Transport Coefficient of Gallium-doped Bi2Te3 Single Crystals

Gallium-doped Bi₂Te₃ single crystals were prepared by means of a modified Bridgman method. Temperature dependences of the Hall constant R_H( B ∥ c ), electrical conductivity σ⊥c and Seebeck coefficient S(Δ T ⊥ c) were measured on the samples of these crystals in the temperature interval of 100–400 K. The variations of the investigated transport coefficients with increasing Ga content in the samples showed that Ga atoms in the Bi₂Te₃ crystal lattice behave as donors. This effect is qualitatively explained on the basis of a model of point defects in Bi₂Te₃(Ga) crystals; singly ionised gallium atoms in interstitial sites; Ga_i, are considered to be the most probable defects.     

Interface of GaN grown on Ge(1 1 1) by plasma assisted molecular beam epitaxy

Early efforts to grow GaN layers on germanium substrates by plasma assisted molecular beam epitaxy led to GaN domains, rotated by 8° relative to each other. Increased insight in the growth of GaN on germanium resulted in the suppression of these domain and consequently high quality layers. In this study the interface of these improved layers is investigated with transmission electron microscopy. The GaN layers show high crystal quality and an atomically abrupt interface with the Ge substrate. A thin, single crystalline Ge₃N₄ layer is observed in between the GaN layer and Ge substrate. This Ge₃N₄ layer remains present even at growth temperatures (850 °C) far above the decomposition temperature of Ge₃N₄ in vacuum (600 °C). Triangular voids in the Ge substrate are observed after growth. Reducing the Ga flux at the onset of GaN growth helps to reduce the triangular defect size. This indicates that the formation of voids in the Ge substrate strongly depends on the presence of Ga atoms at the onset of growth. However complete elimination was not achieved. The formation of voids in the germanium substrate leads to diffusion of Ge into the GaN layer. Therefore we examined the diffusion of Ge atoms into the GaN layer and Ga atoms into the Ge substrate. It was found that the diffusion of Ge into the GaN layer and Ga into the Ge substrate can be influenced by the growth temperature but cannot be completely suppressed. Our results suggest that Ga atoms diffuse through small imperfections in the Ge₃N₄ interlayer and locally etch the Ge substrate, leading to the diffusion of Ga and Ge atoms.     

Theoretical investigation on the decomposition process of GaN(0 0 0 1) surface under a hydrogen atmosphere

The activation energies for Ga and N desorption from a GaN surface were calculated using the density functional theory to understand the detailed decomposition process of the hydrogen terminated GaN(0 0 0 1) Ga and N surfaces under a hydrogen atmosphere. It was found that the Ga atoms on the hydrogen terminated GaN(0 0 0 1) Ga surface desorbed as GaH molecules from the surface while the N atoms on the hydrogen terminated GaN(0 0 0 1) N surface desorbed as NH₃ molecules from the surface. The desorption energies of GaH and NH₃ on the hydrogen terminated surface were more consistent with the previous experimental values than those on the ideal surface. These results suggest that the initial surface structure of the GaN(0 0 0 1) surface is terminated with hydrogen.     

Spatial distribution of rare-earth ions in Se-based chalcogenide glasses with or without Ga

Measured lifetime of the (³F₃, ³F₄) level of Pr³⁺ ions doped in representative Se-based chalcogenide Ge–Sb–Se glasses peaked at the mean coordination number of ∼2.67, indicating that inter-ionic distance between Pr³⁺ dopants became maximized on the average at such particular host compositions. However, this phenomenon was not observed when Ga was further introduced. It has been verified from Ga K-edge EXAFS spectroscopic analysis that a Ga atom has four Se atoms as its nearest neighbors in the Ge–Sb–Se glasses doped with Pr. The Ga–Se bonds are less covalent than other heteropolar chemical bonds, and thereby positively ionized rare-earth elements would be located preferentially next to the GaSe₄ units, acting as charge compensators or network modifiers. We presume that there is a negligible structural and chemical correlation between a rare-earth ion and its next-nearest neighbors on their sites in the typical covalent chalcogenide glasses without Ga, and as a result spatial distribution of rare-earth ions would depend on changes of the free volume accompanied with the structural phase transitions on a medium-range scale from two-dimensional layers to three-dimensional networks. In selenide glasses with Ga, however, the empirical connection disappears due to the structural correlation between rare-earth ions and the GaSe₄ tetrahedra.     

