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The roughening behavior of 10-layer Si/SiGe heterostructures under 1 keV O 2 + sputtering at 44° incidence angle is accounted for using a modified MRI model. The MRI curve-fitted roughness parameters obtained from the Ge depth profiles are compared with the root-mean-square roughness from AFM scans of crater bottom topography. This comparison indicates that the surface roughening behavior during ion sputtering can be reasonably described by the MRI model. The MRI curve-fitted roughness parameter is found to be systematically larger than the AFM roughness values, and this is explained by additional roughness contributions from mixing straggling and interface roughness during the surface morphology evolution under O 2 + sputtering.

Arrays of nanoscale depressions were observed during sputtering of Si/SiGe heterostructures using 500 eV O2+ ions at angles of incidence between 46° and 56° with simultaneous sample rotation in a secondary ion mass spectrometry system. Both the atomic force microscope and transmission electron microscope results suggest that these depressions were produced as a result of preferential sputtering of Ge and only occur when there are undulations present at the Si/SiGe interface. Formation of the interface undulation appears to occur in tandem with the increase in the Ge surface segregation at the Si/SiGe interface for layers grown at elevated temperatures.

Epitaxial Si 1− x Ge x layers have a wide range of applications in microelectronic and optoelectronic devices. One of the possible SiGe configurations involves the growth of Si 1− x Ge x on a virtual substrate (VS). The VS is grown by linearly grading the Ge composition of the Si 1− x Ge x layer up to the final desired Ge composition for subsequent device growth. Such VSs are designed to accommodate the misfit strain between the substrate and the overlying active layer. However, a cross-hatch surface morphology often results which affects subsequent device layer growth on the VS. In this paper, we report on high-resolution channeling contrast microscopy (CCM) on such VSs. CCM measurements give both lateral and depth-resolved information on the cross-hatch features observed. The channeling RBS maps reveal a slight lattice plane bending in adjacent bands of width of ∼10 μm, consistent with the relaxation of the SiGe layer. In addition, the Ge concentration distribution was imaged by proton induced X-ray emission, in order to check if the cross-hatch is associated with increased Ge concentrations.

Si 1− y C y (0 0 1) and Si 1− x − y Ge x C y (0 0 1) layers were grown using gas source molecular beam epitaxy (GSMBE) with three precursors; methylsilane (CH 3 SiH 3 ), disilane (Si 2 H 6 ) and germane (GeH 4 ). The effect of carbon on growth was studied by observing the change in period of the intensity oscillations of reflection high energy electron diffraction (RHEED) during the formation of Si/Si 1− y C y heterojunctions. Si 1− x − y Ge x C y layer thickness (growth rate) changes were measured ex situ using X-ray diffraction (XRD). A decrease in growth rate was measured in the presence of carbon and evidence for carbon segregation was collected. Temperature programmed desorption (TPD) was used to study the limiting effect carbon has on H 2 desorption and disilane adsorption. The flux dependence of the growth rate was modelled and compared to experiment to highlight the relative importance of the sticking probability of disilane to the hydrogen desorption rate constant. The growth rate decrease during Si 1− y C y (0 0 1) growth compared to that of Si(0 0 1) was shown to be dominated by a decrease in the sticking probability of disilane.

In situ X-ray photoelectron spectroscopy (XPS) reveals that deposited Co atoms at room temperature react with Si at the growth front to form a thin “CoSi 2 -like” layer on both clean and hydrogen passivated Si(0 0 1) and Si(1 1 1) surfaces. Improvement in the silicides crystallinity upon annealing was characterised by an appreciable decrease in the full width at half maximum of the Co 2p 3/2 XPS spectra and a shift in binding energy towards Co in the bulk CoSi 2 crystal structure. Unlike the Si(0 0 1) substrate, the presence of hydrogen on the Si(1 1 1) surface appears to delay the decrease in the peak area ratio of Co 2p 3/2 /Si 2p as annealing temperatures increase. Ex situ surface morphology imaged by atomic force microscopy suggests a reduced adatom mobility on the Co/H-passivated Si surface compared to the Co/clean Si surface, as evidenced by a higher and smaller size CoSi 2 islands density observed on Co deposited/H-terminated Si surfaces after annealing to 700 °C.

