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Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS2, WS2 and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different hosts, explaining a persistent photoconductivity above 400 K., The optical and electronic properties of semiconducting transition metal dichalcogenides are strongly dependent on native defects within their crystal lattice. Here, the authors use transient spectroscopy to identify defect-related deep levels in the bandgap of MoS2, WS2 and their alloys.

Technological opportunities are explored to enhance detection schemes in transmission electron microscopy (TEM) that build on the detection of single-electron scattering events across the typical spectrum of interdisciplinary applications. They range from imaging with high spatiotemporal resolution to diffraction experiments at the window to quantum mechanics, where the wave-particle dualism of single electrons is evident. At the ultimate detection limit, where isolated electrons are delivered to interact with solids, we find that the beam current dominates damage processes instead of the deposited electron charge, which can be exploited to modify electron beam-induced sample alterations. The results are explained by assuming that all electron scattering are inelastic and include phonon excitation that can hardly be distinguished from elastic electron scattering. Consequently, a coherence length and a related coherence time exist that reflect the interaction of the electron with the sample and change linearly with energy loss. Phonon excitations are of small energy (

Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS

We report the growth of layered GaSexTe1–x mesostructures across the whole composition range. For compositions up to x = 0.32 (the Te-rich region), mesocrystals form predominantly in the monoclinic structure, similar to naturally occurring GaTe. However, the hexagonal crystal structure, similar to naturally occurring GaSe, begins growing at the x = 0.28 composition and grows almost exclusively in the range of x = 0.32 to pure GaSe, establishing a region of composition where both monoclinic and hexagonal crystals exist. While the optical bandgap of the monoclinic phase increases linearly from 1.65 to 1.77 eV with increasing Se content, the incorporation of Te in the hexagonal phase reduces the optical gap from 2.01 (pure GaSe) to 1.38 eV (x = 0.28). Specifically, a bandgap difference of ∼0.35 eV between monoclinic and hexagonal crystals is observed in the composition range where both crystal structures can be grown. These observations are in good agreement with direct-gap trends calculated by density funct...

Layered two-dimensional crystal systems can exhibit complex interlayer interactions, which are influenced by local crystal structure and/or electronic variations. Here, we study the influence of defects in twisted bilayer graphene (TBG) using Raman spectroscopy. We explore the varied influence of defects on three characteristic Raman modes of both fully-defected TBG, with defects introduced in both layers, and half-defected TBG, with defects introduced in only a single layer. The resonance condition responsible for a strong enhancement of the G peak is sensitive to structural disorder and is quenched within a radius ∼3 nm of defects, while the twist-angle dependence of the 2D peak is influenced only at the site of structural disorder (∼1 nm radius).

We demonstrate the selective, pulsed-laser deposition of hexagonal GaS and monoclinic Ga2S3 films on sapphire substrates from a single Ga2S3 target in high-vacuum conditions. Growth at substrate temperatures below 550 °C causes GaS film formation, which indicates non-stoichiometric transfer from the target to the film. Surprisingly, stoichiometric transfer occurs at substrate temperatures above 650 °C with monoclinic Ga2S3 as the preferred, higher S-content phase. Through a series of growth and annealing experiments, we show that GaS nucleation under S-deficient conditions leads to the preferred growth of this layered, hexagonal phase below 550 °C. Furthermore, GaS films annealed above 650 °C under high vacuum are transformed to Ga2S3, reflecting the greater stability of the monoclinic phase. By first growing Ga2S3 at a higher temperature and subsequently growing GaS at a lower temperature, we can fabricate GaS/Ga2S3 heterostructures in a single growth process.

We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) Ga0.98Mn0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimentaldata,aswellastheoreticalcalculations,tounderstandtheroleoftheMndopantintheemergenceof ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between Ga0.98Mn0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The Ga0.98Mn0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of Ga0.97Mn0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012)], demonstrating the strong similarity between these two materials. The Mn 2p and 3s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

We have used low-energy electron microscopy (LEEM) and diffraction (LEED) to examine the significance of lattice orientation in graphene growth on Cu(0 0 1). Individual graphene domains undergo anisotropic growth on the Cu surface, and develop into lens shapes with their long axes roughly aligned with Cu〈1 0 0〉 in-plane directions. The long axis of a lens-shaped domain is only rarely oriented along a C〈1 1〉 direction, suggesting that carbon attachment at “zigzag” graphene island edges is unfavorable. A kink-mediated adatom attachment process is consistent with the behavior observed here and reported in the literature. The details of the ridged moire pattern formed by the superposition of the graphene lattice on the (0 0 1) Cu surface also evolve with the graphene lattice orientation, and are predicted well by a simple geometric model. Managing the kink-mediated growth mode of graphene on Cu(0 0 1) will be necessary for the continued improvement of this graphene synthesis technique.

