Gain Layer Research Articles

Interface atomic structure of epitaxial ErAs layers on (001) In0.53Ga0.47As and GaAs

High-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy was used to determine the atomic structure of interfaces between epitaxial ErAs layers with the cubic rock salt structure and In0.53Ga0.47As and GaAs, respectively. All layers were grown by molecular-beam epitaxy. We show that the interfacial atomic arrangement corresponds to the so-called chain model, in which the zinc blende semiconductor is terminated with a Ga layer. Image analysis was used to quantify the expansion between the first ErAs plane and the terminating Ga plane. In the HAADF images, a high intensity transfer from the heavy Er columns into the background was observed in the ErAs layer, whereas the background in In0.53Ga0.47As was of much lower intensity.

Compositionally induced phase transition in the Ca 2MnGa 1− xAl xO 5 solid solutions: Ordering of tetrahedral chains in brownmillerite structure

The Ca 2MnGa 1− x Al x O 5 solid solutions ( 0.2 ⩽ x ⩽ 1.0 ) with brownmillerite-type structure were synthesized by solid state reaction at 1250 °C for 40 h in Ar flow. The structures of the solid solutions were studied using X-ray powder diffraction, transmission electron microscopy and high resolution electron microscopy. Replacing Ga by Al introduces a phase transformation from the brownmillerite structure with the P n m a space symmetry ( x ⩽ 0.5 ) to a structure with I 2 m b space symmetry ( x > 0.5 ). The structures differ by the ordering pattern of the mirror-related tetrahedral chains (L and R): in the primitive structure the L and R chains form alternating layers whereas in the body-centered phase all chains are of the same type. The crystal structure of Ca 2MnGa 0.5Al 0.5O 5 was refined from X-ray powder diffraction data (space group P n m a , a = 5.25175 ( 5 ) Å , b = 15.1426 ( 2 ) Å , c = 5.46029 ( 6 ) Å , R I = 0.042 , R P = 0.017 ). A specific feature of this structure is disorder in the Ga layer with intermixing of the L and R chains in a ≈ 2 : 1 ratio. The disorder is related to the formation of numerous antiphase boundaries (APBs) with R = 1 / 2 [ 111 ] as a displacement vector, which produces two adjacent tetrahedral layers with the same type of chains in the initial L R L R L layer sequence of the P n m a phase. The density of APBs increases with increasing x resulting in the formation of slabs of the I 2 m b phase up to a complete phase transformation. Dipole–dipole interactions between the tetrahedral chains are discussed as a possible driving force causing various patterns of tetrahedral chain ordering.

Shadow spectral imaging of absorbing layers in a transversely heated graphite atomizer: Part 1. Analyte atoms

The technique of shadow spectral imaging was used to investigate dynamics of formation and dissipation of Ag, In, Ga, Bi, Mn, Cu and Tl atomic layers in a transversely heated graphite tube atomizer (THGA) with and without integrated platform under gas-stop and gas-flow conditions. It is shown that non-uniform heating of the tube walls and platform surface in the radial cross section is the main reason for analyte transfer from atomizer bottom to less heated sides of the tube and platform before atomization temperature is reached. This transfer in the atomizer transverse cross section can be an additional factor that reduces matrix interferences in the THGA. In all the investigated cases, the atomic absorbing layers are not spatially uniform. Absorbance gradients grow up to 0.2 mm − 1 even in the case of chemically inert silver atomization. Inverse atomization of In, Bi, Ga and Tl when atoms first appear in the atomizer's upper part was detected in THGA with platform. The effect of the internal gas flow on the spatial structure of analyte atoms is less pronounced in the transversely heated atomizer as compared to the end-heated furnaces.

Structural characterization of Al xGa 1− xSb films grown at low temperatures by liquid phase epitaxy

High resolution X-ray diffraction (HRXRD) and far-infrared reflectivity techniques were applied to characterize Al x Ga 1− x Sb alloys. Layers of Al x Ga 1− x Sb grown by the liquid phase epitaxy technique and deposited on GaSb (100) substrates were obtained in the temperature range of 250 to 450 °C. From the HRXRD measurements it can be inferred that the films have good structural characteristics, this is because the lattice mismatch values were no bigger than 0.02% and from the rocking curves the Al concentration was ranged from 0.04 to 0.058. The presence of the ternary alloy in the films was confirmed by reflectivity. A change of the conductivity type in the film was observed for films grown at temperatures lower than 350 °C.

