Correlation between morphological, electrical and optical properties of GaN at all stages of MOVPE Si/N treatment growth

GaN has been grown using Si/N treatment growth by MOVPE on sapphire (0001) in a home-made vertical reactor. The growth was monitored by in situ laser reflectometry. The morphological, electrical and optical properties of GaN are investigated at all the growth stages. To this aim, the growth was interrupted at different stages. The obtained samples are ex situ characterized by scanning electron microscopy (SEM), room temperature Van der Pauw–Hall electrical transport and low temperature (13 K) photoluminescence (PL) measurements. The SEM images show clearly the coalescence process. A smooth surface is obtained for a fully coalesced layer. During the coalescence process, the electron concentration (n) and mobility (μ) vary from 2×10¹⁹ cm⁻³ to 2×10¹⁷ cm⁻³ and 12 cm²/V s–440 cm²/V s, respectively. The PL maxima shift to higher energy and the FWHM decreases to about 4 meV. A correlation between PL spectra and Hall effect measurements is made. We show that the FWHM follows a n^2/3 power law for n above 10¹⁸ cm⁻³.     

Pyroelectric properties in three phases coexistence Pb[(Mn0.33Nb0.67)0.5(Mn0.33Sb0.67)0.5]0.08(ZrxTi1−x)0.92O3 lead ceramics

The bulk dense Pb[(Mn_0.33Nb_0.67)_0.5(Mn_0.33Sb_0.67)_0.5]_0.08(Zr_xTi_1−x)_0.92O₃ pyroelectric ceramics have been successfully prepared by the conventional solid method. The effect of three phases coexistence in the ceramics is studied. When x = 0.95 and 0.85 in the ceramics, the maximum pyroelectric coefficient peaks appear at 23 °C and 45 °C, and the maximum values are 26.5 × 10⁻⁴ C/m² °C and 25.5 × 10⁻⁴ C/m² °C, respectively. The maximum pyroelectric coefficient appears large while the peaks widths are small. When the two kinds of ceramic powders mixed with the mol ratio of 2:1, the pyroelectric coefficient of the ceramics is above 10.0 × 10⁻⁴ C/m² °C in a broad temperature range from 20 °C to 55 °C. The possible physical mechanism of the temperature broadened phenomenon is briefly discussed.     

Effects of deposition temperature on the surface roughness and the nonlinear optical susceptibility of sprayed deposited ZnO:Zr thin films

Zirconium doped zinc oxide thin films were deposited by reactive chemical pulverization spray pyrolysis technique on heated glass substrates at 400 °C, 450 °C and 500 °C using zinc and zirconium chlorides as precursors. The effect of zirconium dopant and surface roughness on the nonlinear optical properties was investigated using atomic force microscopy (AFM) and third harmonic generation (THG). The best value of susceptibility χ⁽³⁾ was obtained from the doped films with less roughness. A strong third order nonlinear optical susceptibility χ⁽³⁾ = 20.49 × 10⁻¹² (esu) of the studied films was found for the 5% doped sample at 450 °C.     

Growth and characterization of DyP/GaAs and DyAs/GaAs-based heterostructures and superlattices

Details of the structural and electrical properties of epitaxial DyP/GaAs and DyAs/GaAs is reported. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01%. DyAs, on the other hand, has a mismatch of nearly 2.4%. Both DyP and DyAs have been grown by solid source MBE using custom designed group V thermal cracker cells and group III high-temperature effusion cells. High-quality DyP and DyAs epilayers, as determined by XRD, TEM, and AFM analysis, were obtained for growth temperatures ranging from 500°C to 600°C with growth rates between 0.5 and 0.7 μm/h. The DyP epilayers are n-type with measured electron concentrations of the order of 3×10²⁰ to 4×10²⁰ cm⁻³, with room temperature mobilities of 250–300 cm²/V s, and with a barrier height of 0.75 eV to GaAs. The DyAs epilayers are also n-type with concentration of 1×10²¹ to 2×10²¹ cm⁻³, with mobilities between 25 and 40 cm²/V s. DyP is stable in air with no apparent oxidation taking place, even after months of ambient exposure to untreated air.     

