Showing papers in "Semiconductor Science and Technology in 2012"

Introduction to topological superconductivity and Majorana fermions

Abstract: This short review paper provides a pedagogical introduction to the rapidly growing research field of Majorana fermions in topological superconductors. We first discuss in some detail the simplest ‘toy model’ in which Majoranas appear, namely a one-dimensional tight-binding representation of a p-wave superconductor, introduced more than 10 years ago by Kitaev. We then give a general introduction to the remarkable properties of Majorana fermions in condensed matter systems, such as their intrinsically non-local nature and exotic exchange statistics, and explain why these quasiparticles are suspected to be especially well suited for low-decoherence quantum information processing. We also discuss the experimentally promising (and perhaps already successfully realized) possibility of creating topological superconductors using semiconductors with strong spin–orbit coupling, proximity-coupled to standard s-wave superconductors and exposed to a magnetic field. The goal is to provide an introduction to the subject for experimentalists or theorists who are new to the field, focusing on the aspects which are most important for understanding the basic physics. The text should be accessible for readers with a basic understanding of quantum mechanics and second quantization, and does not require knowledge of quantum field theory or topological states of matter.

Half-Heusler compounds: Novel materials for energy and spintronic applications

Abstract: Half-Heusler compounds are an impressive class of materials with a huge potential for different applications such as future energy applications and for spintronics. The semiconducting Heusler compounds can be identified by the number of valence electrons. The band gap can be tuned between 0 and 4 eV by the electronegativity difference of the constituents. Magnetism can be introduced in these compounds by using rare-earth elements, manganese or ‘electron’ doping. Thus, there is a great interest in the fields of thermoelectrics, solar cells and diluted magnetic semiconductors. The combination of different properties such as superconductivity and topological edge states leads to new multifunctional materials, which have the potential to revolutionize technological applications. Here, we review the structure, the origin of the band gap and the functionalities of semiconducting half-Heusler compounds.

Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices

Abstract: Growth of InGaN/GaN light-emitting devices on nonpolar or semipolar planes offers a viable approach to reducing or eliminating the issues associated with polarization-related electric fields present in c-plane III-nitride heterostructures. Although progress in device performance has been rapid since the introduction of high-quality free-standing nonpolar and semipolar GaN substrates, a full appreciation of the materials challenges unique to nonpolar and semipolar III-nitride semiconductors has been slower to emerge. Only recently have researchers begun to understand issues such as the origins of the pyramidal hillocks typically observed on nominally on-axis m-plane GaN films, the effects of m-plane substrate misorientation on surface morphology and device performance, the mechanics of anisotropic cracking in tensile strained m-plane AlGaN films, the formation of basal-plane stacking faults in long-wavelength m-plane InGaN quantum wells, and the mechanisms for stress relaxation in semipolar AlGaN and InGaN films. In this paper, we review the materials and growth issues unique to high-performance nonpolar and semipolar light-emitting devices grown on high-quality free-standing GaN substrates and provide an outlook for the opportunities and challenges that lie ahead. (Some figures in this article are in colour only in the electronic version)

Atomic layer deposition for photovoltaics: applications and prospects for solar cell manufacturing

Abstract: Atomic layer deposition (ALD) is a vapour-phase deposition technique capable of depositing high quality, uniform and conformal thin films at relatively low temperatures. These outstanding properties can be employed to face processing challenges for various types of next-generation solar cells; hence, ALD for photovoltaics (PV) has attracted great interest in academic and industrial research in recent years. In this review, the recent progress of ALD layers applied to various solar cell concepts and their future prospects are discussed. Crystalline silicon (c-Si), copper indium gallium selenide (CIGS) and dye-sensitized solar cells (DSSCs) benefit from the application of ALD surface passivation layers, buffer layers and barrier layers, respectively. ALD films are also excellent moisture permeation barriers that have been successfully used to encapsulate flexible CIGS and organic photovoltaic (OPV) cells. Furthermore, some emerging applications of the ALD method in solar cell research are reviewed. The potential of ALD for solar cells manufacturing is discussed, and the current status of high-throughput ALD equipment development is presented. ALD is on the verge of being introduced in the PV industry and it is expected that it will be part of the standard solar cell manufacturing equipment in the near future.

