Showing papers in "Semiconductor Science and Technology in 2016"

Recent progress in Ga2O3 power devices

Abstract: This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga2O3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal–semiconductor field-effect transistors, metal–oxide–semiconductor field-effect transistors and Schottky barrier diodes.

Resistive switching memories based on metal oxides: mechanisms, reliability and scaling

Abstract: With the explosive growth of digital data in the era of the Internet of Things (IoT), fast and scalable memory technologies are being researched for data storage and data-driven computation. Among the emerging memories, resistive switching memory (RRAM) raises strong interest due to its high speed, high density as a result of its simple two-terminal structure, and low cost of fabrication. The scaling projection of RRAM, however, requires a detailed understanding of switching mechanisms and there are potential reliability concerns regarding small device sizes. This work provides an overview of the current understanding of bipolar-switching RRAM operation, reliability and scaling. After reviewing the phenomenological and microscopic descriptions of the switching processes, the stability of the low- and high-resistance states will be discussed in terms of conductance fluctuations and evolution in 1D filaments containing only a few atoms. The scaling potential of RRAM will finally be addressed by reviewing the recent breakthroughs in multilevel operation and 3D architecture, making RRAM a strong competitor among future high-density memory solutions.

A review of the electrical properties of semiconductor nanowires: insights gained from terahertz conductivity spectroscopy

Abstract: © 2016 IOP Publishing Ltd. Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz (THz) conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using THz spectroscopy. A didactic description of THz time-domain spectroscopy, optical pump-THz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how THz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive THz receivers.

Antimony selenide thin-film solar cells

Abstract: Due to their promising applications in low-cost, flexible and high-efficiency photovoltaics, there has been a booming exploration of thin-film solar cells using new absorber materials such as Sb2Se3, SnS, FeS2, CuSbS2 and CuSbSe2. Among them, Sb2Se3-based solar cells are a viable prospect because of their suitable band gap, high absorption coefficient, excellent electronic properties, non-toxicity, low cost, earth-abundant constituents, and intrinsically benign grain boundaries, if suitably oriented. This review surveys the recent development of Sb2Se3-based solar cells with special emphasis on the material and optoelectronic properties of Sb2Se3, the solution-based and vacuum-based fabrication process and the recent progress of Sb2Se3-sensitized and Sb2Se3 thin-film solar cells. A brief overview further addresses some of the future challenges to achieve low-cost, environmentally-friendly and high-efficiency Sb2Se3 solar cells.

Thermal transport in amorphous materials: a review

Abstract: Thermal transport plays a crucial role in performance and reliability of semiconductor electronic devices, where heat is mainly carried by phonons. Phonon transport in crystalline semiconductor materials, such as Si, Ge, GaAs, GaN, etc, has been extensively studied over the past two decades. In fact, study of phonon physics in crystalline semiconductor materials in both bulk and nanostructure forms has been the cornerstone of the emerging field of ‘nanoscale heat transfer’. On the contrary, thermal properties of amorphous materials have been relatively less explored. Recently, however, a growing number of studies have re-examined the thermal properties of amorphous semiconductors, such as amorphous Si. These studies, which included both computational and experimental work, have revealed that phonon transport in amorphous materials is perhaps more complicated than previously thought. For instance, depending on the type of amorphous materials, thermal transport occurs via three types of vibrations: propagons, diffusons, and locons, corresponding to the propagating, diffusion, and localized modes, respectively. The relative contribution of each of these modes dictates the thermal conductivity of the material, including its magnitude and its dependence on sample size and temperature. In this article, we will review the fundamental principles and recent development regarding thermal transport in amorphous semiconductors.

Grain boundaries in CdTe thin film solar cells: a review

Abstract: The current state of knowledge on the impact of grain boundaries in CdTe solar cells is reviewed with emphasis being placed on working cell structures. The role of the chemical composition of grain boundaries as well as growth processes are discussed, along with characterisation techniques such as electron beam induced current and cathodoluminescence, which are capable of extracting information on a level of resolution comparable to the size of the grain boundaries. Work which attempts to relate grain boundaries to device efficiency is also assessed and gaps in the current knowledge are highlighted.

