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ABSTRACT: 9th International Conference on Defects: Recognition, Imaging and Physics in Semiconductors. Rimini, Italy - September 24-28, 2001
The talk presents X-ray diffractometric results on group III-nitride layers containing Al, Ga or In:
- Structural properties of (Al,Ga,In)-nitride films addressed by X-ray diffraction techniques
- X-ray determination of strain and composition in AlxGa1-xN and Ga1-xInxN layers using
the extended Bond method
- Evaluation of the defect structure of a layer using Reciprocal Space Maps
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M. Himmerlich,
A. Knübel,
R. Aidam, L. Kirste,
A. Eisenhardt,
S. Krischok,
J. Pezoldt,
P. Schley,
E. Sakalauskas,
R. Goldhahn,
R. Félix,
J. M. Mánuel,
F. M. Morales,
D. Carvalho,
T. Ben,
R. García,
G. Koblmüller
Journal of Applied Physics 01/2013; 113:033501. · 2.17 Impact Factor
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ABSTRACT: Thermoelectric compounds based on doped bismuth telluride and its alloys have recently attracted increasing interest. Due
to their structural features they show increased values of the thermoelectric figure of merit (ZT). A promising approach to improve the thermoelectric properties is to manufacture nanocomposite materials exhibiting lower
thermal conductivities and higher ZT. The ZT value of compounds can be shifted reasonably to higher values (>1) by alloying with IV-Te materials and adequate preparation
methods to form stable nanocomposites. The influence of PbTe and Sn on the thermoelectric properties is studied as a function
of concentration and preparation methods. Melt spinning and spark plasma sintering were applied to form nanocomposite materials
that were mechanically and thermodynamically stable for applications in thermoelectric devices. The structural properties
are discussed based on analysis by transmission electron microscopy and x-ray diffraction.
Journal of Electronic Materials 04/2012; 38(7):1450-1455. · 1.47 Impact Factor
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ABSTRACT: In this work we report on microstructures made of AlN for the use as piezoelectric microgenerators in energy harvesting applications. Experimental results on the mechanical and piezoelectrical properties of sputtered AlN thin films deposited on Si(001) substrates as well as results on vibrometry of fabricated microcantilevers are presented. It is shown that due to the constant d33 piezo coefficients of > 5 pm/V, AlN films are well suited for the integration in cantilever structures for powering of low-consumption sensor networks (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 11/2010; 8(2):476 - 478.
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ABSTRACT: For high frequency field effect transistors, one of the promising approaches is to grow a very thin (≤10 nm) InN channel pseudomorphically with low defect density between low lattice mismatched InGaN layers. The present work provides a comprehensive analysis of such structures by varying width of the InN well. Both experimental and theoretical approaches have been applied in order to optimize the InN potential well structure for a transistor operation. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 10/2010; 8(2):485 - 487.
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ABSTRACT: Nickel doped single crystal diamond layers were grown by microwave-plasma enhanced chemical vapour deposition. Optical emission spectroscopy (OES) utilised during growth proved that the organometallic compound nickelocene is an applicable nickel source. It was possible to produce stable and adjustable nickel OES signals during growth by altering the flux of the nickelocene carrier gas. The successful incorporation of nickel into the diamond layers was verified by secondary ion mass spectrometry. Cathodoluminescence (CL) was applied to reveal optically active defects related to nickel. The signature of substitutionally incorporated nickel, namely the nickel related 1.4 eV centre, as well as the 2.369 and 1.563 eV centres were observed in CL. The latter is supposed to be a single-photon emitter on the basis of a nickel–nitrogen defect centre.
Physica Status Solidi (A) Applications and Materials 08/2010; 207(9):2054 - 2057. · 1.46 Impact Factor
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ABSTRACT: GaN-based high electron mobility transistors (HEMTs) with a nearly strain-free high-Al-content quaternary barrier and electron mobilities up to 1590 cm<sup>2</sup>/Vs have been grown on 4H-SiC using molecular beam epitaxy (MBE). The processed devices with 150-nm gate length exhibit a high dc performance with a maximum current density of 2.3 A/mm and an extrinsic transconductance up to 675 mS/mm that is among the highest values reported until now for any III-N transistor. We further present, to our knowledge, the first power measurements at 10 GHz of MBE-grown GaN HEMTs with nearly lattice-matched InAlGaN barrier achieving 47% power-added efficiency at 10 V and an output power density of 5.6 W/mm at 30-V bias.
