Showing papers in "Semiconductor Science and Technology in 2014"

An introduction to InP-based generic integration technology

Abstract: Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets. Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology.

If it’s pinched it’s a memristor

Abstract: This chapter consists of two parts. Part I gives a circuit-theoretic foundation for the first four elementary nonlinear 2-terminal circuit elements, namely, the resistor, the capacitor, the inductor, and the memristor. Part II consists of a collection of colorful “Vignettes” with carefully articulated text and colorful illustrations of the rudiments of the memristor and its characteristic fingerprints and signatures. It is intended as a self-contained pedagogical primer for beginners who have not heard of memristors before.

Atomic layer deposition of ZnO: a review

Abstract: Due to the unique set of properties possessed by ZnO, thin films of ZnO have received more and more interest in the last 20?years as a potential material for applications such as thin-film transistors, light-emitting diodes and gas sensors. At the same time, the increasingly stringent requirements of the microelectronics industry, among other factors, have led to a dramatic increase in the use of atomic layer deposition (ALD) technique in various thin-film applications. During this time, the research on ALD-grown ZnO thin films has developed from relatively simple deposition studies to the fabrication of increasingly intricate nanostructures and an understanding of the factors affecting the fundamental properties of the films. In this review, we give an overview of the current state of ZnO ALD research including the applications that are being considered for ZnO thin films.

Development and future of ultraviolet light-emitting diodes: UV-LED will replace the UV lamp

Abstract: Ultraviolet light-emitting diodes (UV-LEDs) have started replacing UV lamps. The power per LED of high-power LED products has reached 12?W (14 A), which is 100?times the values observed ten years ago. In addition, the cost of these high-power LEDs has been decreasing. In this study, we attempt to understand the technologies and potential of UV-LEDs.

High power AlGaN ultraviolet light emitters

Abstract: We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.

Recent progress in metal-organic chemical vapor deposition of $\left( 000\bar{1} \right)$ N-polar group-III nitrides

Abstract: Progress in metal-organic chemical vapor deposition of high quality N-polar (Al, Ga, In)N films on sapphire, silicon carbide and silicon substrates is reviewed with focus on key process components such as utilization of vicinal substrates, conditions ensuring a high surface mobility of species participating in the growth process, and low impurity incorporation. The high quality of the fabricated films enabled the demonstration of N-polar (Al, Ga, In)N based devices with excellent performance for transistor applications. Challenges related to the growth of high quality N-polar InGaN films are also presented.

TiO2-based memristors and ReRAM: materials, mechanisms and models (a review)

Abstract: The memristor is the fundamental nonlinear circuit element, with uses in computing and computer memory. Resistive Random Access Memory (ReRAM) is a resistive switching memory proposed as a non-volatile memory. In this review we shall summarize the state of the art for these closely-related fields, concentrating on titanium dioxide, the well-utilized and archetypal material for both. We shall cover material properties, switching mechanisms and models to demonstrate what ReRAM and memristor scientists can learn from each other and examine the outlook for these technologies.

Hexagonal boron nitride for deep ultraviolet photonic devices

Abstract: This paper provides a brief overview on recent advances in tackling the doping and optical polarization issues involved in the development of high performance deep ultraviolet (DUV) light emitting devices. In particular, recent developments in the exploitation of a novel DUV emitter layer structure based on a hexagonal boron nitride (hBN) and AlGaN p–n junction and doping engineering to potentially overcome the intrinsic problem of low p-type conductivity (or low free hole concentration) in Al-rich AlGaN are summarized. By implementing the wide bandgap and highly conductive hBN p-type layer strategy in nitride DUV emitters, p-type conductivities and DUV transparency of the electron blocking layer and p-type contact layer will be dramatically increased. This will significantly improve the free hole injection and quantum efficiency, reduce the operating voltage and heat generation, and increase the device operating lifetime. The growth of undoped and Mg-doped p-type hBN via a metal organic chemical vapor deposition technique has been studied. Furthermore, p-hBN/n-AlGaN p–n junctions have been fabricated and characterized to demonstrate the feasibility and potential of p-hBN/n-AlGaN p–n heterostructure based DUV light emitting devices. Further improvements in material quality, p-type conductivity control and device processing procedures would enhance the properties of these p–n structures, which could ultimately pave the way towards the realization of high efficiency nitride DUV photonic devices.

