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N Layers (n + layer)
Selected AbstractsGaN growth on LiNbO3 (0001) , a first-principles simulationPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010Simone Sanna Abstract The growth of GaN on the LiNbO3 (0001) surface is simulated by means of first-principles total-energy calculations. Firstly the adsorption of single N and Ga monolayers is investigated and then the layer-by-layer growth of GaN on the polar substrate within different orientations is modeled. While adsorbing a N layer does not heavily affect the substrate morphology, the adsorption of a Ga layer causes a rearrangement of the atomic structure. Furthermore the N layer is more strongly bound to the substrate than the Ga layer. On the basis of our results, we propose a microscopic model for the GaN/LiNbO3 interface. The GaN and LiNbO3 (0001) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [100]GaN II [110]. In this way the (0001) calculated in-plane lattice mismatch between GaN and LiNbO3 is minimal and equal to 6.79% of the GaN lattice constant. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] TEM investigations of (In,Ga)N/GaN quantum structuresPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2008P. Manolaki Abstract The paper reports on the influence of the growth temperature on the structural and chemical properties of (In,Ga)N quantum wells (QWs) on GaN. Two different samples A and B were fabricated. The QWs of the sample A were grown at a constant temperature of 600 °C. For the QWs of the sample B the temperature was 530 °C, while for the GaN barrier it was raised to 600 °C. The chemical and structural properties were studied by electron diffraction contrast imaging using the 0001 and 0002 reflection, respectively. Sample A exhibits homogeneous (In,Ga)N QWs. For sample B some undulated strain contrast of the QWs is visible hinting to the formation of quantum dots (QDs). The self-organisation of (In,Ga)N QDs in sample B is also evidenced by composition sensitive STEM-HAADF imaging, where the individual (In,Ga)N layers exhibit inhomogeneous intensity as well as varied thickness. Moreover, energy dispersive X-ray spectroscopy yielded enrichment of indium at QD sites. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photoluminescence and Hall studies of GaN:Fe and (Ga,Fe)N:Mg layersPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2007M. Wegscheider Abstract Temperature dependent photoluminescence (PL) in the ultraviolet (UV) regime and Hall measurements at room temperature have been performed on Metal-Organic-Chemical-Vapour-Deposition (MOCVD) grown GaN:Fe and (Ga,Fe)N:Mg layers. PL measurements were employed in order to study the dopants' influence on the near-band edge excitonic emission and their tendency to provoke the formation and suppression of defects or incorporation of impurities. For their identification and for the understanding of the PL spectra the evaluation of the free carrier concentrations via Hall measurements were necessary. Depending on the iron concentration of the (Ga,Fe)N layers, the near-band edge emission goes through two different stages: at low Fe-concentration no excitonic emission can be seen whereas with higher doping levels, excitonic features develop. The (Ga,Fe)N films exhibit n-type behaviour. The Mg codoped samples show strong Mg and defect related luminescence bands, whose occurrence and intensity also strongly depends on whether high or low Fe concentration is present. The (Ga,Fe)N:Mg layers were semi insulating. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Strain-engineered novel III,N electronic devices with high quality dielectric/semiconductor interfacesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003M. Asif Khan Abstract Since the early demonstration of 2D-electron gas [M. A. Khan et al., Appl. Phys. Lett. 60, 3027 (1992)] and a heterojunction field effect transistor (HFET) [M. Asif Khan et al., Appl. Phys. Lett. 63, 1214 (1993)] in III,N materials, rapid progress has been made to improve the DC and RF performance of GaN,AlGaN based HFETs. Stable and impressive microwave powers as high as 4,8 W/mm have been reported for device operation frequencies from 10 to 35 GHz. The key reason for these high performance numbers is an extremely large sheet carrier densities (>1 × 1013 cm,2) that can be induced at the interfaces in III,N hetereojunction [A. Bykhovsk et al., J. Appl. Phys. 74, 6734 (1993); M. Asif Khan et al., Appl. Phys. Lett. 75, 2806 (1999)]. These are instrumental in screening the channel dislocations thereby retaining large room temperature carrier mobilities (>1500 cm2/Vs) and sheet resistance as low as 300 ,/sq. These numbers and the high breakdown voltages of the large bandgap III,N material system thus enable rf-power approximately 5,10 times of that possible with GaAs and other competitor's technologies. We have recently introduced a unique pulsed atomic layer epitaxy approach to deposit AlN buffer layers and AlN/AlGaN superlattices [J. Zhang et al., Appl. Phys. Lett. 79, 925 (2001); J. P. Zhang et al., Appl. Phys. Lett. 80, 3542 (2002)] to manage strain and decrease the dislocation densities in high Al-content III,N layers. This has enabled us to significantly improve GaN/AlGaN hetereojunctions and the device isolation. The resulting low defect layers are not only key to improving the electronic but also deep ultraviolet