Electrical conductivity of Ag/Na ion-exchanged titanosilicate glasses

The Ag₂O–TiO₂–SiO₂ glasses were prepared by Ag⁺/Na⁺ ion-exchange method from Na₂O–TiO₂–SiO₂ glasses at 380–450 °C below their glass transition temperatures (T_g), and their electrical conductivities were investigated as functions of TiO₂ content and the ion-exchange ratio (Ag/(Ag+Na)). In a series of glasses 20R₂O·xTiO₂·(80−x)SiO₂ with x=10, 20, 30 and 40 in mol%, the electrical conductivities at 200 °C of the fully ion-exchanged glasses of R=Ag were in the order of 10⁻⁵ or 10⁻⁴ S cm⁻¹ and were 1 or 2 orders of magnitude higher than those of the initial glasses of R=Na. The glass of x=30 exhibited the highest increase of conductivity from 3.8×10⁻⁷ to 1.3×10⁻⁴ S cm⁻¹ at 200 °C by Ag⁺/Na⁺ ion exchange among them. When the ion-exchange ratio was changed in 20R₂O·30TiO₂·50SiO₂ system, the electrical conductivity at 200 °C exhibited a minimum value of 7.6×10⁻⁸ S cm⁻¹ around Ag/(Ag+Na)=0.3 and increased steeply in the region of Ag/(Ag+Na)=0.5–1.0. When the ion-exchange temperature was changed from 450 to 400 °C, the conductivity of the ion-exchanged glass of x=30 decreased. The infrared spectroscopy measurement revealed that the ion-exchange temperature of 450 °C induced a structural change in the glass of x=30. The T_g of the fully ion-exchanged glass of x=30 was 498 °C. It was suggested that the incorporated silver ions changed the average coordination number of titanium ions to form higher ion-conducting pathway and resulted in high conductivity in the titanosilicate glasses.     

Preparation and electrical properties of a pyrochlore-related Ce2Zr2O8−x phase

A pyrochlore-related Ce₂Zr₂O_8−x phase has been prepared in a reduction reoxidation process from Ce_0.5Zr_0.5O₂ powders. Ce₂Zr₂O_8−x, based on a cubic symmetry with a=1.053 nm, decomposes in nitrogen at 800 °C, but remains stable up to 900 °C in air. It shows mixed oxygen ionic and electronic conductivity. The bulk conductivity at 700 °C is 4×10⁻⁴ S cm⁻¹ in air and 1×10⁻² S cm⁻¹ in nitrogen, and the activation energy is 1.27 eV in air. In nitrogen, the Arrhenius law is not obeyed, and a curved plot was obtained from 400 to 700 °C; then, the conductivity decreased rapidly due to the thermal decomposition of Ce₂Zr₂O_8−x.     

Structural and optical properties of nearly stress-free m-plane ZnO film on (1 0 0) γ-LiAlO2 with a GaN buffer layer by metal-organic chemical vapor deposition

m-plane ZnO film was epitaxially deposited on (1 0 0) γ-LiAlO₂ by metal-organic chemical vapor deposition at 600 °C with a GaN buffer layer. The epitaxial relationships between ZnO and GaN, GaN and (1 0 0) γ-LiAlO₂ were determined by X-ray diffraction Φ-scans. There exhibits very small decrease for the E₂ mode shift (0.3 cm⁻¹) of ZnO in the Raman spectrum, which indicates the epitaxial ZnO film was under a slight tensile stress (5.77 × 10⁷ Pa). Unlike the highly strained a-plane ZnO, temperature dependent photoluminescence spectra show that the free A exiton emission was observed with the temperature ≤138 K.     