The incorporation behaviour of phosphorus (P2) during growth by molecular beam epitaxy of InP thin films on InP(0 0 1) substrates has been studied in situ by reflection high energy electron diffraction. The incorporation coefficient of P2 decreases from 0.94 at 360 °C to 0.54 at 470 °C. This behaviour is attributed to the increasing fraction of the incident P2 flux that desorbs from the surface at higher temperatures and does not contribute to layer growth. The low- and temperature-dependent incorporation coefficients imply the need for high P2:In flux ratios and low substrate temperatures for the preparation of smooth InP epitaxial layers.

This paper reports on the study of the growth dynamics of Si and SiGe containing a small fraction of carbon from the hydrides of Si, Ge and methylsilane. Dynamic changes in the growth rate across heterojunctions show that the presence of carbon on the (0 0 1) surface of Si or SiGe supresses growth rate and the carbon concentration could not be changed abruptly at an interface due to the surface segregation of carbon during growth. Temperature programmed desorption of molecular hydrogen from the Si(0 0 1) surface with various adsorbates shows that the presence of carbon increases the barrier to the desorption of molecular hydrogen and reduces the sticking probability of disilane. Both effects may lead to a reduction in the growth rate but the disilane supply rate (flux) dependence of growth rate on clean and carbon containing Si(0 0 1) surface shows that the primary cause of the growth rate decrease is the reduced sticking probability under the growth conditions used.

Using plane wave pseudopotential density functional theory calculations we have identified for the first time precursor states for hydrogen atom chemisorption on the Si(100)-(2×1) surface. These states exist above clean, partially, and fully monohydride-adsorbed surface dimers. In all three cases the dimer bond is broken in the trapped state. A study of the energetics for atomic desorption, abstraction, chemisorption, and migration was carried out. We find that “hot” hydrogen atoms of energies up to approximately 1.3–1.9 eV can be trapped on the surface. These atoms are highly mobile, and we obtained energetics consistent with experimental data from which precursor-mediated adsorption mechanisms have been inferred. The existence of these states provides an understanding of the non-Langmuirian atomic hydrogen adsorption probability, and also underscores the importance of lattice distortions in the interactions of hydrogen with the silicon surface.

The ultrahigh vacuum scanning tunneling microscope (UHV-STM) was employed to investigate the evolution of GaAs(0 0 1)-( n ×6) surface reconstructions. The ( n ×6) surface reconstructions have been referred to as As-rich transitional phases comprising of (1×6), (2×6), (3×6) and (6×6) reconstructions. During the evolution from ( n ×6) to the Ga-rich (4×2) reconstruction, our dual-bias ( n ×6) STM images reveal one alternating single As dimer chain with a periodicity giving n =2; and a pair of Ga dimer rows between the As chains with a periodicity of n =3, which has not been previously reported. A new (6×6) reconstruction model is proposed based on the atomic resolution dual bias STM images obtained; and we discuss whether such (6×6) reconstructions exist in the light of all STM data published to date.

He Rutherford Backscattering Spectrometry (RBS) depth resolution is limited by detector energy resolution, He ion energy loss in the sample material, energy straggling and geometry of the experiment. Examples of experimental results are shown for GeSi/Si super-lattice investigation for random and channeling RBS analysis in different geometry of detection. Different detection geometries are discussed and compared. For the case of axial channeling experiments, the incident beam direction is restricted to typically low index axes only. It is shown how to improve the depth resolution and sensitivity in this type of experiments. Effects of sample material as well as contributions from different physical processes including multiple scattering effects are discussed in some detail.

High resolution channeling contrast microscopy (CCM) and channeling measurements were carried out to characterize SiGe quantum well structures on micron thick graded layers (i.e. virtual substrates). The virtual substrates were grown by gas source molecular beam epitaxy at a pressure of 10 −5 mbar and low pressure chemical vapor deposition at 10 −2 mbar on boron doped Si(0 0 1) substrates respectively. A homoepitaxial silicon buffer layer was grown prior to the deposition. The nominal structure is a 20 nm Si 0.75 Ge 0.25 layer at the surface, followed by 10 nm pure Si, 500 nm Si 0.75 Ge 0.25 and a 1000 nm thick graded SiGe (0–26%) layer. RBS was used to measure the depth profiles, and angular scans around the (1 0 0) axis were carried out to assess crystal and interface quality. CCM was used to acquire depth resolved images of micron-sized lateral inhomogenities (`cross-hatch') present on both samples.

