Raman scattering and photoluminescence study of porous silicon formed on n-type silicon

We report Raman scattering and photoluminescence studies on porous silicon film formed on n-type silicon. The Raman spectra over the sample surface exhibit considerable variation whereas the photoluminescence spectra are practically identical. Our results indicate that, well inside the film surface, it consists of spherical nanocrystals of typical diameter ≈ 100Å, while on the edge these nanocrystals are ? 300Å. We further observe that there is no correlation between the photoluminescence peak position and the nanocrystal diameter. This suggests that the origin of the photoluminescence is due to radiative recombination between defect states in the bulk as well as on the surface of the nanocrystal.     

Phonon Mechanism of High-Tc Superconductivity

For a long time the majority view has been that phonons are irrelevant to the mechanism of high-temperature superconductivity. However, recent experimental results, including the neutron inelastic scattering measurements reviewed here, seriously challenge this view. We point out that the electron–phonon coupling in the cuprates can be substantially different from that in simple metals because of the covalency and strong electron correlation. In particular certain phonon branches induce substantial intersite charge transfer that can result in a negative electronic dielectric susceptibility. Such a strong electron–phonon coupling of certain modes could form the basis for the phonon mechanism of superconductivity in the cuprates.     

Phonon Modes in CMR Manganites at Elevated Temperatures

Phonon modes of the colossal magnetoresistance manganites La_0.7Ca_0.3MnO₃ and La_0.8Li_0.2MnO₃ have been investigated by far-infrared (100–700 cm^–1) reflectivity spectroscopy at elevated temperatures (300–800 K). The three principal optical phonons all move systematically to lower frequency as temperature increases, in accordance with a Grüneisen relationship. The La(Ca,Li)–(MnO₃) vibration (170 cm^–1) varies with cation mass as expected. The Mn–O stretching mode (580 cm^–1) has a higher frequency in the Li-doped compound than in the Ca-doped, whereas the O–Mn–O bending mode )340 cm^–1) occurs at the same frequency in each, demonstrating that the cation strongly influences the Mn–O– length but not the bond angle.     

The modification of the properties of n-type conductivity porous silicon by argon ion irradiation

The results of the research into the influence of argon ion irradiation at 3 keV on the composition and structure of porous silicon are presented. At a certain angle of incidence of the particles relative to the surface of the monocrystallites, an undulating λ∼60 nm nanorelief is formed, while the crystallite sizes and structure remain unchanged. The IR-spectroscopy data show that SiH groups are mainly localized in a thin 120 nm near-surface layer. During exposure of the samples to the air in the dark, monohydride groups are removed from the surface within a month. Dihydride groups, located in deeper layers, are oxidized considerably more slowly than the monohydride ones. The experimental data show that the 0.1 μm-thick surface layer serves as a diffusion barrier preventing oxygen from penetrating deep into the porous silicon.     

Influence of pressure annealing on electrical properties of Mn implanted silicon

The effect of annealing at 610-720 K under enhanced hydrostatic pressure (HP) on electrical properties of manganese implanted Czochralski grown silicon (CzSi:Mn) and floating zone grown silicon (FzSi:Mn) (doses up to 1×10¹⁶ cm⁻¹, E=160 keV) was investigated by electrical C-V, I-V and admittance measurements. Mn⁺ implantation produces both donor-like and acceptor-like implantation-induced defects. The stress-induced decrease in the hole concentration was detected in CzSi implanted by Mn⁺ and annealed at temperatures 610-670 K. The effect of changing initial conductivity of CzSi:Mn samples of high concentration of interstitial oxygen, from p type to n type, due to thermal donors (TDs) generation has been observed. The electron concentration after type conversion depends on annealing conditions and Mn⁺ dose. The TDs generation has not been detected in FzSi:Mn samples containing lower oxygen concentration, however, the FzSi:Mn samples annealed at 720 K under 10⁵ Pa for 10 h indicate at the implanted side the increased carrier concentration due to the defects produced by Mn⁺ implantation. The heat treatment of FzSi:Mn under HP at 720 K for 10 h results in further increase of carrier concentration due to the defect generation.     

