Table of contents

We perform magnetic focussing of high mobility holes confined in a shallow GaAs/Al_0.33Ga_0.67As quantum well grown on a (100) GaAs substrate. We observe ballistic focussing of holes over a path length of up to 4.9 μm with a large number of focussing peaks. We show that additional structure on the focussing peaks can be caused by a combination of the finite width of the injector quantum point contact and Shubnikov–de Haas oscillations. These results pave the way to studies of spin-dependent magnetic focussing and spin relaxation lengths in two-dimentional hole systems without complications of crystal anisotropies and anisotropic g-tensors.

Engineering applications for printed electronics demand solution processable electrically conductive materials, in the form of inks, to realize interconnections, piezoresistive pressure sensors, thermoresistive temperature sensors, and many other devices. Polyaniline is an intrinsically conductive polymer with modest electrical properties but clear advantages in terms of solubility and stability with temperature and in time. A comprehensive study, starting from its synthesis, primary doping, inkjet printing and secondary doping is presented, with the aim of elucidating the doping agent effects on its morphology, printability and electronic performance.

Recent advances in large-area optoelectronics research have demonstrated the tremendous potential of copper(I) thiocyanate (CuSCN) as a universal hole-transport interlayer material for numerous applications, including transparent thin-film transistors, high-efficiency organic and hybrid organic-inorganic photovoltaic cells, and organic light-emitting diodes. CuSCN combines intrinsic hole-transport (p-type) characteristics with a large bandgap (>3.5 eV) which facilitates optical transparency across the visible to near infrared part of the electromagnetic spectrum. Furthermore, CuSCN is readily available from commercial sources while it is inexpensive and can be processed at low-temperatures using solution-based techniques. This unique combination of desirable characteristics makes CuSCN a promising material for application in emerging large-area optoelectronics. In this review article, we outline some important properties of CuSCN and examine its use in the fabrication of potentially low-cost optoelectronic devices. The merits of using CuSCN in numerous emerging applications as an alternative to conventional hole-transport materials are also discussed.

The versatility of printing technologies and their intrinsic ability to outperform other techniques in large-area deposition gives scope to revolutionize the photovoltaic (PV) manufacturing field. Printing methods are commonly used in conventional silicon-based PVs to cover part of the production process. Screen printing techniques, for example, are applied to deposit electrical contacts on the silicon wafer. However, it is with the advent of third generation PVs that printing/coating techniques have been extensively used in almost all of the manufacturing processes. Among all the third generation PVs, dye sensitized solar cell (DSSC) technology has been developed up to commercialization levels. DSSCs and modules can be fabricated by adopting all of the main printing techniques on both rigid and flexible substrates. This allows an easy tuning of cell/module characteristics to the desired application. Transparency, colour, shape, layout and other DSSC's features can be easily varied by changing the printing parameters and paste/ink formulations used in the printing process. This review focuses on large-area printing/coating technologies for the fabrication of DSSCs devices. The most used and promising techniques are presented underlining the process parameters and applications.

Flexible and biodegradable electronics are currently under extensive investigation for biocompatible and environmentally-friendly applications. Synthetic plastic foils are widely used as substrates for flexible electronics. But typical plastic substrates such as polyethylene naphthalate (PEN) could not be degraded in a natural bio-environment. A great demand still exists for a next-generation biocompatible and biodegradable substrate for future application. For example, electronic devices can be potentially integrated into the human body. In this work, we demonstrate that the biocompatible and biodegradable natural silk fibroin (SF) films embedded with silver nanowires (AgNWs) mesh could be employed as conductive transparent substrates to fabricate flexible organic light emitting diodes (OLEDs). Compared with commercial PEN substrates coated with indium tin oxide, the AgNWs/SF composite substrates exhibit a similar sheet resistance of 12 Ω sq⁻¹, a lower surface roughness, as well as a broader light transmission range. Flexible OLEDs based on AgNWs/SF substrates achieve a current efficiency of 19 cd A⁻¹, demonstrating the potential of the flexible AgNWs/SF films as conductive and transparent substrates for next-generation biodegradable devices.

In this contribution we demonstrate an integrated photoactive switch employing a fully-printed planar photodetector and complementary Schmitt trigger. A photoactive switch is fundamental to several light driven systems, such as twilight sensors or industrial machinery control devices. This paper explores a fabrication methodology that enables reliable complementary logic building blocks and photodetectors with a fully-printed, all-polymer approach, resulting in a semi-transparent integrated system on a single plastic foil.

Photodetectors convert light pulses into electrical signals and are fundamental building blocks for any opto-electronic system adopting light as a probe or information carrier. They have widespread technological applications, from telecommunications to sensors in industrial, medical and civil environments. Further opportunities are plastic short-range communications systems, interactive large-area surfaces and light-weight, flexible, digital imagers. These applications would greatly benefit from the cost-effective fabrication processes enabled by printing technology. While organic semiconductors are the most investigated materials for printed photodetectors, and are the main focus of the present review, there are notable examples of other inorganic or hybrid printable semiconductors for opto-electronic systems, such as quantum-dots and nanowires. Here we propose an overview on printed photodetectors, including three-terminal phototransistors. We first give a brief account of the working mechanism of these light sensitive devices, and then we review the recent progress achieved with scalable printing techniques such as screen-printing, inkjet and other non-contact technologies in the development of all-printed or hybrid systems.

