Ni纳米晶制备及其在MOS电容中存储特性研究

优化了Ni纳米晶的制备工艺参数,得到了分布均匀,形状为球形,平均尺寸5nm,密度2×1012/cm2的Ni纳米晶。在此基础上,制备了包含Ni纳米晶的MOS电容结构。利用高频电容-电压(C-V)和电导-电压(G-V)测试研究了其电学性能,证明该MOS电容结构的存储效应主要源于金属纳米晶的限制态。电容-时间(C-t)测试曲线呈指数衰减趋势,保留时间600s,具有较好的保留性能。     

Surface states at steam-grown silicon-silicon dioxide interfaces

A method of determining the energy distribution of surface states at silicon-silicon dioxide interfaces by using low-frequency differential capacitance measurements of MOS structures is described. Low-frequency measurements make it possible to determine the silicon surface potential as a function of MOS voltage directly from the experimental data without requiring knowledge of the Si doping profile. No graphical differentiations are required to determine the surface state density from the experimental curves, and errors introduced by uncertainties in the silicon doping density are reduced. Also, it is shown that the measurements can be used to determine the relative lateral uniformity in the characteristics of the oxide and interface under the MOS field plate. Nonuniformities can result in large errors in the surface-state density derived from MOS capacitance measurements. Measurements are presented and interpreted for both n- and p-type silicon samples prepared by bias-growing the oxide in steam.     

The effect of NO<Subscript>2</Subscript> adsorption on optical and electrical properties of porous silicon layers

The Raman spectra and current-voltage characteristics of porous silicon layers are studied before and after exposure to NO₂. It is shown that spherical nanocrystallites with the diameter of approximately 6–8 nm are present in the samples’ structure. The effect of NO₂ brings about a decrease in the resistance of porous Si by two-three orders of magnitude. An increase in the conductance of the structures at gas concentrations as high as 2000 ppm and a drastic decrease in this conductance if the concentration exceeds the above value are observed. This effect is explained in the context of the model that implies the formation of additional defects of the type of dangling silicon bonds at the Si/SiO₂ interface as a result of oxidation of the porous silicon surface. These defects are traps for holes and reduce the increase in the hole concentration.     

Carrier transport in layered semiconductor (<Emphasis Type="Italic">p</Emphasis>-GaSe)-ferroelectric (KNO<Subscript>3</Subscript>) composite nanostructures

The current-voltage characteristics and frequency dependences of the impedance of composite nanostructures fabricated on the basis of layered anisotropic semiconductor p-GaSe and ferroelectric KNO₃ are studied. Multilayer nanostructures were obtained by introducing nanoscale pyramidal ferroelectric inclusions into a layered GaSe matrix. Hysteresis phenomena in current-voltage characteristics and abrupt changes in the conductance and capacitance in frequency dependences of the impedance are detected. These phenomena are associated with the collective effect of electric polarization switching in nanoscale 3D ferroelectric inclusions in the layered matrix, features of its local deformation, and polytype phase transitions in this matrix. X-ray, atomic-force microscopy, and impedance studies in a low (B < 400 mT) magnetic field show that the electrical characteristics of nanostructures are associated with the Maxwell-Wagner effect in nanostructures, the formation of quantum wells in GaSe during deformation of crystals in the region of nanoscale inclusion localization, and carrier tunneling in the structures.     

Effect of Epitaxial Alignment on Electron Transport from Quasi-Two-Dimensional Iron Silicide α-FeSi2 Nanocrystals Into p-Si(001)

Self-assembled growth of α-FeSi₂ nanocrystal ensembles on gold-activated and gold-free Si(001) surface by molecular beam epitaxy is reported. The microstructure and basic orientation relationship (OR) between the silicide nanocrystals and silicon substrate were analysed. The study reveals that utilisation of the gold as catalyst regulates the preferable OR of the nanocrystals with silicon and their habitus. It is shown that electron transport from α-FeSi2 phase into p-Si(001) can be tuned by the formation of (001)—or (111)—textured α-FeSi2 nanocrystals ensembles. A current-voltage characteristic of the structures with different preferable epitaxial alignment (α-FeSi₂(001)/Si(100) and α-FeSi₂(111)/Si(100)) shows good linearity at room temperature. However, it becomes non-linear at different temperatures for different ORs due to different Schottky barrier height governed by a particular epitaxial alignment of the α-FeSi₂/p-Si interfaces.