Thermal annealing effects on I–V–T characteristics of sputtered Cr/n-GaAs diodes

Sputtered Cr/n-GaAs Schottky diodes have been prepared and annealed at 200 and 400 °C. The current–voltage (I–V) characteristics of the as-deposited and annealed diodes have been measured in the temperature range of 60–320 K with steps of 20 K. The effect of thermal annealing on the temperature-dependent I–V characteristics of the diodes has been investigated experimentally. The ideality factor and barrier height (BH) values for 400 °C annealed diode approximately remain unchanged from 120 to 320 K, and those of the as-deposited sample from 160 to 320 K. The departures from ideality at low temperatures have been ascribed to the lateral fluctuations of the BH. The BH values of 0.61 and 0.74 eV for the as-deposited and 400 °C annealed diodes were obtained at room temperature, respectively. A Richardson constant value of 9.83 A cm⁻² K⁻² for 400 °C annealed Schottky diode, which is in close agreement with the known value of 8.16 A cm⁻² K⁻² for n-type GaAs. Furthermore, T₀ anomaly values of 15.52, 10.68 and 5.35 for the as-deposited and 200 and 400 °C annealed diodes were obtained from the nT versus T plots. Thus, it has been seen that the interface structure and quality improve by the thermal annealing at 400 °C.     

Water incorporation into the Ba2(In1 − xMx)2O5‪ (M = Sc 0 ≤ x < 0.5 and M = Y 0 ≤ x < 0.35) system and protonic conduction

Ba₂(In_1 − xM_x)₂O_{5 − y / 2}(OH)_y‪□_{1 − y / 2} (y ≤ 2; M = Sc³⁺ 0 ≤ x < 0.5 and M = Y³⁺ 0 ≤ x < 0.35) compounds were prepared by reacting Ba₂(In_1 − xM_x)₂O₅‪ phases with water vapor. This reaction is reversible. Analyses of the hydration process by TG and XRD studies show that the thermal stability of hydrated phases increases when x increases and that the incorporation of water is not a single-phase reaction inducing either a crystal system or space group modification. Fully hydrated (y = 2) and dehydrated (y = 0) samples have been stabilized at room temperature and characterized for all compositions. In wet air, all phases show a proton contribution to the total conductivity at temperatures between 350 and 600 °C. At a given temperature, proton conductivity increases with the substitution ratio and reaches at 350 °C, 5.4 10^− 3 S cm^− 1 for Ba₂(In_0.65Sc_0.35)₂O_4.2□_0.2(OH)_1.6.     

The influence of the annealing conditions on the photoluminescence of ion-implanted SiO2:Si nanosystem at additional phosphorus implantation

The influence of P ion doping on the photoluminescence (PL) of the system of nanocrystals in SiO₂ matrix (SiO₂:Si) both without annealing and after annealing at various temperatures (provided before and after additional P implantation) is investigated. The Si and P implantation was carried out with ion energies of 150 keV and doses Φ_Si=10¹⁷ cm⁻² and Φ_P=(0.1–300)×10¹⁴ cm⁻² (current density j3 μAcm⁻²). The system after Si implantation was formed at 1000°C and 1100°C (2 h). For the case of SiO₂:Si system as-implanted by P, the intensity of PL was drastically quenched, but partially retained. As for the step-by-step annealing (at progressively increased temperatures) carried out after P implantation, the sign and degree of doping effect change with annealing temperature. The possible mechanisms of these features are discussed.     

Rotational absorption spectrum of methylene fluoride in the 20–100 cm region

The pure rotational spectrum of CH₂F₂ was recorded in the 20–100 cm⁻¹ spectral range and analyzed to obtain rotation and centrifugal distortion constants. Analysis of the data yielded rotation constants: A = 1.6392173 ± 0.0000015, B = 0.3537342 ± 0.00000033, C = 0.3085387 ± 0.00000027, τ_aaaa = −(7.64 ± 0.46) × 10⁻⁵, τ_bbbb = −(2.076 ± 0.016) × 10⁻⁶, τ_cccc = −(9.29 ± 0.12) × 10⁻⁷, T₁ = (4.89 ± 0.20) × 10⁻⁶, and T₂ = −(1.281 ± 0.016) × 10⁻⁶cm⁻¹.     

