Growth temperature dependence in molecular beam epitaxy of gallium arsenide

Epitaxial layers of GaAs were grown on GaAs(100) at substrate temperatures ranging from 400° to 600°C by molecular beam epitaxy. Surface structures of the substrate and the epitaxial layers were investigated by means of low-energy electron diffraction. Two new structures of c(4 × 4) and c(8 × 8) were observed from layers grown at the low temperature of 400°C. The electrical and optical properties of layers doped with Si were investigated by measurement of Hall effect and photoluminescence as a function of growth temperature. It is found that a semi-insulating layer is grown below a critical temperature, and the layer is useful as a buffer layer for GaAs FET's. Variation of carrier concentration was observed near the interface between layers grown at different temperatures under a constant Sn beam flux. The effect is attributed to defect-induced segregation of Sn.     

Solid-phase epitaxy for the formation of superlattices based on A3B5 compounds

To form superlattices based on A³B⁵ compounds, it is suggested to use solid-phase epitaxy (SPE) which includes recystallization of super-thin films of stoichiometric composition deposited by laser evaporation on single-crystal plates or films with the same or close composition. The recrystallization temperature is shown to be reduced considerably for the film grains with its curvature differing significantly from zero and which are in contact with their own single-crystal seed having practically zero curvature. The single-crystal multilayer films GaAs-(Al, Ga)As … GaAs were formed at 300°C. The recrystallization rate at this temperature was 40 Å/min. The autocatalytic action of the single-crystal seed strongly reduced the activation energy of the SPE process, which may be due to diffusion in solid phase.     

Hopping conduction in Cr-doped amorphous gallium arsenide films

Amorphous Cr-doped GaAs has been prepared by rf sputtering and the dc conductivity measured at fields up to 10⁷ V/m and temperatures down to 80 K. The results are explained in terms of field-assisted hopping. Effects of isothermal annealing at temperatures up to 200°C are studied. The studies are repeated and the results compared with undoped a-GaAs. Room temperature resistivities of a-GaAs: Cr, a-GaAs and polycrystalline GaAs are also compared. The recrystallization temperature for a-GaAs films is determined to be between 320°C and 340°C.     

Defect Structure in Te-doped GaAs single Crystals after Plastic Deformation (I). Twins and Stacking Faults

At temperatures above the brittle-to-ductile transition (490 °C) in Te-doped GaAs three types of predominant defect configurations have been observed after uniaxial compression along a [001] direction: (i) twins and stacking faults (500 … 520 °C), (ii) slip zones of dislocations (≈ 550 °C) and (iii) dislocation cells (580 … 590 °C). In Part I quantitative details of the appearance of twins and stacking faults are given. Most frequently found were 30° partials in twins and stacking faults.     

Defect Structure in Te-doped GaAs Single Crystals after Plastic Deformation (II). Dislocation Slip and Cell Formation

At temperatures above the brittle-to-ductile transition (490 °C) in Te-doped GaAs three types of predominant defect configurations have been observed after uniaxial compression along a [001] direction: (i) twins and stacking faults (500 … 520 °C), (ii) slip zones of dislocations (≈550 °C) and (iii) dislocation cells (580 … 590 °C). In Part II quantitative details of the appearance of slip and cell formation are given. Leading segments of gliding half loops are mainly of 60° type. Cell walls were formed by multiple slip of perfect dislocations.     

Influence of selenization temperature on synthesis,morphology, and properties of nanostructured CoSe2 films

Nanostructured cobalt selenide (CoSe₂) thin films were deposited on a glass substrate using the selenization of Co films at different selenization temperatures (300 °C, 400 °C, and 500 °C) in a pure Se vapor for two hours. The morphology and structure of the as‐deposited films shows that the film morphology and crystallinity are affected by the selenization temperature. Increasing the selenization temperature from 300 °C to 400 °C and 500 °C results in a change in the surface and cross sectional morphology. At 300 °C, the Co films have an almost amorphous structure, while at temperatures of 400 and 500 °C, the Co films have a crystalline nanostructure with bilayered morphology. Optical analyses of the CoSe₂ films at 500 °C show a large absorption (α > 1.0 × 10⁵ cm⁻¹) and a direct band gap (∼1.0 eV).     

