Electronic band structure and optical properties of silicon nanoporous pillar array

Silicon nanoporous pillar array (Si-NPA) is fabricated by hydrothermally etching single crystal silicon (c-Si) wafers in hydrofluoric acid containing ferric nitrate. Microstructure studies disclosed that it is a typical micron/nanometer structural composite system with clear hierarchical structures. The optical parameters of Si-NPA were calculated by general light-absorption theory and Kramers–Kronig relations based on the experimental data of reflectance and the variations compared with the counterparts of c-Si were analyzed. The features of the electronic band structure deduced from the optical measurements strongly indicate that Si-NPA material is a direct-band-gap semiconductor and possesses separated conduction sub-bands which accords with conduction band splitting caused by silicon nanocrystallites several nanometers in size. All these electronic and optical results are due to the quantum confinement effect of the carriers in silicon nanocrystallites.     

Structural and optical properties of porous silicon prepared from a p +-epitaxial layer on n-Si(111)

The structural and optical properties of porous silicon prepared by anodic etching of an n-Si(111) wafer with a p ⁺-homoepitaxial layer on one side are studied by scanning electron microscopy and multiple-crystal X-ray diffraction. A considerable difference between the microstructures on the sides of the wafer is found. Upon aging for 4.5 months, diffraction peaks of the por-Si structures shift from that of the substrate by δθ = ?42″ for the n-Si porous layer and ?450″ for the p ⁺-Si porous layer. The photoluminescence band associated with the p ⁺-layer is twice as narrow as the band associated with the n-layer and is shifted toward shorter wavelengths (higher energies) by 0.4 eV, with the intensities of the bands being the same.     

Influence of water vapor adsorption on the <Emphasis Type="Italic">C-V</Emphasis> characteristics of heterostructures containing porous silicon

Porous silicon (por-Si) is prepared by the electrochemical etching of single-crystal n-silicon in an aqueous-alcoholic solution of hydrofluoric acid in the presence of hydrogen peroxide oxidizer. The dependence of the high-frequency C-V characteristics of Al/por-Si/Si heterostructures on the relative humidity is studied. A model of capacitor structure is proposed, and a method of analyzing its capacitance as a function of the water vapor partial pressure in terms of the adsorption isotherm is elaborated. Within the framework of this model, the porosity of the material, the effective fraction of silicon dioxide in the por-Si, the fraction of intercommunicating porosity, the micropore-to-mesopore volume ratio, and the mesopore size distribution are determined. The porous silicon prepared in this work seems promising as a sensitive layer in capacitance-type humidity sensors.     

Formation of SiH bonds on the surface of microcrystalline silicon covered with SiOx by HF treatment

Infrared absorption due to the surface chemical bonds on microcrystalline silicon (μc-Si) was investigated with recycling procedures of thermal oxygenation accompanied with dehydrogenation and HF etching. SiH bonds were reformed on the surface of μc-Si by HF etching of oxygenated μc-Si. The mechanism of the SiH bond formation was proposed. It was suggested that formation of =SiH₂,

SiH₂

_x or -SiH₃ group depended on the kind of a crystalline plane.     

Synchrotron investigations of Si/Mo/Si…c-Si (100) multilayer nanoperiodic structures

This paper presents the results of the investigation of c-Si/[Si/Mo] _n /Si multilayer nanoperiodic structures by X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation. Changes in the fine structure of XANES MoL _2,3 spectra confirm the formation of the silicide phase on heterophase interfaces Si/Mo/Si due to the solid-phase interactions between silicon and molybdenum layers.     

Laser assisted electrochemical preparation of micro and nanopores in Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>P

We report on the preparation of Ga(x)In(1−x)P/GaAs lattice matched heterostructures by liquid phase epitaxy and their electrochemical etching to prepare porous networks. We show that the process of formation of pores strongly depends on the electrolyte, on the applied current/voltage anodization regime, and that illumination is a crucial factor in obtaining fine porous structures with a high degree of self-organization.     