Review of carbon nanotube nanoelectronics and macroelectronics

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

A direct comparison of CVD-grown and exfoliated MoS 2 using optical spectroscopy

Abstract: MoS2 is a highly interesting material, which exhibits a crossover from an indirect band gap in the bulk crystal to a direct gap for single layers. Here, we perform a direct comparison between large-area MoS2 films grown by chemical vapor deposition (CVD) and MoS2 flakes prepared by mechanical exfoliation from mineral bulk crystal. Raman spectroscopy measurements show differences between the in-plane and out-of-plane phonon mode positions in CVD-grown and exfoliated MoS2. Photoluminescence (PL) mapping reveals large regions in the CVD-grown films that emit strong PL at room-temperature, and low-temperature PL scans demonstrate a large spectral shift of the A exciton emission as a function of position. Polarization-resolved PL measurements under near-resonant excitation conditions show a strong circular polarization of the PL, corresponding to a valley polarization.

Survey of ultraviolet non-line-of-sight communications

Abstract: The unique characteristics of the atmospheric propagation of deep ultraviolet (UV) radiation make possible the novel capability of establishing non-line-of-sight (NLOS) optical communication links. Although NLOS UV communications (UVC) has been studied for decades, early work focused on the use of lasers and flash lamps as sources. Recent advances in device technology, including UV light-emitting diodes and solar-blind optical filters, suggest that compact low-power systems may soon be feasible, and, as a result, research into the effective use of this rapidly maturing technology has accelerated. In this paper, we review the NLOS UVC literature, examining a range of topics from channel modelling and experimentation through system analysis and prototype development. The breadth of this research not only indicates the growing interest in UVC technology but also suggests the existence of many avenues for continued exploration.

Compositional dependence of the direct and indirect band gaps in Ge1−ySny alloys from room temperature photoluminescence: implications for the indirect to direct gap crossover in intrinsic and n-type materials

Abstract: The compositional dependence of the lowest direct and indirect band gaps in Ge1−ySny alloys has been determined from room-temperature photoluminescence measurements. This technique is particularly attractive for a comparison of the two transitions because distinct features in the spectra can be associated with the direct and indirect gaps. However, detailed modeling of these room temperature spectra is required to extract the band gap values with the high accuracy required to determine the Sn concentration yc at which the alloy becomes a direct gap semiconductor. For the direct gap, this is accomplished using a microscopic model that allows the determination of direct gap energies with meV accuracy. For the indirect gap, it is shown that current theoretical models are inadequate to describe the emission properties of systems with close indirect and direct transitions. Accordingly, an ad hoc procedure is used to extract the indirect gap energies from the data. For y < 0.1 the resulting direct gap compositional dependence is given by ΔE0 = −(3.57 ± 0.06)y (in eV). For the indirect gap, the corresponding expression is ΔEind = −(1.64 ± 0.10)y (in eV). If a quadratic function of composition is used to express the two transition energies over the entire compositional range 0 y 1, the quadratic (bowing) coefficients are found to be b0 = 2.46 ± 0.06 eV (for E0) and bind = 1.03 ± 0.11 eV (for Eind). These results imply a crossover concentration yc = , much lower than early theoretical predictions based on the virtual crystal approximation, but in better agreement with predictions based on large atomic supercells.

High performance, transparent a-IGZO TFTs on a flexible thin glass substrate

Abstract: We investigated electrical properties of transparent amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with amorphous indium zinc oxide (a-IZO) transparent electrodes on a flexble thin glass substrate. The TFTs show a high field-effect mobility, a good subthreshold slope and a high on/off ratio owing to the high temperature thermal annealing process which cannot be applied to typical transparent polymer-based flexible substrates. Bias stress instability tests applying tensile stress concurrently with the bending radius of up to 40 mm indicated that mechanically and electrically stable a-IGZO TFTs can be fabricated on the transparent thin glass substrate.