Epitaxial growth of Si/SiGe from hydride precursors has been well established as the preferred technique for preparation of ‘virtual substrate’ and active layers of heterojunction metal-oxide-semiconductor devices as well as other Si/SiGe heterojunction based device structures. This paper reviews briefly the role of hydrogen in the growth mechanisms and establishes the basic reaction pathways in the growth process. The surface related segregation of Ge and As are examined in the context of formation of sharp SiGe/Si heterojunctions and sharp doping profiles. The influence of the growth environment on these surface segregation processes are revealed by the extended rate equation based ‘two-site’ exchange model. The optimisation of growth conditions are clearly identified from the trend gathered from secondary ion mass spectrometry measurements. The strain relaxation process during the growth of virtual substrates often leads to bunching and a high density of threading dislocations and requires substantially thicker grading layers. Some preliminary work shows that incorporation of ‘stresser’ layers in the grading region substantially reduces these detrimental features enabling relatively thin grading layers to be used.

Arrays of ZnO/CdSSe core/shell nanowires with shells of tunable band gaps represent a class of interesting hybrid nanomaterials with unique optical and photoelectrical properties due to their type II heterojunctions and chemical compositions. In this work, we demonstrate that direct focused laser beam irradiation is able to achieve localized modification of the hybrid structure and chemical composition of the nanowire arrays. As a result, the photoresponsivity of the laser modified hybrid is improved by a factor of ~3. A 3D photodetector with improved performance is demonstrated using laser modified nanowire arrays overlaid with monolayer graphene as the top electrode. Finally, by controlling the power of the scanning focused laser beam, micropatterns with different fluorescence emissions are created on a substrate covered with nanowire arrays. Such a pattern is not apparent when imaged under normal optical microscopy but the pattern becomes readily revealed under fluorescence microscopy i.e. a form of Micro-Steganography is achieved.

Surface modification or decoration of ultrathin MoS2 films with chemical moieties is appealing since nanointerfacing can functionalize MoS2 films with bonus potentials. In this work, a facile and effective method for microlandscaping of Au nanoparticles (NPs) on few-layer MoS2 films is developed. This approach first employs a focused laser beam to premodify the MoS2 films to achieve active surface domains with unbound sulfur. When the activated surface is subsequently immersed in AuCl3 solution, Au NPs are found to preferentially decorate onto the modified regions. As a result, Au NPs can be selectively and locally anchored onto designated regions on the MoS2 surface. With a scanning laser beam, microlandscapes comprising of Au NPs decorated on laser-defined micropatterns are constructed. By varying the laser power, reaction time and thickness of the MoS2 films, the size and density of the NPs can be controlled. The resulting hybrid materials are demonstrated as efficient Raman active surfaces for the detection of aromatic molecules with high sensitivity.

Formation of Sn wires on Ge 0.83 Sn 0.17 layer during annealing was discovered. The phenomenon observed may be explained by surface undulation, Sn segregation and aggregation.

Highly strained Ge 1-x Sn x films were epitaxially grown on Ge(001), with x up to 0.17. Good crystalline quality and a relatively flat surface were obtained. Critical temperatures exist for Sn segregation for various x. XPS shows that the Sn surface segregation could occur at temperatures as low as 400 °C for Ge 0.932 Sn 0.068 .

Highly strain-relaxed Ge 1-x Sn x layers were grown directly on Si(100) by MBE firstly. XRD and AFM measurements demonstrate a good crystalline quality and a relatively flat surface. Ge with 0.82% tensile strain were grown on the Ge 0.895 Sn 0.105 /Si, and the tensile-strain value decreases with increasing thickness of Ge epilayers.

Solid silicon nanowires and their luminescent properties have been widely studied, but lesser is known about the optical properties of mesoporous silicon nanowires (mp-SiNWs). In this work, we present a facile method to generate greenish-blue photoluminescence (GB-PL) by fast scanning a focused green laser beam (wavelength of 532 nm) on a close-packed array of mp-SiNWs to carry out photo-induced chemical modification. The threshold of laser power is 5 mW to excite the GB-PL, whose intensity increases with laser power in the range of 5–105 mW. The quenching of GB-PL comes to occur beyond 105 mW. The in-vacuum annealing effectively excites the GB-PL in the pristine mp-SiNWs and enhances the GB-PL of the laser-modified mp-SiNWs. A complex model of the laser-induced surface modification is proposed to account for the laser-power and post-annealing effect. Moreover, the fast scanning of focused laser beam enables us to locally tailor mp-SiNWs en route to a wide variety of micropatterns with different optical functionality, and we demonstrate the feasibility in the application of creating hidden images.

