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ABSTRACT: A modelling approach for carrier transport in a Gaussin density of states is presented which takes into account that the energetic distribution of carriers moving via Miller-Abrahams rates is affected by an electric field. This reorganisation of the energetic carrier distribution can be described by virtual carrier heating to an effective temperature. We show that combining this approach with an existing percolation model reproduces the field dependence found in computer studies in literature for uncorrelated Gaussian disorder. Comparing to our experimental results, we also demonstrate that the parameterizations from these publications do not hold at low temperatures. We produced samples of 4,4′,4″-tris-(2-methylphenyl phenylamino)triphenylamine (m-MTDATA) doped into N,N'- diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB) in the ratio 1:1 with four different thicknesses from 50 nm to 400 nm and measured the IV curves in a temperature range from T=77 K to 346 K. We successfully demonstrate a unified simulation of the samples’ IV characteristics over the entire temperature range under the assumption of carrier heating, which is not possible with published models for correlated and uncorrelated Gaussian disorder.
SPIE Organic Photonics+ Electronics; 01/2012
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ABSTRACT: The development of GaInN quantum well structures with nonpolar crystal orientation for light-emitting diodes and semiconductor lasers is currently one of the main foci of III-nitride-based optoelectronics research. One of the advantages of nonpolar orientations is the absence of polarization fields perpendicular to the quantum well plane. As a consequence, radiative recombination rates are higher compared to quantum wells on polar surfaces. However, due to high densities of threading dislocations and basal plane stacking faults in the case of heteroepitaxially grown nonpolar layers, and due to band gap inhomogeneities in the GaInN quantum wells, characterization of radiative and nonradiative recombination mechanisms is a complex challenge. So far, most published data about band gap fluctuations, charge carrier localization and internal quantum efficiency in nonpolar quantum wells are ambiguous. Here, we present temperature and excitation power density-dependent photoluminescence data featuring multiple characteristics related to strong charge carrier localization in m-plane (1–100) GaInN quantum wells. Thermally activated redistribution of charge carriers between localization sites in these quantum wells is weaker than in polar c-plane ones. The localization strength increases with higher indium concentration in the quantum wells. In the heteroepitaxially grown quantum well structures, the internal quantum efficiency is reduced even at low temperatures (T = 10 K) and especially for m-plane quantum wells with high indium mole fractions.
Semiconductor Science and Technology 09/2011; 26(10):105017. · 1.72 Impact Factor
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ABSTRACT: We investigated and modeled the electrothermal behavior of a 5 × 5 cm2 organic light-emitting diode (OLED) with electrothermal measurements and finite-element simulation. A hybrid electrothermal model consisting of finite and lumped elements was proposed. Heat distribution of large-area OLED was measured by infrared spectroscopy. We have achieved an excellent agreement of measured and simulated results. The simulation confirms a strong influence of temperature on current distribution for large-area OLED. It turns out that the design of homogeneous devices requires knowledge about electrical and thermal aspects. Another result anticipates that the switching behavior of OLED strongly correlates with thermal relaxation. The model is a valuable tool to simulate luminance distribution and local aging allowing strong improvement of device development.
Organic Electronics 01/2011; 12(8):1399-1405. · 4.05 Impact Factor
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L. Rahimzadeh Khoshroo,
N. Ketteniss,
C. Mauder,
H. Behmenburg,
J. F. Woitok,
I. Booker,
J. Gruis, M. Heuken,
A. Vescan,
H. Kalisch,
R. H. Jansen
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ABSTRACT: We report on AlInGaN/AlN/GaN heterostructures on 2′ sapphire substrates with different compositions grown by metal organic vapor phase epitaxy (MOVPE). High-resolution X-Ray diffraction (HRXRD) measurements allowed a reliable thickness determination and indicated sharp interfaces. HRXRD mappings showed good quality and single-phase growth in all barrier layers. At room temperature, the samples yielded average two-dimensional carrier densities in the range of 1.51 – 1.81 × 1013 cm-2, systematically decreasing with rising GaN content in the barrier layer. Room temperature (RT) mobilities up to 1790 cm2/Vs were measured. When cooled to 77 K, the sheet resistivity decreased to 37% of the RT value at no noticeable change of ns. Atomic force microscopy reveals very low roughness and improved step-flow morphology. Further processing yielded high electron mobility transistors exhibiting a transconductance of around 265 mS/mm for all samples and drain currents ranging from 848 mA/mm to 1051 mA/mm at 0 V gate voltage (Lg = 1 μm) (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 04/2010; 7(7‐8):2001 - 2003.
