In-situ monitoring and control of hydrogenated amorphous silicon–germanium band-gap profiling during plasma deposition process

In-situ germanium content monitoring and its characteristics in SiH₄/GeH₄/H₂ plasmas was studied during hydrogenated amorphous silicon–germanium (a-SiGe:H) film depositions. Since an appropriate band-gap profiling in a-SiGe:H deposition is very important to achieve high efficiency solar cell, the accurate monitoring and control of Ge contents are required. In this work, we found the spectral intensity ratio of silicon atom (288.2 nm) and germanium atom (303.9 nm) emission has strong relation with Ge content in plasmas. In typical, band-gap energy of films was decreased with the increasing of gas flow ratio GeH₄/SiH₄. However, at different total flow rate of GeH₄, the band-gap was different for same gas flow ratio cases because the Ge content in plasmas was changed due to the changes of electron temperature by hydrogen dilution. On the other hand, the emission intensity ratio Ge/Si detected the band-gap variation. Using this method, therefore, we measured and control Ge/Si to make a U-shape band-gap profile which was proved by an ellipsometer and Auger electron spectroscopy depth profile analysis.     

Near infrared absorption of Si nanoparticles embedded in silica films

The absorption coefficient of Si nanoparticles embedded in a silica matrix obtained through thermal annealing at 1000 °C of SiO thin films has been determined by a combination of ellipsometry and photothermal deflection spectroscopy. The high absorption level below 2 eV was explained by the superposition of the contribution of: (i) extended states and distorted bond states (Urbach tail), giving rise to an exponential regime of the variation of the absorption coefficient on energy and (ii) point defect states. The value of the characteristic energy of the exponential regime was found above 200 meV. This high value was partly related to the high stress present at the np-Si/SiO₂ interface. The point defects were attributed to dangling bonds and induced an additional absorption band located near 1.2 eV contributing to above 100 cm⁻¹ to the absorption at this energy.     

High resolution Si 2p gas phase photoelectron spectra of model silicon compounds: implications for hydrogen adsorbed on a silicon surface

High resolution Si 2p gas phase photoelectron spectroscopy has been used to measure the vibrational energy spacing in SiH₄ (0.295 eV) and SiD₄ (0.212 eV). These values are compared with those measured for the chemical shift (0.30 eV/hydrogen atom) of model compounds which mimic the effects of zero, one, two and three hydrogen atoms bonded to a silicon atom on a silicon surface. Implications for the interpretation of surface photoelectron spectra are discussed.     

Improvement of transport properties for polycrystalline silicon prepared by plasma-enhanced chemical vapor deposition

The carrier transport property of polycrystalline silicon (poly-Si:H:F) thin films was studied in relation to film microstructure, impurity, in situ or post-annealing treatments to obtain better carrier transport properties. Poly-Si:H:F films were prepared from SiF₄ and H₂ gas mixtures at temperatures <300 °C. Dark conductivity of the films prepared at high SiF₄/H₂ gas flow ratio (e.g., 60/3 sccm) exhibits a high value for intrinsic silicon and its Fermi level is located near the conduction band edge. The carrier incorporation is suppressed well, either by in situ hydrogen plasma treatment or by post-annealing with high-pressure hot-H₂O vapor. It is confirmed that weak-bonded hydrogen atoms are removed by the hot-H₂O vapor annealing. In addition, evident correlation between impurity concentrations and dark conductivity is not found for these films. It is thought that the carrier incorporation in the films prepared at high SiF₄/H₂ gas flow ratios is related to grain-boundary defects such as weak-bonded hydrogen. By applying hot-H₂O vapor annealing at 310 °C to a 1-μm-thick p-doped (400)-oriented poly-Si:H:F film, Hall mobility was improved from 10 cm²/Vs to 17 cm²/Vs. Received: 7 August 2000 / Accepted: 2 March 2001 / Published online: 20 June 2001     

Room-temperature deposition of nanocrystalline PbWO 4 thin films by pulsed laser ablation

