Sanjay Kumar Jana

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...

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.

Evolution and analysis of nitride surface and interfaces by statistical techniques: A correlation with RHEED through kinetic roughening

AbstractIn-situ RHEED and ex-situ AFM characterizations have been employed to investigate transformations of surface topography with the thickness of PAMBE grown AlGaN and InGaN on GaN. The ternary alloys have been grown with identical growth-front roughness as confirmed by XRR and RHEED observations. The spottier RHEED has been observed with increased thickness of the InGaN as opposed to streakier behavior of AlGaN. We have noticed incremental nature of RMS roughness, skewness and kurtosis of InGaN surface compared to GaN or AlGaN from AFM as evident by final spotty RHEED for InGaN. However, the analyzed fractal dimension is lower for InGaN as opposed to AlGaN (DfAlGaN>DfGaN>DfInGaN). From the kinetic roughening perspective of adatoms, the experimental evidences lead to the high correlation between binding energy of the cluster atoms (EbAlN>EbGaN>EbInN) and the modified DDA growth model with dissociation and evaporation to confirm the efficacy of the study. The initial streaky and spotty RHEED of InGaN and AlGaN, respectively, can be attributed to their Eb that causes smoothing and roughening of the GaN surface due to adatoms surface mobility behavior. Therefore, the fractal description reveals the fact during formation of nitride hetero-interface while other AFM results describe the top surface.

Reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure

The reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure is investigated by current-voltage measurement in temperature range from 298 K to 423 K. The Higher electric field across the AlGaN barrier layer of AlGaN/InGaN/GaN double heterostructure due to higher polarization charge is found to be responsible for strong Fowler-Nordheim (FN) tunnelling in the electric field higher than 3.66 MV/cm. For electric field less than 3.56 MV/cm, the reverse bias leakage current is also found to follow the trap assisted Frenkel-Poole (FP) emission in low negative bias region. Analysis of reverse FP emission yielded the barrier height of trap energy level of 0.34 eV with respect to Fermi level.

Growth and Characterization of Self-Assembled InAs Quantum Dots on Si (100) for Monolithic Integration by MBE

High-density, self-assembled InAs quantum dots were grown on Si (100) substrates without any buffer layer by solidsource molecular beam epitaxy, following Volmer-Weber growth mode. The reconstructed (2 × 1) Si surface and growth transition from two-dimensional (2-D) to three-dimensional (3-D) were observed by in situ reflection high energy electron diffraction (RHEED). Surface morphology, shape, and chemical composition of these dots were investigated through scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive X-ray (EDX) analysis, respectively. The dot densities were found to be 4 × 1014 m-2 for the samples grown at 380°C with a V/III ratio of 13. Optical properties of InAs quantum dots were observed by photoluminescence (PL) spectroscopy both at room and low temperatures. Room-temperature PL (RTPL) peak was observed at 1.016 eV (~1220 nm) and 1.05 eV (~1180.95 nm) presumably originating from the ground state (GS) and excited state (ES), respectively. The 3-D carrier confinement rather than the strain is found to have the dominating role, which causes blueshift of 0.662 eV. The theoretical PL spectrum of dots was obtained by using Gaussian size distribution where the dots were assumed to be cubic in shape. The low-temperature PL (LTPL) peak position was observed at 1.16 eV. The experimentally observed variation of the bandgap with temperature was in close proximity with both Varshini and Vina model. Raman spectroscopic analysis confirms the carrier confinement in the InAs quantum dots by the downward frequency shift at 216.57 and 237.96 cm-1 corresponding to the transverse optical (TO) and longitudinal optical (LO) phonon modes, respectively. The qualitative strain analysis was investigated by high-resolution X-ray diffractions (HRXRD) where the strain state was observed from the reciprocal space mapping in symmetric (004), as well as grazing exit and grazing incidence mode at (224) plane.

An Alternative X-ray Diffraction Analysis for Comprehensive Determination of Structural Properties in Compositionally Graded Strained AlGaN Epilayers

In this letter, a standard deviation based optimization technique has been applied on High Resolution X-ray Diffraction symmetric and asymmetric scan results to accurately determine the Aluminum molar fraction and lattice relaxation of Molecular Beam Epitaxy grown compositionally graded Aluminum Gallium Nitride (AlGaN)/Aluminum Nitride/Gallium Nitride (GaN) heterostructures. Mathews–Blakeslee critical thickness model has been applied in an alternative way to determine the partially relaxed AlGaN epilayer thicknesses. The coupling coefficient determination has been presented in a different perspective involving sample tilt method by offset between the asymmetric planes of GaN and AlGaN. Sample tilt is further increased to determine mosaic tilt ranging between 0.01° and 0.1°.Graphical Abstract

Strain Effects on Band Structure of Wurtzite InGaN/GaN Quantum Well on Si Substrate

Heteroepitaxial crystal growth of InGaN/GaN Quantum Well (QW) on Si substrate is necessary for the development of visible and ultraviolet (UV) Light Emitting Diodes (LEDs). In this work we accumulate the efforts that have been made to focus the analysis of strain effects on the band structure of GaN quantum well systems on Si substrate due to its potential application. We limit this investigation on the effect of pseudomorphic GaN quantum wells on Si substrate grown at different GaN planes. It is observed that the deformation potential constants and anisotropic strain separate Heavy Hole (HH) and Light Hole (LH) band and eventually making LH the top most bands.

Photoluminescence study based prediction on visible luminescence from n-Zno/p-GaAs heterojunction

Semiconductor heterojunction is an alternate approach in device fabrication where p-n homojunction could not be made possible due to nonavailability of a particular type of doping with required stability and carrier concentration as a fall out of self-compensation arising out of the non-stoichiometry. ZnO is one such material. In the present investigation a heterojunction of n-ZnO with p-GaAs has been made to investigate the light emission properties of the heterojunction. From the room temperature photoluminescence spectrum of the heterojunction, efforts have been made to predict the colour of the light emitted from it based on spectral response. It is observed that n-ZnO/p-GaAs heterojunction could emit in the purplish red region though the band gap of the constituent materials do not have the requisite band gap for such an emission. The color temperature is found to be less than 1000 K.

Fabrication of n-ZnO/p-GaAs Heterojunction and Prediction of Its Luminescence Based on Photoluminescence Study

Self-compensation arising out of non-stoichiometry makes ZnO as-grown n-type. Nonavailability of stable p-type doping with required carrier concentration limits the formation of homojunction of ZnO. Fabrication of semiconductor heterojunction is thus an alternative approach in device fabrication. In the present study n-ZnO has been grown on p-GaAs substrates using MOCVD technique. The colour of the light which is supposed to be emitted from the said heterojunction has been predicted to be purplish red from the room temperature photoluminescence study. The corresponding colour temperature is found to be less than 1,000 K. Efforts have been made to explain the prediction on the basis of band diagram.