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S. G. Zhang,
X. W. Zhang,
Z. G. Yin,
J. X. Wang,
J. J. Dong,
Z. G. Wang,
S. Qu,
B. Cui, A. M. Wowchak,
A. M. Dabiran,
P. P. Chow
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ABSTRACT: The effects of the growth temperature and thickness of AlN layer on the electroluminescence (EL) performance of n-ZnO/AlN/p-GaN devices have been systematically investigated. It is found that the higher growth temperature of AlN layer (TAlN) may facilitate the improvement of EL performance of the device, which is attributed to that the crystalline quality of AlN layer improves with increasing growth temperatures TAlN. Besides the crystallinity of AlN layer, the thickness of AlN barrier layer plays an important role on the performance of the device. The thinner AlN layer is not enough to cover the whole surface of GaN, while the thicker AlN layer is unfavorable to the tunneling of carriers and many of electrons will be captured and recombined nonradiatively via the deep donors within the thick AlN layer. We have demonstrated that the AlN layer at the growth temperature of 700 °C with an optimized thickness of around 10 nm could effectively confine the injected carriers and suppress the formation of interfacial layer, thus, the EL performance of n-ZnO/AlN/p-GaN device could be significantly improved.
Journal of Applied Physics 05/2011; 109(9):093708-093708-6. · 2.17 Impact Factor
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J. B. You,
X. W. Zhang,
S. G. Zhang,
J. X. Wang,
Z. G. Yin,
H. R. Tan,
W. J. Zhang,
P. K. Chu,
B. Cui, A. M. Wowchak,
A. M. Dabiran,
P. P. Chow
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ABSTRACT: n-ZnO/p-GaN heterojunction light-emitting diodes with and without a sandwiched AlN layer were fabricated. The electroluminescence (EL) spectrum acquired from the n-ZnO/p-GaN displays broad emission at 650 nm originating from ZnO and weak emission at 440 nm from GaN, whereas the n-ZnO/AlN/p-GaN exhibits strong violet emission at 405 nm from ZnO without GaN emission. The EL intensity is greatly enhanced by inserting a thin AlN intermediate layer and it can be attributed to the suppressed formation of the GaOx interfacial layer and confinement effect rendered by the AlN potential barrier layer.
Applied Physics Letters 05/2010; 96(20):201102-201102-3. · 3.84 Impact Factor
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S. Taking,
A. Banerjee,
H. Zhou,
X. Li,
A.Z. Khokhar,
R. Oxland,
I. McGregor,
S. Bentley,
F. Rahman,
I. Thayne,
A.M. Dabiran, A.M. Wowchak,
B. Cui,
E. Wasige
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ABSTRACT: A simple method is reported for fabrication of AlN/GaN MOS-HEMTs. Ultra-thin Al<sub>2</sub>O<sub>3</sub>, which is formed using thermal oxidation of evaporated Al, was used for surface passivation and as a gate dielectric. Prior to formation of Al<sub>2</sub>O<sub>3</sub>, the Al protects the very sensitive AlN epilayer from exposure to processing liquid chemicals. Fabricated two-finger AlN/GaN MOS-HEMTs with 3 m gate length and 200 m gate width showed good gate control of drain currents up to a gate bias of 3 V and achieved a maximum drain current, I<sub>DSmax</sub>, of ~900~mA/mm. The peak extrinsic transconductance, G<sub>max</sub>, of the device is ~100~mS/mm at V<sub>DS</sub> = 8=V. Capacitance-voltage ( C - V ) characteristics of Al<sub>2</sub>O<sub>3</sub>/AlN/GaN circular test MOS structures were observed and measured. They exhibited no hysteresis, indicating the good quality of the thermally grown Al<sub>2</sub>O<sub>3</sub> for realising AlN/GaN MOS-HEMTs for high power and high frequency applications.
