Effect of Mn concentration and growth temperature on nanostructures and magnetic properties of Ge1−xMnx grown on Si

The nanostructures and magnetic properties of Ge_1−xMn_x thin films grown on Si substrates by molecular beam epitaxy, with different nominal Mn concentrations (1−4%) and different growth temperatures, have been systematically investigated by transmission electron microscopy and superconducting quantum interference device. It was discovered that when Ge_1−xMn_x thin films were grown at 70 °C, with increase in Mn concentration, Mn-rich tadpole shaped clusters started to nucleate at 1% Mn and become dominate in the entire film at 4% Mn. While for the thin films grown at 150 °C, tadpoles was firstly seen in the film with 1% Mn and subsequently Mn-rich secondary precipitates became dominant. The magnetic properties show specific features, which are mainly related to the nature and amount of Mn-rich clusters/precipitates within these thin films.     

Energetics of CdSxSe1−x quantum dots in borosilicate glasses

The thermodynamics of CdSe quantum dots embedded in a glass matrix is of great interest because of the numerous applications as optical materials. In this study, the energetics and stability of CdSe quantum dots in a borosilicate glass matrix is investigated as a function of size using high-temperature oxide melt solution calorimetry. CdS_0.1Se_0.9 nanoparticles (1-40 nm) embedded in glass were analyzed by photoluminescence spectroscopy, electron microprobe, X-ray fluorescence, high-energy synchrotron X-ray diffraction, and (scanning) transmission electron microscopy using both electron energy loss and energy dispersive X-ray spectroscopy. As CdSe particles coarsen, their heat of formation becomes more exothermic. The interfacial energy of CdSe QDs embedded in a borosilicate glass, determined from the slope of enthalpy of drop solution versus calculated surface area, is 0.56 ± 0.01 J/m².     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

Self-formation of dielectric layer containing CoSi2 nanocrystals by plasma-enhanced atomic layer deposition

Dielectric layer containing CoSi₂ nanocrystals was directly fabricated by plasma-enhanced atomic layer deposition using CoCp₂ and NH₃ plasma mixed with SiH₄ without annealing process. Synchrotron radiation X-ray diffraction and X-ray photoelectron spectroscopy results confirmed the formation of CoSi₂ nanocrystal. The gate stack composed of dielectric layer containing CoSi₂ nanocrystals with ALD HfO₂ capping layer together with Ru metal gate was analyzed by capacitance–voltage (C–V) measurement. Large hysteresis of C–V curves indicated charge trap effects of CoSi₂ nanocrystals. The current process provides simple route for the fabrication of nanocrystal memory compatible with the current Si device unit processes.     

Structural characterization of CdTe layers grown on (0 0 0 1) sapphire by MOCVD

Crystalline ZnO nanoparticles were synthesized on Si substrates with or without a Au catalyst by a chemical vapor deposition (CVD) method using ZnS as the source material. The average sizes are in the range of 40–200 nm and the densities of 10⁴–10¹⁰ cm⁻². In the absence of an Au catalyst, the average nanoparticle size firstly decreases and then increases with increasing substrate temperature while the nanoparticle density decreases as the substrate temperature increases. In the presence of an Au catalyst, ZnO nanoparticles only grow when the substrate temperature is higher than 300°C and the higher the substrate temperature the denser the nanoparticles are deposited. The density of the ZnO nanoparticles grown on a Si (1 1 1) substrate is higher than that on a Si (1 0 0) substrate with or without Au catalyst.     

Raman scattering in amorphous films of In1−xSex alloys

Raman scattering (RS) in amorphous films of In_1−xSe_x with 0.67±x?0.38 has been studied in backscattering geometry with the use of a microscope. Recorded RS spectra are revealing a mixed vibrational density-of-states and molecular character. The spectra spread from the Rayleigh line up to 200-250 cm⁻¹. The bands superimposed on the continua are related to zone center modes of the relevant crystal counterpart, Se-Se or In-In vibrations. The RS spectra suggest the structure of the In_1−xSe_x alloys to be the continuous random network built up of In centered tetrahedral clusters with In and Se atoms at the corners. The structure of the Se-rich alloys is similar to 4-2 networks with dominant InSe_4/2 clusters and two-fold coordination of Se bridging atoms. That of the In-rich alloys is expected to resemble 4-3 network with rather strong involvement of In atoms at corner of the In-centered tetrahedral clusters and Se atoms being linked to three In ones.     

