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.     

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.     

A study on structural formation and optical property of wide band-gap Be0.2Zn0.8O layers grown by RF magnetron co-sputtering system

Wide band-gap BeZnO layers were grown on Al₂O₃ (0 0 0 1) substrate using radio-frequency magnetron co-sputtering. The rate of Be_xZn_1−xO crystallized as a hexagonal structure was x=0.2. From the X-ray photoelectron spectroscopy measurement, the O–Zn bonds relating the crystal structure and the Be–O bonds related to the deviation of the stoichiometry in the BeZnO layer were caught at 530.4 and 531.7 eV in the O 1s spectrum, respectively. Thus, the observance on the Be 1s peak of 113.2 eV associated with the bonding Be–O indicates that the sputtered Be atoms are substituted for the host-lattice site in ZnO. This Be–O bonding shows a relatively low intense and broadening spectrum caused by large fluctuation of Be content in the BeZnO layer. From the photoluminescence and transmittance measurement, the free exciton and the neutral donor-bound exciton (D⁰, X) emissions were observed at 3.7692 and 3.7313 eV, respectively, and an average transmittance rate over 95% was achieved in a wide ultraviolet (UV)–visible region. Also, the binding energy for the (D⁰, X) emission was extracted to be 37.9 meV. Through the wide band-gap material BeZnO, we may open some possibilities for fabricating a ZnO-based UV light-emitting diode to be utilized as a barrier layer comprised of the ZnO/BeZnO quantum well structure and/or an UV light emitting material itself.     

Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.     

Thermodynamics of the Al–Ga–N2 system

The thermodynamic properties of the Al–Ga–N₂ system under high N₂ pressure up to 10 kbar and 1800 °C are investigated. On the basis of the experimental p–T growth conditions for (Al,Ga)N crystals, the standard Gibbs free energy as well as the standard enthalpy and entropy of formation of the Al_xGa_1−xN crystals as a function of composition x were calculated. The a_N2–T and x–T phase diagrams for (Al,Ga)N are presented.     

On the isothermal closed space sublimation growth of CdSe using a mixed source for selenium

The use of a selenium–tellurium (SeTe) mixed source in the isothermal close space sublimation growth of CdSe epilayers is considered. The epitaxial growth was performed in flowing helium by sequential exposures of the substrate to vapors of the mixed SeTe source and elemental cadmium at temperatures within 350–410 °C. In spite of the mixed source (proposed to decrease the partial pressure of Se), tellurium incorporation was small and CdSe_xTe_(1−x) (x∼0.98) epilayers were obtained. X-ray diffraction reciprocal space mapping shows the existence of hexagonal inclusions mainly on the (1 1 1) facets of the cubic phase. Material deposition on areas of the graphite crucible exposed to the sources, contamination of the Cd source and large growth rates suggest the existence of a selenium transport process via graphite. This transport might be the result of the combination of selenium deposition on graphite with a subsequent activated desorption of selenium under cadmium exposure. It affects Cd source purity and growth kinetic bringing on a modification of the usual atomic layer deposition regimen; however, a reproducible growth rate of the epilayers was obtained.     

Boron content dependance of magnetoimpedance in Fe91−xZr5BxNb4 alloys

A systematic investigation of the influence of B content on the magnetoimpedance (MI) effect in melt-spun Fe_91−xZr₅B_xNb₄ (FZBN, 0 ? x ? 30) ribbons has been performed within a frequency range, f ∼ 310-1110 kHz and under a varying dc magnetic field (H_dc) up to 70 Oe. The MI effect is not observed in the sample with x ? 5 but within the range 8 ? x ? 30. A distinct MI effect has been observed with a maximum change of 180% at around 1.1 MHz in the sample with x = 20, coincident with a saturation magnetic field of 66 Oe and a field sensitivity of about 7%/Oe. Magnetic measurements reveal that the MI effect and B content dependence of the effect are closely related to coercivity of the FZBN alloy series, except for the sample with 20 at.%. The drastic MI ratio observed in the sample with x = 20 is ascribed to its special microstructure. The mechanism of the MI effect in FZBN alloys and of the significant MI value appearing at a B content of x = 20 is discussed in this paper.     

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.     

