Characterization of epitaxial GaAs MOS capacitors using atomic layer-deposited TiO2/Al2O3 gate stack: study of Ge auto-doping and p-type Zn doping

Electrical and physical properties of a metal-oxide-semiconductor [MOS] structure using atomic layer-deposited high-k dielectrics (TiO2/Al2O3) and epitaxial GaAs [epi-GaAs] grown on Ge(100) substrates have been investigated. The epi-GaAs, either undoped or Zn-doped, was grown using metal-organic chemical vapor deposition method at 620°C to 650°C. The diffusion of Ge atoms into epi-GaAs resulted in auto-doping, and therefore, an n-MOS behavior was observed for undoped and Zn-doped epi-GaAs with the doping concentration up to approximately 1017 cm-3. This is attributed to the diffusion of a significant amount of Ge atoms from the Ge substrate as confirmed by the simulation using SILVACO software and also from the secondary ion mass spectrometry analyses. The Zn-doped epi-GaAs with a doping concentration of approximately 1018 cm-3 converts the epi-GaAs layer into p-type since the Zn doping is relatively higher than the out-diffused Ge concentration. The capacitance-voltage characteristics show similar frequency dispersion and leakage current for n-type and p-type epi-GaAs layers with very low hysteresis voltage (approximately 10 mV)

Effect of embedded nanoparticle surface chemistry on plasmonic organic photovoltaic devices

The effect of noble metal nanoparticle (NP) synthesis method on the plasmonic enhancement of organic photovoltaic device performance by these NPs is reviewed. For direct incorporation into a polymer fullerene bulk heterojunction (BHJ) active layer, chemically synthesized colloidal Au or Ag NPs with organic ligands are generally ineffective. Due to the tendency of the ligands in causing exciton quenching, carrier trapping and recombination, the device power conversion efficiency (PCE) can be lower than a BHJ device without NPs. Ligand-free metal NPs prepared by physical methods such as pulsed laser ablation and electron beam evaporation can enhance the PCE when introduced into the BHJ. An alternative effective approach for both polymer and small molecule BHJ devices is core shell metal–silica NPs. Regardless of synthesis method, the NP size should be controlled to the range of ~50–100 nm to increase light trapping due to scattering and achieve synergistic enhancement. A non-spherical NP morphology with tunable dual localized surface plasmon resonance peaks at visible wavelengths is highly desirable. For core shell metal–silica NPs, the dielectric shell thickness must be optimized to ensure significant localized field enhancement at the surface of the NP.Published versio

Time dependent dielectric breakdown in copper low-k interconnects : mechanisms and reliability models

The time dependent dielectric breakdown phenomenon in copper low-k damascene interconnects for ultra large-scale integration is reviewed. The loss of insulation between neighboring interconnects represents an emerging back end-of-the-line reliability issue that is not fully understood. After describing the main dielectric leakage mechanisms in low-k materials (Poole-Frenkel and Schottky emission), the major dielectric reliability models that had appeared in the literature are discussed, namely: the Lloyd model, 1/E model, thermochemical E model, E1/2 models, E2 model and the Haase model. These models can be broadly categorized into those that consider only intrinsic breakdown (Lloyd, 1/E, E and Haase) and those that take into account copper migration in low-k materials (E1/2, E2). For each model, the physical assumptions and the proposed breakdown mechanism will be discussed, together with the quantitative relationship predicting the time to breakdown and supporting experimental data. Experimental attempts on validation of dielectric reliability models using data obtained from low field stressing are briefly discussed. The phenomenon of soft breakdown, which often precedes hard breakdown in porous ultra low-k materials, is highlighted for future research.Published versio

Low dielectric constant materials for multilevel interconnect applications

Low dielectric constant (low-k) constitutive dielectrics for interconnects have been fabricated by both plasma deposition and solution synthesis from -organosilicon precursors. Fluorinated silicon oxide (FSG) was deposited from an inductively coupled high density plasma. The mechanisms of dielectric constant reduction and moisture uptake at high fluorine concentration were determined. Carbon doped silicon oxide (SiOCH) was deposited by glow discharge plasma from trimethylsilane and the effect of deposition parameters on film properties were systematically studied. Two synthesis routes for nanoporous inorganic-organic hybrid low-k dielectrics were developed. Organically modified nanoporous silica was synthesized by a multiple step sol gel process from two precursors and a dielectric constant of 2 was realized. Nanoporous hydrogen methylsilsesquioxane was prepared by thermal curing of the precursor and a dendrimer sacrificial template followed by thermal decomposition of the template

Epitaxy and X-ray lithography of silicon germanium semiconductors

Three series of silicon germanium (SiGe) films totaling fourty seven wafers had been grown by chemical vapor deposition (CVD) technique on Si(100) and Si(111) substrates. The first series (series 1) consists of nominally undoped SiGe single layers with Ge content of 10 at.% and 30 at.% . The second series (series 2) consists of undoped as well as boron and phosphorus doped SiGe single layers with Ge content of 10, 20 and 30 at.%. The third series (series 3) are SiGe heterostructures and the multilayer stacks include single SiGe quantum well, coupled multiple quantum wells and modulation doped high mobility SiGe layers

