High Electronic Quality Research Articles

Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene.

Atomically thin MoS2/graphene heterostructures are promising candidates for nanoelectronic and optoelectronic technologies. Among different graphene substrates, epitaxial graphene (EG) on SiC provides several potential advantages for such heterostructures, including high electronic quality, tunable substrate coupling, wafer-scale processability, and crystalline ordering that can template commensurate growth. Exploiting these attributes, we demonstrate here the thickness-controlled van der Waals epitaxial growth of MoS2 on EG via chemical vapor deposition, giving rise to transfer-free synthesis of a two-dimensional heterostructure with registry between its constituent materials. The rotational commensurability observed between the MoS2 and EG is driven by the energetically favorable alignment of their respective lattices and results in nearly strain-free MoS2, as evidenced by synchrotron X-ray scattering and atomic-resolution scanning tunneling microscopy (STM). The electronic nature of the MoS2/EG heterostructure is elucidated with STM and scanning tunneling spectroscopy, which reveals bias-dependent apparent thickness, band bending, and a reduced band gap of ∼0.4 eV at the monolayer MoS2 edges.

Origin of High Electronic Quality in Structurally Disordered CH3NH3PbI3 and the Passivation Effect of Cl and O at Grain Boundaries

The organic–inorganic hybrid perovskite CH3NH3PbI3 exhibits more structural disorder but more electronic order compared to inorganic CdTe. The grain boundaries of CH3NH3PbI3 could induce defect levels around the VBM and thus increase hole effective mass. Cl and O are found to be able to spontaneously segregate into the GBs, passivate the defect state, and improve carrier transportation.

Platinum-Enhanced Electron Transfer and Surface Passivation through Ultrathin Film Aluminum Oxide (Al₂O₃) on Si(111)-CH₃ Photoelectrodes.

We report the preparation, stability, and utility of Si(111)-CH3 photoelectrodes protected with thin films of aluminum oxide (Al2O3) prepared by atomic layer deposition (ALD). The photoelectrodes have been characterized by X-ray photoelectron spectroscopy (XPS), photoelectrochemistry (Fc in MeCN, Fc-OH in H2O), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) simulation. XPS analysis of the growing Al2O3 layer affords both the thickness, and information regarding two-dimensional versus three-dimensional mode of growth. Impedance measurements on Si(111)|CH3|Al2O3 devices reveal that the nascent films (5-30 Å) exhibit significant capacitance, which is attenuated upon surpassing the bulk threshold (∼30 Å). The Al2O3 layer provides enhanced photoelectrochemical (PEC) stability evidenced by an increase in the anodic window of operation in MeCN (up to +0.5 V vs Ag) and enhanced stability in aqueous electrolyte (up to +0.2 V vs Ag). XPS analysis before and after PEC confirms the Al2O3 layer is persistent and prevents surface corrosion (SiOx). Sweep-rate dependent CVs in MeCN at varying thicknesses exhibit a trend of increasingly broad features, characteristic of slow electron transport kinetics. Simulations were modeled as slow electron transfer through a partially resistive and electroactive Al2O3 layer. Lastly, we find that the Al2O3 ultrathin film serves as a support for the ALD deposition of Pt nanoparticles (d ≈ 8 nm) that enhance electron transfer through the Al2O3 layer. Surface recombination velocity (SRV) measurements on the assembled Si(111)|CH3|Al2O3-15 device affords an S value of 4170 cm s(-1) (τ = 4.2 μs) comparable to the bare Si(111)-CH3 surface (3950 cm s(-1); τ = 4.4 μs). Overall, the results indicate that high electronic quality and low surface defect densities can be retained throughout a multistep assembly of an integrated and passivated semiconductor|thin-film|metal device.

Electron-beam induced nano-etching of suspended graphene.

Besides its interesting physical properties, graphene as a two-dimensional lattice of carbon atoms promises to realize devices with exceptional electronic properties, where freely suspended graphene without contact to any substrate is the ultimate, truly two-dimensional system. The practical realization of nano-devices from suspended graphene, however, relies heavily on finding a structuring method which is minimally invasive. Here, we report on the first electron beam-induced nano-etching of suspended graphene and demonstrate high-resolution etching down to ~7 nm for line-cuts into the monolayer graphene. We investigate the structural quality of the etched graphene layer using two-dimensional (2D) Raman maps and demonstrate its high electronic quality in a nano-device: A 25 nm-wide suspended graphene nanoribbon (GNR) that shows a transport gap with a corresponding energy of ~60 meV. This is an important step towards fast and reliable patterning of suspended graphene for future ballistic transport, nano-electronic and nano-mechanical devices.

2D materials. Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage.

