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ABSTRACT: We report the results from detailed optical spectroscopy from MOCVD grown GaN/AlGaN multiple quantum wells (MQWs), as opposed
to most previous studies where MBE was employed by means of photoluminescence (PL) technique. In this paper we will present
theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in
GaN/AlGaN MQWs affect the emission peak energy, PL line shape, as well as the emission line width. Theoretically estimated
fields in this work are consistent with experimental data. Transition energy of the heavy hole and electron ground state Ee-hh
in GaN/AlGaN MQWs were calculated and it is found that it stays in good agreement with the experimental data.
Opto-Electronics Review 04/2012; 17(4):293-299. · 0.97 Impact Factor
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ABSTRACT: Porous fluorescent (f-) SiC containing N and B was produced by anodic oxidation. The average crystal size can be controlled by adding K2S2O8 as an oxidant to dilute HF solution during the anodic process. With the reduction of the average crystal size of the porous crystal, the PL peak wavelength becomes shorter, and its intensity increases. Such behavior can be explained by the quantum size effect of donor-acceptor-pair recombination. Finally, we confirmed the superior fluorescent property of blue light emission in porous f-SiC crystal (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 06/2010; 7(10):2459 - 2462.
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B. Monemar,
P. P. Paskov,
G. Pozina,
C. Hemmingsson,
J. P. Bergman,
H. Amano, I. Akasaki,
S. Figge,
D. Hommel,
T. Paskova,
A. Usui
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ABSTRACT: Photoluminescence spectra of c -plane Mg doped GaN samples grown by MOVPE on bulk GaN templates reveal previously unknown properties, like the presence of several Mg-related acceptors. The use of unstrained samples allows a study of both bound exciton (BE) and donor-acceptor pair (DAP) spectra. Two main acceptors A1 and A2 are observed strongly in BE spectra as well as in DAP spectra, they have similar binding energies, i.e. about 220 meV. The common assignment of the deeper blue PL emission at 2.8–3.0 eV to a deep donor-shallow acceptor transition is questioned, and discussed in connection with the compensation problem in p-GaN. It seems like the Fermi level in p-GaN is controlled by a set of Mg-related acceptors at energies 0.2–0.6 eV from the valence band top. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 05/2010; 7(7‐8):1850 - 1852.
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ABSTRACT: The threshold voltage (Vth) of normally off-mode AlGaN/GaN junction heterostructure field-effect transistors with a p-type GaN gate can be successfully controlled by inserting a SiNx insulator between the p-GaN and a Ni/Au electrode. The Vth can be controlled from +1 V to above +8 V. Moreover, the gate leakage current of transistors decreases and their gate voltage at which gate current steeply increases becomes higher. The mechanism of the threshold voltage change is analyzed by the equivalent circuit model. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 05/2010; 7(7‐8):1980 - 1982.
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ABSTRACT: We report on a new technology for growing low-dislocation-density AlGaN in which a Mg-doped AlN underlying layer is used. By growing AlxGa1-xN on AlN:Mg with AlN molar fractions x of 0.3, 0.5 and 0.7, the density of misfit dislocations is much reduced compared with that in the case of growing AlGaN on undoped AlN. In addition, the surface becomes atomically flat. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 05/2010; 7(7‐8):2101 - 2103.
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ABSTRACT: Microstructural analysis was carried out to clarify the compositional dependence of the generation of dislocations in AlxGa1-xN on an underlying AlN layer grown by metalorganic vapor phase epitaxy. When the film thickness is less than 1.5 μm, the threading dislocation density (TDD) increases with decreasing AlN molar fraction. However, when the film thickness exceeds 1.5 μm, TDD becomes maximum at x around 0.5. The growth of AlGaN on a grooved AlN template is effective in reducing TDD for all AlN molar fractions. TDD in AlGaN, which is close to binaries such as GaN and AlN, is a few 107 cm–2, while for the intermediate composition with x around 0.5, TDD is still at mid 108 cm–2. The activation energy of Mg in AlGaN is found to show a strong Mg concentration dependence with a negative one-third power law in Al0.25Ga0.75N and Al0.5Ga0.5N as well as in GaN. Overdoping of Mg causes an increase in the activation energy for every composition; from this, the optimum Mg concentration for realizing the highest hole concentration can be deduced. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 10/2009; 6(12):2621 - 2625.
