Sudhakar Bharatan

Nitrogen incorporation and optical studies of GaAsSbN∕GaAs single quantum well heterostructures

In this work, the effects of N incorporation on the optical properties of GaAsSbN∕GaAs single quantum wells (SQWs) have been investigated using temperature, excitation, and magnetic dependencies of photoluminescence (PL) characteristics. These layers were grown in an elemental solid source molecular beam epitaxy system with a rf plasma N source. The N concentrations in the range of 0.5%–2.5% were investigated in this study. The SQW with N∼0.5% exhibits a behavior similar to that in an intermediate regime where the contributions from the localized states in the band gap are dominant. The temperature and excitation dependencies of the PL characteristics indicate that for the N concentration of 0.9% and above, the alloy behavior is analogous to that of a regular alloy and the changes in optical properties are only marginal. The conduction band effective mass (meff) values computed from the magnetophotoluminescence spectra using a variational formalism and the band anticrossing model are in good agreement and...

Annealing effects on the temperature dependence of photoluminescence characteristics of GaAsSbN single-quantum wells

In this work we investigate the effects of ex situ annealing in N ambient and in situ annealing in As ambient on the temperature dependence of photoluminescence (PL) spectral characteristics of GaAsSbN∕GaAs single-quantum-well heterostructures. The focus of this work is on three representative nitride samples grown by molecular-beam epitaxy. The widths of the quantum wells (QWs) varied from 8 to 9 nm and the concentrations of nitrogen and antimony as determined from high-resolution x-ray diffraction and secondary-ion-mass spectroscopy were in the range of 0.8%–1.4% and 26%–33%, respectively. One sample was ex situ annealed in N ambient at 700 °C for 10 min. Two other samples were in situ annealed in As ambient at 650 and 700 °C, respectively, also for 10 min. Excitonic transitions in the QWs exhibit the well-known “S-curve” behavior in the temperature dependence of the PL peak energy. In addition, the variation of the full width at half maximum with temperature exhibits an “inverted S-curve” behavior. The...

Molecular beam epitaxial growth and characterization of InSb1 − xNx on GaAs for long wavelength infrared applications

Recent research progress and findings in InSbN have attracted great attention due to its use in long wavelength infrared applications. A large bandgap reduction in InSb resulting from high N incorporation with minimal crystal defects is challenging due to relatively small atomic size of N. Hence optimization of growth conditions plays an important role in the growth of high-quality InSbN epilayers for device purposes. In this paper, we report on the correlation of structural, vibrational, electrical, and optical properties of molecular beam epitaxially grown InSbN epilayers grown on GaAs substrates, as a function of varying growth temperatures. Two dimensional growths of InSb and InSbN were confirmed from dynamic reflection high energy electron diffraction patterns and growth parameters were optimized. High crystalline quality of the epilayers is attested to by a low full width at half maximum of 200 arcsec from high resolution x-ray diffraction (HRXRD) scans and by the high intensity and well-resolved In...

The effects of annealing on the structural, optical, and vibrational properties of lattice-matched GaAsSbN/GaAs grown by molecular beam epitaxy

The structural, optical, and vibrational properties of a GaAsSbN epilayer lattice matched to GaAs with a band gap of 1eV have been investigated using a variety of characterization techniques. These layers have potential applications in GaAs based tandem solar cells that utilize the near infrared region of the solar spectrum. The epilayers were grown in an elemental solid source molecular beam epitaxy system with a rf plasma nitrogen source. The Sb and N compositions of the nearly lattice-matched layers are 6.8% and 2.6%, respectively, as determined by high resolution x-ray diffraction and secondary ion mass spectroscopy (SIMS) analysis. The high crystalline quality of the layers is attested by the presence of well resolved Pendellosung fringes on a triple axis (004) x-ray scan and dynamical truncation rods observed on the corresponding (004) reciprocal space map. The effects of in situ annealing in As ambient and ex situ annealing in N ambient on the low temperature photoluminescence (PL) characteristics ...

Study of molecular beam epitaxially grown InGaAsSbN/GaSb single quantum wells

InGaAsSbN quantum wells (QWs) have been investigated for potential light-emitting devices in the midinfrared region. This paper presents the growth and properties of molecular beam epitaxially grown InGaAsSbN single QWs using a variety of characterization techniques. A 10 K photoluminescence emission at 2.27 μm, with a lowest full width at half maxima of 5 meV which shifted to 2.30 μm on in situ annealing, has been observed. The presence of well resolved Pendellosung fringes in high resolution x-ray diffraction and sharp abrupt interfaces in the corresponding transmission electron microscope (TEM) images are indications of the high quality of these QWs. Raman spectroscopy studies reveal the presence of well resolved Raman peaks with higher intensity, along with the presence of sharp second order modes of GaSb, further attesting to the high quality of the QW structures grown. Investigation of the annealed samples using Z-contrast scanning TEM images reveals atomic interdiffusion between the QW and surround...