Graphene and related two-dimensional crystals and hybrid systems showcase several key properties that can address emerging energy needs, in particular for the ever growing market of portable and wearable energy conversion and storage devices. Graphene's flexibility, large surface area, and chemical stability, combined with its excellent electrical and thermal conductivity, make it promising as a catalyst in fuel and dye-sensitized solar cells. Chemically functionalized graphene can also improve storage and diffusion of ionic species and electric charge in batteries and supercapacitors. Two-dimensional crystals provide optoelectronic and photocatalytic properties complementing those of graphene, enabling the realization of ultrathin-film photovoltaic devices or systems for hydrogen production. Here, we review the use of graphene and related materials for energy conversion and storage, outlining the roadmap for future applications.

Steric spacing of molecular linkers on passivated Si(111) photoelectrodes.

Surfaces with high photoelectrochemical and electronic quality can be prepared by tethering small molecules to single-crystalline Si(111) surfaces using a two-step halogenation/alkylation method (by Lewis and co-workers).1-7 We report here that the surface coverage of custom-synthesized, phenyl-based molecular linkers can be controlled by varying the steric size of R-groups (R=CH3, C6H11, 2-ethylhexyl) at the periphery of the linker. Additionally, the linkers possess a para triflate group (-O2SCF3) that serves as a convenient analytical marker and as a point of covalent attachment for a redox active label. Quantitative X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface coverage systematically varies according to the steric size of the linker: CH3 (6.7±0.8%), CyHex (2.9±1.2%), EtHex (2.1±0.9%). The stability of the photoelectrochemical cyclic voltammetry (PEC-CV) behavior was dependent on an additional methylation step (with CH3MgCl) to passivate residual Si(111)-Cl bonds. Subsequently, the triflate functional group was utilized to perform Pd-catalyzed Heck coupling of vinylferrocene to the surface-attached linkers. Ferrocene surface coverages measured from cyclic voltammetry on the ferrocene-functionalized surfaces Si(111)-8a/CH3-Fc (R=CH3) and Si(111)-8c/CH3-Fc (R=2-EtHex) are consistent with the corresponding Fe 2p XPS coverages and suggest a ∼1:1 conversion of surface triflate groups to vinyl-Fc sites. The surface defect densities of the linker/CH3 modified surfaces are dependent on the coverage and composition of the organic layer. Surface recombination velocity (SRV) measurements indicated that n-Si(111)-8a/CH3 and the ferrocene coupled n-Si(111)-8a/CH3-Fc exhibited relatively high surface carrier lifetimes (4.51 and 3.88 μs, respectively) and correspondingly low S values (3880 and 4510 cm s(-1)). Thus, the multistep, linker/Fc functionalized surfaces exhibit analogously low trap state densities as compared to the fully passivated n-Si(111)-CH3 surface.

Fully porous GaN p-n junction diodes fabricated by chemical vapor deposition.

Porous GaN based LEDs produced by corrosion etching techniques demonstrated enhanced light extraction efficiency in the past. However, these fabrication techniques require further postgrown processing steps, which increases the price of the final system. Also, the penetration depth of these etching techniques is limited, and affects not only the semiconductor but also the other elements constituting the LED when applied to the final device. In this paper, we present the fabrication of fully porous GaN p-n junctions directly during growth, using a sequential chemical vapor deposition (CVD) process to produce the different layers that form the p-n junction. We characterized their diode behavior from room temperature to 673 K and demonstrated their ability as current rectifiers, thus proving the potential of these fully porous p-n junctions for diode and LEDs applications. The electrical and luminescence characterization confirm that high electronic quality porous structures can be obtained by this method, and we believe this investigation can be extended to other III-N materials for the development of white light LEDs, or to reduce reflection losses and narrowing the output light cone for improved LED external quantum efficiencies.

Transport Properties of Monolayer MoS2 Grown by Chemical Vapor Deposition

Recent success in the growth of monolayer MoS2 via chemical vapor deposition (CVD) has opened up prospects for the implementation of these materials into thin film electronic and optoelectronic devices. Here, we investigate the electronic transport properties of individual crystallites of high quality CVD-grown monolayer MoS2. The devices show low temperature mobilities up to 500 cm(2) V(-1) s(-1) and a clear signature of metallic conduction at high doping densities. These characteristics are comparable to the electronic properties of the best mechanically exfoliated monolayers in literature, verifying the high electronic quality of the CVD-grown materials. We analyze the different scattering mechanisms and show that the short-range scattering plays a dominant role in the highly conducting regime at low temperatures. Additionally, the influence of optical phonons as a limiting factor is discussed.

