Discrete tunneling of holes in porous silicon

It is pointed out that in the partial oxidation of porous silicon (PS) formed on heavily doped crystals, the topology of the pores can result in the formation of an anisotropic material with strings of nanometersized silicon granules embedded in insulating silicon dioxide SiO₂. In this range of granule sizes the correlation effects in the tunneling of electrons (holes) are strong on account of their Coulomb interaction. This should be manifested as discrete electron and hole tunneling at temperatures comparable to room temperature. The room-temperature current-voltage characteristics of n ⁺-PSp ⁺-p ⁺ diode structures with a PS interlayer on p ⁺-Si, which exhibit current steps on the forward and reverse branches, are presented. The current steps are attributed to discrete hole tunneling along the silicon strings in SiO₂. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 794–797 (25 May 1998)     

Electrical characteristics of top contact pentacene organic thin film transistors with SiO<Subscript>2</Subscript> and poly(methyl methacrylate) as gate dielectrics

Organic thin film transistors (OTFTs) were fabricated using pentacene as the active layer with two different gate dielectrics, namely SiO₂ and poly(methyl methacrylate) (PMMA), in top contact geometry for comparative studies. OTFTs with SiO₂ as dielectric and gold deposited on the rough side of highly doped silicon (n⁺-Si) as gate electrode exhibited reasonable field effect mobilities. To deal with poor stability and large leakage currents between source/drain and gate electrodes in these devices, isolated OTFTs with reduced source/drain contact area were fabricated by selective deposition of pentacene on SiO₂/PMMA through shadow mask. This led to almost negligible leakage currents and no degradation in electrical performance even after 14 days of storage under ambient conditions. But, the field effect mobilities obtained were lower than 10⁻³ cm² V⁻¹ s⁻¹, whereas by using PMMA as gate dielectric with chromium deposited on the polished side of n⁺-Si as gate electrode, improved field effect mobilities (>0.02 cm² V⁻¹ s⁻¹) were obtained. PMMA-based OTFTs also exhibited lower leakage currents and reproducible output characteristics even after 30 days of storage under ambient conditions.      

Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloy nanoparticles embedded SiO<Subscript>2</Subscript> films: synthesis and structure

Cu–Ni fcc alloy nanoparticles (NPs) of tunable atomic ratios were generated in SiO₂ films. The films were prepared using the Cu(NO₃)₂ and Ni(NO₃)₂ co-doped inorganic–organic hybrid silica sols by single dipping. Transparent, crack-free, glassy SiO₂ films of 310 ± 10 nm in thickness embedded with high mol percent of Cu–Ni alloy NPs were yielded after annealing at 750 °C in 10% H₂-90% Ar atmosphere. Nominal compositions of the films were 20 mol% (Cu–Ni)-80 mol% SiO₂. Optical spectral study of the heat-treated films showed disappearance of Cu plasmon bands due to Cu–Ni alloy formation. Grazing incidence X-ray diffraction (GIXRD) studies revealed the formation of Cu–Ni alloy (2:1, 1:1 and 1:2) NPs inside the SiO₂ film. GIXRD showed a systematic shifting of the diffraction peaks with respect to the fcc Cu–Ni alloy composition, maintaining the nominal ratios. Transmission electron microscopy (TEM) studies of the representative Cu_0.5Ni_0.5-doped film showed existence of homogeneously dispersed Cu–Ni alloy NPs of average size 6.35 nm inside the SiO₂ matrix. The energy dispersive X-ray scattering (EDX) analysis of the individual NPs using the nano-probe (scanning TEM mode) confirmed the presence of both the Cu and Ni with the desired atomic ratio.     

Growth mechanisms of spatially separated copper dendrites in pores of a SiO2 template

Abstract

n-Si/SiO₂(Cu) structures with different morphologies of copper deposits are produced using the template synthesis method. Deposition of Cu nanostructures and their structural features is thoroughly studied, and mechanisms for the formation of compact and dendrite-like deposits in the pores of the SiO₂ template on the n-type single-crystal silicon substrate are established. Spatially separated copper dendrites are obtained and used to investigate the efficiency of Raman scattering enhancement using the aqueous solution of the Rhodamine 6G dye.     