Challenges and future perspectives in HVPE-GaN growth on ammonothermal GaN seeds

Abstract: Homoepitaxial growth of high structural quality and high-purity thick gallium nitride layers by crystallization from vapor phase (hydride vapor phase epitaxy (HVPE)) on 1, 1.5, and 2 inch substrates obtained by a solution (ammonothermal) growth method is presented. Advantages and disadvantages of both growth technologies are described in detail. Structural, optical, electrical, and thermal properties of gallium nitride grown from the vapor phase are demonstrated and compared to properties of ammonothermally grown material. It is shown that a synergy of these two methods can create new opportunities for an efficient production of bulk gallium nitride crystals and then substrates. It is also shown that free-standing (products of slicing procedures) gallium nitride crystals obtained from growth by vapor phase on ammonothermal substrates can be successfully used as seeds for the next growth process by both discussed methods. Factors limiting HVPE and making it a 'wafer to wafer' technology are presented, clarified, and analyzed. Intentional introduction of silicon to growth of gallium nitride by HVPE and crystals with a high free carrier concentration and high structural quality are demonstrated. First electronic and optoelectronic devices fabricated on the free-standing gallium nitride substrates are shown.

Reliability and parasitic issues in GaN-based power HEMTs: a review

Abstract: Despite the potential of GaN-based power transistors, these devices still suffer from certain parasitic and reliability issues that limit their static and dynamic performance and the maximum switching frequency. The aim of this paper is to review our most recent results on the parasitic mechanisms that affect the performance of GaN-on-Si HEMTs; more specifically, we describe the following relevant processes: (i) trapping of electrons in the buffer, which is induced by off-state operation; (ii) trapping of hot electrons, which is promoted by semi-on state operation; (iii) trapping of electrons in the gate insulator, which is favored by the exposure to positive gate bias. Moreover, we will describe one of the most critical reliability aspects of Metal-Insulator-Semiconductor HEMTs (MIS-HEMTs), namely time-dependent dielectric breakdown.

CMOS-compatible spintronic devices: a review

Abstract: For many decades CMOS devices have been successfully scaled down to achieve higher speed and increased performance of integrated circuits at lower cost. Today’s charge-based CMOS electronics encounters two major challenges: power dissipation and variability. Spintronics is a rapidly evolving research and development field, which offers a potential solution to these issues by introducing novel ‘more than Moore’ devices. Spin-based magnetoresistive random-access memory (MRAM) is already recognized as one of the most promising candidates for future universal memory. Magnetic tunnel junctions, the main elements of MRAM cells, can also be used to build logic-in-memory circuits with non-volatile storage elements on top of CMOS logic circuits, as well as versatile compact on-chip oscillators with low power consumption. We give an overview of CMOS-compatible spintronics applications. First, we present a brief introduction to the physical background considering such effects as magnetoresistance, spin-transfer torque (STT), spin Hall effect, and magnetoelectric effects. We continue with a comprehensive review of the state-of-the-art spintronic devices for memory applications (STT-MRAM, domain wallmotion MRAM, and spin–orbit torque MRAM), oscillators (spin torque oscillators and spin Hall nano-oscillators), logic (logic-in-memory, all-spin logic, and buffered magnetic logic gate grid), sensors, and random number generators. Devices with different types of resistivity switching are analyzed and compared, with their advantages highlighted and challenges revealed. CMOScompatible spintronic devices are demonstrated beginning with predictive simulations, proceeding to their experimental confirmation and realization, and finalized by the current status of application in modern integrated systems and circuits. We conclude the review with an outlook, where we share our vision on the future applications of the prospective devices in the area.

Topical Review: Development of overgrown semi-polar GaN for high efficiency green/yellow emission

Abstract: The most successful example of large lattice-mismatched epitaxial growth of semiconductors is the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great success in developing InGaN-based blue emitters. However, the majority of achievements in the field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001) sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still a great challenge to develop longer wavelength devices such as green and yellow emitters. One clear way forward would be to grow III-nitride device structures along a semi-/non-polar direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium incorporation into GaN and reduced quantum confined Stark effects. This review presents recent progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates, the two kinds of major substrates which are cost-effective and thus industry-compatible, and also demonstrates the latest achievements on electrically injected InGaN emitters with long emission wavelengths up to and including amber on overgrown semi-polar GaN. Finally, this review presents a summary and outlook on further developments for semi-polar GaN based optoelectronics.

Analysis of the scattering mechanisms controlling electron mobility in β-Ga2O3 crystals

Abstract: Electron density and Hall mobility data were simultaneously analyzed in the frame of the relaxation time approximation in order to identify the main scattering mechanisms that limit the carrier mobility in β-Ga2O3 single crystals. The Hall factor correction was self-consistently included in the fitting procedure. The analysis indicates that low-energy optical phonons provide the main scattering mechanism, via lattice deformation. In this regard, a deformation potential of about 4 × 109 eV cm−1 was estimated. Furthermore, it is shown that the Hall coefficient and mobility can be measured by the usual experimental geometry, and the standard transport theory can be applied when off-diagonal elements of the resistivity tensor at zero magnetic field are negligible with respect to the diagonal ones. This directly follows from the analysis of the magneto-resistive tensor of a semiconductor with monoclinic structure. Such a requirement is satisfied under the hypothesis of nearly spherical energy surfaces, as has been reported to occur at the Γ minimum of the conduction band of β-Ga2O3.