Understanding the chemical and electronic nature of molecules attached to semiconductors is of great importance in the study of molecule-based electronic devices. Resolving individual molecular orbitals using scanning tunnelling microscopy is a straightforward approach but remains challenging on the semiconductor surfaces because of their highly reactive dangling bonds. Here we show that hybridized molecular orbitals of pyridazine molecules covalently attached to Ge(100) surfaces can be resolved by scanning tunnelling microscopy. Pyridazine binds to Ge(100) through single/double dative bond(s) and presents two types of features with three and four lobes. These features resemble the lowest unoccupied molecular orbitals of free pyridazine, which are hybridized by the surface states in the adsorbed state. The adsorbing sites, binding mechanisms, orientations and electronic properties of the adsorbed molecules are convincingly determined. Our results indicate that orbital resolution of molecules covalently attached to semiconductors is accessible despite of their high reactivity.

Sulfur mono-layer doped In0.53Ga0.47As films were investigated by infrared spectroscopic ellipsometry. The complex dielectric function of doped layers shows free carrier response which can be described by a single Drude oscillator. Electrical resistivities, carrier relaxation times, and active carrier depths are obtained for the shallow n-In0.53Ga0.47As films. Our results indicate that sub-10 nm sulfur-doped layers with active carrier concentration as high as 1.7 × 1019 cm−3 were achieved. Sheet resistances estimated from infrared spectroscopic ellipsometry are in good agreement with those obtained by electrical methods. Published version

A tailored distribution of ion induced defects in p-type silicon allows subsequent electrochemical anodization to be modified in various ways. Here we describe how a low level of lattice amorphization induced by ion irradiation influences anodization. First, it superposes a chemical etching effect, which is observable at high fluences as a reduced height of a micromachined component. Second, at lower fluences, it greatly enhances electrochemical anodization by allowing a hole diffusion current to flow to the exposed surface. We present an anodization model, which explains all observed effects produced by light ions such as helium and heavy ions such as cesium over a wide range of fluences and irradiation geometries.

The oxygen ion-beam induced surface roughening observed during SIMS (secondary ion mass spectrometry) depth profiling of both the Si (001) and Si (111) surfaces has been studied by atomic force microscopy (AFM) as a function of both sputtered-depth and by varying the incident angles of the primary [Formula: see text] ion beam in the range of 44°–56°. Our result shows a surface orientation dependence of surface roughening whereby the transition roughening depth as well as the sputtering rate were observed to be greater in Si (111) than in Si (001). The orientation dependence was particularly obvious when increasing the incident angle of the [Formula: see text] ion beam from 44° to 56°. In addition, prominent elongated triangular and rectangular features are observed from sputtered Si (001) and Si (111) surfaces respectively at 56° incidence. These features may be associated with the formation of {111} and {001} facets on the sputtered Si (001) and Si (111) surfaces respectively.

Assessment of surface arsenic coverage during epitaxial growth of n-type doped Si is essential to the understanding of dopant incorporation and segregation behaviour. This particular problem is investigated using reflectance anisotropy spectroscopy (RAS) and reflection high energy electronic diffraction (RHEED) intensity oscillation technique as in situ probes. Initial measurement and analysis show that RAS is extremely sensitive to the presence of As on vicinal Si(0 0 1) surfaces and that the changes in the RA response at 2.6 eV are approximately linear to the As surface coverage produced by repeated cycles of low temperature adsorption of arsine followed by thermal desorption of hydrogen. For singular Si(0 0 1) surfaces where RAS cannot be applied, RHEED intensity oscillations were used to follow the change in As surface concentration during the growth of an Si capping layer over pre-deposited As. Using a model of the influence of As on the Si growth rate from Si hydrides, a surface As concentration profile was obtained and compared with that predicted by a model of As surface segregation. The results were in good agreement with secondary ion mass spectrometry measurements of a similar sample.

