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Thesis; Thesis information not provided; 1 Feb 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "JLAB-PHY-03-38" "DOE/ER/40150-2762" Bin Zhang. 02/01/2003. Report is also available in paper and microfiche from NTIS.

This thesis is concerned with the growth, electronic properties and modification of hydrogenated amorphous carbon films of a thickess range of 50-300 nm, which have been deposited using rf plasma-enhanced chemical vapour deposition. These films may be subdivided into two types according to the electrode on which they are grown and the resulting film properties. These are polymer-like amorphous carbon or PAC, and diamond-like amorphous carbon or DAC. PAC possesses a wide optical band gap (2.7 eV), high resistivity (1014 - 10 15 Ocm) and low density of paramagnetic defects (~ 10 17 spins cm-3). The dominant current transport mechanism at room temperature has been observed to be hopping conduction at low electric fields and space-charge-limited current at high electric fields. The addition of nitrogen gas to the plasma to incorporate nitrogen within the film has been shown to move the Fermi level by 1 eV, towards midgap. A mechanism of doping due to the introduction of aromatic nitrogen-containing sites has been postulated. The boron, carbon and nitrogen ion implantation of PAC has resulted in the controllable increase in conductivity from 1015 to 106 O cm as a function of ion dose, from 2 x 1012 to 2 X 1016 ions cm-2. At low ion doses (up to 6 x 1014 ions cm-2) this occurs without any change in band gap; however, at higher doses the band gap collapses as a result of graphitisation. The dependence on the implant ion shows that it is possible to move the Fermi level towards the valence band with the implantation of boron, and towards midgap with the implantation of nitrogen. A hysteresis effect is observed at intermediate ion doses, which is attributed to the trapping of holes resulting in an increase in electron current. Implanting part of the thickness of the film at this ion dose has resulted in rectification, which has not previously been reported for this type of structure in amorphous carbon. DAC has been shown to possess a smaller band gap (0.7 eV), higher density of defects (~ 1020 spins cm-3) and lower resistivity (~ 1013 O cm) than PAC. The room-temperature current transport is governed by band-tail conduction at fields below 105 V cm-1, and the Poole-Frenkel effect at higher fields. The addition of nitrogen of up to 8 at. % has been observed to increase the band gap from 0.7 to 1.0 eV and therefore decrease the magnitude of the Poole-Frenkel conductivity. The Fermi level remains pinned at midgap, however. Therefore, it appears that PAC shows advantages over DAC in terms of future device applications.

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Des puits quantiques à base d'hétérostructures In0.53 Ga0.47 As/In0.52 Al0.48 As, fabriqués par épitaxie par jets moléculaires sur substrats InP(111)A, sont étudiés par microscopie et spectroscopie à effet tunnel à basse température et sous ultra-vide. La première partie est consacrée à une étude de la surface épitaxiée (111)A de In0.53 Ga0.47 As de type n. Il est découvert que le niveau de Fermi de surface est positionné dans la bande de conduction, à proximité du niveau de Fermi de volume, et peut être partiellement contrôlé en variant la concentration d'impuretés de type n dans le volume. Ce résultat est confirmé en déterminant la relation de dispersion de la bande de conduction en surface. Un tel dépiégeage partiel du niveau de Fermi de surface indique que la densité d'états de surface accepteurs est faible. Il est proposé que ces états proviennent de défauts ponctuels natifs localisés à la surface. La deuxième partie, basée sur les résultats obtenus dans la première partie, est consacrée à une étude de puits quantiques In0.53 Ga0.47 As de surface, déposés sur des barrières In0.52 Al0.48 As selon la direction (111)A. Les mesures sont conduites sur la surface épitaxiée (111)A du puits quantique In0.53 Ga0.47 As, de manière à pouvoir sonder à l'échelle du nanomètre la distribution de densité locale d'états électroniques dans le plan du puits quantique. Il est confirmé que des sous-bandes électroniques sont formées dans le puits quantique, et que la concentration d'électrons dans le puits peut être contrôlée du fait du dépiégeage partiel du niveau de Fermi de surface. Il est découvert qu'un phénomène de percolation d'états localisés survient dans la queue de chaque sous-bande, ce qui indique la présence d'un potentiel désordonné dans le puits quantique. Les seuils de percolation sont déterminés en utilisant un modèle semi-classique. L'origine du potentiel désordonné est attribuée à une distribution aléatoire des défauts ponctuels natifs à la surface du puits quantique. Il est également découvert qu'un état lié apparaît au bas de chaque sous-bande à proximité d'un défaut ponctuel natif de type donneur. L'énergie de liaison et le rayon de Bohr des états liés peuvent être directement déterminés. De plus, il est démontré que l'énergie de liaison et le rayon de Bohr sont fonctions de l'épaisseur du puits quantique, en accord quantitatif avec des calculs variationnels d'impuretés dans le modèle de l'atome d'hydrogène.

19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 1936" Jongik Park. 12/19/2004. Report is also available in paper and microfiche from NTIS.

Defects in the functional oxides play an important role in electronic devices like metal oxide semiconductor field effect transistors (MOSFETs) and resistive random-access memories (ReRAMs). The continuous scaling of CMOS has brought the Si MOSFET to its physical technology limit and the replacement of Si channel with Ge channel is required. However, the performance of Ge MOSFETs suffers from Ge/oxide interface quality and reliability problems, which originates from the charge traps and defect states in the oxide or at the Ge/oxide interface. The sub-oxide layers composed of GeII states at the Ge/GeO2 interface seems unavoidable with normal passivation methods like hydrogen treatment, which has poor electrical properties and is related to the reliability problem. On the other hand, ReRAM works by formation and rupture of O vacancy conducting filaments, while how this process happens in atomic scale remains unclear. In this thesis, density functional theory is applied to investigate the defect behaviours in oxides to address existing issues in these electronic devices. In chapter 3, the amorphous atomic structure of doped GeO2 and Ge/GeO2 interface networks are investigated to explain the improved MOSFET reliability observed in experiments. The reliability improvement has been attributed to the passivation of valence alternation pair (VAP) type O deficiency defects by doped rare earth metals. In chapter 4, the oxidation mechanism of GeO2 is investigated by transition state simulation of the intrinsic defect diffusion in the network. It is proposed that GeO2 is oxidized from the Ge substrate through lattice O interstitial diffusion, which is different from SiO2 which is oxidized by O2 molecule diffusion. This new mechanism fully explains the strange isotope tracer experimental results in the literature. In chapter 5, the Fermi level pinning effect is explored for metal semiconductor electrical contacts in Ge MOSFETs. It is found that germanides show much weaker Fermi level pinning than normal metal on top of Ge, which is well explained by the interfacial dangling bond states. These results are important to tune Schottky barrier heights (SBHs) for n-type contacts on Ge for use on Ge high mobility substrates in future CMOS devices. In chapter 6, we investigate the surface and subsurface O vacancy defects in three kinds of stable TiO2 surfaces. The low formation energy under O poor conditions and the +2 charge state being the most stable O vacancy are beneficial to the formation and rupture of conducting filament in ReRAM, which makes TiO2 a good candidate for ReRAM materials. In chapter 7, we investigate hydrogen behaviour in amorphous ZnO. It is found that hydrogen exists as hydrogen pairs trapped at oxygen vacancies and forms Zn-H bonds. This is different from that in c-ZnO, where H acts as shallow donors. The O vacancy/2H complex defect has got defect states in the lower gap region, which is proposed to be the origin of the negative bias light induced stress instability.