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ABSTRACT: This contribution reports on properties and characterization of InAlN/GaN structures prepared by metal organic chemical vapour deposition (MOCVD) using secondary ion mass spectroscopy (SIMS). The SIMS revealed the vertical cross section of the InAlN/GaN sample structures on SiC substrate and also visualizes the different growth procedure results. The SIMS comparison of the structures shows the Al, In and Si incorporation to the neighbouring layers and has influence on the in-plane stress in AlN layer and measured electrical properties of the fabricated HEMT devices.
01/2010;
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L. Rahimzadeh Khoshroo,
C. Mauder,
H. Behmenburg,
J. Woitok,
W. Zander,
J. Gruis,
B. Reuters,
J. Schubert,
A. Vescan, M. Heuken,
H. Kalisch,
R. H. Jansen
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ABSTRACT: In this paper we report on the growth and charcterization of quaternary AlInGaN/AlN/GaN heterostructures, spanning a range from 26% to 82% AlN, 4% to 18% InN and 0% to 70% GaN in the barrier layer. All samples were grown in AIXTRON metal organic vapor phase epitaxy (MOVPE) reactors on 2″ sapphire substrates. Sample characterisation was performed using high resolution X-ray diffraction (HRXRD), X-ray reflection (XRR), photoluminescence spectroscopy (PL), atomic force microscopy (AFM) and Rutherford backscattering (RBS). Inductive sheet resistance measurements and Hall measurements in van der Pauw geometry provided electrical characterization. AFM measurements show closed layers, in HRXRD measurements pendellösung oscillations are visible even in asymmetric reciprocal space maps. Mobility and sheet carrier concentration reach state of the art values currently achieved for ternary structures showing the high degree of tunability in the quaternary nitride system. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 05/2009; 6(S2):S470 - S473.
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L. Rahimzadeh Khoshroo,
C. Mauder,
W. Zhang,
M. Fieger,
M. Eickelkamp,
Y. Dikme,
J. Woitok,
P. Niyamakom,
A. Vescan,
H. Kalisch, M. Heuken,
R. H. Jansen
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ABSTRACT: We report on the growth and characterisation of 190 nm thick Al0.83In0.17N layers and Al0.83In0.17N/GaN high electron mobility transistor (HEMT) structures on GaN/sapphire. High-resolution X-ray diffraction rocking curve measurements (HRXRD) exhibit full width at half maximum (FWHM) values of 68 and 175 arcsec for the GaN (0002) and (10-15) reflections, respectively. The corresponding Al0.83In0.17N reflections yield 321 and 582 arcsec on 190 nm thick layers. XRD simulations of HEMT structures match well with measured data and with the intended heterostructure stack. Layer thickness fringes up to the 3rd order indicate a good heterointerface and enable precise barrier thickness determination. Further, no spinodal decomposition is detected for AlInN in both symmetrical and asymmetrical reciprocal space map measurements. Electrical characterisations of the HEMT at room temperature yielded a charge carrier density of ns = 2 x 1013 cm–2 and a mobility of µ = 1460 cm2/Vs underlined by processed devices demonstrating maximum drain current densities of 1.2 A/mm at VGS = 2 V and extrinsic transconductances of 243 mS/mm (LG = 1 µm, WG = 50 µm, 5 µm source-drain spacing). (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 04/2008; 5(6):2041 - 2043.