Band engineering in dilute nitride and bismide semiconductor lasers

Abstract: Highly mismatched semiconductor alloys such as GaNxAs1 − x and GaBixAs1 − x have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spin-orbit-splitting energy with increasing Bi composition. We review here the electronic structure of such alloys and their consequences for ideal lasers. We then describe the substantial progress made in the demonstration of actual GaInNAs telecommunication (telecom) lasers. These have characteristics comparable to conventional InP-based devices. This includes a strong Auger contribution to the threshold current. We show, however, that the large spin-orbit-splitting energy in GaBiAs and GaBiNAs could lead to the suppression of the dominant Auger recombination loss mechanism, finally opening the route to efficient temperature-stable telecomm and longer wavelength lasers with significantly reduced power consumption.

ALD grown zinc oxide with controllable electrical properties

Abstract: The paper presents results for zinc oxide films grown at low-temperature regime by atomic layer deposition (ALD). We discuss electrical properties of such films and show that low-temperature deposition results in oxygen-rich ZnO layers in which free carrier concentration is very low. For the optimized ALD process it can reach the level of 1015 cm−3, while the mobility of electrons is between 20 and 50 cm2 V−1 s−1. Electrical parameters of ZnO films deposited by ALD at low-temperature regime are appropriate for constructing the ZnO-based p–n and Schottky junctions. We demonstrate that such junctions are characterized by a rectification ratio high enough to fulfill requirements of 3D memories and are deposited at temperature 100 °C which makes them appropriate for deposition on organic substrates.

Silicon spintronics with ferromagnetic tunnel devices

Abstract: In silicon spintronics, the unique qualities of ferromagnetic materials are combined with those of silicon, aiming at creating an alternative, energy-efficient information technology in which digital data are represented by the orientation of the electron spin. Here we review the cornerstones of silicon spintronics, namely the creation, detection and manipulation of spin polarization in silicon. Ferromagnetic tunnel contacts are the key elements and provide a robust and viable approach to induce and probe spins in silicon, at room temperature. We describe the basic physics of spin tunneling into silicon, the spin-transport devices, the materials aspects and engineering of the magnetic tunnel contacts, and discuss important quantities such as the magnitude of the spin accumulation and the spin lifetime in the silicon. We highlight key experimental achievements and recent progress in the development of a spin-based information technology.

Semipolar GaN grown on foreign substrates: a review

Abstract: Non- and semipolar GaN-based optoelectronic device structures have attracted much attention in recent years. Best results have been obtained on small bulk substrates cut from thick c-plane epi-wafers. However, owing to the limited size of such substrates, it is very attractive to study hetero-epitaxial approaches on foreign substrates. In this paper, we review the current state of such studies which eventually lead to large area non- or semipolar nitride structures. The simplest approach is to use planar sapphire or SiC wafers of non-c-plane orientations on which potentially less polar GaN can be grown. However, typically huge dislocation and in particular stacking fault densities evolve. More sophisticated approaches make use of the good GaN growth performance in the c-direction, eventually leading anyway to large area non- or semipolar structures. Several such approaches are discussed in this paper.

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

Abstract: InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0 0) and on semipolar (1 0 2), (1 1 2), (1 0 1) as well as (2 0 1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0 1) < (1 1 2) = (0 0 0 1) < (2 0 1) < (1 0 0) = (1 0 2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0 0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0 0) and the semipolar (1 0 2) and (2 0 1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1 2) and (1 0 1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

Germanium surface passivation and atomic layer deposition of high-k dielectrics—a tutorial review on Ge-based MOS capacitors

Abstract: Due to its high intrinsic mobility, germanium (Ge) is a promising candidate as a channel material (offering a mobility gain of approximately??2 for electrons and??4 for holes when compared to conventional Si channels) However, many issues still need to be addressed before Ge can be implemented in high-performance field-effect-transistor (FET) devices One of the key issues is to provide a high-quality interfacial layer, which does not lead to substantial drive current degradation in both low equivalent oxide thickness and short channel regime In recent years, a wide range of materials and processes have been investigated to obtain proper interfacial properties, including different methods for Ge surface passivation, various high-k dielectrics and metal gate materials and deposition methods, and different post-deposition annealing treatments It is observed that each process step can significantly affect the overall metal?oxide?semiconductor (MOS)-FET device performance In this review, we describe and compare combinations of the most commonly used Ge surface passivation methods (eg epi-Si passivation, surface oxidation and/or nitridation, and S-passivation) with various high-k dielectrics In particular, plasma-based processes for surface passivation in combination with plasma-enhanced atomic layer deposition for high-k depositions are shown to result in high-quality MOS structures To further improve properties, the gate stack can be annealed after deposition The effects of annealing temperature and ambient on the electrical properties of the MOS structure are also discussed

