Table of contents

Carbon nanotubes have the potential to spur future development in electronics due to their unequalled electrical properties. In this article, we present a review on carbon nanotube-based circuits in terms of their electrical performance in two major directions: nanoelectronics and macroelectronics. In the nanoelectronics direction, we direct our discussion to the performance of aligned carbon nanotubes for digital circuits and circuits designed for radio-frequency applications. In the macroelectronics direction, we focus our attention on the performance of thin films of carbon nanotube random networks in digital circuits, display applications, and printed electronics. In the last part, we discuss the existing challenges and future directions of nanotube-based nano- and microelectronics.

We investigated the electrical properties of solution processed high-k Bi_0.5Na_0.5TiO₃(BNT)-BaTiO₃(BT) on n-GaN with Au electrode. Higher barrier height is obtained for Au/BNT-BT/n-GaN structure compared to Au/n-GaN structure. Thin interfacial layer is formed in between BNT-BT and n-GaN confirmed by TEM results. The interface state density of Au/BNT-BT/n-GaN structure is lower than that of Au/n-GaN structure due to the existence of interfacial layer (Ga-O) at the interface. It is observed that the frequency dispersion is decreased in the Au/BNT-BT/n-GaN structure. Poole–Frenkel mechanism is found to dominate the reverse leakage current in both Au/n-GaN and Au/BNT-BT/n-GaN structures.

The interest in GaN for logic applications is increasing. With complementary logic architectures requiring the lowest power consumption, the need for GaN-based p-channel transistors is growing. Yet, the knowledge and the maturity of p-channel devices is far behind those of their n-channel counterparts. By analysing p-channel transistors with a high on/off ratio and a low subthreshold swing under elevated temperatures, we attempt to improve this situation. This is the first report on transistor operation at temperatures as high as 175 °C.

Wet etching of InAl(Ga)N/GaN structures has been studied in detail by means of Rutherford backscattering spectroscopy, x-ray diffraction, atomic force microscopy and capacitance–voltage profiling (C–V). The samples used for the study were grown on three different substrates (sapphire, silicon carbide and silicon(111)). Nearly lattice-matched compositions were measured for all the samples. We obtained different etching rate depending on the homogeneity and root-mean-square roughness of the surface as well as the underlying substrate, attributing the difference possibly to the presence of threading dislocation in the sample. The study interest is correlated to the possibility to control at a very precise level the thickness etching of the material, making it possible to fabricate normally-off recessed gate high-electron-mobility-transistors.

The work shows a successful fabrication of AlGaN/GaN high electron mobility transistor (HEMT) structures on the bulk GaN substrate grown by ammonothermal method providing an ultralow dislocation density of 10⁴ cm⁻² and wafers of size up to 2 inches in diameter. The AlGaN layers grown by metalorganic chemical vapor phase epitaxy method demonstrate atomically smooth surface, flat interfaces with reproduced low dislocation density as in the substrate. The test electronic devices—Schottky diodes and transistors—were designed without surface passivation and were successfully fabricated using mask-less laser-based photolithography procedures. The Schottky barrier devices demonstrate exceptionally low reverse currents smaller by a few orders of magnitude in comparison to the Schottky diodes made of AlGaN/GaN HEMT on sapphire substrate.

GdScO₃ was deposited by pulsed laser deposition on two different templates suitable for III-N growth: metalorganic vapour phase epitaxial GaN (0 0 0 1) on sapphire and molecular beam epitaxial Y₂O₃ on Si (1 1 1). The structure and crystallinity of the layers were determined as well as the band gap and permittivity of the material. It was found that GdScO₃ grows epitaxially and crystallizes hexagonally in contrast to the usually found orthorhombic or amorphous phases. A band gap and permittivity κ of 5.2 eV and 24 were found, respectively, making GdScO₃ a promising epitaxial gate dielectric for III-N transistor applications.