Phonon Mediated Helium Atom Transmission through Superfluid Helium Four

We report results of experiments in which pulses of helium vapor with translational energies of 3 K are directed at a thin film of superfluid helium at a temperature of about 0.2 K suspended over a cesium covered orifice in a platinum film. The response of the superfluid film was detected by a super-conducting titanium bolometer placed on the side of the film opposite to that of the source. For films of approximately 1 mm in thickness we find no response of the bolometer within the limits of our detector. However, for films of less than 100μ thickness, we find a response which is of the same temporal shape, but smaller in amplitude, than that of the orifice when it is not covered with superfluid helium. We interpret these results to mean that we are seeing phonon mediated transmission in the thin films. Roton and condensate mediated transmission amplitudes for these conditions are apparently too small for us to detect in any of the films. This result is consistent with the theoretical results of Sobnack and Inkson [M. B. Sobnack, J. C. Inkson, and J. C. H. Fung, Phys. Rev. B 60, 3465 (1999)] concerning the amplitude of roton to atom and photon to atom conversion as a function of the atomic energy.     

Thermal conductivity of pure monoisotopic silicon

The thermal conductivity of pure monoisotopic silicon is estimated by two methods, which give similar results. One estimate, based on the observed thermal conductivity of monoisotopic germanium, yields a maximum of 66 W · cm^–1 · K^–1 at 22 K. The other estimate, based on measurements of natural silicon and on the theoretical isotope scattering rate, yields 75 W · cm^–1 · K^–1 at 22 K, an increase of only 45% over the natural crystal. These values are for crystals of approximately 0.5 cm diameter; smaller crystals yield lower values of the maximum conductivity and smaller isotope effects. Silicon cooled to liquid hydrogen temperature seems promising for high-irradiance laser mirrors. The small gain obtained by using monoisotopic silicon would be substantially greater in cases when the generated phonon distribution is athermal and weighted to higher frequencies. The effective heat transport could then be increased by as much as a factor 60 through the use of monoisotopic silicon.     

Phonon scattering enhancement in silicon nanolayers

Dimensional confinement in silicon nanowires (NWs) is well-known for enhancing phonon scattering, thus leading to a pronounced reduction of thermal conductivity κ with respect to bulk material. The effect of confinement on phonon scattering in nanolayers (NLs), however, has not been fully understood. In this work, thermal conductivity on polycrystalline silicon NLs with roughened surfaces and thicknesses ranging from 30 to 100 nm has been experimentally investigated. For measurement purposes, the nanostructures were fabricated with a dedicated surface nano-machining process, thus producing vertical silicon nanostructures suspended on Al/Si electrodes on a silicon substrate, using SiO₂ as a sacrificial layer. By designing such structures in a four-terminal configuration, their κ could be determined by the current-voltage method. Boron doped silicon NLs were examined, at resistivity ranging between 2 and 10 m $\Upomega$ cm. We found an increase of phonon scattering from the confinement, since κ decreased steadily with the thickness from values typical of thick films (around 30 W m^?1 K^?1) down to <15 W m^?1 K^?1. Compared to NWs, NLs had displayed figures of merit smaller by one order of magnitude. However, due to the larger filling factor, they were able of generating more than five times the electric power per area unit that could be obtained with high-density stacks of top-efficiency NWs.     

Shallow and deep donors in n-type ZnO characterized by admittance spectroscopy

Shallow and deep energy levels in n-type ZnO materials grown by a pulsed laser injection method are investigated. We report thermal ionization energies for residual shallow donors of about E _d  = 15 meV. Annealing in nitrogen ambient further lowers these ionization energies. We attribute these residual donors to native defects such as Zn interstitials and oxygen vacancies. Using optical admittance spectroscopy we also identify deep defects with optical ionization energies E _o1  = 2.5 eV and E _o2  = 2.1 eV. One of these centers is bistable with large capture barrier energy and therefore accounts for a significant part of the persistent photoconductivity in the material.     