In this work we address three-dimensional heterojunctions, demonstrating that photoluminescence from defect-free, Ge/SiGe multiple quantum well (MQW) micro-crystals grown on deeply patterned Si(001) and Si(111) substrates exhibit similar radiative intensity and analogous spectral shape. The finite lateral size and faceted top morphology of the micro-crystals guarantee the absence of dislocations threading through the MQW structure and the dominance of radiative recombination at slanted $\{113\}$ facets. Our approach yields superior optical quality in comparison to state-of-the-art MQWs grown on SiGe/Si(001) linearly graded buffers.

We study the properties of edge states in in-plane heterostructures made of adjacent zigzag graphene and Boron Nitride (BN) ribbons. While in pure zigzag graphene nanoribbons, gapless edge states are nearly flat and cannot contribute significantly to the conduction, at graphene/BN interfaces the properties of these states are significantly modified. They are still strongly localized at the zigzag edges of graphene, but they exhibit a high group velocity up to 4.3 × 10⁵ m s⁻¹ at the B-C interface and even 7.4 × 10⁵ m s⁻¹ at the N-C interface. For a given wave vector the velocities of N-C and B-C hybrid interface states have opposite signs. Additionally, in the case of the asymmetric structure BN/graphene/BN, a bandgap of about 207 meV is open for sub-ribbon widths of 5 nm. These specific properties suggest new ways to engineer and control the transport properties of graphene nanostructures.

We present a measurement technique to study barrier height inhomogeneity (BHI) in III-N materials. This technique is enabled by a hot electron transistor (HET), a vertical, unipolar device that works by injecting hot electrons over the emitter barrier and into a very thin base layer. After traversing the base, high energy electrons surmount the collector barrier and contribute to the collector current while low energy electrons reflect off the collector barrier and become the base current. The prevailing theory of BHI prescribes the replacement of a constant emitter barrier height with one that depends on both bias and temperature (i.e. ${\phi }_{{\rm{B}}}\to {\phi }_{{\rm{B}}}^{\prime }(V,T)$). Because the magnitude of the collector current is a strong function of the emitted electron energy and, therefore, the emitter barrier height, measuring the change in collector current with emitter bias and temperature allows us to determine the dependence of ${\phi }_{{\rm{B}}}^{\prime }(V,T)$ on these quantities. This advance will help provide a more thorough understanding of the physical sources of BHI in the III-Ns and assist in the diagnosis of key device nonidealities.

With the growing need for large area display technology and the push for a faster and cheaper alternative to the current amorphous indium gallium zinc oxide (a-IGZO) as the active channel layer for pixel-driven thin film transistors (TFTs) display applications, gallium tin zinc oxide (GSZO) has shown to be a promising candidate due to the similar electronic configuration of Sn⁴⁺ and In³⁺. In this work TFTs of GSZO sputtered films with only a few atomic % of Ga and Sn have been fabricated. A systematic and detailed comparison has been made of the properties of the GSZO films annealed at two temperatures: 140 °C and 450 °C. The electrical and optical stabilities of the respective devices have been studied to gain more insight into the degradation mechanism and are correlated with the initial TFT performance prior to the application of stress. Post deposition annealing at 450 °C of the films in air was found to lead to a higher atomic concentration of Sn⁴⁺ in these films and a superior quality of the film, as attested by the higher film density and less surface and interface roughness in comparison to the lower annealed temperature device. These result in significantly reduced shallow and deep interface traps with improved performance of the device exhibiting V_ON of −3.5 V, I_ON/I_OFF of 10⁸, field-effect mobility (μ_FE) of 4.46 cm² V⁻¹s⁻¹, and sub-threshold swing of 0.38 V dec⁻¹. The device is stable under both electrical and optical bias for wavelengths of 550 nm and above. Thus, this work demonstrates GSZO-based TFTs as a promising viable option to the IGZO TFTs by further tailoring the film composition and relevant processing temperatures.

A single sheet of high-density InGaAs quantum dots (QDs) is used as a gain medium of InGaAs–GaAs–AlGaAs lasers. The devices operate at high power in the continuous mode beyond 160 °C with an emission wavelength up to ∼1.27 μm. At short cavity lengths a strong broadening (>300 nm) of the electroluminescence spectrum is observed at high current densities, permitting light sources for broadly wavelength tuneable and multi-wavelength infrared lasers based on a single gain chip, and related frequency conversion devices for the whole visible spectrum range. High power cw operation (>2 W) limited by catastrophic optical mirror damage is realized.