Chlorine-based dry etching of β-Ga2O3

Abstract: Reactive ion etching (RIE) and inductively coupled plasma (ICP) etching techniques were used to determine the optimal dry etch conditions for β-Ga2O3. RF power and chamber pressure were examined to study their effects on etch rate and surface roughness for three crystallographic planes, i.e., (100); (010); and ( by RIE. BCl3 etch rate calibrations were performed on all β-Ga2O3 planes studied, in comparison to Cl2. RIE yielded moderate etch rates (<20 nm min−1), and surface roughness showed no clear trend with RF power. Moreover, the effect of bias power, plasma power, and the choice of etchant were studied using ICP. The etches performed by ICP were shown to be superior to RIE in both etch rate and surface roughness, due to the much higher plasma densities and uniformities possible with plasma powers beyond those realized in RIE. The maximum etch rate of 43.0 nm min−1 was achieved using BCl3 in ICP. SF6/BCl3 mixtures, which yield high GaN etch rates, were also studied. However, in contrast to GaN etching, SF6/BCl3 was found to be far less effective than pure BCl3 in etching β-Ga2O3.

First-principles calculations of thermal, electrical, and thermoelectric transport properties of semiconductors

Abstract: The transport properties of semiconductors are key to the performance of many solid-state devices (transistors, data storage, thermoelectric cooling and power generation devices, etc). An understanding of the transport details can lead to material designs with better performances. In recent years simulation tools based on first-principles calculations have been greatly improved, being able to obtain the fundamental ground-state properties of materials (such as band structure and phonon dispersion) accurately. Accordingly, methods have been developed to calculate the transport properties based on an ab initio approach. In this review we focus on the thermal, electrical, and thermoelectric transport properties of semiconductors, which represent the basic transport characteristics of the two degrees of freedom in solids—electronic and lattice degrees of freedom. Starting from the coupled electron-phonon Boltzmann transport equations, we illustrate different scattering mechanisms that change the transport features and review the first-principles approaches that solve the transport equations. We then present the first-principles results on the thermal and electrical transport properties of semiconductors. The discussions are grouped based on different scattering mechanisms including phonon-phonon scattering, phonon scattering by equilibrium electrons, carrier scattering by equilibrium phonons, carrier scattering by polar optical phonons, scatterings due to impurities, alloying and doping, and the phonon drag effect. We show how the first-principles methods allow one to investigate transport properties with unprecedented detail and also offer new insights into the electron and phonon transport. The current status of the simulation is mentioned when appropriate and some of the future directions are also discussed.

Spin–orbital coupling effect on the power factor in semiconducting transition-metal dichalcogenide monolayers

Abstract: The electronic structures and thermoelectric properties of semiconducting transition-metal dichalcogenide monolayers (M = Zr, Hf, Mo, W and Pt; X = S, Se and Te) are investigated by combining first-principles and Boltzmann transport theory, including spin–orbital coupling (SOC). It is found that the gap decrease increases from S to Te in each cation group when the SOC is opened. The spin–orbital splitting has the same trend with the gap reducing. The calculated results show that SOC has a noteworthy detrimental effect on the p-type power factor, while it has a negligible influence in n-type doping except for the W cation group, which can be understood by considering the effects of SOC on the valence and conduction bands. For (X = S, Se and Te), SOC leads to an observable enhanced power factor in n-type doping, which can be explained by SOC-induced band degeneracy, namely the bands converge. Among all of the cation groups, the Pt cation group shows the highest Seebeck coefficient, which leads to the best power factor, if we assume that the scattering time is fixed. The calculated results show that (M = Zr, Hf, Mo, W and Pt) have the best p-type power factor of all the cation groups, and that (M = Zr and Hf), and (M = Mo and Pt) have a more excellent n-type power factor in their respective cation group. Therefore, these results may be useful for further theoretical prediction or experimental research of excellent thermoelectric materials from semiconducting transition-metal dichalcogenide monolayers.

