Small Lattice Mismatch Research Articles

Optimization of ruthenium as a buffer layer for non-collinear antiferromagnetic Mn3X films

Two thin film deposition routes were studied for the growth of high quality single crystalline Ru (0001) epitaxial films on c-Al2O3 substrates using radio frequency-magnetron sputtering. Such films are very important as buffer layers for the deposition of epitaxial non-collinear antiferromagnetic Mn3X films. The first route involved depositing Ru at 700 °C, leading to a smooth 30 nm thick film. Although, high resolution x-ray diffraction revealed twinned Ru film orientations, in situ post-annealing eliminated one orientation, leaving the film orientation aligned with the substrate, with no in-plane lattice rotation and a large lattice mismatch (13.6%). The second route involved the deposition of Ru at room temperature followed by in situ post-annealing at 700 °C. Transmission electron microscopy confirmed a very high quality of these films, free of crystal twinning, and a 30° in-plane lattice rotation relative to the substrate, resulting in a small in-plane lattice mismatch of –1.6%. X-ray reflectivity demonstrated smooth surfaces for films down to 7 nm thickness. 30 nm thick high quality single-crystalline Mn3Ga and Mn3Sn films were grown on top of the Ru buffer deposited using the second route as a first step to realize Mn3X films for antiferromagnetic spintronics applications.

Epitaxial Growth of Nanorod Meshes from Luminescent Organic Cocrystals via Crystal Transformation.

Two-dimensional (2D) nanorod meshes made of benzoperylene-1,3-dicyanotetrafluorobenzene (BP-1,3-DTFB) were formed via crystal transformation of the pre-existing BP microsheets. The transformation was driven by a cooperative effect of intermolecular charge-transfer and arene-perfluoroarene interactions. Epitaxial growth of cyan-emitting BP-1,3-DTFB nanorod meshes was directed by small lattice mismatch between BP and BP-1,3-DTFB, followed by the consumption of BP and the formation of BP-1,3-DTFB. Such a crystal transformation strategy can also be used to guide the formation of BP-1,4-dicyanotetrafluorobenzene (BP-1,4-DTFB) nanorod meshes. The present work reports a simple yet effective approach for the realization of aligned organic nanorod superstructures.

Growth of Single-Layer and Multilayer Graphene on Cu/Ni Alloy Substrates

ConspectusGraphene, a one-atom-thick layer of carbon with a honeycomb lattice, has drawn great attention due to its outstanding properties and its various applications in electronic and photonic devices. Mechanical exfoliation has been used for preparing graphene flakes (from monolayer to multilayer with thick pieces also typically present), but with sizes limited typically to less than millimeters, its usefulness is limited. Chemical vapor deposition (CVD) has been shown to be the most effective technique for the scalable preparation of graphene films with high quality and uniformity. To date, CVD growth of graphene on the most commonly used substrates (Cu and Ni foils) has been demonstrated and intensively studied. However, a survey of the existing literature and earlier work using Cu or Ni substrates for CVD growth indicates that the bilayer and multilayer graphene over a large area, particularly single crystals, have not been obtained.In this Account, we review current progress and development in the CVD growth of graphene and highlight the important challenges that need to be addressed, for example, how to achieve large single crystal graphene films with a controlled number of layers. A single-layer graphene film grown on polycrystalline Cu foil was first reported by our group, and since then various techniques have been devoted to achieving the fast growth of large-area graphene films with high quality. Commercially available Cu/Ni foils, sputtered Cu/Ni thin films, and polycrystalline Cu/Ni foils have been used for the CVD synthesis of bilayer, trilayer, and multilayer graphene. Cu/Ni alloy substrates are particularly interesting due to their greater carbon solubility than pure Cu substrates and this solubility can be finely controlled by changing the alloy composition. These substrates with controlled compositions have shown the potential for the growth of layer-tunable graphene films in addition to providing a much higher growth rate due to their stronger catalytic activity. However, the well-controlled preparation of single crystal graphene with a defined number of layers on Cu/Ni substrates is still challenging.Due to its small lattice mismatch with graphene, a single crystal Cu(111) foil has been shown to be an ideal substrate for the epitaxial growth of graphene. Our group has reported the synthesis of large-size single crystal Cu(111) foils by the contact-free annealing of commercial Cu foils, and single crystal Cu/Ni(111) alloy foils have also been obtained after the heat-treatment of Ni-coated Cu(111) foils. The use of these single crystal foils (especially the Cu/Ni alloy foils) as growth substrates has enabled the fast growth of single crystal single-layer graphene films. By increase of the Ni content, single crystal bilayer, trilayer, and even multilayer graphene films have been synthesized. In addition, we also discuss the wafer-scale growth of single-layer graphene on the single crystalline Cu/Ni(111) thin films.Recent research results on the large-scale preparation of single crystal graphene films with different numbers of layers on various types of Cu/Ni alloy substrates with different compositions are reviewed and discussed in detail. Despite the remarkable progress in this field, further challenges, such as the wafer-scale synthesis of single crystal graphene with a controlled number of layers and a deeper understanding of the growth mechanism of bilayer and multilayer graphene growth on Cu/Ni substrates, still need to be addressed.