Gallium nitride high electron mobility transistors (HEMTs) have shown the potential to be highly resistant to radiation damage, making them ideal for use in microwave power amplifiers and DC/DC converters in space-based applications. To investigate the mechanisms of radiation-induced degradation in AlGaN/GaN HEMTs, 2MeV protons were used to simulate the space radiation environment, and to probe the second order effect of inherent material defects on the response the initial material quality was intentionally varied by studying HEMTs on sapphire, Si, and SiC substrates. Irradiation was performed incrementally up to a fluence of 6x1014 cm-2. The Hall mobility and 2DEG density was measured on Van der Pauw structures before irradiation and at each dose. The decrease in mobility can be attributed to increased carrier scattering in the 2DEG as a result of radiation-induced defects while the decrease in sheet carrier density is attributed to screening of the 2DEG from charged trap formation [1]. The magnitude of change in the 2DEG density is the same for all substrate materials (1x1012 cm-2), though the percent change varies due to different initial values. Note that mobility begins to degrade at lower fluence than sheet carrier density. FET I-V analysis indicated increased ON-resistance and decreased saturation current, consistent with the reduced mobility and sheet carrier density observed in the Hall measurements. The off-state leakage current decreased and threshold voltage was shifted slightly positive, again due to the reduced 2DEG density. The trapping mechanisms were further probed using pulsed I-V measurements at varying quiescent bias conditions. While the dynamic ON-resistance degradation under quasistatic conditions was consistent with DC measurements, the degradation under off-state stress conditions initiated at a lower fluence, and a culminated with a 10X higher degradation at the final fluence. This implies that dynamic ON-resistance, a parameter related to charge trapping, is much more sensitive to radiation than DC characteristics and must be considered when evaluating the radiation hardness of HEMT devices. TEM imaging was employed to directly probe the mechanisms suggested by the electrical measurements. Process-induced defects were visible in both reference and irradiated samples, notably an overflow of Au in the metal stack at the gate edge, possibly related to PECVD deposition of passivation layers. In addition a new, radiation-induced void at both edges of the gate in the Ni region of the Ni/Au gate metallization was revealed [2]. EDS line scans indicate that, after radiation, the 20 nm Ni layer diffused up into the Au, and to a lesser extent, into the AlGaN, leaving voids protruding ~300 nm under the gate edges. The mechanism of the void formation shows all features of Ni/Au inter-diffusion through vacancy exchange, known as the Kirkendall effect. The exact process is still under investigation, but the voids result in reduced gate area, which is responsible for the degraded off-state performance. It is particularly rare that such pronounced diffusion occurs at room temperature. Since the voids only occur at the gate edges, an additional electrochemical or strain-induced driving force is likely present. Therefore the role of the SiNx passivation layer, as well as strain, at the gate edge will be factor in this ongoing research. 1. T.J. Anderson, et. Al. IEEE Electron Dev. Lett. 35 (8), 826-828 (2014) 2. A.D. Koehler, et. Al. IEEE Electron Dev. Lett. 35 (10) (2014)

Intermediate-band semiconductors have attracted much interest in the photovoltaic community, as they are expected to allow higher solar-cell efficiencies than traditional absorbers. The authors directly measure minority-carrier lifetimes in Ga(P,As,N), only to find that these times are too short for efficient solar cells. The observed lifetimes seem not to be fundamental material properties, though, but rather the result of recombination at defects. This identifies material quality as possibly the most important issue preventing the development of efficient solar cells based on these semiconductors.

Stoichiometric gallium oxide sulfide Ga2(O1 − xSx)3 thin-film alloys were synthesized by pulsed-laser deposition with x ≤ 0.35. All deposited Ga2(O1 − xSx)3 films were found to be amorphous. Despite the amorphous structure, the films have a well-defined, room-temperature optical bandgap tunable from 5.0 eV down to 3.0 eV. The optical absorption data are interpreted using a modified valence-band anticrossing model that is applicable for highly mismatched alloys. The model provides a quantitative method to more accurately determine the bandgap as well as an insight into how the band edges are changing with composition. The observed large reduction in energy bandgap with a small sulfur ratio arises from the anticrossing interaction between the valence band of Ga2O3 and the localized sulfur level at 1.0 eV above the Ga2O3 valence-band maximum.

