R. Pachipulusu
University of South Carolina
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Featured researches published by R. Pachipulusu.
Applied Physics Letters | 2002
J. P. Zhang; A. Chitnis; Vinod Adivarahan; S. Wu; R. Pachipulusu; M. Shatalov; Grigory Simin; J. Yang; M. Asif Khan
We report on AlGaN multiple-quantum-well (MQW)-based deep ultraviolet light-emitting diodes over sapphire with peak emission at 278 nm. A new buffer layer growth process was used to reduce the number of defects and hence the nonradiative recombination. The improved material quality and carrier confinement resulted in pulsed powers as high as 3 mW at 278 nm and a significantly reduced deep-level-assisted long-wavelength emission.
Applied Physics Letters | 2002
A. Chitnis; J. Sun; R. Pachipulusu; S. Wu; Mikhail Gaevski; Vinod Adivarahan; J. P. Zhang; M. Asif Khan; Andrei Sarua; Martin Kuball
We present a detailed high-pump-current study of self-heating effects in ultraviolet light-emitting diodes (LEDs) grown on sapphire. For deep ultraviolet LEDs on sapphire, our results establish self-heating to be a primary cause of premature power saturation under dc pumping. Even the flip-chip packaged devices undergo a steady-state temperature rise to about 70 °C at a dc pump current of only 50 mA (at 8 V) resulting in a significant decrease in LED output. Temperature rise values estimated from peak emission wavelength shifts and from micro-Raman mapping of the active devices were in good agreement.
Applied Physics Letters | 2003
A. Chitnis; J. P. Zhang; Vinod Adivarahan; M. Shatalov; S. Wu; R. Pachipulusu; M. Asif Khan
We report on AlGaN multiple-quantum-well light-emitting diodes over sapphire with peak emission at 325 nm. A pulsed-atomic-layer-epitaxy growth process was used to improve the material quality of the AlN buffer and the AlN/AlGaN strain-relief layers for reducing the nonradiative recombination. In addition, a modified device epilayer structure was used to improve the carrier confinement and the hole injection. A 40% improvement of external quantum efficiency is obtained, resulting in record high optical powers of 10.2 mW at a pulsed pump current of 1 A.
Applied Physics Letters | 2002
Vinod Adivarahan; S. Wu; A. Chitnis; R. Pachipulusu; M. Shatalov; J. P. Zhang; M. Asif Khan; G. Tamulaitis; A. Sereika; I. Yilmaz; M. S. Shur; R. Gaska
We report on AlGaN single-quantum-well light-emitting diodes (LEDs) on sapphire with peak emission at 285 nm. A study is presented to identify the key material parameters controlling the device quantum efficiency. At room temperature, for a 200 μm×200 μm square geometry mesa type device, we obtain a power as high as 0.25 mW for 650 mA pulsed pumping. The LEDs show significantly higher output powers at temperatures below 100 K.
Japanese Journal of Applied Physics | 2002
Vinod Adivarahan; Jianping Zhang; A. Chitnis; Wu Shuai; Jie Sun; R. Pachipulusu; M. Shatalov; Muhammad Asif Khan
Using low defect density n+-Al0.4Ga0.6N buffer layers we fabricated AlGaN p-n junction light emitting diodes over sapphire substrates with peak emission at 285 nm. Powers as high as 0.15 mW were measured at 400 mA pulse pumping.
Applied Physics Letters | 2003
M. Shatalov; A. Chitnis; R. Pachipulusu; J. P. Zhang; Vinod Adivarahan; S. Wu; Grigory Simin; M. Asif Khan; G. Tamulaitis; A. Sereika; I. Yilmaz; M. S. Shur; R. Gaska
We present a study on the time evolution of the electroluminescence (EL) spectra of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) under pulsed current pumping. The EL spectra peaks at 285 nm and 330 nm are found to result from recombination involving band-to-band and free carriers to deep acceptor level transitions. The 330 nm long-wavelength transitions to deep acceptor levels in the p-AlGaN layer as well as the nonradiative processes significantly influence the LED internal quantum efficiency.
IEEE Electron Device Letters | 2002
M. Shatalov; Grigory Simin; Jianping Zhang; Vinod Adivarahan; Alexei Koudymov; R. Pachipulusu; M. Asif Khan
A novel GaN/AlGaN p-channel inverted heterostructure junction field-effect transistor (HJFET) with a n/sup +/-type gate is proposed and demonstrated. A new superlattice aided strain compensation techniques was used for fabricating high quality GaN/AlGaN p-n junction. The p-channel HJFET gate leakage current was below 10 nA, and the threshold voltage was 8 V, which is close to that of typical n-channel HFETs. This new HJFET device opens up a way for fabricating nitride based complimentary integrated circuits.
Japanese Journal of Applied Physics | 2002
M. Shatalov; Grigory Simin; Vinod Adivarahan; A. Chitnis; S. Wu; R. Pachipulusu; Kirill Simin; J. P. Zhang; Jin Wei Yang; Muhammad Asif Khan
We report on a study of current crowding in AlInGaN multiple quantum well based deep ultra-violet light emitting diodes. For lateral geometry devices on sapphire substrates, our study concludes that the thickness and doping level of the high Al-content buffer and the cladding n-AlGaN layers is a key contributor to the lateral resistance and hence current crowding at the mesa edges. The inhomogeneous pumping of large area devices results in increased differential resistance causing a pronounced localized overheating. This degrades the device performance under high dc current operation. We also show stripe geometry to be a better choice for high power deep ultra-violet light emitting diodes on sapphire.
Applied Physics Letters | 2002
A. Chitnis; R. Pachipulusu; M. Shatalov; E. Kuokstis; J. P. Zhang; Vinod Adivarahan; S. Wu; Grigory Simin; M. Asif Khan
We present a study of the electrical and optical characteristics of 285 nm emission deep ultraviolet light-emitting diodes (LED) at temperatures from 10 to 300 K. At low bias, our data show the tunneling carrier transport to be the dominant conduction mechanism. The room-temperature performance is shown to be limited mostly by poor electron confinement in the active region and a pronounced deep level assisted recombination but not by the hole injection into the active region. At temperatures below 100 K, the electroluminescence peak intensity increases by more than one order of magnitude indicating that with a proper device design and improved material quality, milliwatt power 285 nm LED are viable.
Japanese Journal of Applied Physics | 2002
A. Chitnis; Vinod Adivarahan; M. Shatalov; Jianping Zhang; Michael Gaevski; Wu Shuai; R. Pachipulusu; J. Sun; Kirill Simin; Grigory Simin; Jinwei Yang; Muhammad Asif Khan
We report on sub-milliwatt operation of ultra-violet light emitting diodes on sapphire with a peak emission wavelength of 315 nm having quaternary AlInGaN multiple quantum well (MQW) active region. A significant increase in the device-emitted power was achieved using an interdigitated finger geometry design, which reduces the differential resistance and eliminates current crowding. The flip-chip mounting of the diced chips with sapphire substrates improves thermal management and increases the light extraction efficiency from the device active area. For 315 nm emission an optical power as high as 0.23 mW was obtained with 800 mA pulsed pumping current.