Epitaxial Er-doped Y2O3 on silicon for quantum coherent devices

Rare-earth ions (REIs) have incomplete 4f shells and possess narrow optical intra-4f transitions due to shielding from electrons in the 5s and 5p orbitals, making them good candidates for solid-state optical quantum memory. The emission of Er3+ in the telecom C-band (1530 nm–1565 nm) makes it especially attractive for this application. In order to build practical, scalable devices, the REI needs to be embedded in a non-interacting host material, preferably one that can be integrated with silicon. In this paper, we show that Er3+ can be isovalently incorporated into epitaxial Y2O3 thin films on Si (111). We report on the synthesis of epitaxial, single-crystalline Er:Y2O3 on Si with a narrow inhomogeneous linewidth in the photoluminescence (PL) spectra, 5.1 GHz (<100 mK), and an optical excited state lifetime of 8.1 ms. The choice of Y2O3 was driven by its low nuclear spin and small lattice mismatch with Si. Using PL and electron paramagnetic resonance, we show that Er3+ substitutes for Y in the crystal lattice. The role of interfacial SiOx, diffusion of silicon into the film, and the effect of buffer layers on the inhomogeneous PL linewidth are examined. We also find that the linewidth decreased monotonically with film thickness but surprisingly exhibits no correlation with the film crystalline quality, as measured by the x-ray rocking curve scans, suggesting other factors at play that limit the inhomogeneous broadening in Y2O3 films.

Structural properties of ZnSe/InSe/ZnSe heterostructures grown by molecular beam epitaxy on GaAs(001) substrates

The paper reports on molecular beam epitaxy of ZnSe/InSe/ZnSe quantum well (QW) heterostructures grown on GaAs(001) substrates as well as studies of their structural properties. The structures were characterized by reflection high energy electron diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy techniques. The evolution of surface morphology of QW heterostructures as a function of the InSe thickness has been studied. The quasi van der Waals growth of ZnSe on the InSe(0001) surface has been demonstrated, with the ZnSe(111) plane being oriented parallel to the (0001) plane of InSe because of a small lattice mismatch between InSe and ZnSe(111).