We observe laser-induced grain morphology change in silicon nanopillars under a transmission electron microscopy (TEM) environment. We couple the TEM with a near-field scanning optical microscopy pulsed laser processing system. This novel combination enables immediate scrutiny on the grain morphologies that the pulsed laser irradiation produces. We find unusual transformation of the tip of the amorphous or polycrystalline silicon pillar into a single crystalline domain via melt-mediated crystallization. On the basis of the three-dimensional finite difference simulation result and the dark field TEM data, we propose that the creation of the distinct single crystalline tip originates from the dominant grain growth initiated at the apex of the non-planar liquid–solid interface. Our microscopic observation provides a fundamental basis for laser-induced conversion of amorphous nanostructures into coarse-grained crystals.

Semiconductor nanowire (NW) synthesis methods by blanket furnace heating produce structures of uniform size and shape. This study overcomes this constraint by applying laser-localized synthesis on catalytic nanodots defined by electron beam lithography in order to accomplish site- and shape-selective direct integration of vertically oriented germanium nanowires (GeNWs) on a single Si(111) substrate. Since the laser-induced local temperature field drives the growth process, each NW could be synthesized with distinctly different geometric features. The NW shape was dialed on demand, ranging from cylindrical to hexagonal/irregular hexagonal pyramid. Finite difference time domain analysis supported the tunability of the light absorption and scattering spectra via controlling the GeNW shape.

Transmission Electron Microscopy of Mg doped GaN1-xAsx samples, grown by MBE at low temperatures, show substantial structural changes for samples that are semi-insulating and those with high or low conductivity. The conductive samples show p-type conductivity as evidence from the positive thermopower values. All the Mg doped samples show phase segregation: cubic GaAs and GaN grains (a mixture of cubic and some hexagonal) phases within an amorphous matrix. The best conductive samples show cubic GaAs grains with high density of stacking faults embedded into an amorphous matrix. The samples that are less conductive have lower ratio of the amorphous to the crystalline phase of the samples and much lower density of stacking faults. Higher Mg concentration is expected in the amorphous parts of the samples The semi-insulating samples that have either low Mg concentration or low As show grains of GaAs and GaN attached to each other with no evidence of the amorphous phase between them. There are no SFs in these grains. It is possible that the presence of the GaN between the GaAs grains lead to semi-insulating material properties since p-type doping of GaN is more difficult. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertically aligned carbon nanotube (CNT) arrays are promising candidates for advanced thermal interface materials (TIMs) since they possess high mechanical compliance and high intrinsic thermal conductivity. However, the overall thermal performance of CNT arrays often falls short of expectations when used as TIMs, and the underlying reasons have yet to be fully understood. In this work, the volume fraction of CNT arrays is demonstrated to be the key factor in determining the CNT array thermal transport properties. By increasing the array volume fraction, both the CNT array effective thermal conductivity and the CNT array–glass thermal contact conductance were experimentally found to increase monotonically. One interesting phenomenon is that the increasing rate of thermal conductivity is larger than that of array volume fraction. Compressive experiments verified that the CNT arrays with lower volume fractions suffer from severe buckling, which results in a further decreasing trend. By understanding the underlying reasons behind this trend, the overall thermal performance of vertically aligned CNT arrays can be further increased.

A giant bandgap reduction in layered GaTe is demonstrated. Chemisorption of oxygen to the Te-terminated surfaces produces significant restructuring of the conduction band resulting in a bandgap below 0.8 eV, compared to 1.65 eV for pristine GaTe. Localized partial recovery of the pristine gap is achieved by thermal annealing, demonstrating that reversible band engineering in layered semiconductors is accessible through their surfaces.

We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than ∼100 °C under both Ga-rich and N-rich growth conditions. The formation of amorphous GaN1−xBix alloys is promoted by growth under Ga-rich conditions. The amorphous matrix has a short-range order resembling random crystalline GaN1−xBix alloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1−xBix alloys. Despite the fact that the films are pseudo-amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of ∼1 eV.