Improvement of <Emphasis Type="Italic">n</Emphasis>-ZnO/<Emphasis Type="Italic">p</Emphasis>-Si photodiodes by embedding of silver nanoparticles

The photo-current of n-ZnO/p-Si heterojunction photodiodes was improved by embedding Ag nanoparticles in the interface (ZnO/nano-P_Ag/p-Si), and the ratio between photo- and dark-current increased by about three orders more than that of a n-ZnO/p-Si specimen. The improvement in the photo-current resulted from the light scattering of embedded Ag nanoparticles. The I–V curve of n-ZnO/p-Si degraded after thermal treatment (A-ZnO/p-Si) because the silicon robbed the oxygen from ZnO to form amorphous silicon dioxide and left an oxygen vacancy. Notably, the properties of ZnO/nano-P_Ag/p-Si were better in the time-dependent photoresponse under 10 V bias. Ag nanoparticles (15–20 nm) scattered the UV light randomly and increased the probability for the absorption of ZnO to enhance the properties of the photodiode.     

Nanostructures of Si/SiO<Subscript>2</Subscript>/metal systems with tracks of fast heavy ions

Structures based on the SiO₂/n-Si and SiO₂/p-Si systems, with nanopores in silicon dioxide layers filled with Cu and Ni nanoparticles, have been prepared and investigated using the fast heavy ion technology, which includes irradiation with ¹⁹⁷Au²⁶⁺ ions, chemical etching of ion tracks, and subpotential electrochemical deposition. The selectivity of filling nanopores with metals and cluster character of their formation in tracks is shown.     

n+-Si与p-Si衬底上含纳米硅的SiO2膜电致发光

对于Au/富Si-SiO₂/p-Si和Au/富Si-SiO₂／n⁺-Si这两种结构，研究并比较了它们的电致发光特性．对于前者，当正向偏压大于4V时发射红光，而加反向偏压时不发光；对于后者，加正向偏压不发光，而当反向偏压大于3.5V时发射红光．着重讨论了Au/富Si-SiO₂/n⁺-Si的电致发光机制 关键词：     

Effects of oxygen plasma pre-treatments on the characteristics of n-ZnO/p-Si heterojunction diodes

We examine the effects of the oxygen plasma pre-treatments on the material properties of n-ZnO grown on p-Si and characterize the electrical properties of n-ZnO/p-Si heterojunction diodes. The lattice spacing of ZnO becomes larger when the ZnO thin film is grown on the oxygen plasma pre-treated Si substrate. This might be relevant to the growth of (101) ZnO onto the ultra-thin SiO₂ interfacial layer, which is formed during the oxygen plasma pre-treatment onto the Si substrate. The formation of SiO₂ gives rise to the increase in the donor-like defect Zn interstitial, and the increased grain size improves the carrier mobility. Because of all the above, the differential conductance at the on-state is increased for the n-ZnO/p-Si heterojunction diode.     

Local structure of porous silicon studied by means of X-ray emission spectroscopy

2,3 X-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si-structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of n^-- and p⁺-Si-wafers. Whereas for the P-Si processed from p⁺-Si the presence of some amorphous silicon is detected, the X-ray emission spectra of porous Si prepared from n^--Si display a higher content of SiO₂. For spark-processed Si the Si L_2,3 X-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the center of the processed area is close to that of silicon dioxide, and it has an influence on the photoluminescence energy. Specifically, green photoluminescent sp-Si shows a higher degree of oxidation than the blue luminescent specimen. However, the depth of oxidation consistently decreases in areas with weak or no PL. Possible origins of the observed photoluminescence are discussed. Accepted: 6 March 1997     

Au/(SiO2/Si/SiO2)纳米双势垒/n+-Si结构的电致发光研究

利用射频磁控溅射方法,在n⁺-Si衬底上淀积SiO₂/Si/SiO_{2纳米双势垒单势阱结构,其中Si层厚度为2至4nm,间隔为0.2nm,邻近n⁺-S i衬底的SiO2层厚度固定为1.5nm,另一SiO2层厚度固定为3nm.为了 对比研究,还制备了Si层厚度为零的结构,即SiO2(4.5nm)/n⁺-Si 结构.在经过600℃氮气下退火30min,正面蒸上半透明Au膜,背面也蒸Au作欧姆接触后,所 有样品都在反向偏置(n^＋-Si的电压高于Au电极的电压)下发光,而在正向偏压 下不发光.在一定的反向偏置下,电流和电致发光强度都随Si层厚度的增加而同步振荡,位 相相同.所有样品的电致发光谱都可分解为相对高度不等的中心位于2.26eV(550nm)和1.85eV (670nm)两个高斯型发光峰.分析指出该结构电致发光的机制是：反向偏压下的强电场使Au/( SiO2/Si/SiO2)纳米双势垒/n⁺-Si结构发生了雪崩击穿 ,产生大量的电子-空穴对,它们在纳米SiO2层中的发光中心(缺陷或杂质)上复 合而发光. 关键词： 电致发光 纳米双势垒 高斯型发光峰 雪崩击穿}     