Towards monolithic integration of germanium light sources on silicon chips

Abstract: Germanium (Ge) is a group-IV indirect band gap semiconductor, and therefore bulk Ge cannot emit light efficiently. However, the direct band gap energy is close to the indirect one, and significant engineering efforts are being made to convert Ge into an efficient gain material monolithically integrated on a Si chip. In this article, we will review the engineering challenges of developing Ge light sources fabricated using nano-fabrication technologies compatible with Complementary Metal-Oxide-Semiconductor (CMOS) processes. In particular, we review recent progress in applying high-tensile strain to Ge to reduce the direct band gap. Another important technique is doping Ge with donor impurities to fill the indirect band gap valleys in the conduction band. Realization of carrier confinement structures and suitable optical cavities will be discussed. Finally, we will discuss possible applications of Ge light sources in potential photonics-electronics convergent systems.

The annealing temperature dependence of anatase TiO2 thin films prepared by the electron-beam evaporation method

Abstract: In this paper, we report on titanium dioxide (TiO2) thin films deposited by an electron beam evaporation method on quartz glass substrates (15 × 15 × 2 mm3 in size), followed by post-annealing at 300 °C to 600 °C for an annealing time of up to 2 h. The substrate temperature during the film deposition was kept at 150 °C. The effect of post-growth thermal annealing on the structural and optical properties of TiO2 thin films were systematically studied as a function of annealing temperature. We found that the as-deposited TiO2 films are amorphous in structure, while the films started to crystallize into the anatase phase when annealed at temperatures ≥450 °C. An increase in annealing temperature results in a decrease of transmittance percentage and also in optical band gap energy. The refractive indices of the films were evaluated from the measured transmittance spectra using the envelope method. An increase in the refractive index with an increase of annealing temperature was observed.

Enhanced resistive switching performance for bilayer HfO2/TiO2 resistive random access memory

Abstract: We prepared bilayer HfO2/TiO2 resistive random accessory memory (RRAM) using magnetron sputtering on an ITO/PEN flexible substrate. The switching voltages (V SET and V RESET) were smaller for the Pt/HfO2/TiO2/ITO device than for a Pt/HfO2/ITO memory device. The insertion of a TiO2 layer in the switching layer was inferred to act as an oxygen reservoir to reduce the switching voltages. In addition, greatly improved uniformity was achieved, which showed the coefficient of the variations of V SET and V RESET to be 9.90% and 6.35% for the bilayer structure RRAM. We deduced that occurrence of conductive filament connection/rupture at the interface of the HfO2 and TiO2, in combination with the HfO2 acting as a virtual cathode, led to the improved uniformity. A multilevel storage capability can be obtained by varying the stop voltage in the RESET process for bilayer HfO2/TiO2 RRAM. By analyzing the current conduction mechanism, we demonstrated that the multilevel high resistance state (HRS) was attributable to the increased barrier height when the stop voltage was increased.

From single III-nitride nanowires to piezoelectric generators: New route for powering nomad electronics

Abstract: Ambient energy harvesting using piezoelectric nanomaterials is today considered as a promising way to supply microelectronic devices. Since the first demonstration of electrical energy generation from piezoelectric semiconductor nanowires in 2006, the piezoelectric response of 1D-nanostructures and the development of nanowire-based piezogenerators have become a hot topic in nanoscience. After several years of intense research on ZnO nanowires, III-nitride nanomaterials have started to be explored thanks to their high piezoelectric coefficients and their strong piezo-generation response. This review describes the present status of the field of piezoelectric energy generation with nitride nanowires. After presenting the main motivations and a general overview of the domain, a short description of the main properties of III-nitride nanomaterials is given. Then we review the piezoelectric responses of III-N nanowires and the specificities of the piezo-generation mechanism in these nanostructures. Finally, the design and performance of the macroscopic piezogenerators based on nitride nanowire arrays are described, showing the promise of III-nitride nanowires for ultra-compact and efficient piezoelectric generators.

Analysis of temperature dependent forward characteristics of Ni/ β-Ga2O3 Schottky diodes

Abstract: In this paper, the temperature dependence of the turn-on characteristics of Schottky barrier diodes fabricated on oriented n-type β-Ga2O3 is reported. The barrier height (qΦbn) and ideality factor (n) for Ni- β-Ga2O3 was found to be 1.08 ± 0.05 eV and 1.19 respectively at room temperature. The effective Richardson constant (A **) is determined to be 42.96 A cm−2 K−2, in close agreement with the theoretical value. At low temperatures (85–273 K), the current–voltage characteristics reveal a strong temperature dependence of Schottky barrier heights and ideality factors and a corresponding deviation from the barrier height extracted from capacitance–voltage measurements. A detailed analysis is presented, which suggest that these effects can be attributed to a large barrier inhomogeneity at metal/β-Ga2O3 interfaces, possibly resulting from interfacial defects, which can be modeled using a potential fluctuation model.