Comparative study of BeMgZnO/ZnO heterostructures on c-sapphire and GaN by molecular beam epitaxy

Growth of ZnO directly on c-plane sapphire, with a large in-plane lattice mismatch of 18%, is relatively well understood for O-polar variety. However, the two-dimensional (2D) growth of Zn-polar ZnO on c-sapphire, needed for 2D electron gas formation, with low background electron density is in its infancy. While Zn-polar ZnO can be grown on GaN with the resulting small lattice mismatch (1.8%), the parallel conduction through GaN (bulk and/or surface) in fabricated BeMgZnO/ZnO heterostructure field effect transistors (HFETs) limits its utility for the time being. In this contribution, the authors report on the growth of high-quality Zn-polar BeMgZnO/ZnO HFET structures directly on sapphire substrates, in an effort to avoid the aforementioned parallel conduction, by employing an MgO and low-temperature ZnO buffer stack. The residual 2D equivalent concentration in the HFET structure on GaN is ∼2 × 1012 cm−2, which is detrimental, while highly resistive (>100 Ω.cm) Zn-polar ZnO layers with smooth surfaces on c-sapphire have been obtained via controlling the buffer stack growth conditions, which is vital to the realization of HFET device structures. Compared with the highest room temperature electron mobility of ∼250 cm2/V s in BeMgZnO/ZnO HFETs on GaN templates, a slightly lower electron mobility of ∼220 cm2/V s was achieved on c-sapphire due to a somewhat lower overall crystalline quality.

Vertical α/β-Ga2O3 phase junction nanorods array with graphene-silver nanowire hybrid conductive electrode for high-performance self-powered solar-blind photodetectors

Gallium oxide semiconductor, with a bandgap energy of 4.9 eV, is regarded as a hopeful candidate for next-generation solar-blind photodetectors. However, the construction of photodetector based on Ga2O3 pn junction is still challengeable due to immature doping technology of p-Ga2O3. By considering the small lattice mismatch (＜3%) and similar band gap structure of α-Ga2O3 and β-Ga2O3, a well-designed solar-blind photodetector was built, based on vertical α/β-Ga2O3 junction nanorod arrays (NRAs) with a graphene-silver nanowires (Ag NWs) hybrid top electrode. Thanks to the high conductivity and optical transmittance of graphene-Ag NWs top electrode, the photodetector based on α/β-Ga2O3 junction NARs displayed excellent photoelectric performance. Meanwhile, the matched band structure of Π type formed at the interface of α/β-Ga2O3 junction promoted the automatic separation of photogenerated carriers and their transfer to the corresponding electrodes. A fast photoresponse time of 0.54 s, a high light-to-dark ratio about 2000, and a high rejection ratio (R254/R365) of 2.7 × 103 under the zero bias, were realized. The excellent photoelectrical performance of α/β-Ga2O3 junction photodetector even in vacuum environment forecasts its potential application in some low air density fields, such as missile early warning and tracking and ozone hole monitoring.

Self-Powered Solar-Blind Photodetectors Based onα/βPhase Junction ofGa2O3

Self-powered ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$-based solar-blind photodetectors have received attention recently due to the increased demand for energy saving, miniaturization, and high efficiency in devices. An ideal device structure consisting of a ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$-based p-n junction is still difficult to obtain, since p-type doping is a major challenge. Although self-powered devices based on heterojunction are promising, there are two fatal disadvantages: (1) photosensitivity of the non-solar-blind region, on account of the narrower band gap of the heterojunction materials; and (2) poor quality of the epitaxial film due to lattice mismatch. In view of the various polymorphs of ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, we propose constructing a structure consisting of a ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ phase junction with \ensuremath{\alpha} and \ensuremath{\beta} phases (\ensuremath{\alpha}/\ensuremath{\beta} phase junction) for self-powered solar-blind photodetectors. The small lattice mismatch and similar band gap between \ensuremath{\alpha}- and \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ will solve the two problems outlined above. The formation of \ensuremath{\alpha}- and \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is expected to result in a type-II band alignment, promoting separation of photogenerated carriers, which transfer through the junction to the corresponding electrodes. Herein, the \ensuremath{\alpha}/\ensuremath{\beta} phase junction of ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ vertically aligned nanorod arrays with a thickness-controllable \ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ shell layer are fabricated by a low-cost and simple process of hydrothermal and postannealing treatment. Two different types of self-powered \ensuremath{\alpha}/\ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ phase junction-based photodetectors, in the form of solid-state type and photoelectrochemical type, are constructed and realized. Our analysis shows that the constructed photodetectors are capable of highly efficient detection of solar-blind signal without any bias voltage. This work demonstrates the usefulness of using the \ensuremath{\alpha}/\ensuremath{\beta}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ phase junction in a self-powered solar-blind photodetector, which is not only energy efficient, but also potentially workable in outer space, at the south and north pole, and other harsh environments without external power for a long time.