Semiconducting polymers for optoelectronic and microlithographic applications

A variety of organic electroluminescent materials have been synthesized for the fabrication of organic light emitting diodes (OLED). The materials studied consist of two categories: conjugated (semiconducting) polymers and small organic molecules.RG 53/9

Double heterojunction crystalline silicon solar cells: from doped silicon to dopant-free passivating contacts

Carrier-selective passivating contacts for effective electron and hole extraction are crucial to the attainment of high efficiency in crystalline silicon (Si) solar cells. In this comprehensive review, the principle of carrier extraction and recombination mechanisms in conventional industrial Si solar cells are discussed first. Passivating contacts based on (i) amorphous hydrogenated Si and (ii) polysilicon/silicon oxide are next reviewed, with emphasis on carrier selectivity mechanisms including contact layer band alignment with silicon, and localized carrier transport in ultrathin oxides. More recent developments in dopant-free passivating contacts deposited by lower-cost fabrication processes with lower thermal budget are then described. This third category of non-Si based electron- and hole-selective passivating contacts include transition metal oxides, alkali/alkali earth metal fluorides and organic conjugated polymers. The photovoltaic performance of asymmetric double heterojunction Si solar cells fabricated using these non-Si passivating contacts and their stability in damp heat conditions are discussed and compared with Si based passivating contacts.Published versio

Enhancement of power conversion efficiency in solution processed organic photovoltaic devices by embedded plasmonic gold-silica core-shell nanorods

Chemically synthesized gold-silica nanorods were incorporated into the active layer of solution processed organic photovoltaic devices to enhance the absorption of light by the surface plasmon resonance effect in metallic nanoparticles. Solution processed polymer:fullerene and small molecule:fullerene bulk heterojunction devices were studied. The polymer donors include regioregular poly(3-hexylthiophene) (P3HT) and low bandgap poly[2,6-(4,4-bis-(2-ethylhexyl)- 4N-cyclopenta[2,1-b:3,4-b’] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). For the small molecule device, 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']-dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl) benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) was used as the donor. The donors are blended with either [6,6]-phenyl- C61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). The gold-silica nanorods have an aspect ratio (length/diameter) of 3.2 and 2.3 and a shell thickness of ~10 nm. Prior to spin coating, the nanorods were added directly to the donor:acceptor blend solution in either chlorobenzene or dichlorobenzene at different weight percentage of the total donor:acceptor weight. The transverse and longitudinal surface plasmon resonance peaks of the gold-slica nanorods overlap with the absorption spectra of all three donor:acceptor blends to differing degrees. As a result, the power conversion efficiency of optimized plasmonic P3HT:PC60BM and PCPDTBT:PC70BM devices with conventional structure under AM1.5G illumination at 100mW/cm2 were increased by 9.3% (to 3.42%) and 20.8% (to 4.11%) respectively relative to the control device without nanorods. For the p-DTS(FBTTh2)2:PC70BM device, the relative improvement as compared to the control device was 24.2% (to 8.01%).Published versio

High-performance low-cost sulfide/selenide thermoelectric devices

Thermoelectric (TE) materials with high figure-of-merit ZT are critical to the development of energy harvesting devices that directly convert heat to electricity. This chapter surveys recent developments in Earth-abundant high ZT metal selenides and sulfides. Key physical mechanisms leading to high ZT are discussed first including anharmonicity, band structure engineering by doping, nanoscale size effects and topological defects. The impact of synthesis and consolidation processes on the thermoelectric performance of a range of chalcogenides is described. In particular, factors including changes in stoichiometry, phase separation and precipitation arising from volatilization of the chalcogen at elevated temperatures are discussed. The significant challenges presented by devices constructed from copper chalcogenides exhibiting phonon-liquid electron-crystal (PLEC) behaviour are outlined, and strategies to ameliorate the deleterious effects of ionic diffusion in PLEC-based devices reviewed. Material synthesis, spark plasma sintering and emerging TE processing techniques such as screen-printing are described

Dielectric relaxation in AC powder electroluminescent devices

The dielectric properties of AC powder electroluminescent devices were measured and analyzed using complex impedance spectroscopy to determine the relaxation processes occurring within the devices. The relaxation processes identified were ascribed to the electrode polarization caused by ion accumulation at the electrode/resin interfaces, the Maxwell-Wagner-Sillars effects at the (ZnS or BaTiO3) particle/resin interfaces, and the dipolar reorientation of polymer chains in the resin matrix. Each relaxation process was represented by its corresponding equivalent circuit component. Space charge polarization at the electrodes were represented by a Warburg element, a resistor, and a constant phase element. The resin matrix, ZnS/resin and BaTiO3/resin interfaces could each be modeled by a resistor and a capacitor in parallel. The simulated equivalent circuits for three different printed structures showed good fitting with their experimental impedance results.Accepted versio