In-Situ Investigation of Surface Stoichiometry During InGaN and GaN Growth by Plasma-Assisted Molecular Beam Epitaxy Using RHEED-TRAXS

AbstractReflection high-energy electron diffraction total-reflection-angle x-ray spectroscopy (RHEED-TRAXS) uses high-energy electrons from RHEED to excite x-ray fluorescence. Monitoring characteristic x-rays of selected elements thus allows study of surface coverage of materials. In this study, surface coverage of Ga and In during growth of GaN and InGaN was probed using this technique. Evolution of the surface layer of Ga on GaN during growth and deposition of Ga on static GaN at room temperature were studied. RHEED-TRAXS measurements were performed during growth of InGaN by measuring the ratio of the In Lα to Ga Kα intensity. A significant surface coverage of In was observed at all temperatures investigated regardless of actual In incorporation.

Growth of silica nanowire arrays by reaction of Si substrate with oxygen using Ga as catalyst

Silica nanowire arrays were grown by oxidizing Si substrates with Ga catalyst in temperatures of 520–900 °C. The Si substrates, painted with a layer of molten Ga, were placed on a quartz boat, and heated up in a tube furnace. At high temperatures, Ga atoms condense into spheres, along with a small amount of silicon atoms. Si–O–Ga then formed on the surface of Ga–Si alloy sphere, and silica nanowire arrays were eventually grown with typical diameters of about 15–20 nm. A growth model based on extended vapor–liquid–solid mechanism is suggested.

Electronic structure and magnetism of diluted magnetic semiconductors—a first principles study

We present first-principles electronic structure calculations within density functional theory using a plane-wave pseudopotential method for Mn-doped diluted magnetic semiconductors. In particular, we show here a comparative study of Mn-doped GaAs and GaN systems and conclude that the origin of ferromagnetic long-range order is different for the two cases. Finally, we show theoretical simulations of scanning tunelling microscope (STM) images in Mn-doped GaAs. The simulated images with Mn in second Ga layer from a GaAs(1 1 0) surface are in excellent agreement with the experimental images.

In situ investigation of liquid Ga penetration in Al bicrystal grain boundaries: grain boundary wetting or liquid metal embrittlement?

The phenomenon of grain boundary penetration (GBP) of liquid Ga along grain boundaries (GB) of Al bicrystals is investigated by synchrotron radiation X-ray microradiography. From the three different types of bicrystals studied, only the one with the highest GB energy showed GBP in the absence of applied external stress. In situ observations of the penetration process reveal a linear propagation of the penetration front, accompanied by a continuous thickening of the wedge-shaped Ga layer in the GB. The experimental results demonstrate that GBP kinetics are strongly influenced by very weak levels of stress and tend to indicate that such stresses may be a prerequisite for the formation of nanometric penetration layers.

Defect characteristics of ZnO film grown on [formula omitted] sapphire with an ultrathin gallium wetting layer

The defect characteristics of ZnO film grown on ( 0 0 0 1 ) sapphire substrate using an ultrathin Ga wetting layer are investigated by transmission electron microscopy and X-ray diffractometry compared to that of the ZnO film without Ga. It was found that the defects of the ZnO film with a Ga layer were prominently reduced and a high-quality film was formed. Within this ZnO film, most defects near the interface are mixed-type dislocations that interact strongly, leading to a remarkable reduction of dislocations in the upper part of the epitaxial film with a total dislocation density of as low as 8×10 8 cm −2. Almost no pure screw dislocations were observed. Furthermore, the film exhibits a single domain structure, and no inversed domains were found. The role of the ultrathin gallium layer in the defect reduction and inversion domain suppression is discussed.

Droplet heteroepitaxy of GaN quantum dots by metal-organic chemical vapor deposition

Optically active GaN quantum dots on conductive AlGaN templates are synthesized by droplet heteroepitaxy, whereby the Ga droplets are converted to GaN islands in the presence of ammonia at 600°C. We have investigated the evolution of metallic Ga layers on AlGaN, obtaining the optimal surface densities and size distribution of the Ga droplets. The stability of GaN islands is influenced by the surface kinetics and the initial droplet size; the condition of Ga deposition and subsequent nitrogen exposure is identified, which preserves the initial density of the Ga droplets. A nitrogen-rich environment is identified as a necessary condition for maintaining the optimal GaN morphology by suppressing the Ga surface diffusion and preventing two-dimensional layer growth.