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.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

A sharp thermally reversing window in As45Te55−xIx chalcohalide glasses revealed by alternating differential scanning calorimetry and photo thermal deflection studies

The composition dependence of different thermal parameters such as glass transition temperature, non-reversing enthalpy, thermal diffusivity etc., of bulk As₄₅Te_55−xI_x chalcohalide glasses (3 ? x ? 10), has been evaluated using the temperature modulated Alternating Differential Scanning Calorimetry (ADSC) and Photo Thermal Deflection (PTD) studies. It is found that there is not much variation in the glass transition temperature of As₄₅Te_55−xI_x glasses, even though there is a wide variation in the average coordination number . This observation has been understood on the basis that the variation in glass transition temperature of network glasses is dictated by the variation in average bond energy rather than . Further, it is found that both the non-reversing enthalpy (ΔH_nr) and the thermal diffusivity (α) exhibit a sharp minimum at a composition x = 6. A broad hump is also seen in glass transition and crystallization temperatures in the composition range 5 ? x ? 7. The results obtained clearly indicate a sharp thermally reversing window in As₄₅Te_55−xI_x chalcohalide glasses around the composition x = 6.     

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.     

Growth and optical properties of high-density InN nanodots

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10¹⁰ cm⁻² at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH₃ flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V_In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E_a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.     

Thermal-induced phase transition and assembly of hexagonal metastable In2O3 nanocrystals: A new approach to In2O3 functional materials

This paper reports on the thermal-induced performance of hexagonal metastable In₂O₃ nanocrystals involving in phase transition and assembly, with particular emphasis on the assembly for the preparation of functional materials. For In₂O₃ nanocrystals, the metastable phase was found to be thermally unstable and transform to cubic phase when temperature was higher than 600 °C, accompanied by assembly as well as evolution of optical properties, but the two polymorphs coexisted at the temperature ranging from 600 to 900 °C, during which the content of product phase and crystal size gradually increased upon increasing temperature. The assembly of In₂O₃ nanocrystals can be developed to fabricate In₂O₃ functional materials, such as various ceramic materials, or even desired nano- or micro-structures, by using metastable In₂O₃ nanocrystals as precursors or building blocks. The electrical resistivity of In₂O₃ conductive film fabricated by a hot-pressing route was as low as 3.72×10⁻³ Ω cm, close to that of In₂O₃ single crystal, which is important for In₂O₃ that is always used as conductive materials. The findings should be of importance for both the wide applications of In₂O₃ in optical and electronic devices and theoretical investigations on crystal structures.     

Structural and optical properties of Zn(Mg,Cd)O alloy films grown by remote-plasma-enhanced MOCVD

We report the structural and optical properties of wurtzite-structure Zn(Mg,Cd)O ternary alloys. Wurtzite (0 0 0 1) Zn_1−xCd_xO and Mg_yZn_1−yO films were grown on (11–20) sapphire substrates using remote-plasma-enhanced metalorganic chemical vapor deposition. The large bowing parameters of Zn_1−xCd_xO and Mg_yZn_1−yO ternary alloys are 3.0 and 3.5, respectively, which reflects the large difference of each binary’s electronegativity. We have analyzed the broadening of photoluminescence (PL) in Zn(Mg,Cd)O alloys on alloy content by taking into account the statistical alloy fluctuation and the localization of the exciton, and have clarified that the localization of the exciton strongly affects to PL full-width at half-maximum (FWHM) in Zn(Mg,Cd)O alloys. The alloy broadenings in steady-state PL of Zn(Mg,Cd)O alloys are in good agreement with the calculated tendency by the theoretical model based on the statistical alloy fluctuation, while PL FWHM of Zn_1−xCd_xO is three times larger than the calculated results. Moreover, as another way to confirm alloy broadening, we also have done time-resolved PL measurements and derived the localized depth of the exciton in ZnO-based system, indicating a good agreement with the tendency of PL FWHM broadening.     

Sodium tracer diffusion in glasses of the type (CaO·Al2O3)x(2 SiO2)1−x

Tracer diffusion coefficients of the radioactive isotope Na-22 were measured in glasses of the type (CaO·Al₂O₃)_x(2 SiO₂)_1−x to study the diffusion of sodium as a function of glass composition, x, temperature and initial water content. The diffusion of Na-22 in glasses diffusion-annealed in dry air can always be well described by a single tracer diffusion coefficient, but sometimes not in samples annealed in common air. It was found that the sodium tracer diffusion coefficient decreases by about six orders of magnitude when the glass composition x changes from 0 to 0.75 at 800 °C. The temperature dependence of the diffusion of sodium seems to decrease as the silica content increases. Variations of the initial water content in some of the glasses investigated did not very significantly influence the rate of the tracer diffusion of sodium.     