In this work, a new hybrid sputtering–evaporation system providing a scalable process for deposition of Cu(In,Ga)Se₂ (CIGS) layers is presented. The growth apparatus has been designed and realized to fit a size suitable for direct industrial transfer. In this process the metal precursors are first of all sputtered on rotating transfer devices, then evaporated on the substrate by local heating in a Se atmosphere. The desired thickness and composition of the CIGS film are obtained by repeated sputtering–evaporation cycles. The cylindrical geometry of the deposition chamber has been designed to accommodate different types of flexible substrates with a maximum size of 20 × 120 cm² in a roll-to-roll configuration. Several techniques, including secondary ion mass spectrometry, Raman and photoluminescence spectroscopies, x-ray diffraction, scanning electron microscopy, external quantum efficiency, and I–V under 1 Sun illumination, have been used to test both the as-grown CIGS layers and the solar cell devices based on them. A significant performance and good control of Ga grading and Na content were obtained for solar cells grown at 450 °C on polyimide substrates with high deposition rates. In spite of the fact that the present efficiency record for CIGS solar cells on polyimide substrates is 20.4%, the 10.1% obtained using the hybrid method presented in this work is significant because the growth apparatus meets the requirements for direct industrial transfer. In fact, this process is being transferred in a 1 MW production line, where standard CIGS layers are deposited at low temperature on flexible substrates in a single-step process with a 1 mm sec⁻¹ substrate velocity.

We have demonstrated enhancement mode operation of AlInN/GaN (MIS)HEMTs on Si substrates using the fluorine treatment technique. The plasma RF power and treatment time was optimized to prevent the penetration of the fluorine into the channel region to maintain high channel conductivity and transconductance. An analysis of the threshold voltage was carried out which defined the requirement for the fluorine sheet concentration to exceed the charge at the dielectric/AlInN interface to achieve an increase in the positive threshold voltage after deposition of the dielectric. This illustrates the importance of control of both the plasma conditions and the interfacial charge for a reproducible threshold voltage. A positive threshold voltage of +3 V was achieved with a maximum drain current of 367 mA mm⁻¹ at a forward gate bias of 10 V.

Si–Ge interdiffusion with different P doping configurations was investigated. Significant interdiffusion happened when the Ge layers were doped with P in high 10¹⁸ cm⁻³ range, which resulted in a SiGe alloy region thicker than 150 nm after defect annealing cycles. With high P doped Ge, Si–Ge interdiffusivity is enhanced by 10–20 times in the x_Ge > 0.7 region compared with the control sample without P doping. We attribute this phenomenon to the much faster P transport towards the Ge seeding layers from the Ge side during the Ge layer growth, which increases the negatively charged vacancy concentrations and thus the interdiffusivity due to the Fermi effect in Si–Ge interdiffusion. This work is relevant to Ge-on-Si type device design, especially Ge-on-Si lasers.

In this study, which is aimed at assessing a possible relationship between the recoverable and permanent components of negative bias temperature instability (NBTI) degradation, we investigate NBTI in commercial IRF9520 p-channel VDMOSFETs (vertical double diffused MOSFETs) stressed under particular pulsed bias conditions by varying the pulse on-time while keeping the off-time constant and vice versa. The stress-induced threshold voltage shifts are found to be practically independent of duty cycle when the pulse on-time is kept short or the off-time is kept long, and are found to start increasing with duty cycle only when the on-time is increased or the off-time is decreased beyond specific values. These results, which are discussed in terms of dynamic recovery effects and the mechanisms leading to NBTI degradation, point to the existence of an important correlation between the recoverable and permanent components of degradation.

We report a 1.5 μm and 10 Gb s⁻¹ etched mesa buried heterostructure λ/4-shifted distributed feedback laser diode (DFB-LD) for the low-cost application of WDM–based datacenter networks. To reduce the threshold current and improve the modulation bandwidth in a conventional p-/n-/p-InP current blocking structure, a thin undoped-InP (u-InP) layer was inserted between the side walls of the active region and the p-InP layer (i.e., a u-/p-/n-/p-InP structure), and the region containing the active region and the current blocking structures was etched in a mesa form (i.e., an etched mesa). From this work, it was found that a 300 μm long anti-reflection (AR)-AR DFB-LD with a mesa width of 8 μm is reduced by about 25% while a side mode suppression ratio is >50 dB and a 3 dB bandwidth is >10 GHz at a current of 40 mA; in addition, it shows a clear eye-opening with a dynamic extinction ratio of >4.5 dB at 10 Gb s⁻¹, and a power penalty of <1 dB after a 2 km transmission.

An InAsSb nBn detector structure was grown on both GaAs and native GaSb substrates. Temperature dependent dark current, spectral response, specific detectivity $(D*)$ and noise spectral density measurements were then carried out. Shot-noise-limited $D*$figures of $1.2\times {10}^{10}\;{\rm{Jones}}$ and $3.0\times {10}^{10}\;{\rm{Jones}}$ were calculated (based upon the sum of dark current and background photocurrent) for the sample grown on GaAs and the sample grown on GaSb, respectively, at 200 K. Noise spectral density measurements revealed knee frequencies of between 124–337 Hz and ∼8 Hz, respectively. Significantly, these devices could support focal plane arrays capable of operating under thermoelectric cooling.