In this work, we revisit the requirement of higher channel doping (≥10¹⁹ cm⁻³) in junctionless (JL) double gate MOSFETs. It is demonstrated that moderately doped (10¹⁸ cm⁻³) ultra low power (ULP) JL transistors perform significantly better than heavily doped (10¹⁹ cm⁻³) devices. JL MOSFETs with moderate doping results in the spreading out of carriers across the entire silicon film instead of being localized at the center of the film. This improves gate controllability leading to higher on–off current ratio and lower intrinsic delay for ULP subthreshold logic applications. Additional benefits of using a channel doping concentration of 10¹⁸ cm⁻³ instead of conventional heavily doped design is the significant reduction in threshold voltage sensitivity values (by ∼70–90%) with respect to film thickness and gate oxide thickness. Further improvement in ULP performance metrics can be achieved by limiting the source/drain implantation away from the gate edge. This design, specifically for ULP, allows the requirement of gate workfunction to be reduced from p⁺-poly (∼ 5.1 eV) to near about midgap values (∼ 4.8 eV). On–off current ratio and intrinsic delay for optimized JL devices are compared for low standby power projections of the technological roadmap. A 6T-SRAM cell operating at 0.8 V with 25 nm JL devices exhibits a static noise margin of 151 mV with gate workfunction offset of 0.2 eV with respect to midgap value (4.72 eV). The results highlight new viewpoints for realizing improved low power JL transistors.

Gallium and aluminum co-doped ZnO (GAZO) thin films were deposited on glass substrate by using a facing targets sputtering system under various oxygen atmosphere, and the effect of oxygen on their structural, optical and electrical properties was investigated. All as-deposited GAZO thin films under oxygen atmosphere exhibited smooth surface and the lowest value of root-mean-square was 0.6 nm at oxygen 1 sccm, this value is lower than that of film deposited at pure argon atmosphere. The (0 0 2) peak intensity was increased with increase in oxygen flow rate, the peak maximized at oxygen 1 sccm. All films indicated high transmittance above 85% in the visible range and the lowest resistivity of 8.9 × 10⁻⁴ Ω · cm was obtained at pure argon atmosphere.

This work presents a study of the illuminated to dark ratio (IDR) of lateral SOI PIN photodiodes. Measurements performed on fabricated devices show a fivefold improvement of the IDR when the devices are biased in accumulation mode and under high temperatures of operation, independently of the anode voltage. The obtained results show that the doping concentration of the intrinsic region has influence on the sensitivity of the diodes: the larger the doping concentration, the smaller the IDR. Furthermore, the photocurrent and dark current present lower values as the silicon film thickness is decreased, resulting in a further increase in the illuminated to dark ratio.

Engineered or 'virtual' substrates are of interest to extend the range of epitaxially-grown semiconductor heterostructures available for device applications. To this end, elastically strain-relaxed square features up to 30 µm in size and having an in-plane lattice constant as much as 0.49% larger than the lattice constant of GaAs were fabricated from MOCVD-grown GaAs/In_0.08Ga_0.92As/GaAs heterostructures by the in-place bonding method, using either AlAs or Al_0.7Ga_0.3As as the sacrificial layer. TEM images show that the solution-bonded interface is flat with a network of sessile edge dislocations that accommodates the different in-plane lattice constants of the feature and the GaAs substrate and a small rotation of the bonded features. Micro-Raman spectroscopy, which has a spatial resolution of ∼1 µm, was shown to be useful for characterizing lattice mismatch strain ≥ 0.0023, i.e. with an order of magnitude lower sensitivity than high-resolution XRD.

We report on the quantized conductance through side- and top-gated InAs quantum point contacts and discuss its dependence on the temperature and on a magnetic field applied perpendicular to the sample plane. Even in the absence of a magnetic field we observe besides the integer steps in units of 2e²/h spin-resolved steps in units of e²/h up to the highest occupied mode. A conductance anomaly at 0.7 × 2e²/h is found as well.