Coexistence properties of phase separation and CuPt-ordering in InGaAsP grown on GaAs substrates by organometallic vapor phase epitaxy

CuPt-ordering and phase separation were directly investigated in In_1-xGa_xAs_yP_1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 °C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 °C showed less phase separation than the 690 °C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 °C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2×4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface.     

Growth and Raman spectroscopic characterization of As4S4 (II) single crystals

As described by Kutoglu (1976 [16]), single crystals of As₄S₄ (II) phase have been grown using a new two-step synthesis that drastically increases the reproducibility that is attainable in synthetic experiments. First, through photo-induced phase transformation, pararealgar powder is prepared as a precursor instead of AsS melt. Then it is dissolved and recrystallized from CS₂ solvent. Results show that single crystals of the As₄S₄ (II) phase were obtained reproducibly through the dissolution–recrystallization process. Single crystals of As₄S₄ (II) obtained using this method were translucent and showed a uniform yellow-orange color. The crystal exhibits a platelet-like shape as a thin film with well-developed faces (0 1 0) and (0 1¯ 0). The grown crystals are as large as 0.50×0.50×0.01 mm. They were characterized using powder and single crystal X-ray diffraction techniques to confirm the phase identification and the lattice parameters. The As₄S₄ (II) phase crystallizes in monoclinic system with cell parameters a=11.202(4) Å, b=9.954(4) Å, c=7.142(4) Å, β=92.81(4)°, V=795.4(6) Å³, which shows good agreement with the former value. Raman spectroscopic studies elucidated the behavior of the substance and the relation among phases of tetra-arsenic tetrasulfide.     

On the Bridgman growth of lead–tin selenide crystals with uniform tin distribution

The application of T–x–y phase diagram for the control of the crystal composition of alloy crystals during Bridgman growth is illustrated using (Pb_1−xSn_x)_1−δSe_1+δ solid solutions as an example. Within the framework of the proposed approach, the axial segregation can be minimised if the feed composition is chosen in the regions of phase diagram where the iso-mole-fraction lines are perpendicular to isotherms. For the system Pb–Sn–Se the component distribution in crystals grown from a melt with δ=0, a Se-rich melt (δ>0) and a metal-rich melt (δ<0) are discussed. Uniform crystals with x=(0.03–0.15)±0.01 can be obtained from Se-rich melt (Se enrichment is 12–5 at%Se, T=1280–1200 K). The use of metal-rich melt allows obtaining crystals with moderate segregation of 3–5 mol% SnSe for x=0.13–0.23 at 3–9 at% of metal enrichment and T=1270–1200 K and for x=0.35–0.45 at 25–35 at% of metal enrichment and T=1120–1050 K.     

Thermal stability of thin InGaN films on GaN

The thermal stability of ∼200-nm-thick InGaN thin films on GaN was investigated using isothermal and isochronal post-growth anneals. The In_xGa_1−xN films (x=0.08–0.18) were annealed in N₂ at 600–1000 °C for 15–60 min, and the resulting film degradation was monitored using X-ray diffraction (XRD) and photoluminescence (PL) measurements. As expected, films with higher indium concentration showed more evidence for decomposition than the samples with lower indium concentration. Also for each alloy composition, decreases in the PL intensity were observed starting at much lower temperatures compared to decreases in the XRD intensity. This difference in sensitivity of the PL and XRD techniques to the InGaN decomposition suggest that defects that quench luminescence are generated prior to the onset of structural decomposition. For the higher indium concentration films, the bulk decomposition proceeds by forming metallic indium and gallium regions as observed by XRD. For the 18% indium concentration film, measurement of the temperature-dependent InGaN decomposition yields an activation energy, E_A, of 0.87±0.07 eV, which is similar to the E_A for bulk InN decomposition. The InGaN integrated XRD signal of the 18% film displays an exponential decrease vs. time, implying InGaN decomposition proceeds via a first-order reaction mechanism.