High-output-power 255/280/310 nm deep ultraviolet light-emitting diodes and their lifetime characteristics

Abstract: 255/280/310 nm deep ultraviolet light-emitting diodes (DUV LEDs) suitable for high-current operation are reported. Newly developed 1 mm sized chips are installed in a commercial package with a two-series configuration. At a forward current of 350 mA, we measured powers of 45.2, 93.3, and 65.8 mW for the 255, 280, and 310 nm LEDs, respectively. The corresponding external quantum efficiencies per serial circuit were 1.3, 3.0, and 2.4%, and successful chip scalability was demonstrated. The 50% lifetime of the 280 nm LED die was estimated to be 3000 h at a junction temperature of 30 °C.

Bulk AlN growth by physical vapour transport

Abstract: The process technologies of AlN growth by physical vapour transport are reviewed in this paper with a focus on the growth parameters, crucible materials, and the type of seeding/nucleation. In this context the three growth strategies for the first generation of AlN seeds, (i) grain selection, (ii) heteroepitaxially seeding on SiC, and (iii) spontaneous nucleation, are evaluated regarding their impact on the structural properties and the sizes of the grown AlN crystals. Major issues for subsequent homoepitaxial growth runs with controlled diameter enlargement, such as thermal field design and seed fixation, are addressed. Furthermore, the influences of the growth conditions on the main optical absorption bands in AlN are discussed.

Comparative theoretical and experimental studies of two designs of high-power diode lasers

Abstract: Design and technology developments targeted at increasing both power conversion efficiency and optical output power of GaAs-based diode lasers are under intense study worldwide, driven by the demands of commercial laser systems The conversion efficiency at the operation point is known to be limited by electrical and optical losses in the p-side waveguide In this paper an ‘extreme, double asymmetric’ design to mitigate the impact of the p-side waveguide is studied and compared with a more conventional design An increase of the efficiency at the highest power is demonstrated, but it is less than expected from simulations

Metal?nanocarbon contacts

Abstract: To realize nanocarbons in general and carbon nanotube (CNT) in particular as on-chip interconnect materials, the contact resistance stemming from the metal?CNT interface must be well understood and minimized. Understanding the complex mechanisms at the interface can lead to effective contact resistance reduction. In this study, we compile existing published results and understanding for two metal?CNT contact geometries, sidewall or side contact and end contact, and address key performance characteristics which lead to low contact resistance. Side contacts typically result in contact resistances >1 k?, whereas end contacts, such as that for as-grown vertically aligned CNTs on a metal underlayer, can be substantially lower. The lower contact resistance for the latter is due largely to strong bonding between edge carbon atoms with atoms on the metal surface, while carrier transport across a side-contacted interface via tunneling is generally associated with high contact resistance. Analyses of high-resolution images of interface nanostructures for various metal?CNT structures, along with their measured electrical characteristics, provide the necessary knowledge for continuous improvements of techniques to reduce contact resistance. Such contact engineering approach is described for both side and end-contacted structures.

Characterization of doped PEDOT: PSS and its influence on the performance and degradation of organic solar cells

Abstract: The present work is a detailed study of the optical, morphological and electrical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, films doped with ethylene glycol (EG) and multi-walled carbon nanotubes (MWCNT). The conductivity of PEDOT:PSS films doped with EG and MWCNT is higher than pristine PEDOT:PSS film. The optical transparency of PEDOT:PSS film decreases insignificantly after addition of MWCNT and EG. The films were further studied using atomic force microscopy, ?x-ray diffraction, Raman spectroscopy and Kelvin probe work function measurement, after which films of PEDOT:PSS with EG and MWCNT were optimized for the fabrication of solar cells. The optimized film was used as a hole extracting layer in a typical ITO/PEDOT:PSS/P3HT:PCBM/Al solar cell. The suitable concentration for an optimized film was found to be 4% MWCNT and 1:4 ratio of EG to PEDOT:PSS. The performance of the device with doped PEDOT:PSS was found to improve in terms of short circuit current density (JSC) and efficiency (?). The solar cell with a doped PEDOT:PSS layer showed higher JSC?and ? due to the increase in the interchains among PEDOT chains along with the introduction of MWCNT channels in PEDOT:PSS matrix. The degradation behavior of the cells was studied and it was found that both pristine and doped PEDOT:PSS cells showed similar trends of degradation. The performance degradation with time was also studied under variable environmental conditions, which showed different aging rates for the two devices.

