Growth and characterization of polycrystalline Ge1−xCx by reactive pulsed laser deposition

Polycrystalline thin films of Ge-C were grown on Si (1 1 1) substrates by means of reactive pulsed laser deposition with methane pressure of 100 mTorr. Effect substrate temperature, T_s, on C incorporation to substitutional sites (x) in Ge_1−xC_x was investigated systematically by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyzes. The substrate temperatures were ranging from 250 to 400 °C. The substitutional C composition x in the films by XRD were estimated using the Vegard's linear law. The maximum value of x calculated by XRD was 0.032 for T_s of 350 °C. The position of the C 1s peak at 283.4 eV in the XPS spectrum confirmed the germanium-carbon alloys. XRD measurements indicated that x increased with T_s from 250 °C to 350 °C. At T_s = 400 °C, the estimation of x was lowered. However, the C content calculated by XPS analyzes increased with T_s being more these values than substitutional C composition x. XPS and XRD analyzes demonstrate that the remaining C atoms are incorporated to interstitial sites. The use of the T_s plays important roles in the incorporation of substitutional C and in restraining C-cluster formation in the reactive pulsed laser deposition growth of Ge-C/Si.     

Growth of Si whiskers by MBE: Mechanism and peculiarities

We analyzed the stress-driven mechanism of MBE Si whisker growth. It is shown that the driving force for MBE whisker growth is determined by the relaxation of elastic energy stored in the overgrown layer L_s due to gold intrusion. In this case the supersaturation is determined by the interplay between elastic stresses and surface energy. The latter is considerably decreased due to decoration of the Si surface by gold resulting in formation of thin liquid Si/Au eutectic layer. This suggests that in our case the Si supersaturation is not an independent growth parameter as it is in the chemical vapor deposition growth method. Instead it is determined by stress in the overgrown Si layer. This approach allows us to explain quite well the growth kinetic and the relationship between the radius and the length of the whiskers. The whisker growth in our case can be considered as a stress relaxation mechanism, where the stress relaxation occurs due to transition from the two-dimensional system to the three-dimensional one.     

Effect of lower growth temperature on C incorporation in GeC epilayers on Si(0 0 1) grown by MBE

Effect of lower growth temperature T_s on C incorporation to substitutional sites in Ge_1−xC_x/Si(0 0 1) grown by molecular beam epitaxy was investigated. To enhance the non-equilibrium growth condition, the temperature T_s was lowered from 600°C down to 300°C. The C incorporation into substitutional sites of GeC epilayers was very sensitive to T_s. X-ray diffraction (XRD) measurement indicated that the substitutional C composition x increased with decrease in T_s from 600°C to 400°C. At T_s350°C, the estimation of x by the XRD analysis was impossible because of polycrystallization. The Raman shift measurement enables to estimate x for T_s350°C, as consequently larger x than that grown at T_s=400°C was verified. The enhancement of non-equilibrium growth condition by decreasing T_s was important to increase x.     

超导Nb-Ge溅射膜的俄歇电子能谱及电子能谱化学分析

利用表面分析仪器俄歇电子能谱(AES)仪和电子能谱化学分析(ESCA)仪对超导Nb-Ge溅射膜进行表面和深度剖面分析,以探讨在溅射沉积成膜过程中A15结构Nb₃Ge能够稳定生长的机制,以及影响其转变温度T_c的因素。分析结果表明,超导Nb-Ge膜中含有O,C和Al等杂质,其中O的含量对T_c有重要影响。表面上未发现有聚集的Ge。Nb/Ge密度比由表面层的3.1向内部逐渐降至2.26。X射线光电子能谱(XPS)的分析表明:Nb,Ge,C和O的谱峰沿深度均有不同程度的化学位移和峰形变化,这明这些元素的化学态在膜内是复杂的。但其中Nb的变化最小,可能对高T_cA15Nb₃Ge相的生长起稳定作用。 关键词：     

Growth, microstructure and electrical properties of sputter-deposited hafnium oxide (HfO2) thin films grown using a HfO2 ceramic target