In situ cleaning of GaN(0001) surfaces in a metalorganic vapor phase epitaxy environment

The presence of ammonia in a metalorganic vapor phase epitaxy (MOVPE) system configured for the growth of GaN films is necessary and sufficient to remove adsorbed O- and C-containing species from the (0001) surface of this compound without measurable decomposition of this surface. Chemical and microstructural evidence in support of this statement has been obtained from investigations concerned with the extension of a previously developed in situ process route for cleaning heated GaN(0001) surfaces in a molecular beam epitaxy environment using flowing ammonia to the higher pressure regime of MOVPE. Thin films of GaN were deposited under 20Torr total pressure on GaN templates, previously exposed to the laboratory ambient, and heated in either a nitrogen∕hydrogen or an ammonia∕hydrogen mixture to the deposition temperature of 1020°C. Secondary ion mass spectroscopy of these samples revealed significant concentrations of carbon and oxygen at the GaN∕GaN interface in the former and the absence of these contaminants above the detection limits of the instrument in the latter. The surfaces of the templates heated in the nitrogen∕hydrogen atmosphere also decomposed sufficiently to form a very thin liquid Ga layer that reacted with ammonia to form a GaN-containing film either at the outset of film growth or on cooling in an ammonia∕nitrogen atmosphere. Atomic force microscopy (AFM) showed a smoother surface for the GaN films deposited on templates heated and cleaned in the ammonia∕hydrogen mixture relative to films deposited on templates heated in the nitrogen∕hydrogen mixture. The latter surface contained both a higher density of step terminations, indicative of a higher density of threading dislocations having screw and mixed character, and pits.

Defect structure ofGa1−xMnxAs: A cross-sectional scanning tunneling microscopy study

We have studied the atomic scale structure of molecular beam epitaxy grown Ga1-xMnxAs compounds with various Mn concentrations by cross-sectional scanning tunneling microscopy and first principles calculations. Only bright protrusions close to the top layer As atoms are directly correlated with the bulk Mn concentration. Atomically resolved filled and empty states images of this defect are compared to images derived from first principles calculations. We identify the Mn related defect as substitutional Mn in the second layer Ga site. Surprisingly no substitutional Mn is observed in the top-most Ga layer. The experimental results are consistent with the energetics of our first principles total energy calculations.

The structure, morphology, and composition of ultrathin Fe films on CoGa(100) at 550 K

The growth, structure, morphology, and composition of Fe films deposited on CoGa(100) at 550 K was studied by thermal energy helium atom scattering (TEAS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). The misfit between the lattice constants of Fe and CoGa is very small (<0.5%) and, thus, at 550 K, a layer-by-layer growth is found. The TEAS and LEED experiments show that iron films grow at 550 K with the bcc-Fe(100) plane parallel to the substrate surface. On the bcc-Fe(100) surface, a c(2×2) structure is observed, which is explained by Ga atoms floating on top of the surface. The Ga layer could be removed by Ar + ion sputtering, and a clean (1×1)−Fe(100) structure could be established.

TOF–LEIS analysis of ultra thin films: Ga and Ga-N layer growth on Si(1 1 1)

In the paper the ability of TOF–LEIS to analyse the thickness and growth modes of Ga and Ga-N ultra thin films was demonstrated. It is shown that multiple scattering, which usually complicates the spectra evaluation, can be successfully used for the thickness determination of ultra thin films. A linear dependence of the half-width of the Ga peak on the Ga-surface coverage within the range of 1–9 ML in the TOF–LEIS spectrum of Ga layers grown on Si(1 1 1) was found. Additionally, the development of Ga- and Si-peak heights with the Ga-surface coverage suggests the Ga layers grow in the way close to the layer-plus-island growth mode.

Core-level photoemission and near-edge x-ray absorption fine-structure studies of GaN surface under low-energy ion bombardment

We have investigated compositional changes on GaN surfaces under low-energy Ar ion bombardment using synchrotron-based high-resolution core-level photoemission measurements and near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around the N 1s core level reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more detail by x-ray absorption measurements at the N K-edge. We have proposed a mechanism for the relocation and loss of nitrogen during ion bombardment, in agreement with some recent experimental and theoretical studies of defect formation in GaN. We have also demonstrated that photoemission spectroscopy and NEXAFS provide a powerful combination for studying compositional changes and the creation of point defects at GaN surface.