     

Diffusion-Controlled growth of Ge nanocrystals in SiO<Subscript>2</Subscript> films under conditions of ion synthesis at high pressure

The growth of Ge nanocrystals in SiO₂ films is studied in relation to the dose of implanted Ge⁺ ions and the annealing temperature at a pressure of 12 kbar. It is established that the dependences of the nanocrystal dimensions on the content of Ge atoms and the annealing time are described by the corresponding root functions. The nanocrystal radius squared is an exponential function of the inverse temperature. The dependences correspond to the model of the diffusion-controlled mechanism of nanocrystal growth. From the temperature dependence of the nanocrystal dimensions, the diffusion coefficient of Ge in SiO₂ at a pressure of 12 kbar is determined: D = 1.1 × 10^–10 exp(–1.43/kT). An increase in the diffusion coefficient of Ge under pressure is attributed to the change in the activation volume of the formation and migration of point defects. Evidence in favor of the interstitial mechanism of the diffusion of Ge atoms to nanocrystal nuclei in SiO₂ is reported.     

Behavior of germanium ion-implanted into SiO<Subscript>2</Subscript> near the bonding interface of a silicon-on-insulator structure

The properties of germanium implanted into the SiO₂ layers in the vicinity of the bonding interface of silicon-on-insulator structures are studied. It is shown that, under conditions of high-temperature (1100°C) annealing, germanium nanocrystals are not formed, while the implanted Ge atoms segregate at the Si/SiO₂ bonding interface. It is established that, in this case, Ge atoms are found at sites that are coherent with the lattice of the top silicon layer. In this situation, the main type of traps is the positive-charge traps; their effect is interpreted in the context of an increase in the surface-state density due to the formation of weaker Ge-O bonds. It is found that the slope of the drain-gate characteristics of the back MIS transistors increases; this increase is attributed to an increased mobility of holes due to the contribution of an intermediate germanium layer formed at the Si/SiO₂ interface.     

Charging effect of Si nanocrystals in gate oxide near gate on MOS capacitance

MOS structure with Si nanocrystals embedded in the gate oxide close to the gate has a much larger capacitance compared to a similar MOS structure without the nanocrystals. However, charge trapping in the nanocrystals reduces the capacitance dramatically, and after most of the nanocrystals are charged up the capacitance is much smaller than that of the MOS structure without nanocrystals. An equivalent-capacitance model is proposed to explain the phenomena observed.     

Tunneling in MIS structures—II Experimental results on M---SiO2---Si

Experimental results for MOS tunnel structures are discussed and compared to the theory developed in the preceding paper. The low frequency (d.c.) conductance yields a well width wider than the band gap of silicon. For 10 ω-cm p-type silicon, with an oxide thickness of 46 Å, V_w=1.96 eV, in good agreement with the theory, including the effects of interface states. Similar results are obtained for n- and p-type non-degenerate silicon. Structure appears in the conductance well for higher frequency signals. The origin of these peaks is discussed. It is shown that one peak is due to a geometric effect while the second is due to interface states. The conductance due to interface states can be accounted for by the time lag in charge exchange between the majority carrier band in the semiconductor and the interface states; tunneling from the metal to these interface states is not observed. This work indicates a localized state at 0.27 eV above the valence band for 10 ω-cm p-type silicon. The concentration is 7.4 × 10¹¹ cm⁻² and the capture cross-section is 5.0 × 10⁻¹⁸ cm². Capacitance measurements indicate another localized state at 0.14 eV above the valence band which does not contribute to the conductance measurements because of its long time constant.     

Influence of Si nanocrystal distributed in the gate oxide on the MOS capacitance

In this paper, the authors have studied the influence of silicon nanocrystal (nc-Si) distributed in the gate oxide on the capacitance for the circumstances that the nc-Si does not form conductive percolation tunneling paths connecting the gate to the substrate. The nc-Si is synthesized by Si-ion implantation. The effective dielectric constant of the gate oxide in the nc-Si distributed region is calculated based on a sublayer model of the nc-Si distribution and the Maxwell-Garnett effective medium approximation. After the depth distribution of the effective dielectric constant is obtained, the MOS capacitance is determined. Two different nc-Si distributions, i.e., partial and full nc-Si distributions in the gate oxide, have been considered. The MOS capacitance obtained from the modeling has been compared to the capacitance measurement for a number of samples with various gate-oxide thicknesses, implantation energies and dosages, and an excellent agreement has been achieved for all the samples. A detailed picture of the influence of implantation energy and implantation dosage on the MOS capacitance has been obtained.     