Hafnium oxide (HfO₂) thin films have been made by radio-frequency (rf) magnetron-sputtering onto Si(1 0 0) substrates under varying growth temperature (T_s). HfO₂ ceramic target has been employed for sputtering while varying the T_s from room temperature to 500 °C during deposition. The effect of T_s on the growth and microstructure of deposited HfO₂ films has been studied using grazing incidence X-ray diffraction (GIXRD), and high-resolution scanning electron microscopy (HR-SEM) coupled with energy dispersive X-ray spectrometry (EDS). The results indicate that the effect of T_s is significant on the growth, surface and interface structure, morphology and chemical composition of the HfO₂ films. Structural characterization indicates that the HfO₂ films grown at T_s < 200 °C are amorphous while films grown at T_s > 200 °C are nanocrystalline. An amorphous-to-crystalline transition occurs at T_s = 200 °C. Nanocrystalline HfO₂ films crystallized in a monoclinic structure with a (−1 1 1) orientation. An interface layer (IL) formation occurs due to reaction at the HfO₂-Si interface for HfO₂ films deposited at T_s > 200 °C. The thickness of IL increases with increasing T_s. EDS at the HfO₂-Si cross-section indicate that the IL is a (Hf, Si)-O compound. The electrical characterization using capacitance-voltage measurements indicate that the dielectric constant decreases from 25 to 16 with increasing T_s. The current-voltage characteristics indicate that the leakage current increases significantly with increasing T_s due to increased ILs.     

Quantitative evaluation of substrate temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy

Combined Hall effect and low-temperature photoluminescence measurements have been used to perform a thorough evaluation of the growth temperature dependence of Ge incorporation in GaAs during molecular beam epitaxy (MBE) over the entire substrate temperature range (400°≦T _s ≦600[°C]) practicable forn-type layer growth. Using a constant As₄ to Ga flux ratio of two, growth below 500°C yieldsn-GaAs: Ge films having electrical and optical properties rapidly deteriorating with decreasingT _s . Growth at 500° ≦T _s ≦600[°C] produces high-qualityn-GaAs: Ge films (N _D /N _A ≈4) with C as well as Ge residual acceptors competing on the available As sites. The amount of Ge atoms on As sites [Ge_As] increases with substrate temperature, whereas simultaneously the amount of C atoms on As sites [C_As] decreases thus leading to the well-establishednonlinear behaviour of the (N _A /N _D vs. 1/T _s plot. Counting the incorporated Ge impurities separately, however, yields alinear behaviour of the ([Ge_As]/[Ge_Ga]) vs. 1/T _s plot which has exactly the same slope as the (P _{As 2}/P _Ga) vs. 1/T _s plot derived from vapour pressure data of As₂ and Ga over solid GaAs surfaces. The important result is, therefore, that the incorporation behaviour of Ge in GaAs during molecular beam epitaxy is directly correlated with theevaporation behaviour of the growing GaAs surface.     

Dopant patterning in three dimensions during molecular beam epitaxial growth using anin-situfocused ion gun

Conventional molecular beam epitaxial (MBE) growth of III-V semiconductor material systems use a thermal silicon effusion cell as the n-type dopant source. In the technique described here, the thermal source is replaced with a scanning Si focused ion gun. It is therefore possible to directly write dopant patterns into the semiconductor waferit as it is being grown. The precise control over the elemental composition in the growth (z) direction permitted by MBE, combined with high spatial resolution of the focused dopant ion beam in the lateral (xy) plane, allows direct registration of fully integrated, three-dimensional semiconductor structures and devices otherwise unobtainable via conventional lithographic techniques. In this paper, we present electrical measurements of high mobility GaAs/AlGaAs modulation doped heterostructures fabricated using this technique.Fabrication of novel undoped channel FET structures is also discussed, where the dopant beam is used to form extended contacts to the induced electron gas. FET action is demonstrated for the first time using this technique, eliminating the constraint for self-alignment in undoped structures.     

Study of the features of BaF<Subscript>2</Subscript> heteroepitaxy on CaF<Subscript>2</Subscript>/Si(100) layers obtained in the high-temperature growth mode

The surface morphology of BaF₂ epitaxial films grown by MBE (molecular beam epitaxy) in various modes on the surface of CaF₂/Si(100) is investigated by AFM. The CaF₂ layers on Si(100) are obtained in the high-temperature growth mode (Т _S = 750°C). It is shown that the epitaxy of BaF2 at a temperature of 600°C at the initial stage of growth leads to the formation of defects such as perforations in the epitaxial film, while epitaxy at a temperature of 750°C provides a defect-free film with a surface morphology suitable for the subsequent growth of semiconductors of IV–VI type and solid solutions based on them.     