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ABSTRACT: We report on the growth of high quality AlN films on sapphire by MOVPE in an AIX2400G3-HT planetary reactor. Specific reactor hardware modifications were conducted to facilitate growth temperatures of up to 1600 °C and to obtain reduced parasitic gas phase reactions. Growth was optimized regarding growth rate and surface morphology as well as optical and structural properties of the AlN layers on sapphire. With increasing growth temperature we observe a transition from an AlN surface with a high density of large pits to a smooth pit-free morphology. The improvement in material quality with growth temperature is confirmed by X-ray diffraction, AFM, SIMS and Raman measurements. The impact of residual or intentionally introduced Ga during growth on AlN material properties is discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 04/2008; 5(6):1799 - 1801.
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H. Behmenburg,
T. C. Wen,
Y. Dikme,
C. Mauder,
L. Rahimzadeh Khoshroo,
M. M. C. Chou,
M. V. Rzheutskii,
E. V. Lutsenko,
G. P. Yablonskii,
J. Woitok,
H. Kalisch,
R. H. Jansen, M. Heuken
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ABSTRACT: We present a study of growth and properties of m -plane GaN/InGaN/AlInN structures on LiAlO2 substrates grown by metal organic vapour phase epitaxy (MOVPE). A buffer structure, including an m -plane AlInN interlayer prior to GaN growth, has been developed. Quantum well structures on top of this buffer showed absence of polarization-induced electric fields verified by room temperature photoluminescence (RT PL) measurements with different excitation intensities. Different samples with peak emission wavelength between 433 nm and 495 nm exhibited stable peak position even for high excitation intensities of 500 kW/cm2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 04/2008; 245(5):893 - 895. · 1.32 Impact Factor
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M. Fieger,
M. Eickelkamp,
W. Zhang,
L. Rahimzadeh Khoshroo,
C. Mauder,
Y. Dikme, M. Heuken,
A. Noculak,
H. Kalisch,
R. H. Jansen,
A. Vescan
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ABSTRACT: Lattice-matched Al0.83In0.17N/GaN high electron mobility transistors (HEMTs) grown by MOCVD on sapphire substrate were investigated. The transport properties of the 2DEG at room temperature were characterised by sheet carrier concentrations up to 2.0×1013 cm–2 at a mobility of 1450 cm2/Vs. Unpassivated devices with gate lengths of LG = 1 µm and a gate-drain spacing of 5 µm exhibit maximum drain current densities of 1.2 A/mm at VGS = 2 V and extrinsic transconductances of 243 mS/mm, respectively. The small-signal behaviour is characterised by a current gain cut-off frequency fT = 13 GHz and a maximum frequency of oscillation fmax= 29 GHz, respectively. Pulsed I-V measurements from different quiescent bias points reveal dispersion effects resulting in a partial drain current collapse. A SiN passivation was found not to alleviate these effects. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2008; 5(6):1926 - 1928.
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C. Mauder,
L. Rahimzadeh Khoshroo,
M. V. Rzheutski,
E. V. Lutsenko,
G. Yablonskii,
V. I. Kozlovsky,
J. Woitok,
Y. Dikme, M. Heuken,
H. Kalisch,
R. H. Jansen
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ABSTRACT: We report about the MOVPE growth and characterisation of Al0.90In0.10N single layers and multiple quantum well structures (MQW) containing 8 periods of Al0.90In0.10N/AlN quantum wells. All samples show good quality in high-resolution X-ray diffractometry (HRXRD) evaluation, superlattice fringes up to the 2nd order can be seen for the MQW sample. In optical transmission experiments, an absorption edge for the Al0.90In0.10N single layer is detected at 4.9 eV. Due to the low intensity of emission and limited sensitivity of our setup in this spectral region, photoluminescence (PL) emission above 4 eV is only visible for the MQW sample. Both PL and cathodoluminescence (CL) measurements show an emission peak at longer wavelength for the Al0.90In0.10N/AlN MQW sample. When increasing the excitation current density in CL, the emission at lower wavelength is clearly enhanced, while it is slightly reduced at higher wavelength. We believe this to be a saturation effect of a defect-related emission. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2008; 5(6):1553 - 1555.