Oxide ion conductivity and relaxation in CaREZrNbO7 (RE = La, Nd, Sm, Gd, and Y) system

CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ca²⁺–RE³⁺)_av/r(Zr⁴⁺–Nb⁵⁺)_av were larger than 1.34. Thus, the La, Nd, and Sm compounds have a cubic P-type structure and the Gd and Y ones have a defect F-type structure. The electrical conductivity was measured using complex-plane impedance analysis over a wide temperature (300–750 °C) and frequency (1 Hz–1 MHz) ranges. The conductivity relaxation phenomenon was observed in these compounds and the relaxation frequencies were found to show Arrhenius-type behavior and activation energies were in good agreement with those obtained from high temperature conductivity plots. These results support the idea that the relaxation process and the conductivity have the same origin. The ionic conductivity of CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system showed the maximum at the phase boundary between the F-type and P-type phases. On the other hand, the activation energy for the conduction decreased in the F-type phase and increased in the P-type phase with increasing ionic radius ratio. Among the prepared compounds, CaGdZrNbO₇ showed the highest ionic conductivity of 9.47 × 10^− 3 S/cm at 750 °C which was about twice as high as that observed in Gd₂Zr₂O₇ (4.2 × 10^− 3 S/cm at 800 °C). The grain morphology observation by scanning electron microscope (SEM) showed well-sintered grains. AC impedance measurements in various atmospheres further indicated that they are predominantly oxide ion conductors at elevated temperatures (> 700 °C).     

Effect of oxygen partial pressure on the structural and optical properties of dc reactive magnetron sputtered molybdenum oxide films

Molybdenum oxide films (MoO₃) were deposited on glass and crystalline silicon substrates by sputtering of molybdenum target under various oxygen partial pressures in the range 8 × 10⁻⁵–8 × 10⁻⁴ mbar and at a fixed substrate temperature of 473 K employing dc magnetron sputtering technique. The influence of oxygen partial pressure on the composition stoichiometry, chemical binding configuration, crystallographic structure and electrical and optical properties was systematically studied. X-ray photoelectron spectra of the films formed at 8 × 10⁻⁵ mbar showed the presence of Mo⁶⁺ and Mo⁵⁺ oxidation states of MoO₃ and MoO_3−x. The films deposited at oxygen partial pressure of 2 × 10⁻⁴ mbar showed Mo⁶⁺ oxidation state indicating the films were nearly stoichiometric. It was also confirmed by the Fourier transform infrared spectroscopic studies. X-ray diffraction studies revealed that the films formed at oxygen partial pressure of 2 × 10⁻⁴ mbar showed the presence of (0 k 0) reflections indicated the layered structure of α-phase MoO₃. The electrical conductivity of the films decreased from 3.6 × 10⁻⁵ to 1.6 × 10⁻⁶ Ω⁻¹ cm⁻¹, the optical band gap of the films increased from 2.93 to 3.26 eV and the refractive index increased from 2.02 to 2.13 with the increase of oxygen partial pressure from 8 × 10⁻⁵ to 8 × 10⁻⁴ mbar, respectively.     

Nanoscale anglesite growth on the celestite (0 0 1) face

In situ atomic force microscopy (AFM) was used to study the growth behaviour of anglesite (PbSO₄) monolayers on the celestite (0 0 1) face. Growth was promoted by exposing the celestite cleavage surfaces to aqueous solutions that were supersaturated with respect to anglesite. The solution supersaturation, β_ang, was varied from 1.05 to 3.09 (where β_ang = a(Pb²⁺) · a(SO₄²⁻)/K_sp,ang). In this range of supersaturation, two single anglesite monolayers (3.5 Å in height each) from pre-existent celestite steps were grown. However, for solution supersaturation β_ang < 1.89 ± 0.06, subsequent multilayer growth is strongly inhibited. AFM observations indicate that the inhibition of a continuous layer-by-layer growth of anglesite on the celestite (0 0 1) face is due to the in-plane strain generated by the slight difference between the anglesite and celestite lattice parameters (i.e. the linear misfits are lower than 1.1%). The minimum supersaturation required to overcome the energy barrier for multilayer growth gave an estimate of the in-plane strain energy: 11.4 ± 0.6 mJ/m²_. Once this energy barrier is overcome, a multilayer Frank–Van Der Merwe epitaxial growth was observed.     