We have performed density functional calculations using cluster models containing two dimers along a dimer row of the Si(100)-(2×1) surface. Calculations were performed for half-monolayer, one-monolayer and two-monolayer coverages of hydrogen, fluorine and coadsorbed hydrogen and fluorine, the latter at a 1:1 stoichiometry. In all three cases, we found a rather strong attractive pairing-interaction between adsorbed atoms on the same dimer. We also found a weak attractive clustering-interaction between pairs of dimers with two adsorbed atoms each. For paired-fluorine dimers, this attractive clustering interaction is approximately 0.1 eV. In the case of coadsorption, we find that mixed dimers are energetically favored. Thus, our calculations suggest that, in the case of coadsorption, the hydrogen and fluorine atoms do not segregate.

Segregation of Ge during the formation of SiGe/Si heterojunctions in thin film epitaxial growth is perhaps the largest obstacle to the formation of the desirable atomically sharp interfaces in device structures. This process has been studied experimentally using reflection high-energy electron diffraction (RHEED), secondary ion mass spectrometry (SIMS) and numerically using a two-site exchange model. In this paper, the current two-site exchange model is modified to incorporate the growth process explicitly in the rate equation. This allows the extraction of analytical solutions in the dilute regime and provides a better insight into the influence of temperature and other parameters on the eventual composition profiles across the Si/SiGe heterojunction. Using numerical methods, the rate equation is solved for practical conditions and compared with experimental measurements by RHEED and SIMS. The dependence of composition profiles on the growth rate is demonstrated by the model and good agreements with experimental data without modifying the Gibbs energy of segregation.

Optical second harmonic generation (SHG) from Si surfaces has the advantage of surface sensitivity and can be applied in-situ during epitaxial growth process such as gas source molecular beam epitaxy (GSMBE) or chemical vapour depositions (CVD). This enables a number of processes occurring on the surface during epitaxial growth to be investigated in real time. Two processes are of particular interest in the epitxial growth of Si and SiGe structures: (1) oscillatory processes on the surface due to layer-by-layer growth for growth rate measurements and (2) surface segregation of Ge during formation of Si/SiGe heterojunctions. In this paper, we report on the oscillatory SHG response from the Si(0 0 1) surface during homo-epitaxy of Si and variations of SHG response across the SiGe/Si heterojunction during GSMBE growth. The oscillatory response is attributed to the domain coverage variation during layer-by-layer growth analogous to the origin of reflectance anisotropy oscillations. The changes in SHG during formation of SiGe/Si heterojunction is attributed to the gradual variation of surface Ge concentration due to segregation of Ge. The contribution from buried interfaces is shown to be negligible at the photon energy used.

In this paper, we make use of the hydrogen passivation concept to demonstrate that the pyrolysis of disilane (Si 2 H 6 ) results in two silicon atoms and all six hydrogen atoms remaining on the surface and silane (SiH 4 ) produces one silicon atom and four hydrogen atoms on the surface. The dominant reaction pathways are identified and implications for Si film growth are discussed. The adsorption kinetics and subsequent dissociation of silane, disilane and germane during gas source molecular beam epitaxy on the Si(0 0 1) surface are studied in situ using modulated beam mass spectrometry (MBMS), temperature programmed desorption (TPD) and reflection high-energy electron diffraction (RHEED). These were also combined with the growth of various epitaxial layers involving repeated cycles of silane or disilane adsorption and hydrogen desorption with the layer thicknesses determined ex situ using X-ray diffraction (XRD) rocking curve measurements and computer simulations.

Reflection high energy electron diffraction (RHEED) studies of the growth of GaAs on the (2×2) reconstructed GaAs(1 1 1)A surface show re-entrant behaviour in which the specular beam oscillations are present at high and low temperatures, but disappear at intermediate temperatures (∼400°C). Scanning tunnelling microscopy images indicate a two-dimensional growth mode for conditions when the oscillations are present, but a very different morphology in the re-entrant temperature window (∼10°C), with the surface becoming highly stepped with few two-dimensional islands present on the terraces. The results indicate a new mechanism for re-entrant behaviour which does not involve the formation of three-dimensional islands, but involves step propagation.