A comprehensive model for the I –V characteristics of metal-Ta 2 O 5 /SiO 2 -Si structures

The I–V characteristics of metal-Ta₂O₅/SiO₂-Si structures are precisely described with a comprehensive model, for both polarities, in the whole measurement range where there is no noticeable degradation and over seven orders of magnitude of the current. Hopping conduction and tunneling were elucidated to be the dominant conduction mechanisms in the SiO₂ layer and Poole–Frenkel internal field-assisted emission in the Ta₂O₅ layer. Other possible relevant mechanisms were discussed and subsequently discarded, based on their minor contribution. Theoretical calculations are made with fitted values of the defect related constants for hopping conduction of SiO₂ and Poole–Frenkel emission of Ta₂O₅ and the thickness of the SiO₂ layer. For gates positively biased, tunneling of electrons from the silicon conduction band through the SiO₂ is considered, while for gates negatively biased, tunneling of holes from the silicon valence band. Approximations for practical use are proposed and thus introduced limitations of the model discussed. The model is demonstrated on Al-insulator-Si structures containing thermally grown Ta₂O₅, previously studied in terms of microstructural, dielectric and electrical properties. The experimental results suggest that at higher current densities (>10 nA/cm²) an effect of compensation of the existing oxide charges by accumulated charges occurs. PACS 73.40.Qv; 73.50.-h; 73.61.-r; 77.55.+f     

Precise determination of?optical properties of?pentacene thin films grown on various substrates: Gauss–Lorentz model with effective medium approach

Spectroscopic ellipsometry measurements are performed on thin pentacene films grown on glass, SiO₂, and n-Si substrates. The Gauss–Lorentz oscillator model is shown to be effective in modeling the π–π ^∗ transitions found in organic compounds. The effective medium approximation that considers the surface roughness of the films, which can be significant in case of pentacene, is also shown to be a key factor in precisely determining their dielectric functions. The proposed method reveals that there are some quantitative differences in the optical properties of the pentacene films prepared on different substrates.     

Electronic properties of ultrathin films based on pyrrolofullerene molecules on the surface of oxidized silicon

Results of the investigation into the interface formation during the deposition of the films based on aziridinylphenylpyrrolofullerene (APP-C₆₀) up to 8 nm thick on the surface of the oxidized silicon substrate are presented. The procedure of detecting reflection of testing low-energy electron beam from the surface implemented in the total current spectroscopy mode with a change in the incident electron energy from 0 to 25 eV is used. The structure of maxima in the total current spectra induced by the APP-C₆₀ deposited film is established, and the character of interrelation of these maxima with π* and σ* energy bands in the studied materials is determined. It is revealed due to analyzing the variation in intensities of the total current spectra of the deposited APP-C₆₀ film and the (SiO₂)n-Si substrate that the APP-C₆₀ film is formed at the early deposition stage with the coating thickness thinner than one monolayer without the formation of the intermediate modified organic layer. As the APP-C₆₀/(SiO₂)n-Si interface is formed, the work function of the surface increases by 0.7 eV, which corresponds to the transfer of the electron density from substrate (SiO₂)n-Si toward the film APP-C₆₀. The optical absorption spectra of the APP-C₆₀ films are measured and compared with the spectra of films of unsubstituted C₆₀.     

Characterization of epitaxial n-GaP/p-PSi heterojunctions

The liquid phase epitaxy technique was used to fabricate heterojunction devices of n-GaP thin films grown on porous p-Si substrates. XRD and SEM techniques were used to investigate the structure of the GaP grown. Measurements of the current–voltage (I–V) and the capacitance–voltage (C–V) characteristics were evaluated to identify the mechanisms of barrier formation and consequently current flow. The results showed that forward current involves tunneling. It was explained in light of a multi-step tunneling recombination model due to a high density of interface defects. On the other hand, the reverse current seems to be limited by the generation mechanism. The C–V characteristics indicated an abrupt heterojunction model. In addition, various junction parameters were determined from the I–Vand C–V characteristics.     