Molecular doping in amorphous selenium

The transport properties of vacuum evaporated selenium doped with small quantities of atoms (Li, Ga, In, Cl, etc.), and compounds (LiCl, GaF₃, In₂O₃, etc.) are examined using time-of-flight technique. When doped with certain compounds (for example, GeO₂), electrons and holes show comlex transport behavior different from the compensation effect due to co-doping of the component atoms (Ge, O). This suggests that these compounds are taken into selenium in the “molecular form”.     

Crystal structure of Ga<Subscript>0.5</Subscript>In<Subscript>1.5</Subscript>Se<Subscript>3</Subscript> solid solution

A solid solution of the GaIn₃Se₆ (2Ga_0.5In_1.5Se₃) composition with a hexagonal lattice (a = 7.051(3) Å, c = 19.148(2) Å, sp. gr. P6₁, z = 6, V = 824.4332(4) ų, ρ = 5.379(2) g/cm³) has been synthesized as a result of alloying Ga, In, and Se elements with a metal ratio of 1: 3. It was established that six out of nine In atoms in the lattice are located in a trigonal bipyramid, while the other three In atoms and three Ga atoms have a tetrahedral coordination.     

Structure and electric properties of InTe1-x Sex,In1-x GaxTe, and In1-x Tlx Te solid solutions

The local environment of Ga, Tl, and Se atoms in InTe-based solid solutions have been studied by the method of EXAFS spectroscopy. It is shown that these atoms can be regarded as substitutional impurities occupying the In(1), In (2), and Te positions in the InTe structure. Electric measurements showed that the In_1-x Ga_xTe and InTe_1-x Se_x solid solutions are semiconductors at x > 0.24 and x > 0.15, respectively.     

Ga and In incorporation rates in Ga1−xInxAs growth by chemical beam epitaxy

GaAs, InAs and Ga_1?xIn_xAs layers were grown by chemical beam epitaxy (CBE) using triethylgallium, trimethylindium and tertiarybutylarsine as precursors for Ga, In and As, respectively. The growth rate during the homoepitaxial growth of GaAs and InAs, deduced from the frequency of reflection high-energy electron diffraction intensity oscillations, was used to calibrate the incorporation rates for the III elements. The In content of the Ga_1?xIn_xAs layers was measured by Rutherford backscattering spectrometry and compared with the value predicted from the above calibration data; while the measured In mole fraction is close to the predicted value for the samples grown for low In to Ga flux ratios (x<0.2), the In incorporation is enhanced for larger values of this ratio. The results obtained on layers grown at different substrate temperatures show that In mole fraction is almost constant at growth temperatures in the range 400–500 °C, but a strong dependence on the substrate temperature has been found outside this range. The above results, not observed for samples grown by solid source molecular beam epitaxy, indicate that some interaction between Ga and In precursors at the sample surface could take place during the growth by CBE.     

X-ray diffraction studies of interdiffusion in InAs—GaAs powder blends

Interdiffusion in the pseudobinary system In_xGa_1−xAs is investigated by means of X-ray diffractometry of annealed powder blends. The interpretation of the experimental results by means both of the concentric spheres and concentric cubes model yields for In_0.43Ga_0.57As an activation energy of (5.39 ± 0.11) eV. This value shows that the diffusion mechanism could be a vacancy one as in the pure compounds. According to the lower atomic radius the Ga atoms within the temperature interval of 550–625 °C diffuse more easily than the In atoms.     