Pulsed laser ablation (PLA) was applied to synthesize nanocrystalline PbWO₄ thin films onto glass substrates. The effects of Ar background gas pressure on phase evolution, microstructures and optical characteristics of PbWO₄ thin films were investigated in detail. The PLA processes were carried out at room temperature without substrate heating or post-annealing treatment. XRD and HR-TEM results revealed that the PbWO₄ thin films are composed of nanocrystalline and amorphous phases. Moreover, the films contained a high density of lattice defects such as twin boundaries and edge dislocations. The crystallite size and crystallinity increased, which were associated with a change in surface morphology as the Ar pressure increased. Reduced tungsten states W⁵⁺ or W⁴⁺ induced by oxygen vacancies were observed at 10 Pa and the atomic concentration of all constituent element was almost stoichiometric, especially the [Pb]/[W] ratio, which was nearly unity above 50 Pa. The optical energy band-gap was 3.03 eV at 50 Pa and increased to 3.35 eV at 100 Pa, which are narrower than the reported value (4.20 eV). This optical band-gap narrowing could be attributed to localized band-tail states and new energy levels induced by the amorphous structure and inherent lattice defects. PACS 81.15.Fg; 78.20.-e; 68.55.-a; 73.22.-f     

The effects of Xe irradiation on He and H co-implanted Si

A combination of X-ray diffraction, cross-sectional transmission electron microscopy (XTEM), and Raman spectroscopy was used to study the effects of irradiation with swift heavy ions on helium and hydrogen co-implanted silicon.<100>-oriented silicon wafers were co-implanted with 30 keV helium to a dose of 3×10¹⁶He⁺/cm² and 24 keV protons to a dose of 2×10¹⁶ H⁺/cm². Moreover, selected helium and hydrogen co-implanted Si wafers were irradiated with 94 MeV xenon. After He and H co-implantation and Xe-irradiation, the wafers were annealed at a temperature of 673 K for 30 min. The damage region of the wafers was examined by the XTEM analysis. The results reveal that most of the platelets are aligned parallel to the (100) plane in the He and H co-implanted Si. However, majority of the platelets lie in<texlscub>111</texlscub>planes after Xe irradiation. Blisters do not occur on the sample surface after Xe irradiation. Raman results reveal that the intensities of both SiH₂ and V₂H₆ modes increase with the increase in the dose of Xe. A possible explanation is that strong electronic excitation during Xe irradiation produces annealing effect, which reduces both lattice damage and the out-of-plane tensile strain.     

Band alignment of SiO2/Si structure formed with nitric acid vapor below 500 °C

A relatively thick (i.e., ∼9 nm) SiO₂ layer can be formed by oxidation of Si with nitric acid (HNO₃) vapor below 500 °C. In spite of the low temperature formation, the leakage current density flowing through the SiO₂ layer is considerably low, and it follows the Fowler-Nordheim mechanism. From the Fowler-Nordheim plots, the conduction band offset energy at the SiO₂/Si interface is determined to be 2.57 and 2.21 eV for HNO₃ vapor oxidation at 500 and 350 °C, respectively. From X-ray photoelectron spectroscopy measurements, the valence band offset energy is estimated to be 4.80 and 4.48 eV, respectively, for 500 and 350 °C oxidation. The band-gap energy of the SiO₂ layer formed at 500 °C (8.39 eV) is 0.68 eV larger than that formed at 350 °C. The higher band-gap energy for 500 °C oxidation is mainly attributable to the higher atomic density of the SiO₂ layer of 2.46 × 10²²/cm³. Another reason may be the absence of SiO₂ trap-states.     

Generation of high-energy secondary pulsed molecular beams

A method is suggested for generating high-intensity secondary pulsed molecular beams in which the kinetic energy of molecules can be controlled by an intense laser IR radiation through the vibrational excitation of molecules in the source. High-intensity [≥10²⁰ molecule/(sr s)] SF₆ molecular beams with a kinetic energy of ?1.0 eV without carrier gas and of ?1.9 and ?2.4 eV with carrier He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) gases, respectively, were obtained.     

Evidence of the primary-color photoluminescence in ion (H,H,C) irradiated and thermal annealed SrTiO3

We have measured photoluminescence (PL) spectrum of (1) thermal-annealed SrTiO₃/Si thin film and undoped SrTiO₃ single crystal; (2) SrTiO₃ single crystal irradiated by high energy (3 MeV) proton, deuterium, and He ion beams and (3) SrTiO₃ single crystal irradiated by low energy (60 keV) H⁺ and C⁻ ions. Two PL emissions are induced in (1) and (2) at visible frequencies 3 and 2.45 eV, while another PL peak is induced at 2 eV in (3). When compared with our previous PL experiments on high-temperature annealed SrTiO₃/SiO₂/Si thin film and 3 MeV proton (H⁺) irradiated STO single crystal, these results confirm that the three PL emissions with blue (3 eV), green (2.45 eV), and red-orange (2 eV) frequencies originate indeed from SrTiO₃. These primary-color PL effect induced at room-temperature makes STO a strong candidate material for future oxide-based optoelectronic application.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.     