A new compact model for bipolar RRAMs based on truncated-cone conductive filaments—a Verilog-A approach

Abstract: A new model for bipolar resistive random-access memories (RRAMs) is presented in this article. Redox and diffusion processes are used to describe in detail the physics behind the filamentary resistive switching (RS) mechanisms of the RRAMs under study. The model includes truncated-cone shaped filaments which are known to be close to the real conductive filament (CF) geometry and a detailed thermal approach, where two temperatures are considered to describe the rupture process at the CF's narrowest part and also the main CF body's electrical conductivity variations. Ti/ZrO2/Pt RRAM devices have been fabricated and measured, and the model has allowed us to reproduce the experimental data for all the cases analyzed. Finally, the model has been implemented in Verilog-A code within the ADS circuit simulator, and the response of a device to pulsed external voltages within a characterization circuit has been simulated, producing good results when compared with experimental measurements.

Electron channeling in TiO2 coated Cu layers

Abstract: Electron transport in metal conductors with ~5-30 nm width is dominated by surface scattering. In situ transport measurements as a function of surface chemistry demonstrate that the primary parameter determining the surface scattering specularity is the localized surface density of states at the Fermi level N(Ef). In particular, the measured sheet resistance of epitaxial Cu(001) layers with thickness dCu = 9-25 nm increases when coated with dTi = 0.1-4.0 monolayers (ML) of Ti, but decreases again during exposure to 37 Pa of O2. These resistivity changes are a function of dCu and dTi and are due to a transition from partially specular electron scattering at the Cu surface to completely diffuse scattering at the Cu-Ti interface, and the recovery of surface specularity as the Ti is oxidized. X-ray reflectivity and photoelectron spectroscopy indicate the formation of a 0.47±0.03 nm thick Cu2O surface layer on top of the TiO2-Cu2O during air exposure, while density functional calculations of TiOx cap layers as a function of x = 0-2 and dTi = 0.25-1.0 ML show a reduction of N(Ef) by up to a factor of four. This reduction is proposed to be the key cause for the recovery of surface specularity and results in electron confinement and channeling in the Cu layer upon Ti oxidation. Transport measurements at 293 and 77 K confirm the channeling and demonstrate the potential for highconductivity metal nanowires by quantifying the surface specularity parameter p = 0.67±0.05, 0.00±0.05, and 0.35±0.05 at the Cu-vacuum, Cu-Ti, and Cu-TiO2 interfaces.

Growth and characterization of α-phase Ga2−x Sn x O3 thin films for solar-blind ultraviolet applications

Abstract: Ga2−x Sn x O3 thin films on m-plane (300) sapphire substrates were deposited by laser molecular beam epitaxy technology. Corundum-structured α-phase Ga2−x Sn x O3 thin films with different Sn content were obtained at 850 °C in vacuum pressure of 5 × 10−5 Pa. The band-gap energy of the α-phase Ga2−x Sn x O3 thin films, determined from the absorption spectrum, decreases linearly with increasing Sn content. A metal–semiconductor–metal structured solar-blind photodetector based α-phase Ga2−x Sn x O3 thin films was fabricated, and excellent solar-blind ultraviolet characteristics were demonstrated. The ratio of I 254/I dark was up to 1.40 × 102 and the responsivity increased to 9.55 × 10−2 A W−1. The results suggest that α-phase Ga2−x Sn x O3 thin films are promising candidates for use in solar-blind photodetectors.

Preparation of p-type Na-doped Cu2O by electrodeposition for a p-n homojunction thin film solar cell

Abstract: In this work, a method of enhancing the electrical properties of the electrodeposited p-type Cu2O film is described. Sodium doped Cu2O was achieved by adding sodium aluminate complex solution to the electrodeposition alkaline Cu (II) lactate electrolyte. The optimal Na content [Na at% atomic ratio] incorporated in the Cu2O film was found to be approximately 1.34 at.%. The XPS result shows that the binding energy at 1072.4 ± 0.2 eV corresponds to the presence of sodium in sodium oxide. The optimized resistivity and the hole concentration were approximately 291 Ω cm and 2.13 × 1018 cm3, respectively. A Cu2O p-n homojunction solar cell with 2.05% efficiency was fabricated using a Cl-doped n-type Cu2O film and an optimized Na-doped Cu2O film.

Combining graphene with silicon carbide: synthesis and properties – a review

Abstract: Being a true two-dimensional crystal, graphene possesses a lot of exotic properties that would enable unique applications. Integration of graphene with inorganic semiconductors, e.g. silicon carbid ...