Synthesis of lustering two-dimensional α-MoO3 van der Waals crystals by TiO2 assisted selective facet passivation

Large-sized α-MoO3 nanoplates with 10−100 μm were synthesized by introduction of titanium tetra isopropoxide as TiO2 precursor in hydrothermal solution chemistry. During the hydrothermal synthesis, the TiO2 can be selectively deposited on the (001) facets of h-MoO3 hexagonal rods and on the (001) and/or (010) facet of α-MoO3 re-growing crystals due to the small lattice mismatch between them. Owing to the TiO2 passivation of the surface of h-MoO3 rods, the h-MoO3 rods are changed to MoO3 tubes because the h-MoO3 are dissolved from the inner part of the TiO2 passivated h-MoO3 rods. The TiO2-passivated (001) facets of h-MoO3 rods act as templates for the re-growth of α-MoO3 and consequently produced rectangle-shaped α-MoO3 nanoplates. Simultaneously, the TiO2 passivated on the (001) and/or (010) facet of the re-growing α-MoO3 crystals inhibits the preferred crystal growth to [001] direction and promoted the crystal growth to [100] direction, thereby yielding plate-shaped α-MoO3. The small α-MoO3 nanoplates and rectangle-shaped ones are further grown to larger nanoplates by attachment process and/or continuous growth.

Nanointerface Chemistry: Lattice-Mismatch-Directed Synthesis and Application of Hybrid Nanocrystals.

In the past decades, great strides have been made in the synthesis of hybrid nanocrystals (HNCs) consisting of two or more disparate subunits (such as metals and/or semiconductors) joined through nanointerfaces, which are intriguing due to their exceptional functionalities that cannot be achieved by single-component nanosystems. The promising and versatile applications of these HNCs are closely dependent on the structural and electronic properties of the nanointerface between subunits. This is because the compatibility of the lattice structures between subunits not only determines the synthetic accessibility and growth mechanisms of the HNCs on the thermodynamic basis but also influences their interfacial characteristics (atomic arrangement, lattice mismatch-induced strain or defects), configurations, crystallinity, and the synergistic interplay of different subunits at the nanoscale. As a result, nanointerface chemistry has attracted intense scientific endeavors worldwide and spurred the rapid development of the lattice-mismatch-directed precise synthesis. This review gives an overview of the main strategies developed for delicate design and fabrication of core-shell HNCs under different degrees of lattice mismatch (from 0.2% to larger than 50%), including epitaxial seeded growth, nanoscale cation exchange, cation exchange-facilitated nonepitaxial growth, etc. Moreover, as for the core-shell HNCs with small (<5%) or moderate lattice mismatch (∼5-20%), the significance of the lattice-strain control at the nanointerface in maneuvering their functions toward desired applications are discussed in detail. Regarding the core-shell HNCs with large lattice mismatch (>20%), the challenges in precise synthesis, the promising solutions enabled by cation exchange-facilitated nonepitaxial growth, and the enhanced applications of the resulting HNCs with strain-free nanointerface are elaborated. We conclude with a personal perspective on the significance and urgency of fully harnessing the effects of lattice mismatch to further advance the science of synthesis and application of HNCs.

