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Effects of laterally overgrown n-GaN thickness on defect and deep level concentrations
Polyakov, A. Y.,Smirnov, N. B.,Govorkov, A. V.,Markov, A. V.,Yakimov, E. B.,Vergeles, P. S.,Lee, In-Hwan,Lee, Cheul Ro,Pearton, S. J. American Vacuum Society 2008 JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B - Vol.26 No.3
Polyakov, A.Y.,Jeon, D.W.,Govorkov, A.V.,Smirnov, N.B.,Sokolov, V.N.,Kozhukhova, E.A.,Yakimov, E.B.,Lee, I.H. Elsevier Sequoia 2013 Journal of alloys and compounds Vol.554 No.-
Nanopillar structures were prepared by dry etching of maskless epitaxial lateral overgrowth (MELO) GaN samples using a mask of Ni nanoparticles formed upon annealing thin Ni films deposited on top of SiO<SUB>2</SUB>/GaN. Under our experimental conditions the average nanopillars dimensions were close to 170nm, with the nanopillars density close to 10<SUP>9</SUP>cm<SUP>-2</SUP>. The nanopillars formation was random and not correlated with the threading dislocation density in MELO GaN, as evidenced by comparing the size and density of nanopillars in the wing and seed regions of MELO GaN differing in dislocation density by an order of magnitude. After dry etching the luminescent intensity of nanopillars became actually lower than the intensity from the unetched matrix due to the impact of defects introduced in the sidewalls during nanopillars formation. The intensity greatly increased, together with a decrease in the leakage current of Schottky diodes, after rapid thermal annealing of nanopillar structures at 900<SUP>o</SUP>C and further increased after additional etching in KOH solution. These changes are attributed to annealing of radiation defects introduced by dry etching and further removal of the damaged region by KOH etching. The results suggest that, in nanopillar structures produced by dry etching, some increase of internal quantum efficiency alongside improvement of light extraction efficiency are responsible for the observed luminescence intensity changes.
Electrical and luminescent properties and deep traps spectra of N-polar GaN films
Polyakov, A.Y.,Smirnov, N.B.,Govorkov, A.V.,Sun, Q.,Zhang, Y.,Cho, Y.S.,Lee, I.H.,Han, J. Elsevier 2010 Materials science and engineering B. Advanced Func Vol.166 No.1
Electrical and luminescent properties of N-polar undoped GaN films grown using low temperature GaN buffers on on-axis and miscut sapphire and on-axis AlN buffers are compared to the properties of Ga-polar films grown on low temperature GaN buffers. It is shown that the concentration of residual donors increases by about an order of magnitude for on-axis N-polar growth and by two orders of magnitude for off-axis growth compared to Ga-polar films. On-axis films for both Ga-polar and N-polar polarities show the presence of n<SUP>+</SUP> interfacial layers greatly influencing the apparent electron concentration and mobility deduced from capacitance-voltage C-V measurements. These interfacial layers are much less prominent in the miscut N-polar films. Growth on N-polar greatly increases the concentration of electron traps with activation energy of 0.9eV possibly related to Ga-interstitials.
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
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 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.
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>