Monolithic Ge-on-Si lasers for large-scale electronic–photonic integration

Abstract: A silicon-based monolithic laser source has long been envisioned as a key enabling component for large-scale electronic–photonic integration in future generations of high-performance computation and communication systems. In this paper we present a comprehensive review on the development of monolithic Ge-on-Si lasers for this application. Starting with a historical review of light emission from the direct gap transition of Ge dating back to the 1960s, we focus on the rapid progress in band-engineered Ge-on-Si lasers in the past five years after a nearly 30-year gap in this research field. Ge has become an interesting candidate for active devices in Si photonics in the past decade due to its pseudo-direct gap behavior and compatibility with Si complementary metal oxide semiconductor (CMOS) processing. In 2007, we proposed combing tensile strain with n-type doping to compensate the energy difference between the direct and indirect band gap of Ge, thereby achieving net optical gain for CMOS-compatible diode lasers. Here we systematically present theoretical modeling, material growth methods, spontaneous emission, optical gain, and lasing under optical and electrical pumping from band-engineered Ge-on-Si, culminated by recently demonstrated electrically pumped Ge-on-Si lasers with >1 mW output in the communication wavelength window of 1500–1700 nm. The broad gain spectrum enables on-chip wavelength division multiplexing. A unique feature of band-engineered pseudo-direct gap Ge light emitters is that the emission intensity increases with temperature, exactly opposite to conventional direct gap semiconductor light-emitting devices. This extraordinary thermal anti-quenching behavior greatly facilitates monolithic integration on Si microchips where temperatures can reach up to 80 °C during operation. The same band-engineering approach can be extended to other pseudo-direct gap semiconductors, allowing us to achieve efficient light emission at wavelengths previously considered inaccessible.

Room-temperature 3.73 µm GaSb-based type-I quantum-well lasers with quinternary barriers

Abstract: Room-temperature operation of GaSb-based type-I lasers with emission wavelength of 3.73 ?m is reported for the first time. Ridge-waveguide devices with five quantum wells and quinternary barriers for improved hole confinement lase in pulsed operation with a low transparency threshold current density of 676 A cm?2?for infinite stripe width. The presence of additional loss mechanism is observed above 20 ?C, changing the characteristic temperature from 38 K to 16 K and limiting the maximum operating temperature to 32 ?C.

Atomic-scale simulation of ALD chemistry

Abstract: Published papers on atomic-scale simulation of the atomic layer deposition (ALD) process are reviewed. The main topic is reaction mechanism, considering the elementary steps of precursor adsorption, ligand elimination and film densification, as well as reactions with substrates (particularly Si and SiO2) and CVD-like decomposition at the surface. Density functional theory is the first principles method generally applied to these mechanistic questions. The most popular subject for modelling is the ALD of oxides and nitrides, particularly the high-k dielectrics HfO2, ZrO2?and Al2O3, due to their importance in semiconductor processing.

Experimental and theoretical analysis of the dominant lateral waveguiding mechanism in 975 nm high power broad area diode lasers

Abstract: For maximum fibre-coupled power, high power broad area diode lasers must operate with small lateral far field angles at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with low beam quality and wide emission angles. In order to experimentally determine the origin of the low beam quality, spectrally resolved near and far field measurements were performed for a diode laser with 50 µm stripe width. Within the range measured (CW optical output powers to 1.5 W) the laser is shown to operate in just six stable lateral modes, with spatially periodic profiles. Comparisons of the measured profiles with the results of two-dimensional modal simulation demonstrate that current-induced thermal lensing dominates the lateral waveguiding, in spite of the presence of both strong built-in index guiding and gain guiding. No evidence is seen for filamentation. Building on the diagnosis, proposals are presented for improvements to beam quality.

Thermoelectric transport in topological insulators

Abstract: Thermoelectric transport in topological insulators (TIs) is theoretically studied. TIs have gapless edge states in two dimensions, and do surface states in three dimensions. Both of the states have backscattering-free nature, and they remain gapless in the presence of nonmagnetic impurities. In particular, the edge states in two-dimensional TIs form perfect conducting channels. In this study, we calculate system-size dependence of thermoelectric properties in two-dimensional TIs, and evaluate the inelastic scattering length of the edge states by phonons, which affects the thermoelectric properties sensitively. We also study thermoelectric transport in three-dimensional (3D) TIs and compare with two dimensions. In both two- and three-dimensional TIs, there is a competition between the surface/edge and bulk transports in the thermoelectric phenomena. The surface transport in 3D TIs is relatively weak compared with the bulk transport due to impurities. Furthermore, we also study gapped 3D TIs in thin slab geometry and show large values of the figure of merit in the gapped system. This result is consistent with the previous work.