Gallium doped SiO2: Towards a new luminescent material

We show how the interaction between Ga atoms and silica samples in a hot environment gives rise to permanent inclusions of Ga inside the silica matrix which, in turn, produce typical luminescence features. The Ga doped silica is analyzed via laser induced fluorescence, photoluminescence and electron paramagnetic resonance spectroscopies. The results evidence the presence of modifications induced by the Ga inclusions inside the silica matrix and some preliminary hypothesis on their nature are advanced. Possible applications of such Ga doped silica are also mentioned.     

X-ray absorption and Raman characterizations of TeO2-Ga2O3 glasses

The thermodynamic properties of transparent glasses prepared in the TeO₂-Ga₂O₃ system were investigated by differential scanning calorimetry. The change of the thermal parameters as a function of the chemical composition is discussed. Raman and both Te L_III and Ga K edge X-ray absorption spectroscopies at room temperature were used to examine the short range order. Analyses of the spectra suggest that the addition of Ga₂O₃ content to the TeO₂ glass matrix induces the transformation of trigonal bipyramids (TeO₄E, E=lone electronic pair 5s² of Te) to trigonal pyramids (TeO₃E) with formation of Te-O-Ga bridging bonds. Furthermore, Ga K edge XANES and EXAFS studies show that Ga atoms exhibit both tetrahedral (GaO₄) and octahedral (GaO₆) environments.     

Structural dependence of ultrafast third-order optical nonlinearity of Ge–Ga–Ag–S chalcogenide glasses

Ultrafast third-order optical nonlinearity of Ge–Ga–Ag–S chalcogenide glasses at the wavelength of 820 nm has been measured using femtosecond time-resolved optical Kerr (OKE) technique. The results show that Ge–Ga–Ag–S glasses have large third-order optical nonlinear susceptibility, χ⁽³⁾ and the response time is also subpicosecond, which are predominantly due to the ultrafast distortion of electron cloud surrounding the balanced positions of Ge, Ga, Ag and S atoms. What’s more, a strong dependence of χ⁽³⁾ on the composition and microstructure of these glasses was found which shows that [GeS₄] and [GaS₄] tetrahedra play an important role on the third-order optical nonlinearity. These Ge–Ga–Ag–S chalcogenide glasses would be expected as promising materials applied on all-optical switching devices.     

On the Reasons Leading to an Appearance of Correlated and Anticorrelated Dependences of the Minority Carrier Lifetime on the Dislocation Density in Undoped Semi-Insulating “Stoichiometric” GaAs Crystals

It is shown that in undoped semi-insulating GaAs crystals grown under the stoichiometric conditions both correlated and anticorrelated dependences of the minority carrier lifetime τ on the dislocation density N_d could be observed. The above-pointed effect is connected with a slight excess of Ga atoms (then the correlated dependence τ vs Nd appears) or of As atoms (then the anticorrelated dependence τ vs N_d appears) which inevitably exists even in “stoichiometric” GaAs crystals (i.e. in GaAs crystals of “stoichiometric” composition).     

Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods

The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO₂/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO₂/Si heterostructures, which were previously formed by the modified “hot wall” technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO₂/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride.     

Ti-adsorption on Se-prereacted GaAs (110) surfaces studied by SXPS

This study is embedded in the broader context of reactive metal overlayers and passivation on GaAs. High resolution synchrotron-radiation photoemission experiments for Ti coverages on Se-reacted GaAs (110) surfaces show that, contrary to clean Ti-reacted GaAs (110) interface, there is no initial disruption (submonolayer reaction) of the surface involving both Ga and As atoms during early stage of interface formation. However, a delayed Ti involved reaction appears at a trigger coverage of about 1 ML involving only As atoms from the prereacted As Se interface configuration continued by Stranski-Krastanov growth mode. A second reacted phase starts to form two Ga Ti involved configurations replacing Ga Se bonds near a Ti coverage of 4 ML. The preferential chemical trapping of Ti by Se atoms is associated with a smaller interface thickness very likely <10 ML instead of 50 ML in the case of clean Ti/GaAs (110) interface caused by mobile As atoms.     