Infrared spectral study of molecular vibrations in amorphous, nanocrystalline and AlO(OH) · αH2O bulk crystals

Amorphous, nanocrystalline, and bulk AlO(OH) · xH₂O crystals have six fundamental modes (FM) of vibration in a nonlinear AlO(OH) molecular structure. Most of them appear in groups of four IR and Raman bands. Their positions and relative intensities differ significantly in three specimens. The nanocrystals (monoclinic structure with z=8 molecules per unit cell) have four OH stretching bands at values enhanced by up to 360 cm⁻¹ at 3120, 3450, 3560 cm⁻¹ in comparison to those in bulk crystals or amorphous specimens. The first two bands are broad, bandwidth Δν_1/2200 to 350 cm⁻¹, while the other two are sharp, Δν_1/290 cm⁻¹. The sharp bands shift to 3525 and 3595 cm⁻¹ after heating the sample at 100°C. They no longer appear after heating at 300 or 500°C for 2 h (the specimen decomposes to Al₂O₃), leaving behind only two bands at 3100 and 3400 cm⁻¹. A Δν_1/2 value of 500 cm⁻¹ appears in the 3400 cm⁻¹ in a delocalized distribution of H atoms. Two bands also occur at 3098 and 3300 cm⁻¹ in bulk crystals (orthorhombic structure with z=4) or at 2990 and 3515 cm⁻¹ in an amorphous sample. More than one bands appear in a FM vibration in occurrence of sample in more than one conformers. The amorphous sample has approximately the same conformer structure as the bulk crystals. An amorphous surface structure exists in nanocrystals with a group of three bands at 1420, 1510 and 1635 cm⁻¹ in an interconnected network structure. It encapsulates the nanocrystals in an amorphous shell. Its volume fraction, 33% estimated from the integrated intensity in three bands, determines 2.2 nm thickness in the shell in spherical shape of nanocrystals in 35 nm diameter.     

Optical investigation of ferromagnetic Ga1−xMnxN layers grown on sapphire (0 0 0 1) substrates

We have investigated the temperature-dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x≈0.1–0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. All the layers doped with manganese exhibited n-type conductivity with Curie temperature over 350 K. The efficient PL are peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. It was found that the blue band at 3.29 eV is mostly associated with the formation complexes between donors (e.g., N vacancy) and Mn acceptors, which results in forming donor levels at 0.23 eV below the conduction band edge. The yellow band is attributed to intrinsic gallium defects. The broad band at 1.86 eV is attributed to inner ⁵D state transition (T₂ to E) of Mn ions.     

Structural properties of 10 μm thick InN grown on sapphire (0001)

The structural properties of a 10 μm thick In-face InN film, grown on Al₂O₃ (0001) by radio-frequency plasma-assisted molecular beam epitaxy, were investigated by transmission electron microscopy and high resolution x-ray diffraction. Electron microscopy revealed the presence of threading dislocations of edge, screw and mixed type, and the absence of planar defects. The dislocation density near the InN/sapphire interface was 1.55×10¹⁰ cm⁻², 4.82×10⁸ cm⁻² and 1.69×10⁹ cm⁻² for the edge, screw and mixed dislocation types, respectively. Towards the free surface of InN, the density of edge and mixed type dislocations decreased to 4.35×10⁹ cm⁻² and 1.20×10⁹ cm⁻², respectively, while the density of screw dislocations remained constant. Using x-ray diffraction, dislocations with screw component were found to be 1.2×10⁹ cm⁻², in good agreement with the electron microscopy results. Comparing electron microscopy results with x-ray diffraction ones, it is suggested that pure edge dislocations are neither completely randomly distributed nor completely piled up in grain boundaries within the InN film.     