A study on low-power, nanosecond operation and multilevel bipolar resistance switching in Ti/ZrO 2/Pt nonvolatile memory with 1T1R architecture

Abstract: Low-power, bipolar resistive switching (RS) characteristics in the Ti/ZrO2/Pt nonvolatile memory with one transistor and one resistor (1T1R) architecture were reported. Multilevel storage behavior was observed by modulating the amplitude of the MOSFET gate voltage, in which the transistor functions as a current limiter. Furthermore, multilevel storage was also executed by controlling the reset voltage, leading the resistive random access memory (RRAM) to the multiple metastable low resistance state (LRS). The experimental results on the measured electrical properties of the various sized devices confirm that the RS mechanism of the Ti/ZrO2/Pt structure obeys the conducting filaments model. In application, the devices exhibit high-speed switching performances (250 ns) with suitable high/low resistance state ratio (HRS/LRS > 10). The LRS of the devices with 10 year retention ability at 80 °C, based on the Arrhenius equation, is also demonstrated in the thermal accelerating test. Furthermore, the ramping gate voltage method with fixed drain voltage is used to switch the 1T1R memory cells for upgrading the memory performances. Our experimental results suggest that the ZrO2-based RRAM is a prospective alternative for nonvolatile multilevel memory device applications.

Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy

Abstract: For nonpolar and semipolar orientations of GaN heteroepitaxially grown on sapphire substrates, the development of growth procedures to improve surface morphology and microstructure has been driven in a largely empirical way. This work attempts to comprehensively link the intrinsic properties of GaN faceted growth, across orientations, in order to understand, design and control growth methods for nonpolar (1 1 2 0) GaN and semipolar (1 1 2 2) GaN on foreign substrates. This is done by constructing a comprehensive series of kinetic Wulff plots (or v-plots) by monitoring the advances of convex and concave facets in selective area growth. A methodology is developed to apply the experimentally determined v-plots to the interpretation and design of evolution dynamics in nucleation and island coalescence. This methodology offers a cohesive and rational model for GaN heteroepitaxy along polar, nonpolar and semipolar orientations, and is broadly extensible to the heteroepitaxy of other materials. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and blocking of stacking faults. The status and outlook of semipolar and nonpolar GaN growth on sapphire substrates will be presented.

Nitrogen-induced improvement of resistive switching uniformity in a HfO2-based RRAM device

Abstract: The effect of nitrogen doping by the NH3 plasma treatment approach on the resistive switching properties of a HfO2-based resistive random access memory (RRAM) device is investigated. Test results demonstrate that significantly improved performances are achieved in the HfO2-based RRAM device by nitrogen doping, including low operating voltages, improved uniformity of switching parameters, satisfactory endurance and long retention characteristics. Doping by nitrogen is proposed to suppress the stochastic formation of conducting filaments in the HfO2 matrix and thus improve the performances of the Pt/Ti/HfO2/Pt device.

A TCAD-based modeling of GaN/InGaN/Si solar cells

Abstract: Theoretical efficiency potential of GaN/InGaN/cSi tandem solar cells is investigated using two-dimensional numerical computer simulation (i.e. technology-based computer aided design tool: TCAD). With double-junction GaN/InGaN/cSi tandem design, a conversion efficiency of 27% is achieved using a 1.0 µm In0.5Ga0.5N absorber of top cell over crystalline silicon (cSi) bottom cell. This efficiency is further improved to 29.0% with grading of the InxGa1−xN absorber layer close to the top heterointerface (p+-GaN/n−-InxGa1−xN) of the solar cell. A maximum conversion efficiency is obtained when the band discontinuity ratio (i.e. ΔEC:ΔEV) is set to 0.65:0.35. While efficiency remains approximately constant with moderate n-doping (up to 5 × 1016 cm−3) in the top InGaN absorber layer, sensitivity of the efficiency to the interface trap density and trap cross-section (when traps are located only at the heterointerfaces) shows degraded behavior with increasing trap density and trap cross-section. A temperature coefficient for open-circuit voltage (efficiency) of −0.15 (−1.72 × 10−3 °C−1), −0.09 (−0.95 × 10−3 °C−1) and −0.2 (−2.38 × 10−3 °C−1)%/°C for single heterojunction (SHJ), double-heterojunction (DHJ) and tandem-graded design is predicted from the numerical simulations.