AES and XPS investigations of the surface layers of porous silicon with Fe,Co, and Ni embedded pores

A Composite material of porous silicon with pore embedded d-metals por-Si(Me) can be used for fabricating memory cells. Por-Si layers have been produced by electrochemical etching of n-type silicon (100) according to the conventional process. Fe, Co, and Ni galvanic deposition in por-Si has been performed using aqueous solutions of corresponding sulfate salts. The Auger profiles of the obtained por-Si(Fe), por-Si(Co), por-Si(Ni) nanocomposites have shown that its surface layers (up to 40 nm thick) contained about 10% Fe and no more than 1% Co and Ni. These facts confirm data that Co and Ni, unlike Fe, penetrate deep into the silicon pores. The value of the magnetic moment for the nanocrystalline Ni atom incorporated into the por-Si(Ni) has been estimated based on the dependence of the relative intensity of the maxima for the 3s multiplet splitting of the X-ray phase spectra on the number of uncoupled d-electrons in systems with 3d metals. The obtained value ∼2.4 μB exceeds the atomic magnetic moment in bulk metallic Ni.     

Mechanism of primary self-organization in porous silicon with a regular structure

A mechanism for self-organization of a regular system of pores in porous silicon is proposed. According to this mechanism, the self-organization obeys the general kinetic laws for a system of charge carriers. The mean interpore spacing in porous silicon prepared from p-Si and the anodizing current density required for synthesizing porous silicon through anodic etching are evaluated in terms of the proposed mechanism. The results obtained are in good agreement with the available experimental data. The dependence of the mean interpore spacing on the carrier concentration in the initial silicon is predicted to be similar to the function L(n) ～ n^?1/2. The validity of the proposed mechanism is confirmed by computer simulation.     

Visible photoluminescence in porous GaAs capped by GaAs

A typical porous structure with pores diameters ranging from 10 to 50 nm has been obtained by electrochemical etching of (1 0 0) heavily doped p-type GaAs substrate in HF solution. Room temperature photoluminescence (PL) investigations of the porous GaAs (π-GaAs) reveal the presence of two PL bands, I₁ and I₂, located at 1.403 and 1.877 eV, respectively. After GaAs capping, the I₁ and I₂ PL bands exhibit opposite shift trends. However, the emission efficiency of these two bands is not strongly modified. Low temperature PL of capped porous GaAs versus injection levels shows that the I₁ PL band exhibits a red shift while the I₂ PL band exhibits a blue shift with increasing injection levels. The I₂ PL band intensity temperature dependence shows an anomalous behaviour and its energy location shows a blue shift as temperature increases. The observed PL bands act independently and are attributed to electron – hole recombination in porous GaAs and to the well-known quantum confinement effects in GaAs nanocrystallites. The I₂ PL band excitation power and temperature dependencies were explained by the filling effect of GaAs nanocrystallites energy states.     

Optical properties of porous silicon processed in tetraethyl orthosilicate

We investigate the change in the composition and optical properties of porous silicon (por-Si) obtained by electrochemical etching of a palate made of n-type (111) silicon single crystal under high-temperature annealing and processing in tetraethyl orthosilicate (TEOS). It is shown that TEOS processing and annealing prevent contamination of a sample stored for a long time in atmosphere. The processing of por-Si in TEOS does not change the position of the photoluminescence (PL) peak and suppresses PL to a smaller extent as compared to annealing of por-Si. This increases the reliability of optoelectronic devices based on por-Si.     

Surface characterization of porous silicon after pore opening processes inducing chemical modifications

In this paper, we present a study on the porous silicon surface with the aim of filling porous silicon layers with organics. We discuss on two processes used to remove the outer parasitic layer created during the porous silicon formation. We demonstrate that these etching processes influences the surface properties, in particular wetting ability. By XPS and infrared absorption spectroscopy studies, we show that a SF₆ plasma treatment does not modify irreversibly the chemistry of porous silicon surface, nor the surface morphology. We also point out that NaOH etching does bring significant morphological modifications and influences the hydrophilicity of the porous silicon surface. This last treatment increases the polar groups (SiO) concentration on the pore surface and therefore allows a better filling of a porous silicon layer with organics, like dibromo-EDOT which can be thermally converted into PEDOT.     