Electronics Letters 03/2010; · 0.96 Impact Factor
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ABSTRACT: Electrical and optical properties of AlGaN grown by molecular beam epitaxy were studied in the Al composition range 15%–45%. Undoped films were semi-insulating, with the Fermi level pinned near E <sub> c </sub>-0.6–0.7 eV . Si doping to (5–7)×10<sup>17</sup> cm <sup>-3</sup> rendered the 15% Al films conducting n-type, but a large portion of the donors were relatively deep (activation energy 95 meV), with a 0.15 eV barrier for capture of electrons giving rise to strong persistent photoconductivity (PPC) effects. The optical threshold of this effect was ∼1 eV . Doping with Fe to a concentration of ∼10<sup>17</sup> cm <sup>-3</sup> led to decrease in concentration of uncompensated donors, suggesting compensation by Fe acceptors. Addition of Fe strongly suppressed the formation of PPC-active centers in favor of ordinary shallow donors. For higher Al compositions, Si doping of (5–7)×10<sup>17</sup> cm <sup>-3</sup> did not lead to n-type conductivity. Fe doping shifted the bandedge luminescence by 25–50 meV depending on Al composition. The dominant defect band in microcathodoluminescence spectra was the blue band near 3 eV, with the energy weakly dependent on composition.
Journal of Applied Physics 02/2010; · 2.17 Impact Factor
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Byung Hwan Chu,
C. Y. Chang,
Kevin Kroll,
Nancy Denslow,
Yu-Lin Wang,
S. J. Pearton,
A. M. Dabiran, A. M. Wowchak,
B. Cui,
P. P. Chow,
Fan Ren
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ABSTRACT: Endocrine disrupters are known to have negative effects on the environment and human health. Real time detection of vitellogenin, an endocrine disrupter biomarker, was demonstrated using AlGaN/GaN high electron mobility transistors (HEMTs). Anti-vitellogenin antibodies were chemically anchored to the gold-coated gate area of the HEMT and immobilized with thioglycolic acid. The potential difference that occurs from the vitellogenin antigen-antibody interaction-induced caused a drain current change in the HEMT. The HEMT sensor was tested for vitellogenin detection both in phosphate buffer saline and largemouth bass serum.
Applied Physics Letters 01/2010; 96(1):013701-013701-3. · 3.84 Impact Factor
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ABSTRACT: The electrical properties, admittance spectra, microcathodoluminescence, and deep trap spectra of p -AlGaN films with an Al mole fraction up to 45% grown by both metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were compared. The ionization energy of Mg increases from 0.15 to 0.17 eV in p -GaN to 0.3 eV in 45% Al p -AlGaN. In p -GaN films grown by MBE and MOCVD and in MOCVD grown p -AlGaN, we observed additional acceptors with a concentration an order lower than that of Mg acceptors, with a higher hole capture cross section and an ionization energy close to that of Mg. For some of the MBE grown p -AlGaN, we also detected the presence of additional acceptor centers, but in that case the centers were located near the p -AlGaN layer interface with the semi-insulating AlGaN buffer and showed activation energies considerably lower than those of Mg.
Journal of Applied Physics 11/2009; · 2.17 Impact Factor
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ABSTRACT: In this letter, a novel process for recessed-gate AlGaN/GaN high-electron-mobility transistors using an Al<sub>2</sub>O<sub>3</sub>/SiN x dielectric has been developed. The Al<sub>2</sub>O<sub>3</sub>/SiN x dielectric bilayer was used as a recess etch-mask for short-gate-footprint definition. Recessed-gate devices with a gate length of 70 nm have been fabricated on a molecular-beam-epitaxy-grown layer structure using this process. After the removal of the dielectric layers, excellent dc and small-signal results, a high drain-current density of 1.5 A/mm, a unity gain cutoff frequency of 160 GHz, and a maximum frequency of oscillation of 200 GHz were obtained.
