N. Kaluza

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (28)51.35 Total impact

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    ABSTRACT: The influence of the growth temperature in metal organic vapor-phase epitaxy (MOVPE) on intentional Cr incorporation during GaN growth is studied in the range 900–1125°C. The structural properties are investigated and compared to those obtained from undoped layers deposited at the same growth conditions and using the same growth procedure; Whereas, the best surface morphology and the best crystal quality are found at 1125°C for undoped GaN layers, the best structural and morphological properties are obtained for the Cr-doped GaN (GaN:Cr) layers grown at 950°C. GaN:Cr layers deposited at this temperature additionally exhibit the highest Cr concentration in the series as well as a Cr accumulation in the beginning stage of GaN:Cr growth. The accumulation may be responsible for the changed growth mode and the improved structural characteristics as well as for the observed V-shaped defects usually associated with strain relaxation. Thermo-remanent magnetization and a hysteresis loop were observed even above room temperature. The magnetic properties correlate to the structural properties of GaN:Cr layers.
    Journal of Crystal Growth 01/2009; 312(1):1-9. · 1.55 Impact Factor
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    ABSTRACT: We report on a characteristic photoluminescence feature of the substitutional Mn in high quality GaMnN layers. The lattice site was identified using atom localization by channeling enhanced microanalysis with a transmission electron microscope. It shows that 96.5%±5.0% of the Mn atoms are incorporated on the substitutional Ga site. In photoluminescence a feature appears at 1.41 eV with a phonon sideband related to the GaN matrix. The temperature evolution is characteristic of an intra-atomic transition and it is assigned to the internal transition 5E→5T2 of the Mn3+ ion. The assignment is supported by absorption experiments. The persistence of the clear PL signal up to about 1% Mn concentration is proposed to be a fingerprint of high quality diluted GaMnN.
    Journal of Applied Physics 03/2007; 101(6):063504-063504-3. · 2.21 Impact Factor
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    ABSTRACT: This paper reports on new approaches for growth control of GaN-based heterostructures for high frequency and high power application. First in situ methods are presented and their further development discussed [1]. The development leads to a greatly improved observation of growth parameter influences in the MOVPE of GaN. A new growth process is introduced which enhances growth reproducibility [2]. This new growth process is then optimized with respect to the envisaged application. To this end process modeling will be employed. The application envisaged is the AlxGa1-xN/GaN high electron mobility transistor (HEMT). At last device results will be presented. All in all it will be shown how fundamental research can drive technology and how basic knowledge can be employed for process development with respect to device applications.
    Applied Physics A 01/2007; 87(3):491-498. · 1.69 Impact Factor
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    ABSTRACT: This paper discusses the results of a combined modeling and experimental analysis of AlGaN deposition in the horizontal two-flow AIX 200/4 RF-S reactor. The purpose of this study is to examine conventional and inverted supply of the precursors into the reactor with respect to the growth reproducibility and efficiency of the aluminum (Al) incorporation. It has been found that the use of the inverted inlet improves the reproducibility of the growth process and provides a good control of AlGaN deposition. At the same time, the Al content appears to be somewhat lower for the inverted inlet configuration. A good agreement between the experimental data and model predictions allows us to use the modeling results for interpretation of the experimental findings.
    Journal of Crystal Growth - J CRYST GROWTH. 01/2007; 298:413-417.
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    ABSTRACT: Undoped GaN epilayers were grown on sapphire (0001) substrates using different H2/N2 carrier gas ratios (0%, 50%, 70%, 80%, and 100% N2 content in the mixtures) in metal organic vapor phase epitaxy (MOVPE). Growth was observed in situ by reflectometry. Additionally experiments were carried out in which growth was stopped in the high-temperature growth step for the same carrier gas mixtures. The morphology development was correlated with the structural and electrical characteristics of the layers.
    Journal of Crystal Growth 01/2007; 307(1):6-13. · 1.55 Impact Factor
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    ABSTRACT: AlN growth was performed on c-plane sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) using trimethylaluminium (TMAl) and ammonia. The influence of the carrier gases H2 and N2 and their mixtures on the surface morphology and structural characteristics was investigated as well as the influence of the growth temperature of the total source partial pressure and of the TMAl substrate pretreatment. It was found that smoother layers and better structural characteristics are obtained for lower source partial pressures. Decreasing of growth temperature led to improvement of surface morphology, and increasing of nitrogen in gas phase led to improvement structural quality of the layer. A pretreatment of the substrate leads to rougher layers.