Surface-state spectra from thick-oxide MOS tunnel junctions

The conductance and capacitance of thick-oxide MOS tunnel junctions (SiO₂ thickness $\text{40～65}\text{A}\text{?}$)have been measured from 35 Hz to 210 kHz. It is demonstrated that the use of a thick-oxide MOS tunnel junction enables one to obtain the surface-state data throughout the whole silicon band-gap with better resolution and better sensitivity than the conventional MOS capacitance techniques. A slight departure from equilibrium may occur in the voltage range where large tunnel current flows. Corrections to the energy scale must be made in this voltage range. A method for the evaluation of the junction quality is discussed. The simplified equivalent circuits necessary for the calculation of surface-state data are constructed under various bias conditions by an approach different from a previously published work. The present work supports the model that at least some of the observed surface states are a consequence of the diffusion of contact metals into the oxide.     

Checking the electron equilibrium in the semiconductor and at the oxide/semiconductor boundary of a silicon metal/oxide/semiconductor structure with mobile ions in the oxide film

The differential capacitance of a silicon metal/oxide/semiconductor (MOS) structure with mobile ions in the oxide film is measured as a function of temperature and the applied voltage in the frequency range 1–100 kHz. The time relaxation of capacitance after interruption of the voltage sweep is considered. It is found that, in a certain temperature-dependent frequency range, no frequency dispersion of the structure capacitance is observed; i.e., the capacitance is quasi-equilibrium with respect to charges located in the bulk and in the fast surface states of the semiconductor and depends only on its surface potential. In this interval, mobile ions and other “slow” charges in the oxide film cannot keep pace with the varying voltage and make zero contribution to the capacitance. It is shown that, when the voltage is ramped at a rate of ～10 mV/s at a temperature of about 200°C, the reaction of charges in the film noticeably lags behind the voltage change.     

Electrical and gas sensing properties of <Emphasis Type="Italic">por</Emphasis>-Si/SnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposite layers

The electrical characteristics and chemical reactant sensitivity of layers of heterogeneous nanocomposites based on porous silicon and nonstoichiometric tin oxide por-Si/SnO _x , fabricated by the magnetron sputtering of tin with subsequent oxidation, are studied. It is shown that, in the nanocomposite layers, a system of distributed heterojunctions (Si/SnO _x nanocrystals) forms, which determine the electrical characteristics of such structures. The sensitivity of test sensor structures based on por-Si/SnO _x nanocomposites to NO₂ is determined. A mechanism for the effect of the adsorption of NO₂ molecules on the current-voltage characteristics of the por-Si(p)/SnO _x (n) heterojunctions is suggested.     

Routes to Achieving High Quantum Yield Luminescence from Gas‐Phase‐Produced Silicon Nanocrystals

Plasma‐synthesized silicon nanocrystals with alkene ligands have shown the potential to exhibit high‐efficiency photoluminescence, but results reported in the literature have been inconsistent. Here, for the first time, the role of the immediate post‐synthesis “afterglow plasma” environment is explored. The significant impact of gas injection into the afterglow plasma on the photoluminescence efficiency of silicon nanocrystals is reprorted. Depending on the afterglow conditions, photoluminescence quantum yields of silicon nanocrystals synthesized under otherwise identical conditions can vary by a factor of almost five. It is demonstrated that achieving a fast quenching of the particle temperature and a high flux of atomic hydrogen to the nanocrystal surface are essential for a high photoluminescence quantum yield of the produced silicon nanocrystals.     

Kinetics of polarization current in a Pb<Subscript>3</Subscript>O<Subscript>4</Subscript> wide-gap photoconductor

The results of the study of isothermal polarization of Pb₃O₄ high-resistivity semiconductor layers in a dc electric field of various intensities with measurements without and with illumination are presented. To interpret the data obtained, the relay mechanism of charge transport over local states in the band gap is introduced. The values of the parameters describing the relaxation phenomena such as contact capacitance, width of the charge accumulation region, and effective charge mobility are obtained.     