IEEE Electron Device Letters 10/2009; · 2.85 Impact Factor
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ABSTRACT: Enhancement mode AlN/GaN high electron mobility transistors (HEMTs) were fabricated from originally depletion-mode structures using oxygen plasma treatment on the gate area prior to the gate metallization. Starting with a depletion mode AlN/GaN HEMT, the threshold voltage of the HEMT could be shifted from −3.2 to 1 V depending on the oxygen plasma treatment time to partially convert the AlN barrier layer into Al oxide. The gate current was reduced and the current-voltage curves show metal-oxide semiconductor diodelike characteristics after oxygen plasma treatment.
Applied Physics Letters 06/2009; 94(26):263505-263505-3. · 3.84 Impact Factor
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A. Y. Polyakov,
N. B. Smirnov,
A. V. Govorkov,
A. V. Markov,
T. G. Yugova,
A. M. Dabiran, A. M. Wowchak,
B. Cui,
A. V. Osinsky,
P. P. Chow,
S. J. Pearton,
K. D. Scherbatchev,
V. T. Bublik
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ABSTRACT: The electrical and structural properties of AlN/GaN heterostructures grown by molecular beam epitaxy on sapphire are compared with those of AlGaN/GaN heterostructures. The structural characteristics as assessed by x-ray diffraction show little difference but the electron density in the two-dimensional electron gas is about twice higher for AlN/GaN structures with only slightly lower mobility than in AlGaN/GaN. By proper choice of the Fe doping in GaN(Fe) and the thickness of unintentionally doped GaN layers, the composite buffer of the structure can be made semi-insulating. The current through the AlN/GaN structures is determined by tunneling through the AlN barrier and is much higher than that for AlGaN/GaN films due to the lower thickness of AlN compared to AlGaN. Increasing the thickness of AlN from 3 to 4 nm decreases the leakage current by about an order of magnitude.
Journal of Applied Physics 10/2008; · 2.17 Impact Factor
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ABSTRACT: Low defect AlN/GaN high electron mobility transistor (HEMT) structures, with very high values of electron mobility (>1800 cm2/V s) and sheet charge density (>3×1013 cm−2), were grown by rf plasma-assisted molecular beam epitaxy (MBE) on sapphire and SiC, resulting in sheet resistivity values down to ∼ 100 Ω/◻ at room temperature. Fabricated 1.2 μm gate devices showed excellent current-voltage characteristics, including a zero gate saturation current density of ∼ 1.3 A/mm and a peak transconductance of ∼ 260 mS/mm. Here, an all MBE growth of optimized AlN/GaN HEMT structures plus the results of thin-film characterizations and device measurements are presented.
Applied Physics Letters 08/2008; 93(8):082111-082111-3. · 3.84 Impact Factor
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ABSTRACT: Three periods of Al<sub>0.1</sub>Ga<sub>0.9</sub>N/Al<sub>0.15</sub>Ga<sub>0.85</sub> N multiple quantum wells (MQWs) were used as the active region of a p-i-n diode fabricated on 6H-SiC substrate. Electroluminescence (EL) of these MQWs has been investigated in both injection and avalanche modes. Band-to-band luminescence of the Al<sub>0.1</sub>Ga<sub>0.9</sub>N wells was found to peak at 364 nm in the injection mode and in the range of 364-372 nm in the avalanche mode. The most striking phenomenon is that band-to-band EL of the Al<sub>0.15</sub>Ga<sub>0.85</sub>N barriers has also been observed in the injection mode, while it is not seen in the avalanche mode. This is explained by considering different sources of carriers and different carrier transportation mechanisms in the two modes. The luminescence intensity I <sub>EL</sub> has a power-law dependence on the current I by I <sub>EL</sub> prop I <sup>2</sup> in the injection mode and by I <sub>EL</sub> prop I <sup>4</sup> in the avalanche mode.