Electrical and microstructural properties of thermally annealed Ni/Au and Ni/Pt/Au Schottky contacts on AlGaN/GaN heterostructures

Abstract: High work-function metals such as Ni, Pt, and Au in the form of multilayer structures, Ni/Au and Ni/Pt/Au, have been investigated as Schottky metallizations on AlGaN/GaN heterostructures under thermal annealing. As-deposited Ni/Pt/Au had slightly higher Schottky barrier height than its Ni/Au counterpart. Schottky barrier heights for Ni/Au diodes on AlGaN/GaN increased by about 20% from 1.02 eV for as-deposited to 1.21 eV after annealing at 500 ?C for 2 min. Similar trends were observed for Ni/Pt/Au Schottky diodes. Thermal stability study for these devices showed that the interposition of Pt in Ni/Au systems improved the characteristics of the Schottky diodes after short-term anneal but cause significant degradation after long-term anneal at 500 ?C. Ni/Au Schottky contacts exhibited excellent leakage response under thermal annealing for long periods. Microstructural studies were carried out on Ni/Pt/Au and Ni/Au Schottky contacts to elucidate the role of the Pt interlayer in the degradation of the Ni/Pt/Au metallization under long-term thermal anneal.

The effect of the inversion channel at the AlN/Si interface on the vertical breakdown characteristics of GaN-based devices

Abstract: GaN-on-Si transistors attract increasing interest for power applications. However, the breakdown behavior of such devices remains below theoretical expectations, for which the Si substrate is typically made responsible. In this work, the effect of the thickness of an aluminum nitride buffer layer on the vertical breakdown voltage, measured relative to a grounded silicon substrate, has been investigated. A voltage-polarity-dependent breakdown mechanism has been observed. It has been found that the breakdown in the positive bias voltage regime is initiated by carrier injection, for which the carriers originate from an inversion channel formed between the epitaxial layers and the p-silicon substrate. TCAD simulations have confirmed the proposed explanations, and suggest that appropriate modification of the electronic structure at the AlN/silicon interface could significantly improve the vertical breakdown voltage.

Ferecrystals: non-epitaxial layered intergrowths

Abstract: The synthesis, structure, and selected physical properties of a new class of layered intergrowth materials, referred to as ferecrystals, are reviewed. In these layered intergrowths, size and composition can be controlled at atomic length scales by utilizing structural interfaces between component compounds that lack an epitaxial relationship. Opportunities to observe and control size-dependent phenomena in intergrowths of technologically important compound semiconductors are discussed.

Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon–germanium thermoelectrics

Abstract: Nanocrystalline silicon and silicon-germanium alloys are promising thermoelectric materials that have achieved substantially improved figure of merits compared to their bulk counterparts. This enhancement is typically attributed to a reduction in lattice thermal conductivity by phonon scattering at grain boundaries. However, further improvements are difficult to achieve because grain boundary scattering is poorly understood, with recent experimental observations suggesting that the phonon transmissivity may depend on phonon frequency rather than being constant as in the commonly used gray model. Here, we examine the impact of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium alloys in a realistic 3D geometry using frequency-dependent variance-reduced Monte Carlo simulations. We find that the grain boundary may not be as effective as predicted by the gray model in scattering certain phonons, with a substantial amount of heat being carried by low frequency phonons with mean free paths longer than the grain size. Our result will help guide the design of more efficient thermoelectrics.