The influence of pre-heating temperature on cracked–selenized Cu(In, Ga)Se₂ (CIGS) films' structure, growth kinetics, and photovoltaic performance is investigated. The 'large island grains' on the upper surface are the precursors of Cu_2−xSe and finally evolve into Cu_2−xSe as the pre-heating temperature increases to 400 °C. The 'large island grains', as well as In₂Se₃, are considered to be two decisive factors in forming CIGS as they facilitate the diffusion of cracked-Se into the thin films, because they make the films more incompact and suppress the fast formation of complete single CuInSe₂ (CIS) during the 2nd heating. Stoichiometric CIGS thin films without a bi-layer structure and phase separation can be achieved by adjusting the appropriate pre-heating temperature. The performance of the solar cells is mainly influenced by the current leakage caused by small grains and cavities near the CIGS/Mo back contact.

The cause of drain current (I_D) drift in graphene field-effect transistors is analyzed and a method to suppress the drift is proposed. By analyzing I_D-time characteristics, a condition of reasonable gate, drain and source biases (V_G, V_D, and V_S) is proposed to suppress I_D drift. Based on this result, we find a condition for V_G during off-time (V_base), V_D, and V_S in pulsed I-V measurement to obtain the intrinsic I_D-V_G curves, and analyze the effect of V_base on the Dirac point shift. Through an analysis of I_D-time characteristics depending on V_G, I_D drift according to the range of V_G is explained.

In this work, a study has been performed to understand the gradual reset in Al₂O₃ resistive random-access memory (RRAM). Concentration of vacancies created during the forming or set operation is found to play a major role in the reset mechanism. The reset was observed to be gradual when a significantly higher number of vacancies are created in the dielectric during the set event. The vacancy concentration inside the dielectric was increased using a multi-step forming method which resulted in a diffusion-dominated gradual filament dissolution during the reset in Al₂O₃ RRAM. The gradual dissolution of the filament allows one to control the conductance of the dielectric during the reset. RRAM devices with gradual reset show excellent endurance and retention for multi-bit storage. Finally, the conductance modulation characteristics realizing synaptic learning are also confirmed in the RRAM.

A fully relaxed In_0.1Ga_0.9N layer was grown by plasma-assisted molecular beam epitaxy on c-plane GaN using a grading technique. The growth of the graded InGaN layer in the intermediate regime enabled a smooth surface without the accumulation of In droplets. Transmission electron microscopy images show that the relaxation occurs through the formation of a high density of threading dislocations (TDs). Despite the presence of these TDs, relaxed InGaN films were then successfully used as a pseudo-substrate for the growth of InGaN/GaN quantum wells which luminesced at room temperature.

Experimental investigation of electron transport along a two-dimensional channel confined in an InGaN alloy of Al${}_{0.82}$In${}_{0.18}$N/AlN/In${}_{0.1}$Ga${}_{0.9}$N/GaN structure was performed at room temperature under near-equilibrium thermal-bath temperature. A soft damage was observed at a threshold electric field applied in the channel plane. The threshold current for soft damage and the supplied electric power were lower in the channels with a higher electron density. The results are interpreted in terms of plasmon-assisted heat dissipation. In agreement with ultra-fast decay of hot phonons in the vicinity of the resonance with plasmons, the electron drift velocity acquires a highest value of ∼2 × 10⁷ cm s⁻¹ at 180 kV cm⁻¹ in channels with 1 × 10¹³ cm⁻² and decreases as the electron density increases. No negative differential resistance is observed. The effective hot-phonon lifetime is estimated as ∼ 2 ps at 1.6 × 10¹³ cm⁻² at low electric fields and is found to decrease as the field increases.

In this paper, an ultra-thin-body thin-film transistor (UTB-TFT) with a raised source/drain structure is demonstrated and compared with a conventional thin-film transistor. A significant suppression of leakage current and an improvement in subthreshold swing (SS) resulting from the reduced body thickness is observed. The minimum current can be decreased from 245 pA to 42 pA as the channel film thickness is scaled down from 60 nm to 10 nm. However, an ultra-thin-channel film constrains the average grain size and severely impacts the saturation current. Fortunately, by decreasing the gate oxide thickness from 20 nm to 10 nm, the saturation current of a UTB-TFT can be significantly increased from 13 μA to 25 μA. Experimental results suggest that UTB-TFTs with a sub-10 nm gate oxide display great promise for future low-power, high-performance three-dimensional integrated circuits.

The increase in energy demands is leading to growing interest in new thermoelectric inorganic materials, such as the chalcogenides. The recently synthesized quaternary chalcogenide, Tl₂PbXY₄ (X = Zr, Hf and Y = S, Se), compounds were investigated using the full potential linear augmented plane wave method based on density functional theory. We used the generalized gradient approximation plus the optimized effective Hubbard parameter U to treat the exchange correlation. The existence of heavy metals (Tl, Pb and Hf) required the application of relativistic spin–orbit coupling via a second variational procedure. Tl₂PbHfS₄, Tl₂PbHfSe₄, Tl₂PbZrS₄ and Tl₂PbZrSe₄ compounds were found to be semiconductors with indirect band gaps of 0.911, 0.659, 0.983 and 0.529 eV, respectively. The types of carriers and electrical transport properties of Tl₂PbXY₄ (X = Zr, Hf and Y = S, Se) are attributed to the Tl-d and S/Se-s electronic states near the Fermi level. Optical properties were investigated via the calculation of dielectric function and reflectivity. Using Boltzmann theory, we compared the thermoelectric properties and we found that Tl₂PbHfS₄ could be a good candidate for thermoelectric devices.