Zur Epitaxie von Galliumnitrid auf nichtstöchiometrischem Spinell im System GaCl/NH3/He

The crystalline perfection and the epitaxial relationships of GaN layers prepared by the reaction of GaCl and NH₃ in He carrier gas on {111} and {100} spinel substrates have been determined by RHEED measurements. The epitaxial relationships were found to be The best crystalline perfection could be achieved at growth temperatures of 1000…︁1050°C and growth rates more than 1 μm/min. – The growth direction 〈101 1〉 has been found to be more suitable for rapid growth than the direction 〈0001〉.     

Impurity band conduction effects in liquid phase epitaxial Te-doped GaP grown from tin solution

GaP layers were grown by liquid phase epitaxy from tin solution on semi-insulating GaAs substrates with various amounts of Te added to the melt (x_Te = 10⁻⁴ …︁ 3 · 10⁻²). The Sn and Te concentrations in the layers were determined by chemical analysis as function of x. An analysis of the electrical measurements shows that the carrier transport in the layers is essentially determined by impurity band conduction effects.     

Textural properties of AISb films

Textural studies have been carried out in AlSb films deposited by coevaporation method under high vacuum at different substrate temperatures between 450° to 625 °C. The films have been examined by X-ray and electron diffraction techniques. It was observed that the films deposited around 550 °C were found to exhibit [110] and [111] textures and the films deposited at higher substrate temperatures (above 600 °C) were found to exhibit only [111] textures.     

Spray pyrolysis deposition and characterization of highly (100) oriented magnesium oxide thin films

Transparent dielectric thin films of MgO has been deposited on quartz substrates at different temperatures between 400 and 600°C by a pneumatic spray pyrolysis technique using Mg(CH₃COO)₂·4H₂O as a single molecular precursor. The thermal behavior of the precursor magnesium acetate is described in the results of thermogravimetry analysis (TGA) and differential thermal analysis (DTA). The prepared films are reproducible, adherent to the substrate, pinhole free and uniform. Amongst the different spray process parameters, the substrate temperature effect has been optimized for obtaining single crystalline and transparent MgO thin films. The films crystallize in a cubic structure and X‐ray diffraction measurements have shown that the polycrystalline MgO films prepared at 500°C with (100) and (110) orientations are changed to (100) preferred orientation at 600°C. The MgO phase formation was also confirmed with the recorded Fourier Transform Infrared (FTIR) results. The films deposited at 600°C exhibited highest optical transmittivity (>80%) and the direct band gap energy was found to vary from 4.50 to 5.25 eV with a rise in substrate temperature from 500 to 600°C. The measured sheet resistance and the resistivity of the film prepared at 600°C were respectively 10¹³Ω/□ and 2.06x10⁷Ω cm. The surface morphology of the prepared MgO thin films was examined by atomic force microscopy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Changes during Post-Ageing of an Al-Zn (15 at.%) Alloy between 125 °C and 215 °C

The structural changes going on in an Al Zn (15 at.%) alloy postaged between 125 °C and 215 °C after pre-ageing between 20 °C and 90 °C (till start r_s = (20…︁ 55) Å are obtained) were followed by means of XSAS-investigations. The main results are the following:

• 1 An increase of r_s does not favour the growth of the precipitates during the reversion treatment.
• 2 At later reversion times the happening is obviously independent of the pre-history of the samples.
• 3 The integral intensity is decreasing with increase of T_rev, but the mean distance (d)-size (r_G) relation between the precipitates is not changed remarkably up to T_rev ≈ 200 °C in the early reversion stage.

Conclusions from this behaviour are drawn.     

Bi incorporation in GaAs(100)-2×1 and 4×3 reconstructions investigated by RHEED and STM

Bi acts as a surfactant in molecular beam epitaxy (MBE) growth on GaAs(100). Incorporation is achieved by disequilibrium at growth temperatures below ～450 °C. Bi can however affect the static reconstruction up to 600 °C. Two reconstructions are considered in this work: dynamic (2×1) and static c(8×3)/(4×3), which are shown to be the dominant reconstructions for GaAsBi MBE. Bi storage in these two reconstructions provides an explanation of RHEED transitions that cause unintentional Bi incorporation in the GaAs capping layer. Finally dynamic observations of the (2×1) reconstruction are used to explain growth dynamics, atomic ordering and clustering observed in GaAsBi epilayers which have a direct influence on photoluminescence linewidth broadening in mixed anion III–V alloys.     