Elucidating the charge carrier separation and working mechanism of CH3NH3PbI3−xClx perovskite solar cells

Developments in organic-inorganic lead halide-based perovskite solar cells have been meteoric over the last 2 years, with small-area efficiencies surpassing 15%. We address the fundamental issue of how these cells work by applying a scanning electron microscopy-based technique to cell cross-sections. By mapping the variation in efficiency of charge separation and collection in the cross-sections, we show the presence of two prime high efficiency locations, one at/near the absorber/hole-blocking-layer, and the second at/near the absorber/electron-blocking-layer interfaces, with the former more pronounced. This 'twin-peaks' profile is characteristic of a p-i-n solar cell, with a layer of low-doped, high electronic quality semiconductor, between a p- and an n-layer. If the electron blocker is replaced by a gold contact, only a heterojunction at the absorber/hole-blocking interface remains.

Electrical Properties Study of Three Highly Purified CdTe Ingots

Abstract Three high resistivity CdTe ingots have been prepared with highly purified materials. The three starting Te components were purified through three different purification processes: horizontal zone refining, vacuum distillation and by a combination of both processes. The three crystals growths were achieved by the THM method. The purity studies were achieved by the determination of the concentration of 21 impurities, using the Atomic Absorption (AASGF) method. The resistivity ρ measurements along the length of the three CdTe ingots at room temperature showed a nearly constant ρ value within the range of 109 - 1010 Ω.cm. This represents a motivating result, in net advance with the general behavior of classic THM materials, where it began by high ρ at the starting point, and then it decreased by few decades. Values of drifting motilities μ were measured for all ingots and showed interesting high values: μe =1100 cm2/Vs for electrons and μh =90 cm2/Vs for holes. The position in the band gap and eventually the nature of the corresponding electrical defects levels, involved in the trapping process was studied by using the PICTS method. The uniformity of ρ, the high mobility μ and the low concentration values of impurities in all our CdTe ingots confirm the high electronic grade quality of these CdTe materials, which is better than 6N purity.

Epitaxial stabilization of ultra-thin films of EuNiO3

We report on the synthesis of ultra-thin films of highly distorted EuNiO3 (ENO) grown by interrupted pulse laser epitaxy on YAlO3 perovskite (YAP) substrates. Samples were then investigated with reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, reciprocal space mapping, and x-ray absorption spectroscopy. Combined, the measurements revealed the samples exhibited high structural and electronic quality that is of critical importance to the observed electronic and magnetic properties of the rare-earth nickelates. Growth of ultra-thin films of this highly distorted nickelate system in precisely controlled environments provides the ability to thoroughly investigate electronic phases with decoupled metal-to-insulator/charge-order and anti-ferromagnetic transitions.

Electrochemical hydrogen storage of the graphene sheets prepared by DC arc-discharge method

Graphene, as the fundamental two-dimensional carbon structure with exceptionally high crystal and electronic quality, had attracted increasing interest for its promising potential applications. In this article, the few-layer graphene sheets have been achieved by the direct current (DC) arc-discharge under hydrogen atmosphere, using pure graphite rods as the electrodes. The influence of hydrogen pressures (e.g. 0.03MPa, 0.05MPa, 0.07MPa) on the products and their electrochemical properties for hydrogen storage were investigated. It is indicated that the graphene sheets were less than 10 layers and with the sizes in range of 200–500nm. Among all graphene sheet samples, the highest capacity for electrochemical hydrogen storage was 147.8mAh/g. Mechanism for electrochemical hydrogen insertion/desertion of such graphene sheet was discussed with respect to the single-layer graphene and multi-layer graphite.

Mechanical cleaning of graphene

Contamination of graphene due to residues from nanofabrication often introduces background doping and reduces electron mobility. For samples of high electronic quality, post-lithography cleaning treatments are therefore needed. We report that mechanical cleaning based on contact mode atomic force microscopy removes residues and significantly improves the electronic properties. A mechanically cleaned dual-gated bilayer graphene transistor with hexagonal boron nitride dielectrics exhibited a mobility of ∼36 000 cm2/Vs at low temperature.

Optically-aligned visible/near-infrared dual-band photodetector materials and devices on GaAs using metamorphic epitaxy

A monolithic, epitaxially-integrated, vertically-aligned, multi-band photodetector architecture has been demonstrated via the successful growth and fabrication of metamorphic back-to-back n-i-p/p-i-n In0.61Ga0.39P/In0.14Ga0.86As visible/near-IR dual-detector devices. The back-to-back diode design enables simultaneous and independent operation of detectors in both bands with low optical cross talk (<−10 dB outside the 690–720 nm range) and complete electrical isolation between the sub-detectors. The high electronic quality of the resultant metamorphic materials was confirmed via deep level transient spectroscopy, which revealed total trap concentrations of 5 × 1012 cm−3 for the In0.14Ga0.86As and 2 × 1014 cm−3 for the In0.61Ga0.39P sub-detectors, enabling low, room temperature reverse bias (−2 V) dark current densities of 4 × 10−8 A cm−2 and 7 × 10−12 A cm−2, respectively. High responsivity and specific detectivity values, at a working bias of −2 V, were measured: 0.41 A/W and 8.6 × 1011 cm Hz1/2/W for the In0.14Ga0.86As sub-detectors (at 980 nm), and 0.30 A/W and 2.0 × 1014 cm Hz1/2/W for the In0.61Ga0.39P sub-detectors (at 680 nm). The successful integration of high-quality lattice-mismatched materials, combined with the excellent sub-detector performances, demonstrate the potential for extending such a multi-band photodetector technology to achieve simultaneous detection of a wide range of wavelength bands with tunable cut-off wavelengths.