Ohmic contacts to p- and n-type 4H-SiC using refractory alloyed W:Ni thin films were investigated. Transfer length measurement test structures to p-type 4H-SiC (N_A = 3 × 10²⁰ cm⁻³) revealed Ohmic contacts with specific contact resistances, ρ_c, of ∼10⁻⁵ Ω cm² after 0.5 h annealing in argon at temperatures of 1000 °C, 1100 °C, 1150 °C, and 1200 °C. Contacts fabricated on n-type 4H-SiC (N_D = 2 × 10¹⁹ cm⁻³) by similar methods were shown to have similar specific contact resistance values after annealing, demonstrating simultaneous Ohmic contact formation for W:Ni alloys on 4H-SiC. The lowest ρ_c values were (7.3 ± 0.9) × 10⁻⁶ Ω cm² for p-SiC and (6.8 ± 3.1) × 10⁻⁶ Ω cm² for n-SiC after annealing at 1150 °C. X-ray diffraction shows a cubic tungsten–nickel–carbide phase in the Ohmic contacts after annealing as well as WC after higher temperatures. Auger electron spectroscopy depth profiles support the presence of metal carbide regions above a nickel and silicon-rich region near the interface. X-ray energy dispersive spectroscopy mapping showed tungsten-rich and nickel-rich regions after annealing at 1100 °C and above. W:Ni alloys show promise as simultaneous Ohmic contacts to p- and n-SiC, offering low and comparable ρ_c values along with the formation of W_xNi_yC.

Planar (In,Ga)N layers were grown on nanostripe arrays composed of InGaN/GaN multi quantum wells (MQWs) by metal–organic chemical vapor deposition. The MQW nanostripe arrays with height to width aspect ratios of about 0.5 and 1 were fabricated from planar coherently strained InGaN/GaN MQW samples. Independent of their aspect ratio, the nanostripes exhibited elastic relaxation perpendicular to the stripe direction after pattern fabrication, resulting in an a_⊥ lattice constant perpendicular to the stripe direction larger than that of the GaN base layer. In a subsequent step, (In,Ga)N layers were grown on top of the nanostripe arrays, leading to the formation of planar films with a similar a_⊥ lattice constant as the MQW stripes beneath. Bright luminescence was recorded from the planar, partially relaxed re-grown (In,Ga)N layers grown on the stripe arrays with an aspect ratio of 1. Plastic relaxation of the MQW stripes was observed after (In,Ga)N regrowth for samples with a stripe aspect ratio of 0.5, leading to luminescence quenching.

Several types of vertical power Schottky barrier diodes (SBDs) using a high-k (Hk) insulator, which have a low specific on-resistance (R_on) close to that of the superjunction (SJ) SBD with the same breakdown voltage (BV), are studied. By introducing an N⁺-poly-region in the Hk insulator at the anode side, the reverse leakage current of the Hk-SBD can be largely reduced without increasing the forward voltage drop. Simulation results based on Sentaurus show that, by using the N⁺-poly-region, the reverse leakage current of the 400 V Hk-SBD can be lowered by more than 40 times with R_on (=3.13 mΩ · cm²) equaling about eight times smaller than that of the conventional SBD having almost the same BV. It is also found that the reverse recovery behavior of the Hk-SBDs is much softer than that of the SJ-SBD.

The temperature dependent current–voltage (I–V) and 1/f noise characteristics of the Fe/GaN ferromagnetic metal–semiconductor (FM/SC) Schottky barrier diode are presented. At 300 K, an ideality factor of 1.3 and a barrier height of 0.92 eV suggested thermionic emission as the dominant current transport mechanism at the FM/SC interface. The ideality factor increased to 3.4 while the barrier height decreased to 0.36 eV at 100 K, indicating the presence of other current transport mechanisms in addition to thermionic emission. The spectral power density of current fluctuations, ${S}_{{\bf{I}}},$ is measured as a function of frequency in the temperature range 100–300 K, where ${S}_{{\bf{I}}}$ followed $1/{f}^{\gamma }$ with γ close to unity. A 1/f type of noise spectrum at each temperature in the range 100–300 K is attributed to the existence of barrier inhomogeneities at the Fe/GaN interface. The inhomogeneous nature of Fe/GaN is also revealed from the temperature dependent I–V characteristics. From temperature dependent I–V and 1/f noise measurements, it is found that current transport is limited by diffusion currents below 200 K, resulting in an increase in noise with temperature. Above 200 K, thermionic emission is the dominant current transport mechanism and noise behavior is affected by barrier inhomogeneities, resulting in a decrease in noise with an increase in temperature.

