Linköping Linnaeus Initiative for Novel Functional Materials
LiLi-NFM is a coordinated laboratory for interdisciplinary research on advanced materials. It is supported by the Swedish Research Council (VR) for a 10-year period until 2016 by a Linnaeus Grant. The research environment constitutes the back-bone of materials research at Linköping. It consists of ~150 researchers from 9 divisions of IFM. Read more...
Read highlights 2014 here:
Carbon vacancy in SiC: Electron trap for lifetime control
SiC is a semiconductor for high-voltage power devices. However, carrier lifetime in pure SiC is still too short to meet the requirement for blocking voltages above several kV. Carrier lifetime in SiC is known to be limited by the Z1/Z2 center–a common defect acting as an efficient trap of electrons. Z1/Z2 traps two electrons and changes its charge state, e.g. from neutral to double negative. It was suggested that Z1/Z2 is a defect related to a missing carbon atom – a carbon vacancy (VC). However, the exact origin of Z1/Z2 was not clarified. Using n-type 4H-SiC layers irradiated by 250 keV, we were able to identify the negatively charged VC by electron paramagnetic resonance (EPR) and to perform EPR and electrical measurements on the same samples, providing a one-to-one correlation between Z1/Z2 and VC in concentration and energy levels. The results are further supported by large-scale supercell calculations, allowing a conclusive identification of the Z1/Z2 center, including high-lying energy levels, to be the double and single negative charge states of VC at the hexagonal and cubic site, respectively. With such energy level arrangement, VC is a suitable defect for carrier lifetime control and also an excellent carrier compensation center to be used for achieving high-purity semi-insulating SiC.
X.T. Trinh, K. Sz á sz, T. Hornos, K. Kawahara, J. Suda, T. Kimoto, A. Gali, E. Janz é n, N.T. Son, Phys. Rev. B 88, 235209 (2013)
AlN: resolving the n-type doping limitations
The debut of the very first deep-UV light-emitting diodes (LEDs) based on AlN (Nature, 2006) is followed ever since by reports on record-breaking performance characteristics from research groups and companies in Japan and USA. The Al(Ga)N materials suffer n-type doping limitations. Issues of major relevance to the n-type conductivity of Al0.77Ga0.23N associated with Si and O incorporation, their shallow donor or deep donor level behavior, and carrier compensation are elucidated by allying (i) study of Si and O incorporation kinetics at high process temperature and low growth rate, and (ii) electron paramagnetic resonance measurements (EPR). The Al0.77Ga0.23N composition correlates to that Al content for which a drastic reduction of the conductivity of Al(Ga)N materials is commonly reported. We note the incorporation of carbon, the role of which for the transport properties of Al(Ga)N has not been widely discussed. EPR spectrum of a shallow donor is observed at low temperatures in darkness in Si-doped Al0.77Ga0.23N epitaxial layers. It is shown from the temperature dependence of the donor concentration on the neutral donor state measured by EPR that Si is a DX (or negative-U) center but behaves as a shallow donor due to a small separation of only 3 meV between the neutral state Ed and the lower-lying negative state EDX. The neutral state is found to follow the effective mass theory with Ed ~52–59 meV.
A. Kakanakova-Georgieva, D. Nilsson, X.T. Trinh, U. Forsberg, N.T. Son, and E. Janzén, Appl. Phys. Lett. 102, 132113 (2013)
X.T. Thang, D. Nilsson, I.G. Ivanov, E. Janzén, A. Kakanakova-Georgieva, and N.T. Son, Appl. Phys. Lett. 103, 042101 (2013)
Reactivity of adducts relevant to the deposition of hexagonal BN from first-principles calculations
Boron nitride (BN) is being established as a semiconductor material of direct wide-band-gap properties for deep-UV emission (Nature Materials, 2004), and more recently as 2D complement to graphene. Most importantly, the achievement of BN has been approached by MOCVD, which is the paramount deposition technology for the fabrication of device structures on large wafer scale. Our original computational work addresses for the first time the gas-phase chemistry of BN. It proceeds at the high process temperature of ~1300oC and involves precursors such as triethylborane (C2H5)3B and ammonia NH3, being prone to parasitic gas-phase interactions with ultimately adverse effect regarding the efficiency/reproducibility of the deposition process and the material quality. We present the essential outcome of investigating the stability of the adduct formed by the initial precursor molecules and its subsequent reactivity, which is acknowledged as being of key importance to advance the understanding and optimization of MOCVD processes for device-quality structures. We discuss several consistent gas-phase reaction pathways in the context of formation of adduct-derived species and with direct B-N bonding, which are of relevance for the actual deposition mechanism underlying the MOCVD of BN.
R.R.Q. Freitas, G.K. Gueorguiev, F. De Brito Mota, C.M.C. De Castilho, S. Stafström, and A. Kakanakova-Georgieva, Chem. Phys. Lett. 583 (2013) 119
Surface potential effect on excitons in Al-GaN/GaN quantum well structures
AlGaN/GaN quantum well (QW) heterostructures grown on sapphire and on GaN substrates fabricated at Linköping University by G. Pozina and C. Hemmingsson have been studied by time-resolved photoluminescence (PL). A dominant contribution of the exciton radiative lifetime is observed in homoepitaxial samples even at enhanced temperatures up to 100 K. The QW-related emission is found to be more sensitive to the near surface built-in electric field in the homoepitaxial samples, revealed as a red shift of the QW exciton energy with decreasing the cap layer thickness. Absence of such shift in the heteroepitaxial samples suggests an increased polarization field due to substantial residual compressive stress.
