Microstructural and Compositional Characterisation of Electronic Materials

Microstructural and compositional characterisation of electronic materials in support of the development of GaAs, GaN and GaSb based multilayer device structures is described. Electron microscopy techniques employing nanometer and sub-nanometer scale imaging capability of structure and chemistry have been widely used to characterise various aspects of electronic and optoelectronic device structures such as InGaAs quantum dots, InGaAs pseudomorphic (pHEMT) and metamorphic (mHEMT) layers and the ohmic metallisation of GaAs and GaN high electron mobility transistors, nichrome thin film resistors, GaN heteroepitaxy on sapphire and silicon substrates, as well as InAs and GaN nanowires. They also established convergent beam electron diffraction techniques for determination of lattice distortions in III-V compound semiconductors, EBSD for crystalline misorientation studies of GaN epilayers and high-angle annular dark field techniques coupled with digital image analysis for the mapping of composition and strain in the nanometric layered structures. Also, in-situ SEM experiments were performed on ohmic metallisation of pHEMT device structures. The established electron microscopy expertise for electronic materials with demonstrated examples is presented

Silidstate lighting

Lighting is an integral part of human life. Globally, about 15% of the generated power is spent for the lighting applications. In this context, the recent advances in 'solid state lighting (SSL)' are very important to promote energy savings and reduce green house gas emissions. This article presents an overview of advancements in SSL with focus on GaN light emitting diode (LED), its impact on energy savings and relevance to India

The role of surface roughness on dislocation bending and stress evolution in low mobility AlGaN films during growth

The bending and interaction of threading dislocations are essential to reduce their density for applications involving III-nitrides. Bending of dislocation lines also relaxes the compressive growth stress that is essential to prevent cracking on cooling down due to tensile thermal expansion mismatch stress while growing on Si substrates. It is shown in this work that surface roughness plays a key role in dislocation bending. Dislocations only bend and relax compressive stresses when the lines intersect a smooth surface. These films then crack. In rough films, dislocation lines which terminate at the bottom of the valleys remain straight. Compressive stresses are not relaxed and the films are relatively crack-free. The reasons for this difference are discussed in this work along with the implications on simultaneously meeting the requirements of films being smooth, crack free and having low defect density for device applications. Published by AIP Publishing

Optical polarization anisotropy of a-plane GaN/AlGaN multiple quantum well structures grown on r-plane sapphire substrates

A series of nonpolar a-plane GaN/AlGaN multiple quantum well structures of varying quantum well width have been studied by polarization resolved photoluminescence and photoluminescence excitation spectroscopy at low temperature. The photoluminescence spectra from all the structures show two features that are observed to blueshift with reducing well width. The lower energy feature is associated with the recombination of carriers in regions of the wells intersected by basal-plane stacking faults, while the higher energy line is attributed to localized exciton recombination involving only the quantum wells. Using excitation spectroscopy with polarized light, we were able to resolve exciton features associated with both the |Y〉 and |Z〉 valence sub-bands. The observed polarization dependence of the transitions is consistent with a modification to the valence band-edge states due to anisotropic biaxial compressive strain in the quantum well. We were also able to determine the exciton binding energies directly from the photoluminescence excitation spectra, which were found to increase from 36 to 76 meV as the quantum well width reduced from 60 to 35 Å

Optical properties of GaN/AlGaN quantum wells grown on nonpolar substrates

In this paper we report on the optical properties of a series of GaN/AlGaN multiple quantum well structures grown on a-plane (110) GaN, which had been deposited on r-plane (102) sapphire substrates, compared to a reference GaN template of the same orientation. The low temperature photoluminescence spectrum of the template layer is dominated by two emission bands, which we attribute to recombination involving excitons in the bulk of the layer and electrons and holes trapped at basal-plane stacking faults, designated X1 and X2, respectively. The photoluminescence spectra from the quantum well structures show similar emission bands except that both X1 and X2 shift to higher energy with decreasing quantum well thickness. The shift to higher energy is due to the effects of quantum confinement on carriers trapped at the stacking faults that intersect the quantum wells, as well as those excitons that are localized within the quantum wells. This assignment is based partly on excitation spectroscopy that reveals exciton transitions associated with electrons from the n = 1 and n = 2 quantum well confined states

Microstructural and Compositional Characterisation of Electronic Materials

Microstructural and compositional characterisation of electronic materials in support of the development of GaAs, GaN and GaSb based multilayer device structures is described. Electron microscopy techniques employing nanometer and sub-nanometer scale imaging capability of structure and chemistry have been widely used to characterise various aspects of electronic and optoelectronic device structures such as InGaAs quantum dots, InGaAs pseudomorphic (pHEMT) and metamorphic (mHEMT) layers and the ohmic metallisation of GaAs and GaN high electron mobility transistors, nichrome thin film resistors, GaN heteroepitaxy on sapphire and silicon substrates, as well as InAs and GaN nanowires. They also established convergent beam electron diffraction techniques for determination of lattice distortions in III-V compound semiconductors, EBSD for crystalline misorientation studies of GaN epilayers and high-angle annular dark field techniques coupled with digital image analysis for the mapping of composition and strain in the nanometric layered structures. Also, in-situ SEM experiments were performed on ohmic metallisation of pHEMT device structures. The established electron microscopy expertise for electronic materials with demonstrated examples is presented