Type II InSb/InAs quantum dots (QDs) were successfully grown on GaAs substrates using three different metamorphic buffer layer (MBL) designs. The structural properties of the resulting metamorphic InAs buffer layers were studied and compared using cross-sectional transmission electron microscopy and high resolution x-ray diffraction measurements. Photoluminescence (PL) originating from the InSb QDs was observed from each of the samples and was found to be comparable to the PL of InSb QDs grown onto homo-epitaxially deposited InAs. The 4 K PL intensity and linewidth of InSb QDs grown onto a 3 µm thick InAs buffer layer directly deposited onto GaAs proved to be superior to that from QDs grown onto an InAs MBL using either AlSb or GaSb interlayers. Light-emitting diode structures containing ten layers of InSb QD in the active region were subsequently fabricated and electroluminescence from the QDs was obtained in the mid-infrared spectral range up to 180 K. This is the first step towards obtaining mid-infrared InSb QD light sources on GaAs substrates.

Electrodeposition technique is very useful for depositing n-type Cu₂O thin films on various substrates. However, most of the reported n-type Cu₂O thin film electrodes exhibit not only n-type photoactivity but also p-type photoactivity in photoelectrochemical cells. In this study, current–voltage characteristics and zero bias spectral response measurements were employed to investigate the possibilities to remove/minimize this unwanted p-type behaviour of n-type Cu₂O thin films electrodeposited on Ti substrate. For this, prior deposition of Cu thin films on Ti substrate, low temperature annealing of Cu₂O films in air and optimization of deposition bath pH were investigated. Growth of a very thin Cu film improved the n-type photosignal significantly and reduced the p-type photoresponse of the films. Films electrodeposited using an acetate bath of pH 6.1 produced only the n-type photoresponse. Low temperature annealing of Cu₂O films in air improved the n-type photoresponse and it was found that annealing at 100 °C for 24 h produces the best result. These methods will be very useful to obtain electrodeposited Cu₂O thin film with improved n-type photoactivity suitable for applications in thin film solar cells and other devices.

The effect of the lattice-mismatch strain and of the charge carrier confinement profile, on the optical properties of thermally annealed self-assembled In_xGa_1−xAs/GaAs quantum dots (QDs), is theoretically analyzed by using a recently developed 40-band k.p model. First, to evaluate the composition and size of QDs as a function of thermal annealing conditions, we model the In/Ga interdiffusion by a Fickian diffusion. Second, we investigate the decrease of the strain effects on the carrier confinement potentials with annealing by solving the Schrödinger equation separately for electrons and holes. It is clearly found that the strain strongly modifies the QD potential profile, leading to a different electron and hole energy distribution. Finally, we carry on a comparison between theoretical calculations and photoluminescence (PL) experimental results performed in thermal annealed samples. A good agreement is obtained for the energy blueshift and the linewidth narrowing of the PL spectra measured on annealed QD ensemble. These results prove the relevance of the present approach to describe the optoelectronic properties of the nanostructures through the post-growth thermal annealing treatment.

Based on the multiple subbands quasi-ballistic transport theory, we investigate the electronic transport of nano size In_0.53Ga_0.47As nFinFETs with Al₂O₃ gate dielectric, emphasizing the saturation current region. 1D mobile charge density and gate capacitance density are introduced for the first time to describe the nano-FinFET transport property under volume inversion. With the extracted effective channel mobility of electrons in the linear region from our experiments, the electron mean free path λ in the channel with the value of 5–9 nm is obtained. With only one fitting parameter α = 0.31 for the critical length $l=L{{\left( \frac{kT/q}{{{V}_{d}}} \right)}^{\alpha }}$ in the quasi-ballistic transport theory, the calculated drain current can fit all experimental data for various gate voltage V_g, source–drain voltage V_d, and temperature (240–332 K) in overall very good agreement. The backscattering coefficient r in the saturation region is larger than 0.8, indicating a large room for improvement for the present InGaAs FinFET technology and performance.

