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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 (

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.

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.

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)

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.

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.

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.

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.

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.

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.

A copper‐related shallow acceptor complex has been discovered in germanium single crystals grown in vacuum and doped with arsenic and copper. Photothermal ionization spectroscopy of samples quenched from 673 K reveals two sets of hydrogenic lines with ground state binding energies of 9.15 and 10.05 meV. The line intensity ratios between corresponding transitions (1s‐np) of the two hydrogenic series follow a Boltzmann dependence. This shows that the two series belong to the same impurity complex with a split ground state. Taking into account the crystal growth conditions together with the changes in the donor concentration deduced from variable temperature Hall effect measurements, we conclude that arsenic and substitutional copper form the new acceptor complex A(Cus,As) since copper is the only fast‐diffusing species at this low annealing temperature. This complex is expected to have C3v symmetry in agreement with preliminary piezospectroscopy measurements.

Alloys from ZnO and ZnS have been synthesized by radio-frequency magnetron sputtering over the entire alloying range. The ZnO1−xSx films are crystalline for all compositions. The optical absorption edge of these alloys decreases rapidly with small amount of added sulfur (x ∼ 0.02) and continues to red shift to a minimum of 2.6 eV at x = 0.45. At higher sulfur concentrations (x > 0.45), the absorption edge shows a continuous blue shift. The strong reduction in the band gap for O-rich alloys is the result of the upward shift of the valence-band edge with x as observed by x-ray photoelectron spectroscopy. As a result, the room temperature bandgap of ZnO1−xSx alloys can be tuned from 3.7 eV to 2.6 eV. The observed large bowing in the composition dependence of the energy bandgap arises from the anticrossing interactions between (1) the valence-band of ZnO and the localized sulfur level at 0.30 eV above the ZnO valence-band maximum for O-rich alloys and (2) the conduction-band of ZnS and the localized oxygen le...

In molecular-beam epitaxy a monolayer of Pb on the Si(111) surface induces single-crystal growth at temperatures well below those required for similar growth on a bare surface. We demonstrate that the suppression of dopant segregation at the lower temperatures attainable by Pb-mediated growth allows the incorporation of As donors at concentrations reaching a few atomic percent. When Pb and Si are deposited on an As-terminated Si(111) substrate at 350 °C, the Pb segregates to the surface without doping the Si film while the As is buried within nanometers of the substrate–film interface. The resulting concentration of electrically active As, 1.8×1021 cm−3, represents the highest concentration of As donors achieved by any delta-doping or thin-film deposition method.

A monolayer of Pb mediates high-quality homoepitaxial growth on Si (111) surfaces at temperatures where growth with other overlayer elements or on bare surfaces leads to amorphous or highly defective crystalline films. Nearly defect-free epitaxy proceeds for film thicknesses up to 1000 A with no sign that this is an upper limit. The minimum temperature for high-quality epitaxy depends on the substrate miscut. For a 0.2° miscut, the minimum temperature is 340 °C. Films grown on substrates miscut 2.3° towards [112] show good crystalline quality down to 310 °C.

Proton-induced damage in AlGaN/GaN HEMTs was investigated using energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM), and simulated using a Monte Carlo technique. The results were correlated to electrical degradation using Hall measurements. It was determined by EDS that the interface between GaN and AlGaN in the irradiated HEMT was broadened by 2.2 nm, as estimated by the width of the Al EDS signal compared to the as-grown interface. The simulation results show a similar Al broadening effect. The extent of interfacial roughening was examined using high resolution TEM. At a 2 MeV proton fluence of 6 × 1014 H+/cm2, the electrical effects associated with the Al broadening and surface roughening include a degradation of the ON-resistance and a decrease in the electron mobility and 2DEG sheet carrier density by 28.9% and 12.1%, respectively.