Electron‐beam crystallized large grained silicon solar cell on glass substrate

ABSTRACTThin film hetero‐emitter solar cells with large‐grained poly‐silicon absorbers of around 10 µm thickness have been prepared on glass. The basis of the cell concept is electron‐beam‐crystallization of an amorphous or nanocrystalline silicon layer deposited onto a SiC:B layer. The SiC:B layer covers a commercially well available glass substrate, serving as diffusion barrier, contact layer and dopand source. For silicon absorber deposition a low pressure chemical vapour deposition was used. The successively applied e‐beam crystallization process creates poly‐silicon layers with grain sizes up to 1 × 10 mm2 with low defect densities. The high electronic quality of the absorber is reflected in open circuit voltages as high as 545 mV, which are realized making use of the well‐developed a‐Si:H hetero‐emitter technology. Copyright © 2011 John Wiley & Sons, Ltd.

Structural coherency of epitaxial graphene on 3C–SiC(111) epilayers on Si(111)

Graphene has emerged as a promising nanoelectronic material in electronic devices applications and studying two-dimensional electron gases with relativistic dispersion near Dirac point. Nonetheless, the control of the preparation conditions for homogeneous large-area graphene layers is difficult. Here, we illustrate evidence for high structural and electronic quality epitaxial graphene on 3C–SiC(111). Morphology and electronic structure of the graphene layers have been analyzed with low energy electron microscopy and angle resolved photoemission spectroscopy. Using scanning tunneling microscopy and scanning transmission electron microscopy, we show that graphene exhibits remarkably continuity of step edges suggesting the possibility of growing large scale graphene layer.

ChemInform Abstract: Graphene‐Based Materials in Electrochemistry

AbstractReview: 224 refs.

Graphene-based materials in electrochemistry

Graphene, as the fundamental 2D carbon structure with exceptionally high crystal and electronic quality, has emerged as a rapidly rising star in the field of material science. Its sudden discovery in 2004 led to an explosion of interest in the study of graphene with respect to its unique physical, chemical, and mechanical properties, opening up a new research area for materials science and condensed-matter physics, and aiming for wide-ranging and diversified technological applications. In this critical review, we will describe recent advances in the development of graphene-based materials from the standpoint of electrochemistry. To begin with, electron transfer properties of graphene will be discussed, involving its unusual electronic structure, extraordinary electronic properties and fascinating electron transport. The next major section deals with the exciting progress related to graphene-based materials in electrochemistry since 2004, including electrochemical sensing, electrochemiluminescence, electrocatalysis, electrochemical energy conversion and FET devices. Finally, prospects and further developments in this exciting field of graphene-based materials are also suggested (224 references).

Nanostructured semiconductor heterojunctions from quantum dot layers

We report the deposition of conformal thin layers of 10–50 nm thickness from II-VI quantum dot suspensions on ZnO nanowire substrates. Smooth polycrystalline films of high electronic quality can be obtained from CdSe quantum dots after annealing at moderate temperatures. The electronic properties are adequate for detector and solar cell applications. The growth and annealing temperatures permit deposition on light-weight and flexible substrates. Some elemental diffusion of Se across the CdSe/ZnO interface occurs in the film formation. A comparison with CdS/ZnO junctions indicates that the low Se diffusion rates are essential for efficient charge transfer.

Electronic Properties and pH Stability of Si(111)/Alkyl Monolayers

The electronic properties of hydrogenated and alkyl-grafted silicon surfaces have been investigated by photoluminescence, surface photovoltage and electrochemical capacitance measurements, which are sensitive methods for monitoring the presence of electronic defects. On p-type silicon, the electronic quality of the interface was found to depend critically on the surface preparation and was studied with special care. The thermal grafting of one monolayer of linear organic chains on atomically flat silicon allows the formation of stable alkyl-grafted p-Si surfaces, exhibiting a very low density of recombination centers comparable to those measured on hydrogenated silicon surfaces. Photoluminescence measurements show that the high electronic quality of the grafted surfaces is preserved on a month scale in air, indicating a much more efficient long term passivation by comparison with hydrogenated Si surfaces. The stability of the silicon/organic layer interface in aqueous buffer solution has been assessed by in situ photoluminescence and surface photovoltage measurements. Some degradation of the electronic properties was evidenced during exposure in basic solution of pH > 9.