ZnO nanostructures were synthesized on porous Si (PSi) substrates using the thermal catalytic-free immersion method. Crack-like ZnO nanostructures were formed on the bare, sponge-like PSi structures. An approach to fabricate chemical sensors based on the ZnO/PSi nanostructure arrays that uses an electrochemical impedance technique is reported. Sensor performance was evaluated for ethanol solutions by the morphology and defect structures of the ZnO nanostructure layer. Results indicate that the ZnO/PSi nanostructure chemical sensor exhibits rapid and high response to ethanol compared with a PSi nanostructure sensor because of its small particle size and an oxide layer acting as a capacitive layer on the PSi nanostructure surface.

We convert the surface of ${\rm{S}}{{{\rm{m}}}_{2}}{{{\rm{O}}}_{3}}$ pieces to ${\rm{Sm}}{{{\rm{B}}}_{6}}$ films by means of Mg-assisted boronization. ${\rm{S}}{{{\rm{m}}}_{2}}{{{\rm{O}}}_{3}}$ lumps and ${\rm{Mg}}{{{\rm{B}}}_{2}}$ powder are sealed into a quartz ampule in vacuum. By utilizing thermally decomposed Mg from ${\rm{Mg}}{{{\rm{B}}}_{2}}$ as a catalyst, ${\rm{Sm}}{{{\rm{B}}}_{6}}$ films are produced at temperatures as low as 700 °C. The fabrication method hence enables low-temperature synthesis of ${\rm{Sm}}{{{\rm{B}}}_{6}}$ films without using hazardous substances. We evaluate the structural properties of the films using x-ray diffraction and Raman spectroscopy.

In this paper, the characteristics of a novel device structure, uniformly doped ultra-deep-submicron poly-Si barrier modulated thin film transistor (BM-TFT), are investigated and compared with conventional poly-Si TFT. Use of uniform doping provides a solution from problems associated with random dopant fluctuations. The suppression of the leakage current of the TFT by introducing barrier modulation is verified and presented. The device is optimized with respect to channel length, doping of channel, spacer dielectric and gate dielectric material. Simulations resulted in I_OFF of ~2 × 10⁻¹¹ A μm⁻¹, I_ON of ~2mA μm⁻¹, I_ON/I_OFF of 10⁸, subthreshold slope of 144 mV/dec and DIBL of 119 mV V⁻¹ for PolyGate/HfO₂/Poly-Si coplanar BM-TFT at temperature. of 300 K, gate length of 60 nm, oxide thickness of 5 nm, film thickness of 10 nm, low-k spacer thickness of 20 nm and V_DD of 2.5 V.

Studying the temperature dependence of the electrical properties of Ohmic contacts formed on ion-implanted SiC layers is fundamental to understand and to predict the behaviour of practical devices. This paper reports the electrical characterization, as a function of temperature, of Ni-based Ohmic contacts, simultaneously formed on both n- or p-type implanted 4H-SiC. A structural analysis showed the formation of the Ni₂Si phase after an annealing leading to Ohmic behaviour. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism (TFE) dominates the current transport for contacts formed on p-type material, while a field emission (FE) is likely occurring in the contacts formed on n-type implanted SiC. The values of the barrier height were 0.75 eV on p-type material and 0.45 eV on n-type material. The thermal stability of the current transport mechanisms and related physical parameters has been demonstrated upon a long-term (up to 95 h) cycling in the temperature range 200–400 °C.

We report the investigation of threshold voltage (V_th) instability of AlGaN/GaN metal–insulator–semiconductor (MIS) HEMTs with SiN gate dielectric under forward gate bias stress. A systematic step stress-recovery experiment is implemented to study the charging and discharging kinetics of pre-existing defects. A two-step trapping process is identified: electron fast trapping into SiN/ GaN interface by tunneling through the thin GaN/AlGaN layer followed by the slow dynamics featuring a logarithmic time-dependence of the V_th shift. Full and fast recovery of the V_th instability is induced when devices are subjected to a large negative gate bias, while a slow and incomplete detrapping process occurs at V_g = 0 V. In addition, pulsed current–voltage measurements are developed to estimate trap densities and monitor defect generation before and after stressing.