    Journal of Physics and Chemistry of Solids 01/2007; 68(5):1131-1134. · 1.53 Impact Factor
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    ABSTRACT: The magnetotransport in a set of identical parallel AlxGa1-xN/GaN quantum wire structures is investigated. The width of the wires ranges between 1110 and 340nm . For all sets of wires, clear Shubnikov-de Haas oscillations are observed. We find that the electron concentration and mobility are approximately the same for all wires, confirming that the electron gas in the AlxGa1-xN/GaN heterostructure is not deteriorated by the fabrication procedure of the wire structures. For the wider quantum wires, the weak antilocalization effect is clearly observed, indicating the presence of spin-orbit coupling. For narrow quantum wires with an effective electrical width below 250nm , the weak antilocalization effect is suppressed. By comparing the experimental data to a theoretical model for quasi-one-dimensional structures, we come to the conclusion that the spin-orbit scattering length is enhanced in narrow wires.
    Physical review. B, Condensed matter 01/2007; 76(20). · 3.77 Impact Factor
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    ABSTRACT: We studied the suppression of weak antilocalization in an AlxGa1-xN/GaN two-dimensional electron gas in the presence of an additional in-plane magnetic field. By comparing our experimental data to a theoretical model, we concluded that the suppression can be attributed mainly to the Zeeman effect, while the contribution due to disorder at the AlxGa1-xN/GaN heterointerface is considerably smaller. Furthermore, our results give further evidence for the value of spin-orbit scattering length determined from weak antilocalization measurements.
    Physical review. B, Condensed matter 01/2007; 75(19). · 3.77 Impact Factor
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    ABSTRACT: This paper reports on the growth of Cr-doped GaN layers by metal organic vapor phase epitaxy (MOVPE) and their characterization for possible spintronic applications. We have used bis (cyclopentadienyl)chromium (Cp2Cr) to intentionally incorporate chromium (Cr) during GaN layer growth. The effects of the carrier gas, hardware setup, and growth temperature on the growth of Cr-doped GaN were investigated. A linear dependence between mole fraction of Cp2Cr in the gas phase and incorporated Cr in the solid phase was found. The surface morphology was mostly influenced by carrier gas and growth temperature. A remanent magnetization was observed even above room temperature. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 12/2006; 204(1):72 - 77. · 1.46 Impact Factor
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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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    ABSTRACT: In the present paper, studies on the state of strain in single and ensembles of nanocolumns investigated by photoluminescence spectroscopy will be presented. The GaN nanocolumns were either grown in a bottom-up approach or prepared in a top-down approach by etching compact GaN layers grown on Si(111) and sapphire (0001) substrates. Experimental evidence for strain relaxation of the nanocolumns was found. The difference and development of the strain value for different nanocolumns could be verified by spatially resolved micro-photoluminescence on single nanocolumns separated from their substrate. A common D0X spectral position at 3.473 eV was found for all separated single GaN nanocolumns independent of the substrate or processing technique used, as expected for a relaxed system.
    Nano Letters 05/2006; 6(4):704-8. · 13.03 Impact Factor
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    ABSTRACT: This version is available at http://dx.doi.org/10.1063/1.2162871 Copyright © American Institute of Physics Spin-orbit scattering in a polarization-doped Al0.30Ga0.70N/GaN two-dimensional electron gas with one occupied subband is studied at low temperatures. At low magnetic fields weak antilocalization is observed, which proves that spin-orbit scattering occurs in the two-dimensional electron gas. From measurements at various temperatures the elastic scattering time ...tr, the dephasing time ... , and the spin-orbit scattering time ...so are extracted. Measurements in tilted magnetic fields were performed, in order to separate spin and orbital effects.
    Applied Physics Letters 01/2006; · 3.79 Impact Factor
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    ABSTRACT: Metalorganic vapor phase epitaxy (MOVPE) of GaN was performed using hydrogen (H2), nitrogen (N2) and H2/N2mixtures thereof as the carrier gas in the high temperature buffer growth range. The effect of carrier gas on the structural and morphological characteristics of the epilayers was systematically studied using interference and atomic force microscopy (AFM), photoluminescence (PL) measurements at 2 K, Raman spectroscopy and X-ray diffraction (XRD). The higher the N2 content in the carrier gas, the more pinholes are observed, the lower compressive strain and the higher dislocation density in the layers. A carrier gas composition range was defined at which GaN layers with acceptable structural and morphological characteristics are achieved. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2006;
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    ABSTRACT: Weak antilocalization was studied in an Alx Ga1-x N/GaN two-dimensional electron gas as a function of temperature for various gate voltages. By fitting the weak antilocalization measurements by a theoretical model we found that the spin-orbit scattering length does not vary upon changing the carrier concentration or the temperature. The occurrence of spin-orbit coupling was attributed to the crystal inversion asymmetry. The presence of beating patterns observed in the Shubnikov-de Haas oscillations were not assigned to the presence of spin-orbit coupling but rather to structural inhomogeneities in the Alx Ga1-x N/GaN crystal.