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B Monemar,
P P Paskov,
G Pozina,
C Hemmingsson,
J P Bergman,
T Kawashima,
H Amano, I Akasaki,
T Paskova,
S Figge,
D Hommel,
A Usui
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ABSTRACT: The optical signatures of Mg-related acceptors in GaN have been revisited in samples specifically grown on bulk GaN templates, to avoid strain broadening of the optical spectra. Bound-exciton spectra can be studied in these samples for Mg concentrations up to [Mg] approximately 2 x 10(19) cm(-3). Contrary to previous work it is found that instabilities in the photoluminescence spectra are not due to unstable shallow donors, but to unstable Mg-related acceptors. Our data show that there are two Mg-related acceptors simultaneously present: the regular (stable) substitutional Mg acceptor, and a complex acceptor which is unstable in p-GaN.
Physical Review Letters 07/2009; 102(23):235501. · 7.37 Impact Factor
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ABSTRACT: InGaN growth has been performed on nearly lattice-matched ZnO (0001) substrates by metalorganic vapor phase epitaxy at relatively low temperatures below 600 °C. The grown layers were confirmed to be single crystalline by X-ray diffraction and reflection high energy electron diffraction analyses. By annealing the ZnO substrate under oxygen atmosphere prior to the growth, scratches due to polishing were completely removed, as observed by atomic force microscopy. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 05/2008; 5(9):3023 - 3025.
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ABSTRACT: Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on GaN substrates produced by the halide vapor phase technique demonstrate metastability of the near-band-gap photoluminescence (PL). The acceptor bound exciton (ABE) line possibly related to the C acceptor vanishes in as-grown samples within a few minutes under UV laser illumination. Annealing activates the more stable Mg acceptors and passivates C acceptors. Consequently, only the ABE line related to Mg is dominant in PL spectra for the annealed samples. The temporal changes in PL are permanent at low temperatures; however, they can be recovered after heating to 100 K or higher.
Applied Physics Letters 04/2008; 92(15):151904-151904-3. · 3.84 Impact Factor
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T. Kawashima,
T. Hayakawa,
M. Hayashi,
T. Nagai,
D. Iida,
A. Miura,
Y. Kasamatsu,
M. Iwaya,
S. Kamiyama,
H. Amano, I. Akasaki
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ABSTRACT: GaInN-based light-emitting diodes having m-plane heterostructures and multi quantum well (MQW) were fabricated on m-plane SiC by sidewall-seeded epitaxial lateral overgrowth. Defects such as stacking faults or dislocations strongly affect the emission wavelength and efficiency of MQW. Control of the V/III ratio during growth is found to be very effective for growing GaN from only one sidewall, by which we can reduce the area of the high-defect-density region. A light emitting diode (LED) with high internal quantum efficiency and an almost single peak LED was successfully fabricated. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2008; 5(6):2145 - 2147.
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Naofumi Kato,
S. Sato,
H. Sugimura,
T. Sumii,
N. Okada,
M. Imura,
M. Iwaya,
S. Kamiyama,
H. Amano, I. Akasaki,
H. Maruyama,
T. Takagi,
A. Bandoh
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ABSTRACT: The growth of thick AlGaN on a c-plane sapphire substrate covered with an AlN layer was conducted by high-temperature (HT-) metalorganic vapor phase epitaxy. The composition of AlGaN can be controlled by adjusting the growth temperature and V/III ratio. The use of an HT-AlN template extremely improves the crystalline quality of the thick AlGaN layer. In addition, the crystalline quality measured by X-ray diffraction and TEM showed that the FWHMs of X-ray rocking curves (XRCs) for both tilt and twist distributions and the dislocation density in AlGaN are almost the same as those in the underlying HT-AlN template for the Al composition from 0.2 to 1. A thick and crack-free Al0.89Ga0.11N layer with the FWHMs of XRCs for tilt and twist distributions as narrow as 177 arcsec and 457 arcsec, respectively, were successfully grown. An extremely smooth and atomic-size step with a RMS surface roughness as small as 0.35 nm over a 5 µm × 5 µm scan was observed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2008; 5(6):1559 - 1561.
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ABSTRACT: The normally-off-mode junction HFETs with a p-type GaN gate contact showing high drain current and an extremely large on/off ratio were successfully fabricated by MOVPE. The drain currents were found to be very sensitive to the surface of the u-AlGaN barrier exposed by RIE etching. A reproducible and stable high drain current was achieved using a very thin SiN passivation layer. The maximum drain current was 1.58×10–1 A/mm at VGS = 4 V, while the drain current at VGS = 0 V was as low as 1.45×10–8 A/mm. Therefore, an on/off ratio of 107 has been achieved. The sub-threshold swing was as small as 90 mV/dec.. The on resistance was 3.4 mΩcm2, the threshold voltage was +0.45 V, and the breakdown voltage was over 325 V. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2008; 5(6):1906 - 1909.
