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Polyakov, A. Y.,Jang, Lee-Woon,Smirnov, N. B.,Govorkov, A. V.,Kozhukhova, E. A.,Yugova, T. G.,Reznik, V. Y.,Pearton, S. J.,Baik, Kwang Hyeon,Hwang, Sung-Min,Jung, Sukkoo,Lee, In-Hwan American Institute of Physics 2011 JOURNAL OF APPLIED PHYSICS - Vol.110 No.9
<P>The electrical properties, presence of deep electron and hole traps and photoluminescence spectra were measured for undoped a-GaN films grown by metal-organic chemical vapor deposition (MOCVD) in a two-stage process using a high V/III ratio at the first stage and low V/III ratio at the second stage. Growth was performed on r-sapphire substrates with a high temperature GaN nucleation layer. The films showed a full width at half maximum of 450-470 arcseconds for the (11-20) x-ray rocking curve with little anisotropy with respect to the sample rotation around the growth direction. The stacking fault (SF) density determined by selective etching was similar to 5 x 10(4) cm(-1). The residual donor concentration was 10(14)-10(15) cm(-3), with a very low density (2.5 x 10(13) cm(-3)) of electron traps located at E-c - 0.6 eV, which are believed to be one of the major non-radiative recombination centers in nonpolar GaN. Consequently, the films showed a high intensity of bandedge luminescence with negligible contribution from defect bands associated with SFs. In contrast to previously studied nonpolar GaN films, the a-GaN layers showed a high concentration of gallium-vacancy-related acceptors near E-v + 1 eV and a strong yellow luminescence band, both indicating that growth conditions were effectively N-rich. a-AlGaN/GaN heterojunctions with thin heavily Si doped AlGaN barriers made on a-GaN substrates showed two-dimensional electron gas (2DEG) concentrations of 1.2 x 10(13) cm(-3), with 2DEG mobility of 80 cm(2)/Vs. Capacitance-voltage profiling of Schottky diodes on these HJs suggest that the 2DEG is fully depleted by the built-in voltage of the Schottky diode. (C)2011 American Institute of Physics. [doi: 10.1063/1.3658026]</P>
Electrical properties and deep traps spectra of a-plane GaN films grown on r-plane sapphire
Polyakov, A.Y.,Smirnov, N.B.,Govorkov, A.V.,Markov, A.V.,Sun, Q.,Zhang, Y.,Yerino, C.D.,Ko, T.S.,Lee, I.H.,Han, J. Elsevier 2010 Materials science & engineering. B, Advanced funct Vol.166 No.3
Electrical properties, deep traps spectra and luminescence spectra were studied for two undoped a-plane GaN (a-GaN) films grown on r-plane sapphire using metalorganic chemical vapor deposition and differing by structural perfection. For sample A, the a-GaN film was directly deposited on AlN buffer. A two-step growth scheme was implemented for sample B, including an initial islanding growth stage and a subsequent enhanced lateral growth. Preliminary detailed X-ray analysis showed that the stacking faults density was 8x10<SUP>5</SUP>cm<SUP>-1</SUP> for sample A and 1.7x10<SUP>5</SUP>cm<SUP>-1</SUP> for sample B. Electrical properties of a-GaN films were largely determined by deep traps with a level near E<SUB>c</SUB> -0.6eV, with other prominent traps having the activation energy of 0.25eV. The Fermi level was pinned by the E<SUB>c</SUB> -0.6eV deep traps for sample A, but shifted to the vicinity of the shallower 0.25eV traps for sample B, most likely due to the reduced density of the 0.6eV traps. This decrease of deep traps density is accompanied by a very pronounced improvement in the overall luminescence intensity. A correlation of the observed improvement in deep traps spectra and luminescence efficiency with the improved crystalline quality of the films is discussed.