    physica status solidi (c) 01/2006;
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    ABSTRACT: This version is available at http://link.aps.org/abstract/PRB/v73/e241311 Copyright © The American Physical Society Weak antilocalization and the Shubnikov–de Haas effect were investigated in AlxGa1–xN/GaN two-dimensional electron gases. The weak antilocalization measurements on a gated sample revealed a constant spin-orbit scattering length, which does not change if the Al content or the thickness of the AlxGa1–xN barrier layer is varied. The occurrence of spin-orbit coupling is assigned to the lack of crystal inversion symmetry. Although for some of the samples a beating pattern was observed in the Shubnikov–de Haas oscillations, its presence was not attributed to spin-orbit coupling but rather to inhomogeneities in the AlxGa1–xN barrier. The authors thank M. Marso ISG-1 for performing the CV measurements and H. Kertz ISG-1 for assistance during the low-temperature measurements. Fruitful discussions with R. Winkler Northern Illinois University and Yu. Lyanda-Geller Purdue University are gratefully acknowledged.
    01/2006;
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    ABSTRACT: Starting out from our well established process for AlGaN/GaN HFETs, the authors discuss ways to enrich the process in order to fabricate metal-oxide-semiconductor HFETs (MOSHFETs) with a gadolinium scandate (GdScO3) insulation layer. In particular, adequate processing orders, various etching procedures and possible drawbacks of the GdScO3 deposition process on ohmic contacts are discussed. Making use of the gained knowledge, the authors fabricated GdScO3-MOSHFETs for the first time. Compared to a conventional HFET the new device shows a higher saturation drain current and a lower gate leakage current. Nevertheless, the potential insulating properties of GdScO3 are not fully exploited yet and further optimization of the deposition process is needed
    01/2006;
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    ABSTRACT: Weak antilocalization and the Shubnikov-de Haas effect were investigated in AlxGa1-xN/GaN two-dimensional electron gases. The weak antilocalization measurements on a gated sample revealed a constant spin-orbit scattering length, which does not change if the Al content or the thickness of the AlxGa1-xN barrier layer is varied. The occurrence of spin-orbit coupling is assigned to the lack of crystal inversion symmetry. Although for some of the samples a beating pattern was observed in the Shubnikov-de Haas oscillations, its presence was not attributed to spin-orbit coupling but rather to inhomogeneities in the AlxGa1-xN barrier.
    Physical Review B 01/2006; 73(24). · 3.66 Impact Factor
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    ABSTRACT: This paper reports on how the observation of the morphology development and growth by in situ optical methods as well as the determination of substrate temperature can be employed to tailor the characteristics of GaN and to control growth in MOVPE (metalorganic vapor phase epitaxy). Furthermore for the first time a method will be demonstrated that allows the difficult determination of an alloy composition – here (Al$_{\rm x}$Ga$_{\rm 1-x}$)N – independent on the perfection and roughness of the developing layer.
    Journal de Physique IV (Proceedings) 01/2006; · 0.29 Impact Factor
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    ABSTRACT: In this paper we will first report on the use of real-time determination of wafer temperature for transparent substrates. With this method we will study the unintentional influence of growth parameter variations on the surface temperature. The effect on nitride growth optimization will be discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (b) 09/2005; 242(13):2581 - 2586. · 1.49 Impact Factor
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    ABSTRACT: We report results on growth studies of Fe on GaN, in particular with respect to structural and magnetic properties. The growth of GaN has been carried out by molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE) on Si(111) and Al2O3 substrates, respectively. Fe depositions of different thicknesses were performed in ultra high vacuum (UHV) at room temperature using an electron-beam evaporation set-up. X-ray diffraction analysis shows that the iron films are crystalline and indications of a (110) bcc orientation of the film are observed. By means of scanning tunneling microscopy (STM) epitaxial islands of Fe on the GaN(0001) surface, on a scale of 500 × 500 nm2, have been observed. The experimentally determined magnetic hysteresis loops, with the magnetic field applied parallel to the sample surface, show a coercive field that decreases as the temperature increases; at 300 K and 50 K we measure a coercive field of 12 G and 36 G, respectively. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 03/2005; 202(5):754 - 757. · 1.46 Impact Factor