AES and XPS studies of a por-Si/SnOx nanocomposite formed using a powerful ion beam of nanosecond duration

The results of the X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM) investigations of tin-oxide nanolayers on samples of por-Si/SnO _x composites with varying matrix porosity, formed using a powerful ion beam of nanosecond duration, are presented. It is shown that rapid melting and crystallization of the surface leads to the formation of Si nanoparticles with a maximal size of 200 nm. It is established that tin is included in the structure of the nanocomposite in an oxidized state with a small inclusion of metallic β tin. With increasing porosity, the phase composition of the tin nanolayers becomes closer to the state corresponding to the highest tin oxide (SnO₂). It is also shown that, upon an increase in the porosity, the intensity of the tin 4d subvalent line increases, which is, apparently, associated with an enhanced degree of hybridization of tin and oxygen atoms. The changes in the elemental composition of the composite and the depth of tin penetration are estimated from the results of ion etching.     

Nanoroughness control of Al-Doped ZnO for high efficiency Si thin-film solar cells

The Al-doped ZnO (ZnO:Al) front transparent conducting oxide (TCO) for high efficiency Si thin-film solar cell has been developed using RF magnetron sputtering deposition and chemical wet etching. Microscopic surface roughness of the as-deposited ZnO:Al film estimated by spectroscopic ellipsometry is closely related to the compactness of the TCO film, and shown to be a straightforward and powerful tool to optimize the deposition conditions for the proper post-etched surface morphology. Wet-etching time is adjusted to form the U-shaped craters on the surface of the ZnO:Al film without sharp etch pits that can cause the crack-like defects in the overgrown microcrystalline Si-absorbing layers, and deteriorate the V_oc and FF of the Si thin-film solar cells. That is to say, the nanoroughness control of the as-deposited TCO film with proper chemical etching is the key optimization factor for the efficiency of the solar cell. The a-Si:H/a-SiGe:H/μc-Si:H triple junction Si thin-film solar cells grown on the optimized ZnO:Al front TCO with anti-reflection coatings show higher than 14% conversion efficiency.     

Ar ion milling of InSb for manufacturing single electron devices

Ar⁺ ion milling of InSb for manufacturing single electron devices was studied. It is shown that pyramidal structures (porous) are created on the (1 1 1) surface of InSb wafers by anisotropic etching. Also it was shown the axis of the pyramidal structure is a function of the angle of the Ar⁺ incident beam and does not depend on the energy of the beam. EDX measurement results show In_xO_y and Sb_xO_y were not created on the surface after milling process. FTIR measurement results show that the surface reflection was decreased and less than 0.3 V flat band voltage was seen in capacitance voltage measurement results. SEM images show that the etching has approximately vertical profile. Therefore the Ar⁺ milling technique can be used as a dry etching technique for manufacturing mesa and/or porous structures of InSb. Since the surface is porous and of near-pyramidal morphology, one can simulate the surface by a set of needles each of which is a nanometer-size capacitance (i.e. single electron device). We showed, the threshold voltage of this single electron device is 0.3 V approximately, and therefore it can be used for studying single-electron or Coulomb blockade effects.     

Anomalies of elastic properties of layered materials

Acoustic investigations of layered crystals KY(MoO₄)₂ and glassy alloys Si₂₀Te₈₀ (with inclusions of nanocrystallites) are performed with the purpose of elucidating the character of binding forces in layered materials. The absorption and velocity of sound, as well as the spatial evolution of the spectrum of acoustic fluxes in various directions in wide ranges of temperatures (90–300 K), frequencies (14–1800 MHz), and intensities (0.04–100 W/cm²) of sound, are measured. Acoustooptical and pulse-echo methods were used for the measurements. A theoretical analysis of the data obtained has revealed anomalously large values of the nonlinear elastic coefficients and anharmonicity constants of longitudinal phonon modes that are determined by the anharmonicity of binding forces across the layers and at boundaries with nanocrystallites. It is shown that the anisotropy of the mechanical strength of layered crystals is to a large extent determined by the anharmonicity of binding forces.     