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physica status solidi (c) 03/2008; 5:1768-1770.
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International Symposium on Integrated Optoelectronic Devices “Photonics West 2008“; 02/2008
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ABSTRACT: Metastability of near band gap UV emissions in Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on thick GaN templates grown by halide vapor phase epitaxy has been studied by cathodoluminescence (CL). The CL spectrum changes its initial shape within a few minutes under electron irradiation. The acceptor bound exciton line vanishes while the emissions related to the stacking faults (SFs) of different geometry rise significantly. The increase of the defect-related luminescence is likely caused by recombination enhanced SF formation under electron irradiation. The CL spectrum transformation is permanent at low temperatures; however, the metastable process is reversible if samples are heated to room temperature.
Applied Physics Letters 11/2007; 91(22):221901-221901-3. · 3.84 Impact Factor
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ABSTRACT: We report results from optical spectroscopy such as photoluminescence (PL) and time resolved photo-luminescence (TRPL) techniques
from different well width MOCVD grown GaN/Al0.07Ga0.93N MQW samples. There is evidence of localization at low temperature in all samples. The decay time of all samples becomes
non-exponential when the detection energy is increased with respect to the peak of the emission. Localization of carriers
(excitons) is demonstrated by the “S-shape” dependences of the PL peak energies on the temperature. The time-resolved PL spectra
of the 3-nm well multi quantum wells reveal that the spectral peak position shifts toward lower energies as the decay time
increases and becomes red-shifted at longer decay times. There is a gradient in the PL decay time across the emission peak
profile, so that the PL process at low temperatures is a free electron-localized hole transition.
Opto-Electronics Review 08/2007; 15(3):163-167. · 0.97 Impact Factor
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M. Imura,
N. Kato,
N. Okada,
K. Balakrishnan,
M. Iwaya,
S. Kamiyama,
H. Amano, I. Akasaki,
T. Noro,
T. Takagi,
A. Bandoh
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ABSTRACT: We report on the growth of Mg-doped AlxGa1–xN alloys and characterization of their optical properties. Under the low Mg concentration in the range 2×1017 to 1×1018 cm–3 of AlxGa1–xN, the activation energy of Mg acceptor became higher and higher with increase in the Al content of AlxGa1–xN. The experimental results for activation energies of Mg acceptor fitted quite well with the hydrogen atom like model. By the Haynes' rule, relationship between bound-excition energy and activation energy of Mg acceptor was investigated, which coincided with our experimental results. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 05/2007; 4(7):2502 - 2505.
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ABSTRACT: Low defect density a -plane GaN films were successfully grown by sidewall epitaxial lateral overgrowth (SELO) technology. Using this technology, a -plane GaN films with atomically flat surface were grown. The threading dislocation and stacking fault densities in the overgrown regions were lower than 106 cm−2 and 103 cm−1, respectively. We also fabricated and characterized a -plane-GaN-based LEDs using SELO technology. The light output power of a blue-green LED was shown to monotonically increase with decreasing of threading dislocation density. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 05/2007; 204(6):2005 - 2009. · 1.46 Impact Factor
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K. Iida,
H. Watanabe,
K. Takeda,
T. Nagai,
T. Sumii,
K. Nagamatsu,
T. Kawashima,
K. Balakrishnan,
M. Iwaya,
S. Kamiyama,
H. Amano, I. Akasaki,
A. Bandoh
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ABSTRACT: Crack-free and low-dislocation-density AlGaN was successfully grown on grooved AlN layer. UV light-emitting diodes (LEDs) with a peak wavelength of 330 nm on these AlGaN films were fabricated. The efficiency of the UV LEDs is found to be strongly affected by the crystalline quality of the underlying AlN. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 05/2007; 204(6):2000 - 2004. · 1.46 Impact Factor
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ABSTRACT: We succeeded in growing low-defect-density m -plane GaN on grooved m -plane GaN with SiO2 masks on the terrace region. By changing the V/III ratio, we were able to increase the growth rate of GaN on one sidewall, thereby achieving one-sidedwall lateral growth. Dislocations and stacking faults were decreased markedly over the whole area. The densities of dislocations and stacking faults were 1.3 × 107 (cm–2) and <2.6 × 104 (cm–1), respectively. The photoluminescence intensity was 213 times higher than that of an m -plane GaN template on an m -plane 4H-SiC substrate. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 04/2007; 244(6):1848 - 1852. · 1.32 Impact Factor
