Ge/Si薄膜材料生长的偏压效应研究

利用超高真空磁控浅射系统生长了不同偏压下Ge／Si薄膜材料,所生长的材料采用了不加偏压和加偏压的生长环境。通过X射线小角衍射分析表明,加一定偏压的Ge／Si薄膜材料的层状远比不加偏压的材料好,并且加偏压可有效降低材料的生长温度。在加15－25V偏压明,获得了300℃的生长温度下,层状优良,粗糙度小的薄膜材料。     

Low temperature interdiffusion in Au/In thin film couples

Interdiffusion in Au/In thin film couples was studied by in situ backscattering spectrometry. The substrate temperature was varied in the range from -170 to +50 °C. It was found that a uniform layer of AuIn₂ grows at the same rate as that at which gold condenses onto an indium film for substrate temperatures down to -50 °C. This is the fastest formation of intermetallic phases that has been reported. By lowering the substrate temperature the formation of an AuIn₂ layer during evaporation is suppressed. In this case the temperature of the thin film couple has to be raised considerably (to about 20 °C) to obtain interdiffusion within a reasonable time, and the final state of the thin film couple is different from that obtained when the phase formation is completed during evaporation. A possible explanation for this behaviour is discussed. The formation and growth of AuIn₂ after evaporation may be characterized by an activation energy of 0.23 eV.     

Room temperature interactions in Ni/metal thin film couples

Interactions in Ni/metal thin film couples at room temperature were studied. It was found that nickel reacts with gallium, indium and tin to form NiGa₄, Ni₂In₃, Ni₁₀In₂₇ and NiSn, but no reaction was observed with bismuth, cadmium, germanium, magnesium, manganese, antimony, samarium and zinc under the conditions employed.     

Polycrystalline Si1-x Ge x thin film deposition by rapid thermal chemical vapor deposition

Polycrystalline silicon germanium (poly-Si_1−xGe_x) thin films on a-Si film have been deposited by rapid thermal chemical vapor deposition (RTCVD) with SiH₄–GeH₄–H₂. Effect of GeH₄/SiH₄ and deposition temperature on stoichiometry (x), Si-Ge binding character, composition, hydrogen configuration, crystallinity, preferred orientation, grain size, and surface roughness of poly-Si_1−xGe_x films has been investigated. Poly-Si_1−xGe_x deposited on the substrate with amorphous silicon buffer layer on oxide shows better crystallinity and contains the less amount of oxygen than the one deposited directly on the oxide surface. At low temperature region, the Ge–H bond with the small amount of Si–H₂ bond is dominant but all hydrogen bonds are desorbed at high temperature. All films have polycrystalline phase and the grain size and (111) orientation increased with increasing deposition temperature in which Ge content also increases at the fixed gas flow rate of GeH₄ to total source gas. Poly-Si_1−xGe_x/Si thin film transistors (TFT) are fabricated and hydrogen during post-hydrogenation process preferentially is attached to Ge dangling bond and the TFT characteristics could be improved.     

Interdiffusion in PbIn thin film couples

Interdiffusion in PbIn thin film couples has been examined by an X-ray diffraction technique. Initial results indicate that the interdiffusion in such couples is significantly faster than that expected from reported bulk data.     

Interface transformations in thin film aluminum-gold diffusion couples

Interface phase transformations in 1 μm aluminum + 1 μm gold thin film couples upon 24-h isothermal annealing at 125, 150 and 175 °C were studied. It is shown that temperature change does not affect the type of phases formed but only alters their growth rates. The layered structures consisting of multiple intermetallics were identified by X-ray diffraction and electron microscopy. Interface tracking using secondary ion mass spectroscopy depth profiling showed that Au is the predominant diffusant in this system. Intermetallic thickness growth measurements were made and the activation energy calculated to be 33.602 kJ/mol.     

Real time spectroscopic ellipsometry of Ag/ZnO and Al/ZnO interfaces for back-reflectors in thin film Si:H photovoltaics

Real time spectroscopic ellipsometry (RTSE) has been applied to analyze the optical characteristics of Ag/ZnO and Al/ZnO interfaces used in back-reflector (BR) structures for thin film silicon photovoltaics. The structures explored here are relevant to the substrate/BR/Si:H(n-i-p) solar cell configuration and consist of opaque Ag or Al films having controllable thicknesses of microscopic surface roughness, followed by a ZnO layer up to ~ 3000 Å thick. The thicknesses of the final surface roughness layers on both Ag and Al have been varied by adjusting magnetron sputtering conditions in order to study the effects of metal film roughness on interface formation and interface optical properties. The primary interface loss mechanisms in reflection are found to be dissipation via absorption through localized plasmon modes for Ag/ZnO and through intraband and interband transitions intrinsic to metallic Al for Al/ZnO.     