Oxygen and hydrogen accumulation at buried implantation-damage layers in hydrogen- and helium-implanted Czochralski silicon

Oxygen and hydrogen accumulations at buried implantation-damage layers were studied after post-implant-ation annealing of hydrogen- and helium-implanted Czochralski (Cz) silicon. Hydrogen implantation was carried out at energies E=180 keV and doses D=2.7×10¹⁶ cm^-2, and helium implantation at E=300 keV and D=10¹⁶ cm^-2. For comparison hydrogen implantation was also done into float-zone (Fz) silicon wafers. Post-implantation annealing at 1000 °C was done either in H₂ or N₂ atmosphere. Hydrogen and oxygen concentration profiles were measured by secondary ion mass spectroscopy (SIMS). It is shown that the ambient during annealing plays a significant role for the gettering of oxygen at buried implantation-damage layers in Cz Si. For both hydrogen and helium implantations, the buried defect layers act as rather effective getter centers for oxygen and hydrogen at appropriate conditions. The more efficient gettering of oxygen during post-implantation annealing in a hydrogen ambient can be attributed to a hydrogen-enhanced diffusion of oxygen towards the buried implantation-damage layers, where a fast oxygen accumulation occurs. Oxygen concentrations well above 10¹⁹ cm^-3 can be obtained. From the comparison of measurements on hydrogen-implanted Cz Si and Fz Si one can conclude that at the buried defect layers hydrogen is most probably trapped by voids and/or may be stable as immobile molecular hydrogen species. Therefore hydrogen accumulated at the defect layers, and is preserved even after high-temperature annealing at 1000 °C. Received: 3 July 2000 / Accepted: 11 July 2000 / Published online: 22 November 2000     

中子辐照含氢FZ-Si的红外吸收研究

对于中子辐照的n-FZSi(H₂),利用红外吸收光谱研究了由于辐照所产生的各种与氢有关的缺陷态。在未辐照的样品和辐照的样品中分别发现了未曾报道的1992cm^{-1和1857cm-1吸收峰。对于在n-FZSi(H₂)所引起的本征吸收峰和辐照损伤吸收峰,进行了讨论和指派。2150cm-1吸收峰则被认为是由于氢施主所引起的。 关键词：}     

The influence of argon ion bombardment on the electrical and optical properties of clean silicon surfaces

The effect of low energy noble gas ion bombardment on the electrical and optical properties of Si(211) surfaces has been investigated by surface conductivity and field effect measurements, ellipsometry and AES. With this combination of techniques, information is obtained concerning the electrical properties, the chemical composition and the damage of the surface layer. Upon ion bombardment in the energy range of 500–2000 eV, ellipsometry shows the formation of a damaged surface layer with optical properties close to those of an evaporated amorphous silicon film. In order to measure the conductivity changes as sensitive as possible, nearly intrinsic silicon crystals were used. For the clean, 5200 Ω cm Si(211) surface, bombarded only with a mass-analyzed argon ion beam, a small increase in conductivity is found to occur after a small ion dose (saturation after 5 × 10¹⁴ ions cm^?2 while after 5 × 10¹³ ions cm^?2 already half of the increase has occurred). The effect was found to be independent of ion energy between 500 and 2000 eV. As the field effect signal did not change after this treatment, it is concluded that the surface state density in the neighbourhood of the Fermi level shows a slight decrease.     

In situ size measurement of Si nanoparticles and formation dynamics after laser ablation

We have developed a technique to detect Si nanoparticles selectively and to measure size in situ. Applying the technique, we have investigated formation process of Si nanoparticles after pulsed laser ablation of Si targets in Ar gas. Time-resolved photoluminescence (PL) spectroscopy revealed that PL only from Si nanoparticles is observed below 2.4 eV while PL from Si nanoparticles as well as defects in SiO₂ is observed above 2.4 eV. Therefore, Si nanoparticles can be detected selectively by excitation light with a photon energy below 2.4 eV. It is found that the onset of the PL from Si nanoparticles is delayed by approximately 0.3 ms from that of the defects and smaller Si nanoparticles. A size can be estimated by a band gap, which is roughly equal to the lowest photon energy at which Si nanoparticles can be excited. Thus, we estimated the sizes of growing Si nanoparticles. PACS 61.46.+w; 78.66.w; 07.60.Yi     