Thin films of GaNBi alloys with up to 12.5 at.% Bi were grown on sapphire using low-temperature molecular beam epitaxy. The low growth temperature and incorporation of Bi resulted in a morphology of nanocrystallites embedded in an amorphous matrix. The composition and optical absorption shift were found to depend strongly on the III:V ratio controlled by the Ga flux during growth. Increasing the incorporation of Bi resulted in an increase in conductivity of almost five orders of magnitude to 144 Ω-cm−1. Holes were determined to be the majority charge carriers indicating that the conductivity most likely results from a GaNBi-related phase. Soft x-ray emission and x-ray absorption spectroscopies were used to probe the modification of the nitrogen partial density of states due to Bi. The valence band edge was found to shift abruptly to the midgap position of GaN, whereas the conduction band edge shifted more gradually.

Lasers are efficient tools in a variety of micro/nanoscale material processing applications. Even though optical imaging techniques offer convenient in-situ monitoring, their spatial resolution is frequently not sufficient for inspecting the detailed phenomena occurring in micro/nano structures, hence requiring additional characterization tools. Besides the inconvenience, critical processing parameters cannot be readily determined ex situ. In this study, an example of an in-situ monitoring technique for micro/nanoscale laser processing is demonstrated by combining the optical near-field apparatus with a scanning electron microscopy (SEM). In-situ process monitoring under true optical near-field configuration is realized through orthogonal probe manipulation and combined probe-sample translation and tilting apparatus. Catalyst behavior under a chemical vapor deposition (CVD) gas environment coupled with near-field illumination is monitored in an environmental SEM.

Aberration-corrected electron microscopy opens new ways for material characterization. In catalyst research it will enable the observation of single atom arrangements, such as the location of promoter atoms on catalyst particles. However, quantitative procedures must be developed to account for dynamic contrast changes resulting from beam-sample interactions and incoherent instrument aberrations. We demonstrate that at low acceleration voltage (80 kV), for which knock-on damage is suppressed, the residual intensity fluctuations can be attributed to the presence of phonons resulting in 3D low frequency atom displacements. For rhodium [110] oriented particles it was found that the catalysts are platelets with an aspect ratio of about 0.2 and a surface roughness of ±1 atom. Observation of single surface atoms requires minimization of phonon-induced motion.

Author(s): Heyman, JN; Weiss, EM; Rollag, JR; Yu, KM; Dubon, OD; Kuang, YJ; Tu, CW; Walukiewicz, W | Abstract: THz Time-Resolved Photoconductivity is used to probe carrier dynamics in the dilute III-V nitride GaP0.49As0.47N0.036. In these measurements a femtosecond optical pump-pulse excites electron-hole pairs, and a delayed THz pulse measures the change in conductivity. We find the photoconductivity is dominated by localized carriers. The decay of photoconductivity after excitation is consistent with bimolecular electron-hole recombination with recombination constant r = 3.2 0.8 10-8 cm3 s-1. We discuss the implications for applications in solar energy.

We have observed the growth of monolayer graphene on Cu foils using low-energy electron microscopy. On the (100)-textured surface of the foils, four-lobed, 4-fold-symmetric islands nucleate and grow. The graphene in each of the four lobes has a different crystallographic alignment with respect to the underlying Cu substrate. These "polycrystalline" islands arise from complex heterogeneous nucleation events at surface imperfections. The shape evolution of the lobes is well explained by an angularly dependent growth velocity. Well-ordered graphene forms only above ∼790 °C. Sublimation-induced motion of Cu steps during growth at this temperature creates a rough surface, where large Cu mounds form under the graphene islands. Strategies for improving the quality of monolayer graphene grown on Cu foils must address these fundamental defect-generating processes.

Phase change materials are essential to a number of technologies ranging from optical data storage to energy storage and transport applications. This widespread interest has given rise to a substantial effort to develop bulk phase change materials well suited for desired applications. Here, we suggest a novel and complementary approach, the use of binary eutectic alloy nanoparticles embedded within a matrix. Using GeSn nanoparticles embedded in silica as an example, we establish that the presence of a nanoparticle/matrix interface enables one to stabilize both nanobicrystal and homogeneous alloy morphologies. Further, the kinetics of switching between the two morphologies can be tuned simply by altering the composition.