Mechanisms of interdiffusion in Pd-Cu thin film diffusion couples

Interdiffusion in sputter-deposited polycrystalline Pd-Cu bilayers (thickness of each sublayer: 50 nm) was studied in the temperature range 175 °C-250 °C by sputter-depth profiling in combination with Auger electron spectroscopy. X-ray diffraction and transmission electron microscopy investigations revealed that the layers are polycrystalline, consisting of columnar grains separated by grain boundaries oriented more or less perpendicularly to the film surface. Considerable diffusional intermixing occurred in the studied temperature range, which was accompanied by the sequential formation of (ordered) phases Cu₃Pd and CuPd. Volume interdiffusion coefficients were determined using the so-called ‘centre-gradient’ and ‘plateau-rise’ methods. Grain-boundary diffusion coefficients of Pd through Cu grain boundaries were determined by the Whipple-Le Claire method and grain-boundary diffusion coefficients of Cu through Pd grain boundaries were determined by the Hwang-Balluffi method. It was found that both volume and grain-boundary diffusion coefficients decreased roughly exponentially with annealing time. Activation energies were determined which pertain to the same (defect) microstructure at each temperature. The differences with literature results for macroscopic diffusion couples were discussed.     

5. VCNR type characteristic in thin film amorphous TeGeAsSi films

In the hope of further understanding the memory and switching effects of amorphous evaporated semi-conductor films, the current-voltage curve of the quarternary system TeGeAsSi was experimentally examined and our findings are discussed below.     

Numerical modeling of SiH4 discharge for Si thin film deposition for thin film transistor and solar cells

Amorphous and microcrystalline silicon thin films are used in solar cells as a multi-junction photovoltaic device. Plasma enhanced chemical vapor deposition is used and high deposition rate of a few nm/s is required while keeping film quality. SiH₄ is used as a precursor diluted with H₂. Electron impact processes give complex interdependent plasma chemical reactions. Many researchers suggest keeping high H/SiH_x ratio is important. Numerical modeling of this process for capacitively coupled plasma and inductively coupled plasma is done to investigate which process parameters are playing key roles in determining it. A full set of 67 volume reactions and reduced set are used. Under most of conditions, CCP shows 100 times higher H/SiH₃ ratio over ICP case due to its spatially localized two electron temperature distribution. Multi hollow cathode type CCP is also modeled as a 2 × 2 hole array. For Ar, the discharge is well localized at the neck of the hole at a few Torr of gas pressure. H₂ and SiH₄ + H₂ needed higher gas pressure and power density to get a multi hole localized density profile. H/SiH₃ was calculated to be about 1/10.     

Low temperature interdiffusion in the Au-Pd and Au-Rh thin film couples

Interdiffusion profiles in thin polycrystalline multilayer films of Pd-Au and Ti-Rh-Au at temperatures up to 490°C have been measured by Rutherford backscattering. Room temperature grain boundary diffusion of Au into Rh was observed and analyzed to give D_B = 3.5 × 10^-17 cm² sec^-1. The Whipple analysis is applied to our data for the diffusion of Au in Pd; using the lattice diffusivity of Neukam, an activation energy for grain boundary diffusion of 0.9 eV is found. The diffusion of Pd in Au has also been analyzed using the Whipple model, which gives a grain boundary activation energy of 0.6 eV.     

Quenched aging of thin film deposits by chopping

We studied the aging of thin films of MgF₂, of cryolite and of mixtures of these compounds prepared by codeposition with and without chopping of the incident beams. Ellipsometric measurements indicated that chopping of the films during deposition reduces the aging of the films. Spectrophotometric data showed that the transmittances of the single films change owing to aging much more than those of mixed films.     

One-step Ge/Si epitaxial growth

Fabricating a low-cost virtual germanium (Ge) template by epitaxial growth of Ge films on silicon wafer with a Ge(x)Si(1-x) (0 < x < 1) graded buffer layer was demonstrated through a facile chemical vapor deposition method in one step by decomposing a hazardousless GeO(2) powder under hydrogen atmosphere without ultra-high vacuum condition and then depositing in a low-temperature region. X-ray diffraction analysis shows that the Ge film with an epitaxial relationship is along the in-plane direction of Si. The successful growth of epitaxial Ge films on Si substrate demonstrates the feasibility of integrating various functional devices on the Ge/Si substrates.     

Oxidation of sputtered Zr thin film on Si substrate

Oxidation of sputtered Zr thin film on Si substrate has been investigated by varying oxidation times (5–60 min) at 500 °C. Fourier transform infrared spectroscopy indicated the existence of ZrO₂ by showing spectra of Zr–O. Vibration mode of Si–O and Zr–O–Si are also detected for all samples oxidized at different duration. This suggested the existence of SiO _x and Zr _x Si _y O _z compounds and they might be located at interfacial layers (ILs) between ZrO₂ and Si. Cross-sectional image of high resolution transmission electron microscopy taken from 60-min oxidized sample showed that both ZrO₂ and IL thickness is ~3.5 nm. Time-of-flight secondary-ion-mass spectroscopy suggested that Zr _x Si _y O _z may be formed after oxidized for 15 min. The proposed IL is consisted of a mixture of Zr _x Si _y O _z and SiO _x . A physical model has been established to explain the observation. Electrical characterization shows that capacitance–voltage curves have small hysteresis and their flatband voltages are shifted to a negative bias. Effective dielectric constant values of the investigated oxides are in the range of 4.22–5.29. Leakage current density–breakdown voltage characteristic shows that 5-min oxidized sample has the lowest dielectric breakdown voltage if compared with the other samples.     