Influence of transparent conductive oxides on passivation of a-Si:H/c-Si heterojunctions as studied by atomic layer deposited Al-doped ZnO

Abstract: In silicon heterojunction solar cells, the main opportunities for efficiency gain lie in improvements of the front-contact layers. Therefore, the effect of transparent conductive oxides (TCOs) on the a-Si:H passivation performance has been investigated for Al-doped zinc oxide (ZnO:Al) layers made by atomic layer deposition (ALD). It is shown that the ALD process, as opposed to sputtering, does not impair the chemical passivation. However, the field-effect passivation is reduced by the ZnO:Al. The resulting decrease in low injection-level lifetime can be tuned by changing the ZnO:Al doping level (carrier density = 7 × 1019–7 × 1020 cm−3), which is explained by a change in the TCO workfunction. Additionally, it is shown that a ~10–15 nm ALD ZnO:Al layer is sufficient to mitigate damage to the a-Si:H by subsequent sputtering, which is correlated to ALD film closure at this thickness.

AlGaN/GaN HEMT structures on ammono bulk GaN substrate

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

MIEC (mixed-ionic-electronic-conduction)-based access devices for non-volatile crossbar memory arrays

Abstract: Several attractive applications call for the organization of memristive devices (or other resistive non-volatile memory (NVM)) into large, densely-packed crossbar arrays. While resistive-NVM devices frequently possess some degree of inherent nonlinearity (typically 3?30? contrast), the operation of large ( 1000?1000 device) arrays at low power tends to require quite large ( 1e7) ON-to-OFF ratios (between the currents passed at high and at low voltages). One path to such large nonlinearities is the inclusion of a distinct access device (AD) together with each of the state-bearing resistive-NVM elements. While such an AD need not store data, its list of requirements is almost as challenging as the specifications demanded of the memory device. Several candidate ADs have been proposed, but obtaining high performance without requiring single-crystal silicon and/or the high processing temperatures of the front-end-of-the-line?which would eliminate any opportunity for 3D stacking?has been difficult.We review our work at IBM Research?Almaden on high-performance ADs based on Cu-containing mixed-ionic-electronic conduction (MIEC) materials [1?7]. These devices require only the low processing temperatures of the back-end-of-the-line, making them highly suitable for implementing multi-layer cross-bar arrays. MIEC-based ADs offer large ON/OFF ratios (), a significant voltage margin (over which current nA), and ultra-low leakage ( 10 pA), while also offering the high current densities needed for phase-change memory and the fully bipolar operation needed for high-performance RRAM. Scalability to critical lateral dimensions 30 nm and thicknesses 15 nm, tight distributions and 100% yield in large (512 kBit) arrays, long-term stability of the ultra-low leakage states, and sub-50 ns turn-ON times have all been demonstrated. Numerical modeling of these MIEC-based ADs shows that their operation depends on mediated hole conduction. Circuit simulations reveal that while scaled MIEC devices are suitable for large crossbar arrays of resistive-NVM devices with low ( 1.2 V) switching voltages, stacking two MIEC devices can support large crossbar arrays for switching voltages up to 2.5 V.

Hard x-ray emission spectroscopy: a powerful tool for the characterization of magnetic semiconductors

Abstract: This review aims to introduce the x-ray emission spectroscopy (XES) and resonant inelastic x-ray scattering (RIXS) techniques to the materials scientist working with magnetic semiconductors (e.g. semiconductors doped with 3d transition metals) for applications in the field of spin-electronics. We focus our attention on the hard part of the x-ray spectrum (above 3 keV) in order to demonstrate a powerful element- and orbital-selective characterization tool in the study of bulk electronic structure. XES and RIXS are photon-in/photon-out second order optical processes described by the Kramers–Heisenberg formula. Nowadays, the availability of third generation synchrotron radiation sources permits applying such techniques also to dilute materials, opening the way for a detailed atomic characterization of impurity-driven materials. We present the Kβ XES as a tool to study the occupied valence states (directly, via valence-to-core transitions) and to probe the local spin angular momentum (indirectly, via intra-atomic exchange interaction). The spin sensitivity is employed, in turn, to study the spin-polarized unoccupied states. Finally, the combination of RIXS with magnetic circular dichroism extends the possibilities of standard magnetic characterization tools.