IEEE Electron Device Letters 08/2010; · 2.85 Impact Factor
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ABSTRACT: Quaternary InAlGaN semiconductors with AlN mole fractions between 40% and 81% and respective InN contents of 7% and 19% including InAlN as a ternary border case have been grown by plasma-assisted molecular beam epitaxy. The electron mobility in InAlGaN-based heterostructures increases with Ga concentration in the barrier up to 1460 cm2/V s at a sheet electron density of 1.9×1013 cm−2. An advanced spacer comprising an AlN/GaN/AlN triple-layer sequence is inserted between the GaN-buffer and the InAlGaN-barrier. Transistors with thin quaternary barrier show a large current density of 2.3 A/mm and an excellent transconductance of 675 mS/mm.
Applied Physics Letters 06/2010; 96(25):252108-252108-3. · 3.84 Impact Factor
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ABSTRACT: A high-resolution X-ray diffraction and X-ray reflectivity study of the structural properties of AlInN/GaN and AlGaInN/GaN high electron mobility transistor structures deposited by molecular beam epitaxy on metal organic chemical vapor deposition GaN/Al2O3 and GaN/SiC templates is presented. A new AlN/GaN/AlN triple-interlayer is implemented to improve the interface properties between barrier layer and GaN buffer for a higher mobility of the polarization induced two-dimensional electron gas. Layer properties and structural parameters like concentration, interface quality, layer thickness, strain and crystalline perfection are analyzed. Best structural properties are achieved for an AlGaInN layer with AlN/GaN/AlN interlayer deposited on a GaN/4H-SiC (00.1) template.
Physica Status Solidi (A) Applications and Materials 05/2010; · 1.46 Impact Factor
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K. Kohler,
S. Muller,
R. Aidam,
P. Waltereit,
W. Pletschen, L. Kirste,
H. P. Menner,
W. Bronner,
A. Leuther,
R. Quay,
M. Mikulla,
O. Ambacher,
R. Granzner,
F. Schwierz,
C. Buchheim,
R. Goldhahn
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ABSTRACT: The influence of the growth method on the surface potential and thus on the sheet carrier concentration of GaN capped Al <sub>x</sub> Ga <sub>1-x</sub> N / GaN heterostructures was evaluated. Nominally undoped low pressure metal-organic vapor-phase (MOVPE) and plasma-assisted molecular beam epitaxial (PA-MBE) grown structures with an Al-content between 12% and 30% yield carrier concentrations from 3.6×10<sup>12</sup> to 1.2×10<sup>13</sup> cm <sup>-2</sup> . A difference of the concentrations for a fixed Al-content was found between the different epitaxial techniques. This result indicates unambiguously different surface potentials determined quantitatively from the carrier concentration, and is verified in addition by the results of photoreflectance spectroscopy. The GaN surface potentials of MOVPE and PA-MBE grown samples amounts to (0.26±0.04) and (0.61±0.10) eV irrespective of the Al-content of the barrier layer. After device fabrication, we find that due to the identical surface potential defined by the Ni Schottky gate, the threshold voltage for a given Al-content is the same for samples grown with different techniques. Thus, the interplay between epitaxy and process technology defines the threshold voltage.
Journal of Applied Physics 04/2010; · 2.17 Impact Factor
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ABSTRACT: InxGa1−xN films with 0.4 ≤ x ≤ 1 are analyzed using electrolyte-based capacitance-voltage technique. In-rich InxGa1−xN for x>0.4 samples exhibit a strong surface electron accumulation. At x = 0.4, the Fermi level at the surface is pinned to the conduction band edge indicating a crossover from surface accumulation to depletion. The measured Mott–Schottky plots are fitted using a model based on a Schrödinger–Poisson solver. By comparing the measured data to the fitting results, we conclude that a subsurface layer of ∼ 15 nm thickness with remarkably lower donor defect concentration is formed during the growth of InGaN films.