Symmetrical current?voltage characteristic of a metal?semiconductor?metal structure of Schottky contacts and parameter retrieval of a CdTe structure

Abstract: Symmetrical, non-linear and current–voltage (I–V) characteristics of a metal–semiconductor–metal (M-S-M) structure of two metallic Schottky contacts fabricated to a p-type semiconductor were modeled by treating the semiconductor as a resistor sandwiched between two identical head-to-head Schottky barriers. The voltage distributions along the M-S-M structure were numerically determined and found that the voltage drop across the reverse-biased Schottky barrier is dominating at the low bias voltage, and the dominant range depends on the value of the resistor of the semiconductor bulk. The field dependence of barrier height due to the image force was proposed to be the mechanism for the current through the M-S-M structure when the voltage drop across the reverse-biased barrier is dominating. The proposed model was applied to the I–V curves measured at different temperatures on low-resistivity p-type CdTe with Au contacts and the density of the effective acceptors calculated, and the zero-field Schottky barrier height and the Richardson constant were extracted using the activation energy method. The extracted parameters fitted well with that published for the same material structure.

Ge-on-Si films obtained by epitaxial growing: edge dislocations and their participation in plastic relaxation

Abstract: Pure edge 90° misfit dislocations (MDs) are the most effective linear defects that combine the substrate and the film with different lattice parameters. A system consisting of a nonstressed film and a substrate approaches the perfect case in terms of the structural transition from one lattice parameter to the other if imperfections in the form of an ordered network of edge MDs are located exclusively at the interface, while threading dislocations are practically absent. The path to this perfect case goes through studying the possibilities of creating such an ordered network of edge MDs. The mechanism of formation of edge MDs proposed previously by Kvam et al (1990 J. Mater. Res. 5 1900) is discussed. This mechanism involves induced formation of a complementary pair of 60° MDs whose coalescence at the interface creates an edge MD. Some publications are presented, which demonstrate on the basis of experimental data that this mechanism under certain conditions can be the basic mechanism responsible for plastic relaxation of Ge-on-Si films. A cardinal method for decreasing the number of defects at the initial stages of growth of Ge/Si heterosystems is a set of procedures that allow a specified number of MDs to be inserted into the stressed film earlier than conditions of spontaneous nucleation of MDs from the film surface in the 2D–3D transition occur. When the low-temperature/high-temperature strategy of growth is used, the low-temperature GeSi seed layer tuned with respect to the growth temperature, composition and thickness can serve as a source of 60° dislocations, which facilitate earlier formation of edge MDs at the initial stage of plastic relaxation of the GeSi or Ge main layer. Results of some recent publications that report reaching high structural perfection of thin (∼1 µm and less) Ge-on-Si films are discussed. The proposed explanation of these results is based on postulates of controlled insertion of MDs and formation of edge MDs by the model of induced nucleation.

Properties and fabrication of high-order Bragg gratings for wavelength stabilization of diode lasers

Abstract: We report further progress in the design and fabrication of high-order Bragg gratings defined by I-line projection lithography that are implemented in a high-power diode laser. Simulations of surface Bragg gratings with large duty cycles predict large reflectivities even for the 25th Bragg order. We implemented such Bragg gratings in high-power diode lasers and compared the spectral and electro-optical properties with diode lasers having seventh-order Bragg gratings. Details of the fabrication and encapsulation of the Bragg gratings will also be presented.

ZnO-based ultra-violet light emitting diodes and nanostructures fabricated by atomic layer deposition

Abstract: We have investigated ZnO-based light-emitting diodes (LEDs) fabricated by atomic layer deposition (ALD), demonstrating that ALD is one of the noteworthy techniques to prepare high-quality ZnO required for ultraviolet (UV) photonic devices. Here, we review our recent investigations on different ZnO-based heterojunction LEDs such as n-ZnO/p-GaN LEDS, n-ZnO:Al/ZnO nanodots-SiO2 composite/p-GaN LEDS, n-ZnO/ZnO nanodots-SiO2 composite/p-AlGaN LEDs, n-ZnO:Al/i-ZnO/p-SiC(4H) LEDs, and also on ZnO-based nanostructures including ZnO quantum dots embedded in SiO2 nanoparticle layer, ZnO nanopillars on sapphire substrates, Al-doped ZnO films on sapphire substrate and highly (0 0 0 1)-oriented ZnO films on amorphous glass substrate. The latest investigation also demonstrated p-type ZnO:P films prepared on amorphous silica substrates, which allow us to fabricate ZnO-based homojunction LEDs. These devices and structures were studied by x-ray diffraction and various analytical electron microscopy observations as well as electric and electro-optical measurements.