IEEE Journal of Selected Topics in Quantum Electronics 08/2008; 14(4):1010-1013. · 3.78 Impact Factor
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A. Y. Polyakov,
N. B. Smirnov,
A. V. Govorkov,
T. G. Yugova,
A. V. Markov,
A. M. Dabiran, A. M. Wowchak,
B. Cui,
J. Xie,
A. V. Osinsky,
P. P. Chow,
S. J. Pearton
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ABSTRACT: The electrical properties of Al Ga N / Ga N high electron mobility transistor structures grown on composite Ga N ( Fe )/ Ga N buffers by molecular beam epitaxy were reported. The concentration of Fe in the GaN(Fe) layer ranged from 8×10<sup>16</sup> to 3×10<sup>17</sup> cm <sup>-3</sup> as established by secondary ion mass spectrometry. The thickness of the undoped GaN layer of the buffer was varied from 2.2 to 4.1 μ m . For thinner buffers and higher Fe concentration, the buffer was semi-insulating, with the Fermi level pinned near E<sub>c</sub>-0.57 eV . For thicker buffers and lower Fe concentration, the top part of the buffer was conducting. Admittance spectra measured in conducting buffers also showed a prominent contribution from E<sub>c</sub>-(055–0.6) eV electron traps. Despite the universal prominence of these traps in all our films, the behavior of their concentration with Fe doping and with increased distance from the GaN ( Fe )/ Ga N boundary is not compatible with the assumption that they are due to substitutional Fe acceptors. Possible compensation mechanisms in the studied structures were discussed.
Applied Physics Letters 02/2008; · 3.84 Impact Factor
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ABSTRACT: The origin of hysteresis in capacitance-voltage (C-V) characteristics was studied for Schottky diodes prepared on Al Ga N / Ga N transistor structures with GaN (Fe) buffers. The application of reverse bias leads to a shift of C-V curves toward higher positive voltages. The magnitude of the effect is shown to increase for lower temperatures. The phenomenon is attributed to tunneling of electrons from the Schottky gate to localized states in the structure. A technique labeled “reverse” deep level transient spectroscopy was used to show that the deep traps responsible for the hysteresis have activation energies of 0.25, 0.6, and 0.9 eV . Comparison with deep trap spectra of GaN buffers and Si doped n- GaN films prepared on GaN buffers suggests that the traps in question are located in the buffer layer.
Applied Physics Letters 01/2008; · 3.84 Impact Factor
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ABSTRACT: The effects of atomic-layer-deposited (ALD) Al<sub>2</sub>O<sub>3 </sub> passivation layer on AlGaN/GaN HEMTs on SiC were studied. Improved pulsed I-V characteristics and a relatively small decrease in the unity gain cutoff frequency (f<sub>T</sub>) and the maximum frequency of oscillation (f<sub>max</sub>) were observed in the devices passivated using a 45 nm-thick ALD Al<sub>2</sub>O<sub>3</sub> layer. For 0.12 mum gatelength devices, f<sub>T</sub> (f<sub>max</sub>) decreased to 92 GHz (115 GHz) from 120 GHz (140 GHz), while for 0.25 mum devices, f<sub>T</sub> (f<sub>max</sub>) decreased to 58 GHz (120 GHz) from 65 GHz (137 GHz). At a drain bias of 15 V, an output power of 3 W/mm with an associated gain of 5.0 dB and PAE of 33% were obtained for the 0.25 mum gatelength devices
Electronics Letters 02/2007; · 0.96 Impact Factor
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S. K. Zhang,
W. B. Wang,
A. M. Dabiran,
A. Osinsky, A. M. Wowchak,
B. Hertog,
C. Plaut,
P. P. Chow,
S. Gundry,
E. O. Troudt,
R. R. Alfano
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ABSTRACT: AlGaN p-i-n diodes were fabricated on 6H-SiC substrate using Al0.1Ga0.9N/Al0.15Ga0.85N multiquantum wells (MQWs) for the active region. Avalanche breakdown and breakdown luminescence of these AlGaN MQW diodes were experimentally investigated. Breakdown electroluminescence from the MQW active layers was observed for unusually low bias values of 9 V. A continuous red shift of the interband luminescence resulting from the Stark effect was observed with increasing reverse bias. The breakdown threshold was found to be as low as 9 V. Polarization-induced electric fields in the Al0.1Ga0.9N well layers were found to have the same direction as the applied field. These polarization fields greatly enhance the ionization coefficient of electrons and help lower the threshold for avalanche breakdown. Substantial enhancement of the ionization coefficient produced by the polarization fields is quantitatively demonstrated with GaN/AlGaN quantum wells by considering a polarization field with the same direction as the applied field within the GaN well layers.