Giant polarization charge density at lattice-matched GaN/ScN interfaces

Rock-salt ScN is a semiconductor with a small lattice mismatch to wurtzite GaN. Within the modern theory of polarization, ScN has a nonvanishing formal polarization along the [111] direction. As a result, we demonstrate that an interface between (0001) GaN and (111) ScN exhibits a large polarization discontinuity of −1.358 Cm−2. Interfaces between ScN and wurtzite III-nitrides will exhibit a high-density electron gas at the (0001¯) GaN interface or a hole gas at the (0001) GaN interface, with carrier concentrations up to 8.5×1014 cm−2. The large polarization difference and small strain make ScN a desirable choice for polarization-enhanced tunnel junctions within the III-nitride materials system. The large sheet carrier densities may also be useful for contacts or current spreading layers.

ZnSxSe1-x Alloy Passivation Layer for High-Efficiency Quantum-Dot-Sensitized Solar Cells.

Interface modification is an important means for improving the performance of almost all optoelectronic devices. In quantum-dot-sensitized solar cells (QDSCs), effective surface modification of photoanode also has a critical impact on photovoltaic performance. At present, ZnS and ZnSe wide band gap semiconductors are the mainstream materials used for photoanode/electrolyte interface passivation in QDSCs. However, the problem with these two materials is that the passivation effect and the lattice match with TiO2/QD are difficult to be balanced. Although ZnS can form a larger energetic barrier due to the higher conduction band edge, its lattice mismatch with TiO2 and QD (such as CdSe and CuInSe2) is large, leading to the formation of additional defect states. On the contrary, ZnSe has a small lattice mismatch with TiO2 and QD but a relatively lower conduction band edge. Herein, we propose a strategy to employ ZnSxSe1-x alloy materials as a passivation layer for the first time to solve the drawbacks of single-component passivation layers. The ZnSxSe1-x alloy passivation layer was deposited on the Zn-Cu-In-Se (ZCISe) QD-sensitized TiO2 film electrode via successive ionic layer adsorption and reaction (SILAR) method. A stable polyselenosulfide/sulfide mixed anions were served as anion precursor for the formation of ZnSxSe1-x alloy passivation layer. Experimental results revealed that the alloy passivation layer is more favorable for the suppression of charge recombination at the photoanode/electrolyte interface. In addition, the ZnSxSe1-x alloy passivation layer can significantly improve the photogenerated electron extraction efficiency compared to the current classical ZnS passivation layer as confirmed by the transient absorption (TA) measurement. Consequently, the average efficiency of QDSCs was improved from 12.17 to 13.08% with the replacement of traditional ZnS passivation layer by ZnSSe-10 under AM 1.5G one full sun illumination.

Interfacial Silicide Formation and Stress Evolution during Sputter Deposition of Ultrathin Pd Layers on a-Si.

Synchrotron experiments combining real-time stress, X-ray diffraction, and X-ray reflectivity measurements, complemented by in situ electron diffraction and photon electron spectroscopy measurements, revealed a detailed picture of the interfacial silicide formation during deposition of ultrathin Pd layers on amorphous silicon. Initially, an amorphous Pd2Si interlayer is formed. At a critical thickness of 2.3 nm, this layer crystallizes and the resulting volume reduction leads to a tensile stress buildup. The [111] textured Pd2Si layer continues to grow up to a thickness of ≈3.7 nm and is subsequently covered by a Pd layer with [111] texture. The tensile stress relaxes already during Pd2Si growth. A comparison between the texture formation on SiOx and a-Si shows that the silicide layer serves as a template for the Pd layer, resulting in a surprisingly narrow texture of only 3° after 800 s Pd deposition. The texture formation of Pd and Pd2Si can be explained by the low lattice mismatch between Pd(111) and Pd2Si(111). The combined experimental results indicate a similar interface formation mechanism for Pd on a-Si and c-Si, whereas the resulting silicide texture depends on the Si surface. A new strain relaxation mechanism via grain boundary diffusion is proposed, taking into account the influence of the thickness-dependent crystallization on the material transport through the silicide layer. In combination with the small lattice mismatch, the grain boundary diffusion facilitates the growth of Pd clusters, explaining thus the well-defined thickness of the interfacial silicide layer, which limits the miniaturization of self-organized silicide layers for microelectronic devices.

