Splitting of transverse optical phonon modes localized in GaAs quantum wires on a faceted (311)A surface

The energy splitting of fundamental localized transverse optical (TO1) phonon modes in GaAs/AlAs superlattices and quantum wires grown by molecular-beam epitaxy on a faceted (311)A GaAs surface is observed by Raman spectroscopy. The form of the Raman scattering tensor makes it possible to observe the TO_x and TO_y modes separately, using different scattering geometries the y and x axes are the directions of displacement of the atoms and are directed parallel and transverse to the facets on the (311)A surface). Enhancement of the splitting of the TO1_x and TO1_y modes is observed as the average thickness of the GaAs layers is decreased from 21 to 8.5 Å. The splitting is probably due to the effect of the corrugation of the GaAs/AlAs (311)A hetero-interface on the properties of localized phonon modes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 45–48 (10 July 1997)     

Deep microrelief measurement and stereo photography in scanning electron microscopy

Fundamental differences between standard photography and micro-object imaging in a scanning electron microscope are analyzed. Basic ways of taking stereograms with a scanning electron microscope by the object rotation method and the observing technique are described. It is shown that stereo photography correctly estimates the spatial arrangement of features in objects with an intricate microrelief and makes it possible to very accurately calculate the depth of this relief (which sometimes exceeds the field of vision at large magnifications) in wide limits (from 0.5 to 100.0 μm). Using the developed surface of Li₄Ti₅O1₂ oxide and GaAs whiskers as examples, a stereographic computational method based on linear measurements of the same area on the object before and after rotation is demonstrated.     

Effect of gamma irradiation on photoconversion characteristics of metal/gallium arsenide barrier structures with textured interface

The effect of ⁶⁰Co gamma irradiation at doses of 10³-2×10⁵ Gy on the photoconversion and dark I-V characteristics of Au/GaAs surface-barrier solar cells (SCs) is studied. The morphology of the interface microrelief is varied to reach the highest photoconversion efficiency. Of the two types of microrelief morphology (dendritic and quasi-grating) obtained by the chemical anisotropic etching of n-(100)GaAs, the latter is more promising, particularly for SCs designed for space application, since the associated SCs offer higher efficiency and radiation resistance.     

A comparison of the effectiveness of GaAs surface passivation with sodium and ammonium sulfide solutions

To explore the possibility of improving the effectiveness of chemical and electronic passivation, a study has been made of the properties of GaAs surface treated with solutions of inorganic sulfides [Na₂S and (NH₄)₂S] in various amphiprotic solvents (water, alcohols). X-ray photoelectron and photoluminescence spectroscopy shows that the efficiency of both chemical and of electronic passivation of GaAs surface increases with decreasing dielectric permittivity of the solvent. The degree of this increase reached with solutions of the sulfide of a strong base (Na₂S) is larger than that of a weak-base sulfide [(NH₄)₂S]. Fiz. Tverd. Tela (St. Petersburg) 39, 63–66 (January 1997)     

Optics of excitons in systems with sharp heteroboundaries. Approximation of a strongly localized exciton wave function

Optical reflection spectra from a Ga_0.7Al_0.3As/GaAs heteroboundary are calculated using the approximation of a strongly localized exciton wave function. The calculation is based on electron Γ₆ and hole Γ₈ kp-Hamiltonians with position-dependent parameters. Fiz. Tverd. Tela (St. Petersburg) 40, 872–874 (May 1998)     

Photoemission study of the adsorption of O2, CO and H2 on GaAs(110)

Ultraviolet photoemission spectroscopy with hv < 12 eV has been used to study O₂, CO, and H₂ adsorption on the cleaved GaAs(110) face. It was found that O₂ exposures above 10⁵ L(1LM = 10^?6 Torr sec) were required to produce changes in the energy distribution curves. At O₂ exposures of 10⁶ L on p-type and 10⁸ L on n-type an oxide peak is observed in the EDC's located 4 eV below the valence band maximum. On p-type GaAs, O₂ exposures cause the Fermi level at the surface to move up to a point 0.5 eV above the valence band maximum, while on n-type GaAs O₂ exposures do not remove the Fermi level pinning caused by empty surface states on the clean GaAs. CO was found to stick to GaAs, but to desorb over a period of hours, and not to change the surface Fermi level position. H₂ did not affect the EDC's, but atomic H lowered the electron affinity and raised the surface position of the Fermi level on p-type GaAs. A correlation is found in which gases which stick to the GaAs cause an upward movement of the Fermi level at the surface on p-type GaAs, while gases which stick only temporarily do not change the surface position of the Fermi level.     