Stress development and impurity segregation during oxidation of the Si(1 0 0) surface

We have studied the segregation of P and B impurities during oxidation of the Si(1 0 0) surface by means of combined static and dynamical first-principles simulations based on density functional theory. In the bare surface, dopants segregate to chemically stable surface sites or to locally compressed subsurface sites. Surface oxidation is accompanied by development of tensile surface stress up to 2.9 Nm⁻¹ at a coverage of 1.5 monolayers of oxygen and by formation of oxidised Si species with charges increasing approximately linearly with the number of neighbouring oxygen atoms. Substitutional P and B defects are energetically unstable within the native oxide layer, and are preferentially located at or beneath the Si/SiO_x interface. Consistently, first-principles molecular dynamics simulations of native oxide formation on doped surfaces reveal that dopants avoid the formation of P-O and B-O bonds, suggesting a surface oxidation mechanism whereby impurities remain trapped at the Si/SiO_x interface. This seems to preclude a direct influence of impurities on the surface electrostatics and, hence, on the interactions with an external environment.     

Range evaluation in SIMS depth profiles of Er-implantations in silicon

In the last decade ion implantation of common dopants in silicon has been almost full characterised. However, data of inner transition elements are based on few measurements or even extrapolations. Our investigations focus on erbium, an upcoming dopant in photonic applications. Some of us have previously found errors of 20% in the projected range of Er in Si and SiO₂ when comparing the range profiles measured with SIMS and simulations using SRIM, T2D, and our own binary collision simulator IMSIL. Because of the far-reaching consequences, we have performed additional, more precise experiments to confirm our previous results.Equal doses of Er has been implanted into SIMOX wafers with energies of 100, 200, 300, 400, 500, and 600 keV. Profiles have been measured with secondary ion mass spectrometry (SIMS). Relative sensitivity factors (RSF) were gathered from low-energy implantations, remaining within the Si top layer. We used the Si/SiO₂ interface at exactly 217.7 nm to calibrate the depth scale of all profiles. In addition dynamical Monte-Carlo simulations of the sputter process were taken to correct the depth scale and the interface position.     

The effect of arsenic vapour species on electrical and optical properties of GaAs grown by molecular beam epitaxy

Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As₂) vs tetrameric (As₄) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As₂ and As₄ sources, respectively, and from a single source providing an adjustable As₂/As₄ flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As₄/As₂ flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As₂ molecular beam species which provide a higher steady-state arsenic surface population.     

Intermixing-controlled epitaxy for Si–Ge heterostructure devices

To fabricate high quality SiGe/Si heterostructures, control of intermixing between Si and Ge is essential during crystal growth. This paper describes the recent progress of ‘intermixing-controlled epitaxy’. A combined method of MBE (molecular beam epitaxy) and SPE (solid-phase epitaxy) was developed and used to fabricate a new heterostructure (n-Si_0.8Ge_0.2/Si channel/Si_1 − xGe_xbuffer layer/Si substrate). Observation by TEM demonstrated that the hetero-interface obtained by SPE was atomically flat. This interface provides the ultrahigh mobility of a two-dimensional electron gas (2DEG). In addition, the influence of atomic-hydrogen irradiation during MBE on Ge dispersion in the SiGe mixed crystal is examined. Results indicate that the number of Ge–Ge pairings was decreased by hydrogen irradiation. Such a decrease deformed the local symmetry of the Si–Ge bond from tetrahedral symmetry. As a result, photoluminescence intensity was sucessfully increased.     

Formation of rippled surface morphology during Si/Si （100） epitaxy by ultrahigh vacuum chemical vapour deposition

The Si epitaxial films are grown on Si (100) substrates using pure Si2H6 as a gas source using ultrahigh vacuum chemical vapour deposition technology. The values of growth temperature T_g are 650 ℃, 700 ℃, 730 ℃, 750 ℃, and 800 ℃. Growth mode changes from island mode to step-flow mode with T_g increasing from 650 ℃ to 700 ℃. Rippled surface morphologies are observed at T_g = 700 ℃, 730 ℃, and 800 ℃, but disappear when T_g = 750 ℃. A model is presented to explain the formation and the disappearance of the ripples by considering the stability of the step-flow growth.     

