We have fabricated two kinds of n-type microcrystalline silicon carbon (μc-SiC) / porous silicon (PS) / p-type crystalline silicon (c-Si) pn junctions and demonstrated a visible light emission from them. We have observed three types of visible light emission; an uniform red light emission at a forward current above 12mA/cm2 for the pn junction using a 0.2–0.4 Qcm c-Si substrate, and a very weak white light emission at a forward current of about 90 mA/mm2 and a strong orange-red light emission at a forward current from 200 to 619 mA/mm2for the pn junction using a 3.5–4.5 Ωcm c-Si substrate.

Two components of the electroluminescence (EL) from porous silicon light emitting diode (LED) devices have been observed. A slower component and a faster component have been identified. The slower component has a spectral peak shifted to the red from the corresponding photoluminescence (PL) spectrum. The faster component has a spectral peak well in the infrared (IR). Optical and electrical measurements of these two components are discussed. The temperature dependence of the two EL components are presented and contrasted. Our measurements demonstrate that the two EL components and the PL result from recombination in different parts of the porous silicon structure. As the temperature is reduced below room temperature the slower EL exhibits a decrease in intensity at relatively high temperatures, suggesting a freeze out of electrical carriers due to quantum confinement, resulting in a much reduced electrical excitation of the EL.

We utilized the conventional planar fabrication technique and the electrochemical etching method to prepare porous Si layers in the p-type region of a p/n junction, which could make the study on the transverse transport property of this material possible. The junctions were fabricated by low energy ion-implantation, with porous Si formed perpendicular to the junction and between two metal contacts. This structure confines currents to the direction parallel to the surface. Distinct features on current-voltage (I-V) curves has been observed.

We report on the results of our investigation of using porous Si to enhance the performance of crystalline silicon photovoltaic solar cells. Possible approaches include using the porous Si for (1) surface texturing to enhance light trapping, (2) front or back surface fields because of its wider bandgap, and (3) photon color conversion of blue light to longer wavelengths that have higher quantum efficiency in a Si solar cell. In our surface texturing study, a porous-Si-covered single-crystal Si wafer showed an integrated reflectance of only 1.4% at 500-nm wavelength compared to about 40% for a polished Si surface. For our solar cell study, we used a point-contact cell structure with diffused p+ and n+ point contacts on the back of the cell. This cell structure allows us to form the porous Si on the front surface after both the junction formation and the evaporation and alloying of metal contacts.

Organic-inorganic junctions were formed between porous silicon and various conjugated conducting polymers, poly(3-methylthiophene) and polypyrrole. Schottky type barriers were observed between the conducting polymers in their doped state and p and n-type porous silicon. In their undoped state the conducting polymers behave like p-type semiconductors. Consistent with this, ohmic contacts were observed between undoped conducting polymers and p-type porous silicon while rectifying behavior typical of a p-n junction was observed for conducting polymers deposited onto n-type porous silicon. During characterization of the porous silicon substrate, an investigation of the surface chemistry revealed a strong correspondence between solution pH and the luminescence intensity of porous silicon. Surface titration experiments were performed on p and n-type porous silicon and the results indicate that a monoprotic surface acid with a pKa between 3–4 is a primary component in the luminescence mechanism of porous silicon.

Recent observation of efficient luminescence in porous silicon has stimulated interest in the electronic and optical properties of Si quantum wires [1,2,3]. If silicon becomes a material suitable for optical applications, techniques for fabricating silicon wires reliably and uniformly will be needed. Once this is achieved, there will be interest not only in optical properties of silicon wires but also transport properties. For instance, to determine the properties of a hypothetical Si LED, one needs to know about both transport and optical properties.

In this paper, we present theoretical studies of electronic, optical and transpon properties of silicon quantum wires ranging in size from 7.7Ä to 31Ä. The electronic and optical properties are treated in an empirical tight-binding approach with excitonic effects included in the effective mass approximation. Carrier transport is treated in a Boltzmann transport framework with nonpolar deformation potential acoustic phonon scattering being the dominant scattering mechanism.