Controlling the nanodot formation on GaAs surface during focused ion beam processing

GaAs processed using gallium-focused ion beams for the fabrication of photonic devices mostly results in gallium nanodots on the surface. These gallium nanodots may produce unwanted effects and deteriorate the optical and electrical properties of the devices. We have investigated the FIB processing of GaAs with and without exposure to an insulator-enhanced etching precursor gas (XeF₂) to explore the use of XeF₂ during GaAs processing. It is reported that without the gas, FIB processing results in nanodots on the surface that vary in size and density depending on processing parameters such as incident energy, beam current, angle and dwell time. Processing with insulator (XeF₂)-enhanced etching gas irrespective of the process parameters eliminates the nanodots and results in a smooth surface, as characterized by scanning electron microscopy and atomic force microscopy. This method will be useful for surfaces which require dry processing without exposure to any wet chemical etching.     

Effect of dissipation on the properties of surface polaritons in GaAs/AlGaAs heterojunctions in a quantizing magnetic field

A study is reported of nonradiative surface and bulk polaritons in GaAs/Al_xGa_1−x As real heterojunctions under conditions favoring integer-quantum Hall effect (IQHE) and in the presence of dissipation in a two-dimensional electron layer. The conditions of their existence, the spectrum, and damping have been determined. It is shown that under IQHE conditions all aspects of surface and bulk polaritons are quantized. It is found that, as the wave number is varied, surface and bulk polaritons can transform continuously into one another. The possibilities of experimental observation of nonradiative polaritons are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 705–711 (April 1999)     

Single-layer antireflection coatings for GaAs solar cells

Optimal parameters of antireflection coatings for GaAs solar cells are simulated and calculated by the optical matrix approach method. It is shown that by using single-layer Ta₂O₅ and ITO antireflection coatings it is possible to reduce essentially the reflectance from the surface of GaAs solar cells.     

Spin-wave spectroscopy and application of its methods to heterostructures of silicon dioxide with Co nanoparticles on a GaAs substrate

A method has been developed for determining magnetic and electrical characteristics of film nanostructures containing magnetic nanoparticles from dispersion curves of surface spin waves propagating in these nanostructures. The dispersion curves of spin waves are determined by the dynamics of the spin component described by the generalized Landau-Lifshitz equations and an alternating electromagnetic field induced by a spin wave. Since spin waves are very sensitive to inhomogeneity of magnetic parameters, spin disorder, and conductivity of an object near or inside which these waves propagate, they can be used for determining magnetic and electrical characteristics of the objects under investigation. The developed calculation method, which can be employed both in spin-wave spectroscopy and in analysis of dispersion curves obtained by other methods, has been used for determining parameters of heterostructures consisting of a SiO₂ film with Co nanoparticles on a GaAs substrate. It has been found from the shape of dispersion curves of the surface spin waves that, in the film near the interface, spins of the nanoparticles are close to a ferromagnetic ordering, whereas near the free surface, the spin orientation of nanoparticles is more chaotic. It has been revealed that a conducting layer is formed in GaAs, and the SiO₂(Co) film near the interface has an increased conductivity.     

Morphology and optical properties of p-type porous GaAs(1 0 0) layers made by electrochemical etching

Porous GaAs layers were formed by electrochemical etching of p-type GaAs(1 0 0) substrates in HF solution. A surface characterization has been performed on p-type GaAs samples using X-ray photoelectron spectroscopy (XPS) technique in order to get information about the chemical composition, particularly on the surface contamination. According to the XPS spectra, the oxide layer on as-received porous GaAs substrates contains As₂O₃, As₂O₅ and Ga₂O₃. Large amount of oxygen is present at the surface before the surface cleaning.Compared to untreated GaAs surface, room temperature photoluminescence (PL) investigations of the porous layers reveal the presence of two PL bands: a PL peak at ∼871 nm and a “visible” PL peak at ∼650-680 nm. Both peak wavelengths and intensities varied from sample to sample depending on the treatment that the samples have undergone. The short PL wavelength at 650-680 nm of the porous layers is attributed to quantum confinement effects in GaAs nano-crystallites. The surface morphology of porous GaAs has been studied using atomic force microscopy (AFM). Nano-sized crystallites were observed on the porous GaAs surface. An estimation of the mean size of the GaAs nano-crystals obtained from effective mass theory and based on PL data was close to the lowest value obtained from the AFM results.     

不同生长环境下砷化镓纳米颗粒的应变场模拟

对于埋嵌在薄膜材料中的纳米颗粒,在其生长过程中总是不可避免地伴随着应变场的产生,而这种应变场的分布能反映纳米颗粒的结构变化,纳米颗粒结构与它的物理性能有重要的关系.研究埋嵌在不同薄膜材料中的纳米颗粒生长过程中的应变场分布对于调控纳米颗粒的物理性能有着重要的意义.本文利用有限元算法分别计算了埋嵌在非晶氧化铝薄膜和非晶二氧化硅薄膜材料中的砷化镓纳米颗粒生长过程中的应变场分布.砷化镓纳米颗粒在以上两薄膜材料生长过程中都受到非均匀偏应变作用.对于埋嵌在氧化铝薄膜中的砷化镓纳米颗粒,其生长过程中,纳米颗粒内部受到的应变大于纳米颗粒表面受到的应变;而对于埋嵌在二氧化硅薄膜中的砷化镓纳米颗粒,纳米颗粒内部受到的应变小于纳米颗粒表面受到的应变.选择砷化镓纳米颗粒生长的薄膜材料可以调控纳米颗粒生长过程中的应变场分布,从而进一步调控纳米颗粒的晶格结构和形貌及其物理性能.     