First Principles Calculations of the Energetic, Structural, Electronic, and Magnetic Properties of Fe/Ir(100) System

Although the Fe/Ir(100) system has been extensively investigated, contradictory conclusions were drawn from different experimental and theoretical studies about its magnetic ground state. In this work, density functional theory calculations are performed for the adsorption of Fe on the Ir(100) surface with different coverages to resolve this contradiction. The obtained adsorption energy decreases as the coverage increases, which reflects the effect of repulsive interaction between the adsorbates. Antiferromagnetic (AFM) configuration is found to be more stable than the ferromagnetic (FM) configuration for the 0.50 and 1.00 monolayer (ML) coverages. Fe atoms start to form a body centered cubic (BCC) structure, with the basis of a lattice parameter of Ir and have a pseudomorphic growth. The FM configuration is found to be more stable than the AFM configuration for Fe bilayer on Ir(100) surface with c/a ≈ 0.53, which predicts a BCC Fe precursor due to the small lattice mismatch between Fe and Ir surface. The electronic properties predict that the reactivity of Ir surface decreases beyond Fe coverage of 0.50 ML since the d band center of 5d Ir shifts to the left of Fermi energy as Fe coverage increases.

Lattice-matched GaN(0002)/LSAT(111) hetero-structures grown by PLD and analyzed by first-principles calculations

La0.3Sr1.7AlTaO6(LSAT), a desired substrate for GaN epitaxial growth with a lattice mismatch as small as 1%, has not been applied in growth of GaN films due to hetero-interface rotation in epitaxial growth. In this paper, high-quality GaN epitaxial films with a small FWHM for GaN(0002) X-ray rocking curve of 0.06° have been grown by pulsed laser deposition (PLD) with a lattice mismatch as small as 1% between GaN and LSAT, and the corresponding mechanism avoiding interface rotation has also been investigated by first-principles calculation. On the one hand, we found that GaN grown on LSAT by PLD shows an in-plane relationship of GaN[2-1-10]//LSAT[1-10], revealing a small lattice mismatch of 1%. On the other hand, the first-principles calculation based on density function theory reveals that GaN growth along GaN[2-1-10]//LSAT[1-10] shows a higher interface energy barrier compared with that along GaN[1-100]//LSAT[1-10] with a lattice mismatch of 12.5%. The high interface energy barrier can only be provided by PLD. Moreover, this work is of paramount importance for understanding the hetero-interface formation mechanism in a GaN/substrates system and will be beneficial to the development of GaN-based devices that require high-quality GaN epitaxial films.

Database-driven high-throughput study of coating materials for hybrid perovskites

We developed a high-throughput screening scheme to acquire candidate coating materials for hybrid perovskites. From more than 1.8 million entries of an inorganic compound database, we collected 93 binary and ternary materials with promising properties for protectively coating halide-perovskite photoabsorbers in perovskite solar cells. These candidates fulfill a series of criteria, including wide band gaps, abundant and non-toxic elements, water-insoluble, and small lattice mismatch with surface models of halide perovskites.

Graphene grain size-dependent synthesis of single-crystalline Sb2Te3 nanoplates and the interfacial thermal transport analysis by Raman thermometry