The dilute GaNxAs1-x alloys (with x up to 0.05) have exhibited many unusual properties as compared to the conventional binary and ternary semiconductor alloys. We report on a new effect in the GaNxAs1-x alloy system in which electrically active substitutional group IV donors and isoelectronic N atoms passivate each other's activity. This mutual passivation occurs in dilute GaNxAs1-x doped with group IV donors through the formation of nearest neighbor IVGa- NAs pairs when the samples are annealed under conditions such that the diffusion length of the donors is greater than or equal to the average distance between donor and N atoms. The passivation of the shallow donors and the NAs atoms is manifested in a drastic reduction in the free electron concentration and, simultaneously, an increase in the fundamental bandgap. This mutual passivation effect is demonstrated in both Si and Ge doped GaNxAs1-x alloys. Analytical calculations of the passivation process based on Ga vacancy mediated diffusion show good agreement with the experimental results.

We report the realization of a new multi-band-gap semiconductor. The highly mismatched alloy Zn1-yMnyOxTe1-x has been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn1-yMnyTe host. When only 1.3% of Te atoms is replaced with oxygen in a Zn0.88Mn0.12Te crystal (with band gap of 2.32 eV) the resulting band structure consists of two direct band gaps with interband transitions at ~1.77 eV and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model in which the interactions between the oxygen-derived band and the conduction band are considered. With multiple band gaps that fall within the solar energy spectrum, Zn1-yMnyOxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%., Comment: 12 pages, 4 figures

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

We have detected stimulated far-infrared emission from copper-doped germanium single crystals. By varying the magnetic field between 1 and 2.3 T, we have achieved emission in the range of 70–120 cm−1. Laser action was observed for crystals with a copper acceptor concentration as high as 1.5×1015 cm−3, a doping level that is considerably higher than that of any single or double acceptor doped Ge laser. Stimulated emission from Ge crystals with such a high Cu concentration is possible because only a small fraction of the copper acceptors is ionized during operation.

We have synthesized CdxZn1−xO alloys across the full composition range. The structural mismatch of the two endpoint compounds splits the alloy into two regions of distinct optical and electrical behavior. The wurtzite phase alloys at compositions 0

We have grown N-rich, dilute Sb GaN1−xSbx alloys by low temperature molecular beam epitaxy. At low growth temperature of 4 at. %. Despite the different microstructures found for GaN1−xSbx alloys with different composition, the absorption edge shifts continuously from 3.4 eV (GaN) to close to 1 eV for samples with Sb content >30 at. %. GaN1−xSbx alloys with less than 5 at. % Sb show sufficient bandgap reduction (∼2 eV), making them suitable for photoelectrochemical applications.

We present a systematic study on the influence of growth conditions on the incorporation and activation of Mg in GaN layers grown by plasma-assisted molecular beam epitaxy. We show that high quality p-type GaN layers can be obtained on GaN-on-silicon templates. The Mg incorporation and the electrical properties have been investigated as a function of growth temperature, Ga : N flux ratio and Mg : Ga flux ratio. It was found that the incorporation of Mg and the electrical properties are highly sensitive to the Ga : N flux ratio. The highest hole mobility and lowest resistivity were achieved for slightly Ga-rich conditions. In addition to an optimal Ga : N ratio, an optimum Mg : Ga flux ratio was also observed at around 1%. We observed a clear Mg flux window for p-type doping of GaN : 0.31% < Mg : Ga < 5.0%. A lowest resistivity of 0.98 Ω cm was obtained for optimized growth conditions. The p-type GaN layer then showed a hole concentration of 4.3 × 1017 cm−3 and a mobility of 15 cm2 V−1 s−1. Temperature-dependent Hall effect measurements indicate an acceptor depth in these samples of 100 meV for a hole concentration of 5.5 × 1017 cm−3. The corresponding Mg concentration is 5 × 1019 cm−3, indicating approximately 1% activation at room temperature. In addition to continuous growth of Mg-doped GaN layers we also investigated different modulated growth procedures. We show that a modulated growth procedure has only limited influence on Mg doping at a growth temperature of 800 °C or higher. This result is thus in contrast to previously reported GaN : Mg doping at much lower growth temperatures of 500 °C.