light-emitting diode devices. For deep UV LED's they enabled us to obtain peak optical powers as high as 10 mW and 3 mW for wavelengths as short as 320 nm and 278 nm. Building on our past work [M. Asif Khan et al., Appl. Phys. Lett. 77, 1339 (2000); X. Hu et al., Appl. Phys. Lett. 79, 2832 (2001)] we have now deposited high quality SiO2/Si3N4 films over AlGaN with low interface state densities. They have then been used to demonstrate III,N insulating gate transistors (MOSHFET (SiO2) and MISHFET (Si3N4) with gate leakage currents 4,6 order less than those for conventional GaN,AlGaN HFETs. The introduction of the thin insulator layers (less then 100 Å) under the gate increases the threshold voltage by 2,3 V. In addition, it reduces the peak transconductance gm. However the unity cut-off frequency, the gain and the rf-powers remain unaffected as the gm/Cgs (gate-source capacitance) ratio remains unchanged. In addition to managing the defects and gate leakage currents we have also employed InGaN channel double heterojunction structures (AlInGaN,InGaN,GaN) to confine the carriers thereby reducing the spillover into trappings states. These InGaN based MOS-DHFETs exhibited no current-collapse, extremely low gate leakage currents (<10,10 A/mm) and 10,26 GHz rf-powers in excess of 6 W/mm. We have also demonstrated the scalability and stable operation of our new and innovative InGaN based insulating gate heterojunction field effect transistor approach. In this paper we will review the III,N heterojunction field-effect transistors progress and pioneering innovations including the excellent work from several research groups around the world. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Comparative Studies on Mo,Cr,N and Al,Cr,N Coatings Obtained by PVD Dual Magnetron SputteringPLASMA PROCESSES AND POLYMERS, Issue S1 2009Yacine Benlatreche Abstract Recently, several studies have shown that the addition of a secondary element likes Al, Si, etc. to nitride and carbide binary systems such as Ti,N and Cr,N improved their structural and mechanical properties and also their thermal stability. In this study, we realized a comparison between the effects of aluminium or molybdenum addition on the properties of the Cr,N system. The (Cr,Al)N and (Cr,Mo)N films were deposited by RF dual magnetron sputtering. To control the aluminium and molybdenum contents in (Cr,Al)N and in (Cr,Mo)N films, respectively, we modified the Cr, Al and Mo target bias. The structural, morphological and composition analyses of the deposited films were carried out using X-ray diffraction (XRD) and SEM equipped with an energy dispersive spectroscopy (EDS) microanalysis. The variation of the residual stresses with the Al and Mo contents has been studied using the Newton's rings method. The obtained Al contents in (Cr,Al)N deposited films varied between 0 and 51,at.% while the Mo contents in (Cr,Mo)N layers varied between 0 and 42,at.%. A morphological change from amorphous to columnar films has been observed with the addition of Al in the case of (Cr,Al)N coatings, while all the (Cr,Mo)N films presented a columnar structure. The residual stresses of the (Cr,Mo)N coatings are higher than the (Cr,Al)N ones but they exhibited a similar behaviour for both coatings. [source] Synthesis, structure and photoluminescence of a 3D pillared heterometallic coordination polymer containing 2D inorganic cadmium-potassium-oxide layer subunitsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 3 2009Qing-Yan Liu Abstract Self-assembly of Cd(NO3)2 with o -phthalic acid monopotassium salt (KHphth) in the presence of ethylenediamine (en) produced a new heterometallic coordination polymer [CdK2(phth)2(en)0.5(H2O)]n (1). Single-crystal X-ray analyses reveal that it crystallizes in a monoclinic space group P 21/c. a = 11.6707(6) Å, b = 8.1019(4) Å, c = 20.9503(11) Å, , = 94.6640(10)o. The complex displays an en-pillared 3D framework, which is constructed from 2D [CdK2(phth)2(H2O)]n layers featuring uncommon inorganic cadmium-potassium-oxide layers containing potassium-oxide layers. In the solid state, complex 1 shows photoluminescence with the maximum emission intensities at 355 nm upon excitation at 312 nm. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Analysis of multilayer substrates by multilayer contribution of wave concept iterative processMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 6 2007El Amjed Hajlaoui Abstract An efficient iterative technique based on the concept of wave is presented for computing multilayer substrates. This article presents an extensible approach of the iterative method to study substrates with n layers. To validate our theory, we intend to study multilayer dielectric substrates. The approach involves the mixed magnetic and electric filed equation technique and the multilayer contribution of wave concept iterative process, which involves S-parameters extraction technique based on a simple form of Matched Load Simulation. In this sense, a program in FORTRAN has been elaborated to determine different parameters Sij characterizing the studied structure. The numerical results are compared with the published data. Good agreement will be important to establish the validity and usefulness of the iterative method given in this article. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1439,1445, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22406 [source] | ||||||||||