Mismatch of dielectric constants at the interface of nanometer metal-oxide-semiconductor devices with high-<Emphasis Type="Italic">K</Emphasis> gate dielectric impacts on the inversion charge density

The comparison of the inversion electron density between a nanometer metal-oxide-semiconductor (MOS) device with high-K gate dielectric and a SiO₂ MOS device with the same equivalent oxide thickness has been discussed. A fully self-consistent solution of the coupled Schr?dinger–Poisson equations demonstrates that a larger dielectric-constant mismatch between the gate dielectric and silicon substrate can reduce electron density in the channel of a MOS device under inversion bias. Such a reduction in inversion electron density of the channel will increase with increase in gate voltage. A reduction in the charge density implies a reduction in the inversion electron density in the channel of a MOS device. It also implies that a larger dielectric constant of the gate dielectric might result in a reduction in the source–drain current and the gate leakage current.     

Formation of the buffer layer of silicon suboxides SiOx in the Si/SiO2 low-dimensional heterosystem after Si+ ion implantation: Si L2, 3 X-ray emission spectra

Nanosized heterostructures n-Si/SiO₂ with different thicknesses of the oxide film (20, 500 nm) after implantation by Si⁺ ions with energies of 12 and 150 keV have been investigated using Si L _{2, 3} X-ray emission spectroscopy (the Si 3d3s → Si 2p _{1/2, 3/2} electronic transition). The ion-beam modification of the interface has been revealed and studied for the heterostructure with a silicon dioxide thickness of 20 nm. An analysis of the Si L _{2, 3} X-ray emission spectra has demonstrated that the Si⁺ ion implantation leads to the self-ordering of the structure of the initially amorphous SiO₂ film 20 nm thick due to the effect of high doses. A mechanism of ion-beam modification of the insulator-semiconductor interface has been proposed. No substantial transformation of the atomic and electronic structures of the heterostructure with a silicon dioxide thickness of 500 nm has been revealed after the ion implantation.     

Electron-irradiated n+-Si as hole injection tunable anode of organic light-emitting diode

Traditionally, n-type silicon is not regarded as a good anode of organic light emitting diode (OLED) due to the extremely low hole concentration in it; however, when doped with Au element which acts as carrier generation centers, it can be, as shown in our previous work. In this study, we demonstrate a new kind of carrier generation centers in n⁺-type silicon, which are the defects produced by 5 MeV electron irradiation. The density of carrier generation centers in the irradiated n⁺-Si anode can be controlled by tuning the electron irradiation time, and thus hole injection current in the OLEDs with the irradiated n⁺-Si anode can be optimized, leading to their much higher maximum efficiencies than those of the OLEDs with non-irradiated n⁺-Si anode. For a green phosphorescent OLED with the irradiated n⁺-Si anode, the current efficiency and power efficiency reach up to 12.1 cd/A and 4.2 lm/W, respectively.     

Acoustic spectroscopy and electrical characterization of SiO2/Si structures with ultrathin SiO2 layers formed by nitric acid oxidation

Ultrathin silicon dioxide (SiO₂) layers formed on Si substrate with nitric acid have been investigated using both acoustic deep-level transient spectroscopy (A-DLTS) and electrical methods to characterize the interface states. The set of SiO₂/Si structures formed in different conditions (reaction time, concentrations of nitric acid (HNO₃), and SiO₂ thickness [3–9 nm]) was prepared. The leakage current density was decreased by post-oxidation annealing (POA) treatment at 250°C in pure nitrogen for 1 h and/or post-metallization annealing (PMA) treatment at 250°C in a hydrogen atmosphere for 1 h. All structures of the set, except electrical investigation, current-voltage (I - V), and capacitance — voltage (C - V) measurements, were investigated using A-DLTS to find both the interface states distribution and the role of POA and/or PMA treatment on the interface-state occurrence and distribution. The evident decreases of interface states and shift of their activation energies in the structures with PMA treatment in comparison with POA treatment were observed in most of the investigated structures. The results are analyzed and discussed.      

Silicon nitride/silicon oxide interlayers for solar cell passivating contacts based on PECVD amorphous silicon

This Letter demonstrates improved passivating contacts for silicon solar cells consisting of doped silicon films together with tunnelling dielectric layers. An improvement is demonstrated by replacing the commonly used silicon oxide interfacial layer with a silicon nitride/silicon oxide double interfacial layer. The paper describes the optimization of such contacts, including doping of a PECVD intrinsic a‐Si:H film by means of a thermal POCl₃ diffusion process and an exploration of the effect of the refractive index of the SiN_x. The n⁺ silicon passivating contact with SiN_x /SiO_x double layer achieves a better result than a single SiN_x or SiO_x layer, giving a recombination current parameter of ～7 fA/cm² and a contact resistivity of ～0.005 Ω cm², respectively. These self‐passivating electron‐selective contacts open the way to high efficiency silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)