In situ characterization of MBE grown GaAs and Al x Ga1−x As films using RHEED,SIMS, and AES techniques

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and Al _x Ga_1−x As films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and Al _x Ga_1−x As by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.     

Microrelief dependent on the temperature and external field on the free surface of a smectic-<Emphasis Type="Italic">A</Emphasis> liquid crystal

A homeotropically oriented smectic-A film on the surface of a solid substrate with a periodic microrelief is considered. Relationships are derived for describing the dependences of the microrelief-induced deformation of the free surface of the film on the film thickness, the temperature, and the external magnetic (electric) field strength. It is shown that, for a specific choice of the microrelief shape on the substrate surface, a variation in the sample temperature and the external magnetic (electric) field strength makes it possible to control not only the microrelief depth but also the shape of the microrelief formed on the free surface of the smectic-A film.     

Tunneling resonances in structures with a two-step barrier

Electron transport through an asymmetric heterostructure with a two-step barrier N⁺GaAs/N⁻GaAs/Al_0.4Ga_0.6As/Al_0.03Ga_0.97As/N⁻GaAs/N⁺GaAs was investigated. Features due to resonance tunneling both through a size-quantization level in a triangular quantum well, induced by an external electric field in the region of the bottom step of the barrier (Al_0.03Ga_0.97As layer), and through virtual levels in two quantum pseudowells of different width are observed in the tunneling current. The virtual levels form above the bottom step or above one of the spacers (N⁻GaAs layer) as a result of interference of electrons, in the first case on account of reflection from the Al_0.4Ga_0.6As barrier and a potential jump at the Al_0.03Ga_0.97As/N⁻GaAs interface and in the second case — from the Al_0.4Ga_0.6As barrier and the potential gradient at the N⁻GaAs/N⁺GaAs junction, reflection from which is likewise coherent. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 814–819 (25 May 1998)     

GaAs(100)表面硫钝化的新方法:CH3CSNH2/NH4OH处理

建立了一种硫钝化GaAs(100)表面的新方法,即CH₃CSNH₂/NH₄OH溶液处理,应用同步辐射光电子能谱(SRPES)和X射线光电子能谱(XPS)表征了该钝化液处理的n-GaAs(100)表面的成键,特性和电子态.结果表明,经过处理的n-GaAs(100)表面,S既与As成键也与Ga成键,形成S与GaAs的新界面,并且Ga和As的氧化物被移走,这标志着CH₃CSNH₂/NH_{4关键词：}     

P2S5/NH4OH处理GaAs(100)表面的电子能谱研究

采用俄歇电子能谱(AES)和X射线光电子能谱(XPS)研究了P₂S₅/NH₄OH钝化液处理的GaAs(100)表面的微观特性。AES测量表明,在钝化膜和GaAs衬底之间的界面处无O组分,只有P和S组分。XPS测量分析指出,经过P₂S₅/NH₄OH溶液处理后,GaAs表面处Ga₂O₃和As₂O_{3关键词：}     

Optical characteristics of highly ordered gallium oxide nano-islands on GaAs

A periodic array of Ga oxide islands was obtained by annealing the highly ordered Ga nano-droplets on GaAs surface at 400°C under an oxygen atmosphere for 7 hours. These Ga oxides are a mixture of α-Ga₂O₃ and β-Ga₂O₃ confirmed by Raman spectroscopy study. Enhanced optical transmission of GaAs with such ordered Ga oxide nano-islands was obtained. Both dielectric and dimensional confinement effects were considered in analysis of the electromagnetic characteristics of the nanostructured materials. Finite-difference time-domain method was used to numerically study the light transmission through the patterned Ga oxide on GaAs surface. Based on the calculated results, the light transmission enhancement is attributed to the formation of the ordered nano Ga oxides.     

The formation and thermal stability of multilayer ohmic contacts to n-GaAs with TiBx and Mo diffusion barriers

Thermal degradation of Au/Mo/TiB_x/AuGe multilayer ohmic contacts with Mo and TiB _x diffusion barriers was studied. The contacts were employed in Gunn-effect diodes. Depth profiling of the components in the contacts was performed using Auger electron spectroscopy. The microrelief of the metal/semiconductor interface and contact surface morphology were examined with atomic force microscopy and scanning electron microscopy, respectively. The measurements were taken before and after argon annealing at T=400, 600, or 800°C for 60 s. The resistance of the Gunn diode mesa was also measured. Annealing at 400°C is shown not to affect the sandwich structure of the contacts. Annealing at 600°C causes structure rearrangement in the layers up to cracking. It is found that the thermal threshold of degradation of the Au/Mo/TiB_x /AuGe/GaAs structure depends on the resistance of the TiB_x layer to thermal effects. Reasons for the degradation of Mo and TiB_x antidiffusion properties are discussed.