The adsorption kinetics and subsequent dissociation of disilane during gas source molecular beam epitaxy on Si(001) surface is studied in situ using modulated beam mass spectrometry, thermal desorption spectroscopy, reflection high energy electron diffraction and growth of epitaxial layers involving repeated cycles of disilane adsorption and hydrogen desorption. The dissociation of disilane molecules is found to occur sequentially and the major intermediate reaction products are ${\mathrm{SiH}}_{2}$ and SiH. At temperatures above $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ disilane dissociates readily to give two silicon atoms and all six hydrogen atoms and forms the monohydride $(2\ifmmode\times\else\texttimes\fi{}1)+(1\ifmmode\times\else\texttimes\fi{}2)$ surface. The formation of a Si-monohydride surface also passivates against further adsorption and dissociation of disilane. The main reaction pathway for the decomposition of ${\mathrm{SiH}}_{2}$ to SiH is identified and studied as a function of incident flux and growth temperature. This process is found to be controlled by the number of unsaturated dangling orbitals.

A quantitative estimate of the In/Ga surface concentration ratio in ultrathin (In, Ga)As strained layers, grown by molecular-beam epitaxy on a GaAs(001) substrate, is obtained using grazing incidence x-ray diffraction and diffuse-scattering measurements. The commensurate $2\ifmmode\times\else\texttimes\fi{}3$ reconstruction is interpreted as due to cation ordering in the surface unit cell, locking the surface composition at the value ${\mathrm{In}}_{2/3}{\mathrm{Ga}}_{1/3}\mathrm{As}.$ Incommensurate $2\ifmmode\times\else\texttimes\fi{}n$ reconstructions with $nl3$ $(ng3)$ are described in terms of indium-depleted (-enriched) surface layers characterized by a statistical distribution of faults in the ideal $2\ifmmode\times\else\texttimes\fi{}3$ atomic arrangement. Within a defined temperature range 450--490 \ifmmode^\circ\else\textdegree\fi{}C, a unique correspondence between the incommensurability parameter n and the indium surface fraction is established on the basis of a formulation of the diffuse scattering distribution.

The surface evolution of GaAs(1 1 0) thin films grown under different conditions by molecular beam epitaxy has been studied by reflection high-energy electron diffraction, in situ scanning tunnelling microscopy and ex situ Nomarski interference optical microscopy. For various conditions of substrate temperature and As/Ga flux ratio, STM and Nomarski microscope images of 0.5 and 1000 ML depositions have been correlated with different growth regimes identified by RHEED intensity oscillation studies. The surfaces exhibit very different morphologies specific to the growth regime studied. For the more important growth modes (monolayer-by-monolayer, bilayer-by-bilayer and step decoration), STM has also been used to follow the evolution of the surface in the early stages of homoepitaxy (0–2 ML). The results indicate that the smoothest films are grown under As-rich conditions and at low substrate temperatures, and this correlates with a step decoration growth mode.

We have studied the relationship between the surface morphology, Si doping behavior, and arsenic incorporation kinetics for GaAs(110) thin films grown on singular substrates by molecular beam epitaxy. To obtain films with good surface morphology, homoepitaxial growth requires low substrate temperatures and high As:Ga flux ratios. Under these conditions, the Si-doped layers exhibit n-type behavior. Growth at higher temperatures and lower As:Ga flux ratios produces films with a poorer morphology, the n-type layers become increasingly compensated, and p-type layers are eventually formed. This growth-related site switching behavior and corresponding variation in surface morphology can be attributed to a low arsenic surface population, a consequence of the small and temperature-dependent arsenic incorporation coefficient for growth on GaAs(110).

We have studied the relationship between the arsenic incorporation kinetics and the surface morphology and Si doping behaviour in GaAs(110) films grown by molecular beam epitaxy (MBE). The homoepitaxial growth of GaAs(110) requires low substrate temperatures and high As/Ga flux ratios to obtain films with good surface morphology, and under these conditions Si doped layers exhibit n-type behaviour. At higher growth temperatures and lower As/Ga flux ratios, the epitaxial films are highly faceted and the Si doped layers are p-type. We show that this growth related site switching behaviour and variation in surface morphology is due to a decrease in the As coverage arising from a small and temperature dependent incorporation coefficient of arsenic on this surface.

We have successfully demonstrated the formation of lateral p - n junctions via single-step Si-doped GaAs MBE growth over patterned (110) GaAs substrates. Current - voltage measurements indicate that the quality of the p - n junctions is position dependent; those located at the lower facet-flat boundary displayed superior rectification properties compared with the upper facet-flat junctions. Analysis of the lower junction forward characteristics revealed two types of current transport, each dominant over separate voltage ranges. Under low applied voltage, electron tunnelling via mid-gap states is proposed as the main mechanism, while diffusion and generation - recombination currents appear to dominate the device characteristics at higher voltages. In contrast, only tunnelling conduction was seen in the forward characteristics of the upper junctions. These effects are interpreted as due to compensated incorporation of Si in the region of the junctions, resulting from strong Ga diffusion from the (110) plane to the (100) surface. Series resistance values for the upper and lower junctions are consistent with this model, as are the dependences on growth temperature.