Plasma-assisted molecular beam epitaxy of Al(Ga)N layers and quantum well structures for optically pumped mid-UV lasers on c-Al2O3

Abstract: This paper reports on novel approaches developed for plasma-assisted molecular beam epitaxy of Al-rich AlGaN epilayers and quantum well heterostructures on c-sapphire, which allowed us to fabricate low-threshold optically-pumped separate confinement heterostructure lasers emitting in the mid-UV spectral range (258–290 nm) with the threshold power density below 600 kW cm−2. The optimum buffer structure has been developed which provides lowering the near-surface threading dislocation density down to 1.5 × 108 and 3 × 109 cm−2 for screw and edge types, respectively, and improving the surface morphology (rms < 0.7 nm at the area of 3 × 3 μm−2). It comprises the high-temperature (780 °C) migration enhanced epitaxy growth of a (30–70) nm thick AlN nucleation layer on c-Al2O3, followed by a 2 μm thick AlN buffer grown under the metal-rich conditions in the Al-flux modulation mode and containing several (up to 6) ultra-thin (~3 nm) GaN interlayers grown at N-rich conditions. Proper strain engineering in AlGaN single quantum well heterostructure grown atop of the AlN buffer layer enables one to preserve dominant TE polarization of both spontaneous and stimulated emission even at shortest obtained wavelength (258 nm). The threshold power density of stimulated emission as low as 150 kW cm−2 at 289 nm for a single quantum well laser structure has been demonstrated.

Layered oxychalcogenide in the Bi–Cu–O–Se system as good thermoelectric materials

Abstract: Since 2010, we have evidenced the very promising thermoelectric properties of layered oxychalcogenides, with parent compound BiCuSeO, which could be used in thermoelectric conversion systems in the 300?600??C temperature range. These materials, that were first studied in the early 2000s for their optoelectronic properties, exhibit thermoelectric figure of merit values around 1.4 at 650??C, which makes them the best lead- or tellurium-free p-type thermoelectric materials ever reported to date. In this paper, we will review the chemical, structural and physical properties of this family of materials, with an emphasis on the links between crystal structure, electronic structure and functional properties.

Thermal stability of the current transport mechanisms in Ni-based Ohmic contacts on n- and p-implanted 4H-SiC

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

Structural analysis, electronic and optical properties of the synthesized Sb2S3 nanowires with small band gap

Abstract: We report a simple colloidal synthesis of two types of Sb2S3 nanowires with small band gap and high aspect ratio. Field-emission scanning electron and transmission electron microscopies confirmed formation of high aspect ratio Sb2S3 nanowires, separated in the form of bundles and coalesced with each other in long bars. Diffuse reflectance and absorption spectroscopies revealed that the optical band-gap energies of the synthesized nanowires separated in the form of bundles are 1.56 and 1.59 eV, and coalesced with each other in long bars are 1.36 and 1.28 eV, respectively. The structure refinement showed that Sb2S3 powders belong to the orthorhombic structure with space group Pnma (no. 62). It was found that Sb2S3 nanowires separated in the form of bundles predominantly grow along the [0 1 0] direction being in the needle-like shape. The nanowires coalesced with each other in long bars rise in the form of long bars, are ribbon-like in shape and have expressed {1 0 1} facets which grow along the [0 1 0] direction. No peaks in photoluminescence spectra were observed in the spectral range from 250 to 600 nm. In order to shed more light on the experimental results concerning the band-gap energies and, in the literature generally poorly investigated electronic properties of the synthesized material, we performed theoretical calculations of the electronic structure and optical properties of the Sb2S3 samples synthesized here. This was done on the basis of density functional theory with the generalized gradient approximation, and also with an improved version of the exchange potential suggested recently by Tran and Blaha. The main characteristic is the significant improvement of the band gap value.