Temperature effects during the growth of InxGa1−xN films through the whole compositional range by plasma-assisted molecular beam epitaxy

Substrate temperature rises of over 200 °C have been observed for growth of InN and In-rich InGaN on GaAs substrates. We present a model to show that it is not the narrow bandgap that is responsible for the large temperature rises observed during growth of InN, but the large bulk background carrier concentration. We also show how the substrate temperature rise during growth increases as a function of increasing indium composition and the effects of controlling the substrate temperature on film quality.     

Study on the properties of CuInSe2 ingots grown from the melt using stoichiometric and non-stoichiometric charges

CuInSe₂ (CIS) ingots have been prepared by direct reaction of stoichiometric and non-stoichiometric proportions of high-purity Cu, In and Se. Two approaches, namely the one-ampoule process (quartz crucible) and two-ampoule process (graphite crucible) were investigated to grow the crystals, using starting charges with excess copper, and (nearly stoichiometric and with excess indium), respectively. The effect of deviation from stoichiometry in the charge on the physical properties of the resulting polycrystals is presented. Compositional analysis of the best part of the ingots with starting metals ratio (Cu/In) greater than or equal to 1 showed that the matrix preserved the original character of the charge and evidenced that the CIS chalcopyrite structure, -CIS, tolerates well a large In excess. In contrast, the composition of the crystal prepared with a 10% Cu excess was nearly-stoichiometric, with chemical images revealing the formation of heterogeneous phases besides -CIS. The inclusions precipitation was found to increase toward the ingot base. Interestingly, powder X-ray diffraction measurements revealed the presence of secondary phases rather in all the samples. The corresponding diffraction peaks were however few and very weak, with intensities of less than 3% the maximum value recorded for the CIS (1 1 2) plane.     

Stacking pattern of multi-layer InAs quantum wires embedded in In0.53Ga0.47−xAlxAs matrix layers grown lattice-matched on InP substrate

Multi-layer InAs quantum wires were grown on, and embedded in In_0.53Ga_0.47−xAl_xAs (with x=0, 0.1, 0.3 and 0.48) barrier/spacer layers lattice matched to an InP substrate. Correlated stacking of the quantum wire arrays were observed with aluminum content of 0 and 0.1. The quantum wire stacks became anti-correlated as the aluminum content was increased to 0.3 and 0.48. The origin of such stacking pattern variation was investigated by finite element calculations of the chemical potential distribution for indium on the growth front surface of the capping spacer layer. It is shown that the stacking pattern transition is determined by the combined effect of strain and surface morphology on the growth front of the spacer layers.     

Study of effects of polishing and etching processes on Cd1−xZnxTe surface quality

The effect of surface preparation on CdZnTe properties was investigated. Surface etching using bromine solutions enhances Te elemental composition, resulting in a Te rich surface layer that is prone to oxidize. This oxidation degrades the performance of the fabricated CZT gamma detector. Roughness results were identical for samples polished with 1 and 3 μm and subsequently etched in 2% Br-MeOH. The optimal concentration of etching was 2% Br-MeOH.     

Structural and optical properties of CuGaSe2 layers grown by the hot wall epitaxy method

Copper gallium selenide (CuGaSe₂, CGS) layers were grown by the hot wall epitaxy method. The optimum temperatures of the substrate and source for growth turned out to be 450 and 610 °C, respectively. The CGS layers were epitaxially grown along the 1 1 0 direction and consisted of Ga-rich components indicating the slight stoichiometric deviations. Based on the absorption measurement, the band-gap variation of CGS was well interpreted by the Varshni's equation. The band-gap energies at low temperatures, however, had a higher value than those of other CGS. It suggests that the band-gap increase is influenced by the slightly Ga-rich composition. From the low-temperature photoluminescence experiment, sharp and intensive free- and bound-exciton peaks were observed. By analyzing these emissions, a band diagram of the observed optical transitions was obtained. From the solar cell measurement, an 11.17% efficiency on the n-CdS/p-CGS junction was achieved.     