InAs-based intersubband quantum cascade (QC) lasers with an improved waveguide configuration are proposed. Calculations and analyses are presented to show that the waveguide configuration will enhance the optical mode confinement and reduce optical absorption loss. We also discuss how these QC lasers could be constructed. It is expected that the performance of these InAs-based QC lasers, with the proposed waveguide configuration and optimizations, will be significantly improved.

This paper performs an experimental comparative study of the total ionizing dose effects due to the x-ray radiation between the silicon-on-insulator (SOI) metal-oxide semiconductor field-effect transistors (MOSFETs) manufactured with octagonal gate geometry and the standard counterpart. Our main focus is on integrated transceivers for wireless communications and smart-power dc/dc converters for mobile electronics, where the transistor is used as the key switching element. It is shown that this innovative layout can reduce the total ionizing dose (TID) effects due to the special characteristics of the OCTO SOI MOSFET bird's beak regions, where longitudinal electrical field lines in these regions are not parallel to the drain and source regions. Consequently, the parasitic MOSFETs associated with these regions are practically deactivated.

We show how single quantum dots, each hosting a singlet–triplet qubit, can be placed in arrays to build a spin quantum cellular automaton. A fast (∼10 ns) deterministic coherent singlet–triplet filtering, as opposed to current incoherent tunneling/slow-adiabatic based quantum gates (operation time ∼300 ns), can be employed to produce a two-qubit gate through capacitive (electrostatic) couplings that can operate over significant distances. This is the coherent version of the widely discussed charge and nano-magnet cellular automata, and would increase speed, reduce dissipation, and perform quantum computation while interfacing smoothly with its classical counterpart. This combines the best of two worlds—the coherence of spin pairs known from quantum technologies, and the strength and range of electrostatic couplings from the charge-based classical cellular automata. Significantly our system has zero electric dipole moment during the whole operation process, thereby increasing its charge dephasing time.

This work presents a comprehensive optical characterization of Zn_1−xMg_xO thin films grown by spray pyrolysis (SP). Absorption measurements show the high potential of this technique to tune the bandgap from 3.30 to 4.11 eV by changing the Mg acetate content in the precursor solution, leading to a change of the Mg-content ranging from 0 up to 35%, as measured by transmission electron microscopy-energy dispersive x-ray spectroscopy. The optical emission of the films obtained by cathodoluminescence and photoluminescence spectroscopy shows a blue shift of the peak position from 3.26 to 3.89 eV with increasing Mg incorporation, with a clear excitonic contribution even at high Mg contents. The linewidth broadening of the absorption and emission spectra as well as the magnitude of the observed Stokes shift are found to significantly increase with the Mg content. This is shown to be related to both potential fluctuations induced by pure statistical alloy disorder and the presence of a tail of band states, the latter dominating for medium Mg contents. Finally, metal–semiconductor–metal photodiodes were fabricated showing a high sensitivity and a blue shift in the cut-off energy from 3.32 to 4.02 eV, i.e., down to 308 nm. The photodiodes present large UV/dark contrast ratios (10² − 10⁷), indicating the viability of SP as a growth technique to fabricate low cost (Zn, Mg)O-based UV photodetectors reaching short wavelengths.

The thermal transport properties of self-organized Ge nanostructures on Si were studied by means of ultrafast surface sensitive time-resolved electron diffraction. The thermal boundary resistance was determined from the temperature response of the Ge nanostructures upon impulsive heating by fs-laser pulses. The transient temperature was determined through the Debye–Waller effect. Epitaxial growth of Ge hut and dome clusters was achieved by in-situ deposition of 8 monolayers of Ge on Si(001) at 550 °C under ultra-high vacuum conditions. Time-resolved spot profile analysis of different orders of diffraction spots was used to distinguish between the thermal response of hut and dome clusters. Dome clusters of 6 nm height and 50 nm width cool with a time constant of $\tau =150\;{\rm{ps}}$ which agrees well with numerical simulations calculated in the framework of the diffuse mismatch model. The much smaller hut clusters with a height of 2.3 nm and width of 23 nm exhibit a cooling time of $\tau =55\;{\rm{ps}}$, which is a factor of 2 slower than predicted by theory.