Formation of oxygen related donors in step-annealed CZ-silicon

The effect of step-annealing necessitated by the difficulties being faced in the long duration annealing treatments to be given to CZ-silicon has been studied. One pre-anneal of 10 h followed by annealing of 10 h causes a decrease in the absorption coefficient for carbon (α_c). Oxygen and carbon both accelerate thermal donor (TD) formation process but oxygen plays a dominating role. Three anneals of 10 h each followed by one anneal of 10 h support the view that carbon suppresses the donor formation. The absorption coefficient for carbon decreases after a few number of step-anneals resulting in the transformation of TD to new donor (ND) as brought about by annealing at temperature, > 500°C. It is quite logical to conclude that step-annealing may bring about the same results as obtained on continuous annealing for a longer duration. The results have been fully supported by proper interpretation in the light of existing theories.     

Correlation of structural and optoelectronic properties of thin film silicon prepared at the transition from microcrystalline to amorphous growth

Photovoltaic properties of 4 µm thick microcrystalline silicon p–i–n solar cells have been studied, over a range of crystallinity determined using Raman spectroscopy. Low-crystallinity material (below 10%) appears to absorb disproportionately strongly in the infrared, possibly due to increased light scattering or to relaxation of the crystal momentum selection rule. A minimum in solar cell efficiency is observed under AM1.5 illumination when V_OC ≈ 580 mV, with blue response most strongly affected. This is consistent with a reduction in electron mobility to a value below that of amorphous silicon for low-crystallinity material, in agreement with time-of-flight measurements.     

Fabrication of n-type mesoporous silicon nanowires by one-step etching

In general, n-type mesoporous silicon nanowires (mp-SiNWs) are exclusively created by the two-step metal-assisted chemical etching (MACE). This work first reports that one-step MACE (in HF and AgNO3) is also capable of producing the n-type mp-SiNWs, and the developed formula is generally adapted to generate SiNWs by etching n-Si(100) with electrical resistivity over a range of 10(-3)-10(1) Ω·cm. Integrating the contribution of silicon intrinsic properties in the existing MACE mechanism explicitly accounts for the new findings and contradictions with previous studies. The as-generated mesoporous structures emit red light under laser excitation at room temperature. The red-color emission sensitively varies with temperature over a range of 16-300 K, attributed to a temperature-dependent photoluminescent mechanism.     

超高真空电子束蒸发在多孔硅上外延单晶硅

采用超高越空电子束蒸发的方法在用阳极氧化制备的多孔硅衬底上外延单晶硅,研究了不同多孔硅衬底对外延质量的影响。采用高能电子衍射表征外延层的晶体结构,截面透射电镜上材料的微结构,原子力镜表征外延层表面的粗糙度,卢瑟福背散射／沟道表征外延层晶体质量,扩展电阻表征材料的电学性能。一系列的测试结果表明对在5mA／cm^2电流密度下阳极氧化10min形成的多孔硅衬底,可用超高真空电子束蒸发的方法外延出质量良好的单晶硅。     

高阻硅掩膜选择性生长多孔硅阵列

介绍一种在低阻P型硅衬底上用氢离子注入技术形成局部高阻硅掩膜,用电化学腐蚀选择性生长多孔硅微阵列的工艺流程。结果证明,用高阻硅掩膜选择性生长多孔硅具有很好的掩蔽效果,生成的多孔硅阵列的有序性和完整性良好。     

n型补偿直拉单晶硅的电子迁移率

通过霍尔效应测量、二次离子质谱等手段研究了n型硼-磷补偿直拉单晶硅的电子迁移率与掺杂浓度的关系。通过比较迁移率的实验测量值和由Klaassen迁移率模型得到的计算值,发现Klaassen模型适用于掺杂浓度在1018cm-3数量级的补偿单晶硅电子迁移率的计算,但对掺杂浓度在1017cm-3数量级的补偿单晶硅而言则明显高估了电子迁移率。分析认为这是由于该模型未充分考虑低掺杂浓度情形下自由载流子对电离杂质的屏蔽作用由于杂质补偿受到的削弱效应。根据实验结果,修正了Klaassen模型,使之在低掺杂浓度的情况下获得的电子迁移率计算值也与实验值吻合得相当好。     