The Molecular and Crystal Structure of p-methyl-phenyl-glyoxyacid-p-chloranilide

Using X-ray crystal structure analysis of the title compound the positions of all the atoms (including all H atoms) in the monoclinic unit cell with the parameters a = 13.90₀ Å, b = 5.13₈ Å, c = 17.95₆ Å and β = 91.05° and the space group P 2₁/n were localized. The existence of an intramolecular N—H(N)…︁ O(1)-bridge was inferred, whose H-bridge acceptor predominantly is the π-electron density of the carbonyl group. At the same time the H(N) atom participates in the intermolecular bridge N H(N)…︁ O′(1) to the symmetry-equivalent neighbouring molecule. H-bridge-like interactions of the two phenyl hydrogen atoms H(6) and H(15) to the carbonyl oxygen O(2) have also been inferred from the torsion angles.     

MOVPE growth of spontaneously ordered (GaIn) and (AlIn)P layers lattice matched to GaAs substrates

A systematic study of the metal-organic vapour-phase epitaxial growth of (GaIn)P and (AlIn)P layers deposited on GaAs substrates with (001) and (110) orientation is presented. Special attention has been paid to the growth on (001)-oriented wafers with different misorientations to the growth direction. The influence of the growth conditions on the properties of the epitaxial layers such as lattice mismatch, alloy composition, photoluminescence (PL) wavelength, FWHMs of PL peaks and atomic ordering is discussed. Layers with mirrorlike surfaces and various degrees of order could be deposited at growth temperatures T_g ranging from 595 °C to 750 °C for (GaIn)P and 720 °C to 800 °C for (AlIn)P. In addition to the influence of T_g on the Ga incorporation during the (GaIn)P growth we found the Ga distribution coefficient k_Ga to be affected by the misorientation of the substrates. k_Ga correlates presumably with the number of kinks and steps on the substrate surface. Transmission electron diffraction (TED) and PL investigations show that the degree of order — often described by the ordering paramter η — depends strongly on T_g the ordering is more pronounced when the layers are deposited on substrates misoriented towards the (1 11) lattice plane. Strong ordering has been observed for (GaIn)P samples grown at 680 °C on substrates 2° misoriented towards the [1 10] direction and at 650 °C on substrates 6° misoriented towards the same direction. For the (AlIn)P samples striking ordering has been found when they were grown at 720 °C.     

Fabrication and characterization of transparent conductive ZnO:Al thin films prepared by direct current magnetron sputtering with highly conductive ZnO(ZnAl2O4) ceramic target

Using a highly conductive ZnO(ZnAl₂O₄) ceramic target, c-axis-oriented transparent conductive ZnO:Al₂O₃ (ZAO) thin films were prepared on glass sheet substrates by direct current planar magnetron sputtering. The structural, electrical and optical properties of the films (deposited at different temperatures and annealed at 400°C in vacuum) were characterized with several techniques. The experimental results show that the electrical resistivity of films deposited at 320°C is 2.67×10⁻⁴ Ω cm and can be further reduced to as low as 1.5×10⁻⁴ Ω cm by annealing at 400°C for 2 h in a vacuum pressure of 10⁻⁵ Torr. ZAO thin films deposited at room temperature have flaky crystallites with an average grain size of ∼100 nm; however those deposited at 320°C have tetrahedron grains with an average grain size of ∼150 nm. By increasing the deposition temperature or the post-deposition vacuum annealing, the carrier concentration of ZAO thin films increases, and the absorption edge in the transmission spectra shifts toward the shorter wavelength side (blue shift).     

On the Chemical Mechanism of the Zinc Diffusion into GaAs from SiO2 Spin-on Films

The chemical processes which take place during annealing were studied in both model substances and thin films deposited onto GaAs wafers by thermal analysis, X-ray powder diffraction, IR spectroscopy, XPS and TEM. It was found that at 600°C only about 45% of the film was changed to SiO₂ glass. At 700 °C begins the formation of zinc silicates. In this form the zinc seems to be scarcely able to diffuse into GaAs. Obviously, the exothermic DTA effect at 800 °C arises from this zinc silicate formation connected with a recrystallization of the amorphous film.     