NixCd1-xO has a ∼3 eV band edge offset and bandgap varying from 2.2 to 3.6 eV, which is potentially important for transparent electronic and photovoltaic applications. We present a systematic study of the electronic band structure of NixCd1-xO alloys across the composition range. Ion irradiation of alloy samples leads to a saturation of the electron concentration associated with pinning of the Fermi level (EF) at the Fermi stabilization energy, the common energy reference located at 4.9 eV below the vacuum level. The composition dependence of the pinned EF allows determination of the conduction band minimum (CBM) energy relative to the vacuum level. The unusually strong deviation of the CBM energy observed from the virtual crystal approximation is explained by a band anticrossing interaction between localized 3d states of Ni and the extended states of the NixCd1-xO alloy host. The resulting band structure explains the dependence between the composition and the electrical and optical properties of the allo...

Nanoparticle solutions are considered promising for realizing low cost printable high performance flexible electronics. In this letter, excimer laser annealing (ELA) was employed to induce melting of solution-deposited ZnO nanoparticles and form electrically conductive porous films. The properties of the films were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, DC conductance, and photoluminescence measurements. Thin-film field-effect transistors have been fabricated by ELA without the use of conventional vacuum or any high temperature thermal annealing processes. The transistors show n-type accumulation mode behavior with mobility greater than 0.1 cm2/V s and current on/off ratios of more than 104. Optimization and control of the laser processing parameters minimized thermal impact on the substrate. This technique can be beneficial in the fabrication of metal oxide based electronics on heat sensitive flexible plastic substrates using low-cost, large-area solution processing combined with direct printing techniques.

The organization of semiconductor nanostructures into functional macroassemblies remains a fundamental challenge in nanoscience and nanotechnology. In the context of semiconductor epitaxial growth, efforts have focused on the application of advanced substrate patterning strategies for the directed assembly quantum-dot islands. We present a comprehensive investigation on the use of simple metal patterns to control the nucleation and growth of heteroepitaxial islands. In the Ge on Si model system, a square array of metal dots induces the assembly of Ge islands into an extensive two-dimensional lattice. The islands grow at sites between the metal dots and are characterized by unique shapes including truncated pyramids and nanorods, which are programmed prior to growth by the choices of metal species and substrate orientation. Our results indicate that ordering arises from the metal-induced oxidation of the Si surface; the oxide around each metal dot forms an array of periodic diffusion barriers that induce island ordering. The metals decorate the island surfaces and enhanced the growth of particular facets that are able to grow as a result of significant intermixing between deposited Ge and Si substrate atoms.

We have carried out mid-infrared magneto-optical studies of Ga1−x Mn x As films grown using two different techniques: low temperature molecular beam epitaxy (LT-MBE) and a combination of ion-implantation and pulsed laser melting (II-PLM). The second method is a high temperature growth technique which minimizes the formation of Mn interstitials. Both samples exhibit a ferromagnetic response with similar Curie temperatures (T C). The samples show qualitatively similar behavior in below bandgap Faraday and Kerr measurements, which probe the valence band structure of the materials. This suggests that the same mid-infrared transitions are dominating the magneto-optical response of both samples.

Highly mismatched alloys (HMAs) are dilute alloys whose fundamental properties are dramatically modified through the substitution of a relatively small fraction (up to ∼5%) of host atoms with a very different element. Specific examples are dilute III–V nitrides (e.g. GaN x As 1− x ) and II–VI oxides (e.g. ZnO x Te 1− x ) in which the conduction bands are strongly modified by the anticrossing interaction between localized states of O or N and the extended states of the semiconductor matrix. Using a highly non-equilibrium method: the combination of ion implantation and pulsed laser melting (II-PLM) we have synthesized a wide variety of III-N x -V 1− x and II-O x -VI 1− x alloys with N or O content up to 2%. The structural and optical properties of these HMAs have been systematically investigated. In particular, we demonstrated that the Zn 1− y Mn y O x Te 1− y dilute oxide is a multiband alloy that can be used for the realization of the single junction, high efficiency intermediate band solar cells.