Deep traps and enhanced photoluminescence efficiency in nonpolar a-GaN/InGaN quantum well structures
Polyakov, A. Y.,Jang, Lee-Woon,Jo, Dong-Seob,Lee, In-Hwan,Smirnov, N. B.,Govorkov, A. V.,Kozhukhova, E. A.,Hyeon Baik, Kwang,Hwang, Sung-Min American Institute of Physics 2012 JOURNAL OF APPLIED PHYSICS - Vol.111 No.3
Quantum Barrier Growth Temperature Affects Deep Traps Spectra of InGaN Blue Light Emitting Diodes
Polyakov, A. Y.,Smirnov, N. B.,Shchemerov, I. V.,Yakimov, E. B.,Yakimov, E. E.,Kim, Kyu Cheol,Lee, In-Hwan The Electrochemical Society 2018 ECS journal of solid state science and technology Vol.7 No.5
<P>Electroluminescence (EL) efficiency, deep electron and hole traps spectra, microcathodoluminescence (MCL), electron beam induced current (EBIC) imaging, and MCL spectra were studied for blue GaN/InGaN multi-quantum-well (MQW) light emitting diodes differing by the temperature at which the GaN barriers of the MQW active region were grown. It was found that increasing the growth temperature from 850 to 920 degrees C very strongly suppressed the formation of deep electron traps with level at E-c-1 eV in the GaN barriers and of the hole traps with levels at E-v+0.7eV in InGaN QWs. The suppression of the formation of the E-c-1 eV electron trap, a known prominent nonradiative recombination center in n-GaN, improved the carrier injection efficiency into the InGaN QWs and increased the external quantum efficiency by about 9%. EBIC and MCL imaging showed that the density of threading dislocations and terminating them V-pits was relatively low and similar for both studied growth temperatures, close to 10(8) cm(-2) . The cross-sectional dimensions of the V-pits were measurably higher for increased growth temperature. However, the rather low dislocation density and rather high dimensions of the V-pits were believed to result in minor contribution of these defects to the observed EL efficiency changes. (C) 2018 The Electrochemical Society.</P>
Deep hole traps in undoped n-GaN films grown by hydride vapor phase epitaxy
Lee, In-Hwan,Polyakov, A. Y.,Smirnov, N. B.,Govorkov, A. V.,Usikov, A. S.,Helava, H.,Makarov, Yu. N.,Pearton, S. J. American Institute of Physics 2014 JOURNAL OF APPLIED PHYSICS - Vol.115 No.22
Deep hole traps were studied in bulk free-standing GaN crystals and in thinner (10-20 mu m) GaN films prepared by hydride vapor phase epitaxy (HVPE) on sapphire. Six hole traps in different combinations were detected in these crystals, H1 (activation energy 0.92-0.94 eV), H2 (0.55 eV), H3 (0.65-0.7 eV), H4 (0.85-0.9 eV), H5 (1.1-1.2 eV), and H6 (0.95-1.05 eV). The dominant traps in all samples were the H5 and H6 traps that were attributed, respectively, to gallium vacancy complexes with oxygen (V-Ga-O) and substitutional carbon related centers. We associate the H5 hole traps with the red luminescence bands, the H4 hole traps with the green luminescence bands, and the H6 hole traps with the yellow luminescence bands often observed in HVPE GaN. These attributions are based on the low energy thresholds of the deep traps optical excitation spectra and the depth of the respective trap levels. (C) 2014 AIP Publishing LLC.
Alpha particle detection with GaN Schottky diodes
Polyakov, A. Y.,Smirnov, N. B.,Govorkov, A. V.,Markov, A. V.,Kozhukhova, E. A.,Gazizov, I. M.,Kolin, N. G.,Merkurisov, D. I.,Boiko, V. M.,Korulin, A. V.,Zalyetin, V. M.,Pearton, S. J.,Lee, I.-H.,Dabir American Institute of Physics 2009 JOURNAL OF APPLIED PHYSICS - Vol.106 No.10
Kruglov, S.L.,Polyakov, A.V.,Shutova, D.I.,Topeshkin, D.A. The Korea Institute of Applied Superconductivity a 2020 한국초전도저온공학회논문지 Vol.22 No.4
Here we present the comparative experimental study of the stability of the superconducting state in 4 mm YBCO tapes with copper lamination against local heat disturbances at 77 K. The samples are either directly cooled by immersing a bare YBCO tape into a liquid nitrogen pool or operate in nearly-adiabatic conditions when the tape is covered by a 0.6 mm layer of Kapton insulation. Main quench characteristics, i.e. minimum quench energies (MQEs) and normal zone propagation (NZP) velocities for both samples are measured and compared. Minimum NZP currents are determined by a low ohmic resistor technique eligible for obtaining V - I curves with a negative differential resistance. The region of transport currents satisfying the stationary stability criterion is found for the different cooling conditions. Finally, we use the critical temperature margin as a universal scaling parameter to compare the MQEs obtained in this work for YBCO tapes at 77 K with those taken from literature for low-temperature superconductors in vacuum at 4.2 K, as well as for MgB2 wires cooled with a cryocooler down to 20 K.