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V. Darakchieva,
T. Paskova,
M. Schubert,
H. Arwin,
P. P. Paskov,
B. Monemar,
D. Hommel, M. Heuken,
J. Off,
F. Scholz,
B. A. Haskell,
P. T. Fini,
J. S. Speck,
S. Nakamura
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ABSTRACT: We report optical phonon frequency studies in anisotropically strained c-plane- and a-plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN E1(TO), E1(LO), and E2 phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials cE1(TO), cE1(LO), and cE2 are determined. A distinct correlation between anisotropic strain and the A1(TO) and E1(LO) frequencies of a-plane GaN films reveals the aA1(TO), bA1(TO), aE1(LO), and bE1(LO) phonon deformation potentials. The aA1(TO) and bA1(TO) are found to be in very good agreement with previous results from Raman experiments [ V. Yu. Davydov et al. J. Appl. Phys. 82 5097 (1997)]. Our aA1(TO) and aE1(LO) phonon deformation potentials agree well with recently reported theoretical estimations [ J.-M. Wagner and F. Bechstedt Phys. Rev. B 66 115202 (2002)], while bA1(TO) and bE1(LO) are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented.
Physical Review B 04/2007; 75:195217. · 3.69 Impact Factor
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Journal of Crystal Growth 01/2007; 300:32. · 1.73 Impact Factor
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V. Darakchieva,
T. Paskova,
M. Schubert,
H. Arwin,
P. P. Paskov,
B. Monemar,
D. Hommel, M. Heuken,
J. Off,
B. A. Haskell,
P. T. Fini,
J. S. Speck,
S. Nakamura
Journal of Crystal Growth 01/2007; 300:233. · 1.73 Impact Factor
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V. Darakchieva,
M. Schubert,
T. Paskova,
P.P. Paskov,
H. Arwin,
B. Monemar, M. Heuken,
S. Figge,
D. Hommel,
B. Haskell,
P. Fini,
S. Nakamura
Physical Review B 01/2007; 75:195217. · 3.69 Impact Factor
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H. Hardtdegen,
N. Kaluza,
Z. Sofer,
Y. S. Cho,
R. Steins,
H. L. Bay,
Y. Dikme,
H. Kalisch,
R. H Jansen, M. Heuken,
A. Strittmatter,
L. Reißmann,
D. Bimberg,
J.-T. Zettler
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ABSTRACT: This paper reports on the in situ determination of the Al-content in AlxGa1–xN layers deposited by MOVPE on sapphire and silicon substrates by means of optical reflectance. The accuracy of the conventional in situ method which utilizes the dependence of the refractive index and the extinction coefficient on Al-content is compared to that of the new dispersion approach. The limits and possibilities of the new approach will be discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 05/2006; 203(7):1645 - 1649. · 1.46 Impact Factor
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E. V. Lutsenko,
V. N. Pavlovskii,
A. V. Danilchyk,
K. A. Osipov,
N. V. Rzheutskii,
V. Z. Zubialevich,
A. L. Gurskii,
G. P. Yablonskii, T. Malinauskas,
K. Jarašiūnas,
K. Kazlauskas,
S. Juršėnas,
S. Miasojedovas,
A. Žukauskas,
Y. Dikme,
H. Kalisch,
R. H. Jansen,
B. Schineller, M. Heuken
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ABSTRACT: Optical properties and carrier dynamics were investigated in a set of samples grown on Si utilizing different layer combinations between the topmost GaN layer and the Si substrate, including AlN/GaN distributed Bragg reflector (DBR) superlattices as well as AlGaN/GaN strain-reducing layers. The use of the AlN/AlGaN superlattice acting both as buffer layer and as DBR opens the way to control the strain in the upper epitaxial layer in GaN-based heterostructures grown on Si. It also can lead to a decrease of the concentration of non-radiative defects in the upper GaN layers, which, in turn, approximately doubles the carrier lifetime and increases the concentration of non-equilibrium carriers. Carrier lifetimes at high excitation energy densities increased from 50–78 ps to 75–200 ps in the layers with DBR. These effects lead to improved PL efficiency and to an increase of the value of optical gain from 300 cm–1 up to 4000 cm–1. The use of DBR reduces the absorption of light in the Si substrate and may improve the properties of the laser waveguide. All these effects result in a reduction of the laser threshold from 700 to 270 kW/cm2. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 05/2006; 203(7):1759 - 1763. · 1.46 Impact Factor