We demonstrate planar patterning techniques to produce isolated regions of ferromagnetic Ga1–xMnxAs embedded in a GaAs host. One technique consists of ion-implantation through lithographically defined resist, followed by pulsed-laser melting. Planar Hall-bar structures of Ga1–xMnxAs formed by this process show a large anomalous Hall signal along with negative magnetoresistance associated with the well-known hole-mediated ferromagnetic phase. In another technique patterning is achieved by selective hydrogenation of Ga1–xMnxAs, rendering exposed regions electrically insulating and magnetically inactive. The planar nature of these Hall-bars allows for tuning the ferromagnetism by the integration of lateral gates to modulate the exchange-mediating hole population in the Ga1–xMnxAs region, hence creating junction-FET functionality. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The valence band anticrossing model (VBAC) is applied to explain the composition-dependent trends in dilute GaSbxAs1–x and GaBixAs1–x alloys. Photomodulated reflectance spectroscopy (PR) of GaSbxAs1–x shows a reduction of the fundamental bandgap energy as well as an increase of the spin-orbit splitting energy with increasing x. The VBAC model predicts that these shifts are the result of a hybridization of the extended valence band states of GaAs with the localized states of the impurity atoms, which induce an upward movement of the valence band edge. Extrapolation of the model to other III-(Sb,Bi)-V alloys is also discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have used low-energy electron diffraction and microscopy to compare the growth of graphene on hydrogen-free Ge(111) and Ge(110) from an atomic carbon flux. Growth on Ge(110) leads to significantly better rotational alignment of graphene domains with the substrate. To explain the poor rotational alignment on Ge(111), we have investigated experimentally and theoretically how the adatom reconstructions of Ge interact with graphene. We find that the ordering transition of the Ge(111) adatom reconstruction is not significantly perturbed by graphene. Density functional theory calculations show that graphene on reconstructed Ge(110) has large-amplitude corrugations, whereas it is remarkably flat on reconstructed Ge(111). We argue that the absence of corrugations prevents graphene islands from locking into a preferred orientation.

We report the magnetic and transport properties of Ga1-xMnxP synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga1-xMnxAs, Ga1-xMnxP displays a monotonic increase of the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 C leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga1-xMnxAs, Ga1-xMnxP is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x., To be published in Solid State Communications

We demonstrate a simple but effective route for the synthesis of ferromagnetic semiconductors that combines Mn ion implantation (II) and pulsed-laser melting (PLM). By this process Ga1−xMnxAs films exhibiting a ferromagnetic Curie temperature (TC) exceeding 130 K have been synthesized. These materials possess electrical and magnetic characteristics similar to molecular beam epitaxy-grown (MBE-grown) films that are tunable through experimental parameters such as implantation dose, laser energy fluence, and co-doping with Te donors. We have also used II–PLM to form ferromagnetic single-crystalline Ga1−xMnxP films. These films display magnetic properties similar to their arsenide counterparts despite remaining non-metallic and becoming strongly insulating at low temperatures. Unlike as-grown MBE films, both Ga1−xMnxP and Ga1−xMnxAs formed by II–PLM do not contain Mn at interstitial sites; instead, non-substitutional Mn atoms are found to be randomly located, perhaps as clusters. As a result, post-PLM thermal annealing at temperatures higher than 300 °C leads to a decrease in TC as Mn atoms leave substitutional sites.

We present results on the growth of Ge islands into two-dimensional arrays by molecular beam epitaxy on Au-patterned Si. For a square array of Au dots, a two-dimensional square lattice of Ge islands extending over hundreds of microns has been produced. Islands in the patterned region grow between Au dots and have strikingly different shapes than islands on Au-free Si. For example, square base truncated pyramids grow on Si(0 0 1) while rod-shaped islands grow on Si(1 1 0). Characterization of island development on Si(0 0 1) shows three island shape transitions during growth with the final shape converging to that of the super-dome structure. For both Si(0 0 1) and Si(1 1 0) islands initially grow as “lens-like” structures before nucleating steep {1 1 1} side-facets and transforming into truncated pyramids or rods, respectively.

Using low-energy electron microscopy, we find that the mechanisms of graphene growth on Ir(111) depend sensitively on island orientation with respect to Ir. In the temperature range of 750–900 °C, we observe that growing rotated islands are more faceted than islands aligned with the substrate. Further, the growth velocity of rotated islands depends not only on the C adatom supersaturation but also on the geometry of the island edge. We deduce that the growth of rotated islands is kink-nucleation-limited, whereas aligned islands are kink-advancement-limited. These different growth mechanisms are attributed to differences in the graphene edge binding strength to the substrate.

Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, ‘ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene–substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials., Understanding the factors that control island orientation in the growth of two-dimensional materials is likely to improve their quality. Here, using low-energy electron microscopy, the authors show that post-nucleation annealing of 2D materials can improve rotational order.