G. Pozina, C. Hemmingsson, H. Amano, and B. Monemar, Appl. Phys. Lett. 102, 082110 (2013)
Correlation between Si doping and stacking fault related luminescence in homoepitaxial m-plane GaN
Si-doped GaN layers grown on m-plane GaN substrates were investigated by low-temperature cathodoluminescence (CL). Stacking fault (SF) related emission in the range of 3.29-3.42 eV has been observed for samples with moderate doping, while for the layers with high concentration of dopants, no such CL lines have been detected. Perturbation of the SF potential profile by neighboring impurity atoms was suggested to explain the exciton localization at SFs, while this effect would vanish at higher doping levels due to screening.
S. Khromov, B. Monemar, V. Avrutin, H. Morkoc, L. Hultman, and G. Pozina, Appl. Phys. Lett. 103, 192101 (2013)
Optically detected magnetic resonance studies of point defects in quaternary GaNAsP epilayers grown by vapor phase epitaxy
Defect properties of quaternary GaNAsP/GaP epilayers grown by vapor phase epitaxy (VPE) are studied by photoluminescence and optically detected magnetic resonance (ODMR) techniques. Incorporation of more than 0.6% of nitrogen is found to facilitate formation of several paramagnetic defects which act as competing carrier recombination centers. One of the defects (labeled as Gai-D) is identified as a complex defect that has a Ga interstitial (Gai) atom residing inside a Ga tetrahedron as its core. A comparison of Gai-D with other Gai-related defects known in ternary GaNP and GaNAs alloys suggests that this defect configuration is specific to VPE-grown dilute nitrides.
D. Dagnelund, Jan Stehr, A. Yu. Egorov, W. M. Chen and I. A. Buyanova Appl. Phys. Lett. 102, 021910 (2013)
Broken symmetry induced band splitting in the Ag2Ge surface alloy on Ag(111)
The interaction between group IV elements and the Ag(111) surface results in strong spin splits of the surface bands in the case of Pb, while the lightest group IV elements, Si and C, form technologically interesting two-dimensional structures in the form of silicene and graphene, respectively. Our studies of the Ge/Ag(111) surface alloy with ARPES and STM reveal an intriguing band structure as illustrated in the figure. Instead of the expected single band, there are four branches indicated by the arrows. The complicated band structure originates from periodic lateral distortions of the alloy, with a possible spin splitting of the bands indicated by the arrows, which calls for an investigation by spin-ARPES.
W. Wang, Hafiz M. Sohail, Jacek R. Osiecki, and R. I. G. Uhrberg, Phys. Rev. B 89, 125410 (2014).
Toughness Enhancement in Hard Ceramic Thin Films by Alloy Design, Toughness Enhancement in Hard Ceramic Thin Films by Alloy Design
Enormous effort has been dedicated within the last decades to enhancing hardness in ceramic coatings. However, hardness alone, typically accompanied by brittleness leading to cracks, is not sufficient to prevent failure in ceramic films exposed to high thermo-mechanical stress, erosion, corrosion, and/or wear. Using VN as a model system, we demonstrate with density functional theory and experiment that refractory VMoN alloys, via manipulation of electronic d-t2g state occupancy, exhibit not only enhanced hardness, but dramatically increased ductility. V0.5Mo0.5N hardness is 25% higher than that of VN. In addition, while nanoindented VN and TiN reference samples suffer from severe cracking typical of brittle ceramics, V0.5Mo0.5N films do not crack. Instead, they exhibit material pile-up around nanoindents, characteristic of plastic flow in ductile materials. Moreover, the wear resistance of V0.5Mo0.5N is considerably higher than that of VN. These results provide insight into the electronic origin of toughness in brittle materials and a pathway toward design of new tough, hard coatings.
H. Kindlund, D.G. Sangiovanni, L. Martínez-de-Olcoz, J. Lu, J. Jensen, J. Birch, I. Petrov, J.E. Greene, V. Chirita and L. Hultman, APL Materials 1, 042104 (2013)
Tunable Interface Properties between Pentacene and Graphene on the SiC Substrate
Understanding energy-level alignment and molecular growth characteristics of an organic semiconductor on the graphene surface is crucial for graphene-related device performance. Here we demonstrate that tunable interface properties and molecular orientation can be achieved by modifying graphene films on a SiC substrate with monolayer copper-hexadecafluorophthalocyanine (F16CuPc) molecules. On clean graphene, pentacene molecules form a tilted configuration even at very low coverage (one or two monolayers) rather than flat-lying as on the graphite surface. Pentacene molecules prefer to grow with a (022) plane parallel to the clean graphene surface. With increasing coverage, X-ray adsorption data indicate there is no obvious change of molecular stacking orientation. The corresponding hole injection barrier is about 0.7 eV. On the modified graphene where thin (one or two monolayers) F16CuPc molecules are flatlying on graphene, an almost perfect up-standing molecular stacking of pentacene film was formed on the modified surface. A low hole injection barrier of 0.3 eV was observed. Furthermore, the interface of dirty graphene upon pentacene was also discussed.
Xianjie Liu, Alexander Grüneis, Danny Haberer, Alexander V. Fedorov, Oleg Vilkov, Wlodek Strupinski, and Thomas Pichler J. Phys. Chem. C, 117,3969 (2013).
KTH, School of ICT, Department of Materials and Nanophysics, Kista, Sweden. Thermal transport in suspended Si membranes. Read abstract.
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