In this work the investigations of the argon inductively coupled plasma sputtering of the Pb_1−xSn_xTe thin films with the composition variation of x = 0.16–0.95 grown by hot wall deposition technique on glass substrates were carried out. As-grown films had a columnar polycrystalline structure with the grain lateral dimensions of 0.2–5.0 μm, and the dependence of the lattice constant on composition x had a linear behaviour described by the Vegard's law. Energy dispersive x-ray microanalysis showed the presence of 5–8 at.% of oxygen in the films, which can be accumulated from the ambient air or from the substrate in the inter-grain boundaries. A phenomenon of a sputtering rate decrease for the polycrystalline lead tin telluride films in comparison to the single-crystal films is discussed. A novel important phenomenon of the formation of nanostructure arrays on the surface of the Pb_1−xSn_xTe thin films with the dependence on the sputtering rate during plasma treatment is reported.

We search for optimum growth conditions to realize flat Bi$_{2}$Te$_{3}$ layers on InP(111)B by hot wall epitaxy. The substrate provides a relatively small lattice mismatch, and so (0001)-oriented layers grow semicoherently. The temperature window for the growth is found to be narrow due to the nonzero lattice mismatch and rapid re-evaporation of Bi$_{2}$Te$_{3}$. The crystalline qualities evaluated by means of x-ray diffraction reveal deteriorations when the substrate temperature deviates from the optimum not only to low temperatures but also to high temperatures. For high substrate temperatures, the Bi composition increases as Te is partially lost by sublimation. We show, in addition, that the exposure of the Bi$_{2}$Te$_{3}$ flux at even higher temperatures results in anisotropic etching of the substrates due, presumably, to the Bi substitution by the In atoms from the substrates. By growing Bi$_{2}$Te$_{3}$ layers on InP(001), we demonstrate that the bond anisotropy on the substrate surface gives rise to a reduction in the in-plane epitaxial alignment symmetry.

We analyze high order of nongeminate recombination in organic donor–acceptor bulk heterojunction solar cells. The model of recombination where an exciton annihilates on an electron–hole Langevin bound pair near donor–acceptor interface has been applied in our studies. We obtained satisfactory agreement between experimental results and theoretical calculations for the concentration dependences of several parameters characterizing photovoltaic cells, such as the mobility of charge carriers, the recombination coefficient and the recombination time. The influence of carriers from deep states on the mobility has been taken into account.

The effects of B and P single doping and co-doping on the substitutional impurities energies, electronic and optical properties of Si_1 − xGe_x nanowires (NWs) are investigated using first-principles calculations. We demonstrated that B-P co-doping is energetically favored with respect to single B or P doping. The shifts of band edges are found: compared with undoped wires, B-doped Si_1 − xGe_x NWs have the valence band edge (VBE) moving significantly towards higher energy, while in P-doped Si_1 − xGe_x NWs, the conduction band edge (CBE) is downward shifted. Further analysis confirms that conduction band minimum (CBM) of doped wires is strongly localized on P atom, and valence band maximum (VBM) on B atom. Finally, optical properties of Si_1 − xGe_x NWs with different dopants and various Ge concentrations are understood. Our results imply the possibility of impurity based engineering of electronic and optical properties of Si_1 − xGe_x NWs.

Different-indium-content quaternary InAlGaN multi-quantum-well (MQW) structures with emission wavelength around 290–310 nm were grown by metalorganic chemical vapor deposition, and their emission characteristics including the indium-segregation effect were investigated by using not only experimental evaluation but also applying simulation technique, on the basis of the weakly localized exciton model. The value of an effective localized level in the quaternary InAlGaN MQWs was estimated to be around 70 meV from the relationship between photoluminescence lifetimes and photon energies. The simulation study, which was conducted by fitting the emission spectra, also derived the value of around 50 meV. The present study also indicated that the quaternary InAlGaN MQW structures with the indium segregation have clear advantages over ternary AlGaN MQW ones especially in the range of dislocation density larger than approximately 1 × 10⁸ cm⁻².