The photoluminescence (PL) of Er-doped silica films containing Ge nanoclusters synthesized by ion implantation was investigated. The area of the 1540 nm Er3+ PL peak was enhanced by up to a factor of 200 by the addition of Ge nanoclusters. The PL enhancement was found to be proportional to the concentration of Ge atoms. Control experiments with argon ion implantation were used to show that the enhancement is due to the presence of Ge and not radiation damage. Furthermore, the Er3+ PL was found to be strongly influenced by the postgrowth annealing and the crystallinity of the Ge nanoclusters.

We present a systematic investigation on the band structure of the GaNxAs1−x alloys synthesized using nitrogen ion implantation followed by pulsed-laser melting and rapid thermal annealing. The evolution of the nitrogen-concentration depth profile is consistent with liquid-phase diffusion, solute trapping at the rapidly moving solidification front, and surface evaporation. The reduction of the Schottky barrier height of the Γ-like threshold at nitrogen composition up to x=0.016 is studied by ballistic electron emission microscopy (BEEM) and determined quantitatively using the second voltage derivative BEEM spectra to be −191±63 meV per x=0.01, which is close to the corresponding slope for samples grown by low-temperature molecular beam epitaxy. This slope is also consistent with the bandgap narrowing measured on the same samples by photomodulated reflectance and is consistent with the band anticrossing model for the splitting of the conduction band in the GaNxAs1−x alloys. Lithographically patterned GaNxA...

We have synthesized GaN{sub x}As{sub 1-y}P{sub y} alloys (x {approx} 0.3-1% and y = 0-0.4) using nitrogen N ion implantation into GaAsP epilayers followed by pulsed laser melting and rapid thermal annealing techniques. As predicted by the band anticrossing model, the incorporation of N splits the conduction band (E{sub M}) of the GaAs{sub 1-y}P{sub y} substrate, and strong optical transitions from the valence band to the lower (E{sub -}) and upper (E{sub +}) conduction subbands are observed. The relative strengths of the E{sub -} and E{sub +} transition change as the localized N level E{sub N} emerges from the conduction band forming narrow intermediate band for y > 0.3. The results show that GaN{sub x}As{sub 1-x-y}P{sub y} alloys with y > 0.3 is a three band semiconductor alloy with potential applications for high-efficiency intermediate band solar cells.

Wei and Su recently published a letter in this journal on low-temperature homoepitaxial growth of silicon mediated by thin overlayers of Pb. A significant portion of the experimental work reported in their letter was performed in our laboratory ~depositions, Rutherford backscattering analysis, and electron microscopy!. Those experiments were poorly controlled and produced major inconsistencies that made the work unfit for publication. A subsequent, more systematic, investigation revealed the origin of these inconsistencies and established the precise conditions for high-quality growth. Our report on the latter work appeared in this journal several months prior to submission of Ref. 1. The purpose of this comment is to correct the factual errors reported in Ref. 1. Foremost among these is the claim that high-quality Si films can be grown on vicinal Si~111! for Pb coverages of 0.8–1.0 ML (1 ML57.83310 atoms cm) by first depositing the Pb overlayer and subsequently growing the Si film without continuously supplying the sample surface with additional Pb. As reported in Ref. 2, the growth of arbitrarily thick, high-quality Si films requires a Pb coverage of 1.0 60.1 ML which cannot be achieved under the conditions outlined in Ref. 1 because Pb desorbs from the surface of the growing film. We have observed evaporation of Pb from vicinal surfaces including those from the same substrate material used by Wei while working in our laboratory. To maintain a constant Pb coverage at substrate temperatures of 280 °C or higher, one must deposit Pb during the growth of the Si film. For example, at 295 °C a Pb flux of 0.12 ML min is needed to maintain a Pb coverage of 1.0 ML on the sample surface. The Pb coverages reported in Ref. 1 were measured at the end of the deposition of Si. By wrongly assuming that the final Pb coverages they measured represent the actual coverages during growth, Wei and Su have overlooked the effect of Pb desorption on their results. The misinterpretation of ion channeling spectra in Ref. 1 undermines the authors’ claim that high-quality films can be