We report the development of a new method to systematically and controllably achieve very high carrier concentrations in As-doped germanium using ultra-low temperature, high efficiency routes based on the structurally and chemically compatible inorganic hydrides As(SiH₃)₃ and As(GeH₃)₃. The Ge n-layers are grown on Ge-buffered Si(100) using in situ depositions of the compounds with Ge₃H₈ at 330 °C. The as-grown films are found to exhibit excellent crystallinity, defect-free interfaces, atomically smooth surfaces and flat doping profiles with abrupt edges. The active carrier densities are measured to be in the range of 1 × 10¹⁹–8.4 × 10¹⁹ cm⁻³ irrespective of the precursor type. These carrier densities are in close agreement with atomic As concentrations measured by secondary ion mass spectrometry, indicating that the growth mechanism promotes the nearly complete substitutional incorporation of dopant atoms while suppressing the formation of non-active clusters and defects. In spite of the lower solubility of As in Ge relative to that of P, the maximum carrier concentrations obtained with As(SiH₃)₃ and As(GeH₃)₃ are roughly 30% higher than those found with the analogous P(SiH₃)₃ and P(GeH₃)₃. This result, along with the close similarity in band gap narrowing observed for the two methods, suggests that the As-doping route may be advantageous for optical devices that require the highest possible carrier concentrations to populate the conduction band valley associated with direct gap emission. On the other hand—due to the inherently shorter carrier relaxation times in As-doped Ge—the lowest observed resistivity of 5 × 10⁻⁴ Ω cm is slightly higher than the lowest resistivity from P-doped analogs. Finally, optical responsivity, electroluminescence and I–V properties of photodiodes fabricated using As(SiH₃)₃ and As(GeH₃)₃ are found to be on par with those observed from Ge-on-Si reference analogs, indicating that the chemistry approach described here represents a viable and straightforward route to doping and activation of device-quality materials.

The interfaces of InAs/GaSb superlattices (SLs) were studied with the goal of improving interband cascade infrared photodetectors (ICIPs) designed for the long-wavelength infrared region. Two ICIP structures with different SL interfaces were grown by molecular beam epitaxy, one with a ∼1.2 monolayer (ML) InSb layer inserted intentionally only at the GaSb-on-InAs interfaces and another with a ∼0.6 ML InSb layer inserted at both InAs-on-GaSb and GaSb-on-InAs interfaces. The material quality of the ICIP structures was similar according to characterization by differential interference contrast microscopy, atomic force microscopy, and x-ray diffraction. The performances of the ICIP devices were not substantially different despite the different interface structure. Both ICIPs had a peak detectivity of >3.7 × 10¹⁰ Jones at 78 K with a cutoff wavelength near 9.2 μm. The maximum operation temperatures of both ICIPs were as high as ∼250 K, although the structures were not fully optimized. This suggests that the two interface arrangements may have a similar effect on structural, optical and electrical properties. Alternatively, the device performance of the ICIPs may be limited by mechanisms unrelated to the interfaces. In either case, the arrangement of dividing a thick continuous InSb layer at the GaSb-on-InAs interface into thinner InSb layers at both interfaces can be used to achieve strain balance in SL detectors for longer wavelengths. This suggests that with further improvements ICIPs should be able to operate at higher temperatures at even longer wavelengths.

The electronic properties of In(AsN) before and after post-growth sample irradiation with increasing doses of atomic hydrogen have been investigated by photoluminescence. The electron density increases in In(AsN) but not in N-free InAs, until a Fermi stabilization energy is established. A hydrogen ^+/− transition level just below the conduction band minimum accounts for the dependence of donor formation on N, in agreement with a recent theoretical report highlighting the peculiarity of InAs among III–V compounds. Raman scattering measurements indicate the formation of N–H complexes that are stable under thermal annealing up to ∼500 K. Finally, hydrogen does not passivate the electronic activity of N, thus leaving the band gap energy of In(AsN) unchanged, once more in stark contrast to what has been reported in other dilute nitride alloys.

We propose an InP-based upright five-junction (5J) solar cell structure for high conversion efficiency under concentration. In the structure, three bottom subcells are composed of lattice-matched (LM) InGaAsP materials, while two top subcells employ metamorphic InGaP materials. The two InGaP subcells are designed to have the same Ga composition of 30%. The first InGaP subcell is thinned so as to transmit half of the photon flux to the second InGaP subcell, thus forming an upright 5J InGaP(1.64 eV)/InGaP(1.64 eV)/InGaAsP(1.3 eV)/InGaAsP(1.02 eV)/InGaAs(0.74 eV) solar cell structure on the InP substrate. The subcell bandgap energies are chosen in such a way that a current matching condition can be achieved. Because no Al- or N-contained materials are used in the absorbers and only one metamorphic growth is required (with a lattice mismatch of 2.1%), the novel InP-based solar cell architecture is considered practically achievable with current growth technology. By comparing it with a InGaP/GaAs/Ge reference cell and adding additional nonideal factors in the modeling, an efficiency as high as 46.2% is estimated under concentration at ∼1500 suns.

An advanced tunnel oxide layer process for 65 nm NOR-type floating-gate flash memory is proposed to improve tunnel oxide quality by an additive sacrificial oxide layer growth. The sacrificial oxide layer process effectively controls the thickness variation of tunnel oxide and improves the flatness of the SiO₂/Si interface across the active area. The interface traps' generation during program/erase cycling of flash cells is found to be reduced, and the reliability property is significantly improved as compared to flash cells without the sacrificial oxide layer process. The technology is applicable to further scaled floating-gate flash memories.