Optical studies of molecular beam epitaxy grown GaAsSbN∕GaAs single quantum well structures

In this work, the authors present a systematic study on the variation of the structural and the optical properties of GaAsSbN∕GaAs single quantum wells (SQWs) as a function of nitrogen concentration. These SQW layers were grown by the solid source molecular beam epitaxial technique. A maximum reduction of 328meV in the photoluminescence (PL) peak energy of GaAsSbN was observed with respect to the reference GaAsSb QW. 8K and RT PL peak energies of 0.774eV (FWHM of ∼25meV) and 0.729eV (FWHM of ∼67meV) (FWHM denotes full width at half maximum) corresponding to the emission wavelengths of 1.6 and 1.7μm, respectively, have been achieved for a GaAsSbN SQW of N∼1.4%. The pronounced S-curve behavior of the PL spectra at low temperatures is a signature of exciton localization, which is found to decrease from 16to9meV with increasing N concentration of 0.9%–2.5%. The diamagnetic shift of 13meV observed in the magnetophotoluminescence spectra of the nitride sample with N∼1.4% is smaller in comparison to the value of...

Growth and Properties of Lattice Matched GaAsSbN Epilayer on GaAs for Solar Cell Applications

The growth and properties of GaAsSbN single quantum wells (SQWs) are investigated in this work. The heterostructures were grown on GaAs substrates in an elemental solid source molecular beam epitaxy (MBE) system assisted with a RF plasma nitrogen source. A systematic study has been carried out to determine the influence of various growth conditions, such as the growth temperature and the source shutter-opening sequence, on the quality of the grown layers and the incorporation of N and Sb. The effects of ex situ and in situ annealing under As overpressure on the optical properties of the layers have also been investigated. Substrate temperature in the range of 450-470 C was found to be optimum. Simultaneous opening of the source shutters was found to yield sharper QW interfaces. N and Sb incorporations were found to depend strongly upon substrate temperatures and source shutter opening sequences. A significant increase in PL intensity with a narrowing of PL line shape and blue shift in emission energy were observed on annealing the GaAsSbN/GaAs SQW, with in situ annealing under As overpressure providing better results, compared to ex situ annealing.

Annealing studies of heteroepitaxial InSbN on GaAs grown by molecular beam epitaxy for long-wavelength infrared detectors

We report the effect of annealing on the structural, vibrational, electrical, and optical properties of heteropepitaxially grown InSbN epilayers on GaAs substrate by molecular beam epitaxy for long-wavelength infrared detector applications. As-grown epilayers exhibited high N incorporation in the both substitutional and interstitial sites, with N induced defects as evidenced from high resolution x-ray diffraction, secondary ion mass spectroscopy, and room temperature (RT) micro-Raman studies. The as-grown optical band gap was observed at 0.132 eV (∼9.4 μm) and the epilayer exhibited high background carrier concentration at ∼1018 cm−3 range with corresponding mobility of ∼103 cm2/Vs. Ex situ and in situ annealing at 430 °C though led to the loss of N but improved InSb quality due to effective annihilation of N related defects and other lattice defects attested to enhanced InSb LO phonon modes in the corresponding Raman spectra. Further, annealing resulted in the optical absorption edge red shifting to 0.12...

Parallel multi level three phase voltage source inverters using three limb coupled inductors

This paper presents Parallel Voltage Sourced Inverters to improve the efficiency in high current applications. Generally when two inverters are paralleled, they have unmatching currents flowing between them and to reduce that, we go for inductors. These inductors reduces the circulating currents flowing between the inverters and these currents are produced by common mode voltage differences between the output terminals of the two inverters. Standard Discontinuous PWM techniques when applied to parallel connected inverters with three separate inductors naturally produce high frequency common mode voltage differences. So an Interleaved Discontinuous PWM scheme along with sinusoidal reference is proposed to eliminate these voltage differences in parallel connected inverters. By using this technique, a three phase parallel inverter can be operated using a single three limb coupled inductor instead of three separate inductors. And this significantly reduces the high frequency differential current ripple and improves the efficiency of the system. Analysis are done for common mode voltage differences and for the common mode circulating currents using the above mentioned two techniques and it is shown that the common mode circulating currents (Icm) and the common mode voltage differences (Vcm) are reduced by using Interleaved Discontinuous PWM technique. This is fed to an Induction motor load and the output currents are sinusoidal with less current ripples. Computer simulations are done using PSIM for Standard Discontinuous PWM and for Interleaved Discontinuous PWM and their respective waveforms are compared and it is shown that Interleaved Discontinuous PWM is efficient.

Influence of Growth Parameters and Annealing on Properties of MBE Grown GaAsSbN SQWs

In this paper we report the growth of GaAsSbN/GaAs single quantum well (SQW) heterostructures by molecular beam epitaxy (MBE) and their properties. A systematic study has been carried out to determine the effect of growth conditions, such as the source shutter opening sequence and substrate temperature, on the structural and optical properties of the layers. The substrate temperatures in the range of 450-470 C were found to be optimal. Simultaneous opening of the source shutters (SS) resulted in N incorporation almost independent of substrate temperature and Sb incorporation higher at lower substrate temperatures. The effects of ex-situ annealing in nitrogen ambient and in-situ annealing under As ovepressure on the optical properties of the layers have also been investigated. A significant increase in photoluminescence (PL) intensity with reduced full width at half maxima (FWHM) in conjunction with a blue shift in the emission energy was observed on 10 annealing the samples. In in-situ annealed samples, the PL line shapes were more symmetric and the temperature dependence of the PL peak energy indicated significant decrease in the exciton localization energy as exhibited by a less pronounced S-shaped curve. The inverted S-shaped curve observed in the temperature dependence of PL FWHM is also discussed. 1.61 micrometer emission with FWHM of 25 meV at 20K has been obtained in in-situ annealed GaAsSbN/GaAs SQW grown at 470 C by SS.