Optical properties of hydrogenated silicon clusters are investigated by density functional pseudopotential calculation. Transitions between the band-edge orbitals are allowed, in contrast to the indirect gap in bulk silicon. The energy gaps of hydrogenated silicon particles of 15 to 30 Å in diameter are estimated to be 2.0 to 1.5 eV. When the cluster is dehydrogenated, localized states related to dangling bonds appear in the mid-gap, which decrease the photoluminescence intensity. These results agree with much experimental evidence and suggest that the photoluminescence of porous silicon is attributable to hydrogenated silicon particles.

A large number of experiments on porous silicon has reliably demonstrated that the onset of optical absorption is shifted to energies significantly above the band edge of bulk Si. This increased transparency of the small nanometer-sized crystallites with their H-covered surfaces is a fact that asks for theoretical interpretation. Handwaving arguments about quantum size effect can only be a qualitative guide.

We present here a tight binding calculation of a Si slab with nanometer dimensions covered with hydrogen. This is a model system for one-dimensional confinement. We consider the effect on the electron energy structure, the total and local densities of states of Si covered with hydrogen in two phases: monohydride - Si : H (2×1) symmetric dimer, and dihydride phases - Si : Hi (1×1) A total energy minimization method in the framework of the self-consistent tight binding theory has been used to investigate the structural reconstruction of the Si -surface after the adsorption of hydrogen. We find, that the band gap of the slab covered with H on both sides (monohydride phase) shifts to higher energies (typically ∼1.8 eV for 1.16 nm thick slab). The adsorption of hydrogen removes all the electronic states from the gap for both phases investigated. In nanometer sized slabs the lowest electronic states in the conduction band are localized on the surface Si—atoms, in contrast to thicker slabs. We discuss the implication of this model calculation to light emission in porous Si.

The reduction of the dielectric constant due to quantum confinement is studied both experimentally and theoretically. Angle resolved ellipsometry measurements with Ar- and He-Ne-lasers give values for the index of refraction far below what can be accounted for from porosity alone. A modified Penn model to include quantum size effects has been used to calculate the reduction in the static dielectric constant (ε) with extreme confinement. Since the binding energy of shallow impurities depends inversely on ε2, the drastic decrease in the carrier concentration as a result of the decrease in ε leads to a self-limiting process for the electrochemical etching of porous silicon.

The microstructure and electrical properties of μc-Si and μc-Si,C prepared by remote plasma-enhanced chemical-vapor deposition, PECVD, are reviewed. The microstructure has been characterized by transmission electron microscopy, TEM, infrared, IR, absorption and Raman scattering. The electrical properties were characterized by temperature-dependent dark-conductivity measurements. These studies have explained significant quantitative differences between the carrier transport properties of μc-Si and μc-Si,C alloys in terms of a band offset model for the interfacial potential steps between the amorphous and crystalline constituents of these material systems.

The growth of microcrystalline silicon (μc-Si), deposited by a succession of silane and hydrogen plasmas, is investigated in situ by ellipsometry in the visible and near UV-range. It is found that the amorphous tissue is more affected by the hydrogen etching than the crystallites. The model of “selective etching” emerges from these measurements. Although this model is compatible with the “partial chemical equilibrium” of Vep̌ek, it is somewhat more general and explains the porous nature of the (μc-Si) as well as the many atomic layers deposition-etching sequences.

Materials issues central to the application of microcrystalline silicon (μc-Si) doped layers in a-Si:H based solar cells are discussed, which include: (1) characterization of ultra-thin layers to be incorporated in the device, and (2) methods to promote nucleation of μc-Si on desired substrates within a thickness on the order of ∼100Å. Successful application of (μc-Si) in multijunction a-Si:H based solar cells are demonstrated.

Doped and compensated microcrystalline silicon prepared by Very High Frequency Glow Discharge has been investigated by optical spectroscopy and by temperature dependence of the electrical conductivity. Raman spectroscopy confirms the microcrystalline nature of all samples with only small changes of the crystalline volume fraction upon doping. Strong absorption in the infrared region, which correlates with the conductivity, is attributed to free carrier absorption. As a function of temperature the conductivity of all samples shows a deviation from a purely activated behaviour. Consequences of this observation for transport mechanisms are discussed.