The electrical and thermal transport at the interface of hetero-structures plays a critical role in device performance. Here, we successfully grew single-crystalline Sb2Te3 nanoplates (NPs) on graphene by the solvothermal method due to the small lattice mismatch (∼4%) between c-axis aligned rhombohedral Sb2Te3 and bridge sites of graphene. The growth of Sb2Te3 NPs was bound by the graphene grain boundary, and the average size of Sb2Te3NPs increased from 2.2 to 8.8 μm, with a maximum of 21 μm, as the graphene grain size increased from 2.2 to 9.4 μm. The Ohmic contact was realized between the Sb2Te3 NP and graphene. However, a limited thermal boundary conductance (KBD, average and maximum:13.45 and 17.69 MW m−2 K−1) was observed by Raman thermometry due to the large acoustic mismatch (Debye temperature difference = 1935 K). Both small lattice and acoustic mismatch was required to achieve high KBD. The electrical and thermal contact was poor (average and minimum KBD: 0.16 and 0.08 MW m−2 K−1) when Sb2Te3 NPs were separately synthesized and drop-deposited on graphene. This demonstrated that the direct growth of Sb2Te3 on graphene was very important, rather than the simple transfer method, to achieve good electrical and thermal contact at the interface of hetero-structures.

First-principles study of the complex magnetism in Fe16N2

Magnetic exchange interactions in pure and vanadium (V)-doped Fe16N2 are studied within the framework of density functional theory (DFT). The Curie temperatures were obtained via both mean field approximation (MFA) and Monte Carlo (MC) calculations based on interactions that were obtained through DFT. The Curie temperature (TC) for pure Fe16N2 that was obtained under MFA is substantially larger than the experimental value, suggesting the importance of thermal fluctuations. At zero field, the calculated magnetic susceptibility shows a sharp peak at T = TC that corresponds to the presence of localized d-states. From the nature of the exchange interactions, we have determined the reason for the occurrence of the giant magnetic moment in this material, which remained a mystery for decades. Finally, we posit that Fe16N2 can also act as a satisfactory spin injector for III–V semiconductors, in addition to its application as a permanent magnet, since it has very high spin polarization (compared to elemental ferromagnets) and smaller lattice mismatch (compared to half-metallic Heusler alloys) with conventional III–V semiconductors such as GaAs and InGaAs. We demonstrate this application in the case of Fe16N2(001)/InGaAs(001) hetero-structures, which exhibit substantial spin polarization in the semiconductor (InGaAs) region. PACS number: 82.65.My, 82.20.Pm, 82.30.Lp, 82.65.Jv.

Residual strain in free-standing CdTe nanowires overgrown with HgTe

We investigate the crystal properties of CdTe nanowires overgrown with HgTe. Scanning electron microscopy and scanning transmission electron microscopy confirm that the growth results in a high ensemble uniformity and that the individual heterostructures are single-crystalline, respectively. We use high-resolution X-ray diffraction to investigate strain, caused by the small lattice mismatch between the two materials. We find that both CdTe and HgTe show changes in the lattice constant compared to the respective bulk lattice constants. The measurements reveal a complex strain pattern with signatures of both uniaxial and shear strains present in the overgrown nanowires.

Fully depleted emitter layers: a novel method to improve band alignment in thin-film solar cells

The interface between the emitter and absorber layers in a thin-film solar cell must satisfy two important criteria, namely a small lattice mismatch and electron barrier height. It is shown that the barrier height is lowered when the emitter is fully depleted of free electron carriers by making the layer thinner than its space charge region, thereby enhancing thermionic emission of the photocurrent across the interface. Lattice matching is therefore the only requirement for a fully depleted emitter. The concept is applied to a lattice matched ZnS-Cu2ZnSnS4 (CZTS) interface which has a large intrinsic barrier height. Recent experimental evidence however suggest that ZnS becomes current unblocking when sufficiently thin. The theoretical efficiency for fully depleted ZnS is as high as 16.1%, due to the combination of a large open circuit voltage (1.0 V) from lattice matching and reasonable short circuit current density (24 mA cm−2). Lattice matched GaP and AlP are also potential CZTS emitter layers in the fully depleted configuration. The possibilities for exploring new materials combinations are therefore greater with fully depleted emitters. Furthermore, the concept can in principle be applied to any thin-film solar cell, making it highly versatile.