Heterojunctions Of Materials Research Articles

Two-Dimensional Heterojunctions from Nonlocal Manipulations of the Interactions.

We propose to create lateral heterojunctions in two-dimensional materials based on nonlocal manipulations of the Coulomb interaction using structured dielectric environments. By means of ab initio calculations for MoS2 as well as generic semiconductor models, we show that the Coulomb interaction-induced self-energy corrections in real space are sufficiently nonlocal to be manipulated externally, but still local enough to induce spatially sharp interfaces within a single homogeneous monolayer to form heterojunctions. We find a type-II heterojunction band scheme promoted by a laterally structured dielectric environment, which exhibits a sharp band gap crossover within less than 5 unit cells.

Surface-directed Nanoepitaxy on a Surface with an Irregular Lattice.

Understanding and developing metrics on how nanocrystals respond to local external surface stimuli at their interfaces during growth or operation is a key step in advancing scalable and deterministic approaches for fabricating functional one- and two-dimensional (1D and 2D) nanoscale networks. Here, we present early results on a general approach for surface-directed nanocrystal epitaxy on a surface with an irregular lattice constant. We show that patches of lattice matched areas as small as 7 nm in a background of surface lattice disorder could satisfy the condition for epitaxial growth of a crawling nanocrystal over the disordered region. Threshold of failure in nanocrystal epitaxy is found to depend on the spacing between the patches and their total surface area. Results indicate nanoepitaxy on a disordered surface occurs if it contains patches of lattice matched regions with at least 20% of surface coverage, illustrating the remarkable tolerance of this type of growth to surface lattice disorder. By adjusting this threshold, it is possible to scalably restrict nanocrystal growth, filter out single nanowires and partition nanowire heterojunctions into segments with different orientations or modulate their electronic structures. This approach is expected to impact epitaxy of highly-mismatched semiconductors and lead to realization of ultrathin heterojunctions of 1D-2D materials.

Radial Nanowire Light-Emitting Diodes in the (AlxGa1-x)yIn1-yP Material System.

Nanowires have the potential to play an important role for next-generation light-emitting diodes. In this work, we present a growth scheme for radial nanowire quantum-well structures in the AlGaInP material system using a GaInP nanowire core as a template for radial growth with GaInP as the active layer for emission and AlGaInP as charge carrier barriers. The different layers were analyzed by X-ray diffraction to ensure lattice-matched radial structures. Furthermore, we evaluated the material composition and heterojunction interface sharpness by scanning transmission electron microscopy energy dispersive X-ray spectroscopy. The electro-optical properties were investigated by injection luminescence measurements. The presented results can be a valuable track toward radial nanowire light-emitting diodes in the AlGaInP material system in the red/orange/yellow color spectrum.

Photo-Carrier Injection and Magnetoresistance Behavior in the Strongly Correlated Electronic Material Heterojunction

Photo-Carrier Injection and Magnetoresistance Behavior in the Strongly Correlated Electronic Material Heterojunction

Rethinking Band Bending at the P3HT–TiO2 Interface

The advancement of solar cell technology necessitates a detailed understanding of material heterojunctions and their interfacial properties. In hybrid bulk heterojunction solar cells (HBHJs), light-absorbing conjugated polymers are often interfaced with films of nanostructured TiO2 as a cheaper alternative to conventional inorganic solar cells. The mechanism of photovoltaic action requires photoelectrons in the polymer to transfer into the TiO2, and therefore, polymers are designed with lowest unoccupied molecular orbital (LUMO) levels higher in energy than the conduction band of TiO2 for thermodynamically favorable electron transfer. Currently, the energy level values used to guide solar cell design are referenced from the separated materials, neglecting the fact that upon heterojunction formation material energetics are altered. With spectroelectrochemistry, we discovered that spontaneous charge transfer occurs upon heterojunction formation between poly(3-hexylthiophene) (P3HT) and nanocrystalline TiO2. It was determined that deep trap states (0.5 eV below the conduction band of TiO2) accept electrons from P3HT and form hole polarons in the polymer. This equilibrium charge separation alters energetics through the formation of interfacial dipoles and results in band bending that inhibits desired photoelectron injection into TiO2, limiting HBHJ solar cell performance. X-ray photoelectron spectroscopic studies quantified the resultant vacuum level offset to be 0.8 eV. Further spectroelectrochemical studies indicate that 0.1 eV of this offset occurs in TiO2, whereas the balance occurs in P3HT. New guidelines for improved photocurrent are proposed by tuning the energetics of the heterojunction to reverse the direction of the interfacial dipole, enhancing photoelectron injection.

