S Rajasingam
University of Bristol
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Publication
Featured researches published by S Rajasingam.
Applied Physics Letters | 2003
Martin Kuball; S Rajasingam; Andrei Sarua; M.J. Uren; T. Martin; Bt Hughes; Keith P. Hilton; R.S. Balmer
The temperature distribution in multifinger high-power AlGaN/GaN heterostructure field-effect transistors grown on SiC substrates was studied. Micro-Raman spectroscopy was used to measure channel temperature with 1 μm spatial resolution, not possible using infrared techniques. Thermal resistance values were determined for four different device layouts with varying number of fingers, finger width, and spacing. The experimental thermal resistance was in fair agreement to that predicted by three-dimensional finite difference heat dissipation simulations. Uncertainties in thermal properties of this device system made simulation less reliable than experiment.
IEEE Electron Device Letters | 2004
S Rajasingam; James W Pomeroy; Martin Kuball; M.J. Uren; T. Martin; Dc Herbert; Keith P. Hilton; R.S. Balmer
Temperature profiles in the source/drain (S/D) opening of a single finger AlGaN-GaN heterostructure field-effect transistor were studied at increasing S/D voltages by micro-Raman spectroscopy with <1 /spl mu/m spatial resolution. These profiles imply high field regions near the gate edge of length /spl sim/0.4 /spl mu/m for S/D voltages between 45 and 75 V. Electric field strengths of /spl sim/1.2 and /spl sim/1.9 MV/cm are estimated for 45 and 75 V S/D voltage. The experimental results are in excellent agreement with 2-D Monte Carlo simulations.
Journal of Applied Physics | 2002
Lianghong Liu; Bin Liu; James H. Edgar; S Rajasingam; M. Kuball
The stress distribution in bulk AlN crystals seeded on 6H–SiC was theoretically modeled and also determined experimentally from Raman peak positions. The full width at half maximum of the AlN Raman peaks showed the crystal quality improved as its thickness increased. The theoretical frequency shifts of the E1 (transverse optical) mode calculated from model-predicted stress were in good agreement with experimental values taken along the edges of crystal samples. The stress was linearly distributed along the depth of the samples, and changed from compressive at the growing surface to tensile at the interface between AlN and SiC for thickness range of several hundred micrometers. Large tensile stresses, up to 0.6 GPa, were detected in the AlN at the interface. The effects of growth temperature and sample thickness were investigated. It is predicted that the AlN on 6H–SiC must be at least 2 mm thick to prevent it from cracking while cooling down the sample from a growth temperature of 2000 °C.
MRS Proceedings | 2003
Andrei Sarua; S Rajasingam; Martin Kuball; N. Garro; O Sancho; A. Cros; A. Cantarero; D Olguin; B Liu; D Zhuang; James H. Edgar
Raman scattering and photoluminescence (PL) spectroscopy with sub-bandgap excitation has been applied to explore tracing of common impurities (in particular of oxygen) in AlN. Bulk AlN crystals grown by the high temperature sublimation method were studied. PL bands have been observed at around 375 nm and at 560–660 nm and have been attributed to oxygen and to nitrogen vacancy/aluminium excess defects, respectively. The 375 nm UV PL band was found to shift with oxygen concentration. Micro-Raman spectra of the bulk AlN samples were measured in different polarisations. Besides normal Raman modes of AlN the presence of additional vibrational modes was detected. The modes were discussed and tentatively attributed to oxygen and silicon local vibrational modes (LVMs) in AlN.
Journal of Applied Physics | 2003
S Rajasingam; Andrei Sarua; M. Kuball; A Cherodian; Mervyn J Miles; C M Younes; B Yavich; Wang N Wang; N. Grandjean
The effect of high-temperature annealing on stress in AlxGa1-xN in different ambients and at different temperatures was studied using ultraviolet micro-Raman spectroscopy. Low (x=0.08) and high (x=0.31 and x=0.34) composition AlGaN, grown by metalorganic chemical vapor deposition (MOCVD) and molecular-beam epitaxy (MBE), were compared. Compositional and morphological changes were monitored using Auger electron spectroscopy (AES) and atomic force microscopy (AFM), respectively. The Raman results demonstrate that all samples exhibit maximum stress changes in the compressive direction when annealed in an air ambient. AES confirms this to be due to higher oxygen incorporation after annealing in the air ambient, and shows higher oxygen incorporation in the vicinity of cracks and defects. MOCVD and MBE samples of a similar composition were found to reach the same biaxial stress, despite differences in initial stress and growth temperature. Relaxation of a parabolic intercrack stress profile to homogeneous stress was observed with annealing in all ambients for cracked samples. AFM results on cracked samples show an increase in width of the primary cracks along the directions, and the formation of secondary cracks along the directions
MRS Proceedings | 2002
M. Kuball; S Rajasingam; Andrei Sarua; M.J. Uren; T. Martin; R.S. Balmer; K. P. Hilton
We report on the in-situ measurement of temperature, i.e., self-heating effects, in multi-finger AlGaN/GaN HFETs grown on SiC substrates. Optical micro-spectroscopy was used to measure temperature with 1m spatial resolution. Thermal resistance (temperature rise per W/mm) was measured as a function of device pitch and gate finger width. There is significant thermal cross talk in multi-finger AlGaN/GaN HFETs and this needs to be seriously considered for device performance and ultimately device reliability. A comparison with theoretical modeling is presented. Uncertainties in modeling parameters currently make modeling less reliable than experimental temperature assessment of devices.
Journal of Crystal Growth | 2002
James H. Edgar; Lianghong Liu; Bin Liu; D Zhuang; J. Chaudhuri; M. Kuball; S Rajasingam
Physica Status Solidi (a) | 2005
M. Kuball; James W Pomeroy; S Rajasingam; Andrei Sarua; M.J. Uren; T. Martin; Alfred Lell; Volker Härle
Physica Status Solidi (c) | 2003
Andrei Sarua; S Rajasingam; Martin Kuball; Cm Younes; B Yavich; Wang N. Wang
Wiley - VCH Verlag GmbH | 2002
Andrei Sarua; S Rajasingam; Martin Kuball; Cm Younes; B Yavich; Wang N. Wang