Applied Physics Letters 02/2010; 96(8):082106-082106-3. · 3.84 Impact Factor
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P. Waltereit,
W. Bronner,
R. Quay,
M. Dammann,
R. Kiefer,
W. Pletschen,
S. Müller,
R. Aidam,
H. Menner, L. Kirste,
K. Köhler,
M. Mikulla,
O. Ambacher
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ABSTRACT: We present an overview on epitaxial growth, processing technology, device performance, and reliability of our GaN high electron mobility transistors (HEMTs) manufactured on 3- and 4-in. SiC substrates. Epitaxy and processing are optimized for both performance and reliability. We use three different gate lengths, namely 500 nm for 1-6 GHz applications, 250 nm for devices between 6 and 18 GHz, and 150 nm for higher frequencies. The developed HEMTs demonstrate excellent high-voltage stability, high power performance, and large DC to RF conversion efficiencies for all gate lengths. On large gate width devices for base station applications, an output power beyond 125 W is achieved with a power added efficiency around 60% and a linear gain around 16 dB. Reliability is tested both under DC and RF conditions with supply voltage of 50 and 30 V for 500 and 250 nm gates, respectively. DC tests on HEMT devices return a drain current change of just about 10% under IDQ conditions. Under RF stress the observed change in output power density is below 0.2 dB after more than 1000 h for both gate length technologies.
International Journal of Microwave and Wireless Technologies 01/2010;
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ABSTRACT: High electron mobility transistor material and device properties based on AlxGa1−xN/GaN heterostructures with an Al-content ranging from 12 to 35% are presented. The Al-content of the low pressure metal-organic vapor-phase epitaxy grown samples on semi-insulating SiC and sapphire was measured by high resolution X-ray diffraction; carrier concentration and mobility by Hall effect. The sheet carrier density in the nominally undoped structures increases with the Al-content from 3.6 × 1012 to 1.4 × 1013 cm−2 due to spontaneous and piezoelectric polarization. Comparison of the electrical data with a model based on polarization and strain induced effects in dependence of the Al-composition shows good agreement. Further discussion and presentation on devices is restricted to data on SiC substrates. We quantitatively relate threshold voltage and extrinsic transconductance of FETs to the carrier concentration und thus to the Al-content in the barrier. Excellent large signal performance is achieved over the AlxGa1−xN-composition range from 14 to 30%.
Physica Status Solidi (A) Applications and Materials 10/2009; 206(11):2652 - 2657. · 1.46 Impact Factor
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P. Waltereit,
S. Muller,
K. Bellmann,
C. Buchheim,
R. Goldhahn,
K. Kohler, L. Kirste,
M. Baeumler,
M. Dammann,
W. Bronner,
R. Quay,
O. Ambacher
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ABSTRACT: We systematically investigate Al <sub>0.22</sub> Ga <sub>0.78</sub> N / GaN high electron mobility transistors with GaN cap layer thicknesses of 0, 1, and 3 nm. All samples have electron mobilities around 1700 cm <sup>2</sup>/ V s and sheet carrier concentrations around 8×10<sup>12</sup> cm <sup>-2</sup> as determined by Hall effect measurements. From photoreflectance measurements we conclude that the electric field strength within the AlGaN barrier increases with GaN cap layer thickness leading to a broadening of the transition peaks as determined by spectroscopic ellipsometry. The surface potential as determined by photoreflectance varies in the range between 0.585 and 0.249 eV dependent on the thickness of the GaN cap. Device results show a significant decrease in Ohmic contact resistance, an increase in ideality factor, a decrease in gate and drain leakage currents, an increase in gain, and an increase in power added efficiency with increasing cap layer thickness. Finally, devices with GaN cap show an improved direct current reliability compared to their counterparts without GaN cap.