Single-nanowire photodetectors have potential applications in integrated optoelectronic devices and systems. Here, bandgap-engineered GaAs_0.26Sb_0.74 alloy nanowires were synthesized via a chemical vapor deposition method. The synthesized nanowires are single crystals grown along the [111]_B direction with length up to 50 μm and diameter ranging from 40 to 500 nm. Photodetectors are built based on these single-alloy nanowires, which show a wide response in the near-infrared region with a high response peak located in the optical communication region (1.31 μm), as well as an external quantum efficiency of 1.62 × 10⁵%, a responsivity of 1.7 × 10³ A W⁻¹ and a short response time of 60 ms. These novel near-infrared photodetectors may find promising potential applications in integrated infrared photodetection, thermal imaging, information communication and processing.

The formation of recess etched Au-free ohmic contacts to an InAlN/AlN/GaN heterostructure is investigated. A Ta/Al/Ta metal stack is used to produce contacts with contact resistance (R_c) as low as 0.14 Ω mm. It is found that R_c decreases with increasing recess depth until the InAlN barrier is completely removed. For even deeper recesses R_c remains low but requires annealing at higher temperatures for contact formation. The lowest R_c is found for contacts where the recess etch has stopped just above the 2D electron gas channel. At this depth the contacts are also found to be less sensitive to other process parameters, such as anneal duration and temperature. An optimum bottom Ta layer thickness of 5–10 nm is found. Two reliability experiments preliminary confirm the stability of the recessed contacts.

We report the observation of a thermally activated resonant tunnelling feature in the current–voltage characteristics (I(V)) of triple barrier resonant tunnelling structures (TBRTS) due to the alignment of the n = 1 confined states of the two quantum wells within the active region. With great renewed interest in tunnelling structures for high frequency (THz) operation, the understanding of device transport and charge accumulation as a function of temperature is critical. With rising sample temperature, the tunnelling current of the observed low voltage resonant feature increases in magnitude showing a small negative differential resistance region which is discernible even at 293 K and is unique to multiple barrier devices. This behaviour is not observed in conventional double barrier resonant tunnelling structures where the transmission coefficient at the Fermi energy is predominantly controlled by an electric field, whereas in TBRTS it is strongly controlled by the 2D to 2D state alignment.

Semiconductor nanowires have been identified as a viable technology for next-generation infrared (IR) photodetectors with improved detectivity and detection across a range of energies as well as for novel single-photon detection in quantum networking. The GaAsSb materials system is especially promising in the 1.3–1.55 μm spectral range. In this work we present band-gap tuning up to 1.3 μm in GaAs/GaAsSb core–shell nanowires, by varying the Sb content using Ga-assisted molecular beam epitaxy. An increase in Sb content leads to strain accumulation in shell manifesting in rough surface morphology, multifaceted growths, curved nanowires, and deterioration in the microstructural and optical quality of the nanowires. The presence of multiple PL peaks for Sb compositions ≥12 at.% and degradation in the nanowire quality as attested by broadening of Raman and x-ray diffraction peaks reveal compositional instability in the nanowires. Transmission electron microscope (TEM) images show the presence of stacking faults and twins. Based on photoluminescence (PL) peak energies and their excitation power dependence behavior, an energy-band diagram for GaAs/GaAsSb core–shell nanowires is proposed. Optical transitions are dominated by type II transitions at lower Sb compositions and a combination of type I and type II transitions for compositions ≥12 at.%. Type I optical transitions as low as 0.93 eV (1.3 μm) from the GaAsSb for Sb composition of 26 at.% have been observed. The PL spectrum of a single nanowire is replicated in the ensemble nanowires, demonstrating good compositional homogeneity of the latter. A double-shell configuration for passivation of deleterious surface states leads to significant enhancement in the PL intensity resulting in the observation of room temperature emission, which provides significant potential for further improvement with important implications for nanostructured optoelectronic devices operating in the near-infrared regime.

The influence of different C-doping locations in a GaN/Si structure with a GaN/AlN superlattice (SL) buffer on the material and electrical properties of GaN/Si was studied. The introduction of C doping can remarkably degrade the crystal quality of the buffer. C-doping of a top GaN buffer can introduce compressive stress into the top GaN due to the size effect, while C-doping in a SL buffer can impair the compressive stress provided from the SL buffer to the top GaN. It is found that introducing high-density carbon into the whole buffer can result in a more strain-balanced GaN/Si system with small deterioration of the 2DEG channel. Furthermore, the whole buffer C-doping method is an effective and easy way to achieve a thin buffer with low leakage current and high breakdown voltage (266 V@1 nA mm⁻¹; 698 V@10 μA mm⁻¹; 912 V@1 mA mm⁻¹). By using the whole-buffer C-doping method, a 2.5 μm-thick AlGaN/GaN HFET with a breakdown voltage higher than 900 V was achieved, and the breakdown voltage per unit buffer thickness can reach 181 V μm⁻¹.

An analytical study of field emission from microstructures is presented that includes position-dependent electric field enhancements, quantum corrections due to electron confinement and fluctuations of the workfunction. Our calculations, applied to a ridge microstructure, predict strong field enhancements. Though quantization lowers current densities as compared to the traditional Fowler–Nordheim process, strong field emission currents can nonetheless be expected for large emitter aspect ratios. Workfunction variations arising from changes in electric field penetration at the surface, or due to interface defects or localized screening, are shown to be important in enhancing the emission currents.