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ABSTRACT: Two optical techniques for the determination of a bipolar diffusion length LD of optically excited carriers in GaN epitaxial layers, namely a time-resolved picosecond four-wave mixing (FWM) on free carrier grating and time-integrated photoluminescence (PL) are presented and examined. The PL technique is based on time-integrated photoluminescence (PL) spectra measurements from the front and back sides of the sample under cw and nanosecond pulsed laser excitation. The another method utilizes time-resolved picosecond four-wave mixing (FWM) at various light-induced grating periods to extract diffusion coefficient and carrier recombination lifetime. The value of the diffusion length derived by means of FWM decreases with GaN layer thickness from LD = 260 nm (for 1.7 μm-thick layer) to LD = 100 nm (for 0.3 μm-thick layer). The integral PL measurements give the value of LD = 120-130 nm for the 620 nm layer under pulsed excitation intensities up to 200 kW/cm2. It increases to 150-170 nm at the excitation intensity enhancement to 1 MW/cm2. These values are close to the value of the diffusion length equal to 160 nm obtained using FWM for this layer thickness evidencing the compatibility of both methods. The changes in the value of LD are discussed in terms of the defect distribution in the epitaxial GaN layer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 05/2006; 3(6):1935 - 1939.
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ABSTRACT: Using the hydride vapour phase epitaxy technique, we have grown 2-inch diameter bulk GaN material with a thickness up to 2 mm. The growth was performed in a vertical hot-walled reactor at atmospheric pressure. In this geometry, the process gases are distributed from the bottom upwards through the reactor. We present recent results on growth and characterization of the bulk GaN material. The structural and optical properties of the layers have been studied using decorative etching, optical microscopy, scanning electron microscopy, X-ray diffraction, cathodoluminescence, and low temperature photoluminescence.
Superlattices and Microstructures 04/2006; 40:205. · 1.49 Impact Factor
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A. L. Gurskii,
E. V. Lutsenko,
V. N. Pavlovskii,
V. Z. Zubialevich,
G. P. Yablonskii,
R. Aleksiejūnas,
K. Jarašiūnas,
F. Letertre,
B. Faure,
B. Schineller,
A. Alam, M. Heuken,
Y. Dikme,
H. Kalisch,
R. H. Jansen
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ABSTRACT: Carrier dynamics in highly excited GaN/Si heterostructures have been investigated by means of time-resolved picosecond four-wave mixing (FWM), carrier lifetimes at high excitation energy densities being about 50-150 ps were estimated. The excitation energy density corresponding to the saturation of the FWM diffraction efficiency is correlated with the values of laser thresholds in the investigated samples, whereas the carrier lifetimes and the value of the FWM diffraction efficiency show an agreement with the photoluminescence (PL) intensity below the threshold. This study allowed to clarify the observed laser threshold behaviour of the investigated samples by a difference in carrier lifetimes resulting in a difference of the excited volume of the epitaxial layers. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2005; 2(7):2724 - 2727.