Ternary and quaternary dilute II–VI oxides were synthesised using a highly non-equilibrium method: the combination of O ion implantation and pulsed-laser melting. CdOxTe1−x thin films have been produced with x up to 0.015 and with the energy gap reduced by 0.15 eV. Optical transitions corresponding to both the lower (E−) and upper (E+) conduction sub-bands, resulting from the anticrossing interaction between the localised O states and the extended conduction states of the matrix, are clearly observed in quaternary Cd0.6Mn0.4OxTe1−x and Zn0.88Mn0.12OxTe1−x layers. These results have important implications for the existing theoretical models of the electronic structure of the highly mismatched alloys. In Zn1−xMnxTe, where the O level lies below the conduction band edge, it was demonstrated that incorporation of a small amount of oxygen leads to the formation of a narrow, oxygen-derived band of extended states located well below the conduction band edge of the ZnMnTe matrix. The three absorption edges of this material (∼0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for multiband, single-junction, high-efficiency photovoltaic devices.

We have synthesized ternary and quaternary diluted II-VI oxides using the combination of O ion implantation and pulsed laser melting. CdO{sub x}Te{sub 1-x} thin films with x up to 0.015, and the energy gap reduced by 150 meV were formed by O{sup +}-implantation in CdTe followed by pulsed laser melting. Quaternary Cd{sub 0.6}Mn{sub 0.4}O{sub x}Te{sub 1-x} and Zn{sub 0.88}Mn{sub 0.12}O{sub x}Te{sub 1-x} with mole fraction of incorporated O as high as 0.03 were also formed. The enhanced O incorporation in Mn-containing alloys is believed to be due to the formation of relatively strong Mn-O bonds. Optical transitions associated with the lower (E{sub -}) and upper (E{sub +}) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of the host are clearly observed in these quaternary diluted II-VI oxides. These alloys fulfill the criteria for a multiband semiconductor that has been proposed as a material for making high efficiency, single-junction solar cells.

Using oxygen ion implantation and pulsed laser melting, we have synthesized thin films of highly mismatched ternary (y = 0) and quaternary (y = 0.12) Zn 1-y Mn y O x Te 1-x alloys with oxygen content in excess of x ∼ 0.01. We show that incorporation of a small amount of isoelectronic oxygen leads to the formation of a narrow, oxygen-derived band of extended states located well below the conduction band edge of the Zn 1-y Mn y Te matrix. The structure of the conduction band is well described by the anticrossing interaction between O localized states and the extended states of the host semiconductor matrix. As a result the conduction band splits into two subbands with distinctly non-parabolic dispersion relations. The three absorption edges of this material (-0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for the multi-band, single junction, high efficiency photovoltaic devices.

We demonstrate the mutual passivation of electrically active group IV donors and isovalent N atoms in the GaN x As 1− x alloy system. This phenomenon occurs through the formation of a donor-nitrogen bond in the nearest neighbor IV Ga –N As pairs. In Si-doped GaInN 0.017 As 0.983 the electron concentration starts to decrease rapidly at an annealing temperature of 700°C from ∼3×10 19 cm −3 in the as-grown state to less than 10 16 cm −3 after an annealing at 900°C for 10 s. At the same time, annealing of this sample at 950°C increases the gap by about 35 meV, corresponding to a reduction of the concentration of the active N atoms by an amount very close to the total Si concentration. We also show that the formation of Si Ga –N As pairs is controlled by the diffusion of Si via Ga vacancies to the nearest N As site. The general nature of this mutual passivation effect is confirmed by our study of Ge-doped GaN x As 1− x layers formed by N and Ge co-implantation in GaAs followed by pulsed laser melting.

Using ion implantation followed by pulsed-laser melting (PLM), we have synthesized ferromagnetic films of Ga 1− x Mn x As. Ion-channeling experiments reveal that these films are single crystalline and have high Mn substitutionality while variable temperature resistivity measurements reveal the strong Mn-hole interactions characteristic of carrier-mediated ferromagnetism in homogeneous Diluted magnetic semiconductors. We have observed Curie temperatures ( T C s) of approximately 80 K for films with substitutional Mn concentrations of x =0.04. The use of n-type counter doping as a means of increasing Mn substitutionality and T C is explored by co-implantation of Mn and Te into GaAs . In Ga 1− x Mn x P samples synthesized using our technique, the implanted layer regrows as an epitaxial single crystal capped by a highly defective surface layer. These samples display ferromagnetism with T C ≈23 K.