本征α－Si∶H MIS结构电导机制

利用本征α－Ｓｉ：Ｈ薄膜材料制成了Ａｌ－Ｓｉ（１－ｘ）Ｎｘ－ａ－Ｓｉ：Ｈ金属－绝缘体－半导体结构，用高精度差分电容谱仪在５０℃和暗背景条件下测量了这种结构的变频电容谱及变频电导谱。讨论了该ＭＩＳ结构中可能存在的电导机制，计算了禁带态能谱并根据修正后的实验结果得到了其俘获电子的时间常数及截面。     

The formation of silicon nanocrystals in SiO<Subscript>2</Subscript> layers by the implantation of Si ions with intermediate heat treatments

The effect of heat treatments at 1100°C on an ion-beam synthesis of Si nanocrystals in SiO₂ layers is studied. The ion-implanted samples are subjected either to a single heat treatment after the total ion dose (10¹⁷ cm^?2 has been implanted, two heat treatments (a heat treatment after the ion implantation of each half of the total dose), or three heat treatments (a heat treatment after each third of the dose). The total duration of the heat treatments is maintained at 2 h. It is found that the intermediate heat treatments lead to a shift of the Raman spectrum of the nanocrystals to longer wavelengths and to a shift of the photoluminescence spectrum to shorter wavelengths. Study using electron microscopy shows that the size of the nanoprecipitates decreases, which is accompanied by the disappearance of the characteristic features of crystallinity; however, the features of photoluminescence remain characteristic of the nanocrystals. The experimental data obtained are accounted for by a preferential drain of Si atoms to newly formed clusters, which is consistent with the results of a corresponding numerical simulation. It is believed that small nanocrystals make the main contribution to photoluminescence, whereas the Raman scattering and electron microscopy are more sensitive to larger nanocrystals.     

Resonance transfer of charge carriers in Si/CaF<Subscript>2</Subscript> periodic nanostructures via trap states in insulator layers

A model of the resonance-tunneling transport of charge carriers via discrete-level traps in insulator layers in Si/CaF₂ periodic low-dimensional structures is proposed. Upon application of the external bias voltage to the structure, the resonance-tunneling transport occurs in the cases when the energy of the charge carriers in Si wells coincides with the energy of the trap level in CaF₂ layers. It is shown that filling of the traps and violation of the conditions for the resonance-tunneling transport of charge carriers via the trap level, which take place when the energy of the carriers in the wells exceeds the energy of the trap state in the insulator, result in a drop in the current through the structure; thus, a region of negative differential resistance is formed in the current-voltage characteristics of Si/CaF₂ periodic structures. Simulation of this effect shows that devices based on these structures may operate in a wide range of temperatures from 77 to 300 K. Another advantage is their compatibility with silicon integrated-circuit technology.     

Surface states at the nGaAsSiO2 interface from conductance and capacitance measurements

Conductance and capacitance measurements have been used to investigate the interface properties of MOS capacitors formed by depositing an insulating layer of SiO₂ on n-type GaAs. The surface potential as a function of applied bias is evaluated using results of high and low frequency capacitance measurements and the two methods are found to yield roughly similar results. The interface state density evaluated from the conductance and capacitance measurements is found to peak near the center of the band gap and also near the conduction band edge. Plots of conductance vs frequency indicate the presence of both fast and slow interface states in these materials.     

Tunneling transport through passivated CdS nanocrystal arrays grown by the Langmuir-Blodgett method

Tunneling electron transport through CdS nanocrystal arrays fabricated by the Langmuir-Blodgett method are studied by scanning electron spectroscopy. The effect of the matrix-annealing atmosphere on tunneling transport through the nanocrystal arrays is studied. Electron capture at traps in the case of nanocrystals annealed in vacuum is detected by tunneling current-voltage characteristics analyzed using a model relating the data of tunneling spectroscopy, photoluminescence, and quantum-mechanical calculation. Analysis shows that the nanocrystal surface is passivated by an ammonia monolayer upon annealing in an ammonia atmosphere. It is found that the substrate and surrounding non-passivated nanocrystals have an effect on the electron polarization energy.