Nitrogen-dependent optimum annealing temperature of Ga(As,N)

We have investigated the optimum annealing temperature of Ga(As,N) layers with nitrogen concentrations ranging from 0.06% to 6.1%. Photoluminescence (PL) measurements show a strong blueshift and an increase in the PL intensity with higher annealing temperatures. The optimum annealing temperature was determined according to the highest PL intensity of the annealed Ga(As,N) samples. We have found that the optimum annealing temperature is nitrogen-dependent and decreases with increasing nitrogen concentration. Raman spectroscopy reveals growth-induced defects in Ga(As,N) that are removed during the thermal treatment. We have also observed degradation of the structural quality during the annealing process by means of X-ray diffraction measurements. From transmission electron microscopy, we were able to directly measure nitrogen diffusion. However, this nitrogen diffusion is not pronounced enough to explain the structural degradation determined by X-ray diffraction.

Formation of GaAs by annealing of two-layer Ga-As electrodeposits

Polycrystalline GaAs was obtained by heat-treatment of sequentially electrodeposited As and Ga layers. The single layers were electrodeposited galvanostatically from alkaline Ga and acidic As solutions. The structural, morphological and compositional evolution of two-layers brought about by heat-treatment at various temperatures as a function of time was studied by SEM-EDX, laser profilometry and X-ray diffractometry. The formation of GaAs occurs by reaction-diffusion. The value of the Ga diffusion coefficient was determined in the temperature range 80–190°C and correspondingly resulted to vary from 1.4×10−21 to 4.8×10−18m2s−1. Furthermore the activation energy (100kJmol−1) and the frequency factor (9.1×10−7m2s−1) of the process were evaluated. The results are discussed in terms of structural and morphological factors and related to the results on the Ga-Sb, In-As and In-Sb systems previously studied in our group.

Role of gallium wetting layer in high-quality ZnO growth on sapphire (0001) substrates

A Ga wetting layer was used to modify the surface structure of sapphire (0001) substrate to prepare high-quality ZnO film by radio frequency plasma-assisted molecule beam epitaxy. We found that this Ga layer plays a crucial role in eliminating 30° rotation domains, controlling polarity and decreasing defect density in ZnO epilayers, as demonstrated by in situ reflection high energy electron diffraction,ex situ high resolution X-ray diffraction and high resolution cross-sectional transmission electron microscopy. Zn-polar film of ZnO was determined by convergent beam electron diffraction. A Ga bilayer model is proposed to understand the effects of the Ga wetting layer on high-quality ZnO growth.

Compositional changes on GaN surfaces under low-energy ion bombardment studied by synchrotron-based spectroscopies

We have investigated compositional changes on GaN surfaces under Ar-ion bombardment using synchrotron-based high-resolution x-ray photoemission (PES) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around N 1s core levels reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more details by x-ray absorption measurements at N K edge. We have demonstrated that PES and NEXAFS provide a powerful combination for studying the compositional changes on GaN surfaces. A mechanism for the relocation and loss of nitrogen during ion bombardment in agreement with some recent experimental and theoretical studies of defect formation in GaN has been proposed.

Oxygen and Sulfur Adsorption Effects on Electronic States of GaAs(100) Surfaces Studied with Discrete Variational Xα Method

The electronic states of Ga-terminated and As-terminated GaAs(100)-(1×1) surfaces and those surfaces with adsorbed O and S are studied by molecular orbital calculation using small cluster models and the discrete variational Xα method. Without adsorption of any foreign atoms, the Ga-terminated surface has 4p-related surface states in the middle of the forbidden band. While, the As-terminated surface has 4p-related surface states in the lower half of the forbidden band. The adsorption sites of chalcogen atoms on Ga- and As-terminated surfaces are determined ab initio by using density functional theory (Gaussian 98). With adsorbed chalcogen atoms, the density of the surface states in the forbidden band of the surface Ga layer markedly decreases, and that of the surface As layer increases. The adsorbates have states in the lower half of the band gap (Ga-terminated surface) or in the valence band (As-terminated surface). The results suggest that a chalcogen adsorbed Ga-terminated and bare As-terminated surface have low surface densities in the upper half of the band gap.