We present a systematic investigation on the formation of the highly mismatched alloy GaN{sub x}As{sub 1-x} using N{sup +}-implantation followed by a combination of pulsed laser melting and rapid thermal annealing. Thin films of GaN{sub x}As{sub 1-x} with x as high as 0.016 and an activation efficiency of the implanted N up to 50% have been synthesized with structural and optical properties comparable to films grown by epitaxial deposition techniques with similar substitutional N content. The effects of N{sup +} implantation dose, laser energy fluence and rapid thermal annealing temperature on the N incorporation as well as optical and structural properties of the GaN{sub x}As{sub 1-x} films are discussed.

Device reliability is a diverse topic, and influenced by a variety of factors. Device reproducibility which relies heavily on the specific device process, but also depends on local material quality is commonly focused on by industry. Device performance changes under operation and/or under specific environments, such as elevated temperatures, radiation or chemistry (often water-related) may have to be taken into account as well. The investigation of device performance changes can profit from accompanying TEM analysis, but only if the necessary TEM sample preparation and the imaging / spectroscopy technique do not produce their own artifacts. Examples of a device structure characterization include inter-diffusion phenomena at interfaces, diffusion of metal atoms from the device contacts, oxidation / corrosion effects and the study of extended defects as well as their impact on diffusion. AlGaN/GaN:Si HEMTs and GaN-based vertical power device structures are currently studied. This presentation will demonstrate how advanced TEM characterization can significantly improve device performance and reliability, which is enabled through advanced TEM sample preparation and electron dose control during imaging, which is particularly important at locally defective device areas.

Intercalated molecules can dramatically modify the electronic band structure of layered semiconductors, significantly altering the optical properties of the material. In the layered monochalcogenide Gallium Telluride (GaTe), exposure to air induces a nearly 1 eV reduction of its band gap due to the interlayer diffusion and chemisorption of oxygen. The effect of oxygen chemisorption at the Te-terminated surfaces on the structure of GaTe, however, is much less known. To better understand the structure–property relationship of intercalated GaTe, a systematic, long-term, X-ray diffraction study has been performed on GaTe exfoliated crystals exposed to ambient conditions. It is observed that the structural changes are not limited to a previously observed short-term increase in lattice expansion. Over the course of months and even years after exfoliation, the oxygen adsorbates continue to modify the structure of GaTe, inducing significant disorder and grain reorientation. It is estimated that approximately one out of every two grains is slightly displaced by the intercalating oxygen, demonstrating a significant increase in grain mosaicity, while still maintaining the original {−2 0 1} out-of-plane texture. Correlating these structural transformations to observed changes in electrical and optical properties will enable capitalization of the use of adsorbates to engineer novel properties in these layered materials.

Copper impurities affect optical and electronic properties in a diverse range of materials. Using IR spectroscopy, we provide evidence for a shallow Cu acceptor in Si. The observation of ${P}_{1/2}$ excitation lines indicates a Cu acceptor with a hole ionization energy of only 27 meV, which is shallower than any of the group-III acceptors. The observed center has an IR spectrum that is characteristic of a hydrogenic acceptor. Variable-temperature Hall-effect measurements confirm this result. The observation of shallow, hydrogenic behavior is consistent with a universal level model, in which the Cu acceptor level is fixed relative to vacuum, irrespective of the host.

Semiconductors known as highly mismatched alloys (HMAs) have unique properties that make them useful in the fabrication of intermediate band solar cells (IBSCs). Now a theoretical and experimental study of GaNPAs with ~40% phosphorus and up to 2% nitrogen finds that this HMA has an electronic band structure well suited for IBSC applications.

We have studied hopping conduction in uniaxially stressed Cu-doped Ge. The electronic ground state of neutral Cu acceptors, normally accommodating three 1s holes, undergoes a transformation to a 1s22s1 configuration with the application of uniaxial stress greater than 4 kbar. Upon undergoing this transformation, the copper acceptors form a system of lithium-like impurities in which hopping transitions of carriers take place through 2s rather than 1s states. The significant overlap between the more extended 2s wavefunctions located on different sites results in a decrease of the resistivity in the hopping regime by eight orders of magnitude. Good agreement was obtained between the experimental results and our calculations.