Effect of substrate temperature on the properties of vacuum evaporated indium selenide thin films

Thin films of InSe were obtained by thermal evaporation techniques on glass substrates maintained at various temperatures (T_sb = 30°, 400°C). X‐ray diffraction analysis showed the occurrence of amorphous to polycrystalline transformation in the films deposited at higher substrate temperature (400°C). The polycrystalline films were found to have a hexagonal lattice. Compositions of these films have been characterized by EDAX and the surface analysis by scanning electron microscopy. Optical properties of the films, investigated by using spectrophotometer transmittance spectra in the wavelength range (300 – 1100 nm), were explained in terms of substrate temperatures. Films formed at room temperature showed an optical band gap (E_g^opt) 1.56 eV; where as the films formed at 400°C were found to have a E_g^opt of 1.92 eV. The increase in the value of E_g^opt with T_sb treatment is interpreted in terms of the density of states model as proposed by Mott and Davis. The analysis of current ‐Voltage characteristics, based on space charge limited currents (SCLC) measurements, confirms the exponential decrease of density of states from the conduction band edge towards the Fermi level for both the amorphous and polycrystalline films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The transition layers in AlGaAs/GaAs and InGaAsP/GaAs heterostructures grown by LPE

AlGaAs/GaAs and InGaAsP/GaAs heterostructues were grown by liquid phase epitaxy (T_o = 800–770 °C), the transition layers (TLs) were measured with Auger profiling. The Auger profiling of the InGaAsP/GaAs heterostructures after a long heating (1–6 hours, 770 °C) has been made too. The analysis of the experimental data led to the conclusion that the fundamental values of the TL in above mentioned systems are about 10 and 1 to 2 nm, correspondingly. This difference between the minimum widths of the TL in AlGaAs/GaAs and InGaAsP/GaAs system is connected with the different speed of solid state diffusional processes which occur at the initial stpeps of liquid – solid contact.     

TEM-Untersuchungen von Gitterfehlern in ZnSiP2-Einkristallen

Results are dealt with concerning TEM investigations of lattice defects in ZnSiP₂ single crystals. After the crystal growth dislocations or stacking faults were found in a few cases only. More frequently twins were present in the microstructure. The crystallographic elements of twinning are {112} 〈111〉. After plastic deformation by bending at 900 °C local dislocation arrangements with high defect density (N_v ≈ 10⁶…︁ 10⁷ cm⁻²) were observed. By means of the diffraction contrast one Burgers vector b = 1/2 〈111〉 could be identified. In some cases the crystals also contained wide deformation stacking faults, which were limited by partial dislocations. The density of twins was not increased under the conditions of deformation reported here. As it can be concluded from investigations of Oettel et al. and from the results of the twin analysis, slip and generation of stacking faults take place on {112}-planes in ZnSiP₂ crystals. Crystallographic considerations on both processes are dealt with.     

Misfit Strain Relaxation by Dislocations in InAs Islands and Layers Epitaxially Grown on (001)GaAs Substrates by MOVPE

The misfit dislocation configurations in InAs islands as well as in more or less continuous layers grown on (001) oriented GaAs substrates were studied by weak-beam and high-resolution electron microscopy. The islands are confined by {101} and {111} facets where the aspect ratio (height/lateral extension) can be affected by the growth conditions. It is possible to grow well-defined islands as well as relatively continuous layers by MOVPE under As-stabilized conditions. At constant deposition parameters the growth is characterized by islands of different sizes (but with constant aspect ratio) in various strain states depending on their dislocation content. Coherently strained islands without any dislocation can be observed for heights up to 23 ML InAs, or otherwise, up to a maximal island extension of about 12 nm (for the particular aspect ratio ≈︂0.585). With further increase of island height and lateral extension, the introduction of dislocations becomes favourable. Independent of the island size, the layer thickness and the dislocation density, a residual elastic strain of about ε^r = —0.8% remains after relaxation. This means, about 88% of the total misfit strain of ε = —6.686 × 10^—2 were compensated by Lomer dislocations. These sessile Lomer dislocations lie in the island interior only, where single 60° dislocations were observed exclusively in their near-edge regions. With increasing island size and/or layer thickness some close-spaced 60° dislocations occur additionally within the interfacial region. The Lomer dislocations that are always located 4 monolayers (ML) above the InAs/GaAs interfacial plane result from the well-known fusion of two 60° slip dislocations. These 60° dislocations have been nucleated 7 … 8 ML above the interface at surface steps on the {111} facets confining the islands. Based on our experimental observations a new mechanism is proposed that explains the origin of these 60° dislocations. Their further fusion to sessile Lomer dislocations that compensate the misfit strain most efficiently occurs in the way as commonly accepted.