Subhashis Das

Electrical trimming of ion-beam-sputtered polysilicon resistors by high current pulses

Phosphorus doped polysilicon resistors have been fabricated from microcrystalline silicon films which were deposited by ion beam sputtering using an argon ion beam of diameter 3 cm, energy 1 keV and current density 7mA/cm/sup 2/, with a deposition rate of 100-120 /spl Aring//min. The resistors, having a sheet resistance of 70 /spl Omega//square and a carrier concentration of 7.5/spl times/10/sup 19/ cm/sup -3/, were stressed with current pulses of width 10 /spl mu/s and duty cycle 0.6% for 5 min. There was a steady decrease of resistance with increasing pulse current density above a threshold value 5/spl times/10/sup 5/ A/cm/sup 2/. A maximum fall of 27% was observed for a 95 /spl mu/m long resistor. The current-voltage characteristics were also recorded during the trimming process. The trimming characteristics were simulated using a small-signal resistivity model of Lu et al. (1983). and the I-V characteristics by a large-bias conduction model. A close fitting of the experimental data with the theoretical values needed an adjustment of some grain boundary parameters for the different pulse current densities used for stressing. The nature of variation of the grain boundary parameters indicates that the rapid Joule heating of the grain boundaries due to current pulses passivates the grain boundary interfaces, at lower currents above the threshold, and then, at higher values of currents, causes zone melting and gradual recrystallization of the disordered boundary layers and subsequent dopant segregation. It confirms the mechanism suggested in the physical model of Kato et al. (1982). The role played by the field-enhanced diffusivity and electromigration of dopant ions, due to the high instantaneous temperature of the grain boundaries, has also been discussed. The pulse trimming technique is simple and does not cause damage to the adjacent components on a monolithic chip. >

Investigation of cross-hatch surface and study of anisotropic relaxation and dislocation on InGaAs on GaAs (001)

There exist discrepancies between reports on cross-hatch (CH) behaviour and its interaction with interfacial misfit dislocations in the literature. In this work, a thorough CH analysis has been presented by use of conventional and statistical analysis of AFM data. It has been shown that correlation between cross-hatch and misfit dislocation depends on the growth conditions and residual strain. Anisotropic relaxation and dislocations, composition and epitaxial tilt have been studied by HRXRD analysis. To illustrate these findings, molecular beam epitaxy (MBE) grown metamorphic InGaAs on GaAs (001) samples have been used. Reciprocal space mapping has been used to characterize the composition and relaxation while epilayer tilt and dislocation have been investigated by HRXRD rocking curve. A better understanding of CH pattern can enable us to minimize the surface roughness for metamorphic electronic devices and to fully utilize the quasi-periodic undulation in cross-hatch in applications, like ordered quantum dot growth.

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.

Highly Sensitive Acetone Sensor Based on Pd/AlGaN/GaN Resistive Device Grown by Plasma-Assisted Molecular Beam Epitaxy

Highly sensitive acetone sensing performance of Pd/AlGaN/GaN resistive devices in the temperature range of 100 °C–250 °C and in the detection range of 100–1000 ppm was reported. A plasma-assisted molecular beam epitaxy was used to grow the AlGaN/GaN heterostructure on Si (111) substrate. Structural characterization of the grown epilayers was performed through double-crystal X-ray diffraction whereas atomic force microscopy was used to obtain the roughness of the sensing surface. Resistive mode configuration of the sample was tested toward acetone in the detection range of 100–1000 ppm and in the temperature range of 100 °C–250 °C. The optimum temperature was found to be 150 °C with response magnitude ~95% for the acetone concentration of 1000 ppm. The sensor response time and recovery time were found to be in the range of ~18–44 s and ~25–109 s, respectively. The cross-sensitivity of the device with other interfering species such as butanone, benzene, toluene, and xylene attributed to good acetone selectivity of the devices. Acetone sensing as well as current transport of the Pd/AlGaN/GaN devices was illustrated with effect including Langmuir adsorption–desorption kinetics and Schottky barrier height between Pd/AlGaN interfaces.

Evolution of lateral V-defects on InGaN/GaN on Si(111) during PAMBE: the role of strain on defect kinetics

In this article, a unique correlation has been established between the defect kinetics of III-nitride adatoms and strain during plasma assisted molecular beam epitaxial (PAMBE) growth of InGaN/GaN heterostructures on silicon(111) for the first time. This association identifies the possible causes for the evolution of V-defects in nitrides. While cross-sectional observations using transmission electron microscopy (TEM) exhibit common vertical V-defects, a planar view of the heterostructures reveals novel lateral V-defects under a field emission scanning electron microscope (FESEM). Additionally, the density and size of both types of V-defects were found to become altered dramatically above the critical thickness of the InGaN. Asymmetric (105) reciprocal space mapping (RSM) is used to represent the gradual minimization of strain with the increase of InGaN thickness. The autocorrelation length, as obtained by power spectral density (PSD) analysis of atomic force microscopy (AFM) topography, quantifies coalescence of the pits and defects with the relaxation of InGaN on GaN. It is proposed that primary non-threading dislocation (TD) terminated mobile hexagonal pits with varying depth coalesced to form the lateral V-defects. The vertexes of these lateral V-defects on the surface are likely to be connected to TD emanating from the buried GaN. Strain relaxation also accounts for anisotropy in lateral InGaN growth, which consequently leads to alloy inhomogeneity of InGaN as detailed by energy dispersion spectroscopy (EDS). The defects may be used to texture the InGaN surface in situ for photonic applications.

Temperature dependent etching of Gallium Nitride layers grown by PA -MBE

Future of microwave, power and photonics industry is focused on GaN due to its extraordinary material properties such as wide and direct band gap, large thermal and chemical stability, high breakdown voltage, high saturation velocity. Formation of devices for these applications requires a material selective etching which is performed via wet-etching process. In this paper, temperature dependent etching properties of GaN have been revealed. Molten KOH has been employed as an etchant, to etch 2 μm MBE grown GaN layer on Silicon (111). To verify temperature dependence of GaN etching, etching has been performed at a fixed concentration and etching time. Optimum temperature to etch GaN completely has been determined from Arrhenius plot of etch rate vs temperature. Etch depth has been determined from AFM, whereas, morphology has been confirmed using SEM.