Controlling the carriers of topological insulators by bulk and surface doping

Abstract: We report a systematic study of bulk and surface chemical doping effects on single Dirac cone topological insulator Bi2Se3 and Bi2Te3. By bulk doping, we were able to achieve full range control of charge carrier types and concentration, with the exact Fermi energy measured by angle-resolved photoemission spectroscopy (ARPES). Due to the unusual robustness of the topological surface state, we further realized the bi-polar control of the surface carriers by gaseous or alkaline surface doping without affecting the topological nature of these materials. The doping progress monitored by in situ ARPES study clearly demonstrated the switching between different carrier types through the Dirac point. The ability to control the carrier types and the concentration of topological insulators will greatly facilitate future applications.

Terabit/s class InP photonic integrated circuits

Abstract: In this paper, we review recent developments in the area of terabit/s?class monolithically integrated, transmitter and receiver photonic integrated circuits for the implementation of coherent, polarization-multiplexed, quadrature phase shift keying and higher order modulation formats.

On impedance spectroscopy analysis of nonideal heterojunctions

Abstract: A quantitative analysis of the impedance of nonideal heterojunctions was carried out taking into consideration the effects of series resistance, shunt resistance, parasitic inductance and electrically active interface traps. A new approach is proposed to determine the energy distribution of surface state density and to calculate the actual value of barrier capacitance of heterojunctions on the basis of the analysis of their complex impedance–voltage characteristics.

Defect reduction methods for III-nitride heteroepitaxial films grown along nonpolar and semipolar orientations

Abstract: Heteroepitaxial nonpolar and semipolar III-nitride films contain numerous structural defects which prevent their use for the development of efficient optoelectronic devices. After a description of the microstructure of such films, this paper reviews the different techniques which have been developed to reduce the densities of the different types of defects. The choice of a well-adapted substrate is discussed. The effect of the introduction of an interlayer is reported. Then, techniques based on the introduction of a 3D growth stage are evaluated. Finally, the original methods based on the growth on inclined facets are described and assessed.

ZnO films grown by atomic layer deposition for organic electronics

Abstract: In this review, we analyze the properties of zinc oxide (ZnO) films grown by atomic layer deposition (ALD) crucial for organic electronics applications. We show the examples of such applications in hybrid organic-ZnO p–n junctions as well as in devices where conductive ZnO- and aluminum-doped ZnO films are used as transparent electrodes. Additionally, the relevant issues like energy-gap engineering in ZnO:Al films alloyed with magnesium or the long time stability of organic devices with ZnO passivation layers will be considered.

Terahertz magneto-optical spectroscopy in HgTe thin films

Abstract: Thin films of HgTe have been systematically investigated using continuous frequency terahertz spectroscopy in external magnetic fields. In these experiments, full control of the polarization state can be achieved including Faraday rotation and ellipticity. We present the details of the experimental procedure and of the data analysis. Besides the cyclotron resonance, an additional mode is observed in the zero-gap sample. The results at high temperatures can be well understood within the classical Drude model of the dynamic conductivity. Possible dimensionality of the charge dynamics at low temperatures is discussed.

Analysis of buffer-impurity and field-plate effects on breakdown characteristics in small-sized AlGaN/GaN high electron mobility transistors

Abstract: The two-dimensional analysis of breakdown characteristics in field-plate AlGaN/GaN high electron mobility transistors with a relatively short gate length and short gate-to-drain distances is performed by considering a deep donor and a deep acceptor in a buffer layer. It is shown that when the acceptor density in the buffer layer is high, the breakdown voltage is determined by the impact ionization of carriers, and it can decrease with increasing the field-plate length. This is because the distance between the field-plate edge and the drain becomes very short and the electric field there becomes very high. On the other hand, when the acceptor density in the buffer layer is relatively low, the buffer leakage current becomes very large and this can determine the breakdown voltage, which becomes very low. In this case, the breakdown voltage increases with increasing the field-plate length.