A simple synthesis of large-scale SiC–SiO2 nanocables by using thermal decomposition of methanol: Structure, FTIR, Raman and PL characterization

Large-scale SiC nanocables were synthesized on a Ni(NO₃)₂-catalyzed Si substrate by using a simple and cheap method based on thermal decomposition of methanol. Based on X-ray diffraction and high-magnification transmission electron microscopy, the as-grown nanocables consisted of crystalline SiC cores and amorphous SiO₂ shells. The diameters of SiC cores were 5.7–10 nm and the thicknesses of SiO₂ shells were 9–20 nm. Dividing of nanocables was observed and its origin was investigated. An asymmetric feature of SiC TO band with a shoulder at the high-frequency side was attributed to the contribution of SiC TO mode. The nanocables displayed strong violet–blue emission. A possible growth mechanism was proposed.     

Specific features of formation and propagation of 60° and 90° misfit dislocations in GexSi1−x/Si films with x>0.4

The dislocation structure at the initial stage of relaxation of Ge_xSi_1−x films (x∼0.4–0.8) grown on Si (0 0 1) substrates tilted at 6° to the nearest (1 1 1) plane is studied. The use of Si substrates tilted away from the exact (0 0 1) orientation for epitaxial growth of Ge_xSi_1−x films (x≥0.4) allowed finding the basic mechanism of formation of edge dislocations that eliminate the mismatch stresses. Though the edge dislocations are defined as sessile dislocations, they are formed in accordance with the slipping mechanism proposed previously by Kvam et al. (1990). It is highly probable that a 60° misfit dislocation (MD) propagating by the slipping mechanism provokes the nucleation of a complementary 60° MD slipping in a mirror-like tilted plane (1 1 1). The reaction between these dislocations leads to the formation of an edge MD that ensures more effective reconciliation of the discrepancy. Comparative estimation of the slip velocities of the primary and induced 60° MDs and also of the resultant 90° MD is fulfilled. The slip velocity of the induced 60° MD is appreciably greater than the velocity of the primary 60° MD. Therefore, the induced MD “catches up” with the second front of the primary MD, thus forming a 90° MD propagating to both sides due to slipping of the 60° MDs forming it. The propagation velocity of the 90° MD is also greater than the slip velocity of a single 60° MD. For these reasons, 90° MDs under certain conditions that favor their formation and propagation can become the main defects responsible for plastic relaxation of GeSi films close to Ge in terms of their composition.     

Ab initio calculation of vibrational frequencies of GexPxS1−2x glass

The vibrational frequencies of GeS₄, GeP₄, Ge₂S₆, GeP₃, Ge₃P, Ge₂P₂, P₂S₂, P₃S, P₄S₃, α-P₄S₄, β-P₄S₄, α-P₄S₅, β-P₄S₅, P₄S₇, P₄S₉ and P₄S₁₀ are theoretically computed from the first principles. The Raman frequencies of Ge_xP_xS_1−2x glass are obtained for x varying from 0.05 to 0.019. The computed fundamental frequencies of clusters are compared with those experimentally found. In this way, we are able to identify the vibrating clusters in the real glass. The clusters identified in the real glass are found to be Ge₂P₂, P₄S₃, α-P₄S₄, β-P₄S₄, β-P₄S₅, P₄S₇, P₄S₉, β-P₄S₅, Ge₂S₆, Ge₃P.     

Catalytic growth of In2O3 nanobelts by vapor transport

Indium oxide (In₂O₃) nanobelts have been fabricated by thermal evaporation of metallic indium powders with the assistance of Au catalysts. The as-synthesized nanobelts are single-crystalline In₂O₃ with cubic structure, and usually tens of nanometers in thickness, tens to hundreds of nanometers in width, and several hundreds of micrometers in length. The room temperature photoluminescence spectrum of In₂O₃ nanobelts features a broad emission band at 620 nm, which could be attributed to oxygen deficiencies in the as-synthesized belts. The formation of In₂O₃ nanobelts follows a catalyst-assistant vapor—liquid–-solid growth mechanism, which enables the controlled growth of individual belts on predetermined sites.     

