusion barrier and Pd/Ge and Pt/Ti/Pt/Cu as the ohmic contacts.

Effect of band offset on carrier transport and infrared detection in InP quantum dots/Si nano-heterojunction grown by metalorganic chemical vapor deposition technique

Epitaxy of III-V semiconductors on Si gets recent interest for next generation system on heterogeneous chip on wafer. The understanding of band offset is thus necessary for describing the charge transport phenomenon in these heterojunctions. In this work, x-ray photoemission spectroscopy has been used to determine the band offsets in a heterojunction made of InP quantum dots on Si. The valence and conduction band offset was found to be 0.12 eV and 0.35 eV, respectively, with a type-II band lineup. Deviation from theoretical prediction and previously published reports on quasi similar systems have been found and analyzed on the basis of the effect of strain, surface energy, shift in the electrostatic dipole and charge transfer at the interface. The carrier transport mechanisms along with different device parameters in the heterojunction have been studied for a temperature range of 180–300 K. This heterojunction is found to behave as an efficient infrared photodetector with an ON/OFF ratio of 21 at a reverse bias of 2 V. The corresponding rise and decay time was found to be 132 ms and 147 ms, respectively.

Health care social media: expectations of users in a developing country.

Background Affordability, acceptability, accommodation, availability, and accessibility are the five most important dimensions of access to health services. Seventy two percent of the Indian population lives in semi-urban and rural areas. The strong mismatched ratio of hospitals to patients, rising costs of health care, rapidly changing demographics, increasing population, and heightened demands in pricing for technological health care usage in emerging economies necessitate a unique health delivery solution model using social media. A greater disease burden lies in the health care delivery in developing country like India. This is due to the lack of health care infrastructure in the majority of semi-urban and rural regions. New techniques need to be introduced in these regions to overcome these issues. In the present scenario, people use social media from business, automobiles, arts, book marking, cooking, entertainment, and general networking. Developed and advanced countries like the United States have developed their communication system for many years now. They have already established social media in a number of domains including health care. Similar practice incidences can be used to provide a new dimension to health care in the semi-urban regions of India. Objective This paper describes an extended study of a previous empirical study on the expectations of social media users for health care. The paper discusses what the users of social media expect from a health care social media site. Methods Multiple regression analysis was used to determine the significance of the affect of four factors (privacy, immediacy, usability, and communication) on the usage of health care social media. Privacy, immediacy, usability, and communication were the independent variables and health care social media was the dependant variable. Results There were 103 respondents who used the online questionnaire tool to generate their responses. The results from the multiple regression analysis using SPSS 20 showed that the model is acceptable, with P=.011, which is statistically significant on a P<.05 level. The observed F value (2.082) in ANOVA was less than the given value in the F table (2.61), which allowed us to accept the hypothesis that the independent variables influence the dependant variable. The users of social media in India expect that they can best utilize social media through emergency service information. They want to be able to learn the operations of the social media site quickly and expect to know about health camps and insurance collaborations. However, people like to become friends with people with similar interests based on their interests identified. Conclusions Health care social media requires intelligent implementation in developing economies. It needs to cater to the expectations of the users. The people in India, especially those in urban and semi-urban regions, are very interested in accepting the system.

Self-assembled In0.22Ga0.78As quantum dots grown on metamorphic GaAs/Ge/SixGe1-x/Si substrate

Self-assembled In0.22Ga0.78As quantum dots (QDs) grown on Si substrate with Ge∕SiGe as buffer layer grown by metal organic vapor phase epitaxy were investigated. Transmission electron microscopy and atomic force microscopy images were used to observe the size and space distribution of the In0.22Ga0.78As QDs grown on the GaAs∕Ge∕GeSi∕Si layer structure. The influence of the growth temperature on the QDs size and density distribution was investigated. For QDs grown at 450°C, the density of the In0.22Ga0.78As dots was estimated to be 1×1011cm−2 and the In0.22Ga0.78As QDs thickness was 5 ML (monolayer) thick.

Effects of threading dislocations on drain current dispersion and slow transients in unpassivated AlGaN/GaN/Si heterostructure field-effect transistors

Current transient analysis combined with response to pulsed bias drives have been used to explore the possibilities of threading dislocations affecting the current dispersion characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs). A growth strategy is developed to modulate the dislocation density among the heterostructures grown on silicon by plasma-assisted molecular-beam epitaxy. Slow pulsed I-V measurements show severe compressions and appear to be significantly dependent on the threading dislocation density. By analyzing the corresponding slow detrapping process, a deep-level trap with emission time constant in the order of seconds was identified as the cause. Among the specimens, both in the epilayers and at the surface, the number of dislocations was found to have a notable influence on the spatial distribution of deep-level trap density. The observations confirm that the commonly observed degraded frequency performance among AlGaN/GaN HFETs in the form of DC-radio frequency dispersions can at least partly be correlated with threading dislocation density.

Comprehensive strain and band gap analysis of PA-MBE grown AlGaN/GaN heterostructures on sapphire with ultra thin buffer

In this work, cluster tool (CT) Plasma Assisted Molecular Beam Epitaxy (PA-MBE) grown AlGaN/GaN heterostructure on c-plane (0 0 0 1) sapphire (Al2O3) were investigated by High Resolution X-ray Diffraction (HRXRD), Room Temperature Raman Spectroscopy (RTRS), and Room Temperature Photoluminescence (RTPL). The effects of strain and doping on GaN and AlGaN layers were investigated thoroughly. The out-of-plane (‘c’) and in-plane (‘a’) lattice parameters were measured from RTRS analysis and as well as reciprocal space mapping (RSM) from HRXRD scan of (002) and (105) plane. The in-plane (out-of plane) strain of the samples were found to be −2.5 × 10−3(1 × 10−3), and −1.7 × 10−3(2 × 10−3) in GaN layer and 5.1 × 10−3 (−3.3 × 10−3), and 8.8 × 10−3(−1.3 × 10−3) in AlGaN layer, respectively. In addition, the band structures of AlGaN/GaN interface were estimated by both theoretical (based on elastic theory) and experimental observations of the RTPL spectrum.

Comparison of trap characteristics between AlGaN/GaN and AlGaN/InGaN/GaN heterostructure by frequency dependent conductance measurement

Frequency dependent conductance measurement is carried out to observe the trapping effect in AlGaN/InGaN/GaN double heterostructure and compared that with conventional AlGaN/GaN single heterostructure. It is found that the AlGaN/InGaN/GaN diode structure does not show any trapping effect, whereas single heterostructure AlGaN/GaN diode suffers from two kinds of trap energy states in near depletion to higher negative voltage bias region. This conductance behaviour of AlGaN/InGaN/GaN heterostructure is owing to more Fermi energy level shift from trap energy states at AlGaN/InGaN junction compare to single AlGaN/GaN heterostructure and eliminates the trapping effects. Analysis yielded interface trap energy state in AlGaN/GaN is to be with time constant of (33.8–76.5) μs and trap density of (2.38–0.656)  × 1012  eV−1 cm−2 in −3.2 to −4.8 V bias region, whereas for AlGaN/InGaN/GaN structure no interface energy states are found and the extracted surface trap energy concentrations and time constants are (5.87–4.3...