Scanning tunneling microscopy (STM) data are presented for all three phases of the GaAs(111)B surface prepared in situ by molecular beam epitaxy (MBE). The nature of the transitional phase is revealed for the first time by atomic resolution STM and shown to consist of structural units derived from both of the well known (2 × 2) and (√19 × √19)R23.4° surfaces. Individual ringlike units of the Ga-rich surface are stable structures on the (2 × 2) reconstructed surface. By varying the population of these features, the surface has access to a continuous stoichiometric pathway between the (2 × 2) and (√19 × √19)R23.4° reconstructions.

Reflection high-energy electron diffraction (RHEED) intensity oscillations have been used to obtain the arsenic incorporation coefficients for the homoepitaxial growth of GaAs on the (001) surface at different substrate temperatures. The incorporation coefficients of As 2 and As 4 are both temperature-dependent and saturate at maximum values of 1.0 and 0.5 at low temperatures. The results have been modelled using a kinetic scheme which assumes that the incorporation process using either As 2 or As 4 occurs via the formation of a molecularly adsorbed As∗ 2 precursor. The variation of the incorporation coefficients with temperature can be attributed to the fraction of As∗ 2 which does not participate in the incorporation process as the temperature is increased. The final step leading to growth and the formation of GaAs depends only on the incorporation of arsenic from this intermediate, and the implication is that the final incorporation step is independent of the arsenic species used in growth.

Reflection high energy electron diffraction (RHEED) intensity oscillations have been used to obtain the arsenic incorporation coefficient for the growth of GaAs on GaAs(110) by molecular beam epitaxy (MBE) using either As 2 or As 4 as the source of arsenic. In both cases, the incorporation coefficients decrease with increasing growth temperature, with the values obtained for As 2 being approximately twice those of As 4 . The results are modelled within a kinetic scheme based on the assumption that the incorporation processes are precursor mediated and in both cases involve a molecularly adsorbed As 2 intermediate. The difference in the activation energies for the desorption and incorporation of the As 2 intermediate is the same when using either As 2 or As 4 . The implication is that the final incorporation step in the growth of GaAs(110) is independent of the arsenic species used.

The coadsorption of hydrogen and transition metal dimers Fe2, Co2, Ni2, and FeCo on graphene is investigated using density functional theory calculations. Our work is motivated by observations that the magnetic moments of these transition metal dimers are large and that hydrogen adsorption partitions the graphene lattice into magnetic subdomains. Thus, we expect the magnetic dimers to interact strongly with the lattice. Our results show that the majority-spin direction of the lattice electronic states depends upon the dimer identity, the lattice spin polarization being in the same direction as the dimer spin polarization for Fe2 and FeCo, but opposite for Co2 and Ni2. We can understand this by examining the electronic density of states of the dimer and the lattice. We also show that coadsorption significantly increases the adsorption energies of both dimer and hydrogen leading to a more strongly-adsorbed dimer, while the bond length and magnetic moment of the upper dimer atom, the latter important for potential magnetic storage applications, are negligibly changed. Our work shows that the coadsorbed hydrogen and metal dimer interact over a long-range, this interaction being mediated by the hydrogen-induced spin-polarization of the graphene lattice. We obtain general insight into how the elemental identity of these magnetic dimers determines the spin-polarized states on the hydrogenated graphene lattice. These results could be important for potential applications of magnetic properties of decorated graphene lattices.

Germanium-tin (GeSn) n-channel MOSFETs with low temperature (370 °C) Si surface passivation were demonstrated for the first time. With Si passivation, a higher drive current is achieved for Ge0.976Sn0.024 nMOSFETs as compared to devices with GeSnO2 passivation. In addition, the effect of forming gas anneal (FGA) was investigated. FGA improves the gate dielectric interface quality, leading to improvement of subthreshold swing S.

We have synthesized high-quality, micrometer-sized, single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the I-V curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects-induced in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ∼5.3 × 10(2) % at 4 V) at a wavelength of 808 nm. The significant photoresponse and good responsitivity (∼3.5 A W(-1)) suggests its potential applications as photodetectors.