Surface optical phonon-assisted electron Raman scattering in a free-standing quantum wire with ring geometry

We present a theoretical calculation of the differential cross section for the electron Raman scattering process associated with surface optical phonon modes in semiconductor quantum wires (QWs) with ring geometry. Electron states are considered to be confined within the QWs. We consider the Fröhlich electron-phonon interaction in the framework of the dielectric continuum approach. Some singularities for the ring with various sizes in the Raman spectra are found and interpreted. A discussion of the phonon behavior for the QWs with large and small sizes is presented. The numerical results are also compared with those of experiments.     

Porous silicon and aluminum co-gettering experiment in p-type multicrystalline silicon substrate

The lifetimes of non-equilibrium minority carriers, which bound with the diffusion length, are considered as two important parameters of the low-quality multicrystalline silicon (mc-Si) substrate. Its value defines the quality of the initial substrate. It is also subjected to change as a result of many high-temperature operations during the device fabrication. Therefore, it is necessary to incorporate certain processing steps that either improve or preserve the electronic quality of the mc-Si substrate. In this study, a novel porous silicon and aluminum co-gettering experiment has been applied as a beneficial approach to improve the electronic quality of the low-resistivity mc-Si substrates. Porous silicon layers were prepared by anodization of the n⁺ silicon region by a simple electrochemical etching process using an aqueous HF-based electrolyte, which leads to the creation of porous silicon microcavities. Besides making porous silicon and aluminum co-gettered samples, both phosphorous and aluminum alloy-gettered samples and reference samples were made. The gettering-induced lifetime enhancement in the test samples was monitored by measuring the lifetime/diffusion length of the test samples using two independent methods such as photoconductivity decay (PCD) measurement and the photocurrent generation method (PCM), respectively. The result in both the measurements has shown a reasonably good agreement with each other. Therefore, it is inferred that the applied co-gettering experiment has a synergetic effect to improve the lifetime of the mc-Si substrate.     

Porous silicon and aluminum co-gettering experiment in p-type multicrystalline silicon substrate

The lifetimes of non-equilibrium minority carriers, which bound with the diffusion length, are considered as two important parameters of the low-quality multicrystalline silicon (mc-Si) substrate. Its value defines the quality of the initial substrate. It is also subjected to change as a result of many high-temperature operations during the device fabrication. Therefore, it is necessary to incorporate certain processing steps that either improve or preserve the electronic quality of the mc-Si substrate. In this study, a novel porous silicon and aluminum co-gettering experiment has been applied as a beneficial approach to improve the electronic quality of the low-resistivity mc-Si substrates. Porous silicon layers were prepared by anodization of the n⁺ silicon region by a simple electrochemical etching process using an aqueous HF-based electrolyte, which leads to the creation of porous silicon microcavities. Besides making porous silicon and aluminum co-gettered samples, both phosphorous and aluminum alloy-gettered samples and reference samples were made. The gettering-induced lifetime enhancement in the test samples was monitored by measuring the lifetime/diffusion length of the test samples using two independent methods such as photoconductivity decay (PCD) measurement and the photocurrent generation method (PCM), respectively. The result in both the measurements has shown a reasonably good agreement with each other. Therefore, it is inferred that the applied co-gettering experiment has a synergetic effect to improve the lifetime of the mc-Si substrate.     

Phonon excitation of quasiparticles in niobium and tantalum superconducting tunnel junction photon detectors

We report phonon pulse measurements of quasiparticle (qp) self-recombination and trapping by Abrikosov vortices in Nb and Ta superconducting tunnel junctions. The quasiparticle loss times, determined from the shape of the tunnel junction output pulse, were found to be strongly correlated with flux trapping. The measured results were compared with the model of Golubov and Houwman. It was found that qp self-recombination, as measured from the dependence of responsivity on absorbed phonon energy, was also modified by the presence of traps. A process of phonon-mediated de-trapping is proposed to explain the observed effects.