Embedded-Ge source and drain in InGaAs/GaAs dual channel MESFET

We report the first demonstration of n-type III–V metal-semiconductor field-effect transistors (nMESFETs) with IV group material hetero-junction source and drain (S/D) technology. A selective epitaxial growth of germanium (Ge) in the recessed gallium arsenide (GaAs) S/D regions is successfully developed using ultra-high vacuum chemical vapor deposition (UHVCVD) system. The dual channel structure includes an additional 10-nm higher mobility n-In0.2Ga0.8As layer on n-GaAs channel and is introduced to further improve the device performance. The n-MESFET, combining embedded-Ge S/D with In0.2Ga0.8As/GaAs channel, exhibits good transfer properties with a drain current on/off ratio of approximately 103. Due to the small barrier height of Ti/In0.2Ga0.8As Schottky contact, a lattice-matched wide bandgap In0.49Ga0.51P dielectric layer is also integrated into the device architecture to build a higher electron Schottky barrier height (SBH) for gate leakage current reduction. The Ti/In0.49Ga0.51P/n-In0.2Ga0.8As Schottky diode shows a comparable leakage level to Ti/n-GaAs with 2 × 10−2 A/cm2 at a gate voltage of −2.0 V.

Fermi-Level Effect on Group III Atom Interdiffusion in III-V Compounds: Bandgap Heterogeneity and Low Silicon-Doping

ABSTRACTHeavy n-doping enhanced disordering of GaAs based III-V semiconductor superlattice or quantum well layers, as well as the diffusion of Si in GaAs have been previously explained by the Fermi-level effect model with the triply-negatively-charged group III lattice vacancies identified to be the responsible point defect species. These vacancies have a thermal equilibrium concentration proportional to the cubic power of the electron concentration n, leading to the same dependence of the layer disordering rate. In this paper, in addition, we take into account also the electric field effect produced by the material bandgap heterogeneity and/or hetero-junctions. In heavily n-doped or long time annealing cases, this effect is negligible. At low n-doping levels and for short annealing times, the layer disordering rate can be enhanced or reduced by this effect. Available experimental results of low Si-doped and very short-time annealed samples have been satisfactorily fitted using the Fermi-level effect model.

Reliability issues for III-V heterojunction bipolar transistors

This paper presents an overview of known and potential issues limiting heterojunction bipolar transistor stability and reliability based on material properties, heterojunction interfaces, device fabrication, and electrical and thermal stresses. While there has been remarkable progress in recent years in demonstrating the superior capabilities of GaAs- and InP-based heterojunction bipolar transistors, there remain important, unanswered questions with regard to device stability and reliability due to the immature nature of the technology in these materials systems. Based on published reports, the principal modes of device degradation observed to date in these transistors, such as reduction in the current gain due to Be outdiffusion from the base, will be summarized and the current understanding of their origins discussed. In addition, potential sources of device instabilities will be discussed, such as degradation phenomena observed in other related devices, such as semiconductor lasers and FETs, and reports of instabilities in material properties. The aim of this paper is to provide an improved understanding of the current limits imposed by degradation phenomena to device design, fabrication and operation, and thereby enable the development of more reliable, high performance devices and circuits.

Misfit Dislocations at Mismatched Epitactic Heterojunctions

ABSTRACTThe formation and structures of misfit dislocations are significant factors in understanding heteroepitaxy of lattice-mismatched materials. In this study, GaAs/Si, CoGa/GaAs and ErAs/GaAs heterojunctions in materials grown by molecular-beam epitaxy have been characterized using transmission electron microscopy. Different types of misfit dislocations have been generated at these interfaces. The different dislocation configurations are discussed, along with interactions between 60° and 90° dislocations in GaAs/Si heterojunctions; the 60° dislocations might be associated with surface steps or edges of islands. The growth of antiphase boundary structures in the CoGa and ErAs grown on GaAs are proposed.

Germanium-Gallium Arsenide Heterojunctions [Letter to the Editor

Abrupt monocrystalline junctions between two different semiconductor materials (heterojunctions) 1 have been made by depositing germanium epitaxially on gallium arsenide substrates. The purpose of this communication is to summarize some tentative results obtained in a study of the electrical characteristics of these junctions. A more detailed paper is under preparation.