Structural and optical properties of In0.5Ga0.5As/GaAs quantum dots in an In0.1Ga0.9As well using repeated depositions of InAs/GaAs short-period superlattices for the application of optical communication

We report structural and optical properties of In_0.5Ga_0.5As/GaAs quantum dots (QDs) in a 100 Å-thick In_0.1Ga_0.9As well grown by repeated depositions of InAs/GaAs short-period superlattices with atomic force microscope, transmission electron microscope (TEM) and photoluminescence (PL) measurement. The QDs in an InGaAs well grown at 510 °C were studied as a function of n repeated deposition of 1 monolayer thick InAs and 1 monolayer thick GaAs for n=5–10. The heights, widths and densities of dots are in the range of 6–22.0 nm, 40–85 nm, and 1.6–1.1×10¹⁰/cm², respectively, as n changes from 5 to 10 with strong alignment along [1 −1 0] direction. Flat and pan-cake-like shape of the QDs in a well is found in TEM images. The bottoms of the QDs are located lower than the center of the InGaAs well. This reveals that there was intermixing—interdiffusion—of group III materials between the InGaAs QD and the InGaAs well during growth. All reported dots show strong 300 K-PL spectrum, and 1.276 μm (FWHM: 32.3 meV) of 300 K-PL peak was obtained in case of 7 periods of the QDs in a well, which is useful for the application to optical communications.     

Effects of rapid thermal annealing on the energy levels of InAs/InP self-assembled quantum dots

We have studied the effects of thermal annealing on the electrical properties of InAs/InP self-assembled quantum dots (QDs) using deep level transient spectroscopy (DLTS). It was found from the DLTS measurements that the activation energy of the QD signal varied from 0.47 to 0.60 eV and the emission cross section changed from 1.01×10⁻¹⁵ to 9.63×10⁻¹⁴ cm² when the annealing temperature increased up to 700 °C. As a result of the thermal annealing process at the temperature ranging from 500 to 600 °C, the higher activation energy and the larger emission cross section of the QD related signal were observed for the annealed samples compared to those for the as-grown sample. On the basis of the capture barrier height for the QDs structure being lowered from 0.24 to 0.06 eV at the annealing temperature of 700 °C, thermal damage was considered as the reason. The appropriate annealing process can provide a clue for the engineering of the energy levels in the self-assembled QD structures.     

Elementary reactions of formyl (HCO) radical studied by laser photolysis—transient absorption spectroscopy

Several elementary reactions of formyl radical of combustion importance were studied using pulsed laser photolysis coupled to transient UV–Vis absorption spectroscopy: HCO → H + CO (1), HCO + HCO → products (2), and HCO + CH₃ → products (3). One-pass UV absorption, multi-pass UV absorption as well as cavity ring-down spectroscopy in the red spectral region were used to monitor temporal profiles of HCO radical. Reaction (1) was studied over the buffer gas (He) pressure range 0.8–100 bar and the temperature range 498–769 K. Reactions (2a), (2b), (2c), (3a) and (3b) as well as the UV absorption spectrum of HCO, were studied at 298 and 588 K, and the buffer gas (He) pressure of 1 bar. Pulsed laser photolysis (308, 320, and 193 nm) of acetaldehyde, propionaldehyde, and acetone was used to prepare mixtures of free radicals. The second-order rate constant of reaction (1) obtained from the data at 1 bar is: k₁(He) = (0.8 ± 0.4) × 10⁻¹⁰exp(−(66.0 ± 3.4) kJ mol⁻¹/RT) cm³ molecule⁻¹ s⁻¹. The HCO dissociation rate constants measured in this work are lower than those reported in the previous direct work. The difference is a factor of 2.2 at the highest temperature of the experiments and a factor of 3.5 at the low end. The experimental data indicate pressure dependence of the rate constant of dissociation of formyl radical 1, which was attributed to the early pressure fall-off expected based on the theory of isolated resonances. The UV absorption spectrum of HCO was revised. The maximum absorption cross-section of HCO is (7.3 ± 1.2) × 10⁻¹⁸ cm² molecule⁻¹ at 230 nm (temperature independent within the experimental error). The measured rate constants for reactions (2a), (2b), (2c), (3a) and (3b) are: k₂ = (3.6 ± 0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ (298 K); k₃ = (9.3 ± 2.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹(298 and 588 K).