Crystallization of AgGaS2 melts enriched with Ag2S and Ga2S3

In this work we consider experiments with directed crystallization of melts with slight deviation from stoichiometric composition of AgGaS₂. The evolution of melt composition was calculated for both experiments using measured chemical composition of the solid phase. Also the difference in unit cell parameters and luminescence spectra for AgGaS₂ grown from the melts was found. Both monocrystal samples and samples synthesized from elementary Ag, Ga and S were used for thermal analyses. Obtained data suggests that different ways of preparing the samples are responsible for defects concentration and thus for so wide range (70°C) of melting temperatures reported in scientific literature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transmission electron microscopy observation of GaAs/Al0.3Ga0.7As T-shaped quantum well structure fabricated by glancing angle molecular beam epitaxy on GaAs(100) reverse-mesa etched substrates

GaAs/Al_0.3Ga_0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only an the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature (T_s) as high as 630°C, under a V/III ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision.     

Structure of acetato(meso-5,10,15,20-tetraphenylporphyrinato)gallium(III): Ga(tpp)(OAc)

The title compound, Ga(tpp)(OAc), crystallizes in the space group P2₁/n witha=10.628(2),b=16.176(2),c=20.750(2), Å, =92.64(2)°, andZ=4. The gallium atom has a squarepyramidal coordination geometry formed by the four nitrogen atoms of the porphyrinato group (Np) and the acetate ion. The geometry around the gallium center of the Ga(tpp)(OAc) molecule has Ga–O(1)=1.874(4) Å and average Ga–Np(porphyrin nitrogen)=2.056(4) Å. The acetate group is unidentately coordinated to the gallium (III) atom. Ga-71 NMR spectroscopy provides a complementary method for investigation of the acetate ligand. Combining the calculated value of _c, 29.0 ps, with the measured Ga-71 quadrupolar relaxation time, T_1Q=11.1 s, the quadrupole coupling constant of Ga-71 is 28.1 MHz. This further supports that Ga is covalently coordinated to the acetate and tpp in Ga(tpp)(OAc). The methyl and carbonyl carbons of the acetate group are separately located at 20.4 and 168.8 ppm at 24°C for the Ga(tpp)(OAc).     

The Characteristics of GaP Window Layer Grown by Indium‐added Liquid Phase Epitaxy

The effects of added Indium on the growth characteristics of GaP were systematically studied using conventional liquid phase epitaxy technique with Ga‐rich GaP source melt. The GaP growth rate uniformly increase with the source melt of increasing Indium addition against Gallium solvent. These epilayers have mirror‐like surface morphology examined by optical microscope except several grown films with large amount of Indium addition meet a terrible interface. The surface morphologies examined by AFM showed the ripples in samples of R_0.05 and R_0.4 and distinct islands with elliptical base shape in the sample of R_0.7 (R‐In ratio). The epitaxial layer with incorporation of Indium addition during growth had good performance on the carrier concentrations and resistivity. The composition of compound semiconductor become to InGaP at higher amount of In addition to Ga was examined by double‐crystal X‐ray diffraction and the distribution of Indium examined by SIMS also provided the evidences in sample of R_0.7.     

A difference between initial growth stages of the AlGaAs/GaAs and GaAs/AlGaAs heterostructures produced by contact replacement of solutions

The method of liquid epitaxial growth of GaAs/AlGaAs/GaAs heterostructures when the change of solutions occurs due to pushing off one melt by another is discussed. It has been shown theoretically and experimentally that the initial stages of film growth after the change of the binary Ga As melt on the ternary Al Ga As melt differ from that when the Ga As solution pushes off the Al Ga As liquid. The difference is caused by the inequality of the diffusion coefficients of As and Al in a multicomponent Al Ga As liquid (D_Al > D_As). As a result, the growth of an AlGaAs film begins immediately in the case when the Al Ga As solution pushes off the Ga As liquid but in the opposite case the dissolution of an underlying AlGaAs solid is unavoidable and depends little on degree of a saturation of the Ga As washing solution. These peculiarities must be taken into account in discussions of abruptness and other properties of LPE-grown AlGaAs/GaAs and GaAs/AlGaAs heterojunctions.