Study of group-III binary and ternary nitrides using X-ray absorption fine structure measurements

It is demonstrated that the NEXAFS spectra are a “fingerprint” of the symmetry and the composition of the binary nitrides GaN, AlN and InN, as well as of their ternary alloys In_0.16Ga_0.84N and Al_yGa_1−yN. From the angular dependence of the N-K-edge NEXAFS spectra, the hexagonal symmetry of the under study compounds is deduced and the (p_x, p_y) or p_z character of the final state is identified. The energy position of the absorption edge (E_abs) of the binary compounds GaN, AlN and InN is found to red-shift linearly with the atomic number of the cation. The E_abs of the Al_yGa_1−yN alloys takes values in between those corresponding to the parent compounds AlN and GaN. Contrary to that, the E_abs of In_0.16Ga_0.84N is red-shifted relative to that of GaN and InN, probably due to ordering and/or phase separation phenomena. The EXAFS analysis results reveal that the first nearest-neighbour shell around the N atom, which consists of Ga atoms, is distorted in both GaN and Al_xGa_1−xN for x<0.5.     

Anisotropic vapor phase growth of Ga2O3 crystalline nanobelts

Ga₂O₃ nanobelts were synthesized by gas reaction at high temperature in the presence of oxygen in ammonia. X-ray diffraction and chemical microanalysis revealed that the nanostructures were Ga₂O₃ with the monoclinic structure. Electron microscopy study indicated the nanobelts were single crystalline with broad (0 1 0) crystallographic planes. The nanostructures grew anisotropically with the growth direction of . Statistical analysis of the anisotropic morphology of the nanobelts and electron microscopy investigation of the nanobelt tips indicated that both vapor–solid and vapor–liquid–solid mechanisms controlled the growth process. The anisotropic nature of crystallographic morphology is explained in terms of surface energy.     

Intermediate-range order in glassy GexSe1−x around the stiffness transition composition

X-ray scattering measurements on glassy Ge_xSe_1−x were performed in a concentration range x=0.07-0.333 in fine steps of 0.005-0.05, in order to explore the relation between the intermediate-range order (IRO) and the stiffness transition in this glassy system. The oscillations beyond the first peak around 20.5 nm⁻¹ in the structure factor, S(Q), remain almost unchanged or damp very slowly with decreasing the Ge concentration x, suggesting the preserve of the short-range order. On the other hand, the pre-peak around 10-12 nm⁻¹, indicating the existence of IRO, systematically changes with decreasing x; its Q position shifts towards the higher Q values and its area decreases. Especially near the onset of the stiffness transition, x=0.20, the peak position starts to deviate from the linear relation. The origin of the pre-peak is discussed in connection with results of a recent anomalous X-ray scattering measurement indicating two contributions of Ge-Ge and Se-Se correlations. Around the onset composition of the stiffness transition, the area of the pre-peak associated with the Ge-Ge correlation has a plateau-like gradual decrease with x followed by a rapid decrease at x<0.18, which is in good agreement with Raman data.     

The incorporation behavior of arsenic and antimony in GaAsSb/GaAs grown by solid source molecular beam epitaxy

GaAsSb ternary epitaxial layers were grown on GaAs (0 0 1) substrate in various Sb₄/As₂ flux ratios by solid source molecular beam epitaxy. The alloy compositions of GaAs_1−ySb_y were inferred using high-resolution X-ray symmetric (0 0 4) and asymmetric (2 2 4) glance exit diffraction. The non-equilibrium thermodynamic model is used to explain the different incorporation behavior between the Sb₄ and As₂ under the assumption that one incident Sb₄ molecule produces one active Sb₂ molecule. It is inferred that the activation energy of Sb₄ dissociation is about 0.46 eV. The calculated results for the incorporation efficiency of group V are in good agreement with the experimental data.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.