Evolution of infrared spectra and optical emission spectra in hydrogenated silicon thin films prepared by VHF-PECVD

A series of hydrogenated silicon thin films with varying silane concentrations have been deposited by using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The deposition process and the silicon thin films are studied by using optical emission spectroscopy (OES) and Fourier transfer infrared (FTIR) spectroscopy, respectively. The results show that when the silane concentration changes from 10% to 1%, the peak frequency of the Si—H stretching mode shifts from 2000 cm ^{- 1} to 2100 cm ^{- 1}, while the peak frequency of the Si—H wagging—rocking mode shifts from 650 cm ^{- 1} to 620 cm ^{- 1}. At the same time the SiH^*/H_α intensity ratio in the plasma decreases gradually. The evolution of the infrared spectra and the optical emission spectra demonstrates a morphological phase transition from amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). The structural evolution and the μc-Si:H formation have been analyzed based on the variation of H_α and SiH^* intensities in the plasma. The role of oxygen impurity during the plasma process and in the silicon films is also discussed in this study.     

Formation of surface stabilized Si nanocrystal by pulsed laser ablation in hydrogen gas

We prepared silicon nanocrystallites by pulsed laser ablation (PLA) of a Si target in hydrogen background gas. A mixture of hydrogen and helium was used as a background gas and the hydrogen partial pressure was varied. The deposited nanocrystal-film system shows a hierarchical structure composed of surface hydrogenated silicon nanocrystallites as the primary structure and aggregates of the nanocrystallites as the secondary structure. The size of the primary particles was not sensitive to the hydrogen partial pressure, while the porosity of the secondary structure constituted by the aggregation of the primary particles increased with increasing hydrogen partial pressure. This indicates that the surface is stabilized and that aggregation of the primary structure is depressed by surface hydrogenation. The optical gap energy of the deposits shifted to higher energy with increasing hydrogen partial pressure due to the formation of well-isolated nanocrystallites by surface stabilization. These results indicate that PLA in hydrogen gas is a promising technique to prepare surface stabilized and controlled silicon nanocrystallites.     

The interaction of water with SiMo interface. A core and valence photoemission investigation

Core level photoemission with MgKα radiation (hv = 1253.6 eV) and valence photoemission (hv = 21.2 eV) are presented for Si(1 1 1)-Mo interfaces at various Mo coverages (up to 3.2·10¹⁵ at cm⁻²) exposed to 6000 L of H₂O. The metal deposition strongly enhances the oxidation of Si with the complete H₂O dissociation and the formation of a SiO₂-like compound. The results are consistent with an interface growth with the formation of silicide islands.     

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C-V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10^?3 eV wide with acceptor type states 0.18 eV below the intrinsic level E_i and a density of 10¹⁷ states/cm² eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E_i, and a density of 4 × 10¹⁴ states/cm²eV.     

Protons’ generation by laser irradiation at 5 × 10 W/cm from silicon dielectric targets containing an excess of hydrogen

A study of silicon plasma generated in vacuum by 532 nm Nd:YAG laser at intensities of about 5 × 10⁹ W/cm² from dielectric targets containing a relatively huge quantity of hydrogen was presented.Time-of-flight technique was employed to measure the particles’ energy and the relative yield with respect to other ion species. Plasma-accelerated ions show Coulomb-Boltzmann-shifted distributions depending on their charge state.Mass quadrupole spectrometry allowed the estimation of the relative hydrogen amount inside the different samples considered: silicon (Si), silicon nitride (Si₃N₄) and hydrogenated annealed silicon (Si(H)) as a function of the ablation depth and irradiation time.Depth profiles of the laser craters permit to calculate the ablation yield at the used laser fluence. The plasma temperature and density was evaluated by the experimental data. A special regard is given to the protons’ generation process occurring inside the plasma, due to the possible influence of the hydrogen excess on the treated samples in comparison to the not-hydrogenated silicon ones.     

硅基超晶格Si1-xSnx/Si的能带结构

由于Si基发光材料能与现有的Si微电子工艺兼容,其应用前景被广泛看好. 设计具有直接带隙的Si基材料,备受实验和理论研究者的关注. 本文根据芯态效应、电负性差效应和对称性效应设计了Si基超晶格Si_1－xSn_x/Si. 其中Si_0.875Sn_0.125/Si为直接带隙材料. 在密度泛函框架内,采用平面波赝势法计算表明,Si_0.875Sn_{0.125相似文献}