Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.

Organic Thin Film Transistors for Large Area Electronics

Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

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Quantum dot solar cells

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Transport properties of electrons confined in ultrafine wire structures are studied theoretically. The scattering probability of such size-quantized electrons is calculated for Coulomb potential and is shown to be suppressed drastically because of the one-dimensional nature of the electronic motion in the wire. Mobilities are estimated to be well beyond 106 cm2/Vs for a properly-designed GaAs wire at low temperatures. The feasibility of preparing such ultrafine structures with and without ultrafine lithography is discussed.

https://iopscience.iop.org/article/10.1143/JJAP.19.L735/pdf

Scattering Suppression and High-Mobility Effect of Size-Quantized Electrons in Ultrafine Semiconductor Wire Structures

We study experimentally and theoretically the influence of interface roughness on the mobility of two‐dimensional electrons in modulation‐doped AlAs/GaAs quantum wells. It is shown that interface roughness scattering is the dominant scattering mechanism in thin quantum wells with a well thickness Lw<60 A, where electron mobilities are proportional to L6w, reaching 2×103 cm2/V s at Lw∼55 A. From detailed comparison between theory and experiment, it is determined that the ‘‘GaAs‐on‐AlAs’’ interface grown by molecular beam epitaxy has a roughness with the height of 3–5 A and a lateral size of 50–70 A.

Interface roughness scattering in GaAs/AlAs quantum wells

Electron relaxation in a GaAs quantum dot is investigated to second order in electron-phonon interactions. Calculation of relaxation rate, as a function of level separation, indicates the significant contribution of LO\ifmmode\pm\else\textpm\fi{}LA processes, which create a window of rapid (subnanosecond) relaxation around the longitudinal-optical phonon energy. This result may provide a possible solution to the problem of photoluminescence degradation in small quantum dots.

Electron relaxation in a quantum dot: Significance of multiphonon processes.

We describe our systematic study of the two-dimensional electron mobilities \ensuremath{\mu} of n-type ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As/GaAs heterojunctions, in particular their dependence on the electron concentration ${N}_{s}$ and the temperatures T, in a variety of field-effect transistors in which the impurity locations are precisely controlled to vary \ensuremath{\mu} over the wide range of 5\ifmmode\times\else\texttimes\fi{}${10}^{3}$ to 1.5\ifmmode\times\else\texttimes\fi{}${10}^{6}$ ${\mathrm{cm}}^{2}$/V s. It is found that \ensuremath{\mu} increases with increasing ${N}_{s}$ in the temperature range of 10 to 300 K. The measured mobilities are compared with theoretical calculations. The observed dependence of \ensuremath{\mu} on ${N}_{s}$ at low temperatures is shown to be in excellent agreement with the theory of ionized-impurity scattering, whereas the high-temperature data disagree with the existing theory of polar-optical phonon scattering. A quantitative study has been successfully made on the effect of an undoped ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As spacer layer, which enhances not only \ensuremath{\mu} itself but also the slope in log\ensuremath{\mu}-log${N}_{s}$ characteristics. The presence of both positive and negative temperature dependences of \ensuremath{\mu} at low temperatures (T40 K) is noted, and its connection with the effects of nondegeneracy and lattice scattering is also discussed.

Mobility of the two-dimensional electron gas at selectively doped n -type AlxGa1-xAs/GaAs heterojunctions with controlled electron concentrations.

We have measured dc transport through a GaAs/AlGaAs quantum dot in the presence of a microwave signal of frequency $f$. We find features related to the photon energy $\mathrm{hf}$ whose positions in gate voltage are independent of the microwave power buy vary linearly with frequency. The measurements demonstrate photon-assisted tunneling in the mesoscopic regime. A comparison is made with a model that extends Coulomb blockade theory to include photon-assisted tunneling.

Hiroyuki Sakaki

Papers

Scattering Suppression and High-Mobility Effect of Size-Quantized Electrons in Ultrafine Semiconductor Wire Structures

Interface roughness scattering in GaAs/AlAs quantum wells

Electron relaxation in a quantum dot: Significance of multiphonon processes.

Mobility of the two-dimensional electron gas at selectively doped n -type AlxGa1-xAs/GaAs heterojunctions with controlled electron concentrations.

Observation of Photon-Assisted Tunneling through a Quantum Dot