Polymerization of methyl methacrylate (MMA) has been studied in ternary microemulsions which were stabilized by tetradecyltrimethylammonium bromide (TTAB) or stearyltrimethylammonium chloride (STAC). The sizes of MMA-swollen polymer particles (Rh) increased continuously during polymerization. This is in contrast to that of the styrene system, where Rh increased very rapidly to a maximum and then decreased continuously towards a constant value. The continuous growth of PMMA particles at 60°C are discussed. The stability of PMMA latexes increased with increasing the hydrophobic chain length of the cationic surfactant used. Traces of the coagulations of PMMA particles in the TTAB system can be seen from TEM. © 1995 John Wiley & Sons, Inc.

On page 4090, B. Özyilmaz, C. H. Sow, and co-workers use a focused laser beam to modify the surface of a phosphorene device. With a simple focused laser beam, a part of the phosphorene can be scanned and converted into phosphorene-suboxide species, leaving behind a functional and active phosphorene-phosphorene suboxide junction in the device. Once the junction is formed, the photoresponsivity and photocurrent distribution of the device can be significantly altered with a qualitative difference in behavior. Photovoltaic-like behavior is observed, which is not found in the pristine sample.

The thermal stability and germanium–tin (Ge–Sn) interdiffusion properties were studied in epitaxial Ge/GeSn multiple-quantum-well (MQW) structure. No obvious interdiffusion was observed for annealing temperatures of 300 °C or below, while observable interdiffusion occurred for annealing temperatures of 380 °C and above. High-resolution x-ray diffraction was used to obtain the interdiffusion coefficient by analyzing the decrease rate of Ge/GeSn periodic satellite peaks. The interdiffusion coefficient is much higher, and the activation enthalpy of 1.21 eV is substantially lower in Ge/GeSn MQW structure than that previously reported in silicon–germanium (Si–Ge) systems. When the annealing temperature is increased to above 500 °C, Ge–Sn interdiffusion becomes severe. Some small pits appear on the surface, which should be related to Sn out-diffusion to the Ge cap layer, followed by Sn desorption from the top surface. This work provides insights into the Ge–Sn interdiffusion and Sn segregation behaviors in Ge/GeSn MQW structure, and the thermal budget that may be used for fabrication of devices comprising Ge/GeSn heterostructures.

The in-situ Ga doping technique was used to form heavily p-type doped germanium-tin (Ge1−xSnx) layers by molecular beam epitaxy, avoiding issues such as Sn precipitation and surface segregation at high annealing temperatures that are associated with the alternative implant and anneal approach. In this way, an electrically active Ga concentration of up to ∼3.2 × 1020 cm−3 can be realized for Ge1−xSnx. The impacts of varying the Ga concentration on the crystalline quality and the mobility of p-type Ge1−xSnx were investigated. High crystalline quality Ge0.915Sn0.085 can be realized with an active Ga concentration of up to ∼1.2 × 1020 cm−3. More than 98% of the Sn atoms are located on substitutional lattice sites, although the substitutionality of Sn in p-type Ge1−xSnx decreases with an increasing Ga concentration. When the Ga concentration introduced is higher than 3.2 × 1020 cm−3, excess Ga atoms cannot be substitutionally incorporated, and segregation of Ga and Sn towards the surface during growth is obser...

Ge0.94Sn0.06 films with high tensile strain were grown on strain-relaxed InyGa1−yP virtual substrates using solid-source molecular beam epitaxy. The in-plane tensile strain in the Ge0.94Sn0.06 film was varied by changing the In mole fraction in InxGa1−xP buffer layer. The tensile strained Ge0.94Sn0.06 films were investigated by transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. An in-plane tensile strain of up to 1% in the Ge0.94Sn0.06 was measured, which is much higher than that achieved using other buffer systems. Controlled thermal anneal experiment demonstrated that the strain was not relaxed for temperatures up to 500 °C. The band alignment of the tensile strained Ge0.94Sn0.06 on In0.77Ga0.23P was obtained by high resolution x-ray photoelectron spectroscopy. The Ge0.94Sn0.06/In0.77Ga0.23P interface was found to be of the type I band alignment, with a valence band offset of 0.31 ± 0.12 eV and a conduction band offset of 0.74 ± 0.12 eV.