Journal of Applied Physics 08/2009; · 2.17 Impact Factor
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ABSTRACT: This paper presents an x-ray study of GaN, which is grown on nominally undoped and oxygen-doped AlN nucleation layers on sapphire substrates by metal organic vapor phase epitaxy. Without additional oxygen doping a trimodal nucleation distribution of AlN is observed leading to inhomogeneous in-plane strain fields, whereas in oxygen-doped layers a homogeneous distribution of nucleation centers is observed. In both types of nucleation layers extremely sharp correlation peaks occur in transverse ω -scans which are attributed to a high density of edge-type dislocations having an in-plane Burgers vector. The correlation peaks are still visible in the (0002) ω -scans of 500 nm GaN which might mislead an observer to conclude incorrectly that there exists an extremely high structural quality. For the undoped nucleation layers depth-sensitive measurements in grazing incidence geometry reveal a strong thickness dependence of the lattice parameter a , whereas no such dependence is observed for doped samples. For oxygen-doped nucleation layers, in cross-sectional transmission electron microscopy images a high density of stacking faults parallel to the substrate surface is found in contrast to undoped nucleation layers where a high density of threading dislocations is visible. GaN of 2.5 μ m grown on top of 25 nm AlN nucleation layers with an additional in situ SiN mask show full widths at half maximum of 160<sup>″</sup> and 190<sup>″</sup> in (0002) and (10–10) high-resolution x-ray diffraction ω -scans, respectively.
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
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ABSTRACT: We report on the plasma-assisted molecular beam epitaxy (PAMBE) of AlGaN/GaN high electron mobility transistors (HEMTS). Growth was performed on MOCVD grown GaN Templates and 4H-SiC(0001) wafers with 3” diameter. In a detailed study of GaN growth conditions the growth rate was varied between 0.28 and 0.5 µm/h. Under optimized conditions, the surface root mean square roughness was less than 0.6 nm. Low temperature PL measurements in the range 1.9 to 3.8 eV showed only the exciton-emission with a FWHM of 12 meV. AlGaN-HEMT structures on GaN-templates exhibited room temperature Hall mobilities of 1600 cm2/Vs and sheet electron concentration of 8 × 1012/cm2. On SiC mobilities of 1220 cm2/Vs were achieved. Current-voltage output characteristics of PAMBE grown AlGaN/GaN HEMTs on SiC showed a drain current density about 0.8 A/mm for a gate bias of +1V and only small dispersion. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2008; 5(6):1902 - 1905.
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ABSTRACT: The surface potential of GaN:Si, of fundamental interest for knowledge of the electrostatic potential and electric field strength, is determined for Si doping in the device relevant range from 6×1017 cm−3 to 2.3×1019 cm−3, in layers grown by low-pressure metal-organic vapor-phase epitaxy. Comparing the Si doping concentration, measured by secondary ion mass spectrometry, to the free electron concentration, measured by Hall effect, shows that all Si doping atoms are activated. We used the sheet resistance of the samples with different thicknesses measured by eddy current, a nondestructive, contactless method, to determine the depleted region. From the width of the depletion layer, which is dependent on the doping concentration, we obtained the GaN:Si surface potential on the basis of the depletion approximation. The surface potential decreases with increasing carrier concentration from about 1.5 eV to 0.2 eV. A qualitative explanation based on the theoretical description of two surface states, an occupied surface state at 1.7 eV and an empty state 0.6 eV below the conduction band edge, is given.
Journal of Applied Physics 01/2008; 103(2):023706-023706-5. · 2.17 Impact Factor
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physica status solidi (RRL) - Rapid Research Letters 09/2007; 1(6):238 - 240. · 2.22 Impact Factor
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physica status solidi (RRL) - Rapid Research Letters 08/2007; 1(6):235 - 237. · 2.22 Impact Factor
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ABSTRACT: The growth of AlN on c-plane sapphire substrates is reported for a growth temperature of 1100 °C in a radio frequency plasma assisted molecular beam epitaxy system. The high growth temperature in combination with Al-rich growth conditions resulted in a high crystalline quality (FWHM of the 0002 x-ray reflection of 650'') and low O incorporation. Furthermore, the incorporation of Si and Mg as n- and p-type dopants, respectively, has been studied for these growth conditions. For Si doping the corresponding cell temperature was varied between 1300 and 1350 °C. Secondary ion mass spectrometry (SIMS) showed a homogeneous Si depth profile up to a concentration of 7 × 1020 cm−3, but the Si doped layers remained highly resistive. Incorporation of Mg was observed only at a low growth temperature of 830 °C; at higher growth temperatures SIMS revealed a strong surface segregation effect while the amount of Mg incorporated into the AlN layer remained below the detection limit. The build-up of a Mg accumulation layer at the growth surface due to segregation was found to cause a significant reduction in growth rate.
Journal of Physics D Applied Physics 10/2006; 39(21):4616. · 2.54 Impact Factor