Plasma oxidation of Al thin films on Si substrates

In this study, Al thin films deposited on silicon wafers by direct current magnetron sputtering were oxidized under radio frequency 13.56 MHz O₂ plasma at temperatures up to 550 °C. During oxidation, plasma powers as well as oxidation temperature and time were varied to investigate the oxidation behavior of the Al films. X-ray photoelectron spectroscopy and Auger electron spectroscopy results show that the apparent alumina could be observed after O₂ plasma treatment with powers above 200 W as well as at temperatures above 250 °C. However, no alumina increment could be discerned after individual either heat treatment at 550 °C or plasma treatment at room temperature. The thickness of alumina layers increased remarkably with plasma power and could reach about 60 nm when undergone 400 W O₂ plasma treatment at 550 °C for 2 h. Moreover, the thickness of alumina increased parabolically with time during plasma oxidation aided by thermal treatment. The deduced activation energy of such plasma oxidation was 19.1 ± 0.5 kJ/mol.     

Evaluation of depth distribution and characterization of nanoscale Ta/Si multilayer thin film structures

A multilayer thin film structure of ten alternate Ta and Si layers with approximately 18 nm thickness for the combined (Ta + Si) layer, was evaluated to explore the individual layer thickness and the interface mixing behavior using different surface characterization techniques like Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS), X-ray Reflectometry (XRR) and Kelvin Probe Force Microscopy (KPFM). These results were compared with measurements performed earlier using cross section Transmission Electron Microscopy (TEM). The TOFSIMS depth profile results indicate the individual thickness of Si and Ta layers to be 8.1 nm and 5.9 nm respectively which are less than the corresponding actual thickness measured by cross section TEM as 10.5 nm and 7.5 nm. The difference in thickness measurement has been explained in the light of ion bombardment induced atomic mixing in the interface during sputter depth profiling. A scanning electron micrograph shows the actual crater and its edges created due to the sputtering including the multilayer for real view of the structure. The XRR observations however reveal better agreement with the cross section TEM data, both being non-destructive in nature. Attempts were made to characterize the multilayer using KPFM technique which clearly elucidated the grating type cross section of the structure.     

The structure and electrical characteristics of Si/Ge heterojunctions II: The electrical characteristics of Si/Ge heterojunctions

The I–V characteristics of p-Ge/nn⁺ -Si and p-Ge_xSi_1?x/nn⁺ -Si heterojunctions produced by the vacuum epitaxy of germanium onto nn⁺ -Si were determined. The mechanisms of the d.c. and reverse current were investigated.     

Preparation and phase transformations of melt-spun Al–Ge–Si brazing foils

A series of Al-Ge-Si alloys was melt spun into ribbons of about 40 μm thickness. The alloy compositions were selected so as to be suitable as filler metals with brazing temperatures <500°C. In the as-quenched state the foils were relatively brittle due to the occurrence of metastable phases. After appropriate annealing treatments between 300–400°C the metastable phases were transformed into a fine-grained microstructure of β-Ge(Si) particles within the α-Al matrix. This led to considerably improved mechanical properties, which are manifested in decreased microhardness levels near 100 HV_0.02 and bend radii <1 mm. The transformation process of foils on annealing was investigated by differential scanning calorimetry, transmission electron microscopy and X-ray diffraction methods. The intensity of X-ray reflections of the metastable and equilibrium phases as well as the lattice parameters of α-Al and β-Ge(Si) were evaluated as functions of the annealing temperature. Differences in the transformation behaviour of binary Al-Ge and ternary Al-Ge-Si alloys, in particular, the decreased transformation temperature for the decay of metastable phases in ternary alloys, were revealed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electron microscope observations of interdiffusion and ordering in copper-gold thin film diffusion couples

Observation of interdiffusion and ordering in thin polycrystalline sandwich layers of copper and gold has been carried out by means of electron diffraction. Samples of total thickness 800 and 1000 Å with thickness ratios corresponding to AuCu₃, AuCu and Au₃Cu respectively were vacuum deposited onto cleavage surfaces of NaCl. The diffusion experiments were performed in a temperature range around 200 °C. A preferred formation of the disordered AuCu₃ alloy was found during diffusion, in agreement with previous X-ray investigations. The ordering was much slower than the interdiffusion process, resulting in the coexistence of ordered and disordered crystallites even after equalized concentration. More complete ordering was obtained only after extended annealing. The interdiffusion coefficient D was evaluated from the diffraction patterns using a new method and was found to be 3 × 10^-4 exp (25 000/RT).