Segregation in In x Ga1− x As/GaAs Stranski–Krastanow layers grown by metal–organic chemical vapour deposition

Using quantitative high-resolution transmission electron microscopy we studied the chemical morphology of wetting layers in In _x Ga_{1? x} As/GaAs quantum dot structures which were optimized for applications to optical devices operating around 1.3?µm. The samples are grown by low-pressure metal–organic chemical vapour deposition on GaAs substrates. The In concentration profiles of the wetting layers are evaluated with the composition evaluation by lattice fringe analysis method. The profiles reveal a clear signature of segregation. A fit of the profiles with the Muraki et al. model for segregation reveals a segregation efficiency R?=?0.65?±?0.05 at the growth temperature of 550°C, which is significantly lower than segregation efficiencies observed in samples grown by molecular beam epitaxy at similar temperatures.     

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.     

Phenomenological description of the microwave surface impedance and complex conductivity of high-T c single crystals

Measurements of the microwave surface impedance Z _s (T) = R _s (T) + iX _s (T) and the complex conductivity σ_s(T) in the ab-plane of high-quality high-T _c YBCO, BSCCO, TBCCO, and TBCO single crystals are analyzed. Experimental data of Z _s (T) and σ_s(T) are compared with calculations based on a modified two-fluid model that includes a temperature-dependent quasiparticle scattering and a unique temperature variation of the density of superconducting carriers. We describe the agreement and disagreement of our analysis with the salient features of the experimental data. We review the existing microscopic models based on unconventional symmetry types of the order parameter and on novel quasiparticle relaxation mechanisms.     

Growth of fullerene on Ag and hydrogen-passivated Si substrates: Effect of electron beam exposure on growth modes

We have used Auger electron spectroscopy (AES) to investigate the effect of electron beam exposure on growth modes of fullerene (C₆₀) on substrates like Ag and hydrogen-passivated Si(1 1 1). The electron beam comprises of 3.4 keV electrons, which are used in the AES study. To investigate the effect, Auger signal (AS) vs. deposition time (t) measurements were conducted in a sequential mode, i.e., alternating deposition of C₆₀ and analysis using the electron beam. Duration of AES data collection after each deposition was the duration of exposure to electron beam in this experiment. For the growth study of C₆₀ on Ag, three AS-t plots were recorded for three different durations of exposure to electron beam. Changes in the AS-t plot, depending on the duration of exposure to the electron beam, reflect the electron beam-induced damage. Electron beam-induced damages of C₆₀ produce carbon materials of different densities and consequently transmission coefficient (α) of Auger electron through this material changes. In order to fit the AES (AS vs. t) data a model has been used which simultaneously provides the growth mode and the transmission coefficient. Observation of an increasing transmission coefficient with the increasing duration of exposure to the electron beam from α=0.34 to 0.60 indicates the change of the nature of the carbon material due to the partial damage of C₆₀.     

Kinetic limitations to surface segregation during MBE growth of III–V compounds: Sn in GaAs

The incorporation of Sn as a dopant in GaAs has been studied in the temperature range of 500°–650° C, over a wide range of Ga and As fluxes, the latter being incident as either As₄ or As₂ molecules. The results are explained in terms of a surface segregation model in which the behaviour at high growth temperatures (above 600 °C) approaches thermal equilibrium, but growth at lower temperatures involves a kinetic limitation to the segregation process.     

Coevaporation phosphorus doping in Si grown by molecular beam epitaxy

Phosphorus-doped Si epilayers with bulk-like mobilities were grown by molecular beam epitaxy (Si-MBE) by coevaporation of phosphorus from a tin phosphide source. The behaviour of P doping as a function of growth parameters and of potential enhanced doping indicates a non-unity, almost growth-temperature independent incorporation efficiency with negligible surface segregation -a unique combination among coevaporated dopants in Si-MBE.     

Composition depth profiles of Bi3.15Nd0.85Ti3O12 thin films studied by X-ray photoelectron spectroscopy

In the present work, X-ray photoelectron spectroscopy (XPS) was used to investigate the composition depth profiles of Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) ferroelectric thin film, which was prepared on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by chemical solution deposition (CSD). It is shown that there are three distinct regions formed in BNT film, which are surface layer, bulk film and interface layer. The surface of film is found to consist of one outermost Bi-rich region. High resolution spectra of the O 1s peak in the surface can be decomposed into two components of metallic oxide oxygen and surface adsorbed oxygen. The distribution of component elements is nearly uniform within the bulk film. In the bulk film, high resolution XPS spectra of O 1s, Bi 4f, Nd 3d, Ti 2p are in agreement with the element chemical states of the BNT system. The interfacial layer is formed through the interdiffusion between the BNT film and Pt electrode. In addition, the Ar⁺-ion sputtering changes lots of Bi³⁺ ions into Bi⁰ due to weak Bi-O bond and high etching energy.