A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

Quantum confinement in Si and Ge nanostructures: Theory and experiment

Electron transport in GaAs/AlGaAs quantum cascade lasers operating in midinfrared is calculated self–consistently using an intersubband scattering model. Subband populations and carrier transition rates are calculated and all relevant electron-LO phonon and electron–electron scatterings between injector/collector, active region, and continuum resonance levels are included. The calculated carrier lifetimes and subband populations are then used to evaluate scattering current densities, injection efficiencies, and carrier backflow into the active region for a range of operating temperatures. From the calculated modal gain versus total current density dependencies the output characteristics, in particular the gain coefficient and threshold current, are extracted. For the original GaAs/Al0.33Ga0.67As quantum cascade structure [C. Sirtori et al., Appl. Phys. Lett. 73, 3486 (1998)] these are found to be g=11.3 cm/kA and Jth=6±1 kA/cm2 (at T=77 K), and g=7.9 cm/kA and Jth=10±1 kA/cm2 (at T=200 K), in good agreeme...

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Self-consistent scattering theory of transport and output characteristics of quantum cascade lasers

Hollow nanoarchitectured materials with straight channels play a crucial role in the fields of renewable energy, environment and biotechnology due to their one-dimensional morphology and extraordinary properties. The current challenge is the difficulty on tailoring hollow nanoarchitectures with well-controlled morphology at a relatively low cost. As a conventional technique, electrochemistry exhibits its unique advantage on machining nanostructures. In this review, we present the progress of electrochemistry as a valuable tool in construction of novel hollow nanoarchitectures through pulse/step anodization, such as surface pre-texturing, modulated, branched and multilayered pore architectures, and free-standing membranes. Basic principles for electrochemical engineering of mono- or multi-ordered nanostructures as well as free-standing membranes are extracted from specific examples (i.e. porous silicon, aluminum and titanium oxide). The potential of such nanoarchitectures are further demonstrated for the applications of photovoltaics, water splitting, organic degradation, nanostructure templates, biosensors and drug release. The electrochemical techniques provide a powerful approach to produce nanostructures with morphological complexity, which could have far-reaching implications in the design of future nanoscale systems.

Electrochemical engineering of hollow nanoarchitectures: pulse/step anodization (Si, Al, Ti) and their applications

A new qualitative mechanism of pore nucleation and initial stages of porous silicon (por-Si) growth was proposed. The emphasis was on the charge exchange between Si2+ ions generated by electrolytic or chemical oxidation of initial silicon (disproportionation reaction). The mechanism eliminates, to a large extent, the contradictions typical of earlier proposed schemes of por-Si growth; in particular, it explains the morphological features of por-Si produced under various experimental conditions. The impact of light in these processes was also considered.

On the Mechanism of Porous Silicon Formation

Electroluminescence from a quantum-cascade structure comprising 40 periods of GaAs/Al0.15Ga0.85As tunnel-coupled quantum wells (QW) was studied. A terahertz emission band in the range 1.0–1.8 THz is observed under bias exceeding 1.5–2.0 V. The emission band peak shifts linearly to higher frequency with the increasing bias. The effect is accounted for by spatially indirect electron transitions between states in the neighboring QWs.

Electroluminescence from AlGaAs/GaAs Quantum-Cascade Structures in the Terahertz Range

A new method of porous silicon fabrication is suggested which uses as a current source the potential difference arising between the silicon wafer and the platinum counter electrode immersed in an electrolyte solution. Addition of hydrogen peroxide to HF/ethanol electrolyte enables control over the current density in the process and fabrication of photoluminescent layers without an external current source.

Electrolytic Fabrication of Porous Silicon with the Use of Internal Current Source

Photoelectric and electroluminescent properties of silicon-〈porous silicon〉 structures with chemically deposited metal contacts were investigated. The large specific surface area of the contact and selective metal deposition only on the macrocrystalline elements of the structure provide better photoelectric performance of the photodiodes compared to the structures with evaporated contacts, especially in the short-wavelength spectral range. The obtained electroluminescence spectra are explained by metal-silicon barrier properties under forward bias and by double carrier injection into nanocrystallites under reverse bias.

Photoresponse and electroluminescence of silicon-〈porous silicon〉-〈chemically deposited metal〉 structures

The spectral characteristics of the photoresponse of heterostructures with layers of densely packed amorphous silicon nanoparticles produced by laser electrodispersion are studied. The structures exhibit rectifying properties. Annealing in air results in the appearance of silicon oxide nanoparticles in the layers and, in addition, there occurs partial crystallization of the nanoparticles. The spectral characteristics of the photoresponse of the heterostructures have a number of specific features. Compared with standard silicon photodiodes, the sensitivity spectra of the structures under study are shifted to shorter wavelengths, with the shift becoming more pronounced upon annealing. The structures with an annealed layer of nanoparticles exhibit sensitivity in the spectral range 350–600 nm, which exceeds by more than an order of magnitude the sensitivity of unannealed structures. This effect can be attributed to a transistor-like effect in the structure.

Olga M. Sreseli

Papers

On the Mechanism of Porous Silicon Formation

Electroluminescence from AlGaAs/GaAs Quantum-Cascade Structures in the Terahertz Range

Electrolytic Fabrication of Porous Silicon with the Use of Internal Current Source

Photoresponse and electroluminescence of silicon-〈porous silicon〉-〈chemically deposited metal〉 structures

Spectral features of the photoresponse of structures with silicon nanoparticles