Applied Physics Letters 12/2005; 87(26):262113-262113-3. · 3.84 Impact Factor
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ABSTRACT: A report is made on the DC, RF and large-signal pulsed characteristics of unpassivated AlGaN/InGaN/GaN high electron-mobility transistors (HEMTs) grown by molecular beam epitaxy on sapphire substrates. The devices with a 0.5 μm gate-length exhibited relatively flat transconductance (g<sub>m</sub>) with a maximum drain current of 880 mA/mm, a peak g<sub>m</sub> of 156 mS/mm, an f<sub>T</sub> of 17.3 GHz, and an f<sub>MAX</sub> of 28.7 GHz. In addition to promising DC and RF results, pulsed I-V measurements reveal that there is little current collapse in the AlGaN/InGaN HEMTs. These results indicate that the output power of InGaN channel HEMTs should not be limited by surface-state-related current collapse.
Electronics Letters 07/2004; · 0.96 Impact Factor
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ABSTRACT: We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer
to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are
completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas
Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts,
we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers
with carrier concentrations of 1.2 × 1017, 1.5 × 1017 and 4.7 × 1017 cm−3 are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained
in this study is with Co/Au at 0.0081 ohm-cm2. In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from
300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the
most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner
than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily
defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.
Journal of Electronic Materials 04/1999; 28(5):572-579. · 1.47 Impact Factor
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ABSTRACT: RF atomic nitrogen plasma molecular beam epitaxy (MBE) was used to deposit gallium nitride (GaN) p-i-n junction photovoltaic detectors on (0001) sapphire. The detectors consisted of a bottom contact layer n-type silicon doped to 5 × 1018 cm−3. The intrinsic layer was undoped and possessed an n-type background carrier concentration of 1 × 1016 cm−3. The top /p-GaN layer was doped with magnesium to give a Hall concentration of 5 × 1017 cm−3. The p-type GaN cathodoluminescence (CL) spectra showed a strong 372 nm emission level in contrast to the 430 nm level observed in MOCVD samples. These layers were fabricated into 1 × 10 element detector arrays using a chlorine-based reactive ion etch (RIE) and refractory metal ohmic contacts. Peak responsivity of 0.11 AAV on detectors without anti-reflection coating were obtained at the GaN bandedge of 360 nm. The ultraviolet (UV) to visible rejection ratio was greater than 103 − 104 and was accredited to the reduction of the yellow defect levels in MBE material. Preliminary results on AlxGa1−xN detectors with responsivity peaks at 313 and 343 nm are presented as well.
MRS Proceedings. 12/1995; 449.
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ABSTRACT: The group III nitrides have attracted increasing interest because of their significant potential in optoelectronics such as blue emitter and high temperature electronics. In order to take advantage of bandgap engineering of heterostructures such as GaN/AlN, a controllable growth technique is highly desired. The growth of group III nitrides by MBE (molecular beam epitaxy) requires using energetic nitrogen species which can be generated by techniques such as ion, RF (radio-frequency) and ECR (electron cyclotron resonance) sources. We report nitride results using an RF source to achieve epitaxy on sapphire. The nitrogen flux has been carefully characterized and related to the GaN quality. Schottky diode and ultra-violet (UV) photo-response are also reported.
Journal of Crystal Growth.
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