Photocurrent saturation and negative differential photoconductivity in Mn4Si7-Si〈Mn〉-Mn4Si7 and Mn4Si7-Si〈Mn〉-M heterojunctions

A mechanism behind the saturation of the photocurrent and occurrence of negative differential photoconductivity in Mn₄Si₇-Si〈Mn〉-Mn₄Si₇ and Mn₄Si₇-Si〈Mn〉-M heterojunctions is found. Mn₄Si₇-Si〈Mn〉-Mn₄Si₇ and Mn₄Si₇-Si〈Mn〉-M structures are studied with a model of back-to-back diodes. Photocurrent-voltage characteristics are taken at high constant and pulsed applied biases. It is found that the nonlinearity of the photocurrent-voltage characteristics and photoconductivity kinetics are due to the quenching of photoconductivity by Joule self-heating.     

Optical spectroscopy of the surface of nanoporous diamond films

We have studied the IR absorption spectra of samples of porous ultrananocrystalline diamond (UNC diamond) obtained by selective etching of the sp ² phase in UNC diamond films. We show that the surface of porous UNC diamond is polyfunctional. We have studied the behavior of surface hydride, carbonyl, carboxyl, and hydroxyl groups as a function of annealing temperature in air and the time kept under normal conditions for UNC diamond films previously oxidized at 430°C–450°C. In the range from a few minutes to a few months, we studied the kinetics for establishment of the steady state for the functional adsorbed layer on the diamond surface under normal conditions. The observed growth in the intensity of the transmission bands due to hydride (CH _x ) and other hydrogen-containing functional groups is explained by dissociation of water molecules on the surface of the UNC diamond films.     

High-resolution analytical electron microscopy of catalytically etched silicon nanowires

We report on the characterization of hexagonally ordered, vertically aligned silicon nanowires (SiNW) by means of analytical transmission electron microscopy. Combining colloidal lithography, plasma etching, and catalytic wet etching arrays of SiNW of a sub-50 nm diameter with an aspect ratio of up to 10 could be fabricated. Scanning transmission electron microscopy has been applied in order to investigate the morphology, the internal structure, and the composition of the catalytically etched SiNW. The analysis yielded a single-crystalline porous structure composed of crystalline silicon, amorphous silicon, and SiO _x with x≤2.     

Temperature behaviour of optical properties of Si+-implanted SiO2

Silicon nanocrystals were prepared by Si⁺-ion implantation and subsequent annealing of SiO₂ films thermally grown on a c-Si wafer. Different implantation energies (20-150 keV) and doses - cm ^-2 ) were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration of Si atoms of 5, 7, 10 and 15 atomic%. The presence of Si nanocrystals was verified by transmission electron microscopy. The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of μs at room temperature. The changes of PL in the range 70-300 K can be well explained by the exciton singlet-triplet splitting model. We show that all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si⁺-implanted SiO₂ films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials. Received 1st September 1998 and Received in final form 7 September 1999     

Room Temperature Spin Accumulation Effect in Boron Doped Si Created by Epitaxial Fe<Subscript>3</Subscript>Si/<Emphasis Type="Italic">p</Emphasis>-Si Schottky Contact

To study spin-dependent transport phenomena in Fe₃Si/p-Si structures we fabricated 3-terminal planar microdevices and metal/semiconductor diode using conventional photolithography and wet chemical etching. I?V curve of prepared diode demonstrates rectifying behavior, which indicates the presence of Schottky barrier in Fe₃Si/p-Si interface. Calculated Schottky barrier height is 0.57 eV, which can provide necessary conditions for spin accumulation in p-Si. Indeed, in 3-terminal planar device with Fe₃Si/p-Si Schottky contact Hanle effect was observed. By the analysis of Hanle curves spin lifetime spin diffusion length in p-Si were calculated, which are 145 ps and 405 nm, respectively (at T = 300 K). Spin lifetime strongly depends on temperature which can be related to the fact that spin-dependent transport in our device is realized via the surface states. This gives a perspective of creation of spintronic devices based on metal/semiconductor structure without need for forming tunnel or Schottky tunnel contact.