Samples of μc-Si1-xCx:H with different degree of crystallinity and different carbon content were deposited by Plasma Enhanced Chemical Vapor Deposition and characterized by means of optical, electrical and structural measurements. The correlation between the degree of crystallinity and the mechanism of conductivity and optical transitions in both amorphous and crystalline phases and at the crystalline-amorphous interfaces is reported and discussed in terms of a band structure model.

Plasma-enhanced chemical vapor deposited boron carbide (B1-xCx) thin films are shown to be a potential electronic material suitable for high temperature devices. The boron carbide films make excellent p-n heteroj unction diodes with /i-type silicon substrates. The B1-xCx/Si heteroj unction diodes are demonstrated to have rectifying properties at temperatures above 200°C and reverse current is strongly dependent on the energy of the band gap of the boron carbide films.

Microcrystalline silicon with high crystallinity was fabricated on a glass substrate at a rather low temperature (320 C) by alternately repeating the deposition of Si thin layer 10 nm thick from fluorinated precursors and the treatment with atomic hydrogen. Hydrogen content was reduced to 0.5 at% or less. According to the in situ ellipsometric observation, the sticking of precursors followed the reactions for the construction of the ordered structure with the aid of atomic hydrogen. In addition, the defects were passivated efficiently with the treatment down to 4×1016 spins/cm3. A marked improvement was simultaneously verified in the efficiency of the substitutional P-doping in the films fabricated by this layer-by-layer technique.

We discuss a process for selective area deposition of microcrystalline silicon (μc-Si) using plasma enhanced chemical vapor deposition at low substrate temperature (<300°C) using time modulated silane flow in a hydrogen plasma. We discuss selectivity and deposition rate on a variety of substrates with process conditions important for manufacturing applications, and show a distinct microstructural evolution in the initial nucleation layers using Raman spectroscopy that correlates with the transition from selective to non-selective growth. Atomic hydrogen discriminates between different degrees of bond strain in the nucleii formed on different substrates, and can increase the crystallinity fraction in films deposited at low temperatures by modifying the kinetics of bulk-like bond formation.

We have used real-time, in situ spectroscopie ellipsometry (SE) and infrared reflectance (IR) to study microcrystalline silicon (μc-Si) formation in reactive magnetron sputtering (RMS). μc-Si growth occurs at high hydrogen partial pressures and moderate substrate temperatures. We use IR studies and SE studies of film growth on rough surfaces, respectively, to show that these conditions lead to high hydrogen coverage of the film surface and high effective surface diffusivity. The interface region is amorphous and its thickness decreases with deposition rate. For a fixed growth flux, we observe a 30% decrease in the deposition rate of μc-Si relative to the amorphous interface region. This could be due to increased etching or decreased sticking coefficients during microcrystalline growth.

Both B- and P- doped silicon films deposited by Low Pressure Chemical Vapor Deposition (LPCVD) at 300 °C (p-type) and 420 °C (n-type) have been characterized by optical absorption, Photothermal Deflection Spectroscopy (PDS), resistivity, Elastic Recoil Detection Analysis (ERDA), Transmission Electron Microscopy (TEM), Convergent-Beam Electron Diffraction (CBED) and Raman spectroscopy measurements. P-doped films, deposited at large PH3 flux rates, show a high degree of microcrystallinity, indicating that P activates the nucleation process even at low temperatures. In this case, values of activation energy of resistivity as low as 0.007 eV were obtained. Both TEM and RAMAN results confirm a volume percentage of micro crystallinity above 30%. On the contrary, B-doped samples are not microcrystalline at least in the doping range investigated, and show a behaviour not different from samples deposited by PECVD.

To analyse the influence of the grain boundaries (gb) on the transport of carriers in hydrogenated microcrystalline silicon (μC-Si:H) the ambipolar diffusion length (LLMB) was measured by SSPG. In addition, the films were characterised by photo-conductivity, dark conductivity activation energy, Urbach energy (determined by CPM), hydrogen effusion, Raman spectroscopy, X-ray scattering and optical transmission.

The sample series was prepared by PECVD of SiH4 diluted with increasing H2 content. Taking the structural information by Raman spectra and X-ray into account, we explain our optical and activation energy measurements within a three-phase-model (amorphous phase, crystalline phase, gb) and a Fermi level pinning in μc-Si:H.